Organic mediators

Antibiosis Inhibition or lysis of an organism mediated by metabolic products of the antagonist these products include lytic agents, enzymes, volatile compounds, and other toxic substances. 

Rajendren G. and Moss R. (1994). Vomeronasal organ-mediated induction of fos in the CNS pathways of repetitively mated female rats. Brain Res Bull 34, 53-59. 

Wysocki C.J., Katz Y. and Bernard R. (1983). Male vomeronasal organ mediates female induced testosterone surges in mice. Biol Reprod 28, 917-922. 

Fig, 8. Reduction of oxo acids with Proteus vulgaris, with methyl viologen (MV) as organic mediator. Display of the bench-scale rig... 

Mediators can be classified into two categories (1) mediators fixed to the electrode surface and (2) mediators that dissolve in the solution. A representative of the first category is the nickel hydroxide electrode (NHE) and the second category involves inorganic (metal and halide ions) and organic mediators. 

Organic Mediators Organic mediators are very useful in electroorganic chemistry since their structural modification may increase the selectivity of the oxidation. The first organic mediator exploited for oxidation in synthesis was thio-anisole. Since then, there appeared a variety of other organic mediators such as tris-arylamines, tetramethylpiperidinyl-1 -oxy (TEMPO), and Ai-hydroxyphthalimides (NHPIs). 

Thioanisolc. A system utilizing thio-anisole as an organic mediator was developed for the oxidation of secondary alcohols to ketones (Fig. 5 2-octanol to 2-octanone 99%, menthol to menthone 92%, cyclododecanol to cyclododecanone 75%) [43]. The use of 2,2,2-trifluoroethanol as a solvent in the mediatory system improved the yields [44]. 

Ammonification. This process proceeds in steps (1) The breakage of the peptide linkage between amino acids A and B, followed by (2) The deamination of the amino acids and formation of NHJ. The electron carrier (e.g., NAD+, NADP+) involved in such reactions depends on the type of protein undergoing degradation and the species of organism mediating the reaction. These reactions are catalyzed by enzymes. 

Anodic oxidation is used to promote the recycling of palladium(il) in the Wacker process for the conversion terminal alkenes to methyl ketones. Completion of the catalytic cycle requires the oxidation of palladium(O) back to the palla-dium(li) state and this step can be achieved using an organic mediator such as tri(4-bromophenyljamine. The mediator is oxidised at the anode to a radical-cation and... 

Organic redox catalysts in their active forms, mostly radical ions, quite often are not sufficiently stable in many media. The number of stable organic mediators is therefore somewhat limited. However, recently, especially in this field, considerable advances could be made showing that also organic compounds have good pro-sp ctives in their application as redox catalysts. 

Thus, as compared with direct electrolysis, not only a gain in energy can be obtained but also the selectivities can be enhanced. The selectivity of the reaction in this case is determined by the potential differences between the different functional groups of the substrate and the mediator in combination with the rate of the respective follow-up reaction. A typical example of this type is the cleavage of a carbon-sulfur bond using tris(4-bromophenyl)amine as organic mediator ) (Eq. (12)). 

The most important condition for the suitability of a compound to act as a mediator (redox catalyst) is its stability in all oxidation states which are involved in the reaction. Otherwise a fast loss of catalytic activity will be encountered. This prerequisite is difficult to fulfill by organic molecules because their active forms mostly are radical ions which are irreversably attacked in many media. Only recently the number of known stable organic mediators has considerably increased. 

Table 8. Reduction potentials of organic mediators for indirect electroreductions...

Indirect Electrochemical Oxidations Using Other Types of Organic Mediators... 

In several cases A-hydroxyphthalimide has been used as an organic mediator for the oxidation of alcohols to ketones, of benzyl ethers to benzoates , of alkyl aromatics to aryl ketones , and of 4-phenyl-l,3-dioxolanes to unprotected ketones... 

Other organic mediators act as hydride atom-abstracting agents. This is true, for example, with 2,2-dichloro-5,6-dicyano-p-benzoquinone (DDQ) and the oxoammonium ion which is anodically accessible from 2,2,6,6-tetramethylpiperidyl oxide (TEMPO). DDQ has been electrochemically regenerated either externally or internally The in situ electrochemical oxidation, of TEMPO to the active oxoammonium ion is performed in lutidine-containing acetonitrile. Thus, primary alcohols can be oxidized to the aldehydes, while secondary ones are stable Primary amines are transformed to nitriles. If water is present, the amines are cleaved via the Schiff bases to the corresponding carbonyl compounds... 

Some examples of indirect reduction by means of inorganic and organic mediators in different types of reactions will be discussed briefly indirect oxidations may also be performed. 

Similarly, when Bockris and Wass used various organic mediators in a DMF solution with 5% water and p-CdTe as the photocathode [120], the bare p-CdTe produced a faradaic efficiency for CO of 92%. Subsequently, by using their best catalysts, namely 15-crown-5 ether and 18-crown-6 ether, these authors were able also to produce methanol with faradaic efficiencies of 14% and 13%, respectively, with the remaining current going to CO production. Although the potential at which these electrolyses were carried out was not reported, the onset potential for C02 reduction was shifted some 400 mV anodically in the presence of the organic mediators. 

In the SECM measurements shown schematically in Fig. 4a, four stages of the overall process may influence the tip current organic mediator diffusion... 

Electrochemical (EC) techniques provide an alternative way to detect sulfur containing molecules. Earlier methods of EC detection involve the application of a gold/mercury electrode.15 Platinum and gold electrodes have also been used for anodic detection of thiols,16 but this requires high oxidation potentials, which complicates analytical applications. Thus, chemically modified electrodes with inorganic or organic mediators have been employed to facilitate electron-transfer between the electrode and the analyte, and therefore reduce the oxidation potential. Recently, pyrroloquinoline quinone (PQQ) modified electrodes have been developed for detection of endo- and exogenous thiols.17... 

It is remaikable that carbon-carbon double bonds are completely inert in diese oxidations. Some organic mediators, e.g. (25), (26) and (27), have been exploited for the oxidation of alcohols. 

Figure 9 Model representation of organic-mediated dehalogenation reactions in the presence (oval) and absence of...

From the assumptions previously listed, the rate of reaction depends only upon the functional characteristics of the organisms mediating the reaction and upon the activities of the reactants and products for the reaction. The microorganisms mediating reaction B are a diflFerent species from those mediating any other reaction. 

In the following, examples of indirect electrochemical reactions (mainly redox catalysis) with an organic mediator are given. 

Table 2 Examples of Indirect Conversions by Means of Organic Mediators — Reductive Redox Catalysis... 

Oxidation carried out in the presence of iodide and bromide ions have been extensively developed during the last two decades. They may often be used with advantage within a potential range in which other kinds of catalyst are not numerous (e.g., organic mediators) or need to be used ex-cell. Moreover, oxidized species obtained from I and Br are of a very broad applicability and are very active, even in water, toward substrates that are more difficult to oxidize than the halides. In most cases it has been demonstrated that such electrooxidized species often play a specific role at the anode (or close to it), whereas the same halogens in bulk solution are much less efficient. Only reactions offering that advantage are reviewed here. 

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