Hey guys. In this video, I'm going to go over a specific type of sigmatropic shift that's called a Cope rearrangement. So what is the Cope rearrangement? Well, it's a heat-activated [3,3] sigmatropic shift that involves only hydrocarbons. Okay? So it's a specific type of [3,3] sigmatropic shift that involves only hydrocarbons, meaning you can't have oxygen involved, no heteroatoms, and I just want to remind you that this means that all the rules of pericyclic reactions still apply. This is concerted, it's non-ionic, it's reversible, all of that. But on top of that, it just happens to be a very specific subset of sigma tropic shifts, okay? Now, depending on how many pericyclic reactions you've had to learn at this point, you might have a lot of different actions in your head. It really depends on how much your professor is putting on you right now, if they want you to just know Cope or if they want you to know a bunch of them. But I'm here to tell you that it's actually very easy to distinguish the Cope rearrangement from a lot of other different types of pericyclic reactions because this is one of the few that happens without any conjugation at all. Notice that my first molecule, the thing that I'm starting with, is not conjugated. This is not a typical diene. This is, I mean, it's a diene, but it's an isolated diene. It's not a conjugated diene. So when you see this type of rearrangement happening and it doesn't have a conjugated beginning point, the beginning point is not conjugated, and it's hydrocarbons, you know that it's a Cope rearrangement. So I'm just trying to give you some clarity on how to think about recognizing this. Also, just so you guys know, this molecule, the starting reactant, may require some rotation to visualize the [3,3] location, meaning that right now I have it very conveniently aligned for you so that it's very easy to visualize but sometimes your homework or your professor could give it to you in a way that's like linear, and you're going to have to kind of rotate it yourself to visualize what the resulting mechanism would look like. Okay. Cool. So why is it called a [3,3]? Let's just go over this one more time. We have a bond breaking here between the ones. We have a bond that's being formed between the threes. So if you count it around, that means that you're forming a new bond between the [3,3]. Once again, this is hydrocarbons only, so it's called a Cope rearrangement. Also, I just want to remind you guys of the mechanism. The mechanism would just be something, there are multiple ways you could draw it but just something that makes sense where you're breaking a bond and you're making a new bond. So what I would draw is something like this. Awesome. So that being said, let’s go ahead and do this example. So provide the mechanism and final product for the following reaction. So notice here, guys, that I'm given an isolated diene that's only hydrocarbons, but it's not lined up in a way that's easy for me to react because it's written out in a linear structure. So, like I said before, I can even decide what this is, let's try rotating it so they can face each other and so we can get a better idea of what we're looking at. So what I’m going to try to do, here and maybe in this space right here, is I’m going to redraw the molecule in such a way so that I can see what it looks like. So let's go ahead and draw it. This can just be a circle. Actually, let's just draw it anyway. Cool. And then what I'm going to draw is I’m going to draw this double bond facing the same direction but then everything else wrapping underneath it. So I’m going to put this single bond here. Now instead of the next double bond going up I’m going to draw it down. Instead of the next one going like off to the left, off to the right I’m going to put it to the left and then there appears to be one more double bond that I can face this way. Cool, and now I have something that I can look at, in fact, I drew it too small. I mean, I can work with it but let’s make it a little bigger so it’s easier to look at. Cool. Awesome. So now that we have this molecule that's rotated correctly, we can think, is this like what type of reaction is this? Well, it’s not conjugated. It's an isolated diene, so there are really no other pericyclic reactions that could happen here. It has to be a sigmatropic shift. And specifically, it’s, there are only hydrocarbons involved so this looks like it’s going to be a Cope rearrangement which is a [3,3]. So let’s go ahead and, draw the mechanism and then provide the product. So the mechanism would be that I break the bond and make a new double bond, then this double bond comes and I make a new single bond and then this one comes around as well. So what this is going to give me is a new compound that looks like this where now at the bottom what I have is a double bond here, a single bond here, a single bond here, and a double bond here. Cool? Just so you know, the final product here is actually the same exact product that we started with, okay? Because the fact this is a very it happened to be a very simple Cope arrangement where there were not a lot of substituents, so the end product turned out to be the same exact thing that we started off with. That’s totally fine, that happens with sigma tropic shifts sometimes. So just you know, just so you are aware, if you ever get the same product, be sure to be careful but it’s okay. That happens with sigma tropic shifts times. Okay? So that is our product and once again we already know it's a Cope rearrangement, but if we had to name it, the way we would name it is by counting this is the 1, this is the 2, and this is the 3, and then realizing this is going to be a [3,3] Cope rearrangement. Awesome. So that's it for this concept and example. Let’s see if you guys can do the practice problem yourselves.
16. Conjugated Systems
Cope Rearrangement
16. Conjugated Systems
Cope Rearrangement - Online Tutor, Practice Problems & Exam Prep
Ready to learn a specific type of sigmatropic shift? The cope rearrangement can be differentiated from other pericyclic reactions due to its lack of conjugation.Â
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concept
Definition of Cope Rearrangement
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Problem
ProblemProvide the mechanism and final product for the following reaction
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PRACTICE PROBLEMS AND ACTIVITIES (4)
- (•) Predict the product of the Cope reactions shown. (a)
- (•) Predict the product of the Cope reactions shown. (c)
- Predict the product of the following sigmatropic rearrangements. Be sure to rationalize the stereochemical out...
- Predict the product of the following sigmatropic rearrangements. Be sure to rationalize the stereochemical out...