On the Mechanism of the Oxidative Cleavage of N-Heterocyclic-Carbene-Palladium Bonds with Iodine

Abstract

The excellent performance of N-heterocyclic carbenes (NHCs) in various palladium-catalyzed reactions has been explained by their strong donor character, which grants remarkable stability to palladium complexes often under extreme conditions and imparts adjustable steric demand. However, despite such a strong stability, the oxidative cleavage of palladium–carbene bonds by iodine was observed in NHC–palladium complexes. The oxidative cleavage of a palladium(II) iodide complex supported by the NHC with the smallest steric profile, 1,3-dimethylimidazol-2-ylidene (IMe), is demonstrated here. The syntheses and characterization of various palladium –IMe complexes are reported along with a study of the mechanism of the oxidative cleavage of Pd–IMe bonds with iodine. Owing to the small steric profile of IMe, the important features of structure and bonding in these complexes are preserved, but reactivity with the relatively weak oxidant iodine is facilitated. Further computational mechanistic insights for this oxidative cleavage of the Pd–NHC bond were demonstrated, and both dissociative oxidative cleavage and metathesis of the Pd–NHC bond with iodine are feasible on the basis of DFT calculations.