Synthesis and Structure of a Functionalized [9]Cycloparaphenylene Bearing Three Indeno[2,1-a]fluorene-11,12-dione-2,9-diyl Units

Article abstract of DOI:10.1021/acs.orglett.7b01866

A synthetic pathway to a functionalized [9]cycloparaphenylene bearing three indeno[2,1-a]fluorene-11,12-dione-2,9-diyl units in the macrocyclic ring structure ([3]CIFO) has been developed. The ¹H and ¹³C NMR spectra show that only the anti rotamer (anti-[3]CIFO) is produced. DFT calculations indicate that the anti rotamer is thermodynamically more stable than the syn rotamer by 4.3 kcal/mol, and the rotational barrier from the anti to syn rotamer is estimated to be 23.3 kcal/mol. The UV-vis and fluorescence spectra and cyclic voltammogram of anti-[3]CIFO were investigated.

Full text of DOI:10.1021/acs.orglett.7b01866

Letter
pubs.acs.org/OrgLett
Synthesis and Structure of a Functionalized [9]Cycloparaphenylene
Bearing Three Indeno[2,1‑a]fluorene-11,12-dione-2,9-diyl Units
Shuangjiang Li, Merfat Aljhdli, Haresh Thakellapalli, Behzad Farajidizaji, Yu Zhang, Novruz G. Akhmedov,
Carsten Milsmann, Brian V. Popp, and Kung K. Wang*
C. Eugene Bennett Department of Chemistry, West Virginia University, Morgantown, West Virginia 26506-6045, United States
S
* Supporting Information
ABSTRACT: A synthetic pathway to a functionalized [9]-
cycloparaphenylene bearing three indeno[2,1-a]fluorene-11,12-dione-
2,9-diyl units in the macrocyclic ring structure ([3]CIFO) has been
1
developed. The H and 13C NMR spectra show that only the anti
rotamer (anti-[3]CIFO) is produced. DFT calculations indicate that
the anti rotamer is thermodynamically more stable than the syn
rotamer by 4.3 kcal/mol, and the rotational barrier from the anti to syn
rotamer is estimated to be 23.3 kcal/mol. The UV−vis and
fluorescence spectra and cyclic voltammogram of anti-[3]CIFO were investigated.
ttaching functional groups to cycloparaphenylenes (CPPs)
consisting of benzene units connected at para positions
Scheme 2. Synthesis of Indeno[2,1-a]fluorene-11,12-dione
A
(4) from Dimethyl 1,1′:4′,1″-Terphenyl-2′,3′-dicarboxylate
provides opportunities to modify their molecular structures and
photophysical and electrochemical properties.1 Synthetic path-
ways have been reported to attach functional groups, such as
carbonyl,2 amino,3 aryl,4 boryl,2b silyl,2b and vinyl5 groups, to
the CPP frameworks. We recently reported a synthetic pathway
to macrocycle 1,2a which is a functionalized [9]CPP containing
three dimethyl 1,1′:4′,1″-terphenyl-2′,3′-dicarboxylate-4,4″-diyl
units (Scheme 1). We now have converted 1 to anti-2 bearing
(3)
Scheme 1. Transformation of 1 to anti-2
moderate yields of 4, ranging from 20 to 61%.6,7 Upon
exposure of 1 to concentrated sulfuric acid at 140 °C for 5 min,
anti-2 rotamer was produced only in trace amounts, and the
syn-2 rotamer was not observed. In addition, the reaction result
was not reproducible under such a harsh reaction condition.
Treatment of the corresponding carboxylic acid bearing six
carboxyl groups with sulfuric acid at 140 °C for 5 to 10 min also
failed to produce anti-2 and/or syn-2. It was apparent that a
more efficient synthetic method would be needed to produce
anti-2 and/or syn-2 from 1.
We observed that upon exposure of 3 to a mixture of
trifluoromethanesulfonic acid (TfOH), trifluoroacetic anhy-
dride (TFAA), and zinc bromide at rt for 2 h and then at 90 °C
for 1.5 h, the acylation reactions proceeded efficiently to give 4
in 98% isolated yield (Scheme 2). On the other hand, when 3
three indeno[2,1-a]fluorene-11,12-dione-2,9-diyl (IFO) units
in the macrocyclic ring structure with two of the units tilting
above the macrocyclic ring and the third below the ring (anti-
[3]CIFO). The activation barrier (ΔG‡) for conversion to the
syn rotamer (syn-2, syn-[3]CIFO) was estimated to be 23.3
kcal/mol by DFT calculations. The properties of anti-2 were
investigated by UV−vis and fluorescence spectroscopy and
cyclic voltammetry.
It was reported previously that exposure of dimethyl
1,1′:4′,1″-terphenyl-2′,3′-dicarboxylate (3) to concentrated
sulfuric acid at 140 °C for 5 min produced indeno[2,1-
a]fluorene-11,12-dione (4) in only 12% isolated yield (Scheme
2).6 Other indirect synthetic routes also produced low to
Received: June 19, 2017
© XXXX American Chemical Society
A
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was treated with a mixture of TfOH and TFAA at rt for 3 h,
only monoacylation occurred to furnish 2-carbomethoxy-3-
phenyl-9-fluorenone (5) in 98% isolated yield. Similarly, 5 was
obtained in 75−88% isolated yield by subjecting 3 to a mixture
of trifluoroacetic acid (TFA), TFAA, and a small amount of
methanol at 75 °C for 30 h. Upon treatment of 5 with a 1:10
mixture of P2O5 and methanesulfonic acid (MsOH) (Eaton’s
reagent)8 at 75 °C for 12 h, 4 was obtained in 95% isolated
yield.
However, attempts to use the mixture of TfOH, TFAA, and
ZnBr2 to convert 1 to anti-2 and/or syn-2 were unsuccessful,
and 1 decomposed upon exposure to the mixture at rt for 2 h
and then at 90 °C for 1.5 h. When 1 was treated with the
mixture of TfOH and TFAA in dichloromethane at rt for 10 h,
C3 symmetric 6 as determined by NMR spectroscopy was
obtained in 44% isolated yield (Scheme 3). Interestingly, upon
Scheme 3. Synthesis of anti-2 from 1
Figure 1. Temperature-dependent 400 MHz 1H NMR spectra of anti-
6 and syn-6 in CD2Cl2.
one additional broad peak at δ 3.90 can be clearly discerned. At
−80 °C, three singlets of equal intensity appear at δ 4.01, 3.42,
and 3.27, and a singlet with three times the intensity also
appears at δ 3.37. These observations indicate that at −80 °C
the rate of equilibration between the anti and syn rotamers of 6
with a 1:1 mol ratio between them is slower than the NMR
time scale. The signal at δ 3.37 can be attributed to the three
equivalent methoxy groups of the syn rotamer (syn-6), while the
signals at δ 4.01, 3.42, and 3.27 can be attributed to the three
nonequivalent methoxy groups of the anti rotamer (anti-6).
The rotational barrier (ΔG‡) was estimated to be 10.5 kcal/mol
at −40 °C. For 7 at 25 °C, three singlets for the three
nonequivalent methoxy groups were observed.
treatment of 1 with the mixture of TFA, TFAA, and a small
amount of methanol at 75 °C for 30 h, 6 was isolated as the
minor product in 15% yield, and 7, without a C3 axis as
determined by NMR spectroscopy, was isolated as the major
product in 47% yield. The reason for the selectivity in
producing only 6 by TfOH/TFAA is not clear at this time.
Attempts to use Eaton’s reagent to convert 6 to anti-2 and/or
syn-2 at 75 °C for 12 h were unsuccessful, and unreacted 6 was
recovered. Prolonged heating at higher temperatures resulted in
decomposition. To our surprise, treatment of 7 with Eaton’s
reagent at 75 °C for 12 h was successful in producing anti-2 in
50% isolated yield. It was also possible and operationally
simpler to treat the crude mixture of 6 and 7, without further
separation and purification, with Eaton’s reagent to produce
anti-2 from 1 in 25% overall yield over two steps.
The molecular structure of anti-2 containing only a plane of
symmetry, depicted in Figure 2 after DFT optimization, is
1
supported by H and 13C NMR spectroscopy. Unlike syn-2,
which has a C3 axis and can be expected to exhibit only four 1H
1
NMR signals, anti-2 without a C3 axis showed a total of 12 H
NMR signals, including a singlet at δ 7.57 for Hd, a doublet at δ
7.51 for Hh, and a doublet at δ 7.46 for Hi. Three different
carbonyl signals were also observed on the 13C NMR spectrum.
Heating the NMR sample in DMSO-d6 at 110 °C did not show
line broadening or coalescence of the 1H NMR signals,
indicating that the rotation rates of the three IFO units are
relatively slow on the NMR time scale. DFT calculations show
that the rotational barrier (ΔG‡) from anti-2 to syn-2 is 23.3
kcal/mol, which is beyond the range for dynamic NMR studies.
The rotational pathway to equilibrate anti-2 and syn-2 involves
tilting the keto carbonyls of the IFO unit anti to the two other
units toward the inner plane of the macrocyclic ring (Figure 2).
The two carbon−carbon single bonds connecting the rotating
unit to the two other units are more severely bent during the
rotating process. DFT calculations also indicate that anti-2 is
thermodynamically more stable (ΔG°) than syn-2 by 4.3 kcal/
mol.
1
The H NMR spectrum of 6 in CD2Cl2 at 25 °C exhibits a
singlet at δ 3.55 for the three methoxy groups and only one set
of signals for the benzene and 9-fluorenone-2,7-diyl units
(Figure 1). At 0 °C, the signals of the AB pattern from the
benzene units at δ 7.35 and 7.28, the isolated hydrogens of the
9-fluorenone-2,7-diyl units at δ 7.01, and the methoxy groups at
δ 3.53 show the signs of line broadening, indicating slowing
rotations of the 9-fluorenone-2,7-diyl units. The signals
continue to broaden at lower temperatures, and at −60 °C
B
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workers.11 The rates of rotation were found to be influenced by
the chemical structures of the units and the connectivity among
them.
The DFT-optimized structure of anti-2 shows that the
torsional angles between benzene rings B and C, B′ and C′, and
E and E′ are approximately 44.8°, 44.8°, and 0°, respectively
(Figure 2). On the other hand, the torsional angles between the
adjacent benzene rings in syn-2 range from 0.6 to 2.3°,
substantially smaller than those between rings B and C and
between rings B′ and C′ in anti-2. As a result, syn-2 suffers from
more nonbonded steric interactions between IFO units, which
contribute to its lower thermodynamic stability than anti-2. The
keto groups on the CDE and C′D′E′ units of anti-2 cant
toward the inner plane of the [9]CPP circle at an angle of 66.0°
on average away from the plane, and the keto groups on the
ABB′ unit also cant toward the inner plane at an angle of 65.4°
on average.
The IR stretching absorption of the keto carbonyls of anti-2
occurs at 1730 cm−1, which is 9 cm−1 higher than that of 4 at
1721 cm−1. The hoop-like aromatic structure of anti-2 with a
radially π-conjugated system diminishes the extent of its
conjugation with the carbonyl π bonds, which is perhaps
partially responsible for the higher absorption frequency.12 The
effect of the macrocyclic ring structure on the bond angles of
the keto carbonyl groups may also contribute to the higher
absorption frequency.
The UV−vis and fluorescence spectra of anti-2 and 4 in
CH2Cl2 (Figure 3) show that the UV−vis absorption maxima
Figure 2. DFT-optimized structures of anti-2, syn-2, and the transition
state (TS-2) and their relative energies and torsional angles.
Yamago and co-workers reported the synthesis and rotational
barriers of a [6]CPP derivative bearing three 9,9-dimethyl-9H-
fluorene-2,7-diyl (FR) units ([3]CFR), an [8]CPP derivative
bearing four FR units ([4]CFR),9 and related carbazole3b and
dibenzothiophene systems.10 DFT calculations indicate that
anti-[3]CFR is thermodynamically more stable than syn-
[3]CFR by 4.8 kcal/mol, and the rotational barrier from anti-
[3]CFR to syn-[3]CFR is 58.4 kcal/mol. In comparison with
anti- and syn-2, the differences of thermodynamic stabilities
between the anti and syn rotamers are comparable, but [3]CFR
has a much higher rotational barrier. The smaller macrocyclic
ring size of [3]CFR as a [6]CPP derivative causes severe
bending of the three FR units and the single bonds connecting
them together during the rotating process, which is largely
responsible for the higher rotational barrier. For [4]CFR, four
rotamers were calculated to be the local minima, with the
rotamer having anti relationship between the adjacent units
being thermodynamically most stable, and the other three
rotamers are higher in energies by 4.5, 5.3, and 7.5 kcal/mol.
The rotational barrier from the all-anti rotamer to the next
most stable rotamer with one unit anti to the other three was
calculated to be 18.2 kcal/mol, low enough to allow rapid
equilibration of the rotamers at rt. It is worth noting that
[4]CFR, which is an [8]CPP derivative, has a smaller
macrocyclic ring size than [3]CIFO, which is a [9]CPP
derivative, but [4]CFR has a lower rotational barrier. The fact
that [4]CFR contains four FR units with the arc angle of 90° on
average for each unit, instead of three IFO units in [3]CIFO
with the arc angle of 120°, diminishes the extent of bond
distortion during the rotating process, lowering the rotational
barrier. The rotational barriers of several macrocycles bearing
chrysenylene and anthanthrenylene units and related carbon
nanohoops were extensively investigated by Isobe and co-
Figure 3. UV−vis (solid lines) and fluorescence (dashed lines) spectra
of anti-2 (red) and 4 (green).
(λabs) of 4 appear at 289 with two weaker bands at 375 and 392
nm, whereas those of anti-2 occur at 288, 379, and 408 nm.
Upon excitation of 4 at 380 nm, the fluorescence maximum
(λem) appears at 567 nm. For anti-2, upon excitation at 395 nm
two fluorescence maxima appear at 448 and 606 nm. In
comparison, the λabs of 9-fluorenone in CH2Cl2 appears at 258
nm, which can be assigned to a π−π* transition, and a weaker
band at 380 nm, which can be attributed to an n−π*/π−π*
transition.13 It is worth noting that while 9-fluorenone shows
only one broad band for the n−π*/π−π* transition anti-2 and
4 both exhibit two absorption maxima. The λem of 9-fluorenone
in CH2Cl2 appears at 504 nm. For 1 in DMSO, λabs and λem
appear at 327 and 464 nm, respectively.2a The parent [9]CPP
exhibits λabs at 340 nm and λem at 494 nm.14 The longest λem of
anti-2 is significantly red-shifted from those of 1, 4, 9-
fluorenone, and the parent [9]CPP.
The cyclic voltammograms of 9-fluorenone, anti-2, and 4 in
CH2Cl2 (Figure S1 of the Supporting Information) were
recorded versus the ferrocene/ferrocenium (Fc/Fc+) redox
couple. The reduction potentials of anti-2 and 4 at −1.47 and
C
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−1.43 V, respectively, indicate that they are more easily reduced
than 9-fluorenone at −1.83 V.15 The presence of one or more
electron-withdrawing keto carbonyls in anti-2 and 4 is
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S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.7b01866.
Complete experimental details; spectral data; UV−vis
and fluorescence spectra; cyclic voltammograms; compu-
tational coordinates and energetic details of DFT
calculations (PDF)
(5) Golder, M. R.; Colwell, C. E.; Wong, B. M.; Zakharov, L. N.;
Zhen, J.; Jasti, R. J. Am. Chem. Soc. 2016, 138, 6577−6582.
AUTHOR INFORMATION
■
̀
(6) Thirion, D.; Poriel, C.; Rault-Berthelot, J.; Barriere, F.; Jeannin,
Corresponding Author
*E-mail: kung.wang@mail.wvu.edu.
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Brian V. Popp: 0000-0001-6367-1168
Kung K. Wang: 0000-0001-7039-2984
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This material is based upon work supported by the National
Science Foundation under CHE − 1464026. The NMR
spectrometer system on which this work was carried out was
supported by the Major Research Instrumentation Program of
the NSF under CHE − 1228336. B.V.P. acknowledges
computing resources available on the Super Computing System
(Spruce Knob) at West Virginia University, which are funded in
part by the NSF EPSCoR RII #1003907, WVEPSCoR via the
Higher Education Policy Commission, and West Virginia
University.
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