Phen-2NO, a new C2-symmetric rigid-featured tetradentate ligand, and its application in the asymmetric alkylation reaction of indoles

Abstract

As phenanthroline (phen) possesses great coordination ability with various ions, the development of new chiral phen ligands is in great demand in asymmetric catalysis. Herein, we rationally designed and developed an entirely new class of C₂-symmetric rigid-featured chiral phen ligands that provide the N,N,O,O-tetradentate coordination of the phen moiety and two additional N,N-dioxides. The chiral ligands, termed Phen-2NO ligands, incorporate the advantages of both the phen skeleton and the pyrroloimidazolone-based N-oxide moiety, and their electronic and steric properties can be readily tuned through the incorporation of substituents in the aromatic rings of the amide group. These ligands presented a superior performance in the Ni(II)-catalyzed asymmetric Michael-type Friedel–Crafts alkylation reaction of indoles with β,γ-unsaturated α-ketoesters. Excellent yields (up to 93%) and high enantioselectivities (up to 99% ee) are obtained for a wide range of substrates under mild conditions. These results demonstrated the promising potential of the Phen-2NO ligand as an efficient chiral phen ligand.