Catalyst-free sulfenylation of indoles with sulfinic esters in ethanol

Article abstract of DOI:10.1039/c8gc01764f

A novel catalyst-free method for the synthesis of structurally diverse indole thioethers in moderate to excellent yields has been developed. In this reaction, sulfinic esters serve as new sulfur electrophiles.

Full text of DOI:10.1039/c8gc01764f

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Catalyst-free sulfenylation of indoles with sulfinic esters in
ethanol
Received 00th January 20xx,
Accepted 00th January 20xx
Xiuqin Yang, Yishu Bao, Zonghao Dai, Qingfa Zhou,* and Fulai Yang*
DOI: 10.1039/x0xx00000x
www.rsc.org/
A novel catalyst-free method for synthesis of structurally diverse
In the course of exploring new methods to synthesize organic
indole thioethers in moderate to excellent yields has been sulfides, we found sulfonyl hydrazides could serve as sulfur
developed. In this reaction, sulfinic esters serve as new sulfur electrophiles through the cleavage of sulfur-oxygen bonds and
electrophiles.
sulfur-nitrogen bond,16e and this protocol requires iodine as
catalyst. Inspired by this work, along with our interest in developing
new sulfenylating agents, we envisioned that sulfinic esters which
are readily accessible and free of unpleasant odor might serve as
potential sulfenylating agents through the cleavage of sulfur-oxygen
bonds to react with indoles to give indole thioethers under catalyst-
free conditions.
Introduction
The selective formation of carbon-sulfur bonds has emerged as a
significant field of research in organic chemistry because of their
prevalence in the pharmaceutical industry and material science.1 In
recent years, much attention has been paid to the synthesis of
organosulfur compounds.2 Among them, efficient methods for
indole thioethers formation from chemical feedstocks are highly
sought after, because some of them can serve as potent agents to
treat HIV,3 cancer,4 heart disease,5 and allergies.6 Due to the
electron-rich nature of indole rings, many methods have recently
been developed via the direct sulfenylation of indoles with
sulfenylating agents, such as sulfenyl halides,7 N-thioimides,8
sulfoniumsalts,9 thiols,10 disulfides,11 quinone mono-O,S-acetals,12
arylsulfonyl chlorides,13 sodium sulfinates,14 sulfinic acids,15 sulfonyl
hydrazides,16 and N-hydroxy sulfonamides.17 Nevertheless, these
reactions have some well-known disadvantages: (a) some of the
sulfenylating agents are unstable to air and moisture, are difficult to
be prepared, or possess unpleasant smelling; (b) many of these
reactions involve the use of metal catalysts. It is of great
importance to develop efficient processes that do not require a
metal catalyst in consideration of the low threshold residual
tolerance of metals for pharmaceuticals; (c) some of these reactions
require excess additives, high temperature, or yield byproducts
unfriendly to the environment. To address such issues, it is highly
desirable to develop new sulfenylating agents and simple reaction
conditions for the sulfenylation of indoles.
Results and discussion
The model reaction of indole (1a) with sulfinic ester (2a) was
performed in MeOH at 80℃ (Table 1, entry 1). To our delight, when
the reaction was carried out in the absence of catalyst, the desired
sulfenylation reaction can take place and give indole thioether (3a)
in 51% yield. Encouraged by this result, a number of common
solvents were examined, and a best yield was achieved when using
EtOH as solvent (Table 1, entries 2-12). Further examination
showed that temperature has a greater impact on the reaction
(Table 1, entries 13-14). Lower yields were obtained when the
reaction was run at 70℃, while increasing the reaction temperature
to 90℃afforded a better yield. For comparison, the reaction was
performed under nitrogen and oxygen (Table 1, entries 15-16),
unfortunately, both of them gave 3a in a slightly lower yield. Finally,
screening the ratio of starting materials showed a high yield was
obtained when the ratio of 1a, 2a was adjusted to 1.8:1. Thus, the
optimal reaction conditions were 1.8 equiv of 1a, 1.0 equiv of 2a in
EtOH at 90℃.
The development of such a simple route for the formation of
indole thioethers was highly appealing, we first investigated the
substrate scope of indoles (Table 2). Under the optimized reaction
conditions, a series of indoles smoothly underwent sulfenylation
with sulfinic esters 2a to give the corresponding indole thioethers in
good yields. It was found that either electron-donating groups Me,
Bn, Ph, and OMe (3b-3i) or electron-withdrawing groups F, Cl, and
Br (3m-3p) on the N-1, C-2, C-4, C-5, C-6 or C-7 positions of the
indole rings could give the corresponding 3-arylthioindoles in better
†Electronic Supplementary Information (ESI) available: Experimental procedures,
characterization data, and copies of 1H NMR and 13C NMR spectra. See DOI:
10.1039/x0xx00000x
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yields. Lower yields were obtained when strong electron desired products was obtained in 66% and 42% yields, respectively,
withdrawing groups, such as carboethoxy group, nitro group, and suggesting that sulfinic ester might decompose to give sulfinic acid
nitrile group were present at the C-6, C-5 positions of the indole upon heating. In addition, a gram-scale synthesis of indole thioether
rings (3j-3l). To our delight, when the C-3 position was occupied by 3a was successfully performed under the standard reaction
methyl group, the sulfenylation reaction took place at the C-2
conditions (1.14 g, 79% yield).
Subsequently, the scope of sulfinic esters was tested (Table 3).
The sulfenylation reaction of indole with various sulfinic esters give
indole thioethers (4a-4m) in moderate to good yields. Various
electron-donating substituents such as t-Bu and OMe (4b-4c) and
electron-withdrawing substituents such as F, Cl, Br, I, and CF3 (4d-
4h) were well tolerated in this reaction. It should be noted that
steric effect had little influence on the reaction, more bulky
substrates such as 2,4,6-trimethylbenylsulfinic ester, 1-
naphthylsulfinic ester,
Table 1. Optimization of Reaction Conditions 3aa
Entry Solvent
T (oC)
80
80
80
80
80
80
80
80
80
80
80
80
70
90
90
90
90
90
90
1a/2a
1.5/1
1.5/1
1.5/1
1.5/1
1.5/1
1.5/1
1.5/1
1.5/1
1.5/1
1.5/1
1.5/1
1.5/1
1.5/1
1.5/1
1.5/1
1.5/1
1.8/1
1.2/1
1/1.5
Time (h) Yield (%)b
1
MeOH
EtOH
12
12
12
12
12
12
12
12
12
12
12
12
24
12
12
12
12
12
12
51
73
Table 2. Scope of Indoles 1a,b,c
2
3
DMF
34
4
DMSO
MeCN
MeNO2
Dioxane
EtOAc
CH2Cl2
CHCl3
DCE
Trace
62
5
6
67
7
64
8
63
9
59
10
11
12
13
14
15c
16d
17
18
19
50
57
Toluene
EtOH
59
52
EtOH
81
EtOH
59
EtOH
62
EtOH
88
EtOH
52
EtOH
Trace
aReaction conditions: 1 (0.36mmol), 2a (0.2 mmol), ethanol (0.50
aReaction conditions: 1a (0.36mmol), 2a (0.2 mmol), in solvent (0.50
mL), at 90 oC (oil bath), For 12 h. bIsolated yield. cAt 120 °C.
o
b
c
mL), at 90 C (oil bath), For 12 h. Isolated yield. The reaction was
performed under nitrogen. dThe reaction was performed under
oxygen.
and 2-naphthylsulfinic ester all efficiently reacted with 1a and gave
the product in good yields (4i-4k). To our delight, 2-
thiophenylsulfinic ester could also afford corresponding product in
moderate yield (4l). In addition, cyclopropanylsulfinic ester also
could couple with indole to give the desired product in 54% yield
position of the indole ring (3q). However, when N-protected indoles
(Boc, Bz) were used as substrates, indole thioether 3a instead of the
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(4m), which indicating that the reaction might not involve sulfide sulfonothioate 5 and sulfinic acid 6,16a, 16e both of which undergo a
radicals.18
electrophilic reaction with indole 1 to give intermediate 9 and
To get an insight into the mechanism of the sulfenylation intermediate 11. And then anion 10, resonance of intermediate 9
process, several control experiments were performed (Scheme 1). can react with hydrogen ions to form sulfinic acid 6, which also can
Under the optimized reaction conditions, sulfinic ester (2a) react with indole 1 to afford intermediate 11. The intermediate 11
decomposed to give sulfonothioate (5) and sulfinic acid (6) in 72% finally transforms into the desired product 3 or 4.
and 20% yields (Scheme 1a), respectively, while no disulfide was
detected. The decomposition of sulfinic ester (2a) and sulfinic ester
(8) both gave acetophenone as oxidation product, which
substantially demonstrates that sulfinic ester is reduced by ethanol
to serve as sulfur electrophile. (Scheme 1b, 1c) Replacing sulfinic
ester with sulfonothioate gave the desired product (3a) in 42%
yield, which suggests that sulfonothioate might be
a major
intermediate in this reaction. Indole (1a) could react with sulfinic
acid 6 to give final product in 56% yield. Addition of free radical
inhibitor 2,6-di-tert-butyl-p-cresol (BHT) to the reaction mixture of
indole and sulfinic ester had no effect on the reaction efficiency,
indicating that the process might not involve radical intermediates.
Table 3. Scope of Sulfinic Esters 2a,b
Scheme 1. Control Experiments
Scheme 2. Proposed Reaction Pathways
Conclusions
In summary, we have developed
a simple and efficient
sulfenylation reaction of indoles with sulfinic esters. Under catalyst-
free conditions, a range of aryl-, heteroaryl-, and alkylsulfinic esters
smoothly reacted with indoles to give structurally diverse
thioethers in moderate to good yields. This reaction is carried out
under environmentally benign without any catalyst, additive, and
ligand.
aReaction conditions: 1a (0.36mmol), 2 (0.2 mmol), ethanol (0.50
mL), at 90 oC (oil bath), For 12 h. bIsolated yield.
According to the above experimental results and previous
relevant studies,16a we propose the major reaction pathway
depicted in Scheme 2 for catalyst-free sulfenylation of indoles with
sulfinic esters. Initially, sulfinic esters 2 is reduced by ethanol to give Conflicts of interest
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There are no conflicts to declare.
Acknowledgements
We are grateful for the financial support from the National
Natural Science Foundation of China (21502182, 21102179 and
21572271), Qing Lan Project of Jiangsu Province, National Found
for Fostering Talents of Basic Science (J1030830), the
Fundamental Research Funds for the Central Universities
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A novel catalyst-free method for synthesis of structurally diverse indole
thioethers in moderate to excellent yields.
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