Electroreduction aromatic hydrocarbons

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 +. 

In the electroreduction of aromatic hydrocarbons, nitro compounds, and quinones in aptotic solvents, the first step is the transfer of an electron from the electrode to form a radical anion. Once the radical anion is formed, electron repulsion will decrease the facility with which a second electron transfer occurs. But solvation and ion pairing diminish the effect of electron repulsion and tend to shift the reduction potential for the addition of the second electron to more... 

Kariv-Miller and coworkers have developed indirect electroreductive cyclizations with the dimethyl-pyrrolidinium ion (DMP") as a mediator. Preparative electrolysis of 6-hepten-2-one (9) at a graphite cathode afforded cu-dimethylcyclopentanol (10) in 90% yield (equation 5). The reduction is believed to occur via the ketyl radical anion, which cyclizes onto the alkenic bond. In the absence of DMP simple reduction to 6-hepten-2-ol takes place.Very recently it was shown that instead of DMP several aromatic hydrocarbons can be used as mediators to initiate the cyclization reaction. The carbonyl group can also be cyclized onto an alkynic bond and even an aromatic ring. - ... 

Dicyanobenzene was converted into 3-cyanobenzoic acid by using the bacterium Rhodococcus rhodochrocus ". Intramolecular electron transfer involving isomeric forms of dicyanobenzene was investigated in detaiP and the mechanisms of electroreduction of all three isomers were studied. When dicyanobenzene was used as catalyst in photoamination reactions of arenes, the first step of the reaction was the formation of an aryl radical cation by electron transfer from the aromatic hydrocarbon to m-dicyano-benzene. ... 

Aromatic hydroxyl groups are also difficult to reduce however, the transformation of hydroxyl groups to phospho-nate esters followed by electroreduction affords the corresponding hydrocarbons (Scheme 2) [6],... 

Hydrocarbons can be obtained in satisfactory yields by the electroreduction of simple aliphatic aldehydes thus acetone may be reduced to propane at a Cd cathode in quantitative yields. Aromatic aldehydes and ketones can also give rise to hydrocarbon products when a Zn cathode is employed. ... 

---