Synthesis of Novel N-Heterocyclic Carbene-Oxazoline Palladium Complexes and Their Applications in Suzuki–Miyaura Cross-Coupling Reaction

Abstract

A series of novel N-heterocyclic carbene-oxazoline ligands were synthesized in six steps. Palladium complexes were obtained by deprotonation of the benzimidazole salts and subsequent ligation with Pd(OAc)₂ in THF. Different types of cyclic bis- and tetrapalladium complexes were achieved by modifying substituent of oxazoline group. The structures of these palladium complexes were characterized by NMR and X-ray diffraction analysis. Catalytic properties of these Pd-complexes were tested by Suzuki–Miyaura cross-coupling reaction.

Key words

N-heterocyclic carbene - oxazoline - palladium complex - Suzuki–Miyaura cross-coupling reaction

• References and Notes

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• 19 General Procedure of the NHC-Pd Catalyzed Suzuki–Miyaura Cross-Coupling Reaction: NHC-Pd (1.25 or 2.5 μmol, 1 mol% Pd), aryl halide (0.5 mmol, 1 equiv), arylboronic aid (0.6 mmol, 1.2 equiv), potassium tert-butoxide (0.75 mmol, 1.5 equiv) were added into a tube under argon, and the solvent i-PrOH (2 mL) was added into the tube via a syringe. The mixture was stirred for 16 h at 80 °C. After the reaction completed, H₂O was added to the mixture, and the aqueous phase was extracted with EtOAc (3 ×). The combined organic solvent was dried over anhyd Na₂SO₄ and removed in vacuo. The residue was purified by flash column chromatography on silica gel column with petroleum ether as an eluent to give the desired biaryl products 8 in good to excellent yields.