One-pot synthesis of 4,8-dialkylbenzo[1,2-b:4,5-b']dithiophenes

Article abstract of DOI:10.3987/COM-11-12390

Two-step, one-pot syntheses are described for the preparation of 4,8-dialkylbenzo[1,2-b:4,5-b']dithiophenes (BDTs) from alkyllithium or alkyl Grignard reagents. The yields for various alkyl groups used in this procedure are comparable with or exceed those of previously reported methods. The electronic and redox properties of dialkyl-BDTs are compared with the dialkoxy-BDT, 4,8-bis(hexyloxy)benzo[1,2-b:4,5-b']dithiophene. The stabilized HOMO levels of dialkyl-BDTs suggest they are promising building blocks for use in organic photovoltaics such as bulk heterojunction solar cells.

Full text of DOI:10.3987/COM-11-12390

HETEROCYCLES, Vol. 85, No. 2, 2012
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HETEROCYCLES, Vol. 85, No. 2, 2012, pp. 355 - 364. © 2012 The Japan Institute of Heterocyclic Chemistry
Received, 13th November, 2011, Accepted, 26th December, 2011, Published online, 10th January, 2012
DOI: 10.3987/COM-11-12390
ONE-POT
SYNTHESES
OF
4,8-DIALKYLBENZO[1,2-b:4,5-b]-
DITHIOPHENES
Ted M. Pappenfus,1,* Daniel T. Seidenkranz,1 and Eric W. Reinheimer2
1Division of Science and Mathematics, University of Minnesota, Morris,
2
Minnesota 56267, United States. W.M. Keck Foundation Center for Molecular
Structure, California State University System, United States. Email:
pappe001@morris.umn.edu
Abstract – Two-step, one-pot syntheses are described for the preparation of
4,8-dialkylbenzo[1,2-b:4,5-b]dithiophenes (BDTs) from alkyllithium or alkyl
Grignard reagents. The yields for various alkyl groups used in this procedure are
comparable with or exceed those of previously reported methods. The electronic
and redox properties of dialkyl-BDTs are compared with the dialkoxy-BDT,
4,8-bis(hexyloxy)benzo[1,2-b:4,5-b']dithiophene. The stabilized HOMO levels of
dialkyl-BDTs suggest they are promising building blocks for use in organic
photovoltaics such as bulk heterojunction solar cells.
INTRODUCTION
Recently, benzo[1,2-b:4,5-b]dithiophene (BDT) based materials have emerged as leading candidates for
a number of organic electronic applications including thin-film transistors1 and organic solar cells.2 The
high performance in these devices has been attributed to (a) the planar structure of BDT which may lead
to improved intermolecular interactions (i.e., - stacking) and (b) reduced steric hindrance between BDT
and adjacent rings since substitution on the BDT generally occurs at the 4 and 8 positions.2g The
replacement of alkoxy with alkyl in these building blocks will reduce the electron donating effect of the
solubilizing group and has been shown to lower HOMO energy levels of the resulting polymers.3 This is
attractive for polymers in OPV bulk heterojunction devices as a low-lying HOMO energy level (relative
to the acceptor material) can provide a larger open circuit voltage (VOC) which assists in achieving
maximum power conversion efficiency.4 Our present work describes a one-pot synthetic strategy toward
4,8-dialkyl-BDTs from alkyllithium or alkyl Grignard reagents including reagents with branched alkyl
chains for the preparation of more soluble building blocks.

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Although synthetic strategies towards 4,8-dialkoxy-BDTs have been well established,5 approaches toward
4,8-dialkyl-BDTs are less efficient. Current literature approaches toward 4,8-dialkyl-BDTs are outlined in
Scheme 1.6 This three step process begins with the addition of an alkynyllithium or alkynylmagnesium
reagent to benzo[1,2-b:4,5-b']dithiophene-4,8-dione followed by reduction with tin(II) chloride. This
alkynyl intermediate is isolated and reduced with hydrogen gas.
R
R
O
1)
M
R
S
S
S
(M = Li or MgX)
H2/Pd
S
S
S
2) SnCl2/HCl
O
R
R
Scheme 1. Previous approach for the preparation of 4,8-dialkyl-BDTs
We felt a more direct approach toward 4,8-dialkyl-BDTs would be to add the appropriate alkyllithium or
alkyl Grignard reagent to the benzoquinone (followed by reduction with tin(II) chloride) in place of the
alkynyl reagents to avoid the need for the subsequent hydrogen reduction step. Herein we describe the
results of this alternative approach (Scheme 2) using a number of different alkyllithium reagents.
Extension of this method to Grignard reagents has also been tested.7
R
O
1)
M R
S
S
(M = Li or MgX)
S
S
2) SnCl2/HCl
R
O
Scheme 2. Preparation of 4,8-dialkyl-BDTs in this investigation (Methods A and B)
RESULTS AND DISCUSSION
The results of our synthetic approach to 4,8-dialkyl-BDTs are summarized in Table 1. Our initial
approach (Method A) involved the addition of excess alkyllithium (3.2 equiv) to commercially available
benzo[1,2-b:4,5-b']dithiophene-4,8-dione (1.0 equiv) at room temperature under anhydrous conditions.
After an additional hour of stirring at room temperature, the reaction is quenched with water, then tin(II)
chloride (4.2 equiv) is added in the presence of aqueous HCl. This mixture is refluxed for three hours and
is subjected to a normal workup and purification by silica gel column chromatography. We initially
focused on n-alkyllithium reagents including methyllithium, n-butyllithium, and n-hexyllithium which
provide 4,8-dialkyl-BDTs in low to modest yields (Table 1, Entries 1-3). As a comparison to our one-pot,
two step procedure, the three step process outlined in Scheme 1 has reported overall yields of 40%.1d

HETEROCYCLES, Vol. 85, No. 2, 2012
357
Table 1. Synthetic outcomes for 4,8-dialkyl- and 4,8-dialkoxybenzo[1,2-b:4,5-b]dithiophenes
Method
Entry
Alkyl Reagent
Product
(A, B or C):
Yield (%)
S
1
Li Me
DM-BDT
A: 30
A: 42
S
S
DB-BDT
DH-BDT
Li
2
3
4
5
6
S
S
A: 35
B: 45
Li
S
S
A: 21
Li
S
S
BEH-BDT
S
A: 15
B: 30
BrMg
BEH-BDT
S
O
S
DHO-BDT
Br
C: 79
O
Method A conditions: (i) THF, R-Li or RMgX (3.2 equiv), rt, 1 h (ii) SnCl2 (aq) (4.2 equiv), reflux, 3 h.
Method B conditions: (i) THF, R-Li or RMgX (3.2 equiv), reflux, 1 h (ii) SnCl2 (aq) (4.2 equiv), reflux, 3 h.
Method C conditions: (i) H2O, Zn (3 equiv), NaOH (1.9 equiv), reflux, 2 h. (ii) R-Br (3.0 equiv),
tetrahexylammonium bromide (3%), reflux, 16 h.
Given the widespread use of branched alkyl groups to generate more soluble polymers,2 we next
investigated the use of 2-(ethylhexyl)lithium in our reaction procedure. The desired 4,8-dialkyl-BDT,
BEH-BDT, was generated in even lower yield than the linear alkyllithium reagents (Table 1, Entry 4).

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We then investigated extending our initial method to Grignard reagents – specifically using
(2-ethylhexyl)magnesium bromide (Table 1, Entry 5). Unfortunately, even lower yields resulted using
this particular organometallic reagent.
In an attempt to increase our reaction yields, we modified our initial procedure (Method A) by refluxing
the reaction mixture in the 1 hour period after addition of the organometallic reagent (referred to as
Method B). In the case of (2-ethylhexyl)magnesium bromide, reaction yields doubled as a result of this
simple modification. Application of this method to alkyllithium reagents was tested with n-hexyllithium,
which also resulted in an improved reaction yield (Table 1, Entry 3). The remaining alkyllitihium reagents
were not subjected to Method B, but similar results are anticipated. We did, however, investigate the
preparation of the 4,8-dialkoxy-BDT, DHO-BDT, using a slightly modified procedure from the literature
(Scheme 3 and Table 1, Entry 6, Method C).5a This molecule also serves as a comparison for electronic
and redox properties of this series of materials (vide infra).
R
O
O
S
S
1) Zn/NaOH (aq)
S
S
2) R-Br, THAB (cat.)
O
O
R
Scheme 3. Preparation of 4,8-dihexyloxy-BDT in this investigation (Method C)
The structure and purity of the materials were confirmed with NMR, mass spectrometry, and/or
combustion analysis. Although we have been unsuccessful in obtaining suitable crystals for many of these
materials for single crystal X-ray analysis, we have been able to determine the X-ray structure of
DM-BDT. The ORTEP plot of DM-BDT is shown in Figure 1. A crystallographic inversion center exists
within the molecule. A search of the Cambridge Structural Database (CSD) found two structures with
Figure 1. ORTEP diagram for DM-BDT (with arbitrary numbering)

HETEROCYCLES, Vol. 85, No. 2, 2012
359
substituents in the
4
and
8
positions of the benzodithiophene core,1c,1d including
4,8-didodecylbenzo[1,2-b:4,5-b]dithiophene.1d Whereas the didodecyl-BDT molecule shows close
face-to-face - stacking, DM-BDT does not (packing views of the molecule are found in the
Supplementary Data). This demonstrates that the nature of the alkyl group clearly influences the crystal
morphology in these materials.
Table 2. Spectroscopic and electrochemical data
Eo ,
abs
ox
molecule
Vb
(nm)a
346
347
347
348
352
DM-BDT
DB-BDT
DH-BDT
BEH-BDT
DHO-BDT
1.21
1.19
1.18
1.17
0.86
aLowest energy absorption maxima measured in CH2Cl2. bPotentials vs Ag/AgCl in 0.1 M TBAPF6/CH2Cl2 solution
determined from square-wave voltammograms similar to reference 8.
Solution absorption and electrochemical data of the molecules in this investigation are summarized in
Table 2. The UV-Vis absorption spectra are rather complex and possess significant fine structure likely
due to the planar structure of the molecules (spectra are provided in the Supplementary data). The lowest
energy absorption maxima for the dialkyl-BDTs, as expected, are very similar and are all within 2 nm of
each other. Upon alkoxy substitution (as with DHO-BDT) a slight redshift is observed as anticipated for
the more electron-donating group (Figure 2).
2
DH-BDT
DHO-BDT
1.5
1
0.5
0
240 260 280 300 320 340 360 380 400
Wavelength (nm)
Figure 2. Solution absorption spectra for DH-BDT and DHO-BDT in dichloromethane

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Similar trends are observed in the electrochemical data as the oxidation potentials of the dialkyl oligomers
are within 0.04 V of each other. A representative cyclic voltammogram (CV) is presented in the
Supplementary data and illustrates quasi-reversibility on the CV timescale. Noteworthy is the significant
decrease in oxidation potential upon alkoxy substitution. Figure 3 includes square wave voltammograms
of DH-BDT and DHO-BDT which have oxidation potentials of 1.18 and 0.86 V respectively. This
increase in oxidation potential upon alkyl substitution is directly related to the stabilization of the HOMO
level of DH-BDT relative to DHO-BDT and has important implications for utilizing these monomers in
materials for OPV applications (vide supra).
0
-5
-10
DH-BDT
DHO-BDT
-15
-20
-25
1.6 1.4 1.2
1
0.8 0.6 0.4 0.2
0
Potential (V)
Figure 3. Osteryoung square wave voltammograms for DH-BDT and DHO-BDT
In summary, we have explored a more direct route for the preparation of 4,8-dialkyl-BDTs using a two
step, one-pot procedure. The yields for various alkyl groups used in this procedure are comparable with or
exceed the yields of previously reported methods. We have also prepared the 4,8-dialkoxy-BDT molecule,
DHO-BDT, which is compared to the 4,8-dialkyl-BDTs using spectroscopic and electrochemical
methods. The physical data suggests dialkyl substitution may have advantages over dialkoxy substitution
for use of these molecules in materials applications such as organic photovoltaics.
EXPERIMENTAL
General Methods. Synthetic procedures were carried out under an inert atmosphere of nitrogen.
Tetrahydrofuran was distilled from Na/benzophenone. Grignard and alkyllithium reagents were purchased
from Aldrich and titrated prior to use in the reactions using a previously reported procedure.9
Benzo[1,2-b:4,5-b']dithiophene-4,8-dione was purchased from SunaTech Inc. and used as received.
NMR spectra were recorded on a JEOL Eclipse 300 MHz instrument. The chemical shifts are reported in

HETEROCYCLES, Vol. 85, No. 2, 2012
361
ppm and referenced to the residual chloroform peak (7.26 or 77.1 ppm). Mass spectra were obtained on
an Agilent 5973 MSD mass spectrometer and elemental analyses were performed by Atlantic Microlab,
Inc., Norcross, GA. UV-Vis absorption measurements were recorded on an Ocean Optics USB4000 fiber
optic spectrometer and room temperature electrochemical measurements were performed with a BAS
100B electrochemical analyzer using methods previously described.10 Details for the X-ray data
collection of DM-BDT are found in the Supplementary data.
Typical procedure for the preparation of 4,8-dialkyl-BDTs (Method A). To a 250 mL two-neck flask
with large stir bar was added benzo[1,2-b:4,5-b']dithiophene-4,8-dione (1.00 g, 4.54 mmol) and freshly
distilled THF (30 mL). To this stirred suspension, a solution of the appropriate Grignard or alkyllithium
reagent (14.5 mmol) was added dropwise over 10 min at room temperature. The mixture was stirred for
an additional hour at room temperature and was then quenched with 1 mL water. SnCl2 (4.30 g, 19.1
mmol) in 10 mL of 3M HCl was then added with stirring. The mixture was refluxed for 3 h, then cooled
and 15 mL of brine followed by 45 mL of hexanes were added. The organic phase was separated and the
aqueous phase was extracted once with hexanes. The organic phases were combined, dried with MgSO4
and concentrated using rotary evaporation. Purification was performed by dissolving the crude material in
acetone and adsorbing on silica gel. Column chromatography was performed with 100% hexanes
followed by 5% EtOAc in hexanes.
Typical procedure for the preparation of 4,8-dialkyl-BDTs (Method B). To a 250 mL two-neck flask
with large stir bar was added benzo[1,2-b:4,5-b']dithiophene-4,8-dione (1.00 g, 4.54 mmol) and freshly
distilled THF (30 mL). To this stirred suspension, a solution of the appropriate Grignard or alkyllithium
reagent (14.5 mmol) was added dropwise over 10 min at room temperature. The mixture was stirred for
an additional hour under reflux conditions, cooled down, and then quenched with 1 mL water. SnCl2
(4.30 g, 19.1 mmol) in 10 mL of 3M HCl was then added with stirring. The mixture was refluxed for 3 h,
then was cooled and 15 mL of brine followed by 45 mL of hexanes were added. The organic phase was
separated and the aqueous phase was extracted once with hexanes. The organic phases were combined,
dried with MgSO4 and concentrated using rotary evaporation. Purification was performed by dissolving
the crude material in acetone and adsorbing on silica gel. Column chromatography was performed with
100% hexanes followed by 5% EtOAc in hexanes.
Preparation of 4,8-bis(hexyloxy)benzo[1,2-b:4,5-b']dithiophene, DHO-BDT (Method C). To a 250
mL three-neck flask with large stir bar was added in the following order: benzo[1,2-b:4,5-b']-
dithiophene-4,8-dione (1.00 g, 4.54 mmol), Zn dust (0.890 g, 13.6 mmol), 100 mL water, and NaOH (4 x
0.880 g, 8.80 mmol). After stirring the mixture for 2 h under reflux, 1-bromohexane (2.24 g, 13.6 mmol)
and tetrahexylammonium bromide (0.059 g, 0.136 mmol) was added. After an additional 16 h reflux, the
mixture was cooled and extracted with CH2Cl2 (3 x 30 mL). The organic layers were combined, dried

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with MgSO4 and concentrated via rotary evaporation. The compound was purified by dissolving in 10 mL
CH2Cl2 and 10 mL EtOHl. This solution was subjected to rotary evaporation until the formation of a
suspension. The product was filtered and washed with cold EtOH to provide 1.41 g (79%) of DHO-BDT
as an off-white solid.
Analytical data for BDT molecules
4,8-dimethylbenzo[1,2-b:4,5-b]dithiophene (DM-BDT).11 1H NMR (300 MHz, CDCl3): δ 7.50 (d, 2H,
13
J = 5.5 Hz), 7.47 (d, 2H, J = 5.5 Hz), 2.82 (s, 6H). C NMR (75.6 MHz, CDCl3): δ 137.66, 136.15,
126.12, 123.60, 122.10, 18.15. EI-MS m/z (relative intensity) 218 (100), 203 (33), 184 (13), 171 (12).
Anal. Calcd for C12H10S2: C, 66.01; H, 4.62. Found: C, 66.05; H, 4.65.
1
4,8-dibutylbenzo[1,2-b:4,5-b]dithiophene (DB-BDT). H NMR (300 MHz, CDCl3): δ 7.49 (d, 2H, J =
5.5 Hz), 7.45 (d, 2H, J = 5.8 Hz), 3.21 (t, 4H), 1.81 (m, 4H), 1.49 (m, 4H), 0.99 (t, 6H). 13C NMR (75.6
MHz, CDCl3): δ 137.48, 135.99, 129.06, 125.94, 121.99, 33.34, 31.96, 23.19, 14.14. EI-MS m/z (relative
intensity) 302 (60), 259 (100), 217 (14), 203 (16), 184 (11), 171 (8). Anal. Calcd for C18H22S2: C, 71.47;
H, 7.33. Found: C, 71.60; H, 7.40.
4,8-dihexylbenzo[1,2-b:4,5-b]dithiophene (DH-BDT).1b 1H NMR (300 MHz, CDCl3): δ 7.48 (d, 2H, J
= 5.8 Hz), 7.45 (d, 2H, J = 5.5 Hz), 3.19 (t, 4H), 1.82 (m, 4H), 1.48 (m, 4H), 1.34 (m, 8H), 0.91 (t, 6H).
13C NMR (75.6 MHz, CDCl3): δ 137.45, 135.97, 129.10, 125.95, 121.96, 33.63, 31.82, 29.78, 29.75,
22.74, 14.23. EI-MS m/z (relative intensity) 358 (98), 287 (100), 217 (18), 203 (16), 184 (9), 171 (6).
Anal. Calcd for C22H30S2: C, 73.68; H, 8.43. Found: C, 73.79; H, 8.61.
4,8-bis(2-ethylhexyl)benzo[1,2-b:4,5-b']dithiophene (BEH-BDT).7b,7c 1H NMR (300 MHz, CDCl3): δ
7.47 (d, 2H, J = 5.8 Hz), 7.43 (d, 2H, J = 5.5 Hz), 3.13 (m, 4H), 1.98 (m, 2H), 1.45-1.19 (m, 16H),
0.96-0.82 (m, 12H). 13C NMR (75.6 MHz, CDCl3): δ 138.19, 136.61, 128.50, 125.83, 122.31, 40.14,
38.28, 33.20, 29.03, 26.41, 23.21, 14.24, 11.21. EI-MS m/z (relative intensity) 414 (100), 315 (92), 217
(70), 203 (5), 184 (8), 171 (5). Anal. Calcd for C26H38S2: C, 75.30; H, 9.24. Found: C, 75.58; H, 9.42.
4,8-bis(hexyloxy)benzo[1,2-b:4,5-b]dithiophene (DHO-BDT).5a 1H NMR (300 MHz, CDCl3): δ 7.48 (d,
2H, J = 5.5 Hz), 7.37 (d, 2H, J = 5.5 Hz), 4.28 (t, 4H), 1.87 (m, 4H), 1.58 (m, 4H), 1.37 (m, 8H), 0.93 (t,
6H). 13C NMR (75.6 MHz, CDCl3): δ 144.63, 131.72, 130.28, 126.10, 120.42, 74.08, 31.77, 30.61, 25.86,
22.77, 14.20. EI-MS m/z (relative intensity) 390 (43), 306 (8), 221 (100). Anal. Calcd for C22H30O2S2: C,
67.65; H, 7.74. Found: C, 67.81; H, 7.74.
ACKNOWLEDGEMENTS
The authors acknowledge the following: (i) seed funding from the University of Minnesota Initiative for
Renewable Energy and the Environment, (ii) University of Minnesota, Morris (UMM) Faculty Research

HETEROCYCLES, Vol. 85, No. 2, 2012
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Enhancement Funds supported by the University of Minnesota Office of the Vice President for Research,
and the UMM Division of Science and Mathematics for financial assistance, (iii) Prof. Nancy Carpenter
for review of the manuscript, (iv) Matthew Lovander for experimental assistance, and (v) the National
Science Foundation for funding of the Crystallography for Chemists (and others!) workshop (Award #
1022895).
SUPPLEMENTARY DATA
Supplementary data associated with this article, including crystallographic data for DM-BDT,12 can be
found in the online version.
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