Disproportionation of Unsymmetrical Carbonates

Abstract

Mixed ethyl and aryl or aralkyl carbonates disproportionate when heated with suitable catalysts, particularly metal alkoxides. Usually, diethyl carbonate is formed, and often the other symmetrical carbonate can be isolated. However, loss of carbon dioxide and other side reactions can take place. The course of the reactions depends on the structure of the carbonates and on the catalyst. Side reactions of the symmetrical carbonates include polymerization, loss of carbon dioxide, and formation of olefins, alcohols, and ethers. Of the carbonates, the benzyl structure is the one most prone to side reactions. While the β-phenyl- ethyl unit is somewhat more stable, it will degrade to styrene during long reaction times. Phenyl carbonates are also unstable. Cinnamyl and furfuryl derivatives are exceptionally unstable, and readily yield polymers. Tetrahydrofurfuryl carbonates are stable to polymerization, and the symmetrical carbonate is obtained in this case. The allyl system does not disproportionate at its boiling point. It was expected that the 3-phenylpropyl unit would possess enhanced stability, but it was cleaved to the alcohol. The β-phenoxyethyl unit is the most stable carbonate studied with regard to degradation, both during and after the formation of the symmetrical carbonate. The more alkaline catalysts, such as sodium methoxide and lithium aluminum ethoxide, promote more side reactions than does titanium butoxide, the catalyst of choice. © 1959, American Chemical Society. All rights reserved.