Combining Chiral Aldehyde Catalysis and Transition-Metal Catalysis for Enantioselective α-Allylic Alkylation of Amino Acid Esters

A chiral aldehyde is rationally combined with a Lewis acid and a transition metal for the first time to form a triple catalytic system. This cocatalytic system exhibits good catalytic activation and stereoselective-control abilities in the asymmetric alpha-allylation reaction of N-unprotected amino acid esters and allyl acetates. Optically active alpha,alpha-disubstituted a-amino acids (alpha-AAs) are generated in good yields (up to 87%) and enantioselectivities (up to 96% ee). Preliminary mechanism investigation indicates that the chiral aldehyde 3f acts both as an organocatalyst to activate the amino acid ester via the formation of a Schiff base, and as a ligand to facilitate the nucleophilic attack process by coordinating with pi-allyl Pd(II) species.