A Guidebook to Mechanism in Organic Chemistry

Addition to «&-Unsaturated Carbonyl Compounds

With more pronouncedly nucleophilic reagents, e.g. Grignard

reagents, eCN, etc., overall 1:4-addition will take place without need

for initial protonation of the carbonyl oxygen atom:

\c=^c^c^o > \c—C=C—O© -^T \c—C=C—OH

u

(Y=R.C>etc.) \C-CH-C=0

Y

This occurs readily with ^-unsaturated ketones, but with <xj3-unsatur-
ated aldehydes direct attack on the carbonyl carbon atom (1 ^-addi­
tion) also takes place because of the more positive character of this
atom in aldehydes as compared with ketones (p. 159);
With even more powerful nucleophiles, e.g.^0 OH, addition at the
jS-carbon atom takes place, even with a/S-unsaturated aldehydes.
This can lead, under suitable conditions, to reversal of the aldol/
dehydration reaction (p. 175):

H H • H

R • C=^CH^-C=Q> R • C—CH^-O^O^- R

& I II

^OH H OH H

H H
I e I Ot
RC+ CH 2 —C?=0 "—r RC-i-CHs—C=0
II I lt> I
O H O^9 ^ H
Amines, mercaptans, etc., will also add to the /3-carbon atom of
OT/J-unsaturated aldehydes, ketones and esters. The most important
addition reactions of a/J-unsaturated carbonyl compounds, however,
are with carbanions in which carbon-carbon bonds are formed.

• (i) Michael reaction
  The most frequently employed carbanions are probably those de­
  rived from diethyl malonate, ethyl acetoacetate, ethyl cyanoacetate