Role of addition compounds in the halogenation of benzofurans and benzothiophens

Abstract

The bromination of benzofuran and its 2-methyl, 3-methyl, and 2, 3-dimethyl derivatives and the chlorination of 2, 3-dimethylbenzothiophen were investigated in CS₂ at –40 and –50° by n.m.r. spectroscopy. In all cases the formation of an adduct between the halogen and the heteroaromatic derivative is observed. Decomposition of the adduct leads to ring halogenated products with benzofuran and the monomethylbenzofurans, and to side-chain halogenated compounds with the 2, 3-dimethyl derivatives. These results are discussed in terms of mechanism of both conventional and non-conventional electrophilic aromatic substitution.