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Author: Subject: need help of a reaction's mechanism
ZHANGNIUBI
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sad.gif posted on 6-1-2012 at 08:51
need help of a reaction's mechanism


It‘ s about aminofurazan's diazotization(with HNO2--mix of NaNO2/HOAc). when the reaction occurs, a-Hydroximinoacetonitriles were common products.
The problem is how can a C+ becomes a C- . My point is by oxizdation of HNO2, but it seems to be weired. At least, I ve never seen a reaction like that before. My friend thought that it may form a structure like C-N=N-O- and lose N2O to form a C-, but from my point of view, it's wrong again, 'cause usually, a C-N=N-O- is obtained in strong base like conc.KOH.
Does any body know the answer? Thank you!

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[*] posted on 6-1-2012 at 19:13


。。。
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[*] posted on 6-1-2012 at 19:45


This reaction is not known in the literature, or at least not the way you've drawn it. I would not recommend you invoke anions under acidic conditions, this is a fairly universal rule. So, I disagree with your friend as well.

As you pointed out, the starting material is in a higher oxidation state than the product, and thus must undergo a reduction. A good idea when drawing mechanisms is to consider the oxidation state of starting materials and products to determine whether or not it is a redox reaction (see scheme A).
- rings = +1
- heteroatoms = +1
- pi bonds = +1

diazotization.bmp - 565kB

So others know what you're talking about, nitrous acid is formed, and dissociates into the nitrosonium cation (scheme B). This would lead to diazotization by a mechanism such as shown in C, but cannot proceed to product due to lack of a reducing agent. According to literature precedent, this diazonium salt leads to a nitrofurazan in the presence of sodium nitrite. Obviously it could add any nucleophile present.

If any one of these intermediates were reduced, monosubsitututed 1,2,5-oxadiazoles decompose spontaneously, or under acid/basic conditions to give alpha-oximinonitriles by cycloreversion - you know this it seems.

Other options might include radical fragmentation or intramolecular hydride delivery, but I'm not sure exactly how.


[Edited on 7-1-2012 by Arrhenius]
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[*] posted on 6-1-2012 at 20:55


Quote: Originally posted by Arrhenius  
This reaction is not known in the literature, or at least not the way you've drawn it. I would not recommend you invoke anions under acidic conditions, this is a fairly universal rule. So, I disagree with your friend as well.

As you pointed out, the starting material is in a higher oxidation state than the product, and thus must undergo a reduction. A good idea when drawing mechanisms is to consider the oxidation state of starting materials and products to determine whether or not it is a redox reaction (see scheme A).
- rings = +1
- heteroatoms = +1
- pi bonds = +1



So others know what you're talking about, nitrous acid is formed, and dissociates into the nitrosonium cation (scheme B). This would lead to diazotization by a mechanism such as shown in C, but cannot proceed to product due to lack of a reducing agent. According to literature precedent, this diazonium salt leads to a nitrofurazan in the presence of sodium nitrite. Obviously it could add any nucleophile present.

If any one of these intermediates were reduced, monosubsitututed 1,2,5-oxadiazoles decompose spontaneously, or under acid/basic conditions to give alpha-oximinonitriles by cycloreversion - you know this it seems.

Other options might include radical fragmentation or intramolecular hydride delivery, but I'm not sure exactly how.


[Edited on 7-1-2012 by Arrhenius]


well, this reaction actually was found on :

http://www.docin.com/p-43518848.html?bsh_platform=renren

on the page 131, scheme 130. the article part shows some papers(index), I m not sure if those can help. because I can't read them...maybe you can use your school's online library to figure it out. and I ll be very grateful if you can share some of them
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[*] posted on 7-1-2012 at 00:35


Okay, thanks. I found it, but I can only get a few of the papers cited - most are foreign journals, and I don't read Russian anyway. Of the one I do get, and ran into before I posted, they claim to obtain the oximinonitrile byproduct when attempting a Sandmeyer reaction. They used 1eq of copper 1 or 0, and got poor yield of methyl oximinonpropionitrile. Copper is serving as the reducing agent. They draw both the radical (as I've shown) and a cation, which is bloody confusing, because I thought they meant radical cation for a moment, but it could be the cation as well, which ultimately must undergo one electron reduction and hydrogen abstraction (e.g. from solvent) or a second one electron reduction and a proton abstraction. Maybe you could get a good yield with the proper stoichiometry of copper - i.e. >2eq.

diazotization.bmp - 217kB
Rakitin, O.A. et al, Russian Chem. Bull., (1993) 42(11) 1865.

Oy. Here's another idea from a paper that yields a similar product (also cited in the book). Not sure I buy this one, but you decide. I don't even know what to call NO2+ :(.

nitrosation.bmp - 145kB
Churakov, A.M. et al, Chem Heterocycl Comp. (1988)24, 1378.

[Edited on 7-1-2012 by Arrhenius]
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[*] posted on 7-1-2012 at 06:10


Quote: Originally posted by Arrhenius  
Okay, thanks. I found it, but I can only get a few of the papers cited - most are foreign journals, and I don't read Russian anyway. Of the one I do get, and ran into before I posted, they claim to obtain the oximinonitrile byproduct when attempting a Sandmeyer reaction. They used 1eq of copper 1 or 0, and got poor yield of methyl oximinonpropionitrile. Copper is serving as the reducing agent. They draw both the radical (as I've shown) and a cation, which is bloody confusing, because I thought they meant radical cation for a moment, but it could be the cation as well, which ultimately must undergo one electron reduction and hydrogen abstraction (e.g. from solvent) or a second one electron reduction and a proton abstraction. Maybe you could get a good yield with the proper stoichiometry of copper - i.e. >2eq.


Rakitin, O.A. et al, Russian Chem. Bull., (1993) 42(11) 1865.

Oy. Here's another idea from a paper that yields a similar product (also cited in the book). Not sure I buy this one, but you decide. I don't even know what to call NO2+ :(.


Churakov, A.M. et al, Chem Heterocycl Comp. (1988)24, 1378.

[Edited on 7-1-2012 by Arrhenius]



Wow, the second one seems to be what I need..
thank you very much!!
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