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 KO t Bu 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.