Tuning of the structures of chiral phosphane-phosphites: application to the highly enantioselective synthesis of alpha-acyloxy phosphonates by catalytic hydrogenation

A family of new chiral phosphane-phosphites 5 has been prepared and employed in the synthesis of rhodium complexes of formulation [Rh(cod)(5)]BF4 (7). The use of bulky phosphane or phosphite groups in the preparation of 7 avoids the formation of undesired disubstituted complexes, one of which (9 a) has been isolated and characterized. Ligands 5 display important differences from the bulkier phosphane-phosphites 1: complexes 7-unlike their rigid [Rh(cod)(1)]BF4 counterparts-show fluxional behaviour in solution, consistent with backbone oscillation around the coordination plane. A detailed screening of ligands 1 and 5 in catalytic asymmetric hydrogenations of enol phosphonates 12 demonstrated a critical influence of the steric characteristics of the phosphane-phosphite in the course of the reaction, and optimization of the two phosphorus functionalities resulted in the production of versatile and efficient catalysts for this class of hydrogenations: enantioselectivities of up to 98% ee were thus obtained with substrates bearing an alkyl substituent in the beta-position, while for their challenging aryl counterparts values of up to 92% ee were achieved. The coordination mode of phosphonate 12 a towards a Rh phosphane-phosphite fragment has also been investigated and a preference of the olefin fragment to occupy the position cis to the phosphite group has been observed. From this observation an interpretation of the configurations of the hydrogenated phosphonates has also been made.