Chiral iridium spiro aminophosphine complexes: Asymmetric hydrogenation of simple ketones, structure, and plausible mechanism

Article abstract of DOI:10.1002/asia.201000716

The iridium complexes of chiral spiro aminophophine ligands, especially the ligand with 3,5-di-tert-butylphenyl groups on the P atom (1 c) 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)-1 a and 1 c 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)-1 c complex and H₂ by means of ESI-MS and NMR spectroscopy indicated that the early formed iridium dihydride complex with one (R)-1 c ligand was the active species, which was slowly transformed into an inactive iridium dihydride complex with two (R)-1 c ligands. A plausible mechanism for the reaction was also suggested to explain the observations of the hydrogenation reactions. The chiral iridium complexes of spiro aminophosphine ligands were demonstrated to be highly efficient catalysts for the hydrogenation of aromatic ketones, providing chiral alcohols in high enantioselectivities (up to 97%ee) with high turnover number (TON up to 10 000) and turnover frequency (TOF up to 3.7× 10⁴ h^-1).