Page 378 - Mechanism and Theory in Organic Chemistry

1,2-Elimination Reactions 365

Winstein-Parker spectrum is the E2 transition state (42), in which the base pulls
off the proton and pushes off the leaving group sim~ltaneously.~~

B

When a hard base is used as catalyst, the reaction will be more E2H-like,
whereas a soft base will cause it to be E2C-like. Weakly acidic substrates and
good leaving groups also shift the reaction path to a more E2C-like mechanism.
When hard bases are the catalysts, the rate of elimination of a compound
depends on the proton basicity of the catalyst as shown in Equation 7.34 (where
kE is the rate constant for bimolecular elimination) :89
log kE = logpK, + constant (7.34)
Conversely, when soft bases are used, elimination rates, as would be expected
from transition state 43, show no such correlation. Instead there is a relationship
between the rate of elimination and the rate of S,2 substitution by the base as
shown in Equation 7.35 (where kS is the rate constant for bimolecular substitution
and X is a constant) :
log kE = X log kS + constant (7.35)
For example, Figure 7.3 shows a plot of log kE vs. log kS for cyclohexyl tosylate
with a number of soft bases.g0
Abstraction of the ,8 proton in E2C reactions has a low isotope effect. For
example, Reaction 7.36 has a k,/k, of only 2.391 This is consistent with the iso-

Bunnett has contended that weak base-catalyzed eliminations do not involve bonding between the
base and C, but considers them to be part of the E, spectrum of which 39 and 41 are extremes.
Bunnett suggests that weak hydrogen bases are good catalysts only when the leaving group is so
good as to make possible a transition state in which the C,-H bond breaking is very small. As
evidence for his point of view he cites, for example, the facts that: (1) the rate of theophenoxide-
catalyzed elimination of HBr from 1 is approximately five times faster than from 2; (2) in contrast,
S,2 substitution is predominant with 2 but undetectable with 1 [J. F. Bunnett and D. L. Eck, J.

Amer. Chem. Soc., 95, 1897, 1900 (1975)l. Bunnett maintains that if the E,C transition state involves
partial bonding of the base to C,, steric effects on the E,C transition state should be similar to those
on the SN2 transition state. It is difficult to assess this argument because in the looser E,C transition
state (see p. 366) the nucleophile would be farther away from the t-butyl group. For a further dis-
cussion of this controversy, see W. T. Ford, Accts. Chem. Res., 6, 410 (1973).
a9 This is the Bronsted relationship.
80 A. J. Parker, M. Ruane, G. Biale, and S. Winstein, Tetrahedron Lett., 21 13 (1968).
O1 G. Biale, A. J. Parker, I. D. R. Stevens, J. Takahashi, and S. Winstein, J. Amer. Chem. Soc., 94,
2235 (1972).