Highly active water-soluble olefin metathesis catalyst

A novel water-soluble ruthenium olefin metathesis catalyst supported by a poly(ethylene glycol) conjugated saturated 1,3-dimesityl-4,5-dihydroimidazol-2-ylidene ligand is reported. The catalyst displays improved activity in ring-opening metathesis polymerization, ring-closing metathesis, and cross-metathesis reactions in aqueous media.     

Dichlorotetracarbonyltungsten as a catalyst for olefin metathesis

The photolysis of hexacarbonyltungsten (I) in carbon tetrachloride yields a catalytic mixture for olefin metathesis. One of the principal products of this transformation is dichlorotetracarbonyltungsten (II). Chemical preparation of II from I and chlorine was carried out. Authentic II was shown to give catalysis of 2-pentene metathesis by either heating in chlorobenzene or chloroform or irradiation in chlorobenzene or carbon tetrachloride. The Dubois report of phosgene formation in the original irradiated mixture of I in carbon tetrachloride was shown to be an artifact.     

Olefin metathesis catalyst bearing a chelating phosphine ligand

An improved synthetic procedure for the complex (SPY-5-34)-dichloro-(κ²(C,P)-diphenyl-(2-benzylidene)-phosphine)-(1,3-bis(2,4,6-trimethylphenyl)-4,5-dihydro-imidazol-2-ylidene)-ruthenium (2) was elaborated and the title compound was tested as latent initiator in Ring Opening Metathesis Polymerization (ROMP) and as catalyst for Ring-Closing Metathesis (RCM) at elevated temperatures. While not particularly suited as latent initiator for ROMP, exhibiting a switching temperature of only 42 °C in the polymerization of a typical norbornene derivative, 2 shows an appealing performance in RCM of α,ω-dienes at higher temperatures.     

In situ catalyst systems for ring-opening metathesis polymerization

Several new in situ tungsten catalyst systems for ring-opening metathesis polymerizations (ROMP) by reaction injection molding (RIM) have been developed by adding BF₃ promoter to binary catalyst systems, by using metal hydride cocatalysts, and by altering the ligands on the procatalyst metal center. BF₃ etherates improved catalyst efficiency and reduced induction times for formation of active catalysts from reaction of aryloxytungsten complexes [e.g., (ArO)_y(WX_x)] with organotin hydrides. Coordinatively unsaturated cationic intermediates, such as [(ArO)_yWX_x-1]⁺ BF₃X⁻, are proposed to facilitate formation of the active catalysts. Tougher poly(dicyclopentadiene) (polyDCPD) composites were produced using < 5 wt % of styrene-butadiene block copolymers due to formation of small “shell-core” rubber morphologies when BF₃ promoter was added to the catalyst system. Nonalkylating metal hydrides besides R₃SnH, including (PPh₃)₂CuBH₄, (PPh₃CuH)₆, and Cp₂ZrClH, were shown to be cocatalysts. The optimum 2 : 1 stoichiometric ratio of organotin hydride cocatalyst to tungsten, revealed by BF₃-promoted catalyst systems, and W^V EPR resonances (g ∼ 1.7) observed in the reaction of aryloxytungsten with organotin hydride are consistent with an overall reduction and reoxidation mechanism for formation of the active metathesis catalysts. Some tungsten complexes derived from 9-hydroxyfluorene, 2,2′-(and 4,4′)-biphenols, and 1,4-hydroquinones were found to be very reactive procatalysts, even in the absence of cocatalyst in some cases. These procatalysts also were paramagnetic, characterized by unusual EPR spectra consistent with W^V (g = 1.6–1.9) and “ligand-centered” (g = 2.003) resonances. Valence tautomeric species, analogous to catecholate-semiquinonate complexes, are proposed. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35 : 3027–3047, 1997     

Structure-reactivity relationship in alkane metathesis using well-defined silica-supported alkene metathesis catalyst precursors

Alkane metathesis can be performed by using well-defined silica-supported alkene metathesis catalyst precursors as long as the coordination sphere of the metal centre contains both alkyl and alkylidene groups, such as in [([triple chemical bond]SiO)Mo([triple chemical bond]NAr)(=CHtBu)(CH2tBu)]. This system transforms mainly linear alkanes, from propane to hexane, into their lower and higher homologues. Mechanistic studies clearly show that alkene metathesis is a key step of this reaction and also suggest that this system is a single-site single-component system, namely, alkenes are formed and transformed on one site, which is in contrast to that observed with a mixture of catalysts.     

Highly active ruthenium-based catalyst for metathesis of cyano-contained olefins

Ruthenium benzylidene complex (H₂IMes)(2-CH₃-C₅H₄N)(Cl)₂RuCHPh [H₂IMes = 1,3-bis(2,6-dimethylphenyl)-4,5-dihydroimidazol-2-ylidene] (4), which introduced ortho substituted pyridine as dissociating ligand to weaken Ru-N bond and accelerate initiation through steric hindrance, was prepared by the reaction of (H₂IMes)(PPh₃)(Cl)₂RuCHPh (1) with 2-methylpyridine and proved to exhibit enhanced catalytic activity for cyano-contained olefin metathesis.     

Kinetic investigation of a ruthenium metathesis catalyst

The complex [(IMesH₂)(PPh₂Cy)Cl₂RuCHPh] was synthesised and shown to be an active catalyst in ring-closing metathesis of a diallylmalonate. Its phosphine exchange was investigated in C₆D₆ using magnetisation transfer ³¹P NMR spectroscopy and it was found to operate via a dissociative mechanism with k³⁵³ = 4.1 ± 0.9 s⁻¹, ΔH^‡ = 84 ± 10 kJ mol⁻¹ and ΔS^‡ = 4 ± 28 J mol⁻¹ K⁻¹.     

Ring-opening metathesis polymerization of exo-dicyclopentadiene: Reversible crosslinking by a metathesis catalyst

exo-Dicyclopentadiene has been polymerized by several well characterized, single-component metathesis catalysts to provide soluble, linear, poly exo-dicyclopentadiene. Under the best circumstances the polymerizations proceed in a living manner. The crosslinking of polymer chains which can occur under high monomer concentration has been investigated and determined to result from the secondary metathesis of the cyclopentene substituent.     

Synthesis of coumarins by ring-closing metathesis using Grubbs’ catalyst

A novel generally applicable synthesis of coumarins from phenolic substrates utilizing ring-closing metathesis is described. This sequence involves O-allylation of phenols followed by ortho-Claisen rearrangement, subsequent based-induced isomerization affording 2-(1-propenyl)phenols, acylation with acryloyl chloride, and finally ring-closing metathesis (RCM) with Grubbs’ second generation catalyst.     

Synthetic applications with use of a silica-supported alkyne metathesis catalyst

A highly active and durable fumed silica-supported heterogeneous molybdenum(VI) catalyst was applied to ring closing alkyne metathesis and cyclooligomerization reactions to give high yields of metathesis products near room temperature conditions.     

Homologation of ethylene without metathesis on silica supported ruthenium catalyst

A 1:1 mixture of ¹²C₂-C₂H₄ (H₂C=CH₂) and ¹³C₂-C₂H₄ (H₂ ¹³C=¹³CH₂) was contacted on Ru/SiO₂at 200^oC. Propylene (homologation) was observed without ¹³C₁-C₂H₄ formation (metathesis reaction). This might at least be corroborating evidence for discrimination of active sites in regard with the homologation and the metathesis. This revised version was published online in August 2006 with corrections to the Cover Date.     

Efficiency of a ruthenium catalyst in metathesis reactions of sulfur-containing compounds

[reaction: see text] 1,3-Dimesitylimidazol-2-ylidene ruthenium benzylidene catalyst (RuCl2(=C(H)Ph)(PCy3)(IMes)) has been successfully employed in ring-closing metathesis reactions of acyclic diene sulfides, disulfides, and dithianes and in self-cross metathesis reactions of ene-sulfides, thioethers, and thiols.     

New efficient ruthenium metathesis catalyst containing chromenyl ligand

A synthesis of new Hoveyda-Grubbs-type catalyst with chromenyl ligand was described herein. The new catalyst was tested in model RCM and CM reactions. The catalyst proved to be quite efficient. It showed activity comparable or superior to that of commercially available Grubbs second-generation complexes.     

In situ conversion of a Ru metathesis catalyst to an isomerization catalyst

Addition of alcohols and substoichiometric amounts of a base to a metathesis reaction induces conversion of the metathesis-active carbene catalyst to an isomerization-active hydride species.     

A ROMP-derived, polymer-supported chiral Schrock catalyst for enantioselective ring-closing olefin metathesis

A permanently polymer-immobilised version of Schrock's molybdenum catalyst was realised via polymerisation of chiral 5,5'-bis(norborn-5-ene-2-ylmethyleneoxymethyl)-3,3'-di-tert-butyl-6,6'-dimethylbiphen-2,2'-diol followed by reaction with the catalyst precursor Mo(N-2,6-i-Pr2C6H3)(CHCMe2Ph)(OSO2CF3)2(CH3OCH2CH2OCH3); using this 5-, 6- and 7-membered heterocycles were synthesised via ring-closing metathesis (RCM) from (pro-)chiral substrates in high yields and ee with short reaction times; the catalyst shows low loss of molybdenum during RCM, and was easily separated and recycled.     

Optimizing the preparation of an alumina-supported rhenium catalyst for olefin metathesis

The processes involved in the formation of the alumina-supported rhenium catalyst for olefin metathesis, from the impregnation of the support (thermally activated alumina) with ammonium perrhenate to thermal activation, are studied. The monolayer coverage of the Al₂O₃ surface is observed at a rhenium content of 10 wt % (on Re₂O₇ basis), and the surplus rhenium is sublimed as heptoxide from the support upon thermal activation. In the metathesis of both linear α-olefins and methylenecyclobutanes, the optimum supported rhenium content of the catalyst is 10 wt % on Re₂O₇ basis.