Cholinium Pyridinolate Ionic Pair-Catalyzed Fixation of CO₂ into Cyclic Carbonates

A halide-free ionic pair organocatalyst was proposed for the cycloaddition of CO₂ into epoxide reactions. Cholinium pyridinolate ionic pairs with three different substitution positions were designed. Under conditions of temperature of 120 °C, pressure of 1 MPa CO₂, and catalyst loading of 5 mol %, the optimal catalyst cholinium 4-pyridinolate ([Ch]⁺[4-OP]^-) was employed. After a reaction time of 12 h, styrene oxide was successfully converted into the corresponding cyclic carbonate, and its selectivity was improved to 90%. A series of terminal epoxides were converted into cyclic carbonates within 12 h, with yields ranging from 80 to 99%. The proposed mechanism was verified by ¹H NMR and ¹³C NMR titrations. Cholinium cations act as a hydrogen bond donor to activate epoxides, and pyridinolate anions combine with carbon dioxide to form intermediate carbonate anions that attack epoxides as nucleophiles and lead to ring opening. In summary, a halide-free ionic pair organocatalyst was designed and the catalytic mechanism in the cycloaddition of CO₂ into epoxides reactions was proposed.