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TRANSCRIPT

This video is about Sigmatropic Rearrangements, which involve migration of a sigma bond around a system like this. In a Sigmatropic Rearrangement, overall, there's no change in the number of sigma or pi bonds. We need to look for structural features within the molecule on the left versus the molecule on the right to see which side the equilibrium may favor. But first, how do we name these reactions? If you follow the curly arrows through to the product, we can see that the bond is being broken here and it's being formed here. We can see that there's sort of two halves to the molecule. The first half, if we number that like so, we see that there's three atoms in that continuous set of atoms that make up the ring at the transition state. Similarly, there's three atoms on this side.

There's three atoms on either side of the sigma bond that's being formed and the sigma bond that's broken. That's called a [3,3]sigmatropic rearrangement. That particular reaction is called a Cope Rearrangement. How do we predict which side the equilibrium favors? It favors this compound because it has more substituted double bond. It's normally favorable for double bonds to have more alkyl and less hydrogen substituents. This molecule has a Mono substituted Alkene here and a Monosubstituted Alkene here. There's three hydrogens and one alkyl group. Whereas, over here, we have a Disubstituted Alkene and a Monosubstituted Alkene. This molecule is more stable because we have one Alkene with extra substitution.

Let's look at this example here. Once again, curly arrows, we can show this hydride migrating, move these pi electrons around the ring, and we get to our product. There's the sigma bond that's breaking, here's the sigma bond that is forming. On one side, the group that is migrating is only one atom in size. Whereas on the other side, we've got five contiguous atoms that are forming up the chain that is involved in the sigma bond that's breaking and the sigma bond that's forming. Forget the other substituents here, they wouldn't get counted. We just look at the atoms in that chain that is part of the transition state. So this is a [1,5]sigmatropic rearrangement.

It's a hydrogen that's moving so it can also call it a 1,5hydride shift. Now where does the equilibrium lie for this compound for this reaction? Once again it lies towards the right hand side. If we assume that the R group is some kind of alkyl group like a methyl group. Because here we've got a more highly substituted, double bond than we have in this molecule with, but two disubstituted whereas here we have a trisubstituted and a disubstituted. The key concept amongst these sorts of rearrangements, is that the more substituted alkene, are favored. And so we can use that general rule to predict which side that equilibrium will favor. That's the Cope arrangement.

A very important type of sigmatropic rearrangement is the Claisen rearrangement. Here is the simplest case. This involves a vinyl Allyl Ether. This is also a [3,3]sigmatropic rearrangement. This one lies towards the right hand side because carbonyl groups are particularly strong functional groups, particularly energetically favorable.

Therefore this lies over towards the right hand side where we have that carbonyl group. A particular example of a Claisen rearrangement that is really important is the rearrangement of phenyl allyl ethers. This is where we have a benzene ring connected to an allyl group through an oxygen atom. This can undergo exactly the same sort of mechanism, electrons move around in a circle like this. We've got the new bond being formed here, the old bond being broken there. This is also a [3,3]sigmatropic rearrangement. However, the product has this ketone in a very special arrangement. You can think of this as being not a good thing because we're losing our aromaticity of this benzene ring, but it's somewhat made up for by the fact that carbonyl groups particularly strong, the Claisen rearrangement for these sorts of compounds doesn't stop there because we know that ketones are um, they can tautomerize, to an enol form and there's always going to be that possibility.

Enol, keto tautomerism, so this is the keto form, and this is the enol form, however, it's a very special enol form. It's a phenol and phenols benefit from aromaticity. So this is going to be the major product by far at equilibrium between these two. Overall the Claisen rearrangement of a phenyl allyl ether you goes to give this initially, but you don't see that because it tautomerizes to give the ortho allyl phenol tautomerizes.