Metathesis of Electron-Rich Olefins:  Structure and Reactivity of Electron-Rich Carbene Complexes

The addition of excess H2CC(H)ER to (PCy3)2Cl2RuC(H)R (1a,b) afforded a series of well-defined ruthenium carbene complexes, (PCy3)2Cl2RuC(H)ER (ER = OEt (5), SEt (6), SPh (7), N(carbazole) (8), N(pyrrolidinone) (9)) in yields ranging from 66 to 90%. Such complexes containing an electron-donating group on the carbene carbon are often referred to as Fischer-type carbenes. Replacement of one phosphine ligand with 1,3-dimesitylimidazolylidene (IMes) afforded the respective mixed-ligand complexes (IMes)(PCy3)Cl2RuC(H)ER (11−14) in 48−89% yield. Alternatively, addition of H2CC(H)OEt to (H2IMes)(PCy3)Cl2RuC(H)Ph (3a; H2IMes = 1,3-dimesityl-4,5-dihydroimidazolylidene) afforded (H2IMes)(PCy3)Cl2RuC(H)OEt (15) in 93% yield. The crystal structures of complexes 5, 7−9, and 11 were determined and found to be structurally similar to the parent ruthenium alkylidene ([Ru]C(H)R) complexes. In solution, the chemical shift of the [Ru]C(H)ER resonance in 1H NMR spectra was found to be inversely related to the electronegativity of the α-heteratom; however, no trends were evident in the 31P or 13C NMR spectra. Intramolecular coordination of the pendant amide carbonyl group to the Ru center established a temperature-dependent equilibrium between complexes 9 and 14 and their cyclometalated forms. All Ru electron-rich complexes initiated the ring-opening metathesis polymerization (ROMP) of strained cyclic olefins and the ring-closing metathesis (RCM) of diethyl diallylmalonate. A general trend in the relative reactivities and thermal stabilities of the (PCy3)2Cl2RuC(H)ER complexes followed the order C > N > S > O. In addition, complexes coordinated with an N-heterocyclic carbene ligand (e.g., (IMes)(PCy3)Cl2RuC(H)ER) displayed enhanced activities in olefin metathesis and were thermally more stable than their bis(phosphine) analogues. Finally, the thermal decomposition product of (PCy3)2Cl2RuC(H)OEt was isolated and determined by X-ray analysis to be (PCy3)2ClRu(H)CO (10).