Wiechell

FRITSCH BUTTENBERG - WIECHELL Acetylene Synthesis Alpha elimination from haloethylenes leading via carbene rearrangement to acetylenes... 

Without the dilution, (Z)-l,4-diphenyl-l-buten-3-yne is detected, apparently formed from the cross-coupling with phenyl acetylide, derived from lithiation of g-bromostyrene, followed by E2cB elimination or Fritsch-Butlenberg-Wiechell type rearrangement. 

Another feature of carbenoid-type reactivity is the cyclopropanation (reaction c). Again, this reaction does not only take place in vinylidene but also in alkyl carbenoids . On the other hand, the intramolecular shift of a /3-aryl, cyclopropyl or hydrogen substituent, known as the Fritsch-Buttenberg-Wiechell rearrangement, is a typical reaction of a-lithiated vinyl halides (reaction d) . A particular carbenoid-like reaction occurring in a-halo-a-lithiocyclopropanes is the formation of allenes and simultaneous liberation of the corresponding lithium halide (equation 3). ... 

This stereochemical outcome of the Fritsch-Buttenberg-Wiechell rearrangement is well compatible with the crystal structure of the carbenoid 3 (Figure 1, Scheme 4). The aryl moiety trans to the vinylic chlorine atom is bent towards Cl (C1-C2-C9 116.5°). Thus, migration of the fraw -aryl group with simultaneous liberation of lithium chloride becomes evident. The free vinylidene carbene can be ruled out as the intermediate. 

A final carbenoid-type reactivity of a-oxygen-substituted vinylidene carbenoids has been reported for the carbamate 111. When wanned up to temperatures higher than —40 °C, a Fritsch-Buttenberg-Wiechell reanangement takes place to give the alkyne 112 (equation 60). Below that temperature, the lithium compound 111 maintains its nucleophilic reactivity . 

A. Generation and Nucleophilic Property of Magnesium Alkylidene Carbenoids, the Fritsch-Buttenberg-Wiechell Rearrangement... 

Alkylidene carbenoids are sp carbenoids and are known to be very interesting reactive intermediates. The most famous reaction of alkylidene carbenoids is the Fritsch-Buttenberg-Wiechell rearrangement (equation 33) . ... 

The Fritsch-Buttenberg-Wiechell rearrangement of the carbenoid gave the alkyne 131, which was further metallated in the reaction mixture with the excess of the afkyhnetal to... 

This carbenoid undergoes a Fritsch-Buttenberg-Wiechell rearrangement84 when warmed at room temperature, to give the internal acetylene derivative (equation 58)85. 

The intermediary formed sp2-geminated organodimetal in equation 59 also undergoes the Fritsch-Buttenberg-Wiechell rearrangement, once transformed into alcohol, to give diastereomerically enriched alkynol (equation 60)85. 

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