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What is the mechanism of the attached reaction? How could nosylate do the dehydrogenate the species?

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The reaction is from this synthesis (46th IChO preparatory problem 26): enter image description here

ringo
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EJC
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    I think this is quite a complex mechanism, but I would assume that the nitro group is being reduced. Could you add a reference as a starting point? – Martin - マーチン Jan 06 '15 at 08:09
  • Edited my original post. – EJC Jan 06 '15 at 09:53
  • Thanks, that will help put it into context, but I am still thinking that a suitable mechanism is hard to propose. I tried tracking down the original paper, which they used as source, but was unsuccessful due to language barrier. – Martin - マーチン Jan 06 '15 at 11:20
  • So I guess that it would be rather written as a standard red-ox reaction. – EJC Jan 06 '15 at 12:28
  • @Marko The method of using 3-nitrobenzenesulfonate for that kind of dehydrogenation was invented by Gerhard Bachmann http://www.google.com/patents/US3487081 I don't know the mechanism, but if you can get access try looking at the following article and references cited http://onlinelibrary.wiley.com/doi/10.1002/jhet.5570320114/abstract I don't have access. – DavePhD Jan 06 '15 at 13:41
  • I don't have access either. – EJC Jan 06 '15 at 14:19
  • I checked Cignarella's J. Heterocyclic Chem. paper (1995, 32, 79). There is no mention of the mechanism. They cite a conference presentation by Bachmann as the first report of the method (no help there) and a couple of other med chem papers from the Cignarella group. I checked one of these (J. Med. Chem. 1989, 32, 2277), and there is no mention of mechanism. – jerepierre Jan 06 '15 at 17:21
  • Was that a question on the Olympiad or are you just curious? – Jori Jan 06 '15 at 18:56
  • I am just curious. – EJC Jan 06 '15 at 19:00
  • @jerepierre They usually don't bother with the reaction mechanism, as long as it is working. I would not even know where to start investigating this. Dehydrogenation often starts with a proton abstraction, but is a sulfonyl group or nitro group really strong enough to do that? – Martin - マーチン Jan 07 '15 at 11:44
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    @Martin Without knowing the fate of the sulfonate reagent, I don't feel confident in any mechanism. If there were some transition metal to mediate the dehydrogenation, the nitro group could be the terminal reductant. The reaction is under basic conditions (hydroxide) which makes me think it's going through an enolate. I wonder if somehow the O-sulfonate is formed, and then an elimination occurs to form a carbonyl with formation of sulfinate. That's my best guess, but there are no similar reactions in SciFinder. – jerepierre Jan 07 '15 at 16:16

1 Answers1

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This is the reaction mechanism that I propose for this reaction. Though it does not show the complete reduction of the 3-nitrobenzene sulfonate, it could be reduced further through the nitroso-hydroxylamine pathway. This means that in total, a single nitrosyl anion could oxidize three molecules of the starting material.

Extraction of a hydrogen by the nitro group. The hydrogen is extremely acidic compared to an ordinary aliphatic $\ce{C-H}$ bond due to the massive conjugation in the resulting ion:

step 1

Amide-imidic acid tautomerization:

step 2

Resonance structures of the molecule, showing its extensive conjugation, and hence stability:

step 3

Radical reaction pathway and formation of the product. Every atom in product is $\mathrm{sp^2}$ hybridized and is conjugated over the entire system. It is much more stable than the starting material, and is the driving force of the entire reaction:

step 4

ringo
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