Unsymmetrical Chiral PCN Pincer Palladium(II) and Nickel(II) Complexes with Aryl-Based Aminophosphine-Imidazoline Ligands: Synthesis via Aryl C-H Activation and Asymmetric Addition of Diarylphosphines to Enones

Chiral 3-(2'-imidazolinyl)anilines 3a-c were easily synthesized by converting the carboxyl and nitro groups in commercially available 3-nitrobenzoic acid to chiral imidazoline and amine, respectively. The one-pot phosphorylation/metalation reaction of 3-(2'-imidazolinyl)anilines 3a-c, where the amino group in 3a-c was first phosphorylated by reaction with PPh2Cl, followed by metalation with PdCl2 or anhydrous NiCl2 in situ, afforded the three unsymmetrical chiral PCN pincer Pd(II) complexes 4a-c and the Ni(II) complex 5a with aryl-based aminophosphine-imidazoline ligands via aryl C-H bond activation of the related ligands. All of the new compounds were characterized by elemental analysis (HRMS for ligand precursors), H-1 and C-13 NMR, P-31{H-1) NMR (for pincer complexes), and IR spectra. The molecular structures of Pd complexes 4a,c and that of Ni complex 5a have been determined by X-ray single-crystal diffraction. Each complex adopts a typical distorted-square-planar geometry. The potential of the obtained chiral pincers in the asymmetric addition of diarylphosphines to beta-substituted enones or trans-beta-nitrostyrene was investigated. The Pd pincer 4c, the chirality of which originated from L-phenylglycinol, was found to be an effective catalyst for the asymmetric addition of diphenylphosphine to a series of beta-aryl enones with high enantioselectivities (12 examples, 51-94% ee's). In particular, excellent yields and enantioselectivities (>90% yields and >= 90% ee's) were obtained when beta-aryl groups in the enones bore an electron-withdrawing group such as p-NO2.