Highly enantioselective reduction of ketones in air catalyzed by Rh-based macrocycles

Abstract The asymmetric transfer hydrogenation (ATH) of ketones catalyzed by Rh-based macrocycles proceeded smoothly in the presence of air with high catalytic activity and enantioselectivity. Even though the S/C ratio (substrate to catalyst molar ratio) was increased up to 2000:1, the ATH of ketone still afforded 92% isolated yield with 92% ee. Notably, the Rh-based macrocycles could be successfully used to catalyze the ATH of ketones without any need of inert atmosphere, which further highlighted its advantage over those generally air-sensitive transition metal catalysts. The addition of NH 4 I greatly improved both the catalytic activity and enantioselectivity. On the basis of NMR evidence, we postulate that the NH 4 I significantly enhanced the coordination between chiral macrocyclic ligands and rhodium center.