Electrochemical-In-Situ-Oxidative Sulfonylation of Phenols with Sulfinic Acids as an Access to Sulfonylated Hydroquinones

Article abstract of DOI:10.1002/adsc.202100366

The electrochemical in-situ oxidative sulfonylation of phenols with sulfinic acids access to sulfonylated hydroquinones has been developed. A series of sulfonylated hydroquinones were prepared under mild mediator-, catalyst- and exogenous-oxidant-free con

Full text of DOI:10.1002/adsc.202100366

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DOI: 10.1002/adsc.202100366
Electrochemical-In-Situ-Oxidative Sulfonylation of Phenols with
Sulfinic Acids as an Access to Sulfonylated Hydroquinones
Xue Sun,+a Fanjun Zhang,+a Kelu Yan,a Wenfeng Feng,a Xuejun Sun,a
Jianjing Yang,a,* and Jiangwei Wena,*
a
Institute of Medicine and Materials Applied Technologies, College of Chemistry and Chemical Engineering,
Qufu Normal University,
Qufu, Shandong 273165, People’s Republic of China
E-mail: wenjy@qfnu.edu.cn; jjyang@whu.edu.cn
+
These authors contributed equally to this work.
Manuscript received: June 3, 2021; Revised manuscript received: June 3, 2021;
Version of record online: ■■■, ■■■■
Supporting information for this article is available on the WWW under https://doi.org/10.1002/adsc.202100366
compounds A and B act as potential inhibits of β-
Abstract: The electrochemical in-situ oxidative
Ketoacyl ACP-synthase III (FabH),[3] an essential
sulfonylation of phenols with sulfinic acids access
condensing enzyme in bacterial fatty acid biosynthesis.
to sulfonylated hydroquinones has been developed.
Moreover, the alkylsulfuryl diverged hydroquinone C[4]
A series of sulfonylated hydroquinones were pre-
pared under mild mediator-, catalyst- and exoge-
nous-oxidant-free conditions. Notably, such an
electrochemical in-situ-oxidative synthetic strategy
exhibited good activities as selective thyroid receptor β
agonists, and D[5] is the inhibitor of coenzyme Q
systems.
As a consequence, the advantage of sulfone-
presented wide functional group tolerance and
containing hydroquinones has expedited the explora-
amenability to gram-scale synthesis with 62–80%
tion of practical and eco-friendly approaches for their
yields.
preparation.[6] The traditional approach often involves
the conjugate addition of arylsulfinic acid/salt to
benzoquinone under acidic conditions, or in ionic
Keywords: Electrochemical in-situ oxidative; Sulfo-
liquid solvents (Scheme 2a).[7] In 2017, Wu[8] and co-
nylation; Phenols; Sulfonylated Hydroquinones
workers developed a Cu and Mn synergistic catalytic
strategy for the sulfonation of arylquinones with
sulfonyl hydrazides as the sulfone source (Scheme 2b).
Functionalized hydroquinone derivatives are important Recently, a one-pot method for the synthesis of
structural motifs and omnipresent in many bioactive sulfonylated hydroquinones/ naphthalenediols employ-
pharmaceuticals, agricultural drugs, dyestuffs, antiox- ing hypervalent iodine as the oxidizing agents have
idants, and materials.[1] In particular, the sulfone- been reported by Reheman and Chen’s groups
containing hydroquinone derivatives are not only (Scheme 2c).[9] However, these methods have obvious
versatile intermediates in organic synthesis but also drawbacks, including the use of transition metals,
one of the common building blocks in biologically strong oxidants, and harsh acidic treatments, which
active molecules.[2] As depicted in Scheme 1, both makes them environmentally unfriendly and poorly
functional group compatible. Therefore, it is essential
to develop a facile, highly efficient, and greener
approach to constructing sulfonylated hydroquinones.
Electrochemical organic synthesis with electrons as
extremely powerful redox reagents has become an
efficient and widely applied green synthetic strategy, a
series of achievements have been gained.[10] In partic-
ular, the construction of functionalized phenol deriva-
tives by electrochemical oxidative cross-coupling strat-
egy has attracted much attention from organic
Scheme 1. Examples of bioactive sulfone-containing naphthale-
nediols and hydroquinones.
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of phenols with sulfinic acids access to sulfonylated
hydroquinones under the mediator-, metal- and oxi-
dant-free conditions.
Initially, 4-methoxyphenol (1a) and 4-meth-
ylbenzenesulfinic acid (2b) was chosen as the model
substrates to optimize the reaction conditions (Table 1).
By optimizing key reaction parameters, the best results
were obtained by performing the electrolysis at room
temperature under a constant current of 10 mA in an
undivided cell equipped with graphite rod as the anode
and platinum plate as the cathode conducting lithium
perchlorate (LiClO4) as electrolytes in the solvent of
CH3CN. Under optimal conditions, the desired product
3ab can be obtained with a high yield and selectivity
without the formation of 3ab’ and 3ab’’ (Table 1,
entry 1). Undoubtedly, the control experiments con-
firmed that electricity should play a key role in the
sulfonylation of phenols with sulfinic acids access to
sulfonylated hydroquinones (Table 1, entry 2). The
experimental results show that the diluted 4-meth-
ylbenzenesulfinic acid (2b) is not conducive to the
improvement of 3ab yield (Table 1, entry 3). There-
fore, detailed cyclic voltammetry (CV) experiments
were performed to gain a deeper understanding of the
reasons behind (Figure 1). First, the anodic peaks of
single species (1a (1.1 V vs Ag/AgCl, corresponding
cathodic peak at 0.27 V vs Ag/AgCl), 2b (1.69 V vs
Ag/AgCl), or 3ab (1.16 V vs Ag/AgCl)) and mixtures
(1a with 2b) were collected and labelled by CV
(Figure 1a). The CV results from the mixture (1a with
Scheme 2. Synthetic strategies for sulfonylated naphthalene-
diols or hydroquinones.
chemistry in recent years.[11] For example,
Nematollahi’s[12] group developed an electrooxidation
sulfonation of hydroquinones with sulfinic acids under
the divided cell, and three examples of sulfonylated
hydroquinones were synthesized (Scheme 2d). Subse-
quently, Waldvogel[11e] and co-workers provided a
direct metal- and reagent-free sulfonylation of phenols
with sodium sulfinates by electrosynthesis (Sche-
me 2e). Despite the above achievements so made, the
effective methods towards the sulfonylation of phenols
access to sulfonylated hydroquinones are scarce and
remain a challenge under the mediator-, catalyst- and
oxidant-free conditions. In the continuation of our
efforts in the construction of sulfone-containing
compounds,[13] here, we wish to report a mild electro-
chemical-in-situ-oxidative sulfonylation of phenols
with sulfinic acids to access sulfonylated hydroqui-
nones (Scheme 2f).[14] Neither metal catalysts nor
external chemical oxidants are required in this trans-
formation. In terms of substrate scope, various phenols
and sulfinic acids were compatible in the present
protocol, generating the desired products in up to 95%
yields. To our delight, the established synthesis
protocol can be scaled up to access the desired
products with a favorable yield at room temperature.
To the best of our knowledge, this is the first success
on the electrochemical-in-situ-oxidative sulfonylation
Table 1. Optimization of the Reaction Conditions.[a]
Entry Deviation from standard conditions Yield of 3ab[b](%)
1
2
3
4
5
6
7
8
None
Without current
1a:2b=1:1, 1:2
15 mA instead of 10 mA, 4 h
5 mA instead of 10 mA, 12 h
nBu4NClO4 instead of LiClO4
nBu4NBF4 instead of LiClO4
nBu4NPF6 instead of LiClO4
0.1 M LiClO4 (Ecell =2.27–2.5 V)
H2O (0.2 mL)
95 (0, 0)[c]
n. d.
45, 71
79
41
47
55
61
45
n. d.
54
9
10
11
12
Pt (+)jC(À )
Pt (+)jPt (À )
34
[a] Standard conditions: C as the anode, Pt as the cathode,
constant current=10 mA, Ecell =1.6–1.9 V, 1a (0.25 mmol),
2b (0.75 mmol), LiClO4 (2.0 mmol), MeCN (10.0 mL), r.t.,
N2, 6 h. n.d.=not detected.
[b] Isolated yields.
[c] Yield of 3ab’ and 3ab’’.
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Table 2. Scope of the sulfinic acids.[a]
Figure 1. Cyclic Voltammetry at glass carbon as work elec-
trode, Pt (1.5×1.5 cm) as counter electrode in 0.2 M LiClO4,
CH3CN (10.0 mL), scan rate 100 mV/s. (a) 1a (0.25 mM), 2b
(0.75 mM), 3ab (0.25 mM). (b) Concentration of 2b (1 eq:
0.25 mM, 2 eq: 0.5 mM, 3 eq: 0.75 mM, 4 eq: 1.0 mM).
2b) showed that electron transfer occurred between 1a
and 2b to form the product 3ab, and a lower reduction
potential was exhibited (0.17 V vs Ag/AgCl). Subse-
quently, the influence of the concentration of 2b on the
cyclic voltammogram of 1a was investigated, and it
was found that the ratio of reduction potential to the
peak current was the lowest at a concentration of
0.75 mM 2b. These indicate that the oxidation of 3ab
is more difficult than 1a due to the presence of the
electron-withdrawing benzenesulfonyl groups, and pre-
vents 3ab from being further oxidized to 3ab’ under
current conditions (Figure 1b).[12] Slightly decreased
reaction yields could still be obtained either increasing
or decreasing the operating current (Table 1, entries 4,
and 5).
Moreover, the desired product 3ab was obtained
with a yield of 47–61% when LiClO4 was replaced
with quaternary ammonium salts (Table 1, entries 6–8).
The desired product 3ab was only delivered in a 45%
yield due to the high voltage caused by the low
concentration of LiClO4 (Table 1, entry 9). However,
the desired product was not observed when a small
amount of water was present in the reaction (Table 1,
entry 10). Afterward, the influence of the electrode
materials was also investigated. Lower yields were
[a] Standard conditions Standard conditions: C as the anode, Pt
as the cathode, constant current=10 mA, 1a (0.25 mmol), 2
(0.75 mmol), LiClO4 (2.0 mmol), MeCN (10.0 mL), r. t., N2,
6 h.
[b] Isolated yields.
[c] nBu4NBF4 (1.0 mmol).
obtained when the graphite rod (anode) and platinum important compounds. Besides, the polysubstituted
(cathode) were replaced by platinum (anode) and aromatic sulfinic acid and naphthalene-2-sulfinic acid
graphite rod (cathode) or platinum (anode) and could be tolerated under the established conditions to
platinum (cathode) (Table 1, entries 11, and 12).
give the desired products 3al and 3ak in 53% and
With the optimized conditions described above, the 76% yields, respectively. Moreover, when the hydro-
scope and limitations of the reactions of various gen at the m- or ο-position of the benzene ring was
arylsulfinic acids were explored, with the results substituted separately by Cl, Br, and methyl groups,
summarized in Table 2. The arylsulfinic acids contain- the corresponding products can also be obtained with
ing electron-donating or electro-withdrawing substitu- moderate yields (3am–3ao). Unfortunately, aliphatic
ents on the aryl groups can be well applied to this sulfinic acids were not suitable for this transformation.
established condition (3aa–3aj). In particular, various
Subsequently, the applicable scope of phenols with
substituted functional groups such as Ph, F, Cl, Br, different substituents was explored under the estab-
CF3, and acetamido are more compatible with this lished conditions (Table 3). Initially, the influence of
protocol, affording the corresponding products in good the leaving group at the p- position of the phenol was
to excellent yields (3da–3ha), which can be used for investigated. It should be emphasized that the desired
further functionalization to synthesize other kinds of product can also give a moderate to good yield when
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Table 3. Scope of Phenols.[a]
delight, 4-methoxy-1-naphthol was also compatible
with the current protocol, and the corresponding
product 3ob can obtain a yield of 76%, which is
considered as a potential inhibits of β-Ketoacyl ACP-
synthase III (FabH). Interestingly, the sulfonated
products 2,4-dimethoxy-5-tosylphenol (3pb) and 2-
methoxy-4-methyl-5-tosylphenol (3qb) at the C5 posi-
tion of 1p and 1q can be obtained with 57% and 62%
yields, respectively. However, the dominant C4 sulfo-
nated guaiacol was obtained when 2-methoxyphenol
was performed under the current procedure (3rb).
Furthermore, 1a or 1o and 2b were chosen as
substrates to estimate the synthetic applicability of this
method on a gram scale. As shown in Scheme 3, the
desired products 3ab and 3ob were obtained in 80%
and 62% yields, respectively. These results confirmed
that the present protocol can sever as a practical and
efficient strategy to synthesize the sulfonylated naph-
thalenediols/hydroquinones from 4-methoxy-1-naph-
thol or p-methoxyphenol with sulfinic acids.
To gain some insights into the mechanism for this
electrochemical oxidative CÀ H sulfonylation of pho-
nels, several control experiments were carried out
(Scheme 4). The sulfonylation reaction was inhibited
when 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO)
or 2,6-di-tert-butyl-4-hydroxytoluene (BHT) was
added into the present reaction system (Scheme 4a,
[c] Standard conditions: C as the anode, Pt as the cathode,
constant current=10 mA, 1 (0.25 mmol), 2b (0.75 mmol),
LiClO4 (2.0 mmol), MeCN (10.0 mL), r. t., N2, 6 h.
[b] Isolated yields.
Scheme 3. Gram-scale Experiment.
the Me group was replaced by Et, Ph, Bn, and CF3
(3bb–3eb). The desired products can be obtained with
yields of 42% and 36% when 1f and 1g are performed
under the standard reaction conditions, respectively.
Secondly, the C2 position of p-methoxyphenol contain-
ing substituents such as Me, F, Cl, and Br can also be
compatible with the present reaction conditions with a
moderate to good yield. The NMR spectrum results
confirmed that the C5 sulfonated hydroquinone was
the main product (3hb–3kb). Under standard reaction
conditions, the main components of C5 sulfonated
hydroquinone derivatives can also be generated with
excellent yield and selectivity (3lb–3nb). To our
Scheme 4. Control Experiments.
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and 4b). At the same time, pure product 4 was obtained candidate products can be produced in good to high
in 19% yield under the described conditions in the yields under the present electrochemical conditions.
presence of BHT (Scheme 4b, Fig. S1). These results Mechanism studies confirmed that the electrochemical
indicate the sulfonyl radical A is a reasonable in-situ oxidation and sulfonation of phenols underwent
intermediate. Moreover, the desired product 3ab can the radical cross-coupling process.
be obtained in 52% and 45% yields when hydro-
quinone and benzoquinone were used as substrates
Experimental Section
instead of 1a, respectively (Scheme 4c, and 4d).[15]
Furthermore, benzoquinone can also be obtained with
a yield of 76% when 1a was implemented under the
present protocol (Scheme 4e). These results confirm
that hydroquinone and benzoquinone should be suit-
able intermediates for this transformation.
In an oven-dried undivided three-necked flask (25 mL)
equipped with a stir bar, phenols 1 (0.25 mmol), sulfinic acids 2
(0.75 mmol), and lithium perchlorate (LiClO4, 2.0 mmol,
212.0 mg) were combined and added. The flask was equipped
with a graphite rod (diameter: 6 mm) as the anode platinum
plate (1.5×1.5 cm2) as the cathode and was then charged with
nitrogen. Under the protection of nitrogen, CH3CN (10.0 mL)
was slowly injected into the reaction flask. The reaction mixture
was stirred and electrolyzed at a constant current of 10 mA
under room temperature for 6 h. When the reaction was
finished, the solution was concentrated in a vacuum. The pure
product was obtained by flash column chromatography on silica
gel (petroleum: ethyl ether=1:1 - 10:1).
Base on the above preliminary results and previous
reports,[8,11c,d,g] a possible reaction pathway was out-
lined and described in Scheme 5. Initially, the inter-
mediate benzoquinone can be obtained by the electro-
chemical oxidation and demethylation of 1a on the
surface of the anode. Subsequently, the hydroquinone
produced by electrochemical in-situ oxidation 1a can
be used to oxidize 2b to generate sulfone radical A
and benzoquinone. At the same time, radical B can be
produced from benzoquinone being oxidized on the
surface of the anode. Finally, the cross-coupling of
radical A and C could furnish the desired product 3ab
(Path A). In addition, another mechanism approach
that cannot be ignored involves anodic oxidation of
hydroquinone to form benzoquinone followed by
Michel addition with 2b to obtain the final product
3ab (Path B).[11e,h,i,12]
In conclusion, a synthesis of sulfonylated hydro-
quinones was developed via electrochemical in-situ
oxidative sulfonylation reaction between phenols and
sulfinic acids for the first time. Compared with the
previous methods for sulfonylation of phenols, the
mediators, metals, and chemical oxidants are shielded
from this electrochemical in-situ-oxidation strategy,
and it has a wide functional group tolerance and good
gram-scale yields. A series of inhibitors and new drug
Acknowledgments
This work is supported by the National Natural Science
Foundation of China (No. 21902083), Natural Science Founda-
tion of Shandong Province (No. ZR2020QB130, ZR2019BB078,
ZR2018MB014), Shandong, P. R. China. College Students
Innovation and Entrepreneurship Training Project of Shandong
Province (No. S202010446066). This work is also supported by
the Talent Program Foundation of Qufu Normal University
(NO. 6132 and 6125).
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Adv. Synth. Catal. 2021, 363, 1–7
6
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COMMUNICATIONS
Electrochemical-In-Situ-Oxidative Sulfonylation of Phenols
with Sulfinic Acids as an Access to Sulfonylated Hydroqui-
nones
Adv. Synth. Catal. 2021, 363, 1–7
X. Sun, F. Zhang, K. Yan, W. Feng, X. Sun, J. Yang*, J.
Wen*