Synthesis of chiral amines catalyzed homogeneously by metal complexes

This review describes developments in catalytic asymmetric hydrogenation of prochiral imines. The homogeneous systems were initially dominated by ones based on Rh complexes containing chiral, chelating diphosphine ligands, although related Ru-and Ir-based systems are becoming more prominent; a very recent, extremely effective hydrogen transfer system (from formic acid), based on Ru catalysts containing chiral 1,2-diamine ligands, is especially significant. A fundamentally different type involving an early transition-metal catalyst (a chiral titanocene) has been reported. Enantiomeric excess (e.e.) values in the range of 90-100% have now been achieved with certain substrates. Emphasis is given to some Rh and Ru catalysts developed in the Department of Chemistry, The University of British Columbia (UBC). Factors discussed include: dependence of conversions, rates and e.e. values on substrate and catalyst type; kinetic dependences; and mechanistic insights, especially possible roles of intermediate metal-hydride and -imine species.