8 - Substitution at Carbon by Organomagnesium Compounds

Abstract

This chapter describes the substitution at carbon by organomagnesium compounds. For organic synthesis, a good general method for creating carbon-carbon bonds by formal nucleophilic substitution would be invaluable. Unfortunately the apparently obvious methods, such as reactions of organometallic compounds with organic halides, are often far from satisfactory. Accordingly, many variations on both the nucleophilic component and the leaving group have been tried. In this chapter, an attempt is made to recapitulate the circumstances, in which acceptable yields of the preferred products have been achieved, without comprehensive discussion of reaction mechanisms. The nature of the phosphine ligand may be critical in certain cases, and some success has been achieved in inducing asymmetry by the use of a chiral ligand. In contrast, cross-coupling between arylmagnesium chlorides and aryl bromides is catalyzed by nickel(ll) chloride, without any bidentate ligand. Palladium-catalyzed cross-coupling of organomagnesium compounds with alkyl halides is rarely useful, but a satisfactory nickel catalyzed coupling with a neopentyl halide has been reported recently. Various methods for direct homo-coupling are now available, such as palladium-induced methods, so that prior conversion of the halide into an organomagnesium reagent is often superfluous.