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48.1.2.3.2 Variation 2: Kolbe Electrolysis

DOI: 10.1055/sos-SD-048-00012

Kantchev, E. A. B.; Organ, M. G.Science of Synthesis, (20094885.

The oxidative dimerization of two molecules of alkyl carboxylate to yield alkanes 63 with concomitant loss of two molecules of carbon dioxide through anode oxidation is referred to as Kolbe electrolysis.[‌127‌,‌128‌] As the reaction only employs electricity (no external reagents required) and takes place in benign solvents (most often methanol or even water), it is well suited for large-scale and industrial applications. A drawback is the necessity to deprotonate the carboxylic acid so that the salts produced act as electrolytes. Often, the addition of an external electrolyte is also required. A recent development is the use of piperidine supported on silica gel as solid phase, a recyclable electrolyte that can be simply filtered off after the reaction (Table 20).[‌129‌] As the reaction takes place at the anode, the reduction of protons to hydrogen gas takes place at the cathode. If a platinum electrode is employed as the cathode, unwanted hydrogenation of double bonds present in the substrate could take place. To avoid this, a steel cathode should be used.[‌130‌] Kolbe electrolysis proceeds through one electron oxidation of the carboxylate to a carboxyl radical that loses carbon dioxide to produce an alkyl radical. Dimerization of two of these radicals then forms an alkane. Most likely, the radicals formed are free but as they are generated very close to the anode surface, dimerization is the dominant pathway as a consequence of high local concentration. Platinum anodes, low reaction temperature, high reactant concentration, and high current density maximize the local concentration of these radicals, leading to improved alkane yields.[‌129‌,‌131‌] Conducting the electrolysis under high-power ultrasonication conditions allows good results to be achieved with other electrode materials.[‌132‌] The dimerization of two identical carboxylic acids usually proceeds in good yields, and affords a direct route to products that would require multi-step syntheses if prepared by other methods.[‌129‌,‌130‌,‌133‌‌135‌] The electrolysis of two different carboxylic acids yields a mixture of homo and hetero dimers.[‌131‌,‌134‌,‌136‌‌139‌] Generally moderate to low yields and sometimes difficult to separate mixtures are often obtained in the mixed Kolbe electrolysis. The targeted hetero dimer can be obtained in higher yields if one of the carboxylic acids (e.g., acetic acid, propanoic acid, or tridecanoic acid) is inexpensive and can be used in excess (512 equivalents). Despite that, the reaction has great value when at least one of the starting carboxylic acids is readily available and inexpensive or the side products can be separated easily. Double bonds in the substrate situated in the proximity of the radical generated can undergo 5-exo radical cyclizations, leading to more complex products (e.g., 64 and 65, Scheme 22).[‌140‌]

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References