Magnesium Rieke methods

Alkali metal reductions of metallic salts using an arene as electron carrier, lithium being the most used metallic component (Rieke method) . Although not belonging to this group of metals, magnesium-anthracene has found some apphcations in the activation of other metals . 

An important aspect of the active magnesium by Rieke methods is its convenient preparation. The apparatus required is very inexpensive and simple. The reductions are normally carried out in a two-neck flask equipped with a condenser (if necessary), magnetic stirrer, under an argon atmosphere. In general, it is not possible to prepare anhydrous... 

Substituted allyl Grignard reagents were synthesized in high yields from the corresponding chlorides and Mg-slurry, obtained by evaporation, activated by equilibration with its anthracene adduct in THF [30]. The authors compared this magnesium with Rieke-magnesium and found, on careful experimentation, that also the Rieke-method gave satisfactory results. 

As was the case with magnesium, also highly reactive forms of zinc were made through the Rieke method and through its intercalation compounds with graphite. 

Freshly distilled THF (10 ml) is added to the active magnesium (black powder), followed by the appropriate substrate. This is the current method of choice for the preparation of Rieke magnesium [54]. 

Another interesting variation of the potassium-magnesium chloride reduction is to carry out the reduction in a mixed solvent system of THF and triethylamine (ratio 1 1). This method also yields a form of magnesium that is nearly as reactive as the lithium reduction procedure. Table 3 shows reactions of p-chlorotoluene with Rieke magnesium prepared in THF-Et N. Few reports have appeared using this form of highly reactive magnesium. 

The synthesis of spiro y-lactones has been extended to the synthesis of spiro -lactones. This one-pot transformation of 1,3-dienes reacted with epoxides, mediated by Rieke magnesium to spiro -lactones in good isolated yields, was recently reported [15]. Much work has been done to elucidate novel synthetic routes for these types of molecules [16]. In particular, -substituted 5-lactones have recently attracted considerable attention, mainly because molecules of this class include many natural products that exhibit substantial biological activity [17]. However, most methods for the synthesis of these molecules require... 

Highly activated magnesium, prepared using Rieke s method (see 10.2.3.2.1 and 10.2.3.2.3), reacts with 1,3-dienes, including 1,3-butadiene, isoprene, myrcene and ( ),( )-1,4-diphenyl-1,3-butadiene . 

I A new method was introduced for the preparation of highly reactive, finely dispersed metals by reduction of their halides with the aid of alkali metals. These metals are named Rieke metals (such as Rieke magnesium. Mg, Rieke zinc, Zn, etc.) after the inventor of the process [53-56]. Their reactivity is extremely high and reactions have been made possible which were simply unthinkable before. 

Little is known of alkyl and arylcadmium halides. They cannot be obtained by reacting cadmium metal with organic halides, unless active cadmium is prepared by Rieke s method (p. 18). The dialkyls are usually prepared by the Grignard method. Their main use is to convert acyl halides into ketones. Pure Et Cd does not react with benzoyl chloride, but it does so in the presence of the magnesium salts which are by-products in the Grignard synthesis. 

We have developed a direct synthetic method for the one-pot synthesis of lactones, spirolactones, di-spirolactones, tertiary alcohols, and even 1,2-diols from the corresponding conjugated diene-magnesium reagents mediated by Rieke magnesium [122], The overall lactonization procedure can be considered... 

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