Diazenyl radical

The radical and the anion, R-N2 and R-N2, derived (formally) from a diazonium ion by addition of one and two electrons respectively, are named as diazenyl ( radical at the end is not necessary ) and diazenide (IUPAC, 1993). The radical derived formally from a diazoalkane by addition of a hydrogen atom (R=N-NH) is named diazanyl . In order to be consistent with the nomenclature of diazonium ions, the name of the parent compound should precede the words mentioned, e. g., benzenediazenyl for C6H5 - NJ (the term phenyldiazenyl radical is, however, used by Chemical Abstracts). 

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. 

The authors formulate the mechanism in two steps, first an electron transfer from phenoxide ion to diazonium ion forming a radical pair, followed by attack of the diazenyl radical at the 4-position of the phenoxy radical and a concerted proton release, i. e., without involving the o-complex. Admittedly, there is no experimental evidence against such a concerted process, but also none for it It seems that those authors wanted only to demonstrate the occurrence of radical intermediates, but did not consider the question of the mechanism of the proton release. 

Diazenyl radical, see Aryldiazenyl radical Diazo acetates (covalent) 30, 115, 138 f.,... 

Diazenyl radicals have also been detected in related systems. The rapid rearrangement of 1,3,5-triarylpentazadienes [equation (47)] involves intermediate triazenyl-diazenyl radical pairs, as indicated by the appearance in emission of the n.m.r. transitions of the -methyl protons of the starting material when Ar = Ar =j -CHg.C6H4 (Hol-laender and Neumann, 1970). The weak emission of benzene which accompanies a much more intense emission due to toluene when the 1,3-diaryltetrazene 6 decomposes in acetone at 50° has been interpreted... 

Hollaender and Neumann, 1971) as supporting a free-radical induced decomposition of intermediate phenyldi-imide (phenyldiazene) by way of the diazenyl radical [equation (48) cf. Hoffmann and Guhn, 1967]. 

The photoelimination of nitrogen from azocycloalkanes is of interest both from the synthetic and mechanistic point of view. Acyclic azoalkanes appear to undergo elimination of nitrogen by a stepwise process involving an intermediate diazenyl radical, but the photoreactions observed in azocycloalkanes are to some extent dependent on ring size. 

By single electron transfer from an electron donor, e.g. a transition metal ion, a trivalent phosphorous derivative or a base, followed by dissociation of the intermediate diazenyl radical in an aryl radical and dinitrogen. The aryl radical reacts with the solvent or with added reagents in various ways, as shown by the relatively large number of classical named reactions (Sandmeyer, Pschorr, Gomberg-Bachmann, Meerwein reactions). 

Although cyclic azoalkanes are well known as biradical precursors [159] they have been used as 1,2- and 1,3-radical cation precursors only recently [160-164]. Apart from the rearrangement products bicyclopentane 161 and cyclopentene 163, the PET-oxidation of bicyclic azoalkane 158 yields mostly unsaturated spirocyclic products [165]. Common sensitizers are triphenyl-pyrylium tetrafluoroborate and 9,10-dicyanoanthracene with biphenyl as a cosensitizer. The ethers 164 and 165 represent trapping products of the proposed 1,2-radical cation 162. Comparison of the PET chemistry of the azoalkane 158 and the corresponding bicyclopentane 161 additionally supports the notion that the non-rearranged diazenyl radical cation 159 is involved (Scheme 31). 

Thermal decomposition of cis-27 leads then to the geminate radical pair 28 giving the products a-methylstyrene 29, benzene and 30. The main products after isolation are biphenyl (20%) and dicumyl (48%). This study shows that the decomposition of azo compounds proceeds via a diazenyl radical in the singlet reaction (see also 40>). No such studies are available at the present time for the triplet. 

The singlet leads to the configurationally inverted forms, whereas the triplet mainly affords endo-55. This result indicates that rotation is inhibited in the diazenyl radical formed primarily. In the triplet diradical the formed ring closure is obviously slower than in the singlet diradical. 

Since czls-azoalkanes exhibit dipole moments of ca. (7... 10) 10 Cm (2... 3 D) [194], this solvent effect is best rationalized by assuming a decrease and final loss of the dipole moment during activation. Due to their dipole moments, czls-azoalkanes are more stabilized by polar solvents than the less dipolar activated complexes. The activation process corresponds to a synchronous, two-bond cleavage, probably accompanied by widening of the C—N=N bond angles [193]. A two-step, one-bond cleavage process via short-lived diazenyl radicals has been discussed [567], but this mechanism seems to be important only in the case of unsymmetrical azoalkanes, in particular arylazoalkanes [192]. 

The Pschorr reaction is the intramolecular coupling of arenes involving aryl radicals generated by the reduction of aryl diazonium salts [68]. The reaction was extensively studied during the last 100 years [69]. This Cu(I)-catalysed reaction proceeds via an aryl diazenyl radical intermediate (Scheme 45) [70]. Non-protic solvents from which competitive hydrogen abstraction is impossible, are to be preferred [71]. 

It is appropriate to start the discussion on electron-transfer processes to and from diazo compounds with polarographic results on electron additions to a-diazo ketones. For such a reaction one expects the formation of a diazo anion radical, in which the negative charge is localized mainly on the O-atom and to give diazenyl radical character to the diazo group (9.51). 

It seems to us that this dediazoniation is indeed slower than those of aromatic diazenyl radicals (see Zollinger, 1994, Sect. 8.6, p. 189ff.). 

The observation (Porter etal., 1972) that added BrCCla almost completely suppresses the polarization of the olefin, while leaving the polarization of trans-4 unaffected, points to the secondary radical pair as the principal immediate precursor of a-methylst5Tene. A rate constant for the decomposition of the diazenyl radical of lO -lO sec has been estimated. Cage collapse and free-radical formation are also thought to occur and appropriately polarized products have been identified (see above). 

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