Origin of Enantioselectivity in Palladium-Catalyzed Asymmetric Allylic Alkylation Reactions Using Aminophosphine Ligands

Abstract

The asymmetric induction observed in palladium-catalyzed allylic alkylation reactions with typical substrates was investigated using three structurally related chiral PN ligands, which all contain a binaphthyl unit and either a benzyl (1) or ferrocenylmethyl (2, 3) fragment bridging phosphorus and nitrogen complexation sites. The intermediate allyl palladium(II) complexes of ligands 1−3 have been isolated and spectroscopically characterized. For two of them, 1K and 3K, the crystal structure has been determined. A rationalization of the experimental findings on a molecular level is proposed based on the results of DFT (ADF) and molecular mechanics (CERIUS2) calculations. Studies on both the entire molecules and simpler models indicate that the stereochemical outcome is not determined by electronic effects, but mostly by a delicate balance of steric repulsions. In particular, the presence of a benzyl or a ferrocenylmethyl group in the chelate chain has a strong effect on its arrangement, thereby directly influencing asymmetric induction.