From Fenestrindane towards Saddle-Shaped Nanographenes Bearing a Tetracoordinate Carbon Atom

Article abstract of DOI:10.1002/anie.201707505

Two saddle-shaped polycyclic aromatic compounds (8 a and 8 b) bearing an all-cis-[5.5.5.5]fenestrane core surrounded by an o,p,o,p,o,p,o,p-cyclooctaphenylene belt were synthesized and characterized by NMR spectroscopy and mass spectrometry. The key step o

Full text of DOI:10.1002/anie.201707505

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Accepted Article
Title: From Fenestrindane Toward Saddle-Shaped Nanographenes
Bearing a Tetracoordinate Carbon Atom
Authors: Wai-Shing Wong, Chun-Fai Ng, Dietmar Kuck, and Hak-Fun
Chow
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From Fenestrindane Toward Saddle-Shaped Nanographenes
Bearing a Tetracoordinate Carbon Atom
Wai-Shing Wong,[a] Chun-Fai Ng,[a] Dietmar Kuck*,[b] and Hak-Fun Chow*,[a]
Dedication ((optional))
Abstract: Two saddle-shaped polycyclic aromatic compounds 8a
and 8b bearing an all-cis-[5.5.5.5]fenestrane core surrounded by an
o,p,o,p,o,p,o,p-cyclooctaphenylene belt were synthesized and
characterized by NMR spectroscopy and mass spectrometry. The
key step featured four Scholl-type cycloheptatriene formation steps
of the corresponding 1,4,9,12-tetraarylfenestrindane derivatives 7a
and 7b bearing electron-rich substituents. The structural details of
the D2d-symmetrical saddle 8a were determined by X-ray
crystallography, and the photoelectronic and cyclovoltammetric
properties of 8a and 8b were studied.
6, which would preserve the possibility of functionalization at the
bridgeheads, and fully unsaturated, fenestrindene-based
variants (cf. 5) can be envisioned. In turn, it would be of high
interest to elucidate the extent of planarization (“flattening”) of
the central carbon atom in such graphene networks.
Fenestranes have been considered exotic in organic chemistry
for a long time.[1] Nevertheless, they have attracted special
interest owing to the presence of a quaternary, tetracoordinate
carbon atom shared by all of the four rings of the
centrotetracyclic framework.[2-7] Besides the fact that naturally
occurring fenestranes are known and of increasing interest,[5,8]
the intriguing possibility to synthesize purely organic compounds
that would bear a planar tetracoordinate carbon atom with anti-
Van’t-Hoff-Le-Bel geometry has inspired scientists over several
decades.[1,9-15] Small-ring fenestranes have been studied by
experiment,[16] while fenestranes bearing a highly unsaturated -
electron periphery[11,12,13a] and fenestrane units embedded in the
center of a rigidified three-dimensional carbon framework have
Figure 1. Known bridgehead-saturated [5.5.5.5]fenestranes 1, 2 and 4, yet
unknown unsaturated congeners 3 and 5 (“fenestrindene”) and the central
motif of a hypothetical fenestrane-based graphene sheet (6). The colored
extension in 6 emphasizes the novel four-fold bay-bridged fenestrindane
framework achieved in the present work.
In the present work, we disclose the synthesis of the first
fully bay-bridged fenestrindane derivatives, 8a and 8b, via a
four-fold non-classical Scholl-type cycloheptatriene ring
formation reaction[22] from two fenestrindanes, 7a and 7b,
respectively, that bear electron-rich aryl groups at four of the
eight inner (ortho-) positions of the bays (Figure 2). Compounds
8, on the one hand, can be considered as rigidified derivatives of
o,p,o,p,o,p,o,p-cyclooctaphenylene 9,[23] on the other hand, due
to their unique molecular geometry, they also represent
important intermediates on the way to saddle-shaped
fenestrane-based graphenes as well as potential building blocks
for porous covalent organic frameworks.
been investigated by various theoretical approaches.[14]
A
number of low-strain [5.5.5.5]fenestrane hydrocarbons, such as
the parent hydrocarbons 1[17] and 2,[18] and their four-fold
benzoannelated analog 4 (fenestrindane)[19] are known, whereas
the fully unsaturated congeners 3 and 5 (“fenestrindene”) are not.
Notably, the fenestrane motif has never been incorporated into
the network of graphene sheets, as suggested in the
hypothetical structure 6 (Figure 1).
Owing to the unique topography of the fenestrindane
framework,[4,19a] fenestrane-based graphene sheets would adopt
a highly warped profile[20,21] and suffer a strong perturbation of
the extended conjugated -electron system. Therefore, from the
viewpoint of materials science, they would offer an
unprecedented variation of the electronics of such
chromophores. Both saturated fenestrane cores, as depicted in
Compounds
7
were prepared from tetramethoxy-
fenestrindane 10, a key intermediate which in turn was obtained
in a multistep synthesis according to our established strategy
[a]
[b]
W.-S. Wong, Dr. C.-F. Ng, Prof. H.-F. Chow
Department of Chemistry, State Key Laboratory of Synthetic
Chemistry and Institute of Molecular Functional Materials
The Chinese University of Hong Kong, Shatin (Hong Kong SAR)
E-mail: hfchow@cuhk.edu.hk
Prof. D. Kuck
Department of Chemistry and Center for Molecular Materials (CM2)
Bielefeld University, 33615 Bielefeld (Germany)
E-mail:dietmar.kuck@uni-bielefeld.de
Supporting information for this article is given via a link at the end of
the document.
Figure 2. Fenestrindane derivatives 7, bay-bridged fenestrindane compounds
8 and o,p,o,p,o,p,o,p-cyclooctaphenylene 9.
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[see Supporting Information (SI) for details].[19,24] Compound 10
was then converted into the corresponding bis-hydroquinone 11
in 82% yield using an excess of BBr3 (8 equiv.), followed by
treatment of 11 with triflic anhydride to furnish the corresponding
tetrakis(triflate) 12 in 85% yield (Scheme 1). Suzuki-Miyaura
cross-coupling reactions of compound 12 with 3,4-
spectrum of 14 also showed the presence of an AAʹBBʹ spin
system (δ 6.77 and 5.95) similar to that found for the precursor
molecule 7a (δ 6.76 and 6.05), which is only consistent with the
structure of this C-shaped regioisomer. Alongside with 2D 1H-1H
ROESY and COSY techniques, the constitution of compound 14
could be established unambiguously (see SI for details). The
1
dimethoxyphenylboronic
acid
(13a)
and
with
2,3,4-
highly symmetrical four-fold cyclization product 8a gave H and
13C NMR spectra that were greatly simplified relative to 7a
trimethoxyphenylboronic acid (13b) in the presence of Pd(OAc)2,
SPhos and K3PO4 then furnished the target 1,4,9,12-tetraaryl-
substituted fenestrindanes 7a and 7b in 95 and 94% yield,
respectively. The structures of all new compounds were
determined by 1H and 13C NMR spectroscopy and by mass
spectrometry. The structure of 7a was further confirmed by X-ray
crystallographic analysis (see Figure S11 in SI).[25] Compound
7b showed signal broadening in both its 1H and 13C NMR
spectra recorded at room temperature. This phenomenon was
attributed to the restricted rotation of the 2,3,4-trimethoxyphenyl
residues about the newly formed aryl-aryl bonds due to the
presence of the ortho-methoxy groups. The signals were
sharpened when the spectra were recorded at 45 °C (see SI for
details).
(Figures 3a and 3c). In solution, 8a may adopt either D2
d
symmetry or it may exist as two degenerate S4-conformers
under rapid interconversion at room temperature.[4,19a,27] Hence,
the four benzhydrylic protons and eight methoxy groups
appeared as sharp singlets at δ 4.65 and 4.04, respectively,
whereas the two different sets of aromatic protons gave rise to
two singlets at
δ
7.76 and 48. The unexpected
monohydroxylated product 15 was unequivocally confirmed by
various techniques including 2D 1H-1H ROESY (see SI for
details). In particular, the hydroxyl proton resonance at δ 2.06
vanished upon H/D exchange with D2O. In the ESI mass
spectrum of compound 15, the peaks at m/z 943.2865 ([M +
Na]+) and 903.2937 ([M + H – H2O]+) pinpointed the particularly
facile loss of H2O from the protonated molecule in the gas phase.
Independent experiment showed that the monohydroxylated
compound 15 could be obtained in 39% yield when pure 8a was
treated with DDQ (2 equiv.) and triflic acid (100 equiv.) in CH2Cl2
for 2 h. Plausibly, the fully cyclized product 8a might undergo
hydride abstraction at a benzhydrylic position by the slight
excess of DDQ[28] to produce a fairly stable tropylium ion which,
upon aqueous workup, led to compound 15 as an oxidation side
product.
Scheme 1. Synthesis of 1,4,9,12-tetraarylfenestrindanes 7.
Subsequently, the tetrakis(3,4-dimethoxyphenyl)-substituted
compound 7a was treated with 2,3-dichloro-5,6-dicyano-p-
benzoquinone (DDQ, 4.2 equiv.) in the presence of triflic acid
(100 equiv.)[26] to furnish the C-shaped two-fold cyclization
product 14 (10%), the four-fold cyclization product 8a (45%) and
the bridgehead-monohydroxylated four-fold cyclization product
15 (4%) after chromatography (Scheme 2). Use of a slightly
excess of DDQ (4.6 equiv.) altered the product distribution to 14
(5%), 8a (50%) and 15 (7%). In any attempts, neither products
of single nor triple cyclization were observed. The three
compounds were structurally elucidated by 1H and 13C NMR
spectroscopy and mass spectrometry (see SI for details). The 1H
NMR spectrum of the two-fold cyclization product 14 showed
two distinct singlets at δ 5.70 and 4.72 due to the two
nonequivalent pairs of benzhydrylic protons (Figure 3b). The
latter signal corresponds to those incorporated in the
cycloheptatriene rings, in line with the previously reported upfield
shift of bridgehead protons after Scholl cyclization in the TBTQ
series.[22c,d] A priori, two-fold cyclization of 7a could give rise to
three possible regioisomers, yet the C-shaped heptaphenylene
14 was obtained exclusively, presumably because of the more
rapid Scholl-type coupling between the two more electron-rich
Scheme 2. DDQ-mediated Scholl-type oxidative cycloheptatriene formation
with 1,4,9,12-tetraarylfenestrindanes 7.
1
arene moieties formed by the first cyclization step. The H NMR
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(113.0, 114.2°) but were very close to those of the parent all-cis-
fenestrindane 4 (116.4, 116.5°).[19a] Thus, the introduction of four
ortho-phenylene units across the bays of 4 gives rise to only
marginal flattening at the central quaternary carbon. As a
possible consequence, the 13C resonance of the central
quaternary carbon shifted upfield from compound 7a (δ 71.1) to
compound 14 (δ 68.3) and to compound 8a (δ 66.4). A similar
upfield shift was found in the cyclization of tetrakis-(2,3,4-
trimethoxyphenyl) compound 7b (δ 72.3) to form compound 8b
(δ 65.4). More interestingly, the two opposite veratrole units
within each moiety of structure 8a possessed nearly orthogonal
(88.9°) orientation to each other (Figure 4c),[31] suggesting that
the molecule may be used to construct large-size molecular
squares via tetramerization[32] or to form porous covalent organic
frameworks.[33] On the other hand, the packing of molecules 8a
revealed that there is no π–π stacking interaction in the solid
state (see SI for details).
Figure 3. Stacked partial 1H NMR spectra (CDCl3, 22 °C) of (a) 8a (400 MHz),
(b) 14 (700 MHz), and (c) 7a (400 MHz). The red squares and green triangles
indicate the 1H signals due to the benzhydrylic protons before and,
respectively, after cycloheptatriene formation in the corresponding bay. The
blue circles indicate the AAʹBBʹ spin system(s) of the unreacted benzo unit(s).
See Scheme 2 for signal assignments.
With the tetrakis(2,3,4-trimethoxyphenyl) analogue 7b, four-
fold cyclization proceeded smoothly to produce the desired
product 8b (52%) and the corresponding monohydroxylated
derivative 16 (20%). Partially cyclized compounds were not
observed in this case. The total four-fold cyclization yield of 7b
(72%) was superior to that of 7a (49–57%) although the four
additional ortho-methoxy groups in the former might incur
torsional strain upon cyclization. It is believed that the lower
oxidation potential of the trimethoxyphenyl group, as compared
to that of the dimethoxyphenyl group, facilitates the oxidative
cyclization of 7b.[29] The structures of compounds 8b and 16
were again confirmed by 1H and 13C NMR spectroscopy and
mass spectrometry (see SI for details). Notably, the broad NMR
signals found in the case of the precursor molecule 7b were
sharpened upon conversion into 8b and 16 by virtue of the
rigidification of the para-terphenyl substructures.
Figure 4. Crystal structure of compound 8a: (a) top view, (b) oblique view
showing the depth of the saddle, and (c) front view showing the nearly
orthogonal disposition (88.9°) of two opposite veratrole moieties.
The saddle-shaped topography of compound 8a was
unambiguously confirmed by X-ray crystallography using a
single crystal obtained from a 1,2-dichloroethane solution by
slow evaporation.[25] The molecule assumes a conformation with
D2 molecular symmetry (Figure 4a). The depth of the saddle
d
Compounds 8a and 8b are soluble in most organic solvents,
resulting in orange solutions with weak violet fluorescence when
irradiated with UV light. Their photoelectronic and cyclic
voltammetric (CV) data are collected in Table 1. For the
octamethoxy compound 8a, the long-wavelength absorption
edge was estimated to be at 382 nm (see SI Figure S13a),
which translates into an optical gap of 3.25 eV. The value for
compound 8a is noticeably smaller than that for the doubly
cyclized compound 14 (3.79 eV) and the starting material 7a
(3.94 eV). In addition, the fluorescence wavelength of 8a (424
nm) is red shifted from those of 14 (390 nm) and of 7a (370 nm)
(see SI Figure S14a). Together with the drastic increase in
fluorescence intensity accompanying cyclization, both the
absorption and emission data are consistent with the more
extended π–conjugation in 8a. The CV profile of 8a in CH2Cl2
exhibited one quasi-reversible oxidation wave with a half-wave
oxidation potential of +0.76 V vs the ferrocene/ferrocenium
was found to be 3.41 ± 0.02 Å,[30] and the dihedral angles
between two neighboring aromatic rings were in the range of
28.9–35.4° (Figure 4b). When compared to the parent
o,p,o,p,o,p,o,p-cyclooctaphenylene 9, which has a saddle depth
of 3.74–3.97 Å and dihedral angles ranging within 33.2–65.5°,[23]
compound 8a possesses a shallower saddle structure. The C–C
bond lengths between two adjacent aromatic rings in 8a were
found to be 1.48–1.51 Å, and thus typical for sp2–sp2 carbon–
carbon single bonds. Among the six C–C–C bond angles around
the central quaternary carbon, four are internal angles of the
pentagons, the average of which is defined as . The remaining
two bond angles (α and β) quantify the extent of planarization of
the tetracoordinate carbon atom in a fenestrane (Figure 4a).[3,5,6]
The larger the values of α and β, the more flattened is the
central carbon. In 8a, angles α and β were found to be 116.4
and 117.0°, respectively. These values were slightly larger than
the corresponding bond angles of the precursor molecule 7a
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(Fc/Fc+) couple. The HOMO and LUMO energies of 8a were
estimated to be −5.90 and −2.65 eV, respectively (see SI for
details). In comparison to 8a, the dodecamethoxy derivative 8b
[2]
[3]
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has
a similar absorption edge (371 nm, 3.33 eV) and
fluorescence wavelength (416 nm) (see SI Figure S13b and
S14b), and a similar half-wave oxidation potential (+0.75 V).
Likewise, the HOMO and LUMO levels of compound 8b were
estimated to be −5.91 and −2.58 eV, respectively.
[5]
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Compound
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8b
Low-energy absorption edge, λedge (nm)
Fluorescence emission (nm)[a]
HUMO-LUMO gap (eV)[b]
382
371
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Half-wave oxidation potential (V)[c]
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+0.76
+0.80
–5.90
–2.65
+0.75
+0.81
–5.91
–2.58
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In summary, we reported here the synthesis of two novel
saddle-shaped polycyclic aromatic compounds 8a and 8b based
on the fenestrindane framework using a four-fold Scholl-type
cycloheptatriene formation reaction. The structural features of
these compounds and their optical and electronic properties
were examined. With the peripheral methoxy groups being
available for further functionalization and C–C cross coupling
reactions, these molecules should provide access to various
novel π–extended saddle-shaped nanographenes.
Acknowledgements
This work is supported by the RGC-CRF (project no: C4030-
14G) and UGC-AoE (project no: AoE/P-03/08) of HKSAR.
Keywords: fenestrane • polyaromatic compounds • Scholl
cyclization • nanographene • saddle-shaped structures
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10.1002/anie.201707505
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Wai-Shing Wong,[a] Chun-Fai Ng,[a]
Dietmar Kuck*,[b] and Hak-Fun Chow*,[a]
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From Fenestrindane Toward Saddle-
Shaped Nanographenes Bearing a
Tetracoordinate Carbon Atom
Two four-fold bay-bridged fenestrindane derivatives were prepared in good yields
using a non-classical Scholl-type cyclization, paving the way to saddle-shaped
nanographenes
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