Olefin metathesis

Olefin metathesis or transalkylidenation (in some literature, a disproportionation) is an organic reaction which involves redistribution of olefinic (alkene) bonds.^[1] 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, and Richard R. Schrock shared the 2005 Nobel Prize in Chemistry for "the development of the metathesis method in organic synthesis".^[2]

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:

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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.

The Grubbs' catalyst is a ruthenium carbenoid,^[3] while molybdenum or tungsten catalysts are known as Schrock carbenes^[4]. These catalysts can also perform alkyne metathesis and related polymerizations.

Reaction mechanism

Hérison and Chauvin first proposed the widely accepted mechanism of transition metal alkene metathesis.^[5] The direct [2+2] cycloaddition of two alkenes is formally symmetry forbidden and thus has a very high activation energy. The Chauvin mechanism involves the [2+2] cycloaddition of an alkene double bond to a transition metal alkylidene to form a metallocyclobutane intermediate. The metallocyclobutane produced can then cyclorevert 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.

Metathesis chemistry

Some important classes 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

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 Hoveyda-Grubbs Catalyst:^[6]

The metathesis reaction of 1-hexene with the WCl₄(OAr)₂ catalyst yields 5-decene^[7] plus many byproducts from secondary metathesis reactions.

References

Further reading

1. (2002) "Olefin Metathesis: Big-Deal Reaction". Chemical & Engineering News 80 (51): 29-33.
2. (2002) "Olefin Metathesis: The Early Days". Chemical & Engineering News 80 (51): 34-38.
3. Schrock, R. R. (1990). "Living ring-opening metathesis polymerization catalyzed by well-characterized transition-metal alkylidene complexes". Acc. Chem. Res. 23 (5): 158–165. doi:10.1021/ar00173a007.
4. Schrock, R. R.; Hoveyda, A. H. (2003). "Molybdenum and Tungsten Imido Alkylidene Complexes as Efficient Olefin-Metathesis Catalysts". Angew. Chem. Int. Ed. 42 (38): 4592–4633. doi:10.1002/anie.200300576.
5. Trnka, T. M.; Grubbs, R. H. (2001). "The Development of L2X2Ru=CHR Olefin Metathesis Catalysts: An Organometallic Success Story". Acc. Chem. Res. 34 (1): 18–29. doi:10.1021/ar000114f.
6. Grubbs, R. H.; Chang, S. (1998). "Recent advances in olefin metathesis and its application in organic synthesis". Tetrahedron 54 (18): 4413–4450. doi:10.1016/S0040-4020(97)10427-6.
7. Grubbs, R. H. (2004). "Olefin metathesis". Tetrahedron 60 (34): 7117–7140. doi:10.1016/j.tet.2004.05.124.

See also

• Metathesis reaction (chemistry)