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

The addition of excess H 2 C C(H)ER to (PCy 3 ) 2 Cl 2 Ru C(H)R ( 1a, b ) afforded a series of well-defined ruthenium carbene complexes, (PCy 3 ) 2 Cl 2 Ru 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)(PCy 3 )Cl 2 Ru C(H)ER ( 11 − 14 ) in 48−89% yield. Alternatively, addition of H 2 C C(H)OEt to (H 2 IMes)(PCy 3 )Cl 2 Ru C(H)Ph ( 3a; H 2 IMes = 1,3-dimesityl-4,5-dihydroimidazolylidene) afforded (H 2 IMes)(PCy 3 )Cl 2 Ru 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 1 H NMR spectra was found to be inversely related to the electronegativity of the α-heteratom; however, no trends were evident in the 31 P or 13 C 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 (PCy 3 ) 2 Cl 2 Ru C(H)ER complexes followed the order C > N > S > O. In addition, complexes coordinated with an N-heterocyclic carbene ligand (e.g., (IMes)(PCy 3 )Cl 2 Ru 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 (PCy 3 ) 2 Cl 2 Ru C(H)OEt was isolated and determined by X-ray analysis to be (PCy 3 ) 2 ClRu(H)CO ( 10 ).