Kolbe reaction relation

The most important examples from both a fundamental and practical point of view are cathodic evolution from acidic or alkaline water, anodic evolution of O2 from similar solutions, and anodic Clj evolution from Cr ion in melts or in solution. Other related examples are anodic generation of Br2, I2, and (CN)2 from solutions of the corresponding anions, and an interesting case is the Kolbe reaction arising from discharge and decomposition of car-boxylate anions, followed by recombinative coupling of the resulting alkyl radicals. These processes intimately involve chemisorbed intermediates and... 

In the Kolbe reaction and in anodic carboxylation reactions, salts of carboxylic acids function as both substrate and electrolyte. Usually the presence of other anions diminishes the yield of the Kolbe reaction, whereas the presence of anions that are oxidizable with difficulty, like bicarbonate or perchlorate ions, favors the related Hofer-Moest reaction. 

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... 

Today, environmental concerns have impelled regained interest in this area of chemistry, and the Kolbe and related reactions are gradually being rejuvenated. It is the purpose of this brief entry to summarize some of the most important achievements in this electrochemical process. Of course, in view of its brevity, only key features of this transformation can be presented. For the interested reader, a few pertinent reviews are mentioned in the literature section [9-12]. 

Kolbe and Related Reactions, Scheme 1 The Kolbe reaction... 

Electrosynthesis of Fine Chemicals - Kolbe and Related Reactions... 

The electrochemical oxidation mechanism of carboxylates is closely related to the Kolbe reaction." " The mechanism (Fig. 16.12)," consists of a first oxidation leading to the formation of RCOO type radical species which upon decarboxylation will lead to the formation of R. The rate of the decarboxylation process is... 

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... 

The applicability of the Reimer-Tiemann reaction is limited to the formylation of phenols and certain reactive heterocycles like pyrroles and indoles. Yields are usually below 50%. In contrast to other formylation procedures, the Reimer-Tiemann reaction is ort/zo-selective it is therefore related to the Kolb e-Schmitt reaction. 

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. 

The Kolbe-Schmitt reaction is related to enzymatic carboxylations as of D-ribulose 1,5-diphosphate with carbon dioxide, a key step in photosynthesis (Section 20-9). The overall result is C—C bond formation by addition of C02 to an enolate salt or its enamine equivalent. 

H. Kolb aptly noted, nature does not limit itself with performing variations on four themes water, hydrogen, hydrogen chloride, and ammonia . In fact, the number of types had to be later considerably increased, so that the theory had lost much of its initial simplicity and clearness. Moreover, it could not cover many facts related to the addition and elimination reactions and to isomerism. 

The Kolbe-Schmitt reaction[l] has long history related with aspirin and has been a name reaction used for the longest period in an industrial process. While the demand for the manufacturing aromatic hydroxycarboxyhc acids is still successively coming out today with a number of patents, the mechanism of the reaction has remained unsolved. The present nmr spectroscopic studies have proved a [substrate CO2] complex or an intermediate prior to the formation of carboxyhc acids. Another puzzling question about the unstable complex even to moisture, is why the carboxylation of polyhydroxybenzenes, such as resorcinol, should proceed in aqueous solutions. Herein also reported are kinetic studies on the carboxylation of resorcinol in aqueous solutions of alkali hydrogencarbonates. 

The classical Kolbe-Schmidt reaction treats alkali metal phenoxides and carbon dioxide at higher than atmospheric pressure, giving salicylic acid. Hirao and Kato developed several modifications for industrial production ". Recently, phenol phosphate was enzymatically carboxylated, giving p-hydroxybenzoic acid ". As for related reactions, Sartori and coworkers conducted o-carbamoylation of aluminum or boron phenoxides with alkyl isocyanate ", and Adachi and Sugasawa o-cyanated phenols using methyl thioisocyanate in the presence of BCI3 (equation 54). ... 

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