Palladium-catalyzed reaction of butadiene and carbon dioxide

The selective syntheses of the δ-lactone 2-ethylidene-6-hepten-5-olide and of the γ-lactone 2-ethyl-2,4-heptadien-4-olide by the palladium-catalyzed reaction of carbon dioxide and butadiene are described. Mechanistic studies showed that the formation of the γ-lactone proceeds by a consecutive reaction via the δ-lactone. The products are accounted for in terms of a reaction path involving an allylic palladium carboxylate complex as a key intermediate.