Facilitation of Electrochemical Oxidation of Dialkyl Sulfides Appended with Neighboring Carboxylate and Alcohol Groups

The electrochemical oxidation of variously 2-substituted 6-(methylthio)bicyclo[2.2.1]heptanes in acetonitrile was studied by using cyclic voltammetry. Three compounds, endo acid salt 1h, endo primary alcohol 1c, and endo tertiary alcohol 1g, were found to oxidize much more easily in the presence of trace amounts of bromide ion. Controlled potential electrolysis of endo acid salt 1h in the presence of 2,6-di-tert-butylpyridine and a small amount of water gave endo acid sulfoxide 5a. Such oxidation in the presence of ¹⁸O-labeled water led to the incorporation of the label into the oxygen atoms of both the sulfoxide and carboxylate moieties of endo acid sulfoxide 5a. This result suggested the intermediacy of acyloxysulfonium salt 6. This salt was prepared by bromine oxidation of endo acid salt 1h at low temperatures, characterized spectroscopically, and hydrolyzed to endo acid sulfoxide 5a. Controlled potential electrolysis of endo primary alcohol 1c and endo tertiary alcohol 1g in the presence of 2,6-di-tert-butylpyridine produced the corresponding alkoxysulfonium salts 7a and 7b, respectively. These data are interpreted in terms of bromide catalysis of the thioether oxidation with neighboring carboxylate or alcohol participation.