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Author: Subject: reductive amination using ammonium acetate/NaBH4
lullu
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[*] posted on 22-7-2012 at 11:24
reductive amination using ammonium acetate/NaBH4


Anyone here tried forming the imine of a ketone in anhydr. MeOH with ammonium acetate following the reduction via NaBH4? I think this should work quite well with dried and purified ketone/solvents or do I miss something here?
When using silica gel to absorb the water will there be a problem to let it stay while adding NaBH4?


I cant find any references on it as sodium cyanoborohydride is obviously the way to go.
Maybe someone tried it or knows a ref I did not find.

best regards
lullu

[Edited on 22-7-2012 by lullu]
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lullu
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[*] posted on 23-7-2012 at 06:17


To make myself a bit clearer I wonder about how to form the imine correctly.
Because molecular sieves and silica gel will both absorb NH3 which would then require ammonium acetate in excess.

But this would lead to an acetic reaction enviroment which has maybe positive influence on the reaction speed but it will maybe also need more NaBH4 because on lower pH NaBH4 is not stable.

Will the absorbing of NH3 by the drying agent be an issue?
Is there a optimal pH so I can measure the ratio correctly without needing too much NaBH4?
Should I just use dryed NH3 rather than ammonium acetate?

thanks
lullu
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xwinorb
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[*] posted on 23-7-2012 at 12:03
With ammonium acetate


My suggestion to you :

Forget the dessicant.

Workup :

Add about same ammount of water, MAKE IT BASIC, then extract in toluene and then gas or titrate. Remember, here you will have the acetate salt.

It works, but yields are typically low ( around 20 % ).

I would suggest also, posts like yours are better in the Begginings section.

Review the theory and recipe you have as carefully as you can.


Good luck.


xw.
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lullu
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[*] posted on 24-7-2012 at 00:01


Excuse me but did you even read the thread?
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DJF90
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[*] posted on 24-7-2012 at 00:42


I've got some advice for you. Generally speaking with regards to imine formation, stirring the ketone and amine together overnight in methanol with excess (5 eq.) MgSO4 (anhydrous) is generally quantitative. While cyanoborohydride is traditionally used for reductive amination with in situ formation of the imine, triacetoxyborohydride is a more modern approach without the need to mess about with cyanide bearing liquors in the workup. I've used this reagent myself with various anilines and benzaldehyde, affording some reasonable yields (the anilines featured de-activating ring substituents, and DCM was used as solvent. DCE is supposedly alot better). You may find the attached paper an interesting read.

Attachment: STAB reductive amination.pdf (412kB)
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lullu
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[*] posted on 24-7-2012 at 02:23


Thanks DJF90,
this STAB really looks promising except the preparation which seems to take its time to
not risking decomposition.
Have you prepared it in situ or did you isolate it first?
Any experience in storing it regarding decomposition?

I might give it a try in ethylacetate and compare it with NaBH4/imine
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DJF90
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[*] posted on 24-7-2012 at 02:40


I used STAB that was bought in from Aldrich (when I was at an institution). It appears to keep fairly well, just try and keep it dry and in a tightly closed bottle. As far as I recall, I've seen a preparation detailing the addition of glacial acetic acid to a suspension of sodium borohydride in benzene. Due to the large quantities of hydrogen gas given off, it might be wise to vent this outside/to the top of your hood, or use it simultaneously in another reaction/collect it over water for later use.

As an aside, its interesting to note that the mono-acetoxyborohydride (and particularly the analogous species derived from TFA) is a reagent for the reduction of amides to amines. Perhaps a more obtainable replacement for LAH?
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chemrox
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[*] posted on 24-7-2012 at 17:57


You can easily make STAB in situ with NaBH4 and GAA.



"When you let the dumbasses vote you end up with populism followed by autocracy and getting back is a bitch." Plato (sort of)
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lullu
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[*] posted on 25-7-2012 at 01:03


Keep in mind that it is important to have NaBH4 in solution and cooled while dripping Ac2O otherwise you will loose product.

A selective preparation of sodium monoacetoxyborohydride seems impossible to me, still it looks like a nice reagent.
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Nicodem
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