New chiral phosphine-phosphite ligands in the enantioselective palladium-catalyzed allylic alkylation

A series of chiral phosphine-phosphite ligands 1-6 have been synthesized an d used in the enantioselective palladium-catalyzed reaction of rac-1,3-diph enyl-2-propenyl acetate with dimethyl malonate as nucleophile. Ligands la, 2, 3, 5a, 6a, and 6b have been synthesized starting from racemic tert-butyl phenylphosphinoborane. The use of dynamically resolved Li phosphide (-)spar teine provided the optically pure ligands. Crystals of the allylpalladium ( 6a) complex were obtained, suitable for X-ray crystal structure determinati on. The X-ray crystal structure of the allylpalladium (6a) complex revealed a longer palladium-carbon bond distance trans to the phosphine moiety indi cating that the attack of the nucleophile takes place at the carbon trans t o the phosphine moiety. This was confirmed by the fact that the phosphine m oiety did not affect the enantioselectivity directly. Under mild reaction c onditions, enantioselectivities up to 83% were obtained (25 degrees C) with ligand le. Systematic variation of the ligand bridge and the phosphite moi ety showed that the configuration of the product is controlled by the atrop isomerism of the biphenyl substituent at the phosphite moiety. The conforma tion of the biphenyl group, in turn, is controlled by the substituent at th e chiral carbon in the bridge. Ligands with large bite angles yielded highe r enantioselectivities.