Ketones electrochemical oxidation

Despite the fact that the electrochemical oxidation of most of the nonconjugated dienes generally does not give products which result from interaction of the double bonds with one another, the anodic oxidation l-acetoxy-l,6-heptadienes gives intramolecularly cyclized products, that is, the cyclohexenyl ketones (equation 15)13. The cyclization takes place through the electrophilic attack of the cation generated from enol ester moiety to the double bond. 

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]. 

Electrochemical oxidation of easily accessible alkyl aryl ketones (58) in trimethyl orthoformate containing a small amount of iodine or organo-iodo compounds gave methyl a-alkylalkanoates (59) in high yields (Scheme 22) [92]. The products (59) are precursors for drugs possessing antiinflammatory and analgesic activities. 

Benzyl alcohol can be smoothly converted to benzaldehyde by electrochemi-cally recycled BrO as an oxidizing catalyst in an emulsion system prepared from a mixture of water, amyl acetate, and 2% BU4NHSO4 [60]. A different hypobromite reagent is provided by an electrochemical oxidation of a cross-linked poly-4-vinylpyridine in an MeCN-H20-HBr-(Pt) system [61]. Secondary alcohols can be oxidized by this method to give ketones in high yields. The electrooxidation of A-monoalkyltosylamides (42) in a two-phase system consisting of cyclohexane... 

The oxidation of propargyl alcohol to the acid and of but-2-yne-l,4-diol to acetylene dicarboxylic acid is carried out on a technical scale at a lead dioxide anode in sulphuric acid [4, 5]. Electrochemical oxidation of acetylenic secondary alcohols to the ketone at lead dioxide in aqueous sulphuric acid [4], gives better results than the cliromic acid based process of Jones [6], Oxidation of aminoalkan-1-ols to the amino acid at a lead dioxide anode in sulphuric acid is achieved in 31 -73 % 5delds [7]. This route is applied to the technical scale production of (l-alanine from 3-aminopropanol in an undivided cell [8]. 

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]. 

Open chain ketone phenylhydrazones take part in a number of cyclization reactions to form heterocyclic rings. Many of the electrochemical reactions describe can also be achieved using chemical oxidising agents. In general, electrochemical oxidation proceeds to the two-electron level to generate an electrophilic species... 

The molecule has been found to be an efficient electron carrier in electrochemical oxidation, converting secondary alcohols to ketones. Daicel of Tokyo has used NHPI in the development of custom production in proprietary air-oxidation technology , and it can also be used to oxidize cyclohexane to adipic acid and p-xylene to p-toluic acid in the presence of Mn + or Co + salts. The new process produces no nitrogen oxides, is more environmentally friendly and does not require the use of denitration equipment. 

Usually, electrochemical oxidative hydrolysis of ketoximes affords the corresponding ketones. Hence, 2-octanone (equation 13) and acetophenone were obtained from the corresponding ketoximes in 90% and 97% yields, respectively. However, camphor oxime was transformed into the ring-cleaved nitrile (equation 14). ... 

The salen-Cu complex 5a was shown to oxidize a selected number of secondary alcohols (e.g. l-phenylethanol) to the corresponding ketones, with a wider range of primary alcohols being further oxidized to the analogous carboxylic acids, in the presence of 5-15 equiv. of H2O2 as oxidant, while molecular oxygen proved inefficient as oxidant [152], The derivative 5b has been reported to catalyze the electrochemical oxidation of primary alcohols (but not secondary alcohols) into the corresponding aldehydes, with turnovers > 30 [153]. 

The efficient formation of diaryliodo-nium salts during the electrolysis of arylio-dides has been reported by Peacock and Fletcher [166]. The electroiodination of a 3D-aromatic molecule, dodecahydro-7,8-dicarba-nido-undecaborate has also been reported [167]. The iodination (and bromi-nation) of dimedone has been reported to yield 2-iododimedone, which formally is an electrophilic substitution reaction [123]. In a similar process, the indirect electrochemical oxidation of aliphatic ketones in an alkaline Nal/NaOH solution environment has been shown to yield a,a-diiodoketones, which rapidly rearrange to give unsaturated conjugated esters [168]. Dibenzoylmethane has been converted into dibenzoyliodomethane [169]. Terminal acetylenes have been iodinated in the presence of Nal. However, this process was proposed to proceed via oxidation of the acetylene [170]. 

The direct electrochemical oxidation of aliphatic alcohols occurs at potentials which are much more positive than 2.0 V w. SCE. Therefore, the indirect electrolysis plays a very important role in this case. Using KI or NaBr as redox catalysts those oxidations can be performed already at 0.6 V vs. SCE. Primary alcohols are transformed to esters while secondary alcohols yield ketones In the case of KI, the iodo cation is supposed to be the active species. Using the polymer bound mediator poly-4-vinyl-pyridine hydrobromide, it is possible to oxidize secondary hydroxyl groups selectively in the presence of primary ones (Table 4, No. 40) The double mediator system RuOJCU, already mentioned above (Eq. (29)), can also be used effectively Another double mediator system... 

Related photochemistry has also been examined with other functional groups such as phthalimides, which also abstract nearby hydrogens with the photoexcited carbonyl group. - Furthermore, since the hydrogen abstraction is performed by the half-vacant nonbonding orbital of a photoexcited ketone carbonyl, related chemistry is observed if the electron is removed electrochemically, not just photoexcited into a Tr -orbital. Electrochemical functionalization of nearby carbons has been reported in which, after hydrogen atom abstraction by an oxidized ketone, the resulting radical is electrochemically oxidized further to the carbon cation, which reacts with solvent (Scheme 7). ... 

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... 

A third possibility of chemical modification is conversion into an acylsilane which reduces the oxidation potential of the corresponding ketone by approximately 1 V. A peak potential of 1.45 V (relative to Ag/AgCl) for the oxidation of undecanoyltrimethylsilane has been reported. Preparative electrochemical oxidations of acylsilanes proceed in methanol to give the corresponding methyl esters. A two-step oxidation process must be assumed because of the reaction stoichiometry —oxidation of the acylsilane results in the carbonyl radical cation which is meso-lytically cleaved to give the silyl cation and the acyl radical, which is subsequently oxidized to give the acyl cation as ultimate electrophile which reacts with the solvent. A variety of other nucleophiles have been used and a series of carboxylic acid derivatives are available via this pathway (Scheme 49) [198]. 

Electrocatalysis is manifested when it is found that the electrochemical rate constant, for an electrode process, standardized with respect to some reference potential (often the thermodynamic reversible potential for the same process) depends on the chemical nature of the electrode metal, the physical state of the electrode surface, the crystal orientation of single-crystal surfaces, or, for example, alloying effects. Also, the reaction mechanism and selectivity 4) may be found to be dependent on the above factors in special cases, for a given reactant, even the reaction pathway [4), for instance, in electrochemical reduction of ketones or alkyl halides, or electrochemical oxidation of aliphatic acids (the Kolbe and Hofer-Moest reactions), may depend on those factors. 

Electrochemical oxidation of bis(4-chlorophenyl)diselenide in MeCN-H20 containing 3-hydroxyalkynes affords o -arylseleno-o, j6-unsaturated aldehydes or ketones [64] ... 

Amino-3-(4 -nitrophenyl)propan-l,3-diol in its optically active form has been selectively photooxidised to the corresponding ketone,and a study of the oxidation of podophyllotoxin (27) by sodium persulfate using laser flash photolysis has enabled rate constants to be determined for the formation and decay of transients. Photoinduced electrochemical oxidation of benzyl alcohol to benz-aldehyde has been achieved with 100% product selectivity and 100% current efficiency using visible radiation in the presence of riboflavin 2, 3, 4, 5 -tetraace-tate. ... 

Oxidation. The system is best km ketones (Wacker oxidation). It has been e glycosides directly or after photolysis of droisobenzofuran is oxidized to phthalidc An electrochemical version of the Vk tris(4-bromophenyl)amine as mediator im... 

The electrochemical oxidation of 2-furyl-2-thienylmethane in methanol resulted in loss of aromaticity of the furan ring and gave 70 (R = 2-thienyl). The reaction took a different course with 2,2 -dithienylmethane, which oxidized at the methylene group to give methoxybis(2-thienyl)methane (71) and di-2-thienyl ketone.106... 

Incineration is cited exclusively as a method of destruction, applicable to neat compounds or waste solvents. Other thermal methods, such as molten metal salt treatment, which involves intimate contact with a molten salt, such as AI2O3 (Shultz 1985), are suitable. Chemical processes that may be effective are wet air oxidation, electrochemical oxidation, and catalytic destruction. Ketones in aqueous wastes can be altered to innocuous gases by heating at 300-460°C (572-860°F) and 150-400 atm pressure with or without catalyst. Ni and Fc203 were found to be effective catalysts in such thermal treatments (Baker and Sealock 1988). 

Indirect electrochemical oxidative carbonylation with a palladium catalyst converts alkynes, carbon monoxide and methanol to substituted dimethyl maleate esters (81). Indirect electrochemical oxidation of dienes can be accomplished with the palladium-hydroquinone system (82). Olefins, ketones and alkylaromatics have been oxidized electrochemically using a Ru(IV) oxidant (83, 84). Indirect electrooxidation of alkylbenzenes can be carried out with cobalt, iron, cerium or manganese ions as the mediator (85). Metalloporphyrins and metal salen complexes have been used as mediators for the oxidation of alkanes and alkenes by oxygen (86-90). Reduction of oxygen and the metalloporphyrin generates an oxoporphyrin that converts an alkene into an epoxide. 

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