Synthesis, structures, and properties of disubstituted heteroacenes on one side containing both pyrrole and thiophene rings

Article abstract of DOI:10.1021/jo800622y

(Chemical Equation Presented) A new series of ladder-type heteroacenes containing both pyrrole and furan rings, 5,6-disubstituted diindolo[3,2-b:4,5- b′]thiophenes (DITs), were effectively synthesized from N-functionalized 3,3′-dibromo-2,2′-biindoles unde

Full text of DOI:10.1021/jo800622y

Synthesis, Structures, and Properties of Disubstituted Heteroacenes
on One Side Containing Both Pyrrole and Thiophene Rings
Ting Qi,† Wenfeng Qiu,‡ Yunqi Liu,*,‡ Hengjun Zhang,† Xike Gao,† Ying Liu,† Kun Lu,†
Chunyan Du,† Gui Yu,‡ and Daoben Zhu‡
Graduate School of Chinese Academy of Sciences, Beijing 100190, China; Beijing National Laboratory for
Molecular Sciences, Organic Solids Laboratory, Institute of Chemistry, Chinese Academy of Sciences,
Beijing 100190, China
liuyq@mail.iccas.ac.cn
ReceiVed March 20, 2008
A new series of ladder-type heteroacenes containing both pyrrole and furan rings, 5,6-disubstituted
diindolo[3,2-b:4,5-b′]thiophenes (DITs), were effectively synthesized from N-functionalized 3,3′-dibromo-
2,2′-biindoles undergoing intramolecular cyclization with bis(phenylsulfonyl) sulfide and organolithium.
Single-crystal X-ray results demonstrate that 5,6-dipropyldiindolo[3,2-b:4,5-b′]thiophene (4b) forms a
herringbone-type of packing motif and 5,6-di(p-tolyl)diindolo[3,2-b:4,5-b′]thiophene (4d) forms a parallel
packing motif. Both of them have S-S contacts, enhancing the electronic transport between molecules.
Their photophysical properties suggest that the skeleton of diindolo[3,2-b:4,5-b′]thiophene is more favorable
to aggregate in solid than that of indolo[3,2-b]carbazole. The large band gaps and low-lying HOMO
energy levels could result in much better stability.
Introduction
applied in devices, such as poor solubility, limited stability in
ambient conditions,4 and unfavorable stacking in the solid
state.5
One strategy to overcome these shortcomings is to introduce
heteroatoms in the fused-ring system to construct more stable
ladder-type fused molecules. New π-extended pentacene-like
heteroacenes, e.g., 5,7,12,14-tetraazapentacene,6 indole[3,2-
b]carbazole,7 pentathieoacene,8 anthradithiophene,9 benzo[1,2-
b:4,5-b′]bis[b]benzothiophene,10 tetraceno[2,3-b]thiophene,11
The design and synthesis of new π-conjugated organic
semiconductors are attracting more and more attention due to
their potential applications for various optoelectronic devices.1
In particular, the linearly fused acenes have applications in
organic field-effect transistors (OFETs), light emitting diodes
(LEDs), and photovoltaic cells.2 Recent research interest has
primarily focused on pentacene and its derivatives, in which
pentacene exhibits outstanding charge carrier mobility of 1.5
cm2 V-1 s-1 for OFETs (on chemically modified SiO2/Si
substrate).3 However, pentacene has some shortcomings when
(3) (a) Lin, Y.-Y.; Gundlach, D. J.; Nelson, S. F.; Jackson, T. N. IEEE
Electron DeVice Lett. 1997, 18, 606. (b) Nelson, S. F.; Lin, Y.-Y.; Gundlach,
D. J.; Jackson, T. N. Appl. Phys. Lett. 1998, 72, 1854.
(4) Weidkamp, K. P.; Afzali, A.; Tromp, R. M.; Hamers, R. J. J. Am. Chem.
Soc. 2004, 126, 12740.
† Graduate School of Chinese Academy of Sciences.
‡ Chinese Academy of Sciences.
(1) Dimitrakopoulos, C. D.; Malenfant, P. R. L. AdV. Mater. 2002, 14, 99.
(2) (a) Anthony, J. E. Chem. ReV. 2006, 106, 5028. (b) Sun, Y.; Liu, Y.;
Zhu, D. J. Mater. Chem. 2005, 15, 53.
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Sheraw, C. D.; Jackson, T. N.; Eaton, D. L.; Anthony, J. E. AdV. Mater. 2003,
15, 2009.
4638 J. Org. Chem. 2008, 73, 4638–4643
10.1021/jo800622y CCC: $40.75 2008 American Chemical Society
Published on Web 05/22/2008

Disubstituted Heteroacenes
and dibenzo[d,d′]thieno[3,2-b:4,5-b′]dithiophene,12 have been
developed and tested as active semiconducting channels in
OFETs. We found that the reported molecules mainly introduced
sulfur or nitrogen as bridging atoms. To the best of our
knowledge, the introduction of sulfur and nitrogen together in
the pentacene-like system as OFET materials has still not been
reported. The incorporation of sulfur as thiophene substructure
would exhibit a variety of intra- and intermolecular interactions,
such as van der Waals interactions, π-π stacking, and
sulfur-sulfur interactions, essential to achieve high charge
mobility.13 Furthermore, Ong et al. recently reported that 5,11-
disubstituted indolo[3,2-b]carbazole displayed the high mobility
of 0.12 cm2 V-1 s-1 and good environmental stability, and the
introduction of nitrogen atoms can well increase the stability
of materials.7b In addition, the research on crystal structure of
organic semiconductors is very important to explore struc-
ture-property relationships. Notably, substituents can affect
molecular solid-state order.9a,14 At present, we found that the
substituents of the linearly fused acenes studied mainly lie in
one or two terminals of molecular backbone or two sides of
molecular central rings. However, the crystal packing about the
substituents of acenes on one side of central rings is rarely
studied. Recently, only the groups of Bao,15 Nuckolls,16 and
Nakano17 studied crystal packing of monosubstituted acenes on
one side, respectively. This paper will focus on disubstituted
crystal packing of acenes on one side needed for complementary
study of structure-property relationships.
SCHEME 1
N-functionalized 3,3′-dilithio-2,2′-biindole (Scheme 1). The
lithium-halogen exchange reaction of bromoindole with orga-
nolithium conveniently produces aryllithiums,19 and 3,3′-di-
bromo-2,2′-biindole is easily available by bromination of 2,2′-
biindole directly. Thus, these reactions enable us to design an
efficient synthetic route to different substituent ladder-type
heteroacenes. Different substituents for structural modification
may not only improve the solubility and the thin film packing,
but also exert subtle influences on the molecular physical
properties. Structural and physical properties of the synthesized
heteroacenes are discussed.
Results and Discussion
Synthesis. The synthetic route to DITs 4a-d is outlined in
Scheme 2. We used N-functionalized indole as the starting
material, because the substituents can not only improve the
solubility of compounds, but also make compounds subjected
to the lithiation in the next steps. Four substituents were selected,
and alkyl (propyl20 and hexyl20) and aryl (p-tolyl21) were
introduced under different reaction conditions, respectively,
according to the literature. Indole easily reacted with various
alkyl bromides in the presence of KOH to give N-alkyl-
substituted products 1b and 1c. The aryl group was introduced
onto the indole nitrogen by treating indole with aryl iodide in
the presence of potassium carbonate and CuO to give N-aryl-
substituted product 1d. N-Functionalized indoles (1a-d) were
then reacted with n-butyllithium at reflux and oxidized by CuCl2
to produce symmetric 2,2′-coupling dimers 2a, 2b, 2c, and 2d
in moderate yields.22 Bromination of 2a-d (bromine, DMF)
gave rise to 3,3′-dibromo-2,2′-biindole (3a-d) in high yield
directly.23 The final ring closure reaction succeeded by using
bis(phenylsulfonyl) sulfide in generating the appropriate anions
during the lithiation to give the target products 4a, 4b, 4c, and
4d in 51%, 53%, 56%, and 55% yields, respectively. As
expected, the solubility of N-alkylated compounds (4a-c)
increases with the lengthening alkyl chains. Overall, all of these
new compounds exhibit good solubility in common solvents
such as CHCl3, CH2Cl2, THF, toluene, and chlorobenzene, thus
enabling purification by column chromatography.
In this work, we designed and synthesized a new series of
ladder-type pentacene-like heteroacenes 4a-d containing pyr-
role and thiophene rings, diindolo[3,2-b:4,5-b′]thiophene (DIT),
motivated by aforementioned characteristics. The reaction of
bis(phenylsulfonyl)sulfide with 3,3′-dilithiodiaryl generated from
biaryl bromide has been a common method for the preparation
of dithieno[3,2-b:2′,3′-d]thiophene.18 We envisioned that sulfur-
bridged N-functionalized 2,2′-biindole could be synthesized in
a similar way by the reaction of bis(phenylsulfonyl)sulfide with
(6) Ma, Y.; Sun, Y.; Liu, Y.; Gao, J.; Chen, S.; Sun, X.; Qiu, W.; Yu, G.;
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Crystal Packing Studies: X-ray Analyses of DITs 4b
and 4d. Most studies indicate that the linearly fused acenes
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R.; Bre´das, J.-L. Chem. ReV. 2007, 107, 926. (b) Lemaur, V.; da Silva Filho,
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J. Org. Chem. Vol. 73, No. 12, 2008 4639

Qi et al.
SCHEME 2. Synthesis of Compounds 4a-d
commonly adopt herringbone or parallel packing motif. We
obtained colorless block single crystals of 4b and 4d by slow
evaporation of hexane and vacuum sublimation, respectively.
The detailed crystallographic data can be found in the Supporting
Information. X-ray analyses display that the size of substituents
has an effect on molecular solid-state packing.
The tolyl-substituted DIT 4d prefers to pack in a parallel
pattern (Figure 2). As shown in Figure 1, due to the steric
hindrance that the tolyl groups impose, the molecules in a stack
are alternately rotated by 180°. This similar anti conformation
also exists in the 4b crystal structure (Figure S1, Supporting
Information). Possibly due to the DIT backbone having a
molecular dipole moment, the orientation of molecules alternates
between layersr to minimize the dipolar interactions.24 There-
fore, the interlayer adjacent two molecules form a dimer. The
tolyl groups are inclined at approximately 70° with respect to
the acene plane. Short C-H · · · π intermolecular interactions
exist between pendant tolyl and the conjugated plane at 2.77
Å. Notably, this compound forms a face-face slipped π-stack-
ing structure. The short C-C contacts (3.47 Å) exist in an edge-
to-edge manner, shown with dashed lines in Figure 2. The
distance between adjacent full overlapped molecules in the
column is 9.83 Å (Figure S4, Supporting Information). Fur-
thermore, intermolecular S-S (3.63 Å) and S · · · H (2.91 Å)
interactions exist between columns consisting of dimers (Figure
S6, Supporting Information).
The propyl-substituted DIT 4b prefers to pack in a her-
ringbone pattern (Figure 3). However, different from commonly
reported herringbone arrangement, the crystal packing structure
of 4b is more complicated. As shown in Figure 3, there are
three independent molecular configurations in the crystal to
construct three types of stacking columns arranged in an edge-
to-face fashion. Therefore, three types of herringbone patterns
are interlaced in the 4b crystal structure. Similar to 4d, due to
the dipolar interactions, the interlayer adjacent two molecules
also form a dimer, but they are interlaced because of alkyl
cooperation (Figure S1, Supporting Information). The two
adjacent molecules in a stacking column are rotated by almost
180° (column a and column b) and 157° (column c), respectively
(Figure S2, Supporting Information). Short C-H· · ·π (2.71 Å
average) intermolecular interactions between pendant propyl and
the conjugated plane and S-S (3.66 Å in columns a and c and
3.56 Å in column b) and S · · ·H (3.00 Å average) contacts exist
along the c-axis. Meanwhile, three kinds of herringbone packing
patterns show three kinds of C-H· · ·π interactions existing
between adjacent edge-to-face molecules along the a-axis (at
2.74 Å between columns a and b, 3.07 Å between columns a
and c, and 3.26 Å between columns b and c, respectively). The
two types of herringbone arrangements with much longer
C-H· · ·π interactions show a trend to parallel packing, indica-
tive of the transition from herringbone to parallel packing. The
mean distance between adjacent mostly overlapped molecules
in the column is 7.69 Å shorter than that of 4d (9.83 Å),
indicating that 4b has a tighter packing structure because its
FIGURE 1. Crystal cell packing structure of 4d, with hydrogen atoms
removed for clarity.
(24) (a) He, M.; Zhang, F. J. Org. Chem. 2007, 72, 442. (b) Miao, Q.;
Lefenfeld, M.; Nguyen, T. Q.; Siegrist, T.; Kloc, C.; Nuckolls, C. AdV. Mater.
2005, 17, 407.
FIGURE 2. Crystal packing of 4d, view down the b-axis, with
hydrogen atoms and tolyl groups removed for clarity.
4640 J. Org. Chem. Vol. 73, No. 12, 2008

Disubstituted Heteroacenes
FIGURE 4. The molecular structure of 5,11-dimethylphenylindolo[3,2-
b]carbazole (5).
longest maxima around 340-355 nm and emission bands with
the maxima around 385 nm due to their identical skeletons
(Figure 5), the diindolo[3,2-b:4,5-b′]thiophene unit. Shown in
Figure 6 are the spectra of vacuum-deposited films of 4a-d on
quartz substrates. All compounds show sharp band edges
suggesting little structural disorder. Compared with those in
solution, broadening of the absorption spectrum and a 13-17
nm red-shift of all peaks in pristine film were observed because
of the formation of the aggregates. Such a pronounced change
of the absorption spectrum is a result of the delocalization of
the exciton within a cofacial stack induced by the π-π
interaction, which was also evidenced by a related 6-18 nm
red-shift of the photoluminescence spectrum shown in Table 1.
It is worth noting that obvious absorbance enhancement of the
onset peak relative to the second was observed in the alkyl-
substituted compounds, and the relative intensity increased with
the decreasing length of the alkyl chain. This is the result of
intermolecular interactions, suggesting that the shorter the length
of the alkyl chain, the more favorable π-π stacking is. It is
identical with the degree of relative red-shift of emission maxima
from solution to films, in the order 4a > 4b > 4c.
The absorption spectrum of a dilute solution of 5 showed
red-shift relative to 4d. Their structural difference is the central
fused benzene ring of the backbone for 5 changed to thiophene
for 4d, resulting in spectral change. Matzger et al. stated that
adding the sulfur atom acts to widen the HOMO-LUMO gap
induced by LUMO destabilization.26 This could be the reason
that 4d displays a higher energy band compared to 5. The
absorption spectra of 4d display a 13 nm red-shift from a dilute
solution to a film, evidently larger than that of 5, indicating
that the skeleton of diindolo[3,2-b:4,5-b′]thiophene is more
favorable to aggregate in solid than that of indolo[3,2-b]carba-
zole. The crystal structure of 4d also confirmed this conclusion
further. The optical band gaps estimated from the absorption
edges of the thin-film spectra for 4a-d are around 3.2 eV, larger
than 5 (2.8 eV). Semiconductors with large band gaps, which
absorb in the UV or near-UV regions, are suitable for fabricating
transparent OTFT circuits used for optoelectronic devices
requiring circuit transparency.27
FIGURE 3. Crystal packing of 4b, view down the b-axis, with
hydrogen atoms and propyl groups removed for clarity (on the bottom).
TABLE 1. Photophysical Data of Compounds 4a-d and 5
solution
filmd
c
b
c
Eg
λlum
Eg
a
b
compd
λabs (nm)
λlum (nm) (eV) λabs (nm) (nm) (eV)
4a
4b
4c
4d
5e
344, 353
345, 353
344, 353
343, 351
340, 388, 410
370, 384
372, 385
372, 385
371, 383
3.30 352, 370
3.31 350, 370
3.32 348, 366
3.32 348, 364
341, 415
397
393
391
401
3.25
3.14
3.18
3.16
2.8
a Measured in a dilute CH2Cl2 solution (2 × 10-5 M). b Excited at the
absorption maxima. c Estimated from the onset of absorption (Eg
)
1240/λonset). d Films were vacuum deposited (50 nm, quartz substrate).
e From ref 7b.
steric hindrance of substituents is much smaller. We could
predict that 4a with the smallest substituents may have the
tightest packing structure. Unfortunately, crystals of 4a suitable
for single-crystal analysis have not been obtained.
The substituents of this synthesized series are all on the same
side of the central rings of molecular conjugated backbone. This
particular position of substituents makes molecules have
particular configurations, which form dimers more easily. Their
crystal structures show that not only the size of the substituents
but also the molecular dipole moment has a dramatic effect on
the solid-state packing. Meanwhile, the S-S interactions
enhance the electronic transport between molecules. Compared
with the crystal structure of 5,11-diphenylindolo[3,2-b]carbazole,
which has little overlap between π-planes of the fused-ring
framework,25 4d obviously bears enhanced π-stacking in the
crystal lattice because of the incorporation of a thienyl group
(Figure 2). The research would yield complementary and useful
information to study structure-property relationships about
OFET semiconductors.
Optical Properties. The photophysical properties of DITs
4a-d are tabulated in Table 1, together with the data of 5,11-
dimethylphenylindolo[3,2-b]carbazole (5) for comparison (Fig-
ure 4).
For this series of compounds, although the substituents are
different, the variations of absorption and emission wavelengths
in a dilute solution are much less pronounced. They all exhibit
similar vibronic π-π* transition absorption bands with the
Electrochemical Properties. Cyclic voltammetry of DITs
4a-d showed a reversible oxidation wave (Figure 7). The
oxidation peaks (E1/2 values) of 4a, 4b, 4c, and 4d were found
at +0.93, +0.94, +0.94, and +0.98 V vs Ag/AgCl, respectively.
Given the ferrocene/ferrocenium (Fc/Fc+) couple was used as
the internal standard,28 HOMO levels of 4a, 4b, 4c, and 4d
were estimated by using oxidation onsets -5.29, -5.31, -5.29,
(26) Zhang, X.; Matzger, A. J. J. Org. Chem. 2003, 68, 9813.
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Science 2003, 300, 1269.
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J. Org. Chem. Vol. 73, No. 12, 2008 4641

Qi et al.
FIGURE 5. (a) Absorption and (b) emission spectra (2 × 10-5 M) of compounds 4a-d in CH2Cl2.
FIGURE 6. (a) Absorption and (b) emission spectra for films of compounds 4a-d on the quartz substrate.
TABLE 2. Cyclic Voltammogram Data and Optical Band Gaps of
4a-d
a
b
c
d
compd
E1/2 (V)
EHOMO (eV)
ELOMO (eV)
Eg (eV)
ox
4a
4b
4c
4d
0.93
0.94
0.94
0.98
-5.29
-5.31
-5.29
-5.35
-1.99
-2.00
-1.97
-2.03
3.30
3.31
3.32
3.32
a Performed in Bu4NPF6/CH2Cl2 solution, V ) 100 mV/s. b Calculated
by using the empirical equation: HOMO -(4.44
Eoxonset).
c Calculated from Eg and EHOMO d Estimated from the onset of
absorption in CH2Cl2 (Eg ) 1240/λonset).
)
+
.
eV, Eg ) 2.21 eV),29 which should indicate better stabilities
FIGURE 7. Cyclic voltammograms of 4a-d (c ) 10-3 M) in CH2Cl2
containing Bu4NPF6 (0.01 M) as supporting electrolyte at room
temperature, with a scan rate of 100 mV/s.
against oxygen under ambient conditions.
Conclusion
In summary, we report here a new family of ladder-type
heteroacenes, the 5,6-disubstituted diindolo[3,2-b:4,5-b′]thio-
phenes 4a-d, prepared by an efficient method based on
intramolecular cyclization of 3,3′-dibromo-2,2′-biindoles.
Single-crystal XRD study demonstrates that tolyl-substituted
DIT 4d adopts a slipped cofacial π-π packing motif, and
propyl-substituted DIT 4b adopts three types of herringbone
packing interlaced patterns. Their crystal structures show that
the size of substituents and the molecular dipole moment have
effects on the solid-state packing simultaneously, and the S-S
interactions enhance the electronic transport between mol-
ecules. Coupled with optical properties, the skeleton of
diindolo[3,2-b:4,5-b′]thiophene is more favorable to inter-
molecular π-stacking in the solid than that of indolo[3,2-
and -5.35 eV from vacuum, respectively. On the basis of the
HOMO energy levels and the optical band gaps (Eg) evaluated
from the onset wavelengths of UV-vis absorption spectra,
LUMO energy levels were calculated (Table 2). In the produced
heteroacenes, the introduction of different substituents has little
influence on the HOMO and LUMO energy levels. However,
it should be noted that 4d has the relative highest oxidation
peak potential and the lowest HOMO and LUMO energy levels,
which can be attributed to sufficient resonance stabilization to
injected hole carriers for aryl substitution. These presented
compounds practically all have lower lying HOMO energy
levels and larger band gaps than pentacene (EHOMO ) -4.60
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4642 J. Org. Chem. Vol. 73, No. 12, 2008

Disubstituted Heteroacenes
1.75-1.65 (m, 4H), 0.78 (t, J ) 7.40 Hz, 6H); 13C NMR (75 MHz,
CDCl3, ppm) δ 137.1, 131.4, 128.0, 122.2, 121.1, 120.0, 110.4, 104.8,
45.9, 23.5, 11.5; EI-MS m/z (%) 316 (M+, 100%). Anal. Calcd for
C22H24N2: C, 83.50; H, 7.64; N, 8.85. Found: C, 83.49; H, 7.67; N,
8.75.
b]carbazole. 4a-d all exhibit low-lying HOMO energy levels
and large band gaps, indicative of good stabilities against
oxygen under ambient conditions. Further studies on the
applications of these materials to OFETs are in progress in
our laboratory.
3,3′-Dibromo-1,1′-dipropyl-1H,1′H-2,2′-biindole (3b). The 2,2′-
biindole 2b (2.18 g, 6.9 mmol) was dissolved in 50 mL of DMF and
a solution of bromine (0.8 mL, 15.6 mmol) in 20 mL of DMF was
added dropwise with stirring at room temperature. The mixture was
stirred for 15 min and then poured into 100 mL of ice/water containing
ammonia (0.5%) and sodium metabisulfite (0.1%), then extracted with
CH2Cl2. The organic layer was washed with water, dried over
anhydrous MgSO4, and filtered, then the solvent was removed in vacuo.
The crude product was purified by column chromatography (silica gel,
eluent: petroleum) to obtain 3.07 g (94%) of 3b as a white solid: 1H
NMR (400 MHz, CDCl3, ppm) δ 7.69 (d, J ) 7.93 Hz, 2H), 7.44 (d,
J ) 8.27 Hz, 2H), 7.35 (t, J ) 7.58 Hz, 2H), 7.26 (t, J ) 6.92 Hz,
2H), 4.09-4.02 (m, 2H), 3.81-3.74 (m, 2H), 1.73-1.63 (m, 4H), 0.78
(t, J ) 7.43 Hz, 6H); 13C NMR (100 MHz, CDCl3, ppm) δ 136.7,
127.4, 127.3, 123.8, 120.7, 120.2, 110.7, 95.8, 47.1, 23.4, 11.6; EI-
MS m/z (%) 474 (M+, 100%). Anal. Calcd for C22H22Br2N2: C, 55.72;
H, 4.68; N, 5.91. Found: C, 55.93; H, 4.71; N, 5.84.
Experimental Section
1-Propyl-1H-indole (1b). To a suspension of KOH (11.20 g, 0.20
mol) in 100 mL of DMF was added indole (4.69 g, 0.04 mol). After
the solution was stirred for 1 h, 1-bromopropane (5 mL, 0.055 mol)
was added. The reaction mixture was then warmed to 50 °C and stirred
overnight. The resulting suspension was poured into water and
extracted with CH2Cl2. The combined organic layers were washed with
water, dried over anhydrous MgSO4, and filtered, and the solvent was
removed in vacuo. The crude product was purified by column
chromatography (silica gel, eluent: petroleum) to obtain 5.41 g (85%)
of 1b as a yellowish oil: 1H NMR (400 MHz, CDCl3, ppm) δ 7.63 (d,
J ) 7.84 Hz, 1H), 7.35 (d, J ) 8.24 Hz, 1H), 7.19 (t, J ) 7.52 Hz,
1H), 7.10-7.07 (m, 2H), 6.48 (d, J ) 2.72 Hz, 1H), 4.08 (t, J ) 7.08
Hz, 2H), 1.90-1.84 (m, 2H), 0.93 (t, J ) 7.40 Hz, 3H); 13C NMR
(100 MHz, CDCl3, ppm) δ 136.7, 129.3, 128.4, 121.9, 121.5, 119.8,
110.1, 101.4, 48.4, 24.0, 12.0; EI-MS m/z (%) 159 (M+, 100%).
1-Hexyl-1H-indole (1c).20 This compound as a yellow oil was
prepared from indole and 1-bromohexane in 87% yield according to
the procedure for 1b: 1H NMR (400 MHz, CDCl3, ppm) δ 7.63 (d, J
) 7.72 Hz, 1H), 7.34 (d, J ) 8.20 Hz, 1H), 7.19 (t, J ) 8.63 Hz, 1H),
7.10-7.08 (m, 2H), 6.48 (s, 1H), 4.09 (t, J ) 7.16 Hz, 2H), 1.82 (t,
J ) 6.59 Hz, 2H), 1.29 (s, 6H), 0.87 (s, 3H); 13C NMR (100 MHz,
CDCl3, ppm) δ 136.5, 129.2, 128.2, 121.8, 121.4, 119.6, 109.9, 101.3,
46.7, 31.9, 30.7, 27.1, 23.1, 14.5; EI-MS m/z (%) 201 (M+, 100%).
1-p-Tolyl-1H-indole (1d). To a suspension of indole (4.69 g, 0.04
mol), K2CO3 (22.11 g, 0.16 mol), CuO (6.36 g, 0.08 mol), and 18-
crowm-6 (1.06 g, 4 mmol) in 100 mL of DMF was added 4-iodot-
oluene (13.08 g, 0.06 mol) at reflux with stirring for 32 h. The reaction
mixture was filtered through a pad of silica gel, and the solvent of the
filtrate was removed in vacuo. The crude product was purified by
column chromatography (silica gel, eluent: petroleum/ethyl acetate )
5,6-Dimethyldiindolo[3,2-b:4,5-b′]thiophene (4a). To a solution
of 3a (2.02 g, 4.3 mmol) in 150 mL of diethyl ether at -78 °C was
added dropwise a solution of n-BuLi (3.8 mL, 9.5 mmol) under argon.
After the solution was stirred for 1 h at -78 °C, (C6H5SO2)2S (1.66
g, 5.3 mmol) was added. The reaction mixture was then warmed to
room temperature and stirred overnight. Water (100 mL) was added,
and the ether layer was washed with water. The combined aqueous
layers were washed with diethyl ether, and the separated organic layers
were dried over anhydrous MgSO4 and filtered. The solvent was
removed in vacuo. The crude product was purified with column
chromatography (silica gel, eluent: petroleum) to obtain 636 mg (51%)
1
of 4a as a light yellow solid: H NMR (400 MHz, CDCl3, ppm) δ
7.76 (d, J ) 7.76 Hz, 2H), 7.45 (d, J ) 8.24 Hz, 2H), 7.32 (t, J )
7.50 Hz, 2H), 7.21 (t, J ) 7.42 Hz, 2H), 4.18 (s, 6H); 13C NMR (75
MHz, CDCl3, ppm) δ 141.7, 130.9, 123.0, 122.5, 119.6, 118.4, 109.9,
33.8; EI-MS m/z (%) 290 (M+, 100%). Anal. Calcd for C18H14N2S:
C, 74.45; H, 4.86; N, 9.65. Found: C, 74.11; H, 5.11; N, 9.31.
5,6-Dipropyldiindolo[3,2-b:4,5-b′]thiophene (4b). This com-
pound as a white solid was prepared from 3b in 53% yield according
to the procedure for 4a: 1H NMR (400 MHz, CDCl3, ppm) δ 7.74 (d,
J ) 7.84 Hz, 2H), 7.44 (d, J ) 8.28 Hz, 2H), 7.29 (t, J ) 7.44 Hz,
2H), 7.21-7.17 (m, 2H), 4.45 (t, J ) 7.78 Hz, 4H), 2.00-1.91 (m,
4H), 1.00 (t, J ) 7.40 Hz, 6H); 13C NMR (75 MHz, CDCl3, ppm) δ
141.3, 130.3, 123.2, 122.5, 120.1, 119.7, 118.4, 110.3, 48.0, 24.1, 11.5;
EI-MS m/z (%) 346 (M+, 100%). Anal. Calcd for C22H22N2S: C, 76.26;
H, 6.40; N, 8.08. Found: C, 75.84; H, 6.54; N, 7.95.
1
4:1) to obtain 6.55 g (79%) of 1d as a yellowish oil: H NMR (400
MHz, CDCl3, ppm) δ 7.69 (d, J ) 7.76 Hz, 1H), 7.54 (d, J ) 8.14
Hz, 1H), 7.40 (d, J ) 8.05 Hz, 2H), 7.32-7.31 (m, 3H), 7.23-7.14
(m, 2H), 6.67 (d, J ) 3.04 Hz, 1H), 2.44 (s, 3H); 13C NMR (75 MHz,
CDCl3, ppm) δ 136.1, 135.1, 134.8, 129.0, 128.1, 126.9, 123.1, 121.1,
119.9, 119.1, 109.4, 102.1, 19.9; EI-MS m/z (%) 207 (M+, 100%).
1,1′-Dipropyl-1H,1′H-2,2′-biindole (2b). To a stirred solution
of 1b (4.61 g, 29 mmol) in 150 mL of diethyl ether at room temperature
was added dropwise a solution of n-BuLi (14.5 mL, 36 mmol) under
argon atmosphere over 20 min. The reaction mixture was heated at
reflux with stirring under an inert gas atmosphere for 4 h and then
allowed to cool to room temperature. Anhydrous copper(II) chloride
(3.9 g, 29 mmol) was then added in three portions and the mixture
was again heated under reflux for 2 h. After being allowed to cool to
room temperature, the mixture was left to stand for 1 h before being
poured into ice/water. The dirty brown-green precipitate was filtered
off, the organic layer was separated, and the aqueous phase was
extracted twice with diethyl ether. The precipitate was washed twice
with CH2Cl2 and the filtrate combined with the ether phase. The
combined organic phases were dried with anhydrous MgSO4 then
concentrated, and the residue was purified by column chromatography
(silica gel, eluent: petroleum) to obtain 3.16 g (69%) of 2b as a white
solid: 1H NMR (400 MHz, CDCl3, ppm) δ 7.68 (d, J ) 7.80 Hz, 2H),
7.41 (d, J ) 8.20 Hz, 2H), 7.28 (dd, J ) 15.21 Hz, J ) 6.60 Hz, 2H),
7.15 (t, J ) 7.40 Hz, 2H), 6.61 (s, 2H), 4.02 (t, J ) 7.54 Hz, 4H),
Acknowledgment. The present research was financially
supported by the National Natural Science Foundation (60671047,
50673093, 60736004, 20573115, 20721061), the Major State
Basic Research Development Program (2006CB806203, 2006-
CB932103), and the Chinese Academy of Sciences.
Supporting Information Available: General experimental
1
procedures and H and 13C spectra for all new compounds, as
well as crystallographic data for 4b and 4d presented in CIF
format. This material is available free of charge via the Internet
at http://pubs.acs.org.
JO800622Y
J. Org. Chem. Vol. 73, No. 12, 2008 4643