Thermal and photochemical studies of 9,10-BIS(trifluoroethenyl) phenanthrene and perfluoro-E,Z,E- and E,E,E-4,5-dimethyl...

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Title:
Thermal and photochemical studies of 9,10-BIS(trifluoroethenyl) phenanthrene and perfluoro-E,Z,E- and E,E,E-4,5-dimethyl-2,4,6-octatriene and thermal studies of terminally fluorinated 1,5-dienes
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ix, 197 leaves : ill. ; 29 cm.
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Palmer, Keith W., 1966-
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Thesis:
Thesis (Ph. D.)--University of Florida, 1993.
Bibliography:
Includes bibliographical references (leaves 189-196).
Statement of Responsibility:
by Keith W. Palmer.
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Typescript.
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Vita.

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THERMAL AND PHOTOCHEMICAL STUDIES OF
9,10-BIS(TRIFLUOROETHENYL)PHENANTHRENE AND
PERFLUORO-E,Z,E- AND E,E,E-4,5-DIMETHYL-2,4,6-OCTATRIENE AND
THERMAL STUDIES OF TERMINALLY FLUORINATED 1,5-DIENES













By

KEITH W. PALMER


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

1993
UNIVERSITY OF FLORIDA LIBRARIES

























TO DAD, MOM, JACQUELINE, HELEN AND KARYN













ACKNOWLEDGEMENTS

A complete list of all of the friends I am indebted to for assisting me to this

point would require a dissertation in itself. Hopefully in this short space I can
give justice to a few, with the many not mentioned being assured that they will

always be in my thoughts.
I wish to express my gratitude for the excellent guidance and friendship
of my advisor, Professor William R. Dolbier, Jr. Besides attempting to attain a
fraction of his vast knowledge of chemistry, and perhaps more important, I leave
with an appreciation for his understanding, and his methods of motivating those
around him in a positive fashion. With Professor Dolbier, the chemistry is
always surmountable and one comes away with a positive feeling towards
oneself and the tasks ahead. Such an attitude is refreshing in times of stress
and leads to an enjoyable and rewarding experience.
The love and support of my parents and sisters was and will always be
essential. No matter where they live, it is always "home" when I make it away to
see them and the time spent together is cherished. The valuable lessons

learned at a younger age--from Dad's "Pick up the tools!" to Mom's "Stop
procrastinating, and get to your homework!"--echo in my head from time to time
and raise my level of productivity.
The friends and colleagues I have met since at the University of Florida
Department of Chemistry are numerous. Greatly appreciated are the
exceptional faculty members I have had the opportunity to interact with and
whose courses helped to form the core of my chemical intuition. Due to my
extended stay in the Dolbier group, thanks are required to a number of








labmates: Sarah Weaver for her help in learning the ropes around the lab and
roller-blading companionship; Conrad Burkholder for numerous stimulating
discussions; Jeff Keaffaber, Laurent Wedlinger, Hania Wotowicz, Lech
Celewicz, Mohammed Alli Asghar, Hua Qi Zhang, He Qi Pan, Wen Juan Cao,
Xiao Xin Rong, and more recently Martin McClinton, Mike Bartberger and
Michelle Fletcher, all for providing valuable friendships. Last and certainly not
least, special thanks go to Dr. Henryk Koroniak. A true friend, Henryk is greatly
appreciated for teaching me a variety of technical skills and returning more than
once at just the right time to give me a fresh charge of enthusiasm.
Outside the Dolbier group, I will always remember my pistachio addicted
roommates and true friends Kevin Kinter and Brent Kleintop. The
companionship and good times we had together helped maintain proper
perspective and kept me from spending too much time in the lab.













TABLE OF CONTENTS


ACKNOWLEDGEMENTS......................................................................................... iii

ABSTRACT ......................................................................................................................vii

CHAPTER

1 AN OVERVIEW OF FLUORINE SUBSTITUENT EFFECTS IN
ORGANIC SYSTEMS .......................................................................................... 1

Introduction ............................................................................................................ 1
Fluoroalkanes.......................................................... ............................................ 2
Fluoroalkenes............................................................. .......................................... 5
Fluorine Non-bonded Electron Interactions.................................... .............. 8
Fluorine Steric Effects...................................................................................... 11
The Thermal Cyclobutene/1,3-Diene Electrocyclic Process ......................... 13
Conclusions................................................................................................. 22

2 THERMAL AND PHOTOCHEMICAL REARRANGEMENTS OF
9,10-BIS(TRIFLUOROETHENYL)PHENANTHRENE AND
PERFLUORO-E,E,E-(AND E,Z,E-)4,5-DIMETHYL-2,4,6-OCTATRIENE........23

Introduction ........................................................................................................ 23
Development of a Suitable 1,3(Z),5-Triene/1,3-Cyclohexadiene
System .......................................................................................... .....................27
Thermal Study of 9,10-Bis(trifluoroethenyl)phenanthrene............................ 31
Thermal Study of Perfluoro-E,Z,E(E,E,E)-4,5-dimethyl-2,4,6-
octatriene.................................................. .......... ..........................................33
Photochemical Rearrangements of 9,10 Bis(trifluoroethenyl)phenan-
threne and Perfluoro-E,Z,E(E,E, E)-4,5-dimethyl-2,4,6-octatriene ............40
D iscussio n.................................................................................................................4 5
C onclusions..................................... ...... ............................................................77

3 [3,3]-SIGMATROPIC REARRANGEMENTS OF TERMINALLY
FLUORINATED 1,5-DIENES............................... 79

Introduction ................................................... .......... ...........................................79
The [3,3]-Sigmatropic Shift of 1,5-Dienes: The Cope Rearrangement .........80
Fluorinated Cope Systems ............................................................................... 82
Synthesis and Thermolysis of Terminally Fluorinated Cope Systems..........86
1,1,6,6-Tetrafluoro-1,5-hexadiene................................................................ 86
1-Difluoromethylidene-4-methylidenecyclohexane................ ............. 89








1,4-Di(difluoromethylidene)cyclohexane................................................ 91
meso- and d,-1 -(2-Difluoromethylidenecyclopentyl)-2-
difluoromethylidenecyclopentane ................................................ ......94
Discussion........................................................................................................... 95
Conclusions ................................................................................................109

4 EXPERIMENTAL............................................................................................... 111

General Methods ............................................................................................. 111
Experimental Procedures................................................................................ 112

APPENDIX A: GAS PHASE THERMOLYSIS APPARATUS................................160

APPENDIX B: SELECTED 19F NMR SPECTRA ....................................................163

REFERENCES ........................................................................................................ 189

BIOGRAPHICAL SKETCH .......................................................................................197
































vi













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

THERMAL AND PHOTOCHEMICAL STUDIES OF
9,10-BIS(TRIFLUOROETHENYL)PHENANTHRENE AND
PERFLUORO-E,Z,E- AND E,E,E-4,5-DIMETHYL-2,4,6-OCTATRIENE AND
THERMAL STUDIES OF TERMINALLY FLUORINATED 1,5-DIENES

By

Keith W. Palmer

August 1993



Chairman: William R. Dolbier, Jr.
Major Department: Chemistry

Thermolysis of 9,10-bis(trifluoroethenyl)phenanthrene was examined in
solution between 1400C and 1930C, and led to formation of 1,2,2,3,3,4-
hexafluoro-2,3-dihydrotriphenylene and 1,4,4,5,6,6-hexafluoro-2,3-(9,10-
phenanthro)bicyclo[3.1.0]hex-2-ene by irreversible, competitive first-order
processes. The competitive formation of 1,4,4,5,6,6-hexafluoro-2,3-(9,10-
phenanthro)bicyclo[3.1.0]hex-2-ene is virtually unprecedented in 1,3,5-triene
thermal chemistry with only one similar cyclization found in the hydrocarbon
literature. Thermolysis of 1,4,4,5,6,6-hexafluoro-2,3-(9,10-phenanthro)-
bicyclo[3.1.0]hex-2-ene led to further rearrangement and was examined in
solution between 1800C and 1930C. This material was found to rearrange to 4-
difluoromethylidene-3,3,5,5-tetrafluoro-1,2-(9,10-phenanthro)cyclopent-1-ene








and 1,2-(9,10-phenanthro)-3,5,5-trifluoro-4-trifluoromethyl- 1,3-cyclopentadiene
through irreversible, competitive first-order processes.
Photolysis of 9,10-bis(trifluoroethenyl)phenanthrene in solution led to

1,4,4,5,6,6-hexafluoro-2,3-(9,10-phenanthro)bicyclo[3.1.0]hex-2-ene as the
major product with 1,2,2,3,3,4-hexafluoro-2,3-dihydrotriphenylene and [2n+2K]

cycloaddition products 1,4,5,5,6,6,-hexafluoro-2,3-(9,10-phenanthro)-
bicyclo[2.1.1]hex-2-ene and 1,4,5,5,6,6-hexafluoro-2,3-(9,10-phenanthro)-
bicyclo[2.2.0]hex-2-ene being observed in minor amounts.
Thermolysis of perfluoro-E,Z,E- and E,E,E-4,5-dimethyl-2,4,6-octatriene

was studied in solution between 154C and 202C and initially found to

undergo C4-C5 double bond isomerization. At higher temperatures, an
equilibrium between the perfluorooctatriene and perfluoro-cis- and trans-1,3,4-
trimethyl-4-(E-1-propenyl)cyclobutene is established requiring an 47r electron,

conrotatory electrocyclic process.
Photolysis of perfluoro-E,Z,E- and E,E,E-4,5-dimethyl-2,4,6-octatriene in

solution leads to formation of perfluoro-trans-2,3,5,6-tetramethyl-1,3-
cyclohexadiene by a 6t electron, conrotatory electrocyclic process. This
material was found to undergo further photo-cyclization to perfluoro-trans-
2,3,5,6-tetramethylbicyclo[2.2.0]hex-2-ene through a 4x electron, disrotatory

electrocyclic process.
Reluctance of these perfluorinated-1,3(Z),5-trienes to undergo the
thermal, 67 electron, disrotatory electrocyclization via the required boat

transition state was evident, as was the facility of the photoprocess to occur
through the photo-allowed 67c electron, conrotatory electrocyclization via a chair

transition state. This disparity was believed to arise from a detrimental
interaction between terminal cis fluorines as the 1,3(Z),5-triene approaches the
boat-like disrotatory transition state and required further study.







The system chosen to probe this effect was the thermal [3,3]-sigmatropic
rearrangement of terminally gem-difluorinated 1,5-dienes. Synthesis and gas
phase thermal study of 1,1,6,6-tetrafluoro-1,5-hexadiene, 1-difluoro-
methylidene-4-methylidenecyclohexane, 1,4-di(difluoromethylidene)cyclo-
hexane, and solution phase thermal study of meso- and d,-l--(2-
difluoromethylidenecyclopentyl)-2-difluoromethylidenecyclopentane, were
carried out and the transformations found to occur by well-behaved, irreversible,
first-order processes to the respective [3,3]-shift products. Comparison of the
measured activation parameters for these processes and values for the
corresponding hydrocarbon and partially fluorinated 1,5-dienes from the
literature indicates that terminal gem-difluorination accelerates thermal
processes occurring through chair transition states while inhibiting processes
occurring through boat transition states.













CHAPTER 1

AN OVERVIEW OF FLUORINE SUBSTITUENT EFFECTS
IN ORGANIC SYSTEMS

Introduction

The isolation of fluorine by Henri Moissan on June 26, 1886, created
considerable academic interest, and led to rapid advances in the field of
fluorine chemistry.1 In 1930, Midgely and Henne developed CF2CI2 as a cheap
and safe refrigerant to replace the toxic gas ammonia which elevated fluorine
chemistry out of it's status as an academic novelty and initiated the organo-
fluorine industry. Due to the Manhattan Project and nuclear energy applications
in the early 1940s, large scale production of elemental fluorine became
necessary. The post-World War Two era saw intense interest in organo-fluorine
chemistry develop in industrial and academic sectors. This interest was
primarily due to the fascinating properties exhibited by fluoroorganic materials
developed during the concentrated research directed towards production of the
atomic bomb. Fluorine exhibits remarkable effects when utilized as a
substituent in organic systems and a variety of factors intrinsic to the fluorine
atom are responsible. The purpose of this introduction is to address factors
which will be pertinent within this current study by reviewing fluorine effects in a
series of simple and well characterized organic systems.
The effects exhibited by fluorine as a substituent are due to three intrinsic
characteristics of the fluorine atom; extreme electronegativity, non-bonded

electron pairs, and small relative size. Fluorine is the most electronegative of all








elements with a Pauling scale value of 4.10 as compared with oxygen (3.50),
chlorine (2.83), bromine (2.74), carbon (2.50), and hydrogen (2.20).2 Strong
polarization of fluorinated molecules through the a bonding framework and

through space (field effects) are results of fluorine's large electronegativity. The
atom is mono-valent and accommodates three non-bonded electron pairs in
orbitals of similar dimension to hybridized orbitals on carbon.3 Because of
these two preceding factors, fluorine exhibits an interesting donor/acceptor
contradiction under certain circumstances in that the strong removal of electron
density from a bound atom can be offset due to back donation of density from

the non-bonded electrons. The van der Waals radius of fluorine is 1.47 A.4
Compared with the other halogens, carbon, and hydrogen (van der Waals radii:
Cl, 1.73 A; Br, 1.84 A; I, 2.01 A; Calphaic, 1.70 A; H, 1.20 A),4 fluorine should
exhibit minimal spatial requirements as a substituent, a fact which has allowed
for complete substitution of hydrogen by fluorine in many hydrocarbon systems.

This has enabled the completely synthetic field of perfluorocarbon chemistry to

be developed and exploited by industry and academics with substantial
financial and scholarly success.
The effects of fluorine as a substituent in organic systems have been the
subject of a number of reviews,1,5,6,7,8 those of Smart5s6 being most insightful.
This introduction will demonstrate the ways in which fluorine substitution
perturbs the structure and reactivity of simple hydrocarbons.

Fluoroalkanes

The series of fluorinated methanes show an interesting trend in C-F
bonding. Table 1-15 illustrates the strengthening and incremental shortening of
the C-F bond in this series. This trend of bond strengthening with increased







Table 1-1. C-F Bond Lengths and Dissociation Energies in Fluoromethanes.


Fluoromethane

CH3F
CH2F2

CHF3
CF4


r (C-F) (A
1.385
1.358

1.332
1.317


Do(C-F) (kcal/mol
109.0
122

128.0
129.7


Table 1-2. Bond Lengths and Dissociation Energies in Fluoroethanes.


Fluoroethane

CH3-CH3

CH3-CH2F
CH3-CHF2
CH3-CF3

FCH2-CF3
CF3-CF3


r (C-C) (A)
1.532

1.502
1.498
1.494

1.501

1.545


D(C-C) (kcal/monl
90.4

91.2
95.6
101.2

94.6

98.7


D(C-F) (kcalmol)


107.7
Unknown
124.8

109.4 (CH2F)
126.8


substitution is unique to fluorine among the halogens. The series of chlorinated
methanes exhibits a similar bond shortening but is accompanied by an
incremental weakening; 83.7 kcal/mol down to 72.9 kcal/mol per first C-CI bond

homolysis in the series from CH3CI to CC14.5
Table 1-26 illustrates the effect of successive fluorination on the bond

lengths and strengths in the case of ethane. Geminal fluorination leads to
strengthening and shortening of the C-C bond in the series CH3-CH3 to CH3-
CF3. The C-C bond lengths increase upon vicinal fluorination from CH3-CF3 to
CF3-CF3 while the C-C bond strength decreases. The C-F bond strengths in the







series of ethanes follows a similar trend of strengthening with increased
geminal fluorination as observed in the series of methanes.

As of this time, the trends in C-C bond strengths and lengths with various
degrees of fluorination has not been fully explained. However, valence bond

arguments have been used to rationalize the observed trends in C-F bonding in

alkanes. As previously mentioned, fluorine contains three non-bonded electron
pairs in orbitals of dimension which can accommodate appreciable overlap with
orbitals of other period two elements. It is rationalized that for carbon
substituted with two or more fluorines, double-bond no-bond resonance
structures, or negative hyperconjugation, as shown in the classical sense by

Figure 1-1, lead to increased bond order between the carbon and fluorine.3,9,10


F.F
ll F lil F+


Figure 1-1. Fluorine Double-bond, No-bond Resonance.


As the degree of geminal fluorination increases, the number of valence bond
structures involving doubly bound fluorine increases and the C-F bonds are
increasingly shorter and stronger. Theoretical calculations at the ab initio level

have confirmed such a bonding scheme where it is found that the stabilizing
interaction arises from back-donation of a fluorine lone pair into an antibonding
a*C-F orbital.9,1011 This explanation based on fluorine non-bonded electron

interactions to rationalize the observed bonding and geometry characteristics in
fluoro-organics is complemented by other arguments which inherently do not
involve the non-bonding electrons on fluorine. One such argument suggests

that when carbon is bound to more electronegative elements, atomic p
character concentrates in orbitals directed towards the electronegative species








since p electrons are less tightly bound than s electrons.12,13 Carbon

rehybridization then assists in accounting for bonding and geometry trends in
fluoro-organics. Another argument arising from ab initio level theoretical study
attributes bond shortening to Coulombic interactions between oppositely
charged fluorine and carbon.14 In effect, an increase in C-F bond ionic

character is predicted as the degree of fluorination increases. Calculations

indicate the trend arises from negligible change in charge on fluorine but

considerable increase in positive charge on carbon as the degree of
fluorination increases. The variety of rationalizations for C-F bonding in such

simple systems illustrates the complexities in theoretically and certainly
qualitatively explaining bonding trends in fluoroorganic systems.

Fluoroalkenes

Fluorine substitution at a vinylic carbon leads to substantial changes in
alkene geometry and reactivity. The data in Table 1-45,6 reveal that
fluoroethylenes have shorter C=C bonds than ethylene and the C-F bond
lengths are shorter than similarly geminal or vicinal fluorinated alkanes. The

Table 1-4. Structural Aspects of Fluoroethylenes.

CH=Hz HQ CH_=CHF CH2=C~Fi. CHF=CF_ CF-=CF5
r(C-C) A 1.339 1.333 1.315 1.309 1.311
r(C-F) A 1.348 1.323 1.32 1.319
H-C-H deg 117.8 120.4 121.8 -
H-C-F deg 115.4 116.2 -

F-C-F deg 109.3 112.2 112.5
Dn kcal/mol 59.1 Unknown 62.1 Unknown 52.3







geminally difluorinated olefins contain FCF bond angles which are much
smaller than ethylene and very close to the tetrahedral value of 109.470.
Theoretical ab initio level calculations show the C-F bond shortening can be
attributed to fluorine non-bonded electron delocalization into the c(c=c)

molecular orbital as depicted in a valence bond fashion by Figure 1-2.9,15,16 FCF
bond angle contraction in this case may be rationalized by attraction between

the charged and neutral fluorine atoms.16


F F+

F F

Figure 1-2. Fluorine Non-bonded Electron Pair Delocalization in Alkenes.

Fluoroalkene Reactivities

Reactivities in fluoroalkene series lead in some cases to clear trends as
the degree of olefin fluorination is changed, but are often found to be specific to
the olefin and transformation in question. Generally, reactions involving
transformation of unsaturated, fluorine substituted carbon to a saturated state
are more exothermic than the process for a similar hydrocarbon. Table 1-56
illustrates the increasing exothermicity for hydrogenations in the series of
fluoroethylenes, CH2=CHF clearly deviating from the trend. Other reactions
involving saturation of CF2=CF2's double bond such as bromination,

chlorination, HX (X = Br, Cl, I) addition, and polymerization are all in excess of
10 kcal/mol more exothermic than the corresponding reaction with ethylene.6
Cyclobutene to butadiene isomerizations (Table 1-66) illustrate a reverse
in thermal stability between the hydrocarbon (1) and perfluorinated (2) case.
Perfluoro-1,3-butadiene (2) is found to be 11.7 kcal/mol less stable than
perfluorocyclobutene, a result which is in line with the increased exothermicity








Table 1-5. Fluoroalkene Heats of Hydrogenation.


Alkene

CH2=CH2

CH2=CHF

CH2=CF2

CHF=CF2


AHoH (kcal/mol)

-32.6

-29.7

-38.8

-45.7


Table 1-6. Fluorinated Cyclobutene/1,3-Diene Thermal Isomerizations.


A


F F


AHo (kcal/mol

-9.7

11.7


H F F


H F

H F


revealed by AHoH2 upon transcending the series of fluoroethylenes in Table 1-5.

Although AHoH2 for CH2=CF2 is 6.2 kcal/mol more exothermic than ethylene,

isomerization in the case of 3,3,4,4-tetrafluorocyclobutene (3) favors the diene,

indicating the fluoroalkene is lower in energy. Simple alkyl substitution at C1 in

3,3,4,4-tetrafluorocyclobutene as seen with 4 and 5, dramatically shift the

equilibrium towards the cyclobutene, creating doubt as to the usefulness of this

system in demonstrating the thermodynamic influence of fluorine on an olefin.

Other systems as illustrated in Figure 1-3,17 indicate gem-difluoroolefins are


C


AB C
D


A

H

F

H

CH3

CH2CH3


9000

0.0056

77.5

0.50

0.24


B D
D








destabilized relative to the saturated species. Results pertaining to the stability

of monofluorinated alkenes are contradictory but it is generally accepted that

monofluorination stabilizes a double bond relative to the saturated state.5,6


A, 12
F2C=CHCH3 HCF2CH= CH2
AH = -2.5 kcal/mol

(^CF2 A CF2
1 AH0 = -5.1 kcal/mol J

Figure 1-3. Equilibria Involving Gem-difluoro Alkenes.


Since the enthalpy of reaction is a relative energy change between

reactant and product, it is not entirely established whether the favorable driving

force for transformation of a trifluoro- of gem-difluoroolefin to a saturated
fluoroalkane is due to I bond destabilization in the fluoroolefin or stability as a

result of rehybridization of the fluorinated carbon from sp2 in the fluoroolefin to
sp3 in the fluoroalkane. Arguments for both factors are offered and it appears

that both are important in these systems with 7 bond destabilization being the

major contributor.5.6

Fluorine Non-bonded Electron Interactions

As the stabilizing influence of fluorination upon alkanes has been

offered, a discussion on fluorine's non-bonded electron interactions with

adjacent occupied and non-occupied orbital systems is warranted in light of the

aforementioned question of fluoroolefin destabilization.
Destabilization of n systems has related precedent in the case of a-fluoro

carbanions. Such systems are found to be destabilized in situations where the

carbon bearing the negative charge and fluorine are planar.' Figure 1-4







OH OH

6 6
F
10.59 10.49
pKa in 30% EtOH at 25.0C


NaOCH3
7 U^^U
CH3OH
DX HX


X Exchange kRpla
D 1
F 0.125
Cl 400
Br 700

Figure 1-4. Destabilization in Planar a-Fluorocarbanions.


illustrates the decrease in acidity in 4-fluorophenol (6)18 relative to phenol and
rate inhibition in isotope exchange in 9-fluorofluorene relative to fluorene-9-d2
(7)19and other 9-halogenofluorenes. Conjugative destabilization is invoked in
these cases between the fluorine non-bonded electron pairs and the planar
carbanion.
A variety of experimentally observed situations occur with fluorine bound
to sp2 hybridized carbon for which perturbation of an adjacent neutral n system
is induced by interaction with fluorine non-bonded electrons. Fluorine is found
to be an ortho and para director and frequently a net activator in electrophillic
aromatic substitutions.7 Along the same line of thought, fluorine and oxygen are
found to strongly influence the distribution of t electron charge in aromatic








Table 1-3. 13C NMR Shifts (STMS) for Heteroatom Substituted Benzene.

SX Geminal
ortho
meta
para


X Geminal ortho meta .ata
F 163.8 114.4 129.6 124.3

OH 154.9 115.4 129.7 121.0

CI 134.3 128.6 129.8 126.5

SH 130.7 129.2 128.9 125.4

13C NMR Shift for C6H6 21: 128.5


systems leading to the development of partial negative charge at the ortho and

para positions. This is revealed by the observed shielding of carbons at these

positions in the 13C NMR spectra of representative substituted benzenes as

illustrated in Table 1-3.20 The larger third period analogies, chlorine and sulfur

respectively, show a minimal effect as might be expected due to poorer overlap

of their non-bonded electrons with the aromatic n system.

Direct evidence for the stabilization of carbocations by geminally

substituted fluorine has been obtained by a gas phase, ion cyclotron resonance

technique. This study has revealed that the ascending order of stability in the

series of fluoromethyl carbocations is +CH3 < +CF3 < +CH2F < +CHF2.22,23

Furthermore, the +CF3 cation has been generated by matrix photoionization of

trifluoromethyl halides and exhibits an infrared spectrum which is consistent
with extensive i(p-p) bonding.24 Generally, the degree to which carbocations are

stabilized by hydrogen, fluorine, and alkyl will be found to follow the order +CH
< +CF < +CR.5








a-Fluorine changes the geometry of methyl radical from planar to

pyramidal which is proposed to be due to repulsion between the radical and
fluorine non-bonded pairs.25,26 Stability of a and p-fluoro radicals as

established5 from bond dissociation energies is relatively unchanged from
hydrocarbon analogs and thermal rearrangement of 8 to 10 occurs with
activation parameters which are almost identical when X = H or F.27 The overall

effect of fluorination on the stability of free radicals is believed to be minimal.5,6


x x



8 9 10

Figure 1-5. Thermal Rearrangement of 6-Methylidenebicyclo[3.2.0]heptane (8).

Fluorine Steric Effects

It is often assumed that when considered alone, the small differences

between hydrogen and fluorine in size and bond length to carbon will lead to
minimal or no effect on the conformation and reactivity in a hydrocarbon upon

substitution of C-F for C-H. This is often the case and has allowed for synthesis

and study of many poly and perfluorinated hydrocarbons, a situation which is
not available for any other atom to the extent to which it is for fluorine. Although
this is true, there remain a number of situations in which substitution of fluorine
for hydrogen in a hydrocarbon leads to a profound effect on the conformation
and (or) reactivity in a system due solely to the relative size and charge density
of fluorine versus hydrogen.

The potential energy barrier for rotation of the C-C bond in CH3-CH3 is
2.8 kcal/mol whereas in CF3-CF3 it is increased to 3.9 kcal/mol.28 1,3-Repulsion








between fluorines in perfluoro-n-alkanes leads to a twisting in the carbon

backbone. Such an effect is said to be evidenced by polytetrafluoroethylene,

which below 190C contains a 3600 twist in the carbon backbone per 26 CF2
units.29 This is in marked contrast to polyethylene, where the carbon backbone

maintains a zig-zag structure with all of the C-C bonds in the same plane and all

of the hydrogen atoms in straight rows.29 Examples of fluorine influencing

conformational processes are offered in Figure 1-6.30 In these systems,

conformational barriers develop upon substitution of fluorine for hydrogen

arising from electrostatic repulsion between fluorine and the group moving past.

Figure 1-7 shows a persistent radical (14) which was able to be formed up to

88% (weight) in solution and could be diluted in the open air and dissolved in

good hydrogen donor solvents like toluene, or heated to 1000C without





11 HF) Ring Flip kH/kF 1011 at 25C



S (CH S
F
12 V Ring Flip AG* = 23.5n=4, 15.3n5, 10.5n=6 kcal/mol



Figure 1-6. Influence of Fluorine on Conformational Processes.




F2 F2



13 14 15


Figure 1-7. A Persistent Perfluoroalkyl Radical (14).







destroying the ESR signals.1 The stability of this species was attributed to the
sheltering of the radical center provided by the perfluoroethyl and
perfluoroisopropyl groups. From Taft Es values, the CF3 group is found to be
larger than CH(CH3)2 and the CF(CF3)2 group is similar in size to C(CH3)3.31
Steric effects attributed to fluorine are most occurrent and documented in
the case of perfluorinated systems. For fluorine to exhibit a steric effect in a
mono or partially fluorinated system, the molecule must exist with very small
spatial tolerance, whereby substitution of hydrogen by fluorine leads to
destabilization. This would be the result of attempted direct overlap of nuclei or,
more likely, electrostatic repulsion between a substituent and fluorine's high
negative charge density. As shown, steric effects due to fluorine are most
frequently documented for conformational processes occurring in rigid systems
or in the sheltering of a reaction site by a perfluoroalkyl group.

The Thermal Cyclobutene/1.3-Diene Electrocyclic Process

In the previous discussions, examples of the novelty of fluorine
substituent effects were offered and rationalized based on intrinsic
characteristics of the fluorine atom. With this basic groundwork in mind, the
effect of fluorine on the thermal cyclobutene to 1,3-diene electrocyclic
interconversion will be discussed. Understanding the studies of this system is
imperative since the results lead to a hypothesis which this author's initial
project (Chapter 2) was developed to further address.
An electrocyclic rearrangement is a subset of the pericyclic class of
reactions which involve bonding changes in a concerted fashion through a
closed cycle of atoms.32 The electrocyclic rearrangement involves the formation
of a o bond between the termini of a conjugated linear i system which results in
the formation of a ring containing one fewer n bond.33 The reaction is potentially







reversible, a fact which will depend on the relative thermodynamics of the

specific system in question.

Woodward and Hoffmann identified the thermal cyclobutene to butadiene
interconversion as occurring through a concerted, conrotatory pathway.34

Concertedness in a process, a situation where the energetic of bond breaking
assist bond making,33 is evidenced in these systems by low activation energies
relative to the energy of a corresponding homolytic or heterolytic process, low

activation entropies, and stereoselectivity in product formation. The
stereoselectivity in the product butadiene (or cyclobutene for the reverse

reaction) is a result of the conrotatory nature of this process. Woodward and
Hoffmann proposed that conservation of orbital symmetry from reactants to
products is the lowest energy path by which the process may occur.34 To
maintain symmetry for the thermal 4n process, C3 and C4 of cyclobutene (or

terminal carbons in butadiene) must rotate in a similar direction upon breaking
of the a bond; hereby defining a conrotatory process.34 Two equivalent,

stereodistinct, conrotatory processes are allowed by orbital symmetry for the 47

thermal reaction. As shown in Figure 1-8, each leads to a different conjugated
diene. Theoretical studies show that a concerted transition structure does not
exist for the thermally forbidden disrotatory process and estimates the non-

concerted path involving the allylmethylene diradical to be 9-11 kcal/mol above
the concerted conrotatory transition state.35


1 2 1 2 1 2 1 2


B C A C A A D


Figure 1-8. Conrotatory 4n Electrocyclic Process.








Stereochemistry of butadiene products from early thermal studies of C3
and C4 alkylated cyclobutenes were rationalized based on a steric argument.
The C3 and C4 methylated cyclobutenes (Figure 1-9) yielded butadienes in
which the bulkier substituent had stereospecifically rotated outward to form the


CH3 H3
16 --- X-- A- "'CH3


CH3 CH3

17 H C- C
H3C "CH3

References: 1636, 1737

Figure 1-9. Thermal Ring Opening of Methyl Substituted Cyclobutenes.


E-alkenes, away from the breaking C3-C4 a bond in the concerted transition

state. In 1980 Curry and Stevens reported a series of 3,3-disubstituted
cyclobutenes which yielded products contrary to those which would have been
predicted on steric grounds.38 Figure 1-10 illustrates their results in which ethyl,
n-propyl, and -propyl favor inward rotation over methyl to predominately form Z-
butadienes and, surprisingly, t-butyl yields 32% of the product where this very
bulky group has rotated inward. More intriguing examples have followed as
illustrated in Figure 1-11. In each case, the reaction is 100% stereoselective
and occurs contrary to expectations based on steric interactions.

Thermal Study of the Fluorinated Cyclobutene/1.3-Diene Interconversion

The unquantified nature of the system and an interest in fluorine
substituent effects led Dolbier et al. to investigate the thermodynamics and








kinetics of the process for a series of fluorinated materials in the mid-1980s.42,43

Fairly rapidly, studies in the fluorocarbon systems showed drastic deviations

from the corresponding hydrocarbons. One of the major differences is that the

relative thermodynamics of the perfluorocarbon systems are reversed from the

hydrocarbons; at equilibrium, mainly perfluorocyclobutenes exist.42,44 The


R
EtCH3


A


R

oOCH3 +


Ratio of Products
R Z E

Ethyl 68 32

n-Propyl 62 38

i-Propyl 66 34

t-Butyl 32 68

Figure 1-10. Thermal Ring Opening of 3,3-Dialkylcyclobutenes.


CHO

18 E


A


OCH3
-19 C(CH3)3 A
19 | 1


CHO



C(CH3)3

HOCH3


20 E
CO2H


COOH
c. Cl


References: 1839, 1940, 2041
Figure 1-11. Contrasteric Stereoselective Thermal Ring Openings in
Substituted Cyclobutenes.


CH3

R
E








hydrocarbon cyclobutene ring opening is found to occur with AHt = 32.0
kcal/mol,45 ASS = 0.1 cal/molxdeg,45 and is irreversible at reasonable

temperatures due to an exothermicity of 9.7 kcal/mol.46 The exothermicity of the
hydrocarbon process arises roughly from differences in release of cyclobutene
ring strain47 (34.0 kcal/mol) and overall bonding change of a 7 (=61 kcal/mol)33

for a a (=79 kcal/mol)33 bond. This exothermicity is offset in the case of the
perfluorinated species by the preference of fluorine to be bound to carbon
orbitals hybridized with maximal p character. This factor amounts to 2 5
kcal/mol upon conversion of a gem-difluoroalkene to alkane as discussed
earlier (Figure 1-3). Figure 1-12 shows some of the systems reported by
Dolbier et al. In all of the systems, fluorine kinetically prefers outward rotation
and in some cases, outward rotation of fluorine is favored even at the steric
expense of rotating the bulkier CF3 group inward. Formation of the Z-alkenes
(23, 26) from thermolysis of 22 and 25 occur with activation energies 12.9 and
27.1 kcal/mol respectively lower than the alternate processes leading to the E-
alkenes (21, 24). This corresponds to ratios of rates for Z and E-diene
formation (kz/kE) of 7219 and 4.3x106 respectively for these two systems at
200.0C. With only one possible butadiene available from both conrotatory
processes, 27 undergoes ring opening to stereospecifically yield 28 with
relative normal activation parameters. The partially fluorinated cyclobutenes
29, 31, and 34 were investigated to quantitatively determine the effect of a
single fluoro or trifluoromethyl substituent. 3-Fluorocyclobutene (29) was found
to open stereospecifically to 30 with an activation energy 16.9 kcal/mol lower
than that required for ring opening of 34, the large difference in activation
energies arising from inward rotation of a fluorine in 34 versus 29. To
demonstrate that thermodynamic factors were not contributing to the
stereospecificities in ring opening of 29 and 31, iodine catalyzed thermal










F F' 49.6(5.5)*
SCF3 39.2(-5.3)
F F
21


F CF
F F


36.7(-3.5)

28.5(-10.0)


F CF3
F F
F F
23


10.4------------------------------ 0----------------------------------------- 8.2

F (
FF F3 29.1(-5.4)
F3C 0 CF 3 50.6(7.0) cF 29.(-5.4)
F F) "-- 'IFc F3 13 .) OF3 F
F F 43.6(-4.5) F F 23.5(-13.2) C3
24 25 26
5.6 -------- ------------ ---- 0 ---------- --------------------...---------- 6.8


F PCF3
" IF 38.1(2.0)

F 7CF3 32.7(-10.5)
27


F F
F 'CF3
CF3 F
28


0 ----------------... .--------------..-- 5.7

F

2E 28.1(-3.5)
29 30


CF3
EV*


F


34


36.3(3.9)


45.0(8.1)


CF3

^-'CF3 + -
32 33
97.8% (at 200.00C) 2.2%


F
4o 1-F


*Format is, Ea in kcal/mol (ASt, calmolxdeg); Relative AHo in kcal/mol is shown below
structures for 21-23, 24-26, 27-28. References: (21-28)42, (29-35)43

Figure 1-12. Thermal Fluorocyclobutene/Fluoro-1,3-Diene Interconversions
Studied by Dolbier et al.







equilibria of each butadiene system was examined. Z-1-Fluoro-1,3-butadiene

was found to be more stable than the E-diene 30 with K(Z/E) of 1.77 at 60 oC and
E-5,5,5-trifluoro-1,3-pentadiene (32) was more stable than Z-33 with AHo = 2.5
kcal/mol for the E--Z equilibrium. No significant thermodynamic difference

between the Z and E-alkenes was observed in any of the cases examined, as
evidenced by these small differences in AHo values. Therefore in these

systems, the observed kinetic difference must derive from substituent effects on

the relative transition state stability for the two competing conrotatory processes.
Considering all of the data, it was determined that in thermal conrotatory
cyclobutene ring openings, inward rotation of fluorine raises the activation
energy for the process by 10 kcal/mol while outward rotation lowers it by 4
kcal/mol.43

Theoretical Study of the Thermal Cyclobutene/1.3-Diene Interconversion

From the variety of examples and the nature of substituents examined for
this 47, thermal process, it was obvious that the stereoselectivities observed

were not steric in origin except in a minor number of cases. Rather, the
stereoselectivities originate from a strong electronic effect involving interaction
of cyclobutene C3 and C4 position substituents with molecular orbitals of the 4n
transition state. Substituents at the C3 and C4 positions able to act as n

electron donors such as F, Cl, OCH3, and OCOCH3, kinetically favored outward
rotation whereas n electron acceptors such as CHO and COOH favored inward

rotation.
Rondan and Houk developed a hypothesis around results obtained by
theoretical ab initio level calculations on the cyclobutene/1,3-diene thermal
conrotatory process.48 Figure 1-13 is a representation of their proposed HOMO
(Highest Occupied Molecular Orbital) and LUMO (Lowest Unoccupied










./ o* (LUMO)



Stabilization




,4- a (HOMO)
Destabilization in




Figure 1-13. Representation of Donor Orbital Interactions with the HOMO and
LUMO of the Cyclobutene Conrotatory Transition Structure.


Molecular Orbital) for the cyclobutene thermal electrocyclic, conrotatory ring
opening transition structure with the donor orbital system indicated for either
outward or inward rotation. Upon inward rotation, a donor orbital (~CH3 on CH3

or a lone pair of electrons on a heteroatom) destabilizes the transition state due
to a repulsive interaction with the occupied bonding a orbital, while minimal
interaction exists with a* as the donor orbital is directed at the nodal surface.
Outward rotation of a donor is slightly stabilizing due to overlap with the a* and
the fact that the donor orbital experiences less repulsion with a upon rotating
out. Interaction of acceptors containing a 7 bond is more complicated due to the
interactions of occupied -t bonding and unoccupied n* antibonding orbitals with
the a and a* orbitals of the cyclobutene transition state. The maximal stabilizing
interaction in the case of acceptors containing a 7 bond is proposed to occur

upon inward rotation. The two electron interaction between the cyclobutene








C3-C4 a orbital and the x* orbital on the acceptor leads to a small preference

for inward rotation. Experimentally, acceptors show little preference for inward
or outward rotation, the reason being that the stabilizing T*-a interaction is

countered by a repulsive interaction between the occupied r orbital and C3-C4
a orbital of the cyclobutene transition structure. Rondan and Houk suggested a

powerful electron acceptor such as the empty p orbital on a BH2 group would
favor inward rotation due to the strong interaction with the occupied cyclobutene
a bonding orbital. Table 1-748 illustrates calculated AEa's for inward versus

outward rotation for BH2 and other donor and acceptor substituents which
further demonstrate strong rotational selectivities.


Table 1-7. Calculated AEa's for Inward versus Outward Rotation in Substituted
Cyclobutenes.

Substituents Eaout Ein
trans-3,4-diboryl +13

trans-3,4-dimethyl -13
trans-3,4-dichloro -22
trans -3,4-difluoro -29
trans-3,4-dihydroxy -31.6


The cleavage of the a bond in the cyclobutene system biased by the

electronic character of an attached substituent was dubbed "torquoselectivity"
by Houk.49 He argues that in this system, the torque about the stretching a bond

is controlled by interaction with geminal electron donor or acceptor substituents.
It is further stated that such electronic based selectivities should be observable
in a variety of other organic pericyclic processes.





22

Conclusion

The preceding discussions have introduced the variety and fashion in
which fluorine substitution can perturb hydrocarbon chemistry and aid in

mechanistic interpretation. The body of this thesis will address experimental
results obtained from the study of fluorine as a substituent in thermal and
photochemical 67 electrocyclic (Chapter 2) and thermal [3,3]-sigmatropic

rearrangements (Chapter 3).












CHAPTER 2

THERMAL AND PHOTOCHEMICAL REARRANGEMENTS OF
9,10-BIS(TRIFLUOROETHENYL)PHENANTHRENE AND
PERFLUORO-E,E,E- AND E,Z,E-4,5-DIMETHYL-2,4,6-OCTATRIENE

Introduction

The concept of torquoselectivity, developed by Rondan and Houk to
rationalize the strong electronic control of C3 and C4 substituents upon ring
opening of cyclobutenes, was suggested to be operating in a variety of
pericyclic processes. The potential was discussed for various processes such
as 6n electrocyclic reactions, cyclopropane isomerizations, and sigmatropic
shifts, to undergo a bond cleavage in a stereoselective fashion biased by

overlap with geminally bound acceptor or donor substituents.48,49 Disrotatory
electrocyclic processes are expected to exhibit rotational preferences to a
smaller degree than those observed for conrotatory cases. This is due to a
smaller difference in overlap (Figure 2-1) between the breaking a bond and
geminal substituent in inward and outward rotational configurations for
disrotatory as compared to conrotatory processes.


Outward Inward
----------- ( f Disrotatory -----------

j ----- ---- Conrotatory -----------


Figure 2-1. Geminal Substituent Overlap With c Bond in Outward and Inward
Rotation for Disrotatory and Conrotatory Processes.







Surprisingly, probing of torquoelectronic effects through suitably
designed systems has received minimal attention outside of the thermal
cyclobutene ring opening examples discussed in Chapter 1. Torquoselectivity
in the electrocyclic conversion of benzocyclobutenes to o-xylylenes has been
studied through Diels-Alder trapping experiments.50 A few examples involving
cyclopropane ring cleavage, such as: solvolysis of cyclopropyl halides,51 1,3-

sigmatropic shifts of substituted methylenecyclopropanes,49 and retro-ene
reactions of methyl vinylcyclopropanes,49 have been rationalized assisted by
torquoelectronic arguments.
In light of the torquoelectronic effects observed in the thermal 4n

electrocyclic fluorinated cyclobutene/1,3-diene system and the relatively
unexplored question of applicability to other pericyclic processes, a study of the
thermal 6n electrocyclic fluorinated 1,3(Z),5-triene/1,3-cyclohexadiene

interconversion was proposed. Woodward and Hoffman described the thermal
6x 1,3(Z),5-triene to 1,3-cyclohexadiene reaction as occurring through an

orbital symmetry allowed disrotatory pathway.34 Experimentally, it is found that
the cyclized products are formed in a stereospecific fashion in line with a
disrotatory closure involving rotation of terminal 1,3(Z),5-triene substituents in

an opposite sense as illustrated by the examples in Figure 2-2.52









Figure 2-2. Stereospecific Thermal Disrotatory 6x Electrocyclizations.







For hydrocarbon 1,3(Z),5-trienes, a wide variety of substituted systems have
been investigated to establish the relationship between triene structure and
reactivity towards cyclization.53
Observing torqueoelectronics in thermal ring opening of this 67 system is
unlikely, as the thermodynamics for conversion of 1,3-cyclohexadiene (38) to Z-
1,3,5-hexatriene (37) are unfavorable. The enthalpy diagram in Figure 2-354
illustrates the conversion of 37 to 38 is 15 kcal/mol exothermic due to loss of




44.8
29.0
(kcaVmol) ----------- -----* ---------- --
(kcal/mol)37 (40.6)
36 (39.5) 11 15.2
38 (25.4)



S-0C-0
36 37 38

Figure 2-3. Enthalpy Diagram for E and Z-1,3,5-Hexatriene (36, 37) and
1,3-Cyclohexadiene (38).


the conjugated triene double bond and formation of a a bond in 1,3-
cyclohexadiene. 1,3(Z),5-Triene Z--E isomerization (37-436) does not
compete with 67t cyclization at lower temperatures as such a process involves
homolytic cleavage of the central double bond, occurring with a transition state
16 kcal/mol higher in energy than that required for the electrocyclic process.
The large exothermicity observed for Z-1,3,5-hexatriene cyclization leads to an
estimated AH* = 44 kcal/mol for ring opening of 1,3-cyclohexadiene. This

magnitude of an energy barrier for ring opening also accommodates a variety of








sigmatropic shift processes. Retrocyclizations of substituted 1,3-
cyclohexadienes have been observed at higher temperatures but
stereochemical analysis of the formed 1,3(Z),5-trienes in search of

torquoelectronic effects is futile due to competing rearrangements. Ring
opening of 39 (Figure 2-4) at 5600C led to complete equilibration of the labels
and was rationalized as occurring through retrocyclizations and [1,7]-H(D)


HAC CH3 CH2
D D D C(CH3)2 H3C DH2C CD3
D 2

D3Cj CD3 D3C CD3 Dl D
D D CD3

39/
CH3 CH3

D3C CH2D D3C y CD

DH D

Figure 2-4. Thermal Ring Opening of a 1,3-Cyclohexadiene.


shifts.55 1,3,5-Triene Z+->E isomerizations will also be competitive at these

elevated temperatures and create another source of stereochemical
scrambling.56.57
It is obvious from the preceding discussion that productive ring opening
and observation of torquoelectronics in substituted 1,3-cyclohexadienes will
require an adjustment in the thermodynamics of this system. In effect, reduction
or reversal of the enthalpy difference between a 1,3(Z),5-triene and 1,3-
cyclohexadiene would allow retrocyclization to be observed. The achievement
of such a situation would allow for ring opening of a 1,3-cyclohexadiene system
to occur at a reasonable temperature and allow study of torquoelectronic effects








by observing the 1,3(Z),5-triene stereochemistry and activation parameters
required.

Development of a Suitable 1.3(Z).5-Triene/1.3-Cyclohexadiene System

In theory, the exothermicity of 1,3(Z),5-triene cyclization may be offset by
ground state stabilization of the 1,3(Z),5-triene, destabilization of the 1,3-
cyclohexadiene, or some combination of both.
A system in which the thermodynamics seem favorable for observing ring
opening of a 1,3-cyclohexadiene is one in which the 1,3(Z),5-triene/1,3-
cyclohexadiene system shares a 7 bond with an aromatic ring as illustrated for

a general case in Figure 2-5. Cyclization in this case is impeded by loss of the
aromatic ring n bond, in effect destroying the resonance energy for that ring in

the aromatic system. By selecting the appropriate aromatic system and




Aromatic Non-aromatic

Figure 2-5. Aromatic Annulated 1,3(Z),5-Triene/1,3-Cyclohexadiene System.


placement, 1,3-cyclohexadiene ring opening can become a competitive
process with 1,3(Z),5-triene cyclization at lower temperatures. Building a
fluorinated 1,3(Z),5-triene system into an phenanthrene ring was believed to be
the best suited entry into thermal study of 1,3-cyclohexadiene ring opening.
As previously discussed, Z-1,3,5-hexatriene cyclization is exothermic by
15 kcal/mol and terminal gem-difluorination or trifluorination of an pendant
alkene will increase this value by 2-5 kcal/mol per alkene. In line with the C-C a

bond strengthening trend observed for the series of fluorinated ethanes
(Chapter 1, Table 1-2), the C5-C6 a bond in the cyclohexadiene product will be








stronger than the corresponding C5-C6 a bond in the hydrocarbon. Such an

effect is difficult to quantitize due to the nonlinear energy changes in geminal
bond strengths upon successive fluorination. An increase in the exothermicity
of the cyclization process from formation of the tetrafluorinated C5-C6 a bond
can be estimated to have an upper limit of 8 kcal/mol obtained from the
difference in C-C a bond strength between CH3-CH3 and CF3-CF3 (Chapter

One, Table 1-2). This yields an potential enthalpy of reaction range of -19

kcal/mol to -33 kcal/mol for cyclization of the hypothetical 1,1,2,5,6,6-hexafluoro-

1,3(Z),5-hexatriene. Although conceptually straightforward, estimation of this
AHR using AHF of each species through Benson58 type group values cannot

be performed because of the missing groups; CD-(CD)(F), CD-(C)(F), and C-

(C)(CD)(F)(F).59 Likewise, theoretical computations at any level less than ab
initio lead to woefully incorrect energy parameters for fluorinated materials.60
The resonance energy for phenanthrene is observed to be 91 kcal/mol.61

Loss of phenanthrene's C9-C10 double bond is accompanied by an 20
kcal/mol increase in enthalpy, or the difference in resonance energy between
phenanthrene and biphenyl.61,62
Considering the energetic of the 1,3(Z),5-fluorotriene cyclization and
disruption of phenanthrene resonance energy by C9-C10 i bond cleavage


F F F F FF F F
F F


40 41

Figure 2-6. Proposed Thermal Electrocyclic Interconversion between 9,10-
Bis(trifluoroethenyl)phenanthrene (40) and 1,2,2,3,3,4-Hexafluoro-2,3-dihydro-
triphenylene (41).








together in a single system such as 9,10-bis(trifluoroethenyl)phenanthrene (40)
and 1,2,2,3,3,4-hexafluoro-2,3-dihydrotriphenylene (41) as illustrated in Figure
2-6, an enthalpy of reaction range of 1 kcal/mol to -13 kcal/mol for the
cyclization may be predicted based on the previous arguments. The upper limit
of such an enthalpy difference is suitable to allow for potential study of forward
and reverse reactions under reasonable conditions and, ultimately, observation
of torquoelectronic effects in a higher substituted system such as illustrated in
Figure 2-7. Due to a straightforward synthetic route available into these
perfluoroalkenyl substituted aromatics, 40 was prepared for thermal study.


F CF
F FF CF3 F F F
F F
F FF F F
CF


42 43 44

Figure 2-7. Unsymmetrically Substituted, Phenanthrene Annulated, 1,3(Z),5-
Triene/1,3-Cyclohexadiene Interconversion for Torquoelectronic Study.

Synthesis of 9.10-Bis(trifluoroethenyl)phenanthrene (40)

9,10-Bis(trifluoroethenyl)phenanthrene (40) was synthesized in four
steps (Figure 2-8) and isolated in a 1% overall yield. The first two steps
involved literature procedures; bromination63 of phenanthrene (45) to yield 9-
bromophenanthrene (46, 90%), then nitration64 to yield 9-bromo-10-
nitrophenanthrene (47, 13%). The actual yield of 47 from this reaction was
higher. The isolated yield reflects some difficulty in obtaining this material pure
from the other major nitration product, 9-bromo-3-nitrophenanthrene. The next
step involved nucleophillic attack of iodide on the brominated C9 of 47 to yield







Br O0N Br

P Br2 HN03, CH3CO2H
A, CCl4 (CH3CO)20,A -
45 46 (90%) 47 (13%)

ex. Nal, DMF, A
F FFF
NO2
F F
X- n CXZnCF=CF2 (X = I, -CF=CF2) /0 I/
-= cat. Pd(P(C6Hs)3)4, -
40(16%) Triglyme, 110C 48 (42%)

Figure 2-8. Synthesis of 9,10-Bis(trifluoroethenyl)phenanthrene (40).


9-iodo-O1-nitrophenanthrene (48, 42%). The best literature procedure found
for preparation of 48 involved five steps and produced the target in 1% yield
from phenanthrene.65 Our procedure is a significant improvement even in this
unoptimized state involving three steps and producing 48 in 5% isolated yield
from phenanthrene. Preparation of 40 at this point was rather fortuitous. A
Pd(P(C6H5)3)4 catalyzed coupling between iodo aryls and XZnCF=CFY (X = I or
-CF=CFY, Y = F or Z(E)-CF3) was carried out on 48.66 From the reaction
mixture, 16% of 40 could be isolated on average. It was intended to isolate 9-
nitro-10-trifluoroethenylphenanthrene by this procedure, which through
subsequent steps could be converted to 9-iodo-10-trifluoroethenyl-
phenanthrene. This material would then provide the desired 40, 42, and 44
through the appropriate coupling procedure. Preparation of 40 by this coupling
procedure with 48 was found to be reproducible over a number of runs.
Although 9-nitro-10-trifluoroethenylphenanthrene was tentatively identified as a
component in the reaction mixture by 19F NMR, it was never isolated. Having








40 in hand a few steps earlier than anticipated, a study of it's thermal chemistry
was initiated.

Thermal Study of 9.10-Bis(trifluoroethenvl)Dhenanthrene (40)

The thermolysis 40 was studied from 1400C up to 1930C as 0.1 M

solutions in C6D6. Upon thermolysis of 40, four products could be observed in
solution and were isolated pure by preparative GLPC for characterization.
Figure 2-9 illustrates the percent composition of all reaction components versus
time for thermolysis at 180.0C. The reaction is quantitative with regard to
formation of CisH8F6 structural isomers through 19 hours of thermolysis at
180.0C and after 235 hours, a 78% yield of C18H8F6 isomers is obtained as a
21:1 mixture of 1,2-(9,10-phenanthro)-3,5,5-trifluoro-4-trifluoromethyl-1,3-
cyclopentadiene (51) and 1,2,2,3,3,4-hexafluoro-2,3-dihydrotriphenylene (41).
To develop the scheme of overall transformations as shown in Figure 2-

9, it was necessary to thermolyze each of the intermediate materials; 41,
1,4,4,5,6,6-hexafluoro-2,3-(9,10-phenanthro)-bicyclo[3.1.0]hex-2-ene (49), and
4-difluoromethylidene-3,3,5,5-tetrafluoro- 1,2-(9,10-phenanthro)cyclopent-1 -ene

(50), alone under conditions which had been used for the parent triene 40.
Thermolysis of 41 at 180.0C as a 6:1 purified mixture of 41:51 led to non-
productive decomposition of 41 with no reaction of 51. Triene 40 was not
observed to be reversibly formed from the cyclized product 41, and neither 49
nor 50 were observed either. Thermolysis of purified 49 was observed at three
temperatures (180C, 185.0C, 192.50C) and found to form only 50 and 51.
The conversion of 49 to 50 and 51 is quantitative at the temperatures and
times observed. Figure 2-10 illustrates the percent composition of all reaction
components versus time for thermolysis of 49 at 180.00C. Thermolysis of 50,
as one can see at longer times in thermolysis of 40 (Figure 2-9, after 140 hours)








is observed to slowly form 51 in a near quantitative process. The possibility of
fluoride catalysis affording such a rearrangement was demonstrated by the
rapid conversion of 50 to 51 at 800C in the presence of added trace amounts of
CsF.
The possibility of fluoride catalysis effecting the reaction course of 40 at
early times was ruled out by thermolyzing a sample of 40 in DMF containing

CsF at 1150C. After 3.5 hours, 70% of 40 had been consumed and three other
products were observed. Analysis of this mixture by GLPC and 19F NMR
showed that this fluoride catalyzed reaction process and the thermal process
had no products in common and was not further investigated.
The disappearance of 40 and 49 were both found to follow first-order

kinetics, and the corresponding first-order plots are given in Figure 2-11 and

Figure 2-12 respectively. Upon thermolysis of 40 up to 85% conversion, the
ratio of (49+50+51)/41 was maintained at 4.42 0.19 at 180.00C, 5.03 0.11
at 184.50C, and 5.15 0.16 at 193.0C. Upon thermolysis of 49 through 70%
conversion, the ratio of (50/51) was maintained at 1.62 0.12 at the three
temperatures (180.00C, 185.00C, 192.50C) examined. As previously
mentioned, the reactions of 40 and 49 were found to be irreversible in
formation of their respective products. Assuming these reactions occurred via
irreversible, competitive first-order processes, rate constants for formation of 41
and 49 from 40, and 50 and 51 from 49 were obtained from the observed rate

constants for loss of 40 and 49, and the constant ratios of respective products
formed.67 Both the observed and separated rate constants for 40 are reported
in Figure 2-11, and for 49 in Figure 2-12.
The activation parameters (AH*, AS*) for the individual processes were
kT A RTL S
obtained from the Eyring expression68, k = -e rearranged to the
h








form Ln(k/T) = -AH*/RT + ASt/R + Ln(k/h), where k = rate constant at absolute

temperature T, k= Boltzmann constant (1.381x10-23 J/K), h = Planck's constant
(6.626x10-34 Jxs), and R = ideal gas constant (1.9872 calmolxK). Linear least-
squares regression plots of Ln(k/T)versus 1/T yielded AHt and ASt from the

slope and intercept respectively of the fitted line for each system. Fit of the
separated rate constant k41 (Figure 2-13) at the three observed temperatures
yielded AHt = 29.9 0.1 kcal/mol and ASt = -19.6 0.3 cal/molxdeg, and for

k49 (Figure 2-13) yielded AH* = 34.4 2.8 kcal/mol and AS* = -6.6 6.0
cal/molxdeg. Fit of the separated rate constant kso (Figure 2-14) yielded AH* =
31.3 0.4 kcal/mol and ASt = -12.7 0.9 cal/molxdeg, and for k5s (Figure 2-14)
yielded AH* = 31.4 0.1 kcal/mol and AS* = -13.4 0.1 cal/molxdeg.

Quantitative thermal studies of fluorinated 1,3,5-trienes have no
precedent in the literature, and the formation of bicyclo[3.1.0]hex-2-ene ring
structures from 1,3,5-triene thermolyses have no precedent in fluorocarbon
literature. There has been but one such case observed in the hydrocarbon
literature, and this will be discussed later. The novelty of the chemistry
presented above, along with the general absence of thermal studies of
fluorinated 1,3,5-trienes, led to examination of the system which will be
discussed next.

Thermal Study of an Acyclic Perfluorinated 1.3.5-Triene

The unexpected thermal results obtained in the case of 40 led to our
questioning as to whether an acyclic perfluorinated 1,3(Z),5-triene would also
undergo bicyclo[3.1.0]hex-2-ene ring formation in preference to the thermal
disrotatory 6K electrocyclic process. At the time this project began, the literature

contained only one relevant reference. Perfluoro-1,3,5-hexatriene had been
reported to afford an 84% yield of perfluoro-1,3-cyclohexadiene upon pyrolysis












100





75



.2
g 50
E
0



25





0.


0 50 100 150 200


Hours


F F



40


F F
F F F F
F F F F




49 50


\ /


41 51

Figure 2-9. Thermolysis of 9,10-Bis(trifluoroethenyl)phenanthrene (40) at
180.0C as a Solution in C6D6.


250













100
--- 49







05
.............. 5 0

75 ....-o.... 5 1





50






2 5 -. .. ... .... . . .. 0
S----- ....-o


.o--' *-- '


0 5 10 15

Hours

FF

F F
F F





CF3

/ \ / CF3

49 F F




51

Figure 2-10. Thermolysis of 1,4,4,5,6,6-Hexafluoro-2,3-(9,10-phenanthro)-
bicyclo[3.1.0]hex-2-ene (49) at 180.0C as a Solution in CeD6.
























0 45000 90000 135000 180000 225000 270000
Seconds



T (oc) k40 (x10s sec-1) k41 (x10s sec-1) k49 (x105 sec1)

180.0 0.991 0.12 0.183 0.808

184.5 1.55 0.01 0.257 1.29

193.0 2.93 0.06 0.476 2.45


S50+ 51


Figure 2-11. First-Order Plots and Rate Constants for Loss of 9,10-
Bis(trifluoroethenyl)phenanthrene (40) and Derived First-Order Rate Constants
for Formation of 1,2,2,3,3,4-Hexafluoro-2,3-dihydrotriphenylene (41) and
1,4,4,5,6,6-Hexafluoro-2,3-(9,10-phenanthro)bicyclo[3.1.0]hex-2-ene (49).
























0 10000 20000 30000 40000 50000 60000


Seconds


k5o


Figure 2-12. First-Order Plots and Rate Constants for Loss of 1,4,4,5,6,6-
Hexafluoro-2,3-(9,10-phenanthro)bicyclo[3.1.0]hex-2-ene (49) and Derived
First-Order Rate Constants for Formation of 4-Difluoromethylidene-3,3,5,5-
tetrafluoro-1,2-(9,10-phenanthro)cyclopent-1-ene (50) and 1,2-(9,10-
Phenanthro)-3,5,5-trifluoro-4-trifluoromethyl- 1,3-cyclopentadiene (51).


T (C) k49(x105 sec:') kso(x 05 sec-1) ksi (x105 sec-1)

180.0 1.96 + 0.02 1.21 0.751

185.0 2.92 + 0.01 1.81 1.11

192.5 5.14 + 0.08 3.17 1.97









-16.5


-17


-17.5


-18.5


-19


-19.5---
2.14E-03


Figure 2-13. Eyring Plots for Separated Rate Constants k41 and k49.


-16
a 50

51
-16.5




S-17


Figure 2-14. Eyring Plots for Separated Rate Constants kso and ks1.








in a flow system at 4500C.69 At this temperature, it is questionable whether this

is the primary process involved in this system. A lower temperature, more

quantitative study of an acyclic perfluorinated 1,3,5-triene was required.

A system whose synthetic approach potentially allowed for adaptation

into a variety of terminally substituted perfluoro-1,3,5-trienes was perfluoro-

E,Z,E- (and E,E,E-)4,5-dimethyl-2,4,6-octatriene (Z-56(E-56)). This system had

been previously synthesized and Figure 2-15 illustrates the chemistry

involved.70 This procedure was repeated and the isolated yield and product

composition obtained (60%, E,Z,E:E,E,E= 1.17:1) were similar to those reported

(68%, E,Z,E:E,E,E= 1.63:1).

Initially, an attempt was made to synthesize perfluoro-3,4-dimethyl-1,3,5-

hexatriene (58) by similar methodology; a process which had been discussed


F !CF3 Cd, DMF, RT F CF3
>= 30 min.
I F ICd F


CF3 F F CF F CF3
FI F
CF3 CF3 F DMF, RT, 1 Hr
56(60/%)
E,ZE/E, E = 1.17


CuBr, DMF, RT F = F
1 Hr CU' F
Cu F
54
CF3 -=-CF3 /


Cu\

CF3


F F F F

F F
CF3 CF3
58 (-5%)


F F

DMF RT 1 Hr
DMF, RT, 1 Hr


Similar to Above
SFP F
Cu JF

CF3 C7 F3


*Perfluoro-Z-1-iodopropene (52) was synthesized in four steps from perfluoropropene.66

Figure 2-15. Synthesis of Perfluoro-E,Z,E(E,E,E)-4,5-dimethyl-2,4,6-octatriene
(Z-56(E-56)).







only to the point of the chain extended copper reagent 57 in the reference.
Following the reaction progress by 19F NMR, formation of 57 was observed, but
addition of a further equivalent of iodotrifluoroethylene afforded a 5% or less
yield of 52 in the reaction mixture, which for the purpose of this project was
synthetically useless. Not having obtained useful quantities of 58, the synthesis
of 56 was repeated and it's thermal chemistry investigated.

The thermolysis of 56 was studied from 1490C to 2020C as a 0.17 M
solution in n-pentane. Due to difficulty in separation of the triene E,Z,E and
E,E,E isomers, the typical synthetic mixture of isomers (E,Z,E:E,E,E ratio of
1.17:1) was used in all studies. At temperatures around 1500C, E,Z,E->E,E,E

isomerization was observed to occur. After longer times, one new product was
evident, being identified by 19F NMR in the reaction solution as a mixture of
perfluoro-cis(and trans)-1 ,3,4-trimethyl-4-(E-1 -propenyl)cyclobutene (59).
Figure 2-16 shows the percent composition versus time for thermolysis of 56 at
1540C and then at 2020C. E,Z,E--E,E,E isomerization is observed initially

followed by appearance of 59 after 50 hours at 154C. Raising the temperature
of the sample to 202C sets up an equilibrium mixture of E-56/59/Z-56 =
11.4/7.6/1. Thus, this system represents a second example where a highly
fluorinated 1,3(Z),5-triene underwent thermal chemistry other than the 6n
disrotatory formation of a 1,3-cyclohexadiene. Since the thermal 6r closure
seemed to be disfavored, we were interested to see whether the photochemical
6x conrotatory formation of 1,3-cyclohexadienes would occur in these
fluorinated systems.

Photochemical Rearrangements of 9.10-Bis(trifluoroethenyl)phenanthrene (40)
and Perfluoro-E.ZE(EE.E.E)-4.5-dimethyl-2.4.6-octatriene (Z-56(E-56))

Upon photochemical excitation, ground state 1,3,5-trienes may undergo





















0o 0-

g 40- \ ...----........
0




20-





0 p:
0 50 100 150 200 250 300

Hours






CF3 F F CF3
F F
CFF CFC
Z-56 F CF3
F
CF3 F
CF F F3C F
CCF3
F F F CF3
CF359
F CF3
E-56

Figure 2-16. Thermolysis of Perfluoro-E,Z,E(EE,E)-4,5-dimethyl-2,4,6-
octatriene (Z-56(E-56)) as a Solution in n-Pentane.








x-n* transformation to singlet excited states.71 These singlet excited states are

biradical in character and create a symmetry change in the 1,3,5-triene HOMO

from symmetric in the ground state to antisymmetric in the singlet state.
Woodward and Hoffman recognized that concerted photochemical 1,3,5-triene

cyclizations and 1,3-cyclohexadiene ring openings occurring by conrotatory

processes are an artifact of this change in the HOMO symmetry.34 More recent
theoretical study also recognizes a preferred photochemical conrotatory mode
of reaction.72 Experimentally it is found that the process occurs

stereospecifically in a conrotatory fashion as shown in Figure 2-17.73


CH3 CH3
HaC CH3 Hn3C CH3
I HAC
H3C CH3 H3C
CH3 CH3
C6H5 C6H5
CH3

'CH3
C6H5H
CeHs

Figure 2-17. Photochemical 67 Conrotatory Processes.


Simple observation of 1,3(Z),5-triene/1,3-cyclohexadiene photo-
processes are complicated in many cases by other reaction pathways available
to the high energy excited state. Figure 2-18 illustrates the variety of products
which have been observed in various 1,3,5-triene/1,3-cyclohexadiene
photochemical systems.74 A variety of factors are important in dictating the
product distribution and will be discussed later.




















Figure 2-18. Photoproduct Variation in 1,3,5-Triene/1,3-Cyclohexadiene
Systems.


Having not observed the thermal 67r disrotatory process to any significant

extent in the cases of 40 and Z-56 at the temperatures studied, it was of interest
to determine if the photochemical 67 conrotatory process would occur. A
question of the relative ability of the two 6r transition states (thermal disrotatory

versus photochemical conrotatory) to accommodate fluorine emerged, and a
qualitative study of the two systems photo-processes was undertaken.



Photolysis of 9.10-Bis(trifluoroethenyl)phenanthrene (40)

The photolysis of 40 was studied as a 0.05 M solution in n-pentane and
the samples were irradiated through Pyrex at room temperature by a heated-
cathode, low pressure mercury lamp. Four products are observed to arise from
the photolysis of 40. Figure 2-19 shows the percent composition of all reaction
components versus solution irradiation time. As found in the thermolysis of 40,
the bicyclo[3.1.0]hex-2-ene 49 is the major product from photolysis of 40. The
67 electrocyclic product 41 is formed in small and relatively constant
concentration throughout the photolysis. Two other [2,+2n] type products,








1,4,5,5,6,6,-hexafluoro-2,3-(9,10-phenanthro)bicyclo[2.1.1]hex-2-ene (60) and

1,4,5,5,6,6-hexafluoro-2,3-(9,10-phenanthro)bicyclo[2.2.0]hex-2-ene (61) are
formed roughly in a ratio of 4.5(60):1(61) and account for 30% of the reaction
mixture after 93% conversion of 40. Independent photolysis of pure 41 under
similar conditions used for 40 led to formation of 61 in low yields; one hour of
photolysis of 41 yields 53% 41 remaining and 28% of 61 formed with a 19%
decrease in mass balance. Photolysis of 41 for two hours leads to decrease in
amounts of all materials and 60% reduction in mass balance. Reversibility of
41 back to triene 40 was not observed to any extent in these studies. Overall
photolysis of the parent triene is quite clean and after 21 hours with the
aforementioned light source, an 11% decrease in mass balance is observed
with 8% 40 remaining. Some extent of polymerization is occurring under these
conditions and is revealed by a small amount of solid white film appearing on
the walls of the photolysis vessel.
Attempts to isolate 60 and 61 failed. The preparative packed column
GLPC conditions necessary to elute these phenanthrene derivatives led to
decomposition of 60 and 61. It was found that thermolysis of a benzene
solution containing 49, 60, and 61 at 1840C for 83 minutes led to a mixture
containing only 41, 49, 50, and 51 with no 60 or 61 remaining and no new
products evident. This process was not quantified and other attempts at
isolation of 60 and 61 by TLC failed as the isomers could not be separated by
this technique.

Photolysis of Perfluoro-E.Z.E(E.E.E)-4.5-dimethyl-2.4.6-octatriene (Z-56(E-56)

The photolysis of 56 was studied as a 0.17 M solution in n-pentane and
the samples were irradiated through Pyrex at room temperature using a heated-
cathode, low pressure mercury lamp. Two products were observed and








isolated upon photolysis of 56. Figure 2-20 shows percent composition of all
materials versus solution irradiation time. Perfluoro-trans-2,3,5,6-tetramethyl-
1,3-cyclohexadiene (62) is the major photoproduct and is accompanied by
triene E,Z,E->E,E,E C4-C5 double bond isomerization. Perfluoro-trans-2,3,5,6-

tetramethylbicyclo[2.2.0]hex-2-ene (63) is formed in the reaction mixture after
longer times and in small quantity when photolysis is carried out through Pyrex.
Photolysis of pure 62 through quartz under similar conditions used for 56 leads
exclusively to 63, and ring opening to 56 is not observed to any extent.
Attempts to obtain Diels-Alder adducts between 62 and dimethyl
acetylenedicarboxylate or N-phenyltriazoline dione in n-pentane at
temperatures up to 200C showed no reaction and 62 was also found to be
stable at 2020C in n-pentane for 18 hours showing no rearrangement or
decomposition.

Discussion

The experimental data which have been presented point to an unique
disparity in the thermal and photochemical processes occurring in
perfluorinated 1,3,5-triene systems. While the observed photochemical
products will be shown to have related precedence in hydrocarbon 1,3,5-triene
transformations, the thermal rearrangements show little resemblance to those of
analogous hydrocarbons. It is recognized that upon such a drastic change as
perfluorination, anticipating similar results in these systems as those seen for
hydrocarbons is a dangerous assumption. As mentioned earlier, our motivation
for study of these perfluorinated systems was that they were easiest to obtain
synthetically while offering the widest variety of substitution possibilities, and
related precedence had been set in thermal studies of the 4R perfluorinated 1,3-

diene/cyclobutene system. The results from photochemical and thermal studies






46


100-
-- 40 -*--- 60

-.....-.-.- 41 --- --- 61

.... ---.... 49
75-





S50- ....
0








0-I .... ...-...;."........-...--..-;.."
0* ---" --. ---...........-...., ....- ..........-..... .. ........... ... ..............

0 5 10 15 20
Hours

F
F FF FF F \ F FF FF
F F
F F F F F F





49 / 40 \
F F F F F FF
49 4060



F F F- F




41 61

Figure 2-19. Photolysis of 9,10-Bis(trifluoroethenyl)phenanthrene (40) as a
0.05 M Solution in n-Pentane by Low Pressure Mercury Lamp through Pyrex.











100
---- Z-56 ----o---- 62
Pyrex Quartz
------ E-56 --*--- 63


75..**





d 50Is

25-



0 a ,

0



0 10 20 30 40 50
Hours





F CF3 F CF3

F3 F 3 FC3 CF F3 F
F F 'IOilF F CF3
F F 'ICF
CFY F3C FaC: F FaC
SF F F
F3C F F" CF3 F3C
E-56 Z-56 62 63

Figure 2-20. Photolysis of Perfluoro-E,Z,E(E,E,E)-4,5-dimethyl-2,4,6-octatriene
(Z-56(E-56)) as a 0.17 M Solution in n-Pentane by Low Pressure Mercury
Lamp.








of perfluoro-E,E,E(E,Z,E)-4,5-dimethyl-2,4,6-octatriene (Z-56(E-56)) and 9,10-
bis(trifluoroethenyl)phenanthrene (40) will now be discussed in turn.

Discussion of the Photochemical Studies

Early studies in the photochemistry of Vitamin D and it's many isomers
set the precedent for the possible complexities of 1,3,5-triene photochemistry.74
Previously, Figure 2-18 illustrated some of the types of products which have
been observed elsewhere in 1,3,5-triene photochemistry.
It is generally accepted that 1,3,5-triene photoproduct composition can
be directly related to the ground state conformational distribution of the system
by a principle known as the Non-Equilibration of Excited Rotamers (NEER).74
This principle states that in the excited state, an enhanced barrier for rotation
exists about the bonds which in the ground state are single bonds. Due to it's
limited lifetime, the acquiring of sufficient thermal energy by the excited state to
overcome such a barrier is unlikely. In effect, the photoproduct composition
should reflect a quantitative view of the ground state conformational equilibrium
which was irradiated. As illustrated in Figure 2-21, the ground state
conformations of Z-1,3,5-hexatriene will upon irradiation, lead in a specific
fashion to NEER dictated products.74
Although a seemingly trivial concept, the NEER principle turns out to
have good predictive value. Methods such as 1H NMR and UV spectroscopy
have been used to establish the preferred conformation in some hydrocarbon
1,3,5-trienes such as 64,75,76 and 65-67,74.77 and the corresponding 254 nm
primary photoproducts are shown in Figure 2-22. Absolute proof of an
mechanistic rationale is difficult and more so in the case of NEER because of
the number of relevant untested variables and alternate mechanistic proposals.
The possibility of equilibration of excited conformers has been addressed and





tZc
A-,


/H
I I


I
0


I


Figure 2-21. Three "Planar" Conformations of Z-1,3,5-Hexatriene and
Corresponding Photoproducts.


Major
C
(H3C)3C-(- C(CH3)3---
64
cZc


Product Distribution


-0-
65
cZc


66
tEt

67
cZt


Figure 2-22. Examples of 1,3,5-Triene Preferred Ground State Conformations
and 254 nm Primary Photoproduct Distribution.


I.....


Minor


j


P


xEI

Xd


-0, 1 *- (J-







dismissed by a study of the influence of wavelength on the photoproduct

distribution at low percentage conversion.74,78

The preferred ground state conformation of acyclic 1,3,5-trienes changes

considerably from the hydrocarbon to perfluorocarbon systems. A variety of
studies (UV and photoelectron spectra79, theoretical consideration60,80) point
toward perfluoro-1,3-butadiene (75) existing in a cis-skew structure with torsion
angle 0 = 420 (Figure 2-23), whereas 1,3-butadiene (73) exists in a planar trans

conformation with 0 = 1800. A significant hypsochromic shift is observed as the

series of fluoroethylenes (68 72) is transversed. This trend is also observed

between 73, 74, and 75, but here along with photoelectron spectral data, is


Xmax (nm) Emax (L/molxcm) 0 (deg)
68 H2C= CH2 165 10,000

69 H2C=CHF 167 10,000

70 H2C--CF2 165 7,900

71 FHC=CF2 162 6,800

72 F2C CF2 139 11,370 -


73 210 22,300 1800

/CF2
74 C2 200 19,500 1800
F2C 0

75 -CF 162 6,800 420
F2C 2

References: 6821, 69-7281, 7382, 7479, 7582

Figure 2-23. UV Data for Fluoroethylenes and Fluorobutadienes.








proposed to be due to non-planarity in 75 relative to 73 and 74.79 Perfluoro-

1,3-butadiene (75) is observed to have a UV spectrum identical with HFC=CF2

(71), further indicating there is little interaction between the C1-C2 and C3-C4 7

systems. This non-planarity is displayed by models where a disadvantageous

interaction is observed between fluorines on C1 and C3, and C2 and C4 in the

planar trans conformation.83 Hypsochromic shifts attributed to non-planarity

have been observed in other non-planar polyenes such as 2,3-di-t-
butylbutadiene, where ,max = 186 nm.84

Perfluoro-E-1,3,5-hexatriene (E-76) has been subjected to theoretical

conformational study at the ab initio level.60 Two torsional angles (81, 02) exist

in this molecule and local minima were located for two structures with syn and

anti relations of the pendant alkenes relative to the plane formed by C2-C3-C4-

C5. Figure 2-24 illustrates the conformers and the relative energies calculated.

The lowest energy conformer of E-76 was found to be a syn-skew structure as

is observed with perfluoro-1,3-butadiene (Figure 2-23, 75).


e1

1

1

1




Figure 2-24. Relative Calc
76).


F F F



F F F
E-76

(deg) 01 (deg) AE (kcal/m

80 180 3.10

46 -146 2.29

46 146 2.01

53 -53 0.10

52 52 0.00

ulated Minima for Perfluorc


ol)










>-E-1,3,5-hexatriene (E-


_.___







Photochemical Study of Perfluoro-EZ. E-(andE.E.E)4.5-dimethyl-2.4.6-
octatriene (Z-56(E-56))
Photolysis of perfluoro-E,Z,E-4,5-dimethyl-2,4,6-octatriene (Z-56) and
perfluoro-E,E,E-4,5-dimethyl-2,4,6-octatriene (E-56) yielded only two products;
perfluoro-trans-2,3,5,6-tetramethyl-1,3-cyclohexadiene (62) and perfluoro-
trans-1,2,3,4-tetramethylbicyclo[2.2.0]hex-2-ene (63) in good yield. The overall
transformation is illustrated in Figure 2-25. Triene E-56 has two possible
modes of reaction available by NEER type reasoning; 4n disrotatory ring

closure or C4-C5 double bond isomerization. The lowest energy conformer for
E-56 will most likely be similar to that of E-76, a skewed non-planar structure.
The 4n disrotatory photo-process leading to a 3-propenylcyclobutene would
require a tEc triene conformer which would be disfavored due to a repulsive
interaction between C2 fluorine and C5 trifluoromethyl groups. Therefore, the
observed process for E-56 is E,E,E--E,Z,E isomerization about the C4-C5


F CF3 F CF3
1 I FF
FaC F FC FaC CF3 F3 FF
F F F IF F CF
F F C1CF3
S CF3 F3C FC F F3C
F3C F F CF3 FaC
E-56 Z-56 62 63

Figure 2-25. Photolysis of Perfluoro-E,Z,E(E,E,E)-4,5-dimethyl-2,4,6-octatriene
(Z-56(E-56)).


double bond to form Z-56. Cis triene Z-56 is found to undergo only 67
conrotatory closure to yield 62. The lowest energy conformer of Z-56 most
likely involves skewing of the pendant E-perfluoropropenyl groups relative to
the C3-C4-C5-C6 plane, but whether it exists cis or trans skewed about the C3-
C4 and C5-C6 single bonds is unknown. It is noticed that a cis skewed







structure of Z-56 (Figure 2-26) is perfectly aligned with minimal repulsion to
undergo allowed 61 conrotatory bond formation between C2 and C7 leading

directly to 62.
















Figure 2-26. Cis-Skewed Conformer of Perfluoro-E,Z,E-4,5-dimethyl-2,4,6-
octatriene (Z-56).


Perfluoro- trans-2,3,5,6-tetramethyl-1,3-cyclohexadiene (62) undergoes a
further formal 4x disrotatory closure to form perfluoro-trans-1,2,3,4-tetramethyl-
bicyclo[2.2.0]hex-2-ene (63) in preference to ring opening to perfluoro-E,Z,E-
4,5-dimethyl-2,4,6-octatriene (Z-56). Using experimental results and NEER
type reasoning, it has been established for hydrocarbon photochemical 1,3-
cyclohexadiene processes that the preferred ground state conformation of the
system will control whether the 6r conrotatory ring open 1,3(Z),5-triene or 47r
disrotatory ring closed bicyclo[2.2.0]hex-2-ene is observed.85 Studies have
shown that planar or half-boat type conformers (Figure 2-27) undergo
disrotatory closure to bicyclo[2.2.0]hex-2-enes and half-chair type conformers
prefer conrotatory ring opening to 1,3(Z),5-trienes.









Planar Half-boat


bicyc
bicyclo[2.2.O]hex-2-ene


<:7-


Half-chair


Half-chair


Z-1,3,5-Triene


Figure 2-27. Conformationally Controlled Photo-processes of 1,3-
Cyclohexadienes.

With this precedent, it is surprising that cyclohexadiene 62 does not
undergo ring opening. Model studies83 indicate the most favored conformer of
62 (Figure 2-28) is one in which trifluoromethyl steric repulsions are minimized
in a half chair ring orientation with di-pseudoaxial C5 and C6 trifluoromethyl
groups and a C1-C2-C3-C4 torsion angle larger than that found in 1,3-
cyclohexadiene.


0


Figure 2-28. Favored Half-Chair Conformer of Perfluoro-trans-2,3,5,6-
tetramethyl-1,3-cyclohexadiene (62).


11 _


.4


\^


*0'3







The photochemical results obtained with fluorinated triene 56 are
consistent with the few other fluorinated examples found in the literature.
Recently, it has been reported that perfluoro-Z-1,3,5-hexatriene (Z-76)
undergoes double bond isomerization and 4n and 6K photoclosures (Figure 2-
29) to yield a mixture of perfluoro-E-1,3,5-hexatriene (E-76), perfluoro-1,3-
cyclohexadiene (77) and perfluoro-3-ethenylcyclobutene (78).86 At low percent
conversions of Z-76, only E-76 and 77 are identified in the reaction mixture.
When the reaction is carried out to completion a mixture containing 25% 77,
40% 78, and 35% perfluoro[2.2.0]bicyclohex-2-ene (79) is obtained.
An inconsistency exists in that the formation of the perfluoro-3-
ethenylcyclobutene (78) ring structure was not observed in the case of 56. As
already discussed, E-56 will not populate the tZc conformation for steric
reasons and undergoes E,EE-EE,Z,E isomerization only. It is difficult to see
why Z-56 is not observed to form 47 cyclization products as are observed in the



> II/ F
Fe- F8 FTe F8

77 79
E-76 Z-76
\ /


78

Figure 2-29. Photoproducts Obtained from Perfluoro-Z-1,3,5-hexatriene (Z-76).


case of Z-76. Models indicate the cZt conformation of Z-56 may exist as it
must for Z-76 with little crowding of the trifluoromethyl substituents. A possible
explanation arises if one assumes only the fluorinated E-trienes are undergoing








the 4x cyclization. In this case, cEt E-76 can undergo 4n closure or E-4Z

isomerization, whereas tEt E-56 affords only Z-56. It is unproved whether such

an argument applies, but if so, it would be unique to these perfluorinated 1,3,5-
trienes as both E and Z hydrocarbon 1,3,5-trienes are observed to form 3-
alkenylcyclobutenes as primary photoproducts (Figure 2-22).

Photolysis of perfluoro-1,3-cyclohexadiene (77) using a low pressure
mercury lamp has been previously reported to quantitatively yield 79 as the

sole photoproduct.87 In contrast to the previous fluorinated 1,3-cyclohexadiene

results, photolysis of perfluorotricyclo[6.2.2.02,7]dodeca-2,6,9-triene (Figure 2-
30, 80) yields bicyclo[3.1.0]hex-2-ene type isomers 82 and 83. These products

are proposed to be originating from the ring opened triene 81, a species which

was never observed in this study.88 Due to the rigidity of the bicyclohexene ring
structure of 80, the cyclohexadiene ring will be very nearly planar. The

formation of 1,3,5-hexadiene 81 goes counter to the NEER predicted product

for this system, which would be formation of a bicyclo[2.2.0]hex-2-ene (Figure 2-
27) type ring structure.





F4 F4
SEF14 1
/ 182
80 -1 14
a-i 1 +


Figure 2-30. Photolysis of Perfluorotricyclo[6.2.2.02,7]dodeca-2,6,9-triene (80).








Photochemical Study of 9.10-Bis(trifluoroethenyl)phenanthrene (40)

Photolysis of 9,10-bis(trifluoroethenyl)phenanthrene (40) led to formation

of four structural isomers; 41, 49, 60, and 61 as illustrated in Figure 2-31.


F
F F F F F F F

F F F F



49 40 60

F F F F
F F F F



41 61

Figure 2-31. Photolysis of 9,10-Bis(trifluoroethenyl)phenanthrene (40).


9,10-Bis(trifluoroethenyl)phenanthrene (40) was observed by 19F NMR to

exist as a pair of torsional diastereomers with a substantial energy barrier to

interconversion, a discussion of which has been published.89 The 19F NMR

spectrum of 40 at 250C showed signals corresponding to two types of non-

equivalent trifluoroethenyl groups. Such a spectrum is believed to arise from a

substantial thermal barrier due to restricted rotation between conformers
involving an syn and anti relationship of the trifluoroethenyl substituents relative
to the plane of the phenanthrene ring. Observation of the 19F NMR spectra of

40 over the temperature range of -170C to 84C and application of classical

theory with respect to equilibrium and NMR spectra allowed an estimation of
AG* = 15 kcal/mol for interconversion of the isomers. Molecular mechanics








calculations estimated AHORxn = 0.7 kcal/mol for anti-40-+syn-40 with AH* =

16.01 kcal/mol.90 Local minima for syn (syn-40) and anti (anti-40) type

conformers were located (Figure 2-32) with trifluoroethenyl torsional angles

relative to the aromatic ring plane (tZt structure) for the anti conformer of 50 and

-750, and for the syn conformer of 620 and 1190.


anti-40


K(syn/ant = 0.48, CDCl3, 25C


syn-40

Figure 2-32. Conformational Equilibrium of 9,10-Bis(trifluoroethenyl)-
phenanthrene (40).








The formation of the observed photoproducts from 40 has some
precedent from the hydrocarbon literature. Photochemical studies of 1,2-
diethenylbenzene (84, Figure 2-33) have been reported by a few authors.91,92.93
One study found that photolysis of 84 with a medium pressure mercury arc
through Pyrex yielded benzobicyclo[3.1.0]hex-2-ene (83) as the major product
in a low overall yield process (20% max) with smaller amounts of tetralin, 1,2-
dihydronaphthalene, and naphthalene observed arising from 2,3-
dihydronaphthalene (85).91 1,2-Diethenylbenzene-d4 with four terminal
methylene deuterons was also studied to establish a carbon skeletal
rearrangement via 86 to 83, disproving a mechanism involving hydrogen
migration.91


STetralin
S1,2-Dihydronaphthalene
Naphthalene
85 X

84

86 83

Figure 2-33. Photolysis of 1,2-Diethenylbenzene (84).


Photolysis of 9,10-diethenylphenanthrene has been reported and studies
under a variety of conditions did not lead to the observation of any cyclization.94
In this case, polymerization was the only process which was observed.
The photoproducts observed from 9,10-bis(trifluoroethenyl)phenanthrene
(40) are consistent with the above hydrocarbon system results and they can
potentially be seen as arising from a NEER dictated process. Figure 2-34
shows the proposed overall primary and secondary processes. Conformer
anti-40 can form three primary photoproducts; 41, 60, and 88.








Bicyclo[2.1.1]hex-2-ene 60 may be formed from the pendant alkenes reacting in
a photo-allowed 2,s+2,s cycloaddition and cyclohexadiene 41 may arise from

a 67 conrotatory electrocyclization. Initial formation of bicyclo[3.1.0]hex-2-ene

88 may occur through either the anti-40 or syn-40 conformers and the true
nature of the photochemical mechanism involving formation of

bicyclo[3.1.0]hex-2-enes has been the subject of much debate.


A

syn/anti 0.5


anti-40


hv


60 41

88


41 (2%)
I hv


60 (24%)


syn-40

hv


49 (48%)


61 (5%)

1


*All bonds to fluorine except for the 1 through 8 positions of the phenanthrene ring systems.

Figure 2-34. Primary and Secondary Processes and Final Product Distribution
for Photolysis of 9,10-Bis(trifluoroethenyl)phenanthrene (40).


Photochemical formation of bicyclo[3.1.0]hex-2-enes are formally
Woodward and Hoffman allowed 4,s+2na or 4,a+2,s processes.34 Establishing


61

S 88 j








the true nature of the mechanism requires labels with which to follow the
stereochemical course of the process. While there are examples of 4,s+2na
and 4,a+2ns photoisomerizations,95,96 there are also examples of disallowed
4,a+2na processes.97 Further, an explanation exists based on "cross-

bicyclization in linear conjugated polyenes" where the author's rationalization
allows for concerted 4,a+2na photo-processes.98 Other arguments have

involved a stepwise process which involves a concerted conrotatory closure of
the three membered ring followed by closure of the five membered ring,85,99 and
sudden polarization of the 1,3(Z),5-triene from a C3-C4 twisted diradical to a
charge separated zwitterion which undergoes closure.100,101,102
Primary photoproduct 88 was never observed most likely due to thermal
instability caused by strain in this spiro-fused system and loss of aromaticity
from the central phenanthrene ring. Under the reaction conditions, 88 most
likely undergoes a vinylcyclopropane-cyclopentene rearrangement to 49 as
rapidly as it is formed. Photolysis of 40 was attempted at -500C looking for 88
by low temperature 19F NMR but only 49 was observed. Lower temperature
studies were abandoned due to equipment difficulties. Bicyclo[2.2.0]hex-2-ene
61 may be formed by a photo-allowed 2,s+2,s cycloaddition from syn-40 or a
secondary process involving a 4n disrotatory electrocyclization of 41. This was

found to be occurring as photolysis of pure samples of 41 led in low yield to 61
and showed no reversibility to the parent triene 40.

Discussion of Thermal Studies

The initial assumption that a suitably tailored fluorinated 1,3(Z),5-triene
system would allow for probing of the torquoelectronic effect in triene 67 thermal

chemistry turned out to be incorrect in the case of the phenanthrene annulated
system. Investigation of the thermal rearrangements of 9,10-bis(trifluoro-








ethenyl)phenanthrene (40) and perfluoro-E,Z,E(and E,E,E)-4,5-dimethyl-2,4,6-
octatriene (Z-56(E-56)) revealed that rearrangement pathways for these
systems have little in common with the corresponding hydrocarbons.


Thermolysis of Perfluoro-E.Z.E(and EE.EE)-4.5-dimethyl-2.4.6-octatriene (Z-
56(E-56)
Thermolysis of the mixture of Z-56 and E-56 led to initial C4-C5 ZIE
double bond isomerization at temperatures above 1500C. The hydrocarbon
analog, Z-1,3,5-hexatriene, forms 1,3-cyclohexadiene with Ea = 29.3 kcal/mol,
while C3-C4 E to Z double bond isomerization requires temperatures in excess
of 2500C with Ea = 45.5 kcal/mol.57 Thus, the hydrocarbon Z-triene undergoes
the 67 electrocyclic process exclusively due to the AEa 16 kcal/mol difference

between it's concerted ring closure and it's C3-C4 double bond isomerization.
Although alkyl substitution can lower the Ea for C3-C4 double bond
isomerization of 1,3,5-hexatriene by approximately 6 kcal/mol57 and
perfluorination of 2-butene leads to a lowering of the E*->Z isomerization by 6.4

kcal/mol versus the hydrocarbon,103 it is inconceivable that the fluorination of Z-
56 will lead to lowering in energy of the C4-C5 double bond Z--E isomerization
so as to make this process exclusively preferred over the 6n ring closure.
The next process observed to occur is a 4n cyclization of triene 56. An

equilibrium ratio of 1.64:1 was established between 56 (E and Z) and 59 at
2020C in n-pentane as illustrated in Figure 2-35.
Perfluoro-1,3,5-hexatriene (76) has been reported in a patent to afford
an 84% yield of perfluoro-1,3-cyclohexadiene (77) upon pyrolysis at 4500C in a
flow system.69 Concurrent to this thesis project, a thermal study of perfluoro-
1,3,5-hexatriene at lower temperatures was reported.86 These authors offered
similar results to those obtained in our study of 56. Thermolysis of










CF3F F CF3
F: F
Z-56 F CF3

CCF F
CF3 F F) CF
C CF3

F CF3
E-56

Figure 2-35. Mixture Obtained from Thermolysis of Perfluoro-E,Z,E(E,E,E)-4,5-
dimethyl-2,4,6-octatriene (Z-56(E-56)) at 202.00C in n-Pentane.


perfluoro-1,3,5-hexatriene at 1600C established an equilibrium mixture

consisting of 9% perfluoro-1,3,5-hexatriene and 90% perfluoro-3-
ethenylcyclobutene (78). Thermolysis of perfluoro-1,3,5-hexatriene at 2200C
was reported to irreversibly yield perfluoro-1,3-cyclohexadiene.

In the fluorinated 1,3,5-triene systems, formation of 3-
alkenylcyclobutenes in preference to 1,3-cyclohexadienes upon thermolysis

leads to the conclusion that the energy surfaces for the hydrocarbon and

fluorocarbon systems are quite different. It is informative to observe the
enthalpy diagram (Figure 2-36)57 for the hydrocarbon C6H8 system. Formation
of 3-ethenylcyclobutene (89) by a 4n conrotatory electrocyclic process is not

observed in hydrocarbon 1,3,5-triene thermal studies. E-1,3,5-Hexatriene (36)
isomerizes to the Z-triene (37) which then undergoes the 6n ring closure to 1,3-
cylohexadiene (38). Both processes are preferred over the 47 cyclization.

Perfluoro-1,3,5-hexatriene is seen to form perfluoro-1,3-cyclohexadiene at
higher temperatures and it is likely that perfluoro-E,Z,E-4,5-dimethyl-2,4,6-
octatriene (Z-56) would have exhibited a similar reaction path had the system
been investigated at higher temperatures.








100-
>90.3
8 ............ .... .. 83.9
80-

69.6
AHo
(kcal/mol) 60- 57.5


89
40- 39.5 40.6

36 37 254

20- 3

Figure 2-36. Enthalpy Diagram for C6H8 Transformations.



The data obtained from these perfluoro-1,3,5-triene systems, although

qualitative in nature, require changes in the energy profile for fluorinated 1,3,5-

triene transformations (Figure 2-37). The thermal equilibrium rich in perfluoro-

3-alkenylcyclobutenes in the cases of perfluoro-1,3,5-hexatriene (76) and

perfluoro-E,Z,E(and E, E,E)-4,5,-dimethyl-2,4,6-octatriene (Z-56(E-56)) lead to

a lowering of the AHOF of the perfluoro-3-alkenylcyclobutenes relative to the

perfluoro-1,3,5-trienes. Such a change in the relative enthalpy has precedent

in the case of the earlier discussed fluorinated cyclobutene/1,3-diene

interconversions (Chapter One, Table 1-6 and Figure 1-12). In these examples,

it was found that the perfluorinated cyclobutenes were lower in energy than the

perfluoro-1,3-dienes with AAHOR on the order of 17 kcal/mol between the

hydrocarbon and fluorocarbon systems. Irreversible formation of perfluoro-1,3-

cyclohexadienes from the fluoro-1,3(Z),5-trienes only in the high temperature

runs leads to these species having the most negative AHOF of the isomers, but

with a raised energy barrier to the 6n disrotatory transition state.



















F j. F-Q


Figure 2-37. Enthalpy Diagram for Perfluoro-1,3,5-triene Transformations.

Thermolysis of 9.10-Bis(trifluoroethenyl)phenanthrene (40)

Thermolysis of 9,10-bis(trifluoroethenyl)phenanthrene (40) was found to
form the desired 1,2,2,3,3,4-hexafluoro-2,3-dihydrotriphenylene (41), but only
as a small component in a very complicated overall reaction process. A virtually
unprecedented process for 1,3(Z),5-triene thermal chemistry, leading to
bicyclo[3.1.0]hex-2-ene 49 and subsequent rearrangement of this species, was
observed to comprise the major reaction pathway of 40.

Thermal studies of 1,2-dialkenyl aromatics are only rarely encountered in
the literature. Any attempt to investigate the electrocyclic chemistry of such
systems would be expected to be complicated by the instability of the cyclized
non-aromatic products under the reaction conditions and the facility of these
products to undergo further rearrangement. Thermal studies of a series of 1,2-
dipropenylbenzenes has been reported.104 These systems first show pendant
alkene isomerization which was proven to occur by [1,7]-sigmatropic hydrogen
shifts. Higher temperatures yield 1,2-dihydronaphthalenes and alkyl 1,2-
dihydronaphthalenes which arise from disrotatory ring closure followed by an








[1,5]-sigmatropic hydrogen shift. The mechanistic nature of these processes
was established by studying suitably deuterium labeled species, and a
mechanistic rationale was proposed as seen in Figure 2-38.104



.ill 6n disrotatory

1,5]H [1,7]H

0 : 6n disrotatory.

[1,5]H [1,7]H1
6m disrotatory i f*c




6ic disrotatory. [1,5]H



Figure 2-38. Thermal Processes Observed for 1,2-Dipropenylbenzenes.


One instance of the thermolysis of 9,10-diethenylphenanthrene at 2100C
has been reported to yield 35% triphenylene, apparently arising from formation
of the unstable 2,3-dihydrotriphenylene, which undergoes loss of hydrogen.94
From the thermodynamic argument offered early in this chapter, it was
believed that 9,10-diperfluoroalkenylphenanthrenes offered a chance to
observe a reversible thermal 67r electrocyclization in a 1,3(Z),5-triene system.
The thermodynamics seemed favorable and the potential for relatively
unprecedented fluorine shifts nonexistent due to the greater strength of the C-F
versus C-H bond. In this light, it was initially discouraging to observe such a
complex mixture upon thermolysis of 40.








Thermolysis of 9,10-bis(trifluoroethenyl)phenanthrene (40) led to
formation of two primary products and two secondary products as illustrated in

Figure 2-39. One of the primary thermal products, 1,2,2,3,3,4-hexafluoro-2,3-

dihydrotriphenylene (41), was observed to be formed to no more than 15% in
the reaction mixture over the temperatures examined. Contrary to expectations,

thermolysis of purified 41 under identical conditions used for cyliization of the

parent triene demonstrated that the cyclization was irreversible. The cyclized


F F
F FF F F F F F


X-
40 49 50


F FF CF3
F
F F F F


41 51

Figure 2-39. Transformations Observed Upon Thermolysis of 9,10-
Bis(trifluoroethenyl)phenanthrene (40).


product 41 was predicted to be in an enthalpy range of 1 to -13 kcal/mol relative

to the parent triene 40. The fact that reversibility is not observed to any extent in
this system leads to the conclusion that this process is occurring with AHR < -5

kcal/mol. A higher temperature study of this material was not undertaken as

significant non-productive decomposition was found to be occurring at

temperatures used to study 40. As discussed earlier, having observed a good

fit to first-order theory for the loss of 40 and a constant ratio of (49+50+51)/41
over the individual runs examined, activation parameters of AHt = 29.9 0.1







kcal/mol and AS* = -19.6 0.3 cal/molxdeg were obtained for formation of 41
from 40. Such parameters for the formation of 41, being formally a 6r
disrotatory process from 40, are difficult to rationalize as no comparative kinetic
data exists for cyclization of any other fluorinated 1,3(Z),5-triene system.
Formation of 1,4,4,5,6,6-hexafluoro-2,3-(9,10-phenanthro)bicyclo[3.1.0]-
hex-2-ene (49) was observed to be the major process early in the reaction of
40. This thermal cyclization of a fluorinated 1,3(Z),5-triene to form a fluorinated
bicyclo[3.1.0]hex-2-ene is unprecedented in fluorocarbon literature and only
one example, accompanied by minimal discussion, has been found in the
corresponding hydrocarbon literature and is illustrated in Figure 2-40.10o As


NHC(CH3)3 NHC(CH3)3 NHC(CH3)3
hv A Aorhv
6 A Z-- 0

Figure 2-40. Thermal and Photoproducts of E-1-(t-Butylamino)-1,3(Z),5-
hexatriene.


discussed earlier, the activation parameters obtained for formation of 49 from
40 are AH* = 34.4 2.8 kcal/mol and AS* = -6.6 + 6.0 cal/molxdeg. The
unprecedented nature of this transformation does not allow for conjecture as to
the mechanistic significance of these values. Thus, potential processes
involved will be offered, with rationalization of the activation parameters left to
the future.
The thermal closure of a 1,3(Z),5-triene system to a bicyclo[3.1.0]hex-2-
ene ring is formally a Woodward and Hoffman allowed 47ta + 2=ca or 4is + 2xs
process.34 Determining the true stereochemical nature of the process requires
at a minimum, labels at each terminus of the reacting 1,3,5-triene system. Due
to the unprecedented nature of this thermal process, and the lack of







stereochemical labels in the aforementioned and 40 systems, the question of
symmetry conservation can not begin to be addressed. The formation of 49
from 40 involves a further complication as the necessary primary intermediate
88 (Figure 2-41) has still to undergo a vinylcyclopropane-cyclopentene
rearrangement to afford 49. Figure 2-41 offers a representation of the potential
processes involved in formation of 41 and 49. Direct 4xa + 27ta or 4sc + 2ns


/


anti-40


Figure 2-41. Mechanistic Rationale for Formation of 41 and 49.


cyclization of anti-40 could yield the primary intermediate 88. As previously
discussed, this material was also assumed to be a primary intermediate upon
photolysis of 40. Since this intermediate was not observed under the much
lower temperature photolysis conditions, it would certainly not be observable
under the thermolysis conditions. A vinylcyclopropane-cyclopentene







rearrangement must spontaneously occur as 88 is formed to relieve the strain
in this spiro-fused system and restore aromaticity to the central phenanthrene
ring. An alternate diradical process from anti-40 may occur with formation of
the intermediate 90. In this case, closure of the terminus of one trifluoroethenyl
substituent on the second at its carbon attached to the phenanthrene ring would
yield the biradical 90 which may then recombine to 88 and rearrange to the
observed product 49.
1,4,4,5,6,6-Hexafluoro-2,3-(9,10-phenanthro)bicyclo[3.1.0]hex-2-ene
(49) was observed to further rearrange under the thermolysis conditions to form
50 and 51 (Figure 2-39). Since 49 was unreactive with trace fluoride at lower
temperatures and since disappearance of 49 follows first-order kinetics and
yields linear Eyring plots over the temperatures and through the 70% extent of
reaction examined, a mechanism involving fluoride catalysis can effectively be
ruled out.
A hydrocarbon analog, bicyclo[3.1.0]hex-2-ene (91, Figure 2-42), is
known to undergo thermal C1-C5 cyclopropane bond homolysis to yield
biradical 92, in which 1,2-H shifts may occur in two possible directions leading
to the observed products 1,4- and 1,3-cyclohexadiene (93, 94).54






91 92 93 94


Figure 2-42. Thermolysis of Bicyclo[3.1.0]hex-2-ene (91).








Thermolysis of benzobicyclo[3.1.0]hex-2-ene (95, Figure 2-43) has been
studied by the flash vacuum technique.106 Over the high temperatures
investigated (500-9000C), it was found that the major primary product was






O ^7 FVT 96 (30%) 84 (5%)
S700oC '() + Naphthalene (12%)


97(8%) 98 (1%)

Figure 2-43. Flash Vacuum Thermolysis of Benzobicyclo[3.1.0]hex-2-ene (95).


1,2-dihydronaphthalene (96) with a variety of minor products (84, 97, 98) also
observed. These products were proposed as arising from homolytic cleavage
of the appropriate cyclopropane ring bond followed by a hydrogen shift (96, 97,
98) and electrocyclic process (84).
Gas and solution phase thermal rearrangement of
perfluorobenzobicyclo[3.1.0]hex-2-ene (100, Figure 2-44) has been reported in
a study addressing the reaction of perfluoroindene (99) with sources of
difluorocarbene.107.108 Perfluoroindene (99) was found to react with
difluorocarbene generated from thermolysis of hexafluoropropylene oxide
(HFPO). At lower temperatures, perfluorobenzobicyclo[3.1.0]hex-2-ene (100)
was found to be the major product with smaller amounts of
perfluorodihydronaphthalenes (101, 102) and perfluoro-2-methylindene (103)
also being observed. Thermolysis of 100 was also independently investigated.
Thermolysis of neat 100 at 230C yielded an 1:1:7 mixture of 99:102:103 and
thermolysis at 670C in a flow system yielded an 3:1 mixture of 99:103.









HFPO, 6700C


Fe Flo Flo Flo Flo
1HFPO oCQ r n r
190-1950C
99 100 (40%) 101 (15%) 102 (10%) 103 (5%)
2300C, 7 Hours
F8 Flo FIO



99(10%) 102(12%) 103(71%)

Figure 2-44. Reaction of Perfluoroindene (99) with Difluorocarbene and
Thermolysis of Perfluorobenzobicyclo[3.1.0]hex-2-ene (100).


Elimination of difluorocarbene6 can be a facile process in highly
fluorinated cyclopropanes and was observed in the thermal study of
perfluorobenzobicyclo[3.1.0]hex-2-ene (100), albeit at higher temperatures
than were involved in our study of 49. This process was revealed by formation
of significant amounts of perfluoroindene 99 upon thermolysis of 100. The
analogous compound in the case of 49 was never observed in any of the
thermolysis runs with 40 or 49. On these grounds, a mechanism involving
difluorocarbene can be ruled out.
Considering the above results, the observed thermal rearrangement of
49 to form 50 and 51 is not out of character. Figure 2-45 illustrates the
possible mechanistic route involved. Homolytic cleavage of only one out of the
three cyclopropane bonds in 49 is found to productively lead to products.
Cleavage of bonds a (104) or b (105) leads to biradicals which in both cases

















F
F F
F F


104


\c
X

F F
F.
F F


106


Figure 2-45. Mechanistic Rationale for Thermal Decomposition of 1,4,4,5,6,6-
Hexafluoro-2,3-(9,10-phenanthro)bicyclo[3.1.0]hex-2-ene (49).


have one benzylic center; the other being on a secondary (104) and primary
(105) carbon. Appreciable overlap with the C9-C10 phenanthrene n system

early in cleavage of bond b may also facilitate such a process relative to

cleavage of bond a or c. Geminal difluorination of cyclopropanes is known to

weaken the C-C bond opposite to the fluorinated site and those adjacent to the

fluorinated site, although the weakening of the opposite bond is significantly

greater.6 Such an effect may be thought to possibly lead to facilitation of

cleavage of bond a. Here this factor is unlikely to be significant since this
cyclopropane system also contains fluorine at sites (3 to the geminally

difluorinated site which would counter such a disparity in bond strengths. The








final observed products require a 1,2-fluorine atom shift from the intermediate
biradical. A 1,2-fluorine atom shift in the case of 104 will be a higher energy
process than one in 105 due to the cleavage of the stronger C-F bond at the
difluorinated site in the former. This fact together with the stereoelectronic
preference for bond b cleavage and benzylic stabilization in 105 leads to
observation of products arising only from cleavage of 49 to 105. The other
possible ring cleavage route cto form biradical 106 is disfavored as such a

system affords no benzylic stabilization. The final observed products 50 and
51 then arise from 1,2-fluorine atom shifts of Fa (Figure 2-45) in one of two
directions in 105. The slightly favored product 50 is formed by shift towards the
stabilized radical center.
Fluorine atom shifts lack unambigous precedent in the literature.
Thermal [1,3] and [1,5]-fluorine atom shifts have been invoked in studies
involving isomerizations of dihydrohexafluorocyclohexa-1,3-dienes'09 and
perfluoroisoindenes.110 Photochemical [1,5]-fluorine atom shifts have been
invoked in the isomerization of perfluoroindene to perfluoroisoindene"' and a
rearrangement of perfluorotricyclo[5.2.2.02,6]undeca-2,5,8-triene (107) to
perfluorotricyclo[5.2.2.02,6]undeca-2,3,8-triene (108).88




F12 F12




107 108


Figure 2-46. Photochemical [1,5]-Fluorine Atom Shift in Perfluorotricyclo-
[5.2.2.02,6]undeca-2,5,8-triene (107).







Steric Effects in Thermal 1.3(Z).5-Triene Electrocyclizations

The rule of orbital symmetry conservation for a concerted process
requires that C1-C6 bond formation in the thermal rearrangement of a 1,3(2),5-
triene to a 1,3-cyclohexadiene occur by overlap of n orbitals on the same side of
the triene plane; hence defining a disrotatory process. A consequence of this
type of process is that 1,3(Z),5-triene C1 and C6 cis substituents are forced into
a more crowded position in the transition state than in the ground state. It is
found that higher activation energies are required to bring C1 and C6 within
bonding distance in the transition state due to this crowding, and often the
energy of the 6n process will be sufficiently high so that alternate processes are
observed to varying extents.
Activation parameters for a variety of systems in the literature
demonstrate this steric effect and a few examples are given in Figure 2-47.
Many of the examples found (109,52 110,52 111112 112112) show little change in
activation entropy, indicating similar transition state geometry and timing is
being maintained, therefore, the strain being built into the transition state is
manifesting itself as a measurable increase in activation enthalpy. Sufficiently
hindered systems (11373) often do not undergo 67 cyclization from the initial

1,3(Z),5-triene, but rearrange by other processes such as hydrogen shifts to
trienes which are more suited for the disrotatory process.
An interesting result has been reported for the thermal study of 6n ring
closure in a C1 and C6 deuterated Z-1,3,5-hexatriene (Figure 2-48).113
Secondary isotope effects (kH/ko) of 1.05 for 1E,5E-D2 (114) and 0.88 for 1Z,5Z-
D2 (115) were observed. The two transition states involved are
diastereomerically related with respect to the deuterium substituents. Due to
the different stereochemical environments in the transition state, an increase in








intramolecular non-bonding interactions was proposed to increase the force

constants in the case of the terminally cis-deuterated material 115 and give rise

to the inverse kH/kD effect observed .


AH* (kcal/mol) ASS (eu)


28.6


109


110


111


112

CsH5

113

C6H5


28.2


C*c6H5
S j ~~H2CH3
C6Hs


Figure 2-47. Activation Parameters for Non-Hindered versus Hindered 1,3(Z),5-
Triene Cyclizations.


1

-D
114


115


Figure 2-48. 1 E,5E-D2 (114) and 1Z,5Z-D2-1,3(Z),5-Hexatriene (115).


1,3(Z),5-Triene cyclizations have received considerable theoretical
interest. In calculated transition states for the disrotatory 6r 1,3(Z),5-triene

electrocyclization, a definite steric crowding in is found between the C1 and C6







cis substituents.72,113,114,115,116 The transition state is a boat-like ring conformation
(Figure 2-49) with C1 and C6 cis hydrogens twisted in and separated by less
than the sum of their van der Waal's radii.












Figure 2-49. Approximate Representation of the Calculated 1,3(Z),5-Triene
Disrotatory Transition State.

Conclusions

It is believed that for the cases of the fluorinated 1,3(Z),5-trienes
examined in this study and found in the literature; 9,10-
bis(trifluoroethenyl)phenanthrene (40), perfluoro-E,Z,E-4,5-dimethyl-2,4,6-
octatriene (Z-56), and perfluoro-Z-1,3,5-hexatriene (76), significant repulsion
must develop between the C1 and C6 1,3(Z),5-triene cis fluorines upon
approach to the boat-like disrotatory transition state. This repulsion leads to an
increase in the energy barrier for this process and subsequently, increased
potential for occurrence of competing processes. The observation of
cyclobutene products from 4n conrotatory ring closure in these systems seems
reasonable in light of this increased barrier. Closure in a 4re conrotatory

manner is proposed to have a transition state which does not contain crowding
of the terminal cis substituents as illustrated in Figure 2-50.117.118 This together
with the observed reversal of the relative thermodynamics in the perfluorinated








and hydrocarbon cyclobutene/1,3-diene systems lead to 4- closures as facile

and primary processes in the thermolysis of fluorinated 1,3(Z),5-trienes.











Figure 2-50. Approximate Representation of the Calculated 1,3-Diene
Conrotatory Transition State.


An analogous process to the reported 67 thermal closure of perfluoro-Z-
1,3,5-hexatriene at higher temperatures was never observed in thermal studies
of perfluoro-E,Z,E-4,5-dimethyl-2,4,6-octatriene (Z-56) most likely because
sufficiently high temperatures were not used in the study of this material.
The fact that terminal cis fluorines impede the 1,3(Z),5-triene disrotatory
process should not be surprising in light of the previous discussions. It is
surprising though, in terms of the magnitude of the effect. The fact is that in
these fluorinated systems, 4r cyclization is favored over 6c, and occurring
roughly at temperatures necessary for the hydrocarbon 67 rearrangement. This

large deviation in the thermal chemistry of fluorocarbon from hydrocarbon
precedent upon terminal 1,3(Z),5-triene cis fluorination created the impetus for
further study of fluorine's effect on thermal processes involving disrotatory and
conrotatory transition states. Out of this interest, a strategy designed to provide
more insight to this seeming steric influence of fluorine was developed and it's
study is described in Chapter 3.













CHAPTER 3

[3,3]-SIGMATROPIC REARRANGEMENTS OF
TERMINALLY FLUORINATED 1,5-DIENES

Introduction

The observation that 67 thermal disrotatory closure of perfluorinated

1,3(Z),5-trienes is disfavored to such an extent that other primary processes
occur exclusively, is unprecedented and encouraged further attention. The
usual strong electronic influence of fluorine on a reaction process seemed in
the disrotatory process to be offset by an factor more steric in origin. As
discussed in Chapter 1, such influences are rarely responsible for the course of
reaction in fluorinated organic systems due to the small difference between
fluorine and hydrogen in size and bond length to carbon. Nevertheless, the
tightly bound nature of the concerted transition state which is required for
1,3(Z),5-triene disrotatory cyclization leads to a twisting in and crowding of the
terminal cis substituents which is believed to be at least partially responsible for
the observed deviation of perfluorocarbon chemistry from that of analogous
hydrocarbons. While precedent from the hydrocarbon literature indicated
destabilization of such a transition state by groups substantially larger than
hydrogen at the 1,3(Z),5-triene terminal cis positions, it seemed surprising that
upon substitution of hydrogen by fluorine at these positions, rather than
observing an higher energy disrotatory process, alternate rearrangements were
observed. It was then the intention to study another system with a lesser degree
of fluorination rearranging from a specific conformation which would allow a








more quantitative understanding of the effect terminal fluorination has on
pericyclic processes.

The [3.31-Sigmatropic Shift of 1.5-Dienes: The Cope Rearrangement

A hydrocarbon system which has been thoroughly scrutinized

experimentally and theoretically is the Cope Rearrangement. This process is
formally a subset of the family of sigmatropic shifts, which involve migration of a
a bonded atom or n system from one terminus of a conjugated n system to the

other in a concerted fashion. The all carbon [3,3]-sigmatropic shift was

discovered by Hurd119 and later by Cope120 and the most simple case involving
the degenerate rearrangement of 1,5-hexadiene is illustrated in Figure 3-1.


2 2
3 C N' 31 1

4 6 43 6
5 5

Figure 3-1. The Cope Rearrangement of 1,5-Hexadiene.


Orbital symmetry considerations dictate that the process occur in a [3s,3s]
fashion, meaning that bonding occurs from the same face at the terminus of
each three carbon fragment.34 Such a restriction still allows for the
rearrangement to occur through a number of viable conformations of the 1,5-
diene system. Among the conformations available to this system, it has been
experimentally demonstrated that the Cope rearrangement occurs preferentially
through a chair conformation transition state except in cases where the system
is geometrically constrained so as to make the chair inaccessible. A variety of
studies involving stereochemically labeled 1,5-diene systems have








demonstrated the exclusive nature of this process and estimate the chair is
favored over a boat conformation transition state by at least 6 kcal/mol in
enthalpy.54 Semiempiricall21 and more recent ab initio22,123 level theoretical
studies confirm by similar barriers the experimental preference found for the
chair over the boat transition state conformation. The energy difference and
hence the exclusive chair transition state for this process is rationalized as
being due to a through space, destabilizing antibonding interaction between
orbitals on C2 and C5 in the boat conformation.34,124 Although there is no
question as to the preferred conformation of the transition state, the aspect of
synchronicity or bond timing in this concerted process has been under
continuous debate. Such mechanistic finepoints will be addressed later in
discussion but at this point, only the aspect of favored conformation will be
considered.
The Cope system appears to be ideal for study of the effect of terminal
fluorination, as the transition state geometries (Figure 3-2) contain the same
bulk structural features which were rationalized as influencing the fluorinated
thermal electrocyclic processes discussed in Chapter 2. The calculated
transition state conformation for the thermal 67r disrotatory process113 is
structurally similar to a boat Cope process121,122 and the photochemical 6x

conrotatory process'25 is similar to the chairCope process.121,122 The Cope
system surpasses the electrocyclic process in terms of utility for thermal study in
that both the boat and chair Cope processes are orbital symmetry allowed
thermal processes, whereas the electrocyclic disrotatory and conrotatory
processes are allowed only for thermal and photochemical excitation
respectively. The fundamental differences between the ground and excited
state processes in the 6n electrocyclic study do not allow for easy comparison of
activation parameters or discussion of steric and electronic effects of terminal








substituents between the two conformations. Seeing as both conformations may
undergo symmetry allowed thermal Cope processes, design of appropriate
terminally fluorinated 1,5-dienes and measurement of the activation parameters
for Cope rearrangement would allow for a more quantitative understanding as
to the effect of terminal fluorination on these transition state conformations.







6x Disrotatory Boat Cope







6n Conrotatory Chair Cope

Figure 3-2. 6n Electrocyclic and Cope Transition State Conformations.


Fluorinated Cope Systems

The thermal study of fluorinated 1,5-dienes has received little qualitative
or quantitative attention. Two recent reviews offer the minimal literature
available addressing qualitative aspects of [3,3]-sigmatropic processes in
fluorinated 1,5-dienes,126 allylvinyl ethers,126 and carbanions.127

The observation of the effect of terminal fluorine on a chair constrained
thermal Cope process was most easily studied by synthesizing terminally
fluorinated dienes which would complement hydrocarbon systems for which
reliable activation parameters had been previously reported. Due to the
degeneracy in the system, the parent 1,5-hexadiene had been studied as








1,1-dideutereo-1,5-hexadiene (Figure 3-3, 116).128 The partially fluorinated E-

and Z-1-fluoro-1,5-hexadiene129 (118, 119), and 1,1-difluoro-1,5-hexadiene129

(121) thermal studies had already been reported in the literature with reliable

activation parameters. To complete the simple 1,5-hexadiene series required

synthesis and thermal study of 1,1,6,6-tetrafluoro-1,5-hexadiene (123).



116 C DJ 117


118 12

120
119


121 FCF2 122


2 CF2 ? C2F2
123 CF2 ---F2 124


Figure 3-3. Fluorinated 1,5-Hexadiene Cope Processes of Interest.


The experimental study of boat constrained Cope systems has received
a variety of interest, and again, systems were chosen for mechanistic novelty,

activation parameter reliability of the reported hydrocarbons, and synthetic
simplicity. Two hydrocarbon systems (Figure 3-4) which have been

investigated where geometrical constraints force a boat conformation Cope
rearrangement are 1,4-dimethylenecyclohexanesl13.131 such as 125, and meso-
1 -(2-methylidenecyclopentyl)-2-methylidenecyclopentane132 (meso-126). An
artifact of the synthetic route into meso-126, the d,l-1-(2-methylidene-

cyclopentyl)-2-methylidenecyclopentane isomer d,1-126 is also afforded. This








diastereomer is constrained to undergo the Cope rearrangement through a

chair conformation, creating another system for comparison with those of Figure

3-3.


CCD2



CD2
125


H


meso-126



H 127


d,-126

Figure 3-4. Cope Rearrangements in Conformationally Constrained Systems.


There is a subtle but important difference between the two boat

conformations involved with 125 and meso-126. Due to the symmetry of the

system, the boat type transition state for 125 will constrain the two terminal

methylenes to approach one another in a coplanar eclipsed fashion with no

twisting in of terminal substituents. In contrast, the meso-

bismethylenecyclopentane meso-126 will be able to accommodate a true boat

transition state which involves a significant turning in of the two terminal cis

substituents. A thermal study of 1,4-dideuteriomethylidenecyclohexane (125)

has been reported as has a study of the hydrocarbon d,I- and








meso-bismethylenecyclopentanes (d,1-126, meso-126), and all are reported
with reliable activation parameters.
The materials then of interest in this study are illustrated in Figure 3-5. To

probe the effect of terminal fluorination in these different systems, the synthesis
of a variety of materials was required to complete each series from
hydrocarbon, to terminally gem-difluorinated, to terminally bis-gem-difluorinated
1,5-diene. Having reliable reported activation parameters for 116, 121, 125,
and meso and d,1-126, it was necessary to carry out syntheses and thermal
studies of 123, 128, 129, meso and d,1-130, and meso and d,1-131.

Although interest existed in meso and d,1-130, this system was not studied
due to time limitations. The synthetic routes and thermal results for 123, 128,
129, and meso and d,1-131 will now be discussed.




D2 C CF2 C CFF

116 121 123
CD2 CF2 CF2



CD2 CF2


meso & d,-126 meso & d,-130 meso & d,-131


Figure 3-5. Three Hydrocarbon to Terminally Fluorinated 1,5-Diene Series.








Synthesis and Thermolysis of Terminally Gem-difluorinated 1.5-Diene Systems


1.1.6.6-Tetrafluoro-1 .5-hexadiene (123)

1,1,6,6-Tetrafluoro-1,5-hexadiene (123) was synthesized in six steps
from 1,4-butanediol in an overall isolated yield of 6%. The reaction sequence is
illustrated in Figure 3-6. 4-[(Tetrahydro-2H-pyran-2-yl)oxy]-1-butanal (132) was
prepared by a literature procedure133 from 1,4-butanediol in two steps and

subjected to a Wittig-type fluoroolefination134 to yield 5-[(tetrahydro-2H-pyran-2-

yl)oxy]-1,1-difluoropent-1-ene (133). Deprotection to 5,5-difluoro-4-penten-1-ol
(134) and pyridinium dichromate (PDC) oxidation yielded 5,5-difluoro-4-

pentenal (135). Wittig-type fluoroolefination then afforded the desired 1,1,6,6-
tetrafluoro-1,5-hexadiene (123). Initial attempts to obtain 123 through Wittig-
type fluoroolefination of 1,4-butanedial did not afford any amount of fluoroolefin
and was not further pursued.


P(N(CH3)2)3 Acid activated
0 CF2Br2 CF2 Ion exc. resin, CF2
OTHP THF, 00C-RT, 4 hr OTHP 1,4-butanediol OH
H
132 133(58%) PDC 134(83%)
CH20C12
P(N(CH3)2)3 /
CF2 CF2Br2 CF2
CF2 THF, 0C-RT, 4 hr
135(75%)
123(45%) 135(75

Figure 3-6. Synthesis of 1,1,6,6-Tetrafluoro-1,5-hexadiene (123).


Due to appreciable volatility, the thermolysis of 123 was examined in the
gas phase as described in Appendix A. Quantitative conversion of 123 to









4.75



4.5



4.25


3.75


0 5000 10000 15000
Seconds


C F2
SCF2

123


a 207.2C

216.2C

o 224.40C

A 228.8C

235.7C

241.6C






20000


I
12CF

124


Figure 3-7. First-Order Rate Plots and Rate Constants for Thermolysis of
1,1,6,6-Tetrafluoro-1,5-hexadiene (123).


Temperature (oc) k (xl o0 sec1) R2

207.2 2.24 0.01 0.9999

216.2 4.13 0.05 0.9993

224.4 6.88 + 0.07 0.9996

228.8 8.87 0.12 0.9993

235.7 13.68 + 0.07 0.9999

241.6 19.76 0.10 0.9999








3,3,4,4-tetrafluoro-1,5-hexadiene (124, Figure 3-7) was observed. The

thermolysis was examined at six temperatures from 207.2C to 241.60C and the

conversion was observed to follow first-order kinetics with no degree of

reversibility observed. Figure 3-7 offers the first-order plots and rate constants

for the temperatures examined.
The activation parameters (AH*, AS*) for the cyclization were obtained
kT
using the Eyring expression,6 k = -e RT e rearranged to the form

Ln(k/T) = -AHt/RT + AS*/R + Ln(k/h), where k = rate constant at absolute

temperature T, k = Boltzmann constant (1.381x10-23 J/K), h = Planck's constant

(6.626x10-34 Jxs), and R = ideal gas constant (1.9872 calmolxK). A linear least-

squares regression plot of Ln(k/T)versus 1/T, as illustrated in Figure 3-8, yielded
AH* = 29.9 0.2 kcal/mol and ASt = -18.5 0.5 cal/molxdeg for the Cope

rearrangement of 123 to 124 with the errors reported as one standard

deviation.


-14.5


-15 -


-15.5-

--J


1.90E-03 1.95E-03 2.00E-03 2.05E-03 2.10E-03
1/T
Figure 3-8. Eyring Plot for Thermolysis of 1,1,6,6-Tetrafluoro-1,5-hexadiene
(123).








1 -Difluoromethylidene-4-methylidenecyclohexane (128)

1-Difluoromethylidene-4-methylidenecyclohexane (128) was prepared

in three steps from 1,4-cyclohexanedione mono-ethylene ketal (136) in a 37%

isolated yield. The reaction sequence is illustrated in Figure 3-9.

15% H2S04
S0. (C6sH)3PCH3Br 0 Silica Gel
O n-BuLi, THF, o0C CH2 CH2C2 CH2 O
0 0 138 (90%)
136 137 (69%)
P(N(CH3)2)3
CF2Br2
CH2 CF2 THF, OC-RT, 4 hr

128 (38%)


Figure 3-9. Synthesis of 1 -Difluoromethylidene-4-methylidenecyclohexane
(128).


4-Methylidene-l-cyclohexanone ethylene ketal (137) was prepared from 1,4-

cyclohexanedione mono-ethylene ketal (136) by a Wittig reaction135 then

deprotected136 to yield 4-methylidenecyclohexanone (138). This material was
subjected to Wittig-type fluoroolefination134 to yield the desired 128.
Due to appreciable volatility, the thermolysis of 128 was examined in the

gas phase as described in Appendix A. Quantitative conversion of 128 to 1,1-

difluoro-2,5-dimethylidenecyclohexane (139, Figure 3-10) was observed. The

thermolysis was examined at six temperatures from 279.90C to 309.10C. The
conversion was observed to follow first-order kinetics and no degree of
reversibility was observed. Figure 3-10 offers the first-order plots and rate
constants for the temperatures examined.



























0 5500 11000 16500
Seconds


a 279.7C

287.8C

o 292.1 C

a 297.60C

m 303.0C

309.1 C



22000


Temperature (oc) k (xio sece1) R2

279.9 3.78 0.09 0.9976

287.8 6.72 0.04 0.9998

292.1 8.88 + 0.19 0.9977

297.6 12.51 + 0.19 0.9989

303.0 18.12 0.15 0.9996

309.1 25.72 0.16 0.9998

CF2

CF2




128 139

Figure 3-10. First-Order Plots and Rate Constants for Thermolysis of
1-Difluoromethylidene-4-methylidenecyclohexane (128).





91


A linear regression plot of Ln(k/T) versus 1/T (Figure 3-11), as described

in the case of 123, yielded AH* = 40.8 0.5 kcal/mol and ASt = -6.1 0.9

cal/molxdeg for the Cope rearrangement of 128 to 139.


-14.5



-15


-15.5


-16.5---
1.70E-03


1.72E-03 1.75E-03 1.77E-03 1.80E-03 1.83E-03
1/T


Figure 3-11. Eyring Plot for Thermolysis of 1-Difluoromethylidene-4-
methylidenecyclohexane (128).


1.4-Di(difluoromethylidene)cyclohexane (129)


1,4-Di(difluoromethylidene)cyclohexane (129) was synthesized in one

step (Figure 3-12) from 1,4-cyclohexanedione (140) by a Wittig-type

fluoroolefination134 and isolated in an 36% yield.


P(N(CH3)2)3
CF2Br2
THF, O0C-RT, 4 hr


140


CF2 = == CF2

129 (36%)


Figure 3-12. Synthesis of 1,4-Di(difluoromethylidene)cyclohexane (129).