Kolbe coupling

In aqueous solution an elevated pressure favors the Kolbe-coupling against non-Kolbe products [37]. A possible explanation is that high pressure aids the formation of a lipophilic medium at the electrode surface that prevents the adsorption of water and thus disfavours the formation of carbenium ions. 

The conventional synthesis of trans-2,5-dialkyl phospholanes starting from a chiral 1,4-diol is shown in Scheme 24.1. Originally, these 1,4-diols were obtained via electrochemical Kolbe coupling of single enantiomer a-hydroxy adds [25], but this method proved to be commercially impracticable and has since been replaced by more viable biocatalytic routes [26]. Reaction of the chiral 1,4-diol with thionyl chloride followed by ruthenium-catalyzed oxidation with so-... 

On the other hand, trifluoromethylation through crossed Kolbe coupling is also known (Scheme 7.5) [74]. 

These reactions are notable because a-branched carboxylic acids usually do not undergo efficient Kolbe coupling. Similarly, Kubota et al. have achieved highly efficient homo and crossed coupling reactions using trifluoromethylated carboxylic acids as shown in Scheme 7.7 [77,78]. Notably, the protection of the hydroxy group of the acids 12 is not necessary. 

Kolbe coupling of half esters Monomethyl adipate - Dimethyl sebacate Europe, India, Japan n/a Pt anode in methanol... 

The coupling reactions are in general limited to primary carboxylic acids (RCH2CO2H). a-Branched carboxylic acids lead to non-Kolbe products. However, carboxylic acid (74) with the electron-attracting trifluoromethyl group in the a-position yields the Kolbe coupling product (75) (Scheme 26) [99,100]. 

Experimental evidence indicates that the alkyl radical intermediates from the anodic oxidation of carboxylates are generated in free solution and have no memory of the configuration of the carboxyl group that was eliminated. Where the carboxylic acid function is attached to an asymmetric carbon atom as in 15, the Kolbe coupling reaction leads to complete racemization [67]. Anodic oxidation of (+)-2-... 

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... 

Alkene bonds more remote from the carboxyl function do not interfere wiA the normal Kolbe coupling reaction. An example of this point during a polyene synthesis is given by the formation of the insect pheromone 19 [100]. 

Becking and Schafer have shown that mixed Kolbe coupling reactions can provide useful yields (40-60%) of cyclic products.142 In the example provided in equation (4), 1 equiv. of acid (51) and 4 equiv. of acid (52) are electrochemically cooxidized, and the cyclic cross adduct (53) is formed in 53% yield. Because the rates of oxidation of (51) and (52) are similar, the concentration of radicals derived from (52) is higher. Thus, radicals derived from (51) are more likely to cross couple than to self couple. The strength of the mixed Kolbe method is that two carbon-carbon bonds are formed rather than one because the cyclic radical is removed by radical/radical coupling. 

The reaction principle of the Kolbe synthesis can be extended both to higher carboxylic acids (e.g. methyl suberate228)) and to the dimerization of two different carboxylic acids (cross Kolbe coupling). A few examples of syntheses studied on the laboratory scale are listed below. 

Entry to trans-2,5-disubstituted phospholanes 3 was first achieved conveniently through the use of chiral 1,4-diol intermediates 1 (Scheme 13.1).8 Originally, a series of 1,4-diols was prepared via electrochemical Kolbe coupling of enantiomerically pure a-hydroxy acids.9 Commercially, the Kolbe procedure was not practical and more attractive routes involving biocatalytic methodologies currently are used to produce the requisite 1,4-diols.10... 

Form Supplied in colorless oil commercially available. Analysis of Reagent Purity optical rotation NMR spectroscopy. Preparative Methods the preparation of (5, 5)-ethyl-DuPHOS is based on (3R,61 )-octane-3,6-diol as an enantiomerically pure starting compound. The latter is synthesized by a three-step procedure starting from methyl 3-oxopentanoate, which is transformed to methyl (l )-3-hydroxypentanoate (99% ee) by enantioselective hydrogenation with a Ru-(R)-BINAP catalyst, followed by hydrolysis to the hydroxy acid. The subsequent electrochemical Kolbe coupling reaction leads to (3R,6R)-octane-3,6-diol in a protocol that can be scaled up to multigram quantities (eq 1). ... 

Back in 1929 Fichter and Spiegelberg electrolyzed sodium methyl ci5-cyclopropane-1,2-dicarboxylate in MeOH and in H2O. In each case they obtained a low yield of an ester corresponding to the Kolbe-coupled product. Their suggested structural assignment for the ester was apparently wrong since others reinvestigated the reaction and found the ester to be dimethyl biscyclopropyl-2,2 -dicarboxylate (45), among other products. 

Again, in aqueous solution, the reaction proceeds on oxidized noble metal surfaces and, at the potentials at which it takes place, the reactant anion, RCOO , is strongly adsorbed. The R must be aliphatic at the a carbon as the Kolbe reaction does not proceed if, for example, benzoic acid is the reactant however, fi-, or y-aryl alkyl carboxylic acids, for example, phenylacetic acid, will undergo the Kolbe coupling reaction but with rather poor efficiency. The reaction will also proceed on nonoxidized noble metal surfaces, for example, Pt in anhydrous CH3COOH or CF3COOH, gettered with acetic anhydride... 

Some results on Kolbe couplings are listed in Table 3 [42,44-51]. The electrolysis of carboxylic acids is usually carried out in an alkaline protic solvent-(Pt electrodes) system. Most frequently, MeOH-MeONa, Me0H-H20-Na0H, MeOH-KOH, MeOH-Py-Et N, and MeOH-Py-Na salt systems are used as electrolysis media. Aprotic media, such as DMF-KOH, MeCN-Et3N, MeCN-Et4NBF4, and Py-H20-Et3N, are also utilized and, in particular cases, acetone-H20-NaOH is usable. 

Temperature has only a minor effect on the Kolbe electrolysis. In the Kolbe coupling, temperatures higher than 50 C should be avoided because they favor unwanted side reactions such as disproportionation or the esterification of the carboxylic acid. °... 

FIGURE 9.4 Homo- and cross- Kolbe-coupling of cyclic perfluorinated acids. 

Bull, S.D., Davies, S.G., Walton, D.J., and Ryley, S. (2001) Emulsion electrosynthesis in the presence of power ultrasound Biphasic Kolbe coupling processes at platinum and boron-doped diamond electrodes. J. Electroanal. 

C2F5. At platinum, trifluoro-acetate gave the Kolbe coupling reaction with formation of C2F5 but at only moderate efficiency with about 25% oxygen evolution. The coulombic efficiencies for the anodic decarboxylation of formate in anhydrous formic acid... 

Ketones from carboxylic acids by Kolbe coupling page 380... 

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