Decarboxylations aromatic carboxylic acids, benzene

Reductive alkylation of N-methylacridinium (87) occurs when it is irradiated with carboxylic acid salts. The reaction is thought to proceed by electron transfer from the carboxylate to the excited acrldinium ring followed by decarboxylation of RCOO coupling of the alkyl radical produced with the acridinyl radical then gives (88). A very similar sequence probably occurs in a reaction proposed as a synthetic procedure for decarboxylation of carboxylic acids.In this case an aza-aromatic compound such as acridine is irradiated with a carboxylic acid in benzene in the presence of tert-butyl thiol. The authors propose that a hydrogen bonded acridine-acid complex is excited and that adiabatic proton transfer is followed by electron transfer. This produces RCOO which decarboxylates, and reduction of the alkyl radical then ensues. The major fate of the acridine is coupling to (89) if the reaction is perfonned in the absence of oxygen. 

Benzoic acid has been analyzed by Wiberg and Stewart (1955) by refluxing a sample (c. 125 mg.) in dry quinoline (2 ml.) containing copper powder (50 mg.). The benzene evolved is removed in a dry ice-acetone trap. This method can presumably be used for the analysis of other aromatic carboxylic acids. Aromatic acids can also be decarboxy-lated by heating to 750°C, (Gragerov and Ponomarchuk, 1959). Aromatic amino acids are readily decarboxylated by heating to 260°C. in a mixture of diphenylamine and diphenylmethane (Rittenberg and Ponticorvo, 1956). Other amino acids are decarboxylated more slowly in this solvent. 

The decarboxylation of aromatic acids is most often carried out by heating with copper and quinoline. However, two other methods can be used with certain substrates. In one method, the salt of the acid (ArCOO ) is heated, and in the other the carboxylic acid is heated with a strong acid, often sulfuric. The latter method is accelerated by the presence of electron-donating groups in ortho and para positions and by the steric effect of groups in the ortho positions in benzene systems it is... 

Decarboxylation of alkanoic acids means of LTA in benzene as solvent is hindered by die formation of all lbenzenes as by-products. This side reaction is especially pronounced widi radicals derived from primary acids or odier acids from which the radical is not easily oxidized LTA. In some cases, such as that of apocamphane-l-carboxylic acid (equation 54), good yields of alkylbenzene can be obtained. Intramolecular versions of this reaction in which the radical cyclizes onto an aromatic nucleus at the appropriate position in the chain have also been observed. 

Analogous decarboxylations, sometimes in the presence of Ca(OD)2, have been used for preparation of labeled aromatic hydrocarbons from the corresponding carboxylic acids,76 e.g.9 [D6]benzene from mellitic acid,77 deuteriochlo-roform from trichloroacetic acid,78 and [6-DJuracil from orotic acid.24... 

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