title: Chiral iridium spiro aminophosphine complexes: asymmetric hydrogenation of simple ketones, structure, and plausible mechanism.
abstract: The iridium complexes of chiral spiro aminophophine ligands, especially the ligand with 3,5-di-tert-butylphenyl groups on the P atom (1c) were demonstrated to be highly efficient catalysts for the asymmetric hydrogenation of alkyl aryl ketones. In the presence of KOtBu as a base and under mild reaction conditions, a series of chiral alcohols were synthesized in up to 97% ee with high turnover number (TON up to 10,000) and high turnover frequency (TOF up to 3.7×10(4)  h(-1)). Investigation on the structures of the iridium complexes of ligands (R)-1a and 1c by X-ray analyses disclosed that the 3,5-di-tert-butyl groups on the P-phenyl rings of the ligand are the key factor for achieving high activity and enantioselectivity of the catalyst. Study of the catalysts generated from the Ir-(R)-1c complex and H(2) by means of ESI-MS and NMR spectroscopy indicated that the early formed iridium dihydride complex with one (R)-1c ligand was the active species, which was slowly transformed into an inactive iridium dihydride complex with two (R)-1c ligands. A plausible mechanism for the reaction was also suggested to explain the observations of the hydrogenation reactions.
authors.affiliation: State Key Laboratory and Institute of Elemento-organic Chemistry, Nankai University, Weijin Road 94, Tianjin 300071, China.
authors.forename: Jian-Bo
authors.initials: JB
authors.lastname: Xie
authors.forename: Jian-Hua
authors.initials: JH
authors.lastname: Xie
authors.forename: Xiao-Yan
authors.initials: XY
authors.lastname: Liu
authors.forename: Qian-Qian
authors.initials: QQ
authors.lastname: Zhang
authors.forename: Qi-Lin
authors.initials: QL
authors.lastname: Zhou