Addition reactions of benzo[b] thiophen. Part 3. Addition and ring-opening reactions with phenolic ethers

Abstract

In the presence of aluminium chloride, anisole, phenetole, diphenyl ether, and (methylthio)benzene are added rapidly across the 2,3-bond in benzo[b]thiophen to give, inter alia, a mixture of 2- and 3-(p-substituted aryl)-2,3-dihydrobenzo[b]thiophens (3) and (4). Contrary to expectation, the 2-isomer predominates in all cases, and the addition is irreversible. Reaction with anisole also leads to a novel ring-opened product, viz.(E)-4-methoxy-2′-methylthiostilbene (9a), which was synthesised unambiguously. It is believed that benzo[b]thiophen is S-methylated by PhOMe–AlCl₃in a ‘push–pull’ reaction, and that the resulting positive charge is delocalised into the ring, thus allowing the 2-position to participate in electrophilic attack on a second molecule of anisole. The resulting quadrivalent sulphur intermediate achieves stabilisation by means of a rapid ring-opening reaction. Other aromatic methyl ethers give analogous products, but phenetole, diphenyl ether, and (methylthio)benzene do not promote ring-opening. The ring-opening reaction is aided by the addition of MeBr– or EtBr–AlCl₃; even phenetole will then undergo ring-opening. Under these conditions the S-alkyl group in the ring-opened product comes from the starting ether, and not from the added MeBr or EtBr.