Stable Axial Chirality in Metal Complexes Bearing 4,4′-Substituted BIPHEPs: Application to Catalytic Asymmetric Carbon-Carbon Bond-Forming Reactions

Not only electronic but also steric effects of 4,4'-substituents in BIPHEP derivatives and metal (Pd, Pt, and Au) complexes are shown to influence the stability of the biphenyl single bond rotation. While electron-donating or sterically demanding substituents on the 4,4'-positions destabilize the axial chirality of BIPHEP derivatives, electron-withdrawing or sterically less demanding ones on the 4,4'-positions stabilize the axis chirality. Particularly, the axial chirality of palladium dichloride complexes bearing BIPHEP with t-Bu and CF3 substituents on the 4,4'-positions is most labile and stable, respectively (Delta G not equal = 29.22 and 30.49 kcal mol(-1) at 300K; t(1/2) = 7 and 56 years at 300 K). These enantiopure dicationic BIPHEP-Pd complexes can be employed for catalytic enantioselective arylation, alkenylation, and ene reactions to give the corresponding products in good-to-excellent yields and enantioselectivities. Significantly, in the carbonyl-ene reaction of trifluoropyruvate with isobutene, the turnover frequency (TOF) reached 58200h(-1). The remarkable effects of 4,4'-substituents in BIPHEP derivatives can be employed as a guiding principle in the design of versatile and efficient ligands.