Synthesis, characterization, and reactivity of (π-allyl)palladium(II) wrap-around complexes with 1,3-dienes

Synthesis of a series of cationic “wrap-around” complexes, η3-, η2- (CH2–CH–CHR–CH2–CH2–CHCHX) Pd(II)L+ (R = H, CH3; X = H, Cl, CO2Me; L = PPh3, P(C4H4N)3), is described. These chelate complexes were prepared by exposure of π-allyl chloride dimers, (η3-(CH2-CX-CH2)PdCl)2, to either 1,3-butadiene or isoprene to yield π-allyl chloride dimers of the type (η3-CH2CHCRCH2CH2CH = CH(X)PdCl)2 which result from insertion of the diene into each π-allyl unit. Abstraction of chloride with either AgSbF6 or NaB(ArF)4 in the presence of L gives the cationic wrap-around complexes in high yields. Single crystal X-ray diffraction studies of 8a (R = –CH3, X = –Cl, L = PPh3) and 9a (R = –H, X = –Cl, L = PPh3) show that Pd(II) adopts essentially a square planar geometry and the chelate arm occupies a syn orientation with respect to the allyl unit. Exposure of these wrap-around complexes to nitriles of differing basicities displaces the chelated alkene to varying extents and allows assessment of the relative strengths of chelation as a function of substituents, X and R. Initial rapid displacement of the chelated alkene yields a syn-π-allyl isomer which equilibrates with the anti-π-allyl isomer which cannot close to form a chelate. Treatment of 8b with 1,3-butadiene gives not polybutadiene but 2-chloro-4-methyl-1,E-4,6-heptatriene and 2-chloro-4-methyl-1,Z-4,6-heptatriene. Formation of these trienes is first-order in butadiene. This reaction serves as a model for chain-transfer in the polymerization of butadiene.