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-(oxazolinyl)­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), 1H and 13C 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).