Reduction versus rearrangement of 6,13-dihydro-6,13-diarylpentacene-6,13- diols affording 6,13- and 13,13′-substituted pentacene derivatives

Article abstract of DOI:10.1055/s-2004-836055

Pentacene has excellent semi-conducting properties but its practical use in organic thin film transistors (OTFTs) gives rise to a lot of problems caused by its sensitivity to oxygen and its very low solubility. In order to solve the problems involved in t

Full text of DOI:10.1055/s-2004-836055

LETTER
217
Reduction versus Rearrangement of 6,13-Dihydro-6,13-diarylpentacene-
6,13-diols Affording 6,13- and 13,13¢-Substituted Pentacene Derivatives
S
ubstituted Penta
c
a
enes and Pe
t
ntace
n
h
ones alie Vets, Mario Smet, Wim Dehaen*
Department of Chemistry, K.U. Leuven, Celestijnenlaan 200F, 3001 Leuven, Belgium
Fax +32(16)327990; E-mail: Wim.Dehaen@chem.kuleuven.ac.be
Received 14 June 2004
directly to the solution of the organolithium component
thus obtained. The resulting diols 2a–d6 could be reduced
by sodium iodide and sodium hypophosphite in acetic
acid to afford the corresponding pentacenes 3a–d7
Abstract: Pentacene has excellent semi-conducting properties but
its practical use in organic thin film transistors (OTFTs) gives rise
to a lot of problems caused by its sensitivity to oxygen and its very
low solubility. In order to solve the problems involved in the use of
pentacene, different aryl substituents were introduced on the 6- and (Scheme 1).
13-positions. Formation of the pentacene ring system may be ac-
companied by the rearrangement of the starting dihydropentacene-
diol to a 13,13¢-disubstituted pentacen-6-one for electron-rich aryl
substituents. 6-Monosubstituted pentacenes were also prepared.
O
O
Key words: substituted pentacenes, OTFTs, thiophene substitu-
ents, rearrangement, pentacenones
1
1)
Ar Li
2) HCl 1 M
THF
–78 °C
Ar =
OMe
a
b
During the last few years, the development of organic thin
film transistors (OTFTs) has attracted much interest. The
production of OTFTs has been studied because organic
molecules offer the opportunity of deposition over large
surface areas and are compatible with flexible plastic sub-
strates. A number of organic compounds have been pro-
posed for use in light-emitting diodes, field-effect
transistors and photovoltaic cells.1 Pentacene, which is
commercially available but expensive, has shown good
field effect mobilities on vacuum deposition.2 However,
pentacene has some drawbacks. Firstly, its poor solubility
necessitates the use of vacuum systems for deposition.
Secondly, pentacene is very sensitive to oxidation, lead-
ing to rapid degradation of the material unless manipulat-
ed under rigorous exclusion of oxygen. One can overcome
these problems by introducing substituents on the aromat-
ic core which will increase both stability and solubility.3
Moreover, it has been shown that substituents can affect
the self-assembly of pentacene moieties resulting in a
closer packing which can improve the electric proper-
ties.3a In this letter, we will discuss the synthesis of a num-
ber of new pentacene derivatives where (benzo)thienyl
and para-substituted phenyl substituents were introduced
at the 6- and 13-positions.
Ar OH
COMe
S
S
c
d
Ar OH
2a–e
NaI, NaH2PO2
HOAc
reflux
OMe
S
e
Ar
Ar
3a–d
Scheme 1
Bromoanisole and protected bromoacetophenone were
used to prepare diols 2a,b which were obtained in moder-
ate to low yields. The glycol acetal was used as the protec-
tive group. Only the trans-isomers were formed. The
reduction to the corresponding pentacenes 3a,b went rea-
sonably to very well (Table 1). Products 2c,d were syn-
thesized starting from thiophene or benzothiophene. In
both cases, cis- and trans-isomers were isolated in a ratio
of 4:6 and 3:7, respectively. The overall yields for the two
steps leading to pentacenes 3c,d are higher than for the
phenyl-substituted derivatives (Table 1). The solubility of
these substituted pentacenes 3a–d (10–3 M in THF) are
two orders of magnitude higher than in the case of the
parent pentacene. The aforementioned addition reactions
were all worked up with hydrochloric acid (1 M).
The introduction of substituents on the middle ring of
pentacene could be realized by reaction between organo-
lithiums and pentacenequinone (1).4 The quinone is con-
veniently prepared by condensation of o-phthalaldehyde
and 1,4-cyclohexanedione which is a high yielding
reaction.5 The lithium derivative of the desired substituent
was generated with n-BuLi. Pentacenequinone is added
When the same procedure was used in order to prepare
diol 2e, a rearrangement was observed, resulting in penta-
cenone 46 (Scheme 2). The asymmetric structure of
product 4 was confirmed by the characteristic pattern in
SYNLETT 2005, No. 2, pp 0217–0222
0
1.
0
2.
2
0
0
5
Advanced online publication: 02.12.2004
DOI: 10.1055/s-2004-836055; Art ID: G20904ST
© Georg Thieme Verlag Stuttgart · New York

218
N. Vets et al.
LETTER
Because of this strong tendency for rearrangement, reduc-
tion of 2e in acetic acid was anticipated to be impossible.
Reduction of analogous anthracene derivatives was
achieved by treatment with sodium cyanoborohydride and
zinc iodide in 1,2-dichloroethane, avoiding the rearrange-
ment.8 However, this method was not successful with pen-
tacenediol 2e as the formation of the reduced
rearrangement product 59 was observed (Scheme 3). Indi-
cating that in this case, the rate of rearrangement must be
higher than that of reduction. So far, we did not succeed
in the synthesis of 6,13-bis(5-methoxythien-2-yl)penta-
cene (3e) because of the extreme tendency to rearrange-
ment of diol 2e.
Table 1 Yields of Two-Step Synthesis of 6,13-Diarylpentacenes 3
Substituent
Yield of diol 2 (%) Yield of pentacene 3 (%)
34
11
58
87
91
58
87
59
OMe
a
COMe
b
S
c
We observed that the kind of substituent has a great influ-
ence on the tendency to rearrange to a 6,13-dihydro-
13,13¢-diarylpentacen-6-one. We were able to induce the
analogous rearrangement only after treatment of diol 2c
with the strong Lewis acid BF3·OEt2 (Scheme 4).10 Clear-
ly, electron-rich substituents favor rearrangement, in
agreement with the behavior of anthracene analogs.11
Ketone 610 was formed in moderate yield. In contrast,
treatment of the 4-methoxyphenyl substituted diol 2a with
BF3·OEt2 mainly resulted in the formation of pentacene
3a. Only trace amounts of the rearranged product 710
could be detected (Scheme 4). Also diol 2d was reacted
under the same conditions yielding the rearranged ketone
S
d
80a
–
OMe
S
e
a Work up with NH4Cl instead of HCl and addition of 0.5% of Et3N to
the eluents for column chromatography on silica gel.
MeO
S
Li
1)
O
O
O
THF
–78 °C
8
10 (Scheme 4).
HCl 1 M
2)
4
S
O
OH
OH
S
S
OMe
BF3·OEt2
MeO
CH2Cl2
r.t.
Scheme 2
2c
S
6
S
S
the 1H NMR spectrum (two singlets, two doublets and one
multiplet for the pentacene unit). The 13C NMR clearly re-
vealed the carbon of the carbonyl group (d = 167.2 ppm).
45%
O
O
By work-up with ammonium chloride, the rearrangement
could be avoided. However, upon purification of the crude
diol 2e by column chromatography on silica gel, the re-
arrangement again took place. This problem could easily
be solved by adding half a percent of triethylamine to the
eluent.6
S
7
8
S
OMe
MeO
~1%
18%
Scheme 4
OMe
S
O
HO
5
2e
OH
4
S
S
S
S
S
OMe
OMe
MeO
MeO
MeO
NaBH3CN, ZnI2
1,2-dichloroethane
reflux
Scheme 3
Synlett 2005, No. 2, 217–222 © Thieme Stuttgart · New York

LETTER
Substituted Pentacenes and Pentacenones
219
Not only could symmetrically substituted pentacenes be Two other 6-pentacenones 9b,c were synthesized.15 These
made, but asymmetric products were also possible. When products were prepared by a Grignard addition as organo-
quinone 1 was treated with only one equivalent of thienyl- lithium derivatives where found to lead to disubstitution
lithium, selective mono-addition occurred. In this manner, exclusively, even when only one equivalent was used.
the 13-hydroxy-13¢-thien-2-ylpentacen-6-one (9a) was Both products 9b,c were obtained in low yields (8%).
obtained (Scheme 5).12 When a second addition with an- Upon reduction, again the overreduced products 12b,c14
other lithium component, in our case 4-methoxyphenyl- were isolated (Scheme 6), albeit in lower yields (Table 2).
lithium, was carried out, we obtained an asymmetric
diol.12 Diol 10 was converted into pentacene 11 by using
Table 2 Yield of Reduction of 6,13-Dihydro-13-aryl-13¢-hydroxy-
pentacen-6-ones
the reduction we described earlier (Scheme 5).12
Aryl substituent
Yield of dihydropentacene 12 (%)
S
O
O
O
Li
92
S
1 equiv
THF
–78 °C
12a
OMe
12b
Cl
1
9a
OH
50
51
S
39%
OMe
THF
–78 °C
12c
OMe
Li
As side products, the unstable 6-hydroxy-13-phenylpen-
tacene derivatives 14b,c (Figure 1) were detected. Most
likely, the aromatization step is too slow because of the
lower aromatization energy of pentacene compared to an-
thracene. Several mild oxidants were tested to convert the
dihydro derivatives to pentacenes. Only with DDQ, pen-
tacene 13a could be isolated.16 All other attempts (using
p-chloranil or triphenylcarbonium tetrafluoroborate as the
oxidant) resulted in oxidation of the product to an uniden-
tified mixture of products during work up. DDQ may sta-
bilize pentacene derivative 13a by formation of a complex
in solution.
MeO
NaI,
NaH2PO2
OH
acetic acid
reflux
11
10
OH
S
S
36%
58%
Scheme 5
6-Pentacenone 9a gave us the opportunity to explore 6-
monosubstituted pentacenes as well. In contrast with the
behavior of the analogous anthracene derivatives,13 the re-
duction of 9a with NaBH3CN in the presence of ZnI2 did
not give the desired corresponding pentacene 13a. In-
stead, the starting material 9a was overreduced to 6,13-di-
hydro-6-thien-2-ylpentacene (12a) in a very high yield
(Scheme 6).14
It seems not unreasonable to conclude that 6-monosubsti-
tuted pentacenes in general are hard to prepare due to their
facile oxidative degradation. Hence, they may not meet
the most important requirement, that of stability, for the
use in transistors.
S
a
Ar =
OH
Ar
NaBH3CN,
ZnI2
1,2-dichloro-
ethane
OMe
Cl
O
H
H
14
c
reflux
12a–c
Ar
H
DDQ
9
Ar
OH
Figure 1
dioxane
reflux
S
Ar =
a
The reduction of products 9b,c lead also to the interesting
but very unstable side products 14b,c (Figure 1). In case
of the 2-thienyl-substituted analog 9a, the formation of
the monohydroxypentacene 14a was not observed. How-
ever, when the reduction of 9a was carried out in toluene
instead of 1,2-dichloroethane, 6-hydroxy-13-thien-2-yl-
pentacene (14a) was formed besides 12a. On decreasing
the amount of ZnI2 (to about 0.8 equiv), the yield of 6-hy-
OMe
Cl
b
c
13a
Ar
Scheme 6
Synlett 2005, No. 2, 217–222 © Thieme Stuttgart · New York

220
N. Vets et al.
LETTER
droxy-13-thien-2-ylpentacene (14a)17 was found to in-
crease (12–79%). Products 14a–c could not be completely
purified, because of rapid oxidation resulting in a complex
mixture of unidentified products. The reduction of ketone
9a was also carried out with NaBH4 and LiAlH4, but in
these cases pentacene 14a was isolated in lower yields
(3% and 11%, respectively) than with NaBH3CN.
(6) Synthesis of 6,13-Dihydro-6,13-dithien-2-ylpentacene-
6,13-diol(2c):
n-BuLi (4.0 mL, 9.8 mmol) was added to a solution of
thiophene (0.78 mL, 9.8 mmol) in THF (75 mL) at –78 °C
under argon atmosphere. After 10 min pentacenequinone
(1.0 g, 3.4 mmol) was added and the mixture was allowed to
warm up to r.t. and stirred overnight. The reaction was
worked up with 1 M HCl (50 mL), extracted with CH2Cl2
(150 mL) and washed with H2O (3 × 50 mL). After drying
over MgSO4 and evaporation in vacuo, the crude product
was purified by column chomatography over silica gel using
petroleum ether–CH2Cl2 (15:85). The product was isolated
in 58% yield as a mixture of the cis- and trans-isomers (4:6);
mp 134 °C. 1H NMR (300 MHz, CDCl3): d trans-isomer =
3.3 (s, 2 H), 6.8 (d, 2 H), 7.0 (t, 2 H), 7.3 (d, 2 H), 7.5 (m, 4
H), 7.9 (m, 4 H), 8.0 (m, 4 H), 8.2 (s, 4 H), 8.6 (s, 4 H); d cis-
isomer = 3.1 (s, 2 H), 5.9 (d, 2 H), 6.3 (t, 2 H), 6.9 (d, 2 H),
7.6 (m 4 H). 13C NMR (75 MHz, CDCl3): d = 72.6, 124.2,
125.5, 126.2, 126.4, 127.2, 128.3, 132.8, 138.4, 149.0. MS
(CI) [MH+]: m/z = 477 (and 459: rearranged product).
Synthesis of 6,13-Dihydro-6,13-bis(4-methoxy-
The electronic properties of the 6,13-diarylpentacenes
3a–d described here are now being investigated. While
searching for the optimal deposition technique, some low
mobilities were already measured for 6,13-bis(4-
methoxyphenyl)pentacene (3a, 10–8–10–7 cm2/Vs) and
6,13-bis-(4-acetylphenyl)pentacene (3b, 10–8 cm2/Vs) al-
though the materials were used without additional purifi-
cation. It was found possible to spincoat 6,13-bis(4-
methoxyphenyl)pentacene (3a) out of THF. The other
pentacene derivatives did not result in useful spincoated
films because of their low crystallinity. Other deposition
techniques as well as further purification methods are cur-
rently under investigation in order to obtain higher mobil-
ities. Additional purification was carried out by train
sublimation of 6,13-bithien-2-ylpentacene.
phenyl)pentacene-6,13-diol (2a):
The procedure is the same as for 2c except purification was
done by washing the crude product with MeOH (30 mL).
The pure product was collected by filtration. Yield 34%, mp
232 °C. 1H NMR (300 MHz, CDCl3): d = 3.1 (s, 2 H), 3.5 (s,
6 H), 6.1 (d, 4 H), 6.5 (d, 4 H), 7.5 (m, 4 H), 7.9 (m, 4 H), 8.4
(s, 4 H). 13C NMR (75 MHz, CDCl3): d = 55.0, 76.4, 112.6,
124.8, 126.3, 128.1, 129.0, 132.7, 134.7, 139.6, 158.2. MS
(CI) [MH+]: m/z = 525.
We can conclude that 6,13-diarylpentacenes are relatively
easy to obtain and are interesting for further investigation
with respect to their potential use in transistors. In con-
trast, 6-monosubstituted pentacenes seem more difficult
to prepare and more sensitive to oxidation. Further re-
search towards other substituents on the 6- and 13-posi-
tions and the electronic properties of these compounds is
ongoing.
Synthesis of 6,13-Dihydroxy-6,13-(4-acetyl)phenyl-
pentacene-6,13-diol (2b):
The procedure is the same as for 2c, the bromoacetophenone
was protected with 1,2-ethanediol, except column
chromatography over silica gel was carried out with EtOAc–
petroleum ether–CH2Cl2 (2:4:4). Yield 11%; mp 229–
232 °C. 1H NMR (300 MHz, CDCl3): d = 2.4 (s, 6 H), 2.5 (s,
2 H), 6.9 (d, 4 H), 7.3 (d, 4 H), 7.6 (m, 4 H), 8.0 (m, 4 H), 8.4
(s, 4 H). 13C NMR (75 MHz, CDCl3): d = 26.6, 76.4, 125.7,
127.1, 127.4, 128.3, 132.9, 135.7, 139.0, 147.4, 197.8. MS
(CI) [MH+]: m/z = 549 (and 531: rearranged product).
Synthesis of 6,13-Dihydro-6¢,13¢-bis(benzothien-2-
yl)pentacene-6,13-diol (2d):
The procedure is the same as for 2c except column
chromatography over silica gel was carried out with
petroleum ether–CH2Cl2 (1:9). Yield 87%; mp trans: 258–
260 °C, cis: 296–300 °C. 1H NMR (300 MHz, CDCl3): d
trans-isomer = 3.0 (s, 2 H), 7.1 (s, 2 H), 7.3 (m, 4 H), 7.5 (m,
4 H), 7.6–7.7 (m, 2 H), 7.7–7.8 (m, 2 H), 7.9 (m, 4 H), 8.3 (s,
4 H); d cis-isomer = 3.2 (s, 2 H), 5.9 (s, 1 H), 6.6 (d, 2 H),
6.7 (t, 2 H), 6.9 (t, 2 H), 7.3 (d, 2 H), 7.6 (m, 4 H), 8.0–8.1
(m, 4 H), 8.6 (s, 4 H). 13C NMR (75 MHz, CDCl3): d trans-
isomer = 75.4, 122.9, 124.5, 125.0, 125.1, 127.5, 127.6,
128.8, 133.7, 138.9, 139.8, 141.1, 153.0; d cis-isomer = 53.7,
121.2, 123.6, 124.5, 126.9, 128.4, 133.1, 138.5, 139.9. MS
(CI) [MH+]: m/z 559 (rearranged product).
Acknowledgment
We thank P. Heremans for stimulating scientific discussions and S.
Steudel and K. Myny for the implementation of the electric measu-
rements. N.V. and M.S. thank the IMEC/IWT [Institute for the Pro-
motion of Innovation through Science and Technology in Flanders
(IWT-Vlaanderen)] and the FWO Vlaanderen, respectively, for a
doctoral and a postdoctoral fellowship. The University of Leuven
and the Ministerie voor Wetenschapsbeleid are thanked for finan-
cial support.
References
(1) Katz, H. E.; Bao, Z.; Gilat, S. L. Acc. Chem. Res. 2001, 34,
359.
(2) Lin, Y.-Y.; Gundlach, D. J.; Nelson, S. F.; Jackson, T. N.
IEEE Transactions on Electron Devices 1997, 44, 1325.
(3) (a) Anthony, J. E.; Eaton, D. L.; Parkin, S. R. Org. Lett.
2002, 4, 15. (b) Vogel, K. M.; Vogel, D. E.; Terrance, S. P.
US 0105365, 2003. (c) Meng, H.; Bendikov, M.; Mitchell,
G.; Helgeson, R.; Wudl, F.; Boa, Z.; Siegrist, T.; Kloc, C.;
Chen, C. H. Adv. Mat. 2003, 13, 1090. (d) Eaton, D. L.;
Parkin, S.; Anthony, J. E. US 6690029, 2004.
Synthesis of 6,13-Dihydro-13,13¢-bis(5-methoxythien-2-
yl)pentacene-6-one (4):
The procedure is the same as for product 2c except column
chromatography over silica gel was carried out with
petroleum ether–CH2Cl2 (1:9). Yield 55%; the yield rose to
77% when the work up was done with NH4Cl. Mp 117–
120 °C. 1H NMR (300 MHz, CDCl3): d = 3.8 (s, 6 H), 5.9 (d,
2 H), 6.2 (d, 2 H), 7.5–7.6 (m, 4 H), 7.9 (d + s, 4 H), 8.0–8.1
(d, 2 H), 8.9 (s, 2 H). 13C NMR (75 MHz, CDCl3): d = 53.2,
60.1, 102.4, 126.7, 127.2, 128.5, 128.8, 129.8, 130.2, 132.3,
135.3, 136.8, 143.0, 167.2. MS (CI) [MH+]: m/z = 519.
(4) (a) Maulding, D. R.; Roberts, B. G. J. J. Org. Chem. 1969,
34, 1734. (b) Wolak, M. A.; Jang, B. B.; Palilis, L. C.;
Kafafi, Z. H. J. Phys. Chem. B 2004, 18, 5492.
(5) Bruckner, V.; Tomasz, J. Acta Chim. Hung. 1961, 28, 405.
Synlett 2005, No. 2, 217–222 © Thieme Stuttgart · New York

LETTER
Substituted Pentacenes and Pentacenones
221
Synthesis of 6,13-Dihydro-6¢,13¢-bis-(5-methoxythien-2-
CDCl3): d = 52.8, 125.2, 126.2, 126.4, 127.1, 128.3, 128.4,
128.8, 128.9, 129.7, 130.0, 132.2, 135.2, 143.6, 152.0,
184.8. MS (CI) [MH+]: m/z = 459.
Synthesis of 13,13¢-Bis(4-methoxyphenyl)pentacene-6-
one (7):
yl)pentacene-6,13-diol (2e):
The procedure is the same as for product 2c except work up
with NH4Cl and column chromatography over silica gel was
carried out with petroleum ether–CH2Cl2 (1:9) and 0.5%
Et3N. Yield 80%; mp 85–95 °C. 1H NMR (300 MHz,
CDCl3): d = 2.9 (s, 2 H), 6.0 (d, 2 H), 6.4 (d, 2 H), 7.5 (m, 4
H), 7.9 (m, 4 H), 8.2 (s, 4 H). 13C NMR (75 MHz, CDCl3):
d = 60.1, 74.5, 103.1, 125.0, 126.4, 126.7, 128.2, 133.0,
136.7, 139.0, 167.3. MS (CI) [MH+]: m/z = 519.2
(rearranged product), 559.2 (Na adduct).
The procedure is the same as for 6 except column
chromatography over silica gel was carried out with using
CH2Cl2–petroleum ether (5:5); yield 1%. 1H NMR (300
MHz, CDCl3): d = 3.7 (s, 6 H), 6.7, 6.8 (d, d, 8 H), 7.5 (s +
m, 6 H), 7.7 (d, 2 H), 8.0 (d, 2 H), 8.8 (s, 2 H). 13C NMR (75
MHz, CDCl3): d = 55.2, 113.2, 126.7, 128.1, 128.2, 128.5,
129.0, 129.1, 129.3, 129.5, 131.5, 131.6, 131.7, 134.9,
138.6, 144.9, 158.1, 186.0. MS (CI) [MH+]: m/z = 507.
Synthesis of 6,13-Dihydro-13,13¢-bis(benzothien-2-
yl)pentacene-6-one (8):
The procedure is the same as for 6 except column
chromatography over silica gel was carried out with
CH2Cl2–petroleum ether (5:5). Yield 18%, mp >300 °C. 1H
NMR (300 MHz, CDCl3): d = 6.9 (s, 2 H), 7.3 (m, 4 H), 7.6
(m, 6 H), 7.7 (d, 2 H), 7.8 (d, 2 H), 8.0 (s, 2 H), 8.1 (d, 2 H),
8.9 (s, 2 H). 13C NMR (75 MHz, CDCl3): d = 56.9, 116.9,
118.4, 118.8, 119.1, 120.5, 121.4, 122.3, 122.8, 123.5,
123.6, 125.8, 128.3, 131.4, 133.2, 134.3, 142.7, 171.7. MS
(CI) [MH+]: m/z = 559.
(7) Synthesis of 6,13-Dithien-2-ylpentacene (3c):
A suspension of diol 2c (2.0 g, 4.2 mmol), NaI (4.3 g, 29
mmol) and NaH2PO2 (4.8 g, 40 mmol) in HOAc (100 mL)
was refluxed for 1 h. The product 3c was isolated by
filtration and washed with H2O (3 × 50 mL) and MeOH
(2 × 25 mL). After drying in vacuo, 6,13-dithien-2-
ylpentacene was obtained in 87% yield; mp 300–305 °C. 1H
NMR (300 MHz, CDCl3): d = 7.3 (m, 4 H), 7.4 (d, 2 H), 7.5
(t, 2 H), 7.8 (m, 6 H), 8.5 (s, 4 H). 13C NMR (75 MHz,
CDCl3): d = 125.4, 125.5, 127.1, 127.4, 128.6, 130.0, 131.4,
139.5. MS (CI) [MH+]: m/z = 443.
Synthesis of 6,13-Bis(4-methoxyphenyl)pentacene (3a):
The procedure is the same as for 3c. Yield 91%; mp 303 °C.
1H NMR (300 MHz, CDCl3): d = 4.1 (s, 6 H), 7.2 (m, 4 H),
7.3 (d, 4 H), 7.6 (d, 4 H), 7.8 (m, 4 H), 8.4 (s, 4 H). MS
(MH+): m/z = 491.
Synthesis of 6,13-Bis(4-acetylphenyl)pentacene (3b):
The procedure is the same as for 3c. Yield 58%; mp
>300 °C. 1H NMR (300 MHz, CDCl3): d = 2.8 (s, 6 H), 7.3
(m, 4 H), 7.7 (m, 4 H), 7.8 (d, 4 H), 8.2 (s, 4 H), 8.3 (d, 4 H).
MS [MH+]: m/z = 515.
Synthesis of 6,13-Bis(benzothien-2-yl)pentacene (3d):
The procedure is the same as for 3c. Yield 59%; mp
>300 °C. 1H NMR (300 MHz, CDCl3): d = 7.5 (m, 2 H), 7.6
(m, 6 H), 7.7 (m, 2 H), 7.9 (m, 4 H), 8.0 (m, 4 H), 8.6 (m, 4
H). MS [MH+]: m/z = 542.
(11) Smet, M.; Van Dijk, J.; Dehaen, W. Tetrahedron 1999, 55,
7859.
(12) Synthesis of 13-Hydroxy-13¢-thien-2-ylpentacen-6-one
(9a):
n-BuLi (2.0 mL, 4.9 mmol) was added to a solution of
thiophene (0.39 mL, 4.9 mmol) in THF (20 mL) under argon
atmosphere at –78 °C. After 10 min, this solution was added
dropwise to a suspension of pentacenequinone (2 g, 6.5
mmol) in THF (120 mL), which was cooled to –78 °C. The
mixture was allowed to warm up to r.t. and stirred during the
night. The reaction mixture was worked up with HCl (1 M,
25 mL). The precipitate was filtered, washed with H2O
(3 × 30 mL), MeOH (2 × 20 mL), and dried in vacuo. The
filtrate was extracted with CH2Cl2 (75 mL), washed with
H2O (3 × 50 mL), dried over MgSO4 and evaporated in
vacuo. After column chromatography over silica gel using
petroleum ether–CH2Cl2 (3:7) on both fractions separately,
the total yield was 39%; mp 248 °C. 1H NMR (300 MHz,
CDCl3): d = 3.3 (s, 1 H), 6.3–6.4 (d, 1 H), 6.7 (t, 1 H), 7.1 (d,
1 H), 7.5–7.6 (m, 4 H), 7.9 (d, 2 H), 8.0 (d, 2 H), 8.5 (s, 2 H),
8.8 (s, 2 H). MS [MH+]: m/z = 393.
(8) Smet, M.; Van Dijk, J.; Dehaen, W. Synlett 1999, 495.
(9) Synthesis of 6,13-Dihydro-6,6¢-bis(5-methoxythien-2-yl)-
13,13¢-dihydropentacene (5):
A solution of diol 2e (0.50 g, 0.90 mmol), ZnI2 (0.86 g, 2.7
mmol) and NaCNBH3 (0.57 g, 9.0 mmol) in 1,2-
dichloroethane (50 mL) was refluxed during the night. After
filtration of the salts, the filtrate was acidified, extracted with
CH2Cl2 (50 mL) and washed with H2O (2 × 30 mL). After
drying over MgSO4 and evaporation in vacuo. The crude
product was purified by two successive column
Synthesis of 6,13-Dihydro-6-thien-2-yl-13-(4-methoxy-
phenyl)pentacene-6,13-diol (10):
chromatographies over silica gel using CH2Cl2 and CH2Cl2–
petroleum ether (5:5), respectively. The product was
characterized to be 6,13-dihydro-6,6¢-bis(5-methoxythien-2-
yl)-13,13¢-dihydropentacene; mp >300 °C. 1H NMR (300
MHz, CDCl3): d = 3.8 (s, 6 H), 4.1 (s, 2 H), 6.0 (d, 2 H), 6.2
(d, 2 H), 7.4–7.5 (m, 6 H), 7.8 (m, 6 H). 13C NMR (75 MHz,
CDCl3): d = 36.9, 55.2, 60.1, 102.5, 125.4, 125.5, 126.3,
126.6, 127.0, 127.4, 128.5, 132.1, 132.8, 135.2, 135.3,
142.1, 166.6. MS (CI) [MH+]: m/z = 505.
n-BuLi (0.60 mL, 1.4 mmol) was added to a solution of 4-
bromoanisole (0.27 g, 1.5 mmol) in THF (30 mL) under
argon atmosphere at –78 °C. After 10 min, ketone 9a (0.38
g, 0.97 mmol) was added. The mixture was allowed to warm
up to r.t. and stirred during the night. The reaction mixture
was worked up with HCl (1 M, 10 mL), was extracted with
CH2Cl2 (50 mL), washed with H2O (3 × 50 mL), dried over
MgSO4 and evaporated in vacuo. After purification by
column chromatography on silica gel using petroleum ether–
CH2Cl2 (2:8), diol 10 was used in the next reaction. Yield
after column chromatography was 58%. MS [MH+]: m/z =
501.
(10) Synthesis of 13,13¢-dithien-2-ylpentacene-6-one (6):
A solution of diol 2c (0.26 g, 0.55 mmol) and BF3·OEt (0.10
mL, 8.2 mmol) in CH2Cl2 (20 mL) was stirred at r.t. during
the night. After adding NaOH (1 M, 20 mL), the reaction
mixture was extracted with CH2Cl2. The organic layer was
washed with H2O (3 × 100 mL), dried with MgSO4 and
evaporated in vacuo. The product 6 could be obtained after
column chromatography over silica gel using CH2Cl2, with a
yield of 44.5%; mp >300 °C. 1H NMR (300 MHz, CDCl3):
d = 6.6 (d, 2 H), 6.9 (t, 2 H), 7.3 (d, 2 H), 7.6 (m, 4 H), 7.8 (s
+ d, 4 H), 8.1 (d, 2 H), 8.9 (s, 2 H). 13C NMR (75 MHz,
Synthesis of 6-Thien-2-yl-13-(4-methoxy-phenyl)penta-
cene (11):
A suspension of diol 10 (0.09 g, 0.2 mmol), NaI (0.38 g, 2.5
mmol) and NaH2PO2 (0.38 g, 4.3 mmol) was refluxed in
HOAc (20 mL) during 1 h. The mixture was allowed to cool
down and the product was filtered. The residue was washed
with H2O (3 × 20 mL) and MeOH (2 × 10 mL). After drying
in vacuo, 6-thien-2-yl-13-(4-methoxyphenyl)pentacene was
Synlett 2005, No. 2, 217–222 © Thieme Stuttgart · New York

222
N. Vets et al.
LETTER
obtained with a yield of 36%; mp 272–274 °C. 1H NMR
(300 MHz, CDCl3): d = 4.0 (s, 3 H), 7.2, 7.3 (m, 6 H), 7.4 (d,
1 H), 7.5 (t, 1 H), 7.55 (d, 2 H), 7.7, 7.8 (m, 5 H), 8.3 (s, 2
H), 8.5 (s, 2 H). 13C NMR (75 MHz, CDCl3): d = 55.5, 114.1,
125.2, 125.4, 125.8, 127.0, 127.4, 128.2, 128.5, 128.9,
130.0, 130.2, 132.7, 138.3, 159.3. MS [MH+]: m/z = 467.
(15) Synthesis of 6,13-Dihydro-13-hydroxy-13-(4-
methoxy)phenylpentacene-6-on (9b):
Bromoanisole (2.8 mL, 22 mmol) in dry THF (10 mL) was
added dropwise over a period of 30 min to a suspension of
Mg (0.56 g, 23 mmol) in dry THF (10 mL) in the presence
of a catalytic amount of iodine and bromoanisole. This
mixture was than added dropwise to a suspension of
pentacenequinone (6.0 g, 20 mmol) in THF (120 mL). The
reaction mixture was stirred during the night and acidified
with HCl (1 M). The mixture was extracted with CH2Cl2,
washed with H2O and dried over MgSO4. After evaporation
in vacuo, the crude product was purified by column
chromatography on silica gel using petroleum ether–CH2Cl2
(4:6). The desired pentacene derivative 9b was obtained in
8% yield; mp 289–291 °C. 1H NMR (300 MHz, CDCl3): d =
3.0 (s, 1 H), 3.7 (s, 3 H), 6.7 (d, 2 H), 7.2 (d, 2 H), 7.5–7.6
(m, 4 H), 7.9 (d, 2 H), 8.0 (d, 2 H), 8.3 (s, 2 H), 8.9 (s, 2 H).
13C NMR (75 MHz, CDCl3): d = 55.2, 113.7, 126.5, 126.9,
128.1, 128.8, 129.0, 129.2, 129.8, 132.3, 136.0, 151.8,
155.6. MS (CI) [MH+]: m/z = 417.
(13) Smet, M. unpublished results
(14) Synthesis of 6,13-Dihydro-6¢-thien-2-yl-13,13¢-dihydro-
pentacene (12a):
A suspension of ketone 9a (0.1 g, 0.25 mmol), ZnI2 (0.24 g,
0.75 mmol) and NaBH3CN (0.16 g, 2.5 mmol) in 1,2-
dichloroethane (40 mL) was refluxed overnight. The salts
were filtered off and the filtrate was acidified with HCl (3 M,
20 mL). After extraction with CH2Cl2 (20 mL) and washing
with H2O (3 × 50 mL), the organic layer was dried over
MgSO4 and evaporated in vacuo. The product was purified
by column chromatography over silica gel using CH2Cl2–
petroleum ether (5:5). The title compound was obtained as a
white solid in 92% yield; mp 228–230 °C. 1H NMR (300
MHz, CDCl3): d = 4.2 (s, 2 H), 5.8 (s, 1 H), 6.5 (d, 1 H), 6.8
(t, 1 H), 7.1 (d, 1 H), 7.5 (m, 4 H), 7.8–7.9 (m, 4 H), 8.0 (s,
2 H). 13C NMR (75 MHz, CDCl3): d = 48.4, 63.1, 124.8,
125.1, 125.5, 125.9, 126.0, 126.2, 126.6, 127.3, 127.6,
129.5, 132.8, 135.5, 137.8. MS (CI) [MH+]: m/z = 363. The
same reaction in toluene gives a lower yield (44%) and 6-
hydroxy-13-thien-2-ylpentacene (12%) as a side-product.
Synthesis of 6,13-Dihydro-6¢-(4-methoxyphenyl)-13,13¢-
dihydropentacene (12b):
The procedure is the same as for 12a. Yield 51%; mp
155 °C. 1H NMR (300 MHz, CDCl3): d = 3.7 (s, 3 H), 4.1 (s,
2 H), 5.5 (s, 1 H), 6.7 (d, 2 H), 7.0 (d, 2 H), 7.4 (m, 4 H), 7.8
(m, 8 H). 13C NMR (75 MHz, CDCl3): d = 51.9, 55.6, 114.1,
125.9, 126.1, 127.0, 127.6, 129.7, 132.9, 133.0, 134.1,
136.3, 139.1, 158.5. MS (CI) [MH+]: m/z = 387.
Synthesis of 6,13-Dihydro-6¢-(4-chlorophenyl)-13,13¢-
dihydropentacene (12b):
6,13-Dihydro-13-hydroxy-13-(4-chloro)phenyl-
pentacene-6-one (9c):
The procedure is the same as for 9b except column
chromatography over silica gel using petroleum ether–
CH2Cl2 (2:8); yield 8%. 1H NMR (300 MHz, CDCl3): d = 3.2
(s, 1 H), 7.2 (s, 3 H), 7.3 (s, 3 H), 7.5 (d, 5 H), 7.8 (d, 2 H),
8.0 (m, 2 H), 8.2 (2 H), 8.9 (m, 2 H). MS (CI) [MH+]: m/z =
421. Melting point and 13C NMR data were not obtained, due
to unremovable contaminating pentacenequinone.
(16) Synthesis of 6-Thien-2-ylpentacene (13a):
Dihydropentacene 12a (90 mg, 0.25 mmol) was refluxed for
3 h in the presence of DDQ (64 mg, 0.28 mmol) in dioxane
(20 mL). After cooling and evaporating in vacuo, the product
was purified by column chromatography over silica gel
using CH2Cl2. The yield of the impure product was 31%. 1H
NMR (300 MHz, CDCl3): d = 6.3 (m, 1 H), 6.8 (m, 1 H), 7.3
(m, 1 H), 7.5 (m, 4 H), 7.9 (d + s, 2 H + 2 H), 8.0 (d, 2 H),
8.2 (1 H), 8.4 (2 H). MS (CI) [MH+]: m/z = 361.
The procedure is the same as for 12a. Yield 50%; mp
174 °C. 1H NMR (300 MHz, CDCl3): d = 4.0 (s, 1 H), 4.1 (s,
1 H), 5.5 (s, 1 H), 7.0 (d, 2 H), 7.1 (d, 2 H), 7.4 (m, 4 H), 7.8
(m, 8 H). 13C NMR (75 MHz, CDCl3): d = 36.8, 51.7, 125.2,
125.5, 125.7, 126.0, 126.8, 127.4, 127.7, 128.5, 129.7,
132.5, 132.8, 135.8, 136.0, 138.0, 140.4. MS (CI) [MH+]:
m/z = 391.
(17) Spectral data: 1H NMR (300 MHz, CDCl3): d = 6.0 (s, 1 H),
6.7 (d, 1 H), 6.8 (t, 1 H), 7.0 (d, 1 H), 7.4–7.5 (m, 4 H), 7.8
(d, 2 H), 7.9 (s, 2 H), 8.0 (d, 2 H), 8.8 (s, 2 H). MS [MH+]:
m/z = 377.
Synlett 2005, No. 2, 217–222 © Thieme Stuttgart · New York