Chiral NHC Ligands for Enantioselective Gold(I) Catalysis Under Aerobic Conditions: the Importance of Conformational Flexibility and Steric Hindrance of NHC Ligand on Reactivity

Abstract

AbstractGold(I) catalysis has been recognized as a valuable tool for the unique transformation of multiple carbon‐carbon bonds. Enantioselective π‐catalysis based on gold(I) complexes is, however, still underdeveloped due to lack of privileged ligands. Herein, we present an accessible method to a new family of stable yet catalytically active chiral NHC−Au(I)−Cl complexes. The key to preserving a simultaneous fine balance between reactivity and stability in this newly developed family appears to be sterically hindered, but conformationally flexible NHC ligands. These could be easily accessed on a multigram scale by merging sterically hindered anilines with commercially available amino alcohols and amines via a four‐steps synthetic sequence without the need for chromatographic purification. Further investigations of the catalytic activity of NHC−Au−Cl complexes identified the OH functionality incorporated into the NHC core as crucial for the level of enantioselectivity as well as the TsO− anion responsible for the activation of NHC−Au(I)−Cl. Finally, NMR studies and X‐ray investigations revealed for the first time that the widely accepted ion metathesis (NHC−Au−Cl to NHC−Au−OSO2R) responsible for the activation of NHC−Au−Cl complexes does not take place (or it is very slow) in commonly used MeNO2 in contrast to DCM.