Concept: Concept: Birch Reduction Mechanism5m
Hey guys! In this video, we're going to talk about a specific type reduction reaction that can happen with benzene. That's called the birch reduction.
Let's just take a look at the general reaction for a second. What a birch reduction does is it combines elemental sodium within an amine and alcohol to turn a benzene into what we call an isolated diene. Specifically, if this were to happen with an unsubstituted benzene like we have here, our product would be an isolated cyclohexadiene, two double bonds that are far apart from each other in a 1,4 position on a cyclohexane.
If you take a closer look at these reagents, they might look familiar because these are very similar to the reagents that we use on a dissolving metal reduction. This is a reaction from Orgo 1 that we learned a long time ago that worked with alkynes. It was a radical mediated mechanism. It turns out that this mechanism is really the same exact mechanism, except it’s going to work with benzene instead of with an alkyne. Let's get right into it.
The mechanism for this reduction is going to proceed through elemental sodium which means it’s going to possess just one electron. When that one electron donates to any of the carbons, we're going to have to break a bond. But this is going to be a mechanism where we have a combination up half-headed arrows and normal arrows just like the dissolving metal reduction, how there were some arrows that moved one radical and some arrows that moved a lone pair. When we make that bond, we have to break this bond in order to make room for the radical.
In order to keep these charges as far away from each other as possible or these intermediates as far away from each other as possible, this double bond is going to ionize into a lone pair on to the very bottom. Basically the furthest position possible from the radical, we're going to get an anion. Let’s go ahead and draw the product of this first step. What we're now going to get is a single radical at the top, double bonds on both sides and now a lone pair at the bottom which is going to be a carbanion.
This intermediate is called a radical anion which makes sense because that's what it is. It's a radical and it’s an anion. This is where our ethanol comes in. Our ethanol is going to serve as a propagating agent. Just so you know, ethanol isn’t the only alcohol you can use. Some text use tert-butanol. It doesn't matter. It’s the source of hydrogen. That's the biggest deal.
EtO-H my anion is going to grab the H and give a negative charge. What I’m going to get is a molecule that looks like this. I got my two double bonds. I still have my radical. But now I have two H’s at the bottom because I have one originally and now I just added a second which is the one that came from the ethanol.
At this point, I react with another equivalent of my elemental sodium. That elemental sodium is going to donate electrons to that same location. Now I'm going to get a lone pair anion. This is just a carbanion intermediate. This reaction just repeats itself. That's one thing about maybe dissolving metal reduction if your recall. It was the same thing twice.
Here, we would react again with another equivalent of ethanol. We would wind up getting our isolated diene because now I’ve got two H’s in the bottom. I've got two H’s on the top. I’ve got my isolated diene which is this molecule here.
For this reason and the fact that it reacts twice, sometimes you might see professors actually write ethanol times two or alcohol times two. It doesn't matter. It’s just going to have enough equivalents to make the reaction go to completion.
That's really it. That's the mechanism for Birch reduction. Now what we're going to do is we’re going to talk about specific regiochemistry that you have to consider with a Birch reduction.
Concept: Concept: Regiospecific products3m
Problem: Predict the major product from the Birch Reduction2m
Problem: Which is NOT a possible product of the reaction?2m
Predict the product of the following reaction.
Which species is reduced in the following Birch reduction?
a. None of them
Predict the major product for the following reaction paying attention to the regio- and stereochemistry.
Consider the detailed mechanism of the following reaction. Choose the structure below that is not a possible intermediate in the mechanism.
The following highly selective reaction featured in a synthesis of morphine by Rice and co-workers. Provide a complete mechanism.