Chiral (phosphine)-(imidazoline) PCN pincer palladium(ii) complexes: synthesis and application in asymmetric hydrophosphination of 2-alkenoylpyridines with diphenylphosphine

Abstract

A series of new chiral PCN pincer Pd(II) complexes 3a–l with aryl-based (phosphine)-(imidazoline) ligands were conveniently synthesized from readily available starting materials with the key step being phosphination/C–H palladation reaction. All of the new complexes were characterized by ¹H NMR, ¹³C{¹H} NMR, ³¹P{¹H} NMR, and elemental analysis. In addition, the molecular structures of complexes 3c, 3e, 3i, and 3k have been determined by X-ray single crystal diffraction analysis. The potential of the obtained pincer Pd(II) complexes in catalytic enantioselective hydrophosphination reaction was examined. With a loading of 5 mol% and in the presence of a KOAc base, complex 3e acted as an active and stereoselective catalyst for hydrophosphination of various 2-alkenoylpyridines with diphenylphosphine, providing optically active pyridine-containing phosphine derivatives with structural diversity in excellent yields with moderate to good enantioselectivities (up to >99 yield and 87% ee).