An X-ray Study of the Effect of the Bite Angle of Chelating Ligands on the Geometry of Palladium(allyl) Complexes: Implications for the Regioselectivity in the Allylic Alkylation

X-ray crystal structures of a series of cationic (P-P)palladium(1,1-(CH3)2C3H3) complexes (P-P = dppe (1,2-bis(diphenylphosphino)ethane), dppf (1,1‘-bis(diphenylphosphino)ferrocene), and DPEphos (2,2‘-bis(diphenylphosphino)diphenyl ether)) and the (Xantphos)Pd(C3H5)BF4 (Xantphos = 4,5-bis(diphenylphosphino)-9,9-dimethylxanthene) complex have been determined. In the solid state structure, the phenyl rings of the ligand are oriented
in the direction of the nonsymmetrically bound [1,1-(CH3)2C3H3] moiety. An increase of the bite angle of the
chelating ligand results in an increase of the cone angle. In complexes containing ligands having a large cone
angle, the distances between the phenyl rings and the allyl moiety become small, resulting in a distortion of the
symmetry of the palladium−allyl bond. In solution, two types of dynamic exchange have been observed, the
π−σ rearrangement and the apparent rotation of the allyl moiety. At the same time, the folded structure of the
ligand changes from an endo to an exo orientation or vice versa. The regioselectivity in the palladium-catalyzed
allylic alkylation of 3-methyl-but-2-enyl acetate is determined by the cone angle of the bidentate phosphine ligand.
Nucleophilic attack by a malonate anion takes place preferentially at the allylic carbon atom having the largest
distance to palladium. Ligands with a larger cone angle direct the regioselectivity to the formation of the branched
product, from 8% for dppe (1) to 61% found for Xantphos (6). The influence of the cone angle on the regioselectivity
has been assigned to a sterically induced electronic effect.