Aryl copper

To date, the organometallic chemistry of copper, in terms of isolation and structural characterization of compounds, is essentially limited to the Cu(I) oxidation state. Only a very few examples of other oxidation states are known. The older literature offers a reported synthetic procedure for the synthesis of bis(aryl)copper(II) compounds [33, 34] (see Scheme 1.2), but this result has never been reproduced by others. 

Fig. 1.4. Molecular orbitals describing the bridging aryl copper bonding (see text).

Allylic substitutions with nonstabilized C-nucleophiles are an important domain of organocopper chemistry [51]. However, on close inspection of the literature, it becomes apparent that regioselectivity in favor of the branched allylic alkylation products is only obtained with alkyl copper compounds, while aryl copper compounds mainly give the linear alkylation products. This observation was an incentive for Alexakis et al. [52] to probe the reactions of aryl zinc hahdes in the Ir-catalyzed allylic substitution (Scheme 9.18). 

Reactions with hthium or Grignard reagent yield alkyl or aryl copper(I) derivatives, respectively. Such organocopper compounds containing Cu-Cu bonds are formed only by Cu+ and not Cu2+ ions. 

Aryl halides are excellent substrates for metal-halogen exchange by highly activated Rieke copper, affording aryl coppers at or below room temperature in fair to excellent yields. Functional groups such as nitriles, ketones and fluoride substituents are tolerated,... 

The Ullmann reaction (Figure 13.4) represents another synthesis of substituted biphenyls. In this process an aryl iodide or—as in the present case—an aryl iodide/aryl chloride mixture is heated with Cu powder. It is presumed that under standard conditions the aryl iodide reacts in situ with Cu to form the aryl copper compound. Usually, the latter couples with the remaining aryl iodide and a symmetric biphenyl is formed. In a few instances it is also possible to generate asymmetric biaryls via a crossed Ullmann reaction. In these cases one employs a mixture of an aryl iodide and another aryl halide (not an iodide ) the other aryl halide must exhibit a higher propensity than the aryl iodide to couple to the arylcopper intermediate. It is presumed that the mechanism of the Ullmann reaction parallels the mechanism of the Cadiot-Chodkiewicz coupling, which we will discuss in Section 13.4. 

Isocyanide complexes of phenylethynyl-, indenyl-, and cyclopentadi-enyl copper have been isolated 86, 172, 246). Organocopper-isocyanide complexes are assumed to be intermediates in certain organic syntheses (245-252). Isocyanides also insert into the aryl-copper bond 281d) [Eq. (35)]. 

The formation of copper(I) fluoride in reaction (45) seems unlikely, but low yields of aryl copper compounds are obtained owing to pyrolysis during preparation. 

The decomposition of alkyl and aryl copper compounds has been the subject of much debate. Initially free radical mechanisms were advanced, such theories being supported by the reduced yields of hydrocarbons in the presence of benzoquinone or other free-radical scavengers (24, 150). However, under all conditions very poor yields of dimeric alkanes are obtained. These would be the likely products of a free-radical decomposition. 

This reaction is known as the Ullmann coupling. It is believed to involve the intermediacy of aryl copper complexes rather than radical species. The reaction is best suited to the preparation of symmetrical biaryls ( homo-coupled products). Attempts to couple two different halides (Ar X and Ar X) in this way can lead to mixtures of the desired cross-coupled product (Ar -Ar ) and the two homo-coupled species (Ar -Ar and Ar -Ar ). 

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 quinohne 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 quinohne and that aryl-copper compounds are intermediates that can be isolated in some cases. Metallic silver has been used in place of copper, with higher yields. ... 

The coupling of aryl halides with copper is called the Ullmann reaction The reaction is clearly related to 13-9, but involves aryl copper intermediates. The reaction is of broad scope and has been used to prepare many symmetrical and unsym-metrical biaryls." When a mixture of two different aryl hahdes is used, there are three possible products, but often only one is obtained. For example, picryl chloride and iodobenzene gave only 2,4,6-trinitrobiphenyl." The best leaving group is iodo, and the reaction is most often done on aryl iodides, but bromides, chlorides, and even thiocyanates have been used. 

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

Ni complexes are used in place of copper metal and 2) for the preparation of highly substituted biaryls the use of preformed aryl copper species has been successful (Ziegler modification). °... 

The exact mechanistic pathway of the Ullmann coupling is not known. There are two main pathways possible 1) formation of aryl radicals or 2) the formation of aryl copper [ArCu, ArCu " and ArCu " ] intermediates. Currently the most widely accepted mechanism assumes the formation of aryl copper intermediates, since many of these species can be isolated and they can react with aryl halides to give biaryls. 

Arylations. Copper(II) acetate catalyzes the reaction of arylboronic acids with phenols and amines (including imidazole and amides) to provide diaryl ethers (e.g., for synthesis of thyroxine) and arylamines, respectively. The reactions are quite erratic, although some preparatively useful cases have been found. 

The inhibiting effect of nitrobenzene on the reaction [1] is in agreement with the involvement of radicals in the rate limiting step [18]. However Ais effect is not significant enough to exclude the possibility of a mechanism in which free radicals would not exist as free species such as a mechanism involving aryl copper complexes [5, 13, 14] which could also be considered [1]. 

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