Solvent-free lipase-mediated synthesis of six-membered cyclic carbonates from trimethylolpropane and dialkyl carbonates
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Six-membered cyclic carbonates with hydroxyl and/or alkoxycarbonyloxy groups, as potential monomers for polyurethanes and polycarbonates, were prepared by the reaction between trimethylolpropane (TMP) with dimethyl carbonate (DMC) or diethylcarbonate (DEC) mediated by immobilized Candida antarctica lipase B, NovozymR (R) 435 in a solvent-free medium. The dialkyl carbonate served as a solvent for the reaction. The solubility of TMP at 50 degrees C was 565 mg mL(-1) in DMC, and 64.8 mg mL (1) in DEC. Reactions using biocatalyst concentrations of 10, 20 and 40% (w/w of TMP) showed similar profiles, with a linear increase in the conversion of TMP to 90% within 24 h, and complete conversion within 48 h. At biocatalyst concentrations of 2.5 and 5% (w/w of TMP), the main products were mono-carbonated TMP (3) and/or cyclic carbonate (4), while di and tri-carbonated TMP (5, 7) were obtained at lipase concentrations of 20 and 40%. The reactivity of DEC was lower than that of DMC, but led to higher selectivity in production of 3 or 4. A large fraction of the linear carbonates in the product mixture were cyclized by disproportionation involving heating at 60-80 degrees C without any catalyst. The total yield of cyclic carbonates was about 85% after thermal treatment at 80 degrees C. This process, consisting of lipase-catalyzed transesterification and thermal disproportionation, provides a novel and more environmentally friendly approach for the synthesis of cyclic carbonates without using toxic organic solvents, phosgene or isocyanate.