Cs2CO3 promoted direct C-H bond sulfenylation of imidazo[1,2-a]pyridines and related heteroarenes in ionic liquid

Article abstract of DOI:10.1039/c4ra01240b

An efficient and novel method was developed for the synthesis of 3-sulfenyl imidazo[1,2-a]pyridines in good to excellent yields via a Cs₂CO ₃ promoted direct sulfenylation of imidazo[1,2-a]pyridines. The reaction proceeds smoothly with a wide range of structurally diverse heteroarenes and disulfides. This protocol is environmentally friendly because it is free of transition-metal catalysts and utilizes aryl-substituted imidazolium-based ionic liquid rather than volatile organic solvents.

Full text of DOI:10.1039/c4ra01240b

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Cs2CO3 promoted direct C–H bond sulfenylation of
imidazo[1,2-a]pyridines and related heteroarenes in
ionic liquid†
Cite this: RSC Adv., 2014, 4, 19891
*
Zhaochang Gao, Xun Zhu and Ronghua Zhang
An efficient and novel method was developed for the synthesis of 3-sulfenyl imidazo[1,2-a]pyridines in good
to excellent yields via a Cs2CO3 promoted direct sulfenylation of imidazo[1,2-a]pyridines. The reaction
proceeds smoothly with a wide range of structurally diverse heteroarenes and disulfides. This protocol is
environmentally friendly because it is free of transition-metal catalysts and utilizes aryl-substituted
imidazolium-based ionic liquid rather than volatile organic solvents.
Received 12th February 2014
Accepted 17th April 2014
DOI: 10.1039/c4ra01240b
www.rsc.org/advances
activation strategy has also been used for the selective C–S bond
formation.10 In view of the synthetic strategies as indicated
Introduction
above, a number of procedures have been devised for sulfeny-
lation of electron-rich aza-aromatics with transition-metal
catalysts,11 new sulfur reagents12 and stoichiometric amounts of
promoters.13
Sulfur-containing substances are an important class of
compounds, which exist in a variety of natural products and
synthetic drugs and play a signicant role in medicinal chem-
istry for their biological activities.1 Sulfenyl aza-aromatics are
especially valued due to their comprehensive therapeutic value
against a variety of diseases, such as tubulin polymerization,
human breast cancer and other tumors,2,3 and vascular4 and
respiratory disorders.5
Imidazopyridine and its derivatives exist in a variety of
natural products and have attracted much attention due to their
important biological activities and broad utilization in the
pharmaceutical industry. Generally, the pharmacological
prole is mainly dependent on the nature of the substitutional
groups and introduction of sulfenyl groups on the aza-aromatic
rings could impart markedly biological properties to the
compounds, for example, 3-sulfenyl indoles, 3-sulfenyl pyrroles
and 3-sulfenyl imidazopyridines are of considerable therapeutic
value against a variety of diseases.6
Since the discovery of the important applications of sulfur-
containing agent, many strategies for the construction of
carbon–sulfur bond have been developed in recent years.
Research on copper-catalyzed Ullmann-type carbon–sulfur
couplings have made great progresses in recent years.7 Chan–
Evans–Lam-type reactions are also very efficient for C–S bond
formation.8 Base-promoted procedure via reductive coupling of
tosylhydrazones with thiols under metal-free conditions has
been reported recently.9 As an important and attractive method,
Friedel–Cras reaction combined with selective C–H bond
In recent years, rare earth metal catalysts were extensively
used in organic synthesis.14 Cerium(III) chloride has emerged as
a cheap, nontoxic, water-tolerant and very useful Lewis acid
catalyst, which can promote the reactions of electron-rich aza-
aromatics with several electrophiles to form carbon–sulfur,
carbon–nitrogen, carbon–oxygen, and carbon–carbon bonds.15
In compliance to green chemistry principles, “ionic liquids”
(ILs) constitute an attractive alternate in the eld of organic
synthesis, electrochemistry and separation process due to
catalyst recycling, improve selectivity and ease in product
isolation. Henceforth, imidazolium based ILs have been
utilized widely in current organic chemistry.16
Although a number of synthetic protocols have been devel-
oped for the sulfenylation of electron-rich aza-aromatics, it is
still desirable to explore new methods avoid using metal catalyst
and complex sulfur-containing reagent under environmentally
friendly reaction condition. Towards research in developing
new methods for the carbon–heteroatom bond forming reac-
tions, an efficient and simple base promoted direct C–H bond
sulfenylation of imidazo[1,2-a]pyridines and related hetero-
arenes with disuldes in aryl-substituted imidazolium-based
ionic liquid has been developed (Scheme 1).
Department of Chemical Engineering, Yancheng Institute of Industry Technology,
Yancheng 224005, P. R. China. E-mail: rong_hua_zhang@163.com; Fax: +86 (0515)
88583630
† Electronic supplementary information (ESI) available: See DOI:
10.1039/c4ra01240b
Scheme 1 Base promoted 3-sulfenylation of imidazo[1,2-a]pyridines
in ionic liquid.
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Table 1 Optimization of reaction conditions for sulfenylation of imi-
dazo[1,2-a]pyridinesa
Results and discussion
Inspired by the outcomes of recent reports on base promoted
direct C–H bond sulfenylation in volatile organic solvents17 and
the decisive role of ionic liquids in organic synthesis,18 we
designed aryl-substituted 2-ethyl-imidazolium-based ionic
liquid 1-benzyl-3-butyl 2-ethyl-imidazolium tetrauoroborate
([BnEBIm]BF4) and 1-benzyl-3-octyl 2-ethyl-imidazolium tetra-
uoroborate ([BnEOIm]BF4) and investigated their solvent
effect for the present reaction (Scheme 2).16g
We started our studies by reacting phenyl disulde with 2-
phenylimidazo[1,2-a]pyridine in the presence of 2 equivalents
of Cs2CO3 in DMSO at 80 ꢀC under an air atmosphere. However,
only 32% yield of product was detected aer 15 hours reaction.
Encouraged by this result, the reaction was optimized and the
results are summarized in Table 1.
Entry
Base
Solvent
Temp (ꢀC)
Yieldb (%)
1
2
3
4
5
6
7
8
Cs2CO3
K2CO3
KHCO3
KF
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
EtOH
CH3CN
DMC
Toluene
IL1
IL2
IL3
IL4
IL3
IL3
IL3
IL3
IL3
80
80
80
80
80
80
80
80
80
80
80
80
80
80
80
80
100
50
80
32
11
0
0
NaOH
tBuOK
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
—
23
15
36
31
Trace
Trace
56
68
89
91
77
53
89
20
0
9
At rst, several kinds of bases were tested and it was found
that other bases such as K2CO3, KHCO3, KF, NaOH and tBuOK
were less effective compared to Cs2CO3 due to their higher or
lower pKa value (Table 1, entries 1–6). Subsequently, several
solvents such as DMSO, EtOH, CH3CN, DMC, toluene and ionic
liquids were tested for the reaction (Table 1, entries 7–14).
Results show that ionic liquids can give moderate to excellent
yield of the desired product and the highest yield was obtained
in IL4, which may because the aryl and alkyl substituted groups
of ionic liquids increased the interaction and solubility of
reactants. However, when IL3 was used as the solvent, work-up
manipulation is easier than IL4 because the melting point of IL4
is about 60 ꢀC. Reducing the dosage of base from 2 equivalents
to 1.5 and 1.0 equivalent led to decrease of the yield of 3a from
89% to 77% and 53% (Tabꢀle 1, entries 15–16). When the reac-
10
11
12
13
14
15c
16d
17
18
19
a
Reaction conditions: 1a (0.5 mmol), 2a (0.5 mmol), base (1 mmol),
b
solvent (1 mL), 15 h, 80 ꢀC in a 25 mL ask. Yield of isolated
c
product aer column chromatography. 1.5 equivalent of base was
used. d 1.0 equivalent of base was used.
ꢀ
tion was carried out at 50 C instead of 80 C, the yield of 3a
Electron-donating and electron-withdrawing groups on the
benzene rings of imidazo[1,2-a]pyridines were tolerated.
Benzene rings of imidazo[1,2-a]pyridines with electron-
donating groups (Table 2, entries 3b–d), such as CH3 and OCH3,
gave a slightly higher yield than those bearing electron-with-
drawing groups, such as Cl, Br and NO2 (Table 2, entries 3e–g).
Benzene rings of imidazo[1,2-a]pyridines bearing CH3 group at
the ortho-position of the benzene ring afforded the corre-
sponding product in 87% yield, compared with the yield of 91%,
when the CH3 group was located at the para-position of the
benzene ring (Table 2, entries 3b–c). The result represented a
slight ortho-position effect of imidazo[1,2-a]pyridines. Under
the present reaction conditions, imidazo[1,2-a]pyridines
bearing furan group can be easily converted to the desired
product 3i in 87% yield (Table 2, entry 3i). To our delight, imi-
dazo[1,2-a]pyridines bearing alkyl group can also be trans-
formed to the desired product 3h in 84% yield (Table 2, entry
3h). On another hand, various disuldes used for the direct C–H
bond sulfenylation reaction were also investigated. Disuldes
bearing electron-donating groups on R (Table 2, entries 3k–l)
gave better yields than those bearing electron-withdrawing
groups (Table 2, entries 3m–o). It was worth noting that
dibenzyl disulde could also be converted to the corresponding
product 3r in 82% under the presence reaction condition (Table
2, entry 3r). However, alkyl disuldes could not afford the
decreased from 89% to 20% (Table 1, entries 17–18). Blank
experiment show that the reaction cannot proceed without the
use of base (Table 1, entry 19).
In order to extend the scope of this methodology, a variety of
imidazo[1,2-a]pyridines were allowed to reacted with disuldes
under optimized conditions. As can be seen from Table 2, the
direct C–H bond sulfenylation reactions of imidazo[1,2-a]pyri-
dines with disuldes generated the corresponding products in
good to excellent yields.
Scheme 2 Simple routes for the synthesis of aryl-substituted imida-
zolium-based ionic liquids.
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Table 2 The scope of imidazo[1,2-a]pyridines and disulfides for the
synthesis of 3-sulfenyl imidazopyridines under the optimum
conditionsa,b
desired product under the optimized reaction conditions (Table
2, entry 3q).
Other electron-rich heterocycles such as indoles and pyrroles
were also tested under the present optimized reaction condi-
tion. To our delight, they can both transformed to the desired
products in good to excellent yield aer simple work-up
manipulation steps and the results was listed in Table 3. Under
the present reaction condition, diphenyl diselenide can also
react with 1-H indole to form the desired product in 85%.
With respect to the reaction mechanism we assume that
deprotonation of the heterocycle afforded an anion was a key
reaction step. Then the anion reacted with disulde by nucle-
ophilic attack, resulting in C–S bond formation.17,18
Conclusions
In conclusion, we have developed an efficient and environ-
mentally friendly Cs2CO3 promoted direct C–H bond sulfeny-
lation of imidazopyridines, indoles, and pyrroles with disuldes
in aryl-substituted imidazolium-based ionic liquid rather than
volatile organic solvents. Further more, this protocol is free of
transition-metal catalyst and ionic liquid designed above can be
reused via an easy work-up manipulation. A wide range of
heterocycles with both electron-releasing and electron-with-
drawing groups were tested and the corresponding sulfenyl
heterocycles were obtained in good to excellent yields.
Experimental
All chemicals (AR grade) were obtained from commercial
sources and were used without further purication. Petroleum
ꢀ
a
ether (PE) refers to the fraction boiling in the 60–90 C range.
Reaction conditions: 1a (0.5 mmol), 2a (0.5 mmol), Cs2CO3 (1 mmol),
IL3 (1 mL), 15 h, 80 ꢀC in a 25 mL ask. b Yield of isolated product aer
column chromatography.
The progress of the reactions was monitored by TLC (silica gel,
Polygram SILG/UV 254 plates). Column chromatography was
performed on Silicycle silica gel (200–300 mesh). Melting points
were obtained using a Yamato melting point apparatus Model
MP-21 and are uncorrected. IR spectra were recorded on a
Shimadzu spectrophotometer using KBr discs. 1H and 13C NMR
spectra were obtained using a Bruker DRX 500 (500 MHz)
spectrometer in CDCl3 or DMSO-d6 with TMS as the internal
standard. All the products are known compounds and they were
identied by comparison of their physical and spectral data
with those reported in the literature.
Table 3 The scope of other electron-rich heterocycles and disulfides
for direct C–H bond sulfenylation reactiona,b
General procedure for the synthesis of aryl-substituted
imidazolium-based ionic liquid
(1) Sodium hydride (60% suspension, 40 mmol) was placed in a
two-necked ask and 2-ethyl imidazole (20 mmol) in DMF (20
mL) was added under nitrogen atmosphere at À15 ꢀC. The
reaction mixture was stirred for another 30 min at À15 ꢀC, and
then benzyl chloride (20 mmol) was added. The temperature of
ꢀ
the reaction was raised to À5 C and the reaction mixture was
stirred for another 3 hours under N2. Aer completion, reaction
was quenched with methanol (5 mL) and solvent was evapo-
rated to give the crude product which was puried using silica
gel column chromatography to obtain B in 92% yield.
a
Reaction conditions: 1a (0.5 mmol), 2a (0.5 mmol), Cs2CO3 (1 mmol),
IL3 (1 mL), 15 h, 80 ꢀC in a 25 mL ask. b Yield of isolated product aer
column chromatography.
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(2) Alkyl bromide (25 mmol) was added to B (20 mmol) in
THF (20 mL) and the reaction was stirred for 4–6 hours at 80 ꢀC.
The reaction mixture was dried under reduced pressure and
column chromatography over silica gel provided the desired
compounds C and D in the yield of 91% and 90%.
(3) To the solutions of the imdazolium halides C and D (20
mmol) in water (20 mL) was added NaBF4 (21 mmol) and the
reaction mixture was stirred for 1.5 h at room temperature.
Reaction mixture was extracted with dichloromethane (3 Â 10
mL) and puried by column chromatography to obtain ionic
liquid IL3 and IL4 in the yield of 95% and 94%.
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A mixture of heterocycle (0.5 mmol), disulde (0.5 mmol) and
ꢀ
Cs2CO3 (1 mmol) were added in IL3 (1 mL) at 80 C in a ask.
The reaction was carried out under an air atmosphere for about
15 h until complete consumption of starting material as
monitored by TLC. The solution was extracted with ethyl ether
(3 Â 10 mL). And then the organic layer was separated and
concentrated under vacuum and the crude product was puried
by column chromatography (PE : EtOAc, 15 : 1) or recrystalli-
zation (PE : EtOAc, 5 : 1) to provide the analytically pure
product. The ionic liquid layer was washed with water (2 Â 2
mL) and separated via simple work-up manipulation. And the
ionic liquid can be easily recycled aer drying under vacuum.
´
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Acknowledgements
This project was funded by the joint innovation and research
funding of Jiangsu province, joint research project (no.
BY2013060).
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