This is the last of three ways to add alcohol to a double bond. This reaction creates alcohols that are much different from the first two methods, so pay attention to the differences!
General properties of hydroboration-oxidation.
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Hey, guys. So just to catch you up now, we've learned to different addition reactions that both add alcohol to double bonds. The first one was hydration. And if you remember, that was a Markov Nick. Aww, alcohol that could rearrange. Then we had Oxy Mercure ation. Remember that Oxy Merck was also a mark ogni cough alcohol, but it couldn't rearrange because it didn't have a car mechanic. Intermediate. Now what that's gonna do is that's gonna bring us to a third way to add alcohol towards old Bond. But this is just gonna be weird. It's just gonna have a lot of differences from the other two. And that's why it's gonna be really useful to us, because it's a very unique reaction. The name of this reaction is hydro beret shin oxidation. So let's go ahead and just learn the general features first. Okay, so first of all, the intermediate for hydro operation oxidation is not gonna be a cargo cat, and it's actually gonna be its own thing. It's gonna be what we call a four member concerted intermediate. Okay, now, I know that already sounds terrible. Okay, I'm gonna explain to us how to draw it, but it is kind of weird. Is very unique. Looking intermediate. Okay, by the way, this isn't considered one more thing. It's not considered an immediate. It's actually considered a transition state. Okay, so that's another thing that you want to keep in mind. It's actually not an intermediate. It's a transition state. Okay, now, the stereo chemistry for this is going to be very unique. It's actually going to be sin addition. Okay, now, remember that I told you guys for Oxy Merck, that anti edition mint that you get trans products. Well, it's the same kind of thing for Sin Edition. All Sin Edition means is that you're going to get sis products. Okay? And once I show you the mechanism, that will make more sense, OK? And then finally, what's the product gonna be? Why? I already told you. You're taking into a bond and you're making an alcohol out of it. So that part hasn't changed. The only thing is that the stereo chemistry is changing and the intermediate or the transition state is changing. Okay, What else? Well, can this rearrange? Remember that? What kind of intermediates like to rearrange carbon Catalans? Do I have one? No. So we're not gonna get any rearrangements here, Okay? Finally. This is probably the most interesting part of hydro gration. Hydration is gonna be one of the only one of only two reactions were gonna learn or go one that are anti Markov makov. Okay, so what that means is that it's gonna prefer to add my alcohol to the least substituted position on. That's really Onley. Gonna make sense one to explain the mechanism. Okay, But the reason this is important is because later on, when we get into synthesis, synthesis is all about taking molecules and making them into what you want. So, having a reaction, that anti mark is gonna allow me to add two branches, It's gonna allow me to add two things that are on the peripheral or on the outside of the molecule, instead of the things that towards the center. So you're going to see what I mean by that later? But this is a very important reaction. Just keep that in mind. All right? So let's go ahead and look at the general product of this reaction. I have that double bond that I've been using forever. Same double bond and notice that I'm adding some interesting re agents. What I'm adding here is bh three or says other boron source. Okay, like Oxy, Mercury ation. This is also gonna be a two part reaction. We're in the first part. I'm gonna add my boron, and that's gonna be the part that I call Hydro Beret Shin Hydro aberration. The word bore comes from the word boron. So when I see that, I immediately no. Oh, there should be a boron in this reaction. Okay? And that's the way that you can recognize it. Kind of like we used for Oxy. Merck that had a Mercury Hydro aberration has a boron. Okay, Now, the thing that is a little bit complicated about hydro Beret Shin is that different textbooks and different professors might have their own source of boron that they want to use. Okay, So most typically that source of born is gonna be H three or another one. That's very common is B two h six. Okay, now you'll What you'll notice is that the empirical formula of B two h six is the same as BH three. That's just a dime. Er, that just means you have to be h threes together. And it makes be two h six. All right, if you see those things over the same thing but some professors to get a little bit creative with this and they'll use some other re agents. So some other ones that I've seen that you should just be on the lookout for in case you have any online homework or in case you just want to look this up with an online resource is one of them is cat ical boring. This actually looks like this thing right here. Okay? You don't even you don't need to know how to draw it. Just recognize that cata cold boring is one of the one of the regions that is used for this. Now, I said you don't need to draw unless this is one your professor uses. If your professor is always using cata cool, boring, then obviously you should learn how to draw that. Okay, Another one is a molecule called nine bbn, which honestly, I'm not going to draw. But it is a molecule that has boron on it. And then finally, any molecule that has the molecular formula are to be H Okay, so that just means any source of boron that has a boron with to our group sticking off of it. Okay, all of these could be used for the hydro operation step. The one that you're gonna use is just go to class and just make sure that you know which one you're Professor talked about in class. Okay, so that's the hydro operation step. Now, we're gonna go to the second step after we hydro Boeri. We're gonna oxidize, okay? And we're gonna use hydrogen peroxide with a base as our oxidative oxidizing step. Okay, so we don't know a whole lot about oxidation reduction yet, so I'm just gonna leave it right there for now. Now check out what my end products are. What I've got now is that even without knowing the mechanism, I can predict with my products and look like Remember that we said that I'm going to get alcohol. That's Ante Markov, niqab and that sin. So what that means is that my alcohol should go down here to the less substituted carbon, and it should be sis to the hydrogen that it adds. Okay, So notice that, sis, notice that right here I had a metal group. Okay, if that metal group, the reason that that method group is being faced up towards the wedge here is because on the double bond, I added to things. Remember that every distraction adds to single bonds to the dole Bon. I'm gonna get a H on one side and an O. H on the other. And those two need to be cyst to each other because it's sin addition. So that's why I've drawn it like this. Okay, so does that make sense so far? Basically got Ante Markov Nick off alcohol that has Sin Edition, all right?
Acid-catalyzed hydroboration-oxidation mechanism
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So now we've got nothing left to do than to just go through the mechanism. And this is the part that's a little bit interesting. So let's go ahead and get started. Something that I told you guys that's very unique about Boron early in the semester. Actually, in the very first, in the very first chapter, I talked about the way Boron looks, okay. And something that's unique. All boron is that it has an empty P orbital empty p orbital since as the empty P orbital, that's gonna make it really good at doing what? Just remember, it's gonna be an amazing electron pair. Except, er In fact, BH three is an extremely strong Lewis acid. Okay, remember that the definition off a Lewis acid is a good electron pair, except, er okay, remember that Bronson Laurie acid meant that I don't donate protons. Lewis means that I accept electrons. This is a really good electron pair, except er So when my double bond sees that empty P orbital, it's gonna be like, Hey, I want to give my electrons to that p orbital. Okay, but my boron is gonna have to different choices. My born is going to say, Well, could I go closer? Like the born Basically my dull bond is gonna attack the empty orbital and my born is gonna have two choices. Either it could go down here and basically make a bond to that carbon. Or it could go down here to the more substituted position, make a bond to that carbon. Okay? And it turns out that the one that is gonna prefer is gonna be the one that is the least Eric Lee hindered or the one that is the easiest to approach. So what that means is that my boron is actually gonna choose toe Orient. It's P orbital right underneath the least substituted carbon. Alright, What that means is that I'm going to get a transition state that looks like this where basically I got a partial bond to the B and might be h Q is there? Then I've got my h over here, and I'm gonna have a partial bond to my H and this is gonna have a partial bond there. Okay, so it's basically gonna happen. Let me just show you the mechanism Really, really quick is going to be a cycle ization reaction. So my BH three goes like this. This is my bh three and I have a double bond here, right? And that double one says, Okay, I'm gonna give my electrons to the empty orbital. And then this single bond says I'm gonna give my electrons to this bond right here. So what that does is it's gonna make a transition state that looks like this where now I have my metal group there, and I'm gonna have a partial bond to be partial bond to age, partial bond to carbon and then a partial double bond. So all of these bonds are being broken and created at the same time. Okay, so that's what my transition state looks like. Now, does that make sense? Kind of how the Dole bond donates its electrons to the B, and then the H donates its electrons to the bond that is breaking on the on the more substituted side. Okay, so that's why I get a transition state that looks like this. Now what you're gonna notice is that the B H two and H are on the same side of the ring there, sis. And the reason is because since it's making a ring. A ring can either be on the top or it could be on the bottom, but it can't be. Transit can't be like one of them is at the top, and one of them is that the one of them is at the top of one of them is at the bottom. That wouldn't make sense. Okay, so what that means is that that's why we get sin addition with hydro operation because of this four member intermediate on camera, this four member transition state. Okay, so is that making sense so far? Cool. So that's what my intermediate looks like. Now let's go to the oxidation step. So basically, what happens after the transition state is that these bonds fully formed. So what that means is that this bond fully forms and this bond fully forms giving me just a single bond to be H two on one side and a single bond to H on the other. Does that make sense? So basically, the transition state just showed when all the bonds were being broken and made at the same time. Now my oxidation step is gonna have the final hydro operation done at the end Okay, so now we're gonna do is we're gonna do the oxidation step. And it turns out that for this step, just like Oxy Merck, you don't need to know the mechanism. Okay? The reason is because the mechanism is really, really long. It goes through what's called a try borrow Esther. And it's just a very, very long mechanism that professors don't require you to draw the whole thing. Okay, so all you're gonna need to know is that you're going to use an oxidation. Agent oxidizing agent H H 202 to turn this into an alcohol. Okay. And the base is gonna help is well, all right. So what that means is that at the end, I'm gonna get something that looks like this. I'm going to get an alcohol in the least stable position. I'm sorry. You're in the least substituted position, and I'm going to get an h that assists to that. And then this is where my metal group would go. Okay? And if I were to draw this out in an actual plainer structure, what you would see is that it's gonna look like the one that we had up above Where Basically, what we have is an O. H. Towards the back and age towards the back. This has to do with this in addition. Okay? And if those were in the back, that means that my metal group must be going towards the front. And that means that this h must be going towards the front as well. Okay. And if you look at this and if you look at this product, appear that I drew does the same thing. Okay, so basically what I was just drawing was the entire mechanism off the top general reaction. Does that make sense? Now, keep in mind that this could have happened with any source of born. It didn't just have to you Ph three. The only difference would have been that I just have a different looking group in my intermediate In my transition state, I'm gonna have a slightly different looking group. But the bond is still gonna have that p orbital that coordinates with the double bond. Alright, so I hope this mechanism wasn't too confusing, but it is supposed to be a little bit hard. Okay, This is one of the trickier mechanisms that we deal with in this chapter. So I hope that you guys didn't get too freaked out by it. Let's go ahead and move on.
1. Electrophilic Addition
Indentify the mechanism of reaction and predict the product.
Note: @ 3:40 there should be an OH group, not BH2.:)