Anion-Radicals Containing a Thiophene Ring

No persistent anion-radical of thiophene itself has been reported. On the contrary, Gerdil and Lucken reported failure to form the radical under conditions which permit the formation of dibenzothiophene anion-radical (vide infrdf -, and recently it has been found that, in aqueous radiolytic conditions, the first product to be observed following attachment of an electron to thiophene is 2-hydrothienyl radical, which results from a rapid protonation of the anion-radical. However, Soviet workers have described 

The anion-radical of benzo[Z ] thiophene has transient occurrence in the exciplex which results when the heterocycle is irradiated in amine solvents, and the reactivity of the exciplex is explicable in terms of the distribution of spin population calculated for the anion-radical.The anion-radical 106 of benzo[c]thiophene has been formed by reduction of the parent heterocycle with potassium in Its ESR spectrum was measured and assigned. 

The extended conjugation present in dibenzothiophene permits the formation of a stabilized anion-radical 107 from this heterocycle. It was reported by several groups in the early The hyper- 

Adam and Kepford, confirmed by deuteration experiments and obtained by potassium reduction in DME the early literature contained a mis-assignment. Bard and co-workers have correlated homogeneous electron-transfer rates between 107 and its parent heterocycle with heterogeneous rate constants for electroreduction of the heterocycle/ and Japanese workers have described optical absorption spectra for 107 and analogs. They conclude that the heteroatom lias minimal perturbing effect on the 7c-elec-tronic structure of the biphenyl skeleton consistent with the heteroatom lying at a node in the singly occupied 

Other anion-radicals containing the thiophene heterocycle in an otherwise hydrocarbon molecular environment and whose ESR spectra have been analyzed are 110 (for which hyperfine splittings are given in gauss), the anionic counterpart of 94, and the anion-radical of 963 2,343. thieno[2,3-ZjJthiophene (111) failed to give a persistent radical by alkali metal reduction. 

The same Bologna group, who have contributed much to our knowledge of thiophene radicals, have also investigated the anion-radicals of isomeric rraw.s-dithienylethylenes, 1-(2-thienyl)-2-phenylacetylene, and trans- - -thienyl)-2-phenylethylene. The ESR spectra indicate that rotation of the aryl groups in these molecules is slow on the time scale of the ESR experiment, and separate rotamers have been detected for the radical from 

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