Redox mediators of hydrogen sulfide oxidation in reactions with cycloalkanes

Article abstract of DOI:10.1134/S0012500815120058

An indirect electrochemical method for thiolation of cycloalkanes C₅-C₇ has been suggested. The method is based on a new approach to activation of hydrogen sulfide by redox mediators.

Full text of DOI:10.1134/S0012500815120058

ISSN 0012ꢀ5008, Doklady Chemistry, 2015, Vol. 465, Part 2, pp. 295–298. © Pleiades Publishing, Ltd., 2015.
Original Russian Text © N.T. Berberova, E.V. Shinkar’, I.V. Smolyaninov, K.P. Pashchenko, 2015, published in Doklady Akademii Nauk, 2015, Vol. 465, No. 6, pp. 683–686.
CHEMISTRY
Redox Mediators of Hydrogen Sulfide Oxidation
in Reactions with Cycloalkanes
N. T. Berberovaa, E. V. Shinkar’a, I. V. Smolyaninova, b, and K. P. Pashchenkoa
Presented by Academician G.G. Matishov May 27, 2015
Received June 16, 2015
Abstract—An indirect electrochemical method for thiolation of cycloalkanes C5–C7 has been suggested. The
method is based on a new approach to activation of hydrogen sulfide by redox mediators.
DOI: 10.1134/S0012500815120058
In recent years, there has been an increasing trend
Recent studies have shown that anodic activation
toward using electrosynthesis as one of the “green of hydrogen sulfide in the presence of cyclopentane
chemistry” methods for production of various valuable proceeds at a rather high oxidation potential of H2S in
organic compounds. Replacement of toxic reagents by
electric current enables the in situ generation of reacꢀ
tive intermediates, which is favorable for carrying out
reactions under relatively mild conditions and makes
the process more environmentally friendly [1].
dichloromethane (1.70 V) and leads to the formation of
substrate thiolation products at room temperature [9].
The aim of the present work was to study the possiꢀ
bility of using redox mediators for indirect H2S activaꢀ
tion in the reaction with inert cycloalkanes at lower
anodic potential values.
Much attention focuses on the search for the ways
to reduce the electrode overvoltage in the course of
electrosynthesis. To solve this problem, indirect elecꢀ
trosynthesis is increasingly often used where heterogeꢀ
neous electron transfer between the electrode and the
substrate is replaced by a homogeneous reaction of the
reagent (substrate) with the mediator generated at a
lower potential. Among the advantages of mediators is
also their ability for regeneration at the electrode,
which makes it possible to use them in catalytic
amounts [2]. Inorganic mediators have been rather
well studied [3]; however, little is known about their
reactions with hydrogen sulfide.
The following redox pair were considered as H2S
oxidation mediators: Br–/Br• (tetrabutylammonium
bromide) (I)
;
QH2/Q (3,5ꢀdiꢀtertꢀbutylꢀ1,2ꢀdihydroxyꢀ
benzene/3,5ꢀdiꢀtertꢀbutylꢀoꢀbenzoquinone) (II).
Commercially available mediators—tetrabutylamꢀ
monium bromide (Aldrich, 98%), 3,5ꢀdiꢀtertꢀbutylꢀ
oꢀ
benzoquinone (Fluka, 98%), and 3,5ꢀdiꢀtertꢀbutylꢀ
1,2ꢀdihydroxybenzene (Fluka, 98%)—were used.
Electrochemical measurements were carried out with
an IPCꢀPro potentiostat at a Pt anode in CH2Cl2 in the
presence of 0.1 M nꢀBu4NClO4. Microelectrolysis of
We have demonstrated that direct electrooxidation
of H2S in reactions with unsaturated and aromatic
hydrocarbons leads to the quantitative formation of
organic sulfur compounds [4, 5]. In [6, 7], the authors
have studied the possibility of using mediators based
on triarylamines and metal complexes with redoxꢀ
active organic ligands to synthesize aliphatic thiols,
sulfides, and disulfides widely applied in industry and
agriculture [8].
an H2S –cycloalkane mixture was carried out with the
use of a PIꢀ50.1 potentiostat in potentiostatic mode at
a potential of 1.04 V (redox mediator I) or 1.45 V
(redox mediator II). The reference electrode was a satꢀ
urated silver chloride electrode (Ag/AgCl/KCl) with a
waterproof diaphragm. Chromatographic analysis of
the synthesized organic sulfur compounds was perꢀ
formed on a KristallꢀLux 4000M gas chromatograph
with a flame photometric detector. The IR spectra
were recorded on an FSM 1201 FT spectrophotomeꢀ
ter (400–5000 cm–1); chromatography/mass spectra
were recorded on an Agilent Technologies 6890N GC
System equipped with an Agilent Technologies 5973N
MSD massꢀselective detector. Xꢀray fluorescence
analysis was carried out on an ASEꢀ1 energy dispersive
sulfur analyzer. Quantumꢀchemical calculations were
a Astrakhan State Technical University, ul. Tatishcheva 16,
Astrakhan, 414056 Russia
eꢀmail: berberova@astu.org
b Toxicology Research Group, Southern Scientific Center,
Russian Academy of Sciences, ul. Tatishcheva 16, Astrakhan,
414025 Russia
295

296
BERBEROVA et al.
performed with the Gaussian 03 program by the denꢀ
sity functional theory method (B3LYP, 6ꢀ31++G(d,p)
basis set). The solvent effect was considered using the
polarized continuum model (PCM).
SH
•
HS•
+ HS•
H2S
I; II, S8
The thiyl radical was generated from H2S by using
selected redox mediators at room temperature by
Scheme 1:
I; II
2C6H11SH
(C6H11S)2
(C6H11)2S3
Scheme 2.
At the first stage, a hydrogen atom is eliminated
from cycloalkane with subsequent recombination of
the cycloalkyl and thiyl radicals. This leads to cycloalꢀ
kanethiols, which are oxidized to the corresponding
I
Br–
Anode
H2S
disulfides in the presence of redox mediators I and II.
Chemical oxidants of different strength or an anode
are applied to synthesize organic triꢀ and tetrasulfanes
[12, 13]. In all of these reactions, direct oxidation of
H2S leads to cyclooctasulfur as a side product, whatꢀ
ever the nature of substrates and redox mediators. The
presence of S8 among the reaction products causes the
growth of the sulfide chain in RSSR to give trisulfanes.
The current yield and composition of the reaction
products can be changed by varying the hydrogen sulꢀ
fideꢀtoꢀcycloalkane concentration ratio. The increase
in H2S content promotes preferential formation of
trisulfanes through disproportionation of tetraꢀ and
pentasulfanes.
e–
H2S+•
HS•
Br•
–H+
II
Anode
2H2S
2HS•
QH2
2e–
2H+
2e–
Q
2H+
Scheme 1.
The results of the reactions of H2S with cycloalꢀ
kanes with the use of redox pairs I and II are presented
in the table. In the series of unsubstituted cycloalkanes
С5–С7, the overall yield of organosulfur reaction
products depends on the type of redox mediator; in
The electrochemical oxidation of redox mediators
proceeds at the anodic potential 0.92 V for Br–/Br•
and 1.24 V for QH2/Q. The Br–/Br• redox pair actiꢀ
vates H2S to an unstable radical cation through elecꢀ
tron transfer, whereas оꢀbenzoquinone (Q), a product
of oxidation of 3,5ꢀdiꢀtertꢀbutylꢀ1,2ꢀdihydroxybenꢀ
zene (QH2), plays the role of the dehydrogenating
this context, redox pair
I is most promising.
For redox pair , with increasing the ring size of
I
unsubstituted cycloalkanes, the yield of corresponding
cycloalkanethiols and disulfides increases while the
thiol/trisulfane ratio decreases; for the initial hydroꢀ
carbons, this ratio is 2.8 for C5, 2.2 for C6, and 1.9
for C7. With an increase in the ring size of the C5–C7
substrates, the ratio of the forming cycloalkanetiols
changes from 1 to 4.3 and for trisulfane, from 1 to 2.6,
which points to the decrease in the probability of furꢀ
ther transformations of thiols for larger hydrocarbon
rings.
agent with respect to H2S
.
The dimerization of thiyl radicals leads to disulfane
(0.3 V), and its oxidation under electrolysis conditions
leads to a further growth of the sulfide chain with subꢀ
sequent conversion of linear polysulfanes (H2Sn,
2–8) into sulfur (–1.20 V) [5]. Unlike QH2/Q [10, 11],
redox pair has not been considered earlier by us. As
the reagent concentration increases, the reaction of
redox pairs and II with hydrogen sulfide leads to an
n =
I
I
increase in the anodic peak current of the mediator,
which is evidence of the catalytic effect in the course
of the electrochemical process.
When redox mediator
I is used, the presence of
alkyl substituents in cyclohexane has little effect on the
overall yield of organic sulfur derivatives (table). For
The possibility to generate thiyl radical at room cyclohexane and its alkylꢀsubstituted derivatives, the
temperature with the use of redox mediators was studꢀ yield of thiols is almost independent of the substituent
ied for their reactions with unsubstituted and alkylꢀ nature (4.3–5.2%). Secondary transformations of
substituted cycloalkanes С5–С7. The reactions proꢀ cyclohexanethiols leads to disulfide and then to trisulꢀ
fanes in the presence of elemental sulfur; their overall
yield varies within 16.5–18.8%. This fact confirms
that the reactivity of cycloalkanethiols depends on the
ring size. The decrease in the yield of S8 as compared
with unsubstituted cycloalkanes is evidence of the
ceed through a series of successive stages by the radical
mechanism (Scheme 2), which leads to the formation
of a mixture of reaction products in an anaerobic
medium: cyclic thiols (RSH), disulfides (RSSR), and
trisulfanes (RSSSR).
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REDOX MEDIATORS OF HYDROGEN SULFIDE OXIDATION
297
Yields of sulfurꢀcontaining organic compounds and molecular
sulfur in reactions of H2S with C5–C7 cycloalkanes in the
presence of redox mediators I and II
preferential content of trisulfanes in the mixture of
reaction products.
With redox mediator II, the reactions of cycloalꢀ
kanes C5–C7 yield thiols (2.5–6.7%) and disulfides
(5.0–8.3%), while trisulfanes are absent among the
reaction products, which is caused by the inertness of
I
II
current yield current current yield current
of reaction yield of reaction yield
products, % of S8, % products, % of S8, %
Substrate
the generated oꢀquinone toward disulfides. Generaꢀ
tion of organic trisulfanes is typical of methylꢀ and
ethylcyclohexane, which is due to the direct electroꢀ
chemical oxidation of the resulting disulfides (1.48–
1.50 V) at the anode under the electrolysis conditions.
Cyclopenꢀ
tane
Cyclohexane
Cyclohepꢀ
tane
12.7
42.0
10.0
27.0
22.8
51.5
40.0
28.0
7.5
15.8
33.5
29.5
When redox pair I is used, bromide ion is oxidized
to atomic bromine capable of reacting with both H2S
and cycloalkanes. This is responsible for the possibility
of an alternative stage of generation of the cycloalkyl
radical through the Br radical attack at the substrate,
which leads to an increase in the yield of sulfurꢀconꢀ
taining products as compared with the reactions in the
presence of redox mediator II. The reactivity of the
Methylcyꢀ
clohexane
Ethylcycloꢀ
hexane
25.3
23.8
22.2
23.7
32.7
25.5
35.8
22.5
noticeable effect of the substrate nature on the oxidaꢀ
tion of H2S and H2Sn to S8, since the yields of sulfur in
all the reactions are comparable. The observed tenꢀ
dency is explained by the high propensity of the thiyl
radical to dimerize into disulfane and inorganic
polysulfanes. The introduction of alkyl groups into the
ring leads to the increase in the yield of sulfur comꢀ
pounds, which is evidently due to the possibility of
thio substitution reactions in the side group of the subꢀ
strate.
selected substrates in the presence of redox mediator
I
is independent of the ring size (decreases only slightly
in the series C7 > C6 > C5), which correlates with quanꢀ
tumꢀchemical calculation results. From the thermoꢀ
dynamic standpoint, the probability of the cycloalkyl
radical being generated by the attack of the thiyl radiꢀ
cal is comparable for the C5–C7 cycloalkanes, and the
heat (ΔH) of this stage varies within 35.7–45.7 kJ/mol.
According to quantumꢀchemical calculations, in
the reaction of H2S with cycloalkanes in the presence of
The yield of sulfur for the Br–/Br• redox pair is
higher as compared with that for QH2/Q, which is
explained by the higher regeneration rate of the mediꢀ
ator owing to the absence of an additional chemical
stage (deprotonation) in the catalytic cycle of redox
transformations.
redox mediator
dehydrogenation (
the probability of elimination of a hydrogen atom
from the substrate ( = 17.3–27.2 kJ/mol). Redox
mediator is active to thiols according to the experiꢀ
mental electrochemical data and calculated heats of
I, the thermodynamic probability of H2S
Δ
H
= –18.5 kJ/mol) is higher than
Δ
H
I
The content of organosulfur compounds obtained
in the reaction of hydrogen sulfide with cycloalkanes
also depends on the substrate nature and the type of
redox mediator (figure).
formation of cycloalkylthiyl radicals ( H = –33.5 ...
Δ
–27.1 kJ/mol).
The cycloalkyl radical is generated only by the
attack of the thiyl radical at the substrate when redox
pair II is used. This leads to a decrease in the overall
yield of organosulfur reaction products as compared
According to the figure, redox pair II turned out to
be more efficient in the reaction of methylcyclohexane
with H2S, since the overall yield of sulfurꢀcontaining
products is the highest one at comparable disulfide,
thiol, and trisulfane contents.
with the activation of the reagent by redox pair I. In
the presence of redox mediator II, cyclohexane turns
out to be thermodynamically most stable. This is conꢀ
sistent with the data on the reactions of H2S with the
С5–С7 cycloalkanes carried out under conditions of
direct anodic activation of the reagent and is associꢀ
ated with the negligible ring strain for cyclohexane.
The reaction with methylcyclohexane leads to the
highest yield of disulfide, trisulfane, and molecular
sulfur. The accumulation of S8 in sufficient amounts in
the course of reaction also promotes the formation of
organic polysulfanes. In the reactions of the С5–С7
In electrolysis in the presence of redox mediator I,
its concentration decreases by 10%, as distinct from
QH2/Q. The insignificant consumption of redox
mediator
I is caused by the inevitable formation of
hydrogen bromide at the stage of substrate dehydrogeꢀ
nation in the presence of atomic bromine. However,
brominated derivatives of the substrate are absent
among the products of electrolysis of an H2S–cycloꢀ
hexane mixture in the presence of redox mediator
I.
This fact is consistent with the results of the photooxyꢀ
cycloalkanes with hydrogen sulfide (table), there is no genation reaction of cyclohexane in the presence of
DOKLADY CHEMISTRY Vol. 465
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298
Current yield, %
BERBEROVA et al.
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Δ
Е(I)
≈
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II
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≈
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ACKNOWLEDGMENTS
This work was supported by the Russian Science
Foundation (project no. 14–13–00967).
Translated by G. Kirakosyan
DOKLADY CHEMISTRY Vol. 465
Part 2
2015