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

Abstract

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 (QBE₁–QBE₈) 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 °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 °C and then under 1.6 MPa and 100 °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.