Aromatic rings electrochemical reductions

The direct electrochemical reduction of carbon dioxide requires very negative potentials, more negative than —2V vs. SCE. Redox catalysis, which implies the intermediacy of C02 (E° = —2.2 V vs. SCE), is accordingly rather inefficient.3 With aromatic anion radicals, catalysis is hampered in most cases by a two-electron carboxylation of the aromatic ring. Spectacular chemical catalysis is obtained with electrochemically generated iron(0) porphyrins, but the help of a synergistic effect of Bronsted and Lewis acids is required.4... 

Triple bonds in side chains of aromatics can be reduced to double bonds or completely saturated. The outcome of such reductions depends on the structure of the acetylene and on the method of reduction. If the triple bond is not conjugated with the benzene ring it can be handled in the same way as in aliphatic acetylenes. In addition, electrochemical reduction in a solution of lithium chloride in methylamine has been used for partial reduction to alkenes trans isomers, where applicable) in 40-51% yields (with 2,5-dihydroaromatic alkenes as by-products) [379]. Aromatic acetylenes with triple bonds conjugated with benzene rings can be hydrogenated over Raney nickel to cis olefins [356], or to alkyl aromatics over rhenium sulfide catalyst [54]. Electroreduction in methylamine containing lithium chloride gives 80% yields of alkyl aromatics [379]. 

It was previously mentioned1 that cinnoline and 3-substituted cinnolines (94) could be prepared from the condensation products (95) between an o-nitrobenzaldehyde and a nitroalkane by electrochemical reduction. The reaction has been further studied,138 and it was noticed that when the reduction was carried out stepwise, anthranils (96) were formed, especially at elevated temperatures. The final ring closure was catalyzed by traces of oxygen, whereas too much oxygen produced the cinnoline JV-oxide (97) the ring closure was believed to be a radical chain reaction where the formation of the aromatic cinnoline was part of the driving force [Eq. (76)]. 

Polycyclic aromatics with more than two aromatic rings, or more than one heteroatom are relatively easy to reduce and several reviews have summarized works on their electrochemical behavior. Bicyclic heteroaromatics with one heteroatom are reduced close to or beyond the decomposition of the electrolyte unless acidic solutions are used. Very few compounds of this kind have been preparatively reduced in neutral media. Their cathodic reduction could be carried out at mercury cathodes with TA A+ electrolytes. Depending on the heteroatom and the amount of charge transferred, hydrogenated and/or reductive cleavage products were obtained. 

Electrochemical reduction of aromatic halides and subsequent intramolecular reaction of the resulting aromatic cr-radical with another aromatic ring have also been shown to be applicable to formation of a six membered hetero-ring containing nitrogen, though the products are not always directly related with natural alkaloids 33). 

According to this model, the first stage in the treatment of nitrophenols aqueous wastes was the release of the nitro group from the aromatic ring. As a consequence, phenols or quinones were formed. These organic compounds were oxidized first to carboxylic acids (maleic and oxalic) and later to carbon dioxide. Also the cathodic reaction steps were considered in the global process when the electrochemical cell was undivided at the cathode, the reduction of the nitro to the amine group and the transformation of nitrate into ammonia were observed. In alkaline media, aminophe-nols were polymerised and transformed into a dark brown solid. 

Extensive investigations have been made into further methods for the reduction of aromatic rings based on the use of dissolving metals in other solvents, especially the lower molecular weight amines (the Benkeser reduction), electrochemical methods (cathodic reductions), photochemical methods and the reaction of radical anions with silylating reagents rather than proton sources. The aim of much of this work has been to produce the normal Birch products more conveniently or cheaply, but very often the outcome has been quite distinct. The alternative method may then provide access to products which are not so easily obtained by the standard metal-liquid ammonia methodology. 

Arylsilanes serve as a typical example of this system. The reduction potentials of arylsilanes are slightly less negative than those of the parent aromatic hydrocarbons [199-203]. This seems to be attributed to the dj -pj interaction between the aromatic ring and the silicon atom. The electrochemical behavior of silyl-substituted cyclooctatetraene is interesting [204]. The second reduction potential becomes less negative by the silyl substitution. The stabilization of the dianion (aromatic 10 7r-system) by dj -p interaction seems to be responsible for this phenomenon. 

The electrochemical reduction of carboxylic acids and their associated esters is only possible both in the presence of a strong acid and an efficient activation (Ar accounts for an aromatic ring such as pyridine). The reduction leads successively [69]... 

Unpaired electrons can be present in ions as well as in the neutral systems that have been considered up to this point. There are many such radical cations and radical anions, and we consider some representative examples in this section. Various aromatic and conjugated polyunsaturated hydrocarbons undergo one-electron reduction by alkali metals. Benzene and naphthalene are examples. The ESR spectrum of the benzene radical anion was shown earlier in Figure 11.2a. These reductions must be carried out in aprotic solvents, and ethers are usually used for that purpose. The ease of formation of the radical anion increases as the number of fused rings increases. The electrochemical reduction potentials of some representative compounds are given in... 

As shown in the course of this chapter, the electrochemical activity concerns almost exclusively the reduction of those derivatives. The reduction is also nearly always associated with a cleavage reaction (two-electron scission). The reduction is strongly favoured when the electron transition allowing the occupation of a n orbital is made easier. A decrease in the LUMO energy may correspond, at least, to the introduction of an aromatic ring when associated to the S02 or S03 group. In other words, the reduction of... 

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