1,2-Diaminocyclohexane-derived chiral tetradentate ligands for Mn(i)-catalyzed asymmetric hydrogenation of ketones.

A series of (R,R)-1,2-diaminocyclohexane-based chiral PNNP and SNNS tetradentate ligands were successfully employed as chiral chelating ligands for the asymmetric hydrogenation (AH) of substituted acetophenones (13 examples) with good activity and good enantioselectivity (up to 85% ee). In particular, two types of manganese(i) complexes (Mn1 and Mn2) with a "C[double bond, length as m-dash]N" or "NH" group were isolated, and their comparative performance as catalysts revealed Mn1 as more effective in AH of ketones with a maximum enantiomeric excess (ee) value of 85%. DFT calculations revealed that the formation of the major S-type 1-phenylethanol by Mn1 was significantly influenced by steric repulsion between the substrate and ligand.