Chiral PCN pincer Ni(ii) complex-catalyzed asymmetric hydrophosphination of 2-alkenoylpyridines with diphenylphosphine

Abstract

Herein, nine chiral PCN pincer Ni(II) complexes 2 with (phosphine)-(imidazoline) ligands and two complexes 5a and 5b bearing (phosphinite)-(imidazoline) ligands were successfully synthesized via a “one-pot” phosphination(phosphorylation)/nickelation reaction. All the new complexes were characterized using elemental analysis and NMR spectroscopy. Additionally, the molecular structures of complexes 2a, 2e and 5a were elucidated using X-ray single-crystal diffraction analysis. Their efficacy as enantioselective catalysts for the asymmetric hydrophosphination of 2-alkenoylpyridines was investigated. Using 5 mol% of complex 2a as the catalyst in the presence of Et₃N, various 2-alkenoylpyridines reacted smoothly with diphenylphosphine to afford structurally diverse chiral pyridine-containing phosphine derivatives in yields up to 99% with an enantioselectivity up to 98% ee. Further transformations of the catalysis products were also studied.