Silver carboxylates, decarboxylation

Silver carboxylates 1 can be decarboxylated by treatment with bromine, to yield alkyl bromides 2 in the so-called Hunsdiecker reaction. ... 

Involvement of radical pathways in thermal decarboxylation is possible where no solvent or a nonpolar solvent is used. The pyrolysis of silver carboxylates in the absence of solvent under nitrogen gives radicals but no organometallics (21). 

When colloidal selenium was heated with mercuric trifluoroacetate or silver trifluoroacetate, bis(trifluoromethyl)diselenide was formed (43). Later work with selenium/silver carboxylate, RC02Ag (R = CF3, C2F5, or C3F7), mixtures at 280° C in a vacuum produced a mixture of the bis(perfluoroalkyl)selenide and the bis(perfluoroalkyl)diselenide (44). Formation of a polyselenium trifluoroacetate, which decarboxylates to produce the trifluoromethylselenides, was the proposed mechanism for R = CF3 (44). However, silver trifluoroacetate is a source of trifluoromethyl radicals when heated above 260° C (21), hence the trifluoromethylselenides may be formed by reaction of trifluoromethyl radicals with selenium, as in the reaction of CF3I with selenium [Eq. (34)] (45). 

The usual sources used for the homolytic aromatic arylation have been utilized also in the heterocyclic series. They are essentially azo- and diazocompounds, aroyl peroxides, and sometimes pyrolysis and photolysis of a variety of aryl derivatives. Most of these radical sources have been described in the previous review concerning this subject, and in other reviews concerning the general aspects of homolytic aromatic arylation. A new source of aryl radicals is the silver-catalyzed decarboxylation of carboxylic acids by peroxydisulfate, which allows to work in aqueous solution of protonated heteroaromatic bases, as for the alkyl radicals. 

Acyl hypohalites are usually prepared in situ by reaction of a metal salt of die carboxylic acid widi a halogen (equation 3). Classically the silver salt is used, but problems associated widi die preparation of dry silver carboxylates, as well as the more obvious economic factor, have led to the development of methods using mercury and thallium salts. Evidently, those functional groups which react readily widi halogens are not compatible with this approach. A major limitation of the acyl hypohalites is the readiness with which they transfer halogen atoms to alkyl radicals this property essend ly limits their use to decarboxylative halogenation reactions. 

The silver-catalyzed decarboxylation of a-oxo acids (carboxylic acids " ) by peroxy-disulfate leads to acyl " (alkyl radicals, which can effect selective homolytic acylation (alkylation of quinoxaline. This procedure is effective in monoacylation when multiple positions of high nucleophilic reactivity are available in the heterocyclic ring. " ... 

Decarboxylation of silver carboxylates is a well known thermal process and is involved in the Hunsdiecker76 or Kolbe77 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 678. 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... 

The first step of the Hunsdiecker reaction is quite straightforward. The reaction between silver carboxylate 1 and bromine gives rise to insoluble silver bromide along with acyl hypobromite 3. The unstable acyl hypobromite 3 undergoes a homolytic cleavage of the O-Br bond to provide carboxyl radical 4. Carboxyl radical 4 then decomposes via radical decarboxylation to release carbon dioxide and alkyl radical 5, which subsequently reacts with another molecule of acyl hypobromite 3 to deliver alkyl bromide 2, along with regeneration of carboxyl radical 4. Because of the radical pathway, chirality is often lost for the chiral carbon atom immediately adjacent to the carboxylic acid. 

One particular radical decarboxylation reaction, which is used in the synthesis of alkyl or aryl bromide (Hunsdiecker reaction), involves reaction of the silver salt of a carboxylic acid with bromine, and results overall in loss of CO2 to form the corresponding alkyl or aryl bromide (Scheme 4.47). When silver carboxylate is treated with I2 ester formation occurs (Simonini reaction). 

Generation of tert-Butyl Radicals via Oxidative Decarboxylation. Silver-catalyzed decarboxylation of carboxylic acids by persulfate generates alkyl radicals, which have been used for ho-molytic alkylation of aromatic bases. Pivalic acid is a source of f-butyl radicals in this process. 2-f-Butylquinoline is formed re-gioselectively by this method (eq 3) 6-r-butylnicotine has been prepared in a similar way. ... 

A reasonable entry point to the chemistry would be to convert the carboxylic acids into sulfinate salts [12]. Alternatively, a silver-catalyzed decarboxylative thiolation will generate sulfone precursors [16]... 

Liu C, Wang X, Li Z, Cui L, Li C (2015) Silver-catalyzed decarboxylative radical azidation of aliphatic carboxylic acids in aqueous solution. J Am Chem Soc 137(31) 9820-9823... 

Isoquinoline reacts with aliphatic carboxylic acids photolyticaHy or with a silver catalyst to give excellent yields of alkylation products by decarboxylation (155). This method is useful in the synthesis of 2-benzoyhsoquinolines bearing a variety of aromatic substituents in the 1-position (156). 

Practically all pyridazine-carboxylic and -polycarboxylic acids undergo decarboxylation when heated above 200 °C. As the corresponding products are usually isolated in high yields, decarboxylation is frequently used as the best synthetic route for many pyridazine and pyridazinone derivatives. For example, pyridazine-3-carboxylic acid eliminates carbon dioxide when heated at reduced pressure to give pyridazine in almost quantitative yield, but pyridazine is obtained in poor yield from pyridazine-4-carboxylic acid. Decarboxylation is usually carried out in acid solution, or by heating dry silver salts, while organic bases such as aniline, dimethylaniline and quinoline are used as catalysts for monodecarboxylation of pyridazine-4,5-dicarboxylic acids. 

The pyrolysis of perfluoro carboxylic salts can result both in mono and bimolecular products At 210-220 °C, silver salts give mostly the coupled products, at 160-165 °C in A -methylpyrrolidinone, the corresponding copper salts also give the simple decarboxylated compounds in nearly equal amounts The decomposition of the copper salts m the presence of lodobenzene at 105-125 °C results m a phenyl derivative, in addition to the olefin and coupled product [94] (equations 60-62)... 

The Hunsdiecker reaction is the treatment of the dry silver salt of a carboxylic acid with bromine in carbon tetrachloride. Decarboxylation occurs, and the product isolated is the corresponding organic bromide 16). Since dry silver salts are tedious to prepare, a modification of the reaction discovered by Cristol and Firth (77) is now... 

More recently, radical additions to fluoroethenes have attracted attention. Eguchi et al. [125] applied the Barton decarboxylation procedure to add a range of alkyl radicals to l,l-dichloro-2,2-difluoroethene. Addition was regioselective and the terminal carbon could be hydrolysed to a carboxyl group with silver(I) mediation (Eq. 39). The fluoroalkene is effectively an equivalent for either difluoroacetyl anion or cation synthons, because the adding radical can be approached from either polarity manifold. 

Alkyl radicals for such reactions are available from many sources such as acyl peroxides, alkyl hydroperoxides, particularly by the oxidative decarboxylation of carboxylic acids using peroxy-disulfate catalyzed by silver. Pyridine and various substituted pyridines have been alkylated in the 2-position in high yield by these methods. Quinoline similarly reacts in the 2-, isoquinoline in the 1-, and acridine in the 9-position. Pyrazine and quinoxaline also give high yields of 2-substituted alkyl derivatives <74AHC(16)123). 

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