Page 391 - Mechanism and Theory in Organic Chemistry

0 CH2CH2-(=N
0 0 II I
11 11 (I) addition
CH,=CH--CkN + CH3C--CH-C-OEt - -YH (7.52)lZ2
C=O
I
OEt
CH3CrCCF3 + -OCH3 -
CF3\
/H
(I) addition
(2) protonation /C=C \ (7.53)'23
O\ CF3
CH3
In general, the mechanisms of nucleophilic additions to double bonds have
not been as much studied or systemized as those of electrophilic addition.
Reactions 7.51 and 7.52 are examples of the very useful Michael condensation,
in which a carbanion adds to an a,Sunsaturated carbonyl or nitrile compound.
The usefulness of these reactions arises from the fact that the number of ways of
building longer carbon chains from smaller ones is limited.
The mechanism of the Michael condensation is not actually a 1,2-addition
as implied in Equations 7.51 and 7.52, but rather a 1,4-addition as shown in
Equation 7.54. Protonation occurs first on the oxygen atom because 64b contri-
butes more to the overall structure of the anion than 64a. The stereochemistry of
1,2-addition in the Michael condensation is therefore irrevelant to the mechanism
of the condensation.lZ4 Other nucleophilic additions to alkeneslZ5 and alkynes lZe
go either syn or anti depending on the particular reaction.

The rate-determining step in nucleophilic additions is usually nucleophilic
attack on the multiple bond.lZ7 For example, the entropy of activation of a
Michael condensation is always a large, negative quantity. This arises from the
fact that in the transition state the five atoms, O=C-C-C=O of the anion
and the four atoms, C=C-C=O (or C=C-C=N) of the a,gunsatura.ted
carbonyl (or nitrile) system are all restricted to one plane to allow maximum
lZZ See note 12 1.
lZ3 E. K. Raunio and T. G. Frey, J. Org. Chem., 36, 345 (1971).
lZ4 R. A. Abramovitch, M. M. Rogid, S. S. Singer, and N. Venkateswaran, J. Org. Chem., 37, 3577
(1972), and references therein.
lZ5 See note 118, p. 377.
lZ6 For example, see (a) E. Winterfeldt and H. Preuss, Chem. Ber., 99,450 (1966) ; (b) K. Bowden and
M. J. Price, J. Chem. Soc., B, 1466 (1970).
lZ7 See note 118, p. 377.