Alkylmagnesium alkoxides

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],... 

This method has been applied to the preparation of alkylmagnesium alkoxides (with sodium alkoxides) [29], carboxylates (with sodium carboxylates) [30], acetylacetonates (with sodium acetylacetonate) [29],... 

With a dialkylmagnesium compound, one equivalent of the proton donor gives an alkylmagnesium alkoxide (thiolate, amide) (see Section 3.4.4), whereas 0.5 equivalent gives the magnesium alkoxide (thiolate, amide) ... 

Similar behaviour has also been observed in the case of magnesium and other alkaline earth metal alkoxides. The normal dialkoxides of magnesium, calcium, strontium, and barium are insoluble and nonvolatile "" whereas the alkylmagnesium alkoxides are soluble in common organic solvents and some of these (especially methyl and ethyl derivatives) can be volatilized in usually accompanied by... 

The final step is reaction between alkylmagnesium alkoxide and magnesium halide, regenerating Grignard reagent ... 

Though, as mentioned earlier, four is the lowest degree of association of RMOR allowing both metal and oxygen to become 4-co-ordinate (in absence of donor solvents), higher degrees of association are also possible but would be less favoured by the entropy factor. Several alkylmagnesium alkoxides are known which are 6-8 fold associated. 

Detailed reviews on the structures of organomagnesium compounds and the constitution of their solutions have been published [D, 1, 2]. These references, especially the first two, also cover alkylmagnesium hydrides, amides, alkoxides and thiolates. However, while all of these compounds are of intrinsic interest, they are little used in synthesis. 

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. 

Although other methods are available for synthesizing alkylmagnesium dialkylamides, alkoxides and thiolates, the reaction of a dialkylmagnesium compound with a secondary amine, an alcohol, or a thiol, provides a convenient general method. Some examples are summarized in Table 3.11. 

---