Sciencemadness Discussion Board

Preparation of Zn(BH4)

lullu - 20-5-2012 at 03:24

I have a few question regarding the preparation of this compound.
In literature it is prepared by stirring NaBH4 and ZnCl2 in dry THF under nitrogen.
After 72h (really?) the clear supernatant layer is used for reduction.

Why does it form two layer?
Is 72h really needed to complete it?
Has anyone experience using it to reduce aromatic nitrocompounds to their corresponding amines (eg is it possible to reduce the double bond on nitroalkenes and the nitro group to the amine with it)? Maybe its better if someone could move this to the organic chem board.
I would be glad for first hand experiences.

Best regards
lullu

[Edited on 20-5-2012 by lullu]

[Edited on 20-5-2012 by lullu]

kristofvagyok - 20-5-2012 at 10:41

Quote: Originally posted by lullu  
I have a few question regarding the preparation of this compound.

Why does it form two layer?
Is 72h really needed to complete it?
Has anyone experience using it to reduce aromatic nitrocompounds to their corresponding amines (eg is it possible to reduce the double bond on nitroalkenes and the nitro group to the amine with it)? Maybe its better if someone could move this to the organic chem board.
I would be glad for first hand experiences.

Best regards
lullu


The 72 hours is an optimal time to get a high yielded reaction. Here the most time is needed for the reaction of the zinc-chloride and the sodium-borohydride what is a heterogen reaction, because zinc chloride is almost insoluble in THF.

If you take it down after 2 hours, than most of the NaBH4 will remain in the solution.

The two layers could be a saturated layer with NaCl and another one with Zn(BH4)2. But there is a bigger chance for an upper layer with Zn(BH4)2 and a lower with solid NaCl what is not so soluble in THF.
-LiCl is a bit soluble in THF so it's not recommended to use LiBH4 as a starting compound.

And your last question: reducing "aromatic alkenes" to the corresponding amines.... That kind of chemistry is for dumb people. Find out something smarter.

Cheers.

lullu - 20-5-2012 at 23:00

Quote: Originally posted by kristofvagyok  

The two layers could be a saturated layer with NaCl and another one with Zn(BH4)2. But there is a bigger chance for an upper layer with Zn(BH4)2 and a lower with solid NaCl what is not so soluble in THF.


Thanks for your reply you made it very clear to me why the 72h are needed.
I would imagine after 72h there will be only one layer of THF solution of Zn(BH4), NaCl and solid NaCl.
The word supernatant layer just suggests two layer of THF which made me wonder. But I guess the best thing is to just try it.
Quote: Originally posted by kristofvagyok  

And your last question: reducing "aromatic alkenes" to the corresponding amines.... That kind of chemistry is for dumb people. Find out something smarter.


I appreciate your opinion, neverless I think the reaction would be interesting as it was tried with NaBH4+BF3.Et2O but never with this Zinc Borohydride as far as I know.

lullu - 21-5-2012 at 00:40

So from literature the hydrolysis of the conjugated double bonds within the nitroalkene via Zn(BH4)2 reduction seems to work in very good yield [0].
But what I find a bit odd is that the paper does not mention anything regarding amines forming as a product as stated in [1] which describes the reduction of nitrogroups with Pyridine Zinc Tetrahydroborato again with very good yield.

In [0] they use a 1:1 molar ratio whereby they use a ratio of 1:1.35 (regarding ZnBH4 not pyridine which is just used for a stabeling the compound). So what am I missing here to me it seems like the hydrolysis and the reduction to the amine is possible within the same reaction, but it wasnt mentioned in any way by [0].


[0] Brindaban C. Ranu, Rupak Chakraborty (Reduction of conjugated nitroalkenes with zinc borohydride. A mild method for converting monosubstituted nitroalkenes to nitroalkanes and disubstituted ones to oximes); Tetrahedron Volume 48, Issue 25, 1992

[1] B. Zeynizadeh, K. Zahmatkesh (First and Efficient Method for Reduction of Aliphatic and Aromatic Nitro Compounds with Zinc Borohydride as Pyridine Zinc Tetrahydroborato Complex, a New Stable Ligand-Metal Borohydride); J. Chin. Chem. Soc. 2003, 50, 267-271.

DJF90 - 21-5-2012 at 04:28

Not meaning to be rude but neither of you seem to have any clear ideas about this reaction. For starters, you're both oblivious to the fact that this is a heterogenous reaction, being that NaBH4 isn't soluble in THF. The long reaction time is a consequence of this, and can be dramatically reduced by using mechanical activation (e.g. mechanical stirring with glass beads). The NaCl product is also insoluble. The supernatant liquid is THF containing the Zn borohydride, and is used as such after decanting from the insoluble NaCl.

lullu - 22-5-2012 at 04:52

Quote: Originally posted by DJF90  
Not meaning to be rude but neither of you seem to have any clear ideas about this reaction. For starters, you're both oblivious to the fact that this is a heterogenous reaction, being that NaBH4 isn't soluble in THF. The long reaction time is a consequence of this, and can be dramatically reduced by using mechanical activation (e.g. mechanical stirring with glass beads). The NaCl product is also insoluble. The supernatant liquid is THF containing the Zn borohydride, and is used as such after decanting from the insoluble NaCl.


NaBH4 is slightly soluble in THF (0.1 g/100 mL, 20 °C) as is ZnCl (in the paper I read they use 250ml dry THF and 11g NaBH4) which is in my opinion (the slightly solubility) a requirement to get it done within 72h.
Anyway you and kristofvagyok are both correct stating that in the end there will be solid NaCl and a solution of Zn(BH4)2.
thanks lullu