Problem finding Transition state of Meinwald reaction

zako

Hello, how is everyone going ?
I started a computational study on Meinwald reaction or Meinwald rearrangement catalysed by Bronsted acid. I have optimized all the stationary points, and i also ran an IRC calculation to confirm the transition states, but the TS of the reaction step (encercled in red) is good, but the IRC doesn't give the correct product at the end.




The strategy i used to find the TS is by scanning the two bonds 1-2 and 1-3, and taking the highest energy point as a guess of the TS. The optimization of the TS was successful and i get only one imaginary frequency.


Please, I ask for your kind assistance, and thank you very much in advance.


TS2.log (266.66 KB, 下载次数 Times of downloads: 0)

2023-9-28 19:25 上传 Uploaded

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IRC-TS2.zip (432.07 KB, 下载次数 Times of downloads: 2)

2023-9-28 19:29 上传 Uploaded

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zako

Dear researchers,
is there any suggestions ? I really need some help

wzkchem5

zako 发表于 2023-9-28 12:49
Dear researchers,
is there any suggestions ? I really need some help

Maybe the red encircled step is actually a two-step reaction. The first step is as what you calculated, i.e. the C3-C4 bond is broken. The second step is the electrophilic attack of the allylic carbocation C4 onto the electron rich enol beta carbon C2.

zako

wzkchem5 发表于 2023-9-28 21:35
Maybe the red encircled step is actually a two-step reaction. The first step is as what you calcul ...

Thank you very much for your reply and suggestion.
I have read and followed what's in the article "Theoretical Study on the BF3-Catalyzed Meinwald Rearrangement Reaction, DOI 10.1021/jo5003888", and it turns out that the transition state looks like concerted one.
I verified by recalculate a reaction from the article and i get the same result as the article, but i failed in my reaction.

wzkchem5

zako 发表于 2023-9-28 15:59
Thank you very much for your reply and suggestion.
I have read and followed what's in the article ...

It is very common for one reaction to have a concerted mechanism, while a very similar reaction has a stepwise mechanism. The reason is simple. Consider the stepwise mechanism R -> TS1 -> IM -> TS2 -> P, where the energies are E(R)=E1, E(TS1)=E2+x, E(IM)=E2, E(TS3)=E2+y, E(P)=E3. Now suppose that x is a very small positive number. Then, even if you change the system only slightly (e.g. changing the BF3 to H+ as in your case), you may reduce x to zero, so that TS1 disappears and you get a concerted mechanism.