High Turnover Numbers with Ruthenium-Based Metathesis Catalysts

Effective turnover numbers (TON's) and turnover frequencies (TOF's) of unprecedented magnitude have been obtained for the metathesis of various substrates in the presence of ruthenium-based catalysts, without the use of an additional solvent. For the self-metathesis of 1-octene, the 2nd generation Grubbs' catalyst (IMesH2)(PCy3)(Cl)2RuCHPh (2a) (IMesH2=1,3-dimesityl-4,5-dihydroimidazol-2-ylidene) gave TON's 5 times higher than those of the 1st generation catalyst (PCy3)2(Cl)2RuCHPh (1) while maintaining a high degree of selectivity for the formation of the product 7-tetradecene. Moreover, the catalyst (IPrH2)(PCy3)(Cl)2RuCHPh (2b), bearing the extremely bulky 1,3-bis(2,6-diisopropylphenyl)-4,5-dihydroimidazol-2-ylidene ligand (IPrH2), gave TON's 6 times higher (TON >640,000) and initial TOF's 20 times higher than those of 2a at ambient temperature. At elevated temperatures, TOF's were twice as high and initial TOF's were 6 times as high for 2b relative to 2a, and with nearly 100% selectivity. Initial TOF's exceeding 3,800 s1 could be achieved with 2b at 60 °C. Under similar conditions, comparable TON's for the self-metathesis of trans-4-decene and methyl oleate, and the ring-closing metathesis of diethyl diallylmalonate were also obtained.