Asymmetric palladium-catalyzed allylic alkylation using bis(oxazoline) ligands: Phenomenal reversal of enantioselectivity with a single chiral backbone

Chiral bis(oxazoline) ligands with four stereogenic centers were tested in the asymmetric paladium-catalyzed allylic alkylation of the rac-1,3-dipheny l-2-propenyl esters with sodium dimethyl malonate. A remarkable effect on t he enantioselectivity was observed: the dihydroxy bis(oxazoline) Ligands ga ve the (S)-product with 92% ee, while the diester led to the (R)-product wi th 90% ee. This change in direction of chiral induction sense with the dihy droxy ligand is due to the regioselection of the nucleophilic attack on the palladium(II) pi-allyl complex. Implication of the interaction of a hydrox y group with the dimethyl malonate anion is considered to explain the chang e in the regiochemistry of the nucleophilic attack. X-ray structures of the eta(3)-1,3-diphenylallyl-[(2,2-bis[2-((4S)-((1S)-1-hydroxyl-1-phenylmethyl )-1,3-oxazolinyl)]propane)-N,N']palladium(II) tetrafluoroborate 7 and the e ta(3)-1,3-diphenylallyl-[(2,2-bis[2-((4S)-((1S)-1-methyloxy-1-phenylmethyl) -1,3-oxazolinyl)]propane)-N,N']palladium(II) tetrafluoroborate 8 are presen ted, which clearly show the occurrence of hydrogen bonding in complex 7.