Decarboxylation aliphatic acids

Strong acceptors such as tetracyanobenzene can oxidize and decarboxylate aliphatic acids (see Equation 4.5) as well as t-butyl esters. Indeed, simple alkanes also undergo photosensitized oxidation and are deprotonated via the radical cation under these conditions. The radicals are identified from the alkylation of the sensitizer (in this case, TCB, see Equation 4.29). ... 

Carboxylic acids react with xenon difluoride to produce unstable xenon esters The esters decarboxylate to produce free radical intermediates, which undergo fluonnation or reaction with the solvent system Thus aliphatic acids decarboxylate to produce mainly fluoroalkanes or products from abstraction of hydrogen from the solvent Perfluoro acids decarboxylate in the presence of aromatic substrates to give perfluoroalkyl aromatics Aromatic and vinylic acids do not decarboxylate [91] (equation 51)... 

Similar ease of decarboxylation is seen in HaljCCH2C02e, 2,4,6-(N02)3 C6H2C02e, etc., but the reaction is not normally of preparative value with the anions of simple aliphatic acids other than MeCO20. 

The kinetic study of the decarboxylation of aliphatic acids in co-oxidation with cumene showed the following two chemical channels of C02 production [104],... 

Decarboxylativehalogenation (12,417). The Hunsdiecker reaction is not useful for aromatic acids, but decarboxylative halogenation of these acids can be effected in useful yield by radical bromination or iodination of the thiohydroxamic esters, as reported earlier for aliphatic acids.1 Thus when the esters 2 are heated at 100° in the presence of AIBN, carbon dioxide is evolved and the resulting radical is trapped by BrCCl3 to provide bromoarenes (3). Decarboxylative iodination is effected with iodoform or methylene iodide as the iodine donor. 

In another type of oxidative decarboxylation, arylacetic acids can be oxidized to aldehydes with one less carbon (ArCH2COOH — ArCHO) by tetrabutylammonium periodate. 23l< Simple aliphatic carboxylic acids were converted to nitriles with one less carbon (RCH2COOH — RC=N) by treatment with trifluoroacetic anhydride and NaNCU in FjCCOOH.239 See also 4-39. 

Another type of elimination reaction favoured under plasma conditions is the decarboxylation. Carbocyclic acids easily lose carbon dioxide to form the parent hydrocarbons. In acid anhydrides decarboxylation is followed by a decar-bonylation. Cyclic or bicyclic anhydrides fragment forming unsaturated compounds, a reaction which has been studied with phthalie anhydride 24>. This anhydride decomposes to dehydrobenzene which, in the absence of other compounds, dimerizes, trimerizes or polymerizes. Orientation experiments indicated similar results for aliphatic acid anhydrides. 

Radiolytic e.s.r. studies of the reactions of SO4, Cl2, and Brf radicals with organic compounds are currently being carried out in this laboratory by Fessenden et al. One of the interesting findings is the selective decarboxylation by SO4 radicals of certain aliphatic and aromatic carboxylic acids, whereas earlier studies with OH had shown that decarboxylation is not important in such cases. For example, it has been reported that the reaction of OH with malonic acid results mainly in hydrogen abstraction, with only 10% decarboxylation in acid solution and <0-5% in alkaline solution (Behar et al., 1973). 

In flavonoids acylated with aliphatic acids, the most common acids are acetic and malonic. In the MS fragmentation of the dicarboxylic acids (as malonic acid), a first loss of 44 mass units is observed (loss of the carboxylic radical, CO2), and this is due to decarboxylation. 

The majority of reactions discussed are readily applicable to simple primary, secondary and tertiary aliphatic acids. The decarboxylation of aryl- and vinyl-carboxyl radicals is a much more difficult process which limits the application of many of the methods described to aliphatic acids. As such, particular attention is drawn in the text to examples of aryl and vinyl decarboxylations. 

Decarboxylation of aliphatic acids by means of their derived 0-acyl thiohydioxaniates in die presence of an electron deficient terminal alkene results in the overall addition of an alkyl radical and a 2-pyri dylthiyl radical across the double bond (equation 48). 

Reduction of the carbethoxy group to the hydroxymethyl group with lithium aluminum hydride at — 35° and Claisen condensation with ethyl acetate are known to take place with pyridazinecar-boxylic acids. 6-Oxo-l,6-dihydro-2-pyridazinyl aliphatic acids, having the pyridazinonyl residue attached at the a-position of the aliphatic radical, readily undergo decarboxylative acylation with acid anhydrides in the presence of pyridine to form the corresponding 2-alkanones (107). 

The decarboxylation of simple aliphatic acids by fusion of their sodium salts with sodium hydroxide does not give pure hydrocarbons. By heating the barium salts of 1-phenylcycloalkane-l-carboxylic acids with dry sodium methoxide, 1-phenylcycloalkanes are obtained in 6-64% yields, the yield increasing with the size of the alicyclic ring. The coupling of the... 

Decarboxylation—elimination of the —COOH group as CO2—-is of limited importance for aromatic acids, and highly important for certain substituted aliphatic acids malonic acids (Sec. 26.2) and j8-keto acids (Sec. 2. 3). It is worthless for most simple aliphatic acids, yielding a complicated mixture of hydrocarbons. 

The decarboxylation of aliphatic acids may take place as an aliphatic electrophilic substitution but also in some cases can be regarded as an elimination reaction using a cyclic mechanism as described in Section 2.1. 

Another example of this type of reaction is the decarboxylation of aliphatic acids. For the decarboxylation to proceed easily there must be an electron withdrawing group capable of stabilising the negative charge. An example would be an a-ketoacid, e.g. RCOCOOH. Suggest a mechanism for this reaction. 

In the majority of cases thermal cleavage of carbon-carbon bonds consists of decarboxylation of a carboxylic acid RCOOH the tenacity with which the carboxyl group is retained varies within wide limits. Aliphatic acids can normally be decarboxylated only under rather extreme conditions, and the same applies to simple aromatic carboxylic acids unless the attachment of the carboxyl group is weakened by, e.g., ortho- or /rara-hydroxyl groups or by a hetero-ring atom (at a suitable distance from the carboxyl group). On the other hand, many carboxylic acids are known that lose carbon dioxide at or relatively little above room temperature either spontaneously or under the influence of acidic or basic catalysts. In most cases, the decarboxylation occurs by a polar mechanism, in an SE reaction ... 

Carboxylic acids that are difficult to decarboxylate comprise in particular the aliphatic acids and simple aromatic carboxylic acids. They can be decarboxylated only by pyrogenic decomposition of their salts in admixture with an alkali hydroxide or lime. In such mixtures an additional charge produced on the carboxyl group makes it possible, by induction, to remove the group R as an anion ... 

Only moderate yields of hydrocarbon are obtained in this way from salts of aliphatic acids here the main side reaction is formation of the ketone by only partial decarboxylation. This ketone formation becomes the main reaction if the calcium salt is not mixed with an excess of lime but is submitted alone to dry distillation ... 

Decarboxylation of an aliphatic acid to the hydrocarbon is best effected by the so-called salt degradation method. That method is to treat the silver or mercury salt of, preferably, an aliphatic or alicyclic carboxylic acid with bromine in an inert solvent a halogenated hydrocarbon is then formed, together with carbon dioxide and the metal bromide, usually in an exothermic reaction, and the bromine can then usually be readily removed either cata-lytically or by means of a Grignard reagent (the Hunsdiecker reaction) ... 

For <%-halo aliphatic acids it is the rule that the ease of loss of carbon dioxide increases with the atomic weight of the halogen. For example, trichloroacetic acid can be distilled unchanged and decomposes only at temperatures above 200°, and then slowly triiodoacetic acid, however, is decarboxylated at its melting point, 150°. 

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