Palladium-Catalyzed Direct Conversion of Carboxylic Acids into Ketones with Organoboronic Acids Promoted by Anhydride Activators

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Abstract
Various carboxylic acids are catalytically converted into ketones on treatment with organoboron compounds in the presence of activators and palladium catalysts. Detailed examination of factors influencing the yield of ketone formation revealed the following characteristics of the reactions: (a) Palladium complexes containing tertiary phosphine ligands, particularly triphenylphosphine and tricyclohexylphosphine, are most effective among the palladium complexes; (b) Dioxane and THF are suitable as the solvent; (c) Dimethyl dicarbonate is required as the activator to obtain ketones in high yields. The process provides a general, versatile, synthetic method to produce various symmetrical and unsymmetrical ketones with aromatic, aliphatic, and heterocyclic groups. The mechanism proposed for the catalytic process involves (i) exchange reaction of the carboxylic acid employed with dimethyl dicarbonate added as the activator to form a mixed anhydride of the carboxylic acid and mono methyl ester of carbonic acid; (ii) oxidative addition of the mixed anhydride to zerovalent palladium species to give an (acyl)(methyl carbonato)palladium species; (iii) decarboxylation of the methyl carbonato ligand to give (acyl)(methoxo)palladium species; (iv) transmetallation of the (acyl)(methoxo)palladium species with an arylboronic acid to give an (acyl)(aryl)palladium species; and (v) reductive elimination of the (acyl)(aryl)palladium species to liberate the coupling product of the acyl and aryl ligands with regeneration of the Pd(0) species. Not only homogeneous catalyst systems but also heterogeneous systems were found to give ketones under mild conditions. On the basis of the present study involving the acyl(methoxo)palladium complexes generated in the reaction mixture of carboxylic acids with dimethyl dicarbonate on reaction with Pd(0) complexes, we also developed a new catalytic process to convert carboxylic acids with terminal alkynes into α, β-alkynyl ketones under similar conditions.