1.2- Dithiins, tautomers

Tile a-dithione 91, generated by photolysis of 92, is transformed into the dithiin 93 (417o) in the absenee of a trapping agent. The eonversion was proposed to proeeed by a [4 + 2] eyeloaddition of 91 with its dithiete tautomer 94 leading to the dithietane 95, whieh was followed by loss of S2 (85JOC1550). Sueh [4 + 2] dimerizations are often eneountered in the ehemistry of 1,2-dithietes as diseussed later. 

Tile preparation of beiizo-l,2-dithiete (264) had been claimed by oxidation of 1,2-benzenedithiol (25JIC318). However, later work has shown that the reaction product was probably a polymeric mixture (61JOC4782). Subsequently, compound 265 was irradiated to give a mixture of CO, sulfur, and dithiin and thiophene derivatives, which could, at least in part, be explained by the formation of 266 (72JHC707). Results of the thermolysis of 267 were also rationalized in terms of the intermediacy of o-dithiobenzo-quinone (the tautomer of 264) (78JOC2084). 

From Other Sulphur Heterocycles. The diazo-ketone (24), prepared from 27f-thiopyran-3(6//) Oi c and phenyl azide, yields (25) upon heating, which apparently is the stable tautomer of 3-hydroxy-4-phenylaminomethylthiophen, and (26). The structures of both compounds were proven by 2f-ray crystallography. The thermal and photochemical reactions of 1,4-dithiin sulphoxides... 

Dithiins, Oxathiins, and Thiazines. Six-membered cyclic compounds with one or two sulphur atoms have a non-planar structure. Thus, in general, ir-type calculations take into account the twisting of the p,-orbitals. HMO and PPP calculations have provided a negative ir-bond order between the sulphur atoms in 1,2-dithiin (107). EHT calculations, however, predicted a (T-bond between the sulphur atoms. Consequently the cyclic structure should be more favoured than acyclic valence tautomers. This was supported by spectral data the weak long-wavelength absorption is thought to correspond to a ir <- ir transition of (107) rather than a ir n transition of acyclic thione structures. ... 

---