Synthesis and reactivity of oxygen chelated ruthenium carbene metathesis catalysts

The rate of initiation of Hoveyda catalysts is affected by the electronic and steric effects that act upon the Ru⋯O coordination. In order to boost the activity of Hoveyda catalysts, a series of new oxygen chelated ruthenium carbene metathesis catalysts containing an N-heterocyclic carbene (NHC) and a carbonyl group has been developed, and their catalytic activities for olefin metathesis reactions were investigated. The aliphatic end groups of complexes (H2IMes)(Cl)2RuC(H)[(C6H3X)OCH(Me)(C(O)OEt)(X = H, OMe, Me, NO2)] were functionalized by the attachment of a straight-chain ester. The X-ray structures of complex (H2IMes)(Cl)2RuC(H)[(C6H4)OCH(Me)(C(O)NMe2)] showed that the carbonyl oxygen of the amide and the terminal oxygen of the benzylidene ether are both coordinated to the metal to give an octahedral structure. However, the carbonyl oxygen of complexes (H2IMes)(Cl)2RuC(H)[(C6H3X)OCH(CH2C(O)OCH2)(X = H, OMe)] does not coordinate to the metal due to the steric effect of the lactone. All these complexes were used as catalysts for olefin metathesis reactions and all exhibited excellent performances for the ring-closing metathesis (RCM) of diethyl diallymalonate at 30 °C. The initiation rate of these catalysts was higher than that for the Hoveyda catalyst ((H2IMes)(Cl)2RuC(H)(C6H4-2-OiPr)) and these complexes are also active for cross metathesis (CM).