Electrochemical Direct Thiolation of Lactams with Mercaptans: An Efficient Access to N-Acylsulfenamides

Article abstract of DOI:10.1002/ejoc.202100924

An efficient and eco-friendly electrochemical methodology for the oxidative cross coupling hydrogen evolution (CCHE) reactions of lactams with thiols is presented. Various electron deficient N-acylsulfenamides are smoothly produced in modest to excellent yields without using any external oxidant. Elementary mechanistic insight supports a possible free radical process and hydrogen is the only side product. This approach provides a safe, convenient, and economical preparation of synthetically important N-thiophthalimides on a gram scale.

Full text of DOI:10.1002/ejoc.202100924

European Journal of Organic Chemistry
Accepted Article
Accepted Article
Title: Electrochemical Direct Thiolation of Lactams with Mercaptans: An
Efficient Access to N-Acylsulfenamides
Authors: Zhaoxin Wei, Renjie Wang, Yonghong Zhang, Bin Wang, Yu
Xia, Ablimit Abdukader, Fei Xue, Weiwei Jin, and Chenjiang
Liu
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To be cited as: Eur. J. Org. Chem. 10.1002/ejoc.202100924
Link to VoR: https://doi.org/10.1002/ejoc.202100924
01/2020

10.1002/ejoc.202100924
European Journal of Organic Chemistry
COMMUNICATION
Electrochemical Direct Thiolation of Lactams with Mercaptans:
An Efficient Access to N-Acylsulfenamides
Zhaoxin Wei,[a] Renjie Wang,[a] Yonghong Zhang,[a] Bin Wang,[a] Yu Xia,[a] Ablimit Abdukader,[a] Fei
Xue,[a] Weiwei Jin,*[a] and Chenjiang Liu*[a]
[a] Z. Wei, R. Wang, Prof. Dr. Y. Zhang, B. Wang, Prof. Dr. Y. Xia, Prof. Dr. A. Abdukader, F. Xue, Prof. Dr. W. Jin* and Prof. Dr. C. Liu*
Urumqi Key Laboratory of Green Catalysis and Synthesis Technology, Key Laboratory of Oil and Gas Fine Chemicals, Ministry of Education & Xinjiang Uygur
Autonomous Region, State Key Laboratory of Chemistry and Utilization of Carbon Based Energy Resources, College of Chemistry, Xinjiang University, Urumqi
830046, P. R. China
E-mail: wwjin0722@xju.edu.cn; pxylcj@126.com
† Electronic Supplementary Information (ESI) available: Experimental details, compound characterization, NMR spectra. See DOI: 10.1039/x0xx00000x
Abstract: An efficient and eco-friendly electrochemical methodology
for the oxidative cross coupling hydrogen evolution (CCHE)
reactions of lactams with thiols is presented. Various electron
deficient N-acylsulfenamides are smoothly produced in modest to
excellent yields without using any external oxidant. Elementary
mechanistic insight supports a possible free radical process and
hydrogen is the only side product. This approach provides a safe,
convenient, and economical preparation of synthetically important N-
thiophthalimides in gram scale level.
Generally, traditional methods towards N-acylsulfenamides
are mainly based on the reaction of lactams with unstable
o
sulfenyl chlorides/bromides under bases at 0 C or the reaction
of N-chloro(bromo) lactams with thiols at high temperature
(Scheme 2a-b).[6] Recently, Cu- and Co-catalyzed direct aerobic
oxidative cross dehydrogenative coupling (CDC) of amides with
thios under O2 at 100 oC have been well developed by Jang and
Yang, respectively.[7] Although some remarkable achievements
have been obtained using these strategies, some obvious
drawbacks can not be eliminated such as the involvement of
transition metal catalysts, prefunctionalization of starting
materials, tedious reaction steps, harsh reaction conditions,
limited substrate scope, and utilization of toxic and sensitive Cl2
or Br2. So, the development of clean, robust, environmentally
friendly, and practical routes for the synthesis of N-
acylsulfenamides is still highly desirable.
The selective construction of heteroatom-heteroatom bond,
exactly as the formation of carbon-carbon and carbon-
heteroatom bond, is one of the core topics in current
synthetic organic chemistry.[1] Sulfenamides are a class of
important sulfur-containing organic compounds which
characterized as the possess of N−
S bonds.[2] Among them,
N-acylsulfenamides as one kind of electron deficient
sulfenamides have been prevalent in the field of drug and
pesticidal development, fine chemicals production, and
organic synthesis. For example, Folpet and Captan have
been used as commercial organosulfur pesticides, attribute
to
their
high
antifungal
activities.[3]
N-
(Cyclohexylthio)phtalimide (CTP) and its analogues are
widely employed in rubber industry as anti-scorching agents
to accelerate the vulcanization process.[4] Some N-
thioimides,
(phenylthio)pyrrolidine-2,5-dione,
such
as
N-(phenylthio)phthalimide,
and
1-
N-
(trifluoromethylthio)phthalimide, have been extensively
applied as important and commercial available electrophilic
thiolation reagents to form organosulfur compounds
(Scheme 1).[5]
Scheme 2. General methods for thiolation of lactams.
In recent years, organic electrosynthesis especially the
electrochemical cross coupling hydrogen evolution reaction has
emerged as an ideal alternative to C−H activation, visible light
catalysis, and classical transition metal catalyzed cross coupling
reaction.[8,9] Some impressive advances on the construction of
X−X (X = N, O, S, P etc.)[10] bonds and synthesis of heterocycles
under electrochemical conditions have been developed.[11] With
regard to the electrochemical preparation of sulfenamides, some
pioneered works have been established via the electrochemical
direct N−H/S−H cross coupling. In 2019, Wu group realized the
production of sulfinamides following this tactics with secondary
amines as N−H source, sulfenamides were demonstrated as
reaction intermediates.[12] In the same year, Yang and co-
Scheme 1. Selected and significant N-acylsulfenamide compounds.
1
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10.1002/ejoc.202100924
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COMMUNICATION
workers developed a practical and scalable electrochemical
dehydrogenative coupling of S−H and N−H to sulfenamides
under oxidant and metal free conditions.[13] In 1979, Torii
reported the electrolytic coupling of imides with disulfides to N-
acylsulfenamides in mixed supporting electrolytes for 18-22 h.[14]
To the best of our knowledge, the synthesis of N-
acylsulfenamides through the electrocatalytic direct CCHE
reactions of lactams and thiols has not been explored.
Compared to amines, the nucleophilicity of lactams may
hampered its direct cross coupling with thiols.
Driven by the high significance of N-acylsulfenamides and
our recent success on electrochemical N−P bond formation[15]
and eco-friendly synthetic chemistry,[16] we present herein our
recent advance on the build of N−S bond through coupling
reactions of lactams and thiols under mild electrosynthetic
conditions in an undivided cell with the constant current
electrolysis (Scheme 2c).
used to simultaneously increase the solubility and nucleophilicity
of KBr.17 Among other tested solvents, only DCM could produce
the desired product in a very low yield of 15% (entries 7-10).
Increasing or reducing the dosage of KBr both showed negative
influence to this reaction (entries 11-12). The yield had little
change when the constant current was switched to 7 mA, but it
decreased obviously with the lowing of electricity to 3 mA
(entries 13-14). The best reaction time was determined as 2 h
(entries 15-16). The reaction efficiency was slightly enhanced
through the adjustment of the amount of phthalimide 1a to 1.5
equivalent and the loading of 18-crown-6 to 1.0 equivalent (entry
17). To our delight, this electrolytic reaction proceeded smoothly
under air which remarkably simplified the reaction operation, and
the yield of the desired product 3a was raised to 88% (entry 18).
Table 2. Substrate scope for electrochemical thiolation of phthalimides with
mercaptans.[a,b]]
Table 1. Optimization of the reaction conditions.[a,b]]
Entry
Electrolyte
(eq.)
Solvent
Current
(mA)
t (h)
Yield
(%)[b]
1
2
3
4
5
6[c]
7
8
9
10
11
12
13
14
15
16
17[d,e]
18[d,e,f]
NaBr (1.0)
KBr (1.0)
KI (1.0)
KBr (1.0)
KBr (0)
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeOH
CH2Cl2
DMF
DMSO
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
5
5
5
0
5
5
5
5
5
5
5
5
7
3
5
5
5
5
2
2
2
2
2
2
2
2
2
2
2
2
2
2
3
1
2
2
69
79
32
n.d.
n.d.
16
n.d.
15
trace
trace
68
KBr (1.0)
KBr (1.0)
KBr (1.0)
KBr (1.0)
KBr (1.0)
KBr (1.5)
KBr (0.5)
KBr (1.0)
KBr (1.0)
KBr (1.0)
KBr (1.0)
KBr (1.0)
KBr (1.0)
42
73
53
75
38
84
88
[a] Reaction conditions: 1a (0.4 mmol), 2a (0.2 mmol), electrolyte, 18-crown-6
(0.1 mmol), solvent (4.0 mL), Pt plate (10 mm x 10 mm x 0.1 mm) anode, Ni
gauze (10 mm x 10 mm x 1.0 mm) cathode, the distance between the
electrodes (10 mm), constant current, room temperature, undivided cell, 0.1
MPa N2. [b] Isolated yields. [c] without 18-crown-6. [d] 1a (0.3 mmol). [e] 18-
crown-6 (0.2 mmol). [f] under air.
Initially, the reaction of phthalimide 1a and 4-
chlorobenzenethiol 2a was selected as the model reaction to
screen the optimal electrochemical reaction conditions for the
preparation of N−S bond coupling product 3a (Table 1). The
target product was successfully obtained using inexpensive
alkali metal salt as electrolytes, among which KBr afforded a
satisfactory isolated yield in 79% (entries 1-3). Control
experiments revealed the significance of electricity and
electrolyte, both of them were indispensable factors to enable
this cross coupling reaction (entries 4-5). Crown ethers, which
were widely used as chelating agents, played obvious promotion
to the electrochemical reaction efficiency, the yield of 3a
decreased dramatically from 79% to 16% in the absence of 18-
crown-6 (entry 6). It enabled the high reaction efficiency and was
[a] Reaction conditions: 1 (0.3 mmol), 2 (0.2 mmol), KBr (0.2 mmol), 18-crown-
6 (0.2 mmol), MeCN (4.0 mL), Pt plate (10 mm x 10 mm x 0.1 mm) anode, Ni
gauze (10 mm x 10 mm x 1.0 mm) cathode, the distance between the
electrodes (10 mm), 5 mA, room temperature, 2 h, undivided cell, under air. [b]
Isolated yield. [c] Phenyl disulfide (0.1 mmol). [d] 2-Thienyldisulfide (0.1 mmol).
[e] Benzyl disulfide (0.1 mmol).
As shown in Table 2, the generality of phthalimides 1 and
arylthiols 2 was then primarily explored in the light of the
optimized reaction conditions has been established. In principle,
a
sequence of thiols with different electronic and steric
2
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10.1002/ejoc.202100924
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COMMUNICATION
properities were smoothly transformed into their corresponding
cross dehydrogenative coupling products under electrochemical
conditions in moderate to excellent yields. When monohalogen
thiophenols substituted with electron-deficient F, Cl, and Br at C-
2 or -4 positions of the phenyl ring were used as starting
materials, the desired products were obtained with up to 97%
isolated yields (3a−3f). Polyhalogeno atoms substituted
substrates underwent the cross coupling reaction with
phthalimide successfully to yield the arylthiolation product 3g
and 3i in 91 and 56% yields, respectively. The introduction of
strong electron withdrawing trifluoromethyl group on the phenyl
ring slightly deteriorated the reaction efficiency, a 70% yield of
3h was obtained. Considering the foul smell of thiophenol,
phenyl disulfide was selected to instead of the electroneutral
thiophenol and the commercial available electrophilic thionating
reagent 3j was afforded in good yield. Aryl mercaptan with
electron-donating groups such as methyl, iso-propyl, tert-butyl,
methoxyl, and methylthio were viable starting materials,
furnishing the corresponding products 3k−3r in 71-95% yields.
Intriguingly, heteroaromatic and aliphatic disulfide compounds
also could be involved in this reaction and generated the direct
arylthiolation products 3s and 3t in quite good yields. The
phthalimides with electron-withdrawing groups underwent this
reaction smoothly to assemble their respective coupling
products 3s and 3t.
but a variety of N−S coupling products 5a−5p were also
smoothly assembled in satisfactory yields ranging from 30% to
85%. Arylthiols with Cl, F, Me, iso-propyl, MeO, and MeS
substituents on the aromatic ring were compatible, leading to the
desired products in moderate to good yields (5a−5i). The steric
effect of arylthiols had little influence to this reaction (5d−5f). For
substituted isatin derivatives, they were reacted well with various
electrical thiol substrates to produce their respective products
5j−5p in moderate to good yields.
Scheme 3. Gram scale reactions.
By slightly adjust partial reaction parameters and keep other
conditions unaltered, the model reaction could be easily scaled
up to gram scale and kept the similar reaction efficiencies
(Scheme 3), which demonstrated that this metholodogy has
good practicability, high dependability, and potential
amplification manufacturing capability on a much bigger scale.
Table 3. Substrate scope for electrochemical arylthiolation of isatins with
mercaptans.[a,b]
[a] Reaction conditions: 1 (0.3 mmol), 2 (0.2 mmol), KBr (0.2 mmol), 18-
crown-6 (0.2 mmol), MeCN (4.0 mL), Pt plate (10 mm x 10 mm x 0.1 mm)
anode, Ni gauze (10 mm x 10 mm x 1.0 mm) cathode, the distance between
the electrodes (10 mm), 5 mA, room temperature, 2 h, undivided cell, under air.
[b] Isolated yield.
Scheme 4. Control experiments.
Control experiments were exerted to shed some light on the
mechanism of this electrochemical cross coupling reaction. A
possible radical process was preliminary demonstrated by the
radical trapping experiments employing TEMPO or BHT
(Scheme 4a). Under standard conditions, 4-chlorothiophenol 2a
Subsequently, the direct cross dehydrogenative coupling of
isatins and aryl mercaptans under this electrochemical strategy
was also studied, as illustrated in Table 3. On the whole, the
reaction activities of isatins 4 were lower than phthalimides 1,
3
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10.1002/ejoc.202100924
European Journal of Organic Chemistry
COMMUNICATION
Scheme 5. Proposed mechanism.
was transformed to disulfide 6 in almost quantative yield
(Scheme 4b). Treatment of 1a with
6
underwent the
arylthiolation reaction to produce the product 3a in 93% yield
(Scheme 4c). The intermediates 2-bromoisoindoline-1,3-dione 7
and 4-chlorophenyl hypobromothioite 8 were converted into 3a
in 92% and 53% yields, respectively (Scheme 4d-e). These
reaction shows that compounds 6, 7, and 8 are possible reaction
intermediates for this electrochemical cross coupling reaction.
According to our research findings mentioned above and
relevant reports,[14,18] a plausible mechanism is proposed as
shown in Scheme 5. Bromide anion is firstly oxidized at anode to
Br2, which reacts with nitrogen anion intermediate I generated
through the cathodic reduction of 1a to form the intermediate 7.
In the meanwhile, 2a is reduced to sulfur radical intermediate II,
which dimerizes to the disulfide 6. The interaction of 6 with Br2
could produce the intermediate 8. Subsequently, one way is the
cross coupling of 7 with 6 to deliver the target product 3 (red
arrow). Another way is the nucleophilic attack of 8 by the
intermediate I (blue arrow). H2 is releasd as the byproduct
through the reduction of proton at cathode.
In conclusion, we have reported a practical and ideal
electrocatalytic method for the construction of electron deficient
N-acylsulfenamides via direct N−H/S−H cross coupling hydrogen
evolution reaction which avoids the use of preactivation
substrates and noxious chlorine gas or liquid bromine. The
advantages of this protocol including commercial available raw
materials, mild reaction conditions, broad substrate scope, good
compatibility of functional groups, and handy operation. Notably,
gram scale reactions indicate good economics and efficiency for
large scale production of the commercialized and widely used N-
(arylthio)phthalimides and their analogues.
Figure 1. Cyclic voltammetry of 1a, 2a and their mixtures in CH3CN with KBr
(0.05 M) and 18-crown-6 (0.05 M) under air. The glassy carbon-disk (R = 5.5
mm, h = 10 mm) was used as the working electrode. The Pt disk (R = 5.5 mm,
h = 10 mm) and Ag/AgCl (R = 5.0 mm, h = 10 mm) was used as counter and
reference electrode, respectively. The scan rate was 100 mV/s.
Experimental Section
General Procedure for the Synthesis of 3a: Under air, a mixture of
isoindoline-1,3-dione 1a (44 mg, 0.3 mmol), 4-chlorobenzenethiol 2a (29
mg, 0.2 mmol), KBr (24 mg, 0.2 mmol), 18-crown-6 (53 mg, 0.2 mmol)
and MeCN (4.0 mL) were added in an oven-dried undivided bottle (10
mL). The bottle was equipped with Pt plate (10 mm x 10 mm x 0.1 mm)
as anode and Ni gauze (10 mm x 10 mm x 1.0 mm) as cathode, the
distance between the electrodes was 10 mm. The resulting mixture was
stirred and electrolyzed at a constant current of 5 mA at ambient
temperature for 2 h. When the reaction was finished, the resulting
Next, we conducted cyclic voltammetry (CV) experiments to
find the redox potentials of the reactants and their mixtures
(Figure 1). The potassium bromide had an oxidant peak at 0.06
V (red curve), it indicated that bromide ion could be oxidized at
anode. A reduction peak of 1a in acetonitrile was observed at -
1.28 V (blue curve), it suggested that 1a was reduced at cathode. mixture was evaporated under reduced pressure. The resultant residue
was purified by silica gel column chromatography (eluent: petroleum
At the same time, an oxidation peak of 2a could be observe at
0.57 V (pink curve). When 1a and 2a were added together
(green curve), a remarkable increase in the catalytic current and
ether (60-90 oC)/EtOAc = 10:1, v/v) to afford the desired product 3a as a
white solid (51 mg, 88% yield).
the new oxidation peak at 0.59 V were observed, reflecting the
chemical interaction between these two compounds.
Acknowledgements
We are grateful to the National Natural Science Foundation of
China (Grant Nos. 21572195, 21702175, 21961037 and
22061040) for support of this research.
Keywords: lactams • mercaptans • thiolation • N-
acylsulfenamides • N−S bonds
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European Journal of Organic Chemistry
COMMUNICATION
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5
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COMMUNICATION
Entry for the Table of Contents
A variety of N-acylsulfenamides are produced by the electrochemical enabled cross coupling of readily available feedstocks under
standard conditions. This protocol is practical and has wide substrate scope with good reaction efficiency (38 examples, up to 97%
yield). A possible free radical mechanism is preliminarily demonstrated.
6
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