Oxidative decarboxylation aliphatic carboxylic acids

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. 

The persulfate ion S2OI-, with or without various transition metal ions, is a particularly effective oxidant, especially for the decarboxylation of carboxylic acids.535 In the presence of silver(I), persulfate oxidation to silver(II) readily occurs and for aliphatic carboxylic acids the decarboxylation mechanism given in Scheme 4 has been established. The aliphatic radicals produced may then disproportionate, abstract hydrogen or be further oxidized to an alcohol. In... 

The oxidative decarboxylation of aliphatic carboxylic acids is best achieved by treatment of the acid with LTA in benzene, in the presence of a catalytic amount of copper(II) acetate. The latter serves to trap the radical intermediate and so bring about elimination, possibly through a six-membered transition state. Primary carboxylic acids lead to terminal alkenes, indicating that carbocations are probably not involved. The reaction has been reviewed. The synthesis of an optically pure derivative of L-vinylglycine from L-aspartic acid (equation 14) is illustrative. The same transformation has also been effected with sodium persulfate and catalytic quantities of silver nitrate and copper(II) sulfate, and with the combination of iodosylbenzene diacetate and copper(II) acetate. ... 

Esters 106 (R = Me, Et or Pr = Et, Pr, r-Bu or PhCHi) of aliphatic carboxylic acids react with lithium acetylides 107 (R = H, C5 Hi i or Ph) in the presence of boron trifluoride etherate in THE to give acetylenic ketones 108 (equation 18). Palladium-[tetrakis(triphenylphosphine)]-copper(I) iodide catalyses the oxidative addition-decarboxylation of propargyl methyl carbonates, e.g. 109, with terminal alkynes to yield 1,2-dien-4-ynes (allenylacetylenes) 110. The regiochemistry of the palladium-catalyzed addition of phenylacetylene to the allenic ester 111 depends on the nature of the catalyst used palladium(III) acetate-triphenylphosphine yields a 81 19 mixture of adducts 112 and 113, while in the presence of tetrakis(carbomethoxy)palladacyclopentadiene-tris(2,4,6-trimethoxyphenyl)phosphine the ratio is reversed to 9 91 k... 

Scheme 26 Oxidative decarboxylative alkynylation of aliphatic carboxylic acids...

Lee and Rochelle (25) have investigated the effects of various additives on the degradation of carboxylic acids used as buffers in flue gas desulfurization systems, comparing the rates of decarboxylation to sulfite oxidation. It is apparent from Table 3 that, of the S0X radicals, only is likely to react with aliphatic... 

Whatever the routes of formation, these oxidizing species, and in particular OH radicals, are known to react rapidly with most organic solutes. In aromatic compounds, hydroxylation of aromatic moiety occurs and successive oxidation / addition steps lead to ring opening. Resulting aldehydes and carboxylic acids (formed also in the reactions with aliphatics) are further decarboxylated, and finally produce CO2. 

A significant amount of kinetic data exists for the decarboxylation and oxidation of carboxylic acids. However, a relatively small fraction of these results deals with n-C2 to n-C4 aliphatic mono- and dicarboxylic acids under conditions pertinent to geological interests. For example, the early studies of the decarboxylation kinetics of acetic acid utilized flow-though silica tubes in which the anhydrous gas was exposed to very high temperatures for only seconds (Bamford and Dewar 1949 Blake and Jackson 1968, 1969). Nevertheless, it is useful to consider all of these results because it reveals trends common for structural classes of carboxylic acids. In this background discussion, a brief introduction to the subject of isokinetic relationships is given, as well as an overview of the decarboxylation and oxidation of carboxylic acids in which isokinetic relationships are used to establish trends and gross variations in reaction mechanisms between structural classes of acids. 

An alternative oxidation mechanism (Shock 1988, 1989) would be for the reaction in Eq. (23) to proceed directly without involving CO2 or H2O intermediates as required in the coupled oxidation reactions discussed above. The trimolecular interaction of acetic acid molecules is statistically unlikely instead, the reaction in Eq. (23) would have to occur as a series of reaction steps taking place on a surface or as a chain reaction in solution. Reaction involving cleavage of the carboxyl carbon-alkyl carbon bond in aliphatic monocarboxylic acids is likely to require a catalyst for the same structural reasons as discussed in the context of decarboxylation. Although it is possible that propionate absorbed on a surface could more readily lose an alkyl group to form acetate, it is unlikely that the reverse reaction could occur more rapidly than decarboxylation for the following reason. This... 

As pointed out previously, controlled degradation reactions are very difficult with aliphatic or alicyclic hydrocarbons, and most of the relabeling work has been concentrated on aromatic reaction products. Procedures have been extensively described by Pines and co-workers (e.g., 97, 96, also 87, 89-98, 95, 98). For the present purpose, it suffices to note that the 14C contents of the methyl side-chains and the rings in aromatic reaction products are readily estimated by oxidation of the methyl to carboxyl, followed by decarboxylation, while ethyl side-chains may be oxidatively degraded one carbon atom at a time. Radiochemical assays may be made on CO2 either directly in a gas counter, or after conversion to barium carbonate, while other solid degradation intermediates (e.g., benzoic acid or the phthalic acids) may be either assayed directly as solids or burned to CO2. Liquids are best assayed after burning to CO2. 

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