Rieke metals

Not to be confused with Raney nickel.

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 surface area and lack surface oxides which retard reaction.

Preparation

Rieke metals are usually prepared by a reduction of a THF suspension of an anhydrous metal chloride with an alkali metal. Typical alkali metals used in this method are potassium, sodium, and lithium. 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. 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 or naphthalene. 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 reagents, often with considerable selectivity. 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.

Safety

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

• BASF Organozinc Compounds
• Rieke Metals, the owner of the Rieke brand