Synthesis of Phenanthro[9,10-c]thiophenes by 2,3-Dichloro-5,6-dicyano-1,4-benzoquinone (DDQ)-Promoted Cyclo-Oxidation

Article abstract of DOI:10.1002/cplu.201900099

A new method to prepare phenanthro[9,10-c]thiophenes has been developed. In the presence of triflic acid, 3,4-diaryl thiophenes undergo 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ)-promoted cyclo-oxidation. NMR and computational studies indicate that p

Full text of DOI:10.1002/cplu.201900099

Communications
DOI: 10.1002/cplu.201900099
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Synthesis of Phenanthro[9,10-c]thiophenes by
2,3-Dichloro-5,6-dicyano-1,4-benzoquinone
(DDQ)-Promoted Cyclo-Oxidation
Han Gao,[a] David M. Connors,[a, b] and Nancy S. Goroff*[a]
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A new method to prepare phenanthro[9,10-c]thiophenes has
been developed. In the presence of triflic acid, 3,4-diaryl
thiophenes undergo 2,3-dichloro-5,6-dicyano-1,4-benzoquinone
(DDQ)-promoted cyclo-oxidation. NMR and computational
studies indicate that protonation of the thiophene plays a key
role in this reaction. The reaction can be used to prepare
phenanthro[9,10-c]thiophene, as well as derivatives with alkyl,
bromo, and methoxy substituents. However, the yields and
selectivity of the reaction depend on both the nature and
location of the substituents. Bis(3-methoxyphenyl)thiophene
reacts under these conditions to give the desired product in
Figure 1. Benzo[c]thiophene, phenanthro[9,10-c]thiophene, and their conju-
gated polymer products.
57% yield, while bis(4-methoxyphenyl)thiophene gives no
product. Bis(3-bromophenyl)thiophene did not react, but cyclo-
oxidation of bis(4-bromophenyl)thiophene provides the desired
product in 34% yield.
characterized and processed. However, there have been very
few reported syntheses of phenanthro[9,10-c]thiophene or its
derivatives.
Arene-fused thiophenes have been used in polymers for
applications in organic field-effect transistors (OFETs), light-
emitting diodes (OLEDs) and solar cells (OSCs).[1] The bandgap
of poly(isothianaphthene) (Figure 1), for example, has been
measured at 1.0–1.2 eV, which is almost 1 eV smaller than that
of poly(3-hexylthiophene) (P3HT), due to the increased aromatic
stabilization in the quinoidal form of the polymer.[2]
Phenanthrene-fused thiophenes have aromatic stabilization
energies intermediate between those of thiophene and benzo
[c]thiophene.[3] making them promising starting materials for
conjugated polymers. Kathirgamanathan and Shepherd exam-
ined the polymerization of phenanthro[9,10-c]thiophene (1)
under electrochemical conditions, and obtained a red conduct-
ing material.[4] They reported that this polymeric material
changes color depending on oxidation state. The polymer was
not soluble, and there was no further characterization beyond
the electrochemical properties. These results led us to target
soluble derivatives of this polymer that can be more completely
The most straightforward approach to the phenanthrothio-
phenes is by oxidative cyclization of diaryl thiophenes (2,
Scheme 1); this route also benefits from the synthetic accessi-
bility of diaryl thiophenes with a variety of substituents.[5]
However, Schnapperelle and Bach attempted the cyclo-oxida-
tion of compound 2 under a wide variety of conditions, and
found no reaction.[6] To overcome this problem, they prepared
the more complicated desymmetrized chlorodiarylthiophene 3
and successfully converted it to phenanthrothiophene 1 by
irradiation with 254-nm light. Other approaches that have been
used to make phenanthro[9,10-c]thiophene and derivatives
include Hindsburg condensation of a diester sulfide with
phenanthroquinone;[7] ring closure using sodium sulfide and a
phenanthryl dihalide[8] or tetrahalide;[4] and Stille coupling of a
9-stannafluorene with a dibromothiophene.[9] To develop a
more efficient and versatile synthesis for substituted phenan-
threnes, we decided to reexamine the simple cyclo-oxidation of
diarylthiophenes.
Diaryl thiophene derivatives can be synthesized by a variety
of methods.[4,6,8,10] Dang and Chen reported the efficient syn-
[a] H. Gao, Dr. D. M. Connors, Prof. N. S. Goroff
Department of Chemistry
Stony Brook University
Stony Brook, NY 11794-3400 (USA)
E-mail: nancy.goroff@stonybrook.edu
[b] Dr. D. M. Connors
Department of Chemistry
Georgia State University
Atlanta, GA 30302 (USA)
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/cplu.201900099
This article is part of a Special Issue on “Novel Aromatics”.
Scheme 1. Approaches to synthesize phenanthro[9,10-c]thiophene
ChemPlusChem 2019, 84, 1–5
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the reaction of compound 2b (Table 1), based on Rathore’s
conditions for conversion of activated terphenyls to
triphenylenes.[11] Methanesulfonic acid and fluoroboric acid
provide only trace amounts of the desired product 1b, while
Lewis acid BF3-OEt2 gives none. Triflic acid, however, works well,
especially when an excess of DDQ is used. Reducing the
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temperature to À 40 C improves the yield further, as does
increasing the dilution to discourage dimer formation. Under
optimized conditions, this reaction can be carried out in 47%
isolated yield.
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To further investigate the scope of this cyclo-oxidation,
several different diaryl thiophenes have been studied (Table 2).
These experiments demonstrate that both the substituents and
the regiochemistry affect the success of the reaction. Compar-
ing the methoxy-substituted substrates 2c–2e, bis(4-methox-
yphenyl) thiophene (2d) gives none of the desired phenanthro-
thiophene (1d), with recovery of 43% of starting material, while
bis(3-methoxyphenyl) thiophene (2c) undergoes reaction in
good yield, although forming an inseparable mixture of both a
symmetric and asymmetric product (Scheme 3); bis(3,4-dime-
thoxyphenyl) thiophene (2e) reacts in high yield to give
product 1e. For the bromine-substituted compounds, the
selectivity is the opposite: bis(4-bromophenyl) thiophene (2g)
reacts in reasonable yield, but reaction of bis(3-bromophenyl)
thiophene (2f) leads to no identifiable products and recovery of
40% of the starting material.
Scheme 2. Synthetic route to diaryl thiophenes.
thesis of diaryl thiophenes in three steps from the bromoaceto-
phenone (4).[5e] Based on their approach, we prepared a range
of diarylthiophenes (Scheme 2). First, the bromoacetophenone
(4a–g) reacts with sodium sulfide to provide the diketone
sulfide (5a–g) in high yield. Intramolecular McMurry coupling of
the diketone leads to a mixture of diaryl dihydrothiophene and
the desired diaryl thiophene. Dang and Chen used CuBr2 to
oxidize this mixture fully to the diaryl thiophene, but we found
that subjecting the crude product mixture to 2,3-dichloro-5,6-
dicyano-1,4-benzoquinone (DDQ) oxidation leads to clean
production of the diaryl thiophene (2a–g) in good yield.
Table 2. Reaction scope.
To find conditions for the DDQ-promoted cyclo-oxidation of
the diaryl thiophenes, several acid catalysts were screened in
Table 1. Cyclo-oxidation reaction conditions.
Aryl
57%[a]
No Product
Conc. of 2b
[mmol/L]
Acid
Name
DDQ
[eq.]
T
[h]
Temp. Yields
Yield
Aryl
34%
80%
47%
Vol%
10
°
1
2
3
10
10
10
MsOH
1.0
1.0
1
1
1
0 C
Trace
1b
Trace
1b
°
HBF4À OEt2 10
0 C
°
BF3À OEt2
10 eq. 1.0
0 C
Recover
2b
Yield
No Product
34%
15%[b]
35%[b]
36%[b]
39%[c]
20%[c]
°
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10
5
20
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TfOH
TfOH
TfOH
TfOH
TfOH
TfOH
TfOH
TfOH
TfOH
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10
10
10
10
10
5
1.0
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1
1
0 C
[a] Total yield for a mixture of 1c and 1c’, as shown in Scheme 3.
°
0 C
°
48 0 C
1
°
0 C
°
1
0 C
24 À 40 C 47%[c]
°
24 À 40 C 42%[c]
°
24 À 40 C 26%[c]
°
1
10
°
24 À 78 C Recover
2b
Scheme 3. Reaction of bis(3-methoxyphenyl)thiophene yields an inseparable
mixture of two phenanthrothiophenes, the symmetric product 1c and the
asymmetric product 1c’.
[a] The volume of dichoromethane (DCM) is 9 mL. [b] Yield determined by
NMR spectroscopy. [c] Yield of isolated product.
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Investigations of the mechanism of the Scholl reaction of
diaryl benzenes by King and co-workers implicated a proto-
nated arenium cation as a key intermediate.[12] To investigate
whether a similar mechanism might be operative here, we
calculated the proton affinity of diphenyl thiophene 2a at
various positions. DFT calculations (B3LYP/6-311+ +G**) indicate
that the carbon next to sulfur has the largest proton affinity
(Figure 2). According to these calculations, the proton affinity of
phenanthro[9,10-c]thiophene is 14 kcal/mol greater than that of
o-terphenyl.
CDCl3 leads to the appearance of a new peak at 5.66 ppm,
consistent with a protonated thiophene. In addition, there is an
evident loss of symmetry in the molecule, with the peaks
corresponding to the protons on the benzene rings splitting
into separate signals. These peaks also move to higher
frequency (7.5–7.7 ppm vs 7.1–7.3 ppm), consistent with being
adjacent to a protonated thiophene. In contrast, addition of the
much weaker acid methanesulfonic acid[13] to compound 2b in
chloroform leads to no change in the proton NMR peaks,
consistent with the poor catalyzing power of MsOH.
The calculations and NMR data are consistent with a ring-
closing mechanism that starts with protonation of the
thiophene ring at the 2-position to form arenium cation i
(Scheme 4). Protonation increases the electrophilicity of the
benzene ring ortho to the protonated site, leading to
nucleophilic attack by the other benzene ring to form
intermediate ii. Oxidation by DDQ and deprotonation give the
final phenanthro[9,10-c]thiophene.
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NMR experiments support the presence of a protonated
arenium intermediate in the triflic acid-catalyzed reaction (Fig-
ure 3). Adding 10% triflic acid to a solution of compound 2b in
In summary, 3,4-diaryl thiophenes can be converted directly
into phenanthro[9,10-c]thiophenes when subjected to DDQ
oxidation in the presence of triflic acid. This route allows access
to
a variety of substituted phenanthro[9,10-c]thiophenes.
Figure 2. Relative proton affinity (kcal/mol) at each unique position in
diphenyl thiophene (2a). Based on DFT calculations (B3LYP/6-311+ +G**).
Computational and NMR studies suggest that protonation of
the thiophene ring is a key step in the reaction. Further
exploration of the reaction scope and mechanism is ongoing.
Acknowledgements
We acknowledge the Stony Brook Foundation for funding of this
research.
Conflict of Interest
The authors declare no conflict of interest.
Keywords: arenes · cyclization · heterocycles · oxidation ·
polycycles
Figure 3. 1H NMR spectra (11.0–5.0 ppm) of compound 2b (a) in CDCl3; (b) in
10% methanesulfonic acid in CDCl3; and (c) in 10% trilflic acid in CDCl3.[14]
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[3] DFT calculations (B3LYP/6-311+ +G**) indicate the aromatic stabilization
energy for phenanthro[9,10-c]thiophene is 2.6 kcal/mol greater than
that of thiophene.
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Scheme 4. Proposed mechanism for the formation of phenanthro[9,10-c]
thiophene.
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[14] Full 1H NMR spectra can be found in the Supporting Information,
Figures S1–3.
Manuscript received: February 8, 2019
Revised manuscript received: April 25, 2019
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COMMUNICATIONS
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Unexpected pleasure: Diaryl thio-
phenes, previously reported to be
inert to 2,3-dichloro-5,6-dicyano-1,4-
benzoquinone (DDQ), undergo cyclo-
oxidation to the corresponding
phenathro[9,10-c]thiophenes in the
presence of DDQ and triflic acid. The
reaction works in good yield with a
variety of substituents on the aryl
groups (Br, OMe, t-Bu), but it is
sensitive to the regiochemistry of
substitution.
H. Gao, Dr. D. M. Connors, Prof. N. S.
Goroff*
1 – 5
Synthesis of Phenanthro[9,10-c]thi-
ophenes by 2,3-Dichloro-5,6-
dicyano-1,4-benzoquinone (DDQ)-
Promoted Cyclo-Oxidation
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