Selected Recent Developments in the Enantioselective Reduction of Imines by Asymmetric Transfer Hydrogenation

The asymmetric transfer hydrogenation is a highly versatile tool for the stereoselective reduction of C=O and C=N prochiral groups. Carbonyl compounds have been the most frequently used as the synthetic substrates but imines were much less popular despite their importance as the sources of chiral amines. This review summarizes recent development in a search for more effective ligands and conditions of the reduction procedure. Particularly useful appear the mono-tosylated 1,2-diamines as the chirality inductors in the presence of a mixture of formic acid and triethylamine as a hydrogen source. Also, the structure of the substrates plays very important role. The cyclic endogenous imines usually give highest optical yield of the products.