In Olefin Metathesis

In Olefin Metathesis-89
In other words, it occurs through the concerted interaction of one molecule with another.In petroleum refining, heating alkenes over metal oxide surfaces results in the formation of longer-chain alkenes. Metal atoms inside the metal oxides are bridged together by oxygen atoms.A variety of catalysts have been developed for olefin metathesis.

When presented with a racemic mixture of substrates, this catalysts will select one substrate preferentially over the other.

However, the Schrock catalysts can be intolerant of heteroatom functional groups.

Olefin metathesis, or alkene metathesis, is an important process in petroleum refining and in the synthesis of important compounds such as pharmaceuticals.

The mechanism of olefin metathesis is related to pericyclic reactions like Diels Alder and [2 2] reactions.

The [2 2] addition and retro-[2 2] reactions occur in equilibrium with each other.

Each time the metallacyclobutane forms, it can form two different pairs of double bonds through the retro reaction.In contrast to an orbital picture for an alkene, an orbital picture for an alkylidene pi bond suggests orbital symmetry that can easily interact with the LUMO on an alkene.That's because a metal-carbon pi bond likely involves a d orbital on the metal, and the d orbital has lobes alternating in phase like a pi antibonding orbital.The reaction, it turns out, involves a [2 2] cycloaddition of an alkene to a metal alkylidene (to the metal-carbon double bond).This reaction results in a four-membered ring, called a metallacyclobutane. DOI: 10.1055/sos-SD-224-00002 "Simple activation by acid of latent Ru-NHC-based metathesis initiators bearing 8-quinolinolate co-ligands." , 154-165. "Chelating Ruthenium Phenolate Complexes: Synthesis, General Catalytic Activity, and Applications in Olefin Metathesis Polymerization." Kozłowska, A.; Dranka, M.; Zachara, J.; Pump, E.; Slugovc, C.; Skowerski, K.; Grela, K. DOI: 10.1002/chem.201403580 "Two commercially available initiators for the retarded ring-opening metathesis polymerization of dicyclopentadiene." Leitgeb, A.; Wappel, J.; Urbina-Blanco, C. DOI: 10.1007/s00706-014-1249-y "Impact of Electronic Modification of the Chelating Benzylidene Ligand in cis-Dichloro-Configured Second-Generation Olefin Metathesis Catalysts on Their Activity." Pump, E.; Poater, A.; Zirngast, M.; Torvisco, A.; Fischer, R.; Cavallo, L.; Slugovc, C. DOI: 10.1002/macp.201300561 "Mixed N-heterocyclic carbene/phosphite ruthenium complexes: the effect of a bulkier NHC." Urbina-Blanco, C. This mechanism is called the Chauvin mechanism, after its first proponent, Yves Chauvin of the French Petroleum Institute.Chauvin's proposal of this mechanism shortly after the discovery of metal alkylidenes by Dick Schrock at Du Pont earned him a Nobel Prize in 2005.The Schrock catalysts are based on tungsten or, more commonly, molybdenum.These catalysts are among the most efficient available, operating at very high turnover frequencies.

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