N-Heterocyclic Carbene-Catalyzed Asymmetric Functionalization of Aldehyde C–H Bonds
Asymmetric C–H bond functionalization is an extremely attractive research area. The unique catalytic ability of N-heterocyclic carbene (NHC) provides a conventional access to various types of synthetically useful compounds by activation of aldehyde C–H bonds. Since the groundbreaking mechanistic explanation on 1958 by Breslow and the unequivocally established isolation and characterization of stable NHC by Arduengo in 1991, the conception of umpolung of NHC catalysis has been widely accepted and this chemistry of asymmetric activation of aldehyde C–H bonds has attracted much attention as an important subfield in organocatalysis thereafter. This chapter will focus on the recent developments on NHC-catalyzed asymmetric functionalization of aldehyde C–H bonds (contributions mainly after 2010). Based on the diverse catalytic modes of NHC with different functionalized aldehyde substrates, the sections of the chapter are arranged in the following order: acyl anions, homoenolate equivalents, enolate equivalents, and α,β-unsaturated acyl azolium intermediates.