- Electrocyclic – Disrotatory ring closure/opening
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Electrocyclic – Disrotatory ring closure/opening
So, what that means is that this one's rotating to the right and this one is also rotating to the right, okay? Because they both rotate the same direction, what's going to end up happening is that the positives are going to overlap, does that make sense? So, orbitals rotate the same direction is conrotatory but what about a 6 pi system or 3 double bond system, if we were to try to make a ring out of this one, which is actually the example that we had at the top, that would actually be an example of this rotatory because in that case, when you draw the HOMO orbital, what you end up finding is that your orbitals are perfectly symmetric on both sides of the terminal ends.
So, if you rotate conrotatory they're actually going to get the opposites interacting.
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So, this rotatory rotation happens when they rotate in opposite directions, one is clockwise and one is counterclockwise to create that same type of overlap that would lead to a single bond, okay? Now, why is this important, it sounds like no matter what we can get an electrocyclic reaction to happen thermal, no matter what if these are going to rotate conrotatory or disrotatory, but what matters is the substituents because if you have any substituents located on the terminal ends, let's say here or here, let's say that I put an alcohol here, right?
Well, that's why you have to look at the rotation type, the only way you can predict it is to know the rotation type, so the focus of our electrocyclic problems is not actually going to be on forming the ring because that's the easy part, we're always going to assume that the ring conform the focus is going to be on the stereochemistry because if you can predict the stereochemistry accurately that means you understand the molecular orbital rotation that's occurring okay?
So, in the next video we're going to do an example including stereochemistry. Predict the product in the following electrocyclic reaction, label the reaction as either conrotatory or disrotatory. So, guys I just want to emphasize here that we know a reaction is going to take place no matter what, we know that we're going to get a mechanism that looks something like this, where, let's say this dual bond makes a single bond here and then this double bond goes here.
So, what we're going to expect to get is some kind of square, right? With a double bond here and with methyl groups here, does that make sense so far? So, to figure out if these are facing up or down, we're going to have to look at the molecular orbitals. So, where do I think is a good place to start, we're starting from scratch here, what do you think we should do first?
So, probably a good place to start is let's start drawing our molecular orbitals of a diene, so that means I have to draw four of four, right? So, 1, 2, 3, 4 it's not bad, I'm going to use my copy-paste feature and you guys can always pause the video, if you need to.
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So, now we can go ahead and draw in our molecular orbital names. So, we know, this is psi 1, psi 2, psi 3 and psi 4 and then we can shade these in, you guys should be pretty good at shading in dienes by now, why don't we just do this very quickly, it would be that all the bottom is filled, it would be that there's one node on the second one. So, then this is what it looks like because there's one node and the last one has to flip, for 3 and 4, the first one doesn't change, for 3 and 4 the back one keeps flipping then this would go down and then this would go up, cool?
And then finally, we need two nodes and three nodes, that means that these two go up here and then this one goes up and down, cool? So, those are our Mo's, we should then fill in with the number of pi electrons we have, which is 4, cool? So, now what do we do, what's the next step, this was important, we did need to do this but what's the next step, guys, we need to analyze the HOMO. So, that's a good thing, if the in terms of the frontier orbital, there's less to worry about it's just the HOMO, so the HOMO happens to be this orbital here and this is the one that we have to figure out if it's going to be conrotatory or disrotatory, okay?
Photochemical vs Thermal Electrocyclic REACTIONS
Now, the way that we tend, that we typically do, this is to actually try to do it in 3d. So, we can figure out, what it's going to look like at the end. So, what I'm going to try to do is I'm going to try to draw it like this with the orbitals facing up.
So, we can see how they interact with each other, okay? Something like that. So, let's go ahead and draw in our orbitals, we're going to draw 1, 2, 3, 4, does that make sense? So, what I'm trying to do is I'm trying to actually keep it similar to that one but now I'm going to be involving what we know about these orbitals here, let's go ahead and shape them in, let's say that on this psi right here, it's the beginning of my LUMO on the other side, so that means that what it would look like is shade the bottom, shade the bottom, shade the top and shape the top, cool?
Now, don't forget, we have our substituents that we need to include.
Marvell, Thermal Electrocyclic Reactions, 1e
So, where are our substituents going? Well, they should go on the inside. So, that means is that I'm going to draw these as methyls, I should draw a substituent going in here and a substituent going in here, does that make sense so far? Awesome guys. So, now we have to determine, if it's going to be conrotatory or disrotatory, what do you guys think?
Because notice that my lobes are in opposite ends.
So, what that means is they both have to rotate the same direction that means this one, let's say it has to rotate clockwise, this one also needs to rotate clockwise or if you pick counterclockwise they should both rotate counterclockwise. So, what that means for my product is that what it's actually going to look like is like this.
So, I have my, let's try to draw this in 3d still, in three dimensions. So, I have my new square my new cyclobutane, I know that I'm going to form a double bond here and then we have to look at with this rotation where would these groups go, where would the methyl groups go.
So, what I would see is that, let's start off with the first one, the first one at the bottom, it was a, it's going into the page but after I rotate it it's going to rotate down, right? It's going to go down because of the fact that, that thing is rotating clockwise, the way that I drew it it's rotating clockwise. So, that mean it's going to rotate down. So, I would expect this methyl group to go down like this, I hope that's making sense so far.
Now, the other one, that's facing this way, that's actually coming out of the page like, in fact, one way to write this could be that you would write this one into the page and this one out of the page because one is going into the page, one is going out of the page, right? So, then that one when it rotates it's actually going to face up, right? Because it's rotating clockwise then this methyl group would go up, which means that my product is actually going to be a trans dimethyl, where this one faces up and this one faces down.
Now, guys it turns out that this is actually in enantiomer, this is not a meso compound.
So, we should actually draw the other enantiomer as well, meaning that we actually get two products for this reaction, we get the two different trans pop products that are possible, which is this one and this one and you might be saying, we'll Johnny how would you get the other one? So, I guess now we just have to label it and the rotation turned out to be conrotatory, cool? So, guys we did it, great job, we got our products, just so you know, it's not always going to be two products, if they had been facing the same direction that would then maybe a meso compound because it has a plane of symmetry and then you just get one product but since there was a symmetry here we had to draw both products as our answer.
So, remember guys, when you're drawing electrocyclic you can't just draw them on the plane, you need to include stereochemistry because that's really, what your professor, what your homework is going to care about, making the ring is the easy part, the hard part is using HOMO-LUMO frontier orbitals to figure out the stereochemistry, great job, let's move on to the next video.
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vercmiginsi.cf Learn Additional Practice. Next Section Photochemical Electrocyclic Reactions. Mark as complete. Report issue. Transcript Now it's time to learn about a new type of pericyclic reaction called thermal electrocyclic reactions. ACS Catalysis , 6 9 , Accounts of Chemical Research , 49 6 , Qian Peng and Robert S. Accounts of Chemical Research , 49 5 , Donald A.