Preparation of dialkylmagnesium compounds

The following procedure gives the useful hydrocarbon-soluble metallating reagent referred to in Section 3.2.1 (see also Table 3.1, p. 22). It is more complicated than procedures described earlier [18], but gives a reagent which is less liable to precipitation on storage. 

Preparation of dialkylmagnesium compounds from organoalkali metal compounds 

Organoalkali metal compound Magnesium component0 Product Ref. 

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Tertiary amines have been comparatively little used as solvents for organomagnesium compounds, though triethylamine shows some promise [2], TMEDA is more commonly used as an additive than as a solvent one molar equivalent is normally sufficient to exert the required activating effect. Pyridine is less reactive towards organomagnesium compounds than towards organolithium compounds (see p. 96), and has been used as an alternative to 1,4-dioxane in the preparation of dialkylmagnesium compounds (see p. 65). 

Asymmetric synthesis of alcohols. This ligand is elfective for asymmetric addition of an alkyllithium to an aldehyde. In more recent work it has been found that the lithium anion (2) of 1, prepared with n-butyllithium, is particularly eifective for asymmetric addition of dialkylmagnesium compounds to aldehydes. AU the alcohols have the (R)-configuration. Toluene was found to be a superior solvent for this reaction lower temperatures increasfe optical yields. Only one example of use of an aliphatic aldehyde was reported only a low optical yield was obtained. ... 

Interestingly, the first alkylmagnesium fluorides have been prepared in high yield by reaction of alkyl fluorides with magnesium in the presence of catalysts e.g. iodine) and by reaction of dialkylmagnesium compounds with boron trifluoride etherate both types of reaction are carried out in THF, and the latter can be applied to diphenylmagnesimn to provide phenylmagnesium fluoride. 

The reaction of dialkylmercury compounds with magnesium is valuable for preparing dialkylmagnesium compounds, free from traces of halide, and either as solids or in a variety of solvents. 

More recently, a related reaction has been reported, in which active forms of magnesium hydride, prepared in situ or pre-prepared, undergo addition to alkenes, catalysed by titanium or zirconium(iv) halides, to give dialkylmagnesium compounds [56, 59, 64] ... 

For application in synthesis, the most important of these are those where X = halogen and Y = alkyl the transformation of Grignard reagents into dialkylmagnesium compounds. Other transformations of this type are valuable for preparing alkylmagnesium hydrides (Y = H), alkoxides (Y = OR ), carboxylates (Y = OCOR ), amides (Y = NR 2, thiolates (Y = SR ), etc., but since most of these products are not extensively employed in synthesis these transformations are summarized only briefly. 

A convenient alternative procedure for alkylmagnesium alkoxides, which avoids the sometimes tedious preparation of a dialkylmagnesium compound, is the reaction of an alkyl chloride, magnesium and an alcohol, in a medium (such as a hydrocarbon) in which magnesium chloride is insoluble [28],... 

Occasionally the presence of the halide in a Grignard reagent may cause complications, but these can be avoided by the use of the corresponding dialkylmagnesium compound. The above preparation failed with methylmagnesium bromide. 

The reaction of dialkylrnercury compounds with magnesium is valuable for preparing dialkylmagnesium compounds, free from traces of halide, and either as solids or in a variety of solvents (Scheme 3.7). It has been used particularly to prepare samples for physicochemical studies. The reaction can be performed in the absence of solvent, in which case the product is extracted from the magnesium amalgam formed by use of a suitable solvent. In a solvent such as diethyl ether the reaction may be very slow, but proceeds in virtually quantitative yield. 

Solutions of dialkylmagnesium that are free of halide ion can be prepared from alkylmercury compounds. This is an example of a general class of reactions in... 

Hallwachs and Schafarik 1859, Cahours 1860. Ten years after Frankland s discovery of the organozinc compounds Hallwachs and Schafarik [23] and then Cahours [24], were able to prepare and identify dialkylmagnesium compounds. 

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