Riecke zinc

The Riecke zinc, obtained by reduction of a zinc halide with sodium or potassium in THF, is commonly used for the... 

Direct Reaction of Zn with Alkyl Halides. The direct insertion see Insertion) reaction of Zn metal into alkyl halides - alkyl iodides being the ideal snbstrates - is a nseful reaction to prepare simple or polyfunctional organozinc halide compounds (equation 1). With primary alkyl iodides, the reaction requires an excess of Zn dnst (ca. 3 eqniv), previonsly treated with few mol % of 1,2-dibromoethane and TMSCl, and a temperature of 40 °C in THF. In these conditions, secondary alkyl iodides react at room temperatnre and benzylic and allylic bromides at 0 °C. The insertion see Insertion) into less activated C-X bonds may reqnire more reactive forms of zinc (Riecke zinc), higher temperatures, or the use of polar see Polar Compounds) solvent or cosolvent. 

Heteroaryl zinc compounds are particularly useful in palladium-catalysed coupling, being compatible with many functional groups. They are often prepared in situ via lithiation, followed by reaction with zinc halides, but direct zincation of halides can be carried out, using either Riecke zinc, as in the example below, or, more conveniently, ordinary zinc dust, with various means of activation. 

The second pathway is analogous to the reaction of an activated metal, similar to those generated using Riecke s methodology, with alkyl halides. However, Luche reports that pre-activation of the zinc gave poor yields of organozinc under sonochemical conditions. 

This second method of preparation is particularly simple and practical. Zinc powder can be made more reactive if necessary, using various means including sonication or preparation by reduction of ZnClj (1.5h, 20°C) using lithium and naphthalene as electron-transfer mediator in THF (Riecke Zn noted... 

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