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