Acid-induced chemoselective arylthiolations of electron-rich arenes in ionic liquids from sodium arylsulfinates: The reducibility of halide anions in [Hmim]Br

Article abstract of DOI:10.1039/c6ob02823c

An acid-induced protocol for the chemoselective arylthiolations of electron-rich arenes in ionic liquids from sodium arylsulfinates is introduced. The chemistry, in which [Hmim]Br (heaxylmethylimidazolium bromide) is used as both a solvent and a reducer, provides several advantages including odorless and simple operation, inexpensive reagents, recyclable solvents and gram-scale synthesis.

Full text of DOI:10.1039/c6ob02823c

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Takeharu Haino et al.
Solvent-induced emission of organogels based on tris(phenylisoxazolyl)
benzene
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DOI: 10.1039/C6OB02823C
Organic & Biomolecular Chemistry
PAPER
Acid-induced chemoselective arylthiolations of electron-rich
arenes in ionic liquids from sodium arylsulfinates: The reducibility
of halide anions in [Hmim]Br
Received 00th January 20xx,
Accepted 00th January 20xx
Zhu-bing Xu, Guo-ping Lu,* Chun Cai
DOI: 10.1039/x0xx00000x
An acid-induced protocol for the chemoselective arylthiolations of electron-rich arenes in ionic liquids from sodium
arylsulfinates is introduced. The chemistry, in which [Hmim]Br (heaxylmethylimidazolium bromide) is applied as both a
solvent and reducer, provides several advantages including odorless and simple operation, cheap reagents, recyclable
solvent and gram-scale synthesis.
www.rsc.org/
Introduction
Ionic liquids (ILs) as multipurpose materials have been
widely applied in organic synthesis, catalysis, chemical
separation, material and energy science, due to their unique
chemical or biological properties.1 Among them, the
imidazolium-based ILs are one of the first to find applications
in organic synthesis on an industrial scale.2 These ILs have
several unique properties, such as hydrogen bonding,3 self-
assemble4 and outstanding solvation potential,5 which may
promote some transformations.6 Several anions of ILs have
been employed in organic synthesis as the reagents, such as
nucleophilic halogenating reagents, nitrating reagents,7 but
no report notes that the potential reducibility of halide anions
(Br- and I-) in these ILs, which may make them play a dual role
(solvent and reducer) in some transformations.
Recently, sodium arylsulfinates are a kind of stable, odorless
and easy-to-handle sulfur compounds that have been widely
applied as the sulfur sources under reduction conditions for
the construction of C-S bonds.8 Arylsulfinic acid can be reduced
by n-Bu4NI to form the corresponding sulfenyl iodide.9
Therefore, we envisioned that [Hmim]Br may serve as
potential reducing agents to deoxidize sodium arylsulfinates
for the synthesis of organosulfurs through in situ generation of
thiyl cation species (Scheme 1). On the other hand, diaryl
sulfides are a common structural motif in natural products,
pharmacologically active compounds and organic materials.10
Several attempts have been made to employ sodium
arylsulfinates instead of smell thiols for the generation of
diaryl sulfides from electron-rich arenes in water.11
Scheme 1 Work hypothesis: the potential reducibility of halide anions in ILs.
Nevertheless, stoichiometric toxic reducing agents (PPh3 and
HCOOH) and iodine are norm, and the reaction mediums are
unrecyclable in these protocols. Although the environmental
fate of ILs is a complex situation, such as their modes of
toxicity, biodegradation pathways and behavior concerning
biosorption,12 the imidazolium-based ILs are still considered to
be a greener solvent than other organic solvents owing to their
negligible vapour pressure, non-flammability and recyclability.2
Along this line, we disclose an acid-induced approach for the
chemoselective arylthiolations of electron-rich arenes from
sodium arylsulfinates in ILs. This chemistry, in which [Hmim]Br
(heaxylmethylimidazolium bromide) is used as both a solvent
and reducer and the reaction system can be recycled by simple
extraction, is a simpler and greener route for the formation of
diaryl sulfides.
Results and discussion
As a representative example, the reaction of naphthalen-2-
ol 1a and sodium benzenesulfinate 2a was chosen as the
model reaction. Acid is necessary for the reaction (Table 1,
entries 1 vs 2). After screening various acids, strong acids
(entries 2, 6, 10) provided excellent yields, and HCl was the
best option owing to its low price. Lower temperature proved
to inhibit the reaction (entry 2). Compared with other solvents
(entries 11, 14-21), [Hmim]Br resulted in the highest yield.
Other bromides (NH4Br and TBAB) were also added in water as
Chemical Engineering College, Nanjing University of Science &Technology
Xiaolingwei 200, Nanjing, 210094 (China)
* E-mail: glu@njust.edu.cn
† Electronic Supplementary Information (ESI) available: More experimental details,
copies of NMR spectra of all products. See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
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Table 1 Optimization of reaction conditionsa
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DOI: 10.1039/C6OB02823C
entry
1
2
3
4
5
6
7
8
additive
/
HCl
citric acid
H3BO3
HCOOH
TsOH
solvent
yieldb
0
94, 11c
24
trace
trace
93
trace
trace
43
91
12
15
9
27
6
0
trace
12
23
40
37
27
19
27
23
[Hmim]Br
[Hmim]Br
[Hmim]Br
[Hmim]Br
[Hmim]Br
[Hmim]Br
[Hmim]Br
[Hmim]Br
[Hmim]Br
[Hmim]Br
H2O
H2O
H2O
Toluene
THF
DMSO
AcOH
lactic acid
9
H2C2O4•2H2Od
d
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
H2SO4
HCl
HCl/NH4Bre
HCl/TBABe
HCl
HCl
HCl
HCl
HCl
HCl
HCl
HCl
HCl
HCl
HCl
HCl
HCl
HCl
HCl
HCl
HCl
DMF
MeCN
Dioxane
DCE
EtOH
a
Scheme
2
The reactions of sodium arylsulfinates with electron-rich arenesa
Conditions: 1 0.50 mmol, 2 0.75 mmol, HCl 0.75 mmol, [Hmim]Br 0.5 mL, 80 oC, 6 h.
Isolated yields. b 120 oC.
[Hmim]Cl
[Hmim]OAc
[Hmim]HSO4
[Hmim][MeSO3]
[Hmmim]Br
[Hmim]Br
[Hmim]Br
[Hmim]I
[Hmim]Br
81
95f, 85g, 82h
93f,i, 67f,j
92, 94k
93k
a Conditions: 1a 0.50 mmol, 2a 1.0 mmol, additive 1.50 mmol, solvent 0.5 mL, 80
oC, 6 h. b GC yields. c At 40 oC. d 1.5 equiv. e 3.0 equiv. f 1.5 equiv of 2a was used. g
1.2 equiv of 2a was used. h 1.0 equiv of 2a was used. i 1.5 equiv of HCl was used. j
1.0 equiv of HCl was used. k Under N2 atmosphere
Scheme 3 The reactions of sodium arylsulfinates with arylaminesa a Conditions: 1 0.50
the potential reducers, but no obvious promotion was
observed indicating that both cation and anion of [Hmim]Br
might be beneficial to the transformation.
b
mmol, 2 0.75 mmol, HCl 0.75 mmol, [Hmim]Br 0.5 mL, 80 oC, 6 h. Isolated yields. 120
oC.
Other ILs containing different anions were also tested in the
reaction (entries 22-25, 29). Only [Hmim]Br and [Hmim]I could
provide the excellent yields of 3a. It could be concluded that
the reductibility of anion play an important role in the reaction.
[Hmimm]Br (1,2-dimethyl-3-octyl-1H-imidazol-3-ium bromide)
was also employed as the solvent to confirm the effects of C-2
hydrogen of imidazole. The results suggested that a slight
influence was found during the process (entries 2 vs 26). The
amount of 2a and HCl were also optimized, and the best
option was using 1.5 equiv of 2a and 1.5 equiv of HCl (entries
27, 28). The reaction can also take place under N2 atmosphere
(entries 29, 30), so sodium arylsulfinates (rather than air) are
the oxidants, which have enough oxidizability to oxidize both
Br- and I- to Br2 and I2 respectively under acidic conditions.
Therefore, the reduction potential of I2/I- and Br2/Br- system
are no obvious influence on the reaction.
With the optimized conditions in hands, a series of sodium
arylsulfinates
applied in the reaction to establish the scope and generality of
this protocol (Scheme 2). Both sodium arenesulfinates with
electron withdrawing and donating groups could react with
naphthalen-2-ol or 2-methoxynaphthalene smoothly (3a 3n),
2 and various electron-rich arenes 1 were
2
-
and only 1-substituted products were detected in all cases.
Aryl ethers were also afforded the desired products with
excellent regioselectivity and yields (3o-3t). A moderate yield
of 3u was derived in the case of phenol with an electron
donating group (t-butyl) by raising temperature to 120 oC,
however phenol and phenol with electron withdrawing groups
failed to yield the final products.
Likewise, aromatic amines underwent arylthiolation
smoothly, giving moderate to excellent yields (Scheme 3). Both
2-sulfanyl product 4a and 2,6-disulfanyl product 4a’ are
2 | J. Name., 2012, 00, 1-3
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obtained in the coupling of sodium p-tolylsulfinate with 3,5-
dimethoxyaniline. As expect, anilines with electron-
withdrawing groups failed to yield the desired products in the
approach. Higher temperature was needed in the case of N,N-
dimethylaniline and 2,6-diisopropylaniline to produce 4c and
4d. To further demonstrate the potential of the strategy, the
sulfanylations of a more complex arenol (estrone) and
heteroarenes also worked under similar conditions (Scheme 4).
However, coumarin, benzofuran and benzothiophene failed to
yield the desired products in the protocol.
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DOI: 10.1039/C6OB02823C
Further investigations were also conducted to assess the
potential for recycling of [Hmim]Br in the model reaction.
Meanwhile, we scaled up the reaction to 10 mmol to show the
possibility for large-scale operation. After completion of the
reaction, the product underwent in-flask extraction with
minimum amounts of an organic solvent (MTBE). The phase of
[Hmim]Br was separated by simple extraction and reuse for
next run. The process could be repeated 10 times without an
Scheme 5 Control experiments
Scheme 4 The arylsulfanyltion of estrone and heteroarenes in [Hmim]Br. Conditions:
1.0 mmol estrone or heteroarene, 1.5 mmol 2b, 1.5 mmol HCl, 0.5 mL [Hmim]Br, 80 °C,
a
b
c
6 h, isolated yields. 120 oC, 12 h. 3.0 mmol 2b and 3.0 mmol HCl were used. GC
yield.
100
90
80
70
60
50
40
30
20
10
0
Scheme
[Hmim]Br
6
A tentative pathway for the arylthiolation of electron-rich arenes in
obvious change in yields, but an additional distillation process
was required for the dehydration of [Hmim]Br after reusing
five times (in 4th run and 9th run) (Figure 1).
To probe the mechanism, further control experiments were
also performed (Scheme 5). A disulfide
sodium p-toluenesulfinate 2b under optimized conditions, and
only trace of disulfide was detected in the absence of HCl,
suggesting that the acid play a curial role in the reduction of 2b
(eq. 1). However, fail to react with 1a to yield the final
5 was generated from
0
1
2
3
4
5
6
7
8
9
10
5
run time
Fig. 1 Recycle studies. Conditions: 10 mmol 1a, 15 mmol 2a, 15 mmol HCl, 10 mL
[Hmim]Br, 80 °C, 6 h. Isolated yields.
6
product in presence of bromine (eq. 2), so the reaction
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pathway was not including the formation of sulfenyl bromide
from disulfide. A bromide
N-Methylimidazole 40 mmol, 1-halohexane 4V8iewmAmrticolelOanlninde
was also observed as a by-product ethyl acetate 10 mL were heated under reflux for 24 h. The
DOI: 10.1039/C6OB02823C
7
in the reaction of 1,3-dimethoxybenzene and sodium 4- biphasic system obtained was separated and the upper organic
methoxybenzenesulfinate (eq. 3), and an excellent yield of phase discharged. The bottom product phase was washed with
7
could be obtained when sodium trifluoromethanesulfinate ethyl acetate (3 × 20 mL), and dried under vacuum to give 1-
instead of sodium 4-methoxybenzenesulfinate was employ. hexyl-3-methylimidazolium halide (Cl, Br and I) as a colourless
The result was an evidence for the generation of bromine liquid. [Hmim]OAc, [Hmim][CH3SO3] and [Hmim]HSO4 were
during the process.
Based on these results,
proposed (Scheme 6). Firstly, arylsulfinic acid derived from
under acidic conditions, is further reduced by acid and
bromide anion to yield 8 9,13
The arylsulfanol may also react
with bromine to produce path A). Both and may
synthesized by exchanging the bromide ion of [Hmim]Br with
-
-
AcO-, CH3SO3 and HSO4 with NaOAc, CH3SO3Na and H2SO4
a tentative mechanism was
1
respectively.
General procedures for the direct sulfenylation reactions of
electron-rich arenes
.
8
9
(
8
9
A 15 mL oven-dried reaction vessel was charged with
sodium benzenesulfinate (0.75 mmol), arene (0.50 mmol) and
[Hmim]Br (0.5 mL). Concentrated HCl (as a 37wt.% HCl
aqueous solution, 0.75 mmol) was added to the mixture and
the resulting mixture was stirred at 80 °C for 6 h. After
completion, the mixture was extracted with MTBE. The organic
phase was collected and filtered through a bed of silica gel
layered over Celite to dry the organic layer and remove
inorganic salts or other undissolved substances in the organic
layer. The volatiles were removed in vacuo to afford the crude
product. Further column chromatography on silica gel
(EtOAc/petroleum ether, v/v = 1/19 or 1/9) or recrystallization
was needed to afford the pure product. In the cases of
aromatic amines, an additional neutralization step was
required.
undergo the aromatic electrophilic substitution with arenes to
form the p-arylsulfanyl arene (or o-arylsulfanyl arene) as the
major path. The enhancement of [Hmim]Br’s cation on the
pathway may be attributed to the formation of Hydrogen
bonds between the C-2 hydrogen and X (X = OH, Br), and the
charge-charge interaction of the quaternary nitrogen atom and
X, resulting in an electrophilic activation of the S-X bond’s
sulfur.6,14
The chemoselectivity of the arylthiolation can be explained
by resonance theory. The transition states are the most stable
when the electrophilic sulfur intermediates (8 and 9) attack 2
or 4 position of electron-rich arenes, so the reaction has high
chemoselectivity (Scheme 6).8,11 Moreover, arylsulfinic acid
can also afford the electrophilic sulfur intermediate 11 through
dehydration condensation and tautomerizm (path B),13 which
can also react with electron-rich arenes to form the desired
product as the minor path. This can explain that the
arylthiolation still occur in other organic solvents with poor
yields.
The procedure of recycling [Hmim]Br
After reaction completion, the mixture was then extracted
with MTBE (3
×10 mL). The organic layer was collected and
filtered through a bed of silica gel layered over Celite to dry the
organic layer and remove inorganic salts or other undissolved
substances in the organic layer. The volatiles were removed in
vacuo to afford the crude product 3a. To the phase of IL, HCl
15.0 mmol was added, followed by 1a 10.0 mmol and 2a 15.0
mmol at room temperature and the reaction stirred for 6 h at
80 oC. The extraction cycle was then repeated for the
Experimental
General
All chemical reagents were obtained from commercial
suppliers and used without further purification. Melting points
were determined uncorrected. All known compounds were
identified by appropriate technique such as such as 1H NMR
and 13C NMR and compared with previously reported data. All
separation of 3a
.
unknown compounds were characterized by H NMR, 13C NMR,
1
Conclusions
GC-MS (or MS) and elemental analyses. Analytical thin-layer
chromatography was performed on glass plates pre-coated
with silica gel impregnated with a fluorescent indicator (254
nm), and the plates were visualized by exposure to ultraviolet
light. 1H NMR and 13C NMR spectra were recorded on an
AVANCE 500 Bruker spectrometer operating at 500 MHz and
125 MHz and chemical shifts were reported in ppm. GC
analyses were performed on an Agilent 7890A instrument
(Column: Agilent HP-5: 30 m × 320 μm × 0.25 μm, carrier gas:
H2, FID detection). GC-MS analyses were performed on a
Thermo Trace 1300 and ISQLT instrument (Column: TR-35 MS:
30 m X 0.25 mm X 0.25 μm, ionization mode: EI, carrier gas:
He). Elemental analyses were performed on a Yanagimoto
MT3CHN recorder.
In summary, we develop a practical strategy for the
chemoselective synthesis of diaryl sulfides under acidic
conditions, in which odorless, easy-to-handle sodium
arylsulfinates and recyclable IL are used as the sulfur source
and solvent respectively. The reducibility of Br- in [Hmim]Br is
found, so no additional reducing agent is needed in the
process. The chemistry features simple operation, gram-scale
synthesis, inexpensive reagents and recyclable solvent,
thereby making it more environmentally friendly and suitable
for large-scale operations. Other transformations in ILs are
ongoing in our group.
Synthesis of imidazolium-based ILs
Acknowledgements
4 | J. Name., 2012, 00, 1-3
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12 M. Petkovic, K. R. Seddon, L. P. N. Rebelo and C.VSieiwlvAartPicelerOenirliane,
We gratefully acknowledge the Natural Science Foundation of
China (21402093) and Jiangsu (BK20140776), Chinese
Postdoctoral Science Foundation (2016T90465, 2015M571761)
for financial support.
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DOI: 10.1039/C6OB02823C
odorless solid
reducibility
Nu
Nu
HCl, [Hmim]Br
Ar
+
SO2Na
S
Ar
EDG
EDG
Nu = OH, OR, NH2, NR2
EDG = electron-donating group
recyclable solvent
cheap reagents
gram-scale synthesis
simple operation
An odorless approach for arylthiolations of arenes is introduced, in which [Hmim]Br is applied as
both a solvent and reducer.