Synergistic Homogeneous\nAsymmetric Cu Catalysis with\nPd Nanoparticle Catalysis in Stereoselective Coupling of Alkynes with\nAldimine Esters

Understanding the nature of a transition-metal-catalyzed\nprocess,\nincluding catalyst evolution and the real active species, is rather\nchallenging yet of great importance for the rational design and development\nof novel catalysts, and this is even more difficult for a bimetallic\ncatalytic system. Pd(0)/carboxylic acid combined system-catalyzed\nallylic alkylation reaction of alkynes has been used as an atom-economical\nprotocol for the synthesis of allylic products. However, the asymmetric\nversion of this reaction is still rather limited, and the in-depth\nunderstanding of the nature of active Pd species is still elusive.\nHerein we report an enantioselective coupling between readily available\naldimine esters and alkynes using a synergistic Cu/Pd catalyst system,\naffording a diverse set of α-quaternary allyl amino ester derivatives\nin good yields with excellent enantioselectivities. Mechanistic studies\nindicated that it is most likely a synergistic asymmetric molecular\nCu catalysis with Pd nanoparticle catalysis. The Pd catalyst precursor\nis transformed to soluble Pd nanoparticles <i>in situ</i>, which are responsible for activating the alkyne to an electrophilic\nallylic Pd intermediate, while the chiral Cu complex of the aldimine\nester enolate provides chiral induction and works in synergy with\nthe Pd nanoparticles.