Alkanes electrochemical oxidation

The equilibrium (1) at the electrode surface will lie to the right, i.e. the reduction of O will occur if the electrode potential is set at a value more cathodic than E. Conversely, the oxidation of R would require the potential to be more anodic than F/ . Since the potential range in certain solvents can extend from — 3-0 V to + 3-5 V, the driving force for an oxidation or a reduction is of the order of 3 eV or 260 kJ moR and experience shows that this is sufficient for the oxidation and reduction of most organic compounds, including many which are resistant to chemical redox reagents. For example, the electrochemical oxidation of alkanes and alkenes to carbonium ions is possible in several systems... 

The complexes [Ru(bpy)2L]+ (HL = acetylacetone, trifluoroacetylacetone, hexafluoroacetylace-tone, tropolone or dibenzoylmethane) have been prepared and characterized they act as catalysts for the oxidation of alcohols, 3,5-di-tert-butylcatechol and alkanes in the presence of appropriate co-oxidants." Perchlorate salts of [Os(bpy)2L] in which HL = salicylaldehyde, 2-hydroxyaceto-phenone or 2-hydroxynaphthaldehyde, are formed from reactions of [Os(bpy)2Br2] with HL. The structure of the salicylaldehyde derivative has been determined. Chemical and electrochemical oxidations of [Os(bpy)2L]" yield the corresponding low-spin Os species from which [Os(bpy)2L]+ can be regenerated." " [Ru(bpy)2L]" (HL = salicylic acid) has been prepared and structurally characterized as the tetrahydrate. Absorptions at 590 nm, 400 nm, and 290 nm in the electronic spectrum have been assigned to Ru bpy CT transitions electrochemical oxidations of the complex have been investigated." ... 

In acetonitrile, carbonium ions combine with the solvent to form a nitrillium ion. The latter reacts with added water to form the N-substituted acetamide, often in good yield [5, 6, 7]. Thus electrochemical oxidation of alkanes in acetonitrile is a route for the introduction of an amino-substituent. Some carbonium ions are inefficiently quenched by acetonitrile and eliminate a proton to form an alkene. This alkene is readily oxidised at the anode potentials used and oxidation products contribute to electrode fouling. Pulsing of the anode potential to +0.3 V vs, see helps to... 

Hydrocarbons undergo related reaction.s in the super-acid media, such as fluorosuiphuric acid and antimony pentachloride. It has been suggested that the initial one-electron processes during the electrochemical oxidation of alkanes in fluorosuiphuric acid involve a protonated carbon-hydrogen bond with formation of a carbon radical and release of two protons [15]. 

In a subsequent study Devynck and co-workers81,82 studied the electrochemical oxidation of alkanes and alkenes in triflic acid monohydrate. The acidity of CF3SO3H H20 was found to be intermediate between that of aqueous acid media and superacidity. Alkanes undergo two-electron oxidation, whereas alkenes are protonated to yield carbenium ions in this medium. In addition to various transformations characteristic of carbenium ions [Eqs. (5.36)—(5.38)], they undergo a reversible disproportionation to give an alkane and an aldehyde [Eqs. (5.40)]. 

C-H transformation of alkanes by SET is still a developing area of preparative organic chemistry. Generation of cr-radical cations from alkanes in solution requires strong oxidants, and is achieved by photochemical and electrochemical oxidation. Under these conditions even unstrained strained alkanes may be functionalized readily. The C-H substitution is selective if the hydrocarbon forms a radical cation with a definite structure and/or deprotonation from a certain C-H position of the radical cation dominates. Overoxidations are the most typical side reactions that lead to disubstituted alkanes. This can usually be avoided by running the reactions at low alkane conversions. 

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... 

Indirect electrochemical oxidative carbonylation with a palladium catalyst converts alkynes, carbon monoxide and methanol to substituted dimethyl maleate esters (81). Indirect electrochemical oxidation of dienes can be accomplished with the palladium-hydroquinone system (82). Olefins, ketones and alkylaromatics have been oxidized electrochemically using a Ru(IV) oxidant (83, 84). Indirect electrooxidation of alkylbenzenes can be carried out with cobalt, iron, cerium or manganese ions as the mediator (85). Metalloporphyrins and metal salen complexes have been used as mediators for the oxidation of alkanes and alkenes by oxygen (86-90). Reduction of oxygen and the metalloporphyrin generates an oxoporphyrin that converts an alkene into an epoxide. 

More recently, the electrochemical oxidation of lower alkanes in the HF solvent system has been investigated by Devynck and coworkers over the entire pH range. Classical and cyclic voltammetry show that the oxidation process depends largely on the acidity level. Isopentane (2-methylbutane, M2BH), for example, undergoes two-electron oxidation in HFiSbFs and HFiTaFg solutions (equation 18). ... 

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. 

In weaker superacids such as neat CF3SO3H, alkanes that have no tertiary hydrogen are isomerized only very slowly, as the acid is not strong enough to abstract hydride to form the initial carbocation. This lack of reactivity can be overcome by introducing initiator carbenium ions in the medium to start the catalytic process. For this purpose, alkenes may be added, which are directly converted into their corresponding carbenium ions by protonation, or alternatively the alkane may be electrochemically oxidized (anodic oxidation) (equation 22). Both methods are useful to initiate isomerization and cracking reactions. The latter method has been studied by Commeyras and coworkers. ... 

The effects of molecular structure, electrolyte and temperature on the rate and extent of adsorption and electrochemical oxidation of hydrocarbons (alkanes, alkenes, alkynes) have been reviewed . ... 

While alkanes are oxidized by an iodosylbenzene/Fe(TPP) system [43-48], higher valent intermediates observed in these systems are one electron oxidized Fe(IV) complexes such as (MeO)2Fe(IV) and 0=Fe(IV) species [103-107]. Similar 0=Fe(IV) porphyrin complexes are also observable in the peroxidase reactions (so-called compound II) as shown in Scheme 2 [4]. Fe complex of synthetic porphyrin (TPP) was first characterized in the electrochemical oxidation of Fe (TPP)Cl [108]. On the other hand, Phillippi and Goff obtained Fe(III) porphyrin 7C-cation radicals by the electrochemical one-electron oxidation [109]. Balch et al. also prepared 0=Fe(IV) complexes by introducing nitrogen bases into p-peroxo dimmer of Fe(III) porphyrin complexes at low temperature as shown in Scheme 3 [110, 111]. 0=Fe (TPP)(Base) was shown to oxidize triphenylphosphine at -80°C in toluene solution over a period of several hours [112]. 

Dicarbonyl Compounds.— 2,3-Dihydroxy-l,4-dioxan functions as a stable synthetic equivalent to glyoxal, particularly in the synthesis of a variety of heterocycles. The dioxan overcomes the prerequisite preparation of pure glyoxal immediately prior to its use (because of its tendency to polymerize) and offers an alternative available for reaction under non-aqueous conditions. Monoprotected a-keto-aldehydes are seldom available by selective derivatization of the parent dicarbonyl compound. However, 1,1,2,2-tetramethoxy-alkanes, readily prepared from a,a-dichloro-aldehydes, undergo regioselective hydrolysis to give l,l-dimethoxyalkan-2-ones. The sequence from the dichloro-aldehyde may be carried out without isolation of the tetramethoxylated intermediate. a,a-Bis(phenylseleneno)-aldehydes may be prepared from aldehydes or the intermediate a-(phenylseleno)-aldehyde by treatment with morpholinophenyl-selenenamide. a-(Arylseleno)-ap-unsaturated aldehydes result from the electrochemical oxidation of 3-hydroxyalkynes in the presence of a diaryl diselenide [equation (42)]. Treatment of a,a -dibromo-ketones with primary... 

Yoshida, J., Murata, X, and Isoe, S., Electrochemical oxidation of organosihcon compounds. Electrochemical oxidation of 1-phenylthio-l-trimethylsilyl alkanes, Chem. Lett., 631, 1987. 

The perfluoroalkane sulfonic acids were fkst reported ki 1954. Trifluoromethanesulfonic acid was obtained by the oxidation of bis(ttifluoromethyl thio) mercury with aqueous hydrogen peroxide (1). The preparation of a series of perfluoroalkanesulfonic acids derived from electrochemical fluotination (ECF) of alkane sulfonyl haUdes was also disclosed ki the same year (2). The synthetic operations employed when the perfluoroalkanesulfonic acid is derived from electrochemical fluotination, which is the best method of preparation, are shown ki equations 1—3. 

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