Chiral <scp>Phenol‐2NO</scp> Ligand Cooperation with Achiral Organic Base in the Zn(<scp>II</scp>)‐Catalyzed Asymmetric Alkylation Reaction of Indoles
Ke‐Lan Xu, Yuheng Wang, Xirui Wang, Pan Hu, Bo‐Wen Pan, Wenjing Zhang, Zi‐Yue Chen, Ying Zhou, Xiong‐Li Liu
Abstract
Comprehensive Summary The privileged C 2 ‐symmetric rigid phenol‐type ligand is more attractive but challenging in asymmetric catalysis. Herein, we designed and synthesized a class of rigid‐featured chiral tridentate Phenol‐2NO ligands, that incorporate the advantages of both the phenol skeleton and pyrroloimidazolone‐based N ‐oxide moiety, from readily available L ‐prolinamides in operationally simple two steps and up to 44% overall yield. More importantly, using an achiral quinoline derivative as an additive, the newly developed Phenol‐2NO ligand could serve as the anioic ligand upon deprotonative activation to coordinate to Zn(II) to form a highly enantioselective catalyst for the asymmetric Michael‐type Friedel‐Crafts alkylation reaction of indoles with 2,3‐dioxopyrrolidines. Excellent yields (up to 90%) and high enantioselectivities (up to 99% ee) are obtained for a wide range of substrates under mild conditions. Experiments and DFT calculations revealed the reaction mechanism and the origins of the enantioselectivity. This also represented the first activation of phenol‐type ligand/metal complex by an achiral organic base as the additive in asymmetric catalysis.