The Reaction of Carbon Dioxide with Palladium-Allyl Bonds

A family of palladium allyl complexes of the type (2-methylally1)(2)Pd(L) (L = PMe3 (1), PEt3 (2), PPh3 (3), NHC (4); NHC = 1,3-bis(2,6-diisopropylphenyl)-1,3-dihydro-2H-imidazol-2-ylidene) have been prepared through the reaction of (2-methylally1)2Pd with the appropriate free ligand. Compounds 1-4 contain one eta(1)- and one eta(3)-2-methylallyl ligand, and 3 was characterized by X-ray crystallography. These complexes react rapidly with CO, at low temperature to form well-defined unidentate palladium carboxylates of the type (eta(3)-2-methylallyl)Pd(OC(O)C4H7)(L) (L = PMe3 (6), PEt3 (7), PPh3 (8), NHC (9)). The structure of 9 was elucidated using X-ray crystallography. The mechanism of the reaction of 1-4 with CO2 was probed using a combination of experimental and theoretical (density functional theory) studies. The coordination mode of the allyl ligand is crucial, and whereas nucleophilic eta(1)-allyls react rapidly with CO2, eta(3)-allyls do not react. We propose that the reaction of palladium eta(1)-allyls with CO2 does not proceed via direct insertion of CO2 into the Pd-C bond but through nucleophilic attack of the terminal olefin on electrophilic CO2, followed by an associative substitution at palladium.