Chiral CNN Pincer Palladium(II) Complexes with 2-Aryl-6-(oxazolinyl)pyridine Ligands: Synthesis, Characterization, and Application to Enantioselective Allylation of Isatins and Suzuki-Miyaura Coupling Reaction

A series of chiral 2-aryl-6-(oxazoliny)pyridine (aryl = phenyl or 1-naphthyl) ligands 2a-f were conveniently prepared from commercially available 6-bromo-2-picolinaldehyde in two steps. Reaction of 2a-f with PdCl2 in toluene in the presence of sodium bicarbonate afforded the corresponding CNN pincer Pd(II) complexes 3a-f via aryl C-H bond activation of the related ligands. All of the new compounds have been fully characterized by elemental analysis (MS for ligands), H-1 and C-13 NMR, and IR spectra. In addition, the molecular structures of Pd(II) complexes 3c-f have been determined by X-ray single-crystal diffraction. The obtained chiral pincer catalysts were successfully used in the asymmetric allylation of isatins with allyltributyltin, giving the corresponding 3-allyl-3-hydroxyoxindoles in high yields with enantioselectivities of up to 86% ee. These pincers could also catalyze the asymmetric Suzuki-Miyaura coupling reaction to provide the axially chiral biaryl products in good yields with good stereoselectivities (up to 68% ee).