The [2 2] addition and retro-[2 2] reactions occur in equilibrium with each other.
Each time the metallacyclobutane forms, it can form two different pairs of double bonds through the retro reaction.
When presented with a racemic mixture of substrates, this catalysts will select one substrate preferentially over the other.
However, the Schrock catalysts can be intolerant of heteroatom functional groups.
In contrast to an orbital picture for an alkene, an orbital picture for an alkylidene pi bond suggests orbital symmetry that can easily interact with the LUMO on an alkene.
That's because a metal-carbon pi bond likely involves a d orbital on the metal, and the d orbital has lobes alternating in phase like a pi antibonding orbital.There are other variations, including some designed for alkyne metathesis rather than alkene metathesis.There is also a commercially-available Schrock-Hoveyda catalyst; that one is chiral, and can be used to carry out olefin metathesis enantioselectively.Grubbs catalysts are more tolerant than Schrock catalysts, although they do not operate at nearly the speed that the molybdenum and tungsten ones can achieve.Because they are less sensitive to air and moisture, they are more commonly used for small-scale, benchtop reactions. Send corrections to [email protected] material is based upon work supported by the National Science Foundation under Grant No. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.But remember, these reactions occur in equilibrium. If a cyclic alkene can be converted into a diene, then under the right conditions, a diene can be converted to a cyclic alkene. This mechanism is called the Chauvin mechanism, after its first proponent, Yves Chauvin of the French Petroleum Institute.Chauvin's proposal of this mechanism shortly after the discovery of metal alkylidenes by Dick Schrock at Du Pont earned him a Nobel Prize in 2005.For example, a cyclic alkene that undergoes olefin metathesis forms two new double bonds, but these parts of the molecule are still connected to each other. Thus, an olefin metathesis between a cyclic alkene a chain alkene might produce a diene.