Preparation with Rieke Magnesium

Numerous procedures have been used to activate the magnesium surface often 

Examples of 1 and 3 have already been described above. Activation of magnesium by amalgamation or by alloying with copper or different transition metals has been reported, but these means of activation rarely offer advantages over other methods, and involve toxicity hazards and/or the risk of promoting side reactions. They are, therefore, not usually recommended, although the amalgamation method is occasionally useful. 

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Reactivity in Grignard reagent formation varies in the sequence I > Br > Cl > F. Fluorides are particularly unreactive, but Grignard preparation has been reported with metal vapor and matrix-isolation techniques. Mg anthracene, and most efficiently, with Rieke magnesium. Side reactions with iodides, and to a lesser extent bromides, may lower the yield of the reagent. 

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. 

Table 3 Reactions of p-Chlorotoluene with Rieke Magnesium Prepared in THF-Et3N...

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. 

Rieke magnesium ( 0.020 mmol) is prepared as described on p. 24 in a 200 ml three-necked round-bottomed flask equipped with a PTFE-coated magnetic stirrer bar, stopper, septum and reflux condenser, and furnished with an atmosphere of argon, and allowed to cool to room temperature. 1-Chloronorbornane (1.25 g, 9.58 mmol) is injected by means of a warm syringe. The mixture is heated under reflux for 6 h, and allowed to cool to room temperature. 

The reactions of Rieke magnesium with various halides and dihalides, some of which react with difficulty under the conditions of normal Grignard preparations, were investigated [58]. The results are summarized in Table 4. The yields obtained by GC after hydrolysis, and the yields of carboxylic acids obtained after CO2 quench are shown. 

The so-called Rieke magnesium [111], which is prepared by reduction of magnesium bromide with potassium, is highly reactive and allows preparation of Grignard reagents... 

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. 

After nearly 70 years, in 1972 Reuben Rieke showed that even Idnetically inert alkyl fluorides reacts with activated magnesium - the latter is prepared by in situ... 

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