Cerium catalyst promoted C-S cross-coupling: Synthesis of thioethers, dapsone and RN-18 precursors

Article abstract of DOI:10.1039/c9ob02171j

In this work, we present a novel, efficient and green methodology for the synthesis of thioethers by the C-S cross-coupling reaction with the assistance of [Ce(l-Pro)₂]₂Ox as a heterogeneous catalyst in good to excellent yields. A scale-up of the protocol was explored using an unpublished methodology for the synthesis of a dapsone-precursor, which proved to be very effective over a short time. The catalyst [Ce(l-Pro)₂]₂Ox was recovered and it was shown to be effective for five more reaction cycles.

Full text of DOI:10.1039/c9ob02171j

Organic &
Biomolecular Chemistry
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Cerium catalyst promoted C–S cross-coupling:
synthesis of thioethers, dapsone and RN-18
precursors†
Cite this: Org. Biomol. Chem., 2019,
17, 10103
a
José M. da C. Tavares Junior, a Caren D. G. da Silva,a Beatriz F. dos Santos,
Nicole S. Souza,a Aline R. de Oliveira,a Vicente L. Kupfer,b Andrelson W. Rinaldib and
a
Nelson L. C. Domingues
*
In this work, we present a novel, efficient and green methodology for the synthesis of thioethers by the
C–S cross-coupling reaction with the assistance of [Ce(L-Pro)2]2Ox as a heterogeneous catalyst in good
to excellent yields. A scale-up of the protocol was explored using an unpublished methodology for
the synthesis of a dapsone-precursor, which proved to be very effective over a short time. The catalyst
[Ce(L-Pro)2]2Ox was recovered and it was shown to be effective for five more reaction cycles.
Received 7th October 2019,
Accepted 6th November 2019
DOI: 10.1039/c9ob02171j
rsc.li/obc
R2S2) can poison a catalyst, deactivating the catalyst and irre-
versibly limiting the reaction.2
Introduction
Cross-coupling reactions are considered to be a powerful strat-
On the other hand, in recent decades the biological and
egy for the formation of carbon–carbon and carbon–hetero- medicinal properties of organochalcogen compounds have
atom bonds.1 This type of reaction has dramatically changed been studied owing to their antioxidant, antimicrobial, antitu-
the way that organic chemists build a wide variety of organic mor, anti-inflammatory, and antiviral properties.8 Thioethers
compounds.2 However, to perform a cross-coupling reaction, it belong to an important class of compounds that are the build-
is necessary to have an organometallic compound and an ing blocks of biological and therapeutic molecules.9 Typically
organic electrophile (halides and pseudo halides compounds) the preparation of thioethers involves the coupling of organic
in the presence of a catalyst containing palladium,3 nickel4 or halides with thiols or disulfides.10 Owing to the importance of
copper.5 Since the first publication about the cross-coupling this reaction in organic procedures, the transition metal-cata-
reaction a lot of improvements have been made over time, in lysed carbon–heteroatom bond has been receiving more atten-
particular that nucleophiles6 participate well as catalysts. That tion in recent years.11
is why several efficient catalytic systems have been developed
C–S bonds are present in drug-molecules across many
for cross-coupling reactions.7 Despite the elegance of the health areas, such as HIV,12 skin disease (e.g. leprosy)13 and
cross-coupling reaction, and owing to the wide application of Alzheimer’s disease.14 Owing to this, and focusing on the pre-
different nucleophiles such as chalcogenides (ROH, RSH and viously mentioned information about C–S bond formation, it
RSe), some drawbacks existed. Specifically, for C–S cross-coup- is necessary to develop less aggressive procedures, as well as
ling, until recently, the construction of C–S bonds using a cata- more profitable, low cost and efficient methodologies.15 The
lyst containing transition-metals (e.g. Pd, Ni and etc.) remained first report published on C–S cross-coupling was by Migita16
relatively rare compared to the other nucleophiles. It is known and co-workers using tetrakis(triphenylphosphine)palladium
that sulfur species such as thiols and disulfides (RSH and for the preparation of aryl sulfides. Recently, Nejat et al. have
designed recyclable catalysts for the formation of thioethers
via C–S cross-coupling.17 The improvements reported by
Migita et al. highlighted palladium catalysts. However, palla-
dium, iridium, rhodium, and ruthenium are costly transition
metals and are generally toxic.18 However, in contrast to the 4d
and 5d catalysts, Nageswar19 and co-workers reported the first
protocol for C–S cross-coupling using lanthanides. In this pro-
cedure, the authors described the synthesis of thioethers
using aryl halides, thiols and lanthanum(III) oxide, as a hetero-
geneous catalyst and they proved that it was possible to recover
aOrganic Catalysis and Biocatalysis Laboratory – LACOB,
Federal University of Grande Dourados – UFGD, Dourados, MS, Brazil.
E-mail: nelsondomingues@ufgd.edu.br; Tel: +55 (67) 3410-2081
bMaterial Chemistry and Sensores Laboratory – LMSen, State University of Maringá,
Maringá, PR, Brazil
†Electronic supplementary information (ESI) available: Experimental pro-
cedures, spectroscopic details regarding MEV and EDS analysis (gold metalliza-
tion, aluminum base metallization and without metallization), and all the
spectra (1H and 13C NMR) for the products. See DOI: 10.1039/c9ob02171j
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improve the yield for thioethers, we increased the cerium cata-
lyst up to 10 mol% (entry 5, Table 1) which afforded a yield of
more than 99%. Additionally, we performed a solvent assess-
ment employing solvents such as water and toluene and we
concluded that the reaction was compatible with polar solvents
furnishing the best yields (entries 4 and 5, Table 1). To our
delight, the methodology established by us, in comparison
with others,24 presented some important advantages like the
use of green and low cost solvent such as EtOH, and the possi-
bility of using an open flask reaction. Many reports in the lit-
erature reported the C–S cross-coupling reaction using an inert
atmosphere owing to catalyst instability. Herein, the catalyst
proved to be an air-stable catalyst and furthermore, was reusa-
ble. Both properties are important for the industrial appli-
cation of the process.
With this valuable data in hand, we tried to decrease the
amount of thiol in the C–S cross-coupling reaction to make the
procedure even more practicable (Table 2). Interestingly, when
the pattern reaction was performed using 1 equivalent of ben-
zenethiol, we observed a decrease of the yield in the C–S cross-
coupling (40%, entry 1, Table 2). However, when we performed
the reaction using 1.5 equivalents of thiol, we observed an
increase in the yield (78% yield, entry 2, Table 2). Notably,
when we conducted the reaction using 2 equivalents of benze-
nethiol, we achieved an excellent yield of >99% (entry 3,
Table 2).
Fig. 1 Strategies for C–S cross-coupling using lanthanides as recover-
able catalysts.
and reuse the catalyst. Motivated by Nageswar and co-author’s
work on lanthanides, we aimed to perform the C–S cross-coup-
ling reaction using a cost-effective, easy to handle, and reusa-
ble cerium(III) catalyst. In 2016 our research group designed a
cerium(III) heterogeneous catalyst which was applied in various
procedures such as the Kabachnik–Fields reaction,20 pyrazoles
synthesis21 and a one-pot 2,3-dihydroquinazolin-4(1H)-ones
synthesis.22 In a continuation of our work, here we report an
efficient, cost-effective, low cost, short reaction time, green pro-
cedure and scale-up of Migita’s reaction using thiols and aryl
halides and applying [Ce(L-Pro)2]2Ox as a heterogeneous cata-
lyst (Fig. 1), as well as the synthesis of the bioactive compound
RN-18 and a dapsone precursor.23
It is important to note that the standard process affords the
compound in a reduced time compared to many other
reports.25 Again, the above described catalyst efficiency in the
C–S cross-coupling should be noted. To assess the scope for
the synthesis of thioethers using this method and standard
conditions we extended the protocol to different aryl halides
and thiophenols (Table 3). The presence of electron-donating
groups on the phenols (entries 3–9, Table 3) furnished excel-
lent yields 71 → 99%. On the other hand, thiophenols with
electron-withdrawing groups (entries 10–13, Table 3) afforded
lower yields 31–82%. However, for thiosalicylic acid (entries 15
and 16, Table 3) we obtained excellent yields >99 and 86%,
respectively. Regarding entry 15, a RN-18 precursor, a novel
and potent Vif antagonist for HIV treatment26 was produced in
a higher yield, which is an improvement for RN-18 synthesis.
With the change of substrates on the aryl halide (entries
17–19, Table 3) moderate tolerance of substituents was
observed. To demonstrate once more the efficiency and appli-
Results and discussion
Initially, we performed the C–S cross-coupling using a similar
methodology to that described by Domingues and co-
workers,23 selecting a pattern reaction involving 1-iodo-4-nitro-
benzene and thiophenol in ethanol. The first evaluation was
performed without a catalyst (blank reaction entry #1, Table 1)
and we only observed a 30% yield for the compound of inter-
est. Furthermore, the same reaction was carried out using
5 mol% of the cerium catalyst (entry 2, Table 1) over 6 h in
ethanol and resulted in 80% of the product (4-nitrophenyl)
(phenyl)sulfane. After this, we performed the reaction using
7.5 mol% (entry 3, Table 1) and we obtained a 90% yield. To
Table 1 Optimization of reaction conditionsa
Table 2 Study of the amount of thiol in the C–S cross-coupling
reactiona
Entry
Catalyst
Solvent
Yieldb
1
2
3
4
5
6
—
5
7.5
10
10
10
EtOH
EtOH
EtOH
EtOH
H2O
30%
80%
90%
>99%
91%
35%
Entry
Thiol (mmol)
Yieldb
1
2
3
0.5
0.75
1.0
40%
78%
>99%
Toluene
a Reaction conditions: 1-Iodo-4-nitrobenzene (0.5 mmol), benzenethiol a Reaction conditions: 1-Iodo-4-nitrobenzene (0.5 mmol), benzenethiol
(1.0 mmol), K2CO3 (2.5 mmol), and 4 mL of solvent. b Isolated yields (x mmol), K2CO3 (2.5 mmol), and [Ce(L-Pro)2]2Ox (10 mol%) in EtOH
using column chromatography.
(4 mL). b Isolated yields using column chromatography.
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Table 3 Scope of the C–S cross-coupling of thiols with aryl halides.a
Entry Thiol
1
Aryl hallide Product
Yielda (%)
>99
2
90
3
4
5
97
71
80
Fig. 2 The structure of the bioactive compound dapsone, for the treat-
ment of skin disease.
(5 mmol) affording an 83% yield (1.02 g), in 9 h. In this pro-
cedure, with an increase in the molar equivalent of the starting
materials (from 6 to 9 h), the procedure was completed in
comparatively less time, indicating that this procedure can be
easily applied in industry (Scheme 1).
Furthermore, the recyclability of the catalyst was studied
(entry 1, Table 3) and it was found to be active in several cycles
without exhibiting any significant loss for up to five cycles
(Table 4).
We also found an increase in the catalyst weight per cycle
(see Table 4). To explain the catalyst mass increment, we per-
formed scanning electron microscopy (SEM) and energy-dis-
persive X-ray spectroscopy (EDS) analysis (Fig. 3 and 4) on the
pre and post used catalyst.
6
>99
88
89
96
82
75
33
31
65
>99
7
8
9
10
11
12
13
14
15
A good similarity was observed between the SEM and EDS
analysis of the pre (Fig. 3a and b) and post-reaction samples
16
17
18
19
86
36
55
15
Scheme 1 Gram-scale synthesis of
[Ce(L-Pro)2]2Ox.
a dapsone precursor using
a Reaction conditions: Aryl halide (0.5 mmol), thiol (1.0 mmol), K2CO3
(2.5 mmol), and 4 mL of ethanol, 6 h, at 80 °C.
Table 4 Recycling studies of [Ce(L-Pro)2]2Oxa
Cycle (#)
Yieldb (%)
Catalyst recovery (%)
cability of the catalyst [Ce(L-Pro)2]2Ox we proposed the syn-
thesis of an intermediary of the bioactive compound dapsone
(Fig. 2).
For the Dapsone precursor (shown in Table 3, entry 6), the
reaction was conducted using 4-aminothiophenol and 1-iodo-
4-nitrobenzene affording a 96% yield. The same reaction invol-
ving a dapsone-precursor was carried out at a multi-gram scale
1
2
3
4
5
>99
90
86
81
76
100 + (58)
100 + (48)
100 + (45)
100 + (38)
100 + (24)
a Reaction conditions: 1-Iodo-4-nitrobenzene (0.5 mmol), benzenethiol
(1.0 mmol), and K2CO3 (2.5 mmol) in EtOH (4 mL). b Isolated yield
using column chromatography.
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Experimental
All chemical reagents and solvents were used without any
specific treatment. The respective reactions were monitored by
thin layer chromatography (TLC) MACHEREY-NAGEL (SIL
G/UV254). The purification of the compounds was performed
by column chromatography on silica gel using appropriate
quantities of hexane and acetyl acetate. 1H and 13C NMR
spectra were recorded in CDCl3 or DMSO on a Bruker
(300 MHz and 75 MHz respectively) spectrometer.
Synthesis of the [Ce(L-Pro)2]2Ox catalyst
[Ce(L-Pro)2]2Ox was obtained using proline (5.0 mmol) in
methanol (15 mL) and aqueous sodium hydroxide solution
(5.0 mmol in 3 mL) at room temperature for 10 min.
CeCl3·7H2O (2.5 mmol in 1 mL), was added to the reaction,
and the solution was left stirring overnight. Sodium oxalate
(0.1 g mL−1) was used as a precipitating agent. The solid was
filtrated off, washed with methanol and dried (92%).
General procedure for the C–S cross-coupling of thiols with
aryl halides
Aryl halide (0.5 mmol) and thiol (1.0 mmol) were placed in a
tube with ethanol (4 mL), K2CO3 (2.5 mmol) was added in the
presence of 10 mol% of cerium catalyst, and the mixture was
heated at 80 °C for 6 h. TLC was used to monitor the progress
of the reaction. After the reaction was completed, the catalyst
was filtered off and the solvent was removed under a vacuum.
The residue was extracted with chloroform (2 × 20 mL) and
dried over MgSO4. The solvent was removed under vacuum
and the crude product was purified using column chromato-
graphy on silica gel with appropriate quantities of hexane and
ethyl acetate.
Fig. 3 SEM image of [Ce(L-Pro)2]2Ox before the reaction (a) and after
five cycles (b).
C–S cross-coupling of thiosalicylic acid with aryl halides
(entries 15 and 16)
The cerium catalyst (10 mol%), aryl halide (0.5 mmol) and
thiosalicylic acid (1.0 mmol) were placed in a tube and K2CO3
(2.5 mmol) in ethanol (4 mL) was added, the mixture was
heated at 80 °C for 6 h. The progress of the reaction was moni-
tored using TLC (EtOAc : hexane, 1 : 1). After the reaction was
completed, the catalyst was filtered off, the solvent was
removed under vacuum and then ice water was added to the
reaction mixture, followed by acidification using 5 N HCl to
obtain the crude product. The solid was filtered off and
washed several times with petroleum ether and recrystallized
using ethanol to afford the product.
Fig. 4 EDS for [Ce(L-Pro)2]2Ox before the reaction (A) and after five
cycles (B).
(Fig. 3b and 4b), performed using gold based metallization.
The EDS analysis revealed a difference between the pre and
post-reaction catalyst in the atomic percentage of cerium,
carbon, oxygen, and sodium.
C–S cross-coupling of thiols with 2-iodobenzoic acid (entries
17 and 18)
The cerium catalyst (10 mol%), 2-iodobenzoic acid (0.5 mmol)
This remarkable difference is due to the mechanism of the and the thiols (1.0 mmol) were placed in a tube and K2CO3
C–S cross-coupling, driven by [Ce(L-Pro)2]2Ox, affording a new (2.5 mmol) in ethanol (4 mL) was added, the mixture was
complex demonstrated by the insertion of sulfur and aryl heated at 80 °C for 6 h. The progress of the reaction was moni-
halide which strongly indicates a C–S cross-coupling catalytic tored using TLC (EtOAc : hexane, 1 : 1). After the reaction was
cycle has occurred (see spectroscopy data in the ESI†).
complete, the catalyst was filtered off, the solvent was removed
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under vacuum and then ice water was added to the reaction
mixture, followed by acidification using 5 N HCl to obtain the
crude product. The solid was filtered off and washed several
times with petroleum ether and recrystallized using ethanol to
afford the product.
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General procedure for the synthesis of the dapsone-precursor
Synthesis of the dapsone-precursor was carried out in the
gram-scale and the reaction was performed using 1-iodo-4-
nitrobenzene (5 mmol) and 4-aminothiophenol (10 mmol) in
ethanol (4 mL), K2CO3 (25 mmol) was added in the presence
of 10 mol% of cerium catalyst, and the mixture was heated at
80 °C for 9 h. TLC was used to monitor the progress of the
reaction. After the reaction was complete, the catalyst was fil-
tered off and the solvent was removed under vacuum. The
residue was extracted with chloroform (2 × 20 mL) and dried
over MgSO4. The solvent was removed under vacuum to give
the crude products, which were purified using column chrom-
atography on silica gel using appropriate quantities of hexane
and acetyl acetate (83% yield, 1.02 g).
Conclusions
In conclusion, [Ce(L-Pro)2]2Ox was proved to be a highly
efficient reusable catalyst for the synthesis of thioethers and
derivatives in moderate to excellent yields under green con-
ditions. It was possible to reuse the catalyst for five catalytic
cycles. In addition, this methodology can be applied for the
synthesis of two bioactive compounds, a dapsone precursor
(entry 9, Table 3) (in scale-up) and RN-18 (entry 15, Table 3),
which means that this methodology is applicable in the
pharmaceutical industry.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
The authors (N. L. C. D. and J. M. da C. T. Jr.) thank Fundação
de Apoio ao Desenvolvimento do Ensino, Ciência e Tecnologia
do Estado de Mato Grosso do Sul (FUNDECT/Brazil) for finan-
cial support (Chamada FUNDECT/CNPq no. 15/2014
–
PRONEM MS) and the Conselho Nacional de
–
Desenvolvimento Científico e Tecnológico (Chamada CNPq
no. 12/2017 – Bolsas de Produtividade em Pesquisa – PQ) for
financial support and fellowship.
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10108 | Org. Biomol. Chem., 2019, 17, 10103–10108
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