Dialkylmagnesium compounds

Aside from alkyllithium compounds, only dialkylmagnesium compounds gave high optical yields of the alcohols using ligand (94) as chiral auxiliary109c,114). 

Reaction of dialkylmagnesium compounds with 2,6-bis(imino)pyridines results in quantitative A-alkylation of the pyridine skeleton (equation 15). 

The only dialkylmagnesium compound whose enthalpy of formation has been measured is dineopentyl magnesium (s) —236.8 7.2 kJmol and (g) —74.3 7.6 kJmol. Unfortunately there are no enthalpies of formation for any of its isomers or homologs. We cannot even calculate the enthalpy of the Schlenk equilibrium because, although the enthalpy of formation of neopentylmagnesium bromide is for the ether solution, that for dineopentylmagnesium is not, and there is no experimental value for the enthalpy of solution. 

EIGURE 6. Dialkylmagnesium compounds form unusually strong adducts with sparteine. NMR data indicate that sparteine adopts cisoid configuration on co-ordination... 

Cyclopropenes possess appreciable high reactivity toward carbomagnesiation . For example, dialkylmagnesium compounds react with spiro[2.4]hept-l-ene (75) to give the carbometalated products 76 (Scheme 53) . The iyw-addition has been confirmed by D2O quenching of the reaction. 

Carbon-13 shifts of Grignard reagents (Table 4.73) [479]) in ether solutions provide evidence that the Schlenck equilibrium is shifted in favor of the dialkylmagnesium compound There is practically no shift difference between dibenzylmagnesium and benzyl-magnesium chloride. 

The incorporation of TEMPO alkoxy amine at the end of a PE chain has been achieved [99]. The dialkylmagnesium compound in ethylene polymerization was adopted as a chain transfer agent, as mentioned above. It was also reported that PE-TEMPO and terminally N-(2-mcthyl-2-propyl)-N-(l-diethylphosphono-2,2-dimethylpropyl)-N-oxyl PE were synthesized by the reaction with di-polyethylene magnesium produced in ethylene polymerization. They were used for CRP of n-butyl acrylate, leading to PE-h-PnBA. 

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. 

The compounds obtained by reactions of magnesium with dienes (and anthracene) are classified as special cases of dialkylmagnesium compounds see, for example, p. 40. 

Chelating diethers such as DME and 1,4-dioxane are useful, but their use tends to result in precipitation of a magnesium halide complex this phenomenon is made use of in the most commonly used procedure for obtaining solutions of dialkylmagnesium compounds. 

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). 

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. 

The metallation of cyclopentadienes and related compounds by Grignard reagents is slow and often requires high temperatures [A, D], though it is faster in the presence of solvents such as hexametapol [11]. Dialkylmagnesium compounds are more reactive, and metallation of cyclopentadiene itself by the n-butyl-s-butylmagnesium reagent occurs at a convenient rate at 25°. 

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] ... 

Preparation of dialkylmagnesium compounds from organoalkali metal compounds... 

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. 

If this type of equilibrium is displaced to the right by selective precipitation of magnesium halide, a solution of the dialkylmagnesium compound should result. Such selective precipitation does sometimes occur, notably from hydrocarbon solutions (see, for example, p. 6 [1]), but it is rarely sufficiently selective and/or complete to provide a useful method for obtaining dialkylmagnesium compounds. However, certain additives have been found to achieve the desired result. Of these, pyridine [2] is not recommended because of the risk of attack on the pyridine ring... 

Exchange of organic ligands between dialkylmagnesium compounds occurs more or less rapidly, depending on the conditions and on the presence of, for example, metal salts [15] ... 

The pure hydrides may be obtained by reactions of dialkylmagnesium compounds with active magnesium hydride, and isolated as solvated solids [21]-... 

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. 

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],... 

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) ... 

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. 

Unlike dialkylmagnesium compounds, normal dialkylzincs do not react with epoxides. However, the m situ reagents 2RMgX + ZnXa readily give secondary alcohols without primary alcohols (108). Since... 

Trialkylaluminum and alkylaluminum hydrides associate with alkyl or hydride bridges. Since there are no available lone-pair electrons with which to form bridges by standard two-center two-electron interactions, multicenter bonding is invoked in the same manner as for electron-deficient boranes (see Boron Hydrides), alkyllithium (see Alkali Metals Organometallic Chemistry), dialkylberyllium and dialkylmagnesium compounds (see Beryllium Magnesium Organometallic Chemistry). 

Using a variety of approaches (3), hydrocarbon-soluble R Mg were discovered, including the so-called "unsymmetrical" dialkylmagnesium compounds (RMgR, where R and R are to n-C alkyl groups). In fact, RMgR compounds are today the most important of the commercially available dialkylmagnesiums used in manufacture of polyethylene ... 

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. 

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