Highly reactive metal powders convenience

Since 1972, we have published many reports describing convenient methods for the generation of highly reactive metal powders and their use in organic as well as organometallic synthesis [28-34]. Most of the active metals prepared by our group have been prepared in ethereal solvents [35]. Our initial report on the preparation of active uranium (I) employed 1,2-dimethoxyethane (DME) as a solvent (Scheme 13.1) [31]. We have since developed a method for preparing active uranium (3) in hydrocarbon solvents, which leads to a much cleaner and more controllable chemistry compared to 1 [34, 36]. 

However, when the reductions were carried out with lithium and a catalytic amount of naphthalene as an electron carrier, far different results were obtained(36-39, 43-48). Using this approach a highly reactive form of finely divided nickel resulted. It should be pointed out that with the electron carrier approach the reductions can be conveniently monitored, for when the reductions are complete the solutions turn green from the buildup of lithium naphthalide. It was determined that 2.2 to 2.3 equivalents of lithium were required to reach complete reduction of Ni(+2) salts. It is also significant to point out that ESCA studies on the nickel powders produced from reductions using 2.0 equivalents of potassium showed considerable amounts of Ni(+2) on the metal surface. In contrast, little Ni(+2) was observed on the surface of the nickel powders generated by reductions using 2.3 equivalents of lithium. While it is only speculation, our interpretation of these results is that the absorption of the Ni(+2) ions on the nickel surface in effect raised the work function of the nickel and rendered it ineffective towards oxidative addition reactions. An alternative explanation is that the Ni(+2) ions were simply adsorbed on the active sites of the nickel surface. 

The zinc metal is typically activated before use and methods for accomplishing this have been reviewed. The use of highly reactive forms of zinc (Reike powders), obtained by reduction of zinc salts with an alkali metal, detracts from the convenience of the classical procedure but much higher yields have been obtained, at least with the simple substrates that have so far been examined. One of the most convenient preparations of a Reike powder uses sodium naphthalide, as shown in Scheme 4. Reactive zinc powders so allow the use of a-chloro esters which are unsatisfactory with the usual forms of... 

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