Application of zirconacyclopentadienes (metalla-heterocycles) and cross-coupling for the convenient preparative method of 6,13-disubstituted pentacene

Article abstract of DOI:10.3987/COM-12-S(N)130

Iodination of zirconacyclopentadiene derivative gave diiododiene derivative. The product was lithiated with t-BuLi and treated with diiodonaphthalene successively to afford 6,13-bis(trimethylsilyl)-5,14- dihydropentacene. A 6,13-diiodo-5,14-dihydropentacene was synthesized by iodination of 6,13-bis(trimethylsilyl)-5,14-dihydropentacene with ICl. This diiododihydropentacene was used for the introduction of substituent at 6 and 13 positions by the cross-coupling reactions with Pd catalyst. After aromatization by a combination of DDQ and γ-terpinene or triethylamine, 6,13-disubstituted pentacene derivatives were synthesized.

Full text of DOI:10.3987/COM-12-S(N)130

HETEROCYCLES, Vol. 86, No. 2, 2012
1495
HETEROCYCLES, Vol. 86, No. 2, 2012, pp. 1495 - 1506. © 2012 The Japan Institute of Heterocyclic Chemistry
Received, 11th October, 2012, Accepted, 15th November, 2012, Published online, 28th November, 2012
DOI: 10.3987/COM-12-S(N)130
APPLICATION
OF
ZIRCONACYCLOPENTADIENES
(METALLA-HETEROCYCLES) AND CROSS-COUPLING FOR THE
CONVENIENT PREPARATIVE METHOD OF 6,13-DISUBSTITUTED
PENTACENE
Zhiying Jia,a Shi Li,a Kiyohiko Nakajima,b Ken-ichiro Kanno,a Zhiyi Song,a
and Tamotsu Takahashia, *
a
Catalysis Research Center, Hokkaido University, Kita 21, Nishi 10, Sapporo
001-0021, Japan
b
Department of Chemistry, Aichi University of Education, Igaya, Kariya, Aichi
448-8542, Japan; e-mail: tamotsu@cat.hokudai.ac.jp
Abstract – Iodination of zirconacyclopentadiene derivative gave diiododiene
derivative. The product was lithiated with t-BuLi and treated with
diiodonaphthalene successively to afford 6,13-bis(trimethylsilyl)-5,14-
dihydropentacene. A 6,13-diiodo-5,14-dihydropentacene was synthesized by
iodination of 6,13-bis(trimethylsilyl)-5,14-dihydropentacene with ICl. This
diiododihydropentacene was used for the introduction of substituent at 6 and 13
positions by the cross-coupling reactions with Pd catalyst. After aromatization by
a combination of DDQ and γ-terpinene or triethylamine, 6,13-disubstituted
pentacene derivatives were synthesized.
1. INTRODUCTION
Zirconacyclopentadienes are versatile metalla-heterocyclic compounds for aromatic compounds
formation such as benzene derivatives,1 pyridine derivatives,2 and multi-aromatic compounds.3
Zirconacyclopentadienes are conveniently prepared from Cp2ZrBu2 (Negishi reagent) and two alkynes or
diynes.4 We have reported coupling reaction of zirconacyclopentadienes with diiodobenzene or
tetraiodobenzene in the presence of CuCl for extension of aromatic rings as shown in eq. 1.5 However,
trimethylsilyl-substituted zirconacyclopentadienes did not react with diiodobenzene even in the presence
of CuCl.

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HETEROCYCLES, Vol. 86, No. 2, 2012
I
I
SiMe3
SiMe3
SiMe3
R
R
R
R
CuCl
R
R
R
R
R
R
R
Cp2Zr
(1)
Cp2Zr
DMPU
R
SiMe3
no reaction
In order to circumvent this difficulty, we developed an alternative route for the coupling reaction of
trimethylsilyl-substituted zirconacyclopentadienes with diiodobenzene. We have preliminarily reported
the alternative method which involves diiodination, lithiation and coupling with diiodobenzene in the
presence of CuCl (eq. 2).6 In this paper, we would like to report the combination of this alternative
method and cross-coupling reaction for the application to a convenient preparative method of
6,13-disubstituted pentacene derivatives.
I
SiMe3
SiMe3
SiMe3
SiMe3
R
R
R
R
R
2 equiv. I2
CuCl
I
I
Li
Li
4 equiv. t-BuLi
I
Cp2Zr
(2)
Et2O
R
R
R
SiMe3
CuCl
SiMe3
SiMe3
SiMe3
Pentacene derivatives, are known for good p-type semiconductor because of the high charge carrier
mobility, have been extensively studied as organic materials7 and in particular, 6,13-disubstituted
pentacene derivatives have been prepared by the quinone method (eq. 3).8 The reaction of Grignard
reagents with pentacene quinone sometimes did not occur at the carbonyl moiety but at the next ring like
Michael addition to ,-unsaturated ketone moiety to give undesired products.9 In the case of addition of
thienyl groups, it was observed that the second thienyl group migrated to the unexpected position to
afford the by-products.10
A quinone method for synthesis of 6,13-disubstituted pentacene
R
O
R-MgX
or RLi
(3)
R
O
This situation prompted us to develop novel method to prepare 6,13-disubstituted pentacene. For this
purpose, we combine the coupling reaction of silyl-substituted tricyclic zirconacyclopentadienes with
diiodonaphthalene giving disilyldihydropentacene, and introduction of substituents by cross-coupling
reaction such as Negishi coupling, Suzuki-Miyaura coupling and so on, after iodination of trimethylsilyl
substituents of disilyldihydropentacene. These reactions are shown in Scheme 1.

HETEROCYCLES, Vol. 86, No. 2, 2012
1497
SiMe3
SiMe3
I2
CuCl
SiMe3
SiMe3
Cp2ZrBu2
4 equiv. t-BuLi
I
I
ZrCp2
SiMe3
SiMe3
SiMe3
I
2
3
1
SiMe3
I
I
Li
Li
ICl
4
SiMe3
SiMe3
I
5
6
R
R
cross-coupling
RM, Pd cat.
aromatization
DDQ
8
R
R
7
Scheme 1. Synthesis of 6,13-disubstituted pentacene via transformation of zirconacyclopentadienes
2. RESULTS AND DISCUSSION
2.1. Synthesis of a 6,13-diiodo-5,14-dihydropentacene
We have recently reported the formation of 6,13-bis(trimethylsilyl)-5,14-dihydropentacene 5 by
Zr-mediated method.11 It is worth noting that two trimethylsilyl groups of which are capable of further
functionalizing. Encouraged by this, we synthesized a 6,13-diiodo-5,14-dihydropentacene 6 from
compound 5 by a simple iodination with 2.2 equiv of iodine monochloride (Scheme 2).
TMS
I
ICl
TMS
I
5
6
Scheme 2. Synthesis of 6,13-diiodo-5,14-dihydropentacene
This iodo compound could be converted to a variety of pentacene derivatives basically via two steps:
coupling and aromatization.
Recently Chi and co-workers prepared multi-substituted dibromopentacene derivatives from
hexabromide.12

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HETEROCYCLES, Vol. 86, No. 2, 2012
2.2. Coupling reactions of 6,13-diiodo-5,14-dihydropentacene
There are two choices due to the order of two steps, coupling with metal reagents followed by
aromatization or aromatization followed by coupling. Since a diiodopentacene, which should be formed
by doing the aromatization first, was anticipated with poor solubility and poor stability, subsequently, the
coupling reactions of compound 6 were firstly carried out. Alkyl, aryl, alkynyl, and alkenyl groups were
introduced by cross-coupling reactions in the presence of Pd catalyst (Scheme 3).
I
R
R-M
Pd cat.
I
R
6
7
Scheme 3. Pd-catalyzed cross-coupling reaction of 6,13-diiodo-5,14-dihydropentacene 2
2.2.1 Negishi Cross-Coupling Reaction
6,13-Diiodo-5,14-dihydropentacene 6 was treated with three equiv. of trimethylaluminium or
triethylaluminium in the presence of 5mol% PdCl2(PPh3)2 catalyst in THF at reflux for 24 h to give
products 7a or 7b in 86 or 84% yield, respectively (Scheme 4). When compound 6 reacted with Zinc
reagents in the presence of Pd(PPh3)4 catalyst, 6,13-phenyl-, thienyl-, bis-thienyl-substituted
5,14-dihydropentacenes 7c-e were synthesized successfully by the typical Negishi coupling reaction.
R
I
conditions A) or B)
R
I
6
7
NMR yield (isolated yield)
86% (82%)
7a R = Me
A) 3 equiv.
5 mol% PdCl2(PPh3)2, THF, reflux 24 h
R3Al,
Et
84% (75%)
7b
7c
Ph
74% (70%)
94% (72%)
86% (70%)
B) 3 equiv. RZnCl, 5 mol% Pd(PPh3)4, THF, 50 °C 12 h (for 3c, reflux 12 h)
Scheme 4
7d thienyl
7e bisthienyl
2.2.2 Sonogashira Cross-Coupling Reaction
Sonogashira coupling reaction was carried out to form alkynyl group substituted products (Scheme 5).
Five equiv. of trimethylsilylacetylene or triisopropylsilylacetylene were empolyed in these reactions
to ensure that all the starting compound 6 was consumed. Corrossponding producs 7f and 7g were
produced in high yields of 92 and 96%, respectively. The isolated yields were 84 and 87%.

HETEROCYCLES, Vol. 86, No. 2, 2012
1499
SiR'
3
5.0 equiv.
SiR'
H
3
I
5 mol% PdCl2(PPh3)2
5 mol% CuI
Et3N, 50 °C, 6 h
NMR yield(isolated yield)
I
7f R' = Me
7g
92% (84%)
96% (87%)
7
6
i-Pr
SiR'
3
Scheme 5
2.2.2 Suzuki Cross-Coupling Reaction
Alkene substituted dihydropentacene could also be obtained by the coupling reaction of the diiodo
compound 6. One example was examined in this issue as shown in Scheme 6.
Pr
Pr
3.0 equiv.
O
B
O
I
I
5 mol% Pd(PPh3)4.
NaOH, refluxing, 24 h
92% (NMR yield), 54% (isolated yield)
6
7h
Pr
Scheme 6
Suzuki-Miyaura coupling was applied to synthesize 6,13-dialkenyl-5,14-dihydropentacene 7h in 92%
NMR yield. Isolated yield of this case was only 54%, which is possibly due to its polymerization on the
silica column.
2.3. Formation of 6,13-disubstituted pentacenes
As we planned, thus obtained 6,13-disubstituted-5,14-dihydropentacenes 7 were aromatized by DDQ
and 50 equiv. of γ-terpinene or triethylamine at 80 °C for 3 h to give 6,13-disubstituted pentacenes 8.
The results are shown in Scheme 7. Dimethylpentacene and diethylpentacene could be prepared in 45
and 42% yields, respectively. Both of them were isolated and characterized by NMR successfully.
Such alkyl substituted pentacene derivatives are very rear.13 Aryl substituted pentacenes, such as
phenyl, thienyl or bis-thienyl, were also synthesized in 72, 67 and 70% yields, respectively.
Moveover, alkynyl-substituted pentacenes were prepared in high yields of 81 and 90%, respectively.
The aromatization of dialkenyl dihydropentacene 7h was failed under our typical conditions.

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HETEROCYCLES, Vol. 86, No. 2, 2012
R
R
R
R
1) DDQ
2)

-terpinene or triethylamine
8
7
Me
8a (45%)
Et
R = alkyl
R = aryl
8b (42%)
S
S
S
8c (72%)
8d (67%)
8e (70%)
TMS
R = alkynyl
TIPS
8f (81%)
8g (90%)
Scheme 7. Formation of the 6,13-disubstituted pentacene derivatives by aromatization.
3. CONCLUSIONS
In summary, 6,13-diiodo-5,14-dihydropentacene was synthesized from a 6,13-bis(trimethylsilyl)-5,14-
dihydropentacene. Introduction of alkyl, aryl, alkynyl, alkenyl groups was carried out by the palladium
catalyzed cross-coupling reaction. One additional step of aromatization gave the 6,13-disubstituted
pentacene derivatives in good yields.
4. EXPERIMENTAL
All reactions involving air- or moisture-sensitive organometallic reagents were carried out under dry
nitrogen. THF, toluene, pentane, and diethyl ether were distilled over sodium and benzophenone.
1,3-Dimethyl-3,4,5,6-tetrahydro-2(1H)-pyrimidinone (DMPU; Aldrich, 98%) was dried over calcium
hydride and distilled under reduced pressure. Zirconocene dichloride (Tokyo Chemical Industries, Ltd.),
copper(I) chloride (Wako Pure Chemical Industries, Ltd., 99.9%), 2,3-dichloro-5,6-dicyano-1,4-
benzoquinone (DDQ, Kanto, 98%), γ-terpinene (Wako Pure Chemical Industries, Ltd.), and
trans-1-penten-1-ylboronic acid pinacol ester (Aldrich, 97%) were used as received. Zinc dichloride was
dried by heat gun in vacuo. 6,13-Bis(trimethylsilyl)-5,14-dihydropentacene was synthesized by our
reported method.11 PdCl2(PPh3)2 is commercially available catalyst. Pd(PPh3)4 was prepared according to
literature.14 1H and 13C NMR spectra were recorded in CDCl3 or C6D6 (containing 1% TMS) solutions on
a JEOL JNM-ECX400 or a JEOL JNM-ECA600 spectrometer.
4.1. Preparation of 6,13-diiodo-5,14-dihydropentacene 6

HETEROCYCLES, Vol. 86, No. 2, 2012
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To a THF solution (5 mL) of 6,13-bis(trimethylsilyl)-5,14-dihydropentacene (424 mg, 1.0 mmol) was
slowly added ICl (1.0M CH2Cl2 solution, 2.2 mL, 2.2 mmol) at -78 °C. After stirring at this temperature
for 3 h, the reaction mixture was quenched with saturated aqueous Na2S2O3, and extracted with CHCl3.
The combined organic phase was washed with brine. After removal of the solvent, CHCl3 was added
again to appear large amount of precipitate. After filtration, filter cake was collected as a brown solid
(255 mg, 48% yield).
1
6: H NMR (CDCl3, Me4Si) δ 4.47 (s, 4 H), 7.25-7.29 (m, 2 H), 7.43-7.44 (m, 2 H), 7.52-7.55 (m, 2 H),
13
8.09-8.12 (m, 2 H), 8.85 (s, 2 H); C NMR (CDCl3, Me4Si) δ 45.5, 106.7, 126.5, 126.9, 127.2, 128.1,
132.1, 132.8, 133.0, 136.3, 139.8. HRMS (EI) calcd for C22H14I2: 531.9185. Found: 531.9199.
4.2. Coupling reactions of 6,13-diiodo-5,14-dihydropentacene 6
4.2.1. Preparation of 6,13-dimethyl-5,14-dihydropentacene 7a and 6,13-diethyl-5,14-dihydro-
pentacene 7b
To a solution of 6,13-diiodo-5,14-dihydropentacene 2 (161 mg, 0.30 mmol) and PdCl2(PPh3)2 (10 mg,
0.015 mmol) in THF (10 mL) was added AlMe3 (1.0 M hexane solution, 0.9 mL, 0.9 mmol), and the
mixture was heated to reflux for 1 d. After cooling to room temperature, the mixture was quenched with 3
M aqueous HCl and extracted with CHCl3. Removal of the solvent, the residue was purified by column
chromatography (hexane : EtOAc = 10 : 1) to afford the title compound 7a (76 mg, 82% isolated yield) as
pale yellow solid.
7a: pale yellow solid. 1H NMR (CDCl3, Me4Si, 600 MHz) δ 2.78 (s, 6 H), 4.10 (s, 4 H), 7.14-7.16 (m, 2
H), 7.28-7.30 (m, 2 H), 7.34-7.36 (m, 2 H), 7.92-7.93 (m, 2 H), 8.50 (s, 2 H); 13C NMR (CDCl3, Me4Si) δ
15.1, 34.4, 123.0, 125.1, 126.4, 127.1, 127.3, 128.3, 130.8, 131.0, 132.6, 137.3. HRMS (EI) calcd for
C24H20: 308.1565. Found: 308.1565.
Compound 7b was prepared by the same way as described for 7a in 75% yield.
7b: yellow solid. 1H NMR (CDCl3, Me4Si, 600 MHz) δ 1.40 (t, J = 7.1 Hz, 6 H), 3.38 (q, J = 7.1 Hz, 4 H),
4.15 (s, 4 H), 7.22-7.23 (m, 2 H), 7.37-7.38 (m, 2 H), 7.41-7.43 (m, 2 H), 8.00-8.01 (m, 2 H), 8.62 (s, 2
13
H); C NMR (CDCl3, Me4Si) δ 15.0, 22.0, 34.0, 122.9, 125.0, 126.5, 127.0, 128.4, 129.9, 131.0, 132.4,
133.4, 137.9. HRMS (EI) calcd for C26H24: 336.1878. Found: 336.1868.
4.2.2. Preparation of 6,13-diaryl-5,14-dihydropentacene 7c-e
General procedure:
In a Schlenk tube, the arylzinc reagent was prepared in situ by reaction of aryllithium (0.14 mL, 0.27
mmol) with dried ZnCl2 (52 mg, 0.4 mmol) in THF (2 mL). To which 6,13-diiodo-5,14-dihydropentacene
6 (50 mg, 0.09 mmol) was added, and the reaction mixture was degassed by freeze-pump-thaw cycle for

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HETEROCYCLES, Vol. 86, No. 2, 2012
three times. Then a Pd(PPh3)4 catalyst (5.2 mg, 0.0045 mmol) was added and the mixture was heated at
50 °C for 12 h. After cooling to room temperature, the reaction mixture was quenched with 3 N HCl and
extracted with CHCl3. The combined organic phase was washed with water, saturated aqueous NaHCO3,
and brine in successive. The solution was dried over anhydrous MgSO4, filtered, and distilled at reduced
pressure to remove the solvent. The residual solid was purified by a silica gel column chromatography or
GPC to afford the title compound.
7c: 70% isolated yield, 74% NMR yield, yellow solid. 1H NMR (CDCl3, Me4Si, 400 MHz) ) δ 3.87 (s, 4
H), 7.14 (s, 4 H), 7.30-7.33 (m, 2 H), 7.44-7.46 (m, 4 H), 7.56-7.66 (m, 6 H), 7.75-7.77 (m, 2 H), 7.97 (s,
13
2 H); C NMR (CDCl3, Me4Si) δ 35.5, 125.2, 125.3, 126.4, 127.1, 127.5, 128.3, 128.8 (two peaks were
overlapped), 130.7, 130.8 (two peaks were overlapped), 131.0, 133.1, 136.0, 137.7, 139.8. HRMS (EI)
calcd for C34H24: 432.1878. Found: 432.1881.
1
7d: 72% isolated yield, 94% NMR yield, yellow solid. H NMR (CDCl3, Me4Si) δ 4.00 (s, 4 H),
13
7.18-7.25 (m, 6 H), 7.37-7.40 (m, 4 H), 7.65-7.66 (m, 2 H), 7.85-7.87 (m, 2 H), 8.19 (s, 2 H); C NMR
(CDCl3, Me4Si) δ 35.6, 125.1, 125.5, 126.5, 126.6, 127.2, 127.5, 128.4, 128.6, 129.4, 131.38, 131.43,
135.9, 137.3, 139.7. HRMS (EI) calcd for C30H20S2: 444.1006. Found: 444.1003.
7e: 70% isolated yield, 86% NMR yield, yellow solid. 1H NMR (CDCl3, Me4Si) δ 4.08 (s, 4 H), 7.07 (d, J
= 2.4 Hz, 2 H), 7.08-7.10 (m, 2 H), 7.17-7.19 (m, 2 H), 7.24-7.25 (m, 2 H), 7.29 (d, J = 3.6 Hz, 2 H), 7.31
(d, J = 2.4 Hz, 2 H), 7.38-7.39 (m, 2 H), 7.42 (d, J = 2.4 Hz, 2 H), 7.87-7.89 (m, 2 H), 8.32 (s, 2 H); 13C
NMR (CDCl3, Me4Si) δ 35.7, 124.1, 124.2, 124.7, 125.2, 125.7, 126.7, 127.3, 128.1, 128.5, 129.3, 129.6,
131.4, 131.8, 136.1, 137.2, 137.7, 138.9, 139.0. HRMS (EI) calcd for C38H24S4: 608.0761. Found:
608.0759.
4.2.3. Preparation of 6,13-bis(trimethylsilylethynyl)-5,14-dihydropentacene 7f and 6,13-
bis(triisopropylsilylethynyl)-5,14-dihydropentacene 7g
A 20-mL Schlenk flask was charged with trimethylsilylacetylene (70 uL, 0.5 mmol), compound 6 (53 mg,
0.1 mmol), PdCl2(PPh3)2 (3.51 mg, 0.005 mmol), and CuI (1 mg, 0.005 mmol) under a steam of nitrogen.
Freshly distilled triethylamine (3 mL) was added into the flask via syringe, and the reaction mixture was
degassed by freeze-pump-thaw cycle for three times. Then the reaction mixture was stirred at 50 °C for 6
h. The solvent was evaporated in vacuo, the resulting residue was dissolved in CHCl3 (25 mL) then
washed with saturated aqueous NaHCO3 and brine then dried over anhydrous Na2SO4. After removal of
solvent, the residue was purified by silica gel chromatography (hexane : EtOAc = 50 : 1) to afford the title
compound 3f (40 mg, 84% isolated yield) as a yellow solid.
1
7f: H NMR (CDCl3, Me4Si) δ 0.48 (s, 18 H), 4.40 (s, 4 H), 7.26-7.28 (m, 2 H), 7.41-7.42 (m, 2 H),
7.50-7.51 (m, 2 H), 8.07-8.08 (m, 2 H), 8.90 (s, 2 H); 13C NMR (CDCl3, Me4Si) δ 0.4, 35.8, 102.0, 106.7,

HETEROCYCLES, Vol. 86, No. 2, 2012
1503
118.4, 125.4, 125.9, 126.6, 127.7, 128.6, 129.8, 132.1, 136.1, 139.0. HRMS (EI) calcd for C32H32Si2:
472.2043. Found: 472.2045.
The title compound 3g (56 mg) was prepared by the same way as described for 3f in 87% yield from 2
(53 mg, 0.1 mmol).
7g: 1H NMR (CDCl3, Me4Si) δ 1.31 (s, 6 H), 1.32 (s, 36 H), 4.46 (s, 4 H), 7.24-7.27 (m, 2 H), 7.34-7.36
13
(m, 2 H), 7.48-7.51 (m, 2 H), 7.99-8.02 (m, 2 H), 8.99 (s, 2 H); C NMR (CDCl3, Me4Si) δ 11.5, 18.9,
35.8, 103.0, 103.6, 118.4, 125.4, 125.7, 126.5, 127.5, 128.4, 129.9, 131.9, 136.1, 138.8. HRMS (EI) calcd
for C44H56Si2: 640.3921. Found: 640.3920.
4.2.4. Preparation of 6,13-di-pent-1-enyl-5,14-dihydropentacene 7h
The trans-1-penten-1-ylboronic acid pinacol ester (170 mg, 1.5 mmol), compound 6 (266 mg, 0.5 mmol)
were added to a 50 mL Schlenk flask in THF (10 mL). Aqueous NaOH solution (2 M, 1 mL) was added
under N2, and the reaction mixture was degassed by freeze-pump-thaw cycle for three times. Then
Pd(PPh3)4 (17 mg, 0.015 mmol) was added, and the reaction was heated under reflux. The reaction was
mornitored by NMR and worked up after 1 d. The cooled mixture was extracted with CHCl3, washed
with brine and then dried over Na2SO4. The crude product was purified by silica gel chromatography
(hexane) to afford the title compound 7h (152 mg, 54% isolated yield) as a yellow solid.
7h: 1H NMR (C6D6, Me4Si) δ 1.17 (t, J = 7 Hz, 6 H), 1.70-1.79 (m, 4 H), 2.49-2.55 (m, 4 H), 4.23 (s, 4 H),
5.94 (dt, J = 16 Hz, J = 6.8 Hz, 2 H), 6.92 (d, J = 16 Hz, 2 H), 7.21-7.23 (m, 2 H), 7.33-7.35 (m, 2 H),
13
7.42-7.44 (m, 2 H), 7.97-8.00 (m, 2 H), 8.61 (s, 2 H); C NMR (CDCl3, Me4Si) δ14.1, 22.9, 35.4, 35.8,
124.4, 125.0, 126.3, 126.7, 127.1, 128.4, 130.1, 131.0, 131.9, 132.7, 138.1, 138.4. HRMS (EI) calcd for
C32H32: 416.25040. Found: 416.24912.
4.3. Synthesis of 6,13-disubstituted pentacenes via aromatization
4.3.1. Synthesis of 6,13-dimethylpentacene 8a via a 6,13-dimethylpentacene-DDQ adduct 7aa
Cl
O
Cl
CN
Me
Me
Me
O
NC
DDQ
Me
Me
Me
7a
7aa
8a
Scheme 8

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HETEROCYCLES, Vol. 86, No. 2, 2012
In a 20 mL Schlenk tube, 7a (30 mg, 0.1 mmol) and DDQ (45 mg, 0.2 mmol) were mixed in toluene (2
mL). This reaction mixture was degassed and stirred at room temperature for 2 h. The end of reaction was
detected by TLC. The solvent was removed by a rotary evaporator, and the residue was purified by silica
gel column chromatography (hexane: EtOAc = 2:1) to afford the corresponding adduct 7aa (30 mg, 48%
isolated yield and 86% NMR yield) as a red solid (Scheme 8).
DDQ adduct 7aa (533 mg, 1.0 mmol) was mixed with triethylamine (3 mL) in toluene (3 mL). The
mixture was degassed and heated to 80 °C for 3 h. After cooling to room temperature, the solvent was
removed in vacuo. The residue was washed by MeOH (degassed, 15 mL) to afford the title compound 8a
(184 mg, 45% yield) as a blue solid.
7aa: yellow solid. 1H NMR (CDCl3, Me4Si, 400 MHz) δ 2.39 (s, 6 H), 7.57-7.59 (m, 4 H), 7.76 (s, 2 H),
7.82-7.84 (m, 2 H), 7.92-7.94 (m, 2 H), 8.07 (s, 2 H); 13C NMR (CDCl3, Me4Si) δ 15.5, 50.6, 63.4, 113.7,
123.9, 124.0, 127.6, 128.2 (two peaks were overlapped), 128.5, 132.8, 132.9, 134.5, 135.2, 142.3, 178.5.
HRMS (FAB) calcd for C32H18O2N2Cl2Na (M+Na+): 555.0643. Found: 555.0618.
8a: blue solid. 1H NMR (C6D6, Me4Si) δ 3.12 (s, 6 H), 7.18-7.20 (m, 4 H), 7.84-7.87 (m, 4 H), 8.86 (s, 4
H); 13C NMR (CDCl3, Me4Si) δ 15.0, 124.4, 125.4, 128.6, 129.0, 129.2, 131.5. UV-vis (CHCl3) λmax/nm:
604, 555, 523. HRMS (EI) calcd for C24H18: 306.14085. Found: 306.13859.
4.3.2. Preparation of 6,13-diethylpentacene 8b
To a degassed toluene solution (3 mL) of 7b (88 mg, 0.2 mmol) was added DDQ (90 mg, 0.4 mmol). The
reaction mixture was stirred at room temperature for 5 h. Corresponding DDQ adduct was formed in
around 93% NMR yield. This reaction mixture was added with degassed γ-terpinene (1.6 mL, 10 mmol)
and heated at 80 °C for 6 h. Again the reaction mixture was cooled to room temperature. The solvent was
removed under reduced pressure to leave a solid, which was washed with degassed MeOH (15 mL) for 3
times. The solid was then recrystallized from hexane (degassed) to afford the title compound 8b (28 mg,
0.084 mmol) as blue crystals in 42% yield.
1
8b: H NMR (C6D6, Me4Si) δ 1.50 (t, J = 7.0 Hz, 6 H), 3.83 (q, J = 7.1 Hz, 4 H), 7.14-7.16 (m, 4 H),
7.81-7.83 (m, 4 H), 8.92 (s, 4 H); 13C NMR (C6D6, Me4Si) δ 15.8, 22.4, 124.0, 125.4, 128.4, 129.1, 131.8,
135.5. UV-vis (CHCl3) λmax/nm: 604, 557, 518. HRMS (EI) calcd for C26H22: 334.1721. Found:
334.1712.
4.3.3. General procedure for preparation of 6,13-diphenylpentacene 8c-g
To a degassed toluene solution (3 mL) of dihydropentacene derivative 7 (0.1 mmol) was added DDQ (0.2
mmol). The reaction mixture was stirred at room temperature for 1 h and heated to 50 °C for 3 h. The
corresponding DDQ adducts were formed in high yields. It was then treated with γ-terpinene (0.8 mL, 5

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mmol) at 80°C for 6 h before remove the solvent, the remained solid was washed by degassed MeOH (15
mL) for 3 times and analyzed by 1H NMR spectroscopy to give NMR yield of 8. Compounds 8c,15 8d,15
8e,16 8f,17 8g18 were identified according to data reported in references.
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