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Alkenes and alkynes react readily with trithiazyl trichloride 1 to give 1,2,5-thiadiazoles 2 in one step. Thus 3-amino-1,2,5-thiadiazole 5 is now readily available from N-vinylphthalimide and 1 in two steps. Cyclic alkenes react similarly to give fused thiadiazoles (7, 12) and phenanthrene gives the phenanthrothiadiazole 16. Tetra-substituted alkenes such as 13 appear to give the analogous 3,4-dihydrothiadiazoles (e.g. 14) which spontaneously ring open to give readily hydrolysed bis(methyleneamino) sulfides (e.g. 15). A simple set of mechanisms is proposed for all of these reactions.

Treatment of the 2,3-unsubstituted pyrrole 2 and indoles 7 with trithiazyl trichloride 1 in hot tetrachloromethane gives the 2,3-fused 1,2,5-thiadiazolo derivatives 3 and 8. 2,3,4,5-Unsubstituted pyrroles 4 are similarly converted directly into the pyrrolobis(thiadiazoles) 6, without detection of the presumed, highly reactive, bicyclic intermediate 5. This ring fusion process is not prevented by the presence of bromine, chlorine, a carboxylic acid or carboxylic ester group at the α- or β-positions; indeed prior substitution by bromine or chlorine results in faster and cleaner reactions with trithiazyl trichloride to give the fused systems 3, 6 and 8 in high yield. Possible mechanisms are proposed to explain these results.

Activated allylic compounds react with trithiazyl trichloride, (NSCl)3, to give 1,2,5-thiadiazoles 1 and isothiazoles 2. An allylic 2-substituent normally prevents formation of an aromatic 1,2,5-thiadiazole, and isothiazole formation becomes the major pathway. Simple allylic compounds are not very reactive towards (NSCl)3 but a terminal electron withdrawing group (CO2Et) enhances the reactivity. With unsymmetrical allylic compounds, isothiazole formation is regiospecific placing the more electron withdrawing group adjacent to the ring sulfur. 1,3-Diketones give 3-acyl-1,2,5-thiadiazoles; unsymmetrical 1,3-diketones give these thiadiazoles regiospecifically, explicable by cyclisation of an intermediate onto the more reactive carbonyl group. 1,4-Diketones give 3,4-diacyl-1,2,5-thiadiazoles; thus 1,2-dibenzoyl-ethane, -ethene and -ethyne all give 3,4-dibenzoylthiadiazole (40–44%). Many of these trithiazyl trichloride reactions provide attractive one-step routes to 1,2,5-thiadiazoles and isothiazoles.

Trithiazyl trichloride 1 converts 1,2,5-triphenylpyrrole 5 into its 3,4-dichloro derivative together with the isothiazole imine 6 and the imine hydrolysis product, the ketone 3. The best yield of the isothiazole 6 is obtained in the presence of 4 A molecular sieves (Table 1). Conversion of the pyrrole 5 into the isothiazole 6 is exactly analogous to the reaction of 1 with 2,5-diphenyl-furan and -thiophene. Other N-aryl and the related 2,5-diphenylpyrroles 8 give similar results (Table 2). However, 1-methyl-2,5-diphenylpyrrole 11 reacts with 1 in an entirely different way to give 4,4′-diphenyl-3,3′-bi-1,2,5-thiadiazole 12, in which two thiadiazole rings have been fused onto the pyrrole and the CH3N unit has been excised as HCN. The same product 12 is formed, in similar yields, by reaction of 1 with 1,4-diphenylbuta-1, 3-diyne and 1,4-diphenylbut-1-en-3-yne. Other N-alkyl 2,5-diphenylpyrroles 16 react similarly (Table 3), giving the best yield (70%) of bi-thiadiazole 12 in the presence of 4 A molecular sieves (Table 4). 1-Methyl- and 1-ethyl-3,4-dibromo-2,5-diphenylpyrrole also give 12, together with 3-(benzoyldichloromethyl)-4-phenyl-1,2,5-thiadiazole 21 in high combined yield. The formation of bi-1,2,5-thiadiazole 12 from N-alkylpyrroles represents a new dissection of the pyrrole ring and a new and very short route to an aromatic biheterocyclic system. Mechanisms which rationalise the different pathways observed are proposed for all of these reactions.

Alkenes and alkynes react readily with trithiazyl trichloride 1 to give 1,2,5-thiadiazoles 2 in one step. Thus 3-amino-1,2,5-thiadiazole 5 is now readily available from N-vinylphthalimide and 1 in two steps. Cyclic alkenes react similarly to give fused thiadiazoles (7, 12) and phenanthrene gives the phenanthrothiadiazole 16. Tetra-substituted alkenes such as 13 appear to give the analogous 3,4-dihydrothiadiazoles (e.g. 14) which spontaneously ring open to give readily hydrolysed bis(methyleneamino) sulfides (e.g. 15). A simple set of mechanisms is proposed for all of these reactions.

Activated allylic compounds react with trithiazyl trichloride, (NSCl)3, to give 1,2,5-thiadiazoles 1 and isothiazoles 2. An allylic 2-substituent normally prevents formation of an aromatic 1,2,5-thiadiazole, and isothiazole formation becomes the major pathway. Simple allylic compounds are not very reactive towards (NSCl)3 but a terminal electron withdrawing group (CO2Et) enhances the reactivity. With unsymmetrical allylic compounds, isothiazole formation is regiospecific placing the more electron withdrawing group adjacent to the ring sulfur. 1,3-Diketones give 3-acyl-1,2,5-thiadiazoles; unsymmetrical 1,3-diketones give these thiadiazoles regiospecifically, explicable by cyclisation of an intermediate onto the more reactive carbonyl group. 1,4-Diketones give 3,4-diacyl-1,2,5-thiadiazoles; thus 1,2-dibenzoyl-ethane, -ethene and -ethyne all give 3,4-dibenzoylthiadiazole (40–44%). Many of these trithiazyl trichloride reactions provide attractive one-step routes to 1,2,5-thiadiazoles and isothiazoles.