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(NHC)(PCy)(═CHPh) (Ru-2: NHC = HIMes, a; IMes, b; HIPr, c; IPr, d; HITol, e) and indenylidene complexes RuCl(NHC)(PCy)(═CH) (NHC = HIMes, Ru-2f; IMes, Ru-2g). All of these precatalysts form methylidene complex RuCl(NHC)(═CH) Ru-3 as the active species in metathesis of terminal olefins, and generate RuCl(NHC)(PCy)(═CH) 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]Cl A by net loss of three ligands (PCy, 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(CHPCy)(NHC) (py) resulting from nucleophilic attack of free PCy on the methylidene ligand was undetectable for the HIMes 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 HO) 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 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(HIMes)(PCy)(═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.

An inner-sphere synthetic cycle for the hydrophosphination of alkenes is proposed, based on observed [2 + 2] cycloaddition of a wide range of alkenes at a coordinatively unsaturated RuPR 2 complex. Key intermediates in the cycle were prepared, and their reactions with various organic acid/ base pairs were examined to identify both new ruthenium precursors and base cocatalysts that allow turnover of the proposed cycle. Two new cationic ruthenium indenyl phosphine complexes were isolated and structurally characterized. Although preliminary screening studies show the moderate activity of these and related neutral phosphido complexes for catalytic hydrophosphination of acrylonitrile by both HPPh 2 and HPCy 2 , and comparable activity for the hydrophosphination of tert-butyl acrylate by HPPh 2 , no activity was observed for the analogous hydrophosphination of 1-hexene. This is attributed to strong binding of the substrate phosphine to the unsaturated, planar RuPR 2 fragment generated in situ, which inhibits the innersphere, alkene cycloaddition mechanism. An alternative, outer-sphere Michael addition process, involving a saturated complex with a strongly nucleophilic pyramidal Ru−PR 2 ligand, is proposed to rationalize the observed selectivity for catalytic hydrophosphination of activated, but not simple, alkenes. Implications for further catalyst development are discussed.

The Grubbs methylidene RuCl2(PCy3)2(CH2) GIm is immediately decomposed by the small N-heterocyclic carbene (NHC) tetramethylimidazol-2-ylidene (IMe4), even at −80 °C. Labeling experiments with RuCl2(PCy3)2(13CH2) reveal capture of the methylidene ligand as ethylimidazolium chloride [13CH3 13CH2–IMe4]­Cl, A*. Density functional calculations indicate that IMe4 first abstracts methylidene from five-coordinate GIm (a pathway unavailable to bulkier NHCs), generating the N-heterocyclic olefin (NHO) H2CIMe4. The latter attacks a second GIm; protonolysis of highly basic [Ru]–CH2CH2IMe4 then liberates A. Identical, slightly slower, reactivity is seen for RuCl2(H2IMes)­(PCy3)­(CH2), GIIm. The high efficiency of this process opens up opportunities for tandem metathesis enabled by sterically accessible NHCs and NHOs.

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