Kolbe electrolytic reaction

Kolbe electrolytic reaction " for the synthesis of alkanes also involves the radicals as intermediates. For example, when a solution of diphenylacetic acid (2.41) is electrolyzed in DMF (dimethylformamide), the product 2.42 is obtained in 24% yield. 

The reaction is likely to proceed by a radical-chain mechanism, involving intermediate formation of carboxyl radicals, as in the related Kolbe electrolytic synthesis. Initially the bromine reacts with the silver carboxylate 1 to give an acyl hypobromite species 3 together with insoluble silver bromide, which precipitates from the reaction mixture. The unstable acyl hypobromite decomposes by homolytic cleavage of the O-Br bond, to give a bromo radical and the carboxyl radical 4. The latter decomposes further to carbon dioxide and the alkyl radical 5, which subsequently reacts with hypobromite 3 to yield the alkyl bromide 2 and the new carboxyl radical 4Z... 

Suitable starting materials for the Kolbe electrolytic synthesis are aliphatic carboxylic acids that are not branched in a-position. With aryl carboxylic acids the reaction is not successful. Many functional groups are tolerated. The generation of the desired radical species is favored by a high concentration of the carboxylate salt as well as a high current density. Product distribution is further dependend on the anodic material, platinum is often used, as well as the solvent, the temperature and the pH of the solution." ... 

Scheme 7 Synthesis of iV -tert-Butoxycaibonyl-D-a-aminosuberic Acid a-re/7-Butyl Ester to-Methyl Ester by the Kolbe Electrolytic Decarboxylation Dimerization Reaction 271 NHBoc...

After Michael Faraday revealed the fundamental experiments of electrolytic reactions in 1834, Kolbe carried out the electrolysis of salts of monobasic aliphatic acids producing hydrocarbons, which was the first application of the method to organic synthesis (1854). It needed another hundred years before Wilson found that some acrylic ester derivatives were polymerized at the cathode instead of being reduced in... 

The new 23-hydroxy-epimers of cholesterol have been prepared by boro-hydride reduction of the 23-ketone, and Grignard reactions on the cyanohydrin of pregnenolone acetate have been used to prepare both epimers of 20a,22-dihydroxycholesterol. Using optically pure half-esters of methyl succinic acid in Kolbe electrolytic coupling reactions with various bile acids the corresponding 25-d- and 25-L-cholestanoic acids have been prepared. ... 

Koch-Haaf Carboxylations Kochi Reaction Koenigs-Knorr Synthesis Kolbe Electrolytic Synthesis Kolbe-Schmitt Reaction Komer-Contardi Reaction Kostanecki Acylation Krafft Degradation Krapcho Decarbalkoxylation... 

Kolbe Electrolytic Synthesis Crum Brown-Walker Reaction... 

Kolbe electrolysis is a powerful method of generating radicals for synthetic applications. These radicals can combine to symmetrical dimers (chap 4), to unsymmetrical coupling products (chap 5), or can be added to double bonds (chap 6) (Eq. 1, path a). The reaction is performed in the laboratory and in the technical scale. Depending on the reaction conditions (electrode material, pH of the electrolyte, current density, additives) and structural parameters of the carboxylates, the intermediate radical can be further oxidized to a carbocation (Eq. 1, path b). The cation can rearrange, undergo fragmentation and subsequently solvolyse or eliminate to products. This path is frequently called non-Kolbe electrolysis. In this way radical and carbenium-ion derived products can be obtained from a wide variety of carboxylic acids. 

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. 

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 ... 

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... 

The simplest design of electrochemical cell has two electrodes dipping into the solution containing the substrate and the supporting electrolyte. A cell of this type is suitable for the Kolbe oxidation of carboxylate ions (see p. 316) where the anode reaction is given by Equation 1.1 and the cathode reaction is the evolution of hydrogen (Equation 1.2). Both the substrate and the hydrocarbon product are inert... 

The second Kolbe reaction, proceeding via a carbocation, was found by Hofer and Moest (1902) to be promoted by the addition of an indifferent electrolyte [52]. Using a 1 2 molar solution of the carboxylate, addition of potassium hydrogen... 

It is therefore intriguing to understand what is the particular role of the platinum/electrolyte interface in the Kolbe synthesis favoring that reaction path—Eqs. (39a)-(39c)—which is thermodynamically disfavored and unlikely to occur. A closely related reaction whose kinetics are easier to investigate with conventional electrode kinetic methods is the anodically initiated addition of N3 radicals to olefins, discovered by Schafer and Alazrak (275). The consecutive reactions, which follow the initial generation of the reactive intermediate, an Na radical, are somewhat slower than that of the Kolbe radicals, so that their rate influences the shape and potential of the current voltage curves which can be evaluated in terms of reaction rates and rate laws. 

Several authors (23,24) have reported photoassisted reactions between carbon and water yielding some hydrogen and C02. Photo-Kolbe reactions of carboxylates have also been demonstrated (25). However, neither addition of acetate to 0.08M nor an unintentional gross contamination of the 10M NaOH electrolyte with charred epoxy residue caused significant acceleration of hydrogen production in our experiments. The presence of carbon monolayers on SrTi03 shows the need for caution in evaluating photoreactions where the total product yield is on the order of one monolayer or less. 

Despite these precautions, marked corrosion was still observed on some, but not all, of the n-SrTi03 photoanodes obtained from four different sources. The corrosion appeared to be most severe after several experiments (totalling typically 20 hours or more of use as an electrode) had been conducted under photo-Kolbe reaction conditions. A fine white film was also observed to form gradually on the irradiated areas of n-SrT103 when the acid electrolyte (typically 2N H2SO4) was used in the absence of added acetic acid. 

Where there is direct overlap with the valence band edge, the electron transfer process may be so facile as to give rise to the Hofer-Moest reaction (.2), in which the intermediate alkyl radical is itself oxidized (while it is still adsorbed to the electrode surface) to give a carbonium ion. The reaction of this carbonium ion with the aqueous electrolyte would then yield water-soluble products such as methanol, in keeping with our observation that anodic gas evolution is suppressed under these conditions. In acidic solutions, where the Kolbe reaction is energetically allowed, its kinetic competition with the other reactions on SrTiC>3 thus depends on the absence of defect surface states which are present in some electrode crystals and not in others. 

Many O-centered radicals undergo facile P-fragmentation. For example, acyl-oxyl radicals which are intermediates in the electrolytic oxidation of acids (Kolbe electrolysis), rapidly decompose into alkyl radicals and C02 [reaction (1)]. The rate of these reactions is in the order of 109 s 1 and increases with increasing branching of the alkyl substituent, i.e., decreasing C-C02 bond energy (Table 7.1). 

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