An X-ray study of the effect of the bite angle of chelating ligands on thegeometry 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)( 2)C3H3) complexes (P-P = dppe (1,2-bis(diphenylphosphino)ethane), dppf (1,1 ' -bis(diphenylphosphino)ferrocene), and DPEphos (2,2 ' -bis(diphenylphosp hino)diphenyl ether)) and the (Xantphos)Pd(C3H5)BF4 (Xantphos = 4,5-bis(dip henylphosphino)-9,9-dimethylxanthene) complex have been determined. In the solid state structure, the phenyl rings of the ligand are oriented in the d irection of the nonsymmetrically bound [1,1-(CH3)(2)C3H3] moiety. An increa se 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, resul ting in a distortion of the symmetry of the palladium-allyl bond. In soluti on, two types of dynamic exchange have been observed, the pi-sigma rearrang ement 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 o r vice versa. The regioselectivity in the palladium-catalyzed allylic alkyl ation 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% foun d for Xantphos (6). The influence of the cone angle on the regioselectivity has been assigned to a sterically induced electronic effect.