Page 573 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

554 The indenyl (Entry 6) and fluorenyl (Entry 7) ring systems have been studied carefully.
Note that these are cases where (aromatic) anionic stabilization could potentially
CHAPTER 5
stabilize an anionic intermediate. However, the elimination reactions show E2 charac-
Polar Addition teristics. The reaction in Entry 7 shifts to an E1cb mechanism if the leaving group is
and Elimination
Reactions made less reactive.
Because of their crucial role in the ionization step, solvents have a profound effect
on the rates of E1 reactions. These rates for a number of tertiary halides have been
determined in a variety of solvents. For t-butyl chloride there are huge differences
in the rates in water (log k =−1 54), ethanol (log k =−7 07), and diethyl ether
(log k =−12 74). 278 Similarly, the rates of the E1 reaction of 1-methylcyclopentyl
bromide range from 1×10 −3 −1 in methanol to 2×10 −9 −1 in hexane. Polar aprotic
s
s
solvents such as DMSO (k = 2×10 −4 −1 −5 −1
s are also
s ) and acetonitrile (k = 9×10
conducive for ionization. 279 The solvent properties that are most important are polarity
and the ability to assist leaving group ionization. These, of course, are the same features
that favor S 1 reactions, as we discussed in Section 3.8.
N
The details of the mechanism as well as the stereochemistry and regiochemistry
also depend on the identity and degree of aggregation of the base. This is affected
by variables such as the nature of the solvent, the cationic counterions, and the
presence of coordinating ligands. 280 Under given reaction conditions, there may be an
equilibrium involving a number of different species, which, in turn, have different rates
for inducing elimination. The nature of the TS in elimination reactions also controls
the regiochemistry of ß-elimination for compounds in which the double bond can be
introduced at one of several positions. These effects are discussed in the next section.

5.10.2. Regiochemistry of Elimination Reactions

Useful generalizations and predictions regarding regioselectivity in elimination
reactions can be drawn from the variable transition state theory. As we saw earlier
in Figure 5.11, this theory proposes that the TSs in E2 reactions can vary over a
mechanistic range between the E1 and E1cb extremes. When the base is present at
the TS, the reaction will exhibit second-order kinetics and meet the other criteria
of an E2 mechanism. There is no intermediate. The cleavage of the C−H and the
C−X bonds is concerted, but not necessarily synchronous. The relative extent of the
breaking of the two bonds at the TS may differ, depending on the nature of the
leaving group X and the ease of removal of the -hydrogen as a proton. If there
are several nonequivalent -hydrogens, competition among them determines which
one is removed and the regiochemistry and stereochemistry of the reaction. If one
compares E1 and E1cb eliminations, it is seen that quite different structural features
govern the direction of elimination. The variable transition state theory suggests that
E2 elimination proceeding through an “E1-like” TS will have the regiochemistry of
E1 eliminations, whereas E2 eliminations proceeding through an “E1cb-like” TS will
show regioselectivity similar to E1cb reactions. It is therefore instructive to consider
these limiting mechanisms before discussing the E2 case.

278
M. H. Abraham, R. M. Doherty, M. J. Kamlet, J. M. Harris, and R. W. Taft, J. Chem. Soc., Perkin
Trans. 2, 913 (1987).
279 E. A. Ponomareva, I. V. Koshchii, T. L. Pervishko, and G. F. Dvorko, Russ. J. Gen. Chem., 70, 907
(2000).
280
R. A. Bartsch and J. Zavada, Chem. Rev., 80, 453 (1980).