Ruthenium Olefin Metathesis Catalysts Research Articles

Protection of Ruthenium Olefin Metathesis Catalysts by Encapsulation in a Self‐assembled Resorcinarene Capsule

AbstractCatalyst encapsulation is examined as a means of increasing the productivity of olefin metathesis catalysts. Commercially available, cationic ruthenium metathesis catalysts were incorporated into a supramolecular resorcin[4]arene capsule. Encapsulation increased catalyst stability in water‐saturated toluene, delivering higher metathesis yields than the parent, non‐encapsulated Hoveyda catalyst in the same reaction medium.

Preparation of Ruthenium Olefin Metathesis Catalysts Immobilized on MOF, SBA-15, and 13X for Probing Heterogeneous Boomerang Effect

Promoted by homogeneous Ru-benzylidene complexes, the olefin metathesis reaction is a powerful methodology for C-C double bonds formation that can find a number of applications in green chemical production. A set of heterogeneous olefin metathesis pre-catalysts composed of ammonium-tagged Ru-benzylidene complexes 4 (commercial FixCat™ catalyst) and 6 (in-house made) immobilized on solid supports such as 13X zeolite, metal-organic framework (MOF), and SBA-15 silica were obtained and tested in catalysis. These hybrid materials were doped with various amounts of ammonium-tagged styrene derivative 5—a precursor of a spare benzylidene ligand—in order to enhance pre-catalyst regeneration via the so-called release-return “boomerang effect”. Although this effect was for the first time observed inside the solid support, we discovered that non-doped systems gave better results in terms of the resulting turnover number (TON) values, and the most productive were hybrid catalysts composed of 4@MOF, 4@SBA-15, and 6@SBA-15.

In a Quest for Selectivity Paired with Activity: A Ruthenium Olefin Metathesis Catalyst Bearing an Unsymmetrical Phenanthrene-Based N-Heterocyclic Carbene.

Robust, selective, and stable in the presence of ethylene, ruthenium olefin metathesis pre-catalyst, {[3-benzyl-1-(10-phenyl-9-phenanthryl)]-2-imidazolidinylidene}dichloro(o-isopropoxyphenylmethylene)ruthenium(II), Ru-3, bearing an unsymetrical N-heterocyclic carbene (uNHC) ligand, has been synthesized. The initiation rate of Ru-3 was examined by ring-closing metathesis and cross-metathesis reactions with a broad spectrum of olefins, showing an unprecendented selectivity. It was also tested in industrially relevant ethenolysis reactions of olefinic substrates from renewable feedstock with very good yields and selectivities.

Synthesis and Activity of Six-Membered Cyclic Alkyl Amino Carbene–Ruthenium Olefin Metathesis Catalysts

Ru–cyclic alkyl amino carbene (Ru–CAAC) olefin metathesis catalysts perform extraordinarily in metathesis macrocyclization and ethenolysis, but previous studies have been limited to the use of five-membered CAAC (CAAC-5) ligands. In this work, we synthesized a different group of ruthenium catalysts with more σ-donating and π-accepting six-membered CAAC (CAAC-6) ligands, and their metathesis activity was probed through initiation studies, ring-closing metathesis (RCM), cross-metathesis, and ethenolysis. These catalysts display higher initiation rates than analogous Ru–CAAC-5 complexes but demonstrate lower activity in RCM and ethenolysis.

Vortex Fluidic Ethenolysis, Integrating a Rapid Quench of Ruthenium Olefin Metathesis Catalysts

Ruthenium-catalysed ethenolysis occurs in a vortex fluidic device (VFD) – a scalable, thin-film microfluidic continuous flow process. This process takes advantage of the efficient mass transfer of gaseous reagents into the dynamic thin film of liquid. Also reported is the rapid quenching of the ruthenium-based olefin metathesis catalyst by the addition of a saturated solution of N-acetyl-l-cysteine in MeCN, as a convenient alternative to previously reported quenching methods.

A Gentler Touch: Synthesis of Modern Ruthenium Olefin Metathesis Catalysts Sustained by Mechanical Force

AbstractMechanochemical synthesis of nine contemporary ruthenium catalysts used for olefin metathesis is described, being the first reported example of formation of Ru carbene organometallic complexes in solid state. Three key organometallic transformations commonly used in the synthesis of the second and third generations of Ru catalysts in solution—phosphine ligand (PCy3) exchange with in situ formed N‐heterocyclic carbene (NHC) ligand, PCy3 to pyridine ligand replacement, and benzylidene ligands interchange—were proved to work under mechanochemical conditions, affording the targets in high purity. Mechanochemical approach not only requires less amounts of organic solvent (null for synthesis, only for purification) and is scalable, but also allows for transformations that were reported impossible in the solution phase.

Ruthenium Catalyzed Ethenolysis of Internal Olefin Generated from Fischer-Tropsch Process to Produce α-Olefin

Ruthenium olefin metathesis catalysts were evaluated for the ethenolysis of trans-5-decene, a model compound of internal olefins, to produce α-olefin. N-heterocyclic carbene (NHC) ruthenium metathe...

Ethyl Lactate: A Green Solvent for Olefin Metathesis

Compatibility of selected, commercially available ruthenium olefin metathesis catalysts with ethyl lactate as solvent was evaluated using a range of substrates and conditions. In addition, the preparation of a metathesis catalyst in simplified manner by using the advantages of ethyl lactate was accomplished. The application of ethyl lactate facilitates product isolation (also allowing for lower ruthenium contamination in crude metathesis products) and improves the overall green angle of olefin metathesis.

Highly active ruthenium metathesis catalysts enabling ring-opening metathesis polymerization of cyclopentadiene at low temperatures

Development of versatile ruthenium olefin-metathesis catalysts with high activity, stability, and selectivity is a continuous challenge. Here we report highly controllable ruthenium catalysts using readily accessible and versatile N-vinylsulfonamides as carbene precursors. Catalyst initiation rates were controlled in a straightforward manner, from latent to fast initiating, through the facile modulation of the N-vinylsulfonamide ligands. Trifluoromethanesulfonamide-based catalysts initiated ultrarapidly even at temperatures as low as −60 °C and continuously propagated rapidly, enabling the enthalpically and entropically less-favored ring-opening metathesis polymerizations of low-strained functionalized cyclopentene derivatives, some of which are not accessible with previous olefin-metathesis catalysts. To our surprise, the developed catalysts facilitated the polymerization of cyclopentadiene (CPD), a feedstock that is easily and commonly obtainable through the steam cracking of naphtha, which has, to the best of our knowledge, not been previously achieved due to its low ring strain and facile dimerization even at low temperatures (below 0 °C).

Simple and Robust Immobilization of a Ruthenium Olefin Metathesis Catalyst Inside MOFs by Acid–Base Reaction

The first successful immobilization of a transition-metal catalyst inside MOFs by an acid–base reaction has been described. Simple absorption of a commercially available, amino-tagged, Hoveyda–Grub...

Specialized Ruthenium Olefin Metathesis Catalysts Bearing Bulky Unsymmetrical NHC Ligands: Computations, Synthesis, and Application

Second-generation ruthenium olefin metathesis catalysts were investigated with systematic variation of the unsymmetrical uNHC ligands. Depending on the uNHC steric bulk, the catalysts exhibited dif...

Heterogeneous Removal of Water-Soluble Ruthenium Olefin Metathesis Catalyst from Aqueous Media Via Host-Guest Interaction

Heterogeneous Removal of Water-Soluble Ruthenium Olefin Metathesis Catalyst from Aqueous Media Via Host-Guest Interaction

Heterogeneous Removal of Water-Soluble Ruthenium Olefin Metathesis Catalyst from Aqueous Media Via Host-Guest Interaction.

A highly efficient transition metal catalyst removal method is developed. The water-soluble catalyst contains a newly-designed NHC ligand for the catalyst removal via host-guest interactions. The new NHC ligand possesses an adamantyl (guest) tethered linear ethylene glycol units for hydrophobic inclusion into the cavity of a β-cyclodextrin (β-CD) host compound. The new NHC ligand was applied to a Ru-based olefin metathesis catalyst. The Ru catalyst demonstrated excellent activity in representative ring-closing metathesis (RCM) and ring-opening metathesis polymerization (ROMP) reactions in aqueous media as well as organic solvent, CH2Cl2. After the reaction was complete, the lingering Ru residue was removed from the aqueous solution with the efficiency of more than 99% (53 ppm of Ru residue) by simple filtration utilizing a host-guest interaction between insoluble silica-grafted β-CD (host) and the adamantyl moiety (guest) on the catalyst. The new Ru catalyst also demonstrated high removal efficiency via extraction when the reaction is run in organic solvent by partitioning the crude reaction mixture between layers of diethyl ether and water. In this way, the catalyst stays in aqueous layer only. In organic layer, the residual Ru amount was only 0.14 ppm in the RCM reactions of diallyl compounds.

Ruthenium Olefin Metathesis Catalysts Systematically Modified in Chelating Benzylidene Ether Fragment: Experiment and Computations

Five Hoveyda–Grubbs‐type complexes bearing cyclopropyl to cycloheptyl ether moieties in the chelating benzylidene ligand have been synthesized and investigated by spectroscopic and crystallographic methods. Their experimentally measured catalytic activity in model ring‐closing metathesis (RCM) reactions was studied at catalyst loading down to 10 ppm and compared with DFT calculations. The latter can be used as a reliable and accurate guidance in the fast and inexpensive design of new metathesis catalysts, as we were able to find a good correlation between the initiation rate and free‐energy barrier of the reaction.

An Initiation Kinetics Prediction Model Enables Rational Design of Ruthenium Olefin Metathesis Catalysts Bearing Modified Chelating Benzylidenes.

Rational design of second-generation ruthenium olefin metathesis catalysts with desired initiation rates can be enabled by a computational model that depends on a single thermodynamic parameter. Using a computational model with no assumption about the specific initiation mechanism, the initiation kinetics of a spectrum of second-generation ruthenium olefin metathesis catalysts bearing modified chelating ortho-alkoxy benzylidenes were predicted in this work. Experimental tests of the validity of the computational model were achieved by the synthesis of a series of ruthenium olefin metathesis catalysts and investigation of initiation rates by UV/Vis kinetics, NMR spectroscopy, and structural characterization by X-ray crystallography. Included in this series of catalysts were thirteen catalysts bearing alkoxy groups with varied steric bulk on the chelating benzylidene, ranging from ethoxy to dicyclohexylmethoxy groups. The experimentally observed initiation kinetics of the synthesized catalysts were in good accordance with computational predictions. Notably, the fast initiation rate of the dicyclohexylmethoxy catalyst was successfully predicted by the model, and this complex is believed to be among the fastest initiating Hoveyda-Grubbs-type catalysts reported to date. The compatibility of the predictive model with other catalyst families, including those bearing alternative NHC ligands or disubstituted alkoxy benzylidenes, was also examined.

Disentangling Ligand Effects on Metathesis Catalyst Activity: Experimental and Computational Studies of Ruthenium-Aminophosphine Complexes.

Second-generation ruthenium olefin metathesis catalysts bearing aminophosphine ligands were investigated with systematic variation of the ligand structure. The rates of phosphine dissociation ( k1; initiation rate) and relative phosphine reassociation ( k-1) were determined for two series of catalysts bearing cyclohexyl(morpholino)phosphine and cyclohexyl(piperidino)phosphine ligands. In both cases, incorporating P-N bonds into the architecture of the dissociating phosphine accelerates catalyst initiation relative to the parent [Ru]-PCy3 complex; however, this effect is muted for the tris(amino)phosphine-ligated complexes, which exhibit higher ligand binding constants in comparison to those with phosphines containing one or two cyclohexyl substituents. These results, along with X-ray crystallographic data and DFT calculations, were used to understand the influence of ligand structure on catalyst activity. Especially noteworthy is the application of phosphines containing incongruent substituents (PR1R'2); detailed analyses of factors affecting ligand dissociation, including steric effects, inductive effects, and ligand conformation, are presented. Computational studies of the reaction coordinate for ligand dissociation reveal that ligand conformational changes contribute to the rapid dissociation for the fastest-initiating catalyst of these series, which bears a cyclohexyl-bis(morpholino)phosphine ligand. Furthermore, the effect of amine incorporation was examined in the context of ring-opening metathesis polymerization, and reaction rates were found to correlate well with catalyst initiation rates. The combined experimental and computational studies presented in this report reveal important considerations for designing efficient ruthenium olefin metathesis catalysts.

Removable Water-Soluble Olefin Metathesis Catalyst via Host-Guest Interaction.

A highly removable N-heterocyclic carbene ligand for a transition-metal catalyst in aqueous media via host-guest interactions has been developed. Water-soluble adamantyl tethered ethylene glycol in the ligand leads a hydrophobic inclusion into the cavity of β-cyclodextrin. Ruthenium (Ru) olefin metathesis catalyst with this ligand demonstrated excellent performance in various metathesis reactions in water as well as in CH2Cl2, and removal of residual Ru was performed via filtration utilizing a host-guest interaction and extraction.

Forged and fashioned for faithfulness-ruthenium olefin metathesis catalysts bearing ammonium tags.

In this article, the synthesis and applications of selected ammonium tagged Ru-alkylidene metathesis catalysts were described. Because of the straightforward synthesis, the first generation of onium-tagged catalysts have the ammonium group installed in the benzylidene ligand. Such catalysts usually give relatively pure metathesis products, and are used in polar solvents and water, or immobilised on various supports. Later, catalysts tagged in the N-heterocyclic carbene ligand (NHC) were developed to offer higher stability and even lower metal contamination levels. Due to minimal leaching, the non-dissociating ligand tagged systems were successfully immobilised on various supports, including zeolites and Metal Organic Frameworks (MOFs) and used in batch and in continuous flow conditions.

Stable ruthenium olefin metathesis catalysts bearing symmetrical NHC ligands with primary and secondary N-alkyl groups.

Four novel stable Hoveyda-Grubbs-type catalysts containing N,N'-dineopentyl- and N,N'-dicyclohexyl-substituted N-heterocyclic carbene (NHC) ligands with syn and anti phenyl groups on the ring backbone were synthesized and fully characterized. The catalytic potential of these complexes was investigated in metathesis reactions of both standard and renewable substrates. Compared to the Hoveyda-Grubbs second generation catalyst (HGII), all of the new catalysts showed high performances in most of the examined metathesis transformations. In particular, N,N'-dicyclohexyl catalysts gave improved results in the challenging ring-closing metathesis (RCM) reaction to form tetrasubstituted olefins, while catalysts with neopentyl N-groups were found to be more active and Z-selective in cross-metathesis (CM) reactions. Modest enantioselectivities in the asymmetric ring-closing metathesis (ARCM) of achiral trienes with different steric hindrance were observed in the presence of catalysts bearing chiral C2-symmetric NHC ligands.

Recent Advancements in Stereoselective Olefin Metathesis Using Ruthenium Catalysts

Olefin metathesis is a prevailing method for the construction of organic molecules. Recent advancements in olefin metathesis have focused on stereoselective transformations. Ruthenium olefin metathesis catalysts have had a particularly pronounced impact in the area of stereoselective olefin metathesis. The development of three categories of Z-selective olefin metathesis catalysts has made Z-olefins easily accessible to both laboratory and industrial chemists. Further design enhancements to asymmetric olefin metathesis catalysts have streamlined the construction of complex molecules. The understanding gained in these areas has extended to the employment of ruthenium catalysts to stereoretentive olefin metathesis, the first example of a kinetically E-selective process. These advancements, as well as synthetic applications of the newly developed catalysts, are discussed.