Exogenous-oxidant-free electrochemical oxidative C-H sulfonylation of arenes/heteroarenes with hydrogen evolution

Article abstract of DOI:10.1039/c8cc06451b

An efficient and environmentally benign electrochemical oxidative radical C-H sulfonylation of arenes/heteroarenes was developed in this work. A series of significant diarylsulfones were prepared under mild catalyst- and exogenous-oxidant-free reaction conditions, which efficiently avoid the issues of desulfonylation or over-reduction of sulfonyl groups.

Full text of DOI:10.1039/c8cc06451b

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Exogenous-oxidant-free electrochemical oxidative
C–H sulfonylation of arenes/heteroarenes
with hydrogen evolution†
Cite this:DOI: 10.1039/c8cc06451b
Received 8th August 2018,
Accepted 7th September 2018
ab
Yong Yuan,
‡
Yi Yu,‡a Jin Qiao,a Pan Liu,a Banying Yu,a Wukun Zhang,a
Huilin Liu,a Min He,a Zhiliang Huang*b and Aiwen Lei
*
ab
DOI: 10.1039/c8cc06451b
rsc.li/chemcomm
An efficient and environmentally benign electrochemical oxidative
radical C–H sulfonylation of arenes/heteroarenes was developed in
this work. A series of significant diarylsulfones were prepared under
mild catalyst- and exogenous-oxidant-free reaction conditions, which
efficiently avoid the issues of desulfonylation or over-reduction of
sulfonyl groups.
Arylsulfones are not only versatile intermediates in organic
synthesis,1 but also one of the common structural units in
biologically active molecules.2 Diarylsulfones, in particular, are
found in a wide range of pharmaceutically active molecules,
which exhibit intriguing biological properties.3 As a consequence,
Scheme 1 The pathways to synthesize diarylsulfones.
the development of practical and efficient approaches for the have to be applied as the sacrificial reagents for taking the surplus
preparation of diarylsulfones has gained significant attention electrons, which lead to some wasteful byproducts or oxidation
among medicinal and synthetic organic chemists. Traditionally, side reactions. Moreover, under these reaction conditions, con-
these valuable compounds are synthesized by oxidation of trolling desulfonylation or over-reduction is also a challenging
sulfides,4 Friedel–Crafts sulfonylation of arenes,5 or transition task because the sulfonyl groups are easily desulfonylated or over-
metal catalysed cross-coupling reactions (Scheme 1a).6 However, reduced in the presence of certain catalysts.9,10 Thus, it is highly
these methods usually suffer from significant drawbacks, such as desirable to develop a more practical and efficient protocol to
the use of strong oxidants or prefunctionalized coupling partners construct diarylsulfones.
(aryl halides, aryl triflates, aryl boronic or diaryliodonium salts),
Electrochemical synthesis has been recognized as an efficient
harsh acidic treatments, high reaction temperatures and low and environmentally benign synthetic strategy and has become
selectivity, which make them environmentally unfavorable and a growing research field in chemical syntheses.11,12 Recently,
result in poor functional group compatibility. In recent years, our group has been interested in the electrochemical oxidative
oxidative C–H/X–H (XQC, N, O, S, etc.) cross-coupling reactions C–C and C–heteroatom bond formation.13 Herein, we report an
have been developed as an appealing and powerful strategy for the efficient and environmentally benign electrochemical oxidative
construction of C–X bonds.7 For example, oxidative cross-coupling radical C–H sulfonylation between arenes/heteroarenes and
reactions between arenes/heteroarenes and sodium sulfinates or sulfonyl hydrazides (Scheme 1c). By employing the constant
sulfonyl hydrazines have been developed to construct diaryl- current instead of an exogenous oxidant, a series of significant
sulfones (Scheme 1b).8 In these reactions, stoichiometric oxidants diarylsulfones were prepared under mild exogenous oxidant- and
catalyst-free reaction conditions, which efficiently avoid the issues
of desulfonylation or over-reduction of sulfonyl groups.
a National Research Center for Carbohydrate Synthesis, Jiangxi Normal University,
Our investigation was commenced with 4-chlorobenzene-
Nanchang 330022, P. R. China. E-mail: aiwenlei@whu.edu.cn
b College of Chemistry and Molecular Sciences, Institute for Advanced Studies (IAS),
sulfonyl hydrazide (1a) and 3-phenylbenzofuran (2a) as starting
materials, K2CO3 as base, nBu4NBF4 as electrolyte and CH3CN/H2O
Wuhan University, Wuhan 430072, P. R. China. E-mail: zlhuang@whu.edu.cn
as co-solvent. As shown in Table 1, by employing a two-electrode
system with a graphite rod as an anode and a nickel plate as a
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c8cc06451b
‡ Yong Yuan and Yi Yu contributed equally to this work.
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Table 1 Optimization of the exogenous-oxidant-free electrochemical
oxidative C–H sulfonylationa
Table 2 Substrate scope of the exogenous-oxidant-free electrochemical
oxidative C–H sulfonylation with various sulfonyl hydrazidesa
Entry
Variation(s) from the standard conditions
Yieldb (%)
1
2
None
6 mA, 6 h
87
74
3
4
5
6
7
8
18 mA, 2 h
45
88
58
Trace
57
C(+)|Pt(À)
C(+)|Fe(À)
C(+)|C(À)
nBu4NPF6
nBu4NClO4
75
9
Na2CO3
NaHCO3
1.2 equiv. of K2CO3
2.0 equiv. of K2CO3
CH3CN/H2O = 9/1
CH3OH/H2O
No electrical current
77
85
77
82
46
40
n.d.
10
11
12
13
14
15
a
Standard conditions: C anode, Ni cathode, undivided cell, constant
current = 12 mA, 1 (1.5 equiv.), 2a (0.25 mmol), K2CO3 (1.5 equiv.),
nBu4NBF4 (0.1 mmol), MeCN (9.5 mL), H2O (0.5 mL), RT, N2, 3 h,
a
Reaction conditions: C anode, Ni cathode, undivided cell, constant
current = 12 mA, 1a (1.5 equiv.), 2a (0.25 mmol), K2CO3 (1.5 equiv.),
nBu4NBF4 (0.1 mmol), MeCN (9.5 mL), H2O (0.5 mL), RT, N2, 3 h.
b
isolated yields. NaHCO3 as base.
b
Isolated yields.
(Table 2, 3ae–3ag). By contrast, strong electron-rich benzene-
sulfonyl hydrazides exhibited decreased reaction efficiency
cathode, the desired C–H sulfonylated 3aa was produced in 87% (Table 2, 3ah and 3aj). In addition, benzenesulfonyl hydrazide
yield with a 12 mA constant current in an undivided cell (entry 1). afforded the desired product in 81% yield (Table 2, 3ai). Besides
Either decreasing or increasing the operating current led to aryl-substituted sulfonyl hydrazide, 2-thienyl sulfonyl hydrazide
decreased reaction yields (entries 2 and 3). When the platinum could also furnish the desired product (Table 2, 3al). However,
plate and iron plate were used as the cathode materials, the desired aliphatic sulfonyl hydrazides were not suitable for this
products 3aa were isolated in 88% and 58% yields (entries 4 and 5), transformation.
respectively. A much worse result was obtained while the reaction
Next, the scope with respect to the benzofurans was explored.
was performed with a graphite rod cathode (entry 6). As for the As shown in Table 3, benzofurans bearing electron-neutral sub-
n
n
choice of electrolyte, Bu4NPF6 and Bu4NClO4 showed decreased stituents such as methyl or tert-butyl at the 3-phenyl moiety
reaction efficiencies (entries 7 and 8). A slight loss of yield was showed good reaction efficiency (Table 3, 3ba and 3ca). However,
observed when Na2CO3 or NaHCO3 was employed instead of K2CO3 halide substituents, or electron-withdrawing or electron-donating
(entries 9 and 10). By increasing or decreasing the amount of groups at the 3-phenyl moiety of benzofurans decreased the
K2CO3, a lower yield was obtained (entries 11 and 12). The effect of reaction efficiency (Table 3, 3da–3ga). Note that the C-3 substi-
solvent was explored as well. When 1.0 mL of water was used, the tuent has a significant effect on the reaction efficiency. When
reaction showed decreased efficiency (entry 13). Using methanol 3-naphthylbenzofurans were used as the reaction partners,
instead of acetonitrile also showed decreased reaction efficiency desired C–H sulfonylation products were isolated in moderate
(entry 14). As was expected, no reaction could be observed in the yields (Table 3, 3ha and 3ia). Besides 3-arylbenzofurans,
absence of the electrical current (entry 15). Note that this reaction 3-alkylbenzofurans such as 3-methylbenzofuran also delivered
required 1.5 equiv. of sulfonyl hydrazide, probably due to the the corresponding products in good yield (Table 3, 3ja). However,
self-reactions of sulfonyl hydrazide under the current reaction when there was no substituent in the C-3 position, benzofuran
conditions. The by-products thiosulfonate and disulfide could be delivered the corresponding product in low yield (Table 3, 3ka).
detected by GC-MS in the reaction mixture.
Notably, 3-phenylbenzofurans bearing electron-neutral or electron-
With the best reaction conditions established, the scope of donating groups at the benzofuran ring were also suitable
this electrochemical oxidative C–H sulfonylation was explored. substrates for the reaction, affording the desired products in
Firstly, different sulfonyl hydrazides (1) were applied as the 39–95% yields (Table 3, 3la, 3ma, 3oa).
C–H sulfonylation partners (Table 2). Benzenesulfonyl hydrazides
Besides benzofurans, other electron-rich heteroarenes/
bearing halide substituents including F, Cl, and Br showed good arenes were also applied as substrates in this electrochemical
reaction efficiency, affording the C–H sulfonylation products in reaction (Table 4). Electron-rich thiophenes, such as 3-phenyl-
54–87% yields (Table 2, 3aa–3ad). Similarly, benzenesulfonyl benzothiophene and 3,4-ethylenedioxythiophene, delivered the
hydrazides bearing strong electron-withdrawing trifluoromethyl C–H sulfonylation products in 54% and 58% yields (Table 4,
groups or weak electron-donating methyl substituents were also 5a and 5b), respectively. The electron-rich pyrrole and imidazo-
suitable, and the desired products were obtained in 86–91% yields pyridine derivatives were also suitable for this transformation
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Table 3 Substrate scope of the exogenous-oxidant-free electrochemical
3,5-dimethoxytoluene were used, the corresponding C–H sulf-
onylation products were produced in 51% and 56% yields
(Table 4, 5e and 5f), respectively. By contrast, less electron rich
pentamethylbenzene showed slightly low reactivity in this trans-
formation (Table 4, 5g). In addition to electron rich benzenes,
naphthalene and its derivatives were also compatible with the
reaction conditions, providing the products in moderate yields
(Table 4, 5h and 5i). Unfortunately, indole derivatives, such as
1-methylindole, are not amendable to this procedure. The main
reason for this may because most indole derivatives are well-
known electroactive compounds that are readily oxidized than 1a.
To gain some insights into the mechanism for this electro-
chemical oxidative C–H sulfonylation reaction, cyclic voltam-
metry experiments on the reactants were carried out (see the
ESI,† Fig. S2). In the absence of K2CO3, the first oxidation peaks
of 1a and 2a were observed at 2.21 and 1.99 V, respectively,
whereas under the conditions of adding K2CO3, the oxidation
peaks of 1a and 2a were observed at 1.80 and 1.88 V, respectively.
These results indicated that 1a was likely to be first oxidized under
the electrolytic conditions and K2CO3 as a base played a crucial
role in promoting the oxidation of sulfonyl hydrazine. A radical-
trapping experiment was also conducted by employing N-methyl-
N-phenylmethylacrylamide (Scheme 2). When 1.0 equiv. of
N-methyl-N-phenylmethylacrylamide was added in the standard
reaction, a trace amount of sulfonyl radical trapping compound
was detected; on the other hand, the reaction of 1a with N-methyl-
N-phenylmethylacrylamide in the absence of 2a under the standard
reaction conditions gave the trapping compound in 29% yield.
This result indicates that the reaction presumably proceeds
through a sulfonyl radical intermediate and electron rich 2a is
a better sulfonyl radical receptor than N-methyl-N-phenyl-
methylacrylamide under the current reaction conditions.
According to the above experimental results, a plausible
reaction mechanism is proposed in Scheme 3. Firstly, 4-chloro-
benzenesulfonyl hydrazide (1a) was converted to the corres-
ponding sulfonyl radical B in the presence of base via three
times single electron transfer (SET) oxidation and deprotona-
tion as well as further release of molecular nitrogen. Addition of
the sulfonyl radical B to 3-phenylbenzofuran (2a) formed the
key carbon-centered radical C. Subsequently, intermediate C
underwent further single electron transfer (SET) oxidation and
finally resulted in the desired product 3aa after a deprotonation
process. Concomitant cathodic reduction of protons led to hydrogen
liberation. Note that for 2-phenylbenzofuran (Table 3, 3na) and
some less electron-rich arenes (Table 4, 5g and 5h) the radical
addition processes are difficult, so a large excess of sulfonyl
oxidative C–H sulfonylation with various benzofuransa
a
Standard conditions: C anode, Ni cathode, undivided cell, constant
current = 12 mA, 1a (1.5 equiv.), 2 (0.25 mmol), K2CO3 (1.5 equiv.),
nBu4NBF4 (0.1 mmol), MeCN (9.5 mL), H2O (0.5 mL), RT, N2, 3 h,
b
c
d
isolated yields. NaHCO3 as base. C anode, Pt cathode. 1a (3.0 equiv.).
and afforded the corresponding products in 36% and 62%
isolated yields (Table 4, 5c and 5d), respectively. To further
establish the scope of this transformation, electron-rich arenes
were also tested under the electrolysis conditions. For example,
when the highly electron rich 1,3,5-trimethoxybenzene and
Table 4 Substrate scope of the exogenous-oxidant-free electrochemical
oxidative C–H sulfonylation with various (hetero)arenesa
a
Standard conditions: C anode, Ni cathode, undivided cell, constant
current = 12 mA, 1a (1.5 equiv.), 4 (0.25 mmol), K2CO3 (1.5 equiv.),
nBu4NBF4 (0.1 mmol), MeCN (9.5 mL), H2O (0.5 mL), RT, N2, 3 h,
b
isolated yields. The yield was determined by 1H NMR spectroscopy
c
with CH2Br2 as the internal standard. Reaction conditions: C anode,
Ni cathode, constant current = 12 mA, 1a (0.25 mmol), 4 (5.0 equiv.),
K2CO3 (1.0 equiv.), nBu4NBF4 (0.1 mmol), MeCN (9.5 mL), H2O (0.5 mL),
RT, N2, 2 h.
Scheme 2 The radical-trapping experiment.
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hydrazides or arenes is often required to increase the probability
of radical addition.
In summary, we have disclosed an electrochemical oxidative
radical C–H sulfonylation of arenes/heteroarenes towards the
synthesis of diarylsulfones. A series of C–H sulfonylated products
were produced in moderate to high yields. During the reaction, the
constant current is utilized instead of an exogenous oxidant, and
hydrogen and nitrogen are produced as the only side-products.
Moreover, mild catalyst-free electrochemical conditions are
employed, which efficiently avoid the issues of desulfonylation
or over-reduction of sulfonyl groups.
This work was supported by the National Natural Science
Foundation of China (21390402, 21520102003, 21702150), the
China Postdoctoral Science Foundation (BX201600114,
2016M602340), and the Hubei Province Natural Science Foun-
dation of China (2017CFA010). The Program of Introducing
Talents of Discipline to Universities of China (111 Program) is
also acknowledged. This paper is dedicated to Professor Xiyan
Lu on the occasion of his 90th birthday.
Conflicts of interest
There are no conflicts to declare.
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