Ligand-Promoted Carbene Insertion into the Aryl Substituent of an <i>N</i>-Heterocyclic Carbene Ligand in Ruthenium-Based Metathesis Catalysts

Addition of L = carbon monoxide or aryl isocyanides to the Grubbs second-generation carbene complexes Ru(H2IMes)(CHR)(PCy3)Cl2 (H2IMes = 1,3-dimesityl-4,5-dihydroimidazol-2-ylidene; R = Ph, Me, H, CH=CMe2) triggers carbene insertion into an aromatic ring of the N-heterocyclic carbene supporting ligand, forming Ru{1-mesityl-3-(-7′-R-2′,4′,6′-trimethylcycloheptatrienyl)-4,5-dihydroimidazol-2-ylidene}L2(PCy3)Cl2. Insertions are also promoted for other PR3 substituted complexes by carbon monoxide and aryl isocyanides, and for the phosphine-free Hoveyda−Blechert complex Ru(H2IMes)(CH(i-PrOC6H4))Cl2 by aryl isocyanides and small phosphites but only after initial displacement of the coordinated ether. Heteroatom substituted carbenes do not undergo CO-promoted insertion unless poorer electron donor phosphine (PPh3) and carbene (CH(OC6H4-p-NO2) ligands are both present. Insertion depends on the added ligand, the carbene substituent, and to a lesser degree on the PR3 ligand trans to the N-heterocyclic carbene.