Decarboxylation of acid

In 1869 Berthelot- reported the production of styrene by dehydrogenation of ethylbenzene. This method is the basis of present day commercial methods. Over the year many other methods were developed, such as the decarboxylation of acids, dehydration of alcohols, pyrolysis of acetylene, pyrolysis of hydrocarbons and the chlorination and dehydrogenation of ethylbenzene." ... 

The final decarboxylation of mevalonate 5-diphosphate appears unusual because decarboxylations of acids do not typically occur except in /3-keto acids and malonic acids, in which the carboxylate group is two atoms away from an additional carbonyl group (Section 22.7). The function of this second carbonyl group is to act as an electron acceptor and stabilize the charge resulting from loss of CC>2- In fact, though, the decarboxylation of a /3-kelo acid and the decarboxylation of mevalonate 5-diphosphate are closely related. 

The mechanism of decarboxylation of acids containing an amino substituent is further complicated by the possibility of protonation of the substituent and the fact that the species NH2ArCOOH is kinetically equivalent to the zwitterion NHj ArCOO. Both of these species, as well as the anion NH2 ArCOO" and even NH3 ArCOOH must be considered. Willi and Stocker644 investigated by the spectroscopic method the kinetics of the acid-catalysed decarboxylation of 4-aminosalicyclic acid in dilute hydrochloric acid, (ionic strength 0.1, addition of potassium chloride) and also in acetate buffers at 20 °C. The ionisation constants K0 = [HA][H+][H2A+] 1 (for protonation of nitrogen) and Kx = [A"][H+] [HA]-1, were determined at /i = 0.1 and 20 °C. The kinetics followed equation (262)... 

A reaction mechanism with Fe304 as catalyst has been proposed [68], in agreement with previous work concerning decarboxylation of acids in the presence of a metal oxide [83]. After the transient formation of iron(II) and iron(III) carboxylates from the diacid and Fe304 (with elimination of water), the thermal decarboxylation of these salts should give the cyclic ketone and regeneration of the catalyst. 

Dauben, W. G., and P. Coad Oxydation in Decarboxylation of Acids with... 

The decarboxylation of acids of this type should be catalyzed by magnesium. 

Thus, methyl (5-acetyl-2-thienyIthio)acetate (62) was prepared by acetylation of 61. Addition of a second acetyl group in the presence of excess AlClj led to methyl (3,5-diacetyl-2-thienylthio)acetate (63), which, on heating in ethanolic sodium ethoxide yielded about 95% of 5-acetyl-3-methylthieno[2,3-6]thiophene-2-carboxylic acid (64) reduction of acid 64 resulted in 5-ethyl-3-methylthieno[2,3-6] ophene 2-carboxylic add (65), identical with the acid obtained by cyclization of methyl (3-acetyl-5-ethyl-2-thienylthio)acetate. Decarboxylation of acid 64 gave 5-acetyl-3-methylthieno[2,3-6]thiophene (66) [Eq. (25)1. 

Oxidative decarboxylation of acids to alkenes is often accompanied by alkene rearrangement. Lukas... 

Another useful aspect of the radical chemistry associated with thiocarbonyl compounds is the decarboxylation of acids. Typically the acyl derivatives (2) of /V-hydroxypyridine-2-thione (1) are prepared from the appropriate acid [243], and treated with tributyltin hydride. 

R. A. Sheldon and J. K. Kochi, Oxidative Decarboxylation of Acids by Lead Tetraacetate, Organic Reactions 19, 279 (1972). 

The thermal decarboxylation of acids over a metal oxide catalyst (Expts 5.92 and 5.93). 

Scheme 9 Decarboxylation of acids under the conditions of ozone bleaching...

Decarboxylation of acids is of special interest, since the free radicals produced may combine with iodine, heteroaromatic bases or electron-deficient alkenes affording useful products in clean reactions. 

THE THERMAL DECARBOXYLATION OF ACIDS OVER A METAL OXIDE CATALYST... 

The possibility of abstracting a hydrogen atom from the carboxylic group may be expected to be even more limited than O—H bond fission of alcohols. The existence of the process was suggested to account for the decarboxylation of acids during the oxidation of hydrocarbons. The reactivity of the acidic hydrogen, however was estimated to be 100-times... 

Stannyl- (and -silyl-) carboxylic acids undergo oxidative decarboxylation with LTA under mild conditions to provide the corresponding alkenes. This represents an improvement on the well-known alkene-forming decarboxylation of acids with LTA, which requires thermtd or photochemical conditions, for example. The directing metal effect leads to improved yields and regioselectivity. However, stereo-specific alkene formation did not occur and this could imply free radical involvement or transmetallation (Pb for Sn) (stereochemistry ) followed by cation formation, see for example Scheme 27. 

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. 

Oxidative deearboxyUttian of acids (1, 554-557 2, 235-237 3, 168-169). The oxidative decarboxylation of acids by lead tetraacetate has been reviewed by Sheldon and Kochi. ... 

Benzophenone-sensitized decarboxylation of acids of the type RXCH2COOH (where X = O, S, and NH) [165-167] appears to go by electron transfer from the... 

Imidazoles normally undergo free-radical reactions at the 2-position. For example, homolytic free-radical alkylation of histidines and histamines yields 2,3-disubstituted histidines and histamines. In these reactions, the free radical was generated via silver-catalyzed oxidative decarboxylation of acids with peroxydisulfate 433 (Scheme 103) <2001BML1133>. 

The first-order decarboxylation of acids will be considered in this section. As it was indicated in the introduction, the effect of solvent imposes a difficulty in the... 

It was shown many years ago that the decarboxylation of acids that meet these requirements are first-order in the acid - ". However, part of the decarboxylation arises from the anion of the acid without violation of first-order kinetics . The rate law for decarboxylation of )S-ketoacids has the form " ... 

Table 53 lists the activation parameters for decarboxylation of acids in a variety of solvents. Clark, whose work dominates this table, usually employs a 20° temperature range to obtain the activation parameters. For certain acids in Table 53, where a reasonably large numer of solvents have been used, we have correlated the data using eqn. (5). A single line least squares fit was attempted, except in certain instances where a correlation was clearly better with two lines. These data are presented in Table 54 along with the correlation coefficient (/ ). Location of suspect activation parameters is facilitated by the data in Table 54. For the decarboxylation of oxalic acid in propylene glycol, with AH = 10.3 kcal.mole , the AA value is calculated to be —46.5+1.2 eu compared to the reported value of —49.1 eu.

---