Aldehydes electrochemical oxidation

N.A. Anastasijevic, H. Baltruschat, and J. Heitbaum, On the hydrogen evolution during the electrochemical oxidation of aldehydes at lb metals, Electrochim. Acta 38(8), 1067-1072 (1993). 

Reactions of partial electrochemical oxidation are of considerable interest in the electrosynthesis of various organic compounds. Thus, at gold electrodes in acidic solutions, olefins can be oxidized to aldehydes, acids, oxides, and other compounds. A good deal of work was invested in the oxidation of aromatic compounds (benzene, anthracene, etc.) to the corresponding quinones. To this end, various mediating redox systems (e.g., the Ce /Ce system) are employed (see Section 13.6). 

The electrochemical oxidation is often more sensitive to the reaction conditions than to the substituents. Platinum electrodes are recommended for methoxylation and the equivalent acetoxylation procedures.290 In acetonitrile buffered by hydrogen carbonate ion, 3,4-diethylfuran affords the 2,5-dihydroxy-2,5-dihydro derivative (84%) and Jones oxidation readily leads to diethylmaleic anhydride in what is claimed to be the best general method for such conversions.291 In unbuffered methanol and under current density control, the oxidation of 2-methylfuran appears to eliminate the methyl group since the product is the acetal-ester 111 also obtained from methyl 2-furoate.292 If sodium acetate buffer is used, however, the methyl group is retained but oxidized in part to the aldehyde diacetate 112 in a... 

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. 

A similar electrochemical oxidation of aromatic aldehydes to their corresponding methyl esters was achieved by using flavo-thiazolio-cyclophane as a mediator [87]. 

Electrochemical oxidation of higher aldehydes (56) in the presence of secondary... 

The electrochemical oxidation of amines to imines and nitriles typically utilize a chemical mediator. The use of both Al-oxyl radicals [12, 13] and halogens has been reported for this process [14]. For example, the conversion of benzyl amine (14a) into nitrile (15a) and aldehyde (16a) has been accomplished using the M-oxyl radical of a decahydroquinoline ring skeleton as the mediator (Scheme 5). The use of acetonitrile as the solvent for the reaction generated the nitrile product. The addition of water to the reaction stopped this process by hydrolyzing the imine generated. A high yield of the aldehyde was obtained. In the case of a secondary amine, the aqueous... 

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 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]. 

Other organic mediators act as hydride atom-abstracting agents. This is true, for example, with 2,2-dichloro-5,6-dicyano-p-benzoquinone (DDQ) and the oxoammonium ion which is anodically accessible from 2,2,6,6-tetramethylpiperidyl oxide (TEMPO). DDQ has been electrochemically regenerated either externally or internally The in situ electrochemical oxidation, of TEMPO to the active oxoammonium ion is performed in lutidine-containing acetonitrile. Thus, primary alcohols can be oxidized to the aldehydes, while secondary ones are stable Primary amines are transformed to nitriles. If water is present, the amines are cleaved via the Schiff bases to the corresponding carbonyl compounds... 

The electrochemical oxidation of aromatic aldehydes (1) must be studied in strongly alkaline media. Acidity functions for strongly alkaline aqueous solutions of alkali metal and quaternary ammonium hydroxides, corresponding to dissociation of proton (H ), are well established (2, 3). Substituted anilines and diphenylamines (4,5) and indoles (6) were used as acid-base indicators for establishment of such scales, but whether an acidity scale based on one type of indicator can be rigorously applied to acid-base equilibria involving structurally different acidic groups for reactions in strongly alkaline media remains questionable. For substituted anilines, behavior both parallel (7) and nonparallel (8) to the H scale based on indole derivatives has been reported. The limited solubility of anilines in aqueous solutions of alkali metal hydroxides, the reactions of the aniline derivative with more than one hydroxide ion, irreversible substitution reactions (9), and the possibility of hydroxide ion addition rather than... 

Acetaldehyde undergoes electrochemical oxidation in methanolic ammonia to give 2,4,6-trimethyl-l,3,5-triazine in better than 20% yield (Scheme 91). The reaction works very poorly with aromatic aldehydes (76CB1346). 

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)]. 

If the acetic acid is replaced by methanol in the electrolyte, the electrochemical oxidation yields the corresponding benzaldehyde dimethyl acetals in excellent yields. This reaction which was discovered by BASF181), can also be used for the industrial synthesis of aromatic aldehydes. 

Since aldehydes can generally be converted to the corresponding carboxylic acids in very good yields and in a simple manner by catalytic air oxidation, electrochemical syntheses are not of much interest to the chemical industry. The controlled electrochemical oxidation of glyoxal to glyoxylic acid is an exception 279 280> ... 

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. 

An electrochemical oxidation route to tetrahydrofuran and tetrahydropyran rings has been described, in which a silyl-substituted enol ether reacts with a regioselectivity that is reversed from the normal polarity of enol ethers (Scheme 61) <2000JA5636>. Aldol reactions of a-diazo-/ -ketoesters with aldehydes produce adducts which undergo Rh(ii)-catalyzed 0-H insertion reactions to yield highly substituted tetrahydrofurans <1997TL3837>. 

Hydrocarbon oxidations are also possible at Pt electrodes at elevated temperatures, for example, 250°C in phosphoric acid (92). For aliphatic hydrocarbons it is of some special interest that electrochemical oxidations all the way to CO2 and H2O or H can be achieved at Pt (61). Oxidation of olefins is also possible, but under some conditions, for example, at Pd, aldehydes are a product (62, 93). The fact that aliphatic hydrocarbons can be oxidized largely to CO2 plus H2O indicates that the intermediate stages in such multielectron oxidations must proceed successively on the electrode surface with a series of intermediates remaining chemisorbed, as otherwise aldehydes or carboxylic acids would appear in solution, which is not normally observed. Interesting attempts were made by Bruckenstein (94) to identify some of the intermediates by reductive desorption from porous electrodes into a mass spectrometer. 

It can be prepd by the action of Ba peroxide on an ethereal soln of propionic aldehyde at ca 0°. Another method is electrochemical oxidation on a platinum anode of a soln of propionic acid or its salts Refs 1) Beil 2, 243, (108) 15401 2) A.M. 

By electrochemical oxidation of enamines, in situ prepared from an aldehyde and a secondary amine, using KI as mediator system, )6-ketoamines were formed in 30 to... 

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

The electrochemical oxidation of phenyl styryl sulfides in MeCN-2% HoO on a Pt plate electrode using a divided cell produces sulfanylated aldehydes in good yields determined by gas-liquid chromatography (GLC), as in Eq. (75) [128]. 

Some 4-substituted aromatic aldehydes, such as anisic aldehyde, toluic aldehyde, or t-butylbenzaldehyde, are produced commercially. The electrosynthesis can be accomplished by several different methods and is a good example of the versatility of selective electrochemical oxidations [91]. 

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