Olefin metathesis is a chemical reaction in which two carbon-carbon double bonds (olefins) come together and exchange with one another, forming new olefinic products in the process.
In the simple example below, symmetrical disubstituted olefins (each with blue or red substituents) are converted to the non-symmetrical olefins (with one blue and one red substituent).
This reaction won’t occur without a catalyst, and since catalysts work in both directions, you’ll usually end up with a thermodynamic mixture of products. In the above example, you’ll end up with 50% of the red/blue product and 25% each of the red/red and blue/blue compounds. There are ways to coax a metathesis reaction to give predominantly one product, however. The most common technique is shown in the example below, where Dinger and Mol used a ruthenium catalyst for the self-metathesis of 1-octene. Ethylene is produced in this reaction, and because it’s a gas, it escapes the reaction vessel, the reverse reaction is suppressed, and the process goes to completion (Le Chatelier’s principle, anyone?).