N,N′-Bisaryl Substituted Chiral Linker -Bridged Bis(N-Heterocyclic Carbene) Palladium Complexes: Design, Synthesis, and Catalytic Properties

A series of novel N,N'-bisaryl bis(NHC) (NHC = N-heterocyclic carbene) precursors 5a-e, 12a-d, and 13a-c and their palladium complexes 14a-e, 15a-d, and 16a,b with (1R,2R)-cyclohexene and (1R,2R)-diphenylethylene linkers have been designed, synthesized in good yields, and fully characterized by NMR, HRMS, and elemental analysis. The X-ray single crystal analysis of 15a showed that these complexes adopted a slightly distorted square-planar geometry around the Pd(II) center. These N,N'-bisaryl-bis(NHC)-Pd complexes exhibit excellent catalytic activity in asymmetric aryl aryl cross-coupling reactions of arylboronic acids and aryl halides. The axial chiral biaryl compounds could be achieved in high yields (up to 74%) and good enantioselectivities (up to 60% ee) within 24 h. The results show that for this kind of bis(NHC) palladium catalysts the structure characteristics of the chiral linkers have decisive effect on the enantioselectivities of cross-coupling reactions.