Ligand-promoted carbene insertion into the aryl substituent of an n-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)(PCy ₃)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}-L ₂(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(H ₂IMes)(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.