Diazonium salts, aryl anions

The first widely used intermediates for nucleophilic aromatic substitution were the aryl diazonium salts. Aryl diazonium ions are usually prepared by reaction of an aniline with nitrous acid, which is generated in situ from a nitrite salt.81 Unlike aliphatic diazonium ions, which decompose very rapidly to molecular nitrogen and a carbocation (see Part A, Section 4.1.5), aryl diazonium ions are stable enough to exist in solution at room temperature and below. They can also be isolated as salts with nonnucleophilic anions, such as tetrafluoroborate or trifluoroacetate.82 Salts prepared with 0-benzenedisulfonimidate also appear to have potential for synthetic application.83... 

Two mechanisms are among those that have been postulated for decomposition of aryl diazonium salts in aqueous solution containing nucleophilic anions, A ... 

On the other hand, Hartman and Biffar (1977) reported that decomposition of arenediazonium tetrafluoroborates in dichloromethane in the presence of copper metal is catalysed by dicyclohexyl- 18-crown-6. Electron-withdrawing substituents in the aryl ring enhance the rate of the reaction. The main function of the crown ether is probably to solubilize the salt. The effect of the complexation on the rate was not investigated in detail. Similar enhanced solubilization of diazonium salts in apolar solvents was reported and used by Martin and Bloch (1971) in pyrolysis experiments aimed at the generation of the dehydrocyclopentadienyl anion. 

Protonic initiation is also the end result of a large number of other initiating systems. Strong acids are generated in situ by a variety of different chemistries (6). These include initiation by carbenium ions, eg, trityl or diazonium salts (151) by an electric current in the presence of a quartenary ammonium salt (152) by halonium, tri aryl sulfonium, and triaryl selenonium salts with uv irradiation (153—155) by mercuric perchlorate, nitrosyl hexafluorophosphate, or nitryl hexafluorophosphate (156) and by interaction of free radicals with certain metal salts (157). Reports of "new" initiating systems are often the result of such secondary reactions. Other reports suggest standard polymerization processes with perhaps novel anions. These latter include (Tf)4Al (158) heteropoly acids, eg, tungstophosphate anion (159,160) transition-metal-based systems, eg, Pt (161) or rare earths (162) and numerous systems based on triflic acid (158,163—166). Coordination polymerization of THF maybe in a different class (167). 

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-... 

Reactions involving diazo groups are also affected by heavy metals such as cuprous ion. For example, in the well known Sandemeyer reaction, cuprous ion can catalyze the decomposition of aryl diazonium salts. Although the exact mechanism is unknown and may involve free radical processes, the anion of the cuprous salt replaces the nitrogen of the diazonium salt. 

As was shown for TV-aminobenzimidazoles, /V-aminoazoles are easily deaminated by the action of diazonium salts (79ZOR1108). Unfortunately, this interesting reaction has been poorly investigated so far. Its value lies not so much with deamination but with the formation of aryl azides under mild conditions in high yields. Probably, the intermediates in this reaction are tetrazenes 260 cleaved (probably, via the N-anion) as shown in Eq, (64). 

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. 

Aryl diazonium salts are considered more reactive toward oxidative addition than aryl iodides. Furthermore, the presence of the anion eliminates the need for an additional base [62]. The reactions of various aryl diazonium fluoroborate salts with styrene mediated by Pd(OAc)2/bis-(2,6-diisopropylimidazolium chloride) proceeds in almost all solvents at room temperature. In the opti-... 

Under sonication at room temperature, the reaction takes place in high yields in a few minutes.67 xhe thermal procedure requires heating for several hours. The cleavage of the carbon-nitrogen bond in aromatic diazonium salts produces radicals which can be trapped by phenyl f-butyl nitrone (PBN). Besides the aryl radicals, species derived from the counter-anion were detected. 

The photocatalytic generation of aryl radicals was also successfully applied to the formation of carbon heteroatom bonds. The aryl pinacolboronates 141 can be easily achieved by visible light irradiation of a solution of aryl diazonium salts 142 and diboron pinacol ester 143 containing 5mol% of eosin (Scheme 29.24) [89]. The proposed meehanism involves the addition of aryl radical 144 to the complex 145 that is generated by interaction of tetrafluoroborate anion and diboran pinacol ester 143. This process leads to the formation of the aryl pinacolboronates 141 and the radical anion intermediate 146. The oxidation of this intermediate by eosin radical cation completes the catalytic cycle. 

Diazonium salts " have a strong absorption in the region 2300-2130cm (4.35-4.69 pm) which is due to the stretching vibration of the N=N group. This band is dependent on the nature of the ring substituents but is less dependent on the nature of the anion, a shift of about 40 cm" at most being observed for different anions. Aryl diazonium salts may be represented by the resonance structures... 

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