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3.1.8.2.2 Variation 2: From an Organomagnesium Reagent

DOI: 10.1055/sos-SD-003-00002

Knochel, P.Science of Synthesis, (2004355.

In 1926, the reaction of organomagnesium compounds with a dry zinc halide was described for the first time.[‌194‌,‌195‌] Since then, the reaction has been changed in numerous ways. The use of dry, water-free zinc halide (X=Cl, Br, I) is essential to carry out the reaction. Usually, a 1M solution of the zinc halide in diethyl ether is added to the Grignard solution at room temperature under vigorous stirring. The dialkylzinc is then isolated from the resulting mixture by fractional distillation. For example, dibutylzinc(II) is prepared from butylmagnesium chloride and zinc(II) chloride in 57% yield. Di-tert-butylzinc(II) is accessible by this route and cannot be prepared by direct reaction of zinc with tert-butyl iodide or bromide. The yields are only moderate, because the dialkylzinc is not easily separated from its complex with diethyl ether. With diethylzinc(II) and higher boiling dialkylzincs it is possible to remove completely the diethyl ether solvent by heating to 80°C under vacuum for several hours. With this method, even optically active diorganozinc compounds are accessible, with at least one methylene group as a spacer to the metal.[‌196‌] Using the Schlenk trick (precipitation of magnesium salts from ethereal solution by the addition of 1,4-dioxane), mixtures of a Grignard reagent and 0.5 equivalents of zinc(II) chloride in diethyl ether can be converted into dialkylzincs 129 (Scheme 69). The precipitated magnesium halide can be removed by simple filtration under argon in order to achieve a clear, salt-free ethereal solution of the dialkylzinc.[‌18‌] This procedure represents an easy access to salt-free dialkylzinc reagents for routine use.

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