[mana]

hi all
in this reaction why benzyne react as showed here?
why it can not react with formaldehyde 2+2 
and also does OMe group effect on regiochemistry on reaction by electron donating or not?

[clarkstill]

You need to think about how the butyl group is incorporated into the final product. Perhaps the benzyne could do a [2+2] with formaldehyde, but something else happens first (presumably much more quickly).

I'm guessing the regiochemistry is mostly due to acidification of the adjacent CH bond by the presence of the fluorine?

[mana]

You need to think about how the butyl group is incorporated into the final product. Perhaps the benzyne could do a [2+2] with formaldehyde, but something else happens first (presumably much more quickly).

I think may be this mechanism has happened, but I'm not sure

[clarkstill]

That looks reasonable to me. When you ask about regiochemistry, do you mean the regiochemistry of benzyne formation, or of addition of Bu- to the benzyne? You probably need to account for both.

As for why it doesnt do a [2+2] - benzynes are electron-deficient so will tend to react with the best available nucleophile, which here is BuLi.