Cathodic Reduction of Aromatics

The electrochemical reduction of aromatics to the corresponding 1,4-dihydro compounds can be performed by a procedure similar to the Birch reduction. It has been studied intensively in industry. Two different process modifications are to be distinguished  

This reaction principle was also extended to a number of substituted benzene derivatives 386.39°-39i) xhus, the reduction of p-xylene (Hg cathode, N-methylpyrroli-done-H20—Bu4NBr electrolyte) yields 1,4-dimethylcyclohexadiene with a current efficiency of about 80% 393). A number of papers have also been published on the reduction of naphthalene 394 397  

Cell Cathode Catholyte (Electrolyte) Conversion benzene (%) Selec- tivity (%) Current efficiency (%) Ref. 

Experiments using Pb cathodes in an undivided cell (NH3 oxidation as the anode process) indicate rapid deactivation of the cathodes. By optimizing the electrolyte, the operating times can be increased to about 40 hours without deactivation 395). However, this operating time is still far from sufficient from the industrial point of view. 

The reduction of alkoxynaphthalenes 398) was used by Hoechst for the synthesis of P-tetralones  

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Electrolytic reductions generally caimot compete economically with chemical reductions of nitro compounds to amines, but they have been appHed in some specific reactions, such as the preparation of aminophenols (qv) from aromatic nitro compounds. For example, in the presence of sulfuric acid, cathodic reduction of aromatic nitro compounds with a free para-position leads to -aminophenol [123-30-8] hy rearrangement of the intermediate N-phenyl-hydroxylamine [100-65-2] (61). 

Scheme 13 Cathodic reduction of aromatic esters to benzyl alcohols e.g. X 3-OH, yield 91.5%.

Scheme 14 Cathodic reduction of aromatic esters to aromatic aldehydes X= H, alkyl, yields 0-70%.

Scheme 23 Cathodic reduction of aromatic carboxylic acids to benzyl alcohols or benzaldehydes.

Scheme 31 Influence of sonication on the selectivity in the cathodic reduction of aromatic ketones diol 10-23% (without sonication), 36-42% (with sonication).

Cathodic reduction of aromatic hydrocarbons gives 7T-radical anions, which are possible EGBs. However, the PBs normally have low solubilities in polar aprotic solvents, relatively low reduction potentials. 

Chlorotrimethylsilane (CTMS) has been shown to promote various cathodic hydrocoupling reactions. Thus, cathodic reduction of a mixture of carbonyl compounds and activated olefins122 or imines123 in the presence of CTMS provides carbon-carbon cross-ed-coupling products efficiently. The cathodic reduction of aromatic esters and an amide to produce aldehydes was also promoted in the presence of CTMS124. 

Cathodic Reduction of Aromatic and Heterocyclic Halogen Compounds... 

The cathodic reduction of aromatic carboxamides and imides in acid solution at lead or mercury cathodes generally leads to amines and isoindolines, respectively 7,9 These reactions are of great preparative interest but their mechanism have not been examined. Cpe of isonicotinic amide and 2-thiazolecarboxaldehyde in acid solution gives the corresponding aldehydes in good yields 139>141)... 

Cathodic Reduction of Aromatic Compounds Conversion of Aromatic Compounds to Cyclohexadienes... 

Cathodic reduction of aromatic dihalides also undergoes to form conducting polymers on electrode. Moreover, a,a,a ,a -tetralffomo-/7-xylene can be electropolymerized to afford poly(p-phenylene-vinylene) by cathodic reaction [4]. 

Similar electrodes may be used for the cathodic hydrogenation of aromatic or olefinic systems (Danger and Dandi, 1963, 1964), and again the cell may be used as a battery if the anode reaction is the ionization of hydrogen. Typical substrates are ethylene and benzene which certainly will not undergo direct reduction at the potentials observed at the working electrode (approximately 0-0 V versus N.H.E.) so that it must be presumed that at these catalytic electrodes the mechanism involves adsorbed hydrogen radicals. 

The reduction of aromatic sulphoxides into the corresponding thioethers appears to be general it occurs at a lead cathode, in alcoholic sulphuric acid solution and also in the presence of tetraalkylammonium salts - Data in DMF are also available, when phenol is used as a proton donor. 

Thus, electrochemical data involving both thermodynamic and kinetic parameters of hydrocarbons are available for only olefinic and aromatic jr-systems. The reduction of aromatics, in particular, had already attracted much interest in the late fifties and early sixties. The correlation between the reduction potentials and molecular-orbital (MO) energies of a series of aromatic hydrocarbons was one of the first successful applications of the Hiickel molecular orbital (HMO) theory, and allowed the development of a coherent picture of cathodic reduction [1], The early research on this subject has been reviewed several times [2-4],... 

Since the electroreduction of ketones shown in Scheme 29 has been well established [1-3, 12, 62-65], one more recent interest in the electroreduction of carbonyl compounds is focused on the stereo-selective reduction of ketones. For example, the diastereo-selective cathodic coupling of aromatic ketones has been reported. In the presence of a chiral-supporting electrolyte, a low degree of enantioselectivity has been found [66] (Scheme 30). 

Aptotic solvents can be used for the reduction of aromatic hydrocarbons, particularly the condensed ring systems. Solvents used for the conversion of benzene to cyclohexa-1,4-diene at a mercury cathode under constant current conditions include dimethoxyethanc [45] and N-medtylpyrrolidone [46]. Each solvent contained water as a proton source and tetraethylammonium bromide as supporting electro-... 

Aromatic and heteroaromatic esters can be electrochemically reduced to benzyl alcohols, similar to the carboxylic acids. One example is the cathodic reduction of dimethyl terephthalate531 . 

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. 

Experiments designed to elucidate the role of S in cathodic reduction tend to be just as ambiguous as their anodic counterparts, unless certain precautions are taken. The possible intervention of S in the reduction of aromatic hydrocarbons (Asahara et al., 1968 Benkeser and Kaiser, 1963 Benkeser et al., 1964 Sternberg et al., 1963, 1966, 1967, 1969) in SSEs made up of amines or HMPA (to which up to 65% ethanol can be added without impairing the stability of HMPA too much) as compared to the possible direct processes taking part in protic solvents illustrates the problem. 

Table 11 shows some representative results from the cathodic reduction of some aromatic hydrocarbons. These include cases with Ei j2 near the cathodic limit or in the discharge region of the SSE (benzene, toluene) and cases with Ex j2 at considerably more positive potential (naphthalene, anthracene again we must anticipate the discussion of reactivity and refer to Table 21). Reactions nos. 1, 2, 6, and 7 immediately demonstrate one difficulty with such studies in that the catholyte of a divided cell becomes strongly basic as electrolysis progresses. In sufficiently basic medium, the initial product, a 1,4-dihydro derivative (cf. the Birch reduction Birch and Subba Rao, 1972), will rearrange to a conjugated system which, in contrast to the 1,4-dihydro derivative, is further reducible to the tetrahydro product (nos. 1 and 6). In a non-divided cell the acid production at the anode balances the base production and thus only a little rearrangement occurs. It is therefore not a trivial problem to find out if the tetrahydro product is formed from the conjugated dihydro product, formed directly or by rearrangement [eqn (78)].

This method was then extended by a supplementary patent,1 according to which the reduction of aromatic mtro-bodies is carried out in aqueous alkaline suspension instead of in the presence of alkali-soluble oxides of the heavy metals and the use of such metal cathodes the oxides of which are soluble in alkalies. 

The hydroxylation of the aromatic nucleus by hydroxyl radicals, generated by decomposition of hydrogen peroxide in the presence of iron(II) ions, may be applied to the electrochemical synthesis of phenol fiom benzene, since the concentration of the iron(II) ions can be controlled by the cathodic reduction of iron(III) ions formed by oxidation of iron(II) ions wiA H2O2. 

The pathway for the cathodic reduction of acetophenone can be profoundly altered when the aromatic ring is complexed inside the hydrophobic cavity of p-cyclodextrin. Products resulting from ortho and para coupling to the carbonyl carbon are isolated in excellent yield, as shown in equation (2). 

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. 

Electrochemical methods for the reduction of aromatic substrates utilizing ammonia and amines as solvents with lithium salts as electrolytes have been successful. Toluene was reduced to the 2,5-dihydro derivative in 95% yield in methylamine-lithium chloride if an undivided cell was used, while a 53 47 mixture of 3- and 4-methylcyclohexenes was formed in a divided cell.. Of greater interest, however, are attempts to achieve these reductions in aqueous media. In one experiment utilizing a two-phase mixture of substrate in aqueous tetra-n-butylammonium hydroxide and a mercury cathode, anisole was reduced on a preparative scale (15 g) to its 2,5-dihydro derivative in 80% yield. The optimal temperature for most reductions appeared to be 60 °C and under these conditions, even suspensions of high molecular weight substrates could be successfully reduced, e.g. steroid (226) afforded a >90% chemical yield of (227). Much higher coulombic yields were obtained when a small amount of THE was added to the mixture, however. 

The Birch reduction of aromatic hydrocarbons or of double bonds with alkali metals in liquid ammonia or amines [37] resembles in yield and selectivity the cathodic reduction with lithium halide as supporting electrolyte (for example, Ref. 38). 

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