Mechanism and origins of enantioselectivity for [BMIM]Cl ionic liquids and ZnCl2 co-catalyzed coupling reaction of CO2 with epoxides

Abstract

Aiming at gaining more insight into the high catalytic activity of ZnCl2/[BMIM]Cl co-catalysts and elucidating the origination about the product enantioselectivity for the coupling reaction of CO2 with epoxides, a mechanistic study has been conducted by performing density functional theory calculations. The calculated results indicate a new stable complex [BMIM]ZnCl3 is probably formed via the dissociation of the in situ generated [BMIM]2ZnCl4 complex in the reaction system. This complex combined with another Cl− jointly assists the break of CO bond of propylene oxide (PO), which is the rate-determining step for the coupling reaction, and the corresponding barrier (28.0kcalmol−1) is effectively lowered in comparison with the reaction promoted only by ZnCl2 (65.9kcalmol−1) or [BMIM]Cl (33.1kcalmol−1). [BMIM]+ takes part in the reaction by directly or indirectly stabilizing the intermediates and transition states via hydrogen bonding interaction with O of PO or Cl− in the reaction system. The observed product enantioselectivity probably originates from the formation of an interesting intermediate which provides nearly equal opportunities for inserted CO2 to attack the chiral carbon atom of PO on both sides and hence facilitates the formation of both R-product and S-product.