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

Gold(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.