Magnesium compared with lithium

As a first approximation one can view metallation and electrolytic reduction as a single class of reactions differing only in the ease with which electrons are transferred to the substrate. Ordinarily mercury metal does not react with alkyl halides because of its high ionization potential of 240 kcal mol as compared with 124,176 and 216 kcal mol for lithium, magnesium and zinc, respectively. However, if one places a potential across mercury then it will readily react with alkyl halides in an electrolytic reaction. 

It should be noted that this reported chemistry was performed with Rieke magnesium prepared by the MgCl2—KI—K—THF system. Rieke magnesium prepared by the safer lithium naphthalenide reduction approach should be expected to give the same or even improved results compared with what is reported here. 

Equation (10) exemplifies a Hoerner olefination of phosphonic acid esters, using a Grignard reagent or magnesium alkoxide [Mg(OEt)2, not shown] as the base [13]. Magnesium bases gave superior yields with aliphatic aldehydes, compared with sodium and lithium bases. 

Magnesium-eneamides can be used in the synthesis of a chiral oxazoline [29 Eq. (22)]. r-Butylmagnesium bromide was used to deprotonate the starting eneamine. The chelating nature of the magnesium resulted in partial racemization, giving lower optical yields compared with lithium-eneamine. 

The majority of the metallated hetero-aromatic compounds are obtained by direct metallation [1, 2, 5]. The hetero-atom effect (inductive, coordinative and polarizability influence) in most cases directs the metallation to the position a- to the hetero atom and gives rise to an easier deprotonation compared with that of benzene [1]. In some cases, bromine-lithium exchange offers the possibility of introducing the metal in a position that is not accessible by direct deprotonation. In incidental cases, Grignard compounds have been prepared from halogenated hetero aromates and magnesium. 

As we noted in Section 22.1, a second-period element is often considerably different from the other elements in its column. Those differences become more pronounced as you progress to the right in the periodic table. We briefly noted differences in properties of lithium from those of the other alkali metals, though those differences are not great. Beryllium, however, shows rather marked differences from the other alkaline earth elements. We have already noted that beryUium differs in its lack of reactivity compared with the other Group IIA metals. Another notable diffoence is in the properties of the hydroxides. Whereas those of the elements magnesium to barium are basic, beryllium hydroxide is amphoteric, reacting with both acids and bases. 

In a later extension of this methodology, Itami and coworkers extended the ranges of both sets of coupling partners, and examined the effect of replacement of lithium t-butoxide in dioxan with magnesium t-butoxide in dimethylformamide (DMF). Systematically comparing the two sets of conditions, they found that haloarenes 23... 

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