Benzoin and aza-benzoin

The benzoin condensation consists of the addition of an acyl anion equivalent to a carbonyl derivative to afford an α-hydroxyketone. When the electrophilic partner is replaced by an imine, the analogous aza-benzoin reaction results in the formation of an α-aminoketone. These processes require an inversion of polarity (umpolung) of an aldehyde, which can be achieved following three main strategies: (1) the multistep elaboration of stoichiometric acyl anion equivalents; (2) the catalysis by cyanide ions; and (3) the organocatalysis by N-heterocyclic carbenes (NHCs). Advantages and drawbacks of each method are discussed, especially in terms of chemo- and stereoselectivities. Recently developed alternative enantioselective catalytic approaches along with miscellaneous other methods are also presented