What Is Metathesis In Chemistry

Achieving The Difficult Reactions

Sometimes synthetic chemistry does not do what you want. Any organic chemist will tell you that when pressed on the viability of chemical reactions. This can be caused by the favoured submission of side reactions, or by the fact that the reaction in question does not lead to the desired product in a high enough yield.

Of course, alternative routes for the reaction can be devised. One such alternative could involve additional steps that approach the problem from a different angle. However, this workaround could prove to be an unnecessary complication that could easily be avoided. Instead, by using olefin metathesis catalysts, new synthetic routes for substrates can potentially be devised. One example is in the synthesis of multi-functional polycyclic lactams which have potential applications in medicinal chemistry. Here, Grubbs catalysts have shortened the number of steps by achieving a hindered synthesis in a high-yielding reaction.3

The two-step reaction to form complex polycyclic lactam products

Bifunctional Aldehyde Derivatives Via Self

The aldehyde functionality is often reached in industry via hydroformylation of alkenes, and allows the access to a large variety of functional compounds by selective catalytic transformations. Alkene metathesis catalysis, especially promoted by well-defined homogeneous ruthenium alkylidenes and indenylidenes, is offering functionalization by double bond exchange and with the advantage of tolerating a large variety of functional groups including the formyl group. The previous successful results of cross-metathesis motivated our exploration of the cross-metathesis of unsaturated aldehydes arising from derivatives of plant oils, especially via cross-metathesis with acrylonitrile or via self-metathesis for the production of linear diol precursors or amino alcohols.

The 10-undecylenic aldehyde 26 is a renewable derivative obtained from thermal treatment of castor oil . Its self-metathesis was attempted to produce the C20 dialdehyde a key intermediate for the production of symmetrical diol or diamine and the formation of polyesters and polyamides. The self-metathesis of aldehyde 26 was attempted at 40°C in toluene first with catalysts II and IV, containing a N-heterocyclic carbene ligand. They showed high catalytic activity but also yielded isomerization by-products. The catalysts I and III containing the PCy3 ligand offered better production of dialdehyde 27. The catalyst III afforded 79% conversion of 26 with only 9% of self-metathesis by-product .

Design Of New Generations Of Metathesis Catalysts

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Chemical Change In Which A Pair Of Molecules Exchange Electronic Patterns Of Bonding

• Salt metathesis reaction, exchange of bonds between two reacting chemical species
• Olefin metathesis, redistribution of olefinic chemical bonds
• Alkane metathesis, redistribution of alkane chemical bonds
• Alkyne metathesis, redistribution of alkyne chemical bonds

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The Quest For Greener Chemistry

Catalysis is one of the twelve principles of green chemistry. It replaces stoichiometric activation methods with pathways that lead to reduced waste and often milder conditions. For instance, when two olefin substrates are joined by metathesis, the byproduct is ethene . Contrast this to a Wittig olefination where the byproduct is triphenylphosphine oxide and 10 times the mass per mole of waste is generated. Even other catalytic reactions that lead to olefin products such as the Heck reaction rely on the preactivation of one of the substrates, for instance with a halide leaving group. However, this preactivation results in the generation of a halide byproduct in the waste stream. Even in this comparison, olefin metathesis may be the most atom-efficient option, covering another of the twelve green principles: convert as many reactant atoms into product atoms as you can.

Over the years, olefin metathesis has proved to be a useful synthetic tool in improving reaction productivity, reducing the amount of waste while also increasing the yield of the desirable compound or opening new reaction pathways that were previously considered impossible. Olefin metathesis opens up new possibilities for greener methods of chemical synthesis with the added bonus of highly active catalysts and potentially reduced energy consumption or carbon footprint.

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Applications Of Olefin Metathesis In Chemical Biology

Olefin metathesis reactions and their application to chemical biology. Ring-opening metathesis polymerization can produce highly-functionalized and well-defined macromolecules, such as this carbohydrate-displaying polymer . Cross metathesis links two alkene fragments to create increasingly complex molecules, including this prolineglycine dipeptide mimic .

Ring-closing metathesis is an effective reaction for producing cyclic molecules with constrained conformations, such as this oxytocin hormone mimic .

Mechanism Of Olefin Metathesis

The ring-closing metathesis reaction is common among all the metathesis reactions. The reaction mechanism involves a series of cycloadditions and cycloreversions among the reactant alkenes in the presence of catalytic metal carbenes.

Step 1: Initiation step that occurs through substitution of the catalysts alkene ligand with the substrate.

Step 2: The suspended olefin then coordinates the metal and undergoes a formal cycloaddition reaction to give an intermediate metallacyclobutane.

Step 3: The alkylidene undergoes an intramolecular cycloaddition with the second reactive terminal alkene on the same molecule.

Step 4: Cycloreversion of the metallacyclobutane intermediate forms the desired product and regenerates the metal-carbene catalyst that reenters the catalytic cycle.

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Catalyse De Mtathse Dolfines : Une Cl Pour Des Transformations Dhuiles Vgtales Insatures Et De Substances Renouvelables

Pierre H. Dixneuf*, Christian Bruneau and Cédric Fischmeister

Institut des Sciences Chimiques de Rennes, UMR 6226 CNRS-Université de Rennes, Organometallics: Materials and Catalysis, Centre for Catalysis and Green Chemistry, Campus de Beaulieu, 35042 Rennes France

* Corresponding author

Abstract

Résumé

Cet article présente limportance de la réaction de métathèse croisée des oléfines par catalyse au ruthénium pour la transformation de dérivés de la biomasse en produits utiles pour lindustrie. Il constitue une revue des principaux travaux réalisés dans ce domaine par les auteurs dans leur équipe de catalyse à Rennes en coopération avec lindustrie chimique. La métathèse croisée dune variété doléfines fonctionnelles issues des huiles végétales avec lacrylonitrile et le fumaronitrile, suivie dune hydrogénation tandem conduit à des dérivés damino acides linéaires, précurseurs de polyamides par polycondensation. Lexploration de la métathèse croisée de nitriles insaturés bio-sourcés avec des acrylates a conduit à des dérivés d,-amino acides, tandis que les aldéhydes gras ont permis un accès rapide à des aldéhydes bifonctionnalisés à longue chaîne et à des diols saturés.

This is an Open Access article distributed under the terms of the Creative Commons Attribution License , which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Sequential Metathesis/elimination Reaction: Straightforward Access To Conjugated Dienes

Following our works on cross-metathesis from natural products, we explored the possibility of associating two RL transformations in a sequential manner to produce conjugated bio-sourced dienes. Indeed, we have recently shown that catalyzed elimination reactions from allylic carbonates to generate dienes via an allylic activation/elimination mechanism . Thus we applied a 2 step strategy involving first cross-metathesis with allylic chloride followed by HCl elimination as shown in Scheme 25.

The cross-metathesis step was achieved using 6 equivalents of allylic chloride in refluxing dichloromethane in the presence of 2 mol% of the Umicore M51 catalyst VIII . Under these conditions, no isomerization of fatty aliphatic chain and only trace amounts of self-metathesis of the natural substrate were observed. Five natural products and two allylic chlorides were used to exemplify the generality of the transformation. 1-Decene, methyl 10-undecenoate and 10-undecenenitrile as fatty acid derivatives, citronellal as a terpene and eugenyl acetate were cross-metathesized with allyl and 2-methylprop-2-enyl chlorides. The corresponding internal olefins were isolated in good yields as mixtures of Z– and E-isomers with the E-isomer as the major product .

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Devising A Sustainable Future

The environmental impact of the chemical industry has recently received attention across multiple fronts, for example the impact of solvent use on sustainability. Nowadays, industries and institutions must endeavour to ensure that any process is not just cost-effective and efficient, but also environmentally friendly. The examples discussed here illustrate just three cases where metathesis catalysts have been applied in academic research but industrial applications are also viable. The challenge then lies in creating and optimising a plant-scale level of production in the most productive and sustainable manner.

New catalyst applications are continuously being uncovered by researchers. One example, recently published in Polymer Chemistry, applied Umicore catalysts to facilitate and simplify the previously difficult process of recycling natural rubber, all made possible through the process of metathesis.6 This is a single example of the role that metathesis catalysts can play in driving future research and industrial innovations.

The carboncarbon bond is ubiquitous and universal. Its extensive utility will make it an essential part of chemistry in the years to come, and the metathesis reaction could help bring it here in a greener way.

Jessica Gomes-Jelonek is the technical sales manager for homogeneous catalysis at UmicorePMC

What The Olefin Metathesis Is

Olefin metathesis is a chemical reaction in which molecules containing carbon-carbon double bonds, also known as olefins, exchange their substituents to yield new value-added products.

The broad applicability of the reaction and its significance for both academic research and the development of commercially viable compounds were recognized soon after its discovery and resulted in the 2005 Nobel Prize in Chemistry awarded to Prof. Grubbs, Prof. Chauvin, and Prof. Schrock for the development of the metathesis method in organic synthesis.

Metathesis is used daily in the chemical industry, mainly in the development of pharmaceuticals and of advanced plastic materials. Thanks to the Laureates contributions, synthesis methods have been developed that are more efficient , simpler to use and environmentally friendlier .

Nobel Prize Press Release

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Selective Hydrogenation Of Nitrile Esters With Ruthenium Alkylidene Catalysts

A tandem metathesis/hydrogenation process in the presence of ruthenium alkylidene catalyst has already been used to prepare saturated polymers . Thus, we have explored whether rutheniumbenzylidene and rutheniumindenylidene olefin metathesis catalysts could also perform hydrogenation of fatty alkyl nitriles and could be more efficient than the residual ruthenium arising from cross-metathesis.

The hydrogenation of methyl 11-cyano undec-10-enoate 10 , was first performed in the presence of catalyst IV and 30 mol% of tBuOK, under hydrogen pressure of 20 bar at 80°C for 39 h. It quantitatively provided the saturated C12 ,amino ester 18, which was isolated in 89% yield . The presence of tBuOK not only created the hydrogenation catalyst but was shown to inhibit the formation of secondary amines.

The catalyst activity of ruthenium-alkylidenes I, II, and VIII-X was evaluated on the hydrogenation of 10, in the presence of 30 mol% of tBuOK. The best results were obtained with catalyst X and II requiring the presence of only 15 mol% of tBuOK to get quantitative formation of 18 after 16 h. More importantly these two catalysts could perform the quantitative formation of 18 by hydrogenation of nitrile ester 10 at 80°C for 16 h .

Ring Opening Polymerization Of Cyclic Olefins

Metathesis of cyclic olefins results in ring opening polymerization. Some NattaZiegler catalyst systems appear to promote ring opening in competition with normal polymerization.2

Polypentenamer , formed from cyclopentene, shows promise as an elastomer.17 Its potential commercial value has provided the incentive for much of the work on ring opening polymerization. This work has provided many of the important details of the metathesis reaction.

Olefin metathesis can show high stereoselectivity and the double bonds of the polymer can be either highly cis or trans, depending on the catalyst mixture. All-trans-polypentenamer shows the mentioned elastomer properties, while the cis-polymer does not show promising characteristics.18 Acyclic olefins show some stereospecificity but do not approach that obtainable with cyclic olefins. Possible sources of this stereospecificity will be discussed in a later section.

If the polymerization is stopped early in the reaction, the average molecular weight of the polymeric fraction is near that of a completed reaction.19 Some fractions of material are present as large cyclic molecules.

The ring opening polymerization of unsaturated lactones produces unsaturated polyesters with a defined repeating structure .24 Materials produced by these reactions should have useful properties.

K. ukowska, K. Grela, in, 2014

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Accelerating And Selective Efficiency

Cross-metathesis reactions, where two unconnected alkenes undergo metathesis, have received a somewhat notorious reputation as uncontrollable reactions. While this was certainly true for the first-generation of metathesis catalysts, modern innovations have overcome this problem by fine-tuning the reaction conditions and ligands. By changing properties such as the steric bulk the physical space occupied by the ligand the catalyst complex can influence the reaction pathway directing product formation. Changing such properties also has a strong effect on the yield of a reaction. To illustrate the effect various catalysts have on a given reaction, a series of competitive studies have been performed by US researchers with a variety of metathesis catalysts.4

Comparative yields for the cross-metathesis reaction using two different Grubbs catalysts

In this case, the smaller N-heterocyclic carbene ligand directs the reaction down a specific pathway, favouring the formation of the desired product. In addition to improving yields, olefin metathesis can also potentially offer synthetic solutions to those reactions where the stereoselectivity the precise direction of chemical bonds must be controlled. Reactions that produce a mixture of compounds require extensive removal techniques, typically involving wasteful quantities of solvent, in addition to reducing the total volume of useful compound synthesised.

Unfavoured cis-alkenes can now be synthesised with ease

Metathesis: The Green Method Of Chemical Synthesis

Sponsored by Umicore, by Jessica Gomes-Jelonek2018-06-15T09:00:00+01:00

Getting the most out of your reactions is a crucial part of green chemistry. The bond-forming prowess of metathesis catalysts can help

Underpinning all organic chemistry, the simple carboncarbon bond is one of the most stable covalent bonds known. Chemical synthesis relies on the reaction of reagents to form products, with bond breaking being as essential a component as bond formation. The utility and ubiquity of carboncarbon bonds make them vital to modern life: from industrial and agricultural chemicals, to biological processes and pharmaceutical products. The stability of the carboncarbon bond makes it essential to life as we know it, but also means that bond breaking is energetically demanding and specific reaction pathways are needed. Many methods of forming carboncarbon bonds rely on breaking these bonds first, a difficult process for such a stable bond.

A general scheme for the metathesis reaction

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Derivatives Of Copper Silver And Gold

Metathesis of perfluoroisopropylcadmium with the salts CuX, where X = Cl,Br,I, has provided two types of perfluoroisopropyl copper species 2CFCu and 2Cu in quantitative yields. The distribution of the two types depended on which copper salt was used for the reaction and also on the ratio of the copper salt to the cadmium reagent < 92JFC341> .

Perfluoroisopropyl silver was formed by codeposition of silver vapour with 2CFI at 196 °C followed by matrix warm-up < 76JFC95> . Treatment of CF2CFCF3 with silver trifluoroacetate and CsF produced 2CFAg in good yield < 73JOM423> . This procedure was claimed to be more satisfactory than the direct addition of AgF to CF2CFCF3< 68JA7367> . 2CFAg was shown to be heterolytically labile in solution existing in equilibrium with solvated Ag+ and the complex Ag2< 86JA5359> .

The gold dimer 2Br was prepared in good yield by the action of CH2ClBr on 2< 85CC1278> . Interaction of Ph3PAuCl with CH2N2 afforded Ph3PAuCH2Cl in 80% yield < 77IZV2417> .

Robert H. Grubbs, … Melanie S. Sanford, in, 2003

Predicting The Products Of Metathesis Reactions

AB + CD –> AD + CB

Predict reaction products given reactants Na2SO4 and AlCl3:

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Other Types Of Metathesis Reaction

Salt metathesis or precipitation reaction is used in wastewater treatment. A chemical is added to wastewater which precipitates the impurities. The imparities are then filtered out, leaving pure water.

The precipitation reaction is also used in metallurgy. For example, oxalic acid is mixed with seawater and brine water to extract calcium and magnesium through precipitation.

The acid-base neutralization reaction between vinegar and baking soda has many applications. It can be applied to clean surfaces, extinguish a fire, and make a chemical volcano.

Tandem Alkene Metathesis / Hydrogenation Catalyses: From Plant Oil Unsaturated Esters To Linear Amino Esters

The hydrogenation of aliphatic nitriles into primary amines requires more efficient catalysts operating in more drastic conditions. Takemoto et al. and Li et al. reported examples of homogeneous hydrogenation of nitriles into primary amines when alkoxide were added to ruthenium catalyst. Das et al. and Enthaler et al. also showed that addition of tBuOK and phosphine to simple ruthenium complex led to the fast hydrogenation of various nitriles and by replacing the phosphine ligand by a N-heterocyclic carbene, milder conditions could be operating .

We have now shown that, after performance of the ruthenium alkylidene-catalyzed cross-metathesis of alkenes with acrylonitrile, when the catalytic hydrogenation of the resulting nitrile ester with the residual ruthenium catalyst was carried out in the presence of tBuOK or even KOH, the hydrogenation of the nitrile group into primary amine could be obtained, without formation of secondary amine, and that the ,-linear amino ester, a precursor of polyamides, could be obtained in excellent yield .

The evaluation of other basic additives such as potassium hydroxide, sodium hydroxide, or cesium carbonate showed that KOH could be efficiently used but in 60 mol%. Using only 30 mol% of KOH led to 56% yield of 18 together with 40% yield of nitrile ester 16 .

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