NEW CHIRAL N,S-LIGANDS BASED ON OXAZOLINE-THIOGLUCOSE DONORS - PALLADIUM(II)-CATALYZED ENANTIOSELECTIVE ALLYLIC ALKYLATION

New bidentate ligands containing both chiral oxazoline and thiosugar e lements, 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-beta-D-glucop yranose moiety. These N,S-oxazoline-thioglucose ligands afford excelle nt ee's (90.2-96.9%) in the model enantioselective allylic alkylation reaction involving a 1,3-diphenylallyl precursor. H-1 and C-13 NMR spe ctra for the Pd compounds show that they exist in solution as a mixtur e of(syn/syn) exo and endo diastereomeric complexes. It is suggested t hat attack of the dimethyl malonate nucleophile pseudo-trans to the th ioether donor is preferred for electronic reasons, whereas selective a ttack on the endo diastereomer, as opposed to the exo isomer, arises d ue to steric effects in combination with allyl rotation. The organic p roduct is formed by preferential reaction of a minor component. Since the exo and endo isomers are shown to be in equilibrium, via 2-D excha nge spectroscopy, the depleted endo diastereomer can be rapidly replac ed.