Aromatic compounds electrochemical oxidation

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... 

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). 

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 3. Indirect electrochemical oxidation of aromatic compounds to quinones using metal salts as redox catalysts... 

Nuclear oxidations of aromatic compounds to form quinones (Table 3) have been performed technically in the case of anthraquinone using electrochemically regenerated Ce(IV) or Cr(VI) as redox catalysts This technique has, however, become less... 

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... 

Highly selective formation of phenyl acetate was observed in the oxidation of benzene with palladium promoted by heteropoly acids.694 Lead tatraacetate, in contrast, usually produces acetoxylated aromatics in low yields due to side reac-tions. Electrochemical acetoxylation of benzene and its derivatives and alkoxylation of polycyclic aromatics789 790 are also possible. Thermal or photochemical decomposition of diacyl peroxides, when carried out in the presence of polycyclic aromatic compounds, results in ring acyloxylation.688 The less reactive... 

Oxidation to Quinones. Direct oxidation of arenes to quinones can be accom-plished by a number of reagents. Very little is known, however, about the mechanism of these oxidations. Benzene exhibits very low reactivity, and its alkyl-substituted derivatives undergo benzylic oxidation. Electrochemical methods appear to be promising in the production of p-benzoquinone.797 In contrast, polynuclear aromatic compounds are readily converted to the corresponding quinones. 

Removal of two electrons from the formal cyclic 87r-electron structures serves to produce potential Hiickel 4n + 2 aromatic systems. The loss of one electron to form a radical cation was referred to in Section 2.26.2.1, and removal of a second electron by electrochemical oxidation, leading to dicationic structures, has also been achieved for a wide range of unsaturated compounds with heteroatoms in the 1,4 positions (70ZC147, 73JA2375). The oxidations are discussed further in Section 2.26.3.1.5, where tabulated data are presented. An interesting feature is the stability of certain salts of the dications, some of which have been isolated. 

In the 1980s, Rudenko et al.988,989 made pioneering studies on the electrochemical oxidation of various aromatic compounds in HSO3F and HSO3F- Sbl 5. Recently,... 

The possibility to functionalize aromatic compounds by electrochemical methods is of great interest to chemical industry. Therefore, considerable efforts were made to develop the electrochemical oxidation of benzene to p-benzoquinone to the industrial scale thus forming a basis for a new hydroquinone process. The electrochemical oxidation of benzene in aqueous emulsions containing sulfuric acid using divided cells and Pb02 anodes formsp-benzoquinone. The product can then be reduced cathodically to yield hydroquinone in a paired synthesis. 

However, this is not always the case. As an example, electrochemical cyanation 5 5-6 of an aromatic compound can be carried out by anodic oxidation in methanol-sodium cyanide (Eq. (16) ). The current yield (the yield of cyanation product based on the amount of... 

Wellmann J, Steckhan E. Indirect electrochemical processes 2. Electro-catalytic direct oxidation of aromatic compounds by hydrogen peroxide. Chem Ber 1977 110 3561-3571. 

According to this model, the first stage in the treatment of nitrophenols aqueous wastes was the release of the nitro group from the aromatic ring. As a consequence, phenols or quinones were formed. These organic compounds were oxidized first to carboxylic acids (maleic and oxalic) and later to carbon dioxide. Also the cathodic reaction steps were considered in the global process when the electrochemical cell was undivided at the cathode, the reduction of the nitro to the amine group and the transformation of nitrate into ammonia were observed. In alkaline media, aminophe-nols were polymerised and transformed into a dark brown solid. 

Fenton reagent generated in situ — Indirect electrochemical oxidation of aromatic compounds (e.g., benzene to phenol) proceeds with the Fenton reagent generated in situ electrochemically at the cathode by the reduction of ferric to ferrous salt and by the reduction of oxygen to hydrogen peroxide... 

The electrochemical oxidation of aromatic compounds in the presence of ammonium nitrate or N2O4 results in the nuclear nitration shown in equation (34). ... 

---