Origin of enantioselectivity with heterobidentate sulfide-tertiary amine (sp3) ligands in palladium-catalyzed allylic substitution

A series of new chiral heterobidentate sulfide-tertiary amine (sp3) ligands 3a–c, 6 were readily prepared from cheap and easily available (R)-cysteine and l-(+)-methionine. A Pd-catalyzed asymmetric allylic alkylation of 1,3-diphenyl-2-propenyl acetate with dimethyl malonate was used as a model reaction to examine the catalytic efficiencies of these aziridine sulfide ligands, and ligand 3b afforded the enantioselectivity of up to 91% ee. The origin of enantioselectivity for heterobidentate sulfide-tertiary amine (sp3) ligands was first rationalized based on X-ray crystallographic data, and NMR spectroscopic data for relevant intermediate palladium allylic complexes. Our results demonstrated that the sulfur atom was a better π-allyl acceptor than the nitrogen atom for heterobidentate sulfide-tertiary amine (sp3) ligands, and the steric as well electronic properties of the palladium allylic complexes were crucial for the enantioselectivity.