Triarylborane-Catalyzed Formation of Cyclic Organic Carbonates and Polycarbonates

Effective utilization of carbon dioxide as a C1 feedstock is an ongoing challenge for chemists. The catalytic reaction of epoxides and carbon dioxide to produce cyclic or polycarbonates has become an important reaction that continues to be dominated by metal-based catalysts. Metal-free catalysts have shown promise as an alternative for these transformations, but this area remains quite underdeveloped. In this work, we show that arylboranes, BPh3 and B(C6F5)(3), can be used as catalysts, in the presence of a suitable cocatalyst or as a preformed Lewis acid/base adduct, to prepare either the cyclic organic carbonate [e.g., a turnover number (TON) of 2960 was obtained for propylene oxide to propylene carbonate] or polycarbonate product (e.g., copolymerization of vinylcyclohexene oxide gave a polycarbonate with 99+% carbonate linkages, M-n 6270 g mol(-1), D 1.03). Selectivity toward cyclic or polymer product is dependent on the substrate used. Lower activity was observed using B(C6F5)(3) due to its increased Lewis acidity. Kinetic studies of this "metal-free" reaction reveal a process that is first-order in all reagents with the surprising exception of carbon dioxide, for which an inverse dependence was discovered. This means reactions can be performed at atmospheric pressure (TON 3960 for glycidyl chloride to cyclic carbonate at P-CO2 1 atm). In terms of polycarbonate formation, when a bicyclic epoxide containing a vinyl functional group was employed as a substrate, the vinyl functionality could be cross-linked (both intra- and intermolecularly) or part of a functional monomer, leading to polycarbonates with T-g values of 184 and 122 degrees C, respectively. These data highlight that a wide range of sustainable, organic carbonate materials can be produced at modest pressures using arylborane catalysts, the reactivity of which can be modified by adjustment of electronics and potentially sterics.