Isothiouronium salts as useful and odorless intermediates for the synthesis of thiaalkylimidazolium ionic liquids

Article abstract of DOI:10.1016/j.tetlet.2019.02.013

A simple and odorless route for the synthesis of monocationic and dicationic thiaalkylimidazolium ionic liquids (ILs) is reported. Our approach starts with the selective monoalkylation of dihalogenated substrates by methylimidazole derivatives, followed b

Full text of DOI:10.1016/j.tetlet.2019.02.013

Accepted Manuscript
Isothiouronium salts as useful and odorless intermediates for the synthesis of
thiaalkylimidazolium ionic liquids
Gabriela I. Matiello, Alessandra Pazini, Kácris I.M. da Silva, Rafaela G.M. da
Costa, Günter Ebeling, Jairton Dupont, Jones Limberger, Jackson D. Scholten
PII:
S0040-4039(19)30134-0
https://doi.org/10.1016/j.tetlet.2019.02.013
TETL 50607
DOI:
Reference:
Tetrahedron Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
23 January 2019
4 February 2019
6 February 2019
Please cite this article as: Matiello, G.I., Pazini, A., da Silva, K.I.M., da Costa, R.G.M., Ebeling, G., Dupont, J.,
Limberger, J., Scholten, J.D., Isothiouronium salts as useful and odorless intermediates for the synthesis of
thiaalkylimidazolium ionic liquids, Tetrahedron Letters (2019), doi: https://doi.org/10.1016/j.tetlet.2019.02.013
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Tetrahedron Letters
Isothiouronium salts as useful and odorless intermediates for the synthesis of
thiaalkylimidazolium ionic liquids
Gabriela I. Matielloa, Alessandra Pazinia,b, Kácris I. M. da Silvaa, Rafaela G. M. da Costab, Günter
Ebelinga, Jairton Duponta, Jones Limbergerb* and Jackson D. Scholtena*
a Laboratory of Molecular Catalysis, Institute of Chemistry, UFRGS, Av. Bento Gonçalves, 9500, CEP 91501-970, Porto Alegre-RS, Brazil.
b Department of Chemistry, Pontifical Catholic University of Rio de Janeiro, Rua Marquês de São Vicente, 225, Gávea, 22451-900, Rio de Janeiro-RJ, Brazil.
*e-mail: limberger@puc-rio.br, phone: +55 21 3527 1673; jackson.scholten@ufrgs.br, phone: +55 51 3308 9633.
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
A simple and odorless route for the synthesis of monocationic and dicationic thiaalkylimidazolium
ionic liquids (ILs) is reported. Our approach starts with the selective monoalkylation of
dihalogenated substrates by methylimidazole derivatives, followed by the synthesis of odorless
isothiouronium salts via reaction with thiourea. The target ILs are obtained after sequential
hydrolysis-alkylation of the isothiouronium salts followed by anion metathesis in water. After
extraction, the novel thiaalkylimidazolium ILs are obtained with high purity, without the
requirement of additional purification steps. In order to demonstrate their applicability, two of
these task-specific ILs were employed as ligands in Ullmann and Suzuki couplings and also as
charged probes to detect copper intermediates via ESI(+)-MS.
Available online
Keywords:
Ionic liquids
Sulfur compounds
Isothiouronium salts
Cross-coupling
2009 Elsevier Ltd. All rights reserved.
Imidazolium-based ionic liquids (ILs) are among the most
studied media for organic and inorganic reactions.1 The facile
modulation of the physical-chemical properties either by anion
exchange or alkyl chain transformations has allowed a huge
growth in the applications of these ionic fluids.2 These compounds
are widely applied for a plethora of purposes, as media for
homogeneous and nanoparticle catalysis,3-6 CO2 capture,7
electrochemistry.8 Moreover, these features in combination with
their very low vapor pressure, non-flammability and recyclability
render these materials ideal media for green chemistry
transformations.9
odor of the synthetic intermediates, thiols or haloalkylsulfides
(Scheme 1).24, 25, 28 Taking into account these drawbacks, herein we
describe a simple and odorless route to produce various
monocationic and dicationic thiaalkylimidazolium ILs. Our
approach is based on the selective monoalkylation of
dihalogenated substrates (1,2-dibromoethane and 1,3-
dibromopropane) by methylimidazole derivatives followed by the
synthesis of odorless isothiouronium salts. These sulfur-based
salts can be hydrolyzed and alkylated in situ, furnishing the
corresponding thiaalkylimidazolium salts. Finally, after in situ
anion metathesis and extraction, the target ILs can be obtained
with reasonable to high yields and high purity, without additional
purification steps.
In recent years, the development of task-specific ILs has been
reported.10-12 These compounds can be used as task specific
acidic13 and basic14 ILs, ionophilic ligands and/or reaction media
for various catalytic transformations, including cross-coupling
16
19
reactions,15, oxidation reactions,17 olefin metathesis,18, and
hydrogenation reactions.20, They also allow the detection of
21
catalytic intermediates via electrospray ionization mass
spectrometry (ESI-MS) experiments.22 Besides their applications
in homogeneous catalysis, N-, O-, P- and S-substituted
imidazolium ILs have also been employed to modulate the stability
and the reactivity of metallic nanoparticles.23
Scheme 1. Strategies used to synthesize thiaalkylimidazolium ILs: this
work versus the literature.
After preparation, in order to illustrate the applicability of these
compounds, the thiaalkylimidazolium ILs were applied as ligands
in Ullmann and Suzuki coupling reactions.
In particular, thiaalkylimidazolium ILs have been applied in the
extraction of heavy metal cations from water,24 in electrochemical
studies,25 as recyclable and odorless Swern reagents26 and as
ligands for transition-metal complexes.27 However, the synthesis
of these compounds has some drawbacks, mainly related to the
limited synthetic routes for their production and the toxicity and

2
Tetrahedron Letters
Results and Discussion
In our approach to prepare the thiaalkylimidazolium ILs,
initially 1-methylimidazole or 1,2-dimethylimidazole were reacted
with 1,2-dibromoethane or 1,3-dibromopropane in order to obtain
the monoalkylated products. Selective monoalkylation was only
achieved using specific conditions for each imidazolium/alkyl
bromide pair (Scheme 2).
Scheme 3. Synthesis of isothiuronium imidazolium salts 2a-c.
For the synthesis of the desired thiaalkylimidazolium ILs,
sequential hydrolysis-alkylation reactions were performed in
water followed by in situ anion metathesis. With respect to
dicationic ILs (Scheme 4), initially, 2a-c were heated at reflux with
KOH in water for 15 minutes. Afterwards, 1,2-dibromoethane,
1,3-dibromopropane
or
bis(2-chloroethyl)ammonium
hydrochloride were added and the mixture was heated at reflux for
an additional 15 minutes. For ILs 3a-c, the final solution was
adjusted to pH = 6 and the water was evaporated before extraction
of the ILs with methanol. For ILs 4a-c, 5b, 6b and 7b, LiNTf2 (or
KPF6 for IL 5a) was added to the reaction mixture and the ILs were
extracted from water with ethyl acetate, affording the
thiaalkylimidazolium ILs in 69-99% yield. Regarding the
monocationic ILs (Scheme 5), after hydrolysis, monoalkylation
and anion exchange afforded compounds 8b and 9b in 74% and
52% yield, respectively, after direct extraction.
Scheme 2. Synthetic procedure for the preparation of bromo-substituted
imidazolium salts 1a-c.
In order to convert the brominated imidazolium salts into
isothiouronium imidazolium salts 2, intermediates 1 were reacted
with thiourea in ethanol at reflux. Under these conditions,
compounds 2a-c were obtained in 65-81% yield (Scheme 3). It is
noteworthy that the isothiouronium salts are odorless compounds
and the work-up is very simple, since the addition of diethyl ether
to the reaction mixture induces product crystallization.
Scheme 4. Isothiouronium salts as intermediates in the synthesis of dicationic thiaalkylimidazolium ILs. a Unless to 3a-c.
Scheme 5. Isothiouronium salts as intermediates in the synthesis of monocationic thiaalkylimidazolium ILs.
In terms of physical-chemical properties, ILs 3a-c are highly
hygroscopic, due the nature of bromine anion and also the NH
group in the center of the structure, which enables hydrogen
bonding between water and the ILs. The NH group also induces
Tm temperature. This behavior can be regarded as being due to the
methyl group, which hampers the formation of H-bonds between
the cations and anions of the IL. The influence of chain length 4a
(2C) and 4c (3C) cannot be verified because the Tm and Tf for 4c
were not obtained from DSC analysis.
-
the hygroscopicity in ILs with anions such as NTf2 , a classical
anion of hydrophobic ILs.
ILs 4a-c are liquids at room temperature, while 5a exhibit wax
aspect at the same temperature. As shown in Table 1, ILs 4a and
5a have a Tm above 100 ºC, although they are liquid and wax
respectively at room temperature even after being dried at 100 ºC
under vacuum for several hours. This can be explained by the
hygroscopicity of these ILs because of strong H-bonds involving
water and both the NH of the side chain and the C2H on the
imidazolium ring. This behavior allows the confinement of water
Differential scanning calorimetry (DSC) analysis was
performed to determine the influence of the ILs structure on their
-
thermal behavior. The anion influence is observed when 4a (NTf2
) and 5a (PF6-) are compared, where the PF6- anion confers a lower
-
Tm value to the IL than NTf2 . The influence of a substituent at the
C2 position of the imidazolium ring was also evaluated by
comparing 4a (H) and 4b (CH3), where compound 4b has a lower

molecules that are trapped inside the ionic network constituted of
contact ion pairs on the IL and hampers the removal of water even
under vacuum.29 The Tm for IL 4b is lower than the Tm for 4a and
5a due to the absence of H-bonds since the C2 position of the
imidazolium ring of 4b is protected by a CH3 substituent.
Thermogravimetric analysis (TGA) of the ILs were performed in
order to determine the thermal stability of the compounds (Table
1). As depicted in Table 1, the thermal stability was 6b ≥ 4c > 4b
> 7b > 4a > 8b > 5a. In general, monocationic ILs displayed lower
Tonset than dicationic ones. This result is in agreement with the
literature since the observed higher thermal stability of dicationic
ILs is attributed to the greater charge and intermolecular
interactions, their higher molecular weight, higher density, smaller
free volume and higher shear viscosity.30 The reversibility of the
breakdown reactions allows recombination of the fragments which
do not move away due to this “cage effect”.
MS after two hours (ESI, Fig. S32). One peak was observed related
to a structure with the characteristic isotope distribution of a
copper species (m/z = 340). This peak was attributed to the copper-
nucleophile species [Cu(6b)OAr]+I-, which has a double positive
charge due to the ionophilic probe. Copper-nucleophile species
have been reported as being capable of reacting with aryl halides
under mild conditions.31-37 This finding suggests that the formation
of copper-nucleophilic species precedes activation of the aryl
halide, as observed in the literature.38
Taking into account that various organosulfur ligands have
been synthesized and applied in Suzuki cross-coupling reactions39
with advantages (thermal stability and air/moisture insensitivity)
over classic phosphines or N-heterocyclic carbenes (NHCs), the
applicability of the thiaalkylimidazolium ILs was extended to the
Suzuki reaction. Initially, the Pd precursor [PdCl2(6b)] was
prepared by reacting PdCl2 with LiCl to generate the [PdCl4]2-
anion, and then the dicationic ligand was added to form the desired
complex (ESI, Fig. S34). The [PdCl2(6b)] complex was applied as
the catalyst precursor to promote the C-C coupling between
bromotoluene and phenylboronic acid (Table 3).
Table 1. Thermal properties of ILs obtained by DSC and TGA
LI
4a
5a
4b
4c
7b
6b
8b
Tonset (ºC)a
286
Tf (ºC)b
133.5
Tm (ºC)b
168.5
124.2
53.7
268
305
340
289
350
<300
Table 3. Suzuki coupling reaction between 4-bromotoluene and phenylboronic
acid promoted by [PdCl2(6b)] as the catalyst precursor
89.6
n.o.
-
-
-
n.o.
-
-
-
aTGA; bDSC; n.o. = not observed.
In order to evaluate the potential applicability of these
thiaalkylimidazolium ILs, selected compounds were applied as
ionophilic ligands for stabilizing the active species in copper-
catalyzed Ullmann C-O coupling and as charged probes to detect
copper intermediates in the same reaction. The coupling between
4-bromoacetophenone and 4-tert-butylphenol in the presence of
CuI and ligand 8b were used as a model (Table 2). Initially,
conditions screening was performed and reasonable results were
attained using Cs2CO3 in toluene at 100 C (Entry 3). Excellent
conversion and selectivity were observed with ligand 6b (Entry 5,
Table 2), while low conversion was attained without the ligand
(Entry 6, Table 2). Therefore, the presence of an IL as ligand
provides a significant positive effect on the catalytic process.
Entry
Solvent
Base
Temp.
(C)
80
80
100
80
100
100
100
100
time
(h)
2
2
18
2
2
18
2
Conv.
(%)
0
0
>99
99
99
>99
99
1
2
3
4
5
H2O
THF
THF
KOH
KOH
K2CO3
KOH
K2CO3
K2CO3
Toluene
Toluene
Toluene
Toluene KF·2H2O
Toluene K2CO3
6
7
8a
18
77
Reagents and conditions: bromotoluene (0.25 mmol), phenylboronic acid
(0.275 mmol), base (1.0 mmol), [PdCl2(6b)] (0.5 mol%), degassed solvent,
anisole as an internal standard. Conversion determined by GC. aPd(OAc)2
without ligand and diphenylether as an internal standard.
Table 2. Coupling reaction between 4-bromoacetophenone and 4-tert-
butylphenol in the presence of CuI employing thiaalkylimidazolium ILs as
ligands
As observed in Table 3, except for the reactions in THF and
water at 80 C (Entries 1 and 2), all experiments showed
satisfactory conversion. In addition, a control experiment using
Pd(OAc)2 showed lower conversion (Entry 8) when compared to
the reactions employing the IL ligand (see Entries 5 and 6). Indeed,
by comparing entries 5 and 8 it is clear that the complex
[PdCl2(6b)] allowed almost full conversion in only 2 h whereas
Pd(OAc)2 led to only 77% conversion over 18 h. These results
indicate that the sulfur-based IL ligand has an important role for
the efficiency of the reaction. This is probably related to the
stabilizing effect provided by the IL ligand avoiding the
aggregation of Pd particles in the reaction medium. Further
substrates were evaluated with the intention to widen the scope of
the reaction. Indeed, the catalytic system produced the coupling
products with high conversions and, in particular, could activate
the C-Cl bond in chloroacetophenone (Table 4).
Entry
Ligand
Base
Solvent
Conv.
(%)
10
31
64
71
90
11
Sel.a
(%)
>99
>99
>99
94
1
2
8b
8b
8b
8b
6b
-
K3PO4
K3PO4
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Dioxane
Toluene
Toluene
Toluene
Toluene
Toluene
3
4b
5
>99
>99
6
Reagents and conditions: 4-bromoacetophenone (0.5 mmol), 4-tert-
butylphenol (0.6 mmol), base (1 mmol), ligand (10 mol%), CuI (10 mol%),
solvent (2 mL), 100 °C, 24 h. Conversion and selectivity determined by GC.
aBy-products: homocoupling and aryl halide reduction. bReaction performed at
125 C in a resealable Schlenk tube.
Table 4. Suzuki coupling reaction of different substrates and phenylboronic
acid promoted by [PdCl2(6b)] in toluene
A reaction under the optimized conditions (Entry 5, Table 2)
was monitored by ESI-MS experiments in positive mode for the
detection of intermediates with ligand 6b as an ionophilic probe.
First, the solution of CuI and ligand 6b was analyzed and the free
ligand with one counter-anion unity (m/z = 621), the doubly
charged free ligand (m/z = 170) and the ligand coordinated at the
Cu metal center (m/z = 964) were detected (ESI, Fig. S31).
Continuing the experiment, the base, 4-bromoacetophenone and
tert-butylphenol were added and the solution was analyzed by ESI-
Entry
Substrate
Bromoacetophenone
Bromoanisole
Conv. (%)a
1
2
3
99
99
97
Bromobenzotrifluoride

4
Tetrahedron Letters
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4
5
Chloroacetophenone
Chloroanisole
99
16
99-121.
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Chem., 5 (2007) 671-678.
Reagents and conditions: aryl halide (0.25 mmol), phenylboronic acid (0.275
mmol), K2CO3 (1.0 mmol), [PdCl2(6b)] (0.5 mol%), 100 C, 18 h, degassed
solvent (2 mL), anisole as an internal standard. Conversion determined by GC.
Due to these promising results, imidazolium-based ILs were
used as media for the Suzuki coupling reaction. In these tests, the
system from entry 6 (Table 3) was employed under biphasic
conditions (toluene/IL). Among the ILs tested, the best conversion
was achieved using BMI·BF4, but it was less active when
compared to the homogeneous reaction conducted in toluene
(compare Tables 3 and 5). Unfortunately, attempts to recycle the
biphasic catalytic system failed, since the conversion dropped in
the second run (Entry 1, Table 5).
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Gozzo, S.M. Tamborim, A.L. Parize, P.A.Z. Suarez, B.A.D. Neto,
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Adv. Synth. Catal., 349 (2007) 1803-1807.
Table 5. Suzuki coupling reaction between 4-bromotoluene and phenylboronic
acid promoted by [PdCl2(6b)] under biphasic conditions
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Asymmetry, 22 (2011) 769-774.
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Entry
Ionic liquid
BMI·BF4
BMI·NTf2
BMI·PF6
Conv. (%)a
1) 88; 2) 0
1) 58; 2) 41; 3) 18
1) 34; 2) 32; 3) 29
1
2
3
Reagents and conditions: bromotoluene (0.25 mmol), phenylboronic acid
(0.275 mmol), K2CO3 (1.0 mmol), [PdCl2(6b)] (0.5 mol%), toluene (1.0 mL),
IL (0.5 mL), 100 C, 18 h, degassed solvent, anisole as an internal standard;
aConversion determined by GC after each run.
In summary, we have described a practical, useful and odorless
synthesis of mono and dicationic sulfur-containing imidazolium-
based ILs. Our strategy based on a key isothiouronium salt
intermediate allowed the generation of ten new ILs with yields
ranging from 52% to 99%, which represents an excellent
alternative to the protocols already described in literature for the
synthesis of thiaalkylimidazolium ILs. Moreover, these
compounds were applied as ligands and ionic tags for the detection
of reaction intermediates in Ullmann and Suzuki coupling
reactions, opening new possibilities to apply these ILs in several
catalytic transformations. A more comprehensive scope evaluation
as well as the application of these ILs in other metal-catalyzed
reactions are underway in our groups and it will be published in
due course.
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Ed., 49 (2010) 2185-2189.
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Acknowledgments
We thank the CNPq (449758/2014-1), FAPERGS (16/2551-
0000373-4 and PRONEX-FAPERGS), VRAC-PUC-Rio (J.L.)
and INCT-Catálise for financial support. This study was financed
in part by the Coordenação de Aperfeiçoamento de Pessoal de
Nível Superior - Brasil (CAPES) - Finance Code 001. We also
thank the CAPES (G.I.M.) and the CNPq (A.P.; R.G.M.C.) for
scholarships.
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- Synthesis of thiaalkylimidazolium ionic liquids
from odorless isothiouronium salts.
- Mono and dicationic thiaalkylimidazolium ionic
liquids were prepared and characterized.
- These ionic compounds were applied as ligands in
Ullman and Suzuki coupling reactions.
- They can also be employed as charged probes to
detect intermediates by ESI-MS.

GraphicHalNAbstract
2
SPACER
S
S
NH2
SPACER
X
X
( )
( )
N
N
n
( )
S
n
n
N
N
N
A
N
A
R
R
dicationic
thiaalkylimidazolium ILs
2A
R
SPACER
X
OR
Odorless synthesis
SPACER
S
OR
( )
N
n
Ionically tagged ligands
N
A
R
Ligands for metal-catalyzed C-C and
C-O couplings
monocationic
thiaalkylimidazolium ILs

Scheme 1

Scheme 2

Scheme 3

Scheme 4

Scheme 5