Do et al.
JOCArticle
The correlation was best when solvent was included in the
calculation. The chemical shifts for the fluorenyl systems are
found in the region between 4.7 and 5.5 ppm and support the
antiaromaticity of 3e2þ
.
The most stable dication from para-substituted 3-phenyl
benzylidene fluorenes would have been the methoxy-substituted
one, 3e2þ, which was the only one to form cleanly. It is not
surprising that dication 42þ could not be cleanly formed by
chemical oxidation because we were unable to record good
data for its electrochemical formation. It was of course
disappointing to fail to form other dications cleanly, but as
the instability of the species increases, we are going to have
more difficulty with their preparation.
We suspected that part of the complexity of the spectra of
the other oxidation reaction mixtures was due to the in-
stability of the benzylidene fluorene dication and subsequent
cyclization to benz[e]acephenanthrylenes and their probable
subsequent oxidation, shown below. To support this con-
jecture, we quenched the reaction of 3a2þ and were able to
identify starting material 3a as well as downfield peaks that
were consistent with the formation of the substituted benz-
[e]acephenathrylene25 (see the Supporting Information).
FIGURE 2. Comparison of oxidation potentials for 3a-d2þ
,
1a-e2þ, and 2b-e2þ with the corresponding differences in energy
between the dication and its neutral precursor. See Table 2 for
details of the calculations.
Evaluation of Antiaromaticity through Magnetic Measures.
The primary experimental measures of aromaticity and
antiaromaticity that probe a ring current and are considered
1
magnetic measures are the diatropic shifts in the H NMR
spectra of aromatic species and paratropic shifts in the NMR
spectra of antiaromatic species. The use of NMR chemical
shifts for evaluation of aromaticity has been called into
question because the out-of-plane component of the mag-
netic tensor is necessary for evaluation of the ring current.
The chemical shift, which is an average of all the magnetic
tensors, can mask the contribution of the out-of-plane
component.24 We will use the nucleus-independent chemical
shift as the primary tool for the evaluation of antiaromaticity
through magnetic measures because we can include only the
out-of-plane component of the magnetic tensor. However, as
mentioned previously, the comparison of experimental chemical
shifts to the chemical shift obtained in the same calculation
as the NICS values helps to validate the NICS calculations.
1H NMR Chemical Shifts. The formation of dications 32þ
and 42þ by oxidation of the corresponding substituted
benzylidene fluorenes with SbF5/SO2ClF resulted in com-
plex changes in the 1H NMR spectra for the majority of the
precursors. Spectra for reaction mixtures that could contain
32þ were cleaner than the reaction mixtures for oxidation to
give 42þ, but only the oxidation to give 3e2þ gave a spectrum
that was primarily due to a benzylidene fluorene dication
(Figure 3). Assignments of chemical shifts were made on the
basis of COSY spectra and by comparison with calculated
spectra (see the Supporting Information). Note that while
the upfield chemical shifts of the fluorenyl system appear
symmetrical, the phenyl ring must possess restricted rotation
on the NMR time scale, giving rise to four individual proton
signals. While there is more splitting for those signals than we
might anticipate, the COSY spectrum supports this assign-
ment. In addition, the assignment from calculated shifts is in
good agreement with the experimental shifts (see Figure 4).
Nucleus-Independent Chemical Shifts (NICS). The linearity of
the experimental and calculated chemical shifts for 3e2þ suggests
that calculations of NICS using the same basis set should be
reliable. The NICS values were calculated with the GIAO
method in Gaussian 0326 or Gaussian 0927 with B3LYP/6-
˚
311þg(d,p) on ghost atoms 1 A above and below the center of
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