Rieke powder

A very reactive form of a finely divided metal is a so-called Rieke powder [79]. These materials are produced as fine powders by chemical precipitation during the reduction of various metal halides ivith potassium metal in refluxing tetrahydrofuran. Obviously this is a potentially hazardous laboratory procedure and ultrasound has provided an alternative method of preparation of these extremely valuable reagents [80]. The sonochemical technique involves the reduction of metal halides with lithium in TH F at room temperature in a cleaning bath and gives rise to metal powders that have reactivities comparable to those of Rieke powders. Thus powders of Zn, Mg, Cr, Cu, Ni, Pd, Co and Pb were obtained in less than 40 min by this ultrasonic method compared with reaction times of 8 h using the experimentally more difScult Rieke method (Tab. 3.1). 

These sonically prepared Rieke powders show enhanced reactivity in organic synthesis involving metals e.g. in the preparation of organosUicon compounds (Scheme 3.11) [81]. 

Rieke powders are normally prepared by reducing the corresponding metal halides with potassium metal in refluxing tetrahydrofuran, which is clearly a hazardous procedure. Much of the hazard can be avoided if lithium metal is used in place of potassium and the reaction is carried out at room temperature... 

RIEKE ET AL. Preparation of Highly Reactive Metal Powders... 

At this time the applicability of organozinc chemistry had been limited to those zinc reagents that could be prepared by insertion of zinc powder into the corresponding alkyl iodide.20 Further developments, notable the use of highly reactive zinc (Rieke zinc), obtained by the reduction of zinc halides with lithium naphthalenide, allowed the preparation of zinc reagents from otherwise unreactive organic substrates, such as aryl iodides and aryl bromides (Scheme 1.7).21... 

R. D. Rieke, Preparation of Organometallic Compounds from Highly Reactive Metal Powders, Science 1989, 246, 1260-1264. 

The zinc-mediated Reformatsky reaction is one of the classical methods for carbon-carbon bond formation. To date, various main group metals and transition metals have been used for this reaction. Rieke s activated indium powder mediates readily the coupling of ethyl a-bromoacetate and a variety of carbonyl compounds yielding /3-hydroxy esters in good yields (Scheme 87).3 Later, commercially available indium powder has been found to be equally effective for the indium-based Reformatsky reaction in THF.28 This indium Reformatsky reaction is accelerated by ultrasound irradiation (Scheme 88).322,323 Indium(i) iodide also mediates the Reformatsky reaction of aldehydes and ketones to give /3-hydroxy esters, presumably via organoindium(m) diiodide (Scheme 89).27... 

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]. 

Walborsky and Rachon have also reported on the use of Rieke magnesium to obtain reaction with (S)-(+)-l-bromo-l-methyl-2,2-diphenylcyclopropane at —65°C to afford a chiral Grignard reagent that is 33-43% optically pure [73]. In 1961, Young and Walborsky [74] demonstrated that the reaction of chiral (S)-(+ )-l-bromo-l-methyl-2,2-diphenyl-cyclopropane (Table 5 1) with magnesium powder resulted in the formation of a chiral Grignard reagent as evidenced by the fact that on carbonation of the reaction mixture an optically active acid was obtained with an optical purity of approximately 12-18%. 

Dihydrodisilaanthracenes 150 [Eq. (66)] were successfully prepared from the aryl di-Grignard reagents 149 synthesized using Rieke magnesium and bis(o-chlorophenyl) methylsilane or bis(o-chlorophenyl)dimethylsilane [120,121]. These aryl chlorides were unreactive toward granular or powdered magnesium. 

For the synthesis of some functionalized benzylic organocadmium bromides by using highly reactive cadmium metal powder, see E. R. Burkhardt and R. D. Rieke, J. Org. Chem., 1985, 50, 416. 

Rieke, R. D., Li, P. T.-J., Burns, T. P., Uhm, S. T. Preparation of highiy reactive metai powders. New procedure forthe preparation of highiy reactive zinc and magnesium metai powders. J. Org. Chem. 1981, 46, 4323-4324. 

RIeke, R. D., Rhyne, L. D. Preparation of highly reactive metal powders. Activated copper and uranium. The Ullmann coupling and preparation of organometalllc species. J. Org. Chem. 1979, 44, 3445-3446. 

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