Design, Synthesis, and Application of a Family of Chiral Non-C2-Symmetric NHCs with a Fused Sidechain

Abstract

Although a considerable number of chiral nitrogen heterocyclic carbenes (NHCs) have been developed yet it is highly necessary to develop new NHCs bearing multiple sites for facile modifications of both electronic nature and steric hindrance. Herein, we uncover a new family of chiral non-C₂-Symmetric NHCs with a fused sidechain, whose precursors are synthesized by a simple five-step route. The synthesis includes Pd-catalyzed cross-coupling or nucleophilic addition/oxidation, chiral phosphoric acid-catalyzed asymmetric reduction of 2-aryl-quinolines, bromination at C8, Buchwald–Hartwig amination, and cyclization with methyl orthoformate. Among nine prepared NHCs, YC-NHC8 is the optimal ligand for Cu(I)-catalyzed asymmetric S_N2′ silylation, YC-NHC3 works as the best ligand for Cu(I)-catalyzed enantioselective conjugate silylation of simple α,β-unsaturated amides, and YC-NHC9 serves as the most suitable ligand for copper(I)-catalyzed asymmetric silylation of azadienes. Remarkably, these three reactions are successfully run under a catalyst loading of 0.1 mol%, indicating that YC-NHCs may have the potential to be broadly used in efficient asymmetric transition metal catalysis.