Anion radicals from heterocycles

Gerson and co-workers °- have prepared anion-radicals from the macrocyclic heterocycles 113 and 114 and the sulfur analog of 5. Conformational and ion-pairing effects in the ESR spectra were investigated, and in the more recent work solution ENDOR spectroscopy was employed. 

Gerson and co-workers ° have formed both cation- and anion-radicals from the heterocycle dithieno[3,4-6 3, 4 -e][l,4]dithiin-l,3,5,7-tetraone... 

The polarographic half-wave reduction potential of 4-nitroisothiazole is -0.45 V (pH 7, vs. saturated calomel electrode). This potential is related to the electron affinity of the molecule and it provides a measure of the energy of the LUMO. Pulse radiolysis and ESR studies have been carried out on the radical anions arising from one-electron reduction of 4-nitroisothiazole and other nitro-heterocycles (76MI41704). 

However, there are some cases when an unpaired electron is localized not on the n, but on the o orbital of an anion-radical. Of course, in such a case, a simple molecular orbital consideration that is based on the n approach does not coincide with experimental data. Chlorobenzothiadiazole may serve as a representative example (Gul maliev et al. 1975). Although the thiadiazole ring is a weaker acceptor than the nitro group, the elimination of the chloride ion from the 5-chlorobenzothiadiazole anion-radical does not take place (Solodovnikov and Todres 1968). At the same time, the anion-radical of 7-chloroquinoline readily loses the chlorine anion (Fujinaga et al. 1968). Notably, 7-chloroquinoline is very close to 5-chlorobenzothiadiazole in the sense of structure and electrophilicity of the heterocycle. To explain the mentioned difference, calculations are needed to clearly take into account the o electron framework of the molecules compared. It would also be interesting to exploit the concept of an increased valency in the consideration of anion-radical electronic structures, especially of those anion-radicals that contain atoms (fragments) with available d orbitals. This concept is traditionally derived from valence-shell expansion through the use of d orbital, but it is also understandable in terms of simple (and cheaper for calculations) MO theory, without t(-orbital participation. For a comparative analysis refer the paper by ElSolhy et al. (2005). Solvation of intermediary states on the way to a final product should be involved in the calculations as well (Parker 1981). 

Analogously, although the enhanced reactivity of quinoline derivatives (relative to pyridine counterparts) in nucleophilic additions such as the formation of pseudobases and Reissert compounds probably relates to the relatively lower energy of the LUMO in the quinoline derivatives, the radicals from these heterocycles, in which the corresponding orbitals are singly occupied, are anionic thus, reactivity toward nucleophiles is also unlikely to be important. 

When cation-radicals from A3-phosphorins such as 188 are formed in the presence of nucleophiles and excess of oxidant, further reaction takes place to give A5-phosphorin derivatives, e.g., 191, generated in the presence of methanol by oxidation of 188 with mercuric acetate.611-614 A crystal and molecular structure determination of a A5-phosphorin shows a planar heterocycle with spd-hybridized phosphorus.615 Anion- and cation-radicals are also obtainable from the A5-phosphorin system.606,611,612... 

With compounds of rather negative reduction potential such as dimesityl-ketone or 2,2 -bipyridine the reaction produced only Ti(III) species in low amount. On the other hand, good n acceptors such as o-quinones or azo-con taining heterocycles yielded large amounts of Ti(IV) complexes of the anion radicals [89] according to Eq. (5) [88]. Similar Ti(IV) semiquinone complexes were reported from the reactions of photogenerated CpTiCl2 with o-quinones [90]. 

The extended conjugation present in dibenzothiophene permits the formation of a stabilized anion-radical 107 from this heterocycle. It was reported by several groups in the early The hyper-... 

Some nucleophilic displacement reactions (particularly those involving ketone enolates and other carbon nucleophiles) proceed routinely in low yield, with poor material balance. There is increasing evidence that these reactions are actually radical chain processes catalyzed, in principle, by a single electron transfer from the anion to the heterocycle. In such cases... 

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