Studies of heterocyclic compounds. Part 28. Condensation of 3-substituted 5-phenyl-1,2-dithiolylium salts with 2-amino-N-heterocycles

Abstract

3-Chloro-5-phenyl-1,2-dithiolylium chloride condenses in ethanol with 2-amino-N-heterocycles at the aminogroup to yield yellow compounds; in many instances reaction occurs also at the ring nitrogen atom yielding orange compounds. 2-Amino-Δ²-thiazoline, 2-aminothiazole, 2-aminopyridine and its monomethyl derivatives, 2-amino-4,6-dimethylpyridine, 2-aminopyrimidine, and 2-aminobenzimidazole all underwent reaction at the aminogroup to give the corresponding yellow 2-(5-phenyl-1,2-dithiol-3-ylideneamino)-N-heterocycles. 2-Amino-Δ²-thiazoline, 2-aminothiazole, 2-aminopyridine, 2-amino-3-methylpyridine, 2-amino-4-methylpyridine, and 2-amino-5-methylpyridine also underwent reaction at the ring nitrogen atom to give in low yield 4,5-dihydro-3-thiobenzoylmethylene-3H-thiazolo[2,3-c][1,2,4]thiadiazole, 3-thiobenzoylmethylene-3H-thiazolo[2,3-c][1,2,4]thiadiazole, and 3-thiobenzoylmethylene-3H-pyrido[2,1-c][1,2,4]thiadiazole and its 7-, 6-, and 5-methyl derivatives, respectively, as polar orange compounds. The condensations of 3-methoxy-5-phenyl-1,2-dithiolylium fluorosulphonate with 2-amino-Δ²-thiazoline, 2-aminothiazole, and 2-amino-4-methylpyridine were studied. Condensation of 4-methyl-2-trichloromethylthioaminopyridine with benzoylacetic acid in dimethylformamide in the presence of triethylamine gave 6-methyl-3H-pyrido[2,1-c][1,2,4]thiadiazol-3-one (main product) and 6-methyl-3-benzoylmethylene-3H-pyrido[2,1-c][1,2,4]thiadiaiole. Thionation of the latter with tetraphosphorus decasulphide in pyridine gave 6-methyl-3-thiobenzoylmethylene-3H-pyrido[2,1-c][1,2,4]thiadiazole. The results of ¹H n.m.r. spectral studies, and of X-ray crystallographic studies by other workers, are discussed in relation to the structure of the yellow and the orange series of condensation products. It is proposed that the 3-thiobenzoylmethylene-3H-thiazolo[2,3-c][1,2,4]thiadiazoles and the 3-thiobenzoylmethylene-3H-pyrido[2,1-c][1,2,4]thiadiazoles, whose formation involves an S–S → S–N bond switch, are formed via 3H-6,6aλ⁴-dithia-1,3-diazapentalenes which are higher energy intermediates or transition states. 3-Chloro-5-phenyl-1,2-dithiolylium chloride condensed with 2,6-diaminopyridine in a 2 : 1 ratio to give an orange and a polar red product for which structures are proposed.