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

Novel metathesis catalysts that are derivatives of 8-vinylquinoline, (H2IMes)(Cl)2Ru(CH-κ2(C,N)-8-C9H6N) (1a; H2IMes = 1,3-dimesityl-4,5-dihydroimidzol-2-ylidene), and 5-vinylquinoxaline, (H2IMes)(Cl)2Ru(CH-κ2(C,N)-5-C8H5N2) (2a), which possess five-membered chelate rings, were synthesized in
ligand exchange reactions with (H2IMes)(PCy3)(Cl)2Ru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 1H NMR to give the rate constants k1a→1b = 3.2 × 10-2 h-1 and
k2a→2b = 5.5 × 10-3 h-1 in dichloromethane-d2 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.