Dediazoniation mechanism

The experimental work of the groups of Swain and Zollinger on the dediazoniation mechanism of arenediazonium ions, which started in 1975, provided good evidence for the existence of aryl cations as steady state intermediates (see Sec. 8.3). These results also initiated theoretical work on aryl cations, in part combined with further calculations on the structure and reactivity of arenediazonium ions. Publications that contain data on arenediazonium ions and aryl cations will therefore be discussed in the chapter on dediazoniation reactions (Sec. 8.4). In the rest of this section we will concentrate on investigations that are concerned with the geometries and electron densities of diazonium ions but not, or only marginally, with energetics of the dediazoniation reaction. 

The dediazoniation of aromatic diazonium ions has been found to involve a variety of mechanisms. Three typical examples should suffice to show that seemingly slight modifications in the reaction system can lead to entirely different reaction products these suggest fundamentally different dediazoniation mechanisms ... 

As discussed in Section 5.1, at pH = Km the diazonium-diazoate equilibrium shows a maximum for the concentration of both the diazohydroxide and the diazoanhydride (Ar-N2-0-N2-Ar), which may be present in three possible configurations, the (Z),(Z), the (Z),(E), and the (E),(E) (see Scheme 5-15). The diazoanhydrides are also, like the diazoates, compounds that can be formed in water, but not in (anhydrous) alcoholic solutions. This may explain differences between dediazoniation mechanisms and products in water and in alcohols. 

I will now discuss the development of the elucidation of the dediazoniation mechanism in terms of Kuhn s cycle normal science -> crisis -> revolution -> normal science . After Crossley et al. postulated the aryl cation as key intermediate of dediazoniation in 1940 and the strong support given to that hypothesis in Hammett s book Physical Organic Chemistry (1940), work in the area of dediazoniation... 

Before I proceed with the discussion of the dediazoniation mechanism, it is necessary to spend some paragraphs considering the definition of the term crisis as used by Kuhn. As already discussed in Section 8.3 the crisis was terminated by the experiments which demonstrated that the first step in Scheme 9-2 is reversible (mechanism B), or in other words that a simple organic compound, the phenyl cation, does react with N2 molecules. 

More accurate information on k3 is obtainable if the equilibrium constant K is also determined at various crown ether concentrations, as shown by Nakazumi et al. (1981, 1983). The results with benzenediazonium tetrafluoroborate and 3- and 4-substituted derivatives demonstrate that k3 is not unmeasurably small, but that ky-values are generally 1-2% of k2 for complexation with 18-crown-6, 0.1-0.5% of k2 with 21-crown-7, and 2-10% of k2 with dicyclohexano-24-crown-8. A dual substituent parameter (DSP) analysis of A 3-values (Nakazumi et al., 1987) showed that the dediazoniation mechanism of the complexed diazonium ions does not differ appreciably from that of the free diazonium ions. 

This book contains what I call an interlude on the logic, the psychology, and the serendipity of scientific discoveries. Readers may wonder what the correlation is between that short Chapter 9 and diazo chemistry. The specific reason for including it was to elucidate the dediazoniation mechanism of aromatic diazonium ions, but I expanded this mechanistic discussion (Sec. 8.3) in the interlude by including general aspects originating in the philosophy of science as developed by Karl Popper and Thomas S. Kuhn, ideas which, in my opinion, should be better known by all scientists working in chemical research. 

Scheme 1. Dediazoniation mechanism. R = (CH2CH) monomer unit for NaPAA, CH3 for acetate ion.

Scheme 1 and Figs. 1 and 2 illustrate the essential elements of the chemical trapping method and the assumptions required to interpret product yield results. The chemical trapping reagent is prepared as the 4-alkyl-2,6-dimeth-ylarenediazonium tetrafiuoroborate, z-ArN2Bp4, and it is the z-ArNj ion that functions as the chemical probe. Water soluble l-ArNj (z = 1, alkyl = CH3) is used for studies in aqueous solution in the absence of micelles (Fig. 2). Water-insoluble Ih-ArNj (z = 16, alkyl = C16H33) is used in micelles because its hydrocarbon chain binds the probe to the micellar core and its cationic head group orients the reactive diazonio group in the interfacial region of the micelles (Figs. 1 and 2). We use a preassociation model, Scheme 1, a modified form of the heterolytic dediazoniation mechanism proposed by...

Zollinger [15], to interpret the product yield results. The yields of phenol product, z-ArOH, from reaction with water and halo product, z-ArX, from reaction with Br or Cl are assumed to be determined primarily by the position of equilibrium between the ion-molecule and ion-ion pairs. The dediazoniation rate constant for each pair is probably of secondary importance for these dediazoniations because dediazoniation reactions are notoriously insensitive to the polarity of the reaction medium (see later). For several decades the basic consensus on the dediazoniation mechanism has been the rate-determining loss of N2 to give a highly reactive aryl cation intermediate that is trapped extremely rapidly and competitively by available nucleophiles [15]. However, more recent ah initio calculations provide support for a bimolecular mechanism in which C—N bond cleavage is almost complete and bond formation with the nucleophile has barely begim [16]. Because both mechanisms lead to the same definition for the selectivity of the reaction toward competing nucleophiles [Eq. (1)], the bimolecular pathway for dediazoniation is not included in Scheme 1. 

Dediazoniation refers to all those reactions of diazo and diazonium compounds in which an N2 molecule is one of the products. The designation of the entering group precedes the term dediazoniation, e. g., azido-de-diazoniation for the substitution of the diazonio group by an azido group, or aryl-de-diazoniation for a Gomberg-Bachmann reaction. The IUPAC system says nothing about the mechanism of a reaction (see Sec. 1.2). For example, the first of the two dediazoniations mentioned is a heterolytic substitution, whereas the second is a homolytic substitution. 

Thus the DSP treatment provides a reasonable interpretation of substituent effects on the basis of the DN + AN mechanism, i. e., rate-limiting formation of an aryl cation in aromatic dediazoniations, but not on the basis of a bimolecular mechanism. 

Important additional evidence for aryl cations as intermediates comes from primary nitrogen and secondary deuterium isotope effects, investigated by Loudon et al. (1973) and by Swain et al. (1975 b, 1975 c). The kinetic isotope effect kH/ki5 measured in the dediazoniation of C6H515N = N in 1% aqueous H2S04 at 25 °C is 1.038, close to the calculated value (1.040-1.045) expected for complete C-N bond cleavage in the transition state. It should be mentioned, however, that a partial or almost complete cleavage of the C — N bond, and therefore a nitrogen isotope effect, is also to be expected for an ANDN-like mechanism, but not for an AN + DN mechanism. 

Dediazoniation does not show a significant solvent isotope effect ( h2o/ d2o = 0.98 0.01 Crossley et al., 1940 Swain et al., 1975 a). This result is definitely not consistent with a mechanism in which charge is built up on oxygen in the rate-limiting transition state, as expected for an ANDN-like process. 

The second test for the mechanism shown in Scheme 8-8 is to apply equation (c) to (8-10) with kinetic data for dediazoniations with varying concentrations [N2] of molecular nitrogen. As the solubility of N2 is quite low in most solvents, kinetic measurements must be made under N2 pressure. The dediazoniation reaction has a... 

These results demonstrate that, within experimental error, the corresponding reaction constants for the two reactions, solvolysis and rearrangement, are the same. In other words, the two reactions have the same dependence on substituent effects, which is consistent with Scheme 8-10 because the transition state for rearrangement is identical to the first transition state in the mechanism of solvolytic dediazoniation. 

There seem to have been only two investigations on dediazoniations in a protic solvent, where the observed products indicate that, in addition to DN + AN solvolysis, an aryne is likely to be present as a metastable intermediate. Broxton and Bunnett (1979) have found that 3-nitroanisole is formed in the dediazoniation of 2-nitroben-zenediazonium ions in methanol in the presence of methoxide ions. This has to be interpreted as a product arising from 3-nitro-l,2-benzyne as an intermediate. The occurrence of the aryne mechanism in poly (hydrogen fluoride)-pyridine mixtures, as discovered by Olah and Welch (1975), is mentioned in Section 8.2. 

As an alternative to electrochemical or radiolytic initiation, homolytic dediazoniation reaction products can be obtained photolytically. The organic chemistry of such photolyses of arenediazonium salts will be discussed with regard to mechanisms, products, and applications in Section 10.13. In the present section photochemical investigations are only considered from the standpoint that the photolytic generation of aryldiazenyl radicals became the most effective method for investigating the mechanisms of all types of homolytic dediazoniations —thermal and photolytic —in particular for elucidating the structure and the dissociation of the diazenyl radicals. 

The results were interpreted on the basis of a mechanism that starts with the photolytic formation of a radical cage consisting of an aryldiazenyl and and arylthiyl (Ar - S ) radical, followed by diffusion of both radicals out of the cage. Three reactions of the aryldiazenyl radical are assumed to occur bimolecular formation of the azoarene and N2, or of biphenyl and N2 (Scheme 8-37), the monomolecular dediazoniation (Scheme 8-38), and recombination with the thiyl radical accompanied by dediazoniation (Scheme 8-39). In addition, two radicals can react to form a di-phenyldisulfide (Scheme 8-40). 

A variety of complex mechanisms operate in thermal dediazoniations of arenediazonium salts leading to a wide range of products. It is useful to summarize some representative results relating to solvolysis products from the literature on this subject (Table 8-1). One recognizes from these data ... 

Dediazoniations that follow a homolytic mechanism are, however, always (as far as they are known today) faster than heterolytic dediazoniations. A good example is afforded by the rates in methanol. In a careful study, Bunnett and Yijima (1977) have shown that the homolytic rate is 4-32 times greater than the heterolytic rate, the latter being essentially independent of additives and the atmosphere (N2, 02, or argon). In water the rate of heterolytic dediazoniation, measured at pH <3, is lower than that of the homolytic reactions that take place in the range pH 8-11 (Matrka et al., 1967 Schwarz and Zollinger, 1981 Besse and Zollinger, 1981). 

Szele and Zollinger (1978 b) have found that homolytic dediazoniation is favored by an increase in the nucleophilicity of the solvent and by an increase in the elec-trophilicity of the P-nitrogen atom of the arenediazonium ion. In Table 8-2 are listed the products of dediazoniation in various solvents that have been investigated in detail. Products obtained from heterolytic and homolytic intermediates are denoted by C (cationic) and R (radical) respectively for three typical substituted benzenediazonium salts and the unsubstituted salt. A borderline case is dediazoniation in DMSO, where the 4-nitrobenzenediazonium ion follows a homolytic mechanism, but the benzenediazonium ion decomposes heterolytically, as shown by product analyses by Kuokkanen (1989) the homolytic process has an activation volume AF = + (6.4 0.4) xlO-3 m-1, whereas for the heterolytic reaction AF = +(10.4 0.4) x 10 3 m-1. Both values are similar to the corresponding activation volumes found earlier in methanol (Kuokkanen, 1984) and in water (Ishida et al., 1970). 

The observation of a maximum rate of dediazoniation at pH = p m can therefore be explained just as well in terms of a mechanism involving a diazoanhydride. 

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