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A reaction I've tried several times in the past is the hydrogenation of nitroimidazole derivative shown below. The amino product is extremely unstable and rapidly decomposes.

This instability is noted in may pieces of literature and the explanation tends to only go far as saying 'ring fission' occurs.

Question: What is the mechanism of this ring fission? To be clear, I am not interested in getting this reaction to work.... It is no longer a target of mine.

Perhaps the entire mechanism is not determinable, but I presume the amine of one molecule attacks another molecule's imidazole ring? Where would this be? How is fission occuring?

EDIT: Possibly something to do with the apparently favoured tautomerism between these two forms:

https://i.stack.imgur.com/AhNsE.jpg

Reduction

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  • $\begingroup$ I think this product should be stable, but would react with substrate in this case. IMO condensation of amino and nitro group could occur. $\endgroup$
    – Mithoron
    Apr 22, 2017 at 22:01
  • $\begingroup$ @Mithoron I've found evidence in some old textbooks that the decomposition takes place via a Van Slyke determination mechanism? Mechanism is below. I don't see how it applies given I have no acid in my mixture. imgur.com/a/GbyKc $\endgroup$
    – Hazinga
    Apr 23, 2017 at 7:34
  • $\begingroup$ Maybe -HCN after lone pair donation from amine. This may cyclicize with the phenyl. Look at 123 triazole decomposition it will probably be similar. $\endgroup$
    – AS_1000
    Dec 14, 2017 at 11:43

3 Answers 3

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I'm afraid the answer to how this ring fission occurs is... it doesn't, you just made some mistake.

Here and here the authors used the same reaction as you and got the product in good yields. I think you might not realized that the product is a very hydrophilic amine (or salt, depending on workup). If you did an extraction of your reaction mixture then the product surely ended up in the aqueous phase.

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  • $\begingroup$ Can anyone explain why this answer gets downvoted? I provided the publications that back my claim and it is an explanation of what the question author observed. $\endgroup$ Jan 16, 2018 at 10:30
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    $\begingroup$ I'm sure you already had an idea, but it's most likely the language. We're not trying to be prudes, but it unnecessarily detracts from the point you are trying to make. $\endgroup$ Jan 16, 2018 at 20:12
  • $\begingroup$ OK, I'm sorry. I'll remember it and not use such languange in future. $\endgroup$ Jan 16, 2018 at 20:26
  • $\begingroup$ I surmise that if you edit it the downvotes may be retracted, since you have a quite valid point. $\endgroup$ Jan 16, 2018 at 20:45
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    $\begingroup$ @Hazinga perhaps a poor choice of word on my part, I only meant to say that from a moderation POV the answer itself should be allowed to stand (whether it is true or not, I do not know. I did not fact check). I have had similar difficulties isolating 1,2-diaminobenzenes (after hydrogenation of o-nitroanilines) so I am not surprised that aminoimidazoles are problematic, but I don't know anything about the exact nature of the problem. $\endgroup$ Jan 19, 2018 at 20:52
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This is just a guess, but it could be worth considering carbanions. The nitrogen atom containing the phenyl group, when donating its electrons into the ring (in the image at the top, the donation I am talking about is to the left of the donating nitrogen atom) the nitro group has enough electron withdrawing power to accept the donation from the adjacent bond. Thel amine formed from the reduction of this nitro group, may be basic enough to deprotonate any remaining nitro compound (imidazoles are amphoretic). If this is true, when the electron donation occurs, a carbanion may form. With a carbanion could accelerate decomposition.

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One explanation could be that the imadazole is hydrogenated as well, leading to a decomposition of the starting material to phenylamine, ethylamine and ethane: enter image description here

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  • $\begingroup$ Or something similar, e.g. formation of phenylamine and 2-aminopentane. $\endgroup$ May 15, 2017 at 20:04
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    $\begingroup$ I see you are getting downvoted. Can you supply empirical or literature evidence to back the claim? $\endgroup$ May 15, 2017 at 20:58
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    $\begingroup$ Well, its just an idea. If you think its wrong, then explain why/provide an alternative. $\endgroup$ May 15, 2017 at 21:18
  • $\begingroup$ While over reduction of the pyrrole ring is conceivable, reduction of the pyrrolidine ring is without merit. $\endgroup$
    – user55119
    Jan 16, 2018 at 2:51

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