Dialkylmagnesium compounds synthesis

Magnesium can form one or two covalent bonds to carbon. In the former case, the other valency is available to form bonds to a variety of other groups. (For a discussion of coordination to Mg(n), see Section 2.1.2.) Some of the resulting types of organomagnesium(n) compounds are listed in Table 2.1. Of these compounds, only the chlorides, bromides and iodides, and to a lesser extent the dialkylmagnesium compounds, are widely used in synthesis, and almost all the examples described in this book involve these classes. (Some of the others may be important as intermediates in certain reactions see, for example, Section 6.1.) Accounts of the chemistry of the other classes of compounds may be found in General Ref. [D] and in Refs [1] and [2] other references are given in Table 2.1. 

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. 

In a 1985 review article, it was declared that dialkylmagnesium compounds are of "limited practical importance" (2). Though this statement may have been accurate at that time, it is certainly not true today. In the early 2P century, the value of polyethylene produced from R Mg-derived Ziegler-Natta catalysts contributes substantially to the "practical importance" of these materials. Globally, millions of tons of polyethylene are manufactured each year using such catalysts. Dialkylmagnesium compounds are used in Ziegler-Natta catalyst synthesis in two fundamentally different ways discussed in sections 4.3.2 and 4.3.3 below. 

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. ... 

Two principal approaches to the synthesis of an optically pure chiral secondary or tertiary alcohol from the reaction of an organometallic reagent with an aldehyde or ketone respectively are of current interest. In the first approach an alkyllithium or dialkylmagnesium is initially complexed with a chiral reagent which then reacts with the carbonyl compound. In this way two diastereo-isomeric transition states are generated, the more stable of which leads to an enantiometic excess of the optically active alcohol. This approach is similar in principle to the asymmetric reductions discussed in Section 5.4.1 (see also p. 15). Two chiral catalysts may be noted as successful examples, (10) derived... 

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