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.