Decarboxylation of free carboxylic acids

The decarboxylation of carboxylic acid in the presence of a nucleophile is a classical reaction known as the Hunsdiecker reaction. Such reactions can be carried out sometimes in aqueous conditions. Man-ganese(II) acetate catalyzed the reaction of a, 3-unsaturated aromatic carboxylic acids with NBS (1 and 2 equiv) in MeCN/water to afford haloalkenes and a-(dibromomethyl)benzenemethanols, respectively (Eq. 9.15).32 Decarboxylation of free carboxylic acids catalyzed by Pd/C under hydrothermal water (250° C/4 MPa) gave the corresponding hydrocarbons (Eq. 9.16).33 Under the hydrothermal conditions of deuterium oxide, decarbonylative deuteration was observed to give fully deuterated hydrocarbons from carboxylic acids or aldehydes. 

Except for some activated acids, for example aryl carboxylic acids, decarboxylation of free carboxylic acids is often difficult [17]. Under the reaction conditions used for of the deuteration in Scheme 6 (i.e. Pd/C catalyst, in deuterium oxide at 250 C/4-5 MPa), decarboxylation was observed - a carboxyl group is exchanged with a D atom, accompanied by H-D exchange reaction on all carbons [18]. 

The decarboxylation of free carboxylic acids is often difficult except for some activated acids such as aryl carboxylic acid. Conversion of the free carboxylic acids into proper derivatives snch as acid anhydrides or esters makes the transition metal-catalysed decarboxylation reaction easy and these have also been applied to various coupling reactions. Water in a snpercritical stage (374°C, 22 MPa) was used for decarboxylation of free carboxylic acids using 10 wt% Pd on active carbon in a closed pot (Matsnbara et al., 2004) (Scheme 3.14). 

Yoshimi, Y, Hayashi, S., Nishikawa, K., Haga, Y, Maeda, K., Morita, T., Itou, T., Okada, Y, Ichinose, N. and Hatanaka, M. 2010. Influence of solvent, electron acceptors and arenes on photochemical decarboxylation of free carboxylic acids via single electron transfer (SET). Molecules. 15(4) 2623-2630. 

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See also in sourсe #XX -- [ ]

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