Insight into the reaction mechanisms between CO2 and epoxides over Zn(II) phenoxide catalytic system – A DFT study

Abstract

The reaction mechanisms between CO2 and different epoxides including cyclohexene oxide (CHO), propylene oxide (PO), styrene oxide (SO) and epichlorohydrin (ECH) over Zn(II) phenoxide catalytic system were investigated using density functional theory (DFT). It was revealed that the reaction between CO2 and CHO over the catalytic system produced alternating copolymer. The higher polymerization activity of the catalyst was usually associated with the higher electron-deficiency and the lower steric hindrance of the zinc center, and the formation of cyclic carbonate from CHO was a two-step elimination reaction process with high activation energy barrier due to the high steric hindrance of CHO. However, the reaction of CO2 with the other three epoxides (PO, SO and ECH) provided only cyclic carbonate via one-step elimination reaction with low activation energy barrier. The catalyst efficiency of zinc phenoxide catalyst was predicted to increase in the following sequence: ECH < PO < SO. In addition, the ring-opening of PO and SO tended to occur at the methine CCH-R–O bond, whereas, the ring-opening of ECH was preferred thermodynamically at the unsubstituted methylene CCH2–O bond. These theoretical results rationalized well the experimental reports by Darensbourg.