Olefin metathesis
Encyclopedia : O : OL : OLE : Olefin metathesis
Olefin metathesis or transalkylidenation (in some literature, a disproportionation) is an organic reaction which involves redistribution of olefinic (alkene) bonds.[#endnote_astruc] Since its discovery, olefin metathesis has gained widespread use in research and industry for making products ranging from medicines and polymers to enhanced fuels. Its advantages include the creation of fewer sideproducts and hazardous wastes. Yves Chauvin, Robert H. Grubbs, Richard R. Schrock shared the 2005 Nobel Prize in Chemistry for "the development of the metathesis method in organic synthesis".
The reaction is catalyzed by metals such as nickel, tungsten, ruthenium and molybdenum. The reaction consists of an alkene double bond cleavage, followed by a statistical redistribution of alkylidene fragments. The general scope is outlined by the following equation:
Overview
Olefin metathesis was first used in petroleum reformation for the synthesis of higher olefins in the Shell higher olefin process (SHOP) under high pressure and high temperatures. Many traditional catalysts are derived from a reaction of the metal halides with alkylation agents for example WCl6-EtOH-EtAlCl2. A metathesis reaction is a chain reaction that begins when a metallocarbene and an olefin react to form a metallacyclobutane. This intermediate then reacts further, decomposing into a new olefin (the product) and a new metallocarbene, which can then be recycled through the reaction pathway.
A metal carbene based on ruthenium is Grubbs' catalyst[#endnote_dragutan], a Schrock carbene is a molybdenum catalyst.
Metathesis chemistry
The complete family of metathesis chemistry:
- Cross-metathesis (CM)
- Ring-closing metathesis (RCM)
- Enyne metathesis (EM)
- Ring opening metathesis (ROM)
- Ring opening metathesis polymerisation (ROMP)
- Acyclic diene metathesis (ADMET)
- Alkyne metathesis (AM)
- Alkane metathesis
- Alkene metathesis
Alkene metathesis is generally driven by the evolution of gaseous ethylene; and alkyne metathesis is driven by the evolution of acetylene. These are both dominated by the entropy gained by the net release of gas. Enyne metathesis cannot evolve a simple gas, and for that reason is usually disfavored unless there are accompanying ring-opening or ring-closing advantages. 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 highly energetically favorable; although these reactions tend to also evolve ethylene. RCM has been used to close larger macrocycles, in which case the reaction may be kinetically controlled by running the reaction at extreme dilutions.
Alkane metathesis is synthetically equivalent to ozonolysis followed by the Wittig reaction (or vice versa).
Examples
An example of enantioselective olefin metathesis: [#endnote_vanveldhuizen]
The metathesis reaction of 1-hexene with the WCl4(OAr)2 catalyst yields 5-decene[#endnote_baibich] plus many byproducts from secondary metathesis reactions.
External links
- Olefin Metathesis: Big-Deal Reaction Chemical & Engineering News, December 23, 2002 Volume 80, Number 51 p. 29-33 [link]
- Olefin Metathesis: The Early Days Chemical & Engineering News, December 23, 2002 Volume 80, Number 51 p. 34-38 [link]
- Development of the metathesis method in organic synthesis Royal Swedish Academy of Sciences [link]
Further reading
- Trnka, T. M.; Grubbs, R. H. Acc. Chem. Res. 2001, 34, 18-29. DOI [10.1021/ja003582t].
- Grubbs, R. H.; Chang, C. Tetrahedron 1998, 54, 4413-4450.
References
From Wikipedia, the Free Encyclopedia. Original article here. Support Wikipedia by contributing or donating.
All text is available under the terms of the GNU Free Documentation License See Wikipedia Copyrights for details.