Azobenzene radical anion

The relative basicity and nucleophilicity of the azobenzene radical-anion has been assessed by comparing the relative effectiveness of methyl iodide and water in quenching the reversibility of the first reduction wave For HMPA solution it was found that alkylation competed favourably with protonation although inter-... 

Although not strictly to be included in this section it is worth comparing the azobenzene-derived EGB s with those generated from azines. The azine-derived bases are of similar strength to the azobenzene radical-anions, as evidenced by their ability to deprotonate phosphonate esters(Table 15, p. 160). 

One-electron reduction of azobenzene yields the azobenzene radical anion. Its Z-form thermally isomerizes fast. This explains the cleavage/recom-bination mechanism reported for azosulfides and the reduction/oxidation mechanism of azobenzene derivatives in Langmuir-Blodgett monlayers... 

Scheme 5 summarizes some recent examples of the unambiguous use of azobenzene radical anion [25-28]. The reactions in Scheme 6 are probably initiated by the dianions of either hexa-t-propylazobenzene or 2,2 -di-t-butylazobenzene on the basis of the weak acidity of the substrates involved. However, the reactions were actually run [29-31] at potentials controlled in the range of —1.8 to —2.7 V against the silver wire pseudo reference electrode, which is notoriously variable. The last value almost certainly corresponds to reduction to the dianion, but for the electrolysis at —1.8 V (vs. Ag wire) the blue color of the radical anion was observed throughout. This implies that the radical anion is not rapidly protonated, and it may be that protonation actually occurs via disproportionation to the dianion. 

Nitrogen heteroaromatics are expected to be useful probases. The cathodic reduction of phenazine resembles closely that of azobenzene [41,42], and studies of the kinetics of proton transfer indicate [43] that the kinetic basicity of phenazine radical anion is comparable to that of azobenzene radical anion. Phenazine as probase does not appear to have been used in electrosynthesis. 

Table 3 lists a selection of rate constants for proton transfer in DMSO solution, which indicate not only the range of rates involved in common EGB reactions but also that structural features operate in a qualitatively predictable manner. It is safe to conclude from these data that, kinetically, azobenzene radical anion is probably a stronger base than the phenazine radical anion and that the dianions of the 9-fluorenylidine derivatives are relatively weak bases. 

FIGURE 1,10 Product ion mass spectra of a tryptic lectin glycopeptide derived from (a) transmission mode ETD of the triply-charged peptide with azobenzene radical anions and (b) beam-type CID of the triply-charged peptide. (Reproduced from Han, H. Xia, Y. Yang, M. McLuckey, S.A., Anal. Chem. 2008, 80, 3492-3497. With permission from American Chemical Society.)... 

Azobenzenes, (29), and analogous heteroaromatic azo compounds, (30), are in aprotic solvents reduced in two sequential one-electron steps to the radical anion and the dianion [61-66]. Disproportionation of the radical anion to the dianion is favored by the presence of Li+ [67]. The dianion is considerably more basic than the radical anion, and the dianion is only stable in very dry nonacidic solvents [64, 65, 67, 68]. Both the dianion and the radical anion derived from (29) have been used as EGBs. The anion resulting from protonation of the dianion is less basic (by several pK units) than the dianion but more basic than... 

Lund and Iversen first showed that azobenzene was an effective probase it is reduced to radical-anion at a low potential (—0.9 V vi. Ag/AgCl) and the reduced form is sufficiently basic to deprotonate benzylphosphonium salts. Its usefulness as an alternative to conventional bases was illustrated by the near quantitative production of stilbene by electrolysis of azobenzene in the presence of benzaldehyde and benzyl-triphenylphosphonium bromide (Table 2, entry 1). However, the concomitant formation of the carcinogenic benzidene, by acidic work-up of a product mixture which contains hydrazobenzene, is a severe drawback for this system. 

Alkenes may be activated toward electrochemical reduction by electron-withdrawing sucstituents. Thus acrylonitrile and acrylate esters are easily reduced and, depending among other factors on the proton availability of the medium, they undergo either hydrogenation or hydrodimerisation. The basic character of the radical-anions of such substrates has been put to use in EGB promoted Michael additions of the type outlined in Scheme 15 the case where the probase is azobenzene has already been discussed. 

Derivatives of the general formula (7) in Table 6 have been successfully used as probases and their properties in this context are being further explored. In common with the azobenzenes and ethenetetracarboxylate esters, the fluoren-9-ylidene derivatives usually display two reversible one-electron peaks in cyclic voltammetric experiments. Although disproportionation is possible (cf. Scheme 12) it is the dianions which are the effective bases. It was shown early on that the radical-anions of such derivatives are long-lived in relatively acidic conditions (e.g. in DMF solution the first reduction peak of Ph C -.QCN) remains reversible in the presence of a 570-fold molar excess of acetic acid, at 0.1 V s ). Even the dianions are relatively weak bases, useful mainly for ylid formation from phosphonium and sulphonium salts (pKj s 11-15) they are not sufficiently basic to effect the Wittig-Homer reaction which involves deprotonation of phosphonate esters... 

This could be explained in terms of disproportionation of the radical-anion to dianion with subsequent protonation. However, a much more complete explanation followed the realisation that, in most cases, the radical-anion acts not only as a base but also as a single electron-transfer agent (the so-called DISP mechanisms). In particular a comparison of observed cyclic voltammetric behaviour of substituted azobenzenes in the presence of weak acids with that predicted using digital simulation based on various mechanistic possibilities has established the DISPl route given in Eq. (3) (reactions 1-4). 

Stilbenes are an important class of compounds with a broad range of applications in basic and applied research [321]. The isoelectronic ( )-stilbene 105 and ( )-azobenzene 106 belong to the basic organic compounds of which radical anions were first investigated by EPR 30 years ago [322], and since then repeatedly studied by EPR and ENDOR techniques [323]. Radical anions of 107 and 108 are not persistent because they rapidly isomerize to 105 and 106 respectively. Nevertheless, 107 could be characterized by hyperfine data under specific conditions [324], whereas its azo counterpart 108 has hitherto escaped detection by EPR. 

The 7t-spin distribution in the radical anions of stilbene series is only moderately sensitive to deviations of the -system from planarity, the radical anions of the azobenzene series respond to steric strain by shifting the K-spin population from the benzene rings to the azo group. This is impressively demonstrated by similar hy-perfine data for 110 and 111 which contrast with the strongly differing one for their azo counterparts 112 and 113 , as well as by the corresponding values for sterically highly hindered 114 [330]. 

Electrochemical reduction of azobenzene leads to very fast isomerization of the resulting radical anion. Thus, there is no difference in the polaro-graphic behavior of the two isomers. ... 

The system azobenzene-hydrazobenzene is one of the rather few organic redox couples that behave reversibly or very nearly so at the dropping mercury electrode [187]. In nonaqueous solvents, like DMF, azobenzenes are reduced in two, one-electron steps The first one produces the radical anion, whereas the second one yields a hydrazo-benzene dianion. This dianion is easily protonated it has a tendency to decompose into arylhydrazine [188]. 

One peculiar feature of the monoprotonated dianion of azobenzene (Ph-NH-N -Ph) is that it is oxidized at the potential of radical anion oxidation and not, as expected, in a separate step anodic of the radical anion reoxidation [15,17]. This behavior has been explained as catalyzed oxidation of the anion [15]. 

Those EGBs for which proton-transfer rates are easily measured are radical anions derived by one-electron electrochemical reduction from azobenzenes (Sec. III.A.l), aromatic (Sec. III.C.3), and heteroaromatic hydrocarbons (Sec. III.A.3), and dioxygen (Sec. III.B.l). In those cases the protonated EGB is removed in a fast disproportionation reaction (cf. Sec. II.B, Eq. 2-4), and the proton-transfer step therefore is made effectively irreversible. In CV experiments with addition of an acidic substrate, protonation of the already mentioned radical anions is observed as an increase in the cathodic peak current (change from a one-electron to a two-electron process) and a decrease in the anodic peak current. Where the proton transfer reaction is fast compared to the time scale of the CV experiment, the cathodic peak current is doubled and the anodic peak completely vanishes. If the CV at low scan rates is unchanged after addition of (an excess of) acidic substrate, the EGB is too weak a base to deprotonate the substrate at a reasonable rate. 

As a consequence of the limitations just mentioned, attempts to characterize the kinetic basicity of preparatively used EGBs by determination of proton-transfer rates have mainly been done on the first-mentioned group of radical anions, particularly radical anions derived from azobenzenes, and the group of dianions derived from activated alkenes. At the same time, these type of measurements have been used to compare kinetic and thermodynamic acidities of weak organic acids. 

In that same year, a rather revolutionary publication appeared [35] on The Scope of the Reaction Between Carhanions or Nitron ions and Unsaturated Electron Acceptors. Many carbanions and nitranions seemed to react with unsaturated molecules, such as nitroaromatics, azobenzene, and diaryl ketones, to form the radical anions derived from the unsaturated compounds. The same effect was observed with n-butylmagnesium bromide and n-butyllithium. The radical anions were observed with the aid of electron spin resonance... 

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