Oxidative benzylic substitution of azulene-1-azo(4′-methylbenzene)s

Abstract

Treatment of azulene-1-azo(4′-methylbenzene) with position 3 blocked by different substituents X (2) or by steric hindrance (3), with FeCl₃ in benzene, resulted in the substitution of one or two benzyl hydrogens by phenyl groups. Starting either from 2 (X = CH₃, Cl) or from 3, triarylmethane derivatives 7 or 10 and corresponding alcohols 9 or 11 are the main separated products while starting from 2 (X = OCH₃), diarylmethanol 8 as well as the related benzophenone and ether, 13 and 14, respectively, are isolated as major products. Diphenylmethane and triphenylmethane also result from almost all reactions. Conversion of the starting materials was good and the products that were separated and characterised are obtained in 43.5–60.5% overall yields. Single-electron transfer oxidation was thought to be the first reaction step with the generation of the radical cation 2^·+. As a result of the proton and electron transfer or hydrogen atom transfer from 2^·+, the cation 16 with some benzylic character was formed. The cation 16 alkylated benzene in the subsequent reaction and diarylmethane derivatives were formed. A similar reaction pathway can be proposed for triarylmethane products generation.