Olefin Metathesis Activity Research Articles

Synthesis and Application of Robust Spiro [Fluorene-9] CAAC Ruthenium Alkylidene Complexes for the "One-Pot" Conversion of Allyl Acetate to Butane-1,4-diol.

A series of a novel CAAC ligands featuring a spiro-fluorene group have been synthesized and complexed with ruthenium alkylidenes, yielding the corresponding Hoveyda-type derivatives as a new family of olefin metathesis catalysts. The novel complexes have been characterized by XRD, HRMS and NMR measurements. The synthetised complexes were tested in catalysis and showed good activity in olefin metathesis, as demonstrated on diethyl diallylmalonate and allyl acetate substrates. The unique backbone in the ligand with the large, yet inflexible condensed system renders interesting properties to the catalyst, exemplified by the good catalytic performance and improved Z-selectivity. In addition, the complex can also serve as a hydrogenation catalyst in a consecutive (one-pot) reaction. The latter reaction can convert allyl acetate to butane-1,4-diol, a valuable chemical intermediate for biodegradable polybutylene succinate (PBS).

Active Site Descriptors from 95Mo NMR Signatures of Silica-Supported Mo-Based Olefin Metathesis Catalysts.

The olefin metathesis activity of silica-supported molybdenum oxides depends strongly on metal loading and preparation conditions, indicating that the nature and/or amounts of the active sites vary across compositionally similar catalysts. This is illustrated by comparing Mo-based (pre)catalysts prepared by impregnation (2.5-15.6 wt % Mo) and a model material (2.3 wt % Mo) synthesized via surface organometallic chemistry (SOMC). Analyses of FTIR, UV-vis, and Mo K-edge X-ray absorption spectra show that these (pre)catalysts are composed predominantly of similar isolated Mo dioxo sites. However, they exhibit different reaction properties in both liquid and gas-phase olefin metathesis with the SOMC-derived catalyst outperforming a classical catalyst of a similar Mo loading by ×1.5-2.0. Notably, solid-state 95Mo NMR analyses leveraging state-of-the-art high-field (28.2 T) measurement conditions resolve four distinct surface Mo dioxo sites with distributions that depend on the (pre)catalyst preparation methods. The intensity of a specific deshielded 95Mo NMR signal, which is most prominent in the SOMC-derived catalyst, is linked to reducibility and catalytic activity. First-principles calculations show that 95Mo NMR parameters directly manifest the local strain and coordination environment: acute (SiO-Mo(O)2-OSi) angles and low coordination numbers at Mo lead to highly deshielded 95Mo chemical shifts and small quadrupolar coupling constants, respectively. Natural chemical shift analyses relate the 95Mo NMR signature of strained species to low LUMO energies, which is consistent with their high reducibility and corresponding reactivity. The 95Mo chemical shifts of supported Mo dioxo sites are thus linked to their specific electronic structures, providing a powerful descriptor for their propensity toward reduction and formation of active sites.

On the use of 17O NMR for understanding molecular and silica-grafted tungsten oxo siloxide complexes.

17O-labelled tungsten siloxide complexes [WOCl2(OSitBu3)2] (1-Cl) and [WOMe2(OSitBu3)2] (1-Me) were prepared and characterized by 17O MAS NMR, with input from theoretical calculations of NMR parameters. Guidelines linking 17O NMR parameters and the coordination sphere of molecular and silica-grafted tungsten oxo species are proposed. The grafting of 1-Me on SiO2-700 afforded material 2, with surface species [(SiO)WOMe2(OSitBu3)] as shown by elemental analysis, IR and 1H and 13C MAS NMR. The DFT calculations of the grafting mechanism are in line with the observed reactivity. They indicate the occurrence of several isomeric species of close energy for the grafted W centers, precluding efficient 17O MAS NMR studies. The lack of catalytic activity in olefin metathesis and ring-opening olefin metathesis polymerization indicates that initiation by α-H elimination is not operative in 2, contrary to related tungsten surface species, which illustrates the crucial influence of the nature of the metal coordination sphere.

Reactivity regulation for olefin metathesis-catalyzing ruthenium complexes with sulfur atoms at the terminal of 2-alkoxybenzylidene ligands.

For regulating the olefin metathesis (OM) activity of the Hoveyda-Grubbs second-generation complex (HG-II), the structural modification of the benzylidene ligand is a useful strategy. This paper reports the effect of a chalcogen atom placed at the end of the benzylidene group on the catalytic properties of HG-II derivatives, using complexes with a thioether or ether component in the benzylidene ligand (ortho-Me-E-(CH2)2O-styrene; E = S, O). Nuclear magnetic resonance and X-ray crystallographic analyses of the complex with a thioether moiety (E = S) proved the (O,S)-bidentate and trans-dichlorido coordination for the complex. A stoichiometric ligand exchange between HG-II and the benzylidene ligand (E = S) produced the corresponding complex with an 86% yield, confirming higher stability of the complex (E = S) than that of HG-II. Despite the bidentate chelation, the complex (E = S) exhibited OM catalytic activity, indicating the exchangeability of the S-chelating ligand with an olefinic substrate. The green solution color, a characteristic of HG-II derivatives, was retained after the complex (E = S)-mediated OM reactions, indicating high catalyst durability. Conversely, the complex (E = O) rapidly initiated OM reactions; however, it showed low catalyst durability. In the OM reactions conducted in the presence of methanol, the complex (E = S) exhibited higher yields than the complex (E = O) and HG-II: the S-coordination increased the catalyst tolerance to methanol. A coordinative atom (such as sulfur) placed at the terminal of the benzylidene ligand can precisely regulate the reactivity of HG-II derivatives.

Unexpected Latency of Z-Stereoretentive Ruthenium Olefin Metathesis Catalysts Bearing Unsymmetrical N-heterocyclic Carbene or Cyclic(alkyl)(amino)carbene Ligands.

A set of ruthenium complexes bearing a CAAC or uNHC ligand and a dithiocatechol fragment have been obtained and characterized spectroscopically. The activity and Z-selectivity of the newly obtained catalysts were studied in selected model CM, self-CM, and RCM olefin metathesis reactions. Intriguingly, and in contrast to structurally related NHC-bearing catalysts Ru4a and Ru4b, the CAAC and uNHC analogues showed no or only very little activity in olefin metathesis. Interestingly, despite being not productive in metathesis reactions conducted in solution, Ru8 enabled the synthesis of a model 16-membered macrocyclic lactone of valuable musk smell with excellent chemoselectivity (no C-C double-bond migration was observed) at a concentration 40 times higher than that typically used by organic chemists in similar macrocyclizations (200 mM instead of 5 mM) with excellent Z-selectivity. Unfortunately, also here the conversion was low.

Synthesis and catalytic olefin metathesis activity of amberlyst-15 supported cyclic and bicyclic alkyl amino carbene ruthenium complexes

Amberlyst-15 supported cyclic alkyl amino carbene and bicyclic alkyl amino carbene ruthenium olefin metathesis catalysts for sustainable catalytic applications have been synthesized by the well-known wet impregnation method utilizing ionic complex/support interaction. Surface coverages are as high as 4 and 7 wt% were achieved in the case of the significantly higher pore volume Amberlyst-15, compared to Amberlyst-36. These phase separable catalysts show high activity in cross metathesis, ring closing metathesis and ethenolysis reactions compared to the reported heterogenized olefin metathesis catalysts. Leeching tests revealed no more than 1.5 ppm ruthenium content for the investigated metathesis reactions, which is well below the accepted 10 ppm limit in case of consumer products.

Increasing Olefin Metathesis Activity of Silica‐Supported Molybdenum Imido Adamantylidene Complexes through E Ligand σ‐Donation

AbstractMolybdenum imido adamantylidene complexes with different substituents on the imido ligand (dipp=2,6‐diisopropylphenyl, ArF5=C6F5, and tBu) having distinct electron donating abilities were investigated for the metathesis of internal and terminal olefins, for both molecular and silica‐supported species using standardized protocols. Here we show that surface immobilization of these compounds results in dramatically increased activity compared to their molecular counterparts. Additionally, we show that electron withdrawing imido groups increase the activity of the compound towards terminal olefins while they simultaneously decrease the ability to metathesize internal olefins. Furthermore, these systems also show high stability when used as initiators in olefin metathesis, although the species that display higher initial activity deactivate faster than those that show more a more moderate reaction rate at first. Our catalytic studies, augmented by DFT calculations, show that all investigated compounds have a remarkably small energy difference between the trigonal bipyramidal (TBP) and square planar (SP) configurations of the metallacyclobutane intermediates, which has previously been linked to high activity.

Synthesis and micellar property of amphiphilic brush-arm star copolymers via living ROMP

Norbornene macromonomers (MMs) with polystyrene (PS) or poly(tert-butyl acrylate) (PtBA) side chains were synthesized by a combination of atom-transfer radical polymerization (ATRP) and click reactions. A series of amphiphilic linear and star-shaped brush block copolymers were obtained through sequential ring opening metathesis polymerization (ROMP) of MMs by the use of the highly active olefin metathesis Blechert's catalyst and tri-functionalized Blechert's catalyst respectively, followed by hydrolysis reaction. These well-defined diblock molecular brushes can undergo further self-assembly to form spherical micelles in solution, which have been characterized using DLS, AF4-MALLS, AFM, TEM and fluorescence spectra. The dynamic diameter increases as the hydrophilic backbone content increases for the vast majority of the linear and star copolymers. Brush-arm star diblock amphiphilic copolymers have smaller hydrodynamic diameter and aggregation number than the corresponding linear analogues. Moreover, the critical micelle concentration (CMC) values of star diblock copolymers show less sensitive to the variety of composition.

Air-stable 18-electron adducts of Schrock catalysts with tuned stability constants for spontaneous release of the active species

Schrock alkylidenes are highly versatile, very active olefin metathesis catalysts, but their pronounced sensitivity to air still hinders their applications. Converting them into more robust but inactive 18-electron adducts was suggested previously to facilitate their handling. Generating the active species from the inactive adducts, however, required a high-temperature Lewis acid treatment and resulted in an insoluble by-product, limiting the practicality of the methodology. Herein, we introduce an approach to circumvent the inconvenient, costly, and environmentally taxing activation process. We show that 18-electron adducts of W- and Mo-based Schrock catalysts with finite stability constants (typically K = 200–15,000 M−1) can readily be prepared and isolated in excellent yields. The adducts display enhanced air-stability in the solid state, and in solution they dissociate spontaneously, hence liberating the active alkylidenes without chemical assistance.

Insights on alkylidene formation on Mo2C: A potential overlap between direct deoxygenation and olefin metathesis

There is evidence that there is a potential overlap between olefin metathesis and deoxygenation chemistries on molybdenum carbide catalysts. Ketones and aldehydes are known to undergo selective deoxygenation on Mo2C to form surface alkylidenes, and metal alkylidenes are initiator and propagator sites in olefin metathesis reactions. Moreover, a recent report correlated increased olefin metathesis activity with molybdenum carbide or oxycarbide formation on pre-treatment of a supported molybdenum oxide catalyst in methane. Taken together, these three strands prompt a DFT study of the elementary carbonyl bond scission step of selected aldehydes and ketones to form surface alkylidenes on Mo2C. This theoretical study is extended to an analysis of previously reported experimental reflectance infrared data for the interaction of cyclopentanone and acetaldehyde with a polycrystalline bulk hexagonal Mo2C sample.

Well-Defined Silica-Supported Tungsten(IV)-Oxo Complex: Olefin Metathesis Activity, Initiation, and Role of Brønsted Acid Sites.

Despite the importance of the heterogeneous tungsten-oxo-based olefin metathesis catalyst (WO3/SiO2) in industry, understanding of its initiation mechanism is still very limited. It has been proposed that reduced W(IV)-oxo surface species act as precatalysts. In order to understand the reactivity and initiation mechanism of surface W(IV)-oxo species, we synthesized a well-defined silica-supported W(IV)-oxo species, (≡SiO)WO(OtBuF6)(py)3 (F6@SiO2-700; OtBuF6 = OC(CH3)(CF3)2; py = pyridine), via surface organometallic chemistry (SOMC). F6@SiO2-700 was shown to be highly active in olefin metathesis upon removal of pyridine ligands through the addition of tris(pentafluorophenyl)borane (B(C6F5)3) or thermal treatment under high vacuum. The metathesis activity toward olefins with and without allylic C-H groups, namely β-methylstyrene and styrene, respectively, was investigated. In the case of styrene, we demonstrated the role of surface OH groups in initiating metathesis activity. We proposed that the presence of strong Brønsted acidic OH sites, which likely arises from the presence of adjacent W sites in the catalyst as revealed by 15N-labeled pyridine adsorption, can assist styrene metathesis. In contrast, initiation of olefins with allylic C-H groups (e.g., β-methylstyrene) is independent of the surface OH density and likely involves an allylic C-H activation mechanism, like the molecular W(IV)-oxo species. This study indicates that initiation mechanisms depend on the olefinic substrates and reveals the synergistic effect of Brønsted acidic surface sites and reduced W(IV) sites in the initiation of olefin metathesis.

Electronic effects in mixed N-heterocyclic carbene/phosphite indenylidene ruthenium metathesis catalysts.

Five new complexes [RuCl2(SIMes)(Ind)(O-pXC5H4)] bearing different para-substituted triphenylphosphites (X = H, OCH3, CF3, Cl, SF5 and CN) were synthesised and used to study the effect of the electronic properties of the phosphite on olefin metathesis activity. Investigations of the physical properties of the new ligands and complexes were performed using physicochemical and DFT calculations. The catalytic activity of the complexes was benchmarked in challenging ring closing metathesis transformations featuring the formation of tetra-substituted double bonds. Complex [RuCl2(SIMes)(Ind)P(O-pCF3C5H4)3] (3c) exhibited a particularly high catalytic activity, superior to state-of-the-art catalysts, and was further tested on a wide range of substrates.

MoO3 on zeolites MCM-22, MCM-56 and 2D-MFI as catalysts for 1-octene metathesis.

Highly active olefin metathesis catalysts were prepared by thermal spreading MoO3 and/or MoO2(acac)2 on MWW zeolites (MCM-22, delaminated MCM-56) and on two-dimensional MFI (all in NH4+ form). The catalysts‘ activities were tested in the metathesis of neat 1-octene (as an example of a longer chain olefin) at 40 °C. Catalysts with 6 wt % or 5 wt % of Mo were used. The acidic character of the supports had an important effect on both the catalyst activity and selectivity. The catalyst activity increases in the order 6MoO3/HZSM-5(25) (Si/Al = 25) << 6MoO2(acac)2/MCM-22(70) < 6MoO3/2D-MFI(26) < 6MoO3/MCM-56(13) < 6MoO3/MCM-22(28) reflecting both the enhancing effect of the supports‘ acidity and accessibility of the catalytic species on the surface. On the other hand the supports‘ acidity decreases the selectivity to the main metathesis product C14 due to an acid-catalyzed double bond isomerization (followed by cross metathesis) and oligomerization. 6MoO3/2D-MFI(26) with a lower concentration of the acidic centres resulting in catalysts of moderate activity but with the highest selectivity.

Promoting Terminal Olefin Metathesis with a Supported Cationic Molybdenum Imido Alkylidene N‐Heterocyclic Carbene Catalyst

AbstractSilica‐supported cationic Mo‐imido alkylidene N‐heterocyclic carbene catalysts, prepared by surface organometallic chemistry, display contrasting olefin metathesis activity for terminal and internal olefins. The high metathesis activity towards terminal alkenes is attributed to the strong σ‐donating property of the NHC ancillary ligand, which disfavors the formation of the parent square‐planar metallacyclobutane, an off‐cycle reaction intermediate resulting from the reaction with ethylene, one of the metathesis products. This tailored ligand environment also allowed the first trigonal bipyramidal (TBP) metallacyclobutane reaction intermediate for supported Mo metathesis catalysts to be identified.

Promoting Terminal Olefin Metathesis with a Supported Cationic Molybdenum Imido Alkylidene N-Heterocyclic Carbene Catalyst.

Silica-supported cationic Mo-imido alkylidene N-heterocyclic carbene catalysts, prepared by surface organometallic chemistry, display contrasting olefin metathesis activity for terminal and internal olefins. The high metathesis activity towards terminal alkenes is attributed to the strong σ-donating property of the NHC ancillary ligand, which disfavors the formation of the parent square-planar metallacyclobutane, an off-cycle reaction intermediate resulting from the reaction with ethylene, one of the metathesis products. This tailored ligand environment also allowed the first trigonal bipyramidal (TBP) metallacyclobutane reaction intermediate for supported Mo metathesis catalysts to be identified.

Correlation of Active Site Precursors and Olefin Metathesis Activity in W-Incorporated Silicates

SiO2-supported tungsten catalysts (WO3/SiO2) are currently used in industry to make propene via metathesis of butene and ethylene. Reliable information on structure–activity relationships, aimed at improved catalyst design, continues to be an active area of investigation. On the basis of ex situ XPS and solid-state NMR spectroscopy of calcined W-incorporated mesoporous silicates (W-KIT-6), complemented by computational studies and activity evaluation in a fixed-bed reactor, we demonstrate that W–O–Si species are the active site precursors for 2-butene + ethylene metathesis to produce propene at 450 °C. This conclusion is supported by evidence that the population of W–O–Si species on the calcined W-KIT-6 catalysts is linearly correlated with 2-butene conversion, regardless of W loading. We further show that for a given W loading, the accessible W–O–Si species and thereby the propene yield, can be enhanced by delaying the tungsten source addition during catalyst synthesis. Our results are consistent with previously reported in situ mechanistic studies using complementary analytical tools.

Ruthenium Amide Complexes – Synthesis and Catalytic Activity in Olefin Metathesis and in Ring‐Opening Polymerisation

A set of olefin metathesis catalysts bearing a ruthenium amide moiety was synthesised. In the ruthenium amide form these complexes exhibit very low activity in standard metathesis reactions. However, a dramatic increase of activity was observed upon in situ activation with trimethylsilyl chloride or HCl, allowing successful application of such catalysts in a number of model ring‐closing metathesis, cross‐metathesis and enyne transformations. Moreover, such activated complexes proved to be very effective catalysts for bulk polymerisation of dicyclopentadiene (DCPD). The influence of factors such as temperature and the nature of additives on the properties of poly‐DCPD was examined.

Carbamato-benzylidene ruthenium chelates – Synthesis, structure and catalytic activity in olefin metathesis

New carbamato-κN-benzylidene ruthenium chelates were synthesized from the first [RuCl2(PCy3)2(=CHPh)] and second [RuCl2(SIMes)(PCy3)(=CHPh)] generation Grubbs catalysts by metathetic exchange of benzylidene ligand for tert-butyl (2-vinylphenyl)carbamates bearing benzylidene ligand substituted in the position para to carbamato functionality with methyl or trifluoromethyl group. In all metathetical transformations tested, i.e. in ROMP of cycloocta-1,5-diene, RCM of diethyl diallylmalonate and diethyl 2-allyl-2-(2-methylallyl)malonate and cross-metathesis of allylbenzene with Z-1,4-diacetoxybut-2-ene, the complexes behave like latent catalysts. Complexes remain completely inactive until they are activated by the addition of ethereal solution of HCl. The presence of the electron-withdrawing group results in a slight increase in the catalytic activity of the activated form of the catalyst relative to a similar form of the unsubstituted complex or the least active complex carrying the electron donating group.

Ruthenium Olefin Metathesis Catalysts Featuring a Labile Carbodicarbene Ligand

Ruthenium benzylidene complexes containing a carbodicarbene (CDC) ligand are reported. Mechanistic studies indicate that the CDC ligand can dissociate under relatively mild conditions to afford active olefin metathesis catalysts. These catalysts were found to be effective at ring-closing metathesis (RCM) and ring-opening metathesis polymerization (ROMP) reactions.

Synthesis of molybdenum-incorporated mesoporous silicates by evaporation-induced self-assembly: Insights into surface oxide species and corresponding olefin metathesis activity

Molybdenum-incorporated mesoporous silicates with different Si/Mo ratios were synthesized via the Evaporation Induced Self-Assembly (EISA) method. SAXS and nitrogen physisorption results confirm the ordered mesoporosity. The BET surface area, pore volume and pore size distribution generally decrease with Mo loading while preserving the mesoporosity. Highly dispersed monomeric and oligomeric MoOx species (Mo6+ oxidation state) were evident from diffuse reflectance UV–Vis and XPS spectra. H2-TPR results confirm the existence of polymeric MoOx and bulk crystalline MoO3 species. At lower loadings, Mo exits in both tetrahedral and octahedral symmetry as established by Mo L3-edge XANES studies. Mo-EISA catalysts with highly dispersed isolated MoOx sites display stable activity for 2-butene + ethylene metathesis that is comparable to those reported for WO3/SiO2 catalysts.