Carboxylic acids anodic oxidation

Faraday, in 1834, was the first to encounter Kolbe-electrolysis, when he studied the electrolysis of an aqueous acetate solution [1], However, it was Kolbe, in 1849, who recognized the reaction and applied it to the synthesis of a number of hydrocarbons [2]. Thereby the name of the reaction originated. Later on Wurtz demonstrated that unsymmetrical coupling products could be prepared by coelectrolysis of two different alkanoates [3]. Difficulties in the coupling of dicarboxylic acids were overcome by Crum-Brown and Walker, when they electrolysed the half esters of the diacids instead [4]. This way a simple route to useful long chain l,n-dicarboxylic acids was developed. In some cases the Kolbe dimerization failed and alkenes, alcohols or esters became the main products. The formation of alcohols by anodic oxidation of carboxylates in water was called the Hofer-Moest reaction [5]. Further applications and limitations were afterwards foimd by Fichter [6]. Weedon extensively applied the Kolbe reaction to the synthesis of rare fatty acids and similar natural products [7]. Later on key features of the mechanism were worked out by Eberson [8] and Utley [9] from the point of view of organic chemists and by Conway [10] from the point of view of a physical chemist. In Germany [11], Russia [12], and Japan [13] Kolbe electrolysis of adipic halfesters has been scaled up to a technical process. 

Hydroxy- and amino carboxylic acids can be dimerized in good to moderate yields, when the substituents are not in the a- or P-position and when they are additionally protected against oxidation by acylation (Table 2, No. 17-19). 2-Alkenoic acids cannot be dimerized but lead to more or less extensive passivation of the anode due to the formation of polymer films [136]. 3- and 4-Alkenoic acids give moderate yields when they are neutraUzed with BU3N or EtjN [136]. 3-Alkenoic acids with the structure... 

The photo-Kolbe reaction is the decarboxylation of carboxylic acids at tow voltage under irradiation at semiconductor anodes (TiO ), that are partially doped with metals, e.g. platinum [343, 344]. On semiconductor powders the dominant product is a hydrocarbon by substitution of the carboxylate group for hydrogen (Eq. 41), whereas on an n-TiOj single crystal in the oxidation of acetic acid the formation of ethane besides methane could be observed [345, 346]. Dependent on the kind of semiconductor, the adsorbed metal, and the pH of the solution the extent of alkyl coupling versus reduction to the hydrocarbon can be controlled to some extent [346]. The intermediacy of alkyl radicals has been demonstrated by ESR-spectroscopy [347], that of the alkyl anion by deuterium incorporation [344]. With vicinal diacids the mono- or bisdecarboxylation can be controlled by the light flux [348]. Adipic acid yielded butane [349] with levulinic acid the products of decarboxylation, methyl ethyl-... 

Anodic Oxidation of Carboxylic Acids Without Decarboxylation... 

Carboxylic acids can be converted by anodic oxidation into radicals and/or carbo-cations. The procedure is simple, an undivided beaker-type cell to perform the reaction, current control, and usually methanol as solvent is sufficient. A scale up is fairly easy and the yields are generally good. The pathway towards either radicals or carbocations can be efficiently controlled by the reaction conditions (electrode material, solvent, additives) and the structure of the carboxylic acids. A broad variety of starting compounds is easily and inexpensively available from natural and petrochemical sources, or by highly developed procedures for the synthesis of carboxylic acids. 

Photoluminescence (PL) of anodic aluminum oxides was first investigated in films formed in organic acids, the most intense PL being in those formed in oxalic acid. Tajima, in his comprehensive review315 on electro- and photoluminescence in anodic oxide films, concluded that PL centers are carboxylate anions incorporated into the oxide. On the other hand, Eidel berg and Tseitina316 proposed... 

Various nucleophiles other than methanol can be introduced onto the carbonyl carbon. Anodic oxidation of acylsilanes in the presence of allyl alcohol, 2-methyl-2-propanol, water, and methyl /V-methylcarbamate in dichlorometh-ane affords the corresponding esters, carboxylic acid, and amide derivatives (Scheme 24) [16]. Therefore, anodic oxidation provides a useful method for the synthesis of esters and amides under neutral conditions. 

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... 

For the dehydrogenation of CH—XH structures, for example, of alcohols to ketones, of aldehydes to carboxylic acids, or of amines to nitriles, there is a wealth of anodic reactions available, such as the nickel hydroxide electrode [126], indirect electrolysis [127, 128] (Chapter 15) with I , NO, thioanisole [129, 130], or RUO2/CP [131]. Likewise, selective chemical oxidations (Cr(VI), Mn02, MnOJ, DMSO/AC2O, Ag20/Celite , and 02/Pt) [94] are available for that purpose. The advantages of the electrochemical conversion are a lower price, an easier scale-up, and reduced problems of pollution. 

In fluorosulfonic acid the anodic oxidation of cyclohexane in the presence of different acids (RCO2H) leads to a single product with a rearranged carbon skeleton, a 1-acyl-2-methyl-1-cyclopentene (1) in 50 to 60% yield (Eq. 2) [7, 8]. Also other alkanes have been converted at a smooth platinum anode into the corresponding a,-unsaturated ketones in 42 to 71% yield (Table 1) [8, 9]. Product formation is proposed to occur by oxidation of the hydrocarbon to a carbocation (Eq. 1 and Scheme 1) that rearranges and gets deprotonated to an alkene, which subsequently reacts with an acylium cation from the carboxylic acid to afford the a-unsaturated ketone (1) (Eq. 2) [8-10]. In the absence of acetic acid, for example, in fluorosulfonic acid/sodium... 

The anode is an ideal reagent to oxidize organic substrates such as oxygen-containing compounds (alcohols, carbonyl compounds, and carboxylic acids). Thereby these substrates can be converted avoiding chemical reagents, which simplifies the reaction conditions and the work-up. Additionally, the electron transfer leads selectively to a variety of reactive species, which can find further use in organic synthesis. 

Scheme 19 Anodic oxidation of aldehydes to carboxylic acids at the nickel oxide electrode.

Depending on the anode material used, the oxidation of a carboxylic acid can lead to a carbenium ion [R ], which is trapped by a nucleophile (NuH) to afford a one-carbon-shortened product (Scheme 33) [112]. 

While anodic amide oxidations have found the most synthetic use to date, the oxidation of nitrogen-containing molecules is not limited to amide substrates. A variety of amine oxidations have been studied, and the Kolbe electrolysis of carboxylic acids has been used to generate nitrogen-based reactive intermediates. Many of these reactions also offer unique synthetic advantages (Sects. 10.2 and 10.3). 

One of the best-known anodic oxidation reactions is still the Kolbe electrolysis [38]. These reactions typically involve the decarboxylation of a carboxylic acid and... 

Recently, Ohmori and coworkers have used an anodic oxidation reaction to promote the reduction of an acid [34]. In this experiment, the anodic oxidation of triphenyl- or tributylphosphine in the presence of a carboxylic acid led to the formation of an acyl phosphonium ion. The acyl phosphonium ion was then reduced at the cathode to form an ylide which then trapped a second carbonyl... 

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