Bidirectional Synthesis, Photophysical and Electrochemical Characterization of Polycyclic Quinones Using Benzocyclobutenes and Benzodicyclobutenes as Precursors

Article abstract of DOI:10.1002/ejoc.202100848

Quinones have widespread applications in view of their interesting chemical and photophysical features. On the other hand, benzocyclobutenes (BCBs) are generally masked reactive dienes suitable for the [4+2] cycloaddition reactions. Here, benzocyclobutenes and benzodicyclobutenes (BDCBs) were prepared and further reacted with benzoquinone and naphthoquinone in order to obtain some new polycyclic quinones with highly extended π systems, namely, 6-bromo-5,8-dimethoxyanthracene-1,4-dione, 2,9-dibromo-1,4,8,11-tetramethoxypentacene-6,13-dione, 9-bromo-7,10-dimethoxytetracene-5,12-dione, 3,10-dimethoxycyclobuta[b]anthracene-1,5,8(2H)-trione, 6,10,17,21-tetramethoxynonacene-1,4,8,12,15,19-hexaone, and 3,12-dimethoxycyclobuta[b]tetracene-1,5,10(2H)-trione. In addition to their spectroscopic characterization the new compounds are investigated by UV and fluorescence spectroscopy, cyclic voltammetry, and DFT calculations.

Full text of DOI:10.1002/ejoc.202100848

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Bidirectional Synthesis, Photophysical and Electrochemical
Characterization of Polycyclic Quinones Using
Benzocyclobutenes and Benzodicyclobutenes as Precursors
[a, b]
[b]
Amr Mohamed Abdelmoniem,
Holger Butenschön*
Ismail Abdelshafy Abdelhamid, and
[a]
In Memory of Professor Klaus Hafner
Quinones have widespread applications in view of their
interesting chemical and photophysical features. On the other
hand, benzocyclobutenes (BCBs) are generally masked reactive
dienes suitable for the [4+2] cycloaddition reactions. Here,
benzocyclobutenes and benzodicyclobutenes (BDCBs) were
prepared and further reacted with benzoquinone and naphtho-
quinone in order to obtain some new polycyclic quinones with
oxyanthracene-1,4-dione,
2,9-dibromo-1,4,8,11-tetrameth-
oxypentacene-6,13-dione, 9-bromo-7,10-dimethoxytetracene-
5,12-dione, 3,10-dimethoxycyclobuta[b]anthracene-1,5,8(2H)-tri-
one, 6,10,17,21-tetramethoxynonacene-1,4,8,12,15,19-hexaone,
and 3,12-dimethoxycyclobuta[b]tetracene-1,5,10(2H)-trione. In
addition to their spectroscopic characterization the new
compounds are investigated by UV and fluorescence spectro-
scopy, cyclic voltammetry, and DFT calculations.
highly extended
π systems, namely, 6-bromo-5,8-dimeth-
Introduction
The Diels-Alder cycloaddition is one of the most important
strategies for the regio- and stereoselective synthesis of organic
[1–7]
homo- and hetero-polycycles.
Over the years, the reaction
has proven to be a powerful tool for the selective synthesis of
[8–12]
complex natural products and biologically active molecules.
Quinones were found to be typical electron poor dienophiles in
[13–17]
cycloaddition reactions.
In addition, quinone structures are
widely abundant in redox-active natural compounds associated
with a number of biological processes like photosynthesis in
[
18–20]
[21–24]
plants and bacteria,
coenzyme Q functions,
vitamins
[25–29]
[30–32]
[33–34]
K
and E
activities. They show marked anti-oxidant,
[
35–37]
[38–41]
[42–45]
anti-inflammatory,
antibiotic,
and antitumoral
ac-
tivities as well. Some FDA-approved quinone drugs are depicted
in Figure 1.
[a] Dr. A. Mohamed Abdelmoniem, Prof. Dr. H. Butenschön
Institut für Organische Chemie
Leibniz Universität Hannover
Schneiderberg 1B, 30167 Hannover, Germany
E-mail: holger.butenschoen@mbox.oci.uni-hannover.de
https://www.oci.uni-hannover.de/de/arbeitsgruppen/ag-butenschoen/
b] Dr. A. Mohamed Abdelmoniem, Prof. Dr. I. Abdelshafy Abdelhamid
Department of Chemistry
Figure 1. Some FDA-approved quinone drugs.
[
Faculty of Science, Cairo University
Generally, several approaches have been elaborated for the
synthesis of extended quinones. Among them, the bidirectional
synthesis of extended quinones seems to be particularly
attractive. Figure 2 summarizes some selected important bidir-
ectional syntheses of quinones, including, A: the condensation
12613 Giza, A. R. Egypt
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/ejoc.202100848
Part of the “Special Collection in Memory of Klaus Hafner”.
©
2021 The Authors. European Journal of Organic Chemistry published by
Wiley-VCH GmbH. This is an open access article under the terms of the
Creative Commons Attribution Non-Commercial NoDerivs License, which
permits use and distribution in any medium, provided the original work is
properly cited, the use is non-commercial and no modifications or adap-
tations are made.
reaction of aromatic o-dicarboxaldehydes with 1,4-
[46–49]
cyclohexanedione,
with a quinone,
B: the 1,4-cycloadditions of bis-dienes
and C: the cycloaddition of aromatic
[50–52]
quinones with in situ formed dienes from tetra- or hexa
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Figure 3. Benzocyclobutene as precursor of o-xylylene in the Diels-Alder
reaction with a cis-dienophile followed by oxidation to the acene.
impeding the possible formation of aromatized products.
However, the use of an internal oxidant appears not to be
practical as it may react with the sensitive BCB ring. In this
context, here we report on the reactivity of BCBs and BDCBs
towards benzoquinone and naphthoquinone, which serve as
both, dienophile and internal oxidant.
Results and Discussion
Synthesis and characterization
The synthetic procedures of starting materials BCB 18, BDCBs
1
9 and 20 are depicted in Scheme 2. 1,4-Dibromo-2,5-dimeth-
oxybenzene (12) was reacted with an excess of ketene dimeth-
ylacetal (13) (5 equiv.) in the presence of sodium amide
(
4 equiv.) in THF at reflux for 24 h according to our previously
[60]
reported procedure. After a careful chromatographic separa-
tion, three main products 1, 14, and 6 were obtained in 5%,
Figure 2. Some bidirectional synthetic approaches to extended quinones.
5
5% and 25% yield, respectively, besides other byproducts
(16%). To circumvent the poor yield of 1, this compound was
prepared under modified reaction conditions by reaction of 12
with ketene dimethylacetal (3) (1.25 equiv.) in presence of
sodium amide (1 equiv.). In this case, compound 1 was obtained
in 55% yield. BCB 18 was prepared from BCB 1 via hydrolysis to
(
bromomethyl)benzene to form linear or star-shaped
[53–55]
quinones.
On the other hand, BCBs can be considered as masked
[60]
thermally stabilized o-xylylenes via in situ electrocyclic ring
opening (Figure 3).
In a previous work,
methylmaleimide forming some interesting benzo[1,2-f:4,5-f’]
diisoindoles was studied (Scheme 1). The reaction of BCB 1 with
N-methylmaleimide (2) afforded the desired benzo[f]isoindole-
BCB 15
followed by reduction with sodium borohydride.
[56–58]
Similarly, BDCDs 19 and 20 were synthesized starting from
[59]
[60]
[60]
the reactivity of BCBs towards N-
BDCBs 14 and 6 via hydrolysis to 16 and 17 respectively,
followed by reduction. The chemical constitutions of BCB 1,
BDCBs 19, and 20 were confirmed spectroscopically. Surpris-
1
ingly, H NMR spectroscopy revealed that BDCB 19 appeared as
1
,3(2H)-dione 5 in low yield (15%) besides more than 60% yield
an inseparable mixture of diastereomers while BDCB 20
furnished two distinct diastereomers of different solubilities:
20a (partially insoluble in chloroform) and 20b (soluble in
chloroform).
This method has the advantage of obtaining BCBs and
BDCBs in good yields. However, the difficult availability of
ketene precursor 3 and the tedious chromatographic purifica-
tion are problematic. Adopting a similar procedure to that
of unaromatized products 3 and 4. In case of the reaction of
benzodicyclobutene (BDCB) 6 with N-methylmaleimide (2), the
targeted benzo[1,2-f:4,5-f’]diisoindole derivative 11 could only
be isolated in 10% yield along with the formation of other
monoaddition and unaromatized dicycloaddition products 7–
1
0 in a total yield of 68%. The main reason for the low yields of
the desired products clearly is the lack of an internal oxidant
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[
59]
Scheme 1. The reaction of BCB 1 and BDCB 6 with N-methylmaleimide (2).
Scheme 2. Syntheses of starting materials BCB 18, BDCBs 19 and 20.
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[
61]
reported by Dong et al.
we managed to obtain BCB 18
The origin of regioselectivity can roughly be rationalized as
the dipole moments of both CÀ OMe cancel each other while
that of CÀ Br (-I inductive effect) is not cancelled. Thus, a partial
positive charge is generally generated at the carbon in the para
position, while a partial negative charge originates at the meta
carbon atom (Figure 4A). This is further evidenced by DFT
calculations, which show that the more nucleophilic terminus of
the C3-C4 bond is the carbon atom C3 at the meta position to
Br atom. The optimized structure of the benzyne intermediate
directly in a single operation by the action of butyllithium and
lithium 2,2,6,6-tetramethylpiperidide (LiTMP) on a solution of 12
In this reaction, the cycloreversion of
lithiated THF 21 dissociates ethene (22) giving the intermediate
enolate 23. The subsequent [2+2] cycloaddition reaction of 23
3 equiv.) with dibromo compound 12 (1.0 equiv.) induced by
LiTMP (1.5 equiv.) furnishes 24, which is then hydrolyzed to
form BCB 18 in 61% yield.
[62]
in THF (Scheme 3).
(
(Figure 4B) appears as a resonance contributor, in which the
electron density at C3-C4 is delocalized across the ring affecting
the electron densities at C2-C3 and C4-C5 as anticipated from
their bond lengths (Table 1). The natural electronic configura-
tion of valence orbitals of carbon atoms of the ring explains the
2
idealized sp hybridization of all carbon atoms. The nucleophilic
addition preferably occurs at the more distorted carbon atom
C4 (angle C3-C4-C5=129.5°) according to the aryne distortion
[63]
model.
The cycloaddition reaction of BCB 18 with BQ (25) was then
investigated. Two products were successfully isolated in pure
form after chromatographic separation: 6-Bromo-5,8-dimeth-
oxyanthracene-1,4-dione (26), which results from the single
cycloaddition of BCB 18 to one double bond of BQ (25), and
either 2,9- or 2,10-dibromo-1,4,8,11-tetramethoxypentacene-
6
,13-dione, formed via the syn or the anti twofold cycloaddition
of two equivalents of BCB 18 to either one of the double bonds
of BQ (25), respectively (Scheme 4).
This reaction was examined in several solvents under
different reaction conditions of conventional and microwave
Scheme 3. An alternative method for the synthesis of BCB 18.
(μW) heating (Table 2). The best conditions found were the
conventional heating in toluene at 120°C in a sealed tube for
2
4–48 h and the microwave heating in toluene in a sealed tube
Figure 4. Regioselectivity of aryne [2+2] cycloaddition. A) The -I effect of Br
on the triple bond. B) The optimized calculated structure of the benzyne
intermediate at B3LYP/6-31+ +(d,p) level of theory.
Table 1.
Atom
NPA
charge
Natural
electron configuration
Idealized
Hybridization
2
0.97
3.07
0.01
0.01
2
C6
C5
C4
C3
C2
C1
À 0.051
0.052
0.244
À 0.331
À 0.160
0.226
1 s 2 s 2p 4 s 4p
sp
2
0.95
2.98
0.01
0.01
2
1 s 2 s 2p 4p
sp
2
0.84
2.89
0.02
2
1 s 2 s 2p 3d 4p
sp
2
0.97
3.33
0.01
0.02
0.01
2
1 s 2 s 2p 4p
sp
2
0.97
3.17
2
1 s 2 s 2p 3p
sp
2
0.84
2.90
0.01
0.01
2
1 s 2 s 2p 3 s 3d 4p
sp
Bond lengths [Å]
Bond angles [°]
C5-C6
C2-C3
C4-C5
C5-C6
C1-C6
C1-C2
1.25
1.39
1.39
1.41
1.41
1.41
C1-C6-C5
C4-C5-C6
C3-C4-C5
C2-C3-C4
C1-C2-C3
C2-C1-C6
125.6
129.5
109.4
121.9
123.9
110.1
Scheme 4. The reaction of BCB 18 with 1,4-benzoquinone (25). Reaction
conditions: BCB 18 (1 mmol), BQ (25) (4.6 mmol), toluene, 120°C, sealed
tube, 48 h, (26: 38%, 27: 22%) or: μW, 140°C, 270 W, 1 h, (26: 52%, 27: 26%).
Reduction of 27: 27 (0.017 mmol), NaBH
38%).
4
(0.105 mmol), EtOH, 25°C, 3 h, (28:
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[
a]
Table 2. Optimization of the reaction of BCB 18 with 1,4-benzoquinone (25).
[
b]
[c]
[c]
Entry
Solvent
Conditions
T [°C]
t [h]
Yield
2
Yield
6 [%]
27 [%]
1
2
3
4
5
6
7
8
9
THF
Benzene
Toluene
Toluene
1,2-dichlorobenzene
1,2-dichlorobenzene
Nitrobenzene
Nitrobenzene
Toluene
Toluene
Toluene
reflux
reflux
60
80
8
8
–
–
–
–
–
22
6
17
3
18
sealed tube, conventional heating
sealed tube, conventional heating
sealed tube, conventional heating
sealed tube, conventional heating
sealed tube, conventional heating
sealed tube, conventional heating
sealed tube, μW, 270 W
sealed tube, μW, 270 W
sealed tube, μW, 270 W
sealed tube, μW, 270 W
sealed tube, μW, 270 W
120
120
140
140
180
180
140
140
140
140
140
160
190
24
48
24
48
24
48
0.25
0.5
0.75
1
49
38
45
35
27
35
15
24
43
52 (54)
47
48 (49)
38 (40)
–
7
10
11
12
13
14
15
16
26 (24)
22
24 (23)
23 (21)
[
d]
[d]
Toluene
Toluene
1,2-dichlorobenzene
Nitrobenzene
1.5
1
1
[
[
d]
d]
[d]
[d]
sealed tube, μW, 270 W
sealed tube, μW, 270 W
[a] The molar ratio of reactants: BCB 14 (1 mmol), 1,4-BQ (25, 4.6 mmol). [b] Heating bath temperature. [c] Isolated yield. [d] added LiCl (40 mol%).
at 140°C for 1 h. Increasing the temperature further in other
solvents like nitrobenzene or 1,2-dichlorobenzene did not lead
to a noticeable increase in yield. Instead, an unidentified charry
substance was obtained probably due to thermal decomposi-
tion of products. Also, changing the microwave irradiation time
from 0.25 h to 1.5 h (Entries 9–13) was tried, and the best time
was 1 h. Addition of LiCl (40 mol%) led only to a slight increase
in the yield of the mono-cycloaddition product 26 under
microwave (μW) irradiation.
the carbonyl carbon atoms would appear if the bromo
substituents were located at the 2,9 positions, while two signal
would be expected they were located at the 2,10 positions.
1
3
Unfortunately, a well-resolved C NMR spectrum could not be
obtained due to the low solubility of compound 27 in almost all
available deuterated solvents (CDCl , acetone-d , DMSO-d )
3
6
6
even at elevated temperature (40°C). Attempts to obtain a
single crystal for compound 27 were also in vain. Therefore, the
reduction of compound 27 was tried with sodium borohydride
in order to get a reduction product 28 of a better solubility. The
Changing the molar ratio of BCB 18 to 1,4-benzoquinone
1
(25) in a gradient from 2.5:1 to 0.75:1 was tried. A gradual
H NMR spectrum did not show the peak of the hydroxy
increase in the yield of the monocycloaddition product 26 was
observed until its maximum at the ratio 1:1 (Table 3).
protons and showed only one signal integrated for 12 protons
for the methoxy groups. However, the reduction of the carbonyl
groups in 27 was confirmed by the IR and C NMR spectra,
1
3
2
6 was characterized spectroscopically. The mass spectrum
+
+
of 26 displayed peaks (1:1) at m/z=346 [M] and 348 [M+2]
which indicated the absence of carbonyl groups. In addition,
1
3
indicating the presence of one Br atom, in addition to a base
peak at m/z=331, 333 corresponding to the fragmentation of a
methyl group. The IR spectrum revealed a characteristic band at
the C NMR spectrum displayed two different signals for
methoxy groups and one signal for C-OH at δ=153.6 ppm, and
thus it is in accord with a 2,9-dibromo substitution in 28 as well
as in 27.
À 1
1
v˜=1671 cm for the C=O stretching vibration. The H as well
1
3
as the C NMR spectrum are in full accord with the constitution
of 26. On the other hand, the presence of two bromine atoms
in 27 is verified from the isotopic molecular ion cluster at m/z=
Generally, the reaction is greatly facilitated if conducted in a
sealed tube at a temperature higher than toluene reflux
temperature at normal pressure. A plausible mechanism hence
involves the ring-opening of the cyclobutene ring in BCB 18 to
the o-xylylene 29, which undergoes [4+2] cycloaddition
reaction with BQ (25) to form the adduct 30, which eliminates
water and is further oxidized to form the mono-cycloaddition
product 26. Anthra-1,4-quinone 26 can similarly undergo a
subsequent [4+2] cycloaddition reaction with another o-
xylylene intermediate 29 giving the pentacene-6,13-dione 27
(Scheme 5). The oxidant seems to be either oxygen from the air
or benzoquinone itself. Use of freshly distilled toluene under
argon and purging the reaction mixture with argon before
conventional or microwave heating did not affect the fully
aromatized nature of products 26 and 27. This, in addition to
the presence hydroquinone impurities during chromatographic
separation, suggests that the oxidizing agent is benzoquinone.
It is noteworthy that anthracene-1,4-dione is a common
fragment in various quinoid natural products such as rufooliva-
5
84, 586 and 588 (1:2:1). The IR spectrum showed a character-
À 1
istic C=O stretching band at v˜=1673 cm . However, the only
available spectroscopic tool to identify whether the bromo
substituents are located at the 2,9 or at the 2,10 positions in
1
3
compound 27 was C NMR spectroscopy, where one signal for
Table 3. Effect of the molar ratio of reactants (BCB 18 : BQ 25) on the yield
of the monocycloaddition product 26.
Yield [%]
Conventional heating
Entry
BCB : BQ
Microwave
(
120°C, 48 h)
(140°C, 1 h)
2
2:1
1.5:1
1:1
0.75:1
.5:1
49
51
56
64
57
52
54
57
66
58
2
3
4
5
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It should be noted that tetracene derivatives possess
interesting optoelectronic properties, e.g. as organic light-
[71–73]
emitting field-effect transistors (OLETs).
Tetracene-5,12-
dione is the oxidized form of the core structure of anthracycline
[74–75]
antibiotics
such as doxorubicin and daunorubicin, which
were clinically proven to be effective antitumor reagents
against acute leukemia, breast carcinomas, Hodgkin’s disease,
[76–77]
sarcomas and lymphomas.
Encouraged by these results, we tried conducting this
reaction scheme using BDCBs 19 and 20 aiming at higher
acenes. Thus, the cycloaddition reaction of either BDCB 19 or
2
0 with benzoquinone (25) was carried out under conventional
heating as well as under microwave irradiation conditions and
was anticipated to finally yield pentacene-1,4,9,12-tetraone (33).
A careful chromatographic separation of the crude product
afforded different fractions. Besides the residual fractions of BQ,
[
50]
and BDCBs and their oxidized products (ca. 10% yield), two
interesting fractions could be separated and characterized.
However, none of the isolated fractions corresponds to the
target product 33.
The main fraction after isolation and purification was
characterized spectroscopically as cyclobuta[b]anthracene-
1
,5,8(2H)-trione derivative 36. The IR spectrum clearly showed
the absence of hydroxy groups and presence of two close C=O
À 1
1
stretching bands at v˜=1658, 1665 cm . The H NMR spectrum
of this compound showed a characteristic singlet at δ=5.00
Scheme 5. A plausible mechanism for the formation of 1,4-anthraquinone 26
and pentacene-6,13-dione 27.
(
2H) ppm assigned to the cyclobutene CH protons. It displayed
2
also two singlets at δ=3.97 (3H) ppm and δ=4.07 (3H) ppm
due to the two methoxy groups, two singlets at δ=7.12 ppm
(
2H) for H6 and H7, and at δ=8.85 ppm (2H) for H4 and H9.
[
64–65]
cin B, presengulone, and sengulone.
Such compounds are
The formation of the cyclobuta[b]anthracene-1,5,8(2H)-trione 36
can be rationalized by a [4+2] cycloaddition of in situ
generated o-xylylene 34 to BQ (25) followed by loss of water
and oxidation to 35 and further by the oxidation of the
secondary alcohol group to yield the ketone 36 in up to 41%
yield (Scheme 7).
shown to prevent macromolecule synthesis such as the biosyn-
thesis of RNA or polypeptides in living cells and evoke
interesting bioactivity as antiplasmodial, antibiotics, and anti-
[66–68]
tumor agents.
On the other hand, pentacenes in virtue of
their remarkable electronic properties are benchmark in the
[
69–70]
1
field of organic electronic devices.
Another fraction could be isolated and identified. The H
Analogously, tetracene-5,12-dione 32 was obtained as the
sole product by ring opening / cycloaddition of BCB 18 with
NMR spectrum featured a singlet at δ=4.02 ppm (12H) for
methoxy protons besides a set of singlets at δ=6.92 (4H), 7.08
(4H), and 9.02 (4H) ppm for aryl protons. The IR spectrum
showed two characteristic C=O stretching bands at v˜=1614,
1
(
,4-naphthoquinone NQ (31) at elevated temperature
Scheme 6). The constitution of 32 was confirmed spectroscopi-
cally. The mass spectrum shows molecular ion peaks (1:1) at m/
À 1
1665 cm . A proposed structure is nonacene-1,4,8,12,15,19-
+
+
z=396 [M] and 398 [M+2] due to the presence of one Br
atom. The IR spectrum revealed a characteristic C=O stretching
band at v˜ =1672 cm . The H and C NMR spectra are in full
hexaone (40) (Scheme 8). The highly symmetric constitution of
1
3
40 could further be verified by its C NMR spectrum, which
showed a set of 11 signals: One characteristic signal for the
methoxy carbon atoms at δ=55.9 ppm; two signals at δ=173.6
À 1
1
13
accord with the proposed constitution.
[
C8(19)], 184.8 [C1(4, 12, 15)] ppm assigned to carbonyl carbon
atoms; three signals for aromatic CÀ H carbon atoms at δ=
07.5 [C7(9, 18, 20)], 123.5 [C5(11, 16, 22)], 140.0 [C2(3, 13, 14)]
ppm; signals for quaternary aromatic carbon atoms at δ=127.6
C6a(9a, 17a, 20a)], 128.1 [C5a(10a, 16a, 21a)], 130.7 [C4a(11a,
5a, 22a)], 135.2 [C7a(8a, 18a, 19a)], and 150.9 [C6(10,17,20)]
1
[
1
ppm. On the other hand, the EI mass spectrum of 40 did not
show a molecular ion peak. However, it revealed a peak at m/
+
z=663 [M+HÀ C H ] corresponding to the fragmentation of
2
2
Scheme 6. Synthesis of tetracene-5,12-dione 32. Reaction conditions: 18
+
(
1.0 mmol), NQ (31) (4.6 mmol), toluene, 120°C, sealed tube, 48 h (47%) or:
an ethyne molecule from [M+H] .
μW, 140°C, 270 W, 1 h (58%).
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goes loss of water and oxidation to diol 38. Ring-opening of 38
affords bis-diene 39, which undergoes further [4+2] cyclo-
additions to two equivalents of BQ (25) to form nonacene-
1
,4,8,12,15,19-hexaone (40). It has to be noted that we could
observe the formation of 40 in a very low yield (5–7%) only in
the case of microwave heating. This reflects the unlikeness of
the five-component successive cycloaddition reaction of two
equivalents of BDCB 17 or 19 with three equivalents of BQ (25)
in a tandem way. The reaction comprises the formation of four
new benzene rings in a single operation. Trying to conduct this
reaction using a molar ratio of BDCB to BQ (2:3) led to a very
slight increase of 2%. in the yield of 40. It was also found that
the formation of 40 is greatly affected by dilution of the
reaction mixture. We could not detect the formation of 40 on
dilution from 1.00 mmol to 0.01 mmol scales of BDCB 17 or 19
in the same amount of solvent. We note that the synthesis of
another nonacene derivative containing quinone moieties has
[78]
recently been published by Bunz et al.
Furthermore, the cycloaddition reaction of BDCB 19 or 20
with NQ (30) afforded similarly the cyclobuta[b]tetracene-
1
,5,10(2H)-trione 41 (Scheme 9). 41 was characterized spectro-
scopically. Its IR spectrum showed two characteristic bands at
À 1
1
v˜=1738, 1672 cm for C=O stretching vibrations. The H NMR
spectrum showed two distinct singlet signals for methoxy
protons at δ=4.00, 4.10 ppm, a singlet at δ=5.01 ppm (2H) for
methylene protons, and a set of peaks for the aryl protons at
1
3
their expected chemical shifts. Also, the C NMR spectrum fits
well with the constitution of 41 and displays three characteristic
peaks at δ=182.1, 182.87, 182.94 ppm for the three C=O
groups.
Scheme 7. Synthesis of cyclobuta[b]anthracene-1,5,8(2H)-trione 36. Reaction
conditions: 19 or 20 (1.0 mmol), BQ (25) (2.5 mmol), toluene, 120°C, sealed
tube, 48 h (34%) or: μW, 140°C, 270 W, 1 h (41%).
The formation of 40 can be rationalized by a tandem
reaction involving firstly the bis-cycloaddition reaction of two
equivalents of o-xylylene intermediate 33 to either double
bonds of BQ (25) to form diol 37, which subsequently under-
Photophysical properties
The UV-VIS absorption spectra of compounds 26, 27, 32, 36, 40
and 41 in THF at of 10 μM concentration are shown in Figure 5,
Figure 5. A) The UV-VIS absorption spectra of compounds 26, 27, 32, 36, 40 and 41 in THF at a concentration of 10 μM. B) onset of the lowest energy band.
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Scheme 8. A plausible mechanism for formation of 6,10,17,21-tetramethoxynonacene-1,4,8,12,15,19-hexaone (40). Reaction conditions: 19 or 20 (1.0 mmol),
BQ (25) (2.5 mmol), μW, 140°C, 270 W, 1 h (5%) or: 19 or 20 (2.0 mmol), BQ (25) (3.0 mmol), μW, 140°C, 270 W, 1 h (7%).
did not adopt a systematic behavior. The extended nonacene
derivative 40 showed the largest red shifted absorption at
λmax =471 nm while bromotetracenedione 32 and cyclobuta[b]
tetracenetrione 41 displayed the largest blue shift for each at
λmax =427 nm. Increasing the number of quinone rings lead to
red shifted absorptions; thus, the maximum wavelength of 40 is
more than both 1,4-anthraquinone 26 and pentacenetetraone
Scheme 9. Synthesis of 3,12-dimethoxycyclobuta[b]tetracene-1,5,10(2H)-tri-
one (41). Reaction conditions: 19 or 20 (1.0 mmol), NQ (31) (25 mmol),
toluene, 120°C, sealed tube, 48 h (38%); or μW, 140°C, 270 W, 1 h (40%).
2
7. Surprisingly, the largest lowest energy molar absorptivity
was displayed by cyclobuta[b]anthracenetrione 36 (ɛmax
=
4
À 1
À 1
8
.6x10 L·mol ·cm ), while the lowest one was that of cyclo-
4
À 1
À 1
and the data are listed in Table 4. They show a typical behavior
of quinone structures with absorption bands at λ=260-360 nm
for π-π* transitions and for n-π* transitions at λ=400–450 nm.
The lowest energy absorption bands of the tested compounds
buta[b]tetracenetrione 41 (ɛmax =2.3x10 Lmol cm ).
The fluorescence emission spectra are depicted in Figure 6A.
The unfitted fluorescence decay curves are shown in Figure 6B.
The fluorescence spectra were measured in THF at a concen-
À 6
tration of 1×10 M. The data shown in Table 6 allow to
distinguish between bromine containing compounds 26, 27
and 32 on the one hand and compounds 36, 40, and 41 on the
other hand. The known quenching effect of bromine
Table 4. Photophysical properties of compounds 26, 27, 32, 36, 40 and
4
1.
[79]
substituents
is reflected by the significantly shorter
λ
max
ɛ
max
λ
[nm]
em
Δν
[cm
Φ
[%]
f
τ
[ns]
4
À 1
À 1 [a]
[b]
À 1 [c]
[b]
[b]
fluorescence life times of the first three compounds as
compared to those of the second group. The presence of
bromo substituents in 26, 27 and 32 induces a clear heavy
[
nm]
[x 10 M ·cm
]
]
2
2
3
3
4
4
6
7
2
6
0
1
441
435
427
438
471
427
3.1
3.8
3.4
8.6
3.4
2.3
564
487
578
479
475
602
4954
2297
6118
1954
179
<1
<1
<1
<1
3.6
1.79
1.46
1.42
3.56
4.14
2.08
atom effect that decreases the fluorescence by increasing the
[80–82]
intersystem crossing (ISC).
While there are some reports
concerning the use of 9,10-anthraquinones in the context of
organic light-emitting diodes (OLEDs) through thermally acti-
6808
<1
[
a] Molar absorptivity measured in THF at 10 μM concentration. [b]
Fluorescence measurements (fluorescence quantum yield Φ and lifetime
. [c] Stoke’s shift Δν
[83–86]
vated delayed fluorescence (TADF),
fluorescence properties
f
τ) in THF at 10 μM concentration; quantum yield Φ
[
f
1,4-anthtraquinones such as 26 have much less been
À 1
7
[87–90]
cm ]=[(1/λabs)-(1/λem)]×10 .
investigated.
A derivative of 2,9-dibromopentacene-6,13-
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Figure 6. A) The fluorescence emission spectra of compounds 26, 27, 32, 36, 40 and 41 at a concentration of 1 μM in THF. B) The unfitted fluorescence decay
curves of the same compounds
[a]
Table 5. Cyclovoltammetry data of compounds 26, 27, 32 and 40.
shows the strongest fluorescence at λem =475 nm with the
Epa [V]
E
pc [V] ΔE
E
[V]
1/2
[c]
E
[eV]
HOMO
[d]
E
[eV]
LUMO
[d]
ΔEredox
[eV]
highest fluorescence intensity, the lowest Stokes shift (Δν=
[b]
[d]
[
V]
À 1
1
79 cm ), the largest quantum yield (Φ =3.6%), and the
f
2
2
3
4
6
7
2
0
À 1.11 À 1.80 0.69
À 1.64 À 2.31 0.71
À 0.80 À 1.57 0.77
À 1.29 À 1.84 0.55
À 0.65 À 1.63 0.92
À 1.46 À 2.37 0.91
À 1.39 À 1.52 0.13
À 1.83 À 1.97 0.14
À 2.07 À 2.20 0.13
À 1.46 À 6.03
À 1.98
À 3.30
À 3.10
À 3.30
À 3.41
2.73
2.90
2.69
2.88
longest fluorescence lifetime (τ=4.14 ns) in the series. All other
compounds have lower quantum yields (Φ <1 %). This could
f
À 1.19 À 6.00
À 1.57
partly be attributed to the presence of methoxy groups causing
À 1.14 À 5.99
À 1.92
efficient intramolecular charge transfer (ICT) to the aromatic
[92–95]
rings in the excited state.
The fluorescence lifetime values
À 1.49 À 6.29
À 1.90
of all compounds are small (τ=1.42-3.65 ns) which is in accord
À 2.14
with the efficient non-radiative ICT processes. In addition, large
À 1
+
–
Stokes shifts in the range of 5000–7000 cm were displayed by
[
a] Cyclovoltammetry in a 0.1 M solution of Bu
00 mVs scan rate versus FcH/FcH as an internal reference at 25°C. [b]
4
N
PF
6
in THF at
À 1
+
1
all compounds except 40. This could be attributed to efficient
n
n
n
n
ΔE (V)=Epa -Epc (n=1, 2, 3). [c] Half-wave potential E1/2 [V]=(Epa +Epc )/2.
[96–98]
ICT and structural relaxation in the excited state.
onset
onset
) eV, ΔEredox =ELUMO-EHOMO.
red
[99]
[d] EHOMO =À (4.8-Eox ) eV, ELUMO =À (4.8+E
Electrochemical characterization
Table 6. DFT-calculated HOMO and LUMO energies and optical energy
gaps of compounds 26, 27, 32, 36, 40, 41.
The study of the electrochemical redox behavior was limited
herein to the extended quinones anthraquinone 26, pentacene-
dione 27, tetracenedione 32, and nonacenehexaone 40 in order
to avoid the interfering redox behavior of cyclobutenone rings
in 36 and 41. While the first three compounds bear bromo
substituents and are less symmetric, nonacenehexaone 40 is a
highly symmetric compound. The cyclic voltammograms of
anthraquinone 26, pentacenedione 27, and tetracenedione 32
(see SI) show two oxidation waves and two reduction waves
lacking clear reversibility. In contrast, the cyclic voltammogram
of nonacenehexaone 40 (Figure 7) displays three quasi-rever-
sible redox waves. Examining the oxidation potentials (Table 5)
of the first anodic process and the first reduction wave
potentials of the studied compounds shows that the order of
these compounds with respect to their ease of oxidation is 40>
26�27>32 reflecting the extended π system in 40. In addition,
Table 5 presents the HOMO and LUMO energies as calculated
E
HOMO
a]
E
[eV]
LUMO
[a]
ΔE
[eV]
ΔEopt
[b]
[
[a]
[
eV]
[eV]
2
2
3
3
4
4
6
7
2
6
0
1
À 5.72
À 6.10
À 6.17
À 6.11
À 6.16
À 6.02
À 3.59
À 2.89
À 3.05
À 3.43
À 3.60
À 3.08
2.13
3.21
3.12
2.68
2.56
2.94
2.39
2.60
2.60
2.43
2.18
2.56
[
a] Theoretical HOMO, LUMO energies and energy gap calculated from
DFT [B3LYP/6-31+ +(d,p)]. [b] optical energy gap ΔEopt [eV]=1240/
[
101]
λ .
onset
dione 27 could further be functionalized and investigated for
[91]
the application as molecular wires. The data for the cyclo-
butanone-anellated compounds 36 and 41, the latter being the
benzo anellated homologue of 36, do not differ significantly,
which may possibly be due to the quinone moiety separating
the benzoid π systems in 41 in a cross-conjugated way. The
compound with the by far largest π system, nonacene 40,
[99]
from the CV data.
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to those obtained from the CV measurements. Among the
bromine free derivatives the most highly extended π system 40
shows the smallest HOMO-LUMO gap while the opposite is the
case for the bromine containing 27 as compared to 26 and 28.
While the HOMO energies of 27 and 40 are quite similar, the
main difference lies in their LUMO energies. While the LUMO of
2
7 shows the largest coefficients at the central quinone moiety,
this is the case for the two terminal quinone moieties in 40.
Conclusion
Figure 7. Cyclic voltammogram of nonacenehexaone 40.
In conclusion, it has been shown that benzocyclobutenes as
well as benzodicyclobutenes serve as useful building blocks for
the syntheses of a number of π-extended acenes and cyclo-
butaacenes by reactions with quinones followed by water
elimination and oxidation. The possibility of benzodicyclobu-
tenes and of benzoquinone to react in a bidirectional way
makes the approach particularly attractive as shown by the
synthesis of a nonacene derivative incorporating three quinone
subunits. The compounds obtained were characterized spectro-
scopically and were investigated by cyclovoltammetry,
fluorescence spectroscopy as well as by DFT calculations.
Further investigations will head at higher yielding syntheses as
well as towards possible applications of these compounds.
DFT calculations
The DFT calculations were performed using the Gaussian 16
[100]
program
at B3LYP/6-31+ +(d,p) level of theory in order to
get some insight into the geometry and the electronic
structures of the synthesized compounds. Geometry optimiza-
tion was performed in the gas phase. The optimized structures
of compounds 26, 27b, 32, 36, 40 and 41 showed all planarity
over the region of the extended aromatic systems in addition to
the bond lengths with an average of 1.4ꢀ0.01 Å and bond
angles of about 120ꢀ2°. Distortions from these values occur in
quinone or cyclobutenone moieties. For comparison, the Experimental Section
calculated HOMO and LUMO orbitals of the compounds are
General: Solvents were dried and distilled before use. Toluene and
depicted in Figure 8, and the relevant numerical values can be
found in Table 6. Inspection of the HOMO and LUMO distribu-
tions of the quinones reveals that HOMO orbitals are mainly
located at electron-rich aromatic rings and away from the
quinone rings while the LUMO orbitals are preferentially located
at the quinone rings. The calculated energies correspond well
THF were distilled from sodium wire/benzophenone under argon
prior to their use, while petroleum ether (PE), dichloromethane
(DCM) tert-butyl methyl ether (TBME), and ethyl acetate were dried
with calcium chloride. Absolute EtOH was supplied by Sigma-
Aldrich and used as received.
Figure 8. The HOMO and LUMO energy diagram of compounds 26, 27, 32, 36, 40, 41. [DFT, B3LYP/6-31+ +(d,p)].
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Analytical TLC was performed with Merck 60F-254 silica gel thin
to reach room temperature (25°C) and kept at this temperature for
layer plates. Column chromatography was carried out using silica
gel (Macherey-Nagel, 40–63 μm, Silica M) as the stationary phase
24 h. Meanwhile, under argon another Schlenk tube B was charged
with TMP (0.68 mL, 4.0 mmol, 1.0 equiv) and THF (10 mL) and then
cooled to 0°C. Butyllithium (1.6 mL, 2.5 M in hexane, 4.0 mmol,
[
102]
and indicated eluents by flash chromatography.
Melting points:
Electrothermal IA 9200 Series Digital Melting Point Apparatus. IR:
Shimadzu IRAffinity-1S with quest ATR unit (32 scans). Intensity of
signals are determined as s=strong, m=middle, w=weak, br=
1.0 equiv) was added dropwise with stirring, and the mixture was
maintained at this temperature for 0.5 h. Schlenk tube A was cooled
to 78°C in an acetone-dry ice bath. 1,4-Dibromo-2,5-dimeth-
1
13
broad. NMR: Bruker AVS 400 ( H: 400.1 MHz, C: 100.6 MHz) and
oxybenzene (12, 1184 mg, 4 mmol, 1.0 equiv) in THF (5 mL) was
added dropwise. Afterwards, the content of tube B (LiTMP solution)
was added dropwise. After the reaction finished as monitored by
1
13
AVS 500 ( H: 500 MHz, C: 125.7 MHz) instruments. Chemical shifts
δ refer to δTMS =0.00 ppm or to residual solvent signals. The
multiplicities of the signals were determined by ATP measurements
TLC, saturated aqueous NH Cl (10 mL) was added. The mixture was
4
13
and specified as CH , CH, CH or C. Assignment of the C signals
slowly warmed to 25°C. The crude mixture was diluted with water
(30 mL) and then extracted with DCM (3 x 50 mL). The combined
organic layer were washed with brine (50 mL) and dried with
3
2
was made based on 2D NMR spectra (HSQC, HMBC) and in
1
3
comparison to calculated
0.1.0.110). LC–MS (ESI): Micromass LCT premier spectrometer with
lock-spray unit (ESI), loop mode, HPLC Alliance 2695 column
Waters). Matrix-assisted laser desorption/ionization (MALDI): 5800
MALDI TOF/TOF (ABSciex) using 4-Chloro-α-cyanocinnamic acid
CHCA) as matrix. HRMS: VG-Autospec or Micromass LCT
C
chemical shifts (ChemDraw®
2
MgSO . The solvent was evaporated at reduced pressure and
4
purified by recrystallization from TBME/PE (1:5, v/v) to give 18
(630 mg, 2.4 mmol, 61%) as colorless crystals [Rf=0.72 (CHCl3/
acetone 4:1), m. p. 108–110°C].
(
(
a
spectrometer with direct insertion probe; 70 eV electron energy
and 250°C source temperature. UV/VIS spectra were recorded with
a Horiba Dual-FL spectrometer. All samples were diluted with THF
and measured in quartz cuvettes with a path length of 1 cm.
Fluorescence spectra were measured in dilute solutions in 1 cm
quartz cuvettes from Hellma Analytics employing
a Horiba
Fluoromax-4 spectrometer with excitation at the absorption
maximum. Fluorescence quantum yields were measured in a Horiba
Dual-FL instrument with a Horiba Quanta-Phi integrating sphere
with an excitation wavelength of 375 nm by comparison of the area
below the scattered excitation peak and the emission peak for pure
solvent and sample. Fluorescence lifetimes were measured by time
correlated single photon counting using a Horiba Fluoromax-4
coupled with a Fluorohub and a NanoLED with 370 nm wavelength,
pulse width of 1.2 ns and 1 MHz repetition rate. Cyclovoltammetry
IR (neat): v˜=3201 (br, m, OH), 2936 (w), 2344 (w), 1488 (s), 1418 (s),
1
9
383 (m), 1339 (m), 1244 (s), 1159 (m), 1118 (s), 1039 (s), 983 (m),
À 1
1
26 (w), 833 (m), 797 (m), 666 (m) cm . H NMR (CDCl , 400 MHz):
3
2
δ=2.31 (d, 1H, JOH-H7 =10.2 Hz, OH), 3.04 (dd, 1H, J
JH8-H7 =1.8 Hz, H8), 3.66 (dd, 1H, J
H8’), 3.81 (s, 3H, 2-OCH ), 4.10 (s, 3H, 5-OCH ), 5.36 (ddd, 1H, J
1
=À 14.2 Hz,
=4.6 Hz,
H8’-H7
H8-H8’
2
= À 14.2 Hz, J
H8’-H8
=
C
3
3
H7-OH
13
0.2 Hz, JH7-H8 =1.8 Hz, JH7-H8’ =4.6 Hz, H7), 7.02 (s, 1H, H3) ppm.
NMR (CDCl , 100 MHz): δ=41.5 (CH , C8), 56.5 (CH , 2-OCH ), 58.4
3
2
3
3
(
CV) measurements were carried out with a Gamry Reference 600
(
CH , 5-OCH ), 70.6 (CH, C7), 109.6 (C, C4), 120.4 (CH, C3), 125.9 (C,
3 3
Potentiostat/Galvanostat/ZRA. 0.01 mM of the sample compound in
freshly distilled THF, and tetrabutylammonium phosphate (TBAP,
C1), 131.7 (C, C6), 145.8 (C, C5), 148.8 (C, C2) ppm. MS (70 eV): m/z
+
+
+
(
(
%)=260 (43) [M+2] , 258 (51) [M] , 245 (76) [M+2À CH ] , 243
3
0
.387 g, 98% purity) was added corresponding to a concentration
+
+
100) [MÀ CH ] . HRMS (ESI): Calcd. for C H BrO [M] 257.9892,
+
3
10 11
3
of 0.1 M. The reference electrode was a Ag/Ag (AgNO ) electrode
3
found 257.9858.
2,7-Dimethoxytricyclo[6.2.0.0 ]deca-1,3(6),7-triene-4,10-diol (19):
in acetonitrile with 0.01 M AgNO and 0.1 M of TBAP. A 0.25 mm
3
3,6
and a 0.1 mm thick platinum wire served as counter and working
electrodes, respectively. The scan rate was 100 mV/s. Freshly
sublimed ferrocene (FcH) was used for calibration; potentials refer
3
,6
GP1, 2,7-dimethoxytricyclo[6.2.0.0 ]deca-1,3(6),7-triene-4,10-dione
(16) (2180 mg, 10.0 mmol), sodium borohydride (2282 mg,
60 mmol). 19 was obtained as a diastereomeric mixture (3:2)
+
to the FcH/FcH redox couple.
(
4
1444 mg, 0.65 mmol), pale yellow solid [Rf=0.63 (CHCl /acetone
:1), m.p. 150–152°C (mixture)].
3
General procedure 1 (GP1) for the synthesis of 18, 19and 20:
Sodium borohydride was added to a solution of aromatic ketones
(15, 16 or 17) (10 mmol) in ethanol (30 mL), and the solution was
stirred at 25°C for 30 min. The resulting solution was treated with
aq. HCl (1%, v/v, 100 mL) and then extracted with chloroform (3×
5
0 mL). The collected organic layers were washed with aq. NaHCO
3
(0.1 M, 100 mL), brine and water and dried over anhydrous MgSO4.
The solvent was removed at reduced pressure to give practically
pure product.
4
-Bromo-2,5-dimethoxybicyclo[4.2.0]octa-1(6),2,4-trien-7-ol (18):
IR (neat): v˜=3259 (br, m, OH), 2919 (m), 2367 (w), 1487 (s), 1415
GP1, bicyclo[4.2.0]octa-1(6),2,4-trien-7-one (15) (2560 mg, 10 mmol),
sodium borohydride (1141 mg, 30 mmol). The obtained product
was recrystallized from TBME/PE (1:5, v/v) to give 18 (1935 mg,
(
(
s), 1314 (m), 1264 (s), 1199 (m), 1177 (m), 1109 (m), 1067 (m), 1041
s), 977 (m), 924 (m), 892 (w), 802 (m), 779 (m), 612 (m) cm . H
À 1
1
NMR (CDCl , 400 MHz): δ=2.33-2.43 (d, 2H, J
2 OH), 2.98+3.21 [dd, 2H, J
=JOH-H10 =10.2 Hz,
3
OH-H4
2
2
7.5 mmol, 75%) as colorless crystals [Rf=0.72 (CHCl /acetone 4:1),
3
= J
=À 14.2 Hz, J
=JH9-H10
=J
H9’-
=
H5-H5’
H9-H9’
H5-H4
m. p. 108–110°C].
2
2
1
.8 Hz, H5(9)], 3.54 [dd, 2H, J
= J
= À 14.2 Hz, J
H5’-H5
H9’-H9
H5’-H4
=
1.8 Hz, H5’(9’)], 3.63+3.85 (2 s, 3H, 7-OCH ), 4.01+4.05 (2 s,
H10
3
One-step synthesis of 18starting from 1,4-dibromo-2,5-dimeth-
oxybenzene (12): Under argon THF (20 mL) was added to a 100 mL
flamed-dried Schlenk tube A. The tube was inserted into an ice
bath and the temperature decreased to 0°C before butyllithium
3
H, 2-OCH ), 5.26+5.35 [2ddd, 2H, J
=JH10-OH =10.2 Hz, J
=
3
H4-OH
H4-H5
13
JH10-H9 =1.8 Hz, JH4-H5’ =JH10-H9’ =4.6 Hz, H4(10)] ppm. C NMR (CDCl3,
00 MHz): δ=43.1 [CH , C5(9)], 57.3 (CH , 7-OCH ), 57.8 (CH , 2-
1
2
3
3
3
OCH ), 71.0 [CH, C4(10)], 124.4 [C, C6(8)], 127.5 [C, C1(3)], 128.3 (C,
3
(
2.4 mL, 2.5 M in hexane, 6 mmol, 1.5 equiv) was added dropwise
+
C7), 128.4 (C, C2) ppm. MS (70 eV): m/z (%)=224 (34) [M+2] , 222
with continuous stirring. Upon completion, the system was allowed
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+
(
1
36) [M] , 207 (85), 205 (28), 191 (91), 179 (88), 177 (46), 163 (50),
(20 mL) was added to the resulting mixture, which was then filtered
at reduced pressure and washed thoroughly with hot MeOH (3×
20 mL). The combined organic filtrates were evaporated at reduced
pressure and then purified by column chromatography.
48 (31), 135 (25), 133 (17), 121 (15), 105 (31), 91 (100), 77 (83).
HRMS (ESI): calcd. for C H O [M] 222.0892, found 222.0874.
+
12
14
4
3,6
2
,7-Dimethoxytricyclo[6.2.0.0 ]deca-1,3(6),7-triene-4,9-diol (20):
3,6
GP1, 2,7-dimethoxytricyclo[6.2.0.0 ]deca-1,3(6),7-triene-4,9-dione
17) (2180 mg, 10.0 mmol), sodium borohydride (2282 mg,
0 mmol). After extraction with chloroform the combined extracts
6-Bromo-5,8-dimethoxyanthracene-1,4-dione (26) and 2,9-dibro-
mo-1,4,8,11-tetramethoxypentacene-6,13-dione (27): GP 2, Meth-
od A. Benzocyclobutenol 18 (260 mg, 1.0 mmol), benzoquinone (25)
(500 mg, 4.6 mmol). Column chromatography (TBME/PE 1:3) gave
(
6
were concentrated to one third of its volume. During this time, an
insoluble white solid precipitated, was collected and dried under
vacuum to give diastereomer 20a (777 mg, 3.5 mmol, 35%) as a
pure 26 (132 mg, 0.4 mmol, 38%) as red crystals [R =0.52 (TBME/PE
f
1:3), m. p. 214-216 °C.]. The insoluble part in MeOH was recrystal-
colorless solid [Rf=0.44 (CHCl /acetone 4:1), m. p: 192–194°C]. The
lized from hot DMSO to give an orange solid which is filtered at
reduced pressure, washed thoroughly with MeOH, dried at reduced
pressure overnight to afford 27 (128 mg, 0.22 mmol, 22%); orange
solid [R =0.30 (CHCl /PE 1:3), m. p.>300°C].
3
mother liquor was evaporated under reduced pressure to give
diastereomer 20b (622 mg, 0.3 mmol, 28%) as a pale-yellow solid
[
R =0.51 (CHCl /acetone 4:1), m. p. 152–154°C].
f
3
f
3
Method B: Benzocyclobutenol 18 (260 mg, 1.0 mmol), benzoqui-
none (25) (500 mg, 4.6 mmol). Column chromatography (TBME/PE
1
:3) gave pure 26 (180 mg, 0.5 mmol, 52%) as red crystals [R =0.52
f
(
TBME/PE 1:3), m. p. 214-216 °C.]. The insoluble part in MeOH was
recrystallized from hot DMSO to give an orange solid which is
filtered off, washed thoroughly with MeOH, dried at reduced
pressure overnight to afford 27 (152 mg, 0.3 mmol, 26%); orange
solid [R =0.30 (CHCl /PE 1:3), m. p. >300°C].
f
3
2
1
9
4
2
0a: IR (neat): v˜=3262 (br, m, OH), 2928 (w), 1491 (s), 1422 (s),
325 (m), 1262 (s), 1202 (w), 1182 (m), 1110 (m), 1073 (m), 1045 (s),
À 1
1
93 (w), 945 (w), 891 (w), 796 (m), 628 (m) cm . H NMR (CDCl ,
3
00 MHz): δ=2.16 (d, 2H, JOH-H4 =JOH-H9 =10.2 Hz, 2 OH), 2.98 [dd,
H, JH5-H5’ =JH10-H10’ =À 14.2 Hz, J
=JH10-H9 =1.8 Hz, H5(10)], 3.59
H5-H4
[
3
dd, 2H, JH5’-H5 =J
.97 (s, 6H, 2 OCH ), 5.27 [ddd, 2H, J
=À 14.2 Hz, J
=JH10’-H9 =1.8 Hz, H5’(10’)],
H10’-H10
H5’-H4
=JH9-OH =10.2 Hz, J
=
NMR
3
H4-OH
H4-H5
13
JH9-H10 =1.8 Hz, JH4-H5’ =JH9-H10’ =4.6 Hz, H4(9)] ppm.
C
(
[
acetone-d , 100 MHz): δ=39.7 [CH , C5(10)], 56.8 (CH , OCH ), 69.1
6
2
3
3
2
6: IR (neat): v˜ =2918 (w), 1715 (w), 1671 (m), 1598 (m), 1576 (m),
1462 (m), 1432 (m), 1375 (m), 1323 (m), 1297 (s), 1209 (m), 1149
m), 1116 (m), 1055 (m), 1008 (m), 949 (m), 928 (w), 845 (s), 802 (m),
CH, C4(9)], 126.5 [C, C1(6)], 134.0 [C, C3(8)], 146.3 [C, C2(7)] ppm.
+
+
MS (70 eV): m/z (%)=222 (66) [M] , 207 (100) [MÀ CH ] , 192 (10),
3
(
1
79 (13), 177 (17), 164 (19), 147 (23), 121 (13), 91 (43), 77 (38). HRMS
À 1
1
+
773 (w), 727 (m), 705 (m) cm . H NMR (CDCl
, 400 MHz): δ=4.03
3
(ESI): Calcd. for C H O [M] 222.0892, found 222.0884.
12 14 4
(s, 3H, OCH ), 4.05 (s, 3H, OCH ), 7.09 (s, 1H, H7), 7.10 (s, 2H, H2, H3),
3
3
1
3
2
1
9
0b: IR (neat): v˜=3152 (br, m, OH), 2930, (m), 2359 (w), 1487 (s),
8.80 (s, 1H, H9), 9.00 (s, 1H, H10) ppm. C NMR (CDCl , 100 MHz):
δ=56.2 (CH , 8-OCH ), 62.1 (CH , 5-OCH ), 111.9 (CH, C7), 117.1 (C,
C6), 122.8 (C, C10), 124.2 (CH, C9), 127.1 (C, C10a), 128.0 (C, C9a),
129.5 (C, C4a), 130.6 (C, C8a), 140.0 (CH, C3), 140.1 (CH, C2), 148.5
(C, C5), 153.6 (C, C8), 184.3 (C, 4-C=O), 184.5 (C, 1-C=O) ppm. UV/Vis
3
454 (m), 1424 (m), 1329 (w), 1292 (m), 1261 (s), 1202 (w), 1044 (s),
3
3
3
3
–1 1
87 (m), 966 (w), 944 (m), 890 (w), 796 (w), 663 (w) cm . H NMR
=JOH-H9 =10.2 Hz, 2 OH),
(CDCl , 400 MHz): δ=2.42 (d, 2H, J
3
OH-H4
2
5
1
1
.93 [dd, 2H, JH5-H5’ =JH10-H10’ =À 14.2 Hz, J
=JH10-H9 =1.8 Hz, H-
H5-H4
(10)], 3.57 [dd, 2H, JH5’-H5 =J
=À 14.2 Hz, J
=JH10’-H9
=JH9-OH
H4-OH
=
=
(THF): λ
(ɛ)=441 (31.5), 307 (38.0), 295 (39.0), 283 (40.6) nm
H10’-H10
H5’-H4
max
À 1
À 1
.8 Hz, H5’(10’)], 3.97 (s, 6H, 2 OCH ), 5.24 [ddd, 2H, J
(mM cm ). Fluorescence (THF): λex=268 nm; λem=564 nm. MS
(70 eV): m/z (%)=348 (31) [M+2] , 346 (35) [M] , 333 (99) [M+
3
+
+
0.2 Hz, JH4-H5 =JH9-H10 =1.8 Hz, JH4-H5’ =JH9-H10’ =4.6 Hz, H4(9)] ppm.
1
3
+
+
C NMR (CDCl , 100 MHz): δ=40.7 [CH , C5(10)], 57.6 (CH , OCH ),
2À CH ] , 331 (100) [MÀ CH ] , 305 (11), 307 (12). HRMS (ESI): Calcd.
3
2
3
3
3
3
+
6
9.8 [CH , C4(9)], 126.7 [C, C1(6)], 133.8 [C, C3(8)], 143.9 [C, C2(7)]
for C H BrO [M] 345.9841, found 345.9831.
16 11 4
3
+
+
ppm. MS (70 eV): m/z (%)=224 (19) [M+2] , 222 (48) [M] , 207 (81)
+
[
1
MÀ CH ] , 193 (33), 177 (52), 163 (34), 147 (25), 135 (22), 121 (25),
3
+
19 (19), 91 (89), 77 (100). HRMS (ESI): Calcd. for C H O [M]
12 14 4
222.0892, found 222.0884.
General Procedure 2 (GP2)
Method A (conventional heating): A mixture of benzocyclobutene
derivative 18 or benzodicyclobutene derivatives 19 or 20 and
benzoquinone (25) or naphthoquinone (31) in anhydrous toluene
2
1
9
7: IR (neat): v˜=2937 (w), 1673, (s), 1597 (s), 1457 (m), 1429 (m),
(
(
5 mL) was heated in a sealed pressure tube at 140 °C for 48 h. PE
410 (m), 1381 (m), 1318 (s), 1268 (s), 1231 (m), 1136 (s),1024 (s),
À 1
1
20 mL) was added to the resulting mixture, which was then filtered
75 (m), 952 (s), 935 (s), 830 (s), 796 (s), 745 (s), 720 (s) cm . H
at reduced pressure and washed thoroughly with hot MeOH (3×
NMR (CDCl , 400 MHz): δ=4.00 [s, 6H, 1(8)-OCH ], 4.10 [s, 6H, 4(11)-
OCH ], 7.42 [s, 2H, H3(10)], 8,87 [s, 2H, H5(12)], 9.05 [s, 2H, H7(14)]
ppm. UV/Vis (THF): λ (ɛ)=435 (38.3), 313 (185.5), 267 (113.7) nm
3
3
20 mL). The combined organic filtrates were evaporated at reduced
3
pressure and then purified by column chromatography.
max
À 1
À 1
(
(
(
(
mM cm ). Fluorescence (THF): λex=269 nm; λ 487 nm. MS
em=
81 + 81 79 +
Method B (microwave heating): A mixture of benzocyclobutene
derivative 18 or benzodicyclobutene derivatives 19 or 20 and
benzoquinone (25) or naphthoquinone (31) in anhydrous toluene
70 eV): m/z (%)=588 (28) [M, 2 Br] , 586 (56) [M, Br, Br] , 584
7
9
+
+
+
28) [M, 2 Br] , 573 (14) [M+4À CH ] , 571 (28) [M+2À CH ] , 569
3
3
+
+
14) [MÀ CH ] . HRMS (ESI): Calcd. for C H Br O [M] 583.9470,
3
26 18
2
6
(2 mL) was placed in a microwave vessel, flushed with argon and
found 583.9431.
then subjected to microwave irradiation (60 min, 120°C, 200 W). PE
Eur. J. Org. Chem. 2021, 1–16
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12
© 2021 The Authors. European Journal of Organic Chemistry published
by Wiley-VCH GmbH
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Full Papers
doi.org/10.1002/ejoc.202100848
2
,9-dibromo-1,4,8,11-tetramethoxypentacene-6,13-diol (28):
mixture of 27 (10.0 mg, 0.017 mmol) and a large excess of NaBH4
8.0 mg, 0.211 mmol) was heated in MeOH (10 mL) for 3 h. The
A
deep red solid, R =0.68 (CHCl ), m. p. 242–244°C] and 40 (only
f
3
method B: 34 mg, 0.05 mmol, 5%. Bluish red solid (34 mg,
(
0.05 mmol, 5%), R =0.56 (CHCl ), m. p. 274–276°C].
f
3
solution was then treated with 0.1 N HCl (10 mL), diluted with
distilled water (20 mL) and extracted with chloroform (2×15 mL).
The combined organic extracts were washed with NaHCO solution
3
(
0.2 N, 10 mL), brine and then water followed by drying over
MgSO . The organic solvent was removed at reduced pressure and
4
dried under vacuum to obtain analytically pure compound 28
(
3.8 mg, 0.006 mmol, 38%) as a yellow solid (m. p. >300°C), which
deepens in color on standing and in CDCl during NMR measure-
ment forming 27.
3
3
1
1
6:. IR (neat): v˜=2954 (w), 2362 (w), 2086 (w), 2018 (w), 1665 (s),
596 (s), 1449 (m), 1427 (m), 1374 (m), 1323 (m), 1294 (s), 1145 (s),
074 (s), 994 (s), 931 (m), 850 (s), 778 (w), 737(m), 704 (m), 653 (m)
À 1
1
cm . H NMR (CDCl , 400 MHz): δ=3.97 (s, 3H, 3-OCH ), 4.07 (s, 3H,
3
3
1
0-OCH ), 5.00 (s, 2H, H2), 7.12 (s, 2H, H6, H7), 8.85 (s, 2H, H4, H9)
3
13
ppm. C NMR (CDCl , 100 MHz): δ = 57.4 (CH , C2), 62.2 (CH , 3-
3
2
3
OCH ), 64.4 (CH , 10-OCH ), 123.6 (CH, C4), 124.0 (CH, C9), 128.1 (C,
3
3
3
C2a), 128.7 (C, C10a), 128.7 (C, C3a), 130.2 (C, C8a), 132.1 (C, C9a),
1
33.5 (C, C4a), 140.1 (CH, C6), 140.1 (CH, C7), 152.4 (C, C3), 152.9 (C,
1
H NMR (CDCl , 400 MHz): δ=4.09 (s, 12H, 4 OCH ), 7.09 [s, 2H,
3
3
C10), 183.8 (C, C5), 184.6 (C, C8), 184.6 (C, C1) ppm. UV/Vis (THF):
λmax (ɛ)=438 (85.7), 308 (154.3), 297 (144.4), 267 (135.4) nm (mM
cm ). Fluorescence (THF): λex=268 nm; λem=479 nm. MS (70 eV): m/
z (%)=310 (88) [M+2] , 295 (100) [(M+2)-CH ] , 280 (68) [(M+2)-
2
3
13
À 1
H3(10)], 9.11 [s, 2H, H5(12)], 9.29 [s, 2H, H7(14)] ppm. C NMR [d6-
DMSO/CDCl (1:1), 500 MHz]: δ=56.1 (CH , 4,11-OCH ), 60.2 (CH ,
1
À 1
3
3
3
3
+
+
,8-OCH ), 111.8 [CH, C3(10)], 112.7 [C, C2(9)], 117.1 [C, C4a(11a)],
3
3
+
+
1
23.6 [CH, C7(14)], 125.1 [CH, C5(12)], 127.6 [C, C5a(12a)], 130.9 [C,
CH ] . HRMS (EI): Calcd. for C H O [M+2H] 310.0841, found
3
18 12
5
C6a(13a)], 131.4 [+, C7a(14a)], 140.7 [+,C1(8)], 148.2 [+, C4(11)],
1
+
10.0841.
53.6 [+, C6(13)] ppm. HRMS (ESI): Calcd. for C H Br NaO [M+2H
26
22
2
6
+
Na] 610.9681, found 610.9761.
8
-Bromo-7,10-dimethoxytetracene-5,12-dione (32): GP2. Benzocy-
clobutenol 18 (260 mg, 1.0 mmol), naphthoquinone (31) (727 mg,
.6 mmol). The reaction mixture was directly subjected to chroma-
tographic purification (PE/TBME 3:1) to give pure 32 (Method A:
86 mg, 0.5 mmol, 47%. Method B: 230 mg, 0.6 mmol, 58%), dark
4
1
orange solid [R =0.60 (TBME/PE 1:3), m. p. 244–246°C].
f
4
1
1
0: IR (neat): v˜=2922 (s), 2851 (s), 1665 (s), 1614 (m), 1472 (m),
456 (m), 1439 (m), 1387 (m), 1302 (s), 1273 (s), 1215 (m), 1148 (m),
167 (w), 1117 (m), 1086 (m), 964 (w), 849 (m), 822 (m), 758 (s)
À 1
1
cm . H NMR (CDCl , 400 MHz): δ 4.02 (s, 12H, 4 OCH ), 6.92 [s, 4H,
3
3
H2(3,13,14)], 7.08 [s, 4H, H7(9,18,20)], 9.02 [s, 4H, (H5(11,16,22)]
1
3
ppm. C NMR (CDCl , 100 MHz): δ 55.9 (CH , 4 OCH ), 107.5 [CH,
3
3
3
C7(9,18,C20)], 123.5 [CH, C5(11,16,22)], 127.6 [C, C6a(9a,17a,20a)],
28.1 [C, C5a(10a,16a,21a)], 130.7 [C, C4a(11a,15a,22a)], 135.2 [C,C7a
1
IR (neat): v˜=2941 (m), 1672 (s), 1591 (s), 1463 (m), 1431 (m), 1415
(8a,18a,19a)], 140.0 [CH, C2(3,13,14)], 150.9 [C, C6(10,17,20)], 173.6
(
(
(
m), 1380 (m), 1324 (s), 1278 (s), 1242 (m), 1140 (m), 1100 (m), 1052
m), 975 (m), 950 (s), 929 (m), 854 (m), 841 (m), 797 (m), 711 (s), 672
[C, C8(19)], 184.81 [C, C1(4,12,15)] ppm. UV/Vis (THF): λmax (ɛ)=471
(98.4), 268 (33.4) nm (mM cm ). Fluorescence (THF): λex=268 nm;
À 1
À 1
À 1
1
m), 633 (w), 611 (w) cm . H NMR (CDCl , 400 MHz): δ=4.05 (s, 3H,
λ
C
em=475 nm. LCÀ MS (ESI, 70 eV): m/z (%)=1348 (11) [2(M+H-
3
1
0-OCH ), 4.06 (s, 3H, 7-OCH ), 7.06 (s, 1H, H9), 7.83-7.86 (m, 2H, H2,
H
2
2
)+Na], 686 (48) [M+H+NaÀ C
H
2
2
], 685 (100) [M+NaÀ C
H ],
2 2
3
3
+
H3), 8.40-8.42 (m, 2H, H1, H4), 9.02 (s, 1H, H11), 9.20 (s, 1H, H6)
663 (10) [M+NaÀ C
2
H
2
À CH
10 [M+HÀ C
[M+6HÀ 6COÀ 2 C
3
]. MALDI-TOF: m/z=688 [M] . HRMS (EI):
13
ppm. C NMR (CDCl , 100 MHz): δ=56.2 (CH , 10-OCH ), 62.2 (CH ,
Calcd. for C40
H
4
23
O
H ] 663.1280, found 663.4554. Calcd.
2 2
+
3
3
3
3
7
-OCH ), 111.6 (CH, C9), 118.3 (C, C8), 123.5 (CH, C6), 124.9 (CH,
for
446.1543.
C
H
30
22
O
H
2
À 2CH
2
]
3
446.1518, found
3
C11), 127.50 (+, C6a), 127.53 (CH, C1), 127.55 (CH, C4), 129.4 (C,
C11a), 130.91 (C, C5a), 130.93 (C, C10a), 134.16 (C, C3), 134.26 (C,
C2), 134.34 (C, C4a), 134.42 (C, C12a), 148.4 (C, C7) 153.6 (C, C10),
3
,12-Dimethoxycyclobuta[b]tetracene-1,5,10(2H)-trione (41): GP2,
benzodicyclobutene 19 or 20 (224 mg, 1 mmol), naphthoquinone
(31) (395 mg, 2.5 mmol), column chromatography (TBME/PE 1:3)
gave 41 (method A: 136 mg, 0.38 mmol, 38%; method B: 143 mg,
1
3
82.5 (C, C12), 182.8 (C, C5) ppm. UV/Vis (THF): λmax (ɛ)=427 (34.4),
03 (74.7), 293 (74.1), 294 (74.7) nm (mM cm ). Fluorescence
À 1
À 1
(
THF): λex=268 nm; λ 578, 776 nm. MS (70 eV): m/z (%)=398 (11)
em=
+ + + +
0.40 mmol, 40%) as a brown solid [R =0.71 (CHCl ), m. p. 110–
f
3
[
3
M+2] , 396 (11) [M] , 383 (30) [M+2À CH ] , 381 (30) [M–CH ] ,
3
3
1
12°C].
22 (17), 219 (38), 131 (30), 111 (38), 97 (70), 83 (66), 71 (100). HRMS
+
(
EI): Calcd. for C H BrO [M] 395.9997, found 396.0025.
20 13 4
3
6
,10-Dimethoxycyclobuta[b]anthracene-1,5,8(2H)-trione (36) and
,10,17,21-Tetramethoxynonacene-1,4,8,12,15,19-hexaone (40):
GP2, benzodicyclobutene derivative 19 or 20 (224 mg, 1 mmol),
benzoquinone (25) (270 mg, 2.5 mmol). Direct column chromatog-
raphy of the reaction mixture (TBME/PE 1:6) gave 36 (Method A:
105 mg, 0.34 mmol, 34%; method B: 126 mg, 0.41 mmol, 41%);
Eur. J. Org. Chem. 2021, 1–16
www.eurjoc.org
13
© 2021 The Authors. European Journal of Organic Chemistry published
by Wiley-VCH GmbH
��
These are not the final page numbers!

Full Papers
doi.org/10.1002/ejoc.202100848
IR (neat): v˜=2922 (s), 2852 (s), 1738 (m), 1672 (m), 1591 (m), 1459
[15] H. Ohara, H. Kiyokane, T. Itoh, Tetrahedron Lett. 2002, 43, 3041–3044.
[
[
[
16] M. Potowski, M. Schürmann, H. Preut, A. P. Antonchick, H. Waldmann,
Nat. Chem. Biol. 2012, 8, 428–430.
17] M. D. Rozeboom, I. M. Tegmo-Larsson, K. N. Houk, J. Org. Chem. 1981,
(
1
m), 1379 (m), 1324 (m), 1282 (s), 1212 (m), 1144 (m), 1067 (m),
003 (s), 968 (m), 923 (m), 797 (m), 715 (s), 614 (w) cm . H NMR
À 1
1
(
CDCl , 400 MHz): δ=4.00 (s, 3H, 12-OCH ), 4.10 (s, 3H, 2-OCH ), 5.01
3
3
3
46, 2338–2345.
(
s, 2H, H2), 7.85–7.88 (m, 2H, H7, H8), 8.41–8.44 (m, 2H, H6, H9), 9.08
s, 2H, H4, H11) ppm. C NMR (CDCl , 100 MHz): δ 57.5 (CH , C2),
18] J. Breton, E. Nabedryk, Biochimica Biophysica Acta 1996, 1275, 84–90.
13
(
3
2
[19] T. S. Kashey, D. D. Luu, J. C. Cowgill, P. L. Baker, K. E. Redding, Photo-
synth. Res. 2018, 138, 1–9.
6
2.2 (CH , 3-OCH ), 64.5 (CH , 12-OCH ), 124.3 (CH, C4), 124.7 (CH,
3 3 3 3
C11), 127.5 (CH, C6), 127.5 (CH, C9), 129.1 (C, C2a), 129.5 (C, C12a),
30.2 (C, C10a), 130.6 (C, C3a), 131.8 (C, C11a), 133.4 (C, C4a), 134.2
CH, C7), 134.2 (CH, C8), 134.5 (C, C5a), 134.5 (C, C9a), 152.3 (C, C3),
[20] W. Lubitz, Pure Appl. Chem. 2003, 75, 1021–1030.
[21] C. Cleren, L. Yang, B. Lorenzo, N. Y. Calingasan, A. Schomer, A. Sireci,
E. J. Wille, M. F. Beal, J. Neurochem. 2008, 104, 1613–1621.
1
(
[
22] S. Pepe, S. F. Marasco, S. J. Haas, F. L. Sheeran, H. Krum, F. L. Rosenfeldt,
1
52.8 (C, C12), 182.1 (C, C5), 182.87 (C, C10), 182.94 (C, C1) ppm.
À 1
Mitochondrion 2007, 7, S154-S167.
[23] R. Nageswara Rao, M. V. N. K. Talluri, D. D. Shinde, J. Pharm. Biomed.
Anal. 2008, 47, 230–237.
[24] T. C. Rodick, D. R. Seibels, J. R. Babu, K. W. Huggins, G. Ren, S. T.
Mathews, Nutr. Diet. Suppl. 2018, 10, 1–11.
[25] O. Benzakour, Thromb. Haemostasis 2008, 100, 527–529.
UV/Vis (THF): λ (ɛ)=427 (23.2), 305 (83.0), 297 (81.5) nm (mM
cm ). Fluorescence (THF):
max
À 1
λex=267 nm; λem=415, 602 nm. MS
+
+
(
70 eV): m/z (%) 360 (3) [M+2] , 347 (9) [M+2À CH ] , 329 (81) [M-
3
OCH ]. HRMS (ESI): Calcd. for C H O 360.0998, found 360.0963 [M
3
22 14
+
+
+
2] ; calcd. for C H O [MÀ CH =O] 329.0814, found 329.0788.
21
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Keywords: Benzocyclobutenes · Bidirectional acene synthesis ·
Cycloaddition · Photophysical properties · Quinones
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© 2021 The Authors. European Journal of Organic Chemistry published
by Wiley-VCH GmbH
��
These are not the final page numbers!

FULL PAPERS
Dr. A. Mohamed Abdelmoniem,
Prof. Dr. I. Abdelshafy Abdelhamid,
Prof. Dr. H. Butenschön*
1
– 16
The synthesis of some new extended
polycyclic quinones was achieved
through a one-pot procedure com-
prising the cycloaddition of benzocy-
clobutene and benzodicyclobutene
derivatives with benzoquinone or
naphthoquinone. Notably, a highly
symmetrical nonacene-hexaone could
be prepared, albeit in low yields (5–
7%).
Bidirectional Synthesis, Photo-
physical and Electrochemical
Characterization of Polycyclic
Quinones Using Benzocyclobu-
tenes and Benzodicyclobutenes as
Precursors