Directed oxidative coupling of thiols in the synthesis of unsymmetrical disulfides

Article abstract of DOI:10.1007/s11172-020-2860-1

Oxidative coupling of two different thiols bearing aliphatic, alicyclic, aromatic, and hetero-aromatic moieties promoted by mild oxidizing agents, viz., sterically hindered o-benzo(imino)-quinones, carried out in N-methylpyrrolidone at room temperature led to unsymmetrical disulfides. Among the studied oxidizers, the most active was 3,6-di-tert-butyl-o-benzoquinone, which, in contrast to 3,5-di-tert-butyl-o-benzoquinone, was not involved in the Michael addition. Under the optimal reaction conditions, the yields of the target unsymmetrical disulfides reach 81%.

Full text of DOI:10.1007/s11172-020-2860-1

Russian Chemical Bulletin, International Edition, Vol. 69, No. 5, pp. 990—995, May, 2020
990
Directed oxidative coupling of thiols in the synthesis of
unsymmetrical disulfides*
D. A. Burmistrova,a I. V. Smolyaninov,a,b and N. T. Berberovaa
aAstrakhan State Technical University,
16 ul. Tatishcheva, 414056 Astrakhan, Russian Federation.
E-mail: burmistrova.da@gmail.com
bFederal Research Center the Southern Scientific Center of the Russian Academy of Sciences,
41 prosp. Chekhova, 344006 Rostov-on-Don, Russian Federation
Oxidative coupling of two different thiols bearing aliphatic, alicyclic, aromatic, and hetero-
aromatic moieties promoted by mild oxidizing agents, viz., sterically hindered o-benzo(imino)-
quinones, carried out in N-methylpyrrolidone at room temperature led to unsymmetrical di-
sulfides. Among the studied oxidizers, the most active was 3,6-di-tert-butyl-o-benzoquinone,
which, in contrast to 3,5-di-tert-butyl-o-benzoquinone, was not involved in the Michael addition.
Under the optimal reaction conditions, the yields of the target unsymmetrical disulfides reach 81%.
Key words: disulfides, thiols, oxidative coupling, o-benzoquinones, o-iminobenzoquinones,
N-methylpyrrolidone.
Oxidative coupling is widely used in organic synthesis
for the formation of the C—C, C—S, and S—S bonds.1—4 The
reactive species generated during the reaction of oxidizing
agents and substrates are further transformed into the coup-
ling products. The reaction can be performed using both
inorganic and organic oxidizers5 as well as can be realized
under the electrochemical conditions.6—8 The nature of
the oxidizing agent and the solvent exerts strong effects
on the yields and the ratio of the resulting disulfides.9
Disulfides have found applications in manufacturing
pharmaceuticals, dyes, fungicides, and food additives.10
The presence of two different moieties bonded by the
disulfide bridge significantly widened the spectrum of
potential biological activity of these compounds.11 Several
methods are known for synthesizing disulfides by thiol
oxidative couplings, viz., catalysis with the metal ions,12—14
oxidation with either inorganic (pyridinium chlorochro-
mate, potassium ferricyanide, manganese(II) acetate/
carbonate) or organic (tert-butyl hydroperoxide, 2,3-hydro-
peroxide, 2,3-dichloro-5,6-dicyano-1,4-benzoquinone
(DDQ), and gallic acid) compounds,15—17 and electro-
chemical activation.18
Q1—Q5
Q1
R1
H
H
F
Cl
R2
But
H
F
H
R3
H
Q2
But
But
But
But
Q3
Q4
Q5
OMe
OMe
pounds were successfully applied for oxidative activation
of H2S. The aim of the present work is to study the oxida-
tive coupling of thiols in the presence of o-benzo(imino)
quinones, to evaluate the solvent effect, and to optimize
the reaction conditions. The use of low-molecular-weight
aliphatic thiols present in the crude oil and petroleum
products in the reactions with thiols of another structure
will give access to practically valuable organic disulfides.
In the present work, sterically hindered o-benzoquin-
ones (Q1—Q6) and o-iminobenzoquinone (imQ) were used
as the oxidizers.
Results and Discussion
The selected o-benzo(imino)quinones are milder oxi-
Oxidizing agent Q reacts with thiol at room tempera-
ture to give symmetrical disulfide and the corresponding
catechol or o-aminophenol (QH2) (Scheme 1).
To reveal the effect of the position of the tert-butyl
groups in the o-quinone ring and the replacement of the
dizing agents than DDQ.19,20 Earlier,21,22 these com-
* Based on the materials of the XXI Mendeleev Congress
on General and Applied Chemistry (September 9—13, 2019,
St. Petersburg, Russia).
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 0990—0995, May, 2020.
1066-5285/20/6905-0990 © 2020 Springer Science+Business Media LLC

Unsymmetrical disulfides
Russ. Chem. Bull., Int. Ed., Vol. 69, No. 5, May, 2020
991
Scheme 1
Table 1. Effect of the solvents and oxidizers on the yield of
unsymmetrical disulfide 2 (GC/MS data)a
Oxidizer
Yield of disulfide 2 (%)
NMP
CH2Cl2
Hexane
Toluene
b
b
b
Q1
Q2
imQ
3
29
20
—
—
—
oxygen atom in the quinone structure with nitrogen atom
on the efficiency of the thiol oxidative coupling, we carried
out the model reactions of butane-1-thiol and thiophenol
in the presence of quinones Q1, Q2, and iminobenzo-
quinone imQ. In the presence of quinone Q1, the reaction
predominantly proceeds as the Michael addition to give
pyrocatechol sulfides 1a (93%) and 1b (96%) and small
amounts of the target disulfides (Scheme 2). These results
are in agreement with the previous data on the synthesis
of thioethers with antioxidant activity.23—26
b
14
12
—
3
b
9
—
a
Reaction conditions: PrSH (1 equiv.), PhSH (1 equiv.),
quinone (2 equiv.), solvent, 20 C, 3 h.
b No disulfide 2 was detected in the reaction product.
oxidative coupling is a nearly complete absence of unsym-
metrical disulfides in the model Q1-promoted reaction.
To study the effect of the solvent on the selectivity of
the thiol oxidative coupling in the presence of Q2, we
performed the model reaction of 4-methoxythiophenol
with propane-1-thiol at an oxidizer : thiol molar ratio of
1 : 1.2 (Scheme 4, Table 2). Analysis of the reaction prod-
ucts obtained by the Q2-promoted thiol oxidative coupling
showed predominant formation of symmetrical disulfide
(4-MeOC6H4)2S2 (32—53%) in toluene, propan-2-ol,
dioxane, chloroform, and dichloromethane. The only
exception is NMP that favors the formation of the target
unsymmetrical disulfide 3 (51%) along a small amount
(15%) of the symmetrical product.
Scheme 2
R = Bu (1a), Ph (1b)
The reactions of the thiols with Q2 and imQ lead to the
corresponding symmetrical disulfides. Note that in this
case, no formation of the products of S-functionalization
of the aromatic ring was detected, which is an advantage
of the use of o-benzo(imino)quinones Q2 and imQ as the
selective oxidizing agents.
Scheme 4
The effect of the oxidizing agents Q1, Q2, and imQ on
the yield of unsymmetrical 2-propyl-1-phenyldisulfane
(2) was studied on the example of the reaction of propane-
1-thiol with thiophenol in different solvents (N-methyl-
pyrrolidone (NMP), dichloromethane, hexane, and tolu-
ene) (Scheme 3, Table 1). In NMP, the reaction of these
thiols in the presence of Q2 gave the target disulfide 2 in
29% yield and in the presence of imQ the yield of disulfide 2
was 20%. In dichloromethane, hexane, and toluene, the
yields of the target product 2 did not exceed 14% due to
the predominant formation of symmetrical disulfides Pr2S2
(12—24%) and Ph2S2 (62—82%). An additional evidence
of the domination of the Michael addition over the thiol
Reagents and conditions: 4-MeOC6H4SH (1 equiv.), PrSH
(2 equiv.), Q2 (2.5 equiv.), solvent, 20 C, 3 h.
These results are in agreement with the data on selec-
tivity of NMP in the removal of sulfur-containing com-
pounds from hydrocarbons.27,28 Due to high excess
electron density localized predominantly on the oxygen
atom, NMP is able to form stable hydrogen bonds with
the compounds capable of extracting the proton (water,
phenols, alcohols, and thiols).29
To compare the reactivity of the oxidizers in the reac-
tion with thiols, we also used the electrochemical methods.
It is known that the reduction potential of DDQ in MeCN
(relative to Ag/AgCl) is more positive (0.55 V)20 than the
reduction potentials of the studied o-benzoquinones Q1
(–0.48 V) and Q2 (–0.78 V).21,30,31 Cyclic voltammetry
showed the reduction peaks of o-benzo(imino)quinones
in NMP at a Pt electrode. According to the measured
Scheme 3
Reagents and conditions: Q1, Q2 or imQ (2 equiv.), PrSH
(1 equiv.), PhSH (1 equiv.), solvent, 20 C, 3 h.

Burmistrova et al.
992
Russ. Chem. Bull., Int. Ed., Vol. 69, No. 5, May, 2020
Table 2. Effect of the solvents on the yields of symmetrical and
unsymmetrical disulfides formed in the Q2-promoted reaction
of oxidative coupling of 4-methoxythiophenol (ArSH) with
propane-1-thiol (GC/MS data)a
provided the content of disulfide 3 of 16 and 10%, re-
spectively.
According to the obtained result, the further syntheses
of unsymmetrical disulfides by oxidative coupling of two
different thiols were carried out in NMP in the presence
of the most active oxidizing agent, 3,6-di-tert-butyl-o-
benzoquinone (Q2). Similarly to the reaction of sterically
hindered quinones with H2S,21 the reaction between thiols
and Q2 apparently follows the same radical mechanism
via sulfur-centered radicals that recombine to give disul-
fides of various structures and 3,6-di-tert-butylpyrocatechol
(3,6-QH2) (Scheme 5).
Run
Solvent
Yield (%)
Conversion of ArSH
(%)
3
Ar2S2
1
2
3
4
5
6
7
Toluene
NMP
Propane-2-ol
Dioxane
MeCN
6
53
15
44
51
43
32
32
62
72
46
69
47
51
54
51
b
—
14
—
17
17
b
CHCl3
CH2Cl2
Scheme 5
a
Reaction conditions: ArSH (1 equiv.), PrSH (2 equiv.), Q2
(2.5 equiv.), solvent, 20 C, 3 h.
b Trace amounts.
values of the first quasi-reversible cathodic potentials (Ecp),
the studied oxidizers can be arranged in the following
order: Q6 (0.13 V)  Q4 (–0.44 V)  Q3 (–0.45 V)  Q1
(–0.51 V)  Q2 (–0.53 V)  Q5 (–0.74 V)  imQ (–0.82 V).
These data indicated that the studied quinones are milder
oxidizing and dehydrating agents than DDQ. Therefore,
these quinones do not give rise to the overoxidized thiol
species and require neither low-temperature nor high-
temperature reaction conditions.16,17 Cyclic voltammetry
also provided the information about the oxidation peak
potential (Eop) of the thiols involved in the coupling reac-
tions in NMP at the Pt anode. Thiophenol and heptane-
1-thiol are oxidized closely to solvent discharge (1.7 V).
The anodic peak potentials of the low-molecular weight
thiols (propane-1-thiol, propane-2-thiol, and tert-butyl-
mercaptane) are within the 1.3—1.4 V range, cyclopentane-
thiol and cyclohexanethiol are oxidized at 1.5 and 1.6 V,
respectively. Aromatic thiols (4-methoxythiophenol,
2,6-di-tert-butyl-4-mercaptophenol, and 2-mercapto-
thiazole) are oxidized at the lower anodic potentials of
1.4, 1.0, and 0.9 V, respectively, while the oxidation po-
tential of 4-mercaptopyridine is even lower being 0.6 V.
The difference between the oxidation potentials of aro-
matic thiols and the reduction potential of the oxidizing
agent Q is the lowest, therefore, arylthiols are most sus-
ceptible to oxidation among the considered thiols.
Synthesis of disulfides 3—14 is shown in Scheme 6.
Compound 3 and 2,6-di-tert-butyl-4-(propyldisulfanyl)-
phenol (4) were synthesized by the reaction of the excess
of propane-1-thiol (0.8 equiv.) with aromatic thiols
(0.4 equiv.) in the yields of 69 and 19%, respectively. The
low yield of disulfide 4 can be explained in the terms of
better stability and lower reactivity of the radical generated
from 2,6-di-tert-butyl-4-mercaptophenol known for its
antioxidant activity.32 4-(Propyldisulfanyl)pyridine 5
bearing the heteroaromatic and aliphatic moieties was
obtained in 18% yield. Coupling of cycloalkanethiols with
heteroaromatic derivatives gives the products of oxidative
coupling 6—8 (46—79%); this fact is in agreement with
the easiness of the electrochemical oxidation of the hetero-
aromatic moiety.
It should be noted that both the nature and the struc-
ture of the starting thiols affect the yields of the unsym-
metrical disulfides. The thiols bearing thio(oxy) group at
the secondary carbon atom react to give disulfides 9 and
10 in the yields of 81 and 72%, respectively. The replace-
ment of the cycloalkyl groups by the n-heptyl one leads
to the decrease in the content of the target disulfide 11 up
The efficiency of oxidizers Q2—Q6 and imQ was
evaluated on the example of the reaction of 4-methoxy-
phenol with propane-1-thiol to give 1-(4-methoxyphenyl)-
2-propyldisulfane (3) (see Scheme 4). Among the studied
o-quinones, the most active was Q2, which ensured the yield
of the target disulfide 3 of 69% within 10—12 h. In
the presence of the halo-substituted quinones Q3, Q4,
and Q6, two-fold decrease in the product yield was ob-
served (22—32%). Quinones imQ and Q5 with less ac-
ceptor characters were the least active; these compounds

Unsymmetrical disulfides
Russ. Chem. Bull., Int. Ed., Vol. 69, No. 5, May, 2020
993
Scheme 6
to 59%. Oxidative coupling of thiophenol with different
aliphatic (alicyclic) thiols gives predominantly diphenyl
disulfide due to very fast recombination of the phenylthiyl
radicals. Only in the case of the coupling of thiophenol
with cyclohexanethiol, the product, 1-cyclohexyl-2-
phenyldisulfane 12, was obtained in the yield (30%) higher
than that of disulfides 13 and 14.
To explain the obtained experimental results on the
yields of unsymmetrical disulfides, we preformed the
quantum chemical calculations of the heat effects of the
recombination of sulfur-centered radicals (Table 3).
For compounds 5—7 bearing the pyridine moiety,
formation of disulfides 6 and 7 with cyclopentyl and cyclo-
hexyl substituents are the most energetically favorable. In
the case of compounds 8, 10, and 12, recombination of
the radicals with the aliphatic groups to give disulfide is
thermodynamically preferable. Enthalpies of formation of
unsymmetrical disulfides 9—11 via the radical mechanisms
are nearly the same thus indicating an energy gain in the
reaction between two thiols with aliphatic substituents.
The oxidative coupling of thiol bearing the aliphatic
(alicyclic) and aromatic (heteroaromatic) moieties allows the
construction of disulfides with a wide spectrum of biological
activity. The structure of the thus formed hybrid molecules
include the physiologically active group and the lipophilic
fragment bridged by the disulfide moiety that can poten-
tially serve as an endogenous source of hydrogen sulfide.33
In summary, the detailed studies of the directed oxidative
coupling of thiols allowed us to choose the 3,6-di-tert-butyl-
o-benzoquinone as the selective oxidizing agent and NMP
as the solvent. Under the optimized conditions, the yields of
the target unsymmetrical disulfides reached 81%. A series of
disulfides bearing the aromatic, heteroaromatic, aliphatic,
and alicyclic moieties were synthesized. In the presence of
Conditions: NMP, 20 C, 10—12 h.
Table 3. Heat effect* (H) of the radical pair recombination to give R1SSR2 (5—12)
Radical pairs
Product
–H/kJ mol–1
5
191.55
6
7
8
231.35
232.37
217.73
с-C5H9S• + PriS•
с-C6H11S• + PriS•
n-C7H15S• + PriS•
PhS• + с-C6H11S•
9
229.47
228.86
223.60
189.29
10
11
12
* Quantum chemical calculations were performed by density functional theory (DFT)
method at the B3LYP/6-31++G(d,p) level using HyperChem 8.0.8 software.

Burmistrova et al.
994
Russ. Chem. Bull., Int. Ed., Vol. 69, No. 5, May, 2020
column SPB-SULFUR (30 m0.32 mm) was used (tmax
=
3,5-di-tert-butyl-o-benzoquinone, pyrocatechol sulfides are
formed in 93—96% yields. The suggested approach can be
used to convert the undesirable and toxic low-molecular
weight thiols present in the distillation fractions of crude oil
into the mono- and disulfides with potential biological ac-
tivities.
= 320 C), carrier gas was helium. Column temperature was
programed as follows: T0 = 50 C (isotherm 1 min), T1 = 200 C
(isotherm 10 min), T2 = 280 C (isotherm 60 min), total analy-
sis time  = 82 min. The products were identified by comparing
the recorded mass spectra with those from the NIST14 (2014
version) mass spectrometry database (http://www.nist.gov)
implemented in the Shimadzu GCMS-QP2010 Ultra instrument
software.
Experimental
The mass spectrometry data are given in Table 4.
Redox potentials of the compounds (C = 5 mmol L–1) were
measured in NMP under anaerobic conditions by cyclic volt-
ammetry (CV) using a three-electrode cell and a VersaSTAT
potentiostat. The working electrode was a stationary platinum
electrode with a diameter of 3 mm, the auxiliary electrode was
platinum plate (S = 32 mm2). The reference electrode was
Ag/AgCl/KCl with porous membrane. Potential sweep rate was
0.2 V s–1. Supporting electrolyte was 0.15 M Bu4NClO4 (99%,
Acros) twice crystallized from 50% aqueous EtOH and dried
under vacuum at 50 C for 48 h.
Commercially available propane-1-thiol, propane-2-thiol,
butane-1-thiol, cyclopentanethiol, cyclohexanethiol, heptane-
1-thiol, thiophenol (98%, Alfa Aesar), 2-methylpropane-2-
thiol (99%, Sigma Aldrich), 4-methoxythiophenol (97%, Sigma
Aldrich), 2,6-di-tert-butyl-4-mercaptophenol (Sigma Aldrich),
4-mercaptopyridine (95%, Sigma Aldrich), 2-mercaptothiazole
(98%, abcr GmbH), 3,5-di-tert-butyl-o-benzoquinone (Q1)
(98%, Alfa Aesar), and 3,4,5,6-tetrachloro-o-benzoquinone
(Q6) (97%, Sigma Aldrich) were used as purchased. 3,6-Di-tert-
butyl-o-benzoquinone (Q2), 3,6-di-tert-butyl-4,5-difluoro-o-
benzoquinone (Q3), 3,6-di-tert-butyl-4-chloro-o-benzoquinone
(Q4), 3,6-di-tert-butyl-4,5-dimethoxy-o-benzoquinone (Q5),
and 2,4-di-tert-butyl-6-(2,6-diisopropylphenylimino)quinone
(imQ) were kindly provided by the staff of the Laboratory of the
Metal Complexes with Redox-Active Ligands of the G. A.
Razuvaev Institute of Organometallic Chemistry of the Russian
Academy of Sciences (Nizhny Novgorod). Synthesis and physi-
cochemical properties of quinones have been earlier de-
scribed.34—37 The solvents were purified and dried following the
standard procedures.38
Quantum chemical calculations were performed by the DFT
method at the B3LYP/6-31++G(d,p) level of theory using
HyperChem 8.0.8 software.
Synthesis of unsymmetrical disulfides 3—14 (general proce-
dure). To a solution of Q2 (0.025 g, 0.11 mmol) in NMP (2 mL),
a solution of the appropriate thiol (C = 0.11—0.22 mmol) in
NMP (1 mL) was added. The mixture was stirred at room tem-
perature for 10—12 h using an orbital shaker. After discoloration
of the mixture, the reaction was considered complete. The prod-
uct mixture was analyzed by GC/MS. Unsymmetrical disulfides
3—8 were synthesized by the reaction of R1SH (0.05 mmol)
with R2SH (0.082 mmol) in the presence of Q2 (0.11 mmol) at
a ratio Q2 : RSH of 1 : 1.2. Unsymmetrical disulfides 9—14 were
obtained by oxidative coupling of R1SH (0.132 mmol) with R2SH
(0.088 mmol) in the presence of Q2 (0.11 mmol) at a ratio
Q2 : RSH of 1 : 2.
The products of oxidative coupling of thiols with o-benzo-
quinones and o-iminobenzoquinone were analyzed by gas chro-
matography/mass spectrometry (GC/MS) using a Shimadzu
GCMS-QP2010 Ultra instrument that combines gas chroma-
tography and mass spectrometry detection (electron impact
ionization, energy of ionizing electrons of 70 eV). Capillary
Table 4. Mass spectrometry data for the synthesized di- and monosulfides 1—13
Compound
m/z (I (%))
ret/min
1аa
1ba
310 [M]+ (2), 294 (50), 279 (100), 251 (57), 237 (6), 223 (6), 207 (26), 195 (18), 91 (14), 77 (8), 57 (93)
330 [M]+ (7), 315 (20), 285 (3), 279 (58), 251 (26), 222 (10), 208 (18), 192 (9), 125 (12),
149 (24), 109 (4), 91 (14), 77 (8), 57 (60)
48.461
50.112
2b
3
184 [M]+ (43), 142 (67), 109 (20), 78 (100), 65 (15), 43 (49)
12.150
20.061
26.027
10.389
21.757
13.819
22.683
10.524
12.208
13.063
16.259
12.933
214 [M]+ (51), 172 (17), 139 (100), 124 (23), 108 (87), 96 (44), 77 (18), 63 (16), 47 (18), 41 (62)
314 [M]+ (5), 313 (8), 312 (43), 297 (13), 269 (3), 237 (8), 119 (5), 73 (6), 57 (29), 43 (100)
185 [M]+ (21), 143 (16), 110 (5), 79 (20), 75 (5), 51 (23), 46 (20)
211 (5), 143 (22), 110 (7), 98 (7), 78 (12), 69 (56), 51 (26), 41 (100)
225 (7), 143 (23), 115 (3), 110 (11), 98 (12), 83 (53), 78 (25), 55 (100)
231 (2), 149 (45), 117 (19), 83 (36), 72 (40), 58 (41), 55 (100), 45 (43), 41 (67)
176 [M]+ (30), 108 (80), 99 (2), 69 (88), 43 (100)
4
5c
6
7c
8
9
10
11
12
13
190 [M]+ (29), 114 (3), 108 (100), 83 (55), 47 (3), 43 (73)
206 [M]+ (23), 164 (13), 131 (18), 108 (8), 47 (3), 43 (67)
224 [M]+ (27), 142 (100), 109 (18), 78 (39), 83 (31), 65 (12), 55 (64)
198 [M]+ (14), 142 (62), 109 (11), 78 (32), 57 (100)
a Column temperature program: T0 = 30 C (isotherm 5 min), T1 = 50 C (isotherm 5 min), T2 = 150 C (isotherm 10 min), T3 = 200 C
(isotherm 10 min), T4 = 250 C (isotherm 5 min), T5 = 280 C (isotherm 40 min),  = 88 min.
b For compounds 2 and 14, identical m/z values were recorded.
c Column temperature program: T0 = 50 C (isotherm 5 min), T1 = 280 C (isotherm 70 min),  = 82 min.

Unsymmetrical disulfides
Russ. Chem. Bull., Int. Ed., Vol. 69, No. 5, May, 2020
995
22. N. T. Berberova, E. V. Shinkar, Russ. Chem. Bull., 2000, 49,
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23. I. V. Smolyaninov, O. V. Pitikova, E. S. Rychagova, E. O.
Korchagina, A. I. Poddel´sky, S. A. Smolyaninova, N. T.
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s11172-016-1669-4.
This work was financially supported by the Russian
Foundation for Basic Research (Project No. 18-29-24001).
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Received August 19, 2019;
in revised form January 15, 2020;
accepted January 24, 2020