Copper-free and amine-free Sonogashira coupling in air in a mixed aqueous medium by palladium complexes of N/O-functionalized N-heterocyclic carbenes

Highly convenient copper-free and amine-free Sonogashira coupling of aryl bromides and iodides with terminal acetylenes under amenable conditions in air and in a mixed aqueous medium are reported using several new, user friendly and robust palladium precatalysts (1-5) of N/O-functionalized N-heterocyclic carbenes (NHCs). In particular, the precatalysts, 1 and 2, were synthesized from the imidazolium chloride salts by the treatment with PdCl₂ in pyridine in presence of K₂CO₃ as a base while the precatalysts, 3-5, were synthesized from the respective silver complexes by the treatment with (COD)PdCl₂. The DFT studies carried out on the 1-5 complexes suggest the presence of strong NHC-Pd σ-interactions arising out of deeply buried NHC-Pd σ-bonding molecular orbitals (MOs) that account for the inert nature of the metal-carbene bonds and also provide insights into the exceptional stability of these precatalysts.