Alkaline autoxidation of 1,3-diarylsubstituted thioureas and cyclic thioureas

Abstract

Alkaline autoxidation of various 1,3-diarylsubstituted thioureas and cyclic thioureas such as 2-mercaptobenzimidazole and naphth [2,3-d]imidazole-2-thione were carried out in the presence of excess molecular oxygen with potassium tertiary butoxide in tertiary butanol and/or dimethyl sulfoxide at 40-50℃. The reaction of 1,3-diarylsubstituted thioureas with $O_2$-t-BuOK-BuOH(and/or-DMSO) afforded 1,3-diarylureas as major products and 1,2,3-triarylguanidines as minor products. The conversion of 1,3-diphenylthiourea to the 1,3-diphenylurea underwent more smoothly in $O_2$-t-BuOK-t-BuOH than in $O_2$-t-BuOK-DMSO. The formation of 1,3-diarylureas appears to be initiated by one-electron transfer between the substrate and oxygen molecule, and then to form a peroxysulfur intermediate which has neither been isolated nor detected yet. While, the treatment of 2-mercaptobenzimidazole with $O_2$-t-BuOK-t-BuOH at 40-50℃ gave both benzimidazole-2-sulfonic acid and benzimidazole which might be formed via desulfurization from the benzimidazole-2-sulfinate intermediate which are not yet confirmed due to its unstability. Similarly, the oxidation of naphth[2,3-d]imidazole-2-thione with $O_2$-t-BuOK-t-BuOH afforded corresponding desulfurized product and sulfonic acid.