Dications of Fluorenylidenes
J . Org. Chem., Vol. 63, No. 9, 1998 3021
results of the multiple linear regression analysis are
given in eq 2. Although the correlation is excellent,
larger than the inductive term, giving a positive value
for SCS and an increase in the H-f′ chemical shift
(decrease in paratropicity/antiaromaticity). The crucial
conclusion to be drawn from this analysis is that an
increase in electron density, either via π polarization or
through resonance, results in a decrease in the magni-
SCS ) -16.05FI + 20.07FR
(2)
ideally the data set should consist of values from com-
pounds with electron-donating substituents, such as
OCH3, and electron-withdrawing substituents, such as
NO2. As described previously, 1f2+ was not accessible,
presumably due to complexation. Oxidation of other
electron-deficient compounds was unsuccessful.35 Thus,
the dications represented reflect the greatest possible
variation in substitution in these systems.
The signs of the transmission coefficients, FI and FR,
show the relative inductive and resonance effects of the
substituent. The negative sign for FI classifies it as a
“reverse” substituent chemical shift effect36,37 resulting
from a π polarization mechanism38 in which the substitu-
ent acts to polarize the π cloud.29 Polarization by an
electron-withdrawing substituent, for example, would be
greatest at the substituent and would act to make that
position the most electron-deficient. The π cloud would
respond by making positions remote to that position
correspondingly more electron-rich. A substituent like
F would polarize the π cloud in such a way as to increase
the electron density at a site remote to the location of
the substituent, whereas an electron-donating substitu-
ent like CH3 would have the opposite effect.
The positive value for the transmission coefficient
associated with σR means that an electron-donating
substituent, with a negative value for σR, would contrib-
ute to a smaller value for the chemical shift. Since a
paratropic shift results in a smaller value for the chemi-
cal shift, an electron-donating substituent on the ring
would therefore increase the antiaromaticity of the ring
system through resonance. Analogously, an electron-
withdrawing substituent, with a positive value for σR,
would increase the magnitude of the shift through
resonance and, therefore, decrease the antiaromaticity
of the system.
1
tude of the H NMR shift and an increase in antiaroma-
ticity.
Con clu sion
The paratropic shifts of protons of the fluorenyl sys-
tems of 1a -d 2+ have been shown to be due to the
existence of antiaromatic ring currents. Substantial
variation is seen in the 1H shifts of hydrogens on carbons
which do not demonstrate differences in geometry, hy-
bridization, or charge density. The interaction between
the two orthogonal ring systems is due to σ-p donation
from one fluorenyl system to the other, resulting in an
upfield shift of carbons a and a′. The variance of the
upfield shift of carbons a and a′ is small, in contrast to
other systems studied,10 presumably due to the lack of
conformational differences in 1a -d 2+. Use of the dual
substituent parameter method on the unsubstituted ring
system reveals no correlation between the chemical shift
of the probe protons H-e and H-f and σR/σI, presumably
because the inductive effect evaluated by σI reflects a
composite of inductive effects. Use of the dual substitu-
ent parameter method on the substituted ring showed
excellent correlation with σm+, suggesting that the pri-
mary effect of the substituent was a “normal” one, and
that the variations in antiaromaticity seen were not due
to variation in electron donation from the unsubstituted
fluorenyl cationic substituent. In addition, the method
revealed a reverse inductive effect, attributed to π
polarization by the substituent. Careful examination of
the effects of each substituent demonstrated that when
the electron density of the substituted ring is increased,
either through a dominant resonance term or through π
1
polarization, the H chemical shift is decreased (becomes
more paratropic), showing the increased antiaromaticity
of the substituted ring system.
Although the resonance and inductive effects act in
opposition, it is possible to evaluate the dominant effect
for each substituent by calculating the magnitude of the
inductive and resonance effects, using eq 2. For fluorine,
with positive values for both σI and σR, the inductive term
is of greater absolute magnitude, giving a negative value
for SCS and a decrease in the magnitude of the chemical
shift of H-f′ compared 1a 2+ (increase in paratropicity/
antiaromaticity). For methyl, with negative values for
both σI and σR, the resonance term is of greater absolute
magnitude, again giving a negative value for SCS and a
decrease in the magnitude of the chemical shift of H-f′
(increase in paratropicity/antiaromaticity). Since the
absolute magnitude of the σ coefficients for methyl are
smaller than those for fluorine, the decrease in the
Exp er im en ta l Section
Starting material 1a was synthesized according to literature
procedures.11 Antimony pentafluoride was obtained from
Ozark Mahoning (Tulsa, OK) and triply-distilled prior to use.
SO2ClF was prepared by the method of Olah et al.39 or was
purchased from Aldrich Chemical Co. NMR spectra of the
dications were obtained with a Varian VXR-300 or Varian
Inova 400 spectrometer at temperatures from -80 up to -30
°C. See also Supporting Information.
9-(9H-Flu or en -9-yliden e)-2,7-dim eth yl-9H-flu or en e (1b).
n-Butyllithium (32.0 mL, 2.40 M, 13.2 mmol) was added to
fluorene (2.00 g, 12.0 mmol) in 25 mL of THF at -78 °C and
allowed to stir for 15 min under 1 atm of Argon. A solution of
2,7-dimethylfluorenone17 (2.50 g, 12.0 mmol) in 25 mL of THF
was added, and the solution was allowed to warm to 0 °C. The
solution was added to 50 mL of saturated aqueous NaCl, and
the solution was extracted with 2 × 25 mL portions of ether.
The ether layer was dried with MgSO4, and the solvent was
removed. The alcohol product was isolated by chromatography
with hexanes on silica gel. Yield: 3.37 g, 75.0%. The alcohol
(3.37 g, 9.01 mmol) was dissolved in 25 mL of acetic anhydride
with 2 drops of concentrated H2SO4. The solution was gradu-
ally warmed to 50 °C, giving a red solution as the fulvalene
chemical shift for H-f′ in 1b2+ is smaller than in 1d 2+
.
Finally, for chlorine, with positive values for both σI and
σR, the absolute magnitude of the resonance term is
(34) Brown, H. C.; Kelly, D. P.; Periasamy, M. Proc. Natl. Acad. Sci.
U.S.A. 1980, 77, 6956-6960.
(35) Malandra, J . L.; Mills, N. S., unpublished results.
(36) Reynolds, W. F.; Dais, P.; MacIntyre, D. W.; Hamer, G. K.; Peat,
I. R. J . Magn. Reson. 1981, 43, 81-89.
(37) Brownlee, R. T. C.; Craik, D. J . J . Chem. Soc., Perkin Trans. 2
1981, 760-4.
(38) Bromilow, J .; Brownlee, R. T. C.; Craik, D. J .; Fiske, P. R.; Rowe,
J . E.; Sadek, M. J . Chem. Soc., Perkin Trans. 2 1981, 753-9.
(39) Olah, G. A.; Bruce, M. R.; Welch, J . Inorg. Chem. 1977, 16, 2637.