| 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 RuClâ(NHC)(PCyâ)(î»CHPh) (Ru-2: NHC = HâIMes, a; IMes, b; HâIPr, c; IPr, d; HâITol, e) and indenylidene complexes RuClâ(NHC)(PCyâ)(î»Cââ
Hââ) (NHC = HâIMes, Ru-2f; IMes, Ru-2g). All of these precatalysts form methylidene complex RuClâ(NHC)(î»CHâ) Ru-3 as the active species in metathesis of terminal olefins, and generate RuClâ(NHC)(PCyâ)(î»CHâ) 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 [MePCyâ]Cl A by net loss of three ligands (PCyâ, 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 Ï-alkyl intermediate RuClâ(¹³CHâPCyâ)(NHC) (py)â resulting from nucleophilic attack of free PCyâ on the methylidene ligand was undetectable for the HâIMes system, but was spectroscopically observable for the IMes system. The relevance of this pathway to decomposition of catalysts Ru-2aâg 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 HâO) 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 RuClâ(HâIMes)(PCyâ)(î»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.
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