Anions paratropicity

The reader may gain better appreciation of the many basic differences responsible for the division into different classes of heteronin by comparing certain representative members, directly or through appropriate models, in terms of the information presented in Table II. First, one notes that the classification of oxonin (24a) as atropic, jV-methylazonine (27a) as nondescript, and 1 //-azonine or its anion as diatropic, originally proposed on the basis of NMR chemical shifts (data shown in first three rows), was confirmed by the determination of solvent shift character (S values)38 39 that revealed 1//-azonine to possess significant diatropic influence (comparable to that of naphthalene +1.3538), the V-methyl counterpart to exhibit a far weaker effect in the same direction, and oxonin to be atropic or mildly paratropic under this criterion, its S value being closely similar to that of the family s 8 --electron polyenic model, all-cis-cyclononatetraene (24 X = CH2). Major differences between oxonin and parent azonine are also seen to exist in terms of thermal stability and 13C NMR and UV spectroscopy, all of which serve further to emphasize the close structural similarity of oxonin with n-... 

Magnetic properties of polycyclic anions serve as a probe for the mode of delocalization of the added electrons. These anions gain stability either by minimizing paratropicity or by... 

This unusual charge distribution in these Ann-conjugated dianions can be interpreted in terms of minimization of paratropicity (antiaromaticity) and seems to be a general property of such anions. By forcing the charge into the anthracene moiety, the phenanthrene moiety remains almost neutral and aromatic and thus the total paratropicity is reduced and destabilization due to antiaromaticity is minimized. 

Paracyclophane-l,9,17,25-tetraene (42)62 can be reduced using lithium to yield a stable dianion (422 ) followed by a tetraanion (424-)63. Both anions show pronounced anisotropy effects that are manifested by their 111 NMR signals. The signals of the inner protons of the diatropic species, 422-, absorb at high field, whereas the peripheral ones are shifted to low field. 424, which is paratropic, shows an opposite effect. In contrast to 412, the lithium and potassium salts of 422 exhibit very similar ll and 13C NMR spectra. However, the potassium salt of 424 is afforded only after a long contact with the metal62. The opposite behavior is found in compound 41. While the reduction with... 

The polycyclic anions were first prepared by metal reduction in 1914 by Schlenk et al.5 a) and studied later by Schlenk and Bergmann 5 b). This class of conjugated anions opened a new era in carbanion chemistry by pointing out the electron transfer process as a source for charged species. The mechanism of the metal reduction of polycyclic hydrocarbons has been investigated and is well established 1,215 18-68>. The addition of two electrons to the fully conjugated (4n + 2)n-molecules yields 4mt paratropic systems 2°. 137 139>. The chemistry of this reaction is simple, with electrons initially on the alkali metal going to 7t-molecular orbitals associated with the aromatic hydrocarbon molecule (Eq. 13). 

The dependence of NMR patterns upon states of solvation should not be overlooked. This aspect of polyanion chemistry has been discussed (Sect. 4). The influence of the counter cation upon the dianion though unaccounted for by simple calculations is supposed to vary from system to system. The different topologies of the anions and different modes of delocalization may be responsible for the deviations from a linear dependence of the correlation. From Table 9 one can arrive at conclusions on the relative antiaromaticity of the various anions. For example, benzanthracene dianion 49 ) is less antiaromatic than chrysene dianion (542 ), and dibenzo-anthracene dianion (J/2-) is more antiaromatic than the pentacene dianion (472 ). It is also proposed that H NMR paratropic shifts may be used to calibrate the LUMO-HOMO energy gap. 

Reduction of 1,5-bisdehydro- and 1,5,9-trisdehydro[12]annulenes (80] and of octadehydro[24]annulene [81] with alkali metal.s provides radical anions and dianions. These dianions are diatropic, having gained two electrons compared to the paratropic parent dehydroannulenes, and, as with the corresponding annulenes, there is a dramatic change in the chemical shifts. For example, in the case of 1,5-hisdehydro[12]annulene the signal for the inner hydrogen shifts from 5 17.6 to 5 -6.88 on reduction. 

Reaction of azulene with lithium gives an azulenide dianion, probably formed by disproportionation of an intermediately formed radical [142,143], This product is remarkably stable for a charged paratropic species and can be kept in solution in tetrahydrofuran in a sealed n.m.r. tube for weeks. Exposure to dry air leads immediately to the reformation of azulene [143]. N.m.r. spectra show the para-tropicity of this anion [142,143],... 

Reduction of acepleiadylene with lithium at -80 leads in turn to the formation of a paratropic dianion ( H-n.m.r., 6 1.53 to -2.05) and a diatropic tetra-anion ( H-n.m.r., d S) [294], Both ions are stable in sealed tubes, even at room temperature. Reoxidation at any stage regenerates acepleiadylene. Reaction of these ions with dimethyl sulphate leads to the formation, respectively of dimethylated and unstable tetramethylated derivatives. 

The systems l,5-bisdehydro[12]annulene (19) [94] and l,5,9-tridehydro[12j-annulene (20) [95] are paratropic, nearly planar and contain 12 out-of-plane n-electrons. These two systems were reduced with K, and in both cases a radical anion was observed by EPR and a dianion by NMR [96]. The ERR spectra appeared immediately after reduction at low temperature and it was shown that the radical anion is in equilibrium with the neutral molecule. This is in contrast with what was observed for 3 and its radical anion, and reflects the fact that both 19 and 20 do not have to overcome a barrier of ring flattening when reduced. The NMR spectra of both dianions, which contain 14 7c-electrons, clearly show their aromaticity. 

The conformational preferences of 9-alkylthioxanthenes and their. S-oxides have been investigated by n.m.r. techniques, including solvent shifts. The magnitude of the chemical shifts of the 9-protons in the xanthene and thioxanthene anions indicates that their central rings are paratropic. ... 

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