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I could not think of this reaction fitting into any of the reactions I know except the aryne mechanism. And hence I came up with the following mechanism after seeing the answer (which I've written in the picture next to question). Please tell if my proposed mechanism is correct.

Also, I am highly unsure of the fact that the last species formed will be able to extract a hydrogen atom from somewhere in the solution.

Question:

enter image description here

This is my proposed mechanism:

enter image description here

Source: Joint Entrance Exam (JEE) 1997 India

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    $\begingroup$ If the product is indeed correct (diphenylacetylene), I would suggest alpha-elimination to form a carbene which rearranges with phenyl shift to give the alkyne product $\endgroup$
    – orthocresol
    Apr 3, 2018 at 20:48
  • $\begingroup$ I have never seen any alkene form a carbene before. Could you please give an example of a reaction where you have seen such a thing. Or tell, why would it happen.. Please.. $\endgroup$
    – Jeevesh Juneja
    Apr 3, 2018 at 20:55
  • $\begingroup$ @orthocresol I believe you are correct. Deprotonation of the bromo-alkene to give the C(K)Br species will certainly give rise to a carbene (or metallocarbene) see en.wikipedia.org/wiki/Carbene and the phenyl shift seems the obvious route to the product $\endgroup$
    – Waylander
    Apr 3, 2018 at 21:03

1 Answer 1

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Here is a flowchart of what the commenters have stated. An α-elimination occurs stepwise 1 --> 2 --> 3. There is a phenyl migration (bridged?) to afford tolane 4. Look here for this chemistry.

enter image description here

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    $\begingroup$ Try this: J. Am. Chem. Soc., (1936), 58, 2310. pubs.acs.org/doi/abs/10.1021/… $\endgroup$
    – user55119
    Apr 3, 2018 at 23:24
  • $\begingroup$ Can this mechanism be concerted? That is, the abstraction of proton, migration of phenyl and elimination of Br all in a single step? $\endgroup$
    – V.G
    May 20, 2021 at 18:38
  • $\begingroup$ Is the reaction concerted? Perhaps. I haven't researched it. I suspect that the deprotonation may be reversible. There are alkylidene carbenes that undergo intramolecular C-H insertion. www1.udel.edu/chem/valhalla/alkylidene.html $\endgroup$
    – user55119
    May 20, 2021 at 19:44
  • $\begingroup$ I have a doubt in the migration step. I would have thought that the bottom phenyl migrates, not the top one - similar to how in Beckmann rearrangement, the group trans to the leaving $\ce{-OH2^+}$ migrates - isn't this reaction similar with $\ce{-OH2^+}$ replaced by $\ce{-Br}$, and a carbon in place of the nitrogen? $\endgroup$
    – TRC
    Aug 6, 2021 at 13:37

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