Synthesis, structures, DFT calculations, and catalytic application in the direct arylation of five-membered heteroarenes with aryl bromides of novel palladium-N-heterocyclic carbene PEPPSI-type complexes

Abstract

A new series of Pd-catalysts based on an N-heterocyclic carbene PEPPSI-type ligand (PEPPSI = pyridine enhanced precatalyst preparation stabilization and initiation) with the general formula [Pd(II)Br2(NHC)(pyridine)] was synthesized and fully characterized via spectroscopic analytical methods. Further, the structural characterization of 3a, 3b, and 3d was performed via single-crystal X-ray diffraction, which supported the proposed structures and offered a more detailed structural characterization. Additionally, the electronic, vibrational, thermodynamic, and optical properties of 3a, as a representative molecule, were confirmed utilizing density functional theory (DFT) calculations. The experimental molecular geometry of the ground state of complex 3a was compared with the minimized structure obtained from DFT calculations. The B3LYP functional in conjunction with the LANL2DZ basis set for the palladium atom and the 6-311G(d,p) basis set for the hydrogen, carbon, nitrogen, and bromine atoms were used for all calculations. The frontier molecular orbitals and molecular electrostatic potential were also analyzed and discussed. Due to the significant interest in halo-substituted arylated heteroarenes in organic chemistry, the catalytic activity of these Pd(II)–NHC PEPPSI–type complexes were evaluated via the direct arylation of five-membered heteroaromatics such as thiophene, furan, and thiazole derivatives with various (hetero)aryl bromides in the presence of 1 mol% catalyst. The results showed that all new Pd–NHC complexes are effective catalysts, which exhibited very good catalytic activity and gave C-H activation selectively at the C(5)-position of 2-acetyl furan and 2-acetyl thiophene.