Short synthesis of alkyl-substituted acenes using carbonyl-directed C-H and C-O functionalization

Article abstract of DOI:10.1021/ol301608m

A convenient method for the synthesis of tetraalkylanthracenes and -pentacenes by means of ruthenium-catalyzed regioselective C-H alkylation of the corresponding acenequinones was developed. Dialkyldiarylpentacene was also synthesized using chemoselective tandem C-H alkylation/C-O arylation of dimethoxypentacenequinone. It was suggested that a tetraalkylpentacene is stable under air in the dark and possesses an appropriate HOMO level as active material for p-type organic field-effect transistors (OFETs).

Full text of DOI:10.1021/ol301608m

ORGANIC
LETTERS
2012
Vol. 14, No. 15
3882–3885
Short Synthesis of Alkyl-Substituted
Acenes Using Carbonyl-Directed CꢀH
and CꢀO Functionalization
Daiki Matsumura,† Kentaroh Kitazawa,† Seiya Terai,† Takuya Kochi,† Yutaka Ie,‡
Masashi Nitani,‡ Yoshio Aso,‡ and Fumitoshi Kakiuchi*,†
Department of Chemistry, Faculty of Science and Technology, Keio University,
3-14-1 Hiyoshi, Kohoku-ku, Yokohama, Kanagawa, 223-8522, Japan,
and The Institute of Science and Industrial Research (ISIR), Osaka University,
8-1,7 Mihogaoka, Ibaraki, Osaka 567-0047, Japan
kakiuchi@chem.keio.ac.jp
Received June 11, 2012
ABSTRACT
A convenient method for the synthesis of tetraalkylanthracenes and -pentacenes by means of ruthenium-catalyzed regioselective CꢀH alkylation
of the corresponding acenequinones was developed. Dialkyldiarylpentacene was also synthesized using chemoselective tandem CꢀH alkylation/
CꢀO arylation of dimethoxypentacenequinone. It was suggested that a tetraalkylpentacene is stable under air in the dark and possesses an
appropriate HOMO level as active material for p-type organic field-effect transistors (OFETs).
Linear acenes have widely received much attention due
to their remarkable potential as organic semiconducting
materials.1 In particular, pentacene shows high charge mobi-
lity and is considered as one of the most promising materials.2
Semiconducting properties of pentacene fabricated by vapor
deposition have been extensively studied, but its poor solubi-
lity in almost all common organic solvents made it difficult to
prepare OFET devices by solution processes such as spin-
coating. In this context, a variety of substituents have been
introduced to the pentacene framework to improve its solu-
bility in organic solvents,3ꢀ7 and some of the pentacene
derivatives actually displayed OFET properties.4
While convenient methods are available for some pen-
tacene derivatives with aryl5 and alkynyl6 groups, penta-
cenes bearing alkyl groups on the central three rings are
† Keio University.
‡ Osaka University.
(1) (a) Anthony, J. Angew. Chem., Int. Ed. 2008, 47, 452. (b) Yamashita,
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Zagorska, M.; Djurado, D.; Demadrille, R. Chem. Soc. Rev. 2010, 39, 2577.
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r
10.1021/ol301608m
Published on Web 07/13/2012
2012 American Chemical Society

more difficult to prepare.7 For example, Takahashi and
co-workers have developed systematic methods using zir-
conacyclopentadienes to install alkyl groups at desired
positions during multistep construction of the pentacene
cores,7bꢀg but more efficient methods for preparation of
alkyl-substituted pentacenes are still desired.
Recently, we reported a short synthesis of tetraarylated
anthracenes using catalytic CꢀH arylation of anthraqui-
none with arylboronates.8,9 We envisioned that tetraalkyla-
cenes can also be prepared in a few steps, if the ruthenium-
catalyzed CꢀH/olefin coupling developed by our group,10
instead of the CꢀH arylation, is used for acenequinones,
common starting materials for acene syntheses (eq 1).
180 °C in an autoclave improved the yield to 82% (entry
3).12 The high-temperature condition was also effective for
the coupling with other olefins 2cꢀe. Allyltrimethylsilane
(2c) reacted with 1 to give 4c in 89% yield (entry 4). Both
aliphatic (entry 5) and aromatic olefins (entry 6) can be
used for the tetraalkylation reaction. In these cases, the
coupling products were isolated in excellent yields, but the
alkylation proceeded with 96:4 linear/branch selectivity.
Table 1. Reaction of Anthraquinone with Olefinsa
entry
2
R
4
isolated yield (%)
1
2a
2b
2b
2c
2d
2e
Et3Si
4a
4b
4b
4c
4d
4e
78
2
Me3Si
21
3b
4b,c
5b,c
6b
Me3Si
82
Me3SiCH2
C4H9
89
92d
quantd
Herein we report a short synthesis of tetraalkylated
anthracenes and pentacenes using ruthenium-catalyzed
CꢀH alkylation of acenequinones with olefins. Catalytic
chemoselective tandem CꢀH alkylation/CꢀO arylation11
of dimethoxypentacenequinone was also successfully ap-
plied for synthesis of a pentacene with two alkyl and two
aryl groups. Optical and electronic properties of the pen-
tacene products were also investigated.
First, we examined tetraalkylation of anthraquinone (1)
with olefins (Table 1). When the reaction of 1 with 10
equivalents (2.5 equiv to each ortho CꢀH bond) of
triethylvinylsilane (2a) was carried out at 115 °C in toluene
using RuH2(CO)(PPh3)3 (3) as a catalyst, tetraalkylated
anthraquinone 4a was obtained in 78% isolated yield
(entry 1). However, the coupling usingtrimethylvinylsilane
(2b) at 115 °C afforded the tetraalkylation product 4b only
in 21% yield (entry 2). Conduction of the reaction at
o-MeC6H5
a Reaction conditions: anthraquinone (1) (1 mmol), olefin (10 mmol),
RuH2(CO)(PPh3)3 (3) (0.1 mmol), toluene (1 mL), 115 °C, 48 h.
b Performed in mesitylene (1 mL) at 180 °C. c 20 equiv of olefin was
used. d Linear/branch = 96:4.
Anthraquinone derivative 4a was converted to the cor-
responding tetraalkylated anthracene 5a by repeating the
cycle of reduction with LiAlH4 and treatment with hydro-
chloric acid twice.13 Tetraalkylated anthracene 5a was
obtained in 68% isolated yield (eq 2).
(7) (a) Clar, E. Chem. Ber. 1949, 82, 495. (b) Takahashi, T.; Kitamura, M.;
Shen, B.; Nakajima, K. J. Am. Chem. Soc. 2000,122, 12876. (c) Takahashi, T.;
Li, S.; Huang, W.; Kong, F.; Nakajima, K.; Shen, B.; Ohe, T.; Kanno, K.-i.
J. Org. Chem. 2006, 71, 7967. (d) Takahashi, T.; Li, Y.; Hu, J.; Kong, F.;
Nakajima, K.; Zhou, L.; Kanno, K.-i. Tetrahedron Lett. 2007, 48, 6726.
(e) Takahashi, T.; Kashima, K.; Li, S.; Nakajima, K.; Kanno, K.-i. J. Am.
Chem. Soc. 2007, 129, 15752. (f) Li, S.; Li, Z.; Nakajima, K.; Kanno, K.-i.;
Takahashi, T. Chem. Asian. J. 2009, 4, 294. (g) Zhou, L.; Nakajima, K.;
Kanno, K.-i.; Takahashi, T. Tetrahedron Lett. 2009, 50, 2722.
Next, we applied the CꢀH alkylation/aromatization
strategy to the synthesis of tetraalkylpentacenes. The
reaction of pentacenequinone (6) with 2a at 115 °C gave
the tetraalkylated pentacenequinone 7a in 36% isolated
yield (Scheme 1). The low yield of 7a compared to that of
anthraquinone 5a may be attributed to the poor solubility
of 6 in toluene. The reaction was also attempted at 180 °C,
but significant dehydrogenative CꢀH alkenylation was
(8) Kitazawa, K.; Kochi, T.; Sato, M.; Kakiuchi, F. Org. Lett. 2009,
11, 1951.
(9) Use of catalytic CꢀH arylation for synthesis of acene derivatives:
(a) Yasukawa, T.; Satoh, T.; Miura, M.; Nomura, M. J. Am. Chem. Soc.
2002, 124, 12680. (b) Fukutani, T.; Hirano, K.; Satoh, T.; Miura, M.
J. Org. Chem. 2011, 76, 2867. (c) Mochida, K.; Kawasumi, K.; Segawa,
Y.; Itami, K. J. Am. Chem. Soc. 2011, 133, 10716. See also ref 4g.
(10) (a) Kakiuchi, F.; Sekine, S.; Tanaka, Y.; Kamatani, A.;
Sonoda, M.; Chatani, N.; Murai, S. Bull. Chem. Soc. Jpn. 1995, 68, 62.
(b) Kakiuchi, F.; Murai, S. Acc. Chem. Res. 2002, 35, 826.
(11) Ueno, S.; Mizushima, E.; Chatani, N.; Kakiuchi, F. J. Am.
Chem. Soc. 2006, 128, 16515.
(12) RuH2(CO)(PPh3)3-catalyzed CꢀH/olefin coupling at high tem-
perature: Nakazono, S.; Imazaki, Y.; Yoo, H.; Yang, J.; Sasamori, T.;
Tokitoh, N.; Cedric, T.; Kageyama, H.; Kim, D.; Shinokubo, H.;
Osuka, A. Chem.;Eur. J. 2009, 15, 7530.
(13) Vets, N.; Smet, M.; Dehaen, W. Tetrahedron Lett. 2004, 45, 7287.
Org. Lett., Vol. 14, No. 15, 2012
3883

observed as a side reaction. The CꢀH alkylations of 6 with
2d and 2e at 180 °C gave the corresponding tetraalkylated
pentacenequinones 7d and 7e in 69% and 74% isolated
yields, respectively, and only linear isomers were observed.
20-methoxyacetophenones using olefins and organoboron-
ates.11 This reaction enables selective introduction of two
different groups on aromatic rings in a single step. There-
fore, we examined the tandem CꢀH alkylation/CꢀO
arylation using dimethoxy-substituted acenequinones in
order to prepare acenes bearing two different substituents
in short sequences.
Scheme 1. Regioselective Tetraalkylation of Pentacenequinone
The tandem alkylation/arylation was first examined
with 1,4-dimethoxyanthracene-9,10-dione (9) (Table 2).
When the reaction of 9 with vinylsilane 2a and p-tolylboron-
ate 10a was carried out using catalyst 3 under toluene
refluxing conditions for 24 h, the methoxy groups and
hydrogens at ortho positions of carbonyls were selectively
converted to p-tolyl and 2-(triethylsilyl)ethyl groups, re-
spectively, and the desired anthraquinone derivative 11a
was isolated in 74% yield (entry 1). Other olefins such as
2b,d,e and 3,3-dimethyl-1-butene (2f) were also applicable
to the CꢀH alkylation/CꢀO arylation, and the corre-
sponding dialkyldiarylanthraquinones were obtained in
31ꢀ79% isolated yields (entries 2ꢀ5). The molecular
structure of product 11f was established by X-ray diffrac-
tion analysis, and an ORTEP drawing of 11f is shown in
Figure S1 (Supporting Information). The X-ray structure
of 11f clearly shows that two p-tolyl groups are introduced
on one of the benzene rings, while the other benzene ring
possesses two alkyl groups.
The chemoselective tandem CꢀC bond formation was
applied to synthesize a dialkyldiarylpentacene using 5,14-
dimethoxypentacene-6,13-dione (12) (Scheme 2). The re-
action of 12 with 2a and phenylboronate 10b under toluene
refluxing conditions proceeded chemoselectively to give
the desired pentacenequinone 13 in 60% isolated yield.
Aromatization of 13 using LiAlH4 and HCl aq provided
diphenyldialkylpentacene 14 in 42% isolated yield.
Conversion of pentacenequinone derivatives 7 to tetra-
alkylpentacenes 8 were then investigated. When 7a was
reacted with LiAlH4 in TBME and subsequently treated
with hydrochloric acid, the desired pentacene 8a was formed
without repeating the reduction/dehydration cycle and iso-
lated in 45% yield as a blue oil (eq 3). Tetrahexylpentacene
8d was prepared in 39% yield in a similar manner as a blue
solid. As we expected, the obtained tetraalkylpentacenes 8a,d
show high solubility in common organic solvents including
hexane. In order to prepare pentacene derivative 8e, the
reduction of 7e with LiAlH4 was also attempted but did not
proceed probably due to the low solubility of 7e in TBME.
After screening of various reaction conditions, it was found
that reduction the carbonyls of 7e with LiBH4 in THF,
Table 2. Reaction of 1,4-Dimethoxyanthracene-9,10-dione (9)
with p-Tolylboronate 10 and Olefinsa
followed by reductive aromatization using SnCl2 2H2O
3
provides pentacene 8e in 51% isolated yield over two steps
as a deep blue solid (eq 4).5e
entry
2
R
11
isolated yield (%)
1
2a
2b
2d
2e
2f
Et3Si
11a
11b
11d
11e
11f
74
51
31
79
58
2b
3b
4
Me3Si
C4H9
o-MeC6H5
t-Bu
5b
a Reaction conditions: 9 (1 mmol), phenylboronate 10a (5 mmol),
olefin 2 (5 mmol), RuH2(CO)(PPh3)3 (3) (0.1 mmol), toluene (2 mL),
reflux, 24 h. b Performed with 10 mmol of 2 in mesitylene (2 mL) at 180 °C.
UVꢀvis absorption spectra of synthesized tetraalkyl-
ated pentacenes 8a,d,e were measured in THF at room
Previously, we reported ruthenium-catalyzed chemo-
selective tandem CꢀH alkylation/CꢀO arylation of
3884
Org. Lett., Vol. 14, No. 15, 2012

Scheme 2. Synthesis of Dialkyldiarylpentacene 14
Figure 1. Normalized UVꢀvis spectra of tetraalkylpentacenes
8a,d,e in THF.
temperature under air (Figure 1). The absorption maxima
of the pentacenes were observed at 626 (8a), 623 (8e), and
619 nm (8d). The observed absorption bands are highly
red-shifted compared to that of parent pentacene in THF
(λmax = 574 nm),14 but the substituents on the alkyl group
only slightly affected the absorption behavior.
The stability of 8d in THF under air was also evaluated by
monitoring the UVꢀvis spectra (Figure 2). For the first 2 h,
the absorption spectra was acquired in the dark, and the
result suggested that most of 8d remained undecomposed.15
After 2 h, the solution was exposed to ambient light, and the
absorption intensities of 8d were significantly decreased
within 60 min. These observations indicate that 8d is some-
what stable under air in the dark. However, under ambient
atmosphere and light, 8d is easily photooxidized as was
reported for 5,7,12,14-tetraphenylpentacene.5d
Cyclic voltammetry (CV) of 8d was performed in di-
chloromethane containing 0.1 M n-Bu4NPF6. This mole-
cule exhibited irreversible oxidation wave at þ0.66 V.
Based on this value, the highest occupied molecular orbital
(HOMO) level of 8dwas estimated tobeꢀ5.46eV, which is
an appropriate level as active material for p-type OFETs.
In summary, we developed a new convenient method for
the synthesis of alkyl-substituted acenes including pentacenes
using our ruthenium-catalyzed CꢀH alkylation or tandem
CꢀH alkylation/CꢀO arylation. The RuH2(CO)(PPh3)3-
catalyzed alkylations of anthraquinone and pentacenequi-
none with olefins afforded the corresponding tetraalkylated
acenequinones in high yields.
Figure 2. Absorbance change depending on exposing air in the
dark or in the room light.
nequinones to the corresponding acene derivatives were
achieved within two steps. CV analysis suggests that 8d
possesses an appropriate HOMO level as active material
for p-type OFETs.
Acknowledgment. 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.”
The chemoselective tandem CꢀH alkylation/CꢀO aryl-
ation was applicable to acenequinones to give dialkyldiar-
ylacenequinones. Conversion of tetrasubstituted pentace-
Supporting Information Available. Experimental pro-
cedures, characterization, and X-data/CIF. This material
is available free of charge via the Internet at http://pubs.
acs.org.
(14) Debad, J. D.; Lee, S. K.; Qiao, X.; Pascal, R. A., Jr.; Bard, A. J.
Acta Chem. Scand. 1998, 52, 45.
(15) (a) Maliakal, A.; Raghavachari, K.; Katz, H.; Chandross, E.;
Siegrist, T. Chem. Mater. 2004, 16, 4980. (b) Palayangoda, S. S.;
Mondal, R.; Shah, B. K.; Neckers, D. C. J. Org. Chem. 2007, 72,
6584. (c) De Angelis, F.; Gaspari, M.; Procopio, A.; Cuda, G.;
Di Fabrizio, E. Chem. Phys. Lett. 2009, 468, 193.
The authors declare no competing financial interest.
Org. Lett., Vol. 14, No. 15, 2012
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