Contraction of π-Conjugated Rings upon Oxidation from Cyclooctatetraene to Benzene via the Tropylium Cation

Article abstract of DOI:10.1002/chem.201704008

We have serendipitously discovered a unique transformation of a cyclooctatetraene derivative 1 into a cycloheptatriene spirolactone 3 upon oxidation, which is the first such transformation reported in 60 years. Product 3 could be reversibly interconverted into the aromatic tropylium cation 3H⁺ by acid/base treatment, which was accompanied by drastic spectroscopic changes. The resultant cycloheptatriene could be further converted into benzene upon oxidation. We characterized all the key structures by X-ray studies. Eventually, the π-conjugated ring size shrinks from 8 to 7, then finally to 6 upon oxidation, in the direction of the stronger aromatization.

Full text of DOI:10.1002/chem.201704008

A Journal of
Accepted Article
Title: Contraction of π-Conjugated Rings upon Oxidation from
Cyclooctatetraene to Benzene via Tropylium Cation
Authors: Akira Tamoto, Naoki Aratani, and Hiroko Yamada
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Contraction of π-Conjugated Rings upon Oxidation from
Cyclooctatetraene to Benzene via Tropylium Cation
Akira Tamoto,[a] Naoki Aratani*[a] and Hiroko Yamada*[a]
Dedication
Abstract: We have serendipitously discovered the unique
transformation of a cyclooctatetraene derivative 1 for the first time in
60 years, to a cycloheptatriene spirolactone 3 upon oxidation. The
Recently, another unique skeletal rearrangements of COT
derivatives under oxidative conditions were independently
reported by Tobe[5] and Kawase.[6] Tobe’s group reported the
product
3
could be reversibly interconverted to the aromatic
skeletal rearrangements of a twisted polycyclic aromatic
tropylium cation 3H+ by acid/base accompanying drastic spectral
changes. The resultant cycloheptatriene could be further converted
to benzene upon oxidation. We could characterize all the key
structures by X-ray analysis. Eventually, the π-conjugated ring size
shrinks from 8 to 7, then finally 6 upon oxidation, directing to the
stronger aromatization.
hydrocarbon (PAH) to afford a product with a seven-membered
ring and then tetrabenzocoronene with all six membered rings.[5]
The rearrangement was interpreted in terms of the acid-
catalyzed
isomerization
of
9,9’-bifluorenylidene
into
dibenzo[g,p]chrysene moieties. Scott and Kawase reported that
the bicyclo[6.3.0]undecapentaenyl cation afforded azulene upon
oxidation followed by decarbonylation.[6]
Tetra[(1.8)-naphthylene]-1,3,5,7-cyclooctatetraene
(or
tridecacyclene; 1), originally reported by Scott et al. in 2000 as a
byproduct for making a part of fullerene fragment,[7] has COT
unit at the center of molecule (Scheme 2). Recently it was
shown that two-electron reduction of compound 1 gave its
aromatic dianion by Whalley et al.[8] This is the analogous 10π
Hückeloid.[9] We have been interested in the aromaticity
relocation in the polycyclic aromatic hydrocarbons (PAHs) upon
the oxidation and demonstrated the sterically hindered pyrenes
and perylenes could be oxidized with SbCl5 to give the persistent
cations and dications.[10] We have also succeeded in synthesis
of the carbon-nanosheet, tetrabenzoperipentacene, which was
obtained by the oxidative annulation reaction of the
tetranaphthylpyrene derivative.[11] During our research to make a
new COT containing carbon-nanosheet 2 from the naphthyl
Introduction
60 Years ago, Ganellin and Pettit discovered the transformation
of cyclooctatetraene (COT) to the tropylium cation upon the
oxidation.[1] Although whole reaction mechanism and
intermediates were unclear especially for the structure of acid at
that time, they considered that this reaction includes pinacol-
pinacolone rearrangement and 2e– transfer processes (Scheme
1). Since then, this chemistry has been left untouched to date
except for a few related reports.[2]
H
H
CHOH
compound 1 upon oxidative conditions, we happened to
discover the unique transformation of COT unit.
OH
H
COOH
FeCl3/
MeNO2
Scheme 1. Ganellin and Pettit’s proposal for the reaction of cyclooctatetraene
upon oxidation.
+
3
From the point of view of aromatic characteristics, this
transformation has implications of their structural stability.
Oxidation of 8π COT that is non-planar non-aromatic compound
results in the formation of 6π (formally stable) aromatic tropylium
cation[3] accompanying with carbon-skeleton rearrangement. On
the other hand, Olah et al. reported direct 2e– oxidation of
methylated COT to 6π aromatic COT dication at low
temperature.[4]
1
2
Scheme 2. Synthetic attempt to make nanographene 2 from compound 1.
Results and Discussion
We tried the annulation reaction of tridecacyclene 1 with iron
chloride to synthesize compound 2 with planar COT structure.
After several attempts, however, we failed to obtain the
compound 2 and could isolate unexpected compound 3 in 69%
(Scheme 2). High-resolution spiral-MALDI time-of-flight mass
spectroscopy detected the parent ion peak at m/z = 633.1874,
[a]
A. Tamoto, Prof. Dr. N. Aratani and Prof. Dr. H. Yamada
Graduate School of Materials Science
Nara Institute of Science and Technology (NAIST)
8916-5 Takayama-cho, Ikoma 630-0192 (Japan)
E-mail: aratani@ms.naist.jp, hyamada@ms.naist.jp
Supporting information for this article is given via a link at the end
of the document.
1
which was larger by 32 mass units than that of 1. The H NMR
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spectrum indicates lowering of symmetry of the product
compared with 1, featuring 10 doublet peaks and 5 triplet peaks
that correspond to the 1,3- and 2-naphthyl protons, respectively,
thus indicating the generation of the C2 symmetric structure
(Supporting Information, SI; Figure S1). The final structural
determination of 3 was unambiguously obtained by X-ray
analysis (Figure 1a).[12] Surprisingly, the number of carbon
atoms at the central ring reduced from 8-membered ring to 7-
membered one, and the compound 3 has cycloheptatriene
skeleton with spirolactone. To the best our knowledge, this is the
first spiro[5.6]lactone cycloheptatriene compound.
oxygen induces the bond exchanges and the pinacol-type
rearrangement generates cycloheptatriene (Scheme S1).[14] We
confirmed that this conversion was accelerated under the O2
atmosphere and suppressed under an inert atmosphere or
without FeCl3.
O2
FeCl3
1
CH2Cl2
CH3NO2
O
OH
H+
O
O
a)
O
O
H
O
O
=
O
3 O
Scheme 3. A plausible reaction mechanism of 1 to 3: Dioxirane formation-
decomposition route.
3
b)
The spirolactone structure is a bicyclic ester that is typically
contained in fluoresceins.[15] This can be performed as a
reversibly switching unit with close/open ring, responding the
ambient environment (ex. pH etc.). Thus, we expected that the
addition of an acid to the compound 3 would open the
spirolactone to form tropylium cation accompanying drastically
change of its physical properties.
We measured UV-vis absorption and fluorescence spectra of
the spirolactone 3 in THF (Figure 2). The compound 3 shows the
featureless absorption at around 500 nm and the absorption
peaks at 348 and 360 nm. Fluorescence of 3 exhibits at 603 nm
and the fluorescence quantum yield (ΦF) is 3%. Upon the
addition of 30 eq. of trifluoromethanesulfonic acid (TfOH), the
protonated compound 3 (3H⋅OTf) showed new absorption peaks
at 466 and 532 nm. The fluorescence of 3H⋅OTf exhibits
intensive and slightly red shifted peak at 620 nm. The ΦF of
3H⋅OTf is 15%, becoming five times larger than that of the
neutral compound 3. The addition of HBF4 into 3 also gave
similar result (Figure S9). The spectral change was recovered by
the addition of 30 eq. of 1,8-diazabicyclo[5.4.0]-undec-7-ene
(DBU) as a base.
Figure 1. a) Single crystal X-ray structure and chemical structure of 3. b)
Single crystal X-ray structure of 3H⋅BF4.Thermal ellipsoids are scaled at 50%
probability. Solvent molecules are omitted for clarity.
Although the detailed reaction mechanism is uncertain at this
moment, we believe that Scheme 3 can explain a plausible
mechanism for the rearrangement of tridecacyclene
1 to
compound 3. First, the compound 1 was oxidized by FeCl3 to
form the cation radical, which reacts with molecular oxygen. The
rearrangement occurs as illustrated, then, dioxirane formation-
decomposition[13] generates cycloheptatriene with spirolactone.
Another possibility includes that the oxidation with molecular
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aromaticity (HOMA)[19] also suggests its enough aromaticity
(0.59).
O
HOOC
TfOH
DBU
O
To further understand the electronic features, the density
functional theory (DFT) and the time-dependent (TD)-DFT
calculations both at the B3LYP/6-31G(d) level using the
Gaussian 09 software package were carried out (Figure 4).[20]
The HOMO and LUMO are non-degenerative and the coefficient
distribution patterns in these frontier MOs appear delocalized
over the entire molecule. The longest band of 3H+ at 532 nm
mainly comprises the transition from HOMO to LUMO (oscillator
strength, f = 0.046), whereas the main absorption band at
around 466 nm is composed of transitions from HOMO–1 to
LUMO+1 (f = 0.12) and from HOMO–2 to LUMO+1 (f = 0.14).
The transition energies and oscillator strengths simulated by TD-
DFT calculations showed a good agreement with the observed
absorption spectrum of 3H⋅OTf (Figure S16).
Ha
3
3H
1.384(4)
1.441(6)
a)ꢀ
1.445(4)
1.397(6)
1.440(4)
HOMAꢀ 1.394(6)
–ꢀ
0.59ꢀ
1.387(3)
1.447(6)
1.388(3)
1.451(6)
1.523(3)
1.406(6)
1.538(4)
1.408(6)
HOOC
Figure 2. UV-vis (solid line) and fluorescence (broken line) spectra of 3 (black)
and 3H⋅OTf (red) in THF. [3] = 1.6 × 10–5 M. For 3H, 30 eq. of TfOH was added.
Fluorescence spectra were taken for excitation (λex = 500 nm) with the
absorbance adjusted at 0.1.
b) ꢀ
Rꢀ
R =
NICS / ppmꢀ
(+2)ꢀ
(+1)ꢀ
(0)ꢀ
–6.78ꢀ
To confirm the close/open ring structures, 1H NMR spectrum
of 3 with TfOH in THF-d8 was measured at 183 K (Figure S3).
The spectrum of 3H⋅OTf drastically changed especially for
naphthalene ester moiety compared to that of the compound 3.
The proton of the carboxylic acid was detected at 11.33 ppm,
reflecting the hydrogen bonding. The diatropic ring current of the
central tropylium was indicated by the down-field shift of Ha
denoted in Figure 2 (7.78 ppm; Δδ from 3 = 0.4 ppm). These
observations clearly prove the formation of the aromatic
tropylium structure. The structure of the tropylium salt 3H⋅BF4
was determined by single crystal X-ray diffraction analysis
(Figure 1b).[16] The crystal structure of 3H⋅BF4 showed opened
spirolactone. Also the compound 3H⋅BF4 in the crystal forms the
dimer via hydrogen bonding between carboxylic acid. Based on
the crystal structure of 3, we confirmed the C=C bonds of the 7-
membered ring are localized at three positions and two single
bonds are connected to the spiro carbon (Figure 3). On the other
hands, the C-C bond distances of the tropylium 3H⋅BF4 are
delocalized at all the 7-membered ring. The mean-plane
deviation of the tropylium part is 0.12 Å that is much smaller
than that of 3 (0.26 Å). To evaluate the aromatic character of the
7-membered rings of 3 and 3H+, we calculated the nucleus
independent chemical shift (NICS)[17] based on the crystal
structures at the HF/6-31G+(d,p) calculation level (Figure 3).[18]
While the NICS values of the compound 3 are negligible, those
of the tropylium 3H+ vary from –4.26 ppm (NICS(0)) to –7.15
ppm (NICS(+1)), clearly indicating the aromatic character.
Another indicator of aromaticity, harmonic oscillator model of
–7.15ꢀ
–4.26ꢀ
–6.84ꢀ
–4.17ꢀ
R
(–1)ꢀ
(–2)ꢀ
Figure 3. a) Bond lengths and HOMA values of 3 (black) and 3H⋅BF4 (red)
based on the X-ray structures. b) NICS values of 3H+ calculated at the HF/6-
31G+(d,p) level.
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Scheme 4. Further oxidation of 3 with FeCl3 along with a plausible reaction
mechanism to 4 and 5.
In this reaction, the aromatic tropylium cation (7-membered
ring) shrinks to 6-membered ring by division to release 5. The
driving force of this unique splitting could be the achievement of
the stronger aromaticity.
LUMO+1
–5.43 eV
LUMO
–5.55 eV
Conclusions
In summary, we discovered the transformation of the non-
aromatic COT into the cycloheptatriene spirolactone upon the
oxidation, which could be reversibly interconverted to the
aromatic tropylium cation by acid/base. This transformation of
COT was operated not only by chemical reaction but also by
HOMO
–8.35 eV
electrochemical manipulation;
a
CH2Cl2 solution of the
compound 1 was applied to voltage at 0.48 V vs Fc/Fc+ for 2 h
and the absorption spectrum changed to that of 3H+ (Figure
S18). The original phenomenon of the ring-shrinking was
observed by pristine COT, but we could characterize all the key
molecular structures by X-ray analysis. In addition, the resultant
cycloheptatriene could be converted to benzene upon further
oxidation. Consequently, the π-conjugated ring size shrinks from
HOMO–1
–8.49 eV
Figure 4. MO diagram of 3H+ based on calculations at the B3LYP/6-31G(d)
level.
Curiously precedents teach us that the oxidation of cy-
cloheptatriene acids (thus tropylium cations) gives ben-
zaldehyde or benzoic acid.[1,21] According to these reports, we
8
to
7 then 6 upon oxidation, directing to the stronger
aromatization.
tried further oxidation of 3. To
a CH2Cl2 solution of the
compound 3 at room temperature was added 5 equiv. of FeCl3
at once. We monitored reaction profiles by an atmospheric
pressure chemical ionization (APCI) mass spectroscopy, then
detected [M]+ = 451.22 and 198.70, which correspond to
decacyclene 47 and 1H,3H-benzo[de]isochromene-1,3-dione (5),
respectively (Scheme 4, and Figure S10). These products 4 and
5 were confirmed by 1H NMR spectrum (Figure S11) and we
determined the conversion yield of 4 in 40%. A plausible
reaction mechanism is shown in Scheme 4.
Experimental Section
1. Instrumentation and Materials
1H NMR (400 MHz or 300 MHz) and 13C NMR (100 MHz) spectra were
recorded with JEOL JNM-ECX 400 and 300 spectrometers at ambient
temperature by using tetramethylsilane as an internal standard. The high-
resolution MS were measured by
spectrometer and a BRUKER Autoflex II MALDI TOF MS. The high-
resolution APCI MS were performed on BRUKER DALTONICS
a
JEOL JMS-700 MStation
a
micrOTOF using positive and negative ion modes. UV/Vis absorption
spectra were measured with a JASCO UV/Vis/NIR spectrophotometer V-
570. X-ray crystallographic data for 3 were recorded at 100 K on a
Rigaku R-AXIS RAPID/S using Mo-Kα radiation, and the data for 3H⋅BF4
were recorded at 90 K with a BRUKER-APEXII X-Ray diffractometer
equipped with a large area CCD detector. CV measurements were
conducted in a solution of 0.1 M TBAPF6 in dry dichloromethane with a
scan rate of 100 mV/s at room temperature in an argon-filled cell. A
glassy carbon electrode and a Pt wire were used as a working and a
counter electrode, respectively. An Ag/Ag+ electrode was used as
reference electrodes, which were normalized with the half-wave potential
of ferrocene/ferrocenium+ (Fc/Fc+) redox couple. TLC and gravity column
chromatography were performed on Art. 5554 (Merck KGaA) plates and
silica gel 60N (Kanto Chemical), respectively. All other solvents and
chemicals were reagent-grade quality, obtained commercially, and used
without further purification. For spectral measurements, spectral-grade
chloroform was purchased from Nacalai Tesque.
O
O
O
O
FeCl3/
MeNO2
O
+
4
5
3
O2
O
O
O
O
O
O
FeCl3
O
FeCl3
FeCl3
2. Experimental Section
=
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3H-Spiro[benzo[de]isochromene-1,10'-cyclohepta[1,2-a:3,4-a':5,6-
a'']triacenaphthylen]-3-one (3): Compound 2 (10 mg, 0.016 mmol) was
dissolved in CH2Cl2 (5 ml) under atmosphere. FeCl3 (32 mg, 0.16 mmol)
dissolved in CH3NO2 (0.5 ml) was added dropwise. After the reaction
mixture was stirred at r.t. for 4 h, it was added to water and extracted with
CH2Cl2. The organic phase was dried over Na2SO4 and concentrated.
The crude product was separated by silica gel column chromatography
(AcOEt/Hex=1:3) to give the compound 3 as red solid (7.0 mg, 0.011
mmol, 69%). The product was further purified by recrystallization
(CH2Cl2/Hex). 1H-NMR (600 MHz, CDCl3): δ = 8.85 (d, J = 7.6 Hz, 2H),
8.46 (dd, J = 7.6, 1.4 Hz, 1H), 8.19 (d, J = 6.9 Hz, 4H), 8.02 (d, J = 8.2
Hz, 2H), 7.91 (d, J = 8.2 Hz, 1H), 7.84 (t, J = 7.8 4H), 7.71 (q, J = 7.8 Hz,
4H), 7.61-7.49 (m, 3H), 7.41 (d, J = 8.2 Hz, 1H), 7.36 (d, J = 7.3 Hz, 1H)
and 6.47 (t, J = 7.8 Hz, 1H) ppm; 13C-NMR (100 MHz, CDCl3): δ =166.06,
138.07, 137.66, 137.48, 137.38, 136.50, 133.78, 132.31, 131.51, 129.49,
129.29, 129.13, 128.79, 128.77, 128.63, 128.23, 128.14, 128.02, 127.91,
127.87, 127.57, 127.03, 125.93, 125.58, 125.29, 124.99, 124.91, 120.91
and 89.39 ppm; HR-MS (Spiral MALDI): m/z: calcd for C48H25O2,
633.1849 [M+H]+; found: 633.1874; UV-vis (THF): λmax (ε [M–1 cm–1]) =
348 (31997), 360 (32000), and 491 (6637) nm; m.p. > 300°C.
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3H: 1H-NMR (400 MHz, THF-d8 with 1 µl of TfOH): δ = 9.16 (d, J = 7.3 Hz,
2H), 9.09 (d, J = 7.3 Hz, 2H), 8.70 (d, J = 8.1 Hz, 2H), 8.65 (d, J = 8.4 Hz,
1H), 8.53 (d, J = 8.1 Hz, 2H), 8.44 (d, J = 8.4 Hz, 1H), 8.30 (d, J = 8.1 Hz,
2H), 8.20 (q, J = 7.4 Hz, 4H), 7.95 (d, J = 7.7 Hz, 1H), 7.80 (d, J = 7.0 Hz,
1H), 7.65 (d, J = 7.7 Hz, 1H), 7.56 (d, J = 7.0 Hz, 1H), 7.46 (t, J = 7.9 Hz,
2H) and 6.18 (d, J = 7.7 Hz, 2H) ppm; 13C-NMR (100 MHz, THF-d8) δ =
169.32, 151.16, 150.69, 150.50, 147.69, 135.66, 135.36, 135.11, 134.83,
134.52, 133.73, 133.70, 133.36, 132.71, 132.12, 132.09, 131.91, 131.85,
131.77, 131.62, 131.30, 131.06, 130.88, 129.88, 129.28, 129.20, 128.99,
128.76, 128.38, 126.86, 126.14, 121.32 and 119.20 ppm; m.p. > 300°C.
[12] Crystals of 3 were grown by hexane vapor to diffuse into its CH2Cl2
solution. Crystallographic data for 3: C48H24O2⋅CH2Cl2, M = 717.65,
monoclinic, space group P21/c (#14), a = 17.5462(4), b = 12.1503(2), c
= 16.0238(3) Å, β = 107.905(8)°, V = 3250.69(18) Å3, T = 103(2) K, Z =
4, reflections measured 30669, 5937 unique. The final R1 was 0.0518 (I
> 2σ(I)), and the final wR on F2 was 0.1287 (all data), GOF = 1.088.
CCDC 1546499.
Compound 3 (10 mg, 0.016 mmol) was dissolved in CH2Cl2 (4 ml) under
atmosphere. FeCl3 (18 mg, 0.080 mmol) dissolved in CH3NO2 (0.5 ml)
was added dropwise. After the reaction mixture was stirred at r.t. for 4 h,
it was added to water and extracted with CH2Cl2. The organic phase was
dried over Na2SO4 and concentrated. The crude product was separated
by silica gel column chromatography (AcOEt/Hex=1:3) to give the
unreacted 3 (5.4 mg), the compound 4 as yellow solid (2.9 mg, 0.006
mmol, 40% in conversion yield), and compound 5 as pale yellow solid
(1.9 mg, 0.010mmol, 60% in conversion yield).
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7265–7270.
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Chem. 2015, 127, 16057–16061; Angew. Chem. Int. Ed. 2015, 54,
15831–15835.
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Acknowledgements
[16] Crystals of the tropylium 3H⋅BF4 was grown by the diffusion of hexane
vapor to CH2Cl2 solution of
Crystallographic data for 3H⋅BF4: C48H25O2⋅BF4⋅2CH2Cl2, M = 1610.83,
Orthorhombic, space group P212121 (#19), 15.3978(11),
17.4586(12), c = 26.7858(18) Å, V = 7200.7(9) Å3, T = 90(2) K, Z = 4,
reflections measured 42544, 14148 unique. The final R1 was 0.0480 (I
> 2σ(I)), and the final wR on F2 was 0.1191 (all data), GOF = 1.022.
CCDC 1546500.
3 with 48% HBF4⋅Et2O in H2O.
This work was supported by CREST JST (JPMJCR15F1) and
JSPS KAKENHI Grant Numbers JP17H03042, JP16H02286,
JP26105004 and JP15H00876 'AnApple', and the program for
promoting the enhancement of research universities in NAIST
supported by MEXT. We thank Ms. Y. Nishikawa, and Mr. S.
Katao, NAIST, for the mass spectroscopy measurements and X-
ray analysis, respectively.
a
=
b =
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Keywords: Cyclooctatetraene • Oxidation • π-Conjugation •
Aromaticity • Rearrangement
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A few metal catalyzed transformations of COT oxide were reported.
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Entry for the Table of Contents
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We found that the modified
Akira Tamoto, Naoki Aratani* and Hiroko
Yamada*
“C”ꢀ
cyclooctatetraene transforms into a
cycloheptatriene spirolactone upon
oxidation, which could be converted to
benzene upon further oxidation.
Consequently, the π-conjugated ring
size shrinks from 8 to 7 then 6,
R
Page No. – Page No.
R = COOH
Oxidationꢀ
Contraction of π-Conjugated Rings
upon Oxidation from
Cyclooctatetraene to Benzene via
Tropylium Cation
Non Aromaticꢀ
Weakly Aromaticꢀ
“C”ꢀ
directing to the stronger aromatization.
Strongly Aromaticꢀ
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