Dual behavior of iodine species in condensation of anilines and vinyl ethers affording 2-methylquinolines

Article abstract of DOI:10.3390/molecules21070827

A metal-free, mild and efficient method for the synthesis of 2-methylquinolines was successfully developed by condensation of anilines with vinyl ethers in the presence of catalytic amount of iodine. Modification of both pyridine and benzene moieties was easily achieved by changing only the vinyl ether and aniline. In this reaction, the iodine species was revealed to show dual behavior; molecular iodine serves as an oxidant, while its reduced form, hydrogen iodide, activates the vinyl ether. The redox reaction between these iodine species enables the use of a catalytic amount of iodine in this synthetic method.

Full text of DOI:10.3390/molecules21070827

molecules
Article
Dual Behavior of Iodine Species in
Condensation of Anilines and Vinyl Ethers
Affording 2-Methylquinolines
Song Thi Le , Chisa Yasuoka , Haruyasu Asahara 2,3,* and Nagatoshi Nishiwaki 2,3,
1
2
*
1
Faculty of Applied Sciences, Ton Duc Thang University, 19 Nguyen Huu Tho Street, Tan Phong Ward,
District 7, Ho Chi Minh City 700000, Vietnam; lethisong@tdt.edu.vn
School of Environmental Science and Technology, Kochi University of Technology, Tosayamada, Kami,
Kochi 782-8502, Japan; chorogi.max.x@gmail.com
Research Center for Material Science and Engineering, Kochi University of Technology, Tosayamada,
Kami, Kochi 782-8502, Japan
2
3
*
Correspondence: asahara.haruyasu@kochi-tech.ac.jp (H.A.); nishiwaki.nagatoshi@kochi-tech.ac.jp (N.N.);
Tel.: +81-887-57-2258 (H.A.); +81-887-57-2517 (N.N.)
Academic Editor: Wim Dehaen
Received: 10 May 2016; Accepted: 21 June 2016; Published: 25 June 2016
Abstract: A metal-free, mild and efficient method for the synthesis of 2-methylquinolines was
successfully developed by condensation of anilines with vinyl ethers in the presence of catalytic
amount of iodine. Modification of both pyridine and benzene moieties was easily achieved by
changing only the vinyl ether and aniline. In this reaction, the iodine species was revealed to show
dual behavior; molecular iodine serves as an oxidant, while its reduced form, hydrogen iodide,
activates the vinyl ether. The redox reaction between these iodine species enables the use of a catalytic
amount of iodine in this synthetic method.
Keywords: 2-methylquinoline; iodine-mediated reaction; aniline; vinyl ether; redox reaction
1
. Introduction
Iodine-catalyzed reactions have attracted much attention as environmentally sustainable
alternatives to transition metal catalysis in industrial chemistry for producing commodity and specialty
chemicals, foods, medicines, and pharmaceuticals [ ]. Iodine undergoes oxidative addition, ligand
exchange, reductive elimination, and ligand coupling, playing a role similar to that of transition
metal catalysts [ ]. In contrast to poisonous and expensive transition metals, molecular iodine is
1
2
an environmentally friendly, inexpensive, and readily available reagent. The mild Lewis acidity of
iodine also facilitates its use in organic synthesis, from stoichiometric levels to catalytic amounts. Thus,
iodine-mediated reactions have been explored as a powerful method for the synthesis of many organic
compounds [3–8].
Meanwhile, the importance and usefulness of quinoline derivatives has considerably increased
in the pharmaceutical industry. Among them, 2-methylquinoline derivatives have versatile
pharmacological properties such as antibacterial [9], antimalarial [10,11], anti-tumor [12] and
anti-HIV [13 14] properties, and can act as nociceptin receptor antagonists [15]. 2-Methylquinolines
,
can also be used as precursors in the synthesis of styrylquinolines, which are potential inhibitors of
HIV-1 integrase and the replication of HIV-1 in cell culture [16]. Various quinolines for the treatment
of protozoal and retroviral co-infection are also synthesized from 2-methylquinoline [17]. Furthermore,
the 7-methoxy derivative is known to be a non-peptide bradykinin B receptor antagonist [18], and
2
the 8-methoxy derivative is used as an anti-neurodegenerative agent, a radioprotective [19] and an
antibiotic against Staphylococcus aureus [20]. In addition to the aforementioned pharmaceutical uses,
Molecules 2016, 21, 827; doi:10.3390/molecules21070827
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Molecules 2016, 21, 827
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substituted 2-methylquinolines are often employed as precursors for electronic and optoelectronic
materials [21,22].
Despite their great importance, 2-methylquinolines are commonly prepared using traditional
methods developed by Doebner-von Miller [23], Skraup [24], Conrad-Limpach-Knorr [25],
Friedlaender [26] and Pfitzinger [27]. Unfortunately, these methods suffer from several disadvantages
such as low yields due to side reactions; harsh reaction conditions, including the use of strong acids;
multi-step reactions; and low regioselectivity. Although an acid-free approach using transition metal
catalysts overcomes these disadvantages [28
metal contaminants from the product. Furthermore, limited functional group tolerance diminishes
the generality of this method [28 30]. Several researchers have reported synthetic methods using
inexpensive Lewis acids such as iron(III) chloride, magnesium perchlorate, and zinc chloride [26 31 36];
–30], an additional purification step is required to remove
–
,
–
however, these methods still have drawbacks, including the need for harsh reaction conditions, difficult
work-up procedures, low yields, and high catalyst loadings.
Recently, an iodine-mediated reaction was applied to the synthesis of quinoline derivatives.
Wang and co-workers reported a molecular iodine-catalyzed reaction for the synthesis of
substituted quinolines from imines and aldehydes [37]. Furthermore, they reported the highly
efficient multi-component synthesis of quinoline derivatives using catalytic amounts of molecular
iodine [38–41]. Later, Wu et al. improved this method for the synthesis of quinolines and polycyclic
quinolines [42]. Owing to the numerous advantages of molecular iodine over transition metals, an
iodine-catalyzed quinoline synthesis was also reported in which anilines were condensed with cyclic
vinyl ethers, such as 2,3-dihydrofuran and 3,4-dihydro-2H-pyran [43]. In this reaction, iodine was
reported as serving only as a Lewis acid. In contrast, we report here a metal-free and efficient method
for the synthesis of substituted 2-methylquinolines using low toxicity, low cost, commercially available
iodine, anilines, and acyclic ethers under mild reaction conditions. We also suggest that the molecular
iodine serves as an oxidant, which has not been proposed previously. In addition, a reaction mechanism
is proposed, which includes the dual behavior of iodine species.
2
. Results and Discussion
2
.1. Iodine-Catalyzed Synthesis of 6-Substituted 2-Methylquinolines
Initially, the reaction of p-methoxyaniline 1a with ethyl vinyl ether 2a in dichloromethane was
used as model reaction to optimize conditions (Table 1). The reaction did not proceed in the absence
of iodine, with 88% of 1a recovered (entry 1). On the other hand, quinoline 3a 44] was successfully
[
synthesized in 44% yield when the reaction was carried out in the presence of 5 mol % iodine, indicating
that iodine was necessary for the construction of quinoline 3a (entry 2). Among the three solvents
tested, benzene was found to be the most suitable for this reaction (entries 2–4). While 5 mol % iodine
was enough for the reaction to operate efficiently, the yield of quinoline 3a decreased along with a 71%
recovery of 1a, when the catalyst loading was reduced to 1 mol % (entries 4 6). Consequently, the
´
reaction conditions used in entry 4 were determined to be the optimal conditions.
With the optimized conditions in hand, we applied the reaction to the syntheses of
2-methylquinolines 3b–h [29,45–50] using anilines 1b–h (Table 2). The reaction was influenced by
substituents on the benzene ring. When m-methoxyaniline 1b and o-methoxyaniline 1c were used, the
reactions proceeded in a similar fashion, affording corresponding quinolines 3b and 3c in lower yields,
presumably due to the electron-withdrawing inductive effect and steric hindrance of the methoxy
group (entries 2 and 3). In contrast, aniline possessing both o- and m-methoxy groups (1d) had a higher
product yield, indicating that high electron density on the benzene ring overcomes the aforementioned
disadvantages (entry 4). In the case of aminophenol 1e, no detectable 3e was produced due to side
reactions, such as the oxidation of 1e (entry 5). Anilines connected to another electron-donating
group, 1f and 1g, reacted efficiently to afford aminoquinoline 3f and methylquinoline 3g, respectively

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(entries 6 and 7). Unsubstituted aniline 1h was also a suitable substrate in this reaction, but with
somewhat lower efficiency.
Table 1. Synthesis of 6-methoxy-2-methylquinoline 3a.
MeO
MeO
I2
+
OEt
Solvent
NH2
N
80 °C, 2 h
1a
2a
3a
Entry
I2/mol %
Solv.
CH Cl
Yield/%
Recovery of 1a/%
1
2
3
4
5
6
0
5
5
5
10
1
0
88
22
24
3
11
71
2
2
2
CH Cl
44
43
64
55
11
2
MeCN
PhH
PhH
PhH
Table 2. Modification of the benzene ring of quinoline 3.
R
R
I2 (5 mol%)
OEt
2a
+
PhH
80 °C, 2 h
NH2
N
1
3
Entry
Aniline
Product
Yield of 3/%
Recovery of 1/%
MeO
MeO
1
64
3
NH2
N
1
a
3
a
2
3
30
36
30
0
MeO
NH2
MeO
N
1
b
3b
NH2
N
OMe
OMe
1
c
3c
OMe
OMe
4
55
12
NH2
OMe
N
OMe
1
d
3d
HO
HO
5
6
0
0
0
NH2
N
1
e
3e
N
N
83
NH2
N
1
f
3f
Me
Me
7
8
53
37
18
7
NH2
N
1
g
3g
NH2
N
1
h
3h

Molecules 2016, 21, 827
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To further demonstrate the scope of this reaction, various vinyl ethers, 2a
–
f
, were subjected to
the reaction with p-methoxyaniline 1a (Table 3). Interestingly, quinoline 3a was produced despite
the ether alkoxy group being changed from ethoxy to isobutoxy (entries 1 and 2), indicating that the
pyridine moiety in quinolines
-ethyl-3-methylquinoline
corrosion inhibitors, sorbents, and cyanine dyes [52]. When 2,3-dihydrofuran 2d was used, a quinoline
possessing acetal functions, , was synthesized (See the Supplementary Materials). Desired quinolines
3
was derived from the vinyl group. Indeed, propenyl ether 2c afforded
2
4 [51] in good yield, which is employed in the preparation of metal acid
5
6
and
7
were not detected when electron-poor vinyl ethers 2e and 2f were employed (entries 5 and 6).
Table 3. Modification of the pyridine ring of quinoline 3.
R1
MeO
I2
MeO
+
R1
(5 mol%)
OR2
R1
N
NH2
PhH
_
1
a
2
3
7
˝
Entry
Temp./ C
Time/h
Vinyl Ether
Product
Yield/%
OEt
64 1
1
80
2
3a
2
a
OBui
54 1
64
2
3
80
2
3a
4
2
b
Me
OEt
120
14
2
c
O
4
5
6
120
120
2
2
5
6
45
0
2
d
NC
OMe
2
e
O
120
14
MeO
OMe
7
0
2
f
1
Recovery of 1a: Entry 1 (3%), Entry 2 (33%).
O
MeO
MeO
Me MeO
Me
CN MeO
CN
O
MeO
O
O
OMe
OMe
N
N
N
N
N
3
a
4
6
O
O
5
7
2
.2. Study on the Mechanism
In the reaction of o-methoxyaniline 1d with vinyl ether 2a, several products were formed. In order
to gain insight into the mechanism, the reaction mixture was subjected to column chromatography
and the products were identified as 4-ethoxy-1,2,3,4-tetrahydro-8-methoxy- 2-methylquinoline (
8
)
as single diastereomer [53] and N-ethyl-2-methoxyaniline (
9
) [54] (Scheme 1). Although elucidation
1
of the stereochemistry of compound
8
was attempted by H-NMR, it was not achieved due to two
protons at the 3-position having the same coupling constant (12.0 Hz) with a proton at the 2-position.
Considering steric repulsion, the stereochemistry was predicted to be the cis-isomer because the
ethoxy and methyl groups are located at the equatorial position. Indeed, DFT calculation using B3LYP
6-31G(d,p) revealed that the cis-isomer was more stable than the trans-isomer, with 1.887 kcal/mol.
Interestingly, tetrahydroquinoline was converted to 3d in 77% yield by heating in the presence of
8

Molecules 2016, 21, 827
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iodine. In contrast, no reaction was observed for
9
under the same conditions. These results suggest
that 8 is an intermediate in the formation of 3d, and that N-alkylaniline 9 is a by-product (Scheme 1).
1
The reaction of 1a with vinyl ether 2a was monitored by H-NMR (Figure 1). As starting material
1
a
was consumed, product 3a formed. Just after the reaction started, the formation of 10
[55] was
observed, which gradually decreased over time. In addition, a small amount of 11 54] was formed, but
[
the amount was unchanged, even after 72 h. Thus, it was confirmed that 10 was a reaction intermediate
and 11 was a by-product.
OEt
I
2
(5 mol%)
+
+
+
OEt
NH
2
Benzene
N
N
N
8
0 °C, 2 h
H
H
OMe
OMe
OMe
OMe
2
a
1
d
3
d (40%)
8 (21%)
9 (20%)
OEt
I2 (5 mol%)
H
Benzene
N
H
N
EtO
80 °C
OMe
OMe
3
4
8
3d (77%)
H
H
N
1
Me
2
I
2
(5 mol%)
H
+
OEt
no reaction
MeO
N
H
Benzene
80 °C 
8
H
OMe
2a
9
Scheme 1. Reactions of by-products 8 and 9 with iodine under the same conditions.
1
Figure 1. Monitoring the reaction of 1a and 2a by H-NMR.
In addition, hydriodic acid and p-toluenesulfonic acid were employed as catalysts in this reaction
instead of iodine (Scheme 2). Hydriodic acid catalyzed this reaction, but gave a lower yield of 3a
.
˝
˝
p-Toluenesulfonic acid did not catalyze the reaction at 80 C, and it was necessary to heat at 120 C.
Although the reaction proceeded in the presence of an acid catalyst, iodine was necessary to accelerate
the reaction smoothly under milder conditions.

Molecules 2016, 21, 827
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MeO
MeO
Acid
HI
19% (80 °C)
0% (80 °C)
37% (120 °C)
+
OEt
Benzene
p-TsOH
NH2
N
3a
1
a
2
a
Scheme 2. Synthesis of quinoline 3a in the presence of acid catalyst.
Based on these experimental results, the reaction mechanism in Scheme 3 was proposed. At
first, iodine generates a trace amount of hydrogen iodide by reacting with water present in the
reaction mixture [56], initiating the reaction. Aniline 1h attacks the activated vinyl group of 2a
,
leading to an N,O-acetal, from which ethanol is eliminated to afford iminium intermediate 12
.
Iminium 12 is considered a common intermediate for both N-ethylaniline 14 and tetrahydroquinoline
. Tetrahydroquinoline 15 is formed by the attack of another molecule of 2a to iminium ion 12
1
5
followed by intramolecular cyclization, while dihydroquinoline 16 is obtained as a result of ethanol
elimination. Oxidation of 16 by iodine affords the final product, quinoline 3h. In this process, iodine is
reduced to hydrogen iodide, which then returns to iodine upon contact with air. Indeed, when the
reaction was conducted under Ar atmosphere, the product was obtained in lower yield. Thus, oxygen
may assist the reaction. Therefore, a catalytic amount of iodine is enough to produce this reaction.
In contrast, when intermediate 12 is reduced by dihydroquinoline 16, N-alkylaniline 14 is formed
as a by-product.
H
Et
NH2
O
1h
EtOH
N
H
1
6
OEt
H O
2
Et
N
H
N
H
I2
1
2
14
OEt
2a
OEt
2
a
O2
OEt
HI
N
3
h
N
H
H O
2
1
5
I2
H
Et
HI
O
H
H
N
H
H
N
H
1
6
EtOH
Scheme 3. A plausible mechanism.
3
. Materials and Methods
3
.1. General Information
1
All the reagents and solvents were commercially available and used as received. The H-NMR
spectra were measured on a Bruker Ascend-400 (Bruker, Billerica, MA, USA) at 400 MHz with TMS as
13
an internal standard. The C-NMR spectra were measured on a Bruker Ascend-400 at 100 MHz, and
assignments of 13C-NMR spectra were performed by DEPT experiments. The high resolution mass
spectra were measured on a AB SCIEX Triple TOF 4600 (AB Sciex, Framingham, MA, USA).

Molecules 2016, 21, 827
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3
.2. Procedures
Iodine-Mediated Synthesis of 2-Methylquinolines 3
To a solution of vinyl ether 2a (192 L, 2 mmol) in benzene (10 mL), were added p-methoxyaniline
1a, 123.2 mg, 1 mmol) and iodine (12.7 mg, 0.05 mmol), and the resultant mixture was heated at 80 C
for 2 h. The reaction mixture was washed with saturated sodium thiosulfate solution (1 10 mL) to
µ
˝
(
ˆ
remove unreacted iodine, and dried over magnesium sulfate. After removal of solvent, the residue
was subjected to silica gel column chromatography (eluent: hexane/ethyl acetate = 95/5) to afford 3a
(110.7 mg, 0.64 mmol, 64%).
The reactions of the aniline 1 with other vinyl ether 2 were performed in a similar manner.
3
.3. Compound Characterizations
-Methoxy-2-methylquinoline (3a) [44]: H-NMR (400 MHz, CDCl ):
1
6
δ
7.94 (d, J = 8.3 Hz,1H), 7.91(d,
3
J = 9.1 Hz,1H), 7.33 (dd, J = 9.1, 2.8 Hz, 1H), 7.23 (d, J = 8.3 Hz, 1H), 7.04 (d, J = 2.8 Hz, 1H), 3.91 (s,
3
H), 2.70 (s, 3H).
1
7
-Methoxy-2-methylquinoline (3b) [45]: H-NMR (400 MHz, CDCl ):
δ 7.95 (d, J = 8.2 Hz, 1H), 7.63 (d,
3
J = 8.8 Hz, 1H), 7.37 (s, 1H), 7.14 (d, J = 8.2 Hz, 1H), 7.13 (d, J = 8.8 Hz, 1H), 3.93 (s, 3H), 2.71 (s, 3H).
1
8
-Methoxy-2-methylquinoline (3c) [46]: H-NMR (400 MHz, CDCl ):
δ
8.00 (d, J = 8.4 Hz, 1H), 7.38 (dd,
3
J = 8.4, 8.1 Hz, 1H), 7.34 (d, J = 8.1 Hz, 1H), 7.31 (d, J = 8.4 Hz, 1H), 7.03 (d, J = 8.4 Hz, 1H), 4.08 (s, 3H),
2
.80 (s, 3H).
1
5
,8-Dimethoxy-2-methylquinoline (3d) [47]: H-NMR (400 MHz, DMSO): δ 7.45 (d, J = 8.8 Hz,1H), 7.09
(
d, J = 8.8 Hz,1H), 6.87 (d, J = 6.8 Hz, 1H), 6.66 (d, J = 6.8 Hz, 1H), 3.90 (s, 3H), 3.75 (s, 3H), 2.68 (s, 3H).
1
6
-(N,N-dimethylamino)-2-methylquinoline (3f) [49]: H-NMR (400 MHz, DMSO):
δ
7.98 (d, J = 8.4 Hz,
1
1
H), 7.76 (d, J = 9.2 Hz,1H), 7.37 (dd, J = 9.2, 2.8 Hz, 1H), 7.22 (d, J = 8.4 Hz, 1H), 6.88 (d, J = 2.8 Hz,
H), 2.97 (s, 6H), 2.56 (s, 3H).
1
2
,6-Dimethylquinoline (3g) [29]: H-NMR (400 MHz, CDCl ):
δ
7.96 (d, J = 8.3 Hz, 1H), 7.95 (d, J = 8.3 Hz,
3
1
H), 7.52 (s, 1H), 7.51 (d, J = 8.3 Hz, 1H), 7.24 (d, J = 8.3 Hz, 1H), 2.74 (s, 3H), 2.51 (s, 3H).
1
2
-Methylquinoline (3h) [50]: H-NMR (400 MHz, CDCl ):
δ
8.04 (d, J = 5.0 Hz, 1H), 8.02 (d, J = 5.0 Hz,
3
1
H), 7.77 (d, J = 8.0 Hz, 1H), 7.68 (dd, J = 8.4, 7.0 Hz, 1H), 7.48 (dd, J = 8.0, 7.0 Hz, 1H), 7.28 (d, J = 8.4 Hz,
1
H), 2.75 (s, 3H).
1
2
-Ethyl-6-methoxy-3-methylquinoline (
4
) [27]: H-NMR (400 MHz, CDCl ):
δ
7.91 (d, J = 9.2 Hz, 1H), 7.69
3
(
s, 1H), 7.25 (dd, J = 9.2, 2.8 Hz, 1H), 6.94 (d, J = 2.8 Hz, 1H), 3.87 (s, 3H), 2.94 (q, J = 7.5 Hz, 2H), 2.43 (s,
3
H), 1.35 (t, J = 7.5 Hz, 3H).
1
Compound
1
(
5
: H-NMR (400 MHz, CDCl ):
δ
7.89 (d, J = 9.2 Hz, 1H), 7.81 (s, 1H), 7.27 (d, J = 9.2 Hz,
3.93 (m, 1H), 3.91 (s, 3H), 3.88 3.67 (m, 6H), 3.54 3.48
2.06 (m, 2H), 2.03
1.78 (m, 8H); 13C-NMR (100 MHz, CDCl ) 23.4
CH ), 23.5 (CH ), 29.3 (CH ), 32.1 (CH ), 32.3 (two CH signals overlapped), 32.4 (CH ), 55.4 (CH ),
3
H), 6.99 (s, 1H), 5.14 (t, J = 2.0 Hz, 2H), 4.01
´
´
´
m, 1H), 3.07 3.00 (m, 4H), 2.13
´
´
´
3
(
2
2
2
2
2
2
3
6
6.8 (four CH signals overlapped), 103.8 (CH), 103.9(CH), 104.7 (CH), 121.0 (CH), 127.9 (C), 130.0
2
(
CH), 130.9 (C), 134.6 (CH), 142.8 (C), 157.2 (C), 158.9 (C); HRMS Calcd for C H NO : 402.2275.
23 31 5
Found: 402.2272.
1
Cis-4-Ethoxy-1,2,3,4-tetrahydro-8-methoxy-2-methylquinoline (8) [29]: H-NMR (400 MHz, CDCl ): δ 7.00
3
(dd, J = 4.8, 4.8 Hz, 1H), 6.64 (d, J = 4.8 Hz, 2H), 4.70 (dd, J = 10.4, 5.6 Hz, 1H), 4.1 (br, 1H), 3.82 (s, 3H),
3.69–3.55 (m, 1H), 3.51 (dq, J = 9.2, 7.2 Hz, 2H), 2.21 (ddd, J = 12.0, 5.6, 2.4 Hz, 1H), 1.70 (ddd, J = 12.0,
1
0.4, 2.4 Hz, 1H), 1.28 (d, J = 7.0 Hz, 3H), 1.27 (dd, J = 7.2, 7.2 Hz, 3H).

Molecules 2016, 21, 827
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1
N-Ethyl-2-methoxyaniline (
9
) [30]: H-NMR (400 MHz, CDCl ):
δ
6.87 (dd, J = 7.6, 7.6 Hz, 1H), 6.76 (d,
3
J = 7.6 Hz, 1H), 6.65 (dd, J = 7.6, 7.6 Hz, 1H), 6.60 (d, J = 7.6 Hz, 1H), 4.08 (br s, 1H), 3.84 (s, 3H), 3.16 (q,
J = 7.2 Hz, 2H), 1.28 (t, J = 7.2 Hz, 3H).
4
. Conclusions
In conclusion, we have successfully developed an environmentally benign and efficient method
for the construction of quinolines from substituted anilines and vinyl ethers in the presence of an
3
1
2
iodine catalyst. This protocol can be performed with simple manipulations in one step under mild
conditions for both the reaction and work-up. Furthermore, no transition metals are used, which
eliminates the need for a product decontamination step, thus considerably reducing the cost. Hence,
this is a new synthetic method for quinoline derivatives that can be applied in various fields.
Furthermore, the roles of iodine species were also studied, finding that the iodine species had
dual behavior, with molecular iodine serving as an oxidant, and its reduced form, hydrogen iodide,
activating the vinyl ether. The redox reaction between these iodine species enables the use of a catalytic
amount of iodine in this reaction.
Supplementary Materials: Supplementary materials can be accessed at: http://www.mdpi.com/1420-3049/21/
7
/827/s1.
Author Contributions: Le, S.T. wrote the draft and measured HRMS and did a part of experiments; Yasuoka, C.
designed experiments; Asahara, H. performed DFT calculation; Asahara, H. and Nishiwaki, N. analyzed data and
discussed with other authors; all authors contributed to the revision.
Conflicts of Interest: The authors declare no conflicts of interest.
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