Linker‐Free,Silica‐Bound Olefin‐Metathesis Catalysts: Applications in Heterogeneous Catalysis

A set of heterogenized olefin‐metathesis catalysts, which consisted of Ru complexes with the H₂ITap ligand (1,3‐bis(2′,6′‐dimethyl‐4′dimethyl aminophenyl)‐4,5‐dihydroimidazol‐2‐ylidene) that had been adsorbed onto a silica support, has been prepared. These complexes showed strong binding to the solid support without the need for tethering groups on the complex or functionalized silica. The catalysts were tested in the ring‐opening–ring‐closing‐metathesis (RO‐RCM) of cyclooctene (COE) and the self‐metathesis of methyl oleate under continuous‐flow conditions. The best complexes showed a TON>4000, which surpasses the previously reported materials that were either based on the Grubbs–Hoveyda II complex on silica or on the classical heterogeneous Re₂O₇/B₂O₃ catalyst.     

Electrogenerated Chemiluminescence of Tris(2,2′‐bipyridyl)ruthenium(II) Immobilized in Humic Acid‐Silica‐Poly(vinyl alcohol) Composite Films

In this paper, we report the first attempt to use humic acid (HA) as modifiers to prepare the organic‐inorganic hybrid modified glassy carbon electrodes based on HA‐silica‐PVA (poly(vinyl alcohol)) sol‐gel composite. Electroactive species of tris(2,2′‐bipyridyl)ruthenium(II) (Ru(bpy) ) can easily incorporate into the HA‐silica‐PVA films to form Ru(bpy) modified electrodes. The amount of Ru(bpy) incorporated in the composite films strongly depends on the amount of HA in the hybrid sol. Electrochemical and electrogenerated chemiluminescence (ECL) of Ru(bpy) immobilized in HA‐silica composite films coated on a glassy carbon electrode have been studied with tripropylamine (TPA) as the coreactant. The analytical performance of this modified electrode was evaluated in a flow injection analysis (FIA) system with a homemade flow cell. The as‐prepared electrode showed good stability and high sensitivity. The detection limits (S/N=3) were 0.050 μmol L^?1 for TPA and 0.20 μmol L^?1 for oxalate, and the linear ranges were from 0.10 μmol L^?1 to 1.0 mmol L^?1 for TPA and from 1.0 μmol L^?1 to 1.0 mmol L^?1 for oxalate, respectively. The resulting electrodes were stable over two months.     

ESIMS Studies and Calculations on Alkali‐Metal Adduct Ions of Ruthenium Olefin Metathesis Catalysts and Their Catalytic Activity in Metathesis Reactions

Electrospray ionization mass spectrometry (ESIMS) and subsequent tandem mass spectrometry (MS/MS) analyses were used to study some important metathesis reactions with the first‐generation ruthenium catalyst 1 , focusing on the ruthenium complex intermediates in the catalytic cycle. In situ cationization with alkali cations (Li⁺, Na⁺, K⁺, and Cs⁺) using a microreactor coupled directly to the ESI ion source allowed mass spectrometric detection and characterization of the ruthenium species present in solution and particularly the catalytically active monophosphine–ruthenium intermediates present in equilibrium with the respective bisphosphine–ruthenium species in solution. Moreover, the intrinsic catalytic activity of the cationized monophosphine–ruthenium complex 1 a ?K⁺ was directly demonstrated by gas‐phase reactions with 1‐butene or ethene to give the propylidene Ru species 3 a ?K⁺ and the methylidene Ru species 4 a ?K⁺, respectively. Ring‐closing metathesis (RCM) reactions of 1,6‐heptadiene ( 5 ), 1,7‐octadiene ( 6 ) and 1,8‐nonadiene ( 7 ) were studied in the presence of KCl and the ruthenium alkylidene intermediates 8 , 9 , and 10 , respectively, were detected as cationized monophosphine and bisphosphine ruthenium complexes. Acyclic diene metathesis (ADMET) polymerization of 1,9‐decadiene ( 14 ) and ring‐opening metathesis polymerization (ROMP) of cyclooctene ( 18 ) were studied analogously, and the expected ruthenium alkylidene intermediates were directly intercepted from reaction solution and characterized unambiguously by their isotopic patterns and ESIMS/MS. ADMET polymerization was not observed for 1,5‐hexadiene ( 22 ), but the formation of the intramolecularly stabilized monophosphine ruthenium complex 23 a was seen. The ratio of the signal intensities of the respective with potassium cationized monophosphine and bisphosphine alkylidene Ru species varied from [I _4a ]/[I ₄ ]=0.02 to [I _23a ]/[I ₂₃ ]=10.2 and proved to be a sensitive and quantitative probe for intramolecular π‐complex formation of the monophosphine–ruthenium species and of double bonds in the alkylidene chain. MS/MS spectra revealed the intrinsic metathesis catalytic activity of the potassium adduct ions of the ruthenium alkylidene intermediates 8 a , 9 a , 10 a , 15 a , and 19 a , but not 23 a by elimination of the respective cycloalkene in the second step of RCM. Computations were performed to provide information about the structures of the alkali metal adduct ions of catalyst 1 and the influence of the alkali metal ions on the energy profile in the catalytic cycle of the metathesis reaction.     

Bottom‐Up Design of a Copper–Ruthenium Nanoparticulate Catalyst for Low‐Temperature Ammonia Oxidation

A novel nanoparticulate catalyst of copper (Cu) and ruthenium (Ru) was designed for low‐temperature ammonia oxidation at near‐stoichiometric mixtures using a bottom‐up approach. A synergistic effect of the two metals was found. An optimum CuRu catalyst presents a reaction rate threefold higher than that for Ru and forty‐fold higher than that for Cu. X‐ray absorption spectroscopy suggests that in the most active catalyst Cu forms one or two monolayer thick patches on Ru and the catalysts are less active once 3D Cu islands form. The good performance of the tuned Cu/Ru catalyst is attributed to changes in the electronic structure, and thus the altered adsorption properties of the surface Cu sites.     

Alkyne Metathesis with Silica‐Supported and Molecular Catalysts at Parts‐per‐Million Loadings

Improvement of the activity, stability, and chemoselectivity of alkyne‐metathesis catalysts is necessary before this promising methodology can become a routine method to construct C≡C triple bonds. Herein, we show that grafting of the known molecular catalyst [MesC≡Mo(OtBu_F6)₃] ( 1 , Mes=2,4,6‐trimethylphenyl, OtBu_F6=hexafluoro‐tert‐butoxy) onto partially dehydroxylated silica gave a well‐defined silica‐supported active alkyne‐metathesis catalyst [(≡SiO)Mo(≡CMes)(OtBu_F6)₂] ( 1 /SiO_2‐700). Both 1 and 1 /SiO_2‐700 showed very high activity, selectivity, and stability in the self‐metathesis of a variety of carefully purified alkynes, even at parts‐per‐million catalyst loadings. Remarkably, the lower turnover frequencies observed for 1 /SiO_2‐700 by comparison to 1 do not prevent the achievement of high turnover numbers. We attribute the lower reactivity of 1 /SiO_2‐700 to the rigidity of the surface Mo species owing to the strong interaction of the metal site with the silica surface.     

Oxygen‐chelated indenylidene ruthenium catalysts for olefin metathesis

Olefin metathesis is a transition metal‐mediated transformation that rearranges the carbon atoms of the carbon–carbon double bond of olefins. This reaction has become one of the most important and powerful reactions. Therefore development of new, well‐defined, highly active and selective catalysts is very desirable and a valuable goal. This mini‐review mainly introduces the development of ruthenium catalysts in olefin metathesis highlighting oxygen‐chelated indenylidene ruthenium catalysts. Applying an alkoxyl group on the indenylidene ligand fragment can generate the Ru ? O chelating bond. Additionally, various modifications of the ligand as well as the catalytic activity for ring‐closing metathesis reaction and selectivity of cross metathesis reaction are overviewed. Finally, the perspectives on the development of new catalysts are summarized. Copyright © 2015 John Wiley & Sons, Ltd.     

Comparing Grubbs‐, Werner‐, and Hofmann‐Type (Carbene)ruthenium Complexes: The Key Role of Pre‐Equilibria for Olefin Metathesis

A gas‐phase comparison of intrinsic olefin metathesis rates for (carbene)ruthenium complexes by means of electrospray‐ionization tandem mass spectrometry reveals a reversal of the reactivity trends observed in solution. The solution‐phase ordering of reactivity is accordingly attributed to a more favorable pre‐equilibrium, producing the metathesis‐active species in the case of the Hofmann‐ and Werner‐type complexes relative to those of the Grubbs type.     

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.     

Ruthenium‐Catalyzed Transfer‐Hydrogenative Cyclization of 1,6‐Diynes with Hantzsch 1,4‐Dihydropyridine as a H2 Surrogate

The transfer‐hydrogenative cyclization of 1,6‐diynes with Hantzsch 1,4‐dihydropyridine as a H₂ surrogate was performed in the presence of a cationic ruthenium catalyst of the type [Cp′Ru(MeCN)₃PF₆]. Exocyclic 1,3‐dienes or their 1,4‐hydrogenation products, cycloalkenes, were selectively obtained, depending on the substrate structure and the reaction conditions.     

A phase‐separable second‐generation hoveyda‐grubbs catalyst for ring‐opening metathesis polymerization

Polyisobutylene‐supported second‐generation Hoveyda‐Grubbs catalyst is shown to be an effective nonpolar phase tag for ring‐opening metathesis polymerization (ROMP). The catalytic activities of the supported Ru–carbene complex in ROMP are comparable to those of their homogeneous counterparts. The separability of these catalysts leads to lower Ru contamination (0.5 ppm levels) in the polymer products in comparison to the nonsupported Hoveyda‐Grubbs catalyst (10 PPM). © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Ruthenium Nanoparticles on Nano‐Level‐Controlled Carbon Supports as Highly Effective Catalysts for Arene Hydrogenation

The reaction of three types of carbon nanofibers (CNFs; platelet: CNF‐P, tubular: CNF‐T, herringbone: CNF‐H) with [Ru₃(CO)₁₂] in toluene heated at reflux provided the corresponding CNF‐supported ruthenium nanoparticles, Ru/CNFs (Ru content=1.1–3.8 wt %). TEM studies of these Ru/CNFs revealed that size‐controlled Ru nanoparticles (2–4 nm) exist on the CNFs, and that their location was dependent on the surface nanostructures of the CNFs: on the edge of the graphite layers (CNF‐P), in the tubes and on the surface (CNF‐T), and between the layers and on the edge (CNF‐H). Among these Ru/CNFs, Ru/CNF‐P showed excellent catalytic activity towards hydrogenation of toluene with high reproducibility; the reaction proceeded without leaching of the Ru species, and the catalyst was reusable. The total turnover number of the five recycling experiments for toluene hydrogenation reached over 180 000 (mol toluene) (mol Ru)^?1. Ru/CNF‐P was also effective for the hydrogenation of functionalized benzene derivatives and pyridine. Hydrogenolysis of benzylic C? O and C? N bonds has not yet been observed. Use of poly(ethylene glycol)s (PEGs) as a solvent made possible the biphasic catalytic hydrogenation of toluene. After the reaction, the methylcyclohexane formed was separated by decantation without contamination of the ruthenium species and PEG. The insoluble PEG phase containing all of the Ru/CNF was recoverable and reusable as the catalyst without loss of activity.     

Ruthenium Carbenes Supported on Mesoporous Silicas as Highly Active and Selective Hybrid Catalysts for Olefin Metathesis Reactions under Continuous Flow

In the search for a highly active and selective heterogenized metathesis catalyst, we systematically varied the pore geometry and size of various silica‐based mesoporous (i.e., MCM‐41, MCM‐48, and SBA‐15) and microporous (ZSM‐5 and MWW) versus macroporous materials (D11‐10 and Aerosil 200), besides other process parameters (temperature, dilution, and mean residence time). The activity and, especially, selectivity of such “linker‐free” supports for ruthenium metathesis catalysts were evaluated in the cyclodimerization of cis‐cyclooctene to form 1,9‐cyclohexadecadiene, a valuable intermediate in the flavor and fragrance industry. The optimized material showed not only exceptionally high selectivity to the valuable product, but also turned out to be a truly heterogeneous catalyst with superior activity relative to the unsupported homogeneous complex.     

Silica‐Supported Molybdenum Oxo Alkylidenes: Bridging the Gap between Internal and Terminal Olefin Metathesis

Grafting a molybdenum oxo alkylidene on silica (partially dehydroxylated at 700 °C) affords the first example of a well‐defined silica‐supported Mo oxo alkylidene, which is an analogue of the putative active sites in heterogeneous Mo‐based metathesis catalysts. In contrast to its tungsten analogue, which shows poor activity towards terminal olefins because of the formation of a stable off‐cycle metallacyclobutane intermediate, the Mo catalyst shows high metathesis activity for both terminal and internal olefins that is consistent with the lower stability of Mo metallacyclobutane intermediates. This Mo oxo metathesis catalyst also outperforms its corresponding neutral silica‐supported Mo and W imido analogues.     

Comparing Intrinsic Reactivities of the First‐ and Second‐Generation Ruthenium Metathesis Catalysts in the Gas Phase

An experimental comparison of the gas‐phase reactivity of the 14‐electron reactive intermediates produced by phosphine dissociation from the first‐ and second‐generation ruthenium metathesis catalysts, (L)Cl₂Ru?CHR (L=PCy₃ or NHC), supports Grubbs's contention that the second‐generation catalysts show hundred‐fold higher phenomenological activity despite a slower phosphine dissociation because of a much more‐favorable partitioning of the 14‐electron active species towards product‐forming steps. The gas‐phase study finds, in ring‐opening metathesis of norbornene as well as acyclic metathesis of ethyl vinyl ether, that the first‐generation systems display evidence for a higher barrier above that for phosphine dissociation; the second‐generation systems, on the other hand, behave as if there is no significantly higher barrier.     

Polymer‐Bound,Amphiphilic Hoveyda‐Grubbs‐Type Catalyst for Ring‐Closing Metathesis in Water

Summary: We report on the synthesis of a new amphiphilic, polymer‐bound variant of the Hoveyda‐Grubbs catalyst via the coupling reaction of a carboxylic acid‐functionalized poly(2‐oxazoline) block copolymer with 2‐isopropoxy‐5‐hydroxystyrene and subsequent reaction of the resulting macroligand with a second generation Grubbs catalyst. For the benchmark, the substrate diethyl diallylmalonate was studied in the ring‐closing metathesis (RCM) reaction and a turn‐over number (TON) of up to 390 in water was achieved. To the best of our knowledge, this is the highest value for any aqueous RCM reaction to date. For the first time, recycling of a ruthenium initiator in an aqueous RCM reaction has been successful to some extent. In addition, the micellar conditions accelerate the conversion of the hydrophobic diene and at the same time stabilize the active alkylidene species, although competing decomposition of the catalyst in water still impairs the catalyst performance. Residual ruthenium content was determined to be below 1 ppm in the product suggesting a very low leaching of the polymeric catalyst system.

Simplified chemical structure of the amphiphilic, polymer‐bound Grubbs‐Hoveyda catalyst.     

Sustainable synthesis of sulfonamides using supported ionic liquid phase catalyst containing Keggin‐type anion

A silica‐supported ionic liquid phase catalyst containing Keggin‐type anion has been prepared by covalent grafting of ferrocene‐tagged ionic liquid in a matrix of silica followed by anion metathesis reaction. This novel catalyst served as a robust heterogeneous catalyst for the synthesis of biologically active sulfonamides from 4‐toluenesulfonyl chloride and amines. Additionally, recycling experiments showed that the catalyst could be reused five times. Copyright © 2015 John Wiley & Sons, Ltd.     

A Light‐Activated Olefin Metathesis Catalyst Equipped with a Chromatic Orthogonal Self‐Destruct Function

A sulfur‐chelated photolatent ruthenium olefin metathesis catalyst has been equipped with supersilyl protecting groups on the N‐heterocyclic carbene ligand. The silyl groups function as an irreversible chromatic kill switch, thus decomposing the catalyst when it is irradiated with 254 nm UV light. Therefore, different types of olefin metathesis reactions may be started by irradiation with 350 nm UV light and prevented by irradiation with shorter wavelengths. The possibility to induce and impede catalysis just by using light of different frequencies opens the pathway for stereolithographic applications and novel light‐guided chemical sequences.     

Tandem Z‐Selective Cross‐Metathesis/Dihydroxylation: Synthesis of anti‐1,2‐Diols

A stereoselective synthesis of anti‐1,2‐diols has been developed using a multitasking Ru catalyst in an assisted tandem catalysis protocol. A cyclometalated Ru complex catalyzes first a Z‐selective cross‐metathesis of two terminal olefins, followed by a stereospecific dihydroxylation. Both steps are catalyzed by Ru, as the Ru complex is converted to a dihydroxylation catalyst upon addition of NaIO₄. A variety of olefins were transformed into valuable, highly functionalized, and stereodefined molecules. Mechanistic experiments were performed to probe the nature of the oxidation step and catalyst inhibition pathways. These experiments point the way to more broadly applicable tandem catalytic transformations.     

Transfer hydrogenation of ketones catalysed by half‐sandwich (η6‐p‐cymene) ruthenium(II) complexes incorporating benzoylhydrazone ligands

Neutral half‐sandwich η⁶‐p ‐cymene ruthenium(II) complexes of general formula [Ru(η⁶‐p ‐cymene)Cl(L)] (HL = monobasic O, N bidendate benzoylhydrazone ligand) have been synthesized from the reaction of [Ru(η⁶‐p ‐cymene)(μ‐Cl)Cl]₂ with acetophenone benzoylhydrazone ligands. All the complexes have been characterized using analytical and spectroscopic (Fourier transform infrared, UV–visible, ¹H NMR, ¹³C NMR) techniques. The molecular structures of three of the complexes have been determined using single‐crystal X‐ray diffraction, indicating a pseudo‐octahedral geometry around the ruthenium(II) ion. All the ruthenium(II) arene complexes were explored as catalysts for transfer hydrogenation of a wide range of aromatic, cyclic and aliphatic ketones with 2‐propanol using 0.1 mol% catalyst loading, and conversions of up to 100% were obtained. Further, the influence of other variables on the transfer hydrogenation reaction, such as base, temperature, catalyst loading and substrate scope, was also investigated.     

Highly active, immobilized ruthenium catalysts for oxidation of alcohols to aldehydes and ketones. Preparation and use in both batch and flow systems

A novel, highly active immobilized ruthenium catalyst, which can be successfully used in oxidation of alcohols to aldehydes and ketones, has been developed. In contrast to most immobilized catalysts, the Ru catalyst has activity that is higher than that of the original non-immobilized catalyst. In a batch system, the Ru catalyst was recovered and reused several times without loss of activity. The catalyst was also applied to a flow system, in which excellent conversions and yields were demonstrated. No leaching of Ru was observed in both cases.