Continuous Acid-Catalyzed Methylations in Supercritical Carbon Dioxide: Comparison of Methanol, Dimethyl Ether and Dimethyl Carbonate as Methylating Agents

The development of high-yielding, "greener" chemistry-based routes for the continuous synthesis of methyl ethers are reported in this study. Ethers have been efficiently produced using a methodology which eliminates the use of toxic alkylating agents and reduces the waste generation that is characteristic of traditional etherification processes. For the first time it is shown that the use of acidic heterogeneous catalysts can successfully achieve etherification when using scCO(2) as a reaction medium. Furthermore, the relative efficiencies of three alternative methylating agents, dimethyl carbonate, dimethyl ether and MeOH, have been compared and contrasted for the methylation of I-octanol. Dimethyl carbonate has proven to be the superior methylating agent, demonstrating higher conversion and seleetivity. Successful methylation of secondary alcohols, diols, carboxylic acids and amines using dimethyl carbonate in supercritical carbon dioxide has also been shown. Substrate structure was found to influence the temperature required to maximize the yield of the desired product, substrates with multiple hydroxyl groups requiring the highest temperatures.