2. Background
The Fritsch–Buttenberg–Wiechell (FBW) rearrangement,
named for Paul Ernst Moritz Fritsch (1859–1913), Wilhelm
Paul Buttenberg, and Heinrich G. Wiechell, is a chemical
reaction whereby a 1,1-diaryl-2-bromo-alkene rearranges
to a 1,2-diaryl-alkyne by reaction with a strong base such
as an alkoxide.
2
6. Mechanism
Benzyne
Intermediate
Traces at High temp.
Elimination of Possibility of Route A
As the Diels-Alder Adduct is formed only in traces at high
temperature, the formation of benzyne intermediate is ruled
out which eliminates the possibility of Route A mechanism
6
7. Mechanism
Elimination of Possibility of Route C
If Route C mechanism is followed, the Grignard reaction will
also lead to similar ring cyclization through carbanion
intermediate formation, which never happens.
H2O
7
8. Mechanism
Only Possibility of Route B
[through Anionic 1,2-Sigmatropic Rearrangement]
But, as per Woodward-Hoffmann symmetry rule, the anionic 1,2-
Sigmatropic rearrangement is 4-electron process and is
FORBIDDEN.
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9. Mechanism
Orbital Symmetry conversion in a pericyclic reaction
[Woodward-Hoffmann Rule]
9
1,2-Sigmatropic
rearrangement may
be symmetrically
FORBIDDEN but
thermodynamically it
is a ALLOWED
process
14. 14
If reaction follows
Route B,
it must have:
Strong Base
Vinyl Anion
Halogen
Non-Polar Solvents
[A] Bromine attach to terminal carbon
Route B (Forbidden Route):
[B] Bromine partially attach to vinyl group
Mechanism