Ruthenium Olefin Metathesis Research Articles

Double CH Activation of an N‐Heterocyclic Carbene Ligand in a Ruthenium Olefin Metathesis Catalyst

Having a breakdown: Decomposition of the olefin metathesis catalyst [(biph)(PCy3)Cl2RuC(H)Ph] (biph= N,N′-diphenylbenzimidazol-2-ylidene, Cy=cyclohexyl) results in benzylidene insertion into an ortho CH bond of an N-phenyl group of the biph ligand. The ruthenium center further inserts into another ortho CH bond of the other N-phenyl ring to give a new RuC bond as a part of a five-membered metallacycle (see scheme).

Decomposition of Ruthenium Olefin Metathesis Catalysts

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)2Ru=CH(2-C6H4-O-i-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)2Ru=CH2, is also reported.

A New Concept for the Noncovalent Binding of a Ruthenium-Based Olefin Metathesis Catalyst to Polymeric Phases: Preparation of a Catalyst on Raschig Rings

A new concept for noncovalent immobilization of a ruthenium olefin metathesis catalyst is presented. The 2-isopropoxybenzylidene ligand of a Hoveyda-Grubbs carbene is further modified by an additional amino group (7) and immobilization is achieved by treatment with sulfonated polystyrene forming the corresponding ammonium salt. In this novel strategy for the immobilization of ruthenium-based metathesis catalysts, the amino group plays a two-fold role, being first an active anchor for immobilization and second, after protonation, activating the catalysts (electron donating to electron withdrawing activity switch). The polymeric support was prepared by precipitation polymerization which led to small bead sizes (0.2-2 microm) and large surface areas. Compared to commercial resins this tailor-made phase showed superior properties in immobilization of complex 7. This concept of immobilization was applied to glass-polymer composite megaporous Raschig rings. Ru catalyst 7 on Raschig rings was used under batch conditions in various metathesis reactions, including ring-closing (RCM), cross- (CM) and enyne metathesis, to give products of high chemical purity with very low ruthenium contamination levels (21-102 ppm). The same ring can be used for up to 6 cycles of metathesis.

Mechanistic comparison of ruthenium olefin metathesis catalysts: DFT insight into relative reactivity and decomposition behavior

The reaction mechanism and substrate-induced decomposition behavior of three ruthenium olefin metathesis catalysts, viz. first- and second-generation catalysts and the recently developed Phoban catalyst (“Phobcat”) are compared by constructing Δ G surfaces at 298.15 K and 1 atm for the complete ligand systems. From these calculations fundamental insight is gained into the reactivity and stability observed experimentally for the three catalysts. In particular, the higher conversions obtained for the first-generation derived Phobcat catalyst, compared to conventional first-generation catalysts, is attributed to its similarity to the second-generation catalysts instead of first-generation catalyst. Important differences between the calculated Δ G surfaces and previously reported total electronic energy (Δ E) surfaces for the metathesis mechanism with complete ligand complexes are discussed.

New air-stable ruthenium olefin metathesis precatalysts derived from bisphenol S

Synthesis and screening of catalytic activity of novel mono- and diruthenium carbene complexes 7a and 7b prepared from inexpensive Bisphenol S via Claisen rearrangement–isomerisation route is described. These catalysts constitute an excellent tool for ring-closing metathesis by combining high stability with increased catalytic activity as compared with the parent Hoveyda–Grubbs catalyst.

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.

The Isomerization Equilibrium between Cis and Trans Chloride Ruthenium Olefin Metathesis Catalysts from Quantum Mechanics Calculations

The cis-trans chloride isomerization of a ruthenium olefin metathesis catalyst is studied using quantum mechanics (B3LYP DFT), including the Poisson-Boltzmann (PBF) continuum approximation. The predicted geometries agree with experiment. The energies in methylene chloride, lead to DeltaG = -0.70 kcal/mol and a cis:trans ratio of 76:24, quite close to the experimental value of DeltaG = -0.78 kcal/mol or c:t 78:22. In contrast, we predict that in benzene c:t = 4:96 in agreement with the experimental observation of only the trans isomer. Our calculated relative activation energies explain the observed difference in initiation rates and suggest that each isomer should be isolable in high ratio by simply changing solvent.

A Convenient System for Improving the Efficiency of First-Generation Ruthenium Olefin Metathesis Catalysts

The performance of certain olefin metathesis reactions catalyzed by Grubbs catalysts has been enhanced by the simple addition of phenol. Addition of phenol to self-metathesis reactions catalyzed by 1 produced very small quantities of unwanted byproducts and allowed for room-temperature metathesis at high substrate:catalyst loadings. The efficiency of cross-metathesis reactions between methyl acrylate and 1-decene catalyzed by 2 was also significantly increased by addition of p-cresol to the reaction mixture. Mechanistic studies, including NMR spectroscopy and DFT calculations, established that phenol is playing a number of positive roles in the active metathesis cycle, including altering the relative rates of phosphine loss and rebinding, activating the carbene carbon for reaction with olefinic substrate, and hemilabile stabilization of the key 14-electron intermediate species.

Ring-Opening Metathesis Polymerization of Functionalized Low-Strain Monomers with Ruthenium-Based Catalysts

A detailed study of the ring-opening metathesis polymerization of low-strain monomers with ruthenium catalysts is reported. The effects of monomer concentration and catalyst dependence are described for unsubstituted cycloolefins. The ROMP of low-strain olefins with polar substituents is also examined with ruthenium olefin metathesis catalysts, and a predictive model for ROMP feasibility is proposed.

Synthesis of Ruthenium Olefin Metathesis Catalysts with Linear Alkyl Carbene Complexes

Second-generation ruthenium olefin metathesis catalysts with linear alkyl carbenes were needed to install a linear alkyl end group on ROMP polymers. The ethylidene complexes RuCl2(PCy3)(SIMes)CHMe (PCy3 = tricyclohexylphosphine; SIMes = 1,3-bis(mesityl)imidazolidin-2-ylidene) and RuCl2(3BP)2(SIMes)CHMe (3BP = 3-bromopyridine) were readily accessible by reaction of RuCl2(PCy3)(SIMes)CHPh with 2-butene. Synthesis of higher alkyl analogues, such as propylidene and heptylidene, are complicated by the intervention of olefin isomerization. Although slightly less reactive than the parent complex, these complexes are suitable catalysts for both ADMET and ROMP.

Rapidly Initiating Ruthenium Olefin‐Metathesis Catalysts.

AbstractFor Abstract see ChemInform Abstract in Full Text.

A Ruthenium Olefin Metathesis Catalyst with a Four-Membered N-Heterocyclic Carbene Ligand

The first ruthenium olefin metathesis catalyst bearing a four-membered N-heterocyclic carbene (5) was synthesized and characterized by NMR spectroscopy and X-ray crystallography. The synthesis of a (carbene)carbonylrhodium complex (6) was also reported, allowing the study of the electronic properties of this new carbene ligand. The complex 5 was screened toward different olefin metathesis reactions, and it showed a slow reactivity.

Rapidly Initiating Ruthenium Olefin‐Metathesis Catalysts

Rapidly Initiating Ruthenium Olefin‐Metathesis Catalysts

Rapidly Initiating Ruthenium Olefin‐Metathesis Catalysts

Rapidly Initiating Ruthenium Olefin‐Metathesis Catalysts

A New Ruthenium-Based Olefin Metathesis Catalyst Coordinated with 1,3-Dimesityl-1,4,5,6-tetrahydropyrimidin-2-ylidene: Synthesis, X-ray Structure, and Reactivity

The new ruthenium olefin metathesis catalyst 4 bearing a 5,5‘-dimethyl-1,3-dimesityl-1,4,5,6-tetrahydropyrimidin-2-ylidene ligand was first synthesized from (PCy_3)_2(Cl)_2Ru ═ CHPh (1). The X-ray crystal structure of complex 4 has been determined and shows that the N-mesityl group of the six-membered carbene ligand and the benzylidene moiety are in close proximity (2.9 A). This catalyst demonstrates moderate reactivity for both ring-closing olefin metathesis and ring-opening metathesis polymerization.

Polymerization of Acetylene with a Ruthenium Olefin Metathesis Catalyst

Polymerization of Acetylene with a Ruthenium Olefin Metathesis Catalyst

Mechanism and activity of ruthenium olefin metathesis catalysts: the role of ligands and substrates from a theoretical perspective.

The reaction mechanism of olefin metathesis by ruthenium carbene catalysts is studied by gradient-corrected density functional calculations (BP86). Alternative reaction mechanisms for the reaction of the "first-generation" Grubbs-type catalyst (PCy(3))(2)Cl(2)Ru=CH(2) (1) for the reaction with ethylene are studied. The most likely dissociative mechanism with trans olefin coordination is investigated for the metathesis reaction between the "first-" and the "second-generation" Grubbs-type catalysts 1 and (H(2)IMes)(PCy(3))Cl(2)Ru=CH(2) (2) with different substrates, ethylene, ethyl vinyl ether, and norbornene, and a profound influence of the substrate is found. In contrast to the degenerate reaction with ethylene, the reactions with ethyl vinyl ether and norbornene are strongly exergonic by 8-15 kcal/mol, and this excess energy is released after passing through the metallacyclobutane structure. While the metallacyclobutane is in a deep potential minimum for degenerate metathesis reactions, the energy barrier for the [2+2] cycloreversion vanishes for the most exergonic reactions. On the free energy surface under typical experimental conditions, the rate-limiting steps for the overall reactions are then either metallacyclobutane formation for 1 or phosphane ligand dissociation for 2.

Orbital Interactions in the Ruthenium Olefin Metathesis Catalysts

The structural and bonding interactions of the ruthenacyclobutane 4 suggested its nonclassical nature. A strong bonding interaction between the orbitals of the −CH2CH2CH2− fragment and the dy2 orbital on the metal in 4 was found that indicated the presence of two α-CC agostic interactions in the complex. Further, in the transition state corresponding to the rate-determining step of the metathesis, mutually benefiting π interactions exist between the RuCH2 and the olefin fragments. These features are important in the metathesis activity of Grubbs first-generation catalyst because they make the potential energy surface of the M(CH2)(C2H4) ↔ M(C2H4)(CH2) region flat.

Ruthenium Olefin Metathesis Catalysts with Modified Styrene Ethers: Influence of Steric and Electronic Effects.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Ruthenium olefin metathesis catalysts with modified styrene ethers: influence of steric and electronic effects

A series of olefin metathesis catalysts with modified isopropoxybenzylidene ligands were synthesised, and the effects of ligands on the rate of metathesis investigated. Increased steric hinderance ortho to the isopropoxy group enhanced reaction rates. In the case of N-heterocyclic carbene complexes, decreasing electron density at both the chelating oxygen atom and the RuC bond accelerated reaction rates appreciably. Catalysts containing a tricyclohexylphosphane ligand, followed the same trend with regard to benzylidene electrophilicity, while higher electron density at oxygen enhanced reaction rates.