Highly Enantioselective Synthesis of (2-Pyridyl)phosphine Based C-Chiral Unsymmetrical P,N-Ligands Using a Chiral Palladium Complex

The organopalladium complex containing ortho-metalated (R)-(1-(dimethylamino)ethyl)naphthalene as the chiral auxiliary has been used to promote the asymmetric hydrophosphination reactions of diphenylphosphine with (E)-1-phenyl-3-pyridin-2-yl-2-propenone and (E)-1-methyl-3-pyridin-2-yl-2- propenoate in high regio- and stereoselectivities under mild conditions. Hydrophosphination of (E)-1-phenyl-3-pyridin-2-yl-2-propenone with diphenylphosphine generated two stereoisomeric products in a ratio of 8:1 as five-membered 2-pyridylphosphine P−N bidentate chelates on the chiral naphthylamine palladium template. Using the same chiral metal template, the corresponding hydrophosphination reaction of (E)-1-methyl-3-pyridin-2-yl-2-propenoate gave only one product as a six-membered P−N bidentate chelate. The naphthylamine auxiliary could be removed chemoselectively from the template product by treatment with concentrated hydrochloric acid to form the corresponding optically pure neutral complexes. Subsequent ligand displacement from the palladium achieved using aqueous potassium cyanide generated the optically pure keto- and ester-functionalized chiral 2-pyridylphosphines ligands in high yields. The absolute configuration and the coordination properties of the 2-pyridylphosphines have been established by single-crystal X-ray analysis.