Alkene Metathesis: Study with Video Lessons, Practice Problems & Examples

Alkene metathesis involves the exchange of carbon atoms between two alkenes, producing a mixture of E and Z isomers, facilitated by Grubbs Catalyst, which contains ruthenium. The process can yield cycloalkenes through ring-closing metathesis, particularly effective for large rings. Mechanistically, it occurs in two phases: the first phase forms intermediates via cycloaddition, while the second phase generates products through further reactions. This reaction is driven to completion by the volatility of ethane gas, enhancing yields, especially with terminal alkenes.

Alkene Metathesis Concept 1

Everyone. In this video, we're going to take a look at alkene metathesis. Now, in alkene or otherwise known as olefin metathesis, 2 alkenes interchange their carbon atoms from their carbon double bond carbon bond. The product result is a mixture of E and Z isomers. In this process, we use a catalyst called Grubbs Catalyst.

Now, Grubbs Catalyst uses our Ruthenium transition metal in the center. It is connected to 2 Chlorines, 2 ligands. It is double bonded to CH₂ and that is connected to Phenyl. Now, when we're talking about alkene metathesis, we're going to say here that our alkenes can be either aligned parallel to one another or opposite to one another. When aligned parallel to one another, we're going to have the cutting of these bonds here.

And remember, cutting through these 2 double bonds just imagine that the double bond forms between these 2 and these 2. This will result in different possible alkenes as products. Now, remember, we said it creates a mixture of E and Z isomers, meaning our 2 R groups could be aligned on the same side giving us a Z isomer, or they can be aligned opposite to one another giving us an E isomer. And then here, we would just make a non-substituted alkene based on the initial alkenes used. When they're opposite to one another, again, we're cutting through here.

And then just imagine that you're making double bonds here. In this case, each new alkene only has 1 R group. So one would be here and one would be here. Now, here we're going to say that alkene metathesis is an equilibrium process, and it gives best yields when terminal alkenes are used. We're going to say, ethane gas is highly volatile and bubbles out of solution and drives the reaction to the right.

It drives it to completion in order to make our products. Right? So this is basically the simplified version of what's happening here in this particular reaction. So keep that in mind as we delve deeper into these types of reactions.

Alkene Metathesis Example 1

Hey, everyone. In this video, it says, draw the products of the following olefin metathesis reaction. So here, they're just showing us one alkene, but remember, there really are two of them. And they can either align parallel to one another or opposite of one another. So here, we would say, if we are going to connect these two together, how could they orient themselves?

Well, they could orient themselves to give us an E alkene. So just imagine here. And let's say we had another one just like in another copy of it. Right? So it would look like this.

Right? And again, the R groups of both, which is this and this, they're either aligning together on the same side to give us a Z alkene or they're aligning opposite to one another to give us an E alkene. Here, I'm showing them aligning themselves opposite to one another. So if we do that, we're going to say now to give us this particular one. This would be our E.

Or they can align themselves on the same side to give us a Z. So let's see. We can do this. We're going to say, if they're going to do Z. Right, so they're both connected to one carbon, and that carbon's connected to another carbon, which is connected to two methyl groups.

So connected here, connected to another group, carbon with two methyl groups. So this would be our Z. Okay. So these will be our two possible products. One as an E product and one as a Z product.

So these will be our two final answers.

Alkene Metathesis Concept 2

In this video, we take a look at ring closing metathesis. We're going to say intramolecular alkyne metathesis is an important synthetic method to prepare cyclo alkenes. It's used to make large rings, which are otherwise difficult to synthesize. If we consider this structure, we have a 0.5 millimolar concentration of our Methylene Chloride solution. We're using Grubbs catalyst, and our temperature is 40 degrees Celsius for 20 hours.

This length of time allows the ring closure to happen. Remember, we have our two alkenes here, and we're cutting through them too. Just imagine a double bond forming between these two and a double bond forming between these two. That would mean that the carbon here and the carbon here would double bond to each other to give us one product, which we're not really concerned with. We're interested in making a large ring.

We would say that this carbon here and this carbon here, these two carbons here, will also double bond to each other. This is a way to make a considerable yield, so we are going to have an adequate yield of this being formed.

The process is carried out in very dilute solutions to avoid intermolecular metathesis from happening. So, this is just a way of us making a good amount of this product, and an adequate yield of this is achieved. This is the method you can use to approach questions in order to make large rings.

Alkene Metathesis Example 2

Everyone, in this example question, it says, "Draw one of the products of the following ring closure metathesis reaction." So here we have dodeca-1,11-diene used with Grubbs catalyst. Now, the way you approach this question is that we're going to draw these two double bonds near each other. So here goes, one double bond this way and another double bond that way.

What we're going to say here is the remaining chain is 1, 2, 3, 4, 5, 6, 7, and 8. So we're going to say, 1, 2, 3, 4, 5, 6, 7, and 8. Alright. So if we number this, these long chains can be difficult, but just realizing the way we're trying to create our product at the end, we're kind of aligning them in a way that looks like we're making 6-membered rings. And remember, we're going to cut through these bonds here.

As a result, these two carbons would double bond, and then these two carbons would double bond. This would give us, plus this alkene as a side product. We're really interested in the ring closure. So this would be one way. Now, here we could also in this case, we see that around this double bond, the bonds are heading in the same direction.

But we could also do it in a way where they look more trans or E actually in terms of their isomer. So, how do we do something like that? In that case, if you're trying to make a more E-type isomer, this is how you'd arrange them. You'd have one double bond this way, another double bond that way, and still count to 8. So 1, 2, 3, 4, 5, 6, 7, 8, and then we connect right here.

In this case, again, remember you're cutting through. When you do that, you're going to create another ring. In this case, now here goes our ring. And here now, the double bonds would be more like this. Taking more of an E-type of isomer, where it looks like around this double bond, one bond orient this way, one orient that way.

Plus our simple non-substituted alkene. Alright. So these are our two rings that are possible based on our starting material. Alright. So these will be our two answers for the large rings that are created.

Identify the alkene that undergoes metathesis to yield the following product.

• A.
• B.
• C.
• D.

1,4-Divinylcyclohexane has two isomers. One isomer undergoes ring-closing metathesis to form a bicyclic compound. Draw the structure of that compound.

• A.
• B.
• C.
• D.

Alkene Metathesis Concept 3

Everyone. In our continued discussion of alkene metathesis, we're going to take a look at it mechanistically. We're going to say that it takes place in two phases. So, in phase 1, we'll take a look at Grubbs' reactant or Grubbs' catalyst, which reacts with the alkene to form two intermediates. And once we've learned that, we're going to move on to phase 2, where the intermediates will react with the alkene to yield our products.

So just remember, as we take a look at it mechanistically, remember we have phase 1 and then we have phase 2. Now that we have that out of the way, click on the next video, and let's take a look at phase 1.

Alkene Metathesis Concept 4

Everyone, we're now going to take a look at phase 1 mechanism. First, before we do that though, we're going to talk about a couple of terms. When we say 2+2 cycloaddition, this is just a synthetic way to make a cyclobutane ring. And when we say the term cycloreversion, this is just the reverse of a cycloaddition.

So, we make a ring and then we break that ring down. Alright. So this is the general process of this phase 1. We're going to say phase 1 is composed of 3 steps. We're going to stay in step 1a, the alkene replaces one of the ligands (L) in the Grubbs catalyst.

Here is the Grubbs catalyst up here, which is going to interact with our alkene, which has one R group on it, so it is a mono-substituted alkene. The way we can look at it is we're going to interact Grubb's catalyst, its double bond is going to interact with the alkene. The alkene is also going to interact in turn. So in essence, we're connecting them together. Same thing here, we're going to have this interacting, interacting so that we can connect them together. In step 1b, we're going to say the catalyst undergoes a 2+2 cycloaddition in order to make our cyclobutane ring, with the alkene to form a metacyclobutane. Within our cyclobutane ring, we're going to have our ruthenium transition metal. This process creates this and this. And we're going to say we're going to make the RN Chloride Ligands for simplicity.

Because remember, we have ligands and we have these chlorides here. We're going to remove them just to make things simpler. Then we're going to say, in step 1c, the metallocyclobutane undergoes cycloreversion to form 2 intermediates. So, we're basically going to break this down. So what can happen here is this bond would snap here, and this bond here would snap here, that would give us this alkene here that we are concerned with.

We'd also get this here, but we're not really paying attention to it. Then, over here, the same thing would occur where this bond would break here, and this bond would break here, giving us these two as another product. Plus, we're not concerned with this other alkene. We're really concerned with the ones that have the ruthenium on there because that's going to go into phase 2. So, the overall reaction, if we want to think about it, is we're going to split this alkene and just going to put a ruthenium on each of these double-bonded carbons.

This double bonded carbon here gets a ruthenium, this double bonded carbon here gets a ruthenium. That's the simplified overview of phase 1. Here we looked at it mechanistically, and now we're just looking at it as a general overall reaction. Split the alkene bond, put a ruthenium on each of those alkene carbons.

Alkene Metathesis Example 3

Everyone, in this example, it says, "draw the organometallic intermediates formed when the following alkene reacts with Grubbs catalyst." Here, I didn't ask for the mechanism. I'm just asking for the organometallic intermediates. So think about the overall reaction.

In the overall reaction, all that's occurring is that we're cutting the connection between the 2 double bonded carbons, and we're going to say that each double bonded carbon would then be connected to ruthenium. So just put Ru here and Ru right here. These will be our 2 organometallic intermediates. Again, they're not asking for the mechanism. We just need to show the intermediates, so it'll sever the connection between the alkene carbons and add ruthenium to each one of them.

And that's it.

Alkene Metathesis Concept 5

Everyone. Now let's take a look at the phase 2 mechanism. Here we're going to say each of the intermediates undergoes a cycle of addition, which is 2a, and a cycle of revision, which is 2b, reaction cycle with the alkene. We're going to say one alkene product, and another intermediate is produced in each reaction. So here, if we take a look, we can say that this can react here, and this one can react here.

We're going to say here that this alkene can react here, and this one can react here. And then through our 2+2 cycloaddition, which again makes a cyclobutane product, gives us this ring. And then we can say that this bond here breaks to form here, and this bond here breaks to form here. Giving us, as our alkenes, the cis or z isomer. And then here we'd have our ruthenium here.

Oh, actually, ruthenium is there. And we can also have e or z isomers, so the Rs can be on the same side, or they can be on opposite sides. In this one, this interacts here, and this interacts here, giving us this cyclobutane ring. Again, this bond can break here, and this bond can break here. This will give us just one possible alkene since there's only one R group on this alkene.

This can react here, and this could react here, giving us this structure. This bond here would break here, and this bond would break here, giving us these two. This bond would break here, and this would break here, giving us this structure, and here to here, giving us these possible products. Alright. So this is the way we can set up phase 2 in terms of the mechanisms involved.

Alkene Metathesis Example 4

Everyone. In this example question, it says, "Draw the meta cyclobutane intermediate that leads to the formation as a metathesis product." Alright. So here, here's our pi bond. Here's our pi bond.

We can show that this pi bond breaks and attaches here, and this one breaks and attaches right here. This would give us, if we take a look, we're going to say we have our Ruthenium here. Its pi bond is being used to make this bond here, which should be connected to this carbon right here. That carbon is still connected to this carbon. And that carbon is still connected to this cyclohexane ring.

We can then say also that this carbon, which is this carbon, is using its pi bonds to connect to this carbon right here. That carbon is still connected to this ruthenium right there. So, this is our 2+2 cycloaddition, and then that carbon in blue is still connected to its cyclohexane ring. So this will represent our metacyclobutane intermediate when we have these 2 compounds interacting with each other. So this will be our final product in this particular example question.

Predict the products of the following metathesis reaction. Assume there is no self-metathesis and the product is unreactive under reaction conditions.

• A.
• B.
• C.
• D.