Kolbe electrolysis solvents

The yield and selectivity of Kolbe electrolysis is determined by the reaction conditions and the structure of the carboxylate. The latter subject is treated in chaps 3, 4. Experimental factors that influence the outcome of the Kolbe electrolysis are the current density, the temperature, the pH, additives, the solvent, and the electrode material. 

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. 

A mixture of water/pyridine appears to be the solvent of choice to aid carbenium ion formation [246]. In the Hofer-Moest reaction the formation of alcohols is optimized by adding alkali bicarbonates, sulfates [39] or perchlorates. In methanol solution the presence of a small amount of sodium perchlorate shifts the decarboxylation totally to the carbenium ion pathway [31]. The structure of the carboxylate can also support non-Kolbe electrolysis. By comparing the products of the electrolysis of different carboxylates with the ionization potentials of the corresponding radicals one can draw the conclusion that alkyl radicals with gas phase ionization potentials smaller than 8 e V should be oxidized to carbenium ions [8 c] in the course of Kolbe electrolysis. This gives some indication in which cases preferential carbenium ion formation or radical dimerization is to be expected. Thus a-alkyl, cycloalkyl [, ... 

Hydroxy-L-prolin is converted into a 2-methoxypyrrolidine. This can be used as a valuable chiral building block to prepare optically active 2-substituted pyrrolidines (2-allyl, 2-cyano, 2-phosphono) with different nucleophiles and employing TiQ as Lewis acid (Eq. 21) [286]. Using these latent A -acylimmonium cations (Eq. 22) [287] (Table 9, No. 31), 2-(pyrimidin-l-yl)-2-amino acids [288], and 5-fluorouracil derivatives [289] have been prepared. For the synthesis of p-lactams a 4-acetoxyazetidinone, prepared by non-Kolbe electrolysis of the corresponding 4-carboxy derivative (Eq. 23) [290], proved to be a valuable intermediate. 0-Benzoylated a-hydroxyacetic acids are decarboxylated in methanol to mixed acylals [291]. By reaction of the intermediate cation, with the carboxylic acid used as precursor, esters are obtained in acetonitrile (Eq. 24) [292] and surprisingly also in methanol as solvent (Table 9, No. 32). Hydroxy compounds are formed by decarboxylation in water or in dimethyl sulfoxide (Table 9, Nos. 34, 35). 

Non-Kolbe electrolysis of alicyclic p-hydroxy carboxylic acids offers interesting applications for the one-carbon ring extension of cyclic ketones (Eq. 35) [242c]. The starting compounds are easily available by Reformatsky reaction with cyclic ketones. Some examples are summarized in Table 13. Dimethylformamide as solvent and graphite as anode material appear to be optimal for this reaction. 

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)... 

Exercise 18-17 At higher voltages than normally used in the Kolbe electrolysis, salts of carboxylic acids in hydroxylic solvents produce (at the anode) alcohols and esters of the type ROH and RC02R. Explain how this can occur. 

Kolbe electrolysis also allows some comparisons with analogous homogeneous reactions with regard to dimerization, substitution, or addition reactions of the generated radicals. Photolytic or thermal decarboxylation of diacylperoxides is a source of alkyl radicals similar to those afforded by the Kolbe electrolysis. The anodic oxidation of propionate has been compared with the thermal decomposition of dipropionyl peroxide [28]. Examination of the yields shows that reaction between radicals is favored in the electrochemical process, whereas in peroxide decomposition hydrogen atom abstraction from the solvent or the substrate occurs to a higher extent. This illustrates the effect of the higher radical concentration at the electrode. 

The electrolysis products of different carboxylates have been compared with the ionization potentials of the intermediate radicals. From this it appeared that alkyl radicals with gas-phase ionization potentials smaller than 8 eV mainly lead to carbenium ions. Accordingly, a-substituents such as carboxy, cyano or hydrogen support the radical pathway, whilst alkyl, cycloalkyl, chloro, bromo, amino, alkoxy, hydroxy, acyloxy or aryl more or less favor the route to carbenium ions. Besides electronic effects, the oxidation seems also to be influenced by steric factors. Bulky substituents diminish the extent of coupling. The main experimental factors that affect the yield in the Kolbe electrolysis are the current density, the pH of the electrolyte, ionic additives, the solvent and the anode material. 

Methanol is the solvent of choice for the Kolbe electrolysis. The following electrolytes with methanol as solvent have been used methanol/sodium carboxylate, methanol/sodium methoxide/carboxylic acid, methanol/water/sodium hydroxide/carboxylic acid, methanol/triethylamine/pyridine/carbox-ylic acid. An increasing amount of water often decreases the yield of dimer. 

In the Kolbe electrolysis of acid salts in the usual solvents, water or methanol, the yield of dimer RR is limited by side reactions affording ethers, alcohols, esters, RH, and olefins. Yields are improved substantially by use of the nonreacting and highly polar solvent dimethylformamlde in combination with triethylamine as... 

Dimethylformamide is also a suitable solvent [50], it has, however, the disadvantage of being oxidized at fairly low potentials to A-acyloxy-iV-methyl formamide [51]. The influence of the composition of the ternary system water/methanol/dimethyl-formamide on the material and current yield has been systematically studied in the electrolysis of co-acetoxy or -acetamido substituted carboxylates [32]. Acetonitrile can also be used, when some water is.added [52]. The influence of various solvents on the ratio of Kolbe to non-Kolbe products is shown in Table 1 [53]. 

Electro-organic chemistry is the study of the oxidation and reduction of organic molecules and ions, dissolved in a suitable solvent, at an anode and cathode respectively in an electrolysis cell, and the subsequent reactions of the species so formed. The first experiment of this type was reported in 1849 by Kolbe, who described the electrolysis of an aqueous solution of a carboxylate salt and the isolation of a hydrocarbon. The initial step involves an anodic oxidation of the carboxylate anion to a radical which then dimerises to the alkane. 

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. 

The Fuchigami group have recently proposed a different type of two-phase system with the objective of making electrolysis more convenient and green . For this, they employed solid silica gel-supported piperidine in a methanol medium for the Kolbe reaction [48]. In the solvent, the piperidine protonates to give the medium some conductivity the medium then produces almost quantitative yields for the Kolbe dimer, for example ... 

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... 

A weakly acidic medium favors the Kolbe product. Therefore, the carboxylic acid is partially neutralized to the extent of 2-5 %. The concentration of the carboxylate anion remains constant during the whole electrolysis process since the base is regenerated at the cathode at the same rate as the carboxylate is consumed at the anode. While water has been used before, methanol or aqueous methanol is now the solvent of choice. Obviously, the selection of the best solvent rests mostly on experimental investigations. Temperature is usually not a critical... 

Decarboxylation reactions may be induced (depending on the acid) in a variety of ways thermally, bacteriologically, photochemically, or even by electrolysis as in the anodic reaction of the Kolbe synthesis. Thermally induced decarboxylation of many carboxylic acids in solution proceeds by a bimolecular mechanism involving addition of a nucleophilic solvent molecule to an electrophilic carbon atom on the root molecule - preferably at a carbon adjacent to (a) or one removed from (P) the carboxyl carbon (Fraenkel et al. 1954 Clark 1958, 1969). An electrophile-nucleophile pair is formed in the transition state, which subsequently undergoes heterolytic fission (i.e., decomposition of a molecule into two ions of opposite charge) to yield CO2, a proton, and a carbanion the latter two species are reactive intermediates, which then combine rapidly (Brown 1951). The solvent molecule departs unaffected and in this sense the solvent may be considered... 

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