A Hydrogen-Bonding Phosphonium/HFIP Catalyst for Efficient CO2 Conversion into Cyclic Carbonates

Converting carbon dioxide (CO2) into carbonates is a promising strategy for mitigating carbon emissions. In this study, a series of phosphonium salts/hexafluoroisopropanol (HFIP) catalysts were synthesized and employed in the cycloaddition of CO2 to epoxides to produce cyclic carbonates under mild reaction conditions. The experimental results demonstrated that, at 25 °C and 1 atm CO2 pressure, the catalyst methyltriphenylphosphonium bromide/hexafluoroisopropanol 1/2 (Ph3MePBr/HFIP 1/2) achieved very high yields for monosubstituted (86–96%) as well as 1,1- and 1,2-disubstituted epoxides (66–88%). Density functional theory (DFT) calculations and NMR experiments were employed to elucidate the structure of the Ph3MePBr/HFIP 1/2 catalyst as well as the underlying reaction mechanism. The DFT mechanistic analysis indicated a hydrogen-bond-assisted SN2 pathway, in which the HFIP···Br– pair activates the epoxide and enables efficient CO2 fixation under mild conditions.