Synthesis of functionalized pentacenes from isobenzofurans derived from C-H bond activation

Article abstract of DOI:10.1021/ol102349r

The synthesis of unsymmetric functionalized pentacenes from 1,4-anthraquinones and functionalized isobenzofurans, which were prepared by transformation via C-H bond activation, was successfully accomplished. Examples of the synthesis of pentacenes with functional groups at the 5-position are still rare. These obtained functionalized pentacenes are highly soluble in hexane, toluene, and THF.

Full text of DOI:10.1021/ol102349r

ORGANIC
LETTERS
2010
Vol. 12, No. 22
5287-5289
Synthesis of Functionalized Pentacenes
from Isobenzofurans Derived from C-H
Bond Activation
Yoichiro Kuninobu,* Takayuki Seiki, Shunsuke Kanamaru, Yuta Nishina,† and
Kazuhiko Takai*
DiVision of Chemistry and Biochemistry, Graduate School of Natural Science and
Technology, Okayama UniVersity, Tsushima, Kita-ku, Okayama 700-8530, Japan
kuninobu@cc.okayama-u.ac.jp; ktakai@cc.okayama-u.ac.jp
Received September 29, 2010
ABSTRACT
The synthesis of unsymmetric functionalized pentacenes from 1,4-anthraquinones and functionalized isobenzofurans, which were prepared by
transformation via C-H bond activation, was successfully accomplished. Examples of the synthesis of pentacenes with functional groups at
the 5-position are still rare. These obtained functionalized pentacenes are highly soluble in hexane, toluene, and THF.
Polyacenes are important compounds in material science.1
Among them, pentacenes have recently received much
attention because such compounds are useful as organic
semiconductors, organic electron luminescent materials,
organic photodiodes, and components of field effect transis-
tors (FET) and organic solar cells.1 To investigate the effect
of substituents on the pentacene skeletons, several types of
substituted pentacenes have been synthesized by various
methods. These include nucleophilic addition of 2 equiv of
an organometallic reagent to pentacene-6,13-dione followed
by reduction,2 reduction of pentacene-6,13-dione derivatives,3
reduction of pentacene-5,7,12,14-tetraone derivatives,4
Diels-Alder reaction between 2 equiv of isobenzofuran
derivatives and p-benzoquinone followed by reduction,5 zinc-
mediated reaction of pentacene-6,13-diol with 2 equiv of
thiols followed by oxidation,6 and zirconium-mediated
synthesis of polyalkylated pentacenes.7 However, reports on
the synthesis of unsymmetrical pentacenes with substituents
in the 5-position are still rare.8-11 Our strategy for the
(2) (a) Anthony, J. E.; Eaton, D. L.; Parkin, S. R. Org. Lett. 2002, 4,
15–18. (b) Vets, N.; Smet, M.; Dehaen, W. Synlett 2005, 217–222. (c) Kim,
Y.; Whitten, E. J.; Swager, M. T. J. Am. Chem. Soc. 2005, 127, 12122–
12130. (d) Lehnherr, D.; Murray, A. H.; McDonald, R.; Ferguson, M. J.;
Tykwinski, R. R. Chem.sEur. J. 2009, 15, 12580–12584.
(3) (a) Wartini, A. R.; Staab, H. A.; Neugebauer, F. A. Eur. J. Org.
Chem. 1998, 1161–1170. (b) Chan, S. H.; Lee, H. K.; Wang, Y. M.; Fu,
N. Y.; Chen, X. M.; Cai, Z. W.; Wong, H. N. C. Chem. Commun. 2005,
66–68.
(4) Benard, C. P.; Geng, Z.; Heuft, M. A.; VanCrey, K.; Fallis, A. G. J.
Org. Chem. 2007, 72, 7229–7236.
† Present address: Research Core for Interdisciplinary Sciences, Okayama
University, Tsushima, Kita-ku, Okayama 700-8530, Japan.
(1) For reviews, see: (a) Dimitrakopoulos, C. D.; Malenfant, P. R. L.
AdV. Mater. 2002, 14, 99–117. (b) Bendikov, M.; Wudl, F.; Perepichka,
D. F. Chem. ReV. 2004, 104, 4891–4945. (c) Anthony, J. E. Chem. ReV.
2006, 106, 5028–5048. (d) Anthony, J. E. Angew. Chem., Int. Ed. 2008,
47, 452–483.
(5) Rainbolt, J. E.; Miller, G. P. J. Org. Chem. 2007, 72, 3020–3030.
(6) Kobayashi, K.; Shimaoka, R.; Kawahata, M.; Yamanaka, M.;
Yamaguchi, K. Org. Lett. 2006, 8, 2385–2388.
(7) (a) Takahashi, T.; Kitamura, M.; Shen, B.; Nakajima, K. J. Am.
Chem. Soc. 2000, 122, 12876–12877. (b) Li, S.; Li, Z.; Nakajima, K.; Kanno,
K.-i.; Takahashi, T. Chem. Asian J. 2009, 4, 294–301.
10.1021/ol102349r  2010 American Chemical Society
Published on Web 10/20/2010

synthesis of unsymmetric functionalized pentacenes is as
follows (Figure 1): (1) Rhenium-catalyzed synthesis of
Table 1. Synthesis of Several Pentacene Derivatives 7a
Figure 1. Strategy for the synthesis of functionalized pentacenes.
unsymmetric isobenzofurans from aromatic ketimines and
aldehydes12 and (2) Diels-Alder reaction between the
formed isobenzofurans and 1,4-anthraquinone followed by
reduction to give unsymmetric functionalized pentacenes.
Scheme 1. Synthetic Route of Pentacene 7a
a 2 (2.0 equiv). b After the reaction, 4 (0.5 equiv) was added, and the
reaction mixture was stirred for an additional 2 h. c SnCl2·2H2O (2.0 equiv)
was used instead of SnCl2 (2 equiv) and H2O (2.0 equiv). d 2 h. After the
reaction, the reaction mixture was quenched with H2O and extracted with
diethyl ether. The organic layer was concentrated in vacuo, and the residue
was treated with NaBH4 (2.0 equiv) and methanol at 75 °C for 2 h. e H2O
(4.0 equiv). f [ReBr(CO)3(thf)]2 (0.50 mol %), 120 h, 70%. g Without
addition of H2O. h pinB ) 4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl. i 12
M HCl (1.4 equiv) and AcOH (0.1 M) instead of Me3SiOTf and toluene,
j
2 h. 1H NMR yield. k The ratio between 3g and 3g′ is given in square
brackets. l 5g′ was obtained in 54% yield.
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Org. Lett., Vol. 12, No. 22, 2010

First, we synthesized a pentacene derivative bearing an
alkoxy group (Scheme 1). Treatment of aromatic ketimine
1a with an aromatic aldehyde having an alkoxy group at the
para-position of the aromatic ring, 2a, in the presence of a
rhenium catalyst, [ReBr(CO)3(thf)]2, and molecular sieves
in toluene at 115 °C for 24 h gave an isobenzofuran with an
electron-donating group, 3a, in 68% yield (step 1).12 This
step consists of rhenium-catalyzed C-H bond activation at
the ortho-position of the imino group, insertion of an
aldehyde into the formed rhenium-carbon bond, intramo-
lecular nucleophilic cyclization, reductive elimination, and
the elimination of aniline.12a By the reaction of isobenzofuran
3a with 1,4-anthraquinone (4) in the presence of trimethyl-
silyl triflate, the Diels-Alder reaction and dehydration
proceeded, and pentacenequinone 5a was obtained in 81%
yield from isobenzofuran 3a (step 2). Reduction of penta-
cenequinone 5a with sodium borohydride gave pentacenediol
6a in 94% yield (step 3). After pentacenediol 6a was treated
with SnCl2, reduction of 6a proceeded, and a pentacene
derivative having an alkoxy group, 7a, was produced in 88%
yield (step 4). In general, one problem in dealing with
pentacenes is their poor solubility in many organic solvents.
Pentacene 7a is, in contrast, highly soluble in several organic
solvents, including hexane, toluene, and THF.
Next, we investigated the synthesis of several pentacene
derivatives following the procedure shown in Scheme 1
(Table 1). An electron-withdrawing group could be intro-
duced into the pentacene skeleton using an aromatic aldehyde
bearing a trifluoromethyl group at the para-position (entry
1). Pentacenes bearing methoxycarbonyl, bromo, or boryl
groups, 7c-7e, were synthesized using the corresponding
aromatic aldehydes, 2c-2e (entries 2-4). The functional
groups remained intact throughout the synthesis of the
pentacenes. These functional groups are useful because more
complex pentacene derivatives can be prepared by transes-
terification13 or cross-coupling reaction.14 By using this
method, pentacene having a heteroaromatic substituent, such
as a thiophenyl group, was obtained (entry 5). Next, we
investigated the synthesis of an unsymmetric difunctionalized
pentacene derivative with two different functional groups
(entry 6). By the reaction of the aromatic ketimine bearing
a methoxy group, 1b, with an aromatic aldehyde having a
trifluoromethyl group, 2b, in the presence of a rhenium
catalyst, [ReBr(CO)3(thf)]2, a mixture of unsymmetric di-
functionalized isobenzofurans 3g and 3g′ was obtained. In
this reaction, aldehyde 2b inserted selectively into the C-H
bond on the aromatic ring with the methoxy group. After
converting the mixture of isobenzofurans 3g and 3g′ to
pentacenequinones 5g and 5g′ (5g, 15%; 5g′, 54%), the two
isomers were separated by silica gel column chromatography.
Subsequent steps were carried out as described in the
equation in Table 1. As a result, a difunctional pentacene,
which has both electron-withdrawing and -donating groups,
7g, was produced. Pentacenes 7b-7g are also highly soluble
in toluene and THF.
In summary, we have succeeded in the synthesis of
unsymmetric pentacene derivatives bearing a functional
group, such as alkoxy, trifluoromethyl, methoxycarbonyl,
bromo, boryl, or thiophenyl groups on the aromatic ring at
the 5-position of the pentacene skeletons. The pentacenes
were synthesized from functionalized isobenzofuran deriva-
tives, which were derived by rhenium-catalyzed C-H bond
transformation. These pentacenes are highly soluble in
several organic solvents. We hope that this transformation
will become a useful method to synthesize functionalized
pentacene derivatives.
Acknowledgment. Financial support from the Ministry
of Education, Culture, Sports, Science, and Technology of
Japan is gratefully acknowledged.
Supporting Information Available: Typical experimental
procedure, characterization data for isobenzofurans 3, pen-
tacenequinones 5, pentacenediols 6, and pentacenes 7. This
material is available free of charge via the Internet at
http://pubs.acs.org.
OL102349R
(8) For a synthesis of a pentacene derivative bearing a sulfinyl group at
the 5-position, see: Lin, Y.-C.; Lin, C.-H. Org. Lett. 2007, 9, 2075–2078.
(9) For the synthesis of 5,14-dimethoxypentacenes, see: Palayangoda,
S. S.; Mondal, R.; Shah, B. K.; Neckers, D. C. J. Org. Chem. 2007, 72,
6584–6587.
(10) For the synthesis of perfluoropentacenes that include functional
groups at the 5- and other positions, see: (a) Sakamoto, Y.; Suzuki, T.;
Kobayashi, M.; Gao, Y.; Fukai, Y.; Inoue, Y.; Sato, F.; Tokito, S. J. Am.
Chem. Soc. 2004, 126, 8138–8140. (b) Vets, N.; Dilie¨n, H.; Toppet, S.;
Dehaen, W. Synlett 2006, 1359–1362.
(11) For the synthesis of functionalized pentacenes with functional
groups at the 6- and 13-positions, see: (a) Li, Y.; Wu, Y.; Liu, P.; Prostran,
Z.; Gardner, S.; Ong, B. S. Chem. Mater. 2007, 19, 418–423. (b) Lehnherr,
D.; McDonald, R.; Tykwinski, R. R. Org. Lett. 2008, 10, 4163–4166.
(12) (a) Kuninobu, Y.; Nishina, Y.; Nakagawa, C.; Takai, K. J. Am.
Chem. Soc. 2006, 128, 12376–12377. (b) Kuninobu, Y.; Nishina, Y.; Takai,
K. Tetrahedron 2007, 63, 8463–8468.
(13) For a review, see: Otera, J., Ed. Esterification; Wiley-VCH:
Weinheim, 2003.
(14) de Meijere, A.; Diederich, F., Eds. Metal-Catalyzed Cross Coupling
Reactions, 2nd Completely Revised and Enlarged Ed.; Wiley-VCH:
Weinheim, 2004; Vols. 1 and 2.
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