Kolbe reaction decarboxylation

A large variety of organic oxidations, reductions, and rearrangements show photocatalysis at interfaces, usually of a semiconductor. The subject has been reviewed [326,327] some specific examples are the photo-Kolbe reaction (decarboxylation of acetic acid) using Pt supported on anatase [328], the pho-... 

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]. 

The photo-Kolbe reaction is the decarboxylation of carboxylic acids at tow voltage under irradiation at semiconductor anodes (TiO ), that are partially doped with metals, e.g. platinum [343, 344]. On semiconductor powders the dominant product is a hydrocarbon by substitution of the carboxylate group for hydrogen (Eq. 41), whereas on an n-TiOj single crystal in the oxidation of acetic acid the formation of ethane besides methane could be observed [345, 346]. Dependent on the kind of semiconductor, the adsorbed metal, and the pH of the solution the extent of alkyl coupling versus reduction to the hydrocarbon can be controlled to some extent [346]. The intermediacy of alkyl radicals has been demonstrated by ESR-spectroscopy [347], that of the alkyl anion by deuterium incorporation [344]. With vicinal diacids the mono- or bisdecarboxylation can be controlled by the light flux [348]. Adipic acid yielded butane [349] with levulinic acid the products of decarboxylation, methyl ethyl-... 

Electrolysis of carboxylate ions, which results in decarboxylation and combination of the resulting radicals, is called the Kolbe reaction or the Kolbe electrosynthesis. 

Decarboxylation of p-lactones (see 17-27) may be regarded as a degenerate example of this reaction. Unsymmetrical diacyl peroxides RCO—OO—COR lose two molecules of CO2 when photolyzed in the solid state to give the product RR. Electrolysis was also used, but yields were lower. This is an alternative to the Kolbe reaction (11-37). See also 17-29 and 17-40. 

A variety of photocatalyzed decarboxylation reactions on Ti02 powder including the decomposition of acetate to methane and carbon dioxide and the breakdown of benzoic acid yielding predominantly CO2 have been reported by Bard and coworkers (23,24). Evidence for the occurrence of these "photo-Kolbe" reactions has stimulated the search for other organic reactions that might be photochemically initiated by excitation of semiconductors and extensive work in this area is in progress (25). 

The radical produced from the oxidative decarboxylation may also be trapped intramolecularly to form five- and six-membered rings (Scheme 17). The Kolbe protocol avoids the use of the toxic organ-otin reagents that are commonly used in the formation of radicals. Moreover, when alkyltin hydride reagents are used, a C—H bond is formed. The Kolbe reaction protocol, on the other hand, allows the radical formed after cyclization to be captured by a different radical in a coelectrolysis experiment, rather than being reduced. This tandem sequence of events has been exploited in the construction of prostaglandin precursor (70) [37-41]. Here, the cyclized... 

This reaction resembles decarboxylation of carboxylates during electrode one-electron oxidation (Kolbe reaction). Kolbe reaction also consists of one-electron oxidation, decarboxylation, and culminates in dimerization of alkyl radicals just after their formation at the electrode surface. When the sulfate radical acts as a one-electron oxidant, the caboradical dimerization is hampered. The radicals can be used in preparative procedures. One typical example is alkylation of heterocyclic nitrogen bases (Minisci et al. 1983). This difference between Kolbe reaction and the reaction with the help of a dissolved electrode (the sulfate radical) deserves some explanation. The concentration of the one-electron oxidation products in the electrode vicinity is significantly higher than that in the bulk of the solution. Therefore, in the case of anode-impelled reactions, the dimerization of radicals produced from carboxylates proceeds easily. Noticeably, 864 secures the single electron nature of oxidation more strictly than an anode. In electrode reactions, radical intermediates can... 

Electron donating a-substituents favour the non-Kolbe reaction but the radical intermediates in these anodic processes can be trapped during co-electrolysis with an alkanoic acid. Anodic decarboxylation of sugar uronic acids leads to formation of the radical which is very rapidly oxidised to a carbonium ion, stabilised by the adjacent ether group. However, in the presence of a tenfold excess of an alkanoic acid, the radical intermediate is trapped as the unsymmetrical coupling product [101]. Highly functionalised nucleotide derivatives such as 20 will couple successfully in the mixed Kolbe reaction [102], Other examples include the co-electrolysis of 3-oxa-alkanoic acids with an alkanoic acid [103] and the formation of 3-alkylindoles from indole-3-propanoic acid [104], Anodic oxidation of indole-3-propanoic acid alone gives no Kolbe dimer [105],... 

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...

Decarboxylative Dimerization. The Kolbe Reaction De-carboxylide-coupling... 

Electrolysis of carboxylate ions, which results in decarboxylation and combination of the resulting radicals, is called the Kolbe reaction. 30 It is used to prepare symmetrical RR, where R is straight- or branched-chained, except that little or no yield is obtained when there is a branching. The reaction is not successful for R = aryl. Many functional groups... 

Kolbe reactions of /V-heterocyclic compounds have been studied in only a few cases. It appears that the oxidative decarboxylation involves the removal of electrons through the heterocyclic 7i-system and is of the pseudo... 

Tetrahydroisoquinoline-l-carboxylic acids have been anodically decarboxylated in MeOH-NaOMe on a graphite felt anode, giving 3,4-di-hydroisoquinolines (50-90%)417 This may be an example of a pseudo-Kolbe reaction in support of Hahn s theory of the biosynthesis of isoquinoline alkaloids by providing a laboratory analogy for the crucial decarboxylation step. 

This chapter does not include electrochemical decarboxylation processes (the Kolbe reaction. Volume 3, Chapter 2.9) and transition metal catalyzed decarbonylation reactions. 

The mechanism of the Kolbe reaction involves electrochemical decarboxylation-dimerization via radicals (Scheme 2.34). 

The decarboxylation of acids may take place by both radical and ionic processes. Radical processes involving the electrolytic discharge of a carboxylate anion (the Kolbe reaction ) may give rise to dimeric products. 

Brown and Walker first proposed the generally accepted mechanism of the Kolbe reaction, which involves the initial discharge of carboxylates at the anode followed by decarboxylation and subsequent combination of the resulting radicals, leading to the Kolbe dimer [3]. The radical formed may also undergo disproportionation to afford olefins and alkanes as the result of hydrogen abstraction [Eq. (6)]. Actually, olefins and alkanes are found as by-products. 

Experimental variables affecting the course of the electrolytic decarboxylation of carboxylic acids are summarized in Table 2. For the Kolbe dimerization, the conditions specified for a one-electron process are recommended otherwise the reaction through carbenium ion (non-Kolbe reaction) may occur predominantly. It should be emphasized that even under the conditions most favorable for the Kolbe dimerization, the cation-derived products are usually formed to some extent or, in particular cases, as a major product, depending on the structure of the employed carboxylic acid. 

A -Acylated amino acids are anodically oxidized in methanol or acetic acid solution under decarboxylative methoxylation or acetoxylation via the intermediate A-acyliminium ion in the course of a Non-Kolbe reaction (Hofer-Moest reaction) according to Scheme 8, path b. This type of reaction has been used intensively for amidoalkylation reactions by Mori, Seebach, and Steckhan. These reactions were based on the results of Iwasaki applying N-acyl aminomalonic acid half esters [Eq. (46)] [239]. 

Kolbe reactions of heterocyclic compounds have been studied in only a few cases. Anodic oxidation of l-azabicyclo[2.2.2]octane-2-carboxylic acid under Kolbe conditions produced 2-methoxy-l-azabicyclo[2.2.2]octane [454]. The primary radical, formed by loss of an electron from the carboxylate ion, decarboxylates and is oxidized further to a carboca-tion, which is attacked by a methoxide ion. A similar pseudo-Kolbe reaction is found in the anodic decarboxylation of 1,2,3,4-tetrahydroisoquinoline-l-carboxylic acid derivatives to 3,4-dihydroisoquinolines [455]. 

Another source of single electrons is the anode of an electrolytic cell. In the Kolbe reaction, an alkylcarboxylate salt, RC02, is decarboxylated and the resulting alkyl radicals couple to form the dialkyl product, R-R. Suggest the pathway that is followed by this reaction. 

This is a good method of synthesising symmetrical alkanes. The Kolbe reaction is usually performed using the potassium or sodium salt of the carboxylic acid. If the silver salt is reacted with bromine, then decarboxylation occurs again, but this time the alkyl bromide is formed. This reaction is called the Hunsdiecker reaction. The first step involves the formation of an acyl hypohalite, RC02X. 

Kolbe reaction The electrolytic decarboxylative dimerisation of the potassium salts of two carboxylate anions, used to prepare symmetrical R-R alicyclic compounds from RC02 K+. 

Decarboxylation of silver carboxylates is a weU known thermal process and is involved in the Hunsdiecker or Kolbe reactions. The Hunsdiecker reaction is the thermal decarboxylation of silver salts of acids and is used for the formation of bromoalkanes and related compounds, while the Kolbe process involves electrolysis of carboxylates as a route to decarboxylated radicals that can dimerize. Silver carboxylates are also photochemically reactive and the irradiation has been described as a facile process for the formation of alkyl radicals, as illustrated in equation 6. Later experimentation has shown that the irradiation of silver trifluoroacetate can serve as a route to trifluoromethyl radicals. This development uses irradiation of silver trifluoroacetate in the presence of titanium dioxide as a photocatalyst. The reaction follows the usual path with the formation of metallic silver and the formation of radicals. However, in this instance the formation of metallic... 

A classical and long-established electrochemical synthesis is the formation of alkanes (R-R) from the dimerisation of radicals generated via electro-decarboxylation of carboxylate salts known as the Kolbe reaction. The radical species are produced through a single electron process and... 

Oxidative decarboxylation, as in the Kolbe reaction, is one of the oldest of all electrochemical oxidations. Whether such reactions parallel any reactions that occur in nature is still in question. Actually, the reaction can take two courses loss of an electron from a carboxylate and decarboxylation to form a radical which dimerizes or reacts with another radical, or loss of an electron followed by decarboxylation and loss of a second electron to form a carbocation (The Hofer-Moest reaction, 33). The carbocation may then be neutralized by reaction with nucleophile or another source of electrons. 

Efficient electron transfer from axial ligand to the porphyrin chromophore was observed in azafenocene-coordinated Co porphyrin [372] (Fig. 30). Electron transfer from axial ligand carboxylate to the excited Fe" porphyrin induced a Kolbe-type decarboxylation reaction [787]. 

The earliest electrochemical synthesis is the so-called Kolbe reaction involving the oxidation of carboxylic acids in forming decarboxylated coupling products (alkanes). At present, the electrochemical synthesis has become an independent discipline. A large number of organic reactions (synthesis) have been achieved by this technique. The essential requirement for conducting an electrochemical reaction is the conductivity of the reaction medium. The most commonly... 

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... 

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