New Chiral N,S-Ligands Based on Oxazoline−Thioglucose Donors. Palladium(II)-Catalyzed Enantioselective Allylic Alkylation

Abstract

New bidentate ligands containing both chiral oxazoline and thiosugar elements, and also their 1,3-diphenylallyl Pd(II) complexes, have been prepared. The sugar is based on a 2,3,4,6-tetra-O-acetyl-β-d-glucopyranose moiety. These N,S-oxazoline−thioglucose ligands afford excellent ee's (90.2−96.9%) in the model enantioselective allylic alkylation reaction involving a 1,3-diphenylallyl precursor. 1H and 13C NMR spectra for the Pd compounds show that they exist in solution as a mixture of (syn/syn) exo and endo diastereomeric complexes. It is suggested that attack of the dimethyl malonate nucleophile pseudo-trans to the thioether donor is preferred for electronic reasons, whereas selective attack on the endo diastereomer, as opposed to the exo isomer, arises due to steric effects in combination with allyl rotation. The organic product is formed by preferential reaction of a minor component. Since the exo and endo isomers are shown to be in equilibrium, via 2-D exchange spectroscopy, the depleted endo diastere...