Diazonium ions substitution

This effect is observable within a series of very similar electrophiles. Zollinger27 found that in reactions of diazonium ions substituted with 4-C1, 3-C1, and 3-N02 substituents (i.e. the reactivity and electron-withdrawing power of the ion increased along the series) the respective kinetic isotope effects were 6.55, 5.48 and 4.78. 

Aryl diazonium ions prepared by nitrous acid diazotization of primary arylamines are substantially more stable than alkyl diazonium ions and are of enormous synthetic value Their use m the synthesis of substituted aromatic compounds is described m the following two sections... 

Nucleophilic substitution reactions that occur imder conditions of amine diazotization often have significantly different stereochemisby, as compared with that in halide or sulfonate solvolysis. Diazotization generates an alkyl diazonium ion, which rapidly decomposes to a carbocation, molecular nitrogen, and water ... 

Because of the limited range of aromatic compounds that react with diazonium ions, selectivity data comparable to those discussed for other electrophilic substitutions are not available. Because diazotization involves a weak electrophile, it would be expected to reveal high substrate and position selectivity. 

Aniline, PI1NH2, reacts with sodium nitrite, NaN02, and aqueous acid to give phenyl diazonium ion, PhN2. This ion can be isolated, but it also reacts readily with certain nucleophiles to give substitution products, e.g. 

Diazonium coupling reactions are typical electrophilic aromatic substitutions in which the positively charged diazonium ion is the electrophile that reacts with the electron-rich, ring of a phenol or arylamine. Reaction usually occurs at the para position, although ortho reaction can take place if the para position is blocked. 

Compounds containing the neutral (formally zwitterionic) group =N2 attached by one atom to carbon are named by adding the prefix diazo- to the name of the parent compound (Rule 931.4), e.g., diazomethane, ethyl diazoacetate. Diazo is a so-called characteristic group appearing only as a prefix in substitutive nomenclature. Chemical Abstracts and Beilstein indexing of diazo compounds is analogous to that mentioned above for diazonium ions and salts, but Diazo compounds is not... 

In the context of this section it is important that Ruchardt and Tan (1970 a) found that (solid) benzenediazonium fluoroborate gave benzyne adducts with potassium acetate in the presence of aryne trapping agents such as tetracyclone or anthracene. This is, however, not the case if water is present (Cadogan, 1971). As a consequence of these observations, Cadogan et al. (1971) simplified the formation of arynes from diazonium ions by converting aniline or its substitution products into arynes in a... 

Based on observations by Bamberger, Bucherer, and Wolff at the turn of the century, Matrka et al. (1967) described experiments which show that alkaline solutions (pH 8.5-9.2) of substituted benzenediazonium chlorides form nitrite ions and triazenes. The latter is obviously the reaction product of the amine formed in a retro-diazotization with the diazonium ion that is still present. The yield of nitrite formed was between 0.5% (benzenediazonium ion) and 50.2% (2-nitrobenzenediazonium ion). 

In the literature discussing these results, the coincidence of the NN bond lengths in diazonium ions with that in dinitrogen seems always to be regarded with complete satisfaction. In the opinion of the present author this close coincidence is somewhat surprising, firstly because of the fact that in diazonium ions one of the nitrogen atoms is bonded to another atom in addition to the N(2) atom, and secondly because work on dual substituent parameter evaluations of dediazoniation rates of substituted benzenediazonium ions clearly demonstrates that the nx orbitals of the N(l) nitrogen atom overlap with the aromatic 7t-electron system (see Sec. 8.4). 

The most important results are given in Figure 4-1. The oxygen atom lies 244 pm from the N(l) atom of the diazonio group, well within the sum of the van der Waals radii. The diazonio group deviates by 10.4° from linearity. It seems that the 0(1) N(l) interaction is attractive, as indicated by the angle of 169.6° (instead of 180°) at N(l), but the 0(1) N(2) interaction is not. The NN distance (109.9 pm) is, however, not different from normal values found in diazonium ions. The same authors demonstrated later (Wallis et al., 1993) that this result is not unique for the quino-line-8-diazonium-l-oxide salt, as it was found also for two 1-naphthalenediazonium tetrafluoroborates substituted in the 8-position with the electron donors -SCH3 and -N(CH3)2 and - perhaps unexpectedly - for 8-nitronaphthalene-l-diazo-... 

Much earlier information on the structure of diazonium ions than that derived from X-ray analyses (but still useful today) was obtained by infrared spectroscopy. The pioneers in the application of this technique to diazonium and diazo compounds were Le Fevre and his school, who provided the first IR evidence for the triple bonds by identifying the characteristic stretching vibration band at 2260 cm-1 (Aroney et al., 1955 see also Whetsel et al., 1956). Its frequency lies between the Raman frequency of dinitrogen (2330 cm-1, Schrotter, 1970) and the stretching vibration frequency of the C = N group in benzonitrile (2255 cm-1, Aroney et al., 1955). In substituted benzenediazonium salts the frequency of the NN stretching vibration follows Hammett op relationships. Electron donor substituents reduce the frequency, whereas acceptor substituents increase it. The 4-dimethylamino group, for example, shifts it by 103 cm-1 to 2177 cm-1 (Nuttall et al., 1961). This result supports the hypothesis that... 

Klasinc and Schulte-Frohlinde (1968), who also used HMO for the investigation of substituted benzene diazonium ions, were the first to realize that the remarkable distortion of bond lengths and angles of the benzene ring by the diazonio group (Sec. 4.2) has a significant influence on its reactivity. They found that the sp2-hybridization at the carbon atoms of the benzene ring varied from 1.75 for C(l) to 2.16 for C(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). 

In a classic study in 1940, Crossley and coworkers demonstrated that the rates of nucleophilic substitution of the diazonio group of the arenediazonium ion in acidic aqueous solution were independent of the nucleophile concentration, and that these rates were identical with the rate of hydrolysis. Since that time it has therefore been accepted without question that these reactions proceed by a DN + AN mechanism, i.e., that they consist of a rate-determining irreversible dissociation of the diazonium ion into an aryl cation and nitrogen followed by rapid reactions of the cation with water or other nucleophiles present in solution (Scheme 8-6). 

Xu and Li (1989) investigated H — CIDNP spectra of fifteen substituted benzene-diazonium ions during reduction with NaBH4. The spectra are consistent with a mechanism in which the first step is the addition of a hydride ion to the diazonium ion. The diazene formed (Ar - N2 - H) is assumed to dimerize and disproportionate into a radical pair [Ar-N-NH2 N = N — Ar] which loses one equivalent of N2 yielding [Ar—N —NH2 Ar] and recombines to give the diarylhydrazine. A proportion of the aryl radicals escape and form the hydro-de-diazoniation product. 

These reactions in weakly alkaline solutions are faster than the heterolytic (Dn + AN)-like hydroxy-de-diazoniation, which, for most diazonium ions, (depending on their electrophilicity), is dominant below pH 2-4. As shown by Ishino et al. (1976), an increase in rate, corresponding to the occurence of other mechanisms in addition to the heterolytic hydroxy-de-diazoniation, is observable at pH 3.7-7.0. The increase is dependent on the substituent in the specifically substituted benzenediazo-nium ion. The slope d(log )/d(pH) was found to be in the range 0.22-1.09 (see summary of the work of Ishino et al. by Zollinger, 1983, p. 624). 

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