Metathesis of electron-rich olefins: Structure and reactivity of electron-rich carbene complexes

The addition of excess H2C=C(H)ER to (PCy3)(2)Cl2Ru=C(H)R (1a,b) afforded a series of well-defined ruthenium carbene complexes, (PCy3)(2)Cl2Ru=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 (H(2)IMes)(PCy3)Cl2Ru=C(H)Ph (3a; H(2)IMes = 1,3-dimesityl-4,5-dihydroimidazolylidene) afforded (H(2)IMes)(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 H-1 NMR spectra was found to be inversely related to the electronegativity of the alpha-heteratom; however, no trends were evident in the P-31 or C-13 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)(2)Cl2Ru=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)(2)Cl2Ru=C(H)OEt was isolated and determined by X-ray analysis to be (PCy3)(2)ClRu(H)CO (10).