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.

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.

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