Download or read book Novel Olefin Metathesis Catalysts Bearing a Hemi labile NHC Ligand written by Nicolas Cena and published by . This book was released on 2019 with total page 79 pages. Available in PDF, EPUB and Kindle. Book excerpt: The field of olefin metathesis has grown appreciatively in recent decades. Elucidation of the mechanism and a deeper understanding of the key intermediates have enabled chemists to design catalysts, which exhibit greater activity, stability, and selectivity towards a variety of substrates. However, the economic impact of performing this reaction on the industrial scale is often governed by the high cost of the catalysts in comparison to modest turnover numbers (TON). The lifetime of an active catalyst species depends on the stability of intermediates in the catalytic cycle, and this ultimately determines the TON. One of the remaining goals in the field is to further stabilize the intermediates of the catalytic cycle in order to prolong the catalyst lifetimes and increase turnover numbers (TON). The ruthenium catalytic species cycle through several electron deficient intermediates, which lead towards decomposition pathways. These decomposition pathways are directed towards reactions that provide the metal with additional electron density. The primary focus of this thesis project was to develop a new generation of olefin metathesis catalysts employing a tridentate N-heterocyclic carbene (NHC) ligand bearing a hemi labile pyridine arm in the ortho position of the aromatic ring. The rationale behind incorporating these functional groups was to stabilize the reactive intermediates of the catalytic cycle via electron donation from the ether O → Ru and the pyridine N → Ru. These ligands increase the likelihood of stabilization of the metal center by chelation of electron-donating substituents from the O and N, thus adding electron density back into the electron-deficient metal center. The ligand not only donates electron density back to the metal but also shields the sterically open position trans to the NHC. The hypothesis was that stabilizing these reactive intermediates would prolong the lifetime of the active catalyst and thus increase TON and allow for a much lower catalyst loading for industrial applications, thus vastly impacting the economical aspect of olefin metathesis processes. A novel set of two catalysts bearing tridentate NHC ligands with hemi labile pyridine arms were synthesized. The ligands differed in one aromatic ring containing either a 2,6 diisopropyl phenyl (DIPP) or mesityl (Mes) moiety. The result of the x-ray crystallographic analysis revealed the NHC ligand coordinated in the proposed tridentate meridional fashion around the central Ru atom. This coordination was proposed in order to affect a hinge-like mechanism in which the pyridine arm's hemi-labile nature would be in close proximity to the electron deficient metal center, so that it could bind reversibly in order to satiate the metal's desire for electron density while still allowing reactivity upon dissociation. NMR spectroscopy revealed information about the proposed structure in solution and revealed that the ligand was bound in solution to the metal center in one orientation, owing to the coordination of the O and N to the metal center. Catalyst decomposition studies were performed using the methylidene variant of the catalysts at elevated temperatures under inert conditions as well as under an atmosphere of ethylene gas. The purpose of intentionally decomposing the catalyst was to generate the electron deficient Ru center and probe the stabilizing effects of the pyridine arm coordination. These reactive intermediates are electronically and sterically more similar to the 14-electron active catalyst than the 16-electron pre-catalyst, giving insight into how the catalysts behave in solution upon metathesis active conditions. Decomposition products of the DIPP variant were analyzed by NMR and x-ray crystallography, giving insight into possible decomposition pathways for these novel catalysts. The catalysts were screened for metathesis activity in ring-closing metathesis (RCM) and ring opening metathesis polymerization (ROMP). Both showed noticeable differences from previous generations of Grubbs and Hoveyda-Grubbs catalysts in overall efficacy. The prolonged lifetimes of these new catalysts were competitive with commercially available catalysts in terms of lifetime and TON. Though slightly lower in TON, these catalysts lasted much longer in solution at elevated temperatures than their predecessors, thus indicating much more stabilized reactive intermediates. The data gathered from decomposition studies and metathesis activity along with NMR and x-ray crystallography allowed for potential structure-activity relationships and mechanistic pathways to be proposed. The insight into olefin metathesis catalyst structure, performance, and design provided by these studies may assist in future endeavors in the field of olefin metathesis catalyst design and employment.

Download or read book Longer lived Olefin Metathesis Catalysts Based on Molybdenum and Ruthenium written by Joseph Yoon and published by . This book was released on 2020 with total page 91 pages. Available in PDF, EPUB and Kindle. Book excerpt: The field of olefin metathesis has seen considerable growth in the recent past. Some of the earliest milestones in the field include the synthesis of well-defined catalysts based on molybdenum, tungsten, and ruthenium. The efficiencies of these catalysts, however, are limited by their decomposition. Efforts have been made to increase the lifetime of these catalysts by changing the ligand sphere, to stabilize catalytic intermediates. Examples include the employment of the N-heterocyclic carbene (NHC) and the chelating (o-isopropoxy)benzylidene ligand seen in the second-generation Grubbs and Hoveyda catalysts. Processes that utilize the olefin metathesis processes, like those in the petroleum industry and large-scale production of chemicals, are bound by the need for high catalyst loadings which translate to high costs. The work herein presents the pursuit of longer-lived olefin metathesis catalysts based on molybdenum and ruthenium. The first goal of this thesis project was to develop a stable molybdenum-based olefin metathesis catalyst supported by a tridentate PONOP ligand and a chelating (o- x methoxy)benzylidene ligand. Previous attempts in our lab employed nonchelating alkylidene initiators - yielding no success in isolation. The rationale behind this design was that a chelating ether moiety will stabilize the molybdenum-center enough to be isolable. Attempts to isolate the chelating molybdenum-alkylidene species were also unsuccessful. Instead, we probed the in-situ ROMP of norbornene using iPrPONOP MoCl3 as a precatalyst and (2-methoxybenzyl)magnesium chloride as a cocatalyst. This cocatalyst did not lend any improvements to the simpler nonchelating Grignard cocatalysts. The synthesis of a novel dialkyl zirconocene complex is also reported. The second and more heavily pursued endeavor was the development of longer-lived ruthenium olefin metathesis catalysts. Specifically, we aimed at improving the second-generation Hoveyda catalyst with the use of a hemilabile tridentate NHC ligand. Two novel catalysts bearing NHC ligands with a hemilabile ethoxy-pyridyl arm were synthesized along with their unique organic frameworks. The catalyst containing the 2,6-diisopropylphenyl group (C1-Me) was investigated more comprehensively because it was more readily prepared. This complex was characterized by high thermal stability under metathesis conditions and remarkable TONs in the self-metathesis of 1-decene. In our efforts to prepare C1-Me without utilizing a Grubbs I intermediate, a new complex (6) bearing our NHC ligand was isolated and characterized by 1H NMR and single crystal x-ray diffraction spectroscopy. The reaction of C1-Me with ethylene did not produce the desired C1-Me-methylidene variant - however, the same reaction with propylene gave C1-Me-ethylidene with relative ease. Analyzing the active catalytic species under the metathesis of 1-decene revealed that the resting state of the catalyst is not the expected methylidene, but rather the longer chain nonylidene. xi Initiation studies were conducted to compare the rates of initiation for catalyst C1-Me and the nonmethylated C1-H. First, the rate of metathesis was followed in the irreversible reaction with ethyl vinyl ether. Second, ligand exchange equilibrium experiments were carried out to compare the dissociation constants for the pyridyl moieties in both catalysts. The outcome of these studies revealed that catalyst C1-Me, with a methyl group in the phenoxide ring, exhibits a 10-fold increase in initiation versus the nonmethylated C1-H catalyst. The NHC ligand scaffold reported in this work may assist in the development of other inorganic and organometallic catalytic systems, as many rely on the use of ancillary ligands for support. Furthermore, fixing a hemilabile ethoxy-pyridyl arm onto already robust systems, such as ruthenium catalysts bearing a cyclic alkyl amino carbene ligand, may offer even greater catalytic turnover numbers (TONs).

Download or read book Second Generation Hoveyda Grubbs Olefin Metathesis Catalysts Bearing Hemilabile Polyether Arms written by Jordan David Carter and published by . This book was released on 2016 with total page 147 pages. Available in PDF, EPUB and Kindle. Book excerpt: The field of olefin metathesis has grown tremendously in recent decades. Elucidation of the mechanism and an improved understanding of the structure-activity relationship has enabled chemists to design catalysts which exhibit greater activity, stability, and selectivity. However, the economic feasibility of performing this reaction on the industrial scale is often restricted by the high cost of the catalysts in combination with modest turnover numbers (TON). The lifetime of an active catalyst species depends on the stability of intermediates in the catalytic cycle, and this ultimately determines the TON. Therefore, the various decomposition pathways leading to catalyst inactivation represents one of the greatest challenges still remaining in the field. The electron deficient Ru intermediates of the catalytic cycle are susceptible to decomposition through any reaction which provides the metal with additional electron density. The primary objective of this thesis project was to develop 2nd generation Hoveyda-Grubbs catalysts which contain an N-heterocyclic carbene (NHC) bearing hemilabile polyether arms in the ortho position of the aromatic rings. The purpose of incorporating these functional groups was to stabilize the fragile intermediates of the catalytic cycle via O → Ru coordination. The polyether arms have the potential to wrap around Ru via chelation and therefore provide electron density while simultaneously shielding the sterically wide open position trans to the NHC. The hypothesis was that stabilizing these intermediates would prolong the lifetime of the active catalyst, thus leading to increased TON and allowing for lowering loadings. A novel series of 5 catalysts bearing NHC ligands with hemilabile polyether arms were synthesized. The NHC ligands differed by the number of arms (1,2, or 4) and the length of each arm (2 or 3 carbons between the ether oxygens). The structures of the catalysts and NHC ligand precursors were investigated by x-ray crystallography and NMR spectroscopy. The results of x-ray diffraction revealed that the 4-armed ligands adopt a propeller conformation, and their corresponding tetra-substituted catalysts displayed a single ortho O → Ru interaction in the solid state. This coordination forces the NHC to adopt a highly contorted conformation which is twisted far away from the default position seen for the unsubstituted commercial catalyst bearing an NHC with 2 mesitylene rings but no electron donor groups. NMR spectroscopy provided information about the conformation adopted in solution, and revealed that the NHC has restricted rotation once bound to Ru but does not display broken molecular symmetry. The combination of O → Ru coordination in the solid state but retention of symmetry in solution indicates that all of the available polyether arms take turns interacting with the metal faster than the NMR timescale. Catalyst decomposition studies were performed under normal metathesis conditions with a standard alkene substrate or by forming the inactive Fischer carbene by treating the catalyst with a vinyl ether. The purpose of intentional catalyst destruction was to generate an electron deficient Ru center which is stabilized by coordination with the polyether arms. These species are electronically and sterically more similar to the 14 electron active catalyst than the 16 electron pre-catalyst, and therefore provide greater insight into how the catalyst actually behaves in solution upon initiation. Only the decomposition products obtained from the 2C-2arm catalyst 15 displayed chelation in the solid state, while every other catalyst formed a non-chelated dimer upon decomposing. A variety of unexpected degradation products were observed by x-ray crystallography, and a potential mechanism was proposed for each decomposition pathway. The catalysts were screened for ring-closing metathesis (RCM) and ring-opening metathesis polymerization (ROMP) performance. Only the tetra-substituted catalysts exhibited a rate enhancement for RCM when compared with the commercial catalyst bearing 2 mesitylene rings. However, the TON did not differ significantly from the commercial catalyst, thus indicating that the electron deficient intermediates of the catalytic cycle are not stabilized by coordination with the polyether arms. All of the polyether-substituted catalysts were significantly inferior than the commercial catalyst at performing ROMP, exhibiting both decreased TON and slower kinetics. The information gathered from testing catalyst performance in combination with the structural data obtained by analyzing the catalysts and decomposition products via x-ray diffraction and NMR spectroscopy allowed for potential structure-activity relationships to be proposed. The insight into olefin metathesis catalyst structure and performance provided by the studies of this work may assist in future catalyst design.

Download or read book Olefin Metathesis written by Karol Grela and published by John Wiley & Sons. This book was released on 2014-04-11 with total page 608 pages. Available in PDF, EPUB and Kindle. Book excerpt: This is a complete examination of the theory and methods ofmodern olefin metathesis, one of the most widely used chemicalreactions in research and industry. Provides basic information for non-specialists, while alsoexplaining the latest trends and advancements in the field toexperts Discusses the various types of metathesis reactions, includingCM, RCM, enyne metathesis, ROMP, and tandem processes, as well astheir common applications Outlines the tools of the trade—from the importantclasses of active metal complexes to optimal reactionconditions—and suggests practical solutions for commonreaction problems Includes tables with structures of commercial catalysts, andrecommendations for commercial catalyst suppliers

Download or read book Ruthenium based Olefin Metathesis Catalysts Bearing PH responsive Ligands written by Shawna Lynn Balof and published by . This book was released on 2011 with total page 306 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Synthesis of Novel Olefin Complexes of Palladium 0 Bearing Monodentate NHC Phosphine and Isocyanide Spectator Ligands written by and published by . This book was released on 2018 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Lewis Acid Activated Olefin Metathesis Catalysts written by Adam McKinty and published by . This book was released on 2014 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Olefin Metathesis written by Karol Grela and published by . This book was released on 2014 with total page 608 pages. Available in PDF, EPUB and Kindle. Book excerpt: This is a complete examination of the theory and methods of modern olefin metathesis, one of the most widely used chemical reactions in research and industry. Provides basic information for non-specialists, while also explaining the latest trends and advancements in the field to experts Discusses the various types of metathesis reactions, including CM, RCM, enyne metathesis, ROMP, and tandem processes, as well as their common applications Outlines the tools of the trade--from the important classes of active metal complexes to optimal reaction conditions--and suggests practical solutions for common reaction problems Includes tables with structures of commercial catalysts, and recommendations for commercial catalyst suppliers

Download or read book Efficient New Routes to Leading Ruthenium Catalysts and Studies of Bimolecular Loss of Alkylidene written by Craig Day and published by . This book was released on 2019 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Olefin metathesis is an exceptionally versatile and general methodology for the catalytic assembly of carbon-carbon bonds. Ruthenium metathesis catalysts have been widely embraced in academia, and are starting to see industrial uptake. However, the challenges of reliability, catalyst productivity, and catalyst cost have limited implementation even in value-added technology areas such as pharmaceutical manufacturing. Key to the broader adoption of metathesis methodologies is improved understanding of catalyst decomposition. Many studies have focused on phenomenological relationships that relate catalyst activity to substrate structure, and on the synthesis of new catalysts that offer improved activity. Until recently, however, relatively little attention was paid to catalyst decomposition. The first part of this thesis explores a largely overlooked decomposition pathway for "second-generation" olefin metathesis catalysts bearing an N-heterocyclic carbenes (NHC) ligand, with a particular focus on identifying the Ru decomposition products. Efforts directed at the deliberate synthesis of these products led to the discovery of a succinct, high-yielding route to the second-generation catalysts. Multiple reports, including a series of detailed mechanistic studies from our group, have documented the negative impact of phosphine ligands in Ru-catalyzed olefin metathesis. Phosphine-free derivatives are now becoming widely adopted, particularly in pharma, as recognition of these limitations has grown. Decomposition of the phosphine-free catalysts, however, was little explored at the outset of this work. The only documented pathway for intrinsic decomposition (i.e. in the absence of an external agent) was -hydride elimination of the metallacyclobutane (MCB) ring as propene. An alternative mechanism, well established for group 3-7 and first-generation ruthenium metathesis catalysts, is bimolecular coupling (BMC) of the four-coordinate methylidene intermediate. However, this pathway was widely viewed as irrelevant to decomposition of second-generation Ru catalysts. This thesis work complements parallel studies from the Fogg group, which set out to examine the relevance and extent of BMC for this important class of catalysts. First, -hydride elimination was quantified, to assess the importance of the accepted pathway. Even at low catalyst concentrations (2 mM Ru), less than 50% decomposition was shown to arise from -hydride elimination. Parallel studies by Gwen Bailey demonstrated ca. 80% BMC for the fast-initiating catalyst RuCl2H2IMes(=CHPh)(py)2 GIII. Second, the ruthenium products of decomposition were isolated and characterized. Importantly, and in contrast to inferences drawn from the serendipitous isolation of crystalline byproducts (which commonly show a cyclometallated NHC ligand), these complexes show an intact H2IMes group. This rules out NHC activation as central to catalyst decomposition, suggesting that catalyst redesign should not focus on NHC cyclometallation as a core problem. Building on historical observations, precautions against bimolecular coupling are proposed to guide catalyst choice, redesign, and experimental setup. The second part of this thesis work focused on the need for more efficient routes to second-generation Ru metathesis catalysts, and indeed a general lack of convenient, well-behaved precursors to RuCl2(H2IMes). This challenge was met by building on early studies in which metathesis catalysts were generated in situ by thermal or photochemical activation of RuCl2(p-cymene)(PCy3) in the presence of diazoesters. Such piano-stool complexes (including the IMes analogue) have also been applied more broadly as catalysts, inorganic drugs, sensors, and supramolecular building blocks. However, RuCl2(p-cymene)(H2IMes), which should in principle offer access to the RuCl2(H2IMes) building block, has been described as too unstable for practical use. The basis of the instability of RuCl2(p-cymene)(H2IMes) toward loss of the p-cymene ring was examined. Key factors included control over reaction stoichiometry (i.e. limiting the proportion of the free NHC), limiting exposure to light, and maintaining low concentrations to inhibit bimolecular displacement of the p-cymene ring. A near-quantitative route to RuCl2(p-cymene)(H2IMes) was achieved using appropriate dilutions and rates of reagent addition, and taking precautions against photodecomposition. This approach was used to develop atom-economical syntheses of the Hoveyda catalyst, RuCl2(H2IMes)(=CHAr) (Ar = 2-isopropoxybenzylidene) and RuCl2(H2IMes)(PPh3)(=CHPh), a fast-initiating analogue of GII. Related p-cymene complexes bearing bulky, inflexible imidazolidene or other donors may likewise be accessible.

Download or read book The Development of Water soluble Olefin Metathesis Catalysts Containing an N heterocyclic Carbene Ligand written by Jason Paul Jordan and published by . This book was released on 2008 with total page 0 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Novel Catalysts for Olefin Metathesis written by Eyal Tzur and published by . This book was released on 2010 with total page 226 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Ruthenium based Olefin Metathesis Catalysts Coordinated with N heterocyclic Carbene Ligands written by John Philip Morgan and published by . This book was released on 2003 with total page 296 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Iron and Molybdenum Complexes Supported by Pincer Ligands written by Steven Ryan Ruark and published by . This book was released on 2016 with total page 135 pages. Available in PDF, EPUB and Kindle. Book excerpt: Since its discovery in the mid 1950’s, olefin metathesis has become one of the most widely used chemical reactions. Olefin metathesis involves the breaking of carbon-carbon double bonds and the redistribution of the fragments to form new olefins by way of a metal alkylidene.6 It is used in industry to convert cheap plant oils into useful products such as alpha olefins, jet fuel and green diesel. The Elevance BioRefinery has the capacity to run this reaction and produce up to 400 million pounds of products per year. The most expensive part in this refinery process is the catalyst itself. The catalyst currently used is an alkylidene complex of ruthenium—an expensive and rare metal. This has led the Schrodi group to explore the possibility of developing catalysts based on abundant and cheap metals such as iron or molybdenum.40,41 We first attempted to support iron with a tridentate pincer ligand, OiPrPONOP, however the ligand was not robust enough and more than one ligand was required to adequately protect the iron xv center. Ultimately, the ligand was reacted with Fe(PMe3)4 to make (OiPrPONOP)Fe(PMe3)2. This complex is very stable and unreactive, preventing its transformation into any catalytic species. We then turned our attention to a pincer OCO-NHC ligand. This ligand was able to stabilize an iron tricyclohexyphosphine complex, (OC-NHC)FePCy3, However, attempts to react this complex with diazo compounds to form an iron alkylidene (OCO-NHC)Fe=CHR were unsuccessful. Further studies focused on replacing the PCy3 ligand with pyridines, in an attempt to make the complex more labile. However, the resulting species proved much too sensitive to water and was difficult to isolate and characterize. Inspired by the research done by the Chirik group where they reduced several arylpyridinediimine ( ArPDI) ironII complexes into a reduced N2-bridged complex. They reported the bound N2 molecules would readily exchange with 15N2 and ultimately they were able to form an iron alkylidene complex. However, the complex was not metathesis active.54,42 We successfully reduced MesPDIFeBr2 into the bis-N2 complex but the complex refused to react cleanly in attempts to make iron alkylidene species. We also explored the possibility of forming a molybdenum alkylidene supported by a tridentate iPrPONOP ligand. After successfully forming iPrPONOPMoCl3 we tried several strategies to form and isolate a molybdenum alkylidene. We attempted a similar reduction as the iron species trying to access a bis-N2 bridged molybdenum complex but the reaction resulted in decomposition of the complex. We then attempted ‘Schrock type’ chemistry by reacting the iPrPONOPMoCl3 complex with Grignard reagents.81 However, this strategy resulted in decomposition as well. We successfully performed ring opening metathesis polymerization (ROMP) of norbornene by adding Grignard reagents to several different tridentate supported MoCl3 precatalysts. Select polymers were then analyzed for cis content by 1 H NMR to probe for serioregularity. The only precatalyst to have more than 50% cis content was the BinapthPONOPMoCl3 / methyl- and trimetylsilylmethlyl-Grignard reagents but only when run at 25 °C. xvi We were able to perform ROMP of dicyclopentadiene (DCPD) with the molybdenum complex / Grignard reagents. However, while the fully polymerized product is extremely hard and transparent we could only achieve a soft nontransparent product, indicating incomplete polymerization.

Download or read book Improving the Efficiency of Ruthenium catalyzed Olefin Metathesis with Solid supported Catalysts Microfluidic Reactors and Novel X type Ligands written by Matthew Martin Van Wingerden and published by . This book was released on 2013 with total page 288 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Rhenium based Olefin Metathesis Catalysts and Investigation of Ligand Properties written by Yu-Ying Lai and published by . This book was released on 2011 with total page 165 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Industrial Applications of Molecular Simulations written by Marc Meunier and published by CRC Press. This book was released on 2011-11-28 with total page 425 pages. Available in PDF, EPUB and Kindle. Book excerpt: The field of quantum and molecular simulations has experienced strong growth since the time of the early software packages. A recent study, showed a large increase in the number of people publishing papers based on ab initio methods from about 3,000 in 1991 to roughly 20,000 in 2009, with particularly strong growth in East Asia. Looking to the future, the question remains as to how these methods can be further integrated into the R&D value chain, bridging the gap from engineering to manufacturing. Using successful case studies as a framework, Industrial Applications of Molecular Simulations demonstrates the capability of molecular modeling to tackle problems of industrial relevance. This book presents a wide range of various modeling techniques, including methods based on quantum or classical mechanics, molecular dynamics, Monte Carlo simulations, etc. It also explores a wide range of materials, from soft materials such as polymeric blends widely used in the chemical industry to hard or inorganic materials such as glasses and alumina. Features Demonstrates how modeling can solve everyday problems for scientists in industry Provides a broad overview of theoretical approaches Presents a wide range of applications in areas such as materials research, catalysis, pharmaceutical development and electronics Emphasizes the relationship between theory and experiments

Download or read book N Heterocyclic Carbenes in Transition Metal Catalysis written by Frank Glorius and published by Springer. This book was released on 2007-02-05 with total page 240 pages. Available in PDF, EPUB and Kindle. Book excerpt: In this book leading experts have surveyed major areas of application of NHC metal complexes in catalysis. The authors have placed a special focus on nickel- and palladium-catalyzed reactions, on applications in metathesis reactions, on oxidation reactions and on the use of chiral NHC-based catalysts. This compilation is rounded out by an introductory chapter and a chapter dealing with synthetic routes to NHC metal complexes.