Metathesis of Electron-Rich Olefins: Structure and Reactivity of Electron-Rich Carbene Complexes
journal contributionposted on 25.04.2002 by Janis Louie, Robert H. Grubbs
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The addition of excess H2CC(H)ER to (PCy3)2Cl2RuC(H)R (1a,b) afforded a series of well-defined ruthenium carbene complexes, (PCy3)2Cl2RuC(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)Cl2RuC(H)ER (11−14) in 48−89% yield. Alternatively, addition of H2CC(H)OEt to (H2IMes)(PCy3)Cl2RuC(H)Ph (3a; H2IMes = 1,3-dimesityl-4,5-dihydroimidazolylidene) afforded (H2IMes)(PCy3)Cl2RuC(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)2Cl2RuC(H)ER complexes followed the order C > N > S > O. In addition, complexes coordinated with an N-heterocyclic carbene ligand (e.g., (IMes)(PCy3)Cl2RuC(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)2Cl2RuC(H)OEt was isolated and determined by X-ray analysis to be (PCy3)2ClRu(H)CO (10).