Mechanistic Studies of the Palladium-Catalyzed Ring Opening of Oxabicyclic Alkenes with Dialkylzinc

The mechanism of the palladium-catalyzed ring opening of oxabicyclic alkenes with dialkylzinc has been studied. Experiments which rule out a π-allyl mechanism were carried out. Trapping carbometalated products and synthesis and successful reaction of alkyl palladium species provided strong evidence in favor of an enantioselective carbopalladation as the key step in the mechanism. The studies also suggest that a cationic palladium species is responsible for carbopalladation of the alkene. The combination of palladium and dialkylzinc is unique in that the dialkylzinc functions both in the transmetalation to palladium and as a Lewis acid in forming the reactive cationic palladium species.