9-Fluorenyl dications

Benzene-derived dienyl, allylic dications 239 Fluorenyl dications 240... 

The 9-fluorenyl cation, being a 4 -electron system, should be antiaromatic, as has been generally believed. However, Richard and coworkers provided experimental and theoretical data suggesting that this cation is not antiaromatic.Recent preparation of several fluorenyl dications, on the other hand, does prove that the fluorenyl system is antiaromatic in the dicationic systems. Mills and coworkers have generated the fluorenyl dications (115-120) by SbFs-oxidation of the tetraben-zofulvalene derivatives containing fluorenylidene and —five- and seven-membered... 

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-... 

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... 

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. 

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. 

One unusual aspect of these benzidine species is that the dications are less reactive toward solvent than are the monocations. This was not observed for the 2-fluorenyl and 4-biphenylyl nitrenium ions 75g and 75n and their conjugate acids 144g and 144n. The reason for this is not clear, but may be related to the fact that the two positive charges in 144qq and 144rr can be localized onto the two nitrogens on opposite ends of the structures. 

Oxidation of 3-/7-substituted-phenyl-indenylidenefluorenes with SbFs in SO2CIF gave 14 for all substituents except OCH3 which failed to oxidize, presumably because complexation with SbFs resulted in a species whose dication was too unstable to form. The dication for 14, Z=H, is shown in Figure 2. The fluorenyl, indenyl, and phenyl protons are labeled and demonstrate conclusively that the greatest paratropic shift is found for protons on the fluorenyl system. Thus the fluorenyl cation in 14 appears to be more antiaromatic than the indenyl cation. 

The dication (35) is reduced to the cation radical (53) in the reaction with fluorenyllithium, the same was observed with ferrocene <90TL6375>, which indicates that the fluorenyl carbanion acts as a one-electron donor. 

Replacing one heptagon in 21 with a pentagon leads to tetrabenzo[5,7] fulvalene (31), as shown in Figure 4.9, which was studied by the Mills group for the examination of aromaticity/antiaromaticity within the same carbon framework [34,35]. Oxidation of 31 with SbFj in SO2CIF yielded the dication (31 " ), which was found to contain an antiaromatic fluorenyl cation as indicated by a paratropic shift in the NMR spectrum and an aromatic dibenzotropylium cation. Reduction of31 with lithium resulted in the dianion (31 ), which was characterized with NMR... 

Finally, the polymerization of (SBF)P (Scheme 18) is significantly less efiicient than those of (SBF)2P and (SBF>4P as the oxidation first leads to a dimer for which the extension of the process involved only the two non-substituted fluorene units. The deposit may be described as a meshing of SBF wires covered by porphyrin units at their surface. The low polymerization yield observed along (SBF)-por-phyrin oxidation may be explained by (i) the number of SBF units (one for (SBF)P vs. four or two for (SBF)4P or (SBFliP resp.), (ii) the accessibility limitation and (iii) the electronic repulsion between the radical-cation of the fluorenyl unit and the dication of the porphyrin cores. 

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. 

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