Dithiazolyl Radicals

There is not one generic route into 1,3,2-dithiazolyl/ium chemistry. For acyclic derivatives the [4+2] cycloaddition chemistry of [SNS][AsF6] to alkynes has been successfully expoited by Passmore. However for benzo-fused derivatives, the simplest route is via Wolmershauser s method in which 1,2-sulfenyl 

The stable heterocyclic radical 1,2,5-thiadiazolo- 1,3,2-dithiazolopyrazinyl 26 was prepared by treatment of 5,6-dithiolo-l,2,5-thiadiazolo[3,4- )]pyrazine 

The selenium analogues of these 1,3,2-dithiazolyl radicals have not been extensively studied. Benzo-l,3,2-diselenazolium perchlorate 28 can be prepared from 27 in an analogous fashion to the corresponding sulfur compound and its structure determined by X-ray structure analysis. Unlike BDTA (1,3,2-dithiazolyl radical), reduction of 28 afforded an unstable neutral radical. No stable selenium analogues have yet been isolated (Seheme 8). 

Solution EPR measurements on 4 (R=R =CF3) showed that the solution phase dimerisation process was essentially enthalpically neutral. Unlike 2 and 3, there is likely to be a greater tendency for the formation of derivatives with either regular 7i-stacked motifs or other paramagnetic structure. 

This bistability is favoured by structures, which will adopt Jt stacked motifs, i.e. lamellar molecules. In addition the tendency to adopt the planar structure is enforced by the presence of electronegative heteroatoms (currently restricted to N), which lead to a propensity of in-plane electrostatic S- N contacts between rings. These may comprise heterocyclic S- N contacts or S- N contacts to other functional groups such as the pyrazine or cyano nitrogen atoms. 

While the stoichiometry of the reaction appears simple, the reaction mechanism is not well understood, despite numerous studies,77 and the reaction tipically gives rise to a number of by-products. Purification of Herz salts is best achieved via metathesis to one of a number of more soluble salts (A1C14, BF4 , SbCl6-), which can be recrystallised to a high degree of purity.78 

While the Herz reaction provides a convenient route to many dithiazolylium salts, the propensity for chlorination of the aromatic substituent has led to the development of several other approaches to 1,2,3-dithiazolylium salts and 

3- dithiazolyl radicals. A particular convenient route involves the mild condensation reaction between thionyl chloride and ortho-amino-thiophe-nols.79 This methodology can be easily adapted to allow the preparation of all the different selenium-for-sulfur substituted derivatives with a lesser tendency for ring chlorination (see below). 

Reduction of 38 leads to the formation of the corresponding 1,2,3-dithiazolyl radical. The simple benzo-derivatives have long lifetimes and have been studied extensively by EPR spectroscopy, although attempts to isolate radicals to date appear to have been broadly unsuccessful. The only 1,2,3-dithiazolyl which has been structurally characterised is the non-fused 4-chloro-5-pentafluorophenyl- 

3- dithiazolyl.80 Theoretical calculations indicate that the majority of the unpaired spin density resides on the heterocyclic ring, although there is considerable delocalisation onto the fused aromatic substituent. The failure to isolate any of these radicals led to the attempt to add extra stabilisation through the formation of more extended 7i-delocalised systems. 

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Closely related to the 1,3,2-dithiazolyl radicals are the isomeric 1,2,3-dithiazolyl radicals. The benzo-fused derivatives were originally prepared by Herz in 192276 from the reaction of aniline and its derivatives with an excess of S2C12 (Scheme 10). Almost invariably the aromatic ring becomes substituted by chlorine para to the amine N atom. 

Known 1,2,3-dithiazoles are represented by the structures in row (a) of Figure 1. Strictly, these are 1,2,3-dithiazolines, the first one being 1,2,3-dithiazoline-5-one. 1,2,5-Oxathiazoles are represented by l,2,5-oxathiazolidine-4-ones (b) and 3-methylene 1,2,5-oxathiazolines (c). 1,2,3-Oxathiazoles have been obtained only in the form of their S-oxides (d), which should rather be termed -azolidines and -azolidinones. Experimentally prepared S-oxides of 1,2,3-dithiazoles and 1,2,5-oxathiazoles are depicted in row (e) and should be termed dithiazolines and oxathiazolines. 1,2,3-Dithiazolyl radicals (f) are known, as well as 1,2,3-dithiazolium cations (g) which are the only formally aromatic examples of the ring system and which may be represented by a number of resonance structures (h). 

Dithiazoles investigated are given in Figure 1(a) these are 1,2,3-dithiazolium cations 1, 1,2,3-dithiazolyl radicals 2, l,2,3-dithiazole-3-ones 3 and related compounds 4, their 2-oxides 5. 1,2,3-Oxathiazoles have been obtained and investigated in the form of their A-oxides (Figure 1(b) and named as cyclic sulfamidates 7 and 8 and sulfimidates 6, 9, and 10. 

Dispersion of the valence and conduction bands in the bis( 1,2,3-dithiazolyl) radical 14 has been determined by extended Htickel band structure calculations which suggest that the band gap (ca. 0.4 eV) has a value which is considerably smaller than that found in other radical dimer structures and arises from the weakness of the intradimer interaction <1999JA969>. 

The ay, constants and g-values of known 1,2,3-dithiazolyl radicals are listed in Table 1. 

Table 1 ESR g-values, hyperfine coupling constants (aN) of 1,2,3-dithiazolyl radicals and cation radicals, and half-wave potentials of 1,2,3-dithiazolyls...

Various sulfur-nitrogen heterocycles can serve as a source of 1,2,3-dithiazolyl radicals. The thermolysis of benzo-trithiadiazepin 158 and dibenzotetrathiadiazecine 159 in hydrocarbons afforded the 1,2,3-benzodithiazolyl radical 15 (Scheme 30) <2003MG178>. 

Primary enaminones 178 (R1 = Me or PhCH2 R2 = Me, Ph or MeO) are converted into stabilized 1,2,3-dithiazolyl radicals 179 by the joint action of chlorine and sulphur (equation 76)97. 

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