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[(1<i>R</i>,2<i>R</i>)-(<b>1a</b>–<b>i</b>)]Br with different substituent groups and their neutral and cationic diaqua palladium complexes, namely {Pd­[(1<i>R</i>,2<i>R</i>)-(<b>1a</b>–<b>i</b>)]­Br₂} (<b>2a</b>–<b>i</b>), {Pd­[(1<i>R</i>,2<i>R</i>)-(<b>1a</b>)]­X₂} (X = Cl (<b>3</b>), OAc (<b>4-OAc</b>), OC­(O)­CF₃ (<b>4-OCH­(O)­CF</b>_<b>3</b>)), and {Pd­[(1<i>R</i>,2<i>R</i>)-(<b>L</b>^<b>OMe</b>)]­(OH₂)₂}­X₂ (X = OTf (<b>5-OTf</b>), SbF₆ (<b>5-SbF</b>_<b>6</b>)) 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 <b>2a</b> and <b>5-OTf</b> 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