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Topic: Anhydride to Dione - Mechanism Question  (Read 1012 times)

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Offline gpli

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Anhydride to Dione - Mechanism Question
« on: March 04, 2020, 03:31:14 PM »
Hi folks,

I am having a hard time figuring out the mechanism of the following reaction. Can anybody help me here? Great thanks.
« Last Edit: March 04, 2020, 03:49:15 PM by gpli »

Offline OrganicDan96

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Re: Anhydride to Dione - Mechanism Question
« Reply #1 on: March 05, 2020, 05:29:06 AM »
could you show an attempt before we help you?

Offline kriggy

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Re: Anhydride to Dione - Mechanism Question
« Reply #2 on: March 05, 2020, 12:09:26 PM »
Its quite insterseting problem, probably involves some C-C bond cleavege which feels unlikely under the conditions

Offline gpli

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Re: Anhydride to Dione - Mechanism Question
« Reply #3 on: March 05, 2020, 12:47:19 PM »
could you show an attempt before we help you?

I only know the mechanism of the last decarboxylation step. It is an acid-assisted 6-member ring process.

Offline rolnor

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Re: Anhydride to Dione - Mechanism Question
« Reply #4 on: March 05, 2020, 01:17:36 PM »
Can it be something in the scheme thats wrong? Maybe it should be diethylmalonate in the first step?

Offline hollytara

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Re: Anhydride to Dione - Mechanism Question
« Reply #5 on: March 05, 2020, 02:17:01 PM »
Make the enolate between the two carbonyls

This attacks the anhydride to start....

Offline rolnor

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Re: Anhydride to Dione - Mechanism Question
« Reply #6 on: March 05, 2020, 03:45:47 PM »
And then hollytara, what happens next?

Offline hollytara

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Re: Anhydride to Dione - Mechanism Question
« Reply #7 on: March 05, 2020, 11:06:20 PM »
When you make an imide from a cyclic anhydride, you take an amine and make the amic acid - one half of ahydride is amide, the other half is carboxylic acid.  This is treated with acetic anhydride to re-cyclize to the imide - acetic anhydride acts to "dehydrate" and remove the elements of water from the amic acid. 

This is the same, but we are replacing the amine nucleophile with a carbon nucleophile that is based on enolate chemistry. 

Offline rolnor

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Re: Anhydride to Dione - Mechanism Question
« Reply #8 on: March 06, 2020, 03:54:21 AM »
But you need to loose a acetyl group in the first step?

Offline clarkstill

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Re: Anhydride to Dione - Mechanism Question
« Reply #9 on: March 06, 2020, 04:40:38 AM »
Presumably at some point (maybe the final step) there's a retro-Claisen condensation to lose the acetyl? That should be a favourable reaction, since the leaving group is an anion stabilized by the other three carbonyl groups. Not sure what the nucleophile would be, maybe acetate?

Offline rolnor

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Re: Anhydride to Dione - Mechanism Question
« Reply #10 on: March 06, 2020, 05:32:23 AM »
Yes, it has to be something exotic....

Offline pgk

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Re: Anhydride to Dione - Mechanism Question
« Reply #11 on: March 06, 2020, 01:01:10 PM »
Please, take a look at the attached file, below:
« Last Edit: March 06, 2020, 03:19:06 PM by pgk »

Offline rolnor

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Re: Anhydride to Dione - Mechanism Question
« Reply #12 on: March 06, 2020, 04:30:18 PM »
Thanks pgk, that really was something exotic, nice!

Offline hollytara

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Re: Anhydride to Dione - Mechanism Question
« Reply #13 on: March 06, 2020, 04:43:12 PM »
Had time to review old papers.... 

If this follows from the imidization reactions: 

Make enolate, this attacks anhydride.  Result is first product shown by pgk in attached file.

The acetic anhydride reacts with the carboxylate by anhydride exchange: the new carboxylate formed in the first product is now a mixed anhydride with an acetate group, and the other piece of acetic anhydride exits as the acetate ion. 

An enolate is formed again between the three carbonyls - this displaces the acetate of the mixed anhydride to re-form the 5 membered ring. 

So now the product is a 1,3-cyclopentanedione with a 2-acetyl group and a 2-carboxyethyl group. 

This does the retro-Claisen as suggested by clarkstill. 

The second reaction is acid hydrolysis to the acid, which (like acetoacetic acid) readily decarboxylates. 



Offline clarkstill

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Re: Anhydride to Dione - Mechanism Question
« Reply #14 on: March 09, 2020, 11:56:24 AM »
Please, take a look at the attached file, below:

Presumably you meant to show nucleophilic enolates rather than hydride attacks in your mechanism? Regardless, step 2 is not plausible.

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