Thiadiazoles radicals from

Several thiadiazole-fused organic radicals have been characterized by ESR spectroscopy, and significant spin delocalization can be seen from the hyperfine coupling constants (hfcc s) N (Table 5). 

However, there are some cases when an unpaired electron is localized not on the n, but on the o orbital of an anion-radical. Of course, in such a case, a simple molecular orbital consideration that is based on the n approach does not coincide with experimental data. Chlorobenzothiadiazole may serve as a representative example (Gul maliev et al. 1975). Although the thiadiazole ring is a weaker acceptor than the nitro group, the elimination of the chloride ion from the 5-chlorobenzothiadiazole anion-radical does not take place (Solodovnikov and Todres 1968). At the same time, the anion-radical of 7-chloroquinoline readily loses the chlorine anion (Fujinaga et al. 1968). Notably, 7-chloroquinoline is very close to 5-chlorobenzothiadiazole in the sense of structure and electrophilicity of the heterocycle. To explain the mentioned difference, calculations are needed to clearly take into account the o electron framework of the molecules compared. It would also be interesting to exploit the concept of an increased valency in the consideration of anion-radical electronic structures, especially of those anion-radicals that contain atoms (fragments) with available d orbitals. This concept is traditionally derived from valence-shell expansion through the use of d orbital, but it is also understandable in terms of simple (and cheaper for calculations) MO theory, without t(-orbital participation. For a comparative analysis refer the paper by ElSolhy et al. (2005). Solvation of intermediary states on the way to a final product should be involved in the calculations as well (Parker 1981). 

Reactions of 1,2,4-thiadiazoles with radicals and electron-deficient species are virtually unknown. Catalytic and dissolving metal reductions usually cleave the nucleus at its N—S bond by a reaction that may be regarded as the reverse of its synthesis by the oxidative cyclization of amidinothiono structures (Section 4.08.9.4). For example, reduction of the diamino compound (37) gives the amidinothiourea (38) from which it may be prepared by oxidation (Equation (8)). 

A three-step reaction sequence starting from 3,4-dichloro-l,2,5-thiadiazole (81) led to the formation of the stable sulfur-nitrogen radical [l,2,5]thiadiazolo[3,4-fl ][l,3,2]dithiazol-l-ium (14). Treatment of thiadiazole (81) with two equivalents of Na2S afforded the sodium salt (82), which reacted with chlorine gas to furnish the sulfenyl chloride (83). The reaction of this intermediate (83) with trimethylsilyl azide yielded the salt (84), reduction of which afforded the novel pentalene analogue (14) as black needle-like crystals (Scheme 18) <89AG(E)920>. 

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