16. Conjugated Systems
Diels-Alder Reaction
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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 2 different molecules that we're going to go into more depth on. Now the one thing in common between all Diels Alder reactions is that they're always going to yield a 6 membered ring as their product. So you always know that you're going to create one new ring through the formation of a reaction. We need 2 components plus heat to make this happen. We're going to need 1, a 13 diene. 2, we're going to need a dienophile. Now I recognize that these are terms that you're probably not that familiar with, so let's really just dive into what that is. First of all, a 13 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 13, let's say that we used a 14 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. So you can't use a 14 diene. That's isolated. You need to use a conjugated diene. Let's look at some examples. This first one's pretty easy. That looks like a typical diene. You can have any other R groups. The The important thing is that you at least have that 13 diene. Here you see that we actually have a cyclic in the middle. We have a cyclic 13 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 13? It doesn't matter where you start as long as you have diene starting basically 2 carbons away from each other. At the 1 and at the 3, you have 2 diene, 2 double ones starting. So that's another diene. And then here we have another example of a 13 diene. So I'm just trying show you guys how 13 dienes can come in all shapes and sizes. We're just caring about the fact that they're conjugated to each other. Now what's a dienophile? Well, by definition, the word phile means lover. So a dienophile would be a molecule that loves dienes. So 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 is a dienophile. Now notice that this next molecule here has 2 double bonds. Which part of it do you think is the dienophile? Well, I said in the definition it has to be an alkene or an alkyne, So 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, the same molecule to this box? No, I didn't because it turns out that dienes have alkenes in them, right? So 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 and they react together to form a dimer or something that there's 2 of now attached to each other. That's something we're going to need to be aware of. Here's our last example. Triple bonds have pi bonds in them, so this can also be a dienophile. Pretty simple so far. We know that we need a diene, a 13 diene. We need a dienophile. We need heat because I told you it's a heat catalyzed reaction. But it turns out there's few more technicalities you have to go over before you're ready to draw these. So one is that your 13 diene has to be in a certain shape. You can't just use any one 3 diene. The 13 diene has for the mechanism to work, you're going to need to have your 13 diene rotated into what's called the S cis or sigma cis conformation. Now remember from organic chemistry 1 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. We have to make sure that at least it's able to rotate into the S cis conformation momentarily. Trans. Why? Because if you were to draw a 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 on opposite sides. That's what we call S trans because your sigma bond is rotated in such a way that they're on opposite sides. Now 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 Alder 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, let's get it right, 13 diene, right? 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 diene. It just wants to attack it. So 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. So 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, so this one comes over and attaches to the other side of the double bond. This is going to form 2 new sigma bonds. This forms a new single bond here and this forms a new single bond here and then this arrow that's going in between the dienes forms a new pi bond here. Our final product has 2 new bonds and a double bond. As you can see, I now have a 6 membered product. A 6 membered cyclic product. Cool so far? That's the general mechanism. Now, there are a lot of you could get a problem that's just that easy. But Dials Alder can get a little bit more complicated as I'll show you guys. Now 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 13 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, so it's always going to be cis. It can't be 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. Well, when we react this product, we're going to 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. Now did I have to face them up? No. I could've 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 1 r up and 1 r down because that would look like trans and that's not what I began with, right? Awesome. That's pretty straightforward. In the same way, if I began with a trans double bond, as you can see this one's different, then I'm going to get a pair of enantiomers because I'm just going to take myself out of the screen because as you can see, now I'm going to get trans products but there's 2 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 2 different trans products that are possible. This is 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. Okay? So pretty easy so far. But that's not it. There's a few more complications with Diels Alder. So let's go ahead and move on to a few more concepts.