Effects of β-Substituents and Ancillary Ligands on the Structure and Stability of (η3-Allyl)palladium Complexes. Implications for the Regioselectivity in Nucleophilic Addition Reactions

(η 3 -Allyl)palladium complexes possessing various allylic β-substituents (X = Cl, OCH 3, H) and ancillary ligands on palladium (L = Cl -, F -, PH 3 ) have been studied at the second-order Møller−Plesset (MP2) and fourth-order Møller−Plesset (MP4) perturbation theory level including single, double, and quadruple excitations (MP4SDQ). Interactions between the β-substituents and palladium have been analyzed as a function of the conformation and configuration of the complexes. Asymmetric polarization of the electron density by β-substituents leads to significant structural changes of the complexes. The β-substituent effects considerably increase the thermodynamic stability of the complexes and facilitate the heterolytic fission of the C−X bond. The intensity of these effects depends on the nature of the β-substituent, on the conformation and configuration of the complex, as well as on the σ-donor/π-acceptor character of the ancillary ligands on palladium. Since (η 3 -allyl)palladium complexes are key intermediates of some important palladium catalyzed transformations, the β-substituent effects on the regiochemistry of the nucleophilic attack are also discussed. It was concluded that the regioselection is considerably enhanced in the presence of strong β-substituent effects.