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In the landmark synthesis of tropinone by Willstatter in 1901, an interesting step involves the use of $\ce {Br_2}$, quinoline to convert cyclohexa-1,3-diene to cyclohexa-1,3,5-triene. In the scheme below, 7 is first converted into cyclohepta-1,3-diene via a Hofmann elimination. Further reaction with $\ce {Br_2}$, quinoline yields 8. I am quite puzzled as to how this transformation occurs. Wouldn't addition of bromine to the double bonds occur if bromine is reacted with cyclohepta-1,3-diene?

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  • $\begingroup$ It's probably 1,4-addition and later elimination creating two bonds. $\endgroup$
    – Mithoron
    Dec 29, 2021 at 15:05
  • $\begingroup$ @Mithoron 1,4 addition to the diene and then elimination to form the two bonds? Did I get your right? $\endgroup$ Dec 29, 2021 at 15:15
  • $\begingroup$ Yep, pretty simple, but it may be a bit tricky to get it to add bromine selectively in 1,4 manner. $\endgroup$
    – Mithoron
    Dec 29, 2021 at 16:40
  • $\begingroup$ @Mithoron I could not find an example of the bromination of cyclohepta-1,3-diene but the related case of buta-1,3-diene gives a 1:1 mixture of 1,2 and 1,4-bromination see this SE answer chemistry.stackexchange.com/questions/118945/… and chem.libretexts.org/Courses/Purdue/… $\endgroup$
    – Waylander
    Dec 29, 2021 at 17:56
  • $\begingroup$ I think the answer is here: synarchive.com/syn/303. The reagents are slightly different though and I am not sure how at -5°C, there is a 82% yield of the 1,4 adduct because the 1,2 adduct is usually favoured at lower temperatures I believe. $\endgroup$
    – M.L
    Dec 29, 2021 at 19:10

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Thanks to @Mithoron, @Waylander and @M.L for your insightful comments! Willstatter's synthesis is described in greater detail on synarchive, as shared by M.L. Indeed, as Mithoron pointed out, 1,4-addition to the diene formed after Hofmann elimination occurs. Subsequent elimination with the base quinoline yields the triene, as depicted below.

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  • $\begingroup$ I have grave doubts whether this actually gives an 82% yield of the 1,4 addition product. It is very old work and I doubt that the researchers could distinguish between the 1,4 and 1,2 addition products. It probably does not matter, kick the 1,2 dibromo product hard enough (and quinoline at 150C is hard enough) and it will give the di-elimination to the tri-ene. $\endgroup$
    – Waylander
    Dec 31, 2021 at 20:41
  • $\begingroup$ @Waylander Hmmm... Could you elaborate on what you mean by "kick the 1,2 dibromo product hard enough"? I'm guessing your suggesting the triene is the thermodynamic product of this sequence of reactions and that sufficiently high temperature would favour it? $\endgroup$ Jan 1, 2022 at 14:36
  • $\begingroup$ Exactly so. With the 1,2-addition product the pathway to the triene may well pass through a bromodiene intermediate (bromine on an alkenyl carbon) which is harder to eliminate to the triene. However, quinoline at 150C is sufficiently forcing that it will eventually eliminate. $\endgroup$
    – Waylander
    Jan 1, 2022 at 16:16

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