Claisen Rearrangement

Concept: Concept: Definition of Claisen Rearrangement

7m
Video Transcript

Hey everyone. In this video we're going to discuss a specific type of sigmatropic shift called a claisen rearrangement, so that is what is the claisen rearrangement? Well, it's a heat activated heat activated 3, 3 sigmatropic shifts that involves in allyl ether, so this is interesting, it's a specific type of Sigmatropic shift that's a 3, 3 but specifically the reactant needs to have an allyl ether in the original functional group to then transform into a claisen arrangement, if you don't have that allyl ether you're sunk, it's not going to be a claisen rearrangement, okay? I will go ahead and help you identify that allyl ether in a second but let me just read through a few more points first about it. So, depending on how many pericyclic reactions you've had to learn at this point, you might be like kind of confused, how do I identify which type is which. Well, something that's easy about the claisen is that it's one of the few pericyclic reactions that does not begin with any form of conjugation, so notice that we have a diene here, right? But this diene is considered to be isolated because it's not next to each other, right? So, it's really the only pericyclic reaction that could possibly take place between an isolated diene with an allyl ether, if you think about it with those two things in mind you can easily determine this is going to be a claisen rearrangement. Now, something else to keep in mind is that right now I've drawn this molecule very nicely. So, it's very easy to kind of think mechanistically how it would form into this product because all bonds are close to each other but just so you know the way your professor or your homework may give you the problem it may require some Sigma bond rotation for you to get it to look nice and orderly before you do your mechanism. So, I'll show you a few examples later about how we're going to rotate it first into position before we can then draw the mechanism okay? Cool. So, let's go ahead and just identify first of all what do I mean by allyl ether. So, guys an allyl ether is a like I mean, let's just go all the way back to the beginning, what's an ether, an ether isn't R-O-R group ROR, I mean ether, and specifically an allyl ether would mean that your O is attached to a ch2, that's then attached to a double bond, okay? So, that's what an allyl group is, is this group right here, would be allyl, you should be familiar with allyl position by now because we use them a lot in organic chemistry but it means you're not directly attached to a double bond, you're one carbon away. So, what is the allyl ether in the original product, it's actually this, I'm sorry, and I meant reactant, and the original reactant it's this thing, the other thing on the top is actually part of a vinyl ether and that part actually could change, there are claisen rearrangements that happen with vinyl ethers, there are claisen rearrangements that happen with phenol ethers. So don't, you shouldn't worry too much about the top part because that part could change based on the specific type of claisen but the bottom part, that allyl part always has to be there, it always needs an allyl ether, so that's why, that's like the one defining characteristic of a claisen rearrangement. Now, once again, guys you guys should already be familiar with how to number on sigmatropic shifts in general but remember that it has to do with the bond that is being broken and the bond that is being made. So, in this case the bond that is obviously broken is the one between the ones because you can see over here it no longer exists, the bond that's being formed is the one between the three because now that's a new bond over here. So, we would then say that this is in the categories of a 3, 3 sigmatropic shift because a new bond has been created between the threes, cool? And lastly, what would this mechanism look like, in general terms, it would just be any mechanism you control that's pericyclic and concerted that is going to create a bond and break a bond. So, what I would do is something like this, something like, and something like that, cool? And there you have it, that is your final product. Now, guys it turns out that it can get a little bit more complicated than this though and that's because sometimes in your final product you're going to end up with having to decide which tautomer is the most favored because if you'll notice we actually have a carbonyl, we have a carbonyl as our end product here, okay? We have a carbonyl, and something that we learned about carbonyls a long time ago or I mean you've definitely come across it at some point in organic chemistry, is that carbonyls are able to tautomerize into enols. So, they're able to tautomerize into enols and this is, so this is called the keto and this is called the enol. Now, most of the time the keto, and this is called the enol, Now, most of the time the keto is favored, almost all of the time the keto is favored. So, you usually don't need to worry about it, usually you can just leave the product the way it is because ketos are favored but some specific molecules prefer the enol. So, what that means is that, when you come across a molecule that favors the enol position you must tautomerize to the enol in the last step, if you don't tautomerize the enol you get the question wrong because you picked the wrong tautomer, okay? So, what it says here is that in the final tautomerization step is required for molecules in which the enol form is favored, again, not too many molecules favor the enol form but if they do you must tautomerize to the enol, make sense? Cool. So, we're going to do in the next practice problem, in the next example is I'm going to remind you guys of some other properties of keto-enol tautomerism and we're going to pick the most favored one in equilibrium.

Example: Example: When is Enol Tautomer Favored?

4m

Example: Example: Predict the Product

6m