Condensed thiophen ring systems. Part V. Synthesis, reactions, and stability of 2-methyl- and 2-methylthio-3-benzo[b]thienyl-lithium and related compounds.

Abstract

5-Chlorobenzo[b]thiophen on metallation with n-butyl-lithium gave 5-chloro-2-benzo[b]thienyl-lithium, which reacted with carbon dioxide, dimethyl sulphate, and dimethyl disulphide to give, after hydrolysis, the corresponding acid, methyl-, and methylthio-derivatives, respectively. 2,5-Dimethylbenzo[b]thiophen, 2-methylthiobenzo[b]thiophen, and 5-methyl-2-methylthiobenzo[b]thiophen were similarly prepared from 5-methyl-2-benzo[b]-thienyl-lithium or 2-benzo[b]thienyl-lithium. 2-Methyl- and 2-methylthio-benzo[b]thiophen and their 5-methyl- and 5-chloro-derivatives were brominated, and each of the resulting 3-bromo-derivatives was treated with n-butyl-lithium in ether at –70° to give the corresponding 3-lithium compound. These were treated with NN-dimethyl-formamide and carbon dioxide to give the corresponding 3-carbaldehyde or 3-carboxylic acid, respectively. 2-Methyl-3-benzo[b]thienyl-lithium and its derivatives undergo ring-opening reactions in ether at temperatures much in excess of –70°.