C2-Symmetric Hindered "Sandwich" Chiral N-Heterocyclic Carbene Precursors and Their Transition Metal Complexes: Expedient Syntheses, Structural Authentication, and Catalytic Properties

We present here a new family of C-2-symmetric "sandwich" NHC ligands abbreviated as SITLAr and BzITL(Ar) and their complexes. The chiral fragments incorporate bulky yet flexible cyclohexyl groups with alpha-stereocenters joined to nitrogens individually, which set up chiral pockets around pivotal carbene carbons (less than 2.5 angstrom). Full characterization revealed unique structures with roughly parallel aryl moieties above and below imidazoline and benzimidazole planes in the Z-shaped arrangement. Extra hydrogen bonding interactions and steric stress between the square-planar frameworks and aryl groups led to the exceptional confinement and reversed the configurations in the iridium complexes. In accordance with Tolman's electronic parameters and topographic steric maps of well-established metalated complexes, these new ligands possess strong electron-donating properties and exceptionally steric demands. Additionally, the preliminary results employing the BzITL(Ar) ligands allowed appropriate activities and provided potential applications for asymmetric induction in the enantioselective aryl transfer and hydrosilylation reactions with moderate enantioselectivity.