A Highly Efficient and Eco-Friendly Approach for the Synthesis of Triarylpyridine and Novel Triaryl-[1,3]Thiazepine Derivatives Via Ring Transformation and Expansion of Triarylthiopyrylium Salts

Article abstract of DOI:10.1080/10426507.2015.1054482

The paper reports, for the first time, thermally induced ring transformation and expansion of triarylthiopyrylium salts using a novel task-specific dicationic ionic liquid [1,1^′-(butane-1,4-diyl)-bis(3-methylimidazolium)] diazide as a nucleophi

Full text of DOI:10.1080/10426507.2015.1054482

Phosphorus, Sulfur, and Silicon and the Related Elements
ISSN: 1042-6507 (Print) 1563-5325 (Online) Journal homepage: http://www.tandfonline.com/loi/gpss20
A Highly Efficient and Eco-Friendly Approach
for the Synthesis of Triarylpyridine and
Novel Triaryl-[1,3]Thiazepine Derivatives
Via Ring Transformation and Expansion of
Triarylthiopyrylium Salts
Arash Mouradzadegun, Somayeh Elahi & Parastoo Ghanbarzadeh
To cite this article: Arash Mouradzadegun, Somayeh Elahi & Parastoo Ghanbarzadeh
(2015) A Highly Efficient and Eco-Friendly Approach for the Synthesis of Triarylpyridine
and Novel Triaryl-[1,3]Thiazepine Derivatives Via Ring Transformation and Expansion of
Triarylthiopyrylium Salts, Phosphorus, Sulfur, and Silicon and the Related Elements, 190:11,
2031-2039, DOI: 10.1080/10426507.2015.1054482
To link to this article: http://dx.doi.org/10.1080/10426507.2015.1054482
Published online: 02 Dec 2015.
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Date: 14 December 2015, At: 17:05

Phosphorus, Sulfur, and Silicon, 190:2031–2039, 2015
Copyright ꢀC Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426507.2015.1054482
A HIGHLY EFFICIENT AND ECO-FRIENDLY APPROACH
FOR THE SYNTHESIS OF TRIARYLPYRIDINE AND NOVEL
TRIARYL-[1,3]THIAZEPINE DERIVATIVES VIA RING
TRANSFORMATION AND EXPANSION OF
TRIARYLTHIOPYRYLIUM SALTS
Arash Mouradzadegun, Somayeh Elahi,
and Parastoo Ghanbarzadeh
Department of Chemistry, Faculty of Science, University of Shahid Chamran,
Ahvaz, Iran
GRAPHICAL ABSTRACT
Abstract The paper reports, for the first time, thermally induced ring transformation and
expansion of triarylthiopyrylium salts using a novel task-specific dicationic ionic liquid [1,1ꢁ-
(butane-1,4-diyl)-bis(3-methylimidazolium)] diazide as a nucleophile source. This method fur-
nished a straightforward route for the synthesis of valuable 2,4,6-triarylpyridine and 2,5,7-
triaryl-[1,3]thiazepine derivatives, which are widely present as motifs in an assortment of
biologically active molecules.
Keywords Task-specific dicationic ionic liquid; ring transformation and expansion of
thiopyrylium salts; bioactive heterocyclic compound; triarylpyridine; triaryl-[1,3]thiazepine
INTRODUCTION
Reducing the use of hazardous and toxic solvents, reagents, and catalysts in chemical
synthesis and avoiding the generation of waste is one of the most important challenges
presented by the effort to minimize pollution and risks associated with the production of
chemicals.1 Accordingly, organic reactions in ionic liquid have attracted a great deal of
interest in both academic and industrial research because of their synthetic efficiency and
Received 15 January 2015; accepted 20 May 2015.
Address correspondence to Dr Arash Mouradzadegun, Department of Chemistry, Faculty of Science,
University of Shahid Chamran, Ahvaz, Iran. E-mail: arash m@scu.ac.ir
Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/gpss.
2031

2032
A. MOURADZADEGUN ET AL.
environmental friendliness.2 In the last decade, dicationic ionic liquids have appeared as
promising and innovative media due to improved and unique properties such as higher
thermal stability, lower volatility, and more flexibility in tuning the physicochemical prop-
erties compared with corresponding monocationic ionic liquid.3 Modification of the cation
and/or anion with the appropriate functional groups often leads to a pronounced change
in the properties of the ionic liquid. Therefore, ionic liquids often are referred to as tailor-
made or tunable materials and are now expected to be designer liquids with controllable
physical and chemical properties or even specific function liquids such as catalyst and
reagent. Functionalized ionic liquids as organic reagents are very important areas of green
chemistry where current methods can lead to major environmental problems due to the
use of highly toxic reagents. Task-specific nitrite and azide ionic liquids were used for
the efficient one-pot synthesis of 1,2,3-triazoles from aniline derivatives, the use of these
organic reagents and procedure provides a fascinating potent and practical strategy for
the eco and efficient preparation of desired products without handling of highly toxic
chemicals.4
The synthesis of heterocyclic moieties has attracted tremendous attention due to
the utilization of heterocyclic compounds in biological, biochemical, pharmaceutical, and
asymmetric chemical investigations. In this context, six- and seven-membered rings con-
taining nitrogen and/or sulfur atoms are essential constituents of a range of products from
pharmacologically active substances to various materials.
Among this class of compounds, the pyridine ring system is one of the most prominent
six-membered heterocycles present in various natural products and exhibit a board range
of biological activities.5
Moreover, organic compounds containing thiazepine, as seven-membered molecular
scaffold, broadly applied to cure many kinds of diseases. Thiazepine scaffolds can also act
as intermediates for preparing clotiapine and quetiapine, which are atypical antipsychotics
approved for the treatment of depressive disorder, psychosis and bipolar disorder, and
schizophrenia.6
On the one hand, due to the unique position of 2,4,6-triarylpyridine derivatives in
pharmacological chemistry, a great deal of efforts have been drawn to develop new and
efficient synthetic routes for their generation in recent years.7 However, many of these
methods suffer from one or more drawbacks such as expensive catalyst, long reaction
times,7a–e and special care in handling and storing the reagents.7a–e
On the other hand, in spite of aforementioned advances in the synthesis and applica-
tion of thiazepine derivatives, there have been no or few reports on the synthesis of triaryl
substituted of these compounds so far. Hence, it is still of significant interest to explore
novel and efficient synthetic approaches for the preparation of these molecules.
A possible route for the synthesis of triarylpyridine and triarylthiazepine derivatives
is the nucleophilic reaction of triarylthiopyrylium salts with azide anion.8
Previous studies have revealed that the reactivity of azide anion with thiopyrylium
cations at room temperature depends on the substitution of heterocyclic cation. Only ster-
ically hindered thiopyrylium cations lead to the formation of a covalent azide, and tri-
substituted examples give only a donor–acceptor complex.8 The formation of this complex
constitutes a dead-end reaction on normal pathway to covalent azide. In this vein and in
continuation of our success in the development of simple and environment-friendly exper-
imental procedure for various important reactions and transformations,9 besides our new
interest in the synthesis of biologically important heterocyclic compounds,10 herein we re-
port, for the first time, the synthesis of triarylpyridine and triaryl-[1,3]thiazepine derivatives

RING TRANSFORMATION AND EXPANSION OF TRIARYLTHIOPYRYLIUM SALTS 2033
via ring transformation and expansion of tri-substituted thiopyrylium salts using dicationic
ionic liquid as an efficient reaction media, promoter, and reagent.
RESULTS AND DISCUSSION
It was decided in the present work to investigate an expedient route for the synthesis
of triarylpyridine and novel triaryl-[1,3]thiazepine derivatives through the nucleophilic
reaction of triarylthiopyrylium salts with azide anion.
Initially, the reaction of triphenylthiopyrylium perchlorate (1A) with sodium azide
was selected as the model reaction to optimize the requisite reaction condition. The ef-
fect of different protic and aprotic solvents such as ethanol, methanol, dichloromethane,
chloroform, acetonitrile, xylene, and tetrahydrofuran was investigated in model reaction
at room temperature. It was observed that all media remained inactive for the reaction
after prolonged stirring (24 h). Next, in order to evaluate the influence of temperature,
the model reaction was performed in different solvents under reflux condition for 24 h.
The reaction of model compound with sodium azide in boiling acetonitrile gave 2,5,7-
triphenyl-[1,3]thiazepine (3A) and 2,4,6-triphenylpyridine (4A) in low yield (5 and 8%,
respectively) along with considerable amount of starting material 1A and byproduct 2A. To
determine the appropriate concentration of reagent, the reaction of model compound with
varying amounts of sodium azide was investigated. The best result was obtained using two
equivalents of NaN3 per triphenylthiopyrylium perchlorate (1A).
As stated earlier, the formation of charge transfer complex constitutes a dead-end
reaction on the normal pathway to covalent azide and prevents the formation of desired
products.8 It is believed that the byproduct produced as a consequence of the competing
side reaction (I) occurs in parallel to the intended reaction (II). The two pathways are
summarized in Scheme 1.
For environmental concerns and due to the need for green solvent besides the sig-
nificant properties of compounds of type 3 and 4, continuation of the present work was
encouraged to synthesize these privileged compounds in a more efficient method with
the aim to prevent any concentration of the starting material 1A and the byproduct 2A.
In view of this, it was decided to modify the strategy and find suitable alternatives to
conventional organic solvents. Also, considering green credentials of ionic liquid and in
view of the remarkable ability of ionic liquids to efficiently promote a wide range of
conventional reactions, the exploration was started again by synthesizing [1,1’-(butane-
1,4-diyl)-bis(3-methylimidazolium)] dibromide as a highly polar and recyclable dicationic
ionic liquid and unambiguous structural elucidation, accomplished by spectroscopic data.
Following the characterization, the reaction of model compound with sodium azide in the
dicationic ionic liquid as a sustainable medium at different temperature was investigated.
Interestingly, the use of ionic liquid gave encouraging results at 130◦C. As illustrated in
Table 1, the reaction time was dramatically reduced and the yields of desired products
were considerably increased without any unreacted starting material 1A and byproduct
2A (entry 5). This is probably due to the good solubility of triphenylthiopyrylium salt
and sodium azide in the ionic liquid compared with organic solvent. Moreover, the capac-
ity of ionic environments to generate internal pressure and to promote the association of
reactants in solvent cavities renders them excellent media for this reaction. Thereby the
ionic liquid not only acts as a favorable reaction medium but also as a promoter of the
reaction.

2034
A. MOURADZADEGUN ET AL.
Table 1 The reaction of model compound with sodium azide in dicationic ionic liquid at different temperatures
Entry
Temperature (◦C)
Time
Yield (%) (3A, 4A)
1
2
3
4
5
6
80
24 h
24 h
24 h
24 h
40 min
40 min
5, 8
9, 11
10, 11
12, 14
30, 35
30, 35
100
110
120
130
140
It was found that increasing the temperature did not improve the reaction rate (Table 1,
entry 6), while reducing the temperature led to lower yield due to the formation of byproduct
2A (Table 1, entry 1–4).
It is noteworthy that owing to the high toxicity of sodium azide, this method is not
very environment-friendly.
To circumvent this dilemma and reduce the toxicity of azide ion, the attention was
shifted to the design, synthesis, and use of novel azide source that not only possesses
high activity and selectivity, but is also simultaneously benign to the environment. Ac-
cordingly, a new synthetic strategy commenced with the synthesis of novel task-specific
dicationic ionic liquid was carried out. Task-specific ionic liquid [1,1ꢁ-(butane-1,4-diyl)-
bis(3-methylimidazolium)] diazide was prepared by the anion exchange of [1,1ꢁ-(butane-
1,4-diyl)-bis(3-methylimidazolium)] dibromide with NaN3 in the absence of organic sol-
vents at room temperature.
The use of this novel task-specific dicationic ionic liquid as reagent decreases the
reaction time (Table 2, entry 2). A weak interaction between azide anion and imidazolium
cations, which increases the nucleophilicity of azide anion, might contribute to the sup-
pression of byproduct formation and to the rapid progress of nucleophilic reaction.
To the best of our knowledge, this is the first synthetic utility of a task-specific
dicationic ionic liquid as a nucleophile source for the green synthesis of heterocyclic
compounds.
These extraordinary results promoted us to demonstrate the efficiency and the ap-
plicability of the present method. Therefore, the reaction was performed for a variety of
triarylthiopyrylium salts containing electron-withdrawing groups (such as halide and nitro
groups) or electron-donating groups (such as alkyl and methoxy groups) with azide anion
under optimized reaction conditions (Table 3).
As is evident from Table 3, for series of 1B–D, the thiopyrylium ring containing
electron-donating groups reacted well with azide anion to give the corresponding products
3 and 4. It is interesting that the reaction of these substrates (1B–D) was more sluggish
than reaction of model compound (1A). This may be ascribed to the fact that these groups
Table 2 Reaction of the model compound 1A with two different types of reagents
Entry
Solvent
Nucleophile source
Time (min)
1
2
Dicationic ionic liquid
Dicationic ionic liquid
NaN3
Task-specific ionic liquid
40
20

RING TRANSFORMATION AND EXPANSION OF TRIARYLTHIOPYRYLIUM SALTS 2035
Table 3 The conversion of various triarylthiopyrylium salts (1) into the corresponding 2,5,7-triaryl-
[1,3]thiazepine (3) and 2,4,6-triarylpyridine derivatives (4) using task-specific ionic liquid as reagent under
optimized condition
Yield (%) Time
No.
A
Substrate (1)
Product (3)
Product (4)
(3)/(4) (min)
30/35
20
B
C
15/40
30
30/35
30/35
22
85
D
decrease the positive charge at the α-position of heterocyclic ring. The effect of electron-
withdrawing groups was also viewed clearly. The reaction of these substrates with azide
anion gave the corresponding charge transfer complex (2) as the sole product. This can
be rationalized by considering the fact that these groups increase the positive charge of
the heterocyclic ring, thereby increase its tendency for the formation of the charge-transfer
complex (2).
These results confirm that these reactions occur competitively by two pathways
involving the formation of the charge transfer complex I and the nucleophilic attack of
azide anion II. As shown in Scheme 2, the complex formation path is the main route for the
thiopyrylium ring containing electron-withdrawing groups, while the nucleophilic attack
path is the main route for the thiopyrylium ring containing electron-donating groups.
A conceivable mechanism for the synthesis of triaryl-[1,3]thiazepine (3) and tri-
arylpyridine (4) derivatives via a nucleophilic attack route (II) is outlined in Scheme 2.
The first step of the reaction affords 2-azidothiopyran (5), which undergoes ring opening,
leading to acyclic valence tautomer 6. Subsequently, thermal elimination of N2 molecule
from compound 6 followed by intramolecular nitrogen insertion into C-C double bond
(IIa) yields azirine 7, which readily rearranged to the stable 2,5,7-triaryl-[1,3]thiazepine
(3). Another plausible pathway that can occur after the release of N2 molecule from com-
pound 6 is the insertion of nitrogen atom into the C-S double bond (IIb), which affords

2036
A. MOURADZADEGUN ET AL.
Scheme 1 Two pathways for the reaction of model compound with sodium azide in boiling acetonitrile.
Scheme 2 Plausible mechanism for the reaction of triarylthiopyrylium salts (1) with azide nucleophile.
bicyclic thiaziridine 8. Eventually, extrusion of the sulfur atom from compound 8 leads to
the formation of 2,4,6-triarylpyridine (4).
The proposed mechanism is in agreement with the previously reported mechanism
for the nucleophilic attack of azide anion to the sterically hindered thiopyrylium salts.8
The structure of all the products was settled from their physical and spectroscopic
data (1H nuclear magnetic resonance (NMR), 13C NMR and mass spectrum (MS); see
Supplementary Materials, available online).
It is worthy to note that compounds 3 and 4 are of considerable interest since it is
for the first time that the synthesis of these privileged heterocyclics via ring expansion and
transformation of triarylthiopyrylium salts using a task-specific dicationic ionic liquid was
reported.
To confirm the stability of compound 3 under reaction conditions, the isolated product
was heated in the dicationic ionic liquid at 130◦C for a prolonged period. According to
thin-layer chromatography (TLC) analysis, there was no evidence for the transformation
of compound 3.

RING TRANSFORMATION AND EXPANSION OF TRIARYLTHIOPYRYLIUM SALTS 2037
Table 4 Comparison of the present method with literature methods for the synthesis of 2,4,6-Triphenyl-pyridine
via ring expansion
Entry
Reagent
Catalyst
Time
2 h
4 h
2 h
Ref.
1
2
3
NH4OAc
NH4OAc
NH4OAc
Bi(OTf)3
AcOH
7a
7b
7c
Pentafluorophenylammonium
triflate
4
5
6
7
NH4OAc
NH4OAc
NaN3
Ionic liquid bearing azide
anion
H14[NaP5W30O110
HClO4–SiO2
]
3.5 h
4 h
48 h
7d
7e
8
—
—
20 min
Present work
Table 4 confirms that this method presents the best currently available method in
terms of yield and time for the preparation of the 2,4,6-Triphenyl-pyridine.
CONCLUSIONS
As a conclusion from the present study, a convenient method was reported for the
preparation of valuable six- and seven-membered ring heterocycles (triarylpyridine and
triaryl-[1,3]thiazepine derivatives) using the new azide source in ionic solvent. To the best
of our knowledge, this is the first reported example for the formation of 2,4,6-triarylpyridine
and 2,5,7-triaryl-[1,3]thiazepine derivatives starting from thiopyrylium salts, as a simple and
readily available starting materials, in the green solvent. Further studies on the applications
of these compounds, which concluded from the heterocyclic structure, are currently under
way in our laboratory.
EXPERIMENTAL
General
Chemicals were purchased from Fluka, Merck, and Aldrich chemical companies.
Monitoring of the reactions was accomplished by TLC. Infrared (IR) spectra were ob-
tained on a Bomen MB:102 FT-IR spectrophotometer. 1H NMR and 13C NMR spectra were
recorded on a Bruker spectrometer at 400 and 100 MHz, respectively, in CDCl3 or D2O
with tetramethylsilane as an internal standard. MS spectra were measured on an Agilent
5975 Mass Spectrophotometer. The Supplemental Materials contain the complete charac-
terization and sample spectra for 3A–3D (Figures S1–S12). (All supplemental materials,
including Figures S1–S12, are available online under “Supplemental Materials.”)
Synthesis
Synthesis of Triarylthiopyrylium Perchlorates. All triarylthiopyrylium per-
chlorates were synthesized by the method described previously.11
Synthesis of [1,1ꢀ-(butane-1,4-diyl)-bis(3-methylimidazolium)] dibro-
mide. [1,1ꢁ-(butane-1,4-diyl)-bis(3-methylimidazolium)] dibromide was synthesized by
the method described previously.12

2038
A. MOURADZADEGUN ET AL.
Synthesis of [1,1ꢀ-(butane-1,4-diyl)-bis(3-methylimidazolium)] diazide.
[1,1ꢁ-(butane-1,4-diyl)-bis(3-methylimidazolium)] diazide was synthesized by the method
described previously for the synthesis of azide monocationic ionic liquid.4 Briefly, freshly
prepared [1,1ꢁ-(butane-1,4-diyl)-bis(3-methylimidazolium)]dibromide (1 mmol) and NaN3
(2 mmol) were added into deionized water (2 mL), and the mixture was stirred for 24 h at
room temperature. The solvent was removed under reduced pressure at 50◦C to obtain the
crude product containing azide ionic liquid and NaBr, which was washed with acetonitrile
(3 × 10 mL). The remaining acetonitrile was removed under high vacuum to give diazide
as a white solid product. IR spectrum revealed that the ionic liquid conveniently loaded
with the desired azide anion (see Supplemental Materials).
General procedure for the synthesis of 2,5,7-triaryl[1,3]thiazepine
(3A-D) and 2,4,6-triarylpyridine (4A-D) derivatives. A mixture of triarylth-
iopyrylium perchlorates (1 mmol), [1,1ꢁ-(butane-1,4-diyl)-bis(3-methylimidazolium)]
diazide 1 mmol containing azide anion (2 mmol) in [1,1ꢁ-(butane-1,4-diyl)-bis(3-
methylimidazolium)]dibromide (1 g) as a solvent was heated to 130◦C. After the
completion of the reaction, as indicated by TLC (n-hexane/ether, 4/1), the products were
extracted from a quenched reaction mixture with dichloromethane (10 mL), the solvent
was evaporated under vacuum and the residue was adsorbed on silica, transferred to a
silica column, and eluted with a 4/1 mixture of n-hexane/ether. Therefore, the first solid
and the second oil parts containing 2,4,6-triarylpyridine and 2,5,7-triaryl-[1,3]thiazepine
were obtained, respectively. The sampled parts were concentrated and analyzed by NMR
spectroscopy and MS.
FUNDING
This work was supported by the Research Council at the University of Shahid
Chamran.
SUPPLEMENTAL MATERIAL
Supplemental data for this article can be accessed on the publisher’s website at
http://dx.doi.org/10.1080/10426507.2015.1054482.
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