Olefin Metathesis Ru, W, Mo

In the olefin metathesis reaction, the alkylidene fragments of two alkenes (R R2C=CR3r4) are swapped to give R R C CR R and R R C CR R.  

This remarkable reaction is catalyzed by early and middle metals, primarily complexes of Mo, W, and Ru. The active catalytic species in all cases is an alkylidene complex of the metal, (M=CR2). In the case of the W complex, the active catalyst is made by transmetallation from Et4Pb to give (Ar0)9W(0)Et2 followed by a-abstraction to give the alkylidene complex ((Ar0)2W(0)=CHMe). 

The olefin metathesis reaction proceeds by a series of [2 -I- 2] cycloadditions and retro-cycloadditions. No change in metal oxidation state occurs in the course of the reaction. The reaction sets up an equihbrium between all the possible alkenes, but it is possible to drive the equilibrium in one direction or the other, for example, by removal of gaseous ethylene. 

The olefin metathesis reaction has been cleverly applied to organic synthesis. In ring-opening metathesis polymerization (ROMP), a strained alkene such as norbomene is allowed to undergo olefin metathesis with itself to give a polymer. 

Problem 6.11. Draw a mechanism for the following ring-opening metathesis 

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