Iodine/Manganese Catalyzed Sulfenylation of Indole via Dehydrogenative Oxidative Coupling in Anisole

Article abstract of DOI:10.1002/adsc.202000291

This protocol describes an iodine/manganese catalytic system for dehydrogenative oxidative coupling reaction of indoles with thiols in anisole. Particularly, the dual roles of anisole have been first demonstrated as a solvent and as a promoter via the formation of an oxonium ion intermediate to accelerate the generation of products. A series of sulfenylindoles are readily constructed under aerobic mild reaction conditions. In addition, the achievement for preparing anticancer and anti-AIDS drugs testifies the practicability of this approach. The mechanism studies disclose probable alternative pathways and a single-electron transfer process are involved in this transformation. (Figure presented.).

Full text of DOI:10.1002/adsc.202000291

Title: Iodine/Manganese Catalyzed Sulfenylation of Indole via
Dehydrogenative Oxidative Coupling in Anisole
Authors: Weihe Li, Hao Wang, Shengping Liu, Hua Feng, Enrico
Benassi, and Bo Qian
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To be cited as: Adv. Synth. Catal. 10.1002/adsc.202000291
Link to VoR: https://doi.org/10.1002/adsc.202000291

10.1002/adsc.202000291
Advanced Synthesis & Catalysis
FULL PAPER
DOI: 10.1002/adsc.201((will be filled in by the editorial staff))
Iodine/Manganese Catalyzed Sulfenylation of Indole via
Dehydrogenative Oxidative Coupling in Anisole
Weihe Li, a,b,† Hao Wang,b,† Shengping Liu,b Hua Feng,a,* Enrico Benassib,* and Bo
Qianb,*
a
College of Chemistry and Chemical Engineering, Northwest Normal University, Lanzhou, Gansu 730070, China.
E-mail: hfeng@nwnu.edu.cn
State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics, Chinese
b
Academy of Sciences, Lanzhou 730000, P. R. China.
E-mail: ebenassi3@gmail.com, boqian@licp.cas.cn
These authors contributed equally.
†
Received: ((will be filled in by the editorial staff))
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201######.((Please
delete if not appropriate))
Abstract. This protocol describes an iodine/manganese achievement for preparing anticancer and anti-AIDS drugs
catalytic system for dehydrogenative oxidative coupling testifies the practicability of this approach. The mechanism
reaction of indoles with thiols in anisole. Particularly, the studies disclose probable alternative pathways and a single-
dual roles of anisole have been first demonstrated as a electron transfer process are involved in this
solvent and as a promoter via the formation of an oxonium transformation.
ion intermediate to accelerate the generation of products. A
series of sulfenylindoles are readily constructed under
aerobic mild reaction conditions. In addition, the
Keywords: I2/MnO2 Catalysis; Dehydrogenative Coupling;
Aerobic Oxidation; Sulfenylation; Anisole
Introduction
Sulfenylindole moieties, in particular 2- or 3-
substituted sulfenylindoles are prevalent in diverse
natural products, biologically active compounds and
pharmaceuticals, which are required for curing many
diseases, such as cancers, HIV, allergies, respiratory
disorders, etc.[1] (Scheme 1a). Additionally,
sulfenylindoles are also served as reactants or ligands
in metal-catalyzed cross-coupling reaction.[2]
Therefore, a large number of methodologies have
been documented to prepare sulfenylindoles,[3-8] and
the cross-coupling reactions of indoles with
diversified sulfur sources including sulfur, sodium
sulfinates, sulfenyl chlorides, sulfonyl hydrazides,
sulfinic acids, disulfides, thiols, etc. are especially
booming.[4-8] Except thiols, other sulfenylation
reagents should be prefunctionalized, leading to
additional procedures of synthesis. As the result,
directed dehydrogenative oxidative coupling reaction
of indoles with thiols developed rapidly because of
the atom and step economy.[5-8] Among them,
catalytic transformation under aerobic atmosphere is
more fascinating owing to the efficient, gentle and
economic superiorities.[8] Moreover, we recently
reported an iodine/manganese dual catalyzed
dehydrogenative oxidative coupling of amines with
thiols for the formation of N-S bond.[9] The
Scheme 1. Sulfenylindoles Skeleton in Drugs and Our
Strategy for the Synthesis of Sulfenylindoles.
1
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10.1002/adsc.202000291
Advanced Synthesis & Catalysis
mechanism shows that activated MnO2 acts as an
electron transfer bridge, inspiring us to employ
I2/MnO2 dual catalytic system for the construction of
C-S bonds.
employed, almost quantitative 3aa could be obtained
in 99% yield (Table 1, entry 7). Control experiments
were carried out under the optimized reaction
conditions. Sulfenylindole 3aa was sharply reduced
to 12% yield with only I2 as the catalyst, and no
product was observed with MnO2 as the sole catalyst
or without any catalyst (Table 1, entries 8-10). Above
results suggest that both I2 and MnO2 are essential for
the dehydrogenative oxidative coupling reaction.
With the optimized reaction conditions in hand, the
substrate compatibility of thiols 2 was surveyed for
the formation of 3-sulfenylindoles (Table 2). A
variety of thiophenols with electron-donating or
electron-withdrawing substituents were treated with
indole 1a, giving desired products 3ab-3al in
excellent yields. 1-Thionaphthol 2m and 2-
thionaphthhol 2n afforded the products 3am and 3an
in moderate to good yields. Besides aryl substituted
thiols, heteroaryl substituted thiols could also be
tolerated in the transformation, giving the products
3ao and 3ap in 44% and 84% yields, respectively. It
is worth noting that sulfenylindole 3ao has been
applied as a ligand into Pd-catalyzed Suzuki-Miyaura
coupling reaction.[2b] However, the traditional
synthesized method of 3ao is K2CO3 promoted
reaction of indole with 2,2’-dipyridyl disulfide. Alkyl
hydrosulfides with tailing phenyl, ester or hydroxyl
groups exhibited good group tolerance to acquire the
corresponding products 3aq-3au in 74-95% yields.
Fortunately, a diverse catalytic dehydrogenative
oxidative coupling reaction of indoles with thiols
under mild reaction conditions has been realized in
this protocol. The dual functions of anisole are
unprecedentedly revealed for the sulfenylation of
indole with thiol via the coupled I2/MnO2 catalysis
under oxygen atmosphere (Scheme 1b). On the one
hand, anisole can be applied as a solvent in the
formation of sulfenylindoles. On the other hand, more
interestingly, anisole can be used as an accelerator in
the catalytic cycle by forming an oxonium ion
intermediate. Moreover, various sulfenylindoles are
readily synthesized, as well as the anticancer and
anti-AIDS drugs can be facilely synthesized by this
approach. A sequence of mechanistic experiments
and quantum chemical calculations have been carried
out, testifying the possible intermediates and a
feasible single-electron transfer (SET) process.
Results and Discussion
Initially, the reaction of indole 1a with thiol 2a was
run with a oxygen balloon in anisole, when 10 mol%
of iodine and 10 mol% of MnO2 were used as
catalysts at 100 oC for 12 hours. Satisfactorily,
sulfenylindole 3aa was attained in 67% yield (Table
1, entry 1). Sequentially, more dual catalytic systems
were investigated, but no better efficiency was
attained (Table 1, entries 2-4). Decreasing the
Table 2. Substrate Scope of Thiols.[a]
o
o
reaction temperature from 100 C to 80 C did not
reduce the yield of 3aa (Table 1, entry 5). Therefore,
the reaction time was prolonged to 16 hours at 80 oC,
delivering the desired product in 75% yield (Table 1,
entry 6). Dramatically, when excess indole 1a was
Table 1. Optimization of Reaction Conditions.[a]
Entry Catalyst (mol%)
Solvent Yield (%)[b]
1
2
3
4
5
6
7
8
9
I2 (10)/MnO2 (10)
I2O5 (10)/MnO2 (10)
NaI (10)/MnO2 (10)
NH4I (10)/MnO2 (10)
I2 (10)/MnO2 (10)
I2 (10)/MnO2 (10)
I2 (10)/MnO2 (10)
I2 (10)
anisole 67
anisole 60
anisole
0
anisole 11
anisole 67[c]
anisole 75[d]
anisole 99[e]
anisole 12[e]
anisole 0[e]
anisole 0[e]
MnO2 (10)
/
[a] Reaction conditions: 1a (1.0 mmol), 2 (0.5 mmol), I2 (10
10
o
mol%), MnO2 (10 mol%), anisole (2 mL), 80 C, 16 h, O2
[a]
Reaction conditions: 1a (0.5 mmol), 2a (0.5 mmol),
balloon, isolated yield. [b] 24 h.
o
catalyst (10 mol%), solvent (2 mL), 100 C, 12 h, O2
balloon. [b] Isolated yield. [c] 80 ℃. 80 ℃, 16 h. [e] 1a (1.0
[d]
mmol), 2a (0.5 mmol), 80 ℃, 16 h.
2
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10.1002/adsc.202000291
Advanced Synthesis & Catalysis
The scope of indoles was also explored for the
I2/MnO2 catalyzed dehydrogenative oxidative
coupling reaction (Table 3). Disparate electrical
groups on the aromatic rings of indoles 1b-1l
provided access to the 3-sulfenylindoles 3ba-3la in
47-98% yields. Both 2-methyl and 2-ester substituted
indoles could couple with thiol 2a to generate 3ma
and 3na in 93% and 66% yields, respectively. 5-
Bromo-1-methyl-1H-indole was acted as the coupling
partner, providing the 3-sulfenylindoles 3oa in
excellent yield. 2-Sulfenylindoles was smoothly
isolated from the conversion of the 3-sulfenylindole
product 3au, delivering the compound 4 in 25% yield.
Notably, the coupling reaction of indole 1a with thiol
2a could be proceeded on the gram scale under the
optimal reaction conditions, readily preparing 1.18 g
of 3-sulfenylindoles 3aa at 5 mmol scale in 99%
yield (Scheme 2a). To further exploit the synthetic
application of this I2/MnO2 dual catalytic
sulfenylation reaction, the synthesis of anticancer
durg 3pv and anti-AIDS drug 3qw were undertaken
(Scheme 2b and 2c). The sulfenylindoles 3pv and
3qw were accessed by the dehydrogenative oxidative
coupling transfromation in 60% and 97% yields,
respectively (see the SI for details).
Scheme 2. Gram Scale Reaction and Synthesis of Drugs.
Table 3. Substrate Scope of Indoles.[a]
In order to disclose the probable reaction
mechanism for the sulfenylation of indole, a series of
preliminary experiments were completed (Scheme 3
and see the SI for details). Excess amount of free
radical scavenger (2,2,6,6-tetramethyl-piperidin-1-
yl)oxyl (TEMPO) was added into the reaction of
indole 1a with thiol 2a under the standard conditions,
leading to trace amount of 3-sulfenylindole 3aa
detected by GC/MS (Scheme 3a). This outcome
suggests that radical species may be involved into the
reaction. When the transformation was quenched
after five minutes, 28% yield of disulfide 5 could be
isolated and only trace amount of 3aa was detected
(Scheme 3b). Furthermore, if 5 reacted with 1a under
the optimal conditions, product 3aa was formed in 89%
yield (Scheme 3c). The two experiments prove that
disulfide may be an intermediate in this reaction.
However, the low yield of 5 in Scheme 3b excited our
curiosity that whether another intermediate is also
invovled in the reaction. Thus, thiol 2a was severally
run with stoichiometric I2 or MnO2 under N2
atmosphere (Scheme 3d). Surprisingly, 64% yield of
6 and only 23% yield of 5 were isolated in the
presence of 1 equivalent of I2, but 0% yield of 6 and
90% yield of 5 were attained in the presence of MnO2.
Next, we added disulfide 5 into anisole with 1
equivalent of I2 under N2, compound 6 was produced
in 73% yield (Scheme 3e). When 1a reacted with 6
under the standard reaction conditions, 25% yield of
3aa could be generated (Scheme 3f). These results
indicate that 6 may be one of intermediates in the
catalytic cycle.
[a] Reaction conditions: 1 (1.0 mmol), 2a (0.5 mmol), I2 (10
o
mol%), MnO2 (10 mol%), anisole (2 mL), 80 C, 16 h, O2
balloon, isolated yield. [b] 24 h. [c] 40 h.
3
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10.1002/adsc.202000291
Advanced Synthesis & Catalysis
Scheme 4. The Possible Catalytic Cycle and the Values of
ΔG (kJ/mol) in the Steps.
Conclusion
In conclusion, a simple method for the formation
of sulfenylindoles has been established with the
catalysis of I2/MnO2 under mild reaction conditions.
A large number of substituted sulfenylindoles are
successfully prepared in moderate to excellent yields
by this dehydrogenative oxidative coupling
transformation of indoles with thiols. In addition, this
approach affords brief synthetic routes for the
construction
of
antitumor
and
anti-HIV
Scheme 3. Experiments for Mechanistic Studies.
pharmaceuticals. The preliminary mechanistic
experiments and quantum chemical calculations are
employed as the evidence to support the reaction
mechanism. Anisole plays dual roles of a solvent and
a promoter through the formation of an oxonium ion
intermediate. Both I2 and MnO2 are indispensable for
completing the catalytic cycle which may contain
SET procedure.
Based
on the reported works[8,9] and
aforementioned mechanistic studies, a possible
mechanism with alternative pathways has been
hypothesized (Scheme 4). Quantum chemical
calculations has been also carried out to verify the
I2/MnO2 catalytic system (computational details are
reported in the Table S6). The positive values of ΔG
are found to support the corresponding steps as
described in Scheme 4. Initially, thiol 2 is oxidized by
oxidants, speedily generating disulfide that then
reacts with I2 to give R-SI I via a process of SET.
Sequentially, two pathways contain in the catalytic
cycle of iodine transformation. Path a is a treatment
of I with anisole, releasing oxonium ion II which
then reacts with 1a to liberate iminium salt III and
anisole. Path b is a reaction of I with indole 1a,
resulting in the formation of III. The production of
iminium salt III is proved by Hammett-analysis
(Table S5 and Figure S1). Next, the desired product 3
is obtained with HI leaving from III. Both of the
pathways are feasible for the dehydrogenative
oxidative coupling reaction, and path a is more
favorable than path b. Finally, MnO2 plays as an
electron transfer bridge to oxidize HI, which can be
regenerated through the oxidation of Mn(III).
Experimental Section
Iodine (0.05 mmol), activated MnO2 (0.05 mmol), indole
1a (1 mmol), 4-methylbenzenethiol 2a (0.5 mmol) and
anisole (2.0 mL) were added into the reaction tube under
o
air. The mixture was stirred at 80 C for 16 hours with an
oxygen balloon. After cooling to room temperature, the
solvent was removed under reduced pressure. Then the
residue was purified by flash column chromatography on
silica gel and eluted with ethyl acetate/hexane to give the
desired product 3aa.
Acknowledgements
This research was supported by the Chinese Academy of Sciences,
“Light of West China” Program.
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10.1002/adsc.202000291
Advanced Synthesis & Catalysis
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6
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10.1002/adsc.202000291
Advanced Synthesis & Catalysis
FULL PAPER
Iodine/Manganese Catalyzed Sulfenylation of
Indole via Dehydrogenative Oxidative Coupling in
Anisole
Adv. Synth. Catal. Year, Volume, Page – Page
Weihe Li,a,b,† Hao Wang,b,† Shengping Liu,b Hua
Feng,a,* Enrico Benassib,* and Bo Qianb,*
7
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