Magnesium, alkyl derivatives

The reactivity of the zinc alkyl derivatives [TpBut]ZnR is typically lower than that of the corresponding magnesium derivatives (Section III,A,2,c.). For example, whereas [TpBut]MgMe undergoes insertion of C02 into the Mg-C bond at room temperature, no reaction is observed between [TpBut]ZnMe and C02 at 140°C. Similarly, whereas the magnesium alkyl derivatives [TpBut]MgR (R = Me, Et, Pr1, Bul) react immediately with 02 at room temperature to give alkylperoxo derivatives [TpBut]MgOOR, solutions of the zinc derivative [TpBut]ZnEt are stable in the presence of 02 at 100°C. 

Reaction XLIII. (c) Condensation of a-Halogen Fatty Acid Esters with Aldehydes and Ketones by means of Zinc or Magnesium (Reformatsky-Grignard). (C., (1901), I., 1196 II., 30 (1902), I., 856.)—This is an extension of the Grignard and zinc alkyl reactions which enables a-halogen esters to be condensed with carbonyl compounds as if they were simple alkyl halogen compounds. The zinc or magnesium alkyl derivative is neither prepared beforehand nor isolated in the reaction, but there is little doubt that some such compound is transitorily formed. Zinc is the metal... 

II. SYNTHESES AND STRUCTURES OF MONOMERIC FOUR-COORDINATE TRIS(PYRAZOLYL)HYDROBORATO MAGNESIUM ALKYL DERIVATIVES... 

Tris(pyrazolyl)hydroborato magnesium alkyl derivatives [Tp ]MgR are readily prepared by the reactions of [M][Tp ] (M = K, Tl) with solutions of the appropriate dialkyl-magnesium [Eq. (3) 14-17]. The reaction is very general and a series of primary, secondary tertiary, vinyl, and phenyl derivatives have been obtained, as illustrated in Table 1. The magnesium alkyl complexes [Tp ]MgR are all white crystalline solids that are soluble... 

However, it is noteworthy that oxygen atom transfer to the magnesium alkyl derivative does not occur rapidly at room temperature. This is presumably one of the factors that is responsible for the observation that the reactions of [Tp ]MgR with O2 are selective in the formation of alkylperoxo complexes, rather than the alkoxo derivatives. It is likely that this is a consequence of the sterically encumbered nature of the magnesium centers in these complexes. 

II. SyiithcjkO and Structures of Monomeric Four-Coordinate Tris(pyrazolyl)hydroborato Magnesium Alkyl Derivatives... 

Why are dialkylzincs able to mono- or dialkylate tantalnm (V) by reaction with TaClj whereas lithium and magnesium alkyl derivatives, for instance, cannot do so ... 

The direct conversion of 3-methylcyclohex-2-enone into 2-allyl-3-methylcyclohexanone provides an interesting example of the utility of the reduction-alkylation procedure. Synthesis of this compound from 3-methy I cyclohexanone would be difficult because the latter is converted mainly into 2-alkyl-5-methylcyelohexanones either by direct base-catalyzed alkylation11 or by indirect methods such as alkylation of its enamine (see Note 13) or alkylation of the magnesium salt derived from its cyclohexylimine.12... 

Alkyl derivatives of the Group 2 metals (Be, Mg, Ca, Sr, Ba) supported by poly(pyrazolyl)borato ligation have only been isolated for the lighter congeners, beryllium and magnesium. 

As with their beryllium and magnesium counterparts, the zinc alkyl derivatives [TpRR ]ZnR and [BpBut]ZnR are soluble in hydrocarbon... 

Similarly to the alkyl derivatives, the most common route for arylcopper compounds is the reaction of a copper halide and aryllithium compounds (Equation (4)). Organocuprates with aryl groups are obtained by using an appropriate excess of the lithium reagent. Magnesium aryls have also been employed in transmetallation reactions with Cu(l) salts to yield both arylcopper compounds and arylcuprates (Equations (5) and (6)). 

Double-tail derivatives were sometimes prepared by short syntheses thus, dithioacetals 9 were obtained by condensation of a free carbohydrate with the corresponding -alkanethiol [50] whereas the 6,6-di-O-alkyl derivatives 29 were synthesized by the reaction of alkyl magnesium bromide with l,2 3,4-di-0-iso-propylidene-D-galacturonic acid [62]. 

Pyrazolopyrimidine 148 reacted with magnesium to yield the corresponding Grignard reagent 149 that reacted with pivaldehyde to yield the corresponding C-alkylated derivative 150 (Scheme 5) <2003JOC2054>. 

Several years ago, Castro, Villieras and coworkers described the preparation of the magnesium enolate derived from an alkyl a,a-dihaloacetate by halogen-metal exchange between isopropyhnagnesium chloride and alkyl trihaloacetate. THF is required as solvent (equation 7) °. 

This mechanism can be used to account for the solvent dependence, which shows that a strongly coordinating solvent slows or prevents step (4) from occurring and thus retards the rate of exchange. This is similar to the effect observed in Group III alkyl derivatives discussed in Section IV,A. However, if complete coordination of the magnesium alkyl does not occur, it becomes necessary to break a... 

When lithium alkyl catalysts are used in non-solvating media such as aliphatic hydrocarbons, the polymer-lithium bond is not sufficiently ionic to initiate anionic polymerization so that the monomer must first complex with vacant orbitals in the lithium. A partial positive charge is induced on the monomer in the complex, and this facilitates migration of the polymer anion to the most electrophilic carbon of the complexed monomer. This type of polymerization is more appropriately termed coordinated anionic and will be discussed in the next section. There does not appear to be any evidence that alkyl derivatives of metals which are less electropositive than lithium and magnesium can initiate simple anionic polymerization. 

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