Page 945 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

Before we go into these reactions in detail, let us examine vinylcyclopropane itself, 929
which rearranges at high temperature to cyclopentene. 254
SECTION 10.6
Sigmatropic
Rearrangements

4 electrons, 0 nodes
anti-aromatic

The most geometrically accessible TS corresponds to a forbidden 1,3-suprafacial alkyl
shift with retention of configuration. The rearrangement requires a temperature of at

least 200 –300 C. 255 The measured E is about 50 kcal/mol, which is consistent with a

a
stepwise reaction beginning with rupture of a cyclopropane bond and formation of an
allylic fragment. 256 Support for a nonconcerted mechanism comes from the observation
that cis-trans isomerization occurs faster than the rearrangement. This isomerization
presumably occurs by reversible cleavage of the C(1)−C(2) cyclopropane bond.
.
350°C
H H . H H + D H + D
H H D H H
D D
47 % 47 % 6 %
Ref. 257

When this prior stereoisomerization is accounted for, the rearrangement is found
to have resulted from a mixture of all possible suprafacial, antarafacial, inversion,
and retention combinations in roughly equal amounts, indicating lack of stereoselec-
tivity. 258 Thus, the rearrangement of vinylcyclopropane occurs with nearly complete
bond rupture. Computational modeling of the reaction finds no intermediate, and the
TS is diradical in character. 259
A dramatic difference in reactivity is evident when cis-divinylcyclopropane is
compared with vinylcyclopropane. 260 cis-Divinylcyclopropane can only be isolated
at low temperature because it very rapidly undergoes Cope rearrangement to 1,4-
‡
‡

cycloheptatriene. 261 At 0 C H is 18.8 kcal/mol and S is −9 4 eu.
H

H
254
C. G. Overberger and A. E. Borchert, J. Am. Chem. Soc., 82, 1007 (1960).
255
T. Hudlicky, T. M. Kutchan, and S. M. Naqui, Org. React., 33, 247 (1984); T. Hudlicky and J. D. Price,
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256 D. K. Lewis, D. J. Charney, B. L. Kalra, A. M. Plate, M. H. Woodard, S. J. Cianciosi, and J. E. Baldwin,
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258
J. J. Baldwin, K. A. Villarica, D. I. Freedberg, and F. A. L. Anet, J. Am. Chem. Soc., 116, 10845 (1994).
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261
J. M. Brown, B. T. Bolding, and J. F. Stofko, Jr., Chem. Commun., 319 (1973); M. Schneider, Angew.
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