Iridium Complexes with Phosphine−Phosphite Ligands. Structural Aspects and Application in the Catalytic Asymmetric Hydrogenation of N-Aryl Imines

Abstract

A family of modularly designed phosphine−phosphites (P−OP), possessing a C−C−O backbone, has been synthesized and evaluated in the iridium-catalyzed asymmetric hydrogenation of N-aryl imines. The enantioselectivity of this reaction is highly dependent on the nature of the ligand, and catalysts bridged by an oxyethylene fragment have produced significantly higher enantiomeric excesses (Δee > 20%) than their o-oxyphenylene counterparts. Structural studies by X-ray crystallography and NMR spectroscopy of complexes with the formulation [Ir(COD)(P−OP)]BF4 and Ir(Cl)(CO)(P−OP), complemented by DFT calculations of model compounds of the chlorocarbonyls, have shown important differences between complexes bridged by an aliphatic or an aromatic bridge, regarding the iridacycle conformation and the location of phosphine substituents. Catalyst optimization has afforded enantioselectivities from 72 to 85% ee in the hydrogenation of several N-aryl imines.