Metal-Free Oxidative Cross-Coupling Reaction of Thiophene Iodonium Salts with Pyrroles

Article abstract of DOI:10.1002/ejoc.201600791

The hypervalent-iodine-mediated oxidative metal-free cross-coupling reaction of thiophenes with various pyrroles was developed. The corresponding thiophene–pyrrole derivatives were obtained in moderate to high yields (up to 85 %). This method features hig

Full text of DOI:10.1002/ejoc.201600791

A Journal of
Accepted Article
Title: Metal-Free Oxidative Cross-Coupling Reaction of Thiophene Iodonium Salts
with Pyrroles
Authors: Koji Morimoto; Akira Nakamura; Toshifumi Dohi; Yasuyuki Kita
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To be cited as: Eur. J. Org. Chem. 10.1002/ejoc.201600791
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European Journal of Organic Chemistry
10.1002/ejoc.201600791
COMMUNICATION
Metal-Free Oxidative Cross-Coupling Reaction of Thiophene
Iodonium Salts with Pyrroles
Koji Morimoto,[a] Akira Nakamura,[b] Toshifumi Dohi[a] and Yasuyuki Kita*[a],[b]
Abstract: The hypervalent iodine mediated oxidative metal-free
cross-coupling reaction of thiophenes with various pyrroles has been
developed. The corresponding thiophenes-pyrrole derivatives were
obtained in moderate to high yields (up to 85%). This method
features a high efficiency and regioselectivity, and functional-group
tolerance. We further determined the highly planar characteristics of
the EDOT-pyrrole biaryls caused by the intramolecular hydrogen
bonding.
thiophene and pyrrole rings. On the other hand, modern
synthetic methods of the thienyl-pyrrole often rely on the
versatile use of the Pd-catalyzed coupling reaction.11 Thus,
thienyl-pyrroles have been prepared by the Stille or Suzuki
coupling of N-protected pyrrole stanannes or N-protected pyrrole
boronic acid with thienyl-halogens. However, these strategies
also have a limitation due to the substrates, since both the
pyrrole-based metals and halides are unstable and reliant on the
proper selection of the nitrogen-protecting group. Recently, the
Pd-catalyzed direct arylation of heteroaromatics using aryl
halides as coupling partners has been shown to be a very
powerful method for the synthesis of arylated heterocycles.12
Although these methods are more attractive than the other Pd-
catalyzed cross-coupling reactions as they avoid the preparation
of an organometallic derivative, the thienyl-pyrrole synthesis
including the C-H activation method has scarcely reported.13
We now disclose the efficient oxidative synthesis of thiophene-
pyrroles by using an appropriate choice of hypervalent iodine
reagent. In addition, we have extensively characterized the high
planarity of a new EDOT-pyrrole dimer determined by an X-ray
structural analysis (Scheme 1).
Introduction
The biaryl moiety as a key structures was found in many
natural products, bioactive molecules, and functional materials
due to their unique physical properties.1 Therefore, the
development of new and efficient cross-coupling reaction to
prepare biaryls has attracted a great deal of attention.2 Among
them, a large number of straightforward methods that involve the
oxidative coupling processes of two molecules of
unfunctionalized aromatic compounds has been reported using
oxidizing agents to produce these important biaryl compounds.3
Recently, we reported the oxidative biaryl coupling reaction of
aromatic compounds4 and the coupling reaction of electron-rich
heteroaromatic compounds5,6 using hypervalent iodine(III)
reagents. However, the efficient oxidative thienyl-pyrrole
synthesis has not yet been achieved. This is because the control
of the coupling products to avoid any unwanted oligomerization
is particularly difficult due to their particularly lower oxidation
potentials.7
The conventional approach for the thienyl-pyrrole synthesis of
both the thiophene and pyrrole units involves the use of 1,4-
dicarbonyl compounds.8,9 In the case of the thiophenes, the 1,4-
dicarbonyl compounds are reacted with sulfur sources.10 By
similar reactions, the pyrroles have been prepared via the
condensation of 1,4-dicarbonyl compounds with nitrogen
sources, i.e., a reaction known as the Paal–Knorr synthesis.8
These methods, although widely employed, are generally limited
to the 2,5- di- or poly-substituted thienyl-pyrroles at the
Scheme 1. The oxidative metal-free synthesis of functionalized EDOT-
pyrroles of high planarity by an enhanced intramolecular hydrogen bonding
(EWG = electron-withdrawing group).
Results and Discussion
Recently, thienyl-pyrroles and their derivatives have received
much attention as useful precursors for modifying the properties
of corresponding polymers,10 due to the ease of introducing
substituents at the nitrogen atom into the pyrrole ring. On the
other hand, the more successfully conducting polymers are
[a]
Dr. K. Morimoto, Dr. T. Dohi and Prof. Dr. Y. Kita
Ritsumeikan University
poly(3,4-ethylenedioxythiophene)
(PEDOT)14,15
and
College of Pharmaceutical Sciences
1-1-1 Nojihigashi, Kusatsu, Shiga 525-8577, Japan
Fax:(+81)-77-561-5829
polypyrrole.16 The PEDOT shows a rigid coplanar backbone
stabilized by the intramolecular S---O interactions.17 Therefore,
we consider that the possible intramolecular O--H--N hydrogen
bond interaction of the EDOT-pyrrole might lead to finding a
novel precursor of the oligomer and polymer. The development
of a convenient synthesis of EDOT-pyrrole is important in
organic chemistry, as these mixed dimers are expected
precursors for synthesing regioregular conducting polymer.18
Thus, we initially examined the coupling reaction of EDOT 1
using the commercially available pyrrole 2a as a model
E-mail: kita@ph.ritsumei.ac.jp
[b]
Dr. A. Nakamura, Prof. Dr. Y. Kita
Ritsumeikan University
Research Organization of Science and Technology,
1-1-1 Nojihigashi, Kusatsu, Shiga, 525-8577,Japan
Tel. +81-77-561-5829; Fax. +81-77-561-5829
Supporting information for this article is given via a link at the end of
the document.

European Journal of Organic Chemistry
10.1002/ejoc.201600791
COMMUNICATION
substrate under several conditions (Table 1, Eq. 1). The reaction
didn’t proceed using the common inorganic oxidants such as
Fe(III) chloride or Mo(V) chloride.19 We found that the coupling
reaction took place when 2 equivalents of TMSBr and 1
equivalent of PhI(OH)OTs as the oxidant in (CF3)2CHOH (HFIP)
were used. The coupling product was produced in 51% yield.4a
The yield was improved to 65% yield using phenyliodine
bis(trifluoroacetate) (PIFA) as the oxidant (entry 4). We
evaluated other iodine(III) reagents, but the use of PIFA
provided superior results to the use of other iodine reagents,
such as phenyliodine diacetate (PIDA) (entry 5). An another
fluoroalcohol as TFE was less effective than HFIP (entry 6). The
strongly Lewis acidic triflate or TMSI gave poor results (entries 7
and 8). Other ordinary organic solvents, such as CH3CN and
dichloromethane gave no coupling product. The advantages of
the present method include the use of readily accessible starting
materials, complete regiochemical control over the substituent
placement, and metal-free methodologies. We also confirmed
that the reaction did not proceed under other oxidative
conditions which were recently reported using metal catalysts,
i.e., Pd salts with stoichiometric inorganic oxidants, such as
Cu(OAc)2 or AgOAc.20
Encouraged by the results, the syntheses of different
substituted thienyl-pyrroles were performed as shown in Table 2.
Not only the N-H pyrrole 2a, but also the N-methylpyrrole 2b and
N-phenylpyrrole 2c were readily coupled with EDOT 1 in good
yields. In the case of the 3-phenyl pyrrole 2d, the coupling
product 3d was obtained as a single regioisomer. The synthesis
of low band gap (Eg) materials having broad absorption bands
represents one of the most efficient strategies to develop
organic solar cells with higher efficiencies.21 Therefore, we
directed our attention toward the synthesis ofthienyl-
pyrroles having electron-donating group (EDG) and electron-
withdrawing group (EWG) as the substituents. These
approaches to electronic heteroaromatic materials are
desireable for reducing the band gap of polymers which is
influenced by the HOMO and the acceptor LUMO energies.22,23
Therefore, we challenged the coupling reaction of EDOT with
pyrroles having EWG. As a result, pyrroles 2e and 2f having an
electron-withdrawing group gave the desired coupling products
3e and 3f in good yields, respectively. The higher alkyl
substituent 2g did not affect the yield of the product 3g. The
cyano and sulfonyl substituted pyrroles 2i and 2j gave the
coupling products 3i and 3j in moderate yields, respectively.
Table 1. Optimization of reaction conditionsa)
Table 2. Scope of substrates.a, b)
Entry
1
Reaction Condition
Time
24 h
Yieldb) (%)
n.d.
FeCl3
(CF3)2CHOH
(HFIP)
MoCl5
HFIP
PhI(OH)OTs,
TMSBr
HFIP
2
3
24 h
3 h
n.d.
51
PIFA, TMSBr
HFIP
4
//
65
PIDA, TMSBr
HFIP
5
6
7
8
//
41
13
PIFA, TMSBr
TFE
3 h
PIFA, TMSOTf
HFIP
24 h
24 h
n.d.
trace
PIFA, TMSI
HFIP
[a] Performed using EDOT 1 (1 equiv), PIFA (1 equiv), TMSBr (2 equiv) in
HFIP at room temperature. [b] Pyrrole 2 (1.5 equiv) and TMSBr (2 equiv) was
added after EDOT 1 reacted with PIFA in HFIP.
[a] Reaction conditions: EDOT 1 (0.4 mmol), pyrrole 2a (1.2 mmol), TMSBr
(0.8 mmol), and iodine reagent (0.4 mmol) in solvent (4 mL) at room
temperature. [b] Yields are those for the isolated coupling product.

European Journal of Organic Chemistry
10.1002/ejoc.201600791
COMMUNICATION
stirring and then stirred for an additional 3 h under the same conditions,
while the reaction progress was checked by TLC. Saturated aqueous
sodium hydrogen carbonate was added to the mixture when the reaction
completed. The aqueous phase was extracted with CH2Cl2. The extract
was dried over anhydrous Na2SO4 and then evaporated to dryness. The
crude residue was purified by column chromatography on silica-gel
(eluent: n-hexane/ CH2Cl2) to give the pure EDOT-pyrrole dimer 3a (65%
yield)
The crystallographic structure of a single crystal of EDOT-
pyrrole 3f has been analyzed by X-ray diffraction (Figure 1).24
Interestingly, the structure of the molecule is perfectly planar
with the thiophene and pyrrole rings in an anti-conformation as a
result of the strong hydrogen bonding between the EDOT
oxygen and pyrrole NH group. Thus, the N5---O3 distance of
2.764 Å is considerably shorter than the sum of the van der
Waals radii of nitrogen and oxygen (3.07 Å). This result means
that the intramolecular O---N hydrogen bond interaction by the
aid of the electron-withdrawing ester group on the pyrrole ring
leads to the self-planarization and rigidification of the π-
conjugated system.16 The observed downfield shift of the N-H
hydrogen peak of 1H-NMR spectroscopy clearly suggest the
strong hydrogen bond even in organic solvent.
2-(2,3-Dihydrothieno[3,4-b]-1,4-dioxin-5-yl)-1H-pyrrole (3a)
Brown oil; 1H NMR (400 MHz, CDCl3):  9.08 (1H, brs), 6.77-6.78 (1H, m),
6.29-6.30 (1H, m), 6.19-6.21 (1H, m), 6.10 (1H, s), 4.28-4.31 (2H, m),
4.21-4.23 (2H, m) ppm; 13C NMR (100 MHz, CDCl3):  141.5, 135.4,
125.5, 117.5, 111.1, 109.0, 104.6, 94.9, 65.0, 64.6 ppm; IR (KBr): 3427,
3109, 2980, 2929, 2872, 1693, 1551, 1497, 1445, 1366, 1175, 1070, 910,
795, 719, 654 cm-1; HRFABMS: calcd for C10H9NO2S [M]+ 207.0354,
found 207.0357.
d = 2.764 Å
Figure 2. X-ray structure of
highly planer dimer 3f
Methyl
pyrrole-3-carboxylate (3f)
5-(2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)-4-phenyl-1H-
Yellow solid; mp 143-144 oC; 1H NMR (400 MHz, CDCl3): 9.64 (1H, brs),
7.46 (1H, d, J = 3.2 Hz), 7.32-7.41 (5H, m), 5.99 (1H, s), 4.31-4.33 (2H,
m), 4.19-4.21 (2H, m), 3.62 (3H, s) ppm; 13C NMR (100 MHz, CDCl3): 
164.8, 140.8, 136.8, 134.3, 131.1, 128.0, 127.5, 124.2, 123.1, 121.9,
115.3, 109.3, 97.5, 65.2, 64.4, 50.8 ppm; IR (KBr): 3416, 2991, 2945,
2872, 1712, 1520, 1442, 1070, 750 cm-1; HRFABMS calcd for
C18H15NO4S [M]+ 341.0722, found 341.0712.
Figure 1. X-ray structure of highly planer dimer 3f
Acknowledgements
This work was partially supported by Grants-in-Aid for Scientific
Research (A) from the Japan Society for the Promotion of
Science (JSPS), a Grant-in-Aid for Scientific Research on
Innovative Areas ‘‘Advanced Molecular Transformation by
Organocatalysts’’ from The Ministry of Education, Culture,
Sports, Science and Technology (MEXT). K. M. also
acknowledges support from the Grant-in-Aid for Young
Scientists (B) from the JSPS.
Conclusions
In summary, the EDOT-pyrroles 3 were effectively synthesized
by the oxidative coupling reaction using an appropriate
hypervalent iodine reagent. Since this coupling method is quite
simple and practical, it is compatible with the synthesis of a
wide variety of related derivatives. Spectroscopic studies and
thermal characterization revealed that the structure of the
heteroaromatic moiety significantly influences these properties.
Utilizing the high planarity of the EDOT-pyrrole, the oligomer
and polymer of the EDOT-pyrrole would provide novel physical
properties. The electronic properties of the oligomers based on
these new building blocks are under investigation and will be
reported in future publications.
Keywords: Cross-coupling• Iodine • EDOT • Pyrrole
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European Journal of Organic Chemistry
10.1002/ejoc.201600791
COMMUNICATION
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CCDC 1035911 contains the supplementary crystallographic data for
this paper. These data can be obtained free of charge from The
Cambridge
Crystallographic
Data
Centre
via
www.ccdc.
cam.ac.uk/data_request/cif. For more detailed crystallographic data,
see the CIF.
[12]

European Journal of Organic Chemistry
10.1002/ejoc.201600791
COMMUNICATION

European Journal of Organic Chemistry
10.1002/ejoc.201600791
COMMUNICATION
Layout 2:
COMMUNICATION
Biaryl*
Koji Morimoto, [a] Akira Nakamura, [b]
Toshifumi Dohi,[a] and Yasuyuki Kita*[a],
[b]
Page No. – Page No.
The efficient hypervalent iodine induced oxidative cross-coupling reamaction of
3,4-ethylenedioxythiophene (EDOT) with various pyrroles has been succeeded by
optimization of the reagent. The corresponding EDOT-pyrrole derivatives were
obtained in moderate to high yields (up to 85%). This method features a high
efficiency and regioselectivity, and functional-group tolerance.
Metal-Free Oxidative Cross-Coupling
Reaction of Thiophene Iodonium
Salts with Pyrroles
*one or two words that highlight the emphasis of the paper or the field of the study