Salophen-type Organocatalysts for the Cycloaddition of CO and Epoxides under Solvent, Halide, and Metal-Free Conditions.

8-Formyl-7-hydroxycoumarin () and their derived salophen-type organocatalysts , , and were used for the synthesis of cyclic carbonates from carbon dioxide (CO) and epoxides under solvent-, halide-, and metal-free conditions. According to previous optimization tests, and had the best catalytic activity presenting 89 and 92% conversion toward the synthesis of 3-chloropropylene carbonate () using 8 bar CO, 100 °C at 9 h. Therefore, they were used as organocatalysts to complete the catalytic screening with 11 terminal epoxides () exhibiting the highest TOF values of 20 and 22 h using and , respectively. Similarly, they were tested with an internal epoxide, such as cyclohexene oxide () exhibiting 72% conversion, becoming the first salophen organocatalyst to obtain -cyclohexane carbonate () in the absence of a cocatalyst. In addition, a reaction mechanism was proposed for the formation of cyclic carbonates based on experimental data and computational techniques; these contributed in establishing a probable role of CO pressure along the catalysis and the hydrogen bonds that favor the stabilization of the different intermediates of the reaction.