Zinc compounds homocoupling reactions

Functionalized organozinc halides are best prepared by direct insertion of zinc dust into alkyl iodides. The insertion reaction is usually performed by addition of a concentrated solution (approx. 3 M) of the alkyl iodide in THF to a suspension of zinc dust activated with a few mol% of 1,2-dibromoethane and MeaSiCl [7]. Primary alkyl iodides react at 40 °C under these conditions, whereas secondary alkyl iodides undergo the zinc insertion process even at room temperature, while allylic bromides and benzylic bromides react under still milder conditions (0 °C to 10 °C). The amount of Wurtz homocoupling products is usually limited, but increases with increased electron density in benzylic or allylic moieties [45]. A range of poly-functional organozinc compounds, such as 69-72, can be prepared under these conditions (Scheme 2.23) [41]. 

Another efficient method for the preparation of biaryls is Pd-catalyzed homocoupling of biaryliodonium salts in the presence of zinc dust (Scheme In comparison with other methods, this procedure gives excellent yields of the corresponding biaryls however, the efficiency is hampered by the fact that 1 equiv of aryl iodide does not participate in the reaction. This reaction proved to be very sensitive to the catalyst used. High yields are achieved only with Pd(acac)2 the use of other palladium compounds results in considerably lower yields of biaryls, if any. 

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