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Syn dihydroxylation

Reactions that add two hydroxyls to the same face of an alkene double bond as it's converted to a single bond. Created by Jay.

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Video transcript

In the last video, we saw how to add two OH groups on anti to each other. In this video, we'll see how to add them on the same side, or syn addition. So we start with our alkene and to our alkene, we add osmium tetroxide. So this is the OSO4 here. And we could also add water and tert-butanol. And what that does, is that forms your diol over here, adding your two OH groups on the same side for a syn addition. You don't have to use water and tert-butanol. There are several other ways to do it. I've seen hydrogen peroxide and aqueous bisulfide. So just do whatever your professor has for your class. So let's look at the mechanism and figure out why this is a syn addition of the OH. So as you can see over here, we have our alkene and our osmium tetroxide right up here. And in this mechanism, we're going to get a concerted six electron movement. So six electrons are going to move at the same time. So these electrons in here are going to bond to this carbon. That's going to push the electrons in this pi bond off to bond to this oxygen. And then the electrons in this bond are going to move into here onto the osmium. So let's look at what that would give us. So here we have our osmium right here. And then that's double bonded to these oxygens. And then down here, there used to be two bonds to this oxygen. Now there's only one, because the other electrons moved in here to form the bond between the oxygen and the carbon. I'll go ahead and put a wedge and a dash here for this guy. And then this carbon over here on the right has a wedge and a dash bonded to something else. And then it is now bonded to this oxygen on the right here, like that. And then our lone pair of electrons moved in here on the osmium. So let's color coordinate so you can follow along here. So these electrons in here were the ones that moved in here to form our bond. And the electrons in this bond right in here, those move to form this bond. And then finally, these electrons in here. These are the ones that are now a lone pair of electrons on the osmium, like that. So we formed an osmate ester right here. And we can hydrolyze our osmate ester with the addition of water. So if we have add some water to it, so H2O-- or like I said, there are several other things that you can add instead-- the water is going to hydrolyze this osmate ester and give you back your diol up here. It'll give you back this, with your two OH groups on the same side. And the reason why those two OH groups end up on the same side is because of this osmate ester intermediate. The way those two oxygens add-- the way these two oxygens add here-- they add on the same side. They add in a syn fashion. All right, so what's the other product we get? After this osmate ester is hydrolyzed, the osmium is going to be right here bonded to our two oxygens. And that's going to form an OH on either side here, like that. So if you're looking at oxidation states, so if we check out the oxidation state of osmium over here on the left-- so this osmium-- if you remember from general chemistry, doing your oxidation states, you'll see that the osmium is a plus 8 stage over here. So it's plus 8 over here on the left. And after the osmate ester is hydrolyzed, we get this structure over here. And again, from general chemistry, if you assign your oxidation states to that osmium, you're going to get a plus 6. So we have a plus 8 to plus 6. So a decrease or a reduction in the oxidation number. So osmium is reduced while those two carbons are being oxidized, while your alkene is oxidized to a diol. So that's a redox reaction, of course. Let's look at how you would do this in practice, because the problem with osmium is it's very toxic. It's also very expensive. So you don't want to use very much of it. You want to use as little of the osmium as you possibly can. So how do you get around the toxicity and also the cost? So let's look at cyclohexene. If we were to add osmium tetroxide to cyclohexene, it would be OHSO4. We want to add only a catalytic amount, the smallest amount that we absolutely need. So we're going to add a catalytic amount of this, just a very, very small amount of osmium tetroxide. And we're going to add something called NMO, which is N-methylmorpholine-N-oxide. And what this is going to do, is it's going to regenerate your OSO4 by oxidizing the osmium plus 6 back into osmium plus 8. So the NMO is going to take this one over here, the osmium that's in the plus 6, and it's going to oxidize it back into the plus 8 stage so that we can reuse our OSO4 in the reaction. And that way, we can use very, very small amounts of it. So that's what the presence of NMO does. All right. Our solvents-- we use water and tert-butanol, just like we did above. So I'll just write out the abbreviation this time, so TBUOH. And we're going to get a syn addition of OHs. So we're going to get an OHs that add on the same side. OH that adds on the same side. And you might think, "Oh, well, wouldn't we get another product here? I could draw two dashes instead of two wedges. Wouldn't that be a different product?" Well, in reality, these are the exact same molecule. So this is actually a meso compound. So there's a plane of symmetry right here. And you can superimpose these two molecules on each other. Therefore, they're the same thing. So we're only going to get one product for this reaction where we add our two OHs on the same side. Let's look at another way to achieve syn dihydroxylation here. So let's look at a different way to do it. In this case, we're going to use permanganate. So we take our alkene and then we take permanganate, MNO4 minus, so something like potassium permanganate. And usually you do this reaction in cold sodium hydroxide solution-- a cold aqueous solution of sodium hydroxide, so water's in there as well. And what you're going to get is a syn addition of your OHs. You're going to get your two OH groups adding on to the same side, just like we did before. And your other product would be MNO2. So you can follow this reaction pretty easily, because we know that MNO4 minus is purple. And the oxidation state of manganese on the left here is plus 7 when you assign your oxidation states. And over here on the right, it goes to plus 4. So there's a reduction. Manganese has been reduced while those two carbons have been oxidized for this redox reaction. MNO4 minus is purple. MNO2 is kind of a really, really dark brown kind of color. So when you do this reaction, it's pretty much instantaneous. Everything turns from purple to brown and you get your diol. And there's a fast way of figuring out oxidation, if something is an oxidation or a reduction. You could assign oxidation states like I did in the previous video, like I did in one of the earlier videos. Another way to do it is to just count the number of bonds to oxygen. So over here on the left, your alkene-- zero bonds of carbon to oxygen. Over here on the right, each of your two carbons has one bond of carbon to oxygen. So you can tell it's been oxidized that way. That's a fast way of doing it. Why is this a syn dihydroxylation? Why do our 2 Hs add on on the same side? Well, if we draw out the mechanism really fast, so we have MNO4 minus-- so let's draw our permanganate anion-- so here is the dot structure for our permanganate anion-- so negative charge on this oxygen, because it has three lone pairs of electrons around it. So if we can fit in those three lone pairs of electrons, so it's a very, very similar mechanism to before. We take our alkene. We're going to get a concerted six electron movement. So these electrons are going to move into here. These electrons are going to form a bond here, and these electrons are going to bond right here. So it's just like before. It's just like with the osmium. So we get our manganese and these two top oxygens don't do anything, so let's go ahead and leave them like that. This oxygen here gets bonded to a carbon, carbon, oxygen, like that. And then we have our wedge and our dash, like that. And then our lone pair of electrons and our manganese. So it's the exact same thing as before. We can hydrolyze it to get our diol like that. It's the exact same mechanism, which is why our two OHs add syn like that. The problem with using permanganate is permanganate is such a strong oxidizing agent. It's hard to stop it at this first oxidation. So if you do everything really cold, with ice, and then you stop your reaction immediately, it's not too hard to get your diol out. But if you heat things up or let things go for a long time, you won't be able to stop the oxidation at one oxidation. It'll keep going. So let's go ahead and draw what would happen if we added some heat to our oxidation with permanganate. So we add potassium permanganate. So KMNO4 and we add some heat to it this time. So what's going to happen if you add heat to it? It's not going to stop at the diol. It's going to keep oxidizing. So instead of one bond of carbon to oxygen, you're actually going to get two bonds of carbon to oxygen. So let me redraw my alkene here, and let me show you a clever way of doing alkene cleavage. So if I just make this a little bit longer than usual, here's my alkene. The result is going to cleave this bond between my two carbons. So I'm going to actually break it. That's alkene cleavage, so I'm going to go like that. I'll shorten it a little bit right here, like that. And I'm going to put oxygens in there. So to draw my product for this alkene cleavage, I'm going to put an oxygen here and an oxygen here. So there are now two bonds of carbon to oxygen, instead of one bond like we did up there. So it's been oxidized. And this could be a ketone. This could be a carboxylic acid. This could be CO2, and it all depends what kind of reaction that you're doing. So let's do a quick example. So let's start over here. Let's start with this as our starting reactant. So put a methyl group there, a hydrogen here, a methyl group here, and a methyl group here. So we add potassium permanganate and heat. And I'm going to go ahead and redraw my reactant, so we can do that little trick. So I'm going to redraw my reactant. This time I'm going to make these little bit skinnier. And I'll make this a CH3 and make this a hydrogen and put our two methyl groups over here, like that. So alkene cleavage, so you heat it up. You can't stop at the first oxidation. Permanganate is such a strong oxidizer, so it's going to keep on going, and we're going to get alkene cleavage. We're going to break the bond between our two carbons, and then each of these carbons is going to get an oxygen. So now there are two bonds of carbon to oxygen. So I'll go ahead and do that, put my oxygen in there, like that. And it doesn't even stop here. So the molecule on the left, our aldehyde here-- we haven't talked about aldehydes in this course-- but aldehydes are easily oxidized, especially in the presence of something like permanganate. So this aldehyde right here is going to get oxidized again. So we have acid aldehyde. It's going to get oxidized. And if we oxidize an aldehyde, two bonds of carbon to oxygen, we would oxidize to something that has three bonds of carbon to oxygen. So that's our three bonds of carbon to oxygen, like that. So acid aldehyde gets oxidized to acetic acid. So let's just emphasize that. Over here, two bonds of carbon to oxygen. Over here, three bonds of carbon to oxygen. So it's been oxidized. Once again, you could assign oxidation states. We just don't have time in this video to do that. So you're going to get acetic acid as one of your products. And then we had acetone over here as our other product. So alkene cleavage, this reaction isn't very useful, because of these side reactions. It's hard to stop the oxidation. So if your goal is to add two OH groups on the same side, it's probably a little bit better to use the osmium way of doing it, since the permanganate way is not quite as predictable in its products, because of over oxidation. So in the next video, we'll look at another alkene cleavage, ozonolysis.