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Study and utilization of group 6 metal complexes

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Study and utilization of group 6 metal complexes
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Main, Andrea Denise, 1971-
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xii, 120 leaves : ill. ; 29 cm.

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Absorption spectra ( jstor )
Amines ( jstor )
Carbenes ( jstor )
Chemicals ( jstor )
Diamines ( jstor )
Ligands ( jstor )
Organic chemistry ( jstor )
Reactivity ( jstor )
Solvents ( jstor )
Tungsten ( jstor )
Chemistry thesis, Ph.D ( lcsh )
Dissertations, Academic -- Chemistry -- UF ( lcsh )
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bibliography ( marcgt )
theses ( marcgt )
non-fiction ( marcgt )

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Thesis:
Thesis (Ph.D.)--University of Florida, 2000.
Bibliography:
Includes bibliographical references (leaves 108-119).
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Printout.
General Note:
Vita.
Statement of Responsibility:
by Andrea Denise Main.

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STUDY AND UTILIZATION OF GROUP 6 METAL COMPLEXES


By

ANDREA DENISE MAIN













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


2000






























Not many appreciate the ultimate power and potential usefulness of basic knowledge accumulated by obscure, unseen investigators who, in a lifetime of intensive study, may
never see any practical use for their findings but who go on seeking answers to the
unknown without thought of financial or practical gain.


-Eugenie Clark













ACKNOWLEDGEMENTS


First, I am especially thankful for my partner, Dee Dee Mathews, for being there through thick or thin. I would also like to thank Linda Mathews for all of her love and support.

I thank my advisor, Dr. Lisa McElwee-White, for her support and encouragement through the years. Without her knowledge of organometallic chemistry, I would still be explaining metal-mediated reactions as some magical phenomena.

I would like to thank my friends who have helped to make this an extraordinary period of my life: Stacey Huber, Dr. Sue Troutman Lee, Dr. Sunday Brooks, Damon Storhoff, Ann Storhoff, Holly Newman, Dr. Patricia Bottari, Dr. Margaret Kerr, Dr. Yingxia He, Dr. Christine Nixon Lee, Kirsten Johnson, Fang "Annie" Qian, Dr. Mark Tess and Dr. Rene Vieta.

Special thanks go to Larry Lee for all the hours of therapy. Although I think he should have been a psychologist, I am sure he will make a wonderful dentist.

I would like to thank my parents, George and Linda Main, for their love and support throughout my life. I am here today because of their dreams, which helped me to find my own.

I thank my Savior, the Lord Jesus Christ, for being there when things looked bleak. He showed me that life is not to be squandered.















TABLE OF CONTENTS


page

ACKNOWLEDGMENTS ......................................................................1i

L IST O F TA B L E S ............................................................................................................. vii

LIST O F FIG U R E S .......................................................................................................... viii

LIST OF SCHEMES ...................................................................................................... ix

A B ST R A C T ....................................................................................................................... xi

CHAPTERS

1 IN TR O D U C TIO N .......................................................................................................... 1
General Overview of Low-Valent Metal Carbynes and Metal-Catalyzed
Carbonylation R eactions ........................................................................................ 1
B onding ....... ................................................................................................... 2
Protonation of Low-Valent Metal Carbynes ...................................................... 4
Protonation in the Presence of Alkynes ........................................................... 10
Photochem istry .................................................................................................. 12
Carbonylation with Transition Metals ............................................................ 20
Catalytic Carbonylation of Amines with Metal Carbonyl Complexes ............. 21
Nickel Carbonyl Complexes .................................................................. 21
Palladium Carbonyl Complexes ............................................................ 22
Platinum Carbonyl Complexes ............................................................. 22
Cobalt Carbonyl Complexes ................................................................. 23
Rhodium Carbonyl Complexes ............................................................ 25
Iron Carbonyl Catalyst ........................................................................ 26
Ruthenium Carbonyl Complexes .......................................................... 28
Manganese Carbonyl Complexes ........................................................... 30
Rhenium Carbonyl Complexes ............................................................. 31
Chromium, Molybdenum, and Tungsten Carbonyl Complexes ........... 32








C onclusion ......................................................................................................... 37

2 PHOTOPHYSICS OF
LOW VALENT CARBYNES .................................................................................. 38
Introduction ...................................................................................................... 38
Photophysics of Metal Carbyne Complexes .................................................... 39
Synthesis of Cp{P(OMe)3}(CO)W-CR Complexes ........................................ 41
Electronic Structure of Cp {P(OMe)3} (CO)M-CPh (W=37, Mo = 38) ...... 42 Electronic Absorption Spectra ........................................................................... 44
Luminescence Studies ......................................................................................... 48
Transient Absorption Spectroscopy ............................................................... 53
C onclusion ......................................................................................................... 58

3 SYNTHESIS AND PHOTOOXIDATION OF
A TUNGSTEN CYCLOHEXENYL CARBYNE COMPLEX ........................... 59
Introduction ...................................................................................................... 59
Synthesis of (i15-C5H5)(CO){P(OMe)3}W=C(c-C6H9) (44) ........................... 61
Photooxidation of (qi5-C5H5)(CO) {P(OMe)3 }W=C(c-C6H9) (44) ................. 63
Oxidation of (rl5-C5H5)(CO) {P(OMe)3 } W=C(c-C6H9) (44)
in the Presence of Unsaturated Substrates ............. ..................................... 69
Other Reactions with Unsaturated Substrates ................................................. 71
C onclusion ........................................................................................................ 71

4 CATALYTIC OXIDATIVE
CARBONYLATION OF AMINES WITH W(CO)6 ............................................. 73
Introduction ...................................................................................................... 73
Carbonylation of Primary Amines .................................................................... 75
Functional Group Compatibility Study ........................................................... 79
C onclusion ........................................................................................................ 85

5 ALTERNATIVE SYNTHESIS OF CYCLIC UREA
HIV- I PROTEASE INHIBITORS .......................................................................... 87
Introduction ...................................................................................................... 87
Carbonylation of Diamine 85 ........................................................................... 91
Carbonylation of Disubstituted Diamine 91 ................................................... 92
C onclusion ........................................................................................................ 93

6 EXPERIMENTALS .................................................................................................. 94
G eneral ................................................................................................................... 94
General Instrumentation .................................................................................... 94
Photophysical Methods .................................................................................... 95
Syntheses ....................................................................................................... 95
[(CO)5WC(O)(2-napthyl)] [NMe4] (42) ............................................... 95








Cl (CO){P(OMe)3}3W-C-(2-naphthyl) (41) ................................. 96
Cp(CO) {P(OMe)3 }W=C-(2-naphthyl) .................................... 96
(CO)5W=C(OCH3)(c-C6H9) (45) ................................................. 97
[CO(Br){P(OMe)3}3W=X(c-C6H9)] (49) ...................... 97
[Cp(CO){P(OMe)3}W-X(c-C6H9)] (44) .......................................... 98
CpC1{P(OMe)3}W[rl3-CH(c-C6H9)] (51) ........................................ 98
Photooxidation of 44 ............................................................................ 99
General Procedure for the Catalytic Carbonylation
of Benzylamines with W(CO)6 ........................................................... 99
Procedure A ....................................................................................... 99
Procedure B .......................................... ......................................... 100
Procedure C ......................................... ......................................... 100
Preparation of Preparation of N,N'-dibenzylurea 55 ..................... 100
Preparation of NN'-Bis(4-chlorobenzyl)urea 57 .................................. 101
Preparation of N,N'-Bis(4-bromobenzyl)urea 59 .............................. 101
Preparation of N,N'-Bis(3-iodobenzyl)urea 61 ..................................... 101
Preparation of N,N'-Bis(4-methoxybenzyl)urea 63 ..................... 101
Preparation of N,N'-Bis(4-methylthiobenzyl)urea 65 .................... 102
Preparation of N,N'-Bis(4-hydroxymethylbenzyl)urea 67.................... 102
Preparation of N,N'-Bis(ethyl 4-carboxylbenzyl)urea 71 ..................... 102
Preparation of NN'-Bis(4-carboxylic acid benzyl)urea 73 ........... 103
Preparation of N,N'-Bis(4-ethenylbenzyl)urea 75................................. 103
Preparation of N,N'-Bis(4-nitrobenzyl)urea 77 ....................... 104
Preparation of N,N'-Bis(4-cyanobenzyl) urea 79 ................................. 104
Preparation of N,N'-Bis(4-aminobenzyl)urea 81 ........................ 104
Preparation of 3,4-Dihydro-2(1IH)-quinazolinone 83 ............................ 104
Preparation of (4R, 5R)-2,2-Dimethyl-4,5-bisformyl1,3-dioxolane NN-Dimethylhydrazone 88............................................ 105
Preparation of (3R, 4S, 5S, 6R)-2,2-Dimethyl-4,5-bisformyl1,3-dioxolane-3,6-bis(phenylmethyl) N,N-Dimethylhydrazine 89 ........ 105
Preparation of (2R, 3S, 4S, 5R)-2,2-Dimethyl-3,4-bisformyl1,3-dioxolane-2,5-bis(phenylmethyl)diamine 85 ..................................... 106
Preparation of(4R, 5S, 6S, 7R)-Hexahydro-5,6-Oisopropylidene-4,7-bis(phenylmethyl)-2H- 1,3-diazapin-2-one 86......... 106
Preparation of (2R, 3 S, 4S, 5R)-2,2-Dimethyl-3,4-bisformyl- 1,3dioxolane-2,5-bis(phenylmethyl) N,N-Dibenzyldiamine 91 ................... 106
Attempted preparation of (4R, 5S, 6S, 7R)-Hexahydro-5,6-Oisopropylidene-4,7-bis(phenylmethyl)-N,N-dibenzyl1,3-diazapin-2-one 90 ........................................................................... 107

REFEREN CES ................................................. .................................................. 108

BIOGRAPHICAL SKETCH ........................................................................... 120















LIST OF TABLES


Table Vap e

2.1 Absorption Spectra of Cp(CO){P(OMe)3}M-C-R (37-40) in THF Solution ..... 47

2.2 Luminescence and Transient Absorption Data for Cp(CO) {P(OMe) 3}M-CR ........................................................................ 49

4.1 Catalytic carbonylation of primary amines in single/ biphasic solvent system s ................................................................................ 78

4.2 Catalytic carbonylation of substituted benzyl amines ...................................... 81















LIST OF FIGURES


Figure R

1.1 Partial MO diagram for [Cp(CO)2Mn=-C-Me] ....................................................... 3

1.2 Radical cation A prefers bent geometry ........................................................... 20

2.1 Orbital mixing diagram for Cp(CO){P(OMe)3}W C-Ph (37) ............................ 43

2.2 HOMO and LUMO of Cp(CO){P(OMe)3}W-=C-Ph (37) ............................... 45

2.3 Absorption spectrum of Cp(CO){P(OMe)3}W-C-Ph in THF solution at room temperature ................................................................. 46

2.4 Emission and excitation spectra: (a) for 37 and 39 and (b) for 40. Emission spectrum is at right and excitation at left .................................... 51

2.5 Transient absorption difference spectra following laser excitations (355 nm, 10 ns fwhm, 10 mJ/ pulse) .................................................................. 56

3.1 Comparison of 13C NMR data between 51 and known 52 .................67

3.2 Electrospray mass spectral data of 51 ............................................................. 68

3.3 Cyclic voltammogram of 44. Sweep rate: 100 mV/s ......................................... 70

3.4 Investigated reactivity of 53 with unsaturated substrates ......................... 71

5.1 H IV Protease inhibitors ................................................................................... 87















LIST OF SCHEMES


Scheme page

1.1 Protonation of Cp(CO)2W=CTol (7) with HBF4 ............................................ 6

1.2 Protonation of Cp(CO)2W=C-Tol in the presence of alkynes ........................ 11

1.3 Suggested precursors to naphthol complex 18 ................................................. 12

1.4 Photochemical migration of CO to carbyne carbon in Cp(CO)2W-C-Tol (7) ..... 13 1.5 Photochemical isomerization of cis, cis- X(CO)2(PR3)2M-C-Ph ...........14


1.6 Metal-centered reactivity of photooxidized metal carbynes .......................... 16

1.7 Selected organic products from the photooxidation of carbynes ..................... 17

1.8 Mechanisim of photooxidation of Cp(CO) {P(OMe)3} Mo-C(c-C3H5) ...... 18 1.9 Proposed formation of olefins ........................................................................ 19

1.10 Reaction mechanism for the carbonylation of amines with Fe(CO)5 .............. 28

1.11 Proposed two step reaction mechanism .......................................................... 31

1.12 Postulated mechanism for the carbonylation of primary amines in the presence of Mn2(CO)10 .............................................. 32

1.13 Stoichiometric carbonylation of amines in the presence of a tungsten carbamoyl complex .......................................... 34

1.14 Proposed mechanism for the carbonylation of amines in the presence of 32 ..................................................................... 36









3.1 Proposed mechanism for generation of metal carbenes
upon photooxidation of metal carbynes .......................................................... 60

3.2 Synthesis of (Tl-C5H5)(CO){P(OMe)3}W-C(c-C6H9) (44) .......................... 62

3.3 Proposed mechanism for the protonation of carbynes .................................... 65

5.1 Synthesis of protease inhibitor core structure via an amino acid .................... 88

5.2 Preparation of the diamine acetonide 85 ........................................................ 89

5.3 Synthesis of core structure starting from the ester of L-tartaric acid .............. 90

5.4 Dupont process scale-up of cyclic urea 90 ..................................................... 91















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


STUDY AND UTILIZATION OF GROUP 6 METAL COMPLEXES


By


Andrea Denise Main


December 2000


Chairman: Lisa McElwee-White Major Department: Chemistry


This dissertation describes the preparation, reactivity, and photochemistry of a series of low-valent carbyne complexes, Cp(CO){P(OMe)3}M-C-R (M = Mo, W; R = aryl, alkyl). These carbynes were prepared from Mo(CO)6 or W(CO)6 respectively. Photolysis of these carbyne complexes in chlorinated solvents leads to the generation of highly reactive 17 e- species, which are formed by one-electron transfer from the complexes to the solvent. The precursors to these 17 e- species are the excited state of the carbynes. Investigations of the excited states were performed using absorption, emission, and transient absorption spectroscopy.








Previous work done by the McElwee-White group had demonstrated that the photooxidation of carbyne complexes of the type [('rq-C5H5)(CO){P(OMe)3]M-C-R] (M = Mo, W; R = alkyl) results in rearrangement and decoordination of the carbyne ligand to yield organic products. Mechanistic studies suggested that following formation of the 17 e- species mentioned above, H-abstraction took place at the carbyne carbon to yield a carbene. The first direct evidence for the formation of carbene complexes in these reactions was provided by the study of the cyclohexenyl complex [(115C5H5)(CO){P(OMe)3]W-C-C6H9] from W(CO)6. Photooxidation of the carbyne complex afforded the carbene complex [(ir5-C5H5)(Cl){P(OMe)3]W=CH-C6H9].

Following the use of W(CO)6 in the preparation of carbyne complexes, it was then explored as a catalyst for the carbonylation of amines to ureas. Previous McElwee-White group members had demonstrated that W(CO)6 serves as a catalyst for carbonylation of primary and secondary amines to ureas. Primary and secondary diamines can be converted to cyclic ureas by the same process. This dissertation describes the study of functional group compatibility with the catalyst during the carbonylation reaction. A series of substituted benzyl amines were synthesized and carbonylated to the corresponding ureas under the W(CO)6 system. Single and biphasic solvent systems were investigated which revealed that higher yields could be obtained in the presence of the biphasic solvent system, CH2C12 and H20.

This dissertation also describes the synthetic application of the W(CO)6 system towards the synthesis of the cyclic urea core structures of HIV protease inhibitors DMP 323 and DMP 450.













CHAPTER 1
INTRODUCTION



General Overview of Low-Valent Metal Carbynes and Metal-Catalyzed Carbonylation Reactions


Metal carbyne complexes have been extensively explored since their first introduction into the scientific realm in 1973. The first carbyne complexes were lowvalent and of the type X(CO)4M-CR (X = Cl, Br; M = Cr, Mo, W; R = Ph). Although carbynes of groups 5, 7, 8, and 9 have since then been reported as well as carbynes with various R groups, group 6 metal carbynes remain the most common. The constant interest in metal carbyne complexes is due to their potential as organic synthons and their ability to mimic intermediates in catalytic reactions.1,2 Three general reviews by H. Fischer et al.,3 Kim and Angelici,4 and Mayr and Hoffmeister5 have comprehensively covered the synthesis, characterization, and reactivity of metal carbynes up until 1990. Other shorter reviews have covered more specific subjects: low valent group 6 metal carbynes,6"9 bridging carbyne ligands, 10-12 metal carbynes with carbaborane ligands, 13 ruthenium and osmium metal carbynes,14 photochemistry, 15 high valent metal carbynes, 1,16-18 and a survey of the coupling reactions of nt-ligands including carbyne ligands by Mayr and Bastos.19 The most current survey of metal carbyne chemistry is the 1995 review by Engel and Pfeffer.20 Due to the extensive reviews already available








for metal carbynes, this chapter will cover literature after 1995 with emphasis on the reactivity and photochemistry of metal carbyne complexes.

There are a number of different synthetic routes to metal carbyne complexes. One route incorporates a metal carbonyl complex as the starting material.21 Metal carbonyl complexes have not only been useful in preparation of organometallic complexes but also have been found to transform organic substances into other species. Hence, this chapter will also include a review of metal carbonyl complexes that have demonstrated their ability to carbonylate organic compounds, particularly amines.



Bonding
In order to understand the reactivity of low-valent metal carbyne complexes, it is useful to look at the nature of the chemical bonding in these complexes. Fenske analyzed the M-C-R triple bond in neutral and positively charged low-valent carbyne complexes using Fenske-Hal122 approximate SCF calculations.23-25 From these calculations, the a bonds were found to be strong and the 7E bonds degenerate or nearly degenerate between the metal (i.e. chromium, manganese, and iron) and the CR ligand. Kostic and Fenske ascertained from these calculations that the carbyne ligand was a stronger t acceptor than CO resulting in increased electron density on the carbyne carbon.24 A typical MO diagram of 7t interactions is shown in Figure 1.1. The cyclopentadienyl ligand mixes into the former d orbitals to give three non-degenerate molecular orbitals in the metal fragment. However, the HOMO of the resulting complex is still primarily metal-based, and the LUMO is still a t* antibonding orbital.








Further literature includes an ab initio study of a metal carbyne complex, that of trans-Cl(CO)4Cr-CH, along with correlation energies published in 1986 by Poblet et al. These studies showed that the degenerate Cr-C t bond is essentially nonpolar, while the cr bond is polarized toward the carbyne carbon, which leads to a partial negative charge at the carbyne ligand.26 The negative polarity of the carbyne carbon explains the facile protonation of many complexes at the carbyne carbon. Protonation of metal carbynes is said to be charge-controlled. However, in some instances, nucleophiles have been known to attack the carbyne carbon. This mode of reactivity is said to be controlled by frontier molecular orbitals. The HOMO of the nucleophile donates its electrons into the LUMO of the carbyne complex, even though the LUMO is slightly negatively charged.27




-10

-14 -- 3a
-14

ev - a. OM


cp

-22

D
-26


MI. M EI C-Me [-e] 0C 0C ]


Figure 1.1 Partial MO diagram for [Cp(CO)2Mn-C-Me]








Protonation of Low-Valent Metal Carbynes

Following the synthesis of the first metal carbyne in 1973, it was four years before material was published describing the reaction of metal carbyne complexes with electrophilic reagents. Bottrill and Green reported the protonation of the electron rich carbyne [Mo(-CCH2tBu){P(OMe)3}2(rj-C5H5)] (1) with HBF4 which led to the formation of the cationic hydrido species [MoH(=CCH2tBu){P(OMe)3}2(1-C5H5)][BF4] (2) (see equation 1.1).28 The authors were unable to determine the intial site of protonation. Protonation could occur directly on the molybdenum center, or the carbyne carbon could be protonated first, followed by hydrogen transfer to the metal, otherwise known as an ochydride elimination.

+

, t HB4H-- -C-CHtBu(1)

(MeO)3P." C-CH Bu HBF4 - C (1.1)

(MeO)3P (MeO)3P P(OMe)3

2



Numerous reports of protonation reactions followed, showing not only protonation at the metal but also at the carbyne carbon. For example, Holmes and Schrock reported the protonation of [W(-CH)(PMe3)4C1] (3) with CF3SO3H which gave the ligand protonated product [W(=CH2)(PMe3)4C1][ CF3SO3] (4) (equation 1.2). However, by changing the ancillary ligands to the bridging diphosphine Me2PCH2CH2PMe2, protonation occurred at the metal center rather than at the ligand as was seen in the former case (equation 1.3).29













Me3P*.,, 1 .PWe3 Me p I" PMe3
Cl


Me2 Po,,,


Me2


CF3SO3H


H H

Me3 rIt, PMe3 Me3P0""' " %PMe3
Cl 4


CF3SO3H


Me2 1
Cl


Subsequent chemistry revealed that changing the counterion of the acid could greatly influence the course of the reaction. Stone protonated Cp(CO)2W-CTol (7) (Tol = tolyl) with HBF4 to yield the cationic dinuclear pa-alkyne complex 10. The reaction was proposed to proceed through the cationic carbene [Cp(CO)2W=C(H)Tol] (8) (see Scheme 1.1).


CF3SO3+


(1.2)


+CF3SO3


(1.3)










Scheme 1.1 Protonation of Cp(CO)2W=CTol (7) with HBF4


+ BF4-


\
O011W=C-Tol OC


HBF4


\ Tol COH


TOI\ ____ Tol


CP(CO)2W /W(CO)2CP
H


+ BF4Cp(CO)2W=C-Tol


The intermediate 8 was never detected but was inferred by the formation of Cp(CO)2(I)W=C(H)Tol (11) when the reaction was run in the presence of iodide (equation 1.4).30,31


+ BF4-










\7Tol
Oo""CiM -To1 HBF4 0" OC I H (1.4)
I

7 11



However, protonation of Cp(CO)2W-CTol in the presence of HCl instead of HBF4 resulted in double protonation of the carbyne carbon to form an alkyl ligand that subsequently yielded the il2-acyl complex 12 upon CO insertion (equation 1.5).32,33



2 HCl OC--j(15
4/ CH2Tol (1.5)
OC C1

7 12



Another example of counterions affecting the reaction pathway is the protonation of Cp {P(OMe)3}2Mo-CCH2tBu (1) with CF3COOH. Unlike the reaction of 1 with HBF4 (equation 1.1), CF3COO" coordinates to the molybdenum center resulting in loss of the carbyne ligand.34

It was also discovered that protonation could take place across the face of the carbyne complex resulting in an agostic interaction of the C-H bond with the metal (for example, see equation 1.6).35,36 This type of reactivity is better understood by taking into account that the probability of hydrogen migration from the carbyne carbon to the








metal center increases when there is little nt-acceptor character in the ligands. If hydrogen migration did occur in complexes with strong t acceptor ligands like CO, the formation of the localized M-H bond would eliminate it backbonding from the metal center back into the t acid ligand, resulting in ligand loss. Thus, it-acceptor ligands inhibit full migration of the hydrogen to the metal center due to competition for the d electrons resulting in agostic interactions.6



Co 0
,PMe3 .PMe3 Me
C--- C-Me HCl C1- _ e (1.6) Me3 Me H
PMe3 Cl
13 14



Consistent with the explanation above, McElwee-White reported the protonation of the carbyne CpL2Mo-CBu (15) [L = CO, P(OR)3] with HBF4 (equations 1.7-1.9)37 which resulted in a systematic shift of the hydrogen from the metal center to the face to the carbyne ligand as the number of carbonyl groups were increased from zero to two. These results are consistent with determination of the final protonation site by the electron density at the metal. As mentioned before, when it-backbonding to the carbonyl ligands decreases the electron density at the metal, the protonation site shifts from the metal toward the carbyne carbon.













(MeO)3P"'/ CBu (MeO)3P


15a










Oo == c CBu (PhO)3P
15b


HBF4


(MeO)3i P'le= CBu

(MeO)3P


(1.7)


16a


HBF4


Mo CBu

ocPh H (PhO)3P H


+ BF4(1.8)


16b


OCM CBu HBF4 oc


16c


+ BF4(1.9)








Protonation in the Presence of Alkynes



Metal carbenes have found numerous applications in organic synthesis38-41 and one important class of reactions is one that occurs between carbene complexes and alkynes. The product isolated depends on the metal, the ligands, the substituents on the carbene, and the reacting alkyne. Shown in equation 1.10 is a typical example of a metal carbene reacting with an alkyne.



H
/OMe 80CR

(CO)5CF + RC CR heptane + / \heptane R OMe
(1.10)
R
+
P OMe
OMe



The reaction of metal carbenes with alkynes has been extended to the same type of chemistry with metal carbynes. As mentioned earlier, protonation of metal carbynes often leads to the intermediacy of a metal carbene. Analogous to the reaction in equation 1.10, metal carbynes have been protonated in the presence of alkynes to form addition products. For example, when Cp(CO)2W-C-Tol (7) reacts with HBF4 and acetylenes, depending on the R groups on the acetylene and the presence of carbon monoxide, 71allylidene (17) or il4-naphthol (18) products result (Scheme 1.2).42 The naphthol








complex 18 is thought to form by either the 94-ketenyl complex 19a or the metallacycle 19b (Scheme 1.3), both of which are derived from protonation of the metal carbyne at the ligand, followed by insertion of the alkyne into the M-C bond. This is still a relatively new field of chemistry and could lead to a number of different products, depending on the carbyne, the acid, and the unsaturated substrates used.



Scheme 1.2 Protonation of Cp(CO)2W-C-Tol in the presence of alkynes


PhC CPh


O0111 / C-Tol

C


MeOmCMe CO/ 1 atm


L T Tol
OO --\Ph


Ph 17

BF4


Me

WCp(CO)2
HO Me








Scheme 1.3 Suggested precursors to naphthol complex 18.

H

_ / Me Cp(CO)2W Me
19a M + BF4"


p(O)W 0HMWCp(CO)


Me Me

19b

Me
HOO MWCP(CO)2 18




Photochemistry


Relatively little is known about the photochemical properties of metal carbyne complexes. The electronic character of ligands attached to metals can undergo significant change by photoexcitation, especially if the excited states involving those ligands are populated. Carbyne complexes should have a rich photochemistry due to the unsaturated character of the metal-carbyne moiety and the presence of long-lived and low-lying metal-to-carbyne ligand-charge transfer (MLCT) excited states.43 There are a number of examples in literature where photoexcitation of metal carbyne complexes has led to various reaction pathways that would be unavailable to the carbyne in the ground








state. The most common photochemical processes among metal carbyne complexes involve carbonyl ligands. Mayr et al.44 and Geoffroy et al.45 have established that carbonyl-carbyne coupling is an important photochemical reaction pathway in some systems. Photolysis of the electron rich Cp(CO)2W-C-Tol in the presence of phosphines gives 1 I- and n2-ketenyl complexes, depending on the phosphine employed (Scheme

1.4).



Scheme 1.4 Photochemical migration of CO to carbyne carbon in Cp(CO)2W-C-Tol (7)


Cp (CO)2W- C-Tol

7


0


Cp (PPh3)(CO)w20 Tol


I PPh3


hv


0

C
Cp(CO)(dppe)W-/
Tol
21


0


[Cp (CO)2W- C- Tol] *- Cp (CO)W- C.,.

ALCT Excited State 11 -ketenyl


0 A


I
Cp (CO)W- C- Tol Cp (CO)2W3 C- Tol
23


CO


dppe= (Ph)2PCH2CH2P(Ph)2


Tol CO 122
Cp (CO)2W- C-,,
Tol


/+7
0
Toll A -Tol

Cp (CO)2W W(CO)2 Cp

24








The mechanism is not completely understood, but the increased electron density at the carbyne carbon in the MLCT state is thought to facilitate migration of the CO ligand and subsequent formation of a 16 e- ketenyl complex. Further reaction of the ketenyl complex with phosphines leads to the products 20 and 21. In the photoassisted cis-trans isomerization of complexes of the type X(CO)2L2M=C-Ph (X = Cl, Br; L = PMe3, PPh3; M = Mo, W), Mayr has deduced the presence of r92-ketenyl intermediates (Scheme 1.5). In the photolysis of cis 25, the reaction forms a five-coordinate ketenyl species in which the phosphines subsequently rearrange to be trans to each other, minimizing steric forces, followed by migration of the carbonyl back to the metal center to give trans 26.

Scheme 1.5 Photochemical isomerization of cis, cis- X(CO)2(PR3)2M-C-Ph



L1 L
C-Ph X-M= C-Ph

CO CO

25 26


hv




0 Ph


CO CO

0 Ph








Another interesting reaction found in literature is the photolysis of CI(CO)2(PMe)3W-CPh with excess phenylacetylene which gives the ril-ketenyl complex W(l'-PhCCO)CI(CO)(PhCCH)(PMe3)2.45

Photochemical electron transfer has been shown to activate 18 e- complexes toward reactivity. Due to differences in reactivity between open- and closed-shell organometallic complexes, reaction pathways unavailable to neutral 18 e complexes can often be accessed by the ensuing 17 e- radical cations.46-48 This strategy has proven effective for metal carbynes. Over the past several years, the McElwee-White laboratory has been investigating photooxidation reactions involving irreversible electron transfer from a series of tungsten and molybdenum aryl- and alkylcarbynes to halogenated solvents.49 Early experiments involved photolysis of the carbynes CpLIL2M-CR 27 [Li, L2 = CO, P(OMe)3; R = alkyl, aryl; M = Mo, W] in CHC13 solutions. When the reactions were run in the presence of PMe3 to stabilize any unsaturated intermediates, new cationic carbyne complexes resulted as shown in Scheme 1.6.49,50 Mechanistic studies showed the primary photoprocess to be electron transfer from the excited state of the carbyne to CHCI3. Upon reduction, CHC13 undergoes fragmentation to form chloride ion and dichloromethyl radical resulting in irreversible electron transfer. The ensuing 17-electron cationic carbyne 27+� then undergoes metal-centered reactivity involving ligand exchange with the more strongly donating PMe3 to form 28 followed by halogen abstraction to yield the final cationic carbyne 29.








Scheme 1.6 Metal-centered reactivity of photooxidized metal carbynes.


Mo CR L2%"'"'/
L, 27


+ 27"


hv

CHCI3


PMe3


+ 27"


+ CI" + oCHC12


Me3P""'/ CR
PMe3


C- MCR Me3P,-"i
PMe3


+


CI" + .CHC12


In the absence of strongly nucleophilic species such as PMe3, the photolysis of carbyne complexes results in very different reactivity. If the carbyne substituent is a primary or secondary alkyl group, photooxidation yields carbyne ligand-centered reactivity resulting in organic products (Scheme 1.7).


+ CI"


CHC13








Scheme 1.7 Selected organic products from the photooxidation of carbynes.


(MeO)3P
CO







(MeO)3PO"'/J
CO





Mo=C (MeO)3P/
CO


6


hu CDCI3





hu CDC13





hu CDC13


Mechanistic studies revealed that the reactions in Scheme 1.7 share the same first two mechanistic steps. Scheme 1.851 shows these first two steps in the transformation of of Cp(CO){P(OMe)3}Mo-C(c-C3H5) (30) into cyclopentenone. After photoexcitation of 30, the first step involves one electron transfer from the excited state of the carbyne to the solvent, forming the 17-electron radical cation A. In the second step, the radical cation A abstracts a hydrogen from the reaction medium to form the cationic carbene B which undergoes ring expansion to form the metallacycle C. The mechanism then proceeds with insertion of CO into the metal-alkyl bond and coordination of C1- to form D. Reductive elimination of the organic ligand produces the cyclopentenone.







Scheme 1.8 Mechanisim ofphotooxidation of Cp (CO){P(OMe)3}Mo=C(c-C3H5)






ccO e)
P(OMe)3


C CHC13
CHC12. + Cl-


A
oc"" i C
P(OMe)3

A


"He"


OC/ H P(OMe)3


C-


oc e)
P(OMe)3


'/6o
(MeO)3P '
Cl


6





19

Similarly, in the formation of 1-butene from the photolysis of Cp(CO){P(OMe)3}Mo-C-n-C4H9 (31), the first step is electron transfer from the excited state to the solvent, which forms the seventeen-electron radical cation A, followed by Habstraction at the carbyne carbon to yield the cationic carbene complex B (Scheme 1.9).52 However, at this point, B undergoes an H-shift rather than CO insertion to form the cationic Tr 2-olefin complex C, which subsequently eliminates to form the terminal olefin.



Scheme 1.9 Proposed formation of olefins







.MC-CH2CI7 hvoP McP&CCH2C3H] , CHC13 CO
CO 31 A

+ RH
- R+

H, ,H 1 H-shift / H

(MeO)3PM ,C- (MeO)3P'/ CH2CH
A I 2--M C A
L CO C H C3Hi Lo CO BJ

It should be noted that the H-abstraction process (A-+B) for an organometallic radical is highly unusual. Reactivity at an organic ligand is known to occur in metal radicals but reaction at the metal center is much more common.53 When ligand-centered reactions do occur, they are almost always dimerization processes. In order to explain the ligand's affinity for the hydrogen, INDO calculations were carried out on the carbyne








radical cation A as the angle 4 was varied from 1800 to 1200.52 These calculations determined that the radical cation A favors a bent carbyne configuration (Figure 1.2) over the original linear carbyne geometry. The geometry change causes spin density to reside on the carbyne carbon.




&WM oC-C4H 9CN""""-C+ (MeO)3P (MeO)3P

A Preferred geometry Figure 1.2 Radical cation A prefers bent geometry



This type of chemistry has resulted in the formation of a variety of olefins, dienes, dienals and other cycloalkenones from the photooxidation of various alkyl metal carbynes.



Carbonylation with Transition Metals



Carbonylation of organic compounds in the presence of transition metal catalysts has become a topic of increasing interest. One facet of this chemistry is carbonylation involving metal carbonyl complexes.54,55 There are numerous reactions throughout the literature that invoke metal carbonyl complexes in the catalytic carbonylation of organic compounds. There are reviews of this type of chemistry56,57 and a multitude of articles describing catalytic transformations such as olefins to aldehydes58 or carboxylic acids,59








arenes to aromatic aldehydes,60 aryl halides to aromatic acids,61 alkyl halides to esters,62 nitroaromatics to carbamates,63 and alkynylbenzylalcohols64 or epoxides65 to cyclic carbonates. Typically, late transition metals, groups 8-10, are used to catalyze these transformations, although there has been one gold(I) carbonyl catalyst (group 12) reported.66



Catalytic Carbonylation of Amines with Metal Carbonyl Complexes



Further example of these reactions is the catalytic carbonylation of amines with metal carbonyl complexes to form isocyanates, ureas, and carbamates. There is continued interest in developing new synthetic routes for such compounds for multiple reasons: isocyanates are used as precursors for polyurethanes and polyureas;67 ureas are used in the pharmaceutical industry as well as in agricultural products;68 and carbamates not only act as chiral auxilaries, but also are found in pharmaceuticals.69 Furthermore, interest in new synthetic routes continues in an effort to find alternatives to the traditional method of carbonylating amines using phosgene, a highly toxic and corrosive gas.70



Nickel Carbonyl Complexes



Initial studies of the transition metal catalyzed carbonylation of amines date back to World War II where nickel salts were added to amines and the catalytically active species were undoubtedly metal carbonyls.71-73 However, subsequent carbonylation








reports of amines using Ni catalysts are few with very low product yields,71 and none of the catalysts are carbonyl complexes.



Palladium Carbonyl Complexes



Palladium catalysts are the most commonly used transition metal catalysts for the carbonylation of amines. Although the actual catalysts are not carbonyl complexes, a significant amount of the reactions are thought to proceed through palladium carbonyl intermediates. For instance, palladium acetate promoted by iodine can carbonylate amines in the presence of alcohols to carbamates and is thought to proceed through a Pd(O)(carbonyl) complex.74 In the absence of alcohols, ureas are formed.75

Mono- and di-methoxycarbonyl complexes of palladium of formula [PdL2C12. n(COOCH3)n][n = 1 or 2; L2 = bipyridine or 1,10-phenanthroline] have been found to carobnylate amines to ureas at room temperature under CO pressure. In the presence of CuC12, the reaction generates carbamates rather than ureas.76

Furthermore, a number of palladium catalysts that also carbonylate amines to ureas and carbamates have been reported by Kanagasabathy and Chaudhari,77 Alper et al.,78,79 and Valli and Alper.80



Platinum Carbonyl Complexes



The one example in the literature using platinum carbonyls to carbonylate amines is the photolytic reaction of Pt(CO)2(PPh3)2 with morpholine and cyclohexyl iodide under








1 atm of carbon monoxide at room temperature which gives the corresponding amide in 75% yield (see equation 1.12).81


-/ N-H+


hv, Pt(CO)2(PPh3)2
K2CO3

1 atm CO


75%


Cobalt Carbonyl Complexes


The earliest report of a cobalt carbonyl complex carbonylating amines was in 1953 by Sternberg who generated formamides in 60-78% yields from the carbonylation of secondary amines, dimethylamine and piperdine, with Co2(CO)8 under 200 atm of carbon monoxide at 200 �C (equation 1.13).82 The key intermediate in this reaction is assumed to be a carbamoyl species H(CO),Co-CONR2 which produces the formamide upon reductive elimination.


20QNH


3 Co2(CO)8
20 8
200 atm CO


N-C-H + 2 [ColI(piperdine)6 ] [Co2(CO)8]
78% (1.13)


Other reports of Co2(CO)s-catalyzed processes include the carbonylation of allyl amine under even more severe conditions with 300 atm of carbon monoxide at 280 �C which afforded 2-pyrrolidine (equation 1.14). Although run at room temperature,


(1.12)








photolysis of this reaction later proved even more inefficient with low yields, and product mixtures containing 2-pyrrolidone, N,N'-diallylurea, and N-allyl-3-butenamide. The photochemical cleavage of C-N bonds of amines coordinated to Co was proposed.83


,,,NH2 +


Co2(CO)8 (300 atm) 280 �C


H


(1.14)


There are a few examples of a metal carbonyl compound catalyzing the ringopening and carbonylation of aziridines. The ring strain present in three-membered ring compounds suggests ring-opening/ carbonylation could take place under relatively mild conditions. One of those examples is the treatment of 1,2-disubstituted aziridines with 33 atm of carbon monoxide and Co2(CO)8 in DME (1,2-dimethoxyethane) for 24 h at 100 C to yield 1-lactams in up to 95% yield. The reaction is regiospecific with insertion of a carbonyl into the least substituted of the two carbon-nitrogen bonds in the ring.84


R R'
Hll'V.. R"

N I
CH2CH2Ph


Co2(CO)8, DME, CO (33 atm)
100 �C, 24 hr
PhCH2CH2


(1.15)







Rhodium Carbonvl Complexes


When subjected to the rhodium catalyst [Rh(CO)2C1]2, primary amines are converted to the corresponding formamides or ureas, depending on the conditions. For instance, n-butyl amine forms a mixture of N-butylformamide and di-n-butylurea in the presence of the catalyst under 59 atm of carbon monoxide at 160 C (equation 1.16). However, when the reaction is carried out in the presence of PMe3, N-butylformamide is formed in quantitative yield.85


[Rh(CO)2Cl12 60 atm CO
160 �C


BuNHCHO
35%


+ BuNHCONHBu (1.16)
65%


In contrast to the severe temperatures and pressures required for the Co2(CO)8catalyzed carbonylation of allyl amines, Rh4(CO)12 and [Rh(CO)2C1]2 carbonylated butenyl amine under milder conditions to give 2-piperidinone, exclusively (equation 1.17). The conditions used in these rhodium-catalyzed reactions include 400 psi of CO/H2 and temperatures of 40-50 �C. Yields varied from 85-87%.86


+ CO/H2 [Rh] HH


Me

0
H


(1.17)


BuNH2










Pyrrolidinones were isolated as the only products when aliphatic N-allyl- or N(methylallyl)amine was carbonylated in the presence of HRh(CO)(PPh3)3. The reaction requires the addition of NaBH4 and isopropanol, and takes place under 35 atm of carbon monoxide at 100 C. Specifically, N-allylcyclohexylamine afforded N-cyclohexyl-2pyrrolidinone in 78% yield (equation 1.18). The yield decreases appreciably if NaBH4 is substituted by other borohydrides.87





CO, HRh(COXPPh3)3 0 IN (1.18)
NaB-4, i-PIOH, CH2C12
35 atm, 100�C




Additional Rh reports include the catalyst Rh(CO)2(acac) which carbonylates aniline under 41 atm of a CO/ 02 gas mixture, giving N,N'-diphenyl urea (74% selectivity) and N-phenyl carbamate (24.5% selectivity). When Nal, acting as a promoter, is added to the reaction, there is an increase both in the activity of the catalyst and the selectivity for the carbamate.88



Iron Carbonyl Catalyst



Formamide complexes can be obtained by the reaction of piperidine with Fe(CO)5 (equation 1.19). However, this reaction is limited to piperidine. When pyrrolidine or








pyridine is reacted under the same conditions, iron-amine complexes form.89 Furthermore, when the catalyst reacts with ammonia, urea is formed.





6 5 Fe(CO)5 Fe Fe4(CO)3 + 6 CO (1.19)





Dombek and Angelici later repeated this chemistry with the addition of CO and found that they could carbonylate primary (cyclohexylamine) and secondary (piperidine) amines to the corresponding formamides under 95 atm of CO at 200 C in 66-83% yields, respectively. Their proposed mechanism is shown in Scheme 1.10. Reaction A involves the formation of a carbamoyl complex anion, Fe(CO)4[C(=O)NR2]". This reaction is rapid and reversible at room temperature with piperdine, pyrrolidine, and n-butyl amine. Upon heating at 50 0C for 50 hr, the carbamoyl complex decomposes to Fe(CO)4(HNR2) and the formamide, HCONR2. The authors propose that this decomposition occurs via steps B, C, and D. Step B involves proton transfer from the ammonium cation to the iron atom giving the hydride complex which upon reductive elimination, generates the formamide in step C. The resulting coordinatively unsaturated species Fe(CO)4 then reacts with excess amine to the give the final observed product, Fe(CO)4(HNR2). Further reaction of Fe(CO)4(HNR2) in the catalytic reaction with CO could generate Fe(CO)5 as is shown in step E.90








Scheme 1.10 Reaction mechanism for the carbonylation of amines with Fe(CO)5


A
2 HNR2












H NR2

C


OYNR2

OC-Fe- CO 0c I
CO


B
-HNR2



OyNR2




CO


H2NR2 +


HNR2

Fe(CO)4(HNR2)


Ruthenium Carbonyl Complexes


As described earlier, Co2(CO)s and Fe(CO)5 can catalyze the carbonylation of piperdine to the formamide, but under severe reaction conditions. The ruthenium catalyst, Ru3(CO)12, can convert piperdine to the formamide under much milder conditions (75 C, 1 atm CO).91

Typically, secondary amines are more easily converted to formamides than primary amines. Stringent conditions are usually required for primary amines. For


Fe(CO)5





E

CO Fe[CO]4








example, Kealy and Benson reported that carbonylation of the primary amine, cyclohexylamine, in the presence of allene gave N-cyclohexylformamide using Ru3(CO)12 as the catalyst. This transformation, however, required extremely high carbon monoxide pressure (1000 atm).92 Watanabe also used Ru3(CO)12 to catalyze the carbonylation of various primary amines to the corresponding formamides with much lower levels of CO but rather high temperatures (39 atm, 180 oC).93 Again, a carbamoyl species is suggested as the key intermediate in the formation of formamides. It is postulated that the carbamoyl complex is formed by either an intermolecular nucleophilic attack of the amine on the metal carbonyl ligands or by an intramolecular 1,2-shift reaction between coordinated carbon monoxide and amine. The authors could not isolate the carbamoyl species but its presence was evident in the IR.

Aniline can be carbonylated in the presence of various ruthenium carbonyl complexes, such as Ru(CO)313, Bu4N[Ru(CO)313], and Ru(CO)2(py)C1288,94 to give Nbenzylcarbamate or N,N'-diphenylurea. The carbamate is formed in moderate yields with a 13:1 mixture of CO and 02 in methanol at 170 C.

Cyclic amines can be carbonylated to the formamides with ruthenium cluster compounds, [HRu3(CO) i]- and [H3Ru4(CO)12]. Both carbon monoxide and a mixture of hydrogen and carbon dioxide gasses are used. A neutral carbamoyl cluster complex was isolated and thought to be a possible intermediate.80








Manganese Carbonyl Complexes



The manganese carbonyl complexes, Mn2(CO)10 and CH3Mn(CO)5, have also been explored for the carbonylation of amines.95 Again, stringent conditions are required with reactions run between 180 and 200 �C under 130 atm of CO. In these reactions, primary amines gave dialkylureas exclusively, contrary to previous reactions which primarily form formamides. Also, it should be noted that ammonia, secondary and tertiary amines, and aniline react very slowly, with the corresponding urea being the primary product in each case with little formation of the formamide. Calderazzo proposed that the poor yields of formamides in these reactions were due to the over-all reaction occurring in two steps with k2 being probably much larger than ki (Scheme 1.11). If k, were larger than k2, little or no dialkylurea would be formed. Calderazzo concluded that since both steps occur in the coordination sphere of the metal, the condition for the formation of the urea is that the formamide formed in the first step remains on the metal and undergoes a fast nucleophilic attack on the formyl carbon atom by a second molecule of amine.

Dombek and Angelici later explored the reaction mechanism more thoroughly.94 By studying several stoichiometric reactions, they proposed the following mechanism which is detailed in Scheme 1.12. The authors admit that there are possible alternative mechanisms for individual steps but overall, given the supporting evidence, Scheme 1.12 best describes the carbonylation of primary amines with Mn2(CO)1o. There are three key steps in the mechanism. The first describes the formation of the carbamoyl complex from the reaction of the amine with Mn2(CO)lo. The second step depicts the reaction of








the amine and CO with the carbamoyl complex to give the urea via the organic isocyanate. In the third step, Mn2(CO)1O is regenerated from Mn(CO)5- and RNH3'. In addition, HMn(CO)5, an intermediate in the reaction, was also found to catalyze the reaction, probably by a similar mechanism.

Finally, Srivastava and coworkers have used the manganese complex, (rICH3C5H4)Mn(CO)3, to carbonylate primary amines by irradiation with UV light for 100250 hours. The non-catalytic reaction uses the manganese complex simply as a source of CO.72



Scheme 1.11 Proposed two step reaction mechanism



Step 1: H2N-R + CO k, HCONHR



Step 2: HCONHR + H2N-R k2 RNHCONHR + H2



Rhenium Carbonyl Complexes



The only report of a rhenium complex carbonylating amines is the one described by Hieber and Schuster.96 They invoke the use of Re(CO)5C1 in the stoichiometric carbonylation of ammonia to formamide. Such carbonylations with third row transition metals are rare.










Scheme 1.12 Postulated mechanism for the carbonylation of primary amines in the presence of Mn2(CO)o.


(CO~sMn-Mn(CO%


-1/2 H2 HMn(CO)


NH2R

-NH3R+


Mn(CO)5- +


NH2R

M6 OCO-I-, c-fO
CoI
NHR


CO -NH2R


-NH2R


NH3R+


-NH2R


Mn(CO)5- +


Co
OCI/Co 0 NHR


RHN-C-NHR NH2R -NH3R NH2R OC /CO0
R--:C-OOC/ I 0

NR-


Chromium, Molybdenum, and Tungsten Carbonyl Complexes


There are very few examples of Group 6 metals carbonylating amines. Doxsee and Grubbs have described the preparation of dimethylformamide from the stoichiometric reaction of Cr(CO)6 with LiNMe2.97 This example involves nucleophilic








attack at transition-metal-bound carbon monoxide followed by reduction by molecular hydrogen under exceptionally mild conditions (equation 1.20).




LiNMe2 OLi H2
Cr(CO)6 (CO)5Cr - H N (1.20) NMe2



Angelici has reported the reaction of [CpM(CO)4]PF6 (M = Mo, W) with excess CH3NII2 to give 1,3-dimethylurea.98 As shown in Scheme 1.13, when the reaction is run in the presence of two equivalents of amine, the carbamoyl complex CpM(CO)3(CONHCH3) was obtained which suggested that the first step was a nucleophilic attack on a carbonyl ligand. Further reaction of the carbamoyl complex with excess amine gave the corresponding urea. When the reaction is run in excess trimethylamine, an organic isocyanate is obtained. The authors postulate a mechanism similar to the manganese mechanism depicted in Scheme 1.12. The first equivalent of amine deprotonates the carbamoyl ligand producing an isocyanate, which then reacts with another equivalent of primary amine to yield urea.

Finally, the McElwee-White laboratory has reported the carbonylation of secondary amines which produce formamides in the presence of the iodo-bridged dimer [(CO)2W(NPh)2]2 (32) (equation 1.21).99 This stoichiometric reaction proceeds at room temperature for a variety of secondary amines although reported yields are low. When 32 was reacted with primary amines under the same reaction conditions, primary amines








Scheme 1.13 Stoichiometric carbonylation of amines in the presence of a tungsten carbamoyl complex


2 NH2R


OC"W CO OC CO


excess
NEt3



R-N=C=O


OC,"'A l''CO

0-- c CO


R exes H

excess NH2R


R N N ' Nj R

I I
H H



were selectively carbonylated to the corresponding 1,3-disubstituted ureas (equation 1.22). Furthermore, when 32 was reacted with a,o)-diamines under stoichiometric oxidative carbonylation conditions, cyclic ureas were formed. Further research resulted in the carbonylation reactions being catalytic in 32 when the reaction was run under excess carbon monoxide. A possible mechanism for this transformation is shown in









Ih

so""' r Izz" .\\
OCO"O1 Iio" bCO



Ph
32


h




0 C�V" T* iIoe C 0

I j4

Ph
32


/R1
H-

R

CH2C1

[Ox]


R
H
I (1.21)
R


/I
R-N

H

CH2C12

[Ox]


R


H (1.22)


Scheme 1.14. IR spectra of the reaction mixtures were consistent with the presence of a carbamoyl intermediate. Following formation of 33 with four equivalents of amine, the carbamoyl complex 34 was proposed to form after a nucleophilic attack by the amine on a carbonyl ligand followed by deprotonation with an additional equivalent of the amine. Conversion of 34 to the isocyanate 35 is postulated, and is thought to proceed by oxidation at the metal followed by deprotonation with excess amine. However, no free or coordinated isocyanates have been observed in the reaction mixtures. Nucleophilic attack on the isocyanate complex or free isocyanate by another equivalent of amine is thought to








form the urea and the cationic complex 36. Complex 36 now has a free coordination site that in the presence of excess CO would render the reaction catalytic.



Scheme 1.14 Proposed mechanism for the carbonylation of amines in the presence of 32



00,,. V, A I, f .,,CO 2eq NH2R 0(j,
0C40 11K0lf*C OCe1Th*NH2R

Ph
32

1 eq NH2R


CO
---------- ----------*-


MNHR


I1eq NH2R


lox]
1 eq NH2R


33


2 eq NH2R



lh

OG,,.. .,,, NH- A H 34' NHA H 34





37

Conclusion

Although the use of organometallic complexes as organic synthons has been explored for many years, there are multitudes of avenues yet to be investigated. The remainder of this text will focus on investigations of reactive intermediates in the photolysis of metal carbyne complexes as well as examine further applications of metal carbonyl complexes in the catalytic carbonylation of amines.













CHAPTER 2
PHOTOPHYSICS OF LOW-VALENT CARBYNES



Introduction



There is increasing interest in discovering transition metal complexes that will not only luminesce, but also be useful in the preparation of chemical products. Such species are rare. Typically, photolysis of organometallic complexes with carbonyl ligands results in ligand loss as the primary photoprocess due to population of the metal-ligand antibonding orbital via excitation of the d-d manifold.100 However, the "ligand loss" photoprocess is not the only photoprocess available to metal carbyne complexes. There are a number of examples where photolysis of metal carbynes has led to the conversion of the carbyne moiety to other organic ligands.45,101-103 As mentioned in the previous chapter, metal carbyne complexes containing carbonyl ligands are known to undergo migration of the carbonyl to the carbyne ligand, resulting in the formation of rl2-ketenyl complexes (equation 2.1).45 The migration is described as the result of increased electron density on the carbyne carbon due to a MLCT transition.

Prior to the research described herein, it was found that one-electron transfer from the MLCT excited state of a series of tungsten and molybdenum alkylcarbynes to halogenated solvents resulted in unprecedented ligand-centered radical processes.51,52,104-108










0 'W C-Ar + PMe3 0 \ //C (2.1)


Co Ar PMe3


This is highly unusual. Not only is reactivity at an organic ligand in metal radicals far less common than reactions at the metal center,53 but also, if this were 1 e- oxidation of the MLCT excited state, oxidation would take place at the ligand and the ensuing radical would be metal centered. Namely, following oxidation, reactivity would be expected at the metal center.



Photophysics of Metal Carbyne Complexes



In order to understand the unusual reactivity of metal carbyne complexes, there have been a number of photophysical studies of complexes that contain the metalarylcarbyne functionality, M=C-Ar, where M = W(IV), Mo(IV), and Os(IV).109 This family of complexes displays remarkably similar absorption and luminescence properties, despite having a rather wide variety of ancillary ligands in the coordination environment. In general, the absorption of M-=C-Ar complexes is characterized by a weak band in the 400-500 nm region ( c z 102 M'1cm") and a more intense band between 300 and 350 nm (S Z 104 M-cm'). Theoretical and spectroscopic studies indicate that the low energy band is associated with the HOMO -+ LUMO transition, which is dominated by the d(M)

- 1t*(MCAr) configuration.23,24,43,49,110-112 The low intensity of the HOMO -->








LUMO transition indicates that it is forbidden; the transition is likely parity forbidden but spin-allowed (i.e. a singlet-singlet transition). The transition is parity forbidden owing to significant degree of d-character in the HOMO and LUMO. The more intense near-UV band is associated with a spin-allowed n(M-=C-Ar) -+ n*(M-C-Ar) transition, which bears some analogy to the n -+ n* transition of the Ph-C-C-Ph chromophore.

There are also several reports of luminescence from complexes that contain the M-C-Ar functionality.43,49,110,111 For example, complexes of the type X(CO)2(L)2W=C-Ph and [Ph-C=Os(NH3)5]3+ exhibit weak luminescence in the 600-650 nm region which has been attributed to the d(M) -+ n*(M-C-Ar) excited state. The emission lifetimes of these complexes range from 50 to 250 ns, with quantum yields typically on the order of 5 x 10-4. The comparatively long excited state lifetimes coupled with relatively low radiative rates (kr & 103 s-1) strongly imply that the luminescent state has triplet spin character. This assertion has been confirmed in several cases by the observation of triplet energy transfer from a carbyne excited state to organic triplet state acceptors. 110,111 The triplet character of the luminescent state in the carbyne complexes explains the large Stokes shift of the emission relative to the low intensity absorption in the mid-visible region, which is assigned to the spin-allowed (singletsinglet) absorption.

This chapter provides further insight into the excited states of metal carbyne complexes of the type Cp(CO){P(OMe)3}M=C-R, where M = Mo or W and R = aryl (complexes 37-40). These investigations are to provide further understanding of the ligand-centered reactivity at the carbyne ligand. Considering MLCT excitation does not usually lead to reactivity at the ligand, the question to be answered is whether this








photoprocess is best described as MLCT or something else. Answers can be found by examining the electronic structure, the excited states, the emissive states, and lifetimes of these states.






OC"' M C-Ar

P(OMe)3




37: M = W, Ar = phenyl 39: M = W, Ar = o-tolyl

38: M = Mo, Ar = phenyl 40: M = W, Ar = 2-naphthyl





Synthesis of Cp(CO) {P(OMe),} M-C-R Complexes



The carbyne complexes Cp(CO){P(OMe)3}M-C-Ph (W = 37, Mo = 38) and Cp(CO){P(OMe)3}W-C(o-Tol) (39) were prepared as described previously.49,113-115 Preparation of the napthylcarbyne complex Cp(CO){P(OMe)3}W-C(2-Np) (40) involved an analogous route. The tris(phosphite) complex CI(CO){P(OMe)3}3W-C(2-Np) (41) was prepared by reaction of [(CO)sW(CO{2-Np})][NMe4] (42) with oxalyl chloride, followed by addition of excess trimethyl phosphite to give the impure tris(phosphite) complex 41. Reaction of 41 with fresh sodium cyclopentadienide (CpNa) led to displacement of the chloride anion and two P(OMe)3 ligands to yield 40. The crude 40








required more extensive purification than is necessary for 37-39, but was ultimately isolated in 20% yield.



Electronic Structure of Cp(CO) {P(OMe)3_I M=C-Ph (W = 37, Mo = 38)t



As an aid to interpreting the spectra (vide infra) of 37-40, extended Hfickel calculations were performed on 37 and 38.116 The results of these one-electron calculations are useful in visualizing the coefficient distributions in the frontier orbitals and providing a rough sequence of orbital energies for 37-40.

Figure 2.1 shows a partial molecular orbital diagram for the formation of 37 from the fragments Cp(CO){P(OMe)3}W" and CPh , calculated in the same manner as 37. The orbital orderings are similar to those previously reported for the model compound Cp{P(OH)3}2Mo=-CPh,49 although the energetic splittings are somewhat different. These calculations show that the HOMO of the complex (Figure 2.2) is comprised largely of the metal d.2-y2 orbital but is partially devoted to back-bonding into the nt* orbital of the CO ligand. Below it are the two metal-carbon r bonds, their degeneracy broken by conjugation of the lower one with the phenyl ring. The LUMO is the orbital identified as the M-C n* orbital that is conjugated into the phenyl n-system. However, inspection of Figure 2.2 reveals that it too is primarily composed of a metal d orbital delocalized into the n* orbital of the CO ligand. The NLUMO is a nearly pure M=C it' orbital. Extended Hilckel calculations were also performed on the molybdenum compounds 38, and the



t The work in this chapter was done in conjunction with Thomas K. Schoch. Helpful discussions and analysis of spectral data were contributed by Dr. Kirk S. Schanze.











y



Xz

X


d4


d (W-CO ,


d , 2dx2_y - '" .. .. "


S I
* S S I * I S 5


' S W -C


p+Ph n


n(W-C-Ph)


(MeO)3"
oc


[


Figure 2.1. Orbital mixing diagram for Cp(CO){P(OMe)3}W-C-Ph (37). Orbital energies are derived from extended Htickel calculations.


-55-


-65m


n * ( (W-C)


-75 -


- 85









-95 -


L








frontier orbitals obtained were very similar to those calculated for 37. Assignment of the HOMO as a metal d orbital that is nonbonding with respect to the carbyne ligand is consistent with structural information obtained by X-ray crystallography on Br(dmpe)2W= C-Ph and its 1 e- oxidized congener [Br(dmpe)2W=C-Ph][PF6]. 12 Oxidation results in only a slight shortening of the W-C(carbyne) and W-Br bonds (0.024 and 0.042 A) while the W-P bonds are lengthened somewhat. Upon this evidence, the HOMO is assigned as a nonbonding orbital that is primarily dx2-y2 in character. Similar observations have been made for Cl(dppe)2Mo-C(p-Tol) and [Cl(dppe)2Mo-C(pTol)][PF6].l 17

This picture of the electronic structure of carbyne complexes 37 and 38 suggests that the lowest energy transition involves the frontier orbitals depicted in Figure 2.2. This is the transition identified in prior literature as d(M) -+ t*(M-C-Ar) MLCT in nature.43,49,110,111 Although this transition does involve some charge transfer to the carbyne ligand, the earlier designation of "MLCT" suggests a higher degree of charge transfer and higher band intensity than is actually observed (vide infra). Both the HOMO and the LUMO are primarily composed of the metal d orbitals and the carbonyl nt* orbitals. As a result, the lowest energy transition has a considerable degree of d-d character that is consistent with its low extinction coefficient.



Electronic Absorption Spectra

Figure 2.3 illustrates the absorption spectrum of tungsten carbyne 37 in THF solution, which is typical for the series of metal carbynes examined herein, and Table 2.1







C-- -C'"C \C- C








CC










C Cj
/ /


\C





Cc C



Figure 2.2. HOMO and LUMO of Cp(CO){P(OMe)3}W=C-Ph (37). Coefficients are derived from extended Hickel calculations. Hydrogens and phosphite methoxy groups are omitted for clarity. Key: (top) LUMO; (bottom) HOMO.





46

summarizes the data and assignments for the entire set of complexes. The absorption spectra of the aryl-substituted carbynes 37-39 exhibit two primary UV/visible absorption features: a strong band at approximately 330 nm and a weaker broad absorption at about 480 nm. The absorption spectrum of naphthylcarbyne 40 showed slight red shifts of both bands, but otherwise was similar.

The absorption features in Table 2.1 are very similar to those described above for the structurally related arylcarbyne complexes. These features include a weak absorption with Xmax located between 475 and 500 nm which is assigned to the parity forbidden, spin-allowed (i.e. singlet-singlet) d(M) -+ n*(M-C-Ar) transition as well as a moderately intense absorption in the near-UV between 325 and 350 nm which is assigned to the spinallowed t(M-C-Ar) -+t*(M-C-Ar) transition.




1 2 I 5I 0 ' 5


*E9 20 ,






5050
011 1 100 070 0 300 400 500 600 700 Wavelength / nm
Figure 2.3. Absorption spectrum of Cp(CO){P(OMe)3}W-C-Ph in THF solution at room temperature.








The low-energy absorptions are clearly related to those assigned as d -+ n* MLCT for the similar compounds X(CO)2L2W-C-R (R = Ph, tBu; X = Cl, Br; L2 = TMEDA, 2 py, dppe).I 10 These transitions in 37-40 are best described as d to d transitions rather than MLCT. The d-d character of the low-energy absorption in 37-39 (and by inference in 40) is underscored by previous studies from the McElwee-White laboratory which demonstrated that the absorption band is not solvatochromic.49 The more intense absorption at 330 nm is assigned to n -+ n* transitions of the M-C-Ar chromophore. While there is a significant energy gap between the visible and near-UV transitions, the extended Hfickel calculations are in qualitative agreement with these assignments. The slight red shifts for the naphthylcarbyne can be attributed to mixing of the larger t system with the LUMO, lowering its energy.



Table 2.1. Absorption Spectra of Cp(CO){P(OMe)3 }M--C-R (37-40) in THF Solutiona

M R ) .mx/nm(03cm) emaxM'cm"1 assignment

37 W Ph 329 (30.4) 8000 n( W-C-Ph)---n*( W-C-Ph) 483 (20.7) 50 d(W)--n*( W-C-Ph) 38 Mo Ph 328 (30.5) 4000 n( Mo-C-Ph)--7-*( Mo-C-Ph) 477 (20.9) 60 d(Mo)-n*( Mo-C-Ph) 39 W o-Tol 331 (30.2) n( W-C-o-To)---n*( Mo-C-o-Tol)

476 (21.0) d(W)---+n*(W=C-o-ToI) 40 W 2-Np 348 (28.7) 6000 n( W-C-Np)--n*( W-C-Np) 490 (20.4) 70 d(W)--*n*( W-C-Np) aData for 37-39 were previously reported.49 Data for 40 were obtained in this work.










Luminescence Studies



Detailed steady state and time resolved luminescence studies were carried out on complexes 37-40. The fluorescence detection system was corrected for monochromator and photomultiplier response to 800 nm, but owing to the sharp decrease in the efficiency of the detector at wavelengths greater than 800 nm, luminescence data collected beyond this wavelength were not useful. This point is significant, because the emission of the W and Mo carbyne complexes extends well into the near-IR (vide infra).

The luminescence features of the tungsten and molybdenum carbynes studied are also similar to those of related arylcarbyne complexes. Each of the tungsten carbyne complexes luminesces weakly in solution at 298 K with emission lifetimes ranging from 60 to 170 ns. The molybdenum phenylcarbyne complex 38 is not luminescent at room temperature in fluid solution; however, red luminescence is observed when the compound is frozen in a glassy matrix at 77 K. Table 2.2 provides a summary of the emission parameters, including wavelength maxima (Xmx) for the corrected emission bands, and emission quantum efficiencies and lifetimes (Oem and Tem, respectively). Where possible, radiative and nonradiative decay rate constants (kr and k,, respectively) were calculated by the expressions kr = (Dem/Tem and kn = (1/Te - kr). Note that by applying these expressions it is assumed that the luminescent state is reached with unit quantum efficiency.








Table 2.2. Luminescence and Transient Absorption Data for Cp(CO){P(OMe) 3}MMCRa 298 Kb 77 Kc M R i,./nm d r /nSd Iem kr/s' kjr/s' TA/nlS 4aax/nm 17/jJS (103 cm"1) (103) (103 cm-1

37 W Ph 747 (13.4) 141 6.9x104 4.9 7.8x106 129 741(13.5) 3.2 38 Mo Ph e e <10- <2 2.0x107 49 787(12.7) 8.3 39 W o-Tol 745 (13.4) 170 3.6xl04 2.1 5.9x106 735(13.6) 3.6 40 W 2-Np >780 (<12.7) 66 1.5x107 60



'k. = emission maximum; rm, = emission lifetime;


In fluid solution at 298 K the most pronounced emission is observed from tungsten complexes 37 and 39. Emission from these complexes is very similar, appearing as a broad, structureless band in the red with ,max z 745 nm (Figure 2.4a).49 In each case the luminescence efficiency is rather low, with (D'em z 5 X 10-4 (Table 2.2). Emission excitation spectra for 37 and 39 are also illustrated in Figure 2.4a and reveal that the red luminescence is produced by excitation of the visible and near UV absorption bands. Emission lifetimes for 37 and 39 are comparable, and indicate that the luminescent excited state has a lifetime in the 100-200 ns range at room temperature.

The emission of 37 and 39 was also examined at 77 K in a 2-MTHF solvent glass, and the maxima and lifetimes for the low temperature luminescence are listed in Table 2.2. The emission of both complexes at low temperature appears as a broad band which is similar in energy and bandshape to that observed in fluid solution at 298 K. Thus, luminescence "rigidochromism", which is typically observed for charge transfer excited








states, was not apparent for 37 and 39. This effect underscores the small degree of MLCT character in the luminescent excited state for the carbyne complexes. The 77 K emission lifetimes for the two tungsten complexes are similar, and are comparable to the lifetimes observed at 77 K for d-d excited states in W(CO)5L complexes. 118-120

At 298 K in fluid solution the 2-naphthylcarbyne complex 40 exhibits an emission which is substantially red-shifted relative to that of the phenyl and o-tolyl complexes. Owing to the large red shift, the instrument used is only capable of detecting the blue edge of the band (Figure 2.4b), and Xma for 40 is apparently >800 nm. An emission excitation spectrum of 40 obtained with Xem = 790 nm is also illustrated in Figure 2.4b. There is good agreement between the absorption and excitation spectra for 40, which confirms that the near-IR emission of 40 indeed emanates from the complex. The lifetime of the near-IR emission of 40 was determined to be 66 ns, which is slightly shorter than the lifetimes for the phenyl and o-tolyl complexes.

Molybdenum carbyne 38 exhibits no detectable luminescence at 298 K. However, at 77 K in 2-MTHF the complex features a broad, structureless emission band that is similar in bandshape but red-shifted compared to the emission of the tungsten analog 37 (Table 2.2). The emission band decreased in intensity as the temperature of the MTHF solution is increased above the glass-to-fluid transition (100-120 K) at which point the luminescence becomes too weak to detect (Fem < 10-4). The emission lifetime of 38 at 77 K (8.3 pis) is slightly longer than that of the tungsten analog.

By analogy to previously reported arylcarbyne complexes, the luminescence observed from 37-40 is attributed to the d(M) -+ n*(M-C-Ar) triplet state. The triplet





















300 400 500 600 700 800
Wavelength /nm


1.0- A

0.8

0.6

0.4

0.2

0.0 LI a 1300 400 500 600 700 800 Wavelength /nm Figure 2.4 Emission and excitation spectra: (a) for 37 and 39 and (b) for 40. Emission spectrum is at right and excitation spectrum at left.








assignment has been substantiated in 37 by previously reported triplet energy transfer studies.49 The significant Stokes shift of the emission from 37, 39, and 40 relative to the low-intensity visible absorption band (-z7.0 x 103 cm-1) is attributed to the fact that the absorption is due to the singlet-singlet HOMO-LUMO transition. The Stokes shift observed for these carbyne complexes is similar to that reported for 3d-d luminescence in tungsten and molybdenum carbonyl complexes of the type M(CO)5L. 118-120 Of further interest is the fact that the luminescence data presented herein argues against the suggestion made earlier that the large Stokes shift may be partly due to geometry changes associated with excitation of the M-C-Ar functionality.I 10 The lack of a significant difference in the emission energy or bandshape for 37 and 39 in fluid solution compared to in rigid glass at 77 K implies that there is no substantial inner- or outer-sphere reorganization associated with the optical excitation.

Despite the overall similarities of the absorption and luminescence properties of 37-40, there are important distinctions that provide insight concerning the nature of the electronic states involved in the photophysics. First, the visible and near-UV absorption bands and the luminescence of the 2-Np complex 40 are noticeably red-shifted compared to those for the phenyl and o-tolyl analogs. The red-shift of both absorption bands and the luminescence in the 2-Np complex is expected, since the naphthalene ring provides extended conjugation for the nr(M-C-Ar) orbital, which is the LUMO of the complex and is the "acceptor" orbital for the visible and near-UV n, n* transitions.

Second, the tungsten and molybdenum phenylcarbyne analogs 37 and 38 display remarkably similar ground- and excited-state absorption spectra. This likeness points to a similarity in the electronic structure of the two complexes. The one notable difference in








the photophysical properties of the two phenylcarbynes is the lack of observable luminescence from 38 in fluid solution at room temperature. Since the excited state lifetimes of 37 and 38 are not significantly different (i.e. from transient absorption 130 and 50 ns for 37 and 38, respectively vide infra), the lower emission yield of 38 might be attributed to lower spin-orbit coupling (and therefore less singlet-triplet mixing) in the molybdenum complex as compared to the tungsten analog.



Transient Absorption Spectroscopy



Nanosecond laser flash photolysis studies were carried out on carbyne complexes 37, 38, and 40 in argon-degassed THF solutions. All experiments were performed using the third harmonic output of a Nd:YAG laser for excitation (355 nm, 10 ns fwhm, 10 mJ/pulse). As described below, strongly absorbing transients were observed for each complex, and the decay lifetimes for the transient absorptions were determined by global kinetic analysis of the transient absorption decay at > 25 wavelengths. 121

Figure 2.5a illustrates transient absorption difference spectra of 37 at delay times ranging from 0 to 280 ns following laser excitation. The spectra are characterized by a broad absorption band with Xma . 440 nm. There appears to be a hint of structure on the absorption band, with distinct (and reproducible) shoulders at 395, 420, and 460 nm. Global analysis of the transient absorption data indicates that the transient decays with a lifetime of 129 ns, in good agreement with the previously reported emission lifetime (teem = 141 ns). The correspondence of the transient absorption and emission lifetimes strongly implies that the transient absorption is due to the luminescent excited state.








Under the assumption that the lowest excited state 37 is populated with unit efficiency following 355 nm excitation, it is possible to determine the difference molar absorptivity

(As) of this state by using the relative actinometry method.122 This experiment was accomplished by using [fac- 2,2'-bipyridine)Re(CO)3(4-benzylpyridine)]+ as an actinometer (As = 11,200 M'cm1 at 370 nm)123 and led to a value of As = 3000 M'cm'at 440 nm for 37.

Figure 2.5b illustrates transient absorption difference spectra of 40 at delay times ranging from 0 to 120 ns following laser excitation. The spectrum of this complex is characterized by a moderately intense and broad absorption band with Xmax ;Z 445 nm, with distinct (and reproducible) shoulders at 395 and 420 nm. Global analysis of the transient absorption data indicates that the transient decays with a lifetime of 49 ns. The close correspondence between the transient absorption spectra of 37 and 38 points to a similar electronic structure for the absorbing excited states in both complexes.

Figure 2.5c illustrates the transient absorption difference spectra of 38 at delay times ranging from 0 to 160 ns following laser excitation. The spectra are characterized by an intense but featureless transient absorption band with Xmax Z 460 nm. Global analysis of the transient absorption data indicates that the transient decays with a lifetime of 60 ns in very good agreement with the emission lifetime of the complex (66 ns). The agreement between the transient absorption and emission decay lifetimes suggests that for 40 the transient absorption arises from the luminescent excited state.

A distinguishing feature of this work is the observation of strong transient absorption by the d(M) -- 7E*(M-C-Ar) excited states of the tungsten and molybdenum arylcarbynes. This is the first study of transient absorption spectra of photoexcited








Fischer carbyne complexes. There are several important ramifications of the observation of transient absorption signals for the excited states of this family of complexes. First, as noted above, it is clear that the excited state carbynes have comparatively large difference molar absorptivities. This is in contrast with the low molar absorptivity that is typical of d-d excited states of organometallic complexes. 124,125 The moderate to large difference molar absorptivity of the excited states of the arylcarbyne complexes is more in accord with that of dn(M) -+ i*(ligand) MLCT excited states in d6 metal diimine complexes such as Ru(bpy)32 and [(bpy)-Rel(CO)3(py)] 1+ (bpy = 2,2'-bipyridine and py = pyridine). 123,126

An important question concerns the origin of the moderate to strong absorbance of the d -> n* excited states in the arylcarbyne complexes. The absorption of MLCT excited states in d6 metal diimine complexes has been assigned to spin-allowed n* -+ n' and n -+ n transitions of the diimine radical anion chromophore that is produced by MLCT excitation.127 By analogy, the absorption of the d -* n* excited state in the arylcarbyne complexes may be associated with an allowed * -+ 7* transition of the M-C-Ar chromophore. In this connection, it is important to note that the excited state absorption of phenylcarbyne complexes 37 and 38 is quite similar, while that of naphthylcarbyne 40 is slightly red shifted (and qualitatively appears to be more intense). The red shift in the naphthyl system suggests the involvement of the nt to n* levels of the aryl unit in the optical transition detected in the transient absorption experiment.

Finally, the substantial difference absorptivity of the excited state in the arylcarbyne complexes provides an exceedingly useful means to track the excited state dynamics (and reactivity). This is significant, because even though emission studies have















































400 450 500
Waveleng


550 600 650
th/nm


Figure 2.5 Transient absorption difference spectra following laser excitations (355 nm, 10 ns fwhm, 10 mJ/ pulse) of (a) 37 (b) 38 and (c) 40.








provided an opportunity to study the excited states of certain metal carbynes, the method has limitations. For example, among 37-40 and related Cp(CO)2M-C-R and Cp{P(OMe)3}2M-C-R species, only the tungsten aryl carbynes with one phosphite ligand and one CO ligand display easily observable luminescence. Even for these complexes, the luminescence efficiency is low, as evidenced by the quantum yield for emission for 37 and 39. The bis(phosphite) arylcarbynes are not emissive in solution at room temperature,49 and molybdenum complex 38 is also nonluminescent under the same experimental conditions. Comparing the lifetime measurements obtained by absorbance with those observed by the emission method provides assurance that the observed transients are in fact the emissive states. Furthermore, by applying global analysis to the transient absorption data, it is in principle possible to obtain very reliable excited state decay rate data. 121

It should be noted that after the work described in this chapter had been completed, an additional study was undertaken by co-workers that investigated the role of photochemical electron transfer in the reactions of tungsten carbynes, namely, electron transfer from the excited state of Cp{P(OPh)3}(CO)W-CPh (43) to a series of electron acceptors.128 They examined the quenching of the d -+ t* excited state of 43 with a series of pyridinium and nitroaromatic acceptors of varying reduction potential. Laser flash photolysis revealed that quenching is accompanied by the appearance of radical ion products, thereby providing evidence that quenching occurs via electron transfer. The dependence of the bimolecular quenching rate constant on the reduction potentials of the acceptors establishes that the d -+ r* excited state of 43 is a potent reducing agent.








Conclusion



Low-valent carbyne complexes are well-established compounds in organometallic chemistry, yet relatively few photophysical studies on these complexes have appeared in the literature. In this work, the complexes Cp(CO){P(OMe)3}M-C-R (M = Mo, W; R = Ph, o-Tol, 2-Np) (37-40) were examined in an effort to learn more about the excited states of these molecules, as well as to provide some information relevant to the study of photochemical reactions. Extended Hickel calculations and absorbance spectra indicate that 37-40 possess low-lying excited states which can be populated directly via a parityforbidden, singlet-singlet optical transition in the mid-visible region. This transition is better described as a d-d transition rather than MLCT. Emission is observed from the triplet manifold of these excited states, and at room temperature in fluid solution the lifetimes of the triplet state range from 50 ns for the molybdenum phenylcarbyne 38 to 170 ns for the tungsten tolylcarbyne 39. Transient absorbance measurements performed on 37-40 in THF indicate that the triplet excited states can also be detected and monitored by absorption, since the lifetimes obtained in the absorbance experiments match those obtained through emission methods. The observation of strong excited state absorption provides an avenue for future work concerning the photophysics and photochemistry of arylcarbyne complexes. Such work may include the study of excited state resonance Raman to provide further information concerning the nature of the excited states.













CHAPTER 3
SYNTHESIS AND PHOTOOXIDATION OF A TUNGSTEN CYCLOHEXENYL CARBYNE COMPLEX



Introduction


As discussed in Chapter 1, a variety of organic compounds including olefins, dienes, dienals and other cycloalkenones have been synthesized from the photooxidation of various alkyl metal carbynes of the type Cp(CO){P(OMe)3}M-CR [R= alkyl; M= Mo, W1. 106 Mechanistic studies suggested that these reactions shared the following two initial steps (Scheme 3.1): 1) photoinduced electron transfer from the carbyne to the halogenated solvent to form a 17 electron radical cation A and 2) hydrogen abstraction from the reaction medium to produce a 16 electron cationic carbene B.49,51,107,108,129 Despite the usual tendency of organometallic radicals to undergo ligand exchange and halogen abstraction at the metal center as discussed previously,46,47,130 geometric distortions of the carbyne radical cation cause radical reactivity to occur at the carbyne carbon and the H-abstraction takes place at that site.52 The resulting cationic carbene complex B is the reactive species in the reactions that produce organic products. Although mechanistic studies were consistent with the intermediacy of cationic carbene complexes in the formation of organic products from photooxidized carbynes, to this point, carbene complexes had never actually been observed in the reaction mixtures.








This work describes the first direct observation of a hydrogen abstraction product upon photooxidation of a tungsten cyclohexenyl carbyne complex.


Scheme 3.1. Proposed mechanism for generation of metal carbenes upon photooxidation of metal carbynes


00..W C- CHRR2

P(OMe)3


hv CHC13


0,--W C-CHR1R2 P(OMe)3

A


+RH
-R*


\ H

0C W--- C\ CHR1R2
P(OMe)3








Synthesis of (i 5-C H)(CO){P(OMe)i} W-C(C-C6H9) (44)

The cyclohexenyl tungsten carbyne complex 44 was synthesized from the tungsten carbene 45 as shown in Scheme 3.2. There is a literature preparation for the tungsten carbene 45 which reports a yield of 52%. 131 However, for the current study, carbene 45 was synthesized by modifications of a route that was used in the synthesis of the molybdenum analogue of 40.132-134 These modifications resulted in a higher yield than that reported in the literature. The modified route involves the lithiation of the hydrazone 46 with t-butyl lithium to form the cyclohexenyl lithium reagent 47. The immediate reaction of 47 with tungsten hexacarbonyl formed the cyclohexenyl tungsten acyl anion 48. Anion 48 was methylated with trimethyl tetrafluoroborate which gave carbene 45. Once isolated, the crude product 45 was not allowed to stand over-night but was chromatographed immediately on a -78C chilled alumina column giving a purified yield of 75%. Carbene 45 was readily converted to the tris(trimethyl phosphite) carbyne complex 49 using Fischer's method9,21 which involves step-wise addition of BBr3 and P(OMe)3, giving 49 in 65% yield. Reaction of CpNa with 49 and subsequent purification through a -78C chilled alumina column produced carbyne 44 in 41% yield as a reddishorange oil that can be stored for approximately one week at -30 C.











Scheme 3.2. Synthesis of (Tl5-C5H5)(CO){P(OMe)3}W=C(c-C6H9) (44)


HO0
12 11

0


OCH3 (C O )5W = 3

45


0�C W(CO)6 (CH3)30BF4 0- Li+ 48


1) BBr3 CH2C12 2) P(OMe)3


P(OMe)3 I ,"C O / ' '
Br-W-C-=
(MeO)3P P(OMe)3


CpNa THF


oc W-cP(OMe)3


tBuLi

-780C


Li


6








Photooxidation of (D 5-CsHs)(CO) {P(OMe)3 } W=C(c-C6H9) (44)


Photolysis of carbyne 44 at -45 C in CDC13 while monitoring the reaction progress by 'H NMR spectroscopy resulted in observation of downfield shifts of the alkyl peaks and the formation of a new Cp peak. Since the reactivity of related carbyne complexes following photooxidation106 suggested that 44+. had probably abstracted a deuterium atom from the CDC13 solvent, the experiment was repeated in CHC13 at -450C so that the abstracted H-atom could be located by 'H NMR. Upon completion of the photolysis as determined by disappearance of the initial carbyne 44 by TLC of the reaction mixtures, the CHC13 was pumped off at -450C, and the residue was dissolved in cold CDC13 in order to obtain a 'H NMR spectrum. The spectrum revealed a broad singlet at 815.35 which is characteristic of a proton on the carbene carbon of an electrophilic carbene complex. 135

One obvious candidate for the photooxidation product was the cationic carbene complex 50. Electron transfer from 44 to the solvent followed by H-abstraction at the carbyne carbon of 44+' would produce 50 (equation 3.1). However, IR spectra of the reaction mixtures failed to show a new signal for a terminal CO ligand.136,137 Once 50 had been ruled out as the observable species, the next obvious possibility would be displacement of the CO in 44 with CI to yield the neutral carbene 51 (equation 3.2). Reaction mixtures from the photooxidation of 44 would contain C- since it is generated upon reduction of the CHC13 solvent by the excited state of carbyne 44.138










hv CHC13


OC
(MeO)3P


4 J H (MeO)3P

50


(3.1)


Complex 51 is formally the product of protonation of carbyne 44 at the carbyne carbon and C1" substitution. Thus, it was not unreasonable to attempt its preparation by reaction of 44 with HCl, although protonation of metal carbynes is a complicated issue


(3.2)


Cl-


because proton attack can occur either at the metal or at the carbyne carbon.3,29,34,139 Reaction of related (i15-C5H5)(CO) {P(OMe)3} M-=CR complexes with HC1 generally leads to 112-acyl complexes of the type (r)5-C5H5)C12{P(OMe)3}MCOCH2R (for example, see equation 3.3).32,33,49,51,106,107,129,140 However, Kreissl's work on the mechanism of T12-acyl formation points to the intermediacy of a metal carbene intermediate which then undergoes CO insertion (Scheme 3.3). 141 This postulate was supported by the fact that when the aminocarbyne (TI5-C5H5)(CO)2W=CNEt2 was reacted with HCI, the reaction stopped at the carbene. 140 Although in the Kreissl work, the carbene was only


r0


4 P\'












4;x HClEt2O
Mo C- (CHE)3CH3 E (MeO)3pEtO
0C


Scheme 3.3.




OCWOM- C-R
OC


C"/ I CH2(CH2)3CH3
P(OMe)3


(3.3)


Proposed mechanism for the protonation of carbynes.

4 \ CO /H

HX XW- M- C\
R
OC


~HX
0-1A CH2R OC X


o0


Xp.. M \\ C-R
4 I OC H


M= Mo, W R= alkyl, aryl X= CI1, CF3COO, CC13COO



observable for the aminocarbyne case, which is electronically very different from 44, the presence of the cyclohexenyl group in 44 opened the possibility of preventing CO insertion into the carbene ligand of intermediate 50 by chelation of the double bond.

As desired, protonation of the vinyl carbyne 44 with HCl (equation 3.4) did not produce the rn2-acyl complex, but rather yielded the carbene complex 51 as a yellow








powder which is thermally labile but stable at -45 C. Comparison of the IH NMR spectrum of the HC1 product to the 'H NMR spectra of the reaction mixtures from photooxidation of carbyne 44 provided confirmation that both routes had produced the same complex.




HC1.Et2O x H
0C "' W- C- / CI'-C (3.4) (MeO)3P Et20 (MeO)3P 510
44 51



The 1H NMR, 13C NMR, and MS data for 51 support the proposed structure. In the 'H NMR of 51 in CDC13 at -50' C, the proton on the carbene carbon appears as a singlet at 8 15.35, which is characteristic of conjugated electrophilic carbene complexes.135 The 13C NMR chemical shifts of 8 277.8 for the carbene carbon, 8 75.4 for the quaternary vinyl carbon, and 5 93.7 for the tertiary vinyl carbon strongly resemble the literature values for the tungsten 1q-Vinyl carbene complex (r 5C5H5)(CO)(I)W=C(Ph)C(Ph)CHTol (52) (Figure 3.1).42 The electrospray MS of 51 in CH3CN exhibited a molecular ion peak at 502.1, with an isotopic distribution pattern that matched the simulation for the molecular formula of 51, C15H24C103PW (Figure 3.2).








894.4 894.7


8 269.8 68277.8 fb,,, ( Ph c~....~ (H
(< 570.7 -75.4 OC1 /" Ph (MeO)3P~

( Tol %r
691.1

52 51

Figure 3.1. Comparison of 13C NMR data between 51 and known 52.



Photolysis of carbyne 44 at -45 C in CDC13, containing decane as a standard, afforded carbene 51 in 25% yield as determined by integration of the Cp resonance in the 'H NMR spectrum. In order to rule out photochemical formation of HCl as the source of 51 in the photooxidation reactions, 44 was photolyzed in CDC13 to which the hindered base 2,6-di-tert-butylpyridine had been added in order to scavenge any acid. The addition of 2,6-di-tert-butylpyridine is a technique that is used to distinguish radical cation chemistry from acid-induced reactions.51 ,52,107,142,143 The yield of the photolysis product was the same in the presence and the absence of the base, ruling out formation of 51 in the photolysis mixtures by reaction of 44 with HC1.








SIMULATION


501.1 502.1 503.1 504.1 505.1 506.1 507.1 m/z

I I I I I I I


501.1 502.1 503.1 504.1 505.1 506.1 507.1 m/z
I! I I I I I


Figure 3.2 Electrospray mass spectral data of 51


ACTUAL









Oxidation of (n5-CsH)(CO) {P(OMe)3 I W=C(c-C6H9) (44) in the Presence of Unsaturated Substrates


Although high-valent carbyne complexes have found wide use in the areas of olefin metathesis and alkyne polymerization, little is known about the reactivity of low valent metal carbyne complexes with unsaturated substrates. 16,144-146 Efforts to provide further enlightenment on this reactivity involved studying the reactivity of 44+. in the presence of unsaturated species. As discussed earlier, 44+* is formed by photochemical electron transfer from 44 to CHCI3. However, upon reduction, CHC13 fragments to Cl and *CHCI2.138 The *CHC12 is known to undergo unwanted Arbuzov reactions with the P(OMe)3 ligand 147 on similar carbyne complexes which is evident by the formation of CH3C1 in the reaction.51 The use of outer sphere chemical oxidants at low temperatures has been studied and provides an alternative method in which low yields and side reactions from oCHC12 can be avoided.

A cyclic voltammogram of carbyne 44 taken in 0.1 M Bu4NSO3CF3/C2H4CI2 under nitrogen showed the first oxidation wave at +0.55V vs. NHE to be completely irreversible regardless of scan speed or switching potential. The first oxidation peak corresponds to the formation of 44+*. The complete irreversibility of this peak indicates that 44+* is a very reactive species and undergoes immediate chemical reaction on the cyclic voltammogram time scale. Based on the oxidation potential of 44, the chemical oxidant, acetyl ferrocenium, with an oxidation potential of +0.82V148 was chosen for the oxidation investigations.





70


I I I
O9. glo


-26.,,,6 / -A


-48. . V




-Ial, \ \i



- 6 a -9 T
-iE vs. NHE-(U)




Figure 3.3 Cyclic voltammogrm of 44. Sweep rate: 100 mV/s



Initially, carbyne 44 was reacted with acetylferrocenium tetrafluoroborate (AcFc+BF4") at -90 �C and 1H NMR spectra were taken in 10 �C intervals upon warming to room temperature. The reaction mixture upon warming changed color from blue to green to brown. Formation of a hydrogen abstraction product was never observed, unlike the formation of 51, as discussed earlier in the photolysis of 44 in CHC13. Considering the fact that hydrogen abstraction did not readily take place in the chemical oxidation reaction, it seemed feasible to attempt to trap the ensuing 17-electron radical cation with an unsaturated substrate. Torraca et al. have reported the reaction of a butyl molybdenum carbyne with AcFc+BF4 in the presence of phenyl acetylene resulted in C-C coupling of the acetylene to the carbyne carbon. 149 Unfortunately, when carbyne 44 was reacted








with AcFc+BF4 in the presence of various alkynes, olefins, ketenes, and imines, all reactions resulted in intractable material.



Other Reactions with Unsaturated Substrates

The carbene 51 that is formed from the protonation of 44 with HCI is similar in structure to a diene. Efforts to form [4 + 2] or [2 + 2] adducts were attempted in the presence of various olefins, acetylenes, aldehydes, and ketenes. No reactivity was ever observed other than the decomposition of 51 to intractable materials.

It is possible the chelation of the cyclohexenyl group to the metal was inhibiting further reactivity of 51. Attempts to circumvent this problem included protonation of 44 with HBF4 with and without CO pressure in attempts to trap the putative 16-electron cationic carbene 53 (Figure 3.4). Again, all in situ reactions with unsaturated substrates resulted in intractable materials.




H BF4"


(MeO)3P 053

Figure 3.4 Investigated reactivity of 53 with unsaturated substrates.



Conclusion

The photooxidation of the cyclohexenyl carbyne 44 in CHC13 yields carbene complex 51. This experiment represents the first direct observation of a carbene





72

intermediate following single electron oxidation of a carbyne complex and provides further evidence for H-abstraction at the carbyne carbon as a common mechanistic step in previously reported photooxidation reactions. Formation of 51 upon thermal reaction of 44 with HC1 also provides additional support for the Kreissl mechanism for generation of 1 2-acyl complexes from carbyne complexes and HCI by extending the range of observable carbene intermediates from amino derivatives to alkyl derivatives.













CHAPTER 4
CATALYTIC OXIDATIVE CARBONYLATION OF AMINES WITH W(CO)6




Introduction



Substituted ureas have been of recent interest due to appearance of this functionality in drug candidates such as HIV protease inhibitors,150,151 FKBP12 inhibitors,152 CCK-B receptor antagonists153,154 and endothelin antagonists.155 In addition, ureas have found widespread use as agricultural chemicals, resin precursors, dyes, and additives to petroleum compounds and polymers.156 Among the numerous methods for synthesis of N,N'-disubstituted ureas are the reactions of primary amines with isocyanates, phosgene, or phosgene derivatives (equation 4.1).157,158 Yields are usually good to excellent for these reactions, but other aspects can be problematic. Phosgene itself is highly toxic and corrosive while phosgene derivatives can be expensive to use on a large scale. Additional impetus for replacement of phosgene derivatives comes from the standpoint of atom economy159 by which using CO to install a carbonyl moiety into a urea would be preferable to a method using a typical phosgene derivative such as 1,1 -carbonyldiimidazole. The direct metal-catalyzed conversion of amines and CO to ureas provides an alternative to phosgene and its derivatives.








O 0

2 RNH2 + - N N + 2 HX (4.1) I I
H H


/7N
x = Ci, Ci3CO, + N


Although catalytic carbonylation has been investigated over many years,55,85,160 the topic remains of interest. Oxidative conversion of primary amines into ureas has been reported for transition metal catalysts involving Ni,71 Co,73 Mn,73,94,95 Ru161 and, most commonly, Pd.75,162,163 Main group elements such as sulfur164,165 and selenium166-168 can also serve as catalysts. However, neither the transition metal- nor the main group-catalyzed reactions are ideal. The transition metalcatalyzed reactions generally require high temperatures and pressures. In addition, yields for aliphatic amines are generally much lower than those for aromatic cases. Among the main group catalysts, selenium-catalyzed carbonylation reactions can produce high yields of ureas under mild conditions. However, generation of hydrogen selenide as a byproduct and the need for stoichiometric or excess selenium for certain substrates 167,168 are problematic.

Prior to this work, preliminary studies were reported on the catalytic oxidative carbonylation of primary amines to ureas using either [(CO)2W(NPh)2]2 or W(CO)6 as the precatalyst and 12 as the oxidant.99,169 It was found that the W(CO)6/2 catalyst system also works for the conversion of secondary amines to tetrasubstituted ureas170 and the preparation of cyclic ureas directly from primary and secondary ao-








diamines.171 Extensive optimization studies were performed on the oxidative carbonylation of aliphatic primary amines to N,N'-disubstituted ureas using the W(CO)612 oxidative carbonylation system on the substrate n-propyl amine (equation 4.2). Variables such as catalyst, temperature, solvent, CO pressure and quantity of added



0
W(CO)6 R, N N R (4.2) RNHI2 -N N
CO / 12 / K2C03 I I
H H



base were examined. The optimal conditions for the carbonylation of n-propylamine were determined to be 2 mole % W(CO)6, stoichiometric amounts of amine and 12, and 1.5 equiv of K2C03 per equiv amine reacted in CH2Cl2 at 90 C under 80 atm CO pressure. This chapter is a continuation of the work described above. It will include further investigations of optimized procedures, applications of the optimized conditions to other primary amines, as well as a functional group compatibility study of the W(CO)6/12 oxidative carbonylation system.



Carbonylation of Primary Amines



Although CH2CI2 was the best single solvent for the n-propylamine case, solubility problems were encountered when CH2Cl2 was used as solvent for various substituted benzylamines (vide infra), for which either the amine starting material or the hydroiodide salt was insoluble. Note that the stoichiometry of oxidative carbonylation of








amines to ureas dictates that two equiv of the amine hydroiodide (RNH3I) will be produced per equiv of urea (equation 4.3). In the absence of added base, these amine salts are observed in the reaction mixtures. The presence of K2CO3 as a sacrificial base increases the product yield by regenerating the free amine from the salt and returning it to



0
catalyst R R
4 RN c N N + 2 [RNH3]I" (4.3) 12 / CO I I
H H



the substrate pool. However, insolubility of the amine salts will hinder this deprotonation since the K2C03 and the salt will coexist as immiscible solids. To circumvent the inability of solid K2CO3 to deprotonate insoluble amine salts, the two-phase solvent system CH2C12/H20 was investigated. Due to the solubility of the hydroiodide salts in basic water, they could be deprotonated and shuttled back into the methylene chloride layer as the free amine. An added advantage was that the base is soluble in the water layer and stirring problems associated with solid K2C03 were eliminated.

For both n-propylamine and many of the substituted benzylamines used in the functional group compatibility studies (vide infra), room temperature yields increased when the solubility problems were resolved with the two-phase CH2Cl2/H20 solvent system. Although there are several examples of two-phase carbonylation reactions in the literature, a biphasic system for the carbonylation of amines is unprecedented. In fact, carbonylation of amines in aqueous medium has only been reported for conversion of amines to formamides with a water-soluble ruthenium carbonyl catalyst. 172








Although the carbonylation of n-propylamine at room temperature in the twophase CH2CI2/H20 solvent system resulted in 73% yield which was higher than the yield for the single-solvent CH2Cl2 system, repeating the reaction at 90 'C resulted in a 85% yield which was comparable to the 90% yield achieved in the previously described reaction run at 90 �C in CH2C12.

Therefore, a series of primary amines was reacted under the following conditions: 2 mole % W(CO)6, stoichiometric amounts of amine and 12, and 1.5 equiv of K2C03 per equiv amine reacted in CH2Cl2 at 90 'C under 80 atm CO pressure. These reactions gave the corresponding N,N'-disubstituted ureas, as seen in Table 4.1. The ureas were formed in moderate to high yields, ranging from 90% for N,N'-di-n-propylurea to 53% for N,N'di-i-propylurea. The yields are generally lower if the amine bears secondary or tertiary alkyl substituents. For comparison, the amines in Table 4.1 were also run in the CH2C12/H20 solvent system. Analogous to the decrease in yield of the n-propyl urea in the two-phase system at 90 C, the other primary amines also suffered a decrease in yield when carbonylated in the mixed solvent system. It is noteworthy that aniline is unreactive under the reaction conditions, considering aromatic amines are generally better substrates than their aliphatic analogues for the formation of ureas under transition metal-catalyzed carbonylation conditions. 172,173








Table 4.1 Catalytic carbonylation of primary amines in single/ biphasic solvent systems

Amine Product %Yield b %Yieldr
CH2CI2 CH2CI2/H20


NH2


0

N., N N


0
~NHl2 N,, N


0

N''N-


-- NH2


NH2




NH2




-N H 2


0



N-J- N


0




Q-N ) N-Q


a Isolated yield of urea calculated per equivalent of amine b Reaction conditions: amine (14.2 mmol), W(CO)6 (0.28 mmol), 12 (7.1 mmol), K2CO3 (21.3 mmol), CH2C12 (40 mL), 90 �C, 80 atm CO, 24 h. The solvent was CH2Cl2 (35 mL) plus H20 (5 mL). Other conditions are as in footnote b.










Functional Group Compatibility Study

In order to explore applications of this reaction in organic synthesis, a study of functional group tolerance was undertaken. Various substituted benzylamines (equation 4.4) were reacted with CO under similar carbonylation conditions, as seen in Table 4.2. Substituted benzylamines were chosen as model substrates for the compatibility study because the benzene ring provides a rigid backbone which separates the functional group



0
W(CO)6N

R CO / 12 / K2C03 H H(44) from the reacting amine. The possibility of intramolecular interaction is eliminated in these substrates. This first screen of functional groups addresses interference by simple binding of the catalyst to the functional group or competitive reaction of the functional group by oxidation or carbonylation. More complex substrates, such as conformationally flexible amino alcohols in which carbonylation could lead to different types of products, are the subject of further investigation and will not be discussed here.

Initial studies with the parent benzylamine 54 established that N,N'-dibenzylurea 55 was formed in 63% yield upon reaction under the optimized reaction conditions for npropylamine (Table 4.2, CH2CI2). As discussed above, the low yields of ureas for many of the substituted benzylamines were attributed to the insolubility of their respective hydroiodide salts in CH2C12. As shown in Table 4.2, it was found that yields for many of the substituted ureas were significantly higher when the 7:1 CH2Cl2/H20 solvent system was used (Procedure C, Chapter 6).








The effects of aryl halides were examined first. The p-chloro-, p-bromo-, and miodobenzylamines 56, 58, and 60 produced their respective N,N'-disubstituted ureas 57, 59, and 61 in surprisingly low 35%, 30%, and 39% yields when the reaction was run in CH2CI2. Although side reactions resulting from oxidative addition of the aryl halide bonds to the tungsten complex were considered,174,175 the main products of the reactions turned out to be the amine hydroiodides. Upon addition of water to the reaction mixtures, the yields of the ureas increased to 77%, 77%, and 70% respectively.

In experiments with ether and thioether compounds in CH2C12, the pmethoxybenzylamine 62 was carbonylated to urea 63 in 47% yield, while pmethylthiobenzylamine 64 was carbonylated to urea 65 in 24% yield. Under these conditions, the major products were the hydroiodide salts. The solubility problem was again circumvented by addition of H20 to the solvent system resulting in the ureas forming in 70% and 81% yields, respectively.

The hydroxymethyl compound 6 6 produced urea 6 7 with yields of 5% in CH2C12 and 58% in CH2Cl2/H20. Remarkably, the carbonylation reaction was completely selective for the amine over the alcohol, within the limits of detection. None of the corresponding carbamate or carbonate could be observed in the reaction mixtures. This experiment suggests that the carbonylation of amines could be carried out in the presence of unprotected hydroxyl groups. In contrast, thiol 6 8 failed to form the corresponding urea 6 9, undoubtedly due to the sensitivity of thiols to oxidation.









Table 4.2 Catalytic carbonylation of substituted benzyl amines


%Yieldab %Yieldac CH2C12 CH2CI2/H20


63 73


C N. NH2





Br N NH2





I NH2


NH2

Me 62


Nz


58 Br


N. NN
H H




N. xAN'
H H MeFO-


Amine


Product


'N" NH2


54


"Cl 57












%Yield,b %Yieldac
Amine Product CH2C12 CH2CI2/H20


I NH2
MeS. " 64



H NH2

66


HS NH2

68




o
Eto NH12

70


NNH

H NH2

0 72


SMe 65


N N ~ HS H H ~ S SH69


NON
H ORQ~

0






83



%Yieldb %Yieldalc
Amine Product CH2CI2 CH2CI2/H20


N NH2 N NI'N 416
..~ ... I H H I 16
74 75

H N 45 76 02 76 02N H H NO2 77


N N 37 68 N 78 N CN 79


HN NH2 28 14 H2 80 H2 H H NH2 81

N NHH
0 N217 20
NH2 82 H 83 aIsolated yield of urea calculated per equiv of amine b Reaction conditions: amine (7.1 mmol), W(CO)6 (0.14 mmol), 12 (3.5 mmol), K2C03 (10.7 mmol), CH2CI2 (20 mL), 70 C, 80 atm CO, 24 h. C The solvent was CH2Cl2 (21 mL) plus H20 (3 mL). Other conditions are as in footnote b.








The ester-substituted benzylamine 70 was also successfully carbonylated to the urea 71 in 36% yield in CH2Cl2 with 55% yield obtained in CH2CI2/H20. The carboxylic acid analogue 72 is insoluble in CH2C12, which hindered efforts to explore the tolerance of the catalyst to the presence of carboxylic acids. However, the solubility of 72 in basic H20 made it possible to obtain the corresponding urea 73 in 37% yield from the CH2Cl2/H20 system.

Compatibility of the catalyst system with terminal alkenes was then examined. The olefin 74 was reacted under the carbonylation conditions to form the urea 75 in a 41% yield in CH2C12. Although we anticipated possible problems with reactivity at the double bond, there were no identifiable products in which the double bond had been perturbed. The low yields could, however, be indicative of unidentified side reactions. Unlike the previously described reactions of substituted benzylamines, higher yields were obtained when the reaction using CH2C12 as the solvent was diluted to 0.03M in substrate and conducted at room temperature.

In addition, p-nitrobenzylamine (76) was successfully carbonylated to urea 77 in a 45% yield in CH2C12 with 76% yield obtained in CH2CI2/H20. The cyano-substituted benzylamine 78 was also carbonylated to its respective urea (79) in 37% yield in CH2Cl2 and 68% yield in CH2C12/H20. Note that the possibility of the nitrile moiety serving as a ligand for the catalyst did not interfere with the reaction.

Further functional group studies included the reaction of the p-aminobenzylamine 80 under the carbonylation conditions. If both the aliphatic and aromatic amines were to participate in the carbonylation reaction, oligomeric products would be formed. Primary aromatic amines, such as aniline were unreactive under the carbonylation conditions, so








formation of the simple urea 81 was expected to dominate. However the reaction mixture was quite complex and urea 81 was only isolated in 28% yield. In this case, addition of water to the solvent system decreased the yield to 14%.

The apparent participation of the aromatic amine in the reactions of 80 brought up the question of intramolecular carbonylation of the o-amino benzylamine 82 to form 3,4-dihydro-2(1H)-quinazolinone (83) (equation 4.5). Although aromatic amines are apparently not nucleophilic enough to initiate the reaction, they could be sufficiently




2 W(CO)6 N
NH2 CO / 12 / K2C03 N 0k, 0 (4.5)

82 83 H
H



nucleophilic to complete the cyclization following initial reaction of the aliphatic amine. In fact, under the carbonylation conditions 80 did form the cyclic urea, albeit in low yields (17 % yield in CH2C12 and 20% yield in CH2CI2/H20). The results from 80 and 82 suggest that differentiation between aliphatic and aromatic amines is not sufficiently good to obtain selective reactivity in the presence of both.



Conclusion



In summary, aliphatic primary amines can be catalytically carbonylated to 1,3disubstituted ureas in good to high yields using the commercially available, inexpensive and air stable W(CO)6 as the catalyst. The preference of this catalyst for aliphatic amines





86

is complementary to the late transition metal catalysts, which are generally more effective for aromatic amines than for aliphatic ones. Not only does this system provide an alternative to phosgene and phosgene derivatives, but it is also compatible with a variety of functional groups, including unprotected alcohols.














CHAPTER 5
ALTERNATIVE SYNTHESIS OF CYCLIC UREA HIV-1 PROTEASE INHIBITORS



Introduction



Cyclic ureas have recently been of interest as the core structures of potent HIV protease inhibitors such as the clinical candidates DMP 323 and DMP 450.150,151,176 These compounds serve as inhibitors of the HIV- 1-encoded aspartyl protease (HIV-Pr) which plays a fundamental role in the growth of immature virion proteins.[Kaltenbach, 1998 #1088] These cyclic ureas were designed to bind to HIV-Pr with the diol oxygens participating in symmetrical hydrogen bonding to the catalytic Asp 25/25'.




q H2N NH2


N. N N N
Ph Ph Ph,, Ph

HO OH HO OH

DMP 323 DMP 450


Figure 5.1 HIV Protease Inhibitors









There have been a few synthetic routes reported in the literature for compounds like DMP 323 and DMP 450, all of which involve problematic reagents or result in poor yields. 177 Scheme 5.1178 describes the original cyclic urea core structure synthesis which used amino acid starting materials. Disadvantages to this synthesis include the




Scheme 5.1 Synthesis of protease inhibitor core structure via an amino acid


CbzHN H3Cel" COOH


a zHN Me
NIII
H3C OMe
0


b, c


CbzHN NHCbz H3C CH3
HO OH


0

HN NH

H3C CH3

SEMO OSEM



g, h






N N

H3C CH3
HO OH

CORE STRUCTURE


CbzHN NHCbz e, f
, e f H3C-' / _.... CH3


SEMO OSEM


SEM = 2-(Trimethylsilyl)ethoxymethyl


Reagents: (a) i-BuOCOCI, NO-dimethoxyhydroxylamine HCI; (b) LiAIH; (c) VC13(THF)3, Zn-Cu;
(d) SEMCI; (e) cat. Pd(OH)2; (f) 1,1'-carbonyldiimidazole; (g) Benzyl bromide, Nai; (h) HCI, dioxane/MeOH




Full Text

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STUDY AND UTILIZATION OF GROUP 6 METAL COMPLEXES By ANDREA DENISE MAIN 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 2000

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Not many appreciate the ultimate power and potential usefulness of basic knowledge accumulated by obscure, unseen investigators who, in a lifetime of intensive study, may never see any practical use for their findings but who go on seeking answers to the unknown without thought of financial or practical gain. -Eugenie Clark

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ACKNOWLEDGEMENTS First, I am especially thankful for my partner. Dee Dee Mathews, for being there through thick or thin. I would also like to thank Linda Mathews for all of her love and support. I thank my advisor. Dr. Lisa McElweeWhite, for her support and encouragement through the years. Without her knowledge of organometallic chemistry, I would still be explaining metal-mediated reactions as some magical phenomena. I would like to thank my friends who have helped to make this an extraordinary period of my life: Stacey Huber, Dr. Sue Troutman Lee, Dr. Sunday Brooks, Damon Storhoff, Ann Storhoff, Holly Newman, Dr. Patricia Bottari, Dr. Margaret Kerr, Dr. Yingxia He, Dr. Christine Nixon Lee, Kirsten Johnson, Fang "Annie" Qian, Dr. Mark Tess and Dr. Rene Vieta. Special thanks go to Larry Lee for all the hours of therapy. Although I think he should have been a psychologist, I am sure he will make a wonderful dentist. I would like to thank my parents, George and Linda Main, for their love and support throughout my life. I am here today because of their dreams, which helped me to find my own. I thank my Savior, the Lord Jesus Christ, for being there when things looked bleak. He showed me that life is not to be squandered. Ill

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TABLE OF CONTENTS page ACKNOWLEDGMENTS iii LIST OF TABLES vii LIST OF FIGURES viii LIST OF SCHEMES ix ABSTRACT xi CHAPTERS 1 INTRODUCTION 1 General Overview of Low-Valent Metal Carbynes and Metal-Catalyzed Carbonylation Reactions 1 Bonding 2 Protonation of Low-Valent Metal Carbynes 4 Protonation in the Presence of Alkynes 10 Photochemistry 1 2 Carbonylation with Transition Metals 20 Catalytic Carbonylation of Amines with Metal Carbonyl Complexes 21 Nickel Carbonyl Complexes 21 Palladium Carbonyl Complexes 22 Platinum Carbonyl Complexes 22 Cobalt Carbonyl Complexes 23 Rhodium Carbonyl Complexes 25 Iron Carbonyl Catalyst 26 Ruthenium Carbonyl Complexes 28 Manganese Carbonyl Complexes 30 Rhenium Carbonyl Complexes 3 1 Chromium, Molybdenum, and Tungsten Carbonyl Complexes 32 IV

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Conclusion 37 2 PHOTOPHYSICS OF LOW VALENT CARBYNES 38 Introduction 38 Photophysics of Metal Carbyne Complexes 39 Synthesis of Cp{P(OMe) 3 }(CO)WsCR Complexes 41 Electronie Strueture of Cp{P(OMe) 3 }(CO)M=CPh (W=37, Mo = 38) 42 Eleetronie Absorption Spectra 44 Luminescence Studies 48 Transient Absorption Spectroseopy 53 Conclusion 58 3 SYNTHESIS AND PHOTOOXIDATION OF A TUNGSTEN CYCLOHEXENYL CARBYNE COMPLEX 59 Introduction 59 Synthesis of (Ti5-C5H5)(CO){P(OMe)3}W=C(c-C6H9) (44) 61 Photooxidation of (r|5-C5H5)(CO){P(OMe)3}W=C(c-C6H9) (44) 63 Oxidation of (T]5.C5H5)(CO){P(OMe)3}WHC(c-C6H9) (44) in the Presenee of Unsaturated Substrates 69 Other Reaetions with Unsaturated Substrates 71 Conclusion 71 4 CATALYTIC OXIDATIVE CARBONYLATION OF AMINES WITH W(CO)e 73 Introduction 73 Carbonylation of Primary Amines 75 Funetional Group Compatibility Study 79 Conclusion 85 5 ALTERNATIVE SYNTHESIS OF CYCLIC UREA HIV-1 PROTEASE INHIBITORS 87 Introduction 87 Carbonylation of Diamine 85 91 Carbonylation of Disubstituted Diamine 91 92 Conclusion 93 6 EXPERIMENTALS 94 General 94 General Instrumentation 94 Photophysieal Methods 95 Syntheses 95 [(C0)5WC(0)(2-napthyl)][NMe4] (42) 95 V

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Cl (CO){P(OMe)3}3W=C-(2-naphthyl) (41) 96 Cp(CO){P(OMe)3}W=C-(2-naphthyl) (40) 96 (CO)5\V-C(OCH3)(c-C6H9) (45) 97 [CO(Br) {P(OMe) 3 } 3 W=X(c-C 6 H 9 )] (49) 97 [Cp(CO){P(OMe)3}W=X(c-C6H9)] (44) 98 CpCl{P(OMe)3}W[Ti3-CH(c-C6H9)] (51) 98 Photooxidation of 44 99 General Procedure for the Catalytic Carbonylation of Benzylamines with W(CO)6 99 Procedure A 99 Procedure B 100 Procedure C 100 Preparation of Preparation of N,NÂ’-dibenzylurea 55 100 Preparation of N,NÂ’-Bis(4-chlorobenzyl)urea 57 101 Preparation of N,NÂ’-Bis(4-bromobenzyl)urea 59 101 Preparation of N,NÂ’-Bis(3-iodobenzyl)urea 61 101 Preparation of N,NÂ’-Bis(4-methoxybenzyl)urea 63 101 Preparation of N,NÂ’-Bis(4-methylthiobenzyl)urea 65 102 Preparation of N,N'-Bis(4-hydroxymethylbenzyl)urea 67 102 Preparation of N,NÂ’-Bis(ethyl 4-carboxylbenzyl)urea 71 102 Preparation of N,NÂ’-Bis(4-carboxylic acid benzyl)urea 73 103 Preparation of N,NÂ’-Bis(4-ethenylbenzyl)urea 75 103 Preparation of N,NÂ’-Bis(4-nitrobenzyl)urea 77 104 Preparation of N,NÂ’-Bis(4-cyanobenzyl) urea 79 104 Preparation of N,NÂ’-Bis(4-aminobenzyl)urea 81 104 Preparation of 3,4-Dihydro-2(l//)-quinazolinone 83 104 Preparation of (4R, 5R)-2,2-Dimethyl-4,5-bisformyl1.3dioxolane N,N-Dimethylhydrazone 88 105 Preparation of (3R, 4S, 5S, 6R)-2,2-Dimethyl-4,5-bisformyl1.3dioxolane-3,6-bis(phenylmethyl) N,N-Dimethylhydrazine 89 105 Preparation of (2R, 3S, 4S, 5R)-2,2-Dimethyl-3,4-bisformyl1 .3dioxolane-2,5-bis(phenylmethyl)diamine 85 106 Preparation of (4R, 5S, 6S, 7R)-Hexahydro-5,6-(9isopropylidene-4,7-bis(phenylmethyl)-2//1 ,3-diazapin-2-one 86 106 Preparation of (2R, 3S, 4S, 5R)-2,2-Dimethyl-3,4-bisformyl-l,3dioxolane-2,5-bis(phenylmethyl) N,N-Dibenzyldiamine 91 106 Attempted preparation of (4R, 5S, 6S, 7R)-Hexahydro-5,6-0isopropylidene-4,7-bis(phenylmethyl)-N,N-dibenzyl1.3diazapin-2-one 90 107 REFERENCES 108 BIOGRAPHICAL SKETCH 120 VI

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LIST OF TABLES Table page 2.1 Absorption Spectra of Cp(CO){P(OMe) 3 }M=C-R ( 37 40 ) in THF Solution 47 2.2 Luminescence and Transient Absorption Data for Cp(CO){P(OMe) 3 }M=CR 49 4. 1 Catalytic carbonylation of primary amines in single/ biphasic solvent systems 78 4.2 Catalytic carbonylation of substituted benzyl amines 81 vii

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LIST OF FIGURES Figure page I . I Partial MO diagram for [Cp(CO) 2 MnsC-Me]'' 3 1 .2 Radical cation A prefers bent geometry 20 2. 1 Orbital mixing diagram for Cp(CO){P(OMe) 3 } W=C-Ph (37) 43 2.2 HOMO and LUMO of Cp(CO){P(OMe) 3 }WsC-Ph (37) 45 2.3 Absorption spectrum of Cp(CO){P(OMe) 3 }WsC-Ph in THF solution at room temperature 46 2.4 Emission and excitation spectra: (a) for 37 and 39 and (b) for 40 . Emission spectrum is at right and excitation at left 5 1 2.5 Transient absorption difference spectra following laser excitations (355 nm, 10 ns fwhm, 10 mJ/ pulse) 56 3.1 Comparison of '^C NMR data between 51 and known 52 67 3.2 Electrospray mass spectral data of 51 68 3.3 Cyclic voltammogram of 44 . Sweep rate: 100 mV/s 70 3.4 Investigated reactivity of 53 with unsaturated substrates 71 5.1 HIV Protease inhibitors 87 viii

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LIST OF SCHEMES Scheme page 1 . 1 Protonation of Cp(CO) 2 W=CTol (7) with HBF4 6 1 .2 Protonation of Cp(CO) 2 W=C-Tol in the presence of alkynes 1 1 1 .3 Suggested precursors to naphthol complex 18 12 1 .4 Photochemical migration of CO to carbyne carbon in Cp(CO) 2 W=C-Tol (7) 13 1.5 Photochemical isomerization of cis, cisX(CO) 2 (PR 3 ) 2 M=C-Ph 1 4 1 .6 Metal-centered reactivity of photooxidized metal carbynes 1 6 1 .7 Selected organic products from the photooxidation of carbynes 1 7 1.8 Mechanisim of photooxidation of Cp(CO){P(OMe) 3 }MosC(c-C 3 H 5 ) 18 1 .9 Proposed formation of olefins 1 9 1.10 Reaction mechanism for the carbonylation of amines with Fe(CO )5 28 1.11 Proposed two step reaction mechanism 3 1 1.12 Postulated mechanism for the carbonylation of primary amines in the presence of Mn 2 (CO)io 32 1.13 Stoichiometric carbonylation of amines in the presence of a tungsten carbamoyl complex 34 1.14 Proposed mechanism for the carbonylation of amines in the presence of 32 36 IX

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3.1 Proposed mechanism for generation of metal carbenes upon photooxidation of metal carbynes 60 3 .2 Synthesis of (r| ^-C sHjXCO) {P(OMe) 3 } W^CCc-CgHg) ( 44 ) 62 3.3 Proposed mechanism for the protonation of carbynes 65 5.1 Synthesis of protease inhibitor core structure via an amino acid 88 5.2 Preparation of the diamine acetonide 85 89 5.3 Synthesis of core structure starting from the ester of L-tartaric acid 90 5.4 Dupont process scale-up of cyclic urea 90 91 X

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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 STUDY AND UTILIZATION OF GROUP 6 METAL COMPLEXES By Andrea Denise Main December 2000 Chairman: Lisa McElwee-White Major Department: Chemistry This dissertation describes the preparation, reactivity, and photochemistry of a series of low-valent carbyne complexes, Cp(CO){P(OMe) 3 }MsC-R (M == Mo, W; R == aryl, alkyl). These carbynes were prepared from Mo(CO)6 or W(CO)6 respectively. Photolysis of these carbyne complexes in chlorinated solvents leads to the generation of highly reactive 17 e" species, which are formed by one-electron transfer from the complexes to the solvent. The precursors to these 17 e' species are the excited state of the carbynes. Investigations of the excited states were performed using absorption, emission, and transient absorption spectroscopy. XI

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Previous work done by the McElwee-White group had demonstrated that the photooxidation of carbyne complexes of the type [(rj^-C5H5)(CO){P(OMe)3]M=C-R] (M = Mo, W; R = alkyl) results in rearrangement and decoordination of the carbyne ligand to yield organic products. Mechanistic studies suggested that following formation of the 1 7 e' species mentioned above, H-abstraction took place at the carbyne carbon to yield a carbene. The first direct evidence for the formation of carbene complexes in these reactions was provided by the study of the cyclohexenyl complex [(r|^C5H5)(CO){P(OMe)3]W=C-C6H9] Ifom W(CO) 6 . Photooxidation of the carbyne complex afforded the carbene complex [(r|^-C5H5)(Cl){P(OMe)3]W=CH-C6H9]. Following the use of W(CO)g in the preparation of carbyne complexes, it was then explored as a catalyst for the carbonylation of amines to ureas. Previous McElwee-White group members had demonstrated that W(CO )6 serves as a catalyst for carbonylation of primary and secondary amines to ureas. Primary and secondary diamines can be converted to cyclic ureas by the same process. This dissertation describes the study of functional group compatibility with the catalyst during the carbonylation reaction. A series of substituted benzyl amines were synthesized and carbonylated to the corresponding ureas under the W(CO)e system. Single and biphasic solvent systems were investigated which revealed that higher yields could be obtained in the presence of the biphasic solvent system, CH2CI2 and H2O. This dissertation also describes the synthetic application of the W(CO )6 system towards the synthesis of the cyclic urea core structures of HIV protease inhibitors DMP 323 and DMP 450 . xii

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CHAPTER 1 INTRODUCTION General Overview of Low-Valent Metal Carbvnes and Metal-Catalyzed Carbonvlation Reactions Metal carbyne complexes have been extensively explored since their first introduction into the scientific realm in 1973. The first carbyne complexes were lowvalent and of the type X(CO) 4 MsCR (X = Cl, Br; M = Cr, Mo, W; R = Ph). Although carbynes of groups 5, 7, 8, and 9 have since then been reported as well as carbynes with various R groups, group 6 metal carbynes remain the most common. The constant interest in metal carbyne complexes is due to their potential as organic synthons and their ability to mimic intermediates in catalytic reactions. ^ Three general reviews by H. Fischer et al.,^ Kim and Angelici,^ and Mayr and Hoffmeister^ have comprehensively covered the synthesis, characterization, and reactivity of metal carbynes up until 1990. Other shorter reviews have covered more specific subjects: low valent group 6 metal carbynes,6-9 bridging carbyne ligands, ^ O' 12 metal carbynes with carbaborane ligands, 13 ruthenium and osmium metal carbynes, 14 photochemistry,! 3 high valent metal carbynes,! »16-1 8 and a survey of the coupling reactions of n-ligands including carbyne ligands by Mayr and Bastos.19 The most current survey of metal carbyne chemistry is the 1995 review by Engel and Pfeffer.20 Due to the extensive reviews already available 1

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2 for metal carbynes, this chapter will cover literature after 1995 with emphasis on the reactivity and photochemistry of metal carbyne complexes. There are a number of different synthetic routes to metal carbyne complexes. One route incorporates a metal carbonyl complex as the starting material.^ 1 Metal carbonyl complexes have not only been useful in preparation of organometallic complexes but also have been found to transform organic substances into other species. Hence, this chapter will also include a review of metal carbonyl complexes that have demonstrated their ability to carbonylate organic compounds, particularly amines. Bonding In order to understand the reactivity of low-valent metal carbyne complexes, it is useful to look at the nature of the chemical bonding in these complexes. Fenske analyzed the M=C-R triple bond in neutral and positively charged low-valent carbyne complexes using Fenske-Hall22 approximate SCF calculations.23-25 projj^ tpggg calculations, the a bonds were found to be strong and the n bonds degenerate or nearly degenerate between the metal (i.e. chromium, manganese, and iron) and the CR ligand. Kostic and Fenske ascertained from these calculations that the carbyne ligand was a stronger n acceptor than CO resulting in increased electron density on the carbyne carbon.24 a typical MO diagram of n interactions is shown in Figure 1.1. The cyclopentadienyl ligand mixes into the former d orbitals to give three non-degenerate molecular orbitals in the metal fragment. However, the HOMO of the resulting complex is still primarily metal-based, and the LUMO is still a n* antibonding orbital.

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3 Further literature includes an ab initio study of a metal carbyne complex, that of tra«5-Cl(CO)4Ci=CH, along with correlation energies published in 1986 by Poblet et al. These studies showed that the degenerate Cr-C n bond is essentially nonpolar, while the CT bond is polarized toward the carbyne carbon, which leads to a partial negative charge at the carbyne ligand.26 The negative polarity of the carbyne carbon explains the facile protonation of many complexes at the carbyne carbon. Protonation of metal carbynes is said to be charge-controlled. However, in some instances, nucleophiles have been known to attack the carbyne carbon. This mode of reactivity is said to be controlled by frontier molecular orbitals. The HOMO of the nucleophile donates its electrons into the LUMO of the carbyne complex, even though the LUMO is slightly negatively charged.27 Figure 1.1 Partial MO diagram for [Cp(CO) 2 MnsC-Me]^

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4 Protonation of Low-Valent Metal Carbvnes Following the synthesis of the first metal carbyne in 1973 , it was four years before material was published describing the reaction of metal carbyne complexes with electrophilic reagents. Bottrill and Green reported the protonation of the electron rich carbyne [Mo(=CCH2‘Bu){P(OMe)3}2(r|-C5H5)] ( 1 ) with HBF4 which led to the formation of the cationic hydrido species [MoH(=CCH2^Bu){P(OMe)3}2(ri-C5H5)][BF4] (2) (see equation 1 . 1).28 The authors were unable to determine the intial site of protonation. Protonation could occur directly on the molybdenum center, or the carbyne carbon could be protonated first, followed by hydrogen transfer to the metal, otherwise known as an ahydride elimination. -1+ BF, (MeO)3P"^o= C-CH‘Bu (MeO)3P^ HBF^ \ HM^C-CH‘Bu ( 1 . 1 ) (MeO)3P P(OMe)3 Numerous reports of protonation reactions followed, showing not only protonation at the metal but also at the carbyne carbon. For example. Holmes and Schrock reported the protonation of [W(=CH)(PMe3)4Cl] ( 3 ) with CF3SO3H which gave the ligand protonated product [W(=CH2)(PMe3)4Cl][ CF3SO3] (4) (equation 1.2). However, by changing the ancillary ligands to the bridging diphosphine Me2PCH2CH2PMc2, protonation occurred at the metal center rather than at the ligand as was seen in the former case (equation 1 . 3).29

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5 H I C MC3P" I PM 63 Cl H. ^PMc 3 CF3SO3H Mc3l^ Me,P' r ..w.: vsx"' " H ,PMe, PMe, Cl CF 3 S 03 ' + ( 1 . 2 ) H C Mej P'*//,. Men T Cl MC2 *p MC2 CF3SO3H H n CF 3 S 03 ' MC2 C P^ I ^P Men Men H Men ( 1 . 3 ) Subsequent chemistry revealed that changing the counterion of the acid could greatly influence the course of the reaction. Stone protonated Cp(CO) 2 WsCTol (7) (Tol = tolyl) with HBF4 to yield the cationic dinuclear p-alkyne complex 10. The reaction was proposed to proceed through the cationic carbene [Cp(CO) 2 W=C(H)Tol]^ (8) (see Scheme 1.1).

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6 Scheme 1.1 Protonation of Cp(CO) 2 WsCTol (7) with HBF 4 \ qqi.^W=C-To1 oc HBF. OC ""V CO Tol H + BF. 8 Tol. \ /Tol Cp(CO)2W^ /W(CO)2Cp H + BF. 0O/-.,„. \ / ^w=cf CO ^ Tol H Cp(CO)2W=C-Tol + BF. 10 The intermediate 8 was never detected but was inferred by the formation of Cp(CO) 2 (I)W=C(H)Tol (11) when the reaction was run in the presence of iodide (equation

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7 (1.4) However, protonation of Cp(CO) 2 WsCTol in the presence of HCl instead of HBF4 resulted in double protonation of the carbyne carbon to form an alkyl ligand that subsequently yielded the r)^-acyl complex 12 upon CO insertion (equation 1.5).32,33 \ OC'^^^^C-Tol OC 2 HCl CH2T01 (1.5) Cl O'" Cl 12 Another example of counterions affecting the reaction pathway is the protonation of Cp{P(OMe)3}2MosCCH2‘Bu ( 1 ) with CF3COOH. Unlike the reaction of 1 with HBF4 (equation 1.1), CF 3 COO' coordinates to the molybdenum center resulting in loss of the carbyne ligand.24 It was also discovered that protonation could take place across the face of the carbyne complex resulting in an agostic interaction of the C-H bond with the metal (for example, see equation 1.6).25,36 This type of reactivity is better understood by taking into account that the probability of hydrogen migration from the carbyne carbon to the

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8 metal center increases when there is little n-acceptor character in the ligands. If hydrogen migration did occur in complexes with strong 71 acceptor ligands like CO, the formation of the localized M-H bond would eliminate n backbonding from the metal center back into the n acid ligand, resulting in ligand loss. Thus, 71 -acceptor ligands inhibit full migration of the hydrogen to the metal center due to competition for the d electrons resulting in agostic interactions.^ CO ,^PMC3 C1-^>V=C— Me HCl CO .W; .j.PMe3 ^ Me Me3P PMe, H Cl Me3P ( 1 . 6 ) 13 14 Consistent with the explanation above, McElweeWhite reported the protonation of the carbyne CpL 2 MosCBu (15) [L = CO, P(OR) 3 ] with HBF 4 (equations 1.7-1.9)37 which resulted in a systematic shift of the hydrogen from the metal center to the face to the carbyne ligand as the number of carbonyl groups were increased from zero to two. These results are consistent with determination of the final protonation site by the electron density at the metal. As mentioned before, when 7 i-backbonding to the carbonyl ligands decreases the electron density at the metal, the protonation site shifts from the metal toward the carbyne carbon.

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9 \ .M (MeO)3P"y (MeO)3P^ CBu HBF^ (MeO)3P'^• H i (MeO)3P + BF, ^CBu (1.7) 15a 16a 15c 16c

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10 Protonation in the Presence of Alkvnes Metal carbenes have found numerous applications in organic synthesis38-41 anj one important class of reactions is one that occurs between carbene complexes and alkynes. The product isolated depends on the metal, the ligands, the substituents on the carbene, and the reacting alkyne. Shown in equation 1 . 1 0 is a typical example of a metal carbene reacting with an alkyne. The reaction of metal carbenes with alkynes has been extended to the same type of chemistry with metal carbynes. As mentioned earlier, protonation of metal carbynes often leads to the intermediacy of a metal carbene. Analogous to the reaction in equation 1.10, metal carbynes have been protonated in the presence of alkynes to form addition products. For example, when Cp(CO) 2 WsC-Tol (7) reacts with HBF4 and acetylenes, depending on the R groups on the acetylene and the presence of carbon monoxide, q^allylidene (17) or q"-naphthol (18) products result (Scheme 1.2).42 Jhe naphthol

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11 complex 18 is thought to form by either the r|'‘-ketenyl complex 19a or the metallacycle 19b (Scheme 1.3), both of which are derived from protonation of the metal carbyne at the ligand, followed by insertion of the alkyne into the MsC bond. This is still a relatively new field of ehemistry and could lead to a number of different products, depending on the carbyne, the aeid, and the unsaturated substrates used. Scheme 1.2 Protonation of Cp(CO) 2 W=C-Tol in the presence of alkynes 18

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12 Scheme 1.3 Suggested preeursors to naphthol complex 18. 18 Photochemistry Relatively little is known about the photochemical properties of metal carbyne complexes. The electronic character of ligands attached to metals can undergo significant change by photoexcitation, especially if the excited states involving those ligands are populated. Carbyne complexes should have a rich photochemistry due to the unsaturated character of the metal-carbyne moiety and the presence of long-lived and low-lying metal-to-carbyne ligand-charge transfer (MLCT) excited states.43 There are a number of examples in literature where photoexcitation of metal carbyne complexes has led to various reaction pathways that would be unavailable to the carbyne in the ground

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13 state. The most common photochemical processes among metal carbyne complexes involve carbonyl ligands. Mayr et al.44 and Geoffroy et al.45 have established that carbonyl-carbyne coupling is an important photochemical reaction pathway in some systems. Photolysis of the electron rich Cp(CO) 2 \VsC-Tol in the presence of phosphines gives r\ and r| -ketenyl complexes, depending on the phosphine employed (Scheme 1.4). Scheme 1.4 Photochemical migration of CO to carbyne carbon in Cp(CO) 2 \V=C-Tol (7) O 7 Cp(PPh3)(CO)W— 20 . Cp(CO)(dppe)WTol PPhj 21 hv [Cp(CO) 2 W=C-Tol] Cp(CO)W— C.^ r\ MLCT Excited r\ -ketenyl Cp(CO)2W— 22 Tol O O Cp(CO)W— C— Tol CO 23 24

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14 The mechanism is not completely understood, but the increased electron density at the carbyne carbon in the MLCT state is thought to facilitate migration of the CO ligand and subsequent formation of a 16 eÂ’ ketenyl complex. Further reaction of the ketenyl complex with phosphines leads to the products 20 and 21. In the photoassisted cis-trans isomerization of complexes of the type X(CO) 2 L 2 MsC-Ph (X = Cl, Br; L = PMes, PPhs; M = Mo, W), Mayr has deduced the presence of q ^-ketenyl intermediates (Scheme 1.5). In the photolysis of cis 25, the reaction forms a five-coordinate ketenyl species in which the phosphines subsequently rearrange to be trans to each other, minimizing steric forces, followed by migration of the carbonyl back to the metal center to give trans 26. Scheme 1.5 Photochemical isomerization of cis, cisX(CO) 2 (PR 3 ) 2 M=C-Ph CO

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15 Another interesting reaction found in literature is the photolysis of Cl(CO) 2 (PMe) 3 WsCPh with excess phenylacetylene which gives the p'-ketenyl complex W(Ti'-PhCCO)Cl(CO)(PhCCH)(PMe 3)2 45 Photochemical electron transfer has been shown to activate 18 e" complexes toward reactivity. Due to differences in reactivity between openand closed-shell organometallic complexes, reaction pathways unavailable to neutral 1 8 e' complexes can often be accessed by the ensuing 17 e' radical cations.46-48 -phis strategy has proven effective for metal carbynes. Over the past several years, the McElwee-White laboratory has been investigating photooxidation reactions involving irreversible electron transfer from a series of tungsten and molybdenum aryland alkylcarbynes to halogenated solvents.49 Early experiments involved photolysis of the carbynes CpLiL 2 MsCR 27 [Li, L 2 = CO, P(OMe) 3 ; R = alkyl, aryl; M = Mo, W] in CHCI3 solutions. When the reactions were run in the presence of PMe 3 to stabilize any unsaturated intermediates, new cationic carbyne complexes resulted as shown in Scheme 1.6.49,50 Mechanistic studies showed the primary photoprocess to be electron transfer from the excited state of the carbyne to CHCI3. Upon reduction, CHCI3 undergoes fragmentation to form chloride ion and dichloromethyl radical resulting in irreversible electron transfer. The ensuing 17-electron cationic carbyne 27"^* then undergoes metal-centered reactivity involving ligand exchange with the more strongly donating PMe 3 to form 28 followed by halogen abstraction to yield the final cationic carbyne 29.

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16 Scheme 1.6 Metal-centered reactivity of photooxidized metal carbynes. Li 27 hv CHCI3 + 27 + Cl+ •CHCI 2 -I27’ PMe3 PMe3 28 cr CHCI3 28 + cr + •CHCI2 29 In the absence of strongly nucleophilic species such as PMea, the photolysis of carbyne complexes results in very different reactivity. If the carbyne substituent is a primary or secondary alkyl group, photooxidation yields carbyne ligand-centered reactivity resulting in organic products (Scheme 1.7).

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17 Scheme 1.7 Selected organic products from the photooxidation of carbynes. Mechanistic studies revealed that the reactions in Scheme 1.7 share the same first two mechanistic steps. Scheme 1.8^^ shows these first two steps in the transformation of of Cp(CO){P(OMe)3}MosC(c-C3H5) (30) into cyclopentenone. After photoexcitation of 30, the first step involves one electron transfer from the excited state of the carbyne to the solvent, forming the 17-electron radical cation A. In the second step, the radical cation A abstracts a hydrogen from the reaction medium to form the cationic carbene B which undergoes ring expansion to form the metallacycle C. The mechanism then proceeds with insertion of CO into the metal-alkyl bond and coordination of ClÂ’ to form D. Reductive elimination of the organic ligand produces the cyclopentenone.

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18 Scheme 1.8 Mechanisim of photooxidation of Cp (CO){P(OMe) 3 }MosC(c-C 3 H 5 ) 30 "H* \ MoczC OC' / P(OMe )3 B +

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19 Similarly, in the formation of 1 -butene from the photolysis of Cp(CO){P(OMe) 3 }MosC-«-C 4 H 9 (31), the first step is electron transfer from the excited state to the solvent, which forms the seventeen-electron radical cation A, followed by Habstraction at the carbyne carbon to yield the cationic carbene complex B (Scheme 1.9).52 However, at this point, B undergoes an H-shift rather than CO insertion to form the cationic rj^-olefin complex C, which subsequently eliminates to form the terminal olefin. Scheme 1.9 Proposed formation of olefins ,.Mo^C — CH2C^7 (MeO)3P'Y CO 31 hv CHCI3 Mcf=C — CH7C3H7 (MeO)3P' y CO A + RH -R* H H X + H-shift 1 (MeO)3P"^^o^ — n A .MctrC. (MeO)3P i ^CH2C3H7 CO H C3H7 CO c L B J It should be noted that the H-abstraction process (A^B) for an organometallic radical is highly unusual. Reactivity at an organic ligand is known to occur in metal radicals but reaction at the metal center is much more common.^^ When ligand-centered reactions do occur, they are almost always dimerization processes. In order to explain the ligand’s affinity for the hydrogen, INDO calculations were carried out on the carbyne

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20 radical cation A as the angle (|) was varied from 180° to 120°.^2 These calculations determined that the radical cation A favors a bent carbyne configuration (Figure 1 .2) over the original linear carbyne geometry. The geometry change causes spin density to reside on the carbyne carbon. Figure 1.2 Radical cation A prefers bent geometry This type of chemistry has resulted in the formation of a variety of olefins, dienes, dienals and other cycloalkenones from the photooxidation of various alkyl metal carbynes. Carbonylation of organic compounds in the presence of transition metal catalysts has become a topic of increasing interest. One facet of this chemistry is carbonylation involving metal carbonyl complexes.^4,55 There are numerous reactions throughout the literature that invoke metal carbonyl complexes in the catalytic carbonylation of organic compounds. There are reviews of this type of chemistry56,57 and a multitude of articles describing catalytic transformations such as olefins to aldehydes^S or carboxylic acids,^9 A Preferred geometry Carbonylation with Transition Metals

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21 arenes to aromatic aldehydes,^^ aryl halides to aromatic acids, alkyl halides to esters, 62 nitroaromatics to carbamates,63 and alkynylben2ylalcohols64 or epoxides^^ to cyclic carbonates. Typically, late transition metals, groups 8-10, are used to catalyze these transformations, although there has been one gold(I) carbonyl catalyst (group 12) reported.66 Catalytic Carbonylation of Amines with Metal Carbonvl Complexes Further example of these reactions is the catalytic carbonylation of amines with metal carbonyl complexes to form isocyanates, ureas, and carbamates. There is continued interest in deyeloping new synthetic routes for such compounds for multiple reasons: isocyanates are used as precursors for polyurethanes and polyureas;67 ureas are used in the pharmaceutical industry as well as in agricultural products;68 and carbamates not only act as chiral auxilaries, but also are found in pharmaceuticals.69 Furthermore, interest in new synthetic routes continues in an effort to find altematiyes to the traditional method of carbonylating amines using phosgene, a highly toxic and corrosiye gas.20 Nickel Carbonyl Complexes Initial studies of the transition metal catalyzed carbonylation of amines date back to World War II where nickel salts were added to amines and the catalytically actiye species were undoubtedly metal carbonyls.21"23 Howeyer, subsequent carbonylation

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22 reports of amines using Ni catalysts are few with very low product yields,^ 1 and none of the catalysts are carbonyl complexes. Palladium Carbonyl Complexes Palladium catalysts are the most commonly used transition metal catalysts for the carbonylation of amines. Although the actual catalysts are not carbonyl complexes, a significant amount of the reactions are thought to proceed through palladium carbonyl intermediates. For instance, palladium acetate promoted by iodine can carbonylate amines in the presence of alcohols to carbamates and is thought to proceed through a Pd(0)(carbonyl) complex. "74 In the absence of alcohols, ureas are formed. "75 Monoand di-methoxycarbonyl complexes of palladium of formula [PdL 2 Cl 2 n(COOCH 3 )n][n = 1 or 2; L 2 = bipyridine or 1 , 1 0-phenanthroline] have been found to carobnylate amines to ureas at room temperature under CO pressure. In the presence of CuCl 2 , the reaction generates carbamates rather than ureas.^^ Furthermore, a number of palladium catalysts that also carbonylate amines to ureas and carbamates have been reported by Kanagasabathy and Chaudhari,”77 Alper et al.,^8,79 Valli and Alper.^^ Platinum Carbonyl Complexes The one example in the literature using platinum carbonyls to carbonylate amines is the photolytic reaction of Pt(CO) 2 (PPh 3)2 with morpholine and cyclohexyl iodide under

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23 1 atm of carbon monoxide at room temperature which gives the corresponding amide in 75% yield (see equation 1.12).81 Cobalt Carbonyl Complexes The earliest report of a cobalt carbonyl complex carbonylating amines was in 1953 by Sternberg who generated formamides in 60-78% yields from the carbonylation of secondary amines, dimethylamine and piperdine, with Co 2 (CO )8 under 200 atm of carbon monoxide at 200 °C (equation 1.13).^2 i^gy intermediate in this reaction is assumed to be a carbamoyl species H(CO)nCo-CONR2 which produces the formamide upon reductive elimination. 1 atm CO 75% (1.12'l 200 atm CO + 2 [Co”(piperdine)6 ] [Co 2 (CO)g] (1.13) 78% Other reports of Co 2 (CO)s-catalyzed processes include the carbonylation of allyl amine under even more severe conditions with 300 atm of carbon monoxide at 280 °C which afforded 2-pyrrolidine (equation 1.14). Although run at room temperature,

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24 photolysis of this reaction later proved even more inefficient with low yields, and product mixtures containing 2-pyrrolidone, N,N'-diallylurea, and N-allyl-3-butenamide. The photochemical cleavage of C-N bonds of amines coordinated to Co was proposed.^3 There are a few examples of a metal carbonyl compound catalyzing the ringopening and carbonylation of aziridines. The ring strain present in three-membered ring compounds suggests ring-opening/ carbonylation could take place under relatively mild conditions. One of those examples is the treatment of 1,2-disubstituted aziridines with 33 atm of carbon monoxide and Co 2 (CO )8 in DME (1,2-dimethoxyethane) for 24 h at 100 °C to yield P-lactams in up to 95% yield. The reaction is regiospecific with insertion of a carbonyl into the least substituted of the two carbon-nitrogen bonds in the ring.^'^ + CO (300 atm) — (1.14) H R R' R. .R" Co 2 (CO)g, DME, CO (33 atm) '.II R. (1.15) N 100 °C, 24 hr N— \ / CH2CH2Ph PhCH2CH2 O

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25 Rhodium Carbonyl Complexes When subjected to the rhodium catalyst [Rh(CO)2Cl]2, primary amines are converted to the corresponding formamides or ureas, depending on the conditions. For instance, «-butyl amine forms a mixture of Wbutylformamide and di-«-butylurea in the presence of the catalyst under 59 atm of carbon monoxide at 160 °C (equation 1.16). However, when the reaction is carried out in the presence of PMes, Wbutylformamide is formed in quantitative yield. [Rh(CO)2Cl]2 BuNH 2 BuNHCHO + BuNHCONHBu ( 1 . 16 ) 60 atm CO 35 % 65% 160 In contrast to the severe temperatures and pressures required for the Co2(CO)gcatalyzed carbonylation of allyl amines, Rh4(CO)i2 and [Rh(CO)2Cl]2 carbonylated butenyl amine under milder conditions to give 2-piperidinone, exclusively (equation 1.17). The conditions used in these rhodium-catalyzed reactions include 400 psi of CO/H2 and temperatures of 40-50 °C. Yields varied from 85-87%.86 + CO/H2 [Rh] (1.17)

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26 Pyrrolidinones were isolated as the only products when aliphatic iV-allylor N(methylallyl)amine was carbonylated in the presence of HRh(CO)(PPh 3 ) 3 . The reaction requires the addition of NaBH 4 and isopropanol, and takes place under 35 atm of carbon monoxide at 100 °C. Specifically, N-allylcyclohexylamine afforded N-cyclohexyl-2pyrrolidinone in 78% yield (equation 1.18). The yield decreases appreciably ifNaBH 4 is substituted by other borohydrides.^^ CO, HRh(COXPPh3)3 NaBHq, i-PiOH, CH 2 CI 2 35 atm, lOO'^C (1.18) Additional Rh reports include the catalyst Rh(CO) 2 (acac) which carbonylates aniline under 41 atm of a CO/ O 2 gas mixture, giving A,iV-diphenyl urea (74% selectivity) and A-phenyl carbamate (24.5% selectivity). When Nal, acting as a promoter, is added to the reaction, there is an increase both in the activity of the catalyst and the selectivity for the carbamate. Iron Carbonyl Catalyst Formamide complexes can be obtained by the reaction of piperidine with Fe(CO)s (equation 1.19). However, this reaction is limited to piperidine. When pyrrolidine or

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27 pyridine is reacted under the same conditions, iron-amine complexes form.^^ Furthermore, when the catalyst reacts with ammonia, urea is formed. (1.19) Dombek and Angelici later repeated this chemistry with the addition of CO and found that they could carbonylate primary (cyclohexylamine) and secondary (piperidine) amines to the corresponding formamides under 95 atm of CO at 200 °C in 66-83% yields, respectively. Their proposed mechanism is shown in Scheme 1.10. Reaction A involves the formation of a carbamoyl complex anion, Fe(C0)4[C(=0)NR2]'. This reaction is rapid and reversible at room temperature with piperdine, pyrrolidine, and «-butyl amine. Upon heating at 50 °C for 50 hr, the carbamoyl complex decomposes to Fe(CO) 4 (HNR 2 ) and the formamide, HCONR 2 . The authors propose that this decomposition occurs via steps B, C, and D. Step B involves proton transfer from the ammonium cation to the iron atom giving the hydride complex which upon reductive elimination, generates the formamide in step C. The resulting coordinatively unsaturated species Fe(CO )4 then reacts with excess amine to the give the final observed product, Fe(CO) 4 (HNR 2 ). Further reaction of Fe(CO) 4 (HNR 2 ) in the catalytic reaction with CO could generate Fe(CO )5 as is shown in step E.^O

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28 Scheme 1.10 Reaction mechanism for the carbonylation of amines with Fe(CO)5 A 2HNR2 Fe(CO)s — OC— Fe— CO oc I CO H2NR2 ^ E CO B -HNR2 Fe[CO]4 D HNR2 NR2 H CO CO Fe(CO)4(HNR2) Ruthenium Carbonyl Complexes As described earlier, Co2(CO)8 and Fe(CO)s can catalyze the carbonylation of piperdine to the formamide, but under severe reaction conditions. The ruthenium catalyst, Ru3(CO)i2, can convert piperdine to the formamide under much milder conditions (75 °C, 1 atm CO).^l Typically, secondary amines are more easily converted to formamides than primary amines. Stringent conditions are usually required for primary amines. For

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29 example, Kealy and Benson reported that carbonylation of the primary amine, cyclohexylamine, in the presence of allene gave A^-cyclohexylformamide using Ru3(CO)i 2 as the catalyst. This transformation, however, required extremely high carbon monoxide pressure (1000 atm). 92 Watanabe also used Ru 3 (CO)i 2 to catalyze the carbonylation of various primary amines to the corresponding formamides with much lower levels of CO but rather high temperatures (39 atm, 180 °C).93 Again, a carbamoyl species is suggested as the key intermediate in the formation of formamides. It is postulated that the carbamoyl complex is formed by either an intermolecular nucleophilic attack of the amine on the metal carbonyl ligands or by an intramolecular 1,2-shift reaction between coordinated carbon monoxide and amine. The authors could not isolate the carbamoyl species but its presence was evident in the IR. Aniline can be carbonylated in the presence of various ruthenium carbonyl complexes, such as Ru(CO)3l3, Bu4N[Ru(CO)3l3], and Ru(CO)2(py)Cl2^^’94 to give Nbenzylcarbamate or N,N'-diphenylurea. The carbamate is formed in moderate yields with a 13:1 mixture of CO and O 2 in methanol at 170 °C. Cyclic amines can be carbonylated to the formamides with ruthenium cluster compounds, [HRu 3 (CO)n]‘ and [H 3 Ru 4 (CO)i 2 ]’. Both carbon monoxide and a mixture of hydrogen and carbon dioxide gasses are used. A neutral carbamoyl cluster complex was isolated and thought to be a possible intermediate.^®

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30 Manganese Carbonyl Complexes The manganese carbonyl complexes, Mn 2 (CO)io and CH 3 Mn(CO) 5 , have also been explored for the carbonylation of amines. Again, stringent conditions are required with reactions run between 180 and 200 °C under 130 atm of CO. In these reactions, primary amines gave dialkylureas exclusively, contrary to previous reactions which primarily form formamides. Also, it should be noted that ammonia, secondary and tertiary amines, and aniline react very slowly, with the corresponding urea being the primary product in each case with little formation of the formamide. Calderazzo proposed that the poor yields of formamides in these reactions were due to the over-all reaction occurring in two steps with ^2 being probably much larger than k\ (Scheme 1.11). If k\ were larger than ki, little or no dialkylurea would be formed. Calderazzo concluded that since both steps occur in the coordination sphere of the metal, the condition for the formation of the urea is that the formamide formed in the first step remains on the metal and undergoes a fast nucleophilic attack on the formyl carbon atom by a second molecule of amine. Dombek and Angelici later explored the reaction mechanism more thoroughly.^'^ By studying several stoichiometric reactions, they proposed the following mechanism which is detailed in Scheme 1.12. The authors admit that there are possible alternative mechanisms for individual steps but overall, given the supporting evidence. Scheme 1.12 best describes the carbonylation of primary amines with Mn 2 (CO)io. There are three key steps in the mechanism. The first describes the formation of the carbamoyl complex from the reaction of the amine with Mn 2 (CO)io. The second step depicts the reaction of

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31 the amine and CO with the carbamoyl complex to give the urea via the organic isocyanate. In the third step, Mn2(CO)io is regenerated from Mn(CO)5’ and RNHs^. In addition, HMn(CO)5, an intermediate in the reaction, was also found to catalyze the reaction, probably by a similar mechanism. Finally, Srivastava and coworkers have used the manganese complex, (t]CH3C5H4)Mn(CO)3, to carbonylate primary amines by irradiation with UV light for 1 00 250 hours. The non-catalytic reaction uses the manganese complex simply as a source of C0.72 Scheme 1.11 Proposed two step reaction mechanism Stepl: H2N-R + CO — HCONHR Step 2 : HCONHR + H2N-R b. ^ RNHCONHR + H2 Rhenium Carbonyl Complexes The only report of a rhenium complex carbonylating amines is the one described by Hieber and Schuster.96 They invoke the use of Re(CO)5Cl in the stoichiometric carbonylation of ammonia to formamide. Such carbonylations with third row transition metals are rare.

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32 Scheme 1.12 Postulated mechanism for the carbonylation of primary amines in the presence of Mn2(CO)io. (CO)^Mn-Mn(CO:b 1/2 H2 HMn(CO]6 NH2R -NH3R+ NH3R+ NH2R OC XO Mn(CO)5' + .0 OC^ I ^C' CO I NHR CO -NH2R -NH2R NH3R+ -NH2R O II RHN— C — NHR NH2R Mn(C0)5‘ + R— f^G=o CO oc; ^^co oc^l ~^C^ CO I NH2R NHR -NH3R+ OC^ COco Mn /I ^c"^ OC CO T o NRChromium. Molybdenum, and Tungsten Carbonyl Complexes There are very few examples of Group 6 metals carbonylating amines. Doxsee and Grubbs have described the preparation of dimethylformamide from the stoichiometric reaction of Cr(C 0)6 with LiNMc2.97 This example involves nucleophilic

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33 attack at transition-metal-bound carbon monoxide followed by reduction by molecular hydrogen under exeeptionally mild conditions (equation 1 .20). LiNMc2 Cr(CO)6 (CO)5Cr= OLi NMe-i O ( 1 . 20 ) Angeliei has reported the reaction of [CpM(CO) 4 ]PF 6 (M = Mo, W) with excess CH 3 NH 2 to give 1,3-dimethylurea.^^ As shown in Scheme 1.13, when the reaetion is run in the presenee of two equivalents of amine, the earbamoyl eomplex CpM(CO) 3 (CONHCH 3 ) was obtained whieh suggested that the first step was a nucleophilic attack on a carbonyl ligand. Further reaetion of the carbamoyl complex with exeess amine gave the corresponding urea. When the reaction is run in excess trimethylamine, an organic isocyanate is obtained. The authors postulate a mechanism similar to the manganese mechanism depicted in Scheme 1.12. The first equivalent of amine deprotonates the earbamoyl ligand produeing an isoeyanate, which then reacts with another equivalent of primary amine to yield urea. Finally, the McElweeWhite laboratory has reported the earbonylation of seeondary amines whieh produce formamides in the presenee of the iodo-bridged dimer [(CO) 2 W(NPh)l 2]2 (32) (equation 1.21).^^ This stoichiometric reaction proceeds at room temperature for a variety of seeondary amines although reported yields are low. When 32 was reacted with primary amines under the same reaction conditions, primary amines

PAGE 46

34 Scheme 1.13 Stoichiometric carbonylation of amines in the presence of a tungsten carbamoyl complex R-N-C=0 were selectively carbonylated to the corresponding 1,3-disubstituted ureas (equation 1.22). Furthermore, when 32 was reacted with a,oo-diamines under stoichiometric oxidative carbonylation conditions, cyclic ureas were formed. Further research resulted in the carbonylation reactions being catalytic in 32 when the reaction was run under excess carbon monoxide. A possible mechanism for this transformation is shown in

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35 Ph OO/, OC J I w> v\CO CO N 32 Ph / R N \ H CH2CI2 [Ox] R ( 1 . 22 ) Scheme 1.14. IR spectra of the reaction mixtures were consistent with the presence of a carbamoyl intermediate. Following formation of 33 with four equivalents of amine, the carbamoyl complex 34 was proposed to form after a nucleophilic attack by the amine on a carbonyl ligand followed by deprotonation with an additional equivalent of the amine. Conversion of 34 to the isocyanate 35 is postulated, and is thought to proceed by oxidation at the metal followed by deprotonation with excess amine. However, no free or coordinated isocyanates have been observed in the reaction mixtures. Nucleophilic attack on the isocyanate complex or free isocyanate by another equivalent of amine is thought to

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36 form the urea and the cationic complex 36. Complex 36 now has a free coordination site that in the presence of excess CO would render the reaction catalytic. Scheme 1.14 Proposed mechanism for the carbonylation of amines in the presence of 32 r r” OO/,, ,.'K, I 2 eq NH 2 R nc, o(^ i^ir^co ^ , ^ Ph 32 r .j,* OC,„ III ,sNH2R w’ I^^nh2R 36 CO [Ox] OC^ I^'^NH2R 1 eq NH 2 R oa„. N ,„sNH2R 0(T I NH 2 R* 33 2 eq NH 2 R r oa,.f ,„vNH2R leqNH2R T H 34

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37 Conclusion Although the use of organometallic complexes as organic synthons has been explored for many years, there are multitudes of avenues yet to be investigated. The remainder of this text will foeus on investigations of reaetive intermediates in the photolysis of metal carbyne complexes as well as examine further applications of metal carbonyl complexes in the catalytie carbonylation of amines.

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38 CHAPTER 2 PHOTOPHYSICS OF LOWVALENT CARBYNES Introduction There is increasing interest in discovering transition metal complexes that will not only luminesce, but also be useful in the preparation of chemical products. Such species are rare. Typically, photolysis of organometallic complexes with carbonyl ligands results in ligand loss as the primary photoprocess due to population of the metal-ligand antibonding orbital via excitation of the d-d manifold. However, the "ligand loss" photoprocess is not the only photoprocess available to metal carbyne complexes. There are a number of examples where photolysis of metal carbynes has led to the conversion of the carbyne moiety to other organic ligands.45,101-103 ^s mentioned in the previous chapter, metal carbyne complexes containing carbonyl ligands are known to undergo migration of the carbonyl to the carbyne ligand, resulting in the formation of r|^'ketenyl complexes (equation 2.1).45 The migration is described as the result of increased electron density on the carbyne carbon due to a MLCT transition. Prior to the research described herein, it was found that one-electron transfer from the MLCT excited state of a series of tungsten and molybdenum alkylcarbynes to halogenated solvents resulted in unprecedented ligand-centered radical processes.^ 1 >52, 1 041 08

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39 \ CO PMC3 hv This is highly unusual. Not only is reactivity at an organic ligand in metal radicals far the MLCT excited state, oxidation would take place at the ligand and the ensuing radical the metal center. Photophysics of Metal Carbvne Complexes In order to understand the unusual reactivity of metal carbyne complexes, there have been a number of photophysical studies of complexes that contain the metalarylcarbyne functionality, M=C-Ar, where M = W(IV), Mo(IV), and Os(IV). 1 09 family of complexes displays remarkably similar absorption and luminescence properties, despite having a rather wide variety of ancillary ligands in the coordination environment. In general, the absorption of M=C-Ar complexes is characterized by a weak band in the 400-500 nm region ( e » 10^ M'^cm ') and a more intense band between 300 and 350 nm (e « 10'^ M‘’cm'^). Theoretical and spectroscopic studies indicate that the low energy band is associated with the HOMO LUMO transition, which is dominated by the d(M) — > n (M=C-Ar) configuration.23, 24,43,49,1 10-1 12 intensity of the HOMO less common than reactions at the metal center,^^ ^^t also, if this were 1 e' oxidation of would be metal centered. Namely, following oxidation, reactivity would be expected at

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40 LUMO transition indicates that it is forbidden; the transition is likely parity forbidden but spin-allowed (i.e. a singlet-singlet transition). The transition is parity forbidden owing to significant degree of d-character in the HOMO and LUMO. The more intense near-UV band is associated with a spin-allowed Tr(MsC-Ar) -> 7 i*(MsC-Ar) transition, which bears some analogy to the n^n transition of the Ph-CsC-Ph chromophore. There are also several reports of luminescence from complexes that contain the MsC-Ar funetionality.4^’49,1 10,1 1 1 Pqj. example, complexes of the type X(CO) 2 (L) 2 W=C-Ph and [Ph-C=Os(NH 3 )s]^^ exhibit weak luminescence in the 600-650 nm region which has been attributed to the d(M) 7r*(M=C-Ar) excited state. The emission lifetimes of these complexes range from 50 to 250 ns, with quantum yields typically on the order of 5 x 10''*, The comparatively long excited state lifetimes coupled with relatively low radiative rates (kr« 10^ s"') strongly imply that the luminescent state has triplet spin character. This assertion has been eonfirmed in several cases by the observation of triplet energy transfer from a carbyne excited state to organic triplet state acceptors. 110,111 jpg triplet character of the luminescent state in the carbyne complexes explains the large Stokes shift of the emission relative to the low intensity absorption in the mid-visible region, which is assigned to the spin-allowed (singletsinglet) absorption. This chapter provides further insight into the excited states of metal carbyne complexes of the type Cp(CO){P(OMe) 3 }M=C-R, where M = Mo or W and R = aryl (complexes 37-40). These investigations are to provide further understanding of the ligand-centered reactivity at the carbyne ligand. Considering MLCT excitation does not usually lead to reactivity at the ligand, the question to be answered is whether this

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41 photoprocess is best described as MLCT or something else. Answers can be found by examining the electronic structure, the excited states, the emissive states, and lifetimes of these states. OC \ _ Ar P(OMe )3 37: M = W, Ar = phenyl 38: M = Mo, Ar phenyl 39: M = W, Ar = o-tolyl 40: M = W, Ar = 2-naphthyl Synthesis of CptCOllPtOMel^lMsC-R Complexes The carbyne complexes Cp(CO){P(OMe) 3 }MsC-Ph (W = 37, Mo = 38) and Cp(CO){P(OMe)3}WsC(o-Tol) (39) were prepared as described previously.49,1 13-1 15 Preparation of the napthylcarbyne complex Cp(CO){P(OMe)3}W=C(2-Np) (40) involved an analogous route. The tris(phosphite) complex Cl(CO){P(OMe)3}3\V=C(2-Np) (41) was prepared by reaction of [(CO)5W(CO{2-Np})][NMe4] (42) with oxalyl chloride, followed by addition of excess trimethyl phosphite to give the impure tris(phosphite) complex 41. Reaction of 41 with fresh sodium cyclopentadienide (CpNa) led to displacement of the chloride anion and two P(OMe )3 ligands to yield 40. The crude 40

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42 required more extensive purification than is necessary for 37-39, but was ultimately isolated in 20% yield. Electronic Structure of CDtCOMPtOMet^lMsC-Ph tW = 37. Mo = 38t^ As an aid to interpreting the spectra (vide infra) of 37-40, extended Huckel calculations were performed on 37 and 38 . 1 16 jhg results of these one-electron calculations are useful in visualizing the coefficient distributions in the frontier orbitals and providing a rough sequence of orbital energies for 37-40. Figure 2.1 shows a partial molecular orbital diagram for the formation of 37 from the fragments Cp(CO){P(OMe) 3 }W and CPh^, calculated in the same manner as 37. The orbital orderings are similar to those previously reported for the model compound Cp{P(OH)3}2Mo=CPh,49 although the energetic splittings are somewhat different. These calculations show that the HOMO of the complex (Figure 2.2) is comprised largely of the metal dx^-y^ orbital but is partially devoted to back-bonding into the n orbital of the CO ligand. Below it are the two metal-carbon n bonds, their degeneracy broken by conjugation of the lower one with the phenyl ring. The LUMO is the orbital identified as the MsC 71 orbital that is conjugated into the phenyl 7r-system. However, inspection of Figure 2.2 reveals that it too is primarily composed of a metal d orbital delocalized into the 71* orbital of the CO ligand. The NLUMO is a nearly pure M=C tt* orbital. Extended Huckel calculations were also performed on the molybdenum compounds 38, and the TTie work in this chapter was done in conjunction with Thomas K. Schoch. Helpful discussions and analysis of spectral data were contributed by Dr. Kirk S. Schanze.

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43 -55' -65. £? C UJ T3 V '3 *c3 U -75« d -4f^' V \ <• ^ x' il d(W-CO)^|<' dx2-y2 -85. 95. n* (W-C) n* (W-C-Ph) -4f 71 (W-C) IT 7t (W-C-Ph) p -tPh 7t (MeO) 3 F*"’y (MeO)3P""^ *" oc OC Figure 2.1. Orbital mixing diagram for Cp(C0){P(0Me)3} WsC-Ph (37). Orbital energies are derived from extended Hiickel calculations.

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44 frontier orbitals obtained were very similar to those calculated for 37. Assignment of the HOMO as a metal d orbital that is nonbonding with respect to the carbyne ligand is consistent with structural information obtained by X-ray crystallography on Br(dmpe) 2 W =C-Ph and its 1 eÂ’ oxidized congener [Br(dmpe) 2 W=C-Ph][Pp 6 ].^ Oxidation results in only a slight shortening of the W-C(carbyne) and W-Br bonds (0.024 and 0.042 A) while the W-P bonds are lengthened somewhat. Upon this evidence, the HOMO is assigned as a nonbonding orbital that is primarily dx^-y^ in character. Similar observations have been made for Cl(dppe) 2 MosC(>-Tol) and [Cl(dppe) 2 Mo=C(pTo1)][PF6].117 This picture of the electronic structure of carbyne complexes 37 and 38 suggests that the lowest energy transition involves the frontier orbitals depicted in Figure 2.2. This is the transition identified in prior literature as d(M) n*(MsC-Ar) MLCT in nature.4^Â’4^>l 10,1 1 1 Although this transition does involve some charge transfer to the carbyne ligand, the earlier designation of "MLCT" suggests a higher degree of charge transfer and higher band intensity than is actually observed {vide infra). Both the HOMO and the LUMO are primarily composed of the metal d orbitals and the carbonyl n orbitals. As a result, the lowest energy transition has a considerable degree of d-d character that is consistent with its low extinction coefficient. Electronic Absorption Spectra Figure 2.3 illustrates the absorption spectrum of tungsten carbyne 37 in THF solution, which is typical for the series of metal carbynes examined herein, and Table 2.1

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45 Figure 2.2. HOMO and LUMO of Cp(CO){P(OMe)3}W=C-Ph (37). Coefficients are derived from extended Huckel calculations. Hydrogens and phosphite methoxy groups are omitted for clarity. Key: (top) LUMO; (bottom) HOMO.

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46 summarizes the data and assignments for the entire set of complexes. The absorption spectra of the aryl-substituted carbynes 37-39 exhibit two primary UV/visible absorption features: a strong band at approximately 330 nm and a weaker broad absorption at about 480 nm. The absorption spectrum of naphthylcarbyne 40 showed slight red shifts of both bands, but otherwise was similar. The absorption features in Table 2.1 are very similar to those described above for the structurally related arylcarbyne complexes. These features include a weak absorption with X,max located between 475 and 500 nm which is assigned to the parity forbidden, spin-allowed (i.e. singlet-singlet) d(M) n (MsC-Ar) transition as well as a moderately intense absorption in the near-UV between 325 and 350 nm which is assigned to the spinallowed 7r(M=C-Ar) ->7i*(MsC-Ar) transition. 250 200 CO 150 100 o 3 1 50 0 Figure 2.3. Absorption spectrum of Cp(CO){P(OMe) 3 }WsC-Ph in THF solution at room temperature.

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47 The low-energy absorptions are clearly related to those assigned as d -> ti* MLCT for the similar compounds X(CO) 2 L 2 W=C-R (R = Ph, ‘Bu; X Cl, Br; L 2 = TMEDA, 2 Py» dppe).^ These transitions in 37-40 are best described as d to d transitions rather than MLCT. The d-d character of the low-energy absorption in 37-39 (and by inference in 40) is underscored by previous studies from the McElweeWhite laboratory which demonstrated that the absorption band is not solvatochromic.49 The more intense absorption at 330 nm is assigned to 71 ^ 71 * transitions of the M=C-Ar chromophore. While there is a significant energy gap between the visible and near-UV transitions, the extended Hiickel calculations are in qualitative agreement with these assignments. The slight red shifts for the naphthylcarbyne can be attributed to mixing of the larger n system with the LUMO, lowering its energy. Table 2.1. Absorption Spectra of Cp(CO){P(OMe) 3 }M=C-R (37-40) in THE Solution® M R A,n,ax/nm(10^cm‘') emax/M''crn'’ assignment 37 W Ph 329 (30.4) 8000 t:( W=C-Ph)^7i*( W=C-Ph) 483 (20.7) 50 d(W)^7i*( WsC-Ph) 38 Mo Ph 328 (30.5) 4000 7t( MosC-Ph)->7t‘( Mo=C-Ph) 477 (20.9) 60 d(Mo)->7i*( MosC-Ph) 39 W o-Tol 331 (30.2) n( WsC-o-Tol)^7i*( Mo=C-o-Tol) 476 (21.0) d(W)^7i*( WsC-o-Tol) 40 W 2-Np 348 (28.7) 6000 7t( W=C-Np)->7i*( W=C-Np) 490 (20.4) 70 d(W)^7t*( W=C-Np) ®Data for 37-39 were previously reported.49 Data for 40 were obtained in this work.

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48 Luminescence Studies Detailed steady state and time resolved luminescence studies were carried out on complexes 37-40. The fluorescence detection system was corrected for monochromator and photomultiplier response to 800 nm, but owing to the sharp decrease in the efficiency of the detector at wavelengths greater than 800 nm, luminescence data collected beyond this wavelength were not useful. This point is significant, because the emission of the W and Mo carbyne complexes extends well into the near-IR {vide infra). The luminescence features of the tungsten and molybdenum carbynes studied are also similar to those of related arylcarbyne complexes. Each of the tungsten carbyne complexes luminesces weakly in solution at 298 K with emission lifetimes ranging from 60 to 170 ns. The molybdenum phenylcarbyne complex 38 is not luminescent at room temperature in fluid solution; however, red luminescence is observed when the compound is frozen in a glassy matrix at 77 K. Table 2.2 provides a summary of the emission parameters, including wavelength maxima (Xmax) for the corrected emission bands, and emission quantum efficiencies and lifetimes (em/xem and kn = (1/xem K). Note that by applying these expressions it is assumed that the luminescent state is reached with unit quantum efficiency.

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49 Table 2.2. Luminescence and Transient Absorption Data for Cp(CO){P(OMe) sjMsCR® 298 K'’ 77 K' M R ^em Vs'' kjs'' Wns A|nax/nm TeJps (10^ cm-') (10^) (10^ cm-' 37 W Ph 747(13.4) 141 6.9x10"' 4.9 7.8x10^' 129 741(13.5) 3.2 38 Mo Ph e e <10"* <2 2.0x10’ 49 787(12.7) 8.3 39 W o-Tol 745 (13.4) 170 3.6x10"* 2.1 5.9x10^ 735(13.6) 3.6 40 W 2-Np >780 (<12.7) 66 1.5x10’ 60 “^max emission maximum; = emission lifetime; ®em= emission quantum yield; radiative decay rate; /fcnr = nonradiative decay rate; tta = transient absorption decay lifetime. '’Argon-degassed THF solutions. ‘ Argon -degassed 2-MeTHF glasses. ‘'Amax and Tem values for la-c previously reported.' Too weak for accurate determination.^^ In fluid solution at 298 K the most pronounced emission is observed from tungsten complexes 37 and 39. Emission from these complexes is very similar, appearing as a broad, structureless band in the red with Xmax « 745 nm (Figure 2.4a).49 In each case the luminescence efficiency is rather low, with Oem« 5 x 10‘‘* (Table 2.2). Emission excitation spectra for 37 and 39 are also illustrated in Figure 2.4a and reveal that the red luminescence is produced by excitation of the visible and near UV absorption bands. Emission lifetimes for 37 and 39 are comparable, and indicate that the luminescent excited state has a lifetime in the 100-200 ns range at room temperature. The emission of 37 and 39 was also examined at 77 K in a 2-MTHF solvent glass, and the maxima and lifetimes for the low temperature luminescence are listed in Table 2.2. The emission of both complexes at low temperature appears as a broad band which is similar in energy and bandshape to that observed in fluid solution at 298 K. Thus, luminescence "rigidochromism", which is typically observed for charge transfer excited

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50 states, was not apparent for 37 and 39. This effect underscores the small degree of MLCT character in the luminescent excited state for the carbyne complexes. The 77 K emission lifetimes for the two tungsten complexes are similar, and are comparable to the lifetimes observed at 77 K for d-d excited states in W(CO) 5 L complexes.! 18-120 At 298 K in fluid solution the 2-naphthylcarbyne complex 40 exhibits an emission which is substantially red-shifted relative to that of the phenyl and o-tolyl complexes. Owing to the large red shift, the instrument used is only capable of detecting the blue edge of the band (Figure 2.4b), and A.max for 40 is apparently >800 nm. An emission excitation spectrum of 40 obtained with Xgm = 790 nm is also illustrated in Figure 2.4b. There is good agreement between the absorption and excitation spectra for 40, which confirms that the near-IR emission of 40 indeed emanates from the complex. The « lifetime of the near-IR emission of 40 was determined to be 66 ns, which is slightly shorter than the lifetimes for the phenyl and o-tolyl complexes. Molybdenum carbyne 38 exhibits no detectable luminescence at 298 K. However, at 77 K in 2-MTHF the complex features a broad, structureless emission band that is similar in bandshape but red-shifted compared to the emission of the tungsten analog 37 (Table 2.2). The emission band decreased in intensity as the temperature of the MTHF solution is increased above the glass-to-fluid transition (100-120 K) at which point the luminescence becomes too weak to detect (Oem < lO"'*). The emission lifetime of 38 at 77 K (8.3 ps) is slightly longer than that of the tungsten analog. By analogy to previously reported arylcarbyne complexes, the luminescence observed from 37-40 is attributed to the d(M) -> 7i*(MsC-Ar) triplet state. The triplet

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51 Wavelength / nm Figure 2.4 Emission and excitation spectra: (a) for 37 and 39 and (b) for 40. Emission spectrum is at right and excitation spectrum at left.

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52 assignment has been substantiated in 37 by previously reported triplet energy transfer studies.'^^ The significant Stokes shift of the emission tfom 37, 39, and 40 relative to the low-intensity visible absorption band (»7.0 x 10^ cm"’) is attributed to the fact that the absorption is due to the singlet-singlet HOMO-LUMO transition. The Stokes shift observed for these carbyne complexes is similar to that reported for ^d-d luminescence in tungsten and molybdenum carbonyl complexes of the type M(CO) 5 L.l Of further interest is the fact that the luminescence data presented herein argues against the suggestion made earlier that the large Stokes shift may be partly due to geometry changes associated with excitation of the M=C-Ar functionality. 1 ^ ^ ^ significant difference in the emission energy or bandshape for 37 and 39 in fluid solution compared to in rigid glass at 77 K implies that there is no substantial inneror outer-sphere reorganization associated with the optical excitation. Despite the overall similarities of the absorption and luminescence properties of 37-40, there are important distinctions that provide insight concerning the nature of the electronic states involved in the photophysics. First, the visible and near-UV absorption bands and the luminescence of the 2-Np complex 40 are noticeably red-shifted compared to those for the phenyl and o-tolyl analogs. The red-shift of both absorption bands and the luminescence in the 2-Np complex is expected, since the naphthalene ring provides extended conjugation for the n*(MsC-Ar) orbital, which is the LUMO of the complex and is the "acceptor" orbital for the visible and near-UV n, n transitions. Second, the tungsten and molybdenum phenylcarbyne analogs 37 and 38 display remarkably similar groundand excited-state absorption spectra. This likeness points to a similarity in the electronic structure of the two complexes. The one notable difference in

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53 the photophysical properties of the two phenylcarbynes is the lack of observable luminescence from 38 in fluid solution at room temperature. Since the excited state lifetimes of 37 and 38 are not significantly different (i.e. from transient absorption 130 and 50 ns for 37 and 38, respectively vide infra), the lower emission yield of 38 might be attributed to lower spin-orbit coupling (and therefore less singlet-triplet mixing) in the molybdenum complex as compared to the tungsten analog. Transient Absorption Spectroscopy Nanosecond laser flash photolysis studies were carried out on carbyne complexes 37, 38, and 40 in argon-degassed THF solutions. All experiments were performed using the third harmonic output of a Nd:YAG laser for excitation (355 nm, 10 ns fwhm, 10 mJ/pulse). As described below, strongly absorbing transients were observed for each complex, and the decay lifetimes for the transient absorptions were determined by global kinetic analysis of the transient absorption decay at > 25 wavelengths. ^21 Figure 2.5a illustrates transient absorption difference spectra of 37 at delay times ranging from 0 to 280 ns following laser excitation. The spectra are characterized by a broad absorption band with A.max » 440 nm. There appears to be a hint of structure on the absorption band, with distinct (and reproducible) shoulders at 395, 420, and 460 nm. Global analysis of the transient absorption data indicates that the transient decays with a lifetime of 129 ns, in good agreement with the previously reported emission lifetime ('^em =141 ns). The correspondence of the transient absorption and emission lifetimes strongly implies that the transient absorption is due to the luminescent excited state.

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54 Under the assumption that the lowest excited state 37 is populated with unit efficiency following 355 nm excitation, it is possible to determine the difference molar absorptivity (Ae) of this state by using the relative actinometry method. ^ 22 experiment was accomplished by using [/ac-2,2-bipyridine)Re(CO)3(4-benzylpyridine)]^ as an actinometer (As = 1 1,200 M 'cm'' at 370 nm)123 and led to a value of As = 3000 M''cm' ' at 440 nm for 37. Figure 2.5b illustrates transient absorption difference spectra of 40 at delay times ranging from 0 to 120 ns following laser excitation. The spectrum of this complex is characterized by a moderately intense and broad absorption band with Xmax « 445 nm, with distinct (and reproducible) shoulders at 395 and 420 nm. Global analysis of the transient absorption data indicates that the transient decays with a lifetime of 49 ns. The close correspondence between the transient absorption spectra of 37 and 38 points to a similar electronic structure for the absorbing excited states in both complexes. Figure 2.5c illustrates the transient absorption difference spectra of 38 at delay times ranging from 0 to 160 ns following laser excitation. The spectra are characterized by an intense but featureless transient absorption band with Xmax « 460 nm. Global analysis of the transient absorption data indicates that the transient decays with a lifetime of 60 ns in very good agreement with the emission lifetime of the complex (66 ns). The agreement between the transient absorption and emission decay lifetimes suggests that for 40 the transient absorption arises from the luminescent excited state. A distinguishing feature of this work is the observation of strong transient absorption by the d(M) — > n (M^C-Ar) excited states of the tungsten and molybdenum arylcarbynes. This is the first study of transient absorption spectra of photoexcited

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55 Fischer carbyne complexes. There are several important ramifications of the observation of transient absorption signals for the excited states of this family of complexes. First, as noted above, it is clear that the excited state carbynes have comparatively large difference molar absorptivities. This is in contrast with the low molar absorptivity that is typical of d-d excited states of organometallic complexes. ^ 24, 125 moderate to large difference molar absorptivity of the excited states of the arylcarbyne complexes is more in accord with that of d7i(M) — > n (ligand) MLCT excited states in d^ metal diimine complexes such as Ru(bpy) 3 ^’*^ and [(bpy)-Re'(CO) 3 (py)] (bpy = 2,2'-bipyridine and py = pyridine). 123, 126 An important question concerns the origin of the moderate to strong absorbance of the d ^ 71 excited states in the arylcarbyne complexes. The absorption of MLCT excited states in d^ metal diimine complexes has been assigned to spin-allowed n n and 71 — > 7T transitions of the diimine radical anion chromophore that is produced by MLCT excitation. 127 gy analogy, the absorption of the d ^ n* excited state in the arylcarbyne complexes may be associated with an allowed n* n* transition of the MsC-Ar chromophore. In this connection, it is important to note that the excited state absorption of phenylcarbyne complexes 37 and 38 is quite similar, while that of naphthylcarbyne 40 is slightly red shifted (and qualitatively appears to be more intense). The red shift in the naphthyl system suggests the involvement of the ti to ti* levels of the aryl unit in the optical transition detected in the transient absorption experiment. Finally, the substantial difference absorptivity of the excited state in the arylcarbyne complexes provides an exceedingly useful means to track the excited state dynamics (and reactivity). This is significant, because even though emission studies have

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56 Figure 2.5 Transient absorption difference spectra following laser excitations (355 nm, 10 ns fwhm, 10 mJ/ pulse) of (a) 37 (b) 38 and (c) 40.

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57 provided an opportunity to study the excited states of certain metal carbynes, the method has limitations. For example, among 37-40 and related Cp(CO) 2 M=C-R and Cp{P(OMe) 3 } 2 MsC-R species, only the tungsten aryl carbynes with one phosphite ligand and one CO ligand display easily observable luminescence. Even for these complexes, the luminescence efficiency is low, as evidenced by the quantum yield for emission for 37 and 39. The bis(phosphite) arylcarbynes are not emissive in solution at room temperature,49 and molybdenum complex 38 is also nonluminescent under the same experimental conditions. Comparing the lifetime measurements obtained by absorbance with those observed by the emission method provides assurance that the observed transients are in fact the emissive states. Furthermore, by applying global analysis to the transient absorption data, it is in principle possible to obtain very reliable excited state decay rate data.^^l It should be noted that after the work described in this chapter had been completed, an additional study was undertaken by co-workers that investigated the role of photochemical electron transfer in the reactions of tungsten carbynes, namely, electron transfer from the excited state of Cp{P(OPh) 3 }(CO)W=CPh (43) to a series of electron acceptors. 128 examined the quenching of the d ^ ti* excited state of 43 with a series of pyridinium and nitroaromatic acceptors of varying reduction potential. Laser flash photolysis revealed that quenching is accompanied by the appearance of radical ion products, thereby providing evidence that quenching occurs via electron transfer. The dependence of the bimolecular quenching rate constant on the reduction potentials of the acceptors establishes that the d ^ ti* excited state of 43 is a potent reducing agent.

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58 Conclusion Low-valent carbyne complexes are well-established compounds in organometallic chemistry, yet relatively few photophysical studies on these complexes have appeared in the literature. In this work, the complexes Cp(CO){P(OMe) 3 }MsC-R (M = Mo, W; R = Ph, o-Tol, 2-Np) (37-40) were examined in an effort to learn more about the excited states of these molecules, as well as to provide some information relevant to the study of photochemical reactions. Extended HUckel calculations and absorbance spectra indicate that 37-40 possess low-lying exeited states which can be populated directly via a parityforbidden, singlet-singlet optical transition in the mid-visible region. This transition is better described as a d-d transition rather than MLCT. Emission is observed from the triplet manifold of these excited states, and at room temperature in fluid solution the lifetimes of the triplet state range from 50 ns for the molybdenum phenylearbyne 38 to 170 ns for the tungsten tolylcarbyne 39. Transient absorbance measurements performed on 37-40 in THE indicate that the triplet excited states can also be detected and monitored by absorption, since the lifetimes obtained in the absorbance experiments match those obtained through emission methods. The observation of strong excited state absorption provides an avenue for future work concerning the photophysics and photochemistry of arylcarbyne complexes. Such work may include the study of exeited state resonance Raman to provide further information concerning the nature of the excited states.

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59 CHAPTER 3 SYNTHESIS AND PHOTOOXIDATION OF A TUNGSTEN CYCLOHEXENYL CARBYNE COMPLEX Introduction As discussed in Chapter I, a variety of organic compounds including olefins, dienes, dienals and other cycloalkenones have been synthesized from the photooxidation of various alkyl metal carbynes of the type Cp(CO){P(OMe) 3 }M=CR [R= alkyl; M= Mo, W].106 Mechanistic studies suggested that these reactions shared the following two initial steps (Scheme 3.1): 1) photoinduced electron transfer from the carbyne to the halogenated solvent to form a 17 electron radical cation A and 2) hydrogen abstraction from the reaction medium to produce a 16 electron cationic carbene B.49, 5 1,1 07, 108, 129 Despite the usual tendency of organometallic radicals to undergo ligand exchange and halogen abstraction at the metal center as discussed previously,4^Â’47,130 geometric distortions of the carbyne radical cation cause radical reactivity to occur at the carbyne carbon and the H-abstraction takes place at that site.^^ The resulting cationic carbene complex B is the reactive species in the reactions that produce organic products. Although mechanistic studies were consistent with the intermediacy of cationic carbene complexes in the formation of organic products from photooxidized carbynes, to this point, carbene complexes had never actually been observed in the reaction mixtures.

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60 This work describes the first direct observation of a hydrogen abstraction product upon photooxidation of a tungsten cyclohexenyl carbyne complex. Scheme 3 . 1 . Proposed mechanism for generation of metal carbenes upon photooxidation of metal carbynes ^ hv nn W=OCHRiR2 / CHCI3 P(OMe)3 \ ocv"'^w=c-chr,R2 P(OMe)3 +• A + RH -R* \ / P(OMe)3 H CHR1R2 B

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61 Synthesis of fTi5-Cs H. •COHPrOMel2lW^Crr-a H g) (441 The cyclohexenyl tungsten carbyne complex 44 was synthesized from the tungsten carbene 45 as shown in Scheme 3.2. There is a literature preparation for the tungsten carbene 45 which reports a yield of 52%. 131 However, for the current study, carbene 45 was synthesized by modifications of a route that was used in the synthesis of the molybdenum analogue of 40.132-134 xhese modifications resulted in a higher yield than that reported in the literature. The modified route involves the lithiation of the hydrazone 46 with r-butyl lithium to form the cyclohexenyl lithium reagent 47. The immediate reaction of 47 with tungsten hexacarbonyl formed the cyclohexenyl tungsten acyl anion 48. Anion 48 was methylated with trimethyl tetrafluoroborate which gave carbene 45. Once isolated, the crude product 45 was not allowed to stand over-night but was chromatographed immediately on a -78°C chilled alumina column giving a purified yield of 75%. Carbene 45 was readily converted to the tris(trimethyl phosphite) carbyne complex 49 using Fischer’s method9,21 which involves step-wise addition of BBrs and P(OMe)3, giving 49 in 65% yield. Reaction of CpNa with 49 and subsequent purification through a -78°C chilled alumina column produced carbyne 44 in 41% yield as a reddishorange oil that can be stored for approximately one week at -30 °C.

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62 Scheme 3.2. Synthesis of (ii^-C 5 H 5 )(CO){P(OMe) 3 }W=C(c-C 6 H 9 ) (44) 0°C W(CO)6 48 1) BBr3 CH2CI2 2) P(OMe)3 P(OMe)3 I >CO / — \ (MeO)3P p(OMe)3 CpNa THF OC' \ _ P(OMe)3 49 44

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63 Photooxidation of (Ti^-Cs Hs •CO')IPrOMe^ 2 ^^^^^^~^' ^H Q) (44) Photolysis of carbyne 44 at -45 °C in CDCI 3 while monitoring the reaction progress by 'H NMR spectroscopy resulted in observation of downfield shifts of the alkyl peaks and the formation of a new Cp peak. Since the reactivity of related carbyne complexes following photooxidation 1 suggested that 44'^’ had probably abstracted a deuterium atom tfom the CDCI 3 solvent, the experiment was repeated in CHCI 3 at -45°C so that the abstracted H-atom could be located by *H NMR. Upon completion of the photolysis as determined by disappearance of the initial carbyne 44 by TLC of the reaction mixtures, the CHCI 3 was pumped off at -45°C, and the residue was dissolved in cold CDCI 3 in order to obtain a *H NMR spectrum. The spectrum revealed a broad singlet at 515.35 which is characteristic of a proton on the carbene carbon of an electrophilic carbene complex. 1^5 One obvious candidate for the photooxidation product was the cationic carbene complex 50. Electron transfer from 44 to the solvent followed by H-abstraction at the carbyne carbon of 44 would produce 50 (equation 3.1). However, IR spectra of the reaction mixtures failed to show a new signal for a terminal CO ligand. 136,137 Qnce 50 had been ruled out as the observable species, the next obvious possibility would be displacement of the CO in 44 with Cf to yield the neutral carbene 51 (equation 3.2). Reaction mixtures from the photooxidation of 44 would contain Cf since it is generated upon reduction of the CHCI3 solvent by the excited state of carbyne 44.138

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64 Complex 51 is formally the product of protonation of carbyne 44 at the carbyne carbon and Cl substitution. Thus, it was not unreasonable to attempt its preparation by reaction of 44 with HCl, although protonation of metal carbynes is a complicated issue because proton attack can occur either at the metal or at the carbyne carbon.3,29,34,139 Reaction of related (r|5-C5H5)(CO){P(OMe)3}M=CR complexes with HCl generally leads to Ti -acyl complexes of the type (ri^-C5H5)Cl2{P(OMe)3}MCOCH2R (for example, see equation 3.3).^2,33,49,5 1,106,107,129,140 However, KreisslÂ’s work on the mechanism of r] -acyl formation points to the intermediacy of a metal carbene intermediate which then undergoes CO insertion (Scheme 3.3). This postulate was supported by the fact that when the aminocarbyne (Ti5-C5H5)(CO)2W^CNEt2 was reacted with HCl, the reaction stopped at the carbene. 140 Although in the Kreissl work, the carbene was only

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65 riu p. Mo=C-(CH2)3CH3 (MeO)3P / OC HCl*Et20 EtoO ^/ff Mo~Cn cr Cl /i P(OMe )3 CH2(CH2)3CH3 (3.3) Scheme 3.3. Proposed mechanism for the protonation of carbynes. M== Mo, W R= alkyl, aryl Xcr, CF3COO', CC13COO' observable for the aminocarbyne case, which is electronically very different from 44, the presence of the cyclohexenyl group in 44 opened the possibility of preventing CO insertion into the carbene ligand of intermediate 50 by chelation of the double bond. As desired, protonation of the vinyl carbyne 44 with HCl (equation 3.4) did not produce the n^-acyl complex, but rather yielded the carbene complex 51 as a yellow

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66 powder which is thermally labile but stable at -45 °C. Comparison of the 'H NMR spectrum of the HCl product to the 'H NMR spectra of the reaction mixtures from photooxidation of carbyne 44 provided confirmation that both routes had produced the same complex. The 'H NMR, '^C NMR, and MS data for 51 support the proposed structure. In the *H NMR of 51 in CDCI 3 at -50° C, the proton on the carbene carbon appears as a singlet at 5 15.35, which is characteristic of conjugated electrophilic carbene complexes.135 -phe ‘^C NMR chemical shifts of 5 277.8 for the carbene carbon, 6 75.4 for the quaternary vinyl carbon, and 8 93.7 for the tertiary vinyl carbon strongly resemble the literature values for the tungsten ri^-vinyl carbene complex (ri C 5 H 5 )(CO)(I)W=C(Ph)C(Ph)CHTol (52) (Figure 3.1).42 xhe electrospray MS of 51 in CH 3 CN exhibited a molecular ion peak at 502.1, with an isotopic distribution pattern that matched the simulation for the molecular formula of 51, C 15 H 24 CIO 3 PW (Figure 3.2).

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67 5 94.4 8 94 7 52 51 Figure 3.1. Comparison of '^C NMR data between 51 and known 52. Photolysis of carbyne 44 at -45 °C in CDCI 3 , containing decane as a standard, afforded carbene 51 in 25% yield as determined by integration of the Cp resonance in the 'H NMR spectrum. In order to rule out photochemical formation of HCl as the source of 51 in the photooxidation reactions, 44 was photolyzed in CDCI 3 to which the hindered base 2,6-di-tert-butylpyridine had been added in order to scavenge any acid. The addition of 2,6-di-/ert-butylpyridine is a technique that is used to distinguish radical cation chemistry from acid-induced reactions.5U52,107,142,143 jhg yjgijj of the photolysis product was the same in the presence and the absence of the base, ruling out formation of 51 in the photolysis mixtures by reaction of 44 with HCl.

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68 SIMULATION ACTUAL Figure 3.2 Electrospray mass spectral data of 51

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69 Oxidation of (nS-Cs H OrCOMPrOMel.^W^Cfr^-C .Ho^ (44) in the Presence of Unsaturated Substrates Although high-valent carbyne complexes have found wide use in the areas of olefin metathesis and alkyne polymerization, little is known about the reactivity of low valent metal carbyne complexes with unsaturated substrates. 16, 144146 Efforts to provide further enlightenment on this reactivity involved studying the reactivity of 44 '^’ in the presence of unsaturated species. As discussed earlier, 44^’ is formed by photochemical electron transfer from 44 to CHCI 3 . However, upon reduction, CHCI 3 Iragments to Cl and •CHCb.l^^ The •CHCI 2 is known to undergo unwanted Arbuzov reactions with the P(OMe )3 ligand 1^7 qj^ similar carbyne complexes which is evident by the formation of CH 3 CI in the reaction. 61 The use of outer sphere chemical oxidants at low temperatures has been studied and provides an alternative method in which low yields and side reactions Irom •CHCI 2 can be avoided. A cyclic voltammogram of carbyne 44 taken in 0.1 M BU 4 NSO 3 CF 3 /C 2 H 4 CI 2 under nitrogen showed the first oxidation wave at +0.55V vs. NHE to be completely irreversible regardless of scan speed or switching potential. The first oxidation peak corresponds to the formation of 44^*. The complete irreversibility of this peak indicates that 44 is a very reactive species and undergoes immediate chemical reaction on the cyclic voltammogram time scale. Based on the oxidation potential of 44, the chemical oxidant, acetyl ferrocenium, with an oxidation potential of +0.82Vl48 was chosen for the oxidation investigations.

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70 > 120. 00 2. 200 1 . 000 E v». NHE (V) Figure 3.3 Cyclic voltammogram of 44. Sweep rate: 100 mV/s Initially, carbyne 44 was reacted with acetylferrocenium tetrafluoroborate (AeFc BF 4 ) at -90 °C and NMR spectra were taken in 10 °C intervals upon wanning to room temperature. The reaction mixture upon warming ehanged eolor from blue to green to brown. Formation of a hydrogen abstraction product was never observed, unlike the formation of 51, as diseussed earlier in the photolysis of 44 in CHCI 3 . Considering the fact that hydrogen abstraction did not readily take place in the ehemieal oxidation reaetion, it seemed feasible to attempt to trap the ensuing 17-eleetron radical cation with an unsaturated substrate. Torraca et al. have reported the reaction of a butyl molybdenum carbyne with AcFe^BF 4 in the presenee of phenyl acetylene resulted in C-C coupling of the acetylene to the carbyne earbon.149 Unfortunately, when carbyne 44 was reacted

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71 with AcFc BF4 in the presence of various alkynes, olefins, ketenes, and imines, all reactions resulted in intractable material. Other Reactions with Unsaturated Substrates The carbene 51 that is formed from the protonation of 44 with HCl is similar in structure to a diene. Efforts to form [4 + 2] or [2 + 2] adducts were attempted in the presence of various olefins, acetylenes, aldehydes, and ketenes. No reactivity was ever observed other than the decomposition of 51 to intractable materials. It is possible the chelation of the cyclohexenyl group to the metal was inhibiting further reactivity of 51. Attempts to circumvent this problem included protonation of 44 with HBF4 with and without CO pressure in attempts to trap the putative 16-electron cationic carbene 53 (Figure 3.4). Again, all in situ reactions with unsaturated substrates resulted in intractable materials. Figure 3.4 Investigated reactivity of 53 with unsaturated substrates. Conclusion The photooxidation of the cyclohexenyl carbyne 44 in CHCI3 yields carbene complex 51. This experiment represents the first direct observation of a carbene

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72 intermediate following single electron oxidation of a carbyne complex and provides further evidence for H-abstraction at the carbyne carbon as a common mechanistic step in previously reported photooxidation reactions. Formation of 51 upon thermal reaction of 44 with HCl also provides additional support for the Kreissl mechanism for generation of T) -acyl complexes from carbyne complexes and HCl by extending the range of observable carbene intermediates from amino derivatives to alkyl derivatives.

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73 CHAPTER 4 CATALYTIC OXIDATIVE CARBONYLATION OF AMINES WITH W(CO)6 Introduction Substituted ureas have been of reeent interest due to appearanee of this functionality in drug candidates such as HIV protease inhibitors, 150,151 FKBP12 inhibitors, 152 CCK-B reeeptor antagonists 153, 154 endothelin antagonists. 155 in addition, ureas have found widespread use as agrieultural ehemieals, resin precursors, dyes, and additives to petroleum eompounds and polymers. 156 Among the numerous methods for synthesis of N,N'-disubstituted ureas are the reaetions of primary amines with isocyanates, phosgene, or phosgene derivatives (equation 4 . 1 ). 157,158 Yields are usually good to excellent for these reactions, but other aspects can be problematic. Phosgene itself is highly toxie and corrosive while phosgene derivatives can be expensive to use on a large seale. Additional impetus for replacement of phosgene derivatives comes from the standpoint of atom economyl59 by which using CO to install a carbonyl moiety into a urea would be preferable to a method using a typieal phosgene derivative such as 1,1-carbonyldiimidazole. The direet metal-catalyzed eonversion of amines and CO to ureas provides an alternative to phosgene and its derivatives.

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74 O H H + 2 HX (4.1) X = Cl, CI3CO, -^N Although catalytic carbonylation has been investigated over many years, 55iMil60 topic remains of interest. Oxidative conversion of primary amines into ureas has been reported for transition metal catalysts involving Ni,Â’71 Mn,73,94,95 Ru161 a^d, most commonly, Pd.^S, 162,163 jy[ain group elements such as sulfiirl6^Â’165 and selenium 1^6168 gan also serve as catalysts. However, neither the transition metalnor the main group-catalyzed reactions are ideal. The transition metalcatalyzed reactions generally require high temperatures and pressures. In addition, yields for aliphatic amines are generally much lower than those for aromatic cases. Among the main group catalysts, selenium-catalyzed carbonylation reactions can produce high yields of ureas under mild eonditions. However, generation of hydrogen selenide as a byproduct and the need for stoiehiometric or exeess selenium for certain substrates l^^Â’l^^ are problematic. Prior to this work, preliminary studies were reported on the catalytic oxidative carbonylation of primary amines to ureas using either [(CO)2W(NPh)l2]2 or W(CO)6 as the precatalyst and I2 as the oxidant.99,169 jt was found that the W(CO)6/l2 catalyst system also works for the conversion of secondary amines to tetrasubstituted ureas and the preparation of eyclic ureas directly from primary and secondary a,co-

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75 diamines. Extensive optimization studies were performed on the oxidative carbonylation of aliphatic primary amines to N,N’-disubstituted ureas using the W(CO)6/l2 oxidative carbonylation system on the substrate «-propyl amine (equation 4.2). Variables such as catalyst, temperature, solvent, CO pressure and quantity of added W(CO)6 RNH2 CO /I2/ K2CO3 base were examined. The optimal conditions for the carbonylation of w-propylamine were determined to be 2 mole % W(CO)6, stoichiometric amounts of amine and I2, and 1.5 equiv of K2CO3 per equiv amine reacted in CH2CI2 at 90 °C under 80 atm CO pressure. This chapter is a continuation of the work described above. It will include further investigations of optimized procedures, applications of the optimized conditions to other primary amines, as well as a functional group compatibility study of the W(CO)6/l2 oxidative carbonylation system. Carbonylation of Primary Amines Although CH2CI2 was the best single solvent for the « -propylamine case, solubility problems were encountered when CH2CI2 was used as solvent for various substituted benzylamines {vide infra), for which either the amine starting material or the hydroiodide salt was insoluble. Note that the stoichiometry of oxidative carbonylation of O H H

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76 amines to ureas dictates that two equiv of the amine hydroiodide (RNH3I) will be produced per equiv of urea (equation 4 . 3 ). In the absence of added base, these amine salts are observed in the reaction mixtures. The presence of K2CO3 as a sacrificial base increases the product yield by regenerating the free amine from the salt and returning it to catalyst 4 RNf^ I2/CO the substrate pool. However, insolubility of the amine salts will hinder this deprotonation since the K2CO3 and the salt will coexist as immiscible solids. To circumvent the inability of solid K2CO3 to deprotonate insoluble amine salts, the two-phase solvent system CH2CI2/H2O was investigated. Due to the solubility of the hydroiodide salts in basic water, they could be deprotonated and shuttled back into the methylene chloride layer as the free amine. An added advantage was that the base is soluble in the water layer and stirring problems associated with solid K2CO3 were eliminated. For both w-propylamine and many of the substituted benzylamines used in the functional group compatibility studies {vide infra), room temperature yields increased when the solubility problems were resolved with the two-phase CH2CI2/H2O solvent system. Although there are several examples of two-phase carbonylation reactions in the literature, a biphasic system for the carbonylation of amines is unprecedented. In fact, carbonylation of amines in aqueous medium has only been reported for conversion of amines to formamides with a water-soluble ruthenium carbonyl catalyst. 1^2 O R. N H R N H + 2 [RNH3]Y ( 4 . 3 )

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77 Although the carbonylation of «-propylamine at room temperature in the twophase CH 2 CI 2 /H 2 O solvent system resulted in 73% yield which was higher than the yield for the single-solvent CH 2 CI 2 system, repeating the reaction at 90 °C resulted in a 85% yield which was comparable to the 90% yield achieved in the previously described reaction run at 90 °C in CH 2 CI 2 . Therefore, a series of primary amines was reacted under the following conditions; 2 mole % W(CO) 6 , stoichiometric amounts of amine and I 2 , and 1.5 equiv of K 2 CO 3 per equiv amine reacted in CH 2 CI 2 at 90 °C under 80 atm CO pressure. These reactions gave the corresponding N,N’-disubstituted ureas, as seen in Table 4.1. The ureas were formed in moderate to high yields, ranging from 90% for N,N’-di-«-propylurea to 53% for N,N’di-/-propylurea. The yields are generally lower if the amine bears secondary or tertiary alkyl substituents. For comparison, the amines in Table 4.1 were also run in the CH 2 CI 2 /H 2 O solvent system. Analogous to the decrease in yield of the «-propyl urea in the two-phase system at 90 °C, the other primary amines also suffered a decrease in yield when carbonylated in the mixed solvent system. It is noteworthy that aniline is unreactive under the reaction conditions, considering aromatic amines are generally better substrates than their aliphatic analogues for the formation of ureas under transition metal-catalyzed carbonylation conditions.^^2,173

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78 Table 4.1 Catalytic carbonylation of primary amines Amine Product in single/ biphasic solvent systems %Yield“'*’ %YieiF^ CH2CI2 CH2CI2/H2O 90 85 53 78 27 ® Isolated yield of urea calculated per equivalent of amine ’’ Reaction conditions: amine (14.2 mmol), W(CO)6 (0.28 mmol), I 2 (7.1 mmol), K2CO3 (21.3 mmol), CH 2 CI 2 (40 mL), 90 “C, 80 atm CO, 24 h. The solvent was CH 2 CI 2 (35 mL) plus H 2 O (5 mL). Other conditions are as in footnote b.

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79 Functional Group Compatibility Study In order to explore applications of this reaction in organic synthesis, a study of functional group tolerance was undertaken. Various substituted benzylamines (equation 4.4) were reacted with CO under similar carbonylation conditions, as seen in Table 4.2. Substituted benzylamines were chosen as model substrates for the compatibility study because the benzene ring provides a rigid backbone which separates the functional group W(CO)6 CO / I2 / K2CO3 from the reacting amine. The possibility of intramolecular interaction is eliminated in these substrates. This first screen of functional groups addresses interference by simple binding of the catalyst to the functional group or competitive reaction of the functional group by oxidation or carbonylation. More complex substrates, such as conformationally flexible amino alcohols in which carbonylation could lead to different types of products, are the subject of further investigation and will not be discussed here. Initial studies with the parent benzylamine 54 established that N,NÂ’-dibenzylurea 55 was formed in 63% yield upon reaction under the optimized reaction conditions for npropylamine (Table 4.2, CH 2 CI 2 ). As discussed above, the low yields of ureas for many of the substituted benzylamines were attributed to the insolubility of their respective hydroiodide salts in CH 2 CI 2 . As shown in Table 4.2, it was found that yields for many of the substituted ureas were significantly higher when the 7:1 CH 2 CI 2 /H 2 O solvent system was used (Procedure C, Chapter 6).

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80 The effects of aryl halides were examined first. The p-chloro-, p-bromo-, and miodobenzylamines 56, 58, and 60 produced their respective N,NÂ’-disubstituted ureas 57, 59, and 61 in surprisingly low 35%, 30%, and 39% yields when the reaction was run in CH 2 CI 2 . Although side reactions resulting from oxidative addition of the aryl halide bonds to the tungsten complex were considered, the main products of the reactions turned out to be the amine hydroiodides. Upon addition of water to the reaction mixtures, the yields of the ureas increased to 77%, 77%, and 70% respectively. In experiments with ether and thioether compounds in CH 2 CI 2 , the pmethoxybenzylamine 62 was carbonylated to urea 63 in 47% yield, while pmethylthiobenzylamine 64 was carbonylated to urea 65 in 24% yield. Under these conditions, the major products were the hydroiodide salts. The solubility problem was again circumvented by addition of H 2 O to the solvent system resulting in the ureas forming in 70% and 81% yields, respectively. The hydroxymethyl compound 6 6 produced urea 67 with yields of 5% in CH 2 CI 2 and 58% in CH 2 CI 2 /H 2 O. Remarkably, the carbonylation reaction was completely selective for the amine over the alcohol, within the limits of detection. None of the corresponding carbamate or carbonate could be observed in the reaction mixtures. This experiment suggests that the carbonylation of amines could be carried out in the presence of unprotected hydroxyl groups. In contrast, thiol 68 failed to form the corresponding urea 6 9, undoubtedly due to the sensitivity of thiols to oxidation.

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81 Table 4.2 Catalytic carbonylation of substituted benzyl amines Amine Product roYield"Â’*Â’ %YiiiP CH2CI2 CH2CI2/H2O

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82 Amine Product “/oYield^-" %Yield“’‘^ CH2CI2 CH2CI2/H2O O 72 O O 73

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83 Amine Product "/oYield^’” %Yield^" CH2CI2 CH2CI2/H2O 28 14 NH2 8j 17 20 83 Reaction conditions: amine (7.1 mmol), W(CO )6 (0.14 mmol), I 2 (3.5 mmol), K 2 CO 3 (10.7 mmol), CH 2 CI 2 (20 mL), 70 °C, 80 atm CO, 24 h. The solvent was CH 2 CI 2 (21 mL) plus H 2 O (3 mL). Other conditions are as in footnote b.

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84 The ester-substituted benzylamine 70 was also successfully carbonylated to the urea 71 in 36% yield in CH 2 CI 2 with 55% yield obtained in CH 2 CI 2 /H 2 O. The carboxylic acid analogue 72 is insoluble in CH 2 CI 2 , which hindered efforts to explore the tolerance of the catalyst to the presence of carboxylic acids. However, the solubility of 72 in basic H 2 O made it possible to obtain the corresponding urea 73 in 37% yield from the CH 2 CI 2 /H 2 O system. Compatibility of the catalyst system with terminal alkenes was then examined. The olefin 74 was reacted under the carbonylation conditions to form the urea 75 in a 41% yield in CH 2 CI 2 . Although we anticipated possible problems with reactivity at the double bond, there were no identifiable products in which the double bond had been perturbed. The low yields could, however, be indicative of unidentified side reactions. Unlike the previously described reactions of substituted benzylamines, higher yields were obtained when the reaction using CH 2 CI 2 as the solvent was diluted to 0.03M in substrate and conducted at room temperature. In addition, /?-nitrobenzylamine (76) was successfully carbonylated to urea 77 in a 45% yield in CH 2 CI 2 with 76% yield obtained in CH 2 CI 2 /H 2 O. The cyano-substituted benzylamine 78 was also carbonylated to its respective urea (79) in 37% yield in CH 2 CI 2 and 68% yield in CH 2 CI 2 /H 2 O. Note that the possibility of the nitrile moiety serving as a ligand for the catalyst did not interfere with the reaction. Further functional group studies included the reaction of the /7-aminobenzylamine 80 under the carbonylation conditions. If both the aliphatic and aromatic amines were to participate in the carbonylation reaction, oligomeric products would be formed. Primary aromatic amines, such as aniline were unreactive under the carbonylation conditions, so

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85 formation of the simple urea 81 was expeeted to dominate. However the reaction mixture was quite complex and urea 81 was only isolated in 28% yield. In this case, addition of water to the solvent system decreased the yield to 14%. The apparent participation of the aromatic amine in the reactions of 80 brought up the question of intramolecular carbonylation of the o-amino benzylamine 82 to form 3,4-dihydro-2(7//)-quinazolinone ( 83 ) (equation 4.5). Although aromatic amines are apparently not nucleophilic enough to initiate the reaction, they could be sufficiently W(CO )6 CO/I 2 /K 2 C 03 nucleophilic to complete the cyclization following initial reaction of the aliphatic amine. In fact, under the carbonylation conditions 80 did form the cyclic urea, albeit in low yields (17 % yield in CH 2 CI 2 and 20% yield in CH 2 CI 2 /H 2 O). The results from 80 and 82 suggest that differentiation between aliphatic and aromatic amines is not sufficiently good to obtain selective reactivity in the presence of both. Conclusion In summary, aliphatic primary amines can be catalytically carbonylated to 1,3disubstituted ureas in good to high yields using the commercially available, inexpensive and air stable W(CO)e as the catalyst. The preference of this catalyst for aliphatic amines

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86 is complementary to the late transition metal catalysts, which are generally more effective for aromatic amines than for aliphatic ones. Not only does this system provide an alternative to phosgene and phosgene derivatives, but it is also compatible with a variety of functional groups, including unprotected alcohols.

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87 CHAPTER 5 ALTERNATIVE SYNTHESIS OF CYCLIC UREA HIV-1 PROTEASE INHIBITORS Introduction Cyclic ureas have recently been of interest as the core structures of potent HIV protease inhibitors such as the clinical candidates DMP 323 and DMP 450.150,151,176 These compounds serve as inhibitors of the HIV1 -encoded aspartyl protease (HIV-Pr) which plays a fundamental role in the growth of immature virion proteins. [Kaltenbach, 1998 #1088] These cyclie ureas were designed to bind to HIV-Pr with the diol oxygens participating in symmetrical hydrogen bonding to the catalytic Asp 25/25'. Figure 5.1 HIV Protease Inhibitors

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88 There have been a few synthetic routes reported in the literature for compounds like DMP 323 and DMP 450, all of which involve problematic reagents or result in poor yieldsJ^^ Scheme describes the original cyclic urea core structure synthesis which used amino acid starting materials. Disadvantages to this synthesis include the Scheme 5.1 Synthesis of protease inhibitor core structure via an amino acid CbzHN COOH a CbzHN Me N. H,C OMe O b, c CbzHN NHCbz d SEMO OSEM CbzHN NHCbz SEMO OSEM g.h SEM = 2-(TrimethylsiIyl)ethoxymethyl HO OH CORE STRUCTURE Reagents; (a) j-BuOCOCl, A/'.O-dimethoxyhydroxylamine HCl; (b) LiAlH; (c) VCl3(THF)3, Zn-Cu; (d) SEMCl; (e) cat. Pd(OH)2; (f) l.l'-carbonyldiimidazole; (g) Benzyl bromide, NaH; (h) HCl, dioxane/MeOH

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89 tedious preparation of i?-configured amino acids and the intolerance of the vanadium coupling reaction towards heteroatom-containing substituents. Disadvantages in terms of process plant scale-up are the inability to isolate intermediate products without the use of column chromatography, and the disposal of large quantities of the vanadium/zinc mixed waste generated in the pinacol coupling step.^^^ Furthermore, the carbonylation step from the diamine to the cyclic urea uses the toxic and expensive phosgene derivative 1,1carbonyldiimidazole. Attempts were made to find alternative protecting groups that would result in crystalline products and eliminate the need for column chromatography. The diamine acetonide was prepared from diol 84 by protection with 2,2-dimethoxypropane followed by subsequent hydrogenolysis of the bis-Cbz moiety (Scheme 5.2). Problems arose when the carbonylation of diamine 85 resulted in only trace quantities of the cyclic urea 86. The low yield is attributed to the strain of forming a trans fused bicyclo[3.5.0] decane ring system. Under optimized conditions, cyclization can occur under high temperatures and high dilutions in a chlorinated solvent in 67% yield. However, the volume inefficiency of this cyclization and the requirement for high temperature, chlorinated solvents has rendered this conversion unacceptable for process scale-up. Scheme 5.2 Preparation of the diamine acetonide 85 o 85 86

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90 An alternative synthesis to the acetonide diamine 85 has been published which employs the ester 87 of L-tartaric acid as the starting material rather than an amino acid.[Rossano, 1995 #1090; Confalone, 1999 #615] 87 is reduced with DIBAL-H to a dialdehyde followed by addition of 1 , 1 -dimethylhydrazine which affords the bishydrazone 88. Subsequent reaction with benzyl lithium proceeds stereospecifically to afford dihydrazine 89. Diamine 85 is obtained after hydrogenolysis of 89 with Raney nickel. Again, cyclization is achieved by reaction of 85 with 1 , 1'-carbonyldiimidazole. Disubstituted urea 90 is then formed by the deprotonation and alkylation of the urea nitrogens. Although this route is undoubtedly Scheme 5.3 Synthesis of core structure starting from the ester of L-tartaric acid 87 1)DIBAL-H 2) 2-propanol 3) HjNNMcj Ph Ph NaH BnBr Ph MejN NMej 'nh HN RaNi 250 psi H 2 100 “C CD! = l.r-carbonyidiimidazole 90 86 85

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91 more efficient than the amino acid route, the cyclization step to form cyclic urea 86 remains a problem. To date, DuPont's process plant scale-up utilizes the Scheme 5.3 synthesis to produce diamine 85. However, once diamine 85 is formed, it is converted to secondary diamine 91 upon which cyclization takes place in the presence of phosgene to the disubstituted urea 90 (Scheme 5.4). 90 can be isolated in 75% yield. Phosgene is used due to the inability of fl'-carbonyldiimidazole to carbonylate secondary diamines. Scheme 5.4 Dupont process scale-up of cyclic urea 90 This chapter discusses the use of the W(CO)6 / I 2 carbonylation system as a possible alternative to using phosgene or phosgene derivatives in the cyclization of the primary and secondary diamines 85 and 91 to the corresponding cyclic ureas 86 and 90. Carbonylation of Diamine 85 Diamine 85 was synthesized according to the method described in Scheme 5.3. Diamine 85 was reacted under the following conditions: 2 mole % W(CO)6, stoichiometric amounts of diamine and I2, and 2 equiv of K2CO3 relative to diamine reacted in CH2CI2 at room temperature under 80 atm CO pressure. The reaction gave the

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92 corresponding cyclic urea as shown in equation 5.1. Unfortunately, the carbonylation of diamine 85 to the cyclic urea 86 at room temperature resulted in only a 23% yield. Although cyclization could occur without phosgene or phosgene derivatives, this low yield was not an improvement over the yield obtained in Scheme 5.2. However, when the reaction described above was repeated at 80 °C, cyclic urea 86 was formed in 70% yield. Considering these results are unoptimized, the W(CO)6 / U -catalyzed system is an attractive alternative to the synthesis described in Scheme 5.2. Clearly the cyclization to a trans fused ring is not a problem for the W(CO)6 / CO system at 80 °C. Furthermore, keeping in mind the functional group compatibility studies in Chapter 4, it is speculated that future carbonylation of highly functionalized diamine substrates will proceed without tedious functional group protection and deprotection steps. Carbonylation of Disubstituted Diamine 91 Similar to the preceding section, disubstituted diamine 91 was synthesized according to Scheme 5.4. Using the same W(CO)6 / U reaction conditions described for the carbonylation of diamine 85 in equation 5.1, diamine 91 was carbonylated in an attempt to form cyclic urea 90 (equation 5.2). The reaction was run at room temperature

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93 as well as 80 °C. Unfortunately, in both cases, unreacted starting material was isolated after work-up. There was no indication of the presence of cyclic urea 90 in the IR, 'H NMR or C'^ NMR. Further attempts of producing 90 included running the reaction at 90 °C and 1 10 °C as well as changing the solvent to the biphasic solvent system CH 2 CI 2 / H 2 O (90 °C). Again, in all cases, unreacted starting material was isolated. Possibly changing the reaction conditions such as solvent or temperature could render this reaction viable in the future. Conclusion DMP 323 and DMP 450 are representative illustrations of possible synthetic uses of catalytic carbonylation. The cyclic urea core structure 86 of these protease inhibitors can be synthesized in 70 % yield using the W(CO)e / U-catalyzed system which has the potential of being an alternative route to 86, rather than using the previously reported high reaction temperatures and phosgene derivatives. Future work could include applications of this catalytic carbonylation system to highly functionalized diamines as well as further investigations into the cyclization of disubstituted diamine 91 to cyclic urea 90 under catalytic carbonylation conditions.

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94 CHAPTER 6 EXPERIMENTALS General Standard inert atmosphere techniques were used in the syntheses of the metal carbynes and metal carbene. Et20, THE, and toluene were distilled from Na/Ph 2 CO. Hexane, CHCI3 (ethanol free unless stated), and methylene chloride were distilled from CaH 2 . CDCI3 was degassed by three freeze-pump-thaw cycles and stored over 3 A molecular sieves. All other starting materials were purchased in reagent grade and used without further purification. The carbyne complexes 37-39 were synthesized by literature methods.^^’^ ^3-1 15 Syntheses of 85-86 and 88-91 were based on published routes^^^ for which no experimental information was provided. General Instrumentation 'H, '^C, and ^'P NMR spectra were recorded on Varian XL-300 and VXR-300 NMR spectrometers. 'H NMR spectra are referenced to the residual protons of the deuterated solvent. ^'P NMR spectra are referenced to 85% H3PO4 and are proton decoupled. IR spectra were recorded on a Perkin-Elmer 1 600 spectrometer. A HewlettPackard 8452A diode array spectrophotometer was used to obtain UV-visible absorption spectra. Photolyses were performed at -45 °C in 5 mm NMR tubes by irradiation with a Hanovia medium pressure mercury vapor lamp in a Pyrex immersion well.

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95 Photophysical Methods Corrected emission and emission excitation spectra were obtained with a Spex Industries F-112A spectrophotometer. Emission correction factors were generated by using a 1000 W tungsten primary standard lamp. Emission quantum yields are reported relative to aqueous Ru(bpy) 3 ^'^ (<3>em = 0.055). All room temperature measurements were carried out on argon-degassed THE solutions and studies at low temperature were conducted on argon-degassed 2-methyltetrahydrofuran (MTHF) solutions. The instrumentation for the emission lifetime measurement on 39 has been described previously.1^^’1^1 Other emission lifetimes were measured on a Photochemical Research Associates time-correlated single photon counting spectrophotometer, which relies on an H 2 gas-filled spark gap for an excitation source. For emission decay experiments excitation light was filtered using a colored glass filter (Schott, UG-1 1) and emission light was filtered using interference filters at 650, 700 or 750 nm. Emission decay analysis was carried out using the DECAN deconvolution software. Laser flash photolysis experiments were carried out on a system that has been described previously. Global analysis of the multiwavelength transient absorption data was effected using the SPECFIT factor analysis and software. ^^l Syntheses r(COLWaOV2-napthvnirNMeJ (42) 2-Naphthyl bromide (4.0 g, 19 mmol) was dissolved in 60 mL of THF and cooled to -78° C. «-Butyllithium (2.0 M in pentane, 10 mL) was added under nitrogen and a yellow suspension was observed. After the solution was allowed to stir for 1 h, the

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96 entire solution was transferred by cannula to a flask containing W(CO )6 (6.75 g, 19 mmol) in THF at -78° C. The resulting red solution was allowed to stir for 2.5 h at -78° C, and then the THF was removed under vacuum, leaving a red oily-solid which was recrystallized from Et 20 / hexanes to yield a red crystalline solid. The solid was dissolved in 30 mL of degassed H 2 O, and the mixture was filtered through a fritted funnel into a solution containing NMe 4 Br (7.4 g, 48 mmol). The solution separated into two layers, and the top layer was removed via cannula. The remaining red oil was placed under vacuum to remove any additional H 2 O and used without further purification (9.3 g, 88 %). 'H NMR (CDCI3): 6 7.4 8.0 (m, 7H), 2.25 (s, 12H). Cl fCOHP(OMef2 h W=C-t2-naphthvn t41f Acyl complex 42 (9.3 g, 17 mmol) was dissolved in 70 mL of CH 2 CI 2 and cooled to -95° C. Oxalyl chloride (2.2 g, 17 mmol) was added, the solution was allowed to warm to -20° C, and evolution of gas was observed. After the solution was cooled once more to -95° C, trimethyl phosphite was added (16.7 g, 135 mmol), and when it warmed to room temperature, more effervescence occurred. The mixture was then refluxed for 24 h, after which the solvent and excess P(OMe )3 were removed under vacuum. The remaining red oily solid was dissolved in Et 20 and passed through a fritted funnel containing neutral alumina. The Et 20 was removed under vacuum, leaving 41 as a red oil in 58% yield. 'H NMR (CDCI 3 ): 5 7.4 7.9 (m, 7H), 3.6 (m, 27H). Cpf cot f PtOMeL 1 W=C-t2-nanhthvn r40f Tris (phosphite) complex 41 (2.9 g, 3.8 mmol) was dissolved in 45 mL of THF,

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97 and CpNa (2.0 M in THF, 4.0 mL) was added. The dark red solution was refluxed for 24 h, after which THF and P(OMe )3 were removed under vacuum. The resulting dark oil was dissolved in EtaO and passed through a short alumina column to remove unreacted CpNa. After the solvent was removed, the red oil was placed on a longer alumina column and washed with hexane. The yellow eluent was discarded, and further elution with 1 :2 Et20/ hexane gave a red solution which was concentrated to give 40 as a red oil in 20% yield. 'H NMR (CDCI 3 ): 5 7.81 (s, IH), 7.76 (d, IH), 7.69 (d, IH), 7.59 (d, IH, 7.42 (m, 3H), 5.57 (s, 5H, Cp), 3.61 (d, Jhp = 12.0 Hz, 9H, [P(OMe) 3 ]). ‘^C NMR (CDCI 3 ): 6 286.5 (d, /cp = 18.2 Hz, W=C), 235.3 (d, Jcv = 9.7 Hz, CO), 150.0 (s, =C-Q, 133.0, 131.9 (bridge), 127.8, 127.7, 127.2, 126.9, 126.8, 126.4, 125.8 (Np), 90.0 (s, Cp), 52.4 (d, Jcp = 2.4 Hz, P{OMe} 3 ). ^'P NMR (CDCI 3 ): 5 173.0 (Jwp = 662 Hz). IR (CH 2 CI 2 ): 1888 cm ' (vwco)UV (THF): 348 (s = 6000), 490 (e = 70) nm; HRMS (FAB) m/z calcd for M"' (C 20 H 21 O 4 PW), 540.0687; found, 540.0619. (CO)4W=CrOCHAfc-C .H g) (451 Carbene 45 was prepared by a slight modification of the literature method. It was found that the yield could be increased by 20 % if trimethyl tetrafluoroborate is used as the methylating reagent rather than methyl triflate, and if the product is purified by column chromatography on alumina (hexane) under N 2 atmosphere with eluent and column cooled to -78° C. rCO(Br){P(OMe)3 )2 W=Crc-C .H Qftr49f The carbene complex (CO) 5 W=C(OCH 3 )(c-C 6 H 9 ) (45) (0.84 g, 1.9 mmol) was

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98 dissolved in 30 mL of CH 2 CI 2 and cooled to -78° C. A 1.0 M solution of BBrs in CH 2 CI 2 (2.85 mL, 2.85 mmol) was added and the solution was stirred at -78° C for 10 min. The solution was warmed to -45° C, then cooled back down to -78° C and P(OMe )3 (1.34 mL, 15.2 mmol) was added. The solution was refluxed for 10 hr. Removal of solvent in vacuo and washing the product with Et 20 yielded 49 as an orange-yellow oil (0.94 g, 65.3%). 'H NMR (CDCI 3 ): 5 1.63-1.95 (m, 4H), 2.00-2.40 (m, 4H), 3.64 (d, 9H, J=12 Hz), 3.95 (virtual triplet, 18H), 6.23 (hr, IH); IR (CH 2 CI 2 ): vco 1943 cm''. rCp(CO) (P(OMe) 3 l WsCfc-a H oll (44) Complex 49 (0.94 g, 1.2 mmol) was dissolved in 30 mL of THF and 2.0 eq of CpNa (0.21 g, 2.4 mmol) was added. The solution was refluxed for 5 hours and the solvent was removed in vacuo. Chromatography of the resulting orange oil on neutral alumina with 4:1 hexane/Et 20 followed by 100% Et 20 yielded 44 (0.24 g, 40.5%): 'H NMR (CDCI 3 ): 5 1.40-1.60 (m, 4H), 1.73-1.90 (br, 2H), 2.17-2.20 (br, 2H), 3.52 (d, 9H, JPH= 12 Hz), 5.43 (s, 5H), 6.00 (s, IH); '^C (CDCI 3 ): 5 21.5, 22.2, 25.6, 26.6, 52.3 [P(OMe) 3 ], 89.5 (Cp), 129.8 (y-vinyl), 151.4 (P-vinyl), 235.0 (CO), 291.4 (d, W=C, Jpc = 17 Hz); ^'P{'H} NMR (CDCI 3 ) 6 175.1 (Jwp-673 Hz); IR (CH 2 CI 2 ): Vco 1878 cm''; HUMS (FAB) m/z calcd for M^ (C 16 H 23 O 4 PW) 494.0843, found 494.0802. CpCUP(OMeLlWrp^-CHrc-C. H Ql1 (51) Carbyne complex 44 (0.10 g, 0.20 mmol) was dissolved in 2 mL of Et 20 and cooled to -10 "C. Upon addition of 1 equiv of ethereal HCl as a 1.0 M solution (2.2 mL), 51 immediately precipitated as a yellow powder. After further purification by washing

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99 with cold Et 20 , 51 was obtained in 28 % yield. Note: complex 51 decomposes rapidly if not kept below -45 °C during purification and sample preparation. 'H NMR (CDCI 3 ): 5 1.60-2.00 (m, 4H), 2.46-2.80 (m, 4H), 2.82-3.00 (br, IH), 3.78 (d, 9H, Jph = 1 1 Hz), 5.73 (s, 5H), 15.35 (s, IH); (CDCI 3 ): 6 20.5, 24.2, 30.7, 54.1 [P(OMe) 3 ], 75.4 (P-vinyl), 93.7 (y-vinyl), 94.9 (Cp), 277.8 (W=C); NMR (CDCI 3 ) 5 113.0 (Jwp = 409 Hz); MS (electrospray, CH 3 CN) m/z found 502.1 (M+). Isotope distribution pattern matched simulation for C 15 H 24 CIO 3 PW. Photooxidation of 44 Carbyne complex 44 (0.12 g, 0.24 mmol) was dissolved in 10 mL of CHCI 3 and cooled to -45 °C. The solution was photolyzed for 5 hr at -45 °C while the disappearance of 44 was monitored by TLC. Upon completion of the photolysis, the solvent was removed in vacuo at -45 °C leaving a red residue. CDCI 3 that had been cooled to just above its freezing point was cannulated into the remaining residue. The solution was then transferred to a chilled NMR tube and a 'H NMR spectrum was taken at -50 °C. General Procedures for the Catalytic Carbonylation of Benzylamines with W ( CO ) , Procedure A To a stirred solution of W(CO )6 (49 mg, 0.14 mmol) in 20 mL of CH 2 CI 2 in the glass liner of a Parr high-pressure vessel was added 50 equiv of p-methoxybenzylamine (1.0 mL, 7.1 mmol), 75 equiv of K 2 CO 3 (1.47 g, 10.7 mmol) and 25 equiv of iodine (0.89 g, 3.5 mmol). The vessel was then charged with 85 atm of CO, heated to 70 °C and left

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100 to stir under pressure for 24 h. The pressure was released and the maroon solution was filtered. The solid collected on the filter paper was heated in CH 2 CI 2 , filtered, and both filtrates were combined. The combined CH 2 CI 2 layers were washed with a 1.0 M HCl solution followed by washing with a saturated Na 2 S 03 solution. The resulting pale yellow solution was then dried with Na 2 S 04 and filtered. The solution was concentrated to yield a pale yellow solid, which was then rinsed with 1 5 mL of ether. The urea was obtained as a white solid (0.50 g, 1.7 mmol, 47% yield). The solid was identified as N, NÂ’-bis(4-methoxybenzyl)urea by comparison with literature values. Procedure B Procedure B is identical to that of procedure A except the reaction was diluted to O. 03 M in substrate and run at room temperature. Procedure C Procedure C is identical to that of procedure A except 2 1 mL of CH 2 CI 2 and 3 mL of H 2 O were employed as solvent. The product urea precipitated out of the reaction mixture, was filtered, washed with Et 20 and collected. Preparation of N.NÂ’-dibenzvlurea 55 Procedure A afforded 55 in 63% yield. Procedure C afforded 55 in 73% yield. Urea 55 was identified by comparison with literature values. ^

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101 Preparation of N.NÂ’-Bis(4-chlorobenzynurea 57 Procedure A afforded 57 in 35% yield. Procedure C afforded 57 in 77% yield: 'H NMR (DMSO-de) 5 7.37 (d, J = 7.6 Hz, 4H), 7.26 (d, J = 7.8 Hz, 4H), 6.56 (br s, 2H), 4.20 (d, J = 5.4 Hz, 4H); '^C NMR (DMSO-d 6 ) 5 158.0 (C=0), 140.1, 131.0, 128.8, 128.1, 42.3; IR (Nujol) 1567 cm'Â’; HRMS (FAB) calcd for C 15 H 15 CI 2 N 2 O (M+H^) 309.0561, found 309.0561. Preparation of N.NÂ’-Bis(4-bromobenzvBurea 59 Procedure A afforded 59 in 30% yield. Procedure C afforded 59 in 77% yield: 'H NMR (DMSO-de) 5 7.50 (d, J = 8.1 Hz, 4H), 7.20 (d, J = 8.2 Hz, 4H), 6.53 (br s, 2H), 4.19 (d, J = 5.9 Hz, 4H); '^C NMR (DMSO-d 6 ) 6 158.0 (C=0), 140.5, 131.0, 129.2, 119.5, 42.3; IR (Nujol) 1561 cm''; HRMS (FAB) calcd for Ci 5 Hi 5 Br 2 N 20 (M+H^) 396.9551, found 396.9560. Preparation of N.NÂ’-Bis(3-iodobenzvnurea 61 Procedure A afforded 61 in 39% yield. Procedure C afforded 61 in 70% yield: 'H NMR (DMSO-de) 6 7.60 (s, 2H), 7.56 (d, J = 7.7 Hz, 2H) 7.25 (d, J = 7.5 Hz, 2H), 7.10 (t, 2H), 6.49 (t, 2H), 4.19 (d, J = 5.9 Hz, 4H); '^C NMR (DMSO-de) 5 158.0 (C=0), 143.8, 135.5, 135.2, 130.5, 126.4, 94.8, 42.2; IR (Nujol) 1584 cmÂ’Â’; HRMS (FAB) calcd for C 15 H 15 I 2 N 2 O (M+H^) 492.9274, found 492.9271. Preparation of N.NÂ’-Bis('4-methoxvbenzvBurea 63 Procedure A afforded 63 in 47% yield. Procedure C afforded 63 in 70% yield.

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102 Literature reports on 63 do not eontain spectral data. *H NMR (DMSO-de) 5 7.15 (d, J = 8.4 Hz, 4H), 6.85 (d, J = 8.2 Hz, 4H), 6.31 (br s, 2H), 4.13 (d, J = 5.4 Hz, 4H), 3.71 (s, 6H); NMR (DMSO-de) 6 158.04 (C=0), 157.98, 132.8, 128.3, 113.6, 55.0, 42.4; IR (Nujol) 1578 cm'*; HRMS (FAB) calcd for C17H21N2O3 (M+H^) 301.1552, found 301.1557. Preparation of N.N’-Bis(4-methvlthiobenzvl')urea 65 Procedure A afforded 65 in 24% yield. Procedure C afforded 65 in 81% yield: 'H NMR (DMSO-de) 5 7.19 (m, 8H), 6.42 (br t, 2H), 4.16 (d, J= 6.0 Hz, 4H), 2.44 (s, 6H); '^C NMR (DMSO-d6) 5 158.0 (C=0), 137.7, 135.9, 127.8, 126.1, 42.5, 15.0; IR (Nujol) 1584 cm'*; HRMS (Cl) calcd for C17H21N2OS2 (M+H^) 333.1095, found 333.1106. Preparation of N.N'-Bis(4-hvdroxvmethvlbenzvBurea 67 Procedure A afforded 67 in 5% yield. Procedure C afforded 67 in 58% yield. *H NMR (DMSO-de) 5 7.41 (m, 8H), 6.56 (t, 2H), 5.31 (t, 2H), 4.65 (d, J5.4 Hz, 4H), 4.39 (d, J= 6.0 Hz, 4H); ‘^C NMR (DMSO-de) 5 158.0 (C=0), 140.8, 139.2, 126.8, 126.4, 62.7, 42.8; IR (Nujol) 3331, 1555 cm''; HRMS (FAB) calcd for C17H21N2O3 (M+H^) 301.1552, found 301.1533. Preparation of N.N’-Bis(ethvl 4-carboxvlbenzvBurea 71 Procedure A afforded 71 in 36% yield. Procedure C afforded 71 in 55% yield: 'H NMR (DMSO-de) 6 7.91 (d, J = 8.2 Hz, 4H), 7.38 (d, J = 8.2 Hz, 4H), 6.65 (br t, 2H),

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103 4.31 (s, 4H), 4.30 (qt, 4H), 1.32 (t, 6H); NMR (DMSO-de) 6 165.7, 158.1 (C=0), 146.7, 129.1, 128.2, 127.0, 60.6, 42.8, 14.2; IR (CH 2 CI 2 ) 1708, 1678 cm'Â’; HRMS (FAB) calcd for C 21 H 25 N 2 O 5 (M+H^) 385.1763, found 385.1760. Preparation of N.NÂ’-Bis(4-carboxvlic acid benzvBurea 73 Procedure C was employed except that the product 73 did not precipitate out until the reaction mixture was acidified with IN HCl. Urea 73 was obtained in 37% yield; 'H NMR (DMSO-d6) 5 12.89 (s, 2H), 7.88 (d, J= 7.6 Hz, 4H), 7.35 (d, 7.8 Hz, 4H), 6.62 (t, 2H), 4.29 (d, J= 5.9 Hz, 4H); NMR (DMSO-dg) 5 167.2, 158.1(NHC=0), 146.2, 129.3, 126.9, 109.4, 42.8; IR (Nujol) 1684, 1561 cm"'; MS (Electrospray) for C 17 H 16 N 2 O 5 (M+H^) 329.3, calcd 329.3. Preparation of N.NÂ’-Bis(4-ethenvlbenzvnurea 75 When procedure A was employed, no product was isolated. The viscous material obtained suggested the formation of oligomers. Procedure B afforded a 69% yield of 75. When procedure C was employed, 75 was only isolated in 19% yield: 'H NMR (DMSOde) 6 7.40 (d, J = 7.9 Hz, 4H), 7.21 (d, J = 7.9 Hz, 4H), 6.70 (qt, 2H), 6.45 (br s, 2H), 5.78 (d, J = 17.7 Hz, 2H), 5.21 (d, J = 10.9 Hz, 2H), 4.21 (d, J = 5.6 Hz, 4H); NMR (DMSO-de) 5 158.1 (C-0), 140.7, 136.4, 135.5, 127.2, 126.0, 113.7, 42.7; IR (CH 2 CI 2 ) 1678 cm'*; Anal. Calcd for C 19 H 20 N 2 O; C, 78.05; H, 6.89; N, 9.58. Found: C, 74.70; H, 6.95; N, 9.03.

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104 Preparation of N.NÂ’-Bis('4-nitrobenzyllurea 77 Procedure A afforded 77 in 45% yield. Procedure C afforded 77 in 76% yield: 'H NMR (DMSO-de) 6 8.20 (d, J = 8.5 Hz, 4H), 7.51 (d, J = 8.5 Hz, 4H), 6.85 (br t, 2H), 4.34 (d, J = 6.0 Hz, 4H); '^C NMR (DMSO-ds) 5 158.0 (C-0), 149.3, 146.2, 127.8, 123.4, 42.6; IR (Nujol) 1584, 1537 cm'*; Anal. Calcd for C 15 H 14 N 4 O 5 : C, 54.54; H, 4.27; N, 16.96. Found: C, 54.07; H, 4.23; N, 16.66. Preparation of N.NÂ’-Bis(4-cyanobenzvn urea 79 Procedure A afforded 79 in 37% yield. Procedure C afforded 79 in 68 % yield: 'H NMR (DMSO-d 6 ) 5 7.80 (d, J = 7.3 Hz, 4H), 7.44 (d, J 7.0 Hz, 4H), 6.79 (br s, 2H), 4.31 (d, J = 4.9 Hz, 4H); '^C NMR (HMSO-dg) 5 158.0 (C=0), 147.1, 132.2, 127.7, 119.0, 109.2, 42.7; IR (CH 2 CI 2 ) 2214, 1678 cmÂ’'; HRMS (FAB) calcd for Ci 7 H, 5 N 40 (M+H^) 29 1 . 1 246, found 29 1 . 1 249 . Preparation of N.NÂ’-Bis(4-aminobenzvnurea 81 When procedure A was employed, the reaction formed a solid black mass. Procedure B afforded the urea 81 in 28% yield after recrystallization in EtOH. Procedure C afforded 81 in 14% yield. The urea 81 was identified by comparison with literature values. Preparation of 3.4-Dihvdro-2niT)-quinazolinone 83 Procedure A afforded 83 in 17% yield. Procedure C afforded 83 in 20% yield. The urea 83 was identified by comparison with literature values,

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105 Preparation of (4R. 5R')-2.2-DimethvI-4.5-bisformvl-1.3-dioxolane N.NDimethvlhvdrazone 88 To a -40 °C chilled solution of dimethyl 2,3-0-isopropylidene-L-tartrate (2.18 g, 10 mmol) in toluene was added DIBAL-H (22.0 mL, 22.0 mmol) dropwise and allowed to stir at -40 °C for 1 hr. Upon addition of methanol (0.89 mL, 22.0 mmol), the solution was warmed to -10 °C. N,N-dimethylhydrazine (1.67 mL, 22.0 mmol) was then added and allowed to stir for 1 hr at -10 °C. The reaction was quenched with 30 mL of H2O, extracted with Et20, dried over MgS04, and concentrated to give 88 as a white solid (2.35 g, 97 %). 'H NMR (CDCI3) 5 6.37 (s, 2H), 4.46 (s, 2H), 2.85 (s, 12H), 1.47 (s, 6H). Preparation of (3R. 4S. 5S. 6RV2.2-Dimethvl-4.5-bisformvl-1.3-dioxolane-3.6bis(phenvlmethvl') N.N-Dimethvlhvdrazine 89 To a 100 mL Schlenk flask under N2 was added toluene (12.8 mL, 120 mmol) and 5ec-butyl lithium (16.0 mL, 20 mmol). The solution was cooled to -20 °C and THF (5.0 mL, 60 mmol) was added dropwise. Upon formation of yellow crystalline mass, the reaction was allowed to stir at room temperature for 2 hr. The reaction was then cooled to -15 °C and 88 (1.78 g, 7.0 mmol), which was dissolved in 10 mL of toluene, was added dropwise. The reaction stirred for 1 hr at -15 °C and then was quenched with H2O, followed by extraction with Et20. The organic layer was dried over MgS04 and fumaric acid (0.86 g, 7.0 mmol) was added. The mixture stirred overnight upon which the fumaric acid salt of 89 had precipitated. The precipitate was collected, dissolved in H2O, and deprotonated by the addition of a saturated NaHCOs solution. The aqueous mixture was then extracted with Et20, dried over MgS04, and concentrated to give 89 as a colorless oil (2.12 g, 71 %). 'H NMR (CDCI3) 5 7.25 (m, lOH), 4.1 (s, 2H), 3.1 (m, 2H),

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106 2.7 (m, 4H), 2.1 (s, 12H), 1.4 (s, 6H); NMR (CDCI3) 5 139.73, 129.60, 128.47, 126.29, 107.72, 77.53, 59.07, 47.92, 37.61, 27.56. Preparation of ('2R. 3S. 4S. 5R')-2.2-Dimethvl-3.4-bisformvl-1.3-dioxolane-2.5bistphenvlmethvndiamine 85 To a solution of 89 (0.6 g, 1.4 mmol) in methanol contained in a Parr high pressure vessel was added 3 g of wet Raney Ni (washed once with MeOH). The vessel was charged with 500 psi H2 and heated to 80 °C for 18 hr. The reaction was then filtered through celite and concentrated to an oil. The oil was columned on alumina with 99:1 EtOAc/NEts followed by 30:10 EtOAc/MeOH to give 85 (0.11 g, 23 %). 'H NMR (CDCI3) 5 7.20 (m, lOH), 4.05 (s, 2H), 3.10 (m, 2H), 2.80 (m, 2H), 2.61 (m, 2H), 2.50 (s, 4H), 1.41 (s, 6H); ‘^C NMR (CDCI3) 5 138.48, 129.23, 128.55, 126.47, 108.86, 79.60, 53.66,41.34, 27.32. Preparation of t4R. 5S. 6S. 7R')-Hexahvdro-5.6-0-isopropvlidene-4.7-bis(phenvlmethvlV 2//-1.3-diazapin-2-one 86 Procedure A was employed except that 25 equiv of diamine 85, 50 equiv of K2CO3, and 25 equiv of I2 were used. Procedure A at room temperature afforded 86 in 23% yield. Procedure A at 80 °C afforded 86 in 70% yield. The urea 86 was identified by comparison with literature values. Preparation of t2R. 3S. 4S, 5R')-2.2-Dimethvl-3.4-bisformvl-1.3-dioxolane-2.5bistphenylmethyll N.N-Dibenzvldiamine 91 Diamine 85 (50 mg, 0.15 mmol) was refluxed with benzaldehyde (0.03 mL, 0.29 mmol) in toluene for 1 hr. The reaction was cooled followed by the addition of acetic

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107 acid (0.02 mL, 0.29 mmol) and NaBH(OAc )3 (75 mg, 0.35 mmol). The reaction was heated for 2 hr at 35 “C and then quenched with a saturated NaHCOs solution. The mixture was then extracted with EtOAc, dried over MgS 04 and concentrated to give 91 as a white solid (76 mg, 100 %). *H NMR (CDCI 3 ) 5 7.38 (m, 20H), 4.29 (s, 2H), 3.85 (m, 2H), 3.71 (m, 4H), 2.8 (m, 4H), 1.60 (s, 6 H); ‘^C NMR (CDCI 3 ) 5 150.54, 139.46, 129.25, 128.31, 128.10, 126.67, 126.01, 107.87, 77.44, 57.85, 51.53, 38.21, 27.30. Attempted preparation of 14R. 5S, 6 S. 7RVHexahvdro-5.6-(9-isopropvlidene-4.7bistphenvlmethvlVN.N-dibenzvl1 .3-diazapin-2-one 90 The carbonylation of diamine 91 was attempted using procedure A at room temperature, 80 °C, 90°C, and 110 °C. No evidence for product 90 was observed in the IR, 'H NMR, or NMR. The carbonylation of diamine 91 was also attempted using procedure C at 90 °C. Again, no evidence for product 90 was observed in the IR, 'H NMR, or ’^C NMR.

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BIOGRAPHICAL SKETCH Andrea Denise Main was bom on September 24, 1971, and grew up in Winter Garden, Florida. She was home-schooled by her parents from 8th to 12th grades. Upon graduation, she entered the University of Central Florida in Orlando, Florida, in 1989. She spent her spare time working for Dr. John Gupton, her organic research advisor. In 1993, after completing her Bachelor of Science degree in chemistry, she traveled to the Netherlands and spent the summer working for a pharmaceutical company. In the fall of 1993, she entered graduate school at the University of Rochester. Deciding it was extremely cold and weathering Rochester's worst winter in ten years, she transferred to the University of Florida in Gainesville, Florida. She decided to pursue the study of organometallic chemistry under the direction of her advisor. Dr. Lisa McElwee-White. She received her Ph.D. in organic/organometallic chemistry in the summer of 2000. She has obtained a postdoctoral research position at Pacific Northwest National Laboratories in Richland, Washington where she will explore "green" chemistry under the direction of Dr. John Linehan. 120

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I certify that I have read this study and that in my opinion it conforms to acceptable standards of scholarly presentation and is fully adequate, in scope and quality, as a dissertation for the degree of Doctor of Philosophy. Lisa McElweeWhite, Chairman Professor of Chemistry I certify that I have read this study and that in my opinion it conforms to acceptable standards of scholarly presentation and is fully adequate, in scope and quality, as a dissertation for the degree of Doctor of Philosophy. Stewart Associate Professor of Chemistry I certify that I have read this study and that in my opinion it conforms to acceptable standards of scholarly presentation and is fully adequate, in scope and quality, as a dissertation for the degree of Doctor of Philosophy. J. Eric Enholm Associate Professor of Chemistry 1 certify that I have read this study and that in my opinion it conforms to acceptable standards of scholarly presentation and is fully adequate, in scope and quality, as a dissertation for the degree of Doctor of Philosophy. James Boncella Associate Professor of Chemistry I certify that I have read this study and that in my opinion it conforms to acceptable standards of scholarly presentation and is fully adequate, in scope and quality, as a dissertation for the degree of Doctor of Philosophy. Michael Kau: Professor and Engineering Cau^an or Materi! cience

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This dissertation was submitted to the Graduate Faculty of the Department of Chemistry in the College of Liberal Arts and Sciences and to the Graduate School and was accepted as partial fulfillment of the requirements for the degree of Doctor of Philosophy. August 2000 Dean, Graduate School