Rational Study of DBU Salts for the CO2 Insertion into Epoxides for the Synthesis of Cyclic Carbonates

A series of alkylated or protonated bicyclic amidine or DBU organocatalysts were investigated for the synthesis of cyclic carbonates from CO2 and epoxides. The impact of the counteranion was examined in protonated samples where salts featuring halides displayed superior activity as a consequence of the nucleophilicity of the halide. The simple iodide salt of DBU ([HDBU]I) displayed the highest activity for the carboxylation of styrene oxide at 70 °C and 1 atm of CO2 pressure, obtaining 96% conversion after just 4 h. Alkylated salts showed lower catalytic activity than the analogous protonated salts. [HDBU]I also demonstrated broad substrate scope with a number of epoxides successfully converted to the corresponding carbonates, including cyclohexene oxide which is a challenging substrate. Furthermore, the catalyst could be recycled up to 6 times without losing catalytic activity. Molecular modeling was conducted to provide mechanistic insight, and it supported the importance of the nucleophilicity of the counteranion in the insertion of CO2 into epoxides and corroborated the experimental observations.