Modular Synthesis of C2-Symmetric Chiral TPy-BPI Pincer Ligands and Application in Asymmetric Ni(II)-Catalysed Friedel-Crafts Alkylation

Abstract

Terpyridine ligands have been applied as a class of unique ligands due to their rich coordination chemistry in the catalysis. Herein, we developed a new class of C₂-symmetric chiral terpyridine-pyrroloimidazolone ligands (TPy-BPI). Their catalytic activity was evaluated in the asymmetric Friedel-Crafts alkylation of indoles with 2,3-dioxopyrrolidines. Excellent yields (up to 92%) and high enantioselectivities (up to 97% ee) are obtained for a wide range of substrates under mild conditions. In addition to 2,3-dioxopyrrolidines, β,γ-unsaturated α-ketoesters were also compatible in the Ni(OTf)₂-TPy-BPI ligand L1-catalyzed reaction. Control experiments, single crystal structure of two TPy-BPI L1-Ni(OTf)₂ complexes, and DFT calculations revealed the origins of the enantioselectivity. To the best of our knowledge, our work is the first report showing that the terpyridine framework contained only two nitrogen atoms coordinating with the metal, and the additional pyridine unit only acted as stereodirecting element, which is different from the previously reported terpyridine ligands containing all three nitrogen atoms coordinating with the metal.