Transition metal complexes bearing atropisomeric saturated NHC ligands

From achiral imidazolinium salts, chiral transition metal complexes containing an N-heterocyclic carbene (NHC) ligand were prepared (metal = palladium, copper, silver, gold, rhodium). Axial chirality in these complexes results from the formation of the metal-carbene bond leading to the restriction of rotation of dissymmetric N-aryl substituents about the C-N bond. When these complexes exhibited a sufficient configurational stability, a resolution by chiral high-performance liquid chromatography (HPLC) on preparative scale enabled isolation of enantiomers with excellent enantiopurities (>99% ee) and good yields. A study of the enantiomerization barriers revealed the effect of the backbone nature as well as the type of transition metal on its values. Nevertheless, the evaluation of palladium-based complexes in asymmetric intramolecular alpha-arylation of amides demonstrated that the ability to induce an enantioselectivity cannot be correlated to the configurational stability of the precatalysts.

Automated summary

Chiral transition metal complexes with N-heterocyclic carbene ligands were prepared from achiral imidazolinium salts, showing axial chirality. The enantiomers were isolated with high enantiopurities using preparative chiral HPLC. The study revealed the impact of backbone nature and transition metal type on enantiomerization barriers.