Diazonium salts complexation with crown

Interest has been restricted this year to studies of photoelimination of nitrogen from arenediazonium salts and attempts to characterize the resulting aryl cations. Nanosecond laser photolysis techniques have been used to examine the photodecomposition of diazonium tetrafluoroborates. Aryl cations could not directly be detected, but evidence from bleaching experiments indicates that, in water, the phenyl cation has a lifetime of about 500 ps. The isolation of the ether (131) as the sole product of photodecomposition of the diazonium salt (132) in methanol demonstrates that rearrangement of the intermediate aryl cation does not occur. Both heterolytic and homolytic pathways have been described for photodecompositions of arenediazonium salts complexed with crown ethers. A useful synthetic application of this conversion is the formation in 53% yield of ethyl 2,4-difluoroimidazole-5-carboxylate (133) on irradiation of the diazonium fluoroborate (134). ... 

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). 

Complexed arenediazonium salts are stabilized against photochemical degradation (Bartsch et al., 1977). This effect was studied in the former German Democratic Republic in the context of research and development work on diazo copying processes (Israel, 1982 Becker et al., 1984) as well as in China (Liu et al., 1989). The comparison of diazonium ion complexation by 18-crown-6 and dibenzo-18-crown-6 is most interesting. Becker at al. (1984) found mainly the products of heterolytic dediazoniation when 18-crown-6 was present in photolyses with a medium pressure mercury lamp, but products of homolysis appeared in the presence of dibenzo-18-crown-6. The dibenzo host complex exhibited a charge-transfer absorption on the bathochromic slope of the diazonio band. Results on the photo-CIDNP effect in the 15N NMR spectra of isotopically labeled diazonium salts complexed by dibenzo-18-crown-6 indicate that the primary step is a single electron transfer. 

Juri and Bartsch (1979) have studied the coupling of 4-t-butylbenzene-diazonium tetrafluoroborate with N,N-dimethylaniline in 1,2-dichloroethane solution. The addition of one equivalent (based on diazonium salt) of 18-crown-6 caused the rate constant to drop by a factor of 10, indicating that complexed diazonium is less reactive than the free cation. Benzenediazonium tetrafluoroborate complexes of crown ethers are photochemically more stable than the free salt. The decomposition into fluorobenzene and boron trifluoride is strongly inhibited but no explanation has been given (Bartsch et al., 1977). 

The stability of arenediazonium ions in solution and of their salts in the solid state against dediazoniation is increased by complexation with crown ethers2b. Harada and Sugita124a showed recently that the shelf life of photosensitive diazonium salts for diazo imaging processes can be improved by this complexation. 

The first diazonium-salt-crown-ether adduct was isolated and identified as a 1 1 complex by Haymore et al. (1975). Unfortunately Haymore never published the X-ray structural analysis of benzenediazonium hexafluorophosphate with 18-crown-6 which he performed in 1980. ORTEP drawings with measured bond angles and lengths from Haymore s investigation can be found in a review chapter by Bartsch (1983, p. 893). A few data from Haymore s work (e.g., R = 0.064) were also mentioned by Cram and Doxsee (1986, footnote 7). Groth (1981) published the results of his X-ray investigation of 4-methoxybenzenediazonium tetrafluoroborate and 21-crown-7 (R = 0.042) and Xu et al. (1986) those of 4-methoxybenzenediazonium tetrafluoroborate and dibenzo-24-crown-8 (R = 0.086). 

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. 

Substituted Benzenes.— The stability of aryldiazonium fluoroborates under a variety of conditions has been investigated. These compounds are rendered more thermally stable by sodium meta(octa)molybdate, while complexes with 18-crown-6 polyether show enhanced stability over the uncomplexed salts in both thermal and photochemical de-diazoniations. In the latter procedure, products from an ionic mechanism are obtained at 313 nm, but at >330 nm protodedi-azoniation of fluoroborates occurs by a radical mechanism. The effect of solvents on the mechanism of nitrogen loss from diazonium salts has been reviewed. ... 

The conversion of ArN2 salts into ArH takes place in hexamethyl-phosphoramide (by a long radical chain process), and also by the use of the naphthalene anion radical CioHs" in THF. Alternatively, deamination of ArNH2 can be carried out with Bu ONO in DMF. Unsymmetrical biaryls can be formed in a phase-transfer reaction between the diazonium fluoroborate (in an arene solvent) and KOAc complexed with 18-crown-6 polyether. ° Meerwein aryl-ation of fluorinated olefins with ArN2 and copper(ii) halides has been described, but an alternative procedure for the overall addition of ArHal to an alkene involves the use of ArNH2, Bu ONO, and copper(ii) halides in acetone or... 

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). 

Two other types of host for arenediazonium salts were found by Shinkai et al., the calix[ ]arenes, 11.10 (1987 a, 1987 b) and 11.11 (1988). The hexasulfonated calix[6]arenes 11.10 suppress dediazoniation of substituted benzenediazonium ions in aqueous solution much more efficiently than 18-crown-6. The complexation of calix[ ]arenes 11.11 (n = 4, 6, and 8) with 4 -dimethylaminoazobenzene-4-diazonium ions (11.12) was measured, and was found to be weaker than that of 18-crown-6. It may be that the large difference in behavior between these two types of complexation reagents 11.10 and 11.11 is due to the significantly different diazonium ions used as guests for the two types. Electronically the azobenzenediazonium ion (11.12) is... 

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. 

The claim for this aryne is based on the isolation of the appropriate adducts 631 or 632 when a crown-ether complex of the diazonium carboxylate potassium salt 633 is decomposed in the presence of tetracyclone (151) or the diphenyltetrazine (169). As pointed out in Section II.2.B.C, however, both of these diene traps may be ambiguous as aryne probes because of their tendency to react with the aryne precursors by an addition-elimination process via 634 and 635 rather than by an elimination-addition mechanism involving the aryne 617. The failure of several other dienes with unambiguous records as aryne traps (Section II.2.B.b.d), such as anthracene (147), to give aryne adducts with the precursor 633 tends to support the former mechanism. On the other hand. 

---