Desulfurization of 4-nitro-N,2-diphenyl-3-(phenylamino)isothiazol-5(2H)-imine: Formation of a 3-imino-2-nitroprop-2-enamidine

The course of the desulfurization reaction of 4-nitro-N,2-diphenyl-3-(pheny lamino)isothiazol-5(2H)-imine (3) is investigated and the formation of the unstable 3-imino-2-nitroprop-2-enamidine (A) as intermediate is discussed. Addition of amines and thiophenol to the reaction mixture yielded the amidi ne derivatives 5 and the thioimidate 6, respectively via nucleophilic addit ion of the respective reagent to A (Scheme 2). Benzoic acid and thiobenzoic acid afforded the amide 7 and the thioamide 8, respectively, as secondary products of the expected adducts 7a and 8a (Schemes 3 and 4). The presence of (benzylidene)(methyl)amine in the reaction mixture of the desulfurizatio n of 3 led to the 1,2,4-oxadiazole derivative 10, together with the quinoxa line N-oxide 4 as a minor product. Reaction mechanisms involving an interme diate ketene imine and participation of the NO2 group in the reaction leadi ng to 1,2,4-oxadiazole 10 are proposed. Ab initio calculations of model str uctures for the nitroketene imine were performed and the results correlated with the experimental results. The structures of 8 and 10 were established by X-ray crystal-structure analysis.