Synthesis and Structural Characterization of Nickel Complexes Possessing P‑Stereogenic Pincer Scaffolds and Their Application in Asymmetric Aza-Michael Reactions

Novel P-stereogenic pincer-Ni complexes
{κP,κC,κP-3,5-Me2-2,6-(MetBuPCH2)2C6H}­NiCl
(3), {κP,κC,κP-3,5-Me2-2,6-(MetBuPCH2)2C6H}­NiOTf (4),
[{κP,κN,κP-2,6-(MetBuPCH2)2C5H3N}­NiCl]Cl (7), [{κP,κN,κP-2,6-(MetBuPCH2)2C5H3N}­NiCl]­BF4 (8), and [{κP,κN,κP-2,6-(MetBuPCH2)2C5H3N}­Ni­(NCMe)]­(BF4)2 (9) were synthesized in 55–84%
yields and characterized by 1H NMR, 13C­{1H} NMR, 31P­{1H} NMR, 19F­{1H} NMR, and/or single-crystal X-ray diffractions. The ORTEP
diagrams of complexes 3, 7, 8, and 9 show that the coordination geometries around
the Ni center in all these structures are approximately square planar
but have different bond lengths and angles. These complexes were shown
to be active catalysts for the asymmetric aza-Michael addition of
α,β-unsaturated nitriles. For most examples
good to excellent yields (up to 99%) and moderate enantiomeric excesses
(up to 46% ee) were obtained. Notably, the PCP complex 3 exhibited higher catalytic activity in the aza-Michael addition
than the PNP complexes 7, 8, and 9. Two achiral PCP-type pincer-Ni complexes, {κP,κC,κP-3,5-Me2-2,6-(tBu2PCH2)2C6H}­NiCl (11) and {κP,κC,κP-3,5-Me2-2,6-(Ph2PCH2)2C6H}­NiCl (13), were also synthesized and fully characterized in order to reveal
the structural differences between the chiral and achiral complexes.