In this video, we're going to discuss the most famous pericyclic reaction in Organic Chemistry, and that's called the diels alder reaction.
The diels alder reaction is a heat-catalyzed reversible pericyclic reaction between two different molecules that we’re going to go into more depth on. The one thing in common between all diels alder reactions is that they're always going to yield a six-membered ring as their product. You always know that you’re going to create one new ring through the formation of a diels alder reaction.
We need two components plus heat to make this happen. We're going to need one, a 1,3 diene. Two, we're going to need a dienophile. I’ve recognized that these are terms that you’re probably not that familiar with. Let’s really just dive into what that is.
First of all, a 1,3 diene is pretty simple. It sounds like exactly what you’re thinking. It’s a diene that is at the 1 and the 3 position. Basically, another way to say it is that it just has to be a conjugated diene because if it’s not 1,3, let's say that we used a 1,4 diene. Then that would no longer be a conjugated diene. We would actually call that an isolated diene because now the double bonds wouldn’t be next to each other. They would have a space in between. You can't use a 1,4 diene. That’s isolated. You need to use a conjugated diene.
Let’s look at some examples. This first one is pretty easy. That looks like a typical diene. You can add any other R-groups. The important thing is that you at least have that 1,3 diene. Here you see that we actually have a cyclic in the middle. We have a cyclic 1,3 diene because one of them starts at the 1 and one of them starts at the 3.
You might be wondering, ‘Johnny, why are you using those specific atoms to count as 1 and 3?’ It doesn't matter where you start as long as you have diene starting, basically two carbons away from each other, at the 1 and at the 3. You have two diene, two double bonds starting. That's another diene. Here we have another example of a 1,3 diene. I’m just trying to show you guys how 1,3 dienes can come in all shapes and sizes. We're just caring about the fact that they're conjugated to each other.
What’s a dienophile? By definition, the word phile means lover. A dienophile would be a molecule that loves dienes. Dienophiles are actually really easy. All they are is that they are alkenes or alkynes. That's it. It's really that easy. A dienophile could just be a simple cyclohexene, just having that double bond there because it’s a dienophile.
Notice that this next molecule here has two double bonds. Which part of it do you think is the dienophile? I said in the definition it has to be an alkene or an alkyne. This is actually the dienophile, nothing else. The carbonyl doesn't count.
Check this out. That’s weird. Did I make a mistake? Did I drag the wrong molecule at the same molecule to this box? No, I didn’t because it turns out that dienes have alkenes in them. That means both of these double bonds can act as dienophiles. That means you can sometimes see dimerization taking place with these reactions where one molecule reacts as the diene and the other reacts as the dienophile. They react together to form a dimer or something that there's two of them now attached to each other. That's something we need to be aware of.
Here's our last example. Triple bonds have pi bonds in them. This can also be a dienophile. Pretty simple so far. We know that we need a diene, a 1,3 diene. We need a dienophile. We need heat because I told you it’s a heat-catalyzed reaction. But it turns out there's a few more technicalities we have to go over before you’re ready to draw these. One is that your 1,3 diene has to be in a certain shape. You can’t just use any 1,3 diene. For the mechanism to work, you’re going to need to have your 1,3 diene rotated into what's called the s-cis or sigma-sis conformation.
Remember from organic chemistry one that sigma bonds are able to freely rotate as much as they want, meaning that just because it's in that position now doesn't mean it will always stay there. But you have to make sure that at least it’s able to rotate into the s-cis conformation momentarily. Let me show you what I mean.
This diene is not rotated into s-cis. This is what we would call s-trans. Why? Because if you were to draw a dotted line along the single bond or the sigma bond, what you would find is that your R-groups are in opposite sides. That’s what we call s-trans because your sigma bond is rotated in such a way that they’re in opposite sides.
If we were to rotate that sigma bond, what we would find is that now when we draw that same line, now they're on the same side. This is what we would call s-cis. This is the way we need it to be rotated. This would be a big no no. You can’t start off like that. In order to begin your diels alders reaction, you must rotate it first into the s-cis and then you can proceed. Not that bad.
Now let’s look at the general mechanism. The general mechanism is going to be an s-cis diene, specifically s-cis 1,3 diene with a dienophile. Remember I told you guys that a dienophile can be any alkene or alkyne. This molecule right here is a perfect dienophile because it’s just got that double bond.
The cyclization reaction is a 3-membered or 3-arrow reaction where you would get the dienophile initiating because remember it’s like the lover of the dienes. It just wants to attack it. I would go ahead and I would attack one of the edges. But if I make a bond, I have to break a bond because I'm in violation of an octet if I don’t. then this double bond is going to make a new double bond here. Once again, make a bond, break a bond. I’m going to need to break that last diene. This one comes over and attaches to the other side of the double bond.
This is going to form two new single bonds. This forms a new single bond here and this forms a new single bond here. Then this arrow that’s going in between the dienes forms a new pi bond here. Our final product has two new bonds and a double bond. As you can see, I now have a six-membered product, a six-membered cyclic product. Cool so far? That's the general mechanism.
You could get a problem that's just that easy. But diels alder can get a little more complicated as I’ll show you guys. I’m going to start layering on the complications. The first of which being it’s pretty straightforward that the stereochemistry of all substituents must be retained. You have to identify the stereochemistry of your dienophile and your diene so that when you react this together and make a ring, that the stereochemistry is preserved.
Check out this first diels alder. We have a 1,3 diene and a dienophile. But notice that my R-groups on this dienophile are in a cis position. This would be a cis alkene. Remember, double bonds don't twist. It’s always going to be cis. It can't be a trans.
The way we can tell it’s cis is if we were to draw that dotted line or fence that I like to use. They’re both on one side of it. When we react this product, we’re going to draw our arrows. When we react this product, you need to make sure that your R-groups remain cis to each other. They have to remain on the same side of the ring.
Did I have to face them up? No. I could have also faced them down. The important part is that they’re both facing the same exact direction. It wouldn't make sense if I put one R up and one R down because that would like trans. That's not what it began with.
Awesome. That’s pretty straightforward. In the same way if I begin with a trans double bond, as you can see this one’s different. Then I’m going to get a pair of enantiomers because as you can see, now I'm going to get trans products but there's two different ways that those trans products could orient each other. I could get R1 in the front, but I could also get R2 in the front just depending on how the attack works.
In this case, I would have to draw a pair of enantiomers because I have two different trans products that are possible. It’s just really basic stuff that you have to make sure you get right and make sure you pay attention to the stereochemistry in order to get the full credit for this question.
Pretty easy so far, but that's not it. There's a few more complications with diels alder. Let's go ahead and move on to a few more concepts.