Direct arylation of heteroarenes catalyzed by palladium complexes based on N-isopropyl-substituted N-heterocyclic carbenes

A series of seven 1-isopropyl-5,6-dimethylbenzimidazolium chlorides with a range of benzyl substituents on their N3 position (1a–g) were synthesized and used as NHC ligand precursors to prepare the corresponding [PdCl₂(NHC)(Py)] complexes (2a–g). The structural analyses of all novel compounds were done by ¹H and ¹³C{¹H} Nuclear Magnetic Resonance Spectroscopy, Fourier Transform Infrared Spectroscopy, and elemental analysis was also performed to evaluate the purity of the complexes. Also, the solid state structures of complexes 2d and 2g have been determined by the X-ray crystallography technique. Under optimized reaction conditions, the catalytic activity of the palladium complexes was tested by direct arylation of 2-n-propylthiazole, 2-n-propylfuran, and 2-n-propylthiophene with various aryl halides at 120 °C for 1 h and 3 h. It was observed that palladium complexes gave high yields and were selective at the C5-position of heteroaryl derivatives. Complex 2g, with an NHC N3-substituted by a 2,3,4,5,6-pentamethylbenzyl group showed the highest conversion probably due to the flexibility, the electronic and mostly the steric effects of this group.