Fluorenyl cation 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-... 

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. 

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. 

That is, just as oxidation of tetrabenzo[5.5]fulvalene resulted in formation of two antiaromatic fluorenyl cations linked by a single bond, reduction should give two aromatic fluorenyl anions, linked by a single bond, 20. We call this relationship the aromaticity/antiaromaticity continuum and can apply the same measures of aromaticity to 20 as were applied to 3, to evaluate relative aroma-... 

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. 

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... 

Cations structurally related to benzhydryl are anthracenyl (7)75 87 and fluorenyl (8).69 There has been some dispute as to whether or not the flve-membered ring of the fluorenyl cation is antiaromatic. Clearly the antiaromatic character is less than for the indenyl or cyclopentadienyl cations, but current opinion favors antiaromaticity also for the fluorenyl cation.90 92 This is supported by the large difference in pAR from the anthracenyl cation, (although an additional reason for this difference will be noted later in the chapter, p. 61). Again, the brackets indicate correction of a measurement from TFE H20 to water. 

The low reactivity of 9-fluorenyl derivatives was long considered to be due to the antiaromatic character of the An 7t-electron 9-fluorenyl cation 204. 380,424 Later>... 

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 9-phenyl-9-fluorenyl (PhFl, 118, Scheme 56) amino-protecting group has been proposed by Christie and RapoportF as it is significantly more stable to solvolysis in acid conditions than the trityl group because of the antiaromatic character of the fluorenyl cation. In fact, solvolysis of Trt-Cl is 6000 times faster than PhFl-Cl.f The AI -PhFl amino acid derivatives are prepared with PhFl-Br (commercially available or prepared from fluorenone and phen-yUithium and subsequent substitution with HBr) and lead nitrate, which is required to trap the bromide (Scheme 56).F f2i] lead-free alternative using nitromethane as solvent has also been reported. ... 

A numerical scale of calculated values for antiaromaticity was derived from values of the HOMO— LUMO gap for different cations for which a value of 0 indicates no antiaromatic destabilization and with a maximum value of 1.0 for the cyclopentadienyl cation. On this scale of antiaromaticity, the value for the indenyl cation is 0.47 and that for the 9-fluorenyl cation is 0.33. ... 

The cyclopentadienyl and indenyl cations are unambiguously antiaromatic, but the fluorenyl cation 28 is more problematic and for this reason has attracted a great deal of attention. Initial solvolytic studies of 46 indicated that fluorenyl cations 47 are difficult to form (eq 21), and the H NMR spectrum of a substituted fluorenyl cation suggested a diminu-... 

Fluorenyl cations are, however, quite readily formed and observed in flash photolysis experiments. The ease of formation of fluorenyl cations from triplet precursors and the calculated greater stability of triplet compared to singlet cyclopentadienyl cation argue for ground-state destabilization of the fluorenyl cation due to antiaromaticity. 9-Arylfluorenyl cations react with nucleophilic solvents with rate constants approximately 2 orders of magnitude greater than those for the corresponding monosubstituted triaryl cations. Such kinetic instability was one of the early criteria for antiaromaticity, and the parent fluorenyl cation 28 has only been directly observed, on a nanosecond time scale, in the very weakly nucleophilic hexafluoroisopropyl alcohol or in zeolites. ... 

The calculated excitation energy of the longest wavelength absorption of the cyclopentadienyl cation increases upon annelation going to the indenyl and fluorenyl cations, and this is proposed as a characteristic of antiaromatic systems. ... 

RSE of 51 compared to 2-propyl radical was measured as 16 kcaFmol, and flie BDE of diphenyl-methane (81.8 kcal/mol) was essentially the same as that of fluorene. It has been noted radicals centered on 5-membered rings gain little additional stability by conjugation with extended aromatic networks . Considering the doubts raised about antiaromaticity in the fluorenyl cation, it is perhaps surprising there is evidence of significant destabilization in the radical 51. 

The high acidity of 56 in the photoexcited state as well as the efficient photodecarboxylative formation of 58 are paralleled by the ease of photoionization of fluorenol forming the fluorenyl cation 28. These discoveries by Wan et al. - provide an additional useful criterion of antiaromaticity. 

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... 

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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