Kolbe reaction effects

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 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]. 

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... 

A new stereoselective synthesis of 1,2,3-trisubstituted cyclopentanes based on the Wag-ner-Meerwein rearrangement of a 7-oxabicyclo[2.2.1]heptyl 2-cation starts with the Diels-Alder product of maleic anhydride and a furan (78TL2165, 79TL1691). The cycloadduct was hydrogenated and subjected to methanolysis. The half acid ester (47) was then electrolyzed at 0 °C to generate a cationic intermediate via the abnormal Kolbe reaction (Hofer-Moest reaction). Work-up under the usual conditions provided the 2-oxabicyclo[2.2.1]heptane (48) in 83% yield. Treatment of this compound in turn with perchloric acid effected hydrolysis of the ketal with formation of the trisubstituted cyclopentane (49) in nearly quantitative yield (Scheme 11). Cyclopentanes available from this route constitute useful... 

Figure 12.14 shows the effect of ultiasound on the amount of the main products of the Photo-Kolbe reaction. The reaction appears to have been accelerated by ultrasonic irradiation. The product ratio of the sonophotocatalytic reaction, however, was not satisfactory. A reasonable value of methane (CH4) to carbon dioxide (C02) must be 1.0 for Photo-Kolbe reaction, as shown in Eq. (12.14). 

Some additional insight as to possible mechanisms of the photocorrosion process can be gained from a more detailed consideration of the effects of pH on the band levels in SrTiC>3 and on the redox potentials of oxygen formation and the photo-Kolbe reaction. These data, along with the band levels for Ti02, are shown in Figure 5. It is important to remember that the photocorrosion process occurs in com-... 

The effect of concentration gradients in electrode reactions is really not a problem of mechanism but rather a troublesome source of possible systematic ambiguity in the interpretation of the product distributions observed, one of the tasks that lies close to the heart of the organic chemist. To see how this comes about, it is instructive to make the mental experiment that we generate acetoxy radicals by the Kolbe reaction of acetate ion in acetic acid [eqn (52)] at an electrode of 1 cm2 surface area, passing a current of 1 A during... 

Orientation effects by the electrode surface have been invoked in a large number of other cases (for reviews of the stereochemistry of electrode processes, see Eberson and Homer, 1973 Fry, 1972a), for example in halide reduction (for a review, see Casanova and Eberson, 1973), formation of dimers in the Kolbe reaction (Hawkes et al., 1973), reduction of systems with double and triple bonds (Horner and Roder, 1969), and anodic coupling of 1-methylcorypalline... 

Anodic reactions at Pt have been claimed to be dependent upon the surface state of the platinum. The Kolbe reaction is perhaps the best known case (for a review, see Conway and Vijh, 1967) for which a change in the surface composition has been held responsible and indeed necessary for the reaction to occur. Thus, at a low potential, < 0-8 V, acetate in aqueous solution is completely oxidized to carbon dioxide and water on pure platinum sites (i.e. we have in effect a fuel cell electrode). On raising the potential, PtO and adsorbed oxygen begin to cover the surface and oxygen evolution takes place in the range between 1-2- 1-8 V. A further increase in the... 

Formic acid and oxalic acid are green sacrificial reagents because they are easily oxidized and converted into CO2, which is separated from the solvent under acidic conditions. The photocatalytic effect in the presence of formic acid can be attributed to the photo-Kolbe reaction, originally evidenced by Kraeutler and Bard (1978) in the presence of various carboxylic acids ... 

The principal theories of the mechanism of the Kolbe electrosynthesis, as discussed earlier, have been criticized. A satisfactory summary of these has been presented by Dickinson and Wynne-Jones. The main points of disagreement among these theories are, (a) whether the hydroxyl ion or the carboxylate ion is discharged at the anode, (b) the explanation of the effect of the electrode material on the reaction, and (c) the reason for the occurrence of the Kolbe synthesis in preference to oxygen evolution which could occur at lower anode potentials. Recent experimental investigations, particularly those of Dickinson and Wynne-Jones and of Conway and Dzieciuch permit a detailed analysis of the mechanism of the Kolbe reaction, in both aqueous and nonaqueous media. 

Electrode modification can be carried out by methods that vary greatly. A reaction can be affected simply by addition to the electrolysis solution of a substance that is readily adsorbed onto the electrode surface. Thus, additimi of a thiocyanate salt to the medium diverts the anodic oxidatimi of carboxylates frran decarboxylative dimerization (Kolbe reaction) to peracid formation [1]. Often, a polymer solutimi containing an electrocatalyst is placed on a surface, and the solvent evaporated or a monomer is electrochemicaUy polymerized in situ from solution mito the surface. Electrocatalysts deposited in this manner include organometallic electrocatalyst complexes such as vitamin B12 [2], oxidizable heterocycles such as pyrrole or thiophene, or metal ions [3]. Successive layers of complementary materials may be laid down on an electrode to achieve the desired immobilization effect. Thus, a polymer (PDAA polydimethyldiallyl ammonium chloride) bearing... 

Temperature has some effect on Kolbe electrolysis. Higher temperatures seem to support disproportionation against the coupling reaction and intramolecular additions... 

In fl-trimethylsilylcarboxylic acids the non-Kolbe electrolysis is favored as the carbocation is stabilized by the p-effect of the silyl group. Attack of methanol at the silyl group subsequently leads in a regioselective elimination to the double bond (Eq. 29) [307, 308]. This reaction has been used for the construction of 1,4-cyclohexa-dienes. At first Diels-Alder adducts are prepared from dienes and P-trimethylsilyl-acrylic acid as acetylene-equivalent, this is then followed by decarboxylation-desilyl-ation (Eq. 30) [308]. Some examples are summarized in Table 11, Nos. 12-13. 

The study of electrosynthetic reactions is not a new phenomenon. Such reactions have been the study of investigation for more than a century and a half since Faraday first noted the evolution of ethane from the electrolysis of aqueous acetate solutions. This reaction is more well known as the Kolbe electrolysis [51]. Since the report of Kolbe, chemists have had to wait nearly a century until the development, in the 1960 s, of organic solvents with high-dielectric which have been able to vastly increase the scope of systems that could be studied [52]. Added to this more recently is the synergistic effect that ultrasound should be able to offer in the improvement of the expected reactions by virtue of its ability to clean of surfaces, form fresh surfaces and improve mass transport (which may involve different kinetic and thermodynamic requirements)... 

The presence of -OH group In phenols activates the aromatic ring towards electrophilic substitution and directs the Incoming group to ortho and para positions due to resonance effect. Reimer-Tiemann reaction of phenol 5delds sallcylaldehyde. In presence of sodium hydroxide, phenol generates phenoxlde Ion which Is even more reactive than phenol. Thus, In alkaline medium, phenol undergoes Kolbe s reaction. 

Reaction between a siloxycyclopropane and Cu(BF3)2 in ether gives a product due to symmetrical coupling of two homoenolate moieties (Eq. 53, Table 12) [51]. This is particularly noteworthy as a simple route to 1,6-ketones superior to classical approaches such as the Kolbe electrolysis [52], Several lines of evidence suggest the intermediacy of Cu(II) homoenolates. AgBF3 and CuF2 effect the same reaction albeit with lower yields. The reactions with cupric halides give... 

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