Enhancing enantioselectivity of manganese catalyst for asymmetric transfer hydrogenation of ketones through P,N,N-chelation of a cyclooctyl pyridine

Abstract

A new ferrocene-based chiral PNN pincer ligand (L1) was successfully applied in Mn-catalyzed asymmetric transfer hydrogenation (ATH) of ketones (37 examples), leading to high activities (up to 4000 TON with 48 h) and excellent enantioselectivities (up to >99 % ee) for their product alcohols. The combined coordination and catalytic studies led to the conclusion that PNN pincer ligand L1 proved higher enantioselectivity in ATH than that observed by PN bidentate analogue L0. Furthermore, DFT calculations and control experiments highlighted the significance of the cyclooctyl group among these cycloalkyl motifs in the Mn-catalysts, resulting in enhanced enantioselectivity (up to 99 % ee), and disclosed the stereocontrol with the assistance of an attractive aromatic π-π stacking interaction between substrate and catalyst in the catalytic process.