Comparative Study of Bifunctional Mononuclear and Dinuclear Amidoiridium Complexes with Chiral C−N Chelating Ligands for the Asymmetric Transfer Hydrogenation of Ketones

A series of new bifunctional C−N chelating Ir complexes possessing a metal/NH group was synthesized by cyclometalation of optically active primary benzylic amines such as O‐silylated (S)‐2‐amino‐2‐phenylethanols (1 a and 1 a’), (R)‐5‐amino‐6,7,8,9‐tetrahydro‐5H‐benzocycloheptene (1 b), and (R)‐1‐phenyl‐2,2‐dimethylpropylamine (1 c). Although treatment of KOtBu with the amine complexes originating from 1 a and 1 a’ afforded amido‐bridged dinuclear complexes (3 a and 3 a’), more sterically hindered complexes were solely transformed into the coordinatively unsaturated mononuclear amido complexes (3 b and 3 c), which can serve as real catalyst species in the asymmetric transfer hydrogenation. The structural difference in the C−N chelate framework markedly affected the catalytic performance. Among them, amido complex 3 c showed a pronounced ability to catalyze the transfer hydrogenation of acetophenone in 2‐propanol, even at a low temperature of −30 °C. A hydridoiridium complex (4 c) was also identified in the reaction of 3 c in 2‐propanol, which provides mechanistic insights into the enantiodiscriminating step in the hydrogen transfer to prochiral ketones. Hunting the Actual Catalyst: Mono‐ and dinuclear chiral amidoiridium complexes were specifically synthesized by changing the substituents on the C−N chelating ligand. The isolated mononuclear amido complex based on (R)‐1‐phenyl‐2,2‐dimethylpropylamine was easily accessible to the hydrido(amine) complex in 2‐propanol with excellent diastereoselectivity and served as an efficient catalyst for the asymmetric transfer hydrogenation of acetophenone.