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8.8 Claisen Condensation
Let's practice drawing mechanisms for aldol condensations.
Each of the following transformations has a mechanism.
An ester enolate is similar to a regular enolate in that it is nucleophilic and will attack a carbonyl group.
Claisen condensation is a reaction that takes place when an ester is attacked by an enolate.
This product looks very different from the a,b-unsaturated ketones obtained from aldol condensations.
We will see the parallel between the aldol and Claisen condensations when we explore the mechanism.
Both mechanisms are almost the same.
The Claisen condensation uses an alkoxide, and we will discuss the reason for this shortly.
Let's compare the mechanisms for now.
Both mechanisms are essentially the same.
The groups seem to come along for the ride in the Claisen condensation.
The two reactions are different.
We can use our golden rule to understand why.
The oxygen atom needs to be protonsated with a suitable source in order to re-form.
The product of a Claisen condensation looks very different from the product of an aldol condensation.
You can appreciate that these reactions are very similar when you understand the mechanisms.
We made an enolate in the first step.
We used a strong base.
We pointed out that we did not use hydroxide.
We used an alkoxide ion.
We might have seen a competing reaction if we had used hydroxide.
The carbonyl group could be formed again by expelling an alkoxide ion.
We use an alkoxide as our base to avoid this.
It doesn't matter which group gets expelled when the carbonyl group re-forms.
We can avoid unwanted reactions by using an alkoxide ion as the base.
We need to choose the alkoxide ion carefully.
There is a simple reason for this.
We could use ethoxide in this case.
We can avoid side reactions by doing that.
Now that we know what base to choose for a Claisen condensation, let's talk about another special role that the base plays.
The final product is a b-keto ester.
The reaction is performed under basic conditions.
We talked aboutstabilized enolate in the beginning of the chapter.
An alkoxide ion is less stable than this enolate.
It means that the reaction will favor the formation of the product.
This reaction toward the formation of products is pushed by the formation of this stabilizing enolate.
The Claisen condensation gives us a way to make b-keto esters.
There is a clever synthetic trick that you can perform with b-keto esters.
Let's make sure that we have mastered the Claisen condensation.
We are removing an alkoxy group from one of the esters.
The remaining fragments are joined together.
There is a small molecule liberated.
Predicting products is something we should practice.
We will master the mechanism when we come back.
To see the products of a Claisen condensation in an instant, you need to train your eyes.
You will need that skill to propose something.
In order to insert MeOH, we broke the molecule apart.
We have to decide which fragment gets the methoxy group and which fragment gets the protons.
The left fragment has alkoxy group, so it must be the one that gets the protons.
The two esters are the same.
We just need one kind of ester.
It is possible to achieve crossed Claisen condensations, just like we can achieve crossed aldol condensations, but we would have the same concerns as before.
Potential side reactions are something we would have to worry about.
When one of the esters has no alpha protons, a Claisen condensation will be more efficient.
Some of the problems below are the result of crossed Claisen condensations.
Keep an eye out for them.
The formation of this stabilizing anion is the driving force for the reaction and this deprotonation step is important.
That's why we need to show this step.
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