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Klute
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Well, if you want to avoid the Zn oxides sludge, you will need huge amounts of conc. NH3, which is a pain to work with afterwards. Plus, it seems NH3
doesn't seperate the amine as well as alkali hydroxides.
If you directly useNaOh/KOH, you obtain a gel like sludge of Zn oxides, very hard to work with. The best way to handle it is to dilute it to a
workable consistancy, and extract with toluene or another lighter solvent, takin great care not to form an emulsion. It is hard to recover all the
solvent, and easy to mess up. Large amounts, and several (>5) extractions were found necessary to enven remove the smell of the amine from the
sludge.
With Pd/C, you basicly filter, remove alcohol, perform a A/B and you have your amine in solution, often colorless, ready to be crisatillzed or further
reacted.
With somewhat volatil amines, it is adviseable to acidify before removing the alcohol, with GAA (causes less coloration than HCl or H2SO4), then wash
the aq. residu after slight dilution to remove neutral byproducts, basify and extract.
When using K formate, i find it more practical to acidify with GAA before filtering to remove the insoluble bicarbonate.
But amm. formate as proved to be at less as efficient as amm. formate, so I prefer sticking with aq. MeoH at RT for 12h or so.
There alsays is 3-4 impurities apart from the amine by TLC (clean seperation using the eluant I mentionned earlier), only one remains after an A/B
with the amine. Theses must be intermediate reduction compounds, or byproducts from the hydrolsyis of the intermediates (and their eventual subsequent
reduction).
\"You can battle with a demon, you can embrace a demon; what the hell can you do with a fucking spiritual computer?\"
-Alice Parr
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DNA
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Isn't it possible to do a steam distilation right out of the flask when using Zn reactions.
Works fine with Al/Hg reductions, ofcourse your amine needs to be able to get steam distilled.
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smuv
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Maybe this is stupid; but couldn't you just add more Sodium hydroxide to form the soluble sodium zincates, the solution would be very alkaline but I
don't think this should be a problem for most substrates.
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Klute
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I had tried adding NaOH as a conc. solution, then as a solid, but stopped in the middle as so much was needed and I couldn't see any improvement. I'm
sure it could be possible, but it would really require very alrge amoutns of NaOH.
Steam distn I haevn't tried, considering the thick sludge consistancy, and long time required, but could be an option too.
\"You can battle with a demon, you can embrace a demon; what the hell can you do with a fucking spiritual computer?\"
-Alice Parr
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DNA
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As for small amounts <10g steam distillation works fine.
When you go bigger then one would need to distill way too much water to get all the freebase out.
But I think it is a nice method, just start heating it further after adding NaOH to the reaction mixture and replace the condenser by a distillation
setup.
And distill the solvent (recycling), then as the water comes it takes the freebase with it, if you would have H2SO4 in the receiver flask then one can
do immediately an a/b afterworths gives very clean amine so I heard from my 80 year old neighbour.
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Barium
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| Quote: | Originally posted by starman
Welcome Barium! Been wanting to ask about RT ammonium formate/Zinc reduction of nitro compounds.Seems easier/cheaper and faster than anything
discussed in this thread. |
Ask away! I'll try to help if I can.
In my experience, it isn't a generally useful method. It works great with some substrates and terrible with others. I have no explanation why.
Perhaps another thread should be used for Zn/NH4OOCH reductions in general.
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Nicodem
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This thread is already terribly long so I would appreciate if it would be reserved for discussing catalytic transfer hydrogenation (CTH) reductions
only. If anybody wants to discuss other reduction methods for nitro compounds, like Zn/HCOONH4 or other metal/acid based reductions, please do so
in a separate thread.
Edit: The separate thread for Zn/acid based reduction of nitro compounds has been opened here.
[Edited on 27/8/2008 by Nicodem]
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stoichiometric_steve
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Contrary to reports that apparently successfully employ Triethylamine as a base used in conjunction with Formic acid as a hydrogen source, it appears
that Triethylamine itself is a pretty potent catalyst poison for Palladium on Carbon.
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Klute
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Do you mean you added freebase Et3N tot he catalyst, and dripped formic acid in? Would it be better to prepare triethylammonium formate before
introducing the catalyst? I only had 10% yield or somethign when trying out this method... And have found ammonium formate to be more practical and
better yielding, closely followed by potassium formate.
\"You can battle with a demon, you can embrace a demon; what the hell can you do with a fucking spiritual computer?\"
-Alice Parr
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stoichiometric_steve
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I've retried the CTH with 0.1eq Et3N, 5eq. HCOOH and MeOH:H2O 9:1. The bubbling effectively stops after about 2 hours, and a lot of substrate is still
present by OC™ (Olfactory Chromatography™ .
I'm currently looking into alternatives, but it turns out that Hydrazine, one of my favoured candidates, also turns out to be a catalyst poison under
acidic conditions. This could probably be avoided by employing Hydrazinium Monoformate, discussed as a Hydrogen donor elsewhere (Tetrahedron 58
(2002):2211-2213)
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stoichiometric_steve
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It is also critical to make sure when using CTH that before adding any catalyst, the reaction mixture must be a solution. Otherwise, either the
catalyst or the substrate will not come in contact with the hydrogen donor.
I just learnt that the hard way...
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Barium
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In which CTH is that critical Steve?
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stoichiometric_steve
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It certainly is critical when reducing nitro groups to the amine in MeOH/NH4COOH. all i got from this reduction is an unquantified yield of the
ketoxime and some polymerized crap.
As for the Bandil-style CTH, i've previously stated that this kind of 2-phase CTH has never ever worked for me. And it's not very likely to ever do so
since the catalyst has no big chance of picking up any H2 from a KCOOH saturated phase.
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Barium
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I beg to differ Steve. I have even performed several reductions where the substrate have not been practically soluble in the particular solvent used.
Yet the reaction proceeds without a problem, e.g. hydrogenations using only plain water as solvent.
As I have stated earlier. The CTH with a saturated aqueous KCOOH-phase and a alcoholic phase containing the substrate does indeed work.
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stoichiometric_steve
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| Quote: | Originally posted by Barium
I have even performed several reductions where the substrate have not been practically soluble in the particular solvent used. Yet the reaction
proceeds without a problem, e.g. hydrogenations using only plain water as solvent. |
I'd be willing to believe that unless i hadn't tried your writeups with anal detail for quite a few attempts.
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Barium
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One day you ask me to give more details about developments of a particular procedure and the next day you clearly state that you don't believe me when
discussing another. If you are convinced I am lying, why ask me in the first place?
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stoichiometric_steve
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What i meant is that they may work for you, they obviously don't for me. Dog knows why...
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Isomeric_Fred
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I have to support stoichiometric_steve on this. Man, did I cry multiple times when trying out some of your procedures reported on the hive and failing
again and again. Ill be honest though, and say that a few of your ideas work well, but its a small percentage of everything youve posted back in the
day.
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stoichiometric_steve
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just sayin'...this works beautifully on 1mol scale with NH4COOH/5%Pd/C
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