Decomposition, aryl halides

Arylcopper intermediates can be generated from organolithium compounds, as in the preparation of cuprates.95 These compounds react with a second aryl halide to provide unsymmetrical biaryls in a reaction that is essentially a variant of the cuprate alkylation process discussed on p. 680. An alternative procedure involves generation of a mixed diarylcyanocuprate by sequential addition of two different aryllithium reagents to CuCN, which then undergo decomposition to biaryls on exposure to oxygen.96 The second addition must be carried out at very low temperature to prevent equilibration with the symmetrical diarylcyanocuprates. 

The Sandmeyer reaction utilizes a diazonium salt to produce an aryl halide. The process begins by converting an amine to a diazonium salt. Decomposition of the diazonium salt in the presence of a copper halide places the halide ion into the position originally occupied by the amine. The most useful copper halides are CuCl and CuBr in addition, the copper pseudohalides, such as CuCN, also works by placing a nitrile in the position originally occupied by the amine. Figure 13-27 shows an example of the Sandmeyer reaction. 

The reactions were carried out in sealed Pyrex tubes, employing a single-mode microwave cavity. The reagents were added to the polymer-bound aryl halide under a nitrogen atmosphere and the reactions were irradiated for the time periods indicated. Very short reaction times (<4 min) provided almost quantitative conversion, as well as minimal decomposition of the solid support. 

However, if the phenol is first treated with the complex formed from triphenylphosphine and a halogen in acetonitrile solution, an aryloxytriphenylphosphonium halide is formed which on thermal decomposition yields the aryl halide in good yield (e.g. the preparation of p-bromochlorobenzene, Expt 6.30). 

Aryl halides are often susceptible to photochemical degradation. As described in Chapter 10 later in this book, cleavage of the C-X bond occurs with low quantum yield for aryl chlorides (132), higher quantum yields for aryl bromides and iodides (133), and high quantum yields for some aryl fluorides (e.g., fluoroquinolones) (134). Aryl chlorides are photolabile to homolytic and/or heterolytic dechlorination (43). For sertraline hydrochloride, decomposition of the aryl dichloride moiety occurs in solution when exposed to light (ultraviolet and fluorescent conditions). As shown in the following proposed mechanism, the major photochemical decomposition products include mono-chloro- and des-chloro-sertraline via homolytic cleavage (Fig. 91) (86). 

Biphenyls.1 Pd(0)-catalyzed coupling of aryltributyltins and aryl triflates provides a useful route to substituted biphenyls. Lithium chloride is required for reasonable yields it inhibits decomposition of the catalyst. A wide range of functional groups is tolerated on both components. Aryl halides undergo this coupling, but triflates have some advantages because of the many available substituted phenols. 

The possibility of coupling an aryl halide with an unreactive C - H bond opens a plethora of possibilities in the synthesis of biaryl compounds [171, 172]. A (NHC)Pd(II) complex has recently been used to promote the intramolecular direct arylation of aryl chlorides [173]. Whereas a number of complexes with different NHC were screened, (IPr)Pd(OAc)2 was found to be the pre-catalyst of choice (Scheme 22). The use of IPr HC1 as additive led to an enhancement of reactivity, probably due to the preventive effect on the catalyst decomposition at the high reaction temperature. These conditions... 

Apart from direct arylation using aryl halides, aryl thioglycosides can be prepared most efficiently by treatment of the thioaldose with an aryidiazonium salt followed by thermal decomposition of the produced diazo derivative (O Scheme 46) [158]. The procedure is usually very... 

If halide ion is present during hydrolysis of benzenediazonium ion or i -nitro-benzenediazonium ion, there is obtained not only the phenol, but also the aryl halide the higher the halide ion concentration, the greater the proportion of aryl halide obtained. The presence of halide ion has no effect on the rate of decomposition of benzenediazonium ion, but speeds up decomposition of the p-nitrobenzenediazonium ion. 

Depending on the nature of the counterion, triarylsulfonium salts are more or less stable. The most stable compounds have a non-nucleophilic counterion. On the other hand, the presence of a nucleophilic counterion such as a halide (Cl, Br or I) frequently results in the relatively facile decomposition by thermolysis to afford quantitatively the diarylsulfide and the corresponding aryl halide. The observation of the preferential relief of the steric strain in the case of phenyl-p-tolyl-(2,5-dimethylphenyl)sulfonium salts (23) was explained by the intramolecular decomposition of a first formed tetracoordinate intermediate. 35.36... 

Table 5.1 Relative ratios of aryl halides in the thermal decomposition of orf/io-methyl substituted diaryliodonium salts45.47...

The decomposition of triarylselenonium halides was reported to produce aryl halide. jhis reaction is likely to proceed through a ligand coupling mechanism. 

Even superheated (to 260 °C) or supercritical (to 400 °C) water was employed in the Heck reaction with several catalyst precursors and aryl halides with styrene. However, all conversions show large amounts of side products and the yields were in the 5-30% range, indicating radical intermediates and by-products from decomposition of the arene starting material [15]. 

Though the almost unique catalytic effects of cuprous salts, or of metallic copper, in the Sandmeyer reaction have been contested by Hodgson (7), the careful kinetic work of Cowdrey and Davies (8) has established that the anion (CuCh)" does play a special role. These workers, however, suggested that an intramolecular decomposition of complex cuprous salts (At.N2, CuCh) produced the aryl halide. If this concept is correct, then the Sandmeyer reaction should not yield detectable free aryl radicals and should differ from many associated reactions of diazonium salts (9,10) for which the homolytic character is in less doubt. 

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