ASYMMETRIC HOMOGENEOUS HYDROGENATION OF IMINES USING RHODIUM PHOSPHINE SYSTEMS

An efficient, reproducible system for the asymmetric homogeneous catalytic hydrogenation of imines is presented. By optimization of a number of parameters, it has been found that the combination of a chiral bidentate phosphine having a two-carbon backbone such as cycphos (cycphos = 1,2-bis(diphenylphosphino)-1-cyclohexylethane) with a rhodium(I) precursor in methanol-benzene or methanol-toluene at pressures between 1000 and 1500 psi hydrogenates the imine ArNC(CH2OMe)CH3 (Ar = 2,6-dimethylphenyl or 2-methyl-6-ethylphenyl) in quantitative yield (reactions times range between 40 and 70 h) and with good enantioselectivity. The enantioselectivities range from 40 to 69% e.e. depending on the temperature, lower temperatures (-25°C) favouring the higher e.e. Some discussion of the mechanism is presented. © 1990.