Anodic oxidation alkanes

Commeyras and coworkers have also electrochemically oxidized alkanes (anodic oxidation). However, the reaction results in condensation and cracking of alkanes. The results are in agreement with the alkane behavior in superacid media and indicate the ease of oxidation of tertiary alkanes. However, high acidity levels are necessary for the oxidation of alkanes possessing only primary C—H bonds. 

The monofluoromethylene group and difluoromethyl group m 1H perfluoro-alkanes and -cycloalkanes are oxidized at the C-H bond to perfluoroalkyl and perfluorocycloalkyl fluorosulfates by anodic oxidation m fluorosulfonic acid [J, 4] Two modifications of the method are used ox idation by fluorosulfonyl peroxide generated pnor to the reaction [J] (equation 2A) and direct electrolysis m the acid [i, 4] (equabons 2B and 3)... 

The anodic oxidation of the carboxylate anion 1 of a carboxylate salt to yield an alkane 3 is known as the Kolbe electrolytic synthesis By decarboxylation alkyl radicals 2 are formed, which subsequently can dimerize to an alkane. The initial step is the transfer of an electron from the carboxylate anion 1 to the anode. The carboxyl radical species 4 thus formed decomposes by loss of carbon dioxide. The resulting alkyl radical 2 dimerizes to give the alkane 3 " ... 

Compared with the anodic oxidation of a 1,3-diene, the cathodic reduction of a 1,3-diene may be less interesting since the resulting simple transformation to monoolefin and alkane is more conveniently achieved by a chemical method than by the electrochemical method. So far, only few reactions which are synthetically interesting have been studied15. The typical pattern of the reaction is the formation of an anion radical from 1,3-diene followed by its reaction with two molecules of electrophile as exemplified by the formation of the dicarboxylic acid from butadiene (equation 22)16. 

In fluorosulfonic acid the anodic oxidation of cyclohexane in the presence of different acids (RCO2H) leads to a single product with a rearranged carbon skeleton, a 1-acyl-2-methyl-1-cyclopentene (1) in 50 to 60% yield (Eq. 2) [7, 8]. Also other alkanes have been converted at a smooth platinum anode into the corresponding a,-unsaturated ketones in 42 to 71% yield (Table 1) [8, 9]. Product formation is proposed to occur by oxidation of the hydrocarbon to a carbocation (Eq. 1 and Scheme 1) that rearranges and gets deprotonated to an alkene, which subsequently reacts with an acylium cation from the carboxylic acid to afford the a-unsaturated ketone (1) (Eq. 2) [8-10]. In the absence of acetic acid, for example, in fluorosulfonic acid/sodium... 

Tab. 1 Anodic oxidation of alkanes in fluorosulfonic acid, 1.1 M acetic acid...

Alkanes are functionalised by anodic oxidation in acetonitrile, methanol, acetic acid and more acidic solvents such as trifluoracetic acid and fluorosulphuric acid. Reaction requires very positive electrode potentials (see Table 2.1) and platinum has generally been used as anode materials in laboratory scale experiments. On a larger scale carbon is used as anode material. The first stage in these reactions in-... 

The anodic oxidation of alkanes on a platinum anode in anhydrous hydrogen fluoride19 at low potentials is accompanied by exhaustive fluorination, with all of the hydrogen atoms being replaced by fluorine. Thus, the major fluorination products of methane and propane are carbon tetrafluoride and octafluoropropane, respectively. 

Partial anodic oxidation of n-alkanes on a smooth Pt electrode in CF3COOH gives isomeric sec-alkyl trifluoroacetates in 50-80% yield.118... 

The protolytic oxidation of alkanes is also strongly supported by electrochemical studies. In 1973, Fleischmann, Plechter, and co-workers78 showed that the anodic oxidation potential of several alkanes in HSO3F was dependent on the proton donor ability of the medium. This acidity dependence shows that there is a rapid protonation equilibrium before the electron transfer step and it is the protonated alkane that undergoes oxidation (Scheme 5.9). 

Electro-organic chemistry is the study of the oxidation and reduction of organic molecules and ions, dissolved in a suitable solvent, at an anode and cathode respectively in an electrolysis cell, and the subsequent reactions of the species so formed. The first experiment of this type was reported in 1849 by Kolbe, who described the electrolysis of an aqueous solution of a carboxylate salt and the isolation of a hydrocarbon. The initial step involves an anodic oxidation of the carboxylate anion to a radical which then dimerises to the alkane. 

Anodic oxidation of alkanes is a viable alternative means of generating ej-radical cations from alkanes [23], In contrast with oxidations with short-lived photoexcited species, electrooxidation of the substrate absorbed on the anode involves two consecutive ET steps - oxidation of the alkane then deprotonation to an alkyl radical (Eq. 8) and further oxidation of the alkyl radical to a carbocation (Eq. 9). 

A variety of alkylbenzenes undergo anodic acetoxylation, in which the loss of an a proton and solvation of the radical cation intermediate form the basis of side-chain and nuclear acetoxylation, respectively.30Sa b The nucleophilicity of the solvent can be diminished by replacing acetic acid with TFA. The attendant increase in the lifetimes of aromatic radical cations has been illustrated in anodic oxidations.308 Radical cations also appear to be intermediates in the electrochemical oxidation of alkanes and alkenes.309a-c... 

While examples corresponding to (a), (b) and the deprotonation case of (c) can be counted literally in thousands, the effect of substrate protonation in anodic oxidation is less well documented. However, amines and other nitrogen compounds have been thoroughly investigated on this point (Adams, 1969) and found to behave normally, but some recent work on anodic reactions in superacidic media has revealed a theoretically interesting exception to the rule. This concerns the anodic oxidation of alkanes and cycloalkanes in fluoro-sulphuric acid (Table 4, no. 9) with varying concentrations of added base, potassium fluorosulphate and/or acetic acid (Bertram et al., 1971, 1973). 

Direct anodic oxidation of alkanes may be performed if they have ionization potentials lower than about 10 eV. Such oxidations can be classifted into two types of reactions, cleavage of C—bonds (equation 4) and cleavage of C—C bonds (equation S). 

In the anodic oxidation, adamantane is a unique compound among alkanes. It has a rather low oxidation potential, and its anodic oxidation in acetonitrile affords acetamidoadamantane (1 equation 6) in 90% yield.<... 

This anodic oxidation does not show any of the stereochemical limitations usually observed in cleavage reactions by chemical oxidizing reagents. Furthermore, 1,2-dimethoxy- and l-hydroxy-2-methoxy-alkanes are also oxidized with similar current efficiencies. 

The anodic oxidation of polyalkyl substituted cyclopropanes and spiro[2.n]alkanes gave mono- and dimethoxy products which resulted from the cleavage of the most highly substituted C-C bond, in contrast to the results obtained by acid-catalyzed methanolysis of alkyl-substituted cyclopropane by the same authors. Those results suggest that a direct (7-electron transfer from the most strained a bond of cyclopropane to the anode may be the initiation step. 

The anodic oxidation of a series of 1, -halo(alkylthio)-, X(CH2) SR (X = Cl, Br, I), and l, -chloro(alkylsulfinyl)alkanes, Cl(CH2) SOR, was studied by cyclic voltammetry and by controlled potential electrolyses in MeCN-Bu4NPF6 [78]. No influence of the alkyl substituents R nor of the number of methylene groups n with n > 2 upon the ease of sulfur oxidation, a two-electron irreversible process, was observed. Sulfoxides and sulfones are obtained in good yields by electrolysis in aqueous acetonitrile, as in Eqs. (38) and (39). 

Anodic oxidation of n-alkanes in acetonitrile results in mixtures of A -s-alkylacetamides but skeletal rearrangement of the intermediate i-carbenium ions is not observed. Aromatic compounds can undergo direct acetamidation in the ring. Thus, acetophenone, which normally undergoes electrophilic aromatic substitution at the meta position, affords the o- and p-acetamides (Scheme 44). Anthracene is cleanly converted into the acetamide (84) when the reaction is performed in the presence of TFAA as water scavenger (equation 41). ... 

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