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.

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The method usually involves reduction of a THF suspension an anhydrous metal chloride with an alkali metal. Typical alkali metals used in this method are potassium, sodium, and lithium.^[1] 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.^[2] 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^[2] or naphthalene. The coprecipitated alkali metal chloride is usually not separated from the highly reactive metal, which is generally used in situ.

Risks and procedure

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.

Uses

Rieke magnesium reacts with aryl halides at -78 °C to afford the corresponding Grignard reagents, often with considerable selectivity.^[3] Rieke magnesium is famous for enabling the formation of "impossible Grignard reagents" such as those derived from aryl fluorides and from 2-chloronorbornane.^[2]

Rieke zinc reacts with bromoesters to give organozinc reagents of value for the Reformatsky reaction.^[4]

References

1. ^ Rieke, R. D. (1989). "Preparation of Organometallic Compounds from Highly Reactive Metal Powders". Science 246: 1260-1264.
2. ^ ^{a b c} Rieke, R. D.; Bales, S. E.; Hudnall, P. M.; Burns, T. P.; Poindexter, G S. “Highly Reactive Magnesium for the Preparation of Grignard Reagents: 1-Norbornane Acid” Organic Syntheses, Coll. Vol. 6, p.845 (1988); Vol. 59, p.85 (1979). http://www.orgsyn.org/orgsyn/pdfs/CV6P0845.pdf
3. ^ Lee, J.-S.; Velarde-Ortiz, R.; Guijarro, A.; Wurst, J. R.; Rieke, R. D., (2000). "Low-Temperature Formation of Functionalized Grignard Reagents from Direct Oxidative Addition of Active Magnesium to Aryl Bromides". Journal of Organic Chemistry 65: 5428-5430. doi:10.1021/jo000413i.
4. ^ Rieke, R. D.; Hanson, M. V., (1997). "New organometallic reagents using highly reactive metals". Tetrahedron 53: 1925-1956. doi:10.1016/S0040-4020(96)01097-6.