Electrochemical and direct C-H methylthiolation of electron-rich aromatics

Article abstract of DOI:10.1039/d0gc00591f

The electrochemical-induced C-H methylthiolation of electron-rich aromatics has been accomplished via a three component cross-coupling strategy. Potassium thiocyanate (KSCN) as both the supporting electrolyte and sulfur source and methanol as the methylation reagent are used. This protocol is versatile for various (hetero)aromatic compounds such as aniline, anisole and indole. The reaction proceeds under mild conditions without any metal catalyst, exogenous oxidant and highly toxic sulfur reagent. Importantly, such an electrochemical-induced methylthiolated reaction could be easily scaled up with good efficiency.

Full text of DOI:10.1039/d0gc00591f

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ARTICLE
Electrochemical and Direct C-H Methylthiolation of Electron-Rich
Aromatics†
Yaxing Wu,‡ Hongliang Ding,‡ Ming Zhao,* Zhong-Hai Ni and Jing-Pei Cao
Received 00th January 20xx,
Accepted 00th January 20xx
The electrochemical-induced C-H methylthiolation of electron-rich aromatics has been accomplished via a three component
cross-coupling strategy. KSCN as both the supporting electrolyte and sulfur source and methanol as the methylation reagent
are used. This protocol is versatile for various (hetero)aromatic compounds such as aniline, anisole and indole. The reaction
proceeds under mild conditions without any metal catalyst, exogenous oxidant and highly toxic sulfur reagent. Importantly,
such an electrochemical-induced methylthiolated reaction could be easily scaled up with good efficiency.
DOI: 10.1039/x0xx00000x
Several synthetic methods have been reported for the
formation of aryl-methyl sulfide. Conventionally, the most
common pathway is application of methylthiol for the source of
Introduction
Synthesis of organosulfur compounds is gaining more and more
significance since carbon-sulfur bonds are frequently found in
pharmaceuticals1, biological molecules2, agrochemicals3 and
materials4. Aryl-methyl sulfide, sulfoxide, and sulfone motifs are
found in many modern pharmaceutical molecules.
Consequently, they are frequently incorporated as building
blocks in structure-activity relationship (SAR) development of
medicinal entities. Some representative examples are shown in
Figure 1, such as the nonsteroidal anti-inflammatory drug
Firocoxib5, the nonsteroidal anti-inflammatory drug Sulindac6,
the cardiovascular drug Sulmazole7, the proliferative diseases
drug Thiocolchicine8, the basal-cell carcinoma drug Vismodegib9,
and the antipsychotic drug Thioridazine10. In addition, Katz and
co-workers has reported that thioethers can also be used as
biopheromone for animal communication11.
“MeS” functional group, which is introduced through transition
metal catalyzed nucleophilic substitution with only electron-
deficient substrates12 or the methylation of aryl thiols13
(Scheme 1). However, mercaptans are generally highly biotoxic,
and intractable problems associated with oxidation
compatibility, catalysts poisoning, and environmental pollution
have been found in these complex systems14. Recently, a series
of copper-catalyzed methylthiolation using dimethylsulfoxide
as a methylthiolation reagent have been reported, and some
examples of C-H bond activation are also included15.
Nevertheless, most of these methods suffer from some
limitations such as the need for harsh reaction conditions and
the use of metal catalysts. Jiang and co-workers reported an
efficient Pd-catalyzed three-component cross-coupling
approach for methylthiolation with the combination of green
inorganic sulfur source and methylation reagent16. However,
these reactions need to be carried out at a high temperature
and in the presence of a strong base.
During the past few decades, electrochemical-aided
organic synthesis has attracted continuous interest and
attention because the electro-oxidation or reduction of
substrates can sustainably activate these for further
functionalization without stoichiometric amounts of oxidants or
reductants and in an even more energy-efficient fashion than
photochemical-synthesis17. This increased attention has led to
the discovery of new electrochemical processes including C-H
functionalization reactions18, cross-dehydrogenative coupling
reactions19, C-H activation combined with transition metal
catalysis20, as well as synthetic utilization of electro-generated
N- and O- centered radicals21. Recently, some electro-oxidative
cross-coupling reactions have been applied to realize the C-S
bond construction, such as sulfonylation, thiolation and a series
of oxidative cyclization for the formation of C-S bonds22.
However, electrochemical oxidative cross-coupling reactions to
synthesize aryl-alkyl sulfides is rarely reported. Herein, we
H3CS
O
S
CH3
N
O
OH
OCH3
OCH3
O
SCH3
N
S
HN
O
OCH3
F
Thioridazine
Sulindac
Thiocolchicine
O
O
O
Cl
N
S
H3C
NH
N
N
N
H
O
CH3
O
S
O
S
H3CO
O
O
O
CH3
Sulmazole
Vismodegib
Firocoxib
Figure 1 Examples of methylsulfide-containing drug
Key Laboratory of Coal Processing and Efficient Utilization (Ministry of Education),
China University of Mining and Technology, Xuzhou 221116, Jiangsu, China. E-mail:
ming815zhao@163.com
†Electronic Supplementary Information (ESI) available: See DOI: 10.1039/
x0xx00000x
‡ These authors contributed equally to this work.
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Journal Name
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DOI: 10.1039/D0GC00591F
Previous Works
X
SCH3
S
H
SCH3
SCH3
base
[Pd]
Ar
Ar
Ar
Ar
Ar
+
CH3I or (CH3)2SO4
CH S or CH S
Na
3
Ar
Ar
+
H
3
ref. 12
ref. 13
CH3
O
H
SCH3
X
[Pd]
[Cu]
O
O
S
+
Ar
S
Ac +
+
K
H3C
CH3
ref. 16
ref. 15
DG
DG
O
CH3
This Work
S
CN
H
Mg
Ni
Pt
Ni
SCH3
CH
+
3OH
Ar
S
CH
3OH
Ar
+
K
CN +
Ar
Undivided cell
Undivided cell
Constant current
Constant current
Scheme 1 Methylthiolation strategies for aromatic compounds.
reported a protocol for electrochemical and direct C-H conducted and resulted in an better yield without the
methylthiolation of electron-rich aromatic compounds under supporting electrolyte salt tetrabutylammonium
mild conditions, using potassium thiocyanate (KSCN) as both tetrafluoroborate (n-Bu4NBF4) in this transformation (Table 1,
the supporting electrolyte and sulfur source, and methanol as entry 5). In this case, excess KSCN is used as both a sulfur source
methyl source (Scheme 1). Neither chemical oxidants nor metal and an electrolyte, so the cost of the electrolysis has been
catalysts are required in this transformation. Further, we have greatly reduced. In addition, we observed that the yield of 3aa
also studied the cross-coupling between methanol and the decreased from 80% to 74% when the ratio of 1a to 2a was
intermediate aryl-thiocyanate in the above reaction for more increased from 1:5 to 1:3 (Table 1, entry 6), which probably due
efficient synthesis of aryl sulfide. To the best of our knowledge, to the decrease in conductivity. Electrolysis in methanol-free
this is first success on the synthesis of aryl-alkyl sulfide via solvents, such as CH3CN or aqueous mixtures of CH3CN, was not
electrochemical cross-coupling between aryl-thiocyanate and successful (Table 1, entry 7), indicating the key role of CH3OH in
alcohol.
methylthiolation. And both increasing and decreasing the water
content in CH3OH would lead to decreased reaction yields of the
methylthiolated product 3aa (Table 1, entries 8 and 9).
Moreover, the effects of the electrode materials were also
Results and discussion
At the outset of our investigate on the electrochemical cross-
coupling reaction to construct the C-S bond, thiocyanate salt
was selected as a suitable sulfur source due to its stability and
low-toxicity. Electrolysis of SCN- in acetonitrile solvent can
successfully achieve C-H thiocyanation of electron-rich
aromatics. Interestingly, when we changed the solvent to
methanol in this reaction system, three component coupling
reaction occured between aromatics, methanol, and SCN-, and
aryl-methyl sulfide was obtained as the main product. This
discovery prompted us to conduct in-depth research on the
electrochemical C-H methylthiolation.
Table 1 Optimization of methylthiolation conditionsa
I = 5 mA
Pt (+) | Ni foam (-),
S
CH
3OH
+
K
CN
+
NH
NH
CH3OH/H2O, r. t., 20 h
H
H3CS
1a
2a
3aa
Entry Deviation from standard conditions
Yieldb (%)
Initially, diphenylamine (1a) and KSCN (2a) were chosen as
the model substrates to optimize the reaction conditions (Table
1). Upon optimizing key reaction parameters, the best results
were obtained by performing the electrolysis at room
temperature with a constant current of 5 mA in an undivided
cell equipped with a platinum plate as the anode, a nickel foam
plate as the cathode in the solvent of CH3OH/H2O (volume ratio,
9:1). Under these conditions, the desired product 3aa was
obtained in 80% yield (Table 1, entry 1). Methylthiolation was
totally suppressed without electric current (Table 1, entry 2).
And both increasing and decreasing the constant current would
lead to decreased reaction yields under the same reaction
conditions (Table 1, entries 3 and 4). To our delight, compared
to the case where a supporting electrolyte is required for
general electrochemical synthesis, electrolysis could be
1
2
3
4
None
Without current
3 mA instead of 5 mA, 33 h
7 mA instead of 5 mA, 14 h
2a (3 equiv.), n-Bu4NBF4 (0.1 M) instead of
2a (5 equiv.)
2a (3 equiv.) instead of 2a (5 equiv.)
CH3CN or CH3CN/H2O instead of CH3OH/H2O
CH3OH/H2O (3:1) instead of CH3OH/H2O (9:1) 52
CH3OH instead of CH3OH/H2O (9:1)
80
n.d.
78
64
5
75
6
7
8
9
10
11
12
71
n.d.
69
43
Pt (+) | C (-) instead of Pt (+) | Ni foam (-)
Pt (+) | Fe foam instead of Pt (+) | Ni foam (-) 66
C (+) | Ni foam instead of Pt (+) | Ni foam (-)
73
13
a
Divided cell instead of undivided cell
24
Standard conditions: Undivided cell, Pt anode, Ni foam cathode,
constant current = 5 mA, 1a (1.0 mmol), 2a (5 equiv), CH3OH/H2O (9:1,
10.0 mL), r. t., N2, 20 h. n.d. = not detected. Q = 3.73 F/mol. b Isolated
yields.
2 | J. Name., 2012, 00, 1-3
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investigated. Lower reaction yields were obtained when the derivatives, the methylthiolation took place selectively at C-3
View Article Online
DOI: 10.1039/D0GC00591F
platinum plate (anode) was replaced by the graphite rod (anode) position of indoles. Unsubstituted N-H anilines containing an
or the nickel foam (cathode) was replaced by the iron foam electron withdrawing group (-Cl or phenyl) showed better
(cathode) or graphite rod (cathode) (Table 1, entries 10, 11 and reaction efficiency (products 3aa, 3ab) than those containing an
12). The control experiments have shown that the yield of 3aa electron donating group (-CH3) or without group (products 3ac,
was sharply decreased when the undivided cell was replaced by 3ad). N-alkyl anilines afforded the methylthiolated products in
the divided cell (Table 1, entry 13), and this result indirectly that good yield (products 3ae, 3af), which may be used for further
the cathodic reduction of the intermediate is an indispensable functionalization to synthesize other kinds of important
step in the electrochemical methylthiolation process.
complex molecules. The aryl ethers substrates are relatively
With the optimized conditions in hand, we have tested the difficult to cause oxidation side reactions in electrolysis, so we
scope of this electrochemical methylthiolation strategy have expanded the electrolytic current to 15 mA to improve
applying anilines derivatives and other electron-rich arenes as efficiency,
substrates, and the results are summarized in Table 2. Generally, methylthiolation
and
these
process
substrates
smoothly,
underwent
achieving
the
the
electrochemical methylthiolation occurs at para or ortho corresponding product (3ag-3ai) in good yield. The indole
position of anilines and aryl ethers; in the case of indole
derivatives containing electron-donating or electron-
withdrawing substituents, also afforded the desired product
with moderate yields (products 3aj-3an). Interestingly, pyrrole
and indolazine furnished the corresponding bismethylthiolated
product (3ao-3ap) in moderate yield under the optimized
conditions. It should be noted that when we changed the
methanol in the solvent to ethanol or n-propanol and adjusted
the solvent composition appropriately, the corresponding
products ethyl sulfide and propyl sulfide were also obtained in
moderate to good yields (Table 2, products 3ba-3bg). However,
iso-propyl sulfide was not formed when iso-propanol was used
as a solvent, which may due to the larger steric effect of iso-
propyl group. These experimental results directly indicate that
alcohols are the source of alkyl groups in aryl-alkyl sulfides.
The success of electrochemical methylthiolation of
electron-rich aromatics and the presence of a possible
intermediate thiocyanate indicated in control experiment, led
us to furtherly explore the electrochemical cross-coupling
between aryl-thiocyanate and methanol. As shown in Table 3,
cross-coupling reaction was achieved by using magnesium plate
as sacrificial anode and n-Bu4NBF4 as electrolyte, and the
Table 2 Scope of alkylthiolation of electron-rich aromaticsa
SCH3
H
I = 5 mA
Pt (+) | Ni foam (-),
S
CH
3OH
R
Ar
+
K
CN
+
R
Ar
CH3OH/H2O, r. t., 20 h
2a
1
3
SCH3
Cl
SCH3
SCH3
H2N
N
H2N
H
3ab
3ae
, 71%
SCH3
3ac
, 64%
SCH3
3aa
, 80%
SCH3
N
H
H2N
CH3
N
3ad
3af, 73%
, 59%
, 80%
SCH3
SCH3
SCH3
OCH3
H3CO
H3CO
Cl
OCH3
b, c
c
c
3ag
, 63%
3ah
3ai
, 83%
, 67%
SCH3
SCH3
SCH3
Br
N
H
, 55%
N
N
H
H
Table 3 Scope of cross-coupling between aryl-thiocyanates and
3al
3aj
, 31%
3ak
, 46%
methanola
SCH3
SCH3
SCH3
N
S
SCH3
N
H
CN
F
N
I = 5 mA
Mg (+) | Ni foam (-),
CH
3OH
Ar
R
Ar
+
R
SCH3
n-Bu4NBF4 (0.1 M), CH3OH/CH3CN
3am
, 41%
N
3an
,32%
3ao
,37%
r. t., 20 h
4
3
S
CH3
H3CS
S
CH3
CH3
SCH3
N
SCH3
SCH3
SCH3
Cl
N
d
e
3ba
, 63%
3bb
S
3ap
, 55%
,31%
H2N
N
H2N
H
S
CH3
S
CH3
H3CO
Cl
H2N
3ac
, 66%
SCH3
3aa
3ab
, 91%
, 77%
SCH3
SCH3
OCH3
H3CO
OCH3
d
d
e
3bc
3bd
, 75%
, 70%
3be
, 58%
N
N
H
H2N
CH3
3af
3ae
, 83%
SCH3
, 85%
S
CH3
3ad
S
, 67%
CH3
SCH3
SCH3
OCH3
N
H
Br
N
H
OCH3
H3CO
3ah
e
d
N
3bg
, 47%
3bf
, 68%
H
, 91%
3al
, 58%
3ai
, 93%
a Standard conditions: Undivided cell, Pt anode, Ni foam cathode, constant
current = 5 mA, 1 (1.0 mmol), 2a (5 equiv), CH3OH/H2O (9:1, 10.0 mL), r.
t., N2, 20 h, Q = 3.73 F/mol. Isolated yields shown. b 60 ℃ instead of r. t.
c 15 mA, 10 h instead of 5 mA, 20 h. d EtOH/H2O instead of CH3OH/H2O. e
CH3CN/n-PrOH/H2O (7:2:1) instead of CH3OH/H2O (9:1).
a
Standard conditions: Undivided cell, Mg anode, Ni foam cathode,
constant current mA, (1.0 mmol), n-Bu4NBF4 (0.1M),
CH3OH/CH3CN (1:1, 10.0 mL), r. t., 20 h. Q = 3.73 F/mol. Isolated yields
shown.
=
5
4
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J. Name., 2013, 00, 1-3 | 3
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e-
e-
desired corresponding products could be obtained in good
yields. Compared with electrochemical C-H methylthiolation,
the reductive cross-coupling strategy can well avoid the
possible side electro-oxidation of the substrate at the anode.
The scalability of this approach was evaluated by
performing the electrochemical C-H methylthiolation of
electron-rich aromatics and electrochemical cross-coupling
between aryl-thiocyanate and methanol on a gram-scale. The
electrolysis of 7 mmol 1h and 4h led to 71% and 76% isolated
yield of 3ah, respectively (Scheme 2). The results confirm that
the present protocol can serve as an efficient and practical
strategy to obtain products. Moreover, compared with the
traditional electrochemical approach, the double role of
thiocyanated salt in this approach (as both as sulphur source
and supporting electrolyte) has remarkably reduced the cost of
electrolysis, showing a great potential on industrial applications.
In order to gain insights into the mechanism of the
electrochemical C-H methylthiolation, several control
experiments were carried out. Firstly, we performed the radical
inhibition experiment to verify the existence of SCN radical
intermediates23. Since the oxidation potential of 2, 2, 6, 6-
tetramethyl-1-piperidinyloxy (TEMPO) is higher than SCN-, it
cannot be oxidized before SCN- (see ESI † for the
voltammograms), so it could inhibit SCN radical during
electrolysis22a. When an excess amount of TEMPO was added in
the reaction between 1a and 2a under the standard reaction
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DOI: 10.1039/D0GC00591F
D
1/2 H2
H+
-
S
2
(
CN
+ e-
- 2 e-
base
- HOCN
path 1
S
CN)2
S
CH
3
O
R
N
Ar
NH
SCN-
C
A
path 2
+ e-
CH
+
3OH
S
CN
OCN-, 1/2 H2
S
H
C
R
R
Ar
Ar
- H+
SCH3
R
Ar
B
D
Scheme 4 Postulated reaction pathway.
conditions, methylthiolation were suppressed (Scheme 3a).
These results led us to confirm the presence of SCN radical
intermediates during electrochemical oxidation. In fact,
literature reports have already demonstrated a mechanism via
electrophilic
aromatic substitution
for
the
similar
transformation24. Furthermore, in order to prove that the aryl-
thiocyanate produced by electrolysis is the key intermediate
product of this transformation, divided cell equipped with a
proton exchange membrane are used as reaction vessel. In this
case, aryl-thiocyanate 4a and sulfide 3aa were obtained in 53%
and 24% yields, respectively (Scheme 3b). 4a can also be
converted to sulfide at a lower yield under standard conditions
in divided cell. (Scheme 3c). The result indicates that the
electrolytic reaction on the cathode may significantly promote
the methylthiolation. Moreover, no desired product 3aa could
be obtained when potassium thioacetate 2b was used under
the standard reaction conditions (Scheme 3d).
SCH3
H
I = 15 mA
Pt (+) | Ni foam (-),
CH
(a)
S
+
K
CN
3OH/H2O, r. t., 70 h
H3CO
3ah:
OCH3
H3CO
H3CO
OCH3
2a
1h
(7 mmol)
71%, 0.915 g
SCH3
S
CN
I = 15 mA
Mg (+) | Ni foam (-),
(b)
CH
n-Bu4NBF4 (0.1 M),
3OH/CH3CN
H3CO
3ah:
OCH3
OCH3
(7 mmol)
r. t., 70 h
4h
76%, 0.980 g
On the basis of the above results and previous reports22a,
25, a possible reaction pathway was outlined and is
22c, 24,
described in Scheme 4. Initially, SCN- gets oxidized by a single-
electron-transfer process at the anode to generate the SCN
radical. (SCN)2 could be formed through radical coupling, which
could furtherly generate the electrophile SCN+ A. Subsequent
electrophilic substitution of the substrate and deprotonation to
give the aryl-thiocyanate B. Then, the oxathioimine
intermediate C is obtained through the reaction of aryl-
Scheme 2 Gram-scale experiments.
Pt (+) | Ni foam (-), I = 5 mA
CH
(a)
NH
S
CN
+
K
NH
3OH/H2O, r. t., 20 h
TEMPO (6.0 equiv.)
H3CS
H
3aa,
n. d.
1a
2a
thiocyanate
intramolecular
B
and methanol, which may undergo
rearrangement under electrochemical
Pt (+) | Ni foam (-), I = 5 mA
S
+
K
CN
+
(b)
NH
NH
NH
CH
3OH/H2O, r. t., 20 h
Divided cell
deprotonation to give the desired product D, with the removal
of HOCN. The base formed during electrolysis may also catalyze
the transformation, and the role of water in the solvent may be
to reduce the oxidation potential of the SCN-, thereby avoiding
side reactions such as oxidative polymerization of the
H
H3CS
S
NC
4a,
53%
1a
2a
3aa,
24%
Mg (+) | Ni foam (-), I = 5 mA
NH
NH
(c)
CH
3OH/H2O, r. t., 20 h
Divided cell
substrate22a
.
H3CS
S
NC
4a
3aa, 41%
Conclusions
Pt (+) | Ni foam (-), I = 5 mA
CH
(d)
S
K Ac
+
NH
NH
3OH/H2O, r. t., 20 h
In conclusion, we have disclosed a facile and efficient C-H
methylthiolation under electrochemical redox. Compared with
the previously reported methods for the synthesis of aryl-
H3CS
H
3aa,
n. d.
1a
2b
Scheme 3 Control experiments.
4 | J. Name., 2012, 00, 1-3
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methyl sulfide, this electrochemical-based synthetic strategy
can avoid the usage of external oxidants and transition metal 8.
catalysts. Various electron-rich aromatics exhibited good
efficiencies for this transformation. On the other hand, it has
been demonstrated that the cross-coupling of aryl-thiocyanate 9.
and methanol strategies can also efficiently synthesize
sulfides. Importantly, these reactions can be performed on a
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There are no conflicts to declare.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (Grant 21908241).
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