Mediators, indirect electrolysis

The transfer of redox equivalents can be achieved by an electrocatalyst (mediator) or a modified electrode. Indirect electrolysis can lead to a better selectivity due to the specific interaction of the mediator with the substrate. However, low turnovers and the need to separate the mediator from the product are possible disadvantages, as mentioned above. The nickel hydroxide electrode [195,196] is fairly free from these disadvantages. The following mechanism for the oxidation at the nickel hydroxide electrode has been proposed in the literature [195]. 

Similar transformations can be carried out via indirect electrolysis using vitamin Bi2 and Bia analogs as mediators [74]. As shown in the accompanying equations, the methodology lends itself nicely to the formation of both spiro and linearly fused materials (Tables 13 and 14) [75]. 

Anodic oxidation of formazane 18 [17], 1-arylmethylenesemicarbazide 19 [55], p-nitrobenzaldehyde phenylhydrazone 20 [56], and 2-benzoylpyridine phenylhydrazone 21 [57] afforded the corresponding heterocycles in a very good yield (Scheme 14). The homogeneous oxidation of compounds 18-20 was carried out by indirect electrolysis by the mediators generated in situ [58]. 

As schematically demonstrated in Fig. 1, the indirect electrolysis combines a heterogeneous step, that is the formation and regeneration of the redox-catalyst (Med = mediator) in its active form, with the homogeneous redox reaction of the substrate involving the active mediator. 

The direct electrochemical oxidation of aliphatic alcohols occurs at potentials which are much more positive than 2.0 V w. SCE. Therefore, the indirect electrolysis plays a very important role in this case. Using KI or NaBr as redox catalysts those oxidations can be performed already at 0.6 V vs. SCE. Primary alcohols are transformed to esters while secondary alcohols yield ketones In the case of KI, the iodo cation is supposed to be the active species. Using the polymer bound mediator poly-4-vinyl-pyridine hydrobromide, it is possible to oxidize secondary hydroxyl groups selectively in the presence of primary ones (Table 4, No. 40) The double mediator system RuOJCU, already mentioned above (Eq. (29)), can also be used effectively Another double mediator system... 

In a direct electrolysis, the electron is exchanged between the electrode and the substrate, and the rate of the reaction depends on the electrode potential and the rate constant of the heterogeneous electron-transfer reaction. In an indirect electrolysis, the electron is primarily exchanged with a substance (a mediator) that exchanges the electron with the substrate in a chemical reaction, and the rate does not depend on the ability of the substrate to exchange an electron with the electrode. 

Typical examples for type 1 are the anodic cleavages of two carbon-sulfur bonds in 1,3-dithianes [46] or dithiolanes [47]. This reaction is especially effective if performed under the conditions of indirect electrolysis using triarylamine cation radicals as regenerable oxidative mediators [47] ... 

Another important classification of electroorganic reactions is obtained by dividing them into those in which the substrate undergoes direct electron transfer with the electrode (direct electrolysis) and those in which an additional compound (redox catalyst, mediator) transfers the redox equivalents between the substrate and the electrode (indirect electrolysis). 

In indirect oxidation, organic pollutants do not exchange electrons directly with the anode surface but they exchange through the mediation of some electroactive species regenerated there, which act/acts as an intermediary for shuttling electrons between the electrode and the organics. Indirect electrolysis can be a reversible or an irreversible process. 

Indirect and direct electrolysis — Electrolysis is the oxidation or reduction reaction caused by the current flowing through the cell. Direct electrolysis is a -> heterogeneous process and the -> electron transfer proceeds directly from the electrode to the molecule of the substrate. In the case of indirect electrolysis the -> mediator is necessary, which is first heterogeneously reduced (oxidized) at the electrode and then the electron transfer to/from the molecule of the substrate occurs in homogeneous conditions in the bulk of the solution. The intermediate... 

Indirect electrolysis using mediators is an alternative powerfiil electrolytic method. This method is suitable for the following cases ... 

Kariv-Miller and coworkers have developed indirect electroreductive cyclizations with the dimethyl-pyrrolidinium ion (DMP") as a mediator. Preparative electrolysis of 6-hepten-2-one (9) at a graphite cathode afforded cu-dimethylcyclopentanol (10) in 90% yield (equation 5). The reduction is believed to occur via the ketyl radical anion, which cyclizes onto the alkenic bond. In the absence of DMP simple reduction to 6-hepten-2-ol takes place.Very recently it was shown that instead of DMP several aromatic hydrocarbons can be used as mediators to initiate the cyclization reaction. The carbonyl group can also be cyclized onto an alkynic bond and even an aromatic ring. - ... 

Anodic oxidation of azomethine, hydrazone, oxime, formazane, and semicarbazone structures has been used to initiate the intramolecular cyclization [119] under formation of heterocycles like triazoles [126,127], oxadiazoles [128,129], triazolinones [129], benzoxa-zoles [130,131], benzimidazoles [130,131], pyrazoles [132], indazoles [133], furoxanes [134], and tetrazolium salts [135] (see Chapter 18). Some of these reactions can be performed advantageously by indirect electrolysis using tris(4-bromophenyl)amin or 2,3-dihydro-2,2-dimethylphenothiazine-6(l/7)-one as mediators [119,136]. Two examples are given in Eqs. (19) and (20). 

A useful system for indirect electrolysis of sulfonamides was reported to be anthracene as mediator in the presence of ascorbic acid as proton donor [260]. 

Direct electrolysis means oxidation or reduction of the substrate at the electrode via an electron transfer or the stoichiometric attack of an oxidized or reduced species. Indirect electrolyses use mediators, namely, redox systems transferring electrons between the substrate and the electrode. The indirect electrolysis can be accomplished in two ways ... 

When substrates are removed from mediated bioelectrocatalytic systems, the reactions are considered to be indirect electrolysis of enzymes (or more generally proteins) ... 

As industrial important anodic addition reaction can be mentioned the synthesis of chiral 1,2-diols by indirect electrolysis. This reaction can be carried out, using a double mediator system consisting of ferricyanide and osmium tetroxide in the presence of chiral ligands. As an alternative to ferricyanide, electrogenerated iodine may be used. This reaction can be seen as an electrochemical variant of the asymmetric bishydroxylation introduced by Sharpless (Fig. 9-5). 

Organic Pollutants, Direct and Mediated Anodic Oxi- discharge of the water (c) indirect electrolysis, where dation, Fig. 1 Scheme of the electrochemical processes Med is the mediator and Oxid is the electrogenerated for the removal of organic compounds (R) (a) direct oxidant electrolysis (b) via hydroxyl radicals produced by the... 

The indirect electrolysis that uses metallic couples such as Ag(II), Ce (IV), Co(III), and Fe (III) as redox reagents is called Mediated Electrochemical Oxidation (MEO) [4]. In this process, metal ions in acidic solutions are oxidized... 

Electrochemical cofactor reduction can be achieved by direct reduction of the cofactor at the electrode surface, or indirectly by using a mediator molecule to shuttle electrons between the electrode and the cofactor. For details on the direct approach the reader is referred elsewhere [31, 32], since here no transition-metal complexes are involved. One point to be considered in the direct approach is the issue of selectivity. Whereas direct cofactor oxidation can be successfully achieved, special care must be taken to produce enzyme active reduced cofactors by direct electrolysis. 

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