Antiaromaticity dications

We have chosen to examine the problem not by studying more aromatic compounds but instead by looking at antiaromatic species. This approach has been utilized very infrequently (27) because of the perception that antiaromatic species should be so reactive that they would be difficult to study. As we have discovered and will discuss below, antiaromatic dications are very amenable to study. 

NICS methods (NICS(O) and NICS(l)) have found widespread use in evaluating relative aromatic and antiaromatic characters for a variety of systems—aromatic transition states, ° antiaromatic dications,and aromatic bowls ... 

In this scission of the C—-NO2 bond, the homolytic process may be realized in neutral and weakly acid media and the heterolytic one in strong acids In consideration of ion 31) conversions one should also take into account the possibility of the acid-catalyzed heterolytic removal of the X group as X anion with the formation of antiaromatic dication 131) which further loses a proton... 

By the use of the dimethyldihydropyrene experimental probe for aromaticity, the tropy-lium ion is found to be about 50% as aromatic as benzene. Mills reviewed the antiaromatic dications and the effectiveness of various theoretical methods in evaluating the extent of antiaromaticity. Using several criteria, antiaromaticity in fluorenyl cations and dications was found to be related to the amount of charge that is present in the fluorenyl rings. Antiaromaticity was also found to be related to the delocalization, the greater the positive charge in the fluorenyl system, the greater the delocalization. 

In Chapter 11 (the only chapter in the volume that focuses on persistent oxidation dications), N. S. Mills discusses the fluorenylidene dications and their antiaromatic character. 

Fluorenylidene and Indenylidene Dications Insights about Antiaromaticity... 

Fluorenylidene dications, such as the dications of p- and m-substituted diphenylmethylidenefluorenes, show appreciable antiaromaticity. Evidence of antiaromaticity is demonstrated through H NMR shifts, nucleus independent chemical shifts (NICS), magnetic susceptibility exaltation, A, and (anti)aromatic (de)stabilization energies, ASE. Extension of the research to indenylidenefluorene dications shows that, contrary to expectation, the indenyl cation in these dications is less antiaromatic than the fluorenyl cation. The magnitude of the antiaromaticity is evaluated through comparison to the aromaticity of related dianions and reveals that the fluorenylidene dications are more antiaromatic than the fluorenylidene dianions are aromatic. 

Is the behavior of fluorenylidene dications, such as 3 or 5 unusual Comparison of the average chemical shifts of fluorenyl cations 7 with the analogous diphenylmethyl cations shows minimal antiaromaticity in the monocations, with paratropic shifts of 0.5 ppm or less (29). The dramatic para-... 

Oxidation of p- and m-substituted diphenylmethylidene fluorenes resulted in the formation of a suite of dications, 9 and 10, which were shown to be antiaromatic through magnetic and energetic criteria. ... 

The most common evaluation of aromaticity via energetic criteria is done using calculations either a type of isodesmic reaction (34) or comparison of two isomers that differ only through the aromaticity of one (3). We were interested in the possibility of evaluating stability experimentally and the electrochemical formation of dications such as 8 was attractive. In this approach, the redox potential for formation of the dication would be compared to the redox potential for formation of dications which could not be antiaromatic. If 8 was antiaromatic, its redox potential should be larger and more positive than that of the reference system. This approach was applied to the evaluation of the antiaromaticity of 9... 

While the redox potentials for formation of 9 and 10 indicates the greater instability of these dications compared to reference systems, is this instability due to antiaromaticity The plot of the redox potentials vs the sum of the NICS values for the fluorenyl system, Figure 2, next page, shows a reasonable relationship between these energetic and magnetic measures of antiaromaticity. 

Fluorenylidene dications 8 have been shown to be more antiaromatic than fluorenyl monocations 7. When the substituent R on 8 has electron-withdrawing... 

In the majority of the systems examined, the magnetic and energetic criteria used gave similar results, suggesting that either is an effective measure of antiaromaticity in fluorenylidene dications. 

The results from studies on fluorenylidene dications such as 8 demonstrate clearly that species of this type are antiaromatic. However, extension of these studies to indenylidene dications such as 13 has important advantages. The... 

We have not yet prepared the unsubstituted indenylidene fluorene dication 17 but the NICSO) (18) values demonstrate that the fluorenyl cation is still more antiaromatic than the indenyl cation, see Table III. 

To perform a similar evaluation for the indenyl cation, 18, we must examine the bis-indenylidene dication, 19, and the appropriate data for both systems is given in Table IV. While 19 is more antiaromatic than 18 by all measures, the dependence of the measure of aromaticity and presumably antiaromaticity on ring size requires that the values for 19 be divided by 2. When examined in this manner, the indenyl cation 18 is more antiaromatic than 19, in contrast to the behavior of the fluorenyl system in 7 and 3. 

Table IV. Energetic and magnetic measures of antiaromaticity for fluorenyl systems, 7, Y=H, and 3, indenyl systems, 18 and 19, vide infra, and indenylidenfluorene dications... 

The previous sections have demonstrated the antiaromaticity of a variety of indenyl and fluorenyl cations and dications but there has been no attempt to evaluate the degree of antiaromaticity. That is, are these species very antiaromatic or not particularly antiaromatic at all We need a context for the numbers and the corresponding aromatic dianions provides that context. 

We have demonstrated that oxidation of fluorenylidenes and indenylidenes results in the formation of dications with appreciable antiaromaticity. The antiaromaticity of these species was examined for a series of dications from... 

The antiaromaticity of indenyl cations was examined in the dications of indenylidene fluorenes. Contrary to expectation, the fluorenyl cation was more antiaromatic than the indenyl cation in these systems. The sum of NICS(l)zz values was linearly related to global measures of antiaromaticity such as A, suggesting that they could be used to evaluate antiaromaticity in a local sense, for individual rings. 

Once again, we may identify the cis-1,2-difluoroethylene pi dication as a 4N pi electron Hiickel antiaromatic system and the trans isomer as a 4N pi electron non-aromatic system. 

Anti-aromatic 1,2-dithiins 179 display properties opposite to those of 1,4-dithiins 180, whose dications show aromatic stabilization. Unlike other antiaromatic compounds, the 1,2-dithiin derivatives, with eight jr-electrons (such as 181 and 182), appear in... 

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