Generation of Stable Carbenes

It is possible to synthesise stable alkylidene complexes from the reaction between transition metal salts and metal alkyl reducing agents. These conditions are similar to those in some metathesis reactions. For example a Ta alkylidene can be produced in the following manner [27,28]  

Complex 4 has been shown to metathesize 2-pentene to 2-butene and 3-hexene. 

Model reactions have been performed on tungsten carbene complexes, proving that a reaction between a carbene complex and an olefin can produce metathesis products [29]  

In each case, the reaction involves the scission of the alkene with transfer of an alkylidene group via the formation of a metallacyclobutane. 

The products obtained by reaction between a cycloalkene and a symmetric olefin cannot be explained by a pair wise mechanism [30], since the symmetric products 6, 7 are observed immediately the reaction commences and not as a result of further metathesis of 5 after a period of time. Thus theses reactions are good evidence in support of the carbene cyclobutane mechanism. 

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