Rearrangement reactions Fritsch-Buttenberg-Wiechell

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). ... 

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... 

An a-elimination of hydrogen halide or halogene from a sp2 carbon leads formally to a vinyl carbene species which may eventually rearrange to the corresponding acetylene. This reaction was discoverd in 1894 and is called Fritsch-Buttenberg-Wiechell rearrangement159). 

Turning to carbene related reactive species, alkylidene carbenoids like 26 (X = halogen, OR, NR2) are particularly valuable for preparative purposes since they can undergo cycloaddition reactions with olefins (to methylenecyclopropanes), isomerizations (to alkynes by the so-called Fritsch-Buttenberg-Wiechell rearrangement), and dimerization (to [3]cumulenes). Although carbenoids have been studied extensively by NMR spectroscopy [23], the first X-ray structural analysis of a stable carbenoid, 27, as a TMEDA 2THF complex has been reported only recently [24]. 

In addition, this reaction has been extended to occur under electrochemical and photochemical conditions. The rearrangement occurs under photo irradiation is known as photo-Fritsch-Buttenberg-Wiechell rearrangement. ... 

Wiechell (see Fritsch-Buttenberg-Wiechell Rearrangement) Wieland (see Barbier-Wieland Degradation) Willgerodt-Kindler Reaction Williamson Synthesis... 

A remarkable allylic alcohol-alkyne combination reaction, catalysed by a ruthenium complex, is believed to proceed via a vinylidene complex (Scheme 8.47). The proposed mechanism, again, involves a reverse Fritsch-Buttenberg-Wiechell rearrangement, followed by nucleophilic attack on the resulting vinylidene... 

Typical substrates for Fritsch-Buttenberg-Wiechell rearrangements are chloro-ethenes and ge/w-dibromoethenes. They are readily prepared by Wittig reaction from the corresponding ketones. The method offers a versatile entry to aromatic or heterocyclic acetylenes. Moreover, ethynyl groups can embark also on 1,2-migrations, thus, making simple or functionalized diynes and triynes (e.g., 321) accessible (Scheme 1-250). ... 

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