Grubbs-type Catalysts Research Articles

Automated de Novo Design of Olefin Metathesis Catalysts: Computational and Experimental Analysis of a Simple Thermodynamic Design Criterion.

Methods for computational de novo design of inorganic molecules have paved the way for automated design of homogeneous catalysts. Such studies have so far relied on correlation-based prediction models as fitness functions (figures of merit), but the soundness of these approaches has yet to be tested by experimental verification of de novo-designed catalysts. Here, a previously developed criterion for the optimization of dative ligands L in ruthenium-based olefin metathesis catalysts RuCl2(L)(L')(═CHAr), where Ar is an aryl group and L' is a phosphine ligand dissociating to activate the catalyst, was used in de novo design experiments. These experiments predicted catalysts bearing an N-heterocyclic carbene (L = 9) substituted by two N-bound mesityls and two tert-butyl groups at the imidazolidin-2-ylidene backbone to be promising. Whereas the phosphine-stabilized precursor assumed by the prediction model could not be made, a pyridine-stabilized ruthenium alkylidene complex (17) bearing carbene 9 was less active than a known leading pyridine-stabilized Grubbs-type catalyst (18, L = H2IMes). A density functional theory-based analysis showed that the unsubstituted metallacyclobutane (MCB) intermediate generated in the presence of ethylene is the likely resting state of both 17 and 18. Whereas the design criterion via its correlation between the stability of the MCB and the rate-determining barrier indeed seeks to stabilize the MCB, it relies on RuCl2(L)(L')(═CH2) adducts as resting states. The change in resting state explains the discrepancy between the prediction and the actual performance of catalyst 17. To avoid such discrepancies and better address the multifaceted challenges of predicting catalytic performance, future de novo catalyst design studies should explore and test design criteria incorporating information from more than a single relative energy or intermediate.

A Self-Healing System for Polydicyclopentadiene Thermosets.

Self-healing offers promise for addressing structural failures, increasing lifespan, and improving durability in polymeric materials. Implementing self-healing in thermoset polymers faces significant manufacturing challenges, especially due to the elevated temperature requirements of thermoset processing. To introduce self-healing into structural thermosets, the self-healing system must be thermally stable and compatible with the thermoset chemistry. This article demonstrates a self-healing microcapsule-based system stable to frontal polymerization (FP), a rapid and energy-efficient manufacturing process with a self-propagating exothermic reaction (≈200 °C). A thermally latent Grubbs-type complex bearing two N-heterocyclic carbene ligands addresses limitations in conventional G2-based self-healing approaches. Under FP's elevated temperatures, the catalyst remains dormant until activated by a Cu(I) co-reagent, ensuring efficient polymerization of the dicyclopentadiene (DCPD) upon damage to the polyDCPD matrix. The two-part microcapsule system consists of one capsule containing the thermally latent Grubbs-type catalyst dissolved in the solvent, and another capsule containing a Cu(I) coagent blended with liquid DCPD monomer. Using the same chemistry for both matrix fabrication and healing results in strong interfaces as demonstrated by lap-shear tests. In an optimized system, the self-healing system restores the mechanical properties of the tough polyDCPD thermoset. Self-healing efficiencies greater than 90% via tapered double cantilever beam tests are observed.

Properties and Catalytic Activity of Hoveyda–Grubbs-Type Catalysts with an S → Ru Coordination Bond in a Six-Membered Chelate Ring

Properties and Catalytic Activity of Hoveyda–Grubbs-Type Catalysts with an <i>S</i> → Ru Coordination Bond in a Six-Membered Chelate Ring

Application of New Efficient Hoveyda-Grubbs Catalysts Comprising an N→Ru Coordinate Bond in a Six-Membered Ring for the Synthesis of Natural Product-Like Cyclopenta[b]furo[2,3-c]pyrroles.

The ring rearrangement metathesis (RRM) of a trans-cis diastereomer mixture of methyl 3-allyl-3a,6-epoxyisoindole-7-carboxylates derived from cheap, accessible and renewable furan-based precursors in the presence of a new class of Hoveyda–Grubbs-type catalysts, comprising an N→Ru coordinate bond in a six-membered ring, results in the difficult-to-obtain natural product-like cyclopenta[b]furo[2,3-c]pyrroles. In this process, only one diastereomer with a trans-arrangement of the 3-allyl fragment relative to the 3a,6-epoxy bridge enters into the rearrangement, while the cis-isomers polymerize almost completely under the same conditions. The tested catalysts are active in the temperature range from 60 to 120 °C at a concentration of 0.5 mol % and provide better yields of the target tricycles compared to the most popular commercially available second-generation Hoveyda–Grubbs catalyst. The diastereoselectivity of the intramolecular Diels–Alder reaction furan (IMDAF) reaction between starting 1-(furan-2-yl)but-3-en-1-amines and maleic anhydride, leading to 3a,6-epoxyisoindole-7-carboxylates, was studied as well.

Dual Activity of Grubbs-Type Catalyst in the Transvinylation of Carboxylic Acids and Ring-Closing Metathesis Reactions.

The development of a multifunctional catalyst, which mimics the promiscuity of enzymes, that would catalyze more than one chemical transformation in a single reaction vessel is one of the key points of modern sustainable chemistry. The results of our experiments indicated that Grubbs-type catalysts possess such multitask activity, catalyzing the transvinylation reaction of carboxylic acids without losing their original metathetic activity. This new activity of Grubbs catalysts was evidenced on several examples. It allows us to design a transvinylation/ring-closing metathesis (RCM) cascade reaction leading to the formation of endocyclic enol lactones from unsaturated carboxylic acids in an one-pot procedure. This unique ability of Grubbs catalyst to catalyze multiple mechanically distinct cascade reactions in a chemoselective way offers the new possibility for the synthesis of complex compounds from simple, easily accessible substrates.

NHC Ligand Effects on Ru-Catalyzed Cross-Metathesis of Renewable Materials

As petrochemical resources become increasingly scarce and expensive, much attention has been focused on renewable resources from biomass as alternative options for producing basic building blocks for chemical manufacturing. Catalytic olefin metathesis represents a powerful tool to transform biosourced structural motifs in valuable commodity, fine, and specialty chemicals. In that respect, the appropriate choice of the catalyst is the key issue of each metathesis transformation. The current study examines the influence of different N-heterocyclic carbene (NHC) ligands containing one or two N-alkyl substituents on the efficiency of Hoveyda–Grubbs-type catalysts in the cross-metathesis reaction of ethyl oleate with cis-1,4-diacetoxy-2-butene and cross-metathesis of eugenol acetate with cis-1,4-dichloro-2-butene. Interestingly, the introduction of alkyl N-substituents in the NHC ligand was revealed as beneficial for catalytic performances in the examined cross-metathesis (CM) reactions, leading to higher activity and/or selectivity than those observed in the presence of the classical, commercially available Hoveyda–Grubbs second generation catalyst (HGII).

Second-coordination sphere effects on the reactivities of Hoveyda-Grubbs-type catalysts: a ligand exchange study using phenolic moiety-functionalized ligands.

The Hoveyda-Grubbs (HG) second-generation catalyst (HG-II), a Ru complex with a 2-isopropoxybenzylidene ligand, is extensively used for olefin metathesis, the rearrangement of carbon-carbon double bonds. A well-known strategy to control its complex reactivity is to modify the phenyl ring in the ligand, thereby directly influencing the coordination of the phenolic oxygen to the metal center. We, herein, report that a functional group attached to the phenolic moiety in the 2-alkoxybenzylidene ligand can indirectly affect the reactivities of HG-type complexes. In this work, the ligand exchange reactions between HG-II and phenolic moiety-modified 2-alkoxybenzylidene ligands are useful for evaluating the structural effects of the ligands. Specifically, an ethylene amide or an ester group at the terminal phenolic moiety in the benzylidene ligand was found to influence the relative stabilities of HG-type complexes compared to that of the HG-II complex. The structural analyses proved that the observed effects of the functional groups on the complex stabilities originate from the interactions with a chlorido ligand in HG-type complexes without changes in coordination fashions at the metal centers. It was found that the outer-sphere interactions also influence the catalytic activities of HG-type complexes, namely, the properties of HG-type complexes can be controlled by outer-sphere structural factors toward the metal center (i.e., "the second-coordination sphere effect"). In the design of functionalized HG-type complexes, the outer-sphere structural effects need to be considered in addition to the optimization of the metal coordination site.

Formation of active species from ruthenium alkylidene catalysts\u2014an insight from computational perspective

Ruthenium alkylidene complexes are commonly used as olefin metathesis catalysts. Initiation of the catalytic process requires formation of a 14-electron active ruthenium species via dissociation of a respective ligand. In the present work, this initiation step has been computationally studied for the Grubbs-type catalysts (H2IMes)(PCy3)(Cl)2Ru=CHPh, (H2IMes)(PCy3)(Cl)2Ru=CH-CH=CMe2 and (H2IMes)(3-Br-py)2(Cl)2Ru=CHPh, and the Hoveyda-Grubbs-type catalysts (H2IMes)(Cl)2Ru=CH(o-OiPrC6H4), (H2IMes)(Cl)2Ru=CH(5-NO2–2-OiPrC6H3), and (H2IMes)(Cl)2Ru=CH(2-OiPr-3-PhC6H3), using density functional theory (DFT). Additionally, the extended-transition-state combined with the natural orbitals for the chemical valence (ETS-NOCV) and the interacting quantum atoms (IQA) energy decomposition methods were applied. The computationally determined activity order within both families of the catalysts and the activation parameters are in agreement with reported experimental data. The significance of solvent simulation and the basis set superposition error (BSSE) correction is discussed. ETS-NOCV demonstrates that the bond between the dissociating ligand and the Ru-based fragment is largely ionic followed by the charge delocalizations: σ(Ru–P) and π(Ru–P) and the secondary CH…Cl, CH…π, and CH…HC interactions. In the case of transition state structures, the majority of stabilization stems from London dispersion forces exerted by the efficient CH…Cl, CH…π, and CH…HC interactions. Interestingly, the height of the electronic dissociation barriers is, however, directly connected with the prevalent (unfavourable) changes in the electrostatic and orbital interaction contributions despite the favourable relief in Pauli repulsion and geometry reorganization terms during the activation process. According to the IQA results, the isopropoxy group in the Hoveyda-Grubbs-type catalysts is an efficient donor of intra-molecular interactions which are important for the activity of these catalysts.

Alkyne gem-Hydrogenation: Formation of Pianostool Ruthenium Carbene Complexes and Analysis of Their Chemical Character.

Parahydrogen (p-H2 ) induced polarization (PHIP) NMR spectroscopy showed that [CpX Ru] complexes with greatly different electronic properties invariably engage propargyl alcohol derivatives into gem-hydrogenation with formation of pianostool ruthenium carbenes; in so doing, less electron rich CpX rings lower the barriers, stabilize the resulting complexes and hence provide opportunities for harnessing genuine carbene reactivity. The chemical character of the resulting ruthenium complexes was studied by DFT-assisted analysis of the chemical shift tensors determined by solid-state 13 C NMR spectroscopy. The combined experimental and computational data draw the portrait of a family of ruthenium carbenes that amalgamate purely electrophilic behavior with characteristics more befitting metathesis-active Grubbs-type catalysts.

Synthesis of Functional Polyacetylenes via Cyclopolymerization of Diyne Monomers with Grubbs-type Catalysts.

Metathesis cyclopolymerization (CP) of α,ω-diynes is a powerful method to prepare functional polyacetylenes (PAs). PAs have long been studied due to their interesting electrical, optical, photonic, and magnetic properties which make them candidates for use in various advanced applications. Grubbs catalysts are widely used throughout synthetic chemistry, largely due to their accessibility, high reactivity, and tolerance to air, moisture, and many functional groups. Prior to our entrance into this field, only a few examples of CP using modified Grubbs catalysts existed. Inspired by these works, we saw an opportunity to expand the accessibility and utility of Grubbs-catalyzed CPs. We began by exploring CP with popular and commercially available Grubbs catalysts. We found Grubbs third-generation catalyst (G3) to be an excellent catalyst when we used strategies to stabilize the propagating Ru carbene, such as decreasing the polymerization temperature or using weakly coordinating solvent or ligands. Controlled living polymerizations were demonstrated using various 1,6-heptadiyne monomers and yielded polymers with exclusively 5-membered rings (via α-addition) in the polymer backbone. The strategy of stabilizing the Ru carbene was also critical to successful CP with Hoveyda-Grubbs second-generation (HG2) and Grubbs first-generation (G1) catalysts. We found that decomposed Ru species were catalyzing side reactions which could be completely shut down by decreasing the reaction temperature or using weakly coordinating ligands. While HG2 generally led to uncontrolled polymerizations, we found it to be an effective catalyst for monomers with very large side chains. G1 displayed broader functional group tolerance and thus broader monomer scope than G3. We next looked at our ability to change the regioselectivity of the polymerization by using Z-selective catalysts which favor β-addition and the formation of 6-membered rings in the polymer backbone. While modest β-selectivity could be obtained using Grubbs Z-selective catalyst at low temperatures, we found that by using one of Hoveyda and co-workers' catalysts with decreased carbene electrophilicity, we could achieve exclusive formation of 6-membered rings. We also pursued alternative routes to achieve 6+-membered rings in the polymer backbone by using diyne monomers with increased distance between alkynes. We found that optimizing the monomer structure for CP was an effective strategy to achieve controlled polymerizations. By using bulky substituents (maximizing the Thorpe-Ingold effect) and/or using heteroatoms (shorter bonds) to bring the alkynes closer together, controlled living CP could be achieved with various 1,7-octadiyne and 1,8-nonadiyne monomers. Finally, we took advantage of several inherent properties of controlled CP techniques to prepare polymers with advanced architectures and nanostructures. For instance, the living nature of the polymerization enabled production of block copolymers, the tolerance of very large substituents enabled production of dendronized and brush polymers, and the insolubility or crystallinity of some monomers was utilized for the spontaneous self-assembly of polymers into various one- and two-dimensional nanostructures. Overall, the strategies of stabilizing the propagating Ru carbene, modulating the selectivity and reactivity of the Ru carbene, and enhancing the inherent reactivity of monomers were key to improving the utility and performance of CP with Grubbs-type catalysts. The insight provided by these studies will be important for future developments of CP and other metathesis polymerizations utilizing ring-closing steps.

Geographical information system software as in-house chemical indexing database for catalyst screening of alkene metathesis catalysts

Abstract Large amounts of chemical information can be indexed on various databases. If these databases store information in an appropriate format for specific search queries, we can perform a catalyst screening from intelligent searches. However, little to no database applications are flexible enough to allow for simultaneous screening and comparable visualizations. Therefore, for setting up a flexible in-house database, we propose the use of geographical information system (GIS) software. GIS includes an attribute table that can be populated with text and numerical information. From this table, we could selectively screen our data as well as visualize the different values on an intelligently created grid pattern. Consequently, 11 types of alkene metathesis catalysts together with 9 Grubbs-type catalysts were investigated using GIS techniques. GIS proved to be an efficient and versatile tool whereby quick and successful catalyst screening could be done.

Alternating ring-opening metathesis polymerization by Grubbs-type catalysts with N-pentiptycenyl, N-alkyl-NHC ligands.

A Grubbs-Hoveyda type catalyst with a N-pentiptycenyl, N-cyclohexyl-NHC ligand provides poly(nbe-alt-coe) with an excellent degree of alternation while lacking significant activity in the homopolymerization of cyclooctene.

Mechanistic studies on acetylene cyclotrimerization catalyzed by Grubbs first and second generation catalysts

Cyclotrimerization of acetylene by Grubbs first and second generation CC bond metathesis catalysts has been attempted by experimentalists with limited success and lack of reliable information about the mechanism of the reaction hinders further progress in the development of efficient catalysts. Herein, we describe a systematic density functional theory study on the probable mechanisms of this reaction and show that it proceeds through either a fully metathesis pathway (pathway 1) or through an alternate non-metathesis pathway (pathway 2). Pathway 1 consists of four metathesis steps characterized by the formation of metallacyclobutene-like intermediate in each step while pathway 2 passes through a metallacyclopentadiene intermediate. In metathesis pathway, the final ring closing metathesis step for benzene formation competes with another cross metathesis propagation step towards polyene chain expansion. Pathways 1 and 2 show a highly exothermic ∼120kcal/mol reaction profile due to the tremendous energy gain in converting acetylene triple bonds to aromatic bonds. The reaction is highly influenced by the choice of ligands in the catalyst design, particularly, by reducing the steric influence of ligands, cyclotrimerization of acetylene to benzene appears as a feasible process by Grubbs-type catalysts.

Tuning the molecular weight of ROMP polymers by using Grubbs type catalysts and terminal alkynes

In this study, ruthenium vinyl carbene derivatives were obtained in situ by reacting Grubbs 1st and 2nd generation catalysts with terminal alkynes in toluene at 85 °C. These active species were used in ROMP of norbornene derivatives. The effect of alkyne/Ru (mol/mol) ratio and alkyne substituent on number average molecular weights (Mn) of ROMP polymers were investigated in detail. ROMP polymers with Mn values between 75 and 680 kDa were synthesized by varying alkyne/Ru (mol/mol) ratio in the presence of Grubbs type catalysts. The mechanism of the reaction and the role of alkyne in ROMP reactions were investigated in detail by 1H NMR.

Ru complexes of Hoveyda-Grubbs type immobilized on lamellar zeolites: activity in olefin metathesis reactions.

Hoveyda–Grubbs type catalysts with cationic tags on NHC ligands were linker-free immobilized on the surface of lamellar zeolitic supports (MCM-22, MCM-56, MCM-36) and on mesoporous molecular sieves SBA-15. The activity of prepared hybrid catalysts was tested in olefin metathesis reactions: the activity in ring-closing metathesis of citronellene and N,N-diallyltrifluoroacetamide decreased in the order of support MCM-22 ≈ MCM-56 > SBA-15 > MCM-36; the hybrid catalyst based on SBA-15 was found the most active in self-metathesis of methyl oleate. All catalysts were reusable and exhibited low Ru leaching (<1% of Ru content). XPS analysis revealed that during immobilization ion exchange between Hoveyda–Grubbs type catalyst and zeolitic support occurred in the case of Cl− counter anion; in contrast, PF6− counter anion underwent partial decomposition.

Ring-Closing and Cross-Metathesis with Artificial Metalloenzymes Created by Covalent Active Site-Directed Hybridization of a Lipase.

A series of Grubbs-type catalysts that contain lipase-inhibiting phosphoester functionalities have been synthesized and reacted with the lipase cutinase, which leads to artificial metalloenzymes for olefin metathesis. The resulting hybrids comprise the organometallic fragment that is covalently bound to the active amino acid residue of the enzyme host in an orthogonal orientation. Differences in reactivity as well as accessibility of the active site by the functionalized inhibitor became evident through variation of the anchoring motif and substituents on the N-heterocyclic carbene ligand. Such observations led to the design of a hybrid that is active in the ring-closing metathesis and the cross-metathesis of N,N-diallyl-p-toluenesulfonamide and allylbenzene, respectively, the latter being the first example of its kind in the field of artificial metalloenzymes.

Alkynol polymerization catalysed by Grubbs-type and Hoveyda–Grubbs ruthenium alkylidene complexes: A computational study

Density functional theory has been applied to study the mechanism of alkynol (3-butyn-2-ol) polymerization by the third generation Grubbs catalyst and the second generation Hoveyda–Grubbs catalyst. Besides typical linear products that can immediately undergo the next insertion step, cyclic products can be also formed, by oxygen-chelation to the ruthenium centre or as less stable rutenacyclobutene species. The initiation stage is predicted to be faster for the Grubbs-type catalyst, compared to the Hoveyda–Grubbs initiator. It is shown that α-insertion is always kinetically preferred over the β-insertion path. On the other hand, the NMR calculations indicate that the experimentally observed 1H NMR signals can be assigned to alkylidene protons of ruthenium species formed in minor amounts by β-insertion. The choice of the DFT method affects significantly the theoretical values of the 1H NMR chemical shifts. The general conclusions about the kinetic preferences do not depend on the density functional used, although the calculated energy profiles are strongly influenced. The simulation of the solvent effect is less significant.

Effect of Added Salt on Ring-Closing Metathesis Catalyzed by a Water-Soluble Hoveyda–Grubbs Type Complex To Form N-Containing Heterocycles in Aqueous Media

The efficiency of ring-closing metathesis catalyzed by a Hoveyda–Grubbs type catalyst in water can be enhanced by addition of a chloride salt under neutral conditions. UV–vis spectroscopic study showed that a characteristic band of the catalyst around 380 nm remained over 16 h in the presence of KCl, whereas the band distinctly decreased in the absence of KCl. The disappearance of the band is ascribed to a displacement of a chloride ligand by a water molecule or a hydroxide anion. The spectral changes can be related to the metathesis activity. The experimental results indicate that avoidance of the chloride ligand loss is important to maintain the metathesis activity in water.

Monomeric and dendritic second generation Grubbs- and Hoveyda–Grubbs-type catalysts for olefin metathesis

The synthesis and characterization of monomeric and dendritic Grubbs II and Hoveyda–Grubbs II-based complexes are reported. These complexes were synthesized via a route based on the connection of monomeric or dendritic N-alkyl-N′-mesitylimidazol-2-ylidene pre-ligands to Grubbs I or Hoveyda–Grubbs I complexes. The immobilization of a modified Grubbs II type catalyst on a G0 carbosilane dendrimer was successfully carried out. Together with monomeric Grubbs II and Hoveyda–Grubbs-analogs and several commercially available olefin metathesis catalysts, the soluble, homogeneous G0-dendritic Grubbs II complex was tested as catalyst in the ring closing metathesis of diethyl diallylmalonate. The immobilized complex proved to outperform its monomeric analog in this reaction at room temperature, whereas it was found to be slightly slower at reflux temperature.

Grubbs-type catalysts immobilized on SBA-15: A novel heterogeneous catalyst for olefin metathesis

A series of novel heterogeneous olefin metathesis catalysts have been developed by immobilizing Ru species on SBA-15 mesoporous silica with tunable ordered pores and different textural properties. Various techniques such as X-ray diffraction, N2 adsorption–desorption, high resolution transmission electron microscopy have been used to characterize the physicochemical properties of the catalysts. The catalytic activity of thus prepared Ru-based catalysts has been studied in ring closing metathesis (RCM) and other metathesis reactions. Among these immobilized catalysts, the SBA-15 support with the largest pore size gives rise to the highest catalytic activity because the large pores are benefit for the diffusion of reactants and products. The immobilized Ru catalysts prove to be reusable in RCM reactions. Their catalytic activity is closely related to the confinement effect and high hydrophobicity of SBA-15 mesopores.