1,2,4-Thiadiazole 4-oxides

Abstract

Benzamidine reacts with 4,5-dichloro-1,2,3-dithiazolium chloride 1 to give 5-cyano-3-phenyl-1,2,4-thiadiazole 3. Similarly, benzamidoxime† reacts with 1 regiospecifically to give 5-cyano-3-phenyl-1,2,4-thiadiazole 4-oxide 5, the first 1,2,4-thiadiazole N-oxide, as a minor product. O-Acylbenzamidoximes 8b–e give the same N-oxide 5 in somewhat better yield, the best being from the N-methylcarbamate 8e (30%). The derivative 10 of other benzamidoximes, with electron releasing substituents gives the analogous 5-cyano-3-aryl-1,2,4-thiadiazole 4-oxides 11 in comparable yield. The N-oxides are shown to be the 4-isomers by analysis of the NMR and mass spectra of 15N-labelled and unlabelled products and X-ray structure determination of the derived carboxamide 14. This suggests a mechanism in which the amidoximes react with 1 via their oximino, rather than amino, nitrogen atom. Such a mechanism is supported by the isolation of benzonitrile and the novel benzoyloxyiminodithiazole 13 as by-products in the reaction of O-benzoylbenzamidoxime 8c with 1. Thiadiazole 3 has an almost planar structure in the solid state, and the packing of the molecules is controlled by cooperative electrostatic, dipole· · ·dipole, π–π, and aromatic edge-to-face interactions. Compound 13 also has a near-planar conformation and a packing motif exhibiting O· · ·S electrostatic interactions and π–π stepped stacks. In 14 there are only small out-of-plane torsional twists and the packing is dominated by N–H· · ·O hydrogen bonds, weak S· · ·N interactions and π–π stacking of loosely linked tapes.