Alkenes anodic oxidation

Faraday, in 1834, was the first to encounter Kolbe-electrolysis, when he studied the electrolysis of an aqueous acetate solution [1], However, it was Kolbe, in 1849, who recognized the reaction and applied it to the synthesis of a number of hydrocarbons [2]. Thereby the name of the reaction originated. Later on Wurtz demonstrated that unsymmetrical coupling products could be prepared by coelectrolysis of two different alkanoates [3]. Difficulties in the coupling of dicarboxylic acids were overcome by Crum-Brown and Walker, when they electrolysed the half esters of the diacids instead [4]. This way a simple route to useful long chain l,n-dicarboxylic acids was developed. In some cases the Kolbe dimerization failed and alkenes, alcohols or esters became the main products. The formation of alcohols by anodic oxidation of carboxylates in water was called the Hofer-Moest reaction [5]. Further applications and limitations were afterwards foimd by Fichter [6]. Weedon extensively applied the Kolbe reaction to the synthesis of rare fatty acids and similar natural products [7]. Later on key features of the mechanism were worked out by Eberson [8] and Utley [9] from the point of view of organic chemists and by Conway [10] from the point of view of a physical chemist. In Germany [11], Russia [12], and Japan [13] Kolbe electrolysis of adipic halfesters has been scaled up to a technical process. 

Romakhin et al. [49] showed that anodically generated phosphoniumyl radicals can add onto alkenes to yield phosphonylated alkenes through an anodic oxi-dation/addition/anodic oxidation/elimination/nucleophilic attack sequence (Scheme 17). 

Olefins, see also Alkenes specific compounds added, reactions during Fischer-Tiopsch synthesis, 39 251-253 adsorption of, 20 82-84 anodic oxidation, 40 162-165 autoxidation of, 25 281, 282, 305-308 bicyclic... 

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... 

In the addition to nonactivated alkenes, where the direct anodic oxidation is less, satisfactorily good yields can be achieved when Mn(OAc)2 is used as mediator (Table 8, entries 6, 7). Sorbic acid precursors have been obtained in larger scale and high current efficiency by a Mn(III)-mediated oxidation of acetic acid/acetic anhydride in the presence of butadiene [112]. 

Alkenes, as other organic substrates, can be converted by indirect anodic oxidation. 

CH2CI2, sulfolane, THF) in the presence of alkenes, whose oxidation potential is lower than that of the disulfide, results in the addition of the MeS group on the double bond. Since then, the anodic sulfeanylation of multiple bonds received many synthetic applications. 

Azide ions are oxidised at low positive potentials and generate azide radicals. Azide radicals will add to an alkene. Thus the anodic oxidation of enol ethers in... 

Sctiem 2.3. Catalytic cycle for the allylic hydroxytation of alkenes by anodic oxidation in the presence of diphenyl diselenide. 

Anodic oxidation is used to promote the recycling of palladium(il) in the Wacker process for the conversion terminal alkenes to methyl ketones. Completion of the catalytic cycle requires the oxidation of palladium(O) back to the palla-dium(li) state and this step can be achieved using an organic mediator such as tri(4-bromophenyljamine. The mediator is oxidised at the anode to a radical-cation and... 

The anodic oxidation of 4-methoxyphcnols in acetic acid effectively stabilises the phenoxonium ion, in an equlibrium with the acetoxylation product. Tbis allows an intermolecular [5 + 2] bx-cycloaddition processes with some alkenes [110], The cycloaddition process has been used very successfully in the synthesis of a number of natural products [III]. The rate of cycloaddition is sensitive to substituents on the alkene bond and in imfavourable cases other reactions of the phenoxonium ion predominate. 

The delocalised carbonium ion formed by the anodic oxidation of 3,4-dimeth-oxyphenol will take part in intramolecular [5 + 2] cycloaddition with an attached alkene [146, 147], This step has been used in the synthesis of a number of complex diterpenes. Acetic anhydride, containing acetic acid, is the best solvent for these... 

Electrophilic addition to die alkene with the formation of a 5-membered ring occurs on anodic oxidation of 44 [150]. Reaction of the intermediate delocalised carbonium ion with the adjacent hydroxyl function in 45 also results in the formation of a 5-merabered ring [151],... 

The ruthenium tetroxide dioxide catalytic system is effective for the oxidation of alkanols, although it will also react with any alkene groups or amine substituents that are present. The catalyst can be used in aqueous acetonitrile containing tetra-butylammonium hydroxide with platinum electrodes in an undivided cell Primary alcohols are oxidised to the aldehyde and secondary alcohols to the ketone [30]. Anodic oxidation of ruthenium dioxide generates the tetroxide, which is the effective oxidising agent. 

Anodic oxidation of 1,2-dicarboxylic acids as their alkali metal salts in concentrated aqueous solution gives the alkene with the loss of two molecules of carbon dioxide [125]. Succinic acid affords etltene and methylsuccinic acid ptopene [50]. Allene is obtained from itaconic acid and the isomeric methylmaleic and methyl-fumaric acids give propyne... 

Anodic oxidations of the Mo(II) bis-cyclopentadienyl alkyne compound Mo(jj -C5H5)2( i -C2Ph2), and of metal-alkene analogs such as Mo(j -C5H5)2(j7 -C2H4) involve a reversible one-electron oxidation at very facile potentials. The v(CC) alkyne stretch in the IR spectrum of 17-electron Mo-alkyne cation (1824 cm ) was shifted by 50cm to a higher wave number to that of its parent, consistent with a decrease in the metal-alkyne interaction in the Mo(III) cation. 

It is easier to oxidize an alkene electrochemically than to reduce it, because it is easier to reach powerfully oxidizing anode potentials12 (at which one can remove an electron from the HOMO of the alkene) without interference from the solvent or electrolyte than it is with reductions, where one can normally not achieve sufficiently negative cathode potentials to be able to add an electron to the LUMO. Even so, anodic oxidation of alkenes is relatively rarely observed almost always the alkene is part of a conjugated system or it bears an electron-supplying substituent, which raises the HOMO energy. 

Moeller has carried out an extensive series of studies of the electrochemical oxidation of electron-rich w-alkenes. One olefinic component is an enol ether, which is converted into an electrophilic center upon oxidation this center then attacks the other site intramolecu-larly. The anodic oxidation of the bis-enol ethers 21 in methanol25 exemplifies the course of such reactions (Scheme 4). The products are w-acetals (22), formed in 50-70% yield in many cases. The cyclization can be used to produce quaternary25 and angularly fused26 bicyclic and tricyclic structures (equation 11). In its original form, this work involved oxidation of a mono-enol ether bearing a nearby styrene-type double bond27. 

Anodic oxidation of Mn(OAc)2 (catalytic amounts) in the presence of nonacti-vated alkenes and ethyl acetoacetate provides a route to dihydrofurans (cf, 6, 356). This electrooxidation of Mn(OAc)2 has been extended to coupling of activated methylene compounds with alkenes and dienes. 

Baumer US, Schafer HJ (2005) Cleavage of alkenes by anodic oxidation. J Appl Electrochem 35 1283-1292... 

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... 

If the reactions are carried out in a nitrile as solvent, rather than dichloromethane, using triflic acid as catalyst, a modified Ritter reaction takes place, and the intermediate nitrilium ion traps the liberated amine, forming an amidine (Scheme 67). In an earlier reaction cf. Scheme 67) the lithium perchlorate catalyzed reaction of sulfenyl chlorides with alkenes in the presence of nitriles had also given l-amido-2-sulfenyl adducts. Ritter products are also obtained in good yields by anodic oxidation (Pt or C, 1.2-1.4 V) of disulfides in acetonitrile, in the presence of excess alkene, using B114NBF4 as supporting electrolyte (Scheme 68). ... 

As the oxidation potentials of simple alkenes clearly show, carbon-carbon double bonds are usually anodically oxidized unless electron-withdrawing groups located on the alkene carbon atoms attract electrons from the unsaturated systems to shift the oxidation potentials beyond dmse accessible by anodic oxidation. On the other hand, electron-donating groups on Ae unsaturat bonds facilitate oxidation. 

In general, the anodic oxidation of simple alkenes in nucleophilic solvents yields products resulting from both allylic substitution and oxidative addition of nucleophiles. Cyclohexene has been studied extensively as starting compound. The anodic oxidation of cyclohexene in methanol or acetic acid... 

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