Chiral transition metal complexes-catalyzed asymmetric hydrogenation is one of the most efficient methods for the synthesis of optically active compounds, and has been intensively investigated in the past decades. This review presents a brief overview on the progress in the transition metal-catalyzed asymmetric hydrogenation since the beginning of this new century from three aspects: (1) chiral ligands and catalysts; (2) new catalytic asymmetric hydrogenations; (3) new methods and new strategies in asymmetric hydrogenations. Chiral monodentate phosphorus ligands have been a renaissance from the beginning of new century, and many efficient chiral monophosphoramidites such as MonoPhos, SiPhos, DpenPhos have been developed. The chiral phosphine ligands with a chirality on the phosphorus atom (P-chirality), such as BenzP*, ZhanPhos, and TriFer, have also been explored. Chiral ligands with a spiro skeleton have been a highlight of design and synthesis of chiral ligands. The chiral spiro ligands such as SDP, SiPhos, SIPHOX, and SpinPHOX are very efficient in asymmetric hydrogenations. The iridium complexes of chiral spiro pyridine-aminophosphine ligands SpiroPAP is the most efficient molecular catalysts up to now with a turnover number (ratio of converted substrate to catalyst) over 4500000 in the hydrogenation of acetophenone. A number of breakthroughs have been made in the researches on new catalytic asymmetric hydrogenations. Highly enantioslective hydrogenations of unprotected enamines, heteroaromatic compounds, and N-H imines have been developed. New methods and strategies including self-assembled, dendrimerized, and ferromagnetic nanomaterials-loaded chiral catalysts with recyclability and reusability, the catalysts with “mixed” chiral ligands have also been successfully applied in the catalytic asymmetric hydrogenations. However, the chiral transition metal complexes catalyzed asymmetric hydrogenation still has many challenges and is looking forward to new breakthroughs in the future. In addition to the continuous developments of new chiral ligands and catalysts with high efficiency, high enantioselectivity, and high chemoselectivity, new strategies such as co-operative or relay catalysis are becoming focuses of the asymmetric hydrogenations studies.

Key words: asymmetric hydrogenation, chiral ligands, chiral catalyst, chiral compounds, metal catalysts