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Comparative Study of Bifunctional Mononuclear and Dinuclear Amidoiridium Complexes with Chiral C−N Chelating Ligands for the Asymmetric Transfer Hydrogenation of Ketones
Author(s) -
Sato Yasuhiro,
Kayaki Yoshihito,
Ikariya Takao
Publication year - 2016
Publication title -
chemistry – an asian journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.18
H-Index - 106
eISSN - 1861-471X
pISSN - 1861-4728
DOI - 10.1002/asia.201600955
Subject(s) - acetophenone , bifunctional , chemistry , transfer hydrogenation , chelation , steric effects , amine gas treating , catalysis , medicinal chemistry , stereochemistry , organic chemistry , ruthenium
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‐5 H ‐benzocycloheptene ( 1 b ), and ( R )‐1‐phenyl‐2,2‐dimethylpropylamine ( 1 c ). Although treatment of KO t Bu 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.