Dications of 3-Phenylindenylidenefluorenes
maroon oil. The oil was crystallized in 2-propanol, yielding a dark
maroon powdery solid. Recrystallization from isopropyl alcohol
gave 0.39 g of product (42% yield). 1H NMR (400 MHz, CDCl3):
δ 7.22 (t, 1H, H-6), 7.24 (t, 1H, H-7′), 7.28 (t, 1H, H-5), 7.31 (t,
1H, H-6′), 7.33 (t, 2H, H-2′ and H-3′), 7.41 (s, 1H, H-2), 7.52 (d,
J ) 6.5 Hz, 1H, H-4), 7.66 (d, J ) 6.9 Hz, 2H, H-1′ and H-4′),
7.72 (d, J ) 8.2 Hz, 2H, H-o), 7.83 (d, J ) 7.8 Hz, 2H, H-m),
7.95 (d, J ) 7.3 Hz, 1H, H-5′), 8.37 (d, J ) 6.9 Hz, 1H, H-8′),
8.47 (d, J ) 7.8 Hz, 1H, H-7). Anal. Calcd for C29H17F3: C, 82.45;
H, 4.06; F, 13.49. Found: C, 82.34; H, 4.15; F, 13.02.
3-(Phenyl)-9-(1H-indenylidene)-9H-fluorene, 3b. Same pro-
cedure as for 3a, with the following exceptions: purification was
achieved by flash column and then recrystallization in petroleum
ether, and instead of 10 min intervals between each addition, 5
min intervals were used. Yield: 0.27 g (37%). 1H NMR (400 MHz,
CDCl3): δ 7.25 (t, 2H, H-6 and H-7′), 7.27 (t, 1H, H-5), 7.33 (t,
1H, H-6′), 7.34 (t, 2H, H-2′,3′), 7.40 (s, 1H, H-2), 7.43 (t, 1H,
H-p), 7.51 (d, 2H, H-m), 7.61 (d, 1H, H-4), 7.69 (d, 2H, H-1′.4′),
7.78 (d, 2H, H-o), 8.02 (d, 1H, H-5′), 8.40 (d, 1H, H-8′), 8.52 (d,
1H, H-7). Anal. Calcd for C28H18: C, 94.88; H, 5.12. Found: C,
95.03; H, 5.07.
substituent in general follows the trends seen both with the
1
NICS(1)zz calculations and H NMR shifts. As was true for
NICS, 3f2+ is substantially more antiaromatic than 3a-e2+
.
Although the primary focus of this paper is on the use of
magnetic properties to evaluate antiaromaticity, it is appropriate
to briefly consider how other measures of aromaticity/antiaro-
maticity, such as those using the energetic criteria, evaluate the
relative antiaromaticity of the phenyl-substituted dications of
indenylidenefluorene, such as 3b2+ and 3f2+. The aromatic
stabilization energy for formation of 3b2+ and for 3f2+ was
calculated by the isodesmic reaction scheme shown in Figure
1. We have found that radical species are appropriate species
in the isodesmic reaction schemes used to evaluate the desta-
bilization of the fluorenyl cation.1 The energy difference shown
was calculated by subtracting the energies of the reactants from
the energies of the products. Thus the larger and more positive
∆E, the less stable the species examined. The (anti)aromatic
(de)stabilization energy for 3b2+ was 27.44 kcal/mol; that of
3f2+ was 38.41 kcal/mol. Thus 3f2+ is less stable than 3b2+
,
consistent with its greater magnetic susceptibility exaltation and
larger, more positive NICS(1)zz values.
3-(p-Fluorophenyl)-9-(1H-indenylidene)-9H-fluorene,
3c.
1
Yield: 0.525 g (46.2%). H NMR (400 MHz, CDCl3): δ 7.19 (t,
2H, H-6 and H-7′), 7.26 (d, 2H, H-m), 7.27 (t, 1H, H-5), 7.32 (t,
1H, H-6′), 7.34 (t, 2H, H-2′,3′), 7.35 (s, 1H, H-2), 7.54 (d, 1H,
H-4), 7.69 (d, 2H, H-1′,4′), 7.74 (d of d, 2H, H-o), 8.00 (d, 1H,
H-5′), 8.39 (d, 1H, H-8′), 8.50 (d, 1H, H-7). Anal. Calcd for
C28H17F: C, 90.30; H, 4.60; F, 5.10. Found: C, 84.51; H, 5.52.
3-(p-Methylphenyl)-9-(1H-indenylidene)-9H-fluorene, 3d. Same
procedure as for 3a, with the following exceptions: purification
was achieved by column separation and recrystallization in pentane.
Summary
The dications of 3a-c2+ have been characterized experimen-
tally via their 1H NMR shifts, which show satisfactory agreement
with calculated 1H NMR shifts when those calculations include
the effect of solvent. The agreement is markedly poorer for
protons H-2 of the indenyl system and H-1/8 of the fluorenyl
system in the absence of solvent in the calculations. There is
good agreement between experimental and calculated chemical
shifts for the remaining protons of the system, giving support
to the NICS values calculated by the same method and allowing
extension of the examination of antiaromaticity to 3d,e2+. The
chemical shifts and NICS values both show that the indenyl
system is less antiaromatic than the fluorenyl system. The
presence of a phenyl substituent in the 3-position of the indenyl
ring is responsible for a decrease in the antiaromaticity of that
ring system, but even when the phenyl substituent is absent,
the indenyl system of 3f2+ is less antiaromatic than the fluorenyl
system. The greater antiaromaticity of 3f2+ over 3b2+ was
supported by ASE calculations.
1
Yield: 0.35 g (48.4%). H NMR (400 MHz, CDCl3): δ 2.42 (s,
3H, CH3), 7.22 (t, 2H, H-6,7′), 7.23 (d, 2H, H-m), 7.27 (t, 1H,
H-5), 7.31 (t, 2H, H-2′,3′), 7.32 (t, 1H, H-6′), 7.35 (s, 1H, H-2),
7.58 (d, 1H, H-4), 7.68 (d, 4H, H-1′,4′,o), 8.00 (d, 1H, H-5′), 8.37
(d, 1H, H-8′), 8.49 (d, 1H, H-7). Anal. Calcd for C29H20: C, 94.53;
H, 5.47. Found: C, 94.05; H, 5.33.
3-(p-Methoxyphenyl)-9-(1H-indenylidene)-9H-fluorene, 3e.
Same procedure as for 3a with the following exceptions: instead
of 10 min intervals between each addition, 2 h intervals were
1
used. Yield: 0.78 g (46%). H NMR (400 MHz, CDCl3): δ 3.90
(s, 3H, CH3), 7.02 (d, J ) 8.9 Hz, 2H, H-m), 7.23 (t, 2H, H-6,7′),
7.27 (t, 1H, H-5), 7.30 (s, 1H, H-2), 7.31 (t, 2H, H-2′,3′), 7.32 (t,
1H, H-6′), 7.58 (d, J ) 7.5 Hz, 1H, H-4), 7.68 (d, J ) 7.0 Hz, 2H,
H-1′,8′), 7.72 (d, J ) 8.4 Hz, 2H, H-o), 8.01 (d, J ) 7.0 Hz, 1H,
H-5′), 8.37 (d, J ) 7.5 Hz, 1H, H-8′), 8.49 (d, J ) 7.5 Hz, 1H,
H-7). Anal. Calcd for C29H27O: C, 90.60; H, 5.24. Found: C, 87.23;
H, 5.36.
Experimental Section
Preparation of Dications by Chemical Oxidation. SbF5 (∼0.7
mL, ∼9 mmol) was added to a graduated centrifuge tube in a
drybox, and the tube was capped with a septum and placed in an
ice bath. SO2ClF29 (1.3 mL) at -78 °C was transferred by cannula
into the centrifuge tube. The contents were mixed on a vortex stirrer
until homogeneous, and the solution was cooled to -78 °C. The
neutral precursor (∼3 mmol) was added in small portions, followed
by vortex mixing and cooling to -78 °C. Samples for NMR analysis
were kept at -78 °C until needed and transferred by chilled pipet
into a chilled NMR tube. A capillary tube with acetone-d6 was then
inserted into the NMR tube to serve as an external standard and
deuterium lock. At the conclusion of NMR studies, samples were
quenched with 20 mL of saturated K2CO3 in methanol at -78 °C.
The resulting mixture was extracted with CH2Cl2 and solvent
removed under vacuum. The majority of the isolated solid was
starting material, with 60-80% recovery of starting material.
Computational Methods. Geometries were optimized at B3LYP/
6-31G(d) density functional theory levels with the Gaussian 9830
The olefin precursors to 3a-e were synthesized by Peterson
olefination of the appropriate substituted 3-phenylindene with
fluorenone, as described below for the synthesis of 3a. Experimental
details for the synthesis of the appropriate 3-phenylindenes and 1H
NMR data for 3a-e and for 3a-d2+ can be found in the Supporting
Information.
3-(p-Trifluoromethylphenyl)-9-(1H-indenylidene)-9H-fluo-
rene, 3a. To 1-(4-trifluoromethylphenyl)indene (0.559 g, 2.20
mmol) in 30 mL of dry THF at -78 °C was added 2.02 mL of
n-butyllithium (3.23 mmol), giving a dark red solution. After 10
min, trimethylsilyl chloride (0.50 mL, 6.4 mmol) was added to the
reaction mixture. The reaction mixture turned a slightly lighter shade
of deep red. After 10 min, n-butyllithium (2.02 mL, 3.23 mmol)
was added, and the reaction mixture returned to the deeper red color.
After 10 min, 9-fluorenone (0.39 g, 2.20 mmol) in 10 mL of dry
THF was added to the reaction mixture. The mixture was allowed
to stir overnight, warming to room temperature, and was a deep
maroon color the next morning. The reaction mixture was quenched
with water and extracted with 2 × 40 mL of ether and 2 × 40 mL
of water. The solvent was removed under a vacuum, giving a dark
(29) Reddy, V. P.; Bellew, D. R.; Prakash, G. K. S. J. Fluorine Chem.
1992, 56, 195-197.
J. Org. Chem, Vol. 71, No. 21, 2006 7945
Mills, Nancy S.
