Ene-yne metathesis
Before we leave metathesis, and carbenes, we need to introduce one final reaction where metathesis leads to a remarkable transformation. Metathesis works on any C=C π bond, but the π bonds need not be an alkene—it can be an alkyne. The scheme below shows what hap pens: the two C=C double bonds change places. When an alkene reacts with an alkene, the result is two new alkenes, but when an alkene reacts with an alkyne, there is still a single bond remaining from the original alkyne, which ends up linking the two products together as a diene.
The mechanism follows exactly the same sequence of events as before. First the ruthenium carbene catalyst undergoes [2 + 2] cycloaddition with the alkyne. The intermediate is now a metallacyclobutene, and when the reverse [2 + 2] takes place the Ru carbene is still connected to the alkene product.
Now the new carbene can undergo [2 + 2] cycloaddition and reverse [2 + 2] cycloaddition again, this time with the alkene component, and out comes the diene, plus a Ru carbene ready to start the cycle again.
Ene-yne metathesis is therefore a valuable way of constructing dienes—of the type you might require for a Diels–Alder reaction, for example. Unlike more reactive organometallics such as organolithiums and Grignard reagents, the Ru carbenes are fully compatible with acidic NH and OH bonds and with electrophilic carbonyl groups.
