Vinylidene carbenoids reactivity

The particular substitution pattern of lithium carbenoids, the fact that both an electropositive metal and an electronegative substituent X are bound to the same carbon atom, causes the ambiphilic character of this species. The chameleon-like reactivity becomes evident from the resonance formulas of the carbenoid lb (equation 1) Whereas the carbanionic character is expressed by the resonance formula la, the electrophilic character is represented by Ic. In an analogous way, the reactivity of vinylidene carbenoids 2b is expressed by the mesomeric structures 2a and 2c. 

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 . 

The electrophilic reactivity of lithium carbenoids (reaction b) becomes evident from their reaction with alkyl lithium compounds. A, probably metal-supported, nucleophilic substitution occurs, and the leaving group X is replaced by the alkyl group R with inversion of the configuration . This reaction, typical of metal carbenoids, is not restricted to the vinylidene substitution pattern, but occurs in alkyl and cycloalkyl lithium carbenoids as well ". With respect to the a-heteroatom X, the carbenoid character is... 

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). ... 

Another mode of activation of o-alkynylanilines 105, involving an alkyne-vinyl-idene isomerization, was reported by Mc-Donald (Scheme 9.40) [191]. High yields of 1-monosubstituted indoles 106 were obtained upon cydoisomerization of terminal o-alkynylanUines 105 using an in situ generated Et3N Mo(CO)5 catalyst. The authors proposed a mechanism involving the initial alkyne-vinylidene isomerization [192-194] of a terminal acetylene 105 into the reactive carbenoid 107. The... 

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