Indirect electrochemical methods

Electroorganic Synthesis by Indirect Electrochemical Methods New Applications of Electrochemical Techniques ... 

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... 

Indirect electrochemical methods have been intensively studied, especially from the viewpoint of development of innovative synthetic methods in industrial organic chemistry. The indirect procedure is required when the direct method is unsuitable because (1) the desired reaction does not proceed sufficiently because of an extremely slow reaction or a very low current efficiency (2) the electrolysis lacks product-selectivity and thus offers only a low yield (3) tar and products cover the surface of the electrode, interrupting the electrolysis. Indirect electrochemical techniques involve the recycling of mediators (or electron carriers) in a redox system, as depicted in Fig. 1 [1-24]. 

Dimethoxy-2-(3-nitro-1,2,4-triazol-1 -yl)benzene and 1,1,4-trimethoxy-4-(3-nitro-l,2,4-triazol-l-yl)cyclohexane-2,5-diene have been prepared by indirect electrochemical method [598],... 

The anodic oxidation of 2-nitrophenylsulfenamides, which are easily obtained by the reaction of amines or amino acid esters with 2-nitrophenylsulfenyl chloride, to form the corresponding 2-nitrophenylsulfenimines, can be realized either by direct or indirect electrochemical methods. In most cases the direct electrochemical method is preferred because of the convenient reaction procedure and the simple workup [Eq. (34)] [82,169]. 

As mentioned earlier, passivation of the anode due to the formation of nonconductive polymers on the anode takes place commonly during anodic oxidation of organic substrates in the presence of fluoride ions. For example, as shown in Eq. (38), anodic oxidative difluorodesulfurization of dithioacetals does occur however, the current efficiencies are low due to this passivation phenomenon [89]. In order to prevent such passivation, Fuchigami and coworkers have developed an indirect electrochemical method using various mediators [91-95]. Thus, Br /Br" and triarylamine redox mediators have been shown to be effective for selective mono- and difluorodesulfurization of dithoacetals, respectively [91]. Furthermore, triarylamine has recently been shown to be a highly effective mediator for monofluorodesulfurization of )0-lactams [Eq. (39)] [95]. In the absence of triarylamine, severe passivation of the anode takes place during anodic fluorination. 

An indirect electrochemical method developed for nitrite determination may be of general applicability for PAA determination, as shown in equation 13. A nitrite sample is placed into a cell containing a known amount of 3-sulfanilic acid in dilute HC1 at pH 3. After 5 min the diazonium ion formation is complete an excess of catechol (109) is added and the concentration of the remaining 3-sulfanilic acid is determined at +0.12 V with a GCE vs. standard calomel electrode, by measuring the adduct (110) formed between the aromatic amine and the quinone derived from catechol in the diffusion layer of the electrode. The 3-isomer of sulfanilic acid was chosen among the three isomers, aniline and 4-nitroaniline for its highest sensitivity and its lowest LOD, 0.7 pM, with linearity from 20 to 80 pM. A spectrophotometric assay may be carried out for nitrite by measuring at 516 nm the azo dye derived from catechol and the diazonium ion after 3 h ... 

Many enzymes use redox centers to store and transfer electrons during catalysis. These redox centers can be composed of metals such as iron or cobalt, or organic cofactors such as quinones, amino acid radicals, or flavins. In order to fully appreciate the catalytic mechanisms of these enzymes, it is often necessary to determine the free energy required to reduce or oxidize their protein redox centers. This is called the redox potential. The measurement of enzyme redox potentials can be performed by either direct or indirect electrochemical methods. The type of electrochemistry suitable for a particular protein system is simply dictated by the accessibility of its redox center to the electrode surface. Because most reactions catalyzed by enzymes occur within hydrophobic pockets of the protein, the redox sites are often far from the surface of the protein. Unless an electron transfer path exists from the protein surface to the redox center, it is not feasible to use direct electrochemistry to measure the redox potential. Since only a few enzymes (most notably certain heme-containing enzymes) have such electron transferring paths and... 

One advantage of being able to perform the voltammetric analysis of en2ymes is that it is possible to obtain information about the kinetics of electron transfer by varying the scan rate. It is crucial, however, that proper control experiments be performed to demonstrate that interactions between the modified electrode and en2yme are not perturbing the electrochemical properties of the en2yme. In the likely event that voltammetric analysis is not feasible for a particular protein system, indirect electrochemical methods are often successful. 

Many 5rn1 reactions require external stimulation involving the injection of a catalytic amount of electrons, but thermal 5rn1 reactions are known. In these there is no other source of initiating electrons than the nucleophile, which is usually a poor electron donor. A solution to this conundrum has now been proposed. Initiation follows a mechanism in which electron transfer and bond cleavage are concerted. This conclusion is based on a fuU analysis of a model system involving 4-nitrocumyl chloride as the substrate and the 2-nitropropanoate ion as the nucleophile. All pertinent thermodynamic and kinetic parameters were determined by direct or indirect electrochemical methods. 

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