title: A General Decomposition Pathway for Phosphine-Stabilized Metathesis Catalysts: Lewis Donors Accelerate Methylidene Abstraction.
abstract: 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)2resulting from nucleophilic attack of free PCy3on 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 major31P-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.
authors.affiliation: Department of Chemistry and Centre for Catalysis Research & Innovation, University of Ottawa , Ottawa, Ontario K1N 6N5, Canada.
authors.forename: William L
authors.initials: WL
authors.lastname: McClennan
authors.affiliation: Department of Chemistry and Centre for Catalysis Research & Innovation, University of Ottawa , Ottawa, Ontario K1N 6N5, Canada.
authors.forename: Stephanie A
authors.initials: SA
authors.lastname: Rufh
authors.affiliation: Department of Chemistry and Centre for Catalysis Research & Innovation, University of Ottawa , Ottawa, Ontario K1N 6N5, Canada.
authors.forename: Justin A M
authors.initials: JA
authors.lastname: Lummiss
authors.affiliation: Department of Chemistry and Centre for Catalysis Research & Innovation, University of Ottawa , Ottawa, Ontario K1N 6N5, Canada.
authors.forename: Deryn E
authors.initials: DE
authors.lastname: Fogg