Dediazoniation photolytic

Photolytic Dediazoniations and their Applications in Synthesis and in Image Technology 277... 

UV spectra are, however, very useful for the determination of acid-base and ion pair formation equilibria, and for photochemical investigations (e. g., determination of quantum yield in photolytic dediazoniation, Tsunoda and Yamaoka, 1966 fluorescence and phosphorescence at low temperature, Sukigahara and Kikuchi, 1967a). 

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). 

Becker et al. (1985 a) also investigated the photolytic dediazoniation of arenediazonium salts sensitized by zinc tetraphenylporphyrin (ZnTPP). Due to the very different lifetimes of singlet and triplet ZnTPP (1.8 ns and 1.4 ms, respectively) it is possible to differentiate between singlet and triplet electron transfer reactions using different diazonium salt concentrations. 

Horner and Stohr (1952) found that in methanol the photolytic formation of the aryl methyl ether is rather a minor process, the main reaction being a hydro-de-di-azoniation. In a comparison between thermal and photolytic dediazoniation in water, Lewis et al. (1969 b) analyzed the percentages of chloro-de-diazoniations for three arenediazonium chlorides in aqueous solution in the presence of various concentrations of NaCl under both thermal and photolytic conditions. The authors came to the conclusion that these processes do not involve the same intermediates. 

There is a multiplicity of pathways for thermal dediazoniations. An analogous situation is to be expected for photochemical dediazoniations. Based on the general experience that light-sensitive reactions often involve free radical intermediates, it was commonly assumed that all photolytic dediazoniations are free radical reactions. Horner and Stohr s results (1952), mentioned above, could lead to such a conclusion. More sophisticated methods of photochemistry also began to be applied to investigations on arenediazonium salts, e. g., the study of photolyses by irradiation at an absorption maximum of the diazonium ion using broad-band or monochromatic radiation. This technique was advocated by Sukigahara and Kikuchi (1967 a, 1967 b,... 

The observation by Becker et al. (1977 a, b see above) that the photolytic dediazoniation products of arenediazonium ions are dependent on the irradiation... 

For the introduction of fluorine into aromatic and heteroaromatic compounds the photolytic fluoro-de-diazoniation sometimes has advantages compared with the corresponding thermal dediazoniation (Balz-Schiemann reaction, see Sec. 10.4). For aromatic substrates the reaction was studied by Rutherford et al. (1961), Christie and Paulath (1965), Petterson et al. (1971), and Becker and Israel (1979). Hexafluorophos-phates sometimes give better yields than tetrafluoroborates (Rutherford et al., 1961). In analogy to Balz-Schiemann reactions in solution (Fukuhara et al., 1987), photolytic fluoro-de-diazoniations of benzene derivatives with electron-withdrawing substituents give lower yields. 

Although it has been known since the early days of diazo chemistry that arenediazonium salt solutions are sensitive to light, photolytic dediazoniations have only marginal importance in organic synthesis. The recent successful photolytic fluoro-de-diazoniations of Yoneda and coworkers132 were discussed in Section III.C. 

Another relatively recent development is based on mechanistic interest in carbenes, for which the thermal and the photolytic dediazoniation of diazoalkanes has become one of the two major points of access (see Chapt. 8). 

Dediazoniation of diazoalkanes also takes place, however, without primary addition of a Bronsted or Lewis acid at the C-atom. Carbenes are obtained thermally, photolytically, and by transition-metal catalysis (Chapt. 8). 

Yamabe et al. (1980) investigated this problem in more detail and confirmed the assumption that the thermal and photolytic dediazoniation along the path of least motion (a Ciy dissociation) is unfavorable. Nevertheless, it is possible if the symmetry of the system is reduced to C2 or Q. Closed-shell Hartree-Fock MO calculations led to the result that the lowest excited state of diazomethane can initiate the dediazoniation through the bent-in-plane path (C5.). These results were corroborated by Csizmadia s group (Wang et al., 1991) in ab initio calculations with 3-21G and... 

The Balz-Schiemann photolytic dediazoniation of pyrazolodiazonium salts using ultraviolet irradiation at low temperature, in the presence of tetrafluoroboilc acid, was used in order to obtain 3-/5-fluoropyrazole derivatives 2 [1]. Although poor yields were observed in general (Scheme 1), the presence of an electron-withdrawing group at the C-4 (R =N02) improved the process significantly [Ic]. 

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