title: Comparative Study of Bifunctional Mononuclear and Dinuclear Amidoiridium Complexes with Chiral C-N Chelating Ligands for the Asymmetric Transfer Hydrogenation of Ketones.
abstract: 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.
authors.affiliation: Department of Chemical Science and Engineering, School of Materials and Chemical Technology, Tokyo Institute of Technology, 2-12-1-E4-1 O-okayama, Meguro-ku, Tokyo, 152-8552, Japan.
authors.forename: Yasuhiro
authors.initials: Y
authors.lastname: Sato
authors.affiliation: Department of Chemical Science and Engineering, School of Materials and Chemical Technology, Tokyo Institute of Technology, 2-12-1-E4-1 O-okayama, Meguro-ku, Tokyo, 152-8552, Japan. ykayaki@o.cc.titech.ac.jp.
authors.forename: Yoshihito
authors.identifier: http://orcid.org/0000-0002-4685-8833
authors.initials: Y
authors.lastname: Kayaki
authors.affiliation: Department of Chemical Science and Engineering, School of Materials and Chemical Technology, Tokyo Institute of Technology, 2-12-1-E4-1 O-okayama, Meguro-ku, Tokyo, 152-8552, Japan. tikariya@apc.titech.ac.jp.
authors.forename: Takao
authors.initials: T
authors.lastname: Ikariya
keywords: asymmetric catalysis; cooperative effects; hydrogen transfer; iridium; ketones