15.6 : 烯醇化的立体化学效应

Carbonyl compounds bearing hydrogen at the chiral α carbon, when treated with acids or bases, undergo racemization via the formation of an achiral enol.

In the acidic medium, protonation of the C=O oxygen and subsequent deprotonation of the α-carbon atom results in an enol.

In the basic medium, deprotonation of the α-carbon atom leads to the formation of a resonance-stabilized achiral enolate intermediate.

Subsequently, protonation of the enolate oxygen generates the enol.

In both cases, as the enol formed is achiral, reprotonation can occur from either side of the plane of the double bond with equal probability.

This leads to the formation of a racemic mixture—one with retained and the other with inverted stereochemistry.

In case the carbonyl compound bears an additional chiral carbon, enolization changes the stereochemistry of the α carbon to generate a more stable isomer.