Design of rigid chiral bipyridine-2NO tetradentate ligands: application in asymmetric catalysis

Abstract

Expanding the chemical space of chiral N-oxides and chiral 2,2′-bipyridine ligands is in great demand in asymmetric catalysis. Herein, a new class of C₂-symmetric bipyridine-tertiary amine-derived chiral N,N′-dioxide (bipyridine-2NO) ligands, featuring structural fine-tunability, chemical stability, and ready accessibility, was rationally designed and developed, and the effectiveness was demonstrated in a Ni(II)-catalyzed asymmetric Michael-type Friedel–Crafts alkylation reaction of indoles and β,γ-unsaturated α-keto esters. Excellent yields (up to 92%) and high enantioselectivities (up to 99% ee) were obtained for a wide range of substrates under mild conditions. These results demonstrated the promising potential of the chiral bipyridine-2NO scaffold as an efficient type of tetradentate ligand.