N. Vets et al. / Tetrahedron Letters 45 (2004) 7287–7289
7289
1
The thermolysis of adduct 5 was studied by H NMR
spectroscopy. At 100ꢁC, the reaction reaches an equilib-
rium at a conversion of 25%. However, the retro-
Diels–Alder reaction reached a 75% conversion after
approximately 3h at 120ꢁC (Fig. 1).
6. Synthesis of 6,13-dihydropentacene-6,13-diol (2): To a sus-
pension of pentacenequinone (2.0g, 6.5mmol) in dry THF
(
100mL) was added NaBH4 (1.06g, 29mmol). After
refluxing the mixture overnight, it was acidified with acetic
acid while cooling with ice. After extraction with dichloro-
methane (200mL), drying over MgSO
vacuo, the product was isolated by column chromatogra-
phy of the residue on silica gel using CH Cl /EtOAc (95:5).
4
and evaporation in
2
2
The diol was obtained in 51% yield as a mixture of the cis-
3
. Conclusions
1
and trans-isomers (35:65). Melting point: 171ꢁC; H NMR
(
7
300MHz, CDCl
.5 (m H2, H3, H9, H10), 7.9 (m H1, H4, H8, H11), 8.1 (s
C
3
): d trans-isomer 5.8, 6.6 (2 · d H6, H13),
We can conclude that we found a new, environmentally
friendlier method to reduce pentacenequinone to penta-
cene. A Diels–Alder adduct with thiophosgene and pen-
tacene was prepared and thermolysis to regenerate
pentacene was found possible at temperatures exceeding
13
H5, H7, H12, H14); d cis-isomer 6.0, 6.1 (d, d H6, H13);
NMR (75MHz, CDCl ) d 60.4, 125.0, 126.4, 127.9, 133.0,
3
1
Dihydropentacen-6-one (3): NaBH (0.5g, 14.5mmol) was
36.9; mass spectrum (MH+): m/z 313. Synthesis of 6,13-
4
1
20ꢁC.
added to an ice-cooled suspension of pentacenequinone
1.0g, 3.3mmol) in dry THF (50mL) under argon atmos-
(
phere. The suspension was heated at reflux overnight. The
mixture was cooled to room temperature and HCl (6M,
30mL) was added under cooling with ice. The mixture was
then heated at reflux for another 3h. The residue was
filtered, washed with water (2 · 30mL) and dichlorometh-
ane (2 · 50mL). The filtrate was taken aside and the two
layers were separated. The organic layer was washed with
Acknowledgements
We thank P. Heremans for stimulating scientific discus-
sions. N.V. and M.S. thank the IMEC and the FWO
Vlaanderen, respectively, for a doctoral and a postdoc-
toral fellowship. The University of Leuven and the Min-
isterie voor Wetenschapsbeleid are thanked for financial
support.
water (3 · 50mL), dried over MgSO
vacuo. After column chromatography on silica gel using
petroleum ether/CH Cl /EtOAc (55:40:5), 6,13-dihydro-
4
and evaporated in
2
2
pentacen-6-one was isolated in 11% yield.
Melting point: 274ꢁC (Clar, E., Chem. Ber., 1949, 82, 495–
1
14); H NMR (300MHz, CDCl
5
H2, H10), 7.6 (t H3, H9), 7.9 (d H1, H11), 8.0 (s H12, H14),
3
): d 4.7 (s 2 · H13), 7.5 (t
References and notes
1
.1 (d H4, H8), 9.1 (s H5, H7); C NMR (75MHz, CDCl3):
3
8
1
2
. Katz, H. E.; Bao, Z.; Gilat, S. L. Acc. Chem. Res. 2001, 34,
59–369.
. Lin, Y.-Y.; Gundlach, D. J.; Nelson, S. F.; Jackson, T. N.
IEEE Trans. Electron Dev. 1997, 44, 1325–1331.
d 32.6, 126.1, 126.7, 127.1, 128.6, 129.4, 129.9, 130.3, 131.9,
135.6, 185.4; mass spectrum (MH ): m/z 295.
+
3
7. Synthesis of Diels–Alder adduct (5): A suspension of
pentacene (0.46g, 1.6mmol) in thiophosgene (2mL) was
heated at 65ꢁC for 6h. After cooling to room temperature,
dichloromethane (2mL) was added to the reaction mixture
and the unreacted pentacene was removed by filtration. The
filtrate was evaporated. Then toluene (2 · 40mL) was
added and again evaporated in order to remove all
thiophosgene. The product was purified by column chro-
matography on silica gel using CH Cl /petroleum ether
3
4
. Herwig, P. T.; M u¨ llen, K. Adv. Mat. 1998, 11, 480–483.
. Bruckner, V.; Tomasz, J. Acta. Chim. Hung. 1961, 28,
4
05–408.
. Synthesis of pentacene (4): LiAlH
added to a ice-cooled suspension of pentacenequinone
2.0g, 6.5mmol) in dry THF (100mL) under argon atmos-
5
4
(0.98g, 25mmol) was
(
phere. The suspension was refluxed for 30min. The mixture
was cooled to room temperature and HCl (6M, 60mL) was
added under cooling with ice. The mixture was then
refluxed for another 3h. The residue was filtered, washed
with water (2 · 30mL), dichloromethane (2 · 30mL),
MeOH (2 · 30mL) and diethyl ether (2 · 30mL) and after
2
2
(50:50). The white adduct was obtained in 40% yield.
1
Melting point:/decomposition starting at 100ꢁC; H NMR
(300MHz, CDCl ): d 5.5 (s, H13), 5.8 (s H6), 7.5 (m H2,
3
H3, H9, H10), 7.8 (m H1, H4, H8, H11), 7.9 (2 · s H5, H7
1
3
3
and H12, H14); C NMR (75MHz, CDCl ): d 121.8, 125.2,
drying again treated with LiAlH (0.98g, 25mmol). The
126.7, 126.8, 127.8, 127.9, 132.3, 132.6, 134.4, 137.7; Mass
spectrum (MH ): m/z 339.
4
+
same procedure was repeated. The pure pentacene was
filtered and washed with water (2 · 30mL), dichlorometh-
ane (2 · 30mL), MeOH (2 · 30mL) and diethyl ether
8. (a) Afzali-Ardakani, A.; Dimitrakopoulos, C. D.; Breen, T.
L. J. Am. Chem. Soc. 2002, 124, 8812–8813; (b) Afzali-
Ardakani, A.; Dimitrakopoulos, C. D.; Graham, T. O. Adv.
Mater. 2003, 24, 2066.
9. Goodings, E. P.; Mitchard, D. A.; Owen, G. J. Chem. Soc.,
Perkin Trans. 1 1972, 1310–1314.
(
2 · 30mL). After drying in vacuo, pentacene was obtained
in 54% yield.Melting point: >300ꢁC (dec); Mass Spectrum
+
MH ): m/z 279. The spectral characteristics (UV–vis) are
(
in agreement with the literature.
9