Electrochemical oxidation of phenols

The electrochemical oxidation of phenols produces quinones that can be used as dienophiles for the Diels-Alder reaction. A typical example is shown in Scheme 14, where a lithium perchlorate/nitromethane system and an electrode coated with a PTFE [poly-(tetrafluoroethylene)] fiber, to create a hydrophobic reaction layer. 

Electrochemical oxidation of phenol substituted by a side chain containing an oxime group afforded a spiro-isoxazole in a quantitative yield (equation 18) . 

The direct electrochemical oxidation of phenols generates phenoxonium cations which are able to undergo [3-1-2] cycloaddition in the presence of unactivated alkenes to produce benzofurans <1999JOC7654>. Thus, electrolysis of methyl 2,5-dihydroxybenzoate in a solution of lithium perchlorate in nitromethane in the presence of acetic acid and 2-methyl-2-butene produces the dihydrobenzofuran in excellent yield (Equation 88). 

Figure 8. A brief outline of the electrochemical oxidation of phenols showing some typical reaction products...

Sharifian H, Kirk DW. Electrochemical oxidation of phenol. J Electrochem Soc 1986 133 921-924. 

Comninellis C, Pulgarin C. Electrochemical oxidation of phenol for waste-water treatment using Sn02 anodes. J Appl Electrochem 1993 23 108-112. 

Iniesta J, Michaud PA, Panizza M, Cerisola G, Aldaz A, Comninellis C (2001) Electrochemical oxidation of phenol at boron-doped diamond electrode. Electrochim Acta 46 3573-3578... 

ComnineUis, Ch. and Pulgarin, C. (1993) Electrochemical oxidation of phenol for wastewater treatment using tin dioxide anodes. J. Appl. Electrochem. 23, 108-112. 

The electrochemical oxidation of phenol on doped SnC>2 and platinum anodes was studied by Comninellis and Pulgarin (1993). The rate of phenol removal was almost the same for both anodes but the rate of TOC elimination and the ICE were much higher for the Sn02 electrode (Fig. 2.7). 

Fig. 2.8 Variation of the concentration of (1) phenol, (2) 1,4-benzoquinone, and TOC during electrochemical oxidation of phenol on a Ta/Pb02 anode at different temperatures. Initial phenol concentration 0.021 mol dm-3 i = 100 mA cm-2 (Tahar and Savall 1998)...

Awad, Y. M. and Abuzaid, N. S. (1999) Electrochemical oxidation of phenol using graphite anodes. Sep. Sd. Technol. 34,699-708. 

Canizares, P., Lobato, J., Paz, R., Rodrigo, M. A. and Saez, C. (2005), Electrochemical oxidation of phenolic wastes with boron-doped diamond anodes. Water Res., 39(12) 2687-2703. 

Pignatello, J.J. (1992) Dark and photoassisted iron (3+)-catalyzed degradation of chlorophenoxy herbicides by hydrogen peroxide. Environ. Sci. Technol. 26, 944-951 Polcaro, A.M., Mascia, M, Palmas, S. and Vacca, A. (2002) Electrochemical oxidation of phenolic and other organic compounds at boron doped diamond electrodes for wastewater treatment Effect of mass transfer. Ann. Chim. 93, 967-976... 

Other careful electrochemical measurements of the oxidation potentials of 2,4,6-tri-t-butylphenol and 2,6-di-t-butyl-4-methylphenol in acetate buffered ethanol or acetonitrile have been measured by Mauser et al.184). They determined the static potentials using a boron carbide indicator and a mercury/mercury-acetate reference-electrode. Since in this case the oxidation of the phenols and not the phenolates to the phenoxyls has been determined the oxidation potentials cannot be compared with those in Table 12. For other electrochemical oxidations of phenols in buffered aqueous solutions using a graphite electrode see Ref. 185 186>. 

There are, however, many different types of electrochemical oxidations of phenol derivatives possible, the results of which largely depend on the methods used as well as the structure of the different phenols. Secondary chemical reactions of factors including the primary or secondary oxidation products can also occur. The various electrochemical methods used are dependent on solvents, pH values, electrode materials or absorption effects at the electrodes. These all influence the measured potentials. Moreover, the liquid/liquid potentials and the various indicator electrodes can give results, which cannot be safely compared with the general E scala of redox potentials in aqueous solutions. In this review we cannot go into the many details obtained by these methods. For some examples see Ref. 203 . 

Pattenden and coworkers have recently evaluated the relative merits of LTA and electrochemical oxidation of phenolic compounds with particular reference to synthesis of the antiallergic compounds sodium chromoglycate (lOTAL 53) and proxicromil (54), which are used for the prophylactic treatment of asthma. The 2-carboxychromone moieties in the compounds (53) and (54) are synthesized from the appropriate 2, 6 -dihydroxyacetophenones. Oxidation of the 2 -hydroxyacetophenone (55) by LTA in (U-chloromethane gave almost exclusively the quinol acetate (56), which was subsequently converted to the 2, 6 -dihydroxyacetophenone (57), a precursor to proxicromil (54 Scheme 21). By contrast, electro-... 

Influence of Chloride Ions on the Electrochemical Oxidation of Phenol. 1083... 

The need to study the influence of chloride ions on the electrochemical oxidation of phenol was necessitated by the fact that real (industrial) effluents contained chlorides in significantly high concentrations. Chloride ions, in solution, have the ability to produce chlorinated organic products, especially in acidic media. Halocompounds are usually more harmful to the environment than the organic compounds they result from. It has been reported that under certain conditions, electrogenerated chlorine converts to hypochlorite, which is a powerful oxidant but weak chlorinating agent [78]. 

Maluleke MA and Linkov VM. Partial electrochemical oxidation of phenol on ceramic-based flat-sheet t)fpe electromembrane reactors. Sep PurifTechnol 2003 32 377-385. 

From the viewpoint of organic synthesis, nature provides us with a number of target molecules, which have novel structures and a variety of biological activities. As already shown in Section II.A, electrochemical oxidation of phenols has been applied successfully to natural products synthesis. Hypervalent (diacyloxyiodo)benzenes have also been proved to be effective for natural products synthesis. Generally, oxidation of o- and p-methoxyphenols in MeOH provides the corresponding o- and p-quinone monoketals, respectively. They are utilized as promising synthons for natural products and related bioactive compounds, as demonstrated by Swenton . Recently, these quinone monoketals have been utilized for syntheses of terpenoids, neolignans, anthraquinones, alkaloids and related compounds. 

The ICE decreases with time during electrolysis to finally reach a value of about zero. A typical example is given in Fig.l where the evolution of ICE ( from the oxygen flow rate method) during the electrochemical oxidation of phenol (model pollutant) is given.This figure shows that for the experiment conditions used ICE decreases from 0,28 (initial value) to 0,015 after 3,5 h of electrolysis. 

Figure 1. Evolution of the ICE (from the oxygen flowr rate method) during the electrochemical oxidation of phenol, i = 57 mA cm-2, T = 70 °C, pH 12,5 (constant). Initial phenol concentration 23 mmol dm 3...

Fig. 11 gives the influence of current density on the EOI and EOD for the electrochemical oxidation of phenol at platinum anode. 

The influence of anode material on EOI values for the electrochemical oxidation of phenol is given in Fig. 13... 

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