Evaluating metal-free quaternized boronate esters as efficient catalysts for the fixation of CO2 with epoxides to form cyclic carbonates under suitable conditions

The conversion of CO2 into high value-added chemicals is receiving increasing attention from the scientific community, commercial enterprises, and policymakers due to environmental problems like global warming. Herein, metal-free quaternized boronate esters (QBE(1)-QBE(8)) were prepared and then used as potential efficient metal-free catalysts for the chemical valorization of CO2 to organic cyclic carbonates under solvent-free and sustainable green atmospheric and high-pressure conditions (1 atm or 1.6 MPa, 100 degrees C, 2 h) as an alternative to toxic reagents such as phosgene. Analyses performed with various spectroscopic tools (1H, 13C, and 11B NMR, FT-IR, UV-vis, LC-MS/MS, elemental analysis, and melting point measurement together with thermal gravimetric analysis (TGA-DTA)) revealed that the targeted quaternized boronate esters were successfully synthesized. After that, the Lewis acidity of the synthesized quaternized boronate esters was investigated by the traditional Gutmann-Beckett method and found to range from 53.72 to 50.47 ppm, respectively. In the presence of 0.1 mol% metal-free quaternized boronate ester QBE3 and 0.2 mol% co-catalyst DMAP, 4-chloromethyl-1,3-dioxalan-2-one was obtained as a cyclic carbonate in 51.7% yield at 1 atm and 100 degrees C and then under 1.6 MPa and 100 degrees C in an excellent 94.9% yield with 97.9% selectivity in 2 h, allowing us to facilitate the fixation of CO2 into cyclic carbonates rapidly. According to the catalytic findings, the optimum cat./ECH ratio for CO2 cycloaddition reactions is 1/1000.