We’ve been going through some categories of organometallic reactions, so let’s learn one more
before we dive into all sorts of applications. This one is called metathesis, and it may remind
us of some of the electrocyclization reactions we learned in organic chemistry. In these reactions,
two bonds will break and two bonds will form, all in concerted fashion, and they can be any
combination of sigma and pi bonds, so let’s check out some different possibilities now.
Here is a generalized version of sigma-sigma metathesis. We have some metal attached to an
alkyl ligand, and we have an alkane, as separate molecular entities. We can line them up in this
fashion, and as we will see, these two sigma bonds will break, and two new sigma bonds will form with
each group swapping partners, like a little square dance. Because this is concerted, the transition
state will look like this, with partial sigma bond character connecting all four groups in a square,
before generating the products, the metal now with a different alkyl ligand, and a
different alkane. It is important to understand that this works best with metals that are d0,
or possessing no d valence electrons, as it needs open d orbitals to participate, and for the other
reactant, hydrogen must be at least one of the groups if this sigma bond is to break. Here is
a concrete example of sigma-sigma metathesis. We have a zirconium center with two Cp- ligands,
an alkyl ligand, and a halogen. This can react with molecular hydrogen. So one of these hydrogen
atoms will end up coordinated to the metal, and the other will end up coordinated to the alkyl
group, while the metal-carbon bond will break, leaving us with a hydride ligand and an alkane.
So the M-R and H-H sigma bonds broke, and we got new M-H and C-H sigma bonds. In this example,
the zirconium atom is in the +4 oxidation state, and since its ground state electron configuration
ends in 5s24d2, that means its valence shell is empty, and it can accommodate this mechanism.
Now let’s check out sigma-pi metathesis. Looking at the generalized version first, we see a single
bond and double bond oriented in this fashion. We have an alkane, which gives us the single bond,
which is the sigma bond, and we have a metal with an alkylidene ligand,
which gives us the double bond, and therefore the pi bond that will be involved in the metathesis.
So here the sigma bond and the pi bond will break, and we will get two new sigma bonds, one between
the metal and the alkane, and the other between the hydrogen and the carbon from the ligand.
Again, in the transition state we can see partial bond character in this square configuration,
with only the metal-carbon sigma bond remaining intact. With this type of metathesis, again the
metal must be d0 for the same reason as before. And we should also note that the final product is
one molecular entity, rather than two separate entities like with sigma-sigma metathesis.
To see another concrete example let’s bring back that zirconium complex
but this time we have this CH2 substituent. This can react with methane via sigma-pi metathesis,
and this will result in two methyl substituents on the zirconium,
one being from methane, and the other resulting when one hydrogen from methane joined the ligand.
Finally let’s go over pi-pi metathesis.
This involves two double bonds, so here we have a metal with a double bond to an alkylidene ligand,
and then some alkene. Here, the two pi bonds will break, and two new sigma bonds will form,
passing through this transition state, and then yielding this four-membered ring.
Now the fascinating thing here is that while this step is reversible, it is possible for another
metathesis to occur, where the original sigma bonds also break, to form two additional pi bonds,
passing through this other transition state, and yielding these two completely new double bonds.
So the metal has exchanged one alkylidene ligand for another, and the alkene has exchanged one
group. With pi-pi metathesis it is actually the case that the metal does not need to be d0,
so this opens up additional possibilities. We should note that this type of metathesis is
not limited to double bonds. Triple bonds can also be involved, as is the case here
where a carbene reacts with an alkyne. We can see the relevant four-membered ring intermediate
which then gives rise to the unimolecular product with the extended alkyl ligand.
Let’s go over a couple examples to make sure we have the hang of this.
Here’s a hafnium complex with two Cp- ligands and two methanide ligands. This will react
with molecular hydrogen. We have only sigma bonds available, so this will be sigma-sigma metathesis.
Let’s position hydrogen here next to one of these ligands, and we can see that these
sigma bonds will break while these sigma bonds form. As a result, methane has been liberated,
and one of the methanide ligands has been replaced with a hydride.
Now here’s a scandium complex reacting with ethene. There are no pi bonds stemming from
the metal atom, but ethene has a pi bond, so this looks like it will be sigma-pi metathesis.
Recall that in such a scenario we will get a singular molecular entity as the
product. This sigma bond to carbon is the clear candidate for the sigma half of the metathesis,
so let’s position our pi bond nearby, and we can see that these two bonds will break,
one sigma and one pi, while these two bonds form, both sigma. As a result, the ligand
simply lengthens by two carbons, so this is an interesting strategy for ligand modification.
Finally let’s look at this transformation and see if we can explain it using metathesis.
We have a titanium complex with this interesting four-membered ring and a phenyl group on the other
end. The product looks precisely the same, but with the phenyl group on this carbon instead.
How could this be possible? Well let’s remember that pi-pi metathesis can go in either direction.
So two double bonds can yield a four-membered ring, but this can also go
in reverse. Let’s take this ring and split it up by shuffling the electrons this way.
Now we have two double bonds, and all we have to do is flip this vinylbenzene to its other face,
and send pi-pi metathesis in the forward direction again. The pi bonds break and two new sigma bonds
form, and we get the original four-membered ring, just with the phenyl group on the other carbon.
And with that we have wrapped up our introduction to metathesis reactions,
and organometallic reactions in general. Let’s now move forward
and start to get a broader sense of what this chemistry can be used for.