Magnesium Rieke

Rieke magnesium by reduction of magnesium chloride with lithium naphthalene [7,8]. Lithium (224 mg, 3.3 mmol), anhydrous magnesium chloride (1.57 g, 16.5 mmol)1, naphthalene (436 mg, 3.4 mmol) and dry... 

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. 

Saturated ethers do not normally react with magnesium, though THF is cleaved by Rieke magnesium [120] or active magnesium from magnesium hydride [10] to give (2). Allyl and benzyl ethers are more reactive towards... 

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. 

Table 1 Results from the Magnesium-ene Reaction Shown in Scheme 2 Using Vapor-Activated Magnesium. Anthracene-Magnesium and Rieke Magnesium...

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. 

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

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. 

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. 

Table 4 Reactions of Rieke Magnesium with Various Halides and Dihalides...

Significantly, when p-dibromobenzene was treated with one equivalent of Rieke magnesium at — 78X, only the mono-Grignard was formed quantitatively in 15 min [62]. The yield was determined by GC after hydrolysis and was based on the disappearance of starting material and the formation of bromobenzene. This is in stark contrast to the reaction of p-dibromobenzene with ordinary magnesium, which affords a mixture of the mono- and di-Grignards, as well as unreacted starting material [63]. 

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