Co(Salox)-Catalyzed Enantioselective Reduction of α,β-Unsaturated Esters

Co-Salox complexes are suitable catalysts for the reduction of prochiral alpha,beta-unsaturated esters. These ligands can be prepared in a single step from available and inexpensive materials, thus representing an easily accessible alternative to previously reported Co-catalysts. NaBH4 is employed as reducing agent in the presence of EtOH as proton source, leading to the stereoselective formation of chiral esters, amides, and nitriles in up to 99 % yield and 96.5 : 3.5 er. The concentration of the reductant counter cation (Na+) and the solvent polarity have been shown to correlate with reactivity and enantioselectivity, suggesting that a relatively complex mechanistic manifold is in place.