10 - Chemistry of the Alkaline Earth Metal Enolates

Abstract

Aldehydes, ketones, carboxylic esters, carboxylic amides, imines, and N, N-disubstituted hydrazones react as electrophiles at their sp2-hybridized carbon atoms. These compounds also become nucleophiles, if they contain an H atom in α position relative to their C=O or C≡N bonds. In valence bond theory, every enolate can be described by two resonance forms. The negative formal charge is located at a C atom in one of these resonance forms and at an O or an N atom in the other resonance form. It is known that the chemistry of enolates depends on the nature of the metal. Moreover, the metals are an integral part of the structures of enolates. Lithium enolates are most frequently employed, and in the solid state, the lithium cations are definitely associated with the heteroatoms rather than with the carbanionic C atoms. Only one enolate can be generated from aldehydes or their aza analogs, from symmetric ketones or their aza analogs, or from carboxylic esters or carboxylic amides. The chapter describes the condensation of enolates with carbonyl compounds leading to the synthesis of michael acceptors and acylation reactions.