Silyl Ether Protecting Groups

Alcohols are so reactive that sometimes we want to react with another part of the molecule without actually interfering with the alcohol and this is a situation where you want to use a protecting group, OK? So now I want to talk about a type of protecting group called Silyl Ether protecting group, OK? So just you guys know there are two main ways to protect alcohols, you can use a t butyl ether protect group or a Silyl Ether protecting group you may have to learn one or both of these for your professor I'm only going to include the ones that your professor needs, OK? So if you're watching this video and fits in your playlist that means that your professor want you to know this one, alright? So Silyl Ethers I know they sound weird but it just means that it's something made with silicon, OK? It's a molecule made of silicon It looks a lot like an ether except instead of an O it's going to have an SI a silicon, OK? And these are used to protect, remember that the definition of protecting group is something that you can put on the molecule, protect and then take it off later, OK? So just so you guys know there's this weird reagent called TBDMS, OK? TBDMS you absolutely do not need to know what it stands for but you do need to know is that the most common Silyl chloride used in organic chemistry 1 to make a Silyl ether, OK? So just so you guys know this is the structure, TBDMS, OK? And I would advise knowing what the structure looks like, why? Because you need to know the mechanism for this, OK? So how does this work? Well basically what I want to do is let's say that I'm trying to get Alkyl halide, OK? But we know that some reagents that react with alkyl halides also react with alcohols, OK? For example strong bases, strong bases can do elimination on alkyl halide but they can also deprotonate an alcohol so how do we protect that? Well what we could do is you could you expose the alcohol to a Silyl chloride what's going to wind up happening is that this silicon has a pretty strong dipole pulling away from it so there's going to be a partial positive charge right there, you can use your alcohol to attack that silicon but now Silicon just it's like carbon it's actually right under carbon in the periodic table so silicon wants to have 4 bonds, OK? Right now by adding that bond to silicon we're making five so if we make a bond we have to break a bond is there an easy bond here to break? Hell yeah, we can break off the chlorine and cause it to kick out as a leaving group what that's going to give us is a molecule that looks like this where nothing's happened to my Alkyl Halide yet but now I have O-SI with the two methyl groups and the tert butyl group, cool so far? OK We've also got the H that's still present and a positive charge, OK? What we can now do is we can use the chlorine that got kicked off to deprotonate, OK? And what we're going to wind up getting HCl, OK? Because of the fact that we are deprotonating with the CL and we're going to get our protected alcohol, OK? So we got O then you got SI methyl methyl T butyl, OK? Now the reason that this is hopeful is because now if I were to expose this to a region that reacts normally with alcohol this thing will not react with alcohol this is unreactive which is the entire point of a Silyl ether this thing isn't going to do anything, OK? It's not going to react to anything so what that means is that now in the next step there's another step that we're going to skip here we could do whatever we want to this part of the molecule, OK? We could react this with a strong base, we could react it with I don't know a nucleophile or whatever and we would only react with this...Only reacts with the target functional groups and it would not react with the Silyl ether.

So now that's great we can do whatever we want to the alkyl halide but what happens when we want to get that alcohol back? Because that's the whole point of protecting group is you want to make sure you can take the silicon off and get that H back on, OK? Well then, we're going to use another reagent and that reagent is kind of weird it's a nitrogen with butyl groups, OK? So just if you guys want to see what that looks like it would be a nitrogen with four butyl groups, OK? 1 2 3 4, OK? That's going to give it a positive charge and then you're going to have a negative F associated with that, OK? And what's going to happen is that the fluorine from that molecule can wind up kicking out so this is after I've done my reaction to the bromine, OK? Now I'm in my last step I'm going to deprotect, this is the protecting step this is protect 1 is protect then 2 is react, you do your actual reaction, OK? Then 3 is deprotect and when you deprotect the way that the mechanism goes for that is that your F negative is going to wind up coming over here and attacking that silicon, OK? And then kicking out the O because the fact that you can't make that many bonds to silicon you can only react 4 so now we're going to get is something that looks like this where now I still own you know let's say that I reacted with that with that Alkyl halide and I did some kind of SN2 so now I have a nucleophile here, OK? Notice that all I'm doing is I'm saying that some kind of reaction took place where I had a bromine before and now I have something else, OK?

Well now in this last step what I would get is O negative, OK? So what can we do to make an alcohol? Remember there's HCl around, where did that HCl come from? The HCl came from the Cl earlier that deprotonated the H so now my alcohol my OH...I'm sorry my O negative is going to deprotonate HCL and what I get as my final product is my alcohol once again, OK? Now the reason this is so helpful is because now notice that I can target a specific functional group if I wouldn't have protected and deprotected then maybe my nucleophile would have actually reacted with the OH, OK? But since I protected it with a Silyl ether now I can do whatever I want to the other part of the molecule and then I can deprotect it and get that alcohol back, OK? So it's very useful reaction it's one that your professor may want you to know, OK? So what I want you guys to do now is try to do this yourself draw the whole mechanism and try to predict what's going to happen here notice that it's really the same thing we're going to protect, what are you protecting? The alcohol, right? Then you got to react this is up to you have to figure out what that reaction is and then you're going to deprotect, OK? What I'm interested in is that you can draw that mechanism and figure out what the final product is, alright? So go ahead and try to do that and then I'll jump in and I'll show you guys how solve it.