Page 575 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

556 resonance effects of adjacent substituents, and by the degree of steric hindrance to
approach of base to the proton. Alkyl substituents tend to retard proton abstraction
CHAPTER 5 both electronically and sterically. Preferential proton abstraction from less-substituted
Polar Addition carbons leads to the formation of the less-substituted alkene. Carbanion-stabilizing
and Elimination
Reactions substituents control the regiochemistry of E1cb eliminations by favoring deprotonation
at the most acidic carbon.
The preferred direction of elimination via the E2 mechanism depends on the
precise nature of the TS. The two extreme TSs for the E2 elimination resemble
the E1 and E1cb mechanisms in their orientational effects. At the “E1cb-like” end
of the E2 range, a highly developed bond is present between the proton and the
base. The leaving group remains tightly bound to carbon, and there is relatively little
development of the carbon-carbon double bond. When the TS of an E2 reaction has
E1cb character, the direction of the elimination is governed by the ease of proton
removal. In this case, the less-substituted alkene usually dominates. At the “E1-
like” end of the E2 spectrum, the TS is characterized by well-advanced cleavage of
the C−X bond and a largely intact C−H bonds. An “E1-like” TS for E2 reactions
leads to formation of the more highly substituted of the possible alkenes. In a more
synchronous E2 reaction, the new double bond is substantially formed at the TS
with partial rupture of both the C−H and C−X bonds. E2 eliminations usually give
mainly the more-substituted alkene. This is because the TSs leading to the isomeric
alkenes reflect the partial double-bond character and the greater stability of the more-
substituted double bond. Concerted E2 reactions are also subject to the stereoelec-
tronic requirement that the reacting C−H and C−X bond be antiperiplanar. This
requirement makes reactant conformation a factor in determining the outcome of the
reaction.
Prior to development of the mechanistic ideas outlined above, it was recognized
by experience that some types of elimination reactions give the more substituted alkene
as the major product. Such eliminations are said to follow the Saytzeff rule. This
behavior is characteristic of E1 reactions and E2 reactions involving relatively good
leaving groups, such as halides and sulfonates. These are now recognized as reactions
in which C−X cleavage is well advanced in the TS. E2 reactions involving poor
leaving groups, particularly those with quaternary ammonium salts, are said to follow
the Hofmann rule and give primarily the less-substituted alkene. We now recognize
that such reactions proceed through TSs with E1cb character.
The data recorded in Table 5.11 for the 2-hexyl system illustrate two general trends
that have been recognized in other systems as well. First, poorer leaving groups favor
elimination according to the Hofmann rule, as shown, for example, by the increasing
amount of terminal olefin in the halogen series as the leaving group is changed from
iodide to fluoride. Poorer leaving groups move the TS in the E1cb direction. A higher
negative charge must build up on the ß-carbon to induce loss of the leaving group.
This charge increase is accomplished by more complete proton abstraction.
Comparison of the data for methoxide with those for t-butoxide in Table 5.11
illustrates a second general trend. Stronger bases favor formation of the less-substituted
alkene. 282 A stronger base leads to an increase in the carbanion character at the TS
and, thus, shifts it in the E1cb direction. A correlation between the strength of the

282
(a) D. H. Froemsdorf and M. D. Robbins, J. Am. Chem. Soc., 89, 1737 (1967); I. N. Feit and W. H.
Saunders, Jr., J. Am. Chem. Soc., 92, 5615 (1970); (b) R. A. Bartsch, G. M. Pruss, B. A. Bushaw, and
K. E. Wiegers, J. Am. Chem. Soc., 95, 3405 (1973); (c) R. A. Bartsch, K. E. Wiegers, and D. M. Guritz,
J. Am. Chem. Soc., 96, 430 (1974).