Interactive image. PubChem CID. Chemical formula. GHS hazard statements.
Ring Closing Metathesis (RCM)
GHS precautionary statements. Dichloro o -isopropoxyphenylmethylene tricyclohexylphosphine ruthenium II. Handbook of Metathesis 1st ed. Weinheim: Wiley-VCH. In Murahashi, S. Ruthenium in Organic Synthesis. Noels Kirk-Othmer Encyclopedia of Chemical Technology. Tetrahedron Lett. Tetrahedron Letters. Journal of the American Chemical Society. Angewandte Chemie International Edition. Ruthenium compounds. RuCl 3. RuF 5.
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- Ring Closing Metathesis.
RuF 6. RuO 4. The metallacyclobutane produced can then cycloeliminate to give either the original species or a new alkene and alkylidene. Interaction with the d-orbitals on the metal catalyst lowers the activation energy enough that the reaction can proceed rapidly at modest temperatures.
Olefin metathesis involves little change in enthalpy for unstrained alkenes. Product distributions are determined instead by le Chatelier's Principle , i. Cross metathesis and ring-closing metathesis are driven by the entropically favored evolution of ethylene or propylene , which can be removed from the system because they are gases. The reverse reaction of CM of two alpha-olefins, ethenolysis , can be favored but requires high pressures of ethylene to increase ethylene concentration in solution. The reverse reaction of RCM, ring-opening metathesis, can likewise be favored by a large excess of an alpha-olefin, often styrene.
Ring-opening metathesis usually involves a strained alkene often a norbornene and the release of ring strain drives the reaction. Ring-closing metathesis, conversely, usually involves the formation of a five- or six-membered ring, which is enthalpically favorable; although these reactions tend to also evolve ethylene, as previously discussed.
RCM has been used to close larger macrocycles, in which case the reaction may be kinetically controlled by running the reaction at high dilutions. The Thorpe—Ingold effect may also be exploited to improve both reaction rates and product selectivity. Cross-metathesis is synthetically equivalent to and has replaced a procedure of ozonolysis of an alkene to two ketone fragments followed by the reaction of one of them with a Wittig reagent. In a Du Pont research group polymerized norbornene to polynorbornene using lithium aluminum tetraheptyl and titanium tetrachloride  a patent by this company on this topic dates back to  ,.
According to the then proposed reaction mechanism a RTiX titanium intermediate first coordinates to the double bond in a pi complex. The second step then is a concerted SNi reaction breaking a CC bond and forming a new alkylidene-titanium bond; the process then repeats itself with a second monomer:. Only much later the polynorbornene was going to be produced through ring opening metathesis polymerisation. The DuPont work was led by Herbert S. Giulio Natta in also observed the formation of an unsaturated polymer when polymerizing cyclopentene with tungsten and molybdenum halides.
In a third development leading up to olefin metathesis, researchers at Phillips Petroleum Company in  described olefin disproportionation with catalysts molybdenum hexacarbonyl , tungsten hexacarbonyl , and molybdenum oxide supported on alumina for example converting propylene to an equal mixture of ethylene and 2-butene for which they proposed a reaction mechanism involving a cyclobutane they called it a quasicyclobutane — metal complex:. This particular mechanism is symmetry forbidden based on the Woodward—Hoffmann rules first formulated two years earlier.
Cyclobutanes have also never been identified in metathesis reactions, which is another reason why it was quickly abandoned. Then in researchers at the Goodyear Tire and Rubber Company described a novel catalyst system for the metathesis of 2-pentene based on tungsten hexachloride , ethanol , and the organoaluminum compound EtAlMe 2.
The researchers proposed a name for this reaction type: olefin metathesis. In this reaction 2-pentene forms a rapid a matter of seconds chemical equilibrium with 2-butene and 3-hexene. No double bond migrations are observed; the reaction can be started with the butene and hexene as well and the reaction can be stopped by addition of methanol.
The Goodyear group demonstrated that the reaction of regular 2-butene with its all- deuterated isotopologue yielded C 4 H 4 D 4 with deuterium evenly distributed.
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- Olefin metathesis.
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In Chauvin proposed a four-membered metallacycle intermediate to explain the statistical distribution of products found in certain metathesis reactions. Chauvin's experimental evidence was based on the reaction of cyclopentene and 2-pentene with the homogeneous catalyst tungsten VI oxytetrachloride and tetrabutyltin :.
The three principal products C9, C10 and C11 are found in a regardless of conversion.
Grubbs catalyst - Wikipedia
The same ratio is found with the higher oligomers. Chauvin also explained how the carbene forms in the first place: by alpha-hydride elimination from a carbon metal single bond. For example, propylene C3 forms in a reaction of 2-butene C4 with tungsten hexachloride and tetramethyltin C1. In the same year Pettit who synthesised cyclobutadiene a few years earlier independently came up with a competing mechanism.
Experimental support offered by Pettit for this mechanism was based on an observed reaction inhibition by carbon monoxide in certain metathesis reactions of 4-nonene with a tungsten metal carbonyl .
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- Olefin Metathesis, Grubbs Reaction.
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Robert H. Grubbs got involved in metathesis in and also proposed a metallacycle intermediate but one with four carbon atoms in the ring. This reaction was first used in petroleum reformation for the synthesis of higher olefins Shell higher olefin process - SHOP , with nickel catalysts under high pressure and high temperatures. Synthetically useful, high-yield procedures for lab use include ring closure between terminal vinyl groups, cross metathesis - the intermolecular reaction of terminal vinyl groups - and ring opening of strained alkenes.
When molecules with terminal vinyl groups are used, the equilibrium can be driven by the ready removal of the product ethene from the reaction mixture. Ring opening metathesis can employ an excess of a second alkene for example ethene , but can also be conducted as a homo- or co-polymerization reaction.
The driving force in this case is the loss of ring strain. All of these applications have been made possible by the development of new homogeneous catalysts. Shown below are some of these catalysts, which tolerate more functional groups and are more stable and easy to handle. The Schrock catalysts are more active and are useful in the conversion of sterically demanding substrates, while the Grubbs catalysts tolerate a wide variety of functional groups.
The second generation Grubbs catalysts are even more stable and more active than the original versions. Some of these are depicted:.
Mechanism of Olefin Metathesis
Grela, S. Harutyunyan, A. Michrowska, Angew. Hong, D.