Cycloaddition of CO₂ to Terminal Epoxides Catalyzed by NHC–ZnBr₂ System under Mild Conditions

Shi et al. have reported NHC–ZnBr₂ (N-Heterocyclic Carbene; NHC prepared from 1,3-bis(2,6-diisopropylphenyl)imidazolium chloride) catalyzed cycloaddition reactions of CO₂ (0.1 MPa) to terminal epoxides. The previously reported NHC–catalyzed systems needed high pressure and high temperature conditions, whereas this NHC–ZnBr₂ system requires much milder conditions. They suggest a mechanism for the addition of the imidazolium carboxylate formed from CO₂ with NHC to epoxides.