Synthesis of N-Heterocyclic Carbene Complexes by Oxidative Addition of 4-Iodo-imidazolium Salts Followed by an Unusual Rearrangement

Compound 1,3-diethyl-4-iodo-imidazolium iodide (H-1(I)) has been metalated at the C2 position through an oxidative addition/rearrangement reaction sequence. The oxidative addition of the C4–I bond to low-valent transition metals [M(PPh3)4] (M = Pd, Pt) yields presumably complexes bearing an abnormal carbene (aNHC) ligand. These complexes undergo a subsequent intermolecular rearrangement leading via metalation of the C2 position to the classical monometallic NHC complexes [2] and [3]. A plausible mechanism for the formations of [2] and [3] is proposed based on DFT calculations and MS studies. The reaction of hexafluorophosphonium salt H-1(PF6) with [Pd(PPh3)4] yields via oxidative addition of the C4–I bond to Pd0 the expected PdII aNHC complex [4](PF6).