Addition reactions of benzo[b]thiophen. Part 4. Reactions of some acetoxy- and hydroxy-benzo[b]thiophens with benzene or toluene

Abstract

Heating acetoxybenzo[b]thiophens with AICl₃–benzene can give the normal Fries-rearranged products (for the 4-OAc and 7-OAc isomers); these sometimes react further to give their 2,3-dihydro-(for 4-OAc and 7-OAc) or 2-phenyl derivatives (for 6-OAc). Alternatively, benzene can add across the 2,3-double bond of the acetoxycompound, to give the 2-(for 6-OAc) or 3-phenyl-2,3-dihydrobenzo[b]thiophens (for 7-OAc). With AICl₃ in dichloromethane, 6-acetoxybenzo[b]thiophen undergoes intermolecular transfer of an acetyl group to give a mixture of 6-acetoxy-2- and 3-acetylbenzo[b]thiophens and 6-hydroxybenzo[b]thiophen. With AICl₃ in benzene at room temperature, 4-acetoxybenzo[b]thiophen gives a rearranged product, 4,5-dihydro-2,4-diphenylbenzo[b]thiophen-7(6H)-one (3a)(17%).

In the presence of AICl₃ for 0.5 h, 4-, 5-, 6-, and 7-hydroxybenzo[b]thiophens undergo addition of benzene or toluene, to give the appropriate 2-aryl-2,3-dihydrohydroxybenzo[b]thiophens (1). Yields are high for the 4- and 6-hydroxy-isomers (80–-85%), but lower for the 5-(55%) and 7-isomers (10%). In each of these reactions the starting hydroxybenzo[b]thiophen is partly converted into its 2,3-dihydro-derivative. 5- and 7-Hydroxybenzo-[b]thiophen also each give the same 4,5-dihydro-2,4-diarylbenzo[b]thiophen-7(6H)-one (3)(25%) in this reaction. When the reaction period with benzene or toluene is extended to 5 days, the amount of solvent addition product (1) decreases, but all four hydroxy-isomers now give the same rearranged product (3). The mechanism of of this unusual rearrangement is discussed in terms of a spiro-intermediate.