Diels-Alder Forming Bridged Products

Now, let's discuss a few added complexities of the diels-alder reaction. So it turns out that sometimes when you run a diels-alder reaction you get a bicyclic bridged molecule as your products, as shown in this diagram right here, so, how does that happen and why would we get a bicyclic as our product? Well, it turns out that by cyclic bridge products are obtained, when you're s-cis diene cyclic, okay? Remember, that I told you guys that one three dienes could be regular straight chains but they could also be found within rings. So, if your diene is found within a ring it's going to produce, we call a bridge product, okay? So, let's just look at these two different examples, if you're starting off with a normal acyclic diene as, we have here, okay? Then you're just going to wind up getting a six membered ring as your product and we're used to seeing that, however, if you start off with a cyclic diene. 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 six round ring on top of it, we're actually going to get a bycyclic product, there's going to be two rings present not just one because we started with the 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 an acyclic diene but let's see why you get a bridge in the cyclic sign, because notice that the cyclic sign is going to have a diene portion of the molecule and it's also going to have a non diene 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 diene, it's outside of the diene. So, when the dienophile goes to attack with its three mechanistic arrows, you've got the one, two, three, what we find is that one or more of these atoms get pushed out of the way and get pushed above the entire reaction, you can almost imagine it like the dienophile is a diene lover, right? So, imagine, that the diene file and the diene are trying to like 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, hey I need to get out of the way because these guys are 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 going to go ahead and stay out of it and just move right on top of the Ring. So, as, we see you end up still getting the six membered 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 planar representation and this is the 3d 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 membered ring below, okay? So, that's what we call a bycyclic bridge product and that happens when your diene is a ring. So, let's look at this example here, I'm actually going to draw this one, we're just going to 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 dimerization, that means that we have the same molecule acting as the diene and acting as the dienophile, okay? So, how would this happen? Well, first of all is this diene in the right conformation to even react. Remember, that we stated how your one, three diene always has to be in the S cis conformation, is it in the right conformation? yes it is because if you were to draw a line between you would notice that both of the R groups are faced in the same side. So, we want to do is want to rotate that so that it's going to be facing opposite to the dienophile. So, in order to line this up correctly I would actually flip my cyclo pentadiene over to the right so that now, it's going to be able to correctly face the dienophile. Now, the dienophile, I told you guys for cyclopentadiene, either one of these double bonds could be the dienophile so it doesn't really matter which one we pick I'm just going to 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 one two three 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 want to be there. So, we're going to go ahead and draw this product it's going to be a new six membered ring. Now, let me just show you how I do that, you've always got your four carbons from the diene, right? And you've got your fifth and sixth carbon from the diamond file, right? So, that's going to make your six membered ring. Now, we know that in between, this is one, two, three, four, five, six, we know that the double bond should be in between, which carbons 2 and 3 because 2 and 3 are the ones that received an arrow and we know that we're going to need the rest of the Ring attached to carbons 5 and 6. So, just hold for a second 5 and 6, we're going to get a five carbon ring coming off of that and we should have a double bond here since that's the double bond really nothing happens to it from the original cycle penta diene, right? But, we're still missing something, what are we missing? we're missing the bridge. Notice that the bridge was attached to which atoms? it's attached to atoms, let me just highlight this, attached to atoms 1 and 4, okay? So, I'm going to go ahead and draw a bridge between 1 and 4 and that's going to represent the fact, that's going to 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 bicyclic, okay? So, hopefully that made sense so far. Now, it turns out we're not done because bridge compounds add an extra complication to the product of the diels-alder, it turns out that anytime that a bridge product is made, you have to be aware of stereochemistry that we call EXO and endo stereochemistry. Now, what the heck do those words mean? those are words that specifically relate to the diels-alder products and it turns out that this ring actually could have faced two different ways. Now, if you guys just notice this molecule here is the same one that I drew up there. So, it's just drawn more professionally, and what 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 it 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 3D 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 going to do something called a flagpole interaction, where you actually have hydrogens 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 going to be some interaction there, they're not going to be very happy about being so close together, okay? Now, we've got those same hydrogen's here, but now notice that in this situation I face my ring down. So, there's a lot less, you know there's a lot more room for these hydrogen's to exist and this ring is way happier being on the bottom side of the entire molecule. So, when the rings are 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 going to face the in dough direction because you want your ring or your substituents to be away from the bridge, so it doesn't just apply to rings it could apply to any substituents that we're facing off of the dienophile, if it's a bulky substituent, even if it's just a methyl it's going to want to be down, away from the bridge so it 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 going to be endo or is it going to be EXO, and your professor isn't going to be satisfied until you draw that correctly. So, how could we change this product so that it's correct? what I would want to do is I would want to erase those bonds and put them on dashes, okay? That shows that my ring or my substituents is now away from the bridge, okay? Another thing you want to do that you're going to see very often, many times, pretty much all the time, we ignore hydrogen's on hydrocarbons, we don't draw them, they're implied you never have to draw them, unless stereochemistry is involved and then they can be helpful. So, what you'll see many times and what I recommend to do is not only put the ring facing down, put the hydrogen's here facing up, draw those in explicitly, why? Because that's going to show your professor that you know what you're doing and that you agree that the H's should go up closer to the bridge because they're smaller and the ring should face away, this basically shows that you understand what an endo position is and that you know how to draw it, alright? So, remember guys, when do you use endo and EXO only when you have what? A bridge, when you have a bridge, when you have a cyclic diene.

So, you can see all these things are kind of tied together to that entire idea of starting off with a cyclic Diene, if you start off with a cyclic sign you have to worry about EXO and endo, if you start off with a normal diene, like we did in past videos, skip all 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 on to the next topic.