Direct exchange of oxygen and selenium atoms in the 1,2,5-Oxadiazoles and 1,2,5-Selenadiazoles by Action of Sulfur Monochloride

Article abstract of DOI:10.3390/molecules200814522

A short synthetic approach to fused 1,2,5-thiadiazoles from the corresponding 1,2,5-oxadiazoles and 1,2,5-selenadiazoles has been developed. Mono- And bis(1,2,5- thiadiazoles) were selectively obtained in high yields. The pathways for these novel reactions were discussed.

Full text of DOI:10.3390/molecules200814522

Molecules 2015, 20, 14522-14532; doi:10.3390/molecules200814522
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molecules
ISSN 1420-3049
www.mdpi.com/journal/molecules
Article
Direct Exchange of Oxygen and Selenium Atoms in the
1,2,5-Oxadiazoles and 1,2,5-Selenadiazoles by Action of
Sulfur Monochloride
Lidia S. Konstantinova 1,2, Ekaterina A. Knyazeva 1,2 and Oleg A. Rakitin 1,2,
*
1
N. D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, 119991 Moscow,
Russia; E-Mails: konstantinova_ls@mail.ru (L.S.K.); katerina_knyazev@mail.ru (E.A.K.)
Department of Chemistry, South Ural State University, 454080 Chelyabinsk, Russia
2
* Author to whom correspondence should be addressed; E-Mail: orakitin@ioc.ac.ru;
Tel.: +7-499-135-53-27; Fax: +7-499-135-53-28.
Academic Editor: Panayiotis Koutentis
Received: 14 July 2015 / Accepted: 3 August 2015 / Published: 12 August 2015
Abstract: A short synthetic approach to fused 1,2,5-thiadiazoles from the corresponding
1,2,5-oxadiazoles and 1,2,5-selenadiazoles has been developed. Mono- and bis(1,2,5-
thiadiazoles) were selectively obtained in high yields. The pathways for these novel reactions
were discussed.
Keywords: 1,2,5-thiadiazoles; 1,2,5-oxadiazoles; 1,2,5-selenadiazoles; sulfur monochloride
1. Introduction
1,2,5-Thiadiazoles and particularly their benzo-fused derivatives have been known for many years,
and their synthesis and applications in various branches of technology and medicine were extensively
investigated and reviewed [1–5]. Recently, they were found to be an efficient electron acceptor and were
used as the building blocks of many actual or potential molecule-based functional materials for organic
electronics and spintronics [6–12]. Although methods for the preparation of fused 1,2,5-thiadiazoles are
numerous and well elaborated [2,3,5], there is still a lack of suitable preparative approaches to many
interesting derivatives containing electron-deficient heterocycles or electron-withdrawing groups.
A few years ago 3,4-diamino-1,2,5-oxadiazole 1 was found to react with sulfur monochloride and
pyridine in acetonitrile and gave, unexpectedly, [1,2,5]thiadiazolo[3,4-c][1,2,5]thiadiazole 2 in high

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yield (Scheme 1) [10]. The main feature of this transformation is an exchange of the oxygen atom in
the 1,2,5-oxadiazole ring with a sulfur atom in the reaction with sulfur monochloride. To the best of
our knowledge, this is the first example of this unusual reaction. Later on, it was shown that another
oxadiazole derivative, 4-amino-3-nitro-1,2,5-oxadiazole 3, can undergo a similar transformation with
S2Cl2 in lower yield [13].
NH2
NO2
NH2
N
N
N
N
N
N
N
N
Py, MeCN
75%
Py, MeCN
49%
O
S2Cl2 +
O
+ S2Cl2
S
S
NH2
2
3
1
Scheme 1. Synthesis of [1,2,5]thiadiazolo[3,4-c][1,2,5]thiadiazole 2.
Recently, it was discovered that under similar conditions, (5Z,6Z)-[1,2,5]oxadiazolo[3,4-b]pyrazine-
5,6(4H,7H)-dione dioxime 4 gave tricyclic bis([1,2,5]thiadiazolo)-[3,4-b;3′,4′-e]pyrazine 5 in moderate
yield (Scheme 2). In that case, three reactions occurred: exchange of an oxygen atom in a
1,2,5-oxadiazole ring by a sulfur atom, formation of thiadiazole ring from dioxime, and aromatization
of a piperazine ring [14].
H
N
N
NOH
NOH
N
N
N
N
Py
N
N
+ S2Cl2
S
S
O
MeCN
N
N
H
5
, 40%
4
Scheme 2. Synthesis of bis([1,2,5]thiadiazolo)-[3,4-b;3′,4′-e]pyrazine 5.
A specific feature of sulfur monochloride (S2Cl2) is its diverse reactivity [15–18]. The most useful
property of this compound is its sulfurating ability. Apart from this, S2Cl2 is a strong chlorinating agent,
but it is rarely used since there are many other chlorinating agents that afford higher yields of the reaction
products. The oxidative ability of S2Cl2 has been investigated to a much lower extent because the
products of formal oxidation reactions are formed, as a rule, in a course of complex multistage processes
involving chlorination, dehydrochlorination, sulfuration, etc. A direct exchange of a chalcogen atom with
a sulfur atom in the reaction with sulfur monochloride in chalcogen-nitrogen heterocycles has not been
discovered before our works [10,13]. To the best of our knowledge, the only formal analogy of this
exchange is the classical Yuryev reaction performed under very drastic conditions [19–21]. However,
the reaction pathways seem to be very different and its scope is very limited to heterocycles with one
heteroatom, and high yields were achieved by using furan as a starting material; vigorous conditions of
this method did not allow its spread for compounds with labile groups.
In this paper, we report a study of a reaction between a wide range of 1,2,5-oxadiazoles and
1,2,5-selenadiazoles and sulfur monochloride for the synthesis of fused mono- and bis(1,2,5-thiadiazoles).

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2. Results and Discussion
2.1. Conversion of 1,2,5-Oxadiazoles into 1,2,5-Thiadiazoles
We attempted to extend the reaction with sulfur monochloride and pyridine in acetonitrile to
other monocyclic, benzo-, and pyrido-fused 1,2,5-oxadiazoles. It was found that the uncondensed
1,2,5-oxadiazoles, 2,1,3-benzooxadiazole, and 5,6-disubstituted [1,2,5]oxadiazolo[3,4-b]pyrazines,
shown in Scheme 3, did not react with S2Cl2 under these conditions in practically all cases. Employing
1,4-diazabicyclo[2.2.2]octane or triethylamine as a base and changing the solvent to chloroform or to
dimethylformamide at temperatures from −25 to 100 °C also did not lead to 1,2,5-thiadiazole derivatives.
The analysis of these results has driven us to the conclusion that for the successful conversion of
1,2,5-oxadiazoles into 1,2,5-thiadiazoles, one or two NH2 or NH groups attached to oxadiazole ring
are needed.
R1
R2
R1
R2
N
N
N
N
Py
O
+ S2Cl2
S
MeCN
R1 = Me;
Ph;
R2 = Me
Ph
N
N
R
R
R1+R2 =
;
NH2;
NH2;
Me
Ph
R = CH3, C6H5, Cl
CO2H;
CO2H;
CO2Me;
CN;
Me
CO2H
CO2Me
CN
NO2;
NO2
Scheme 3. 1,2,5-Oxadiazoles which did not react with sulfur monochloride.
To continue our attempts to convert the 1,2,5-oxadiazole ring into 1,2,5-thiadiazole, the reaction
of 4H,8H-bis[1,2,5]oxadiazolo[3,4-b:3′,4′-e]pyrazine 6, containing two oxadiazole rings and two NH
groups, with sulfur monochloride was investigated in detail (Scheme 4).
H
N
N
N
N
N
S
S
N
H
S2Cl2:Py (2:3)
8
S2Cl2:Py (6:8)
H
N
N
N
N
N
N
N
N
N
S2Cl2:Py (6:8)
O
O
S
S
N
N
N
S2Cl2:Py (3:4)
H
6
5
S2Cl2:Py (3:4)
H
S2Cl2:Py (6:8)
N
N
O
N
S
N
N
N
H
7
Scheme 4. Reactions of 4H,8H-bis[1,2,5]oxadiazolo[3,4-b:3′,4′-e]pyrazine 6 with sulfur
monochloride in MeCN.

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It was found that the treatment of compound 6 with an excess of sulfur monochloride (6 equiv) and
pyridine (8 equiv) in boiling MeCN gave aromatic bis(1,2,5-thiadiazole) 5 in good yield (Scheme 4,
Table 1, entry 1).
Table 1. Reactions of [1,2,5]oxadiazolo[3,4-b]pyrazines 6–8 with S2Cl2 in MeCN.
Quantity of Reagents for 1 equiv of
Starting
Reaction Reaction Product,
Starting Compound
No.
Тemperature, °С
Compound
Time, h
Yield %
S2Cl2
Pyridine
1
2
3
4
5
6
6
6
6
7
7
8
6
3
6
3
6
2
8
4
8
4
8
3
82
20
20
20
82
82
4
20
20
20
5
5, 76
7, 90
8, 83
8, 66
5, 56
5, 85
2
The formation of thiadiazoles 7 and 8 from bis(1,2,5-oxadiazole) 6 indicated that the successive
formation of two thiadiazole rings occurred at room temperature (entries 2 and 3, Table 1) and preceded
the final aromatization of the piperazine cycle in boiling MeCN (Scheme 4). This was confirmed by the
treatment of oxadiazolothiadiazole 7 with the same mixture at room temperature which led to bis(1,2,5-
thiadiazole) derivative 8 in good yield (entry 4, Table 1), and the conversion of bis(1,2,5-thiadiazole) 8
to aromatic tricycle 5 at high temperature (entry 6, Table 1).
Tricycle 5 can be obtained also from diamine derivative 9 by treatment with S2Cl2 and pyridine in
boiling MeCN (Scheme 5). In that case, two processes take place simultaneously—the formation of the
thiadiazole ring from the diamine moiety and the dehydration of dihydropiperazine into the aromatic
pyrazine ring, presumably by action of sulfur monochloride.
H
N
N
NH2
NH2
N
N
N
N
N
N
S2Cl2:Py (6:8)
MeCN, , 4h
S
O
S
N
N
H
9
5, 81%
Scheme 5. Reaction of 4,7-dihydro[1,2,5]oxadiazolo[3,4-b]pyrazine-5,6-diamine 9 with S2Cl2.
Dehydration of dihydropiperazine to a pyrazine ring is quite a rare process and it was investigated
in detail for bis(1,2,5-thiadiazole) derivative 8. It was found that the treatment with a rank of oxidizers
usually used for similar processes—bromine, DDQ, PhI(OAc)2 and its perfluoro analog PhI(O(C(O)CF3))2
in various solvents (CHCl3, THF, ether, MeCN)—gave no reaction in most cases, and in some it led to
the decomposition of starting material. Aromatic bis-1,2,5-thidiazolopyrazine 5 has been obtained in
the reaction of 8 with sulfuryl chloride, and it was found that to get a high yield of compound 5, it is
necessary to employ eight equivalents of oxidant and hours-long boiling.
The described procedures provide a new synthetic pathway to fused 1,2,5-thiadiazoles from
corresponding oxadiazoles. The key steps may be explained by the sulfurization of the tautomeric form
10 of the tricycle 6 with sulfur monochloride in the presence of the base to give chlorodithio derivative

Molecules 2015, 20
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11, followed by the elimination of hydrogen chloride with the formation of N-thiosulfinylamine
12 (Scheme 6). Further cyclization of compound 12 into 1,2,5-thiadiazole 7 via cycloaddition/
retrocycloaddition with the extrusion of sulfur monoxide (SO), which is thermodynamically unstable
and decomposes very rapidly [22], may occur. A similar formation of the 1,2,5-thiadiazole ring from
N-thiosulfinylamine and nitro groups has recently been proposed [13]. This reaction could then be
repeated to give bis(1,2,5-thiadiazole) 8, which may oxidize with the formation of aromatic tricycle 5.
Bis([1,2,5]thiadiazolo)-[3,4-b;3′,4′-e]pyrazine 5 is of interest as a prominent precursor of persistent
radical anions [6].
Scheme 6. A plausible mechanism for the formation of bis([1,2,5]thiadiazolo)-[3,4-b;3′,4′-e]
pyrazine 5 from 4H,8H-bis[1,2,5]oxadiazolo[3,4-b:3′,4′-e]pyrazine 6.
2.2. Conversion of 1,2,5-Selenadiazoles into 1,2,5-Thiadiazoles
To further explore the possibility of sulfur monochloride to substitute the oxygen atom in
1,2,5-oxadiazoles with a sulfur atom, the reaction of 4,8-dihydro[1,2,5]oxadiazolo[3,4-b][1,2,5]
selenadiazolo[3,4-е]pyrazine 13 with S2Cl2 was investigated. Tricycle 13 was found to be inactive by
treatment with S2Cl2 in MeCN, apparently because it was not dissolved in the reaction mixture. When
MeCN was substituted with DMF, 4,8-dihydro[1,2,5]oxadiazolo[3,4-b][1,2,5]thiadiazolo[3,4-е]pyrazine
7 was isolated in high yield (entry 1, Table 2). It means that a selective exchange of the selenium atom in
the 1,2,5-selenadiazole ring with a sulfur atom has occurred. There is only one example of conversion of
1,2,5-selenadiazoles to 1,2,5-thiadiazoles by the action of hydrogen sulfide to the (1,2,5-selenadiazolo)
porphyrazine derivative [23]. When an excess of sulfur monochloride was used in the reaction of
pyrazine 13, both selenium and oxygen atoms were replaced by sulfur atoms with the formation of
bis(1,2,5-thiadiazole) 8 (entry 2, Table 2). Also, mono- and bis(1,2,5-selenadiazoles) 14 and 15,
respectively, can be involved in this reaction to give bis(1,2,5-thiadiazole) 8 in high yields (Scheme 7).
The most important results are summarized in Table 2.
In an attempt to develop general direct synthesis of 1,2,5-thiadiazoles from selenadiazoles, the latter
were reacted with S2Cl2. Treatment of 1,2,5-selenadiazoles fused with electron-acceptor heterocycles,
such as 1,2,5-thiadiazole (16), 1,2,5-selenadiazole (17), quinoxaline (18), and others (19–20) with S2Cl2
in DMF, gave the corresponding mono- and bis(1,2,5-thiadiazoles) (2, 21–23) at various temperatures
in good yields (Scheme 8, entries 5–9, Table 2).

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Scheme 7. Reactions of [1,2,5]selenadiazolo[3,4-b]pyrazines 13–15 with sulfur monochloride.
Table 2. Reactions of fused 1,2,5-selenadiazoles with S2Cl2 in DMF.
Starting
Quantity of S2Cl2 for 1 equiv
Reaction
Reaction
No.
Тemperature, °С
Compound
of Starting Compound
Time, h Product, Yield %
1
2
3
4
5
6
7
8
9
13
13
14
15
16
17
18
19
20
3
6
3
6
3
6
2
2
2
20
100
20
3
6
7, 91
8, 78
8, 83
8, 85
2, 93
2, 73
21, 75
22, 47
23, 57
20
6
100
20
20
6
100
70
3
20
6
70
2
In all cases, the formation of the characteristic red amorphous precipitate of elemental selenium
was indicated. In the case of 16, this precipitate was isolated in practically quantitative yield and its
structure was confirmed by mass-spectrometry and elemental analysis.
We attempted to extend this reaction to benzo-fused and monocyclic 1,2,5-selenadiazoles. It was
found that 2,1,3-benzoselenadiazole and 3,4-diphenyl-1,2,5-selenadiazole did not react with sulfur
monochloride in organic solvents under forcing conditions (refluxing in acetonitrile or heating in DMF
at 100 °C for 10 h) and were recovered from the reaction mixtures in practically quantitative yields.
To compare the reactivity of 1,2,5-oxadiazoles and 1,2,5-selenadiazoles in the reaction with sulfur
monochloride, it should be noted that in both cases a fused nitrogen-containing heterocycle is needed
for the successful exchange of oxygen or selenium atoms to sulfur. The most plausible mechanism of
the conversion of 1,2,5-selenadiazoles into 1,2,5-thiadiazoles is shown in Scheme 9. The key steps may
include the sulfurization of the selenadiazole ring with sulfur monochloride to give chlorodithio
derivative 24, followed by the recyclization of the selenadiazole ring and the elimination of sulfur
dichloride with the formation of 1,2,5-selenadiazolo-N-selenide 25 (Scheme 9). Further extrusion of
elemental selenium, which is precipitated from the reaction mixture, affords the final 1,2,5-thiadiazole.

Molecules 2015, 20
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Scheme 8. Reactions of fused 1,2,5-selenadiazoles 16–20 with sulfur monochloride.
Scheme 9. A plausible mechanism for the transformation of 1,2,5-selenadiazoles into 1,2,5-thiadiazoles.
The method described here supplemented and enhanced the possibilities for the synthesis of fused
1,2,5-thiadiazoles.
3. Experimental Section
3.1. General Information
Elemental analyses for C, H, and N were performed with Perkin Elmer 2400 Elemental Analyser
(Perkin Elmer, Waltham, MA, USA). Melting points were determined on a Boetius hot-stage apparatus
and are uncorrected.
1H- (300.1 MHz) and 13C- (75.5 MHz) NMR spectra were taken for CDCl3 solutions (unless
otherwise indicated) with a Bruker AM-300 (Bruker AXS Handheld Inc., Kennewick, WA, USA) and
referred to tetramethylsilane (1H and 13C), J values are given in Hz.
MS spectra (EI, 70 eV) were obtained with a Finnigan MAT INCOS 50 (Hazlet, NJ, USA).
High-resolution MS spectra were measured on a Bruker micrOTOF II instrument (Bruker Daltonik
Gmbh, Bremen, Germany) using electrospray ionization (ESI). The measurement was operated in a

Molecules 2015, 20
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positive ion mode (interface capillary voltage −4500 V) or in a negative ion mode (3200 V); mass range
was from m/z 50 to m/z 3000 Da; external or internal calibration was done with Electrospray Calibrant
Solution (Fluka). A syringe injection was used for solutions in acetonitrile, methanol, or water (flow
rate 3 µL·min−1). Nitrogen was applied as a dry gas; interface temperature was set at 180 °C.
IR spectra were measured with a Specord M-80 (Carl Zeiss, Jena, Germany) instrument in KBr pellets.
Fused 1,2,5-oxadiazoles 6 [24], 7 [25], 9 [25] and 1,2,5-selenadiazoles 13–20 [11,26,27] were prepared
according to the published procedures.
3.2. General Procedure for the Reaction of 1,2,5-Oxadiazoles with S2Cl2 and Pyridine in Acetonitrile
Sulfur monochloride (quantity, see Table 1) was added dropwise to a stirred suspension of oxadiazole
6–9 (1.0 mmol) and pyridine (quantity, see Table 1) in dry acetonitrile (10 mL) under argon at −25 °C.
The mixture was stirred at room temperature or heated at the temperature and for the time specified in
Table 1. The precipitate was filtered and washed thoroughly with MeCN and hexane.
Bis([1,2,5]Thiadiazolo)-[3,4-b;3',4'-e]pyrazine 5. Yellow solid, mp 320–323 °С (Lit. [14] mp 320–322 °С).
IR and mass spectra are similar to the literature data [14].
4,8-Dihydrobis([1,2,5]thiadiazolo)[3,4-b:3',4'-e]pyrazine 8. Beige crystals, mp >350 °С. Anal. calcd
for С4Н2N6S2. (198.23): C, 24.24; H, 1.02; N, 42.40; S, 32.35. Found: C, 24.38; H, 0.98; N, 42.26; S,
1
13
32.38. NMR (DMSO-d6), δ, H: 10.11 (s, 2Н, NH); С: 142.8, 155.1. IR, ν, cm−1: 3136 (NH), 1628,
1334, 941, 520. MS, m/z (%): 198 (M+, 100), 171 (14), 131 (7), 46 (9). ESI-MS: found m/z 220.9850;
calc. for С4Н2N6S2 [M + Na]+ 220.9675.
3.3. General Procedure for the Reaction of 1,2,5-Selenadiazoles with S2Cl2 in DMF
Sulfur monochloride (quantity, see Table 2) was added dropwise to a stirred suspension of
selenadiazole 13–20 (1.0 mmol) in dry DMF (10 mL) under argon at −5 °C. The mixture was stirred at
room temperature or heated at the temperature and for the time specified in Table 2. Elemental selenium
was filtered, the reaction mixture was poured into H2O with ice, extracted with EtOAc (4 mL × 20 mL).
Combined extracts were washed with brine, dried over MgSO4, and the solvent was evaporated under
reduced pressure. Yields are given in Table 2.
[1,2,5]Thiadiazolo[3,4-b]quinoxaline 21. Yellow solid, mp 195–197 °С (Lit. [28] mp 199.5–200.5 °С).
IR and mass spectra are similar to the literature data [28].
Dibenzo[f,h][1,2,5]thiadiazolo[3,4-b]quinoxaline 22. Yellow solid, mp 302–304 °С (Lit. [28]
mp 298–300 °С). IR and mass spectra are similar to the literature data [28].
9Н-Inden[1,2-e][1,2,5]selenadiazolo[3,4-b]pyrazin-9-one 23. Yellow solid, mp 286–287 °С. Anal.
calcd for С11Н4N4OS (240.24): С 54.99, Н 1.68, N 23.32. Found: C 55.13, H 1.82, N 23.09. NMR
(DMSO-d6), δ, 1H: 7.84 (m, 1Н, Ph), 7.97 (m, 2Н, Ph), 8.20 (d, 1Н, J = 7.3 Hz, Ph); 13С: 126.6, 127.7,
137.6, 140.8 (4 CH), 133.3, 138.0, 141.7, 143.3, 158.7, 163.9 (6 sp2 tertiary C), 190.3 (C=O). ESI-MS:
found m/z 241.0177; calc. for С11Н4N4OS [M]+ 241.0179.

Molecules 2015, 20
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4. Conclusions
A new reaction, namely that of fused 1,2,5-oxadiazoles and 1,2,5-selenadiazoles with sulfur
monochloride, has been described as a one-pot synthetic route to 1,2,5-thiadiazoles. This procedure is
especially valuable in cases where 1,2,5-oxadiazoles (e.g., 6) are more readily available than the
corresponding 1,2,5-thiadiazoles. These reactions have broadened the utility of sulfur monochloride as
a highly efficient sulfurating reagent. Fused 1,2,5-thiadiazoles are of interest as precursors of persistent
radical anions and components of charge-transfer complexes.
Acknowledgments
This work was financially supported by the Russian Science Foundation (grant no. 15-13-10022).
Author Contributions
LSK and EAK designed research; LSK and OAR performed research and analyzed the data; OAR
wrote the paper. All authors read and approved the final manuscript.
Conflicts of Interest
The authors declare no conflict of interest.
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Sample Availability: Samples of all compounds are available from the authors.
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