Structure and Activity Peculiarities of Ruthenium Quinoline and Quinoxaline Complexes:  Novel Metathesis Catalysts

Novel metathesis catalysts that are derivatives of 8-vinylquinoline, (H 2 IMes)(Cl) 2 Ru(CH-κ 2 ( C, N )-8-C 9 H 6 N) ( 1a; H 2 IMes = 1,3-dimesityl-4,5-dihydroimidzol-2-ylidene), and 5-vinylquinoxaline, (H 2 IMes)(Cl) 2 Ru(CH-κ 2 ( C, N )-5-C 8 H 5 N 2 ) ( 2a ), which possess five-membered chelate rings, were synthesized in ligand exchange reactions with (H 2 IMes)(PCy 3 )(Cl) 2 Ru CHPh. Both 1a and 2a are formed as complexes with the trans -dichloro geometry common for Grubbs-type complexes, while on prolonged storage in dichloromethane they isomerize to the thermodynamically favored cis -dichloro isomers ( 1b and 2b, respectively). The structures of compounds 1a, b were confirmed by X-ray analysis. The kinetics of trans to cis isomerization were measured by 1 H NMR to give the rate constants k 1a → 1b = 3.2 × 10 - 2 h - 1 and k 2a → 2b = 5.5 × 10 - 3 h - 1 in dichloromethane- d 2 at 23 °C. Investigations of the relative activities of these catalysts in model RCM and enyne reactions showed that catalyst 1a was faster than 1b (and 2a faster than 2b ) but that 2a was faster than 1a .