2004 J . Org. Chem., Vol. 67, No. 7, 2002
Mills et al.
as the only uniquely applicable criterion of aromaticity/
antiaromaticity,16 Katritzky, et al.17 and J ug18 have
argued that aromaticity is at least two-dimensional. Thus
an effective examination of aromaticity/antiaromaticity
should consider as many measures of aromaticity as
possible.
ship. We report here our examination of the electrochemi-
cal oxidation to give dications of p-substituted diphenyl-
methylidene fluorenes and discuss their antiaromaticity,
as evaluated through calculation of NICS and HOMA and
the relationship between stability (redox potential) and
antiaromaticity.
The instability predicted for antiaromatic species has
prevented the characterization of a sufficient number of
species to allow an understanding of antiaromaticity to
be developed. We have discovered that dications of
fluorenylidenes 1-3, formed by oxidation of their olefin
Th e Rela tion sh ip betw een Exp er im en ta l a n d
Ca lcu la ted Ch em ica l Sh ift. There are several mea-
sures of ring current that are available through calcula-
tion, including nucleus independent chemical shift and
magnetic susceptibility, which was used to calculate
magnetic susceptibility exaltation. The nucleus indepen-
dent chemical shift (NICS)15 is based on the observation
that protons in the center of an aromatic ring system
experience a dramatic upfield shift because of the effect
of the ring current.27,28 By computing absolute magnetic
shieldings at ring centers, effectively putting a dummy
atom in the center of the ring system to “sense” the effect
of the ring current, one can evaluate both the aromatic
or antiaromatic character through the sign of the shield-
ing, with aromatic species possessing negative NICS, as
well as its relative magnitude through the size of the
shielding. Because the local contributions of the σ bonds29,30
might affect the magnetic shieldings, NICS values are
normally calculated in the plane of the ring and at a
distance, usually 1 Å, above the plane of the ring.
Magnetic susceptibility exaltation refers to an absolute
susceptibility that is larger than that expected on the
basis of an incremental system.11 NICS is usually calcu-
lated with the GIAO method in the Gaussian suite of
programs,31,32 while magnetic susceptibility is most com-
monly calculated using the IGLO program.33
precursors with SbF5 in SO2ClF, possess appreciable
antiaromaticity,19-22 as evidenced by the substantial
paratropic 1H NMR shift of the fluorenyl ring. Calculation
of magnetic susceptibility exaltation12,15 and of nucleus
independent shifts for 1a and 9-substituted fluorenyl
monocations confirmed the antiaromaticity of the fluo-
renylidene dications,23 in contrast to the lack of antiaro-
maticity of the fluorenyl monocation.24-26 We were anx-
ious to identify a second experimental probe of anti-
aromaticity. Redox potential appeared to be a reasonable
method because the stability of the dication, as suggested
by the magnitude of the redox potential for oxidation,
would be inversely correlated to the degree of antiaro-
maticity. In addition, one of the limitations of chemical
oxidation with SbF5, complexation of the Sb cation with
substituents such as methoxy or bromo,21,22 would not
exist in the electrochemical oxidation, allowing the exten-
sion of studies to include these substituents and the
subsequent examination of a linear free energy relation-
The absolute value of either NICS or magnetic sus-
ceptibility is dependent upon the basis set used for the
calculation23 and it seemed prudent to begin with a
comparison of the experimental NMR shifts for those
dications for which measurements exist and their calcu-
lated shifts. The only dications in this group for which
experimental data has been reported are 3c, 3e, and 3f.22
The 13C shifts were calculated with four different methods
(27) Mitchell, R. H. Chem. Rev. 2001, 101, 1301-1316.
(28) Gomes, J . A. N. F.; Mallion, R. B. Chem. Rev. 2001, 101, 1349-
1384.
(29) Fleischer, U.; Kutzelnigg, W.; Lazzeretti, P.; Mu¨hlenkamp, V.
J . J . Am. Chem. Soc. 1994, 116, 5298-306.
(30) Schleyer, P. v. R.; J iao, H.; van Eikema Hommes, N. J . R.;
Malkin, V. G.; Malkina, O. L. J . Am. Chem. Soc. 1997, 119, 12669-
12670.
(16) Schleyer, P. v. R.; J iao, H. Pure Appl. Chem. 1996, 68, 209-
218.
(31) Frisch, M. J .; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J . R.; Zakrzewski, V. G.; Montgomery, J ., J .
A.; Stratmann, R. E.; Burant; Dapprich, S.; Millam, J . M.; Daniels, A.
D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J .; Barone, V.;
Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford,
S.; Ochterski, J .; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma,
K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J . B.;
Cioslowski, J .; Ortiz, J . V.; Baboul, A. G.; Stefanov, B. B.; Liu, G.;
Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R. L.;
Fox, D. J .; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.;
Gonzalez, C.; Challacombe, M.; Gill, P. M. W.; J ohnson, B.; Chen, W.;
Wong, M. W.; Andres, J . L.; Gonzalez, C.; Head-Gordon, M.; Replogle,
E. S.; Pople, J . A. Gaussian 94, A.7 ed.; Gaussian, Inc.: Pittsburgh,
PA, 1998.
(32) Frisch, M. J .; Trucks, G. W.; Schlegel, H. B.; Gill, P. M. W.;
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A.; Montgomery, J . A.; Raghavachari, K.; Al-Laham, M. A.; Zakrzewski,
V. G.; Ortiz, J . V.; Foresman, J . B.; Cioslowski, J .; Stefanov, B. B.;
Nanayakkara, A.; Challacombe, M.; Peng, C. Y.; Ayala, P. Y.; Chen,
W.; Wong, M. W.; Andres, J . L.; Replogle, E. S.; Gomperts, R.; Martin,
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(17) Katritzky, A. R.; Karelson, M.; Sild, S.; Krygowski, T. M.; J ug,
K. J . Org. Chem. 1998, 63, 5228-5231.
(18) J ug, K.; Koster, A. M. J . Phys. Org. Chem. 1991, 4, 163-9.
(19) Malandra, J . L.; Mills, N. S.; Kadlecek, D. E.; Lowery, J . A. J .
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(20) Mills, N. S.; Malandra, J . L.; Burns, E. E.; Green, A.; Unruh,
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(21) Mills, N. S.; Burns, E. B.; Hodges, J .; Gibbs, J .; Esparza, E.;
Malandra, J . L.; Koch, J . J . Org. Chem. 1998, 63, 3017-3022.
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(26) See the following references for studies that support some
degree of antiaromaticity for the fluorenyl monocation: Deno, N.;
J aruzelski, J .; Schriesheim, A. J . Am. Chem. Soc. 1955, 77, 3044-
3051; Breslow, R.; Chang, H. W. J . Am. Chem. Soc. 1961, 83, 3727-
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(33) Kutzelnigg, W.; Schindler, M.; Fleischer, U. NMR, Basic
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