Aromatics electrochemical oxidation

Electropolymerization is also an attractive method for the preparation of modified electrodes. In this case it is necessary that the forming film is conductive or permeable for supporting electrolyte and substrates. Film formation of nonelectroactive polymers can proceed until diffusion of electroactive species to the electrode surface becomes negligible. Thus, a variety of nonconducting thin films have been obtained by electrochemical oxidation of aromatic phenols and amines Some of these polymers have ligand properties and can be made electroactive by subsequent inincorporation of transition metal ions... 

The soluble products are able to form charged high-spin states after chemical and electrochemical oxidation. The high-spin character is the result of the lack of conjugative interaction between the highly distorted, orthogonally arranged aromatic subunits (decoupled rr-systems) [68]. 

Reactions of partial electrochemical oxidation are of considerable interest in the electrosynthesis of various organic compounds. Thus, at gold electrodes in acidic solutions, olefins can be oxidized to aldehydes, acids, oxides, and other compounds. A good deal of work was invested in the oxidation of aromatic compounds (benzene, anthracene, etc.) to the corresponding quinones. To this end, various mediating redox systems (e.g., the Ce /Ce system) are employed (see Section 13.6). 

Intermediates generated at an electrode surface may react while still near the electrode. If so, one side of the intermediate may be wholly or partly shielded from attack by other reactants by the electrode itself. Such behavior is particularly common in the electrochemical oxidation of aromatic compounds since, as we have already seen with coumarin, aromatic compounds are generally tightly adsorbed parallel to the electrode surface at potentials positive of the p.z.c. For example, electrochemical oxidation of the stilbenes in alkaline methanol affords a mixture of dl and meso-1,2 dimethoxy-1,2-diphenylethane (1) 10>. It is found that c/s-stilbene affords a mixture of isomers of 1 in which the... 

Dvorak V, Nemek I, Zyka J (1967) Electrochemical oxidation of some aromatic amines in acetonitrile medium II. benzidine, N,N,N ,N -tetramethylbenzidine, and 1,4-phenylene-diamine derivatives. Microchem J 12 324-349... 

For /8-substituted 7t-systems, silyl substitution causes the destabilization of the 7r-orbital (HOMO) [3,4]. The increase of the HOMO level is attributed to the interaction between the C-Si a orbital and the n orbital of olefins or aromatic systems (a-n interaction) as shown in Fig. 3 [7]. The C-Si a orbital is higher in energy than the C-C and C-H a orbitals and the energy match of the C-Si orbital with the neighboring n orbital is better than that of the C-C or C-H bond. Therefore, considerable interaction between the C-Si orbital and the n orbital is attained to cause the increase of the HOMO level. Since the electrochemical oxidation proceeds by the initial electron-transfer from the HOMO of the molecule, the increase in the HOMO level facilitates the electron transfer. Thus, the introduction of a silyl substituents at the -position results in the decrease of the oxidation potentials of the 7r-system. On the basis of this j -efleet, anodic oxidation reactions of allylsilanes, benzylsilanes, and related compounds have been developed (Sect. 3.3). 

Trisarylamines Trisarylamines have been successfully used as redox catalysts in many indirect electrochemical oxidations. Their advantage is the possibility to adjust their oxidation potential by selection of the substituents on the aromatic rings. 

A similar electrochemical oxidation of aromatic aldehydes to their corresponding methyl esters was achieved by using flavo-thiazolio-cyclophane as a mediator [87]. 

Accordingly, many reactions can be performed on the sidewalls of the CNTs, such as halogenation, hydrogenation, radical, electrophilic and nucleophilic additions, and so on [25, 37, 39, 42-44]. Exhaustively explored examples are the nitrene cycloaddition, the 1,3-dipolar cycloaddition reaction (with azomethinylides), radical additions using diazonium salts or radical addition of aromatic/phenyl primary amines. The aryl diazonium reduction can be performed by electrochemical means by forming a phenyl radical (by the extrusion of N2) that couples to a double bond [44]. Similarly, electrochemical oxidation of aromatic or aliphatic primary amines yields an amine radical that can be added to the double bond on the carbon surface. The direct covalent attachment of functional moieties to the sidewalls strongly enhances the solubility of the nanotubes in solvents and can also be tailored for different... 

Anodic oxidation in inert solvents is the most widespread method of cation-radical preparation, with the aim of investigating their stability and electron structure. However, saturated hydrocarbons cannot be oxidized in an accessible potential region. There is one exception for molecules with the weakened C—H bond, but this does not pertain to the cation-radical problem. Anodic oxidation of unsaturated hydrocarbons proceeds more easily. As usual, this oxidation is assumed to be a process including one-electron detachment from the n system with the cation-radical formation. This is the very first step of this oxidation. Certainly, the cation-radical formed is not inevitably stable. Under anodic reaction conditions, it can expel the second electron and give rise to a dication or lose a proton and form a neutral (free) radical. The latter can be either stable or complete its life at the expense of dimerization, fragmentation, etc. Nevertheless, electrochemical oxidation of aromatic hydrocarbons leads to cation-radicals, the nature of which is reliably established (Mann and Barnes 1970 Chapter 3). 

Furthermore, electrochemical oxidation with TEMPO mediated aromatization of 6-membered cyclic dienes" and transformation of alkenes to alkenones also took place" . 

The aromatic dibenzopyrrocoline dimer 33 was obtained by Hess etal. (22) by electrochemical oxidation of papaverine at a platinum anode in methanolic so-... 

The efficient formation of diaryliodo-nium salts during the electrolysis of arylio-dides has been reported by Peacock and Fletcher [166]. The electroiodination of a 3D-aromatic molecule, dodecahydro-7,8-dicarba-nido-undecaborate has also been reported [167]. The iodination (and bromi-nation) of dimedone has been reported to yield 2-iododimedone, which formally is an electrophilic substitution reaction [123]. In a similar process, the indirect electrochemical oxidation of aliphatic ketones in an alkaline Nal/NaOH solution environment has been shown to yield a,a-diiodoketones, which rapidly rearrange to give unsaturated conjugated esters [168]. Dibenzoylmethane has been converted into dibenzoyliodomethane [169]. Terminal acetylenes have been iodinated in the presence of Nal. However, this process was proposed to proceed via oxidation of the acetylene [170]. 

The electrochemical oxidations of aromatic compounds in the presence of a fluoride ion sources have been widely studied by a number of workers to produce a range of partially fluorinated compounds [9-12]. 

Consequently, the excited 3 Ru(bipy)32+ state can be produced via three different routes (i) Ru(bipy)3+ oxidation by TPrA"+ cation radical, (ii) Ru(bipy)33+ reduction by TPrA" free radical, and (iii) the Ru(bipy)33 + and Ru(bipy)3 + annihilation reaction. The ECL intensity for the first and second waves was found to be proportional to the concentration of both Ru(bipy)32+ and TPrA species in a very large dynamic range with reported detection limits as low as 0.5 pM155 for Ru(bipy)32+ and 10 nM156 for TPrA. In addition to Ru(bipy)32+, many other metal chelates and aromatic compounds or their derivatives can produce ECL in the presence of TPrA as a coreactant upon electrochemical oxidation (cf. Chapter 4 in the Bard s ECL monograph.32). 

Scheme 1. Pathways for the indirect electrochemical oxidation of aromatic compounds by metal salts...

Table 2. Indirect electrochemical oxidation of alkyl aromatics to form benzaldehydes using metal salts as redox catalysts ... 

In addition to the synthesis of saccharin, also a number of other side-chain oxidations have been studied leading to aromatic carboxylic acids by indirect electrochemical oxidation using chromic acid as oxidizing agent. They include the oxidation of p-nitrotoluene 2,4-dinitrotoluene toluene, p-xylene, and p-tolualdehyde... 

A principally different approach for the indirect electrochemical oxidation of aromatic compounds goes via the formation of hydroxyl radicals from cathodically generated hydrogen peroxide and from reductively formed iron(II) ions. The thus in situ formed Fenton reagent can lead to side-chain as well as nuclear oxidations of aromatic compounds. Side-chain oxidations to form benzaldehydes according to Eqs. (18)—(24) can also be initiated by the redox pairs and Cu instead of... 

Indirect electrochemical oxidations using the nitrate ion as redox catalyst proceed via the electro-generated NOj radical. They are useful for the oxidation of secondary alcohols and of alkyl aromatic compounds in the side-chain... 

Technically interesting are the indirect electrochemical oxidations of benzylic alcohols (Table 11, No. 15-18) benzaldehyde dimethylacetals (Table 11, No. 19) and alkyl aromatic compounds (Table 11, No. 20, 21) It could be proven that benzylic alcohols are oxidizable using tris(2,4-dibromophenyl)amine as mediator not only in acetonitrile in a divided cell but also in methanol in an undivided cell... 

Irreversible electrochemical oxidation of dimeric 4//-pyrans 163 to monomeric pyrylium ions at about +0.6 V was observed.225,227,228 The anodic oxidation of the dimers in the presence of aromatic hydrocarbons caused electroluminiscence of the latter.228,472 The polarographic oxidation of carbo-ranyl 4//-pyran 174a (R = Ph) at a platinum microelectrode was found to... 

The electrochemical oxidation of aromatic aldehydes (1) must be studied in strongly alkaline media. Acidity functions for strongly alkaline aqueous solutions of alkali metal and quaternary ammonium hydroxides, corresponding to dissociation of proton (H ), are well established (2, 3). Substituted anilines and diphenylamines (4,5) and indoles (6) were used as acid-base indicators for establishment of such scales, but whether an acidity scale based on one type of indicator can be rigorously applied to acid-base equilibria involving structurally different acidic groups for reactions in strongly alkaline media remains questionable. For substituted anilines, behavior both parallel (7) and nonparallel (8) to the H scale based on indole derivatives has been reported. The limited solubility of anilines in aqueous solutions of alkali metal hydroxides, the reactions of the aniline derivative with more than one hydroxide ion, irreversible substitution reactions (9), and the possibility of hydroxide ion addition rather than... 

The electrochemically oxidized forms of certain tetrahydropteridines,18 o-hydroquinones,244 and aromatic diamines245 are very active NADH oxidants. 

Anodic oxidation of l,3-diaryl-5-methyl-A2-pyrazoline-5-carboxylic acids in CH3CN-Et4NBF4 proceeded with decarboxylation to the aromatized pyrazoles in high yield.414 Similarly, electrochemical oxidation of N-acetyl-2,3-substituted A4-pyrroline-2-carboxylic acids in water-tetrahydrofuran (3 1) containing KOH forms the corresponding pyrroles (80-98%).415... 

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