1,2-Dithiole is a five-membered, unsaturated, planar, sulfur heterocycle with two adjacent sulfur atoms and three  carbon atoms with one double bond between C3-C4  of the ring system. Almost all the 1,2-dithiole derivatives with  a few exceptions are categorized either under dithiolylium cations or compounds in which C3  of the dithiole ring  is linked to exocyclic bivalent electronegative heteroatoms (O,S) or groups such as 1,2-dithiole-3-one, 1,2-dithiole- 3-thione, and amide. These compounds are readily obtained and possess high stability. In 1,2-dithiolylium cations,  each sulfur atom contributes a pair of 3p electrons to the sextet and thus possesses delocalized 6π electrons and obeys  the Hückel (4n+2)π rule for aromaticity. 

Physical Properties

The chemical reactivity of the 1,2-dithiolylium cation depends upon the charge distribution in the ground state. As  per charge distribution on 1,2-dithiolylium cations, the C3  or C5  site is least electron dense and prone to preferential  nucleophilic and free radical attack, but is quite resistant to electrophiles. Based on X-ray studies of 1,2-dithiolylium  cations the bond lengths are S-S 2.00–2.03 ?, C-S 1.67–1.714 ?, and C-C 1.37–1.40 ? and are shorter than the normal  single bond possibly due to π-orbital delocalization of ring atoms, which also reflects the stability of the molecule.  However, these physical constants are dependent on substitution.

UV (ethanol) λnm: 202, 285. 

1 H NMR (TFA-d), δ (ppm): C3 –H, 10.57; C4 –H, 8.87; C5 –H, 10.57. 

13C NMR (TFA-d), δ (ppm): C3 , 176.72; C4 , 142.68; C5 , 176.72.

Chemical Reactivity

Nucleophilic substitution on C3  or C5  is highly facile because of low electron density on both the positions and  is susceptible to ring opening in the presence of a base.

1,2-Dithiolylium cation on reaction with sodium ethoxide gave a ring-opened compound.

3-Halo-1,2-dithiolylium cation reacting with H2 S is transformed to a respective mercapto compound, 1,2-dithiolylium- 3-thione, with elimination of HCl.

3,5-Diphenyl-1,2-dithiolylium cation on reaction with ammonia causes ring opening as the initial step with subsequent cyclization to yield isothiazole in 50% yields. However, on reaction with a primary amine the ring-opened  compound was isolated in lieu of isothiazole.

Among known 1,2-dithioles, 1,2-dithiole-3-thiones are the largest class of compounds, which after alkylation transformed to the 1,2-dithiolylium cation. The various resonating structures for the 1,2-dithiolylium cation and dipolar 3H-1,2-dithiole-3-thione and 3-alkylthio-1,2-dithiolylium cations are depicted in the following  scheme.

Synthesis of 1,2-Dithiolylium Cation

Hexachloropropene on heating with sulfur at 160°C offered 3,4,5-trichloro-1,2-dithiolylium chloride salt in 65%  yields.

A reaction of 1,3-diketone with H2S2  in hydrochloric acid-saturated ether afforded 3,5-disubstituted  1,2-dithiolylium salt.

Sulfurization of 3-morpholino-1-phenylthioxopropan-1-one with P2S5  gave 3-morpholino-5-phenyl-1,2- dithiolylium chloride.

3,3-Bis(methylsufanyl)alk-2-en-1-ones on treatment with P2S5  in carbon disulfide with subsequent addition of  perchloric acid afforded 3-methylsulfanyl-1,2-dithiolylium perchlorate.

α,β-Unsaturated β-aminoketones on thionation with P2S5  are transformed to thioketone, which on subsequent  treatment with perchloric acid afforded 1,2-dithiolylium perchlorate.

Oxidation of diethyl propanedithiolate by iodine in ether gave 3,5-bis(ethylsulfanyl)-1,2-dithiolylium salt in  69% yields.