Remarkable 3-methyl substituent effects on the cyclization reaction of diphenylamine derivative cation radicals in acetonitrile

Abstract

Reactions of methyl substituted diphenylamine cation radicals in acetonitrile were observed using an electron transfer stopped-flow (ETSF) method. In the reactions of the 4-methylphenyl(phenyl)amine cation radical (4M–DPA˙⁺), the main reaction route was the formation of the benzidine dimer, which is similar to the case of the diphenylamine cation radical (DPA˙⁺). The dimerization rate of 4M–DPA˙⁺ was 2.3 × 10⁴ M⁻¹ s⁻¹, which was slower than for DPA˙⁺ (dimerization rate 1.0 × 10⁶ M⁻¹ s⁻¹), due to the 4-methyl substituent. Also, in the cases of 4-methyl and 4,4′-dimethyl DPA˙⁺, the formation of the cyclized dimer compounds was inferred from the presence of a large excess of the neutral molecules acting as a base. In contrast, the formation of the cyclized dimer compounds was characterized for 3-methyl- and 3,3′-dimethyl substituted DPA˙⁺ using cyclic voltammetry. In the ETSF method, the dimerization reaction was determined to be second-order in the cation radical and totally independent of the neutral molecule. The dimerization rates were ca. 1.0 × 10⁷ M⁻¹ s⁻¹, which are faster than the reaction of DPA˙⁺. In spite of the complex pathway of the cyclization reaction, the 3-methyl substituent was found to promote the reaction between the 6-position of the phenyl ring and the nitrogen molecule of 3M–DPA˙⁺ or 3,3′M–DPA˙⁺.