Organometallic Catalysts Research Articles

Transition metal catalyzed glycosylation reactions - an overview.

Carbohydrates are a large class of natural products that play key roles in a number of biological processes such as in cellular communication or disease progression. Carbohydrates are also used as vaccines and pharmaceuticals. Their synthesis through glycosylation reactions is challenging, and often stoichiometric amounts of promoters are required. Transition metal catalyzed glycosylation reactions are far less common, but can have advantages with respect to reaction conditions and selectivity. The review intends to approach the topic from the catalysis and carbohydrate perspective to encourage researchers from both the fields to perform research in the area. The article covers the basics in glycosylation and catalysis chemistry. The catalysts for the reaction can be roughly divided into two groups. In one group, the catalysts serve as Lewis acids. In the other group, the catalysts play a higher sophisticated role, are involved in all elementary steps of the mechanism and remain coordinated to the substrate throughout the whole catalytic cycle. Based on selected examples, the main trends in transition metal catalyzed glycosylation reactions are explained. Lewis acid catalysts tend to require a somewhat higher catalyst load compared to other organometallic catalysts. The reaction conditions such as the temperature and time depend in many cases on the leaving group employed. An outlook is also presented. The article is not meant to be comprehensive; it outlines the most common transition metal catalyzed processes with the intention to bring the catalysis and carbohydrate communities together and to inspire research activities in both areas.

Cooperation of cerium oxide nanoparticles and soluble molecular catalysts for alcohol oxidation

Nano-cerium oxide and organometallic catalysts cooperate in anaerobic and aerobic alcohol oxidations.

Особенности процесса гидрирования п-нитросалициловой кислоты в присутствии органометаллических катализаторов

Особенности процесса гидрирования п-нитросалициловой кислоты в присутствии органометаллических катализаторов

One-pot Synthesis of Unsymmetrical Diaryliodonium Tetrafluoroborate Salts Bearing an Isoxazole Moiety from Aryl Boronic Acids

Hypervalent iodine compounds exhibit attractive features of low cost, mild and selective reagents in organic synthesis. These reagents serve as environmentally benign alternatives to toxic heavy-metal based oxidants and expensive organometallic catalysts. The practical and simple synthesis of unsymmetrical diaryl iodonium tetrafluoroborate salts is described. This synthetic method has allowed the production of isoxazole tetrafluoroborate salts from readily available aryl boronic acids without an extra anion exchange step in acceptable yields of 45% and 50%.

Organocatalytic ring-opening polymerization of disulfide functional macrocyclic carbonates: An alternative strategy to enzymatic catalysis

Ring-opening polymerization (ROP) of macrocyclic carbonates (≥12 ring) is still challenging due to the associated low ring-strain. Although organometallic and enzyme based catalysts have been studied, disadvantages such as organometallic residue and enzymic nature limit their applications. After screening commercially available organocatalysts, we report the use of organocatalysts for ROP of disulfide-containing macrocyclic carbonates (MSS, 16 ring). It was found that organocatalysts (TBD) presented high active and living ROP of MSS, as evidenced by kinetic studies, yielding main chain disulfide-containing polycarbonates with tailor-made structures and predictable molecular weights with low molecular weight distribution. Copolymerizations with trimethylene carbonate (TMC) generated random copolymers with controlled components, regulating the density of disulfide functional groups. By comparing with the behaviors of enzyme catalysis in kinetic studies and (co-)polymerization, it is observed that organic catalyzed ROP showed more efficient (~ ten times faster), milder condition and more controlled behaviors than enzyme catalyzed ROP (N-435). Therefore, we believe this organic catalyzed strategy will provide an alternative to the current enzymatic and organometallic catalyst for ROP of macrocyclic carbonates.

Ethylene polymerization over novel organic magnesium based V/Ti bimetallic Ziegler-Natta Catalysts

A series of novel V/Ti bimetallic polyethylene catalysts were developed successfully through the introduction of vanadium species into the (SiO2/MgR2/MgCl2)·TiClx Ziegler-Natta Catalyst. The organometallic heterogeneous catalysts combined advantages of reducibility of magnesium alkyl on the activation of secondary metal active sites (vanadium active sites), which successfully enhanced average molecular weight in homopolymer products and produced dominantly bimodal molecular weight distribution in copolymer products. Catalyst preparation conditions including calcination temperature and magnesium dosage were investigated to explicit the effect on the relative performance of different metal active centers. Moreover, the factors including hydrogen and ethylene/1-hexene copolymerization were systematically studied. The bimetallic catalysts exhibit 63.9% enhancement on activity, 37.3% better hydrogen response than the original (SiO2/MgR2/MgCl2)·TiClx organic magnesium Ziegler-Natta catalyst. Moreover, a simplified schematic model was proposed to elaborate on the reason why the bimetallic catalysts showed much higher activity than the original one, and each active species performance well. Vanadium active sites react quickly in the initial reaction time on the external surface of the support, which helps titanium active sites crack the catalyst particles together in the fragmentation and activation stage; then the released internal surface active sites, which were accessible to the monomer in the slurry phase, speed up the polymerization reaction during the latter reaction process.

Rapid Degradation of Poly(lactic acid) with Organometallic Catalysts.

Poly(lactic acid) (PLA) is an effective sacrificial material for the creation of vascular networks in thermoset polymers and composites. The high thermal stability of PLA limits its applications as an embedded sacrificial template in high-temperature-resistant thermoset matrices. Here, we demonstrate faster and more efficient PLA degradation at temperatures lower than previously reported using two organometallic catalysts: tin(II) oxalate (Sn(Oxa)) and tin(II) acetate (Sn(Ac)2). We process Sn(Oxa) by two separate methods to obtain a significant difference in the specific surface area (SSA) of the catalyst particles and compare PLA degradation performance in a thermogravimetric analysis (TGA) instrument. Changing the SSA of Sn(Oxa) by a factor of ∼20 reduces the PLA degradation onset temperature by 37 °C. The total degradation time of PLA films also decreases after blending with Sn(Oxa) having a higher SSA. We also find Sn(Ac)2 lowers the degradation onset of PLA by 53 °C compared to Sn(Oxa) with a similar SSA. In addition, Sn(Ac)2 decreases the time for complete degradation of PLA films by an order of magnitude compared to Sn(Oxa) at 200 °C. Films with a significantly lower Sn(Ac)2 concentration compared to Sn(Oxa) degrade much faster at lower temperatures up to 160 °C. Finally, PLA films with different loadings of Sn(Ac)2 are embedded in an epoxy thermoset matrix and subsequently vascularized at elevated temperatures in a vacuum oven. Microchannel formation is observed at 170 °C using Sn(Ac)2, reducing the temperature required for vaporization of embedded sacrificial polymer compared to Sn(Oxa) catalyst. Sn(Ac)2 can potentially reduce the energy, time, and amount of catalyst required for degrading PLA into volatile products for sacrificial applications.

Different Strategies for Designing Dual-Catalytic Enantioselective Processes: From Fully Cooperative to Non-cooperative Systems.

An emerging area of research in chemistry requires that we learn how to manage the characteristics of a pair of co-catalysts so that a transformation proceeds as we wish it to. These are processes during which one catalyst first generates a non-isolable intermediate, which then in situ undergoes a reaction that is promoted by a different catalyst. This scenario raises several design issues. Since co-catalysts often have overlapping functions, what if there is an exchange of ligands between two organometallic catalysts? How can we be certain that a co-catalyst reacts specifically with a particular intermediate? What if the less reactive co-catalyst must engage first, and the one that is more active needs to wait its turn? How might we orchestrate the proper sequence of events? While many dual-catalytic processes have been introduced and reviews are available, there are subtle yet crucial distinguishing attributes that remain unappreciated. While the terms "dual-catalysis" and "cooperative catalysis" are often used interchangeably, on many occasions the catalysts are not entirely cooperative. Here, we will discuss how chemists have been able to harmonize the opposing functions of catalysts to achieve high efficiency and/or stereoselectivity. We will show that the progress achieved thus far is likely the preamble to the future development of non-orthogonal multi-catalytic processes (i.e., transformations involving several catalysts that are not inherently cooperative) where the order with which each catalyst enters the fray will demand additional innovative strategies.

Single-atom gold oxo-clusters prepared in alkaline solutions catalyse the heterogeneous methanol self-coupling reactions.

In an effort to obtain the maximum atom efficiency, research on heterogeneous single-atom catalysts has intensified recently. Anchoring organometallic homogeneous catalysts to surfaces creates issues with retaining mononuclearity and activity, while the several techniques developed to prepare atomically dispersed precious metals on oxide supports are usually complex. Here we report a facile one-pot synthesis of inorganometallic mononuclear gold complexes formed in alkaline solutions as robust and versatile single-atom gold catalysts. The complexes remain intact on impregnation onto supports or after drying in air to give a crystalline powder. They can be used to interrogate the nuclearity of the catalytically active gold site for reactions known to be catalysed by oxidized gold species. We show that the [Au1-Ox]- cluster directs the heterogeneous coupling of two methanol molecules to methyl formate and hydrogen with a 100% selectivity below 180 °C. The reaction is industrially important as well as the key step in methanol steam reforming on gold catalysts.

Stereoselective Cyclopropanation of Electron-Deficient Olefins with a Cofactor Redesigned Carbene Transferase Featuring Radical Reactivity.

Engineered myoglobins and other hemoproteins have recently emerged as promising catalysts for asymmetric olefin cyclopropanation reactions via carbene transfer chemistry. Despite this progress, the transformation of electron-poor alkenes has proven very challenging using these systems. Here, we describe the design of a myoglobin-based carbene transferase incorporating a non-native iron-porphyrin cofactor and axial ligand, as an efficient catalyst for the asymmetric cyclopropanation of electron-deficient alkenes. Using this metalloenzyme, a broad range of both electron-rich and electron-deficient alkenes are cyclopropanated with high efficiency and high diastereo- and enantioselectivity (up to >99% de and ee). Mechanistic studies revealed that the expanded reaction scope of this carbene transferase is dependent upon the acquisition of metallocarbene radical reactivity as a result of the reconfigured coordination environment around the metal center. The radical-based reactivity of this system diverges from the electrophilic reactivity of myoglobin and most of known organometallic carbene transfer catalysts. This work showcases the value of cofactor redesign toward tuning and expanding the reactivity of metalloproteins in abiological reactions and it provides a biocatalytic solution to the asymmetric cyclopropanation of electrodeficient alkenes. The metallocarbene radical reactivity exhibited by this biocatalyst is anticipated to prove useful in the context of a variety of other synthetic transformations.

Cytotoxicity of Catalysed Silicone Resin Coatings for Smart Biomedical Devices

Abstract Equipping medical devices with smart technologies holds great potential for the development of modern medical products. The development requires the identification of new integration strategies and the research of new material combinations due to the miniaturization of systems and increasing production figures. The realization of Smart Biomedical Devices requires a sufficient barrier effect (bioprotection) by appropriate encapsulation of the electronic components. Thinnest polymer coatings have proven to be suitable for conformal encapsulation. The aim of the study was to investigate the fundamental suitability of thin-film lacquers added with catalysts as coating materials for electronic systems with regard to their biological use. Due to long curing times of up to 14 days, eight different catalysts based on different chemical structures were added to the coating materials and their influence on a cytotoxic effect was investigated. A non-cytotoxic effect was observed for the organometallic catalysts based on tin, zirconium, titanium, bismuth, and tertiary amine. Most were resistant to steam sterilization. The curing time of the non-cytotoxic coatings could be significantly reduced by the addition of catalysts. The shortening of process times is an important economic aspect in the production of mass-produced Smart Biomedical Devices.

Synthesis, and molecular modeling of bis(3-(piperazine-1-yl)propyl)tungstate (BPPT) nanoparticles, and its first catalytic application for one-pot synthesis of 4H-chromene derivatives

A novel, nano-sized, bis(3-(piperazine-1-yl)propyl)tungstate (BPPT) is introduced as an efficient and reusable organometallic catalyst which is considered as a heterogeneous Bronsted-Lowry base and applied successfully for one-pot synthesis of methyl 2-amino-4-aryl substituted-4H-chromene derivatives with good to excellent yields. BPPT has been prepared via a two-step route from natrium tungstate salt. At first, the oxygens of Na2WO4 react with 1-bromo-3-chloropropane via nucleophilic substitution to produce bis(3-choloro propyl)tungstate. Then nucleophilic substitution of piperazine with chlorines produced bis(3-(piperazine-1-yl)propyl) tungstate. Bis(3-(piperazine-1-yl)propyl) tungstate, which was called BPPT, characterized by fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), thermal gravimetric analysis (TGA), transmission electron microscopy (TEM) and scanning electron microscope (SEM). The catalyst is heterogeneous, green and recyclable. It is a thermally stable and its handling is easy. Its catalytic activity is very high and leads to the production of 4H-pyran derivatives with good to excellent yields in short reaction times. Furthermore, molecular modeling studies and ADMETox prediction revealed that not only it can inhibit acetylcholinesterase enzyme and act as an anti-Alzheimer agent but also has no variation from Lipinski's rule of five and can be a good candidate as anti-Alzheimer agents. These above-mentioned facts can be countered as advantages of the current protocol.

Reactivity and selectivity descriptors of dioxygen activation routes on metal oxides

The activation of dioxygen at typically isolated two-electron reduced centers can lead to the formation of electrophilic superoxo or peroxo species, providing an essential route to form reactive O2-derived species in biological, organometallic, and heterogeneous catalysts. Alternatively, O2 activation can proceed via outer sphere routes, circumventing the formation of bound peroxo (OO*) species during oxidation catalysis by forming H2O2(g), which can react with another reduced center to form H2O. The electronic and binding properties of metal oxides that determine the relative rates of these activation routes are assessed here by systematic theoretical treatments using density functional theory (DFT). These methods are combined with conceptual frameworks based on thermochemical cycles and crossing potential models to assess the most appropriate descriptors for the activation barriers for each route using Keggin polyoxometalates as illustrative examples. In doing so, we show that inner sphere routes, which form OO* species via O2 activation on the O-vacancies (*) formed in the reduction part of redox cycles, are mediated by early transition states that only weakly sense the oxide binding properties. Outer sphere routes form H2O2(g) via O2 activation on OH pairs (H/OH*) formed by dissociation of H2O on O-vacancies; their rates and activation barriers reflect the rates of the first H-atom transfer from H/OH* to O2. The activation barriers for this H-transfer step depend on the binding energy of more weakly-bound H-atom in H/OH* pairs (HAE2) and on the OOH-surface interaction energy at its product state (Eint0). The Eint0 values are similar among oxides unless a large charge-balancing cation is present and interacts with OOH; consequently, HAE2 acts as an appropriate descriptor of the outer sphere dynamics. HAE2 also determines the thermodynamics of H2O dissociation on O-vacancies, which influence the inner and outer sphere rates by setting the relative coverage of * and H/OH*. These results, in turn, show that HAE2 is a complete descriptor of the reactivity and selectivity of oxides for O2 activation; the O-atoms in more reducible oxides (more negative HAE2) exhibit a greater preference for the inner sphere routes and for the formation of electrophilic OO* intermediates that mediate epoxidation and O-insertion reactions during catalytic redox cycles. Large charge-balancing cations locally modify Eint0 values that determine the outer sphere rates and thus can be used to alter the preference of O-atoms to either inner or outer sphere routes.

In Situ Decoration of Gold Nanoparticles on Graphene Oxide via Nanosecond Laser Ablation for Remarkable Chemical Sensing and Catalysis.

Gold decorated graphene-based nano-hybrids find extensive research interest due to their enhanced chemical catalytic performance and biochemical sensing. The unique physicochemical properties and the very large surface area makes them propitious platform for the rapid buildouts of science and technology. Graphene serves as an outstanding matrix for anchoring numerous nanomaterials because of its atomically thin 2D morphological features. Herein, we have designed a metal-graphene nano-hybrid through pulsed laser ablation. Commercially available graphite powder was employed for the preparation of graphene oxide (GO) using modified Hummers’ method. A solid, thin gold (Au) foil was ablated in an aqueous suspension of GO using second harmonic wavelength (532 nm) of the Nd:YAG laser for immediate generation of the Au-GO nano-hybrid. The synthesis strategy employed here does not entail any detrimental chemical reagents and hence avoids the inclusion of reagent byproducts to the reaction mixture, toxicity, and environmental or chemical contamination. Optical and morphological characterizations were performed to substantiate the successful anchoring of Au nanoparticles (Au NPs) on the GO sheets. Remarkably, these photon-generated nano-hybrids can act as an excellent surface enhanced Raman spectroscopy (SERS) platform for the sensing/detection of the 4-mercaptobenzoic acid (4-MBA) with a very low detection limit of 1 × 10−12 M and preserves better reproducibility also. In addition, these hybrid materials were found to act as an effective catalyst for the reduction of 4-nitrophenol (4-NP). Thus, this is a rapid, mild, efficient and green synthesis approach for the fabrication of active organometallic sensors and catalysts.

Copper Schiff base functionalized polyaniline (Cu‐SB/PANI): A highly efficient polymer based organometallic catalyst for the synthesis of 2‐amino chromene derivatives

Copper Schiff Base functionalized Polyaniline (Cu‐SB/PANI) has been synthesized as an efficient, recyclable and heterogeneous polymer based organometallic catalyst by simple method. The catalyst has been well characterized with different spectroscopic techniques such as FTIR, SEM/EDX, elemental mapping, XRD, TEM, TG, XPS, EPR and ICP‐AES analyses. The catalytic potential of the catalyst has been explored by synthesizing 2‐amino chromene derivatives. The catalyst efficiently catalyzed the reaction affording excellent yield of the products (95–97%) in very short reaction time period (6–8 min). The catalyst could be reused for five times with insignificant loss in catalytic activity.

Simultaneous quantitative structure‐activity relationship analysis of catalyst activity and selectivity in the direct oxidation of C―H bonds

AbstractThe development of green, efficient, and stable organometallic catalysts for the selective direct oxidation of C―H bonds under mild reaction conditions represents a long‐standing challenge for chemists. Can the catalytic activity and selectivity be improved simultaneously through modification of the catalyst structure? To address this question, three one‐pot C―H oxidation reactions catalyzed by a series of structurally diverse metalloporphyrins were studied using mechanistically relevant molecular parameters derived mainly from density functional theory calculations. Particle swarm optimization (PSO) algorithm was performed to search for the best descriptor subsets to obtain simple quantitative structure‐activity relationship models with high correlations. The reaction outcomes (transformation rates and selectivity) of the three reactions were modeled with six multivariate linear regression equations using a pool of 17 parameters representing charges, steric effects, and other parameters. These studies cast new light on the reaction mechanisms and the relationships among the molecular properties that influence the catalytic activity and selectivity of these species and guide the further development of more efficient and selective catalysts through careful design of the molecular structure.

Zn(L-proline)2: An efficient and reusable organocatalyst for the synthesis of polyfunctionally substituted pyrans and 2-amino-4-aryl-8-oxo-4,8-dihydropyrano[3,2-b]pyran derivatives

Efficient synthesis of non-annulated 2-amino-4H-pyrans and 2-amino-8-oxo-4,8-dihydropyrano[3,2-b]pyran derivatives, which are biologically relevant heterocycles is achieved, utilizing a domino three-component reaction of ethyl acetoacetate or kojic acid with aromatic aldehydes and malononitrile catalyzed by Zn(L-proline)2 as reusable organometallic catalyst. The process exhibits high atom economy, short reaction time, simple work up, high yields and environmentally friendly nature. Excellent yields of the targeted molecules have been obtained.

Atomically Dispersed Rh Active Sites on Oxide Supports with Controlled Acidity for Gas-Phase Halide-Free Methanol Carbonylation to Acetic Acid

Acetic acid (AA) is an important bulk commodity chemical produced primarily via methanol carbonylation using homogeneous organometallic catalysts in liquid phase reactors with halides that participate in each catalytic turnover. Here we report a heterogeneous, halide-free, gas-phase process for methanol carbonylation to AA using a catalyst consisting of atomically dispersed rhodium (Rh) active sites on an acidic support. It is demonstrated that active site pairs consisting of atomically dispersed Rh and support acid sites enable highly selective AA production, whereas Rh clusters drive methanol decomposition to CO and CO2. Through a comparison of methanol carbonylation over atomically dispersed Rh species on γ-Al2O3, ZrO2, and Na-modified ZrO2, it was identified that by decreasing the concentration of acidic sites, the production of byproduct dimethyl ether could be minimized. After the acidity of the support was tuned by depositing 5 wt % Na on ZrO2 that contained atomically dispersed Rh, AA selectivity ...

Green, cost‐effective and efficient procedure for Heck and Sonogashira coupling reactions using palladium nanoparticles supported on functionalized Fe3O4@SiO2 by polyvinyl alcohol as a highly active, durable and reusable catalyst

A novel heterogenized organometallic catalyst was synthesized by coordinating palladium with polyvinyl alcohol‐functionalized Fe3O4@SiO2 nanospheres. This novel catalyst was characterized using Fourier transform infrared spectroscopy, X‐ray diffraction, transmission electron microscope, field emission scanning electron microscope, dynamic light scattering, UV–vis spectroscopy, X‐ray photoelectron spectroscopy, energy dispersive X‐ray analysis, thermogravimetric analysis and inductively coupled plasma analysis. The prepared palladium nanoparticles supported on polyvinyl alcohol functionalized Fe3O4@SiO2 nanoparticles were successfully applied as a magnetically recyclable catalyst in Heck and Sonogashira coupling reactions in water. They showed remarkable activity toward aryl halides (I, Br, Cl) using very low palladium loading in excellent yields and demonstrated high TONs (mmol of product per mmol of catalyst). Also, the catalyst could be magnetically separated and reused seven times without any appreciable loss of catalytic activity.

Macroporous scaffolds of cross-linked Poly(ɛ-caprolactone) via high internal phase emulsion templating

A high internal phase emulsion based ring-opening polymerization (HIPE-ROP) of ɛ-caprolactone (CL) was conducted to fabricate macroporous scaffold in a single-step. An oil-in-oil HIPE of CL was formed using high molecular weight olefin (C10-C16) dispersed in continuous medium comprised of CL, a bifunctional cross-linking monomer bis-ɛ-caprolactone-4-yl (BCY) and catalyst stannous octoate (Sn(Oct)2). The emulsion was stabilized using nonionic surfactant, pluronic F127 and polymerization was conducted at 120 °C without any inert atmosphere for a period of 8 h. Complete monomer conversion and cross-linking was achieved and am acroporous scaffold of desired shape was obtained, having in-situ generated porosity and interconnected pores. All chemicals were purposely used without any purification and the high temperature process prevented deactivation of catalyst. The HIPE-ROP developed in this way was found to be highly feasible for conducting otherwise moisture sensitive organo-metallic catalyst based ROP. The morphology of HIPE was directly transferred to macroporous structure of the scaffold in this process where pore size was reproducibly controlled by variation in volume fraction of dispersed phase and cross-link density. The resultant macroporous scaffold showed superior compressive loading and swelling capacity. The porous scaffolds also exhibited excellent osteoblast (MG63) attachment and proliferation. HIPE-ROP based macroporous scaffold of cross-linked PCL thus developed paved a new route to create porous scaffolds of aliphatic polyesters in a single step. The method also reflected high commercial viability by potential conversion into a continuous process.