Aromatic hydrocarbons, anodic substitution

Anodic substitution reactions of aromatic hydrocarbons have been known since around 1900 [29, 30]. The course of these processes was established primarily by a study of the reaction between naphthalene and acetate ions. Oxidation of naphthalene in the presence of acetate gives 1-acetoxynaphthalene and this was at first taken to indicate trapping of the acetyl radical formed during Kolbe electrolysis of... 

Reactions between aromatic hydrocarbon radicabcations and cyanide ions, with few exceptions, give low yields of nuclear substitution products [76], In some cases, better results have been obtained by anodic oxidation of the aromatic compound in an emulsion of aqueous sodium cyanide and dichloromethane with tetra-butylammonium hydrogen sulphate as a phase transfer agent [77, 78]. Methoxy-benzenes give exceptionally good yields from reactions in acetonitrile containing tetraethylammonium cyanide, sometimes with displacement of methoxide [79, 80]... 

As an example, it has been suggested 37-) that many anodic substitution reactions of aromatic hydrocarbons are ECEC processes, as shown below for anodic aromatic acetoxylation Eqs. (2-5) ) ... 

In this equation, E normally corresponds to hydrogen but can also be another group, e.g. -OCH3. The denotation E is used to show the interplay between nucleophiles and electrophiles in electroorganic reactions. Thus, due to the oxidative nature of the process, a nucleophile can be used as a reagent in an otherwise impossible substitution reaction. As an example of an anodic substitution process, electrochemical cyanation of an aromatic hydrocarbon can be mentioned ... 

Similarly, cobaltic and argentic ion have been generated by anodic oxidation of cobaltous and argentous ion, respectively, and used for the oxidation of methyl-substituted aromatic hydrocarbons to aldehydes 18a Electrogenerated mercuric ion can be used for the conversion of propene to acrolein 18al ... 

A reaction that has been discussed in terms of high acid concentrations is the nitration that takes place upon electrolysis of a suspended aromatic hydrocarbon in dilute (1-2 m) nitric acid (for summaries, see Allen, 1958 Fichter, 1942 Tomilov, 1961 Tomilov and Fiochin, 1963). The proposal has been that nitric acid would be formed in high concentration in the anode region, thus causing nitration in an ordinary electrophilic substitution reaction. This is an entirely reasonable idea, as it has since been shown (Cauquis and Serve, 1970) that nitrate ion is oxidized to dinitrogen pentoxide using nitromethane as a solvent (59). In an aqueous solution this... 

This mechanism does not constitute a major reaction pathway for the anodic oxidation of alternant aromatic hydrocarbons (AAH) or alkyl-substituted AAH under common experimental conditions [12,16-19], but has been found to be operative, for example, for tetra-arylethylenes [20-23]. The difference in the reaction pattern of AAH and tetraarylethylenes is related to the rotational freedom around the central C-C bond experienced by the radical cations of the latter [24]. This results in a small or even negative difference between the standard potentials for the first and second one-electron transfer, E and E2, and, accordingly, the value of AT(j,spr [Eq. (15)] may take relatively large values. 

Dithianes and gemdithioacetals could be alternatively oxidized indirectly by means of the redox catalysis method. The technique appeared to be particularly mild and mainly avoided inhibition and adsorption phenomena relative to the anode platinum interface. Thus aromatic hydrocarbons (e.g. 9,10-diphenylanthracene) [83] and judiciously substituted triphenylamines [84] afford quite stable cation radicals used homogeneously as oxidants. Their standard potential, E°x, will determine the rate of electron exchange with the concerned sulfur compound. The cleavage of a C—S bond in any dithiane can be regarded as fast enough to draw the redox catalysis process to the indirect oxidation. 

Anodic Substitution Reactions with Aromatic Hydrocarbons... 

Several synthetically useful substitution reactions can be carried out through anodic oxidation of an aromatic hydrocarbon ... 

This chapter deals with anodic oxidation of saturated hydrocarbons, olefins, and aromatic compounds. Substituted hydrocarbons are included, when the substituents strongly influence the reactivity. Anodic functional group interconversions (FGI) of the substituents are covered in Chapters 6, 8-10 and 15. 

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