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Organic Chemistry

Learn the toughest concepts covered in Organic Chemistry with step-by-step video tutorials and practice problems by world-class tutors.

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16. Conjugated Systems

Diels-Alder Forming Bridged Products

What happens when we add some complexity to our s-cis-1,3-diene? We can form a bridged diels-alder product. It's a lot easier than it sounds, I promise! Let's take a look. 



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Now let's discuss a few added complexities of the deals. Outer reaction. So it turns out that sometimes when you run a deal's alder reaction, you get a bicycle IQ bridged molecule as your product, as shown in this diagram right here. So how does that happen? And why would we get a bicycle IQ as our product? Well, it turns out that bicycling bridge products are obtained when you're s sister dying is cyclic. Okay. Remember that I told you guys that won three dying's could be regular straight chains, but they could also be found within rings. So if you're dying is found within a ring, it's gonna produce what we call a bridge product. Okay, so let's just look at these two different examples. If you're starting off with a normal a cyclic dying as we have here, okay, then you're just gonna wind up getting a six member ID ring as your product, and we're used to seeing that. However, if you start off with a cyclic dying notice that we already have one ring, that one ring is here. So when we go to react, let me just make that a little more clear. One Okay, So when we go to react this and form another 600 ring on top of it, we're actually going to get a bicycling product. There's going to be two rings present, not just one, because we started with a ring to begin with. So now we're layering another ring on top of that. Okay, well, let's just go through this really quick. I know that you guys already know how to draw the cyclic product for a a cyclic dying. But let's see why you get a bridge in the cyclic dying, because notice that the cyclic dying is gonna have a dying portion of the molecule. And it's also gonna have a non dying portion as identified. That's a little star there. There's some portion of this molecule that's in the ring that is not a part of the dying. It's outside of the dying. So when the Diana file goes to attack with its three mechanistic arrows, you've got those 123 What we find is that one of the one or more of these atoms get pushed out of the way and get pushed above the entire reaction. You could almost imagine it like the Diana file is a dying lover, right? So imagine that the dying, the dying, a file on the dying or trying to hook up or something. And there's, like, some third wheel. You're that awkward third wheel in the middle and you're just like hick. I need to get out of the way because these guys were just going at it. Okay, well, that is exactly how this red Carbon is feeling right now. He's feeling super awkward. So instead of getting involved in the mix, he's gonna go ahead and stay out of it and just move right on top of the ring. So as we see you wind up still getting the six member ID ring, there's nothing you can do about that. But now we're going to get a bridge on top of it because this carbon really wanted nothing to do with what was going on. So we went ahead and stayed above the whole situation. Now, the difference between these two molecules here is that they're just represented differently. This is the planer representation, and this is the three D representation, and you should be able to understand both of them. I know they look a little bit weird, but this is essentially the same carbon. And then you have your six member ring below. Okay, so that's what we call a bicycle bridge products. And that happens when you're dying is a ring. So let's look at this example here. I'm actually gonna draw this one. We're just gonna do this as a worked example, since I think that it's still a little too hard for you guys to do this. So how would we draw this product? Well, as you can see, this is going to be a diamond is ation. That means that we have the same molecule acting as the dying, enacting as the dying. Oh, file. Okay, so how would this happen? First of all, is this dying in the right confirmation to even react? Remember that we stated how you're 13 Dying always has to be in the s ist confirmation. Is it in the right confirmation? Yes, it is. Because if you were to draw a line between you would notice that both of the our group's heir faced in the same side. So what we wanna do is want to rotate that so that it's going to be facing opposite to the Diana file. So in orderto line this up correctly, I would actually flip my cyclo Penta dying over to the right so that now it's gonna be ableto correctly face the Diana file. Now, the Diana file. I told you guys for cyclo Penta Diane. Either one of these double bonds could be the Diana file, so it doesn't really matter which one we pick. I'm just gonna line one of them up next to it. Okay, so there we go. Now it's time to draw our arrows, and we're going to see that we get 123 with notice. We've got a bridge because this carbon here is not so happy about what's going on. It doesn't want a part of this. Just doesn't wanna be there. So we're gonna go ahead and draw this product. That's gonna be a new six remembered ring. Now, let me just show you how I do that. You've always got your four carbons from the dying, right? And you've got your fifth and six carbon from the dying of file. Right? So that's gonna make your six member bring. Now we know that in between this is 123456 We know that the double bond should be in between which carbons two and three, because two and three are the ones that received an arrow. And we know that we're gonna need the rest of the ring attached. Toe carbons five and six. So just hold for a second. Five and six. We're going to get a five carbon rings coming off of that. And we should have a double bond here since that double bond. Really? Nothing happens to it from the original cycle. A pen to dying. Right? But we're still missing something. What are we missing? We're missing the bridge. Noticed that the bridge was attached to which atoms it's attached to atoms. Going to just highlight this to touch toe Adams one and four. Okay, so I'm gonna go ahead and draw a bridge between one and four, and that's going to represent the fact that's gonna represent the carbon that didn't want to be there in the first place. That is not participating in the reaction. And it's now above the ring as a bridge. Bicyclists. Okay, so hopefully that made sense so far. Now, it turns out we're not done because bridged compounds add an extra complication to the product of a deal's elder. It turns out that any time that a bridge product is made, you have to be aware off stereo chemistry that we call exo and Endo stereo chemistry. Now, what the heck do those words mean? Those are words that specifically relate to the deals elder products. And it turns out that this ring actually could have faced two different ways. Now, if you guys just noticed this molecule here is the same one that I drew up there, so it's just drawn more professionally on. But you notice is that there's actually two different orientations that ring could have taken either. It could have been faced towards the top or could have been faced towards the bottom. Right? Is one of them preferred? Is one of them not preferred? Okay, well, it turns out that yes, one of them is highly preferred, and one of them is highly not favored. And that would be the direction that is away from the bridge. So that means that basically, we've got two different options. Let's look at three D to see if we can figure this out, we've got one ring that is really close to that bridge. Now, this bridge has hydrogen sticking off of it. So this is gonna do something called a flagpole interaction where you actually have hydrogen zones that are too close to each other. Okay, so you've got this hydrogen. I know it looks like it's kind of far, but still, there's gonna be some interaction there. They're not gonna be very happy about being so close together. Okay, now we've got those same hydrogen is here. But now notice that in this situation I faced my ring down. So there's a lot less. You know, there's a lot more room for these hydrants to exist, and this ring is way happier being on the bottom side of the entire molecule. So when the rings air close together, we call that exo. When the rings are far apart, we call that endo. Well, when a bridge product is made, you're always gonna face the endo direction because you want your ring or your substitute wants to be away from the bridge so it doesn't just apply to rings. It could apply to any substitutes that we're facing off of the Diana file. If it's a bulky substitue in, even if it's just a metal, it's gonna wanna face down away from the bridge so you can avoid that flagpole interaction. Okay, so that means that for the product that I was drawing above, this is actually not drawn correctly yet because I have to be specific about Is this gonna be endo, or is it gonna be excel? And your professor isn't gonna be satisfied until you draw that correctly? So how could we change this product said it's correct. What I would want to do is I would want to erase those bonds and put them on dashes Kane, that shows that my ring or my substitution is now away from the bridge. Okay, Another thing you wanna do that you're going to see very often, many times pretty much all the time. We ignore hydrogen on hydrocarbons. We don't draw them, they're implied. You never have to draw them unless stereo chemistry is involved, and then they can be helpful. So what you'll see many times and what I recommend to do is not on Lee. Put the ring facing down. Put the hydrogen is here facing up. Draw those in explicitly. Why? Because that's gonna show your professor that you know what you're doing and that you agree that the H is should go up closer to the bridge because they're smaller. The ring should face away. This basically shows that you understand what an end opposition is and that you know how to draw it. All right, So remember, guys, when do you use endo and exo Onley? When you have what? Ah, bridge. When do you have a bridge? When you have a cyclic dying so you could see all these things were kind of tied together. Thio that entire idea of starting off with a cyclic dying. If you start off with a cyclic dying, you have to worry about exo and Endo. If you start off with a normal dying like we did in past videos, skip all of this explanation because you don't need it. You only need it for the cyclic version. Okay, Awesome. So let's go ahead and move onto the next topic