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It is found that use of acetone in place of acetic acid in the reaction with isobutyric acid is effective for the synthesis of the cross-ketonization product, methyl isopropyl ketone (MIPK). Rate of MIPK formation is of the same order of magnitude, but slightly lower for acetone compared to acetic acid under similar conditions (Fig. 1). The most active catalyst is KOH-treated titania. The second product of this reaction, DIPK, results from the ketonic decarboxylative condensation of isobutyric acid with itself. The 13C labeled carbonyl group from isobutyric acid is almost exclusively (within the detection error) transferred to the MIPK product (Scheme 2). In the reaction of acetic acid with a more branched ketone, DIPK, only a negligible amount of MIPK is produced with all studied catalysts. Based on the experimental data, the proposed mechanism most likely includes enolization of acetone, followed by its condensation as the nucleophile with isobutyric acid as the electrophile, and completed by the retro-condensation to MIPK (Scheme 2). The order of the enolic components activity, acetic acid ≥ isobutyric acid > acetone >> DIPK, is generally consistent with the order of their adsorption energies on metal oxides. Low or non-branched ketones could be efficiently used in place of one of the acids in the cross-ketonization reaction. Because of the relatively high reaction rate, this process needs to be accounted for in the kinetic scheme of the decarboxylative ketonization.


Presented at the North American Catalysis Society Meeting in Pittsburgh, Pennsylvania, June 19, 2015.

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