The Kolbe Reaction

The Kolbe reaction is usually carried out by allowing sodium phenoxide to absorb carbon dioxide and then heating the product to 125°C under a pressure of several atmospheres of carbon dioxide. The unstable intermediate undergoes a proton shift (a keto-enol tau-tomerization see Section 18.2) that leads to sodium salicylate. Subsequent acidification of the mixture produces salicylic acid. 

Reaction of salicylic acid with acetic anhydride yields the widely used pain reliever 

Another member of the family is daunomycin. Both of these antibiotics are produced in strains of Streptomyces bacteria by a pathway called polyketide biosynthesis. 

Isotopic labeling experiments have shown that dauno-mycin is synthesized in Streptomyces galilaeus from a tetracyclic precursor called akiavinone. Akiavinone, in turn, is synthesized from acetate. When S. galilaeus is grown in a medium containing acetate labeled with carbon-13 and oxy-gen-18, the akiavinone produced has isotopic labels in the positions indicated below. Notice that oxygen atoms occur at alternate carbons in several places around the structure. 

This and other information show that nine C2 units from malonyl-coenzyme A and one C3 unit from propionyl-coen-zyme A condense to form the linear polyketide intermediate shown below. These units are joined by acylation reactions that are the biosynthetic equivalent of the malonic ester synthesis we studied in Section 18.7. These reactions 

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Participation of adsorbed intermediates can also be shown by the prolonged decay of the potential 011 interruption of the current (Conway and Vijh, 1967a) or by measurement of the time-dependence of the formation of products by carrying out the reaction with pulses of potential of controlled duration (Fleischmann et al., 1966). Thus the formation of ethane in the Kolbe reaction of acetate ions in acid solutions is initially proportional to the square of time as would be predicted for the rate of the step (27) (Fleischmann et al., 1965). 

Stabilization of intermediates by strong adsorption will frequently be a necessary precondition for synthesis. Thus, in the case of the Kolbe reaction, further oxidation of the radicals is prevented the formation of metal-carbon bonds in the reduction of alkyl halides (Fleischmann et al., 1971a Galli and Olivani, 1970) or oxidation of Grignard reagents (Fleischmann et al., 1972c) is shown by the isolation of organometallic... 

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. 

Foreign cations can increasingly lower the yield in the order Fe, Co " < Ca " < Mn < Pb " [22]. This is possibly due to the formation of oxide layers at the anode [42], Alkali and alkaline earth metal ions, alkylammonium ions and also zinc or nickel cations do not effect the Kolbe reaction [40] and are therefore the counterions of choice in preparative applications. Methanol is the best suited solvent for Kolbe electrolysis [7, 43]. Its oxidation is extensively inhibited by the formation of the carboxylate layer. The following electrolytes with methanol as solvent have been used MeOH-sodium carboxylate [44], MeOH—MeONa [45, 46], MeOH—NaOH [47], MeOH—EtsN-pyridine [48]. The yield of the Kolbe dimer decreases in media that contain more than 4% water. 

The nature of the cathode material is not critical in the Kolbe reaction. The reduction of protons from the carboxylic acid is the main process, so that the electrolysis can normally be conducted in an undivided cell. For substrates with double or triple bonds, however, a platinum cathode should be avoided, as cathodic hydrogenation can occur there. A steel cathode should be used, instead. 

The current-potential relationship indicates that the rate determining step for the Kolbe reaction in aqueous solution is most probably an irreversible 1 e-transfer to the carboxylate with simultaneous bond breaking leading to the alkyl radical and carbon dioxide [8]. However, also other rate determining steps have been proposed [10]. When the acyloxy radical is assumed as intermediate it would be very shortlived and decompose with a half life of t 10" to carbon dioxide and an alkyl radical [89]. From the thermochemical data it has been concluded that the rate of carbon dioxide elimination effects the product distribution. Olefin formation is assumed to be due to reaction of the carboxylate radical with the alkyl radical and the higher olefin ratio for propionate and butyrate is argued to be the result of the slower decarboxylation of these carboxylates [90]. 

Electrolysis of carboxylate ions, which results in decarboxylation and combination of the resulting radicals, is called the Kolbe reaction or the Kolbe electrosynthesis. 

Decarboxylation of p-lactones (see 17-27) may be regarded as a degenerate example of this reaction. Unsymmetrical diacyl peroxides RCO—OO—COR lose two molecules of CO2 when photolyzed in the solid state to give the product RR. Electrolysis was also used, but yields were lower. This is an alternative to the Kolbe reaction (11-37). See also 17-29 and 17-40. 

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 ... 

The anions of substituted acids will not always react according to this scheme. Some of them, particularly the a-substituted acids, form unsaturated compounds. More complex reactions also occur when dicarboxylate ions COOCRCOO are used. When one of the carboxyl groups in the dicarboxylic acid is esterilied, the Kolbe reaction proceeds without difficulty ... 

M. Faraday was the first to observe an electrocatalytic process, in 1834, when he discovered that a new compound, ethane, is formed in the electrolysis of alkali metal acetates (this is probably the first example of electrochemical synthesis). This process was later named the Kolbe reaction, as Kolbe discovered in 1849 that this is a general phenomenon for fatty acids (except for formic acid) and their salts at higher concentrations. If these electrolytes are electrolysed with a platinum or irridium anode, oxygen evolution ceases in the potential interval between +2.1 and +2.2 V and a hydrocarbon is formed according to the equation... 

As an example we may consider the Kolbe reaction, the oxidation of carboxylic acid and carboxylates of the form R-COOH or R-COO- to form coupled hydrocarbon products of the form R2. Investigation of this reaction in aqueous and non-aqueous solvents has revealed that the processes taking place are very complex indeed. In general, the product R2 is only formed at high current densities on smooth electrodes. At lower current densities, alkenes and non-dimeric products such as R-H are found, and, especially in alkaline solutions, the product R-OH can be formed in good... 

One example of the application of in situ electrochemical epr concerns the study of the Kolbe reaction. As was discussed in section 1.3, the Kolbe reaction involves some extremely complex processes and considerable effort has been expended in the search for the identities of the radical intermediates. Evidence for such intermediates remains sparse but one system that has provided such evidence is the electro-oxidation of triphenyl acetic acid (TPA) at a platinum electrode in acetonitrile (Waller and Compton, 1989). The case history of epr in the study of this system is a very good example of the application of the technique to provide details of a reaction mechanism. In... 

The initial steps of the Kolbe reaction, the oldest organic electrochemical reaction, constitute a good illustration of the loss of an acid moiety upon oxidative electron transfer (Scheme 2.24). The issue of the stepwise versus concerted character of the electron transfer/bond-breaking process in this reaction is discussed in Chapter 3. 

Ti02, with a band gap of 3.2 eV, was successfully used for the photooxidation of acetate ion in acetic acid, a photochemical version of the Kolbe reaction (Kraeutler et al., 1978). The main products formed were methane and carbon dioxide, in addition to small amounts of ethane. The latter is the major product... 

There are also some reactions known, which need - contrary to the normal case - a high current density for a sufficient selectivity, for example, the Kolbe reaction (see Chapter 6). 

The Kolbe reaction is the oldest electroorganic reaction, discovered in the middle of the 19th century [93], but is... 

In order to obtain the coupling product (62) in good yield, factors such as anode material, solvent, pH, and structure of the substrate are critical. In general, the following conditions are recommended for the Kolbe reaction solvent MeOH containing... 

The radical produced from the oxidative decarboxylation may also be trapped intramolecularly to form five- and six-membered rings (Scheme 17). The Kolbe protocol avoids the use of the toxic organ-otin reagents that are commonly used in the formation of radicals. Moreover, when alkyltin hydride reagents are used, a C—H bond is formed. The Kolbe reaction protocol, on the other hand, allows the radical formed after cyclization to be captured by a different radical in a coelectrolysis experiment, rather than being reduced. This tandem sequence of events has been exploited in the construction of prostaglandin precursor (70) [37-41]. Here, the cyclized... 

Radicals prepared by anodic oxidation of anions or by the Kolbe reactions can couple with other radicals or add to double bonds. For instance in Scheme 2 [4, 5], the... 

Today the coupled product is described as being formed by union of two alkyl radicals fonned by loss of one electron and carbon dioxide from the carboxylate ion. Extensive early use of the Kolbe reaction was made for the synthesis of long chain a,co-dicarboxylate esters starting from the half esters of shorter chain a,03-diacids [49]. 

In many cases both Kolbe and non-Kolbe products are isolated from a reaction. Carboxylic acids with an a-alkyl substituent show a pronounced dual behaviour. In these cases, an increase in the acid concentration improves the yield of the Kolbe product. An example of the effect of increased substrate concentration is given in Kolbe s classical paper [47] where 2-methylbutyric acid in high concentration affords mostly a dimethylbexane whereas more recent workers [64], using more dilute solutions, obtained both this hydrocarbon and butan-2-ol. Some quantitative data is available (Table 9.2) for the products from oxidation of cyclohexanecar-boxylic acids to show the extent of Kolbe versus non-Kolbe reactions. The range of products is here increased through hydrogen atom abstraction by radical intermediates in the Kolbe reaction, which leads to some of the monomer hydrocarbon... 

The Kolbe reaction is earned out in an undivided cell with closely spaced platinum electrodes. Early examples used a concentrated, up to 50 %, aqueous solution of an alkali metal salt of the carboxylic acid and the solution became strongly alkaline due to hydrogen evolution at the cathode. Ingenious cells were devised with a renewing mercury cathode, which allowed removal of alkali metal amalgam. These experimental conditions have been replaced by the use of a solution of the carboxylic acid in methanol partially neutralised by sodium methoxide or trieth-... 

A -Enecarboxylic acids do not undergo the Kolbe coupling reaction. A -Ene-carboxylic acids give poor 3uelds in the Kolbe reaction and a mixture of products... 

Protected sugar derived acids, having no a-hydroxy substituent, have also been used in die mixed Kolbe reaction [106]. The Kolbe reaction is used to graft short functionalised alkyl chains onto poly(acryIic acid) [107],... 

The pyrrole anion also undergoes the Kolbe carboxylation reaction The Kolbe reaction is best... 

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