Cooperative calcium-based catalysis with 1,8-diazabicyclo[5.4.0]-undec-7-ene for the cycloaddition of epoxides with CO2 at atmospheric pressure

Abstract

A bifuncational catalytic system consisting of CaBr₂ and 1,8-diazabicyclo[5.4.0]-undec-7-ene (DBU) was developed for the efficient fixation of CO₂ with epoxides to cyclic carbonates. Such a dual catalysis facilitates the reaction to proceed smoothly at atmospheric CO₂ pressure, presumably due to the simultaneous activation of CO₂ by DBU and epoxides by CaBr₂. In addition, the activation role of CaBr₂ was also studied using density functional theory (DFT) calculations. A plausible mechanism involving the DBU–CO₂ adduct-assisted ring opening path and the bromide anion-promoted ring opening path is proposed, in combination with the activation of epoxides by the calcium cation. This process represents a simple, cost-effective and biocompatible route to obtain cyclic carbonates from CO₂ under mild conditions, especially at atmospheric CO₂ pressure.