njl
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Reduction of nitroalkenes to nitroalkanes
I was scrolling through the rhodium archive and I noticed the section on reductions is chock-full of methods for the reduction of nitroalkenes to
oximes, nitroalkanes, and amines. Why would someone bother reducing to the oxime or nitroalkane if their target is the amine anyway? Is it because you
can't reduce the nitroalkene directly to an amine without LAH? For example, there is a method that describes the reduction of a nitrostyrene to the
amine with Zn/HCl. Isnt this a nitroalkene straight to an amine? Does the Zn/HCl for some reason work on nitrostyrenes but not 2-nitropropenes?
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karlos³
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Is your target the amine though?
Then use the novel one-pot method with NaBH4/CuCl2, it replaced the old Al/Hg for us as the gold standard.
And besides those two methods, there still are others that don't use LiAlH4 for the reduction all the way in one-pot.
And it works not just as well as LiAlH4, it works actually even better, both of them.
And those two, NaBH4/CuCl2 and Al/Hg, are the most popular, with a good third place being stoichiometric_steve's NaBH4/Zn one-pot method, in my
opinion.
But I have to add that I am quite well informed on that matter, especially regarding actual developments and how the people are making use of them
Those two borohydride based methods actually use the nitroalkane as intermediate of course, just prepared in-situ.
Yes the Zn/HCl works only on nitrostyrenes but not on a-alkylnitroalkenes.
But on all nitroalkanes it does work very well(thus said mentioned one-pot method using NaBH4/Zn+ammonium formiate), also the one or other method that
achieves this in two separate reactions with an isolation of the intermediate.
But this is also popular to do that as two separate reactions, since the nitroalkanes can be reduced in the very huge scalable method employing a
hydrogenation under CTH conditions with palladium.
And unlike LiAlH4 reductions which require a large volume, this can be scaled very well, but without being as potentially dangerous as that stuff, yet
while resulting in very good yields as well.
Both steps will result in yields of at least 80%, and can be carried out in comparably small reaction vessels because of the mentioned reasons.
If you want to reduce nitroalkenes to -alkanes, there is no other really useful method besides sodium borohydride.
And for those, a preferable method is to do it in a biphasic system with a PTC agent, as this reduces the amount of reducing agent needed, while the
need for an overall excess of borohydride is reduced without an increased risk of dimerisation.
The formation of said dimer is an issue if not enough borohydride is employed.
So what is your real question here, or rather, your intention?
I could answer much more precisely and helpful if I just would know more precisely what you want to do?
Surely I can reply in a more useful manner if you tell what it is exactly you want to know.
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monolithic
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I've seen Fe/HCl used to reduce nitropropenes, I assume Zn/HCl would also work since Zn has a stronger reduction potential than Fe. Reducing agents
like LAH and Red-Al are generally used to reduce nitrostyrenes to amines but CuCl2 and NaBH4 also gets the job done, see http://www.sciencemadness.org/talk/viewthread.php?tid=84596
I'm not sure if Zn/HCl on nitrostyrenes is straight to the amine, there are a number of intermediate steps. Someone uploaded a paper with some theory
on it, very interesting read if you like dissolving metal reactions.  http://www.sciencemadness.org/talk/files.php?pid=603835&... With that said, I've tried it on vanillin nitrostyrene a few times -- it works,
but it's a challenging reaction that requires strict temperature control throughout.
[Edited on 8-31-2020 by monolithic]
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karlos³
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Well, the reduction of nitropropenes with Fe or Zn will result in the oxime, and then in their hydrolysis to the ketone.
So they can be reduced with it, it just wouldn't end up in the desired product.
For nitrostyrenes, as you also say, it works, but I guess under less than ideal conditions quite some other things are produced like the aldoxime and
such, accounting for the low yields that most get when the conditions it is ran under aren't perfectly on point.
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njl
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@Karlos thanks for the info. I don't have any plans for PEAs/Amphs in the near future. My question boiled down to why take multiple steps to get from
nitropropene to amine when Zinc can he had for cheap. I did not realize that Zn/HCl cant reduce a nitropropene due to the alkyl substitution (I'm
still reading the paper monolithic shared). Is that the reason that people dont go straight from nitroalkene to amine (because they can't), or is it
for some other reason? I'd also like to clarify that by straight to the amine I meant that in a one-step reaction the amine could be isolated, instead
of having two reductions from nitro to oxime, and then oxime to amine. I'm still confused as to why zinc cant reduce the nitropropene. According to
mono's paper, it may be because the alkyl substitution stabilizes the intermediate carbocation, but this doesn't seem right because the Zn/HCl system
CAN reduce nitrobenzene to aniline.
@mono thanksfor the links! I will check them both out. I have seen several metal/acid systems for reduction including Fe/HCl and Fe/AcOH.
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njl
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Also karlos https://erowid.org/archive/rhodium/chemistry/oxime.txt at the bottom there is a short ref for oxime reduction to amine with Zn/ACOH. Now I know
better than to trust a 1 paragraph ref in plaintext, but could there be any credence to this since AcOH is milder than HCl?
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Corrosive Joeseph
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Wu, Y., & Ahlberg, P. (1992). Intramolecularity and the function of the side arm base of simple models of transaminases. The Journal of Organic
Chemistry, 57(23), 6324–6327.
is attached...
| Quote: |
could there be any credence to this since AcOH is milder than HCl? |
Who knows, to the best of my knowledge, it has never been tried by anyone on the forums.... Here is another interesting review, this time oxime
reduction to amine with Zn-NH3-MeOH.
https://chemistry.mdma.ch/hiveboard/novel/000413177.html
/CJ
Attachment: Intramolecularity and the function of the side arm base of simple models of transaminases.pdf (600kB)
This file has been downloaded 293 times
[Edited on 31-8-2020 by Corrosive Joeseph]
Being well adjusted to a sick society is no measure of one's mental health
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Corrosive Joeseph
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Slight OT, I posted two references for Zn-AcOH reduction of a,b unsaturated nitro compounds to oxime during the week.
https://www.sciencemadness.org/whisper/viewthread.php?tid=15...
/CJ
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karlos³
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To be honest, I remain absolutely doubtful about this paper and every similar one where oximes are reduced under conditions and reagents that just
seem to be too easy(and thus, not strong enough)..
I reduced too many oximes, trying different have usually only worked well with reagents that are found to be working by much more people/scientific
references.
Oximes aren't reasily reduced, at least ketoximes defnitely are not.
But for your question AcOH will not hydrolyse an oxime, yes.
If you want to isolate it(why should you?), it would be your acid of choice.
But if this is all purely theoretical, I'm out of this discussion.
I'm not among the most well versed people in theoretical stuff, I know the practical stuff much better and well, and know how to do and isolate your
product, but thats it mostly.
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clearly_not_atara
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A long long time ago, in a forum far away, chemists lived in a paradise of cheap palladium. Catalytic hydrogenation was the way. It was easy,
effective, and scalable.
As luck would have it, Pd/C/HCOOH is an excellent reducing system for converting nitroalkanes to amines. It gives good yields and a clean reaction.
Unfortunately, it doesn't work on nitroalkenes; you get oximes.
Faced with these conditions, chemists looked for methods that would convert nitroalkenes to nitroalkanes. Usually this was borohydride but sometimes
Hantzsch ester, benzimidazoline, etc.
And that's why there's so much about it on Erowid. Unfortunately, palladium will probably never be cheap again.
[Edited on 04-20-1969 by clearly_not_atara]
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zed
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Ummm. Never used Palladium. Platinum! Platinum is great.
Still too spendy, but currently much cheaper than Palladium.
Precious Metals are subject to speculation. Gambling actually. Real value, no longer has meaning.
The Stock Market is High, but there is underlying insecurity. This uncertainty breeds high spot metal prices.
People figure if our whole financial system, collapses like a house of cards, precious metals will still have some value.
Platinum hit its lowest price in years, just a few months ago. Didn't stay down though. Bounced right up again.
Platinum is currently about a thousand an ounce. Palladium and Gold, about twice that. Up, up, up!
Back in the day, Palladium was about 20% of the price of Platinum. Cheap! A mere after-thought.
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njl
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Sort of the other end of the spectrum but it's also pretty crazy how high silver has gone. Are the rising prices due to scarcity or more industrial
demand?
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monolithic
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Quote: Originally posted by njl  | | Sort of the other end of the spectrum but it's also pretty crazy how high silver has gone. Are the rising prices due to scarcity or more industrial
demand? |
Fear of inflation and loss of confidence in fiat and equities. People want a place to park their money.
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