Delocalized systems anion radicals

Scheme 1.5 represents case f, that is, an anion-radical belonging to the borderline between moderately and completely delocalized species. Its optical spectra, along with frontier orbital analysis, testifies that in this anion-radical there is a positive overlap between C-N bonds and the psendo-geminal carbons of the opposite rings, as shown by the dashed lines in the strnctnre of Scheme 1.5 (Nelsen et al. 2005). Taken together, the experimental results considered provide direct evidence for the throngh-bond mechanism of electron transfer in these paracyclophane systems.

In contrast, the nitro and ethylenic fragments in trani-4-nitrostilbene form the united conjugation system. Such a conjugation is a necessary condition for the whole-contour delocalization of an unpaired electron in arylethylene anion-radicals. Whether this condition is the only one or there is some interval of allowable strength for the acceptor is a question left to future experiments. 

Low-lying vacant orbitals of alkali metal cations can, consequently, accept an unpaired electron density even if it is delocalized over an extended n system of carbon chains. The anion-radical of 1,4-diphenylbutadiene can exist in i-trans and in -cis forms. The relative amounts of these geometrical isomers appear to depend highly on the counterion/solvent system. Li and K+ were studied as counterions THF, 2-MeTHF, and DME were employed as solvents (Schenk et al. 1991). Interaction between the anion-radical and the cation contributes to a stabilization of... 

One-electron reduction of a norcaradiene derivative produces the corresponding anion-radical. The conditions of the odd-electron delocalization in this anion-radical are less favorable than in its skeletal isomer. According to calculations, the incorporation of the unpaired electron in the nonatetraenyl n system lowers the energy content by 0.62p. However, the anion-radical initially formed is less stable than the benzotropylidene anion-radical. The latter is the end product of the isomerization (Gerson et al. 1978 Scheme 6.33). 

This conversion is directed so as to create the most favorable conditions for the delocalization of the nnpaired electron within the aromatic nucleus. It is worth noting here that thermal treatment (150—190°C) also initiates isomerization of the initial neutral molecule of norcara-diene into the benzotropylidene system. At the same time, the reductive transformation of Scheme 6.33 proceeds smoothly even at negative temperatures. Under comparable reaction conditions (25°C), the rate of conversion of the neutral molecule is 15 orders lower than that of the anion-radical. 

The azulenes 68 and 69 displayed a reversible reduction wave at —1.48 V for 68 and —1.42 V for 69, which have been attributed to the delocalization of the radical anion between the azulene and 1,2-thiazine ring systems (Scheme 9) <2003T4651>. 

Stabilization that takes place by delocalization of electrons in a it bonded system. Cations, radicals, and anions are often stabilized by resonance delocalization, (p. 163)... 

Conjugated olefins, like styrene, butadiene, and isoprene, can be caused to polymerize by cationic and anionic as well as by free-radical processes because the active site is delocalized in all cases. The most practical ionic polymerizations for these species are anionic, because such reactions involve fewer side reactions and better control of the diene polymer microstructure than in cationic systems. Free-radical polymerization of styrene is preferred over ionie proeesses, however, for cost reasons. 

This is the anion radical of l-iodo-2-benzoylnaphthalene. The dashed lines indicate a delocalized tt system. The symbol Ph stands for a phenyl group.)... 

This system is a 14it-system but it does not exhibit a diamagnetic aromatic nature. It seems that the delocalization energy is not sufficient to overcome the energy that has to be invested in the flattening of the system. The radical-anion 14, dianion 142, radical-trianion 143 and tetraanion 14 derived from 14 were prepared and characterized The dianion 142 has a D2h symmetry as deduced from its 13C NMR... 

The somewhat simplified reaction mechanism shown in Scheme 14.2 is based on the photogeneration of electron/hole pairs in PPP. While the holes react with triethylamine present in the system, the electrons remain in the polymer as delocalized anion radicals. They react with benzophenone to form the diphe-nylcarbinol anion, and the latter eventually reacts with CO2. The CO2 fixation is strongly enhanced by the presence of tetraethylammonium chloride. The soft onium cations are thought to stabilize the diphenylcarbinol anion, the precursor of the final product. 

The number of electrons changes stability in a more complex way in three-center systems, i.e. the allyl and related species. In this case, delocalization of charge is much more important than delocalization of spin. For example, rotation around the C-C bond becomes much more difBcult in the allyl cation (-38 kcal/mol) compared to the allyl radical (-13 (calculated), 15.7 (experimental)kcal/mol). Allylic anions have a lower rotation barrier relative to the cation (-23 vs. -38kcal/mol). In the case of anions, additional stabilization to the twisted form (-8-14 kcal/mol) is provided by rehybridization, which partially offsets the lower efficiency of hyperconjugation in the twisted anion than in the twisted cation. The calculated barriers for the allyl system depend strongly on the methods employed, but the trend of cation > anion > radical remains. The same trend is observed for the rotation barriers in the benzyl radical and cation (Figure 3.10). ... 

The actual E of a particular system is often not as informative as is the comparison of the delocalized system with a reference system having localized orbitals. Figure 4.11 shows the reference system for allyl a double bond separated by an imaginary barrier from a p orbital that may have 0 (cation), 1 (radical), or 2 (anion) electrons. The molecular orbital description of that system, then, is simply a sum of the HMOs of the double bond and of the p orbital. Again, it does not matter whether we are talking about the cation, radical, or anion in Figure 4.11. The HMOs of the reference system are simply those of ethene (E = a -H /3, = a — /8) superimposed on the one HMO for an isolated p orbital (E = a). 

Flavin coenzymes can exist in any of three different redox states. Fully oxidized flavin is converted to a semiqulnone by a one-electron transfer, as shown in Figure 18.22. At physiological pH, the semiqulnone is a neutral radical, blue in color, with a A ax of 570 nm. The semiqulnone possesses a pAl of about 8.4. When it loses a proton at higher pH values, it becomes a radical anion, displaying a red color with a A ax of 490 nm. The semiqulnone radical is particularly stable, owing to extensive delocalization of the unpaired electron across the 77-electron system of the isoalloxazine. A second one-electron transfer converts the semiqulnone to the completely reduced dihydroflavin as shown in Figure 18.22. 

In contrast to the allyl system, where the reduction of an isolated double bond is investigated, the reduction of extensively delocalized aromatic systems has been in the focus of interest for some time. Reduction of the systems with alkali metals in aprotic solvents under addition of effective cation-solvation agents affords initially radical anions that have found extensive use as reducing agents in synthetic chemistry. Further reduction is possible under formation of dianions, etc. Like many of the compounds mentioned in this article, the anions are extremely reactive, and their intensive studies were made possible by the advancement of low temperature X-ray crystallographic methods (including crystal mounting techniques) and advanced synthetic capabilities. 

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