Palladium complexes of the N-fused heterocycle derived abnormal N-heterocyclic carbenes for the much-preferred Cu-free and the amine-free Sonogashira coupling in air

摘要: Abstract Abnormal N-heterocyclic carbenes of the N-fused heterocycles were employed in stabilizing a series PEPPSI (Pyridine Enhanced Precatalyst Preparation Stabilization and Initiation) themed complexes type (abnormal-NHC)PdI2(pyridine). Specifically, imidazo[1,2-a]pyridine based abnormal complexes, trans-{(1-ethyl-2-(4-R-phenyl)-imidazol-3-ylidene[1,2-a]pyridine)}PdI2(pyridine) [R = F (1b), Cl (2b), Br (3b) Me (4b)] synthesized from reaction corresponding imidazo[1,2-a]pyridinium iodide salts (1–4)a with PdCl2 pyridine presence K2CO3 as base. The (1–4)b carried out Sonogashira couplings aryl bromide substrates terminal alkynes mixed aqueous medium air under Cu-free amine-free conditions. density functional theory studies revealed that Ccarbene–Pd σ-bonding interaction was significantly stronger than trans Pd–Npyridine interaction.