Pyrazines radical anions

Figure 2.3 ESR spectrum of the potassium salt of pyrazine radical anion simulated using hyperfine couplings from ref. 2.

Pyrazine is exceptional, by comparison with pyridine, pyrimidine, and pyridazine, in forming a relatively stable anion (592) the ion pair association of pyrazine radical anions with alkali metals has been studied using e.s.r. techniques (593-595) and a study has been made of the kinetics of their dimerization (596). 

E.s.r. showed that, X. ray irradiation of tetraalkyldiphosphine diphosphides gave phosphoranyl radicals with t.b.p. structures (39).114 A structure has been assigned to phosphiny1hydrazy1s (40). The dimethy1 ami no radical was particularly persistent.115 The e.s.r. parameters of the electrogenerated pyrazine radical cations (41) have been recorded.116 The spectra of a stable furanyl phosphate radical adduct117 and a phenalene radical anion which involves injection of spin density into half an attached cyclophosphazene ring,11 are reported. 

Thus, ketone enolates easily substitute chlorine in position 2 of the electrophilic nucleus of pyrazine (1,4-diazabenzene), and even in the dark, the reaction proceeds via the Sj l mechanism (Carver et al. 1981). It is expected that the introduction of the second chlorine in the ortho position to 4-nitrogen in the electrophilic nucleus of pyrazine promotes the ion-radical pathway even more effectively. However, 2,6-dichloropyrazine in the dark or subjected to light reacts with the same nucleophiles by Sr.,2 and not S nI mechanism (Carver et al. 1983). The authors are of the opinion that two halogens in the pyrazine cycle facilitate the formation of a-complex to the extent that deha-logenation of anion-radicals in solution and a subsequent nucleophilic attack of free pyrazine radical become virtually impossible. Thus, the reaction may either involve or exclude the intermediate a-complex, and only special identification experiments can tell which is the true one. 

Aromatic 7c-systems bearing two positive charges can accept one electron to form a delocalised radical-cation, which is isoelectronic with the radical-anion from the corresponding aromatic hydrocarbon. The phenanthrene analogue 4 is one such example [30]. Pyrazine is bis-protonated and reduced in acid solution to the... 

Reduction of pyrazine with potassium in 1,2-diethoxyethane at — 70° gives the paramagnetic radical anion. Anions have also been generated from the low-temperature reaction of pyrazine, methyl-pyrazine, 2,5- and 2,6-dimethylpyrazines with sodium or potassium in tetrahydrofuran or 1,2-dimethoxyethane. From measurements of their ESR spectra, ion-pair formation is proposed between the alkali metal cation and the pyrazine anion.96-101... 

Radical anions of 2,3-dimethylpyrazine and 2,5-di-/-butyl-3-isopropylpyrazine have been prepared with metallic potassium in 1,2-dimethoxyethane, and their e.s.r. spectra examined (596a). Heats of hydrogenation of compounds containing isolated and conjugated C=N double bonds have been measured, and the empirical delocalization energy for pyrazine has been determined as 22.3 kcal/mol (93.3 kJ/mol) and corresponds to only 62% of the empirical delocalization energy of benzene (597). 

The polarographic reduction of quinoxaline and its derivatives has been studied by a number of workers. Half-wave electrode potentials are pH dependent and over the pH range of 1 to 10, Ei has been reported to vary from —0.254 to -0.863 The half-wave electrode potential in anhydrous dimethylformamide is — l.bOV for quinoxaline, —1.06 V for cinnoline, —1.41V for phthalazine, and —1.22 V for quinazoline. The benzodiazines are more easily reduced than the corresponding diazines, thus pyrazine has a half-wave electrode potential of -1.57V. Pyridine with a half-wave potential of —2.15 V is still more difficult to reduce. The reversible reduction potential of quinoxaline as determined by cyclic voltammetry is -1.097 V. The observed energy differences between azine and radical anion are well correlated with the results of CNDO and SCF iT-electron calculations. 

The ESR spectra of the radical anions of pyrazino[2,3-b]pyrazine obtained by electrolytic and by chemical reduction have been examined. 

Fig. 35 Pyridine, pyrazine, and bipyridine organoaluimnum(in) radical anions...

Heterocyclic Compounds. All the heterocyclic compounds, unsubstituted by electroactive groups, from which radical ions have been obtained by electrochemical generation are nitrogen heterocycles. Among the numerous systems involved radical anions have been obtained from pyrazine [118], pyridazine [126], and tetrazine [126], and radical cations have been obtained from dihydropyrazine [126-128] and dihydrotetramethylpyrazine [128]. 

Pentadienyl radical, 240 Perturbation theory, 11, 46 Propane, 16, 165 n-Propyi anion conformation, 34 n-Propyl cation, 48, 163 rotational barrier, 34 Propylene, 16, 139 Protonated methane, 72 Pyrazine, 266 orbital ordering, 30 through-bond interactions, 27 Pyridine, 263 Pyrrole, 231... 

Things get a little more complicated when a spin 1 nucleus like 14N is added to the picture, but the same technique works again for the determination of the relative intensities of the ESR lines. Consider, for example, the relative intensities of the hyperfine lines arising from the pyrazine anion radical, whose spectrum is shown in Figure 2.3. Like that of the naphthalene anion radical, the spectrum observed for the pyrazine anion radical2 consists of 25 well-resolved... 

Figure 2.4 Derivation of the relative intensities of the 25 ESR lines of the pyrazine anion radical. See Section 2.5 for details of the method.

All discussed Ksem s are related to the separately solvated members of a two step redox system. In solvents of low polarity the charged forms may form ion pairs. Especially prone to this association are anionic redox systems of Type C (RED + OX = 2 SEM ) since the often used gegenions K , Na and Li tend to form ion pairs with the anions. These exhibit special UV/VIS-, NMR- and ESR-spectra as well as g-values Dimeres of the type (SEM M )2 may also be formed, as demonstrated with the anion radicals of pyrazines ° heptafulvalene and tetracyano-quinodimethanes. Corresponding associations are reported for dian-... 

A further significant consequence of variation in the extent of conjugation in aromatic radicals is demonstrated by a comparison of proton hyperfine splittings in pyrazine and quinoxaline anion-radicals (22,23) and also in the... 

Santos-Veiga in 1962 upon pyrazine, quinoxaline, phenazine, and 1,4,5,8-tetraazanaphthalene.35 They failed, however, to observe the anion of pyridine (1) in its stead they found that of 4,4 -bipyridyl in ethereal solvents the ion-pairs of alkali metals and the pyridine anion-radical rapidly dimerize, yielding ultimately 4,4 -bipyridyl whose anion-radical is observed.30,35... 

The anion-radical 1 was eventually observed, along with those of various of its derivatives, of pyrazine, and of pyrimidine, upon electroreduction in liquid ammonia.29 This solvent was also used for reductions of azines by solvated electrons, formed upon dissolution of sodium metal.71,72... 

The ion-association phenomena manifested by pyrazine anion-radicals have been studied intensively since the first observation.35 Simple ion-pairs form in ethereal solvents on treatment with alkali metals various structures have been assigned which place the counterion adjacent to one heteroatom or symmetrically above the ring.76 1 16,147-154 Some recent work reinterprets aspects of the earlier material.155-157 Red uction of pyrazine and tetramethy 1-pyrazine by alkali metals in ether solvents in the presence of the metal... 

Hydrogen-bonding effects of ethanol, phenol, and water on the e.s.r. spectra of the anion radicals of pyrazine mono- and di-A -oxides have been studied quantitatively (751). 

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