Diazonium ions nucleophilic

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 ... 

FIGURE 22.5 The diazonium ion generated by treatment of a primary alkylamine with nitrous acid loses nitrogen to give a carbocation. The isolated products are derived from the carbocation and include, in this example, alkenes (by loss of a proton) and an alcohol (nucleophilic capture by water). 

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. 

Phenyl diazonium ion, PhN2, reacts with nucleophiles in several ways. Water displaces N2 to give phenol, while dimethylamine adds to the terminal N. 

Next, consider the reactivity of phenyl diazonium ion. Are either of the reactions shown above consistent with nucleophilic attack at the ion s most electron-poor site Examine the lowest-unoccupied molecular orbital (LUMO) of phenyl diazonium ion. What electrophilic sites are identified by the LUMO Are either of the reactions shown above consistent with an orbital-controlled addition ... 

Experimental observations indicate that electron-rich aromatic nucleophiles, such as phenoxide, add to phenyl diazonium ion in the same way as dimethylamine. 

In Eq. (3), the unstable methyl diazonium ion decomposes by S l reaction into nitrogen and a methyl cation w hich combines with the anion Z to give CH3—Z. In Eq. (4) an Sk2 reaction occurs. The loss of the nitrogen from CH3—here takes place only wdth the participation of the anion as nucleophile. 

The volumes of activation for some additions of anionic nucleophiles to arenediazonium ions were determined by Isaacs et al. (1987) and are listed in Table 6-1. All but one are negative, although one expects — and knows from various other reactions between cations and anions — that ion combination reactions should have positive volumes of activation by reason of solvent relaxation as charges become neutralized. The authors present various interpretations, one of which seems to be plausible, namely that a C — N—N bond-bending deformation of the diazonium ion occurs before the transition state of the addition is reached (Scheme 6-2). This bondbending is expected to bring about a decrease in resonance interaction in the arenediazonium ion and hence a charge concentration on Np and an increase in solvation. 

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). 

Structural and Mechanistic Aspects of Additions of Nucleophiles to Diazonium Ions... 

The changes in the substituent constants and in the parameters F and R on going from the diazonium ion to various addition products provide a useful probe for understanding the mechanism(s) of addition of these nucleophiles to arenediazonium ions. Such constants and parameters are listed in Table 7-4. All values are taken from the relevant tables in the paper by Hansch et al. (1991). With the exception of the last three entries, which we shall discuss later, the products of nucleophile additions are arranged in a sequence of decreasing electron-withdrawing capability, as estimated from the values of <7m and op for the substituent corresponding to the nucleophile added. ... 

In the reactions of nucleophiles with diazonium ions the rate-determined product is, in many cases, a (Z)-azo compound, in spite of the fact that the (is)-isomers are... 

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). 

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. ... 

Two inorganic nucleophiles that react easily with arenediazonium ion, namely the nitrite ion and the hydroxide ion, provide good examples of the concept of the nucleophilic homolytic leaving group. By electron transfer to a diazonium ion the... 

In principle it should be possible to predict quantitatively the reactivity of such species containing nucleophilic homolytic leaving groups towards diazonium ions, by using a dual parameter equation. One parameter serves as a measure of the donor property of the particle the other parameter is the redox potential. However, the complex nature of kinetics of homolytic dediazoniations is likely to be a great obstacle in attempts to calculate rate constants referring only to the radical-generation step. 

In conclusion, it is very likely that the influence of solvents on the change from the heterolytic mechanism of dissociation of the C —N bond in aromatic diazonium ions to homolytic dissociation can be accounted for by a mechanism in which a solvent molecule acts as a nucleophile or an electron donor to the P-nitrogen atom. This process is followed by a one- or a two-step homolytic dissociation to an aryl radical, a solvent radical, and a nitrogen molecule. In this way the unfavorable formation of a dinitrogen radical cation 8.3 as mentioned in Section 8.2, is eliminated. 

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. 

Elegant evidence that free electrons can be transferred from an organic donor to a diazonium ion was found by Becker et al. (1975, 1977a see also Becker, 1978). These authors observed that diazonium salts quench the fluorescence of pyrene (and other arenes) at a rate k = 2.5 x 1010 m-1 s-1. The pyrene radical cation and the aryldiazenyl radical would appear to be the likely products of electron transfer. However, pyrene is a weak nucleophile the concentration of its covalent product with the diazonium ion is estimated to lie below 0.019o at equilibrium. If electron transfer were to proceed via this proposed intermediate present in such a low concentration, then the measured rate constant could not be so large. Nevertheless, dynamic fluorescence quenching in the excited state of the electron donor-acceptor complex preferred at equilibrium would fit the facts. Evidence supporting a diffusion-controlled electron transfer (k = 1.8 x 1010 to 2.5 X 1010 s-1) was provided by pulse radiolysis. 

The replacement of an electrofugic atom or group at a nucleophilic carbon atom by a diazonium ion is called an azo coupling reaction. By far the most important type of such reactions is that with aromatic coupling components, which was discovered by Griess in 1861 (see Sec. 1.1). It is a typical electrophilic aromatic substitution, called an arylazo-de-hydrogenation in the systematic IUPAC nomenclature (IUPAC 1989c, see Sec. 1.2). 

In Brown s classification a diazonium ion is a reagent of very low reactivity and correspondingly high substrate selectivity and regioselectivity. This follows from the fact that benzenediazonium salts do not normally react with weakly nucleophilic benzene derivatives such as toluene. More reactive heteroaromatic diazonium ions such as substituted imidazole-2-diazonium ions will even react with benzene (see Sec. 12.5). 

As far as we are aware, the azo coupling of an ethyne derivative was only investigated over half a century ago Ainley and (Sir Robert) Robinson (1937) investigated the reaction of phenylethynes (phenylacetylenes) with diazonium ions (Scheme 12-59). Unsubstituted phenylethyne did not give identifiable products with the 4-nitrobenzenediazonium ion, but with the more nucleophilic 4-methoxyphenyl-ethyne an azo compound (12.119) was formed. On reaction with water it gives an arylhydrazone of an a-ketoaldehyde (12.120). 

---