Thianthrene is a sulfur-containing heterocyclic chemical compound in which two benzene rings are connected by a 1,4-dithiin ring. It is notable for its ease of oxidation.
Structure Like other 1,4-dithiins but unlike its oxygen analog dibenzodioxin, the shape of thianthrene is not planar. It is bent, with a fold angle of 128° between the two benzo groups. The central six-atom ring adopts a boat conformation, as also found in thianthrene's parent heterocycle 1,4-dithiin and its selenium analog selenanthrene.
Synthesis Thianthrene was first synthesized by John Stenhouse by dry distillation of sodium benzenesulfonate. Thianthrene can be prepared by treating benzene with disulfur dichloride in the presence of aluminium chloride..
Reactions Thianthrene has an extensive redox chemistry. It is oxidized by sulfuric acid to a red radical cation. In water-free environments, the metastable thianthrenediium dication may also be observed.
Thianthrene•+ (5-thianthrenyl-5-ylium) has been characterized by electron paramagnetic resonance. Four different publications describe the crystal structure of thianthrene radical cation salts. The cation is nearly planar, depending on the particular salt. In addition to flattening the heterocycle, removal of an electron causes the C−S distances to contract by 3%. These salts have use as one-electron oxidants. The tetrafluoroborate salt is obtained by reacting thianthrene with nitrosonium tetrafluoroborate. It is safer than the perchlorate, a shock-sensitive explosive.
The radical cation reacts with water to give equimolar quantities of thianthrene and its S-oxide, which is hence the product of aqueous oxidation with reagents such as ferric nitrate. The S-oxide is a precursor to aryl thianthrenium salts, which are versatile synthetic intermediates.
The oxidation of thianthrene S-oxide has been used to measure the electrophilicity of oxygen-transfer reagents; electrophilic oxidation yields more cis- and trans- disulfoxides, whereas nucleophilic oxidation yields more of the sulfone. Oxides all the way up to the tetroxide are known.
Thianthrene also can function as a Lewis base.
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