The efficient and reusable imidazolium-organoboron catalysts for green CO2 insertion reactions in solvent-free under atmospheric and high-pressure conditions

Abstract

Organoboron compounds that are available in industrial and scientific research were investigated as organocatalysts for the coupling reaction of CO2 into cyclic carbonates as fine chemicals in solvent-free and under sustainable green atmospheric and high-pressure conditions (1 atm and 1.6 MPa, 100 °C, 2 h). This work synthesized a series of imidazolium-organoboron catalysts (IBC1-IBC8) using the eco-friendly and metal-free methodology and characterized by various analysis techniques. These techniques were NMR (1H, 13C, and 11B), UV–Vis, FT-IR, Mass spectrometry, C/H/N analysis, and melting point measurement together with thermal gravimetric analysis (TGA-DTA), respectively. After that, the Lewis acidity of the synthesized imidazolium-organoboron catalysts, which is important for catalytic CO2 conversion studies, was investigated by the traditional Gutmann-Beckett test, and the Lewis acidity order of the target compounds was found at range 54.96–50.36 ppm. In the presence of 0.1 mol% imidazolium-organoboron catalyst (IBC8) and 0.2 mol% DMAP as co-catalyst, 4-chloromethyl-1,3-dioxalan-2-one was obtained as cyclic carbonate from epichlorohydrin in 44.9 % yield and 97.6 % selectivity (1 atm and 100 °C) and in an excellent 97 % yield with 98.9 % selectivity (1.6 MPa and 100 °C) in 2 h without any solvents. After the imidazolium-organoboron catalyst (IBC8) showed efficient catalytic conversion and high selectivity, the effect of different parameters, including the effect of substrate, Lewis acid-base, reaction time, reaction temperature, CO2 pressure, and catalyst amount, was studied for this catalyst. According to the obtained catalytic conversion results, it was also determined that the optimum Cat./ECH ratio for CO2 cycloaddition reactions is 1/1000.