Carboxylate anions, decarboxylation

When the carboxylate anion decarboxylates, it forms a resonance-stabilized anion 0 =6 " Q -... 

The carbon-carbon bond forming potential inherent m the Claisen and Dieckmann reac tions has been extensively exploited m organic synthesis Subsequent transformations of the p keto ester products permit the synthesis of other functional groups One of these transformations converts p keto esters to ketones it is based on the fact that p keto acids (not esters ) undergo decarboxylation readily (Section 19 17) Indeed p keto acids and their corresponding carboxylate anions as well lose carbon dioxide so easily that they tend to decarboxylate under the conditions of their formation... 

The anodic oxidation of the carboxylate anion 1 of a carboxylate salt to yield an alkane 3 is known as the Kolbe electrolytic synthesis By decarboxylation alkyl radicals 2 are formed, which subsequently can dimerize to an alkane. The initial step is the transfer of an electron from the carboxylate anion 1 to the anode. The carboxyl radical species 4 thus formed decomposes by loss of carbon dioxide. The resulting alkyl radical 2 dimerizes to give the alkane 3 " ... 

Step 2 of Figure 29.11 Decarboxylation The TPP addition product, which contains an iminium ion j8 to a carboxylate anion, undergoes decarboxylation in much the same way that a jB-keto acid decarboxylates in the acetoacetic ester synthesis (Section 22.7). The C=N+ bond of the pyruvate addition product acts... 

The manifestation of noncovalent catalysis as a microsolvent effect is illustrated by cycloamylose-catalyzed decarboxylations of activated carboxylic acid anions. Anionic decarboxylations, as illustrated in scheme VII, are generally assumed to proceed by a rate-determining heterolytic... 

Anodic decarboxylation proceeds via a C—C bond scission of carboxylate anions to afford the Kolbe dimer,197 i.e.,... 

The decarboxylation of carboxylate anions is carried out chemically by a variety of one-electron oxidants such as lead tetraacetate, uranyl nitrate, peroxides, quinones, pyridinium cations, etc.199 Importantly, the carboxylate anion (as... 

The rate of decarboxylation of activated carboxylate anions [e.g. (10)], shows strong solvent dependence. It is not surprising, therefore, that these reactions have been used to probe the microsolvent effects of micelles and CDs (Fendler and Fendler, 1975). In particular, it was anticipated that complexation with a CD might result in catalysis by providing an environment for the reaction that is less polar than water. 

In comparison, photodecarboxylation of various other carboxylic acids has been studied extensively. For example, photodecarboxylation of 23 in water presumably involves electron transfer from the carboxylate anion to the phenyl ring (Scheme 16). The electron transfer is followed by decarboxylation to form the anion 24, which is protonated by the solvent. As shown in Scheme 16, in less polar aprotic solvents, homolytic cleavage leads to decarboxylation subsequent to charge transfer in 23. 

Catalytic decarboxylation processes occur in aliphatic keto acids in which the keto group is in an a-position to one carboxyl group and in a P-relationship to another. Thus, the normal decarboxylation of a p-keto acid is facilitated by metal coordination to the a-keto acid moiety. The most-studied example is oxaloacetic acid and it has been shown that its decarboxylation is catalyzed by many metals following the general order Ca2+ < Mn2+ < Co2+ < Zn2+ < Ni2+ < Cu2+ < Fe3+ < Al3"1".66 67 The overall rate constants can be correlated with the stability constants of 1 1 complexes of oxalic acid rather than oxaloacetic acid, as the uncoordinated carboxylate anion is essential for the decarboxylation. The generally accepted mechanism is shown in Scheme 15. Catalysis can be increased by the introduction of x-bonding ligands, which not only increase the... 

The effects of the addition of sugars, long-tailed n-alkyl pyranosides, n-alkyl glycerol ethers and n-alcohols on the properties of di-n-hexadccyldi methyl ammonium bromide (DHAB) vesicles have been studied.54 Upon addition of most additives, an inhibiting effect on the decarboxylation reaction of 6-nitrobenzisoxazole-3-carboxylate anion has been observed relative to the reaction in vesicles without any additive. The largest inhibition was observed in the case of cholesterol. 

In cases of extensive branching, as for the alkylbenzenesulfonates, the CnH2n+2 1°33 series is significantly perturbed compared to the straight chain alkyl sulfates, but it is sufficiently abundant to show perturbations in the loss pattern. These perturbations may be interpretable for identifying branch points. N-Acylated amino acids also show a suppressed remote charge site loss series because the preferred fragmentations are the decarboxylation of the parent anion and formation of the carboxylate anion of the amino acid (8). 

Finally, mention should be made of the polysoap-catalyzed decarboxylation of 6-nitrobenzisoxazole-3-carboxylate anion (Eg. I) studied by Kunitake and co-workers 21). This reaction is known to proceed faster in apolar solvents than polar ones. 

Aqueous solutions of minium perchlorates were irradiated in the presence of carboxylate anions and alkylated adducts were formed [184, 185]. This was shown to occur through electron transfer quenching of the iminium excited singlet, followed by an efficient decarboxylation of the resulting acyloxy radical. When a-hydroxy carboxylates are used, a reduction product is also formed and probably linked to the formation of a ketyl radical whose reducing properties are known. 

In decarboxylation, the loss of CO2 from the carboxylate anion is believed to involve a carbanion intermediate, which acquires a proton from solvent or other sources. The anion of (3-ketoacids can undergo facile decarboxylation (Scheme 2.19). 

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. 

The effect of solvent upon the activation parameters for decarboxylation has been studied extensively. All of the reactions reported here show first-order kinetics however, the species which undergoes decarboxylation is usually unknown. Whether this species is the free acid, the carboxylate anion or a zwitterion is usually not resolved. The activation parameters may not reflect a particular reaction path, but instead the sum of decarboxylation pathways. In a number of instances linear AH vs. AS plots are realized for a given acid in a variety of solvents. The equation for such a relationship is given by... 

The P keto-esters can be easily hydrolysed and decarboxylated by the methods of Chapter 26 to give the symmetrical cyclic ketone. The carboxylate anion is reasonably stable, but the free acid cannot usually be isolated as it loses carbon dioxide easily and gives the enol of the hnal product. 

Another measurement of the p Ta for ethylene comes from the formation of carbanions in the gas phase by decarboxylation of carboxylate anions. Carbanions that are too basic will not form in this way the corresponding carboxylates do not decarboxylate. From the energy thresholds of such decarboxylations Graul and Squires estimated A//acid of ethylene <401 kcalmoF but this value differs substantially from the accepted value of 409.4 kcalmoF. ... 

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