Allosteric effects in asymmetric hydrogenation catalysis? Asymmetric induction as a function of the substrate and the backbone flexibility of C1-symmetric diphosphines in rhodium-catalysed hydrogenations

Abstract

The new unsymmetrical, optically active ligands 1,2-C₂H₄(PPh₂)(2′R,5′R-2′,5′-dimethylphospholanyl) (L_aa) and 1,3-C₃H₆(PPh₂)(2′R,5′R-2′,5′-dimethylphospholanyl) (L_bb) form complexes of the type [Rh(L)(cyclooctadiene)][BF₄] where L = L_aa (1a) or L_bb (1b), [PtCl₂(L)] where L = L_aa (2a) or L_bb (2b) and [PdCl₂(L)] where L = L_aa (3a) or L_bb (3b). The crystal structures of 2a and 2b show the chelate ligand backbones adopt δ-twist and flattened chair conformations respectively. Asymmetric hydrogenation of enamides and dehydroaminoesters using 1a and 1b as catalysts show that the ethylene-backboned diphosphine L_aa gives a more efficient catalyst in terms of asymmetric induction than the propylene-backboned analogue L_bb. The greatest enantioselectivities were obtained with 1a and enamide substrates with ees up to 91%. Substrate-induced conformational changes in the Rh–diphosphine chelates are proposed to explain some of the ees observed in the hydrogenation of enamides.