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

Abstract

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