Synthesis and effective catalytic performance in cycloaddition reactions with CO2 of boronate esters versus N-heterocyclic carbene (NHC)-stabilized boronate esters

Abstract

A series of boronate esters (1–3) and N-heterocyclic carbene (NHC)-stabilized boronate esters (4–6) were synthesized and characterized by various spectroscopic techniques, including NMR (¹H, ¹³C, and ¹¹B), FT-IR, UV-Vis, LC-MS/MS spectrometry, melting point, and elemental analysis techniques. The boronate esters (1–3) and N-heterocyclic carbene (NHC)-stabilized boronate esters (4–6) (as Lewis acid) rapidly catalyze the coupling of a variety of epoxides and CO₂ to obtain cyclic carbonates in the presence of DMAP (as Lewis base) without the organic solvent. Good to excellent yields of various cyclic carbonates were also achieved under suitable conditions (125 °C, 0.5 h, and 1.6 MPa). After seeing the high catalytic performance of boronate ester catalysts, effects of epoxide, base, temperature, CO₂ pressure, reaction time and amount of catalyst and base were investigated for these catalysts. The boronate ester (1) and DMAP showed the best catalytic activity (94.3%) and selectivity (97.5%) for the coupling of CO₂ and ECH under suitable conditions (125 °C, 0.5 h, and 1.6 MPa).