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Author: Subject: Enolate Alkylation
Filemon
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[*] posted on 19-7-2006 at 08:15
Enolate Alkylation


An enolate preparation needs low temperatures. Why? if it was made to bigger temperatures that it would happen?
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Nicodem
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[*] posted on 19-7-2006 at 09:32


Enolate anions add to the carbonyl groups so a self addition or condensation of the ketone itself is the only thing that happens when adding a strong base to the ketone at room temperature.
At low enough temperatures most such reactions are extremelly slow except for the proton transfers which can still go on. Once all the ketone is enolized and deprotonated it won't add to the carbonyl groups simply because there are no more carbonyl groups around. Thus you must use a base in at least equimolar ratio and several pKa units more basic than the enol (LDA, NaH etc.) or else there will be some ketone present due to the low equilibrium constant.




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Nick F
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[*] posted on 19-7-2006 at 11:21


Well...

Low temperatures and strong bases will favour the kinetic enolate, ie, deprotonation from the least hindered alpha proton. The kinetic product is the one which for whatever reason forms fastest, and since a strong base is used here there can be no equilibrium which could allow the formation of the more stable thermodynamically-favoured product. For example, treating butanone with LDA at -78*C will deprotonate the terminal alpha carbon, since this has three enolisable protons compared to just two at the other alpha position, and so deprotonation at this site is faster.
Higher temperatures and weaker bases, which allow an equilibrium to be established, will favour the thermodynamic enolate equivalent, ie, deprotonation at the most hindered possible site. For example, 2-methylcyclohexanone with trimethylsilyl chloride and DABCO in DMF at 100*C will produce ~78% of the thermodynamically prefered trimethylsilyl enol ether from deprotonation at the tertiary carbon, which can be seperated from the kinetic product by fractional distilation. Weaker bases, to produce the naked enolate, will cause self-condensation as mentioned above.

Enolate chemistry is actually quite rich in detail and very useful, have a read through it if you have a good organic chemistry textbook.

[Edited on 19-7-2006 by Nick F]
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Filemon
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[*] posted on 23-7-2006 at 15:51


t-BuONa is sufficiently strong?
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sparkgap
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[*] posted on 23-7-2006 at 18:15


Compare the pKa's and you can figure it out from there. ;)

sparky (~_~)




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[*] posted on 24-7-2006 at 02:45


Where I can find it? I have not found it in internet.
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Nicodem
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[*] posted on 24-7-2006 at 03:07


Try harder! It's there.



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