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Metathesis and Isomerization Activity of Ruthenium Carbene Catalysts in Acyclic Diene Metathesis Polymerization


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METATHESIS AND ISOMERIZATION ACTIVITY OF RUTHENIUM CARBENE CATALYSTS IN ACYCLIC DIENE METATH ESIS POLYMERIZATION By FLORENCE C. COURCHAY A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLOR IDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 2005

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Copyright 2005 by Florence C. Courchay

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To my parents Elizabeth and Bernard, and grandparents Suzanne, Luce, Aim, and Yvon

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iv ACKNOWLEDGMENTS First and foremost, I would like to tha nk my parents Elizabeth and Bernard, and grandparents, Suzanne, Luce and Yvon. I would have never come this far without them. Their unconditional love and support have meant the world to me. They helped me become the person I am today; there are no word s to describe the respect, the love and the gratitude I feel. These past 4 plus years in graduate sc hool were one of the most rewarding, lifechanging experiences I have ever been thr ough, and none of it would have been possible without the help and guidance of my adviso r Professor Ken Wagener. Arriving in a foreign country is a scary th ing to take on, so I am most thankful for his support and understanding, his words of wisdom, his genuine concern, and his exciting take on science. I would also like to thank all th e members of my supervisory committee (Dr. John Reynolds, Dr. Mike Scott, Dr. Dan Ta lham, and Dr. Tony Brennan), for the time they invested in reading and discussing this document. My work here would not have been completed without the precious help of Dr. Ion Ghiviriga who spent many hours assisting me with NMR spectroscopy. I also wish to thank Dr. Khalil Abboud for his kind help with X-ray crystallography, and Dr James Boncella for helpful discussions. At the University of Florida, I thank all my coworkers on the polymer floor; particularly all the Wagener’s group member s past and present. Among them I would like to thank Dr. Ed Lehman for his expertise in catalysis, and James Leonard for his help with X-ray crystallography and hi s constant enthusiasm in the lab. Special thanks go to

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v John Sworen, my mentor and my friend, who he lped me get started in the lab and much further. I have learned from a tr ue master, I will be forever in debted to him. I would also like to thank my friends and classmates Tr avis Baughman and Piotr Matloka, for all the poking, the laughs, and all our discussions, chemis try-related or not. I will never forget our Eurotrip! This experience would not have been the same without Emilie Galand, Sophie Bernard, Merve Ertas, Kornelia Matloka, and Victor ia Broadstreet. Their friendship helped me through my ups and dow ns and I will miss them immensely, along with the shopping extravaganzas, the late-nig ht talks, and the party-dancing. Pauline Anselme, my roommate during my first year here, was also a wonde rful support as I was settling into my new life as a graduate stude nt. I also wish to thank Armando Coronado, my undergraduate student, for his tremendous help with all the boring stuff I delegated to him. Finally, I would not have succeeded without the support of Josh McClellan, who has been there for me in the worst and the best of times.

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vi TABLE OF CONTENTS page ACKNOWLEDGMENTS.................................................................................................iv LIST OF TABLES.............................................................................................................ix LIST OF FIGURES.............................................................................................................x ABSTRACT.....................................................................................................................xi v CHAPTER 1 INTRODUCTION........................................................................................................1 Introduction to Metathesis............................................................................................1 Early Catalyst Developments.......................................................................................2 Well-Defined Ruthenium Catalysts X2L(PR3)Ru=CHR’31...........................................4 First-Generation Grubbs Catalysts........................................................................4 Second-Generation Grubbs Catalysts....................................................................7 Mechanistic Considerations...................................................................................9 Role of the L-type ligand.............................................................................10 Role of the phosphine ligand (PR3)..............................................................11 Role of the halide ligand (X)........................................................................12 Role of the carbene ligand (R’)....................................................................12 Stereochemical aspects.................................................................................13 Catalyst Decomposition.......................................................................................14 Other Second-Generation Ruthenium Catalysts..................................................15 Introduction to ADMET Polycondensation................................................................17 Catalytic Cycle....................................................................................................17 Kinetics and Equilibrium Considerations............................................................18 Cyclization versus Linear Polymerization...........................................................20 Interchange Reactions and Mo lecular-Weight Distribution................................21 Utility of ADMET...............................................................................................22 Ruthenium Catalysis Applied to ADMET..................................................................24 Ruthenium 1st Generation....................................................................................24 Ruthenium 2nd Generation...................................................................................26 Limitations...........................................................................................................28 Hydrogenation.....................................................................................................29 Kinetics................................................................................................................29 Olefin Isomerization............................................................................................30

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vii 2 METATHESIS ACTIVITY AND STABILITY OF NEW GENERATION RUTHENIUM POLYMERIZATION CATALYSTS................................................32 Introduction.................................................................................................................32 Results and Discussion...............................................................................................35 Kinetic Assessment of Catalyst Activity.............................................................35 Thermal Stability of the Catalysts.......................................................................46 Further Comments on ADMET Polymerization.................................................46 Isomerization of the Olefin Bond........................................................................48 Conclusions.................................................................................................................52 Experimental...............................................................................................................53 General Considerations.......................................................................................53 X-ray Experimental.............................................................................................55 Synthesis of Complex [Ru]11.............................................................................56 Kinetic Study.......................................................................................................57 ADMET Polymerization of 1,9-decadiene..........................................................57 Self-Metathesis Dimerization..............................................................................57 Measurement of the Thermal Stabil ity of Complex [Ru]10 and [Ru]11.............58 3 THE UTILITY OF HOVEYDATYPE CATALYSTS IN ADMET CHEMISTRY: STERICS VS. ELECTRONICS........................................................59 Introduction.................................................................................................................59 Results and Discussion...............................................................................................61 Steric effect..........................................................................................................62 Electronic effect on H2IPr ligated complexes.....................................................65 Temperature effect...............................................................................................69 Conclusions.................................................................................................................70 Experimental...............................................................................................................71 General................................................................................................................71 X-ray Experimental.............................................................................................72 General Procedure for Ligand Synthesis.............................................................73 General Procedure for Carbene Exchange..........................................................73 Kinetic Study.......................................................................................................76 4 ISOMERIZATION BEHAVIOR UNDER METATHESIS CONDITIONS OF NHC-CONTAINING RUTHENIU M CARBENE CATALYSTS.............................77 Introduction.................................................................................................................77 Results and Discussion...............................................................................................79 Synthesis and Characterization............................................................................80 Isomerization Behavior with 1-Octene................................................................81 ADMET Polymerization.....................................................................................86 Conclusions.................................................................................................................87 Experimental...............................................................................................................88 General Considerations.......................................................................................88 Synthesis of Complexes [Ru]18-22.....................................................................89

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viii 5 UNDERSTANDING STRUCTURAL ISOMERIZATION DURING RUCATALYZED OLEFIN METATHESIS: A DEUTERIUM LABELING STUDY..92 Introduction.................................................................................................................92 Results and Discussion...............................................................................................95 Mechanistic Studies.............................................................................................95 Other Mechanistic Considerations....................................................................100 Kinetic Studies...................................................................................................101 Design, Synthesis and Characterization............................................................102 Thermal Stability...............................................................................................106 Catalytic Behavior Unde r Metathesis Conditions.............................................108 Conclusions...............................................................................................................117 Experimental.............................................................................................................118 General..............................................................................................................118 Deuterium Labeling Study with Allyl1,1-d2-methyl ether...............................118 X-ray Experimental...........................................................................................123 Synthesis of Complex [Ru]23...........................................................................124 Thermal Decomposition of Complex [Ru]23....................................................129 Thermal Decomposition of Methylidene Complex [Ru]23’.............................130 Isomerization Experiment.................................................................................130 6 UNDERSTANDING THE EFFECT OF ALLYLIC METHYLS IN OLEFIN CROSS-METATHESIS...........................................................................................131 Introduction...............................................................................................................131 Results.......................................................................................................................1 33 Schrock’s Molybdenum Catalyst......................................................................134 Grubbs’ Ruthenium Catalysts............................................................................138 Discussion.................................................................................................................143 Conclusion................................................................................................................144 Experimental.............................................................................................................145 General..............................................................................................................145 Materials and Methods......................................................................................146 Bulk Polymerization Experiment......................................................................146 NMR Catalyst Experiments...............................................................................146 Synthesis of EP3 Monomer (6-2 to 6-9)............................................................147 APPENDIX CRYSTALLOGRAPHIC DATA...............................................................151 LIST OF REFERENCES.................................................................................................168 BIOGRAPHICAL SKETCH...........................................................................................181

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ix LIST OF TABLES Table page 2-1 Initial rates for complexes [Ru]1-[Ru]11 during the ADMET oligomerization of 1,9-decadiene............................................................................................................35 2-2 Bond Lengths and Angles for Complexes [Ru]9 and [Ru]11 .................................42 2-3 Half-lives for the decomposition of complexes [Ru]10 and [Ru]11 .......................46 2-4 Polymerizations of 1,9-decadiene at 55 C for 120 h..............................................47 3-1 Initial rates in DP s-1 for catalysts [Ru]9 [Ru]11-13 ...............................................63 3-2 Initial rates in DP s-1 for catalysts [Ru]11-[Ru]16 ..................................................67 4-1 Percentage of isomers during the metathesis of 1-octene........................................82 4-2 Results for the ADMET polymeri zation of 1,9-decadiene at 55 C for 120 h.........87 5-1 Selected bond lengths () a nd angles (deg) for complexes [Ru]4 and [Ru]23 .....105 A-1 Crystal data and st ructure refinement for [Ru]11 ..................................................152 A-2 Atomic coordinates ( x 10 4 ) and equivalent isotropic displacement parameters ( 2 x 10 3 ) for [Ru]11 .............................................................................................153 A-3 Crystal data and struct ure refinement for [Ru]13...................................................157 A-4 Atomic coordinates ( x 10 4 ) and equivalent isotropic displacement parameters ( 2 x 10 3 ) for [Ru]13 .............................................................................................158 A-5 Crystal data and st ructure refinement for [Ru]16 ..................................................161 A-6 Atomic coordinates ( x 10 4 ) and equivalent isotropic displacement parameters ( 2 x 10 3 ) for [Ru]16 .............................................................................................162 A-7 Crystal data and st ructure refinement for [Ru]23 ..................................................165 A-8 Atomic coordinates ( x 104) and equivalent isotropi c displacement parameters (2x 103) for [Ru]23 ...............................................................................................166

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x LIST OF FIGURES Figure page 1-1 Olefin metathesis........................................................................................................2 1-2 Mechanism of olefin metathesis.................................................................................3 1-3 Schrock’s alkylidene catalyst.....................................................................................3 1-4 Synthesis of the first generation ruthenium carbene catalysts...................................5 1-5 Alternative route to ruthenium carbene catalysts.......................................................5 1-6 Other well-defined, metathesis-active L2X2Ru=CHR ruthenium carbene catalysts...................................................................................................................... 7 1-7 Synthesis of 2nd generation ruthenium metathesis catalysts.......................................9 1-8 Mechanism of metathesis cat alyzed by ruthenium alkylidene.................................10 1-9 Proposed conformations for olefin -bound Grubbs catalyst intermediate.................13 1-10 Stereochemical preference of the ’-disubstituted metallacyclobutane...............14 1-11 Decomposition products of ru thenium carbene complexes.....................................15 1-12 Other second-generation catalysts............................................................................16 1-13 Hoveyda’s “release-return” mechanism...................................................................16 1-14 Mechanism of productive ADMET..........................................................................18 1-15 Variation of molecular weight with conversion in step polymerization..................19 1-16 Formation of cyclics in ADMET.............................................................................21 1-17 Trans-metathesis in ADMET: a) Interchange with a polymeric alkylidene; b) Interchange with the methylidene from a productive step.......................................22 1-18 Examples of polymeric structures obtained via ADMET polymerization...............23

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xi 1-19 Structural olefin isom erization of 1-octene..............................................................30 1-20 Olefin isomerization du ring ADMET polymerization.............................................31 1-21 Olefin isomerization mechanism: a) -allyl mechanism. b) metal-hydride addition-elimination mechanism..............................................................................31 2-1 Metathesis catalysts..................................................................................................33 2-2 Degree of polymerization (DP) vs. time curves for complex [Ru]1 [Ru]4 and [Ru]10 using 450:1 monomer:catalyst ratio at 30 C...............................................36 2-3 Comparison of the activity of complex [Ru]10 at three different temperatures using a 450:1 monome r:catalyst ratio......................................................................37 2-4 Degree of polymerization (DP) vs. time curves for complexes [Ru]4 and [Ru]10 using 450:1 monomer:catalyst ratio at 45 C...........................................................38 2-5 Degree of polymerization (DP) vs. time data for complex [Ru]10 using 900:1 monomer:catalyst ratio.............................................................................................39 2-6 Degree of polymerization (DP) vs. time data for complex [Ru]9 using 450:1 monomer:catalyst ratio.............................................................................................40 2-7 Synthesis of complex [Ru]11 ...................................................................................41 2-8 ORTEP Diagram of complex [Ru]11 Thermal ellipsoids are drawn at the 30% probability level........................................................................................................42 2-9 Degree of polymerization (DP) vs. time data for complex [Ru]11 using 450:1 monomer:catalyst ratio.............................................................................................43 2-10 Proposed intermediate of complex [Ru]9 during the metathesis of dipentenyl ether.......................................................................................................................... 44 2-11 Degree of polymerization (DP) vs. time data for complexes [Ru]9 and [Ru]11 using dipentenyl ether as the m onomer (450:1 monomer:catalyst).........................45 2-12 Composition of olefin mixture as a function of time for reaction of [Ru]10 with 1-octene at 30 C.......................................................................................................48 2-13 Proposed mechanism for isomerization of 1-octene................................................49 3-1 Olefin metathesis catalysts.......................................................................................59 3-2 Synthesis of complexes [Ru]9 [Ru]11-16 ..............................................................62 3-3 Degree of polymerization (DP) vs. time data for H2IMes versus H2IPr containing catalysts using 450:1 monomer:catalyst ratio at 45oC.............................................64

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xii 3-4 Degree of polymerization (DP) vs. time data for complexes [Ru]11-[Ru]16 using 450:1 monomer:catalyst ratio at 45oC............................................................66 3-5 Degree of polymerization (DP) vs. time data for complexes [Ru]11-[Ru]16 using 450:1 monomer:catalyst ratio at 60oC............................................................69 4-1 Examples of competitive isomerizati on reactions during olefin metathesis............78 4-2 Effect of isomerizati on during ADMET polymerization.........................................79 4-3 Synthesis of [Ru]18-22 ............................................................................................81 4-4 GC chromatogram of the product mixtur e from the self-metathesis of 1-octene: (A) with [Ru]21 at 25oC; (B) with [Ru]22 at 25oC. Retention scale is in minutes.83 4-5 Postulated mechanism of the isomerization process................................................84 4-6 Decomposition products of ru thenium carbene complexes.....................................85 5-1 Olefin metathesis catalysts.......................................................................................92 5-2 Decomposition products of ru thenium carbene complexes.....................................94 5-3 Deuterium shift with a -allyl hydride mechanism..................................................96 5-4 Deuterium shifts with a metal-hydr ide addition-elimination mechanism................97 5-5 Products of the reaction of allyl -1,1-d2-methyl ether with complex [Ru]4 .............98 5-6 Incorporation of hydrogen at C1 by reversible isomerization..............................100 5-7 -H abstraction-elimination mechanism pr oposed for allyl alcohol isomerization by [Ru]1 .................................................................................................................101 5-8 Total concentration of 4-1 versus time. Conditions: 2.3M solution of 4-1 in C6D6 with 0.2 mol% of [Ru]4 at 35oC for 8 hours................................................102 5-9 Synthesis of Complex [Ru]23a...............................................................................103 5-10 X-Ray structure of complex [Ru]23 A) ORTEP Diagram of complex [Ru]23 B) Superposition of the crys tal structures of complex [Ru]4 (orange) and [Ru]23 (blue)..........................................................................................................105 5-11 NMR spectrum of neat 1-octe ne with 0.5 mol% of complex [Ru]23 after 4h at 35oC and 20h at 55oC: a) 1H NMR; b) 2H NMR....................................................109 5-12 Formation of 1-octene isomers catal yzed by a ruthenium deuteride complex.......111 5-13 Complex [Ru]24 ....................................................................................................112

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xiii 5-14 2H NMR spectrum of neat allyl methyl ether with 0.5 mol% of complex [Ru]23 after 4h at 35oC.......................................................................................................113 5-15 Orientation by oxygen precoordination into -H-elimination at the C-3 position.114 5-16 2H NMR spectrum of neat 3,3-dimethyl butene with 0.5 mol% of complex [Ru]23 after 4h at 35oC.........................................................................................115 5-17 1H and 13C chemical shifts assignment in compounds 5-2 cis 5-2 trans 5-3 cis and 5-3 trans ...........................................................................................................119 5-18 Molar fractions of compounds 5-2 cis 5-2 trans 5-3 cis and 5-3 trans in the product (under the compound number) and mo lar fraction of deuteration at each position.a.................................................................................................................119 5-19 Expansion of the proton spectrum, the CH2 region................................................120 5-20 Expansion of the gHMBC spectrum......................................................................121 5-21 Expansion of the carbon spectrum, C3 region.......................................................122 6-1 Olefin metathesis catalysts.....................................................................................131 6-2 Synthesis of monomer 6-9 ......................................................................................133 6-3 ADMET reaction of 6-9 .........................................................................................134 6-4 Cross metathesis of 3-methyl-1-pentene catalyzed by [Mo]1 ................................134 6-5 Catalytic cycle for the metathesis of 3-methyl-1-pentene with a molybdenum carbene catalyst......................................................................................................135 6-6 1H NMR alkylidene region for the reaction of 3-methyl-1-pentene. (a) With catalyst [Mo]1 ; (b) with [Ru]1 ; (c) with [Ru]4 .....................................................137 6-7 CM of 3-methyl-1-pentene catalyzed by [Ru]1 or [Ru]4 ......................................138 6-8 Catalytic cycle for the metathesis of 3-methyl-1-pentene with ruthenium carbene catalyst [Ru]1 or [Ru]4 ..........................................................................................139 A-1 Crystal structure of [Ru]11 (H2IPr)(Cl)2Ru=CH(2 -i PrO)C6H4............................151 A-2 Crystal structure for [Ru]13 (H2IPr)(Cl)2Ru=CH(2 -i PrO)(5-NO2)C6H3..............156 A-3 Crystal structure for [Ru]16 (H2IPr)(Cl)2Ru=CH(2 -i PrO)(5-OCH3)C6H3...........160 A-4 Crystal structure for [Ru]23 (H2IPr)PCy3(Cl)2Ru=CHPh....................................164

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xiv Abstract of Dissertation Pres ented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy METATHESIS AND ISOMERIZATION ACTIVITY OF RUTHENIUM CARBENE CATALYSTS FOR ACYCLIC DIENE METATH ESIS POLYMERIZATION By Florence C. Courchay December 2005 Chair: Kenneth Wagener Major Department: Chemistry Acyclic diene metathesis (ADMET) polymer ization being a step condensation, the catalyst requires long lifetimes and a robus tness that allows it s reactivity at high temperatures. Accordingly, several novel ruthenium metathesis catalysts were investigated through a kinetic study conducte d under ADMET polymerization conditions. Their kinetic behavior was co mpared to the classical 1st and 2nd generation Grubbs’ complexes ( [Ru]1 and [Ru]4 respectively) at various temp eratures. Among the catalysts tested, Hoveyda-type systems were steric ally and electronically modified. The complexes containing bulkier N-heterocyclic carbene (NHC) ligands consistently result in faster initial ADMET rates than any othe r existing phosphine or Hoveyda complexes. Electronic modifications are mo stly dominated by the steric effect of the NHC ligand, especially at low temperatures. Although th ese complexes show promising longevity and stability in small molecule chemistry, th ey do not produce higher molecular weight polymer than catalyst [Ru]4

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xv Soon after, olefin isomerization emerged as an undesirable side reaction induced by NHC-containing catalysts during metathesis proce sses. With fast initiators, isomerization occurs after the metathesis re action, while it becomes competit ive with slower initiators. After several attempts to reduce isomer ization by designing new catalytic systems featuring different phosphine, carbene, or NHC ligands, a comprehensive mechanistic study was undertaken. From a deuteriu m labeling experiment monitored by NMR spectroscopy, olefin isomerization promoted by catalyst [Ru]4 under metathesis conditions was determined to operate via a metal-hydride addition-elimination pathway. Further deuteration of the catalyst itself, by synthesis of analogue [Ru]23 revealed the existence of an exchange process between the NHC ligand and the substrate, suggesting the presence of a ruthenium dihydride. However, the active species could not be identified spectroscopically. Finally, [Ru]1 [Ru]4 and Schrock’s molybdenum catalyst [Mo]1 were individually examined during the metathesis of olefins containi ng allylic methyls. Conversions in cross-metathesis were lim ited to 50% with all catalysts, while ADMET only produced the dimer of the corresponding diene. These results are directly correlated to the orientation of the substrate’s bulk during the metallacyclobutane formation, the alkyl branch being adjacent to the metal center in the case of the molybdenum catalyst [Mo]1 and opposite to it in the case of ruthenium catalyst [Ru]1 and [Ru]4

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1 CHAPTER 1 INTRODUCTION Introduction to Metathesis Olefin metathesis has become a powerful tool for the formation of carbon-carbon bonds.1,2 Discovered inadvert ently in the mid-1950s,3,4 it was not until 1967 that Calderon5,6 recognized the term “olefin metathesis ”, which is understood as the metalcatalyzed redistribution of carbon-carbon double bonds. In 2 005, the metathesis reaction was distinguished by the Nobel prize in chemistry, awarded to Prof. Yves Chauvin who unveiled the mechanism, and Prof. Rich ard Schrock and Prof. Bob Grubbs who developed the catalysts that expanded the vers atility of the reaction. This transformation enjoys a variety of applica tions (Figure 1-1). Ring-closing metathesis (RCM) is the intramolecular reaction of an acyclic diene into an unsatur ated ring. This reaction has particularly been taken adva ntage of in the pharmaceutical industry, where the synthesis of complex cyclic systems used to require several, often tedious steps.7-11 The crossmetathesis (CM) reaction involves the scra mbling of substituent groups between two olefins to form two new products.12-15 The extensive use of cross-metathesis in organic synthesis has also been applied to green ch emistry to convert oleochemical feedstocks into valuable chemical products. Ring-ope ning metathesis is the opening of an unsaturated cycle by cross-coupl ing with a mono-olefin, to form a substituted acyclic diene.16-18 Metathesis has also been activel y used in the production of polymer molecules, either through ring-openi ng metathesis polymerization (ROMP)19,20 or acyclic diene metathesis (ADMET) polymerization.21,22 While ROMP is a chain-addition type of

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2 polymerization driven by the release of ring-strain in th e cyclic monomer, ADMET is a step-condensation whose equili brium is driven by the rel ease of a small molecule, ethylene. ADMET RCM ROMP -C2H4-C2H4X X Xnnn X + R X R R1R2+ CM R1R2+ ROM Figure 1-1. Olefin metathesis Early Catalyst Developments For a long time, the scope of the reaction was limited by its insufficient catalyst performance, using multicomponent homogene ous and heterogeneous systems such as WCl6/Bu4Sn, WOCl4/EtAlCl2, MoO3/SiO2, and Re2O7/Al2O3. It was not until Hrisson and Chauvin23 proposed that the mechanism involved the interconversion of an olefin and a metal alkylidene through a me tallacyclobutane interm ediate that well-defined catalysts were developed, considerably expanding the ra nge of metathesis applications (Figure 12). His work in metathesis was awar ded the Nobel Prize of Chemistry in 2005.

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3 [M] R R2R1[M] R R2R1[M] R R2R1 Figure 1-2. Mechanism of olefin metathesis The first tantalum24,25 and tungsten alkylidenes26 led to the development of the highly active molybdenum and tungsten al kylidenes of the general formula (NAr)(OR’)2M=CHR, where the most widely used is better known as molybdenum Schrock catalyst [Mo]1 with Ar=2,6-iPr2-C6H3, R=CMe2-Ph, and R’=C(CH3)(CF3)2 (Figure 1-3).27,28 M= Mo [Mo]1 W [W]1 M N Ph O O F3C CF3F3C CF3 Figure 1-3. Schrock’s alkylidene catalyst These catalysts were referred to as “well-de fined” in contrast to the “ill-defined” classical catalysts since the cat alytically active species is spectroscopically identifiable. Molybdenum complexes are actua lly more active than their tungsten analog that tend to be trapped into a metallacyclobutane form. Catalyst [Mo]1 allowed the metathesis of terminal and internal olefins, the ROMP of low-strain monomers, as well as the RCM of sterically demanding and el etron-poor substrates.27-30 Schrock carbenes are regarded as X2 ligands that render the metal of these comp lexes in its higher oxidation state (+VI). Thus, these early transition-metal catalys ts have moderate-to-poor functional group tolerance because of the high oxophilicity of the metal centers which also renders them extremely sensitive to oxygen and moisture.

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4 Well-Defined Ruthenium Catalysts X2L(PR3)Ru=CHR’ 31 The impetus for developing olefin metathesis catalysts of the late-transition metals was to make less electrophilic complexes that would preferentially react with olefins in the presence of heteroatomic functionali ties. With this in mind, ruthenium was introduced to metathesis chemistry as RuCl3(hydrate), which effectively promoted the ROMP of norbornene in aqueous solu tion, albeit at low initiation rates.32 Further catalyst screening revealed that Ru(H2O)6(tos)2 (tos= p -toluenesulfonate) had even shorter initiation times and could ring -open functionalized norbornene.33 However, these illdefined ruthenium complexes were limited to the ring-opening of strained olefins, and therefore did not work for ADMET. First-Generation Grubbs Catalysts The real breakthrough came in 1992, with th e synthesis of the first, well-defined, metathesis active ruthenium alkylidene complex Cl2(PPh3)2Ru=CHCHCPh2, based on the same methodology as their tungs ten analogs, starting with RuCl2(PPh3)3 and 3,3diphenylcyclopropene (Figure 1-4).34 The alkylidene proton is visible by 1H NMR as two overlapping triplets at 17.94 ppm. In contrast with Sc hrock’s catalysts, the carbene unit is considered a neutral L-type ligand that renders the metal center in a second oxidation state, although it is sometimes shown as a Ru(IV). The carbene moiety is believed to be an intermediate between a Schroc k and a Fisher type carbene. Phosphine exchange with the larger and more basic PCy3 further enhanced activity as complex Cl2(PPh3)2Ru=CHCHCPh2 was able to promote the ROMP of lower strain monomers and the metathesis of acyclic olefins.35 Even though the catalytic activity was far lower compared to the early transition me tal carbene complexes, the ease of use and the newly-found functional group to lerance of this first genera tion of ruthenium catalysts

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5 rendered them attractive for a variety of a pplications. Of 250 publications on olefin metathesis that appeared between 19 90 and 1999, 75% reported using ruthenium metathesis catalysts. Their synthesis wa s industrialized by using diazoalkanes as the alkylidene precursor, which also produced complex [Ru]1 after phosphine exchange, the most widely used catalyst of th is series and better known as “1st generation Grubbs’ catalyst” (Figure 1-4).36,37 The key to this synthesi s is the insolubility of [Ru]1 in acetone, which allows for a simple and convenient wash of the crude product right out of the reaction pot. The use of rather explosive diazo compoun ds can be bypassed through the rearrangement of vinyl or propar gyl halides in the presence of Ru(H)(H2)Cl(PR3)2 (Figure 1-5). RuCl2(PPh3)3PhPh N2Ph RuPh Ph Cl Cl PPh3PPh3RuPh Ph Cl Cl PCy3PCy3Ru Cl Cl PPh3PPh3Ph Ru Cl Cl PCy3PCy3Ph +PCy3PPh3+PCy3PPh3[Ru]1 Figure 1-4. Synthesis of the first ge neration ruthenium carbene catalysts [RuCl2(COD)]xCl RuCH3CH3Cl Cl PCy3PCy3Ru Cl Cl PCy3PCy3+ 2 PCy3H Cl Ru Cl Cl PCy3PCy3H2H H2 (1.5 atm), NEt3sec -BuOH, 80oC Figure 1-5. Alternative route to ruthenium carbene catalysts

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6 These (PR’3)2Cl2Ru=CHR complexes typically exhib it a distorted square-pyramidal geometry with the alkylidene moiety in th e apical position, and a trans bi(phosphine) ligand arrangement at the base of the s quare pyramid. Although in most cases the carbene is coplanar to the P-Ru-P plane, in the case of bulky phosphine, it is twisted away, almost perpendicular to the P-Ru-P plane. The remarkable tolerance of these catalysts allowed scientists to apply a variety of modifications to the ligand sphere for specifi c conditions. Hence, their use was extended to ROMP and RCM in aqueous media by substituting the PCy3 ligands to water-soluble phosphine such as Cy2P(CH2)2NMe3 +Cl-.38-40 Several supported models were also introduced for heterogeneous catalysis.41-45 Other variations have been reported, including heterobimetallic complexes46 and multidentate ligands, such as Shiff bases,47-48 arenes,49-52 and bidentate phosphines,53,54 in an attempt to tune in activity and/or stability (Complexes [Ru]A-D in Figure 1-6). Pyridi ne coordinated complex [Ru]E features a remarkably high activity but a short lifetime,55 while the alkoxy-chelated complex [Ru]F better known as Hoveyda catalys t, shows exceptional stability, which allow its use in reagent grade solvents and/or in air.56 These first-generation of ruthenium catalyst are air stable and can be handled on the benchtop without significant loss of activity. Solutions prepared in reagent-grade solvents decompose slowly (over several hou rs) through different un clear processes, the primary pathway involving a bimolecular decomposition. Several mechanistic studies that suggested the formation of a highly active mono(phosphine) inte rmediate during the catalytic cycle triggered the development of second-generation ruthenium catalysts.57-63

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7 RuPh Ph ClCl P P Ru Cl O PCy3Ph ButBut tButBu N NO2Pri iPr O2N Ru. .Ph Ph Cl R3P+ X-Rh Ru Cl Cl Cl Ph L Ru PCy3Cl Cl O Ru PCy3Cl Cl N Ph [Ru]A[Ru]B [Ru]C [Ru]D [Ru]E [Ru]F Figure 1-6. Other well-def ined, metathesis-active L2X2Ru=CHR ruthenium carbene catalysts Second-Generation Grubbs Catalysts Attempts to increase the concentratio n of the monophosphine complex by addition of phosphine traps (such as HCl or CuCl) only led to fast decomposition of the catalyst. Even though the activity was greatly enhan ced, increasing the c oncentration of the monophosphine complex was not the way to improve catalytic e fficiency. After Herrmann et al.64 reported complex [Ru]2 (Figure 1-7), researchers turned to Nheterocyclic carbene (NHC) ligands, someti mes referred to as “phosphine mimics” for their strong -donor and weak -acceptor characteristics. Unlike most organic carbenes, these NHC ligands, pioneered by Arduengo65-68 and now used extensively in organometallic chemistry, are ground-state singlet s, although this electronic structure is not fully understood. In any case, these NHC ligands are stronger -donors and much less labile than phosphines. So, while complex [Ru]2 showed little improvement in activity, a mixed ligand complex would ove rcome the problem both ways: promote

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8 phosphine dissociation by trans effect, and stabilize the el ectron-deficient intermediate through the steric bulk and electron donation of the remaining NHC ligand. Indeed, complex [Ru]3 containing a mesityl-substituted NHC ligand (IMes) exhibited greatly enhanced metathesis activity and stability to ward air and a variety of functional groups (Figure 1-7).69,70 Soon after, the saturated version [Ru]4 was discovered by the Grubbs71 group to be even more reactiv e, albeit less stable than [Ru]3 The 1,3-dimesityl-4,5dihydroimidazol-2-ylidene (IMesH2) ligand was thought to be more basic than its C-C unsaturated counterpart because of the reduced conjugation throughout the NHC ring. However, this carbene is not stable in air, and must be formed in situ, either from the imidazolium salt and a base, or throu gh the chloroform adduct. Complex [Ru]3 can then be isolated after a methanol wash and colu mn chromatography, if further purification is desired. The methanol wash, which sometime s contaminates the catalyst with hydrides, can be avoided by running the reaction in anhydrous hexanes.72 These NHC-containing complexes exhibit th e same distorted square-pyramidal geometry. But while two carbene fragments ar e present, they display different Ru-C distances (e.g. in [Ru]2 Ru-C(carbene)=1.841(11), Ru -C(L)= 2.069(11) ).These important metrical parameters clearly dist inguish two metal-carbe ne interactions: a covalently bound benzylidene and a datively bound N-heterocyclic carbene, the latter acting as a two-electron donor, al so reflecting their very poor -acceptor character. The NHC group also appear to be more sterically demanding than PCy3.73 Finally, this new class of catalysts combines the activity of early transition metal complexes and the functional group tolerance of [Ru]1

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9 Ru PCy3Cl Cl Ph Ru PCy3Cl Cl Ph PCy3N N Ru PCy3Cl Cl Ph N N Ru Cl Cl Ph N N CyCy NN Cy Cy [Ru]2 [Ru]3 [Ru]4 [Ru]1 N N MesMes..1 equiv. N N CyCy..2 equiv. N N MesMes H+ Cl-1. KOtBu 2. [Ru]1 in C6H6N N MesMes CCl3 Figure 1-7. Synthesis of 2nd generation ruthenium metathesis catalysts Mechanistic Considerations Mechanistic studies started shortly afte r the discovery of the first ruthenium alkylidene Cl2(PPh3)2Ru=CHCPh2 and played a major role in the development of the 2nd generation of catalysts. For a while, scien tists believed in an associative mechanism where the olefin first coordinated to the metal center to form a stable 18 ecomplex, followed by phosphine dissociation and subsequent formation of the metallacyclobutane.57 Yet, Grubbs et al. reported in 200174,75 that the substitution of a phosphine ligand with and olefinic substrate actu ally proceeds in a dissociative fashion to

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10 generate the four-coordinate 14 especies X2LRu=CHR’ ( IB ) (Figure 1-8). The recoordination of free phosphine is competitive with substrate binding and the catalytic turnovers that the active sp ecies carries out before be ing “quenched” with free PR3 depends on a variety of f actors, detailed below. Ru L X R' X PR3[Ru] R' k2+ olefin olefin k-2[Ru] R' R'k3k-3[Ru] R' R' [Ru] R' R' ( IC ) ( IE ) ( IC ) ( IB ) k2+ olefin k-2olefin k-3k3[Ru] = X2LRu k1PR3+ PR3k-1 Figure 1-8. Mechanism of metathesis catalyzed by ruthenium alkylidene Role of the L-type ligand Conversely to the general idea spread at the time wh en NHC-containing catalysts were developed, phosphine dissociation is mu ch slower than with the bis-phosphine systems. The high activity of [Ru]4 over [Ru]1 for example, can be understood by considering the ratio of phosphine rebinding to olefin binding rates ( k-1/k2), which virtually indicates metathesis activity, the sm aller ratio, the more active catalyst. Data show that the IMesH2 ligand increases selectivity for bi nding olefinic substrates over free phosphine by 4 orders of magnitude.75 This increase in selectivity is due to the excellent electron donation and weak -acidity of NHCs, which promot es and stabilizes metal-toolefin back-bonding to a greater extent than the phosphine ligand, and therefore favors

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11 the formation of intermediate IC .76 For example, [Ru]4 is more active than [Ru]3 in the ROMP of COD, IMesH2 being a better donor than IMes.77,78 In addition, the electron donation of NHC ligands is expected to accele rate the oxidative addition required for the metallacyclobutane formation. The reason for the dramatic decrease in initiation exhibi ted by these NHCcontaining catalysts is sti ll debated. Although both PCy3 and IMesH2 are large ligands, this could be due to a steric effect, as the di stribution of the steric bulk is dramatically different in each case.78 The differential steric distri bution could be the source of stabilizing or destabilizing interactions that would change the activ ation energy required for phosphine loss. An electronic effect co uld also explain these rates difference by considering the weaker -acidity of NHCs compared to phosphines. While the two phosphine ligands have to compete for Ru-P -back bonding in the bis-phosphine complex, the ruthenium-phosphine bond is rein forced in the NHC-containing complex by stronger -backbonding. Unfortunately this is not reflected by the crystal structures where Ru-PCy3 bond distances barely change upon substitution of one PCy3 with IMes (Ru-P= 2.4097(6) and 2.4221(6) in [Ru]1 and Ru-P= 2.419(3) in [Ru]3 ).70 Role of the phosphine ligand (PR3) Changing the phosphine ligand has a dramatic effect on both catalyst initiation and catalyst activity. However, the effect is inverted from the first generation to the second generation of catalysts. Indeed, bis-phos phines complexes are re ndered more active by larger and more electron donating phosphine s, whether it promotes dissociation by trans effect or it helps stabilize the 14 emetallacyclobutane.57,79 On the other hand, less

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12 electron donating phosphines, hence more labi le, tend to increase the activity of NHCcontaining systems by accelerating the dissociation step.75 Role of the halide ligand (X) While larger halides increase initiation rates by forcing phosphine dissociation, the overall activity suffers from the increased bulk around the metal center. For example, with both [Ru]1 and [Ru]4 moving from the di-chloride to the di-iodide complex results in a 100-fold increase in k-1/k2.75 Because the conformation of intermediates B and C are still unclear at this point, th e reasons behind this large shift remain speculative. One possible explanation involves the suggestion that olefin coordination requires a trans isomerization of the X-type ligands, whic h would be less favorable with larger Xligands.57 Role of the carbene ligand (R’) The effect of the carbene ligand is signi ficant because unlike the other ligands, it can change throughout the metathesis reaction. For example ROMP reactions produce an alkylidene complex that becomes the propa gating species, while RCM, CM and ADMET generate a methylidene after the first turnove r. Sterically bulk y and electron-donating R1 groups (e.g. alkyls) lead to hi gher initiation rates because th ey more effectively promote phosphine dissociation.75 In contrast, small and electroni cally neutral groups (e.g. H) are less effective at labilizing the phosphine ligand.80 Indeed, the methylidene complexes, especially those containing NHC ligands, are extremely poor in itiators at room temperature. While the methylidene of [Ru]4 remain an active metathesis catalyst in its phosphine-free form, it is virtually incapable of re-entering the metathesis cycle once trapped with free-PCy3.

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13 Stereochemical aspects As mentioned above the conformation of the olefin-bound complex is still unidentified because these interm ediate are not isolable or observable spectroscopically. Until recently, the nature of the metallacycl obutane was the subject of a theoretical debate, whether it was a reaction intermediate61 or a transition state,81-83 when Piers et al.84 finally isolated a ruthenacyclobutane as a C2v structure. Grubbs and others61,85 have shown that the olefin may bind either trans to a halide (X), which necessitates a rearrangement ( IC-a and IC-b in Figure 1-9), or trans to the L-type ligand ( IC-c ). Although in some cases there seems to be more evidence for intermediate IC-a ,86 the symmetrical C2v structure of the metallacyclobutane suggests that IC-c is the correct conformation. L Ru X R1X IC-a L Ru X R1X L Ru X R1IC-bIC-c X R1Ru L X X metallacyclobutane Figure 1-9. Proposed conformations for olefin-b ound Grubbs catalyst intermediate In the case of monosubstituted dienes, and probably as a general rule, ’disubstituted metallacyclobutane seem to be kinetically favor ed, within the limits of nonsterically demanding ol efins (Figure 1-10).80 This preference is more likely the result of the greater stabilization of the metal cente r by electron-donating substituents in an position rather than in a -position. Conversely, this c onformation only leads to nonproductive metathesis suggesting that the energy difference between the 2 metallacyclobutane is not enough to prevent th e metathesis reaction. However, this may not apply in the case of st erically demanding olefins.

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14 L Ru X R'X [Ru] R' R [Ru] R' R R R L Ru X R'X R L Ru X CH2X R R' Preferred NonProductive + + Productive Figure 1-10. Stereochemical preference of the ’-disubstituted metallacyclobutane Catalyst Decomposition Even though a substantial am ount of effort has been devoted to the understanding of decomposition pathways in solution and unde r metathesis conditions, this area still comports many unanswered questions The thermal decomposition of [Ru]1 has been proposed to occur via phosphine dissociation followed by bimolecular coupling of two 4coordinate ruthenium fragments, the addition of phosphine slowing down the decomposition rate.87 Conversely, the decomposition of the methylidene follows firstorder kinetics which could be the result of an intramolecular C-H activation of an Lligand. In general, NHC-containing comple xes exhibit significantly improved thermal stabilities compared to their bisphosphine counterparts. For example, [Ru]3 shows no sign of decomposition after 1h at 100oC in toluene, while in th e same conditions 75% of [Ru]1 has already decomposed.88 This increased stability is a combination of the stabilizing effect of the NHC ligand on the 14eintermediate a nd of the reduced extent of phosphine dissociation. Nevert heless, only one compound has been identified so far, bimetallic hydride complex [Ru]5 from the thermal decomposition of [Ru]4 methylidene analog.89

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15 Others have studied the be havior of these catalyst under different conditions and isolated other decomposition product s. For example, carbonyl hydrides [Ru]6 and [Ru]7 (Figure 1-11) first isolated during the decomposition of alkoxycarbene complexes,90,91 were identified as products of the reaction of [Ru]1 and [Ru]4 with primary alcohols in the presence of a base.72,92-94 However, the mechanism is still at a speculative stage. Noteworthy, [Ru]8 product of the C-H activation of a mesityl group, was isolated during the synthesis of [Ru]4 but was never observed as a proper decomposition product of [Ru]4 .72 Ru L Cl OC H PCy3[Ru]6 L = PCy3N N CRu Cl Cl Ru N N Mes Mes H [Ru]5 [Ru]7 L = IMesH2N N Cl Ru OC PCy3[Ru]8 Figure 1-11. Decomposition products of ruthenium carbene complexes Other Second-Generation Ruthenium Catalysts A variety of related second-ge neration catalysts have been investigated in olefin metathesis. Hoveyda et al.95 reported a second-genera tion tethered catalyst [Ru]9 which turns out to be a faster initiator than [Ru]4 (Figure 1-12). Even though [Ru]4 and [Ru]9 generate the same propagating sp ecies after a single turnover, [Ru]9 afford considerably higher yields with electron-defi cient olefins, which is proba bly due to the absence of a strong donor ligand.96-98 This observation emphasizes th e importance of ligand rebinding during the metathesis cycle. Bis-pyridine complexes provides similar results, and is often used as an intermediate for phosphine exchange.99

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16 Several modifications of the NHC ligands have also been implemented. Among those [Ru]10 was reported to catalyze terminal olefins much faster than [Ru]4 thanks to the increased bulk of its NHC ligand exerting a stronger steric pressure on the carbene and therefore prompting phosphine dissociation.100,101 Ru Cl Ph N Cl N N Ru Cl O Cl N N N X XX= H, Br[Ru]9Ru Cl Ph PCy3N N Cl[Ru]10 Figure 1-12. Other second-generation catalysts The isopropoxystyrene ligand of [Ru]9 represents an ideal anchor for heterogeneous catalysis applications,102-107 and further ligand modi fication by substitution of electronic and steric groups.108,109 In addition to their extr eme stability towards air and moisture, the release-return mechanism of these Hoveyda-type catalysts confers them an interesting recyclability (Figure 1-13).95 Ru L Cl Cl Ru L CH2Cl Cl Ru Cl O Cl LORu Cl O Cl LO"release""return" Figure 1-13. Hoveyda’s “r elease-return” mechanism

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17 Introduction to ADMET Polycondensation Before continuing into catalyst considerat ions, it is important to understand the requirements of a polycondensation type of r eaction, such as ADMET. This next section will focus on ADMET polymerization, its characteristics and requirements for catalyst modeling, and its applications for polymer synthesis following the development of ruthenium carbene catalysts. Catalytic Cycle ADMET is essentially the self metathesis of a diene to form high polymer, and therefore operates through the Chauvin mech anism. The precatalyst goes through a first turnover to generate the active alkylidene complex A1 Upon coordination of a second monomer molecule, an -metallacyclobutane ( M2 ) is formed to produce a dimer and the methylidene complex A3 A substantial amount of the -metallacyclobutane probably also forms but only leads to non-productive metathesis. The methylidene A3 then reacts with another monomer mo lecule, which upon formation of the metallacyclobutane ( M4 ) releases a molecule of ethylen e and the “starting” alkylidene complex. The cycle then goes on to form trimer, tetramer, etc. until reaching high molecular weigh polymer. For each “coupling” reaction one molecule of ethylene is formed. However, reaction of a polymer chain with ethylene will result in depolymerization. To drive ADMET equilibr ium onto a productive pathway and achieve high molecular weigh polymer, ethylene gas is usually eliminated from the reaction vessel as the polymerization proceeds.

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18 R LnM R' LnM R LnMR R LnMCH2LnM R R H2CCH2+ R R R LnM R R' LnM R' R R'A1 M2 A3 M4 Figure 1-14. Mechanism of productive ADMET Kinetics and Equilibrium Considerations From a kinetic viewpoint, ADMET is a step-growth condensation reaction, meaning that it proceeds by the stepwise reaction between the functional groups of reactants, in this case the olefinic double bond, to produce dime r, trimer, tetramer, and so on until eventually large polymer molecules are formed.110 The size of the polymer molecules continuously increases with time (c onversion) at a relatively slow rate. Therefore, high conversions are required to reach high molecular weight polymer as expressed by the Carothers equation where nX is the number-average degree of polymerization, or the average number of struct ural units per polymer chain, and p is the extent of reaction or conversion of the dien e monomer (Equation 1-1) In other words,

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19 nX is simply given as the total number of monomer molecules M initially present divided by the total number of molecules present at time t Step-polymerizations are characterized by the disappearance of monomer very early in the reaction. For example in most st ep polymerizations, there is less than 1% of monomer left when the average polymer ch ain contains about 10 monomer units. Highmolecular-weight polymer is obtained only near the end of the reaction (>98% conversion). Passed these high conversions, e ach connection that is made involves two high molecular weight molecules, and the average degree of polymerization increases exponentially at this point, as illustrated by Figure 1-15. the catalys t therefore needs to remain highly active throughout the entire polym erization, which sometimes require long reaction times. 0 50 100 150 200 250 020406080100 % ConversionAvg. Degree of Polymerization Figure 1-15. Variation of molecular weight with conversion in step polymerization

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20 Most metathesis catalysts ar e more reactive towards termin al olefin than unstrained internal olefins, mostly because of the reduced steric hindrance of terminal olefins. For this reason, ADMET is conducted with -diene monomers to achieve the highest conversions possible. Incident ally, this generates methylidene complexes, which in the case of ruthenium hinders the rate of metathes is. This point will be further developed in the next section. Of course, any mono-olefi n impurity should avoided at all times, since they will act as end-capping reagents, ther efore limiting the molecular weight of the polymer. However, in some instances, mono-ol efins can be used as chain transfer agents to reach a target molecular weight. Condensation reactions are usually equilibri um processes that require the removal of the condensate molecule to shift the equi librium towards high conversions, i.e. high molecular weights polymers. Industrially, the synthesis of commercial condensation polymers, such as polyesters, polyamides and polycarbonates, liberate molecules a such as water, alcohols or even HCl that often require heat and vacuum to be evacuated.111 In ADMET, the condensate molecule is ethylene gas, which is conveniently removed at room temperature. For this reason, most ADMET polymerizations are run under vacuum, without a solvent, whic h also minimizes cyclization. Cyclization versus Linear Polymerization The production of linear polymers by step polymerization is sometimes complicated by the competitive occurrence of intramolecular cyclization reactions.112 This phenomenon depends on thermodynamics and kinetic considerations of the size of the ring structure that may be formed. Th ermodynamically, the stability of the ring decreases under 4 and over 8 atoms. Ki netically, the probability of ring formation

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21 decreases as the possibility of the 2 functiona l groups encountering each other decreases. Oligomeric cyclics has been observed in ADMET; however, the amount and the form of cyclics were impossible to identify.113 The metathesis reaction allowing the ring-closing of acyclic dienes (RCM), the use of monomers able to form 5-,6-, and 7-membered rings should be avoided in ADMET (Figure 1-16). Cyclization can also be limited by carrying out the polymerization at high concentrations of reactants. Linear polymerization being a bimolecular (intermolecular) proc ess, the reaction ra te increases as the concentration of reactants increase. Typically, ADMET polym erization is run in the bulk, at high temperatures to lower the viscosity as it in creases with the polymer molecular weight. Similarly, the synthesis of large rings (mor e than 20 atoms) has been achieved in high dilution conditions.114 LnM R R R M RR R + M R R R R3 Figure 1-16. Formation of cyclics in ADMET Interchange Reactions and Mo lecular-Weight Distribution Interchange involves reactions between the terminal functional group of one polymer molecule with the interunit repea ting linkage of another polymer molecule. These interchange reactions are quite co mmon in condensation polymers, such as polyamides and polyesters.115 Two polymer chains react to yield one shorter and one longer chain, which does not affect the averag e molecular weight of the polymer. In ADMET, the carbon-carbon double bonds of the pol ymer backbone are still metathesis active and can react with either a polymeric alkylidene complex or a methylidene

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22 complex formed from a productive metathesis cycle (Figure 1-17). This phenomenon is often referred as trans-metathesis and will go on until the system reaches a thermodynamic equilibrium. It allows the perfect randomization of copolymers synthesized by ADMET, whether the two monomers are mixed together or one monomer is incorporated into a different homopolymer.116 M P2P1M P2P3+ P1P3+ M P2M CH2P4+ P2P4+ M CH2P1P3+ P1M P3+ a) b) Figure 1-17. Trans-metathesis in ADMET: a) Interchange wi th a polymeric alkylidene; b) Interchange with the met hylidene from a productive step The product of a polymerization is a mixtur e of polymers molecules of different molecular weights. The polydispersity index (PDI) is a measure of the breadth of this molecular weight distribution, defined as the weight average molecular weight Mw divided by the number average molecular weight Mn.117 Hence, a high PDI will be the indicative of a broad molecular weight dist ribution, which will have various impacts on the polymer mechanical properties. The mol ecular weight distribution has been derived by Flory118 through a statistical approach which pred icts that for step polymerization, the PDI should be close to 2 when the conversions approach 100%. As a matter of fact, most ADMET polymers exhibit a PDI of 2.0, thanks to trans-metathesis. Lower PDIs indicate the presence of competitive mechanisms such as cyclization followed by ROMP. Utility of ADMET Although ADMET is limited by the catalyst lifetime and therefore does not reach molecular weights higher than 80,000 g/mol, the polymers obtained present sufficiently

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23 interesting material properties for applic ation. Actually, many commercial polymers, such as polyesters and polyimides present most interesting properties at molecular weights ranging between 15,000 and 30,000g/mol. The clean mechanism of ADMET has often allowed to bypass some of the most puzzling challenges in the polymer community. For example, the random or alternated copolymerization of ethylene with polar monomers is complicated by the reactivity differences of the 2 monomers, often pr oducing block copolymers through standard polymerization procedures. ADMET provides a unique method to synthesize completely linear polymer analogous to sequence specific or statistical copolymers of ethylene and polar monomers in a wide variety of comonomer compositions. This is achieved by polymerization of the appropriate func tionalized diene followed by exhaustive hydrogenation. The design of the diene monome r can therefore lead to virtually any polymer via ADMET, whether it is used for modeling known structures or developing new materials, such as ami no-acid containing polymers a nd polysiloxanes (Figure 1-18). R x x n x x n R' Hydrocarbon -OSiCl2Si(OCH3)2-OCHPhO-S-OC=O Aromatic -COOH -C=O -Cl -Br -I -OAc -CnH2n+1-NH-AminoAcid R'= C O O N H R C O -O-B-OPh -NPh R= Figure 1-18. Examples of polymeric stru ctures obtained via ADMET polymerization

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24 In addition, the ADMET polymerization of different alkyl branched dienes has provided a family of model ethylene/ -olefin copolymers and allowed the study of the effects of regular microstructures on polym er properties. The structure-property relationships of various ethylene copolymers can be clarified using these model systems, by comparing Tm as a function of the functional group or by examining crystal packing. Ruthenium Catalysis Applied to ADMET The development of ruthenium Grubbs cat alysts greatly expanded the scope of ADMET by allowing the polymerization of functi onalized dienes that are not tolerated by Schrock’s catalysts and by easing polymeriza tion procedures. Even though the first generation of catalysts requi re higher catalyst loadings and afford generally lower molecular weights, it never necessitate ab solute dryness of monomer or oxygen-free conditions as Schrock catalysts do. A polymer formed via ruthenium-catalyzed ADMET has a typical trans content of about 80%., whereas Schrock’s catalysts produce 90 to 95% trans polymer. The preferred trans configuration is not only a corr elation of the metallacyclobutane conformation but is also a th ermodynamic reflection of the trans metathesis equilibrium. The difference between the two types of catalysts is more likely the result of a sterically less flexible metallacyclobutane in the case of Schrock’s catalysts due to its bulky ligand sphere. Ruthenium 1st Generation It was not until the introduction of [Ru]1 that ruthenium became extensively used in ADMET. Indeed, its triphenylphosphi ne and vinylidene analogs only promotes ADMET at a very slow rate at normal catalyst loadings (0.1 to 1.0 mol%).119 Typically, the polymers produced with [Ru]1 have a molecular weight range of 25 to 35 kg/mol and

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25 an 80% trans ratio, at optimal catalyst loadings of 0.25 mol%.119 This catalyst allows the polymerization of monomers containing acetals, alcohols,119 aromatic groups,120 esters both in the main chain121 and as pendant group, ketones,122 ethers,123 boronates,124 germanium,125 silyl chlorides and siloxanes,126 in addition to a vari ety of alkyl branched monomers.127-130 Amine131 and carboxylic acid132 containing olefins can also be polymerized via ADMET but require to be prot ected prior polymerization. However, in the case of hydrocarbon polymers, the molecular weights reached with [Ru]1 are still much lower than what can be obtained with Schrock’s catalysts. For example, an ADMET ethylene-propylene copolymer, with the same methyl branch content, has a Mn of 17,400 g/mol with [Ru]1, versus Mn of 72,000 g/mol with [Mo]1.128 Other model ethylene copolymers, such as ethylene-vinyl chloride,121 ethylene-vinyl acetate,122 ethylene-acrylate,133 and ethylene-styrene copolymers133 were also successfully synthesized with [Ru]1, some even exhibiting unique th ermal characteristics. Other useful structures include vari ous cyclolinear phosphazene polymers,134,135 and liquid crystalline polyesters.136 Numerous block copolymer architectur es have also been devised using functionalized polyoctenamer telech elics via ADMET polycondensation,121,137 for example in the synthesis of novel poly( -benzyl-L-glutamate)b -polyoctenamerb -( benzyl-L-glutamate) triblock copolymers. In th is case, the functiona l group tolerance of [Ru]1 allowed the end-capping of ADMET polymerized polyoctenamer with phtalimido groups, as the first step towards a pot entially bioactive triblock copolymer.138 The only requirement for the monomers is to place the functionality at least 2 carbons away from the double-bond to limit cyc lization and/or catal yst deactivation by

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26 intramolecular coordination leading to the formation of a stable chelate.60 The polymerization of 1,5-hexadien e and polyethylene glycol (P EG) grafted copolymers have been limited to oligomeric structures for this reason.119,139-141 Because these catalysts react rapidly and qua ntitatively with vinyl ethers to form Fisher carbenes that are relatively metathesis inactive,74,91 this provides a convenient way to quench most ADMET reactions by addition of et hyl vinyl ether. This also insures that the ADMET polymer has well-de fined olefinic end-groups. Depolymerization of commercial pol ybutadiene by ADMET can be achieved by applying high pressures of ethylene in the presence of [Ru]1 to yield 1,5-hexadiene.142 In the absence of ethylene a nd solvent the depolymeriza tion only produces cyclics.143 The results are much better than with Schrock’s catalyst thanks to the lesser sensitivity of the ruthenium catalysts. Ruthenium 2nd Generation Complex [Ru]4 has been used extensively for ADMET, for it yields polymers of higher molecular weight than [Ru]1. Overall, the reactivity of [Ru]4 falls in between that of [Ru]1 and [Mo]2.144 For example, the polymeri zation of 1,9-decadiene with [Ru]4 using the same catalyst loadings ( 0.25 mol%) affords polyoctenamer with a Mn of 75,000 g/mol, compared to only 25,000 g/mol with [Ru]1.145 However, all polymers obtained using second-generation catalysts exhi bit irregular microstr uctures as evidenced by a depreciation of the melting temperature Tm. This is due to a competitive isomerization reaction that di sturbs the molecular structur e by migration of the doublebond along the olefin backbone. Therefore th ese catalysts are not suitable for the modeling of precise polymer architectures.

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27 On the other hand, the second generati on of catalysts has allowed the ADMET polymerization of aminoacid containing dienes.146 These reactions need to be conducted in THF solution because of the solid nature of the monomers. The polymers produced have molecular weights of up to 30,000 g/ mol and represent ideal candidates for biological applications.147 Imperfect comb graft copolymers of polyet hylene (PE) and PEG were synthesized with [Ru]4 in higher molecular weights than with [Ru]1 (12,000 vs. 5,500 g/mol), albeit as irregular microstructures.141 Tandem ADMET/ATRP polymerization was used for the preparation of polyethylene grafted pol ystyrene copolymers; however, only low molecular weight materials were obtainable because of the steric hindrance of the polystyrene grafted diene interacting with the catalyst.148 For this particular class of copolymers, ROMP followed by ATRP polym erization was able to afford higher molecular weights.149,150 The use of 2nd generation ruthenium catalysts for the synthesis of conjugated polymers has hardly been explored, and woul d not be expected to promote the vinyl addition reactions that have been problematic in ADMET of electronrich monomers with the more electrophilic molybdenum complexes. Attempts to polymerize 1,4divinylbenzene only resulted in dimers and trimers because the mixture solidified passed a DP of 3.151 Depolymerization reactions with [Ru]4 also affords better results than [Ru]1 or [Mo]1 for polyisoprene and polybutadiene blocks of triblock copolymers of the type polystyrene-polyene-polystyrene.152 Complex [Ru]4 is also able to depolymerize sulfur crosslinked polyisoprene into oligomers (DP=10). This fi eld represents a potentially

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28 useful recycling technique especially if the r obustness and the cost of the catalysts can be improved. Limitations These late-transition metal catalysts are not immune to interaction with functional groups, as exampled by the isolation of multiple chelates of functionalized olefins with ruthenium olefin metathesis catalysts.60,153,154 Hoveyda-type catalyst s are an example of this phenomenon that retains high catalytic reactivity. Concerning the nature of the functional group tolerance the coordination of th e heteroatom to the metal is of central focus. Reversible coordinations will only de lay or slow down the metathesis reaction and maybe prevent high conversions while irreversible coordination will simply deactivate the catalyst towards ADMET.60 A typical example is the ca se of thioethers, which are polymerizable with Schrock’s molybdenum, but not with the [Ru]1.155 A stable Ru-S chelate complex is formed as evidenced by the appearance of a new alkylidene by 31P NMR. The Ru-S bond is expected to be st ronger than the Mo-S bond according to the size and polarizability of the two metals. Ev en though the ruthenium is tolerant towards a greater variety of functional groups than [Mo]1, kinetically, coordina ting functionalities such as ethers and sulfides, influence the rate of polymerization to a much greater extent with [Ru]1 than with [Mo]1.155 Indeed, after phosphine dissociation, heteroatoms can compete with olefins for the open coordination site and thus reta rd the rate of metathesis. Reversible coordination proba bly occurs both intra and in termolecularly in ADMET due to the high monomer concentrations in bulk conditi ons. This effect is not as significant in the case of molybdenum catalysts probably because of the ster ic influence of the bulky ligand sphere.

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29 Hydrogenation The ruthenium catalysts feat ure the possibility to be converted into hydrogenation catalysts, which can be attractive for the onepot synthesis of branched polyethylene by sequenced ADMET polymerization-hydrogenati on. Heterogeneous hydrogenation could be performed by simple addition of chromat ographic silica to the diluted polymerization mixture under 200-500 psi of hydrogen.121 Copolymers of ethylene-styrene, ethylenevinyl chloride and ethylene-acr ylate were prepared this way.133 Even though many examples have shown that ruthenium carbenes [Ru]1 and [Ru]4 could be transformed into homogeneous hydrogenation catalysts, th is strategy has not been tested during ADMET polymerization.156-159 The homogeneous catalyst could then be washed away by precipitation of the saturated polymer. Kinetics Kinetics in ADMET can hardly be monito red by NMR since ADMET requires bulk conditions (to limit cyclization) while NMR ope rates in diluted solu tion of deuterated solvents. So the best way, so far, to meas ure ADMET kinetics is to measure the volume of ethylene released, which is directly correla ted to the degree of polymerization. Even though this method does not reflect ADMET kinetics under vacuum, it allows the comparative study of different catalyst s, which was first implemented with [Mo]1 and [Ru]1 by Brzezinska et al.155 At early conversions, i.e. be fore the viscosity grew so high that the rate becomes diffusion controlle d, ADMET follows a quasi-second order rate. [Mo]1 was found to catalyze ADMET of 1,9-decadiene about 50 times faster than [Ru]1. Nevertheless, [Ru]4 promotes ADMET faster than [Ru]1 at temperatures of 45 and higher.144 An induction period marks the slow phosphine dissociation of this NHC containing catalyst; however, th e greater affinity of the 14ecomplex for -olefinic

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30 substrates combined with a slower phos phine rebinding allow for higher molecular weights polymers. Hence, phosphine dissoci ation/rebinding plays a major role in the ADMET mechanism, the ultimate concern being the molecular weight of the polymer, not the initial metathesis rate. Olefin Isomerization Figure 1-19. Structural olefin isomerization of 1-octene. In spite of their high reactivity in metathesis chemistry, these ruthenium catalysts were suspected to undergo nonmetathesis transformations.160-165 Among them, alkene isomerization has emerged as an important side reaction of ruthenium catalyzed metathesis, disturbing the product's microstr ucture by apparent migration of the double bond along the alkyl chain (Figure 1-19).166 First observed on substrates containing allylic functionalities in combinati on with first generation catalysts,167-171 double bond isomerization has since been reported with 2nd generation catalysts on a broad variety of substrates competitively and sometimes prior to olefin metathesis.114,173-176 More recently, olefin isomerization was evidenced in ADMET catalyzed by [Ru]4, by the MALDI-TOF analysis of an amino-acid branched polymer.177 Whether it occurs on the monomer, or on the polymer followed by tr ans-metathesis, isomerization during ADMET results in irregular microstructures pres enting altered properties (Figure 1-20). Noteworthy, under ADMET conditions, first-generation [Ru]1 was found not to isomerize olefins.178

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31 Figure 1-20. Olefin isomeri zation during ADMET polymerization Olefin isomerization is promoted by a va riety of metal complexes and has been known to operate via two di stinctive pathways: the -allyl hydride mechanism involves a metal in a low oxidation state that forms an 3-allyl intermediate upon coordination of the olefin, while the metal hydride addition-elim ination involves a long-lived metal hydride that forms an alkyl intermediate (Figure 1-21).179 However, in the case of ruthenium metathesis catalysts, the intermediate res ponsible for this undesirable reaction has not been identified yet, although it is often attributed to the fo rmation of a ruthenium hydride in situ, as a decomposition product of the original carbene catalyst.180,181 Figure 1-21. Olefin isomerization mechanism: a) -allyl mechanism. b) metal-hydride addition-elimination mechanism

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32 CHAPTER 2 METATHESIS ACTIVITY AND STABILITY OF NEW GENERATION RUTHENIUM POLYMERIZATION CATALYSTS Reproduced in part with permission from: Cour chay, F. C.; Sworen, J. C.; Wagener, K. B. Macromolecules 2003, 36 8231-8239. Copyright 2003 American Chemical Society Introduction The recent development of ruthenium carbene complexes (Figure 2-1) has increased considerably the utility of olefin metathesis both in the fields of organic synthesis and polymer chemistry. The intr oduction of the well-defined, functional-group tolerant complex (PCy3)2(Cl)2Ru=CHPh [Ru]1 by Grubbs and coworkers37,57,80,87 stimulated researchers’ interest towards late-transition metal-catalyzed metathesis chemistry, for it is a useful catalyst in a vari ety of ring-closing meta thesis (RCM), crossmetathesis (CM), acyclic diene metathesis polymerization (ADM ET) and ring-opening metathesis polymerization (ROMP) reactions.7-10,20,31 Improvements on this “first generation” ruthenium structure have been reported by many gr oups in recent times. For example, the Grubbs research group reported that incorporation of the more bulky and strongly sigma-donating imidazolylidene liga nd leads to higher activity and thermal stability,71 enabling the preparation of -functionalized, di-, triand tetrasubstituted olefins.12 These imidazolylidene based catalysts be have in a manner similar to that of early transition metal catalysts such as Schrock’s molybdenum catalyst [Mo]1,27,28 and remain active at loadings as low as 0.05 mol% for RCM and 0.0001 mol% for ROMP.71

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33 Additional ligand variations have led to further improvements in both catalytic activity and stability. For example, Mol and coworkers101 reported improved turnovers during the cross metathesis of terminal olefins with a modified version of the imidazolylidene liga nd, catalyst (IPrH2)(PCy3)(Cl)2Ru=CHPh [Ru]10, first synthesized by Frstner.100 Also, Hoveyda et al.95 have developed the complex (H2IMes)(Cl)2Ru=CH( o-i PrOC6H4) [Ru]9 which promotes olefin metathesis by a unique “release-return” mechanism, allowing efficient metal recovery without significant loss of activity. In fact, complex [Ru]9 can readily be used in co mbinatorial synthesis in air with reagent-grade solvents. This Ru -complex offers reactivity, chemoand stereoselectivity profiles differing from both catalysts [Ru]1 and [Ru]4, notably during the metathesis of electron-deficient olefins.96-98 The robustness of this complex also allows its use in solid-suppor ted heterogeneous catalysis.102-107,181 Ru Cl Ph PCy3N N Ru PCy3PCy3Ph Cl Cl Ru Cl Ph PCy3Cl Cl[Ru]1[Ru]4 [Ru]10Ru Cl O Cl[Ru]11N N N N Mo N Ph[Mo]1O H3C F3C CF3O H3C F3C F3C Ru Cl O Cl[Ru]9N N Figure 2-1. Metathesis catalysts Acyclic diene metathesis (ADMET) chemis try has enjoyed the benefit of this metathesis catalyst research, resulting in the synthesis of unique architectures and new functionalized materials,121-133,141-143 such as perfectly branched polyethylene,127,128,130

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34 and amino acid containing polyolefins.146,147 The choice of cataly st to perform these transformations is dictated by the monomer structure and the reac tion conditions. For example, Schrock’s catalyst is best used for hydrocarbon dienes and some functionalized olefins, whereas Grubbs’ catalysts are pref erred with highly-func tionalized monomers and at higher temperatures. ADMET, CM and RCM require longer reaction times than ROMP chemistry, and so catalyst entities with longer lives are preferred. Consequently, our group has a particular interest in comple xes that exhibit both long lifetimes and high turnovers ratios. A new cat alyst structure (complex [Ru]11) was prepared by combining the thinking that yielded the Grubbs, Hove yda and Mol complexes, and was found to exhibit improvements in terms of stability under ADMET conditions. A kinetic approach was used to investigate the activity and utility of these ruthenium complexes during ADMET polymeriza tion, work which has led to the design of more efficient ADMET catalyst systems.155 Previously, Lehman and Wagener144 reported a comparative kineti c study of Grubbs’ catalysts [Ru]1 and [Ru]4 during the ADMET polymerization of the benchmark m onomer, 1,9-decadiene, research that showed the effect of temperature on ac tivity and initiation rates. Catalyst [Ru]4 is clearly more active if used at temperatures of 45 C and above. The olefin isomerization activity of catalysts [Ru]1 and [Ru]4 was investigated as well,176 where catalyst [Ru]4 was found to isomerize both external and internal olefins if used at its optimal temperature for ADMET, 60 C. This observation is important to consider when building precise polymer microstructures. Herein the early kinetic work is expanded to include complexes [Ru]10, [Ru]9 and the new complex [Ru]11, comparing them with the 1st and 2nd generation Grubbs'

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35 catalysts.144 Initial rates of ADMET polymeri zation were measured at different temperatures, and the stability and activity of these complexes were examined at longer reaction times. Finally, their propensity to induce olefin isomeriza tion was investigated under polycondensation conditions. Results and Discussion Kinetic Assessment of Catalyst Activity. ADMET polycondensation of -dienes releases ethylene as the condensate, the volume of which can be measured and used to calculate the average degree of polymerization (DP) of the monomer as a func tion of time. The kinetic experiments were run for approximately an hour, focusing on the oligomerization (not polymerization) of the monomer. The monomer 1,9-decadiene was chosen in this study because of its wellunderstood behavior during ADMET chemistry. Monitoring oligomerization by plotting DP versus time allows for an efficient method to compare the catalytic activity for various complexes. Table 1 displays the initial rates of 1,9-decadiene ADMET chemistry, calculated as the initial slope (D P vs. time) of each kinetic curve during the average dimerization (DP=2) of the monomer. Table 2-1. Initial rates for complexes [Ru]1-[Ru]11 during the ADMET oligomerization of 1,9-decadiene Initial Rate (10-3 DP s-1) Catalyst Monomer:Catalyst Ratio 30 C 45 C 60 C [Ru]1a [Ru]4a [Ru]10 [Ru]10 [Ru]9 [Ru]11 450:1 450:1 450:1 900:1 450:1 450:1 1.0 0.5 0.42 0.05 36 1 8.8 4.0 4.8 3 12 2 2.3 0.4 4.4 0.7 39 7 39 1 18 5 42 4 5.1 0.6 23 3 55 10 78 10 68 10 [a] Data obtained from reference 144.

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36 Catalytic activity was measured at three temperatures (30 C, 45 C and 60 C) and two catalyst loadings (450:1 and 900:1 monomer:catalyst), pr oviding a broad basis set to evaluate the catalyst systems tested. This activity was compared to the widely used 1st and 2nd generation ruthenium catalysts, previ ously examined in ADMET conversions by our group .144 01000200030004000500060007000 0 1 2 3 4 5 6 7 8 9 10 11 [Ru]10 Ru Cl Ph PCy3N N Cl[Ru]1 [Ru]4 [Ru]10Average degree of Polymerization (DP)Time (s) Figure 2-2. Degree of polymerization (DP) vs. time curves for complex [Ru]1, [Ru]4 and [Ru]10 using 450:1 monomer:catalyst ratio at 30 C The kinetic runs at 30 C show that the Mol catalyst, complex [Ru]10, exhibits the highest activity of any ruthenium complex to date in th e dimerization of 1,9-decadiene. Figure 2-2 illustrates these data. When compared to the benchmark catalyst [Ru]1, complex [Ru]10 shows a 36-fold increase in activity at 30 C (see Table 1). This low temperature activity makes catalyst [Ru]10 an ideal candidate for controlled organic synthesis and polymerization applications. Fu rther, the initial dimerization rate when using complex [Ru]10 is greater than either complex [Ru]1 or [Ru]4 at both 45 and 60 C. For example, complex [Ru]10 affords a dimerization rate of 55x10-3 DP s-1, twice that when using complex [Ru]4 at 60 C; in fact, the high activity of complex [Ru]10 at

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37 60 C makes it difficult to accurately measure th e evolution of ethylene, resulting in a large error in the measured DP. This higher activity of complex [Ru]10 most likely originates from the increased steric bulk around the ruthenium center, a re sult of the isopropyl groups present on the ligand. Apparently, the interaction of the isopropyl groups with the carbene fragment and/or the chloro groups allows for faster and earlier dissociation of the tricyclohexyl phosphine, which has been commonly accepted as the initial rate-deter mining step of the metathesis reaction. This statement can be confirmed by comparing the 31P NMR spectrum of complex [Ru]10 versus either complex [Ru]1 or [Ru]4. The phosphine resonance is shifte d from 32 ppm for [Ru]4 to 28.1 ppm for [Ru]10, where this upfield shift indicates a weaker Ru-P bond, implying an easier dissociation of the phosphine. Also, the induction period observed with [Ru]4 is not observed with [Ru]10, which is consistent with complex [Ru]10’s faster phosphine dissociatio n. The induction period is thought to correspond to the in itial dissociation of phosphine before the steady-state concentration of the 14 electron (e-) ruthenium alkylidene complex is reached. 0100020003000400050006000 0 2 4 6 8 10 12 Ru Cl Ph PCy3N N Cl60 oC 45 oC 30 oCDPTime (s) Figure 2-3. Comparison of the activity of complex [Ru]10 at three different temperatures using a 450:1 monomer:catalyst ratio

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38 While complex [Ru]10 shows the largest decadiene dimerization rate for any complex to date, increasing temperature has an adverse effect on the lifetime of the catalyst (Figure 2-3). Complex [Ru]10 no longer competes with either [Ru]1 or [Ru]4 at temperatures above 45 C beyond trimerization of 1,9-d ecadiene. Figure 2-4 further illustrates this phenomenon; these lower turnove rs were also reported by Mol at elevated temperature.100,101 This decrease in conversion likely is due to the thermal decomposition of the complex as observed with Grubbs’ catalysts,87 or by way of C-H and/or C-C activation pathways.182,183 Decomposition may occur faster with complex [Ru]10 for 2 reasons: 1) the rate increase in phosphine di ssociation is known to affect the catalyst stability by generating too many unstable 14 especies; 2) the presence of an electron rich methine proton on the isopropyl groups make s it more likely to undergo C-H activation than the methyl protons of complex [Ru]4. Moreover, the additional carbons on each isopropyl group may increase the probability of ruthenium insertion into the C-C bond. 01000200030004000 0 1 2 3 4 5 6 7 8 [Ru]10 [Ru]4DPTime (s) Figure 2-4. Degree of polymerization (D P) vs. time curves for complexes [Ru]4 and [Ru]10 using 450:1 monomer:catalyst ratio at 45 C On the other hand, this temperature trend is inverted when th e monomer-to-catalyst ratio is doubled (Figure 2-5). Indeed, higher conversions are reported when the

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39 temperature is increased from 30 to 60 C with its average DP reaching 4 and 5.8, respectively. In addition, complex [Ru]10 is more active initially at 60 C when using lower catalyst ratios; the data are reported in Table 1. For example, when the catalyst concentration is decreased by a factor of two, the respective rate increases from 55x10-3 to 78x10-3 DP s-1. The increase in the overall activ ity observed at high temperature when lower catalyst concentrations are used may correspond to a decrease in the catalyst decomposition rate, which would in turn support a bimolecular decomposition pathway.87 However, at this point the po ssibility of an internal d ecomposition as outlined above cannot be ruled out. More like ly, this would prove the coex istence of both mechanisms. Further, the reaction rate for complex [Ru]10 at 30 C is decreased by a factor of 4, which is consistent with the lower catalyst ratio as little decomposition should occur at such low temperatures. Besides, this activ ity trend has been reported by Mol101 as well in the dimerization of terminal olefins. 0100020003000400050006000 0 1 2 3 4 5 6 7 8 Ru Cl Ph PCy3N N Cl60 oC 45 oC 30 oCDPTime (s) Figure 2-5. Degree of polymerization (DP) vs. time data for complex [Ru]10 using 900:1 monomer:catalyst ratio

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40 Although known for its particular tolera nce to a variety of substrates,95-98 complex [Ru]9 appears to be unstable once dissolved in 1,9-decadiene, for complex [Ru]9 affords low conversions at 30 C and 45 C (Figure 2-6). Consequently, no experiment was conducted at 60 C with complex [Ru]9. 010002000300040005000 0 1 2 3 4 5 6 7 8 9 10 11 12 Ru Cl O Cl N NTHF, 45 oC 45 oC 30 oCDPTime (s) Figure 2-6. Degree of polymerization (DP) vs. time data for complex [Ru]9 using 450:1 monomer:catalyst ratio The low stability of complex [Ru]9 may be attributed to its release-return mechanism, yielding an unstable 14 especies by the loss of isopropoxy styrene, which in turn produces the unbound ligand and/or its subs equent attachment to the chain end. We assume that the low stability of complex [Ru]9 is due to this highly unstable 14 eintermediate not being re stabilized by the isop ropoxy ligand. This 14 ecomplex may have to be stabilized by coordination to th e substrate (monomer) or solvent, and since 1,9-decadiene lacks any kind of electron donor or Lewis basic group, it is not able to serve as a stabilizing liga nd. Consequently, the catalys t apparently decomposes immediately after activation. To verify this assumption, the same experiment was

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41 conducted by adding a catalytic amount of TH F to the bulk reaction mixture. As is apparent in Figure 2-6, a drop of TH F improved the activity of complex [Ru]9 when polymerizing 1,9-decadiene, which refutes the general belief that ruthenium catalysts exhibit lower metathesis activity in THF.34 These observations led us to synthesize complex [Ru]11, which in effect combines the thinking of Grubbs, Mol and Hoveyda. The complex was obtained in good yields by implementing the same synthetic pro cedure used to generate complex [Ru]9,95 simply substituting complex [Ru]10 as one of the starting compounds (Figure 2-7). Conveniently, [Ru]11 can be purified in the air afford ing a dark green crystalline solid after flash chromatography and precipitati on in hexane, where the copper-phosphine complex must be removed by filtration prior to column chromatography. Ru Cl O Cl N N O [Ru]10 CH2Cl2Ru Cl Cl N N 1.03 equiv CuCl 40 oC, 20 min [Ru]11 97% PCy3 Figure 2-7. Synthesis of complex [Ru]11 The structure was confirmed through singlecrystal X-ray analys is (Figure 2-8). Selected bond lengths and angles of both complexes [Ru]9 and [Ru]11 are provided in Table 2 for comparison. Complex [Ru]11 is characterized by a distorted trigonal bipyramidal coordination about the metal cen ter, with the alkylidene group and the chlorines in the same plan. The overall ge ometry and most of the bond angles and bond lengths are analogous to their related values in complex [Ru]9. As the bond angles show, the geometry of [Ru]11 is a little more distorted, which was expected considering the

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42 bulk brought about by the isopr opyl groups. Noteworthy, the Ru-O bond is much shorter in complex [Ru]11, which indicates a st ronger coordination. Figure 2-8. ORTEP Diagram of complex [Ru]11. Thermal ellipsoids are drawn at the 30% probability level Table 2-2. Bond Lengths a nd Angles for Complexes [Ru]9 and [Ru]11 [Ru]9 [Ru]11 Bond Length () Ru-C(1) 1.828(5) 1.822(5) Ru-C(2) 1.981(5) 1.978(5) Ru-O 2.261(3) 2.247(3) Ru-Cl(2) 2.328 (12) 2.3271(15) Ru-Cl(1) 2.340(12) 2.3409(14) N(1)-C(2) 1.350(6) 1.362(6) N(2)-C(2) 1.351(6) 1.363(6) Bond Angles (deg) C(1)-Ru-C(2) 101.5(14) 102.0(2) C(1)-Ru-O 79.3(17) 78.60(18) C(2)-Ru-O 176.2(14) 171.94(17) C(1)-Ru-Cl(2) 100.1(15) 99.57(15) C(2)-Ru-Cl(2) 90.9(12) 86.24(15) O-Ru-Cl(2) 85.3(9) 85.75(10) C(1)-Ru-Cl(1) 100.2(15) 100.20(15) C(2)-Ru-Cl(1) 96.6(12) 101.29(15) O-Ru-Cl(1) 86.9(9) 86.44(9) Cl(2)-Ru-Cl(1) 156.5(5) 156.84(6) C(2)-N(1)-C(3) 127.2(4) 126.4(4) C(2)-N(2)-C(4) 127.0(4) 124.6(4)

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43 The activity of this new complex was ev aluated using the same kinetic study as done previously with 1,9-decadiene Contrary to its predecessor [Ru]9, complex [Ru]11 catalyzes the ADMET oligomerization of 1,9decadiene, as shown in Figure 2-9. The shape of the kinetic curves is the sa me as observed for the parent complex [Ru]10, and the overall activity is less a f unction of temperature. Ther e is no definite trend, but 45 C seems to be an optimal temperature. On th e other hand, the initial oligomerization rate when using complex [Ru]11 is more sensitive to temperat ure variations than it is for catalyst [Ru]10, reaching a maximum of 68x10-3 DP s-1 at 60 C. Thus, higher temperatures are recommended to get fast initiation using this particular complex. Moreover, catalyst [Ru]11 exhibits faster initial rates than [Ru]9, which is consistent with the introduction of the bulkier isopropyl groups on the imidazo lylidene ligand. In fact, complex [Ru]11 produces high molecular weight poly(octenamer) whereas complex [Ru]9 only yields oligomers, ev en after 120 hours in the bulk. 0500100015002000250030003500 0 1 2 3 4 5 6 7 8 9 10 11 12 Ru Cl O Cl N N60 oC 45 oC 30 oCDPTime (s) Figure 2-9. Degree of polymerization (DP) vs. time data for complex [Ru]11 using 450:1 monomer:catalyst ratio

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44 Interestingly, complex [Ru]11 does not need THF to catalyze the ADMET reaction of 1,9-decadiene. To better understa nd the differences between catalysts [Ru]9 and [Ru]11 and the role of polar reaction conditions on these Hoveyda-type catalysts, kinetic experiments were completed using dipente nyl ether as the monomer instead of 1,9decadiene. Dipentenyl ether was chosen for its inherent elec tron donating ability; it serves as both monomer and solvent, possi bly allowing coordina tion to stabilize the reactive intermediate through a six-membered ring transition state (Figure 2-10). Ru Cl O Cl N N O Ru Cl O Cl N N O [Ru]9 Figure 2-10. Proposed intermediate of complex [Ru]9 during the metathesis of dipentenyl ether. Surprisingly, the activity of complex [Ru]9 with dipentenyl ether is actually lower than with 1,9-decadiene using the same ki netic conditions, suggesting that dipentenyl ether intramolecular complex is not sufficien tly labile to allow coordination of other monomer molecules, thereby pr eventing further continuation of the metathesis process. Experiments conducted using complex [Ru]11 (Figure 2-11) also show a decreased activity of about 1 DP s-1 where the initial rate drops from 42x10-3 DP s-1 (1,9-decadiene) to 15x10-3 DP s-1 (dipentenyl ether). The fact that a similar behavior was obtained with both complexes [Ru]9 and [Ru]11 suggests that these H oveyda-type systems are sensitive to substrate polarit y, and may give better results in solution with solvents of lower polarity. Another possibility could be th at this particular ether is too sterically hindering to effectively stabilize the Ru center.

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45 0500100015002000250030003500 0 1 2 3 4 5 6 7 [Ru]11 (1,9-decadiene) [Ru]11 [Ru]9DPTime (s) Figure 2-11. Degree of polymerization (DP) vs. time data for complexes [Ru]9 and [Ru]11 using dipentenyl ether as the monomer (450:1 monomer:catalyst) Overall, [Ru]11 is a better catalyst than [Ru]9 for the ADMET oligomerization of either 1,9-decadiene or dipentenyl ether, al though slightly less act ive towards the ether monomer. Considering the hypothesis of a 14eintermediate as proposed for the parent complex [Ru]9, the enhanced activity of [Ru]11 can be explained in terms of olefin affinity. The isopropyl groups of the imidazo lylidene should exer t a stronger steric pressure on the carbene moiety and favor the formation of a metallacyclobutane with the incoming olefin. The inherent olefin affin ity offers an ideal stabilization to the 14eintermediate and could account for the higher activity. In fact, the crystal structure of [Ru]11 shows that the Ru-C bond with the N-hetero cyclic carbene is slightly shorter than for complex [Ru]9.95 This suggests a higher affinity of complex [Ru]11 for -acidic olefinic substrates. The Ru-O is also found to be shorter in complex [Ru]11, meaning the coordination of the oxygen to the ruth enium is stronger than in complex [Ru]9. Therefore, the increased stability of [Ru]11 could be due to a higher tendency of the

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46 isopropoxystyrene to come b ack and stabilize the 14eintermediate. While the activity of complex [Ru]11 at low conversion does not compet e with the activity of complex [Ru]10, its long term stability, described below, renders it an attractive metathesis catalyst. Thermal Stability of the Catalysts The decomposition rates of complexes [Ru]10 and [Ru]11 were monitored by 1HNMR in d6-benzene at 55 C and calculated as the time for half the complex to decompose. As observed duri ng the kinetic study, complex [Ru]10 is not resistant to high temperatures, exhibiting a half-life of 90 min at 55 C. On the contrary, complex [Ru]11 remains completely intact at 55 C for more than 3 days, showing no sign of decomposition whatsoever (Table 3). This comp lex is not only stable in air but also in solution even at elevated temperatures. Th is increased stability is desirable for RCM, CM and ADMET chemistry in solution and ma y also be useful for solid state ADMET polymerization.184 Table 2-3. Half-lives for th e decomposition of complexes [Ru]10 and [Ru]11 Complex Temp. Conc. Half-life [Ru]1a 55 C 0.023M 8 days [Ru]10 55 C 0.032M 90 min [Ru]9 55 C 0.032M >3 days [a] Value taken from Grubbs et al. 37 Further Comments on ADM ET Polymerization In order to expand these kinetic oligom erization measurements made using 1,9decadiene, the utility of the complexes was investigated during full-scale polymerization of this monomer. These polymerizations were conducted using standard Schlenk techniques, with complexes [Ru]10 and [Ru]11 at 55 C using both 450:1 and 900:1

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47 monomer:catalyst ratios (Table 4). The resultant poly(octena mer)s were characterized by 1H and 13C-NMR and further analyzed by GPC a nd DSC. Molecular weights were compared to poly(octenamer)s obt ained with the catalyst systems [Ru]1, [Ru]4 and [Mo]1. Table 2-4. Polymerizations of 1,9-decadiene at 55 C for 120 h Catalyst Monomer:Catalyst ratio Mn (g/mol) PDI [Ru]1 [Ru]4 [Mo]1 [Ru]10 [Ru]10 [Ru]11 [Ru]11 450:1 450:1 1200:1 450:1 1000:1 450:1 1000:1 12,000 78,000 15,000 29,000 37,000 21,000 32,000 1.8 1.8 1.7 1.6 1.9 1.6 1.6 When the polymerizations were heated at 55 C for an extended period, poly(octenamer) was obtained with Mn of 37,000 g/mol using complex [Ru]10 and 32,000 g/mol using complex [Ru]11. This is an encouragin g result, as the benchmark Shrock’s molybdenum catalyst (6) for ADMET polymerization produces a polymer half that average mass, or Mn= 15,000 g/mol poly(octenamer) under the same conditions. Both are acceptable molecular weights in step polymerization chemistry, especially for functionalized versions of polyolefins ma de by this route. Endgroup conversion increases from 99.1% to 99.6% in this doubling of molecular weight, clearly illustrating how clean mechanistic chemistry is essential in any step polym erization. Further, the use of complex [Ru]10 or [Ru]11 leads to successful polymeri zation with lower catalyst loadings (0.05 mol%). Complex [Ru]9 was not included in this pol ymerization study since initial experiments at 30 C in the bulk produced only oligomeric poly(octenamer), an observation consistent with the model kinetic study. However, the situation differs when

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48 functionalized dienes are used in a polar me dium as this catalys t was successful in polymerizing amino-acid cont aining monomers in THF.131 Isomerization of the Olefin Bond Olefin isomerization is a ma jor side reaction of Ru-cat alyzed metathesis; however, the specie(s) responsible for such transfor mation, whether it is a decomposition product or the metathesis catalyst itself, have not been fully identified yet.176,185 Studying isomerization therefore occupies a signif icant part in the pr ocess of understanding catalyst decomposition. Besides, under ADMET conditions olefin isomerization generates irregular polymer microstructures, and since part of our group research focuses on precision in polymer synthesis, th e isomerization activity of complexes [Ru]10 and [Ru]11 was also explored in detail. The isomerization study was done using a model compound, 1-octene as the starting reagent, and the reaction was monitored by GC analysis. Typical results are illu strated in Figure 2-12 for complex [Ru]10. In fact, both complexes [Ru]10 and [Ru]11 isomerize the olefinic bonds in the reactant as well as in the metathesis products. Figure 2-12. Composition of olefin mixtur e as a function of time for reaction of [Ru]10 with 1-octene at 30 C

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49 If self-metathesis were the only mechan ism operating in the presence of these catalysts, then this model reaction w ould yield 7-tetradecene exclusively (C14 in the figure above). Figure 2-12 shows that not to be the case, where the concentration of isomerized products increases at the expe nse of the product, 7tetradecene and the starting reagent, 1-octene For example, exposure of 1-octene to complex [Ru]10 at room temperature leads to 94% conversion (1 hour), where the product mixture consists of 82% 7-tetradecene and 18% isomers. By comparison, complex [Ru]4 is less active in this model study, requiring 3 days at room te mperature to generate a similar conversion (86%). Complex [Ru]4 is slower to isomerize olefin s, however, yielding a mixture containing only 3% isomers. Longer run times using complex [Ru]10 at room temperature result in considerably higher quant ities of isomer content, up to 76% of the final product mixture. The starting reagent, 1-octene, is completely consumed, and the distribution of isomerized products (as determ ined by GC analysis) is centered about 7tetradecene. 4 4+ C2H4M I I I TM 1-octene I TM 1-octene3 5 4 3C13+4OR +4C9C7 4 5 3 4+ C15C13 3 5C7C9OR TM 1-octene C12 + C4C10 + C6OR C3+2 6etc. TM3 3 5 5+ C16C12etc. Figure 2-13. Proposed mechanism for isomerization of 1-octene

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50 These observations are consistent wi th isomerization chemistry operating concurrently with metathesis, although at a slower rate than th e initial condensation metathesis chemistry, which yields 7-tetrad ecene. In fact, three different reaction mechanisms operate concurrently throughout the conversion of 1-octene: condensation metathesis (we abbreviate this reaction as “M”), trans-metathesis (involving both internal double bonds and active metal alkylidene ch ain ends) (abbreviated “TM”), and isomerization chemistry ( abbr eviated “I”) (Figure 2-13). These three reactions compete until conde nsation metathesis is complete, after which only trans-metathesis (TM) and isomerizat ion (I) continue. If the catalyst entities possess sufficiently long lives as a ppears to be the case for complex [Ru]10, then the starting reagent, 1-octene, will be completely consumed, and an equilibrium mixture of isomerized products will result. Complex [Ru]11 approaches this situation, with conversions of 98% and a lower percentage of isomerization (23%) after 3 days. These results are consistent with the data of the kinetic study, a nd the product/isomer distribution indicates that is omerization occurs concurren tly with metathesis. The relative rates of condensation metathesis, trans metathesis, and isomerization apparently differ depending upon the identity of the me tal catalyst complex, with isomerization being significantly slower than trans-metathes is, and trans-metathesis being slower than condensation (end group) metathesis. Since isomerization occurs at a slower rate than either metathesis reaction with complex [Ru]10, lower temperatures should suppress this reaction to the advantage of metathesis chemistry. This has been shown to be true in prior research,176 and the same observation is noted here when using catalyst [Ru]10. In fact, the conversion of 1-octene

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51 is quantitative at 0 C in 3 days, with only 30% of the product mixture being isomers, as compared to 76% isomers at room temperature. Recently, Mol et al.92 demonstrated that a possi ble decomposition pathway of catalyst [Ru]1 results in the formation of (PCy3)2(CO)Ru(Cl)(Ph) which appears to be a good precatalyst for olefin isomerization. This carbonyl complex coul d be converted into the corresponding hydride (PCy3)2(CO)Ru(Cl)(H) upon reaction with a terminal olefin and could promote 1-octene isomerization without any trace of metathesis product. Further, Grubbs and coworkers72 witnessed the formation of the hydride complex (H2Mes)(PCy3)(CO)Ru(Cl)(H) by heating complex [Ru]4 in the presence of oxygencontaining substrates. These prior observ ations suggest that the ruthenium hydride equivalent of [Ru]10 could be formed under ADMET reaction conditions and could account for the isomerization chemistry repor ted herein. The rapid decomposition of complex [Ru]10 observed during our prior kinetic study is consistent with isomerization occurring early in the reaction and increasi ng over time, all as a result of increasing ruthenium hydride concentration. However, th e possibility of isom erization catalyzed by the metathesis catalyst itself cannot be ruled out at this point. Complex [Ru]10 is sufficiently active to induc e isomerization of the starting reagent, 1-octene to other C8 isomers (2-, 3and 4-octene), which happens concurrently with metathesis chemistry, and further, complex [Ru]10 appears to induce isomerization of internal bonds at a substantiall y higher rate than other catalysts. Additional evidence for the hi gh propensity of catalysts [Ru]10 and [Ru]11 to isomerize internal olefins is found in the melting behavior of the materials produced when using them to polymerize 1,9-decadiene. Schrock’s molybdenum catalyst

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52 (complex 6), which yields virtually no isomer ization, produces poly(octenamer) that exhibits a melting point of 75 C,176 close to the theoretical maximum for this polymer. On the other hand, catalysts [Ru]10 and [Ru]11 yield poly(octenamer) samples possessing considerably lower melting points, 40 C and 45 C respectively. Isomerization, in effect, creates random c opolymers via migration of internal olefins along the polymer backbone or via monome r isomerization. This decrease in regioregular placement of olefinic groups in the poly(octenamer) re sults in a depression of the polymer peak melting poi nt as observed for both catalyst [Ru]10 and [Ru]11. Conclusions Complex [Ru]10 is highly active at low temperatures for the ADMET polymerization of 1,9-decadiene; loadings as low as 0.1 mol% produced high molecular weight poly(octenamer). In addition, complex [Ru]10 has the highest initiation rate of any phosphine ligated ruthenium complex to date. The catalyst is sensitive to thermal decomposition and therefore becomes less active as the temperature increases. The high initiation rate and activity at low temperatures make complex [Ru]10 an interesting prospect for further metathesis considerati ons, notably for low boiling point reagents or thermally sensitive substrates in both organic synthesis or polymerization. On the contrary, complex [Ru]9 does not exhibit satisfactory activity with non functionalized substrates. Although known for its robustness, complex [Ru]9 appears to require stabilization either from a polar solvent or from a functionalized monomer in order to serve as a viable polyconde nsation catalyst. Complex [Ru]11 exhibits an interesting stability and produces high molecular weight polymers. The increase of steric interactions on the 4,5 dihydroi midazolylidene carbene ligand increases the catalytic

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53 activity, while the incorporation of Hoveyda -type ligand expands the stability of the complex. Complexes [Ru]10 and [Ru]11 both catalyze olefin isomerization in high yields, however, given that both complexes are hi ghly reactive at room temperature their isomerization/metathesis processes ca n be mediated by lowering the reaction temperature. In fact, complex [Ru]10 efficiently metathesizes 1-octene at 0 C resulting in faster metathesis rates than isomeriza tion. This isomerization pathway could be attributed to the hydride complex, whose forma tion and/or catalytic activity is highly dependent on temperature conditions. Therefor e, increasing the catalyst stability towards decomposition seems most important to design catalysts that will produce regular polymers via metathesis chemistry. Nonetheless, complexes [Ru]10 and [Ru]11 are useful catalysts for ADMET polymerization, with out the temperature constraints of either catalysts [Ru]1 or [Ru]4. Experimental General Considerations 1H NMR (300 MHz) and 13C NMR (75 Hz) spectra of the ADMET polymers were recorded in CDCl3 on either a Mercury series or Varian VXR-300 NMR superconducting spectrometer. Chemical shifts were referenced to residual CHCl3 (7.23 for 1H and 77.23 for 13C) with 0.03% v/v TMS as an internal refe rence. All organometallic spectra were recorded in d6-benzene and the chemical shifts were referenced to residual C6H6 (7.16 for 1H and 128.39 for 13C). Gel permeation chromatography (GPC) of the unsaturated ADMET polymers was performed using two 300 mm Polymer Labor atories gel 5m mixed-C columns. The instrument consisted of a Rainin SD-300 pump, Hewlett-Packar d 1047-A RI detector

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54 (254 nm), TC-45 Eppendorf column heater set to 35 oC, and Waters U6K injector. The solvent used was THF at a flow rate of 1.0 mL/min. Polymer samples were dissolved in HPLC grade THF (approximately 0.1% w/v) and filtered before injection. Retention times were calibrated to polystyrene sta ndards from Polymer Laboratories (Amherst, MA). GC analysis was conducted on a Shimadzu GC-17A chromatograph equipped with a HP-5 (Hewlett Packard) 25 m column with FID detection. All amounts of products were calculated by a systematic ratio to th e Decalin peak at 8.72 minutes, using the integrated peak areas. Differential scanning calorimetry (DSC) wa s performed using a Perkin-Elmer DSC 7 at a heating rate of 10 C/min. Thermal calibrations we re made using indium and freshly distilled n-octane as references for thermal transi tions. Heats of fusion were referenced against indium. The samples we re scanned for multiple cycles to remove recrystallization differe nces between samples and the resu lts reported are the second scan cycle. Reported values are given as Tm (melting peak). Complex [Ru]9 was a gift from Materia Co. and was used as received. Complex [Ru]10 was synthesized according to the literature procedure.101 All catalysts were stored in an argon-filled drybox prior to use in ki netic and polymerization experiments. 1,9Decadiene (Aldrich) was distil led from Na/K alloy under reduced pressure into a Kontes flask equipped with a Teflon valve, dega ssed by three freeze-pump-thaw cycles, and stored in an argon-filled drybox. For the kinetic study, 1,9decadiene was portioned into small Teflon-capped vials in the drybox and were removed and stored in a dessicator until use. All polymerizations were conducte d in the bulk using the different monomer-

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55 to-catalyst ratio displayed in the tables 2-Isopropoxystyrene was synthesized by a standard Wittig reaction on 2-isopropoxybenzylaldehyde using (methyl)triphenylphosphine iodide and potassium t -butoxide (Aldrich). 2Isopropoxybenzylaldehyde was synthesized via a Williamson-ether synthesis from salicyl-aldehyde (A ldrich) and 2-bromopropane (Ald rich). Pentenyl ether was synthesized according to the literature,123 distilled over CaH2 and stored in the drybox. All other starting materials were distilled over Na/K alloy before use, except chlorinated compounds which were distilled over CaH2. After distillation, d6-benzene was degassed by three freeze-pump-thaw cycles, an d stored in an argon-filled drybox. X-ray Experimental Crystals suitable for X-ra y structure determination were obtained from slow diffusion of pentane into a saturated solution of [Ru]11 in CH2Cl2 at room temperature. Data were collected at 173 K on a Siemens SMART PLATFORM equipped with A CCD area detector and a graphite monochromator utilizing MoK radiation ( = 0.71073 ). Cell parameters were refined using up to 8192 reflections. A full sphere of data (1850 frames) was collected using the -scan method (0.3 frame width). The first 50 frames were remeasured at the end of data collecti on to monitor instrument and crystal stability (maximum correction on I was < 1 %). Ab sorption corrections by integration were applied based on measured indexed crystal faces. The structure was solved by the Direct Methods in SHELXTL6, and refined using full-matrix least squares. The non-H atoms were treated anisotropically, whereas the hydrogen atoms were calculated in ideal pos itions and were riding on their respective carbon atoms. The asymmetric unit consists of two chemically equivalent but

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56 crystallographically independent Ru complexe s and a half molecule of dichloromethane (located on a center of inversion). A total of 813 parameters were refined in the final cycle of refinement using 22770 reflections with I > 2 (I) to yield R1 and wR2 of 6.48% and 8.38%, respectively. Refinement was done using F2. Synthesis of Complex [Ru]11 In a glovebox, complex [Ru]10 (930 mg, 1.00 mmol) and CuCl (Aldrich) (100 mg, 1.03 mmol) were weighed into a 100 mL Schl enk flask and dissolved in 20 mL of CH2Cl2. 2-Isopropoxystyrene (180 mg, 0.970 mmol) was dissolved in 2 mL of CH2Cl2 and added to the solution of complex [Ru]10 and CuCl at room temperature. The flask was equipped with a condenser and the soluti on was refluxed for 20 minutes. From this point forth, all manipulations we re carried out in air with reagent-grade solvents. The reaction mixture was concentrated in vacuo to a green residue. The unpurified material was dissolved in a minimal volume of 1:1 pentane/CH2Cl2 and loaded onto a plug of silica gel. Insoluble coppe r-phosphine precipitates were removed prior to loading by passing the solution through a second Pasteur pipette containing a pl ug of glass wool. Elution with 1:1 pentane/CH2Cl2 removed a bright green band from the column. The eluant was concentrated and a ddition of hexanes just prior to complete removal of the solvent resulted in spontane ous precipitation of the product. Filtration and drying under high vacuum afforded 691 mg (0.970 mmol 97%) of a pale green powder. 1H NMR (C6D6, 300 MHz): (ppm)=16.4 (s, 1H, Ru=C H Ph), 7.53 (t, 2H, para C H 3JH,H = 7.2Hz), 7.43 (m, 1H, ortho C H ), 7.37 (m, 4H, meta C H ), 6.80 (td, 2H, meta-para C H 3JH,H = 7.4Hz), 6.78 (d, 1H, ortho CH, 2JH,H = 8.4Hz), 4.92 (sept, 1H, (CH3)C H OAr, 3JH,H = 6.3Hz), 4.18(s, 4H, N(C H2)2N), 3.60 (sept, 1H, C H (CH3)2, 3JH,H = 6.6Hz), 1.38 (d, 6H,

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57 OCH(C H3)2, 2JH,H = 6.3Hz), 1.26 (d, 24H, CH(C H3)2, 3JH,H = 7.1Hz); 13C NMR (C6D6, 75 MHz): = 296.7, 215.5, 149.6, 144.79, 130.0, 129.1, 124.8, 122.4, 122.2, 113.2, 112.1, 112.1, 75.2, 54.9, 29.3, 26.8, 23.9, 21.9. Kinetic Study The kinetic study was performed with the same apparatus and following the same procedure as previously described by Wagener.144 Each catalytic run was repeated 5 times for reproducibility. ADMET Polymerization of 1,9-decadiene All glassware was thoroughly cleaned a nd oven-dried. All metathesis reactions were initiated inside the drybox using 50 mL Schlenk flas ks equipped with a Teflon stirbar. The flasks were then brought out of the drybox and placed on a high vacuum line (<10-3 mmHg) while vigorously stirring. Th e polymerization vessel was exposed to intermittent vacuum at room temperature un til the reaction either became highly viscous or solid (stirring ceased). The flask was then placed in 55 C oil bath at high vacuum (<10-3 mmHg) for 120 hours. The polymerization vessel was cooled at room temperature, and finally, the unsaturated polymer was taken up into toluene and precipitated into cold acidic methanol (1 M HCl) to remove catalyst residue. Self-Metathesis Dimerization A clean, dry 50 mL Schlenk fl ask equipped with a Teflon stir bar was charged with complex [Ru]10 or [Ru]11 at the desired ratio in the drybox. The flask was brought out of the drybox and placed on a Schlenk line. T ypically, 4.5 mmol of olefin was injected through an Ar purged syringe. The mixture was stirred vigorously at the desired temperature under Ar and open to an oil bubbl er to maintain ambient pressure. For complex [Ru]10, samples were taken at appropriate intervals under a posi tive pressure of

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58 Ar with a dry pipette and added to a chlo roform solution contai ning traces of 2,6-di-tbutyl-4-methylphenol used as a radical inhibi tor, ethyl vinyl ether used to quench the metathesis catalyst, and Decalin used as a standard internal reference. Measurement of the Thermal Stabilit y of Complex [Ru]10 and [Ru]11 Benzened6 solutions (32 mM) of complexes [Ru]10 and [Ru]11 were heated to 55 C and monitored by 1H NMR spectroscopy. The half-lif e was calculated as the time required for half the ma terial to decompose ( 1/2) through integration of the carbene peak (19.9 ppm for [Ru]10, 16.4 ppm for [Ru]11) which disappears with decomposition of the complex.

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59 CHAPTER 3 THE UTILITY OF HOVEYDA-TYPE CATALYSTS IN ADMET CHEMISTRY: STERICS VS. ELECTRONICS Introduction Over the past decade, olefin metathesis has become a major synthetic method for the formation of C-C bonds,1,2,7-10,20 especially since the discovery of the Grubbs’ functional group tolerant ru thenium carbene catalyst [Ru]1.37,57,80,87 Its reactivity was further enhanced by exchanging one phosphine with an N -heterocyclic carbene (NHC) ligand (complex [Ru]471and [Ru]10100,101). This phosphine mimic, being a strong donor but a weak -acceptor, favors the binding of olef inic substrates to ruthenium, which results in higher turnovers.64,70,71,74,75,101 Since then, other modified complexes have been reported (Figure 3-1), among th em the Hoveyda-type catalysts bearing an isopropoxystyrene ligand ([Ru]9).95 The chelating nature of the ligand provides an exceptional stability to this type of cataly st and allows its recovery after some ringclosing metathesis (RCM) and cross-metathesis (CM) reactions.186 Ru Cl Ph PCy3Cl Ru Cl Ph PCy3Cl Ru Cl O Cl Ru Cl O ClN N N N N N N N[Ru]4[Ru]10 [Ru]9[Ru]11 Figure 3-1. Olefin metathesis catalysts Even though these phosphine-free alkyliden es initiate more slowly than their phosphine analogs, they have gathered much in terest for both their ease of use and their

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60 enhanced reaction rates with electron deficient olefins. In addition, the isopropoxystyrene ligand represents an idea l anchor point for modification studies involving the introduction of differe nt steric and electronic groups108,109,187-190 as well as ionic groups191 and solid supports.102-107 Fine tuning of the ligand structure can be achieved for a desired substrate, as the ini tiation rate and the overall activity of these catalysts have proven very sensitive to th e nature of the isopropoxybenzylidene. As suggested by the dissociative mechanism, th e catalytic activity can be enhanced by 1) increasing the steric hindrance around the isopropoxy group, or 2) decreasing the electron density at both the chelating oxyge n atom and the benzylidene carbon.108,109 Our laboratory routinely uses metathesis catalysts for the synthesis of unique macromolecules via acyclic diene metathesis (ADMET) polymerization.21,22,121-133,146-148 The extended lifetime of this class of catalyst is of particular inte rest, since long-living catalysts are desirable considering the reaction times required by ADMET polycondensation. Typically, the activity of a catalyst in ADMET is evaluated through comparative kinetic experiment s using standard catalysts [Ru]1 and [Ru]4, and 1,9decadiene as the substrate.155,144,192 This method has allowed to find optimal conditions of temperature and concentration for a wide range of metathesis catalysts in ADMET chemistry.144,192 In addition, the effect of structur al perturbations on catalytic activity was investigated in order to model more effi cient catalyst systems. For instance, larger substituents around the NHC ligand ([Ru]10 and [Ru]11) result in faster initial rates while incorporation of is opropoxybenzylidene expands the stability of the complex ([Ru]9 and [Ru]11).192

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61 After developing complex [Ru]11, whose increased activity was more suitable to ADMET polymerization,192 the effect of electronics on the isopropoxy fragment of the catalyst was examinated. Herein, a compre hensive study is presented on a series of ruthenium catalysts bearing different isopropoxybenzilydene ligands and different NHC ligands (Figure 3-2). Their catalyti c activity is measured during the ADMET oligomerization of 1,9-decadiene at different temperatures, and th eir initial rates are compared to standard catalysts for ADMET. Results and Discussion Following the introduction of faster initi ators by increasing the NHC ligand bulk, the focus turned to the isopropoxy fragment of the catalyst, the id ea being to enjoy the inherent stability of the Hoveyda-type catalysts while improving their catalytic activity in ADMET. Although the influence of electronics and sterics has been studied in much detail in small molecule chemistry,108,109,187,190 the catalytic activity was only observed during monocoupling reactions such as cross-me tathesis or ring-closing metathesis. The polycondensation nature of ADMET renders the catalytic profile very different, hence the use of different parameters to measure cata lytic activity in this study. For example, complex [Ru]9 has been successful in many organi c reactions but only results in low conversions in the ADMET oligomerization of 1,9-decadiene.192 Therefore, electronic modifications were applied on complex [Ru]11, which has proven to be an efficient ADMET catalyst. The design of catalysts [Ru]13-[Ru]16 was based on the idea that decreasing the electron de nsity at the chelating i PrO fragment would result in even higher catalytic activities, the ultimate goal being th e elaboration of a Hammett plot. The same modifications were applied to complex [Ru]9 to control the influe nce of sterics on the NHC ligand.

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62 O CuCl in CH2Cl2Ru Cl O Cl L R Ru Cl Ph PCy3Cl L [Ru]4 L=H2IMes [Ru]10 L=H2IPr [Ru]9 L=H2IMes, R=H [Ru]11 L=H2IPr, R=H [Ru]12 L=H2IMes, R=NO2[Ru]13 L=H2IPr, R=NO2[Ru]14 L=H2IPr, R=Cl [Ru]15 L=H2IPr, R=CH3[Ru]16 L=H2IPr, R=OCH3RN N N N H2IMes H2IPr Figure 3-2. Synthesis of complexes [Ru]9, [Ru]11-16 The ligands were obtained from the corresponding substituted ohydroxybenzaldehyde which was subjected to a Williamson substitution followed by a Wittig reaction. The synthesis of complexes [Ru]12-16 then followed the same procedure as for complex [Ru]9 and [Ru]11, affording good to excellent yields (8395%).95 All complexes are stable in solution and in air. Each catalyst was subjected to a systema tic reaction with 1,9-decadiene using a 450:1 monomer:catalyst ratio to reproduce t ypical polymerization conditions. Reaction progress was followed by quantifying ethylen e as it was released, and the degree of polymerization was plotted against time.155 The catalytic activity was evaluated in comparison with [Ru]9 and [Ru]11 at 30o, 45o and 60C. Steric effect Table 1 shows the initial rates for catalysts [Ru]9,11 and [Ru]12,13 at different temperatures, calculated as the initial slope (DP vs. time) of each ki netic curve during the average dimerization (DP=2) of the monomer.193 Rates increase with temperature for all complexes, and a straight line is obtained when the log of the initial rate is plotted against 1/T, verifying the Arrhenius la w. The activation energies Ea are estimated to be 40.2

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63 KJmol-1s-1 and 48.9 KJmol-1s-1 for [Ru]11 and [Ru]13 respectively. Introduction of the nitro group on the isopropoxybenz ylidene results in an increas e of the initial rate for both catalysts, for indeed, the presence of the electron withdrawing substituent para to the isopropoxy group should faci litate the ligand dissociati on by decreasing the electron density on the coordinating oxygen, leading to the active species.95,99,194 However, a closer examination at Table 1 indicates that the electronic effect is only minimal for catalyst [Ru]12, bearing the mesityl NHC ligand, c onversely to what was observed in monocoupling reactions.109,195 Indeed, catalysts [Ru]9 and [Ru]12 exhibit comparable initial rates regardless of th e temperature (around 20 DP s-1 at 45oC, and 42 DP s-1 at 60oC). This observation emphasizes the fundame ntal difference between small molecule metathesis and ADMET, where the initial rate is not necessarily a direct correlation of the isopropoxystyrene dissociation rate, but also involves the stability of the 14 ecomplex. In this case, the propagating species Ru=CH(CH2)6CH=CH2 is the same for catalysts [Ru]9 and [Ru]12. The similarity of the initial rates obtained here, regardless of the dissociating ligand, seems to i ndicate that the propagation st ep is rate determining in ADMET, i.e. that dissociation is fast. Table 3-1. Initial rates in DP s-1 for catalysts [Ru]9,[Ru]11-13. Temperature[Ru]9 [Ru]12 [Ru]11 [Ru]13 30oC 5 3 10 2 13 2 21 4 45oC 20 5 22 4 45 4 48 7 60oC 42 4 42 9 70 10 117 8 In the case of catalysts [Ru]11 and [Ru]13, containing a bulkier NHC ligand, the nitro group seems to have a smaller effect on the initial rate, if any, at 30 and 45oC than at 60oC. For example, at 45oC the nitro complex [Ru]13 only exhibits a 3 DP s-1 rate increase (within the experimental error) compared to parent [Ru]11, while a 50 DP s-1

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64 rate increase is reported at 60oC for [Ru]13. The difference at 30oC does not seem significant and will be further investigated by comparing other electronically modified catalysts [Ru]14-[Ru]16. The higher rates of complexes [Ru]11 and [Ru]13 versus [Ru]9 and [Ru]12 are due to the increase in steric bulk on the NHC ligand brought about by the isopropyl groups, which exert a stronge r steric pressure on the benzylidene and therefore facilitate its dissociation.61 As a consequence, at 60oC complex [Ru]13 exhibits one of the fastest initiation rates (117 DP s-1) ever reported for the ADMET reaction of 1,9-decadiene. 050010001500200025003000 1 2 3 4 5 6 7 DPTime (s) [Ru]11 [Ru]13 [Ru]12 [Ru]9 Figure 3-3. Degree of polymerization (DP) vs. time data for H2IMes versus H2IPr containing catalysts using 450:1 monomer:catalyst ratio at 45oC This substituent effect is not as obvious when examining the overall activity of the catalysts [Ru]9,[Ru]11-13 (Figure 3-3). The ‘tailing off’ of each curve is due to the reaction mixture becoming more viscous as th e DP increases until reaching a solid state (around a DP of 4-5) where the rate becomes diffusion controlled. Even so, the catalysts deviate from the initial trend discussed above before re aching the solid state and after

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65 dimerization. The catalysts bearing bulkier NHC ligands ([Ru]11 and [Ru]13) seem to be unaffected by the presence of the nitro group, which is in striking contrast with analogs [Ru]9 and [Ru]12. Indeed, complex [Ru]12 shows almost a two-fold increase in its overall activity compared to the parent catalyst [Ru]9 (at 30 minutes the DPs are 2.8 and 4.1 for [Ru]9 and [Ru]12, respectively), while [Ru]11 and [Ru]13 show the same catalytic profile. The electronic effect on the isopropoxybenzylidene seems to be dominated by the steric hindrance of the NHC ligand. Following the model of ruthenium-phosphine complexe s, if the presence of H2IPr facilitates dissociation, then it also slows down catalyst deactivation by rebi nding of free phosphine, or in this case, rebinding of the ether-tethered ligand ( i PrO).99 Thus, complexes [Ru]11 and [Ru]13 actually undergo more turnovers before being trapped by i PrO. The dissociation/rebinding rate s ratio appears to appr oach an optimum with H2IPr ligands, a ratio that is hardly disturbed by simply changing the electr on density around the isopropoxystyrene. On the other hand, the rebinding of i PrO is rendered easier in the presence of the less hindered H2IMes ligand. In this case, the electron density of the coordinating oxygen has a greater effect on bot h the dissociation and rebinding rates, which explains the higher reactivity of [Ru]11 whose nitro-substituted ligand will be less likely to recoordinate and ther efore increases the amount of ac tive species able to react. This electronic effect is not evident duri ng the initial rate probably because ligand rebinding only becomes substa ntial after several turnovers. Electronic effect on H2IPr ligated complexes The series of complexes [Ru]11-[Ru]16 should help better understand the importance of dissociation/rebinding of these ether-tethered ligands. All the synthesized complexes are efficient catalysts for th e ADMET oligomerization of 1,9-decadiene

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66 reaching DPs of 4 and 5 under 5 minutes at 45oC. As illustrated in Figure 3-4, even at DP>4 (where the viscosity becomes significan t) the polymerization continues at a steady rate without any sign of extensive decom position of the catalytic center, usually observable by a darkening of th e solution, confirming the robust nature of these catalysts. 05001000150020002500 1 2 3 4 5 6 7 8 DPTime (s) CH3 [Ru]15 OCH3 [Ru]16 H [Ru]11 NO2 [Ru]13 Cl [Ru]14 Figure 3-4. Degree of polymerization (DP) vs. time data for complexes [Ru]11-[Ru]16 using 450:1 monomer:catalyst ratio at 45oC Table 2 gives an overview of th e initial rates for complexes [Ru]11-[Ru]16 at 30o, 45o and 60oC. As observed earlier, initial rates at 30oC and 45oC stay the same regardless of the isopropoxybenzylidene used (around 20 DP s-1 at 30oC, and 45 DP s-1 at 45oC within experimental error) except for parent catalyst [Ru]11, which exhibits a slightly lower initial rate at 30oC. The difference noticed earlier (Table 1) could have been insignificant if the rates of the electronically modified catalysts [Ru]13-[Ru]16 had not been so reproducible. The singularity of the unsubstituted catalyst [Ru]11 proves the existence of an electronic effect, albeit not qua litative. This lack of sensitivity to the electronic nature of the substi tuent is again in significant co ntrast to what was observed

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67 during CM and RCM reactions catalyzed by [Ru]9 analogs; however, other reports have also referred to the unpredictability of other electronically-modified Hoveyda-type catalysts.108,109,196 Here, the steric hindrance present on the NHC ligand seems to overshadow the electronic nature of the dissociating ligand at low temperatures, canceling the effect predicted by sigma values. This proposal finds credence in the X-ray structure of [Ru]11, [Ru]13 and [Ru]16. The Ru-O bond length, usually indicative of the strength of the i PrO->Ru chelation, does not show si gnificant variation (See the X-ray crystal structures in Appendix A) Conversely, nitro-substituted [Ru]13 exhibits a slightly shorter Ru-O bond length (2 .2462(3) ) while methoxy-substituted [Ru]16’s is slightly longer (2.2486(18) ). Table 3-2. Initial rates in DP s-1 for catalysts [Ru]11-[Ru]16. Temperature NO2 [Ru]13 Cl [Ru]14 H [Ru]11 CH3 [Ru]15 OCH3 [Ru]16 0.71 0.37 0 -0.06 0.05 + 0.79 0.11 0 -0.31 -0.78 30oC 21 4 21 3 13 2 22 5 19 4 45oC 48 7 41 8 45 4 47 2 40 5 60oC 117 8 118 15 70 10 136 9 110 7 Values for and + were taken from ref. 198 At 45oC all complexes follow about a similar cata lytic profile, i.e. there is no major improvement brought about by the electronic substituents. However, there is a noticeable distinction between complexes (Figure 3-4) Electron donating groups (EDG) appear to increase the catalytic activity while ewithdrawing groups (EWG) reduce the rate. Since the initial rates at 30oC and 45oC are the same for all catalysts, the dissociation rate of isopropoxybenzylidene cannot be the only dete rmining factor. Theoretically, after dissociation the propagating species is the same for complexes [Ru]11-[Ru]16.

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68 Therefore, the lability of isopropoxystyrene must allow its rebindi ng to the ruthenium center so that it influences the overall catalytic activity. The trend observed at 45oC can be rationalized by cons idering the formation of the unstable 14especies, either the alkylidene or the methylidene complex. In Chapter 2, it was demonstrated that this type of cataly st was quite sensitive to substrate polarity compared to their phosphine analogs.192 In the latter, the phos phine coordinates back to the 14eintermediate to stabilize it and forms a dormant state.61 In Hoveyda-type complexes, this type of stabilization is lessened by the extreme lability of isopropoxy styrene. However, all substrates conventionally used to probe the metathesis activity of any catalyst contain some hetero atom that can serve as a stabilizer, along with the solvent used for the reaction. For example, dur ing the ROMP reaction of diverse oxygencontaining monomers, Khosravi et al.154 showed that Hoveyda-type catalysts were stabilized through the ch elation of an oxygen contained in the monomer unit. In the case of substrates lacking any kind of elec tron donor or Lewis basic group, such as 1,9decadiene, the 14eintermediate is not stabilized; therefore, the catalyst is more susceptible to decomposition, which results in slower rates. With this in mind, it is reasonable to assume that the catalysts with higher dissociation rates and/or slower rebinding rates should have a lower overall ac tivity. According to sigma values, electron withdrawing substituents para to the isopropoxy group should facilitate the ligand dissociation by decreasing th e electron density on the c oordinating oxygen and increase its lability, while electron dona ting substituents should slow the dissociation step and facilitate its rebinding. Simply stated, donating groups should slow down catalyst decomposition. Consequently, complexes 6 and 7 (EDG) show a slightly higher

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69 reactivity than complexes 4 and 5 (EWG), while neutral complex [Ru]11 lies in between (Figure 3-2). Temperature effect When the temperature is raised to 60oC, the singularity of catalyst [Ru]11 over the electronically modified [Ru]13-[Ru]16 is accentuated. While initial rates for catalysts [Ru]13-[Ru]16 are all around 120 DP s-1, parent complex [Ru]11 only affords a rate of 70 DP s-1, broadly deriving from the predicted Arrh enius plot. Electronics now represent a determining factor on catalytic activity, which may be the result of a switch in the mechanism possibly due to a change in cat alyst conformation. Similar temperature activation barriers have been witnesse d with nitrogen-chelated complexes.198 05001000150020002500 1 2 3 4 5 6 7 8 DPTime (s) Cl [Ru]14 CH3 [Ru]15 OCH3 [Ru]16 NO2 [Ru]13 H [Ru]11 Figure 3-5. Degree of polymerization (DP) vs. time data for complexes [Ru]11-[Ru]16 using 450:1 monomer:catalyst ratio at 60oC The same catalytic profile persists as th e reaction continues, and the overall activity is now significantly higher for the modified catalysts [Ru]13-[Ru]16 (Figure 3-5). This suggests that the steric effect of the H2IPr ligand is no longer dominant, even though the

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70 electronic factor cannot be qua lified according to sigma values This apparent lack of sensitivity towards the electronic nature of th e substituents is more likely the result of several mechanistic processes operating simu ltaneously. While el ectron withdrawing groups could facilitate dissociation by w eakening the iPrO->Ru chelation, electron donating groups could equally in crease catalytic activity by re ducing the Lewis acidity of the metal. The rate of decomposition of the methylidene intermediate, which is accelerated at elevated temperatures, is pr obably an important factor. Also, assuming that the H2IPr ligand does not hinder the rebinding of the i PrO moiety, the 14especies can be properly stabilized, but, since the dissociation is quite fast, the catalyst quickly returns to the catalytic cycle affording higher DPs. The diminished steric effect may be due to a reorganization of the NHC ligand, possibly by ring rotation, often seen at high temperatures. Conclusions This report gives a different outlook on the influence of the ligand sphere’s electronics/sterics on Hoveyda -type metathesis catalysts. Unlike most modification studies conducted on these complexes, the ‘s teric factor’ is only located on the NHC ligand (isopropyl vs. methyl groups) wher eas the ‘electronic factor’ is on the isopropoxybenzylidene. All the complexes tested are efficient catalysts for the ADMET polymerization of 1,9-decadiene. However, in the polycondensation conditions of ADMET, the steric hindrance larg ely dominates any electronic effect at all temperatures. Interestingly, unsubstituted complex [Ru]11 curiously stands out at 30oC and 60oC, suggesting some participation of electronics in the catalytic potential of these Hoveydatype complexes. The overall activity of these complexes seems to depend only on the propensity of the isopropoxystyrene ligand to rebind to the ruthenium center and stabilize

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71 it in a similar way as their phosphine analogs, es pecially at low temperatures. As a result, complexes bearing EWGs result in lower DPs than complexes bearing EDGs, the more active catalyst being the better st abilized. In any case, complexes ligated to larger NHC ligands are better catalysts than a ny modified Hoveyda-Grubbs such as [Ru]12, again showing the prevalence of ster ics over electronics. At 60oC, all electronically modified catalysts exhibit comparable initial rates and increased reactivities, while parent complex [Ru]11 constitutes a significant exception. The catalytic profiles of [Ru]13-[Ru]16 are similar and lay 2 DP over [Ru]11’s, implying that the different electronic groups are able to improve activity through different processes. A reorganization of the ligand sphere may occu r at higher temperatures so that the steric bulk of the NHC ligand doe s not hinder the rebinding of isopropoxystyrene anymore. Consequently, catalysts [Ru]13-[Ru]16 represent the fastest initiators ever tested for ADMET polymerization. Further ex periments will be conducted to investigate the importance of ligand rebind ing in these Hoveyda-type cat alysts, which represent an interesting prospect for metathesis polycondensation. Experimental General 1H NMR (300 MHz) and 13C NMR (75 Hz) spectra of the organometallic complexes were recorded in CDCl3 on either a Mercury seri es or Varian VXR-300 NMR superconducting spectrometer. Chemical shif ts were referenced to residual CHCl3 (7.27 for 1H and 77.23 for 13C) with 0.03% v/v TMS as an internal reference. Complex [Ru]9 was a gift from Materia Inc. a nd was used as received. Complex [Ru]4,71 [Ru]10,101 [Ru]11,192 and [Ru]12195 were synthesized accord ing to the literature procedure. All catalysts were stored in an argon-filled drybox prior to use in kinetic and

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72 polymerization experiments. 1,9-Decadiene (Aldrich) was distilled from Na/K alloy under reduced pressure into a Kontes flask equipped with a Teflon valve, degassed by three freeze-pump-thaw cycles, and stored in an argon-filled drybox. For the kinetic study, 1,9-decadiene was portioned into sma ll Teflon-capped vials in the drybox and were removed and stored in a dessicator until use. All other starting material were disti lled over Na/K alloy before use except chlorinated compounds which were distilled over CaH2. After distillation, dchloroform was degassed by three freeze-pump-thaw cycles and stored in an argon-filled drybox. X-ray Experimental Crystals suitable for X-ra y structure determination were obtained from slow diffusion of pentane into a saturated solution of [Ru]13 or [Ru]16 in CH2Cl2 at room temperature. Data were collected at 173 K on a Siemens SMART PLATFORM equipped with A CCD area detector and a grap hite monochromator utilizing MoK radiation ( = 0.71073 ). Cell parameters were refined using up to 8192 reflections. A full sphere of data (1850 frames) was collected using the -scan method (0.3 frame width). The first 50 frames were remeasured at the end of data collection to monitor instrument and crystal stability (maximum correction on I was < 1 %). Absorption corrections by integration were applied based on measured indexed crystal faces. The structure was solved by the Direct Methods in SHELXTL6, and refined using full-matrix least squares. The non-H atoms were treated anisotropically, whereas the hydrogen atoms were calculated in ideal pos itions and were riding on their respective carbon atoms. Refinement was done using F2:

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73 For [Ru]13, a total of 426 parameters were refine d in the final cycle of refinement using 7198 reflections with I > 2 (I) to yield R1 and wR2 of 3.19% and 7.23%, respectively. For [Ru]16, a total of 406 parameters were refine d in the final cycle of refinement using 6677 reflections with I > 2 (I) to yield R1 and wR2 of 3.85% and 11.46%, respectively. The toluene molecule were di sordered and could not be modeled properly, thus program SQUEEZE, a part of the PLAT ON package of crystallographic software, was used to calculate the solvent disorder area and remove its contribution to the overall intensity data. General Procedure for Ligand Synthesis 2-Isopropoxystyrene derivatives were synthesized by a standard Wittig reaction on the 2-isopropoxybenzylaldehyde parent usi ng (methyl)triphenylphos phine iodide and potassium t -butoxide (Aldrich). All the 2-isop ropoxybenzylaldehyde derivatives were synthesized via a Wilkinson-ether synthesi s from the corresponding salicyl aldehyde (Aldrich) and 2-bromopropane (Aldrich). Spectral data for 1-isopropoxy-2-vinyl-4methylbenzene,108 1-isopropoxy-2-vinyl-4-nitrobenzene,195 and 1-Isopropoxy-2-vinyl-4methoxybenzene196 match the literature. General Procedure for Carbene Exchange In a glovebox complex [Ru]4 (300 mg, 0.32 mmol) and CuCl (Aldrich) (32 mg, 0.32 mmol) were weighed into a 100 mL schl enk flask and dissolved in 15 mL of CH2Cl2. 2-Isopropoxy-5-nitrostyrene (133 mg, 0.6 4 mmol) was dissolved in 5 mL of CH2Cl2 and added to the solution of complex [Ru]4 and CuCl at room temperature. The flask was equipped with a condenser and the solution was refluxed for 30 minutes or until

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74 the brown solution turns to a deep green. Fr om this point forth, all manipulations were carried out in air with reagent-grade solvents The reaction mixture was concentrated in vacuo to a green residue. The unpurified mate rial was dissolved in a minimal volume of 5:2 cyclohexane/ethyl acetate and loaded ont o a plug of silica ge l. Insoluble copperphosphine precipitates were removed prior to loading by passing the solution through a second Pasteur pipette containing a plug of glass wool. Elution with 5:2 cyclohexane/ethyl acetate removed a bright green band from the column. The solvent was evaporated and the residue washed with n -pentane. Synthesis of complex [Ru]13. (-NO2) The general procedure was followed to afford 200 mg (0.267 mmol, 83%) of a green powder. 1H NMR (CDCl3, 300 MHz): =16.32 (s, 1H, Ru=C H Ph), 8.40 (d, 1H, para C H 2JH,H = 8.4 Hz ), 7.70 (s, 1H, ortho C H ), 7.58 (t, 2H, para C H 3JH,H = 7.2 Hz), 7.39 (d, 4H, meta C H ), 6.90 (d, 1H, meta C H 2JH,H = 8.4 Hz), 4.98 (sept, 1H, (CH3)2C H OAr, 3JH,H = 6.3 Hz), 4.20 (s, 4H, N(C H2)2N), 3.54 (sept, 4H, C H (CH3)2, 3JH,H = 6.6 Hz), 1.40 (d, 6H, OCH(C H3)2, 2JH,H = 6.3 Hz), 1.24 (d, 24H, CH(C H3)2, 3JH,H = 7.1 Hz); 13C NMR (CDCl3, 75 MHz): = 284.4, 210.6, 156.9, 149.3, 143.9, 143.2, 136.4, 130.3, 124.7, 124.3, 117.0, 113.0, 77.9, 54.8, 29.1, 27.1, 26.8, 21.9. MS (HRMS/EI) calcd for C37H49O3N3Cl2Ru [M]+ 755.2194, found 755.2161. Anal. Calcd for C37H49O3N3Cl2Ru: C, 58.80; H, 6.53; N, 5.56. Found: C, 59.96; H, 7.03; N, 5.20. Synthesis of complex [Ru]14. (-Cl) The general procedure was followed using 2isopropoxy-5-chlorostyrene (125 mg, 0.64 mmol) to afford 202 mg (0.272 mmol, 85%) of a pale green powder. 1H NMR (CDCl3, 300 MHz): =16.28 (s, 1H, Ru=C H Ph), 7.55 (t, 2H, para C H 3JH,H = 7.2 Hz), 7.43 (s, 1H, ortho C H 2JH,H = 8.8 Hz), 7.38 (d, 4H, meta

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75 C H, 2JH,H = 7.4 Hz ), 6.80 (d, 1H, para C H 2JH,H = 2.4 Hz), 6.72 (d, 1H, meta C H 2JH,H = 8.8 Hz), 4.87 (sept, 1H, (CH3)2C H OAr, 3JH,H = 6.2 Hz), 4.19 (s, 4H, N(C H2)2N), 3.58 (sept, 1H, C H (CH3)2, 3JH,H = 6.6 Hz), 1.35 (d, 6H, OCH(C H3)2, 2JH,H = 6.3 Hz), 1.25 (d, 24H, CH(C H3)2, 3JH,H = 7.1 Hz); 13C NMR (CDCl3, 75 MHz): = 286.3, 212.3, 150.9, 149.3, 145.0, 130.0, 128.1, 127.7, 124.6, 121.4, 114.1, 75.9, 54.7, 29.0, 26.7, 23.5, 21.8. MS (HRMS/EI) calcd for C37H49ON2Cl3Ru [M+Na]+ 767.1846, found 767.1941. Anal. Calcd for C37H49ON2Cl3Ru: C, 59.63; H, 6.63; N, 3.76. Found: C, 59.52; H, 7.03; N, 3.46. Synthesis of complex [Ru]15. (-CH3) The general procedure was followed using 2-isopropoxy-5-methylstyrene ( 113 mg, 0.64 mmol) to afford 209 mg (0.288 mmol, 90%) of a pale green powder. 1H NMR (CDCl3, 300 MHz): =16.40 (s, 1H, Ru=C H Ph), 7.55 (t, 2H, para C H 3JH,H = 7.6 Hz), 7.40 (d, 4H, meta C H, 2JH,H = 7.8 Hz ), 7.24 (s, 1H, ortho C H ), 6.68 (d, 1H, para C H 2JH,H = 8.3 Hz), 6.58 (s, 1H, meta C H ), 4.88 (sept, 1H, (CH3)2C H OAr, 3JH,H = 6.0 Hz), 4.18 (s, 4H, N(C H2)2N), 3.62 (sept, 1H, C H (CH3)2, 3JH,H = 6.5 Hz), 2.32 (s, 3H, C H3), 1.36 (d, 6H, OCH(C H3)2, 2JH,H = 6.2 Hz), 1.27 (d, 24H, CH(C H3)2, 3JH,H = 6.8 Hz); 13C NMR (CDCl3, 75 MHz): = 290.8, 214.4, 150.8, 149.5, 144.4, 137.1, 131.5, 129.9, 124.6, 122.6, 112.8, 74.9, 54.8, 29.0, 26.8, 23.6, 22.0, 20.2. MS (HRMS/EI) calcd for C38H52O2N2Cl2Ru [M+Na]+ 747.2395, found 747.2427. Anal. Calcd for C38H52O2N2Cl2Ru: C, 62.97; H, 7.23; N, 3.86. Found: C, 62.67; H, 7.31; N, 3.49. Synthesis of complex [Ru]16. (-OCH3) The general procedure was followed using 2-isopropoxy-5-methoxystyrene (1 23 mg, 0.64 mmol) to afford 229 mg (0.310 mmol, 97%) of a pale green powder. 1H NMR (CDCl3, 300 MHz): =16.33 (s, 1H, Ru=C H Ph),

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76 7.53 (t, 2H, para C H 3JH,H = 7.2 Hz), 7.37 (d, 4H, meta C H ), 7.02 (dd, 1H, meta C H 2JH,H = 8.4 Hz), 6.69 (d, 1H, para C H 2JH,H = 7.4 Hz), 6.37 (s, 1H, ortho C H ), 4.83 (sept, 1H, (CH3)2C H OAr, 3JH,H = 6.3 Hz), 4.18 (s, 4H, N(C H2)2N), 3.72 (s, 3H, OC H3), 3.60 (sept, 1H, C H (CH3)2, 3JH,H = 6.6 Hz), 1.35 (d, 6H, OCH(C H3)2, 2JH,H = 6.2 Hz), 1.24 (d, 24H, CH(C H3)2, 3JH,H = 7.1 Hz); 13C NMR (CDCl3, 75 MHz): = 298.1, 208.9, 152.5, 149.4, 146.2, 131.9, 130.6, 129.9, 124.7, 117.7, 113.2, 111.4, 72.7, 55.8, 32.8, 29.0, 26.8, 23.6, 21.9. MS (HRMS/EI) calcd for C38H52O2N2Cl2Ru [M+Na]+ 763.2344, found 763.2361. Anal. Calcd for C38H52O3N2Cl2Ru: C, 61.61; H, 7.08; N, 3.78. Found: C, 61.86; H, 7.28; N, 3.63. Kinetic Study The kinetic study was performed with the same apparatus and following the same procedure as previously described by Wagener.144,192

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77 CHAPTER 4 ISOMERIZATION BEHAVIOR UNDER M ETATHESIS CONDITIONS OF NHCCONTAINING RUTHENIUM CARBENE CATALYSTS. Introduction In the past few years, alkene isomeri zation has emerged as an important side reaction of ruthenium catalyzed metathesis, disturbing the product's microstructure by apparent migration of the double bond along the alkyl chain.160,166 Figure 4-1 illustrates the versatility of this side -reaction. First observed with [Ru]1 during the ring-closing metathesis of allylic functionalized alkenes,164,167,199,200 olefin isomerization rapidly appear to affect a variety of oxygen and nitrogen-containing compounds.168,169 In some cases, double-bond isomerization even occurred prior to the metathesis reaction.200 Nevertheless, [Ru]1 remains inactive towards isomerization with unsubstituted olefins or unfunctionalized alkenes ev en under harsh conditions.176 However, when the 2nd generation of catalysts were introduced, competitive isomerization became a problem with all kinds of substrates.160,166 Frstner et al.114 reported the synthesis of a 20membered ring in addition to the desired 21membered lactone from the RCM of a 1,22diene with [Ru]3. The ring contraction was the result of initial isomeriz ation of one of the double-bonds followed by RCM and the release of propene. Many other examples appeared in the literature and scientists bega n to use isomerization to their advantage for the synthesis of 5-, 6-, 7-membered cycl ic enol ethers by tandem RCM double-bond isomerization reactions.201

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78 O O O O O O + [Ru]3 20 21 O R O R O Rm m m n n n[Ru]4 N O RO R' OHnN O RO R' OHn[Ru]1 OH OTES OH OTES OH OTES [Ru]1 + 6%7% 65% 10% 46-65% Figure 4-1. Examples of competitive isomeri zation reactions during olefin metathesis The 2 established pathways for transition metal-catalyzed olefin isomerization are the -allyl metal hydride mechanism and th e metal hydride addition-elimination.178 Despite an increasing number of studies, the ruthenium intermediate responsible for this undesirable reaction has not been identified yet, although it is often attributed to the formation of a ruthenium hydride in situ, as a decomposition product of the original carbene catalyst.

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79 Acyclic diene metathesis has been used in our research for the modeling of precise ethylene/ -olefin copolymers.127,128,130 Controlling isomerization is crucial as it results in irregular microstructures, whether it occurs on the monomer or on the product (Figure 42). Furthermore, ADMET conditions are partic ularly favorable to isomerization since polymerizations are typically conducted in neat diene or in concentrated solutions at high temperatures of 40-75oC, for extended periods of time to reach high molecular weight materials. Previously, we have reported the metathesis vs. isomerization activity of fast initiators using the model substrate 1-octene.192 In an attempt to better understand the causes of isomerization, this study was ex tended to other modified complexes, introducing different phosphine and carbene ligands. Prev ious Hoveyda-type complexes Ru]E, [Ru]11, and [Ru]13 were also included in this study. ADMETmRCM R33R3 3R23Rn 3 3R3 3R3 2R isomerization isomerization isomerization and ADMETx yx=0,1,2,3 y=6,5,4,3 "heavier" unit "lighter" unit Figure 4-2. Effect of isomeri zation during ADMET polymerization Results and Discussion The fact that isomerization seemed to occur to a significant extent with fast initiators combined with previous observ ations showing that ruthenium vinylidene

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80 catalysts afforded polymers with singularly high trans content,202 prompted us to investigate a variety of slow initiators. Smaller phosphine are known to dissociate slower.75 Tricyclopentyl phosphine (PCp3) ruthenium dimethylvinyl carbene [Ru]17 being readily available in the la b from previous studies, the PCp3 equivalents of both parent complexes [Ru]4 and [Ru]10 represent ideal candidates to investigate the influence of both the NHC and phosphine li gands. On the other hand, dimethylvinyl carbene ruthenium complexes r eact slowly and incompletely with ethyl vinyl ether even when a phosphine is replaced by an NHC liga nd. Despite incomplete initiation of the catalyst, vinylidene complexes are efficient ca talysts for a variety of cross-metathesis applications,203 which make them potentially suitable for ADMET chemistry. Synthesis and Characterization Complexes [Ru]13-16 were synthesized from the parent catalyst [Ru]10 by addition of the corresponding isopropoxystyrene and copper chloride as described in Chapter 3. The synthesis of complex [Ru]18 was adapted from the literature.203 Noteworthy, the H2IPr analogue proved difficult to synthesize using the same conditions. The additional bulk brought about by th e isopropyl groups of the NHC ligand may interact with the dimethyl vinyl carbene, rendering th e ligand substitution more problematic. Complexes [Ru]21 and [Ru]22 were obtained by phosphine exchange using the intermediate pyridine adduct [Ru]19 and [Ru]20, respectively (Figure 4-3).204 The bis -pyridine complexes were crystallized and reacted with 1.4 equivalents of PCp3. Spectroscopically, the vinylidene protons of [Ru]18 exhibit two characteristic doublets at 19.14 and 7.72 ppm, while [Ru]21 and [Ru]22 showed a single phosphorus signal at 26.30 and 24.27 ppm, respectively. Th e upfield shift relative to PCy3 ligated

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81 parent complexes [Ru]4 and [Ru]10 indicates the presence of the less -basic tricyclopentyl phosphine Ru Cl P Cl N N Ru Cl Ph ClL [Ru]21 L= H2IMes [Ru]22 L= H2IPrP[Ru]18Ru Cl Ph ClLPCy3Ru Cl Ph ClLN NRu Cl P Cl N N P BF4 -KOtBu xs. Pyridine P(Cyp)3[Ru]4 L= H2IMes [Ru]10 L= H2IPr [Ru]19 L= H2IMes [Ru]20 L= H2IPr [Ru]17 NN NN.. ..H2IMes H2IPr Figure 4-3. Synthesis of [Ru]18-22 Isomerization Behavior with 1-Octene The extent of isomerization was tested for complexes [Ru]E, [Ru]11, [Ru]13 and [Ru]17-22, using the model 1-octene, and compared to catalysts [Ru]1, [Ru]4 and [Ru]10. The reaction conditions were typically the same as the original experiment, using 0.1 mol% catalyst in neat 1-oc tene at both room temperature and 55oC for 3 days.176 The product mixture was analyzed by gas chromatography (GC), where each peak corresponds to a linear olefin of a sp ecific carbon length. The results are expressed as the percentage of isomers in the mixture ve rsus the percentage of metathesis product, assuming complete conversion, which is true for all catalysts except [Ru]18 (Table 4-1). The combination of the vinylidene and the smaller phosphine, even though PCp3 is less

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82 -basic than PCy3, render this catalyst extremely slow to initiate at room temperature and the concentration of active species is proba bly not sufficient to drive the reaction to completion. Table 4-1. Percentage of isomers during the metathesis of 1-octene. Entry Catalyst % isomers at 25oC % isomers at 55oC 1a [Ru]1 0% 0% 2 [Ru]E 0% 12% 3a [Ru]4 3% 76% 4b [Ru]10 76% 5 [Ru]11 23% 50% 6 [Ru]13 78% 7 [Ru]18 7% 70% 8 [Ru]21 39% 49% 9 [Ru]22 84% a Data taken from ref. 176; b ref. 193. All catalysts exhibit an isomeric di stribution centered around the metathesis product, indicating that isomerization occurs subsequently to the metathesis process (Figure 4-4). This is in stark contrast with what was observed with Grubbs’ catalyst [Ru]4 where the two reactions occurred competitively at 55oC. Since, [Ru]21 is a slower metathesis catalyst,75 it is tempting to concl ude that substituting PCy3 for PCp3 delays the isomerization reaction but yet does not decrease its extent. The PCp3 ligated complexes [Ru]21, and [Ru]22 are actually better isomerization catalysts than [Ru]4 at room temperature. This could be due to th e slower rate of dissociation of PCp3, combined to the lower -basicity of the phosphine, which probab ly affects the rate of catalytic decomposition, in that it decreases the chances for nucleophilic attack on the methylidene complex.89

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83 Figure 4-4. GC chromatogram of the produc t mixture from the self-metathesis of 1octene: (A) with [Ru]21 at 25oC; (B) with [Ru]22 at 25oC. Retention scale is in minutes. It appears that the rate of isomerization is closely related to the rate of metathesis, as the faster metathesis initi ators, bearing the bulkier H2IPr ligand, result in higher % isomerization products. For example [Ru]13 and [Ru]22 initiate fast and afford more than 75% isomers at room temperature (Figure 4-4A, entries 6 and 9), while [Ru]18 and [Ru]21 are known for their slow initiation and c onsequently yield 7-39% isomers in the same conditions (Figure 4-4B, entries 7 and 8) The breadth of th e isomer distribution increases with the reactivity of the isomer ization catalyst. However, isomerization cannot be correlated as a function of th e dissociation rate. Indeed, complexes [Ru]1 and [Ru]E are known for faster dissociation, still the results show lit tle to no trace of

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84 isomerization. On the other hand, changing the dissociating ligand or the carbene had little effect on complexes bearing NHC lig ands. Accordingly, electronics on the Hoveyda-type complexes ([Ru]11 vs. [Ru]13) did not significantly affect either metathesis or isomerization rates. Even though this study does not allow to conclude on the causes of isomerization it is now clear that NHC ligands play a crucial role in promoting this undesired reaction. Since the metathesis and isomerization act ivity of ruthenium catalysts apparently go hand in hand, it seems logical that isomer ization is promoted by the metathesis catalyst itself or by a species formed in situ during the metathesis cycle. Bourgeois et al.174 proposed the -allyl mechanism illustrated in Figure 4-5, via deprotonation of the allylic position by the carbene moiet y. They postulate that the strong -donation of the NHC ligand increases the basic ity of the carbene, therefor e favoring isomerization. In addition, they proved that tric yclohexyl phosphine oxide, O=PCy3, generated from oxidation of PCy3, serves as an isomerization inhibitor. H RuIMes Ru metathesis IMes R Cl Cl Ru IMes R Cl Cl HH Cl2+ [Ru]3 path a path b [Ru]3 -PCy3N NMes Mes..IMes = R = H, Ph Figure 4-5. Postulated mechanism of the isomerization process.

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85 On the other hand, ruthenium carbene catalysts are known to form hydride complexes as they decompose. Because early metathesis complex Ru(H2O)6(tos)2 was proven to isomerize allylic functionalized substrat es through a metal-hydride intermediate,205 it is tempting to assume that one of the catalyst decomposition product might be responsible for this undesirable reaction. Therefor e, understanding the isomerization process also goes along with the determination of the decomposition pathway. Several hydrido complexes have been identified as decomposition products of catalysts [Ru]1 and/or [Ru]4 under different conditions; however, none reproduces rigorous metathesis conditions.72,89-94 Complexes [Ru]6 and [Ru]7 (Figure 4-6) have been isolated under high oxygen pres sures or in alcohol solvents,92-94 but cannot be obtained under the inert conditi ons typically used in this study. Even though complex [Ru]7 has been isolated as an im purity during the synthesis of [Ru]4, supposedly as a consequence of the methanol wash, it is not a good isomerization catalyst. Grubbs et al.89 have recently reported the hydrido complex [Ru]5, which directly involves the NHC ligand in the decomposition process. Finally, the C-H activation of one of the methyls of the NHC ligand is a common reaction in many ruthenium complexes,183,206-209 even though complex [Ru]8 is the only example so far with a ruthenium carbene and does not contain any hydride ligand.72 Ru L Cl OC H PCy3[Ru]6 L = PCy3N N CRu Cl Cl Ru N N Mes Mes H [Ru]5 [Ru]7 L = IMesH2N N Cl Ru OC PCy3[Ru]8 Figure 4-6. Decomposition products of ruthenium carbene complexes

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86 Therefore, before moving towards new cat alyst designs, further studies will focus on the identification of the isomeriza tion mechanism operating during rutheniumcatalyzed metathesis reactions. ADMET Polymerization The catalysts were also tested fo r their utility in ADMET chemistry by polymerizing benchmark 1,9-decadiene in the bulk (monomer:catalyst ratio 450:1), at 55oC for 5 days. The primary interest was to evaluate the cis:trans ratio of the resulting polyoctenamer since a previous experiment indicated that vinylidene complexes may produce a polymer with a high trans content.202 This is particularly attractive since high cis content reduces both the melting temper ature and the perc ent crystallinity. Molecular weights ( Mn) and cis:trans ratios are given in Table 4-2. All polymers exhibit a trans content of about 80%, which is ty pical for ruthenium catalyzed ADMET polymers. As expected, 1st generation catalysts [Ru]1 and [Ru]E are less active than NHC-containing complexes, as they produce polyoctenamer with Mn lower than 12,000 g/mol, compared to Mn of at least 18,000 g/mol using 2nd generation catalysts. Interestingly, vinylidene [Ru]18 yields similar molecular weight material than benzylidene [Ru]21 and [Ru]22 even though it is known to be a slower initiator. This observation not only illustrate s that vinylidene complexes are efficient ADMET catalysts, but also that faster initiators do not necessarily afford highe r molecular weight polymer. Indeed, the slow initiation of the vinyliden e complex allows the sl ow diffusion of active species in the bulk of ADMET monomer, providing a constant feed of fresh catalyst as the reaction proceeds to highe r DPs. Monomers that prove hard to polymerize under standard ADMET conditions are sometimes dr iven to high molecular weight polymer by repeated additions of fresh cat alyst to the reaction medium.147

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87 Table 4-2. Results for the ADMET polym erization of 1,9-decadiene at 55 C for 120 h Entry Catalyst polyoctenamer cis:trans ratioa Mn (g/mol) 1 [Ru]1 12,000b 2 [Ru]E 20:80 5,000c 3 [Ru]4 17:83 75,000b 4 [Ru]10 18:82 32,000b 5 [Ru]11 17:83 21,000b 6 [Ru]18 23:77 20,000c 7 [Ru]21 21:79 18,000c 8 [Ru]22 16:84 26,000b a Determined by 13C NMR. b Determined by GPC analysis. c Determined by 1H NMR end-group analysis. Phosphine substitution from PCy3 to the smaller PCp3 significantly depresses catalytic activity, as [Ru]4 and [Ru]21 afford polyoctenamers with Mn of 75,000 g/mol and 18,000 g/mol, respectively. This observatio n is harder to rationalize since the phosphine ligand is involved in both the rebi nding step, which deactivates the catalyst, and catalyst decomposition, which is also an important consideration as ADMET is typically run for long reaction times. The smaller PCp3 may rebind more easily, which is problematic since the methylidene complex is not likely to reenter the ADMET cycle after being trapped by free phosphine.75 In any case none of these complexes compete with the activity of second genera tion Grubbs’ catalyst in ADMET. Conclusions A series of ruthenium carbene catalysts wa s synthesized and tested on 1-octene to study the effect of ligand varia tion on olefin isomerization during metathesis reactions. Even though all complexes are efficient cat alysts for the ADMET polymerization of 1,9decadiene, they all promote structural isom erization. The isomerization extent is increased by faster metathesis initiators, and the presence of N-heterocyclic carbene ligand clearly accelerates this undesirable reaction. Otherwise, it is difficult to provide

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88 definite conclusions on the role of other liga nds on the catalyst isomerization behavior. Therefore, before pursuing the design of novel cat alysts to limit olefin isomerization, it is important to identify the mechanism ope rating under these metathesis conditions. Experimental General Considerations 1H NMR (300 MHz) and 13C NMR (75 Hz) spectra of the ADMET polymers were recorded in CDCl3 on either a Mercury series or Varian VXR-300 NMR superconducting spectrometer. Chemical shifts were referenced to residual CHCl3 (7.23 for 1H and 77.23 for 13C) with 0.03% v/v TMS as an internal refe rence. All organometallic spectra were recorded in d6-benzene and the chemical shifts were referenced to residual C6H6 (7.16 for 1H and 128.39 for 13C). Gel permeation chromatography (GPC) was performed using a Waters Associates GPCV2000 liquid chromatography system with its internal differen tial refractive index detector (DRI), internal differential visc osity detector (DP), and a Wyatt Technology 3 angle light scattering detector (LS). The chromatography wa s performed at 45 C using two Waters Styragel HR-5E columns (10 microns PD, 7.8 mm ID, 300 mm length) with HPLC grade tetrahydrofuran as the mobile phase at a flow rate of 1.0 mL/minute. Injections were made at 0.05-0.07 % w/v sample concentration using a 322.5 ml injection volume. In the case of universal calibrat ion, retention times were calibrated against narrow molecular weight polystyrene standard s (Polymer Laboratories; Amherst, MA). All standards were selected to produce Mp or Mw values well beyond the expected polymer's range. GC analysis was conducted on a Shimadzu GC-17A chromatograph equipped with a HP-5 (Hewlett Packard) 25 m column with FID detection. All amounts of products

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89 were calculated by a systematic ratio to th e Decalin peak at 8.72 minutes, using the integrated peak areas. Catalyst [Ru]17 was a gift from Materia and complexes [Ru]18203 and [Ru]19204 were synthesized according to th e literature. All catalysts we re stored in an argon-filled drybox prior to use in kinetic and polymeriza tion experiments. Pyridine (Aldrich) was dried over CaH2 prior to use. Tricyclopentyl pho sphine (Aldrich) was stored in the drybox and used as received. 1,9-Decadiene (A ldrich) was distilled from Na/K alloy under reduced pressure into a Kontes flask equipped with a Teflon valve, degassed by three freeze-pump-thaw cycles, and stored in an argon-filled drybox. All polymerizations were conducted in the bulk using (450:1) m onomer-to-catalyst rati o. Benzene, toluene and pentane were distilled over Na/K alloy be fore use, and methanol was distilled over Mg/I2. After distillation, d6-benzene was degassed by three freeze-pump-thaw cycles, and stored in an argon-filled drybox. Synthesis of Complexes [Ru]18-22 Synthesis of complex [Ru]21. In the drybox, a flame-dr ied Schlenk flask was charged with 0.102 g (0.140 mmol) of catalyst [Ru]19 and dissolved in 3 ml of benzene. A solution of tricyclopentyl phosphine(80 m g, 0.336 mmol, 3.2 eq) in benzene was added to the Schlenk flask and the color immedi ately turned brown-red. The mixture was stirred at room temperature for 10 minut es under argon, and the solvent was then evaporated. The brown residue was wa shed with anhydrous methanol (2 x 25 mL) and pentane (50 mL). Finally, the brown-pink so lid was dried under vacuum to yield 0.100 g (0.124 mmol, 88%) of complex [Ru]21.

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90 1H NMR (benzened6, 300 MHz): = 19.72 (s, 1H, Ru=C H ), 7.12-6.90 (m, 7H, para-meta C H ), 6.22 (t, 2H, ortho C H ), 3.43-3.15 (m, 4H, NC H2C H2N), 2.82 (s, 6H, mesityl C H3), 2.40 (s, 6H, mesityl C H3), 2.32 (m, 3H, Cyp), 2.23 (s, 3H, mesityl C H3), 1.80 (s, 3H, mesityl C H3), 1.49 (m, 18H, Cyp), 1.27 (m, 6H, Cyp); 13C NMR (benzened6, 75 MHz): =296.8, 221.7, 152.4, 139.9, 138.7, 138.1, 137.8, 137.5, 136.2, 131.7, 130.3, 129.7, 52.3, 51.4, 35.9, 35.6, 32.3, 29.9, 26.5, 26.4, 23.4, 21.7, 21.4, 20.9, 19.3, 14.7; 31P NMR (benzened6, 300 MHz): =26.29 ; MS (HRMS/EI) calcd for C43H59N2Cl2PRu [M]+ 806.2840, found 806.2813. Synthesis of complex [Ru]22. The procedure was adapted from the literature.72 In the drybox, a flame-dried Schlenk flask equipped with a magnetic stir bar was charged with 0.130 g (0.128 mmol) of catalyst [Ru]10 and dissolved in 4 ml of toluene. Once out of the drybox, 0.82 ml (10.1 mmol) of degassed pyridine was added to the stirring solution via cannula. The color im mediately changed from brown to bright green. The mixture was stirred for another 10 minutes at room temperature, then added dropwise to 50 ml of cold pentane at –78oC. A green solid precipitated and the solution was filtered out via cannula filtration The green residue, corresponding to [Ru]20 was washed with cold pentane a nd dried under vacuo. A solution of tricyclopentyl phosphine (78 mg, 0.328 mmol, 2.5 eq) in 4 ml of benzen e was added to the Schlenk flask and the color immediately turned brow n. The mixture was stirred at room temperature for 15 minutes under argon, then the solvent was eva porated. The brown residue was washed with anhydrous methanol (2 x 25 mL) and pentane (50 mL), and dried under vacuum to yield 0.693 g of a pink solid (0.83 mmol, 78%).

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91 1H NMR (benzened6, 300 MHz): = 19.85 (s, 1H, Ru=C H ), 8.32 (d, 1H, ortho C H, 2JH,H= 8.4 Hz), 7.30-7.17 (multiple peaks, 5H, aryl C H ), 7.00 (t, 2H, para C H, 3JH,H= 7.4 Hz), 6.72 (s, 2H, aryl C H ), 4.22 (sept, 1H, C H (CH3)2), 3.89-3.67 (m, 4H, NC H2C H2N), 2.30 (s, 3H, C H in PCp3), 1.72 (d, 6H, CH(C H3)2, 2JH,H= 6.5 Hz), 1.58 (d, 6H, CH(C H3)2), 1.45 (m, 16H, Cyp), 1.22 (m, 8H, Cyp), 1.18 (d, 6H, CH(C H3)2, 2JH,H= 6.5 Hz), 1.08 (d, 6H, CH(C H3)2, 2JH,H= 6.5 Hz); 13C NMR (benzened6, 75 MHz): =292.8, 222.7, 150.5, 147.6, 138.8, 136.7, 132.0,130.6, 129.5, 125.1, 124.6, 55.3, 54.7, 35.7, 35.4, 32.3, 29.9, 29.8, 28.6, 27.9, 27.2, 26.5, 26.4, 25.0, 23.4, 14.7; 31P NMR (benzened6, 300 MHz): =26.29 ; MS (HRMS/EI) calcd for C49H71N2Cl2PRu [M]+ 890.3780, found 890.3724.

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92 CHAPTER 5 UNDERSTANDING STRUCTURAL ISOMERIZATION DURING RU-CATALYZED OLEFIN METATHESIS: A DEUTERIUM LABELING STUDY. Introduction The discovery of well-defined, functi onal group tolerant ruthenium carbene catalysts such as Grubbs’ (PCy3)2(Cl)2Ru=CHPh [Ru]137,57,80,87 has considerably broadened the scope of olefin metathesis.1,2,7-10,20,31 The versatility of this class of catalysts has contributed to their growing popularity and mo tivated new developments. A ‘second-generation’ of catalytic systems wa s introduced as one N-heterocyclic carbene replaced a phosphine ligand ([Ru]369,70 and [Ru]471,74,75 in Figure 5-1), further enhancing their catalytic activity and the thermal stab ility. Since then, many ligand modifications have been reported in order to tune in the activity and/or the stab ility of the catalyst according to specific reaction conditions. For example, the exchange of a phosphine for an isopropoxy-tethered benzylid ene conferred an exceptional st ability to the catalyst in solution, which has allowed the catalyst to be recycled after ringclosing metathesis (RCM) or cross-metathesis (CM) reactions.56,95,186 Also, addition of more labile ligands, like pyridine derivatives, enabled fast initiati on rates particularly de sirable for living ringopening metathesis polymerization (ROMP).99,194,204 Ru Cl Ph PCy3Cl PCy3Ru Cl R PCy3Cl N N [Ru]1 [Ru]4 R=Ph [Ru]4' R=H Ru Cl Ph PCy3Cl N N [Ru]3 Figure 5-1. Olefin metathesis catalysts

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93 In spite of their high reactivity in metathesis chemistry, these ruthenium catalysts were suspected to undergo nonmetathesis transformations.160-165 Among them, alkene isomerization has emerged as an important side reaction of ruthenium catalyzed metathesis, disturbing the product's microstr ucture by apparent migration of the double bond along the alkyl chain.166 First observed on subs trates containing allylic functionalities in combination with first generation catalysts,167-171 double bond isomerization has since been reported with 2nd generation catalysts on a broad variety of substrates competitively and sometimes prior to olefin metathesis.113,172-177,192 Promoted by diverse transition metals, ol efin isomerization has been known to operate through two distinctive pathways: either through 3-allyls, or through an al kyl intermediate involving a long-lived metal hydride.178 Mechanistically, the -allyl involves a formal intra molecular 1,3-H shift while the metal hydride operates through an inter molecular addition-elimination with an inherently comp etitive 1,2-H shift. Consequently, isotopic labeling experiments have often been used to probe the nature of the mechanism.205,210-214 However, despite the increasing number of studies, the ruthenium intermediate responsible for this undesirable reaction has ye t to be identified. Although it is often attributed to the formation of a ruthenium hydride in situ as a decomposition product of the original carbene catalyst,179-180 the possibility of a -allyl mechanism cannot be ruled out.174 Apart from standard hydrogenolysis,158,215,216 numerous pathways have been proposed for the transformation of ruthenium carbene into ruthenium hydride complexes. For example, carbonyl hydrides [Ru]6 and [Ru]7 (Figure 5-2) first isolated during the decomposition of alkoxycarbene complexes,90,91 were identified as products of the

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94 reaction of [Ru]1 and [Ru]4 with primary alcohols in the presence of a base.72,92-94 Although the mechanism is st ill at a speculative stage,90,92 both complexes are known to be efficient catalysts fo r isomerization, hydrogenation,92,217-220 and hydrovinylation.221 More recently, Grubbs et al.89 isolated a dinuclear ruthenium hydride [Ru]5 as one of the decomposition products of the methylidene intermediate [R]4’ formed during a metathesis cycle. Other studies include the possibility of a ruthenium insertion into the C-H bond of one of the methyl groups present on the NHC ligand.72,183 This type of activation is common for complexes contai ning NHC ligands and has been witnessed during the preparation of ruthenium carbene [Ru]4; however, not yielding a hydride complex [Ru]8.72 Ru L Cl OC H PCy3[Ru]6 L = PCy3N N Cl Ru OC PCy3N N CRu Cl Cl Ru N N Mes Mes H [Ru]8 [Ru]5N N[Ru]7 L = Figure 5-2. Decomposition products of ruthenium carbene complexes Acyclic diene metathesis (ADMET) has been used for the modeling of precise ethylene/ -olefin copolymers.127,128,130 Therefore, controlling isomerization is crucial since it results in irregular microstructures, whether it occurs on the monomer or on the product (Figure 4-2). Previously, we have reported the metathesis vs isomerization activity of ruthenium carbene catalyst s using the model substrate 1-octene.176,192 Even though metathesis consistently occurred faster isomerization products could represent as much as 80% of the reaction mixture with ca talysts containing NHC ligands. When other modified complexes were used, the isomeriza tion activity appeared to be closely related

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95 to the metathesis activity (Chapter 4). B ecause no definite conclusion could be drawn from catalyst modification, it was time to investigate the mechanism of olefin isomerization under metathesis conditions. The reaction of allyl1,1-d2-methyl ether with catalyst [Ru]4 was monitored by 1H and 2H NMR spectroscopy to watch deuterium scrambling and therefore conclude on the operating isomerization mechanism. In the meantime, a deuterated version of catalyst [Ru]4 (complex [Ru]23 in Scheme 1) was designed to further demonstrate the role of NHC ligand in catalyst decomposition and assess the possibility of a C-H activation, whic h could be responsible for the formation of a metal hydride complex. Again, 2H NMR was used to follow the decomposition of complex [Ru]23 and observe its behavior in the pr esence of olefinic substrates; any deuterium incorporation on the olefin backbone would prove the presence of a ruthenium deuteride complex. Results and Discussion Mechanistic Studies Even though both the -allyl hydride and the metal hydride addition-elimination mechanism result in the same product, isot opic labeling studies can highlight their differences. Indeed the two pathways can be distinguished by looking at the hydrogen shift undergone during the isom erization reaction. The -allyl mechanism, involving a metal that does not contain a hydride ligand, is typically intra molecular. A free olefin coordinates to the metal and undergoes an oxida tive addition of the a llylic proton to form a -allyl metal hydride intermediate. The hydrid e fragment is then transferred to the terminal position (of an -olefin) to yield the isomerized olefin. Hence, a formal 1,3-H shift is observed when the -allyl mechanism is active (Figure 5-3).

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96 M R D D M R D M R D D D M R D D D R D 1,3-deuterium shift Figure 5-3. Deuterium shift with a -allyl hydride mechanism On the other hand, the metal hydride mech anism involves a stable metal-hydride complex already present in th e reaction, and is therefore inter molecular, i.e. hydrogen atoms from one substrate can be transferred to the catalys t on to another substrate molecule. The olefin inserts into the meta l-hydride bond to form either a primary metal alkyl (thermodynamically favored), or a sec ondary metal alkyl that will lead to the isomerized olefin by -elimination, generating another metal-hydride. The occurrence of both Markovnikov and anti-Markovnikov add itions of the metal hydride across the double bond results in a ch aracteristic 1,2-H-shift:213 when a primary metal alkyl undergoes a -elimination of a different hydrogen, the re sulting olefin can reinsert in the metal-hydride bond to form a secondary metal alkyl. Subsequent -elimination of the appropriate hydrogen results in an apparent 1,2-H-shift as illustrated in Figure 5-4 for a deuterated olefin. A formal 1,3-H-shift can also be observed through the formation of a secondary metal-hydride.

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97 M R M R D D D D D M R D D D M R H D D D D M R D D D1,2-deuterium shiftM R DD D M R D D D1,3-deuterium shiftM R H D D D Figure 5-4. Deuterium shifts with a me tal-hydride addition-elimination mechanism Substrates such as allyl -1,1 d2 alcohol205,210,211 and the corresponding methyl ether205,212,213have often been used to probe the nature of the hydrogen shift in metalcatalyzed olefin isomerization, particularly b ecause of the irreversib ility of the reaction. Indeed, allyl -1,1 d2 alcohol will isomerize excl usively into propionaldehyde1,3-d2 in the case of a -allyl hydride mechanism, while a mixture of partially deuterated propionaldehydes with deuterium incorporation at the C-2 position will be expected in the case of the metal-hydride mechanism. Actually this method is often taken as convincing proof for the -allyl hydride mechanism without investigating the inter/intramolecularity of the process. In this particular case, allyl alcohol has been known to react with ruthenium carbenes and form hydride complexes.163,164,222 Therefore only allyl methyl ether will be used, which in the end should provide the sa me set of observations. A 2.3 M solution of allyl -1,1-d2-methyl ether (4-1) in C6D6 was loaded with 0.2 mol% of complex [Ru]4 and heated at 35oC for 3 days to reproduce the conditi ons used in previous isomerization studies.176,192 The composition of the reaction mi xture was solved using 2D NMR and

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98 NOE experiments (See experimental section for details). The resulting sample consists of four compounds shown in Figure 5-5: cis and trans -propenyl methyl ether (5-2cis and 5-2trans ) are the isomerization prod ucts of the starting material and represents 70% of the mixture; the remaining 30%, cis and trans -1,4-dimethoxy-but-1-ene (5-3cis and 53trans ), are the result of isomerization on the metathesis product. Each of these compounds is a mixture of the four isotopom ers yielding from par tial deuteration in positions 2 and 3 of the alkylene ether. Interestingly, position 1 is completely deuterated while position 4 does not contain any deuteriu m. The relative ratios of each compound and the molar fractions of deuteration in each position are given in Figure 5-5. Note: The sum of deuterium for each molecule of product 5-3 and 5-2 does not equal 2.00 (number of deuterium pres ent in one molecule of allyl -1,1-d2-methyl ether) because some deuterium atoms were incorpor ated in the ethylene released during the metathesis of allyl -1,1,3-d3methyl ether. O O O O O O0.14 0.30 0.15 1.00 0.00 0.33 1.00 0.00 0.29 1.00 1.00 1.00 1.00 0.00 0.00 0.00 0.00 0.31 0.31 0.285-2cis (36%) 5-2trans (34%) 5-3trans (13%) 5-3cis (17%) Figure 5-5. Products of the reaction of allyl -1,1-d2-methyl ether with complex [Ru]4 The fact that position 4 is not deuterated at all proves that there is no deuterium exchange with the solvent, i. e. all the deuterium present on the products actually comes from the starting material, which confirms the validity of this mode l study. Therefore, deuterium incorporation at the C2 position clearly indicates that the isomerization

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99 mechanism operating here is a metal-hydride addition-elimination In fact, the relative ratios of deuterium incorporation at pos ition 2 and 3 reflect th e relative rates of Markovnikov (formation of a secondary meta l alkyl) versus anti-Markovnikov addition (formation of a primary metal alkyl) of th e metal hydride across th e olefin bond (Figure 5-5). This ratio is estimated to be about 1:2 for both 5-2cis and 5-2trans, which is consistent with the preferred formation of the thermodynamically favored primary metal alkyl. For example, Cramer et al.223 reported that in the is omerization of 1-butene by rhodium (III) hydride, addition of D to C-2 was 15 times as fast as addition to C-3. The lower ratio obtained with our allyl ether may be a result of the directing effect of the oxygen moiety, as described previously by McGrath and Grubbs205 during the isomerization of allylic alcohols and ethers by RuII(H2O)6(tos)2. In the case of 5-3cis and 5-3trans the relative ratios of de uterium incorporation at C-2 and C-3 are approximately 1:1, which make s sense since isomerization occurs on an internal double bond. Therefor e, the metal-hydride can onl y add across the olefin bond of the metathesis product and forms exclusively a secondary metal alkyl. The complete deuteration of position 1 suggests the irreversibility of the isomerization reaction. If the reaction were reversible, an isomerized olefin, methoxy 1,3-d2propene for instance, should be able to insert into a metal-hydride bond to form a secondary metal alkyl, which could subsequently undergo a -D-elimination to produce methoxy -3-d1propene (Figure 5-6). This transfer of the methylene deuterium to the metal would automatically bring the % deuter ation at C-1 under 100% which is not the case. A similar behavior was witnessed by Krompiec et al.224 with hydride complex RuCl(H)(CO)(PPh3)3. Under ADMET conditions, the reaction of allylic ether N,N -bis[3-

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100 (allyloxy)-2-hydroxypropyl]aniline with [Ru]4 also afforded complete isomerization into the vinylic ether.175 Ru O H D D Ru O D H D Ru O H D D Figure 5-6. Incorporation of hydrogen at C-1 by reversible isomerization The distribution of isomeriz ation vs. metathesis product is a result of several factors. Although metathesis is often believe d to be the faster and therefore primary reaction, the present conditions are set up in a closed system, which limits its progression. Once the system gets saturated with ethylen e, it reaches equilibrium and metathesis chemistry virtually stops. Meanwhile, the isomerization reaction continues to consume starting material and produce a vinyl ether, wh ich can then trap the metathesis catalyst into a stable, low-reactive Fisher carb ene whose bis-phosphine analog is known to thermally decompose into hydride complex [Ru]6.90,91 Observation of a typical color change from pink to bright yellow, indicative of alkoxy carbene decomposition, reinforces the argument for the proposed m echanism. Therefore, not only does the metathesis rate decrease overtime, but the isom erization rate also increases as the hydride concentration increases (if we consider that the source of hydride is related to catalyst decomposition). Hence only 30% of metathes is product, subsequently isomerized to 52cis and 5-2trans Other Mechanistic Considerations Note : The olefin isomerization mechan ism operating here cannot involve a -H shift from an allylic proton to the metal as proposed for the is omerization of allyl alcohols in the presence of [Ru]1.222 Indeed, the mechanism shown in Figure 5-7

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101 involves the stoichiometric consumption of th e isomerization-metathes is catalyst and in our case would have yielded only up to 0.2 mol% of isomerized starting material. Nevertheless, this step may be responsible for the formation of the hydride species involved in the metal-hydride isomerization reaction described above, by reaction of the ruthenium carbene with vinyl ether, and subs equent decomposition of the Fisher carbene hence formed.90,91 Ru O D D Ru Ph H Ru O D D Ru O D DO D D -CH2=CHPh Figure 5-7. -H abstraction-elimination mech anism proposed for allyl alcohol isomerization by [Ru]1 Kinetic Studies The relative rates of isomerization versus metathesis could be measured for allyl 1,1-d2-methyl ether when the system had reached metathesis equilibrium. Indeed, when the reaction was monitored by 1H NMR the starting material qui ckly disappeared to form 1,4-dimethoxy-but-2-ene, but after 60 minutes, the rate of formation of metathesis product became slower until its concentration leveled, indicating metathesis equilibrium at only 7% conversion. (Notewor thy, traces of deuterium in corporation on the olefinic bond of the starting material were also obser ved, suggesting the pr esence of a metal hydride at this early stage of reaction.) From this point on, the steady evolution of isomerized starting material was clearly observed. Considering the rate of metathesis to be negligible and the isomerization of 5-2 to be irreversible, as proven above, the isomerization reaction then followed first-order kinetics. The rate was calculated at approximately 5.59 10-06 s-1 (Figure 5-8). The metathesis equilibrium constant could also

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102 be estimated, at 4.04 10-06, by considering the concentratio ns of metathes is product and starting material at equilibrium. Of course, this does not reflect the rate of the metathesis reaction, but if we consider it to be of the sa me order or larger than the isomerization rate, the reverse metathesis rate with ethylene should be 1 or larger which emphasizes the importance of ethylene removal. 05000100001500020000250003000035000 -20 -18 -16 -14 -12 -10 -8 -6 -4 -2 0 y = -5.59E-6 t 2.448 R2=0.99967Ln [ 9 ]/[ 9 ]0Time (s) Figure 5-8. Total concentration of 4-1 versus time. Conditions: 2.3M solution of 4-1 in C6D6 with 0.2 mol% of [Ru]4 at 35oC for 8 hours Now that olefin isomerization is known to operate via a metal-hydride complex, the question remains what is the structure of this hydride species and what causes its formation? The first step would aim to understand the role of NHC ligands in the isomerization behavior of ruthenium carbene catalysts. Design, Synthesis and Characterization Recent reports have shown that metathes is catalysts bearing NHC ligands undergo olefin isomerization to a considerable extent compared to their phosphine analogs.166,176,192 Ligand variation experiment s on both the phosphine and carbene

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103 ligands have further reinforced the idea that NHC ligands play a major role in the isomerization behavior of ruthen ium carbene catalysts (Chapter 4). In light of the results discussed above, the eventual participation of NHC groups in the production of a metalhydride became the focus of the following experiment. Speculations evoke a possible ruthenium insertion into the C-H bond of one of the mesityl groups. To address this issue, complex [Ru]23 (Figure 5-9) was designe d with fully deuterated ortho methyl groups on the NHC aryl rings. The structur e similarity should guarantee comparable reactivities to parent [Ru]4 while the deuterium atoms should allow its traceability through isotopic labeling experiments. Any sign of ruthenium insert ion should be easily detected by NMR spectroscopy. Br CH3H3C Br COOH HOOC Br CD2D2C OHOH Br CD3D3CiN CD3CD3D3C D3C Nii, iiiiv, vNH2CD3D3C Cl Ru Cl Ph PCy3Cl D3C D3C CD3CD3N N N CD3CD3D3C D3C N H CCl35-A5-B5-C5-D5-E 5-Fvi vii-ix [Ru]235-Gx xi Figure 5-9. Synthesis of Complex [Ru]23a a Reagents and conditions: (i) KMnO4, 50% t BuOH in water, reflux, 16h; 99%. (ii) K2CO3, CH3I, cat. CsCO3, DMF, reflux 12h; 81%. (iii) LAD, THF, 0oC, 20 min; 85%. (iv) CBr4, PPh3, CH2Cl2; 70%. (v) LAD, THF, room temperature; 90%. (vi) (a) Mg, ether, reflux, 2h; (b) TMSDA, room temperature, 3h; 55%. (vii) glyoxal, n -PrOH, water, room temperature to 60oC, 6h; 80%. (viii) NaBH4, 1:2 MeOH:THF, room temperature, 16h, 1M HCl; 85%. (ix) CH(OEt)3, reflux, 4h; 60%. (x) NaH, CHCl3, room temperature 2h; 82%. (xi) [Ru]1, toluene, 60oC, 90 min; 75%. Starting material bromom -xylene (5-A) was sequentially oxidized and reduced to afford deuterated xylene 5-D. Direct reduction of the diacid 5-B by lithium aluminum

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104 deuteride only yielded unidentified deco mposition products. The reaction was rendered successful by switching to th e corresponding diester, but st ill necessitated controlled conditions to reach reasonable yi elds. The alcohol groups of 5-C were substituted to bromine groups to allow further reduction into the fully deuterated bromoxylene 5-D. Deuterated 2,6-dimethyl ani line was obtained by substitution of the bromine group through a Grignard reaction with trim ethylsilylmethyl azide (TMSDA). The corresponding ligand salt 5-F was then easily prepared according to the regular procedure. However, the chloroform adduct 5-G was used for the synthesis of final complex [Ru]23 to ensure a better yield and avoid salt contamination.72 Both 1H and 31P NMR spectra of complex [Ru]23 are very similar to catalyst [Ru]4 showing the same exact carbene shift at 19.63 ppm and the same phosphine at 29.5 ppm.71 No hydride trace was detect ed at this point. The 2H NMR shows 2 singlets at 2.35 and 2.75 ppm, suggesting the asymmetr y of the 2 aryl groups provoked by a distortion of the imidazolylidene ring. The cr ystal structure exhib its a typical squarepyramidal geometry with the carbene moiety in the apical position (Figure 5-10). However, almost the entire molecule is affect ed with an inherent disorder related to the existence of two mirror images within the as ymmetric unit. Representative bond lengths and angles of complexes [Ru]4 and [Ru]23 are reported in Table 1 for comparison.194 Apart from the chlorine atoms, whose bond le ngth to ruthenium vary of about 0.1 , both solid-state structures appear to be very similar. The deviation of the chlorines is just a result of the global molecular disorder. In fact, the Ru-Cl bond length averages at 2.3929 from the 2 disordered structures. This augmented freedom of the NHC ring could be due to the missing para -methyl group on the aryl substituents compared to

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105 complex [Ru]4, but does not disturb the overall charac teristics. The superposition of complex [Ru]4 and one of the mirror image of [Ru]23 shows little variation between the two crystal structures and clearly illust rates their similarity. Since complexes [Ru]4 and [Ru]23 are so related characteristically, it is reas onable to expect a comparable catalytic behavior. A B Figure 5-10. X-Ray structure of complex [Ru]23. A) ORTEP Diagram of complex [Ru]23. B) Superposition of the cr ystal structures of complex [Ru]4 (orange) and [Ru]23 (blue) Table 5-1. Selected bo nd lengths () and angles (deg) for complexes [Ru]4 and [Ru]23 Bond lengths Complex [Ru]4 Complex [Ru]23 Ru-C(1) 2.085(2) 2.076(3) Ru-C(20) 1.835(2) 1.844(3) Ru-Cl(1) 2.3988(5) 2.3644(9) Ru-Cl(2) 2.3912(5) 2.4302(10) Ru-P 2.4245(5) 2.4298(8) Bond angles Complex [Ru]4 Complex [Ru]23 C(20)-Ru-C(1) 100.24(8) 98.37(11) C(1)-Ru-Cl(1) 94.55(5) 88.90(8) C(1)-Ru-Cl(2) 83.26(5) 89.65(8) C(20)-Ru-Cl(1) 89.14(7) 105.17(9) C(20)-Ru-Cl(2) 103.15(7) 87.50(9) Cl(1)-Ru-Cl(2) 167.71(2) 167.32(4) C(1)-Ru-P 163.73(6) 163.29(7) C(20)-Ru-P 95.89(6) 97.33(9)

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106 Thermal Stability When complex [Ru]23 was left at 70oC in an airtight solution of benzene -d6, no sign of ruthenium deuteride was detected by 2H NMR, even after 21 days, where the decomposition process was already at an adva nced stage. In f act, observation of the 31P NMR spectrum indicates that only 4% of the original complex remained. Other typical peaks have emerged at 10.4 ppm, 34.6 pp m, 47.0 ppm and 47.4 ppm, along with what appears to constitute 70% of the phosphorus products centering around 72 ppm. The first signal can be attributed to free PCy3. The peak at 34.6 ppm seems consistent with the phosphonium salt [PCy3CH2Ph]+[Cl]-, observed by Grubbs89 as a by-product of methylidene [Ru]4’ decomposition. Interestingly, the peaks at 47.0 ppm and 47.4 ppm, representing 15% of the phosphorus produc ts, correspond to carbonyl hydride complex [Ru]6 and [Ru]7, respectively, usually formed by reaction of [Ru]4 with primary alcohols.93 Contamination during the methanol wash following the synthesis cannot possibly account for the 15% of hydrid e produced here, especially since 1H NMR only confirmed the presence of [Ru]6 and [Ru]7 (characteristic triplet at -24.25 ppm and a doublet at -24.95 ppm) as traces. Ther efore, some unknown phosphorus compound must appear around the same shifts as the carbonyl compounds. A third uni dentified hydride is visible on the 1H NMR as a doublet at -7.43 ppm (2J=50 Hz) that had also been observed by reaction of [Ru]4 with primary alcohols. Even though the 2H NMR does not indicate the presence of a ruthenium deuteride, a number of deuterated species have formed. Most signals overlap between 1 and 3 ppm collapsing into a broad base, which accounts for the deuterated methyls of the NHC ligand whether it is free or coordinated to a ruthenium atom (i.e. complexes [Ru]7 and

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107 [Ru]23). The 13C NMR did not reflect the presence of any carbonyl compounds nor carbide. Since no compound could be isolat ed, the isomerization behavior of this decomposition mixture was tested on 1-oc tene using typical reaction conditions.192 After 24h at 55oC, a 2.3M solution of catalyst [Ru]4 in neat 1-octene affords 76% isomers. In this case, the catalytic mixture was dried under vacuo once fully decomposed (no more carbene signal on the 1H NMR), and 0.5 g of 1-octene was added. The reaction was allowed to stand at 55oC for 24h, and the isomers content was determined by 1H NMR by integration of the internal olefin peak at 5.40 ppm. No decomposition product of complex [Ru]23 was able to isomerize olefins at the rate observed with the original metathesis catalyst (5% of isomer s vs. 76% obtained with catalyst [Ru]4 in similar conditions).176 Because the methylidene is a key intermed iate in most metathesis reactions, its decomposition was also checked in situ by reacting [Ru]23 with excess ethylene in benzene at 60oC and stirring it for 3 days. When the reaction was sampled for 1H NMR, these conditions led to the comple te transformation of benzylidene [Ru]23 into the methylidene, which decomposed rapidly as indicated by the disappearance of the methylidene signal at 18.40 ppm. But ag ain, no sign of ruthenium deuteride was detectable by 2H NMR, and the same peaks, obser ved during the decomposition of the benzylidene, appeared between 1 and 3 ppm. Interestingly, a singlet at 7.14 ppm indicated the presence of par tially deuterated benzene, th e deuterium content largely exceeding the amount naturally found in benzen e. This suggests the existence of an exchange process since the only original s ource of deuterium in this case was the methylidene of complex [Ru]23. A similar exchange to solvent was observed with

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108 ruthenium dihydride (IMes)Ru(H)2(H2)2PCy3,225 and other dihydrides have shown to undergo exchange processes.206,207 The species responsible for this H/D exchange may also be a ruthenium dihydride, which may not be detectable because of NMR detection/concentration limits. The catalytic mixture once fully decompos ed was again tested for isomerization activity using 1-octene in the same cond itions as previously. Even though the isomerization extent was increased to 25% isomers, the decomposition products of complex [Ru]23’s methylidene did not match the is omerization behavior observed with catalyst [Ru]4 in metathesis conditions, which suggests that the isomerization process involves more than simply decomposition products. Other intermediates of the metathesis cycle must play a role in the form ation of the hydride res ponsible for this sidereaction. For example, Forman et al.226 have described the decomposition of a metallacyclobutane through -H-abstraction to form a ruth enium hydride, and explain the appearance of different alkenes from the metathesis of ethylene with [Ru]1’. Hence, hydride formation in the presence of olefinic su bstrates was further investigated to reflect true metathesis conditions. Catalytic Behavior Under Metathesis Conditions If there was formation of a ruthenium deut eride by metal insertion in one of the methyl bonds, inevitably there would be de uterium incorporation on the olefin, which should be easily detectable by NMR. To a void any solvent contam ination and allow the detection of both catalyst and substrate on the NMR scale, th e experiments were run neat, the only deuterium source being complex [Ru]23. First, 1-octene was used as the substrate since previous isomerizatio n studies were based on this model.176,192 A 0.5

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109 mol% solution of complex [Ru]23 in 1-octene was monitored by 1H and 2H NMR at 35oC for 4 hours. These reaction conditions will ensure significant amounts of both metathesis and isomerization.176,192 The 1H and 2H spectra were systematically superimposed to match chemical shifts, si nce no internal standard was available. Because of the chemical simplicity of the s ubstrate, the distincti on between isomers and metathesis products is virt ually impossible by standard 1H NMR. Still, in the present conditions metathesis is known to occur much faster than the isomerization process. Figure 5-11. NMR spectrum of neat 1-octene with 0.5 mol% of complex [Ru]23, after 4h at 35oC and 20h at 55oC: a) 1H NMR; b) 2H NMR Shortly after the experiment was started, an internal olefin signal appeared on the proton spectrum marking the beginning of th e metathesis process, while the deuterium spectrum only showed the two singlets from the catalyst deuterated methyl groups. However, after 4 hours, several other peaks became visible on the deuterium spectrum,

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110 confirming incorporation of deuterium at va rious positions of the olefin backbone. For easier reading, Figure 5-11 shows the 1H and 2H NMR spectra taken after 24 hours of reaction. A multiplet correspondi ng to the terminal olefin suggests the formation of CHD=CH(CH2)5CH3. Two other vinyl signals at 5.75 and 5.38 ppm, though smaller, evidence deuterium incorporation at the C-2 position of 1-octene and on an internal double-bond probably from an isomer of eith er 1-octene or 7-tetradecene. These observations are consistent with the presence of a metal deuteride, which is further supported by the emergence of p eaks in the ally lic region (around 2.0 ppm) followed by peaks in the alkane region (at 1.30 and 0.90 ppm), supposing that the isomerization reaction progresses along the olefin backbone to produce 1,d12-, -3-, -4-octene and 8,d13-, -2-, -1-octene, for example (Figure 5-12) The formation of the metal alkyl may be fast relative to the decomplexa tion of the olefin, so the same molecule of substrate can undergo several transformations before being released.214 Despite the large excess of 1octene, it is also possible that some of the deuterated products come from isomerization of the metathesis product, although this canno t be quantified. Noteworthy, the alkyl deuteron signals are usually broader and fla tter than their vinyl analogs because of multiple couplings. For example, the methyl signal expected for 1,d12-octene at 1.60 ppm only becomes visible by 2H NMR after longer reacti on times. Nevertheless, deuterium atoms appear to preferentially incorporate at the C-1 position, suggesting a predominant Markovnikov addition of the me tal deuteride across the double bond. When the temperature was raised to 55oC, the concentration of these deuterated terminal olefins seemed to increase even faster. Accord ing to fundamental entropy thermodynamics, decomplexation of the olefin must occur to a greater extent at higher temperatures,

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111 therefore generating more olefins at a lower degree of isomerizati on. Indeed, after 20 hours at 55oC, the signal for the allylic methyl of 1,d12-octene at 1.60 ppm has increased significantly and is now larger than othe r allylic or alkyl signals (Figure 5-11). Ru Ru D Ru D D H D D D etc. RuH D Figure 5-12. Formation of 1-octene isomer s catalyzed by a ruthenium deuteride complex Aside from the signals of the deuterated me thyl groups of the metathesis catalyst, 2 broad singlets arise at 2.25 and 2.50 ppm. This indicates a transformation of the original complex, whether it represents a dissociating NHC ligand or a different organometallic species. Yet, none of the co mplexes identified so far from catalyst decomposition could match our observations. Even though the multiple peaks between 2 and 3 ppm recall hydride complex [Ru]5 formed by thermal decomposition of the methylidene [Ru]4’, it does not account for the fo rmation of the metal deuteride evidenced in this experiment.89 Likewise, the characteristic signal from the Ru-C D2Ar moiety is not observed, which would ha ve probed the presence of complex [Ru]8’s equivalent. Of course, this does not exclude the possibility of a different C-D activation. In fact, hydride complex [Ru]24 shown in Figure 5-13, formed by insertion of ruthenium into the C-H bond of a mesityl group in the pres ence of alkenes, exhibi ts a broad ‘triplet’ at 2.76 ppm from the protons of the Ru-C H2Ar moiety. Other examples of C-H

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112 activation from ruthenium carbenes could par tially match these signals; however these assignments must remain speculations as long as the components of the mixture are not isolated. N N Ph3P Ru OC PPh3CH2H Figure 5-13. Complex [Ru]24 As a matter of fact, no ruthenium deuterid e species was detected even after longer reaction times. The complex responsible fo r the isomerization pr ocess probably reacts faster than the NMR time scale allows for de tection or its concentration is simply too low. But, if there were indeed formation of a ruthenium deuteride complex in sufficient amounts, then for each deuterium atom incorporated onto a substrate molecule, the corresponding ruthenium hydride species would be produced, according to the metalhydride addition-elimination mechanism. So this hydride complex should be visible by 1H NMR as the isomerization process, producing the D/H exchange, goes on. As predicted, a doublet appeared at –20.36 ppm with 2JHP=17.8 Hz, characteristic of a hydride situated trans to an empty coordination site and cis to a phosphine. The same signal had been observed during the decompositi on of the methylidene, and interestingly, only the methylidene remains at this point, impl ying that all the catalyst has either been activated by the olefin or decomposed. Howeve r, the information collected thus far does not allow to conclude whether this hydride complex is in fact the one responsible for olefin isomerization under metathesis condi tions or if it is simply a product of the methylidene decomposition.

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113 The same experiment was performed with allyl methyl ether, which was used to evaluate the isomerization mechanism. D ealing with an irreversible isomerization reaction should facilitate pr oducts identification, which is limited to NMR spectroscopy. Again, 0.5 mol% of catalyst was used to pr ovide sufficient deuterium content for NMR detection. Figure 5-14. 2H NMR spectrum of neat allyl methyl ether with 0.5 mol% of complex [Ru]23, after 4h at 35oC After 4 hours at 35oC, about 30% of metathesis product had formed, with no isomerization product detectable on the 1H NMR, the high concentration of ether favoring the bimolecular metathesis process over the monomolecular isomerization reaction. The 2H NMR spectrum shows deuterium incor poration mostly at the terminal carbon of the olefin, as allyl -3-d1methyl ether, the signal ma tching perfectly the pattern of the 1H spectrum (Figure 5-14). Note: Due to the large excess of allyl methyl ether, it is assumed that the molecules are only monodeuter ated at early reaction times, since they are not likely to react again with another ru thenium deuteride. De uterated ethylene is also present in significant amounts, more likely by metathesis of allyl -3-d1methyl ether.

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114 Again, this is consistent with the presence of a metal deuteride complex. Smaller resonances at 1.68 ppm indicate the formation of isomerized product, as 3-d1-1-methoxypropene, suggesting that the secondary metal-al kyl preferentially eliminates the proton at the C-3 position rather than at the C-1 positi on. This could be due to precoordination of the oxygen atom to the metal center of th e alkyl complex, hence forcing the C-1 hydrogens away from the Ru-C-C plan for -H-elimination (Figure 5-15). Surprisingly, a triplet at 3.13 ppm and a doubl et at 3.72 ppm suggest the presence of deuteron at the C-4 and C-1 position of the starting material (less than 0.3%), respectively. A classical metal-hydride mech anism cannot explain deuteration at these positions, nor can a -allyl mechanism. Hence, an alternate mechanism must be proceeding competitively; however, to a much lower extent. The exchange process observed during the deco mposition of complex [Ru]23’s methylidene may also be operating here. If this is the case, the intr oduction of a non-isomeriz able olefin, such as 3,3-dimethylbutene, should result in the deuteration of every position, and more particularly on the methyl groups. Ru O H1H1RuO D Ru O D D H3H3Ru O H1H1D H3H3Ru O D Figure 5-15. Orientation by oxygen precoordination into -H-elimination at the C-3 position Noteworthy, the two original singlets from the catalyst methyl groups merge into a broad singlet around 2.35 ppm, unlike what was observed when 1-octene was used as the substrate. The catalyst may adopt a more sy mmetrical coordination in the presence of the polar allyl ether solvent.

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115 To look into this potential exchange proce ss, the experiment was repeated with 3,3dimethylbutene for its inherent inability to isom erize. If there was indeed formation of a “classical” ruthenium deuteride, de uterium incorporation would occur exclusively at the C-1 and C-2 position. In addition, the metathes is reaction of olefin s containing allylic methyls is known to proceed at a slow rate, if at all. In fact, the cross-metathesis of 3,3dimethylbutene with [Ru]1 only resulted in an accumulati on of the methylidene complex by non-productive metathesis.80 Accordingly, after 4 hours at 35oC, metathesis conversions merely reached 10%. Figure 5-16. 2H NMR spectrum of neat 3,3-dimethyl butene with 0.5 mol% of complex [Ru]23, after 4h at 35oC Figure 5-16 illustrates the corresponding 2H NMR. Even though there are signs of deuteration on the double bond (signals at 5.70 and 4.70 ppm), most displaced deuteron are on the methyl groups as indicated by the majo r peak at 0.86 ppm. This is clearly the result of an exchange process, although not necessarily direct, between the methyl groups of the NHC ligand and the substrate. It is likely that the deuterium atoms are transferred to the metal center from the NHC methyl groups and then onto the substrate. Again, this type of transfer could very well be the result of ruthenium insertion into the methyl C-D

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116 bond, the cyclometallation of a substituent on a nitrogen-ligated carbene on ruthenium being a fairly common reaction, albeit not well understood.72,183,206-209 Interestingly, ruthenium dihydride (IMes)Ru(H)2(H2)2PCy3 promotes a similar exchange from the mesityl methyl groups to a variety of aromatic compounds, although preferentially exchanging with sp2 rather than sp3 C-H bonds.225 Even though the event of the formation of a dihydride complex from a ruth enium carbene in the absence of dihydrogen may seem unlikely,158,215,216 the reactivity of the phosphine free methylidene offers many possibilities. For instance, Stradiotto et al.227 reported a reversible -H elimination allowing the interconversion of Ru=C and Ru-al kyl species. The elec trophilicity of the methylidene carbon also renders it highl y sensitive to nucleophilic attack.228 In fact, Grubbs et al.89 propose that the first step toward s formation of bimetallic complex [Ru]5 involves a nucleophilic attack by free phosphi ne to form a ruthenium alkyl complex. Certainly, the hypothesis of a ruthenium dihydr ide is not ruled out by the spectroscopy analysis of [Ru]4’ decomposition. One of the observed hydride signals may very well be one of a dihydride. The rate of the exchange process appare ntly depends on the substrate. Indeed, deuterium incorporation is gr eater on the methyls of 3,3dimethylbutene than on allyl methyl ether. The extensive production of the methylidene complex inherent to the metathesis of olefins containing allylic methyls, accelerates the rate of catalyst decomposition, which is probably in direct correlation with the formation of the species promoting the exchange process.

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117 Conclusions A deuterium labeling study was undertaken to determine the mechanism of olefin isomerization during the metathesis reactions catalyzed by 2nd generation Grubbs’ catalyst. The reaction of allyl1,1-d2methyl ether with [Ru]4 at 35oC was followed by 1H and 2H NMR spectroscopy. The evidence of deuterium incorporation at the C-2 position allowed to conclude that a metalhydride addition-elimination mechanism was operating under these conditions. In closed-system, the metathesis rate was calculated at approximately 5.59 10-06 s-1 while the equilibrium c onstant was about 4.04 10-06. The next step was attempted to identify the eventual role of the NHC ligand in the formation of the hydride species responsible for the isomerization reaction. Complex [Ru]23, an analogue of [Ru]4, was synthesized with deuterated omethyl groups on the aromatic rings of the NHC ligand. Its decomposition afforded several metal hydride species; however, no deuteride complex was detected by 2H NMR spectroscopy. The methylidene analogue [Ru]23’ was also subjected to thermal decomposition, but again did not afford any detectable ruthenium deut eride complex. The isomerization activity of the catalytic decomposition mixtures were te sted with benchmark 1-octene, but did not match the isomerization rates observed with [Ru]4 under similar metathesis conditions. This suggests that other metathesis intermed iates, such as the metallacyclobutane, must be involved in the isomerization process. When complex [Ru]23 was tested with 1octene, a variety of deuterated olefins were produced, indicating the presence of a deuteride complex, albeit not isolable spectroscopically. Finally, reaction of [Ru]23 with allyl methyl ether and 3,3-dimethylbutene actually revealed the existence of a competitive exchange process between the NHC ligand and the substrate. The exchange might be promoted by a ruthenium dihydride intermediate whose formation is closely

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118 related to the methylidene d ecomposition. Further studies mu st be conducted to isolate and characterize these hydride complexes. Meanwhile, hydride traps should be used during metathesis reactions catalyzed by ruthenium carbenes in order to prevent competitive isomerization. Experimental General Routine 1H NMR (300 MHz) and 13C NMR (75 Hz) measurements were recorded on either a Mercury series or Varian VXR-300 NMR s uperconducting spectrometer. Chemical shifts are reported in ppm relati ve to tetramethylsila ne (TMS) or residual proton from the solvent. All organom etallic spectra were recorded in d6-benzene, previously dried over Na/K alloy and dega ssed by three freeze-pump-thaw cycles. All materials were purchased from Aldrich and used as received, except for the THF and ether that were previously dried over a catalyst bed. Complex [Ru]4,71 allyl1,1-d2-methyl ether,205 and trimethylsilylmethyl azide (TMSMA)229 were synthesized according to the literature. Allyl1,1-d2-methyl ether was distilled, degassed by three freeze-pump-thaw cycles and stored in a glovebox. Deuterium Labeling Study with Allyl-1,1-d2-methyl ether In the drybox, 4 mg (0.005 mmol) of complex [Ru]4 was dissolved with 0.8 mL of C6D6 or toluened8 in a J. Young Valve NMR Tube, which was then layered with 185 mg (2.5 mmol) of allyl1,1-d2-methyl ether. The tube was shaken right before being loaded in the NMR instrument. NMR spectra were recorded on a Varian Inova spectrometer equipped with a 5 mm indirect detection probe operating at 500 MHz for 1H and at 125 MHz for 13C. The temperature was 35C. Chemical shifts ar e reported in ppm relative to TMS. The

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119 residual methyl signal of the solvent was used as reference (2.09 ppm for 1H and 20.4 ppm for 13C). O O O O O O12.6 1.43 96.4 4.52 147.9 D 55.1 3.17 100.5 4.29 9.3 1.62 147.0 D 58.8 3.18 28.5 2.06 98.7 4.60 148.6 D 55.1 3.17+ 58.0 3.12 73.1 D2 72.0 D2 58.1 3.11 24.9 2.41 102.8 4.36 147.3 D 58.9 3.165-2 trans 5-3 c is 5-3 trans 5-2 cis Figure 5-17. 1H and 13C chemical shifts assignment in compounds 5-2cis 5-2trans 53cis and 5-3trans O O O O O O0.14 0.13 0.13 0.30 0.32 0.30 1.00 0.00 0.21 n.m. 0.29 0.24 0.32 0.31 1.00 0.00 0.00 1.00 1.00 0.00 0.28 0.25 0.28 0.32 0.32 0.31 1.00 0.00 0.15 0.14 0.09 0.33 0.32 0.29 1.000.005-2 trans 5-3 cis 5-3 trans 5-2 cis0.34 0.34 0.36 0.34 0.17 0.19 0.13 0.13 Figure 5-18. Molar fractions of compounds 5-2cis 5-2trans 5-3cis and 5-3trans in the product (under the compound number) and molar fraction of deuteration at each position.a (a) The number on top was based on the proton spectrum, and the second number on the deuterium spectrum. The number on the bottom for the molar fraction of deuteration was

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120 based on the integral of the proton signal at that position and the composition determined in the deuterium spectrum, and it is the most reliable. The sample is a mixture of four compounds, 5-2cis 5-2trans 5-3cis and 5-3trans shown in Figure 5-17. Each of these com pounds is a mixture of the four isotopomers resulting from partial deuteration in positions 2 and 3 of the alkylene ether. Positions 1 and 4 are completely deuterated. The relative ratios of the four compounds, and the molar fractions of deuteration in each pos ition are given in Figure 5-20. Figure 5-19. Expansion of the proton spectrum, the CH2 region The proton spectrum displays in the re gion 4.70-4.20 ppm the signals of a mixture of four compounds (Figure 519), quartets for compounds 5-2cis and 5-2trans and triplets for compounds 5-3cis and 5-3trans All these signals display an extra coupling with deuterium, triplet in a 1:1:1 ratio. These signals present in the gHMBC sp ectrum (Figure 5-20) a one-bond coupling to a carbon in the range 96-102 ppm and two lo ng-range couplings, one with a carbon at 9-28 ppm and one with a carbon at 147-149 ppm. This later carbon disp lays a triplet in f1 consistent to a one-bond coupling with deut erium, and also a long-range coupling to protons at 3.16-3.18 ppm, protons which are on carbons at 55-60 ppm. The protons on the

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121 carbons at 9-28 ppm are in the range 1.43-2.41 ppm and couple with both the carbons at 96-102 and with the carbons at 147-149. Thes e proton-carbon couplings, together with the characteristic chemical shifts, identify the methyl-1-propenyl ether fragment in all four compounds. In compounds 5-3cis and 5-3trans the aliphatic protons at 2.06 and 2.41 couple with carbons at 73.1 and 72.0, resp ectively. Both these carbons carry no protons and couple with th e protons of a methoxy group. The NOESY spectrum reveals nOe’s between the alkene proton and the me thoxy protons of the methyl-vinyl ether fragment in compounds 5-2trans and 5-3trans only, therefore in these compounds the double bond is E. In these compounds the proton-deuterium coupling across the double bond is ca 1.8 Hz, while in the Z compounds 5-2cis and 5-3cis it is ca 0.9 Hz. Figure 5-20. Expansion of the gHMBC spectrum

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122 The carbon spectrum (Figure 5-21) confirms the lack of deuterium on the methoxy groups and the complete deuteration in position 1 of the alkene fragment, for all of four compounds. In compounds 5-3cis and 5-3trans the signals at 73.1 and 72.0 could barely be seen, as broad multiplets, confirming that these carbons carry only deuterium. With two positions with partia l deuteration, compounds 5-2cis 5-2trans 5-3cis and 5-3trans are each present as four isotopomers. This is why the carbons in position 3 of the alkene fragments display a pattern of eight lines – the one at higher field is the H3,H2 isotopomer, followed by the one of the H3,D2 isotopomer. The next six lines are less intense and belong to the tr iplets of the D3,H2 and D3,D2 isotopomers. The carbon chemical shifts in Figure 5-17 ar e those of the H2,H3 isotopomer. Figure 5-21. Expansion of th e carbon spectrum, C3 region The molar fraction of the four compounds in the mixture and the fraction of deuterium substitution at each position were de termined by integration of the signals in both the proton and the deuterium spectra. They are given in Figure 518, as the first and the second sets of values, respectively. Th e spectra were taken at 70C, when the separation of the signals of interest was optim al. In the proton spectrum, the signals of the methoxy groups were used, since there is no de uteration of these gr oups. Four signals could be integrated separately, 3.22-3.20 (5-2cis ), 3.20-3.18 (5-2trans + 5-3cis + 5-

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123 3trans ), 3.14-3.12 (5-3cis ) and 3.12-3.11 (5-3trans ). The separation of the last two signals is marginal, and for this reason the values for the relativ e ratio of the four compounds determined in the proton spectrum ar e considered to be less precise than the ones determined in the deuterium spectrum. F our signals in the deuterium spectrum were used, 6.30-6.12 (position 1 in 5-2trans + 5-3trans ), 5.78-5.62 (position 1 in 5-2cis + 53cis ), 3.30-3.20 (position 3 in 5-3cis ), and 3.20-3.09 (position 3 in 5-3trans ). The fraction of deuteration at positions 2 and 3 was determined in both the proton and the deuterium spectra. The separation of the signals was poorer in the deuterium spectrum, in which also the signal of position 3 in 5-3trans was overlapping the methyl signal of toluene. We consider that the best ap proach is to determine the relative ratios of compounds in the deuterium spectrum and then to use these ratios and the integrals from the proton spectrum to calculate the fraction of deuteration at e ach position. Values determined this way are presente d on the third row in Figure 5-17. X-ray Experimental Crystals suitable for X-ra y structure determination were obtained from slow diffusion of pentane into a saturated solution of [Ru]23 in benzene at room temperature. Data were collected at 173 K on a Siemens SMART PLATFORM equipped with A CCD area detector and a graphite monochromator utilizing MoK radiation ( = 0.71073 ). Cell parameters were refined using up to 8192 reflections. A full sphere of data (1850 frames) was collected using the -scan method (0.3 frame width). The first 50 frames were re-measured at the end of data collecti on to monitor instrument and crystal stability (maximum correction on I was < 1 %). Ab sorption corrections by integration were applied based on measured indexed crystal faces.

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124 The structure was solved by the Direct Methods in SHELXTL6, and refined using full-matrix least squares. The non-H atoms were treated anisotropically, whereas the hydrogen atoms were calculated in ideal pos itions and were riding on their respective carbon atoms. The structure has two disorder s, a minor one where the C2-C3 moiety is refined in two parts with thei r site occupation factors depende ntly refined. The second disorder is a major one involving the tr iphenylphosphine group and the C20 ligand according to a pseudo-mirror symmetry passi ng through Ru, C1 and bisecting the C2-C3 bond. A total of 539 parameters were refined in the final cycle of refinement using 13498 reflections with I > 2 (I) to yield R1 and wR2 of 3.77% and 8.40%, respectively. Refinement was done using F2. Synthesis of Complex [Ru]23 Synthesis of 2-bromo-isophthalic acid (5-B). In a round-bottom flask equipped with a condenser, 25.0g of bromoxylene (0.135 mo l) was dissolved in 200 mL of 1:1 tBuOH:water. Two equivalents of KMnO4 (42.7g, 0.270 mol) were added portionwise, and the solution was refluxed for 4 hours. Upon cooling, 2 more equivalents of KMnO4 were added, and the solution was refluxed overn ight. The mixture was filtered through a celite bed while still hot, and the filtrate was concentrated to 1/3. Concentrated H2SO4 was added slowly until a white precipitate fo rmed. Filtration and drying under vacuum afforded 32.7g (0.133 mol, 99%) of a white powder. (mp>250oC). 1H NMR (CD3OD, 300 MHz): =7.73 (d, 2H, meta C H 2JH,H = 7.6 Hz ), 7.48 (t, 1H, para C H 3JH,H = 7.8 Hz), 5.32 (s, 2H, COO H ); 13C NMR (CD3OD, 75 MHz): = 169.9, 137.7, 138.5, 128.4, 118.3; LRMS/EI C8H5BrO4 = 245, found 245.

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125 Synthesis of 2-bromo-di methyl isophtalate. 2-Bromo-isophthalic acid 5-B (32.7g, 0.133 mol) was dissolved in DMF. Two equivalents (37.0g, 0.270 mol) of K2CO3 and 38.0g (0.27 mol, 2.0 eq) of methyl iodide were added to the solution as well as a catalytic amount of CeCO3. The reaction was stirred overn ight at room temperature and quenched with acidic water until all the base dissolved. The aqueous solution was extracted with diethyl ether. Afterwards the organic layer was washed with water, dried over MgSO4 and concentrated into a yellow oil (30.0g, 0.110 mol, 81%). 1H NMR (CDCl3, 300 MHz): = 7.68 (d, 2H, meta C H 2JH,H = 7.4 Hz ), 7.40 (t, 1H, para C H 3JH,H = 7.8 Hz), 3.94 (s, 3H, COOC H3); 13C NMR (CDCl3, 75 MHz): = 166.6, 135.2, 132.1, 127.1, 118.7, 52.6; MS (LRMS/EI): C10H9BrO4= 273, found 273. Synthesis of 2-bromo-1,3-benzenedimethyl-d4 alcohol (5-C). In a flame-dried round bottom flask immersed in an ice ba th, 9.23g (0.219, 2.0 eq.) of lithium aluminum deuteride were suspended in 200 ml of dry THF. 2-Bromo-dimet hyl isophtalate (30.0g, 0.110 mol) diluted in 20 ml of dry THF was added dropwise at 0oC. After the addition was complete, the slurry was stirred for 20 minutes. The reaction was quenched and worked up following the standard procedure,230 to yield 20.6g (0.093 mol, 85%) of a white powder (mp=152-154oC). 1H NMR (Pyridined5, 300 MHz): = 7.95 (d, 2H, meta C H 2JH,H = 7.2 Hz ), 7.50 (t, 1H, para C H 3JH,H = 7.5 Hz), 5.14 (s, 2H, O H ); 13C NMR (Pyridined5, 75 MHz): = 140.9, 126.3, 125.6, 112.1, 62.2 (q, C D2OH); MS (LRMS/EI): C8H5D4BrO2= 221, found 221. Synthesis of 2-bromo, ’-dibromo-m-xylene-d4. 2-Bromo-1,3-benzenedimethyld4 alcohol 5-C (20.6g, 0.093 mol) was suspended in a solution of CBr4 (71.2g, 0.214 mol, 2.3 eq) in CH2Cl2. Triphenylphosphine (57.4g, 0 .219 mol, 2.35 eq.) was added

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126 portionwise at room temperatur e to avoid overheating. Afte r the addition was complete, the clear solution was precipitated in diethy l ether and filtered. As the solvent was evaporated, any remaining triphenylphosphine was further filtered. The crude product was purified via column chromatography using 5:1 hexanes: ethyl ac etate to yield 22.5g (0.065 mol, 70%) of a white powder. (mp= 85-88oC). 1H NMR (CDCl3, 300 MHz): = 7.41 (d, 2H, meta C H 2JH,H = 7.3 Hz ), 7.28 (t, 1H, para C H 3JH,H = 7.2 Hz); 13C NMR (CDCl3, 75 MHz): = 138.4, 128.4, 127.9, 126.9, 33.9 (q, C D2Br, 5JC,D = 18.9 Hz); MS (LRMS/EI): C8H3D4Br3= 347, found 347. Synthesis of 2-bromo-m-xylene-d6 (5-D). In a flame-dried flask, 2-bromo, ’dibromom -xylened4 (22.5g, 0.065 mol) in THF was added dropwise to a slurry of lithium aluminum deuteride (2.73, 0.065 mol) in dry THF at room temperature. Then, the reaction was quenched and worked up following the standard procedure,230 to yield 11.2g (0.058 mol, 90%) of a colorless liquid. 1H NMR (CDCl3, 300 MHz): = 7.05 (m, 3 Hz ); 13C NMR (CDCl3, 75 MHz): = 138.4, 128.3, 127.8, 126.7, 23.2 (q, C D2Br, 5JC,D = 18.8 Hz); MS (LRMS/EI): C8H3D6Br= 191, found 191. Synthesis of 2,6-dimethyl-d6 aniline (5-E). In a flame-dried three-neck round botton flask equipped with a conde nser, 11.2g (0.058 mol) of 2-bromom -xylened6 (5-D) was added dropwise to a mixture of 1.71g (0.070 mol, 1.2 eq.) of magnesium in 50 ml of anhydrous ether or a minimum amount. Th e solution refluxed upon addition and was subsequently maintained at reflux for 2 hours. After cooling down to room temperature, a 1.2M solution of TMSMA in ether was dripped into the reaction mixture, which was then stirred at room temperature for an additional 3 hours. Water was added to quench the reaction and the etheral phase was extracte d, washed with brine and dried over MgSO4.

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127 The crude product was purified through column chromatography in CH2Cl2 to afford 4.09g (0.032 mol, 55%) of an orange-yellow liquid. 1H NMR (CDCl3, 300 MHz): = 7.45 (d, 2H, meta C H 2JH,H = 7.8 Hz ), 7.19 (t, 1H, para C H 3JH,H = 7.5 Hz), 4.46 (s, 2H, N H2); 13C NMR (CDCl3, 75 MHz): =142.8, 128.4, 121.7, 118.2, 17.4 (q, C D3, 5JC,D = 18.8 Hz) ; MS (LRMS/EI): C8H5D6N= 127, found 127. Synthesis of glyoxal-bis-(2,6-dimethyl-d6 phenyl)imine. The procedure was modified from the literature.67 Under argon, a mixture of 2.12g (0.014 mol) of a 40% aqueous solution of glyoxal, 2.3 ml of n-propa nol and 5.8 ml of water was combined with 4.09g (0.032 mol, 2.2 eq) of 2,6-dimethyld6 aniline (5-E) in 20 ml of n-propanol. The solution was stirred at room temperature for 4 hours and then at 60oC for 2 hours. Upon addition of 30 ml of water, a yellow solid pr ecipitated. The mixture was chilled in ice to allow further precipitation and 3.09g (0.011 mol, 80%) was colle cted by filtration (mp=145-146oC). 1H NMR (CDCl3, 300 MHz): = 8.13 (s, 2H, C H =N), 7.12 (d, 4H, meta C H 2JH,H = 7.4 Hz ), 6.98 (t, 2H, para C H 3JH,H = 7.7 Hz); 13C NMR (CDCl3, 75 MHz): =163.7, 150.1, 128.5, 126.5, 125.0, 17.4 (q, C D3, 5JC,D = 18.9 Hz); (HRMS/EI) calcd for C18H8D12N2 [M+Na]+ 299.2272, found 299.2282. Synthesis of N,N’-bis-(2,6-dimethyl-d6 phenylamino)ethane dihydrochloride. A suspension of 3.09g (0.011 mol) of the glyoxal-bis-(2,6-dimethyld6 phenyl)imine in 45 ml of 2:1 THF:methanol was treated with 1.70 g (0.044 mol, 4.0 eq) of sodium borohydride at room temperature. The mi xture was stirred for 16 hours at room temperature and refluxed for one hour. Upon c ooling, 45 ml of iced-water were added followed by 45 ml of 3M hydrochloric acid. A white solid precipitated, filtered and washed with diethyl ether and acetone Yield : 3.35 g (9.9 mmol, 85%); mp>250oC. 1H

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128 NMR (DMSOd6, 300 MHz): = 7.12 (s, 6H, para-meta C H ), 3.60 (s, 4H, NC H2); 13C NMR (DMSOd6, 75 MHz): =135.5, 131.7, 129.9, 127.9, 17.8 (q, C D3, 5JC,D = 18.8 Hz) ; MS (LRMS/EI): C18H14D6N2Cl2=352, found 352. Synthesis of 1,3-bis-(2,6-dimethyl-d6 phenyl)imidazolinium chloride (5-F). A mixture of 3.35 g (9.9 mmol) of N,N’-bis-(2,6-dimethyld6 phenylamino)ethane dihydrochloride, 40 ml of triethyl orthofor mate, and two drops of 96% formic acid was heated in a distillation apparatus until th e distillation of ethanol ceased. Once the temperature raised towards 130oC, the heating was stopped an d the reaction was cooled to room temperature. At th is point, a white solid precip itated which was isolated by vacuum filtration. Recrys tallization from 2:1 CH3CN:ethanol afforded 1.67 g (5.7 mmol, 58%) of white crystals. 1H NMR (CD3CN, 300 MHz): = 9.38 (s, 1H, NC H N), 7.32 (t, 2H, para C H 3JH,H = 7.7 Hz), 7.24 (d, 4H, meta C H 2JH,H = 7.4 Hz), 4.44 (s, 4H, NC H2); 13C NMR (CD3CN, 75 MHz): =161.3, 137.0, 131.3, 130.1, 118.4, 52.1, 17.4 (q, C D3, 5JC,D = 19.0 Hz) ; MS (HRMS/EI) calcd for C19H11D6N2Cl [M]+ 291.2609, found 291.2621. Synthesis of 1,3-bis-(2,6-dimethyl-d6 phenyl)-2-(trichloromethyl)imidazolidine (5-G). To a solution of chlorine salt 5-F (1.67 g, 5.7 mmol) in dry chloroform was added 0.230 g (5.7 mmol) of a 60% dispersion of sodi um hydride. The reaction was stirred at room temperature for 2 hours and subsequently filtered. The filtrate was evaporated into a yellowish solid, which was flushed through a silica plug with 9:1 hexane:ethyl acetate. The ligand was further purified by recrystal lization in boiling hexane, affording 3.28 g (4.7 mmol, 82%) of a white solid (mp=118-120oC). 1H NMR (CDCl3, 300 MHz): = 7.03 (m, 6H, para-meta C H ), 5.66 (s, 1H, C H CCl3), 3.98 (m, 2H, NC H2), 3.37 (m, 2H,

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129 NC H2); 13C NMR (CDCl3, 75 MHz): = 144.1, 138.4, 134.2, 129.5, 129.3, 125.6, 107.9, 86.1, 51.8, 19.0 (q, C D3, 5JC,D = 19.0 Hz) ; MS (HRMS/EI) calcd for C19H10D12N2 [M+H]+ 291.2609, found 291.2622. Synthesis of complex [Ru]23. The procedure was adapted from the literature.72 In the drybox, a flame-dried Schlenk flask was ch arged with 0.879 g (1.07 mmol) of catalyst [Ru]1 and 1.640 g (2.35 mmol, 2.2 eq) of ligand 5-G. The mixture was dissolved in 25 ml of toluene and stirred at 60oC for 90 minutes under argon. The solvent was then evaporated, and the brown residue was washed with methanol (2 x 25 mL) and pentane (50 mL). Finally, the pink solid was dr ied under vacuum to yield 0.693 g (0.83 mmol, 78%) of complex [Ru]23. 1H NMR (toluened8, 300 MHz): = 19.57 (s, 1H, Ru=C H ), 7.17 (t, 2H, para C H 3JH,H = 7.2 Hz), 7.13-6.94 (m, 9H, para-meta C H ), 6.54 (t, 1H, 3JH,H = 7.3 Hz ), 3.43-3.15 (m, 4H, NC H2C H2N), 2.41 (q, 3H), 1.55 (m, 15H), 1.08 (m, 15H); 13C NMR (toluened8, 75 MHz): =147.8, 129.5, 128.9, 128.6, 127.9, 125.8, 125.1, 32.4, 32.2, 30.3, 28.6, 28.5, 27.0, 20.8 (q, C D3, 5JC,D = 19.7 Hz); 31P NMR (toluened8, 300 MHz): =29.69 ; MS (HRMS/EI) calcd for C44H48D12N2Cl2PRu [M]+ 831.3656, found 831.3648. Anal. Calcd for C44H48D12N2Cl2PRu: C, 64.38; H, 7.49; N, 3.41. Found: C, 64.80; H, 7.39; N, 3.31. Thermal Decomposition of Complex [Ru]23 In the drybox, 32 mg (0.038 mmol) of complex [Ru]23 was dissolved with 0.8 mL of C6D6 in a glass ampoule, which was then equi pped with a swedgelock connected to a Teflon Schlenk valve. The system was ta ken out of the glovebox, and the ampoule was sealed under argon. The ampoule was left in a 70oC bath for 21 days and shaken

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130 regularly. It was then taken back into the drybox and transferred in to a J. Young Valve NMR Tube. Thermal Decomposition of Methylidene Complex [Ru]23’ In the drybox, a Schlenk tube was charge d with 200 mg (0.038 mmol) of complex [Ru]23 dissolved in 10 mL of degassed ben zene. The tube was taken out of the glovebox, connected to argon and immersed in a 60oC oil bath. Ethylene gas was bubbled rapidly through the solution for 30 minut es, resulting in a color change from pink to dark brown. The mixture was sa mpled to confirm full conversion of the benzylidene by 1H NMR. The mixture was then left stirring at 60oC for 3 days, after which the solvent was evaporated under vacuo. The flask was taken back into the drybox a nd transferred into a J. Young Valve NMR Tube. Isomerization Experiment Once no more carbene signal was visible by 1H NMR spectroscopy ( 19.63 and 18.40 ppm for [Ru]23 and [Ru]23’, respectively), the catalytic mixture was considered fully decomposed. The J. Young Valve NMR Tube was fitted onto a vacuum hook-up and the solvent was evaporated. The tube wa s then taken into the drybox, and 0.5 g of 1octene were added to the decomposed mixtur e. The tube was a llowed to stand at 55oC for 24h, and the isomers content was determined by 1H NMR by integration of the internal olefin peak at 5.40 ppm.

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131 CHAPTER 6 UNDERSTANDING THE EFFECT OF ALLYLIC METHYLS IN OLEFIN CROSSMETATHESIS Reproduced in part with permission from: Courchay, F. C.; Baughman, T.W.; Wagener, K. B. Journal of Organometallic Chemistry 2005 accepted. Copyright 2005 Elsevier Introduction Olefin metathesis has been applied to a variety of organic synthetic challenges by allowing the simple formation of carboncarbon double bonds in a single step where other routes would sometimes require several tedious steps.1,2 The development of welldefined metathesis carben e catalysts, such as Schrock’s molybdenum catalyst [Mo]1,27,28,231 and the more recent Gr ubbs’ ruthenium catalysts [Ru]137,57,80,87 and [Ru]4,69 has considerably widened the scope of olefin metathesis in both organic and polymer chemistry (Figure 6-1).7-11,19-20,31 This success has inspired many catalyst modifications to introduce heterogeneous catalysts,41-45,102-107 water-soluble catalysts,3840,105,232,233 recyclable catalysts,56,95,186 slower and faster initiators,75,99,194 so as to accommodate any particular set of conditions. Ru Cl Ph PCy3Cl PCy3Ru Cl Ph PCy3ClN N[Mo]1 [Ru]1 [Ru]4Mo N Ph O O F3C CF3F3C CF3 Figure 6-1. Olefin metathesis catalysts

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132 Despite the broad range of a pplications of these catalysts, challenges remain that must be addressed. On one hand, Schroc k’s highly active carbene complexes are oxophilic which renders them sensitive to air and moisture, and thus inappropriate when used with certain functionalities such as aldehydes and alcohols.12 On the other hand, ruthenium complexes, which possess a much higher functional group tolerance, are less active towards electron-poor or sterically demanding olefins.12,29 Although the 2nd generation catalyst [Ru]4 displays activities comparab le to early transition metal complexes,14,69,77 it also promotes double bond isomeri zation at elevated temperatures, competetively with metathesis, which can be problematic for the synthesis of precise polymer microstructures.166,176,177,192 Part of our group research effort has recently focused on building a family of olefin/ethylene copolymers to better understand the structur e-properties relationship of widely commercialized polyethylene materials.21,22 Using acyclic diene metathesis (ADMET), ethylene-propylene copolymers [EP( n+1)] with exact ethylene run length of n= 4, 6, 8, 14, 18 and 20 carbons were synthesi zed, the methyl branch content being determined during the monomer design.127,128,132 Although much progress has been made, the EP3 diene monomer (6-9 in Figure 6-2) refused to polymerize since allylic methyl groups seem to pose problem. Consequently, the influence of allylic methyls in condens ation metathesis chemistry was investigated using standard catalysts [Mo]1, [Ru]1, and [Ru]4, in both cross-metathesis (CM) and ADMET conditions Herein, is the report of an NMR spectroscopy study highlighting mechanistic f eatures of both molybdenum and ruthenium

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133 metathesis catalysis in this reaction, and th e importance of steric arrangement in the metallacyclobutane intermediate. Results ADMET Monomer 6-9, the target monomer posse ssing allylic groups, was synthesized starting with 3-methyl-4-pentenoi c acid which was reduced and tosylated to yield diester 6-4 by addition to diethyl malonate (Figure 6-2). Saponification and decarboxylation produced acid 6-6, which then was reduced and tosylated. Substitution with LAH afforded monomer 6-9 in 29% overall yield. CH3CH3OOH OH CH3OH CH3O CH3CH3OTs CH3CH3CH3CH3CH3OH OTs CH3LAH Et2O pyridine TsCl CH3CH3O OH O HO CH3CH3O OEt O EtO LAH Et2O LAH Et2O EtOH, H2O NaOH pyridine TsCl 185oC DecalinTMNaH, THF O OEt O EtO6-9 6-8 6-7 6-6 6-5 6-4 6-3 6-2 6-1 Figure 6-2. Synthesis of monomer 6-9 Efforts to polymerize 6-9, 3,6,9-trimethylundeca-1,10-diene, to create unsaturated polyethylene containing met hyl branches on every third carbon led to little if any conversion (Figure 6-3). Color changes during the reaction suggested catalyst decomposition very early in the reaction; ol efin conversion was calculated at 4% for catalyst [Mo]1 and 8% for catalyst [Ru]1 at best. These results warranted a detailed NMR study with catalysts [Mo]1, [Ru]1, and [Ru]4, probing for key intermediates

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134 during the catalytic cycle. To facilitate interpretation and broaden the scope of our study, the polymerization of 6-9 was examined more carefully al ong with the cross metathesis reaction of 3-methyl-1-pentene research which had been pr eviously reported by Grubbs with catalyst [Ru]1 .80 Figure 6-3. ADMET reaction of 6-9 Schrocks Molybdenum Catalyst The experiments described below demonstr ate that the dominant reaction pathway when using catalyst [Mo]1 leads to an accumulation of the nonproductive metallacyclobutane intermedate, rather than full metathesis conversion. The experiments were set up using conditions that would allo w monitoring dynamic catalytic behavior by NMR spectroscopy. For example, when exam ining the cross reaction of 3-methyl-1pentene (the control reaction for ADMET chemistry of monomer 6-9 ), a typical experiment consisted of loading an NMR t ube with a 0.2 M solution of olefin in C6D6 and 30 mol% of complex [Mo]1 (Typical catalyst loading for CM reactions are in the order of 0.1 to 1 mol% for [Mo]1 2 to 10 mol% for [Ru]1 and 1 to 5 mol% for [Ru]4 ). 30 mol% [Mo]1+ + 6A 6-B 6-C 25oC Figure 6-4. Cross metathesis of 3-methyl-1-pentene catalyzed by [Mo]1

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135 After 30 minutes at room temperature, the proton NMR spectrum indicates the presence of the expected metathes is product 3,6-dimethyl-oct-4-ene (6-A), and products of the catalyst initiation ( 1,1-dimethyl-allyl)-benzene (6-B), and (1,1,4-trimethyl-hex-2enyl)-benzene (6-C) (Figure 6-4). However, product 6-A only formed in 46% yield, which is insufficient for ADMET chemistry, again illlustrating the difficulty associated with the homodimerization of this class of olefins. Products 6-B and 6-C, formed by reaction of one molecule of substrate w ith the original molybdenum catalyst (and therefore present in catalytic quantities), nor mally are not observed. They are visible here because of the use of hi gh catalyst concentrations. [Mo] [Mo] [Mo] [Mo] [Mo] [Mo] [Mo] H H [Mo] [Mo] H H [Mo] + + 2,3 2,4 2,4 2,3 2,3 + + 2,4 productive metathesis non-productive metathesis non-productive metathesis + step 1 step 2 step 3 step 4 step 1' step 2' step 5 step 6 Ph Ph Ph Phs t e p 3 '6-A 6-C 6-B Ph step 7 step 8 [Mo] =Mo N O O F3C CF3F3C CF3 Figure 6-5. Catalytic cycle for the metathesis of 3-methyl-1-pentene with a molybdenum carbene catalyst

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136 Figure 6-5 depicts the catalytic cycle invol ved in each products presented in Figure 6-4. For clarity, note that single arrows are drawn instead of equilibrium arrows, but according to the metathesis mechanism it is impl ied that each step actu ally exists as an equilibrium. Further, it should be note d that steps 3’, 7 and 8 do not represent equilibrium conditions, but rather th e non-productive metathesis cycle. As the reaction starts, the first feat ure observed is the major production of CH2=C(CH3)2Ph (6-B) compared to the alternate product 6-C, which indicates that formation of the 2,4-metallacyclobutane is largely favored when 3-methyl-1-pentene coordinates to the original Schrock’s alkylidene (Step 1 in Figure 6-5). The two doublets at 13.18 and 13.23 ppm (2JH,H= 5.6 Hz) confirm the presen ce of the new alkylidene Mo=CHCH(CH3)Et, each signal corresponding to one enantiomer of the racemic mixture (Figure 6-6a).231 Two metallacyclobutanes can form from this intermediate, the 2,3addition leading to productive metathesis (s tep 3), and the 2,4-addition leading to nonproductive metathesis (step 3’). After 10 minutes, two broad singlets appear in equal ratios at –0.17 and –1.06 ppm, characteristic of the two protons of a trigonalbipyramidal metallacyclobutane ring, ther efore implying the fo rmation of the 2,4metallacyclobutane, further confirmed by the two protons singlets at 5.10 and 4.84 ppm.231 The 2,3-metallacycle would have given ri se to a single signal in the upper field region. The fact that it is unde tectable by NMR may be simply a result of its instability, forcing the metallacycle intermediate to deco mpose quickly into the methylidene and the metathesis product (step 4), as proven by the internal olefin resonance at 5.32 ppm. Because the methylidene complex is highl y unstable, its signals cannot be assign unambiguously. Moreover, the few methyliden e complexes that were ever observeable

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137 by NMR were adducts stabilized by a protic so lvent such as dme or THF, which is not possible here.28,234 The most common decomposition routes for high oxidation state (“d0”) alkylidene complexes in the pres ence of olefins are rearrangement of metallacyclobutanes complexes by -hydride mechanism and bimolecular coupling, methylidenes being the most suscep tible to bimolecular decomposition.234 However, the broad singlet at 12.36 ppm, assigned to the N H proton of an amido alkylidyne complex, indicates yet another substant ial decomposition pathway. Th is kind of proton transfer reaction from carbon to nitrog en was observed by Schrock et al.235 during the preparation of Mo(NArCl)(CHCMe2Ph)[Biphen]. Figure 6-6. 1H NMR alkylidene region for the reaction of 3-methyl-1-pentene. (a) With catalyst [Mo]1; (b) with [Ru]1; (c) with [Ru]4.

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138 In any case, the metathesis reaction only reaches 46% conversion after 30 minutes, while the 2,4-metallacyclobutane already constitutes 50% of th e catalyst mixture, which highlights the significant extent of non-produc tive metathesis. The product distribution is similar when 3-methyl-1-pentene is substituted with monomer 6-9, although ADMET conversions are slightly lower probably becau se of the substrate’s longer carbon chain. Grubbs’ Ruthenium Catalysts The NMR experiments with 3-methyl-1 -pentene (CM conditions) and monomer 69 (ADMET conditions) were repeat ed using ruthenium catalysts [Ru]1 and [Ru]4. Unlike what is observed in the case of early transition metals, the ruthenium metallacyclobutane is highly unstable, and it is still debated whether it is an intermediate or a transition state in the metathesis catalytic cycle.61,81,82 As had been done before, an NMR tube was loaded with a 0.2 M so lution of 3-methyl-1-pentene in C6D6 and 30 mol% of complex [Ru]1. 30 mol% [Ru]1 or [Ru]4+ + 6A 6-E 6-D45oC Figure 6-7. CM of 3-met hyl-1-pentene catalyzed by [Ru]1 or [Ru]4 After 30 minutes at 45oC, the proton NMR spectrum indicates the presence of the metathesis product 3,6-dimethyl-oct-4-ene (6-A) in low yields and products of the catalyst initiation: styrene (6-D), and (3-methyl-pen t-1-enyl)-benzene (6-E) that are formed by reaction of one molecule of substr ate with the ruthenium benzylidene (Figure 6-7). Product 6-A is formed in 5% and 30% yields with catalyst [Ru]1 and [Ru]4, respectively, again too low to be useful in ADMET step-growth polymerization. As before, products 6-D and 6-E are only visible because of the high catalyst concentrations.

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139 Figure 6-8 depicts the catalytic cycle involved in each product formation. Downfield in the NMR spectrum, there is a steady disappear ance of ruthenium benzylidene while the methylidene resonance at 19.40 ppm appears almost at a comparable rate. At the end of th e experiment, three times more Ru=CH2 (15%) is present than the expected Ru=CHCH(CH3)Et (5% of the catalyst mixture), evidenced by a doublet at 19.28 ppm (Figure 6-6b). [Ru] Ph PCy3[Ru] Ph [Ru] Ph [Ru] Ph [Ru] [Ru] [Ru] [Ru] H H [Ru] [Ru] H H [Ru] Ph Ph + + 2,3 2,4 2,4 2,3 2,3 + + 2,4 productive metathesis non-productive metathesis non-productive metathesis + step 1 step 2 step 3 step 4 step 1' step 2' step 5 step 6-PCy36-A 6-E 6-D step 7 step 8s t e p 3 '[Ru] = LRuCl2L = PCy3 or 1,3-dimesityl4,5-dihydroimidazol-2-ylidene Figure 6-8. Catalytic cycle for the metathes is of 3-methyl-1-pentene with ruthenium carbene catalyst [Ru]1 or [Ru]4 As observed with the molybdenum catalyst, styrene is produced in much larger quantities than product 6-E, which indicates the preferred formation of the 2,4metallacyclobutane from the ruthenium benz ylidene (step 1 and 1’ in Figure 6-8). Theoretically, the amount of styrene rele ased corresponds to the amount of catalyst

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140 activated for productive metathesis, Ru=CHCH(CH3)Et. Since this alkylidene is not detected by NMR, it must react quickly with a substrate molecule through the intermediate of a 2,3-meta llacyclobutane to afford the metathesis product 6-A and the methylidene (step 3 and 4). This analysis is further supported by the fact that the amount of 6-A equals the amount of styrene generated. At this point, the methylidene continues to increase in concentration, while the metath esis reaction seems to cease, indicating that upon coordination of a new olefin molecu le, the 2,3-metallacyclobutane is formed preferentially to regenerate the methylidene through step 7. Step 1’ and 2’ could also generate the methylidene, but this would involve a build-up of product 6-E, which we do not observe. Further, the satu ration of the reaction vessel wi th ethylene cannot be held entirely responsible for the methylidene build-u p either, since little ethylene is present in solution compared to the excess of starting material. In any case, after 30 minutes, only 5% of starting material has reacted even though more than 20% of the benzylidene precatalys t has been consumed, mostly forming the highly unstable rutheniu m methylidene. Apparently, decomposition of the catalyst system occurs at a faster rate than the metathesis reaction. Hence, complex [Ru]1 can not be regarded as a suitable catalyst for the homodimerization of 3-methyl-1-pentene, and probably not for any other olefin containing allylic methyls.80 Indeed, when the reaction was repeated with the corresponding ADMET diene, similar patterns we re observed; in fact, conversions were even lower. Metathesis products 6-A and 6-E were undetectable, and half less styrene was produced compared to the CM experiment, implying that less metallacyclobutane had formed. After 30 minutes, the methylidene constitutes 13% of the catalyst mixture

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141 and seems to be the only species besides the or iginal benzylidene. These data indicate that with the ADMET monomer the rate of formation of any metallacyclobutane is slower, more likely becaus e of the larger olefin. Catalyst [Ru]4 was subjected to scrutiny under the same conditions. At room temperature, approximately th e same amounts of styrene and 6-E are detected at room temperature, while at 45oC styrene seems to be in slight excess. Phosphine dissociation is so slow at low temperatures that an equilibrium between the 2,3 and 2,4metallacyclobutane is created. When the di ssociation event becomes faster, a greater amount of active catalyst is ge nerated and the rate differen ce between formation of the 2,3or 2,4-metallacyclobutane becomes ev ident because of the higher catalyst concentration. After 30 minutes the methylidene represen ts about 30% of the catalyst mixture whereas Ru=CHCH(CH3)Et is only present in 1%, appearing as a doublet at 17.48 ppm (Figure 6-6c). A quadruple t at 19.0 ppm also indicates the presence of Ru=CHCH3 formed from the metathesis of isomerized starting material. Indeed ruthenium catalysts, particularly those containing N-heterocycl ic carbene ligands (NHC), can isomerize olefins by migration of th e double bond along the backbone.176,177,192 The appearance of a multiplet at 5.19 ppm confirms the presence of 3-methyl-2-pentene by structural isomerization of the starting material. Traces of 2-methyl-1-butene also become visible at 4.67 ppm, formed by reaction of 3-methyl-2-p entene with the ruthenium methylidene. Nevertheless, the expected metathesis product 6-A, 3,6-dimethyl-oct-4-ene, constitutes only 30% of the olefinic mixture, while 50% of starting material still remains. This level of conversion obviates any ADMET chemistry. The reaction does proceed, albeit only at

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142 slow rates. Noteworthy, 34% of the original precatalyst [Ru]4 has already been consumed at this point, which is equivale nt to a 10 mol% catal yst loadings, already surpassing the typical catalytic quantities. Since Ru=CHCH(CH3)Et is not detected in significant amounts, we can still assume that the 2,3-metallacyclobutane is pref erred during step 3. However, the amount of metathesis product formed far exceeds the amount of styrene detected, which implies that productive metathesis does occur, unlike what was observed with catalyst [Ru]1. This observation is consistent with the higher reactivity of NHC -containing complexes towards olefinic substrates, attributed to the stabilizing effect of the NHC ligand on the metallacyclobutane.81 The ligand sphere may also allo w a better arrangeme nt of the alkyl bulk during the formation of the 2,4-metallacyclobutane. As seen with 1st generation complex [Ru]1, the reaction profile of catalyst [Ru]4 with the ADMET monomer 6-9 is identical to the CM r eaction, but affords much lower conversions. Under the same conditions as CM chemistry, 70% of starting material remains even though more catalyst has been activated (46%). The amount of methylidene formed is also higher than dur ing the CM reaction, wh ich suggests that nonproductive metathesis occurs to a larger ex tent through the preferred formation of a 2,3metallacyclobutane. This favored conformation is likely a direct result of the additional sterics brought about by the larger diene monomer. In addition, four hydride complexes have formed during the course of the reac tion indicating the start of the catalytic decomposition process, accelerated by the accumulation of the unstable methylidene complex.

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143 Discussion. Although the conditions used in this st udy do not reflect exact polymerization conditions (where the metathesis equilibrium is driven by removal of ethylene) these experiments reflect a general cat alytic behavior, since they de monstrate a steric conflict rather than a rate problem. In the case of the Schrock’s molybdenum cat alyst, 3-methyl-1-pentene (or its diene analog, monomer 6-9) seem to consistently coordinate to the metal in order to minimize steric interactions between the alkyl branches, hence form ing the 2,4-metallacyclobutane, which, during step 3’ (Figure 6-5), only l eads to non-productive metathesis. Moreover, this somewhat stable intermediate traps the catalyst in an inactive form. Even though productive metathesis occurs to some ex tent, it is insufficient to promote ADMET polymerization, a step-growth process that re quires high conversions (>99%) in order to reach high molecular weigh polymer. A different mechanism operates with ruthen ium catalysts to prevent the metathesis of allylic methyl-containing substrates. Th e ruthenium methylidene accumulates when catalyst [Ru]1 is used with either 3-methyl-1 -pentene or the corresponding ADMET momomer, through the consecutive steps 1 to 4 in Figure 6-8. The original benzylidene first forms a 2,4-metallacyclobutane upon olefin coordination to create a new alkylidene complex (steps 1 and 2 in Figure 6-8), which then forms the 2,3-metallacyclobutane by reacting with another olefin (steps 3 and 4). However, since the alkylidene is the most reactive intermediate, it is impossible to a ssert the amount of n on-productive metathesis occurring through step 3’. From this point on, the methylidene complex only reacts with incoming olefins to promote non-productive meta thesis and regenerate itself (step 7), even though formation of the methy lidene is not kinetically favored.80

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144 It has been suggested that the steric eff ects orient the bulk away from the crowded ruthenium metal center, while the electronic ef fects favor alkyl substitutents adjacent to the metal.80 Consequently, when the olefin reac ts with the benzylidene complex, the benzene ring adjacent to ruthenium may be su fficiently electron-withdrawing to place the alkyl group next to the metal center, minimizi ng in the same time the steric repulsions between the alkyl branch and the phenyl group (step 1). On the other hand, when the methylidene complex is involved, no electroni c effect directs the coordination of the olefin, which positions its steric bulk away from the metal center to form the 2,3ruthenacyclobutane (step 7). The fact that the same reaction with linear olefins affords the inverse conformation80 reveals the directing effect of allylic methyls, more likely by steric interactions with the lig and sphere of the complex. These conclusions also apply to catalyst [Ru]4, although the steric directing effects seem to be diminished in the presence of the NHC ligand. This could be due to a different arrangement of the ligand sphere reducing steric interactions, or to the propensity of the NHC ligand to stabilize the ruthenacyclobutane. De spite better yields, the conditions required by ADM ET polycondensation could not be achieved. In addition, a significant amount of olefin isomerization interferes with the metathesis process and therefore renders this 2nd generation of catalysts unsuita ble for the modeling of precise molecules. Conclusion ADMET polymerization of dienes possessing a llylic methyl groups is not possible, principally due to interaction of this methyl group with the me tathesis catalyst. A series of NMR spectroscopy experiments conduc ted with both the diene monomer (6-9) and model compound 3-methyl-1-pentene, the corr esponding mono-olefin, demonstrated that

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145 the reaction limitations depend on catalyst se lection. With Schrock’s molybdenum catalyst [Mo]1, the reaction led to an accumulation of metallacyclobutane, trapping the catalyst into an inactive form. With Grubbs’ ruthenium catalysts [Ru]1 and [Ru]4, the substrate coordinates to the metal center on ly to yield non-productive metathesis, which results in a build-up of the methylidene complex, more prompt to decomposition. Although the NHC-containing complex [Ru]4 affords better yields than its phosphine analog, this experiment also illustra te the competitive nature of double-bond isomerization during olefin metathesis. These results are dictated by the steric arrangement of the substrate within the catalyst ligand sphere during the metallacyclobutane formation, the alkyl branch being adjacent to the metal center for the molybdenum [Mo]1, and opposite to it in the ca se of ruthenium catalysts [Ru]1 and [Ru]4. Experimental General 1H NMR (300 MHz) and 13C NMR (75 Hz) spectra were recorded on either a Mercury series or Vari an VXR-300 NMR superconducting spectrometer for small molecule structure determination. Chemical shifts were referenced to residual C6H6 (7.15 for 1H and 128.39 for 13C) or CHCl3 (7.27 for 1H and 77.23 for 13C), and the NMR solvents were distilled, degassed by three fr eeze-pump-thaw cycles, and stored in an argon-filled drybox prior to use. Thin la yer chromatography (TLC) was performed on EMD silica gel coated (250 m thickness) glass plates cut to custom sizes. Developed TLC plates were stained with iodine absorbed on silica to produce a visible signature. Reaction conversions and rela tive purity of crude produc ts were monitored by TLC chromatography and NMR. High-resolution mass spectral (HRMS) data were obtained

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146 on a Finnegan 4500 gas chromatograph/mass sp ectrometer using either the chemical ionization (CI) or electrospra y ionization (ESI) mode. Materials and Methods All materials were purchased from Aldrich chemical and used as received unless otherwise specified. Complex [Mo]1,231,27,28 and [Ru]469 were synthesized according to literature procedure. Complex [Ru]1 was a gift from Materia Inc., and was used as received. Catalysts were stored in an argon-filled drybox prior to use. Bulk Polymerization Experiment Monomer 6-9 (1.5 g, 77 mmol) was vacuum transferred from potassium mirror into a schlenk flask and taken into an argon-filled glove box. The catalyst (monomer:catalyst ratio is 1500:1 for [Mo]1, 450:1 for [Ru]1) was added to the monomer in a 50 ml roundbottom flask equipped with a magnetic stirba r, and allowed to react approximately 15 minutes before sealing the reactor with a sc hlenk adaptor and connecting to high vacuum line. Vacuum was applied intermittently for the first 2 hours, then polymerization was heated at 40oC and put under full vacuum (10-3 Torr) for 4 days. NMR samples were taken in d -chloroform directly from the reactor with no purification. Conversions were calculated by the ratio of integral values of the internal olefin ( 5.20 ppm, m, 2H) to the terminal olefin ( 4.95 ppm, m, 2H). NMR Catalyst Experiments NMR spectra were recorded on a Varian Inova spectrometer equipped with a 5 mm indirect detection probe operating at 500 MHz for 1H and at 125 MHz for 13C. The solvent was d6benzene and the temperature was 25C for catalyst [Mo]1 and 45oC for catalysts [Ru]1 and [Ru]4. Chemical shifts were referenced to residual C6H6 (7.15 for 1H and 128.39 for 13C). In the drybox, 0.036 mmol of cat alyst was introduced in an NMR

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147 tube equipped with a Teflon valve. Approximately 0.8 ml of C6D6 was added to the tube so as not to dissolve the cat alyst extensively, and 0.119 mmol of substrate (3-methyl-1pentene or monomer 6-9) was carefully layered on top of the mixture. The NMR tube was sealed and shaken right before being in troduced in the spectrome ter. Proton spectra were recorded every 5 minutes, and quantit ation was obtained by integration of the appropriate peaks against the solvent pea k, which served as internal standard. Synthesis of EP3 Monomer (6-2 to 6-9) 3-Methyl-4-pentene-1-ol (6-2). Under an argon atmosphere a solution of 3methyl-4-pentenoic acid (30.0 g, 263 mmol) in diethyl ether (100 mL) was added dropwise to a suspension of LAH (12.0 g, 342 mmol) in diethyl ether (350 mL) over 45 minutes at 0oC. When the addition was complete, the slurry was stirred cold for 15 minutes, then the reaction was warmed to r oom temperature for 2 hours. The reaction was then cooled to 0oC and quenched by addition of wa ter (200 mL) and concentrated HCl (~30 mL) until the aqueous layer was pH=3. The organic layer was collected, combined with a second ether wash ( 200 mL), and dried with brine and MgSO4. Filtration, followed by distillati on of the filtrate yielded a 21.5 g of colorless oil. 81.6 % yield. bp = 149oC, 760 mmHg. 1H NMR matched reported spectral data.236 13C NMR (300 MHz, CDCl3): (ppm) 20.61, 35.10, 39.48, 61.41, 113.32, 144.45. 3-Ethenyl-1-butanol tosylate (6-3). To a stirred solution of 6-2 (21.45 g, 214 mmol) in pyridine (100 mL) at 0oC was added tosyl chloride (53.06 g, 278 mmol). The suspension was stirred cold for 10 minutes, then the viscous slurry was warmed to room temperature for 2 hours. Addition of wa ter (200 mL) and diethyl ether (200 mL) produced a biphasic mixture. The organic phase was isolated and combined with a

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148 second diethyl ether wash. Washing twice with 1N HCl (200mL), followed by drying over MgSO4, and column chromatography (15% diet hyl ether in hexane) afforded 53.6 g of colorless oil. 98.3% yield. 1H NMR (300 MHz, CDCl3): (ppm) 0.96 (d, 3H), 1.64 (m, 2H), 2.24 (m, 1H), 2.46 (s, 3H), 4.04 (m 2H), 4.90 (m, 2H), 5.55 (m, 1H), 7.35 (d, 2H), 7.79 (d, 2H). 13C NMR (300 MHz, CDCl3): (ppm) 20.27, 21.84, 34.34, 35.40, 69.01, 114.31, 128.10, 130.00, 133.40, 142.70, 144.86. Diethyl 2,2-bis(3-methyl-4-pentenyl) malonate (6-4). A solution of diethyl malonate (15.7 g, 98 mmol) in THF (100 mL ) was added to a stirred solution of 6-3 (53.6 g, 210 mmol) and sodium hydride (5.6 g, 233 mmol) in THF (100mL) over 30 minutes at 0oC. The mixture was stirred cold for 1 hour then warmed to room temperature for 18 hours. The reaction was quenched by addition of water (250 mL), extracted with diethyl ether (300 mL), and dried with brine. Column chromatography (10% diethyl ether in hexane) afforded 18.9 g of co lorless oil. 59.5% yield. 1H NMR (300 MHz, CDCl3): (ppm) 0.95 (d, 6H), 1.14 (q, 4H), 1.24 (t, 6H), 1.85 (m, 4H), 2.08 (m, 2H), 4.17 (q, 4H), 4.96 (m, 4H), 5.64 (m, 2H). 13C NMR (300 MHz, CDCl3): (ppm) 14.33, 20.35, 29.96, 30.87, 38.10, 57.51, 61.16, 113.35, 144.11, 172.06. Elemental analysis calcd. for C19H32O4: 70.33 C, 9.94 H; found: 69.93 C, 9.54 H. 2,2-Bis(3-methyl-4-penteny l)malonic acid (6-5). Potassium hydroxide (26.3 g, 470 mmol) was added to a solution of 6-4 (18.9g, 58 mmol) in ethanol (100 mL) and water (20 mL) which was then brought to re flux for 3 hours. The reaction was cooled, quenched with water (150 mL) and conc. HC l (until pH=3), and washed twice with diethyl ether (200 mL). The ether phase was dried with brine and c oncentrated to 15.0 g of a white solid with no further pur ification necessary. 95.7% yield. 1H NMR (300

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149 MHz, CDCl3): (ppm) 1.01 (d, 6H), 1.25 (m, 4H), 1.83 (m, 4H), 2.11 (m, 2H), 4.98 (m, 4H), 5.65 (m, 2H), 11.18 (br, 2H). 13C NMR (300 MHz, CDCl3): (ppm) 20.40, 31.18, 32.09, 38.14, 57.85, 113.73, 143.75, 177.96. CI/HRMS: [M+H]+ cacld for C15H25O4: 269.1753, found: 269.1749; Elemental analysis calcd. for C15H25O4: 67.14 C, 9.01 H; found: 66.98 C, 8.89 H. 5-Methyl-2-(3-methyl-4-pentenyl )-6-heptenoic acid (6-6). Decalin™ (15 mL, 1:1 wt%) was added to 7 (14.5 g, 54 mmol) and heated to 185oC in a 250mL round bottom flask equipped with an air cooled condenser under nitrogen. Production of CO2 was monitored with a mineral oil bubbler, a nd the reaction was stirred vigorously until gas evolution ceased after about 30 minutes. Upon cooling, Decalin™ was removed via rotary evaporation affording crude acid. Column chromat ography (20% ethyl acetate in hexane) afforded 11.4 g colorless oil. 94.0% yield. 1H NMR (300 MHz, CDCl3): (ppm) 1.01 (d, 6H), 1.33 (m, 4H), 1.50 (m, 2H), 1.61 (m, 2H), 2.12 (m, 2H), 2.31 (m, 1H), 4.96 (m, 4H), 5.67 (m, 2H). 13C NMR (300 MHz, CDCl3): (ppm) 20.34, 20.51, 29.96, 30.06, 34.25, 34.36, 37.96, 38.08, 45.70, 45.86, 46.05, 113.13, 113.17, 144.38, 144.45, 183.01. CI/HRMS: [M+H]+ cacld for C14H25O2: 225.1855, found: 225.1845; Elemental analysis calcd. for C14H25O2: 74.95 C, 10.78 H; found: 74.89 C, 10.68 H. 5-Methyl-2-(3-methyl-4-pentenyl) hept-6-en-1-ol (6-7). Same procedure as for 6-2 using 6-6 as starting material. 8.6g of colorl ess oil was isolated with no further purification needed. 84% yield. 1H NMR (300 MHz, CDCl3): (ppm) 1.00 (d, 6H), 1.201.41 (br, 9H), 1.66 (br, 1H), 2.09 (m, 2H), 3.53 (d, 2H), 4.95 (m, 4H), 5.69 (m, 2H). 13C NMR (300 MHz, CDCl3): (ppm) 20.44, 20.47, 28.55, 28.58, 28.65, 33.97, 38.35, 40.94, 40.99, 65.74, 65.78, 112.78, 144.92, 144.94. CI/HRMS: [M+H]+ cacld for C14H27O:

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150 211.2062, found: 211.2062; Elemental analysis calcd. for C14H26O: 79.94 C, 12.46 H; found: 79.71 C, 12.31 H. 5-Methyl-2-(3-methyl-4-pentenyl )-6-heptenyl tosylate (6-8). Same procedure for 6-3 with 6-7 as starting material, 103% crude yiel d after concentration. No further purification performed. 1H NMR (300 MHz, CDCl3): (ppm) 0.93 (d, 6H), 1.15 (m, 4H), 1.22 (m, 4H), 1.56 (br, 1H), 1.99 (m, 2H), 2.45 (s, 3H), 3.90 (d, 2H), 4.91 (m, 4H), 5.60 (m, 2H), 7.34 (d, 2H), 7.78 (d, 2H). 13C NMR (300 MHz, CDCl3): (ppm) 20.31, 20.39, 21.79, 28.29, 28.36, 28.40, 33.38, 33.41, 33.49, 37.97, 38.08, 38.10, 72.81, 112.94, 112.97, 128.11, 129.97, 133.25, 144.81, 144.43, 144.46. CI/HRMS: [M+H]+ cacld for C21H33O3S: 365.2150, found: 365.2143; Elementa l analysis calcd. for C21H33O3S: 69.19 C, 8.85 H; found: 68.93 C, 8.58 H. 3,6,9-Trimethylundeca-1,10-diene (6-9). Same procedure as for 6-7 with 6-8 as starting material. Isolated 2.5 g of a colorle ss oil after column chro matography (hexane). 81.1% yield. 1H NMR (300 MHz, C6D6): (ppm) 0.87 (d, 3H), 0.98 (d, 6H), 1.12 (m, 2H), 1.30 (7H), 2.03 (m, 2H), 4.97 (m, 4H), 5.68 (2H). 13C NMR (300 MHz, C6D6): (ppm) 19.85, 19.92, 19.98, 20.40, 20.64, 33.34, 33.42, 33.54, 34.48, 34.89, 34.92, 34.99, 35.01, 38.54, 112.65, 112.75, 144.96, 145.07. CI/HRMS: [M+H]+ cacld for C14H26O: 194.2035, found: 194.2037; Elemental analysis calcd. for C14H26O: 86.52 C, 13.48 H; found: 86.32 C, 13.51 H.

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151 APPENDIX A CRYSTALLOGRAPHIC DATA Figure A-1. Crystal structure of [Ru]11, (H2IPr)(Cl)2Ru=CH(2 -i PrO)C6H4

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152 Table A-1. Crystal data a nd structure refinement for [Ru]11. Empirical formula C74.5 H101 Cl5 N4 O2 Ru2 Formula weight 1463.98 Temperature 193(2) K Wavelength 0.71073 Crystal system Triclinic Space group P-1 Unit cell dimensions a = 8.5827(6) = 70.824(2). b = 19.9621(13) = 83.083(2). c = 22.5391(15) = 80.542(2). Volume 3588.6(4) 3 Z 2 Density (calculated) 1.355 Mg/m 3 Absorption coefficient 0.654 mm -1 F(000) 1530 Crystal size 0.34 x 0.11 x 0.06 mm 3 Theta range for data collection 1.66 to 27.50. Index ranges -10 h 11, -19 k 25, -20 l 29 Reflections collected 22770 Independent reflections 15797 [R(int) = 0.0772] Completeness to theta = 27.50 95.8 % Absorption correction Integration Max. and min. transmission 0.9623 and 0.8615 Refinement method Full-matrix least-squares on F 2 Data / restraints / parameters 15797 / 0 / 813 Goodness-of-fit on F 2 0.896 Final R indices [I>2sigma(I)] R1 = 0.0648, wR2 = 0.0838 [7310] R indices (all data) R1 = 0.1709, wR2 = 0.1099 Largest diff. peak and hole 0.747 and -0.734 e. -3 R1 = (||F o | |F c ||) / |F o | wR2 = [ w(F o 2 F c 2 ) 2 ] / w F o 2 2 ]] 1/2 S = [ w(F o 2 F c 2 ) 2 ] / (n-p)] 1/2 w= 1/[ 2 (F o 2 )+(0.0370*p)2+0.31*p], p = [max(F o 2 ,0)+ 2* F c 2 ]/3

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153 Table A-2. Atomic coordinates ( x 10 4 ) and equivalent isotropic displacement parameters ( 2 x 10 3 ) for [Ru]11. U(eq) is defined as one third of the trace of the orthogonalized U ij tensor. x y z U(eq) Ru1 3379(1) 2495(1) 2676(1) 22(1) Cl1 1150(2) 1891(1) 2990(1) 35(1) Cl2 5205(2) 3213(1) 2727(1) 30(1) O1 1667(4) 3509(2) 2423(2) 25(1) N1 6260(5) 1429(2) 2522(2) 22(1) N2 5541(5) 1396(2) 3497(2) 23(1) C1 3408(6) 2708(3) 1824(2) 22(1) C2 2600(6) 3380(3) 1455(2) 22(1) C3 2695(6) 3637(3) 799(3) 28(1) C4 1911(7) 4288(3) 480(3) 36(2) C5 1011(7) 4704(3) 821(3) 37(2) C6 872(6) 4473(3) 1469(3) 30(2) C7 1670(6) 3817(3) 1784(3) 24(1) C8 585(7) 3841(3) 2843(3) 31(2) C9 1133(7) 3482(3) 3491(3) 43(2) C10 -1105(7) 3731(3) 2784(3) 48(2) C11 5113(7) 1689(3) 2892(3) 23(1) C12 7473(6) 904(3) 2894(2) 30(2) C13 7206(6) 1035(3) 3522(3) 30(2) C14 4560(6) 1477(3) 4044(3) 26(1) C15 4889(7) 1943(3) 4346(3) 29(2) C16 3843(9) 2031(3) 4846(3) 46(2) C17 2546(8) 1655(4) 5035(3) 53(2) C18 2313(7) 1168(3) 4762(3) 46(2) C19 3314(7) 1052(3) 4265(3) 31(2) C20 3102(7) 482(3) 3989(3) 35(2) C21 1430(7) 235(3) 4140(3) 49(2) C22 4346(7) -178(3) 4198(3) 49(2) C23 6391(7) 2306(3) 4186(3) 33(2) C24 7678(7) 1844(3) 4620(3) 47(2) C25 6090(8) 3064(3) 4235(3) 49(2) C26 6171(6) 1485(3) 1879(3) 21(1) C27 7011(6) 1979(3) 1410(3) 25(1) C28 6913(7) 2010(3) 793(3) 34(2) C29 6040(8) 1584(3) 643(3) 39(2) C30 5222(7) 1106(3) 1108(3) 40(2) C31 5286(6) 1038(3) 1741(3) 30(2) C32 4438(6) 482(3) 2246(3) 32(2) C33 2675(7) 561(3) 2153(3) 55(2) C34 5233(7) -267(3) 2279(3) 48(2)

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154 Table A-2. continued x y z U(eq) C35 7995(7) 2464(3) 1564(3) 35(2) C36 7468(8) 3253(3) 1233(3) 50(2) C37 9754(7) 2255(3) 1393(3) 58(2) Ru2 11116(1) -3492(1) 2438(1) 22(1) Cl3 9247(2) -4276(1) 2630(1) 30(1) Cl4 13167(2) -3009(1) 2680(1) 36(1) O2 12923(4) -4449(2) 2427(2) 25(1) N3 8535(5) -2705(2) 3010(2) 22(1) N4 8373(5) -2311(2) 1991(2) 23(1) C38 11547(6) -3321(3) 1597(3) 25(1) C39 12586(6) -3850(3) 1370(3) 24(1) C40 12919(7) -3789(3) 734(3) 35(2) C41 13976(7) -4301(3) 549(3) 36(2) C42 14681(7) -4883(3) 1002(3) 34(2) C43 14377(6) -4965(3) 1634(3) 30(2) C44 13337(6) -4441(3) 1819(3) 21(1) C45 13797(6) -4966(3) 2948(3) 30(2) C46 15442(6) -4781(3) 2914(3) 39(2) C47 12838(7) -4943(3) 3549(2) 36(2) C48 9300(6) -2739(3) 2452(3) 21(1) C49 6927(6) -1951(3) 2232(2) 29(2) C50 6896(6) -2345(3) 2931(3) 28(2) C51 9194(6) -2954(3) 3615(3) 24(1) C52 10273(6) -2569(3) 3732(2) 25(1) C53 10914(7) -2818(3) 4315(3) 35(2) C54 10495(7) -3425(3) 4770(3) 39(2) C55 9377(7) -3782(3) 4662(3) 37(2) C56 8698(6) -3564(3) 4082(3) 27(1) C57 10713(7) -1867(3) 3248(3) 31(2) C58 9412(7) -1233(3) 3239(3) 44(2) C59 12272(7) -1668(3) 3360(3) 44(2) C60 7396(7) -3955(3) 3993(3) 32(2) C61 5823(7) -3729(3) 4327(3) 47(2) C62 7824(7) -4780(3) 4252(3) 41(2) C63 8762(6) -2069(3) 1324(3) 22(1) C64 9603(6) -1475(3) 1073(3) 23(1) C65 9945(7) -1246(3) 422(3) 35(2) C66 9428(7) -1557(3) 37(3) 36(2) C67 8568(7) -2126(3) 299(3) 35(2) C68 8219(7) -2397(3) 947(3) 28(2) C69 10157(7) -1102(3) 1483(3) 33(2) C70 9545(8) -292(3) 1264(3) 48(2)

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155 C71 11975(7) -1227(3) 1487(3) 52(2) C72 7258(7) -3012(3) 1220(3) 33(2) C73 5547(7) -2765(3) 1032(3) 62(2) C74 7972(8) -3664(3) 1012(3) 50(2) Cl5 15585(3) -629(1) 483(1) 115(1) C75 13903(14) 21(7) 7(7) 60(5)

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156 Figure A-2. Crystal structure for [Ru]13, (H2IPr)(Cl)2Ru=CH(2 -i PrO)(5-NO2)C6H3

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157 Table A-3. Crystal data and st ructure refinement for [Ru]13. Empirical formula C37 H49 Cl2 N3 O3 Ru Formula weight 755.76 Temperature 173(2) K Wavelength 0.71073 Crystal system Monoclinic Space group I2/a Unit cell dimensions a = 27.1836(18) = 90. b = 12.0491(8) = 95.811(2). c = 22.2557(15) = 90. Volume 7252.1(8) 3 Z 8 Density (calculated) 1.384 Mg/m 3 Absorption coefficient 0.619 mm -1 F(000) 3152 Crystal size 0.28 x 0.16 x 0.08 mm 3 Theta range for data collection 1.51 to 27.49. Index ranges -35 h 35, -15 k 15, -28 l 28 Reflections collected 30787 Independent reflections 8204 [R(int) = 0.0477] Completeness to theta = 27.49 98.5 % Absorption correction Integration Max. and min. transmission 0.9539 and 0.8601 Refinement method Full-matrix least-squares on F 2 Data / restraints / parameters 8204 / 0 / 426 Goodness-of-fit on F 2 1.059 Final R indices [I>2sigma(I)] R1 = 0.0319, wR2 = 0.0723 [7198] R indices (all data) R1 = 0.0393, wR2 = 0.0757 Largest diff. peak and hole 0.338 and -0.543 e. -3 R1 = (||F o | |F c ||) / |F o | wR2 = [ w(F o 2 F c 2 ) 2 ] / w F o 2 2 ]] 1/2 S = [ w(F o 2 F c 2 ) 2 ] / (n-p)] 1/2 w= 1/[ 2 (F o 2 )+(m*p)2+n*p], p = [max(F o 2 ,0)+ 2* F c 2 ]/3, m & n are constants.

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158 Table A-4. Atomic coordinates ( x 10 4 ) and equivalent isotropic displacement parameters ( 2 x 10 3 ) for [Ru]13. x y z U(eq) Ru1 8745(1) 1535(1) 9232(1) 23(1) Cl2 9381(1) 2275(1) 9889(1) 35(1) Cl1 7979(1) 1476(1) 8657(1) 32(1) O1 8320(1) 1460(1) 10046(1) 30(1) O2 8354(1) -3683(2) 10321(1) 65(1) O3 7689(1) -3314(2) 10737(1) 52(1) N1 9031(1) 2623(1) 8169(1) 25(1) N2 9159(1) 851(1) 8057(1) 24(1) N3 8053(1) -3034(2) 10488(1) 42(1) C1 8802(1) 52(2) 9409(1) 26(1) C2 8502(1) -382(2) 9863(1) 26(1) C3 8445(1) -1511(2) 9971(1) 31(1) C4 8129(1) -1844(2) 10389(1) 32(1) C5 7878(1) -1092(2) 10715(1) 34(1) C6 7940(1) 34(2) 10625(1) 32(1) C7 8245(1) 381(2) 10197(1) 27(1) C8 8141(1) 2376(2) 10409(1) 33(1) C9 8162(1) 3419(2) 10032(1) 38(1) C10 8461(1) 2451(2) 11005(1) 44(1) C11 9037(1) 1628(2) 8455(1) 23(1) C12 9281(1) 1349(2) 7485(1) 30(1) C13 9062(1) 2500(2) 7514(1) 33(1) C14 9323(1) -271(2) 8190(1) 26(1) C15 9811(1) -437(2) 8453(1) 29(1) C16 9964(1) -1532(2) 8571(1) 35(1) C17 9654(1) -2410(2) 8416(1) 40(1) C18 9181(1) -2232(2) 8142(1) 37(1) C19 9004(1) -1154(2) 8022(1) 30(1) C20 10168(1) 506(2) 8619(1) 34(1) C21 10633(1) 416(2) 8289(1) 52(1) C22 10311(1) 557(2) 9299(1) 50(1) C23 8478(1) -989(2) 7729(1) 34(1) C24 8412(1) -1494(3) 7093(1) 57(1) C25 8101(1) -1486(2) 8119(1) 49(1) C26 8978(1) 3677(2) 8469(1) 29(1) C27 9399(1) 4115(2) 8807(1) 37(1) C28 9337(1) 5080(2) 9137(1) 51(1) C29 8884(1) 5591(2) 9121(1) 58(1) C30 8484(1) 5198(2) 8757(1) 50(1) C31 8521(1) 4237(2) 8408(1) 37(1) C32 9910(1) 3599(2) 8801(1) 41(1)

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159 Table A-4. continued x y z U(eq) C33 10132(1) 3874(3) 8220(1) 65(1) C34 10282(1) 3909(3) 9334(1) 65(1) C35 8096(1) 3917(2) 7946(1) 39(1) C36 8131(1) 4624(2) 7375(1) 47(1) C37 7584(1) 4060(2) 8164(1) 57(1) U(eq) is defined as one third of the trace of the orthogonalized U ij tensor.

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160 Figure A-3. Crystal structure for [Ru]16, (H2IPr)(Cl)2Ru=CH(2 -i PrO)(5-OCH3)C6H3

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161 Table A-5. Crystal data a nd structure refinement for [Ru]16. Empirical formula C38 H52 Cl2 N2 O2 Ru Formula weight 740.79 Temperature 173(2) K Wavelength 0.71073 Crystal system Monoclinic Space group P2(1)/c Unit cell dimensions a = 11.5767(8) = 90. b = 12.1545(9) = 96.174(1). c = 26.1769(19) = 90. Volume 3662.0(5) 3 Z 4 Density (calculated) 1.344 Mg/m 3 Absorption coefficient 0.608 mm -1 F(000) 1552 Crystal size 0.14 x 0.08 x 0.06 mm 3 Theta range for data collection 1.77 to 27.50. Index ranges -14 h 15, -15 k 15, -16 l 34 Reflections collected 23595 Independent reflections 8270 [R(int) = 0.0436] Completeness to theta = 27.50 98.6 % Absorption correction Integration Max. and min. transmission 0.9470 and 0.8869 Refinement method Full-matrix least-squares on F 2 Data / restraints / parameters 8270 / 0 / 406 Goodness-of-fit on F 2 0.891 Final R indices [I>2sigma(I)] R1 = 0.0385, wR2 = 0.1146 [6677] R indices (all data) R1 = 0.0521, wR2 = 0.1228 Largest diff. peak and hole 1.835 and -0.648 e. -3 R1 = (||F o | |F c ||) / |F o | wR2 = [ w(F o 2 F c 2 ) 2 ] / w F o 2 2 ]] 1/2 S = [ w(F o 2 F c 2 ) 2 ] / (n-p)] 1/2 w= 1/[ 2 (F o 2 )+(m*p)2+n*p], p = [max(F o 2 ,0)+ 2* F c 2 ]/3, m & n are constants.

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162 Table A-6. Atomic coordinates ( x 10 4 ) and equivalent isotropic displacement parameters ( 2 x 10 3 ) for [Ru]16. x y z U(eq) Ru1 8479(1) 1373(1) 1272(1) 23(1) Cl1 7500(1) 1327(1) 445(1) 37(1) Cl2 9717(1) 770(1) 1978(1) 42(1) O1 10089(2) 1569(2) 870(1) 30(1) C11 6964(2) 1205(2) 1548(1) 23(1) C14 6381(2) 3082(2) 1821(1) 28(1) C5 11223(2) 4244(2) 462(1) 36(1) C12 5101(2) 1431(2) 1806(1) 34(1) C31 6936(3) -1366(2) 1012(1) 38(1) C6 11111(2) 3105(2) 504(1) 34(1) C23 5532(3) 3640(2) 912(1) 39(1) C27 7721(2) -1261(2) 1929(1) 37(1) C2 9573(2) 3361(2) 1046(1) 28(1) C19 6033(2) 3904(2) 1455(1) 34(1) C18 6208(3) 4995(3) 1610(1) 46(1) C7 10302(2) 2671(2) 796(1) 28(1) C26 7051(2) -827(2) 1490(1) 30(1) C1 8703(2) 2866(2) 1331(1) 28(1) C13 5203(2) 280(2) 1609(1) 36(1) C4 10515(2) 4942(2) 710(1) 34(1) C15 6872(2) 3328(2) 2317(1) 33(1) C16 7035(3) 4449(3) 2444(1) 46(1) C35 7649(3) -804(3) 2460(1) 43(1) C3 9696(3) 4496(2) 1003(1) 35(1) C32 6014(3) -1064(3) 589(1) 41(1) C30 7630(3) -2293(3) 967(2) 55(1) C28 8383(3) -2195(3) 1853(2) 52(1) C10 12009(3) 771(3) 1077(2) 60(1) C29 8371(3) -2655(3) 1379(2) 69(1) C8 10902(3) 744(2) 715(1) 38(1) C17 6706(3) 5259(3) 2093(1) 53(1) C9 10273(3) -338(3) 715(2) 56(1) C38 11440(3) 6547(3) 426(1) 49(1) C34 4925(3) -1774(3) 635(1) 51(1) C33 6387(4) -1204(3) 43(1) 60(1) C37 8715(4) -1049(4) 2844(2) 71(1) C22 6520(4) 2484(4) 3163(1) 65(1) C36 6578(4) -1249(4) 2683(2) 76(1) C21 8540(3) 2545(4) 2913(2) 71(1) N1 6176(2) 1947(2) 1677(1) 25(1) N2 6445(2) 201(2) 1550(1) 25(1)

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163 Table A-6. continued x y z U(eq) O2 10584(2) 6064(2) 705(1) 48(1) C24 6248(3) 4157(3) 512(1) 52(1) C25 4259(3) 4010(4) 811(2) 56(1) C20 7263(3) 2450(3) 2713(1) 40(1) U(eq) is defined as one third of the trace of the orthogonalized U ij tensor.

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164 Figure A-4. Crystal structure for [Ru]23, (H2IPr)PCy3(Cl)2Ru=CHPh

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165 Table A-7. Crystal data a nd structure refinement for [Ru]23 Empirical formula C44 H61 Cl2 N2 P Ru Formula weight 820.89 Temperature 173(2) K Wavelength 0.71073 Crystal system Triclinic Space group P-1 Unit cell dimensions a = 11.7056(18) = 87.768(3). b = 12.3160(19) = 70.834(2). c = 16.042(3) = 70.927(3). Volume 2058.5(6) 3 Z 2 Density (calculated) 1.324 Mg/m3 Absorption coefficient 0.582 mm-1 F(000) 864 Crystal size 0.20 x 0.11 x 0.06 mm3 Theta range for data collection 1.35 to 27.50. Index ranges -13 h 14, -12 k 15, -20 l 18 Reflections collected 13498 Independent reflections 8977 [R(int) = 0.0287] Completeness to theta = 27.50 95.1 % Absorption correction Integration Max. and min. transmission 0.9530 and 0.8215 Refinement method Full-matrix least-squares on F2 Data / restraints / parameters 8977 / 48 / 539 Goodness-of-fit on F2 1.084 Final R indices [I>2sigma(I)] R1 = 0.0377, wR2 = 0.0840 [7550] R indices (all data) R1 = 0.0496, wR2 = 0.0895 Largest diff. peak and hole 0.775 and -0.792 e.-3 R1 = (||F o | |F c ||) / |F o | wR2 = [ w(F o 2 F c 2 ) 2 ] / w F o 2 2 ]] 1/2 S = [ w(F o 2 F c 2 ) 2 ] / (n-p)] 1/2 w= 1/[ 2 (F o 2 )+(m*p)2+n*p], p = [max(F o 2 ,0)+ 2* F c 2 ]/3, m & n are constants.

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166 Table A-8. Atomic coordinates ( x 104) and equivalent is otropic displacement parameters (2x 103) for [Ru]23. x y z U(eq) Ru -89(1) 7168(1) 2493(1) 25(1) N1 1025(2) 4545(2) 1978(2) 47(1) N2 2500(2) 5347(2) 1581(2) 49(1) C1 1239(3) 5549(2) 2000(2) 34(1) C2 2289(8) 3536(7) 1691(9) 48(3) C3 3297(8) 4094(7) 1418(8) 44(2) C2' 2111(6) 3621(5) 1372(6) 51(2) C3' 3149(6) 4169(6) 1092(6) 52(2) C4 -110(3) 4270(2) 2425(2) 37(1) C5 -307(3) 3931(2) 3283(2) 45(1) C6 -1378(4) 3603(3) 3685(2) 59(1) C7 -2195(4) 3590(3) 3248(3) 65(1) C8 -1974(4) 3902(3) 2404(3) 61(1) C9 -920(3) 4241(2) 1955(2) 47(1) C10 610(4) 3921(3) 3761(3) 73(1) C11 -681(5) 4558(3) 1018(2) 77(1) C12 3196(3) 6137(2) 1417(2) 39(1) C13 3819(3) 6248(3) 2000(2) 52(1) C14 4550(3) 6992(4) 1783(3) 63(1) C15 4661(3) 7562(3) 1023(3) 58(1) C16 4081(3) 7399(3) 453(2) 47(1) C17 3356(3) 6670(3) 627(2) 40(1) C18 3771(4) 5571(4) 2806(3) 83(1) C19 2833(3) 6436(4) -60(2) 67(1) Cl1 603(1) 7049(1) 3733(1) 32(1) Cl2 -397(1) 7433(1) 1063(1) 34(1) P1 -1140(1) 9239(1) 2789(1) 24(1) C20 -1549(3) 6761(2) 2874(2) 28(1) C21 -2409(3) 6577(2) 3731(2) 30(1) C22 -3588(3) 6513(3) 3747(2) 38(1) C23 -4458(3) 6337(3) 4526(3) 47(1) C24 -4172(4) 6222(4) 5287(3) 56(1) C25 -3024(4) 6285(4) 5292(3) 58(1) C26 -2145(3) 6463(3) 4520(2) 43(1) C27 -1672(3) 9694(3) 3988(2) 36(1) C28 -2896(4) 9446(3) 4508(2) 44(1) C29 -3150(5) 9548(4) 5502(2) 65(1) C30 -3190(7) 10719(9) 5799(4) 65(2) C31 -2027(5) 11018(4) 5263(3) 58(1) C32 -1766(4) 10899(3) 4271(2) 51(1) C33 -2458(3) 9892(3) 2337(2) 29(1)

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167 Table A-8. continued x y z U(eq) C34 -3480(3) 9326(3) 2523(3) 37(1) C35 -4329(4) 9806(3) 1948(3) 43(1) C36 -4942(3) 11108(3) 2098(3) 50(1) C37 -3940(4) 11680(3) 1965(3) 51(1) C38 -3088(3) 11204(3) 2529(2) 40(1) C39 -25(3) 10039(2) 2251(2) 28(1) C40 284(3) 10109(3) 1245(2) 43(1) C41 1077(4) 10911(3) 913(3) 45(1) C42 2319(3) 10493(3) 1133(3) 44(1) C43 2033(4) 10360(4) 2118(3) 52(1) C44 1229(3) 9571(3) 2452(2) 40(1) Cl1' 334(6) 6815(6) 3982(4) 39(2) Cl2' -738(6) 7348(5) 1345(4) 33(2) P1' -2101(4) 8630(4) 3482(3) 23(1) C20' 882(18) 8175(16) 2390(13) 37(1) C21' 1129(17) 9065(15) 1838(11) 37(1) C22' 757(18) 9305(16) 1085(11) 37(1) C23' 978(17) 10209(15) 603(12) 37(1) C24' 1590(20) 10847(19) 829(14) 37(1) C25' 2040(20) 10593(18) 1524(14) 37(1) C26' 1750(20) 9749(16) 2075(12) 37(1) C27' -2124(17) 9518(16) 4403(14) 37(1) C28' -3457(15) 10324(16) 4980(12) 37(1) C29' -3380(40) 10800(40) 5813(18) 37(1) C30' -2419(17) 11456(17) 5549(13) 37(1) C31' -1121(17) 10718(16) 4987(11) 37(1) C32' -1136(17) 10151(17) 4178(11) 37(1) C33' -3112(18) 9671(15) 2918(12) 37(1) C34' -3840(20) 9294(18) 2441(18) 37(1) C35' -4736(18) 10340(20) 2151(13) 37(1) C36' -4000(17) 10999(16) 1544(12) 37(1) C37' -3225(19) 11387(15) 1989(13) 37(1) C38' -2430(19) 10446(19) 2396(13) 37(1) C39' -3223(18) 7860(14) 4096(10) 37(1) C40' -3070(20) 6805(16) 3544(12) 37(1) C41' -4090(20) 6260(18) 4001(14) 37(1) C42' -4020(20) 5873(17) 4894(13) 37(1) C43' -4085(18) 6868(16) 5453(11) 37(1) C44' -3079(17) 7432(16) 4986(10) 37(1) U(eq) is defined as one third of the trace of the orthogonalized Uij tensor.

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181 BIOGRAPHICAL SKETCH Florence Catherine Courchay was born on May 26th 1978 in Nice, France. After receiving her baccalaureat in 1996, she spent 2 years in preparatory classes at the International Center of Valbonne (France), where she majored in mathematics, physics and chemistry. In September 1998, she en tered ESCPE Lyon (N ational School of Chemistry and Electronics) and studied chemistr y and chemical engineering for 2 years. During this time, she worked as an undergradu ate research assistant in the Laboratory of Surface Organometallic Chemistry, on a suppor ted chromium catalyst for ethylene polymerization under the supervision of Mrs. Anne Baudoin and Prof. Jean-Marie Basset. Then, in July 2000 she relocated to Rheinmnster, in southwest Germany, to work for Dow Deutschland as an intern in the R&D department, Drytech division of superabsorbent polymers. There, she resear ched optimal conditions for the continuous process of superabsorbent polymer manufact ure, under the supervision of Dr. Herbert Gartner. Finally in August 2001, she arrived at the University of Florida, to start graduate school in chemistry. She immediat ely joined the group of Prof. Ken Wagener, where her research project focused on the design of ruthenium carbene catalyst for acyclic diene metathesis (ADMET) polym erization. In May 2003, she defended her master’s degree with ESCPE Lyon and obtai ned her “Engineer Diploma”. She is currently finishing her Ph.D. studies and plans to return to Europe to work for industry, upon completion of her degree in December 2005.


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Permanent Link: http://ufdc.ufl.edu/UFE0012985/00001

Material Information

Title: Metathesis and Isomerization Activity of Ruthenium Carbene Catalysts in Acyclic Diene Metathesis Polymerization
Physical Description: Mixed Material
Copyright Date: 2008

Record Information

Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
System ID: UFE0012985:00001

Permanent Link: http://ufdc.ufl.edu/UFE0012985/00001

Material Information

Title: Metathesis and Isomerization Activity of Ruthenium Carbene Catalysts in Acyclic Diene Metathesis Polymerization
Physical Description: Mixed Material
Copyright Date: 2008

Record Information

Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
System ID: UFE0012985:00001


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METATHESIS AND ISOMERIZATION ACTIVITY
OF RUTHENIUM CARBENE CATALYSTS
IN ACYCLIC DIENE METATHESIS POLYMERIZATION















By

FLORENCE C. COURCHAY


A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY

UNIVERSITY OF FLORIDA


2005

































Copyright 2005

by

Florence C. Courchay

































To my parents Elizabeth and Bernard, and grandparents Suzanne, Luce, Aime, and Yvon















ACKNOWLEDGMENTS

First and foremost, I would like to thank my parents Elizabeth and Bernard, and

grandparents, Suzanne, Luce and Yvon. I would have never come this far without them.

Their unconditional love and support have meant the world to me. They helped me

become the person I am today; there are no words to describe the respect, the love and the

gratitude I feel.

These past 4 plus years in graduate school were one of the most rewarding, life-

changing experiences I have ever been through, and none of it would have been possible

without the help and guidance of my advisor Professor Ken Wagener. Arriving in a

foreign country is a scary thing to take on, so I am most thankful for his support and

understanding, his words of wisdom, his genuine concern, and his exciting take on

science. I would also like to thank all the members of my supervisory committee (Dr.

John Reynolds, Dr. Mike Scott, Dr. Dan Talham, and Dr. Tony Brennan), for the time

they invested in reading and discussing this document. My work here would not have

been completed without the precious help of Dr. Ion Ghiviriga who spent many hours

assisting me with NMR spectroscopy. I also wish to thank Dr. Khalil Abboud for his

kind help with X-ray crystallography, and Dr. James Boncella for helpful discussions.

At the University of Florida, I thank all my coworkers on the polymer floor;

particularly all the Wagener's group members past and present. Among them I would

like to thank Dr. Ed Lehman for his expertise in catalysis, and James Leonard for his help

with X-ray crystallography and his constant enthusiasm in the lab. Special thanks go to









John Sworen, my mentor and my friend, who helped me get started in the lab and much

further. I have learned from a true master, I will be forever indebted to him. I would also

like to thank my friends and classmates Travis Baughman and Piotr Matloka, for all the

poking, the laughs, and all our discussions, chemistry-related or not. I will never forget

our Eurotrip! This experience would not have been the same without Emilie Galand,

Sophie Bernard, Merve Ertas, Kornelia Matloka, and Victoria Broadstreet. Their

friendship helped me through my ups and downs and I will miss them immensely, along

with the shopping extravaganzas, the late-night talks, and the party-dancing. Pauline

Anselme, my roommate during my first year here, was also a wonderful support as I was

settling into my new life as a graduate student. I also wish to thank Armando Coronado,

my undergraduate student, for his tremendous help with all the boring stuff I delegated to

him. Finally, I would not have succeeded without the support of Josh McClellan, who

has been there for me in the worst and the best of times.
















TABLE OF CONTENTS



A C K N O W L E D G M E N T S ................................................................................................. iv

LIST OF TABLES ........... ............ ........... ........... .... ......... .............. ix

LIST OF FIGURES ............................... ... ...... ... ................. .x

ABSTRACT .............. .......................................... xiv

CHAPTER

1 IN TR OD U CTION ............................................... .. ......................... ..

Introduction to M etathesis ........................................................... ..................
E arly C ataly st D evelopm ents ............................................................ .....................2
Well-Defined Ruthenium Catalysts X2L(PR3)Ru=CHR31.............. .......................4
First-G generation G rubbs C analysts ............................................. .....................4
Second-Generation Grubbs Catalysts ....................... ...................................7
M echanistic Considerations..................... ....... ............................ 9
R ole of the L -type ligand .................................................... .................... 10
R ole of the phosphine ligand (PR 3) ................................................. ...........11
R ole of the halide ligand (X ).............................................. .................. 12
Role of the carbene ligand (R ) ....................................... ............... 12
Stereochem ical aspects........................................... .......................... 13
Catalyst D ecom position ................................ .. ... .......... ........... 14
Other Second-Generation Ruthenium Catalysts...............................................15
Introduction to ADMET Polycondensation............................................. 17
C atalytic C ycle ..................................................... ................. 17
Kinetics and Equilibrium Considerations ................ ..................................18
Cyclization versus Linear Polymerization.............................. ..................20
Interchange Reactions and Molecular-Weight Distribution.............................21
Utility of ADM ET ............ ........ ...................... .... ... ..................22
Ruthenium Catalysis Applied to ADM ET....................................... ............... 24
R uthenium 1st G eneration......................................................... ............... 24
Ruthenium 2nd G eneration........................................................ ............... 26
L im itatio n s ..................................................................................................... 2 8
Hydrogenation ................................. ........................... .... ...... 29
K inetics.................................................. 29
O lefin Isom erization ...................................... ......... ............... ............... 30









2 METATHESIS ACTIVITY AND STABILITY OF NEW GENERATION
RUTHENIUM POLYMERIZATION CATALYSTS ............................................... 32

In tro du ctio n ...................................... ................................................ 3 2
R results and D discussion ................................................... .................... ............ 35
Kinetic Assessm ent of Catalyst Activity .................................. ............... 35
Therm al Stability of the Catalysts ............................................ ............... 46
Further Comments on ADMET Polymerization .............................................46
Isom erization of the Olefin Bond......................................... .........................48
C o n c lu sio n s........................................................................................................... 5 2
E xperim mental ............. ....................................................................................53
G general C considerations ............................................... ............................ 53
X -ray E xperim ental ....................... .. .... ..................... .... .. ........... 55
Synthesis of Com plex [Ru] 11 ....................... ........... ....... .................56
K in etic Stu dy ........... .......... ... .............. ..... .. ............ ............. ... ...... 57
ADM ET Polymerization of 1,9-decadiene....................................................... 57
Self-M etathesis D im erization ............................................ ............................ 57
Measurement of the Thermal Stability of Complex [Ru]10 and [Ru]11.............58

3 THE UTILITY OF HOVEYDA-TYPE CATALYSTS IN ADMET
CHEMISTRY: STERICS VS. ELECTRONICS.................. .......... ............... 59

Introduction ............. ...... .. ............ ... .........................................59
R results and D iscu ssion .............................. ........................ .. ...... .... ...... ...... 6 1
S te ric e ffe ct.......................................................................................................... 6 2
Electronic effect on H2IPr ligated complexes ............................................. 65
Tem perature effect ............................... .... ..................... .. ................. 69
C o n c lu sio n s........................................................................................................... 7 0
E xperim mental ......................................................................................................7 1
G e n e ra l ............................................................7 1
X -ray Experim mental ........................................72
General Procedure for Ligand Synthesis............................... ............... 73
General Procedure for Carbene Exchange .................... ................. ..... 73
K in etic S tu d y .................................................................................. ..............7 6

4 ISOMERIZATION BEHAVIOR UNDER METATHESIS CONDITIONS OF
NHC-CONTAINING RUTHENIUM CARBENE CATALYSTS ..................... 77

Introduction .......................... ........... ............ ................ ............... 77
R results and D discussion ....................... ...... .......... ............... .... ....... 79
Synthesis and Characterization................... ........ .......................... 80
Isomerization Behavior with 1-Octene......................................................81
ADM ET Polym erization .................................. .....................................86
C o n c lu sio n s........................................................................................................... 8 7
E x p e rim e n ta l ..................................................................................................8 8
G general C considerations ........................................ .................. ............... 88
Synthesis of Complexes [Ru]18-22.................. ..................89









5 UNDERSTANDING STRUCTURAL ISOMERIZATION DURING RU-
CATALYZED OLEFIN METATHESIS: A DEUTERIUM LABELING STUDY..92

In tro du ctio n ...................................... ................................................ 9 2
R results and D iscu ssion .............................. ........................ .. ...... .... ...... ...... 95
M echanistic Studies.............. .... ..................... .......... 95
Other M echanistic Considerations ....................................... ............... 100
K inetic Studies ............................................ ................ ......... 101
D esign, Synthesis and Characterization .................................... ............... 102
T herm al Stability ............................... .... ... ....... ...... .. .. .. ....... .... 106
Catalytic Behavior Under M etathesis Conditions............................................108
C o n c lu sio n s......................................................................................................... 1 1 7
E xperim mental .........................................................................................................118
G general ................................................................... ...............118
Deuterium Labeling Study with Allyl-1, 1-d2-methyl ether.............................118
X -ray E xperim ental ........................................... .. .. .. ...... ...............123
Synthesis of Complex [Ru]23 .......................... .. .... ...............124
Thermal Decomposition of Complex [Ru]23............................... ...............129
Thermal Decomposition of Methylidene Complex [Ru]23' .............................130
Isom erization Experim ent ........................................................................... 130

6 UNDERSTANDING THE EFFECT OF ALLYLIC METHYLS IN OLEFIN
CRO SS-M ETATHESIS .................................. ......................................... 131

Introdu action ...................................... ............................... .......... ...... 13 1
R e su lts ............................. ........................... ................ ................ 13 3
Schrock's M olybdenum C atalyst .....................................................................134
G rubbs' Ruthenium Catalysts....................................... ......................... 138
Discussion......................... .........................143
Conclusion ........................ ....................... .... 144
Experimental ............... .............................. 145
G e n e ra l .......................................................................14 5
Materials and Methods ............ ..........................................146
Bulk Polymerization Experiment ........................................146
N M R Catalyst Experim ents ..................................................................... 146
Synthesis of EP3 M onom er (6-2 to 6-9) ...........................................................147

APPENDIX CRYSTALLOGRAPHIC DATA....................................151

LIST OF REFERENCES ............................................. ......... ........168

BIOGRAPHICAL SKETCH ..............................................................................181















LIST OF TABLES


Table page

2-1 Initial rates for complexes [Ru]l-[Ru]ll during the ADMET oligomerization of
1,9 -d ecad ien e ........................... ..................................................... ............... 3 5

2-2 Bond Lengths and Angles for Complexes [Ru]9 and [Ru]ll .............. ...............42

2-3 Half-lives for the decomposition of complexes [Ru]10 and [Ru]11 .......................46

2-4 Polymerizations of 1,9-decadiene at 55 C for 120 h .............................................47

3-1 Initial rates in DP s-1 for catalysts [Ru]9,[Ru]11-13 ...............................................63

3-2 Initial rates in DP s-1 for catalysts [Ru]ll-[Ru]16. .............................................. 67

4-1 Percentage of isomers during the metathesis of 1-octene. ............. ................ 82

4-2 Results for the ADIMET polymerization of 1,9-decadiene at 55 C for 120 h.........87

5-1 Selected bond lengths (A) and angles (deg) for complexes [Ru]4 and [Ru]23.....105

A-i Crystal data and structure refinement for [Ru]ll ............................ .............152

A-2 Atomic coordinates ( x 104) and equivalent isotropic displacement parameters
(A2x 103) for [R u]ll ............................. ............. .... .. .. .. .......... 153

A-3 Crystal data and structure refinement for [Ru]13 ...........................................157

A-4 Atomic coordinates ( x 104) and equivalent isotropic displacement parameters
(A 2x 103) for [R u ]13 ................................................................ .......... .... 158

A-5 Crystal data and structure refinement for [Ru]16. .............................................. 161

A-6 Atomic coordinates ( x 104) and equivalent isotropic displacement parameters
(A 2x 103) for [R u ]16 ................................................................ ................... 162

A-7 Crystal data and structure refinement for [Ru]23 ............................................165

A-8 Atomic coordinates ( x 104) and equivalent isotropic displacement parameters
(A2x 103) for [Ru]23............... ......................... ........166

















LIST OF FIGURES


Figure p

1-1 O lefin m etathesis......... ...................................................................... ........ .. .... 2

1-2 M echanism ofolefin m etathesis......................................... ............................ 3

1-3 Schrock's alkylidene catalyst ........................................................ ...............

1-4 Synthesis of the first generation ruthenium carbene catalysts ................................

1-5 Alternative route to ruthenium carbene catalysts.....................................................5

1-6 Other well-defined, metathesis-active L2X2Ru=CHR ruthenium carbene
c a ta ly sts ............................................................ ................ 7

1-7 Synthesis of 2nd generation ruthenium metathesis catalysts.................. ..........9..

1-8 Mechanism of metathesis catalyzed by ruthenium alkylidene.............................. 10

1-9 Proposed conformations for olefin-bound Grubbs catalyst intermediate.................13

1-10 Stereochemical preference of the ca,a'-disubstituted metallacyclobutane...............14

1-11 Decomposition products of ruthenium carbene complexes .................................15

1-12 Other second-generation catalysts...................................................................... 16

1-13 Hoveyda's "release-return" mechanism ...... ......... ........................................ 16

1-14 M echanism of productive ADM ET...................................... ........................ 18

1-15 Variation of molecular weight with conversion in step polymerization .................. 19

1-16 Form ation of cyclics in AD M ET ........................................ ........................ 21

1-17 Traii-inci, It\i\ in ADMET: a) Interchange with a polymeric alkylidene; b)
Interchange with the methylidene from a productive step .......................................22

1-18 Examples of polymeric structures obtained via ADMET polymerization ..............23









1-19 Structural olefin isomerization of 1-octene............... ....................................30

1-20 Olefin isomerization during ADMET polymerization .......................... ..........31

1-21 Olefin isomerization mechanism: a) 7t-allyl mechanism. b) metal-hydride
addition-elim nation m echanism ..................................................... .... ............... 31

2-1 M etathesis cataly sts .................................................................... ....... .. ......33

2-2 Degree of polymerization (DP) vs. time curves for complex [Ru]l, [Ru]4 and
[Ru]10 using 450:1 monomer:catalyst ratio at 30C .......................................36

2-3 Comparison of the activity of complex [Ru]10 at three different temperatures
using a 450:1 m onom er:catalyst ratio ........................................... ............... 37

2-4 Degree of polymerization (DP) vs. time curves for complexes [Ru]4 and [Ru]10
using 450:1 monomer:catalyst ratio at 45 C ..................................................... 38

2-5 Degree of polymerization (DP) vs. time data for complex [Ru]10 using 900:1
m onom er: catalyst ratio ........ ............................... .............. .. .. ........ .... 39

2-6 Degree of polymerization (DP) vs. time data for complex [Ru]9 using 450:1
m onom er: catalyst ratio .................................... ......... ..................... ............... 40

2-7 Synthesis of com plex [R u]ll ....................... ......... ........................ ............... 41

2-8 ORTEP Diagram of complex [Ru]ll. Thermal ellipsoids are drawn at the 30%
probability level ........................................................................ ......... .................... 42

2-9 Degree of polymerization (DP) vs. time data for complex [Ru]ll using 450:1
m onom er: catalyst ratio .................................... ......... ..................... ............... 43

2-10 Proposed intermediate of complex [Ru]9 during the metathesis of dipentenyl
e th e r ............................................................................. 4 4

2-11 Degree of polymerization (DP) vs. time data for complexes [Ru]9 and [Ru]ll
using dipentenyl ether as the monomer (450:1 monomer:catalyst) .......................45

2-12 Composition of olefin mixture as a function of time for reaction of [Ru]10 with
1-o cten e at 3 00 C .................................................... ................ 4 8

2-13 Proposed mechanism for isomerization of 1-octene ............ .......................49

3-1 O lefin m etathesis catalysts ............................. ....... ......... ... ........ ............. 59

3-2 Synthesis of complexes [Ru]9, [Ru]ll-16 ............. ............................................. 62

3-3 Degree of polymerization (DP) vs. time data for H2IMes versus H2IPr containing
catalysts using 450:1 monomer:catalyst ratio at 450C .......................................... 64









3-4 Degree of polymerization (DP) vs. time data for complexes [Ru]ll-[Ru]16
using 450:1 m onom er:catalyst ratio at 45 C ........................................ ................66

3-5 Degree of polymerization (DP) vs. time data for complexes [Ru]ll-[Ru]16
using 450:1 m onom er:catalyst ratio at 60 C .................................... ..................69

4-1 Examples of competitive isomerization reactions during olefin metathesis ............78

4-2 Effect of isomerization during ADMET polymerization ......................................79

4-3 Synthesis of [R u]18-22 ................................................. ........................................... 81

4-4 GC chromatogram of the product mixture from the self-metathesis of 1-octene:
(A) with [Ru]21 at 25C; (B) with [Ru]22 at 25C. Retention scale is in minutes. 83

4-5 Postulated mechanism of the isomerization process....................... ................84

4-6 Decomposition products of ruthenium carbene complexes ...............................85

5-1 Olefin m etathesis catalysts .............. .. ................. ............... ............... 92

5-2 Decomposition products of ruthenium carbene complexes ...............................94

5-3 Deuterium shift with a x-allyl hydride mechanism .............................................96

5-4 Deuterium shifts with a metal-hydride addition-elimination mechanism................97

5-5 Products of the reaction of allyl-1,1-d2-methyl ether with complex [Ru]4 .............98

5-6 Incorporation of hydrogen at C-l by reversible isomerization .............................100

5-7 /7-H abstraction-elimination mechanism proposed for allyl alcohol isomerization
by [R u]l .............................. ........ ........................................... .......................10 1

5-8 Total concentration of 4-1 versus time. Conditions: 2.3M solution of 4-1 in
C6D6 with 0.2 mol% of [Ru]4 at 350C for 8 hours ............................................. 102

5-9 Synthesis of Com plex [R u]23 ...................................... ............................. 103

5-10 X-Ray structure of complex [Ru]23. A) ORTEP Diagram of complex [Ru]23.
B) Superposition of the crystal structures of complex [Ru]4 (orange) and
[R u ]2 3 (b lu e)......................................................................................... ..... 10 5

5-11 NMR spectrum of neat 1-octene with 0.5 mol% of complex [Ru]23, after 4h at
35C and 20h at 550 C: a) 1H NM R; b) 2H NM R ...................................................109

5-12 Formation of 1-octene isomers catalyzed by a ruthenium deuteride complex....... 11

5-13 Com plex [R u]24 .......................................... ............... .. ........ .... 112









5-14 2H NMR spectrum of neat allyl methyl ether with 0.5 mol% of complex [Ru]23,
after 4h at 350 C .................................................................. ............ 113

5-15 Orientation by oxygen precoordination into P-H-elimination at the C-3 position. 114

5-16 2H NMR spectrum of neat 3,3-dimethylbutene with 0.5 mol% of complex
[Ru]23, after 4h at 35C ............................... .......................115

5-17 1H and 13C chemical shifts assignment in compounds 5-2cis, 5-2trans, 5-3cis
and 5-3trans ...................................... .................... ...... ........ ...... 119

5-18 Molar fractions of compounds 5-2cis, 5-2trans, 5-3cis and 5-3trans in the
product (under the compound number) and molar fraction of deuteration at each
position. .....................................................................119

5-19 Expansion of the proton spectrum, the CH2 region................................... 120

5-20 Expansion of the gHM BC spectrum ........................................... ............... 121

5-21 Expansion of the carbon spectrum, C3 region ..................................... ........122

6-1 Olefin m etathesis catalysts ..............................................................................131

6-2 Synthesis of monomer 6-9 ........................ .........................133

6-3 A D M E T reaction of 6-9 ......................................................................................... 134

6-4 Cross metathesis of 3-methyl-l-pentene catalyzed by [Mo]l..............................134

6-5 Catalytic cycle for the metathesis of 3-methyl-l-pentene with a molybdenum
carbene catalyst .................................... .. .......................... ... .........135

6-6 1H NMR alkylidene region for the reaction of 3-methyl-1-pentene. (a) With
catalyst [M o]l; (b) with [Ru]l; (c) with [Ru]4........... .................................... 137

6-7 CM of 3-methyl-1-pentene catalyzed by [Ru]l or [Ru]4 ..................................... 138

6-8 Catalytic cycle for the metathesis of 3-methyl-1-pentene with ruthenium carbene
cataly st [R u]l or [R u]4 ............ .... ........................................ .......... .. ......... 139

A-1 Crystal structure of [Ru]11, (H2IPr)(C1)2Ru=CH(2-iPrO)C6H4 ............................ 151

A-2 Crystal structure for [Ru]13, (H2IPr)(C1)2Ru=CH(2-iPrO)(5-NO2)C6H3............156

A-3 Crystal structure for [Ru]16, (H2IPr)(C1)2Ru=CH(2-iPrO)(5-OCH3)C6H3 ...........160

A-4 Crystal structure for [Ru]23, (H2IPr)PCy3(Cl)2Ru=CHPh ....................................164















Abstract of Dissertation Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Doctor of Philosophy

METATHESIS AND ISOMERIZATION ACTIVITY
OF RUTHENIUM CARBENE CATALYSTS
FOR ACYCLIC DIENE METATHESIS POLYMERIZATION

By

Florence C. Courchay

December 2005

Chair: Kenneth Wagener
Major Department: Chemistry

Acyclic diene metathesis (ADMET) polymerization being a step condensation, the

catalyst requires long lifetimes and a robustness that allows its reactivity at high

temperatures. Accordingly, several novel ruthenium metathesis catalysts were

investigated through a kinetic study conducted under ADMET polymerization conditions.

Their kinetic behavior was compared to the classical 1st and 2nd generation Grubbs'

complexes ([Ru]l and [Ru]4, respectively) at various temperatures. Among the catalysts

tested, Hoveyda-type systems were sterically and electronically modified. The

complexes containing bulkier N-heterocyclic carbene (NHC) ligands consistently result

in faster initial ADMET rates than any other existing phosphine or Hoveyda complexes.

Electronic modifications are mostly dominated by the steric effect of the NHC ligand,

especially at low temperatures. Although these complexes show promising longevity and

stability in small molecule chemistry, they do not produce higher molecular weight

polymer than catalyst [Ru]4.









Soon after, olefin isomerization emerged as an undesirable side reaction induced by

NHC-containing catalysts during metathesis processes. With fast initiators, isomerization

occurs after the metathesis reaction, while it becomes competitive with slower initiators.

After several attempts to reduce isomerization by designing new catalytic systems

featuring different phosphine, carbene, or NHC ligands, a comprehensive mechanistic

study was undertaken. From a deuterium labeling experiment monitored by NMR

spectroscopy, olefin isomerization promoted by catalyst [Ru]4 under metathesis

conditions was determined to operate via a metal-hydride addition-elimination pathway.

Further deuteration of the catalyst itself, by synthesis of analogue [Ru]23, revealed the

existence of an exchange process between the NHC ligand and the substrate, suggesting

the presence of a ruthenium dihydride. However, the active species could not be

identified spectroscopically.

Finally, [Ru]1, [Ru]4 and Schrock's molybdenum catalyst [Mo]l were

individually examined during the metathesis of olefins containing allylic methyls.

Conversions in cross-metathesis were limited to 50% with all catalysts, while ADMET

only produced the dimer of the corresponding diene. These results are directly correlated

to the orientation of the substrate's bulk during the metallacyclobutane formation, the

alkyl branch being adjacent to the metal center in the case of the molybdenum catalyst

[Mo]l, and opposite to it in the case of ruthenium catalyst [Ru]l and [Ru]4.














CHAPTER 1
INTRODUCTION

Introduction to Metathesis

Olefin metathesis has become a powerful tool for the formation of carbon-carbon

bonds.1'2 Discovered inadvertently in the mid-1950s,3'4 it was not until 1967 that

Calderon5'6 recognized the term olefinn metathesis", which is understood as the metal-

catalyzed redistribution of carbon-carbon double bonds. In 2005, the metathesis reaction

was distinguished by the Nobel prize in chemistry, awarded to Prof Yves Chauvin who

unveiled the mechanism, and Prof. Richard Schrock and Prof. Bob Grubbs who

developed the catalysts that expanded the versatility of the reaction. This transformation

enjoys a variety of applications (Figure 1-1). Ring-closing metathesis (RCM) is the

intramolecular reaction of an acyclic diene into an unsaturated ring. This reaction has

particularly been taken advantage of in the pharmaceutical industry, where the synthesis

of complex cyclic systems used to require several, often tedious steps.711 The cross-

metathesis (CM) reaction involves the scrambling of substituent groups between two

olefins to form two new products.12-15 The extensive use of cross-metathesis in organic

synthesis has also been applied to green chemistry to convert oleochemical feedstocks

into valuable chemical products. Ring-opening metathesis is the opening of an

unsaturated cycle by cross-coupling with a mono-olefin, to form a substituted acyclic

diene.16-18 Metathesis has also been actively used in the production of polymer

molecules, either through ring-opening metathesis polymerization (ROMP)19'20 or acyclic

diene metathesis (ADMET) polymerization.21,22 While ROMP is a chain-addition type of









polymerization driven by the release of ring-strain in the cyclic monomer, ADMET is a

step-condensation whose equilibrium is driven by the release of a small molecule,

ethylene.

RCM
)Zz" -C2H4

ADMET -CH4 /ROMP







S ROM ;X R
+ R




RI CM
+ R.-. R2
R2 1


Figure 1-1. Olefin metathesis

Early Catalyst Developments

For a long time, the scope of the reaction was limited by its insufficient catalyst

performance, using multicomponent homogeneous and heterogeneous systems such as

WCl6/Bu4Sn, WOC14/EtAlC12, MoO3/SiO2, and Re207/A1203. It was not until Herisson

and Chauvin23 proposed that the mechanism involved the interconversion of an olefin and

a metal alkylidene through a metallacyclobutane intermediate that well-defined catalysts

were developed, considerably expanding the range of metathesis applications (Figure 1-

2). His work in metathesis was awarded the Nobel Prize of Chemistry in 2005.









R R R
[M]-=/ [M I [M] ]

R1 R2 R1 R2 R1 R2

Figure 1-2. Mechanism of olefin metathesis

The first tantalum24'25 and tungsten alkylidenes26 led to the development of the

highly active molybdenum and tungsten alkylidenes of the general formula

(NAr)(OR')2M=CHR, where the most widely used is better known as molybdenum

Schrock catalyst [Mo]l with Ar=2,6-iPr2-C6H3, R=CMe2-Ph, and R'=C(CH3)(CF3)2

(Figure 1-3).27,28



N
F3C O- M= Mo [Mo]l
CF3 W [W]1
F3C-
CF3

Figure 1-3. Schrock's alkylidene catalyst

These catalysts were referred to as "well-defined" in contrast to the "ill-defined"

classical catalysts since the catalytically active species is spectroscopically identifiable.

Molybdenum complexes are actually more active than their tungsten analog that tend to

be trapped into a metallacyclobutane form. Catalyst [Mo]l allowed the metathesis of

terminal and internal olefins, the ROMP of low-strain monomers, as well as the RCM of

sterically demanding and eletron-poor substrates.27-30 Schrock carbenes are regarded as

X2 ligands that render the metal of these complexes in its higher oxidation state (+VI).

Thus, these early transition-metal catalysts have moderate-to-poor functional group

tolerance because of the high oxophilicity of the metal centers, which also renders them

extremely sensitive to oxygen and moisture.









Well-Defined Ruthenium Catalysts X2L(PR3)Ru=CHR'31

The impetus for developing olefin metathesis catalysts of the late-transition metals

was to make less electrophilic complexes that would preferentially react with olefins in

the presence of heteroatomic functionalities. With this in mind, ruthenium was

introduced to metathesis chemistry as RuC13(hydrate), which effectively promoted the

ROMP of norbornene in aqueous solution, albeit at low initiation rates.32 Further catalyst

screening revealed that Ru(H20)6(tos)2 (tos=p-toluenesulfonate) had even shorter

initiation times and could ring-open functionalized norbornene.33 However, these ill-

defined ruthenium complexes were limited to the ring-opening of strained olefins, and

therefore did not work for ADMET.

First-Generation Grubbs Catalysts

The real breakthrough came in 1992, with the synthesis of the first, well-defined,

metathesis active ruthenium alkylidene complex Cl2(PPh3)2Ru=CHCHCPh2, based on the

same methodology as their tungsten analogs, starting with RuCl2(PPh3)3 and 3,3-

diphenylcyclopropene (Figure 1-4).34 The alkylidene proton is visible by 1H NMR as two

overlapping triplets at 6 17.94 ppm. In contrast with Schrock's catalysts, the carbene unit

is considered a neutral L-type ligand that renders the metal center in a second oxidation

state, although it is sometimes shown as a Ru(IV). The carbene moiety is believed to be

an intermediate between a Schrock and a Fisher type carbene.

Phosphine exchange with the larger and more basic PCy3 further enhanced activity

as complex C12(PPh3)2Ru=CHCHCPh2 was able to promote the ROMP of lower strain

monomers and the metathesis of acyclic olefins.35 Even though the catalytic activity was

far lower compared to the early transition metal carbene complexes, the ease of use and

the newly-found functional group tolerance of this first generation of ruthenium catalysts









rendered them attractive for a variety of applications. Of 250 publications on olefin

metathesis that appeared between 1990 and 1999, 75% reported using ruthenium

metathesis catalysts. Their synthesis was industrialized by using diazoalkanes as the

alkylidene precursor, which also produced complex [Ru]l after phosphine exchange, the

most widely used catalyst of this series and better known as "1st generation Grubbs'

catalyst" (Figure 1-4).36,37 The key to this synthesis is the insolubility of [Ru]l in

acetone, which allows for a simple and convenient wash of the crude product right out of

the reaction pot. The use of rather explosive diazo compounds can be bypassed through

the rearrangement of vinyl or propargyl halides in the presence of Ru(H)(H2)Cl(PR3)2

(Figure 1-5).

Ph Ph PPh3
Ru Ph +C PCy3
C u PPhh 3 C Ru- Ph
C I PPh3 CIe
PPh3 Ph P Ph
PCy3
RuCI2(PPh3)3
PPh3 PCy3
CI/, C +PCy3 Cl/,,
SRu-\ PPh3 Ru [Ru]1
Nf\ Cl t Ph 3 Cl t Ph
Ph I I
PPh3 PCy3


Figure 1-4. Synthesis of the first generation ruthenium carbene catalysts

CI PCy3
H "' Cl ,,

PCy3 Ci tRuCH3
[RuCI2(COD)]x H2 (1.5 atm), NEt3 \,CI PCy3 CH3
+ H--R--H2
sec-BuOH, 800C CI' t PCy3
2 PCy3 C,


PCy3


Figure 1-5. Alternative route to ruthenium carbene catalysts









These (PR'3)2C12Ru=CHR complexes typically exhibit a distorted square-pyramidal

geometry with the alkylidene moiety in the apical position, and a trans bi(phosphine)

ligand arrangement at the base of the square pyramid. Although in most cases the

carbene is coplanar to the P-Ru-P plane, in the case of bulky phosphine, it is twisted

away, almost perpendicular to the P-Ru-P plane.

The remarkable tolerance of these catalysts allowed scientists to apply a variety of

modifications to the ligand sphere for specific conditions. Hence, their use was extended

to ROMP and RCM in aqueous media by substituting the PCy3 ligands to water-soluble

phosphine such as Cy2P(CH2)2NMe3+C1840 Several supported models were also

introduced for heterogeneous catalysis.41-45 Other variations have been reported,

including heterobimetallic complexes46 and multidentate ligands, such as Shiff bases,47-48

arenes,49-52 and bidentate phosphines,53'54 in an attempt to tune in activity and/or stability

(Complexes [Ru]A-D in Figure 1-6). Pyridine coordinated complex [Ru]E features a

remarkably high activity but a short lifetime,55 while the alkoxy-chelated complex [Ru]F,

better known as Hoveyda catalyst, shows exceptional stability, which allow its use in

reagent grade solvents and/or in air.56

These first-generation of ruthenium catalyst are air stable and can be handled on the

benchtop without significant loss of activity. Solutions prepared in reagent-grade

solvents decompose slowly (over several hours) through different unclear processes, the

primary pathway involving a bimolecular decomposition. Several mechanistic studies

that suggested the formation of a highly active mono(phosphine) intermediate during the

catalytic cycle triggered the development of second-generation ruthenium catalysts.57-63












"4, R02N R 'Pr iCRu Ph
1Rh/ P
Cl C Ru==\Ph Oc Ph R3P Ph
CI0l u I Ph I P Ph
L PCy3
[Ru]A [Ru]B [Ru]C

t PCy3 PCy3
But\ t Bu C, Cl I
Bt, t Cle t Ph CR
BK / tB u '.R uz \ 'R 7
Ru Ph N Ph ci / O
CI C" Ph


[Ru]D [Ru]E [Ru]F

Figure 1-6. Other well-defined, metathesis-active L2X2Ru=CHR ruthenium carbene
catalysts

Second-Generation Grubbs Catalysts

Attempts to increase the concentration of the monophosphine complex by addition

of phosphine traps (such as HC1 or CuC1) only led to fast decomposition of the catalyst.

Even though the activity was greatly enhanced, increasing the concentration of the

monophosphine complex was not the way to improve catalytic efficiency. After

Herrmann et a.64 reported complex [Ru]2 (Figure 1-7), researchers turned to N-

heterocyclic carbene (NHC) ligands, sometimes referred to as "phosphine mimics" for

their strong G-donor and weak 7t-acceptor characteristics. Unlike most organic carbenes,

these NHC ligands, pioneered by Arduengo65-68 and now used extensively in

organometallic chemistry, are ground-state singlets, although this electronic structure is

not fully understood. In any case, these NHC ligands are stronger G-donors and much

less labile than phosphines. So, while complex [Ru]2 showed little improvement in

activity, a mixed ligand complex would overcome the problem both ways: promote









phosphine dissociation by trans effect, and stabilize the electron-deficient intermediate

through the steric bulk and electron donation of the remaining NHC ligand. Indeed,

complex [Ru]3 containing a mesityl-substituted NHC ligand (IMes) exhibited greatly

enhanced metathesis activity and stability toward air and a variety of functional groups

(Figure 1-7).69,70 Soon after, the saturated version [Ru]4 was discovered by the Grubbs71

group to be even more reactive, albeit less stable than [Ru]3. The 1,3-dimesityl-4,5-

dihydroimidazol-2-ylidene (IMesH2) ligand was thought to be more basic than its C-C

unsaturated counterpart because of the reduced conjugation throughout the NHC ring.

However, this carbene is not stable in air, and must be formed in situ, either from the

imidazolium salt and a base, or through the chloroform adduct. Complex [Ru]3 can then

be isolated after a methanol wash and column chromatography, if further purification is

desired. The methanol wash, which sometimes contaminates the catalyst with hydrides,

can be avoided by running the reaction in anhydrous hexanes.72

These NHC-containing complexes exhibit the same distorted square-pyramidal

geometry. But while two carbene fragments are present, they display different Ru-C

distances (e.g. in [Ru]2 Ru-C(carbene)=1.841(11), Ru-C(L)= 2.069(11) A).These

important metrical parameters clearly distinguish two metal-carbene interactions: a

covalently bound benzylidene and a datively bound N-heterocyclic carbene, the latter

acting as a two-electron donor, also reflecting their very poor 7t-acceptor character. The

NHC group also appear to be more sterically demanding than PCy3.73 Finally, this new

class of catalysts combines the activity of early transition metal complexes and the

functional group tolerance of [Ru]1.










2 equiv.

Cy-N N-Cy


PCy3

Rug
CI4 Ph
PCy3
[Ru]l


Cy-N N-Cy

Ru
CIl I Ph
Cy-N N-Cy
[Ru]2
[Ru]2


1 equiv. /- N N

Mes-N N-Mes CI

ClIf Ph
PCy3
[Ru]3


Mes-N N-Mes

CC13


+ Cr


1. KOtBu


2. [Ru]1 in C6H6


[Ru]4

'I


Figure 1-7. Synthesis of 2nd generation ruthenium metathesis catalysts

Mechanistic Considerations

Mechanistic studies started shortly after the discovery of the first ruthenium

alkylidene Cl2(PPh3)2Ru=CHCPh2 and played a major role in the development of the 2nd

generation of catalysts. For a while, scientists believed in an associative mechanism

where the olefin first coordinated to the metal center to form a stable 18 e- complex,

followed by phosphine dissociation and subsequent formation of the

metallacyclobutane.57 Yet, Grubbs et al. reported in 200174,75 that the substitution of a

phosphine ligand with and olefinic substrate actually proceeds in a dissociative fashion to









generate the four-coordinate 14 e- species X2LRu=CHR' (IB) (Figure 1-8). The

recoordination of free phosphine is competitive with substrate binding and the catalytic

turnovers that the active species carries out before being "quenched" with free PR3

depends on a variety of factors, detailed below.

kl k2
S PR3 R' + olefin R'
R R [Ru]= [Ru]
X + PR3 -olefin
PR3 k1 (IB) k2 R (IC)


k-2 k2 k k
olefin + olefin -3 k3

R',,, _
T \R' k3 [Ru R
[R u ]k'_3
k-3
(IC) R' (IE)

[Ru] = X2LRu

Figure 1-8. Mechanism of metathesis catalyzed by ruthenium alkylidene

Role of the L-type ligand

Conversely to the general idea spread at the time when NHC-containing catalysts

were developed, phosphine dissociation is much slower than with the bis-phosphine

systems. The high activity of [Ru]4 over [Ru]1, for example, can be understood by

considering the ratio of phosphine rebinding to olefin binding rates (k_ /k), which

virtually indicates metathesis activity, the smaller ratio, the more active catalyst. Data

show that the IMesH2 ligand increases selectivity for binding olefinic substrates over free

phosphine by 4 orders of magnitude.7 This increase in selectivity is due to the excellent

electron donation and weak 7t-acidity of NHCs, which promotes and stabilizes metal-to-

olefin back-bonding to a greater extent than the phosphine ligand, and therefore favors









the formation of intermediate IC.76 For example, [Ru]4 is more active than [Ru]3 in the

ROMP of COD, IMesH2 being a better o donor than IMes.78 In addition, the electron

donation ofNHC ligands is expected to accelerate the oxidative addition required for the

metallacyclobutane formation.

The reason for the dramatic decrease in initiation exhibited by these NHC-

containing catalysts is still debated. Although both PCy3 and IMesH2 are large ligands,

this could be due to a steric effect, as the distribution of the steric bulk is dramatically

different in each case.78 The differential steric distribution could be the source of

stabilizing or destabilizing interactions that would change the activation energy required

for phosphine loss. An electronic effect could also explain these rates difference by

considering the weaker n-acidity of NHCs compared to phosphines. While the two

phosphine ligands have to compete for Ru-P n-back bonding in the bis-phosphine

complex, the ruthenium-phosphine bond is reinforced in the NHC-containing complex by

stronger nT-backbonding. Unfortunately this is not reflected by the crystal structures

where Ru-PCy3 bond distances barely change upon substitution of one PCy3 with IMes

(Ru-P= 2.4097(6) and 2.4221(6)A in [Ru]l and Ru-P= 2.419(3)A in [Ru]3).70

Role of the phosphine ligand (PR3)

Changing the phosphine ligand has a dramatic effect on both catalyst initiation and

catalyst activity. However, the effect is inverted from the first generation to the second

generation of catalysts. Indeed, bis-phosphines complexes are rendered more active by

larger and more electron donating phosphines, whether it promotes dissociation by trans

effect or it helps stabilize the 14 e- metallacyclobutane.57'79 On the other hand, less









electron donating phosphines, hence more labile, tend to increase the activity of NHC-

containing systems by accelerating the dissociation step.7

Role of the halide ligand (X)

While larger halides increase initiation rates by forcing phosphine dissociation, the

overall activity suffers from the increased bulk around the metal center. For example,

with both [Ru]l and [Ru]4, moving from the di-chloride to the di-iodide complex results

in a 100-fold increase in k/k2.75 Because the conformation of intermediates B and C are

still unclear at this point, the reasons behind this large shift remain speculative. One

possible explanation involves the suggestion that olefin coordination requires a trans

isomerization of the X-type ligands, which would be less favorable with larger X-

ligands.57

Role of the carbene ligand (R)

The effect of the carbene ligand is significant because unlike the other ligands, it

can change throughout the metathesis reaction. For example ROMP reactions produce an

alkylidene complex that becomes the propagating species, while RCM, CM and ADMET

generate a methylidene after the first turnover. Sterically bulky and electron-donating R1

groups (e.g. alkyls) lead to higher initiation rates because they more effectively promote

phosphine dissociation.7 In contrast, small and electronically neutral groups (e.g. H) are

less effective at labilizing the phosphine ligand.80 Indeed, the methylidene complexes,

especially those containing NHC ligands, are extremely poor initiators at room

temperature. While the methylidene of [Ru]4 remain an active metathesis catalyst in its

phosphine-free form, it is virtually incapable of re-entering the metathesis cycle once

trapped with free-PCy3.









Stereochemical aspects

As mentioned above the conformation of the olefin-bound complex is still

unidentified because these intermediate are not isolable or observable spectroscopically.

Until recently, the nature of the metallacyclobutane was the subject of a theoretical

debate, whether it was a reaction intermediate61 or a transition state,81-83 when Piers et

al.84 finally isolated a ruthenacyclobutane as a C2v structure. Grubbs and others61'85 have

shown that the olefin may bind either trans to a halide (X), which necessitates a

rearrangement (IC-a and IC-b in Figure 1-9), or trans to the L-type ligand (IC-c).

Although in some cases there seems to be more evidence for intermediate IC-a,86 the

symmetrical C2v structure of the metallacyclobutane suggests that IC-c is the correct

conformation.

L L L L
I ,,X x X I X
"RV=V X-RRUu r
U R1 R1 R1 R
x -
IC-a IC-b IC-c metallacyclobutane

Figure 1-9. Proposed conformations for olefin-bound Grubbs catalyst intermediate

In the case of monosubstituted dienes, and probably as a general rule, a,a'-

disubstituted metallacyclobutane seem to be kinetically favored, within the limits of non-

sterically demanding olefins (Figure 1-10).80 This preference is more likely the result of

the greater stabilization of the metal center by electron-donating substituents in an ca-

position rather than in a P-position. Conversely, this conformation only leads to non-

productive metathesis suggesting that the energy difference between the 2

metallacyclobutane is not enough to prevent the metathesis reaction. However, this may

not apply in the case of sterically demanding olefins.









Preferred
L
R [Ru] I,,X R Non-
L R X,,Ru=-, + / Productive
L R R

X ,,X R

xR u Ru=CH2 + Productive

R

Figure 1-10. Stereochemical preference of the ca,a'-disubstituted metallacyclobutane

Catalyst Decomposition

Even though a substantial amount of effort has been devoted to the understanding

of decomposition pathways in solution and under metathesis conditions, this area still

comports many unanswered questions. The thermal decomposition of [Ru]1 has been

proposed to occur via phosphine dissociation followed by bimolecular coupling of two 4-

coordinate ruthenium fragments, the addition of phosphine slowing down the

decomposition rate.87 Conversely, the decomposition of the methylidene follows first-

order kinetics which could be the result of an intramolecular C-H activation of an L-

ligand. In general, NHC-containing complexes exhibit significantly improved thermal

stabilities compared to their bis-phosphine counterparts. For example, [Ru]3 shows no

sign of decomposition after Ih at 1000C in toluene, while in the same conditions 75% of

[Ru]l has already decomposed.88 This increased stability is a combination of the

stabilizing effect of the NHC ligand on the 14e- intermediate and of the reduced extent of

phosphine dissociation. Nevertheless, only one compound has been identified so far,

bimetallic hydride complex [Ru]5, from the thermal decomposition of [Ru]4

methylidene analog.89









Others have studied the behavior of these catalyst under different conditions and

isolated other decomposition products. For example, carbonyl hydrides [Ru]6 and [Ru]7

(Figure 1-11) first isolated during the decomposition of alkoxycarbene complexes,90'91

were identified as products of the reaction of [Ru]l and [Ru]4 with primary alcohols in

the presence of a base.72'92-94 However, the mechanism is still at a speculative stage.

Noteworthy, [Ru]8, product of the C-H activation of a mesityl group, was isolated during

the synthesis of [Ru]4 but was never observed as a proper decomposition product of

[Ru]4.72



'Ru R CRu--H ,C
Mes RIu-_R-- Ojt 0ColRO

N H CI PC3 PCy;
Mes
Mes ][Ru]6 L =PCy3
[Ru]5 [Ru]8
[Ru]7 L = IMesH2

Figure 1-11. Decomposition products of ruthenium carbene complexes

Other Second-Generation Ruthenium Catalysts

A variety of related second-generation catalysts have been investigated in olefin

metathesis. Hoveyda et al.95 reported a second-generation tethered catalyst [Ru]9, which

turns out to be a faster initiator than [Ru]4 (Figure 1-12). Even though [Ru]4 and [Ru]9

generate the same propagating species after a single turnover, [Ru]9 afford considerably

higher yields with electron-deficient olefins, which is probably due to the absence of a

strong donor ligand.96-98 This observation emphasizes the importance of ligand rebinding

during the metathesis cycle. Bis-pyridine complexes provides similar results, and is often

used as an intermediate for phosphine exchange.99









Several modifications of the NHC ligands have also been implemented. Among

those [Ru]10 was reported to catalyze terminal olefins much faster than [Ru]4, thanks to

the increased bulk of its NHC ligand exerting a stronger steric pressure on the carbene

and therefore prompting phosphine dissociation.100'101



-N i N-\- N N N
CI/,I C,
Ru! 7, & Ru 11 Ru\
C c N XPh Ct Ph
X N PCy3


[Ru]9 X= H, Br [Ru]lO

Figure 1-12. Other second-generation catalysts

The isopropoxystyrene ligand of [Ru]9 represents an ideal anchor for

heterogeneous catalysis applications,102-107 and further ligand modification by substitution

of electronic and steric groups. 108109 In addition to their extreme stability towards air and

moisture, the release-return mechanism of these Hoveyda-type catalysts confers them an

interesting recyclability (Figure 1-13).95


L L
CI// V Ru.
'Ru ci ,
'R CI
CIO '

^ 0 -o-

S"release" mcai

Figure 1-13. Hoveyda's "release-return" mechanism


L
CI/I."
1Ru=0H2





-0urn"









Introduction to ADMET Polycondensation

Before continuing into catalyst considerations, it is important to understand the

requirements of a polycondensation type of reaction, such as ADMET. This next section

will focus on ADMET polymerization, its characteristics and requirements for catalyst

modeling, and its applications for polymer synthesis following the development of

ruthenium carbene catalysts.

Catalytic Cycle

ADMET is essentially the self metathesis of a diene to form high polymer, and

therefore operates through the Chauvin mechanism. The precatalyst goes through a first

turnover to generate the active alkylidene complex Al. Upon coordination of a second

monomer molecule, an a,3-metallacyclobutane (M2) is formed to produce a dimer and

the methylidene complex A3. A substantial amount of the a,a-metallacyclobutane

probably also forms but only leads to non-productive metathesis. The methylidene A3

then reacts with another monomer molecule, which upon formation of the a-

metallacyclobutane (M4) releases a molecule of ethylene and the "starting" alkylidene

complex. The cycle then goes on to form trimer, tetramer, etc. until reaching high

molecular weigh polymer. For each "coupling" reaction one molecule of ethylene is

formed. However, reaction of a polymer chain with ethylene will result in

depolymerization. To drive ADMET equilibrium onto a productive pathway and achieve

high molecular weigh polymer, ethylene gas is usually eliminated from the reaction

vessel as the polymerization proceeds.










R1R

R L _

R1

R'
H2C=CH2 LnM=

Al








A3
R LM=CH2 R R


Figure 1-14. Mechanism of productive ADMET

Kinetics and Equilibrium Considerations

From a kinetic viewpoint, ADMET is a step-growth condensation reaction,

meaning that it proceeds by the stepwise reaction between the functional groups of

reactants, in this case the olefinic double bond, to produce dimer, trimer, tetramer, and so

on until eventually large polymer molecules are formed.110 The size of the polymer

molecules continuously increases with time (conversion) at a relatively slow rate.

Therefore, high conversions are required to reach high molecular weight polymer as

expressed by the Carothers equation where Xn is the number-average degree of

polymerization, or the average number of structural units per polymer chain, and p is the

extent of reaction or conversion of the diene monomer (Equation 1-1). In other words,










X, is simply given as the total number of monomer molecules M initially present

divided by the total number of molecules present at time t.


S 1 [M]
n p [M]o (1-1)


Step-polymerizations are characterized by the disappearance of monomer very

early in the reaction. For example in most step polymerizations, there is less than 1% of

monomer left when the average polymer chain contains about 10 monomer units. High-

molecular-weight polymer is obtained only near the end of the reaction (>98%

conversion). Passed these high conversions, each connection that is made involves two

high molecular weight molecules, and the average degree of polymerization increases

exponentially at this point, as illustrated by Figure 1-15. the catalyst therefore needs to

remain highly active throughout the entire polymerization, which sometimes require long

reaction times.


250
..
N 200

E
150
0
a.
0
a 100

50-


0
0 20 40 60 80 100
% Conversion


Figure 1-15. Variation of molecular weight with conversion in step polymerization









Most metathesis catalysts are more reactive towards terminal olefin than unstrained

internal olefins, mostly because of the reduced steric hindrance of terminal olefins. For

this reason, ADMET is conducted with a,co-diene monomers to achieve the highest

conversions possible. Incidentally, this generates methylidene complexes, which in the

case of ruthenium hinders the rate of metathesis. This point will be further developed in

the next section. Of course, any mono-olefin impurity should avoided at all times, since

they will act as end-capping reagents, therefore limiting the molecular weight of the

polymer. However, in some instances, mono-olefins can be used as chain transfer agents

to reach a target molecular weight.

Condensation reactions are usually equilibrium processes that require the removal

of the condensate molecule to shift the equilibrium towards high conversions, i.e. high

molecular weights polymers. Industrially, the synthesis of commercial condensation

polymers, such as polyesters, polyamides and polycarbonates, liberate molecules a such

as water, alcohols or even HC1 that often require heat and vacuum to be evacuated.111 In

ADMET, the condensate molecule is ethylene gas, which is conveniently removed at

room temperature. For this reason, most ADMET polymerizations are run under

vacuum, without a solvent, which also minimizes cyclization.

Cyclization versus Linear Polymerization

The production of linear polymers by step polymerization is sometimes

complicated by the competitive occurrence of intramolecular cyclization reactions.112

This phenomenon depends on thermodynamics and kinetic considerations of the size of

the ring structure that may be formed. Thermodynamically, the stability of the ring

decreases under 4 and over 8 atoms. Kinetically, the probability of ring formation









decreases as the possibility of the 2 functional groups encountering each other decreases.

Oligomeric cyclics has been observed in ADMET; however, the amount and the form of

cyclics were impossible to identify.113 The metathesis reaction allowing the ring-closing

of acyclic dienes (RCM), the use of monomers able to form 5-,6-, and 7-membered rings

should be avoided in ADMET (Figure 1-16). Cyclization can also be limited by carrying

out the polymerization at high concentrations of reactants. Linear polymerization being a

bimolecular (intermolecular) process, the reaction rate increases as the concentration of

reactants increase. Typically, ADMET polymerization is run in the bulk, at high

temperatures to lower the viscosity as it increases with the polymer molecular weight.

Similarly, the synthesis of large rings (more than 20 atoms) has been achieved in high

dilution conditions.114


M RR t RR + LnM=R





Figure 1-16. Formation of cyclics in ADMET

Interchange Reactions and Molecular-Weight Distribution

Interchange involves reactions between the terminal functional group of one

polymer molecule with the interunit repeating linkage of another polymer molecule.

These interchange reactions are quite common in condensation polymers, such as

polyamides and polyesters.115 Two polymer chains react to yield one shorter and one

longer chain, which does not affect the average molecular weight of the polymer. In

ADMET, the carbon-carbon double bonds of the polymer backbone are still metathesis

active and can react with either a polymeric alkylidene complex or a methylidene









complex formed from a productive metathesis cycle (Figure 1-17). This phenomenon is

often referred as tra\-inciiith\i\ and will go on until the system reaches a

thermodynamic equilibrium. It allows the perfect randomization of copolymers

synthesized by ADMET, whether the two monomers are mixed together or one monomer

is incorporated into a different homopolymer.116

a) M pi + +p P3 M .P2 + P-pP3



b) M P- + P- M=CH2 + ~pP4


M=CH2 + "p P3, M -P ,. + p-

Figure 1-17. Tra,/\-i'/ i/rthi\ in ADMET: a) Interchange with a polymeric alkylidene;
b) Interchange with the methylidene from a productive step

The product of a polymerization is a mixture of polymers molecules of different

molecular weights. The polydispersity index (PDI) is a measure of the breadth of this

molecular weight distribution, defined as the weight average molecular weight Mw

divided by the number average molecular weight Mn.11 Hence, a high PDI will be the

indicative of a broad molecular weight distribution, which will have various impacts on

the polymer mechanical properties. The molecular weight distribution has been derived

by Flory118 through a statistical approach which predicts that for step polymerization, the

PDI should be close to 2 when the conversions approach 100%. As a matter of fact, most

ADMET polymers exhibit a PDI of 2.0, thanks to trans-metathesis. Lower PDIs indicate

the presence of competitive mechanisms such as cyclization followed by ROMP.

Utility of ADMET

Although ADMET is limited by the catalyst lifetime and therefore does not reach

molecular weights higher than 80,000 g/mol, the polymers obtained present sufficiently










interesting material properties for application. Actually, many commercial polymers,

such as polyesters and polyimides present most interesting properties at molecular

weights ranging between 15,000 and 30,000g/mol.

The clean mechanism of ADMET has often allowed to bypass some of the most

puzzling challenges in the polymer community. For example, the random or alternated

copolymerization of ethylene with polar monomers is complicated by the reactivity

differences of the 2 monomers, often producing block copolymers through standard

polymerization procedures. ADMET provides a unique method to synthesize completely

linear polymer analogous to sequence specific or statistical copolymers of ethylene and

polar monomers in a wide variety of comonomer compositions. This is achieved by

polymerization of the appropriate functionalized diene followed by exhaustive

hydrogenation. The design of the diene monomer can therefore lead to virtually any

polymer via ADMET, whether it is used for modeling known structures or developing

new materials, such as amino-acid containing polymers and polysiloxanes (Figure 1-18).

R'


n

R= -O-B-O- R'= -COOH
Ph -C=O
Hydrocarbon -Ci
-0- -Br
-Br
SiC12 -I
Si(OCH3)2 -OAc
-OCHPhO- -CH2 1
-NH-AminoAcid
-OC=O
Aromatic
-N-
Ph 0

SR H


Figure 1-18. Examples of polymeric structures obtained via ADIMET polymerization









In addition, the ADMET polymerization of different alkyl branched dienes has

provided a family of model ethylene/a-olefin copolymers and allowed the study of the

effects of regular microstructures on polymer properties. The structure-property

relationships of various ethylene copolymers can be clarified using these model systems,

by comparing Tm as a function of the functional group or by examining crystal packing.

Ruthenium Catalysis Applied to ADMET

The development of ruthenium Grubbs catalysts greatly expanded the scope of

ADMET by allowing the polymerization of functionalized dienes that are not tolerated by

Schrock's catalysts and by easing polymerization procedures. Even though the first

generation of catalysts require higher catalyst loadings and afford generally lower

molecular weights, it never necessitate absolute dryness of monomer or oxygen-free

conditions as Schrock catalysts do.

A polymer formed via ruthenium-catalyzed ADMET has a typical trans content of

about 80%., whereas Schrock's catalysts produce 90 to 95% trans polymer. The

preferred trans configuration is not only a correlation of the metallacyclobutane

conformation but is also a thermodynamic reflection of the trans metathesis equilibrium.

The difference between the two types of catalysts is more likely the result of a sterically

less flexible metallacyclobutane in the case of Schrock's catalysts due to its bulky ligand

sphere.

Ruthenium 1st Generation

It was not until the introduction of [Ru] 1 that ruthenium became extensively used

in ADMET. Indeed, its triphenylphosphine and vinylidene analogs only promotes

ADMET at a very slow rate at normal catalyst loadings (0.1 to 1.0 mol%).119 Typically,

the polymers produced with [Ru]l have a molecular weight range of 25 to 35 kg/mol and









an 80% trans ratio, at optimal catalyst loadings of 0.25 mol%.119 This catalyst allows the

polymerization of monomers containing acetals, alcohols,119 aromatic groups,120 esters

both in the main chain121 and as pendant group, ketones,122 ethers,123 boronates,124

germanium,125 silyl chlorides and siloxanes,126 in addition to a variety of alkyl branched

monomers.127-130 Aminel31 and carboxylic acid132 containing olefins can also be

polymerized via ADMET but require to be protected prior polymerization. However, in

the case of hydrocarbon polymers, the molecular weights reached with [Ru]1 are still

much lower than what can be obtained with Schrock's catalysts. For example, an

ADMET ethylene-propylene copolymer, with the same methyl branch content, has aMn

of 17,400 g/mol with [Ru]l, versus Mn of 72,000 g/mol with [Mo]1.128 Other model

ethylene copolymers, such as ethylene-vinyl chloride,121 ethylene-vinyl acetate,122

ethylene-acrylate,133 and ethylene-styrene copolymers133 were also successfully

synthesized with [Ru]l, some even exhibiting unique thermal characteristics. Other

useful structures include various cyclolinear phosphazene polymers,134'135 and liquid

crystalline polyesters.136

Numerous block copolymer architectures have also been devised using

functionalized polyoctenamer telechelics via ADMET polycondensation,121'137 for

example in the synthesis of novel poly(y-benzyl-L-glutamate)-b-polyoctenamer-b-(y-

benzyl-L-glutamate) triblock copolymers. In this case, the functional group tolerance of

[Ru]l allowed the end-capping of ADMET polymerized polyoctenamer with phtalimido

groups, as the first step towards a potentially bioactive triblock copolymer.138

The only requirement for the monomers is to place the functionality at least 2

carbons away from the double-bond to limit cyclization and/or catalyst deactivation by









intramolecular coordination leading to the formation of a stable chelate.60 The

polymerization of 1,5-hexadiene and polyethylene glycol (PEG) grafted copolymers have

been limited to oligomeric structures for this reason.119,139-141

Because these catalysts react rapidly and quantitatively with vinyl ethers to form

Fisher carbenes that are relatively metathesis inactive,74'91 this provides a convenient way

to quench most ADMET reactions by addition of ethyl vinyl ether. This also insures that

the ADMET polymer has well-defined olefinic end-groups.

Depolymerization of commercial polybutadiene by ADMET can be achieved by

applying high pressures of ethylene in the presence of [Ru]l to yield 1,5-hexadiene.142 In

the absence of ethylene and solvent the depolymerization only produces cyclics.143 The

results are much better than with Schrock's catalyst thanks to the lesser sensitivity of the

ruthenium catalysts.

Ruthenium 2nd Generation

Complex [Ru]4 has been used extensively for ADMET, for it yields polymers of

higher molecular weight than [Ru]l. Overall, the reactivity of [Ru]4 falls in between

that of [Ru]l and [Mo]2.144 For example, the polymerization of 1,9-decadiene with

[Ru]4 using the same catalyst loadings (0.25 mol%) affords polyoctenamer with aMn of

75,000 g/mol, compared to only 25,000 g/mol with [Ru]1.145 However, all polymers

obtained using second-generation catalysts exhibit irregular microstructures as evidenced

by a depreciation of the melting temperature T,. This is due to a competitive

isomerization reaction that disturbs the molecular structure by migration of the double-

bond along the olefin backbone. Therefore these catalysts are not suitable for the

modeling of precise polymer architectures.









On the other hand, the second generation of catalysts has allowed the ADMET

polymerization of amino-acid containing dienes.146 These reactions need to be conducted

in THF solution because of the solid nature of the monomers. The polymers produced

have molecular weights of up to 30,000 g/mol and represent ideal candidates for

biological applications.147

Imperfect comb graft copolymers of polyethylene (PE) and PEG were synthesized

with [Ru]4 in higher molecular weights than with [Ru]l (12,000 vs. 5,500 g/mol), albeit

as irregular microstructures.141 Tandem ADMET/ATRP polymerization was used for the

preparation of polyethylene grafted polystyrene copolymers; however, only low

molecular weight materials were obtainable because of the steric hindrance of the

polystyrene grafted diene interacting with the catalyst.148 For this particular class of

copolymers, ROMP followed by ATRP polymerization was able to afford higher

molecular weights.149,150

The use of 2nd generation ruthenium catalysts for the synthesis of conjugated

polymers has hardly been explored, and would not be expected to promote the vinyl

addition reactions that have been problematic in ADMET of electron-rich monomers with

the more electrophilic molybdenum complexes. Attempts to polymerize 1,4-

divinylbenzene only resulted in dimers and trimers because the mixture solidified passed

a DP of 3.151

Depolymerization reactions with [Ru]4 also affords better results than [Ru]l or

[Mo]l for polyisoprene and polybutadiene blocks of triblock copolymers of the type

polystyrene-polyene-polystyrene.152 Complex [Ru]4 is also able to depolymerize sulfur

crosslinked polyisoprene into oligomers (DP=10). This field represents a potentially









useful recycling technique especially if the robustness and the cost of the catalysts can be

improved.

Limitations

These late-transition metal catalysts are not immune to interaction with functional

groups, as example by the isolation of multiple chelates of functionalized olefins with

ruthenium olefin metathesis catalysts.60,153,154 Hoveyda-type catalysts are an example of

this phenomenon that retains high catalytic reactivity. Concerning the nature of the

functional group tolerance the coordination of the heteroatom to the metal is of central

focus. Reversible coordinations will only delay or slow down the metathesis reaction and

maybe prevent high conversions while irreversible coordination will simply deactivate

the catalyst towards ADMET.60 A typical example is the case ofthioethers, which are

polymerizable with Schrock's molybdenum, but not with the [Ru]1.155 A stable Ru-S

chelate complex is formed as evidenced by the appearance of a new alkylidene by 31P

NMR. The Ru-S bond is expected to be stronger than the Mo-S bond according to the

size and polarizability of the two metals. Even though the ruthenium is tolerant towards a

greater variety of functional groups than [Mo] 1, kinetically, coordinating functionalities

such as ethers and sulfides, influence the rate of polymerization to a much greater extent

with [Ru]l than with [Mo]1.155 Indeed, after phosphine dissociation, heteroatoms can

compete with olefins for the open coordination site and thus retard the rate of metathesis.

Reversible coordination probably occurs both intra and intermolecularly in ADMET due

to the high monomer concentrations in bulk conditions. This effect is not as significant in

the case of molybdenum catalysts probably because of the steric influence of the bulky

ligand sphere.









Hydrogenation

The ruthenium catalysts feature the possibility to be converted into hydrogenation

catalysts, which can be attractive for the one-pot synthesis of branched polyethylene by

sequenced ADMET polymerization-hydrogenation. Heterogeneous hydrogenation could

be performed by simple addition of chromatographic silica to the diluted polymerization

mixture under 200-500 psi of hydrogen.121 Copolymers of ethylene-styrene, ethylene-

vinyl chloride and ethylene-acrylate were prepared this way.133 Even though many

examples have shown that ruthenium carbenes [Ru]l and [Ru]4 could be transformed

into homogeneous hydrogenation catalysts, this strategy has not been tested during

ADMET polymerization.156-159 The homogeneous catalyst could then be washed away by

precipitation of the saturated polymer.

Kinetics

Kinetics in ADMET can hardly be monitored by NMR since ADMET requires bulk

conditions (to limit cyclization) while NMR operates in diluted solution of deuterated

solvents. So the best way, so far, to measure ADMET kinetics is to measure the volume

of ethylene released, which is directly correlated to the degree of polymerization. Even

though this method does not reflect ADMET kinetics under vacuum, it allows the

comparative study of different catalysts, which was first implemented with [Mo] 1 and

[Ru]l by Brzezinska et al. 155 At early conversions, i.e. before the viscosity grew so high

that the rate becomes diffusion controlled, ADMET follows a quasi-second order rate.

[Mo]l was found to catalyze ADMET of 1,9-decadiene about 50 times faster than [Ru]l.

Nevertheless, [Ru]4 promotes ADMET faster than [Ru]1 at temperatures of 45 and

higher.144 An induction period marks the slow phosphine dissociation of this NHC

containing catalyst; however, the greater affinity of the 14e- complex for Xt-olefinic









substrates combined with a slower phosphine rebinding allow for higher molecular

weights polymers. Hence, phosphine dissociation/rebinding plays a major role in the

ADMET mechanism, the ultimate concern being the molecular weight of the polymer,

not the initial metathesis rate.

Olefin Isomerization
olefin
isomerization


Figure 1-19. Structural olefin isomerization of 1-octene.

In spite of their high reactivity in metathesis chemistry, these ruthenium catalysts

were suspected to undergo non-metathesis transformations.160-165 Among them, alkene

isomerization has emerged as an important side reaction of ruthenium catalyzed

metathesis, disturbing the product's microstructure by apparent migration of the double

bond along the alkyl chain (Figure 1-19).166 First observed on substrates containing

allylic functionalities in combination with first generation catalysts,167-171 double bond

isomerization has since been reported with 2nd generation catalysts on a broad variety of

substrates competitively and sometimes prior to olefin metathesis.114'173-176 More

recently, olefin isomerization was evidenced in ADMET catalyzed by [Ru]4, by the

MALDI-TOF analysis of an amino-acid branched polymer.177 Whether it occurs on the

monomer, or on the polymer followed by trans-metathesis, isomerization during ADMET

results in irregular microstructures presenting altered properties (Figure 1-20).

Noteworthy, under ADMET conditions, first-generation [Ru]l was found not to

isomerize olefins.178











R [Ru] R
Ar flow -l M C2H4
m
ethylene


RCM R


RR




others
R


Figure 1-20. Olefin isomerization during ADMET polymerization

Olefin isomerization is promoted by a variety of metal complexes and has been


known to operate via two distinctive pathways: the 71-allyl hydride mechanism involves a


metal in a low oxidation state that forms an r?-allyl intermediate upon coordination of the


olefin, while the metal hydride addition-elimination involves a long-lived metal hydride


that forms an alkyl intermediate (Figure 1-21).179 However, in the case of ruthenium


metathesis catalysts, the intermediate responsible for this undesirable reaction has not


been identified yet, although it is often attributed to the formation of a ruthenium hydride


in situ, as a decomposition product of the original carbene catalyst. 180,81


H H

I
H H
R


M HH H H H ,CHH
* M -M"0 _---M
H CH2 RH H H
R


b) H H
LnMH + C
H CHR
R


H H CH3
ML,, LnM
H CH2 CH2
R R


H CH3
LnMH + X
R H

H H
LnMH +
H CHz
I
R


Figure 1-21. Olefin isomerization mechanism: a) 7r-allyl mechanism. b) metal-hydride
addition-elimination mechanism














CHAPTER 2
METATHESIS ACTIVITY AND STABILITY OF NEW GENERATION
RUTHENIUM POLYMERIZATION CATALYSTS

Reproduced in part with permission from: Courchay, F. C.; Sworen, J. C.; Wagener, K.
B. Macromolecules 2003, 36, 8231-8239.
Copyright 2003 American Chemical Society

Introduction

The recent development of ruthenium carbene complexes (Figure 2-1) has

increased considerably the utility of olefin metathesis both in the fields of organic

synthesis and polymer chemistry. The introduction of the well-defined, functional-group

tolerant complex (PCy3)2(Cl)2Ru=CHPh [Ru]l by Grubbs and coworkers37'57'80'87

stimulated researchers' interest towards late-transition metal-catalyzed metathesis

chemistry, for it is a useful catalyst in a variety of ring-closing metathesis (RCM), cross-

metathesis (CM), acyclic diene metathesis polymerization (ADMET) and ring-opening

metathesis polymerization (ROMP) reactions.7-10,20'31 Improvements on this "first

generation" ruthenium structure have been reported by many groups in recent times. For

example, the Grubbs research group reported that incorporation of the more bulky and

strongly sigma-donating imidazolylidene ligand leads to higher activity and thermal

stability,71 enabling the preparation of c-functionalized, di-, tri- and tetrasubstituted

olefins.12 These imidazolylidene based catalysts behave in a manner similar to that of

early transition metal catalysts such as Schrock's molybdenum catalyst [Mo] 1,27,28 and

remain active at loadings as low as 0.05 mol% for RCM and 0.0001 mol% for ROMP.71









Additional ligand variations have led to further improvements in both catalytic

activity and stability. For example, Mol and coworkers101 reported improved turnovers

during the cross metathesis of terminal olefins with a modified version of the

imidazolylidene ligand, catalyst (IPrH2)(PCy3)(Cl)2Ru=CHPh [Ru]10, first synthesized

by Firstner.100 Also, Hoveyda et a.95 have developed the complex

(H2IMes)(Cl)2Ru=CH(o-iPrOC6H4) [Ru]9 which promotes olefin metathesis by a unique

"release-return" mechanism, allowing efficient metal recovery without significant loss of

activity. In fact, complex [Ru]9 can readily be used in combinatorial synthesis in air

with reagent-grade solvents. This Ru-complex offers reactivity, chemo- and

stereoselectivity profiles differing from both catalysts [Ru]l and [Ru]4, notably during

the metathesis of electron-deficient olefins.96-98 The robustness of this complex also

allows its use in solid-supported heterogeneous catalysis.102-107,181



/\ ciy/,, / N-cj *<
Clf "Ph "u= Ru=\
PCy3 CIC 'Ph I u Ph
PCy3 PCy3
[Ru]1 [Ru]4 [Ru]10



NN N- / \ HC
Ru CII Ru F MO|

SCF3
[Ru]9 [Ru]11 [Mo]1

Figure 2-1. Metathesis catalysts

Acyclic diene metathesis (ADMET) chemistry has enjoyed the benefit of this

metathesis catalyst research, resulting in the synthesis of unique architectures and new

functionalized materials,121-133'141-143 such as perfectly branched polyethylene,127'128'130









and amino acid containing polyolefins.146,147 The choice of catalyst to perform these

transformations is dictated by the monomer structure and the reaction conditions. For

example, Schrock's catalyst is best used for hydrocarbon dienes and some functionalized

olefins, whereas Grubbs' catalysts are preferred with highly-functionalized monomers

and at higher temperatures. ADMET, CM and RCM require longer reaction times than

ROMP chemistry, and so catalyst entities with longer lives are preferred. Consequently,

our group has a particular interest in complexes that exhibit both long lifetimes and high

turnovers ratios. A new catalyst structure (complex [Ru]ll) was prepared by combining

the thinking that yielded the Grubbs, Hoveyda and Mol complexes, and was found to

exhibit improvements in terms of stability under ADMET conditions.

A kinetic approach was used to investigate the activity and utility of these

ruthenium complexes during ADMET polymerization, work which has led to the design

of more efficient ADMET catalyst systems.155 Previously, Lehman and Wagener144

reported a comparative kinetic study of Grubbs' catalysts [Ru]l and [Ru]4 during the

ADMET polymerization of the benchmark monomer, 1,9-decadiene, research that

showed the effect of temperature on activity and initiation rates. Catalyst [Ru]4 is clearly

more active if used at temperatures of 45 C and above. The olefin isomerization activity

of catalysts [Ru]l and [Ru]4 was investigated as well,176 where catalyst [Ru]4 was found

to isomerize both external and internal olefins if used at its optimal temperature for

ADMET, 60 C. This observation is important to consider when building precise

polymer microstructures.

Herein the early kinetic work is expanded to include complexes [Ru]10, [Ru]9 and

the new complex [Ru]ll, comparing them with the 1st and 2nd generation Grubbs'









catalysts.144 Initial rates of ADMET polymerization were measured at different

temperatures, and the stability and activity of these complexes were examined at longer

reaction times. Finally, their propensity to induce olefin isomerization was investigated

under polycondensation conditions.

Results and Discussion

Kinetic Assessment of Catalyst Activity.

ADMET polycondensation of a, co-dienes releases ethylene as the condensate, the

volume of which can be measured and used to calculate the average degree of

polymerization (DP) of the monomer as a function of time. The kinetic experiments were

run for approximately an hour, focusing on the oligomerization (not polymerization) of

the monomer. The monomer 1,9-decadiene was chosen in this study because of its well-

understood behavior during ADMET chemistry. Monitoring oligomerization by plotting

DP versus time allows for an efficient method to compare the catalytic activity for

various complexes. Table 1 displays the initial rates of 1,9-decadiene ADMET

chemistry, calculated as the initial slope (DP vs. time) of each kinetic curve during the

average dimerization (DP=2) of the monomer.

Table 2-1. Initial rates for complexes [Ru]l-[Ru]ll during the ADMET oligomerization
of 1,9-decadiene
Monomer:Catalyst Initial Rate (10-3 DP s-1)
Catalyst Ratio 30 oC 45 OC 60 C
[Ru]la 450:1 1.0 + 0.5 2.3 + 0.4 5.1 + 0.6
[Ru]4a 450:1 0.42 + 0.05 4.4 + 0.7 23 + 3
[Ru]10 450:1 36 1 39 7 55 + 10
[Ru]10 900:1 8.8 4.0 39 1 78+ 10
[Ru]9 450:1 4.8 3 18 5
[Rull 450:1 12 2 42 4 68 + 10
[a] Data obtained from reference 144.










Catalytic activity was measured at three temperatures (30 C, 45 C and 60 C) and

two catalyst loadings (450:1 and 900:1 monomer:catalyst), providing a broad basis set to

evaluate the catalyst systems tested. This activity was compared to the widely used 1st

and 2nd generation ruthenium catalysts, previously examined in ADMET conversions by

144
our group.








5 [Ru]1 [Ru]10
l 0



S- N [ N[Ru]4



0 1000 2000 3000 4000 5000 6000 7000
Time (s)


Figure 2-2. Degree of polymerization (DP) vs. time curves for complex [Ru]1, [Ru]4
and [Ru]10 using 450:1 monomer:catalyst ratio at 30C

The kinetic runs at 30 OC show that the Mol catalyst, complex [Ru]10, exhibits the

highest activity of any ruthenium complex to date in the dimerization of 1,9-decadiene.

Figure 2-2 illustrates these data. When compared to the benchmark catalyst [Ru]1,

complex [Ru]10 shows a 36-fold increase in activity at 30 OC (see Table 1). This low

temperature activity makes catalyst [Ru]10 an ideal candidate for controlled organic

synthesis and polymerization applications. Further, the initial dimerization rate when

using complex [Ru]10 is greater than either complex [Ru]l or [Ru]4 at both 45 and 60

C. For example, complex [Ru]10 affords a dimerization rate of 55x10-3 DP s-1, twice

that when using complex [Ru]4 at 60 OC; in fact, the high activity of complex [Ru]10 at










60 C makes it difficult to accurately measure the evolution of ethylene, resulting in a

large error in the measured DP.

This higher activity of complex [Ru]10 most likely originates from the increased

steric bulk around the ruthenium center, a result of the isopropyl groups present on the

ligand. Apparently, the interaction of the isopropyl groups with the carbene fragment

and/or the chloro groups allows for faster and earlier dissociation of the tricyclohexyl

phosphine, which has been commonly accepted as the initial rate-determining step of the

metathesis reaction. This statement can be confirmed by comparing the 31P NMR

spectrum of complex [Ru]10 versus either complex [Ru]l or [Ru]4. The phosphine

resonance is shifted from 32 ppm for [Ru]4 to 28.1 ppm for [Ru]10, where this upfield

shift indicates a weaker Ru-P bond, implying an easier dissociation of the phosphine.

Also, the induction period observed with [Ru]4 is not observed with [Ru]10, which is

consistent with complex [Ru]10's faster phosphine dissociation. The induction period is

thought to correspond to the initial dissociation of phosphine before the steady-state

concentration of the 14 electron (e-) ruthenium alkylidene complex is reached.


12-

10- 30 C

8-

6-
4- /
4 _45 0C c P
4 60 'C pcy"
2


0 1000 2000 3000 4000 5000 6000
Time (s)


Figure 2-3. Comparison of the activity of complex [Ru]10 at three different temperatures
using a 450:1 monomer:catalyst ratio










While complex [Ru]10 shows the largest decadiene dimerization rate for any

complex to date, increasing temperature has an adverse effect on the lifetime of the

catalyst (Figure 2-3). Complex [Ru]10 no longer competes with either [Ru]l or [Ru]4 at

temperatures above 45 C beyond trimerization of 1,9-decadiene. Figure 2-4 further

illustrates this phenomenon; these lower turnovers were also reported by Mol at elevated

temperature.100'101 This decrease in conversion likely is due to the thermal decomposition

of the complex as observed with Grubbs' catalysts,87 or by way of C-H and/or C-C

activation pathways.182,183 Decomposition may occur faster with complex [Ru]10 for 2

reasons: 1) the rate increase in phosphine dissociation is known to affect the catalyst

stability by generating too many unstable 14 e- species; 2) the presence of an electron rich

methine proton on the isopropyl groups makes it more likely to undergo C-H activation

than the methyl protons of complex [Ru]4. Moreover, the additional carbons on each

isopropyl group may increase the probability of ruthenium insertion into the C-C bond.


8-
7- [Ru]4
6-
5-
[Ru]10
4-
3
2
1-

0
0 1000 2000 3000 4000
Time (s)



Figure 2-4. Degree of polymerization (DP) vs. time curves for complexes [Ru]4 and
[Ru]10 using 450:1 monomer:catalyst ratio at 45 C

On the other hand, this temperature trend is inverted when the monomer-to-catalyst

ratio is doubled (Figure 2-5). Indeed, higher conversions are reported when the










temperature is increased from 30 to 60 OC with its average DP reaching 4 and 5.8,

respectively. In addition, complex [Ru]10 is more active initially at 60 OC when using

lower catalyst ratios; the data are reported in Table 1. For example, when the catalyst

concentration is decreased by a factor of two, the respective rate increases from 55x10-3

to 78x10-3 DP s-1. The increase in the overall activity observed at high temperature when

lower catalyst concentrations are used may correspond to a decrease in the catalyst

decomposition rate, which would in turn support a bimolecular decomposition pathway.7

However, at this point the possibility of an internal decomposition as outlined above

cannot be ruled out. More likely, this would prove the coexistence of both mechanisms.

Further, the reaction rate for complex [Ru]10 at 300C is decreased by a factor of 4, which

is consistent with the lower catalyst ratio as little decomposition should occur at such low

temperatures. Besides, this activity trend has been reported by Mol101 as well in the

dimerization of terminal olefins.


8-

7-

6 60 oC

5- N N
645oC
4-30 oC -C ,,Ru
3- Ph
PCy3
2-

1-

0 1000 2000 3000 4000 5000 6000
Time (s)



Figure 2-5. Degree of polymerization (DP) vs. time data for complex [Ru]10 using 900:1
monomer:catalyst ratio







40


Although known for its particular tolerance to a variety of substrates,95-98 complex

[Ru]9 appears to be unstable once dissolved in 1,9-decadiene, for complex [Ru]9 affords

low conversions at 30 C and 45 C (Figure 2-6). Consequently, no experiment was

conducted at 60 OC with complex [Ru]9.






9- CIl


THF, 45 C


3 45 5oC

2-
0 I

0 1000 2000 3000 4000 5000
Time (s)



Figure 2-6. Degree of polymerization (DP) vs. time data for complex [Ru]9 using 450:1
monomer:catalyst ratio

The low stability of complex [Ru]9 may be attributed to its release-return

mechanism, yielding an unstable 14 e- species by the loss of isopropoxy styrene, which in

turn produces the unbound ligand and/or its subsequent attachment to the chain end. We

assume that the low stability of complex [Ru]9 is due to this highly unstable 14 e-

intermediate not being restabilized by the isopropoxy ligand. This 14 e- complex may

have to be stabilized by coordination to the substrate (monomer) or solvent, and since

1,9-decadiene lacks any kind of electron donor or Lewis basic group, it is not able to

serve as a stabilizing ligand. Consequently, the catalyst apparently decomposes

immediately after activation. To verify this assumption, the same experiment was









conducted by adding a catalytic amount of THF to the bulk reaction mixture. As is

apparent in Figure 2-6, a drop of THF improved the activity of complex [Ru]9 when

polymerizing 1,9-decadiene, which refutes the general belief that ruthenium catalysts

exhibit lower metathesis activity in THF.34

These observations led us to synthesize complex [Ru]ll, which in effect combines

the thinking of Grubbs, Mol and Hoveyda. The complex was obtained in good yields by

implementing the same synthetic procedure used to generate complex [Ru]9,95 simply

substituting complex [Ru]10 as one of the starting compounds (Figure 2-7).

Conveniently, [Ru]11 can be purified in the air affording a dark green crystalline solid

after flash chromatography and precipitation in hexane, where the copper-phosphine

complex must be removed by filtration prior to column chromatography.




CT N R 1.03 equiv CuCI C N /R
CIIII.Ru cII,.--
CItr / CH2CI12 Cil \
PCyI ) 40 C, 20min

[Ru]10 [Ru]11
& 97%

Figure 2-7. Synthesis of complex [Ru]ll

The structure was confirmed through single-crystal X-ray analysis (Figure 2-8).

Selected bond lengths and angles of both complexes [Ru]9 and [Ru]ll are provided in

Table 2 for comparison. Complex [Ru]ll is characterized by a distorted trigonal

bipyramidal coordination about the metal center, with the alkylidene group and the

chlorines in the same plan. The overall geometry and most of the bond angles and bond

lengths are analogous to their related values in complex [Ru]9. As the bond angles show,

the geometry of [Ru]ll is a little more distorted, which was expected considering the









bulk brought about by the isopropyl groups. Noteworthy, the Ru-O bond is much shorter

in complex [Ru]11, which indicates a stronger coordination.


Figure 2-8. ORTEP Diagram of complex [Ru]ll.
30% probability level


Thermal ellipsoids are drawn at the


Table 2-2. Bond Lengths and Angles for Complexes [Ru]9 and [Ru]ll


Ru-C(1)
Ru-C(2)
Ru-O
Ru-CI(2)
Ru-Cl(1)
N(1)-C(2)
N(2)-C(2)


C(1)-Ru-C(2)
C(1)-Ru-O
C(2)-Ru-O
C(1)-Ru-Cl(2)
C(2)-Ru-Cl(2)
O-Ru-Cl(2)
C(1)-Ru-Cl(1)
C(2)-Ru-Cl(1)
O-Ru-Cl(1)
Cl(2)-Ru-Cl(1)
C(2)-N(1)-C(3)
C(2)-N(2)-C(4)


[Ru]9 [Ru]ll
Bond I, Agh (A)
1.828(5) 1.822(5)
1.981(5) 1.978(5)
2.261(3) 2.247(3)
2.328(12) 2.3271(15)
2.340(12) 2.3409(14)
1.350(6) 1.362(6)
1.351(6) 1.363(6)

Bond Angles (deg)
101.5(14) 102.0(2)
79.3(17) 78.60(18)
176.2(14) 171.94(17)
100.1(15) 99.57(15)
90.9(12) 86.24(15)
85.3(9) 85.75(10)
100.2(15) 100.20(15)
96.6(12) 101.29(15)
86.9(9) 86.44(9)
156.5(5) 156.84(6)
127.2(4) 126.4(4)
127.0(4) 124.6(4)







43


The activity of this new complex was evaluated using the same kinetic study as

done previously with 1,9-decadiene. Contrary to its predecessor [Ru]9, complex [Ru]ll

catalyzes the ADMET oligomerization of 1,9-decadiene, as shown in Figure 2-9. The

shape of the kinetic curves is the same as observed for the parent complex [Ru]10, and

the overall activity is less a function of temperature. There is no definite trend, but 45 C

seems to be an optimal temperature. On the other hand, the initial oligomerization rate

when using complex [Ru]ll is more sensitive to temperature variations than it is for

catalyst [Ru]10, reaching a maximum of 68x10-3 DP s-1 at 60 OC. Thus, higher

temperatures are recommended to get fast initiation using this particular complex.

Moreover, catalyst [Ru]ll exhibits faster initial rates than [Ru]9, which is consistent

with the introduction of the bulkier isopropyl groups on the imidazolylidene ligand. In

fact, complex [Ru]11 produces high molecular weight poly(octenamer) whereas complex

[Ru]9 only yields oligomers, even after 120 hours in the bulk.








2-
11 r

9- I I.Ru

7- \ -C
4- 604C
300C





0 500 1000 1500 2000 2500 3000 3500
Time (s)



Figure 2-9. Degree of polymerization (DP) vs. time data for complex [Ru]ll using 450:1
monomer:catalyst ratio









Interestingly, complex [Ru]ll does not need THF to catalyze the ADMET reaction

of 1,9-decadiene. To better understand the differences between catalysts [Ru]9 and

[Ru]11 and the role of polar reaction conditions on these Hoveyda-type catalysts, kinetic

experiments were completed using dipentenyl ether as the monomer instead of 1,9-

decadiene. Dipentenyl ether was chosen for its inherent electron donating ability; it

serves as both monomer and solvent, possibly allowing coordination to stabilize the

reactive intermediate through a six-membered ring transition state (Figure 2-10).


F_\ f-\
N N N


ci11



[Ru]9 &

Figure 2-10. Proposed intermediate of complex [Ru]9 during the metathesis of
dipentenyl ether.

Surprisingly, the activity of complex [Ru]9 with dipentenyl ether is actually lower

than with 1,9-decadiene using the same kinetic conditions, suggesting that dipentenyl

ether intramolecular complex is not sufficiently labile to allow coordination of other

monomer molecules, thereby preventing further continuation of the metathesis process.

Experiments conducted using complex [Ru]ll (Figure 2-11) also show a decreased

activity of about 1 DP s-1 where the initial rate drops from 42x10-3 DP s-1 (1,9-decadiene)

to 15x10-3 DP s-1 (dipentenyl ether). The fact that a similar behavior was obtained with

both complexes [Ru]9 and [Ru]ll suggests that these Hoveyda-type systems are

sensitive to substrate polarity, and may give better results in solution with solvents of

lower polarity. Another possibility could be that this particular ether is too sterically

hindering to effectively stabilize the Ru center.











7-

6 [Ru]11 (1,9-decadiene)




4-

3-

2-
[Ru]9
1 -


0 500 1000 1500 2000 2500 3000 3500
Time (s)



Figure 2-11. Degree of polymerization (DP) vs. time data for complexes [Ru]9 and
[Ru]ll using dipentenyl ether as the monomer (450:1 monomer:catalyst)

Overall, [Ru]ll is a better catalyst than [Ru]9 for the ADMET oligomerization of

either 1,9-decadiene or dipentenyl ether, although slightly less active towards the ether

monomer. Considering the hypothesis of a 14e- intermediate as proposed for the parent

complex [Ru]9, the enhanced activity of [Ru]ll can be explained in terms of olefin

affinity. The isopropyl groups of the imidazolylidene should exert a stronger steric

pressure on the carbene moiety and favor the formation of a metallacyclobutane with the

incoming olefin. The inherent olefin affinity offers an ideal stabilization to the 14e-

intermediate and could account for the higher activity. In fact, the crystal structure of

[Ru]ll shows that the Ru-C bond with the N-heterocyclic carbene is slightly shorter than

for complex [Ru]9.95 This suggests a higher affinity of complex [Ru]ll for 21-acidic

olefinic substrates. The Ru-O is also found to be shorter in complex [Ru]ll, meaning the

coordination of the oxygen to the ruthenium is stronger than in complex [Ru]9.

Therefore, the increased stability of [Ru]11 could be due to a higher tendency of the









isopropoxystyrene to come back and stabilize the 14e- intermediate. While the activity of

complex [Ru]11 at low conversion does not compete with the activity of complex

[Ru]10, its long term stability, described below, renders it an attractive metathesis

catalyst.

Thermal Stability of the Catalysts

The decomposition rates of complexes [Ru]10 and [Ru]ll were monitored by 1H-

NMR in d6-benzene at 55 C and calculated as the time for half the complex to

decompose. As observed during the kinetic study, complex [Ru]10 is not resistant to

high temperatures, exhibiting a half-life of 90 min at 55 C. On the contrary, complex

[Ru]ll remains completely intact at 55 C for more than 3 days, showing no sign of

decomposition whatsoever (Table 3). This complex is not only stable in air but also in

solution even at elevated temperatures. This increased stability is desirable for RCM,

CM and ADMET chemistry in solution and may also be useful for solid state ADMET

polymerization.184

Table 2-3. Half-lives for the decomposition of complexes [Ru]10 and [Ru]11
Complex Temp. Conc. Half-life
[Ru]la 55 OC 0.023M 8 days
[Ru]10 55 OC 0.032M 90 min
[Ru]9 55 OC 0.032M >3 days
[a] Value taken from Grubbs et al. 37

Further Comments on ADMET Polymerization

In order to expand these kinetic oligomerization measurements made using 1,9-

decadiene, the utility of the complexes was investigated during full-scale polymerization

of this monomer. These polymerizations were conducted using standard Schlenk

techniques, with complexes [Ru]10 and [Ru]ll at 55 oC using both 450:1 and 900:1









monomer:catalyst ratios (Table 4). The resultant poly(octenamer)s were characterized by

1H and 13C-NMR and further analyzed by GPC and DSC. Molecular weights were

compared to poly(octenamer)s obtained with the catalyst systems [Ru]1, [Ru]4 and

[Mo]l.

Table 2-4. Polymerizations of 1,9-decadiene at 55 C for 120 h
Monomer:Catalyst PDI
Catalyst ratio An (g/mol)

[Ru]l 450:1 12,000 1.8
[Ru]4 450:1 78,000 1.8
[Mo]l 1200:1 15,000 1.7
[Ru]10 450:1 29,000 1.6
[Ru]10 1000:1 37,000 1.9
[Ru]ll 450:1 21,000 1.6
IRull 1000:1 32,000 1.6

When the polymerizations were heated at 55 C for an extended period,

poly(octenamer) was obtained withMn of 37,000 g/mol using complex [Ru]10 and

32,000 g/mol using complex [Ru]ll. This is an encouraging result, as the benchmark

Shrock's molybdenum catalyst (6) for ADMET polymerization produces a polymer half

that average mass, or Mn= 15,000 g/mol poly(octenamer) under the same conditions.

Both are acceptable molecular weights in step polymerization chemistry, especially for

functionalized versions of polyolefins made by this route. Endgroup conversion

increases from 99.1% to 99.6% in this doubling of molecular weight, clearly illustrating

how clean mechanistic chemistry is essential in any step polymerization. Further, the use

of complex [Ru]10 or [Ru]11 leads to successful polymerization with lower catalyst

loadings (0.05 mol%).

Complex [Ru]9 was not included in this polymerization study since initial

experiments at 30 C in the bulk produced only oligomeric poly(octenamer), an

observation consistent with the model kinetic study. However, the situation differs when









functionalized dienes are used in a polar medium as this catalyst was successful in

polymerizing amino-acid containing monomers in THF.131

Isomerization of the Olefin Bond

Olefin isomerization is a major side reaction of Ru-catalyzed metathesis; however,

the species) responsible for such transformation, whether it is a decomposition product

or the metathesis catalyst itself, have not been fully identified yet.176'185 Studying

isomerization therefore occupies a significant part in the process of understanding

catalyst decomposition. Besides, under ADMET conditions olefin isomerization

generates irregular polymer microstructures, and since part of our group research focuses

on precision in polymer synthesis, the isomerization activity of complexes [Ru]10 and

[Ru]ll was also explored in detail. The isomerization study was done using a model

compound, 1-octene as the starting reagent, and the reaction was monitored by GC

analysis. Typical results are illustrated in Figure 2-12 for complex [Ru]10. In fact, both

complexes [Ru]10 and [Ru]ll isomerize the olefinic bonds in the reactant as well as in

the metathesis products.


90%-


I M





C2H4 + Isomers 0r
5 1 20 30 50 so 258
Time (min)

Figure 2-12. Composition of olefin mixture as a function of time for reaction of [Ru]10
with 1-octene at 300C










If self-metathesis were the only mechanism operating in the presence of these

catalysts, then this model reaction would yield 7-tetradecene exclusively (C14 in the

figure above). Figure 2-12 shows that not to be the case, where the concentration of

isomerized products increases at the expense of the product, 7-tetradecene and the

starting reagent, 1-octene. For example, exposure of 1-octene to complex [Ru]10 at

room temperature leads to 94% conversion (1 hour), where the product mixture consists

of 82% 7-tetradecene and 18% isomers. By comparison, complex [Ru]4 is less active in

this model study, requiring 3 days at room temperature to generate a similar conversion

(86%). Complex [Ru]4 is slower to isomerize olefins, however, yielding a mixture

containing only 3% isomers. Longer run times using complex [Ru]10 at room

temperature result in considerably higher quantities of isomer content, up to 76% of the

final product mixture. The starting reagent, 1-octene, is completely consumed, and the

distribution of isomerized products (as determined by GC analysis) is centered about 7-

tetradecene.


M
kN4 + C2H4





TM 1-octene TM1-octene 3


+
STM-octene5 + C 7

etc.

OR
Co + C46 13
+ +


etc.

Figure 2-13. Proposed mechanism for isomerization of 1-octene









These observations are consistent with isomerization chemistry operating

concurrently with metathesis, although at a slower rate than the initial condensation

metathesis chemistry, which yields 7-tetradecene. In fact, three different reaction

mechanisms operate concurrently throughout the conversion of 1-octene: condensation

metathesis (we abbreviate this reaction as "M"), trans-metathesis (involving both internal

double bonds and active metal alkylidene chain ends) (abbreviated "TM"), and

isomerization chemistry ( abbreviated "I") (Figure 2-13).

These three reactions compete until condensation metathesis is complete, after

which only trans-metathesis (TM) and isomerization (I) continue. If the catalyst entities

possess sufficiently long lives as appears to be the case for complex [Ru]10, then the

starting reagent, 1-octene, will be completely consumed, and an equilibrium mixture of

isomerized products will result. Complex [Ru]ll approaches this situation, with

conversions of 98% and a lower percentage of isomerization (23%) after 3 days. These

results are consistent with the data of the kinetic study, and the product/isomer

distribution indicates that isomerization occurs concurrently with metathesis. The

relative rates of condensation metathesis, trans metathesis, and isomerization apparently

differ depending upon the identity of the metal catalyst complex, with isomerization

being significantly slower than trans-metathesis, and trans-metathesis being slower than

condensation (end group) metathesis.

Since isomerization occurs at a slower rate than either metathesis reaction with

complex [Ru]10, lower temperatures should suppress this reaction to the advantage of

metathesis chemistry. This has been shown to be true in prior research,176 and the same

observation is noted here when using catalyst [Ru]10. In fact, the conversion of 1-octene









is quantitative at 0 OC in 3 days, with only 30% of the product mixture being isomers, as

compared to 76% isomers at room temperature.

Recently, Mol et al. 92 demonstrated that a possible decomposition pathway of

catalyst [Ru]l results in the formation of (PCy3)2(CO)Ru(C1)(Ph) which appears to be a

good precatalyst for olefin isomerization. This carbonyl complex could be converted into

the corresponding hydride (PCy3)2(CO)Ru(C1)(H) upon reaction with a terminal olefin

and could promote 1-octene isomerization without any trace of metathesis product.

Further, Grubbs and coworkers72 witnessed the formation of the hydride complex

(H2Mes)(PCy3)(CO)Ru(C1)(H) by heating complex [Ru]4 in the presence of oxygen-

containing substrates. These prior observations suggest that the ruthenium hydride

equivalent of [Ru]10 could be formed under ADMET reaction conditions and could

account for the isomerization chemistry reported herein. The rapid decomposition of

complex [Ru]10 observed during our prior kinetic study is consistent with isomerization

occurring early in the reaction and increasing over time, all as a result of increasing

ruthenium hydride concentration. However, the possibility of isomerization catalyzed by

the metathesis catalyst itself cannot be ruled out at this point.

Complex [Ru]10 is sufficiently active to induce isomerization of the starting

reagent, 1-octene to other Cs isomers (2-, 3- and 4-octene), which happens concurrently

with metathesis chemistry, and further, complex [Ru]10 appears to induce isomerization

of internal bonds at a substantially higher rate than other catalysts.

Additional evidence for the high propensity of catalysts [Ru]10 and [Ru]ll to

isomerize internal olefins is found in the melting behavior of the materials produced

when using them to polymerize 1,9-decadiene. Schrock's molybdenum catalyst









(complex 6), which yields virtually no isomerization, produces poly(octenamer) that

exhibits a melting point of 750C,176 close to the theoretical maximum for this polymer.

On the other hand, catalysts [Ru]10 and [Ru]ll yield poly(octenamer) samples

possessing considerably lower melting points, 40 oC and 45 OC respectively.

Isomerization, in effect, creates random copolymers via migration of internal olefins

along the polymer backbone or via monomer isomerization. This decrease in

regioregular placement of olefinic groups in the poly(octenamer) results in a depression

of the polymer peak melting point as observed for both catalyst [Ru]10 and [Ru]ll.

Conclusions

Complex [Ru]10 is highly active at low temperatures for the ADMET

polymerization of 1,9-decadiene; loadings as low as 0.1 mol% produced high molecular

weight poly(octenamer). In addition, complex [Ru]10 has the highest initiation rate of

any phosphine ligated ruthenium complex to date. The catalyst is sensitive to thermal

decomposition and therefore becomes less active as the temperature increases. The high

initiation rate and activity at low temperatures make complex [Ru]10 an interesting

prospect for further metathesis considerations, notably for low boiling point reagents or

thermally sensitive substrates in both organic synthesis or polymerization. On the

contrary, complex [Ru]9 does not exhibit satisfactory activity with non functionalized

substrates. Although known for its robustness, complex [Ru]9 appears to require

stabilization either from a polar solvent or from a functionalized monomer in order to

serve as a viable polycondensation catalyst. Complex [Ru]11 exhibits an interesting

stability and produces high molecular weight polymers. The increase of steric

interactions on the 4,5 dihydroimidazolylidene carbene ligand increases the catalytic









activity, while the incorporation of Hoveyda-type ligand expands the stability of the

complex.

Complexes [Ru]10 and [Ru]ll both catalyze olefin isomerization in high yields,

however, given that both complexes are highly reactive at room temperature their

isomerization/metathesis processes can be mediated by lowering the reaction

temperature. In fact, complex [Ru]10 efficiently metathesizes 1-octene at 0 OC resulting

in faster metathesis rates than isomerization. This isomerization pathway could be

attributed to the hydride complex, whose formation and/or catalytic activity is highly

dependent on temperature conditions. Therefore, increasing the catalyst stability towards

decomposition seems most important to design catalysts that will produce regular

polymers via metathesis chemistry. Nonetheless, complexes [Ru]10 and [Ru]ll are

useful catalysts for ADMET polymerization, without the temperature constraints of either

catalysts [Ru]l or [Ru]4.

Experimental

General Considerations

1H NMR (300 MHz) and 13C NMR (75 Hz) spectra of the ADMET polymers were

recorded in CDC13 on either a Mercury series or Varian VXR-300 NMR superconducting

spectrometer. Chemical shifts were referenced to residual CHC13 (7.23 for 1H and 77.23

for 13C) with 0.03% v/v TMS as an internal reference. All organometallic spectra were

recorded in d6-benzene and the chemical shifts were referenced to residual C6H6 (7.16 for

H and 128.39 for 13C).

Gel permeation chromatography (GPC) of the unsaturated ADMET polymers was

performed using two 300 mm Polymer Laboratories gel 5tm mixed-C columns. The

instrument consisted of a Rainin SD-300 pump, Hewlett-Packard 1047-A RI detector









(254 nm), TC-45 Eppendorf column heater set to 35 C, and Waters U6K injector. The

solvent used was THF at a flow rate of 1.0 mL/min. Polymer samples were dissolved in

HPLC grade THF (approximately 0.1% w/v) and filtered before injection. Retention

times were calibrated to polystyrene standards from Polymer Laboratories (Amherst,

MA).

GC analysis was conducted on a Shimadzu GC-17A chromatograph equipped with

a HP-5 (Hewlett Packard) 25 m column with FID detection. All amounts of products

were calculated by a systematic ratio to the Decalin peak at 8.72 minutes, using the

integrated peak areas.

Differential scanning calorimetry (DSC) was performed using a Perkin-Elmer DSC

7 at a heating rate of 10 C/min. Thermal calibrations were made using indium and

freshly distilled n-octane as references for thermal transitions. Heats of fusion were

referenced against indium. The samples were scanned for multiple cycles to remove

recrystallization differences between samples and the results reported are the second scan

cycle. Reported values are given as Tm (melting peak).

Complex [Ru]9 was a gift from Materia Co. and was used as received. Complex

[Ru]10 was synthesized according to the literature procedure.101 All catalysts were stored

in an argon-filled drybox prior to use in kinetic and polymerization experiments. 1,9-

Decadiene (Aldrich) was distilled from Na/K alloy under reduced pressure into a Kontes

flask equipped with a Teflon valve, degassed by three freeze-pump-thaw cycles, and

stored in an argon-filled drybox. For the kinetic study, 1,9-decadiene was portioned into

small Teflon-capped vials in the drybox and were removed and stored in a dessicator

until use. All polymerizations were conducted in the bulk using the different monomer-









to-catalyst ratio displayed in the tables. 2-Isopropoxystyrene was synthesized by a

standard Wittig reaction on 2-isopropoxybenzylaldehyde using

(methyl)triphenylphosphine iodide and potassium t-butoxide (Aldrich). 2-

Isopropoxybenzylaldehyde was synthesized via a Williamson-ether synthesis from

salicyl-aldehyde (Aldrich) and 2-bromopropane (Aldrich). Pentenyl ether was

synthesized according to the literature,123 distilled over CaH2 and stored in the drybox.

All other starting materials were distilled over Na/K alloy before use, except chlorinated

compounds which were distilled over CaH2. After distillation, d6-benzene was degassed

by three freeze-pump-thaw cycles, and stored in an argon-filled drybox.

X-ray Experimental

Crystals suitable for X-ray structure determination were obtained from slow

diffusion of pentane into a saturated solution of [Ru]11 in CH2C12 at room temperature.

Data were collected at 173 K on a Siemens SMART PLATFORM equipped with A CCD

area detector and a graphite monochromator utilizing MoKa radiation (X = 0.71073 A).

Cell parameters were refined using up to 8192 reflections. A full sphere of data (1850

frames) was collected using the co-scan method (0.3 frame width). The first 50 frames

were remeasured at the end of data collection to monitor instrument and crystal stability

(maximum correction on I was < 1 %). Absorption corrections by integration were

applied based on measured indexed crystal faces.

The structure was solved by the Direct Methods in SHELXTL6, and refined using

full-matrix least squares. The non-H atoms were treated anisotropically, whereas the

hydrogen atoms were calculated in ideal positions and were riding on their respective

carbon atoms. The asymmetric unit consists of two chemically equivalent but









crystallographically independent Ru complexes and a half molecule of dichloromethane

(located on a center of inversion). A total of 813 parameters were refined in the final

cycle of refinement using 22770 reflections with I > 2o(I) to yield R1 and wR2 of 6.48%

and 8.38%, respectively. Refinement was done using F2

Synthesis of Complex [Ru]ll

In a glovebox, complex [Ru]10 (930 mg, 1.00 mmol) and CuCl (Aldrich) (100 mg,

1.03 mmol) were weighed into a 100 mL Schlenk flask and dissolved in 20 mL of

CH2C12. 2-Isopropoxystyrene (180 mg, 0.970 mmol) was dissolved in 2 mL of CH2C12

and added to the solution of complex [Ru]10 and CuCl at room temperature. The flask

was equipped with a condenser and the solution was refluxed for 20 minutes. From this

point forth, all manipulations were carried out in air with reagent-grade solvents. The

reaction mixture was concentrated in vacuo to a green residue. The unpurified material

was dissolved in a minimal volume of 1:1 pentane/CH2Cl2 and loaded onto a plug of

silica gel. Insoluble copper-phosphine precipitates were removed prior to loading by

passing the solution through a second Pasteur pipette containing a plug of glass wool.

Elution with 1:1 pentane/CH2Cl2 removed a bright green band from the column. The

eluant was concentrated and addition of hexanes just prior to complete removal of the

solvent resulted in spontaneous precipitation of the product. Filtration and drying under

high vacuum afforded 691 mg (0.970 mmol, 97%) of a pale green powder. 1H NMR

(C6D6, 300 MHz): 6 (ppm)=16.4 (s, 1H, Ru=CHPh), 7.53 (t, 2H, para CH, 3JH,H= 7.2Hz),

7.43 (m, 1H, ortho CH), 7.37 (m, 4H, meta CH), 6.80 (td, 2H, meta-para CH, 3HJH=

7.4Hz), 6.78 (d, 1H, ortho CH, 2JH,H = 8.4Hz), 4.92 (sept, 1H, (CH3)CHOAr, 3HJH=

6.3Hz), 4.18(s, 4H, N(CH2)2N), 3.60 (sept, 1H, CH(CH3)2, 3JH,H = 6.6Hz), 1.38 (d, 6H,









OCH(CH3)2, 2JH,H= 6.3Hz), 1.26 (d, 24H, CH(CH3)2, 3JH,H= 7.1Hz); 13C NMR (C6D6, 75

MHz): 6 = 296.7, 215.5, 149.6, 144.79, 130.0, 129.1, 124.8, 122.4, 122.2, 113.2, 112.1,

112.1, 75.2, 54.9, 29.3, 26.8, 23.9, 21.9.

Kinetic Study

The kinetic study was performed with the same apparatus and following the same

procedure as previously described by Wagener.144 Each catalytic run was repeated 5

times for reproducibility.

ADMET Polymerization of 1,9-decadiene

All glassware was thoroughly cleaned and oven-dried. All metathesis reactions

were initiated inside the drybox using 50 mL Schlenk flasks equipped with a Teflon

stirbar. The flasks were then brought out of the drybox and placed on a high vacuum line

(<10-3 mmHg) while vigorously stirring. The polymerization vessel was exposed to

intermittent vacuum at room temperature until the reaction either became highly viscous

or solid (stirring ceased). The flask was then placed in 55 C oil bath at high vacuum

(<10-3 mmHg) for 120 hours. The polymerization vessel was cooled at room

temperature, and finally, the unsaturated polymer was taken up into toluene and

precipitated into cold acidic methanol (1 M HC1) to remove catalyst residue.

Self-Metathesis Dimerization

A clean, dry 50 mL Schlenk flask equipped with a Teflon stir bar was charged with

complex [Ru]10 or [Ru]ll at the desired ratio in the drybox. The flask was brought out

of the drybox and placed on a Schlenk line. Typically, 4.5 mmol of olefin was injected

through an Ar purged syringe. The mixture was stirred vigorously at the desired

temperature under Ar and open to an oil bubbler to maintain ambient pressure. For

complex [Ru]10, samples were taken at appropriate intervals under a positive pressure of









Ar with a dry pipette and added to a chloroform solution containing traces of 2,6-di-t-

butyl-4-methylphenol used as a radical inhibitor, ethyl vinyl ether used to quench the

metathesis catalyst, and Decalin used as a standard internal reference.

Measurement of the Thermal Stability of Complex [Ru]10 and [Ru]11

Benzene-d6 solutions (32 mM) of complexes [Ru]10 and [Ru]11 were heated to 55

C and monitored by 1H NMR spectroscopy. The half-life was calculated as the time

required for half the material to decompose (cl/2) through integration of the carbene peak

(19.9 ppm for [Ru]10, 16.4 ppm for [Ru]11) which disappears with decomposition of the

complex.














CHAPTER 3
THE UTILITY OF HOVEYDA-TYPE CATALYSTS IN ADMET CHEMISTRY:
STERICS VS. ELECTRONICS

Introduction

Over the past decade, olefin metathesis has become a major synthetic method for

the formation of C-C bonds,1,'27-10,20 especially since the discovery of the Grubbs'

functional group tolerant ruthenium carbene catalyst [Ru]1.37,57,80,87 Its reactivity was

further enhanced by exchanging one phosphine with an N-heterocyclic carbene (NHC)

ligand (complex [Ru]471and [Ru]10100'101). This phosphine mimic, being a strong a-

donor but a weak 7T-acceptor, favors the binding of olefinic substrates to ruthenium,

which results in higher turnovers.64'70'71'74'75'101 Since then, other modified complexes

have been reported (Figure 3-1), among them the Hoveyda-type catalysts bearing an

isopropoxystyrene ligand ([Ru]9).95 The chelating nature of the ligand provides an

exceptional stability to this type of catalyst and allows its recovery after some ring-

closing metathesis (RCM) and cross-metathesis (CM) reactions.186



NN NNN NN N N

CI t Ph CIR C Ph
PCy3 PCy3

[Ru]4 [Ru]9 [Ru]10 [Ru]11

Figure 3-1. Olefin metathesis catalysts

Even though these phosphine-free alkylidenes initiate more slowly than their

phosphine analogs, they have gathered much interest for both their ease of use and their









enhanced reaction rates with electron deficient olefins. In addition, the

isopropoxystyrene ligand represents an ideal anchor point for modification studies

involving the introduction of different steric and electronic groups108'109'187-190 as well as

ionic groups191 and solid supports.102-107 Fine tuning of the ligand structure can be

achieved for a desired substrate, as the initiation rate and the overall activity of these

catalysts have proven very sensitive to the nature of the isopropoxybenzylidene. As

suggested by the dissociative mechanism, the catalytic activity can be enhanced by 1)

increasing the steric hindrance around the isopropoxy group, or 2) decreasing the electron

density at both the chelating oxygen atom and the benzylidene carbon.108'109

Our laboratory routinely uses metathesis catalysts for the synthesis of unique

macromolecules via acyclic diene metathesis (ADMET) polymerization.21'22'121-133'14148

The extended lifetime of this class of catalyst is of particular interest, since long-living

catalysts are desirable considering the reaction times required by ADMET

polycondensation. Typically, the activity of a catalyst in ADMET is evaluated through

comparative kinetic experiments using standard catalysts [Ru]l and [Ru]4, and 1,9-

decadiene as the substrate.155'144'192 This method has allowed to find optimal conditions

of temperature and concentration for a wide range of metathesis catalysts in ADMET

chemistry.144,192 In addition, the effect of structural perturbations on catalytic activity

was investigated in order to model more efficient catalyst systems. For instance, larger

substituents around the NHC ligand ([Ru]10 and [Ru]l1) result in faster initial rates

while incorporation of isopropoxybenzylidene expands the stability of the complex

([Ru]9 and [Ru]11).192









After developing complex [Ru]ll, whose increased activity was more suitable to

ADMET polymerization,192 the effect of electronics on the isopropoxy fragment of the

catalyst was examinated. Herein, a comprehensive study is presented on a series of

ruthenium catalysts bearing different isopropoxybenzilydene ligands and different NHC

ligands (Figure 3-2). Their catalytic activity is measured during the ADMET

oligomerization of 1,9-decadiene at different temperatures, and their initial rates are

compared to standard catalysts for ADMET.

Results and Discussion

Following the introduction of faster initiators by increasing the NHC ligand bulk,

the focus turned to the isopropoxy fragment of the catalyst, the idea being to enjoy the

inherent stability of the Hoveyda-type catalysts while improving their catalytic activity in

ADMET. Although the influence of electronics and sterics has been studied in much

detail in small molecule chemistry,108,109,187,190 the catalytic activity was only observed

during monocoupling reactions such as cross-metathesis or ring-closing metathesis. The

polycondensation nature of ADMET renders the catalytic profile very different, hence the

use of different parameters to measure catalytic activity in this study. For example,

complex [Ru]9 has been successful in many organic reactions but only results in low

conversions in the ADMET oligomerization of 1,9-decadiene.192 Therefore, electronic

modifications were applied on complex [Ru]ll, which has proven to be an efficient

ADMET catalyst. The design of catalysts [Ru]13-[Ru]16 was based on the idea that

decreasing the electron density at the chelating iPrO fragment would result in even higher

catalytic activities, the ultimate goal being the elaboration of a Hammett plot. The same

modifications were applied to complex [Ru]9 to control the influence of sterics on the

NHC ligand.










L L
N \ / CuCI in CH2CI2 CIu
H I Ct Ph CO C Ru R
Tv p ^t Ph .o<> 0 b R
H21Mes PCy3 -

/ --\ [Ru]4 L=H21Mes [Ru]9 L=H21Mes, R=H
N N [Ru]10 L=H21Pr [Ru]11 L=H21Pr, R=H
I [Ru]12 L=H21Mes, R=N02
[Ru]13 L=H21Pr, R=N02
H21Pr [Ru]14 L=H21Pr, R=CI
[Ru]15 L=H21Pr, R=CH3
[Ru]16 L=H21Pr, R=OCH3

Figure 3-2. Synthesis of complexes [Ru]9, [Ru]11-16

The ligands were obtained from the corresponding substituted o-

hydroxybenzaldehyde which was subjected to a Williamson substitution followed by a

Wittig reaction. The synthesis of complexes [Ru]12-16 then followed the same

procedure as for complex [Ru]9 and [Ru]11, affording good to excellent yields (83-

95%).95 All complexes are stable in solution and in air.

Each catalyst was subjected to a systematic reaction with 1,9-decadiene using a

450:1 monomer:catalyst ratio to reproduce typical polymerization conditions. Reaction

progress was followed by quantifying ethylene as it was released, and the degree of

polymerization was plotted against time.155 The catalytic activity was evaluated in

comparison with [Ru]9 and [Ru]ll at 300, 450 and 600C.

Steric effect

Table 1 shows the initial rates for catalysts [Ru]9,11 and [Ru]12,13 at different

temperatures, calculated as the initial slope (DP vs. time) of each kinetic curve during the

average dimerization (DP=2) of the monomer.193 Rates increase with temperature for all

complexes, and a straight line is obtained when the log of the initial rate is plotted against

1/T, verifying the Arrhenius law. The activation energies Ea are estimated to be 40.2









KJ-mol-l1s- and 48.9 KJ-mol-ls'1 for [Ru]ll and [Ru]13 respectively. Introduction of

the nitro group on the isopropoxybenzylidene results in an increase of the initial rate for

both catalysts, for indeed, the presence of the electron withdrawing substituentpara to

the isopropoxy group should facilitate the ligand dissociation by decreasing the electron

density on the coordinating oxygen, leading to the active species.95'99'194 However, a

closer examination at Table 1 indicates that the electronic effect is only minimal for

catalyst [Ru]12, bearing the mesityl NHC ligand, conversely to what was observed in

monocoupling reactions.109'195 Indeed, catalysts [Ru]9 and [Ru]12 exhibit comparable

initial rates regardless of the temperature (around 20 DP s-1 at 450C, and 42 DP s-1 at

60C). This observation emphasizes the fundamental difference between small molecule

metathesis and ADMET, where the initial rate is not necessarily a direct correlation of the

isopropoxystyrene dissociation rate, but also involves the stability of the 14 e- complex.

In this case, the propagating species Ru=CH(CH2)6CH=CH2 is the same for catalysts

[Ru]9 and [Ru]12. The similarity of the initial rates obtained here, regardless of the

dissociating ligand, seems to indicate that the propagation step is rate determining in

ADMET, i.e. that dissociation is fast.

Table 3-1. Initial rates in DP s-1 for catalysts [Ru]9,[Ru]ll-13.
Temperature [Ru]9 [Ru]12 [Ru]ll [Ru]13
300C 5 3 10 2 13 2 21 4
450C 20 5 22 4 45 4 48 7
600C 42 4 42 9 70 10 117 8

In the case of catalysts [Ru]11 and [Ru]13, containing a bulkier NHC ligand, the

nitro group seems to have a smaller effect on the initial rate, if any, at 30 and 450C than at

600C. For example, at 450C the nitro complex [Ru]13 only exhibits a 3 DP s-1 rate

increase (within the experimental error) compared to parent [Ru]ll, while a 50 DP s-1







64


rate increase is reported at 600C for [Ru]13. The difference at 300C does not seem

significant and will be further investigated by comparing other electronically modified

catalysts [Ru]14-[Ru]16. The higher rates of complexes [Ru]ll and [Ru]13 versus

[Ru]9 and [Ru]12 are due to the increase in steric bulk on the NHC ligand brought about

by the isopropyl groups, which exert a stronger steric pressure on the benzylidene and

therefore facilitate its dissociation.61 As a consequence, at 600C complex [Ru]13 exhibits

one of the fastest initiation rates (117 DP s-) ever reported for the ADMET reaction of

1,9-decadiene.


7-












1 --- [Ru]9
5-



3-





0 500 1000 1500 2000 2500 3000
Time (s)



Figure 3-3. Degree of polymerization (DP) vs. time data for H2IMes versus H2IPr
containing catalysts using 450:1 monomer:catalyst ratio at 450C

This substituent effect is not as obvious when examining the overall activity of the

catalysts [Ru]9,[Ru]11-13 (Figure 3-3). The 'tailing off of each curve is due to the

reaction mixture becoming more viscous as the DP increases until reaching a solid state

(around a DP of 4-5) where the rate becomes diffusion controlled. Even so, the catalysts

deviate from the initial trend discussed above before reaching the solid state and after









dimerization. The catalysts bearing bulkier NHC ligands ([Ru]ll and [Ru]13) seem to

be unaffected by the presence of the nitro group, which is in striking contrast with

analogs [Ru]9 and [Ru]12. Indeed, complex [Ru]12 shows almost a two-fold increase in

its overall activity compared to the parent catalyst [Ru]9 (at 30 minutes the DPs are 2.8

and 4.1 for [Ru]9 and [Ru]12, respectively), while [Ru]ll and [Ru]13 show the same

catalytic profile. The electronic effect on the isopropoxybenzylidene seems to be

dominated by the steric hindrance of the NHC ligand. Following the model of

ruthenium-phosphine complexes, if the presence of H2IPr facilitates dissociation, then it

also slows down catalyst deactivation by rebinding of free phosphine, or in this case,

rebinding of the ether-tethered ligand (iPrO).99 Thus, complexes [Ru]ll and [Ru]13

actually undergo more turnovers before being trapped by iPrO. The

dissociation/rebinding rates ratio appears to approach an optimum with H2IPr ligands, a

ratio that is hardly disturbed by simply changing the electron density around the

isopropoxystyrene. On the other hand, the rebinding of iPrO is rendered easier in the

presence of the less hindered H2IMes ligand. In this case, the electron density of the

coordinating oxygen has a greater effect on both the dissociation and rebinding rates,

which explains the higher reactivity of [Ru]ll whose nitro-substituted ligand will be less

likely to recoordinate and therefore increases the amount of active species able to react.

This electronic effect is not evident during the initial rate probably because ligand

rebinding only becomes substantial after several turnovers.

Electronic effect on H2IPr ligated complexes

The series of complexes [Ru]ll-[Ru]16 should help better understand the

importance of dissociation/rebinding of these ether-tethered ligands. All the synthesized

complexes are efficient catalysts for the ADMET oligomerization of 1,9-decadiene







66


reaching DPs of 4 and 5 under 5 minutes at 45C. As illustrated in Figure 3-4, even at

DP>4 (where the viscosity becomes significant) the polymerization continues at a steady

rate without any sign of extensive decomposition of the catalytic center, usually

observable by a darkening of the solution, confirming the robust nature of these catalysts.


8-

7-

6-
5- 0

4 -
-- CH [Ru]15
3 -- OCH3 [Ru]16
-- H [Ru]11
2- -*-NO2 [Ru]13
CI [Ru]14
1-
I I I I II
0 500 1000 1500 2000 2500
Time (s)



Figure 3-4. Degree of polymerization (DP) vs. time data for complexes [Ru]ll-[Ru]16
using 450:1 monomer:catalyst ratio at 450C

Table 2 gives an overview of the initial rates for complexes [Ru]ll-[Ru]16 at 300,

450 and 600C. As observed earlier, initial rates at 300C and 450C stay the same regardless

of the isopropoxybenzylidene used (around 20 DP s-1 at 300C, and 45 DP s-1 at 450C

within experimental error) except for parent catalyst [Ru]ll, which exhibits a slightly

lower initial rate at 300C. The difference noticed earlier (Table 1) could have been

insignificant if the rates of the electronically modified catalysts [Ru]13-[Ru]16 had not

been so reproducible. The singularity of the unsubstituted catalyst [Ru]ll proves the

existence of an electronic effect, albeit not qualitative. This lack of sensitivity to the

electronic nature of the substituent is again in significant contrast to what was observed









during CM and RCM reactions catalyzed by [Ru]9 analogs; however, other reports have

also referred to the unpredictability of other electronically-modified Hoveyda-type

catalysts.108,109,196 Here, the steric hindrance present on the NHC ligand seems to

overshadow the electronic nature of the dissociating ligand at low temperatures,

canceling the effect predicted by sigma values. This proposal finds credence in the X-ray

structure of [Ru]ll, [Ru]13 and [Ru]16. The Ru-O bond length, usually indicative of

the strength of the iPrO->Ru chelation, does not show significant variation (See the X-ray

crystal structures in Appendix A). Conversely, nitro-substituted [Ru]13 exhibits a

slightly shorter Ru-O bond length (2.2462(3) A) while methoxy-substituted [Ru]16's is

slightly longer (2.2486(18) A).

Table 3-2. Initial rates in DP s-1 for catalysts [Ru]ll-[Ru]16.
NO2 Cl H CH3 OCH3
Temperature
[Ru]13 [Ru]14 [Ru]ll [Ru]15 [Ru]16
a- 0.71 0.37 0 -0.06 0.05
o+ 0.79 0.11 0 -0.31 -0.78
30C 21 4 21 3 13 2 22 5 19 4
450C 48 7 41 8 45 4 47 2 40 5
600C 117 8 118 15 70 10 136 9 110 7
Values for c- and c+ were taken from ref 198

At 450C all complexes follow about a similar catalytic profile, i.e. there is no major

improvement brought about by the electronic substituents. However, there is a noticeable

distinction between complexes (Figure 3-4). Electron donating groups (EDG) appear to

increase the catalytic activity while e- withdrawing groups (EWG) reduce the rate. Since

the initial rates at 300C and 450C are the same for all catalysts, the dissociation rate of

isopropoxybenzylidene cannot be the only determining factor. Theoretically, after

dissociation the propagating species is the same for complexes [Ru]ll-[Ru]16.









Therefore, the liability of isopropoxystyrene must allow its rebinding to the ruthenium

center so that it influences the overall catalytic activity.

The trend observed at 450C can be rationalized by considering the formation of the

unstable 14e- species, either the alkylidene or the methylidene complex. In Chapter 2, it

was demonstrated that this type of catalyst was quite sensitive to substrate polarity

compared to their phosphine analogs.192 In the latter, the phosphine coordinates back to

the 14e- intermediate to stabilize it and forms a dormant state.61 In Hoveyda-type

complexes, this type of stabilization is lessened by the extreme liability of isopropoxy

styrene. However, all substrates conventionally used to probe the metathesis activity of

any catalyst contain some heteroatom that can serve as a stabilizer, along with the solvent

used for the reaction. For example, during the ROMP reaction of diverse oxygen-

containing monomers, Khosravi et al.154 showed that Hoveyda-type catalysts were

stabilized through the chelation of an oxygen contained in the monomer unit. In the case

of substrates lacking any kind of electron donor or Lewis basic group, such as 1,9-

decadiene, the 14e- intermediate is not stabilized; therefore, the catalyst is more

susceptible to decomposition, which results in slower rates. With this in mind, it is

reasonable to assume that the catalysts with higher dissociation rates and/or slower

rebinding rates should have a lower overall activity. According to sigma values, electron

withdrawing substituents para to the isopropoxy group should facilitate the ligand

dissociation by decreasing the electron density on the coordinating oxygen and increase

its liability, while electron donating substituents should slow the dissociation step and

facilitate its rebinding. Simply stated, donating groups should slow down catalyst

decomposition. Consequently, complexes 6 and 7 (EDG) show a slightly higher







69


reactivity than complexes 4 and 5 (EWG), while neutral complex [Ru]11 lies in between

(Figure 3-2).

Temperature effect

When the temperature is raised to 600C, the singularity of catalyst [Ru]ll over the

electronically modified [Ru]13-[Ru]16 is accentuated. While initial rates for catalysts

[Ru]13-[Ru]16 are all around 120 DP s1, parent complex [Ru]ll only affords a rate of

70 DP s-1, broadly deriving from the predicted Arrhenius plot. Electronics now represent

a determining factor on catalytic activity, which may be the result of a switch in the

mechanism possibly due to a change in catalyst conformation. Similar temperature

activation barriers have been witnessed with nitrogen-chelated complexes.198


8-

7-

6-

5-

4-

3- Cl [Ru]14
S--- CH3 [Ru]15
2 ---- OCH3 [Ru]16
--N02 [Ru]13
1 -- H [Ru]11

0 500 1000 1500 2000 2500
Time (s)



Figure 3-5. Degree of polymerization (DP) vs. time data for complexes [Ru]ll-[Ru]16
using 450:1 monomer:catalyst ratio at 600C

The same catalytic profile persists as the reaction continues, and the overall activity

is now significantly higher for the modified catalysts [Ru]13-[Ru]16 (Figure 3-5). This

suggests that the steric effect of the H2IPr ligand is no longer dominant, even though the









electronic factor cannot be qualified according to sigma values. This apparent lack of

sensitivity towards the electronic nature of the substituents is more likely the result of

several mechanistic processes operating simultaneously. While electron withdrawing

groups could facilitate dissociation by weakening the iPrO->Ru chelation, electron

donating groups could equally increase catalytic activity by reducing the Lewis acidity of

the metal. The rate of decomposition of the methylidene intermediate, which is

accelerated at elevated temperatures, is probably an important factor. Also, assuming

that the H2IPr ligand does not hinder the rebinding of the iPrO moiety, the 14e- species

can be properly stabilized, but, since the dissociation is quite fast, the catalyst quickly

returns to the catalytic cycle affording higher DPs. The diminished steric effect may be

due to a reorganization of the NHC ligand, possibly by ring rotation, often seen at high

temperatures.

Conclusions

This report gives a different outlook on the influence of the ligand sphere's

electronics/sterics on Hoveyda-type metathesis catalysts. Unlike most modification

studies conducted on these complexes, the 'steric factor' is only located on the NHC

ligand isopropyll vs. methyl groups) whereas the 'electronic factor' is on the

isopropoxybenzylidene. All the complexes tested are efficient catalysts for the ADMET

polymerization of 1,9-decadiene. However, in the polycondensation conditions of

ADMET, the steric hindrance largely dominates any electronic effect at all temperatures.

Interestingly, unsubstituted complex [Ru]ll curiously stands out at 30C and 60C,

suggesting some participation of electronics in the catalytic potential of these Hoveyda-

type complexes. The overall activity of these complexes seems to depend only on the

propensity of the isopropoxystyrene ligand to rebind to the ruthenium center and stabilize









it in a similar way as their phosphine analogs, especially at low temperatures. As a result,

complexes bearing EWGs result in lower DPs than complexes bearing EDGs, the more

active catalyst being the better stabilized. In any case, complexes ligated to larger NHC

ligands are better catalysts than any modified Hoveyda-Grubbs such as [Ru]12, again

showing the prevalence of sterics over electronics.

At 60C, all electronically modified catalysts exhibit comparable initial rates and

increased reactivities, while parent complex [Ru]ll constitutes a significant exception.

The catalytic profiles of [Ru]13-[Ru]16 are similar and lay 2 DP over [Ru]ll's,

implying that the different electronic groups are able to improve activity through different

processes. A reorganization of the ligand sphere may occur at higher temperatures so

that the steric bulk of the NHC ligand does not hinder the rebinding of isopropoxystyrene

anymore. Consequently, catalysts [Ru]13-[Ru]16 represent the fastest initiators ever

tested for ADMET polymerization. Further experiments will be conducted to investigate

the importance of ligand rebinding in these Hoveyda-type catalysts, which represent an

interesting prospect for metathesis polycondensation.

Experimental

General

1H NMR (300 MHz) and 13C NMR (75 Hz) spectra of the organometallic

complexes were recorded in CDC13 on either a Mercury series or Varian VXR-300 NMR

superconducting spectrometer. Chemical shifts were referenced to residual CHC13 (7.27

for 1H and 77.23 for 13C) with 0.03% v/v TMS as an internal reference.

Complex [Ru]9 was a gift from Materia Inc. and was used as received. Complex

[Ru]4,71 [Ru]10,101 [Ru]11,192 and [Ru]12195 were synthesized according to the literature

procedure. All catalysts were stored in an argon-filled drybox prior to use in kinetic and









polymerization experiments. 1,9-Decadiene (Aldrich) was distilled from Na/K alloy

under reduced pressure into a Kontes flask equipped with a Teflon valve, degassed by

three freeze-pump-thaw cycles, and stored in an argon-filled drybox. For the kinetic

study, 1,9-decadiene was portioned into small Teflon-capped vials in the drybox and

were removed and stored in a dessicator until use.

All other starting material were distilled over Na/K alloy before use except

chlorinated compounds which were distilled over CaH2. After distillation, d-chloroform

was degassed by three freeze-pump-thaw cycles, and stored in an argon-filled drybox.

X-ray Experimental

Crystals suitable for X-ray structure determination were obtained from slow

diffusion of pentane into a saturated solution of [Ru]13 or [Ru]16 in CH2C2 at room

temperature. Data were collected at 173 K on a Siemens SMART PLATFORM equipped

with A CCD area detector and a graphite monochromator utilizing MoKa radiation (k =

0.71073 A). Cell parameters were refined using up to 8192 reflections. A full sphere of

data (1850 frames) was collected using the co-scan method (0.30 frame width). The first

50 frames were remeasured at the end of data collection to monitor instrument and crystal

stability (maximum correction on I was < 1 %). Absorption corrections by integration

were applied based on measured indexed crystal faces.

The structure was solved by the Direct Methods in SHELXTL6, and refined using

full-matrix least squares. The non-H atoms were treated anisotropically, whereas the

hydrogen atoms were calculated in ideal positions and were riding on their respective

carbon atoms. Refinement was done using F2









For [Ru]13, a total of 426 parameters were refined in the final cycle of refinement

using 7198 reflections with I > 2o(I) to yield R1 and wR2 of 3.19% and 7.23%,

respectively.

For [Ru]16, a total of 406 parameters were refined in the final cycle of refinement

using 6677 reflections with I > 2o(I) to yield R1 and wR2 of 3.85% and 11.46%,

respectively. The toluene molecule were disordered and could not be modeled properly,

thus program SQUEEZE, a part of the PLATON package of crystallographic software,

was used to calculate the solvent disorder area and remove its contribution to the overall

intensity data.

General Procedure for Ligand Synthesis

2-Isopropoxystyrene derivatives were synthesized by a standard Wittig reaction on

the 2-isopropoxybenzylaldehyde parent using (methyl)triphenylphosphine iodide and

potassium t-butoxide (Aldrich). All the 2-isopropoxybenzylaldehyde derivatives were

synthesized via a Wilkinson-ether synthesis from the corresponding salicyl aldehyde

(Aldrich) and 2-bromopropane (Aldrich). Spectral data for 1-isopropoxy-2-vinyl-4-

methylbenzene,108 1-isopropoxy-2-vinyl-4-nitrobenzene,195 and 1-Isopropoxy-2-vinyl-4-

methoxybenzene196 match the literature.

General Procedure for Carbene Exchange

In a glovebox complex [Ru]4 (300 mg, 0.32 mmol) and CuCl (Aldrich) (32 mg,

0.32 mmol) were weighed into a 100 mL schlenk flask and dissolved in 15 mL of

CH2C12. 2-Isopropoxy-5-nitrostyrene (133 mg, 0.64 mmol) was dissolved in 5 mL of

CH2C12 and added to the solution of complex [Ru]4 and CuCl at room temperature. The

flask was equipped with a condenser and the solution was refluxed for 30 minutes or until









the brown solution turns to a deep green. From this point forth, all manipulations were

carried out in air with reagent-grade solvents. The reaction mixture was concentrated in

vacuo to a green residue. The unpurified material was dissolved in a minimal volume of

5:2 cyclohexane/ethyl acetate and loaded onto a plug of silica gel. Insoluble copper-

phosphine precipitates were removed prior to loading by passing the solution through a

second Pasteur pipette containing a plug of glass wool. Elution with 5:2

cyclohexane/ethyl acetate removed a bright green band from the column. The solvent

was evaporated and the residue washed with n-pentane.

Synthesis of complex [Ru]13. (-NO2) The general procedure was followed to

afford 200 mg (0.267 mmol, 83%) of a green powder. 1H NMR (CDC13, 300 MHz):

6=16.32 (s, 1H, Ru=CHPh), 8.40 (d, 1H, para CH, 2JH,H= 8.4 Hz ), 7.70 (s, 1H, ortho

CH), 7.58 (t, 2H, para CH, 3JH,H= 7.2 Hz), 7.39 (d, 4H, meta CH), 6.90 (d, 1H, meta CH,

2JH,H= 8.4 Hz), 4.98 (sept, 1H, (CH3)2CHOAr, 3JHH= 6.3 Hz), 4.20 (s, 4H, N(CH2)2N),

3.54 (sept, 4H, CH(CH3)2, 3JH,H= 6.6 Hz), 1.40 (d, 6H, OCH(CH3)2, 2JH,H= 6.3 Hz), 1.24

(d, 24H, CH(CH3)2, 3JHH= 7.1 Hz); 13C NMR (CDC13, 75 MHz): 6 = 284.4, 210.6, 156.9,

149.3, 143.9, 143.2, 136.4, 130.3, 124.7, 124.3, 117.0, 113.0, 77.9, 54.8, 29.1, 27.1, 26.8,

21.9. MS (HRMS/EI) calcd for C37H4903N3C12Ru [M] 755.2194, found 755.2161.

Anal. Calcd for C37H4903N3C12Ru: C, 58.80; H, 6.53; N, 5.56. Found: C, 59.96; H, 7.03;

N, 5.20.

Synthesis of complex [Ru]14. (-Cl) The general procedure was followed using 2-

isopropoxy-5-chlorostyrene (125 mg, 0.64 mmol) to afford 202 mg (0.272 mmol, 85%)

of a pale green powder. 1HNMR (CDC13, 300 MHz): 6=16.28 (s, 1H, Ru=CHPh), 7.55

(t, 2H, para CH, 3JHH= 7.2 Hz), 7.43 (s, 1H, ortho CH, 2JHH= 8.8 Hz), 7.38 (d, 4H, meta









CH, 2JH,H= 7.4 Hz ), 6.80 (d, 1H, para CH, 2JH = 2.4 Hz), 6.72 (d, 1H, meta CH, 2JHH

8.8 Hz), 4.87 (sept, 1H, (CH3)2CHOAr, 3JH,H = 6.2 Hz), 4.19 (s, 4H, N(CH2)2N), 3.58

(sept, 1H, CH(CH3)2, 3JH,H= 6.6 Hz), 1.35 (d, 6H, OCH(CH3)2, 2JH,H= 6.3 Hz), 1.25 (d,

24H, CH(CH3)2, 3JH,H= 7.1 Hz); 13C NMR (CDC13, 75 MHz): 6 = 286.3, 212.3, 150.9,

149.3, 145.0, 130.0, 128.1, 127.7, 124.6, 121.4, 114.1, 75.9, 54.7, 29.0, 26.7, 23.5, 21.8.

MS (HRMS/EI) calcd for C37H490N2C13Ru [M+Na]+ 767.1846, found 767.1941. Anal.

Calcd for C37H490N2Cl3Ru: C, 59.63; H, 6.63; N, 3.76. Found: C, 59.52; H, 7.03; N,

3.46.

Synthesis of complex [Ru]15. (-CH3) The general procedure was followed using

2-isopropoxy-5-methylstyrene (113 mg, 0.64 mmol) to afford 209 mg (0.288 mmol,

90%) of a pale green powder. 1H NMR (CDC13, 300 MHz): 6=16.40 (s, 1H, Ru=CHPh),

7.55 (t, 2H, para CH, 3JH,H= 7.6 Hz), 7.40 (d, 4H, meta CH, 2JH,H= 7.8 Hz,), 7.24 (s, 1H,

ortho CH), 6.68 (d, 1H, para CH, 2JH,H= 8.3 Hz), 6.58 (s, 1H, meta CH), 4.88 (sept, 1H,

(CH3)2CHOAr, 3JH,H= 6.0 Hz), 4.18 (s, 4H, N(CH2)2N), 3.62 (sept, 1H, CH(CH3)2, 3JH,H

= 6.5 Hz), 2.32 (s, 3H, CH3), 1.36 (d, 6H, OCH(CH3)2, 2JH,H= 6.2 Hz), 1.27 (d, 24H,

CH(CH3)2, 3JH,H= 6.8 Hz); 13C NMR (CDC13, 75 MHz): 6 = 290.8, 214.4, 150.8, 149.5,

144.4, 137.1, 131.5, 129.9, 124.6, 122.6, 112.8, 74.9, 54.8, 29.0, 26.8, 23.6, 22.0, 20.2.

MS (HRMS/EI) calcd for C38H5202N2C12Ru [M+Na]+ 747.2395, found 747.2427. Anal.

Calcd for C38H5202N2C12Ru: C, 62.97; H, 7.23; N, 3.86. Found: C, 62.67; H, 7.31; N,

3.49.

Synthesis of complex [Ru]16. (-OCH3) The general procedure was followed using

2-isopropoxy-5-methoxystyrene (123 mg, 0.64 mmol) to afford 229 mg (0.310 mmol,

97%) of a pale green powder. 1H NMR (CDC13, 300 MHz): 6=16.33 (s, 1H, Ru=CHPh),









7.53 (t, 2H, para CH, 3JHH= 7.2 Hz), 7.37 (d, 4H, meta CH), 7.02 (dd, 1H, meta CH,

2JHH= 8.4 Hz), 6.69 (d, 1H, para CH, 2JH,H= 7.4 Hz), 6.37 (s, 1H, ortho CH), 4.83 (sept,

1H, (CH3)2CHOAr, 3JHH = 6.3 Hz), 4.18 (s, 4H, N(CH2)2N), 3.72 (s, 3H, OCH3), 3.60

(sept, 1H, CH(CH3)2, 3JH,H= 6.6 Hz), 1.35 (d, 6H, OCH(CH3)2, 2JH,H= 6.2 Hz), 1.24 (d,

24H, CH(CH3)2, 3JH,H= 7.1 Hz); 13C NMR (CDC13, 75 MHz): 6 = 298.1, 208.9, 152.5,

149.4, 146.2, 131.9, 130.6, 129.9, 124.7, 117.7, 113.2, 111.4, 72.7, 55.8, 32.8, 29.0, 26.8,

23.6, 21.9. MS (HRMS/EI) calcd for C38H5202N2C12Ru [M+Na]+ 763.2344, found

763.2361. Anal. Calcd for C38H5203N2C12Ru: C, 61.61; H, 7.08; N, 3.78. Found: C,

61.86; H, 7.28; N, 3.63.

Kinetic Study

The kinetic study was performed with the same apparatus and following the same

procedure as previously described by Wagener.144'192














CHAPTER 4
ISOMERIZATION BEHAVIOR UNDER METATHESIS CONDITIONS OF NHC-
CONTAINING RUTHENIUM CARBENE CATALYSTS.

Introduction

In the past few years, alkene isomerization has emerged as an important side

reaction of ruthenium catalyzed metathesis, disturbing the product's microstructure by

apparent migration of the double bond along the alkyl chain.160,166 Figure 4-1 illustrates

the versatility of this side-reaction. First observed with [Ru]l during the ring-closing

metathesis of allylic functionalized alkenes,164'167'199,200 olefin isomerization rapidly

appear to affect a variety of oxygen and nitrogen-containing compounds.168,169 In some

cases, double-bond isomerization even occurred prior to the metathesis reaction.20

Nevertheless, [Ru]l remains inactive towards isomerization with unsubstituted olefins or

unfunctionalized alkenes even under harsh conditions.176 However, when the 2nd

generation of catalysts were introduced, competitive isomerization became a problem

with all kinds of substrates.160,166 Firstner et a. 114 reported the synthesis of a 20-

membered ring in addition to the desired 21-membered lactone from the RCM of a 1,22-

diene with [Ru]3. The ring contraction was the result of initial isomerization of one of

the double-bonds followed by RCM and the release of propene. Many other examples

appeared in the literature and scientists began to use isomerization to their advantage for

the synthesis of 5-, 6-, 7-membered cyclic enol ethers by tandem RCM double-bond

isomerization reactions.201
isomerization reactions.










OH R'
RO -

n


OH R'
[Ru]l RO
o --N ....., n


[Ru]l


O






65%


[Ru]3
O






10%


R o [Ru]4 R 0 n R 0



46-65%

Figure 4-1. Examples of competitive isomerization reactions during olefin metathesis

The 2 established pathways for transition metal-catalyzed olefin isomerization are

the 7t-allyl metal hydride mechanism and the metal hydride addition-elimination.178

Despite an increasing number of studies, the ruthenium intermediate responsible for this

undesirable reaction has not been identified yet, although it is often attributed to the

formation of a ruthenium hydride in situ, as a decomposition product of the original

carbene catalyst.










Acyclic diene metathesis has been used in our research for the modeling of precise

ethylene/a-olefin copolymers.127,128,130 Controlling isomerization is crucial as it results in

irregular microstructures, whether it occurs on the monomer or on the product (Figure 4-

2). Furthermore, ADMET conditions are particularly favorable to isomerization since

polymerizations are typically conducted in neat diene or in concentrated solutions at high

temperatures of 40-75C, for extended periods of time to reach high molecular weight

materials. Previously, we have reported the metathesis vs. isomerization activity of fast

initiators using the model substrate 1-octene.192 In an attempt to better understand the

causes of isomerization, this study was extended to other modified complexes,

introducing different phosphine and carbene ligands. Previous Hoveyda-type complexes

Ru]E, [Ru]ll, and [Ru]13 were also included in this study.

R R
ADMET

R

RC isomerization isomerization



R R
m "lighter" unit isomerization "heavier" unit
and ADMET

R

x yn

x=0,1,2,3
y=6,5,4,3

Figure 4-2. Effect of isomerization during ADMET polymerization

Results and Discussion

The fact that isomerization seemed to occur to a significant extent with fast

initiators combined with previous observations showing that ruthenium vinylidene









catalysts afforded polymers with singularly high trans content,22 prompted us to

investigate a variety of slow initiators. Smaller phosphine are known to dissociate

slower.7 Tricyclopentyl phosphine (PCp3) ruthenium dimethylvinyl carbene [Ru]17

being readily available in the lab from previous studies, the PCp3 equivalents of both

parent complexes [Ru]4 and [Ru]10 represent ideal candidates to investigate the

influence of both the NHC and phosphine ligands. On the other hand, dimethylvinyl

carbene ruthenium complexes react slowly and incompletely with ethyl vinyl ether even

when a phosphine is replaced by an NHC ligand. Despite incomplete initiation of the

catalyst, vinylidene complexes are efficient catalysts for a variety of cross-metathesis

applications,203 which make them potentially suitable for ADMET chemistry.

Synthesis and Characterization

Complexes [Ru]13-16 were synthesized from the parent catalyst [Ru]10 by

addition of the corresponding isopropoxystyrene and copper chloride as described in

Chapter 3. The synthesis of complex [Ru]18 was adapted from the literature.203

Noteworthy, the H2IPr analogue proved difficult to synthesize using the same conditions.

The additional bulk brought about by the isopropyl groups of the NHC ligand may

interact with the dimethylvinyl carbene, rendering the ligand substitution more

problematic. Complexes [Ru]21 and [Ru]22 were obtained by phosphine exchange

using the intermediate pyridine adduct [Ru]19 and [Ru]20, respectively (Figure 4-3).204

The bis-pyridine complexes were crystallized and reacted with 1.4 equivalents of PCp3.

Spectroscopically, the vinylidene protons of [Ru]18 exhibit two characteristic

doublets at 6 19.14 and 7.72 ppm, while [Ru]21 and [Ru]22 showed a single phosphorus

signal at 6 26.30 and 24.27 ppm, respectively. The upfield shift relative to PCy3 ligated









parent complexes [Ru]4 and [Ru]10 indicates the presence of the less nT-basic

tricyclopentyl phosphine


KOtBu


[Ru]17


L
xs. Pyridine "N P(Cyp)3
-N-C P
h cIt Ph


L
Cl// u
SRu=\
t' Ph

0:161


[Ru]4 L= H21Mes [Ru]19 L= H21Mes [KUJZ1 L= H21vIes
[Ru]10 L= H21Pr [Ru]20 L= H21Pr [Ru]22 L= H21Pr

Figure 4-3. Synthesis of [Ru]18-22

Isomerization Behavior with 1-Octene

The extent of isomerization was tested for complexes [Ru]E, [Ru]ll, [Ru]13 and

[Ru]17-22, using the model 1-octene, and compared to catalysts [Ru]l, [Ru]4 and

[Ru]10. The reaction conditions were typically the same as the original experiment,

using 0.1 mol% catalyst in neat 1-octene at both room temperature and 55C for 3

days.176 The product mixture was analyzed by gas chromatography (GC), where each

peak corresponds to a linear olefin of a specific carbon length. The results are expressed

as the percentage of isomers in the mixture versus the percentage of metathesis product,

assuming complete conversion, which is true for all catalysts except [Ru]18 (Table 4-1).

The combination of the vinylidene and the smaller phosphine, even though PCp3 is less


H21Mes


cl/,I
C9Ru=
CI t




[Ru]18


L
CI/ y

PCY3









7t-basic than PCy3, render this catalyst extremely slow to initiate at room temperature and

the concentration of active species is probably not sufficient to drive the reaction to

completion.

Table 4-1. Percentage of isomers during the metathesis of 1-octene.
% isomers % isomers
Entry Catalyst at 250C at 550C
1a [Ru]l 0% 0%
2 [Ru]E 0% 12%
3a [Ru]4 3% 76%
4b [Ru]10 76% -
5 [Ru]ll 23% 50%
6 [Ru]13 78% -
7 [Ru]18 7% 70%
8 [Ru]21 39% 49%
9 [Ru]22 84% -
a Data taken from ref 176; b ref 193.

All catalysts exhibit an isomeric distribution centered around the metathesis

product, indicating that isomerization occurs subsequently to the metathesis process

(Figure 4-4). This is in stark contrast with what was observed with Grubbs' catalyst

[Ru]4 where the two reactions occurred competitively at 550C. Since, [Ru]21 is a slower

metathesis catalyst,7 it is tempting to conclude that substituting PCy3 for PCp3 delays the

isomerization reaction but yet does not decrease its extent. The PCp3 ligated complexes

[Ru]21, and [Ru]22 are actually better isomerization catalysts than [Ru]4 at room

temperature. This could be due to the slower rate of dissociation of PCp3, combined to

the lower K7-basicity of the phosphine, which probably affects the rate of catalytic

decomposition, in that it decreases the chances for nucleophilic attack on the methylidene

complex.89







83


A)















I I I I f 1 I
C12 C13 C14 C15 C16 C17 C18

B)








1 I I I I I I I I I I I
C9 C10 C11 C12 C13 C14 C15 C16 C17 C18 C19 C20 C21

Figure 4-4. GC chromatogram of the product mixture from the self-metathesis of 1-
octene: (A) with [Ru]21 at 25oC; (B) with [Ru]22 at 25TC. Retention scale is
in minutes.


It appears that the rate of isomerization is closely related to the rate of metathesis,

as the faster metathesis initiators, bearing the bulkier H2IPr ligand, result in higher %

isomerization products. For example [Ru]13 and [Ru]22 initiate fast and afford more

than 75% isomers at room temperature (Figure 4-4A, entries 6 and 9), while [Ru]18 and

[Ru]21 are known for their slow initiation and consequently yield 7-39% isomers in the

same conditions (Figure 4-4B, entries 7 and 8). The breadth of the isomer distribution

increases with the reactivity of the isomerization catalyst. However, isomerization


cannot be correlated as a function of the dissociation rate. Indeed, complexes [Ru]l and

[Ru]E are known for faster dissociation, still the results show little to no trace of









isomerization. On the other hand, changing the dissociating ligand or the carbene had

little effect on complexes bearing NHC ligands. Accordingly, electronics on the

Hoveyda-type complexes ([Ru]ll vs. [Ru]13) did not significantly affect either

metathesis or isomerization rates. Even though this study does not allow to conclude on

the causes of isomerization it is now clear that NHC ligands play a crucial role in

promoting this undesired reaction.

Since the metathesis and isomerization activity of ruthenium catalysts apparently

go hand in hand, it seems logical that isomerization is promoted by the metathesis

catalyst itself or by a species formed in situ during the metathesis cycle. Bourgeois et

a. 174 proposed the 7r-allyl mechanism illustrated in Figure 4-5, via deprotonation of the

allylic position by the carbene moiety. They postulate that the strong o-donation of the

NHC ligand increases the basicity of the carbene, therefore favoring isomerization. In

addition, they proved that tricyclohexyl phosphine oxide, O=PCy3, generated from

oxidation of PCy3, serves as an isomerization inhibitor.

CI IMes
CI Ru CI2RulMes
[Ru]3 R-/ IJ path a
H -- metathesis
-PCy3


Spath b

Cl IMes

R u- > "+ [Ru]3
R = H,Ph H H
IMes = Mes-N N-Mes


Figure 4-5. Postulated mechanism of the isomerization process.









On the other hand, ruthenium carbene catalysts are known to form hydride

complexes as they decompose. Because early metathesis complex Ru(H20)6(tos)2 was

proven to isomerize allylic functionalized substrates through a metal-hydride

intermediate,205 it is tempting to assume that one of the catalyst decomposition product

might be responsible for this undesirable reaction. Therefore, understanding the

isomerization process also goes along with the determination of the decomposition

pathway. Several hydrido complexes have been identified as decomposition products of

catalysts [Ru]l and/or [Ru]4 under different conditions; however, none reproduces

rigorous metathesis conditions.72,89-94 Complexes [Ru]6 and [Ru]7 (Figure 4-6) have

been isolated under high oxygen pressures or in alcohol solvents,92-94 but cannot be

obtained under the inert conditions typically used in this study. Even though complex

[Ru]7 has been isolated as an impurity during the synthesis of [Ru]4, supposedly as a

consequence of the methanol wash, it is not a good isomerization catalyst. Grubbs et a.89

have recently reported the hydrido complex [Ru]5, which directly involves the NHC

ligand in the decomposition process. Finally, the C-H activation of one of the methyls of

the NHC ligand is a common reaction in many ruthenium complexes,183'206-209 even

though complex [Ru]8 is the only example so far with a ruthenium carbene and does not

contain any hydride ligand.72



N C"Ru--H Cl
Mes\ Ru C-Ru OCn OC .Ru
c L =C t oc L
N H CI PCy3 PCy3
Mes
Mes ][Ru]6 L = PCy3
[Ru]5 [Ru]8
[Ru]7 L= IMesH2

Figure 4-6. Decomposition products of ruthenium carbene complexes