PEPPSI-type N-heterocyclic carbene palladium(II) complexes as catalysts in the direct C5 arylation of furan and thiophene

Abstract

In this study, a series of amine-functionalized benzimidazolium salts and their PEPPSI-type N-heterocyclic carbene (NHC) palladium(II) complexes 3a-e were synthesized and characterized by FT-IR, ¹H NMR and ¹³C NMR spectroscopy, elemental analysis, and mass spectrometry. The molecular and crystal structure of 3c and 3d was confirmed by the single-crystal X-ray diffraction (SC-XRD) method. Structural analysis reveals that the geometries of the Pd^II centers of the complexes adopt slightly distorted square planar environment. The new palladium complexes were tested as catalysts in the direct C5 arylation of 2-acetylfuran and 2-acetylthiophene with aryl bromides at 120 °C in N,N-dimethylacetamide. The arylation reactions proceeded selectively at the C5 position of the heteroaromatic compounds, and the corresponding coupling products were obtained in moderate to good yields by using 1 mol% of the palladium complex. Also, solid computational validation of the experimental results was achieved by geometric optimizations, frontier molecular orbital and molecular electrostatic potential studies, as well as natural bonding orbital analysis utilizing density functional theory.