Lithium Halomethylcarbenoids: Preparation and Use in the Homologation of Carbon Electrophiles

α-Halomethyllithium carbenoids are useful homologating reagents which – reacting under proper reaction conditions as carbanions – enable the installation via nucleophilic addition of a reactive halomethyl fragment onto a preformed carbon–heteroatom bond. The pronounced thermolability represented – since seminal studies by Köbrich – the Achilles' heel of these reagents: the use of Barbier-type methodologies (i.e., the electrophile should be present in the reaction mixture prior to the formation of the carbenoid) was pivotal in order to suppress decomposition through α-elimination processes. Nowadays, the use of low temperatures (−78 °C) guarantees reliable procedures and, significantly, the employment of microreactor technologies allows external trapping to be performed even at higher temperatures as reported by Luisi. We will discuss the α-halomethyllithium-mediated homologations of a series of carbon electrophiles such as carbonyl compounds, imines, esters, Weinreb amides, and isocyanates.