The Impact of Oxygen on Leading and Emerging Ru-Carbene Catalysts for Olefin Metathesis: An Unanticipated Correlation Between Robustness and Metathesis Activity

The impact of O-2 on olefin metathesis promoted by Ru-NHC and Ru-CAAC catalysts is examined (NHC = N-heterocyclic carbene; CAAC = cyclic alkyl amino carbene). An atmosphere of 8% O-2 in Ar is found to decrease turnover numbers by ca. 15-95% in ring-closing metathesis, including macrocyclization reactions. All H(2)IMes catalysts studied, irrespective of their ease of initiation, exhibit broadly similar O-2-sensitivity, within the middle of this range. Much greater disparities in performance emerge for the CAAC catalysts, which exhibit an unexpected correlation between reactivity and robustness. Fast-initiating CAAC catalysts show greatest resistance to O-2 of all systems studied. Mechanistic studies with the second-generation Grubbs catalyst RuCl2(H(2)IMes)(PCy3)(=CHPh), which contains the useful PCy3 and benzylidene reporter groups, suggest that [2 + 2] cycloaddition of O-2 dominates over the reported attack of O-2 at the benzylidene ligand. At catalyst concentrations compatible with NMR analysis (20 mM Ru), bimolecular coupling of Ru-4 is also a significant contributor to decomposition.