1.2- Dithiete 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. 

Treatment of the thioacetal (34) with phosphoryl chloride in pyridine gave the expected dehydration product and, with longer reaction times, the 2,3-dihydro-1,4-benzodithiin (35). Photolysis of the latter afforded ethylene and the compound (36), the stable dithiet tautomer of a dithio-o-quinone, whose structure was established by A -ray analysis. 

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). 

Dithiolene complex 138 (having a 1 1 ratio) was also formed in the reaction of 3,4-di(l-adamantyl)-l,2-dithiete 135 or its valence tautomer 136 with ethylenebis(triphenylphosphine)platinum(0) 137 (Scheme 16) <2003JA12114>. 

Photochemically induced extrusions of carbon monoxide generate 1,2-dithietes 38 which are in equilibrium with the valence bond tautomers 37 (Scheme 29) <1974JA3502, 2003JA12114>. 

Molecular orbital calculations have been performed on l,2-dithietes. " Electron-releasing substituents favor the valence tautomers, the 1,2-dithiones,... 

The first stable 1,2-dithiete, 3,4-bis(trifluoromethyl)-1,2-dithiete 527, was reported in 1960, as a result of the reaction of hexafluoro-2-butyne with boiling sulfurother fluorinated derivatives are prepared similarly.The strained acetylene 528 reacts with sulfur to give dithietes. Ring-contraction of 529 with loss of ethylene gives yellow needles of the benzo-1,2-dithiete 526. ° Benzo-dithiete (stable to 180°K) has been obtained by thermolysis of 529a-c or photolysis of 529a. The benzodithiete structure proposed for the product of the oxidation of o-benzenedithiol by iodine is incorrect the material is a polymer. " 3,4-Di-t-butyl-l,2-dithiete is obtained by spontaneous cyclization of the dithione valence tautomer. " ... 

A species that was proposed to be the hexathione 284 or its valence tautomer dithiete 285 was generated from 283 as a stable neutral molecule in beam experiments by subjecting CeSe" to neutralization reionization mass spectrometry (89CB2411). Calculation studies predicted that stability of 284-288 decreases in the order of 285 dithiete), 286 D2h quinoid), 287 Dsa chair thione), and 284 Dqh planar thione) (the order of 285 and 286 may depend on the method of calculations) (90AGE1410 96MI3). A yellow CeSe isomer, which could be interconverted photochemically into a colorless second isomer CeSe, was obtained upon matrix photolysis or flash pyrolysis of 283. The structure of these isomers was most likely 285 and 286, as shown by comparison of the experimental vs the calculated IR spectra of the five possible cyclic QSe isomers 284-288 (92CB265). 

The first stable a-dithione, 4,4 -bis(dimethylamino)dithiobenzil, has been synthesized by photolysis of the vinylene dithiocarbonate (55 Ar = p-Me2NCeH4). The dark red, crystalline product was found to have the structure (56) in the solid state, whereas it exists in solution as an equilibrium mixture of the valence tautomers (56) and (57). The equilibrium concentration of the dithiet form (57) was found to depend on the solvent, the temperature, and on the presence or absence of light. A kinetic study showed that the tautomerization process cleanly follows reversible first-order kinetics, i.e. a reversible formation of any dimer can be excluded. The enthalpy and entropy changes for the process (57) -> (56) were determined... 

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