Indirect cathodic reduction

Indirect cathodic reduction of sulphones. Redox catalysis.1014... 

Titanium ions can also he used as redox catalysts for the indirect cathodic reduction of nitro compounds (417). The electroreduction is carried out in an H20-H2S04/Ti(S04)2-(Pb/Cu) system at 45 80°C under 5 20Am . Nitrobenzene, dinitrobenzene, nitrotoluene, 2,4-dinitrotoluene, 2-nitro-m-xylene, nitro-phenol, 2,4-dinitrophenol, nitrophenetole, o-nitroanisole, 4-nitrochlorotoluene, ni-trobenzenesulfonic acid, and 4,4 -dinitro-stilbene-2,2 -disulfonic acid can all be reduced by this procedure to the corresponding amino compounds (418) in good yields (Scheme 146) [513-516]. Tin... 

Bechtold T, Burstcher E, Turcanu E. Anthraquinones as mediators for the indirect cathodic reduction of dispersed organic dyestuff. J Electroanal Chem 1999 465 80-87. 

In the first group the titrant is generated either directly from a participating or active electrode, or indirectly from an inert or passive electrode, in which case it is necessary to add previously an auxiliary substance that generates the titrant by either cathodic reduction or anodic oxidation the end-point detection is usually potentiometric or amperometric. The following selected examples are illustrative of the first group in non-aqueous media ... 

Some of these processes have been developed for technical conversions and have been summarized in Ref. [228, 229]. The anodic technical production of t-butylbenzaldehyde has been coupled with the cathodic reduction of phthahc anhydride to phthalide in a paired electrosynthesis in a capillary gap cell [230]. Indirect oxidations with Mn +/Mn + or as mediators... 

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... 

Amperometric detection is the preferred method for the analysis of nitroaromatic explosives on microchip devices since it offers up to three orders of magnitude higher sensitivity than indirect LIF and it has a great potential for miniaturization and integration on microchip platform. Presence of nitrogroup allows its cathodic reduction to form al-kylhydroxyamines. The reduction mechanism of polynitroaromatic compounds is complex and depends on the number of nitro groups,... 

Under this heading a few borderline cases between direct and indirect electrochemical reactions can be included. Dihalocarbenes are probably intermediates in the cathodic reduction of certain polyhalogenated compounds172) as an example, cathodic reduction of carbon tetrachloride at a mercury cathode in acetonitrile-tetrabutylammonium bromide in the presence of tetramethylethylene gave a low yield of l,l-dichloro-2,2,3,3-tetramethylcyclopropane ... 

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. 

Lead and mercury are deposited as micron-sized clusters, predominantly at intercrystallite boundaries [105] so does lithium from the polyethylene oxide solid electrolyte. What is more, Li intercalates into the sp2-carbon [22, 138], Thus, observations on the Li intercalation and deintercalation enable one to detect non-diamond carbon on the diamond film surface. Copper is difficult to plate on diamond [139], There is indirect evidence that Cu electrodeposition, whose early stages proceed as underpotential deposition, also involves the intercrystallite boundaries [140], We note that diamond electrodes seem to be an appropriate tool for use in the well-known electroanalytical method of detection of traces of metal ions in solutions by their cathodic accumulation followed by anodic stripping. The same holds for anodic deposition, e.g. of, Pb as PbCh with subsequent cathodic reduction [141, 142], Figure 30 shows the voltammograms of anodic dissolution of Cd and Pb cathodically predeposited from their salt mixtures on diamond and glassy carbon electrodes. We see that the dissolution peaks are clearly resolved. The detection limit for Zn, Cd, and Pb is as low as a few ppb [143]. 

Aldoximes may be transformed into nitriles via nitrile oxides, obtained by indirect anodic oxidation of the aldoxime in methanol, followed by cathodic reduction of the nitrile oxide [92]. 

The indirect reduction of tosyl esters can be performed7 in non-aqueous solutions. Thus, for example, the anthracene anion radical formed by cathodic reduction in DMF/TBAB (tetrabutylammonium bromide) electrolyte may reduce tosylates in solution. Similarly, the pyrene anion radical was shown8 (Figure 1) to react also with ethyl tosylate. The redox catalysis general scheme (indirect reduction by a redox P/Q couple) where P is a reducible species and Q its stable reduced form can be written as below ... 

During indirect cathodic EET, the same type of mediators (artificial or naturally produced) as reported in indirect anodic EET studies can be applied or involved [81]. Consequently, the same advantages and possible negative effects apply to the cathode side. However, manganese oxides have also been applid to shuttle electrons for oxygen reduction, and have the advantage that, owing to their solid nature, wash-out is slowed [116]. 

The detailed mechanism is often not fully understood. A blocking of active sites is often postulated for anodic dissolution or cathodic reduction of redox systems. However, the conclusions were mostly drawn indirectly from electrokinetic data, and a detailed investigation with surface analytical methods is still missing. The chemical reaction steps and their inhibition at surface sites should be further investigated systematically by methods like XPS, AES, STM, SFM, etc. Most of the knowledge is still empiric. However, the results of electrochemical investigations are very valuable for a better corrosion protection of materials in industrial applications. Some few principles of inhibition are discussed in the following sections. 

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