Solvent-Free Continuous Flow Synthesis of Cyclic Organic Carbonates Catalyzed by Organic Salts

Cyclic organic carbonates are prepared under intensified continuous flow conditions from the corresponding 1,2-diols. The process utilizes dimethyl carbonate (DMC) as carbonation reagent and proceeds efficiently under solvent-free conditions. Cheap organic salts of the ammonium and phosphonium-types are assessed as homogeneous catalysts for the model carbonation of glycerol. The nature of the anion dramatically influences the catalytic activity, while the nature of the cation is negligible. Under optimized conditions, glycerol carbonate is prepared in 3 min at 180 °C with 3.5 mol% of tetrabutylammonium bromide (TBAB) as catalyst. By contrast to the prior art, the reaction is extended to a wide range of substrates, affording functionalized cyclic organic carbonates in good to excellent yield. The substrate scope notably encompasses biobased starting materials such as glycerol ethers and erythritol-derived diols. On-line NMR is implemented as analytical tool for qualitative real-time monitoring of the process.