Stereochemical and reactivity studies of the 2 +2, 2 +4, and 1,3-dipolar cycloadditions of partially flourinated allenes

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Stereochemical and reactivity studies of the 2 +2, 2 +4, and 1,3-dipolar cycloadditions of partially flourinated allenes
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Allene -- Reactivity   ( lcsh )
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Thesis (Ph. D.)--University of Florida, 1985.
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Includes bibliographical references (leaves 176-180).
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by Gene Ellis Wicks.
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Vita.

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STEREOCHEMICAL AND REACTIVITY STUDIES
OF THE [2+2], [2+4], AND 1,3-DIPOLAR CYCLOADDITIONS
OF PARTIALLY FLUORINATED ALLENES
















By

GENE ELLIS WICKS


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


1985


mmm















TO MOM AND DAD















ACKNOWLEDGEMENTS

It is with much gratitude that I acknowledge the guidance of Professor

William R. Dolbier, Jr., with the chemical adventures recorded herein. He

is both a scholar and a gentleman, and he has been an excellent mentor

and friend throughout my graduate studies.

The invaluable assistance of Jim Rocca from Professor Wallace Brey's

high-resolution NMR lab is also gratefully acknowledged. In addition, Dr.

Simon Sellers from SCM Specialty Chemicals aided in running a few of the

19F NMR spectra in Chapter 3. Professor Gus Palenik and Dr. Marian

Gawron obtained the X-ray crystal structure described in Appendix B, and

several people from the UF Quantum Theory Project provided aid in doing

the IN DO calculations for fluoroallene described in Chapter 4. It is also a

pleasure to thank Seth Elsheimer and Randy Winchester for their helpful

discussions and comradery throughout my graduate studies.

Finally, the support and encouragement of several friends and relatives

during the course of this work has been much appreciated. My parents and

the family of W. Randy Van Ness deserve special mention in this regard.
















TABLE OF CONTENTS


ACKNOWLEDGEMENTS----------------------------------------

ABSTRACT ------------------------------------------- --

CHAPTER


Page

iii

v


1 INTRODUCTION AND BACKGROUND INFORMATION --------- 1

2 [2+2] CYCLOADDITIONS OF 1,1-DIFLUOROALLENE --------- 14

3 STEREOCHEMISTRY OF THE 1,3-DIPOLAR CYCLO-
ADDITIONS OF FLUOROALLENE ------------------------- 37

4 STEREOCHEMISTRY OF THE [2+4] CYCLOADDITIONS
OF FLUOROALLENE ------------------------------------ 63

5 CONCLUSION ------------------------------------------- 74

6 EXPERIMENTAL SECTION ---------------------------------- 76

APPENDICES

A SELECTED SPECTRA -------------------------------------- 103

B X-RAY DIFFRACTION STUDY ------------------------------ 165

C SELECTED RESULTS FROM 'INDO" CALCULATIONS
FOR FLUOROALLENE AND 1,1-DIFLUOROALLENE -------- 169

D ACRONYMS USED IN THIS DISSERTATION ------------------ 174

REFERENCES ---------------------------------------------------- 176

BIOGRAPHICAL SKETCH ---------------------------------------- 181















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


STEREOCHEMICAL AND REACTIVITY STUDIES
OF THE [2+2], [2+4], AND 1,3-DIPOLAR CYCLOADDITIONS
OF PARTIALLY FLUORINATED ALLENES

By

Gene Ellis Wicks

December 1985


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

Cycloadditions involving fluoroallenes are extremely useful mechanistic

probes. Due to the high reactivity of these allenes and the small size of

their fluorine substituents, cycloadditions of fluoroallenes are free from

many of the ambiguities surrounding analogous studies by previous workers.

Styrene and 1,1-difluoroallene, DFA, undergo clean reactions between

600 and 100C to give good yields (060%) of two cycloadducts: 2,2-difluoro-

3-phenyl-l-methylenecyclobutane (major) and 3-phenyl-l-(difluoromethylene)

cyclobutane (minor). Similar experiments comparing the reactions of p-nitro

and p-methoxystyrene with the above results give relative reactivities of 2.1

and 1.3, respectively. The [2+2] cycloaddition of DFA with Z-0-deuterio-

styrene gives the expected products in an 82:18 ratio with stereochemical

ratios of 58:42 and 79:21, respectively, at 800C. A mechanism involving

two kinetically distinguishable diradicals is proposed to rationalize the

results. The reaction of DFA with 1-cyanovinyl acetate, a "capto-dative"









reagent, is also studied and found to give results similar to the styrene

reactions.

N-Methyl, N-phenyl, and N-(2-naphthyl)-C-pheny!nitrones undergo regio-

specific C2-C3 dipolar cycloadditions with monofluoroallene, MFA, in nearly

quantitative yields at 250C. The average stereochemical ratio for the two

4-substituted isoxazolidines produced is 84:16 296 with the product from

syn addition of MFA in excess. The stereochemical assignments of the

products are made with the aid of difference nuclear Overhauser effect

(NOE) experiments and an X-ray crystal structure. Extensive solvent and

kinetic studies show that these are well-behaved second-order kinetic

processes consistent with previously well-studied 1,3-dipolar cycloadditions.

The reaction of MFA with diazomethane-d2 strongly favors the product

from syn addition (88:12 ratio), also. Consideration of transition-state

dipole-dipole interactions and solvation phenomena show that the factors

affecting the stereochemical outcome of all these 1,3-dipolar cycloadditions

are extremely subtle.

Finally, MFA undergoes Diels-Alder reactions with cyclopentadiene,

butadiene, and furan to give a modest preference for the syn adduct (51:49,

59:41, and 63:37, respectively). The results are discussed in terms of a

theoretical study done for MFA using INDO calculations.














CHAPTER 1
INTRODUCTION AND BACKGROUND INFORMATION

Introduction
Partially fluorinated allenes, like fluoroallene (MFA) and 1,1-difluoro-

allene (DFA), have several features that make their use ideal in the study
of cycloaddition reaction mechanisms. For instance, the presence of one or

two fluorines in an allene molecule dramatically increases its reactivity


F F

\ _C ___ .,.H \ ___ 11 i.H
"4H ,iIH


H F

MFA DFA



as compared to the hydrocarbon analog.1 In addition, the small size and
high electronegativity of the fluorine substituents allow a reaction

mechanism to be strongly influenced by the fluorines' electronic effects
while being virtually free from their steric interference. Furthermore, the

cycloaddition products obtained in these reactions have reasonable thermo-

dynamic stability, and they are not generally susceptible to mechanistically

complicating rearrangements under the reaction conditions. These and other
qualities of partially fluorinated allenes allow unique insights to be obtained
from the unambiguous study of cycloadditions which have been under both
scrutiny and dispute for two decades.









First, this dissertation contains a mechanistic investigation of the [2+2]

cycloadditions of 1,1-difluoroallene with olefins. Relative reactivity studies

and a detailed stereochemical study are presented and discussed. Second,

the stereochemistry of addition for fluoroallene in 1,3-dipolar and [2+4]

cycloadditions is examined. Extensive solvent and kinetic studies are pre-

sented for the 1,3-dipolar cycloadditions as well as a theoretical study

encompassing both the [2 +4] and 1,3-dipolar cycloaddition results. An

overview of pertinent background information for the study of these topics

will be given along with a brief review of the salient features of fluorine

as a substituent in organic molecules.

Fluorine as a Substituent
General Considerations

The characteristics of fluorine as a substituent on carbon have most

recently been discussed in Smart's excellent reviews.2,3 The properties of

fluorine are largely derived from its small size, its high electronegativity,

and its nonbonded electron pairs. The atomic orbitals of fluorine are of a

proper size for excellent orbital overlap with carbon since both elements

are in the same period. This allows fluorine to participate in potent a-with-

drawal from carbon and sometimes in Tr-donation. Fluorine's high electro-

negativity combined with its effective orbital overlap result in a C-F bond

which is quite strong, polar, and relatively short (1.36 A for a C-F bond;

1.77 A for a C-C1 bond).4 In addition, a C-F bond is also rich in p-char-

acter as reflected by the changes in hybridization and molecular geometry

that take place relative to C-H substitution. For example, the HCH bond

angle of CH2F2 is 113.70 and the FCF bond angle is 108.30 as compared to

the idealized bond angles of 109.50 for CH4.5 Finally, when a fluorine is


1








bound to an sp2 carbon, the lone pairs of fluorine backbond into the empty

p-orbital of carbon to the extent that a fluorine will actually stabilize a

carbocation in spite of its strong inductive effect.6






'' C--------- F-





In view of the discourse given so far, it is not surprising that even a

single fluorine can drastically alter the properties of a molecule. A few
examples will be given to illustrate the reactivity, thermodynamic, and

steric considerations helpful to understanding the cycloadditions of fluori-
nated allenes. These considerations will have an important bearing on the

discussion of experimental results throughout the dissertation.
Reactivity Considerations

The cycloadditions of cyclopentadiene and acrylonitrile with allene,7,8
fluoroallene,9 and 1,1-difluoroallene9 have been well studied. The [2+2]
cycloadditions of the allenes with acrylonitrile have been observed to give

one or two products in moderate yields as shown in equation (1). In
comparing the reaction conditions in Table 1-1, it is apparent that there is

a significant increase in reactivity by substitution of a single fluorine on
allene. Substitution of two fluorines on allene allows the reaction to

proceed at a temperature 100 C less than allene itself in a comparable
time and yield.







I_
















>

NC y


Table 1-1. [2+2] Cycloadditions with Acrylonitrile


Reaction Product
X Y Conditions Ratio (1:2) % Yield


H H 2100C, 12 h 60%

H F 135C, 21 h 31:69 45%

F F 1100C, 12 h 77:23 64%


The [2+4] cycloadditions of these allenes with cyclopentadiene show a

similar trend for equation (2). Although allene itself requires vigorous con-

ditions to give a modest yield of the [2+4] cycloadduct, Table 1-2 shows

that the fluorinated allenes react to give nearly quantitative yields of

products at 0C.


x



y


+ =\CN













X


Y


Table 1-2. [2+4] Cycloadditions with Cyelopentadiene


X Y Reaction Conditions % Yield

H H 215 C, 6 h, autoclave 49%
H F 0 C, 101 h 90%

F F 0 C, 2 min 99%


Thermodynamic Considerations

The first thermodynamic consideration for the cycloadditions of fluori-
nated allenes is that of product stability. Fluorine forms the strongest
single bonds to carbon of all the elements. The bond dissociation energies

of C-F, C-C1, and C-O bonds are 116, 78, and 86 kcal/mole, respectively.10
Thus, it is not surprising that saturated fluorocarbons have high chemical

stability and are less reactive than alkanes toward nearly all reagents.4
Although partially fluorinated compounds are quite different in this respect,

it is noteworthy that the high reactivity of partially fluorinated allenes, for
example, allows them to undergo cycloadditions under conditions mild

enough to insure the survival of products with reasonable thermodynamic
stability.


-10


~glily







An additional issue to consider here is the thermodynamic stability of
a single fluorine versus a gem-difluoro group. One may have noticed from
the product ratios in equation (1) that the major product is 2 for a single
fluorine but 1 when a gem-difluoro group is involved. This phenomenon has
been well studied. The equilibration of fluoropropenes shows that a single


F\=/


F


A Hrxn
3 4 -3.34 kcal/mole
3 5 -2.68 kcal/mole
4 5 +0.64 kcal/mole

fluorine is thermodynamically favored on an sp2 carbon by about 3
kcal/mole.11 In contrast to this, the Cope rearrangement of 6 to 7 shows
that a gem-difluoro group is thermodynamically favored on an sp3 carbon by


CF 2

K0>


F2

0\)


A Hrxn
-5.1 0.6 kcal/mole


6 7


F -=









5 kcal/mole.11 Thus, in the [2+2] cycloaddition of the fluorinated allenes

with acrylonitrile in equation (1), the more thermodynamically stable

product is favored in each case. This, however, is not what is found for

the [2+4] cycloadditions in equation (2). The conclusion is that the cyclo-

additions in equations (1) and (2) proceed via different mechanisms, a

matter which will be addressed shortly.

Steric Considerations

Excluding hydrogen, fluorine has the smallest Van der Waals radius of

any substituent atom or group. Thus, it is not surprising that the steric

demand of fluorine substituents has been shown to be practically negligible

in the [2+21 and [2+4] cycloadditions of fluorinated allenes.9,12 An

example of this is the reaction of fluoroallene with 1,1-dichloro-2,2-difluoro-

ethylene where 8 and 9 are formed in equal amounts. In contrast to this,

the related results of the cycloadditions of monoalkyl allenes are often

explained in terms of a steric rationale.13,14



F
F

1340C F F
+8h > + +
8h C2 F2 C12 F2 Cl2 F2

CF = CC1
CF2= CC 2 8 9 10


25.3 : 25.4 : 49.3

(45 % Yield)








It is interesting to note that even in the reactions of some perfluori-

nated compounds, which one might expect to be more sensitive to steric

congestion than partially fluorinated allenes, the steric demand of the

fluorines seems minimal. Indeed, the electrocyclic ring opening of trans-

perfluoro-3,4-dimethylcyclobutene, which is governed by strong electronic

factors, proceeds in a remarkable, contrasteric fashion to preferably give 11

instead of 12.15 The activation energy for the ring opening to the Z,Z-

diene 11 is 18 kcal/mole less than for opening to the E,E-diene 12, and at

111.5 OC, kZZ/kEE = 1.9 x 109.


k-zz


kEE

-EE


*CF3

*CF3


CF3
F F


F F
CF3


Mechanism of [2 +2] Versus [2 +4] Cycloadditions
The [2+2] cycloadditions of fluoroallene (MFA) and 1,1-difluoroallene

(DFA) are thought to proceed via a stepwise diradical mechanism while the
[2+4] cycloadditions are thought to be concerted.9,16 In the [2+2] reac-

tions, the central carbon of DFA undergoes bond formation with the






9

unsubstituted carbon of acrylonitrile, for example, to give an allyl- and

cyano-stabilized diradical intermediate 13. The diradical undergoes

competitive ring closures to predominately give the more stable product

with the CF2 endocyclic. This explains the product regiochemistry and the



F 2



-- CF2 NC
2CF2 (major)



CF 2

NCN

13 NC

(minor)



thermodynamic preferences observed for the DFA-acrylonitrile reaction in

Table 1-1. Other experimental results show a rough correlation between

the product ratio and the stability of the diradical intermediate for

reactions of this type. The Hammond postulate can be applied in that the

greater the stability of the diradical intermediate, the later the transition

state to ring closure, and thus, the more the product with greater thermo-

dynamic stability is favored.

Although much is known about the regiochemistry of [2+2] cyclo-

additions with MFA and DFA, no stereochemical study had been done before

the work in this dissertation was initiated. It was expected that the high

reactivity of these allenes and the lack of a fluorine's steric interference in









the ensuing reaction would make such a study unique. This is the rationale

for pursuing the cycloaddition of DFA with Z-B-deuteriostyrene17 discussed

in Chapter 2. The use of styrene as the olefin also allows a Hammett

reactivity study to be performed by the substitution of various electron-

releasing and electron-withdrawing groups in the para-position of the phenyl

ring.

It is interesting to note that the concerted [2+4 ] cycloaddition of

DFA with cyclopentadiene in Table 1-2 is regiospecific to give solely the

product of lesser thermodynamic stability in comparison with the [2+2 ]

reactions. Photoelectron spectra and ab initio 4-31G calculations for DFA

help substantiate the explanation one might intuitively expect.9,18 The

fluorine-substituted C1-C2 ir-bond, which is involved in Tr-donation from the

fluorine lone pairs, is both electron rich and quite unreactive in [ 2 +4 1 cyclo-

additions much like the r-bonds of allene. The C2-C3 IT-bond, however, is

greatly influenced by the a-withdrawing inductive effect of the allylic

fluorines and is very electron deficient. This results in a low C2-C3 r*

orbital energy, the energy of the lowest unoccupied molecular orbital

(LUMO) in the molecule. Thus, [ 2+4 ] cycloadditions of DFA occur exclu-

sively at the C2-C3 Tr-bond in spite of thermodynamic considerations.

The [ 2 +4 ] reactions of MFA display the same character as those of

DFA for presumably similar reasons.9 From this, it would not be surprising

to learn that the 1,3-dipolar cycloadditions of MFA and DFA also occur at

the unsubstituted double bond like the [2+4] cycloadditions.

1,3-Dipolar Cycloadditions

The 1,3-dipolar cycloadditions of DFA with diazoalkanes, nitrones,

nitrile oxides, and carbonyl ylides have been well-studied.19,20 The








1,3-dipoles undergo reaction with DFA exclusively at the C2-C3 w-bond to

give two regioisomers, 14 and 15 for example, for diazoalkanes.


F2 C f


N2A
N=N


+ R2 F
N-N


+
R2C -N =N
14 15



Table 1-3. DFA 1,3-Dipolar Cycloadditions with Diazoalkanes


Reaction Product
R Conditions Ratio (14:15) % Yield


H 0C, 5 min, Et20 100:0 95%

CH3 0C, 5 min, Et20 61:39 99%

Ph 28 C, 5 h, Et20 14:86 95%



Interestingly, Table 1-3 shows that the reaction of DFA with

diazomethane (R=H) is regiospecific with respect to the dipole as well as to

DFA. In contrast, the R=Ph reaction strongly favors product 15, and a

solvent study performed for this reaction demonstrates no significant change

in the 14:86 product ratio for six solvents ranging in polarity from pentane

to dimethylsulfoxide. The rationale for this reversal of product ratios in

Table 1-3 is a competition between frontier molecular orbital and steric

considerations.19 Extended Huckel calculations indicate that secondary

orbital interactions in the low-lying LUMO of DFA reinforce the wave

function at the middle carbon of the allene. Thus, regioisomer 14 is









favored by molecular orbital considerations for substituents of low steric

bulk. When the steric bulk of the R substituents becomes too great,

however, a reversal in product ratio occurs and product 15 is favored by

steric considerations.

In the reaction of fluoroallene (MFA) with 1,3-dipoles, both product

regiochemistry and stereochemistry are important. Indeed, a 1,3-dipole can

add either syn or anti to the fluorine in MFA. The reaction of MFA with




C syn


/ H
H
anti


N,C-diphenylnitrone has been studied by Burkholder.1 It is interesting to

note that the reaction is regiospecific with respect to both the allene and

1,3-dipole like the previously discussed cycloaddition of DFA with



F
F H H F


+ 250C P
CDC1 N-O N-O
Ph 0- CDCl3 / /
P/ Ph
N+

H Ph 16 17

86 : 14

(99 % Yield)









diazomethane. Also, the major product, tentatively although not

unambiguously assigned 16, appears to be that from syn addition of the

allene. This is in marked contrast to the 1,3-dipolar cycloadditions of

methoxy and phenoxyallene wherein the product from anti addition is

favored due to steric considerations.21,22

It was therefore expected that a systematic study of the 1,3-dipolar

cycloadditions of fluoroallene would be very interesting. First, unambiguous

stereochemical assignments had to be made for the products of the nitrone

reaction, preferably by obtaining an X-ray crystal structure for the major

isomer. Then the reactions could be studied kinetically and in different

solvents to see how electrostatic dipole-dipole interactions, the basicitv of

the fluorine, and molecular orbital calculations would affect or give insight

into the stereochemical preference for cycloaddition. The results for these

studies appear in Chapters 3 and 4 of this dissertation along with the

related results of the [2 +4] cycloadditions of fluoroallene.















CHAPTER 2
[2+2] CYCLOADDITIONS OF 1,1-DIFLUOROALLENE

Introduction

The mechanism of allene [2+2] cycloadditions has received consider-

able attention over the years23 with early excitement being generated by

the Moore et al. 1,2-cyclononadiene dimerization studies24 and Hoffmann's

related conjecture that allenes could act as antarafacial cycloaddends in

concerted cycloadditions.25 Early stereochemical studies,26-28 while lending

credibility to the concerted mechanism, did not provide unambiguous

evidence for it. Meanwhile, kinetic studies provided strong evidence for the

involvement of diradicals in such reactions.7,29 In spite of an interesting

theoretical exercise which reasserted the issue of concertedness,30 recent

studies,13,31 including two detailed stereochemical studies on the reactions

of fumarate and maleate esters with 1,1-dicyclopropylallene32 and 1,1-

dimethylallene,14 have been uniformly consistent with the involvement of

nonconcerted mechanisms.

As discussed in the previous chapter, the [2+2] cycloadditions of

1,1-difluoroallene (DFA) are also consistent with the involvement of a

nonconcerted diradical mechanism. The high reactivity of DFA and lack of

potentially complicating steric effects make the cycloadditions of

1,1-difluoroallene uniquely different from all previous mechanistic studies of

allene [2+2] cycloadditions. Thus, the reactions of DFA with variously

substituted styrenes were undertaken to obtain relative reactivity factors as

well as unambiguous stereochemical information about these extremely






15


informative [2+2] reactions. In addition, the reaction of DFA with 1-cyano-

vinyl acetate, a "capto-dative" reagent, was studied.

Results

Para-Substituted Styrene Studies

When the cycloadditions of 1,1-difluoroallene with para-substituted

styrenes were carried out neat under vacuum in sealed Pyrex tubes using a

tenfold excess of the styrene, only two adducts, 18 and 19, were obtained

in the product ratios and yields shown in Table 2-1. Either hydroquinone or

4-tert-butylcatechol were added to inhibit the free radical polymerization of




2 FCF2

--+ >
a, X = H'
b, X = OCH3
Sc, X = NO2
X X X


18a-c 19a-c



the styrene. The styrene and para-methoxystyrene reactions were very

clean, but there was partial polymerization of the para-nitrostyrene under

the reaction conditions. Nevertheless, extensive study of the crude reaction

mixtures by thin-layer chromatography, gas chromatography, and 19F NMR

at 282 MHz demonstrated that there were no other adducts than 18 and 19

except for the oligimers of DFA. Indeed, the lack of higher yields was

undoubtedly due to the competitive oligimerization of the highly self-

reactive DFA.









Table 2-1. Reactions of DFA with Para-Substituted Styrenes.
Yields and Product Ratios.


Product
X Conditionsa Ratio (18:19)b Yield (%)C


H 1000C, 2 h 81.8:18.2 58

H 80 C, 8 h 83.1:16.9 60

H 60C, 32 h 84.9:15.1 59

OCH3 100C, 2 h 83.5:16.5 58

OCH3 80C, 8 h 84.7:15.3 58

OCH3 60C, 32 h 86.2:13.8 57

NO2 100C, 2 h 83.5:16.5d 72

NO2 80C, 8 h 84.5:15.5d 74

aStyrene:allene = 10:1, hydroquinone or 4-tert-butylcatechol used as a free
radical inhibitor.
bError 0.2%; determined by GLC with a FID.
cError + 3%; determined by GLC or 19F NMR.
dError +0.5%; determined by 19F NMR.


The reaction of DFA with styrene was shown to be under kinetic

control. Products 18a and 19a were demonstrated to be both noninter-

converting as well as stable to further cycloadditions under the reaction

conditions. Identical reaction mixtures were prepared and allowed to

undergo reaction at 1000C for 3.0 h, 6.3 h, and 8.3 h. The reaction

mixtures were analyzed by gas-liquid chromatography (GLC), and the

reaction of DFA with styrene was shown to be complete within 3.0 h while

maintaining a constant product ratio of 81.8:18.2 (0.3) over time. Six

more identical mixtures were prepared and allowed to undergo reaction at

70 C for 15 min, 30 min, 1.0 h, 2.0 h, 3.0 h, and 6.0 h. While the yield

of the products gradually increased, there was again no change in the









observed 83.1:16.9 (0.2) product ratio by GLC. Finally, an isolated sample

of the minor isomer 19a was heated to 100 C under vacuum in a sealed

bulb with some heptane for 6.0 h, but it was recovered unchanged.

A slight temperature dependence in the ratio of 18:19 was observed

for the reaction of DFA with the para-substituted styrenes as shown in

Table 2-1. The greatest selectivity was observed at the lowest tempera-

ture. Products 18a-c and 19a-b were characterized by 1H, 13C, and 19F

NMR spectra, IR spectra, low and high resolution MS, and in the cases of

18a-c and 19a by combustion analyses. Product 19c, due to isolation

difficulties, was characterized only by 1H and 19F NMR. The spectral

interpretations were straightforward.

Relative Reactivity Study

Using equimolar mixtures of pairs of styrenes, the relative reactivities

of each were determined at 1000C as shown in Table 2-2. Since the

fluorine resonances of the four products in each competitive study were

resolved at 282 MHz, the product ratios and yields were determined by 19F

NMR using meta-bromobenzotrifluoride as an internal standard. The

reactivities of para-nitro- and para-methoxystyrene relative to styrene were

found to be 2.13 and 1.34, respectively.

Conservative NMR conditions with excellent digitization were chosen

for these analyses. A T1 population inversion experiment demonstrated that

the longest T1 was less than 6 sec, and thus, a 10 sec pulse delay was

used. The product ratios in Table 2-2 obtained by 19F NMR were in

excellent agreement with those of the 100 C experiments in Table 2-1

determined by GLC. Although the error usually quoted for NMR results is

2%,33 the error here seems to be 0.5% at most.









Table 2-2. Competition Reactions of DFA with Para-Substituted Styrenes.


Styrenes Product Reactivity
Runa Used Ratiosb Yields (%)b Ratio


X=OCH3 83.1:16.9 38
1 -j 1.34
X=H 81.4:18.6 28

SX=N02 83.3:16.7 47 2.13
2 2.13
X=H 81.7:18.3 22

XConditions: 100 C, 2.0 h; styrenes:DFA ratio = 5:5:1
bDetermined by 19F NMR


It is important to note that there was no polymerization of the para-

nitrostyrene under the reaction conditions in the competitive olefin experi-

ment. Since the product ratio for the para-nitrostyrene case in Table 2-2

is so close to that at 1000C in Table 2-1, this serves to validate the

earlier results where partial polymerization of the nitrostyrene occurred.

Stereochemical Study

Since styrene was used in large excess in previous reactions with DFA,

it was desired to prepare Z-B-deuteriostyrene in extremely high stereo-

chemical and isotopic purity free from any trace contaminants that might

hinder the interpretation of the cycloaddition results. Both diisobutyl-

aluminum hydride (DIBAH)17 and disiamylborane34 have generally been used

to prepare Z-B-deuteriostyrene from phenylacetylene, but each method has

drawbacks. First, DIBAH requires an excess of the acetylene to prevent bis-

addition, and phenylacetylene and styrene are not conveniently separated in

multigram quantities. Second, although borane reductions proceed in a

stereospecific cis manner, the additions to acetylenes are only regio-

selective35 and not regiospecific as desired. The stringent requirements for









the preparation of Z-6-deuteriostyrene were met with a modified synthesis

of Brown and Gupta.36

Phenylacetylene-d117 20 was smoothly reduced with catecholborane and

the catechol ester was cleaved with water to give, after recrystallization

from water with hot filtration and overnight drying in a vacuum oven, the

trimer of a-deuterio-B-phenylethenylboronic acid 21. The acid underwent

esterification with ethylene glycol in good yield to give 22, and the boronic

ester 22 reacted with acetic acid to give Z-8-deuteriostyrene 23 in a 28%

overall yield from the starting acetylene 20. The 300 MHz 1H NMR

spectrum of 23 agreed with published results17 and none of the trans






(1) O B-H, 2 h, 700C
S(2) H20, 4.0 h, 900C

(3) Vacuum Oven Drying D 3
20 21 45.6 %



Toluene, ,
HO OH

Reflux 20 h



HOAc, 900C, 1.5 h H

0 -H OH B
SOH D O


23 76.5 % 22 79.9 %






20


deuterated isomer was detected.* A comparison of the mass spectrum of

styrene at 12 eV with that of the cis deuterated material suggested an

isotopic purity of 99.3 (0.7)%.

The [2+2] cycloadditions of DFA with Z-B-deuteriostyrene 23 were

carried out neat under vacuum in sealed Pyrex tubes similar to previous

cycloadditions except that smaller styrene:DFA ratios were used, which

resulted in lower yields. The results are shown in Table 2-3. Again, the


S-CF2


A---c-


24-Z


24-E


23
23


25-Z


25-E


*A comparison of the 300 MHz 1H NMR spectra of styrene and 23 appears
in Appendix A in Figure A-23.









stability of the products under the reaction conditions was demonstrated.

The stereochemical ratio of the minor isomer 25 remained the same after

an isolated sample was resubjected to the reaction conditions. Unreacted

Z-S-deuteriostyrene was also able to be recovered with complete retention

of stereochemical integrity. No cycloreversion of 24 or 25 to the starting

materials was observed under the reaction conditions. The product ratios

were determined by GLC analysis of the crude reaction mixtures as before,

and the products were separated by flash chromatography37 and isolated for

the determination of the stereochemical ratios.


Table 2-3. Reaction of DFA with Z-B-Deuteriostyrene.
Stereochemical and Product Ratios.


Stereochemicalb
Ratios
Product 24-Z:24-E
Ratioa and
Styrene:DFA Conditions (24:25) 25Z:25-E Yield (%)


1.0:2.8 100 C, 3 h 81:19 57:43 22%c
79:21
2.2:1 80 C, 7.5 h 82:18 58:42 32%c
79:21
6.8:1 70 C, 15.5 h 83:17 59:41 58%d
79:21
'Error 1%; determined by GLC with a TCD.
bError 1%; determined by 300 MHz 1H NMR (pulse delay: 30s).
elsolated Yields.
dGLC yield.


The critical Z:E ratios were determined by quantitative integration of

the cyclobutane-ring methylene protons in the 300 MHz 1H NMR spectra of

24 and 25. Baseline resolution of Ha and Hb and of He and Hd in the

spectra of 24 and 25, respectively, enabled precise determinations of these









ratios as shown in Figure 2-1. The assignment of the methylene protons to

a particular chemical shift was made on the basis of difference NOE

experiments.* (Difference nuclear Overhauser effect (NOE) experiments can

currently be performed so that Overhauser enhancements of less than 1%

can be detected and accurately quantified.)38-40 Irradiation of Hx in 19a

gave an enhancement of 0.6% at the downfield methylene peak assigned He.

Irradiation of Hc gave enhancements of 0.8% at Hx and 2.0% at Hd.

Finally, irradiation of Hd gave enhancements of 0.6% at Hx and 1.7% at

He. Thus, the results were internally consistent with the proton


product




P H
Ph Hb


undeuterated deuterated




H H an H(b2

H. and Hb (0 ) 0.58:0.42 (24)


Ph Hd


Figure 2-1.


300 MHz 1H N MR Expansions of the Ring Methylene Protons


*The coupling of the methyne proton Hx to the ring methylene protons in
both 18a and 19a was not helpful in the assignment of the asymmetric
protons to a particular chemical shift. The coupling constant JAX was
shown to be nearly equal to JBX by decoupling experiments. Similarly,
JCX = JDX. This problem has been encountered before in the adduct of
styrene and diphenylketene. (See ref. 34.)









assignments given in Figure 2-1.* Difference NOE was less successful for

the structure elucidation of 18a, perhaps due to the proximity of the

geminal fluorines. It was noted, however, that 19a was an excellent model

compound for the assignment of the resonances in the spectra of 18a and

24. Thus, the resonances were assigned in an analogous manner. Worth

noting is the fact that the assignment of resonances by NOE experiments is

in agreement with what one would expect to observe when the cycloaddition

is performed with Z-4-deuteriostyrene. The largest methylene peaks in the

spectra of 24 and 25 are from the resonance of protons in the Z-deuterated

adducts.

Capto-Dative Study

The term "capto-dative" is derived from the extraordinary ability of a

"capto" electron acceptor group and a "dative" electron donor group to

stabilize a radical when both groups are attached to the same carbon.41

Both inductive and resonance effects of the attached groups contribute to


I



c = "capto" electron acceptor group

d = "dative" electron donor group

the stabilization. Geminal capto-dative substitution of an olefin increases

its reactivity in both radical and diradical reactions. The reduced frontier

orbital HOMO-LUMO energy separation makes capto-dative substituted

olefins radicophilic since the singly occupied molecular orbital (SOMO) of a


*The difference NOE spectra of 19a and 25 appear in Appendix A in
Figures A-7 and A-26.









radical is near to both the HOMO and the LUMO of the olefin. Thus, it

was thought that a capto-dative substituted olefin like 1-cyanovinyl acetate

26 might be more reactive toward DFA than the para-substituted styrenes

or any other olefin yet studied.9

The reactions of DFA with 1-cyanovinyl acetate 26 were carried out

the same as the reactions of DFA with the para-substituted styrenes. A

tenfold excess of the olefin was used. Analysis of the crude reaction

mixtures by GLC showed two products 27 and 28, and there was a slight

dependence of the product ratio on temperature as shown in Table 2-4.


-=*-CF2


+
AcO


NC


27 28


Table 2-4. Reactions of DFA with 1-Cyanovinyl Acetate.
Yields and Product Ratios.


Conditions Product Ratio (27:28)b Yield (%)C


100 oC, 2 h 77.0:23.0 53

80 oC, 8 h 78.0:22.0 54

60 oC, 32 h 79.2:20.8 46

aOlefin:allene = 10:1, hydroquinone used as a free radical polymerization
inhibitor.
bError 0.2%; determined by GLC with a FID.
tError 3%; determined by GLC.









The greatest selectivity was observed at the lowest temperature similar to

the results in Table 2-1. Products 27 and 28 were characterized by 1H,

13C, and 19F NMR spectra, IR spectra, low and high resolution MS, and in

the case of 27 by combustion analysis. The spectral interpretations were

straightforward. The reaction of DFA with 1-cyanovinyl acetate was very

clean with no polymerization observed.

The relative reactivity of 1-cyanovinyl acetate compared to styrene

was determined to be 0.82 by GLC as shown in Table 2-5. Surprisingly, the

capto-dative reagent was less reactive toward DFA than any of the styrenes

studied. A comparison of the percent yields for the reactions in Tables 2-1

and 2-4 seems to support the validity of the competitive study.


Table 2-5. Competition Reaction of DFA with Styrene and
1-Cyanovinyl Acetate (26).


Olefins Product Yields Reactivity
Used Ratiosb (%)b Ratio


OAc

76.9:23.1 29

CN

0.82

82.0:18.0 36

aConditions: 1000C, 2.0 h; styrene:26:DFA ratio = 5:5:1.
bDetermined by GLC.


Discussion

Mechanism of [2+2 ] Cycloadditions of 1,1-Difluoroallene

The [2+2] cycloadditions of 1,1-difluoroallene (DFA)42 with the para-

substituted styrenes and with 1-cyanovinyl acetate give results which are









consistent with those of previously studied DFA [2+2] reactions.9 The high

reactivity of DFA allows the cycloadditions to take place at 60-100C in 32-

2 h, respectively, while giving an average 59% yield of two products when

a tenfold excess of the olefin is used. The lack of higher yields is due to

the competitive oligimerization of the highly self-reactive DFA. All

products are completely stable under the reaction conditions with no

competing side reactions. The stereochemical study is also very clean with

no evidence for interconversion of the products once formed nor cleavage

of any intermediate to give starting materials under the reaction conditions.

Thus, the reactions are shown to be under kinetic control.

The [2+2] cycloadditions studied seem superficially consistent with the

type of stepwise diradical mechanism discussed in Chapter 1. In the first

step, an allyl-ethyl diradical intermediate 29 is formed which undergoes

competitive ring closures to predominately give, for example, 18a, the more





F


-.-CF Ph
--*--C2
CF 2 18a

SP-- /P)C




Ph 2

29 Ph


19a









thermodynamically stable product for the styrene reaction. This rationale

adequately explains the observed product ratios and regiochemistry of

cycloaddition in Tables 2-1 and 2-4.

The relative reactivity and stereochemical studies lend further support

to this mechanism. Table 2-3 shows there is a scrambling of the deuterium

label in the products when the reaction is performed with Z-6-deuterio-

styrene. Thus an intermediate must be involved. The relative reactivity

studies of Table 2-2 require an intermediate that is devoid of any polar

character to account for the fact that both electron-rich para-methoxy-

styrene and electron-deficient para-nitrostyrene undergo cycloaddition with

DFA more rapidly than styrene itself. The intervention of HOMO-LUMO

(donor-acceptor) or charge transfer interactions seems unlikely in view of

this, while a diradical intermediate like 29 is quite plausible.

It is interesting to note that although the competition studies are the

first example of a Hammett-type study of a [2+2] reaction, the results are

consistent with a previous substituent effect study of the thermal rearrange-

ments of 2-aryl-3,3-dimethylmethylenecyclopropanes 30.43 Ten different






lOO 100C *

Q isooctane

x x
30 X


X = CN, Br, C1, CF3, CH3, H, OCH3, SCH3 among others









substituents of varying degrees of electron donating or accepting ability

were shown to give increased reactivity relative to a hydrogen for 30.

A close examination of the stereochemical ratios in Table 2-3 reveals

that the common-intermediate diradical mechanism used to explain the [2+2]

cycloadditions of DFA so far may be an oversimplification. The retention

of stereochemistry is quite different in the two products with only a minor

variation of the stereochemical ratios with temperature. If the two

products were formed from the same intermediate, one would expect that

the stereochemical ratios would be nearly the same. Thus, a modification

of the mechanism must be made which accounts for the stereochemical

ratios but which is still consistent with all of the results discussed so far.

A thorough examination of what is currently known about the mechanistic

details of [2+2] reactions, in particular the details of the first mechanistic

step, would be helpful to insure that any modification of the common-

intermediate diradical mechanism would have a proper beginning.

Scrutinization of the First Mechanistic Step

Although evidence continues to accumulate indicating the stepwise

nature of [ 2 +2] cycloadditions in general and allene [ 2 +2] reactions in

particular,14,23,32 little is really known about the mechanistic details of

these reactions. Little theoretical effort has been directed toward [2+2]

cycloadditions.44,45 Not much more is known about factors which drive

these cycloadditions other than the general rules espoused in Roberts and

Shart's review of all known such reactions in 1962.46 The [2+2] reactions

do not appear to be driven by HOMO-LUMO (donor-acceptor) interactions

since many if not most of the best [2 +2 ]'s are dimerizations. In general,






29

classic factors seem to promote [2+2] reactivity like relief of strain and/or

the presence of radical-stabilizing or olefin-destabilizing substituents.

Allenes are both interesting and unique addends in [2+2] cyclo-

additions. While free radical additions to allenes occur at both terminal

and central carbons,47,48 [2+2] cycloadditions of allenes proceed almost

exclusively via central carbon attack23 as shown in Figure 2-2. The

contrast in regiochemistry for these superficially similar processes can be

understood in terms of the differences in energetic between them. The




PhS PhS--

peroxides, 80 %
PhSH
200C

= PhS -* PhS

20 %




1100C 0
o -NC

exclusively

Figure 2-2. Comparison of Free Radical Additions with Cycloadditions
of Allene.


addition of radicals to allene is generally an exothermic process with a low

activation energy and with little temperature dependence on regiochemistry

observed.47 In contrast, the first mechanistic step in a [2+2] cycloaddition

of allenes is endothermic with a significant activation energy. This, com-

bined with the regiospecificity observed in initial bond formation, leads one








to conclude that the transition state for this first step has undergone signi-

ficant rotation of the terminal methylene of the reactive double bond to

give the transition state considerable stabilizing allyl radical character as in

31-TS.* Once intermediate 31 has formed,49 it has only rarely been

observed to revert to starting materials,14 unlike the usual [2+2] diradical

intermediates, because the allyl radical stabilization in 31 must be lost

prior to cleavage. Indeed, no reversibility of this initial bond formation has

ever been observed in the Dolbier et al. studies of the [2+2] cycloadditions

of allene7 or of fluorine-substituted allenes.9



H H ~ H H

\ H

H c""H H



31-TS 31




Unlike the uncertainty present for most other allenes, one can be

reasonably assured that the initially reactive ir-bond of DFA in the first

mechanistic step is the fluorine-substituted one. This assumption is derived

from the observation that fluorine-substituted olefins are much more

reactive in [2 +2] cycloadditions than hydrogen-substituted ones.46 The

comparison of the [2+2] reactivities of methylenecyclopropanes 32a and 32b



*A recent isotope effect study is consistent with such rotation occurring in
the transition state (ref. 31).





31

illustrate this fact. Compounds 32a and 32b are essentially "homoallenes"
and should be considered excellent models of the fluorine-substituted and
hydrogen-substituted r-bonds of DFA.50 Thus, in the DFA-styrene reaction


+ CF2 CC12


+ CF2 CC12


1150C
4 days


1400C
5 days


F2

>C12
F2
45 %




C12
2 F2
22 %


it will be assumed that the initial reaction occurs at the central carbon of
DFA at the C1-C2 ri-bond.
Evidence for Two Kinetically Distinguishable Intermediates
In modifying the common-intermediate diradical mechanism for the
[2+2] cycloadditions of DFA, it should be recalled from Chapter 1 that the
fluorines exert little if any steric demand in [2+2] reactions. The only
substituent making a significant steric demand in the DFA-styrene system
under discussion is the phenyl ring. This is in stark contrast to Pasto's
related stereochemical study of the reactions of alkyl-substituted allenes
with diethyl fumarate and diethyl maleate. In his study, a steric rationale
was solely used to account for the results.14


>=CF2


C-






32


In order for a diradical mechanism to quantitatively rationalize the

stereochemical results in Table 2-3 wherein the minor product 25 is formed

with greater retention of stereochemistry than the major product 24, it is

necessary to invoke at least two kinetically distinct intermediates. The

first, short-lived intermediate or intermediates should be nonregioselective,

but highly stereoselective in the cyclization to 24 and 25. It must also

convert to a more stable intermediate before being able to undergo much

cyclization. The second intermediate should be much longer-lived, should




F2


__ CF2 Ph D Ph D

24-Z 24-E
+A
---^->-=--
CF CF2




23 Ph D Ph D

25-Z 25-E




randomize stereochemically before cyclizing, and should be much more

regioselective in forming the more stable product 24 versus 25. Figure 2-3

gives a reasonable mechanistic sequence of events which could give rise to

the above expectations.

Assuming a lack of steric effects due to the deuterium or fluorine

substituents, 33 and 34 should be formed at similar rates and should have

similar kinetic stabilities in Figure 2-3. They should rotate at similarly









rapid rates to alleviate the likely steric effects due to the phenyl

substituent and form the presumably more stable diradicals 35 and 36.

Because of their high reactivity, it is reasonable to assume a lack of

selectivity for 33 and 34 in their ring closures, although one might expect

that 33, due to its likely mode of rotation to 35, might prefer cyelization

to 25-Z while 34 might to a similar extent prefer cyelization to 24-Z.

Diradicals 35 and 36 in Figure 2-3 should be more stable kinetically

than 33 and 34 due to the relative lack of proximity of their benzyl radical


H






H r24-Z
H ..


11.2 %

.1


- -F


+ 25-Z -

11.0 %


24-Z + 24-E + 25-Z + 25-E

34.9 % 34.9 % 4.0 % 4.0 %



Figure 2-3. Modified Diradical Mechanism Which Quantitatively
Accounts for the Stereochemical Results in Table 2-3. Percent
Yields Given for 100 C Reaction.


33









orbitals to the termini of their allyl radical systems. The allyl radical

systems of 35 and 36 could potentially rotate as shown to form 36' and 35',

respectively, the enantiomeric forms of 36 and 35. The two radical

systems 35 +36' and 36 +35' are not interconvertible, but each should lead

to identical, fully stereorandomized mixtures of 24 and 25. Therefore, the

viability of Figure 2-3 is independent of the relative rate of formation of

33 and 34.

Remarkably, this scheme of mechanistic events has excellent corre-

lation with the experimental results. If one assumes that 33 and 34

undergo ring closure to give only the excess stereochemically-retained Z-

products, the percent yield of products formed from each pathway can be

calculated as shown in Figure 2-3 for the 1000 C reaction. This kinetic

scenario exactly reproduces the 24:25 product ratio (81:19) and the stereo-

chemical Z:E ratios for 24 (57:43) and 25 (79:21) in Table 2-3. One should

verify that the percent yields in Figure 2-3 quantitatively indicate 33 and

34 to be highly stereoselective but nonregioselective, and they indicate 35

and 36 to be highly regioselective but stereorandom in agreement with

previous expectations.

If one applies this same analysis to the lower temperature stereo-

chemical results in Table 2-3, greater selectivity of closure for 33 and 34

is observed as shown in Table 2-6. At 70C, for example, 15% of 24-Z and

10% of 25-Z is formed from 33 and 34 in agreement with thermodynamic

expectations.









Table 2-6. Selectivity of Closure for Intermediates 33 and 34 at
Various Temperatures.


% Excess % Excess
Conditions 24-Z Formed 25-Z Formed


100 oC, 3.0 h 11.2% 11.0%

30 oC, 7.5 h 13.1% 10.4%

70 C, 15.5 h 14.9% 9.9%



It has been assumed, for lack of a better hypothesis, that the pro-

posed diradicals satisfying the stereochemical results are simply conforma-

tional isomers like those depicted in Figure 2-3. However, there is no

direct evidence for the structures of the specific species actually involved,

nor is it even necessary that the two modes of reaction be sequentially

related. They could be parallel pathways. The stereochemical results

merely require two pathways to products 24 and 25, a minor pathway which

is nonregioselective but stereospecific and a major pathway which is regio-

selective but stereorandom. It should be finally stated, however, that the

specific mechanistic scheme presented in this discussion is one which is

consistent with everything that is presently known about allene [2+2]

cycloaddition reactions.

Summary

In summary, the [2 +2] cycloadditions of DFA with para-substituted sty-

renes (H, OCH3, NO2) and the "capto-dative" reagent 1-cyanovinyl acetate*



*It should, perhaps, be mentioned that the reactions of DFA with 1,2-
dichloro-1,2-difluoroethylene (1212) and cis-stilbene were also studied with
little success. In the 1212 reaction, the DFA underwent extensive
oligimerization with no cycloaddition at 147 C, and in the cis-stilbene
reaction, the starting material underwent extensive isomerization to trans-
stilbene under the reaction conditions.









were studied in order to gain a greater mechanistic understanding of allene

[2+2] cycloadditions. The product ratios and regiochemistry were consistent

with previously studied DFA reactions wherein the more thermodynamically

stable product was favored. In addition, a stereochemical study with

Z-B-deuteriostyrene was consistent with a stepwise mechanism since there

was a scrambling of the deuterium label in the products. A relative

reactivity study required an intermediate which was devoid of any polar

character since both para-methoxystyrene and para-nitrostyrene underwent

reaction with DFA more rapidly than styrene itself. These results seemed

consistent with a common-intermediate diradical mechanism for which there

has been substantial literature precedent.

Close examination of the stereochemical results in Table 2-3 suggested

that a common-intermediate diradical mechanism was an oversimplification,

however. The distinctively different retentions of stereochemistry for the

two products in the reaction of DFA with Z-B-deuteriostyrene gave evidence

for two kinetically distinguishable intermediates. The first, highly-reactive

intermediate was thought to be nonregioselective but stereospecific, and the

second, more-stable intermediate was thought to be regioselective but

stereorandom. This helped to account for the fact that there was a

greater retention of stereochemistry in the minor product. The high

reactivity of DFA, the stability of the products under the reaction

conditions, and the preparation of Z-S-deuteriostyrene with an isotopic

purity of 99.3% and greater than 99% Z allowed this stereochemical study

to be completely unambiguous.














CHAPTER 3
STEREOCHEMISTRY OF THE 1,3-DIPOLAR
CYCLOADDITIONS OF FLUOROALLENE

Introduction

Understanding the stereochemical preferences observed in cycloaddition

reactions has been one of the monumental problems of organic chemistry

during the past 50 years. Early excitement was generated by the reactions

of norbornene and related compounds which preferentially underwent exo

attack by a variety of reagents.51'52 Dipolar cycloadditions have played an

important role in the evolution of the stereochemistry problem. Indeed, the

infamous Alder exo rule, published in 1935, was formulated from a study of

the 1,3-dipolar cycloadditions of variously substituted norbornenes with

phenyl azide.53 The focus of this chapter will be on the stereochemical

preferences observed in dipolar cycloadditions of contemporary interest54

and, in particular, on the preferences observed for addition to dipolarophiles

containing an allylic halogen substituent.

Although several 1,3-dipolar cycloadditions of allenes have been

reported,23 only a few stereochemical studies have been done. The

reactions of methoxy and phenoxyallene show a definite preference for anti

attack, presumably due to steric reasons,21,22 while the reaction of

monofluoroallene (MFA) with N,C-diphenylnitrone seems to show a strong

preference for syn addition as discussed in Chapter 1. The use of a

monosubstituted allene to study the stereochemistry of cycloaddition

reactions seems ideal due to the simplicity of the dipolarophile. Thus, an

examination of the reactions of MFA with other nitrones and 1,3-dipoles












syn


MFA C C C


H
anti




would be expected to be quite interesting. In addition, it would be

interesting to do solvent and kinetic studies to obtain a greater under-

standing of the factors affecting the stereochemical outcome of these

reactions. These studies as well as unambiguous structural proofs for the

previously mentioned MFA/diphenylnitrone cycloadducts will be presented.

Similar to past chapters, both the high reactivity of MFA and the lack of a

significant steric demand for the fluorine substituent aid in the attainment

of some extremely interesting and valuable results.

Results

Nitrone Stereochemical Study

When a tenfold excess of MFA42 was allowed to react with each of

three nitrones55-57 37 at 25 C in chloroform, there were two products, 38

and 39, produced in a nearly quantitative yield. The reactions were done in

the dark in NMR tubes which had been sealed under vacuum. The progress

of the reaction was followed by 1H and 19F NMR, and the reactions were

shown to be complete between 6 h and 72 h. The product ratios shown in

Table 3-1 were determined by 19F NMR at 282 MHz, and they were found

not only to be unchanged during the reactions but also to remain unchanged













H F


F





+
Ph \0-



H R
37a-c


250C, CDC13 h
N--O
a) R = CH3
b) R = Ph
c) R = 2-Naphthyl
38a-c


39a-c


Table 3-1. MFA 1,3-Dipolar Cycloadditions with Nitrones.


Product
R Conditions Ratio (38:39)a % Yieldb krel


CH3 72 h 82:18 95 1.0

Ph 10 h 85:15 99 11.6

2-naphthyl 6 h 84:16 90 12.0

aError + 1%
bError 3%; determined by 19F NMR using meta-bromobenzotrifluoride as
an internal standard.


for more than 30 days after the reactions had gone to completion. Further

proof of kinetic control was obtained by heating the minor isomer 39c to

61-64 C for 48 h in chloroform. There was no decomposition or rearrange-

ment of 39c detected. Although 38 and 39 were the sole products found

for the R = Ph and R = 2-naphthyl cases, two additional possible products

were observed by 19F N MR, each in 3% relative yield, for the R = CH3

reaction. They were not identified. The relative rates in Table 3-1 were

determined from the pseudo-first-order rate constants similar to the method

that will be described for the solvent studies.









Products 38 and 39 were separated by flash chromatography37 and

isolated. The R = Ph adducts were previously characterized by Burkholder,

and the 1H and 19F NMR spectra obtained for them were identical to

previous results. The remaining nitrone adducts were characterized by 1H,

13C, and 19F NMR spectra, IR spectra, low and high resolution MS, and

finally by combustion analyses. The regioisomer assignments were straight-

forward as in Burkholder's nitrone adducts, but the stereochemical assign-

ment of the fluorine relative to the phenyl ring was not. The stereo-

chemical assignments were made on the basis of difference NOE experi-

ments for 38a and 39a and by an X-ray crystal structure determination for

38c.

The interpretation of the difference nuclear Overhauser effect (NOE)

experiments39 was forthright. For the irradiation of the PhCH and CH2 in

the 1H NMR, the CHF NOE enhancements were 1.9% and 4.2%, respective-

ly, for 38a. For the analogous irradiations in 39a, the enhancements were



F H H F


Ph Ph

N- N-0
H3C/ H3C/
38a 39a




1.9% and <1%. The difference NOE spectra of 38a and 39a appear in

Figures A-39 and A-44 of Appendix A for the interested reader.

A suitable crystal for an X-ray structure determination was grown by

slow evaporation of solvent from an ethanolic solution of 38c in an









uncapped NMR tube. After diffractometer data collection and structure

refinement, the structure shown in Figure 3-1 was obtained. One can see

that the fluorine is, indeed, sn to the phenyl in the R = 2-naphthyl major

adduct. The nonbonded distances F1--C15 and H4--C15 were 3.19 A and

4.15 A, respectively. (Atom H4, which is not shown in Figure 3-1, is

attached to atom C4.) The details of the X-ray structure determination

along with the bond lengths and bond angles of 38c appear in Appendix B.






C(3)


Figure 3-1. X-Ray Crystal Structure for the R = 2-Naphthyl Major Adduct,
38c.

It was of interest to be able to make firm stereochemical assignments

without having to resort to difference NOE and crystal structure determina-

tions each time a new reaction of MFA was studied. With this in mind,

salient spectroscopic features of stereochemical importance were noted for

the nitrone adducts 38 and 39. These will be discussed using the R = 2-

naphthyl case as an example. First, there was a greater deshielding of cis-








allylic than trans-allylic protons by fluorine. The chemical shift of each

proton cis to the fluorine in the 1H NMR was an average 0.33 ppm further

downfield than that in the other isomer. This is illustrated for 38c and

39c. Also, there was a larger transoid allylic than cisoid allylic H-H


F_ H


6 5.65 H
Ph



00f


6 4.61
nH--


6 5.


4.88
, 6 4.88


coupling constant observed in the 1H spectrum as well as a larger transoid

allylic C-F coupling constant observed in the 13C spectrum. This is shown

in Table 3-2. The average difference between the transoid and cisoid coup-

ling constants was 0.51 Hz for the H-H coupling and 5.4 Hz for the C-F
coupling. In view of this, the C-F coupling constant seemed more helpful

in assigning stereochemistry. Armed with these spectroscopic observations,

one can attempt to elucidate future stereochemical assignments from the

routine spectra obtained for new adducts of MFA.

Table 3-2. 1H and 13C NMR Coupling Constants to CHF for 38c and 39c.

H-H Coupling (Hz) C-F Coupling (Hz)
Compound J(CHPh) J(CH2) J(CHPh) J(CH2)

38c 1.86 1.70 < 1 5.9
39e 1.34 2.19 4.9 < 1


39c









Solvent Study for the Reaction of MFA with N-Methyl-C-phenylnitrone

The reaction of MFA with N-methyl-C-phenylnitrone was studied in

eight solvents of widely different ET values,58 where ET is a measure of

solvent polarity. First, the reaction rate was determined in each solvent at

23-24oC using 1H NMR to follow the progress of the reaction. The

relative rates are shown in Table 3-3. The reactions were run under psuedo-

first-order conditions with a tenfold excess of MFA. Multiple integration

over the nitrone methyl group at 3.95 ppm and the product methyl groups

near 2.65 ppm were obtained for at least five points within the first three

half-lives of the reaction. Plots of log [nitrone] versus time gave good

straight lines with correlation coefficients uniformly greater than 0.999, and

the pseudo-first-order rate constants were determined from them. From

these first-order rate constants, the relative rates of Table 3-3 were cal-

culated. The small spread of the relative rates is in excellent agreement

with the Huisgen et al. classic kinetic studies for nitrones.59

One may wonder why relative rates were reported in Table 3-3 rather

than the more informative second-order rate constants. The reason is that

the [MFA] values upon which the calculation of the second-order rate

constants depends (k2nd = klst/[MFA]) had poor accuracies. This matter is

worth considering further. The concentration of a reactant in excess, like

MFA, is usually well known in a pseudo-first-order kinetic study. Since

MFA is a gas, however, it can partition itself between the solvent and the

evacuated space directly above the solvent in a sealed NMR tube. Thus,

not only is the amount of MFA in the liquid somewhat less than the

amount initially condensed in the tube, but also it is potentially dependent

on the solubility of MFA in the different solvents.60 This can pose

problems. Using para-dioxane as an internal standard, these ideas








Table 3-3. Relative Rates and Product Ratios in Various Solvents for the
Reaction of MFA with N-Methyl-C-phenylnitrone at 23-24 C.


ET (kcal)/ Product
Solvent mole) krela Ratio (38a:39a)b


benzene-d6 34.5 16 80:20

dioxane-d8 36.0 11 75:25

CDC13 39.1 5.4 82:18

CD2C12 41.1 5.3 76:24

acetone-d6 42.2 5.4 60:40

DMSO-d6 45.0 5.2 53:47e

CD3CN 46.0 4.9 61:39

CD30D 55.5 1.0 68:32

aThe pseudo-first-order rate constants are given in the experimental section
in Table 6-1.
bError 1%.
CError 3%.


were tested by 1H NMR. It was found first that for all the solvents of

Table 3-3, there was no significant change in the solubility of MFA within

the uncertainty of the measurement. Second, it was shown that there was

a sevenfold excess of MFA over the nitrone present in the liquid at all

times. This knowledge was sufficient to justify reporting the relative rates

as shown in Table 3-3.

In addition to the relative rates, the product ratio for the reaction of

MFA with N-methyl-C-phenylnitrone was determined in each solvent by 19F

or 1H NMR and confirmed by GLC. These results are also shown in Table

3-3. Surprisingly, rather than to find that the product ratios were all the

same or that there was a good correlation of them with the solvent ET

values, it was found that there was a great deal of scatter in the results.









In spite of this, there was a net decrease in the ratio with increasing ET

value, and the results were reproducible.

A comparison of the benzene-d6, CDC13, and acetone-d6 rates and

ratios in Table 3-3 reveals some interesting results. Although the relative

rate for the MFA/N-methyl-C-phenylnitrone reaction was greater in benzene-

d6 than in CDC13, the product ratios were nearly the same in both

solvents. In comparing the CDC13 and acetone-d6 results, however, the

rates were the same but the product ratios were different. It was decided

to investigate the reaction kinetics in these three solvents more fully.

Kinetic Study for the Reaction of MFA with N-Methyl-C-phenylnitrone

As has been noted in the solvent study, the reaction of MFA with N-

methyl-C-phenylnitrone at 23-24C shows good first-order kinetics with

respect to the nitrone in all of the solvents studied. In separate experi-

ments, the reaction was also shown to be first-order with respect to MFA,

too. The second-order rate constants in CDC13 at 23-250C were calculated

for each of these studies. When MFA was in excess as in the solvent

studies, the rate constant was found to be 8.3 x 10-6 1/mole-sec, and when

the nitrone was in excess, the rate constant was found to be 8.1 x 10-6

1/mole-sec. Since the agreement in these second-order rate constants was

good and since a multitude of experimental errors was best minimized by

keeping [MFA] constant and in large excess, it was decided to continue the

pseudo-first-order kinetic approach in determining the activation parameters

for the reaction of interest.

The kinetics were followed by 1H NMR in the manner previously

described for the solvent studies. Pseudo-first-order rate constants were

obtained for the reaction of MFA with N-methyl-C-phenylnitrone at roughly

0C, 25 C, and 50C in benzene-d6, CDC13, and acetone-d6. The 00C








reactions were done in an ice-water bath inside a tall Dewar, and the 25 oC

reactions were done in water that had equilibrated to room temperature

inside a different Dewar flask. The 500C reactions were done in a well-

stirred oil bath, and separate reactions were used to obtain each point in

the rate plots. All of the reactions were done in the dark to prevent any

trace nitrone rearrangement.61 By carefully controlling the temperatures

and taking most points well within the first two half-lives of the reaction,

good pseudo-first-order rate constants were obtained from lines having

correlation coefficients greater than 0.999.

In order to calculate the second-order rate constants from the avail-

able data, the [MFA] values for each solvent and temperature had to be

determined with good accuracy. Since gaseous MFA partitions itself

between the solvent and the space directly above it in a sealed N MR tube,

this was not a trivial problem. Four samples of MFA in each of the three

solvents were prepared and studied by 1H and 19F NMR at 250C using para-

dioxane and meta-bromobenzotrifluoride as internal standards. The twelve

samples, which were identical to the kinetics samples except for the lack

of nitrone, were then studied by variable-temperature 1H NMR to obtain

good estimates for [MFA] at 0C and 500C as shown in Table 3-4.

Interestingly, although the concentration values seemed to indicate that the


Table 3-4. MFA Concentrations Used to Calculate the Second-Order
Rate Constants.


Solvent [MFA], 0 C [MFA], 250C [MFA], 500C


benzene-d6 1.5 M 1.8 M 2.1 M

CDC13 1.4 M 1.7 M 2.0 M

acetone-d6 1.5 M 1.8 M 2.1 M








solubility of MFA in each of the solvents was about the same, the [MFA]

values were quite dependent on temperature. Taking into account the

instrumental, operator, and calculational errors in the determinations, a

generous [MFA] uncertainty of +0.3 M was used to propagate errors in the

final kinetics results. It is of value to note that the maximum [MFA]

concentration if all of the gas was dissolved in the liquid was calculated to

be 2.6 M.

Using the [MFA] concentrations in Table 3-4 and the previously

determined pseudo-first-order rate constants, the second-order rate constants

were calculated for the reaction of MFA with N-methyl-C-phenylnitrone as

shown in Table 3-5. It was interesting to observe that the half-life for the

reaction in acetone-d6 was less than that in CDC13 at 0C, but that the

opposite was true at 500C.


Table 3-5. Overall Second-Order Rate Constants for the Reaction of MFA
with N -Methyl-C-phenylnitrone.


Rate Constant, ko Half-
Conditions (1/mole-sec) Life, t1/2


benzene-d6, 0.0 C 2.11 x 10-6 60.9 h

benzene-d6, 25.0C 2.11 x 10-5 5.07 h

benzene-d6, 49.7 C 1.44 x 10-4 0.64 h

CDC13, 0.0 OC 6.59 x 10-7 209 h

CDC13, 23.80C 8.29 x 10-6 13.7 h

CDC13, 50.40C 7.20 x 10-5 1.34 h

acetone-d6, 0.0C 7.60 x 10-7 169 h

acetone-d6, 23.8 C 7.39 x 10-6 14.5 h

acetone-d6, 49.7 oC 5.71 x 10-5 1.60 h








The activation parameters for the overall reaction in each solvent

were calculated from the data in Table 3-5. The results are shown in

Table 3-6. The Arrhenius plots had correlation coefficients of 0.99999,

0.9996, and 0.99998 for the benzene-d6, CDC13, and acetone-d6 results,

respectively. Table 3-6 shows that the free energy of activation, AG at

24 C was virtually the same in CDC13 as in acetone-d6, but it was lower

in benzene-d6 as one might have expected from the rate constants and half-


Table 3-6. Overall Activation Parameters for the Reaction of MFA with
N -Methyl-C-phenylnitrone.

Eact (0.5 AGO (0.1 AHV (0.4 AS (1.4
Solvent kcal/mole) kcal/mole)a kcal/mole)a cal/deg-mole)a


benzene-d6 14.9 23.8 14.3 -32.0

CDC13 16.4 24.4 15.8 -28.8

acetone-d6 15.2 24.4 14.6 -32.7

aMean temperature: 24"C.


lives given in Table 3-5. The enthalpies of activation, AH', were relatively

small and the entropies of activation, ASO, were large negative numbers in

keeping with previous kinetic studies of 1,3-dipolar cycloadditions of

nitrones.59 The large uncertainty in [MFA] previously noted was used to

calculate separate tables of activation parameters to determine the

minimum and maximum values for Eact, AG AHV, and AS# in Table 3-6.

The resulting uncertainties are shown in the column headings. Unfortun-

ately, the uncertainties seemed quite large to make reasonable conjecture

about the subtle differences in activation parameters between the solvents.

The product ratio, 38a:39a, for each kinetic study was determined by

19F NMR as shown in Table 3-7. Interestingly, there was little change in



















38a


Table 3-7. Separate Product Second-Order Rate Constants for the Reaction
of MFA with N-Methyl-C-phenylnitrone.


Product k1 for 38a k2 for 39a
Conditions Ratio (38a:39a)a (1/mole-sec) (1/mole-sec)


benzene-d6, 0.000C 80:20 1.69 x 10-6 4.22 x 10-7

benzene-d6, 25.0 C 81:19 1.71 x 10-5 4.01 x 10-6

benzene-d6, 49.7 0C 80:20 1.15 x 10-4 2.88 x 10-5

CDC13, 0.0 C 84:16 5.54 x 10-7 1.05 x 10-7

CDC13, 23.8 C 81:19 6.71 x 10-6 1.58 x 10-6

CDC13, 50.4 C 82:18 5.90 x 10-5 1.30 x 10-5

acetone-d6, 0.0 C 61:39 4.64 x 10-7 2.69 x 10-7

acetone-d6, 23.8 "C 61:39 4.51 x 10-6 2.88 x 10-6

acetone-d6, 49.7 C 63.37 3.60 x 10-5 2.11 x 10-5

aError + 2%, determined by 'IF NMR at 94.1 MHz.


product ratio with temperature within the uncertainty of the experimental

results. The rate constants shown in Table 3-7 for the formation of each

product in the reaction of MFA with N-methyl-C-phenylnitrone were

calculated by multiplying the fractional product ratios by the overall second-

order rate constants in Table 3-5.62 The separate product activation









parameters shown in Table 3-8 were, in turn, calculated from the data in

Table 3-7. Again, the uncertainties in the activation parameters were large

in comparison to the subtle differences between them in the different

solvents. In addition, the activation parameters for the separate products

were also very similar.


Table 3-8. Separate Product Activation Parameters for the
MFA with N-Methyl-C-phenylnitrone.


Reaction of


Product & Eact (0.5 AG (0.1 AH (0.4 AS (1.4
Solvent kcal/mole) kcal/molea kcal/mole)a cal/deg-mole)a


Major Product 38a:

benzene-d6 14.9 24.0 14.3 -32.4

CDC13 16.3 24.5 15.7 -29.5

acetone-d6 15.3 24.7 14.8 -33.3

Minor Product 39a:

benzene-d6 14.9 24.8 14.3 -35.2

CDC13 16.8 25.4 16.2 -30.8

acetone-d6 15.1 24.9 14.5 -35.3

aMean temperature: 24 C.


Ultraviolet (UV) and Infrared (IR) Studies for Nitrones

It was desired to know if there was any significant complexation

occurring between N-methyl-C-phenylnitrone and the more polar solvents of

Table 3-3. The ultraviolet spectrum of N-methyl-C-phenylnitrone was taken

in all eight solvents studied, but the spectra showed little change in Xmax

or molar absorptivity, e The Amax for the longest wavelength absorption

band remained between 299 nm and 295 nm for the first seven solvents,

and it decreased to 288 nm for CD3OD, the hydrogen-bonding solvent.






51

There were no other bands observed between 240 nm and 500 nm in the UV

spectra. The molar absorptivity E was between 1.6 x 104 l/mole-cm and

2.4 x 104 1/mole-cm in each of the solvents studied. The spectral

observations were in excellent agreement with published UV results.63

A possible change in C=N bond length due to complexation was tested

by measuring the C=N infrared stretching frequency, which is known to

appear at 1547 cm-1 for N,C-diphenylnitrone,64 in two key solvents. In

CHC13, the band appeared at 1555 cm-1, and in acetone, the band appeared

at 1551 cm-1. Again, as in the UV studies, there seemed to be little

evidence for significant complexation of the nitrone in more polar solvents.

Reaction of MFA with Diazomethane-d2

Although much effort had been expended in studying the reactions of

MFA with nitrones, it was still unknown whether cycloadditions of MFA

with other 1,3-dipoles also proceeded to predominately give the syn adduct.

Thus, the reaction of MFA with diazomethane-d2 was examined.

Diazomethane-d265 was prepared under nitrogen with due precautions. It

was allowed to warm to 250C from liquid nitrogen temperatures in the

presence of MFA for 55 min. Afterward, the ether was removed by careful


F

F H H F

+ 25C, 55 min D2 + D2(
Et20 N=N N--N
D2C -- N -N
2 40 41


88 : 12

(54 % Isolated Yield)









vacuum transfer to give a 54% isolated yield* of a white solid, which

rapidly melted at 250C. The product was quickly placed in an NMR tube

with cold (<0C) CDC13, and the NMR tube was sealed under vacuum and

stored on dry ice. The product ratio 40:41 was determined to be 88:12 by

300 MHz 1H N MR, and the isotopic purity of the products was shown to be

98%. The reaction was repeated, and the same product ratio was obtained.

A sample N MR spectrum is shown in Figure 3-2. Thus, there are, indeed,

other 1,3-dipoles besides nitrones that prefer to form syn adducts with

MFA.




41 40

H F F H


D D
N=N N=N

12 8 88 %











7.0 5.8 6.6 6.E 6.2 6.0 5.3 :. 5.



Figure 3-2. 300 MHz 1H NMR Spectrum for the MFA/Diazomethane-d2
Adducts.



*Some product likely transferred with the ether to give the less than
quantitative yield.









Discussion

Cycloadditions with Halogen-Substituted Dipolarophiles

The 1,3-dipolar cycloadditions of MFA with nitrones and diazomethane-

d2 show a strong preference for syn addition. The product structural proofs

rest upon difference NOE experiments, an X-ray crystal structure, and

several spectral observations which, when taken together, make the stereo-

chemical assignments completely unambiguous. The reaction of MFA with

N-methyl-C-phenylnitrone has been studied in great detail both kinetically

and in different solvents. The small spread of relative rates in Table 3-3

for solvents of contrasting polarities, the relatively small AH values

obtained, and the large negative ASO values shown in Tables 3-6 and 3-8 all

demonstrate that this reaction is completely consistent with other well-

studied 1,3-dipolar cycloadditions.54 Indeed, the results are in excellent

agreement with the Huisgen et al. classic studies of nitrones,59 which

include an extensive solvent study for the reaction of N-methyl-C-phenyl-

nitrone with ethyl acrylate.

The regiochemistry of these MFA cycloadditions might tentatively be

explained by the molecular orbital (MO) considerations discussed in Chapter

1 for the reactions of 1,1-difluoroallene with diazoalkanes.19 These MO

arguments, which involve secondary orbital interactions due to the fluorines,

would also explain the stereochemical preference observed in the analogous

reaction of MFA with diazomethane-d2. Padwa et al., who have studied

and reviewed the 1,3-dipolar cycloadditions of nitrones with electron-

deficient dipolarophiles like MFA, also use MO arguments to explain the

regiochemical and stereochemical preferences observed in their nitrone

reactions.66 One must be careful in applying these MO explanations,

however. They have often been qualified by steric interactions or other






54


such anomalous factors. Electrostatic MFA-dipole attractions or differential

solvation of the reactants and products may play an important role in

rationalizing the MFA cycloaddition results. Contributions from these

factors may, for instance, explain the scattering of the product ratios with

ET values observed in Table 3-3. Before a complex analysis of these ideas

is undertaken, it is of interest to briefly survey other dipolarophiles with

halogen substituents to determine if syn addition occurs in other cases.

In a 1963 review, Huisgen noted that several 1,3-dipoles underwent

cycloadditions with norbornene 42 and norbornadiene 43a with high stereo-

selectivity to form exo adducts.67 In contrast, other work demonstrated

that the reaction of most halogen-substituted norbornadienes 43b with

diazoethane favored the formation of endo-anti monoadducts,68,69 which in

the case of X = Cl would undergo exo-syn attack to form a bis adduct.

Interestingly, the reaction of 7-fluoronorbornadiene 43c with diazoethane

proceeded differently.70 A careful analysis of the complicated mixture of

mono and bis adducts produced showed that the ratio of initial endo-anti to

initial exo-syn attack by the dipole was approximately 50:50.



X
exo exo-anti- exo-sn




endo endo-anti-- endo-yn

42 43 a) X = H
b) X = I, Br, Cl
c) X = F








More dramatic examples of a halogen helping to induce syn attack on

a double bond are found in the clean cycloadditions of cis-3,4-dichloro-

cyclobutene 44 with diazomethane,71 nitrones,54,72 and nitrile oxides.73 In

many reactions, the product from sy addition is strongly favored. Various





Cl

/ CH 44

H-
anti- H



explanations considered for the contrasteric approach of 1,3-dipoles to 44

include alkene orbital distortion due to the halogens,74 pyramidalization of

the olefinic carbons away from the chlorines,75 dipole-dipole interactions,54

and direct halogen lone-pair interactions with the 1,3-dipole.54,72 Interest-

ingly, studies of the dipolar cycloadditions to allylic mono and disubstituted

cyclopentenes did not show the same stereochemical preferences observed

for 44.75

Factors Affecting the Stereochemical Outcome of MFA Cycloadditions

There seems to be several possible factors that contribute to the

stereochemical outcome of 1,3-dipolar cycloadditions involving halogen-

substituted dipolarophiles. In this chapter, emphasis will be placed on

transition-state dipole-dipole interactions and solvent-reactant or solvent-

product interactions as explanations for the MFA cycloaddition results.

Molecular orbital arguments, which are often invoked to explain such

results, are of questionable value at this point since quite polar transition

states may be involved in the reactions. It would be interesting to see if









the [2+4] reactions of MFA with cyclopentadiene and butadiene, which

involve nonpolar transition states, would also show a preference for syn

addition to MFA. If so, this would give stronger experimental support for

the use of MO arguments in explaining stereochemical cycloaddition

preferences. These ideas are considered in Chapter 4, and a thorough

discussion of the MO arguments for MFA dipolar cycloadditions will be

deferred until then.

As shown in a 1977 review, solvent studies have been performed for

several 1,3-dipolar cycloadditions.54 Although these have often been limited

to rate studies, the regiochemistry of cycloaddition has also been probed for

diazomethanel9,76,77 and some nitrile oxides.78,79 In addition, a few

stereochemically related solvent studies have been reported,73,80,81 and

they give interesting insight into explaining the observed trends in Table

3-3. It has been noted that the greater the dipole moment of a transition

state73 or product,80 the more it is favored in solvents of greater polarity.

The transition states to products 38a and 39a are shown in 45 and 46. As

one can see from 45, the dipole moments in the separate halves of the





F

--
Ph> Ph t,,"
H 'tN' N 05- H Nil +-

Me Me
-i-->-H--->









transition state leading to 38a, the major product, are directly opposed. In

contrast, the dipole moments of 46, which leads to 39a, the minor product,

are less opposed. Thus, the net dipole moment for 46 is somewhat greater

than that for 45, and one would expect that a greater amount of the minor

product would be formed in polar solvents than in nonpolar solvents. This

is consistent with the overall trend observed in Table 3-3.

This transition-state dipole-dipole argument also rationalizes the

regiochemistry and stereochemistry observed in MFA dipolar cycloadditions.

The points of dipole attraction in 45 and 46 are shown by 6+ and 6-. Since

fluorine is such a small element and has such a high electronegativity (i.e.

large 6-), it is not surprising that syn addition is favored for MFA. The

transition-state dipole-dipole argument cannot solely rationalize the

cycloaddition results for two reasons, however. First syn addition of MFA

is favored for all solvents in Table 3-3. If the dipole-dipole explanation

were solely responsible for the results, one would expect that the minor

product 39a would become the major product in more polar solvents.

Second, the trend in product ratios in Table 3-3 is not strictly linear with

ET value as one can see from Figure 3-3. If the dipole-dipole argument

were well-followed, there should be a good correlation.80 Interestingly, if

the CD30D product ratio is ignored, however, the remainder of the points

in Figure 3-3 seem to suspiciously lie on two nearly parallel lines.

There is additional evidence for factors other than dipole-dipole

interactions influencing the reaction of MFA with N-methyl-C-phenylnitrone.

For example, Figure 3-4 shows that there is a rough correlation of the

psuedo-first-order rate constants, which were used to calculate the relative

rates in Table 3-3, with solvent polarity. In general, the rate of reaction












90:10





80:20 -





70:30 -
s y:anti
Product
Ratio

60:40 -


50:50


0 Benzene-d6


0 Dioxane-d8


( CD2C1,


CD3OD G


Acetone-d6 )


1 EnISO-d6
'o


ET (kcal/mole)


Figure 3-3. Correlation of Product Ratios with Solvent ET Values for the
Reaction of MFA with N-Methyl-C-phenylnitrone in Various Solvents.
40-

0 Benzene-d6

20- ~ Dioxane-d8


CD2C12
Log of 10- MTSO-d6
Pseudo- CDC CDOCN
Pseudo 8- 3
First- Acetone-d6
Order k 6-
(106 sec'I)

4-





CD.OD
3)


I I 4I I 6
35 40 45 so SS 60


Figure 3-4.


Correlation of log(k) with Solvent ET Values for the Reaction
of MFA with N-Methyl-C-phenylnitrone.


ET (kcal/mole)









decreases with increasing polarity of the solvent. This trend is easily

explained. The polar reactant N-methyl-C-phenylnitrone is solvated best in

solvents of high polarity, while the less polar products 38a and 39a are

solvated best in solvents of low polarity. Thus, the reaction rate is

greatest in benzene-d6 and least in CD30D.

As one considers this solvation phenomenon for the nitrone and

products, one may wonder why it is not reflected in the activation param-

eters of Table 3-6. One might expect, for instance, that the enthalpy of

activation, AHO would be greater in acetone-d6, where solvent interactions



Polar Solvent Nonpolar Solvent



Transition Transition
State State




A nonsolvated
AH ltd
solvated



Nonsolvated
Nitrone


Solvated
Nitrone



AH > AH
solvated nonsolvated



Figure 3-5. Comparison of AH9 in Polar and Nonpolar Solvents Assuming
Strong Solvent-Nitrone Interactions in the Polar Solvent.









or even complexation of the nitrone might take place, than in benzene-d6.

(See Figure 3-5.) Solvent interactions observed in acetone-d6 might help to

explain the more random product ratios found for more polar solvents in

Table 3-3, too. The actual differences that one would observe between the

activation parameters would be within the uncertainties given for the

kinetics results, however. Table 3-8 shows that the difference in AHO

between the formation of the two products 38a and 39a is at most 1.4

kcal/mole,* irrespective of the fact that 38a is strongly favored in benzene-

d6. Since the spread of relative rates in the solvent study is small, the

differences in AH0 between the solvents would also be expected to be

minimal (<1.5 kcal/mole) in spite of any differences in solvation of the

nitrone. In view of this, it is not surprising that neither the activation

parameters nor the UV and IR studies detected strong solvent interaction or

complexation with the nitrone. The factors affecting the kinetics of the

reaction of MFA with N-methyl-C-phenylnitrone are extremely subtle.

In addition to transition-state dipole-dipole interactions and solvation

of the nitrone and products, there is also the possibility that nucleophilic

solvation of the fluorines may play a role in these dipolar cycloadditions.

The propensity of covalently-bound fluorine to undergo nucleophilic solvation

is reflected by the enormous solvent dependence of 19F NMR chemical

shifts.82 If there were, indeed, greater solvation of the fluorine of MFA in

polar solvents, then this would give another explanation for the

experimentally observed increase in formation of the minor product 39a in

polar solvents for the solvent study. It would be interesting to know how

viable the nucleophilic solvation argument might be. One can obtain a


*There is good literature precedent for this figure (ref. 73).









rough measure of this from the gas-phase basicities,83 which are reflected

by the proton affinities, of fluorine-containing compounds. For example,

the proton affinities, defined as the enthalpies for the gas-phase reaction

BH+-B + H+, of ethane, CH3F, CH3C1, H20, and NH3 are 142, 151, 160,

170, and 205 kcal/mole, respectively.84 From these values, the nucleophilic

solvation argument seems to be of limited value, but it does provide an

interesting explanation for the product ratios in the solvent study results.

Additional work will have to be done to evaluate the contribution of

nucleophilic solvation to the reactions of MFA with 1,3-dipoles.

Summary

To summarize, in spite of one's usual steric prejudices, fluoroallene

(MFA) underwent clean, high-yield 1,3-dipolar cycloadditions with nitrones

and diazomethane-d2 to strongly favor the adduct from syn addition. These

reactions were well-behaved second-order kinetic processes that were com-

pletely consistent with other well-studied 1,3-dipolar cycloadditions. The

products from these reactions were unambiguously characterized using

difference NOE experiments, an X-ray crystal structure, and several spectro-

scopic observations to make the stereochemical assignments.

Although the product regiochemistry and stereochemistry of MFA

cycloadditions could be well explained by molecular orbital arguments, it

was decided to defer this MO discussion until Chapter 4. In Chapter 4, the

[2+4] cycloadditions of MFA with cyclopentadiene and butadiene give

greater insight into how seriously these MO arguments should be considered

for explaining stereochemical preferences. In this chapter, it was shown

that transition-state dipole-dipole interactions were helpful to, in part,

explain the regiochemical and stereochemical preferences observed.






62

However, the reaction of MFA with N-methyl-C-phenylnitrone was also

found to be influenced by the solvation of the nitrone and the cycloadducts.

In addition to these solvation influences, possible nucleophilic solvation of

the fluorines was considered, but more experimental results were needed to

determine its overall contribution to rationalizing the results. In general, it

seemed that all of the factors affecting the stereochemical outcome of

these reactions were extremely subtle in spite of the contrasting product

ratios observed for the solvent study.














CHAPTER 4
STEREOCHEMISTRY OF THE [2+4]
CYCLOADDITIONS OF FLUOROALLENE

Introduction

In the previous chapter, it was shown that an allylic halogen sub-

stituent on a dipolarophile strongly influences the stereochemical outcome

of 1,3-dipolar cycloadditions. Surprisingly, both the past studies of cis-3,4-

dichlorocyclobutene 44 and the current studies of monofluoroallene (MFA)

show a strong preference for syn addition of 1,3-dipoles. In contrast to




Cl


/P C 44
H-
anti- H



this, the [2+4] cycloadditions of 44 have been shown to strongly favor the

product from anti addition.85,86

The [2+4] cycloadditions of MFA with cyclopentadiene, butadiene, and

furan were also studied briefly, but not definitively. The reported syn:anti

addition ratios are approximately 50:50 for reaction with the first two

dienes, but a 70:30 preference for syn addition is claimed for the furan

reaction.12,87 Since the experimental section of this work shows that the

product ratios were not determined from crude reaction mixtures, it was of








interest to repeat these reactions and make more careful studies. Evidence

for syn addition to cyclopentadiene or butadiene would give more credibility

to molecular orbital arguments in rationalizing the stereochemical

preferences observed for the 1,3-dipolar cycloadditions of MFA.

Results

When a tenfold excess of cyclopentadiene was allowed to react with

MFA at 0C for 4.0 days, a quantitative yield of two products, 47 and 48,

was obtained. The product ratio, determined by both 19F N MR and gas-

liquid chromatography (GLC), was found to be 51.0:49.0 + 0.3% with a

miniscule preference for the syn adduct.



F


OOC
4 days

10 eq. 47 48

51.0 : 49.0 (0.3 %)

99 % Yield


When a sevenfold excess of butadiene was allowed to react with MFA

at 110 'C for 38 h in hexane, three adducts 49, 50 and 51 were obtained in

a 44% overall yield. The stereochemical ratio for the [2+4] adducts was

determined to be 59:41 1% by 19F NMR at 282 MHz. The same 49:50

ratio was obtained upon repeating the reaction. Besides the dimers of

butadiene and MFA, there were also three possible minor products observed

by GLC in less than 3% total yield. They were not identified.















1100C, 38 h
hexane


7 eq.


F

49 H


49


59 : 41 (1 %)

38 % Yield


(Also isolated


in 6 % Yield)


The [2+4 ] cycloadducts 49 and 50 were isolated by preparative GLC,

and their 1H and 13C NMR spectra were obtained. The stereochemical

assignments were made on the basis of the 1H NMR chemical shifts given

below and the 13C NMR carbon-fluorine coupling constants shown in Table

4-1. The spectral characteristics were analogous to those of the nitrone


6 2.84


6 2.56


\ "--6 2.12
H


,'--6 2.39








Table 4-1. 13C NMR Coupling Constants to CHF for 49 and 50.


JCF for bis- JCF for
Compound Allylic Carbon, C3 Allylic Carbon, C5


49 3.1 Hz 7.6 Hz

50 7.7 Hz 1.9 (or 5.9) Hza

aThe assignment of C5 and C6 is ambiguous here, but both coupling
constants are smaller than that for C3.


adducts in Chapter 3, whose stereochemical assignments were confirmed by

an X-ray structure and difference NOE experiments.

It should finally be mentioned that a product stability experiment was

performed for 49 and 50. When an isolated mixture of the [2 +4] cyclo-

adducts was heated at 110-115C for 74 h in benzene-d6, there was no

rearrangement and the stereochemical ratio remained the same.

When a tenfold excess of furan was allowed to react with MFA at

51 C for 98 h, two cycloadducts, 52 and 53, were formed in 60% yield.






F
S0 0
\= F H

+510C H
98 h

10 eq. 52 53


63 : 37 (1 %)

60 % Yield









The product ratio was found to be 63:37 + 1% by GLC, and the results

were reproducible in a separate experiment. The stereochemical

assignments were straightforward in view of past spectroscopic studies.

Discussion

Syn-Addition in [2+4 Cycloadditions of MFA

Interestingly, a close examination of the Diels-Alder reactions of MFA

with cyclopentadiene, butadiene, and furan does show some preference for

syn addition of the allene. The preference is not nearly as great as that

observed in the 1,3-dipolar cycloadditions, however. In explaining the stereo-

chemical addition preferences, the cyclopentadiene and furan cases do not

provide strong evidence for the involvement of molecular orbital (MO)

arguments. The syn:anti ratio is practically random for the cyclopentadiene

reaction. Also, the presence of the oxygen in the furan case opens up any

stereochemical arguments to dipole-dipole interactions or other interactions

discussed in Chapter 3. The only reaction which seems supportive of MO

arguments is the one involving butadiene.

The results from the reaction of MFA with butadiene must be treated

with care for several reasons, however. For example, there are two

reasonable pathways to obtain the products. The first is a competing [2+2]

and [2+4] pathway, and the other is a totally diradical mechanism. This

mechanistic problem has been considered at length for the analogous

reaction of 1,1-difluoroallene and butadiene with conclusive evidence support-

ing the former pathway.12,16,88 A more serious consideration is the

possible rearrangement of the cyclobutane adduct 51 to preferentially form

syn adduct 49 under the reaction conditions. This seems unlikely since 54

and 55, which are thermally similar to 51, do not interconvert until

temperatures above 2000 C.88 A final consideration here is leakage from














F

F 0? CF2 >2000: NF

SNC NCL


51 49 54 55





the [2+2] reaction to preferentially form a small amount of 49 from an

intermediate diradical. This, again, seems unlikely since the reaction of

MFA with l,1-dichloro-2,2-difluoroethylenel,9 discussed in Chapter 1

demonstrates that both the formation and ring closure of any intermediate

diradical is completely stereorandom with respect to the fluorine. The

conclusion of this argument is that the reaction of MFA with the less-

reactive, more-selective butadiene does, indeed, give some support to

molecular orbital arguments for explaining the syn preferences observed in

MFA reactions.

Theoretical Study for MFA

Molecular orbital (MO) theory has been enormously successful in

organizing and explaining a vast body of experimental results.89 In fact, it

has been so successful that sometimes simpler explanations for rationalizing

results seem to be overlooked or underplayed in favor of making MO

arguments. An example of this is the invocation of secondary orbital

interactions,74 like the kind possible in MFA, to explain the regiochemistry

and stereochemistry of cycloaddition reactions. This is why the dipole-

dipole interactions and solvation phenomena were considered at length in








Chapter 3. Nevertheless, MO theory does give some interesting insights

into explaining the stereochemical preference of MFA to undergo syn

addition.

It seems clear that the cycloadditions of MFA studied in this

dissertation are members of the class of "concerted" reactions. In the 1,3-

dipolar cycloadditions, the small AH#, large negative ASO, and small spread

of relative rates observed over a wide range of solvent polarities give

testimony to this fact. In the [2+4] reactions, the relatively low tempera-

tures required to produce good yields of the cycloadducts also supports this

statement. The question still remains: What is the origin of the stereo-

chemical preferences observed in the cycloadditions of MFA in terms of MO

theory? Intermediate neglect of differential overlap (INDO) calculations90

for MFA suggest an interesting answer.

The first task in this theoretical study is to determine which orbital

of MFA is involved in the controlling frontier MO interaction. There is

much precedent to suggest that it is the lowest unoccupied MO for MFA.66

Houk et al. have considered the effect of electron-releasing, electron-

withdrawing, and conjugating substituents on the orbitals of dienophiles91

and dipolarophiles.92,93 It is found that, in general, both the highest

occupied molecular orbital, HOMO, and the lowest unoccupied molecular

orbital, LUMO, of the 7r-bond have orbital energies which are lowered

significantly when under the influence of an electron-withdrawing

substituent. A CHF or CF2 group on a carbon-carbon double bond would be

expected to influence the orbitals similar to the electron-withdrawing groups

that Houk considered. Houk also noted that when comparing an electron-

rich dipolarophile with an electron-deficient one, the controlling frontier MO

interaction changed from the HOMO to the LUMO.










These ideas are further illustrated in Figure 4-1. The IN DO

calculations for allene, fluoroallene, and 1,1-difluoroallene indicate that as

more fluorines are placed on allene, the energies of the SHOMO and LUMO

of the C2-C3 rT-bond are both lowered considerably, but the orbital energies

of the HOMO and SLUMO of the nonreactive orthogonal C1-C2 7T-bond

approach each other. This is in excellent agreement with ab initio

calculations for 1,1-difluoroallene.18 Interestingly, the INDO calculations

from Figure 4-1 predict that the orbital energies of MFA are more like

those of difluoroallene than allene itself.


Allene


Fluoroallene


1,1-Difluoroallene


-LU\N0's


- SLLDM

-LUHD





R11)a.


--SUD
-- UN)


- -HOO's


-SHCDO


- SHcIO


Figure 4-1. Comparison of Selected Orbital Energies (Eigenvalues) for
Allene, Fluoroallene, and 1,1-Difluoroallene.


Energy
(in eV)








It should finally be noted that the regiochemistry of nitrone cyclo-

additions is also consistent with having the LUMO of MFA as the

controlling frontier MO interaction. It has been observed that the usual

reactions of nitrones with non-electron-withdrawing dipolarophiles are HOMO-

controlled and 5-substituted isoxazolidines are produced. The 4-substituted

isoxazolidines experimentally observed for the reactions of MFA are found

in LUMO-controlled situations.66
Now that the LUMO of MFA has been established as the controlling

frontier MO for the molecule, it is time to investigate the possibility of

significant secondary orbital interactions in the LUMO wavefunction. A

portion of the eigenvector matrix for MFA has been extracted for the three

carbons in the LUMO as shown in Table 4-2. These eigenvectors show the

proportions in which atomic orbitals mix to form the resultant molecular

orbital. Interestingly, there are significant coefficients of -0.112 and -0.286

for the 2s and 2pz orbitals, respectively, of C1, the carbon bearing the

fluorine. If these orbitals mix as shown, an asymmetric orbital results. If






0 Mix






this asymmetric orbital further mixes with a 2pz orbital on C2, the middle

carbon of MFA, there is greater reinforcement of the wavefunction at C2

on the side of the molecule bearing the fluorine. A computer-generated

graph of the resultant LUMO confirms these expectations as shown in

Figure 4-2. This asymmetric LUMO may give rise to the stereochemical










Table 4-2. Eigenvectors (Coefficients) for the Atomic Orbitals
of the Carbons in the LUMO of Fluoroallene.


Atom Atomic Orbital Eigenveetora


C1 2s -0.112

2Px 0.087

2py 0.014

2pz -0.286

C2 2s 0.034

2px -0.018

2py -0.007

2pz -0.567

C3 2s -0.016

2Px 0.017

2py -0.089

2pz 0.660

aA more complete table of eigenvectors for both MFA and DFA along with
axis orientations are shown in Appendix C.


.--- H-
~I -~, --~N'
1'
i -


Figure 4-2. LUMO of Fluoroallene.


-' :
'~~Y-;---__L-----C--C-- i;
--








preferences observed for concerted cycloadditions of MFA. Indeed, the

preceding argument is completely analogous to that previously made to

explain the regiochemistry of cycloaddition of 1,1-difluoroallene with

diazoalkanes.19

Summary

In summary, it was shown that the syn:anti ratio for the [2+4] cyclo-

addition of MFA with cyclopentadiene was nearly 50:50 while a moderate

preference for syn addition was observed for reactions with butadiene and

furan. Only the butadiene reaction seemed to unambiguously support the

use of molecular orbital arguments in explaining the stereochemical

preferences observed in MFA cycloadditions, however. A theoretical study

of MFA using INDO calculations did suggest that there were secondary

orbital interactions present in the LUMO. A computer-generated graph of

the LUMO showed that the lobe at the middle carbon of MFA was much

larger on the side of the molecule bearing the fluorine. It was thought

that this explanation, in conjunction with past explanations, may help

rationalize the [2+4] and 1,3-dipolar cycloaddition behavior of MFA. One

should probably note the author's comments on this subject in Chapter 5,

however.














CHAPTER 5
CONCLUSION

The mechanistic studies for the thermal cycloadditions of fluoroallene

and 1,1-difluoroallene have been quite fruitful. The small size of the

fluorine substituents and the high reactivity of these allenes have allowed

some very interesting results to be obtained.

A major achievement was made by obtaining conclusive evidence for

two kinetically distinguishable intermediates in a [2+2] cycloaddition

mechanism. This opens the door to research in an area which has not

made significant mechanistic progress for several years. Also, the first, to

our knowledge, Hammett type relative reactivity study of a [2+2] reaction

was described. This may also be a productive area for additional work.

A second major achievement was the conclusive structural proofs for

the adducts obtained in the [2+4] and 1,3-dipolar cycloadditions of fluoro-

allene. It was curiously observed that there seemed to be a marked

preference for syn addition of the allene in spite of one's usual steric

prejudices. Several avenues were explored in trying to rationalize the

allene's cycloaddition behavior, but the studies seemed to prompt more

questions than answers. It is pertinent to briefly elaborate on a few of

these concerns for consideration in future work.

There seemed to be a scattering of product ratios with solvent ET

values for the fluoroallene/N-methyl-C-phenylnitrone solvent study shown in

Table 3-3. It would be interesting to probe more deeply into the solvation

phenomena influencing this reaction to see if a rationalization for the data

could be found. Also, it would be interesting to find a different molecular








system or analytical method to repeat the kinetic studies of Chapter 3.

With reasonable uncertainties for the activation parameters, these studies

could be quite important. Finally, although Chapter 4 proposed that the

syn addition preferences of fluoroallene might be explained by molecular

orbital theory, one should realize that the secondary orbital interactions

observed in semiempirical calculations (like INDO) are not always observed

in higher level calculations.94 Ab initio calculations for fluoroallene would

be quite informative.

In conclusion, there are opportunities for promising research in several

areas here. Continued study of the cycloadditions of fluoroallene and 1,1-

difluoroallene should be productive for many years to come.














CHAPTER 6
EXPERIMENTAL SECTION

General Methods

Infrared (IR) spectra were determined as films between KBr plates or

as CC14 solutions in 0.10 mm matched liquid cells. The IR spectra were

taken on a Perkin-Elmer 283B spectrophotometer and are reported in cm-1.

Ultraviolet (UV) spectra were determined on a Perkin-Elmer 330 spectro-

photometer in the absorbance mode from 240 nm to 500 nm and from 0 to

1.5 absorbance units. Spectro-grade solvents and matched 1.00 cm quartz

cells were used.

Nuclear magnetic resonance (NMR) chemical shifts for 1H and 13C

spectra are reported in parts per million downfield (6) from internal TMS.

Chemical shifts for 19F spectra are reported in parts per million upfield (4)

from internal CFC13. All spectra were taken in CDC13 unless otherwise

specified. The 13C multiplicities were determined with the aid of off-

resonance or INEPT spectra. All NMR spectra were taken on a Varian EM-

360L, a Jeol FX-100, a Jeol FX-90Q, or a Nicolet NT-300 spectrometer.

Mass spectra and exact masses were determined on an AEI-MS 30

spectrometer at 70 eV. Elemental analyses were performed by Atlantic

Microlabs or by the University of Florida microanalysis service. Melting

points and micro-boiling points were determined with a Thomas-Hoover

capillary melting-point apparatus and are uncorrected.

Bulk chromatographic separations were performed by either gas-liquid

chromatography (GLC) or flash chromatography. The GLC separations were









done on a Varian Aerograph A90-P3 gas chromatograph with a thermal

conductivity detector. Flash chromatography was performed with strict

adherence to the published method,37 and 230-400 mesh silica gel was used

in all cases.

The product ratios for the deuterated styrene work were determined

on a Varian Aerograph A90-P3 gas chromatograph. Other product ratios

and percent yields were determined as an average of at least three

injections on a Hewlett-Packard 5790A gas chromatograph with a flame

ionization detector, or by multiple 19F and 1H NMR integration. The

internal standards used to determine the yields are given in the text where

necessary.

Experimental Procedures

2,2-Difluoro-3-phenyl-l-methylenecyclobutane (18a) and 3-Phenyl-l-
(difluoromethylene)cyclobutane (19a).

Into each of three 30-ml pyrex bulbs, each containing 2.75 ml (24.0

mmole) of doubly degassed styrene, was condensed 15.3 mmole of previously

degassed DFA. After sealing under vacuum, the bulbs were placed into a

stirred oil bath at 97-99 C. The bulbs were removed, cooled, and opened

at the end of 3.0 h, 6.3 h, and 8.3 h. The regioisomer ratio 18a:19a was

determined to be 81.8:18.2 (0.3) for each bulb by GLC using a 1/4 in. x 20

ft. 10% DNP column at 128 C (45 ml/min flow), and the reaction was

shown to be essentially complete within 3.0 h by a comparison of the

chromatograms. Further examination of the 3.0 h reaction by TLC, GLC,

and 19F NMR demonstrated that there were no other products other than

the oligimers of DFA. The excess styrene was removed from the 3.0 h

reaction mixture by rotary evaporation at reduced pressure, and the

products were separated by flash chromatography on a 40-mm column with








15 cm of 230-400 mesh silica gel using 25-ml fractions and hexane as the

eluant. In this way, 0.54g (3.0 mmole) of 18a, which was further purified

by preparative GLC at conditions similar to those given above, and 0.04g

(0.2 mmole) of 19a were each isolated as colorless, sweet-smelling oils in a

combined isolated yield of 21%.

18a: Rf=0.34; IR, 3070, 3038, 2938 (w), 1500 (s), 1454, 1432, 1276

(vs), 1155 (vs), 1113 (s), 1050 (vs), 927 (vs), 724 (vs), 694 (vs) cm-1; 1H

NMR (300 MHz) 6 7.32 (m, 5H), 5.60 (m, 1H), 5.30 (m, 1H), 3.99 (d of
virtual quartets, 1H, JHF = 13.4 Hz, JHF = 10.2 HZ, JHH = 10.2 Hz

demonstrated by decoupling experiments), 2.87 (m, 2H); 19F N MR (282 MHz)
91.4 (dddd, 1F, JFF = 209.1 Hz, JHF = 13.6 Hz, JHF = 5.1 Hz, JHF = 2.5

Hz), 107.4 (dd, 1F, JFF = 209.1 Hz, HHF = 10.5 Hz); 13C NMR (75 MHz) 6
144.1 (t, JCF = 20.7 Hz, C1), 135.2 (Ph), 128.5 (Ph), 128.1 (Ph), 127.4 (Ph),

119.0 (dd, JCF = 286.6 Hz and JCF = 278.4 Hz, CF2), 112.2 (olefinic CH2),

49.9 (t, JCF = 21.6 Hz, CHPh), 29.1 (dd, JCF = 14.3 Hz, JCF = 4.0 Hz,

aliphatic CH2); MS, m/e 180 (55), 165 (66), 152 (25), 140 (43), 129 (100),
115 (28), 104 (100), 89 (16), 78 (49), 63 (21), 51 (39), 39 (35); M+ 180.07466

0.00103 (5.7 ppm), calc'd for C11H10F2 180.07506, deviation 0.00039 (2.2

ppm); Anal. Calc'd for C11H10F2: C, 73.31; H, 5.59. Found: C, 73.40; H,
5.59.

19a: Rf=0.46; IR 3070, 3034, 2969, 2931 (s), 1782 (vs), 1496, 1450,

1241 (vs), 1139 (s), 1096 (vs), 749 (vs), 694 (vs) cm-1; 1H NMR (300 MHz) 6

7.25 (m, 5H), 3.58 (virtual pentet, 1H, J = 8.3 Hz), 3.08 (m, 2Htrans), 2.79
(m, 2Hcis); 19F NMR (282 MHz) 4 97.3 (m); 13C NMR (75 MHz) 6 150.8 (t,

JCF = 282.3 Hz, CF2), 144.7 (Ph), 128.6 (Ph), 126.5 (Ph), 126.4 (Ph), 84.0








(t, JCF = 26.2 Hz, C1), 36.1 (CHPh), 31.7 (CH2); MS m/e 180 (33), 165

(36), 159 (31), 152 (19), 140 (11), 129 (73), 115 (20), 104 (100), 91 (12), 78

(60), 63 (17), 51 (42), 39 (23); M+ 180.07468 0.00099 (5.5 ppm), calc'd for

C11Ho1F2 180.07506, deviation 0.00038 (2.1 ppm); Anal. Calc'd for

C11H10F2: C, 73:31; H, 5.59. Found: C, 73.46; H, 5.61.

Similarly prepared samples were heated at 700 for 15 min, 30 min, 1

h, 2 h, 3 h, and 6 h with the ratios (18a:19a = 83.1:16.9 0.2) remaining

constant for all times of reaction even though none of the reactions had

gone to completion.

2,2-Difluoro-3-(4-methoxyphenyl)-l-methylenecyclobutane (18b) and 3-(4-
Methoxyphenyl)-l-(difluoromethylene)cyclobutane (19b)

Into a 15-ml pyrex bulb, which contained 1.50 ml (11.2 mmole) of

doubly degassed para-vinylanisole and 29 mg of hydroquinone, was condensed

4.4 mmole of degassed DFA. After sealing under vacuum, the bulb was

placed into a stirred oil bath at 61-63 0C for 16.0 h. The bulb was cooled

and opened, and the regioisomer ratio 18b:19b was determined to be

86.1:13.9 by GLC using a 1/4 in x 10 ft 10% diisodecylphthalate column at

150 C (73 ml/min flow). The products were separated by flash chromato-

graphy on a 30-mm column using carbon tetrachloride as the eluant. The

colorless products, when isolated, were each purified by preparative GLC at

conditions similar to those given above to afford 121 mg (0.576 mmole) of

18b and 8.0 mg (0.038 mmole) of 19b. The combined isolated yield was

14%.

18b: Rf=0.60; IR, 3007 (w), 2970, 2947, 2847, 1615 (s), 1587, 1517

(vs), 1467, 1445, 1278 (vs), 1252 (vs), 1221 (s), 1182 (s), 1160 (s), 1122 (s),

1080 (s), 1053 (vs), 1037 (s), 933 (s), 835 (s), 812, 782, 745 cm-1; 1H NMR








(100 MHz) 6 7.17 and 6.87 (AB pattern, 4H, JHH = 8.9 Hz), 5.57 (m, 1H),

5.25 (m, 1H), 3.90 (d of virtual quartets, 1H, JHF = 13.5 Hz, JHF = 9.8

Hz, JHH = 9.8 Hz), 3.77 (s, 3H), 2.80 (m, 2H); 19F NMR (282 MHz) ( 91.8

(dddd, 1F, JFF = 208.9 Hz, JHF = 13.5 Hz, JHF = 5.2 Hz, JHF = 2.5 Hz),

107.7 (dd, 1F, JFF = 208.9 Hz, JHF = 10.3 Hz); 13C NMR (25 Mz) 6 158.9

(Ph), 144.2 (t, JCF = 20.8 Hz, C1), 129.1 (Ph), 127.2 (Ph), 119.0 (dd, JCF
286.3 Hz and JCF = 277.7 Hz, CF2), 113.8 (Ph), 112.0 (olefinic CH2), 55.2
(OCH3), 49.2 (t, JCF = 21.3 Hz, CHAr), 29.3 (dd, JCF = 14.7 Hz and JCF

= 4.9 Hz, aliphatic CH2); MS m/e 210 (100), 195 (31), 179 (22), 170 (11),
159 (23), 134 (77), 119 (47), 91 (23), 84 (31), 77 (11), 65 (11), 51 (10); M+

210.08649 0.001179 (5.6 ppm), calc'd for C12H12F20 210.08562, deviation
+ 0.00087 (4.1 ppm); Anal. Calc'd for C12H12F20: C, 68.55; H, 5.76.

Found: C, 68.68; H, 5.79.
19b: Rf=0.73; IR, 2965, 2934, 2840 (w), 1785 (vs), 1613, 1512 (vs),

1461, 1442, 1290, 1250 (vs), 1227, 1179 (s), 1140, 1112, 1096 (s), 1038 (s),

826 (s) cm-1; 1H NMR (100 MHz) 6 7.18 and 6.90 (AB pattern, 4H, JHH =
8.9 Hz), 3.80 (S, 3H), 3.58 (virtual pentet, 1H, JHH = 8.3 Hz), 2.91 (m, 4H);

19F NMR (282 MHz) ( 97.4 (m); 13C NMR (25 MHz) 6 158.2 (Ph), 150.7 (t,

JCF = 282.0 Hz, CF2), 136.8 (Ph), 127.3 (Ph), 113.9 (Ph), 83.9 (t, J = 26.2
Hz, C1), 55.3 (OCH3), 35.4 (CHAr), 31.9 (CH2); MS m/e 210 (100), 195 (26),
179 (18), 159 (22), 134 (100), 119 (61), 91 (41), 84 (44), 77 (11), 65 (22), 49

(53), 39 (12); M+ 210.08584 0.001695 (8.1 ppm), calc'd for C12H12F20
210.08561, deviation + 0.00024 (1.0 ppm).

2,2-Difluoro-3-(4-nitrophenyl)-l-methylenecyclobutane (18c) and 3-(4-
Nitrophenyl)-l-(difluoromethylene)cyclobutane (19c)

Into a 5-ml pyrex bulb, which contained 0.50 ml (3.6 mmole) of
degassed para-nitrostyrene and 20 mg of 4-tert-butylcatechol, was condensed








0.32 mmole of degassed DFA. After sealing under vacuum, the bulb was

placed into a stirred oil bath at 101-103 C for 2.0 h. Several similar

reactions were performed, and the reaction mixtures were combined. The

products were separated by flash chromatography on a 30-mm column using

a 6% ethyl acetate in hexane solution as the eluant. In this way, 110.2 mg

(0.490 mmole) of 18c was obtained as a bright yellow oil. Purification of

19c, which was contaminated with para-nitrostyrene, was attempted by GLC
on a 1/4 in x 10 ft 4% diisodecylphthalate column at 150C (86 ml/min).

This afforded 3.1 mg of a bright yellow oil which contained mostly 19c

with some decomposition products. The combined 18c/19e isolated yield

was 26%.

18e: Rf=0.32; IR, 3085 (w), 2940 (w), 1600 (s), 1514 (vs), 1438, 1348

(vs), 1272 (s), 1218, 1200, 1155 (s), 1119 (s), 1108 (s), 1076 (s), 1048 (s), 930

(s), 850 (s), 730 (s), 692 cm-1; 1H NMR (100 MHz) 6 8.20 and 7.43 (AB

pattern, 4H, JHH = 8.91 Hz), 5.65 (m, 1H), 5.36 (m, 1H), 4.10 (d of virtual


quartets, 1H, JHF = 12.7 Hz, JHF = 9.8 Hz, JHH =

19F NMR (282 MHz) ( 90.8 (dddd, 1F, JFF = 209.7

JHF = 5.0 Hz, JHF = 2.5 HZ), 106.4 (dd, 1F, JFF =
Hz); 13C NMR (25 MHz) 6 147.2 (Ph), 142.9 (t, JCF

(Ph), 129.0 (Ph), 123.5 (Ph), 118.4 (dd, JCF = 288.09

Hz, CF2), 113.2 (olefinic CH2), 49.6 (t, JCF = 21.97

JCF = 13.43 Hz and JCF = 4.88 Hz, aliphatic CH2);
(17), 208 (98), 185 (5), 179 (74), 178 (100), 177 (50),

(86), 149 (22), 146 (19), 128 (74), 119 (60), 103 (60),


9.8 Hz), 2.92 (m, 2H);

Hz, JHF = 12.9 Hz,

209.7 Hz, JHF = 10.1

= 20.75 Hz, C1), 142.6

Hz and JCF = 278.32

Hz, CHAr), 28.9 (dd,

MS m/e 225 (7), 209

164 (42), 160 (25), 159

91 (33), 77 (72), 63


(20), 51 (28), 43 (66), 39 (24), 32 (32); M+ 225.06213 0.00045 (20 ppm),

calc'd for C11H9F2NO2 225.06013, deviation + 0.00199 (8.87 ppm); Anal.









Calc'd for C11H9F2NO2: C, 58.66; H, 4.04; N, 6.22. Found: C, 58.63; H,

4.08; N, 6.20.

19c: Rf=0.50; 1H NMR (100 MHz) 6 8.19 and 7.41 (AB pattern, 4H,

JHH = 8.91 Hz), 3.75 (virtual pentet, 1H, JHH = 8.25 Hz), 3.01 (complex m,
4H); 19F NMR (282 MHz) c 96.4 (m).

Quantitative Individual and Competitive Olefin + 1,1-Difluoroallene
Experiments

The fifteen 5-ml pyrex bulbs used in these experiments were all

washed thrice with distilled water, thrice with acetone, and dried overnight

in a 118C oven. The styrene, p-vinylanisole, and 1-cyanovinyl acetate

starting materials were shown by 100 MHz 1H NMR and GLC to be of

sufficient purity to use directly from their bottles. The p-nitrostyrene was

purified by flash chromatography before use. The DFA was purified by low-

temperature* preparative GLC on a 1/4 in x 20 ft 20% SE-30 column at 640

C (60 ml/min flow) to afford a material of extremely high spectroscopic

purity.

For each of the individual olefin + DFA experiments, 0.500 ml of an

olefin was placed into a 5-ml bulb with either 10 mg of hydroquinone or, in

the case of p-nitrostyrene, 20 mg of 4-t-butylcatechol. After the olefin

had been degassed twice, enough previously degassed DFA was condensed

into the bulb so that an olefin:DFA mole ratio of 10:1.0 was maintained.

The bulb was sealed under vacuum and placed into a well-stirred oil bath at

the reaction conditions shown in Tables 2-1 and 2-4 of Chapter 2. The

bulb was cooled and opened, and the regioisomer ratio and percent yield



*The syringe was cooled on dry ice for at least 15 min before each
injection.








were determined by GLC and/or 19F N MR integration of the reaction

mixture. The method used for the competition experiments was similar

except that all reactions were carried out at 100C for 2.0 h and the

styrene:variable olefin:DFA mole ratio was 5:5:1 with 0.250 ml of styrene

used in each run. The results are given in Tables 2-2 and 2-5.

Examination of these product mixtures by 19F NMR, TLC, and GLC

indicated that no products other than the oligimers of DFA were formed.

Yields were determined by comparison of each reaction mixture to a known

composition of purified major isomer, 18 or 27, in its respective starting

olefin as an external standard. Each reaction mixture and standard-

containing solution was analyzed by GLC (1/8 in x 10 ft, 20% DNP) from 3

to 5 times, with the average being reported.

For all of the reactions involving a-nitrostyrene and for the styrene/2-

methoxystyrene competitive olefin experiment, the regioisomer ratios were

determined by 282 MHz 19F NMR in CDC13. Use of m-bromobenzotri-

fluoride as an internal standard allowed calculation of the % yields. The

samples were all degassed and sealed under vacuum. A T1 population

inversion experiment33 showed that the longest T1 < 6 sec for these

compounds, and thus, a 10 sec pulse delay was used.

(Z)-6-Deuteriostyrene (23)

A solution of 40.2g (0.335 mole) of catecholborane and 36.2 g (0.351

mole) of phenylacetylene-d117 was stirred under nitrogen at 100C for 20

min. During slow warming with an oil bath, an exothermic reaction took

place at 550 C. It was controlled by periodic application of an ice-water

bath. After stirring for 2.0 h at 70C, 700 ml of distilled water was

slowly added and the milky white mixture was stirred at 890C for 4.0 h.









Upon cooling, the filtered solid was washed with hexane and recrystallized

from water using hot filtration. In this way, 36.0 g of white plates were

collected. These were dried at 60 C, in vacuo, overnight to afford 20.0 g

(50.9 mmole) of the trimer of c-deuterio-8-phenylethenylboronic acid 21

(45.6%); m.p. 150-153oC; 1H NMR (acetone-d6) 6 7.50 (m); M+ 393.1982 +

0.0025 (6.4 ppm), calc'd for C28H18D3B303 393.1958, deviation 0.00238

(6.1 ppm); M.S. m/e (relative intensity), 393 (100), 131 (38.5), all others

<15% rel. intensity.

The trimer (20.0 g, 0.134 mole), 8.38 g (0.135 mole) of ethylene

glycol, and 275 ml of toluene were heated at reflux with the azeotropic

removal of water for 20 h. The solvent was removed by rotary evaporation

at 20 torr to leave a liquid, which crystallized upon refrigeration. Re-

crystallization from n-hexane using hot filtration afforded 18.7 g (0.107

mole) of white needles (79.9%) of 2-((Z)-c-deuterio-6-phenyl)ethenyl-l,3,2-

dioxaborolane 22 which was stored in a vacuum dessicator; m.p. 47.5-49.0

C; 1H NMR (CDC13, 100 MHz) 6 7.50 (m, 6H), 4.24 (s,4H).

To a melted solution of 18.5 g (0.106 mole) of the boronic ester and

100 mg of 4-t-butylcatechol at 65 C under nitrogen was added 67.1 g (1.12

mole) of freshly distilled (29C, 4.2 torr) glacial acetic acid dropwise.

After stirring at 90C for 1.5 h, the cooled solution was poured onto a 200

ml ice/100 ml pentane mixture, and after vigorous stirring, the water layer

was extracted with pentane. The combined organic extract was washed

with 0.5 N KOH solution and distilled water, dried, and the solvent was

removed by rotary evaporation at 200 torr. Removal of the last traces of

pentane by a stream of dry nitrogen followed by vacuum transfer of the

colorless residue gave 8.52 g (0.0810 mole) of cis-deuteriostyrene 23

(76.5%). The product was stored under nitrogen at -10 C with









approximately 50 mg of 4-t-butylcatechol added. The 300 MHz 1H NMR

spectrum agreed with the published spectrum17 and none of the trans

deuterated isomer was detected. A comparison of the mass spectrum of

styrene at 12 eV with that of the cis deuterated material suggested an

isotopic purity of 99.3 0.7%.

(E) and (Z)-4-Deuterio-2,2-difluoro-3-phenyl-1-methylenecyclobutane (24) and
(E) and (Z)-2-Deuterio-3-phenyl-l-(difluoromethylene)cvclobutane (25)

A representative procedure for the deuterated runs in Table 2-3 of

Chapter 2 is given below. Into a clean, dry 30-ml pyrex bulb containing a

doubly degassed mixture of 2.00 ml (17.2 mmole) of 99.3% Z-B-deuterio-

styrene and 41 mg of hydroquinone was condensed 7.9 mmol of previously

triply degassed DFA. After sealing under vacuum, the bulb was placed in a

stirred oil bath at 79-810C for 7.5 h. The bulb was cooled and opened, and

the regioisomer ratio 24:25 was determined to be 82:12 by GLC using a 1/4

in x 10 ft 10% DNP column at 1400C (48 ml/min flow). (In the 700C run

of this reaction, the GLC yield was determined at this point with the aid

of a calibration curve.95 A previously purified sample of 24 was used as

the external standard.) The excess Z-B-deuteriostyrene was removed by

vacuum transfer at 12 torr, along with some 25, unfortunately.

Crude separation of the residue by flash chromatography on a 45-mm

column using 14 cm of 230-400 mesh silica gel, 25-ml fractions, and hexane

as the eluant afforded 77 mg (0.42 mmole) of 25 followed by 378 mg (2.09

mmole) of 24 for a combined isolated yield of 32%. All trace impurities

were removed by repeating the flash chromatography for each separated

isomer. The ratios of Z to E isomers of 24 and 25 were determined by

300 MHz 1H NMR using a pulse delay of 30 sec to insure relaxation of the








protons integrated. In this way the stereochemical ratios for the deuterium

Z versus E to the phenyl ring were determined to be 58:42 for regioisomer

24 and 79:21 for 25. Analysis of each isolated regioisomer by GLC at

conditions similar to those given above showed a single peak. Examination

of the recovered Z-l-deuteriostyrene by 100 MHz 1H NMR demonstrated

that there was no isomerization of the starting material under the reaction

conditions.

An additional control whereby 19.5 mg of 25 was heated in heptane at

100C for 6 h showed no isomerization of 25 to the more stable 24 and no

change in the Z to E ratio of recovered 25.

1-Acetoxy-2,2-difluoro-3-methylenecyclobutanecarbonitrile (27) and 1-Acetoxy-
3-(difluoromethylene)cyclobutanecarbonitrile (28)

Into a 15-mi pyrex bulb, which contained 1.50 ml (14.0 mmole) of

doubly degassed 1-cyanovinyl acetate and 30 mg of hydroquinone, was

condensed 2.3 mmole of degassed DFA. After sealing under vacuum, the

bulb was placed into a stirred oil bath at 79-80 C for 8.0 h. The bulb was

cooled and opened, and the regioisomer ratio 27:28 was determined to be

78.7:21.3 by GLC using a 1/4 in x 10 ft 10% diisodecylphthalate column at

137 C (55 ml/min flow). The excess 1-cyanovinyl acetate was removed by

short-path distillation at 12 torr to obtain 150 mg of a brown oil. The

products were separated by preparative GLC at the conditions described

above to afford 33.5 mg (0.179 mmole) of 27 and 4.1 mg (0.022 mmole) of

28 for a combined isolated yield of 8.7%. Both products were colorless

oils. Product 28 was the first eluted.

27: IR 3008 (w), 2948 (w) 1765 (vs), 1421 (s), 1370 (s), 1283 (vs), 1221

(vs), 1192 (vs), 1127 (vs), 1040 (vs), 1000 (s), 980 (s), 960 (s), 936, 907 (s),








836, 840, 730 (s) cm-l; 1H NMR (100 MHz) 6 5.84 (m, 1H), 5.57 (m, 1H)

3.39 (d of m, 1H, J = 17.6 Hz), 3.03 (d of m, 1H, J = 17.6 Hz), 2.21 (s,

3H); 19F NMR (282 MHz) > 98.0 (d of m, 1F, JFF = 212.0 Hz), 108.8 (d of

m, 1F, JFF = 212.0 Hz); 13C NMR (25.1 MHz) 6 168.0 (C=O), 137.1 (t, JCF
= 21.4 Hz, C3), 118.1 (olefinic CH2), 114.4 (dd, JCF = 296.0 Hz and JCF =

289.9 Hz, CF2), 113.5 (CN), 71.7 (t, JCF = 24.4 Hz, C1), 37.7 (dd, JCF =

9.2 Hz and JCF = 4.3 Hz, aliphatic CH2), 19.9 (CH3); MS m/e (no M- 127
(3.5), 109 (0.8), 100 (1.3), 90 (3.2), 82 (1.2), 76 (1.5), 75 (1.9), 64 (2.9), 51

(2.0), 43 (100.0), 39 (3.7), 32 (3.5); Anal. Calc'd for C8H7F2NO2: C, 51.34;
H, 3.78; N, 7.48. Found: C,51.26; H, 3.80; N, 7.40.

28: IR 2932 (w), 1790 (vs), 1762 (vs), 1411 (w), 1368, 1267 (vs), 1204

(vs), 1150, 1083, 1027 (s), 922 (w), 569 (w); 1H NMR (100 MHz) 6 3.53 (d of

m, 2H, J = 16.2 Hz, 3.14 (d of m, 2H, J = 16.2 Hz), 2.16 (s, 3H); 19F
NMR (282 MHz) P 91.7 (m); 13C NMR (75.3 MHz) 6 168.6 (C=0), 151.9 (t,

JCF = 285.5 Hz, CF2), 117.6 (CN), 78.0 (t, JCF = 29.5 Hz, C3), 64.6 (C1),
38.0 (t, JCF = 2.0 Hz, CH2), 20.5 (CH3); MS m/e (no MI), 127 (9.2), 108

(0.6), 100 (1.5), 90 (2.6), 76 (1.5), 75 (2.6), 64 (3.1), 51 (2.2), 43 (100.0), 39
(3.3), 32 (1.8).

N-(2-N aphthyl)-C-phenylnitrone (37e)

Into an Erlynmeyer flask was placed 11.2 g (64.7 mmole) of 2-nitro-

naphthalene, 315 ml of tetrahydrofuran, and 29.2 g of water. To this
rapidly stirred solution was added aluminum amalgam in small pieces over a

period of 1 h. The amalgam was prepared by tearing 3.75 g (139 mmole)
of aluminum foil into approximately 40 pieces and placing the pieces
separately into a 2% mercuric chloride solution for 15 to 30 sec followed









by washing with ethanol and tetrahydrofuran.96 The amalgam was used

immediately. The reduction was exothermic (490C) and a brown precipitate

formed. After all the amalgam had decomposed, the precipitate was

removed by suction filtration, and the solvent was removed by rotary

evaporation at reduced pressure to give an orange solid. The crude product

was recrystallized from chloroform followed by washing with benzene and

hexane to give, when dry, 6.58 g (41.3 mmole) of orange plates for a 63.9%

yield. The N-(2-naphthyl)hydroxylamine had a melting point of 135-1370C.

Into an Erlynmeyer flask was placed 6.23 g (39.1 mmole) of freshly

recrystallized N-(2-naphthyl)hydroxylamine and 70 ml of absolute ethanol.

The mixture was heated to 55 C to dissolve the amine whereupon 4.22 g

(39.8 mmole) of benzaldehyde was added with vigorous swirling. The

solution was set aside in the dark to cool. After 2.0 hours, the crude

product was collected by suction filtration and recrystallized twice from

ethanol to give 5.88 g (23.8 mmole) of pale yellow plates for a 60.9% yield

of 37c.

37c: mp 134-135 C; IR (CHC13) 3062, 2990 (s), 1508, 1445 (s), 1401

(vs), 1172 (s), 1079, 1063 (s), 900, 889, 807 (s), 683 (s), 425 (s) cm-1; 1H

NMR (100 MHz) 6 7.25-8.25 (m); MS m/e 247 (8), 231 (46), 230 (35), 141

(100), 140 (21), 127 (51), 115 (22), 114 (17), 77 (21); M+ 247.0998 0.0015

(6.0 ppm), calc'd for C17H13NO 247.0997, deviation +0.0001 (0.5 ppm).

(E) and (Z)-2-Methyl-3-phenyl-4-(fluoromethylene)isoxazolidines (38a) and
(39a).

Into an NMR tube, which contained a triply degassed solution of 120

mg (0.888 mmole) of N-methyl-C-phenylnitrone55 in 1.5 ml of CDC13, was

condensed 3.2 mmole of degassed MFA. The tube was sealed under vacuum

and kept at room temperature in the dark for 6 days. The progress of the








reaction was followed by 1H and 19F NMR. Upon completion, there were

two products present in an 82:18 ratio by 19F NMR. After the tube was

cooled and opened, the solvent was evaporated using a stream of dry

nitrogen to give 173 mg of an amber oil. The products were separated by

flash chromatography using silica gel and a 5% solution of ethyl acetate in

hexane. In this way, 26 mg (0.14 mmole) of 39a followed by 119 mg (0.616

mmole) of 38a were obtained as pale yellow oils for a combined isolated
yield of 84%. The stereochemical assignments were made, in part, with the

aid of difference NOE experiments as discussed in Chapter 3.

38a: Rf=0.29; IR (neat) 3032, 2960, 2852, 1707 (s), 1490, 1450, 1104

(vs), 1085 (vs), 1019 (s), 748 (s), 693 (vs), cm-l; 1H NMR (300 MHz) 6 7.34

(m, 5H), 6.61 (dm, JHF = 81.8 Hz, 1H), 4.52 (m, 2H), 4.43 (broad s, 1H),
2.68 (s, 3H), 19F NMR (282 MHz) P 131.3 (dm, JHF = 81.8 Hz); 13C NMR

(25 MHz) 6 141.3 (d, JCF = 225.1 Hz, CHF), 138.3 (Ph quat.), 128.4 (Ph),
127.8 (Ph), 126.8 (d, JCF = 8.6 Hz, olefinic quat. C4), 72.2 (CHPh), 65.5 (d,

JCF = 7.3 Hz, CH2), 43.5 (CH3); MS, m/e 193 (49), 148 (78), 147 (100), 146
(29), 133 (37), 116 (65), 115 (46), 77 (21), 51 (22), 42 (34); M+ 193.0899 -

0.0013 (6.7 ppm), calc'd for C11H120NF 193.0903, deviation -0.0004 (2.1

ppm); Anal. Calc'd for C11H120NF: C, 68.37; H, 6.26; N, 7.25. Found:
C, 68.21; H, 6.28; N, 7.20.

39a: Rf=0.46; IR (neat) 3030, 2960, 2850, 1710 (s), 1490, 1452, 1092

(vs), 1022 (s), 755 (s), 698 (vs), cm-1; 1H NMR (300 MHz) 6 7.33 (m, 5H),
6.15 (dm, JHF = 81.5 Hz, 1H), 4.71 (m, 2H, non-first-order AB pattern),

4.01 (broad s, 1H), 2.64 (s, 3H); 19F NMR (282 MHz) c 134.0 (dm, JHF

82.0 Hz); 13C NMR (25 MHz) 6 142.8 (d, JCF = 251.5 Hz, CHF), 137.3 (Ph
quat.), 129.2 (d, JCF = 12.2 Hz, olefinic quat. C4), 128.7 (Ph), 128.4 (Ph),









73.2 (d, JCF = 7.3 Hz, CHPh), 66.4 (d, JCF = 3.7 Hz, CH2), 42.9 (CH3);

MS, m/e 193 (49), 148 (76), 147 (100), 146 (23), 133 (39), 116 (70), 115 (52),

77 (22), 51 (23), 42 (33); M+ 193.0889 0.0019 (9.6 ppm), calc'd for

C11H120NF 193.0903, deviation -0.0014 (7.3 ppm); Anal. Calc'd for

C11H120NF: C, 68.37; H, 6.26; N, 7.25. Found: C, 68.18; H, 6.28; N,

7.21.

(E) and (Z)-2-(2'-N aphthyl)-3-phenyl-4-(fluoromethylene)isoxazolidines (38c)
and (39e)

Into an N MR tube, which contained a triply degassed solution of 217

mg (0.877 mmole) of N-(2-naphthyl)-C-phenylnitrone 37c in 1.5 ml of

CDC13, was condensed 2.9 mmole of MFA. The tube was sealed under

vacuum and kept at room temperature in the dark for 41 h. The progress

of the reaction was followed by 1H and 19F NMR. Upon completion, there

were two products present in an 84:16 ratio by 19F NMR. After the tube

was cooled in a dry ice/isopropanol bath and opened, the solvent was

evaporated using a stream of dry nitrogen to give 383 mg of a dark amber

oil. The products were separated by flash chromatography using silica gel

and a 4% solution of ethyl acetate in hexane. In this way, 32 mg (0.10

mmole) of 39c followed by 220 mg (0.720 mmole) of 38c were obtained for

a combined isolated yield of 93%. The products were recrystallized from

methanol to give snow white solids. A monocrystal suitable for an X-ray

structure determination was grown by slow evaporation of solvent from an

ethanolic solution of 38c in an uncapped NMR tube. The X-ray structure is

shown in Appendix B.

A product stability experiment was performed by heating the minor

isomer 39c at 61-64 C for 48 h in CDC13. There was no decomposition or

rearrangement of 39c detected by 100 MHz 1H NMR.








38c: mp 91-920C; Rf=0.31; IR (CC14) 3067, 3035, 2862, 1709 (s), 1632

(vs), 1599 (s), 1508, 1495, 1467, 1453, 1355, 1098 (vs), 1029 (s), 850 (s), 695
(s) cm-1; 1H NMR (300 MHz) 6 7.45 (m, 12H), 6.49 (ddt, 1H, JHF = 81.5
Hz, JHH = 1.7 Hz, JHH = 1.9 Hz by decoupling experiments), 5.65 (broad s,

1H), 4.61 (m, 2H, non-first-order AB pattern); 19F NMR (282 MHz) c 128.6
(dm, JHF = 81.9 Hz); 13C NMR (75 MHz) 6 147.6 (Ar quat.), 141.2 (d, JCF

= 255.6 Hz, CHF), 139.1 (d, JCF = 1.4 Hz, Ar quat.), 133.8 (Ar quat.),
129.9 (Ar quat.), 128.9 (Ar), 128.6 (Ar), 127.8 (Ar), 127.5 (Ar), 127.2 (Ar),

127.0 (Ar), 126.5 (Ar), 125.3 (d, JCF = 10.4 Hz, olefinic quat. C4), 124.3
(Ar), 116.8 (Ar), 110.6 (Ar), 69.6 (CHPh), 65.8 (d, JCF = 5.9 Hz, CH2); MS,

m/e 305 (100), 256 (18), 148 (36), 147 (69), 143 (16), 133 (24), 128 (21), 127
(56), 115 (59), 77 (17); M+ 305.1212 0.0015 (4.8 ppm), calc'd for

C20H160NF 305.1215, deviation -0.0004 (1.3 ppm); Anal. Calc'd for

C20H160NF: C, 78.67; H, 5.28; N, 4.59. Found: C, 78.79; H, 5.34; N,
4.56.

39c: mp 110-111C; Rf=0.46; IR (CC14) 3067, 3035, 2861, 1713 (s),
1633 (s), 1600 (s), 1508, 1495, 1467, 1455, 1355, 1106 (vs), 1030, 850, 698

(vs) cm-l; 1H NMR (300 MHz) 6 7.46 (m, 12H), 6.47 (ddt, 1H, JHF = 80.7
Hz, JHH = 2.2 Hz, JHH = 1.3 Hz by decoupling experiments), 5.22 (broad s,

1H), 4.88 (m, 2H, non-first-order AB pattern); 19F NMR (282 MHz) 4 129.2
(dm, JHF = 81.3 Hz); 13C NMR (75 MHz) 6 147.0 (Ar quat.), 142.5 (d, JCF
= 256.0 Hz, CFH), 139.1 (Ar quat.), 133.3 (Ar quat.), 129.7 (Ar quat.), 128.4

(Ar), 128.3 (Ar), 127.6 (Ar), 127.1 (Ar), 126.9 (Ar), 126.7 (Ar), 126.3 (d, JCF
= 10.2 Hz, olefinic quat. C4), 126.0 (Ar), 124.1 (Ar), 117.0 (Ar), 111.3 (Ar),
69.5 (d, JCF = 4.9 Hz, CHPh), 66.1 (CH2); MS, m/e 305 (40), 256 (23), 148

(30), 147 (87), 133 (32), 128 (44), 127 (100), 115 (89), 77 (33), 51 (27); M+









305.1210 0.0013 (4.2 ppm), calc'd for C20H160NF 305.1216, deviation

-0.0006 (2.1 ppm); Anal. Calc'd for C20H160NF: C, 78.67; H, 5.28; N,

4.59. Found: C, 78.81; H, 5.49; N, 4.41.

Relative Rate Studies for the Reaction of Fluoroallene with N-Methyl-C-
phenylnitrone in Various Solvents and with Nitrones in General

Into each of eight 9-inch NMR tubes, which each contained a degassed

solution of 20.0 mg (0.148 mmole) of N-methyl-C-phenylnitrone in 0.700 ml

of deuterated solvent, was condensed 1.47 mmole of degassed 99% MFA.

The tubes were sealed under vacuum and allowed to remain at 23-24 C in

the dark. The solutions were agitated by hand periodically. The progress

of the reactions was followed by 60 MHz 1H NMR. At least five integra-

tions of the nitrone methyl group at 3.95 ppm and the two isoxazolidine

product methyl groups at 2.65 ppm were obtained for each of five or more

points within the first three half-lives of the reaction. Plots of log(nitrone)

versus time were constructed from the data obtained for each tube, and the

pseudo-first-order rate constants were determined from them. The results

are summarized in Table 6-1. The reactions were allowed to go to comple-

tion, and the product ratios were determined by 19F NMR at 282 MHz as

shown in Table 3-3 of Chapter 3. The product ratios for the DMSO-d6 and

CD2C12 reactions were also determined by 1H NMR at 100 MHz. (There

was a strong solvent-product interaction observed in the DMSO-d6 19F NMR

spectrum of the isoxazolidine products which made that 19F N MR ratio

ambiguous.) Afterwards, the N MR tubes were cooled in a dry ice/isopro-

panol bath and opened, and the reaction mixtures were analyzed by GLC on

a 1/8 in x 10 ft 10% DNP column at 145C (11 ml/min flow). The product

ratios obtained by GLC were consistent with those obtained by 19F NMR.









Table 6-1. Pseudo-First-Order Rate Constants for the Solvent Study of the
Reaction of MFA with N-Methyl-C-phenylnitrone at 23-240C.

Pseudo-First- Correlation # of Points
Solvent Order k (see-1) Coefficient on Line


Benzene-d6 (2.81 0.04) x 10-5 0.9997 5

Dioxane-d8 (1.95 0.02) x 10-5 0.9996 7

CDC13 (9.58 0.19) x 10-6 0.9994 5

CD2C12 (9.48 0.26) x 10-6 0.9989 5

Acetone-d6 (9.57 0.12) x 10-6 0.9998 5

DMSO-d6 (9.22 0.20) x 10-6 0.9990 6

CD3CN (8.77 0.16) x 10-6 0.9994 6

CD3OD (1.78 0.03) x 10-6 0.9991 7


The relative rate study of the reaction of MFA with various nitrones

was done in a completely analogous way to the solvent studies. The results

are shown in Table 3-1 of Chapter 3. Into each of the 9-inch NMR tubes

was placed 0.150 mmole of nitrone, 0.700 ml of CDC13, and 1.47 mmole of

99% MFA. There was also 6.90 il of meta-bromobenzotrifluoride added to

each tube to calculate the yields by 19F N MR, and 4.70 vl of para-dioxane

added to aid in monitoring the loss of the nitrone in the relative rate

study. The tubes were sealed under vacuum and allowed to remain at 23-

24 C in the dark as before. The progress of the reaction was followed by

60 MHz 1H NMR, and the pseudo-first-order rate constants were determined

from the data as shown in Table 6-2. The reactions were allowed to go to

completion, and the product ratios and yields were determined by 19F NMR

as shown in Table 3-1.









Table 6-2. Pseudo-First-Order Rate Constants for the Reaction of MFA
with Various Nitrones at 23-24 C in CDC13.


Pseudo-First Correlation # of Points
Nitrone Order k (sec-1) Coefficient on Line


37a (N-methyl) (9.58 0.19) x 10-6 0.999 5

37b (N-phenyl) (1.11 0.04) x 10-4 0.999 4

37c (N-(2-naphthyl)) (1.15 0.07) x 10-4 0.996 4



In all of the 19F NMR spectra of these solvent and nitrone studies,

there were some impurities present. A spectrum of MFA alone demon-

strated that all except two of the impurities were present in the starting

material. These two possible products, which were observed only in the N-

methyl-C-phenylnitrone reactions, were each present in an average 3%

relative yield. They were not identified. Even though the MFA was 99%

pure by NMR integration, the impurities were readily apparent in the 19F

NMR spectra since MFA was in tenfold excess compared to the nitrone in

all of the kinetic studies. It was also observed by 19F N MR that the

product ratios did not change during the course of these reactions. In

addition, the ratios remained unchanged for more than 30 days after the

reactions had gone to completion.

Kinetic Studies for the Reaction of Fluoroallene with N-Methyl-C-phenyl-
nitrone in Benzene-dr, Chloroform-d, and Acetone-df

The reaction of MFA with N-methyl-C-phenylnitrone had already been

shown to be first order with respect to the nitrone in the relative rate

studies. The reaction was also shown to be first order with respect to

MFA. With the nitrone in tenfold and sevenfold excess compared to MFA,

the second-order rate constants were determined to be 6.0 x 10-6 1/mole-