Asymmetric Palladium-Catalyzed Allylic Alkylation Using Bis(Oxazoline) Ligands:  Phenomenal Reversal of Enantioselectivity with a Single Chiral Backbone

Abstract

Chiral bis(oxazoline) ligands with four stereogenic centers were tested in the asymmetric palladium-catalyzed allylic alkylation of the rac-1,3-diphenyl-2-propenyl esters with sodium dimethyl malonate. A remarkable effect on the enantioselectivity was observed: the dihydroxy bis(oxazoline) ligands gave the (S)-product with 92% ee, while the diester led to the (R)-product with 90% ee. This change in direction of chiral induction sense with the dihydroxy ligand is due to the regioselection of the nucleophilic attack on the palladium(II) π-allyl complex. Implication of the interaction of a hydroxy group with the dimethyl malonate anion is considered to explain the change in the regiochemistry of the nucleophilic attack. X-ray structures of the η3-1,3-diphenylallyl-[(2,2-bis[2-((4S)-((1S)-1-hydroxy-1-phenylmethyl)-1,3-oxazolinyl)]propane)-N,N‘]palladium(II) tetrafluoroborate 7 and the η3-1,3-diphenylallyl-[(2,2-bis[2-((4S)-((1S)-1-methyloxy-1-phenylmethyl)-1,3-oxazolinyl)]propane)-N,N‘]palladium(II) tetrafluoroborate 8 are presented, which clearly show the occurrence of hydrogen bonding in complex 7.