Metal free sulfenylation and bis-sulfenylation of indoles: Persulfate mediated synthesis

Article abstract of DOI:10.1039/c3ob41601a

A method which avoids metal and halogen for the synthesis of 3-arylthioindoles from indoles and diaryl disulfides using ammonium persulfate in methanol has been presented. Moreover, double C-H sulfenylation of indoles at 2 and 3-positions has also been ac

Full text of DOI:10.1039/c3ob41601a

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Biomolecular Chemistry
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Metal free sulfenylation and bis-sulfenylation of
indoles: persulfate mediated synthesis†
Cite this: Org. Biomol. Chem., 2013, 11,
8036
Ch. Durga Prasad, Shailesh Kumar, Moh. Sattar, Amit Adhikary and Sangit Kumar*
Received 5th August 2013,
Accepted 26th September 2013
DOI: 10.1039/c3ob41601a
www.rsc.org/obc
A method which avoids metal and halogen for the synthesis of
3-arylthioindoles from indoles and diaryl disulfides using
ammonium persulfate in methanol has been presented. Moreover,
double C–H sulfenylation of indoles at 2 and 3-positions has also
been achieved using iodine and ammonium persulfate.
Thiolated indole moieties are an important class of organo-
sulfur heterocyclic compounds having a greater therapeutic
value in the treatment of HIV, cancer, cardiovascular, and bac-
terial diseases.1,2 Among the various indole derivatives known,
3-arylthioindoles have attracted considerable interest (Chart 1).
As a result two synthetic routes have been developed to con-
struct thiolated indoles. One route involves the introduction of
substituents by direct sulfenylation of an existing indole
ring,3–13 whereas the other route involves the cyclization reac-
tions of 2-alkynylanilines14 or N,N-dialkyl-2-iodoanilines.15
Most of the methodologies derived from the first synthetic
route involve the usage of high boiling solvents such as
DMSO3 and DMF,4 a strong base,5 and the presence of tran-
sition metal catalysts6 or halogenating reagents N-bromo-
succinimide4b and trichloroisocyanuric acid6e to accomplish
sulfenylation in indoles using arylsulfenyl halides,7,13 aryl/
alkyl thiols,5a,6a,b,8 diaryl disulfides,3,4c,6c,9 aryl-N-thioimides,10 aryl-
sulfonium salts,11 and sulfonyl hydrazides12 as a source of the
organo-sulfur moiety. Most of the sulfur sources are either
difficult to prepare or air and moisture sensitive and expensive.
The development of a methodology which involves mild
reaction conditions, readily available reagents and substrates
will be highly desirable for the synthesis of sulfenyl indoles.
Mild reaction conditions enable installation of a wide range of
functional groups such as ether, bromide, and most impor-
tantly functionalities having acidic protons such as hydroxyl,
amine, and carboxylic acid which could be helpful in late stage
Chart 1 Biologically important 3-arylthioindoles.
transformation/functionalization for the synthesis of substi-
tuted sulfenyl indoles. Although many methods have been
developed to construct sulfenyl indoles, synthesis of arylthio-
lated indoles having acidic proton functional groups (OH,
NH2, CO2H etc.) has not been well documented to date. Also a
methodology which can cause double C–H sulfenylation in
indoles will be interesting because the double C–H sulfenyla-
tion in indoles has remained elusive. Recently, our group
has launched a program to synthesize organo ethers, sulfur,
selenium and tellurium compounds using new benign reaction
conditions and reagents.16 Here, in continuation of our work
on organochalcogen chemistry, we present an economical and
mild methodology for the regioselective synthesis of diversely
substituted 3-sulfenyl indoles. Moreover, double C–H thiolation
in indoles has been performed to give 2,3-bis-sulfenyl indoles.
Phenyl disulfide and indole were chosen as substrates in
the presence of various oxidants such as hydrogen peroxide,
tert-butyl hydroperoxide, potassium, and ammonium per-
sulfates in methanol at reflux temperature to screen for the
optimal reaction conditions (entries 1–5, Table 1). Among
these, ammonium persulfate (2 equiv.) gave good yield of the
sulfenylated product 1. Increasing or decreasing oxidant
loading resulted in a poor yield of 1. A number of solvents
like THF, dichloromethane, CH3CN, DMF, and DMSO were
examined; nonetheless, better yield could not be obtained
(entries 6–10, Table 1).
Department of Chemistry, Indian Institute of Science Education and Research (IISER),
Bhopal, MP 462 066, India. E-mail: Sangitkumar@iiserb.ac.in
†Electronic supplementary information (ESI) available: Experimental pro-
With the optimized reaction conditions in hand, a variety
of indoles and diaryl disulfides were tested to determine the
scope and limitations of this method. Results presented in
cedures, characterization data, NMR spectra. CCDC 952234 (21) and 953902 (11).
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c3ob41601a
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Table 1 Optimization of the sulfenylation of indolea
At first, we examined the reaction of indole with substituted
diaryl disulfides. Disulfides having electron donating groups like
para-methyl, para-methoxy, ortho-amino, and para-hydroxy
gave the respective sulfenylated products (2–5) with indole in
good yields. Carboxylic acid containing diaryl disulfide also
underwent this reaction smoothly with indole, resulting in
46% yield of 6. Synthesis of hydroxyl substituted 3-arylthio-
indoles has not been explored to date, whereas amino and
acid substituted 3-arylthioindoles are scarcely found in the lit-
erature.13k,17 The present mild methodology gives access to a
series of hydroxyl, amino, and acid substituted sulfenyl indoles
4–6, 9 and 16–18. Phenyl diselenide also reacted with indole
under optimized reaction conditions in methanol to produce
3-selenylated indole 7 in 80% yield.
Next, the sulfenylation reaction of diaryl disulfides with var-
iously substituted indoles was explored. The electronic pro-
perties of the groups attached to the indole have little effect on
the outcome of the reaction as the yields of the sulfenyl
indoles 8–19 remained nearly the same. Indoles with electron
donating methoxy group and electron withdrawing 5-nitro
and 5-bromo groups reacted with diaryl disulfides to give the
sulfenyl indoles 8–13 in 52–94% yield. The structure of the
synthesized 5-nitro-3-(p-tolylthio)-1H-indole 11 was also estab-
lished by an X-ray single crystal structure study (Fig. 1).
Entry
Oxidant
Yield (%)
Entry
Solventb
Yield (%)
1
2
3
4
5
H2O2
20
25
47
61
68
6
7
8
9
10
THF
22
35
43
Trace
Trace
tBuOOH
Oxone
CH2Cl2
CH3CN
DMF
K2S2O8
(NH4)2S2O8
DMSO
a The reaction was carried out on a 1 mmol scale using 2 equiv. of
indole, 1 equiv. of phenyl disulfide, and 2 equiv. of oxidant in 5 mL of
CH3OH at reflux (70 °C) temperature, unless otherwise noted. b 5 mL
of solvent is used instead of CH3OH.
Similarly, the sulfenylation reaction of substituted indoles
with substituted disulfides was tested. N-Methyl indole reacted
with para-methoxy, para-hydroxy, and ortho-aminophenyl di-
sulfides to afford the sulfenyl indoles 14–17 in 70–84% yield.
2-Methyl indole also reacted smoothly with para-methoxy and
ortho-aminophenyl disulfides resulting in the respective pro-
ducts 18 and 19 in 72 and 79% yield, respectively.
Assuming that 2,3-bis-sulfenyl indoles could be generated
by a persulfate mediated reaction in methanol, we have con-
ducted a model reaction with 1 mmol of phenyl disulfide,
1 mmol of indole and 2 mmol of ammonium persulfate under
optimized reaction conditions. Unfortunately, varying the
equiv. of persulfate and/or disulfide did not provide any 2,3-
bis-sulfenyl indole. Interestingly, addition of iodine to the
reaction mixture provided 2,3-bis-sulfenyl indole 20. Varying
Scheme 1 Synthesis of monosulfenyl indoles. The reaction was carried out on
a 1 mmol scale using 2 equiv. of indole, 1 equiv. of phenyl disulfide, and 2
equiv. of (NH4)2S2O8 in 5 mL of CH3OH at reflux (70 °C) temperature, unless
otherwise noted.
Scheme 1 demonstrate that a series of diaryl disulfides under-
went smooth transformations to give the structurally diverse
sulfenyl indoles 1–19 in 46–94% yield.
Fig. 1 X-Ray crystal structure of 11.
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Table 2 2,3-Bis-sulfenylation of indolesa
Entry Indole
ArSSAr
Product
Yield (%)
1
2
20 X = H (44)
21 X = OCH3 (62)
Scheme 2 Proposed mechanism for 3- and 2,3-bis arylthiolation of indole.
3
22 (71)
We have also attempted the bis-sulfenylation reaction in
N-methyl indole. para-Methoxy disulfide reacted with N-methyl
indole in methanol and gave the respective 2,3-bis-sulfenyl
indole 22 in 71% yield (Table 2, entry 3).
Based on the above experimental results and persulfate
mediated reactions16e,18,19 a possible mechanism has been
depicted in Scheme 2.
a The reaction was carried out on a 1 mmol scale using 1 equiv. of
indole, 1 equiv. of phenyl disulfide, 1 equiv. of iodine and 2 equiv. of
(NH4)2S2O8 in 5 mL of CH3OH at reflux (70 °C) temperature, unless
otherwise noted.
First, ammonium persulfate reacts with diaryl disulfide to
form aryl sulfenium ion intermediates I and II. The electro-
philic aryl sulfenium ion could add on to the indole moiety to
produce indole intermediate III, which may undergo deproto-
nation to give the desired monosulfenyl indole 1. For the for-
mation of 2,3-bis-sulfenyl indole 20, the presence of iodine
and persulfate is crucial in the reaction as iodine or persulfate
alone is noticed to be ineffective for 2,3 C–H sulfenylation of
indole. The formation of 2,3-bis thiolated product in the pres-
ence of iodine can be speculated by proposing a less hindered
iodophenylthiolate intermediate, which may attack on the
third position leading to 3,3-bis-substituted indolenium inter-
mediate IV.3c,6c,13h,m Positive charge on nitrogen could enforce
one of the phenylthio groups from position 3 to 2 forming an
episulfonium species V and subsequent release of proton can
give 2,3-bis-sulfenyl indole 20.
In summary, a metal and halogen free, simple and mild
method for the selective sulfenylation of indoles has been
developed in methanol. The present methodology is not moist-
ure sensitive and is carried out in the low boiling solvent
methanol. In addition, both N-protected and unprotected
indoles and disulfides with acidic protons such as amino,
hydroxyl, and acid groups can be tolerated well in this
methodology. Moreover, the methodology can be extended to
synthesize 2,3-bis-sulfenyl indoles with the addition of
stoichiometric amount of iodine, making use of diaryl disul-
fides as the sulfur source.
the loading of iodine yielded the mixture of mono and disulfe-
nyl indoles and one equiv. of iodine was noticed to be
optimum for the maximum yield (44%) of 2,3-bis-sulfenyl
indole 20 (Table 2).
Previously, the synthesis of 2,3-bis-sulfenyl indoles had
been achieved by Hamel et al.13g,h utilizing aryl sulfenyl chlor-
ides as the sulfur source. To the best of our knowledge, there
are no reports in the literature on accomplishing the synthesis
of 2,3-bis-sulfenyl indoles under mild reaction conditions and
using readily available sulfur precursors. Herein, we have
reported the synthesis of 2,3-bis-sulfenyl indoles exploiting
diaryl disulfide substrates. Phenyl disulfide and para-methoxy-
phenyl disulfide reacted smoothly under the optimized reac-
tion conditions to give the respective 2,3-bis-sulfenyl indoles
20–21 in moderate yields (Table 2, entries 1 and 2). 2,3-Bis-sul-
fenyl indole 21 was also characterized by a single X-ray crystal
structure study (Fig. 2).
We are thankful to DAE-BRNS Mumbai, DRDO New Delhi,
and IISER Bhopal for financial support. C. D. P. and M. S. are
grateful to UGC and CSIR New Delhi, respectively, for
fellowships. S. K. is thankful to IISER Bhopal for a fellowship.
Mr. B. S. Bhakuni is acknowledged for assistance in growing
crystals.
Fig. 2 X-Ray crystal structure of 21.
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