Chiral 1,2-Cyclohexane-Bridged Bis-NHC Palladium Catalysts for Asymmetric Suzuki–Miyaura Coupling: Synthesis, Characterization, and Steric Effects on Enantiocontrol

The series of chiral 1,2-cyclohexane-bridged bis-N-heterocyclic carbene ligand precursors H2[(1R,2R)-(1a–i)]Br with different substituent groups and their neutral and cationic diaqua palladium complexes, namely {Pd­[(1R,2R)-(1a–i)]­Br2} (2a–i), {Pd­[(1R,2R)-(1a)]­X2} (X = Cl (3), OAc (4-OAc), OC­(O)­CF3 (4-OCH­(O)­CF3)), and {Pd­[(1R,2R)-(LOMe)]­(OH2)2}­X2 (X = OTf (5-OTf), SbF6 (5-SbF6)) have been prepared in moderate to good yields. These chiral palladium complexes were fully characterized by elemental analysis, high-resolution mass spectra, 1H and 13C NMR, and optical rotation determinations. The crystal structures of the chiral complexes 2a and 5-OTf were further confirmed to adopt a distorted-square-planar coordination geometry around the palladium center. The obtained chiral NHC-Pd compounds were able to catalyze the asymmetric Suzuki–Miyaura couplings of aryl halides with arylboronic acids in good yields (up to 96%) and moderate enantioselectivities (up to 64% ee). The coligand and steric effects were studied carefully. The coligands, including Br–, Cl–, AcO–, CF3COO–, and water molecules, have little influence on the catalytic results. However, a strong steric effect of the two aromatic substituents R on the enantiocontrol has been proved in the catalytic asymmetric Suzuki–Miyaura coupling reaction. The highest enantioselectivity of 64% ee could be achieved under the standard reaction conditions.