Alcohols electrochemical oxidation

As shown below, the basic principles of peroxidase-mimetic sensor appliance operation are developed using the example of model peroxidase reaction of ethyl alcohol electrochemical oxidation to aldehyde. 

Figure 8.1 summarizes the operation principle and the main mechanisms occurring at multiple scales in a DAFC alcohol and water transport in the anode (air and water in the cathode) at the macroscale (within the distributor), mesoscale (within the secondary pores formed by C in the electrodes) and microscale (within the primary pores of the C), nanoscale electrochemical double layer formation around the catalyst nanoparticles, alcohol electrochemical oxidation in the anode and ORR in the cathode. 

Polyquiaolines have been used as polymer supports for transition-metal cataly2ed reactions. The coordinatkig abiUty of polyqukioline ligands for specific transition metals has allowed thek use as catalysts ki hydroformylation reactions (99) and for the electrochemical oxidation of primary alcohols (100). 

Petoxycatboxyhc acids have been obtained from the hydrolysis of stable o2onides with catboxyhc acids, pethydtolysis of acyhinida2ohdes, reaction of ketenes with hydrogen peroxide, electrochemical oxidation of alcohols and catboxyhc acids, and oxidation of catboxyhc acids with oxygen in the presence of o2one (181). 

Ionic polysulfides dissolve only in media of high polarity hke water, liquid ammonia, alcohols, nitriles, amines, and similar solvents. In all of these solvents 8 can be reduced electrochemically to polysulfide anions. On the other hand, the electrochemical oxidation of polysulfide anions produces elemental sulfur ... 

It was seen when analyzing the kinetic data for alcohol oxidation reactions that the catalytic action of nickel oxide is due to a mediator mechanism. Higher oxide forms interact with the adsorbed organic species and oxidize them. In the following step the higher oxide forms are regenerated by electrochemical oxidation of lower oxide forms. 

Eberson and Olofsson, 4> observed exactly the same effect, and advanced the same rationale, in their study of the electrochemical oxidation of 5 in acetonitrile-water mixtures, to afford mixtures of pentamethylbenzyl alcohol (10) and the amide 9. 

Studies on the electrochemical oxidation of silyl-substituted ethers have uncovered a rich variety of synthetic application in recent years. Since acetals, the products of the anodic oxidation in the presence of alcohols, are readily hydrolyzed to carbonyl compounds, silyl-substituted ethers can be utilized as efficient precursors of carbonyl compounds. If we consider the synthetic application of the electrooxidation of silyl-substituted ethers, the first question which must be solved is how we synthesize ethers having a silyl group at the carbon adjacent to the oxygen. We can consider either the formation of the C-C bond (Scheme 15a) or the formation of the C-O bond (Scheme 15b). The formation of the C Si bond is also effective, but this method does not seem to be useful from a view point of organic synthesis because the required starting materials are carbonyl compounds. 

The potentiality of the present methodology is demonstrated by the synthesis of y-undecalactone, as shown in Scheme 18 [37,47], The treatment of the THP-protected cu-hydroxyalkyl iodide with the anion of methoxybis(trimethylsilyl) methane gave the corresponding alkylation product. Acidic deprotection of the hydroxyl group followed by Swern oxidation produced the aldehyde. The aldehyde was allowed to react with heptylmagnesium bromide, and the resulting alcohol was protected as tm-butyldimethylsilyl ether. The electrochemical oxidation in methanol followed by the treatment with fluoride ion afforded the y-undeealactone. 

Because the direct electrochemical oxidation of NAD(P)H has to take place at an anode potential of + 900 mV vs NHE or more, only rather oxidation-stable substrates can be transformed without loss of selectivity—thus limiting the applicability of this method. The electron transfer between NADH and the anode may be accellerated by the use of a mediator. At the same time, electrode fouling which is often observed in the anodic oxidation of NADH can be prevented. Synthetic applications have been described for the oxidation of 2-hexene-l-ol and 2-butanol to 2-hexenal and 2-butanone catalyzed by yeast alcohol dehydrogenase (YADH) and the alcohol dehydrogenase from Thermoanaerobium brockii (TBADH) repectively with indirect electrochemical... 

G. Palmisano, R. Ciriminna and M. Pagliaro, Waste-Free Electrochemical Oxidation of Alcohols in Water, Adv. Synth. Catal., 2006, 348, 2033. 

Fuels cells are of interest both from energetic and environmental considerations. When methanol is fed directly to an anode, as in Direct Methanol Fuel Cells , electric power is generated, making the devices suitable for small and lightweight uses [53], Alternative fuels such as polyhydric alcohols like ethylene glycol and glycerol are much less volatile and toxic, on the one hand, and electrochemically oxidizable on the other [54]. Therefore, the electrochemical oxidation of various polyhydric alcohols has been investigated in acidic as well as in alkaline conditions. 

The electrochemical oxidation of polyhydric alcohols, viz. ethylene glycol, glycerol, meso-erythritol, xilitol, on a platinum electrode show high reactivity in alkaline solutions of KOH and K2C03 [53]. This electro-oxidation shows structural effects, Pt(lll) being the most active orientation. This results from different adsorption interactions of glycerol with the crystal planes [59]. 

This article shows a variety of patterns of electrochemical oxidation of oxygen-containing compounds (alcohols, carbonyl compounds, and carboxylic acids), aiming to be helpful for both electroorganic and organic chemists to cover this field from a synthetic viewpoint. Since there have been excellent books [1-5] published on the subject, this article quotes only some typical and important papers from before 1990. 

Direct electrochemical oxidation is not a convenient way for a preparative production of carbonyl compounds from alcohols due to the unselectivity caused by the high oxidation potentials of alcohols. Thus, there have been only a few compounds (some aliphatic alcohols, glycols, and related alcohols) that have been oxidized by the direct method, while the indirect method has often been used to oxidize selectively a variety of alcohols, since it does not... 

The direct electrochemical oxidation of aliphatic alcohols (1) to carbonyl compounds (2) (Eq. 1) is not a convenient way for synthesis because of the high oxidation potentials of alcohols. The oxidation always competes with the oxidation of a solvent and supporting electrolyte, leading to low current efhdencies and side products. 

Higher alcohols, however, cannot be used as neat liquids in electrolysis. For anodic oxidation those alcohols must be dissolved in appropriate solvents. Acetonitrile is the most frequently used solvent for that purpose. Electrochemical oxidation of n-butyl alcohol to n-butyraldehyde was achieved in moderately dilute acetonitrile solution in a current yield of 77% [9]. 

Alcohols with ring strain can be oxidized by the direct method. Borneol (7) was electrochemically oxidized in MeOH with cleavage of the Ca Cp bond to afford (8) with an endojexo ratio of 97/3 in more than 70% yield at 5 F mol (Scheme 2) [11]. 

The direct electrochemical oxidation of alcohols is in many cases unselective because of the high oxidation potentials of the alcohols. One possible way to avoid this disadvantage is the use of a mediatory system (an indirect oxidation). Thereby a mediator is converted to its oxidized form at a less positive potential than that required for the direct oxidation of the alcohol, and the oxidized form of the... 

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... 

Another heterocyclization is presented by Panifilow et al. Cyclic acetals and ethers are obtained by electrochemical oxidation of the terpenoid alcohol linalool 57 in methanol containing alkaline and sodium methoxide as electrolyt [102]. Anodic oxidation of the C(6)-C 7) double bond of linalool leads to the radical cation 58. In addition to direct methoxylation of the radical cation an attack on the hydroxyl group takes place. After a second one-electron oxidation and following methoxylation the regioisomeric cyclic acetal and a subsequent 1,2-hydride shift, the cyclic acetal 60 and the cyclic ether 61 are finally formed in yields of 16 and 24%, respectively (Scheme 13). As shown by Utley and co-workers bicyclic lactones 65 and 66 can be synthesized by anodic oxidation... 

The complexes [Ru(bpy)2L]+ (HL = acetylacetone, trifluoroacetylacetone, hexafluoroacetylace-tone, tropolone or dibenzoylmethane) have been prepared and characterized they act as catalysts for the oxidation of alcohols, 3,5-di-tert-butylcatechol and alkanes in the presence of appropriate co-oxidants." Perchlorate salts of [Os(bpy)2L] in which HL = salicylaldehyde, 2-hydroxyaceto-phenone or 2-hydroxynaphthaldehyde, are formed from reactions of [Os(bpy)2Br2] with HL. The structure of the salicylaldehyde derivative has been determined. Chemical and electrochemical oxidations of [Os(bpy)2L]" yield the corresponding low-spin Os species from which [Os(bpy)2L]+ can be regenerated." " [Ru(bpy)2L]" (HL = salicylic acid) has been prepared and structurally characterized as the tetrahydrate. Absorptions at 590 nm, 400 nm, and 290 nm in the electronic spectrum have been assigned to Ru bpy CT transitions electrochemical oxidations of the complex have been investigated." ... 

Osmium(VI) hydrazido complexes can be generated by electrochemical oxidation of the corresponding osmium(V) hydrazido complexes (Section 5.6.5.3.1). The complex trans-[0s (tpy)(Cl)2(NN(CH2)40)] " (83) is able to oxidize benzyl alcohol to benzaldehyde. It also oxidizes PPhs to PPh30, and R2S to give R2SO the source of O atoms is presumably H2O in the solvent. 

In aqueous solution, cA-[Ru (0)(pyen)Cl] has also been generated electrochemically from cA-[Ru (pyen)Cl(OH2)] (see Figure 3 for structure of ligand). This Ru 0x0 complex has an ii°(Ru ) value of 1.29 V vs. SCE and it is an active catalyst for the electrochemical oxidation of alcohols and THF. The rate constants for the oxidation of PhCH20H, CH3OH, CH3OD, and... 

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]. 

The direct electrochemical oxidation of alcohols involves removal of one electron from a non-bonding pair on oxygen. Relatively anodic potentials are required and the use of reagents, which can provide another mechanism for the oxidation step, has been extensively explored. Electrochemistry is then involved in the reoxidation of spent reagent and often the system can be adapted so as to require only a catalytic amount of reagent. 

Electrochemically generated nickei(lll) oxide, deposited onto a nickel plate, is generally useful for the oxidation of alcohols in aqueous alkali [49]. The immersion of nickel in aqueous alkali results in the formation of a surface layer of nickel(ll) oxide which undergoes reversible electrochemical oxidation to form nickel(lll) oxide with a current maximum in cyclic voltammetry at 1.13 V vj. see, observed before the evolution of oxygen occurs [50]. This electrochemical step is fast and oxidation at a prepared oxide film, of an alcohol in solution, is governed by the rate of the chemical reaction between nickel oxide and the substrate [51]. When the film thickness is increased to about 0.1 pm, the oxidation rate of organic species increases to a rate that is fairly indifferent to further increases in the film thickness. This is probably due to an initial increase in the surface area of the electrode [52], In laboratory scale experiments, the nickel oxide electrode layer is prepared by prior electrolysis of nickel sulphate at a nickel anode [53]. It is used in an undivided cell with a stainless steel cathode and an alkaline electrolyte. 

Glycols are cleaved by electrochemical oxidation at a carbon electrode using potentials around +2.2 V vs. see. Reaction is carried out in methanol in an undivided cell. Secondary alcohol centres lead to the aldehyde dimethyl acetal while... 

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. 

Electrochemical oxidation of hydroxamic acids in the presence of amines, alcohols or water afforded the corresponding amides, esters or carboxylic acids (equation 19) . ... 

A pioneering electrochemical investigation was undertaken by Masui and coworkers . They showed that HPI is an efficient electron carrier (viz. mediator) in the electrochemical oxidation of alcohols to the corresponding carbonyl compounds. The anodic one-electron... 

Figure 3. Electrochemical oxidation of Poly B-411 in the presence of veratryl alcohol at various concentrations (mM). 1, 0 2, 0.1 3, 1.0 4, 10.

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