The reactivity of thieno[3,2-b]thiophen and thieno[2,3-b]thiophen with phenyl radicals

Abstract

The isomeric thieno[3,2-b]thiophen (I) and thieno[2,3-b]thiophen (II) have been phenylated with phenyl radicals derived from the thermal decomposition of N-nitrosoacetanilide and from the reaction of aniline and pentyl nitrite. The radical substitution is favoured in the 2-position in the case of compound (I), but in structure (II) the 2- and 3-positions have comparable reactivities. The values of the relative reactivities show that for homolytic phenylation the order of decreasing reactivity is thieno[3,2-b]thiophen > thiophen > thieno[2,3-b]thiophen. The experimental results may be rationalised on the assumption that the contribution of the condensed thieno-group to the stabilisation of the intermediates is greater in (I) than in (II).

The syntheses of 2- and 3-phenylthieno[3,2-b]thiophens and of 2- and 3-phenylthieno[2,3-b]thiophens are described.