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) (L-a) and 1,3-C3H6(PPh2)(2' R, 5' R- 2', 5'-dimethylphospholanyl) (L-b) form complexes of the type [Rh(L)( cyclooctadiene)][BF4] where L = L-a ( 1a) or L-b (1b), [PtCl2(L)] where L = L-a (2a) or L-b (2b) and [PdCl2(L)] where L = L-a (3a) or L-b ( 3b). The crystal structures of 2a and 2b show the chelate ligand backbones adopt delta-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-a gives a more efficient catalyst in terms of asymmetric induction than the propylene-backboned analogue L-b. 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.