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

Abstract

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), 1H and 13C NMR, 31P{1H} 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 β-substituted enones or trans-β-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 β-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 β-aryl groups in the enones bore an electron-withdrawing group such as p-NO2.