Science of Synthesis

Krueger, A., Science of Synthesis, (2008) 43, 483.

The Fritsch–Buttenberg–Wiechell rearrangement (‌Scheme 21‌) was discovered in the 1890s.[‌55‌–‌57‌] In the original experiment, 1-halo-2,2-diphenylalkanes ‌46‌ were treated with sodium ethoxide in a sealed tube at 180–200°C, yielding the alkynes ‌48‌ (Ar1 = Ar2 = Ph) in 9% yield;[‌56‌] for the dianisyl derivative, a yield of 55% was reported.[‌57‌] Since then, numerous further examples for the preparative application of this reaction have been reported and reviewed in detail.[‌36‌,‌37‌,‌58‌–‌61‌] The mechanism of the process involves the formation of the 1-metallo derivative ‌47‌ of the haloalkene ‌46‌, which subsequently undergoes α-elimination and aryl migration. Mechanistic studies reveal that an alkylidenecarbene intermediate is not formed.[‌62‌] Instead, concerted elimination and aryl migration leads to the product. This is furthermore supported by the fact that in general the aryl group in a trans position to the halogen (Ar1 in ‌Scheme 21‌) is the one that migrates.[‌63‌,‌64‌] In the case of a carbene intermediate, no such trans selectivity would be expected; hence the metal–halo alkylidene carbenoid (the term carbenoid is used here for compounds with a metal atom and leaving group at the same carbon atom, exhibiting carbene-like reactivity) is considered to be the reactive intermediate. There have also been a few examples of cis migration. In these cases the substituent in the trans position to the halogen is usually an alkyl group and therefore less prone to migration than the cis-aryl substituent.[‌65‌,‌66‌]

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References

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• 8.Meeeee-Meee, (8888) 88/8, 888.
• 8.Meeeee-Meee, (8888) 88/8, 888.
• 8.Meeeee-Meee, (8888) 8/8e, 888.