Intramolecular catalysis by a carboxy-group in the reduction and racemization of sulphoxides by halide ion. 16O/18O exchange as a test for the mechanism

Abstract

¹⁶ O/¹⁸O Exchange and racemization of the sulphinyl group of o-methylsulphinylbenzoic acid have been studied in aqueous perchloric acid both in the presence and in the absence of Br^– and Cl^– ions. Exchange of the sulphinyl oxygen atom is faster than racemization in the presence of halide ions; no exchange occurs at the carboxy-group. These results indicate that, in the reduction and racemization of sulphoxides, intramolecular catalysis by a carboxy-group involves nucleophilic attack by the latter on the sulphur atom of the protonated sulphoxide, with formation of a cyclic acyloxy-sulphonium intermediate.