Synthesis of Benzothiadiazole Derivatives by Applying C-C Cross-Couplings

Article abstract of DOI:10.1021/acs.joc.5b02689

The benzothiadiazole moiety has been extensively exploited as a building block in the syntheses of efficient organic semiconducting materials during the past decade. In this paper, parallel synthetic routes to benzothiadiazole derivatives, inspired by previous computational findings, are reported. The results presented here show that various C-C cross-couplings of benzothiadiazole, thiophene, and thiazole derivatives can be efficiently performed by applying Xantphos as a ligand of the catalyst system. Moreover, improved and convenient methods to synthesize important chemical building blocks, e.g., 4,7-dibromo-2,1,3-benzothiadiazole, in good to quantitative yields are presented. Additionally, the feasibility of Suzuki-Miyaura and direct coupling methods are compared in the synthesis of target benzothiadiazole derivatives. The computational characterization of the prepared benzothiadiazole derivatives shows that these compounds have planar molecular backbones and the possibility of intramolecular charge transfer upon excitation. The experimental electrochemical and spectroscopic studies reveal that although the compounds have similar electronic and optical properties in solution, they behave differently in solid state due to the different alkyl side-group substitutions in the molecular backbone. These benzothiadiazole derivatives can be potentially used as building blocks in the construction of more advanced small molecule organic semiconductors with acceptor-donor-acceptor motifs. (Chemical Equation Presented).

Full text of DOI:10.1021/acs.joc.5b02689

Article
pubs.acs.org/joc
Synthesis of Benzothiadiazole Derivatives by Applying C−C
Cross‑Couplings
,†
‡
†
‡
‡
‡
Juha P. Heiskanen,* Paola Vivo, Niko M. Saari, Terttu I. Hukka, Tuuva Kastinen, Kimmo Kaunisto,
‡
†
Helge J. Lemmetyinen, and Osmo E. O. Hormi
†
Research Unit of Sustainable Chemistry, University of Oulu, P.O. Box 3000, FI-90014 Oulu, Finland
‡
Department of Chemistry and Bioengineering, Tampere University of Technology, P.O. Box 541, FI-33101 Tampere, Finland
*
S Supporting Information
ABSTRACT: The benzothiadiazole moiety has been exten-
sively exploited as a building block in the syntheses of efficient
organic semiconducting materials during the past decade. In
this paper, parallel synthetic routes to benzothiadiazole
derivatives, inspired by previous computational findings, are
reported. The results presented here show that various C−C
cross-couplings of benzothiadiazole, thiophene, and thiazole
derivatives can be efficiently performed by applying Xantphos
as a ligand of the catalyst system. Moreover, improved and
convenient methods to synthesize important chemical building
blocks, e.g., 4,7-dibromo-2,1,3-benzothiadiazole, in good to
quantitative yields are presented. Additionally, the feasibility of
Suzuki−Miyaura and direct coupling methods are compared in
the synthesis of target benzothiadiazole derivatives. The computational characterization of the prepared benzothiadiazole
derivatives shows that these compounds have planar molecular backbones and the possibility of intramolecular charge transfer
upon excitation. The experimental electrochemical and spectroscopic studies reveal that although the compounds have similar
electronic and optical properties in solution, they behave differently in solid state due to the different alkyl side-group
substitutions in the molecular backbone. These benzothiadiazole derivatives can be potentially used as building blocks in the
construction of more advanced small molecule organic semiconductors with acceptor−donor−acceptor motifs.
4
INTRODUCTION
structure. Stille and Suzuki−Miyaura have been the most
extensively used methods to connect different building blocks,
but within the last two decades, many researchers have started
to favor direct coupling instead. The major drawback of both
the Stille and Suzuki−Miyaura methods is the necessity of
preparing the organostannane or boron reagent, which
increases the number of reaction steps and thus, in most
cases, the cost of the method. The Suzuki−Miyaura method has
some important advantages compared to Stille coupling: (1)
boron reagents are safer to use compared to the toxic
organostannanes, and (2) there is a wide variety of boron
reagents available commercially for different applications of
■
Benzothiadiazole (BT) has become a highly important building
block that has been extensively exploited in efficient semi-
1
conducting materials. The BT unit also has one of the top
frequencies of occurrence among the leading candidate
structures in The Harvard Clean Energy Project. This project
computationally assessed candidate structures for organic
2
electronic materials and particularly for organic photovoltaics
(
OPVs). Two example structures are presented in Scheme 1.
OPV devices with the theoretical compounds of Scheme 1 as
donor components were predicted to produce power
conversion efficiencies of 8.0 and 9.1%. In addition, the
3
5
Suzuki−Miyaura couplings. Pd-catalyzed direct coupling6
highest occupied molecular orbital (HOMO) and the lowest
unoccupied molecular orbital (LUMO) energies were also
reported. Inspired by these results, we were interested in
investigating the possibility of synthesizing close analogues of
these theoretical compounds. The plan for the synthetic
pathway to the target molecules is presented in Scheme 1. The
framework of the target molecule was planned so that it can be
constructed from widely commercially available chemical
building blocks by connecting them using palladium-catalyzed
cross-coupling reactions.
mechanistically resembles other cross-coupling reactions.
The main advantage of the method is that the C−H bond is
directly converted to the C−C bond without preceding
halogenation or other conversions, thus reducing previous
reaction steps before the actual coupling reaction. However, the
direct coupling method also has disadvantages of its own. In
some cases, higher yields of the target compound have only
been achieved by using a large excess of another direct coupling
Cross-coupling reactions are efficient tools in the syntheses
of semiconducting organic materials that have a modular
Received: November 24, 2015
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.joc.5b02689
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2
,3
Scheme 1. Model Compounds of the Harvard Clean Energy Project and the Proposed Synthetic Pathway to the Target
Molecules
Scheme 2. Syntheses of 2-(Thiophen-2-yl)thiazole Derivatives
Table 1. Optimization of Suzuki−Miyaura Cross-Coupling of 2-Bromothiazole (1) with Compounds 2a and 2b
a
entry
catalyst
ligand
base
temperature
reaction time
yield (%)
b
c
c
1
2
3
4
5
Pd(PPh3)4
Cs CO3
110 °C
110 °C
60 °C
110 °C
60 °C
3 h
3 h
3a 62
3a 87
3a 89
3b 69
3b >99
2
d
e
4
Pd (dba)
2
P(t-Bu )·HBF
Xantphos
Cs CO
3
3
2
3
f
d
d
Pd(OAc)2
Pd(PPh3)4
K PO4
3
3 h
b
c
3
Cs CO
1.5 h
1.5 h
2
d
d
f
Pd(OAc)2
Xantphos
K PO4
3
a
b
c
d
e
f
Isolated yield. With 5 mol %. With 2.5 equiv. With 2.5 mol %. With 10 mol %. With 3.0 equiv.
7
partner. The regioselectivity of the coupling may be an issue
with the compounds that have more than one C−H bond
available. At least partially, that problem can be solved with a
careful choice of the catalyst system and the reaction
catalyst system in particular cases. Additionally, the feasibility of
Suzuki−Miyaura and direct coupling methods are compared in
the synthesis of target BT derivatives. The optical and
electronic properties of the prepared compounds, which have
been determined both experimentally and computationally, are
also reported. These BT derivatives can potentially be applied
as building blocks in the construction of more advanced
acceptor−donor−acceptor-type organic semiconductors.
8
conditions. On the other hand, the availability of two or
more reactive C−H bonds in the same compound can be a
desired property in polymer synthesis using the direct arylation
9
approach.
In this paper, we present various parallel synthetic routes to
BT derivatives. Particularly, improved and convenient methods
to synthesize important chemical building blocks, e.g., 4,7-
dibromo-2,1,3-benzothiadiazole, in good to quantitative yields
are presented. Moreover, the results of this paper show that
various couplings of BT, thiophene, and thiazole derivatives can
be efficiently done by applying Xantphos as a ligand of the
RESULTS AND DISCUSSION
■
Synthesis. The first target compounds of the synthetic
scheme, 3a and 3b, were synthesized using Suzuki−Miyaura
cross-coupling between commercial 2-bromothiazole (1) and
pinacolboronic ester reagents 2a and 2b (Scheme 2). Previous
efforts to synthesize compound 3a using the Suzuki−Miyaura
B
DOI: 10.1021/acs.joc.5b02689
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1
0
method have afforded the product in 36−43% yields. The
cross-coupling reaction between compound 1 and thienyl
aluminum reagent have been reported to produce product 3a in
our hands. Thus, this reaction route to produce compound 4b
was unsatisfactory.
Singaram et al. have demonstrated that various bromo aryl,
alkyl, and thiophene derivatives react with pinacolborane under
Barbier-type conditions, producing corresponding aryl, alkyl,
1
1
8
9
7% yield, whereas Stille coupling has given product 3a in
9% yield. In our hands, the Suzuki−Miyaura cross-coupling
reaction with Pd(PPh ) as a catalyst gave products 3a and 3b
in 62 and 69% yields, respectively (Table 1, entries 1 and 4).
The use of Pd (dba) with P(t-Bu) ·HBF as a catalyst system
raised the yield of compound 3a to 87% (entry 2). The
bidentate phosphine ligand Xantphos together with air stable
palladium source Pd(OAc) was shown to be very efficient and
gave compounds 3a and 3b in 89% and quantitative yields,
respectively (entries 3 and 5).
Recently, our and other groups have demonstrated that
12
1
5
3
4
and thiophene pinacolboronic esters, respectively. This
approach gave the desired pinacolboronic ester derivatives 7a
and 7b in moderate 47 and 67% yields, respectively (Scheme
2
3
3
4
2
). As a comparison, a trial of Pd-catalyzed borylation of
compound 4b using Pd (dba) together with SPhos as a
2
3
2
catalyst system and reaction conditions presented by Buchwald
16
et al. gave only trace amounts of compound 7b. Furthermore,
the common borylation using n-BuLi and 2-isopropoxy-4,4,5,5-
17
tetramethyl-1,3,2-dioxaborolane failed to give compound 7b.
ultrasound-assisted bromination is an efficient method to
4
,7-Dibromo-2,1,3-benzothiadiazole (9) is an important
13
produce brominated thiophene derivatives. However, bromi-
nation of 2-(thiophen-2-yl)thiazole (3a) with NBS in CHCl3
gave the desired monobrominated regioisomer 4a only in low
heterocyclic starting material that is usually prepared from
18
2
,1,3-benzothiadiazole (8) by using elemental Br in HBr.
2
This approach has been reported to give compound 9 at up to
5% yield. However, the synthetic procedure and workup are
3
8% yield. TLC and MS analyses revealed that the reaction
9
mixture contained mono- and dibrominated products together
with the unreacted starting material. The reactions in toluene
and DMF gave product 4a in lower yields (12 and 29%,
respectively).
The hexyl side-chain analogue 2-(3-hexylthiophen-2-yl)-1,3-
thiazole 3b showed completely different reactivity in bromina-
tion with NBS in CHCl3 compared with compound 3a.
Compound 3b could be monobrominated with high
regioselectivity affording compound 4b in 86% yield (Scheme
). Formation of a small amount of dibrominated product of 3b
was also observed during the reaction. Interestingly, the
reaction of compound 3b with NBS in concentrated H SO4
instead of CHCl gave solely 2-(4,5-dibromo-3-hexylthiophen-
somewhat time-consuming and laborious. Previously, com-
pound 9 was also synthesized from compound 8 by using NBS
19
in anhydrous THF. After 24 h of reaction time, product 9 was
isolated in 50% yield. Our experiments showed that compound
9
8
can be prepared easily and highly efficiently from compound
by using a slight excess of NBS (2.05 equiv) in concentrated
H SO (Scheme 4). Compound 9 was isolated after a 3 h
2
4
reaction and simple workup procedure as an off-white powder
in a quantitative yield (>99%). The product was used without
further purification in the next reaction step.
2
Compound 9 was applied as a starting material in the
synthesis of compound 10 (Scheme 4). The monocoupling
product 10 has been previously synthesized from compound 9
via both the Stille and the Suzuki−Miyaura cross-coupling
2
3
2
-yl)-1,3-thiazole (5) (Scheme 3) without any traces of desired
20
reactions in 59−60% yields. Following similar reaction
conditions as presented in the literature, our first Suzuki−
Miyaura coupling trial with a standard Pd(PPh ) catalyst gave
Scheme 3. Bromination of Compound 3b with NBS in
Concentrated H SO
2
4
3
4
product 10 in 56% yield (Table 2, entry 1). The yield could be
improved to 61% by increasing the excess of compound 9 in
the reaction system (entry 2). The use of Pd (dba) with P(t-
2
3
Bu) ·HBF as a catalyst instead of Pd(PPh ) raised the yield to
3
4
3 4
6
6% (entry 3). TLC and MS analyses revealed that product 10
undergoes consequent cross-coupling with the pinacolboronic
ester reagent, producing an undesired dicoupling byproduct.
Naturally, this side reaction decreases the yield of compound
product 4b or byproducts. Apparently, the protonation of
thiazole nitrogen in concentrated H SO prevents bromination
2
4
10. In the literature, compound 9 has been cross-coupled with
at the thiazole site of compound 3b, thereby directing
bromination to the otherwise more inactive thiophene sites
and giving dibrominated product 5.
various organostannanes using the Stille coupling approach and
Pd(PPh ) as a catalyst. The resulting monocoupling products
3
4
21
have been isolated in 66−70% yields. Respectively, Suzuki−
Miyaura cross-coupling reactions with Pd(PPh ) as a catalyst
have produced the monoarylation products in 30−78% yields.
In some cases, the reaction outcome has been an inseparable
mixture of mono- and dicoupling products. The Dupont’s
palladacyclic catalyst has produced monoarylated Suzuki−
Miyaura coupling products of compound 9 in somewhat higher
yields (71−85%). However, restricted commercial availability
of Dupont’s catalyst makes its utilization difficult in cross-
coupling reactions. Our studies showed that bidentate
Because of the problems with bromination of compound 3a,
we looked for an alternative route to synthesize compounds 4a
and 4b. The study of Strotman et al. demonstrated that
Suzuki−Miyaura cross-couplings of commercially available 2,5-
dibromothiazole (6) with various arylboronic acids can be
conducted regioselectively by applying a suitable phosphine
3 4
22
2
3
24
1
4
ligand. Our experiments revealed that compound 4a can be
efficiently synthesized by cross-coupling compound 6 with
pinacolboronic ester reagent 2a (Scheme 2). Product 4a was
isolated in good 88% yield after a chromatographic separation.
In the case of 4b, this reaction approach gave the full
conversion of the starting material. However, formation of the
undesired dicoupling product was also observed. Furthermore,
the 1:9 mixture of dicoupling product and the desired
monocoupling product 4b were shown to be inseparable in
25
Xantphos ligand together with Pd(OAc)
is capable of
2
producing the desired monocoupling product 10 in 78% yield
(Table 2, entry 4) which could be further improved up to 80%
after solvent, base, and reaction temperature optimizations
(entries 5−8).
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Scheme 4. Syntheses of BT Derivatives
Table 2. Optimization of Suzuki−Miyaura Cross-Coupling of Compound 9 with 5-Hexyl-2-thiopheneboronic Acid Pinacol Ester
a
entry
catalyst
ligand
temperature
reaction time
yield of compound 10 (%)
b
b
c
1
2
3
4
5
6
7
8
Pd(PPh3)4
Pd(PPh3)4
85 °C
85 °C
85 °C
85 °C
60 °C
50 °C
60 °C
60 °C
3 h
3 h
56
61
66
78
d
e
Pd (dba)
2
P(t-Bu) ·HBF
1.5 h
1.5 h
1.5 h
1.5 h
1.5 h
1.5 h
3
3
4
b
b
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Xantphos
b
b
b
b
b
f
Xantphos
80
b
Xantphos
58
b
g
Xantphos
79
b
h
Xantphos
63
Reaction conditions: 5-hexyl-2-thiopheneboronic acid pinacol ester (50 mg), compound 9 (1.5 equiv), Cs CO (2.5 equiv), solvents: toluene (2
2
3
a
b
c
d
e
mL), DMA (2 mL), and H O (0.4 mL). Isolated yield. With 5 mol %. With 1.1 equiv of compound 9. With 2.5 mol %. With 10 mol %.
2
f
g
h
Average of two runs. Solvents: 1,4-dioxane (4 mL) and H O (0.4 mL). Base: K PO (2.5 equiv).
2
3
4
To synthesize a structurally closer analogue of the model
ester reagent 7a. The standard Pd(PPh3)4 catalyst gave
compound 11b in only very low 16% yield. Furthermore, the
compounds of the Harvard Clean Energy Project from the
easily available starting materials, we were interested in cross-
coupling compound 9 with 3-methoxythiophene-2-boronic acid
pinacol ester (Scheme 4). However, the use of the optimized
cross-coupling reaction conditions described above gave desired
Suzuki−Miyaura reaction product 12 only in 7% yield.
Obviously, the quick protodeboronation of pinacol ester
otherwise superior Pd(OAc) /Xantphos catalyst combination
2
gave only partial conversion of starting materials within 21 h in
this case. The Pd (dba) /P(t-Bu) ·HBF catalyst system gave
2
3
3
4
all of the desired products in good 77% (11a), 87% (11b), and
72% (11c) yields.
A drawback of the Suzuki−Miyaura approach is the necessity
of preparing the borylated compounds 7a and 7b, which in
terms of atom economy and synthesis time is a waste of
resources. Potentially, the other possibility to produce the final
target compounds 11a−c is to couple compounds 10 and 12
with compounds 3a and 3b directly. In the literature,
compound 9 has been coupled directly with cyclopentadithio-
phene derivatives to form polymeric materials and with
2
6
reagent occurred, which could be observed by TLC analysis
as quick disappearance of the reagent from the reaction
medium. The solution to the problem was to use XPhos-
containing palladium precatalyst (Scheme 4), which forms the
catalytically active XphosPd(0) rapidly even at room temper-
ature and efficiently catalyzes various Suzuki−Miyaura cross-
26,27
couplings of unstable boronic reagents.
This approach gave
7
b,28
compound 12 in 55% yield. Similarly to the synthesis of
compound 10, the yield of product 12 was limited by the
formation of the undesired dicoupling product. By replacing
K PO (aq) with Cs CO (aq) as a base, the yield could be
thiophene derivatives to form oligomeric products.
Specifically, the drawback of the ligand-free direct coupling
approach is the requirement of a large excess of thiophene
7
b
(15−20 equiv). Direct arylation of thiazole has been
3
4
2
3
8a,b,29
slightly improved to 57%.
researched somewhat more.
After comparing the most
The final target compounds 11a and 11b were synthesized
from compound 10 and pinacolboronic ester reagents 7a and
b using Suzuki−Miyaura cross-coupling (Scheme 4).
Respectively, the third target molecule of this study, compound
1c, was synthesized from compound 12 and pinacolboronic
promising results of these studies, we carried out the
optimization studies of the direct coupling of compound 10
with compound 3b. The optimized reaction conditions were
also tested in direct coupling reaction of compound 10 with
compound 3a. The results are presented in Table 3.
7
1
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Table 3. Optimization of Direct Coupling of Compound 10 with Compounds 3a and 3b
a
entry
ligand
base
additive
amount of 3b/3a
reaction time
yield (%)
b
c
1
2
3
4
5
6
7
8
9
Xantphos
Cs CO
PivOH
1.0 equiv
1.0 equiv
1.0 equiv
1.0 equiv
1.0 equiv
1.0 equiv
1.5 equiv
1.5 equiv
1.5 equiv
1.5 equiv
24 h
24 h
24 h
6 days
24 h
24 h
24 h
5 h
11b 21
11b 45
11b 28
11b 29
11b 43
11b 52
11b 55
11b 55
11a trace
2
3
b
b
c
Xantphos
Xantphos
K CO
PivOH
2
3
3
3
3
3
3
3
3
3
d
K CO
CuI
2
d
c
P(t-Bu) ·HBF
K CO
PivOH
3
4
2
d
c
PCy ·HBF
K CO
PivOH
3
4
2
d
d
c
P(t-Bu) Me·HBF
K CO
PivOH
2
4
4
2
c
P(t-Bu) Me·HBF
K CO
PivOH
2
2
b
c
Xantphos
K CO
PivOH
2
b
c
Xantphos
K CO
PivOH
5 h
2
d
4
c
e
10
P(t-Bu) Me·HBF
K CO
PivOH
5 h
11a 33
2
2
a
b
c
Reaction conditions: compound 10 (30 mg), Pd(OAc) 5 mol %, base (2.5 equiv), DMA (1 mL), 100 °C. Isolated yield. With 5 mol %. With 60
2
d
e
mol %. With 10 mol %. Mixture of structural isomers.
Figure 1. Molecular ground-state geometries of compounds 11a−c optimized in CHCl at the B3LYP/6-311++G(d) level of theory.
3
In the presence of Cs CO base, the direct coupling gave
ligand gave full conversion of starting material 10 within 5 h,
2
3
desired compound 11b in low yield (Table 3, entry 1), and the
formation of an unidentified byproduct was observed that was
not present in other reactions. The use of K CO as a base
improved the yield up to 45% (entry 2). The replacement of
pivalic acid additive with CuI led to decreased yield of
compound 11b (entries 2 and 3). The screening of the ligand
effect for this particular coupling revealed that the decrease of
the steric size of the monodentate ligand improves the yield of
and the product was isolated in 33% yield (entry 10). However,
H NMR and HPLC-HRMS analyses revealed that the product
1
was a mixture of desired product 11a and its structural isomer
in a 9:1 ratio (see Supporting Information). This mixture of
two isomers was shown to be inseparable in our hands. These
experiments reflect the fact that the problems in both reactivity
and selectivity may cause serious limitations to exploit the
direct coupling reaction as an effective synthetic tool.
2
3
the direct coupling product 11b (entries 4−6). P(t-Bu) Me·
2
HBF ligand showed improved and the best performance
COMPUTATIONAL STUDIES
4
■
compared with other monodentate ligands and, moreover,
bidentate Xantphos. The yield of direct coupling product 11b
could be slightly improved by increasing the amount of
coupling partner 3b from 1.0 to 1.5 equiv (entries 6 and 7).
Surprisingly, the use of Xantphos as a ligand and 1.5 equiv of
compound 3b gave compound 11b in the same 55% yield as
The optimized ground-state molecular geometries presented in
Figure 1 (see also Figure S1, Table S1, and the coordinates in
the Supporting Information), the total energies (Table S2), the
energies of the HOMO and LUMO, and the HOMO−LUMO
gaps (Table 4) of compounds 11a−c were computed using
density functional theory (DFT). The final optimized geo-
metries (Figure 1) were confirmed at the same level of theory
to be the minimum energy structures by the absence of the
imaginary frequency. The energetically most favorable con-
formations of compounds 11a−c (Figure S2, Table S2) are
P(t-Bu) Me·HBF ligand (entries 7 and 8). Interestingly, the
2
4
conversion rate of starting material 10 was much higher for the
reaction with Xantphos ligand. The reaction was finished just
after 5 h.
Direct coupling of 3a with compound 10 showed a major
difference compared to the reaction of 3b with compound 10.
The reaction in the presence of Xantphos ligand gave only trace
almost planar both in vacuum and in CHCl . The second hexyl
3
side-chain of compound 11b causes small torsional twists of the
molecular backbone compared to the totally planar structure of
compound 11a. The hexyl side-chain at the end position is
amounts of product 11a (Table 3, entry 9). P(t-Bu) Me·HBF4
2
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Table 4. Experimental and Computational Values of HOMO, LUMO, and Band Gap (E ) Energies of Compounds 11a−c in
g
Electron Volts (eV)
a
,
b
a
,
b
EHOMO−LUMO ,
ab
HOMO ,
ac
LUMO ,
ac
EHOMO−LUMO ,
ac
d
d
d
compound
HOMO
LUMO
HOMO
LUMO
Eg
1
1
1
1a
1b
1c
−5.52
−5.43
−5.39
−3.00
−2.94
−2.88
2.52
2.50
2.51
−5.59
−5.53
−5.47
−3.04
−3.00
2.55
2.53
2.52
−5.70
−5.49
−5.47
−3.36
−3.13
−3.31
2.34
2.36
2.16
−2.95
a
b
c
d
Calculated at the B3LYP/6-311++G(d,p) level of theory. In vacuum. In CHCl . Experimentally measured by DPV.
3
Figure 2. Natural transition orbitals (NTOs) presenting the hole (left) and electron (right) of the S → S transitions of compounds 11a−c
0
1
calculated at the TDDFT/B3LYP/6-311++G(d,p) level of theory in CHCl (isodensity contour = 0.030). The eigenvalue λ is the fraction of the
3
hole−electron contribution to the given excitation which in all of these is 100%.
present in both 11a and 11b, but due to its parallel positioning
with the backbone, it has only a small effect on the dihedral
angles of compound 11a. In the energetically less favorable
conformation of compound 11c, the oxygen atom of the
methoxy substituent is repelled by the nitrogen atom of the BT
unit, which makes the structure nonplanar. On the other hand,
in the most favorable conformation of 11c, the distance
between the methoxy oxygen and a hydrogen atom of the BT
unit is 2.12 Å, implying hydrogen bonding, which stabilizes the
structure and makes it nearly planar. These kinds of highly
planar backbones are important starting points for effective
conjugation.
backbone of 11a−c, whereas the electron is mainly localized
on the central BT acceptor unit. Thus, the lowest singlet
excitation of compounds 11a−c has a clear charge transfer
character.
Electrochemical and Spectroscopic Studies. The
HOMO/LUMO energy levels of compounds 11a−c, calculated
from differential pulse voltammetry (DPV) experiments, and
the HOMO−LUMO gap energies (E ) are presented in Table
g
4 together with the computational values. The voltammograms
(change in current−voltage curves, where voltages are given
with respect to the ferrocene couple) are shown in the
Supporting Information.
30
Natural transition orbitals (NTO) were used to study the
nature of the electronic transitions. The NTOs provide a
compact description for the electronic transition density matrix
and simplify the analysis of the excited states consisting of
mixed electronic configurations, which are usually difficult to
characterize with MOs. The NTOs reduce the description of an
excited-state transition into a single hole-particle excitation
whose fraction to the given transition is given by an eigenvalue
λ, which is 0 ≤ λ ≤ 1. The NTOs of compounds 11a−c for the
S → S transition are illustrated in Figure 2. The S → S
1
Compound 11a undergoes a three-step reduction and a two-
step oxidation. The reduction peaks are detected at −2.2, −1.6,
and −1.5 V, whereas the oxidation peaks appear at +0.9 and
+1.0 V. Compound 11b shows a two-step oxidation and
reduction. The reductions occur at −1.7 and −2.3 V, whereas
the oxidations occur at +0.7 and +0.9 V. Compound 11c has
oxidation peaks at +0.67, + 0.87, and +1.18 V and reduction
peaks at −1.49, −1.69, and −2.21 V. The slightly higher
reduction potential of 11b suggests that it is the strongest
electron donor among the studied compounds. However, all
three compounds, 11a−c, present similar electrochemical
properties, as shown in Table 4. These experimental findings
are supported by the computed HOMO/LUMO levels and
HOMO−LUMO gap values, which show only minor differ-
0
1
0
transitions are mostly HOMO → LUMO in character (Table
S3), and thus the corresponding NTOs are almost identical to
the frontier molecular orbitals (not shown here). In the S → S1
0
transitions, the hole is delocalized along the conjugated
F
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The Journal of Organic Chemistry
Article
Figure 3. Molar absorptivity (ε) of compounds 11a−c in CHCl solution (left); normalized absorption spectra of 11a−c in solid films (right).
3
Table 5. Absorption and Fluorescence Emission Maxima and Stokes Shifts of Compounds 11a−c
a
a
a
εa,b (cm‑1 mol‑1 dm3)
c
c
c
compound
λabs (nm)
λem (nm)
Stokes shift (nm)
λabs (nm)
λem (nm)
Stokes shift (nm)
1
1
1
1a
1b
1c
472
475
593
597
618
121
122
143
30900
35300
471
486
496
626
677
683
155
191
187
475
34300
a
b
c
In CHCl solution. Molar extinction coefficient at absorption maxima. In solid film.
3
Figure 4. Normalized fluorescence emission spectra of compounds 11a−c in solution (left) and solid films (right).
ences between the compounds. However, compound 11c, the
structurally closest analogue of the theoretical Harvard Clean
Energy Project molecules (presented in Scheme 1), has the
closest experimentally determined HOMO level and HOMO−
LUMO gap relative to the theoretical compounds of Scheme 1.
Compared to the HOMO/LUMO levels of the theoretical
molecules, the LUMO levels of compounds 11a−b and also
different types of aggregation in solid films of compounds 11a
(H-aggregates) and 11b and 11c (J-aggregates).
In Figure 4, the normalized fluorescence emission spectra of
11a−c in solution and films are presented, whereas in the
Supporting Information, the non-normalized emission spectra
(Figure S5) together with the fluorescence measurements setup
31
32
are reported. In CHCl solution, the emission spectra of 11a
3
11c are higher and the HOMO−LUMO gap wider. This is
and 11b approximately overlap, and the spectrum of compound
11c has red-shifted 21 nm with respect to 11b. Whereas the
emission spectra of 11b and 11c show similar spectral features
and are considered red-shifted with respect to that of 11a in
solid films. Interestingly, these compounds or their derivatives
may also find potential future applications as coloring
substances. For example, compound 11a, which has the
smallest full width at half-maximum of emission in solution
(95 nm), shows high color purity (98%) of a yellowish-orange
hue with CIE 1931 chromaticity coordinates of (0.54, 0.45).
In organic materials, the emission peak from an electronic
state is red-shifted with respect to the absorption peak by the
Stokes shift. The Stokes shifts for 11a and 11b are nearly the
same in CHCl3 solution and considerably smaller in
comparison to that of 11c (Table 5). In solid films, compounds
11b and 11c have a larger Stokes shifts (191 and 187 nm,
respectively) compared to 11a (155 nm). A large Stokes shift
typically indicates a change in geometry from the ground state
to the excited state. It can be expected that the energy losses
due to reorganization upon excitation are larger for 11b and
11c than for 11a because of the extra hexyl side-chain (11b)
logical because the fused furan structure of the theoretical
molecules extends the conjugation and thus lowers the LUMO
level and simultaneously decreases the HOMO−LUMO gap.
Absorption spectra (on molar absorptivity (ε) scale) of
compounds 11a−c in CHCl solution and absorption spectra in
3
solid films normalized at the Q-transition (S0−S1) absorption
maximum are depicted in Figure 3. The non-normalized spectra
are presented in the Supporting Information. The summary of
optical data is shown in Table 5. The absorption spectra of
compounds 11a−c in solution are almost identical with a 3 nm
red-shift (in the Q-band absorption) for the spectra of
compounds 11b and 11c with respect to 11a. The absorption
spectra of 11b and 11c in solid films are instead bathochromi-
cally (red) shifted 15 and 20 nm, respectively, compared to that
of 11a. It can be further noted that for compound 11a there is a
very small hypsochromic (blue) shift between the absorption
b
maximum in solid films (λ abs) compared to the maximum in
solution (λaabs), whereas for 11b and 11c there are 11 and 21
nm red-shifts, respectively, in the absorption maxima peaks in
solution and solid films. This suggests the possibility for
G
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The Journal of Organic Chemistry
Article
1
3d
and methoxy substituent (11c).
Hence, there is a larger
in the measurements. Samples of 0.34−3.16 mg were placed in 40 μL
Al crucibles, which were sealed with pierced lids.
Synthesis of 2-(Thiophen-2-yl)thiazole (3a). Toluene (2.5 mL),
dipole moment change from the ground state to the excited
state in 11b and 11c compared to 11a in solid films. The
33
ethanol (0.8 mL), distilled water (0.8 mL), and 2-bromothiazole (1)
larger Stokes shift of 11c in CHCl is attributed to the methoxy
3
(
99.0 mg, 0.60 mmol) were deoxygenated with argon for 15 min in a
substituent providing greater reorganization energy loss upon
light absorption.
reaction tube with a magnetic stirring bar. Ground K PO (3 equiv,
3
4
3
89.5 mg, 1.83 mmol), 2-thiopheneboronic acid pinacol ester (2a)
(
1.03 equiv, 130.4 mg, 0.62 mmol), Pd(OAc) (2.6 mol %, 3.5 mg,
2
CONCLUSIONS
15.6 μmol), and Xantphos (2.6 mol %, 9.0 mg, 15.6 μmol) were added.
The sealed tube was evacuated and backfilled with argon five times,
and finally the tube was flushed with argon for 5 min. The reaction
mixture was stirred and heated in an oil bath (60 °C) for 3 h. The
reaction mixture was filtered through a thin pad of silica gel, rinsed
with toluene, and evaporated under reduced pressure. The product
■
Various reaction routes involving 4−6 reaction steps for the
synthesis of three BT derivatives, compounds 11a−c, are
presented. Our synthetic findings show in particular that the
use of bidentate Xantphos ligand as a part catalyst system in
cross-couplings is an effective way to synthesize various
thiophene-thiazole, thiophene-BT, and thiazole-BT coupling
products. For instance, monocoupling product 4-bromo-7-(5-
hexylthiophen-2-yl)-2,1,3-benzothiadiazole (10) can be synthe-
sized in good 80% yield by applying the Xantphos ligand in
Suzuki−Miyaura cross-coupling to overcome the competing
dicoupling. In the final reaction step, Suzuki−Miyaura cross-
coupling was a more powerful tool compared to the direct
coupling method and, in particular, the only way to produce
compound 11a. However, the overall yields of compound 11b
were 39% via the Suzuki−Miyaura pathway (6 reaction steps)
and 43% via the direct coupling pathway (4 reaction steps).
This means that the direct coupling method may offer a good
alternative for Suzuki−Miyaura coupling in particular cases: (i)
the yield of boron reagent, which is needed in Suzuki−Miyaura
cross-coupling, is low, and (ii) the direct coupling does not
suffer regioselectivity problems.
was purified by using flash chromatography (SiO
product 3a was collected as a viscous oil (89.7 mg) in 89% yield. The
, toluene). Isolated
2
12
1
spectral data correspond with that in a previous report. H NMR
(
400 MHz, CD Cl , ppm): δ 7.09 (dd, J = 5.1, 3.6 Hz, 1H), 7.29 (d, J
2 2
=
1
3.3 Hz, 1H), 7.41 (dd, J = 5.1, 1.1 Hz, 1H), 7.53 (dd, J = 3.6, 1.1 Hz,
H), 7.75 (d, J = 3.3 Hz, 1H). 13C NMR (100.6 MHz, CD Cl , ppm):
2 2
δ 118.8, 127.1, 128.1, 128.4, 138.0, 143.7, 162.3. HRMS (ESI+, TOF)
+
m/z: [M + H] calcd for C H NS , 167.9942; found, 167.9934.
7
6
2
2
-(3-Hexylthiophen-2-yl)thiazole (3b). Compound 3b was pre-
pared by using the same synthetic procedure as described above for
compound 3a. The specific amounts of reagents used were toluene
(3.75 mL), ethanol (1.2 mL), distilled water (1.2 mL), compound 1
(
146.6 mg, 0.89 mmol), K PO (3 equiv, 582.4 mg, 2.74 mmol), 3-
3 4
hexyl-2-thiopheneboronic acid pinacol ester (2b) (1.04 equiv, 274.6
mg, 0.93 mmol), Pd(OAc) (2.6 mol %, 5.1 mg, 22.7 μmol), and
2
Xantphos (2.6 mol %, 13.4 mg, 23.2 μmol). Pure compound 3b was
1
isolated as a viscous oil (223.2 mg) in >99% yield. H NMR (200
MHz, CD Cl , ppm): δ 0.90 (t, J = 6.8 Hz, 3H), 1.28−1.50 (m, 6H),
2
2
1
7
.62−1.77 (m, 2H), 2.93 (t, J = 7.5 Hz, 2H), 6.99 (d, J = 5.2 Hz, 1H),
.30 (d, J = 5.2 Hz, 1H), 7.35 (d, J = 3.3 Hz, 1H), 7.79 (d, J = 3.3 Hz,
The prepared target molecules 11a−c closely structurally
resemble the selected molecules (Scheme 1) presented by the
Harvard Clean Energy Project. Computational studies showed
that compounds 11a−c have nearly planar molecular back-
bones, which offer prerequisites for effective conjugation and
intramolecular charge transfer. Electronic and optical properties
of the compounds resemble each other, nevertheless, 11b and
13
1H). C NMR (100.6 MHz, CD Cl , ppm): δ 14.0, 22.5, 29.2, 29.8
2
2
2
(2C), 31.5, 118.0, 126.2, 130.3, 131.4, 142.0, 142.5, 160.9. HRMS (ESI
+
+
, TOF) m/z: [M + H] calcd for C H NS , 252.0881; found,
13 18 2
2
52.0893.
Synthesis of 5-Bromo-2-(thiophen-2-yl)thiazole (4a) from 3a.
Compound 3a (61.9 mg, 0.37 mmol) was dissolved in CHCl (5.5
3
mL), and NBS (1.1 equiv, 73.7 mg, 0.41 mmol) was added. The
reaction mixture was irradiated with ultrasound (42 kHz, 100 W) for
1
1c exhibit stronger red-shift in absorption and emission in film
with respect to those in 11a. In addition, compound 11c shows
more pronounced red-shift in emission also in solution in
comparison to 11a and 11b. Larger Stokes shifts for 11b and
2
4 h. The solvent was evaporated, and the crude product was
subjected to flash chromatography (SiO , toluene). Pure compound 4a
2
1
was isolated as an off-white solid (34.7 mg) in 38% yield. H NMR
1
1c are assigned to a larger dipole moment change upon light
(400 MHz, CHCl , ppm): δ 7.07−7.10 (m, 1H), 7.42 (d, J = 5.1 Hz,
3
13
excitation due to extra hexyl side-chain of 11b and methoxy
substituent of 11c. This is consistent with our previous
observation that the nature of the solubility increasing alkyl
1H), 7.45 (d, J = 3.7 Hz, 1H), 7.65 (s, 1 H). C NMR (100.6 MHz,
CDCl , ppm): δ 107.6, 126.8, 127.9, 128.2, 136.7, 144.4, 163.2. HRMS
3
+
(
ESI+, TOF) m/z: [M + H] calcd for C H NS Br, 245.9047; found,
7
5
2
13d
245.9036. HRMS (ESI+, TOF) m/z of dibrominated byproduct: [M +
side-chains may notably change the solid state properties.
+
H] calcd for C H NS Br , 323.8152; found, 323.8161. HRMS (ESI+,
7
4
2
2
Compounds 11a−c can be potentially applied as building
blocks in the construction of more advanced acceptor−donor−
acceptor-type organic semiconductors in which the compounds
act as the acceptor units. This strategy allows for free variation
of the central donor unit. Our current work continues with that
subject.
+
TOF) m/z of monobrominated byproduct: [M + H] calcd for
C H NS Br, 245.9047; found, 245.9039.
7
5
2
5
-Bromo-2-(3-hexylthiophen-2-yl)thiazole (4b). Compound 4b
was prepared by using the same synthetic procedure as described
above for compound 4a. The specific amounts of reagents used were
compound 3b (97.2 mg, 0.39 mmol), NBS (1.1 equiv, 76.5 mg, 0.43
mmol), and CHCl (6 mL). Pure compound 4b was isolated as a solid
3
1
(
110.3 mg) in 86% yield; mp 44 °C. H NMR (400 MHz, CD Cl ,
2
2
EXPERIMENTAL SECTION
Characterization. All new compounds were purified using flash
■
ppm): δ 0.90 (t, J = 7.0 Hz, 3H), 1.30−1.44 (m, 6H), 1.64−1.71 (m,
2H), 2.86 (t, J = 7.8 Hz, 2H), 6.98 (d, J = 5.3 Hz, 1H), 7.32 (d, J = 5.3
Hz, 1H), 7.67 (s, 1H). 13C NMR (100.6 MHz, CD Cl , ppm): δ 14.4,
chromatography (SiO ). Only fractions that showed a single spot on
2
2
2
TLC were collected. The chemical structures of new compounds were
characterized using H NMR, C NMR, and HRMS techniques. ESI-
23.2, 29.8, 30.4, 30.5, 32.2, 108.2, 127.4, 131.1, 131.7, 143.4, 144.3,
1
13
+
162.8. HRMS (ESI+, TOF) m/z: [M + H] calcd for C H NS Br,
1
3
17
2
1
MS/MS studies were carried out for compounds 11a−c. H NMR
329.9986; found, 329.9986. HRMS (ESI+, TOF) m/z of dibrominated
+
spectra assignments are presented in the Supporting Information. A
heat-flux differential scanning calorimeter was used for measuring the
melting points (reported as onset values) of crystalline compounds 4b,
byproduct: [M + H] calcd for C H NS Br , 407.9091; found,
13
16
2
2
407.9094.
Synthesis of Compound 4a Using Compound 6 as a Starting
Material. 2,5-Dibromothiazole (6) (72.8 mg, 0.30 mmol), 2-
thiopheneboronic acid pinacol ester (2a) (1.07 equiv, 68.0 mg, 0.32
−1
5
, 7b, 11a, 11c, and 12. A heating rate of 20 °C min , temperature
−1
scans from 5 to 300 °C, and a nitrogen flow of 60 mL min were used
H
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The Journal of Organic Chemistry
Article
mmol), Pd(OAc) (3.3 mol %, 2.3 mg, 10 μmol), Xantphos (2.5 mol
130.4, 131.7, 143.0, 152.0, 166.3. HRMS (ESI+, TOF) m/z: [M + H]+
2
%
, 4.3 mg, 7.4 μmol), and K PO (3 equiv, 199.3 mg, 0.94 mmol) were
calcd for C H NO S B, 377.1769; found, 377.1774.
3
4
19 29
2 2
placed in a reaction tube with a magnetic stirring bar. The sealed tube
was evacuated and backfilled with argon five times. Deoxygenated dry
THF (1.5 mL) was added with a syringe. The reaction mixture was
stirred in oil bath (60 °C) for 5 h. The reaction mixture was filtered
through a thin pad of silica gel, rinsed with toluene, and evaporated
under reduced pressure. The product was purified using flash
Synthesis of 4,7-Dibromo-2,1,3-benzothiadiazole (9). 2,1,3-
Benzothiadiazole (8) (501.1 mg, 3.68 mmol) and NBS (2.1 equiv,
1.354 g, 7.61 mmol) were placed in a round-bottom reaction flask.
Concentrated H SO (97%; 5 mL) was added, and the reaction
2 4
mixture was stirred in an oil bath (60 °C) for 3 h. The cooled reaction
flask was transferred to an ice bath. Distilled water (25 mL) was added
dropwise. The product was extracted with 3 × 30 mL of toluene. The
chromatography (SiO , toluene). Isolated product 4a was collected
2
as an off-white solid (65.2 mg) in 88% yield. Dicoupling byproduct of
the coupling reaction between compounds 2b and 6: HRMS (ESI+,
combined organic layers were dried with anhydrous Na SO for 10
2 4
min, filtered, and evaporated under reduced pressure. The procedure
gave compound 9 as an off-white powder (1.08 g) in >99% yield. The
+
TOF) m/z of dicoupling byproduct: [M + H] calcd for C H NS ,
23
32
3
1
8a
1
spectral data corresponds with a previous report.
H NMR (200
4
18.1697; found, 418.1679.
Synthesis of 2-(4,5-Dibromo-3-hexylthiophen-2-yl)-1,3-thiazole
5). Compound 3b (11.6 mg, 46 μmol) was dissolved in concentrated
H SO (0.5 mL) in a round-bottom flask with a magnetic stirring bar.
MHz, CDCl , ppm): δ 7.74 (s, 2H). HRMS (ESI+, TOF) m/z: [M +
3
+
(
H] calcd for C H N SBr , 292.8384; found, 292.8393.
6 3 2 2
Synthesis of 4-Bromo-7-(5-hexylthiophen-2-yl)-2,1,3-benzothia-
diazole (10). Toluene (2 mL), DMA (2 mL), distilled water (0.5 mL),
and 5-hexyl-2-thiopheneboronic acid pinacol ester (49.2 mg, 0.17
mmol) were deoxygenated with argon for 15 min in a reaction tube
with a magnetic stirring bar. Compound 9 (1.5 equiv, 75.1 mg, 0.26
2
4
The reaction mixture was stirred and heated in an oil bath (60 °C).
NBS (1.0 equiv, 8.3 mg, 47 μmol) was dissolved in concentrated
H SO (0.5 mL). The prepared solution was added dropwise to the
2
4
reaction mixture. After an 18 h reaction time, the cooled reaction
mixture was added dropwise in the mixture of aqueous NaOH (1M, 5
mL) and toluene (5 mL). The pH of the aqueous layer was adjusted to
mmol), Cs CO (2.5 equiv, 138.6 mg, 0.43 mmol), Pd(OAc) (5 mol
2
3
2
%
, 1.9 mg, 8.46 μmol), and Xantphos (5 mol %, 5.1 mg, 8.81 μmol)
were added. The sealed tube was evacuated and backfilled with argon
five times, and finally the tube was flushed with argon for 5 min. The
reaction mixture was stirred and heated in an oil bath (60 °C) for 1.5
h. The reaction mixture was filtered through a thin pad of silica gel,
rinsed with toluene, and evaporated under reduced pressure. The
>
7 with solid NaOH. The mixture was extracted with toluene (3 × 5
mL). The combined organic layers were dried with anhydrous Na SO4
2
for 10 min. The solution was filtered and evaporated under reduced
pressure. The crude product was purified with flash chromatography
(
SiO , toluene). Pure product 5 was isolated as a solid (5.4 mg); mp
2
1
product was purified using flash chromatography (SiO , toluene 1:1 n-
2
3
1
3
4 °C. H NMR (400 MHz, CD Cl , ppm): δ 0.88−0.92 (m, 3H),
2
2
heptane). Isolated product 10 was collected as a yellow solid (51.1
.32−1.37 (m, 4H), 1.44−1.51 (m, 2H), 1.56−1.64 (m, 2H), 2.97−
.01 (m, 2H), 7.42 (d, J = 3.3 Hz, 1H), 7.80 (d, J = 3.3 Hz, 1H). 13
mg) in 80% yield. The spectral data corresponds with previous
C
20
1
reports. H NMR (400 MHz, CDCl , ppm): δ 0.91 (t, J = 7.0 Hz,
3
NMR (100.6 MHz, CD Cl , ppm): δ 14.4, 23.2, 29.1, 29.9, 31.4, 32.0,
2
2
3
H), 1.31−1.36 (m, 4H), 1.39−1.44 (m, 2H), 1.75 (quin, J = 7.6 Hz,
1
13.8, 118.8, 119.7, 133.7, 141.1, 143.3, 159.7. HRMS (ESI+, TOF)
+
2H), 2.88 (t, J = 7.7 Hz, 2H), 6.87 (d, J = 3.5 Hz, 1H), 7.61 (d, J = 7.8
m/z: [M + H] calcd for C H NS Br , 407.9091; found, 407.9094.
13
16
2
2
Hz, 1H), 7.80 (d, J = 7.8 Hz, 1H), 7.91 (d, J = 3.5 Hz, 1H). HRMS
Synthesis of 5-(4,4,5,5-Tetramethyl-1,3,2-dioxaborolan-2-yl)-2-
thiophen-2-yl)thiazole (7a). Magnesium powder (1.3 equiv, 22.3
+
(
ESI+, TOF) m/z: [M + H] calcd for C H N S Br, 381.0095;
16
18
2 2
(
found, 381.0094. HRMS (ESI+, TOF) m/z of dicoupling byproduct:
mg, 0.92 mmol) and an iodine crystal were added in a reaction tube
with a magnetic stirring bar. The sealed reaction system was heated
until iodine sublimed. The reaction system was allowed to cool to 25
+
[
M + H] calcd for C H N S , 469.1806; found, 469.1811.
26 33 2 3
General Method for the Syntheses of Compounds 11a−c
Using Suzuki−Miyaura Cross-Coupling. Toluene, DMA, and
distilled water were deoxygenated with argon for 15 min in a reaction
tube with a magnetic stirring bar. Compound 10 or 12, compound 7a
or 7b, Cs CO , t-Bu P·HBF , and Pd (dba) were added. The sealed
°C after which the system was purged with argon for 5 min.
Compound 4a (170.8 mg, 0.70 mmol) was dissolved in dry THF (2
mL) and added through the septum in the reaction system. Under
constant stirring, pinacolborane (1.1 equiv, 0.11 mL, 0.76 mmol) was
added dropwise through the septum. The reaction mixture was stirred
at 25 °C for 2.5 h. The reaction mixture was cooled in an ice bath; 2 M
aqueous HCl (5 mL) was added dropwise, and the mixture was stirred
2
3
3
4
2
3
tube was evacuated and backfilled with argon five times, and finally the
tube was flushed with argon for 5 min. The reaction mixture was
stirred and heated in an oil bath (90 °C) for 2−3.5 h. The reaction
mixture was filtered through a thin pad of silica gel, rinsed with
toluene, and evaporated under reduced pressure. The product was
for 15 min. During that period, the released H gas escaped from the
2
reaction system through the open needle. The reaction mixture was
extracted with toluene (4 × 5 mL). The combined organic layers were
dried with Na SO , filtered, and evaporated to dryness. The product
purified using flash chromatography (SiO , toluene).
2
4
-(5-Hexylthiophen-2-yl)-7-(2-(thiophen-2-yl)thiazol-5-yl)benzo-
2
4
[c][1,2,5]thiadiazole (11a). Compound 11a was prepared by using the
general method described above. The specific amounts of reagents
used were toluene (1.5 mL), DMA (1.5 mL), distilled water (0.3 mL),
compound 10 (49.9 mg, 0.13 mmol), compound 7a (1.4 equiv, 53.0
mg, 0.18 mmol), Cs CO (3.3 equiv, 138.5 mg; 0.43 mmol), t-Bu P·
was isolated using flash chromatography (SiO ). The column was
2
eluated with toluene until the impurities ran out. In the second stage,
the column was eluated with acetone to isolate the desired product.
1
Compound 7a was collected as a solid (96.5 mg) in 47% yield. H
2
3
3
NMR (400 MHz, CHCl , ppm): δ 1.37 (s, 12H), 7.09−7.11 (m, 1H),
3
HBF (15 mol %, 5.8 mg; 20.0 μmol), and Pd (dba) (3.9 mol %, 4.7
4
2
3
13
7
.42 (d, J = 4.9 Hz, 1H), 7.58 (d, J = 3.7 Hz, 1H), 8.16 (s, 1H).
C
mg, 5.1 μmol). Isolated product 11a was collected as a red solid (47.3
1
NMR (100.6 MHz, CDCl , ppm): δ 24.7, 84.5, 127.3, 128.0, 128.2,
3
mg) in 77% yield; mp 108 °C. H NMR (400 MHz, CDCl , ppm): δ
3
+
1
37.2, 152.6, 167.1. HRMS (ESI+, TOF) m/z: [M + H] calcd for
0
.92 (t, J = 6.9 Hz, 3H), 1.32−1.37 (m, 4H), 1.40−1.47 (m, 2H), 1.77
(quin, J = 7.6 Hz, 2H), 2.90 (t, J = 7.6 Hz, 2H), 6.90 (d, J = 3.7 Hz,
1H), 7.14 (dd, J = 5.1, 3.7 Hz, 1H), 7.45 (dd, J = 5.1, 1.1 Hz, 1H), 7.63
C H NO S B, 293.0830; found, 293.0839.
13
17
2 2
2
-(3-Hexylthiophen-2-yl)-5-(4,4,5,5-tetramethyl-1,3,2-dioxaboro-
lan-2-yl)thiazole (7b). Compound 7b was prepared using the same
synthetic procedure as described above for compound 7a. The specific
amounts of reagents used were compound 4b (222.0 mg, 0.67 mmol),
Mg (1.2 equiv, 20.0 mg, 0.82 mmol), pinacolborane (1.1 equiv, 0.11
mL, 0.76 mmol), and THF (6 mL). Pure compound 7b was isolated as
(
1
dd, J = 3.7, 1.1 Hz, 1H), 7.82 (d, J = 7.6 Hz, 1H), 7.86 (d, J = 7.6 Hz,
H), 7.99 (d, J = 3.7 Hz, 1H), 8.65 (s, 1H). C NMR (100.6 MHz,
13
CDCl , ppm): δ 14.1, 22.6, 28.8, 30.3, 31.5, 31.6, 122.2, 124.6, 125.3,
1
1
3
26.3, 126.7, 127.0, 127.9, 128.0, 128.0, 133.7, 136.3, 137.4, 142.1,
+
48.5, 152.1, 152.3, 162.0. HRMS (ESI+, TOF) m/z: [M + H] calcd
1
a solid (161.1 mg) in 67% yield; mp 103 °C. H NMR (400 MHz,
for C H N S , 468.0697; found, 468.0680.
23
22
3 4
CDCl , ppm): δ 0.90 (t, J = 7.1 Hz, 3H), 1.33−1.45 (m, 18H), 1.71
4-(5-Hexylthiophen-2-yl)-7-(2-(3-hexylthiophen-2-yl)thiazol-5-yl)-
benzo[c][1,2,5]thiadiazole (11b). Compound 11b was prepared using
the general method described above. The specific amounts of reagents
3
(
1
quin, J = 7.6 Hz, 2H), 2.95 (t, J = 7.8 Hz, 2H), 6.98 (d, J = 5.1 Hz,
H), 7.32 (d, J = 5.1 Hz, 1H), 8.21 (s, 1H). C NMR (100.6 MHz,
13
CD Cl , ppm): δ 14.1, 22.6, 24.8, 29.2, 29.7, 30.0, 31.7, 84.5, 126.9,
used were toluene (1 mL), DMA (1 mL), distilled H O (0.2 mL),
2
2
2
I
DOI: 10.1021/acs.joc.5b02689
J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Article
+
compound 10 (36.0 mg, 94.4 μmol), compound 7b (1.1 equiv, 38.4
mg, 0.10 mmol), Cs CO (2.5 equiv, 79.2 mg, 0.24 mmol), P(t-Bu) ·
156.9. HRMS (ESI+, TOF) m/z: [M + H] calcd for C H N OS Br,
11 8 2 2
326.9261; found, 326.9263. HRMS (ESI+, TOF) m/z of dicoupling
byproduct: [M + H] calcd for C H N O S , 361.0139; found,
2
3
3
+
HBF (13 mol %, 3.5 mg, 12.1 μmol), and Pd (dba) (3 mol %, 2.7
4
2
3
16 13
2
2 3
mg, 2.9 μmol). Pure compound 11b was isolated as a dark red solid
361.0125.
1
(
0
7
45.2 mg) in 87% yield. H NMR (400 MHz, CDCl , ppm): δ 0.90−
Computational Methods. The DFT and the time-dependent
DFT (TDDFT) methods were applied using the Gaussian 09 (revision
D.01) suite of programs. The ground-state geometries were
optimized and the electronic properties calculated both in vacuum
and CHCl using the B3LYP approximation, i.e., the combination of
the hybrid Becke’s three-parameter exchange functional and the
Lee−Yang−Parr correlation functional. The suitable level of theory
was verified with the polarized split-valence basis sets 6-31G(d), 6-
3
.94 (m, 6H), 1.32−1.44 (m, 12H), 1.73−1.81 (m, 4H), 2.90 (t, J =
.6 Hz, 2H), 3.04 (t, J = 7.8 Hz, 2H), 6.90 (d, J = 3.7 Hz, 1H), 7.02 (d,
3
5
J = 5.1 Hz, 1H), 7.35 (d, J = 5.1 Hz, 1H), 7.82 (d, J = 7.6 Hz, 1H),
.87 (d, J = 7.6 Hz, 1H), 7.99 (d, J = 3.7 Hz, 1H), 8.74 (s, 1H). 13
NMR (100.6 MHz, CDCl , ppm): δ 14.1, 14.1, 22.6, 22.6, 28.8, 29.3,
7
C
3
3
6
3
3
7
2
1
1
9.9, 30.1, 30.3, 31.5, 31.6, 31.7, 122.4, 124.6, 125.3, 126.2, 126.7,
26.8, 127.9, 130.5, 131.8, 133.7, 136.4, 141.7, 142.6, 148.4, 152.2,
+
52.3, 161.2. HRMS (ESI+, TOF) m/z: [M + H] calcd for
3
1G(d,p), 6-311+G(d,p), and 6-311++G(d,p) with the latter two also
C H N S , 552.1636; found, 552.1622.
29
34
3
4
including the diffuse functions. Solvation effects were taken into
account by means of the integral equation formalism of the polarizable
continuum model. The energies of the vertical transitions to the
lowest singlet (S) excited states, the oscillator strengths, and the
4
-(3-Methoxythiophen-2-yl)-7-(2-(thiophen-2-yl)thiazol-5-yl)-
benzo[c][1,2,5]thiadiazole (11c). Compound 11c was prepared using
the general method described above. The specific amounts of reagents
38
used were toluene (1 mL), DMA (1 mL), distilled H O (0.2 mL),
2
electronic configurations were determined with TDDFT for the first
compound 12 (34.2 mg, 105 μmol), compound 7a (1.07 equiv, 32.9
mg, 112 μmol), Cs CO (2.5 equiv, 85.5 mg, 0.26 mmol), P(t-Bu) ·
1
0 excited states both in vacuum and in CHCl at the B3LYP/6-311+
3
2
3
3
+G(d,p) level of theory. In the TDDFT solvent calculations, the
HBF (12 mol %, 3.8 mg, 13.1 μmol), and Pd (dba) (3 mol %, 2.9
4
2
3
ground state geometries optimized with DFT in CHCl3 were
employed in the TDDFT single-point calculations using the same
solvent. The electronic transitions were qualitatively described using
NTOs as a representation for the transition density matrix. Pictorial
data for the geometries and NTOs were generated using Chemcraft
mg, 3.2 μmol). Pure compound 11c was isolated as a red solid (31.1
1
mg) in 72% yield; mp 173 °C. H NMR (400 MHz, CDCl , ppm): δ
3
4
7
1
.04 (s, 3H), 7.03 (d, J = 5.6 Hz, 1H), 7.13 (dd, J = 5.1, 3.7 Hz, 1H),
.43−7.45 (m, 2H), 7.63 (dd, J = 3.7, 1.2 Hz, 1H), 7.90 (d, J = 7.8 Hz,
H), 8.43 (d, J = 7.8 Hz, 1H), 8.66 (s, 1H). C NMR (100.6 MHz,
30
13
1
.7. The figures of the molecular geometries and NTOs were edited
CDCl , ppm): δ 58.7, 115.4, 115.8, 121.7, 126.3, 126.3, 126.7, 126.8,
3
with the Inkscape version 0.91.
1
26.8, 127.8, 128.0, 134.1, 137.6, 142.1, 152.1, 152.8, 157.2, 161.9.
Spectroscopic Measurements. The steady-state absorption
spectra were measured with a UV-3600 Shimadzu UV−vis−NIR
spectrophotometer, and emission spectra were recorded using a Yobin
Yvon-SPEX Fluorolog spectrofluorometer. Absorption and emission
spectra were measured in CHCl solution and in films. The excitation
wavelengths were 475 and 470 nm for solution and solid samples,
+
HRMS (ESI+, TOF) m/z: [M + H] calcd for C H N OS ,
18
12
3
4
4
13.9863; found, 413.9859.
Synthesis of Compound 11a Using Direct Coupling. DMA (1
mL) and compound 3a (1.5 equiv, 20.1 mg, 0.12 mmol) were
deoxygenated with argon for 15 min in the reaction tube with a
magnetic stirring bar. Compound 10 (30.3 mg, 79 μmol), Pd(OAc)2
3
respectively. The solid films were deposited by spin-coating in WS-
(
7
5 mol %, 0.9 mg, 4.0 μmol), P(t-Bu) Me·HBF (10 mol %, 1.9 mg,
2 4
4
00B-6NPP/LITE spin-coater from Laurell Technologies from CHCl3
.7 μmol), K CO (2.5 equiv, 27.7 mg, 0.20 mmol), and PivOH (60
2 3
solution (1700 rpm, 1 min). The concentrations of the spin-coating
solutions are reported in the Supporting Information.
mol %, 5.1 mg, 49.9 μmol) were added. The sealed tube was evacuated
and backfilled with argon five times, and finally the tube was flushed
with argon for 5 min. The reaction mixture was stirred and heated in
an oil bath (100 °C) for 5 h. The reaction mixture was filtered through
a thin pad of silica gel, rinsed with toluene, and evaporated under
reduced pressure. The product was purified using flash chromatog-
Electrochemical Measurements. To determine the HOMO and
LUMO energy levels of the synthesized target compounds, DPV
measurements were realized by employing a potentiostat (Compact-
Stat, Ivium Technologies) and a three-electrode cell configuration.
The DPV experiments were carried out using 0.1 M dry
tetrabutylammonium tetrafluoroborate in dichloromethane (DCM)
as a supporting electrolyte, glass platinum electrode as a working
electrode, platinum wire as a counter electrode, and Ag/AgCl wire as a
pseudo-reference electrode. Ferrocene/ferrocenium (Fc/Fc+) couple
was used as an internal standard reference to scale the measured
potentials against the vacuum level.
For each sample, the background was initially measured for 2.5 mL
of the electrolyte solution after 20 min deoxygenation with N ; 100 μL
of 0.5 mM sample in DCM was then inserted into the electrolyte
solution and, after the system was stabilized by purging N , the
voltammogram was recorded. Finally, 20 μL of ferrocene (3 mM in
DCM) was added to the “electrolyte + sample” solution, and a new
voltammogram was recorded.
Each measurement was carried out between −2.4 and 1.6 V,
scanning in both directions with 2.5 mV steps. The HOMO and
LUMO energy levels were calculated from the oxidation and reduction
potentials observed from the DPV curves according to the equations
raphy (SiO , toluene). Product 11a, with its structural isomer, were
2
34
collected (12.3 mg) in 33% yield.
Compound 11b. Compound 11b was prepared using the same
synthetic procedure as described above for compound 11a. The
specific amounts of reagents used were DMA (1 mL), compound 10
(
30.1 mg, 78.9 μmol), compound 3b (1.5 equiv, 30.2 mg, 0.12 mmol),
3
9
Pd(OAc) (5 mol %, 0.9 mg, 4.0 μmol), Xantphos (5 mol %, 2.5 mg,
2
4
.3 μmol), K CO (2.5 equiv, 27.7 mg, 0.20 mmol), and PivOH (74
2 3
mol %, 6.0 mg, 58.7 μmol). Isolated product 11b was collected (24.1
mg) in 55% yield.
Synthesis of 4-Bromo-7-(3-methoxythiophen-2-yl)-2,1,3-benzo-
thiadiazole (12). 3-Methoxythiophene-2-boronic acid pinacol ester
2
2
(
48.9 mg, 0.20 mmol), compound 9 (1.5 equiv, 89.2 mg, 0.30 mmol),
and Pd precatalyst (3 mol %, 4.6 mg, 5.8 μmol) were dissolved in
deoxygenated THF (2 mL). The sealed tube was evacuated and
backfilled with argon five times. Deoxygenated 1 M Cs CO (aq) (5
2
3
equiv, 1 mL, 1 mmol) was added via syringe. The reaction mixture was
stirred and heated in an oil bath (30 °C) for 70 min. At this point,
toluene (10 mL) and distilled water (3 mL) were added to the
reaction mixture. The aqueous phase was extracted with toluene (3 ×
EHOMO(eV) = −(Eox − EFc/Fc+) − 4.80eV
5
mL). The combined organic layers were dried with Na SO for 10
2 4
min, filtered, and evaporated under reduced pressure. The product was
ELUMO(eV) = (Ered − EFc/Fc+) − 4.80eV
purified by using flash chromatography (SiO , toluene 2:3 n-heptane).
2
Isolated product 12 was collected as a yellow solid (37.7 mg) in 57%
1
yield; mp 126 °C. H NMR (400 MHz, CDCl , ppm): δ 4.02 (s, 3H),
where Eox and Ered are the oxidation and reduction potentials of the
sample and EFc/Fc+ the potential of ferrocene. Eox, Ered, and EFc/Fc+ are
all referred against the Ag/AgCl reference electrode. The value −4.80
eV is the energy level of ferrocene against vacuum.
3
7
1
5
.02 (d, J = 5.6 Hz, 1H), 7.45 (d, J = 5.6 Hz, 1H), 7.86 (d, J = 7.8 Hz,
H), 8.27 (d, J = 7.8 Hz, 1H). 13C NMR (100.6 MHz, CDCl , ppm): δ
3
8.7, 110.5, 114.4, 115.7, 126.2, 126.7, 126.8, 132.6, 152.2, 153.4,
J
DOI: 10.1021/acs.joc.5b02689
J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Article
Hormi, O. E. O.; Lemmetyinen, H. J. RSC Adv. 2014, 4, 8846.
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.joc.5b02689.
■
(c) Manninen, V. M.; Heiskanen, J. P.; Pankov, D.; Kastinen, T.;
*
S
Hukka, T. I.; Hormi, O. E. O.; Lemmetyinen, H. J. Photochem.
Photobiol. Sci. 2014, 13, 1456. (d) Heiskanen, J. P.; Manninen, V. M.;
Pankov, D.; Omar, W. A. E.; Kastinen, T.; Hukka, T. I.; Lemmetyinen,
H. J.; Hormi, O. E. O. Thin Solid Films 2015, 574, 196.
NMR spectra of synthesized compounds, ESI-MS/MS
spectra of compounds 11a−c, and computational and
spectroscopic data (PDF)
(
14) Strotman, N. A.; Chobanian, H. R.; He, J.; Guo, Y.; Dormer, P.
G.; Jones, C. M.; Steves, J. E. J. Org. Chem. 2010, 75, 1733.
15) Clary, J. W.; Rettenmaier, T. J.; Snelling, R.; Bryks, W.; Banwell,
(
J.; Wipke, W. T.; Singaram, B. J. Org. Chem. 2011, 76, 9602.
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Int. Ed. 2007, 46, 5359.
AUTHOR INFORMATION
Corresponding Author
E-mail: juha.heiskanen@oulu.fi.
Notes
■
*
(17) Drozdov, F. V.; Myshkovskaya, E. N.; Susarova, D. K.; Troshin,
P. A.; Fominykh, O. D.; Balakina, M. Y.; Bakirov, A. V.; Shcherbina, M.
A.; Choi, J.; Tondelier, D.; Buzin, M. I.; Chvalun, S. N.; Yassar, A.;
Ponomarenko, S. A. Macromol. Chem. Phys. 2013, 214, 2144.
The authors declare no competing financial interest.
(
18) (a) DaSilveira Neto, B. A.; Sant’Ana Lopes, A.; Ebeling, G.;
ACKNOWLEDGMENTS
■
Goncalves, R. S.; Costa, V. E. U.; Quina, F. H.; Dupont, J. Tetrahedron
2005, 61, 10975. (b) Jin, Y.; Kim, Y.; Kim, S. H.; Song, S.; Woo, H. Y.;
Lee, K.; Suh, H. Macromolecules 2008, 41, 5548. (c) Lee, D. H.; Lee,
M. J.; Song, H. M.; Song, B. J.; Seo, K. D.; Pastore, M.; Anselmi, C.;
The authors thank Mrs. Pai
̈
vi Joensuu for HRMS data. Jenny
and Antti Wihuri Foundation, Magnus Ehrnrooth Foundation,
and Fortum Foundation are greatly acknowledged for funding
for J.P.H. and N.M.S. The Academy of Finland (Decision
Numbers 251823 and 268672) is gratefully acknowledged for
financial support for T.I.H, T.K., and P.V. In addition, T.I.H.
and T.K. acknowledge the support of the Graduate School of
Tampere University of Technology. Computing resources
provided by the CSC−IT Center for Science Ltd., administered
by the Finnish Ministry of Education, are acknowledged.
̈
Fantacci, S.; De Angelis, F.; Nazeeruddin, M. K.; Graetzel, M.; Kim, H.
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Tao, Y.; Wong, M. S. Chem. Commun. 2011, 47, 9471. (e) Kim, J.-H.;
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Bongini, A.; Maccagnani, P.; Ostoja, P.; Derue, G.; Lazzaroni, R.;
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DOI: 10.1021/acs.joc.5b02689
J. Org. Chem. XXXX, XXX, XXX−XXX