Rieke metal powders

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... 

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... 

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., 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. 

Magnesium can take a variety of different physical forms turnings, powder, chips and finely divided metal (such as Rieke-magnesiuin )- Each of these forms offers advantages and disadvantages. as summarized iu Tahle 6.2. This section also considers the issues with mechanical activation of the magnesium metal, while the next section discusses die use of chemical initiators. 

Rieke, R.D. 1977. Preparation of highly reactive metal powders and their use in organic and organometallic synthesis. Acc Chem Res 10 301-306. 

Rieke, R. D. 1989. Preparation of organometallic compounds from highly reactive metal powders. Science 246 1260-1264. 

Finely divided, highly reactive, Rieke metal powders are normally prepared by reducing metal halides with potassium metal in refluxing tetra-hydrofuran. This clearly represents a laboratory safety hazard but sonication can produce these same powders under much milder conditions using the less reactive lithium metal at ambient temperatures [12]. Thus the preparation of Rieke copper, which normally requires an 8 h reflux with potassium in THF, can be achieved in less than 40 min at room temperature with sonication (Scheme 10.3). 

Rieke Magnesium. Rieke metals are highly reactive metal powders prepared by reduction of the corresponding halide by potassium in a suitable solvent. The first publication in this field by Rieke and co-workers appeared in 1972 [38]. For reviews see [39]. 

Rieke Zinc. Highly reactive metal powders are made through the reduction of metal salts, using alkali metals in a suitable solvent [38, 39]. Rieke zinc, Zn, has... 

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