Ketones, electrolytic oxidation

Direct Electron Transfer. We have already met some reactions in which the reduction is a direct gain of electrons or the oxidation a direct loss of them. An example is the Birch reduction (15-14), where sodium directly transfers an electron to an aromatic ring. An example from this chapter is found in the bimolecular reduction of ketones (19-55), where again it is a metal that supplies the electrons. This kind of mechanism is found largely in three types of reaction, (a) the oxidation or reduction of a free radical (oxidation to a positive or reduction to a negative ion), (b) the oxidation of a negative ion or the reduction of a positive ion to a comparatively stable free radical, and (c) electrolytic oxidations or reductions (an example is the Kolbe reaction, 14-36). An important example of (b) is oxidation of amines and phenolate ions ... 

Electrolytic oxidation of tetrahydrofuran Addition of alcohols to aldehydes or ketones... 

Electrolytic oxidation of ketones in methanolic solutions of NaCN in the presence of catalytic amounts of KI affords oxiranecarbonitriles along with small amounts of oxiranecarboximidates aryl ketones, however, lead to benzoylpropanedinitriles <93JOC6l94>. 

The unmasking of 1,3-dithianes by electrolytic oxidation, to generate ketones, has been developed into a synthesis of 1,2-dithiolene 1-oxides. The method requires controlled... 

YA. Gallego, a. Mendes, LM. Madeira, S.P. Nunes, Proton electrolyte membrane properties and direct methanol fuel-cell performance I. Characterization of hybrid sulfonated poly-(ether ether ketone)/zirconium oxide membranes. Journal [Pg.85]

In 1859, Friedel electrolytically oxidized acetone and found a mixture of formic, acetic, and carbonic acids with evolution of carbon dioxide and oxygen at the anode in an acetone-sulfuric acid mixture. Further studies on ketones were not reported until 1931, when a similar study was carried out resulting in the formation of methane, ethane, and unsaturated hydrocarbons, in addition to carbon dioxide and oxygen at platinum anodes. The first anodic oxidation of benzene was reported in 1880, with the observation that the electrolytic oxidation of benzene in an ethanolic-sulfuric acid medium yielded unidentifiable substances. A few years later Gotterman and Friedrichs reported that hydrocarbons were obtained from the anodic oxidation of benzene in alcoholic-sulfuric acid solution at platinum anodes. 

For example, acetyladamantane and isopropyl methyl ketone gave 1-adamantyl-acetamide in 85% yield and N-acetyl-isopropylamine in 46% yield, respectively [79]. A similar a-cleavage was observed in the electrochemical oxidation of the a-branched cyclic ketone (43) using Et3N-5HF as an electrolyte (Scheme 16) [81]. 

Shono et al. (1979) recommend the use of thioanisole as a catalyst that allows lowering the electrode potential in the oxidation of the secondary alcohols into ketones. The cation-radical of thioanisole is generated at a potential of up to +1.5 V in acetonitrile containing pyridine (Py) and a secondary alcohol. (The background electrolyte was tetraethylammonium p-toluene sulfonate.) Thioanisole is recovered and, therefore, a ratio of RXR )CHOH PhSMe = 1 0.2 is sufficient. The yield of ketones depends on the nature of the alcohol and varies from 70 to 100%. 

Early electrochemical processes for the oxidation of alcohols to ketones or carboxylic acids used platinum or lead dioxide anodes, usually with dilute sulphuric acid as electrolyte. A divided cell is only necessary in the oxidation of primary alcohols to carboxylic acids if (he substrate possesses an unsaturated function, which could be reduced at the cathode [1,2]. Lead dioxide is the better anode material and satisfactory yields of the carboxylic acid have been obtained from oxidation of primary alcohols up to hexanol [3]. Aldehydes are intermediates in these reactions. Volatile aldehydes can be removed from the electrochemical cell in a... 

The nickel oxide electrode is generally useful for the oxidation of alkanols in a basic electrolyte (Tables 8.3 and 8.4). Reactions are generally carrried out in an undivided cell at constant current and with a stainless steel cathode. Water-soluble primary alcohols give the carboxylic acid in good yields. Water insoluble alcohols are oxidised to the carboxylic acid as an emulsion. Short chain primary alcohols are effectively oxidised at room temperature whereas around 70 is required for the oxidation of long chain or branched chain primary alcohols. The oxidation of secondary alcohols to ketones is carried out in 50 % tert-butanol as solvent [59], y-Lactones, such as 10, can be oxidised to the ketoacid in aqueous sodium hydroxide [59]. 

Electrochemical oxidation of epoxides in absence of nucleophiles, catalyses a rearrangement to the carbonyl compound. The electrolyte for this process is dichlo-romethane with tetrabutylammonium perchlorate. Reaction, illustrated in Scheme 8.7, involves the initial formation of a radical-cation, then rearrangement to the ketone radical-cation, which oxidises a molecule of the substrate epoxide. The process is catalytic and requires only a small charge of electricity [73]. 

Ketogenesis is an important metabolic function in the liver. It is the result of an increase in lipolysis in the fatty tissue, with a rise in fatty acids. Insulin inhibits ketogenesis, whereas it is accelerated by fasting as well as by glucagons and insulin deficiency. Ketones (acetacetate, 3-hydroxybutyrate, acetone) are synthesized by means of P-oxidation from acetyl-CoA, assuming the production of this substance exceeds the amount required by the hepa-tocytes (and glucose metabolism is simultaneously reduced). The liver itself does not require any ketones acetone is expired, whereas 3-hydroxybutyrate and acetacetate serve as a source of energy. Ketonaemia can lead to metabolic acidosis and electrolyte shifts. 

In chemical oxidation or reduction the redox reagent and the substrate often form a covalent or ionic bond, for example, an ester in chromic acid oxidation [8], a sulfonium methylide in the Swern oxidation [9], cyclic esters in the svn dihydroxylation with OSO4 [10], or in the selenium dioxide oxidation of ketones and aldehydes [11]. In electrochemical processes the substrate must diffuse from the bulk of the solution to the electrode and compete there with other components of the electrolyte by competitive adsorption for a position at the electrode surface [12]. The next step is then generation of the reactive intermediate by electron transfer at the electrode that reacts with a low activation energy to the products. In chemical oxidations or reductions one finds a reductive or oxidative elimination of the intermediate with a higher activation energy. 

Electrolytic formation of carbon bonds during formation of heterocyclic compounds occurs in the reduction of ketones to pinacols, in the hydrodimerization reaction, in some radical coupling reactions, and in the oxidative coupling of activated aromatic systems. 

When aqueous solutions (buffered or unbuffered) are used (and their pH adjusted to a desired process), electrolytic reactions afford hydrogenations at cathodes and oxidations of the diverse functional groups at anodes. In other words, cathodic saturation of double bonds or carbonyl groups is, in principle, achievable under these conditions, while alcohols are anod-ically transformed into ketones or acids. Additionally, catalytic hydrogenations can also be performed in acidic electrolytes at... 

Catalytic supercritical water oxidation is an important class of solid-catalyzed reaction that utilizes advantageous solution properties of supercritical water (dielectric constant, electrolytic conductance, dissociation constant, hydrogen bonding) as well as the superior transport properties of the supercritical medium (viscosity, heat capacity, diffusion coefficient, and density). The most commonly encountered oxidation reaction carried out in supercritical water is the oxidation of alcohols, acetic acid, ammonia, benzene, benzoic acid, butanol, chlorophenol, dichlorobenzene, phenol, 2-propanol (catalyzed by metal oxide catalysts such as CuO/ZnO, Ti02, MnOz, KMn04, V2O5, and Cr203), 2,4-dichlorophenol, methyl ethyl ketone, and pyridine (catalyzed by supported noble metal catalysts such as supported platinum). ... 

Table II shows results for the electro-oxidation of secondary alcohols and ketones. In alkaline electrolyte, secondary butanol was not oxidized to methyl ethyl ketone but was cleaved to acetate. Similarly methyl ethyl ketone was cleaved to acetate, although some CO2 and propionate formed, indicative of cleavage on the other side of the carbonyl group. Butanediol (2 ) went to acetate yielding less CO2. At pH 9 in borax buffer 2 Trtanol went exclusively to methyl ethyl ketone at 89% conversion, suggesting that enolization in alkali is a necessary part of the cleavage process. Cyclohexanol and cyclohexanone were both converted to adipic acid. Figure 12 summarizes the various types of electro-organic oxidations, thus far discussed, which are observed to occur on lead ruthenate in alkaline electrolyte.

Yoshida and co-workers reported the oxidation of alcohols mediated by cross-linked poly(4-vinylpyridininium bromide) in the presence of a small amount of water (Figure 12.5) [16]. The electrolysis can be carried out without intentionally added electrolyte, and therefore ketone products are easily separated by simple filtration and the mediator can be recovered and re-used. This method has also been applied successfully to the oxidation of sulfides, epoxidation of olefins, and side-chain oxidation of alkylbenzenes. A similar polymeric system has also been reported by Zupan and Dolenc [17]. 

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