Amine oxides coupling, anodic

Oxidative coupling has been observed for benzene (52), methyl substituted benzenes (53), triphenylethylene (54), triphenyl-amines (55-59), anilines (57), carbazoles (60,61), iminobibenzyls (62), and heterocyclic phenols (71,72). Intramolecular anodic coupling reactions are used for synthesizing specific ring structures (63-68). Both dimer and octamer of dibenzothiophene have been detected (69,70)... 

Tetraphenylethylene cyclizes anodically to 9,10-diphenylphenanthrene analogously to its photooxidative cyclization. The attempted anodic cyclization of cis- or frans-stilbene to phenanthrene however failed due to electrophilic reaction of the intermediate radical cation with the solvent 37S Primary aromatic amines are oxidized to radical cations which, depending on the pH of the electrolyte couple to aminodiphenylamines (C-N coupling (84) in Eq. (172) ), yield benzidines (85) at low pH (C-C coupling) or dimerize to hydrazobenzene (86) (N-N coupling) which is subsequently oxidized to azobenzene (Eq. (172) ) 2 5,376,377)... 

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]. 

Otherwise, lithium amides of secondary amines undergo anodic dimerization to form hydrazines in moderate yields [12]. Hydrazines are also generated, if secondary amines are electrochemically oxidized in the presence of an alkali hydroxide [27,28]. This reaction is mainly effective if the coupling takes place intramolecularly to give cyclic hydrazine derivatives [28]. If lithium amides of secondary amines are anodically oxidized in tetrahydro-furan (THF) solution in the presence of the free amine, 2-aminotetrahydrofurans are formed in reasonable yields. In contrast, the respective aminomagnesium bromides only gives A, A -coupling products [29]. 

Aliphatic amines are mainly converted to a-substituted products [99,100], whereby especially the a-methoxylation leads to valuable reagents for synthesis. The intermediate iminium salts can be directly trapped by silyl enol ethers to form Mannich bases [108]. If the a-position is blocked or steric conditions favor it, N,N coupling to hydrazo or azo compounds occurs (Table 5, numbers 17-19). 1,1-Disubstituted hydrazines are dimerized to tetrazenes in fair to excellent yields (Table 5, numbers 20-24). The intermediate diaze-nium ions can attack enolizable carbonyl compounds to form aza-Mannich bases [109]. Arylazonaphthols undergo anodic oxidation, producing radical cations. These couple to biphenylbisazo compounds (up to 34%) or can be trapped by anisidine to form azodiphe-nylamines (up to 74%) [110a]. 

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). 

Rate constants for a series of oxidations of amines and alcohols at anodically oxidized Ni, Ag, Cu, and Co electrodes were measured. The oxidations proceed by reaction with the oxide layer coupled with its electrochemical regeneration by an anodic reoxidation step. Reaction with solution-... 

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