Coupling reactions Grignard reagents/copper salts

As can be seen in the following examples, both Cu and Cu" salts such as CuCl. CuBr, Cul, CuCN, Cu(acac)2, and Li2CuCl4 (known as Kochi s catalyst [22]), introduced at 0.2 10% to a Grignard or a substrate are most typical for these reactions. The choice of copper salts is dependent on their intrinsic reactivities, as well as on physical properties, such as solubility and ease of handling. For example, Li2CuCl4 is soluble in tetrahydrofuran (THF) and is frequently quite useful to obtain clean cross-coupled products. An added Cu species is considered to be reduced to Cu in situ by the Grignard reagent to participate in the catalytic cycle [see Eq. (36)]. 

Similar variants of Ullmann-type reactions based on intra- as well as intermolecular homo-couplings of various arylmetallic reagents, beside aryllithiums, also aryl Grignard reagents, arylzincs etc. with copper(II) salts give intermediate diarylcopper(Il) reagents which subsequently produce respective biaryls, usually in good yields, see Chapter 7. 

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