The Redox Catalyst

In indirect electrolyses the redox catalyst occupies a key position, as it takes part in the heterogeneous as well as in the homogeneous redox reaction. To be suitable for both reactions, the mediators have to fulfill the following conditions  

Both redox states must be chemically stable. Even slight side reactions to unregenerable compounds will lead to a fast decay in catalytic activity. 

Electron exchange with the electrode as well as the redox reaction with the substrate have to be rapid and reversible. Inhibition of the electrode reaction or slow homogeneous redox reactions will prolong the time for turnover drastically and thus will afford larger electrode surfaces and thereby larger investments. Besides that, side reactions will often be favored. 

Redox reactions with other compounds like the solvent or the product must not take place or must be suppressed. 

Both redox states must have high enough solubility in the electrolyte (exception two-phase systems). 

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To be able to regenerate NADP(H) by an indirect electrochemical procedure without the application of a second regeneration enzyme system, the redox catalyst must fulfill four conditions ... 

Back electron transfer is at the diffusion limit because the homogeneous electron transfer reaction is uphill, owing to the fact that the standard potential of the redox catalyst is necessarily chosen as positive of the reduction potential of the substrate. 

The reactions of aldehydes at 313 K [69] or 323 K [70] in CoAlPO-5 in the presence of oxygen results in formation of an oxidant capable of converting olefins to epoxides and ketones to lactones (Fig. 23). This reaction is a zeolite-catalyzed variant of metal [71-73] and non-metal-catalyzed oxidations [73,74], which utilize a sacrificial aldehyde. Jarboe and Beak [75] have suggested that these reactions proceed via the intermediacy of an acyl radical that is converted either to an acyl peroxy radical or peroxy acid which acts as the oxygen-transfer agent. Although the detailed intrazeolite mechanism has not been elucidated a similar type IIaRH reaction is likely to be operative in the interior of the redox catalysts. The catalytically active sites have been demonstrated to be framework-substituted Co° or Mn ions [70]. In addition, a sufficient pore size to allow access to these centers by the aldehyde is required for oxidation [70]. 

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. 

Fig. 2. Flow diagram of an indirect electrochemical process with external regeneration of the redox catalyst (ex-cell process)...

On the other hand, the activation of the electrode can take place by its chemical modification. In this case the redox catalyst is fixed to the electrode surface either by adsorption, polymer coating, or covalent binding. This aspect has been treated in several reviews covered separately within this series... 

The selectivity and reactivity in indirect electrochemical syntheses can be enhanced by coordination of the substrate or an intermediate to the redox catalyst, for example through metal centers. In direct electrolyses, however, the selectivity and reactivity is mainly controlled by the difference between the electrode potential and the redox potentials of the different functions within the substrate. 

The redox catalysts can principally be divided into two groups according to the reaction mechanism they undergo ... 

The number of electrons which are exchanged can be controlled by the choice of the redox catalyst. 

Recently, interesting results have been obtained using the electrochemically generated ruthenium(IV) complex [(trpy)(bpy)RuO] in the oxidation ofp-xylene and p-toluic acid to terephthalic acid and of toluene to benzoic acid . The current yield is practically quantitative. After 100 turnovers about 75% of the redox catalyst can be recovered giving a turnover number of about 400 (Eq. (16)). 

The redox catalyst [(trpy)(bpy)RuO]2 + jj oxidize allylic methylene groups... 

Positively polarized selenation reagents for oxyselenation reactions can be substituted by an indirect electrochemical procedure in which the bromide ion acts as the redox catalyst. The active bromine species generates the phenylselenyl cation from the added diphenyl diselenide (Eq. (48) Table 4, No. 21-26)... 

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] ... 

Figure 22.1 Schematic representation of an indirect electrochemical process (given for an oxidation Med0Jt, Medred oxidized and reduced forms of the redox catalyst = mediator Sox, Sred oxidized and reduced forms of the substrate).

In type 2, the homogeneous redox reaction of the electrogenerated and regenerated redox catalyst consists of a chemical reaction. For oxidations, these reactions may be hydride ion or hydrogen atom abstraction, oxygen transfer, or an intermediate complex or bond formation. For reductions, hydride or car-banion transfer from a metal complex is often observed. In all these cases, very large potential differences between the standard potential of the substrate and the redox catalyst may be overcome. The selectivity can be very high and may... 

Qualitatively, the role of the redox catalyst can be seen as a way of accumulating reducing... 

Finally, as an alternative to colloidal metal particles it is possible to use hydrogenase,133,153 186 187-200,202,238"248 normally from Desulphovibrio vulgaris, or a synthetic analogue derived from Fe4S4 clusters248 in the presence of bovine serum albumin, as the redox catalyst for production of hydrogen from MVf and protons. 

The most studied telogen is certainly CCl3Br initiated either by UV, peroxide or by redox systems. However, we have demonstrated that in this last case, the redox catalyst, especially ferric chloride, induces a disproportionation which leads to a mixture of new telogens as follows [230] ... 

H202 formation by tylakoids or isolated PSI was followed in a transparent lucite cuvette as previously described (De la Rosa et al., 1986 Navarro et al., 1987a). Riboflavin at 10 pM final concentration was used as the redox catalyst. Thylakoid and PSI suspensions were prepared from spinach bought in the market following the procedures of Amon and Chain (1977) and Peters et al. (1983), respectively. 

Fig. 16 Structures of the redox catalyst tris(l,10-phenanthroline-5,6-dione) ruthenium(II) perchlorate 1 and the mixed ligand system consisting of l,10-phenanthroline-5,6-dione and one N,N, A-tris(aminoethyl)amine ligand 2...

Alkanesulfonyl chlorides are known to be a good source of alkanesulfonyl radicals or alkyl radicals with the aid of redox catalysts [3]. A series of studies using RuCl2(PPh3)3 as the redox catalyst have been carried out by Kamigata and coworkers (Scheme 13.6) [33-39]. Arenesulfonyl chlorides add to styrene derivatives to form the corresponding adducts, which undergo dehydrochlorination of EtsN to form the unsaturated sulfones [33]. When styrylsulfonyl chlorides are used as the precursor. 

One example illustrating the indirect LSV approach pertains to the reduction of 1-chloronaphthalene with 4-methoxybenzophenone as mediator in DMSO. The experimental parameter measured in the LSV experiments is ip/2Cipd, where C is the excess factor, that is, the concentration ratio of the substrate 1-chloronaphthalene and the redox catalyst 4-methoxybenzophenone, and ip and ipd are the peak currents measured for the mediator with and without the substrate being present. Note that the currents obtained are dependent on the nature of the products formed in Eqs, 118 and 119, that is, the amount of A regenerated. In many situations a reduction process is expected to dominate, but a contribution from other processes has to be looked for and specifically taken into account in the simulation procedure. In the present example, the forward electron-transfer reaction (Eq. 114) is rate-controlling... 

Intermediate enols can also be oxidized by indirect electrosynthesis (vide supra) halide anions are used as oxidation mediators. A multistep reaction converts ketones into a-hydroxylated acetals when oxidized electrochemically in the presence of iodine as the redox catalyst (Scheme 48) [196],... 

For preparative applications, especially in the case of continuous processes, means for the recovery of the valuable enzymes, the cofactors, and the redox catalysts in the reactor must be developed. One attractive possibility is the immobilization of the enzyme and sometimes also the cofactor and the redox catalyst at the electrode surface. However, the formation of enzyme-modified electrodes has also some practical drawbacks ... 

The immobilization of the enzyme, the redox catalyst, and sometimes also the cofactor can also take place at a solid support different from the electrode so that the components can be recovered within a solid-bed reactor (a column filled with the enzyme-containing particles) or by a filter plate or membrane. The immobilization of enzymes at solid supports or by the foraiation of cross-linked enzyme crystals can sometimes also enhance the enzyme stability. This concept has the advantage of the ease of separation but the disadvantage of diffusional limitations due to the heterogeneity of the reactions between the enzyme and the substrate and the cofactor or the redox catalyst. Additionally, the number of available redox centers is usually limited. 

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