14 - Formation of Carbon-Halogen Bonds via Organomagnesium Compounds

Abstract

This chapter explores the formation of carbon–halogen bonds via organomagnesium compounds. The reactions described here are valuable for replacing one halogen by another, when the organomagnesium compound has been prepared from an organic halide, or for introducing a halogen regiospecifically, when the organomagnesium compound has been obtained by other routes. Both types of reactions are particularly useful with aromatic compounds, where halogen exchange is otherwise difficult, and where regiochemistry different from that of electrophilic aromatic substitution may be accessible. In many cases, the reaction of an organomagnesium compound with elemental chlorine, bromine or iodine gives the corresponding organic halogen compound in satisfactory yields. The main limitations are the risk of halogenation of functional groups (particularly unsaturation), and in the case of iodine, coupling of the organic groups. Chlorination of alkylmagnesium bromides or iodides, and bromination of alkylmagnesium iodides, may lead to halogen scrambling. These are relatively nonhazardous and easily stored and handled sources of electrophilic fluorine. Preliminary studies indicate that they react smoothly with Grignard reagents to give the corresponding organic fluorides. When side-reactions are troublesome, other sources of “positive halogen” may be used.