Condensed thiophen ring systems. Part VII. Stability of 3-benzo[b]-thienyl-lithium

Abstract

Ring-cleavage reactions of 3-benzo[b]thienyl-lithium (1) and its 2-methyl derivative are reported. When an ethereal solution of compound (1), prepared by treating 3-bromobenzo[b]thiophen with n-butyl-lithium at –70 °C, was kept for 1 h at room temperature prior to treatment of the product with dimethyl sulphate, it gave a mixture of benzo[b]thiophen, 3-bromo-2-methylbenzo[b]thiophen, o-(methylthio)phenylacetylene, and methyl-[o-(methyl-thio)phenyl]acetylene. A similar reaction mixture kept for 18 h at room temperature prior to methylation of the product gave a mixture of benzo[b]thiophen, the two acetylenes obtained before, and methyl -[o-(n-butylthio)-phenyl]acetylene. Methyl-[o-(methylthio)phenyl]acetylene was also obtained as the major product when an ethereal solution of 2-methyl-3-benzo[b]thienyl-lithium was similarly treated (30 min at room temperature). In contrast, when ethereal solutions of compound (1) were kept at room temperature and the products carboxylated instead of methylated, mixtures of benzo[b]thiophen, 3-bromobenzo[b]thiophen-2-carboxylic acid, and benzo[b]-thiophen-2-carboxylic acid were formed. A mechanism is given to account for these results. An unambiguous synthesis of o-(methylthio)phenylacetylene was attempted and a synthesis of methyl 3-bromobenzo[b]thiophen-2-carboxylate is reported. In tetrahydrofuran ring cleavage of compound (1) does not occur but the transmetallation reactions that predominate in this solvent, even at –70 °C, give rise to mixtures of products following either methylation or carboxylation.