Dimerizations of 2-bromo-3-methyl-1,4-naphthoquinone and 2-methyl-1,4-naphthoquinone in tetra-n-butylammonium bromide

Article abstract of DOI:10.1016/j.tet.2019.130899

2-Bromo-3-methyl-1,4-naphthoquinone and 2-methyl-1,4-naphthoquinone are highly reactive species known to dimerize in various ways. The product obtained in the dimerization of each compound is primarily dependent upon the solvent used. Ionic liquids represent a new class of solvents having non-molecular (ionic) character. Replacement of a conventional organic solvent with an ionic solvent frequently changes the mechanism of a reaction. In this study, reactions of 2-bromo-3-methyl-1,4-naphthoquinone and 2-methyl-1,4-naphthoquinone in ionic liquids were examined. Dimerization of 2-bromo-3-methyl-1,4-naphthoquinone or 2-methyl-1,4-naphthoquinone with N-methylcyclohexylamine in tetra-n-butylammonium bromide (TBAB), an ionic liquid, under an aerobic atmosphere afforded 5,7,12,14-pentacenetetrone and 1-methylKuQuinone, respectively. The use of TBAB as a solvent improved the yields of the products as compared with previously reported methods. The mechanism of each dimerization was also investigated.

Full text of DOI:10.1016/j.tet.2019.130899

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Dimerizations of 2-bromo-3-methyl-1,4-naphthoquinone and 2-methyl-1,4-
naphthoquinone in tetra-n-butylammonium bromide
Mai Onuki, Motohiro Ota, Shoya Otokozawa, Shogo Kamo, Shusuke Tomoshige,
Kazunori Tsubaki, Kouji Kuramochi
PII:
S0040-4020(19)31315-8
https://doi.org/10.1016/j.tet.2019.130899
TET 130899
DOI:
Reference:
To appear in: Tetrahedron
Received Date: 12 November 2019
Revised Date: 17 December 2019
Accepted Date: 19 December 2019
Please cite this article as: Onuki M, Ota M, Otokozawa S, Kamo S, Tomoshige S, Tsubaki K, Kuramochi
K, Dimerizations of 2-bromo-3-methyl-1,4-naphthoquinone and 2-methyl-1,4-naphthoquinone in tetra-n-
butylammonium bromide, Tetrahedron (2020), doi: https://doi.org/10.1016/j.tet.2019.130899.
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Dimerizations of 2-bromo-3-methyl-1,4-
naphthoquinone and 2-methyl-1,4-
naphthoquinone in tetra-n-butylammonium
bromide
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a,1
b,1
a
a,2
a
b
Mai Onuki, Motohiro Ota, Shoya Otokozawa, Shogo Kamo, Shusuke Tomoshige, Kazunori Tsubaki, Kouji
a,*
Kuramochi
a
Department of Applied Biological Science, Faculty of Science and Technology, Tokyo University of Science, 2641
Yamazaki, Noda, Chiba 278-8510, Japan
b
Graduate School for Life and Environmental Sciences, Kyoto Prefectural University, 1-5 Shimogamo Hangi-cho,
Sakyo-ku, Kyoto 606-8522, Japan
*
Corresponding author.
These authors contributed equally to this work.
Present address: Department of Chemistry, University of Pittsburgh, 219 Parkman Avenue, Pittsburgh,
1
2
Pennsylvania 15260, United States.

Tetrahedron
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Dimerizations of 2-bromo-3-methyl-1,4-naphthoquinone and 2-methyl-1,4-
naphthoquinone in tetra-n-butylammonium bromide
a,1
b,1
a
a,2
a
Mai Onuki, Motohiro Ota, Shoya Otokozawa, Shogo Kamo, Shusuke Tomoshige, Kazunori
b
a,*
Tsubaki, Kouji Kuramochi
a
Department of Applied Biological Science, Faculty of Science and Technology, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba 278-8510, Japan
Graduate School for Life and Environmental Sciences, Kyoto Prefectural University, 1-5 Shimogamo Hangi-cho, Sakyo-ku, Kyoto 606-8522, Japan
b
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
2-Bromo-3-methyl-1,4-naphthoquinone and 2-methyl-1,4-naphthoquinone are highly reactive
species known to dimerize in various ways. The product obtained in the dimerization of each
compound is primarily dependent upon the solvent used. Ionic liquids represent a new class of
solvents having non-molecular (ionic) character. Replacement of a conventional organic solvent
with an ionic solvent frequently changes the mechanism of a reaction. In this study, reactions of
Available online
2
-bromo-3-methyl-1,4-naphthoquinone and 2-methyl-1,4-naphthoquinone in ionic liquids were
examined. Dimerization of 2-bromo-3-methyl-1,4-naphthoquinone or 2-methyl-1,4-
naphthoquinone with N-methylcyclohexylamine in tetra-n-butylammonium bromide (TBAB), an
ionic liquid, under an aerobic atmosphere afforded 5,7,12,14-pentacenetetrone and 1-
methylKuQuinone, respectively. The use of TBAB as a solvent improved the yields of the
products as compared with previously reported methods. The mechanism of each dimerization
was also investigated.
Keywords:
Ionic liquid
Dimerization
1
5
1
,4-Naphthoquinone
,7,12,14-Pentacenetetrone
-MethylKuQuinone
2
009 Elsevier Ltd. All rights reserved.
1
. Introduction
,4-Naphthoquinones are known to dimerize in various ways,
3 [5]. When 1 was treated with hexamethylditin, CuI and Na CO
2 3
in the presence of tris(dibenzylideneacetone)dipalladium
1
[
(
Pd (dba) ] and (±)-2,2′-bis-(diphenylphosphino)-1,1′-binaphthyl
BINAP) in DMF, an oxepin 4 was obtained [5]. Treatment of 1
2 3
depending on their structure and the particular set of conditions
in use [1-3]. They serve as electrophiles and participate in 1,2-
addition and 1,4-addition reactions with nucleophiles. In contrast,
with Cs CO in DMF gave a dimeric epoxide 5 [5], whereas
2
3
treatment of 1 with Na CO in MeOH gave a tetrahydropyran 6
2
3
1
,4-dihydroxynaphthalenes, the reduced forms of 1,4-
[
6]. Switching only the solvent also results in different reactivity.
naphthoquinones, act as nucleophiles [1]. Thus, coupling
between naphthoquinones and the corresponding 1,4-
dihydroxynaphthalenes can occur to give dimeric products. 2-
Methyl-1,4-naphthoquinones are known to produce dimeric
products via their o-quinone methides. The o-quinone methides
have both nucleophilic and electrophilic properties because they
have a nucleophilic dienol/enol and an electrophilic α,β-
unsaturated enone/ketone in the same molecule [1]. Baxter and
coworkers reported that treatment of 2-bromo-3-methyl-1,4-
naphthoquinone (1) with N-methylcyclohexylamine in EtOH
afforded 5,7,12,14-pentacenetetrone (2) in 11% yield (Scheme
For example, a 2H-naphtho[1,2-b]pyran 7 was obtained by
treatment of 1 with 1-ethylpiperidine in THF, while treatment of
1
with 1-ethylpiperidine in CH Cl afforded a diquinone 8 [7].
2 2
Baxter and coworkers reported that treatment of 2-methyl-1,4-
naphthoquinone (9) with N-methylcyclohexylamine in EtOH
gave 2 in 14% yield (Scheme 2A) [4]. Although the yield of the
reaction was low, this method was considered useful because 2
can be prepared from inexpensive starting materials in a
straightforward and reproducible way [8]. Our group reported a
biomimetic dimerization of 9 affording a dimeric epoxide 10 in
higher yield by treatment with a 5 M aqueous NaOH solution in
EtOH under an aerobic atmosphere (Scheme 2B) [9].
1
A) [4]. Our group reported the following six different
dimerizations of 1 (Scheme 1B) [5-7]. Treatment of 1 with
hexamethylditin and CuI in the presence of [1,1′-
Ionic liquids, salts that are liquid at temperature lower than
bis(diphenylphosphino)ferrocene]palladium
dichloride
1
50 °C, represent a relatively new class of solvents with non-
[
Pd(dppf)Cl ] in dioxane afforded a 2,2′-dimeric naphthoquinone
2
∗
Corresponding author. Tel. +81-4-7122-9413; fax: +81-4-7123-9767; e-mail: kuramoch@rs.tus.ac.jp
These authors contributed equally to this work.
Present address: Department of Chemistry, University of Pittsburgh, 219 Parkman Avenue, Pittsburgh, Pennsylvania 15260, United States.
1
2

2
Tetrahedron
molecular (ionic) character [10]. They have emerged as useful
In this paper, dimerizations of 1 and 9 in tetra-n-
solvents with high thermal and chemical stability, and have found
several applications in synthetic chemistry. Notably, ionic liquids
can exploit novel and unusual chemical reactivity [10-12]. This
peculiar nature of ionic liquids motivated us to employ them as
solvents for the “chameleon-like” dimerization of 1,4-
naphthoquinones.
butylammonium bromide (TBAB), an ionic liquid, are reported.
Dimerizations of
1 and 9 in TBAB gave 2 and 1-
methylKuQuinone (11), respectively. Both 2 and 11 have
interesting properties. Compound 2 and its derivatives are
promising organic candidates for use as high-capacity cathodes in
rechargeable lithium batteries [13-16]. Additionally, compound 2
has been used as a starting material in the production of a new
class of pentacene-based π-conjugated systems [8,17-21]. 1-
MethylKuQuinone (11) was synthesized by Coletti and
(A) Dimerization of 1 reported by Baxter and coworkers
coworkers
in
6%
yield
via
treatment
of
2-
hydroxynaphthoquinone (12) and 1-bromopropane with Cs CO
2
3
in the presence of a catalytic amount of ferrocene in DMSO at
14 °C (Scheme 3) [22]. This compound possesses interesting
1
absorption, fluorescence and electrochemical properties [23-25].
In addition to its chemical properties, 11 shows cytotoxic
activities against human cancer cells [25]. Thus, a better and
simpler methodology for the synthesis of 11 is needed. Our
present method using TBAB as a solvent for the dimerizations of
(B) Six different dimerizations reported by our group
1
and 9 improved the yields of 2 and 11, respectively. Notably,
the use of TBAB as a solvent and a reagent provided a novel
dimerization mode of 9. The reaction mechanism for the
formation of 11 by dimerization of 9 was also elucidated by
13
determination of the reaction intermediate and C labeling
experiments.
(A) Dimerization of 9 reported by Baxter and coworkers
(B) Dimerization of 9 reported by our group
Scheme 2. Previous Dimerizations of 2-Methyl-1,4-
naphthoquinone (9).
Scheme 3. Previous synthesis of 1-methylKuQuinone (11).
2
. Results and discussion
Scheme 1. Previous dimerizations of 2-bromo-3-methyl-1,4-
naphthoquinone (1).
Dimerization of 2-bromo-3-methyl-1,4-naphthoquinone (1) in
ionic liquids was examined (Table 1). Treatment of 1 with N-

methylcyclohexylamine (2.0 equiv) in TBAB (52 equiv) at 120
C under an aerobic atmosphere gave 2 in 17% yield (entry 1).
The melting point (Mp) of TBAB is 103 °C. Thus, TBAB is a
liquid at the reaction temperature of 120 °C. Because compound
of the bromide ion affords diquinone I, which is in equilibrium
with its dienol form I′. Interaction between the carbonyl oxygen
and the tetra-n-butylammonium cation should activate the
unsaturated ketone and induces the 1,4-addition. Finally,
oxidation of the tautomeric mixture of diquinoids I and I′ by
oxygen yields 2.
°
1
is unstable under basic conditions, the low yield is due to
decomposition of 1 accompanied by formation of 2. Dimerization
of 1 with N-methylcyclohexylamine (2.0 equiv) in TBAB (50
equiv) and refluxing toluene gave 2 in lower (7%) yield (entry 2).
The reaction did not proceed in the absence of N-
methylcyclohexylamine (entry 3). These results indicate that the
use of TBAB and N-methylcyclohexylamine as solvent and base,
respectively, is important for this dimerization. The use of tetra-
n-butylammonium chloride (TBAC; Mp = 70°C), tetra-n-
butylphosphonium bromide (TBPB; Mp = 100–103 °C), tetra-n-
butylphosphonium tetrafluoroborate (n-Bu P·BF ; Mp = 96–99
4
4
°
~
4
=
C), or 1-butyl-3-methylimidazolium chloride ([Bmim]Cl; Mp =
70 °C) as the ionic liquid all decreased the yield of 2 (entries
−7). The reaction in tetra-n-butylammonium iodide (TBAI; Mp
141–143 °C) at 145 °C resulted in decomposition of 1 (entry
). The melting point of TBAI is higher than those of other liquid
8
ions used in this study, suggesting that TBAI induces thermal
decomposition of 1.
Table 1.
Dimerization of 2-bromo-3-methyl-1,4-naphthoquinone (1)
in ionic liquids.
ionic liquid
equiv)
TBAB
52)
TBAB
50)
TBAB
50)
TBAC
50)
TBPB
50)
n-Bu P·BF
50)
Bmim]Cl
50)
TBAI
50)
temp
(°C)
time
(min)
yield
(%)
entry
solvent
a
(
b
1
2
–
120
reflux
120
100
110
110
80
45
60
300
60
30
30
50
50
17
(
b
toluene
7
(
c
d
3
–
–
–
–
–
–
–
(
b
4
5
6
7
8
9
(
b
13
(
4
4
b
10
(
[
b
4
(
e
145
0
(
a
Isolated yield.
b
TLC analysis showed the starting material 1 was fully consumed.
Decomposition of 1 was accompanied by formation of 2.
Without N-methylcyclohexylamine.
TLC analysis showed that formation of 2 was not observed and
unreacted starting material 1 was observed.
c
d
e
Compound 1 decomposed, and 2 was not obtained.
The mechanism for the dimerization of 1 in TBAB is
Scheme 4. Mechanism for formation of 2 from 1.
proposed in Scheme 4. First, the diquinone 8 is formed by
coupling 1 to its o-quinone methide [7]. A base-induced
enolization of 8, an intramolecular 1,4-addition, and elimination

4
Tetrahedron
We examined the conventional solvents for the dimerization
Table 2.
of 1 (Scheme 5). According to the procedure reported by Baxter
and coworkers [4], treatment of 1 with N-methylcyclohexylamine
in EtOH under an aerobic atmosphere gave 2 in 7% yield
Dimerization of 2-methyl-1,4-naphthoquinone (9) in ionic
liquids.
(
Scheme 5A). Surprisingly, treatment of
methylcyclohexylamine in MeOH gave 13 in 19% yield (Scheme
B). Decomposition of 1 was accompanied by formation of 13.
The reaction in 2-propanol gave 2H-naphtho[1,2-b]pyran 7 [7] in
7% yield with 64% recovery of 1 (Scheme 5C). The use of
1
with N-
5
1
THF as a solvent gave 7 in 12% (Scheme 5D). When DMF was
used as a solvent, decomposition of 1 occurred. These results are
the reason why we call the dimerization of 1,4-naphthoquinones
ionic liquid
equiv)
TBAB
11)
TBAB
11)
TBAB
11)
TBAB
10)
TBAB
10)
TBAC
10)
TBAC
10)
TBAB
10)
TBAI
10)
temp
(°C)
time yield
entry
atmosphere solvent
a
“chameleon-like”.
(
(h)
(%)
b
c
1
air
air
–
–
–
130
130
130
1
30
(
c
2
3
4
5
6
7
8
9
7
22
(
c
O
2
O
2
O
2
O
2
O
2
O
2
O
2
19
(
c
toluene reflux
ethanol reflux
6.5
7
17
(
d
0
(
d
–
–
100
130
130
145
7
0
(
c
7
15
(
e
DMF
–
7
0
(
e
9
7
0
(
a
Isolated yield.
b
c
In the presence of N-methylcyclohexylamine (3.0 equiv).
TLC analysis showed the starting material 9 was fully consumed.
Decomposition of 9 was accompanied by formation of 11.
No reaction occurred. Almost all the starting material was recovered.
Compound 9 decomposed, and 11 was not obtained.
d
e
TBAI, decomposition of 9 occurred (entry 9). TBAI induces
thermal decomposition of 9. These results indicate that the
reaction temperature, solvent, and ionic liquid used in this
reaction are considered major factors in this dimerization.
To determine the intermediate in the dimerization of 9, two
hypothetical intermediates 10 and 14 were subjected to the
reaction conditions (Scheme 6). Compound 14 was prepared by
Scheme 5. Dimerization of 1 in EtOH (A), MeOH (B) and 2-
propanol (C), and THF (D).
reduction of the epoxide in 10 [7] with PPh and iodine. Heating
3
1
0 in TBAB at 130 °C under an aerobic atmosphere resulted in
the decomposition of 10. On the other hand, when 14 was heated
in TBAB at 130 °C, 11 was obtained in 55% yield. These results
indicate that 14 is a potential intermediate in the conversion of 9
to 11.
Next, dimerization of 9 in ionic liquids was examined (Table
). When 9 was heated with N-methylcyclohexylamine (2.0
equiv) in TBAB (11 equiv) at 130 °C under an aerobic
atmosphere, 1-methylKuQuinone (11) was obtained in 30% yield
2
(
entry 1). In this reaction, decomposition of 9 was accompanied
To elucidate the reaction pathway and mechanism of the
13
by formation of 11, resulting in the low yield of 11. The reaction
proceeded in the absence of N-methylcyclohexylamine (entry 2)
and under an oxygen atmosphere (entry 3). Dimerization of 9 by
TBAB in a refluxing toluene (bp = 110.6 °C) under an oxygen
atmosphere afforded 11 in 17% yield (entry 4). When the
reaction of 9 with TBAB was performed in a refluxing ethanol
formation of 11 from 9, a C-labeled 1-methylKuQuinone 11′
13
was synthesized from 2-(methyl- C)-1,4-naphthoquinone (9′)
Scheme 7). Compound 9′ was prepared by treatment of 1,4-
naphthoquinone (15) with silver nitrate and potassium persulfate
(
13
in the presence of sodium acetate-2- C (Scheme 7A). Treatment
of 9′ with a 5 M aqueous NaOH solution under an oxygen
atmosphere in EtOH gave dimer 10′ in 37% yield. Reduction of
(bp = 78.4 °C), the formation of 11 was not observed (entry 5).
When the reaction was performed in TBAC at 100 °C, no
reaction occurred (entry 6). However, the reaction in TBAC at
the epoxide in 10′ with PPh and iodine afforded 14′. Heating 14′
3
13
in TBAB under an aerobic atmosphere gave C-labeled 1-
13
1
30 °C yielded 11 in 15% yield (entry 7). These results suggest
that the dimerization might proceed at temperatures higher than
10 °C. But, treatment of 9 with TBAB in DMF at 130 °C gave a
complex mixture (entry 8). When 9 was heated at 145 °C in
methylKuQuinone 11′ in 29% yield. Strong C NMR signals
derived from C-5a (or C-6a) and the methyl group at C-12 of 11′
were observed at 125.4 and 14.2 ppm, respectively (Table S1
and Fig. S21 in the Supplementary material). The coupling
1

Taking the results obtained in Schemes 6 and 7 together, the
mechanism for the dimerization of 9′ is proposed in Scheme 8.
Sequential intermolecular and intramolecular Michael reactions
of 9′, followed by oxidation of the resulting hydroquinone II
with molecular oxygen gives 14′ [26,27]. Further oxidation of the
cyclopentene moiety in 14′ affords III. The addition of bromide
ion to the carbonyl group in III and a C−C bond cleavage
reaction affords an acyl bromide IV. The cyclopentadienyl anion
in IV is stabilized by the three carbonyl groups adjacent to the
cyclopentadienyl ring, and acts as a good leaving group to
promote C−C bond cleavage [27-30]. The ring-closing reaction
occurs at the less hindered unsubstituted carbon in the
cyclopentadienyl anion to give 11′.
Scheme 6. Reactions of 10 and 14 in TBAB under an
aerobic atmosphere.
Scheme 8. Proposed Mechanism for the Formation of 11′ from 9′.
1
3
Scheme 7. Synthesis of C-labeled 1-methylKuQuinone 11′
from 1,4-naphthoquinone (15) (A) and via dimerization of
2
In order to prove the existence of the intermediates III and IV
in Scheme 8, compound 14 was treated with NaOMe under an
oxygen atmosphere in MeOH (Scheme 9). This reaction afforded
a cyclopentadienyl anion 16 in 69% yield. This result strongly
supports the existence of the intermediates III and IV. The
formation of 16 from 14 is explained by the oxidative double
bond formation in 14, followed by methanolysis of the resultant
13
-(methyl- C)-1,4-naphthoquinone (9′) (B).
13
constant between the 3 C-labeled carbons in 11′ is 2.8 Hz,
13
indicating a long-range J coupling. When 2-(methyl- C)-1,4-
C-C
naphthoquinone (9′) was heated in TBAB, compound 11′ was
obtained in 27% yield (Scheme 7B).

6
Tetrahedron
intermediate III′. The driving source of the methanolysis
and 100 MHz, respectively), using chloroform-d (CDCl ) or
3
should be the high stability of the cyclopentadienyl anion in 16.
methanol-d (CD OD). Chemical shift values are expressed in δ
4
3
(
ppm) relative to tetramethylsilane (TMS, δ 0.00 ppm) or the
13 1
solvent resonance (CDCl , δ 77.0 ppm for C{ H} NMR;
CD OD, δ 3.30 ppm for H NMR, δ 49.0 ppm for C{ H}
3
1
13
1
3
NMR). Data are reported as follows: chemical shift, multiplicity
(s = singlet, d = doublet, dd = double doublet, m = multiplet),
coupling constants (J; Hz), and integration. Mass spectra were
obtained by a JEOL high-resolution double-focusing mass
spectrometer (JMS-700) using fast atom bombardment (FAB), or
a Bruker Fourier transformation-ion cyclotron resonance (FT-
ICR) mass spectrometer (solariX) using electrospray ionization
(
(
ESI), or an Applied Biosystems hybrid quadrupole time-of-flight
QTOF) mass spectrometer (API QSTAR pulsar i) using ESI.
Compound 1 was prepared from commercially available 9
according to the reported procedure [31].
4
1
.2. General Procedure for Dimerization of 2-Bromo-3-methyl-
,4-naphthoquinone (1) in an Ionic Liquid (Table 1)
A mixture of 1 (75.0 mg, 299 ꢀmol, 1.0 equiv) and TBAB
(
5.00 g, 15.5 mmol, 52 equiv) was heated in an oil bath under an
aerobic atmosphere in the presence of N-methylcyclohexylamine
80.0 ꢀL, 608 ꢀmol, 2.0 equiv) at 120 °C for 45 min. The mixture
Scheme 9. Formation of cyclopentadienyl anion 16 from
compound 14.
(
was diluted with EtOAc and water. After the layers were
separated, the organic layer was washed with a 1 M aqueous HCl
solution, water, and brine, dried over Na SO , and concentrated.
2
4
3
. Conclusion
The residue was purified by silica gel column chromatography
using toluene as the eluent to afford 2 (8.8 mg, 17%) as a pale
brown solid. The H NMR spectrum of 2 was identical with that
reported for 2 [32].
In this study, dimerizations of 2-bromo-3-methyl-1,4-
1
naphthoquinone (1) and 2-methyl-1,4-naphthoquinone (9) in
TBAB, an ionic liquid, were reported. Treatment of 1 with N-
methylcyclohexylamine in TBAB at 120 °C afforded 5,7,12,14-
pentacenetetrone (2) in 17% yield. The yield of 2 is slightly
higher than that previously reported for the dimerization of 1
using EtOH as a solvent (entry 1 in Table 1 versus Scheme 1A).
The presence of N-methylcyclohexylamine is necessary for
formation of 2. The dimerization of 1 also proceed in other ionic
In entry 2, following the general procedure, the reaction of 1
(
50.0 mg, 199 ꢀmol) in the presence of N-
methylcyclohexylamine (52.0 ꢀL, 395 ꢀmol) and TBAB (3.21 g,
.96 mmol) in a refluxing toluene (6 mL) for 60 min gave 2 (2.3
9
mg, 7%). In entry 3, the reaction of 1 (50.0 mg, 199 ꢀmol) was
performed in TBAB (3.21 g, 9.96 mmol) at 120 °C for 300 min.
No significant TLC changes were observed during 300 min. In
entry 4, the reaction of 1 (75.2 mg, 300 ꢀmol) in the presence of
N-methylcyclohexylamine (80.0 ꢀL, 608 ꢀmol) in TBAC (4.17 g,
liquids including TBAC, TBPB, n-Bu P·BF , and [Bmim]Cl to
4
4
give 2. Treatment of 9 with N-methylcyclohexylamine in TBAB
at 130 °C afforded 1-methylKuQuinone (11) in 30% yield. The
present method greatly improves the yield of 11 compared with
the reported method (entry 1 in Table 2 versus Scheme 3).
Furthermore, using the present method, compound 11 was
purified by repeated centrifugation and washing with MeOH. The
reaction mechanism was elucidated by determination of the
1
5.0 mmol) at 100 °C for 60 min gave 2 (4.6 mg, 9%). In entry 5,
the reaction of 1 (75.5 mg, 301 ꢀmol) in the presence of N-
methylcyclohexylamine (80.0 ꢀL, 608 ꢀmol) in TBPB (5.10 g,
1
6
5.0 mmol) at 110 °C for 30 min gave 2 (6.8 mg, 13%). In entry
, the reaction of 1 (75.3 mg, 300 ꢀmol) in the presence of N-
13
reaction intermediates and C labeling experiments using 2-
methylcyclohexylamine (80.0 ꢀL, 608 ꢀmol) in n-Bu P·BF
4
4
13
(
methyl- C)-1,4-naphthoquinone (9′). The present study
(5.19 g, 15.0 mmol) at 110 °C for 30 min gave 2 (5.1 mg, 10%).
demonstrates not only the potential utility of ionic liquids as
solvents for the dimerization of 1,4-naphthoquinones, but also the
potential utility of 1,4-naphthoquinones as versatile intermediates
for the divergent syntheses of numerous 1,4-naphthoquinone
dimers.
In entry 7, the reaction of 1 (75.1 mg, 299 ꢀmol) in the presence
of N-methylcyclohexylamine (80.0 ꢀL, 608 ꢀmol) in [Bmim]Cl
(2.62 g, 15.0 mmol) at 80 °C for 50 min gave 2 (2.1 mg, 4%).
4
1
.3. General Procedure for Dimerization of 2-Bromo-3-methyl-
,4-naphthoquinone (1) in a Conventional Solvent (Scheme 5)
4
. Experimental section
A solution of 1 (75.0 mg, 299 ꢀmol, 1.0 equiv) and N-
4
.1. General Information
methylcyclohexylamine (80.0 ꢀL, 608 ꢀmol, 2.0 equiv) in EtOH
3 mL) was stirred under an aerobic atmosphere at rt for 7 h. The
solution was concentrated. The residue was purified by silica gel
(
All solvents and reagents were used without further
purification unless otherwise noted. Analytical TLC was
performed using Silica gel 60 F254 plates (0.25 mm, normal
phase) (Merck). Flash column chromatography was performed
using Silica gel 60 (particle size 40–63 ꢀm; 230–400 mesh
ASTM) (SiliCycle). Melting point (Mp) data were determined
using a MM-2 instrument (Shimadzu) and uncorrected. IR
spectra were recorded on a FT-720 spectrometer (Horiba), using
column chromatography using toluene as the eluent to afford 2
(3.5 mg, 7%). The H NMR spectrum of 2 was identical with that
reported for 2.
1
In Scheme 5C, following the general procedure, the reaction
of 1 (50.0 mg, 199 ꢀmol) with N-methylcyclohexylamine (52.0
ꢀ
L, 395 mmol) in 2-propanol (3 mL) at rt for 8 h, and
1
13
13
1
purification by silica gel column chromatography using
hexanes/EtOAc (5/1) as the eluent gave 7 (7.2 mg, 17%) and
KBr pellets. H and proton-decoupled C ( C{ H}) NMR
spectra were recorded on a Bruker Avance 400 spectrometer (400

1
recovered 1 (31.9 mg, 64%). The H NMR spectrum of 7 was
organic layer was collected, washed with water and brine, dried
over Na SO , and concentrated. The residue was purified by
identical with that reported for 7 [7].
2
4
silica gel chromatography using hexanes/EtOAc (10/1) as the
eluent to afford 14 (37.8 mg, 79%) as a red brown amorphous
In Scheme 5D, following the general procedure, the reaction
of 1 (50.0 mg, 199 ꢀmol) with N-methylcyclohexylamine (52.0
solid. Mp = 151–152 °C; IR (KBr) ν
= 2929, 1685, 1664,
max
ꢀ
L, 395 mmol) in THF (3 mL) at rt for 12 h, and purification by
−1
1
1
8
3
618, 1591, 1458, 1431 cm ; H NMR (400 MHz, CDCl ) δ
3
silica gel column chromatography using hexanes/EtOAc (5/1) as
the eluent gave 7 (5.9 mg, 12%).
.13–8.04 (m, 4H), 7.81–7.70 (m, 4H), 3.50–3.41 (m, 2H), 3.14–
13
1
.04 (m, 1H), 1.85 (s, 3H); C{ H} NMR (100 MHz, CDCl ) δ
3
4
.4. Methyl 1-hydroxy-2,2-dimethoxy-3-oxo-2,3-dihydro-1H-
197.2, 195.8, 183.2, 182.3, 149.9, 149.8, 135.1, 134.4, 133.9,
133.7, 133.6, 133.2, 132.4, 127.7, 127.1, 126.8, 126.2, 61.8, 57.1,
indene-1-carboxylate (13) (Scheme 5B)
3
3.7, 22.4 (one aromatic carbon signal is overlapped); HRMS
+
Following the general procedure described in section 4.3, the
reaction of 1 (116 mg, 462 ꢀmol) with N-methylcyclohexylamine
162 ꢀL, 1.23 mmol) in methanol (30 mL) at rt for 8 h, and
purification by silica gel column chromatography using
(ESI/QTOF) m/z: [M+Na] Calcd for C H O Na 365.0784;
22 14 4
Found 365.0792.
(
4
.7. Synthesis of 11 from 14
hexanes/EtOAc (5/1~3/1) as the eluent gave 13 (23.6 mg, 19%)
A mixture of 14 (15.0 mg, 43.8 ꢀmol) and TBAB (0.650 g,
2.02 mmol) was heated at 130 °C in an oil bath under an aerobic
as a pale yellow oil. IR (neat) ν = 3477, 3010, 2953, 2845,
max
−1
1
1
728, 1622, 1604, 1464 cm ; H NMR (400 MHz,CDCl ) δ 7.85
3
atmosphere for 6 h. The mixture was diluted with CHCl and
3
(d, J = 7.6 Hz, 1H), 7.71 (td, J = 7.6, 0.8 Hz, 1H), 7.63 (d, J = 7.6
water. After the layers were separated, the organic layer was
Hz, 1H), 7.54 (td, J = 7.6, 0.8 Hz, 1H), 4.01 (s, 1H), 3.69 (s, 3H),
1
3
1
washed with a 3 M aqueous H
SO solution, water and brine,
2 4
3
1
8
.55 (s, 3H), 3.42 (s, 3H); C{ H} NMR (100 MHz, CDCl ) δ
3
dried over Na SO , and concentrated. The residue was diluted
2
4
93.2, 170.7, 150.0, 135.9, 134.0, 130.1, 124.2, 124.0, 102.1+,
with MeOH (3 mL) and centrifuged (3500 rpm, 1 min). After the
supernatant was removed, the precipitate was washed with
MeOH (3 mL). This washing process was repeated five times to
2.1, 53.6, 51.84, 51.77; HRMS (ESI/FT-ICR) m/z: [M+H]
Calcd for C H O 267.0863; Found 267.0864.
13
15
6
1
4
.5. General Procedure for Dimerization of 2-Methyl-1,4-
give 11 (8.1 mg, 54%). The H NMR spectrum of 11 was
naphthoquinone (9) in an Ionic Liquid (Table 2)
identical with that of our authentic sample.
13
4
.8. 2-(Methyl- C)-1,4-naphthoquinone (9′)
A mixture of 9 (99.8 mg, 580 ꢀmol, 1.0 equiv) and TBAB
(
2.10 g, 6.51 mmol, 11 equiv) was heated in an oil bath under an
aerobic atmosphere in the presence of N-methylcyclohexylamine
230 ꢀL, 1.75 mmol, 3.0 equiv) at 130 °C for 1 h. The mixture
A solution of 1,4-naphthoquinone 15 (436 mg, 2.76 mmol),
13
sodium acetate-2- C (156 mg, 1.88 mmol), and AgNO (160 mg,
3
(
0
.941 mmol) in a 1:1 mixture of MeCN and water (14 mL) was
was diluted with EtOAc and water. After the layers were
separated, the organic layer was washed with a 1 M aqueous HCl
solution, water, and brine, dried over Na SO , and concentrated.
degassed. K S O (2.49 g, 9.21 mmol) was added to the mixture
2
2
8
at rt. The mixture was stirred at 60 °C by heating in an oil bath
under an argon atmosphere for 3 h. The resultant mixture was
diluted with EtOAc and water to give a biphasic solution. The
organic layer was collected, washed with water and brine, dried
2
4
The residue was diluted with MeOH and centrifuged (3500 rpm,
min). After the supernatant was removed, the precipitate was
1
washed with MeOH. This washing process was repeated five
times to give 11 (29.6 mg, 30%). The H NMR data of 11 was
identical with that reported for 11 [22]. The C{1H} NMR data
of 11 was in good agreement with that reported for 11, except for
the chemical shift at the methyl group at C-12 [22]. We assigned
1
over Na SO , and concentrated to give a residue. The residue was
2 4
13
purified via silica gel column chromatography using
hexanes/EtOAc (50/1) as the eluent to give 9′ (108 mg, 33%) as a
yellow solid and recovered 15 (124 mg, 28%). Mp = 98–99 °C;
IR (KBr) νmax = 3064, 3047, 2949, 2922, 2852, 1666, 1622, 1593
1
13
1
all signals in the H and C{ H} NMR spectra of 11 through the
−1
1
1
13
cm ; H NMR (400 MHz,CDCl
3
) δ 8.13–8.05 (m, 2H), 7.75–
analysis of the H– C heteronuclear multiple-bond correlation
HMBC) spectrum of 11 (Table S1 in the Supplementary
material).
7
3
1
.71 (m, 2H), 6.86–6.84 (m, 1H), 2.20 (dd, J = 129.3, 1.5 Hz,
(
13
1
H); C{ H} NMR (100 MHz, CDCl ) δ 135.7, 133.7, 133.6,
3
32.3, 133.2, 126.5, 126.1, 16.5 (The signals derived from other
1
3
In entry 2, following the general procedure, the reaction of 9
50.2 mg, 292 ꢀmol) in TBAB (1.02 g, 3.16 mmol) under an
carbons were not observed, because the signal of the C-labeled
carbon was greatly enhanced); HRMS (FAB/double-focusing
(
+
12
13
aerobic atmosphere at 130 °C for 9 h in the absence of N-
methylcyclohexylamine gave 11 (10.9 mg, 22%). In entry 3, the
reaction of 9 (152.4 mg, 885 ꢀmol) in TBAB (3.00 g, 9.30 mmol)
under an oxygen atmosphere at 130 °C for 7 h gave 11 (28.8 mg,
MS) m/z: [M+H] Calcd for C10 CH O2 174.0636; Found
9
174.0637.
13
4
.9. 11a,12-Dihydro-5b-(methyl- C)-5a,12a-epoxy-11H-
13
dibenzo[b,h]fluorene-5,6,11,13(5bH)-tetrone-12- C (10′)
1
9%). In entry 4, the reaction of 9 (50.6 mg, 294 ꢀmol) in TBAB
935 mg, 2.90 mmol) in a refluxing toluene under an oxygen
atmosphere at 130 °C for 6.5 h gave 11 (8.4 mg, 17%). In entry
, the reaction of 9 (50.0 mg, 290 ꢀmol) in TBAC (806 mg, 2.90
mmol) under an oxygen atmosphere at 130 °C for 7 h gave 11
7.6 mg, 15%).
(
A 5.0 M aqueous NaOH solution (1.00 mL, 5.00 mmol) in
ethanol (20 mL) was bubbled by oxygen gas at −78 °C.
Compound 9′ (100 mg, 0.577 mmol) was added to the solution at
5
−
78 °C, and the mixture was stirred under an oxygen atmosphere.
(
The reaction was warmed up to rt, and the mixture was stirred at
rt for 30 min. The reaction was quenched by the addition of
water, and the resulting mixture was diluted with EtOAc. The
layers were separated, and the aqueous layer was extracted with
EtOAc three times. The combined organic layer was washed with
water and brine, dried over Na SO , and concentrated. The
4
5
.6. 5a-Methyl-12,12a-dihydro-5H-dibenzo[b,h]fluorene-
,6,11,13(5aH)-tetraone (14)
Iodine (72.9 mg, 0.574 mmol) and PPh3 (148 mg, 0.562
mmol) were added to a solution of 10 [9] (50.5 mg, 0.141 mmol)
in MeCN (5 mL) at rt. The mixture was refluxed by heating in an
oil bath for 4.5 h. The reaction was quenched by the addition of
water, and the resulting mixture was diluted with EtOAc. The
2
4
residue was purified via silica gel chromatography using
hexanes/EtOAc (10/1) as the eluent to afford 10′ (37.9 mg, 37%)
as a white solid. Mp = 194–195 °C; IR (KBr) νmax = 3066, 3014,

8
Tetrahedron
3
−1
1
2
941, 1697, 1593, 1464 cm ; H NMR (400 MHz, CDCl ) δ
MeOH (3 mL). This washing process was repeated fourteen
1
8
7
7
1
1
.17–8.15 (m, 1H), 8.10–8.07 (m, 1H), 7.97–7.92 (m, 2H), 7.76–
.68 (m, 4H), 3.63 (dd, J = 129.9, 14.8 Hz, 1H), 2.95 (dd, J = 7.8,
.8 Hz, 1H), 2.81 (ddd, J = 129.9, 14.8, 7.8 Hz, 1H), 1.69 (d, J =
29.9 Hz, 3H); C{ H} NMR (100 MHz, CDCl ) δ 134.6, 134.5,
34.2, 127.52, 127.47, 127.1, 127.0, 29.0, 18.0 (The signals
times to give 11′ (13.4 mg, 27%). The H NMR spectrum of 11′
was identical with that of our authentic sample.
13
1
4.13. Sodium 2-(2-(methoxycarbonyl)benzoyl)-3-methyl-4,9-
dioxo-4,9-dihydro-1H-cyclopenta[b]naphthalenide (16)
3
derived from other carbons were not observed, because the
A 0.5 M NaOMe solution in MeOH (5.00 mL, 2.50 mmol)
was added to 14 (10.0 mg, 29.2 ꢀmol). The resultant mixture was
stirred at rt under an oxygen atmosphere for 30 min. The reaction
was quenched by the addition of water, and the resulting mixture
was diluted with EtOAc. The aqueous layer was extracted with
EtOAc. The combined organic layer was collected, washed with
water and brine, dried over Na SO , and concentrated. The
13
signals of the C-labeled carbons were greatly enhanced);
+
HRMS (FAB/double-focusing MS) m/z: [M+H] Calcd for
12
13
C20 C H O 361.0987; Found 361.0984.
2
15
5
1
3
4
5
.10. 5a-(Methyl- C)-12,12a-dihydro-5H-dibenzo[b,h]fluorene-
13
,6,11,13(5aH)-tetraone-12- C (14′)
2
4
Iodine (37.0 mg, 0.291 mmol) and PPh (75.9 mg, 0.289
residue was purified via silica gel chromatography using
hexanes/EtOAc (2/1) and EtOAc/MeOH (10/1) as the eluents to
afford 16 (8.0 mg, 69%) as an orange amorphous solid. Mp = 180
°C (decomposition); IR (KBr) νmax = 2924, 1722, 1711, 1579,
3
mmol) were added to a solution of 10′ (19.7 mg, 54.7 ꢀmol) in
MeCN (3 mL) at rt. The mixture was refluxed by heating in an
oil bath for 19 h. The reaction was quenched by the addition of
water, and the resulting mixture was diluted with EtOAc. The
organic layer was collected, washed with water and brine, dried
over Na SO , and concentrated. The residue was purified via
−1
1
1550, 1493, 1483, 1444, 1427, 1385 cm ; H NMR (400 MHz,
CD OD) δ 8.06 (d, J = 7.4 Hz,1H), 7.99 (d, J = 7.1 Hz, 1H), 7.93
3
(d, J = 7.7 Hz, 1H), 7.63 (dd, J = 7.7, 7.4 Hz, 1H), 7.55–7.50 (m,
3H), 7.44 (d, J = 7.5 Hz, 1H), 6.71 (s, 1H), 3.69 (s, 3H), 2.81 (s,
2
4
silica gel chromatography using hexanes/EtOAc (10/1) as the
eluent to afford 14′ (13.3 mg, 70%) as a pale brown amorphous
13
1
3H); C{ H} NMR (100 MHz, CD OD) δ 196.7, 182.5, 180.8,
3
solid. IR (KBr) ν = 2956, 2925, 2854, 1685, 1666, 1618, 1593,
169.0, 145.9, 142.0, 139.5, 138.5, 132.9, 132.7, 132.5, 130.9,
130.7, 130.5, 129.8, 129.1, 128.0, 127.1, 126.9, 125.3, 125.1,
52.7, 15.5; HRMS (FAB/double-focusing MS) m/z: [M−Na]
max
−1
1
1
7
456 cm ; H NMR (400 MHz, CDCl ) δ 8.13–8.04 (m, 4H),
.81–7.70 (m, 4H), 3.48 (ddd, J = 130.6, 18.2, 8.6 Hz, 1H), 3.43
3
–
(
m, 1H), 3.06 (ddd, J = 130.6, 18.2, 8.2 Hz, 1H), 1.85 (d, J =
Calcd for C H O 371.0921; Found 371.0920, and m/z:
23
15
5
13
1
+
1
1
2
30.6 Hz, 3H); C{ H} NMR (100 MHz, CDCl ) δ 135.1, 134.4,
[M+Na] Calcd for C H O Na 417.0715; Found 417.0719.
3
23 15 5 2
33.9, 133.6, 127.7, 127.1, 126.7, 126.2, 33.7 (d, J = 1.1 Hz),
2.4 (d, J = 1.1 Hz) (The unassigned signal at 127.3 ppm was
Acknowledgments
also observed. The signals derived from other carbons were not
1
3
observed, because the signals of the C-labeled carbons were
This study was supported by Japan Society for the Promotion
of Science (JSPS) KAKENHI (No. 18K19185) and the
Sumitomo Foundation (No. 170183) to K.K.
greatly enhanced); HRMS (FAB/double-focusing MS) m/z:
M+H] Calcd for C20 C H O 345.1037; Found 345.1037.
+
12
13
[
2 15 4
13
4
5
.11. 6-Hydroxy-12-(methyl- C)-5H-dibenzo[b,h]fluorene-
,11,13-trione-5a- C (11′)
13
References
1
2
.
.
For a review on total syntheses of natural dimeric
naphthoquinones, see: S. Kamo, K. Kuramochi, K. Tsubaki,
Tetrahedron Lett. 59 (2018) 224–230.
R. H. Thomson, Naturally Occurring Quinones IV. Blackie
Academic & Professional, London, 1997.
A mixture of 14′ (14.4 mg, 41.8 ꢀmol) in TBAB (703 mg,
.18 mmol) was heated at 130 °C in an oil bath under an aerobic
2
atmosphere for 6 h. The mixture was diluted with EtOAc and
water. After the layers were separated, the organic layer was
washed with a 3 M aqueous H SO solution, water and brine,
2
4
3. Y. Kumagai, Y. Shinkai, T. Murata, A. K. Cho, Annu. Rev.
Pharmacol. Toxicol. 52 (2012) 221–247.
4
dried over Na SO , and concentrated. The residue was diluted
2
4
.
I. Baxter, D. W. Cameron, R. B. Titman, J. Chem. Soc. C (1971)
253–1256.
with MeOH (3 mL) and centrifuged (4000 rpm, 1 min). After the
supernatant was removed, the precipitate was washed with
MeOH (3 mL). This washing process was repeated fifteen times
to give 11′ (4.1 mg, 29%) as a red purple solid. Mp = >250 °C;
1
5
.
S. Azuma, K. Nishio, K. Kubo, T. Sasamori, N. Tokitoh, K.
Kuramochi, K. Tsubaki, J. Org. Chem. 77 (2012) 4812−4820.
6. S. Azuma, M. Ota, A. Ishida, K. Isozaki, H. Takaya, M. Nakamura,
T. Sasamori, N. Tokitoh, K. Kuramochi, K. Tsubaki, Chem. Lett.
42 (2013) 1531−1533.
7. M. Ota, T. Sasamori, N. Tokitoh, T. Onodera, Y. Mizushina, K.
Kuramochi, K. Tsubaki, J. Org. Chem. 80 (2015) 5687−5695.
8. N. Vets, H. Diliёn, S. Toppet, W. Dehaen, Synlett 2006,
1359−1362.
−1
IR (KBr) ν =3443, 3068, 3030, 2922, 1666, 1622, 1593 cm ;
max
1
H NMR (400 MHz,CDCl ) δ 18.09 (s, 1H), 8.28–8.24 (m, 4H),
3
13
1
8
(
.28–7.71 (m, 4H), 2.96 (d, J = 129.48 Hz, 3H); C{ H} NMR
100 MHz, CDCl ) δ 135.0, 133.0, 127.8, 127.2, 125.4 (d, J = 2.8
3
Hz), 14.2 (d, J = 2.8 Hz) (The unassigned signal at 127.4 ppm
was also observed. The signals derived from other carbons were
not observed, because the signals of C-labeled carbons were
9. S. Maruo, K. Nishio, T. Sasamori, N. Tokitoh, K. Kuramochi, K.
Tsubaki, Org. Lett. 15 (2013) 1556−1559.
13
1
0. P. Wasserscheid, W. Keim, Angew. Chem. Int. Ed. 39 (2000)
772–3789.
11. T. Welton, Chem. Rev. 99 (1999) 2071–2083.
greatly enhanced); HRMS (FAB/double-focusing MS) m/z: [M–
3
–
12
13
H] Calcd for C20 C H O 341.0724; Found 341.0722.
2
11
4
1
1
2. J. P. Hallett, T. Welton, Chem. Rev. 111 (2011) 3508–3576.
3. M. Yao, H. Senoh, T. Sakai, T. Kiyobayashi, Int. J. Electrochem.
Sci. 6 (2011) 2905–2911.
4. M. Yao, S. Yamazaki, H. Senoh, T. Sakai, T. Kiyobayashi, Mater.
Sci. Eng. B. 177 (2012) 483–487.
4
.12. Synthesis of 11′ from 9′
A mixture of 9′ (49.8 mg, 0.288 mmol) in TBAB (0.93 g, 2.89
1
mmol) was heated at 130 °C in an oil bath under an aerobic
atmosphere for 9 h. The mixture was diluted with EtOAc and
water. After the layers were separated, the organic layer was
washed with a 3 M aqueous H SO solution, water and brine,
dried over Na SO , and concentrated. The residue was diluted
with MeOH (4 mL) and centrifuged (3500 rpm, 2 min). After the
supernatant was removed, the precipitate was washed with
15. M. Yao, H. Ando, T. Kiyobayashi, Energy Procedia 89 (2016)
222–230.
1
6. L. Miao, L. Liu, Z. Shang, Y. Li, Y. Lu, F. Cheng, J. Cheng, Phys.
Chem. Chem. Phys. 20 (2018) 13478–13484.
7. D. R. Maulding, B. G. Roberts, J. Org. Chem. 34 (1969) 1734–
2
4
2
4
1
1
736.

1
1
8. I. Kaur, W. Jia, R. P. Kopreski, S. Selvarasha, M. R. Dokmeci, C.
Pramanik, N. E. McGruer, G. P. Miller, J. Am. Chem. Soc. 130
27. H. Lessmann, R. P. Maskey, S. Fotso, H. Lackner, H. Laatsch, Z.
Naturforsch. 60b (2005) 189–199.
28. H. Lessmann, J. Krupa, H. Lackner, K. Schmidt-Bäse, G. M.
Sheldrick, Z. Naturforsch. 48b (1993) 672–682.
29. S. Kamo, K. Kuramochi, K. Tsubaki, Org. Lett. 20(2018) 1082–
1085.
(
2008) 16274–16286.
9. J. Nishida, Y. Fujiwara, Y. Yamashita, Org. Lett. 11 (2009) 1813–
816.
1
2
2
0. E. A. Younes, Y. Zhao, Org. Chem. Front. 4 (2017) 804–810.
1. F. Glӧcklhofer, A. Petritz, E. Karner, M. J. Bojdys, B. Stadlober, J.
Frӧhlich, M. M. Unterlass, J. Mater. Chem. C. 5 (2017) 2603–
30. Y. Xu, X. Qi, P. Zheng, C. C. Berti, P. Liu, G. Dong, Nature 567
(2019) 373–378.
2
610.
31. R. Adams, T. A. Geissman, B. R. Baker, H. M. Teeter, J. Am.
Chem. Soc. 63 (1941) 528–534.
32. P. Costa, M. Linhares, S. L. H. Rebelo, M. G. P. M. S. Neves, C.
Freire, RSC Adv. 3 (2013) 5350–5353.
2
2
2
2
2. A. Coletti, S. Lentini, V. Conte, B. Floris, O. Bortolini, F. Sforza,
F. Grepioni, P. Galloni, J. Org. Chem. 77 (2012) 6873−6879.
3. F. Sabuzi, V. Armuzza, V. Conte, B. Floris, M. Venanzi, P.
Galloni, E. Gatto, J. Mater. Chem. C. 4 (2016) 622−629.
4. M. Bonomo, F. Sabuzi, A. Di Carlo, V. Conte, D. Dini, P. Galloni,
New J. Chem. 41 (2017) 2769−2779.
Appendix A. Supplementary Material
5. F. Sabuzi, S. Lentini, F. Sforza, S. Pezzola, S. Fratelli, O.
Bortolini, B. Floris, V. Conte, P. Galloni, J. Org. Chem. 82 (2017)
Supplementary data to this article can be found online at
https://.
1
0129−10138.
6. A. V. B. Sankaram, A. S. Rao, J. N. Shoolery, Tetrahedron 35
1979) 1777–1782.
2
(

Highlights
·
·
·
·
Dimerizations of 3-methyl-1,4-naphthoquinones in TBAB were developed.
The use of TBAB as a solvent improved the yield of 5,7,12,14-pentacenetetrone.
The use of TBAB as a solvent improved the yield of 1-methylKuQuinone.
The reaction mechanisms for the dimerizations were elucidated.

Declaration of interests
☒
The authors declare that they have no known competing financial interests or personal relationships
that could have appeared to influence the work reported in this paper.
☐
The authors declare the following financial interests/personal relationships which may be considered
as potential competing interests: