Copper-catalyzedortho-selective direct sulfenylation ofN-aryl-7-azaindoles with disulfides

Article abstract of DOI:10.1039/d1ob00106j

A copper-catalyzed direct C-H chalcogenation ofN-aryl-azaindoles with disulfides is described. This transformation was performed using Earth abundant Cu(OAc)₂as a catalyst, benzoic acid as an additive, air as a terminal oxidant, and readily available diaryl and dialkyldisulfides (or diselenide) as chalcogenation reagents. High functional group tolerance and excellent regioselectivity are demonstrated by the efficient preparation of a wide range ofortho-sulfenylation-7-azaindoles.

Full text of DOI:10.1039/d1ob00106j

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Copper-catalyzed ortho-selective direct
sulfenylation of N-aryl-7-azaindoles with
disulfides†
Cite this: Org. Biomol. Chem., 2021,
19, 2901
Received 21st January 2021,
Accepted 4th March 2021
Yu Ru-Jian, Zhang Chun-Yan, Zhou Xiang, Yan-Shi Xiong * and
DOI: 10.1039/d1ob00106j
Xue-Min Duan
*
rsc.li/obc
A copper-catalyzed direct C–H chalcogenation of N-aryl-azain- bonds, such as cross-coupling reaction with thiol, but they
doles with disulfides is described. This transformation was per- always needed the use of pre-functionalized starting materials
formed using Earth abundant Cu(OAc)2 as a catalyst, benzoic acid or harsh reaction conditions and special substrates.8 From the
as an additive, air as a terminal oxidant, and readily available diaryl view point of atom economy, direct C–H activation/functionali-
and dialkyldisulfides (or diselenide) as chalcogenation reagents. zation is one of the most efficient methods for the modifi-
High functional group tolerance and excellent regioselectivity are cation of the 7-azaindole scaffold.2–5 Copper salts, 3d tran-
demonstrated by the efficient preparation of a wide range of sition-metal complexes, are abundant and inexpensive with
ortho-sulfenylation-7-azaindoles.
low toxicity. Recently, considerable efforts have been focused
on C–H functionalization by copper complexes,9 whereas
copper-catalyzed C–S10 and C–Se bond-forming reactions
remain scarce. The pioneering work by Yu’s group involves
copper-mediated direct sulfenylation of inactivated aryl C–H
bonds with the assistance of a 2-pyridine bidentate directing
group (Scheme 1a).10a In 2012, Daugulis and co-workers
reported an elegant copper-catalyzed direct trifluoromethyl-
sulfenylation of (hetero)aryl C–H bonds assisted by N,N-biden-
tate auxiliaries (Scheme 1b).10b Ackermann et al. recently
reported a copper-catalyzed C–H sulfenylation of indolines
and indoles (Scheme 1c),10c and Shi’s group reported copper-
mediated thiolation of inactivated heteroaryl C–H bonds
(Scheme 1d).10d However, the direct C–H/C–S bond formation
still suffered from narrow substrate scope and low catalytic
efficiency. As part of our ongoing research on direct C–S bond
formation11 by base metal catalysts, our group also reported
copper-catalyzed C–H sulfenylation of 1-naphthylamines
(Scheme 1e)12a and nickel-catalyzed C–H sulfenylation of
benzoyl hydrazides with disulfides.12b Herein, we wish to dis-
close the aerobic copper-catalyzed ortho-selective sulfenylation
of 7-azaindole for the preparation of 1-(2-(phenylthio)phenyl)-
1H-pyrrolo[2,3-b]pyridine (Scheme 1f), which is an important
structural motif present in bioactive molecules with anticancer
and antibacterial properties.
Introduction
The 7-azaindole motif is an important scaffold in numerous
natural products and pharmaceuticals.1 Moreover, the diversi-
fication of 7-azaindole has attracted much attention in organic
chemistry. Various modifications of 7-azaindoles reported in
literature studies focused on transition metal catalyzed C–C
bond formation.2 Only a few examples of direct C–O,3 C–Cl,4
and C–N5 bond formations at the ortho- and para-positions of
the N-aryl ring of azaindole have been reported. However, the
more challenging C–S and C–Se bond formations remain
scarce because of the strong coordination of metal catalysts to
the sulfur reagents and the highly reactive C-3 position of
azaindole. Deb’s group reported a rhodium-catalyzed selective
ortho-C–H bond sulfenylation of N-aryl-7-azaindoles with disul-
fides.6 Moreover, they realized the ortho-selective chalcogena-
tion of N-aryl-7-azaindoles with
However, this method needs the use of precious rhodium and
silver salt as a co-catalyst and additive.
The C–S bond is of utmost importance in organic synthesis
and is receiving tremendous attention.7 Various methods have
been developed over the past decades for the formation of C–S
a 7-azaindole auxiliary.
Jiangxi Provincial Key Laboratory of Drug Design and Evaluation, School of
Pharmacy, Jiangxi Science & Technology Normal University, Nanchang,
330013 Jiangxi, P. R.China. E-mail: xiongys1214@163.com, duanxuemin@126.com
†Electronic supplementary information (ESI) available: Copies of 1H, 13C and
19F NMR spectra. See DOI: 10.1039/d1ob00106j
Results and discussion
Our investigation was initiated with N-aryl-7-azaindole (1a)
and diphenyl disulfide (2a) as model substrates for the
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optimization of the reaction conditions (Table 1). We are
delighted that the ortho-C–H bond sulfenylation on the aryl
ring took place and gave the corresponding product 3a in
50% isolated yield in the presence of Cu(OAc)2 (20 mol%) in
DCE (1 mL) under air atmosphere at 120 °C (Table 1, entry
1). Inspired by this result, we then evaluated different metal
catalysts such as Pd(OAc)2, Co(OAc)2, and Ni(OAc)2, and
found them less efficient than Cu(OAc)2 in this transform-
ation (entries 1–4, see the ESI Table S1†). We surprisingly
found that mesitylene was the most effective solvent in this
coupling reaction, providing the desired product 3a in 57%
yield (entry 10), whereas other solvents such as toluene,
DMSO, chlorobenzene, fluorobenzene, and para-xylene were
less effective (entries 5–10). Different additives were examined
subsequently (entries 11–16), and switching the additive to
PhCOOH improved the yield to 70% (entry 14). Further
investigations on the amounts of additives and the reaction
temperatures were conducted (entries 17 and 18), and
20 mol% PhCOOH and 140 °C were found optimal for the
highest yield of 87% (entry 19). In addition, control experi-
ments showed that the reaction did not take place in the
absence of copper salts.
With the optimized reaction conditions in hand (Table 1,
entry 18), the substrate scope and limitations were evaluated
and the results are shown in Tables 2 and 3. A broad range of
N-aryl-7-azaindoles with electron-donating groups such as
methyl (–Me), methoxy (–OMe), and tert-butyl (–tBu) groups,
and electron-withdrawing groups such as halides (–F, –Cl, –Br
and –I), nitro (–NO2), cyanide (–CN), acetyl (–COMe), and tri-
fluoromethyl (–CF3) groups at the para-position of the
Scheme 1 Cu(OAc)2catalyzed direct C–S bond formation with different
directing groups.
Table 1 Optimization of the reaction conditionsa
Table 2 Substrate scope of N-aryl-7-azaindolesa,b
Catalyst
Entry (mol%)
Additive
(2.0 equiv.) Solvent
Yieldb
T (°C) (%)
1
2
3
4
5
6
7
8
CuCl (20)
CuCl2(20)
CuBr2(20)
—
—
—
—
—
—
—
—
—
—
NaHCO3
KHCO3
K2HPO4
DCE
DCE
DCE
DCE
120
120
120
120
120
120
5
13
34
50
55
37
38
24
52
57
23
33
24
70
57
44
77
87
—
Cu(OAc)2(20)
Cu(OAc)2(20)
Cu(OAc)2(20)
Cu(OAc)2(20)
Cu(OAc)2(20)
Cu(OAc)2(20)
Cu(OAc)2(20)
Cu(OAc)2(20)
Cu(OAc)2(20)
Cu(OAc)2(20)
Cu(OAc)2 (20) PhCOOH
Cu(OAc)2(20)
Cu(OAc)2(20)
Cu(OAc)2 (20) PhCOOH
Cu(OAc)2(20) PhCOOH
Toluene
DMSO
Chlorobenzene 120
Fluorobenzene 120
9
para-Xylene
Mesitylene
Mesitylene
Mesitylene
Mesitylene
Mesitylene
Mesitylene
Mesitylene
Mesitylene
Mesitylene
Mesitylene
120
120
120
120
120
120
120
120
120
140
140
10
11
12
13
14
15
16
17c
18d
19e
Piv-OH
AcOH
—
PhCOOH
a Reaction conditions: 1a (0.1 mmol), 2a (0.15 mmol), additive
(0.2 mmol), and solvent (1 mL) under air for 12 h. b Isolated yield. a Reaction conditions: 1 (0.2 mmol), (PhS)2 (0.3 mmol), Cu(OAc)2
c Using 20 mol% PhCOOH. d Using 20 mol% PhCOOH and stirring at (20 mol%), and PhCOOH (20 mol%) in mesitylene (2 mL) at 140 °C
140 °C. e Without Cu(OAc)2.
under air for 12 h. b Isolated yield.
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Table 3 Substrate scope of disulfides 2a,b
went chalcogenation with 7-azaindole to afford product 3ae in
66% yield. In addition to the aromatic disulfide, less reactive
dialkyldisulfides, such as dipropyldisulfide, dicyclohexyl-
disulfide and dibenzyldisulfide were tolerated in this trans-
formation, affording products 3af–3ah in good yields.
Moreover, diphenyl diselenide was efficiently transformed
into the desired selenylated product 3ai, thereby demon-
strating the robustness of the present copper-catalyzed C–H
chalcogenation.
The synthetic utility of the copper-catalyzed sulfenylation is
also demonstrated in Scheme 2. The present transformation
could be smoothly scaled up to 5 mmol under the standard
conditions without an apparent loss of yield (Scheme 2). We
also could remove the drifted group 7-azaindolein in a trace-
less fashion, thereby obtaining diphenylsulfane 4 with 80%
yield.10c
To gain insight into the reaction mechanism, a series of
control experiments was carried out. Firstly, a radical capture
experiment was performed (Scheme 3a). This reaction was sup-
pressed by the radical scavenger 2,2,6,6-tetramethyl-1-piper-
idinyloxy (TEMPO) and 2,6-di-tert-butyl-4-methylphenol (BHT),
suggesting that this reaction might go through a SET pathway.
Moreover, performing the reaction under N2 and O2 atmos-
a Reaction conditions: 1a (0.2 mmol),
2 (0.3 mmol), Cu(OAc)2
(20 mol%), and PhCOOH (20 mol%) in mesitylene (2 mL) at 140 °C
under air for 12 h. b Isolated yield.
N-arylring underwent the reaction efficiently, affording the pheres resulted in 17% and 77% yields, which implied that O2
corresponding products 3b–3l in moderate to high yields. We might act as an oxidant in this reaction. Furthermore, on
are also glad to find that para-phenyl substituted N-aryl-7- using p-toluenethiol 6 instead of disulfide, the reaction did
azaindole 1m was tolerated in this transformation. However, not take place, and p-toluenethiol 6 could not be oxidized to
substrates with meta-substituents offered lower yields than p-tolyldisulfide under standard reaction conditions. We specu-
those with para-substituents (3l vs. 3o; 3c vs. 3p; 3b vs. 3n). We lated that the hemolytic cleavage of disulfides generated the
assumed that steric factors clearly reduced the reaction sulfenyl radicals (Scheme 3b). In addition, no deuterium was
efficiency. Heteroarenes such as thiophene underwent incorporated into the recovered starting material, which indi-
thioetherification reaction efficiently under optimized catalytic cated that an irreversible C–H cleavage event occurred during
conditions and gave up to 85% yield of product 3q. Finally, we the reaction. We carried out an intermolecular kinetic isotope
also examined the thioetherification reaction of substituted effect (KIE) of 1a and 1a–d and observed a kH/kD value of 1.17,
7-azaindoles under the standard reaction conditions, and which indicated that the aromatic C–H bond cleavage might
1-phenyl-1H-pyrazolo[3,4-b]pyridine 1r was proved to be a suit- not be the rate-determining step (Scheme 3c).13 Finally, we
able substrate, providing the corresponding product 3r in evaluated the role of PhCOOH in this reaction. When 20 mol%
good yield. C–Br/C–Cl bonds remained untouched at high Cu(PhCO2)2 was used as a catalyst in this reaction, 5% yield of
temperature. For example, 1s, 1t, and 1u were also well compa- product 3a was observed in the absence of 20 mol% PhCOOH
tible under the standard reaction conditions, affording the as an additive, whereas 12% yield of 3a was obtained in the
products 3s, 3t, and 3u in yields of 77%, 69%, and 86%, presence of 20 mol% PhCOOH. We propose that PhCOOH
respectively, and these products could be used for further
transformations.
To further explore the scope of this transformation, various
disulfides were examined. As shown in Table 3, diaryl disul-
fides bearing either electron-donating groups (–Me, –OMe, and
–t-Bu) or electron-withdrawing groups (–F, –Cl, –Br and –NO2)
on the phenyl ring underwent the reaction efficiently,
affording the corresponding products 3v–3aa in moderate to
high yields. Moreover, the position of the substituent had
some effects on the reaction. Disulfides that bear either elec-
tron-donating groups (–Me) or electron-withdrawing groups
(–F) at the para-position of the phenyl ring were superior to
their ortho-substituted counterparts in isolated yields (3v vs.
3ad; 3y vs. 3ac), showing that steric factors clearly reduced the
Scheme 2 Gram-scale preparation of 3a and removal of the 7-azain-
reaction efficiency. Moreover, dithiophenedisulfides under- dole group.
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Scheme 4 Proposed mechanism.
tive elimination together with the Cu(I) species, which is
further oxidized to Cu(II) by oxygen.
Conclusions
In summary, we developed a copper-catalyzed C(sp2)–H sulfe-
nylation reaction of various N-aryl-7-azaindoles with disulfides
as sulfur sources. The reaction involved the use of the in-
expensive Cu(OAc)2 catalyst, removable directing groups, diaryl
or dialkyldisulfide (or diselenide) reagents, and air as a term-
inal oxidant. The operationally simple method is characterized
by ample substrate scope under aerobic reaction conditions.
Mechanistic studies provide evidence for a SET-type process
and indicate an operative facile C–H cleavage.
Experimental section
General information
Scheme 3 Key mechanistic studies.
Reagents and solvents. All starting materials, which were
purchased from commercial sources, were used without
further purification. Solvents for column chromatography were
of technical standards. Column chromatography was per-
might act as an auxiliary ligand in the catalytic cycle formed with silica gel 200–400 mesh. 1H, 13C and 19F NMR
(Scheme 3d).
spectra were recorded on a Bruker Avance 400 MHz spectro-
Although the mechanism remains unclear at this moment, meter. Chemical shifts in 1H NMR spectra were reported in
based on the mechanistic studies and previous reports,14 we parts per million (ppm) downfield from the internal standard
propose a plausible catalytic cycle for the copper-catalyzed Me4Si (TMS). Chemical shifts in 13C NMR spectra were
C(sp2)-sulfur bond formation (Scheme 4). Firstly, the coordi- reported relative to the central line of the chloroform signal (δ
nation of 7-azaindole 1a with copper acetate in the presence of = 77.0 ppm). Peaks were labeled as singlet (s), doublet (d),
PhCOOH produces the bicyclic Cu(II) intermediate A. triplet (t), quartet (q), and multiplet (m). High resolution mass
Oxidation of intermediate A by a sulfenyl radical (PhS-SPh spectra were obtained with a Shimadzu LCMS-IT-TOF mass
bond hemolytic cleavage) can deliver a Cu(III)-sulfur species spectrometer. Analytical TLC was performed using EM separ-
B.15 The sulfenylation product 3a is obtained through reduc- ations with percolated silica gel 0.2 mm layer UV 254 fluo-
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Into an oven-dried sealed tube was added N-aryl-7-azaindoles 1
(0.2 mmol), disulfide 2 (0.3 mmol), Cu(OAc)2 (7.2 mg,
0.04 mmol), PhCOOH (4.9 mg, 0.04 mmol) and mesitylene
(2.0 mL). The mixture was stirred at 140 °C for 12 hours until
the complete consumption of 1 as monitored by TLC analysis.
The reaction mixture was then diluted with water and extracted
with ethyl acetate. After the combined organic layers were
washed with brine, dried over Na2SO4, and concentrated under
reduced pressure, the residue was purified by flash column
chromatography on silica gel using petroleum ether/ethyl
acetate as the eluent (8 : 1, V/V) to afford the pure product 3.
Conflicts of interest
The authors declare no conflict of interest.
Acknowledgements
This work was financially supported by the Department
Education Science and Technology Research Project of Jiangxi,
China (GJJ190619) and Jiangxi Science & Technology Normal
University (2019BSQD016, JGZD-19-10-9).
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