Electron transfer and subsequent reactions during electrochemical oxidation of aryl- and alkylthio derivatives of mucochloric acid

Article abstract of DOI:10.1007/s11172-009-0114-3

The electrochemical oxidation of aryl- and alkylthio derivatives of mucochloric acid (3,4-dichloro-5-hydroxyfuran-2(5H)-one) in MeCN-Bu ₄NBF₄ (0.1 mol L-1) was investigated. It was shown that all sulfides are electrochemically active, from one to five oxidation steps of sulfur-containing groups were observed for them. The ease and direction of oxidation of the thio group depend on its nature and position in the furanone ring. 3-Substituted 2(5H)-furanones possess the lowest oxidation potential. 4-Substituted 2(5H)-furanones are predominantly oxidized to sulfoxides, 5-aryl- and -alkylthio derivatives undergo fragmentation to give mucochloric acid, and 3-arylthio derivative gives complex unidentified mixture of products. In the case of 3,4-bis(4-methylphenylthio) derivative, the oxidation product of the arylthio group at the 3 position to the corresponding sulfoxide was isolated. Based on the data from cyclic voltammetry with different concentrations of a substrate and water added, the results of preparative electrolysis and quantum chemical calculations, possible mechanisms of electrochemical oxidation of mucochloric acid-derived sulfides are discussed. The initial common step is a reversible single-electron transfer from the substrate molecule to form highly reactive radical cation.

Full text of DOI:10.1007/s11172-009-0114-3

Russian Chemical Bulletin, International Edition, Vol. 58, No. 5, pp. 908—919, May, 2009
908
Electron transfer and subsequent reactions during electrochemical oxidation
of arylꢀ and alkylthio derivatives of mucochloric acid
N. F. Devyatova,а A. R. Kurbangalieva,а V. V. Yanilkin,b and G. A. Chmutovaа
аKazan State University,
18 ul. Kremlevskaya, 420008 Kazan, Russian Federation
Fax: +7 (843) 231 5416. Еꢀmail: Almira.Kurbangalieva@ksu.ru
bA. E. Arbuzov Institute of Organic and Physical Chemistry,
Kazan Research Center of the Russian Academy of Sciences,
8 ul. Arbuzova, 420088 Kazan, Russian Federation
Fax: +7 (843) 273 2253
The electrochemical oxidation of arylꢀ and alkylthio derivatives of mucochloric acid
(3,4ꢀdichloroꢀ5ꢀhydroxyfuranꢀ2(5H)ꢀone) in MeCN—Bu4NBF4 (0.1 mol L–1) was investiꢀ
gated. It was shown that all sulfides are electrochemically active, from one to five oxidation
steps of sulfurꢀcontaining groups were observed for them. The ease and direction of oxidaꢀ
tion of the thio group depend on its nature and position in the furanone ring. 3ꢀSubstituted
2(5H)ꢀfuranones possess the lowest oxidation potential. 4ꢀSubstituted 2(5H)ꢀfuranones are
predominantly oxidized to sulfoxides, 5ꢀarylꢀ and ꢀalkylthio derivatives undergo fragmentation
to give mucochloric acid, and 3ꢀarylthio derivative gives complex unidentified mixture of
products. In the case of 3,4ꢀbis(4ꢀmethylphenylthio) derivative, the oxidation product of the
arylthio group at the 3 position to the corresponding sulfoxide was isolated. Based on the data
from cyclic voltammetry with different concentrations of a substrate and water added, the
results of preparative electrolysis and quantum chemical calculations, possible mechanisms
of electrochemical oxidation of mucochloric acidꢀderived sulfides are discussed. The initial
common step is a reversible singleꢀelectron transfer from the substrate molecule to form highly
reactive radical cation.
Key words: mucochloric acid, sulfides, furanꢀ2(5H)ꢀones, electrochemical oxidation,
quantum chemistry, sulfoxide, fragmentation.
chemical simulation are presented. The pathways of forꢀ
mation of their oxidation products including sulfoxides
24, 25 are discussed.
Aromatic and aliphatic sulfides have been well studied
by electrochemical methods. Both reduction and oxidaꢀ
tion processes of these compounds were investigated in
detail.1,2 At the same time, the current interest in this
class of compounds does not wane due to the synthesis of
new sulfides and, therefrom, sulfoxides and sulfones with
practically useful properties. It appears that the structures
combining a heterocyclic fragment and a sulfinyl or sulfonyl
group will be particularly promising. Also, of great interꢀ
est are optically active sulfoxides widely used in the
enantioselective syntheses.3,4
In recent years, we have synthesized a variety of sulꢀ
fides based on mucochloric acid, 3,4ꢀdichloroꢀ5ꢀhydroxyꢀ
furanꢀ2(5Н)ꢀone (1a).5—7 This stimulated our search for
the selective oxidation reactions of the obtained sulfides
by chemical and electrochemical methods. In the present
communication, the results of the experimental and
theoretical studies on the processes of electrochemical
oxidation (ECO) of sulfides 2—23 in MeCN by cyclic
voltammetry, preparative electrolysis and quantum
Experimental
Cyclic voltammograms (CV curves) were recorded using a
PIꢀ50ꢀ1 potentiostat connected to an N 307/2 XYꢀrecorder.
The working electrode was a glassy carbon disk electrode
(d = 2 mm) pressedꢀfitted into a fluoroplastic casing. Prior to
each measurement, the electrode was mechanically polished.
Potentials were measured relative to the standard potential
of the redox system ferrocene/ferrocenium cation (Fc/Fc+)
(internal standard) using a silver reference electrode Ag/AgNO3
(0.01 mol L–1) in MeCN. Dissolved oxygen was removed by
bubbling argon through solution. The measurements were carried
out at 295 К. The organic solvents were purified and dried
according to the standard procedures.8 The supporting electroꢀ
lytes Et4NBF4 and Et4NClO4 (Aldrich) were used without
further purification. The commercial mucochloric acid (1a) were
recrystallized from water, m.p. 127 °С (see Ref. 9). Ethyl10 (1b)
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 889—899, May, 2009.
1066ꢀ5285/09/5805ꢀ0908 © 2009 Springer Science+Business Media, Inc.

Electrochemical oxidation of sulfides
Russ.Chem.Bull., Int.Ed., Vol. 58, No. 5, May, 2009
909
and isopropyl11 (1c) pseudoesters of mucochloric acid, as well as
chloride10 (1d) were synthesized according to known procedures.
The syntheses of sulfides 2–23 were described in literature.5—7
The synthesis of sulfoxide 24 by treatment of sulfide 3 with
mꢀchloroperbenzoic acid will be described in a separate paper
devoted to the results of chemical oxidation of sulfides derived
from mucochloric acid to the corresponding sulfoxides and
sulfones.
The IR spectra of crystalline samples were recorded in the
wavenumber region 4000—400 cm–1 on a Bruker Tensorꢀ27
Fourier spectrometer at the Optical Research Laboratory of the
Federal Center of Joint Use (Nujol as mulls, KBr plates). The
NMR spectra were recorded on a Bruker Avanceꢀ400 instrument
(400.13 MHz for 1Н and 100.62 MHz for 13С) at 25 °С in
acetoneꢀd6 and DMSOꢀd6. The chemical shifts were measured
relative to the signals of the residual protons of the deuterated
solvents. Analysis by TLC was carried out on Silufol UVꢀ254
plates.
The preparative electrochemical oxidation of furanꢀ2(5Н)ꢀ
ones 2, 3, 6, 9 was performed in an undivided cell in a
galvanostatic mode (I = 40 mA) in the presence of specified
amount of water (see below) using Et4NBF4 as a supporting
electrolyte. The anode was a platinum foil (S = 100 cm2) and the
cathod was a platinum wire gauze. The completeness of reaction
was monitored by TLC. All studied compounds are more difficult
to reduce than molecular oxygen, therefore, the dissolved oxygen
functioned as cathodic depolarizer, which was reduced to water
under these conditions. The formed and specially added water
acted as a nucleophile, which was oxidized on the anode along
with the target compounds. Therefore, for exhaustive oxidation
of the substance, we passed excess quantity of electricity, which
was 2—3 times greater than the quantity calculated for twoꢀ
electron oxidation of the starting compound.
Oxidation of 4ꢀchloroꢀ5ꢀhydroxyꢀ3ꢀ(4ꢀmethylphenylthio)ꢀ
furanꢀ2(5Н)ꢀone (2). A working solution (total volume 30 mL)
was prepared by dissolving furanone 2 (0.13 g, 0.51 mmol),

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Devyatova et al.
Et4NBF4 (0.3256 g, 1.5 mmol), and water (0.02 mL) in MeCN.
The electrolysis was conducted for 4 h. The reaction mixture
was then concentrated under reduced pressure to remove MeCN
completely, and the oily residue was twice heated in benzene
under reflux for better extraction of organic products. The
benzene solution was evaporated to dryness under reduced
pressure. The obtained residue was inseparable mixture of
oxidation products. Their structures were not established.
Oxidation of 3ꢀchloroꢀ5ꢀhydroxyꢀ4ꢀ(4ꢀmethylphenylthio)ꢀ
furanꢀ2(5Н)ꢀone (3). A working solution (total volume 80 mL)
was prepared by dissolving furanone 3 (0.3 g, 1.17 mmol),
Et4NBF4 (0.8682 g, 4 mmol), and water (0.1 mL) in MeCN.
The electrolysis was conducted for 3 h. The reaction mixture
was then concentrated under reduced pressure to remove MeCN
completely, and the oily residue was purified by column
chromatography on silica gel (the eluent was hexane—diethyl
ether, 1 : 3) followed by recrystallization of the main fraction
from carbon tetrachloride. The white precipitate that formed
was identified as 3ꢀchloroꢀ5ꢀhydroxyꢀ4ꢀ(4ꢀmethylphenylsulꢀ
finyl)furanꢀ2(5Н)ꢀone (24) as a 1 : 1 mixture of two diastereoꢀ
mers, m.p. 138 °C, yield 16%, Rf 0.17. Found (%): C, 48.32; H,
3.28; Cl, 13.02; S, 11.60. C11H9ClO4S. Calculated (%):
C, 48.45; H, 3.33; Cl, 13.00; S, 11.76. IR, ν/cm–1: 3148 (ОН);
1789 (С=О); 1629 (С=С); 1591, 1490 (С=Сarom); 1050 (S=O).
1Н NMR (acetoneꢀd6), δ: 2.43 (s, 3 Н, Me); 2.44 (s, 3 Н, Me);
6.40 (d, 1 Н, С(5)Н, J = 8.1 Hz); 6.41 (d, 1 Н, С(5)Н, J = 8.4 Hz);
H, 4.42; S, 17.77. C18H16O4S2. Calculated (%): C, 59.98;
H, 4.47; S, 17.79. IR, ν/cm–1: 3151 (ОН); 1751 (С=О); 1655
1
(С=С); 1597 (С=Сarom); 1077 (S=O). Н NMR (acetoneꢀd6),
δ: 2.37 (s, 6 Н, Me); 2.42 (s, 6 Н, Ме); 5.91 (d, 1 Н, С(5)Н,
J = 8.7 Hz); 5.95 (d, 1 Н, С(5)Н, J = 8.7 Hz); 6.95 (d, 1 Н, ОН,
J = 8.7 Hz); 6.96 (d, 1 Н, ОН, J = 8.7 Hz); 7.29, 7.43 (АА´BB´,
4 Н, Нarom, N = 3JAB + 5JAB´ = 8.4 Hz); 7.29, 7.43 (АА´BB´, 4 Н,
Нarom, N = 3JAB + 5JAB´ = 8.4 Hz); 7.58, 7.70 (АА´BB´, 4 Н, Нarom
,
3
N = JAB
+ ,
5JAB´ = 8.4 Hz); 7.61, 7.73 (АА´BB´, 4 Н, Нarom
N = 3JAB + 5JAB´ = 8.4 Hz). 13С{1H} NMR (DMSOꢀd6), δ: 21.3,
21.4 (Ме); 97.37, 97.40 (C(5)); 111.9 (С(3)); 123.1, 123.2, 124.6,
125.0, 125.2, 125.4, 128.8, 139.3, 139.6, 140.8, 140.9, 141.0,
142.3, 142.4 (Carom); 164.4, 164.4 (C(4)); 169.7, 170.2 (C(2)).
The calculations of the structures of the starting sulfides,
their oxidation products and intermediates were performed with
full geometry optimization without symmetry restrictions by the
density functional methods using the Gaussian´98 (the doubleꢀ
zeta splitꢀvalence basis В3LYP/6ꢀ31G(d,p))12 and Priroda (the
PBE density functional, the tripleꢀzeta complete basis augmented
with polarized functions TZ2P (3z))13 programs. It should be
noted that both calculation methods provide sufficiently close
results for the structures in the gas phase, the calculations
performed by the Priroda program requiring signifficantly less
computing time. Account of the solvation effects was carried out
by the B3LYP/6ꢀ31G(d,p) method within the selfꢀconsistent
reaction field theory using PCMꢀcontinual model (Tomasi
polarizable continuum method14) which is taken as a standard
in the Gaussian´98 program. Calculations were performed with
the fixed molecular geometry optimized in the gas phase (sp)
and/or the geometry further optimized in solution (opt). The
second derivative matrix was calculated for all stationary points.
All discussed structures have only positive frequencies. When
geometry optimization lead to several possible structures fulfilling
the local minimum condition, we analyzed the structure possessing
lower energy.
7.40 (d, 1 Н, ОН, J = 8.1 Hz); 7.42 (d, 1 Н, ОН, J = 8.4 Hz);
3
7.46, 7.79 (АА´BB´, 8 Н, Нarom, N = JAB
+
5JAB´ = 8.2 Hz).
13С{1H} NMR (acetoneꢀd6), δ: 22.50, 22.52 (Me); 97.48, 98.05
(C(5)); 127.71, 127.74 (С(3)); 126.68, 126.83, 131.33, 131.37,
139.01, 139.29, 144.09, 144.20 (Carom); 159.69, 159.74 (C(4));
163.97, 164.04 (C(2)).
Oxidation of 3,4ꢀdichloroꢀ5ꢀ(4ꢀmethylphenylthio)furanꢀ2(5Н)ꢀ
one (6). A working solution (total volume 80 mL) was prepared
by dissolving furanone 6 (0.3 g, 1.01 mmol), Et4NBF4 (0.8682 g,
4 mmol), and water (0.1 mL) in MeCN. The electrolysis was
conducted for 6 h. The reaction mixture was then concentrated
under reduced pressure to remove MeCN completely, and the
oily residue was purified by column chromatography on silica
gel (hexane—diethyl ether, 1 : 3) followed by recrystallization of
the main fraction from hexane—carbon tetrachloride, 3 : 1. The
white precipitate that formed was identified as mucochloric acid
1а, m.p. 126 °С (Ref. 9: m.p. 127 °С). The yield was 12%. The
second fraction contained an inseparable mixture of di(pꢀtolyl)
disulfide and oxidation products of furanone 6.
Oxidation of 5ꢀhydroxyꢀ3,4ꢀbis(4ꢀmethylphenylthio)furanꢀ
2(5Н)ꢀone (9). A working solution (total volume 80 mL) was
prepared by dissolving furanone 9 (0.4 g, 1.16 mmol), Et4NBF4
(0.8682 g, 4 mmol), and water (0.1 mL) in MeCN. The electroꢀ
lysis was conducted for 4 h. The reaction mixture was then
concentrated under reduced pressure to remove MeCN comꢀ
pletely, and the oily residue was twice boiled in benzene under
reflux for better extraction of organic products. The benzene
solution was concentrated to dryness under reduced pressure.
The oily residue obtained was purified by column chromatoꢀ
graphy on silica gel (acetone—benzene, 1 : 4) followed by reꢀ
crystallization of the main fraction from carbon tetrachloride.
The white precipitate that formed was identified as 5ꢀhydroxyꢀ
3ꢀ(4ꢀmethylphenylsulfinyl)ꢀ4ꢀ(4ꢀmethylphenylthio)furanꢀ2(5Н)ꢀ
one (25), a 1 : 1 mixture of two diastereomers, m.p. 132—134 °C,
yield 19%, Rf 0.51 (acetone—benzene, 1 : 4). Found (%): C, 60.11;
Results and Discussion
For sulfides 2—23, we observed from one to five
oxidation steps on a glassy carbon electrode in МеСN—
Вu4NBF4 (0.1 mol L–1) (Table 1). According to the data
from cyclic voltammetry (CV), all peaks of the compounds
studied are irreversible and the oxidation products are not
reduced in the region of positive potential up to 0 V. Acid
1а, its pseudoesters 1b and 1c, and chloride 1d are not
oxidized in the available potential region (≤ 2.2 V relative
to Fc/Fc+), therefore, all observed steps of electrochemiꢀ
cal oxidation of compounds 2—23 relate to the oxidation
of the thio group.
It is known that ECO of sulfides involves a singleꢀ
electron transfer step resulting in radical cations.1,2 The
oxidation of the compounds studied also starts from the
reversible singleꢀelectron transfer to form radical cations,
which was inferred from the value of potential difference
between the peak and halfꢀpeak potentials (Ер – Ер/2 ≈ 60 mV),
which corresponds to the criterion of reversibility.15
Attempts to register radical cations by CV and ESR at
lower temperature (–40 °С) did not succeed even in the

Electrochemical oxidation of sulfides
Russ.Chem.Bull., Int.Ed., Vol. 58, No. 5, May, 2009
911
Table 1. The peak potentials of oxidation of arylthio derivatives of mucochloric acid 2—24 (С = 10–3 mol L–1
,
MeCN—Bu4NBF4 (0.1 mol L–1)) and the theoretical values of the free energies of ionization ΔΔG298 (RC•+ – Molec)*
calculated within the Priroda program for compounds 2—11
Compound
Epox/В
(rel. to Fc/Fc+)
ΔΔG298 (RC•+ – Molec)
Compound
Epox/В
(rel. to Fc/Fc+)
/kcal mol–1
2
1.40; 1.60; 1.80;
1.96; 2.18
173.2; 173.1**
12
13
14
15
16
17
18
19
20
21
22
23
24
1.73; 2.06
1.78; 1.98
1.56; 1.96
1.55; 1.93
1.80; 1.95
1.65; 1.89
1.71; 1.96
1.85
1.75
2.14
0.95; 1.41
1.78; 2.14
1.99
3
4
5
6
7
8
9
1.56; 1.99
1.71; 2.12
1.68; 2.08
1.56; 2.05
1.70; 2.09
1.77; 2.10
180.4; 181.2**
127.9
180.9
178.8; 179.0**
183.1
181.6
163.7
1.20; 1.34; 1.48;
1.72; 1.99
10
1.55; 1.74;
1.87; 2.08
1.67; 2.09
165.5
11
183.0
* RC•+ is a radical cation, Molec is a neutral molecule.
** The values were obtained using the Gaussian´98 program (B3LYP/6ꢀ31G(d,p)).
positions 3 and 4 (compound 9) facilitates oxidation
compared to each of the monosubstituted derivatives
(compounds 2 and 3), but when these substituents are in
positions 3 and 5 of the heterocycle (compound 10), the
value of the first oxidation potential is very close to
the oxidation potential of 5ꢀmonosubstituted derivative 6.
Although the first oxidation potentials are not exact
thermodynamic characteristics of the singleꢀelectron
transfer process, there is a certain correspondence
between these potentials and the values of the free energy
of electron transfer calculated as a difference between the
free energies of the corresponding radical cations RC•+
and neutral molecules Molec (ΔΔG298(RC•+ – Molec),
see Table 1). A comparison of energy characteristics of
the singleꢀelectron transfer act and the morphology of the
voltammograms of compounds 2, 6, 10 with those of
the arylthio derivatives of mucochloric acid where the
hydroxyl group in position 5 of the heterocyclic system is
absent (it is replaced by the group —SR or —OR) suggests
significant difference between the said types of comꢀ
pounds. The reasons for this are discussed below.
As expected, the simulation of the solvent effect on
the singleꢀelectron process by the calculations of the
solvation energy of the neutral molecules and radical
cations showed that the electron transfer in solution is
considerably facilitated compared to that in the gas phase,
since the stabilization of all radical cations due to interacꢀ
tion with MeCN is mush stronger than that of the neutral
molecules (Table 2).
Processes following the singleꢀelectron transfer
step. During oxidation of organic sulfides, the primary
radical cations are most commonly involved in the reacꢀ
tions of deprotonation, fragmentation with the C—S
case of the most easily oxidizable (Ер = 0.95 V) bicyclic
compound 22, whose radical cation had be the most
stable.16 The results obtained evidence that the rate of
irrversible chemical reactions proceeding after the singleꢀ
electron transfer step is sufficiently high. These rapid irreꢀ
versible steps are responsible for the irreversibility of the
total experimental registered peak (vide supra). Therefore,
the experimentally observed values of the oxidation potenꢀ
tials are influenced by the kinetics of the subsequent steps.
Arylthio derivatives of mucochloric acid 2—11. Singleꢀ
electron transfer process. The oxidation of the arylthio
derivatives of mucochloric acid is significantly more diffiꢀ
cult to proceed than that of aromatic sulfides due to the
electronꢀwithdrawing properties of the furanone fragment.
For example, the peak oxidation potential Epox of diphenyl
sulfide is +1.20 V (see Ref. 17), while the smallest value of
the oxidation potential for monothio derivative of mucoꢀ
chloric acid 2 is equal to +1.40 V (see Table 1).
It follows from the data in Table 1 that the substituents
in the aromatic system of the thio derivatives of mucoꢀ
chloric acid influence the oxidation potentials according
to their electronic effects: the replacement of the methyl
group in the paraꢀposition of the aromatic ring by elecꢀ
tronꢀwithdrawing substituents (Cl and Br) substantially
hinder the oxidation of both the 4ꢀarylthio (compounds
3—5) and 5ꢀarylthio (compounds 6—8) derivatives of
furanꢀ2(5Н)ꢀone 1.
The oxidation potential of the arylthio group also
appreciably depends on its position in the furanone ring.
Arylthio derivatives 6—8 containing arylthio group in
position 5 of the lactone ring are more diffucult to oxidize
than the corresponding 3ꢀ and 4ꢀarylthio derivatives
2—5. The presence of two pꢀtolylthio groups in the ring

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Devyatova et al.
was shifted to the less positive values15). It seems that this
conclusion may be applied to the other arylthio derivaꢀ
tives of mucochloric acid studied in the present work, and
this type of transformation of their radical cations may be
excluded from the consideration.
Table 2. Effect of acetonitrile on the stabilization of the neutral
molecules and radical cations of sulfides 2, 3, and 6 (B3LYP/
6ꢀ31G(d,p), sp calculcations)
Comꢀ
pound
ΔΔG298 (gas – MeCN)*/kcal mol–1
Neutral molecule
Radical cation
The independence of the first peak oxidation potential
of aryl sulfides 2—4, 6, 7 of the water concentration,
which was varied in the range 0.05—0.50 mol L–1 (Table 4),
suggests the absence of direct interaction of the radical
cations with water. If such interaction occured, then a
shift of the first peak oxidation potential to the less posiꢀ
tive values should be observed with increasing water conꢀ
centration.5 Since the experiments where water was
present encompassed the arylthio derivatives with differꢀ
ent structures, this conclusion seems to be common for
such type of compounds. Therefore, it can be thought
that the radical cations of the compounds studied that
formed in the first step underwent another irreversible
transformations (deprotonation and fragmentation), whereas
the reaction with water occured in the latter steps of the
process.
2
3
6
–6.79
–6.95
–3.76
–40.1
–44.4
–40.0
* The ΔΔG298 values were calculated with respect to the correꢀ
sponding energies in the gas phase
bond cleavage, dimerization, reaction with nucleoꢀ
philes, etc.1,2
In order to reveal the most probable pathways of conꢀ
version of the primary radical cations, we studied the
concentration effect of the sulfides and water on the morꢀ
phology of the voltammograms and the characteristics
of the ECO peaks for particular representatives of 3ꢀ, 4ꢀ,
and 5ꢀmonosubstituted derivatives of mucochloric acid
2—4, 6, 7. It follows from the data listed in Table 3
that with the increase in the concentration of a subꢀ
stance increase by one order of magnitude (from
1•10–3 to 1•10–2 mol L–1), the peak current is increased
proportionally for all compounds, but in neither case the
first peak oxidation potential changes. This mean that
the homocoupling processes of the compounds studied
do not noticeably occur under the experimental condiꢀ
tions (if the radical cations dimerized, the peak potential
The schemes of oxidation of different aryl sulfides that
are possible in principle are discussed below.
For example, the most probable pathway of sulfoxide
formation in the case of the 4ꢀarylthioꢀsubstituted derivaꢀ
tive of mucochloric acid appears to be as follows:
Ar is an aryl group, Fur is a furanone fragment, R• is an interꢀ
mediate radical.
Table 3. The dependence of peak potential of electrochemical
oxidation (Ерox, relative to Fc/Fc+) of mucochloric acid derivaꢀ
tives on the concentration of studied compound (C)*
Table 4. The dependence of peak potentials of electrochemical
oxidation (Ерox, relative to Fc/Fc+) of mucochloric acid derivaꢀ
tives (С = 5•10–3 mol L–1) on the concentration of the introꢀ
duced water in solution
Comꢀ
pound
Ерox/В* (at different С/mol L–1
)
1•10–3
2•10–3
5•10–3
7•10–3
Comꢀ
pound
Ерox/В (at different СH O/mol L–1
)
2
2
+1.40
+1.80
+1.96
+2.19
+1.56
—**
+1.99
+1.71
+2.12
+1.56
+2.05
+1.40
+1.80
+1.97
+2.17
+1.56
—**
+2.01
+1.72
+2.13
+1.57
+1.72
+2.05
+1.71
+1.41
+1.84
+2.02
+2.25
+1.58
+1.82
+2.01
+1.73
+2.16
+1.61
+1.76
+1.99
+1.75
—
—
—
—
+1.60
+1.84
+2.02
+1.74
+2.18
+1.62
+1.78
+1.99
+1.76
5•10–2
7•10–2
1•10–1
2•10–1
5•10–1
2
+1.39
+1.82
+2.00
+2.17
+1.58
+1.81
+2.04
+1.71
+2.19
+1.61
+1.77
+1.97
+1.75
+2.08
+1.40
+1.82
+2.00
+2.15
+1.57
+1.81
+2.02
+1.73
+2.19
+1.59
+1.76
+1.93
+1.75
+2.07
+1.39
+1.81
+2.01
+2.13
+1.57
+1.80
+2.02
+1.72
+2.19
+1.60
+1.77
+1.92
+1.75
+2.05
+1.39
+1.81
+2.13
–
+1.57
–*
+2.02
+1.71
+2.19
+1.59
+1.77
+1.88
+1.74
+2.02
+1.385
+1.7
+1.99
–
+1.55
–*
+1.99
+1.71
+2.17
+1.60
+1.84
3
3
4
6
4
6
7
+1.70
+2.09
+2.06
+2.08
+2.08
7
+1.74
+1.95
+2.15
* A certain shift of potential towards more positive values
upon increase of the compound concentration is caused by the
increase in the uncompensated voltage drop in solution (iR).
** The second peak is present but is poorly expressed.
* The second peak is present but is overlapped by the third peak.

Electrochemical oxidation of sulfides
Russ.Chem.Bull., Int.Ed., Vol. 58, No. 5, May, 2009
913
Scheme 1
Scheme 1 shows the possible mechanism of oxidation
of the 4ꢀ(pꢀtolylthio) derivative of furanꢀ2(5Н)ꢀone 3
implying two different versions of deprotonation of the
primary radical cation.
Quantum chemical simulation showed that the radiꢀ
cals loose the electron considerably easier than the neuꢀ
tral sulfides (Table 5) so that the whole transformation
chain can be realized at the first peak potential (ЕСЕС
process).
ably, to form the sulfone). This is evidenced by the correꢀ
spondence of the second peak potential and intensity of
sulfide 3 (Fig. 1, а) to a single irreversible peak of oxidaꢀ
tion of sulfoxide 24 (Fig. 1, b). Consequently, under
voltammetry conditions, the major twoꢀelectron oxidaꢀ
tion product of sulfide 3 at the first peak potential is
sulfoxide 24 (see Scheme 1).
We also prepared, isolated and identified sulfoxide 24
under conditions of preparative electolysis (see Experiꢀ
mental). The difference in the peak potentials of oxidaꢀ
tion of sulfoxide 24 and sulfide 3 is ∼400 mV. One can also
suppose that, in the case of other arylthio derivatives of
mucochloric acid studied, the values of the peak potential
differences for sulfoxides and sulfides will be nearly the
same. Indeed, the peaks in the region of potentials ≥2 V,
which can be assigned to oxidation of the corresponding
sulfoxides, were present in the cyclic voltammograms of
all compounds having arylthio group in position 4 (see
Table 1). Since the peak currents of oxidation of the sulfꢀ
oxides and the starting compounds are close to each other,
one can conclude that, for the compounds discussed,
sulfoxides are the major oxidation products in the first
step of electrochemical transformations.
The second oxidation peak observed in cyclic voltꢀ
ammograms in the region of potentials ≤2 V indicates a
twoꢀelectron oxidation process of a sulfoxide (presumꢀ
Table 5. The energies (ΔΔG298) of electron detachment from the
neutral molecules and radicals of compounds 2 and 6 as calcuꢀ
lated by the Gaussian´98 (B3LYP/6ꢀ31G(d,p)) program
Comꢀ
pound
Process of electron
detachment*
ΔΔG298
/kcal mol–1
+
2
6
Moleс → RC•
173.2
151.5
179.0
158.5
R• → C+
+
Molec → RC•
R• → C+
The results of the CV study of the compounds conꢀ
taining arylthio group in position 3 (Fig. 2, see Table 1)
* The designations are used as follows: Molec is a molecule,
RC•+ is a radical cation, R• is a radical, С+ is a cation.

914
Russ.Chem.Bull., Int.Ed., Vol. 58, No. 5, May, 2009
Devyatova et al.
derivatives of mucochloric acid. In the case of compounds
2 and 9 containing pꢀtolylthio group in position 3 of the
heterocycle, as for the 4ꢀ(pꢀtolylthio) derivatives, the main
reaction of the radical cations is deprotonation, wherein
the proton can be removed from both the methyl group
and the С(5) atom of the heterocycle (Scheme 2).
a
I/μА
+2.01
25 μА
+1.56
The first reaction pathway (k1) in Scheme 2 seems
to be preferred than the second one (k2) due to greater
thermodynamic stability of radical А over radical В
(ΔG298(А) = –1507.0836 au, ΔG298(B) = –1507.0569 au,
according to the calculations within the Gaussian´98
program). Subsequent deprotonation reactions and interꢀ
action of transient cations С and D with water result in
ahnydride E and sulfoxide F. Oxidation of sulfoxide F
corresponds to the third peak (Ер = +1.80 V), and oxidaꢀ
tion of anhydride E corresponds to the second and fourth
peaks (Ер = +1.60 and +1.96 V, respectively).
b
+2.01
In the case of derivatives 6—8 containing arylthio
group in position 5 of the heterocycle, only the first oxiꢀ
dation peak is intense, and subsequent peaks have low
intensities (Fig. 3). This allows one to conclude that, upon
oxidation in acetonitrile under voltammetry conditions,
sulfoxides of 5ꢀarylthio derivatives 6—8 are either proꢀ
duced in small amounts or not produced at all. Generally,
compounds that are electrochemicaly inactive in the
potential range studied are produced probably through
other pathways of the oxidation process. Presumably, the
most probable reactions of the primary radical cations of
5ꢀarylthio derivatives 6—8 are the fragmentation of the
heterocyclic and arylthio moieties and deprotonation.
The probability of the first pathway of the oxidation
process is evidenced by the results of the quantum chemiꢀ
cal calculations for the structures of the radical cations of
5ꢀarylthio derivatives of mucochloric acid (Table 6).
As seen from Table 6, the S—C(5) bonds in the radical
cations of 5ꢀarylthio derivatives 6 and 7 are elongated
relative to the similar bonds in the neutral molecules (on
the contrary, in the radical cations of 3ꢀ and 4ꢀarylthio
derivatives of mucochloric acid 2 and 3, the S—C(3) and
0
0.5
1.0
1.5
2.0 E/V (rel. to Fс/Fc+)
Fig. 1. Cyclic voltammograms of oxidation of sulfide 3 (а)
and sulfoxide 24 (b) (С = 2•10–3 mol L–1, MeCN—Bu4NBF4
(0.1 mol L–1), a glassy carbon electrode).
suggest that their oxidation process is more complex. The
cyclic voltammograms of these compounds (see Fig. 2)
also exhibit the peaks of oxidation of sulfoxides, but there
are still some peaks in addition to the aboveꢀmentioned
peaks. It it obvious that sulfoxides are not the single or
predominant oxidation products of the 3ꢀsubstituted thio
I/μА
+1.97
+2.17
+1.80
+1.40
25 μА
I/μА
+1.61
+1.76
+1.99
55 μА
0
0.5
1.0
1.5
2.0 E/V (rel. to Fс/Fc+)
0
0.5
1.0
1.5
2.0 E/V (rel. to Fс/Fc+)
Fig. 2. Cyclic voltammogram of oxidation of sulfide
2
Fig. 3. Cyclic voltammogram of oxidation of sulfide 6
(С = 5•10–3 mol L–1, MeCN—Bu4NBF4 (0.1 mol L–1),
a glassy carbon electrode).
(С = 2•10–3 mol L–1, MeCN—Bu4NBF4 (0.1 mol L–1), a
glassy carbon electrode).

Electrochemical oxidation of sulfides
Russ.Chem.Bull., Int.Ed., Vol. 58, No. 5, May, 2009
915
Scheme 2
S—C(4) bonds are somewhat shorter than in the neutral
molecules).
At the same time, one cannot completely exclude the
process of the proton abstraction from the С(5) atom to
form the radical
The pattern of the geometry changes together with the
analysis of electron density distribution in radical cations
6•+ и 7•+ (based on the data of the calculations by the
Prirodа program, the total spin density is 0.9325 in
the thioaryl fragment of radical cation 6•+ and 0.0669
in the heterocyclic fragment; for radical cation 7•+, the
corresponding values are 0.9032 and 0.0773) evidences
that the fragmentation of the S—C(5) bond to form
just the thioaryl radical and mucochloric acid cation is
favorable. The subsequent reactions with water afford
mucochloric acid, which was isolated upon preparative
electrolysis, and arenesulfonic acids (Scheme 3).
,
which, as a result of detachment of the second electron,
can be transformed to the corresponding cation and
further, after hydration and deprotonation, give the
desired sulfoxide. The fact that we did not observe

916
Russ.Chem.Bull., Int.Ed., Vol. 58, No. 5, May, 2009
Devyatova et al.
Scheme 3
the latter in the products of preparative electrolysis, as
described below, is likely attributed to losses arising durꢀ
ing the workꢀup of the reaction mixtures.
Thio derivatives of mucochloric acid with aliphatic subꢀ
stituents at the sulfur atom. In the series of the alkylthio
derivatives of mucochloric acid, with some exceptions,
the same in principle regularities of the influence of the
position and nature of thio substituents on the peak oxiꢀ
dation potential related to the radical cation formation
are observed as for the arylthio derivatives.
Among the derivatives of mucochloric acid containꢀ
ing the fragment of mercaptoacetic acid, 3ꢀsubstituted
furanꢀ2(5Н)ꢀone 12 is oxidized easier than 4ꢀregioisomer
13 as in the case of the arylthio derivatives (see Table 1).
However, in contrast to the latter, compound 14 with the
thio substituent in position 5, is oxidized considerably
easier (by ∼200 mV) compared to compounds 12, 13
having substituents in positions 3 and 4 of the heteroꢀ
cycle. At the same time, in the other aliphatic thio derivaꢀ
tives of mucochloric acid, the sulfurꢀcontaining fragment
in position 5 of the heterocycle (compounds 15, 19, 21) is
more difficult to oxidize than the thio group in position 4
(compounds 16—18, 20) as in the case of the arylthio
derivatives.
Table 6. The S—C(5) bond lengths in the neutral molecules and
radical cations of the arylthio derivatives of mucochloric acid
as calculated by the Gaussian´98 (B3LYP/6ꢀ31G(d,p)/Priroda
program
Comꢀ
pound
d(S—C(5))/Å
Neutral molecule
Radical cation
The influence of substituents in the alkyl fragment of
the compounds on the first wave potential is in good
agreement with the calculated values of the HOMO
energy (EHOMO) of the corresponding sulfides and of the
2
3
6
7
1.759/1.751
1.750/1.747
1.842/1.846
1.843/1.847
1.743/1.746
1.748/1.742
1.889/1.892
1.892/1.890

Electrochemical oxidation of sulfides
Russ.Chem.Bull., Int.Ed., Vol. 58, No. 5, May, 2009
917
free energies of electron transfer (e.g., for 4ꢀsubstituted
furanꢀ2(5Н)ꢀones 13, 17, and 18, Ep = 1.78, 1.65, and
is also the main process in the oxidation of these
compounds.
ox
1.71 V, EHOMO = –6.89, –6.74, and –6.58 eV, respecꢀ
tively)*. The substitution of the —CH2СН2О— fragment
(compound 21) for —CH2СН2S— in bridging sulfides 19
and 20 also hinders oxidation (by 300 mV) due to the
strong donor ability of the S atom compared to the O atom.
The data from voltammetry and the results of quanꢀ
tum chemical calculations allowed us to reveal certain
differences in the processes of electrochemical oxidation
of the aliphatic and aromatic thio derivatives that are
mostly related to further transformations of the initially
formed radical cations. As for the majority of alkyl sulꢀ
fides,1 the proton elimination from the α-methylene group
of the alkyl substituent, in accordance with Scheme 4 (ED
is dissociative electron transfer step), appears to be typical
of the aliphatic compounds.
In general, the results obtained allow one to predict
the possibility of electrochemical synthesis of sulfoxides
of substituted furanꢀ2(5Н)ꢀones. For example, it is hoped
that the selective electrosynthesis of sulfoxides of 4ꢀthioꢀ
substituted derivatives of mucochloric acid may be realꢀ
ized, whereas the selective electrooxidation of 3ꢀthioꢀ
substituted derivatives is hardly probability, and it is
almost impossible in the case of the 5ꢀthioꢀsubstituted
derivatives.
It is of note that oxidation of pseudoesters of mucoꢀ
chloric acid in all series of the studied sulfides proceeds more
difficult than the oxidation of the corresponding thio
derivatives of mucochloric acid (cf. compounds 11 and 3,
16 and 13, 18 and 17 in Table 1). This does not conform
with the changes in the electronic structure parameters of
the neutral molecules and with the energies of electron
transfer in the gas phase: e.g., for 3 and 11, ЕHOMO = –5.95
and –5.85 eV; and ΔΔG298(RC•+ – Molec) = 7.71 and
7.82 eV, respectively. One can suppose that this is parꢀ
tially connected with polarization of the OH group in
acetonitrile and with certain contribution of the ionic
form to the structure of 5ꢀhydroxy compounds, which
facilitates the process of electron detachment. Quantum
chemical calculations of the structure of 5ꢀhydroxy comꢀ
pounds 3, 13, and 17 do not contradict such assumption
and evidence that the values of the effective charges
on the О and Н atoms of the hydroxy group are larger in
MeCN than in the gas phase (Table 7). The largest changes
were observed in calculations with the use of the discrete
and discreteꢀcontinuum models of solvent19 taking into
acount explicitly the possibility of Hꢀcomplexation of
hydroxy compounds with MeCN: —O—H…N≡CМе.
Scheme 4
Table 7. The values of the polarization of the О—Н bond
(DC(5)(O—H)) in compounds 3, 13, 17 in the gas phase and in
acetonitrile as calculated without geometry optimization (sp)
and with additional optimization in solution (opt) (the atomic
effective charges qО and qН) within the PCM, Gaussian´98,
B3LYP/6ꢀ31G(d,p) programs
For 4ꢀsubstituted aliphatic thio derivatives 13, 16—
18, and 22, as for the corresponding arylthio derivatives,
two oxidation peaks having comparable intensities are
observed, the difference between their potentials is 0.20—
0.40 V. It is obvious that sulfoxides are the major products
of oxidation of these compounds and the second peak is
related to their oxidation.
Comꢀ
pound
Calculation
method
qO
qH
DC(5)(O–H)
/Å
There are no oxidation peaks of the supposed sulfoxꢀ
ides in the cyclic voltammograms of 5ꢀalkylthio derivaꢀ
tives 14, 19, 21, and, according to quantum chemical
calculcations, the bond between the S and C atoms of the
heterocycle in the radical cations is elongated, as in the
case of the aromatic thio derivatives of mucochloric acid.
It appears that cleavage of the С—S bond in sulfides
3
In the gas phase –0.487
0.319
0.342
0.346
0.350
0.350
0.352
0.321
0.344
0.320
0.342
0.969
0.969
0.974
0.976
0.976
0.977
0.969
0/969
0.969
0.969
sp
оpt
–0.507
–0.509
In the gas phase –0.520
3•MeCN*
sp
оpt
–0.527
–0.530
13
17
In the gas phase –0.484
sp –0.506
In the gas phase –0.486
sp –0.507
* It should be noted that the HOMO energies, as calculated
within both DFT methods, are systematically overestimated relative
to the experimental values18, but the relative energy changes caused
by the influence of substituents are provided sufficiently well.
* Complex of compound 3 with acetonitrile.

918
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Devyatova et al.
Preparative electrooxidation of some sulfides of mucoꢀ
We also carried out oxidation of the 3,4ꢀdi(pꢀtolylthio)
derivative 9. The major product of this reaction is sulfꢀ
oxide 25 formed by oxidation of the arylthio group in
position 3 of the lactone ring (see Scheme 5). The strucꢀ
ture of product 25 was confirmed by IR, 1Н and 13С NMR
spectroscopy. The shift of the signal for the С(3) atom by
12.4 ppm in the 13С NMR spectrum of product 25 comꢀ
pared with the position of the С(3) signal in the spectrum
of the starting compound 9 (see Ref. 5) is in favor of
oxidation of the arylthio group in position 3 of the heteroꢀ
cycle. For monosulfoxide 25, the data from Xꢀray difꢀ
fraction analysis have been also obtained. The crystal
structure of the sulfoxides and sulfones of the furanone
series produced under conditions of chemical oxidation
of the corresponding sulfides will be discussed in detail
elsewhere.
chloric acid. For the preparative electrochemical oxidaꢀ
tion (ECO), we selected regioisomeric sulfides 2, 3, 6
containing pꢀtolylthio group in different positions (3, 4,
and 5) of the furanone ring, as well as the 3,4ꢀdisubstiꢀ
tution product 9. The aim of preparative ECO was the
preparation of the corresponding sulfoxides, determinaꢀ
tion of their spatial structures and confirmation of differꢀ
ences in the pathways of electrooxidation of various
regioisomeric sulfides. In all cases, ECO gave complex
mixtures of products. Upon electrooxidation of 4ꢀarylthio
derivative 3, we succeeded in isolating the corresponding
sulfoxide 24 (Scheme 5).
Scheme 5
The authors acknowledge the help by E. A. Berdnikov
and N. V. Nastapova.
This work was performed under financial support of
the Academy of Sciences of the Republic of Tatarstan
(Program for State Support of the Young Scientists of the
Republic of Tatarstan, Grant No. 03ꢀ13/2009 (G)).
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Received July 29, 2008;
in revised form January 22, 2009