Catalytic and structural features of hydroxy and methoxy groups as hemilabile coordinating ligands in chiral (diphosphane)rhodium(I) hydrogenation catalysts

The influence of two three-arranged methoxy or hydroxy groups as hemilabile ligands in seven-membered (diphosphane)Rh-I chelates upon the asymmetric hydrogenation is studied. In comparison to the parent complex based on 1,4-bis(diphenylphosphanyl)butane (DPPB) a lowering of the reaction rate is caused by the hemilabile ligands. The catalyst bearing the hydroxy groups gives significantly higher enantioselectivities than the corresponding methoxy complex (by ca. 35 % ee). X-ray structural analysis reveals that the oxygen atom of the hydroxy group is by 0.9 Angstrom closer to the metal center than that of the methyl ether. P-31-NMR studies give evidence that the dihydroxy diphosphane ligand binds at low temperature exclusively in the eta(3)-coordination mode, whereas for the dimethoxy complex also an eta(2)-coordinated isomer can be found. These differences in complexation could be responsible for the superior enantioselectivities achieved with this and other hydroxy catalysts.