Olefin Metathesis

Olefin Methathesis

Home

Olefin metathesis

Olefin is an old name for alkenes. Olefin metathesis is a reaction where a catalyst cuts the double bonds of two alkenes in half and reassembles them. Yves Chauvin, Robert Grubbs, and Richard Schrock developed the organometallic catalysts that are used in this reaction. They were awarded the 2005 Nobel Prize in Chemistry for this work. Probably the easiest of these catalysts to use was developed by Grubbs. Grubbs’ catalyst uses ruthenium metal in it. The catalysts developed by Chauvin and Schrock are very moisture sensitive. We need to much less careful with Grubbs catalyst.

Grubbs Catalyst

Overall, you can think of the metathesis catalyst as acting like a pair of scissors. It cuts each double bond in half and makes a “sticky” part of a double bond. These “sticky” parts of double bonds can all randomly attach to each other. This sometimes remakes one of the reactants, or they can make new products. As long as the catalyst is around, cutting and reattaching keeps happening. This causes the metathesis reaction to be in equilibrium. To get more of the products, we need a driving force.

Of course, this is not really the mechanism of this reaction, but it is a good way to think about it. The mechanism is really much more elegant, but can be left for another time.

Olefin metathesis

Ring Closing Metathesis

RCM

If there are two alkenes on a molecule, it is possible to form a ring in a ring closing metathesis. When the alkenes are “cut” and become “sticky”, the sticky ends can find each other and form a ring. In the example below, the CH2= sticky parts can find each other, but they would form ethene which is a volatile gas and can bubble away from the reaction. Since the ethene can bubble away, one of the products is removed from the reaction driving the equilibrium to the right making more of the cyclic product.

Here’s how to think about it.

ROMP

Ring Opening Metathesis Polymerization (ROMP)

When the alkene of a cyclic alkene is "cut”, the “sticky” ends can find each other in another molecule and form a polymer. This ring opening metathesis polymerization, which is known by the acronym ROMP, is a powerful way to make polymers, especially those with double bonds in the chain. The following shows what happens when cyclobutene undergoes ROMP. The driving force for this reaction is the relief of ring strain in the reactants.