Arylcopper compounds with

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). 

Actually diaryl compounds are obtained by the reaction of arylcopper compounds with aryl halides. 

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... 

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-... 

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).

Substituted diazomethanes, such as diazoacetic ester, react with some arylcopper compounds 34, 255) by carbene insertion into the C—Cu bond [Eq. (34)]. Isonitriles are also reported 281d) to insert into an aryl—Cu bond [Eq. (35)] in benzene solution at 25°C. 

From the F chemical shifts in the NMR spectra of m- and/>-fluoro-phenylcopper were calculated the substituent parameters for copper. The zero value obtained for oj suggests that copper is electronically neutral, but a large or value indicates that copper is capable of a d interaction with the aromatic nucleus 37). The high stability of penta-fluorophenylcopper tetramer relative to many other arylcopper compounds may thus be the result of interactions between the five fluorine atoms and the benzene ring increasing the charge density of the 7r-elec-tron system 37). 

The infrared spectra of many organocopper compounds have been reported (40, 41, 73, 75, 77) and compared favorably with the respective covalently bound organic halides. Bidentate phosphorus ligands coordinate more strongly with arylcopper compounds than monodentate ligands (41, 75, 281e). Substantial modification of the ligand spectrum is also seen in the infrared spectra of arylcopper complexes with bipy-ridyl or 1,10-phenanthroline (41). 

Arylcopper compounds react with carbon tetrachloride, although no reaction products have been reported (40), and complexes with 1,2-bis-(diphenylphosphino)ethane, (ArCu)a DPPE3, react with 1,2-dichloro-ethane to give vinyl chloride (41). Van Koten and Noltes (281e) have observed a cleavage of the HaC—P bond in a 1 2 reaction of o-(dimetbyl-aminomethyl)phenylcopper with DPPE ([Eq. (76)] the cleavage can be explained by a concerted mechanism which takes place within a complex. A similar mechanism was proposed (281e) to explain the above reaction with 1,2-dichloroethane. 

Arylcopper compounds decompose alkyldiazo compounds [Eq. (34)] 34, 37, 255). for which pentafluorophenylcopper is a very active catalyst. Bis(trifluoromethyl)diazomethane and pentafluorophenylcopper give perfluoro-2-phenylpropene under mild conditions, perhaps via elimination of copper(I) fluoride from an intermediate copper compound 37). Benzenediazonium compounds react in one of two ways 37) (1) diazo coupling with ffi-trifluoromethylphenylcopper to give m-trifluoromethyl-azobenzene in 93% yield or (2) direct displacement with pentafluorophenylcopper to give a 16% yield of 2,3,4,5,6-pentafluorobiphenyl, as well as 75% decafluorobiphenyl and 65% azobenzene. 

Since then, experiments have been reported which indicate that (1) organocopper compounds will couple with aryl halides (2) arylcopper compounds can be oxidatively and thermally dimerized (3) arylcopper compounds are intermediates in the Ullmann reaction (4) organocopper compounds are intermediates in copper-catalyzed decarboxylations and (5) copper-promoted coupling reactions are not restricted to aromatic halides. The copper(I) oxide-promoted coupling reactions, however, have still to yield firm evidence of a copper intermediate. 

An organocopper intermediate was detected by Lewin and Cohen in the reaction of / -iodotoluene with copper in a good complexing solvent (184). Analysis of protonated aliquots from a reaction performed in quinoline indicated an accumulation of />-tolylcopper to a maximum of 43% after 95 hours, at which point the iodide was consumed, and then a slow decrease to by dimerization. Other experiments also indicate the formation of an arylcopper compound in Ullmann reactions (127,141, 210). The isolation of deuterated products, presumably from the decomposition of an intermediate organocopper species in deuterated benzene and cyclohexane, suggested decomposition to free radicals (127). Decompositions of certain o-haloarylcopper intermediates by a benzyne mechanism cannot be totally excluded. The formation of a dichlorobenzene and by-products such as dibenzofuran and triphenylene from only the ortho isomer of the chloroiodobenzenes in Ullmann coupling reactions (210)... 

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