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.

Automated summary

Co-Salox complexes are efficient catalysts for reducing prochiral alpha,beta-unsaturated esters. They can be easily prepared from inexpensive materials, offering a viable alternative to previously reported Co-catalysts. The presence of NaBH4 and EtOH in the reaction leads to the stereoselective formation of chiral esters, amides, and nitriles with high yields and enantioselectivities. The reactivity and enantioselectivity are influenced by the concentration of the reductant counter cation (Na+) and the polarity of the solvent, suggesting a complex mechanistic pathway.