Regioselective C-H arylation of imidazoles employing macrocyclic palladium(II) complex of organoselenium ligand

Abstract

In this report we have presented synthesis of a new air stable bidentate selenium ligand (L1) by the reaction of 1,8-bis(2-(chloromethyl)phenoxy)octane with sodium salt of diphenyl diselenide. The reaction of bidentate selenium ligand (L1) with Pd(CH3CN)2Cl2 in acetonitrile under reflux conditions resulted nineteen membered ring macrocyclic palladium(II) complex. The structure of ligand precursors, ligand, and macrocyclic palladium(II) complex were authenticated by using 1H, 13C{1H} NMR spectroscopy and elemental analysis. The repeated attempts to obtain single crystals of macrocyclic palladium(II) complex were unsuccessful probably due to long flexible aliphatic chain in the molecule. The air and moisture stable, thermally robust macrocyclic palladium(II) complex was employed as catalyst to catalyze the regioselective arylation reaction of imidazole derivatives. The reaction works efficiently without any need of inert atmosphere. The current protocol works under mild reaction conditions with exclusive C-5 regioselectivity. Excellent yields (~73–95%) were obtained by using only 1.5 mol% of C1, with wide range of derivatives and large functional group tolerance. Homogeneous nature of catalysis process was confirmed with the help of mercury and triphenylphosphine poisoning tests. Further, the catalyst can be reused with significant loss (22%) in the efficiency.