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Author: Subject: Problem with Al/Hg reduction- strange ether soluble sulfate
Methyl.Magic
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[*] posted on 13-7-2009 at 06:31
Problem with Al/Hg reduction- strange ether soluble sulfate


Hello !

1 week ago I've done an Al/Hg with very good results (71%) reduced from nitrostyrene (made from BnO and nitromethane with Propylamine in iPrOH, 77% yield).

Quantity :

Nitrostyrène - 7,5 g in 75 ml iPrOH
In the 2l flask : 80g AcOH and 40 ml H2O and 13 g pre-amalgmated Al thick Kitchen foils.
At the end of the reaction I neutralised with NaOH, separated out the IPrOH layer extracted with 2x70ml Et2O. These organic extracts were dried over anhydrous MgSO4 and conc. H2SO4 was added drop by drop until pH7, the sulfate immediatly precipitate out and was filtred through a buchner, and the cake was washed with ether and dried under vacuum to give phenethylamine as amorphous white powder, 71% yield based from the nitrostyrene.

Yesterday I tried the same reaction on the 2,5-dimethoxynitrostyrene substrate.

WORK UP :

17g of the orange 2,5-dimethoxynitrostryène (made from 2,5-dimethoxybenzaldehyde and nitromethane with EDDA as catalyst in iPrOH, yield was 91%) was added in 140ml of iPrOH and 60ml AcOH. The solution was carried at reflux to dissolve all the solid.

In a 2l RBF , 80ml of iPrOH, 100ml of AcOH and 80ml of H2O were placed and heated with a water bath to avoid suddently precipitating the stryrene when I add the solution.

30g of aluminium kitchen foil was added in a 500ml H2O / 100mg HgCl2 solution. 10 min later the solution became grey and the Al was carefully take out of the beaker and washed with 500ml fresh H2O and put into the 2L RBF fitted with a reflux condenser. 5-6 min later, some bubbles appeared in the flask, I immediatly added the hot 2,5-DMNS solution... Though I previously heated the solution, unfortunately the 2,5-DMSN immediatly cristallised. Thanks to the heat envolved by the reaction, the 2,5-DMSN was rapidely re-solubilised. The reaction became very vigorous and i was happy to have chosen a good condenser. 2 minutes later the solution was more calm, so I placed a valve at the top of the condenser to increase up the pressure. 1 hour later, the reaction was quiet. I estimated it was over but only the half part of the Al was dissolved. I add a few ml of HCl to try killing the rest of the Al but with no success. Then, I added 360 g of 30% NaOH (by portion)with good stirring and let the flask cooling for the night. This morning I could see a green/yellow layer at the top of the Al sludge. This layer is separated and the sludge was extracted with 150 ml and 80ml of Et2O. The extracts were combined, washed with 10% NaOH and dried over MgSO4.

There was the problem : I added conc. H2SO4 but I saw no precipitate at all ! But hopefully I added more than 1ml to reach pH 7... This is strange because IMO this sulfate is too ionic to be dissolved in the non-polar Et2O/IPA ... Ok, I said "oulah jai quand même 500ml de solvant, en fait c'est pas mal pour si peu de produit", so I placed the solution to the rotavap, but when it was being heated, I saw some crystals at the bottom of the flask. I immediatly stopped the rotavap and put the flask in the fridge. 2 hour later I had a minimal amount of beautiful yellow cristals. I filtered the solution through a buchner and washed the cake with fresh ether to get beautiful white crystals. The product is still in the dryer. Tomorrow I'll test its melting point.

My question is : what can fuck up the Al/Hg reduction on the methoxylated phenylnitroalkenes ??? All other al/Hg reduction I've made gave good or acceptable yields... Can aluminium lewis acid cleave these methoxyethers with the heat of the reaction ?

I've planned to reduce the 2-methoxynitrostyrène, too but next time I thing i'll go with the LiAlH4 route...

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Klute
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[*] posted on 13-7-2009 at 06:55


I would try NaBH4 reductionb of the double bond and then CTH to the amine, very good results were reported with this substrate before!

Never done such a Al/Hg, so can't comment more on this.. Sulfates can be pretty tricky, and I have found some phenethylamine sulfate to be surprisingly soluble in ether... You should evaporate as much ether as you can and add petroleum ether or similar solvent to crash out th eamine sulfate.

Good luck!




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Foss_Jeane
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[*] posted on 13-7-2009 at 11:18


If you still have "beautiful yellow crystals" at the end, it looks like the reduction failed.

I haven't tried using Al-Hg for reducing nitro styrenes, however, I used the Clemmensen reduction on 3,4 dimethoxyphenylnitrostyrene about ten years ago, or so. That worked like a charm, giving better than 70% yields of the dimethoxyphenylethylamine. (Tried this with both mossy zinc and zinc powder, both worked, though mossy takes a dog's age. Powder goes much faster.) Don't see why Al-Hg wouldn't get the job done just as well. These nitrostyrenes aren't all that difficult to reduce.

The only possibilities for failure I see are dirty aluminum and insufficient amalgamation. Did you clean the foil first? According to Vogel, the foil should be treated with a 10% NaOH solution with warming until you see vigorous hydrogen generation for three minutes. Then strain off the solution and wash the foils with water, then ethanol, and dry by patting with paper towels before adding to the HgCl2 solution.

I see another problem if your 100mG of HgCl2 isn't a typo. That's about an order in magnitude less than what you'd require here. For 30G of aluminum foil, you need more like 1.0G of HgCl2 as a 2.0% (by weight) solution. Insufficient Al-Hg could make that reduction fail, especially if the dimethoxy compound is more resistant to reduction than the unsubstituted phenylnitrostyrene you used for the dry run.
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[*] posted on 14-7-2009 at 08:07


maybe you just added too much H2SO4 and made the organic soluble hydrogen sulfate salt.



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Methyl.Magic
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[*] posted on 20-7-2009 at 06:56


Quote: Originally posted by Klute  
I would try NaBH4 reductionb of the double bond and then CTH to the amine, very good results were reported with this substrate before!

Never done such a Al/Hg, so can't comment more on this.. Sulfates can be pretty tricky, and I have found some phenethylamine sulfate to be surprisingly soluble in ether... You should evaporate as much ether as you can and add petroleum ether or similar solvent to crash out the amine sulfate.

Good luck!


Hello !

I evaporated off all the ether and half of the IPA and placed the flask at -15°C. I only collected a few amount of crystals...
Yield = 20,7% of 2,5-dimethoxyphenethylamine sulfate as white crystals. Next I added petroleum to the IPA but it don't mix with IPA, there were 2 layers... oh shit, I thrown all away after the solution became dark green.

I've learnt something : ACHTUNG when you try to precipitate out some phenethylamines in ether or DCM because some salt such as HCl or sulfate seems to be soluble in these solvent !

I've ever tried the NaBH4/CTH. The double bond reduction worked well but I had a problem with the CTH because I used Pd/BaSO4 insted of Pd/C and it was not sufficient. But perhaps the reaction worked beacause I gassed the freebase with HCl in ether and I've just seen that some salts are soluble in ether...



Quote:

If you still have "beautiful yellow crystals" at the end, it looks like the reduction failed.

I haven't tried using Al-Hg for reducing nitro styrenes, however, I used the Clemmensen reduction on 3,4 dimethoxyphenylnitrostyrene about ten years ago, or so. That worked like a charm, giving better than 70% yields of the dimethoxyphenylethylamine. (Tried this with both mossy zinc and zinc powder, both worked, though mossy takes a dog's age. Powder goes much faster.) Don't see why Al-Hg wouldn't get the job done just as well. These nitrostyrenes aren't all that difficult to reduce.

The only possibilities for failure I see are dirty aluminum and insufficient amalgamation. Did you clean the foil first? According to Vogel, the foil should be treated with a 10% NaOH solution with warming until you see vigorous hydrogen generation for three minutes. Then strain off the solution and wash the foils with water, then ethanol, and dry by patting with paper towels before adding to the HgCl2 solution.

I see another problem if your 100mG of HgCl2 isn't a typo. That's about an order in magnitude less than what you'd require here. For 30G of aluminum foil, you need more like 1.0G of HgCl2 as a 2.0% (by weight) solution. Insufficient Al-Hg could make that reduction fail, especially if the dimethoxy compound is more resistant to reduction than the unsubstituted phenylnitrostyrene you used for the dry run.


Thanks for the tips !

No I didn't clean the foil before the amalgamation, next time I won't skip this step ! But why do you wash with EtOH after H2O ?

I'll use more Hg salt, too. 100mg is effectively a tiny amount !

Thank you foss_jane, I'll post my second attempt here !

bye


Quote:

maybe you just added too much H2SO4 and made the organic soluble hydrogen sulfate salt.


I don't think I made the hydrogenosulfate, beacause I added the H2SO4 drop by drop and at the first drops i didn't see any precipitate at all, just the solution becoming trouble. And the pH was no more than 7 and hydrogenosulfates are acidic.
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Foss_Jeane
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[*] posted on 21-7-2009 at 21:24


Quote: Originally posted by Methyl.Magic  
But why do you wash with EtOH after H2O ?


It has something to do with "activating" the metal to make amalgamation easier. It's also a standard practice to wash amalgamated lead cathodes in ethanol before you put them into service for performing reductions.
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[*] posted on 27-7-2009 at 07:05


Foss_jeane, what condition did you choose to recude your nitrostyrene via Zn/Hg ? Solvent, température, ... Because I'm very interesed, I've just bought Zn powder.
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[*] posted on 28-7-2009 at 04:23


Quote: Originally posted by Methyl.Magic  
what condition did you choose to recude your nitrostyrene via Zn/Hg ? Solvent, température, ... Because I'm very interesed, I've just bought Zn powder.

In my experience the following procedure gives good results:
Amalgamate the zinc dust with a 1% HgCl2-solution (1mass% of Hg is more than sufficient, stir at RT, 30 min). Add 20 or more vol% of THF (solubility of the nitrostyrene increased) to the Zn/Hg/water mixture. Add the nitrostyrene and conc. HCl alternately in portions with very good stirring. Approx. 5 h, after that time 30 min 80°C - water bath. Remove the THF, extract all nonbasic impurities with CHCl3 (not in the case of CHCl3-soluble amine-HCl-salt!). NaOH in huge excess = clear solution + free amine at the top. Standard workup, yields about 80% repeatedly.


[Edited on 28-7-2009 by grind]
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[*] posted on 30-7-2009 at 23:46


Quote: Originally posted by Methyl.Magic  
Foss_jeane, what condition did you choose to recude your nitrostyrene via Zn/Hg ? Solvent, température, ... Because I'm very interesed, I've just bought Zn powder.


Here's how I did it.

0.1M of the phenylnitrostyrene to 1.0L of ethanol

100G Zn to 10G of HgCl2

When using mossy zinc: use "linesman's pliers" and cut the pieces as small as possible. If the mossy looks dirty, clean it by stirring with 2.0% HCl solution for two minutes. Mine looked clean, so I didn't do this step. Next, prepare a 1.0% (by weight) HgCl2 solution. It's best to add the HgCl in small portions since it likes to "bead up" and you don't want to be chasing down a lot of those beads with the stir rod to crush them against the side of the beaker. Stir vigorously, and add the zinc. Stir this for 10 minutes.

Dissolve the notrostyrene in ethanol. (You can do this while letting the Zn/Hg settle out.) Decant as much solution from the Zn and fluffy Zn/Hg, and vac filter with a Buchner funnel. Wash with water, suck the water off, but don't let it dry, and add to the nitrostyrene solution with vigorous stirring.

I had about a quart of technical grade, 37% HCl, and used a glass tube to simply pull up a portion of the HCl. Drop a couple into the nitrostyrene solution until you notice bubbling at the zinc. Here's where it takes a dog's age: wait until you see the bubbling slack off, then add another portion of HCl. For the longest time, it looks like nothing's happening. The end is near when you see a thin ring of zinc chloride just above the liquid. The solution goes clear almost all at once, it seems. (This is about three hours after you start.) Add one final portion of HCl, wait until the bubbling stops, and continue stirring for another 30 minutes.

If using zinc powder, this goes much faster, though the ethanol solution becomes quite opaque. You will also notice the color disappearing much faster, but you have to spot a drop on white paper to see if it's really gone. You might have to let the zinc settle a bit to see if it needs more stirring and HCl to reach the end point (clear solution). Here, you won't see the bubbling due to the stirring, so I added a portion of HCl at intervals of about 90 seconds.

Decant off as much solution without getting any zinc or amalgam in it, and vac filter the rest. Wash the zinc with hot ethanol, and combine the filtrate with the decanted portion. Use a water driven vacuum pump and distill off the ethanol. (The solution should not smell of alcohol.) Basify strongly with concentrated NH4OH, and extract with the solvent of your choice. I used benzene for this, but toluene or ether might also work, depending on what you're trying to extract. Dry the extracts over night with dry MgSO4.

You can get the amine salt out of the extract in the usual ways: extract with small portions of dilute HCl, bubble in dry HCl to precipitate the hydrochloride, shake with very small amounts of concentrated H2SO4 to neutrality. I opted for HCl extraction. This extract was distilled under a water driven vac pump until dry. (Used a water bath here to keep from overheating the product>;) The HCl salt of the resulting 3,4 dimethoxyphenylethylamine recrystalized from ethanol. After filtering, let the filtrate stand over night in the freezer to precipitate more.

Except for the distilling, all these reactions take place at room temperature.

When handling zinc, you definitely want to keep a can of dry sand handy. Damp zinc powder could ignite spontaneously. After filtering the zinc, dispose by covering it with dry sand in a metal paint can. I never had it ignite, but that doesn't mean it couldn't happen.

[Edited on 31-7-2009 by Foss_Jeane]
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Methyl.Magic
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[*] posted on 6-8-2009 at 07:54


Thank you very much !!!

Your work up is very complete and instructive !

But I've already done this reaction on another substrate before read your post. The reaction faild :(

14g of 2-methoxy-b-nitrostyrene is added in 50ml of THF and 100 ml isopropanol. The susppension is gently heated with a water bath until all the solid has been dissolved.

Meanwhile 1,5g of HgCl2 was dissolved in 400ml of H2O. Then 40g of Zn powder ( I used 30 years old Zn powder because the supplier called me to say it's out of stock...) is added in 250ml of water. 8ml of conc Hcl is added with strong stirring. There were some bubbles. 4 minutes later I filtered the Zn on a buchner and washed with H2O. The activated zinc was added to the HgCl2 solution and I let it for 15 minutes. I didn't see any bubbles like the Al/Hg preparation... So I decided to filter the Zn, wash it with H2O and EtOH. Next I added it to the hot nitrostyrene/IPA/THF solution.

The RBF was fited with a reflux condenser and a dropping funnel containing conc. HCl. 10ml of the acid is added to the solution. There was a strong reaction and the solution started boiling. I made the most of the opportunity to place a valve at the top of the condenser to hold an overpressure of H2. I added another 10ml portion after 1 minute and the reaction wasn't as vigorous as the first. Then I added several portion of conc. HCl/ IPA 50:50 until the total volume was about 800ml. The reaction was very slow and the Zn take 1 hour to be completely dissolved. I didn't see any ZnCl2, it was dissolved in the solution. Next I placed a liebig condeser and I boiled off about 200ml of IPA. The solution is brown.

I neutralised the acid with 30% NaOH and the zincate precipitate immediatly. I separated the top layer and extracted the aqueous layer with 2x100ml toluene. The zinc sludge is easier to extract than the Aluminium one.

The combined extracts are dried over MgSO4 and neutralised with conc. H2SO4. Two drop of the acid are sufficient to neutralised the dark brown toluene/IPA. I didn't see any precipitate at all. The reduction failed...

Next time I'll tried the Al/Hg AcOH reduction again because I don't have any clean Zn powder. I'll use THF instead of IPA because it seems much more soluble in THF than IPA.
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Foss_Jeane
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[*] posted on 13-8-2009 at 23:29


Quote: Originally posted by Methyl.Magic  
Thank you very much !!!

Your work up is very complete and instructive !

But I've already done this reaction on another substrate before read your post. The reaction faild :(


That's unfortunate.

Quote:
Meanwhile 1,5g of HgCl2 was dissolved in 400ml of H2O. Then 40g of Zn powder ( I used 30 years old Zn powder because the supplier called me to say it's out of stock...) is added in 250ml of water.


That's your problem right there. I used a weight ratio of 10 : 1 Zn : HgCl2. If you need 40G of Zn, then you need 4.0G of HgCl2, not 1.5G. You probably didn't have enough amalgam in there to do any good.

There is also the problem of different substances. You mentioned 2-methoxyphenylnitrostyrene. I didn't use that, since mine was 3,4 dimethoxyphenylnitrostyrene. Perhaps others don't reduce so easily? There are just esssssss-loads of ways to perform Clemmensen reductions.

Do let us know how the Al/Hg works out. As I said before, I haven't tried that.
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[*] posted on 14-8-2009 at 01:06


I wonder if gallium, heated to its melting-point of about 29ºC, would be a satisfactory, and of course much less toxic and possibly cheaper, substitute for Al-Hg amalgam.
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[*] posted on 14-8-2009 at 20:39


I don't know if gallium will alloy with as wide a range of metals as mercury, but I do know that it does allow with aluminum in the same way, making it much more reactive. In fact, this method has been considered for commercial hydrogen production. However, I believe it is more expensive than mercury.

One problem though is that gallium and glassware don't mix. Gallium, particularly in the liquid phase, tends to diffuse into the glass, resulting in a "snowy" appearance IIRC.





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[*] posted on 14-8-2009 at 21:03


Notice on woelens page that mixing copper sulfate, sodium chloride, and a dash of an acid will get aluminum to react with water, as does a Hg amalgam.

If I remember correctly, a while ago on some thread, someone was making a mixture of metals or the salts, probably containing gallium to have a low melting point for just this thing. I would try and look around a bit more, I bet there is quite a bit of information on it.




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[*] posted on 5-2-2010 at 21:02


Does one know of a good way to clean the leftover hardened aluminium thats left over in flask after the al/hg has finished.
What is a good disolving solvent OTC for this type of substance?
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Methyl.Magic
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[*] posted on 6-2-2010 at 04:16


Aluminium can be killed with NaOH solution.
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[*] posted on 10-2-2010 at 08:43


if the procedure is carried out effectively one should have little to no Al left, unless one has stopped the reaction before completion. If you are referring to the Al-oxides that stick to flasks after Al/Hg's, 5% HCl removes them effectively.



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