A novel and facile synthesis of mesoionic 1,2,4-triazolium-3-thiolate derivatives

Article abstract of DOI:10.1016/j.tetlet.2014.02.107

Mesoionic 1,2,4-triazolium-3-thiolate derivatives were synthesized from the reaction of N-substituted-2-phenylhydrazinocarbothioamides with tetracyanoethylene (TCNE) to give tricyanovinyl intermediates, followed by heterocyclization to afford 5-(1-amino-2,2-dicyano-vinyl)-4-substituted-1- phenyl-4H-1,2,4-triazol-1-ium-3-thiolates in 67-76% yields. The structures of the products have been confirmed unambiguously by single crystal X-ray structure analyses. A rationale for the formation of the products is presented.

Full text of DOI:10.1016/j.tetlet.2014.02.107

Tetrahedron Letters 55 (2014) 2385–2388
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
A novel and facile synthesis of mesoionic 1,2,4-triazolium-3-thiolate
derivatives
a
a
a
b
Alaa A. Hassan a, , Kamal M. A. El-Shaieb , Nasr K. Mohamed , Hendawy N. Tawfeek , Stefan Bräse ,
⇑
Martin Nieger c
a Chemistry Department, Faculty of Science, Minia University, 61519 El-Minia, Egypt
b Institute of Organic Chemistry, Karlsruhe Institute of Technology, Fritz-Haber-Weg 6, 76131 Karlsruhe, Germany
c Laboratory of Inorganic Chemistry, Department of Chemistry, University of Helsinki, PO Box 55 (A.I. Vinasen aukio 1), 00014 Helsinki, Finland
a r t i c l e i n f o
a b s t r a c t
Article history:
Mesoionic 1,2,4-triazolium-3-thiolate derivatives were synthesized from the reaction of N-substituted-2-
phenylhydrazinocarbothioamides with tetracyanoethylene (TCNE) to give tricyanovinyl intermediates,
followed by heterocyclization to afford 5-(1-amino-2,2-dicyano-vinyl)-4-substituted-1-phenyl-4H-
1,2,4-triazol-1-ium-3-thiolates in 67–76% yields. The structures of the products have been confirmed
unambiguously by single crystal X-ray structure analyses. A rationale for the formation of the products
is presented.
Received 26 October 2013
Revised 4 February 2014
Accepted 26 February 2014
Available online 6 March 2014
Keywords:
Ó 2014 Elsevier Ltd. All rights reserved.
1,2,4-Triazolium-3-thiolate derivatives
N-Substituted 2-phenylhydrazinocarbo-
thioamides
Tetracyanoethylene
Electron donor–acceptor interactions are important in the field
of drug receptor binding mechanisms,1 in solar energy storage,2
and in many biological fields,3 and have successfully been utilized
in pharmaceutical analysis,4 and in non-linear optics.5,6
Tetracyanoethylene (TCNE) is a highly reactive pernitrile al-
kene, and is known to be a strong organic electron acceptor. The
high reactivity of TCNE toward electron-rich reagents is frequently
used in addition,7,8 cycloaddition,7,9 and heterocyclization10–12
reactions, and in charge transfer (CT) complexation.13,14
We have previously demonstrated that 4-substituted thiose-
micarbazides reacted with TCNE (2) in ethyl acetate, with admis-
sion of air, to give pyrazolothiadiazole, and thiadiazapine
derivatives.26 Based on the above findings, we decided to investi-
gate the behavior of other hydrazinecarbothioamides, namely N-
substituted-phenylhydrazinocarbothioamides, 1a–c toward TCNE.
Treatment 1a–c with two molar equivalents of TCNE (2) in dry
ethyl acetate, with admission of air at room temperature, resulted
in a pink coloration of the solution that quickly turned into brown
(Scheme 1). This behavior may be explained due to the initial for-
mation of unstable CT-complexes, followed by a chemical reaction.
Monitoring of the reaction by visible spectroscopy failed since
the reaction is fast. Also, at lower concentrations, significant color
changes were no longer observed. The mixture was stirred and
then left to stand for 48 h at room temperature. After concentra-
tion to dryness, the residue was sublimed to remove the excess
TCNE and then subjected to preparative thin-layer chromatogra-
phy. From the one significant zone and by crystallization from
acetonitrile, colorless crystals of 5-(1-amino-2,2-dicyanovinyl)-4-
alkyl/aryl-1-phenyl-4H-1,2,4-triazol-1-ium-3-thiolates 3a–c were
obtained in 67–76% yields. The structures of 3a–c were delineated
from their spectroscopic properties, elemental analyses, and single
crystal X-ray crystallography.
Mesoionic compounds have attracted the attention of chemists
because of the bonding aspects of their unusual structures, which
are strongly stabilized by p
-electron delocalization.15–20 They have
generated interest in non-linear optical studies,21–23 and medicinal
chemistry, where a wide range of biological activities have been
reported.24
Reaction of 2-methyl-4-phenylthiosemicarbazide with ethyl
orthoformate led to the formation of 2-methyl-4-phenyl-1,2,4-
triazolium-5-thiolate and 1-methyl-4-phenyl-1,2,4-triazolium-5-
thione.25 The formation of these mesoionic compounds resulted
from the rearrangement of 2,4-disubstituted thiosemicarbazides
into 1,4-derivatives, which helped to formulate the structures
quite convincingly.25
Absorption bands around 3340–3325 cmÀ1 relating to NH2
groups appeared in the infrared spectra of 3a–c, which were con-
firmed by 1H NMR spectra, which showed a broad singlet in the
⇑
Corresponding author. Tel./fax: +20 862363011.
E-mail address: alaahassan2001@mu.edu.eg (A.A. Hassan).
http://dx.doi.org/10.1016/j.tetlet.2014.02.107
0040-4039/Ó 2014 Elsevier Ltd. All rights reserved.

2386
A. A. Hassan et al. / Tetrahedron Letters 55 (2014) 2385–2388
NC
H2N
CN
NC
NC
CN
CN
H
N
H
N
EtOAc
Ph
N
N
N
H
R
N
R
+
S
S
2
1a-c
3a-c
1,3
R
Yield (%)
76
a
b
c
CH2CH3
CH2-CH=CH2 71
C6H5 67
Scheme 1. Reaction between N-substituted phenylhydrazinocarbothioamides 1a–c
and TCNE (2).
range 10.34–10.18 ppm. Also, the IR spectra of 3a–c clearly showed
CAS stretching at 1292–1286 cmÀ1, and C@N and conjugated cyclic
C@N absorptions at 1495–1486 cmÀ1 27
.
In fact, the 1H NMR spectral studies were not very informative
due to the absence of a proton on the core heterocycle. However,
the 1H NMR spectrum of 3a clearly showed the presence of triplet
and quartet signals centered at 1.28 and 3.81 ppm, respectively,
with the integral ratio 3:2 due to the CH3CH2 group, besides the
aromatic protons.
In 3b, the substituent (allyl group) gave rise to three multiplet
signals at 4.62–4.63 (allylACH2N), 5.23–5.28 (allylACH2@), and at
5.87–5.90 (allylACH@). The presence of the allyl group was evident
from the 13C DEPT-NMR spectrum which exhibited positive signals
at 133.5 ppm (allylACH@) and negative signals at 49.7, 121.4 ppm
for the allylACH2N and allylACH2@, respectively.28
Figure 2. X-ray crystal structure of 3b (ORTEP, displacement parameters are drawn
at the 50% probability level, minor disordered parts are omitted for clarity). The
crystallographic numbering does not reflect the systematic IUPAC numbering.
tra of 3a–c, dicyanovinyl-carbons C1 and C2 resonated at 171.4–
171.2 and 60.5–60.1 ppm, respectively, and were in accord with
the observed trends in the d values for C-atoms in push–pull
alkenes.31,32
The structures of the newly synthesized triazoliumthiolate
derivatives 3a–c were also confirmed by their EI mass spectra.
The mass spectra of 3a–c showed molecular ion peaks at m/z
296, 308, and 344 in conformity with their molecular formulae.
The molecular ion peaks were fairly intense suggesting the pres-
ence of a stable triazoliumthiolate ring system in these cyclized
products.
Moreover, the structures of 3a,b were confirmed unambigu-
ously by single crystal X-ray structure analyses (Figs. 1 and 2 and
Tables S1–S7, S8–S14 in the Supplementary data; note that the
crystallographic numbering does not correspond to systematic IU-
PAC numbering rules).
In contrast to the 1H NMR spectra, the 13C NMR spectra of 3a–c
were crucial for the characterization of the mesoionic ring due to
the large differences in the electronic densities between the two
regions of the triazolium ring.29,30 This difference of electronic den-
sity was apparent through positive and negative shielding of the
carbons of the mesoionic triazolium rings in 3a–c. The C-3 and
C-5 signals were centered around 169.8–169.7 and 151.7–
152.1 ppm. These results are consistent with the assignment of
chemical shifts for triazolium thiolates 3a–c. In the 13C NMR spec-
Figure 1 shows the structure of 5-(1-amino-2,2-dicyanovinyl)-
4-ethyl-1-phenyl-4H-triazol-1-ium-3-thiolate (3a). The C(1)–S(1)
bond length of 1.700(2) Å, is intermediate between the double
bond length of 1.645 Å33 and the single bond length of 1.737 Å.26
The bond lengths C(1)–N(2) 1.333 (3), C(1)–N(5) 1.403 (2), and
C(4)–N(5) 1.338 (3) Å suggest that these bonds have some double
bond character as they are comparable to the CAN
r-bond length
of 1.47 Å. The N(2)–N(3) bond length of 1.374 (2) Å is slightly
shorter than the single-bond value of 1.401 Å. The fact that the
N(2)–N(3) bond length is closer to the single bond length supports
the presence of a mesoionic ring system.34 The S(1)–C(1)–N(2)–
N(3), C(1)–N(2)–N(3)–C(4), C(31)–N(3)–C(4)–C(5), C(41)–C(4)–
N(5)–C(1), and C(1)–N(5)–C(51)–C(52) torsion angles are
À177.42 (15)°, À0.7 (2)°, 177.11 (18)°, 174.90 (18)°, and 83.4(2)°,
respectively.
In
4-allyl-3-(1-amino-2,2-dicyano-vinyl-1-phenyl-4H-1,2,4-
triazol-1-ium-3-thiolate (3b) (Fig. 2), the S(1)–C(1) and N(2)–
N(3) bond lengths are 1.6933 Å (10) and 1.3804 Å (2), respectively
(note that the crystallographic numbering does not correspond to
systematic IUPAC numbering rules), and thus are slightly shorter
than the CAS and NAN
r-bonds. The sum of the angles N(2)–
C(1)–N(5), N(2)–C(1)–S(1), and N(5)–C(1)–S(1) is close to 360° as
are the sums of the angles around N(3), C(4), N(5), C(41), and
C(33) demonstrating planarity around C(1), N(3), C(4), N(5),
C(41), and C(33). However, the sum of the angles N(5)–C(51)–
C(52), N(5)–C(51)–H(51A), and N(5)–C(51)–H(51B) is 329.69°; this
Figure 1. X-ray crystal structure of 3a (ORTEP, displacement parameters are drawn
at the 50% probability level). The crystallographic numbering does not reflect the
systematic IUPAC numbering.

A. A. Hassan et al. / Tetrahedron Letters 55 (2014) 2385–2388
2387
1
2H
2
1 a-c
[ CT-Complex]
1
2
+
+
+
NC
H
CN
NC
N
N
CN
H
N
N
NC
SH
CN
H
CN
NC
CN
CN
CN
NC
CN
CN
Ph
Ph
NH
N
Ph
Ph NH
N
N
R
N
- HCN
N
NH
R
R
4
R
SH
SH
SH
6
5
CN
CN
HN
Ph
HN
CN
H
HN
H2N
HN
CN
CN
CN
CN
Ph
Ph
Ph
CN
R
N
N
N
N
N
N
N
R
N
R
N
R
N
N
S
S
SH
S
7
9
8
3a-c
Scheme 2. Mechanistic rationale for the formation of compounds 3a–c.
angle is not that expected for a typical sp3 hybridized carbon due to
the resonance in the allyl group.
Due to steric hindrance, there is a substantial twisting of the
phenyl group out of the plane of the triazolium ring N(2)–N(3)–
C(31)–C(32) À53.9 (2)°.
Compound 3a: C14H12N6S, Mr = 296.36 g molÀ1, colorless crystal,
size 0.20 Â 0.08 Â 0.06 mm, orthorhombic, P212121 (no.19),
a = 7.117(1) Å, b = 13.743 (2) Å, c = 14.608 (2) Å, V = 1428.8 (3) Å,
Z = 4,
Dcalcd = 1.378 mg mÀ3
,
F(000) = 616,
l ,
= 0.23 mmÀ1
T = 123 K, 28,049 measured reflections (2hmax = 55°), 3265 inde-
In order to rationalize the formation of compounds 3a–c, initial
CT-complexes may (but do not have to) be intermediates. Forma-
tion of tetracyanoethane derivative 4 and elimination of a molecule
of HCN would give the tricyanovinyl compound 5. Intramolecular
nucleophilic attack of PhNH on the C@C bond followed by cycliza-
tion would afford bicyclic 7 via intermediate 6. Compound 7 could
rearrange to form 8 and 9, and finally the highly stable mesoionic
triazolium thiolate derivatives 3a–c (Scheme 2).
pendent [Rint = 0.041], 196 parameters, 2 restraints, R1 [for 3062
I > 2r
(I)] = 0.027, wR2 (for all data) = 0.064, S = 1.08, largest diff.
peak and hole = 0.21 e AÀ3/À0.22 e AÀ3. The absolute structure
was determined by refinement of Parsons parameter x = 0.01(2)
(Parsons, S., Flack, H. D. Acta Crystallogr. 2004, A60, s61).
Compound 3b: C15H12N6S—0.5(C2H3N), Mr = 328.89 g molÀ1, col-
orless crystal, size 0.30 Â 0.15 Â 0.06 mm, monoclinic, C2/c (no.15),
a = 19.988(2) Å, b = 14.058 (1) Å, c = 14.590 (1) Å, b = 120.19(1)°,
In conclusion, novel mesoionic triazoliumthiolate derivatives
containing aliphatic and aromatic groups are prepared from N-
substituted-2-phenylhydrazinecarbothioamides 1a–c and tetracy-
anoethylene (2). The structures of the products were confirmed
by X-ray crystallography.
V = 3543.6 (6) Å, Z = 8, Dcalcd = 1.233 mg mÀ3
,
F(000) = 1368,
measured reflections
(2hmax = 55°), 4050 independent [Rint = 0.032], 204 parameters, 5
l
= 0.19 mmÀ1
,
T = 123 K,
37,848
restraints, R1 [for 3389 I > 2r
(I)] = 0.045, wR2 (for all data) = 0.124,
S = 1.06, largest diff. peak and hole = 1.00 e AÀ3/À0.51 e AÀ3
.
Supplementary data
Acknowledgment
Supplementary data (crystallographic data (excluding structure
factors) for the structures reported in this work have been depos-
ited with the Cambridge Crystallographic Data Centre as supple-
mentary publication nos. 966497 (3a) and 966498 (3b). Copies of
the data can be obtained free of charge on application to the Direc-
tor, CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax: +44 (1223)
336 033: e-mail: deposit@ccdc.cam.ac.uk) associated with this
article can be found, in the online version, at http://dx.doi.org/
10.1016/j.tetlet.2014.02.107.
Alaa A. Hassan is indebted to the AVH-Foundation for the dona-
tion of a Shimadzu 408 IR instrument.
Crystal X-ray structure determination of 3a
Single crystals were obtained by recrystallization from CH3CN.
The single crystal X-ray diffraction study was carried out on a Bru-
ker–Nonius Kappa CCD diffractometer at 123 K using Mo Ka radia-
tion (k = 0.71073 Å). Direct methods (SHELXS-98)35 were used for
structure solution and refinement was carried out using SHELXL-
201335 (full-matrix least-squares on F2). Hydrogen atoms were
localized using the difference Fourier synthesis map and refined
using a riding model [H(N) free]. A semi-empirical absorption cor-
rection was applied.
References and notes
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1190–1191.
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density due to heavily disordered acetonitrile which could not be
refined with split atoms. Therefore the ‘SQUEEZE’ option of the
program package PLATON (Spek, A.L. Acta Crystallogr. 2009, D65,
148–155) was used to create a hkl file taking into account the
residual electron density in the void areas [in the two voids (chan-
nel parallel c-axis) are 2 disordered acetonitriles per void]. Addi-
tionally the allyl moiety is disordered.
9. Fatiadi, A. J. Synthesis 1987, 749–789.

2388
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21. Lira, B. F.; Athayde-Filho, P. F.; Miller, J.; Simas, A. M.; Dias, A. F.; Vieira, M. J.
Molecules 2002, 7, 791–800.
(KBr) m = 3340 (NH2), 2215 (CN), 1645 (C@N), 1588, 1486 (ArAC@C and
conjugated cyclic C@N), 1290 (C@SÀ str.), 1H NMR (400 MHz, CDCl3):
dH = 10.18 (br s, 2H, NH2), 7.40–7.33 (m, 2H, Ar@H), 7.21–7.17 (m, 3H,
ArAH), 3.81 (q, 2H, J = 7.64, CH2), 1.28 (t, 3H, J = 7.64, CH3). 13C NMR (100 MHz,
CDCl3): dc = 171.4 (@CANH2), 169.8 (triazolium-C-3), 151.7 (triazolium-C-5),
138.9 (Ar@C), 131.2, 128.7, 124.2 (Ar@CH), 110.8 (CN), 60.4 (C (CN)2), 37.2
(CH2), 15.2 (CH3). MS (EI): m/z = 296 (M+, 100), 253(26), 147 (34), 105 (51), 77
(64). Anal. Calcd for C14H12N6S (296.35): C, 56.74; H, 4.08; N, 28.36; S, 10.82.
Found: C, 56.87; H, 3.96; N, 28.29; S, 10.95.
4-Allyl-5-(1-amino-2,2-dicyanovinyl)-1-phenyl-4H-1,2,4-triazol-1-ium-3-thiolate
(3b): Colorless crystals (0.219 g, 71%), mp = 190–191 °C, MeCN. IR (KBr)
m
= 3325 (NH2), 2220 (CN), 1648 (C@N), 1590, 1495 (ArAC@C and conjugated
cyclic C@N), 1292 (CASÀ str.), 1H NMR (400 MHz, CDCl3): dH = 10.28 (br s, 2H,
NH2), 7.34–7.22 (m, 2H, ArAH), 7.19–7.10 (m, 3H, ArAH), 5.90–5.87 (m, 1H,
allylACH@), 5.28–5.23 (m, 2H, allylACH2@), 4.63–4.62 (m, 2H, allylACH2N).
13C NMR (100 MHz, CDCl3): dc = 171.2 (@CANH2), 169.8 (triazolium-C-3),
151.1 (triazolium-C-5), 138.6 (ArAC), 133.5 (allylACH@), 131.3, 129.4, 128.5
(ArACH), 121.4 (allylACH2@), 110.9 (CN), 60.5 (C (CN)2), 49.7 (allylACH2N). MS
(EI): m/z = 308 (M+, 100), 253(32), 209 (18), 203 (11), 105 (28), 99 (34), 77 (42),
41 (26). Anal. Calcd for C15H12N6S (308.36): C, 58.43; H, 3.92; N, 27.25; S, 10.40.
Found: C, 58.29; H, 4.06; N, 27.41; S, 10.27.
22. Pilla, V.; Araújo, C. B.; Lira, B. F.; Simas, A. M.; Miller, J.; Athayde-Filho, P. F. Opt.
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therein.
5-(1-Amino-2,2-dicyanovinyl)-1,4-diphenyl-4H-1,2,4-triazol-1-ium-3-thiolate
(3c): Colorless crystals (0.230 g, 67%), mp = 300–302 °C, MeCN. IR (KBr)
m
= 3330 (NH2), 2210 (CN), 1688 (C@N), 1595, 1492 (ArAC@C and conjugated
25. Buccheri, F.; Cusmano, G.; Gruttadauria, M.; Noto, R.; Werber, G. J. Heterocycl.
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Thieme Verlag: Stuttgart, 1997; p 54.
28. General procedure for the preparation of 3a–c: To a stirred solution of TCNE (2)
(256 mg, 2.0 mmol) in dry EtOAc (10 mL) was added dropwise a solution of 1a–
c (1.0 mmol) in dry EtOAc (15 mL), which caused a spontaneous change of
color from yellow to pink and finally to brown. The mixture was stirred for 2 h
and then left to stand for 48 h at room temperature. After concentration to
dryness, the residue was sublimed at 80 °C to remove the excess TCNE and
then subjected to preparative thin-layer chromatography using toluene/EtOAc
(5:1) as the eluent to give numerous colored zones, only one intense zone was
removed and extracted with acetone and recrystallized from acetonitrile to
give pure crystals of 3a–c.
cyclic C@N), 1286 (CASÀ str.), 1H NMR (400 MHz, CDCl3): dH = 10.34 (br s, 2H,
NH2), 7.51–7.44 (m, 2H, ArAH), 7.42–7.35 (m, 6H, ArAH), 7.33–7.28 (m, 2H,
ArAH). 13C NMR (100 MHz, CDCl3): dc = 171.2 (@CANH2), 169.7 (triazolium-C-
3), 151.4 (triazolium-C-5), 138.4, 131.9 (ArAC), 131.6, 131.2, 129.7, 129.5,
128.6, 128.5 (ArACH), 111.2 (CN), 60.1 (C (CN)2). MS (EI): m/z = 344 (M+, 100),
209(37), 135 (71), 105 (28), 77 (53). Anal. Calcd for C18H12N6S (344.39): C,
62.77; H, 3.51; N, 24.40; S, 9.31. Found: C, 62.91; H, 3.38; N, 24.56; S, 9.19.
29. Lira, B. F.; Miller, J.; Simas, A. M.; Athayde Filho, P. F.; Dias, A. F.; Silva, R. O.;
Oliveira, V. C Arkivoc 2004, iv, 12–21.
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2 1994, 2571–2575.
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Stuttgart, 1984; p 121.
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35. Sheldrick, G. M. Acta Cryst. Sect. A 2008, A64, 112–122.
5-(1-Amino-2,2-dicyanovinyl)-4-ethyl-1-phenyl-4H-1,2,4-triazol-1-ium-3-
thiolate (3a): Colorless crystals (0.225 g, 76%), mp = 245–247 °C, MeCN. IR