C (2)-Symmetric atropisomeric N-heterocyclic carbene-palladium(ii) complexes: synthesis, chiral resolution, and application in the enantioselective alpha-arylation of amides

The concept of atropisomeric N-heterocyclic carbene (NHC)-metal complexes was extended to NHCs possessing a C-2-symmetry and implemented to prepare palladium-based complexes. An in-depth study of the NHC precursors and the screening of various NHC ligands enabled us to circumvent the issue associated with the formation of meso complexes. A set of 8 atropisomeric NHC-palladium complexes were prepared and then obtained with high enantiopurities, thanks to an efficient resolution by chiral HPLC at the preparative scale. These complexes displayed good activity in the intramolecular alpha-arylation of amides and various cyclic products were isolated with excellent enantioselectivities (up to 98% ee).