Nucleophilic substitution, lithium carbenoids

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... 

A similar type of substitution, which clearly shows the electrophilic character, occurs in vinylidene carbenoids. In an early example of this reaction, Kobrich and AnsarP observed that the aUcene 70 results when the fi-configurated vinyl lithium compound 68 is treated with an excess of butyllithium and the fithioafkene 69 formed thereby is protonated (equation 41). Obviously, the nucleophilic attack of the butyl residue on the carbenoid takes place with inversion of the configuration. 

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 . 

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