Organocopper and -silver compounds

Me Tlacac 6 Distorted pentagonal bipyramid (one equatorial site vacant) 170 

The structures of several dimethylthallium derivatives have been determined in the solid state by X-ray crystallography. Some results are summarized in Table 3.12 and in Fig. 3.21. Thallium adopts coordination numbers of four, five and six. The trend to higher coordination numbers going down a Group is noted for Si to Pb (p. 102). 

Copper compounds have been used for many years in organic chemistry. The Ullmann synthesis of biphenyls in which an aryl iodide is heated with copper metal to temperatures over 200°C is considered to involve arylcopper intermediates rather than free radicals. The presence of 2- and 4-nitro groups aids coupling 2-nitro-iodobenzene can be coupled by Cu OjSCF, in ammoniacal solution even at room temperature. 

Copper(I) iodide reacts with monoalkyl and arylethynes in aqueous ammonia giving insoluble yellow, red or brown derivatives [Cu—C=CR] . These reactions resemble those of mercury(II) (p. 61). These acetylides couple with aryl iodides 

The most widely used reagents, however, which have been developed during the past 20 years, are the lithium organocuprates, R CuLi. They are prepared in situ, in ether or THF, by addition of two mol of alkyllithium to copper(I) halide below 0°C under a nitrogen atmosphere. It is convenient to use Me S.CuBr as this is more resistant to oxidation in air than CuBr and can thus be introduced without contamination by copper(II). 

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The nature of organocopper reagents appears to be dependent on the method of preparation and the stoichiometry. Specific examples are methylcopper (76, 310), phenylcopper 73), and pentafluorophenyl-copper 34, 37, 147). The best method of preparing pentafluorophenyl-copper of composition CeFsCu appears to be via the addition of copper(I) bromide to pentafluorophenylmagnesium bromide 34, 37), since the lithium reagent and copper iodide gives an ate complex 147). An ate complex was also obtained from pentafluorophenyllithium and silver chloride in equimolar proportions 265). As shown in Table III, many of the isolated copper compounds gave somewhat incorrect or irreproducible analyses, and others contain metal halide and solvent molecules. 

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