Syntheses and properties of linear π-conjugated molecules composed of 1-azaazulene and azulene

Article abstract of DOI:10.1016/j.tet.2019.130658

Two compounds, 6-(1-azaazulen-2-yl)ethynylazulene (8) and 6-(2-azulenyl)ethynylazulene (10), were synthesized using the Sonogashira-Hagihara cross-coupling reaction followed by decarboxylation with concentrated phosphoric acid. Compounds 8 and 10 were cha

Full text of DOI:10.1016/j.tet.2019.130658

Tetrahedron xxx (xxxx) xxx
Contents lists available at ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Syntheses and properties of linear
of 1-azaazulene and azulene
p-conjugated molecules composed
,
Keito Ohtsu a, Ryohei Hayami a, Takuya Sagawa a, Satoru Tsukada a b, Kazuki Yamamoto a,
,
*
Takahiro Gunji a
a Department of Pure and Applied Chemistry, Faculty of Science and Technology, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba 278-8510, Japan
b Department of Materials Science, Graduate School of Engineering, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Two compounds, 6-(1-azaazulen-2-yl)ethynylazulene (8) and 6-(2-azulenyl)ethynylazulene (10), were
synthesized using the Sonogashira-Hagihara cross-coupling reaction followed by decarboxylation with
concentrated phosphoric acid. Compounds 8 and 10 were characterized by 1H and 13C nuclear magnetic
resonance (NMR) spectroscopy, mass spectrometry, ultravioletevisible (UVeVis) spectroscopy, cyclic
voltammetry, and density functional theory (DFT) calculations. Based on the results, both compounds
Received 20 August 2019
Received in revised form
25 September 2019
Accepted 27 September 2019
Available online xxx
were confirmed to have p-conjugation throughout their molecular structures. The acidic responsivity of
compounds 8 and 10 was evaluated using UVeVis and 1H NMR spectroscopy. Compound 8 was found to
be highly sensitive to trifluoroacetic acid, with its 1-azaazulenyl moiety acting as a base. Compound 10
generated azulenium cations when mixed with excess amounts of trifluoroacetic acid.
© 2019 Elsevier Ltd. All rights reserved.
Keywords:
Azulene
1-Azaazulene
Ethynylene
Protonation
Sonogashira-Hagihara cross-coupling
reaction
1. Introduction
groups have been reported very little. Previously, our group re-
ported the syntheses and properties of amino-bridged compounds
Azulene and 1-azaazulene are structural isomers of naphthalene
and quinoline, respectively. Interestingly, their unique properties
include: (i) controllable band gap energies [1,2], (ii) excellent
chromogenic and electronic characteristics [3,4], and (iii) large
dipole moments (azulene: 1.08 D [5]; 1-azaazulene: 3.05 D [6]).
Recently, azulenyl derivatives have gained attention in the appli-
cations of conductive polymers [7,8], near-infrared-light-absorbing
materials [9e12], and organic semiconductors [13e16]. These ma-
terials are composed of subunits which contain azulenyl group
substituents. Specifically, combined azulenyl derivatives, having
substituents in the 2- or 6-position have smaller band gap energies
compared to those with substituents in the 1- or 3-position [17].
This is because they are strongly affected by dipole moments [18].
Linear azulenyl trimers, which are composed of combinations of
azulenes in a head-to-tail manner, were reported by Yamaguchi
and coworkers [14,15] to have unique properties. Comparatively,
hetero-coupled compounds composed of azulenyl and azaazulenyl
composed of 1-azaazulenyl and azulenyl groups [19]. However, the
planarity of the compounds was low because the two groups were
twisted. Hence, the unique properties of azulene and 1-azaazulene
were not utilized.
In this work, ethynylene-bridged linear dimers composed of
azulenyl and 1-azaazulenyl moieties were synthesized. The ethy-
nylene group was selected due to its high p-conjugation ability. To
investigate the properties of linear dimers, 6-(1-azaazulen-2-yl)
ethynylazulene (8) and 6-(2-azulenyl)ethynylazulene (10) were
chosen because of their simple structures. These compounds were
characterized by nuclear magnetic resonance (NMR) spectroscopy,
Fourier-transform infrared (FTIR) spectroscopy, and mass spec-
trometry. Their properties were evaluated by ultravioletevisible
(UVeVis) spectroscopy, cyclic voltammetry, and density func-
tional theory (DFT) calculations (Scheme 1). Moreover, protonation
of compounds 8 and 10 was studied based on changes in the
UVeVis spectra observed.
* Corresponding author.
E-mail address: gunji@rs.noda.tus.ac.jp (T. Gunji).
https://doi.org/10.1016/j.tet.2019.130658
0040-4020/© 2019 Elsevier Ltd. All rights reserved.
Please cite this article as: K. Ohtsu et al., Syntheses and properties of linear
Tetrahedron, https://doi.org/10.1016/j.tet.2019.130658
p-conjugated molecules composed of 1-azaazulene and azulene,

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K. Ohtsu et al. / Tetrahedron xxx (xxxx) xxx
Scheme 1. Syntheses of compounds 6, 8, and 10.
2. Results and discussion
1e5 ppm region, no signals due to an ethoxycarbonyl group were
present for compound 8. Furthermore, in the aromatic region
(7e10 ppm), a doublet at 7.41 and a triplet at 7.92 ppm were present
2.1. Syntheses
3
(coupling constant ¼ 3.5 Hz). This value was attributed to JHeH
In general, the 1- and 3-positions of azulenyl compounds and
the 3-position of 1-azaazulenyl compounds are easily attacked by
nucleophiles; therefore, 1,3-diethoxycarbonyl azulene (compound
2) and 1-ethoxycarbonyl azaazulene (compound 1) were selected
to prevent side reactions. Decarboxylation was performed using
concentrated phosphoric acid, using a method similar to that of
other azulene derivatives [14,15,20]. Compound 5 was synthesized
using the Sonogashira-Hagihara cross-coupling reaction of com-
pounds 1 and 2 in the presence of Pd(PPh3)4, CuI, and triethylamine
in toluene, with a 37% isolation yield. To synthesize compound 8,
compound 5 was decarboxylated using concentrated phosphoric
acid. Based on the 1H NMR spectrum of the product (Fig. 1), a
doublet at 7.43 ppm was observed and assigned to the protons in
the 1,3-positions of the azulene moiety (coupling con-
stant ¼ 3.6 Hz). Additionally, signals representative of an ethox-
ycarbonyl group in the azaazulene moiety appeared. Hence, the
decarboxylation of compound 5 did not produce compound 8, but
instead produced compound 6; furthermore, compound 6 was
contaminated with byproducts. From these results, the decarbox-
ylation of the azaazulenyl moiety was deemed to be very difficult.
In place of compound 1, compound 3 was selected because side
reactions of the Sonogashira-Hagihara cross-coupling reaction are
prevented by the high reactivity of its iodo group. Compound 8 was
synthesized through the reaction of compounds 3 and 2 in the
presence of Pd(PPh3)4, CuI, and triethylamine in toluene, followed
by decarboxylation using concentrated phosphoric acid. The 1H
NMR spectra of compounds 7 and 8 are shown in Fig. 2. In the
coupling interactions between protons of the 1-/3-position and 2-
position. Compound 10 was synthesized using the same method
as compound 8, except compound 4 was used instead of compound
3.
2.2. Properties of compounds 8 and 10
The properties of compounds 8 and 10 were evaluated using
UVeVis spectroscopy, cyclic voltammetry, and DFT calculations.
The UVeVis spectra of compounds 8 and 10 are shown in Fig. 3. The
absorption bands assigned to the transitions from the ground states
to the second excited states (S0/S2) of 1-azaazulene and azulene
occurred at 325 [21e23] and 341 nm [24,25], respectively.
Furthermore, the transitions from the S0 to the first excited states
(S0/S1) of 1-azaazulene and azulene occurred at 440 [21e23] and
580 nm [24e26], respectively. As seen in Fig. 3, the strongest ab-
sorption bands in the visible region for compounds 8 and 10
occurred at 406 and 419 nm, respectively, which suggested S0/S2
transitions. Additionally, the longest absorption bands of com-
pounds 8 and 10 occurred at wavelengths >600 nm and were
assumed to be S0/S1 transitions. Based on these results, the
conjugation of these compounds was obviously extended
compared to that of azulene and 1-azaazulene.
The electrochemical properties of compounds 8 and 10 were
evaluated using cyclic voltammetry (Fig. 4). The oxidation side
(>0 V) was very complicated because compounds
8 and 10
decomposed to compounds such as azulene-1-carbonitriles [27],
Fig. 1. 1H NMR spectra of compounds 5 and 6 in the regions of 7e10 ppm (left) and 1e5 ppm (right).
Please cite this article as: K. Ohtsu et al., Syntheses and properties of linear
Tetrahedron, https://doi.org/10.1016/j.tet.2019.130658
p-conjugated molecules composed of 1-azaazulene and azulene,

K. Ohtsu et al. / Tetrahedron xxx (xxxx) xxx
3
Fig. 2. 1H NMR spectra of compounds 7 and 8 in the regions of 7e10 ppm (left) and 1e5 ppm (right).
Comparatively, compound 10 had two reversible redox peaks
at ꢀ1.30 and ꢀ1.56 V; suggesting the anionic species formed from
this compound was stable.
DFTcalculations of compounds 8 and 10 are shown in Figs. 5 and
6, respectively. The highest occupied molecular orbitals (HOMOs) of
compounds 8 and 10 are distributed in their 6-ethynylazulenyl
moieties, while the lowest unoccupied molecular orbitals
(LUMOs) are distributed throughout the molecules. The orbital
overlaps of the HOMO and the LUMO clearly differ; therefore, the
transition probability from HOMO to LUMO is low (the calculated
oscillator strength is shown in Figs. S1 and S2). This result corre-
sponds to the low molar extinction coefficients of the absorption
bands from 600 to 700 nm observed by UVeVis spectroscopy. It
was proposed that the strongest absorption bands in the visible
regions of compounds 8 and 10 were due to S0/S2 transitions, that
is, transitions from the HOMO to the LUMOþ1. However, the orbital
overlaps of the HOMO and the LUMOþ1 are too different; thus the
transition probability is low. From the DFT calculations, the stron-
gest absorption bands in the visible regions of compounds 8 and 10
are possibly derived from transitions from the HOMOꢀ1 to the
LUMO and the HOMOꢀ2 to the LUMO, respectively. These transi-
tions would be possible because the calculated energy levels be-
tween the HOMO and the HOMOꢀ1 for compound 8, and the
Fig. 3. UVeVis spectra of compounds 8 and 10 in CH2Cl2.
Fig. 4. Cyclic voltammograms of compounds 8 and 10 in CH2Cl2 (working electrode:
glassy carbon; reference electrode: Agþ/Ag; counter electrode: Pt; internal reference:
Fcþ/Fc; and supporting electrolyte solution: 0.1 mol/L n-Bu4NClO4).
which were deposited onto the electrode. Hence, examination was
limited to the reduction side. The voltammogram of compound 8
showed an irreversible reduction peak at ꢀ1.20 V, and two
reversible redox peaks at ꢀ1.46 and ꢀ1.69 V. Due to the presence of
the irreversible peak in the first stage, the anionic species formed
by the reduction of compound 8 was found to be unstable.
Fig. 5. Molecular orbitals, energy levels, and calculated dipole moment of compound
8.
Please cite this article as: K. Ohtsu et al., Syntheses and properties of linear
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Fig. 7. UVeVis spectra of compound 8 (10 mmol/L in CH2Cl2) upon increasing addition
of TFA 0e1 equiv.
Fig. 6. Molecular orbitals, energy levels, and calculated dipole moment of compound
10.
HOMOꢀ2 and the HOMO for compound 10, are relatively close.
The calculated dipole moment of compound 8 was 3.89 D, which
is lower than the summation of the values of azulene (1.08 D [5])
and azaazulene (3.05 D [6]). Comparatively, the calculated dipole
moment of compound 10 was 2.98 D, which is approximately three
times that of azulene.
2.3. Acid protonation of compounds 8 and 10
Azulenyl derivatives generally form azulenium cations
(azuleneeHþ) when
a large excess of strong acid is added
[18,28e30]. The colors of the azulenium cations differ from those of
the original azulenyl derivatives because of changes in their mo-
lecular electronic structures. Using this knowledge, the acid
responsivity of compound 10 was tested using trifluoroacetic acid
(TFA). When a large amount of TFA was added, a change in the
UVeVis spectra of compound 10 was observed (Figs. S3eS6), sug-
gesting the protonation of compound 10 proceeded in a similar
manner to that of azulene [29]. When triethylamine was added to
protonated compound 10, the compound returned to its original
de-protonated form, as supported by its UVeVis and 1H NMR
spectra (Figs. S7 and S8, respectively). Therefore, the changes in the
UVeVis spectra observed were not due to decomposition of com-
pound 10, but due to its protonation.
Fig. 8. UVeVis spectra of compound 8 (10
of TFA 1000e200000 equiv.
mmol/L in CH2Cl2) upon increasing addition
To test the acid responsivity of compound 8, protonation with
TFA was performed. As shown in Fig. 7, compound 8 was found to
be highly sensitive to TFA. Since a similar trend of acid responsivity
was confirmed for a nitrogen-containing heterocycle [31], it can
therefore be suggested that the azaazulenyl moiety acts as a base
[19,32,33]. When a large excess of TFA was added, the UVeVis
spectra changed (Fig. 8). Therefore, similar to compound 10, it
was suggested that in the presence of excess TFA, protonation of
compound 8 at the azulenyl moiety occurred [34]. Reversibility of
the protonation of compound 8 was also observed (Figs. S9 and
S10).
Protonation of compound 8 was also confirmed by 1H NMR
spectroscopy (Fig. 9). A large shift was observed even when only 0.1
equivalent of TFA was added. With increasing equivalents of TFA,
the signals due to the azaazulenyl moiety (f, g, h, i, and j) were
shifted to a lower magnetic field, and the signals due to the azulenyl
moiety (a, b, c, and d) were slightly shifted to a lower magnetic
field. Hence, it was suggested that the cationic species formed by
Fig. 9. 1H NMR spectra of compound 8 upon increasing addition of TFA 0e2.0 equiv. in
CDCl3.
Please cite this article as: K. Ohtsu et al., Syntheses and properties of linear
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5
protonation delocalized the cation across the entire molecule, and
that the protonation of compound 8 must occur at the nitrogen
atom.
(BAS Inc.), platinum wire as the counter electrode, and Ag/AgClO4
as the reference electrode (0.01 M AgClO4 in 0.1 M Bu4NClO4/
dichloromethane, BAS Inc.). After each measurement, ferrocene
was added as an internal standard. The three-parameter Beck-
eeLeeeYangeParr (B3LYP) hybrid exchange-correlation functional
was employed in our theoretical calculations. The 6-31G(d,p) basis
set was used for H, C, O, and N [35e37]. All calculations were
performed using the Gaussian 09W (Revision-A.02) software pro-
gram [38].
Finally, acetic and hydrochloric acid were used instead of TFA.
For acetic acid, no protonation of compound 8 or 10 was observed
since acetic acid is a weak acid compared to TFA. When compound
10 was stored under HCl vapor, no protonation occurred. However,
for compound 8 stored under HCl vapor, a solid was produced
which was insoluble in organic solvents. When the solid was mixed
with triethylamine, compound 8 was regenerated. For comparison,
the protonation of compound 7 under HCl vapor was monitored
using 1H NMR spectroscopy (Fig. S11). Protonated compound 7
regenerated compound 7 when mixed with triethylamine. Based
on these results, compounds 7 and 8 have potential application as
proton sensors.
4.2. Materials
All the solvents were purified using a standard process [39] and
stored over activated molecular sieves. Ammonium chloride and
phosphoric acid were purchased from Tokyo Chemical Industry
(Tokyo, Japan). Copper(I) iodide was purchased from Kanto
Chemical Industry (Tokyo, Japan). Tetrakis(triphenylphosphine)
palladium(0), triethylamine, 85% phosphoric acid, phosphorus(V)
oxide, and trifluoroacetic acid were purchased from FUJIFILM Wako
Pure Chemical Corp. (Osaka, Japan). Ethyl 2-bromo-1-azaazulene-
3-carboxylate (1), diethyl 6-ethynylazulene-1,3-dicarboxylate (2),
2-iodo-1-azaazulene (3), and 2-iodoazulene (4) were prepared by
according to the literature [17,40e51], as described in the sup-
porting information. Silica-gel column chromatography was per-
3. Conclusion
Compounds 5, 7, and 9 were synthesized using the Sonogashira-
Hagihara cross-coupling reaction. Decarboxylation of these com-
pounds was performed using concentrated phosphoric acid to
produce compounds 6, 8, and 10, respectively. The UVeVis spectra
of compounds 8 and 10 showed absorption bands from 700 to
800 nm. The electrochemical properties of compounds 8 and 10
were evaluated in terms of reduction. Compound 8 showed an
irreversible reduction peak at ꢀ1.20 V and two reversible redox
peaks at ꢀ1.46 and ꢀ1.69 V. Compound 10 presented two reversible
redox peaks at ꢀ1.30 and ꢀ1.56 V. Based on the DFT calculations,
the LUMOs of compounds 8 and 10 were distributed throughout the
molecules. Moreover, the dipole moments of compounds 8 and 10
were calculated as 3.89 and 2.98 D, respectively. Protonation of
compounds 8 and 10 was observed using TFA. Formation of singly
protonated compound 10 increased with increasing concentrations
of TFA (500e30000 equivalents). Comparatively, compound 8 was
protonated stoichiometrically by TFA because its 1-azaazulenyl
formed using Wakogel® C-200 (spherical, neutral, 75e150 mm)
purchased from FUJIFILM Wako Pure Chemical Corp.
4.3. Synthesis of diethyl 6-(3-ethoxycarbonyl-1-azaazulen-2-yl)
ethynylazulene-1,3-dicarboxylate (5)
Compound 1 (140 mg, 50 mmol), compound 2 (150 mg, 51 mmol),
Pd(PPh3)4 (29 mg, 5 mol%), CuI (10 mg, 10 mol%), triethylamine
(5 mL), and toluene (20 mL) were mixed and stirred at room tem-
perature for overnight. Then, ammonium chloride aqueous solution
was added, and the solution was extracted with CH2Cl2. The organic
layer was washed with water followed by drying over MgSO4. After
filtration, the solution was concentrated by rotary-evaporator. The
residual solid was pre-purified by silica gel column chromatog-
raphy (AcOEt: CHCl3 ¼ 1: 9 v/v). The solid was purified by recrys-
tallization in toluene; the blackish brown solid was obtained
(92 mg, 37%).
moiety was easily protonated. Protonated compound
8 was
deprotonated by triethylamine to produce compound 8. Moreover,
a double-protonated compound 8 was formed when 200000
equivalents of TFA was added. Furthermore, protonation of com-
pound 8 was confirmed through reaction with HCl vapor.
4. Experimental section
Compound 5: Decomp. 223.2e224.0 ꢁC. 1H NMR (500 MHz,
CDCl3)
d
1.46 (t, J ¼ 7.0 Hz, 6H), 1,54 (t, J ¼ 7.0 Hz, 3H), 4.44 (q,
4.1. Measurements
J ¼ 7.0 Hz, 4H), 4.58 (q, J ¼ 7.0 Hz, 2H), 8.01 (t, J ¼ 10.0 Hz, 1H), 8.03
(t, J ¼ 10.0 Hz, 1H), 8.09 (d, J ¼ 10.0 Hz, 2H), 8.11 (t, J ¼ 10.0 Hz, 1H),
8.83 (s, 1H), 8.88 (d, J ¼ 10.0 Hz,1H), 9.71 (d, J ¼ 10.0 Hz,1H), 9.73 (d,
NMR spectra were recorded using a JEOL Resonance JNM-ECP
300 spectrometer (JEOL, Akishima, Japan) [1H at 300 MHz and 13C
{1H} at 75 MHz] and a JNM-ECP 500 spectrometer [1H at 500 MHz
and 13C{1H} at 126 MHz]. The chemical shifts were reported in ppm
relative to chloroform-d (CDCl3) (1H ¼ 7.26 ppm by the residual
chloroform and 13C{1H} ¼ 77.16 ppm). High-resolution electrospray
ionization time-of-flight mass spectrometry (HR-ESI-TOF MS) was
carried out on a JEOL JMS-T100CS “AccuTOF CS” spectrometer.
UVeVis absorption spectra were recorded on a JASCO V-670 UV-
VIS-NIR spectrophotometer. FT-IR spectra were recorded on a
JASCO FT/IR-6100 spectrometer (JASCO, Hachioji, Japan) using the
KBr method. Elemental analysis was performed using a Perkin
Elmer 2400II Elemental Analyzer. Melting points were recorded
using a Bibbly Stuart Scientific SMP3 instrument; the reported
melting points were uncorrected. Cyclic voltammetry was recorded
with an ALS 650E electrochemical analyzer. Bu4NClO4 was used as a
supporting electrolyte, which was recrystallized from EtOH and
dried under vacuum for 24 h. A series of measurements were per-
formed under an argon atmosphere in a three-electrode cell (BAS
Inc.) using 3 mm-diameter glassy carbon as the working electrode
J ¼ 10.0 Hz, 2H) ppm. 13C{1H} NMR (126 MHz, CDCl3)
d 14.7, 14.8,
60.3, 60.8, 91.6, 100.9, 117.3, 118.0, 133.0, 133.7, 135.3, 137.5, 138.8,
139.0, 140.8, 143.8, 144.4, 146.5, 150.0, 159.3, 163.7, 164.8 ppm. IR
(KBr, cmꢀ1): 2194(
nC≡C), 1690(nC]O). HRMS (APCI, m/z): Cacld. for
C
30H26NO6 [MþH]þ 496.17601; found: 496.17528. Anal. Calcd. for
C30H25NO6: C, 72.72; H, 5.09; N, 2.83; found: C, 72.74; H, 5.06; N,
2.80.
4.4. Decarboxylation of compound 5 using 100% phosphoric acid
This reaction was aimed to complete the decarboxylation of
compound 5 (that is, the synthesis of compound 8); however,
compound 8 was not synthesized but compound 6 was obtained.
This method was established as follows: 3 mL of 85% phosphoric
acid and 2 g of P2O5 were mixed at room temperature for 10 min.
The 100% phosphoric acid was added with compound 5 (38 mg,
7.6 m
mol), followed by heating at 95 ꢁC for 1 h. The solution was
added with ice and neutralized with NaHCO3. The mixture was
extracted with CH2Cl2; the organic layer was washed with water
Please cite this article as: K. Ohtsu et al., Syntheses and properties of linear
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K. Ohtsu et al. / Tetrahedron xxx (xxxx) xxx
followed by drying over MgSO4. After filtration, the solution was
concentrated by rotary-evaporator. The residual solid was sepa-
rated by silica gel column chromatography (AcOEt: CHCl3 ¼ 1: 9 v/
v). The 6-(3-ethoxycarbonyl-1-azaazulen-2-yl)ethynylazulene (6)
was obtained as blackish green solid, but contaminated with some
by-products (6.6 mg, 25%).
CH2Cl2. The organic layer was washed with water followed by
drying over MgSO4. After filtration, the solution was concentrated
by rotary-evaporator. The residual solid was pre-purified by silica
gel column chromatography (CH2Cl2: hexane ¼ 3: 1 v/v). The solid
was purified by recrystallization with toluene; the dark green
needle was obtained (200 mg, 94%).
Compound 6: 1H NMR (300 MHz, CDCl3)
d
1.54 (t, J ¼ 6.9 Hz,
Compound 9: M.p. 181.6 ꢁC. 1H NMR (500 MHz, CDCl3)
d 1.47 (t,
3H), 4.56 (q, J ¼ 6.9 Hz, 2H), 7.43 (d, J ¼ 3.6 Hz, 2H), 7.61 (d,
J ¼ 10.2 Hz, 2H), 7.93e8.08 (m, 4H), 8.32 (d, J ¼ 10.2 Hz, 2H), 8.88 (d,
J ¼ 9.6 Hz, 1H), 9.58 (d, J ¼ 9.9 Hz, 1H) ppm. HRMS (APCI, m/z):
Cacld. for C24H18NO2 [MþH]þ 352.13375; found: 352.13278.
J ¼ 7.0 Hz, 6H), 4.45 (q, J ¼ 7.0 Hz, 4H), 7.22 (t, J ¼ 10.0 Hz, 2H), 7.55
(s, 2H), 7.56 (t, J ¼ 10.0 Hz, 1H), 7.98 (d, J ¼ 10.0 Hz, 2H), 8.31 (d,
J ¼ 10.0 Hz, 2H), 8.78 (s, 1H), 9.68 (d, J ¼ 10.0 Hz, 2H) ppm. 13C{1H}
NMR (126 MHz, CDCl3)
d 14.7, 60.3, 98.2, 98.8, 117.1, 121.3, 124.5,
128.7, 133.5, 136.8, 137.6, 137.8, 138.8, 140.5, 143.5, 143.7, 165.1 ppm.
4.5. Synthesis of diethyl 6-(1-azaazulen-2-yl)ethynylazulene-1,3-
dicarboxylate (7)
IR (KBr, cmꢀ1): 2185(
nC≡C), 1693(nC]O). HRMS (APCI, m/z): Calcd.
for C28H23O4 [MþH]þ 423.15963; found: 423.15975. Anal. Calcd. for
C28H22O4: C, 79.60; H, 5.25; found: C, 79.72; H, 5.16.
Compound 3 (130 mg, 50 mmol), compound 2 (150 mg, 50 mmol),
Pd(PPh3)4 (29 mg, 5 mol%), CuI (10 mg, 10 mol%), triethylamine
(5 mL), and toluene (20 mL) were mixed and stirred at room tem-
perature for overnight. After the reaction, ammonium chloride
aqueous solution was added, and the solution was extracted with
CH2Cl2. The organic layer was washed with water followed by
drying over MgSO4. After filtration, the solution was concentrated
by rotary-evaporator. The residual solid was purified by silica gel
column chromatography (AcOEt: CHCl3 ¼ 1: 4 v/v), and dark red
powder was obtained (85 mg, 40%).
4.8. Synthesis of 6-(2-azulenyl)ethynylazulene (10)
A 3 mL of 85% phosphoric acid and 2 g of P2O5 was mixed and
stirried at room temperature for 10 min. The 100% phosphoric acid
was added with compound 9 (103 mg, 24 mmol), followed by
heating at 95 ꢁC for 1 h. The solution was added with ice and
neutralized with NaHCO3. The mixture was extracted with CH2Cl2;
the organic layer was washed with water followed by drying over
MgSO4. After filtration, the solution was concentrated by rotary-
evaporator. The residual solid was pre-purified by silica gel col-
umn chromatography (CCl4). The solid was purified by recrystalli-
zation with CCl4; the green film-like solid was obtained (23 mg,
34%).
Compound 7: Decomp. 190.5 ꢁC.1H NMR (500 MHz, CDCl3):
d
1.46 (t, J ¼ 7.0 Hz, 6H), 4.44 (q, J ¼ 7.0 Hz, 4H), 7.65 (s, 1H), 7.69 (t,
J ¼ 10.0 Hz, 1H), 7.83 (t, J ¼ 10.0 Hz, 1H), 7.91 (t, J ¼ 10.0 Hz, 1H), 8.03
(d, J ¼ 10.5 Hz, 2H), 8.56 (d, J ¼ 10.0 Hz, 1H), 8.72 (d, J ¼ 10.0 Hz, 1H),
8.79 (s, 1H), 9.68 (d, J ¼ 10.5 Hz, 2H) ppm. 13C{1H} NMR (126 MHz,
Compound 10: Decomp. 272.8 ꢁC. 1H NMR (500 MHz, CDCl3)
CDCl3): d 14.7, 60.3, 91.9, 98.3, 117.3, 119.0, 129.7, 130.6, 133.6, 135.4,
d
7.21 (t, 2H, J ¼ 10.0 Hz), 7.40 (d, 2H, J ¼ 4.0 Hz), 7.50 (d, 2H,
136.7, 137.3, 137.6, 139.2, 143.7, 144.2, 147.0, 148.2, 158.0, 164.9 ppm.
J ¼ 10.0 Hz), 7.53 (s, 2H), 7.58 (t, 1H, J ¼ 10.0 Hz), 7.89 (t, 1H,
J ¼ 4.0 Hz), 8.29 (d, 2H, J ¼ 10.0 Hz), 8.30 (d, 2H, J ¼ 10.0 Hz) ppm.
IR (KBr, cmꢀ1): 2188(
nC≡C), 1685(nC]O). HRMS (APCI, m/z): Calcd.
for C27H22NO4 [MþH]þ 424.15488; found: 424.15484.
13C{1H} NMR (126 MHz, CDCl3)
d 90.6, 100.3, 119.2, 121.0, 124.3,
126.3, 129.9, 132.6, 135.2, 137.2, 137.9, 138.2, 139.9, 140.5 ppm. IR
(KBr, cmꢀ1): 2188(
nC≡C). HRMS (APCI, m/z): Calcd. for C22H15
4.6. Synthesis of 6-(1-azaazulen-2-yl)ethynylazulene (8)
[MþH]þ 279.11738; found: 279.11734. Anal. Calcd. for C22H14: C,
A 3 mL of 85% phosphoric acid and 2 g of P2O5 was mixed and
stirred at room temperature for 10 min. The 100% phosphoric acid
94.93; H, 5.07; found: C, 94.91; H, 4.78.
was added with compound 7 (99 mg, 24 m
mol) and heated at 95 ꢁC
Acknowledgement
for 1 h. The solution was mixed with ice and neutralized with
NaHCO3. The mixture was extracted with CH2Cl2; the organic layer
was washed with water followed by drying over MgSO4. After
filtration, the solution was concentrated by rotary-evaporator. The
residual solid was purified by silica gel column chromatography
(AcOEt: CHCl3 ¼ 1: 9 v/v); the dark orange powder was obtained
(21 mg, 31%).
Professor Emeritus Noritaka Abe (Yamaguchi Univ.) is greatly
acknowledged for his valuable advices.
Appendix A. Supplementary data
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.tet.2019.130658.
Compound 8: Decomp. 150.4 ꢁC. 1H NMR (500 MHz, CDCl3):
d
7.41 (d, J ¼ 3.5 Hz, 2H), 7.56 (d, J ¼ 10.0 Hz, 2H), 7.61 (s, 1H), 7.66 (t,
J ¼ 10.0 Hz, 1H), 7.80 (t, J ¼ 10.0 Hz, 1H), 7.86 (t, J ¼ 10.0 Hz, 1H), 7.92
(t, J ¼ 3.5 Hz, 1H), 8.29 (d, J ¼ 10.0 Hz, 2H), 8.52 (d, J ¼ 10.0 Hz, 1H),
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