Efficient asymmetric hydrogenation with rhodium complexes of C1-symmetric 2,5-dimethylphospholane-diphenylphosphines

Abstract

The unsymmetrical, optically active ligands 1,2-C₆H₄(PPh₂)((R,R)-2,5-dimethylphospholanyl) (1a) and the new 1,1′-Fe(C₅H₄)₂(PPh₂)((R,R)-2,5-dimethylphospholanyl) (1b) form complexes of the type [PtCl₂(diphos)] (2a,b) and [Rh(diphos)(diene)][BF₄] (3a,b). The crystal structure of 2a reveals that only one quadrant is blocked. Asymmetric hydrogenation of acrylic esters and enamides using 3a and 3b as catalysts show that the phenylene-backboned diphosphine gives a more efficient catalyst in terms of asymmetric induction than the more flexible ferrocene-backboned diphosphine. The best results, which were obtained with 3a and enamide substrates, exceeded those obtained with Duphos catalysts. The rate of hydrogenation of the enamides with 3a was 10 times faster than with [Rh(Duphos)(diene)][BF₄]. A quadrant diagram can be used to predict the configuration of the major product, provided it is assumed to be derived from the less sterically congested intermediate.