Various carboxylic acid are catalytically converted into ketones on treatment with organoboron compounds in the presenceof 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 catlaytic process involves (i) exchange reactionof the carboxylic acid employed with diemthyl dicarbonate added as the activator to form a mixed anhydrode of the carboxylic acid and mono methyl ester of carbonic acid;(ii)oxidative addition of the mixed anhydride to zerovalent palladiumspecies to give an (acyl)(methyl carbonato)palladiumspecies;(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 theacyl and aryl ligands with regeneration of the Pd(0) species.Not only homogeneous catlayst systems but also heterogeneous systems were found to give ketones under mild conditions.On the basis of thepresent study involving the acyl(methoxo)palladium complexes generated in the reaction mixture of carboxylic acids with dimethyl dicarbonate on reactionwith Pd(0) complexes,we also developed a new catalytic process to convert carboxylic acids with terminal alkynes into alpha,beta-alkynyl ketones under similar conditions.
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