Activation of electrophilic aromatic substitution by the substituent –CH2Co(dmgH)2py. Products of reaction of benzylcobaloximes with halogens in acetic acid

Abstract

Benzyl-, 2-methylbenzyl-, and 4-methylbenzyl-cobaloxime react rapidly with low concentrations of chlorine and bromine in acetic acid to give the corresponding benzyl and methylbenzyl halides. 3-Methylbenzylcobaloxime undergoes a substantial proportion of ring substitution by both bromine and chlorine in competition with the cleavage of the carbon–cobalt bond. 3,5-Dimethylbenzylcobaloxime undergoes ring substitution before carbon–cobalt bond cleavage and at rates greater than those for the comparable substitution reactions of mesitylene, but comparable with those of 3,5-dimethylanisole. The p-xylylenedicobaloxime does not undergo ring substitution, but m-xylylenedibaloxime undergoes considerable substitution at both the positions para- to the organometallic substituents, before carbon–cobalt bond cleavage occurs. In contrast, iodine causes carbon–cobalt bond cleavage without prior ring substitution in all the cases studied. The results are discussed in terms of the substituent effect of the organometallic group, which is greater than that of the methyl group, but less than that of the methoxy-group.