One-step construction of unsymmetrical thioureas and oxazolidinethiones from amines and carbon disulfide: Via a cascade reaction sequence

Article abstract of DOI:10.1039/c9ra04540f

A concise and versatile method for the construction of unsymmetrical thioureas and oxazolidinethiones from amines and carbon disulfide has been achieved in DMSO without addition of extra reagents. The present protocol is compatible with various secondary amines and primary amines, and suitable for intermolecular and intramolecular reactions. Diverse unsymmetrical thioureas and oxazolidinethiones were efficiently obtained in good to excellent yields via a cascade reaction sequence.

Full text of DOI:10.1039/c9ra04540f

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One-step construction of unsymmetrical thioureas
and oxazolidinethiones from amines and carbon
disulfide via a cascade reaction sequence†
Cite this: RSC Adv., 2019, 9, 26768
*
*
Chaochao Ding, Shaoli Wang, Yaoguang Sheng, Qian Dai, Yunjie Zhao, Guang Liang
*
and Zengqiang Song
A concise and versatile method for the construction of unsymmetrical thioureas and oxazolidinethiones
from amines and carbon disulfide has been achieved in DMSO without addition of extra reagents. The
present protocol is compatible with various secondary amines and primary amines, and suitable for
intermolecular and intramolecular reactions. Diverse unsymmetrical thioureas and oxazolidinethiones
were efficiently obtained in good to excellent yields via a cascade reaction sequence.
Received 17th June 2019
Accepted 17th August 2019
DOI: 10.1039/c9ra04540f
rsc.li/rsc-advances
The third alternative way is the direct reaction of amines and
carbon disulde.8 In this transformation, starting materials are
Introduction
easy to be gained and no extra synthetic steps are required.
Therefore, it is considered as a straightforward route for the
preparation of substituted thioureas. In this regard, several
methods have been developed on the basis of this strategy for
the preparation of symmetrical and unsymmetrical thioureas.
Nevertheless, the reported methods for the synthesis of
unsymmetrical thioureas contain two steps, two kinds of
amines need to be separately added to the reaction system
under basic or microwave conditions (Scheme 1a).8e,8f In 2017,
the Jiang's group reported a novel method for the preparation of
unsymmetrical thioureas with amines, sulfur and chloroform in
Thioureas have attracted signicant interest in medicinal
chemistry due to their broad spectrum of biological activities.
Their derivatives and metal complexes have been demonstrated
to have antimicrobial, analgesic, anticancer and anti-
inammatory activities.1 Some of them have been developed
as drugs, such as hyperthyroidism drugs,1c sedative hypnotics
drugs,1d to treat diseases in clinics. Thioureas are also impor-
tant in pesticide chemistry, a variety of thiourea derivatives have
been used as insecticides, herbicides, rodenticides and fungi-
cides.2 In synthetic chemistry, thioureas are useful blocking
buildings for the construction of many valuable compounds,
such as guanidines, amides and sulfur-containing heterocy-
cles.3 Additionally, they also play an important role in organo-
catalysis as auxiliaries or catalysts.4
On account of the usefulness of thioureas in different elds,
methods for synthesizing this type of compounds have been
widely explored.5–8 Generally, three synthetic strategies are
commonly used for the preparation of substituted thioureas.
One way is transforming carbonyl to thiocarbonyl employing
Lawesson reagents or P2S5.6 The second way is condensation or
substitution of amines with prepared isothiocyanate, 1-(meth-
yldithiocarbonyl)imidazole, thiocarbamoylbenzotriazoles, or
derivatives thereof.7 Based on these two synthetic strategies,
many methods were developed for the synthesis of symmetrical
and unsymmetrical thioureas. However, these protocols require
extra synthetic steps for the preparation of starting materials,
leading to low synthetic efficiency and a poor substrate scope.
Chemical Biology Research Center at School of Pharmaceutical Sciences, Wenzhou
Medical University, Wenzhou, Zhejiang 325035, China. E-mail: songzengqiang09@
163.com; Fax: +86-577-86699396; Tel: +86-577-86699396
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c9ra04540f
Scheme 1 Straightforward methods for the synthesis of unsymmetric
thioureas.
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the presence of potassium tert-butoxide in mixture solvents DMSO was the best solvent for this transformation compared to
(Scheme 1b).9 This transformation also contains two steps. A THF, MeOH, MeCN, HFIP and chlorobenzene (entries 3–9); the
one-step reaction for the synthesis of unsymmetrical thioureas reaction provided the expected product in 89% yield in 1.5 h
was achieved by Nguyen's group with isocyanides, amines and (entry 5). This is probably because the solvent increased the
elemental sulfur (Scheme 1c).10 However, this protocol is only basicity of the amines. Dropping the temperature to 40 C, the
ꢀ
suitable for aliphatic amines. Obviously, the development of reaction time was prolonged and the yield of the product was
convenient and versatile methods for one-step construction of lowered (entry 10). To our delight, the product was obtained in
unsymmetrical thioureas is in high demand.
95% yield when the amounts of carbon disulde and diethyl
Cascade reactions,11 which involve multistep trans- amine were changed to 1.2 equiv. (entry 12). Therefore, the
formations in a one-pot fashion, are attractive in modern optimal conditions for the synthesis of unsymmetrical thio-
synthetic chemistry due to their simplicity and atom-economy. ureas via acascade reaction sequence were gained: carbon
In continuation of our study on the development of efficient disulde (1.2 equiv.), diethyl amine (1.2 equiv.), in DMSO (2 mL)
ꢀ
routes for the preparation or modication of heteroatom- at 70 C for 1 h (entry 12). Moreover, a 10 mmol scale reaction
containing compounds,12 we are of great interest in the explo- was carried out under optimal reaction conditions, affording
ration of new routes for the preparation of unsymmetrical the desired product in 89% yield (entry 14). This result
thioureas. Herein, we wish to disclose a novel protocol for the demonstrated the easy scalability of this transformation.
synthesis of unsymmetrical thioureas via a cascade reaction
sequence (Scheme 1c).
Having identied the optimal parameters of the model
reaction, the substrate scope and generality of this trans-
formation were evaluated. First of all, various secondary amines
were tested with 2-naphthylamine under standard reaction
conditions (Table 2, 4b–4m). Application of N-methylcyclohex-
ylamine was successful, providing the desired product in 94%
yield (4b). Changing dialkylamines to N-benzylethanamine or
dibenzylamine, the corresponding products were afforded with
90% and 95% yields, respectively (4c, 4d). Reactions of 2-
naphthylamine with cyclic amines containing different ring
numbers, substituents or heteroatoms proceeded well, giving
the related products in good to excellent yields (4e–4m).
Subsequently, we turned our attention to explore the reaction of
2-naphthylamines with primary amines under the optimal
conditions. To our delight, non-cyclic and cyclic alkylamines
were suitable for this transformation, the desired products were
obtained in good yields in 24 h (4n, 4o). This protocol was also
applicable for benzylamines. Reactions of benzylamines with
various substituents at different positions of benzene ring
worked well, affording the desired products in good to excellent
yields (4p–4r). Aniline and its derivatives were also examined in
this transformation. Gratifyingly, when methyl or methoxyani-
lines were used as substrates, the expected products were ob-
tained in good yields in 12 h (4s, 4t). However, reactions of
aniline or anilines with electron-withdrawing substituents
provided not only unsymmetrical thioureas but also symmet-
rical thioureas from the reactions of anilines with carbon
disulde, and the formed two products were inseparable due to
their similar polarity. This is probably because of weak nucle-
ophilicity of these anilines. Application of 3-aminopyridine in
this transformation was successful, the desired product was
obtained in 78% yield in 12 h (4u). Intriguingly, this protocol
was also suitable for the reactions of 3-aminoquinoline and
anilines, the desired products were gained in good yields (4v–
4z).13 Symmetrical thioureas were obtained from the reactions
of anilines with weak nucleophilicity, which were consistent
with the results of the reactions of 2-naphthylamine and
anilines. This transformation allows the preparation of
unsymmetrical thioureas using diverse naphthylamines, ami-
noquinolines and aminoisoquinolines (4aa–4ai). Moreover, the
reactions of anilines with benzyl amines were successful, the
Results and discussion
Methods for the preparation of unsymmetrical thioureas using
naphthylamines as substrates are rare.1f,7g,9 Our exploration
started with using 2-naphthylamine (1a), carbon disulde (2)
and diethylamine (3a) as model substrates, all substrates were
added into the reaction system in one step. Initially, the reac-
tion was performed in DCE at 70 ꢀC. Unfortunately, only a trace
amount of the desired product was observed aer 12 h (Table 1,
entry 1). Changing DCE to toluene, the desired product was
obtained in 58% isolated yield (entry 2).
Encouraged by this result, the reaction was examined with
different solvents to improve the yield of 4a. It was found that
Table 1 Optimization of the reaction conditionsa
Entry
Solvent
Time (h)
Yieldb (%)
1
2
3
4
5
6
7
DCE
Toluene
THF
MeOH
DMSO
MeCN
DMF
12
12
12
12
1.5
12
4
Trace
58
72
78
89
81
85
8
9
HFIP
12
12
12
1
Trace
69
62
95
89
Chlorobenzene
DMSO
DMSO
DMSO
10c
12d
14e
12
a
Reaction conditions: 1a (0.2 mmol), 2 (0.4 mmol), 3a (0.4 mmol) in
b
solvent (2 mL) at 70 ꢀC. Yield refers to isolated products aer
c
d
column chromatography. Reaction was performed at 40 ꢀC. 1.2
e
equiv. of 2 and 3a were used. 10 mmol of 1a were used. HFIP ¼
hexauoroisopropanol.
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Table 2 Exploration of substrate scope of aminesa
Table 3 Exploration of intramolecular reactionsa
a
Reaction conditions: 5 (0.2 mmol), 2 (0.24 mmol) in DMSO (2 mL) at
70 ꢀC.
We also investigated the mechanism of this protocol. Radical
trapping experiments were examined in the presence of 1.2
equiv. of TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxy) or BHT
(butylhydroxytoluene) under the standard reaction conditions
(Scheme 2(a) and (b)). Almost no inhibition of the generation of
the desired product 4a was observed. These results indicated
that this process did not involve radicals. In order to under-
stand the sequence of this three components reaction, the
reaction of 2-naphthylamine and carbon disulde was carried
out under the standard reaction conditions. Aer 1 h, almost no
reactions occurred (Scheme 2(c)). A new spot was observed from
the reaction of diethylamine and carbon disulde under the
standard reaction conditions, it was conrmed as dieth-
ylcarbamodithioic acid (Scheme 2(d)).8d–f Subsequently, adding
2-naphthylamine to the reaction system, the desired product
was obtained in 95% yield. These experiments suggested that
this transformation was initiated by the reaction of diethyl
amine and carbon disulde.
a
Reaction conditions: 1 (0.2 mmol), 2 (0.24 mmol), 3 (0.24 mmol) in
b
DMSO (2 mL) at 70 ꢀC. 0.24 mmol of 1 were used, 0.2 mmol of 3
were used.
corresponding products were obtained in excellent yields (4aj,
4ak).
Aer successful application of this novel transformation for
the preparation of unsymmetrical thioureas by intermolecular
reactions of different amines with carbon disulde, we
wondered if this protocol is suitable for the intramolecular
reactions.7c,8c,8d,8g,9 Reactions of naphthylamine, aniline, alkyl-
amine and benzylamine were tested under standard reaction
conditions, all of them gave the desired products in excellent
yields (Table 3, 6a–6d). To our delight, this approach was also
applicable for the construction of oxazolidinethiones (6e–6i),
which are great of importance in organic chemistry and
medicinal chemistry.9,14
Scheme 2 Mechanistic investigation.
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protocol for the preparation of thioureas and oxazolidine-
thiones from amines and carbon disulde via a cascade reac-
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good compatibility with different amines as well as broad
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