Synthesis and reactivity of olefin metathesis catalysts bearing cyclic (alkyl)(amino)carbenes.

The evolution of olefin metathesis into a reaction routinely used to form new carbon–carbon double bonds has been enabled by the development of well-defined transition-metal catalysts. [1,2] Many metathesis catalysts based on the [L2X2Ru=CHR] scaffold have been synthesized in an effort to increase catalyst stability, activity, and substrate scope.[3–10] A significant gain in these areas was achieved after exchanging a single PCy3 ligand of 1 with H2IMes (H2IMes = 1,3-dimesityl-4,5-dihydroimidazol-2-ylidene),an N-heterocyclic carbene (NHC), to produce catalyst 2 (Figure 1).[5] These results are attributed to the increased σ-donor ability of H2IMes over PCy3, which increases the affinity for π-acidic olefins relative to σ-donating phosphines. [11] Additionally, exchange of the remaining PCy3 ligand with a chelating ether moiety provides a more stable complex, catalyst 3.[6]