Synthesis Of Unsymmetrical Benzimidazolyl PCN Pincer Palladium(â…¡) Complexes And Their Application In Catalysis

Three novel unsymmetrical PCN pincer palladium(II) complexes were synthesized according to the classical one-pot phosphorylation/palladation reaction.These new compounds were fully characterized and then applied as catalysts in C–H bond arylation ofazoles with aryl iodides. The main contents are as follows:1. Synthesis and characterization of unsymmetrical PCN pincer palladium complexesFirstly, the new compound 1 was prepared from 2-bromonitrobenzene and 3-hydroxybenzoic acid in five steps. 1 was then stirred in acetic acid under reflux to obtainthe ring-closure product 2, which can be transformed to 3-(N-butylbenzimidazol-2-yl)phenol 3 by deprotection reaction. Compound 3 was stirred with chlorodiphenylphosphine, chlorodicyclohexylphosphine or chlorodiisopropyl phosphine in the presence of triethylamine in refluxing toluene, followed by the addition of palladium chloride without isolating the generated PCN pincer ligand. Eventually, three novel PCN pincer palladium complexes 4a-c were obtained. All the new compounds were characterized by 1H NMR, 13 C NMR, 31 P NMR, DEPT, HQSC. In addition, the metal complexes 4a-c were further characterized by elemental analysis and their molecular structures were determined and confirmed with X-ray single-crystal diffraction.2. Application of unsymmetrical PCN pincer palladium complexes in C-H bond arylation ofazolesThe catalytic activities of the unsymmetrical PCN pincer palladium complexes 4a-c were investigated as catalysts in C-H bond arylation ofazoles with aryl iodides. The reaction time, catalysts, reaction temperature, bases and other conditions were screened respectively, and the optimized conditions were 0.25 mol% 4a and 1.25 mol% Cu I as co-catalyst, Cs2CO3 as base in DMF at 120 oC for 18 h under argon atmosphere. Under this condition, the corresponding 2-arylated products were obtained with the yield up to 99%.