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Author: Subject: TLC monitoring progress of STAB reductive am
chemrox
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[*] posted on 20-11-2008 at 20:43
TLC monitoring progress of STAB reductive am


I'm running an STAB mediated reductive amination of an aniline and a saturated cyclic ketone. I'm using: 10:2:1, DCM: AcOEt:MeOH + drop or two Et3N. It's going slowly.

There's been a bit of smearing and spot overlap but the major constituents are well separated and distinct. Only oddity is the aniline spot which as a core and halo... bizarre (at least to me).. any explanation of the latter phenonmenon or suggestions for improvement much appreciated.
CRX




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[*] posted on 20-11-2008 at 22:28


Your solvent mixture is not suitable for STAB reductions! Methanol destroys STAB quickly and you must not use it!
Usual solvents are toluene, acetonitrile, 1,2-dichloroethane and THF, occassionally DCM and ethyl acetate. Addition of acetic acid accelerates the reaction.
I think the unknown spot is the alcohol, derived from your cyclic ketone. The methanol in your solvent mixture promotes this side reaction.
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[*] posted on 21-11-2008 at 02:49


What's a STAB reduction? Google was useless, only gave me info on how not to get stabbed...



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Klute
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[*] posted on 21-11-2008 at 04:43


Nerro: STAB: Sodium Triacetoxyborohydride, see related thread.

Grind: No, no, he is using the above system as TLC eluant, not reaction solvent.

Chemrox: Sometimes TLC does some weird things.. Some stains do strange patterns, one compound can make two different forms on the same plate, etc... I should worry too much, it can simply be because of the solvent system when it reacts the aniline, elutin g it side ways at different rayes. It's hard to give a valid explanation. Maybe you could try doing a spot of aniline with the same eluant but withut any other compounds and see if it acts the same.




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chemrox
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[*] posted on 21-11-2008 at 13:28


I think I caused a misunderstanding. The solvent system is for TLC plate development. For the reax I'm using DCE.
@Klute: the aniline reference spot didn't do that so it might be a reaction phenomenon...(?) and thanks for helping clear up the solvent misunderstanding.



[Edited on 21-11-2008 by chemrox]




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[*] posted on 21-11-2008 at 15:03


Oh I see I was wrong with the solvent mixture. Sorry....

May be the reaction product is still complexed with the boron compound, that could interfere with the TLC. The safe way is to take an aliquot of the reaction suspension (solution AND solid particles!) and to treat it in the following way (mini workup):
Add diluted HCl, shake until you have 2 clear phases and 5 minutes longer, then basify with NaOH-solution, separate the DCE-Phase, dry the organic phase with Na2SO4 (or omit the drying step). Now you can run your TLC.

Note: DCE is able to react with amines to form 2-chlorethylamines, especially at elevated temperatures! So you can expect (tiny amounts of) products of this side reaction. On the other side, DCE usually gives the highest yields in STAB reductions, compared with toluene, THF and acetonitrile.
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chemrox
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[*] posted on 21-11-2008 at 20:42


"DCE usually gives the highest yields in STAB reductions, compared with toluene, THF and acetonitrile.." I've also read this. Does anyone know why? Why not DCM for example?



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stoichiometric_steve
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[*] posted on 21-11-2008 at 23:41


Even though 1,2-Dichloroethane gives the highest yield, i would still avoid it being pretty carcinogenic.



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[*] posted on 22-11-2008 at 09:19


Different components can spread at different rates in the reaction solvent when you apply the drop to the TLC plate. So if you're getting a halo, that would be from whatever moves the fastest in that solvent. It shouldn't be possible to get a "core and halo" from the same compound...

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chemrox
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[*] posted on 22-11-2008 at 23:30


@steve I'm not drinking it..anyway, I'll do an NaOH workup on a small aliquot and extract with ether. Maybe the aniline halo will disappear then. It's been running a long time anyway. Time to quench and obtain product. I edited this to add a thought I had.. I looked at the reference aniline spot and it has a sort of broad halo as well. I wonder if the Et3N in the development solution is making this? I'll try an Et3N reference spot and see where it ends up.

[Edited on 23-11-2008 by chemrox]




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[*] posted on 25-11-2008 at 16:58
STAB gen proc: HELP wanted


Anyone using STAB for red. ams. please weigh in here. My general procedure calls for "quenching with 1N NaOH, then extract with ether..." after the reax is done. It doesn't provide any guidance on how much solution to use or what to look for. I added 1N NaOH until it looked like no more white solids were being generated. I added ether. I've got an ethereal solution with white solids and a lot of globular looking emulsion. What should I do? What's the recovery?

[Edited on 25-11-2008 by chemrox]




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[*] posted on 26-11-2008 at 00:33


When your reaction mixture is strongly (!) alkaline, you can extract the amine. The NaOH solution destroys residual STAB and acetic acid salts of your amine. You need the free amine to extract it with ether. My suggestion is to test the pH of your mixture and then you add an excess of NaOH-solution. If you can´t get rid of a solid - don´t worry. Separate the solid by filtration and extract the filtrate with ether.
The solid is probably the acetamide of your amine (it´s a secondary amine - right?), arising from a side reaction. Boil the solid with 6N HCl for some hours, make alkaline and extract with ether. That should increase the yield of your amine.
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[*] posted on 26-11-2008 at 02:11


In my experience with STAB, fine white solids that are a pain to work with/filter are formed when quenching with acids, and then dissolve by addition of strong NaOH. Maybe you haven't added enough base, don't be afraid to go to pH 14, they should dissolve. Wash well the organic extract with your amine, otherwise when you add dilute acid some solids reappear and seem to form emulsions very easily.



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chemrox
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[*] posted on 26-11-2008 at 03:57


Thanks! Both of you!



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[*] posted on 26-11-2008 at 04:00


The solid is H3BO3 (borates with basic quench), no acetamide or anything!!

[Edited on 26-11-2008 by stoichiometric_steve]




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[*] posted on 26-11-2008 at 06:24


Quote:
Originally posted by stoichiometric_steve
The solid is H3BO3 (borates with basic quench), no acetamide or anything!!

[Edited on 26-11-2008 by stoichiometric_steve]

If the solution pH is < 7, that´s correct. But normally you add NaOH until the solution is strongly alkaline, then soluble sodium borate is formed.
In one of my reductive aminations with STAB I´ve also got a solid which wasn´t soluble even in an excess of dilute NaOH solution.
Analysis of this solid showed it was the acetamide of the product amine. Saponification of the amide yielded a further crop of product amine.
So I assume it´s very similar in chemrox´case.
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[*] posted on 26-11-2008 at 14:17


I'll run an IR on the stuff. It's all going to keep until Monday. I'm going out of state for the hollidays. Which reminds me how grateful I am for this forum/website/whatever we are.
Peace,
CRX

[Edited on 26-11-2008 by chemrox]




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[*] posted on 3-12-2008 at 18:20


I did check the aqueous layer and found the pH to be ~7 so i added enough base to get it up to ~ 12-14. I re-extracted with the first ether portion and got a gawdawful emulsion that is sitting in the fridge overnight



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[*] posted on 5-12-2008 at 16:20


So does anyone know a coarse filtration medium to reduce the solids and make this mess a little more manageable? I tried glass wool... not very effective....clogged quickly



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[*] posted on 6-12-2008 at 00:43


That was fucking triple posting

use Celite




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[*] posted on 10-12-2008 at 07:04


I filtered my reduction mixture through a simple small glass filter frit (so called G4 frit), that was satisfactory. Or like steve said - you can use a celite pad of approx. 1-2 cm thickness in the frit.
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