Asymmetric Oxindole Synthesis Catalyzed by Palladium Complexes with Unsymmetrical Chiral Bioxazoline-Based Carbene Ligands

摘要

Despite the great success in catalyses that N-heterocyclic Carbene Ligands (NHCs) have enjoyed over the past decade, NHCs exhibiting high enantioselectivites are yet a rarity, especially as homoleptic chiral NHC species. Although monodentate chiral ligands based on phosphorus atom have also demonstrated extremely high ees in many catalytic reactions, some of the results have revealed to be collaboration of a pair of ligands. Catalytic enantioselective alpha-arylation of amide la, giving oxindole 2a, is considered as one of the most difficult examples for gainig high ee values, in which NHCs showed the highest catalytic activity as ligands. In this catalysis, each chiral NHC behaves as a sole monodentate spectator ligand in each active palladium complex. We considered prevoiusly reported not-so-high ees (up to 67% ee, 14% yield or 57% ee, 74% yield with 1a) amazing to be gained without any collaboration with one more ligand. To quantitatively elaborate the steric interactions between (a)chiral substituents on NHCs and substrate ligated on palladium, we modified bioxazoline-based chiral NHC ligands into unsymmetrical ones and applied them to the catalysis to see which substituent on NHCs is critically working for gaining higher enantioselectivity in the catalysis.