Reductions, indirect electrochemical

In contrast to the direct reduction as described above, the indirect electrochemical reduction of perfluoroalkyl halides is a versatile and novel method for generating perfluoroalkyl radicals selectively. Saveant et al. have demonstrated many successful examples. Using terephthalonitrile as a mediator, the indirect reduction of CF3Br in the presence of styrene leads to the dimer of the radical adduct obtained by the attack of CF on styrene. On the other hand, in the presence of butyl vinyl ether, the mediator reacts with the radical adduct obtained by the attack of CF3. on the olefin (Scheme 3.4) [14]. 

For enzymatic reductions with NAD(P)H-dependent enzymes, the electrochemical regeneration of NAD(P)H always has to be performed by indirect electrochemical methods. Direct electrochemical reduction, which requires high overpotentials, in all cases leads to varying amounts of enzymatically inactive NAD-dimers generated due to the one-electron transfer reaction. One rather complex attempt to circumvent this problem is the combination of the NAD+ reduction by electrogenerated and regenerated potassium amalgam with the electrochemical reoxidation of the enzymatically inactive species, mainly NAD dimers, back to NAD+ [51]. If one-electron... 

Aliphatic hydroxylamines are not easily reduced by direct electrochemical reduction. However, they are reduced indirectly by electrochemically generated low-valent Ti(III) or Fe(II) to afford the corresponding amino compounds (equation 1). This electrochemical method has certain advantages it takes place at a less negative potential where hydrogen ions are not reduced and the low concentration of the metal salts facilitates the work-up compared to the reduction by chemical reagents where at least equivalent amounts of reagents must be used. 

In a similar way it is possible to perform the amination of aromatics by indirect electrochemical reduction of hydroxylamine using a combination of the Cu /Cu and redox pairs Thus, aniline and toluidine are formed from benzene... 

Scheme 2. Proposed reaction pathways in the indirect electrochemical reduction of geminal dibromocyclopropanes by chromium (II)...

Indirect Electrochemical Reductions with Transition Metal Complexes... 

The indirect electrochemical reduction of alkyl halides is also possible by use of nickel(I) complexes which may be obtained by cathodic reduction of square planar Ni(n)-complexes of macrocyclic tetradentate ligands (Table 7, No. 10, 11) 2 4-248) Comparable to the Co(I)- and Ni(O)-complexes, the Ni(I)-species reacts with the alkyl halide unter oxidative addition to form an organo nickel(III) compound. The stability of the new nickel-carbon bond dominates the overall behavior of the system. If the stability is low, the alkyl group is lost in form of the radical and the original Ni(II)-complex is regenerated. A large number of regenerative cycles is the result. 

Electro-generated and regenerated bis(bipyridine)rhodium(I) complexes were able to catalyze the selective non-enzymatically coupled electrochemical generation of NADH from NAD . The direct cathodic reduction even at very negative working potentials leads to the formation of large amounts of enzymatically inactive NAD dimers, while the indirect electrochemical reduction via the rhodium complex acting as... 

Indirect Electrochemical Reductions Using Radical Anions or Dianions of Organic Compounds as Mediators... 

In several examples the reductive halide-hydrogen exchange has been studied on a preparative scale. For example, the indirect electroreduction of 2-chloropyridine in DMF using anthracene as mediator gives pyridine in 83-86 % yield 2 . For the dehalogenation of 1-chlorohexane (80% yield), naphthalene is applied as redox catalyst. Similarly, 6-chloro-hexene yields 1-hexene (60%) and methylcyclopentane (25%), which is the product of the radical cyclization . The indirect electrochemical reduction of p- and y-bromocarboxylic esters forms coupling and elimination products besides the dehalogenated products... 

Indirect Electrochemical Reductions Using Viologen Cation Radicals as Redox Catalysts... 

Some nickel(II) tetraaza macrocycles have been proved to act as efficient catalysts for the electrochemical reduction of C02 in H20/MeCN medium. This indirect electroreduction occurs at potentials in the range -1.3 to -1.6 V vs. SCE and mainly produces either CO or a CO/H2 mixture, depending upon the type of complex.2854 The five-coordinate complexes [NiL] (394) formed by some deprotonated dioxopentamine macrocycles have been found to display very low oxidation potentials Nin/Nira in aqueous solution (about 0.24-0.25 V vs. SCE at 25 °C and 0.5 M Na2S04). Air oxidation of the same complexes in aqueous solution yields 1 1 NiL-02 adducts (5 = 1) which are better formulated as superoxo complexes, NimL-02 (Scheme 56). The activation of Ni-bound oxygen is such that it attacks benzene to give phenol.2855... 

Fisher and Eisenberg (107) have reported on the electrocatalytic reduction of carbon dioxide using macrocycle complexes of nickel and cobalt (e.g., complex 27). An indirect electrochemical reduction of C02 was ac-... 

Zn metal can be regenerated by an electroreduction of Zn2+ at the cathode. At 25 °C, current densities greater than 300 A m 2 were found, and the indirect electrochemical reduction of 2 M CFC 113 in 90 vol.% methanol-water mixture was dominated by zinc mediation. 

Anodic stripping voltammetry — Refers to a family of procedures involving a - preconcentration by electrochemical reduction of the - analyte (or a salt or derivative of the analyte) onto (or into) the working electrode prior to its direct or indirect determination by means of... 

Analogous results have been obtained in the electrolysis of the pyridine nucleotides134-141 the dimer has, however, been suggested to be a 4,4 -dimer.139 The coenzyme activity of the reduction product varies with the experimental conditions. Some reductions yielded totally inactive products, whereas others produced partly or fully active material. The highest activity was obtained by an indirect electrolytic reduction of triphosphopyridine nucleotide (TPN).141 Electrochemical reduction of methyl viologen in the presence of ferridoxin-TPN-reductase caused a reduction of TPN to a biochemically active product. 

One useful approach involves indirect electrochemical reduction using an electron transfer catalyst which is selected for inertness to any anion radicals or dianions present. Thus allyiic sulfones (Scheme... 

The catalytic reduction of nitrates using H2 or NH4 as reducing agent was investigated [73-75] using Pd-Cu catalysts supported on y-alumina. The direct electrochemical reduction of nitrate or indirect reduction by electrochemically produced hydrogen has not received much attention and could be an interesting area of research. 

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