A Guidebook to Mechanism in Organic Chemistry

Addition to afi-Unsaturated Carbonyl Compounds

The compound does in fact exist virtually entirely in the enol form:

O

/

CH,,—C

/ \

Me 2 C CH

\ /

CH 2 —C

V

^ OH

Dimedone is of value as a reagent for the differential characterisa­

tion of carbonyl compounds for it readily yields derivatives (XLVIII)

with aldehydes but not with ketones, from a mixture of the two:

OR 0

Me, I ( CH/ \ .Me

Me<\_7 VyVe

OH HO

(XLVIII)

The Michael reaction is promoted by a varjfify of bases, pjjgggnt in

catalytic quantities only, and its synthetic usefulness resides in the

large number of carbanionsand a£-unsaturated carbonyl compounds

that may be employed. The Michael reaction is reversible (cf. the

Claisen ester condensation, p. 176) and the rate-determining step is

believed to be the formation of the carbon-carbon bond, i.e. (XLIII)

-•(XLlV), though this has not been definitely proved.