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A catalytic system comprising Pd(0) and N-heterocyclic carbene (NHC) demonstrated remarkable efficiency in the acyl migration of enol carboxylates via the regioselective cleavage of the Cacyl–O bond. Employing NHC ligands with appropriate steric bulkiness and electron-donating ability enabled the transformation of a diverse array of enol esters containing alkyl-substituted internal and terminal alkenes into unsymmetrical β-diketones. Merging this methodology with π-acid-catalyzed hydroacyloxylation of alkynes delivered a formal catalytic alkyne insertion into the Cacyl–O bond of carboxylic acid. In addition to the essential roles of both Pd and NHC in driving the reaction, the intramolecular aspects were also unveiled through a crossover experiment. Elucidation of mechanistic insights via DFT studies suggested the kinetic preference for oxidative addition of the Cacyl–O bond over the Cenol–O and the Cacyl–Caryl bonds, aligning well with perfect selectivity in the experiments. Tentatively, reductive elimination of the C–C bond occurs directly from the acylpalladium oxa-π-allyl complex without intermediacy of acylpalladium C-enolate.
Michigami et al. (Wed,) studied this question.