Conjugate Addition to α,β-Unsaturated Carbonyl Compounds

Recall that α,β-unsaturated carbonyl compounds are molecules with two functional groups—a carbonyl and an alkene—in conjugation with each other.

The conjugation results in resonance structures, and its hybrid form reveals two possible electrophilic sites: the carbonyl carbon and the β carbon.

Simple nucleophilic addition across the C=O bond, followed by protonation gives the 1,2-addition product.

Addition across the C=C bond, via an attack at the β carbon, generates an enolate intermediate, and subsequent protonation gives an enol.

As protonation occurs at the fourth position from the nucleophilic attack in the conjugated system, this is called a 1,4-addition or conjugate addition.

Finally, tautomerization of the enol restores the carbonyl group.

The type of addition favored depends on the nature of the nucleophile: stronger nucleophiles prefer 1,2-addition while weaker nucleophiles favor 1,4-addition.