- Rieke metals
Rieke metals are highly reactive
metal powders prepared by the methods developed by Reuben D. Rieke. Rieke metals are highly reactive because they have high surfaces areas and lack passivating surface oxides.Preparation
Rieke metals are usually prepared by a reduction of a
THF suspension ananhydrous metalchloride with analkali metal . Typical alkali metals used in this method arepotassium ,sodium , andlithium . [cite journal| author=Rieke, R. D.|title=Preparation of Organometallic Compounds from Highly Reactive Metal Powders|journal= Science|year= 1989|volume= 246| pages= 1260–1264|doi=10.1126/science.246.4935.1260| pmid=17832221] For example, the preparation of Rieke magnesium employs potassium as the reductant::MgCl2 + 2 K → Mg + 2 KCl
More recent reports emphasize the use of the less hazardous lithium metal in place of potassium.OrgSynth | author = Rieke, R. D.; Bales, S. E.; Hudnall, P. M.; Burns, T. P.; Poindexter, G S. | title = Highly Reactive Magnesium for the Preparation of Grignard Reagents: 1-Norbornane Acid | collvol = 6 | collvolpages = 845 | year = 1988 | prep = CV6P0845] Among the many metals that have been generated by this method are Mg, Ca, Ti, Fe, Co, Ni, Cu, Zn, In.
In some cases the reaction is carried out with a catalytic amount of an electron carrier such as
biphenyl OrgSynth | author = Rieke, R. D.; Wu, T.-C.; Rieke, L. I. | title = Highly Reactive Calcium for the Preparation of Organocalcium Reagents: 1-Adamantyl Calcium Halides and Their Addition to Ketones: 1-(1-Adamantyl)cyclohexanol” | collvol = 9 | collvolpages = 9 | year = 1998 | prep = cv9p0009] ornaphthalene . The coprecipitated alkali metal chloride is usually not separated from the highly reactive metal, which is generally used "in situ".Uses
Rieke magnesium reacts with aryl halides at -78 °C to afford the corresponding
Grignard reagent s, often with considerable selectivity. [cite journal|author=Lee, J.-S.; Velarde-Ortiz, R.; Guijarro, A.; Wurst, J. R.; Rieke, R. D., |title=Low-Temperature Formation of Functionalized Grignard Reagents from Direct Oxidative Addition of Active Magnesium to Aryl Bromides|journal= Journal of Organic Chemistry|year= 2000|volume= 65|pages= 5428–5430| doi=10.1021/jo000413i] Rieke magnesium is famous for enabling the formation of "impossible Grignard reagents" such as those derived from aryl fluorides and from 2-chloronorbornane.Rieke zinc reacts with bromoesters to give organozinc reagents of value for the
Reformatsky reaction . [cite journal|author=Rieke, R. D.; Hanson, M. V.,|title= New organometallic reagents using highly reactive metals| journal=Tetrahedron|year= 1997|volume= 53|pages=1925–1956| doi=10.1016/S0040-4020(96)01097-6]afety
The method is straightforward but requires a highly experienced chemist. The starting metal halides must be strictly anhydrous and alkali metals can be very hazardous. The resulting reagents are generally pyrophoric, the expected trade-off for a highly reactive metal.
External links
* [http://www.rznx.basf.com/ BASF Organozinc Compounds]
* [http://www.riekemetals.com/ Rieke Metals] , the owner of the Rieke brandReferences
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