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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. A metal carbene based on ruthenium is Grubbs' catalyst, a Schrock carbene is a molybdenum catalyst. 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. Alkane metathesis is synthetically equivalent to ozonolysis followed by the Wittig reaction (or vice versa). Examples An example of enantioselective olefin metathesis: The metathesis reaction of 1-hexene with the WCl4(OAr)2 catalyst yields 5-decene plus many byproducts from secondary metathesis reactions. Further reading | ||||||||||
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