Olefin and Alkyne Metathesis Ru, W, Mo, Ti

This remarkable reaction is catalyzed by early and middle metals, primarily complexes of Ti, Mo, W, and Ru. All olefin metathesis catalysts either have a M=C 7r bond or are converted into compounds that have a M=C ir bond under the re- 

The olefin metathesis reaction proceeds by a series of [2 + 2] cycloadditions and retro-cycloadditions. No change in metal oxidation state occurs in the course of the reaction. The reaction sets up an equilibrium 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 two Grubbs catalysts undergo dissociation of a Cy3P ligand before the catalytic cycle begins so that coordination of the alkene to the metal may precede each [2 + 2] cycloaddition. 

Ring-closing metathesis has quickly been adopted as a premier method for preparing compounds with very large rings. However, substrates that would lead to 

Problem 6.27. Draw a mechanism for the following ring-closing metathesis reaction. 

There are other variations of olefin metathesis. In the cross-metathesis reaction (CM), two olefins are coupled. This reaction gives good yields and nonsta-tistical mixtures if the two alkenes have electronically different properties. 

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