Reactions Kolbe synthesis

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." ... 

Stabilization of intermediates by strong adsorption will frequently be a necessary precondition for synthesis. Thus, in the case of the Kolbe reaction, further oxidation of the radicals is prevented the formation of metal-carbon bonds in the reduction of alkyl halides (Fleischmann et al., 1971a Galli and Olivani, 1970) or oxidation of Grignard reagents (Fleischmann et al., 1972c) is shown by the isolation of organometallic... 

Faraday, in 1834, was the first to encounter Kolbe-electrolysis, when he studied the electrolysis of an aqueous acetate solution [1], However, it was Kolbe, in 1849, who recognized the reaction and applied it to the synthesis of a number of hydrocarbons [2]. Thereby the name of the reaction originated. Later on Wurtz demonstrated that unsymmetrical coupling products could be prepared by coelectrolysis of two different alkanoates [3]. Difficulties in the coupling of dicarboxylic acids were overcome by Crum-Brown and Walker, when they electrolysed the half esters of the diacids instead [4]. This way a simple route to useful long chain l,n-dicarboxylic acids was developed. In some cases the Kolbe dimerization failed and alkenes, alcohols or esters became the main products. The formation of alcohols by anodic oxidation of carboxylates in water was called the Hofer-Moest reaction [5]. Further applications and limitations were afterwards foimd by Fichter [6]. Weedon extensively applied the Kolbe reaction to the synthesis of rare fatty acids and similar natural products [7]. Later on key features of the mechanism were worked out by Eberson [8] and Utley [9] from the point of view of organic chemists and by Conway [10] from the point of view of a physical chemist. In Germany [11], Russia [12], and Japan [13] Kolbe electrolysis of adipic halfesters has been scaled up to a technical process. 

In this way, the operational range of the Kolbe-Schmitt synthesis using resorcinol with water as solvent to give 2,4-dihydroxy benzoic acid was extended by about 120°C to 220°C, as compared to a standard batch protocol under reflux conditions (100°C) [18], The yields were at best close to 40% (160°C 40 bar 500 ml h 56 s) at full conversion, which approaches good practice in a laboratory-scale flask. Compared to the latter, the 120°C-higher microreactor operation results in a 130-fold decrease in reaction time and a 440-fold increase in space-time yield. The use of still higher temperatures, however, is limited by the increasing decarboxylation of the product, which was monitored at various residence times (t). 

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... 

Using the high-p,T microreactor processing, the Kolbe-Schmitt synthesis was completed within less than 1 min at comparable yields, i.e., a reaction time reduced by a factor of approximately 2,000 was achieved (see Fig. 6). This corresponds to an increase in space-time yield by a factor of 440. 

The hydroquinone process was developed by BASF [12]. Hydroquinone-2,5-di-carboxylic acid is prepared by a modified Kolbe-Schmidt synthesis from hydroquinone and carbon dioxide. Subsequent reaction with arylamine in an aqueous-methanolic suspension in the presence of an aqueous sodium chlorate solution and a vanadium salt affords the product in good yield ... 

Since, in Kolbe s synthesis, as here described, the mono-sodium salicylate reacts to some extent with unchanged sodium phenoxide, producing the di-sodium salt, part of the phenol is liberated and excluded from the reaction. The reaction proceeds to completion if the sodium phenoxide is heated to about 150° for a long time, with carbon dioxide under pressure in the autoclave. This is the technical method of Schmitt. 

Sandmeyer s synthesis of aromatic nitriles is far more elegant than the removal of water from the ammonium salts of carboxylic acids, which latter reaction is also applicable to benzene derivatives. In particular, the former synthesis permits of the preparation of carboxylic acids via the nitriles, and so provides a complete substitute for Kolbe s synthesis (alkyl halide and potassium cyanide), which is inapplicable to aromatic compounds. The simplest example is the conversion of aniline into benzoic add. The converse transformation is Hofmann s degradation (benzamide aniline, see p. 152). 

Scheme 34 1,2-Rearrangement induced by the Non-Kolbe reaction as key step for a muscone synthesis.

In the past few years, however, very efficient new methods of cyclisation proceeding via radical intermediates have been developed and several reviews [19a] and a comprehensive book by Giese [19b] have been published. Rather than reactions involving the dimerisation of two radicals -as in the Kolbe electrochemical synthesis [20] or the radical induced dehydrodimerisation developed by Viehe [21]-more important are the reactions between a radical with a non-radical species. The advantage of this type of reaction is that the radical character is not destroyed during the reaction and a chain-reaction may be induced by working with catalytic amounts of a radical initiator. However, in order to be successful two conditions must be met i) The selectivities of the radicals involved in the chain-reaction must differ from each other, and ii) the reaction between radicals and non-radicals must be faster than radical combination reactions. 

Kolbe hydrocarbon synthesis orgchem The production of an alkane by the electrolysis of a water-soluble salt of a carboxylic acid. kol-bo. hT-dro kar-bon, sin-th3-s3s Kolbe-Schmitt synthesis org chem The reaction of carbon dioxide with sodium phenoxide at 125°C to give salicyclic acid. kol-bo shmit, sin-th3-s3s Konowaioff ruie phys chem An empirical rule which states that in the vapor over a liquid mixture there is a higher proportion of that component which, when added to the liquid, raises its vapor pressure, than of other components., k6-n9 va-lof, rul ... 

Today the coupled product is described as being formed by union of two alkyl radicals fonned by loss of one electron and carbon dioxide from the carboxylate ion. Extensive early use of the Kolbe reaction was made for the synthesis of long chain a,co-dicarboxylate esters starting from the half esters of shorter chain a,03-diacids [49]. 

Nonetheless, a number of electroorganic synthesis reactions are known whose outcome i.e., whose yield and selectivity, is decisively determined by the nature of the electrode so that heterogeneous acceleration of at least one of several competitive reactions of the electrogenerated reactive intermediates might be anticipated. A famous case is the Kolbe reaction, which is essentially the anodic dimerization of alkyl radicals that are generated at platinum anodes by anodic oxidation of the anions of carboxylic acids ... 

A new stereoselective synthesis of 1,2,3-trisubstituted cyclopentanes based on the Wag-ner-Meerwein rearrangement of a 7-oxabicyclo[2.2.1]heptyl 2-cation starts with the Diels-Alder product of maleic anhydride and a furan (78TL2165, 79TL1691). The cycloadduct was hydrogenated and subjected to methanolysis. The half acid ester (47) was then electrolyzed at 0 °C to generate a cationic intermediate via the abnormal Kolbe reaction (Hofer-Moest reaction). Work-up under the usual conditions provided the 2-oxabicyclo[2.2.1]heptane (48) in 83% yield. Treatment of this compound in turn with perchloric acid effected hydrolysis of the ketal with formation of the trisubstituted cyclopentane (49) in nearly quantitative yield (Scheme 11). Cyclopentanes available from this route constitute useful... 

Reaction Parameters and Mechanistic Studies of the Kolbe-Schmitt Synthesis... 

Tommasi et al. recently reported the carboxylation of l,3-dialkyHmidazoHum-2-carboxylates, which can be seen as a remarkable variant of the Kolbe-Schmitt synthesis [27]. In this case, it was shown that l,3-dialkyBmidazolium-2-carboxylates could be synthesized from 1,3-diaIkylimidazolium chlorides and C02 via a Kolbe-Schmitt-type reaction (actually, this was more of a Marasse variant, run in solution). The starting compounds were carboxylated in anhydrous dimeth-ylformamide (DMF) under approximately 5 MPa C02, at temperatures ranging from 353 to 408 K and with Na2C03/C02 as a catalyst, according to Scheme 5.4. 

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