The importance of alkali cations in manganese-catalyzed enantioselective transfer hydrogenation of ketones: An insight into the effect of “NH” and “CN” groups in ligands

Abstract

A family of new ferrocene-based chiral PNN ligands and two novel manganese(I) PNN-pincer complexes (Mn1 and Mn2) with the “NH” or“C=N”group have been established, enabling the asymmetric transfer hydrogenations of (hetero)aryl alkyl ketones (33 examples) with high activity (up to 1250 TON) and excellent enantioselectivity (up to > 96 % ee). The catalytic studies and DFT investigations led to the conclusion that the alkali cation, especially, Li⁺ is an important player in the ligand-assisted hydrogen-transfer step and supports a new preferred NLi/MH instead of NH/MH bifunctional mechanism. Furthermore, humidity sensitivity experiments and DFT calculations highlight the superior binding properties of the imine pincer ligand in Mn2 over its amine-containing counterpart in Mn1. Overall, the lithium effect in the manganese (I) catalytic system has proved useful for enhancing the ATH of ketones.