Salophen-type Organocatalysts for the Cycloaddition of CO₂ and Epoxides under Solvent, Halide, and Metal-Free Conditions

Abstract

High Resolution Image Download MS PowerPoint Slide 8-Formyl-7-hydroxycoumarin ( A ) and their derived salophen-type organocatalysts L1, L2, and L3 were used for the synthesis of cyclic carbonates from carbon dioxide (CO 2 ) and epoxides under solvent-, halide-, and metal-free conditions. According to previous optimization tests, L1 and L2 had the best catalytic activity presenting 89 and 92% conversion toward the synthesis of 3-chloropropylene carbonate ( 2c ) using 8 bar CO 2, 100 °C at 9 h. Therefore, they were used as organocatalysts to complete the catalytic screening with 11 terminal epoxides ( 1a–k ) exhibiting the highest TOF values of 20 and 22 h –1 using 1c and 1b, respectively. Similarly, they were tested with an internal epoxide, such as cyclohexene oxide ( 1l ) exhibiting 72% conversion, becoming the first salophen organocatalyst to obtain cis -cyclohexane carbonate ( 2l ) 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 2 pressure along the catalysis and the hydrogen bonds that favor the stabilization of the different intermediates of the reaction.