Synthesis of dibenz[a,h]anthracenes by Pd-catalyzed intramolecular double-cyclization of (Z,Z)-p-styrylstilbenes

Article abstract of DOI:10.1246/cl.2012.215

Dibenz[a,h]anthracene (1a) and its dimethoxy derivatives 1b and 1c were synthesized by the Pd-catalyzed intramolecular double-cyclization of the corresponding (Z,Z)-p-styrylstilbene derivatives 25, which were readily prepared by the Wittig reaction. The optical properties of the dibenz[a,h]anthracenes 1a1c are also presented.

Full text of DOI:10.1246/cl.2012.215

doi:10.1246/cl.2012.215
Published on the web February 18, 2012
215
Synthesis of Dibenz[a,h]anthracenes by Pd-Catalyzed Intramolecular Double-cyclization
of (Z,Z)-p-Styrylstilbenes
Rui Umeda, Satoshi Miyake, and Yutaka Nishiyama*
Faculty of Chemistry, Materials and Bioengineering, Kansai University,
3-3-35 Yamate-cho, Suita, Osaka 564-8680
(Received December 10, 2011; CL-111183; E-mail: nishiya@kansai-u.ac.jp)
Dibenz[a,h]anthracene (1a) and its dimethoxy derivatives
OHC
CHO
Br
1b and 1c were synthesized by the Pd-catalyzed intramolecular
double-cyclization of the corresponding (Z,Z)-p-styrylstilbene
derivatives 2-5, which were readily prepared by the Wittig
reaction. The optical properties of the dibenz[a,h]anthracenes
1a-1c are also presented.
PPh3
NaOH aq.
PPh3Br
Br
toluene
98%
CHCl3
23%
Br
Br
Br
(Z,Z )-2
Br
OHC
CHO
Br
Br
PPh3Br
Br
NaOH aq.
PPh3
toluene
99%
Fused polycyclic aromatic compounds have been attracting
a great deal of interest in view of the application of organic
materials for electronic devices such as organic field effect
transistors (OFETs) and light-emitting diodes (OLEDs).1 Among
them, linearly fused systems, such as pentacene (Chart 1) and its
derivatives, have been intensively studied for their application to
organic materials for electronic devices.2 Picene, which is an
isostructural analogue of pentacene, contains a condensed zigzag
form with five benzene rings. An FET with a thin film of picene
CHCl3
66%
Br
(Z,Z )-3
BrPh3P
Br
HBr aq.
(CH2O)n
OMe
OMe
OMe
PPh3
toluene
99%
AcOH
78%
MeO
MeO
MeO
PPh3Br
Br
CHO
Br
Br
OMe
¹1
NaOH aq.
shows a high ® value more than 1 cm2 V¹1 s and the FET
properties are clearly improved, not only the ® value but also
the on-off ratio under air/O2 conditions.3a Furthermore, alkali-
metal, potassium or rubidium, doped picene exhibits super-
conductivity below 18 K.3b Compared to pentacene and picene,
little is known for the application of dibenz[a,h]anthracene (1a),
which has angularly fused systems consisting five benzene rings,
to organic materials.4,5 Moreover, examples of the synthetic
methods of dibenz[a,h]anthracene and its derivatives are still
rare; (i) cyclization of naphthyl lactone derivative followed
by sequential transformation,6 (ii) photoinduced cyclization of
substituted 1,4-distyrylbenzenes,7 (iii) ring-closing olefin meta-
thesis of 2,2¤,5¤,2¤¤-tetravinyl-[1,1¤;4¤,1¤]terphenyl by Schrock
and Grubbs catalysts,8 (iv) ruthenium-catalyzed cyclization of
diethynylterphenyl,9 (v) tandem radical cyclization of (Z,Z)-1,4-
bis(2-iodostyryl)benzene derivatives,10 and (vi) ruthenium-cata-
lyzed C-H arylation of aromatic ketones.5 In addition, there are
several drawbacks for these methods, such as lengthy sequences
for their preparation, using expensive and unstable transition
metals, and limitation of the substrates.
CHCl3
43%
MeO
Br
(Z,Z )-4
Br
OHC
CHO
Br
PPh3Cl
Cl
Br
OMe
NaOH aq.
PPh3
toluene
94%
CHCl3
42%
MeO
Br
OMe
OMe
(Z,Z )-5 :(E,E )-5 = 92 : 8
Scheme 1. Synthesis of p-styrylstilbenes 2-5.
C-H arylation in the synthesis of dibenz[a,h]anthracenes. We
now report the synthesis of dibenz[a,h]anthracenes 1 by the Pd-
catalyzed intramolecular double-cyclization of the correspond-
ing (Z,Z)-p-styrylstilbene derivatives 2-5 and investigation of
the optical properties of 1.
The synthesis of p-styrylstilbenes 2-5 as precursors of
dibenz[a,h]anthracenes 1 is shown in Scheme 1.13 The reaction
of o-bromobenzyltriphenylphosphonium bromide and tereph-
thalaldehyde followed by the purification by silica gel chroma-
tography and recrystallization from CHCl3 gave the dibromo-
styrylstilbene (Z,Z)-2. Likewise, (Z,Z)-3 was synthesized from
benzyltriphenylphosphonium bromide and 2,5-dibromotereph-
pentacene
picene
dibenz[a,h]anthracene (1a)
thalaldehyde.
1,4-Bis(bromomethyl)-2,5-dimethoxybenzene,
Chart 1.
which was prepared by the reaction of 1,4-dimethoxybenzene,
HBr aq., and paraformaldehyde, was allowed to react with
triphenylphosphine to give the corresponding phosphonium
compound. The Wittig reaction of the above phosphonium
compound with o-bromobenzaldehyde afforded (Z,Z)-4 after
reprecipitation from a mixed solvent of CHCl3 and hexane. The
Wittig reaction of p-methoxybenzyltriphenylphosphonium chlo-
Pd-Catalyzed C-H arylation is a powerful method for the
construction of polycyclic aromatic carbons.11 Indeed, various
types of polycyclic aromatic carbons, including highly conju-
gated polyarenes, polycyclic heteroarenes, and helicenes, have
been prepared by Pd-catalyzed intramolecular C-H arylation.12
However, there is no example of the application of Pd-catalyzed
Chem. Lett. 2012, 41, 215-217
© 2012 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

216
cat.Pd(OAc)2 (0.01 mmol)
K2CO3 (0.60 mmol)
Table 1. Pd-Catalyzed intramolecular cyclization of (Z,Z)-p-
OMe
styrylstilbene 2
(Z,Z )-4
cat.Pd(OAc)2 (0.01 mmol)
DMF (6 mL)
110 °C, 12 h
(0.06 mmol)
MeO
additive (0.12 mmol)
K2CO3 (0.60 mmol)
Br
1b
41%
DMF (6 mL)
cat.Pd(OAc)2 (0.01 mmol)
K2CO3 (0.60 mmol)
OMe
Br
1a
(Z,Z )-2
(Z,Z )-5 : (E,E )-5 = 92 : 8
DMF (6 mL)
110 °C, 12 h
(0.06 mmol)
(0.06 mmol)
MeO
1c
Time Temp Recovery Yield of
15%
Entry Additive
/h
/°C
of 2/%a
1a/%a
Scheme 3. Synthesis of 7,14- and 2,9-dimethoxydibenz[a,h]-
anthracenes (1b and 1c) by Pd-catalyzed intramolecular cycliza-
tion.
1
2
3
4
5b
LiBr, TBAB
LiBr, TBAB
LiI, TBAB
®
®
1
12
6
6
12
12
80
110
110
110
110
56
®
12
2
®
41
34
81 (79)
®
75
aDetermined by H NMR with internal standard. The number
in parenthesis shows the isolated yield. (E,E)-Isomer 2 was
b
used.
cat.Pd(OAc)2 (0.01 mmol)
K2CO3 (0.60 mmol)
(Z,Z )-3
(0.06 mmol)
DMF (6 mL)
110 °C, 12 h
1a
84%
Scheme 2. Synthesis of dibenz[a,h]anthracene (1a) by Pd-
catalyzed intramolecular cyclization.
ride and 2,5-dibromoterephthalaldehyde gave the isomeric
mixture of styrylstilbene 5 in the ratio of (Z,Z):(E,Z):(E,E) =
49:15:36. Repeated recrystallization from toluene gave a 92:8
mixture of (Z,Z)- and (E,E)-5 in 42% combined yield.
First, the reaction of (Z,Z)-2 (0.06 mmol) in the presence of
a catalytic amount of Pd(OAc)2 (0.01 mmol), K2CO3 (0.60
mmol), LiBr (0.12 mmol), and tetrabutylammonium bromide
(TBAB) (0.12 mmol) at 110 °C gave 1a in moderate yield
(Entry 2 in Table 1). At a lower reaction temperature (80 °C),
the reaction did not proceed and the isomeric mixtures of 2 were
recovered together with uncharacterized polymeric materials
(Entry 1). With the addition of LiI instead of LiBr, the yield of
1a slightly decreased (Entry 3). It is interesting to note that the
reaction smoothly proceeded without additives, TBAB and LiBr,
to give dibenz[a,h]anthracene (1a) in high yield (Entry 4). When
the reaction using (E,E)-2 instead of (Z,Z)-2 was carried out
under the same conditions as Entry 4, the cyclization did not
proceed and the isomeric mixtures of 2 were recovered (Entry 5).
In all cases, the formation of [5]helicene was not observed.14
Next, the Pd-catalyzed intramolecular cyclization of (Z,Z)-3
was examined under the optimized conditions as Entry 4 in
Table 1 (Scheme 2). Dibenz[a,h]anthracene (1a) was formed
in good yield (84%). The treatment of (Z,Z)-4 and (Z,Z)-5,
which contained a small amount of (E,E)-5, under the same
conditions gave 7,14-dimethoxydibenz[a,h]anthracene (1b)15
and 2,9-dimethoxydibenz[a,h]anthracene (1c),16 respectively
(Scheme 3).
Figure 1. UV-vis absorption (top) and fluorescence (bottom)
spectra of dibenz[a,h]anthracenes 1a-1c in CH2Cl2.
Table 2. Optical properties of dibenz[a,h]anthracenes 1a-1c in
a
CH2Cl2
Fluorescence
em/nm
Absorption -abs/nm
¹1
(¾/M¹1 cm
)
-
Φb
1a 288 (46100), 298 (65200),
321 (10300), 334 (8500),
350 (7100), 373 (500), 395 (500)
1b 293 (81000), 303 (91100),
336 (13300), 351 (17300),
369 (16700), 404 (4700)
1c 305 (96500), 329 (17600),
343 (17200), 360 (19500),
384 (2900), 406 (2900)
398, 420, 446 0.035
413, 435, 464 0.118
411, 435, 463 0.083
The UV-vis and FL spectroscopy of dibenz[a,h]anthracenes
1a-1c were measured in CH2Cl2 (Figure 1) and the results of
the photophysical properties of 1a-1c are shown in Table 2. The
UV-vis spectrum of 1a shows the characteristic absorption
bands, such as the ¡-, ¢-, and para-bands, of aromatic
a1a (c = 3.59 © 10¹5 M), 1b (c = 2.96 © 10¹5 M), 1c (c =
b
2.96 © 10¹5 M). Absolute quantum yield measured using an
integrating sphere.
Chem. Lett. 2012, 41, 215-217
© 2012 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

217
hydrocarbons (see also Figure S1 in SI20).17 The absorption
maxima of 1a is bathochromically shifted by about 15 nm in
comparison to picene.18 The FL spectrum of 1a exhibits a
significant vibrational fine structure due to its rigid planarity. In
the case of 1b and 1c, their absorption maxima are slightly red-
shifted and the fluorescence quantum yields of them increased
compared to that of 1a.
In conclusion, we succeeded in the synthesis of dibenz-
[a,h]anthracene (1a) and the 7,14- and 2,9-dimethoxydibenz-
[a,h]anthracenes (1b and 1c) by the Pd-catalyzed intramolecular
double-cyclization of (Z,Z)-p-styrylstilbene derivatives 2-5.19
The optical spectra of dibenz[a,h]anthracenes 1a-1c exhibited
typical features of polycyclic aromatic hydrocarbons and were
significantly influenced by the positions of the substituted
methoxy groups.
8
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This research was supported by a Grant-in-Aid for Science
Research, and Strategic Project to Support the Formation of
Research Bases at Private Universities from the Ministry of
Education, Culture, Sports, Science and Technology of Japan.
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Table 1.
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Chem. Lett. 2012, 41, 215-217
© 2012 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/