Cinchona-Derived Quaternary Ammonium Salts-Improved Asymmetric Cycloaddition of CO2 to Epoxides

A new catalyst system, SalenCoX/cinchona-derived quaternary ammonium salts, was developed to catalyze the cycloaddition of CO2 to epoxides under extremely mild reaction conditions. Chiral propylene carbonate was obtained with good enantioselectivity in the presence of the catalyst. The anions of the catalyst and the cocatalyst evidently affect the activity and enantioselectivity of the reaction. The activity of (S,S)-catalysts in terms of anion is in the order 2-nitrophenoxy > 2,4,6-trinitrophenoxy > NO3− > OAc− > CF3CO2− ≈ Br− > Cl− > OTs−. The cocatalyst with anion Cl− gives the highest ee value for propylene carbonate. In contrast, the cocatalyst with anion Br− has higher activity. When the catalyst (S,S)-(+)-1,2-cyclohexanediamino(N,N-bis(3,5-di-tert-butyl-salicylidene) cobalt(III) acetate ((S,S)-A) was combined with the cocatalyst N,O-dibenzylquininium chloride (1a), it gives the highest 73% ee for propylene carbonate.