Convenient synthesis of Dibenzo[a, h]anthracenes and picenes via C-H arylation of acetophenones with arenediboronates

Article abstract of DOI:10.1246/cl.2011.300

A new convenient method for the synthesis of dibenzo-[a, h]anthracenes and picenes using ruthenium-catalyzed regioselective C-H arylation of aromatic ketones has been developed. Acetophenone derivatives and 1, 4-benzenediboronates were coupled in 2:1 ratios to form p-terphenyl derivatives. Conversion of the acetyl group to an ethynyl group, followed by cycloaromatization provided the desired fused aromatic compounds. An organic field-effect transistor fabricated from one of these products gave moderate hole mobility.

Full text of DOI:10.1246/cl.2011.300

doi:10.1246/cl.2011.300
300
Published on the web February 16, 2011
Convenient Synthesis of Dibenzo[a,h]anthracenes and Picenes
via C-H Arylation of Acetophenones with Arenediboronates
Kentaroh Kitazawa,1 Takuya Kochi,1 Masashi Nitani,2 Yutaka Ie,2 Yoshio Aso,2 and Fumitoshi Kakiuchi*1
1Department of Chemistry, Faculty of Science and Technology, Keio University,
3-14-1 Hiyoshi, Kohoku-ku, Yokohama, Kanagawa 223-8522
2The Institute of Scientific and Industrial Research (ISIR), Osaka University, 8-1 Mihogaoka, Ibaraki, Osaka 567-0047
(Received January 11, 2011; CL-110023; E-mail: kakiuchi@chem.keio.ac.jp)
A new convenient method for the synthesis of dibenzo-
R
R
[a,h]anthracenes and picenes using ruthenium-catalyzed regio-
selective C-H arylation of aromatic ketones has been developed.
Acetophenone derivatives and 1,4-benzenediboronates were
coupled in 2:1 ratios to form p-terphenyl derivatives. Conversion
of the acetyl group to an ethynyl group, followed by
cycloaromatization provided the desired fused aromatic com-
pounds. An organic field-effect transistor fabricated from one of
these products gave moderate hole mobility.
step c
step b
R'
R
R
and/or
cyclization
dehydration
R
R
R
step a
cat.
[RuH2(CO)(PPh3)3]
pinacolone, reflux
R'
O
O
O
H
R'
R
R
+
B
B
C–H arylation
O
B =
B
Fused aromatic compounds have recently received much
attention due to their potential utility for organic optical and
electronic materials.1 For example, linear acenes and their
derivatives such as pentacene2 and rubrene3 are promising
candidates for organic field-effect transistors (OFETs). Develop-
ment of methods to prepare various derivatives of desired fused
aromatic compounds is of great interest because derivatization
of the fused aromatic compounds may adjust their optical and
electronic properties as well as their solubility and packing
structures in the crystals.4 In this context, our group has initiated
efforts toward short syntheses of multisubstituted fused aromatic
compounds using our ruthenium-catalyzed C-H arylation5 of
aromatic ketones with arylboronates. This reaction not only
provides convenient ways to form a biaryl framework but is also
useful for further manipulation because the directing group (the
carbonyl group) can be easily transformed into other classes of
functional groups. Recently, we succeeded in two- and three-
step syntheses of tetra- and hexaarylanthracenes from anthra-
quinone, taking advantage of the C-H arylation method.6
Dibenzo[a,h]anthracenes7 and picenes8 are two classes of
fused aromatic compounds which have recently shown some
promise as OFET materials. However, only limited strategies
have been reported for the syntheses of these compounds,9-12
and convenient methods for the syntheses of various derivatives
are still desired. We envisioned that the use of our ruthenium-
catalyzed C-H arylation would lead to a new method for
efficient synthesis of these compounds (Figure 1). First, aceto-
phenone derivatives may be coupled with arenediboronates
to give teraryl derivatives using our C-H arylation method
(step a).5 The acetyl directing group may then be transformed
into an ethynyl group (step b).13 Finally, cycloaromatization
of the 2-ethynylbiphenyl structures would provide the desired
fused aromatic compounds (step c).14 The cyclization of
2-ethynylbiphenyls is a practical way to construct phenanthrene
ring systems and has been used in various syntheses of
dibenzo[a,h]anthracenes. For example, Swager and co-workers
developed a three-step strategy to construct the dibenzo[a,h]-
O
Figure 1. Retrosynthesis of dibenzo[a,h]anthracene and pi-
cene derivatives.
Miyaura/cyclization sequence, starting from 1,4-dibromo-2,5-
diiodobenzene.9 This method has provided convergent routes
to a variety of dibenzo[a,h]anthracene derivatives,10 but the
position of the substituents at the terminal rings are directed by
the substituents themselves in the cyclization step. Therefore, it
may be difficult to introduce substituents selectively at the 2-, 4-,
9-, or 11-positions. Introduction of substituents at the central
rings may also be difficult using this method.
Herein, we report an efficient synthesis of dibenzo[a,h]-
anthracene and picene derivatives via our ruthenium-catalyzed
C-H arylation method. First, it was found that the C-H arylation
can be applied to the 2:1 coupling of acetophenones with
arenediboronates to form teraryl structures. The teraryl products
were converted to dibenzo[a,h]anthracenes and picene deriva-
tives in two steps. The OFET properties of the fused aromatic
compounds obtained were also studied.
Initially, we examined the synthesis of fused aromatic
systems using 2¤-methylacetophenone (1a) with a 1,4-benzene-
diboronate, 2,2¤-(1,4-phenylene)bis(5,5-dimethyl-1,3,2-dioxa-
borinane) (2a). The ruthenium-catalyzed C-H arylation and
dehydration with LDA and (EtO)2P(O)Cl using Negishi’s
protocol13 afforded the corresponding diethynylterphenyl deriv-
ative in good yield,15 but step c did not proceed selectively and
gave an inseparable mixture of dibenzo[a,h]anthracene and
picene derivatives.
The regioselectivity of the electrophilic intramolecular
cycloaromatization was controlled by placing substituents at
appropriate positions on arenediboronates or using a thiophene
derivative. The arenediboronates used for the selective synthesis
of the fused aromatic compounds are listed in Figure 2, as well
as acetophenones coupled with the diboronates.
First, we explored the synthesis of dibenzo[a,h]anthracene
derivatives using diboronate 2b (Table 1). This diboronate
anthracene cores using
a
Hagihara-Sonogashira/Suzuki-
Chem. Lett. 2011, 40, 300-302
© 2011 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

301
(a) Acetophenone derivatives (1)
Table 1. Three-step syntheses of dibenzo[a,h]anthracene 6 and
picene 7a
O
O
1a: R" = H
1b: R" = Me
1c: R" = OC10H21
step a
step b
1) LDA
step c
Me3Si
2) (EtO)2P(O)Cl
cat. PtCl2
cat. 3
pinacolone
reflux
1
+ 2
4
5
6 or 7
R"
1d
3) LDA
4) H3O+
(b) Arenediboronate (2)
4 (R" = Ac);
5 (R" = C≡CH),
R"
Fused aromatic
compounds 6, 7
B
B
B
B
Entry
1
1
2
B
B
S
2b
2c
2d
1a 2b
1b 2b
Figure 2. (a) Acetophenones and (b) arenediboronates used
for the synthesis of fused aromatic systems.
R"
4ab, 41%;b 5ab, 69%
6ab, 36%
6bb, 38%
R"
possesses C-H bonds para to each other and should give
dibenzo[a,h]anthracenes selectively. The C-H arylation of 1a
with 2b carried out under pinacolone refluxing conditions gave
teraryl product 4ab in 41% yield and treatment of 4ab with LDA
and (EtO)2P(O)Cl provided diethynylterphenyl 5ab in 69%
yield (Entry 1). Platinum-catalyzed intramolecular cycloaroma-
tization of 5ab afforded the desired dibenzo[a,h]anthracene
derivative 6ab in 36% isolated yield.14c Arylation of other
acetophenones, such as 2¤,4¤-dimethyl-, 2¤-methyl-4¤-n-decyl-
oxy-, and 3¤-trimethylsilylacetophenones (1b, 1c, and 1d,
respectively) conducted under the same reaction conditions also
gave the corresponding coupling products in 43-69% yields
(Entries 2-4). Conversion of the acetyl group to ethynyl group in
4bb-4db proceeded smoothly to give 2,2¤¤-diethynyl-1,1¤:4¤,1¤¤-
terphenyls 5bb-5db in good yields. The cycloaromatization of
terphenyls 5bb and 5cb yielded dibenzo[a,h]anthracenes 6bb
and 6cb in moderate yields. The yield of the reaction using 6db
is much lower than other substrates (Entry 4) probably due to
desilylation and/or related side reactions.
Syntheses of picene derivatives were also examined.
Diboronates 2c and 2d were used for this purpose, because
they possess two C-H bonds next to each other on the aromatic
rings. The C-H arylation of 1a using 1,2-dimethylphenyl-3,6-
diboronate 2c afforded teraryl product 4ac in 14% yield
(Entry 5), while thiophene derivative 2d was coupled with 1a
to give the corresponding product 4ad in 38% yield (Entry 6).
The actual cause of the low yield obtained for the coupling with
2c is unclear at present, but the repulsion between two methyl
groups on diboronate 2c may create more severe steric
congestion during the coupling. Transformations of the acetyl
group to the ethynyl group of 4ac and 4ad were then carried out
to form diethynylteraryls 5ac and 5ad in 42% and 61% yields,
respectively. Subsequent cycloaromatizaion using PtCl2 catalyst
provided the desired picene 7ac and thiophene-containing
derivative16 7ad in 51% and 73% yields, respectively. Other
thiophene-containing picene derivatives 7bd and 7cd were
prepared in similar manner using acetophenone derivatives 1b
and 1c with arenediboronate 2d (Entries 7 and 8).
2
3
R"
4bb, 43%; 5bb, 76%
R
R"
R
1c 2b
1d 2b
R"
R = OC10H21
R
R
R = OC10H21
6cb, 26%
4cb, 69%; 5cb, 55%
R"
R
R
R
R
4
R"
R = Me3Si
R = Me3Si
4db, 55%;c 5db, 78%
6db, 3%
R"
R"
5
6
1a 2c
1a 2d
4ac, 14%;d 5ac, 42%
7ac, 51%
R"
R"
S
S
7ad, 73%e
4ad, 38%; 5ad, 61%
R"
R"
S
S
7
8
1b 2d
1c 2d
7bd, 67%
4bd, 38%; 5bd, 44%
R"
R"
S
S
R
R
R
R
R = OC10H21
R = OC10H21
7cd, 67%
4cd, 51%; 5cd, 58%
aConditions: (step a) 1 (4 mmol), 2 (2 mmol), 3 (0.4 mmol),
pinacolone (4 mL), reflux, 24 h; (step b) LDA (2.1 equiv),
THF, ¹78 °C to reflux; (EtO)2P(O)Cl (2.1 equiv), ¹78 °C to rt;
LDA (4.5 equiv), ¹78 °C to rt; 1 M HCl aq.; (step c) PtCl2
(10 mol %), toluene, 120 °C, 24 h. Performed on a 1.75-fold
scale. Performed on a 2.5-fold scale. 1a (9.0 mmol), 2c (4.5
mmol), 3 (0.90 mmol), pinacolone (4.5 mL), toluene (4.5 mL),
b
c
d
Dibenzo[a,h]anthracenes are expected to exhibit good
OFET properties,7 but there are only a limited number of
studies concerning measurements of OFET properties of these
types of compounds. Thus, properties and OFET performance of
6db were preliminarily investigated. The UV-vis absorption
spectrum in chloroform shows an absorption maximum at
317 nm and a broad shoulder between 300 and 390 nm (see
Figure S1 in the Supporting Information; SI17). Cyclic voltam-
metry (CV) was performed in dichloromethane containing 0.1 M
e
reflux, 65 h. Performed at 80 °C.
n-Bu4NPF6. The half-wave oxidation potentials were found at
+0.74 V (Figure 3a). Based on this value, the highest occupied
molecular orbital (HOMO) level of 6db was estimated to be
¹5.54 eV, which is a suitable level as active materials for p-type
OFETs. The performance of OFET of 6db was evaluated by
Chem. Lett. 2011, 40, 300-302
© 2011 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

302
(a)
(b)
2000, 122, 12876. See also ref. 1.
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10–6
10–8
4.0
3.0
2.0
1.0
0
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0
+0.5
+1.0
−80 −60 −40 −20
VGS / V
0
20
E vs. Fc/Fc+ / V
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employing bottom-contact configuration. The active layer was
prepared by spin-coating from 1.0 wt % chloroform solution.
As expected, this compound exhibited typical p-type
characteristics, and the hole mobility is found to be 3.0 ©
¹1
10¹4 cm2 V¹1 s with the on/off current ratio of 1.3 © 107 and
the threshold voltage of 0 V. Its transfer characteristics are shown
in Figure 3b.
In summary, we developed a new convenient method for the
synthesis of fused aromatic compounds such as dibenzo[a,h]-
anthracenes and picenes via our ruthenium-catalyzed C-H
arylation method. The arylation of acetophenones using 3 as a
catalyst was found to be applicable for the construction of teraryl
frameworks using arenediboronates 2. The C-H arylation,
conversion of acetyl group to an ethynyl group, and platinum-
catalyzed intramolecular cycloaromatization constitutes a three-
step procedure to provide dibenzo[a,h]anthracenes 6 and picenes
7. The OFET properties of 6db fabricated on the OFET devices
with a bottom-contact configuration have been measured. This
compound exhibited moderate hole mobility (®FET = 3.0 ©
10¹4 cm2 V¹1 s¹1).
This work was supported in part by a Grant-in-Aid for
Scientific Research from the Ministry of Education, Culture,
Sports, Science and Technology, Japan, by The Science
Research Promotion Fund, and by the Cooperative Research
Program of “Network Joint Research Center for Materials and
Devices.” K. K. acknowledges JSPS Research Fellowships for
Young Scientists.
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17 Supporting Information is available electronically on the CSJ-Journal
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Chem. Lett. 2011, 40, 300-302
© 2011 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/