Side-chain halogenation of 2,3-dimethylbenzofuran. Competition between ionic and free-radical reactions

Abstract

The chlorination and bromination of 2,3-dimethylbenzofuran lead predominantly to the formation of side-chain substituted products. The main product is 3-halogenomethyl-2-methylbenzofuran when the reaction occurs by a heterolytic mechanism and 2-halogenomethyl-3-methylbenzofuran when the reaction occurs by a free-radical mechanism. Competition between the two mechanisms is observed in conditions usually suitable for heterolytic halogenation. The formation of an addition product as a reaction intermediate in heterolytic halogenation can be excluded on the basis of the observed positional orientation. Probably, this reaction occurs by a mechanism similar to that postulated for electrophilic side-chain halogenation of polymethylbenzenes.