Crown ethers, complexation with diazonium ions

In the first paper on arenediazonium salt/crown ether complexes, Gokel and Cram (1973) mention that they were not able to synthesize the rotaxane 11.14 by an azo coupling reaction of the complexed diazonium ion with Af,Af-dimethylaniline. 

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). 

Kuokkanen evaluated a series of constants. Kpeg for substituted diazonium ions with PEG 1000 and found a reaction constant (p = 1.12) comparable to those for complexation with the three crown ethers investigated by Nakazumi et al. (1983), p = 1.18-1.38). It is therefore likely that the host-guest interaction of diazonium ions with acyclic polyethers is basically similar to that with crown ethers. A dual substituent parameter analysis (DSP, see Sec. 8.3) for (Nakazumi et al., 1987)... 

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. 

In the context of Scheme 11-1 we are also interested to know whether the variation of K observed with 18-, 21-, and 24-membered crown ethers is due to changes in the complexation rate (k ), the decomplexation rate (k- ), or both. Krane and Skjetne (1980) carried out dynamic 13C NMR studies of complexes of the 4-toluenediazo-nium ion with 18-crown-6, 21-crown-7, and 24-crown-8 in dichlorofluoromethane. They determined the decomplexation rate (k- ) and the free energy of activation for decomplexation (AG i). From the values of k i obtained by Krane and Skjetne and the equilibrium constants K of Nakazumi et al. (1983), k can be calculated. The results show that the complexation rate (kx) does not change much with the size of the macrocycle, that it is most likely diffusion-controlled, and that the large equilibrium constant K of 21-crown-7 is due to the decomplexation rate constant k i being lower than those for the 18- and 24-membered crown ethers. Izatt et al. (1991) published a comprehensive review of K, k, and k data for crown ethers and related hosts with metal cations, ammonium ions, diazonium ions, and related guest compounds. 

Zollinger and coworkers (Nakazumi et al., 1983) therefore supposed that the diazonium ion and the crown ether are in a rapid equilibrium with two complexes as in Scheme 11-2. One of these is the charge-transfer complex (CT), whose stability is based on the interaction between the acceptor (ArNj) and donor components (Crown). The acceptor center of the diazonium ion is either the (3-nitrogen atom or the combined 7r-electron system of the aryl part and the diazonio group, while the donor centers are one or more of the ether oxygen atoms. The other partner in the equilibrium is the insertion complex (IC), as shown in structure 11.5. Scheme 11-2 is intended to leave the question open as to whether the CT and IC complexes are formed competitively or consecutively from the components. ... 

Complexation with crown ethers increases the notoriously low solubilities of diazonium salts in most solvents (with the obvious exception of water). Therefore, it is possible to carry out phase-transfer reactions with complexed diazonium ions (review Gokel et al., 1985). Useful examples can be found in a paper from Gokel s group (Beadle et al., 1984a) on the Gomberg-Bachmann and Pschorr reactions (see Sec. 10.10). 

Apart from complex formation involving metal ions (as discussed in Chapter 4), crown ethers have been shown to associate with a variety of both charged and uncharged guest molecules. Typical guests include ammonium salts, the guanidinium ion, diazonium salts, water, alcohols, amines, molecular halogens, substituted hydrazines, p-toluene sulfonic acid, phenols, thiols and nitriles. 

The stability of arenediazonium ions in solution and of their salts in the solid state against dediazoniation is increased by complexation with crown ethers . Harada and Sugita showed recently that the shelf life of photosensitive diazonium salts for diazo imaging processes can be improved by this complexation. 

It is also interesting that glymes can retard certain reactions. For example Bartsch and Juri l i a.ve found that glymes slow the thermal decomposition of -tert-butylbenzenediazonium tetrafluoroborate in 1,2-dichloroethane. The dimethylether of PEG 1000 (approximately 22 ethyleneoxide units) was found to be the most effective noncyclic ether, being a factor of five less effective than 18-crown-6. The rate retardation is said to result from formation of a stable complex with the diazonium ion. 

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