Post on 15-Jul-2020
Elsevier Editorial System(tm) for Chinese
Chemical Letters
Manuscript Draft
Manuscript Number: CCLET-D-19-00134R1
Title: A Ketone-Functionalized Aromatic Saddle as a Potential Building
Block for Negatively Curved Carbon Nanobelts
Article Type: SI: To Prof. Henry Wong
Keywords: polycyclic arene; synthesis; negatively curved carbon
allotropes; carbon nanobelts; Scholl reaction
Corresponding Author: Professor Qian Miao, ph.D
Corresponding Author's Institution: The Chinese University of Hong Kong
First Author: Kwan Yin Cheung, PhD
Order of Authors: Kwan Yin Cheung, PhD; Qian Miao, ph.D
Abstract: A novel ketone-functionalized aromatic saddle consisting of 72
sp2 carbon atoms is successfully synthesized and unambiguously identified
with X-ray crystallography. It can, in principle, be used as a building
block for synthesis of negatively curved carbon nanobelts and for a
bottom-up approach to negatively curved carbon allotropes.
Suggested Reviewers:
Opposed Reviewers:
Response to Reviewers: We thank the reviewer for recognizing the value of
this work and providing valuable comments for improvement. Please see the
revision and response after each comment below.
Comment 1: (CD3)2CO was used as solvent in the 13C NMR spectrum of
compound 5, but the text in the SI of the 13C NMR was mistakenly marked
as CDCl3.
Revision: Corrected accordingly.
Comment 2: All new compounds in this manuscript lacks IR
characterization.
Response: According to the guidelines of the American Chemical Society,
"all new compounds, evidence adequate to establish both identity and
degree of purity (homogeneity) should be provided". These evidences
typically include melting point, 1H NMR, 13C NMR and HRMS. We have
already provided these data, and provided single crystal structures for
compound 3 and 5. In fact, the structural information of large polycyclic
aromatics that can be provided from IR is quite limited. Therefore we
think IR is not necessary.
1
Prof. Qian Miao
The Chinese University of Hong Kong,
Department of Chemistry
Shatin, New Territories
Hong Kong SAR, China
Tel: 852-26098127
Fax: 852-26035057
email: miaoqian@cuhk.edu.hk
01/28/2019
Prof. Xuechen Li
Associate Editor, Chinese Chemical Letters
Dear Prof. Li,
Thank you again for inviting me to contribute to the Special Issue of Chinese Chemical
Letters dedicated to Prof. Henry N. C. Wong. In the attachment please find a manuscript entitled
"A Ketone-Functionalized Aromatic Saddle as a Potential Building Block for Negatively Curved
Carbon Nanobelts" for your consideration of publication on Chinese Chemical Letters as a
communication.
Negatively curved carbon allotropes are theoretical nanocarbon materials, which are
predicted to have interesting properties and potential applications on the basis of computational
studies, but are yet to be synthesized. A promising bottom-up approach to negatively curved
nanocarbon materials is synthesis of negatively curved nanographenes, which are saddle-shaped
polycyclic arenes containing seven or eight-membered rings. As inspired by this idea, a few
negatively curved nanographenes have been designed and synthesized by us and other research
groups since 2012. A further step toward negatively curved carbon allotropes is synthesis of
negatively curved carbon nanobelts or nanotubes. Herein, we report synthesis and crystal
structure of a novel ketone-functionalized aromatic saddle, which consists of 72 sp2 carbon
atoms. It is a potential building block for synthesis of negatively curved carbon nanobelts.
Therefore, this work, in my opinion, would be suitable for the readership of Chinese Chemical
Letters.
Sincerely yours,
Qian Miao
Cover Letter
1
Prof. Qian Miao
The Chinese University of Hong
Kong,
Department of Chemistry
Shatin, New Territories
Hong Kong SAR, China
Tel: 852-26098127
email: miaoqian@cuhk.edu.hk
03/21/2019
Editorial Office
Chinese Chemical Letters
Dear Editor:
Thank you for your e-mail on Mar. 18 regarding our manuscript CCLET-D-19-00134,
which is entitled " A Ketone-Functionalized Aromatic Saddle as a Potential Building Block
for Negatively Curved Carbon Nanobelts". The Supporting Information has been revised
following the reviewer's comment. In the attachment please find the revised files for your
further consideration to be published in Chinese Chemical Letters.
We thank the reviewer for recognizing the value of this work and providing valuable
comments for improvement. Please see the revision and response after each comment below.
Comment 1: (CD3)2CO was used as solvent in the 13
C NMR spectrum of compound 5, but the
text in the SI of the 13
C NMR was mistakenly marked as CDCl3.
Revision: Corrected accordingly.
Comment 2: All new compounds in this manuscript lacks IR characterization.
Response: According to the guidelines of the American Chemical Society, "all new
compounds, evidence adequate to establish both identity and degree of purity
(homogeneity) should be provided". These evidences typically include melting
point, 1H NMR,
13C NMR and HRMS. We have already provided these data, and
provided single crystal structures for compound 3 and 5. In fact, the structural
information of large polycyclic aromatics that can be provided from IR is quite
limited. Therefore we think IR is not necessary.
Thanks for further considering the revised manuscript for publication on Chinese
Chemical Letters.
Sincerely yours,
Qian Miao
*Detailed Response to Reviewers
Graphical Abstract
A ketone-functionalized aromatic saddle as a potential building block for negatively curved carbon
nanobelts
Kwan Yin Cheung, Qian Miao
Department of Chemistry, The Chinese University of Hong Kong, Hong Kong, China
Herein, we report the synthesis and crystal structure of a novel ketone-functionalized aromatic saddle,
which is a potential building block for synthesis of negatively curved carbon nanobelts.
Corresponding author.
E-mail address: miaoqian@cuhk.edu.hk
*Graphical Abstract (for review)
Graphical Abstract
A ketone-functionalized aromatic saddle as a potential building block for negatively curved carbon nanobelts
Kwan Yin Cheung, Qian Miao
Department of Chemistry, The Chinese University of Hong Kong, Hong Kong, China
Herein, we report the synthesis and crystal structure of a novel ketone-functionalized aromatic saddle, which is a potential
building block for synthesis of negatively curved carbon nanobelts.
———
Corresponding author.
E-mail address: miaoqian@cuhk.edu.hk
*Manuscript
Communication
A ketone-functionalized aromatic saddle as a potential building block for negatively
curved carbon nanobelts
Kwan Yin Cheung, Qian Miao
Department of Chemistry, The Chinese University of Hong Kong, Hong Kong, China
———
Corresponding author.
E-mail address: miaoqian@cuhk.edu.hk
Negatively curved carbon allotropes are theoretical structures
of sp2 carbon atoms, which present saddle-shaped surfaces as a
result of embedding seven- or eight-membered rings in a
graphitic network. Negatively curved periodical structures of
carbons are known as Mackay crystals or carbon schwarzites
because Mackay together with Terrones in 1991 first proposed
negatively curved carbon allotropes [1], whose topological
model was described by Schwarz, a German mathematician [2].
Although they are predicted to have interesting properties and
potential applications on the basis of computational studies [3-5],
negatively curved carbon allotropes are yet to be synthesized. A
promising bottom-up approach to negatively curved nanocarbon
materials is synthesis of negatively curved nanographenes, which
are saddle-shaped polycyclic arenes containing seven- or eight-
membered rings [6]. They are not only segments of negatively
curved carbon allotropes containing important structural
information, but also are envisioned as templates or monomer
units for synthesis of negatively curved carbon allotropes in a
controlled growth process or by polymerization, respectively
[7,8]. As inspired by this idea, a few negatively curved
nanographenes have been designed and synthesized by us [9,10]
and other research groups [11] since 2012. A further step toward
negatively curved carbon allotropes is synthesis of negatively
curved carbon nanobelts or nanotubes, which present key
segments of Mackay crystals. We envision that a negatively
curved carbon nanobelt can in principle be synthesized by
connecting properly functionalized aromatic saddles, whose
curved structure can facilitate formation of a macrocycle. For
example, as shown in Fig. 1a, nanobelt 1 can in principle be
synthesized by connecting two molecules of ketone-
functionalized aromatic saddle (2) through a McMurry reaction
and subsequent oxidative cyclodehydrogenation or photo-
chemical cyclization. Herein, we report synthesis and crystal
structure of 3 (Fig. 1b), which is an alkoxylated derivative of 2.
Fig. 1. (a) Retrosynthesis of negatively curved carbon nanobelt 1; (b)
structure of ketone-functionalized aromatic saddle 3.
ART ICLE INFO AB ST R ACT
Article history:
Received 28 January 2019
Received in revised form 21 March 2019
Accepted 1 April 2019
Available online
A novel ketone-functionalized aromatic saddle consisting of 72 sp2 carbon atoms is successfully
synthesized and unambiguously identified with X-ray crystallography. It can, in principle, be
used as a building block for synthesis of negatively curved carbon nanobelts and for a bottom-up
approach to negatively curved carbon allotropes.
Keywords:
Polycyclic arenes
Synthesis
Negatively curved carbon allotropes
Carbon nanobelts
Scholl reaction
Scheme 1. (a) Synthesis of 3 and (b) preparation of 5.
As shown in Scheme 1a, the synthesis of 3 started from the
Suzuki coupling of compound 4 and borinic acid 5, which gave
compound 6 in a good yield (80%). Compound 4 was reported
by us earlier [10], while borinic acid 5 is a new compound,
which was synthesized for the first time in this study. As shown
in Scheme 1b, halogen-lithium exchange of 8 [12] with n-
butyllithium followed by quenching with 1.3 equiv. of
triisopropyl borate yielded borinic acid 5 as the major product in
a yield of 84%. The structure of 5 was unambiguously identified
by X-ray crystallography (CCDC 1893914 contain the
supplementary crystallographic data of 5) as shown in Fig. 2a. It
is worth mentioning that the Suzuki coupling of diarylborinic
acid is rare in the literature [13], and using such a reaction to
form an extra ring [14] is even rarer. The two methylene bridges
in 6 were converted to carbonyl groups in 7 by successive
oxidation using potassium permanganate and pyridinium
chlorochromate. When potassium permanganate was used alone,
the corresponding diol was identified by 1H NMR but could not
be separated by column chromatography from the reaction
mixture in a pure form. Finally, the Scholl reaction of 7 with
DDQ and triflic acid gave compound 3 in excellent yield (92%).
However, our attempts to connect two molecules of 3 into a
macrocycle through McMurry coupling reaction or Barton-
Kellogg reaction were not successful.
Compound 3 is soluble in common organic solvents resulting
in red solutions and appears almost nonfluorescent. As shown in
Fig. S1 in Supporting information, the UV-vis absorption of 3
(5×10−6
mol/L in CH2Cl2) exhibits a broad and relatively weak
absorption band in the visible light region with λmax of 495 nm.
The cyclic voltammogram of 3 in CH2Cl2 (Fig. S2 in Supporting
information) exhibits one reversible oxidation wave with a half-
wave oxidation potential of 0.42 V vs. ferrocenium/ferrocene
(Fc+/Fc) and one reversible reduction wave with a half-wave
reduction potential of −1.68 V vs. Fc+/Fc. From the half-wave
oxidation and reduction potentials, the HOMO and LUMO
energy level of 3 are estimated as −5.52 eV and −3.42 eV,
respectively [15], which lead to a HOMO-LUMO gap of 2.1 eV.
It is in good agreement with the optical gap of 2.07 eV as
obtained from the absorption edge at around 600 nm.
Fig. 2. Structures of 5 and 3 in single crystals: (a) top view of 5; (b)
side view of 3; (c) front view of 3; (d) top view of 3. (Carbon,
oxygen and boron atoms are shown with ellipsoids set at 50%
probability, and octyl groups in 3 are removed for clarity.)
Crystals of 3 were characterized with X-ray crystallography
(CCDC 1893913 contain the supplementary crystallographic data
of 3). As shown in Figs. 2b and c, the polycyclic backbone of 3 is
curved like a saddle, which is 12.4 Å wide and 6.7 Å deep in the
upper part, and is 10.1 Å wide and 3.7 Å deep in the lower part.
As a result, 3 presents a deeper saddle shape than closely related
aromatic saddles reported by us earlier [10], and resembles half
of a negatively curved carbon nanobelt. Having a bond length of
1.46-1.48 Å, the C−C bonds shown in blue are significantly
longer than a typical C-C aromatic bond (1.38-1.40 Å) but
resemble C−C single bonds between sp2-sp
2 carbons, which have
a typical bond length of 1.45-1.48 Å depending on the degree of
conjugation [16]. This indicates that the π-bonds are not fully
delocalized in the polycyclic backbone. Instead, the π-bonds are
largely localized on the aromatic sextets following Clar’s rule.
[17].
In summary, a novel ketone-functionalized aromatic saddle
(3) was successfully synthesized and unambiguously identified
with X-ray crystallography. It can, in principle, be used as a
building block for synthesis of negatively curved carbon
nanobelts, although our preliminary attempts to connect two
molecules of 3 into a macrocycle through reactions of carbonyl
groups were not successful.
Acknowledgment
We thank Ms. Hoi Shan Chan (the Chinese University of
Hong Kong) for the single crystal crystallography. This work
was supported by the Research Grants Council of Hong Kong
(No. GRF 14300218). This work is dedicated to Prof. Henry N.
C. Wong.
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1
Supplementary information
A ketone-functionalized aromatic saddle as a potential building block for negatively curved
carbon nanobelts
Kwan Yin Cheung, Qian Miao*
Department of Chemistry, the Chinese University of Hong Kong, Shatin, New Territories, Hong Kong, China
1. Synthesis
General: The reagents and starting materials employed were commercially available and used without any further
purification unless otherwise noted or made following reported methods as indicated. Anhydrous and O2-free
dichloromethane and THF were purified by an Advanced Technology Pure-Solv PS-MD-4 system. NMR spectra were
recorded on a Bruker AVANCE III 400MHz spectrometer (1H NMR: 400 MHz,
13C NMR: 100 MHz). Chemical shift values
(δ) are expressed in parts per million using residual solvent (1H NMR, δH = 7.26 for CDCl3;
13C NMR, δC = 77.16 for
CDCl3; 29.84 for (CD3)2CO) as internal standard. Mass spectra were recorded on Bruker SolariX 9.4T FTICR MS or Bruker
Autoflex speed MALDI-TOF. X-ray crystallography data were collected on a Bruker AXS Kappa ApexII Duo
Diffractometer. UV-vis absorption spectra were recorded on a Varian CARY 1E UV-vis spectrophotometer. Fluorescence
spectra were taken on a Hitachi F-4500 spectrofluorometer. Melting points, without correction, were measured using a
Nikon Polarized Light Microscope ECLIPSE 50i POL equipped with an INTEC HCS302 heating stage. Cyclic voltammetry
was performed on a PAR Potentiostat / Galvanostat Model 263A Electrochemical Station (Princeton Applied Research).
Dibenzo[b,e]borinin-5(10H)-ol (5)
To a solution of bis-(2-bromophenyl)-methane (8)1 (10.8 g, 33 mmol) in anhydrous Et2O (140 ml) cooled with a liquid
N2/ acetone bath under an atmosphere of N2 was added n-BuLi (45.5 ml, 1.6 M hexane solution, 73 mmol) over 5 minutes
by syringe. The reaction mixture was stirred for 30 minutes and the cooling bath was removed. After the reaction mixture
was stirred at room temperature for 1 hour, it was cooled again with a liquid N2/ acetone bath. Triisopropyl borate (10 ml, 43
mmol) was added to the reaction mixture by syringe over 3 minutes. The reaction mixture was allowed to slowly warm to
room temperature overnight and was quenched with saturated NH4Cl (aq). The aqueous layer was extracted with Et2O, the
combined organic layer was washed with brine, dried with anhydrous Na2SO4, and filtered through a pad of silica gel. The
silica gel pad was further washed with Et2O. The filtrate was concentrated under reduced pressure and precipitated by
adding hexane. Filtration gave Dibenzo[b,e]borinin-5(10H)-ol (5) (5.43 g, 84%) as white solid. mp: decompose at around
100-110°C. 1H NMR (CDCl3) δ (ppm): 7.96 (d,
3J = 7.2 Hz, 2H), 7.51 (dt,
3J = 7.2 Hz,
4J = 1.6 Hz, 2H), 7.46 (d,
3J = 7.6 Hz,
2H), 7.37 (dt, 3J = 7.6 Hz,
4J = 0.4 Hz, 2H), 5.86 (s, 1H), 4.39 (s, 2H).
13C NMR (acetone-d6) δ (ppm): 148.6, 132.5, 131.8,
129.0, 126.0, 37.5. 11
B NMR (CDCl3) δ (ppm): 40.2. HRMS (EI+): calcd. for C13H10BO ([M]
+): 193.0822, found: 193.0822.
Compound 6
To a mixture of 4 (1.41 g, 1.0 mmol), 5 (975 mg, 5.0 mmol), K2CO3 (1.67 g, 12 mmol) and Pd(PPh3)4 (232 mg, 0.2 mmol)
in a Schlenk flask under an atmosphere of N2 was added 40 ml of toluene and 10 ml of dioxane/water 3/2 (V/V) solution,
both of which were bubbled with nitrogen for 30 minutes beforehand. The Schlenk flask was then sealed and the reaction
1. T.K. Wood, W.E. Piers, B.A. Keay, M. Parvez, Chem. Eur. J. 16 (2010) 12199–12206.
Support Information
2
mixture was heated to 100oC for 42 hours, cooled to room temperature, and then treated with water. The resulting mixture
was extracted with CH2Cl2, and the organic layer was washed with brine, dried with anhydrous Na2SO4 and the solvent was
evaporated under reduced pressure. The crude mixture was purified by column chromatography on silica gel with
hexane/CH2Cl2 3/1 (V/V) as eluent. 6 (1.14 g, 80%) was obtained as yellow solid. mp: 298-301°C. 1H NMR (CDCl3) δ
(ppm): 9.11 (dd, 3J = 8 Hz,
4J = 1.2 Hz, 4H), 7.40 (t,
3J = 8 Hz, 4H), 7.13 (dd,
3J = 7.6 Hz,
4J = 1.2 Hz, 4H), 7.07 (d,
3J = 7.2
Hz, 4H), 7.06 (s, 4H), 6.77 (t, 3J = 7.6 Hz, 4H), 6.40 (t,
3J = 7.2 Hz, 4H), 6.08 (d,
3J = 7.2 Hz, 4H), 4.23-4.28 (m, 4H), 4.13
(d, 2J = 16.4 Hz, 2H), 4.02-4.07 (m, 4H), 3.69 (d,
2J = 16.8 Hz, 2H), 1.85-1.92 (m, 8H), 1.48-1.52 (m, 8H), 1.25-1.34 (m,
32H), 0.83 (t, 3J = 7.2 Hz, 12H).
13C NMR (CDCl3) δ (ppm): 156.2, 142.6, 139.0, 137.7, 136.7, 133.5, 130.9, 130.0, 129.1,
128.2, 127.4, 126.2, 125.9, 125.6, 125.3, 125.1, 124.9, 110.4, 100.4, 70.1, 37.3, 31.9, 29.6, 29.5, 29.3, 26.4, 22.8, 14.2 .
HRMS (MALDI-TOF): calcd. for C104H102O4 ([M]+): 1415.7806, found: 1415.7755.
Compound 7
To suspension of 6 (1.14 g, 0.81 mmol) in 400 ml acetone was added KMnO4 (6.36 g, 40 mmol) and the mixture was stirred
at room temperature for 1 day. The reaction mixture was then concentrated under reduced pressure and the solid residue was
washed with CH2Cl2 and filtered. The filtrate was concentrated under reduced pressure. 87 mg of pyridinium
chlorochromate and 80 ml CH2Cl2 was added to the residue and stirred at room temperature for 4 hours. The reaction
mixture was concentrated under reduced pressure and purified by column chromatography on silica gel with hexane/CH2Cl2
1/3 to 1/4 (V/V) as eluent. 7 (682 mg, 59%) was obtained as yellow solid. mp: 348-351°C. 1H NMR (CDCl3) δ (ppm): 9.20
(dd, 3J = 8 Hz,
4J = 0.8 Hz, 4H), 7.95 (dd,
3J = 8 Hz,
4J = 1.2 Hz, 4H), 7.44 (t,
3J = 8 Hz, 4H), 7.06-7.09 (m, 10H), 6.70 (dt,
3J = 8 Hz,
4J = 1.2 Hz, 4H), 6.22 (d,
3J = 8 Hz, 4H), 4.29-4.35 (m, 4H), 4.03-4.09 (m, 4H), 1.88-1.95 (m, 8H), 1.50-1.54 (m,
8H), 1.26-1.36 (m, 32H), 0.83 (t, 3J = 7.2 Hz, 12H).
13C NMR (CDCl3) δ (ppm): 186.2, 156.5, 144.5, 141.9, 138.7, 133.0,
130.8, 139.2, 130.2, 129.7, 129.1, 128.0, 127.4, 127.1, 126.1, 125.7, 125.2, 124.8, 124.6, 110.0, 100.2, 70.0, 31.9, 29.6, 29.4,
29.3, 26.4, 22.8, 14.2. HRMS (MALDI-TOF): calcd. for C104H98O6 ([M]+): 1443.7392, found: 1443.7370.
Compound 3
To a stirred solution of 7 (147 mg, 0.10 mmol) and DDQ (115 mg, 0.51 mmol) in 28 ml of anhydrous CH2Cl2 under an
atmosphere of N2 was added 0.7 ml of trifluoromethanesulfonic acid. The mixture was stirred for 1 hour at room
temperature, and then quenched with an aqueous solution of NaHCO3. The resulting mixture was extracted with CH2Cl2, and
the organic layer was washed with brine, dried with anhydrous Na2SO4, and concentrated under reduced pressure. The crude
product was purified by column chromatography on silica gel with CH2Cl2/Et2O 1/0 to 20/1 (V/V) as eluent. 3 (135 mg,
92%) was obtained as red solid. mp: decompose during heating to around 340-350°C. 1H NMR (CDCl3) δ (ppm): 8.98 (d,
3J
= 9.2 Hz, 4H), 8.54 (d, 3J = 7.6 Hz, 4H), 8.50 (d,
3J = 8.4 Hz, 4H), 8.32 (d,
3J = 9.2 Hz, 4H), 7.65 (t,
3J = 7.6 Hz, 4H), 6.82
(s, 2H), 4.14 (broad s, 8H), 1.86 (t, 3J = 7.2 Hz, 8H), 1.47 (t,
3J = 6.8 Hz, 8H), 1.21-1.31 (m, 32H), 0.80 (t,
3J = 6.8 Hz, 12H).
13C NMR (CDCl3) δ (ppm): 185.1, 155.0, 135.0, 132.0, 129.9, 129.4, 129.2, 129.1, 129.0, 128.5, 128.3, 128.0, 127.6, 127.3,
127.2, 125.7, 122.0, 120.3, 108.0, 98.8, 69.5, 31.8, 29.4, 29.3, 29.2, 26.2, 22.7, 14.2. HRMS (MALDI-TOF): calcd. for
C104H90O6 ([M]+): 1435.6766, found: 1435.6778.
2. X-ray crystallography
X-ray crystallography data were collected on a Bruker AXS Kappa ApexII Duo Diffractometer.
3
Table S1
Summary of Crystal Structures of 5 and 3.
5 3
Space Group P3121 P-1
Unit Cell Lengths (Å) a = 13.0362(6)
b = 13.0362(6)
c = 5.0703(3)
a = 15.3414(10)
b = 17.0870(12)
c = 17.3983(12)
Unit Cell Angles (°) α = 90
β = 90
γ = 120
α = 115.120(2)
β = 100.102(2)
γ = 98.707(2)
Cell Volume (Å3) 746.219 3932.75
R factor 0.0421 0.0978
3. Absorption spectrum
UV-vis spectra were recorded with a Varian CARY 5G UV-vis spectrophotometer.
Fig. S1. Absorption spectrum of 3 in CH2Cl2 (5 × 10−6 mol/L).
4. Cyclic Voltammetry
The cyclic voltammetry was performed in a solution of CH2Cl2 with 0.1M Bu4NPF6 as the supporting electrolyte. A
platinum bead was used as a working electrode, a platinum wire was used as an auxiliary electrode, and a silver wire was
used as a pseudo-reference. Ferrocene/ferrocenium was used as the internal standard. Potentials were referenced to
ferrocenium/ferrocene (FeCp2+/FeCp2
0).
Fig. S2. Cyclic voltammogram of 3
4
5. NMR Spectra
1H NMR spectrum of 5 in CDCl3
13C NMR spectrum of 5 in acetone-d6
5
11B NMR spectrum of 5 in CDCl3
1H NMR spectrum of 6
6
13C NMR spectrum of 6
1H NMR spectrum of 7
7
13C NMR spectrum of 7
1H NMR spectrum of 3
8
13C NMR spectrum of 3
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Crystallographic Data (.cif) - aromatic-saddle-3.cif
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Crystallographic Data (.cif) - borinic-acid-5.cif
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