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

Abstract

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).

Enantioselective and Enantiospecific Transition-Metal-Catalyzed Cross-Coupling Reactions of Organometallic Reagents To Construct C–C Bonds

“…Several additional ligands that have proven less successful are shown in Figure 96 . 391 − 398 Enantioenriched dialkoxybinaphthalenes have also been targeted through an asymmetric Suzuki–Miyaura coupling ( Scheme 53 ).…”

Catalytic asymmetric synthesis of biologically important 3-hydroxyoxindoles: an update

“…The palladium catalyst is widely used in organic synthesis and has showed its usefulness in the allylation of isatins. In 2014, Song and co-workers designed the chiral CNN (three atoms attched to the palladium) pincer Pd complexes, which were proved to be efficient in catalyzing the enantioselective allylation of isatins with allyltributytin ( Scheme 1 ) [ 14 ]. 3-Allyl-3-hydroxyoxindoles were obtained in 89–98% yield and with 32–86% ee when the reactions were carried out at −60 °C using 5 mol % of CNN pincer Pd complex (cat.…”

Palladium catalyzed asymmetric Suzuki–Miyaura coupling reactions to axially chiral biaryl compounds: Chiral ligands and recent advances

“…Song and coworkers successively prepared the chiral CNN and PCN pincer palladium (II) complexes 22 and 23a-e (Scheme 20) via C-H bond activation of the related ligands [36][37].…”