Bifunctional transition metal-based molecular catalysts for asymmetric C-C and C-N bond formation.

This paper describes the recent advances in the conceptually new bifunctional Ir and Ru catalysts for asymmetric catalytic reactions. These reactions include the enantioselective Michael addition of 1,3-dicarbonyl compounds to cyclic enones and nitroalkenes, and the enantioselective direct amination of alpha-cyanoacetates with diazoesters. The outcome of these reactions in terms of reactivity and selectivity was delicately influenced by the catalyst structures and the reaction conditions including the solvents used. Even with a 1 : 1 molar ratio of donors to acceptors, the reactions proceeded smoothly to give the corresponding chiral adducts with an excellent yield and enantiomeric excess (ee). Preliminary mechanistic studies showed that the key stage of the catalytic cycle is the interaction of the bifunctional catalyst with a pronucleophilic reagent that leads to stereoselective formation of C-, O-, or N-bound complexes. The resulting protonated catalyst bearing metal-bound nucleophiles readily reacts with electrophiles to provide C--C and C--N bond formation products in a highly stereoselective manner.