Oxidations, indirect electrochemical

Other mediators which have been used in combination with diaphorase for the regeneration of NAD+ are riboflavin and Vitamin K3, which is 2,3-dimethyl-1,4-naphthoquinone. However, especially riboflavin is not stable enough for synthetic applications [40]. Better stability is exhibited by phenanthrolindiones as mediators. In combination with diaphorase, Ohshiro [41] showed the indirect electrochemical oxidation of cyclohexanol to cyclohexanone using the NAD+ dependent HLADH with a turnover frequency of two per hour. For an effective enzymatic synthesis, this turnover frequency, however, would be too small. In our own studies, we were able to accelerate the NAD(P)+ regeneration considerably by lowering the electron density within the... 

The time-dependent formation of cyclohexanone according to Fig. 9 is shown in Fig. 10. We obtained a turnover frequency of 28 turnovers per h. This is larger by a factor of 14 than that reported for the indirect electrochemical oxidation using only the free ligand as mediator [41]. 

Fig. 14. Concentration profiles during the continuous indirect electrochemical oxidation of 4-ethylphenol catalyzed by the enzyme EPMH in the electrochemical enzyme membrane reactor...

The direct and indirect electrochemical oxidation of toluenes and alkoxytoluenes... 

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. 

Indirect electrochemical oxidation of substituted toluenes in methanol affords good yields of side chain substitution products, Tris(2,4-dibromophenyl)amine is... 

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]. 

Scheme 11 Indirect electrochemical oxidation cleavage of alkenes using a double-mediator system consisting of 104 and ruthenium tungstosilicate (taken from Ref 8).

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... 

Metal salts and complexes have also often been used as redox catalysts for the indirect electrochemical oxidation of alcohols. Particularly, the transformation of benzylic alcohols to benzaldehydes has been studies. For this purpose oxoruthe-nium(IV) and oxoruthenium(V) complexes have been applied as redox catalysts. In a similar way, certain benzyl ethers can be cleaved to yield benzaldehydes and the corresponding alcohols using a di-oxo-bridged binuclear manganese complex Electrogenerated 02(804)3 was used to generated 1-naphthaldehyde from 1-naphthylmethanol... 

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... 

Indirect Electrochemical Oxidations Using Triarylamines as Redox Catalysts... 

As mentioned above, it is difficult to find organic compounds which are suitable as redox catalysts for oxidations. This is the case because organic cation radicals, which are mostly the active forms in indirect electrochemical oxidations, are usually easily attacked by nucleophiles, thus eliminating them from the regenerative cycle. Therefore, the cation radicals must be stabilized towards the reaction with nucleophiles. Nelson et al. demonstrated that the cation radicals of triaryl amines and related compounds are very stable if the para positions of the aryl... 

Table 11. Indirect electrochemical oxidations using triarylamines as mediators... 

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... 

Scheme 10. Reactivity pattern for the methoxylation in a-position to oxygen by indirect electrochemical oxidation in MeOH/NaOMe with tris(2,4-dibromophenyl)amine as mediator...

Indirect Electrochemical Oxidations Using Other Types of Organic Mediators... 

A phosgene-free synthesis of alkylisocyanates makes use of the indirect electrochemical oxidation in the alpha-position to nitrogen of formamides. Bromide in methanol solution acts as the redox catalyst, which, presumably, is oxidized to the methyl hypobromite [9] ... 

In several cases, the oxidation of primary alcohols to carboxylic acids is desired as a technical process. Direct and indirect electrochemical oxidations have been developed for this purpose. 

Anodic substitution can also occur easily in the a-position to heteroatoms like nitrogen or oxygen. Thus, the indirect electrochemical oxidation of ethylene glycol dimethyl ether in methanol using tris(2,4-dibromophenyl)amine as redox catalyst leads to the formation of 2-methoxyacetaldehyde dimethylacetal [25] ... 

Cation radicals may be generated by direct or indirect electrochemical oxidation of the molecule of interest, and many such oxidations are synthetically useful. However, several other methods are also available, which fall into two broad categories thermally-induced electron transfer (TIET) and photo-induced electron transfer (PIET). 

The indirect electrochemical oxidation of aldoses to the corresponding aldonic acids 273-27 ), which was carried out industrially as early as about 1930, is still used today for production on the tonne scale by Sandoz 27 S) and in India 276). Specific examples are the anodic oxidation of lactose to calcium lactobionate 275,277 278) ... 

Van Hege, K., Verhaege, M. and Verstraete, W. (2002), Indirect electrochemical oxidation of reverse osmosis membrane concentrates at boron-doped diamond electrodes. Electrochem. Commun., 4(4) 296-300. 

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... 

Indirect electrochemical oxidation encompasses all those processes in which the pollutant is indirectly oxidized, either by the generation of oxidants (e.g., CI2, C10 , O3) or by the presence of a redox couple (Ag /Ag, Fe /Fe, Co /Co " ) that is used as an electron carrier for oxidation. 

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]. 

Steam reforming of small organic molecules, to facilitate indirect electrochemical oxidation via H2, involves some thermodynamic inefficiency as well as formation, usually, of some CO in the H2 produced. Special catalysts for the fuel-cell oxidation of the H2 thus formed are then necessary, namely, catalysts that can effect dissociative adsorption of H from H2 in the presence of small but significant concentrations of CO in the H2. In recent years, such catalysts have been engineered (95) that allow oxidation of H2 at rates of several amperes per square centimeter in the presence of traces of CO. Similarly, a variety of modified noble metal catalysts have been developed that allow CH3OH oxidation to proceed with improved performance with respect to avoidance of self-deactivation behavior. Doping of Pt by Sn02 or Ru has been effective in this direction (96. 97). 

The success of indirect electrochemical oxidation and disinfection lies in the production of safe drinking water especially in rural areas where the necessity for skilled maintenance personnel should be avoided. The simplicity, stability, and low power consumption of devices such as the sodium hypochlorite generator described by Bashtan et al. [71] and the TiN reactor described by Matsunaga et al. [34] make these devices most cost effective for small-scale applications in remote locations. 

Polyelectrolytes and soluble polymers containing triarylamine monomers have been applied successfully for the indirect electrochemical oxidation of benzylic alcohols to the benzaldehydes. With the triarylamine polyelectrolyte systems, no additional supporting electrolyte was necessary [91]. Polymer-coated electrodes containing triarylamine redox centers have also been generated either by coating of the electrode with poly(4-vinyltri-arylamine) films [92], or by electrochemical polymerization of 4-vinyl- or 4-(l-hydroxy-ethyl) triarylamines [93], or pyrrol- or aniline-linked triarylamines [94], Triarylamine radical cations are also suitable to induce pericyclic reactions via olefin radical cations in the form of an electron-transfer chain reaction. These include radical cation cycloadditions [95], dioxetane [96] and endoperoxide formation [97], and cycloreversion reactions [98]. 

Cleavage of oximes to the parent carbonyl compound may be performed under mild conditions by anodic oxidation in CH3CN/H2O [123]. Thus, anodic oxidation of 4-sub-stituted acetophenone oximes in wet acetonitrile furnishes the corresponding acetophenones in high yields, while the oxidation of benzaldoximes is unselective [124] [Eq. (18)]. By indirect electrochemical oxidation using the Mn(II)/Mn(III) mediator system in the... 

In the case of basic amino acids like lysine or ornithine, the regioselectivity of the direct and indirect electrochemical oxidation can be demonstrated. Direct anodic oxidation of A/, A -dimethoxycarbonylated L-lysine and L-ornithine in methanol regioselec-tively resulted in the methoxylation at the xt>-amino group, while indirect... 

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