Study on the reaction of organic sulfur compounds with superoxide anion$(O_2^{-.})$

Abstract

Desulfurization reaction of 1,3-diarylthioureas to the corresponding ureas and triarylguanidines using superoxide radical anion($O_2$-.) was found: when the reaction of 1,3-diarylthioureas with potassium superoxide was carried out in anhydrous dimethyl sulfoxide 1,3-diarylureas were obtained as the main products(70-90\%), while the same reactions in less polar aprotic media such as acetonitrile and tetrahydrofuran yielded 1,2,3-triarylguanidine as the main product(70-90\%). Since treatment of cyclic thioureas such as benzimidazole-2-thione and naphth[2,3-d]imidazole-2-thione with $KO_2$ in dimethyl sulfoxide, tetrahydrofuran, or acetonitrile gave the corresponding sulfonic acid quantitatively, a key step appears to be desulfurization of 1,3-diarylthioureas to the formamidine sulfonate which are quite labile and removable. A major factor is determining the formation of urea or guanidine appears to be depending upon a competitive nucleophilic attack by superoxide anion or thiolate ion on the formamidine sulfonate. The yields of urea derivatives appears to be depended on the concentration of naked superoxide anion, since the yield of 1,3-diphenyl urea was increased by increasing the ratio of 18-crown-6-ether to potassium superoxide. The oxidation of 1,3-diarylthiourea and cyclic thioureas may not involve to form an intermediate of the disulfide, but peroxy sulfur intermediates such as peroxy-sulfenate, -sulfinate, and/or sulfonate. These peroxy sulfur intermediate should react with dimethyl sulfoxide of the trapping reagent for the peroxy oxygen to give dimethyl sulfone. This desulfurization which proceeds readily under mild conditions may suggest a critical role of superoxide in metabolic in vivo.