Bromination and Vilsmeier–Haack formylation of 6,7-dihydrobenzo[b]-thiophen-4(5H)-one

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Abstract
Monobromination of 6,7-dihyrobenzo [b] thiophen-4(5H)-one in aqueous acetic acid or ether gives mainly the 2- or the 5-bromo-derivative respectively. The 2-bromo-derivative (I; R1= Br, R2= H) reacts further with bromine (1 mol.) in aqueous acetic acid to give the 2,3-dibromo-derivative, and in ether to give the 2,5-dibromo-derivative. Reaction of 2,5-dibromo-6,7-dihydrobenzo[b]thiophen-4(5H)-one with an excess of bromine in acetic acid in the presence of a trace of hydrobromic acid gives a phenolic compound which is probably 2,3,5,7-tetrabromo-4-hydroxybenzo[b]thiophen. The 1H n.m.r. and mass spectra of all the bromo-compounds have been examined. Vilsmeier–Haack reaction of 6,7-dihydrobenzo[b]thiophen-4(5H)-one with N-methylformanilde gives a mixture of the 5-formyl derivative [which is better represented as the hydroxymethylene form (II)[, 6,7-dihydro-4-(N-methylanilino)benzo[b]thiophen-5-carbaldehyde (VI) and the corresponding iminium salt (VII), 2-formyl-6,7-dihydrobenzo[b]thiophen-4(5H)-one, and 4-chloro-6,7-dihydrobenzo[b]thiophen-5-carbaldehyde (VIII). The last compound is the major product (58%) of the reaction of 6,7-dihydrobenzo[b]thiophen-4(5H)-one with phosphoryl chloride and dimethylformamide. Several reactions of the hydroxymethylene compound (II) are described.

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