Diazene anion

Sometimes, anion radicals are formed indirectly, by means of special chemical reactions. Photoionization of hydrazine in a mixture of liquid ammonia with THF in the presence of potassium l-butoxide leads to the formation of the diazene anion radical by the sequence of the following reactions (Brand et al. 1985) ... 

Side Note 17.8. Diazene anions have a key role in both the Wolff-Kishner reduction and its alternative, the Reductive Cyanation semicarbazone reduction (Formula D in Figure 17.67 and Formula G in Figure 17.68, respectively) they decompose into elemental nitrogen and an organometallic compound. The italics immediately explain what happens in the second step of the reductive cyanation of a ketone shown in Figure 17.69. 

Fig. 17.69. Two-step sequence for the conversion of a ketone into the homologous nitrile ("reductive cyanation of a carbonyl compound"). In the second step of the reaction the diazene anion G is generated and decomposes in a similar way as the diazene anion D in the Wolff-Kishner reduction of Figure 17.67 and the diazene anion G of the semicarbazone reduction in Figure 17.68.

Fig. 17.70. Alternative II to the Wolff-Kishner reduction reduction of a tosylhydrazone. Due to the lack of a sufficiently strong base, the diazene intermediate D does not react further via a diazene anion, which was encountered in the reactions of the Figures 17.67-17.69, but undergoes a radical reaction. "Non" refers to a nonyl group.

Fig. 17.71. Reduction of a conjugated tosylhydrazone. Since the diazene intermediate D cannot decompose via a diazene anion (cf. Figure 17.67-17.69) for lack of base and the radical decomposition mechanism of Figure 17.70 is not employed either, the third of a total of three mechanisms of the diazene —> hydrocarbon transformation is presented here.

Chiral 2-imidazoline dianions undergo one-electron oxidation in the presence of TEMPO (2,2,6,6-tetramethyl-l-piperidinyloxy) to form a radical anion that is either trapped stereoselectively by TEMPO or undergoes dimerization. Oxidation of bis-diazene oxides leads to novel (9-stabilized 4N/3e radical cations and 4N/2e dications. These were detected by ESR spectroscopy and cyclic voltammetry. B3LYR/6-31G calculations confirmed the nature of the 4N/3e and 4N/2e systems. ... 

Although monosubstituted diazenes and their anions are too unstable to be synthetically useful, the silyldiazines, ArN=NSiMe3, are stable and can be used to prepare organodiazenido complexes (equations 128 and 129). 

A,TV-Dimethyl-A -phenylthiourea has been shown to coordinate to Rh111 as an N—S bidentate involving four-membered chelate ring formation.154 N-Substituted thioamides also may bond in this manner.155 156 l-Amidino-2-thioureas (44) may behave either as N—S or as N—N bidentates, with this donor choice being dependent mainly on pH and the nature of the metal ion.157 As N—S donors they are known to stabilize lower oxidation states.158 As part of a study on Mo—S-containing complexes as models for redox-active molybdoenzymes, Dilworth et al. have shown that some p-(substituted)phenylhydrazines may coordinate as N—S bidentates in three different ways to one metal atom.159 The three diazenido, diazene and hydrazonido forms vary in their degree of deprotonation and therefore their anionic nature. 

The reaction of phenyllithium with [(Tj5-C5H5)Mn(CO)2N2] is unique (305) [Eq. (27)]. It has been proposed that the phenyl anion attacks the a-nitrogen atom, and subsequent protonation of the /3-nitrogen gives the diazene product. 

Oxidation of N1 and N2 cannot be established. However, one-electron reduction is feasible in a straightforward way. Exposure with ahcali-metal mirrors in THE or dimethoxyethane under super dry condidons aUows the detection of EPR spectra attributable to the radical anions Nl and N2. The hfc values of the pairwise equivalent nitrogen nuclei are 0.420/0.394 and 0.430/0.340 mT, respectively. This is approximately half the size of the corresponding values of (mono) diazenes and reveals that the spin is (almost) evenly distributed between the virtually equivalent nitrogen centers. 

Are Nl and N2 really through-space delocalized radical anions as illustrated in Fig. 7.12 A rather clear indication can be derived from the conspicuously large H hfc values of the y protons in NT and N2 of 0.625 and 0.842mT. This size can only be rationalized by a dominating electron density between the formaUy nonbonded diazene units and is in perfect agreement with density functional theory calculations. 

ArN=NH) has been demonstrated,arising from nucleophilic attack by BH4 on the p nitrogen. Such diazenes can be obtained as moderately stable (half-life of several hours) species in solution. It is not entirely clear how the aryldiazene decomposes, but there are indications that either the aryl radical AR or the corresponding anion Ar may be involved. 

The mechanism of the Wharton transposition is very simiiar to that of the Woiff-Kishner reaction. The epoxyhydrazone is first deprotonated, which triggers the faciie and irreversible epoxide ring-opening. The C-N bond of the resuiting vinyi diazene is broken upon another deprotonation, releasing N2 and a vinyl anion, which in turn affords the desired aiiyiic aicohoi. Aiternativeiy, the formation of a vinyi radical has been proposed. ... 

---