Arylcopper compounds

Despite its synthetic importance, the mechanism of the copper-quinoline method has been studied very little, but it has been shown that the actual catalyst is cuprous ion. In fact, the reaction proceeds much faster if the acid is heated in quinoline with cuprous oxide instead of copper, provided that atmospheric oxygen is rigorously excluded. A mechanism has been suggested in which it is the cuprous salt of the acid that actually undergoes the decarboxylation. It has been shown that cuprous salts of aromatic acids are easily decarboxylated by heating in quinoline and that arylcopper compounds are intermediates that can be isolated in some cases. Metallic silver has been used in place of copper, with higher yields. ... 

Organocopper compounds have been trapped by coordination with organic bases. In addition, arylcopper compounds (ArCu) have been independently prepared and shown to give biaryls (ArAr ) when treated with aryl iodides (Ar I). A similar reaction has been used for ring closure. 

Similarly to the alkyl derivatives, the most common route for arylcopper compounds is the reaction of a copper halide and aryllithium compounds (Equation (4)). Organocuprates with aryl groups are obtained by using an appropriate excess of the lithium reagent. Magnesium aryls have also been employed in transmetallation reactions with Cu(l) salts to yield both arylcopper compounds and arylcuprates (Equations (5) and (6)). 

However, reaction of the sterically congested arylcopper compound 89 with the disilylplumbylene 66 yielded the strictly monomeric plumbylene 90 (Equation (30)).100... 

This simplified view can explain stability trends and differences between various organocopper(I) compounds, as well as the influence of bulky or coordinating substituents ortho to the copper-carbon bond on the stability of arylcopper compounds. This interpretation of the copper-carbon bond can also be applied to the binding of sp (alkyl e.g., CH2SiMe3), sp (C=C-R) [40], and other sp (vinyl) groups [41, 42]. 

Complete degradation of organocopper aggregates may occur when they react with tertiary phosphines. This is illustrated by treatment of arylcopper compounds with bis-(diphenylphosphino)methane (DPPM) and with l,2-bis-(diphenylphosphino)-ethane (DPPE) (see Scheme 1.11). 

Scheme 1.11. Reactions between arylcopper compounds and diphosphines.

More recently, several arylcopper compound syntheses that make use of a soluble form of a copper halide precursor, CuBr-DMS (DMS = dimethylsulfide) in DMS as the solvent have been reported. Some of these compounds, such as [Cu4(QH5)4(DMS)2] [61] and [Cu4(C6H4Me-2)4(DMS)2] [62], appeared to be DMS adducts and were fully characterized by X-ray crystal structure determination (see Fig. 1.7). It is interesting to note that these structures contain two- and three-coordinate copper atoms in trans positions. These structures may be envisaged as ion-pairs comprising Cu(Aryl)2 anions bound to Cu(DMS) cations through the Cipso atoms. 

Diarylzinc compounds react with silver salts to give arylsilver compounds of high purity and stability (Scheme 1.15).52 Van der Kerk and coworkers synthesized phenylsilver and a number of methyl-substituted arylsilver compounds via this route, and found that ortho-methy substitution significantly increased the thermal stability of the compound, as is the case for the corresponding arylcopper compounds (Table 1.5).53... 

Neutral arylcopper compounds are accessible from aryllithium compounds by way of transmetalation with one equivalent of Cul (example Figure 16.4). Here, one equivalent of lithium iodide is released. This transmetalation also is a step in the formation of Gilman cuprates from two equivalents of the same organolithium compound and one equivalent of Cul (Figure 10.43, top left). In the latter case, the transmetalation is followed by the addition of a second equivalent of an organolithium compound ArLi + ArCu —> Ar2CuLi. Neutral aryl-... 

In a few instances, it is also possible to generate unsymmetrical biaryls using Ullmann s arylcopper compound generated in situ. In these cases, one employs a mixture of an aryl iodide and another aryl halide (not an iodide ) and the other aryl halide must exhibit a higher propensity than the aryl iodide to couple to the arylcopper intermediate (example Figure 16.5, bottom). This is referred to as a crossed Ullmann coupling. 

Fig. 16.5. Biaryl synthesis via arylcopper compounds II— in-situ preparation of the nucleophile with the classical Ullmann procedure ("variant 1"), with a procedure that may also be applied to aryl bromides ("variant 2") and with a crossed classical Ullmann coupling (bottom-most reaction example).

Fig. 13.3. Biaryl syntheses via arylcopper compounds I preparation of the nucleophile in a separate reaction step.

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