1.3- Dithiolium-4-olates

Robert and co-workers (239,240) discovered novel conversions of 2-amino-1,3-dithiolium-4-olates (348) into other mesoionic heterocycles. For example, reaction of 348 with carbon disulfide, phenyl isocyanate, or phenyl isothiocyanate affords l,3-dithiolium-4-thiolates (349), l,3-thiazolium-4-olates (350), and 1,3-thiazolium-4-thiolates (351), respectively. Some of these reactions proceed via the ring-opened ketene tautomer of 348 (240). 

Reaction with acetylenic dipolarophiles represents an efficient method for the preparation of 2,5-dihydrothiophenes. These products can be either isolated or directly converted to thiophene derivatives by dehydration procedures. The most frequently used dipolarophile is dimethyl acetylenedicarboxylate (DMAD), which easily combines with thiocarbonyl ylides generated by the extrusion of nitrogen from 2,5-dihydro-1,3,4-thiadiazoles (8,25,28,36,41,92,94,152). Other methods involve the desilylation (31,53,129) protocol as well as the reaction with 1,3-dithiolium-4-olates and l,3-thiazolium-4-olates (153-158). Cycloaddition of (S)-methylides formed by the N2-extrusion or desilylation method leads to stable 2,5-dihydrothiophenes of type 98 and 99. In contrast, bicyclic cycloadducts of type 100 usually decompose to give thiophene (101) or pyridine derivatives (102) (Scheme 5.37). 

Dithiolium-4-olat 5-Methyl-2-phenyl- E8a, 638 [Ar-CS-S-CH(CH3)-COOH + Ac2OJ... 

Few IR data of 1,3-dithiolium salts have been reported.1,3-Dithiolium-4-olates have been studied more in relation to their mesoionic and resonance hybrid structure 97. They show a strong absorption band whose position varies between 1575 and 1610 cm according to the substituents. This is also observed in the spectra of other mesoionic... 

Type Syntheses (N-C + C-S-C). Mesoionic 1,3-dithiolium-4-olates (88 R=Ph,4-MeCgHj ) cycloadd with PhNCS to yield, on extrusion of COS, mesoionic 1,3-thiazoliuro-4-thiolates (89)... 

The meso-ionic 1,3-dithiolium 4-olate (283 R = SMe) exists in equilibrium with the dimer (284 R = SMe). Complete regiospecificity is observed in the 1,3-dipolar cycloaddition of styrene to the diphenyl-derivative (283 R = Ph), which gives only compound (285 R = H,R = Ph) in contrast, methyl acrylate yields a mixture of (285 R = C02Me, R = H) and the isomeric adduct (285 R = H, R = C02Me)/ Irradiation of the diphenyl-compound (283 R = Ph) yields a mixture of 4,5-diphenyl-l,2-dithiole-3-thione (286) via the bridged intermediate shown), sulphur, diphenylacetylene, tetraphenylthiophen, and the latter s precursor, the dithiin (287). The dithiin is thought to arise from the dimeric compound (284 R = Ph) by loss of carbon oxysulphide. ... 

Dithiolium salts, e.g. (126), can be made from the dithioester (125) by reaction with acetic anhydride in polar solvents the meso-ionic 1,3-dithiolium-4-olate (127) is readily produced. If dithiocarbamates are used as starting materials, then 2-amino-l,3-dithiolium-4-olates are formed. ... 

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