Rhodium-catalyzed synthesis of unsymmetric di(heteroaryl) compounds via heteroaryl exchange reactions

Article abstract of DOI:10.1080/10426507.2019.1602621

Unsymmetric di(heteroaryl) HetAr–X–HetAr′ compounds have flexible and rigid groups, and are expected to exhibit various biological activities by interacting with proteins and nucleic acids. Then, synthesis of such compounds is critical for the development of drugs. Unsymmetric HetAr-X-HetAr′ compounds were efficiently synthesized by rhodium-catalyzed heteroaryl exchange reactions, which involved equilibrium control by judicious design of organic heteroaryl reagents. This method allows synthesis of unsymmetric HetAr–O–HetAr′, HetAr–S–HetAr′, and HetAr–CH₂–HetAr′ compounds as well as HetAr–F compounds from heteroaryl aryl ethers and heteroaryl reagents. The rhodium-catalyzed heteroaryl exchange reaction was also applied to the synthesis of C–N-linked di(heteroaryl) compounds from N-benzoyl heteroarenes and heteroaryl aryl ethers. The synthesis has a broad applicability, which gives a diversity of novel unsymmetric HetAr–X–HetAr′ and C–N-linked di(heteroaryl)s compounds containing five- and six-membered heteroarenes.

Full text of DOI:10.1080/10426507.2019.1602621

Phosphorus, Sulfur, and Silicon and the Related Elements
ISSN: 1042-6507 (Print) 1563-5325 (Online) Journal homepage: https://www.tandfonline.com/loi/gpss20
Rhodium-catalyzed synthesis of unsymmetric
di(heteroaryl) compounds via heteroaryl exchange
reactions
Mieko Arisawa
To cite this article: Mieko Arisawa (2019): Rhodium-catalyzed synthesis of unsymmetric
di(heteroaryl) compounds via heteroaryl exchange reactions, Phosphorus, Sulfur, and Silicon and
the Related Elements, DOI: 10.1080/10426507.2019.1602621
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PHOSPHORUS, SULFUR, AND SILICON AND THE RELATED ELEMENTS
https://doi.org/10.1080/10426507.2019.1602621
Rhodium-catalyzed synthesis of unsymmetric di(heteroaryl) compounds via
heteroaryl exchange reactions
Mieko Arisawa
Department of Organic Chemistry, Graduate School of Pharmaceutical Sciences, Tohoku University, Aoba, Sendai, Japan
ABSTRACT
ARTICLE HISTORY
Unsymmetric di(heteroaryl) HetAr–X–HetAr0 compounds have flexible and rigid groups, and are
expected to exhibit various biological activities by interacting with proteins and nucleic acids.
Then, synthesis of such compounds is critical for the development of drugs. Unsymmetric HetAr-
X-HetAr0 compounds were efficiently synthesized by rhodium-catalyzed heteroaryl exchange reac-
tions, which involved equilibrium control by judicious design of organic heteroaryl reagents. This
method allows synthesis of unsymmetric HetAr–O–HetAr0, HetAr–S–HetAr0, and HetAr–CH2–HetAr0
compounds as well as HetAr–F compounds from heteroaryl aryl ethers and heteroaryl reagents.
The rhodium-catalyzed heteroaryl exchange reaction was also applied to the synthesis of C–N-
linked di(heteroaryl) compounds from N-benzoyl heteroarenes and heteroaryl aryl ethers. The syn-
thesis has a broad applicability, which gives a diversity of novel unsymmetric HetAr–X–HetAr0 and
C–N-linked di(heteroaryl)s compounds containing five- and six-membered heteroarenes.
Received 28 March 2019
Accepted 29 March 2019
KEYWORDS
Heteroaryl exchange
reaction; rhodium-catalyzed
reaction; di(heteroaryl)
sulfide; di(heteroaryl)
methane; di(heteroaryl)
ether; C–N-linked
bi(heteroaryl)s
GRAPHICAL ABSTRACT
Introduction
Unsymmetric HetAr–X–HetAr0 compounds possess rigid
heteroaryl structures and two rotating flexible sp2–C/sp3–X
bonds, which significantly enhance structural diversity in
binding to proteins and nucleic acids. Chiral pseudohelical
structures are observed for unsymmetric HetAr–X–HetAr0
compounds in the solid state, as shown in our studies. For
example, the ORTEP views of 6-chloro-2-(furylthio)benzoxa-
zole possesses the chiral pseudohelical structure with a syn-
conformation with regard to the sulfur atom in thiophene
and the nitrogen atom in benzoxazole (Figure 3).[3] It is
then expected that various novel bioactive substances can be
developed using this structure of compounds.
Diaryl Ar–X–Ar0 compounds with
a one-atom linker
between two aryl groups, and aryl heteroaryl HetAr–X–Ar
compounds, in which an aryl group in an Ar–X–Ar0 com-
pounds is substituted with a heteroarene, are important for
drugs, often noted as the privileged structure. Examples are
zaltoprofen, dimenhydrinate, XK469, and papaverine
(Figure 1).[1] It is therefore a logical extension to consider
the utility of unsymmetric di(heteroaryl) HetAr–X–HetAr0
compounds with two different heteroarenes.
Unsymmetric HetAr–X–HetAr0 compounds possessing a
one heteroatom linker X, such as oxygen, sulfur, and carbon,
can be important scaffold for functional organic molecules
characterized by structural diversity at their HetAr groups as
well as at their linker X groups (Figure 2).[2]
With regard to synthesis, unsymmetric Ar–X–Ar0 and
HetAr–X–Ar compounds generally have been synthesized by
the nucleophilic substitution reaction of aryl halides and
metalated arylmethyl, arylthio, or aryloxy reagents;[4]
CONTACT Mieko Arisawa
arisawa@m.tohoku.ac.jp
Department of Organic Chemistry, Graduate School of Pharmaceutical Sciences, Tohoku University,
Aoba, Sendai, 980-8578, Japan.
Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/gpss.
Supplemental data for this article can be accessed on the publisher’s website.
ß 2019 Taylor & Francis Group, LLC

2
M. ARISAWA
Scheme 1. Conventional synthesis of unsymmetric HetAr–X–Ar compounds
Figure 1. Pharmaceutical products with Ar–X–Ar’/HetAr–X–HetAr0 structure.
Scheme 2. Rh-catalyzed heteroaryl exchange
Figure 2. HetAr–X–HetAr0 structure.
Figure 4. Equilibrium control using organic co-substrate S0 & co-product P0.
developed
for
the
synthesis
of
unsymmetric
HetAr–X–HetAr0 compounds (Scheme 2). Such a method
exhibited broad substrate applicability, in which reactivities
are relatively unaffected by the structures of hetero-
aryl groups.
As part of our study to develop synthetic methods for
organoheteroatom compounds, transition-metal catalysis was
shown effective to realize the catalytic cleavage and exchange
between two heteroaryl groups, which are not highly react-
ive.[6] Applying this catalytic method, various unsymmetric
HetAr–X–HetAr0 compounds containing five- and six-mem-
bered heteroarenes were effectively obtained.
A catalyst changes the course of a reaction without affect-
ing the relative thermodynamic stability of the substrates S
and products P. Then, the reaction must be exergonic, i.e.,
energetically downhill, to obtain a high product yield and a
reasonable reaction rate.
The exchange reaction of two covalent bonds often
becomes an equilibrium reaction. Therefore, in order to
obtain high chemical yield and reasonable reaction rate, it is
necessary to develop equilibrium control in consideration of
the relative thermodynamic stability of the substrate S and
the product P (Figure 4).
Figure 3. The ORTEP views of 6-chloro-2-(thienylthio)benzoxazole.
alternatively, substitution reaction of arylmetal reagents and
arylmethyl halides or arylthio chlorides has also been
employed.[5] These methods employ metal reagents or bases.
In contrast to unsymmetric Ar–X–Ar and HetAr–X–Ar
compounds, unsymmetric HetAr–X–HetAr0 compounds are
rare owing to the lack of efficient and versatile synthetic
method. The lack was ascribed to diverse reactivities of het-
eroaryl compounds depending on their structure. (1) The
reactivity of heteroaryl halides is significantly affected by
their heteroaryl structure. (2) Heteroaryl compounds have
diverse acidities, and reaction of their conjugate base should
also be diverse. (3) Reactivity differences depending on the
position of heteroatoms in heteroarenes complicate the reac-
tion, and the reaction sites can change depending on the
substrate and reaction conditions. (4) The low solubility and
stability of metalated heteroaryl reagents in organic solvents
generally makes the reaction heterogeneous and complex.
Such difficulties in the synthesis of unsymmetric
HetAr–X–HetAr0 compounds are associated with the use of
metal reagents and metal bases.
The development of a judicious combination of the
organic co-substrate/organic co-product S0/P0 was consid-
ered.[7] The equilibrium between S and P can be shifted to
P in the presence of S0 forming P0, when the relative
thermodynamic stability of the P/P0 system is higher than
that of the S/S0 system (Figure 4). The advantage of the
organic co-substrate/co-product S0/P0 method is that the
Thus,
a
synthetic
method
for
unsymmetric
HetAr–X–HetAr0 compounds utilizing covalent bond
exchange involving cleavage and exchange reactions between
two organoheteroaryl compounds, which does not use metal
reagents or metal bases, is considered attractive. In this art-
icle, rhodium-catalyzed heteroaryl exchange reactions reaction can be tuned using various combinations of S0
between heteroaryl aryl ethers and heteroaryl reagents were and P0.

PHOSPHORUS, SULFUR, AND SILICON AND THE RELATED ELEMENTS
3
Table 1. Synthesis of HetAr–S–HetAr0 compounds.
Scheme 3. Organothiolating reagent
It may also be worth noted here that an inorganic co-
substrate such as metal bases can also be used to make an
exothermic reaction. Since the metal base has a large chem-
ical energy, it becomes a large exothermic reaction, and then
consume a large chemical energy. In contrast, an exothermic
reaction design utilizing an organic co-substrate without
using bases can realize energy-saving reaction.
methanesulfonic acid gave slightly exothermic, and use of
thioanisole or N-(phenylthio)succinimide became an endo-
thermic. The analysis suggests that unsymmetric
HetAr–S–HetAr0 compounds could be synthesized without
using metal reagents or metal bases by the judicious design
of organic substrates.
Unsymmetric HetAr–X–HetAr0 compounds were synthe-
sized using judicious combinations of the substrates S and
organic co-substrates S0: An exothermic reaction is devel-
oped, and equilibrium is shifted to the desired product P.
This is a novel methodology for the catalytic synthesis of
unsymmetric HetAr–X–HetAr0 compounds in an energy-
saving manner. In addition, reactions are relatively insensi-
tive to heteroarene structures, and the reactions have broad
applicability.[2]
Accordingly, unsymmetric HetAr–S–HetAr0 compounds
are obtained by the rhodium-catalyzed heteroaryl exchange
reaction of heteroaryl aryl ethers HetAr–O–Ar and S-(heter-
oaryl) thiobenzoates PhC(O)–SHetAr0 using neutral and sta-
ble organic substrates (Table 1). The method is applied to
various five- and six-membered heteroarenes, including tria-
zyl, quinazolyl, pyridyl, bennzoxazolyl, benzothiazolyl, furyl,
thienyl groups. Esters are formed as the organic co-products,
which can easily be recovered and used for other purposes.
Studies of the biological activities of the unsymmetric
HetAr–S–HetAr0 compounds are under way.
Results and discussion
Synthesis of HetAr–S–HetAr0 compounds
Unsymmetric diaryl sulfides Ar–S–Ar0 are widely used for
materials and drugs, and their heteroaryl derivatives, unsym-
metric di(heteroaryl) sulfides HetAr–S–HetAr0, are an inter-
esting group of multiple heteroatom compounds that can
exhibit notable chemical and biological functions. Synthesis
by conventional aromatic nucleophilic substitution reactions
in the presence of bases, however, is restricted to di(pyridyl)
derivatives.
Synthesis of HetAr–CH2–HetAr0 compounds
Di(aryl/heteroaryl)methane HetAr–CH2–HetAr compounds
are much less common compared with di(heteroaryl)
HetAr–HetAr0 compounds, which can be obtained by cross
coupling methods. The former compounds can be an
important scaffold for functional organic molecules, charac-
terized by a more flexible structure.
In order to design an appropriate combination of sub-
strates and co-substrates to synthesize unsymmetric
HetAr–CH2–HetAr0 compounds, the reactions of diphenyl
ether and different benzylating reagents were examined by
preliminary DFT calculations.[2] The reaction of diphenyl
ether and phenyl benzyl ketone to form diphenylmethane
and phenyl benzoate is considerably exothermic (ꢀ67.4 kJ/
mol) (Scheme 4). Another reaction to form phenyl benzyl
ether and benzophenone is slightly endothermic (þ2.4 kJ/
mol) and unlikely to occur. Use of toluene gave slightly exo-
thermic, and use of thioanisole or N-(benzyl)succinimide
became an endothermic. The analysis suggests that diphenyl-
methane can be synthesized from diphenyl ether and phenyl
Developed in this study was synthesis of unsymmetric
HetAr–S–HetAr0 by
a rhodium-catalyzed heteroarylthio
exchange reaction of heteroaryl aryl ethers and S-(hetero-
aryl) thioesters.[3] The reaction has a broad applicability,
giving diverse unsymmetric di(heteroaryl) sulfides contain-
ing five- and six- membered heteroarenes. No base is
required in this reaction, because of the judicious design of
organic substrates.
It was considered interesting to use the less chemical
energy derived from neutral organic compounds in place of
bases, and the reactions of diphenyl ether with different
phenylthiolating reagents were examined by calculations
(Scheme 3).[2] Thermodynamic analysis by preliminary DFT
calculations shows that the reaction of diphenyl ether and S- benzyl ketone.
It was initially confirmed that a rhodium catalyst can
phenyl thiobenzoate to form diphenyl sulfide and phenyl
benzoate is exothermic (ꢀ39.5 kJ/mol). Use of S-phenyl cleave the C(O)–C bond of di(benzyl) ketones (Scheme 5).

4
M. ARISAWA
Table 3. Synthesis HetAr–O–HetAr0.
Scheme 4. Benzylating reagent
Table 4. Synthesis HetAr–F.
Scheme 5. Rh-catalyzed C(O)–C bond cleavage
Table 2. Synthesis HetAr–CH2–HetAr0.
substrate is used in accordance with Le Chatelier0s principle
(Table 3). Various unsymmetric HetAr–O–HetAr0 com-
pounds containing five- and six-membered heteroarenes
were synthesized.
A rhodium complex cleaved the CO–C bond of dibenzyl
ketone and transferred intermolecularly.[8]
On the basis of the result, the rhodium-catalyzed synthe-
sis of unsymmetric HetAr–CH2–HetAr0 compounds was
developed by the reaction of aryl heteroaryl ethers and het-
eroarylmethyl ketones (Table 2).[9] A wide range of elec-
tron-deficient/electron-rich five/six-membered heteroaryl
ethers and heteroarylmethyl ketones were efficiently
employed. The broad scope of this reaction is due to the use
of stable substrates for both heteroaryl aryl ethers and heter-
oarylmethyl ketones. This method involves cleavage and for-
mation of the C–C bond without requiring the use of metal
reagents or metal bases.
Synthesis of HetAr–F compounds
Heteroaryl fluorides HetAr–F are important as drugs owing
to their unique biological activity derived from the unique
properties of fluorine. A rhodium-catalyzed method was
applied to the synthesis of HetAr–F, which converted the
C–O bond of heteroaryl aryl ethers to the C–F bond.[11] The
reaction of (4-chlorophenylthio)pentafluorobenzene with
heteroaryl aryl ethers provided heteroaryl fluorides and het-
eroaryl (4-chlorophenylthio)tetrafluorophenyl ethers; this
involved the cleavage of a single heteroaryl C–O bond under
equilibrium (Table 4).
Synthesis of HetAr–O–HetAr0 compounds
The reported synthesis of unsymmetric diheteroaryl ethers,
i.e., HetAr–O–HetAr0 compounds, has a quite limited scope
of the heteroaryl moiety such as several pyridyl ethers. On
the basis of the results of the synthesis of unsymmetric
HetAr–S–HetAr0 and HetAr–CH2–HetAr0 compounds
described above, unsymmetric HetAr–O–HetAr0 compounds
were synthesized from heteroaryl aryl ethers and hetroaryl
benzoate.[10] The relative thermodynamic stabilities of the
substrates and products are comparable in this reaction,
and, in order to increase the chemical yield of unsymmetric
Notably, both heteroaryl and aryl C–O bonds in a hetero-
aryl aryl ether were converted to C–F bonds, when 2-fluoro-
benzothiazole 1 was employed as the organic fluorinating
reagent (Scheme 6). The reaction of 1 (2 equiv.) and hetero-
aryl aryl ether 2 gave heteroaryl fluoride 3 and 4-fluoroben-
zonitrile 4, which was accompanied by [2,30 (20H)-
benzothiazol]-20-one 5. Note that the oxygen atom of 1 was
captured by 2, which resulted in the formation of the
dimeric compound 5. The reverse reaction did not occur,
HetAr–O–HetAr0 compounds, an excess amount of a which indicated an exothermic nature of the reaction. The

PHOSPHORUS, SULFUR, AND SILICON AND THE RELATED ELEMENTS
5
Figure 5. Rh-catalyzed synthesis of unsymmetric di(heteroaryl) compounds.
Scheme 6. Use of 2-fluorobenzothiazole
the reactions, aryl benzoates, are readily separated and
recycled. The unsymmetric di(heteroaryl) HetAr–X–HetAr0
compounds obtained in this study are mostly novel com-
pounds and are expected to be used for the development of
new drugs.
Table 5. Synthesis of heteroaryl azoles, azolones, ureas, and imides.
Acknowledgments
Author is grateful thanks to Prof. Masahiko Yamaguchi for advice on
experimental design, and thanks to Assistant Prof. Saori Tanii for her
efforts in this study.
Funding
This work was supported by the Platform Project for Supporting Drug
Discovery and Life Science Research from AMED under Grant
JP18am0101100, and JSPS KAKENHI Grant Nos. 17K19112
and 15H00911.
fluorination of various five- and six-membered heteroaryl
aryl ethers proceeded in high yields.
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Synthesis of C–N-linked di(heteroaryl)s
The rhodium-catalyzed method was applied to the synthesis
of C–N-linked bi(heteroaryl)s, which are often important for
drugs. The reaction involves the covalent bond-exchange
reaction of N–CO and HetAr–O bonds without using metal
reagents or bases, and exhibits a broad applicability, giving
diverse C–N-linked bi(heteroaryl)s containing five- and six-
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azolones, pyridones, cyclic ureas, and cyclic imides using
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Conclusions
Unsymmetric HetAr–X–HetAr0 compounds have flexible
and rigid groups, and are expected to exhibit various bio-
logical activities by interacting with proteins and nucleic
acids. Rhodium-catalyzed heteroaryl exchange reactions are
a novel and efficient method of synthesizing unsymmetric
HetAr–O–HetAr0, HetAr–S–HetAr0, and HetAr–CH2–HetAr0
compounds from heteroaryl aryl ethers and various hetero-
aryl reagents (Figure 5). These reactions can also be used to
synthesize HetAr–F compounds and C-N-linked di(heter-
oaryl) compounds. It is worth noted that the byproducts of

6
M. ARISAWA
[5] For examples, synthesis of diaryl sulfides, (a) Yonova, I. M.;
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[8] Arisawa, M.; Kuwajima, M.; Toriyama, F.; Li, G.; Yamaguchi, M.
Rhodium-Catalyzed Acyl-Transfer Reaction between Benzyl
Ketones and Thioesters: synthesis of Unsymmetric Ketones by
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Org. Lett. 2012, 14, 3804–3807. DOI: 10.1021/ol3017148.
[9] Li, G.; Arisawa, M.; Yamaguchi, M. Rhodium-Catalyzed
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[10] Tanii, S.; Arisawa, M.; Tougo, T.; Horiuchi, K.; Yamaguchi,
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[11] Arisawa, M.; Tanii, S.; Tazawa, T.; Yamaguchi, M. Rhodium-
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[12] Tanii, S.; Arisawa, M.; Tougo, T.; Yamaguchi, M. Catalytic
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[7] Arisawa, M. Synthesis of Organosulfides Using Transition-
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