Iron-catalysed regioselective arylsulfonylation of quinoline N-oxides with sodium arylsulfinates under microwave irradiation

Article abstract of DOI:10.3184/174751917X15125690124246

An efficient protocol for one-pot deoxygenative and regioselective synthesis of 2-arylsulfonylquinolines has been developed via ironcatalysed cross-coupling reaction of quinoline N-oxides with sodium arylsulfinates in moderate to good yields under microwave irradiation. The reactions proceeded over a broad range of substrates with good regioselectivity and functional group tolerance.

Full text of DOI:10.3184/174751917X15125690124246

JOURNAL OF CHEMICAL RESEARCH 2017
VOL. 41 DECEMBER, 705–708
RESEARCH PAPER 705
Iron-catalysed regioselective arylsulfonylation of quinoline N-oxides with
sodium arylsulfinates under microwave irradiation
Wenpeng Mai, Mingxiu Lv, Xiaofeng Zhang and Kui Lu*
School of Materials and Chemical Engineering, Henan University of Engineering, Zhengzhou 450001, P.R. China
An efficient protocol for one-pot deoxygenative and regioselective synthesis of 2-arylsulfonylquinolines has been developed via iron-
catalysed cross-coupling reaction of quinoline N-oxides with sodium arylsulfinates in moderate to good yields under microwave irradiation.
The reactions proceeded over a broad range of substrates with good regioselectivity and functional group tolerance.
Keywords: iron catalysed cross-coupling, quinoline N-oxide, sodium arylsulfinates, sulfonylation, 2-arylsulfonyl quinoline
Heterocyclic aromatic sulfones are a crucial class of scaffolds
O
R2
R1
+
Cl
S
in natural products, pharmaceuticals, material sciences and
biologically active compounds.1–7 In particular, quinoline
sulfone compounds exhibit biological antibacterial activities
and antiproliferative activity.8,9 The development of a simple
and efficient strategy for the formation of C-S bonds is one of
the fundamental challenges for the synthetic organic chemistry
community.10,11 In the past decade, significant contributions have
been devoted to the synthesis of sulfonyl quinoline derivatives.
Traditionally, quinoline sulfones can be synthesised by the
nucleophilic substitution of heteroaromatic halides with thiols
to form a thioether, followed by oxidation to the corresponding
sulfone.12 However, low efficiency and redundant steps restrict
these methodologies for general applications. Thus, the
synthesis of sulfonyl quinolines through novel methods has
attracted attention in medicinal and synthetic chemistry.
In the past few years, great efforts have been made to
synthesise2-sulfonylquinolinescaffolds. Wu’sgroupreportedan
efficient and concise protocol to prepare sulfonylated quinoline
N-oxides via C–H bond activation with arylsulfonyl chlorides
as the sulfonylation reagents. However, to realise the synthesis
of the desired 2-sulfonylated quinolines, a harsh reagent
(PCl3) was required to reduce the N-oxide products in this
transformation.13 Chen’s group described a transition metal-free
one-pot approach for selective synthesis of 2-sulfonylquinolines
via H-phosphite-mediated C–H activation. Although the
reaction was carried out at room temperature under metal-free
conditions, stoichiometric amounts of toxic H-phosphite were
required (Scheme 1, a).14 He’s group independently developed
the synthesis of 2-sulfonylquinolines by the iodide-induced
2-sulfonylation of quinoline N-oxides with sulfonyl hydrazides
in the presence of tert-butyl hydroperoxide (TBHP), but up
to 2.0 equiv. of NaI and 3.0 equiv. of TBHP were needed to
achieve the transformation. The sulfonylation of quinoline
N-oxides induced by iodine was demonstrated by Xiang’s
group, and this reaction could proceed under the conditions of
1.0 equiv. of I2, 2.0 equiv. of NaOH and 3.0 equiv. of hydrogen
peroxide (H2O2) (Scheme 1, b).15,16 Pan’s group synthesised a
variety of 2-sulfonylquinoline derivatives via a Cu-catalysed
C2-sulfonylation of quinoline N-oxides with sodium sulfinates
in the presence of potassium persulfate (K2S2O8);17 however,
the reaction needed 0.5 equiv. of K2S2O8 as a radical initiator
and long reaction times. Zhao’s and Yotphan’s groups achieved
the synthesis of 2-sulfonylquinolines via iodine-mediated
sulfonylation of quinoline N-oxides with sodium sulfinates.18,19
Although this reaction was carried out under mild and metal-
free reaction conditions, this approach had a narrow range of
N
O
O
a
Refs 13,14
R1
R1
R1
N
O
N
O
CuBr2/K2S2O8
O
S
Refs 15,16
+
+
N
b
R2
Refs 17–19
R2
O
S
O
R2
c
O
S
NaO
H2NHN
No base
O
Fe
No stoichiometric additive
Short time
MW
d
O
R2
R1
This work:
+
NaO
S
N
O
Scheme 1 Current strategies for the synthesis of 2-sulfonyl quinolines.
substrates, with moderate yields, or 3.0 equiv. of TBHP was
required (Scheme 1, c). Despite the great success made, it is
still highly desirable to develop new protocols that can tolerate
a broad range of functional groups and have a short reaction
time and air-insensitive reaction conditions.
Transition-metal-catalysed C-H functionalisation has
received extensive attention in organic synthesis,20–29 as it
has been demonstrated to be a powerful and versatile tool
for directly introducing new functionalities via C-H bond
transformation with high atom economy. Quinoline N-oxides
are widely present in biologically active compounds,30–32 and
they are important intermediates for the synthesis of substituted
quinolines as motifs in biologically active compounds.33
Moreover, N-oxides can serve as a directing group and allow
C–H functionalisation to occur at the 2-position.34,35 Guided
by the recent studies on using sodium arylsulfinates as a
sulfonylation source,14–19 we decided to investigate the modular
synthesis of 2-arylsulfonylquinolines based on transition-
metal-catalysed C–H activation of quinoline N-oxides. Herein,
we disclose a novel iron-catalysed synthetic strategy for
synthesising a wide variety of 2-sulfonylquinoline derivatives
by reacting quinoline N-oxides with sodium sulfinates under
microwave irradiation (Scheme 1, d). The advantages of
our methodology are short reaction times, a broad range of
substrates, no base and no additive.
Results and discussion
We initially chose quinoline N-oxide (1a) as a model substrate
to react with sodium phenylsulfinate (2a) using the transition-
metal salt Cu(OAc)2 as a catalyst in DMSO at 120 °C for 20 min
under microwave irradiation. The reaction proceeded to give
access to the desired sulfonylation product 3a, albeit with a low
* Correspondent. E-mail: lukui126@126.com

706 JOURNAL OF CHEMICAL RESEARCH 2017
yield (46%, Table 1, entry 1). The structure of 3a was identified
by IR, NMR and HRMS spectra, and the sulfonylation reaction
proceeded at the C2 position of quinoline N-oxide with high
regioselectivity. Inspired by this result, various catalysts, such
as CuCl2, CuSO4, FeCl3, FeCl2, FeSO4, Fe(Cp)2 and Fe(NO3)3
were screened, and they provided 20–65% yields (Table 1,
entries 2–8). Fe(NO3)3 was superior to the others. This feature
may be due to its special oxidising properties in organic
synthesis, for example, iron(iii) nitrate on montmorillonite —
a reagent known as ‘Clayfen’ — has been employed for the
oxidation of alcohols to aldehydes. It was found that the solvent
played a crucial role in this transformation. Among all the
solvents (DMSO, H2O, CH3CN, 1,4-dioxane, CH3OH and DCE)
examined, DMSO was the best, affording 3a in 65% yield. No
3a was obtained when using H2O, 1,4-dioxane, CH3OH and
DCE as solvents, which suggested that DMSO plays a unique
role in this process (Table 1, entries 9–13). Increasing the
catalyst loading from 10% to 30% showed improved reaction
efficiency, with the starting material consumed completely in
20 min and the yield improved to 73% (Table 1, entry 16). In
addition, the reaction time was also investigated, and 20 min
proved to be the suitable time (Table 1, entries 16 and 17). No
product was obtained in the absence of catalyst (Table 1, entry
18).
With the optimal reaction conditions in hand, we turned our
attention to the substrate scope for this transformation. Several
sodium arylsulfinates were evaluated in the reaction with
quinoline N-oxide 1a (Table 2). A series of functional groups
on the phenyl ring of the sodium arylsulfinates were tested.
Both the electron-donating and electron-withdrawing groups
were well tolerated in this sulfonylation, affording the desired
products in moderate to good yields (48–75%) (3a–k). In
general, the electron-donating substituted sodium arylsulfinates
(3b–d) showed superior reaction efficiency to those of the
electron-withdrawing ones (3e–k). Fortunately, this protocol
was tolerant of synthetically valuable functional groups on the
phenyl moiety (for example, alkoxy, chloro, bromo, and iodo,
etc.), which allowed the opportunity for further transformations
(3d–h). The sulfonylation reaction also proceeded efficiently
with sodium arylsulfinates containing strong electron-
withdrawing groups e.g. –F, –CF3 or –CN, which afforded
the target compounds 3e, 3i and 3j, respectively, in 49–65%
yields. Reactions of sodium o-, and p-chloro phenylsulfinate
proceeded well, and about 70% yields were achieved (3f and
3k, respectively), suggesting that the steric effect of substituents
on aromatic rings is negligible. Unfortunately, aliphatic sodium
sulfinates were found to be unsuitable under the current
conditions.
We next turned our attention to the scope of quinoline
N-oxides. We found that this catalytic system was suitable
for a wide range of substituted quinoline N-oxides (3l–r).
Regardless of whether the quinoline ring was substituted with
electron-donating (–CH3 and –OCH3) or electron-withdrawing
(–Cl and –Br) groups, they all delivered the desired products
Table 2 Synthesis of 2-(arylsulfonyl)quinoline derivatives from quinoline
N-oxides and sodium arylsulfinatesa,b
R2
R2
R3
R1
R1
Fe(NO3)3
DMSO, 120 o
O
S
+
NaO2S
N
N
O
H
C
R3
O
1
2
3
O
O
S
O
S
N
S
N
N
O
O
O
3a
3b
3c
(66%)
(73%)
(72%)
O
O
O
O
S
O
S
N
S
O
N
N
O
O
O
F
l
C
3d
3g
3e
3h
3f
(65%)
(49%)
N
(68%)
Table 1 Effect of solvents, catalysts and temperature in the reactiona
O
S
O
S
N
S
N
O
S
O
O
O
+
NaO2S
N
N
O
1a
H
Br
CF3
I
c
c
O
3i
(53%)
(56%)
N
(59%)
2a
3a
Cl
O
S
O
N
O
S
Temperature
(°C)
Time
(min)
Yield
(%)b
N
O
S
Entry Catalyst (mol%) Solvent
O
CN
Br
1
2
3
4
5
6
7
8
Cu(OAc)2(10)
CuCl2 (10)
CuSO4 (10)
FeCl3 (10)
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
Dioxane
CH3OH
DCE
120
120
120
120
120
120
120
120
120
120
120
120
120
100
120
120
120
120
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
30
20
46
48
20
42
50
52
65
35
0
0
0
0
19
47
70
73
71
0
3j
3k
3l
(65%)
N
(70%)
N
(68%)
C
l
O
O
O
S
O
N
S
S
O
O
O
FeCl2 (10)
c
3m
3p
3n
3o
(54%)
(56%)
(60%)
FeSO4 (10)
Fe(NO3)3 (10)
Fe(Cp)2(10)
Fe(NO3)3 (10)
Fe(NO3)3 (10)
Fe(NO3)3 (10)
Fe(NO3)3 (10)
Fe(NO3)3 (10)
Fe(NO3)3 (10)
Fe(NO3)3 (20)
Fe(NO3)3 (30)
Fe(NO3)3 (30)
–
O2N
O
S
O
S
O
S
N
N
N
O
O
O
9
3q
3r
(0%)
(74%)
(75%)
10
11
12
13
14
15
16
17
18
N
H2O
O
O
S
O
O
S O
CH3CN
DMSO
DMSO
DMSO
DMSO
DMSO
S
O
N
N
C
l
3s
3t
3u
(48%)
(50%)
(72%)
aReaction conditions: quinoline N-oxides 1 (0.3 mmol), sodium arylsulfinates 2
(0.45 mmol), Fe(NO3)3 (0.09 mmol) in DMSO solvent (3.0 mL), 120 °C for 20 min under
aReaction conditions: quinoline N-oxide 1a (0.2 mmol, 29.0 mg), sodium phenylsulfinate
2a (0.4 mmol, 65.6 mg), catalyst and solvent (2.0 mL) under microwave irradiation.
bIsolated yields.
microwave irradiation.
bIsolated yields.
c20% Fe(NO3)3 was used.

JOURNAL OF CHEMICAL RESEARCH 2017 707
reported in terms of chemical shift (δ ppm) multiplicity and coupling
O
S
Standard conditions
constant (Hz). The spectra are referenced against the internal solvent
+
+
NaO2S
(a)
(b)
N
1
(CDCl3, δ H = 7.26 ppm, 13C = 77.0 ppm; DMSO-d6, δ 1H = 2.50 ppm,
N
O
13C = 40.0 ppm). Data are reported as follows: s, singlet; d, doublet;
t, triplet; q, quartet; m, multiplet. ESI MS spectra were recorded on a
Bruker Esquire 3000.
2a
3a,
0%, yield
O
Standard conditions
TEMPO or BHT
NaO2S
N
S
O
N
O
1a
Synthesis of 2-arylsulfonyl quinolines (3a–c); general procedure
Quinoline N-oxides 1 (0.3 mmol), sodium arylsulfinates 2 (0.45 mmol)
and Fe(NO3)3 (0.09 mmol) in DMSO (3.0 mL) were added to a microwave
reaction tube (5.0 mL). The reaction mixture was heated at 120 °C for
20 min under microwave irradiation. After reaction completion, the
solvent was distilled under vacuum. Ethyl acetate (10 mL) was added
to the residue, and washed with saturated sodium chloride solution
(3 × 30 mL). The organic phase was dried over anhydrous NaSO4 and
concentrated under vacuum. The crude product was purified by silica gel
column chromatography to give the desired products 3 using ethyl acetate/
petroleum ether (1:10 to 1:5) as the eluent. All compounds were confirmed
by IR, 1H NMR, 13C NMR and MS.
2a
3a,
70% (TEMPO, 2.0 equiv.)
67% (BHT, 2.0 equiv.)
Scheme 2 Control experiments.
in moderate to good yields (54–75%). However, when 6-nitro
quinoline N-oxide was employed, the desired product 3p
was not observed. The outcome indicated that electronic
variations in the substituents on the quinoline ring have
a remarkable influence on the efficiency of the reaction.
Notably, isoquinoline N-oxide also proceeded well, affording
the corresponding 1-phenylsulfonyl isoquinoline 3s in 50%
yield and no 3-phenylsulfonyl isoquinoline was found. Also
of note, when 4,7-dichloro quinoline N-oxide was employed,
the reaction proceeded smoothly to afford the unexpected
product 3t in 72% yield. Moreover, 3-bromo quinoline N-oxide
was also sulfonylated efficiently, affording the unprecedented
product 3u with moderate yield (48%). Furthermore, when
2-chloroquinoline N-oxide was used as the reactant to explore
the sulfonylation reaction, it was found that the sulfonylation
did not occur.
To clarify the reaction mechanism, some control experiments
were carried out (Scheme 2). When quinoline, instead of
quinoline N-oxide, was used as the reactant under the standard
conditions, no 2-phenylsulfonylquinoline was found, which
indicated that the N–O group plays an important role in this
transformation (Scheme 2, a). Moreover, when a radical
scavenger 2,2,6,6-tetramethylpiperidyl-1-oxyl (TEMPO) or
butylated hydroxytoluene (BHT) was employed in the reaction
of quinoline N-oxide and sodium phenylsulfinate, the reaction
still proceeded well under the optimal conditions (Scheme
2, b). The product 3a was obtained in 70% and 67% yields,
respectively, which suggests that the reaction possibly involves
a non-radical pathway. However, a certain mechanism for this
reaction is not clear at present.
2-(Phenylsulfonyl)quinoline (3a): Light yellow solid; m.p.
157–158 °C (EtOAc); IR (KBr) (υ cm−1): 1587, 1493, 1462, 1317,
1
1161, 1130, 758, 721, 644; H NMR (400 MHz, CDCl3): δ 8.38 (d,
J = 8.5 Hz, 1H), 8.22 (d, J = 8.5 Hz, 1H), 8.18 (d, J = 8.6 Hz, 1H), 8.15
(d, J = 8.6 Hz, 2H), 7.88 (d, J = 8.2 Hz, 1H), 7.79 (t, J = 7.2 Hz, 1H),
7.66 (t, J = 7.4 Hz, 1H), 7.61 (t, J = 7.2 Hz, 1H), 7.54 (t, J = 7.2 Hz, 2H);
13C NMR (100 MHz, CDCl3): δ 158.1, 147.4, 139.1, 138.7 (CH), 133.7
(CH), 131.0 (CH), 130.4 (CH), 129.2 (CH), 129.1 (CH), 129.0 (CH),
128.8, 127.7 (CH), 117.7 (CH). HRMS (ESI) m/z calcd for C15H12NO2S+
[M + H]+: 270.0583: found: 270.0583.
2-Tosylquinoline (3b): Light yellow solid; m.p. 135–136 °C
(EtOAc); IR (KBr) (υ cm−1): 3105, 1591, 1500, 1321, 1173, 1132, 696,
544; 1H NMR (400 MHz, CDCl3): δ 8.36 (d, J = 8.5 Hz, 1H), 8.18 (t,
J = 8.8 Hz, 2H), 8.02 (d, J = 8.3 Hz, 2H), 7.86 (d, J = 8.2 Hz, 2H),
7.78 (td, J = 7.0, 1.4 Hz, 1H), 7.65 (td, J = 8.0, 1.0 Hz, 1H), 7.32 (d,
J = 8.1 Hz, 2H), 2.39 (s, 3H); 13C NMR (100 MHz, CDCl3): δ 158.3,
147.4, 144.8, 138.6 (CH), 136.1, 130.9 (CH), 130.4 (CH), 129.7 (CH),
129.1 (CH), 129.0 (CH), 128.9, 128.8, 127.7 (CH), 117.6 (CH), 21.6
(CH3). HRMS (ESI) m/z calcd for C16H14NO2S+ [M + H]+: 284.0740;
found: 284.0739.
2-[(4-(tert-Butyl)phenyl)sulfonyl]quinoline (3c): Light yellow
solid; m.p. 140–141 °C (EtOAc); IR (KBr) (υ cm−1): 2956, 2854,
1
1331, 1176, 1074, 764, 640; H NMR (400 MHz, CDCl3): δ 8.37 (d,
J = 8.5 Hz, 1H), 8.20 (d, J = 8.5 Hz, 2H), 8.06 (d, J = 8.6 Hz, 2H),
7.87 (d, J = 8.2 Hz, 1H), 7.79 (td, J = 7.7, 1.4 Hz, 1H), 7.65 (td, J = 7.5,
1.0 Hz, 1H), 7.54 (d, J = 8.6 Hz, 2H), 1.30 (s, 9H); 13C NMR (100 MHz,
CDCl3): δ 158.4, 157.7, 147.5, 138.6 (CH), 136.1, 130.9 (CH), 130.4
(CH), 129.1 (CH), 128.9, 128.8 (CH), 127.7 (CH), 126.1 (CH), 117.8
(CH), 35.2, 31.0 (CH3). HRMS (ESI) m/z calcd for C19H20NO2S+ [M +
H]+: 326.1209; found: 326.1209.
In summary, a novel iron-catalysed deoxygenative C2-
sulfonylation reaction of quinoline N-oxides was developed.
This reaction was carried out under microwave-assisted
conditions, which provides a fast and easy pathway for the
preparation of bioactive 2-arylsulfonylquinolines with sodium
arylsulfinate as a sulfonyl precursor. This reaction has several
advantages, including a broad range of substrates, no base, no
additive and a short reaction time, making this method more
practical than traditional methods.
Acknowledgements
Wenpeng Mai and Mingxiu Lv contributed equally to this
work. We gratefully acknowledge the National Natural Science
Foundation of China (No. 21572046), the Youth Doctoral
Funding of Henan University of Engineering (2015017) and the
support of “the 543 Plan”of Henan University of Engineering.
Experimental
All substrates were purchased from J & K Scientific Ltd. and used
without further purification. Column chromatography was performed
using 300–400 mesh silica with the indicated solvent system
according to standard techniques. Reactions were monitored by thin
layer chromatography (TLC) (silica gel 60 F254, Qingdao Haiyang
Chemical Co. Ltd.). Reactions were carried out in a CEM Discover
microwave reactor (0–600W, 2450 MHz, CEM Corp.). Products
Electronic Supplementary Information
The data for products 3d–u are provided in the ESI, which
is available through: http://ingentaconnect.com/content/stl/
jcr/2017/00000041/00000012/art00007
were detected using a UV–Vis lamp (254 nm). The H and 13C NMR
spectra were obtained on a Bruker 400 MHz NMR Fourier transform
1
Received 23 July 2017; accepted 19 November 2017
Paper 1704893
https://doi.org/10.3184/174751917X15125690124246
Published online: 11 December 2017
1
spectrometer. H NMR data are reported as: chemical shift (δ ppm),
multiplicity, coupling constant (Hz) and integration. 13C NMR data are

708 JOURNAL OF CHEMICAL RESEARCH 2017
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