Latent Ruthenium Olefin Metathesis Catalysts That Contain an N-Heterocyclic Carbene Ligand

A new N-heterocyclic carbene containing olefin metathesis catalyst, (sIMes)(Cl)_2Ru(CH(CH_2)_2-C,N-2-C_5H_4N) (4a), was synthesized from (sIMes)(PCy_3)(Cl)_2RuCHPh (1) or (sIMes)(py)_2(Cl)_2RuCHPh (3). When heated at 40 °C in dichloromethane, 4a is slowly converted to its isomer 4b. The X-ray structures of 4a and 4b show that the NHC and pyridine ligands are trans in 4a and cis in 4b. 4a is more latent than 1 and 4b much more latent than 4a in ring-closing metathesis (RCM) and ring-opening metathesis polymerization (ROMP).