Dithietes, formation

Dithiatopazine 73 serves as a sulfur-transfer reagent under thermal conditions. Tlius, heating 73 and diene 130 in toluene at 100°C formed a cyclic disulfide 131 (25%) and a tetrasulfide 132 (28%) with the formation of the alkene 118 (90%). Under similar conditions, the highly strained acetylene 133 was converted to the 1,2-dithiete 134 (65%) and the congested thiophene 135 (12%) (90JA3029). 

All unexpected reaction is the formation of dithiete 175, thiirene 1-oxide 185, and thioketone 186 by the treatment of di-l-adamantylacetylene with S2CI2 (99UPl).Tlie yield of each compound is 30% at most. 

Cycloadditioiis in which 1,2-dithietes acted formally as dienes are among the most typieal reaetions of 1,2-dithietes. Tire dithiete 144 is highly reaetive and eapable of reaetions even with simple alkenes and alkynes (60JA1515 61JA3434,61JA3438).Tlius, 144 reaeted with aeetylene to form 191 and 192 with the initial formation of 193, and with tetramethylethylene to give 194. Other [4 + 2] eyeloadditions of 144 involved those with ethylene, cyelohexene, fra s-stilbene, ethyl vinyl ether, butyl vinyl sulfide, 3-hexyne, and DMAD. 

Tile following photochemical conversions also involve 1,2-dithietes as intermediates whose chemical trapping was reported in most cases. Tlie formation of the dithiin 249 from 250 may best be explained by the formation of the dithiete dimer 251 and the loss of S2 (73ZC424).Tlie formation of 252 and 253 from 254 (78NJC331) should be compared with the sulfuration of the acetylene 182 with elemental sulfur (93BCJ623).Tlie photolysis of 255 provides a rare example when the ejection of a nitrile was employed for the generation of a 1,2-dithiete (73ZC431). 

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). 

The stereospecific formation of 5,6-dihydro-l,4-dithiins from the reaction of the 13-dithiete 60 with alkenes has been shown to proceed through its valence isomer, l,2-bis(methoxycarbonyl)ethane-13-dithione (Scheme 42) <99JOC8489>. 

The condensation of perfluorinated alkyne 228 with elementary sulfur in the presence of iodine carried out at elevated temperature for 6 days led to the formation of the difluoroalkenyl dithiete derivative 229 and a tetrasub-stituted thiophene 230 in 50% and 22% yield, respectively (Equation 49) <2000JFC(102)323>. 

The reaction of 1,2-diadamantylacetylene with disulfur dichloride led to the formation of the dithiete 232 (mentioned above), l,2-diadamantyl-2-thiooxoethanone 233, and the thiirene 1-oxide derivative 234 in 21%, 33%, and 27% yield, respectively (Equation 50) <2000TL8349, 2003JA12114, 1998BCJ1181 >. 

Figure 1. Valence bond description of the various dithiolene ligand forms. Two-electron oxidation of the dianionic ene-1,2-dithiolate results in the formation of the 1,2-dithione and 1,2-dithiete resonance forms.

Russell and co-workers have obtained cation-radicals in the 1,2-dithiete system. Such radicals are cyclic, conjugated within the heterocycle, and possess (4 -P 1) electrons, with = 1 they are consequently heteroaromatic witliin the definition given in the introduction to Part I. 3,4-Dimethyl-l,2-dithiete cation radical 89 (R = R = Me) was obtained by treatment of acetoin in sulfuric acid with sulfide ion. The radical is persistent at ambient temperature and unaffected by oxygen. This evident stability, and its formation from open-chain precursors to the exclusion of acyclic possibilities such as 90, whose oxygen equivalent exists, implies that aromatic stabilization of 89 and similar radicals is a matter of fact and not merely definition. 

The synthesis of the stable 1,2,3-trithiole (120) has been reported by Tokitoh et al. <89TL2955>. The authors have photolyzed (120) in an argon matrix at low temperature while following changes in the electronic spectrum. This experiment revealed that two intermediates are formed, namely the 1,2-dithiete (121) and the thione (122). These intermediates are seen as forming from initial formation of thiosulfoxide (123) which is further transformed to the spirodithiirane (124) which serves as precursor to (121) and (122). 

Typical for disubstituted 1,2-dithietes is their valence isomerization, which results in the formation of... 

As in the case of the corresponding anhydride (23) (Section III.1.C), didehydromaleimide (30) probably does not qualify as a true aryne, but is expected to be sufficiently similar in properties to be covered in this review (Section I.2.C). This species (30) was first suggested as an intermediate along with the 1,2-dithiete (274) in the decomposition of the dithiin (275) in pyridine at 120°C to explain the formation of the aryne trimer (276). When this reaction was carried out in the presence of cyclopentadiene (146b), however, the failure to obtain the appropriate aryne adduct (277) was rationalized as being due to its presumed instability. The only products obtained from this reaction, as well as from butadiene (146c) and anthracene (147), were Diels-Alder adducts such as 278 and 279 of the dithiin (275) and the dithiete (274), respectively. 

Typical for disubstituted 1,2-dithietes is their valence isomerization, which results in the formation of 1,2-dithiones. The equilibrium favors the 1,2-dithiete with electron-withdrawing substituents such as CF3. The reaction with 2,3-dimethylbut-2-ene to give a hexasubstituted 2,3-dihydro-l,4-dithiine proceeds, however, as a (4 + 2) cycloaddition via the 1,2-dithione. 

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