Enantioswitchable catalysts for the asymmetric transfer hydrogenation of aryl alkyl ketones.

A subtle change in the ligand structure, replacing the carbonyl oxygen with sulfur in simple alpha-amino acid amides, resulted in a dramatic activity and selectivity improvement in the rhodium- or ruthenium-catalyzed reduction of ketones under hydrogen transfer conditions. In addition, in most cases, a switch of the product's absolute configuration was observed on going from amides to the corresponding thioamides. Under optimized conditions, we obtained the secondary alcohol products in high yield and enantioselectivity (up to 97% ee) using only 0.25 mol % catalyst loading. [structure: see text]