Alkenes, anodic processes

Acetoxylation proceeds mostly via the radical cation of the olefin. Aliphatic alkenes, however, undergo allylic substitution and rearrangement predominantly rather than addition [224, 225]. Aryl-substituted alkenes react by addition to vic-disubstituted acetates, in which the dia-stereoselectivity of the product formation indicates a cyclic acetoxonium ion as intermediate [226, 227]. In acenaphthenes, the cis portion of the diacetoxy product is significantly larger in the anodic process than in the chemical ones indicating that some steric shielding through the electrode is involved [228]. 

The sacrificial zinc anode process has also been used to perform the electrochemical cyclopropanation of alkenes from dibromomethane or bromochloromethane in CH2C12/... 

For alkenes more difficult to reduce than CO2, such as butadiene (63a), electron transfer from C02 to the alkene may be involved. Cross-coupling of CO2 and 63a in MeCN has been carried out in an undivided flow cell at constant current. Using Et4N salts of formate or oxalate as supporting electrolyte, the anode process is formation of CO2 and H" ", which are both consumed in the cathode process [167]. The outcome (up to 63% total yield) was a mixture of isomers of C5, Cg, and Cjo unsaturated carboxylic acids and diacids. The detailed mechanism is not known, but the products may arise from initial addition of C02 to the unreduced butadiene [167], although electron transfer from C02 to 63a or direct reduction of 63a (present in large excess) cannot be ruled out. Based on the observed influence of experimental parameters on the distribution of the C5, Cg, and C]o acid products, the authors suggest that the reactions take place between adsorbed intermediates [167]. 

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. 

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... 

In these reactions (Scheme 3.1), the first electron addition is to the alkene giving a radical-anion. This interacts with the alkyl halide to transfer an electron, in a process driven by simultaneous cleavage of the carbon-halogen bond. The alkyl radical formed in this manner adds an alkene radical-anion [25]. Aluminium ions generated at the anode are essential to the overall process. They coordinate with the intermediate carbanion, which then interacts with the second halogen substituent in an Sn2 process to form the carbocycle. 

Cyclo-coupling between arylalkenes and an aliphatic ester function is achieved by electrolysis in tetrahydrofuran using cathode and anode both of magnesium in an undivided cell. The first electron addition is to the arylalkene. The bond forming steps involves nucleophilic attack by radical-anions or dianions derived from the alkene. Magnesium ions generated at the anode are essential to the process. The... 

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. 

Generation of the carbon based radical in these processes involves the prior formation of a complex between manganese(lll) and the enol of the carbonyl reactant. Intramolecular electron transfer occurs within this complex. Addition to the olefin then takes place within the co-ordination sphere of manganese. When manganese is present in catalytic amount, the relative values of the equlibrium constants between manganese and both the carbonyl compound and the alkene arc important. If the olefm is more strongly complexed then no radical can form and reaction ceases. Reactions are usually carried out at constant current and the current used must correspond to less than the maximum possible rate for the overall chemical steps involved. Excess current caused the anode potential to rise into a region where Kolbe reaction of acetate can occur and this leads to side reactions [28]. 

Alkyl alkanoates are reduced only at very negative potentials so that preparative scale experiments at mercury or lead cathodes are not successful. Phenyl alkanoates afford 30-36% yields of the alkan-l-ol under acid conditions [148]. Preparative scale reduction of methyl alkanoates is best achieved at a magnesium cathode in tetrahydrofuran containing tm-butanol as proton donor. The reaction is carried out in an undivided cell with a sacrificial magnesium anode and affords the alkan-l-ol in good yields [151]. In the absence of a proton donor and in the presence of chlorotrimethylsilane, acyloin derivatives 30 arc formed in a process related to the acyloin condensation of esters using sodium in xylene [152], Radical-anions formed initially can be trapped by intramolecular addition to an alkene function in substrates such as 31 to give aiicyclic products [151]. 

Finally, the electroiodofluorination of alkynes and alkenes has been reported [179]. The process occurs with high regioselectivity and is based on the anodic formation of an iodonium ion in the presence of iodide and fluoride. 

Annulation of furans via electrochemical oxidation at the anode has become an important process for the synthesis of complex polycycles, and was covered in a review <2000T9527>. Furans tethered at the 3-position to electron-rich alkenes, enol ethers, or vinyl sulfides were converted to [6,5] and [7,5]-fused ring systems <1996JOC1578, 2002OL3763, 2004JOG3726, 2005JA8034>, as illustrated in Scheme 20. Analysis of crude reaction mixtures and side... 

Electron transfer to an anode involves the removal of an electron from the highest occupied molecular orbital and, in the absence of solvent, the ease of this process is reflected in the first ionization potential (I.P.). Electrochemical oxidation must perforce involve a solvent but despite this complication there is a remarkably linear empirical relationship between gas-phase ionization potentials and oxidation halfwave potentials (Ej) referred to the Ag/Ag+ electrode in acetonitrile. For a considerable number and range of organic compounds the best linear plot of Ei vs. I.P. obeys the equation, Ei = 0-92(I.P.) — 6-20. Using this equation and experimental or calculated I.P. values culled from the literature, Ej values for a number of alkenes and alkynes have been calculated and displayed in Table 3. The calculated Ei values... 

The anodic behavior of A -substituted alkenes can be described as the oxidation of an electron-rich double bond. Tetraamino-substituted alkenes are extremely easily oxidized. Tetrakis(dimethylamino)ethylene exhibits two reversible one-electron processes at —0.75 and —0.61 V vs. SCE at a dropping mercury electrode in acetonitrile [140]. The anodic behavior of A, A -dimethylaminoalkenes has been studied intensively by cyclic voltammetry and electron spin resonance (ESR) spectroscopy [141]. The anodically E° = 0.48 V vs. SCE) generated cation radical of l,l-bis(iV,iV-dimethylamino)ethylene is shown to undergo C-C coupling, forming l,l,4,4-tetrakis(A, iV-dimethylamino)butadiene, which subsequently is further oxidized to its dication at —0.8 V [141,142]. With vicinal diamino ethylenes, usually two reversible one-electron oxidations are observed [143], while gem-inal diamino ethylenes exhibit an irreversible behavior [141]. Aryl-substituted vicinal diamino ethylenes (endiamines) can undergo a double cyclization to give an indolo-oxazoline when oxidized at 0.4 V vs. SCE in acetonitrile in the presence of 2,6-lutidine [144] ... 

The electrogenerative mode has also proved successful in direct and indirect electrocatalytic brominations and fluorinations under mild conditions. Vaporized bromine in nitrogen was used at the cathode to brominate alkenes at the anode, in a process similar to the chlorination above (47). Since bromine can form polyhalogen ions in solution. Bra , Br, , which could react nonelectrochemically, the electrolyte was flowed over the catalyst to remove such ions. As with direct chlorination, bromohydrins and dibromoalkanes were formed at platinum anodes. 

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. ... 

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