The effect of styrene α-substituents on the regiochemistry of [2+2] cycloadditions with difluoroallene.

Tetrahedron Letters,Vol.28,No.l4,pp Printed in Great Britain

1491-1492,1987

0040-4039/87 $3.00 + .OO Pergamon Journals Ltd.

'IRE EFFECT OF STYRENE a-SUBST'ITUENTS ON THE REGIOCHEMISTRY OF [2+2] CYCLOADDITIONS WITH DIFLUOROALLENE. William Department

R. Dolbier, Jr.* and Mark Seabury of Chemistry, University of Florida Gainesville, Florida 32611

Summary: Consistent with our hypothesis for a mechanism involving two kinetically-distinct diradical intermediates, the observed effects of styrene a-substituents can be explained as deriving either from steric effects or radical stabilizing effects.

In a detailed (DPA)

with

study

of the

of the

stereochemistry

-.- + t-c

Z-B-deuterpstvrene.

[2+2]

the necessitv

we proposed

cycloaddition

of invoking

of difluoroallene

at least

two

u I

observed

in degree

similar

rationalization.

2

results

jx,+pg; 59%

1

RETENTION intermediates in order

diradical

differences

explaining

pcI=\D *

H

F kinetically-distinct

I

for

A simplified

of retention

in the

cycloadditions

’

79% 3 RETENTION t4.6:1) + to rationalize satisfactorily

two products

1 and

of dimethylallene,

representation

of the proposed

2.'

proposed mechanism

Pasto

the

and

Yang,

in

a similar is presented

below:

F

k TOt._

!k

3a

3b 1

krot.‘kCYCI. 1+2

MAINLY

1

(CA.50:501 In an effort factors

which

reactions, component

we

to probe

the

should

have

an

sought

to perturb

at the a-position.

details

impact the It was

of this

upon

mechanism,

the regiochemical

system

systematically

considered

1491

probable

so as and

to more

through that

fully

stereochemical substitution

an increase

elucidate outcome of the

in radical

those of such styrene

1492

stabilizing

ability

regioselective fraction

would decrease

and highly

of conversion

stereoselective" chlorine

the reactivity

stereoselective"

to the second,

intermediate,

or phenyl would

3b.

Therefore

lead to greater

rotation

"highly-reactive,

it might be expected

reluctance

to be more sterically

the 3a, with a resultant

of regioselectivity.

Regiochemistry

for [2+2] Cycloadditions

H Me Et isobu isopr F Cl Ph The results3 mentioned

which are presented However,

expectations.

the effect of steric bulk at the a-position below.

The relative

concentrations

6

this type must be playing inhibition,

which

vs H steric effects

Acknowledgement. the Petroleum

Without

a role.

Research

consistent

with the above-

equilibria

such as that shown

A and B could exert considerable

e such steric considerations makes

[2+2] Cycloadditions

with our observed

have not been observed

of this research

Fund administered

very sensitive

in yield as R increases

in our cycloaddition

Studies before,

all of the results

in part by the National

by the American

the ratio of 5:6

it clear that a steric effect of

are generally

drop-off

at least to some degree to explain

Support

%6(+0.3%)' 18.9 20.4 29.4 30.3 68.7 20.4 14.5 15.6

model does not take into account

on conformational

The result for R=isopropyl

is consistent

appear to be required

as

from

-sFGzPh-i

upon the ratio of products.

should not go below 50:50.

mechanistic

of conformations

A influence

%5(+0.3%) 81.1 79.6 70.6 69.7 31.3 79.6 85.5 84.4

in the Table are generally

our simplistic

Hence

deriving

of DFA with a-R-Styrenes.

Isolated Yield4 35% 37% 28% 22% 15% 26% 23% 18%

Reaction Temp, OC 100°C. 3hrs 1OOOC; 3hrs 100°C, 3hrs 100°C, 3hrs 100°C, 3hrs lOO'C, 3hrs lOO'C, 3hrs 100°C, 3hrs

R

for 3a to

crowded.

ethyl, etc. one might expect more product

Table

such as

On the other hand, as we increase

regioselectivity.

we were to change R from 8 to methyl, diminishment

its

and less

that substituents

R, one might expect an increased

to form 3b, which would be expected

non-

3a, and hence enhance

more-regioselective

"less reactive,

the steric bulk of the a-substituent, undergo

of the proposed

first intermediate,

Chemical

Science Society

to steric

in size.

F

but they

in this study. Foundation

and by

is acknowledged

by

the authors with thanks. References

and Footnotes.

1.

Dolbier,

W. R. Jr.; Wicks, G. E.

2.

Pasta, D. J.; Yang, S. H.

3. 4.

Products Title

5.

Product

J. Am. Chem. Sot. 1985, 107,

J. Am. Chem. Sot. 1964, 106,

5 and 6 for each R were fully characterized

reaction

is very clean one; low yields

ratios determined

by g.c..

(Received in USA 6 November 1986)

3626.

152.

by 'R, 13C, and "F

caused by competitive

NMR.

oligomerisation

of DFA.