Halomethyllithium Carbenoids: Versatile Reagents for the Homologation of Electrophilic Carbon Units

Monohalomethyllithium carbenoids are synthetically useful methylene transfer agents. They allow the direct incorporation of a (reactive) halomethylenic fragment into an electrophile, thus overcoming additional steps needed for the transformation of intermediate diazoketones or β-oxo dimethylsulfoxonium ylides into the desired adducts. The good nucleophilicity of these lithium species accounts for the exclusive 1,2-addition to a,b-unsaturated systems, thus complementing the 1,4-addition observed with sulfur ylides. Though the preparation of these reagents requires cryogenic temperatures and the use of pyrophoric reagents, the hazardous risks can be considered less problematic than those involving the use of the carcinogenic and explosive diazomethane.