Activation by Formation of Alloys

Cementation, the deposition of small amounts of an auxiliary metal on a support metal, has been used for a long time as an activation method. A common example is surface alloying with mercury, i.e., amalgamation. The properties of the couple largely depend on the redox potentials of both constituents. In general, the base metal is considered to behave as an electron source, while the true chemistry is effected by the superficial auxiliary metal. Several examples illustrate this principle. 

49 A common example is the activation of magnesium by mercuric chloride. See Wakefield, B.J. Organomagnesium Methods in Organic Synthesis, Academic Press, London, 1995. 

The conventional preparation of the zinc-copper couple is long and tedious Sonication of copper (I) iodide with zinc powder in THF or in alcoholic solvents makes it much easier, and a black suspension is produced in a few minutes The method tolerates up to 40% of water in the organic solvents. Besides the applications for conjugate additions to activated olefins (p. 225), this sonochemically prepared couple was used for the carbonylation of iodoalkanes to aldehydes.52 The zinc-nickel couple prepared from nickel salts and zinc is an excellent catalyst for the hydrogenation of olefinic bonds (p. 187).53 Cementation of cobalt on zinc is achieved in water under sonication.54 The temperature must be adjusted to avoid the reduction of water. 

Whether a couple is formed from a sonicated mixture of zinc and magnesium powders was not determined, but because of the tribochemical effects, this possibility should not be excluded. Such a metallic system was used for an intramolecular reductive coupling.55 

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