Exceptional products of the desulfurization of N,2-diaryl-5-(arylimino)-2,5-dihydro-4-nitroisothiazol-3-amines

Abstract

The desulfurization of several N,2-diaryl-5-(arylimino)-2,5-dihydro-4-nitroisothiazol-3-amines 5 with Ph3P led to complex mixtures of products in low yields. For instance, quinoxaline-2-carboxamide 1-oxides of type 6 (Scheme 2) and, in some cases, also 3-nitroquinolines of type 7 (Scheme 5) were isolated. By the desulfurization of the substituted derivatives 5b-e, a rearrangement of the intermediates yielded 6 and 7 with a different substitution pattern from that expected from the starting materials (Scheme 3). The additional formation of two isomeric 1,2,5-oxadiazole-3-carboxamides 8 was observed only in the case of 5d (R1 = R2 = F) (Scheme 6). Under the same reaction conditions, the major product of the desulfurization of 5c was the quinoxaline-2-carboxamide 1-oxide 9 (Scheme 7). Reaction mechanisms involving intermediate ketene imines and O transfer from the NO2 group to the neighboring ketene imine are proposed. The structures of 6a, 6e, 6k, 7b, and 8d were established by X-ray crystallography, while the structure of 9 was elucidated by 2D-NMR spectroscopy and corroborated by X-ray crystallography.