One of the most likely mechanisms suggested for the catalytic decarboxylative cross-ketonization of two carboxylic acids must start by enolizing one of the acids. In the next step, the enolized acid may condense with the carbonyl group of the other acid to form beta-ketoacid which decarboxylates to the final ketone product by releasing CO2. Alternative mechanisms, for example, the concerted one, and the one via beta-ketoacid formation, can be distinguished by quite different kinetic schemes. Therefore, kinetic studies can be of a great importance for elucidating ketonization mechanism. The ability of the ketone products to inhibit decarboxylative ketonization has been previously documented and explained by their competitive adsorption on surface. We would like to add another dimension to the role of ketone products, which is their ability to react with carboxylic acids. Thus, we report that the condensation of acetone with isobutyric acid yields methyl isopropyl ketone, MIPK, (Scheme 1) at a rate slightly lower but still comparable to that of the decarboxylative ketonization of isobutyric acid. These results necessitate an update of the kinetic scheme of the decarboxylative ketonization by including condensation of the ketone product with the starting reagent, carboxylic acid. Experiments with 13C isotopically labeled carbonyl group on isobutyric acid demonstrated that isobutyric acid is the sole source of the carbonyl group in the MIPK. We propose a mechanism of MIPK formation through enolization of acetone on the catalyst surface, followed by the nucleophilic attack of the enolized acetone on the carbonyl group of isobutyric acid, and the retro-condensation of the beta-diketone to MIPK.

Condensation of ketones with carboxylic acids: Extending kinetic scheme for the mechanism of ketonic decarboxylation

249th National Meeting (2015)

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