Development of chiral modular bifunctional C2-symmetric bipyridine/phenanthroline-bipyrroloimidazolone ligands and application in noncovalent interaction-assisted enantioselective catalysis

Abstract

Chiral bipyridine/phenanthroline ligands have been applied as a class of unique ligands due to their rich coordination chemistry in catalysis. Herein, we synthesize 13 new C₂-symmetric chiral phenanthroline-bipyrroloimidazolone ligands (Phen-BPI) and 5 new C₂-symmetric chiral bipyridine-bipyrroloimidazolone ligands (BPy-BPI). These Bpy/Phen-BPI pincer ligands performed successfully in nickel-catalyzed asymmetric Friedel–Crafts alkylation of indoles with 2-enoylpyridine-1-oxides, affording the chiral products with up to 93% yield and 99% ee. The control experiment and metal complex L1–Ni crystal supported the dual activation manner, in which the tridentate Bpy/Phen-BPI ligand contained two nitrogen atoms of the pyridine unit and one tertiary amine group of the pyrroloimidazolone moiety coordinating with Ni(II) to activate the 2-enoylpyridine-1-oxides, and the tertiary amine group of the additional pyrroloimidazolone moiety as the H-bond acceptor activated the indoles through a NH-bonding interaction, resulting in the asymmetric transformation. This represented the first example showing that the Bpy/Phen ligand contained three nitrogen atoms coordinating with the metal, and the additional nitrogen atom acted as the H-bond acceptor.