Synthesis of cyclic carbonates from CO2 and epoxides by novel metal and halide-free chiral boron compounds

Abstract

The production of organic cyclic carbonates as valuable chemicals was carried out using a green and sustainable catalytic strategy for the environmentally friendly metal and halide-free CO₂ fixation. A series of novel chiral boron compounds (B₁ and B_1a-1d with B₂ and B_2a-2d) was synthesized, systematically characterized and applied for the synthesis of cyclic carbonates via CO₂ insertion into epoxides under reasonably mild reaction conditions. The prepared chiral boron compounds were characterized by NMR (¹H, ¹³C, and ¹¹B), FT-IR, UV–Vis spectroscopy, LC-MS/MS spectrometry. The thus characterized chiral boron compounds were used as Lewis acids for the cycloaddition reaction of CO₂ to epoxides in the presence of halide-free 4-dimethylaminopyridine (DMAP) as a nucleophilic catalytic component. Importantly, the performance of the newly synthesized compounds was also compared to that of a series of commercially available boron compounds. Halide-free commercially available boron Lewis acids generally showed lower conversion and/or selectivity under the applied reaction conditions that the newly synthesized compounds. The most active chiral boron compounds, dioxaborinanes (B_1a) and (B_1d), were selective for cyclic carbonate production under atmospheric pressure and were also screened under mild CO₂ pressures to afford higher conversion without significantly affecting cyclic carbonate selectivity.