Triphenylamine, anodic reactions

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. 

Substituted di- and triphenylamines lead to carbazoles (XXII, 0, Y = NR Table 7, number 8). The yields are poor, however, and the cyclization is restricted to tertiary amines with the para position blocked by substituents not susceptible to substitution or elimination. The 2-methyltetramethoxybibenzyl derivative XXIII cyclizes in an anodic 2,6 -coupling reaction, with a subsequent dienone-phenol rearrangement, to give an 88-98% yield of the dihydrophenanthrene XXIV [Eq. (15)], which contains the essential structural elements of the B, C, and D ring of steroids [143]. 

Intramolecular coupling or nucleophilic substitutions sometimes occur upon oxidation of the cation radical the new product frequently then undergoes an additional anodic electron-transfer reaction to yield a new cation radical. For example, tri-p-substituted triphenylamines form stable cation radicals upon oxidation at their first anodic wave. Reynolds et alt (1974) have shown, however, that further oxidation of the cation radicals leads to the appearance of a new reversible redox couple at potentials slightly positive with respect to the first anodic wave. This new couple was shown to occur at potentials where the corresponding carbazole is oxidized, so that the overall process involves conversion of the amine to the carbazole cation radical by reaction scheme (85). 

As outlined in the introduction, quenching of excited states of complexes such as Ru(bipy)3 by reductants such as triphenylamine in a one-electron transfer step (Reaction 3) is a well-established process. With most complexes examined to date, back-electron transfer is both rapid and efficient so that net chemical change is seldom observed. The hydro-phobic Complexes 1, 2, and 3 were found to have absorption spectra, luminescence spectra, and excited-state lifetimes similar to those of Ru(bipy)3 however, their solubility and redox behavior is somewhat different (39, 40). While Ru(bipy)3 is soluble in water and a few polar organic solvents. Complexes 1, 2, and 3 are water insoluble but rather widely soluble in a variety of nonaqueous solvents. Redox potentials of Complexes 1 and 2 are shifted more anodic as shown in Scheme 1 the... 

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