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Topic: Knoevenagel mechanism doubt  (Read 241 times)

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Offline xshadow

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Knoevenagel mechanism doubt
« on: March 13, 2021, 12:52:00 PM »
Hi

I've a doubt about the Knoevenagel mechanism, in the Verley Doebner variation

Mechanism:  ( the one at page 8 of the file)
https://www.google.com/url?sa=i&url=https%3A%2F%2Fwww.tandfonline.com%2Fdoi%2Fpdf%2F10.1080%2F17518253.2020.1851398&psig=AOvVaw3rl58-58X9yq58v1tpSPsG&ust=1615743994318000&source=images&cd=vfe&ved=0CAIQjRxqFwoTCNCOtMzpre8CFQAAAAAdAAAAABAD



Don't understand why the enolate attack the imine and not for example the C=O of the carboxilic group...
Is the imine more electrophilic??

thanks

Offline rolnor

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Re: Knoevenagel mechanism doubt
« Reply #1 on: March 13, 2021, 03:38:25 PM »
The imine is protonated so its very reactive, a carboxylic acid is less reactive.

Offline xshadow

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Re: Knoevenagel mechanism doubt
« Reply #2 on: March 14, 2021, 08:21:00 AM »
The imine is protonated so its very reactive, a carboxylic acid is less reactive.

Ok thanks!!

One last thing:

usually I know that aldehydes are more reactive than ketones that are more reactive than ester that are more reactive than amides

Now where imines are located ?
Are imines more reactive than ketone towards reaction with nucleophile (for example LiAlH4 or NaBH4)??
THANKS

Offline rolnor

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Re: Knoevenagel mechanism doubt
« Reply #3 on: March 14, 2021, 01:31:50 PM »
No, non-protonated imines are less reactive.  The negative charge after addition will be on the nitrogenatom and that is not ideal I think. I am not sure if you can reduce ketones selective when an imine is in the molecule but I think so. If you have an protonated imine this will be more easily reduced by NaBH3CN than a aldehyde, thats why you can do reductive amination with this reagent. It has been debated if reductive amination can be proceed via the hemiaminal and not the imine so, I am not 100% sure about this either.
https://en.wikipedia.org/wiki/Reductive_amination

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