Protonation of Enolates and Tautomerization of Enols

Abstract

Enantioselective protonation reactions are an important tool for synthesis of carbonyl compounds with a stereogenic carbon center in the a-position [1-4]. These compounds are useful building blocks as intermediates for, e.g., pharmaceuticals and fragrance compounds. For the latter, the industrial relevance of these reactions has already been demonstrated [3]. Most protonations are still based on the use of stoichiometric amounts of chiral auxiliary, although catalytic versions are gaining increasing importance. Metal enolates as indicated in Scheme 9.1, Eq. (a) or metal-free analogs as indicated in Scheme 9.1, Eq. (b) have been used as achiral precursors. When metal enolates are used a transition state is formed consisting of the chiral organic molecule and the metal. Thus, although this reaction starts with an organic molecule as ''catalyst'' it can be regarded as a transition metal complex-catalyzed synthesis in which the metal, surrounded by the chiral organic molecule as chiral ligand, plays an important role in the asymmetric induction process. Because this type of process seems to be more a metal complex-catalyzed reaction than a ''pure'' organocatalytic reaction, and taking into account that the subject of enantioselective protonation has already been reviewed extensively