Palladium(II)-catalyzed oxidation of aldehydes and ketones. 1. Carbonylation of ketones with carbon monoxide catalyzed by palladium(II) chloride in methanol

Article abstract of DOI:10.1021/jo005627e

Unsubstituted or alkyl-substituted cyclic ketones react with PdCl₂ in methanol under a CO atmosphere to give mainly acyclic diesters along with some acyclic chloro-substituted monoesters. The monosubstituted cyclic ketones, 2-hydroxy- and 2-methoxycyclohexanone, do not give ring cleavage but rather produce 2-(carbomethoxy)cyclohex-2-en-1-one. ¹³CO labeling experiments indicate one CO is inserted in forming the diester product so the second ester group must arise from the original ketone group. Two mechanisms are possible for the diester reaction. One involves initial Pd(II)-CO₂CH₃ insertion across the double bond of the enol form of the ketone while the second involves initial addition of Pd(II)-OCH₃ followed by CO insertion into the new Pd(II) carbon bond formed. Pd(II) elimination and acid-catalyzed ring cleavage produce the second methyl ester group in both routes. The chloro-substituted monoester is formed by initial Pd(II)-Cl insertion across the double bond followed by the acid-catalyzed ring cleavage. The 2-(carbomethoxy)cyclohex-2-en-1-one must result from elimination of water or methanol from the α-ketoester product formed by the initial methoxycarbonylation of the enol form of the ketone. As expected, the acyclic ketone 2-decanone, formed methyl acetate and a mixture of methyl nonanoate and products.