Novel isothiouronium ionic liquid as efficient catalysts for the synthesis of cyclic carbonates from CO2 and epoxides

•A series of novel functionalized isothiouronium ionic liquids (FITILs) was synthesized.•The FITILs was firstly used as efficient catalysts for the cycloaddition reaction.•The tertiary N atom coordinates reversibly with CO2 to afford carbamate as an activated form of CO2.•A plausible mechanism for cycloaddition reaction over FITILs was proposed.

A series of easily prepared functional isothiouronium ionic liquids (FITILs) was synthesized and employed as efficient catalysts for the synthesis of cyclic carbonates from CO2 and epoxides without any co-solvent and co-catalyst. Notably, almost quantitative yield together with 100% of selectivity of cyclic carbonate were attained with using hydroxyl- and carboxyl-functionalized isothiouronium ILs as the catalysts at 140 °C and 2.0 MPa for 2 h. The effects of catalyst structure and various reaction parameters on the catalytic performance were investigated in detail. It was found that the synergistic effect of polarization induced by hydrogen bonding and nucleophilic attack of halide anion could facilitate the ring-opening of epoxide. In addition, the tertiary N atom of FITILs cation would coordinate with CO2 to afford the carbamate salt as an activated form of CO2. Those structure factors maybe account for the good catalytic activity of FITILs for the synthesis of cyclic carbonate from CO2 and epoxide. A possible catalytic mechanism for the cycloaddition of CO2 to epoxide was also proposed.

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