Copper-Catalyzed Enantioselective Propargylic Amination of Propargylic Esters with Amines: Copper−Allenylidene Complexes as Key Intermediates

The scope and limitations of the copper-catalyzed propargylic amination of various propargylic esters with amines are presented, where optically active diphosphines such as Cl-MeO-BIPHEP and BINAP work as good chiral ligands. A variety of secondary amines are available as nucleophiles for this catalytic reaction to give the corresponding propargylic amines with a high enantioselectivity. The results of some stoichiometric and catalytic reactions indicate that the catalytic amination proceeds via copper−allenylidene complexes formed in situ, where the attack of amines to the electrophilic γ-carbon atom in the allenylidene complex is an important step for the stereoselection. Investigation of the relative rate constants for the reaction of several para-substituted propargylic acetates with N-methylanilines reveals that the formation of the copper−allenylidene complexes is involved in the rate-determining step. The result of the density functional theory calculation on a model reaction also supports the proposed reaction pathway involving copper−allenylidene complexes as key intermediates. The catalytic procedure presented here provides a versatile and direct method for the preparation of a variety of chiral propargylic amines.