Metal–halogen exchange

In organometallic chemistry, metal–halogen exchange is a fundamental reaction that converts an organic halide into an organometallic product. The reaction commonly involves the use of electropositive metals (Li, Na, Mg) and organochlorides, bromides, and iodides. Particularly well-developed is the use of metal–halogen exchange for the preparation of organolithium compounds. Two kinds of lithium–halogen exchange can be considered: reactions involving organolithium compounds and reactions involving lithium metal. Commercial organolithium compounds are produced by the heterogeneous (slurry) reaction of lithium with organic bromides and chlorides: 2 Li + R−X → LiX + R−Li Often the lithium halide remains in the soluble product. Most of this article is about the homogeneous (one-phase) reaction of preformed organolithium compounds: R−Li + R′−X → R−X + R′−Li Butyllithium is commonly used. Gilman and Wittig independently discovered this method in the late 1930s. It is not a salt metathesis reaction, as no salt is produced. Lithium–halogen exchange is frequently used to prepare vinyl-, aryl- and primary alkyllithium reagents. Vinyl halides usually undergo lithium–halogen exchange with retention of the stereochemistry of the double bond. The presence of alkoxyl or related chelating groups accelerates lithium–halogen exchange. Lithium halogen exchange is typically a fast reaction. It is usually faster than nucleophilic addition and can sometimes exceed the rate of proton transfer. Exchange rates usually follow the trend I > Br > Cl. Alkyl- and arylfluoride are generally unreactive toward organolithium reagents. Lithium–halogen exchange is kinetically controlled, and the rate of exchange is primarily influenced by the stabilities of the carbanion intermediates (sp > sp2 > sp3) of the organolithium reagents. Two mechanisms have been proposed for lithium–halogen exchange. One proposed pathway involves a nucleophilic mechanism that generates a reversible “ate-complex” intermediate.