Synthesis of 3,6-diaryl-1,4,5-thiadiazepines from substituted 2-thiocyano acetophenone and investigation of reaction mechanism

Article abstract of DOI:10.1080/17415993.2012.735672

In this work, we have studied the reaction of substituted 2-thiocyanoacetophenone and hydrazine hydrate as a novel and simple pathway for the preparation of the substituted 1,4,5-thiodiazepine ring system. The mechanism of this reaction revealed that in the initial step condensation of hydrazine with carbonyl groups of substituted 2-thiocyanoacetophenons 2a-2f gives the corresponding substituted aromatic dithiocyano azide intermediates which in turn undergo cyclization to1,4,5-thiadiazepines in the presence of hydrazine. This cyclization is a novel method for the preparation of sulfide bond from the reaction of hydrazine and a dithiocyano intermediate. An account of the reaction mechanism is given.

Full text of DOI:10.1080/17415993.2012.735672

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Synthesis of 3,6-diaryl-1,4,5-
thiadiazepines from substituted
2-thiocyano acetophenone and
investigation of reaction mechanism
M. Rahimizadeh a , B. Feizyzadeh a , M. Bakavoli a & H. Eshghi a
a Department of Chemistry, School of Sciences, Ferdowsi
University of Mashhad, Mashhad, 91775-1436, Iran
Published online: 06 Nov 2012.
To cite this article: M. Rahimizadeh , B. Feizyzadeh , M. Bakavoli & H. Eshghi (2013): Synthesis of
3,6-diaryl-1,4,5-thiadiazepines from substituted 2-thiocyano acetophenone and investigation of
reaction mechanism, Journal of Sulfur Chemistry, 34:3, 276-283
To link to this article: http://dx.doi.org/10.1080/17415993.2012.735672
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Journal of Sulfur Chemistry, 2013
Vol. 34, No. 3, 276–283, http://dx.doi.org/10.1080/17415993.2012.735672
Synthesis of 3,6-diaryl-1,4,5-thiadiazepines from substituted
2-thiocyano acetophenone and investigation of reaction
mechanism
M. Rahimizadeh, B. Feizyzadeh, M. Bakavoli* and H. Eshghi
Department of Chemistry, School of Sciences, Ferdowsi University of Mashhad, Mashhad 91775-1436, Iran
(Received 5 November 2011; final version received 27 September 2012)
In this work, we have studied the reaction of substituted 2-thiocyanoacetophenone and hydrazine hydrate
as a novel and simple pathway for the preparation of the substituted 1,4,5-thiodiazepine ring system. The
mechanism of this reaction revealed that in the initial step condensation of hydrazine with carbonyl groups
of substituted 2-thiocyanoacetophenons 2a–2f gives the corresponding substituted aromatic dithiocyano
azide intermediates which in turn undergo cyclization to1,4,5-thiadiazepines in the presence of hydrazine.
This cyclization is a novel method for the preparation of sulfide bond from the reaction of hydrazine and
a dithiocyano intermediate. An account of the reaction mechanism is given.
O
Ar
S
S
N
NH2
N2H4/EtOH
HCl/reflux
SCN
Ar
N
and n't
N
N
Ar
Ar
Keywords: 2-thiocyanoacetophenons; hydrazine; thiocyanic esters; 1,4,5-thiadiazepine, 1,3-oxathiole
1. Introduction
Organic thiocyanates include not only biologically important natural products such as anticancer
agents (1) formed by deglycosylation of glucosinolates derived from cruciferous vegetables, but
are also versatile intermediates for the synthesis of various organosulfur compounds. The unique
combination of a sulfur atom with the cyano group to generate thiocyanic esters provide a versatile
route for cyclization reactions that proceed under mild conditions. For example, thiocyanic esters
are important precursors for the synthesis of thiazoles and fused thiazoles (2). There are few
detailed studies on the chemical properties of organic thiocyano esters and the reaction mechanism
of this group in many reactions is not known. However, in the majority of reported reactions, the
cyano group is attacked by hard nucleophiles while soft nucleophiles attack the sulfur atom to
induce S–CN bond fission (3). For example, treatment of 2-thiocyano carbonyl compounds with
amines involves a nucleophilic attack on the cyano group to generate the thiazole ring system (2, 4).
The reaction of substituted 2-thiocyano acetophenone 2 with aromatic amines is an interesting
example, since the reaction products are dramatically influenced by acidic condition (5). Indeed,
*Corresponding author. Email: mbakavoli@yahoo.com, mbakavoli@um.ac.ir
© 2013 Taylor & Francis

Journal of Sulfur Chemistry 277
Figure 1. Reaction of 2-thiocyano acetophenone derivatives 2 with aromatic amines.
Figure 2. General process for produce 1H-dibenzo[b,f]pyrazolo[1,2-d][1,4,5]thiadiazepine-1,3(2H)-dione G.
NH2NH2 · H2SO4
EtOH, Reflux
Figure 3. Reported reaction of C with hydrazinium sulfate.
in strong acid, nucleophilic attack of amines at the carbonyl group is preferred over nucleophilic
attack on the thiocyano group and in the course of this reaction the substituted thiazole A is
produced. In the presence of a weak acid such as acetic acid, nucleophilic attack of amines on the
thiocyano groups takes a different course and gives a different substituted thiazole B (Figure 1).
1,4,5-Thiadiazepines are important heterocyclic compounds with an array of biological activ-
ities (6). The general route to 1,4,5-thiadiazepines includes cyclization of bis 2-keto sulfides
with hydrazine and substituted hydrazine compounds (7). Another method for the synthesis of
these compounds involves reduction of the nitro groups of bis (2-nitrophenyl) sulfide E to its
hydrazino derivative, which on treatment with substituted malonyl dihalides give the 1,3-dioxo-
1H-dibenzo[b,f]-pyrazolo-[1,2-d][1,4,5]-thiadiazepines G as anti-inflammatory compounds (6).
(Figure 2).
Recently, Saravanan et al. (8) reported the synthesis of 1,4,5-thiadiazepines D from the reaction
of 2-(4,5-dihydrothiazol-2-ylthio)-1-arylethanone C with hydrazinium sulfate (Figure 3).
In this paper, we wish to report the results of our studies on the reaction of substituted 2-
thiocyano acetophenone with hydrazine and give an account of the reaction mechanism.
2. Results and discussion
As a part of our continuing interest in the reaction of hydrazine with functionalized compounds
(9), we became interested in studying the reactions of 2-thiocyano acetophenone with hydrazine

278 M. Rahimizadeh et al.
and substituted hydrazines as a pathway for the synthesis of 1,3,4-thiadiazines 5a–5f. However,
on the basis of X-ray analysis and to our surprise the reaction with hydrazine led to the formation
of 1,4,5-thiadiazepine 4a–4f. Our attempts to prepare the 1,3,4-thiadiazines 5a–5f through the
reaction of substituted alkyl hydrazines such as methyl hydrazine and 2-hydroxy ethyl hydrazine
with 2-thiocyano acetophenone did not give the expected products but instead produced the
corresponding hydrazones similar to the ones which have already been reported in the literature
(10, 11) (Scheme 1).
2a–2f
4a–4f
5a–5f
Scheme 1. Reaction of 2a–2f with hydrazine and substituted hydrazine.
A literature survey revealed that this reaction has been studied previously by Abdelrazek and
Kandeel(9)wheretheyreportedthesynthesisofbis(4-aryl-S-hydro-thiazol-2-ylidene) H fromthe
reaction of substituted 2-thiocyano acetophenone 2a and 2b with hydrazine hydrate as illustrated
in Figure 4.
These authors erroneously assigned structure H for their product based on its spectral and
microanalytical data. They gave a molecular formula of C18H14N4S2(H, Ar = Ph) for their prod-
uct and surprisingly, assigned the wrong molecular weight of 267 for this compound. We have
investigated the same reaction and on the basis of single-crystal X-ray analysis (Figure 5), we
found that a 1,4,5-thiadiazepine ring system 4c is formed which is in obvious contradiction to the
structure reported previously (12).
The 1H NMR data of 4c is exactly the same as has been reported previously (7). In this spectrum,
there are signals assignable to methylene groups at δ 3.23–3.72 ppm and signals showing aromatics
Figure 4. Reported reaction of 2a,b with hydrazine by Abdelrazek.
Figure 5. The X-ray crystal structure of 4c.

Journal of Sulfur Chemistry 279
Table 1. Optimization of reaction conditions for 4a.
N2H4
(equiv.)
HCl
(equiv.)
Temperature
(◦C)
Time
(h)
Yield
(%)
Entry
Solvent
1
2
3
4
5
6
7
8
9
10
11
12
13
14
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
MeOH
AcOH
MeCN
1
1
1
0.5
2
3.5
5
7
8
9
–
rt
rt
8
Nil
7a
0.2
0.2
–
12
12
12
12
12
12
12
12
12
18
18
18
18
78
78
78
78
78
78
78
78
78
66
118
80
22
8a,b
35
47
56
64
68
68
69
54
60
44
0.2
0.3
0.3
0.3
0.4
0.4
0.4
0.4
–
8
8
8
8
0.4
Note: rt, room temperature.
aIsolated after column chromatography.
bHydrazinium sulfate and chloride were used as sources of N2H4 in ethanolic buffer solution at pH = 4.
protons at δ 7.43–7.91 ppm, respectively. The structure of 4c was further confirmed by mass
spectrum and microanalytical data.
To prepare substituted 1,4,5-thiodiazepines, the reaction of 2a with hydrazine was chosen as
the model experiment and the reaction condition was optimized as listed in Table 1. In most cases,
4a is produced in moderate to high yield by increasing the amount of hydrazine.
As can be seen from Table 1, in the absence of HCl as a catalyst at room temperature (Table 1,
Entry 1), the reaction did not proceed while in the presence of this catalyst (0.2 equiv.) 4a was
obtained only in 7% yield (Table 1, Entry 2). The yield of 4a can be increased to 22% with
N2H4 (1 equiv.) and HCl (0.2 equiv.) in refluxing ethanol for 12 h (Table 1, Entry 3). Even, the
reaction of an equimolar amount of 2a with hydrazonium chloride or hydrazonium sulfate gave
4a in low yield and 5a was not produced under these conditions (Table 1, Entry 4). Subsequently,
the amount of N2H4 was increased (Table 1, Entries 5–11) up to 8 equivalents and HCl to 0.4
equivalent and the solution was treated in boiling ethanol for 18 h to give 4a in 69% yield (Table 1,
Entry 11). However, further increase in the amount of hydrazine and HCl did not improve the
yield of 4a (Table 1, Entry 10). Next, the model reaction was performed in several solvents, such
as CH3OH, AcOH or CH3CN, with the result that the corresponding products were obtained in
only 54%, 60% and 44% yields, respectively, even if the reaction time was prolonged to 18 h
(Table 1, Entries 12–14). Thus, it was clear from the experiments that the best conditions for
obtaining 4a includes N2H4 (8 equiv.), HCl (0.4 equiv.) as a catalyst and EtOH as a solvent at
78◦C for 18 h (Table 1, Entry 11). Having established the optimal conditions, we then examined
the efficiency of this protocol using other substrates. Five substituted 2-thiacyanoacetophenones
2b–2f were chosen for reaction with hydrazine under the optimized conditions (Table 2). The
reactions proceeded very smoothly and in most cases gave the corresponding products 4a–4f in
good yields.
A plausible mechanistic pathway explaining the formation of 3,6-diaryl-1,4,5-thiadiazepine
4a–4f is outlined in Scheme 2. In this scheme, it is assumed that the initial nucleophilic addition
of hydrazine to two equivalents of 2-thiocyano acetophenone 2a–2f will give an intermediate I
which upon treatment with hydrazine will lead to 3,6-diaryl-1,4,5-thiadiazepines.
Consistent with the mechanistic proposal presented in Scheme 2, the intermediate I (Ar = Ph),
which was prepared unambiguously (Scheme 3) gave the same product 4a on treatment with
hydrazine.

280 M. Rahimizadeh et al.
Table 2. Synthesis of compounds 4a–4f.
Time
(h)
Yield
(%)
Entry
Compound
Ar
Product
1
2
3
4
5
6
2a
2b
2c
2d
2e
2f
C6H5
18
18
18
18
18
18
4a
4b
4c
4d
4e
4f
69
59
71
75
60
78
−
4-Me C6H4
−
4-Cl C6H4
−
4-Br C6H4
−
4-MeO C6H4
−
3-NO2 C6H4
Scheme 2. Mechanism pathways for the preparation of 4a–4f.
Scheme 3. Step-by-step reactions for the preparation of 4a.
In continuation of this work, we have studied the reaction of some alkyl dithiocyano compounds
6a–6d with hydrazine hydrate to further explore the scope of the reaction of the thiocyano group
and hydrazine. In most cases, the cyclic alkyl sulfides 7a–7c were formed, but in case of 1,2-
dithiocyano ethane 6d the 3-amino-1,3,5-dithiazepane-2,4-dione 7d was obtained. Based on these
observations, it can be suggested that the reaction of hydrazine with dithiocyano compounds will
lead to the formation of a sulfide bond (such as intermediate I) except for compound 6d, which
is not able to form such a bond due to the highly strained ring (Scheme 4).
It is surprising that the reaction of 2-thiocyano-(4ꢀ-nitrophenyl)-acetophenone 2g with
hydrazine hydrate under optimized condition proceeded rapidly to give the unexpected oxathi-
ole 8g. The proposed mechanism for this reaction is depicted in Scheme 5. As can be seen in
the scheme, hydrazine as a base enolizes 2g with subsequent nucleophilic attack of the enolic

Journal of Sulfur Chemistry 281
7a–7c
6a–6d
n
n
Scheme 4. Reaction of some alkyl dithiocyano compounds with hydrazine.
oxygen in intermediate 2ꢀg on the thiocyano group to afford 8g. Likewise, other bases such as
KOH, pyridine, Et3N or N,N-dimethylhydrazine play the same role and gave the same product
8g (Scheme 5). According to our observations, the presence of electron-withdrawing groups such
as NO2 at the para position encourages enolization of 2g instead of nucleophilic addition of
hydrazine to carbonyl moiety.
Scheme 5. Isomerization of 2g in the presence of base.
3. Conclusion
In summary, we have reported a novel and simple route for the preparation of 3,6-diaryl-
1,4,5-thiadiazepine from the reaction of hydrazine and substituted 2-thiocyano acetophenones
in moderate to high yield. This reaction can be extended for the synthesis of many more
thiodiazepines.
4. Experimental section
All the reagents were purchased from local suppliers and used without purification. Compounds
4a–4e and 7a–7c are known compounds and their spectral data are available from Ref. (7) and
web site SDBS (spectral database system for organic compounds), respectively. Chromatography
refers to open column chromatography on silica gel (100–200 mesh). The 1H NMR (300 MHz)
and the 13C NMR (75 MHz) were recorded on a Bruker Avance 300 MHz spectrometer. Chemical
shifts are reported in ppm downfield from TMS as the internal standard; coupling constants J
are given in Hertz. IR spectra were recorded on a Nicolet 510P FT-IR spectrometer and only
major peaks are reported in cm−1. The mass spectra were scanned on a Varian Mat CH-7 at 70 eV.
Elemental analyses were performed on a Thermo Finnigan Flash EA microanalyzer.
4.1. General procedure for the preparation of 3,6-diaryl-2,7-dihydro-1,4,5-thiadiazepine
(4a–4f)
Hydrazine hydrate 80% (0.008 mol) and concentrated HCl (0.3 ml) were added to a mixture of
2-thiocyano acetophenone derivatives 2a–2f (0.001 mol) in ethanol (5 ml); then the resulting

282 M. Rahimizadeh et al.
mixture was heated under reflux for an identical time, and then the reaction mixture was cooled
to room temperature and dripped into 50 ml cold water and acidified with HCl 10% to pH = 6–7.
The residue solid material was filtered and washed with water and recrystallized from ethanol to
produce pure thiadiazepines 4a–4f.
4.1.1. 2,7-dihydro-3,6-diphenyl-1,4,5-thiadiazepine 4a
mp 180–182oC; [Found: C, 72.69; H, 5.32; N, 10.70; S, 11.48. C16H14N2S requires C, 72.15;
H, 5.30; N, 10.52; S, 12.04%]; υmax (KBr) 3036, 1552, 1436, 1323 cm−1; 1H NMR (CDCl3) δH
(ppm) 3.23 (2H, d, J = 12 Hz), 3.81 (2H, d, J = 12 Hz), 7.42–7.81(10H, m, 2Ph); 13C NMR
(CDCl3) δC (ppm) 26.41, 127.13, 128.75, 130.13, 135.17, 151.63; MS (m/z) M+, 266.
4.1.2. 2,7-dihydro-3,6-bis(4ꢀ-methylphenyl)-1,4,5-thiadiazepine 4b
mp 206–208◦C; [Found: C, 73.56; H, 6.02; N, 9.36; S, 10.88. C18H18N2S requires C, 73.43; H,
6.16; N, 9.51; S, 10.89%]; υmax (KBr) 2894, 1552, 1506, 1435, 1318, 890, 823 cm−1; 1H NMR
(CDCl3) δH (ppm) 2.42 (6H, s, 2CH3), 3.43 (2H, d, J = 12 Hz), 3.76 (2H, d, J = 12 Hz), 7.22
(4H, d, J = 8 Hz), 7.82 (4H, d, J = 8 Hz);13C NMR (CDCl3) δC (ppm) 21.30, 26.32, 127.05,
129.49, 132.43, 140.35, 151.52; MS (m/z) M+, 294.
4.1.3. 2,7-dihydro-3,6-bis(4ꢀ-chloro-phenyl)-1,4,5-thiadiazepine 4c
mp 195–196◦ C; [Found: C, 56.96; H, 3.58; N, 8.41; S, 9.43. C16H12Cl2N2S requires C, 57.32; H,
3.61; N, 8.36; S, 9.56%]; υmax (KBr) 2980, 1580, 1490, 1433, 1320, 1089, 833 cm−1; 1H NMR
(CDCl3) δH (ppm) 3.23 (2H, d, J = 13 Hz), 3.72 (2H, d, J = 13 Hz), 7.43 (4H, d, J = 9 Hz), 7.91
(4H, d, J = 8 Hz); 13C NMR (CDCl3) δC (ppm) 26.30, 128.42, 129.05, 133.41, 136.37, 150.63;
MS (m/z) (M + 2)+, 336.
4.1.4. 2,7-dihydro-3,6-bis(4ꢀ-bromophenyl)-1,4,5-thiadiazepine 4d
mp 208–210◦ C; [Found: C, 45.13; H, 2.81; N, 6.64; S, 7.43. C16H12Br2N2S requires C, 45.31; H,
2.85; N, 6.60; S, 7.56%]; υmax (KBr) 2898, 1575, 1435, 1323, 1090, 835 cm−1; 1H NMR (CDCl3)
δH (ppm) 3.24 (2H, d, J = 13 Hz), 3.76 (2H, d, J = 13 Hz), 7.48 (4H, d, J = 8.5 Hz), 7.82 (2H,
d, J = 8 Hz); 13C NMR (CDCl3) δC (ppm) 26.29, 128.57, 132.05, 133.83, 143.37, 150.68; MS
(m/z) (M + 4)+, 426.
4.1.5. 2,7-dihydro-3,6-bis(4ꢀ-methoxyphenyl)-1,4,5-thiadiazepine 4e
mp 214–216◦ C; [Found: C, 66.30; H, 5.51; N, 8.48; S, 9.73. C18H18N2O2S requires C, 66.23; H,
5.56; N, 8.58; S, 9.82%]; υmax (KBr) 2920, 1579, 1555, 1436 cm−1; 1H NMR (CDCl3) δH (ppm)
3.25 (2H, d, J = 13 Hz), 3.82 (2H, d, J = 13 Hz), 3.86 (6H, s, 2OCH3), 6.97 (2H, d, J = 8 Hz),
7.89 (2H, d, J = 8 Hz); 13C NMR (CDCl3) δC (ppm) 26.41, 127.13, 128.75, 130.13, 135.17,
151.63; MS (m/z) M+, 326.
4.1.6. 2,7-dihydro-3,6-bis(3ꢀ-nitrophenyl)-1,4,5-thiadiazepine 4f
mp 180–182◦ C; [Found: C, 53.87; H, 3.38; N, 15.81; S, 8.73. C16H12N4O4S requires C, 53.93;
H, 3.39; N, 15.72; S, 9.00%]; υmax (KBr) 2894, 1552, 1506, 1435, 1318, 890, 823 cm−1; 1H NMR
(CDCl3) δH (ppm) 3.34 (2H, d, J = 12 Hz), 3.86 (2H, d, J = 12 Hz), 7.54–8.72 (8H, m, C6H4);

Journal of Sulfur Chemistry 283
13C NMR (CDCl3) δC (ppm) 26.41, 127.13, 128.75, 130.13, 132.31, 135.17, 151.63; MS (m/z)
M+, 356.
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