Information AboutOlefin Metathesis |
| CATEGORIES ABOUT OLEFIN METATHESIS | |
| carbon-carbon bond forming reactions | |
| organometallic chemistry | |
| homogeneous catalysis | |
| industrial processes | |
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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 from the products (α-olefins) from 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. The Grubbs' Catalyst is a ruthenium carbenoid,1. 105. From Arkivoc . while molybdenum or tungsten catalysts are known as Schrock Carbenes 2 Acc. Chem Res.. These catalysts can also perform Alkyne Metathesis and related polymerizations. METATHESIS CHEMISTRY The complete family of Metathesis chemistry:
Like most organometallic reactions, the metathesis pathway is usually driven by a thermodynamic imperative; that is, the final products are determined by the energetics of the possible products, with a distribution of products proportional to the exponential of their respective energy values. 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. The Thorpe-Ingold Effect may be exploited to improve both reaction rates and selectivity. Alkene 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 . SCOPE One study reported a ring-opening cross-olefin metathesis based on a J. Am. Chem. Soc. ; 2002; 124(18) pp 4954 - 4955; (Communication) The metathesis reaction of 1-hexene with the WCl4(OAr)2 catalyst yields 5-decene3 plus many byproducts from secondary metathesis reactions. FURTHER READING #4 #5 #6 #7 #8 REFERENCES EXTERNAL LINKS
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