5.17 Reduction: Pinacol Coupling

The pinacol coupling reaction, a reductive coupling reaction of carbonyl compounds that proceeds through the formation of ketyl radical intermediates, affords the corresponding 1,2-diols in one step. Chirality of the vicinal stereogenic centers that are formed can be induced or controlled by (1) chiral substrates with center-, axial-, or planar-chiralities or substrates modified by chiral auxiliaries, (2) chiral reducing agents such as low-valent metal complexes with chiral modifiers, or (3) chiral catalysts like transition metal complexes bearing chiral ligands, the low-valent species of which can be recycled in the presence of the appropriate coreductants. Thus, a variety of chiral and nonracemic (i.e., optically active) 1,2-diols were produced through the highly stereoselective pinacol coupling reactions performed either in an inter- or intramolecular way. The reductive homocoupling of imines (imino-pinacol coupling) and the carbonyl-imine heteropinacol coupling also proceeded to give the corresponding 1,2-diamines and 1,2-aminoalcohols, respectively, in high stereoselectivities under certain conditions. Some applications to the synthesis of biologically interesting organic molecules are also presented.