In this reaction, we learn how to use certain agents to add diols to a double bond. This is also known as the 1,2-syn diols reaction.
Concept: General properties of syn vicinal dihydroxylation.5m
So now I'm moving on to yet another addition reaction and this addition reaction is going to be one that adds two alcohols to the same double bond and it's going to do it all in one step. This reaction is called syn vicinal dihydroxylation. So how does this work?
Basically, we have these two very special reagents. We have potassium permanganate and we have osmium tetraoxide, this is KMnO4 and OsO4, that are both highly capable of adding oxygens to double bonds.
Now it turns out that this mechanism, you're not going to need to know the whole thing, but what you do need to know is you need to know what the end product is going to look like. As you can see, it's really easy. All we have is a double bond. We add one of these two reagents. It doesn't matter which one. Sometimes you're going to see catalytic NMO. That's just a catalysis that sometimes comes up, sometimes it doesn't. Don't worry too much about it.
And what you're going to get at the end is what we call diols. Why? Because there's two alcohols on the same molecule. They have a vicinal relationship. What's vicinal mean? It just means that they're next to each other. They have a one-two relationship. And they are syn to each other or they're cis. That means that the reaction is syn.
So how does this actually work? The reason this works is because potassium permanganate and osmium tetraoxide both have a very, very similar structure where it's basically one central atom surrounded with as many oxygens as possible.
Now the way that I like to think of this, maybe just to make it really kind of fun, is I kind of visualize that these look like spaceships. So this is like a flying saucer. And there's a flying saucer. And there's a little cockpit here with an alien inside. But of course, he's got four arms because aliens don't have the same amount of arms as we do. And whatever.
We've got these UFOs and what do they do? Well, they fly down to earth and they go attacking double bonds. So the UFO is like coming. It's swooping down on the double bond. And it decides I'm going to leave humans a gift. I'm going to leave this double bond a gift. What it's going to do is it's just going to add an oxygen to both sides of the double bond. It's basically going to take one of the oxygens here, one of the oxygens there and add them to both sides leaving a diol at the end.
Please, do not tell your professor that I just told you that. I sound like a retard telling you this mechanism where there's like a spaceship and it's abducting cows or whatever. I don't know. I'm just making shit up at this point. But I'm just trying to help you guys remember because I know there's been a lot of reagents today, why these reagents are special and how you could think of what they do. So they're going to leave two alcohols behind and they're going to take off back into space.
Now if you do want to know the mechanism really quick. I am going to show you the first step. You don't need to know the whole thing. But it's going to look like this, where if you have a double bond and you have let's say we're using osmium tetraoxide, so a double bond. What's going to happen is you're going to get a cyclization reaction where my – and there's a methyl there. I'm sorry. Where my double bond grabs one of the O's. These electrons go down onto the osmium and then these electrons go and make a bond to the double bond.
So what winds up happening is that we get the cyclization reaction, you wind up getting two alcohols at the end and then you're OsO4, obviously it's missing two oxygens. That thing just leaves and it goes back into space. So you might need to know the first step, but regardless, you're never going to need to know the whole mechanism, at least I've never seen that. And I've been teaching orgo for a very long time at a lot of universities.
So let's go ahead and do a multi-step reaction here. And what I want to do is just literally have you guys go ahead and try to figure this out by yourself and then I'll go ahead and jump in.
You don’t need to know this entire mechanism, but I would suggest knowing the first step:
Example: Predict the product for the following multi-step reaction.5m
Provide the major product for the alkene reaction below.
Consider the strucutres below and answer the following questions.
d. Which comopunds will each form a pair of enantiomers by reaction with OsO4 / NaHSO3 ?
Give the product, or products, including stereochemistry of the reaction of (Z)-3-methyl-2-pentene with the reagent below. If the products are a pair of enantiomers, you need to draw only one and state that the other enantiomer is formed.
OsO4 / NaHSO3
Suggest a synthetic scheme for the following transformation:
What is the major product of the reaction drawn below? For clarity, the other enantiomer was not drawn for those labeled as racemic.
What will be the major product of the following reaction? Pay careful attention to the stereochemistry of the product.
Draw the organic product of the following reaction, being mindful of stereochemistry. If the reaction forms two enantiomers, draw one of the two enantiomers.