Benzo[b]thiophen derivatives. Part XI. Nitration of 3-acetyl- and 3-formyl-benzo[b]thiophens
The reactions of 3-acetyl- and 3-formyl-benzo[b]thiophens have been studied under three different nitrating conditions. Although treatment of 3-acetylbenzo[b]thiophen in sulphuric acid–acetic acid (1 : 10) with concentrated nitric acid at 60° leads to furoxan formation as had previously been observed for reactions with concentrated nitric acid in refluxing acetic acid, reaction of 3-acetylbenzo[b]thiophen with a molar proportion of potassium nitrate in concentrated sulphuric acid at 0° or with a cold mixture of fuming nitric acid–acetic acid–acetic anhydride gives rise to substitution at all four available positions in the benzene ring. In no case was substitution in the thiophen ring at the 2-position observed, though with the cold fuming nitric acid–acetic acid–acetic anhydride procedure appreciable quantities of 3-nitrobenzo[b]thiophen were obtained in addition to the mixed 4-, 5-, 6- and 7-nitro-3-acetylbenzo[b]thiophens. As in the nitration of the 3-acid, the observed order of preference of substitution in the benzene ring varies with the nitration procedure. Similar results, including replacement of the formyl function on treatment with fuming nitric acid in cold acetic acid–acetic anhydride were found on nitration of 3-formyl-benzo[b]thiophen. Electron density calculations for 3-acetyl- and 3-formyl-benzo[b]thiophens and the 1H n.m.r. spectra of the 4-, 5-, 6- and 7-nitro-3-acetylbenzo[b]thiophens are recorded. G.I.c. retention times are given for the isomeric nitro-3-acetylbenzo[b]thiophens.