Reaction Analysis of 3-Substituted-diphenylamine Cation Radicals in Acetonitrile. Cyclization Reaction vs. Benzidine Formation

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The reactions of 3-substituted-diphenylamine cation radicals in acetonitrile were studied using an electron transfer stopped-flow method. In the reactions of the 3-chloro-diphenylamine cation radicals (mCl-DPA·+), the main reaction route was the formation of the benzidine dimer, which was similar to the case of the diphenylamine cation radical (DPA·+). Although the reaction of DPA·+ proceeded via the cation radical-cation radical coupling as verified from the rate law of −d[DPA·+]/dt=k[DPA·+]2, the present kinetic analysis has revealed that the decay rate of mCl-DPA·+ was dependent on the concentration of the neutral molecules, i.e., the rate law was expressed as −d[mCl-DPA·+]/dt=k[mCl-DPA·+]2 [mCl-DPA]. In contrast, the reaction of the 3-methoxy-diphenylamine cation radical (mMeO-DPA·+) was too fast to be observed using the stopped-flow method, which is quite in contrast to the 4-methoxy-diphenylamine cation radical (pMeO-DPA·+) which was very stable in acetonitrile. In the case of mMeO-DPA·+, the cyclization reaction was confirmed to proceed soon after the generation of mMeO-DPA·+, which is similar to the case of the 3-methyl-diphenylamine cation radical (mMe-DPA·+). Thus, it was found that the substituent on the 3-position changed the reaction pathways of DPA·+ significantly, as well as their reactivity.