The iridium complexes of chiral spiro aminophophine ligands, especially the ligand with 3,5-di-tert-butylphenyl groups on the P atom () 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 3.7×104 h−1). Investigation on the structures of the iridium complexes of ligands (R)- and 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)- complex and H2 by means of ESI-MS and NMR spectroscopy indicated that the early formed iridium dihydride complex with one (R)- ligand was the active species, which was slowly transformed into an inactive iridium dihydride complex with two (R)- ligands. A plausible mechanism for the reaction was also suggested to explain the observations of the hydrogenation reactions.
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