Rearrangement Fritsch-Buttenberg-Wiechell

Treatment of 1,1-diaryl-2-haloethylene with base affords diaryl acetylene via the intermediacy of alkylidene carbene. 

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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. 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... 

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. 

It is unclear why in the carbenoid D it is the H atom in cis-position relative to the bromine that undergoes migration, and not the cyclohexenyl residue, which is in trans-position to the bromine. In the Fritsch-Buttenberg-Wiechell rearrangement of the chlorine-containing vinyl carbenoid it is the aryl residue in trans-position to the chlorine atom that undergoes migration ... 

An a-elimination of hydrogen halide or halogene from a sp carbon Irads 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 rearrangement... 

Organolithium compounds can also induce the rearrangement of 1,1-diaiyl vinyl dichlorides and dibromides (Fritsch-Buttenberg-Wiechell rearrangement) to diarylalkynes p,p -bridged cyclic diaiylal-kynes ° can be prepared via this route (Scheme 59). 

On treatment with n-BuLi, 2-(trimethylsilyl)-4-[(2,2-dibromo)ethenyl]furan undergoes a Fritsch-Buttenberg-Wiechell rearrangement to provide the lithium acetylide, which reacts with... 

When geminal dichloroalkenes obtained from aldehydes are treated with n-BuLi (2 eq) at low temperature, a Fritsch-Buttenberg-Wiechell rearrangement is observed to give lithiated terminal alkynes, which can be readily hydrolyzed or directly reacted with electrophiles such as alkyl halides, trialkylsilyl chlorides, or aldehydes (Scheme 3.88, Table 3.14). 

The addition of allylzinc bromide (allylmagnesium bromide + ZnBr2) to alkynyllithi-ums gives rise to gem-bimetalloalkenes. Chlorination of the latter species with PhSOiCl sets up a Fritsch-Buttenberg-Wiechell rearrangement. ... 

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]. 

The Fritsch-Buttenberg-Wiechell rearrangement provides a method to expand a cyclic ketone to a cyclic acetylene with an additional carbon atom in the ring [34]. The ketone is first converted to a vinyl bromide or a vinylidene dibromide, from which the corresponding vinylidene carbene is produced with butyllithium. The vinylidene carbene then rearranges to a cyclic alkyne (Scheme 8-10). 

When warming these vinyllithinms of type 346 to temperatures above -40 C, a Fritsch-Buttenberg-Wiechell rearrangement, which includes the a-elimina-tion of lithium carbamate and the migration (with retention of configuration) of the alkyl group attached at C-2 to C-1,provides 1-alkynes [199],... 

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. ... 

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