Tert-Butyl Hydroperoxide (TBHP)-Initiated Vicinal Sulfonamination of Alkynes: A Radical Annulation toward 3-Sulfonylindoles

Article abstract of DOI:10.1021/acs.orglett.6b01427

A novel, efficient, and facile vicinal sulfonamination of alkynes by the reaction of accessible 2-alkynyl arylazides with sulfinic acids in the presence of tert-butyl hydroperoxide (TBHP) has been developed. This protocol utilizes sulfinic acids as the su

Full text of DOI:10.1021/acs.orglett.6b01427

Letter
pubs.acs.org/OrgLett
tert-Butyl Hydroperoxide (TBHP)-Initiated Vicinal Sulfonamination of
Alkynes: A Radical Annulation toward 3‑Sulfonylindoles
Fei Chen, Qiang Meng, Shang-Qing Han, and Bing Han*
State Key Laboratory of Applied Organic Chemistry, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou,
730000, P. R. China
S
* Supporting Information
ABSTRACT: A novel, efficient, and facile vicinal sulfonami-
nation of alkynes by the reaction of accessible 2-alkynyl
arylazides with sulfinic acids in the presence of tert-butyl
hydroperoxide (TBHP) has been developed. This protocol
utilizes sulfinic acids as the sulfonating reagent, azidos as the
aminating reagent, and TBHP as the sulfonyl radical initiator.
By using this protocol, a variety of potentially bioactive 3-sulfonylindoles were facilely synthesized via direct annulation.
he indole skeleton as a prevalent structure widely exists in
many bioactive natural products and pharmaceuticals.1
Scheme 1. Strategies for the Synthesis of 3-Sulfonylindoles
T
Among them, 3-sulfonylindole derivatives have served as novel
HIV-1 non-nucleoside reverse-transcriptase inhibitors (e.g., I),2
5-HT6 receptor ligands (e.g., II),3 norepinephrine reuptake
inhibitors and 5-HT2A receptor antagonists (e.g., III),4 and
orexin receptor antagonists (e.g., IV)5 due to their special
chemical and biological properties (Figure 1). Thus, the
difunctionalization attract increasing attention from organic
chemists.10 In this context, this strategy not only presents the
first example of radical vicinal sulfonamination of alkynes
employing sufinic acids as the sulfonating reagent11 and azidoes
as the aminating reagent12 but also realizes a direct annulation
toward structurally important 3-sulfonylindoles. It is note-
worthy that the efficient process can be initiated by a catalytic
amount of environmentally friendly TBHP, which rendered this
approach more attractive to practical synthesis and sustainable
chemistry.
To achieve this idea, the reaction of 1-azido-2-(phenyl-
ethynyl)benzene (1a, 0.2 mmol) with benzenesulphinic acid
(2a, 0.4 mmol) was chosen as a model reaction and stirred in
CH3CN at 80 °C for 24 h under oxidative conditions for the
optimization investigation (Table 1). In the beginning, O2 (1
atm) was used as the initiator, but the desired 2-phenyl-3-
(phenylsulfonyl)-1H-indole 3a was only obtained in less than
5% yield (Table 1, entry 1). To our delight, when a catalytic
amount (0.2 equiv) of peroxides such as benzoyl peroxide
(BPO), di-tert-butyl peroxide (DTBP), and TBHP (5−6 M in
Figure 1. Biologically active 3-sulfonylindole derivatives.
efficient synthesis of 3-sulfonylindoles has drawn much
attention from chemists and pharmacologists. To date, three
general strategies have been achieved, including the indole
annulation of suitable acyclic precursors,6 direct C3-sulfonyla-
tion of indoles,7 and the oxidation of the corresponding sulfides
(Scheme 1).8 However, the majority of these methods relate to
either transition metal catalysts,6a,b,7b−d stepwise operation,6c,d
or harsh conditions.8 Therefore, the development of a
convenient, metal-free, and one-pot approach to synthesize 3-
sulfonylindoles from simple and readily available starting
materials is still in demand.
With our continuing interest in the construction of
structurally important heterocyclic scaffolds via radical cycliza-
tions and annulations,9 herein, we intend to establish a facile
and efficient protocol for the synthesis of bioactive 3-
sulfonylindoles by direct annulation of 2-alkynyl arylazides
and sulfinic acids via a tert-butyl hydroperoxide (TBHP)
initiated radical vicinal sulfonamination of alkynes under metal-
free conditions (Scheme 1). Alkynes are one of the most
common and useful functional groups, and their vicinal
Received: May 17, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.6b01427
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
With the optimized conditions in hand (Table 1, entry 7),
the scope of the reaction with various 2-alkynyl arylazides was
investigated as shown in Scheme 2. First, the substitution
Table 1. Optimization of the Reaction Conditions
ab
,
Scheme 2. Scope of 2-Alkynyl Arylazides
b
entry
initiator
O2 (1 atm)
t (°C)
solvent
yield (%)
1
80
80
100
80
80
80
80
80
80
80
80
80
100
80
80
80
80
80
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
DMF
<5
39
63
69
75
84
87
45
69
51
87
81
82
75
57
75
0
2
BPO (0.2 equiv)
DTBP (0.2 equiv)
TBHP (0.2 equiv)
TBHP (0.3 equiv)
TBHP (0.4 equiv)
TBHP (0.4 equiv)
TBHP (0.4 equiv)
TBHP (0.4 equiv)
TBHP (0.4 equiv)
TBHP (0.5 equiv)
TBHP (0.6 equiv)
TBHP (0.4 equiv)
TBHP (0.4 equiv)
TBHP (0.4 equiv)
TBHP (0.4 equiv)
TBHP (0.4 equiv)
TBHP (4.0 equiv)
3
4
5
6
7
8
DMSO
1,4-dioxane
toluene
DMF
9
10
11
12
13
DMF
DMF
c
14
DMF
d
15
DMF
e
16
f
DMF
17
DMF
g
18
DMF
24
a
All reactions were carried out by using 1a (0.2 mmol), 2a (2.0 equiv),
TBHP (5−6 M in decane), and solvent (1 mL) under argon (1 atm)
b
c
and stirred for 24 h, except as noted. Isolated yield. TBHP (70%
d
e
aqueous). 2a (1.2 equiv) was used. 2a (1.5 equiv) was used.
f
g
PhSO2Na (2.0 equiv) was used instead of 2a. PhSO2NHNH2 (2.0
equiv) was used instead of 2a.
a
All reactions run in DMF (2.5 mL) using 1 (0.5 mmol), 2a (1.0
b
mmol), and TBHP (0.2 mmol) at 80 °C under Ar for 24 h. Isolated
decane) were utilized as the initiator, the reaction took place
smoothly and the results showed that TBHP was more efficient
than others to deliver 3a in 69% yield (Table 1, entries 2−4).
The structure of 3a was confirmed by a single-crystal X-ray
diffraction study (Figure 2). With the increase of the usage
c
yields are shown. 1a (5 mmol) was used, and the corresponding
product 3a was obtained in 1.45 g.
pattern on the aromatic ring of the arylazide moiety was
explored. Using 4-phenyl with a wide range of electronic
properties, substituted 2-phenylethynyl arylazides all proceeded
well in the reaction, affording the desired 3-sulfonylindoles 3a−
f in excellent yields. When the phenyl bearing multisubstituent
was incorporated in 2-phenylethynyl arylazide, for instance, a
4,6-dimethyl group, the reaction also proceeded smoothly and
gave the corresponding products 3g in 94% yield. Next, both
aryl alkynyl and heterocyclic alkynyl substituted azidobenzenes
were examined for the transformation. Phenylethynyl with a
range of electronic properties substituents on the phenyl ring
substituted azidobenzenes all proceeded well in the reaction,
giving rise to the corresponding products 3h−k in excellent
yields. Other aryl alkynyls such as 4-biphenyl ethynyl, 1-
naphthyl ethynyl, 2-pyridinyl ethynyl, and 2-/3-thienylethynyls
incorporated into azidobenzenes were also tolerated well in the
process, delivering the desired products 3l−p in good to
excellent yields. Unfortunately, aliphatic alkynyls such as 1-
heptinyl and trimethylsilyl ethynyls involving substrates 1q and
1r were inert in the reaction, resulting in 0% yields for the
corresponding products 3q and 3r, respectively; they were
recovered in the reaction probably because of the lower stability
of the in situ generated alkenyl radical derived from the
addition of the sulphonyl radical to 1q or 1r. In addition,
methyl propiolate substituted substrate 1s only gave
complicated compounds rather than the desired product 3s.
Notably, the reaction could be carried out in gram scale without
Figure 2. X-ray structure of 3a (thermal ellipsoids are shown with 30%
probability).
amount of TBHP to 0.3 equiv and 0.4 equiv, the yield of 3a was
increased to 75% and 84%, respectively (Table 1, entries 5 and
6). Screening other solvents, such as DMF, DMSO, 1,4-
dioxane, and toluene, revealed that DMF was the best solvent
and the yield of 3a was further improved to 87% (Table 1,
entries 7−10). However, increasing the usage amount of TBHP
or raising the reaction temperature did not give a better result
(Table 1, entries 11−13). The yield of 3a was decreased to 75%
when TBHP (70% aqueous) was used (Table 1, entry 14). In
addition, the effects of the loading amount of sulfinic acid 2a
and other sulfonyl sources were also investigated, but the
results obtained under these conditions were less satisfactory
(Table 1, entries 15−18).
B
DOI: 10.1021/acs.orglett.6b01427
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
any difficulty, as it was demonstrated that the product 3a was
obtained in 87% yield (1.45 g) when 1-azido-2-(phenyl-
ethynyl)benzene (1a, 5 mmol) was used, displaying the
pragmatic application of this method.
Scheme 4. Control Experiment
Having successfully achieved the cascade sequence with 2-
alkynyl arylazides, we shifted our attention to explore the scope
of sulfinic acids 2. The reactions of a variety of sulfinic acids
with 1a were tested, and the results were illustrated in Scheme
3. Arylsulfinic acids bearing substituents such as p-Me, p-Cl, p-
Based on the experimental results and aforementioned
control experiment, a proposed mechanism for this TBHP-
initiated cascade sequence is illustrated in Figure 3. Initially,
a b
,
Scheme 3. Scope of Sulfinic Acids
Figure 3. Proposed mechanism.
sulfinic acid 2 reacts with TBHP under heating to form the
corresponding radical A, which can be drawn as the resonance
structures sulfinic acid and sulfonyl radicals. Subsequently, the
sulfonyl radical adds to the alkynyl moiety of 1 to generate the
intermediate B, which immediately undergoes intramolecular
cyclization of the alkenyl radical with an azido moiety to yield
the N-radical intermediate C along with the release of N2.
Finally, the intermediate C abstracts a hydrogen atom from
sulfinic acid 2 or the surroundings to produce 3-sulfonylindole
3 or 4.
a
All reactions run in DMF (2.5 mL) using 1a (0.5 mmol), 2 (1.0
b
mmol), and TBHP (0.2 mmol) at 80 °C under Ar for 24 h. Isolated
yields. 2h (1.0 mmol) and TBHP (0.25 mmol) were divided into two
equal parts and added per 12 h for 24 h. 2i (2.0 mmol) and TBHP
(0.5 mmol) were divided into four equal parts and added per 12 h for
48 h. 2j−l (3.0 mmol) and TBHP (0.75 mmol) were divided into two
equal parts and added per 12 h for 24 h.
c
d
e
In conclusion, we have demonstrated a facile, metal-free, and
direct annulation approach for access to a variety of structurally
important 3-sulfonylindoles via a TBHP-initiated cascade
sulfonation/cyclization process by the reaction of sulfinic
acids with available 2-alkynyl arylazides. This protocol not
only provides a novel method for the vicinal sulfonamination of
alkynyls but also provides a general approach for the synthesis
of 3-sulfonylindole frameworks that are important in medicinal
and biological chemistry. Further studies of the radical tandem
reactions constructing heterocyclic scaffolds are in progress in
our laboratory.
Ph, and m-Cl on the phenyl ring gave the corresponding 3-
sulfonylindoles 4a−d in excellent yields. However, 2-methyl-
benzenesulfinic acid reacted with 1a to afford 4e in 57% yield,
suggesting that the reaction was influenced by the steric effect.
In addition, naphthalene-2-sulfinic acid was also suitable for this
conversion, as demonstrated in the case of 4f. When
naphthalene-1-sulfinic acid 2h was allowed to react with 1a,
the desired product 4g was obtained in 25% yield accompanied
by the unexpected product 5 in 41% yield, which indicates that
the 1-naphthyl sulfonyl radical derived from sulfinic acid 2h was
more inclined to undergo desulfonylation under this circum-
stance. Furthermore, thiophene-2-sulfinic acid was a good
sulfonating agent as well, providing 4h in 57% yield.
Remarkably, the reactions involving simple aliphatic sulfinic
acids, such as methyl, ethyl, and octyl substituted sulfinic acids,
were also successful and led to the desired products 4i−k in
24−63% yields, demonstrating that aliphatic sulfinic acids were
a good sulfonating reagent in the protocol as well.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.6b01427.
Detailed experimental procedures and spectral data for
all products (PDF)
Crystallographic data for compound 3a (CIF)
To confirm that the reaction went through a radical process,
a radical inhibition experiment was conducted as shown in
Scheme 4. When the radical scavenger 2,2,6,6-tetramethyl-1-
piperidinyl-oxy (TEMPO) was added in the reaction system,
the sulfonamination was absolutely inhibited and 1a was almost
completely recovered, attesting that a radical process is really
involved.
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: hanb@lzu.edu.cn.
Notes
The authors declare no competing financial interest.
C
DOI: 10.1021/acs.orglett.6b01427
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
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ACKNOWLEDGMENTS
■
We thank the NNSFC (21422205 and 21272106), the Program
for New Century Excellent Talents in University (NCET-13-
0258), the Changjiang Scholars and Innovative Research Team
in University (IRT-15R28), the “111” project, and the
Fundamental Research Funds for the Central Universities
(lzujbky-2016-ct02 and lzujbky-2016-ct08) for financial sup-
port.
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DOI: 10.1021/acs.orglett.6b01427
Org. Lett. XXXX, XXX, XXX−XXX