Synthesis of [PdBr2(benzimidazole-2-ylidene)(pyridine)] complexes and their catalytic activity in the direct C-H bond activation of 2-substituted heterocycles

A series of unsymmetrical 1,3-disubstituted benzimidazolium chlorides, 2a-f, having two nitrogen atoms substituted by various alkyl groups were synthesized as N-heterocyclic carbene (NHC) precursors in high yields. The benzimidazolium salts are readily converted into the corresponding PEPPSI-type palladium-NHC complexes 3a-f (PEPPSI = pyridine-enhanced precatalyst preparation, stabilization and initiation). The structures of all the compounds have been characterized by H-1 NMR, C-13 NMR and IR spectroscopy, as well as the X-ray diffraction technique (3a, 3d and 3e), which support the proposed structures. Next, the palladium-NHC-PEPPSI complexes were used as catalysts in the direct C(5)-arylation of 2-acetyl furan and 2-acetylthiophene with various aryl bromides. These complexes exhibited moderate to high catalytic activities and gave C-H activation selectively at the C(5)-position of 2-acetylfuran and 2-acetylthiophene. (c) 2021 Elsevier Ltd. All rights reserved.