In previous chapters, we have examined conjugate addition reactions of a-b-unsaturated ketones and aldehydes, and these reactions are summarized in the first three examples shown below.

The last example utilizes an enolate anion as a nucleophile towards an a-b-unsaturated carbonyl to give, in this example, a branched tri-ketone. The formation of this compound by the conjugate addition follows the mechanism shown below. The enolate simply adds to the b-carbon, and then the intermediate enolate collapses, picking up a proton on the a-carbon to give the final product.

A variety of compounds with acidic hydrogens are suitable nucleophiles for conjugate addition, as shown in the examples below.

Nitroalkanes are also suitable nucleophiles, since the a-carbon is acidified by the adjacent nitrogen cation and the anion is resonance stabilized.

Enamines, likewise, can be utilized as nucleophiles in these reactions, yielding d-di-ketones.