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 hemil abile ligands in seven-membered (diphosphane)Rh-I chelates upon the as ymmetric hydrogenation is studied. In comparison to the parent complex based on 1,4-bis(diphenylphosphanyl)butane (DPPB) a lowering of the r eaction rate is caused by the hemilabile ligands. The catalyst bearing the hydroxy groups gives significantly higher enantioselectivities th an the corresponding methoxy complex (by ca. 35 % ee). X-ray structura l 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 bin ds at low temperature exclusively in the eta(3)-coordination mode, whe reas for the dimethoxy complex also an eta(2)-coordinated isomer can b e found. These differences in complexation could be responsible for th e superior enantioselectivities achieved with this and other hydroxy c atalysts.