Dications of fluorenylidenes. The relationship between redox potentials and antiaromaticity for meta- and para-substituted diphenylmethylidenefluorenes

Article abstract of DOI:10.1021/jo051599u

Electrochemical oxidation of meta-substituted diphenylmethylidenefluorenes (3a-g) results in the formation of fluorenylidene dications that are shown to be antiaromatic through calculation of the nucleus independent chemical shift (NICS) for the 5- and 6-

Full text of DOI:10.1021/jo051599u

Dications of Fluorenylidenes. The Relationship between Redox
Potentials and Antiaromaticity for Meta- and Para-Substituted
Diphenylmethylidenefluorenes
Nancy S. Mills,* Cornelia Tirla, Michele A. Benish, Amber J. Rakowitz, Lisa M. Bebell,
Caroline M. M. Hurd, and Anna L. M. Bria
Department of Chemistry, Trinity University, One Trinity Place, San Antonio, Texas 78212-7200
nmills@trinity.edu
Received July 30, 2005
Electrochemical oxidation of meta-substituted diphenylmethylidenefluorenes (3a-g) results in the
formation of fluorenylidene dications that are shown to be antiaromatic through calculation of the
nucleus independent chemical shift (NICS) for the 5- and 6-membered rings of the fluorenyl system.
There is a strong linear correlation between the redox potential for the dication and both the
calculated NICS and σm. Redox potentials for formation of dications of analogously substituted
tetraphenylethylenes shows that, with the exception of the p-methyl derivative, the redox potentials
for these dications are less positive than for formation of the dications of 3a-g and for dications of
p-subtituted diphenylmethylidenefluorenes, 2a-g. The greater instability of dications of 2a-g and
3a-g compared to the reference system implies their antiaromaticity, which is supported by the
positive NICS values. The redox potentials for formation of the dications of meta-substituted
diphenylmethylidenes (3a-g) are more positive than for the formation of dications of para-
substituted diphenylmethylidenes (2a-g), indicating their greater thermodynamic instability. The
NICS values for dications of 3a-g are more antiaromatic than for dications of 2a-g, which is
consistent with their greater instability of the dications of 3a-g. Although the substituted
diphenylmethyl systems are not able to interact with the fluorenyl system through resonance
because of their geometry, they are able to moderate the antiaromaticity of the fluorenyl cationic
system. Two models have been suggested for this interaction, σ to p donation and the ability of the
charge on the substituted ring system to affect delocalization. Examination of bond lengths shows
very limited variation, which argues against σ to p donation in these systems. A strong correlation
between NICS and σ constants suggests that factors that affect the magnitude of the charge on
the benzylic (R) carbon of the diphenylmethyl cation affect the antiaromaticity of the fluorenyl
cation. Calculated atomic charges on carbons 1-8 and 10-13 show an increase in positive charge,
and therefore greater delocalization of charge in the fluorenyl system, with increasing electro-
negativity of the substituent. The change in the amount of positive charge correlated strongly with
NICS, supporting the model in which the amount of delocalization of charge is related to the
antiaromaticity of the species. Thus, both aromatic and antiaromatic species are characterized by
extensive delocalization of electron density.
Introduction
criteria used to characterize aromaticity fall into three
categories that are derived from properties of benzene:
magnetic, structural, and energetic.4,5 Briefly, magnetic
criteria focus on the consequences of the existence of a
ring current, which include magnetic susceptibility ex-
Aromaticity is one of the fundamental concepts of
organic chemistry; however, there is widespread dis-
agreement about how it is manifested.1-3 In general, the
10.1021/jo051599u CCC: $30.25 © 2005 American Chemical Society
Published on Web 11/18/2005
J. Org. Chem. 2005, 70, 10709-10716
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Mills et al.
altation (Λ),6 nucleus-independent chemical shift (NICS),7
and diatropic shift of protons on the periphery of an
aromatic ring system.8,9 An antiaromatic system would
show values of NICS, Λ, and potentially chemical shift
that are opposite in sign to those of an aromatic system.10
Structural criteria include lack of bond length alternation
and can be evaluated through the harmonic oscillator
model of aromaticity (HOMA).11-13 While some authors
believe that antiaromatic species are characterized by
bond length alternation,10,14 there is growing evidence
that both aromatic and antiaromatic species are charac-
terized by a lack of bond length alternation.15 Energetic
criteria are based on the greater stability of an aromatic
system over a comparable system with localized bonds.
Antiaromatic systems would show a greater thermody-
namic instability compared to an appropriate reference
system.
suite of fluorenylidene dications 122-29 that demonstrate
antiaromatic behavior. These dications are characterized
by a cationic fluorenyl system that is antiaromatic and
a second cationic system that can be modified to system-
atically “adjust” the antiaromaticity of the fluorenyl ring
system. Thus, by a series of rational changes, it is
possible to evaluate the antiaromaticity of the fluorenyl
cation via the three criteria and to see the relationship
between the criteria.
One problem with the assessment of aromaticity/anti-
aromaticity is that many of the criteria involve calculated
rather than measured quantities. Although there is
growing acceptance of the results of calculations, many
chemists are more comfortable when those results are
supported by experimental data. A particular problem
exists in the assessment of the stability of a molecule or
species because the choice of a reference system can be
contentious.
While there appears to be little dissension about the
three criteria, there is a great deal of discussion about
whether the criteria evaluate the same property, that is,
are the criteria related or orthogonal? To date, the
evaluation of these criteria has been primarily with
aromatic systems. Antiaromatic systems, such as the
We have been interested in using the redox potential
for formation of fluorenylidene dications as a measure
of their stability. That is, the more positive the potential
for formation of the dication, the greater its instability.
We demonstrated this approach with the oxidation of a
series of para-substituted diphenylmethylidenefluorenes,
2, which demonstrated a linear relationship between the
antiaromaticity of their dications, as evaluated by NICS
1
calculations and paratropic H NMR shifts, and redox
potential for formation of the dications, with species with
the greatest positive potential showing the largest degree
of antiaromaticity.27 We here report the extension of that
work to meta-substituted diphenylmethylidenefluorenes,
3. Because the manner in which the antiaromaticity of
fluorenyl cation, could allow an alternative approach. By
evaluating the criteria in systems whose antiaromaticity
should make them as far removed from benzene and its
aromaticity as possible, differences in the responses of
the criteria should be enhanced. However, there is some
dispute about the antiaromaticity of the fluorenyl
cation.16-21 We have entered this discussion through a
(1) Schleyer, P. v. R.; Freeman, P. K.; Jiao, H.; Goldfuss, B. Angew.
Chem., Int. Ed. Engl. 1995, 34, 337-340.
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Antiaromaticity; John Wiley and Sons: New York, 1994.
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A. R. Tetrahedron 2000, 56, 1783-1796.
fluorenylidene dications is moderated by substituents
(20) Jiao, H.; Schleyer, P. v. R.; Mo, Y.; McAllister, M. A.; Tidwell,
T. T. J. Am. Chem. Soc. 1997, 119, 7075-7083.
(21) Herndon, W. C.; Mills, N. S. J. Org. Chem 2005, 70, 8492-
8496.
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Am. Chem. Soc. 1994, 116, 11622-11624.
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K. E.; Kadlecek, D. E.; Lowery, J. A. J. Org. Chem. 1997, 62, 9318-
9322.
(24) Mills, N. S.; Burns, E. E.; Hodges, J.; Gibbs, J.; Esparza, E.;
Malandra, J. L.; Koch, J. J. Org. Chem. 1998, 63, 3017-3022.
(25) Mills, N. S.; Malinky, T.; Malandra, J. L.; Burns, E. E.; Crossno,
P. J. Org. Chem. 1999, 64, 511-517.
(26) Mills, N. S. J. Am. Chem. Soc. 1999, 121, 11690-11696.
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2003-2012.
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3990-3998.
(11) Krygowski, T. M.; Cyranski, M. K. Chem. Rev. 2001, 101, 1385-
1419.
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3842.
(13) Krygowski, T. M. J. Chem. Inf. Comput. Sci. 1993, 33, 70-78.
(14) Shurki, A.; Hiberty, P. C.; Dijkstra, F.; Shaik, S. J. Phys. Org.
Chem. 2003, 16, 731-745.
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10710 J. Org. Chem., Vol. 70, No. 26, 2005

Dications of Fluorenylidenes
TABLE 1. Redox Potentials for Cation Radical and
was difficult to determine in the para series (2) in which
the substituent could operate through a combination of
inductive/field and resonance effects, the examination of
meta-substituted diphenylmethylidenefluorene dications
would allow the effect to be restricted to inductive/field
effects only. Our intent was to utilize both the paratro-
Dication of 2a-g, 3a-g, 5a-g, and 6a-g
E1/2, cation
radical
E1/2
,
E1/2, cation
radical
E1/2
,
∆E1/2 dication
2/3-5/6
,
dication
dication
3a
3b
3c
3d
3e
3f
1.24
1.17
1.17
1.15
0.98
0.94
1.25
1.07
1.05
1.00
0.70
0.66
1.53
1.44
1.44
1.41
1.27
1.26
1.54
1.33
1.31
1.26
1.06
1.03
5a
5b
5c
5d
5e
5f
1.16
1.08
1.09
1.09
0.91
0.86
1.18
1.00
0.98
0.93
0.78
0.61
1.44
1.32
1.32
1.33
1.20
1.14
1.45
1.26
1.24
1.19
1.08
0.88
0.09
0.13
0.12
0.08
0.07
0.12
0.09
0.08
0.07
0.07
-0.02
0.14
1
picity of the H NMR shifts for the dications and NICS
calculations to evaluate magnetic properties, but unfor-
tunately, the dications cyclized so rapidly, see below, that
it was impossible to measure their proton spectra. This
was also true for the unsubstituted diphenylmethylidene-
fluorene dication.25 Thus, our comparison of magnetic and
energetic criteria will rest on the calculated NICS values
along with redox potentials.
2a
2b
2c
2d
2f
6a
6b
6c
6d
6f
2g
6g
substituent. We also compare the antiaromaticity of para-
substituted diphenylmethylidenefluorene dications with
the meta-substituted series and propose a mechanism by
which the substituent moderates the antiaromaticity of
these fluorenylidene dications.
Results
A second limitation of our evaluation of the antiaro-
maticity of para-substituted diphenylmethylidenefluo-
rene dications was the choice of reference system used
in the evaluation of the decreased stability of 2. We
utilized tetrakis-4-substituted phenylethylenes, 4, as the
precursor to the dications. The effect of the substituent
is overstated because it appears four times in the
reference system as compared to twice in 2. We report
here the redox potentials for oxidation of 5 and 6 in which
the effects of the substituents are more appropriately
balanced with those of the diphenylmethylidenefluorene
systems.
Synthesis of 3, 5, and 6. The precursors to the
fluorenylidene dications, 32+, and the corresponding
tetraphenyl ethylene dications, 52+ and 62+, were in
general synthesized by Peterson olefination of fluorene
or diphenylmethane, respectively, by the corresponding
substituted benzophenones as shown in the synthetic
scheme below for 3 or 5.
Electrochemical Oxidation of Diphenylmeth-
ylidenefluorenes. The oxidation of the olefin precursors
to 3a-g was examined by cyclic voltammetry in deaer-
ated CH2Cl2. The solvent with supporting electrolyte was
dried by passing it through a column of dried alumina
just prior to use. Oxidation with a 1.6 mm diameter
electrode gave irreversible oxidation to the dication at
all scan rates accessible, presumably due to its reaction
with the traces of residual water in the solution. When
the electrode was changed to a 10 µm diameter micro-
electrode, allowing an increase in scan rates to 25-102
V/s, the cyclic voltammograms showed reversible behav-
ior, as indicated by the invariance of Epa and Epc with
scan rates from 25 to 102 V/s; see the Supporting
Information. The redox potentials for formation of the
cation radical (3a-f)•+ and for formation of the dications
(3a-f)2+ are shown in Table 1, along with the same data
for 2a-g. The cyclic voltammograms were more poorly
resolved than those of the para-substituted diphenyl-
methylidenefluorenes.27 The cyclic voltammogram of 3g
was so poorly resolved that determination of the redox
potential for formation of the cation radical and dication
was unreliable.
In this paper, we report the electrochemical oxidation
of dications of meta-substituted diphenylmethylidene-
fluorenes, their antiaromaticity, as evaluated through
calculation of nucleus-independent chemical shift (NICS),
the relationship between stability (redox potential) and
antiaromaticity, and the relationship between antiaro-
maticity and the electron-withdrawing ability of the
To determine if the substituent was playing an unusual
role in the formation of the dication, a plot was made of
J. Org. Chem, Vol. 70, No. 26, 2005 10711

Mills et al.
the redox potential (E1/2) for formation of both cation
radical and dication of 3a-g vs a variety of substituent
coefficients, σ. The best correlation was with σm,30 which
gave a correlation coefficient (r2) of 0.980 for formation
of the cation radical and 0.926 for formation of the
dication; see the Supporting Information. The slightly
better correlation with σm for formation of the cation
radical suggests that the positive charge is localized on
the diphenylmethylidene portion of the system which
would be more responsive to substituent effects, with the
radical localized on the fluorenyl portion. This behavior
has been seen previously in studies of the cation radical
for 3d and 3f.31 The effect of substituent constants on
the redox potential shows that the more electronegatively
substituted diphenylmethylidenefluorene was oxidized at
a more positive potential, as would be predicted. The
relationship of this potential to measures of antiaroma-
ticity will be discussed in a later section.
Choice of the Reference System, Preparation,
and Electrochemical Oxidation. To evaluate antiaro-
maticity, it is necessary to determine whether the stabil-
ity of the system is less than that of an appropriate
reference system. In this context, this would ask whether
the redox potential observed in the system is more
positive than that for a reference system. The ideal
reference system would be one like 7 in which the phenyl
rings that were to serve as a model for the incompletely
delocalized fluorenyl system were planar, rather than 5/6
in which the phenyl rings would be expected to be
oriented in a propellared geometry.32 Compound 7 was
the redox potential for formation of the dications of the
appropriately substituted diphenylmethylidenefluorene
and the corresponding tetraphenylethylene. The redox
potentials show that, in all cases except for p-methyl-
substituted diphenylmethylidenefluorene, the dications
of diphenylmethylidenefluorenes were formed at more
positive potentials than the dications of tetraphenyl-
ethylenes, suggesting their decreased stability and greater
antiaromaticity. While the redox potential for formation
of the dication of p-methyl-substituted diphenylmeth-
ylidenefluorene is less positive than that of the corre-
sponding tetraphenylethylene, the difference is very
small.
Nucleus-Independent Chemical Shift Calcula-
tions. Nucleus-independent shift calculations (NICS)
calculations are made by GIAO calculation of the chemi-
cal shift of a ghost atom placed 1 Å above the center of a
ring.7,36 A positive value for NICS is associated with
antiaromaticity. Its magnitude is dependent on the basis
set and on the method used.26 To determine the appropri-
ate method for calculation of NICS, we compared the
experimental carbon and proton shifts for 2c2+, d2+, and
f2+ and those calculated using several calculational
methods.37 As we have seen in the past, the most effec-
tive correlation, r equals 0.998 for the carbon shifts
and 0.952 for the proton shifts of the fluorenyl sys-
tem,28 is found when the calculations are done using the
density functional theory method B3LYP with basis set
6-31G(d) on geometries optimized using B3LYP with
basis set 6-31G(d) using the Gaussian 94,38 Gaussian
98,39 or Gaussian 0340 suite of programs. The good
correlation between experimental and calculated shifts
suggests that other magnetic properties, such as NICS,
will also be calculated effectively by this method.
The values for the nucleus independent chemical shift
calculated using density functional theory for the
5-(NICS-5-1) and six-membered rings (NICS-6-1) of
(3a-g)2+ are given in Table 2, along with the values for
(2a-g)2+. Although the NICS values for (2a-g)2+ were
reported previously,27 they were calculated in the plane
not well-behaved in an electrochemical sense; see the
Supporting Information. However, the potentials for
formation of the cation radical and dication were roughly
similar to those of 5e/6e, so we were comfortable with
the use of 5/6 as model systems. A further advantage of
5/6 as reference systems is that electrochemical oxida-
tions of several derivatives of tetraphenylethylene had
been reported in the literature,32-35 so we had some
context for assuming that the oxidations of 5/6 would not
have unexpected outcomes. The unsymmetrical 5/6 were
synthesized primarily by Peterson olefination of diphen-
ylmethane with the appropriately substituted benzophe-
nones. Electrochemical oxidation was performed in an
analogous fashion to that of 3, and the redox potentials
for formation of their cation radicals and dications are
reported in Table 1, along with the difference between
(36) Schleyer, P. v. R.; Manoharan, M.; Wang, Z.-X.; Kiran, B.; Jiao,
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with the GIAO method at RHF/6-31G(d)//RHF/6-31G(d) and B3LYP/
6-31G(d)//B3LYP/6-31G(d) levels and with the IGLO method (Kut-
zelnigg, W.; Schindler, M.; Fleischer, U. NMR, Basic Principles and
Progress: Springer-Verlag: Berlin, Germany, 1990), basis sets DZ and
II, on geometries optimized at B3LYP/6-31G(d).
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Johnson, B. G.; Robb, M. A.; Cheesman, J. R.; Keith, T.; Petersson, G.
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,
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Jr.; 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.; Johnson, B. G.; Chen,
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10712 J. Org. Chem., Vol. 70, No. 26, 2005

Dications of Fluorenylidenes
TABLE 2. NICSa for (3a-g)2+ and (2a-g)2+
NICS-5-1
NICS-6-1
3a2+
3b2+
3c2+
3d2+
3e2+
3f2+
24.4
23.7
23.9
24.1
23.2
22.8
22.6
10.4
9.8
10.1
10.3
8.9
8.6
8.4
3g2+
2a2+
2b2+
2c2+
2d2+
2f2+
24.0
22.6
22.9
22.9
22.2
22.2
10.0
8.6
8.9
9.1
7.5
7.5
2g2+
of the ring rather than 1 Å above. The strongly positive
values for the calculated values of NICS substantiate
the claim of antiaromaticity for these fluorenylidene
dications.
FIGURE 1. Redox potential for formation of the dications of
2/3 vs NICS-5-1 and NICS-6-1 values for those dications.
dynamic stability, correlates well with antiaromaticity,
with the greatest antiaromaticity shown by the dications
with the largest positive redox potential, those that are
most difficult to oxidize.
Discussion
Assessment of Redox Potential as a Measure of
Antiaromaticity. The redox behavior of substituted
tetraphenylethylenes was examined so that the redox
potentials for formation of their dications could be
compared with that of substituted diphenylmethylidene-
fluorenes. As was reported previously, in the majority of
cases the redox potential for formation of the dications
of 2/3 was greater than the potential for formation of
dications of 5/6. If the difference in redox potential is
truly measuring antiaromaticity, it should show a linear
correlation with other measures of antiaromaticity, such
as NICS. The plot of redox potential differences vs NICS-
5-1 and NICS-6-1 showed effectively no correlation. This
is not surprising, however, when one looks at the
magnitude of the differences in redox potential. For
almost all of the compounds examined, the measurement
itself is no more than 1 order of magnitude greater than
the error in the measurement. The primary value of the
electrochemical study of the model systems is to demon-
strate that the dications of 2/3 are indeed less stable than
the dications of 5/6.
Effect of Substituent Position on Redox Potential
and on Antiaromaticity. A comparison of the redox
potentials for formation of the meta-substituted dications
3a-g with the redox potentials for formation of the para-
substituted dications 2a-g, shown in Table 1 shows that
3a-g have more positive redox potentials and are more
difficult to form by oxidation than 2a-g. A similar
comparison of NICS-5 and -6 for 3a-g shows that they
are more antiaromatic than 2a-g. It is apparent from
these data that substitution in the meta position of the
diphenylmethyl cationic substituent results in greater
antiaromaticity than substitution in the para position.
But how is this effect manifested? We have considered
two modes by which the cationic substituent on a
fluorenyl cation can interact with the fluorenyl system
to affect its antiaromaticity. Because the fluorenyl system
and its cationic substituent are not planar, it is not
possible for interaction between the systems to occur
through simple resonance. However, if fluorenyl system
and diphenylmethyl substituent were basically orthogo-
nal, the geometry would be appropriate for σ- to π-dona-
tion, as shown below with the benzene rings of the
fluorenyl cation removed for ease of viewing.41
However, when the redox potentials for formation of
the dications of 2/3 are plotted against the NICS values,
a strong linear correlation is shown, as seen in Figure 1.
The correlation between redox potential for formation of
the dication and NICS suggests that in these closely
related systems, redox potential, a measure of thermo-
(40) Frisch, M. J. T., G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin,
K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone,
V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.;
Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa,
J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene,
M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.;
Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.;
Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.;
Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrze-
wski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.;
Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz,
J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B.
B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.;
Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.;
Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.;
Gonzalez, C.; Pople, J. A. Gaussian 03, revision B.03 ed.; Gaussian,
Inc.: Pittsburgh, PA, 2004.
There is experimental evidence for the perpendicular
geometry of this type of system.22 The ability of the
substituent to donate electron density into the π-sys-
tem of the fluorenyl cation could explain the upfield
shift for carbon 9 in dications such as the dication of
tetrabenzo[5.5]fulvalene in comparison with fluorenyl
(41) Lammertsma, K.; Gutner, O. F.; F., T. A.; Schleyer, P. v. R. J.
Am. Chem. Soc. 1989, 111, 8995-9002.
J. Org. Chem, Vol. 70, No. 26, 2005 10713

Mills et al.
monocations.23 Alternatively, the presence of a positively
charged substituent on the fluorenyl cation might simply
function to force greater electron delocalization on the
fluorenyl system, resulting in greater antiaromaticity. We
had observed such an increased electron delocalization
for the dication of tetrabenzo[5.5]fulvalene, in comparison
with the analogous fluorenyl monocation.26 Schleyer et
al. had explained the lack of antiaromaticity in the
fluorenyl monocation by suggesting that the aromaticity
of the two benzene rings of the fluorenyl system negated
the antiaromaticity of the central 5-membered ring.20 In
such a situation, one would expect substantial delocal-
ization within each benzene ring, as opposed to through-
out the fluorenyl system, with the majority of charge
localized on carbon 9 of the fluorenyl system.
If the primary manner for the diphenymethyl substitu-
ent to interact with the fluorenyl system was σ to p
donation, we would expect that the bond lengths between
the ipso and benzylic (R) carbons would be lengthened
and those between the benzylic carbon of the diphenyl-
methyl system and between carbon 9 of the fluorenyl
system to be shortened, from participation in the hyper-
conjugative resonance forms shown below. The bond
FIGURE 2. NICS-5-1 and NICS-6-1 vs σm/σp+ for (2a-g)2+
and (3a-g)2+
.
the charge on the fluorenyl system. There would be no
reason to expect a correlation between NICS, which
reflects the behavior of the fluorenyl system and a
Hammett substituent constant. The plot of NICS vs
30
σm/σp+ in Figure 2, however, shows such a correlation.
This correlation would be consistent with a modification
in the charge on the R carbon of the diphenylmethyl
system that is then transmitted to the fluorenyl system,
affecting its antiaromaticity.
If greater delocalization of electron density in the
fluorenyl system is occurring as a function of the mag-
nitude of the positive charge on the R carbon, then we
should see that as a change in the atomic charges of the
carbons of the fluorenyl system. It is helpful to consider
Schleyer’s assessment of the lack of antiaromaticity in
the unsubstituted fluorenyl cation20 in greater depth. On
the basis of NICS calculations and calculated magnetic
susceptibility exaltation, he concluded that the aroma-
ticity of the two benzene rings counterbalances the
antiaromaticity of the five-membered ring system, result-
ing in very small magnetic susceptibility exaltation. In
effect, the positive charge would tend to remain localized
on carbon 9, to avoid the decrease in aromaticity of the
benzene rings that delocalization throughout the system
would require. However, if a positively charged substitu-
ent on carbon 9 of the fluorenyl system forced delocal-
ization of charge, the consequence would be to decrease
the aromaticity of the benzene rings and to increase the
antiaromaticity of the entire fluorenyl ring system.26 We
have calculated the atomic charge for each carbon using
natural population analysis42 and the results for each
carbon are tabulated in the Supporting Information. A
plot of average atomic charge for all carbons of the
fluorenyl system vs NICS-5-1 or NICS-6-1 shows no
correlation; see the Supporting Information. However, not
all carbons of the fluorenyl system would respond in the
same way. That is, Schleyer’s analysis would suggest that
in the absence of perturbing influences, carbon 9 of the
fluorenyl system would bear the majority of the positive
charge. As greater delocalization of charge occurs, its
lengths are tabulated in the Supporting Information
along with the analogous bond lengths for the tetra-
phenylethylene dication. The bond lengths show very
little variation with substituent, suggesting that if σ to
p donation is occurring, it is a very subtle effect. When
the bond lengths for the dication of the unsubstituted
diphenylmethylidenefluorene (2e/3e) is compared with
the unsubstituted tetraphenylethylene dication (5e/6e)2+
,
there is no difference in the length of the bond between
the ipso and R carbons. There is a substantial elongation
of the length of the bond connecting the two cationic
systems in the tetraphenylethylene dication compared to
the diphenylmethylidenefluorene dication, 1.5063 Å for
5e/6e2+ vs 1.4848 Å for 2e/3e2+. Although the shorter
bond length in 2e/3e2+ might support σ to p donation, it
might also reflect the consequence of a greater delocal-
ization of positive charge in the fluorenyl system, result-
ing less charge repulsion in the two systems. Kochi et
al. observed a similar elongation of the bond connecting
the diphenylmethyl systems of the p-methoxy- and p-
methyltetraphenylethylene dications.32
The second mechanism for interaction between the
nominally perpendicular ring systems would be through
the greater delocalization in the fluorenyl system forced
by the magnitude of the positive charge on the R carbon
of the diphenylmethyl system. The bond length changes
described above are consistent with this. A second piece
of evidence that supports this may come from the
relationship between the Hammett substituent constants
and NICS. The ability of a substituent to stabilize a
charged species is demonstrated by a linear free energy
relationship. However, the substituents of necessity only
stabilize the charge on the diphenylmethyl system, not
(42) Kar, T.; Angyan, J. G.; Sannigrahi, A. B. J. Phys. Chem. A 2000,
104, 9953-9963.
10714 J. Org. Chem., Vol. 70, No. 26, 2005

Dications of Fluorenylidenes
FIGURE 3. (a) Atomic charge (calculated using natural population analysis in Gaussian O3) vs NICS-5-1 and NICS-6-1 (NICS
calculations on dummy atoms 1 Å above the center of each ring using the GIAO method in Gaussian 03 at B3LYP/6-31G(d)//
B3LYP/6-31G(d)) for carbon 9 of the fluorenyl system of (2a-g)2+ and (3a-g)2+. (b) Average atomic charge for carbons 1-8 and
10-13 of the fluorenyl system of (2a-g)2+ and (3a-g)2+
.
nones and NMR data for 3a-g can be found in the Supporting
atomic charge would become less positive as that of the
remaining atoms of the system becomes more positive.
This is shown clearly in Figure 3a, showing a very strong
inverse correlation between electron density, as shown
by the calculated atomic charges, and antiaromaticity,
as evaluated through NICS. That is, the greater the
positive charge on carbon 9, the smaller the antiaroma-
ticity, as measured through NICS values. Similarly, when
the positive charge is forced to be delocalized throughout
the fluorenyl system, the antiaromaticity of the fluorenyl
system would increase, as shown by the positive correla-
tion between the average calculated atomic charge and
NICS values in Figure 3b. Note that for both plots, the
Y-axis is showing increasing positive charge (decreasing
negative charge).
Information.
Bis(m-trifluoromethylphenyl)methylidenefluorene
(3a). To a solution of 9-trimethylsilylfluorene (0.39 g, 1.65
mmol) in 21 mL of THF at -78 °C was added n-butyllithium
(0.70 mL of a 2.5 M solution in hexanes, 1.7 mmol). The
resulting orange suspension was stirred at -78 °C for 10 min
and then at 0 °C for 1.5 h. The reaction mixture was recooled
to -78 °C, and a solution of 3,3′-bis(trifluoromethyl)benzophe-
none (0.50 g, 1.6 mmol) in 20.5 mL of THF was added via
cannula. The cooling bath was removed, and the orange
solution was stirred for 3 h at room temperature. The reaction
mixture was quenched with 5 mL of 10% aqueous HCl. After
addition of 60 mL of ether, the reaction mixture was extracted
with 10% HCl, water, and saturated NaCl. The solvent was
evaporated under reduced pressure, giving a bright yellow
solid. Recrystallization from ethanol gave 0.41 g of bis(m-
trifluoromethylphenyl)methylidenefluorene as white plates
(61% yield): mp 185-186 °C; 1H NMR (400 MHz, CDCl3) δ
6.50 (t, J ) 7.6 Hz, 2 H), 6.93 (t, J ) 7.6 Hz, 2H), 7.28 (t, J )
7.6 Hz, 2 H), 7.54-7.74 (m, 10 H); 13C NMR (100 MHz, CDCl3)
δ 119.6, 122.6, 124.8, 125.3, 126.8, 128.0, 128.6, 129.7, 131.5,
131.8, 133.3, 137.9, 140.8, 140.9, 143.0. Anal. Calcd for
C28H16F6: C, 72.10; H, 3.46; F, 24.44. Found: C, 71.94; H, 3.27;
F, 24.63.
Summary. Control of antiaromaticity in these and
possibly all fluorenylidene dication systems comes from
control of the pattern of delocalization of the positive
charge of the fluorenyl cation. More importantly, this
helps to establish the principle that both aromatic and
antiaromatic systems are characterized by extensive
delocalization, with the greater degree of aromaticity/
antiaromaticity demonstrated in systems with the great-
est delocalization of electron density. Antiaromatic prop-
erties have been considered as manifested in an opposite
sense to those of aromatic compounds. That is, antiaro-
matic compounds are anticipated to be unstable, have
The tetraphenylethylene 5e is commercially available. The
substituted tetraphenylethylenes 5b-d,f,g and 6a-d,f,g were
synthesized by Peterson olefination of the appropriate ben-
zophenone with diphenylmethane as described below for the
synthesis of 5f. Yield and NMR data for 5b-d,g and 6a-d,f,g
can be found in the Supporting Information.
1
paratropic H NMR shifts and magnetic susceptibility
exaltations of opposite sign, and have bond length
alternation which is a consequence of limited delocaliza-
tion.5,10 This study shows that extensive electron delo-
calization is demonstrated in both aromatic and anti-
aromatic species. Furthermore, it suggests that proper-
ties related to extensive electron delocalization, such as
lack of bond length alternation would be manifested in
both aromatic and antiaromatic species.
1,1-Diphenyl-2,2-di-m-tolylethylene (5f). To a solution
of diphenylmethane (1.68 g, 10 mmol) in 5 mL of THF at -78
°C was added n-butyllithium (8 mL, 1.6 M, 12.3 mmol). The
solution was allowed to stir at room temperature for 1 h. To
the resulting red solution was added chlorotrimethylsilane (1.5
mL, 12,5 mmol), and the resulting solution was stirred at room
temperature for 10 min. The reaction was quenched by the
addition of 10 mL of a saturated aqueous solution of NH4Cl,
and after the addition of 100 mL of CH2Cl2 and 100 mL of
water, the two layers were separated and the organic layer
was extracted with CH2Cl2. The organic layers were dried over
MgSO4 and concentrated in vacuo. The product, trimethyl-
silyldiphenylmethane, was purified by flash column chroma-
tography on silica gel using hexane as eluant to afford 2.13 g.
of a white solid: yield 88.1%; 1H NMR (400 MHz, CDCl3) δ
0.00 (s, 9 H), 3.48 (s, 1H), 7.06-7.13 (m, 2H), 7.18-7.21 (m,
Experimental Section
The olefin precursors to 3a-d,f,g were synthesized by
Peterson olefination of the appropriate benzophenone with
fluorene, as described below for the synthesis of 3a. Experi-
mental details for the synthesis of the appropriate benzophe-
J. Org. Chem, Vol. 70, No. 26, 2005 10715

Mills et al.
8H); 13C NMR (100 MHz, CDCl3) δ 0.0, 47.8, 126.7, 129.9,
130.4, 144.6.
A mixture of n-buthyllithium (0.75 mL, 1.2 mmol, 1.6 M in
hexanes) and TMEDA (0.120 mL, 1.2 mmol) in 1 mL of THF
was added to a solution of of 9,9-dimethyl-10-(trimethylsilyl)-
9,10dihydroanthracene (0.280 g, 1.00 mmol) in 4 mL of THF
at -40 °C under a N2 atmosphere. The reaction mixture was
allowed to stir at -40 °C for 1 h. The reaction mixture was
then cooled at -78 °C, and a solution of benzophenone (0.180
g, 1.00 mmol) in 10 mL of THF was added. The reaction
mixture was allowed to stir at -20 °C for 6 h. The reaction
mixture was quenched with satd aq NH4Cl. Ether was added,
and the layers were separated. The aqueous layer was
extracted with ether, the organic layers were combined and
dried over MgSO4, and the solvent was removed in vacuo. The
product was purified by flash chromatography on silica gel
using hexanes as eluent to afford 235 mg, 63%, of compound
7 as a pale yellow solid: 1H NMR (400 MHz, CDCl3) δ 1.64 (s,
6H), 6.62 (t, J ) 8.8 Hz, 2H), 6.86 (t, J ) 8.6 Hz, 2H 3H), 6.96
(d, J ) 9.0 Hz, 2H), 7.03 (d, J ) 10.0 Hz, 4H), 7.15 (d, J ) 8.4
Hz, 4H), 7.27 (d, J ) 9.6 Hz, 2H); 13C NMR (100 MHz, CDCl3)
δ 40.3, 40.4, 122.9, 125.1, 126.6, 126.7, 128.7, 129.4, 129.5,
135.6, 137.3, 137.7, 143.1, 146.5; mp °C. Anal. Calcd for
C29H24: C, 93.51; H, 6.49; Found: C, 93.00; H, 6.52.
To a solution of trimethylsilyldiphenylmethane (480 mg, 2
mmol) in 2 mL of THF at -78 °C was added n-butyllithium
(1.5 mL, 1.6 M, 2.4 mmol). The solution was allowed to stir at
room temperature for 8 h. To the reaction mixture was then
added a solution of 3,3′-dimethylbenzophenone (420 mg, 2
mmol) in THF (5 mL), and the resulting solution was stirred
at room temperature for other 24 h. The reaction was quenched
by the addition of 2 mL of a saturated aqueous solution of
NH4Cl, and after the addition of 50 mL of CH2Cl2 and 50 mL
of water, the two layers were separated and the organic layer
was extracted with CH2Cl2. The organic layers were dried over
MgSO4 and concentrated in vacuo. The resulting oil was
dissolved in 20 mL of CH2Cl2, and to the resulting solution
was added 4 mL of a concd solution of H2SO4. The reaction
mixture was allowed to stir at room temperature overnight.
After 16 h, 100 mL of CH2Cl2 and 100 mL of saturated aque-
ous solution NaHCO3 were added to the mixture. The two
layers were separated, and the organic layer was extracted
with CH2Cl2. The organic layers were dried over MgSO4 and
concentrated in vacuo. The product was purified by flash
column chromatography on silica gel using hexanes as eluant
to afford 360 mg of a white solid: yield 50.0%; mp 124-126
°C; 1H NMR (400 MHz, CDCl3) δ 2.10 (s, 6H), 6.78 (s, 1H),
6.80-6.81 (m, 3H), 6.84 (d, J ) 7.2 Hz, 2H), 6.91 (dd, J ) 14.8
Hz, J ) 7.6 Hz, 2H), 6.97-6.99 (m, 4H), 7.01-7.03 (m, 6H);
13C NMR (100 MHz, CDCl3) δ 23.0, 127.9, 128.8, 129.1, 129.2,
130.1, 133.0, 133.6, 138.7, 142.3, 142.8, 145.2, 145.6. Anal.
Calcd for C28H24: C, 93.29; H, 6.71; Found: C, 92.69; H, 6.52.
Analytical data for 5a, 6f, 6g, and trimethylsilyldiphenyl-
methane agreed with literature values.43-45 In the case of 5a,
the synthetic pathway involves a Barbier reaction of the
appropriate benzophenone with the bromodiphenylmethane
followed by a dehydration reaction in acidic conditions, as
reported in the Supporting Information.
Electrochemistry. Cyclic voltammetric experiments were
performed with a 10 µm platinum disk in glass, approximately
7 cm long and 0.3 cm in diameter. The Ag/AgNO3 reference
electrode consisted of a silver wire in a saturated aqueous
solution of AgNO3. The counter electrode was a 1 cm square
platinum gauze. For specific details of the electrochemical
procedure, see the Supporting Information.
Computational Methods
Geometries were optimized at RHF/6-31G(d) ab initio and
B3LYP/6-31G(d) density functional theory levels with the
Gaussian 9438, 98,39 and 0340 program packages. The nucleus-
independent chemical shifts (NICS)7 1 Å above the ring centers
were calculated at B3LYP/6-31G(d) using the GIAO approach
with the Gaussian 94, 98, or 03 program packages. Atomic
charges were calculated by natural population analysis in the
Gaussian 03 package.
Model compound 7 was synthesized by Peterson olefination
from 9,9-dimethyl-9,10-dihydroanthracene.
Synthesis of 10,10-Dimethyl-9-diphenylmethylidenan-
thracene, 7. n-Butyllithium (0.75 mL, 1.2 mmol, 1.6 M in
hexanes) was added to a solution of 9,9-dimethyl-9,10-dihy-
droanthracene46 (0.208 g, 1.00 mmol) in 5 mL of THF at 0 °C
under a N2 atmosphere. After 10 min, trimethylsilane (0.120
mL, 1.2 mmol) was added, and the mixture was allowed to
stir at rt for 10 min. The reaction mixture was quenched with
satd aqueous NH4Cl. Ether was added, and the layers were
separated. The aqueous layer was extracted with ether, the
organic layers were combined and dried over MgSO4, and the
solvent was removed in vacuo to afford 280 mg, 99%, of 9,9-
dimethyl-10-(trimethylsilyl)-9,10dihydroanthracene as a pale
yellow solid.
Acknowledgment. We gratefully acknowledge the
Welch Foundation (Grant No. 794) and the National
Science Foundation (Grant No. CHE-9820176, REU site,
and Grant No. CHE-0242227) for their support of this
work.
Supporting Information Available: 1H and 13C NMR
spectra of 3a-c,g, 5b-d,f,g, 6a-d, and 7; cyclic voltammo-
grams of 3a-g, 5a-g, 6a-g, and 7; tables of redox potentials,
∆E°, and current ratios for 3a-g, 5a-g, 6a-g; tables of bond
lengths for (2a-g)2+, (3a-g)2+, and 5e2+; a table of atomic
charges for (2a-g)2+ and (3a-g)2+; calculated total energies
and [x,y,z] coordinates for (2a-g)2+, (3a-g)2+, and 5e2+; plots
of redox potentials vs σm, plots of atomic charges vs NICS. This
material is available free of charge via the Internet at
http://pubs.acs.org.
(43) Binkley, R. W.; Schumann, W. C.; Vashi, D. B.; Ross, J. A. J.
Org. Chem. 1972, 37, 21-24.
(44) Bauer, A.; Miller, M. W.; Vice, S. F.; McCombie, S. W. Synlett.
2001, 254-256.
(45) Katritzky, A. R.; Qi, M. J. Org. Chem. 1997, 62, 4116-4120.
(46) Falshaw, C. P.; Hashi, N. A.; Taylor, G. A. J. Chem. Soc., Perkin
Trans. 1 1985, 1837-1843.
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10716 J. Org. Chem., Vol. 70, No. 26, 2005