Mechanism of asymmetric hydrogenation by rhodium complexes with unsymmetrical P-chirogenic bisphosphine ligands.

Using a series of the rhodium complexes with (1S,2S)-1-(R(1))methylphosphino-2-(R(2))(R(3))phosphinoethane (R(1), R(2) and R(3) = 1-adamantyl, t-butyl, cyclohexyl, cyclopentyl, methyl; abbreviated as unsymmetrical BisP*), very high enantioselectivities were observed when the di- or tri- substituted and tetra-substituted dehydro-alpha-amino acid derivatives were used as the substrates. The main factor to give high enantioselectivity is the repulsive interaction between the functional groups of the substrate and the bulky substituents of the unsymmetrical BisP*. Since the unsymmetrical BisP* has two independent chiral phosphorous atoms in the vicinity of the active site, the higher enantioselectivity than those by the C2 symmetric BisP* complexes can be obtained. Moreover, the fine-tuning to obtain extremely high enantioselectivity may be possible by changing the combination of the substituents on the two phosphorous atoms of the unsymmetrical BisP*.