# Mechanism of Fritsch–Buttenberg–Wiechell rearrangement

The following reaction was given, which uses potassium t-butoxide.

The solution to this problem was given as follows:

Step 1 is the attack of strong base, t-butoxide, on the vinyl proton.

Step 2 seems to have the chlorophenyl group shift to the neighbouring carbon, forcing the cleavage of the $$\ce{C–Br}$$ bond.

Here, the bromine is conjugated to the alkene, since the lone pair on bromine is in resonance with the alkene, leading to partial double bond character between $$\ce{C}$$ and $$\ce{Br}$$. In such a case, the $$\ce{C–Br}$$ bond usually cannot be cleaved.

My question is

• Is this carbanion intermediate responsible for the product?
• What is the mechanism to form 1-chloro-4-(phenylethynyl)benzene?

However, if you carefully read this reference, you'd realize they use special method to generate diradical from gem-dibromo-alkene stepwise. Thus, the proposed mechanism for every FBW-type reactions. Earlier, A. A. Bothner-By (Ref.2) from Harvard reported that in a diaryl-substituted system of a bromoalkene (e.g., see structures 1 and 4 in the scheme 1), the major migrating group is the aryl moiety located anti to the halogen (the fact, which has been proven by $$\ce{^{14}C}$$-labeling experiments). Particularly, this migration of an aryl group between multiply-bonded carbons is one of the several unusual features of the rearrangement (Ref.2):