The role of Zn2+ in enhancing the rate and stereoselectivity of the aldol reactions catalyzed by the simple prolinamide model

The aldol reaction between acetone and 4-nitrobenzaldehyde catalyzed by single l-prolinamide and its zinc complexes has been studied. An increase in the rate and the stereoselectivity of the reaction has been shown by using zinc derivatives. A mechanistic proposal, based on NMR and ESI studies, has been put forward to explain the experimental data: zinc-prolinamide complexes catalyze the reaction following the general mechanism of stereoselective enamine nucleophilic addition to the acceptor aldehyde. Zn2+ prevents the nonspecific base-catalyzed reaction by diminishing the basicity of the amine nitrogen of prolinamide.