Chiral Iridium Spiro Aminophosphine Complexes: Asymmetric Hydrogenation of Simple Ketones, Structure, and Plausible Mechanism

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 x 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.