Diazonium cations decomposition

Salts of diazonium ions with certain arenesulfonate ions also have a relatively high stability in the solid state. They are also used for inhibiting the decomposition of diazonium ions in solution. The most recent experimental data (Roller and Zollinger, 1970 Kampar et al., 1977) point to the formation of molecular complexes of the diazonium ions with the arenesulfonates rather than to diazosulfonates (ArN2 —0S02Ar ) as previously thought. For a diazonium ion in acetic acid/water (4 1) solutions of naphthalene derivatives, the complex equilibrium constants are found to increase in the order naphthalene < 1-methylnaphthalene < naphthalene-1-sulfonic acid < 1-naphthylmethanesulfonic acid. The sequence reflects the combined effects of the electron donor properties of these compounds and the Coulomb attraction between the diazonium cation and the sulfonate anions (where present). Arenediazonium salt solutions are also stabilized by crown ethers (see Sec. 11.2). 

Recently, stable salts of vinyl diazonium cations such as 144 have in fact been prepared (119,120), but their decomposition has not so far been investigated. 

Carbocations may be obtained from the decomposition of other cations, e.g. diazonium cations from the action of NaN02/HCl on RNH2 (cf. p. 119),... 

The value of Av and its sign can be used to discriminate between possible mechanisms for a reaction. An example taken from the literature [14,15] is the decomposition of diazonium cations in water. The mechanism of this reaction could be bimolecular ... 

The reactions of picryl fluoride with (2,3-dihydroxypropyl)-ammonium or -phosphonium compounds have been found120 to yield zwitterionic spiro-cr-adducts such as (31). Aromatic diazonium cations have been shown to add to the 9-position of cr-adducts of 9-nitroanthracene to yield neutral derivatives.121 A study has been reported of the effects of ion pairing and change of solvent on the kinetics of decomposition of some anionic cr-adducts.122... 

For thermal decomposition of aryl diazonium cations in water two mechanism are considered an S l mechanism involving an aryl cation and concerted frontside displacement S Ar. Kinetic, isotopic and theoretical studies delivered arguments for both mechanisms without unambiguous generalization. Application of isotope effects combined with theoretical calculations gave a new view of this problem.96 p-Tolyldiazonium cation 54 was chosen for 13C KIEs studies... 

Diazotization of the amino substituent of 6-amino-4-methylpyrido[2,3-/ >]pyrazin-3(4/7)-one in aqueous solution is followed by decomposition of the diazonium cation to give 4-methy]pyri-do[2,3-h]pyrazine-3,6(4//,5//)-dione.18... 

In this context, [Ga4Lg] coordination cage was used for encapsulation and stabilization of aromatic diazonium cations and tropylium cations (Fig. 9.7) [19], The results indicated that only 4-(diethylamino) benzenediazonium cation, 1, and aromatic tropylium cation, 4, could be encapsulated within a [Ga4L5] coordination cage. Encapsulation protected reactive cations and resulted in deceleration of their decomposition [19]. 

Phenyl cations are formed by thermal decomposition of aryl diazonium ions. The cation is so extremely reactive that under some circumstances it can recrqrture the nitrogen... 

One of the first reports involving vinyl diazonium ions and possible vinyl cations is the work of Newman and co-workers (107) on the alkaline decomposition of 3-nitroso-2-oxazolidones, 132. When an aqueous suspension or... 

In all of the above cases involving decompositions of vinyl diazonium ions, the observed products are consistent with a vinyl cation formulation, but extensive mechanistic studies of these reactions have not been reported. It is difficult, for instance, to establish to what extent reaction proceeds through the diazonium ion via a backside nucleophilic attack and concerted loss of nitrogen rather than through the free vinyl cation. In the absence of kinetic data, it is also difficult to rule out competing or alternative mechanisms not involving vinyl cations. 

The wide utility of aryl diazonium ions as synthetic intermediates results from the excellence of N2 as a leaving group. There are several general mechanisms by which substitution can occur. One involves unimolecular thermal decomposition of the diazonium ion, followed by capture of the resulting aryl cation by a nucleophile. The phenyl cation is very unstable (see Part A, Section 3.4.1.1) and therefore highly unselective.86 Either the solvent or an anion can act as the nucleophile. 

Juri and Bartsch (1979) have studied the coupling of 4-t-butylbenzene-diazonium tetrafluoroborate with N,N-dimethylaniline in 1,2-dichloroethane solution. The addition of one equivalent (based on diazonium salt) of 18-crown-6 caused the rate constant to drop by a factor of 10, indicating that complexed diazonium is less reactive than the free cation. Benzenediazonium tetrafluoroborate complexes of crown ethers are photochemically more stable than the free salt. The decomposition into fluorobenzene and boron trifluoride is strongly inhibited but no explanation has been given (Bartsch et al., 1977). 

The Balz-Schiemann and Wallach reactions The Balz-Schiemann reaction (the thermal decomposition of an aryl diazonium salt. Scheme 46) was for many years the only practical method for the introduction of a fluorine atom into an aromatic ring not bearing electron-withdrawing substituents. This reaction, first reported in the late 1800s, was studied in fluorine-18 chemistry as early as 1967 [214]. It involves the generation of an aryl cation by thermal decomposition, which then reacts with solvent, nucleophiles or other species present to produce a substituted aromatic compound. Use of fluorine-18-labelled... 

Because of the small concentration of the 2 1 complex the last term can be ignored. From the extreme rate values in the absence of zinc and with an excess of zinc, 2i and 22 are determined as 2.4 X 104 min.-1 and 1.57 min.-1 respectively. These values can be combined with the trend in the rate constants to give the stability constant of the reactive complex, presumably Zn(OR)(OAc), as 3 X 107. For the simple zinc complex in water the literature values of the stability constant for the 1 1 complex vary from 2.5 X 108 to 6.3 X 108. The diazo coupling reaction of the complex indicates the smaller effect of coordination vis a vis protonation since this reaction is very sensitive to such effects and does not proceed with phenols. Unfortunately the choice of cations for such a reaction is restricted since the cation should not interfere with the analytical methods used to obtain the kinetic data nor should it introduce additional reactions such as occur with transition metal cations which can catalyze the decomposition of the diazonium salt via a redox process. 

Thermal decomposition of arenediazonium salts involves an aryl cation (vide supra). In the case of diazonium tetrafluoroborates, the aryl cation is postulated to be formed in the ion pair ArNj VBF ",195 and trapping of fluoride has been demonstrated to occur without dissociation of the tetrafluoroborate anion.lld... 

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