Triphenylamine oxidative dimerization

Fleischmann et al s 34 report cyclic voltammetry data for the oxidation of a series of aromatic hydrocarbons in a molten salt electrolyte, AlCl3-NaCl-KCl at 150°. Electrooxidation in this medium occurs at unusually low oxidation potentials. Tris-(p-substituted phenyl)amines, with the exception of tri (p-nitrophenyl) amine, yield very stable radical cations by all electrochemical criteria 380>S42 Mono- and bis-p-substituted triphenylamines, however, dimerize with rate constants ranging from 101 to 10s M 1 sec 1 to benzidines 176 (Eq. (237)), which subsequently are oxidized to the radical cations 177, whose ESR-spectra are observed. Dimerization is fastest with the p-N02 andp-CN-derivative, in accordance with HMO calculations, which predict the highest spin sensity in the p-position of these compounds 542 ... 

The reaction of n radical cations with n nucleophiles usually leads to C-C bond formation, a reaction that can be very fast (cf. pericyclic reactions also), as in the oxidative dimerization of triphenylamine k = 1-10 x 10 M s ) [293], Hence, such a reaction mechanism can even operate in anodic oxidations (4-methoxybiphenyl [294], tetrahydrocarbazole [295], 4,4 -dimethoxystilbene [296] and 9-methoxyanthracene) [297], where the radical cation concentration is very high. 

Example for the mechanism 13a the cation radical produced by the oxidation of triphenylamine rapidly dimerizes to the tetraphenylben-zidine which undergoes further oxidation at the same potential [22]. 

Anodic oxidation of triphenylamine gives the radical-cation which is rapidly converted to the dehydrodimer, tetraphenylbenzidine. A para-substituent prevents this dimerization and gives radical-cations very stable in acetonitrile solution against nucleophiles [155], This class of compound was first prepared as the per-... 

Triphenylamine forms a radical cation on anodic oxidation which dimerizes into tetraphenylbenzidine. The redox potential can be mned by the substituting the aromatic ring. The redox behavior of the PAI can be characterized by cyclic voltametry. Films are cast on an indium tin oxide-coated glass substrate as a working electrode in dry acetonitrile. The electrochromism is examined by an optically transparent thin-layer electrode coupled with a UV-vis spectroscopy. 

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