Imidazol(in)ium-2-Carboxylates as N-Heterocyclic Carbene Ligand Precursors for Ruthenium Metathesis Initiators

Summary: Imidazol(in)ium-2-carboxylates were used as N-heterocyclic carbene (NHC) ligand precursors to convert the RuCl₂(p-cymene)]₂ dimer into three ruthenium-arene complexes of the RuCl₂(p-cymene)(NHC)] type. The decarboxylation of NHC · CO₂ betaines also provided a convenient synthetic path to prepare five well-known ruthenium-NHC catalysts for olefin metathesis and related reactions, including the second generation Grubbs and Hoveyda–Grubbs catalysts, via ligand exchange with phosphine-containing, first generation ruthenium-benzylidene or indenylidene complexes. Both procedures are particularly attractive from a practical point of view, because NHC · CO₂ adducts are stable zwitterionic compounds that can be stored and handled with no particular precautions.