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-kappa(2)(C,N)-8-C9H6N) (1a; H(2)IMes = 1,3-dimesityl-4,5-dihydroimidzol-2-ylidene), and 5-vinylquinoxaline, (H(2)IMes)-( Cl)(2)Ru(CH-kappa(2)(C,N)-5-C8H5N2)(2a), which possess five-membered chelate rings, were synthesized in ligand exchange reactions with (H(2)IMes)(PCy3)(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 H-1 NMR to give the rate constants k(1a -> 1b) = 3.2 x 10(-2) h(-1) and k(2a -> 2b) = 5.5 x 10(-3) h(-1) in dichloromethane-d(2) at 23 degrees 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.