Selective, Green Synthesis of Six-Membered Cyclic Carbonates by Lipase-Catalyzed Chemospecific Transesterification of Diols with Dimethyl Carbonate

A facile and green synthesis of six-membered cyclic carbonates, the potential monomers for isocyanate-free polyurethanes and polycarbonates, was achieved by transesterification of diols with dimethyl carbonate catalyzed by immobilized Candida antarctica lipase B, Novozym (R) 435, followed by thermal cyclization in a solvent-free medium. The difference in the chemospecificity of the lipase for the primary, secondary and tertiary alcohols as acyl acceptors was utilized to obtain a highly chemoselective synthesis of the cyclic carbonate in high yield. In the lipase-catalyzed reaction with diols, the product contained almost equal proportions of mono- and di-carbonates with 1,3-propanediol having two primary alcohols, a higher proportion of mono-carbonate with 1,3-butanediol having a primary and a secondary alcohol, and mainly mono-carbonate with 3-methyl-1,3-butanediol having a primary and a tertiary alcohol. The chemospecificity of cyclic carbonates formed by thermal treatment at 90 degrees C was closely related to the proportion of mono-carbonate. The yield of cyclic carbonate was 99.3% with 3-methyl-1,3-butanediol, 85.5% with 1,3-butanediol, and 43.2% with 1,3-propanediol.