[kriggy]

Hi there, for last week or two I was trying to reduce nitro group in compound containing aromatic nitro group and imine. I was able to reduce it using Na₂S in EtOH at 45°C. I got basicaly pure compound (95% by HPLC) but after workup it seems it decomposed partialy (±40%). The workup was to evaporate solvent, add water and extract with ether. I think it hydrolyzed by the addition of water but since the HPLC sample was 30% water it doesnt seem right.
Anyone experienced working with imines experienced something similar? What can be the cause? Also what are my options to purify the product - I could extract the ethanolic solution directly or run it through short column anything else I am missing?

[Dan]

after workup it seems it decomposed partialy (±40%).

How did you determine this, NMR in CDCl₃?

Trace HCl in chloroform is a real nuisance when characterizing imines. I once lost weeks on this before I realised what was going on. Either use CDCl₃ stored over K₂CO₃, or use a different NMR solvent.

If that's not the cause, have you tried HPLC post workup?

[orgopete]

Is a hydroquinone a possible result?

[kriggy]

Dan: I did HPLC post workup and it showed the decomposition. I wanted to do NMR but didnt because the compound decomposed.

orgopete: I dont follow the hydroquinone there :/ could you please elaborate bit more?

[orgopete]

Yah, never mind. I was thinking you lost the nitrogen of the nitro group. That would be unusual, so I was speculating how that could happen and I thought if you had something like a p-nitrophenol, then a quinoid intermediate might form and attack on its carbon could release the nitrogen. Reading you post again, that isn't what was written, so ...

[Dan]

Another thought: If your molecule contains an imine and an aniline, concentrating it may promote intermolecular reactions between these two groups, forming aminals or other imines.

You could dilute the EtOH solution with ether and wash several times with water. This will also remove the EtOH. If you use a large volume of ether relative to the EtOH (e.g. 4 times volume), you shouldn't lose too much product to the aqueous layer. Salting the water layer also helps if you are losing compound in the aqueous washes.

[kriggy]

Thanks Dan. One of the product is basicaly dimer of the compound (the reduced amino group reacts with the imine of other molecule substituing the amine part) so it could be it.
 We (my supervisor and me) are not sure where we lose the product.
Im going to run it again tomorrow in three separate vials to see where do we lose the product - using different isolation procedures. Fortunately, the reaction is done withing an hour :-D

Thank you guys for ideas and help

[Dan]

Thanks Dan. One of the product is basicaly dimer of the compound (the reduced amino group reacts with the imine of other molecule substituing the amine part) so it could be it.

That's a big clue. I'd try washing it and using it in the next step without concentration, or skip the wash if you can.

Can you tell us what the next step is?

[kriggy]

Diazotation  ;-D So I would realy like it be dissolved in water but I guess I can worry about it later when I get the compound pure.

[kriggy]

If anyone is interested in update.. I did it all over again and analyzed it by HPLC after every step..

1) compound decomposes when I evaporate the ether after extraction
2) its insoluble in water so I just precipitate it by adding water to the reaction.