Alkenes 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... 

Electrochemical oxidation of a mixture of alkenes, NaN02 and NaN03, in water is also a good method for the preparation of nitroalkenes.59... 

An electrochemical oxidation of hydro-quinones can be used to initiate [3 + 2] cycloaddition reactions with alkenes. The... 

The electrochemical oxidation of A, Ai-dibenzyl, Ai -arylhydrazines gives carbocations that react with alkenes affording five-membered heterocycles (Scheme 48) [70]. 

Electrochemical oxidation of alkenes results in the removal on one electron from the alkene function to give a 7t-radical-cation where the electron deficiency is delocalised over tire conjugated system. The majority of alkene radical-cations cannot be characterised because they readily lose an allylic proton in aprotic sol-... 

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... 

Electrochemical oxidation of enol acetates in an undivided cell gives monomeric products in parallel with the reactions of simple alkenes [47, 48]. Thus, in the reaction of menthol enol acetate 23, the a-acetoxyketone product arises from nucleophilic attack of acetate ion on the radical-cation while the enone product... 

Electrochemical oxidation of 2,6-dimethoxy-4-allylphenol in aqueous methanol buffered with sodium hydrogen carbonate gives similar amounts of 2- and 4-methoxy substitution products, fhe 2-methoxylated product readily undergoes a Diels-Alder reaction with itself. The dimer 19, the natural product asatone, is found in some 1 % yield and most of the 2-methoxylated product is lost by addition of the ally alkene bond across the diene system of a second molecule [109]. 

Vinylindote radical-cations, for example that derived from 64, take part in a Diels-Alder reaction with alkenes. Subsequent oxidation of the initial product with loss of two protons and dimethylamine gives the pyrido[l,2a]indoie. Reaction is achieved either by direct electrochemical oxidation or by photochemical electron... 

Furthermore, electrochemical oxidation with TEMPO mediated aromatization of 6-membered cyclic dienes" and transformation of alkenes to alkenones also took place" . 

Scheme 11 Indirect electrochemical oxidation cleavage of alkenes using a double-mediator system consisting of 104 and ruthenium tungstosilicate (taken from Ref 8).

The direct electrochemical oxidation of phenols generates phenoxonium cations which are able to undergo [3-1-2] cycloaddition in the presence of unactivated alkenes to produce benzofurans <1999JOC7654>. Thus, electrolysis of methyl 2,5-dihydroxybenzoate in a solution of lithium perchlorate in nitromethane in the presence of acetic acid and 2-methyl-2-butene produces the dihydrobenzofuran in excellent yield (Equation 88). 

In redox methods, radicals are generated and removed either by chemical or electrochemical oxidation or reduction. Initial and final radicals are often differentiated by their ability to be oxidized or reduced, as determined by substituents. In oxidative methods, radicals are removed by conversion to cations. Such oxidations are naturally suited for the additions of electrophilic radicals to alkenes (to give adduct radicals that are more susceptible to oxidation than initial radicals). Reductive methods are suited for the reverse addition of alkyl radicals to electron poor alkenes to give adducts that are more easily reduced to anions (or organometallics). 

The oxidative method is often conducted on enol (or enolate) derivatives and a simplified mechanism is shown in Scheme 71. Initial chemical or electrochemical oxidation gives an electrophilic radical (68 that may be free or metal-complexed) that is relatively resistant to further oxidation. Addition to an alkene now gives an adduct radical (69) that is more susceptible to oxidation. Products are often derived from the resulting intermediate cation (70) by inter- or intra-molecular nucleophilic capture or by loss of a proton to form an alkene. The concentration and oxidizing potential of the reagent help to determine the selectivity in such reactions. 

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. 

To further explore the limiting structural and energetic prerequisites for the unique o-bishomoconjugation the structurally related seco-l,16-dodecahedradiene 596 and 1,16-dodecahedradiene 597 were also studied. For 596, only the corresponding bisallylic dication could be observed,1051 1052 whereas for 597 the dication could not be observed at all.1053 The o-bishomoaromatic species 599, however, could be generated via electrochemical oxidation of alkene 598.1054... 

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)]. 

The structure of diphosphallenic radical cations, generated from the allene ArP=C=PAr by electrochemical oxidation, has been examined using EPR spectroscopy. Ab initio calculations including correlation effects at the MP2 and MCSCF levels have determined that two rotamers exist compatible with Jahn-Teller distortion of the allene.146 Anodically generated radical cations of alkyl phosphites [(RO P] and silylphosphites [(RO)2POSiMe3] reacted with alkenes by initial attack at the C=C bond followed by electron transfer, deprotonation, and elimination of an alkyl or trimethylsilyl cation to form identical alkyl phosphate adducts.147 The electron ionization-induced McLafferty rearrangement of n-hexylphosphine afford the a-distonic radical cation CTEPH, the distinct reactivity of which suggests there is no... 

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... 

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... 

The essence of the Wacker process is the invention of the reoxidation process for Pd° by using CuCh as a cocatalyst. Cu" salts are good reoxidants, but chlorination of carbonyl compounds takes place with CuCh. For example, chloroacetaldehyde is a by-product of the Wacker process. Chlorohydrin is another by-product from the reaction of ethylene with PdCh and CuCb. - Thus, a number of other reoxidants were introduced. When CuCl, pretreated with oxygen, is used, no chlorination of ketones takes place and the rate of the reaction is higher. - Also Cu(N03)2 and Cu(OAc)2 have been used. Oxidation of cy-clopentene with PdCl2/Fe(C104)3 combined with electrochemical oxidation was carried out. Benzoqui-none was used at first by Moiseev et al and later by many other researchers as a good reoxidant, but a stoichiometric amount is necessary. The oxidation of alkenes can be carried out smoothly with catalytic... 

The first example of a catalytic approach to the selenium promoted conversion was reported by Torii, who described an oxyselenenylation-deselenenylation process using catalytic amounts of diphenyl diselenide [115]. The electrophilic species was produced from the diselenide by electrochemical oxidation in the presence of the alkene 233 in methanol or in water. As indicated in Scheme 36, the addition product is electrochemically oxidized to afford the selenoxide which by elimination gives the allylic ether or alcohol 234 and the phenylselene-nic acid which continues the cycle by adding again to the alkene 233. 

For the electrochemical oxidation and reduction of alkynes and alkenes an analogy may be drawn with their relative reactivities towards electrophilic and nucleophilic attack. Alkynes are the more easily attacked by nucleophiles and are slightly easier to reduce. Alkynes are, however, much less prone to electrophilic attack than alkenes and are correspondingly more difficult to oxidize electro-chemically. 

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... 

Alkene formation by cycloelimination has also been observed for several aminocyclo-propane derivatives. Thus amino acids 526 (equation 135) and 527 (equation 136) were converted into ethene and 1-butene, respectively by electrochemical oxidation or interaction with other oxidizing reagents, e.g. NaOCP aqueous chromic acid ... 

Another type of chain reaction is sometimes observed when electrochemical oxidation of unsaturated hydrocarbons is carried out in the presence of atmospheric oxygen, which may result in the formation of dioxetanes exemplified by the oxidation of biada-mantylidene [Eq. (53)] and related compounds [121-123]. Similar behavior has been observed for other alkenes [124]. 

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