Electrochemical transformations and antiradical activity of asymmetrical RS-substituted pyrocatechols

Article abstract of DOI:10.1007/s11172-018-2299-9

Redox transformations of sulfides 1–8 combining a fragment of sterically hindered pyrocatechol with alkyl, cycloalkyl, and aromatic substituents were studied. The first step of electrooxidation of thioethers affords o-benzoquinones. The introduction of the redox-active thioether group extends the range of redox properties of pyrocatechols. In the second step, the thioether fragment is involved in the quasi-reversible anodic process, and the number of electrons participating in the electrode reaction depends on the structure of the hydrocarbon group bonded to the sulfur atom. The reactivity of compounds 1–8 toward O₂^?– was evaluated on the basis of the electrochemical data. Cyclopentyl, phenyl, or benzyl substituents in the thioether group exert a greater effect on the antiradical activity than the alkyl moieties. The formation of an o-semiquinolate radical anion in the reaction of pyrocatechol thioethers with KO₂ was detected by the ESR method. It was shown using the reaction with the stable 2,2-diphenyl-1-picrylhydrazyl radical as an example that RS-functionalized pyrocatechols show a higher antiradical activity compared to 3,5-di-tert-butylpyrocatechol.

Full text of DOI:10.1007/s11172-018-2299-9

Russian Chemical Bulletin, International Edition, Vol. 67, No. 10, pp. 1857—1867, October, 2018
1857
Electrochemical transformations and antiradical activity
of asymmetrical RS-substituted pyrocatechols
I. V. Smolyaninov,a,b O. V. Pitikova,c A. I. Poddel´sky,d and N. T. Berberovaa
aAstrakhan State Technical University,
16 ul. Tatishcheva, 414056 Astrakhan, Russian Federation.
E-mail: ivsmolyaninov@gmail.com
bSouthern Scientific Center, Russian Academy of Sciences,
41 ul. Chekhova, 344006 Rostov-on-Don, Russian Federation
cIngineering Centre LLC "Gazprom dobycha Astrakhan,"
Building 8, 6 ul. Savushkina, 414056 Astrakhan, Russian Federation
dG. A. Razuvaev Institute of Organometallic Chemistry, Russian Academy of Sciences,
49 ul. Tropinina, 603137 Nizhni Novgorod, Russian Federation
Redox transformations of sulfides 1—8 combining a fragment of sterically hindered pyro-
catechol with alkyl, cycloalkyl, and aromatic substituents were studied. The first step of electro-
oxidation of thioethers affords o-benzoquinones. The introduction of the redox-active thioether
group extends the range of redox properties of pyrocatechols. In the second step, the thioether
fragment is involved in the quasi-reversible anodic process, and the number of electrons par-
ticipating in the electrode reaction depends on the structure of the hydrocarbon group bonded
•–
to the sulfur atom. The reactivity of compounds 1—8 toward O2 was evaluated on the basis
of the electrochemical data. Cyclopentyl, phenyl, or benzyl substituents in the thioether group
exert a greater effect on the antiradical activity than the alkyl moieties. The formation of an
o-semiquinolate radical anion in the reaction of pyrocatechol thioethers with KO2 was de-
tected by the ESR method. It was shown using the reaction with the stable 2,2-diphenyl-1-
picrylhydrazyl radical as an example that RS-functionalized pyrocatechols show a higher anti-
radical activity compared to 3,5-di-tert-butylpyrocatechol.
Key words: cyclic voltammetry, catechol thioethers, redox transformations, antiradical
activity, o-benzoquinones.
Pyrocatechol fragment is contained in the structures
of physiologically active compounds including flavonoids,
amines, and alkaloids. It is known that pyrocatechols and
hydroquinones, on the one hand, are inhibitors of free
radical processes.1,2 On the other hand, they act as gener-
ators of reactive oxygen species (ROS), such as superoxide
radical anion and hydrogen peroxide. Pyrocatechols ex-
hibit antibacterial, antitumor, and cytotoxic activity.3—6
Sulfides are referred to antioxidants involved in the de-
struction of hydroperoxides without radical species forma-
tion.7 Thioethers prevail in the structures of biologically
active compounds and pharmaceuticals.8 Sulfides con-
taining aromatic groups are suitable building blocks for
the synthesis of potent drugs,9 organic materials, and
polymers.10
The introduction of chalcogens (S, Se, and Te) into
the structures of phenolic antioxidants, synthetic deriva-
tives of tocopherols, and flavonoids, including those
containing pyrocatechol moieties, provides new possi-
bilities for the design of polyfunctional antioxidants.11—13
The majority of antioxidants are electrochemically active
compounds and, therefore, the introduction of an addi-
tional redox center as a thioether group leads to the exten-
sion of the range of redox properties. The oxidation of
sulfides forms sulfoxides and sulfones thus favoring an
increase in their biological activity.14
Antioxidants with thioether groups evoke special inter-
est in the recent time.15,16 Phenols and hydroquinones,
being redox-active compounds, participate in electron
transfer reactions. Sulfur-containing o- and p-benzoqui-
nones have biological activity17—19 and unusual physico-
chemical properties,20—23 due to which they are attractive
for using as ligands in coordination chemistry.24,25
Electrochemical methods are universally recognized
for studying the properties and preparation of redox-
modulating agents26,27 and are considered as a useful tool
for modeling biological (metabolic) redox reactions.28,29
Electrochemical approaches make it possible to evaluate
the potential antiradical activity30,31 and also to explain
a possible mechanism of action of the studied com-
pounds.32,33 In the presence of oxygen, cyclic redox
transformations pyrocatechol/o-benzoquinone result in
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 1857—1867, October, 2018.
1066-5285/18/6710-1857 © 2018 Springer Science+Business Media, Inc.

1858
Russ. Chem. Bull., Int. Ed., Vol. 67, No. 10, October, 2018
Smolyaninov et al.
Microelectrolysis of thioethers 1—8 was performed on
a PI-50-1.1 potentiostat at stationary platinum electrodes, which
were plates with a surface area of 30 mm2, in a 5-mL undivided
three-electrode cell under anaerobic conditions. The Ag/AgCl/
KCl electrode with a conducting water-impermeable membrane
was used as a reference electrode. Pyrocatechol was added to an
electrochemical cell containing a solution of the supporting
electrolyte (0.1 М Bu4NClO4) in acetonitrile. Electrolysis was
performed in the potentiostatic mode at a potential of 1.40 V.
Microelectrolysis of thioethers 1—8 in the presence of oxygen
was carried out in the potentiostatic mode at a potential of –1.20 V
for 30 min. Thioethers (С = 3 mmol L–1) were introduced into an
aerated solution of the supporting electrolyte (0.1 М Bu4NClO4)
in acetonitrile.
the anti/prooxidant activity of this redox pair due to the
possibility of formation of superoxide radical anion.34
The present study is aimed at studying the electro-
chemical transformations of 3-alkylsulfanyl-, 3-cyclo-
alkylsulfanyl-, 3-arylsulfanyl-, and 3-benzylsulfanyl-
substituted pyrocatechols 1—8, establishing the mecha-
nism of their oxidation, and evaluating the influence of
the thioether group on the reactivity toward superoxide
radical anion, КО2, and diphenylpicrylhydrazyl.
The reaction with the electrogenerated superoxide radical
anion was performed in acetonitrile under the conditions of CV
experiment as follows. Oxygen was preliminarily passed through
the solution for 10 min to saturation (3 mmol L–1).36 Cyclic
voltammograms of oxygen reduction were detected in the range
from –0.5 to –1.20 V at a potential scan rate of 0.2 V s–1 in the
absence of pyrocatechols for the determination of the primary
current values of the anodic and cathodic peaks. The concentra-
1—8: R = Bun (1), C5H11 (2), C6H13 (3), C7H15 (4), C8H17 (5),
cyclo-C5H9 (6), Ph (7), Bn (8)
tion of pyrocatechols 1—8 was varied from 0.5 to 6.0 mmol L–1
.
The efficiency of superoxide radical anion scavenging by pyro-
catechols was estimated from the IC50 value (concentration of
pyrocatechol necessary for decreasing the superoxide radical
anion concentration by 50% (from the initial level based on cur-
rent)), which depends on the change in the anodic current of
superoxide radical anion reoxidation in the presence of additives of
Experimental
Commercially available reagents, viz., 3,5-di-tert-butyl-o-
benzoquinone (98+%, Alfa Aesar), 3,5-di-tert-butylpyrocatechol
(98%, Aldrich), 1-butanethiol, 1-pentanethiol, 1-hexanethiol,
1-heptanethiol, 1-octanethiol, cyclopentanethiol, thiophenol
(98+%, Alfa Aesar), benzylthiol (99%, Alfa Aesar), 2,2-diphenyl-
1-picrylhydrazyl (DPPH) radical (Aldrich), potassium peroxide
KO2 (Acros Organics), cis-dicyclohexano-18-crown-6 (98%,
Aldrich), and Bu4NOH•30H2O (>99%, Sigma-Aldrich) were
used as received. The solvents used were purified and dehydrated
according to standard procedures.35
pyrocatechols Ipa (Ipa = ((Iipa – I0pa)/I0pa)•100%), where I0
pa
and Ii are the current values of the reoxidation peak of the
superopxaide radical anion to oxygen in the absence of the studied
(I0pa) compound and in the presence of this compound in concen-
tration i (Iipa).37 As the current of the reoxidation peak of the
superoxide radical anion decreases by 50% of the initial value, 50%
of the electrogenerated superoxide radical anion detected at the re-
verse CV branch were consumed in the reaction with pyrocatechols.
The IC50 value was determined from the dependence of Ipa of
the superoxide radical anion on the pyrocatechol concentration.
Spectroelectrochemical studies were carried out in a three-
electrode cell (quartz cuvette). A platinum wire (S = 3 cm2) was
placed in a cuvette containing a 0.1 М solution of Bu4NClO4 and
pyrocatechols 1 or 8 (С = 0.5 mmol L–1), and absorption spec-
tra were recorded in the UV and visible ranges (340—840 nm)
using an SF-103 spectrophotometer. The platinum wire served
as an auxiliary electrode. The Ag/AgCl/KCl electrode with
a conducting water-impermeable membrane was used as a refer-
ence electrode.
1H (200 MHz) and 13C NMR (50 MHz) spectra were re-
corded on a Bruker AVANCE DPX-200 spectrometer using
tetramethylsilane as an internal standard and CDCl3 as a solvent.
IR spectra were measured on an FSM 1201 FTIR spectrometer
in KBr pellets.
Elemental analysis was carried out on Euro EA 3000 (C, H,
N) and Analytik Jena multi EA 5000 (C, S, N, Cl) elemental
analyzers.
ESR spectra were detected on a Bruker EMX spectrometer
(working frequency ~9.5 GHz). Hyperfine coupling constants
were determined using simulation of theoretical spectra by the
WINEPR Simfonia 1.25 program (Bruker). Electronic absorption
spectra were recorded on SF-103 and SF-104 spectrophotometers
(in a range of 300—1100 nm) at room temperature.
The reaction of pyrocatechol 2 (С = 1 mmol L–1) with KO2
(С = 2 mmol L–1) was conducted in anhydrous DMF in the
Electrochemical transformations of compounds 1—8 were
studied by cyclic voltammetry (CV) in a three-electrode cell with
an IPC-pro potentiostat under argon. The supporting electrolyte,
0.1 М Bu4NClO4 (99%, Acros), was twice recrystallized from
aqueous EtOH and dried in vacuo at 50 C for 48 h. The con-
presence of cis-dicyclohexano-18-crown-6 (С = 0.05 mol L–1
)
in order to increase the solubility of KO2, and the formation of
the reaction products was monitored by the ESR method.
Electronic absorption spectra were recorded on an SF-104 spec-
trophotometer in a range of 300—600 nm at room temperature.
The reactions of thioethers 4 and 5 (С = 1 mmol L–1) with
1 equiv. KO2 were carried out in anhydrous DMF in a spectro-
photometric cell in the presence of cis-dicyclohexano-18-crown-6
(С = 0.05 mol L–1).
centration of the studied compounds was 0.5—6.0 mmol L–1
.
A stationary glassy carbon (GC) electrode (d = 2 mm) (or
platinum electrode (Pt) (d = 2 mm)) served as a working elec-
trode, a platinum wire (S = 18 mm2) was the auxiliary electrode,
and Ag/AgCl/KCl with a water-impermeable membrane was the
reference electrode.
The main parameters of the antiradical activity (ЕС50, TEC50,
AE, nDPPH) of the compounds in the reactions with the DPPH

Asymmetrical catechol thioethers
Russ. Chem. Bull., Int. Ed., Vol. 67, No. 10, October, 2018
1859
radical were detected in acetonitrile at 298 K. The EC50 value
(concentration of the antioxidant necessary for decreasing the
amount of the DPPH radical by 50% compared to the initial
value) was determined after achieving the equilibrium state, which
was established in 30—60 min after beginning of the reaction. To
determine ЕС50, the dependence of the residual concentration
of the stable radical on the molar ratio expressed in the number
of moles of the antioxidant per 1 mole of the stable radical was
plotted. The concentration of compounds 1—8 was varied from
C6H1); 7.28 (s, 1 H, OH). 13С NMR, : 14.03, 22.57, 28.87,
28.98, 29.32, 29.42, 31.47, 31.66, 35.10, 36.83, 37.86, 115.88,
115.94, 135.82, 140.55, 143.14, 145.21. Found (%): C, 71.92; H, 9.74;
S, 9.13. C21H34O2S. Calculated (%): C, 71.95; H, 9.78; S, 9.15.
4,6-Di-tert-butyl-3-(octylsulfanyl)benzene-1,2-diol (5). The
yield was 0.72 g (66%). White powder, m.p. 65—67 C. IR,
/cm–1: 3496, 3319 (O—H), 2984, 2957, 2924, 2852 (С—H).
1H NMR, : 0.89 (t, 3 H, CH3, J = 6.1 Hz); 1.20—1.40 (m, 10 H,
CH2); 1.41 (s, 9 H, But); 1.49 (s, 9 H, But); 1.55—1.80 (m, 2 H,
CH2); 2.66 (t, 2 H, CH2S, J = 7.4 Hz); 5.56 (br.s, 1 H, OH);
6.91 (s, 1 H, C6H1); 7.27 (s, 1 H, OH). 13С NMR, : 14.05, 22.61,
29.02, 29.10, 29.17, 29.32, 29.41, 31.47, 31.77, 35.10, 36.83, 37.87,
115.88, 115.94, 135.82, 140.54, 143.15, 145.21. Found (%):
C, 72.50; H, 9.91; S, 8.80. C22H36O2S. Calculated (%): C, 72.48;
H, 9.95; S, 8.79.
4,6-Di-tert-butyl-3-(cyclopentylsulfanyl)benzene-1,2-diol (6).
The yield was 0.67 g (70%). White powder, m.p. 68 C. IR,
/cm–1: 3485, 3275 (O—H), 2957, 2907, 2867 (C—H). 1H NMR,
: 1.42 (s, 9 H, But); 1.50 (s, 9 H, But); 1.55—1.85 (m, 6 H, CH2,
cyclopentyl); 1.90—2.15 (m, 2 H, CH2, cyclopentyl); 3.14 (q, 1 H,
CH, cyclopentyl, 3JH,H = 7 Hz); 5.58 (s, 1 H, OH); 6.92 (s, 1 H,
C6H1); 7.27 (s, 1 H, OH). 13С NMR, : 24.47, 29.31, 31.70, 33.93,
35.08, 36.93, 51.07, 115.80, 115.99, 135.80, 140.43, 143.28, 145.62.
Found (%): C, 71.18; H, 8.76; S, 9.94. C19H28O2S. Calculat-
ed (%): C, 71.21; H, 8.81; S, 10.00.
5 to 50 mol L–1
.
The number of molecules of transformed DPPH radical
(nDPPH) was calculated by the equation nDPPH = С0(DPPH)/2•
•EC50, where C0(DPPH) is the initial concentration of the DPPH
radical (С = 50 mol L–1). The antiradical efficiency (AE) was
calculated by the equation АЕ = 1/(EC50•ТЕС50), where ТЕС50
is the time of achievement of the equilibrium state at the anti-
oxidant concentration equal to EC50
.
Compounds 1—8 were synthesized using the modified known
procedure.38 A solution of thiol (4.5 mmol) in ethanol (10 mL)
was added dropwise to a solution of 3,5-di-tert-butyl-o-benzo-
quinone (0.66 g, 3.0 mmol) in ethanol (20 mL) over 4—5 h at room
temperature under argon to the complete bleaching of the reac-
tion mixture. The solvent was evaporated under reduced pressure.
The formed precipitate was recrystallized from acetonitrile.
4,6-Di-tert-butyl-3-(butylsulfanyl)benzene-1,2-diol (1). The
yield was 0.41 g (44%). White powder, m.p. 65—67 С. IR,
/cm–1: 3502, 3325 (O—H), 2989, 2958, 2930, 2909, 2870 (C—H).
1H NMR, : 0.94 (t, 3 H, CH3, Bun, 3JH,H = 7.2 Hz); 1.40 (s, 9 H,
But); 1.45 (m, 2 H, CH2, Bun); 1.49 (s, 9 H, But); 1.67 (m, 2 H,
CH2, Bun); 2.66 (t, 2 H, CH2, Bun, 3JH,H = 7.4 Hz); 5.56 (br.s,
1 H, OH); 6.90 (s, 1 H, C6H1); 7.28 (s, 1 H, OH). 13С NMR, :
13.67, 22.18, 29.32, 31.47, 35.10, 36.84, 37.58, 115.89, 115.92,
135.85, 140.56, 143.17, 145.23. Found (%): C, 69.96; H, 9.10;
S, 10.31. C18H28O2S. Calculated (%): C, 70.08; H, 9.15; S, 10.39.
4,6-Di-tert-butyl-3-(pentylsulfanyl)benzene-1,2-diol (2). The
yield was 0.42 g (43%). White powder, m.p. 45—77 C. IR,
/cm–1: 3502, 3320 (O—H), 2991, 2954, 2927, 2866, 2870
(C—H). 1H NMR, : 0.92 (t, 3 H, CH3, n-pentyl, 3JH,H = 6.4 Hz);
1.42 (s, 9 H, But); 1.25—1.50 (m, 4 H, CH2, n-pentyl); 1.50
(s, 9 H, But); 1.55—1.85 (m, 2 H, CH2, n-pentyl); 2.67 (t, 2 H,
S—CH2, 3JH,H = 7.4 Hz); 5.58 (s, 1 H, OH); 6.92 (s, 1 H, C6H1);
7.30 (s, 1 H, OH). 13С NMR, : 13.89, 22.30, 29.09, 29.31, 31.15,
31.46, 35.09, 36.83, 37.84, 115.87, 115.93, 135.82, 140.54, 143.13,
145.20. Found (%): C, 70.72; H, 9.33; S, 9.95. C19H30O2S.
Calculated (%): C, 70.76; H, 9.38; S, 9.94.
4,6-Di-tert-butyl-3-(phenylsulfanyl)benzene-1,2-diol (7). The
yield was 0.62 g (63%). White powder, m.p. 85—87 C. IR,
/cm–1: 3536, 3374 (O—H), 3073, 3055 (C—H аром.), 3001,
1
2961, 2913, 2869 (С—H). H NMR, : 1.44 (s, 9 H, But); 1.46
(s, 9 H, But); 5.61 (s, 1 H, OH); 6.82 (s, 1 H, C6H1); 6.88—7.02
(m, 2 H, Ph); 7.06 (s, 1 H, OH); 7.10—7.35 (m, 3 H, Ph). 13С NMR,
: 29.30, 31.30, 35.28, 36.78, 110.89, 116.59, 125.27, 125.48,
129.14, 136.34, 137.28, 140.94, 144.03, 145.44. Found (%):
C, 73.10; H, 7.40; S, 9.74. C20H24O2S. Calculated (%): C, 73.13;
H, 7.37; S, 9.76.
3-(Benzylsulfanyl)-4,6-di-tert-butylbenzene-1,2-diol (8). The
yield was 0.48 g (46%). White powder, m.p. 90—92 C. IR,
/cm–1: 3486, 3259 (O—H), 3089, 3062 (C—H arom.), 2958,
2910, 2870 (С—H). 1H NMR, : 1.43 (s, 9 H, But); 1.53 (s, 9 H,
But); 3.84 (s, 2 H, CH2); 5.56 (br.s, 1 H, OH); 6.95 (s, 1 H,
C6H1); 7.30 (s, 1 H, OH); 7.24—7.36 (m, 5 H, Ph). 13С NMR,
: 29.31, 31.54, 35.15, 36.90, 42.22, 115.30, 116.05, 127.65, 128.79,
128.90, 136.28, 136.84, 140.74, 143.30, 145.40. Found (%):
C, 73.60; H, 7.68; S, 9.30. C21H26O2S. Calculated (%): C, 73.64;
H, 7.65; S, 9.36.
4,6-Di-tert-butyl-3-(hexylsulfanyl)benzene-1,2-diol (3). The
4,6-Di-tert-butyl-3-(pentylsulfanyl)-o-benzoquinone (9) was
synthesized by the oxidation of pyrocatechol 2 (0.3225 g,
0.001 mol) in diethyl ether (40 mL) with an aqueous solution of
K3Fe(CN)6 (5 equiv.) in an alkaline medium of KOH (0.112 g).
The product was extracted with diethyl ether and washed with
water twice. Pure o-benzoquinone 9 was obtained by chromato-
graphy of the reaction mixture using a hexane—ethyl acetate
(1 : 1) mixture as an eluent. The yield was 0.195 g (61%), m.p.
15—17 C. IR, /cm–1: 1655, 1672, 1736 (С=O), 2873, 2931, 2961
yield was 0.50 g (49%). White powder, m.p. 48 C. IR, /cm–1
:
3502, 3275 (O—H), 2991, 2957, 2930, 2917, 2866 (C—H). 1H NMR,
: 0.90 (t, 3 H, CH3, J = 6.2 Hz); 1.20—1.40 (m, 6 H, CH2); 1.41
(s, 9 H, But); 1.50 (s, 9 H, But); 1.55—1.80 (m, 2 H, CH2); 2.66
(t, 2 H, CH2S, J = 7.4 Hz); 5.58 (br.s, 1 H, OH); 6.91 (s, 1 H,
C6H1); 7.29 (s, 1 H, OH). 13С NMR, : 13.99, 22.48, 28.68,
29.31, 29.39, 31.40, 31.47, 35.09, 36.83, 37.87, 115.87, 115.94,
135.82, 140.55, 143.14, 145.20. Found (%): C, 71.32; H, 9.50;
S, 9.50. C20H32O2S. Calculated (%): C, 71.38; H, 9.58; S, 9.53.
4,6-Di-tert-butyl-3-(heptylsulfanyl)benzene-1,2-diol (4). The
1
(C—H). H NMR, : 0.75—0.85 (m, 3 H, CH3); 1.18 (s, 9 H,
But); 1.20—1.35 (m, 6 H, 3 CH2, C5H11); 1.43 (s, 9 H, But);
2.45—2.60 (m, 2 H, SCH2, C5H11); 6.92 (s, 1 H, C6H1). 13С NMR,
: 12.86, 21.11, 26.60, 27.58, 28.73, 30.22, 34.05, 36.09, 40.14,
116.55, 135.91, 146.62, 159.98, 178.81, 181.23. Found (%):
C, 70.81; H, 9.30; S, 9.98. C19H30O2S. Calculated (%): С, 70.76;
Н, 9.38; S, 9.94.
yield was 0.47 g (45%). White powder, m.p. 47 C. IR, /cm–1
:
3495, 3320 (O—H), 2991, 2988, 2957, 2927, 2863 (C—H). 1H NMR,
: 0.90 (t, 3 H, CH3, J = 6.0 Hz); 1.20—1.40 (m, 8 H, CH2); 1.41
(s, 9 H, But); 1.49 (s, 9 H, But); 1.55—1.75 (m, 2 H, CH2); 2.66
(t, 2 H, CH2S, J = 7.4 Hz); 5.57 (br.s, 1 H, OH); 6.91 (s, 1 H,

1860
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Smolyaninov et al.
I/mA
0.16
Results and Discussion
1
2
Many works are devoted to electrosynthesis of pyro-
catechol thioethers containing various heterocyclic frag-
ments.39—43 However, data on the redox properties and
mechanism of electrochemical transformations of similar
compounds are lacking. In this work, a series of asym-
metrical thioethers 1—8 was obtained by the reactions of
3,5-di-tert-butyl-o-benzoquinone with various thiols.44
The electrochemical properties of thioethers 1—8 were
studied by cyclic voltammetry (CV) in acetonitrile at the
GC electrode. The redox potentials of the studied com-
pounds are presented in Table 1. The electrooxidation of
sulfides 1—8 proceeds similarly in two consecutive steps
in acetonitrile at the potential scan to +1.80 V (Fig. 1).
The first two-electron peak is irreversible and corre-
sponds to the oxidation of the pyrocatechol fragment. The
length of the alkyl group does not affect the values of
oxidation potentials of compounds 1—5. On going from
compound 3 to compound 7, the oxidation potential in-
significantly shifts to the anodic region (0.04 V). The
presence of the thioether group in the pyrocatechol frag-
ment favors the anodic shift of the oxidation potentials of
compounds 1—8 compared to 3,5-di-tert-butylpyrocate-
chol (+1.11 V), which indicates its electron-acceptor
character. The cathodic peak observed on the reverse
branches of voltammograms in the range from 0.44 to
0.30 V presumably corresponds to the reduction of the
H+Q—S—R species formed due to the chemical reac-
tion following the electron transfer.45 The ЕСЕ mecha-
nism takes place for pyrocatechols in aprotic organic
solvents, and two EC steps (E is the electrochemical stage,
and C is the chemical stage) converge into one electrode
process.45,46 The electrochemical oxidation of thio-
ethers 1—8 primarily results in the generation of an un-
0.11
0.06
0.01
–0.04
–0.4
0
0.4
0.8
1.2
1.6 E/V
Fig. 1. CV curves of the oxidation of thioethers 4 (1) and 8 (2)
in the potential scan range from –0.7 to +1.8 V (MeCN,
GC anode, Ag/AgCl/KCl, 0.1 M NBu4ClO4 , C = 3 mmol L–1
,
v = 0.2 V s–1, argon).
stable dication undergoing partial deprotonation in a solu-
tion (Scheme 1).
The use of the platinum anode made it possible to
detect the reduction peak for the H+Q—S—R species and
proton (–0.03 V), which was identified by the addition of
concentrated perchloric acid. The values of oxidation
potentials for sulfides 1—8 at the GC or Pt anodes are
almost identical.
The introduction of the thioether group results in the
extension of the range of redox properties of the studied
compounds due to the appearance of an additional redox
transition at 1.54—1.63 V (see Table 1). It is known that
the electrochemical oxidation of sulfides, depending on
the structure, solvent nature, and the presence of a nu-
cleophilic reagent, proceeds as one- or two-electron
process.47 The studied thioethers can be divided into
two groups. The first group contains compounds 1—5
Table 1. Oxidation potentials of compounds 1—9 according to the CV data*
Com-
pound
Eox1
Eox2
Eox
Ic/Ia
–Ered1/2(Q/SQ)/V
E
ox(CatH—S—R)/V
1/2
V
1
2
3
4
5
6
7
8
9
1.20
1.21
1.19
1.20
1.20
1.21
1.23
1.18
1.58
1.60
1.59
1.60
1.59
1.59
1.59
1.63
1.54
—
1.55
0.38
0.33
0.33
0.40
0.39
0.58
0.40
0.70
—
0.40
0.41
0.40
0.39
0.40
0.41
0.37
0.40
0.41
–0.07
–0.06
–0.07
–0.08
–0.08
–0.06
–0.01
–0.05
—
1.54
1.55
1.55
1.54
1.55
1.60
1.51
—
Note. Eox1 and Eox2 are potentials of the anodic peaks, Eox
is the half-wave potential for the
second quasi-reversible oxidation peak, Iс/Ia is the ratio of 1c/u2rrents of the reverse cathodic and
direct anodic peaks, Ered1/2(Q/SQ) is the half-wave potential for the reduction of electrogenerated
o-benzoquinones (Q) to o-benzosemiquinones (SQ), and Eox(CatH—S—R) is the potential of the
oxidation peak of the singly deprotonated form of pyrocatechol thioethers.
* GC electrode, MeCN, v = 0.20 V s–1, 0.1 М NBu4ClO4, С = 3 mmol L–1, Ar, Ag/AgCl/KCl (sat.).

Asymmetrical catechol thioethers
Russ. Chem. Bull., Int. Ed., Vol. 67, No. 10, October, 2018
1861
Scheme 1
Scheme 2
(see Fig. 1, curve 1), which are characterized by the par-
ticipation of two electrons in the second anodic process
(see Scheme 1, dication I), and thioethers 6—8 character-
ized by the one-electron level based on current are in the
second group (see Fig. 1, curve 2).
The low stability of dications I generated by the elec-
trooxidation of compounds 1—5 (see Table 1, ratio Ic/Ia)
indicates the chemical step following the electron transfer:
the C—S bond cleavage and formation of sulfoxides, sul-
fones, or sulfonium salts.47 The formation of relatively
stable radical cations (see Scheme 1, radical cation II) is
detected for the compounds with the cyclopentyl or benzyl
groups. A similar electrochemical pattern was observed
earlier in dimethylformamide at a potential of 1.65 V
(Ic/Ia = 0.53) for 3,6-di-tert-butyl-o-benzoquinone con-
taining annelated 1,2-diethiete ring.22
absorption maximum at 505 nm, whose intensity increases
with time, confirming the formation of the corresponding
o-benzoquinones (Figs 3 and 4).
Compounds containing quinonoid fragment and thio-
ether group are considered as efficient phototriggers, the
mechanism of action of which is based on the intramo-
Microelectrolyses of compounds 1—8 (2 h) at a con-
trolled potential value +1.40 V were conducted to confirm
the formation of o-quinones. A considerable decrease in
the intensity of the first oxidation peak at the pyrocatechol
fragment (conversion reaches 80%) is observed in the CV
curves of the electrolysis products, and the second anodic
peak remains unchanged by current and potential (Fig. 2).
The cathodic region exhibits the quasi-reversible one-
electron peak corresponding to the redox transition
o-benzoquinone/o-benzosemiquinone (Q—S—R/SQ—
S—R) (Scheme 2).
The bands corresponding to stretching vibrations of
the С=О bonds of o-quinones (1640—1670 cm–1) are
observed in the IR spectra of the electrolysis products.
The UV-visible absorption spectrum of the electrolysis
products exhibits an intense absorption maximum at
500—510 nm. The spectroelectrochemical studies of com-
pounds 1 and 8 at an oxidation potential of +1.40 V are
accompanied by the appearance in the visible range of an
I/mA
0.14
0.09
0.04
–0.01
–0.1
0.4
0.9
1.4
1.9 E/V
Fig. 2. CV curve of the oxidation of the product of exhaustive
electrolysis of thioether 2 (2 h) obtained at an electrolysis poten-
tial of 1.40 V (MeCN, GC electrode, Ag/AgCl/KCl, 0.1 M
NBu4ClO4 , C(2) = 3 mmol L–1, v = 0.2 V s–1, argon).

1862
Russ. Chem. Bull., Int. Ed., Vol. 67, No. 10, October, 2018
Smolyaninov et al.
A
the о-quinonoid ring results in the shift of the reduction
potential to the anodic range by 0.04 V compared to 3,5-di-
tert-butyl-o-benzoquinone, which agrees with the elect-
ron-acceptor effect of the heteroatom. The electrochemi-
cal activity of 4,6- di-tert-butyl-3-pentylthio-о-benzo-
quinone (9) obtained by the counter synthesis is analogous
to that of the electrolysis product generated by the electro-
oxidation of compound 2 (see Table 1).
0.6
0.5
0.4
0.3
0.2
0.1
Quinonoid/pyrocatechol (hydroquinone) fragment is
a moiety of many natural compounds with diverse bio-
logical activity. The presence of the sulfide fragment, which
is transformed into sulfone upon oxidation, makes it pos-
sible to enhance the pharmacological activity of quinonoid
compounds.14 The redox potential value of the quinone/
hydroquinone (pyrocatechol) pair is used for the prediction
of anti/prooxidant activity. The ratio of the oxidized/re-
duced forms affects the balance of cytotoxic and cytopro-
tector properties of compounds of this type.49,50 One of
intermediates involved in the redox cycle of quinones is
superoxide radical anion, which includes in the pull of
reactive oxygen species. Superoxide radical anion finds
use in electrocatalysis and synthesis of organic compounds
and is also applied for neutralization of dangerous chem-
ical waste.51,52 Owing to this, the reactions of pyrocatechol
thioethers with superoxide radical anion were studied
under the conditions of CV experiment using spectral
methods (UV-visible and ESR spectroscopy), as well as
their reactions with КО2. The CV method enables one to
monitor changes in oxygen or analyte in the course of the
reaction.
The electrochemical pattern changes upon the intro-
duction of pyrocatechols into the solution (Figs 5 and 6).
A prepeak (A) appears in the direct branch of the voltam-
mogram at the potential shifted to the anodic region. In
the reverse branch of the voltammogram, the anodic peak
of superoxide radical anion oxidation (C) decreases by
current and an additional peak (B) appears in the poten-
tial range from –0.08 to 0.01 V. An increase in the pyro-
catechol concentration decreases the reversibility of oxy-
gen reduction with a decrease in the anodic peak current
(C). This fact is explained by the participation of the
electrogenerated superoxide radical anion in the homo-
geneous chemical reaction in a solution. The changes
observed allow one to evaluate the reactivity of pyrocat-
echol thioethers toward superoxide radical anion from
a change in the IC50 parameter.
440
540
640
740
840 /nm
Fig. 3. Changes in the electronic absorption spectrum of com-
pound 1 during electrolysis (45 min) at a controlled potential of
+1.40 V (Ag/AgCl/KCl) (C = 0.5 mmol L–1, MeCN).
A
0.5
0.4
0.3
0.2
0.1
440
540
640
740
840 /nm
Fig. 4. Changes in the electronic absorption spectrum of com-
pound 8 during electrolysis (60 min) at a controlled potential of
+1.40 В (Ag/AgCl/KCl) (C = 0.5 mmol L–1, MeCN).
lecular redox reaction of benzoquinones with the sulfide
bridge. The broad absorption bands in the visible spectral
range is a characteristic feature of this type of com-
pounds.48 The absorption band in the visible spectral range
for electrogenerated о-benzoquinones corresponds to the
partial intramolecular charge transfer between the bound-
ary redox orbitals (HOMO—LUMO) involving the thio-
ether group and о-quinone fragment.21,22,48 The energy
band gap value (Е) calculated from the spectral data
(parameter of absorption edge of the absorption spectrum)
is 2.2 eV, on the average, for the synthesized о-quinones.
The electrochemical data can be used for the determination
of Е = Eox1/2 – Ered1/2 only for the о-quinones generated
from compounds 6—8, which are characterized by one-
electron oxidation. As a result, values of 1.96, 1.97, and
1.91 eV close to those calculated by the spectral data were
obtained for compounds 6, 7, and 8, respectively.
To estimate the efficiency of O2•– scavenging, we used
the equation proposed earlier37 for the calculation of IC50
from a change in the anodic current of oxidation of the
superoxide radical anion in the presence of pyrocatechol
additives: Ipa = ((Iipa – I0pa)/I0pa)•100%. When the
initial peak current value of the reoxidation of superoxide
radical anion to oxygen (I0pa) decreases by 50% in the
For electrogenerated о-quinones, the nature of the
hydrocarbon group at the sulfur atom exerts no pronounced
effect on the potential values for the redox pair о-quinone/
о-semiquinone. The introduction of the sulfur atom into
•–
presence of pyrocatechol, 50% of formed O2 detected
on the reverse branch of the voltammogram were consumed

Asymmetrical catechol thioethers
Russ. Chem. Bull., Int. Ed., Vol. 67, No. 10, October, 2018
1863
I/mA
The value of conversion of pyrocatechols to o-benzo-
quinones under the microelectrolysis conditions (–1.20 V,
30 min) was used as an additional parameter that charac-
terizes the reactivity of compounds 1—8 toward O2•–. The
conversion of compounds 1—8 was estimated by a decrease
in the anodic peak current at 1.18—1.20 V corresponding
to the oxidation of the pyrocatechol moiety. It was found
that with the elongation of the alkyl group the conversion
of pyrocatechols to o-benzoquinones increases as follows:
44% (1) < 47% (2) < 50% (3) < 59% (4) < 65% (5). The
maximum value equal to 77% was obtained for compound
6 containing the cyclopentane residue. The replacement
of alkyl groups by phenyl (compound 7) or benzyl (com-
pound 8) substituents decreases the conversion to 40%.
The conversion for 3,5-di-tert-butylpyrocatechol is 45%,
which is close to the results obtained for compound 1.
Therefore, the introduction of the thioether group with
linear (С6—С8) or cyclic (cyclopentyl) alkyl substituents
leads to the enhancement of the reactivity of sulfides toward
O2•– compared to 3,5-di-tert-butylpyrocatechol.
0.08
1
2
3
0.04
0
–0.04
–0.08
–1.1 –0.9 –0.7 –0.5 –0.3 –0.1 0.1 0.3
E/V
Fig. 5. CV curves of oxygen reduction in the absence (1) and in
the presence of compound 4 in concentrations of 2 (2) and
4 mmol L–1 (3). Potential scan range from +0.5 to –1.20 V (MeCN,
GC electrode, Ag/AgCl/KCl, 0.1 M NBu4ClO4, v = 0.2 V s–1).
Here and in Fig. 6, arrow shows the direction of an intensity
decrease in the current of superoxide radical anion reoxidation
with an increase in the concentration of introduced pyrocatechol.
The prepeak (A) in the CV curves was earlier detected
for flavonoids containing pyrocatechol fragment and pyro-
catechols with electron-acceptor groups.53,54 The forma-
tion of this peak was explained54 by the formation of
a complex stabilized by hydrogen bonds between the oxy-
gen radical anion and substrate. The quantum chemical
calculations performed recently for unsubstituted pyro-
catechol also confirmed the existence of a similar com-
plex.55 The peak of oxidation of the reaction product (B)
was observed in the present study along with the prepeak
(A) in the CV curves of oxygen reduction in the presence
of pyrocatechol thioethers 1—8.
To elucidate the nature of the formed intermediate (B),
we studied the reactions of compounds 1—8 with tetra-
butylammonium hydroxide. The introduction of 2 equiv.
NBu4OH into a solution of pyrocatechols 1—8 leads to
the disappearance of the redox transition for the pyrocat-
echol fragment and the appearance of a new oxidation
peak in the potential range from 0.01 to –0.08 V. It should
be mentioned that the second anodic step corresponding
to the oxidation at the sulfur atom remains unchanged
(Fig. 7). The introduction of 1 equiv. NBu4OH favors
a twofold decrease in the current of the oxidation peak of
the pyrocatechol group and the appearance of an anodic
peak identical to that appeared upon the addition of
2 equiv. NBu4OH. A similar behavior was earlier observed
for 3,5-di-tert-butylpyrocatechol, which is explained by
monoanion formation.56 The dianion can be formed as
a result of the disproportionation of the о-semiquinone
radical or during its one-electron reduction at the elec-
trode. The highly basic dianion is protonated to the
monoanion by ethanol or trace amounts of water present
in the solvent. The potentials of the oxidation peaks of the
monoanions (see Table 1) are identical to the potentials
of the oxidation peaks (B) detected in the reverse branches
I/mA
0.07
B
C
0.03
–0.01
1
2
A
–0.05
–0.09
3
4
5
6
–0.9
–0.5
–0.1
0.3
E/V
Fig. 6. CV curves of oxygen reduction in the absence (1) and in
the presence of compound 2 in concentrations of 0.5 (2), 1 (3),
2.5 (4), 4 (5), and 5 mmol L–1 (6) (MeCN, GC electrode,
Ag/AgCl/KCl, 0.1 M NBu4ClO4, v = 0.2 V s–1).
in the reaction with pyrocatechol. Therefore, the concen-
tration of pyrocatechols (i) necessary for achieving this
Ipa value can be considered as IC50. The parameter of
IC50 for catechol thioethers 1—5 was registered in the
narrow range from 3.8 to 4.2 mmol L–1, as well as for
3,5-di-tert-butylpyrocatechol (3.8 mmol L–1). The mini-
mum values of IC50 were obtained for sulfides 6 and 7 and
are equal to 3.6 and 3.2 mmol L–1, respectively. For com-
pound 8, IC50 is 4.5 mmol L–1. Sterically hindered pyro-
catechol and its thioethers 1—8 manifest moderate activ-
ity toward O2•–, being at the level of retinoic acid and
biotin and exceeding the data for -tocopherol.37

1864
Russ. Chem. Bull., Int. Ed., Vol. 67, No. 10, October, 2018
Smolyaninov et al.
I/mA
A
1.2
0.12
0.08
0.04
0
1.0
0.8
0.6
0.4
0.2
1
2
–0.3
0.2
0.7
1.2
1.7 E/V
350
400
450
500
550
/nm
Fig. 7. CV curves of the oxidation of pyrocatechol 1 in the absence (1)
and in the presence of a solution of 2 equiv. NBu4OH in EtOH
(2) (MeCN, С(1) = 2 mmol L–1, GC anode, Ag/AgCl/KCl,
0.1 M NBu4ClO4 , v = 0.2 V s–1).
Fig. 9. Changes in the electronic absorption spectrum of com-
pound 5 (С = 1 mmol L–1) during 90 min after the addition of
1 equiv. KO2 in the presence of cis-dicyclohexano-18-crown-6
(С = 0.05 mol L–1) (298 K, solvent DMF).
of the CV curves (see Figs 5 and 6). In the presence of
a base, the pulse potential scan from –0.75 to 0.30 V made
it possible to establish the quasi-reversible redox transition
at a potential of –0.40 V (Fig. 8) corresponding to the
reduction of о-benzoquinone.
The reactions of pyrocatechol thioethers 4 and 5 with
KO2 were studied in dimethylformamide in the presence
of cis-dicyclohexano-18-crown-6. The introduction of
KO2 into a solution of pyrocatechol 4 leads to the color-
ation of the solution within 30—90 min, which is accom-
panied by fast changes in the absorption spectrum (Fig. 9)
in which a broad band at  = 500—510 nm and a shoulder
in a range of 320—330 nm appear. These changes are
similar to those considered above and correspond to the
formation of о-quinone and the corresponding mono-
anion.56
o-Benzoquinones observed in the reactions are the
final products of the reactions of the superoxide radical
anion with pyrocatechols and, hence, the question about
the mechanism of their formation remains principal.
2-Hydroxyphenoxyl radical formed upon hydrogen atom
transfer or o-semiquinone radical anion generated by the
one-electron transfer coupled with the abstraction of two
protons can act as the major intermediates. The reactions
of pyrocatechols with KO2 in DMF were studied by the
ESR method for compound 2 used as an example.
The reaction with potassium peroxide leads to the
oxidation of pyrocatechol to form the corresponding
о-semiquinone derivative, whose ESR spectrum represents
a doublet with the isotropic g factor equal to 2.0049 (Fig. 10).
The hyperfine coupling constant (HFC) of a lone electron
with the proton in position 5 of the aromatic ring of the
о-semiquinone radical is ai(1H) = 2.95 Oe. The spectrum
intensity decreases rapidly with time and becomes lower
by approximately 20 times within 15 min, which is ex-
plained by the disproportionation of the o-semiquinone
radical to o-quinone.
I/mA
0.06
0.04
0.02
0
Thus, this is the o-semiquinone radical anion the
formation of which was detected. This fact together with
the electrochemical and spectral data provides understand-
ing of the mechanism of the reactions of the superoxide
radical anion with the studied pyrocatechols (Scheme 3).
The complex between the superoxide radical anion and
pyrocatechol is primarily formed and detected only during
the CV experiment, and then electron and two protons are
transferred leading to the о-semiquinone radical anion.
The disproportionation of the latter favors the generation
of о-quinone observed in the UV-visible spectrum and in
–0.02
–0.6
–0.4
–0.2
0
0.2
E/V
Fig. 8. CV curves of the oxidation of pyrocatechol 8 in the pres-
ence of a solution of 2 equiv. NBu4OH in EtOH supplying
pulses in the potential scan range from –0.75 to 0.30 V (MeCN,
С(8) = 2 mmol L–1, GC anode, Ag/AgCl/KCl, 0.1 M NBu4ClO4,
v = 0.2 V s–1) (see text).

Asymmetrical catechol thioethers
Russ. Chem. Bull., Int. Ed., Vol. 67, No. 10, October, 2018
1865
dependence exists in the case the stable 2,2-diphenyl-1-
picrylhydrazyl radical scavenging often used for the es-
timation of antiradical activity of mono- and poly-
phenolic compounds.57 The reactions of thioethers 1—8
with DPPH were carried out in a deaerated solution of
MeCN (298 К). The introduction of compounds 1—8 into
the solution containing DPPH radical results in a decrease
in the intensity of the absorption maximum at 517 nm. The
antiradical activity of thioethers was determined from
changes in EC50, ТEC50, and antiradical efficiency (AE)
as a complex parameter capable of estimating both the
ability of a compound to abstract oxygen atom and the
rate of its reaction with DPPH radical. The comparative
data on the antiradical activity are presented in Table 2.
As a whole, pyrocatechol thioethers exhibit a fairly high
antiradical activity comparable with that of gallic acid
esters.58 The minimum ЕС50 values were obtained for
compounds 6—8, which is consistent with the data on their
reactions with the superoxide radical anion.
3439 3440 3441 3442 3443 3444 3445 H/Oe
Fig. 10. ESR spectrum of potassium o-semiquinolate (derivative
of H2Cat—S—C5H11) formed by the reaction of compound 2
(С = 1 mmol L–1) with КО2 (С = 2 mmol L–1) in DMF in the
presence of cis-dicyclohexano-18-crown-6 (С = 0.05 mol L–1
)
(293 K).
An increase in the number of carbon atoms in the
hydrophobic alkyl group for thioethers 1—5 does not lead
to a significant change in ЕС50 and TEC50 parameters.
The presence of the thioether group in the pyrocatechol
structure favors a decrease in the time of achievement of
the equilibrium state compared to 3,5-di-tert-butylpyro-
catechol, which indicates an increase in the antiradical
activity of thioethers. It should be mentioned that a de-
crease in the TEC50 parameter for thioethers 6—8 would
considerably affect their reactivity toward short-lived free
radicals formed in biological systems, which is confirmed
in the reactions with the superoxide radical anion. The
the voltammograms, as well as the dianion easily proton-
ated to the monoanion, the oxidation of which is observed
in the reverse branches of the CV curves. The scheme
proposed for transformations of pyrocatechol thioethers
is consistent with the recent results taking into account
the electron transfer coupled with the abstraction of two
protons.54,55
Various substituents in the thioether group of pyrocat-
echols 1—8 affect their reactivity toward superoxide radi-
cal anion. It seemed interesting to study whether a similar
Scheme 3

1866
Russ. Chem. Bull., Int. Ed., Vol. 67, No. 10, October, 2018
Smolyaninov et al.
Table 2. Parameters of antiradical activity of compounds 1—8
and 3,5-di-tert-butylpyrocatechol in the test with the DPPH
radical (MeCN, 298 K)
radical activity of compounds 1—8 were determined in the
test with the DPPH radical. It was found that the presence
of the thioether group favors a decrease in the reaction
time and, hence, an increase in the radical scavenging
efficiency. Sulfides 1—8 exhibit a higher antiradical activ-
ity than 3,5-di-tert-butylpyrocatechol.
Com-
pound
ЕС50
TEC50
/min
AE•103
/mol L–1
1
2
3
4
5
6
7
8
12.0 0.5
16.0 0.4
15.9 0.7
12.8 0.3
14.5 0.6
11.1 0.9
12.0 0.7
11.5 0.4
13.1 1.3
50
40
40
50
40
35
40
32
60
1.67
1.56
1.57
1.56
1.72
2.57
2.08
2.71
1.33
This work was financially supported by the Russian
Science Foundation (Project No. 17-13-01168).
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number of transformed molecules of diphenylpicrylhydr-
azyl (nDPPH) for compounds 2, 3, and 5 is less than two.
The most part of compounds have the value of nDPPH  2.
The obtained results agree with the electrochemical data
on the number of electrons transferring in the first anodic
step. The AE (antiradical efficiency) parameter enables
one to evaluate the studied compounds in compari-
son. According to the previously proposed classifica-
tion,59 thioethers 1—8 are characterized by a medium AE
(АЕ > 1•10–3). In the case of compounds 6—8, the pres-
ence of the thioether group in combination with cyclo-
pentyl, phenyl, or benzyl substituent results in an increase
in the reactivity of the pyrocatechol moiety toward both
superoxide radical anion and DPPH.
To conclude, in this work we studied the influence of
the thioether fragment on the electrochemical properties
and antiradical activity of the series of asymmetrical pyro-
cathehol thioethers. It is shown that the introduction of
the thioether functional group favors the extension of the
range of redox properties of compounds 1—8 due to the
possibility of sulfide bridge oxidation. In the first step, the
electrooxidation of thioethers affords o-benzoquinones
for which the reduction potentials of the redox pair
o-benzoquinone/o-benzosemiquinone were determined.
The second oxidation step involves the thioether fragment,
and the number of electrons participating in the electrode
reaction depends on the structure of the hydrocarbon
group bonded to the sulfur atom. The reactions of com-
pounds 1—8 with the superoxide radical anion and KO2
were studied using a combination of electrochemical and
spectral methods. A comparative evaluation of the reactiv-
ity enabled us to establish that cyclopentyl, phenyl, and
benzyl substituents in the thioether group exert a higher
effect on the antiradical activity than alkyl fragments do.
The mechanism was proposed for the reaction of the su-
peroxide radical anion with pyrocatechols taking into
account the one-electron transfer coupled with the ab-
straction of two protons. The main parameters of anti-

Asymmetrical catechol thioethers
Russ. Chem. Bull., Int. Ed., Vol. 67, No. 10, October, 2018
1867
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Received April 4, 2018;
in revised form May 28, 2018;
accepted July 26, 2018