Effect of the Dihedral Angle of Biaryl-Bridged Bis(N-Heterocylic Carbene) Ligands on Enantioselectivity in Pd-Catalyzed Asymmetric Aryl–Aryl Cross-Coupling

Abstract

A series of new biaryl-bridged bis­(N-heterocylic carbene) palladium complexes has been synthesized and applied in asymmetric Suzuki cross-coupling of aryl halide and aryl boronic acid. X-ray crystallographic studies of square-planar palladium complexes of four of these bis­(NHC) ligands revealed that the dihedral angle of the bridging biaryl moiety depends on its identity and lies between 79.65° and 92.69°. A linear correlation between the dihedral angle in these Pd complexes and cross-coupling enantioselectivity is first reported. Moreover, larger Ccarbene–Pd–Ccarbene bite angles were found to lead to increased enantioselectivity in a first fast and then slow growing tendency.