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Sterically accessible Lewis donors are shown to accelerate decomposition during catalysis, for a broad range of Grubbs-class metathesis catalysts. These include benzylidene derivatives RuCl 2 (NHC)(PCy 3 )(═CHPh) (Ru-2: NHC = H 2 IMes, a; IMes, b; H 2 IPr, c; IPr, d; H 2 ITol, e) and indenylidene complexes RuCl 2 (NHC)(PCy 3 )(═C 15 H 10 ) (NHC = H 2 IMes, Ru-2f; IMes, Ru-2g). All of these precatalysts form methylidene complex RuCl 2 (NHC)(═CH 2 ) Ru-3 as the active species in metathesis of terminal olefins, and generate RuCl 2 (NHC)(PCy 3 )(═CH 2 ) Ru-4 as the catalyst resting state. On treatment with a 10-fold excess of pyridine, Ru-4a and Ru-4b decomposed within minutes in solution at RT, eliminating [MePCy 3 ]Cl A by net loss of three ligands (PCy 3 , methylidene, and one chloride), and a mesityl proton. In comparison, loss of A from Ru-4a in the absence of a donor requires up to 3 days at 55 °C. The σ-alkyl intermediate RuCl 2 ( 13 CH 2 PCy 3 )(NHC) (py) 2 resulting from nucleophilic attack of free PCy 3 on the methylidene ligand was undetectable for the H 2 IMes system, but was spectroscopically observable for the IMes system. The relevance of this pathway to decomposition of catalysts Ru-2a-g was demonstrated by assessing the impact of pyridine on the in situ-generated methylidene species. Slow initiation (as observed for the indenylidene catalysts) did not protect against methylidene abstraction. Importantly, studies with Ru-4a and Ru-4b indicated that weaker donors (THF, MeCN, DMSO, MeOH, and even H 2 O) likewise promote this pathway, at rates that increase with donor concentration, and severely degrade catalyst productivity in RCM, even for a readily cyclized substrate. In all cases, A was the sole or major 31 P-containing decomposition product. For DMSO, a first-order dependence of decomposition rates on DMSO concentration was established. This behavior sends a warning about the use of phosphine-stabilized metathesis catalysts in donor solvents, or with substrates bearing readily accessible donor sites. Addition of pyridine to RuCl 2 (H 2 IMes)(PCy 3 )(═CHMe) did not result in ethylidene abstraction, indicating that this decomposition pathway can be inhibited by use of substrates in which the olefin bears a β-methyl group.

A General Decomposition Pathway for Phosphine-Stabilized Metathesis Catalysts: Lewis Donors Accelerate Methylidene Abstraction