Redox Activation of Hydrogen Sulfide, Thiols, and Sulfur in Electrosynthesis of Organic Di- and Polysulfides

Article abstract of DOI:10.1134/S107036321904008X

A novel and efficient method for the synthesis of biologically active organic di-, tri- and tetrasulfides has been proposed. Different methods of redox activation of sulfur, hydrogen sulfide, and thiols in the reactions with organic compounds have been considered. Electrochemical initiation of the reactions of the mediator–H₂S–S₈ system with cyclohexane, methylcyclohexane, and benzene has occurred to the formation of polysulfides R₂S_n (n = 2–4). The application of tetrabutylammonium bromide as a mediator of H₂S oxidation has allowed to decrease the anodic overpotential of electrosynthesis. Di- and tetrasulfides have been obtained under anodic activation of the cycloalkanethiols (C₅, C₆) or thiophenol in the reaction with sulfur. Electroreduction of S₈ in the presence of the same thiols has favored the formation of di- and trisulfides. The yield and the ratio of the R₅S_n (n = 2–4) depend on the method of redox activation of the thiolating reagent.

Full text of DOI:10.1134/S107036321904008X

ISSN 1070-3632, Russian Journal of General Chemistry, 2019, Vol. 89, No. 4, pp. 689–696. © Pleiades Publishing, Ltd., 2019.
Russian Text © The Authors(s), 2019, published in Zhurnal Obshchei Khimii, 2019, Vol. 89, No. 4, pp. 552–560.
Redox Activation of Hydrogen Sulfide, Thiols, and Sulfur
in Electrosynthesis of Organic Di- and Polysulfides
a
a
a
a
Е. V. Shinkar’ , I. V. Smolyaninov , V. V. Kuzmin , and N. Т. Berberova *
a
Astrakhan State Technical University, ul. Tatishcheva 16, Astrakhan, 414056 Russia
*
e-mail: nberberova@gmail.com
Received November 15, 2018; revised November 15, 2018; accepted November 22, 2018
Abstract—A novel and efficient method for the synthesis of biologically active organic di-, tri- and
tetrasulfides has been proposed. Different methods of redox activation of sulfur, hydrogen sulfide, and thiols in
the reactions with organic compounds have been considered. Electrochemical initiation of the reactions of the
mediator–H
polysulfides R
has allowed to decrease the anodic overpotential of electrosynthesis. Di- and tetrasulfides have been obtained
under anodic activation of the cycloalkanethiols (С , C ) or thiophenol in the reaction with sulfur.
Electroreduction of S in the presence of the same thiols has favored the formation of di- and trisulfides. The
yield and the ratio of the R (n = 2–4) depend on the method of redox activation of the thiolating reagent.
2
S–S
8
system with cyclohexane, methylcyclohexane, and benzene has occurred to the formation of
2
S
n
(n = 2–4). The application of tetrabutylammonium bromide as a mediator of H S oxidation
2
5
6
8
2 n
S
Keywords: electrosynthesis, organic polysulfides, hydrogen sulfide, cycloalkanethiols, elemental sulfur, redox
activation
DOI: 10.1134/S107036321904008X
Organic di- and polysulfides are widely used in
various fields of industry, especially in food and pharma-
ceutical industry, as potential antifungal, antibacterial,
and antitumor compounds [1–4]. Polysulfides R2Sn
of di-, tri-, and tetrasulfides with various substituents is
an topical issue.
As far as the design of biologically active organic
sulfur compounds is concerned, special attention has
been paid to the methods of the C–S and S–S bonds
formation [8, 9]. The thiolation reactions proceed via
(n = 2–4) are platform compounds to design drug
molecules. High biological activity of organic
polysulfides is related to the possibility of the H2S
formation. Therefore these compounds are often used
as a promising source of the endogenous hydrogen
sulfide (pro-drugs). Recently, hydrogen sulfide has
been recognized as a biologically important molecule
involved in various physiological processes [5, 6].
Recent studies have revealed the ability of hydrogen
sulfide to actively participate in the signal transmission
between the cells in living organisms and to regulate
some processes; hence, it can be considered a gas
transmitter [7]. One of priority directions in the
chemistry of natural and synthetic compounds is the
development of novel approaches to the synthesis of
compounds acting as hydrogen sulfide donors.
Interestingly, there is a correlation between the
structure of R S and the rate of evolution of H S via
3
the activation of the C(sp )–H bond catalyzed by metal-
containing compounds [10, 11] or di-tert-butyl-
peroxide radical initiation [12–14]. Organic tri- and
tetrasulfides have been also prepared via the reaction
of alkyl(aryl)thiols with sulfur dichloride in the
presence of bases in nonpolar solvents [15]. Many
successful examples of the synthesis of di- and
polysulfides have been reported under the conditions
of thermal activation of sulfur have been reported. For
example, symmetrical polysulfides have been formed
in the reaction of sulfur with diallyl disulfide at the
temperature of S
activation of the S
melting (115–120°С) [16]. Thermal
–H S system at 120–180°С is
8
8
2
efficient in the synthesis of organosulfur compounds
due to generation of hydropolysulfide and thiyl
radicals in the melt [17, 18]. Also, the electrochemical
method for the preparation of a mixture of organic
2
n
2
decomposition of organic polysulfides, which should
be considered in the design of drugs. Therefore,
development of efficient ap-proaches to the synthesis
sulfides R S (n = 2–4) at room temperature is known,
2 n
6
89

6
90
SHINKAR’ et al.
based on the reaction of alkyl(aryl)thiols with anode-
generated sulfur dication (2.2 V) [19].
Scheme 1.
+e
−
Br
Pt,
Br
We have earlier proposed an electrochemical
S
−
e
8
HS
HS9
method of the preparation of R S (n = 2–4) based on
H2S
H2S
H+
2
n
ο
E = 1.1 V; 25 C;
the reaction of С –С cycloalkanes with hydrogen
5
8
CH Cl2
2
sulfide under the conditions of direct (anode/cathode)
activation of H S at room temperature and atmospheric
different number of sulfur atoms. Electrolysis at the
oxidation potential of the mediator led to the one-
electron oxidation of H S (0.4–1.5 V) into the HS
2
pressure [20, 21]. In order to decrease the anodic
overpotential of the electrosynthesis, we have also
taken advantage of indirect activation of hydrogen
sulfide in the presence of Вu NBr as mediator of H S
2
n
n
radicals (Scheme 2).
4
2
Scheme 2.
oxidation [22, 23]. The efficacy of tetrabutylammo-
nium bromide is due to its inertness with respect to the
reagent and its ability to oxidation at lower potentials
as well as the possibility of regeneration at the cathode
+e
−
Br
Br
−
e
H
2
S
2
H S
2 2
HS2
2HS
H+
[
24]. Different methods of H S activation in the
2
2
HS2
Prolonged time of the reaction resulted in the
increase of concentration of higher polysulfanes H
(n ≥ 4–8), which were converted into S [26, 27]. The
Med–H S–S system was used for the synthesis of di-
H2S4
reaction with cycloalkanes (C –C ) led initially to the
5
8
products of ring thiolation (cycloalkanethiols and
disulfides). The increase in the time of electrosynthesis
has promoted the forma-tion of inorganic polysulfanes
and sulfur, which are involved in the synthesis of tri-
and tetrasulfides of symmetrical structure [25]. Hence,
the earlier per-formed studies have shown that various
organosulfur compounds (hydrogen sulfide, thiols, and
sulfur) participate in the electrosynthesis of organic di-,
tri-, and tetrasulfides. The major advantages of the
method are mild conditions and ecological safety of
the process. In view of this, the present study aimed to
elaborate efficient methods of involvement of organo-
sulfur compounds in the synthesis of biologically
active sulfides R S (n = 2–4) by means of their
S
2 n
8
2
8
and polysulfides in the reactions with cyclohexane,
methylcyclohexane, and benzene in dichloromethane
at room temperature. In the case of cycloalkanes, the
corresponding cycloalkanethiols were initially formed
(Scheme 3).
Scheme 3.
HS
+
HS
H2S
SH
2
n
preliminary redox activation in organic solvent.
Comparative assessment of the reactivity of the
thiyl and hydropolysulfide radicals НS (n = 2–6) in
n
·
Several methods of redox activation of sulfur-
containing reagents (hydrogen sulfide, cyclopentane-
thiol, cyclohexanethiol, thiophenol, and sulfur) were
employed to perform efficient electrosynthesis of
polysulfides R S (n = 2–4). Electromediate oxidation
the reactions with cycloalkanes using quantum-
chemical calculations showed that abstraction of a
hydrogen atom from the substrate under the action of
thiyl radical was much easier (by ~75 kJ/mol). Another
competiton particle was the bromine radical generated
at the anode. However, the mediator concentration in
the reaction mixture was much lower than that of
hydrogen sulfide, which caused efficient regeneration
of Med in the near-electrode layer. Hence, initiation of
the reaction with cycloalkanes by thiyl radicals was
more probable (Scheme 3).
2
n
(
Вu NBr; Е = 0.9; Еap2 = 1.2 V) of hydrogen sulfide
4 ap1
(Еap = 1.7 V) was used to generation radical cation, to
reduce the potential of electrosynthesis (ΔE = 0.8 V) in
the considered reactions. Generation of the thiyl
radical occurred via cyclic transformations of the
–
·
Вr /Br redox pair at the potential of the first anodic
peak. The reaction of the HS radical with S promoted
8
the formation of hydropolysulfide radicals in the
solution (Scheme 1).
Cycloalkanethiols RSH formed during the
electrolysis were also oxidized by the active form of
mediator into cycloalkylthiyl radicals. The generated
species are able to dimerize in the near-electrode layer
or to react with sulfur in the solution (Scheme 4).
Dimerization of thiyl radicals as the products of
fragmentation of unstable H S radical cation favored to
2
the generation of inorganic polysulfanes (H S ) with
2
n
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REDOX ACTIVATION OF HYDROGEN SULFIDE, THIOLS, AND SULFUR
691
S–
Electromediated oxidation of RSH led to disulfides,
Table 1. Yield of the products in the reaction of Med–H
system with cyclohexane and methylcyclohexane under
different time of electrosynthesis [c(S ) = c(Med) = 5 mmol/L,
2
S
8
whereas unstable cycloalkylhydropoly-sulfides RS H
n
8
were converted into tri- and tetra-sulfides. The process
is accompanied by evolution of hydrogen sulfide and
E
2 2
el = 1.10 V, CH Cl , Pt anode)
sulfur. Polysulfides R S (n = 2–4) were obtained
2
n
Yield, %
(CH
1.5 h
under the electrolysis of cycloalkanes C , C with the
6
7
Value
R2S2
C
6
H
12
3 6 11
)C H
Med–H S–S system at the oxidation potential of the
2
8
1
.5 h
2.5 h
2.5 h
9.8
first anodic peak of the mediator (Table 1). The use of
the Med–H S–S system allowed reducing of the
2
8
5.8
4.9
7.2
5.7
6.8
10.4
18.4
35.6
energy and time costs for the preparation of R S (n =
2
n
R
2
2
S
3
4
11.0
25.4
46.2
2
–4) as compared to direct anodic activation of H S in
2
R
S
16.9
27.6
22.6
35.5
the presence of S8.
а
Σ
The total yield of the reaction products obtained
а
Σ is the total yield of polysulfides.
from C H
and (CH )C H
grew when the
6
12
3
6
11
electrolysis time was increased to 2.5 h. The yield of
R S (n = 2–4) was lowered to 24.6 and 37.7%, when
2
n
sulfides into inorganic polysulfanes. These compounds
are able to decompose with the formation of sulfur.
the process was carried out during 3 h for cyclohexane
and methylcyclohexane, respectively. The concentra-
tion of tetrasulfides upon the electrosynthesis was
decreased to 14.9 and 20.0%, respectively, likely due
to the reaction of R S (n = 2–4) with Н S leading to
The obtained data revealed higher total yield of di-,
tri-, and tetrasulfides in the reaction with methyl-
cyclohexane. The different behavior of this substrate
was explained by participation of the thiyl radicals in
the reaction at the tertiary carbon atom of the ring. The
2
n
2
RSH and RS H, readily occurring in biological
n–1
systems [28]. The values of energy effects of the
increase in the yield of polysulfides R S (n = 2–4) was
reactions of Н S with R S bearing cyclohexyl groups
2 n
2
2 n
due to substantial increase in the content of
tetrasulfides (Table 1). The ratio of the reaction
varied from –2.98 to 17.42 kJ/mol. Noteworthily,
similar transformations involving low-molecular in-
organic sulfanes H S (n = 2, 3) proceed much easier
products was different: R S : R S : R S = 1.0 : 1.3 :
2
2
2
3
2 4
2
n
0
.6, when electromediated synthesis was performing
(ΔН is between –7.75 and 3.77 kJ/mol). The formed
(
1.5 h) in the absence of sulfur in solution. The
unstable intermediate compounds RS H dispropor-
n–1
composition of the reaction mixture was changed in
favor of tetrasulfides, hence, the preliminary intro-
duction of sulfur promoted the increase in their
tionated into di- and trisulfides with elimination of
sulfur. Besides, the presence of bromide ion in the
reaction mixture might favor the decomposition of
concentration. The yield of R S in the reaction of
RS H into thiols and sulfur [29]. That fact was
2 3
2
cyclohexane with the Med–H S–S system was
consistent with the calculated conversion of sulfur,
which only slightly varied in the considered reactions
2
8
insignificant as compared to that in the case of the
methyl derivative. Faster accumulation of disulfides as
compared to trisulfides was explained by high rate of
dimerization of cycloalkylthiyl radicals. The experi-
(
25.8–32.3%). The content of S8 in the reaction
mixture increased when the time of electrosynthesis
was elongated, due to the transformation of hydrogen
Scheme 4.
RSH
Anode
−
Br
−
e
RS
Br
RSH
RS
S8
R S
2 2
−
H+
H2S
HS
RS9
RS H
9
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 89 No. 4 2019

6
92
SHINKAR’ et al.
mental data were consistent with the earlier obtained
results on direct anodic activation of the H S–S system
Scheme 6.
2
8
−2e
in the reactions with C –C cycloalkanes [25].
+
5
7
Br
+2e
−H+
−
Br
H
Besides cyclohexane and its methyl derivative, the
transformations of benzene in the presence of the Med–
+
H2S2
HS
+
+
2
SSH
H S–S8 system were investigated under electrolysis
2
conditions at the potential of the first anodic peak of
SSH
Вu NBr (0.9 V). Under those conditions of
4
−
H+
electrolysis, benzene did not react with activated
hydrogen sulfide, since radical substitution reactions
are not typical of benzene. Therefore, the electrolysis
was performed at the potential of the second oxidation
peak of the mediator (1.2 V), which allowed to
decrease the anodic overpotential of the hydrogen
mixture of products: R S 8.9%, R S 17.3%, and R S
2 4
2
2
2
3
31.2%. The increase in the reaction duration to 2.5 h
led to the increase in the total content of thiolation
products to 66.1% (R S 11.3%, R S 18.8%, R S
2
2
2
3
2 4
sulfide oxidation by 0.5 V. As a result, an electrophile
36.0%), the major contribution to the increased yield
being from the tetrasulfides. That fact was explained
by the oxidation of unstable hydrophenyldisulfane into
R S or the interaction with sulfur to give high-mole-
cular polysulfides. As in the case of cycloalkanes, the
increase in the electrolysis duration to 3 h did not
increase the total yield of the reaction products (52.8%).
+
(Br ) was generated, which attacked hydrogen sulfide
and benzene. That decreased the degree of
regeneration of the mediator by 15–20% with respect
to the transformations of cycloalkanes under similar
conditions (95–98%).
2
4
The formation of disulfane is occurred in both case
of hydrogen sulfide activation at the radical initiation
As for the preparation of polysulfides R S (n = 2–
2
n
(
Scheme 2) or in the interaction with electrophile
4) via electrosynthesis in the absence of a mediator,
two approaches were considered: anodic activation of
thiols in the reactions with sulfur and cathodic
activation of sulfur in the presence of thiols. In the
former case, the thiols were oxidized (redox potential
for cyclopentanethiol, cyclohexanethiol, and thio-
phenol was 1.68, 1.72, and 1.75 V, respectively) under
the electrolysis conditions (E 1.85 V), while sulfur
exhibited higher anodic potential (2.20 V). In the latter
case, electrochemical reduction of sulfur proceeded
under energetically milder conditions (–1.10; –1.40 V)
than cathodic activation of thiols (> –1.80 V). The
anodic activation of RSH, as in the presence of
mediator (Scheme 4), led to the formation of alkylthiyl
radicals reacting with sulfur. Hydropoly-sulfides with
resulted (Scheme 5).
Scheme 5.
Anode
Br
−
H2
2
H2S
−
2e
−
2e
+
Br
2H+
H2S2
Under the conditions of two-electron oxidation of
bromide ion in the reaction with H S, the sulfur-
centered cation HS2 was likely generated, which
2
+
reacted with benzene to afford the intermediate hydro-
phenyldisulfane (Scheme 6).
high molecular mass (RS H) could disproportionate
9
into more stable sulfides R S and R S (Scheme 7).
2
2
2 4
Scheme 7.
2 8
+ R + 3H S + 7/8S
The proposed mechanism of electrothiolation of
benzene was consistent with the earlier studied electro-
chemical transformations of aromatic and heterocyclic
compounds involving thiocyanogene ions [30, 31].
Electrooxidation of the anion promoted the generation
of a reactive intermediate, thiacyanogene (SCN)2,
which attacked the (hetero)aromatic ring to yield the
products of thiocyanation [32].
4
RS
H → R S
9 2 2
2
S
4
The results of electrosynthesis of polysulfides using
anodic activation of thiols in the presence of sulfur are
presented in Table 2.
A special feature of that reaction was the pre-
dominant yield of tetrasulfides (25.0; 30.5%) as
compared to disulfides (16.5; 12.6%) for cyclopentane-
and cyclohexanethiol, respectively. However, in case
The electrolysis of benzene in the presence of the
Med–H S–S system for 1.5 h gave the following
2
8
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REDOX ACTIVATION OF HYDROGEN SULFIDE, THIOLS, AND SULFUR
693
Table 2. Yield of the products of reaction under anodic activa-
tion of RSH in the presence of S [RSH : S = 2 : 1, c(RSH) =
, τ = 1.5 h, Pt anode]
Table 3. Composition and yield of the products under cathodic
8
8
activation of S in the presence of RSH [RSH : S = 2 : 1,
8
8
2
0 mmol/L, Eel = 1.85 V, CH
2
Cl
2
8 el 2 2
c(S ) = 1.5 mmol/L, E = –1.30 V, CH Cl , τ = 1.5 h, Pt anode]
Yield of reaction
products, %
Coulomb efficiency, %
а
а
Compound
δ, %
Compound
δ, %
R
2
S
2
R
2
S
3
2 4
R S
2
R S
2
2 4
R S
cyclo-C
5
6
H
H
9
SH
30.0
43.0
34.6
29.0
39.0
–
–
–
42.0
41.7
51.7
cyclo-C
5
6
H
H
9
SH
16.0
12.3
27.8
25.4
30.0
10.1
20.1
33.5
7.0
cyclo-C
11SH
cyclo-C
11SH
C
6
H
5
SH
49.0
6 5
C H SH
а
а
δ is the degree of conversion of sulfur.
δ is the degree of conversion of sulfur.
of thiophenol, the main pathway of the reaction was
dimerization of phenylthiyl radicals. Similar features
have been observed earlier in the case of the reaction
of the electrogenerated sulfur dication with thiophenol
presence of sulfur. High concentration of the formed
disulfides was due to the reaction of thiols with
nucleophilic sulfur species, leading to oxidation of RS
anions in the undivided cell (Scheme 10).
[33]. The values of sulfur conversion were consistent
Scheme 10.
with low yield of tetrasulfides caused by low reactivity
of phenylthiyl radical towards sulfur. Let us note that
no trisulfides were formed in the reactions of the
studied thiols, due to the absence of sulfur-centered
2
8
–
–
8
–
S
+ RSH → RS
+ RS
−
e
−
RS
2RS
R2S2
intermediates (HS radical or HS cation) generated
To obtain R
in the solution; their recombination with the RS
radicals would result in RS H. The disproportionation
reaction of the latter intermediate compounds would
give the trisulfide, hydrogen sulfide, and sulfur
(Scheme 11).
2 3 3
S , the HS radicals should be formed
2
from hydrogen sulfide in the reaction mixture.
Therefore, when using anodic activation of thiols in
the presence of sulfur, hydrocycloalkyl(phenyl)-
disulfanes which could lead to the formation of
trisulfides via disproportionation were not formed
4
(
Scheme 8) [28].
Scheme 11.
Scheme 8.
RSSH → R + H
·–
·
–
S
3
+ RSH → HS
3
+ RS
2
2
S
3
2
S
·
·
HS
3
+ RS → RS
4
H
2
RS
4
H → R
2
S
3
+ H S + 1/2S
2
8
Let us not consider the preparation of R S (n =
2
n
2
–4) via cathodic activation of S with thiols. The
8
electrochemical reduction of sulfur is known to yield
many reactive intermediates (Scheme 9) [19, 34, 35].
The formation of tetrasulfides in a rather high yield
was observed only in the reaction of thiophenol, in
agreement with higher conversion of sulfur. Con-
sequently, the phenylthiyl radical was active towards
different polysulfide species formed upon sulfur
activation. The advantages of that type of the reaction
initiation were the possibility of targeted electro-
synthesis of di- and trisulfides and higher conversion
of sulfur as compared to anodic activation of
cycloalkanethiols. A special feature of that approach
was the absence of tetrasulfides in the products of the
reaction, since the conditions of anodic activation of
sulfur were outside the range of electrooxidation of
R S (n = 2, 3).
Scheme 9.
+2e
2
8
−
S8
S
2
−
2−
4S
4S
6
+ S
8
8
2
−
−
S6
2S3
Cathodic activation of sulfur in the presence of
thiols allowed preparation of a mixture of different
polysulfides (Table 3). For cycloalkanethiols, the di-
and trisulfides were exclusively formed, whereas
trisulfides were not detected in the products of the
reaction with thiophenol. Such trend was observed in
the case of direct anodic activation of thiophenol in the
2
n
In summary, novel efficient approaches for
electrosynthesis of di-, tri-, and tetrasulfides using
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6
94
SHINKAR’ et al.
different methods of activation of sulfur-containing
reagents in dichloromethane at room temperature were
Microelectrolysis of the Med–H S–S system (1.5 h)
2 8
was performed at controlled-potential conditions on
2
proposed in this study. The use of the Med–H S–S8
platinum electrodes (S = 30 mm ) in a diaphragm-free
2
redox system in the reactions with cycloalkanes (С6,
С ) and benzene allowed to synthesize R S (n = 2–4)
with the high yield by varying the potential of
electrolysis and its duration. Depending on the nature
three-electrode cell (2 mL) in dichloromethane at 25°C
under argon atmosphere. The concentrations of the
mediator (Вu NBr) and S was 5 mmol/L; the
7
2 n
4
8
hydrogen sulfide : hydrocarbon molar ratio equaled 3 : 1,
of the substrate, the use of Вu NBr as mediator
at c(H S) = 30 mmol/L. Hydrogen sulfide was intro-
4
2
facilitated the decrease in anodic overpotential by 0.5–
duced in the reaction mixture after 0.5 h in the form of
0
.8 V. Activation of H S in the presence of S resulted
saturated solution in dichloromethane (20 μL). The
2
8
in the generation of thiyl and hydropolysulfide radicals
involved in the thiolation reaction. The use of this
approach was reasonable for the preparation of
tetrasulfides in high yield when di- and trisulfides were
present in the mixture of the products. Under the
conditions of anodic initiation of the reaction of thiols
with sulfur, di- and tetrasulfides were predominantly
formed, while thw trisulfides was absent. The cathodic
activation of sulfur in the presence of cycloalkane-
thiols favored the formation of di- and trisulfides; in
the reaction with thiophenol, di- and tetrasulfides were
obtained. Thus, the method of activation of the used
reagent (hydrogen sulfide or thiol) in the presence of
sulfur and duration of the reaction were the
determining factors in the targeted electrosynthesis of
polysulfides. The major advantages of electrosynthesis
of R S (n = 2–4) were mild conditions and high
concentration of H S was determined by gravimetry
2
after the reaction with Pb(CH COO) . The value of
3
2
electrolysis potential depended on the nature of the
substrate: for cycloalkanes the potential was
maintained at 1.10 V (the first peak of oxidation of the
mediator), for benzene it was 1.40 V (the second anode
peak of the mediator).
Anodic activation of thiols (5 mmol) was per-
formed at controlled-potential conditions at the
potential 1.85 V, the RSH : S ratio being 2 : 1 during
8
1
.5 h. Cathodic activation of sulfur (1.5 mmol) was
performed at the potential of –1.30 V and the RSH : S8
ratio of 2 : 1 during 1.5 h.
Large-scale electrolysis (10 mL) of the mixture of
hydrocarbons and the Med–H S–S system in dichloro-
2
8
2
n
methane was performed on platinum electrodes (S =
2
ecological safety of the process.
5
5 mm ) during 1.5 or 2.5 h. The rate of intake of
hydrogen sulfide was 2–3 mL/min, providing the
EXPERIMENTAL
required concentration of H S in the electrochemical
2
cell. The molar ratio of hydrogen sulfide : cycloalkane
Commercial cyclohexane, methylcyclohexane, benzene,
cyclopentanethiol, cyclohexanethiol, thiophenol, tetra-
butylammonium bromide (98%, Aldrich), hexane
was 3 : 1 at c(H S) = 30 mmol/L, c(S) = c(Med) =
2
5
mmol/L. The current density was maintained in the
2
range of 5–10 mА/cm during the electrolysis. After
the electrolysis, the reaction mixture was degassed by
bubbling argon during 30 min and then concentrated in
vacuum. Supporting electrolyte and mediator were
precipitated with hexane. The mixture of organic
polysulfides was isolated by three-step extraction with
hexane, and the extract was concentrated in vacuum.
(
95%, Alfa Aesar), and sulfur (99.5%, Sigma-Aldrich)
were used without purification. Hydrogen sulfide was
obtained as described elsewhere [36]. Methylene
chloride (“chemical pure”) was purified via the known
procedure [37]. Bu NClO (99%, Acros) was twice
4
4
crystallized from aqueous EtOH and dried in vacuum
during 48 h at 50ºС.
Cyclic voltammetry (CV) was used for the analysis
of the products mixtures and determination of redox
potentials of the compounds. Electrochemical
experiments were carried out in a three-electrode cell
under argon atmosphere using an IPC-pro potentiostat
Large-scale electrolysis (15 mL) with sulfur and
thiols was performed at controlled-potential conditions
at the potential 1.85 V in dichloromethane on platinum
2
electrodes (S = 50 mm ) during 1.5 h. A specimen of
sulfur (10 mmol) was preliminarily dissolved in
dichloromethane. The solution of sulfur and thiol in
the 1 : 2 ratio was deaerated for 5–7 min. For cathodic
activation of sulfur in the presence of thiol, the
preparative electrolysis was performed at the potential
of –1.30 V. The current density was maintained in the
(
3
working electrode: stationary platinum electrode,
mm in diameter; auxiliary electrode: platinum plate,
2
S = 36 mm ; reference electrode: Ag/AgCl/KCl with a
waterproof diaphragm; potential sweep rate 0.2 V/s;
supporting electrolyte: 0.15 М Bu NClO ).
4
4
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 89 No. 4 2019

REDOX ACTIVATION OF HYDROGEN SULFIDE, THIOLS, AND SULFUR
695
2
+
range of 5–10 mА/cm . After the electrolysis, the
(55); (С Н ) S : 230 (14) [M] , 147 (16), 83 (100), 55
6
11 2 2
+
reaction mixture was concentrated in vacuum. The
background electrolyte was precipitated with hexane.
The mixture of organic polysulfides and thiols was
isolated by three-step extraction with hexane and the
extract was concentrated in vacuum.
(27); (С Н ) S : 262 (16) [M] , 230 (3), 179 (8), 115
6 11 2 3
+
(32), 83 (100), 55 (25); (С Н ) S : 218 (100) [M] , 185
6
5 2 2
(70), 154 (65), 109 (80), 77 (55), 66 (70); (С Н ) S :
250 (8) [M] , 218 (100), 185 (25), 154 (35), 140 (10),
109 (80), 66 (35); (С Н ) S : 260 (13) [M] , 163 (17),
30 (10), 97 (100); (С Н ) S : 291 (14) [M] , 260 (4),
63 (28), 130 (44), 97 (100).
Organic tetrasulfides destruct under the conditions
of chromatography-mass spectrometry analysis. In
view of that, the mixture of R S (n = 2–4) was
analyzed by means of X-ray fluorescence using an
АSE-1 spectrometer to determine the total content of
sulfur. The obtained data allowed to calculate the
content of R S , using the chromato–mass
spectrometry data on the yield of di- and trisulfides.
The determined yields of tetrasulfides were consistent
with the results of electrochemical measurements.
6
5 2 3
+
+
7
13 2 2
+
1
1
7 13 2 3
The obtained organosulfur compounds were
identified using the methods of CV, IR spectroscopy,
chromatography-mass spectrometry, and X-ray
fluorescence analysis. The cyclic voltammograms of
the electrolysis products contained three anodic peaks:
R S (1.50–1.60 V), R S (1.73–1.85 V), and R S
2
n
2
2
2
3
2 4
(
1.90–2.05 V). In the cases of thiols reactions with
sulfur, the peaks of oxidation of the starting thiols were
also observed at 1.62–1.75 V. The yield of di- and
trisulfides was estimated from the data of chromato–
mass spectro-metry. In the reactions involving the
mediator system, the yield of the electrolysis products
was calculated with respect to the reacted hydrogen
sulfide, whereas in the case of redox-activation of the
RSH–S system, the content of R S and R S was
referenced to the reacted thiol. The yield of
tetrasulfides was estimated from the data of CV by
determining the ratio between the anodic peaks current
of di- and tetrasulfides, taking into account molecular
masses of R S (n = 2, 4) oxidized under those
conditions in a single two-electron step. Inorganic
polysulfanes (H S ) with different molecular masses
were observed by CV method at 0.4–1.5 V. The degree
of conversion of sulfur was monitored by means of CV
from the decrease in the value of the cathodic peak
current (–1.15 V).
2
4
Quantum-chemical calculations were performed
using the B3LYP/6-31++G(d,p) density functional
method implemented in Hyper Chem 8.0 software. The
effect of solvent (CH Cl ) was taken into account by
the polarizable continuum model (PCM). The energy
effects of the reactions (ΔH) were calculated as the
difference of total energies of final and initial
structures.
8
2
2
2 3
2
2
2
n
2
n
FUNDING
This study was financially supported by the Russian
Science Foundation (grant no. 17-13-01168).
CONFLICT OF INTERESTS
No conflict of interest was declared by authors.
REFERENCES
IR spectra of the electrolysis products were
registered using a FSM-1201 IR Fourier spectrometer
–
1
(
KBr, 400–4000 cm ). The IR spectra contained the
–
1
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–
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