Diazonium ions, decomposition

Because a neutral molecule is eliminate4 rather than an anion, there is no electrostatic attraction (ion pairing) between the products of the dissociation step. As a result, the carbocations generated by diazonium-ion decomposition frequently exhibit somewhat different behavior from those generated from halides or sulfonates under solvolytic conditions. ... 

Diazotization Diazonium ion decomposition gives a phenol in water and reduction in SDS Abe etal., 1983... 

It is clear from the data in Table 4.2 that the two pairs of stereoisomeric cyclic amines do not form the same intermediate. The collapse of the ions to product is evidently so fast that there is not time for relaxation of the initially formed intermediates to reach a common structure. Generally speaking, we can expect similar behavior for all alkyl diazonium ion decompositions. The low activation energy for dissociation and the neutral and hard character of the leaving group result in a carbocation that is free of direct interaction with the leaving group. Product composition and stereochemistry is determined by the details of the collapse of the solvent shell. 

When we turn to a secondary aliphatic diazonium ion, then loss of nitrogen to yield an unrearranged carbocation is an available pathway, so we get products of both diazonium ion decomposition and carbocation decomposition. We should thus expect to get less rearrangement from secondary than from primary aliphatic diazonium ions because this lower-energy pathway is available to the diazonium ion. 

Reaction pathways in diazonium ion decomposition. (Adapted from reference 149.)... 

The oxidation by cupric ions of aryl radicals generated Y 93%. by diazonium ion decomposition constitutes a promising method for the prepn. of phenols. 

Figure 22 5 shows what happens when a typical primary alkylamine reacts with nitrous acid Because nitrogen free products result from the formation and decomposition of diazonium ions these reactions are often referred to as deamination reactions Alkyl... 

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... 

AUcyl amines also react with NaN02 and aqueous acid, but no diazonium ions can be isolated. Rather, alcohols and alkenes are obtained, products that might result from decomposition of RN2 - If RN2 does indeed form in these reactions (and there is evidence that it may not), why is it so unstable compared to PhN2 ... 

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). 

The rapid formation of the (Z)-diazoate is followed by the slower (Z/J )-isomeri-zation of the diazoate (see Scheme 5-14, reaction 5). Some representative examples are given in Table 5-2. Both reactions are first-order with regard to the diazonium ion, and the first reaction is also first-order in [OH-], i.e., second-order overall. So as to make the rate constants k and k5 directly comparable, we calculated half-lives for reactions with [ArNj ]0 = 0.01 m carried out at pH = 9.00 and 25 °C. The isomerization rate of the unsubstituted benzenediazonium ion cannot be measured at room temperature due to the predominance of decomposition (homolytic dediazoniations) even at low temperature. Nevertheless, it can be concluded that the half-lives for (Z/ )-isomerizations are at least five powers of ten greater than those for the formation of the (Z)-diazohydroxide (reaction 1) for unsubstituted and most substituted benzenediazonium ions (see bottom row of Table 5-2). Only for diazonium ions with strong -M type substituents (e.g., N02, CN) in the 2- or 4-position is the ratio r1/2 (5)/t1/2 (1) in the range 6 x 104 to 250 x 104 (Table 5-2). 

Important mechanistic information can be obtained from the reaction rates of the two diazoates with acid. The older literature, e. g., publications by Grachev (1947 a, 1947 b, 1948), by Porai-Koshits (1960), and by Porai-Koshits et al. (1946, 1960), will not be reviewed here because it is outdated and in some cases the results were not reproducible (see Lewis and Suhr 1958 b, footnote 5). On the basis of the above discussion of the formation of the (Z)-diazoate from the diazonium ion by reactions 1 and 2 of Scheme 5-14, one might assume that the reverse process should be easy to follow experimentally. This is not the case, however, as was first shown simultaneously by Lewis and Suhr (1958 b) and by Passet and Porai-Koshits (1958). The investigation of the acidification of (2i)-4-nitrobenzenediazoate is difficult due to irreversible decomposition, particularly at pH >5. Lewis and Suhr (1958b) observed,... 

The conductometric results of Meerwein et al. (1957 b) mentioned above demonstrate that, in contrast to other products of the coupling of nucleophiles to arenediazonium ions, the diazosulfones are characterized by a relatively weak and polarized covalent bond between the p-nitrogen and the nucleophilic atom of the nucleophile. This also becomes evident in the ambidentate solvent effects found in the thermal decomposition of methyl benzenediazosulfone by Kice and Gabrielson (1970). In apolar solvents such as benzene or diphenylmethane, they were able to isolate decomposition products arising via a mechanism involving homolytic dissociation of the N — S bond. In a polar, aprotic solvent (acetonitrile), however, the primary product was acetanilide. The latter is thought to arise via an initial hetero-lytic dissociation and reaction of the diazonium ion with the solvent (Scheme 6-11). 

On the basis of the nucleophilicity parameters B, NBs, and fi (see Table 8-2) one expects less of the homolytic product in water than in methanol. This is, however, not the case. It has been known for many decades that a very complex mixture of products is formed in the decomposition of diazonium ions, including polymeric products, the so-called diazo tars. In alcohols this is quite different. The number of products exceeds three or four only in exceptional cases, diazo tars are hardly formed. For dediazoniation in weakly alkaline aqueous solutions, there has, to the best of our knowledge, been only one detailed study (Besse et al., 1981) on the products of decomposition of 4-chlorobenzenediazonium fluoroborate in aqueous HCOf/ CO]- buffers at pH 9.00-10.30. Depending on reaction conditions, up to ten compounds of low molecular mass were identified besides the diazo tar. 

The kinetics of the decomposition of the 4-chlorobenzenediazonium ion under strict exclusion of oxygen (< 5 ppb 02, Schwarz and Zollinger, 1981) are compatible with the CIDNP results, subject to the reservation mentioned already, namely that CIDNP as a probe does not necessarily give results for all pathways, whereas kinetic measurements are normally related to the sum of all competitive mechanisms. The first reaction observable with conventional kinetic methods is the formation of the (E )-diazoate (t1/2 ca. 200 min), but it is also first-order with respect to the diazonium ion concentration. 

The complexity of these decomposition reactions makes it understandable that little work has been carried out in this field, in spite of the great technical and synthetic importance of the reactions of diazonium ions in such systems. 

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. 

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. Experimental data42,43 point to the formation of molecular... 

In order to explain some of the peculiarities of the decomposition reaction and isomer distribution of the products 99, 105), a reaction mechanism has been postulated that involves complex formation between At and the diazonium ion, followed by electron transfer, leading to the release of nitrogen, while the phenyl radical recombines with astatine according to Eq. (12). 

The Sandmeyer reaction utilizes a diazonium salt to produce an aryl halide. The process begins by converting an amine to a diazonium salt. Decomposition of the diazonium salt in the presence of a copper halide places the halide ion into the position originally occupied by the amine. The most useful copper halides are CuCl and CuBr in addition, the copper pseudohalides, such as CuCN, also works by placing a nitrile in the position originally occupied by the amine. Figure 13-27 shows an example of the Sandmeyer reaction. 

Alcohols. In a reaction reminiscent of diazonium ion chemistry, 26 is reduced by ethanol to 13 (R = H). The ethanol is oxidized to acetaldehyde (72UP1). Like water, decomposition of 26 in f-butanol gives nitrogen in quantitative yield, but the other product is intractable. 

This point was made in connection with the decomposition of diazoesters by way of diazonium ions.15 See also, Refs. 16 and 17. 

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