The Use of Bis(4-chlorophenyl)Selenide/Lewis Acid Catalysts in the Electrophilic Chlorination of Toluene

Article abstract of DOI:10.1021/jo00300a030

The electrophilic chlorination of toluene has been studied using bis(4-chlorophenyl)selenide/Lewis acids as catalysts.These catalysts generate ortho/para ratios which are considerably lower than those obtained using Lewis acids as catalysts with the ortho/para ratio decreasing as the reaction temperature increases in the range of -30 to 70 deg C.The enhanced para selectivity observed using these catalysts has been ascribed to the intermediacy of a bis(4-chlorophenyl)selenium dichloride/Lewis acid complex which functions as a sterically hindered source of chlorine.Proton NMR studies in acetone-d6 support the existance of bis(4-chlorophenyl)selenium dichloride/Lewis acid complexes which lose chlorine directly from the selenium atom.The loss of para selectivity as the reaction temperature decreases has been ascribed to an increase in the conversion of bis(4-chlorophenyl)selenide to bis(4-chlorophenyl)(4-methylphenyl)selenonium chloride, which does not function as a catalyst in this reaction.Although triarylselenonium chlorides are known to reductively eliminate to produce aryl chlorides, our studies have shown that only where the Lewis acid is aluminium(III)chloride does this occur.Subsequently, with the exception of aluminium(III)chloride, reductive elimination of bis(4-chlorophenyl)(4-methylphenyl)-selenonium chloride is not responsible for the high para selectivity observed under our conditions.