Tetraalkylammonium potential window

Organic electroreductions at mercury cathodes in tetraalkylammonium (TAA+) electrolyte solutions at the limit of the cathodic potential window are described. Aromatic hydrocarbons, fluorides, ethers and heterocycles, as well as aliphatic ketones, alkenes and alkynes have been studied, using both aqueous and non-aqueous solvents. At these very negative potentials neither the TAA+ cation nor the mercury cathode are inert, instead they combine to form TAA-mercury. It is hypothesized, and supporting evidence is presented, that TAA-mercury serve as mediators in the organic electroreductions. The mediated reactions show remarkable selectivity in certain cases and it is shown that this selectivity can be improved by the choice of the TAA +. 

Electrochemical studies are usually performed with compounds which are reactive at potentials within the potential window of the chosen medium i.e. a system is selected so that the compound can be reduced at potentials where the electrolyte, solvent and electrode are inert. The reactions described here are distinctive in that they occur at very negative potentials at the limit of the cathodic potential window . We have focused here on preparative reductions at mercury cathodes in media containing tetraalkylammonium (TAA+) electrolytes. Using these conditions the cathodic reduction of functional groups which are electroinactive within the accessible potential window has been achieved and several simple, but selective organic syntheses were performed. Quite a number of functional groups are reduced at this limit of the cathodic potential window . They include a variety of benzenoid aromatic compounds, heteroaromatics, alkynes, 1,3-dienes, certain alkyl halides, and aliphatic ketones. It seems likely that the list will be increased to include examples of other aliphatic functional groups. 

Polycyclic aromatic hydrocarbons contain extended conjugated rc-electron systems and are electroactive within the cathodic potential window of many organic sol-vent/tetraalkylammonium electrolyte combinations. For many of them reduction potentials were measured and reported. It is important to note that the reduction of polycyclic aromatics involves consecutive electron-transfers and protonations. The nature of the product depends on the number of electrons and protons consumed and higher numbers result in more hydrogenated, less conjugated and thus less electroactive products. A simplified reduction scheme for polycyclic aromatics is shown below. The products are divided into three groups (A) are reduced within the potential window and (B) require drastic reduction conditions as those described for the substrates in Chapt. 3. 

The electrochemical window of electrolytes is very crucial in electrochemical applications. ILs are electrochemically rather stable having an electrochemical window typically from 2 to even 6 V. The electrochemical stability of the ILs depends on redox reactions of both the anion and the cation. Anions such as Tf2N oxidize at relatively high anodic potentials compared with, for example, BF4 resulting in extended stability of ILs containing Tf2N [6, 29]. Also, tetraalkylammonium cation-based ILs show extended electrochemical window on the negative side of the potential scale... 

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