Thieno[2,3-b][1]benzothiophen and thieno[3,2-b][1]benzothiophen. Part II. Substitution reactions

Abstract

Thieno[2,3-b][1]benzothiophen and thieno[3,2-b][1]benzothiophen show similar monosubstitution reactions; each undergoes bromination, Vilsmeier–Haack formylation, Friedel–Crafts acetylation, nitration, and metalation very readily in the 2-position, and the results are in accord with calculated electron-density values. However, 2-bromothieno[2,3-b][1]benzothiophen gives the 2,3-dibromo-compound on further bromination, whereas 2-bromothieno[3,2-b][1]benzothiophen is brominated further in the benzene ring, probably in the 6-position. The Schmidt reaction of 2-acetylthieno[2,3-b]- or [3,2-b]-[1]benzothiophen with hydrazoic acid gives N-methyl-thieno[2,3-b]- or [3,2-b]-[1]benzothiophen-2-carboxamide, and not the expected 2-acetamido-compound. Reduction (LiAlH₄) of thieno[2,3-b][1]benzothiophen-2-carbaldehyde gives bis-(2-thieno[2,3-b][1]benzothienyl)methane. The latter compound is also formed in attempted chloromethylation of thieno[2,3-b][1]benzothiophen, or by treatment of 2-hydroxymethylthieno[2,3-b][1]benzothiophen with a trace of acid.