Synthesis of Unsymmetric Monosubstituted and Disubstituted Dinaphthothiophenes

Article abstract of DOI:10.1002/jhet.3002

Dinaphthothiophenes (DNTs) are a class of compounds with potential uses in organic semiconductors and the synthesis of unsymmetric catalysts. Symmetrical or asymmetrical addition of functional groups to the DNT structure may be desired for steric bulk in binaphthyl catalyst synthesis or tuning the electronic properties of semiconductors. Thus, versatility of functional group addition is a great asset in DNT synthesis. Until now, no versatile and concise methods for the synthesis of unsymmetrically substituted DNTs have been reported. Herein, we report three synthetic routes for the creation of three different classes of DNTs. Each route involves the successive addition of two functionalized styryl groups to a thiophene ring, followed by a photocyclization to form the desired asymmetric DNT. Various novel unsymmetrically monosubstituted and disubstituted dinaphtho[2,1-b:1′,2′-d]thiophenes, dinaphtho[1,2-b:1′,2′-d]thiophenes, and dinaphtho[1,2-b:2′,1′-d]thiophenes were synthesized from 2-bromothiophene,2,4-dibromothiophene, and 3,4-dibromothiophene in three or four steps. These methods can be used to synthesize a wide variety of unsymmetrically functionalized DNTs.

Full text of DOI:10.1002/jhet.3002

Month 2017
Synthesis of Unsymmetric Monosubstituted and Disubstituted
Dinaphthothiophenes
*
John C. Throgmorton, Satyanarayana M. Chintala, and Ryan D. McCulla
Department of Chemistry, Saint Louis University, 3501 Laclede Avenue, Saint Louis, Missouri 63103,
USA
*
E-mail: rmccull2@slu.edu
Received January 19, 2017
DOI 10.1002/jhet.3002
Published online 00 Month 2017 in Wiley Online Library (wileyonlinelibrary.com).
Dinaphthothiophenes (DNTs) are a class of compounds with potential uses in organic semiconductors and
the synthesis of unsymmetric catalysts. Symmetrical or asymmetrical addition of functional groups to the
DNT structure may be desired for steric bulk in binaphthyl catalyst synthesis or tuning the electronic
properties of semiconductors. Thus, versatility of functional group addition is a great asset in DNT synthesis.
Until now, no versatile and concise methods for the synthesis of unsymmetrically substituted DNTs have
been reported. Herein, we report three synthetic routes for the creation of three different classes of DNTs.
Each route involves the successive addition of two functionalized styryl groups to a thiophene ring,
followed by a photocyclization to form the desired asymmetric DNT. Various novel unsymmetrically
monosubstituted and disubstituted dinaphtho[2,1-b:10,20-d]thiophenes, dinaphtho[1,2-b:10,20-d]thiophenes,
and dinaphtho[1,2-b:20,10-d]thiophenes were synthesized from 2-bromothiophene,2,4-dibromothiophene,
and 3,4-dibromothiophene in three or four steps. These methods can be used to synthesize a wide variety
of unsymmetrically functionalized DNTs.
J. Heterocyclic Chem., 00, 00 (2017).
INTRODUCTION
been used as precursors to axially chiral 1,10-binaphthyl
catalysts, which play a large role in asymmetric synthesis
[6–8]. In addition, dibenzothiophene S-oxide (DBTO)
and its derivatives are a class of compounds suggested to
release O(3P) upon irradiation with UV light [9–12].
DNT S-oxides and other fused ring thiophene S-oxides
Dinaphthothiophenes (DNTs) are a class of compounds
structurally similar to thiophene-based organic
semiconductors [1–3] and have shown promise for use in
p-type organic semiconductors [4,5]. DNTs have also
© 2017 Wiley Periodicals, Inc.

J. C. Throgmorton, S. M. Chintala, and R. D. McCulla
Vol 000
have been investigated for their potential to release atomic
oxygen during irradiation at longer wavelengths [13].
Despite their potential applications, few efficient ways to
prepare asymmetrically substituted DNTs have been
reported.
method of Morrison and Musgrave, which is the sole
method to require only one step to achieve the cyclization
precursor, lacks the ability to generate unsubstituted
DNTs and has only been used to make diphenyl
substituted DNT-2112 [19].
All three classes of DNTs have been synthesized from
the flash vacuum pyrolysis of diethynyl/dichlorovinyl-
diphenylthiophenes in 7–89% yield [23]. The flash
Dinaphtho[2,1-b:10,20-d]thiophene
dinaphtho[1,2-b:10,20-d]thiophene
“DNT-2112,”
“DNT-1212,” and
dinaphtho[1,2-b:20,10-d]thiophene “DNT-1221” (Fig. 1)
are three types of DNTs whose syntheses have previously
been reported. DNT-2112 has been synthesized from the
Newman–Kwart rearrangement of dithiocarbamates by
heating the dimethylthiocarbamate of binaphthol neat at
285–310°C to give the DNT-2112 in 20–40% yield
[8,14,15], from dinaphthyl sulfide using an iodine-
catalyzed photocyclization in 85% yield [16] and from
cyclization of alkynes by heating ethynyl sulfides in
benzene at 200°C in a cascade cycloaromatization with
10% yield [17]. Rabindran and Tilak performed the
pyrolysis
method
has
some
capability
for
functionalization of the DNT structure. However, it is
only able to functionalize symmetrically, which limits the
potential for the tuning of the electronic properties of
DNTs by tuning functional groups. Furthermore,
Tedjamulia et al. prepared all three classes of DNTs from
formyl-benzonaphthothiophenes
[24].
A
Horner–
Wadsworth–Emmons reaction was used to add a styrene
unit, followed by an iodine-catalyzed photocyclization
that gave yields of 45–76%. The synthetic route created
here has potential for use in asymmetric DNT
substitution, because the DNT core structure is assembled
one-half at a time; yet controlling the final position of the
functional group would be difficult because of the
variability of products of the final photocyclization step.
This method also has the disadvantage of requiring three
to six steps to reach the photocyclization precursor.
While DNTs have previously been asymmetrically
functionalized, this has always been carried out after the
synthesis of the DNT structure (Fig. 2). Cho et al. have
taken DNT-2112 and functionalized the 6-position of
DNT-2112 by use of tert-butyllithium and iodine [8]. In
addition, n-butyllithium and dimethylformamide (DMF)
have been used to add an aldehyde group, also at the
condensation
of
2-bromo-1-tetralone
with
2-
naphthalenethiol or 1-naphthalenethiol, followed by
cyclization with P2O5 in phosphoric acid and
dehydrogenation with selenium, giving DNT-2112 and
DNT-1212 in 78% and 76% overall yield, respectively
[18]. Morrison and Musgrave used the condensation of
thiophene with 1,2-diphenylethanone to give (E,E)-2,5-
bis(α-phenylstyryl)thiophene [19]. The phenyl-substituted
distyrylthiophene was then photocyclized with iodine to
give the diphenyl substituted DNT-2112 in 10% yield.
The drawbacks of these preparations of DNT-2112 are
that they give low overall yields and take two to four
steps to prepare.
DNT-1221 derivatives have been made from the
reaction of naphthalene-1-sulfonic acid dimethylamide
with n-butyllithium and S8 in 29–37% yield [20]. In
addition, both DNT-2112 and DNT-1221 have been
synthesized from dinaphthyl sulfides using a potassium
tert-butoxide or n-butyllithium induced cyclodehydro-
genation in 18–31% yield [21,22]. These syntheses of
DNT-1221 suffer from low overall yields. While the final
cyclization reactions to synthesize all three varieties of
DNTs usually require only one step, anywhere from one
to six steps may be required to synthesize the precursors
needed for the cyclization reaction from commercially
available materials. In addition, the methods requiring
fewer steps to reach the cyclization precursor tend to
have a more limited scope of synthesis. For example, the
Figure 2. Dinaphthothiophene functionalization.
Figure 1. Example dinaphthothiophenes.
Journal of Heterocyclic Chemistry
DOI 10.1002/jhet

Month 2017
Synthesis of Unsymmetric Monosubstituted and Disubstituted
Dinaphthothiophenes
Scheme 1. Synthesis of dinaphtho[1,2-b:10,20-d]thiophenes.
Table 1
Suzuki–Miyaura reactions.
Entry
Bromothiophene
Product
Equiv R─B(OH)2
Temp. (°C)
Water (% of solvent)
Yield (%)
1
2
3
4
5
6
7
8
9
2,4-Dibromothiophene
2,4-Dibromothiophene
1
2
3
4
5
6
7
8
15
1.1
1.1
1.2
1.1
1.2
1.2
1.1
1.1
1.2
95
90
80
85
55
80
75
75
80
11
20
17
17
17
20
17
17
11
81
29
46
10
48
36
30
31
78
1
1
1
2
2
2
2-Bromothiophene
6-position [25,26]. This indicates that the six (and eight)
positions of DNT-2112 are selectively deprotonated by
bases such as n-butyllithium, leaving the other positions
on the DNT rings unable to be so similarly substituted.
In short, a variety of synthetic routes have previously
been reported to produce unfunctionalized and a few
functionalized DNTs. However, none of these methods
begin with the thiophene ring and, therefore, require a
greater number of steps to reach the DNT structure.
Furthermore, these methods do not provide a simple way
to unsymmetrically incorporate functional groups onto
the DNT structure. Herein are shown straightforward novel
synthetic routes by which to synthesize unsymmetrically
substituted derivatives of DNT-2112, DNT-1212, and
DNT-1221 (Fig. 1), from monosubstituted and dibromine-
substituted thiophenes. Each of these routes involves only
three to four steps in total and contains the potential for the
symmetric and unsymmetric introduction of a wide variety
of functional groups.
d]thiophene, and dinaphtho[1,2-b:20,10-d]thiophene were
created by three different routes. Each route followed the
same general strategy, beginning with two styrene groups
being sequentially added to
dibrominated thiophene by either Suzuki coupling or
Horner–Wadsworth–Emmons reaction. Once the
a monosubstituted or
thiophene was doubly substituted with styrene units, a
photocyclization reaction with iodine as an oxidant was
used to create the final DNT structure. In contrast to other
synthetic routes, unsymmetrically substituted functional
groups have been incorporated into the synthesis of the
DNT structure itself. Methoxy, trifluoromethyl, and
methyl functional groups were used to create substituted
Table 2
2,4-Distyrylthiophene cyclization.
Time
(days)
Yield
(%)
Entry Distyrylthiophene Product
1
2
3
4
5
6
3
4
5
6
7
8
9
0.8
0.6
1.9
0.8
0.8
0.7
24
66
24
31
20
28
10
11
12
13
14
RESULTS AND DISCUSSION
The derivatives of three different classes of thiophenes:
dinaphtho[2,1-b:10,20-d]thiophene, dinaphtho[1,2-b:10,20-
Journal of Heterocyclic Chemistry
DOI 10.1002/jhet

J. C. Throgmorton, S. M. Chintala, and R. D. McCulla
Vol 000
Scheme 2. Synthesis of dinaphtho[2,1-b:10,20-d]thiophenes.
DNTs. These three groups were chosen for their
differences in electronegativity, which could be used to
tune the electronic properties of DNTs.
Table 3
Horner–Wadsworth–Emmons reaction with 2-formyl-5-styrylthiophene.
DNT-1212 derivatives were synthesized using the path
shown in Scheme 1. This synthetic path used for the
synthesis of unsymmetrically substituted DNT-1212
derivatives began with 2,4-dibromothiophene. The
bromine in the 2-position is preferentially substituted
over the 4-position in carbon–carbon coupling reactions
to give an unsymmetric product [27–32]. Therefore, a
Suzuki–Miyaura reaction with one equivalent of [(E)-2-
phenylethenyl]boronic acid or [(E)-2-[4-(methyl)phenyl]
ethenyl]boronic acid was performed to add the first
styrene unit to give 4-bromo-2-styrylthiophenes 1 and 2
(Table 1). A second Suzuki–Miyaura reaction was used
to add a second styryl group in the 4-position. This
second coupling was successful with 4-substituted
styrylboronic acids to give 2,4-distyrylthiophenes 3–8
(Table 1). These Suzuki–Miyaura couplings gave yields
anywhere from 10 to 81% depending on the substituent
on the boronic acid. Reactions were performed at
temperatures ranging from 55°C to 95°C; however, no
significant change in yield was noticed. Unsubstituted
styrylboronic acids gave the highest yields, followed by
trifluoromethyl-substituted styrylboronic acids. Methoxy-
substituted boronic acids gave the lowest yields. In the
next step of this synthetic route, 2,4-distyrylthiophenes
3–8 were irradiated with short-wave ultraviolet (UVC)
light in the presence of iodine and propylene oxide to
fuse the rings, giving DNTs 9–14 (Table 2). This
photoreaction proceeds by an oxidative mechanism: first,
a photoinduced electrocyclization to create the C─C
bond, followed by an oxidative dehydrogenation to
regain aromaticity [33]. The photocyclization of 2,4-
distyrylthiophenes gave yields of 20–31%, with the
exception of 4. The photocyclization resulting in
compound 4 gave a higher yield of 66%, in contrast to
most other reactions performed involving methoxy-
Equiv
phosphonic
ester
Equiv
NaH
Yield
(%)
Entry Aldehyde
Product
1
2
3
16
16
16
17
18
19
1.3
1.9
1.2
5.5
3.3
4.7
58
14
76
Table 4
2,5-Distyrylthiophene cyclization.
Time
(days)
Yield
(%)
Entry Distyrylthiophene Product
1
2
3
17
18
19
20
21
22
6
27
1.5
1
<1%
29
substituted reactants, which had significantly lower yields
on average than those involving different substituents.
The 4-substituted trans-2-(phenylethenyl)boronic acids
used in the Suzuki–Miyaura coupling step often
underwent self-coupling rather than coupling with the
bromothiophene. This created a side product, which was
detected by gas chromatography/mass spectrometry
(GCMS), with an m/z dependent on the boronic acid
used. For example, Suzuki reactions involving
unsubstituted styrylboronic acids in Table 1 (entries 1, 3,
and 6), gave a product with a m/z of 206 and was
believed to be 1,4-diphenyl-1,3-butadiene. The
purification of the desired products was complicated by
the presence of these self-coupled byproducts, especially
in the case of 7. Propylene oxide was added to all
photocyclizations to quench the hydrogen iodide resulting
from the reaction. In every photoreaction, care was taken
not to irradiate the solution past completion, which would
result in both a decreased yield and a white precipitate
Journal of Heterocyclic Chemistry
DOI 10.1002/jhet

Month 2017
Synthesis of Unsymmetric Monosubstituted and Disubstituted
Dinaphthothiophenes
Scheme 3. Synthesis of dinaphtho[2,1-b:20,10-d]thiophenes.
that was insoluble in organic solvents. In the absence of
iodine, most photoreactions still occurred; however, they
proceeded more slowly. In the synthesis of both DNT-
1221 derivatives and DNT-2112 derivatives, reactions
involving a methoxy substituent gave lower yields than
those involving other substituents.
DNT-2112 derivatives 20–22 were created using the
synthetic route shown in Scheme 2. First, 2-
bromothiophene was coupled with a trans-2-(4-Phenyl)
vinylboronic acid using a Suzuki–Miyaura coupling to
create compound 15 in 76% yield [34]. The 5-position of
the thiophene ring was then formylated using n-
butyllithium and DMF, giving compound 16 in 36%
yield [35,36]. The 5-position is preferentially formylated
because of its relatively low pKa (~33) compared with
the three or four positions (~39) resulting from its
location next to the sulfur in the thiophene ring [37]. A
of reactants containing the methoxy group gave lower
yields. In addition, 2,5-distyrylthiophenes gave lower
photocyclization yields than other distyrylthiophenes.
Because DNT-2112 is known to adopt
a twisted
conformation, these lower yields are likely due to sterics
hindering the cyclization of 2,5-distyrylthiophene into the
planar shape of other fused thiophene structures [13].
DNT-1221 derivatives 32–36 were synthesized using
the route shown in Scheme 3. Formylation with n-
butyllithium and DMF was used to convert 3,4-
Table 5
Suzuki–Miyaura reaction with 3-bromo-4-formylthiophene.
Equiv
R─B
Temp. Yield
Entry Bromothiophene Product (OH)2
(°C)
(%)
Horner–Wadsworth–Emmons reaction using
a
4-
1
2
3
23
23
23
24
25
26
1.2
1.5
1.2
95
70
86
77
46
86
substituted phosphonic acid diethyl ester was used to add
a second styryl group to the other side of the thiophene ring
(Table 3) to create 2,5-distyrylthiophenes17–19. The
methoxy-substituted phosphonic acid diethyl ester gave a
14% yield that was significantly lower than the methyl
and trifluoromethyl-substituted phosphonic esters, which
gave yields of 58 and 76%, respectively. The DNTs
20–22 were created via the same oxidative photocyclization
used in the synthesis of DNTs 9–14 (Table 4).
The Horner–Wadsworth–Emmons reaction of 16 to
yield 2,5-distyrylthiophenes17–19 gave unreliable yields.
Different variables, such as molar equivalents of sodium
hydride and temperature during reagent addition, were
changed with no consistent improvement in yield. The
Horner–Wadsworth–Emmons reaction to give 18 (14%
yield) and the subsequent photocyclization to give 21
(<1% yield) gave significantly lower yields than the
reactions to produce 17 and 19. The photocyclization of
18 did not yield enough 21 to completely characterize,
although it was detected by GCMS. This follows the
trend seen in Table 1 (entries 4 and 7) and Table 2
(entry 5) where the Suzuki coupling and photocyclization
Table 6
Horner–Wadsworth–Emmons reaction with 3-formyl-4-styrylthiophene.
Equiv phosphonic
ester
Yield
(%)
Entry Aldehyde
Product
1
2
3
4
5
24
24
24
25
26
27
28
29
30
31
1.5
1.2
1.5
1.5
1.0
16
21
95
32
40
Table 7
3,4-Distyrylthiophene cyclization.
Entry
Distyrylthiophene
Product
Time (h)
Yield (%)
1
2
3
4
5
27
28
29
30
31
32
33
34
35
36
8
7
20
18
9
6.5
7.5
5
3.5
6
Journal of Heterocyclic Chemistry
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J. C. Throgmorton, S. M. Chintala, and R. D. McCulla
Vol 000
Figure 3. Products of 3,4-distyrylthiophene photoreaction.
dibromothiophene to 3-bromothiophene-4-carbaldehyde23
in 77% yield [38,39]. Suzuki–Miyaura coupling was then
used to add the first styryl group to one side of the
thiophene ring to give 3-formyl-4-styrylthiophenes24–26
in 10–77% yield (Table 5), from which unsymmetric
distyrylthiophenes could easily be synthesized. The
trifluoromethyl-substituted styrylboronic acid gave the
highest yields. A Horner–Wadsworth–Emmons reaction
was used to add the second substituted styryl group to the
other side of the thiophene ring (Table 6), creating
3,4-distyrylthiophenes27–31 in yields from 16% to 95%.
The CF3-substituted benzylphosphonic esters used in the
creation of 29 and 30 gave higher yields compared with
those with other substituents. The Suzuki–Miyaura
coupling was performed before the Horner–Wadsworth–
Emmons reaction in this route because the 1,4-
diphenyl-1,3-butadiene byproducts formed from the
Suzuki–Miyaura reaction were easier to separate from the
more polar 3-formyl-4-styrylthiophenes than from 3,4-
distyrylthiophenes. An oxidative photocyclization was
then used in the same manner as the previous routes to
fuse the rings together to give DNT-1221 derivatives
32–36 in yields of 6–20% (Table 7).
d]thiophenes, dinaphtho[2,1-b:10,20-d]thiophenes, and
dinaphtho[1,2-b:20,10-d]thiophenes from 2,4-dibromo-
thiophene,2-bromothiophene, and 3,4-dibromothiophene,
respectively. Distyrylthiophenes were created unsym-
metrically by adding one styrene unit to the thiophene
ring at a time. The first styrene was added by Suzuki–
Miyaura coupling, while the second styrene as added by
either Suzuki–Miyaura or Horner–Wadsworth–Emmons
reactions, depending on the desired DNT. The 2,4-
distyrylthiophenes,2,5-distyrylthiophenes,
and
3,4-
distyrylthiophenes were fused to form their respective
DNTs using an oxidative photocyclization. Using these
methods, a variety of asymmetric monosubstituted and
disubstituted DNTs can be synthesized in three to four
straightforward steps. Whereas only symmetrically
substituted DNTs had been synthesized before, now a
wide variety of unsymmetrically substituted DNTs can be
easily made and tuned for use in organic semiconductors
and chiral 1,10-binaphthyl catalysts.
Acknowledgments. A portion of this work was supported by the
National Science Foundation under CHE-1255270. The authors
thank the National Science Foundation for support under grant
CHE-0963363 for renovations to the research laboratories in
Monsanto Hall. This work was additionally supported by donors
to the Herman Frasch Foundation.
A side product of the photoreaction of 27–31 is
suspected to form by the ring closure of the thiophene
ring as shown in Figure 3, forming side products 37–41.
The side product for Table 7, entry 1 (37), was analyzed
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no significant change in the product ratios. Preparative
thin-layer chromatography was used to separate the
photocyclization reaction products.
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SUPPORTING INFORMATION
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composition, polymerizable or curable composition, and optical material
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Additional Supporting Information may be found online
in the supporting information tab for this article.
Journal of Heterocyclic Chemistry
DOI 10.1002/jhet