Copper® salts reactions with aryl diazonium ions

Replacement of diazonium groups by halide is a very valuable synthetic alternative to direct halogenation for preparation of aryl halides. There are three broad types of procedures decomposition of aryl diazonium-halide adducts with expulsion of nitrogen, copper-mediated redox processes, and thermal processes proceeding via aryl radicals. The first type of process is probably involved in the reaction of aryl diazonium salts with iodide ion. Smooth high-yield transformation takes place in the absence of any metal catalyst. The mechanism of the reaction... 

Introduction of Other Nucleophiles Using Diazonium Ion Intermediates. Cyano and azido groups are also readily introduced via diazonium intermediates. The former involves a copper-catalyzed reaction analogous to the Sandmeyer reaction. Reaction of diazonium salts with azide ion gives adducts that smoothly decompose to nitrogen and the aryl azide.56... 

One cannot distinguish between the analogous copper intermediates involved in oxidative electron-transfer and ligand-transfer reactions. In each the ionization of the ligand to copper(II) has an important role in the formation of carbonium ion intermediates. A reaction analogous to the copper-catalyzed decomposition of peroxides is the copper-promoted decomposition of diazonium salts (178). The diazonium ion and copper(I) afford aryl radicals which can undergo ligand-transfer oxidation with copper(II) halides (Sandmeyer reaction) or add to olefins (Meerwein reaction). 

In the Sandmeyer reaction, the cold diazonium salt solution is run into a solution of the copper(I) halide dissolved in the halogen acid. The complex, which usually separates, is decomposed to the aryl halide by heating the reaction mixture. The mechanism (Scheme 8.16) involves generation of an aryl radical by electron transfer from Cu(I), which then reacts with the halide ion. 

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... 

The formation of aryl chlorides from arenediazonium ions on reaction with copperfi) chloride is known as the Sandmeyer reaction and also involves the intermediacy of aryl radicals [192]. Copper salts are similarly involved in the reactions of diazonium ions with cyanide, nitrite, and sulfur dioxide. The uses of arenediazonium salts in synthetic reactions forming carbon-carbon, carbon-sulfur, and carbon-boron bonds have been summarized recently [193]. 

Aryl bromides and iodides are usually prepared from diazonium salts by a copper-catalyzed process, a reaction commonly known as the Sandmeyer reaction. Under the classic conditions of the Sandmeyer reaction, the diazonium salt is added to a hot acidic solution of the cuprous halide.It is also possible to convert anilines to aryl halides by generating the diazonium ion in situ. Reaction of anilines with alkyl nitrites and cuprous halides in acetonitrile gives good yields of aryl bromides by a copper-mediated process which is mechanistically similar to that occurring under the usual Sandmeyer conditions. Diazonium salts can also be converted to... 

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... 

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