Diazonium salts aryl cations from

Generation of an aryl radical and an aryl cation from a diazonium salt are easy processes. Both species are very reactive and are readily trapped by a wide variety of nucleophiles. Conversion of an amino group into a nitro group involves reaction of a diazonium fluoroborate with aqueous sodium nitrite solution in the presence of copper powder (Scheme 8.22). 

The photochemistry of diazonium salts has considerable industrial interest. Attempts have been made to characterize aryl cations from their decomposition and their quenching in anionic micelles has been meas-... 

Interest has been restricted this year to studies of photoelimination of nitrogen from arenediazonium salts and attempts to characterize the resulting aryl cations. Nanosecond laser photolysis techniques have been used to examine the photodecomposition of diazonium tetrafluoroborates. Aryl cations could not directly be detected, but evidence from bleaching experiments indicates that, in water, the phenyl cation has a lifetime of about 500 ps. The isolation of the ether (131) as the sole product of photodecomposition of the diazonium salt (132) in methanol demonstrates that rearrangement of the intermediate aryl cation does not occur. Both heterolytic and homolytic pathways have been described for photodecompositions of arenediazonium salts complexed with crown ethers. A useful synthetic application of this conversion is the formation in 53% yield of ethyl 2,4-difluoroimidazole-5-carboxylate (133) on irradiation of the diazonium fluoroborate (134). ... 

Primary arylamines like primary alkylammes form diazonium ion salts on nitro sation Aryl diazonium 10ns are considerably more stable than their alkyl counterparts Whereas alkyl diazonium 10ns decompose under the conditions of their formation aryl diazonium salts are stable enough to be stored m aqueous solution at 0-5°C for a rea sonable time Loss of nitrogen from an aryl diazonium ion generates an unstable aryl cation and is much slower than loss of nitrogen from an alkyl diazonium ion... 

All these results are consistent with the hypothesis that aryl cations react in aqueous media at diffusion-controlled rates with all nucleophiles that are available in the immediate neighbourhood of the diazonium ion. On this basis Romsted and coworkers (Chaudhuri et al., 1993a, 1993b) used dediazoniation reactions as probes of the interfacial composition of association colloids. These authors determined product yields from dediazoniation of two arenediazonium tetrafluoroborates containing ft-hexadecyl residues (8.15 and 8.16) and the corresponding diazonium salts with methyl groups instead of Ci6H33 chains. ... 

In each case the mechanism involves generation of an aryl radical from a covalent azo compound. In acid solution diazonium salts are ionic and their reactions are polar. When they cleave, the product is an aryl cation (see p. 852). However, in neutral or basic solution, diazonium ions are converted to covalent compounds, and these cleave to give free radicals ... 

The carbanion is in an sp2 orbital in the plane of the ring. Indeed, this intermediate is very similar to the aryl cation intermediate in the Sjvjl mechanism from diazonium salts. That had no electrons in the sp2 orbital the carbanion has two. 

In appropriately polar solvents, diazonium salts decompose to nitrogen and aryl cations, and formation of arenes may take place by transfer of hydrogen from the reducing agent or from the solvent. Depending on reaction conditions, an ionic mechanism (equation 87)," or a radical mechanism (equation 88) " may be operating, and experimental evidence is claimed for both. However, the radical mechanism seems to have more support. 

Nitrous acid (3), which is generated from sodium nitrite and aqueous acid, undergoes dehydration under the acidic conditions to form nitrosyl cation 4. Reaction of 4 with the aryl amine eventually affords the aryl diazonium salt after tautomerization and dehydration. The addition of fluoroboric acid sets the stage for the second step leading to aryl fluoride. 

The carbanion is in an sp orbital in the plane of the ring. Indeed, this intermediate is very similar to the aryl cation intermediate in the SnI mechanism from diazonium salts. That had no electrons in the sp orbitai the carbanion has two. Why should this proton be removed rather than any other The bromine atom is electronegative and the C—Br bond is in the plane of the sp2 orbital and removes electrons from it. The stabilization is nonetheless weak and only exceptionally strong bases will do this reaction. 

There are many other reactions that possibly involve aryne intermediates. While some are of mechanistic curiosities, some have been studied in detail, although none are generally synthetically useful. Irradiation of 1,2-diiodobenzene can lead to ortho-benzyne, probably via an aryl radical intermediate resulting from cleavage of the weak C-1 bond (Scheme 7.19). Aryl cations, formed by the decomposition of diazonium salts are also possible intermediates to ortho-benzynes provided that a large ortho-substituent is present, loss of a proton to give an aryne becomes competitive with the normal nudeophihc addition to the cation. 

The existence of S /ace-N=N-aryl bonds are more difficult to establish because azo bonds are also formed during the growth of the film. A C(OH)-C-N=N-C6H5 has been observed by TOF-SIMS [126] and assigned to an electrophilic attack of the diazonium cation on an -OH-substituted aromatic ring at the surface of carbon. This is the well-known azo coupling reaction between diazonium salts and, for example, naphthol that has been the basis of the dye chemistry. It is different from the reaction of Figure 3.1 that involves a radical. 

Fluorine can also be introduced onto aromatic rings via diazonium ions. One procedure is to isolate aryl diazonium tetrafluoroborates. These thermally decompose to give aryl fluorides presumably by formation of the aryl cation which abstracts fluoride from the anion.Hexafluorophosphate salts behave similarly.The... 

In some cases, fluorination was photoaccelerated in the reaction of aryl diazonium salts in the presence of etherated boron trifluoride [221], Photodecomposition of diazonium salts in the presence of other halide anions (bromide or chloride) was in practice not explored [222,223]. Although aryl iodides can be obtained thermally from diazonium and iodonium precursors, some photoinduced syntheses were also reported [224,225]. The reaction seemed to involve aryl radicals [225] in the former case and aryl cations in the latter one [224],... 

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