Benzo- and Dibenzo-l,4-dithiins

Benzo- and Dibenzo-l,4-dithiins.—The benzo-l,4-dithiins (51) and (52), when oxidized with hydrogen peroxide, give mono-5-oxides (53) and (54) which on further oxidation are partially desulpjiurized to compound (55).  

The thianthrene cation-radical (56) shows enhanced stability in trifluoro-acetic acid, which is recommended generally as a solvent in which to prepare cation-radicals. Thianthrenium perchlorate (56 C10 as counter-ion) reacts with substituted benzenes PhR at the para position, rapidly where R = MeO, more slowly where R = Me. The product is a sulphonium salt (58). Kinetic studies show that the reaction is second-order in the thianthrene cation-radical (56). The suggested mechanism features the thianthrene dication (57) as the reactive species, formed in low concentration by disproportionation of the thianthrene cation-radical. 

A sufficient number of syntheses of isothiazoles have now been described to make their formal classification desirable. They may be conveniently arranged, as in the case of other heterocyclics, according to the nature of the fragments from which the ring system is constructed. Although the isothiazole nucleus may be built up in numerous ways, the syntheses reported in the present section fall into the following three categories  

When the isothiazole arises from another ring system, the synthesis is classified according to the linear intermediates that are most likely concerned in the final ring closure. It will be noticed that the majority of syntheses reported are of Type C, i.e. the route by which isothiazole was first obtained. 

From a-Amino-ketones or ( -Cyano-enamines and Sulphur Chlorides (Type 

Benzo- and Dibenzo-1,4-dithiins.—An improved synthesis of benzo-l,4-dithiin and -oxathiin is based on a novel intramolecular sulphenylation of the thiol-sulphonates PhY(CHa)2SSOaMe (Y = S or O) in the presence of aluminium chloride. Phenyl-lithium reacts with bromobenzene and carbon disulphide to give 2,3-diphenyl-l,4-benzodithiin via a benzyne intermediate.  

The first thianthrene dication (32) has been isolated. Work on the thianthrene cation radical (33) continues. Its reaction with aliphatic amines yielded the sulphimides (34 X = NR) phenols and aromatic amines were attacked in the pflra-position to afford (34 X = / -HOCeH4 or p-R2NC H4), and enolizable ketones yielded the j8-keto-sulphonium salts (34 X = CHR COR ). The latter compounds reacted with nucleophiles Y to give the a-substituted ketones YCHR COR, and might prove to be useful intermediates for their synthesis. Reinvestigation of the kinetics of the reaction of (33) with phenol and with anisole has provided evidence which appears to disprove the disproportionation mechanism postulated earlier for these reactions. 

Synthesis.—As before (see Vol. 3, p. 541), the synthetic routes to isothiazoles have been classified according to the nature of the fragments from which the heterocyclic ring is ultimately constructed. 

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