Facile Synthesis of Rubicenes by Scholl Reaction

Article abstract of DOI:10.1055/s-0036-1588570

The treatment of 9,10-diphenylanthracenes with DDQ in the presence of TfOH readily gave the corresponding rubicene derivatives in good yields. The effects of oxidant, acid, substituent, and other conditions are discussed. This protocol involving the Schol

Full text of DOI:10.1055/s-0036-1588570

SYNTHESIS
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© Georg Thieme Verlag Stuttgart · New York
2017, 49, A–E
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M. Kawamura et al.
Paper
Synthesis
Facile Synthesis of Rubicenes by Scholl Reaction
Masahiko Kawamura
R
R
Eiji Tsurumaki
Shinji Toyota*
Scholl reaction
DDQ
Department of Chemistry, School of Science, Tokyo Institute of
Technology, 2–12–1 Ookayama, Meguro-ku, Tokyo 152-8551,
Japan
TfOH
CH2Cl2
stoyota@cms.titech.ac.jp
R = H, t-Bu, OMe
53–80% yield
R
R
rubicenes
Received: 26.07.2017
Accepted after revision: 28.08.2017
Published online: 25.09.2017
to form tetrabenzopyracylenes.12,13 To the best of our
knowledge, there are no reports of a similar approach to ru-
bicene from 9,10-diphenylanthracene. We herein report an
efficient synthesis of rubicene and its derivatives by utiliz-
ing the Scholl reaction.
DOI: 10.1055/s-0036-1588570; Art ID: ss-2017-f0485-op
Abstract The treatment of 9,10-diphenylanthracenes with DDQ in the
presence of TfOH readily gave the corresponding rubicene derivatives in
good yields. The effects of oxidant, acid, substituent, and other condi-
tions are discussed. This protocol involving the Scholl reaction is conve-
nient for the preparation of some rubicene derivatives from conven-
tional starting materials.
R
Key words rubicene, Scholl reaction, DDQ, oxidation, arenes
1a: R = H
1b: R = t-Bu
1c: R = OMe
Rubicene (1a, C26H14), a parent aromatic system consist-
ing of five benzene rings and two five-membered rings, has
been known for more than a century, and its name originat-
ed from its characteristic ruby color (Figure 1).1 This aro-
matic compound, which is a substructure of C70,2 has re-
cently drawn the attention of chemists engaged in the mo-
lecular design of novel aromatic compounds, such as
organic electroluminescence and light-emitting diode de-
vices.3 Rubicene was originally synthesized by the reductive
dimerization of 9-fluorenone with Mg,4 and subsequent
synthetic methods include the cyclization of halogenated
9,10-diphenylanthracenes under strongly basic conditions,5
the Heck reaction,6 and the Friedel–Crafts type reaction
(Scheme 1).7,8 Although these reactions gave rubicene,
harsh conditions were required and the yields were not al-
ways high. In order to develop an efficient synthesis of rubi-
cene and its derivatives, we adopted the Scholl reaction9 us-
ing the readily available starting material, 9,10-diphenylan-
thracene,10 a fluorescence standard. The Scholl reaction
involves the dehydrogenative coupling between arene com-
pounds and oxidants or Lewis acids, and is widely utilized
for the construction of highly fused polyaromatic hydrocar-
bons (PAHs).11 Recently, Murata’s group applied this reac-
tion to the cyclization of 5,11-diphenyltetracene derivatives
R
Figure 1 Structure of rubicenes 1
O
KOH
quinoline
Mg
Cl
Condensation
Substitution
F
Pd
[i-Pr3Si]+
Cl
Heck
Friedel–Crafts
1a
F
Scheme 1 Previous synthetic methods of rubicene (1a)
We first reacted 9,10-diphenylanthracene (2a) with FeCl3,
utilizing the reaction conditions for 5,11-diphenyltetracene
(Scheme 2).12a The reaction with 8.0 equivalents of FeCl3 at
room temperature for 24 hours gave only a trace amount of
rubicene (1a) and 8-phenylbenzo[a]fluoranthene (3a), a
© Georg Thieme Verlag Stuttgart · New York — Synthesis 2017, 49, A–E

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M. Kawamura et al.
Paper
Synthesis
monocyclization product, in 25% yield. Another product
was also obtained in 10% yield, and its structure was deter-
mined by single crystal X-ray analysis to be 3-chloro-8-
phenylbenzo[a]fluoranthene (4), as shown in Figure 2. The
formation of this product means that the chlorination by
FeCl3 competes with the second cyclization.14
rubicene was found in the reaction products. This simple
protocol readily gave rubicene in high yield from the con-
ventional starting material.
DDQ or
chloranil
Cl
+
TfOH
CH2Cl2
FeCl3
+
MeNO2/
2a
1a
3a
CH2Cl2
Scheme 3 Scholl reaction of 2a with DDQ or chloranil
2a
3a
25%
4
10%
We could control the cyclization step by changing the
amount of DDQ (Table 1). The reaction with 1.75 equiva-
lents of DDQ gave 3a as the major product (64%). The reac-
tion did not work at all when methanesulfonic acid (MsOH)
was used as the acid instead of TfOH.9c,13a Therefore, a
strong acid is essential for the completion of the cyclization.
This result means that protonation in the arenium ion
mechanism should play an important role in the overall
steps, although the radical cation mechanism cannot be
ruled out.9c–e We were able to use chloranil (tetrachloro-
1,4-benzoquinone) as the oxidant for the rubicene synthe-
sis as well, although the reaction was slower than that using
DDQ under the same conditions. The reaction with 3.0
equivalents of chloranil at 0 °C gave a small amount of 1a in
10 minutes, but 1a became the major product in 4 hours.
The slow reaction with chloranil is consistent with its oxi-
dizing ability being weaker than that of DDQ.16
+
1a
0%
Scheme 2 Scholl reaction of 2a with FeCl3
The above method was then applied to the synthesis of
some substituted rubicenes from the corresponding diphe-
nylanthracene derivatives (Scheme 4). The reaction of 9,10-
bis(4-tert-butylphenyl)anthracene (2b) with DDQ (3.0
equiv) was slow at 0 °C, and the product ratio of 1b and 3b
was 72:28 even at 1 hour. Even though the reactions were
carried out under various conditions, namely, at room tem-
perature, for a long time, or with an increased amount of
DDQ, the product ratios were not improved and the
amounts of unidentified byproducts increased. From the re-
action mixture under the above conditions (0 °C, 1 h, and
3.0 equiv of DDQ), 5,12-di-tert-butylrubicene (1b) was ob-
tained in 53% yield as a red solid. The reaction of 9,10-bis(4-
methoxyphenyl)anthracene (2c) proceeded more slowly
than that of 2b, so the reaction mixture was heated at 60 °C
in CHCl3. The reaction under the above-mentioned condi-
tions was completed in 2 hours, and 5,12-dimethoxyrubi-
cene (1c) was obtained in 76% isolated yield as a purple sol-
id, where most of the loss was attributed to the low solubil-
ity during purification.
Figure 2 An ORTEP drawing of the X-ray crystal structure of 4; thermal
ellipsoids are set at 50% probability
Then, 2,3-dichloro-4,5-dicyano-1,4-benzoquinone (DDQ)
was used as the oxidant in trifluoromethanesulfonic acid
(TfOH) according to the literature method.15 When 2a was
treated with 3.0 equivalents of DDQ in the presence of TfOH
in CH2Cl2, the reaction proceeded smoothly at 0 °C and was
completed within 10 minutes (Scheme 3). Quenching fol-
lowed by column chromatography on silica gel afforded
pure 1a in 80% isolated yield as a deep red solid and no iso-
© Georg Thieme Verlag Stuttgart · New York — Synthesis 2017, 49, A–E

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M. Kawamura et al.
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Synthesis
Table 1 Scholl Reactions of 2a with DDQ or Chloranil under Various Conditionsa
Product ratio (%)b
Entry
Oxidant
Equiv
Acidc
TfOH
Time (Min)
Temp (°C)
1a
3a
2a
1
2
3
4
5
6
7
DDQ
3.00
2.50
1.75
1.00
3.00
3.00
3.00
10
10
0
100 (80)
0
0
DDQ
TfOH
TfOH
TfOH
MsOH
TfOH
TfOH
0
68
32
0
DDQ
10
0
19 (14)
74 (64)
49
7 (5)
DDQ
10
0
2
0
49
DDQ
2880
10
r.t.
0
0
100
54
0
chloranil
chloranil
4
42
240
0
84
16
a The reactions were carried out in CH2Cl2.
b The ratios of the two products and the starting material were determined by 1H NMR analysis. Isolated yields in parentheses.
c The solvent/acid ratio was 19:1 (v/v).
added to H2O (50 mL). The organic materials were extracted with
CH2Cl2 (3 × 40 mL). The combined organic layers were dried (Na2SO4)
and evaporated. The crude products were separated by chromatogra-
phy (silica gel, hexane).
R
R
R
DDQ
TfOH
8-Phenylbenzo[a]fluoranthene (3a)5b
+
[CAS Reg. No. 500310-14-5]
Yield: 4.9 mg (25%); yellowish orange solid; mp 185–187 °C (Lit.5b mp
185–186 °C); Rf = 0.44 (hexane/CH2Cl2 3:2).
R
1H NMR (400 MHz, CDCl3): δ = 7.42 (t, J = 7.6 Hz, 2 H), 7.49–7.62 (m, 7
H), 7.67 (dd, J = 4.0, 8.4 Hz, 2 H), 7.94 (d, J = 8.8 Hz, 1 H), 8.04 (t, J = 7.0
Hz, 2 H), 8.44 (d, J = 7.6 Hz, 1 H), 8.85 (d, J = 8.4 Hz, 1 H).
13C NMR (125 MHz, CDCl3): δ = 120.1, 121.7, 123.8, 124.3, 124.8,
126.4, 126.7, 127.1, 127.2, 127.6, 127.9, 128.1, 128.9, 129.1, 130.0,
131.0, 131.6, 132.4, 136.8, 137.9, 139.0, 139.1, 140.3.
R
R
b: R = t-Bu
c: R = OMe
2
1
3
Scheme 4 Scholl reactions of 2b and 2c with DDQ
In summary, we have found a facile synthetic method
for rubicene and its derivatives by using the DDQ/TfOH sys-
tem under mild conditions. Because some substituted 9,10-
diphenylanthracene derivatives can be prepared from an-
thraquinone or 9,10-dihaloanthracenes, this protocol will
be useful for the synthesis of functionalized rubicenes,
which would be valuable as core structures of functional
materials and the scaffolds for the construction of planar or
curved PAH systems.
HRMS (FAB): m/z calcd for C26H16 [M]+: 328.1252; found: 328.1248.
3-Chloro-8-phenylbenzo[a]fluoranthene (4)
Yield: 2.2 mg (10%); yellow solid; mp 193–195 °C; Rf = 0.48 (hex-
ane/CH2Cl2 3:2).
1H NMR (400 MHz, CDCl3): δ = 7.43 (ddd, J = 0.8, 1.6, 6.4 Hz, 1 H), 7.46
(dd, J = 1.6, 8.0 Hz, 1 H), 7.51 (dd, J = 1.2, 7.2 Hz, 1 H), 7.52 (d, J = 2.4
Hz, 1 H), 7.54–7.63 (m, 4 H), 7.68 (ddd, J = 1.2, 2.0, 6.8 Hz, 1 H), 7.71
(d, J = 8.8 Hz, 1 H), 7.94 (d, J = 9.2 Hz, 1 H), 7.99 (d, J = 2.0 Hz, 1 H), 8.01
(d, J = 6.8 Hz, 1 H), 8.32 (d, J = 8.0 Hz, 1 H), 8.76 (d, J = 8.4 Hz, 1 H).
13C NMR (100 MHz, CDCl3): δ = 120.8, 122.0, 124.1, 124.3, 125.0,
127.0, 127.1, 127.4, 127.4, 127.5, 127.8, 128.1, 129.0, 129.1, 129.9,
130.2, 131.5, 132.2, 132.4, 135.6, 137.7, 138.5, 139.6, 140.5.
Melting points are uncorrected. NMR spectra were measured on a
JEOL JNM-ECS 400 (1H: 400 MHz, 13C: 100 MHz) or JNM-ECZ500R
spectrometer (1H: 500 MHz, 13C: 125 MHz). High-resolution mass
spectra were measured with a JEOL JMS-700 MStation mass spec-
trometer. Column chromatography was carried out with Wako Gel C-
300 (45–75 mesh). 9,10-Diphenylanthracene derivatives were pre-
pared from anthraquinone by a Grignard reaction followed by reduc-
tive aromatization by the general method.17
HRMS (FAB): m/z calcd for
C
26H1535Cl [M]+: 362.0862; found:
362.0819.
X-ray Crystal Structure Analysis of 418
A single crystal of 4 was prepared by recrystallization from CHCl3/
MeOH. Diffraction data were collected on a Rigaku Varimax with Saturn
system equipped with a Rigaku GNNP low-temperature device using
MoKα radiation (λ = 0.71075 Å) to a maximum 2θ value of 55.0° at
123 K. Equivalent reflections were merged and the images were pro-
cessed with the Rigaku CrysAlisPro program. The structure solution
was performed using the Yadokari-XG program19 as a graphical user
interface with SHELX-2013 as a set of structure determination pro-
Reaction of 9,10-Diphenylanthracene (2a) with FeCl3
In a 10 mL flask, FeCl3 (78.5 mg, 484 μmol) and MeNO2 (1 mL) were
added to a solution of 9,10-diphenylanthracene (2a; 20.0 mg, 60.5
μmol) in CH2Cl2 (2 mL). The deep blue solution was stirred for 24 h at
r.t. The reaction was quenched with H2O (5 mL), and the whole was
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D
M. Kawamura et al.
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Synthesis
grams. The structure was solved by the direct method (SHELXS) and
refined by full-matrix least squares method (SHELXL).20 Non-hydro-
gen atoms were refined anisotropically. Hydrogen atoms were includ-
ed in fixed positions. Formula C26H15Cl, M = 362.86, monoclinic, P21/n,
a = 14.7056(9), b = 5.4556(4), c = 22.7969(17) Å, β = 108.011(7)°, V =
1739.3(2) Å3, Z = 4, d = 1.3856 g cm–3, μ(MoKα) = 0.227 mm–1. Number
of reflection 3084 (all data), 2227 [I > 2.0σ(I)], number of reflection
used 3084, R1 = 0.0543, wR2 = 0.1667, GOF = 1.105.
HRMS (FAB): m/z calcd for C34H32 [M]+: 440.2504; found: 440.2459.
5,12-Dimethoxyrubicene (1c)5a
[CAS Reg. No. 219725-13-0]
In a Schlenk flask (10 mL), a solution of 2c22 (47.2 mg, 121 μmol) and
DDQ (83.8 mg, 363 μmol, 3.0 equiv) in anhyd CHCl3 (3.8 mL) was
stirred for 10 min at r.t. under N2. After the addition of TfOH (0.20 mL,
2.26 mmol), the mixture was stirred for 2 h at 60 °C. The mixture was
poured into aq NaHCO3 (50 mL) and rinsed with H2O (50 mL). The
formed precipitate was collected by filtration, and the solid was
washed with H2O (50 mL), MeOH (50 mL), and hexane (50 mL). The
crude product was purified by recrystallization from toluene/MeOH;
yield: 35.3 mg (76%); purple solid; mp 358–360 °C (partly sublimed)
(Lit.5a mp >300 °C); Rf = 0.21 (hexane/CH2Cl2 3:2).
Rubicene (1a); Typical Procedure
[CAS Reg. No. 197-61-5]
In a two-necked flask (10 mL), a solution of 2a (100 mg, 303 μmol)
and DDQ (210 mg, 909 μmol) in anhyd CH2Cl2 (9.5 mL) was stirred for
10 min in an ice bath at 0 °C under N2. After the addition of TfOH (0.50
mL, 5.65 mmol), the mixture was stirred for 10 min at 0 °C. The reac-
tion mixture was poured into aq NaHCO3 (50 mL) and rinsed with H2O
(50 mL). The formed precipitate was collected by filtration, and the
solid was washed with H2O (50 mL), MeOH (50 mL), and hexane (50
mL). The crude product was purified by chromatography (silica gel,
hexane/CH2Cl2 1:1); yield: 78.6 mg (80%); red solid; Rf = 0.37 (hex-
ane/CH2Cl2 3:2).
1H NMR (500 MHz, CDCl3): δ = 3.98 (s, 6 H), 7.00 (dd, J = 2.5, 8.5 Hz, 2
H), 7.54 (d, J = 2.5 Hz, 2 H), 7.76 (dd, J = 2.0, 9.0 Hz, 2 H), 7.99 (d, J = 7.0
Hz, 2 H), 8.21 (d, J = 8.5 Hz, 2 H), 8.56 (d, J = 9.0 Hz, 2 H).
3-Methoxy-8-(4-methoxyphenyl)benzo[a]fluoranthene (3c)
When the above reaction was performed in CH2Cl2 at 40 °C for 14 h,
3c was formed as the major product. The purification of the crude
product by chromatography (silica gel, hexane/CH2Cl2 5:1) gave pure
3c; yield: 19.5 mg (42%); orange solid; mp 201–205 °C; Rf = 0.26 (hex-
ane/CH2Cl2 3:2).
1H NMR (400 MHz, CDCl3): δ = 3.96 (s, 3 H), 3.99 (s, 3 H), 7.03 (dd, J =
2.8, 8.8 Hz, 1 H), 7.13 (dt, J = 2.8, 8.8 Hz, 2 H), 7.40 (ddd, J = 1.2, 6.8, 9.2
Hz, 1 H), 7.44 (dt, J = 2.8, 8.8 Hz, 2 H), 7.55 (dd, J = 6.4, 8.8 Hz, 1 H),
7.60 (d, J = 2.4 Hz, 1 H), 7.63 (ddd, J = 1.2, 6.8, 9.2 Hz, 1 H), 7.72 (d, J =
8.4 Hz, 1 H), 7.96 (d, J = 8.8 Hz, 1 H), 7.99 (d, J = 6.4 Hz, 1 H), 8.30 (d, J =
8.8 Hz, 1 H), 8.75 (d, J = 8.4 Hz, 1 H).
The reactions with DDQ or chloranil under various conditions were
similarly performed starting from 20.0 mg of 2a. After the reaction
mixture was quenched, the organic material in the reaction mixture
was extracted with CH2Cl2.
1H NMR (400 MHz, CDCl3): δ = 7.40 (td, J = 0.8, 7.2 Hz, 2 H), 7.47 (td,
J = 0.8, 7.2 Hz, 2 H), 7.79 (dd, J = 6.6, 8.6 Hz, 2 H), 7.98 (d, J = 7.6 Hz, 2
H), 8.03 (d, J = 6.4 Hz, 2 H), 8.33 (d, J = 7.6 Hz, 2 H), 8.61 (d, J = 8.8 Hz,
2 H).
5,12-Di-tert-butylrubicene (1b)5a
13C NMR (125 MHz, CDCl3): δ = 55.4, 55.7, 108.0, 112.9, 113.5, 119.9,
124.3, 124.3, 124.7, 126.7, 126.9, 127.0, 127.4, 128.4, 128.8, 130.0,
130.2, 131.0, 132.7, 133.5, 136.7, 137.6, 140.9, 159.0, 159.1.
[CAS Reg. No. 219725-19-6]
The reaction was similarly carried out with 2b21 (26.8 mg, 60.5 μmol)
and DDQ (41.9 mg, 182 μmol, 3.0 equiv). The reaction mixture was
stirred at 0 °C for 1 h. The crude product contained 1b and 3b in 72:28
ratio, which was separated by chromatography (silica gel, hex-
ane/CH2Cl2 10:1).
HRMS (FAB): m/z calcd for C28H20O2 [M]+: 388.1463; found: 388.1431.
Funding Information
1b
This work was partly supported by JSPS KAKENHI for Scientific Re-
Yield: 14.7 mg (53%); red solid; mp 385–395 °C (dec.) (Lit.5a mp >300
search (C) (26410060).
S
JPS
K
A
K
E
N
H
I
o
f
r
Secnifit
Research
2(
6
4
1
0
0
6
0)
°C); Rf = 0.57 (hexane/CH2Cl2 3:2).
When 3.2 equiv of DDQ was used, 16.5 mg (62%) of 1b was obtained.
Acknowledgment
1H NMR (400 MHz, CDCl3): δ = 1.47 (s, 18 H), 7.50 (dd, J = 2.0, 7.6 Hz, 2
H), 7.78 (dd, J = 6.4, 8.0 Hz, 2 H), 8.02 (d, J = 2.0 Hz, 2 H), 8.04 (d, J = 6.4
Hz, 2 H), 8.24 (d, J = 7.6 Hz, 2 H), 8.58 (d, J = 8.0 Hz, 2 H).
The authors thank Professor Yasuhiro Mazaki of Kitasato University
for his helpful assistance.
3-tert-Butyl-8-(4-tert-butylphenyl)benzo[a]fluoranthene (3b)
Supporting Information
Yield: 4.8 mg (18%), yellow solid; mp 250–252 °C; Rf = 0.63 (hex-
ane/CH2Cl2 3:2).
Supporting information for this article is available online at
1H NMR (400 MHz, CDCl3): δ = 1.48 (s, 9 H), 1.49 (s, 9 H), 7.41 (t, J = 8.8
Hz, 1 H), 7.46 (d, J = 7.6 Hz, 2 H), 7.52-7.57 (m, 2 H), 7.60 (d, J = 8.0 Hz,
2 H), 7.65 (t, J = 8.8 Hz, 1 H), 7.71 (d, J = 8.4 Hz, 1 H), 7.98 (d, J = 8.8 Hz,
1 H), 8.03 (d, J = 6.4 Hz, 1 H), 8.07 (s, 1 H), 8.33 (d, J = 8.0 Hz, 1 H), 8.81
(d, J = 9.2 Hz, 1 H).
https://doi.org/10.1055/s-0036-1588570.
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© Georg Thieme Verlag Stuttgart · New York — Synthesis 2017, 49, A–E