Abstract

Dimethyl carbonate (DMC) has been widely used as a methylating agent since the 1980s for a wide variety of substrates, for example, O-methylation of phenols.1,2 It is a promising green reagent for replacing conventional methylating agents, such as methyl halides and dimethyl sulphate, which are environmentally unfriendly. By using DMC in methylations, the production of inorganic waste salt can also be avoided resulting in increased atom economy. In this study, a novel catalyst dimethyl sulfide is used to perform DMC methylations and the products are purified simply by rotary evaporation. Besides O-methylation of phenols reported previously, O-methylation of phenolic acids and C-methylation of phenylacetonitriles (eg. 4-nitrobenzoic acid, 4-fluorophenylacetonitrile) were also methylated using the condition (220°C, 4-6 hours) with excellent yields.  GC-MS and 1H-NMR were used to identify and quantify both major and minor products. >90% yield was obtained for almost all the selected substrates. The mechanism of the methylation reaction has also been elucidated. Dimethyl sulfide and DMC reacts at high temperature to form a very reactive ionic species, [Me3S]+[OCO2Me]–. The cation acts as an alkylating agent in the methylation reaction. This has been elucidated by observing phenyl butyl ether/phenyl octyl ether and but-1-ene/oct-1-ene when reacting DMC with dibutyl sulfide and di-n-octyl sulfide.3

Reference:

1. P. Tundo, F. Trotta, G. Moraglio and F. Ligorati, Ind. Eng. Chem. Res., 1988, 27, 1565–1571.
2. Z.-H. Fu and Y. Ono, Catalysis Letters, 1993, 21, 43–47.

3. Y. W. Lui, B. Chan, M. Y. Lui, ChemSusChem 2022, 15, e202102538.