Synthesis of [PdBr2(Benzimidazole-2-Ylidene)(Pyridine)] Complexes and Their Catalytic Activity in the Direct C-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 readily converted into the corresponding PEPPSI-type palladium–NHC complexes 3a-f (PEPPSI = pyridine-enhanced precatalyst preparation, stabilization, and initiation). The structures of all compounds have been characterized by 1 H NMR, 13 C NMR, and IR spectroscopy, as well as X-rays ( 3a, 3d and 3e ) techniques, 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.