Chapterr 7. Ann X-ray study towards the effect of the bite angle of bidentate phosphine ligandss on the geometry of palladium(allyl) complexes: implications for thee regioselectivity in the ally lie alkylation of a di substituted allyl moiety

t X-rayy crystal structures of a series of cationic (P-P)palladium(3,3-(CH3)2-C3H3) complexes (P-P = dppe (1,2bis(diphenylphosphino)ethane),, dppf (l,l'-bis(diphenylphosphino)ferrocene), DPEphos) and the (Xantphos)Pd(C3H5)BF44 complex have been determined. In the solid state structure, the phenyl rings of the ligandss are oriented in the direction of the non-symmetrically bound [3,3-(CH3)2-C3H3] moiety. An increase off the bite angle of the chelating ligand results in an increase of the cone angle. In complexes containing ligandss having a large cone angle, the distances between the phenyl rings and the allyl moiety become small, resultingg in a distortion of the symmetry of the palladium-allyl bond. In solution, two types of dynamic exchangee have been observed, the r|-T|-r| rearrangement and the apparent rotation of the allyl moiety. At thee same time, the folded structure of the ligand changes from an endo to an exo orientation or vice versa. Thee regioselectivity in the palladium catalyzed allylic alkylation of 3-methyl-but-2-enyl acetate is determined byy the cone angle of the bidentate phosphine ligand. Nucleophilic attack by a malonate anion takes place preferentiallyy at the allylic carbon atom having the largest distance to palladium. Ligands with a larger cone anglee direct the regioselectivity to the formation of the branched product, from 8% for dppe (1) to 61 % for Xantphoss (6). Thee influence of the cone angle on the regioselectivity has been assigned to a sterically induced electronic effect. .