Dediazoniation Initiated by an Electron Transfer

In this section a general introduction to homolytic dediazoniation of arenediazonium salts is given, with some representative examples. The following sections of this chapter first describe reaction conditions for observing the changeover from hetero- 

There are three types of electron transfers, firstly the generation of an electron electrochemically, by y-irradiation, or by photolytic dissociation, secondly the transfer of an electron from an inorganic or organic compound, referred to as a nucleophilic homolytic leaving group (Zollinger, 1973 a), and thirdly a transfer from a transition metal or transition metal ion complex. In this section we will discuss the fundamental aspects of these three types. In the following sections and in Chapter 10, specific examples and synthetic applications will be summarized. 

The involvement of the diazenyl radical as an intermediate in radiolytic dediazoni-ations was demonstrated by Becker s group (Brede et al., 1979), when they identified a tetraazadiene (Ar — N2 — N2 — Ar) among the products. Substituent effects in the radiolytically induced reduction have the same sign, but are larger (p = 0.55, Packer et al., 1980) than those for the electrochemical process. 

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. 

Kasukhin et al. (1974) estimated a lifetime of 10-7 s for the phenyldiazenyl radical based on a CIDNP study. Later estimates of the decay rate constant were roughly in agreement with this 3.4 x 106 s 1 from the transient UV absorption in 

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The formation of aryl radicals from benzenediazonium ions, initiated by electron transfer from a nitrite ion, has already been discussed in Section 8.6. It is an excellent example of a dediazoniation assisted by a donor species that is capable of forming a relatively stable species on release of an electron, in this case a nitrogen dioxide radical NO2 (Opgenorth and Rtichardt, 1974). 

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