Oxidation of Unsymmetrical Disulfide and Thiosulfinic S-Esters with Peroxy Acids. Search for Formation of α-Disulfoxide as An Intermediate in the Electrophilic Oxidation of Thiosulfinic S-Ester

Abstract Oxidation of an unsymmetrical disulfide, methyl phenyl disulfide, with peroxy acids takes place mainly at more electron-rich sulfur atom to form thiosulfinic S-ester at the first stage, while oxidation of unsymmetrical thiosulfinic S-ester with peroxy acids gives usually four thiosulfonic S-esters, together with sulfinic and sulfonic acids, further oxidation products. Oxidation of S-methyl benzenethiosulfinate 6 affords, as one of the products, S-phenyl methanethiosulfonate 11, in which the original sulfinyl oxygen is brought into methanesulfonyl group. A small amount of S-phenyl benzenethiosulfonate 9 (or benzenesulfonic acid 15) is obtained in the oxidation of S-phenyl methanethiosulfinate 7. 18O-Label of the starting thiosulfinic S-ester 6 was found to be incorporated to some extents into all four products, i.e. 8–11, while a part of 18O-label of S-phenyl methanethiosulfinate 7 was also observed in S-phenyl benzenethiosulfonate 9 and benzenesulfonic acid 15, although most of 18O-label of 7 was incorporated into S-phenyl methanethiosulfonate 11. Thiosulfinic S-esters are stable and do not undergo disproportionation and 18O-exchange under the conditions. These observations suggest the formation of α-disulfoxide as an intermediate in the oxidation of thiosulfinic S-ester to thiosulfonic S-ester, especially in the oxidation of 6, although no peak corresponding to α-disulfoxide was observed directly in the NMR spectra taken during the oxidations of a few thiosulfinic S-esters with peroxy acids. Mechanism of the oxidation is discussed.