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

Sterically accessible Lewis donors are shown to accelerate decomposition during catalysis, for a broad range of Grubbs-class metathesis catalysts. These include benzylidene derivatives RuCl₂(NHC)(PCy₃)(═CHPh) (Ru-2: NHC = H₂IMes, a; IMes, b; H₂IPr, c; IPr, d; H₂ITol, e) and indenylidene complexes RuCl₂(NHC)(PCy₃)(═C₁₅H₁₀) (NHC = H₂IMes, Ru-2f; IMes, Ru-2g). All of these precatalysts form methylidene complex RuCl₂(NHC)(═CH₂) Ru-3 as the active species in metathesis of terminal olefins, and generate RuCl₂(NHC)(PCy₃)(═CH₂) 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 [MePCy₃]Cl A by net loss of three ligands (PCy₃, 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 RuCl₂(¹³CH₂PCy₃)(NHC) (py)₂ resulting from nucleophilic attack of free PCy₃ on the methylidene ligand was undetectable for the H₂IMes 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 H₂O) 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 ³¹P-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 RuCl₂(H₂IMes)(PCy₃)(═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.