Regioselective C-H sulfenylation ofN-sulfonyl protected 7-azaindoles promoted by TBAI: a rapid synthesis of 3-thio-7-azaindoles

Article abstract of DOI:10.1039/d0ra06635d

This paper describes the regioselective C-3 sulfenylation ofN-sulfonyl protected 7-azaindoles with sulfonyl chlorides. In this transformation, dual roles of TBAI serving as both promoter and desulfonylation reagent have been demonstrated. The reaction proceeded smoothly under simple conditions to afford 3-thio-7-azaindoles in moderate to good yields with broad substrate scopes. This protocol refrains from using transition-metal catalysts, strong oxidants or bases, and shows its practical synthetic value in organic synthesis.

Full text of DOI:10.1039/d0ra06635d

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Regioselective C–H sulfenylation of N-sulfonyl
protected 7-azaindoles promoted by TBAI: a rapid
synthesis of 3-thio-7-azaindoles†
Cite this: RSC Adv., 2020, 10, 31819
a
a
Jingyan Hu, Xiaoming Ji, Shuai Hao,a Mingqin Zhao,a Miao Lai,a Tianbao Ren,a
*
Gaolei Xi,b Erbin Wang,b Juanjuan Wangb and Zhiyong Wu
*
a
This paper describes the regioselective C-3 sulfenylation of N-sulfonyl protected 7-azaindoles with sulfonyl
chlorides. In this transformation, dual roles of TBAI serving as both promoter and desulfonylation reagent
have been demonstrated. The reaction proceeded smoothly under simple conditions to afford 3-thio-7-
azaindoles in moderate to good yields with broad substrate scopes. This protocol refrains from using
transition-metal catalysts, strong oxidants or bases, and shows its practical synthetic value in organic
synthesis.
Received 31st July 2020
Accepted 21st August 2020
DOI: 10.1039/d0ra06635d
rsc.li/rsc-advances
benzene as coupling partners under different conditions. In
addition, the N-oxide-assisted palladium-catalyzed C6–H aryla-
Introduction
tion of 7-azaindoles has also been achieved by Fagnou and co-
workers.12 Das's group17 realized the oxidative C3–H alkenyla-
tion of 7-azaindoles under palladium catalysis. However, the
use of transition-metals may cause potential contamination of
the products, which is particularly signicant in the pharma-
ceutical industry and advanced functional materials. Among
others, the C–H bonds activation reaction under transition-
metal-free conditions has emerged as promising protocols
Indole core structures are the most important nitrogen-
containing aromatic heterocycles, which are widely distrib-
uted in organic synthesis,1 medicinal chemistry,2 natural
products,3 pharmaceutical agents,4 and others.5 Among them, 7-
azaindoles and their synthetic analogues which possess the
same [4.3]-bicyclic indene architecture as indoles (Fig. 1), have
become one of the most widely studied organic templates, in
part probably because of their prevalence in many bio-active
structures6 and functional molecules7 (Fig. 1a–c).
Due to the signicance of such sub-structures in various
elds, chemists are showing an increased interest in developing
effective methods to form 7-azaindole derivatives.8 Tradition-
ally, 7-azaindole derivatives are synthesized starting from ami-
nopyridines through the construction of pyrrole ring.8a,b,9
However, these methodologies suffer from some drawbacks
such as toxic and foul-smelling reagents, prolonged reaction
steps and low atom efficiency, which limit their wide applica-
tions. With the aim to functionalize the 7-azaindoles in a mild
and atom-economical manner, transition-metal catalyzed C–H
bonds activation has been described as an attractive strategy
(Scheme 1a).10 For example, Sames,11 Fagnou,12 DeBoef,13 Das,14
Cao,15 and Laha16 reported independently the palladium-
catalyzed C-2 arylation of 7-azaindole by using aryl iodide or
Fig. 1 Biological activity and material applications of 7-azaindole
derivatives.
aFlavors and Fragrance Engineering & Technology Research Center of Henan Province,
College of Tobacco Science, Henan Agricultural University, Zhengzhou, 450002, China.
E-mail: xiaomingji@henau.edu.cn; smileyongyong062@163.com
bTechnology Center, China Tobacco Henan Industrial Co., Ltd., Zhengzhou, Henan,
450000, China
† Electronic supplementary information (ESI) available. CCDC 2018442. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/d0ra06635d
Scheme 1 Regioseletive C–H functionalization of 7-azaindoles.
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because of their environmental friendliness. For instance, Liu18 desired product was observed with 1,4-dioxane, acetonitrile,
and co-workers developed a regioselective deoxygenative C–H toluene and DCE (entries 7–10, Table 1). Gratifyingly, increasing
thiolation of 7-azaindole N-oxides with I2/PEG as the efficient the reaction temperature resulted in a signicant improving of
and reusable catalytic system (Scheme 1b). Some other specic the product yield (entries 11–13, Table 1) and the highest yield
examples on C-3 sulfenylation of free 7-azaindole have also been (79%) product 3a was observed when the reaction was con-
achieved by Zhang,19 Wang,20 Liu21 and Sinha.22 Despite this ducted at 120 ꢀC for 18 h. The reaction time was also examined
progress, direct C-3 sulfenylation of N-sulfonyl protected 7- (entries 14 and 15, Table 1), and 6 h was found to be the best
azaindoles using TBAI (tetrabutylammonium iodide) both as choice. The nitrogen protected reaction was also carried out,
the promoter and as the desulfonylation reagent has not yet and a similar result was obtained in this reaction compared
been documented. Based on our ongoing interest in the with the reaction in air (entry 17 vs. entry 15, 83% vs. 86%, Table
formation of C–S bond,23 herein, we want to disclose the 1). Based on the detailed investigations, we conrmed that the
regioselective C–H bond sulfenylation of N-sulfonyl protected 7- optimal conditions: TBAI (3.0 equiv.) as the additive in DMF at
ꢀ
azaindoles promoted by TBAI (Scheme 1c).
120 C under air atmosphere for 6 h (entry 15, Table 1).
Based on the optimized conditions presented above, we
subsequently focused on examining the generality and limita-
tions of this protocol (Tables 2 and 3). Firstly, various protecting
groups were surveyed for this transformation, and the results
were summarized in Table 2. Both aryl sulfonyl and alkyl sulfonyl
protected 7-azaindole underwent the reaction smoothly to
provide the corresponding products in moderate to good yields
(3a – 1–6, 54–86%, Table 2). Generally, different type of protecting
groups have some appreciable inuence on the outcome of the
reaction, and tosyl group was conrmed as the best one. Next,
a wide range of substituted aryl sulfonyl chlorides was subjected
to the reaction with N-Ts protected 7-azaindole (1a) to produce
Results and discussion
At the outset of this investigation, we commenced our study on
the model reaction of N-Ts protected 7-azaindole (1a) with tosyl
chloride (2a) to optimize various reaction parameters. The
results were summarized in Table 1. Initially, C-3 sulfenylation
took place in the presence of TBAI (3 equiv.) in DMF under air,
affording product 3a in 35% yield (entry 1, Table 1). The
molecular structure of 3a was conrmed by NMR and HRMS
spectra. Inspired by this result, various additives such as NaI,
KI, I2 and TBAB were screened (entries 2–5, Table 1), however,
no better results were observed with these experiments. The
effect of solvent was also examined, and the results showed that
DMAc gave lower yield of 3a (entry 6 vs. entry 1, Table 1) while no
Table 2 Substrate scope of protecting groups and sulfonyl chlorides
for the sulfenylation reactionsa,b,c
Table 1 Optimization of the reaction conditionsa
Entry
Additive
Solvent
Yieldb (%)
1
2
TBAI
NaI
DMF
DMF
35
17
3
KI
DMF
14
4
5
6
7
8
9
I2
DMF
DMF
N.R.
N.R.
23
N.R.
N.R.
N.R.
N.R.
72
79
82
84
86
TBAB
TBAI
TBAI
TBAI
TBAI
TBAI
TBAI
TBAI
TBAI
TBAI
TBAI
TBAI
TBAI
DMAc
1,4-Dioxane
Acetonitrile
Toluene
DCE
DMF
DMF
DMF
DMF
10
11c
12d
13e
14d,f
15d,g
16d,h
17d,g,i
DMF
DMF
DMF
80
83
a
Reaction conditions: 1 (0.15 mmol), 2 (0.45 mmol), TBAI (3 equiv.),
DMF 1 mL, 120 ꢀC, 6 h, under air. Isolated yields. R1 ¼ p-tolyl for
b
c
a
products 3b–3r.
Reaction conditions: 1a (0.15 mmol), 2a (0.45 mmol), additive (3.0
b
c
equiv.) and solvent (1 mL), 80 ꢀC, 18 h. Isolated yields. Run at 100
d
e
f
g
ꢀC. Run at 120 ꢀC. Run at 140 ꢀC. Run for 12 h. Run for 6 h.
h
i
Run for 3 h. Run under N2 atmosphere. N.R. ¼ no reaction.
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Table 3 Substrate scope of N-Ts protected 7-azaindoles for the sul-
fenylation reactionsa,b
a
Reaction conditions: 1 (0.15 mmol), 2a (0.45 mmol), TBAI (3 equiv.),
Scheme 2 Plausible reaction mechanism.
b
DMF 1 mL, 120 ꢀC, 6 h, under air. Isolated yields.
Conclusions
the corresponding product 3b–3n in moderate to good yields (65–
96%). Notably, benzenesulfonyl chloride and the mono-
substituted (Me, OMe, t-Bu) benzenesulfonyl chlorides have
proven to be suitable substrates for the reaction to provide the
corresponding products (3b–c, 3g and 3j) in synthetic acceptable
yields (73–91%). Substrates bearing electron-withdrawing
substituents also resulted in good yields. Halides such as F, Cl,
and Br afforded the desired products in 80–96% yields (3d–e, 3h–
i, 3k–l), even strong electron-withdrawing groups (CF3) gave quite
good yield of 3f (93%). Aryl sulfonyl chlorides containing a steri-
cally hindered groups, bicyclic moiety naphthalene and
substituted pyridine ring showed good compatibility (3m–p, 62–
87%). Ethanesulfonyl chloride and cyclopropanesulfonyl chlo-
ride reacted as well to give the desired products 3q and 3r with
yields of 52% and 46%, respectively. These results greatly
expanded the substrate scope of this reaction.
In summary, we have described an efficient approach for the
production of 3-thio-7-azaindoles via C–H bond activation
under transition-metal-free conditions. In this protocol, dual
roles of TBAI serving as both promoter and desulfonylation
reagent have been demonstrated, and a series of 3-thio-7-
azaindoles were obtained in moderate to good yields with
high regioselectivity and good functional group tolerance.
Further studies on extending the substrate scope and the
application of the obtained products are underway, and the
results will be forthcoming soon.
Conflicts of interest
There are no conicts to declare.
The compatibility of 7-azaindole derivatives was subse-
quently evaluated in this transformation (Table 3). Not
surprisingly, the reaction of substrate 2a with several
substituted N-Ts protected 7-azaindoles furnished the corre-
sponding products (4a–d) in moderate yields. It is noteworthy
that although relatively low yields were obtained when the
halogenated 7-azaindoles were subjected to the reaction, it may
provide a signicant opportunity for their further trans-
formation by transition-metal-catalyzed coupling reactions,
especially in pharmacological demand.
Acknowledgements
The authors greatly acknowledge the nancial support by The
Education Department of Henan Province (20A210023), Henan
Agricultural University (30500567, 30500701) and Key Science
and the Technology Program of Science and Technology
Department of Henan Province (152102210058, 122102210129).
Notes and references
Considering the experimental results and previously
reports,24 a plausible mechanism was proposed and illustrated
in Scheme 2. We envisioned that the formation of sulfonyl
iodide A by anion exchange of sulfonyl chloride and TBAI would
be the initial step of this transformation. Then, sulfonyl iodide
A is continuously reduced into the intermediates B,21,25 which
undergoes homolytic cleavage to produce the active sulfur
radical C.24a Subsequently, the addition of sulfur radical to 7-
azaindole occurs chemoselectively and generates a key inter-
mediate D, which captured by the iodine radical from inter-
mediates B affording intermediate E. Next, HI elimination takes
place to provide 3-thio-7-azaindole F, which readily undergoes
N-desulfonylation16a,26 to provide the desired products 3.
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