Application of Organic Redox Catalysts for Indirect Electrochemical Syntheses

4 Application of Organic Redox Catalysts for Indirect Electrochemical Syntheses 

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. 

Reductions have been performed with radical anions and dianions of mostly aromatic compounds. It is clear that these mediators can not be used in protic media, while viologens can act as electron transfer agents in protic solvents. Their application, however, is limited to potentials of up to about —1.0 V. 

For oxidations, the cation radicals of aromatic compounds like 9,10-diphenyl-antracene, thiantrene, phenoxathiine, or dibenzodioxine are likely candidates. Their reactivity towards nucleophiles, however, limits their application to media of low nucleophilicity. Sometimes the stability of such cation radicals can be enhanced through blocking the reactive positions by substituents. For example, para-substituted triarylamines deliver cation radicals with often excellent stability even in methanol. The stability is further increased by incorporation of urzAu-substituents. Other mediators which have been applied in indirect electrosyntheses are those which are able to abstract hydrogen atoms or hydride atoms. 

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