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

The new unsymmetrical, optically active ligands 1,2-C2H4(PPh2)(2'R,5'R-2',5'-dimethylphospholanyl) (La) and 1,3-C3H6(PPh2)(2'R,5'R-2',5'-dimethylphospholanyl) (Lb) form complexes of the type [Rh(L)(cyclooctadiene)][BF4] where L = La (1a) or Lb (1b), [PtCl2(L)] where L = La (2a) or Lb (2b) and [PdCl2(L)] where L = La (3a) or Lb (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 La gives a more efficient catalyst in terms of asymmetric induction than the propylene-backboned analogue Lb. 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.