Dithiazoles and related compounds. Part. 4. Preparation of 1,4,2-dithiazolium salts unsubstituted at C-5 including the parent heterocycle, NMR spectroscopic evidence for aromaticity, and some novel reactions

The reaction of 1-(1,4,2-dithiazol-5-ylidene)piperidinium salt 1 and 5-methylthio-1,4,2-dithiazolium salt 5 with sodium borohydride yields 5-piperidino- and 5-methylthio-1,4,2-dithiazoles 2 and 6, which may be solvolysed with perchloric acid in acetic anhydride to give the title salts, 3, including the parent unsubstituted compound 3d. Evidence for aromaticity in these salts is discussed. Similar reduction of the salts 3 yields the first examples of 1,4,2-dithiazoles 10 unsubstituted at C-5. A new synthesis of 1,4,2-dithiazole-5-thiones 4 is reported, and the 3-unsubstituted and 3-methyl derivatives, together with their 5-oxo analogues 9d and 9e, are described for the first time. Reaction of the salts 5 with 2,5-dimethylpyrrole gives dithiadiazafulvalenium salts 14.