Most ring-closing metathesis reactions are carried out at fairly high dilution of the substrate (10 - 50 mM) with catalyst loadings of 5 - 10 mol % and at slightly elevated temperatures (25 - 110 ºC). Molybdenum catalyst 1 exhibits extreme sensitivity to air and water such that use of a glovebox is ideal. On the other hand, ruthenium catalysts are more stable in air and Schlenck tubes are typically used. Standard workup involves concentration of the reaction mixture, aqueous extraction, and purification via silica gel chromatography, recrystallization, or distillation. Because the standard procedure can leave behind traces of ruthenium, more rigorous workup procedures have been developed that use additional ligands, supercritical fluids, and mesoporous silicates to decrease ruthenium concentrations to extremely low levels.
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The decomposition of a series of ruthenium metathesis catalysts has been examined using methylidene species as model complexes. All of the phosphine-containing methylidene complexes decomposed to generate methylphosphonium salts, and their decomposition routes followed first-order kinetics. The formation of these salts in high conversion, coupled with the observed kinetic behavior for this reaction, suggests that the major decomposition pathway involves nucleophilic attack of a dissociated phosphine on the methylidene carbon. This mechanism also is consistent with decomposition observed in the presence of ethylene as a model olefin substrate. The decomposition of phosphine-free catalyst (H2IMes)(Cl)2RuCH(2-C6H4-O--Pr) (H2IMes = 1,3-dimesityl-4,5-dihydroimidazol-2-ylidene) with ethylene was found to generate unidentified ruthenium hydride species. The novel ruthenium complex (H2IMes)(pyridine)3(Cl)2Ru, which was generated during the synthetic attempts to prepare the highly unstable pyridine-based methylidene complex (H2IMes)(pyridine)2(Cl)2RuCH2, is also reported.
Well-defined alkene metathesis catalysts II.
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Ring-opening metathesis polymerization (ROMP) uses metathesis catalysts to generate polymers from cyclic olefins. ROMP is most effective on strained cyclic olefins, because the relief of ring strain is a major driving force for the reaction – cyclooctene and norbornenes are excellent monomers for ROMP, but cyclohexene is very reluctant to form any significant amount of polymer. Norbornenes are favorite monomers for ROMP, as a wide range of monomer functionalities are easily available through Diels-Alder reactions.
Olefin Metathesis, Grubbs Reaction - Organic chemistry
Unsubstituted α,β-unsaturated esters can likewise coordinate to the metal center and prevent reaction. Including a Lewis acid such as titanium(IV) isopropoxide in the reaction mixture does not interfere with metathesis and prevents coordination to the catalytic metal, enabling reactions of acrylates (Eq. 17).
Olefin Metathesis Grubbs Reaction
Unsaturated lactams are a biochemically important class of heterocycles that can be prepared via ring-closing metathesis. Catalyst 1 is effective in the preparation of five- or six-membered lactams, but crotonamides must be used as unsubstituted α,β-unsaturated amides coordinate to molybdenum, preventing reaction (Eq. 16).
METATHESIS | Alkene | Catalysis
The olefin metathesis reaction (the subject of 2005 Nobel Prize in Chemistry).
Olefin metathesis Olefin metathesis or transalkylidenation (in some literature, a disproportionation) is an organic reaction which involves redistribution.
Grubbs’ Catalyst in Paraffin: An Air-Stable Preparation for Alkene Metathesis Douglass F.
alkene-metathesis reactions, ..
Cyclic boronates are formed in cross-metathesis reactions of allylic alcohols and allylboron reagents. Treatment with hydrogen peroxide and sodium hydroxide yields stereodefined allylic diols (Eq. 15).