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

Abstract

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 1H NMR, 13C 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 CH activation selectively at the C(5)-position of 2-acetylfuran and 2-acetylthiophene.