Reactivity of halogenated aromatic radical anions. Electron spin resonance evidence for the dimerization of the 6-halogeno-2H,3H-benzo[b]thiophene-2,3-semidiones

The reactions of the 6-halogeno-W,3H-ben[b]thiophene-2,3-dione(6a- X-BTD; X = F, Cl, Br, I) with potassium or sodium tert-butoxide in Me2S0, with KOH in DMF or with t-BuONa in DME in the presence of crown ether afforded radicals whose ESR spectra indicate that the radical anions of the starting products dimerize. It is suggested that the observed dimeric radicals form through an initial oupling between the oxygen atom at position 3 and the carbon atom at position 6, followed by elimination of halide ions and reduction. The dimerization process is suppressed in t-BuOH or in DME where the halogenated semidiones are present as chelated complexes with the metal cations.