Diazonium ions, aromatic reductive

As shown in other sections of this chapter, overall attention has shifted from diazonium salts as aryl radical sources to bromo- or iodobenzenes. One of the few recent attempts to improve the classical Pschorr cyclization using diazonium ions as starting materials led to the discovery of new catalysts [119]. Results from a first samarium-mediated Pschorr type show the variety of products that can be expected from intramolecular biaryl syntheses under reductive conditions (Scheme 22). Depending on the substitution pattern of the target aromatic core and the reaction conditions, either the spirocycle 60, the biphenyl 61, or the dearomatized biphenyl 62 were formed as major product from 63 [120]. 

In an earlier review (Zollinger, 1994, Chapt. 8) we explained that the dediazoniation of aromatic diazonium ions to aryl cations and dinitrogen, or to aryl radicals and dinitrogen in the presence of suitable electron donors, is based primarily on the high stability of N2. Analogous aliphatic diazonium ions show similar behavior, but the existence of diazenides (R —Nf) and diazoalkanes also opens the possibility of generating carbanions, e.g., in the Wolff-Kishner reduction and related reactions (see the classic book of Cram, 1965, and the monographs of Staley, 1985, and Buncel and Durst, 1980-1987), and carbenes (see Chapt. 8 of this book). 

Now let s draw the forward scheme. The starting material is treated with Bra and a Lewis acid, thereby installing a bromine atom in the meta position. Reduction of the nitro group gives raera-bromoaniline, which is then converted into an aromatic diazonium ion upon treatment with sodium nitrite and HCl. The aromatic diazonium ion is then treated with CuBr to give a Sandmeyer reaction that affords the desired product, as shown ... 

The nature of the azo bond is such that only a very limited number of possible functional groups can be considered to have the necessary features to serve as starting materials for reductive methods of preparation. In a sense, the Bogo-slovskii reaction [17, 18] may be considered a reduction of a diazonium salt by copper(I) ions. However, because the reaction resembles the other condensations of diazonium salts, its classification among the condensation reactions seems appropriate. The direct reduction of azoxy compounds as such is of minor preparative importance except as a method of identification of an azoxy compound. However, in the various bimolecular reduction procedures of aromatic nitro compounds, it has been postulated that an azoxy intermediate forms in the course of the reaction. This intermediate azoxy compound is ultimately reduced to an azo compound. 

Acylamino groups also are useful activating groups and have the advantage that the amino groups obtained after hydrolysis of the acyl function can be removed from an aromatic ring by reduction of the corresponding diazonium salt with hypophosphorous acid, preferably in the presence of copper(I) ions. 

Copper(I) iodide is unsatisfactory for use in the Sandmeyer reaction because of its insolubility. The iodo group is introduced by warming the diazonium salt solution in aqueous potassium iodide solution (Scheme 8.17). This method is one of the best means of introducing iodine into an aromatic ring. A one-electron reduction by the iodide ion is thought to initiate a radical reaction in a similar way to the Cu(I) ion. 

The first example of an intramolecular homolytic aromatic substitution was published by Pschorr more than a century ago [34], Biaryls were prepared by intramolecular homolytic substitution of arenes by aryl radicals which were generated by treatment of arenediazonium salts with copper(I) ions (Pschorr reaction). Later it has been shown that similar reactions can be conducted under basic conditions or by photochemical or thermal decomposition of the diazonium salts [35]. Electrochemical reduction [36], titanium (III) ions [37], Fe(II)-salts [38], tetrathiafulvalene... 

When Pschorr reported more than a century ago on the first intramolecular homolytic aromatic substitution [25], he showed that biaryls could be readily prepared by intramolecular homolytic aromatic substitution using reactive aryl radicals and arenes as radical acceptors. The aryl radicals were generated by treatment of arene-diazonium salts with copper(l) ions. Today, this reaction and related processes are referred to as Pschorr reactions. It was later found that radical biaryl synthesis could be conducted without copper salts by photochemical or thermal generation of the aryl radical from the corresponding diazonium salt [26], Moreover, the reduction of aryl diazonium salts offers another route to generate reactive aryl radicals. Hence, electrochemistry [27], titanium(lll) ions [28], Fe(II)-salts [29], tet-rathiafulvalene [30] and iodide [31] have each been used successfully for the reduction of diazonium salts to generate the corresponding aryl radicals [32]. As an example, the iodide-induced cycUzation of diazonium salt 6 to phenanthrene derivative 7 is presented in Scheme 13.3 [31]. For further information on the... 

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