Substitution reactions of benzo[b]thiophen derivatives. Part V. t-Butylation of benzo[b]thiophen and nitration of 3-t-butylbenzo[b]thiophen-2-carboxylic acid

Abstract

Contrary to previous reports, alkylation of benzo[b]thiophen with 2-methylpropene in the presence of polyphosphoric acid does not give entirely 3-t-butylbenzo[b]thiophen; instead, a mixture containing the 2-(22%) and 3-t-butyl isomers (71%) and an unidentified component (7%) is obtained. Similarly, alkylation with t-butyl alcohol and concentrated sulphuric acid gives mainly 2-(6%) and 3-t-butylbenzo[b]thiophen (89%). The mixtures are best separated by successive treatment with n-butyl-lithium and carbon dioxide. 2-t-Butylbenzo[b]thiophen is then recovered unchanged, but the 3-t-butyl isomer is converted into the 2-carboxylic acid, from which it can be obtained pure by decarboxylation.

Nitration of 3-t-butylbenzo[b]thiophen-2-carboxylic acid in sulphuric acid and acetic acid at 60° or in acetic anhydride and acetic acid at 0° gives differing proportions of the following acidic products in each case: benzo[b]-thiophen-2-carboxylic acid, 3-, 4-, 6-, and 7-nitrobenzo[b]thiophen-2-carboxylic acid, and 3-t-butyl-4-, 3-t-butyl-6-, and 3-t-butyl-7-nitrobenzo[b]thiophen-2-carboxylic acid. The products are identified mainly by n.m.r. spectroscopy; mention is made of the effect of the 3-t-butyl group on the n.m.r. spectra of some benzo[b]thiophen derivatives. Mechanisms are proposed to explain the formation of those of the above products which lack a t-butyl group. Neutral material from the nitration reaction contains, inter alia, 3-t-butyl-2-nitrobenzo[b]thiophen and 2-nitrobenzo[b]thiophen.