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Author: Subject: Zn/HCl reduction-analogous conditions
lowtech
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[*] posted on 9-1-2019 at 01:53
Zn/HCl reduction-analogous conditions


Hi
for some time i have a trouble reducing aromatic nitroalkene substrate, because of its ridiculous insolubility practically in any solvent/solvent mixture i have (AcOH, IPA, toluene, MeOH, EtOH...). Only solvent i find where its somewhat soluble is methylethylketone, but this is nonsense to use in any of these reactions.
I tryied reduction using MeOH/HCl/Zn but it yielded no product at all. IPA/HCl/Zn the same problem.

I was thinking about zinc reduction, where water is not present, so the solubility of substrate will be much improved and, hopefully it will proceed... Something like AcOH/zinc pellets refluxing, or maybe some other metal (Fe, Al) as well.
I dont know if there is any special function of HCl in this reaction except its a strong acid, so could it be substituted with another strong acid, eg H2SO4 (water-free conditions=>better solubility) ?

I was thinking about systems:
AcOH (best "solubility" of substrate)/conc. H2SO4/Zinc pellets or dust
just hot AcOH and zinc (dont know if this is strong enough to reduce it)

did somebody expirienced the same problem, or is worth trying these modifications ? whats your opinion ?

thanks :)


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LabRatNW
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[*] posted on 25-1-2019 at 08:17


You probably need a strong acid to make this run.
Haloacids work well because they form highly stable products with zinc, and you're leveraging the redox of Zn -> ZnX (X = Cl, Br, I) to drive the reaction.

Unless you're performing a redox with Zinc, you've got nothing. Carboxylic acids definitely don't work here.
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Elrik
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[*] posted on 25-1-2019 at 11:51


As far as I know, its the zinc that is the active reducing agent and there is nothing special about hydrochloric acid to make it work while others wouldn't. HCl is just fast and ubiquitous and so its what is normally used.
Harmaline has been reduced to tetrahydroharmine using zinc powder and 5% acetic acid! Its just slower. Van Der Sypt used 7% in his paper 'A harm-reduction approach to the isolation of harmine and its hydrogenated derivatives from Peganum Harmala seed in low-tech settings'.
Granted that is not identical to reduction of a nitroalkene, but it shows that zinc reductions do not require strong acids.
I'd try GAA if your precursor is soluble in it.
Alternately, if your compound is soluble in GAA and the reduction goes too slow or not at all why not use a solution of HCl in glacial acetic acid?
In the cold its soluble to the extent of ~10% HCl
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Corrosive Joeseph
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[*] posted on 25-1-2019 at 13:57


Does your aromatic nitroalkene have a beta-methyl or a beta hydrogen.................?


/CJ




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