A dithionate

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 a-dithione 158 (dithiete 159) underwent a [4 + 2] cycloaddition with fra s-(l,2-dimethoxy)ethylene to give 197 in addition to reactions with HCN and CH2N2, which yielded 198 and 199, respectively (74JA3502). 

These conclusions were ratified in a second paper134 which includes also 2,4-dithiouracil. The appearance of the N1H and/or N3H stretches and the absence of the SH and OH stretches in the spectrum of 2- and 4-thiouracil confirmed that these molecules exist only in the oxo-thione form. Similar findings for 2,4-dithiouracil showed that this molecule exists only as a dithione tautomeric form in an inert matrix. The spectra of S-methylated thiouracils are substantially different they present an absorption in the typical NH stretching range and another one near 3570 cm-1, which is the region characteristic for the OH stretching modes. 

The different nature of the two ligands in the complex allows one of them to be taken mostly as a dithione (L) and the other as a dithiolate (L ) see Section in.C.2, in particular resonance forms 40-42. 

Reaction of disubstituted thiirene-l-oxides 251 with Lawesson s reagent, in dichloromethane at room temperature for 15 or 30 min, gave the corresponding a-dithiones 252 in 20% yield. Only when R = 1-adamantyl was the a-dithione isolated in pure form (Scheme 71) <2003JA12114>. 

When this reaction was carried out in the presence of DMAD in carbon disulfide, thiophene 253 was obtained in 51% yield (Equation 39). A tentative mechanism of formation of a-dithiones is shown in Scheme 72. First, episulfonium ions 254 are formed and then sulfur ylides 255 are obtained. From the ylides the a-dithiones 256 are formed, while the episulfonium ions 254, by reaction with DMAD to produce 257, finally lead to the formation of 253. 

Dicyano-l,2-dithiete is an intermediate in the oxidation of the dianion of cis-2,3-dicyano-2,3-dimercaptoethylene 530 and related compounds.1,2-Dithietes are believed to be in equilibrium with 1,2-dithiocarbonyl compounds, as exemplified by 531. Several of these unstable 1,2-dithietes are observed by their electronic spectra at 77°K. Theoretical considerations indicate that the photochemical conversion of an a-dithione into a 1,2-dithiete is allowed the thermal process is forbidden. ... 

Methylsulfanyl)-5,6-tetra-(and penta)-methylene-l,4,2-dithiazines are thermolyzed in refluxing dioxane to yield methyl rhodanide and 1,4-dithiins, e.g. 2 (see Houben-Weyl, Vol. E9a, p 250, 268). It is assumed that the corresponding thietes or the a-dithiones are intermediates, because thermolysis of 3-(methylsulfanyl)-5,6-tetramethylene-l,4,2-dithiazine in the presence of dimethyl fumarate affords a 19% yield of dimethyl 5,6-tetramethylene-2,3-dihydro-1,4-ditkiin-2,3-di-carboxylate (4).1,2,20... 

The stability of the a-dithiocarbonyl compounds follows closely their potential utility for 4ir participation in Diels-Alder reactions with typical olefinic and acetylenic dienophiles [a-dithioamide > a-dithionoester > a-dithionothioester (dimer equilibrium at 25°C) > a-dithione > a-dithioalde-hyde]. The a-dithiocarbonyl compounds are electron-deficient and consequently react rapidly with electron-rich and strained olefins in inverse electron demand (LUMOd,ene controlled) Diels-Alder reactions and more slowly with unactivated or electron-deficient dienophiles in apparent normal (HOMODiels-Alder reactions (Scheme 8-IX).36 Subtle differences in the reactivity and observed course of reactions of the various a-dithiocarbonyl compounds have been detailed.36... 

A full report on the synthesis of a-dithiones by the photochemical decarbonylation of vinylenecarbonates has been published.124... 

Similarly, l,3-dithiol-2-ones undergo S-acyl bond cleavage followed by CO elimination to afford a-dithiones, dithio- or perthiooxalates, respectively. These compounds can be either isolated directly [84,85] or trapped with dienophiles to afford 1,4-dithiins [86]. An equilibrium between an a-dithione and a 1,2-dithiete has also been observed [84] (Scheme 37). 

In 2.19a, the S-donor behaves as a dithione L2 ligand, making the metal (f Accordingly, the central C-C bond of the dithione is long... 

A new type of thioketone (12), conjugated with an a-ester group, has been obtained by basic cleavage of a thiosulphate (Bunte salt) obtained from the a-chloro-ester. This unstable deep-blue thioketone dimerizes easily to a dithietan. Preparation of the a-dithione (13 R = p-MeaNCgHi) has been reported in detail. In the solid state it exists in the dithione form and in solution in... 

Theoretical considerations indicate that the thermal conversion of an a-dithione into a 1,2-dithiet is allowed photochemically the thermal processes, both forward and reverse, are forbidden. Model calculations on the dithioglyoxal-dithiet equilibrium show that the Hiickel method predicts the dithioglyoxal to be more stable than the dithiet, whereas the CNDO/2 method predicts the opposite. The potential surface for C2S4 was calculated, and energy minima were predicted for (81), (82), (83), and (84). ... 

Bis-(p-dimethylaminophenyl)dithiet exists in the solid state as the a-dithione, but an equilibrium exists in solution between the two forms. The equilibrium is sensitive to light, temperature, and solvent. 

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