Diphenylamine anodic oxidation

The conversion of substituted diphenylamines and triphenylamines to carbazoles at platinum anodes in CH3CN-Et4NC104 takes place if the intermediate cation-radical is fairly stable. Thus the anodic oxidation of (V-ethylbis(p-fert-butylphenyl)amine (87) gave 3,6-di-ferf-butyl-Af-ethyl-carbazole (88) in 15% yield152 [Eq. (72)]. 

Tail-to-tail coupling of radicals obtained in the anodic oxidation of triphenyl-amines results in the formation of tetraphenylbenzidines. Oxidation of triarylamines to the di-cation results in the formation of the carbazoles, as observed for Ai -alkyl-p,p -disubstituted diphenylamines [1-3, 78]. The cation radicals of triarylamines with substituents in the para position of the aryl groups, which can protect them against nucleophilic attack, are very stable and can be used as organic redox catalysts for indirect electrochemical oxidation reactions. Depending on the substitution pattern on the phenyl group the oxidation potentials of the triarylamines can be tuned over a wide range [Eoy. = 0.7-2.0 V) and many of these have been used as redox catalysts in numerous indirect electrochemical reactions [1-3, 79-83]. 

The major pathways for the anodic oxidation of diarylamines are N,N- or aryl,aryl-coupling, A,aryl-coupling, and nucleophilic substitution in the aromatic ring. Which pathway actually will be followed depends on the reaction conditions and the substitution pattern of the substrate. With regard to the latter, one has to distinguish between p-unsubstituted, p-mono- and / -disubstituted diphenylamines. 

Electro-oxidation of diphenylamine systems has received extensive attention. Recent interest has been associated with the preparation of electronically conductive polymers, such as polyaniline. An important role of p-aminodiphenylamine in the anodic oxidation of aniline is well documented and therefore fundamental electrochemical properties... 

Anodic oxidation of 4-aminobiphenyl (68f, X = Ph) in mixtures of hydrochloric acid and ACN results in the same polymer (38) as that obtained from diphenylamine, suggesting68 the conventional head-to-tail coupling, as for other para-substituted anilines. 

The mechanism of the reaction that occurs at a platinum anode in CH3CN-Et4NC104 has been elucidated by showing that iV,N,AMriphenyl-o-phenylenediamine (50) is oxidized quantitatively at + 1.3 V versus SCE to give the dication of (49), which then can be reduced to 49 at —0.7 V versus SCE. Selection rules for the conversion of diphenylamines to dihydrophen-azines are given.109... 

The numerous experiments which led to the formation of dyes at the anode, when aniline, toluidine, methylaniline,. diphenylamine, nethyldiphenylamine and naphthylamine or their salts were electrolyzed, have, however, not been scientifically investigated and, hence, still remain unsolved. The same holds true of Goppelsroder investigations concerning the oxidation of phenol and anthraquinone. The most important discovery is the fact that aniline salts smoothly yield aniline black at the anode the naphthylamine salts give naphtliylamine-violet.1... 

Usually, aromatic amines are very easy to oxidize. Unsubstituted, A-alkyl-, and N,N-dialkylanilines show anodic potentials in the range of 0.4 to 1.0 V (vs. NHE). Diphenylamines cover a broader spectrum of potentials depending on their substitution pattern. Triphenylamines have oxidation potentials between 0.7 and 2.0 V (vs. NHE). For aromatic amines, a large number of potentials has been tabulated [33], The anodic oxida-... 

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