Intermediates anodic oxidation

If the anodic potential is increased, the current density becomes larger than JPS and dissolution occurs via an intermediate anodic oxide film. Hence the reaction can be separated into electrochemical oxide formation according to reaction (4.2) and chemical dissolution of the oxide due to HF, (HF)2 or HF2 [So2] ... 

Other approaches to the generation of the azallyl cation have been found. One of the most useful involves the use of lead tetraacetate (73TL2143). The anodic oxidation of aziridines also leads to the azallyl cation intermediate (75JA1600). 

The procedure described is essentially that of Belleau and Weinberg and represents the only known way of obtaining the title compound. One other quinone acetal, 1,4,9,12-t6traoxadispiro[4.2.4.2]tetradeea-6,13-diene, has been synthesized by a conventional method (reaction of 1,4-cyclohexanedione with ethylene glycol followed by bromination and dehydrobromination ) as well as by an electrochemical method (anodic oxidation of 2,2-(l,4-phenylenedioxy)diethanol ). Quinone acetals have been used as intermediates in the synthesis of 4,4-dimethoxy-2,5-cyclohexadienone,. syw-bishomoquinone, - and compounds related to natural products. ... 

Formation of the first layer (a monolayer) of passivating oxide film on a denuded metal surface occurs very simply by the loss of protons from the adsorbed intermediate oxidation products, such intermediates being common to both dissolution and passivation processes . Thus for example, the first oxidative step in the anodic oxidation of nickel is the formation of the unstable adsorbed intermediate NiOH by... 

The oxidation of hydrazine follows the change in surface completely since it oxidizes rapidly on bare nickel and again on the nickel(III) oxide surface but in the intermediate potential region, where the surface is covered with nickel(II) hydroxide, the anodic oxidation cannot occur (Fleischmann etal., 1972d). 

The electrosynthesis of 4-methoxybenzaldehyde (anisaldehyde) from 4-methoxy-toluene by means of direct anodic oxidation is performed on an industrial scale [69]. Via an intermediate methyl ether derivative, the corresponding diacetal is obtained, which can be hydrolyzed to the target product. The different types of products - ether, diacetal, aldehyde - correspond to three distinct single oxidation steps. 

The set of all intermediate steps is called the reaction pathway. A given reaction (involving the same reactants and products) may occur by a single pathway or by several parallel pathways. In the case of invertible reactions, the pathway followed in the reverse direction (e.g., the cathodic) may or may not coincide with that of the forward direction (in this example, the anodic). For instance, the relatively simple anodic oxidation of divalent manganese ions which in acidic solutions yields tetrava-lent manganese ions Mn +— Mn -l-2e , can follow these two pathways ... 

The anodic oxidation of organic substances is a complex multistep process. The question as to the depth of oxidation required (and sufficient) lias to be answered in each case. Where intermediate oxidation products pose no ecological risk, one can stop at incomplete oxidation. However, in the anodic oxidation of many aromatic substances, the corresponding quinones are formed in the first step, and these are more harmful than the original substances. Upon more profound oxidation, the benzene rings are broken and aliphatic substances are formed that are almost as harmless as carbon dioxide. 

Sidheswatan P, Lai H. 1971. A study of intermediates adsorbed on platinized platinum during the anodic oxidation of formaldehyde. J Electroanal Chem 34 173-183. 

The anodic oxidation of 2-alkyl-3-MeO-CHT followed by hydrolysis of the intermediate... 

On the other hand, Becker et al. also have attempted the anodic oxidation of RfCH2CH2I in acetonitrile and they have achieved the anodic transformation of C8F, 7CH2CH2I to the corresponding acetamide, trifluoroacetate, and benzoate derivatives in good yields [35]. They propose a different reaction mechanism involving a hypervalent iodanyl radical intermediate as shown in Eq. 17. 

Recently, Germain et al. have also shown that the indirect anodic oxidation in fluorosulfuric add of fluorocarbon derivatives of the type RfCF2X (X — H, COOH, S03H, CH2OH, Br), which are not directly oxidizable, leads to fluorosulfates of the type FSG3CF3Rf (Eqs. 18 and 19) [37, 38], in these reactions, the peroxide (FS03)2, partially dissociated in its free radicals, is the in-situ electrochemically produced reactive intermediate as shown in Scheme 5.1. 

The fact that the anodic oxidation of allylsilanes usually gives a mixture of two regioisomers suggests a mechanism involving the allyl cation intermediate (Scheme 3). The initial one-electron transfer from the allylsilane produces the cation radical intermediate [9], Although in the case of anodic oxidation of simple olefins the carbon-allylic hydrogen bond is cleaved [28], in this case the... 

Suda and coworkers described the anodic oxidation of 2-silyl-l,3-dithianes which have two sulfur atoms on the carbon adjacent to silicon [42], In this case, however, the C Si bond is not cleaved, but the C-S bonds are cleaved to give the corresponding acylsilanes (Scheme 12). Although the detailed mechanism has not been clarified as yet, the difference in the anode material seems to be responsible for the different pathway of the reaction. In fact, a platinum plate anode is used in this reaction, although a carbon anode is usually used for the oxidative cleavage of the C-Si bond. In the anodic oxidation of 2-silyl-l,3-dithianes the use of a carbon anode results in a significant decrease in the yield of acylsilanes. The effects of the nature of the solvent and the supporting electrolyte may also be important for the fate of the initially formed cation radical intermediate. Since various 2-alkyl-2-silyl-l,3-dithianes can be readily synthesized, this reaction provides a convenient route to acylsilanes. 

Nitrogen compounds are also effective as nucleophiles in the anodic oxidation of silyl-substituted ethers. The electrochemical oxidation in the presence of a carbamate or a sulfonamide in dry THF or dichloromethane results in the selective cleavage of the C-Si bond and the introduction of the nitrogen nucleophile at the carbon (Scheme 21) [55]. Since a-methoxycarbamates are useful intermediates in the synthesis of nitrogen-containing compounds [44], this reaction provides useful access to such compounds. Cyclic silyl-substkuted ethers such as 2-silyltetrahydrofurans are also effective for the introduction of nitrogen nucleophiles. The anodic oxidation in the presence of a carbamate or a... 

Recently, acylsilanes have been utilized as useful intermediates in organic synthesis [57], For example, treatment of acylsilanes with the fluoride ion generates the corresponding acyl anions which react with electrophiles. On the other hand, by using the electrochemical method, acylsilanes serve as acyl cation equivalents because nucleophiles are introduced at the carbonyl carbon. Chemical oxidation of acylsilanes with hydrogen peroxide which affords the corresponding carboxylic acids has been reported [58], However, the anodic oxidation provides a versatile method for the introduction of various nucleophiles... 

Schafer reported that the electrochemical oxidation of silyl enol ethers results in the homo-coupling products. 1,4-diketones (Scheme 25) [59], A mechanism involving the dimerization of initially formed cation radical species seems to be reasonable. Another possible mechanism involves the decomposition of the cation radical by Si-O bond cleavage to give the radical species which dimerizes to form the 1,4-diketone. In the case of the anodic oxidation of allylsilanes and benzylsilanes, the radical intermediate is immediately oxidized to give the cationic species, because oxidation potentials of allyl radicals and benzyl radicals are relatively low. But in the case of a-oxoalkyl radicals, the oxidation to the cationic species seems to be retarded. Presumably, the oxidation potential of such radicals becomes more positive because of the electron-withdrawing effect of the carbonyl group. Therefore, the dimerization seems to take place preferentially. 

Anodic oxide formation suggests itself as a passivating mechanism in aqueous electrolytes, as shown in Fig. 6.1a. However, pore formation in silicon electrodes is only observed in electrolytes that contain HF, which is known to readily dissolve Si02. For current densities in excess of JPS a thin anodic oxide layer covers the Si electrode in aqueous HF, however this oxide is not passivating, but an intermediate of the rapid dissolution reaction that leads to electropolishing, as described in Section 5.6. In addition, pore formation is only observed for current densities below JPS. Anodic oxides can therefore be excluded as a possible cause of pore wall passivation in PS layers. Early models of pore formation proposed a... 

The oxidation of an amine can benefit from the use of an electroauxiliary [19-28]. Electroauxiliaries are substituents that both lower the initial oxidation potential of the substrate and control the formation of the subsequent reactive intermediates. To this end, the anodic oxidation of the 6-membered ring a-silylamines in the presence of cyanide was shown to afford a net displacement of the silyl... 

Heterocycles are of great interest in organic chemistry due to their specific properties. Many of these cycles are widely present in natural and pharmaceutical compounds. Electrochemistry appears as a powerful tool for the preparation and the functionalization of various heterocycles because anodic oxidations and cathodic reductions allow the selective preparation of highly reactive intermediates (radicals, radical ions, cations, anions, and electrophilic and nucleophilic groups). In this way, the electrochemical technique can be used as a key step for the synthesis of complex molecules containing heterocycles. A review of the electrolysis of heterocyclic compounds is summarized in Ref. [1]. 

Various reactive intermediates have been postulated in the formation of heterocycles by anodic oxidation. 

The anodic oxidation of Ai-methylamides leads to A -acyliminium intermediates that may react with olefins (Scheme 49) [71]. 

As an example, a convenient preparation of methyl (E) and (Z)-4,4-dimethoxybutenoa-tes can be performed by anodic oxidation of furfuryl alcohol, furfural or furoic acid via a dimethoxylated dihydrofuran intermediate (Scheme 132) [157]. 

Anodic oxidation of a mixture of ds-cyclohexane-l,2-diol and cyclohexanol in methanol containing dibutyltin oxide and Et4NBr provides 2-hydroxycyclohexanone selectively, while cyclohexanol is recovered almost completely (Fig. 28) [146]. The cyclic intermediate dibutyl-(l,2-dioxycyclo-hexyl) stannan is oxidized indirectly by Br+. 

Protons, generated indirectly by deprotonation of an intermediate during anodic oxidation of an organic substrate, are obviously produced in stoichiometric amounts. Reactions induced by these protons are typically acid-catalyzed conversions of the initial oxidation product or proton-induced reactions/deactivation of unconverted substrate. 

Similar oxidative cyclization reactions involving the direct oxidation of acyclic 1,3-dicarbonyl compounds have not been reported. However, the generation of radical intermediates by the direct oxidation of cyclic 1,3-dicarbonyl compounds at an anode surface has been reported. Yoshida and coworkers have shown that the anodic oxidation of cyclic 1,3-dicarbonyl compounds in the presence of olefin trapping groups gives rise to a net cycloaddition reaction (Scheme 10) [23]. These cycloaddition reactions proceeded by initial oxidation of the 1,3-dicarbonyl compound at the anode followed by a radical addition to the second olefin. Following a second oxidation reaction, the material then... 

The success of these reactions was intriguing because, under normal reaction conditions, group 14 organometallic compounds serve as carbanion synthons. The anodic oxidation reaction successfully reversed this polarity thereby expanding the overall synthetic utihty of group 14 organometallic intermediates. 

Intramolecular coupling reactions between nucleophilic olefins have also proven to hold potential as synthetically useful reactions. The first example of this type of reaction was reported by Shono and coworkers who examined the intramolecular coupling reaction of an enol acetate and a monosubstituted olefin (Scheme 41) [50]. This reaction was conducted in an effort to probe the nature of the radical cation intermediate generated from the anodic oxidation of... 

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