Electrochemical synthesis of sulfinic esters from alcohols and thiophenols

Article abstract of DOI:10.1016/j.tetlet.2020.151631

Electrochemical oxidative couplings between S[sbnd]H and O[sbnd]H bonds are achieved herein directly from readily-available alcohols and thiophenols, affording a series of diverse sulfinic esters. This strategy can take advantage of 6 equivalents of alcohol, relative to thiophenol, to achieve moderate to good yields, without the assistance of any metallic catalysts, bases, and additional oxidants.

Full text of DOI:10.1016/j.tetlet.2020.151631

Journal Pre-proofs
Electrochemical Synthesis of Sulfinic sters from Alcohols and Thiophenols
Yang He, Jinli Zhang, Liang Xu, Yu Wei
PII:
S0040-4039(20)30044-7
DOI:
https://doi.org/10.1016/j.tetlet.2020.151631
TETL 151631
Reference:
Tetrahedron Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
19 November 2019
9 January 2020
13 January 2020
Please cite this article as: He, Y., Zhang, J., Xu, L., Wei, Y., Electrochemical Synthesis of Sulfinic sters from
Alcohols and Thiophenols, Tetrahedron Letters (2020), doi: https://doi.org/10.1016/j.tetlet.2020.151631
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Graphical Abstract
Electrochemical Synthesis of Sulfinic Esters
from Alcohols and Thiophenols
Leave this area blank for abstract info.
Yang He, Jinli Zhang, Liang Xu, Yu Wei
O
R
S
S
H
O
Ar1
Ar1
H
+
OR
catalyst free
base free
additional oxidant free
21 examples, up to 81% yield
Taking advantage of the electrochemical method, the synthesis of sulfinic esters from alcohols and thiophenols has been
achieved, under catalyst, base, and oxidant-free conditions.

1
Tetrahedron Letters
journal homepage: www.elsevier.com
Electrochemical Synthesis of Sulfinic Esters from Alcohols and Thiophenols
Yang Hea, Jinli Zhang a,b, *, Liang Xu a, *, Yu Wei a, *
a School of Chemistry and Chemical Engineering/Key Laboratory for Green Processing of Chemical Engineering of Xinjiang Bingtuan, Shihezi University, Shihezi, 832003, China.
b Key Laboratory for Systems Bioengineering MOE, Tianjin University; Collaborative Innovation Center of Chemical Science and Chemical Engineering (Tianjin), Tianjin 300072, PR
China
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
Electrochemical oxidative couplings between S–H and O–H bonds are achieved herein directly
from readily-available alcohols and thiophenols, affording a series of diverse sulfinic esters. This
strategy can take advantage of 6 equivalents of alcohol, relative to thiophenol, to achieve
moderate to good yields, without the assistance of any metallic catalysts, bases, and additional
oxidants.
Available online
Keywords:
Electrochemical synthesis
Sulfinic esters
Thiophenols
Organosulphur compounds
S
S
Sulphur-containing structural units are widely present in
synthetic intermediates, natural products and bioactive
molecules, such as agrochemicals and pharmaceuticals.1
Therefore, the development of approaches to construct and
transform the sulphur-containing skeleton has always been
an attractive research topic of academic and industrial
interest in organic chemistry. In particular, among the
established strategies to build up the S–C and S–X (X = N,
O, P, S) bonds, the direct oxidative dehydrogenative
coupling between S–H bonds in thiols/thiophenol and C–H
or X–H bonds may represent the most straightforward one,
since such processed bypass the incorporation of leaving
groups and require no pre-functionalization steps.2 However,
the requirement of the oxidants also renders the control of
sulphur’s oxidation state a tough nut to crack, since
thiols/thiophenols are easily over-oxidised in the presence of
oxidants. In the pursuit of more efficient approaches to
realize the oxidative coupling of S–H bonds,
electrochemical anodic oxidation has proved its utility and
received increasing attention.
(a)
H
R1
Ar1
H
Ar1
+
R1
Ar3
N
H
H
H
H
NHR2
=
Ar2
R1
R3
CO2Et
O
O
S
R1
S
(b)
R1
H
N
R2
H
Ar1
Ar1
N
+
R2
O
O
S
S
S
R
(c)
(d)
H
H
H
H
R
S
Ar1
Ar1
Ar1
Ar1
+
+
S
S
OR
OR
OR
OR
P
P
O
O
Scheme 1. Electrochemical dehydrogenative oxidative couplings of
thiophenols.
More specifically, when it comes to electrochemical
Organic electrosynthesis has been developed for a long
history and is currently gaining its renaissance under the
background of pursuing greener synthetic strategies.3 It is
considered to be environmentally friendly because it
provides an environment where electrons can interact with
the nucleus directly, thus avoiding the use of chemical
oxidants or reductants and reducing waste and pollution.4
Furthermore, such an electrochemical process can be
precisely controlled via the variation of voltage or current.
transformations of S–H bonds,
a series of fruitful
achievements have been made in the past three years. As
shown in Scheme 1, starting from thiophenols,
dehydrogenative oxidative couplings between thiophenols
(S–H) and (hetero)arenes (C–H, Scheme 1a),5 enamines (C–
H, Scheme 1a),6 acetonitrile (C–H, Scheme 1a),7 amines
(N–H, Scheme 1b),8 thiols (S–H, Scheme 1c),9 thiophenols
(S–H, Scheme 1c)10, aryl sulfinic acids (Scheme 1c)11 or
phosphonate (P–H, Scheme 1d)12 have been achieved with
the assistance of electricity.13 Generally, no additional
oxidants or catalysts were required for such transformations,

2
and S–H bonds could be cleaved with retention of
respectively. Furthermore, changing the Bu4NCl to another
commonly used electrolyte Bu4NClO4 induced 17% less
yield (Entry 3). The test of other electrolytes also did not
give better results. Stronger or weaker electrical currents
were both detrimental to the reaction (Entry 5, 6, 7).
Increasement of the current to 10 mA induced 10% less
yield, however, further increment to 14 mA would
dramatically reduce the yield to 16%, which might be
attributed to the rising side reactions caused by the strong
current. As for electrodes, Pt/C and C/C electrode pairs gave
49% (Entry 9) and 55% yield (Entry 11) of product,
respectively.
sulphur’s oxidation state. More recently, the following
oxidation of sulphur atoms was also realized under the
electrochemical environment, which could merge well with
the foregoing dehydrogenative couplings. For instance,
sulfonamides (Scheme 1b)14 and thiosulfonates (Scheme
1c)10 could be obtained from two consecutive oxidations of
sulfonamide and disulfides that were in situ generated via
oxidative couplings of S–H bonds in the same vials,
respectively. Despite such progress, electrochemical
oxidative couplings between S–H bonds of thiophenols and
O–H bonds of alcohols have not been disclosed, to the best
of our knowledge.
Table 1. Optimization of reaction conditions[a]
The disclosed metal-catalysed approaches
+
(1 equiv)
H
RO
CuI (5 mol %), TBD (10 mol %)
2 (1 atm), THF, 65 o
O
S
O
S
R
S
H
S
H
Ar1
O
O
H
O
O
C
+
Ar1
undivided cell
constant current
Me
(3 equiv)
Me
1a
3a
2a
+
(solvent)
H
RO
Solvent Electrolyte Current Yields
Co/N-SiO2-AC (1.46 mol %)
2a
Anode
O
S
Entry
(%)[b]
None
S
H
(mA)
(equiv) /cathode
K2CO3 (20 mol %)
R
R
Ar1
O
O
O
2 (1 atm), 60-80 o
C
6
6
6
6
6
6
6
6
6
6
6
4
8
Pt/Pt
Pt/Pt
Pt/Pt
Pt/Pt
Pt/Pt
Pt/Pt
Pt/Pt
Pt/Pt
Pt/C
DMF
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
Bu4NCl
PivONa
Bu4NClO4
AcOK
10
Ar1
1
2
(1 equiv)
10
10
10
10
14
4
41
50
Our proposal
3
+
RO
H
O
S
S
Trace
57
4
H
Ar1
Ar1
Bu4NCl
Bu4NCl
Bu4NCl
Bu4NCl
Bu4NCl
Bu4NCl
Bu4NCl
Bu4NCl
Bu4NCl
5
(i) Formation of S-O bonds ?
(ii) Oxidation of S atoms ?
16
6
43
7
Scheme 2. Dehydrogenative oxidative couplings of thiophenols
and alcohols.
6
67
8
6
49
9
On the other hand, as an important class of organosulphur
compounds, sulfinic esters with umpolung reactivity can
serve as both electrophiles and nucleophiles in organic
synthesis to access other types of sulphur-containing
molecules.15 Therefore, their synthesis also received
considerable attention. Traditionally, sulfinic esters can be
prepared from sulfinyl halides,16 sulfinic acids,17 sodium
sulfonates,18 sulfonyl hydrazides or disulfides.19 More
recently, the direct oxidative couplings of thiophenols and
alcohols have been realized to afford sulfinic esters via two
independent metal-catalysed reaction systems (Scheme 2).20
With this in mind, we speculate it feasible to access sulfinic
esters via electrochemical oxidative couplings of the same
two commodity chemicals. Such transformations would
avoid the use and preparation of catalysts and would be
particularly useful, given the inexpensive and readily
available starting materials and electricity. However, as
mentioned above, electrochemical oxidative couplings of S–
H/O–H bonds remained unknown at present.21 In addition, a
suitable control of oxidation state of the s ulphur atoms to
suppress the over-oxidised by-products is also key to the
success of such transformations. Herein, we report our
preliminary efforts on this subject.
We commenced the studies using 4-methylthiophenol (1a)
and benzyl alcohol (2a) as reaction components (Table 1).
After extensive experiments, 67% of target sulfinic ester 3a
could be obtained, when 1a and 6.0 equivalents of 2a were
stirred in aerobic MeCN for 10 hours at room temperature,
with the constant current being 6 mA, Pt/Pt as the electrodes
and Bu4NCl as the electrolyte (Entry 8). Variants from
these parameters more or less decreased the yield of 3a.
Increasing or reducing the amount of 2a by 2.0 equiv gave
46% (Entry 13) and 58% (Entry 12) yields of 3a,
Pt/RVC
C/C
6
20
10
11
12
13
6
55
Pt/Pt
Pt/Pt
6
58
10
46
[a] Reaction conditions of Entry 8: Pt/Pt electrodes (10 mm × 10 mm
× 0.20 mm), constant current = 10 6 mA, 0.4 mmol of 1a (1.0 equiv),
2.4 mmol of 2a (6.0 equiv), Bu4NCl (0.15 mmol) in MeCN (2.0 mL),
rt, air, 10 h. Current efficiency, 24%. [b] Isolated yield.
With the optimized reaction conditions in hand, we further
explored the substrate scope of this electrochemical
transformation (Scheme 3). Thiophenols with different
substituents were treated with 6.0 equiv of benzyl alcohol at
first. The electron-donating group on thiophenols rendered
better yields, since methoxy group substituted thiophenol
(3e, 72%) gave a better yield than the methyl (3a, 67%; 3b,
50%), fluorine (3d, 60%) or chlorine (3c, 52%) substituted
ones. Then, variation from benzyl alcohol was explored. It
seemed that the electronic property of this counterpart had a
limited effect on the outcome of the reaction. The
application of 4-trifluoromethyl benzyl alcohol in this
transformation resulted in 70% yield of the corresponding
product 3h. Fluorine and chlorine were also suitable
substituents for the benzyl alcohol, affording 3f and 3i in
65% and 60% yields, respectively. The compatibility with
the halides on both of the counterparts, especially chlorine
(3c, 3i), provided the opportunity for the further elaboration
of the obtained sulfinic esters. Several other types of
alcohols were also competent starting materials for this
transformation, resulting in the formation of the
corresponding sulfinic esters in approximately 60% yields
(3q and 3t). This list included the cyclic secondary alcohol

3
o
cyclohexanol (3q), primary alcohols with appending phenyl
group 2-phenylethan-1-ol (3r) and 3-phenylpropan-1-ol (3s)
and long chain primary alcohol octan-1-ol (3t). It should be
noted that all the above-mentioned reactions were performed
with 6.0 equiv of alcohol involved.
heating the reaction mixture at 100 C for 6 hours would
afford thiosulfonate 7 in 62% yield.
Me
O
a)
S
EtOH
S
O
Me
+
N
90 °C, 12 h
H
Me
N
4
, 75%
3k
H
O
Pt (+) I Pt (-) = 6 mA
S
R
S
OMe
R
H
H
+
O
Ar
O
Ar
Bu4NCl (0.15 mmol)
MeCN (2.0 mL), rt, 10 h
undivided cell
O
b)
O
S
AlCl3
S
Me
OH
O
OH
+
DME, rt, 26 h
O
S
O
S
MeO
c)
O
3j
5
, 72%
Me
S
O
O
O
O
S
O
S
Cl
Me
Me
Me
CsF
Me
3a, 67%
O
3c, 52%
3b, 50%
CF3
TMSCF3
+
100 °C, 12 h
MeO
d)
MeO
3j
6
, 45%
OMe
O
O
O
S
S
S
Pt (+) I Pt (-) = 6 mA
O
O
O
O
S
SH
MeO
F
Bu4NCl
F
100 °C, 6 h
MeO
S
O
3d, 60%
O
3e, 72%
O
3f, 65%
O
MeOH, rt, 10 h
undivided cell
MeO
7
, 62%
Scheme 4. Derivatization of the obtained sulfinic esters to other
S
S
S
Cl
O
O
O
sulphur-containing structures
Me
CF3
Me
3g, 53%
Me
3h, 70%
3i, 60%
To gain more insight into the reaction mechanism, several
control experiments were performed (Scheme 5a). First, the
formation of 3a or 3k was obstructed after addition of 1.0
equiv of TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl) to
the standard standard reaction conditions, which would lead
to 3a or 3k, obstructed the formation of target products.
Then, the direct use of 1,2-diphenyldisulfane in the reaction
with ethanol resulted in the formation of sulfinic ester 8 in
45% yield, which implied the involvement of disulfane in
the reaction mechanism. Interestingly, under the same
conditions, the interaction between 1,2-diphenyldisulfane
and methanol would generate sulfonate 9, the product that
might derive from further oxidation of the sulfinic ester. To
demonstrate this, a reaction between 4-methoxybenzenethiol
and ethanol/methanol was performed under O2 atmosphere.
Such change resulted in sulfonate 10 and 11 in 35% and
40% yields, separately, which also demonstrated the key
role of O2 in the oxidation of sulphur atom.25
O
S
O
S
O
S
Me
O
Me
O
Me
Me
O
MeO
MeO
Me
MeO
3j, 65% [1]
3k, 65% [1]
3l, 75% [1]
O
O
S
Me
O
S
Me
Me
S
Me
O
O
O
MeO
MeO
Me
Cl
3m, 48% [1]
3n, 63% [1]
O
3o, 81% [1]
O
O
S
Me
S
S
O
O
O
Me
3p, 58% [1]
O
3r, 57%
O
3q, 63%
O
S
S
Me
S
7
Me
O
O
O
N
MeO
Cl
Me
3s,61%
3u, 55% [1]
3t, 63%
Scheme 3. Substrate scope of the electrochemical oxidative
couplings of alcohols and thiophenols. [1] alcohol as the solvent.
O
a)
Pt (+) I Pt (-) = 10 mA
S
When it came to the short chain alkyl alcohols, such as
methanol and ethanol, which were much cheaper and more
available than the long chain alcohols and benzyl alcohol,
the direct application of the above-mentioned conditions
resulted in limited success. A brief further optimization was
carried out (see Supporting Information for details). It
turned out that using these alcohols as the reaction solvents
could gave give comparative yields, relative to the above
reaction system. Based on this, several commonly used
alcohol solvents were applied in the reaction, affording the
products in 48-81 yields. Similarly, in these cases, electron-
donating group on thiophenols rendered better yields (3k, -
Me, 65% vs. 3l, -OMe, 75%; 3m, -Me, 48% vs. 3n, -OMe,
63%; 3o, -OMe, 81% vs. 3p, -Cl, 58%). As anticipated,
secondary alcohol gave less product than the primary ones,
probably due to the bulky steric effect during approaching
the coupling counterpart.
O
Me
S
S
+
Me
OH
Bu4NCl (0.15 mmol)
rt, 10 h
undivided cell
8
, 45%
O
S
Pt (+) I Pt (-) = 10 mA
Me
S
O
Me OH
S
+
O
Bu4NCl (0.15 mmol)
rt, 10 h
undivided cell
9
, 40%
O
S
Pt (+) I Pt (-) = 6 mA
SH
R
+
R
OH
O
O
Bu4NCl (0.15 mmol)
O2, 20 h
undivided cell
MeO
b)
MeO
10
11
, R = Et, 35%
, R = Me, 40%
anode
Ar
Ar
O
ArSH
ArS
S
S
S
S
Ar
Ar
B
A
D
C
Taking advantage of the obtained sulfinic esters, other
sulphur-containing structures could be readily accessible
(Scheme 4). When 3k was treated with indole in ethanol,
indole thioether 4 was isolated in 75% yield.22 Diaryl
sulfoxides 5 could be prepared by virtue of the Friedel-
Crafts type reaction between 3j and 2-naphthol, in 72%
yield.23 Trifluoromethyl sulfoxide 6 could also be obtained
via the reaction between 3j and TMSCF3.24 In addition, after
the preparation of 3j, simple removal of the electrodes and
O
S
ROH
Ar
OR
Scheme 5. A proposed reaction mechanism.
Based on the experimental results and previous literature
reports.5-14 A preliminary reaction mechanism is proposed in
Scheme 5b. The thiol radicals B, which could be generated
via oxidation and deprotonation of thiophenol A on the

4
anode, dimerize to form disulfane C. The following
providing direct access to sulfinic esters in moderate to good
yields. This electrosynthesis process bypassed the use of any
metallic catalyst, base, and additive oxidant and was able to
take advantage of 6.0 equiv of alcohol as the coupling
counterpart. Detailed mechanism exploration and further
derivation of this electrochemical system are on the way in
our lab.
oxidation of C by O2 would lead to the generation of
thiosulfinates D, which would go through the nucleophilic
substitution process with alcohol to access target product. In
the meanwhile, the regenerated thiophenol should take part
in the next reaction cycle.
In conclusion, the electrochemical oxidative coupling
between alcohols and thiophenols has been achieved,26
Y. Li, Q. Yang, L. Yang, N. Lei and K. Zheng, Chem. Commun.,
2019, 55, 4981.
Acknowledgments
13. G. Laudadio, A. d. A. Bartolomeu, L. M. H. M. Verwijlen, Y.
Cao, K. T. de Oliveira and T. Noël, J. Am. Chem. Soc., 2019,
141, 11832.
We thank the financial support by the National Natural
Science Foundation of China (No. 21602141), Shihezi University
(funding to Y. W., CXRC201601).
14. G. Laudadio, E. Barmpoutsis, C. Schotten, L. Struik, S. Govaerts,
D. L. Browne and T. Noël, J. Am. Chem. Soc., 2019, 141, 5664.
15. A. Tapia-Pineda, C. Perez-Arrieta, C. Silva-Cuevas, E. Paleo and
J. A. Lujan-Montelongo, J. Chem. Educ., 2016, 93, 1470.
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Wu, Adv. Synth. Catal., 2015, 357, 1393; (d) L. Kadari, P. Radhaꢀ
Krishna and Y. LakshmiꢀPrapurna, Adv. Synth. Catal., 2016, 358,
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17. J. Drabowicz, M. Kwiatkowska and P. Kiełbasiński, Synthesis,
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The following files are available.Compound characterization,
1H and 13C NMR spectra (PDF).
☐The authors declare the following financial
interests/personal relationships which may be
considered as potential competing interests:
Declaration of interests
☒ The authors declare that they have no known
competing financial interests or personal
relationships that could have appeared to influence
the work reported in this paper.

Graphical Abstract
Highlights
Electrochemical Synthesis of Sulfinic Esters
from Alcohols and Thiophenols
Leave this area blank for abstract info.
Yang He, Jinli Zhang, Liang Xu, Yu Wei
O
S
R
S
H
O
Ar1
Ar1
H
+
OR
catalyst free
base free
additional oxidant free
21 examples, up to 81% yield
Taking advantage of the electrochemical method, the synthesis of sulfinic esters from alcohols and thiophenols has been
achieved, under catalyst, base, and oxidant-free conditions.
• An electrochemical approach to the synthesis of
sulfinic esters.
• The reaction starts from readily available alcohols
and thiophenols.
• The process is feasible without the assistance of
any metallic catalysts, bases, and additional
oxidants.