Addition reactions of benzo[b]thiophen. Part 1. Self-addition and addition of simple aromatic hydrocarbons
Benzo[b]thiophen undergoes facile addition reactions across the 2,3-bond when treated with aluminium chloride in an appropriate solvent at 0 or 20 °C. In carbon disulphide or dichloromethane, it undergoes self-addition to give two or more of the four possible 2- or 3-(2- or 3-benzo[b]thienyl)2,3-dihydrobenzo[b]thiophens (3)–(6). In the presence of an aromatic solvent, the dimerisation reaction just mentioned predominates at low temperatures (0 °C or below), or at room temperature if the solvent is benzene, chlorobenzene, t-butylbenzene, isopropylbenzene, or 1,3,5-trimethylbenzene. At room temperature, in toluene, ethylbenzene, and 1,2- or 1,4-dimethylbenzene, solvent addition occurs to give a mixture of the corresponding 2- and 3-aryl-2,3-dihydrobenzene[b]thiophens. At 80 °C, benzene and toluene give the fully aromatic 2-arylbenzo[b]thiophen. The reactions are discussed in terms of an ionic mechanism involving protonation of benzo[b]thiophen by moist aluminium chloride and reaction of the resulting electrophile with benzo[b]thiophen or with an aromatic substrate.