A Novel Dopamine-Based Boronate Esters with the Organic Base as Highly Efficient, Stable, and Green Catalysts for the Conversion of CO2 with Epoxides to Cyclic Carbonates

A series of dopamine-based mono- and dinuclear boronate esters are studied as stable, efficient, and green metal-free catalysts for coupling CO2 with epoxides under suitable and solvent-free conditions. Under the optimized reaction conditions, dinuclear boronate esters compared with those with mononuclear compounds display high catalytic activity and selectivity for the chemical conversion of CO2 into cyclic organic carbonates as a valuable product. Ferrocene group containing boronate ester (B-4a) display the highest activity for the carboxylation of epichlorohydrin at 100 degrees C and 1.6 MPa of CO2 pressure, obtaining 96.3% conversion after just 2 h. After seeing a high catalytic performance of trigonal-planar boronate ester (B-1-B-5) and tetrahedral boronate ester (B-1a-B-5a) catalysts, the effect of ionic liquid, epoxide, base, temperature, CO2 pressure, reaction time, and amount of catalyst/substrate/cocatalyst is investigated for these catalysts. The newly synthesized dopamine-based trigonal-planar and tetrahedral boronate esters are entirely characterized by melting point, elemental analysis, NMR (H-1, C-13, and B-11 NMR) spectroscopy, FT-IR spectroscopy, UV-vis spectroscopy, and LC-MS/MS spectrometry.