Potassium iodide promoted thiolation of pyrazolones and benzofurans using aryl sulfonyl chlorides as sulfenylation reagents

Article abstract of DOI:10.1016/j.tetlet.2016.10.053

An efficient, transition-metal-free method to synthesize pyrazolone thioethers as well as 2-aryl and 3-aryl benzofuran thioethers by employing aryl sulfonyl chlorides as sulfenylation reagents in the presence of triphenylphosphine was developed. Potassium iodide was found to facilitate the transformation for the first time by generating more reactive sulfenyl iodide in situ from sulfenyl chloride.

Full text of DOI:10.1016/j.tetlet.2016.10.053

Accepted Manuscript
Potassium Iodide Promoted Thiolation of Pyrazolones and Benzofurans Using
Aryl Sulfonyl Chlorides as Sulfenylation Reagents
Xia Zhao, Xiaoyu Lu, Aoqi Wei, Xiaoliang Jia, Jun Chen, Kui Lu
PII:
S0040-4039(16)31367-3
DOI:
http://dx.doi.org/10.1016/j.tetlet.2016.10.053
TETL 48223
Reference:
Tetrahedron Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
6 September 2016
14 October 2016
17 October 2016
Please cite this article as: Zhao, X., Lu, X., Wei, A., Jia, X., Chen, J., Lu, K., Potassium Iodide Promoted Thiolation
of Pyrazolones and Benzofurans Using Aryl Sulfonyl Chlorides as Sulfenylation Reagents, Tetrahedron Letters
(2016), doi: http://dx.doi.org/10.1016/j.tetlet.2016.10.053
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Potassium Iodide Promoted Thiolation of
Pyrazolones and Benzofurans Using Aryl
Sulfonyl Chlorides as Sulfenylation Reagents
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Xia Zhao,* Xiaoyu Lu, Aoqi Wei, Xiaoliang Jia, Jun Chen and Kui Lu*

1
Tetrahedron Letters
Potassium Iodide Promoted Thiolation of Pyrazolones and Benzofurans Using Aryl
Sulfonyl Chlorides as Sulfenylation Reagents
Xia Zhaoa,  , Xiaoyu Lua, Aoqi Weia, Xiaoliang Jiab, Jun Chenc and Kui Lu b, 
a College of Chemistry, Tianjin Key Laboratory of Structure and Performance for Functional Molecules, Key laboratory of Inorganic-organic Hybrid Functional
Material Chemistry, Ministry of Education, Tianjin Normal University, Tianjin 300387, China
b College of Biotechnology, Tianjin University of Science & Technology, Tianjin 300457, China
c Beijing Institute of Microchemistry, Beijing 100091, China
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
An efficient, transition-metal-free method to synthesize pyrazolone thioethers as well as 2-aryl
and 3-aryl benzofuran thioethers by employing aryl sulfonyl chlorides as sulfenylation reagents
in the presence of triphenylphosphine was developed. Potassium iodide was found to facilitate
the transformation for the first time by generating more reactive sulfenyl iodide in situ from
sulfenyl chloride.
Available online
Keywords:
Thiolation
Pyrazolone
2009 Elsevier Ltd. All rights reserved.
Benzofuran
Sulfonyl Chloride
Potassium Iodide
1. Introduction
benzofurans8 and electron-rich aromatics9 with sulfonyl
hydrazides as the sulfur source. Herein, we would like to report a
Carbon–Sulfur (C–S) bond-forming reactions play a pivotal
role in the synthesis of organo-sulfur compounds of
pharmaceutical and material interest.1 Therefore, exploration of
new methods for C–S bond generation has attracted much
attention in recent years.2 Although transition-metal-catalyzed C–
S bond formation reactions could avoid harsh condition as well
as toxic polar solvents with high boiling temperatures, expensive
and often air-sensitive catalysts as well as foul-smelling, unstable
and volatile sulfur sources limitted their further application3
Recently, sulfonyl hydrazides have emerged as a novel and
environmental-friendly sulfenylation reagents because they are
generally stable, readily accessible, and odor-free.4 To the best of
our knowledge, the synthetic precursor sulfonyl chlorides have
rarely been reported to be sulfenylation reagents, although they
are used as sulfonylating reagents as well as arylation reagents in
C–C cross coupling reactions. In 2011, You and co-workers
reported an efficient thiolation of indolizines, indoles and
electron-rich benzenes using aryl sulfonyl chlorides as
sulfenylation reagents in combination with PPh3 as reductant.5 In
2012, Zheng and coworkers reported Ru(bpy)3Cl2 catalyzed
visible light-induced 3-sulfenylation of N-methylindoles with
arylsulfonyl chlorides (Scheme 1).6 As a part of ongoing our
interests in exploring new sulfenylation reagents, we have
novel potassium iodide facilitated thiolation reaction of
pyrazolones and benzofurans with aryl sulfonyl chlorides as
sulfenylation reagents.
Scheme 1 Thiolation of indolizines and N-methylindoles using sulfonyl
chlorides as sulfenylation reagents
2. Results and discussion
To probe the thiolation of heterocyclic compounds like
pyrazolones and benzofurans, we employed the reaction of 5-
methyl-2-phenyl-2,4-dihydro-3H-pyrazol-3-one 1a and 4-
methylbenzenesulfonyl chloride 2a as the model reaction and
o
PPh3 as reductant. The reaction in toluene at 70 C gave the
developed an iodine catalyzed thiolation of pyrazolones,7
desired thiolation product 3aa in 49 % yield (Table 1, Entry 1).
To improve the yield, a series of solvents such as 1,2-
———
 Xia Zhao. Tel.: +86 22 23766531; fax: +86 2223766531; e-mail: hxxyzhx@mail.tjnu.edu.cn
 Kui Lu. Tel.: +86 22 60912562; fax: +86 22 60602298; e-mail: lukui@tust.edu.cn

2
Tetrahedron
dichloroethane
(DCE),
1,4-dioxane
and
N,N-
Table 2 Thiolation of pyrazolones with aryl sulfonyl chloride
a
dimethylformamide (DMF) were investigated (Table 1, Entry 2-
4) and 1,4-dioxane gave the best yield. Next, temperature and
in the presence of PPh3
o
o
additive effects were evaluated from 80 C to 100 C. When the
reaction was carried out at 80 oC and 90 oC, the yield increased to
65% (Table 1, Entries 5 and 6). However，further increasing the
reaction temperature to 100 oC led to a diminished yield (Table 1,
Entry 7). Notably, when 0.2 equiv. potassium iodide (KI) was
added, the yield increased dramatically to 90% yield (Table 1,
Entry 8), while the classical iodide anion source tetra-n-
butylammonium iodide (nBu4NI) decreased the yield (Table 1,
Entry 9). Finally, equivalents of 2a, PPh3 and KI as well as the
reaction concentration were investigated (Table 1, Entries 10-15).
For sulfonyl chloride, 1.2 equiv. of 2a gave the highest yield, and
decreasing the amount of 2a led to a diminished yield (Table 1,
Entry 10). For reductant, 2.0 equiv. of PPh3 were sufficient to
obtain a yield of 90%, and further increasing the amount of the
PPh3 did not affect the yield (Table 1, Entry 11). For additive, 0.2
equiv. of KI gave a yield of 90% as shown above. Decreasing the
KI amount to 0.15 equivalent led to a decreased yield (Table 1,
Entry 12), while further increasing the amount did not affect the
yield (Table 1, Entry 13). It was noteworthy that increasing or
decreasing the concentration both diminished the yield (Table 1,
Entries 14 and 15). Therefore, the reaction conditions were
optimized to be as follows: 1a (0.5 mmol), 2a (0.6 mmol), PPh3
(1.0 mmol), KI (0.1 mmol) and 1,4-dioxane (1 mL), at 80 C.
Entry
1
2
3
4
5
6
7
8
1, R1 =, R2 =
1a, Ph, Me
1a, Ph, Me
1a, Ph, Me
1a, Ph, Me
2, Ar =
Yield (%)
3aa, 90
3ab, 78
3ac, 75
3ad, 50
3ba, 55
3be, 70
3bf, 65
3ca, 60
3cb, 64
3da, 80
3ea, 85
3eb, 96
3ec, 87
3ed, 74
3ee, 78
3ef, 83
3eg, 73
3eh, 92
3ei, 71
3ej, 96
3fa, 70
3ga, 68
3ha, 70
2a, p-MeC6H4
2b, p-NCC6H4
2c, 3,5-Cl2C6H3
2d, o-MeC6H4
2a, p-MeC6H4
2e, m-ClC6H4
2f, p-MeOC6H4
2a, p-MeC6H4
2b, p-NCC6H4
2a, p-MeC6H4
2a, p-MeC6H4
2b, p-NCC6H4
2c, 3,5-Cl2C6H3
2d, o-MeC6H4
2e, m-ClC6H4
2f, p-MeOC6H4
2g, Ph
1b, p-MeOC6H4, Me
1b, p-MeOC6H4, Me
1b, p-MeOC6H4, Me
1c, p-NCC6H4, Me
1c, p-NCC6H4, Me
1d, Ph, Ph
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
1e, Me, Me
1e, Me, Me
1e, Me, Me
1e, Me, Me
1e, Me, Me
1e, Me, Me
1e, Me, Me
1e, Me, Me
1e, Me, Me
1e, Me, Me
1f, Me, Ph
1g, Me, Bn
2h, p-BrC6H4
2i, p-FC6H4
2j, p-CF3C6H4
2a, p-MeC6H4
2a, p-MeC6H4
2a, p-MeC6H4
1h, Bn, Me
a
Reaction conditions: 1a–i (0.5 mmol), 2a–j (0.6 mmol), PPh3 (1.0
mmol), KI (0.1 mmol), 1,4-dioxane (1 mL), 80 C, 12 h.
Table 1 Optimization of the thiolation of 1a with 2a in the
a
presence of PPh3
The reaction condition was then explored for the thiolation of
benzofuran derivatives. Considering the relatively less
nucleophilicity of benzofuran versus pyrazolone, the reaction of
2-(4-methoxyphenyl)benzofuran (4a) with 2a was carried out in
the presence of PPh3 and KI at 100 C in 1,4-dioxane, and the
desired thiolation product 5aa was obtained in 58% yield (Table
3, Entry 1). To improve the yield, varied solvents such as DCE,
DMF, toluene, ethanol (EtOH) and isopropanol (i-PrOH) were
tested (Table 3, Entry 2-6), and EtOH gave the highest yield of
80%. Reaction temperatures and KI equivalents were evaluated
Entry KI (equiv)
Solvent
Toluene
DCE
Volume (mL) Yield (%)
T (C)
70
70
70
70
80
90
100
80
80
80
80
80
80
80
80
1
2
-
-
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.5
0.5
49
32
48
55
65
65
37
90
56
79c
90d
78
90
79
71
3
-
DMF
4
-
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
o
as well. Increasing the reaction temperature to 110 C did not
5
-
6
-
affect much on the yield (Table 3, Entry 7), while decreasing the
reaction temperature to 90 oC led to an increase of the yield to
90% yield (Table 3, Entry 8). However, further decreasing the
7
-
8
9
0.2
0.2b
0.2
0.2
0.15
0.25
0.2
0.2
o
reaction temperature to 80 C diminished the yield (Table 3,
10
11
12
13
14
15
Entry 9). Decreasing KI loading to 0.15 equiv. or 0.1 equiv. led
to a decrease of the yield (Table 3, Entries 10 and 11). Finally,
concentration screening demonstrated that decreasing or
increasing reactant concentrations led to a decrease of the yield
(Table 3, Entries 12 and 13). Thus, the optimized reaction
conditions for thiolation of 4a were as follows: 4a (0.5 mmol), 2a
(0.6 mmol), PPh3 (1.0 mmol), KI (0.1 mmol) and EtOH (1 mL),
at 90 C.
a
b
Reaction conditions: 1a (0.5 mmol), 2a (0.6 mmol), PPh3 (1.0 mmol).
c
d
nBu4NI was used. 2a (0.55 mmol) was used. PPh3 (1.1 mmol) was
used.
The substrate scope of the sulfenylation reaction was
investigated under the optimized conditions. As shown in Table
2, a series of substituted pyrazolones could be coupled with
various substituted aryl sulfonyl chlorides to afford the
corresponding pyrazolone thioethers in moderate to excellent
yields.10 For pyrazolones, both aryl groups (1a-1d) and alkyl
groups (1e-1h) on 1-position were well tolerated. Moreover,
pyrazolones bearing phenyl, benzyl and methyl groups on 3-
position could be readily converted to the desired products in
good yields.
Table 3 Optimization of the thiolation of 4a with 2a in the
presence of PPh3
a
Entry KI (equiv) T (C)
Solvent
1,4-dioxane
DCE
DMF
Toluene
EtOH
i-PrOH
EtOH
EtOH
EtOH
EtOH
Volume (mL) Yield (%)
1
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.15
0.1
100
100
100
100
100
100
110
90
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
58
0
trace
0
80
69
79
90
80
87
86
2
3
4
5
6
7
8
For aromatic sulfonyl chloride, both electron-donating and
electron-withdrawing groups, as well as ortho-, meta-, and para-
substitutions (2a-2j) were tolerated in the reaction. Notably, for
steric effect, phenyl sulfonyl chloride with an ortho-methyl group
provided pyrazolone thioether in a relatively low yield compared
with the corresponding para-methyl group (3aa versus 3ad, 3ea
versus 3ed).
9
80
10
11
80
80
EtOH

3
12
13
0.2
0.1
80
80
EtOH
EtOH
0.5
1.5
80
80
Table 5 Thiolation of 3-substituted benzofurans with aryl
sulfonyl chlorides in the presence of PPh3
a
a Reaction conditions: 4a (0.5 mmol), 2a (0.6 mmol), PPh3 (1.0 mmol)
Under the optimized reaction conditions for thiolation of
benzofurans 4a, a series of 2-substituted benzo[b]furans was
coupled with varied of aryl sulfonyl chlorides in the presence of
PPh3 (Table 4). Aryl sulfonyl chlorides with electron-donating or
electron-withdrawing substituents at ortho-, meta- or para-
position were reacted with 4a, and the reactions gave the desired
products in good to excellent yields. However, the reaction of 2-
phenylbenzofuran (4b) with 2a yielded only 20% of the desired
product 5ba, which might be due to the electronic effect on the
benzofuran. When more KI and higher reaction temperature were
employed, the yield of 5ba was improved to 34%. For substituted
2-phenylbenzofurans, the reaction of 2a with 5-methyl-2-
phenylbenzofuran (4c) gave better yields than the unsubstituted
2-phenylbenzofuran (4b). Notably, 2-methylbenzofuran (4d)
could couple with 2a, 2e and 2f smoothly and afforded the
thiolation products 5da, 5de and 5df in 60%, 44% and 70%
yield, respectively.
Table 4 Thiolation of 2-substituted benzofurans with aryl
a
sulfonyl chlorides in the presence of PPh3
Scheme 2 Proposed reaction mechanism
3. Conclusion
In conclusion, we developed a novel and concise method to
synthesize pyrazolone thioethers as well as 2-aryl and 3-aryl
benzofuran thioethers by using aryl sulfonyl chlorides as
sulfenylation reagents. Potassium iodide was demonstrated to
facilitate this transformation possibly by generating more
reactive sulfenyl iodide in situ from sulfenyl chloride. This study
broadened the scope of sulfonyl chloride as novel thiolation
reagents in organic synthesis.
To further probe the scope of this reaction, 3-substituted
benzofurans were examined under the modified optimal reaction
conditions. The results are illustrated in Table 5. 3-
Phenylbenzofuran (6a), 3-(4-methoxyphenyl)benzofuran (6b)
and 3-methylbenzofuran (6c) could all be converted to the
sulfenylation products smoothly with various aryl sulfonyl
chlorides (2a, 2c, 2f and 2k) as sulfenylaiton reagents.
Based on the experiments and previous reports,5 a plausible
mechanism for these thiolation reactions is proposed (Scheme 2).
Initially, aryl sulfonyl chloride 2 is reduced by PPh3 via
intermediates A-E to sulfenyl chloride F, which is attacked by
iodide anion to give sulfenyl iodide G. Electrophilic thiolation of
pyrazolone 1 as well as benzofuran 4 and 6 by sulfenylation
reagent G gives the corresponding pyrazolone thioethers, 2-aryl
and 3-aryl benzofuran thioethers respectively.
Acknowledgments
The authors sincerely thank the financial support from
National Science Foundation of China (Grants 21572158,
21202119, 21202118). The authors also thank Dr. Jing Liu to
polish the English of our manuscript.
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10. General procedure of thiolation of pyrazolones with aryl sulfonyl
chloride in the presence of PPh3: Pyrazolones 5-methyl-2-phenyl-
2,4-dihydro-3H-pyrazol-3-one (1a) (87 mg, 0.5 mmol), 4-
methylbenzenesulfonyl chloride (2a) (114 mg, 0.6 mmol), PPh3
(262 mg, 1.0 mmol) and KI (17 mg, 0.1 mmol) in 1,4-dioxane (1.0
mL) were mixed in a sealed tube. The mixture was stirred at 80 oC
for 12 hours. Then, the solvent was evaporated under reduced
pressure and the residue was purified by silica gel column
chromatography to afford 3-methyl-1-phenyl-4-(p-tolylthio)-1H-
pyrazol-5-ol (3aa) (133 mg, 90%) as a white solid.

5
Highlights
1. Transition-metal-free thiolation of pyrazolones and benzofurans was achieved
2. Aryl sulfonyl chlorides were used as sulfenylation reagents
3. Triphenylphosphine was used as a reducing agent
4. Potassium iodide was found to facilitate the transformation