Cryoelectrochemical reduction of a phenyl sulfide in tetrahydrofuran: Mediated reduction gives different products compared to direct reduction

Article abstract of DOI:10.1002/poc.1138

An electrocatalytic reduction of [(3-{[trans-4-(methoxymethoxy)cyclohexyl] oxy}propyl)thio]benzene (RSPh) in the presence of naphthalene as a mediator is investigated, using steady-state voltammetry at various sized platinum microelectrodes and at low temperature (201 K) in tetrahydrofuran (THF). This mediated process has been found to involve the transfer of one electron, in contrast to the direct electrochemical reduction which involves two electrons. In addition, the mediated reduction proceeds at a potential, some 500mV less negative than the direct electrochemical reduction. The evidence for the proposed mechanism has been obtained from theoretical simulations, using DIGISIM which shows satisfactory fitting to experimental results and allowed the determination of the rate constant for the homogeneous step. In contrast to direct reduction of RSPh where only one product, trans-1-(methoxymethoxy)-4- propoxycyclohexane (RH), has been obtained, the isolation of two products, RH and the dimer, diphenyl disulfide, PhSSPh, following mediated preparative electrolysis of RSPh, in presence of naphthalene shows that this one-electron process may be carried out at the reduction potential of naphthalene at low temperature and has also validated deductions made from voltammetric results. Copyright

Full text of DOI:10.1002/poc.1138

JOURNAL OF PHYSICAL ORGANIC CHEMISTRY
J. Phys. Org. Chem. 2007; 20: 144–150
Published online 5 February 2007 in Wiley InterScience
(www.interscience.wiley.com) DOI: 10.1002/poc.1138
Cryoelectrochemical reduction of a phenyl sulfide in
tetrahydrofuran: mediated reduction gives different
products compared to direct reduction
Alexander V. Burasov,1 Christopher A. Paddon,2 Farrah L. Bhatti,3
3
2
*
Timothy J. Donohoe and Richard G. Compton
1N. D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, 47 Leninsky prosp., 119991 Moscow,
Russian Federation, Russia
2Physical and Theoretical Chemistry Laboratory, University of Oxford, South Parks Road, Oxford, OX1 3QZ, UK
3Chemistry Research Laboratory, University of Oxford, Mansfield Road, Oxford, OX1 3TA, UK
Received 5 September 2006; revised 21 October 2006; accepted 13 November 2006
ABSTRACT: An electrocatalytic reduction of [(3-{[trans-4-(methoxymethoxy)cyclohexyl]oxy}propyl)thio]benzene
(RSPh) in the presence of naphthalene as a mediator is investigated, using steady-state voltammetry at various sized
platinum microelectrodes and at low temperature (201 K) in tetrahydrofuran (THF). This mediated process has been
found to involve the transfer of one electron, in contrast to the direct electrochemical reduction which involves two
electrons. In addition, the mediated reduction proceeds at a potential, some 500 mV less negative than the direct
electrochemical reduction. The evidence for the proposed mechanism has been obtained from theoretical simulations,
using DIGISIM which shows satisfactory fitting to experimental results and allowed the determination of the rate
constant for the homogeneous step. In contrast to direct reduction of RSPh where only one product, trans-1-
(methoxymethoxy)-4-propoxycyclohexane (RH), has been obtained, the isolation of two products, RH and the dimer,
diphenyl disulfide, PhSSPh, following mediated preparative electrolysis of RSPh, in presence of naphthalene shows
that this one-electron process may be carried out at the reduction potential of naphthalene at low temperature and has
also validated deductions made from voltammetric results. Copyright # 2007 John Wiley & Sons, Ltd.
KEYWORDS: phenyl thioether; mediator; cryoelectrochemistry; microelectrode; naphthalene; DIGISIM
INTRODUCTION
structure alkyl-chain ‘tag’, which may be successfully
used for spectroscopic analyses, following bulk electro-
lysis. It was shown by microdisk chronoamperometry
and bulk electrolysis that only one product, trans-1-
(methoxymethoxy)-4-propoxycyclohexane (RH), was
formed in that process and the number of electrons, n
involved in the reduction was 2.
Recently, there has been some interest in mediated
electrochemistry. This has been most noticeable in the area
of biosensor research.8–12 A few ‘aromatic compounds’ are
known to be good mediators in electrochemical catalytic
processes, proceeding according to the so-called EC’
mechanism. Numerous electrochemical processes, involv-
ing mediation by specific organic and inorganic species
have been investigated over the past three decades. For
example, naphthalene has been used frequently as a
reliable mediator and a review by Simonet13 has been
published. More recently, naphthalene has been used in
the dechlorination of various organic compounds such as
chlorobenzene,14 1-chloronaphthalene,15 some organic
pollutants containing chlorine,16 and more complicated
aromatic compounds.17
Organosulfur compounds are important in many areas of
chemistry, including organic electrochemistry. Most of
the electrochemical investigations of sulfur compounds
were originally conducted in aqueous media,1,2 with only
a few reports of electrochemistry in organic solvents.
The reduction of various phenyl sulfides (RSPh) in
N,N-dimethylformamide, DMF at platinum,3 mercury,
and glassy carbon electrodes4–6 are notable examples. In
general, the reduction of thioethers may be used as a
convenient synthetic method for carbon—sulfur bond
cleavage and, therefore, there is interest in such processes.
We have recently reported on the cryoelectrochemical
reduction in tetrahydrofuran, THF of the thioether,
[(3-{[trans-4-(methoxymethoxy)cyclohexyl]oxy}propyl)
thio]benzene7 (Fig. 1). This compound was chosen due to
ease of its synthetic functionalization and presence in the
*Correspondence to: R. G. Compton, Physical and Theoretical
Chemistry Laboratory, University of Oxford, South Parks Road,
Oxford, OX1 3QZ, United Kingdom.
E-mail: richard.compton@chem.ox.ac.uk
Copyright # 2007 John Wiley & Sons, Ltd.
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CRYOELECTROCHEMICAL MEDIATED REDUCTION OF A PHENYL SULFIDE
145
(Goodfellow Cambridge Ltd., Cambridge, UK) used as
the counter electrode. The working electrodes were all
carefully polished on a clean polishing pad (Kemet, UK),
using 1.0 and 0.3 mm aqueous-alumina slurries (Beuhler,
Lake Buff, II., USA), and subsequently rinsed in
de-ionized and doubly filtered water of resistivity, no
greater than 18 MV cm, taken from an Elgastat filter
system (Vivendi, Bucks, UK). The electrodes were
carefully dried prior to use. Before carrying out electro-
chemical experiments, the microdisc radius was electro-
Figure 1. Phenyl thioether [(3-{[trans-4-(methoxymethoxy)-
cyclohexyl]oxy}propyl)thio]benzene, (RSPh)
chemically calibrated using a literature methodology.19
A
Fc/FcRPFS6 reference electrode was developed for use
in THF at low temperature and has been described
previously.20 Tetra-n-butylammonium perchlorate was
used as the supporting electrolyte in all electrochemical
measurements at a concentration of 0.1 M. The solutions
were degassed vigorously for 1 min, using impurity-free
argon (BOC gases, Guildford, Surrey, UK) to remove
any trace oxygen and an inert argon-atmosphere was
maintained throughout all analyses. All solutions were
prepared under an atmosphere of argon, using oven-dried
glassware such as syringes and needles used for the
transfer of moisture sensitive reagents. Low temperature
experiments were conducted, using a Julabo FT902
immersion cooler with temperature control for counter-
cooling (JULABO UK Ltd.).
Herein, we report on the indirect reduction of the
phenyl thioether, [(3-{[trans-4-(methoxymethoxy)cyclo-
hexyl]oxy}propyl)thio]benzene (RSPh) (Fig. 1) at low
temperature in THF, where naphthalene is used as a
mediator. In this case, when the catalytic process is
the subject of investigation, low temperature plays an
important role, since selectivity over reaction product(s),
mechanistic pathways, and kinetics (rate determining
steps) may be affected significantly. Specifically, under
indirect bulk electrolytic methods, n is shown to be 1,
and a different sulfur-containing product, diphenyl
disulfide (PhSSPh) is observed, compared to previous
work in which n ¼ 2. The steady-state waves are shown
for additions of RSPh to a solution containing only
naphthalene at 196 K in THF and the subsequent catalytic
waves fitted using DIGISIM.
Microdisc chronoamperometry. Microdisc chron-
oamperometry permits the simultaneous determination of
the diffusion coefficient, D and the number of electrons,
n transferred to an electroactive species of interest,
provided no coupled chemistry operates on the timescale
of the experiment, and the concentration of the electr-
oactive species is known. The time dependent current
response, I, resulting from a diffusion-controlled reduc-
tive current after a potential step at a microdisc electrode
is given in Eqn (1):
EXPERIMENTAL
Reagents
Tetra-n-butylammonium perchlorate, TBAP (Fluka), was
used as received, without any further purification.
Anhydrous tetrahydrofuran (THF) was purified by
filtration through two columns of activated alumina
(grade DD-2) as supplied by Alcoa, employing the method
of Grubbs et al.18 Naphthalene (Aldrich) was used as
received. The phenyl sulfide, thioether [(3-{[trans-
4-(methoxymethoxy)cyclohexyl]oxy}propyl)thio]benzene,
was prepared as previously described.7
I ¼ ꢀ4nFDCrfðtÞ
where Eqn (2) defines f(t):
(1)
ꢀ1=2
fðtÞ ¼ 0:7854 þ 0:8862tꢀ1=2 þ 0:2146eꢀ0:7823t
and Eqn (3) defines t:
(2)
4Dt
t ¼
(3)
r2
Electrochemical procedures
F is the Faraday constant, r is the radius of the disc
electrode, and t the time. The above approximation (Eqns
1–3) were derived by Shoup and Szabo,21 and describes
the current response to within an accuracy of 0.6% over
all t. Experimentally, the chronoamperometric experi-
ment is run over a time scale, incorporating a transition
from transient, with a I / D dependence, to steady-state
with a I / D dependent behavior. Accordingly, decon-
volution of the parameters D and n is possible from
a single scan. Fitting was achieved via ORIGIN
6.0 (Microcal Software, Inc.) where, having input an
Cyclic voltammetry (CV). Voltammetric measure-
ments were carried out on
a m-Autolab (Eco-
Chemie, Utrecht, Netherlands) PGSTAT 20 potentiostat.
A three- electrode arrangement was used in an air-tight,
three-necked cryoelectrochemical cell. The cryo-cell
with solid electrolyte was dried in vacuo overnight,
before solvent addition and electrochemical experiments.
The working electrodes employed were 1.2, 5.0, and
12.5 mm (radius) platinum electrodes (Cypress Systems,
Inc., Kansas, US), with a large area, shiny platinum wire
Copyright # 2007 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2007; 20: 144–150
DOI: 10.1002/poc

146
A. V. BURASOV ET AL.
accurate value for r and C, the software iterates through
values of D and n, until the fit of the experimental data had
been optimized.
in CDCl3 and referenced to tetramethylsilane (SiMe4),
as an internal standard. Signal positions were recorded in
d (ppm) with the abbreviations s, d, t, q, quint., sept., br,
app., and m denoting singlet, doublet, triplet, quartet,
quintet, septet, broad, apparent, and multiplet, respect-
ively. All coupling constants, J, are quoted in Hz.
Crude electrolysis mixture was analyzed by thin-layer
chromatography (TLC). TLC was performed on Merck
Kieselgel 60 F254 0.25 mm pre-coated aluminum backed
silica plates. Compounds were visualized with UV light and/
or by staining with basic potassium permanganate solution.
Flash chromatography was used to separate the
products, following electrolysis and carried out, accord-
ing to the method described by Still et al. 23 using Merck
Kieselgel 60 (40–63 mm). Column fractions were
monitored by TLC.
Preparative electrolysis. Controlled-potential, bulk
electrolysis was carried out in a two-compartment cell,
the catholyte and anolyte being separated by a sintered
glass frit. The cathode was a rectangular platinum
plate (area ¼ 3.25 cm2) (Goodfellow, UK) and the anode,
a platinum mesh housed within the separate compartment
with 0.1 M TBAP in THF. A Fc/FcRPF6S reference
electrode was used as described above. The electrolysis
was vigorously stirred, using a magnetic stirrer bar and
the temperature maintained at 195 ꢁ 1 K, using an
external system (Julabo FT902, JULABO Labortechnik
GmbH, D-77960 Seelbach/Germany). The potential was
held constant at the reduction potential of naphthalene
(vs. Fc/FcRPFS6 ). Naphthalene was added to the solution
at 195 ꢁ 1 K to minimize any decomposition of the
mediator.
RESULTS AND DISCUSSION
Saturated aqueous ammonium chloride (2 mL)
followed by water (10 mL) was added to the electrolyzed
solution upon reaction completion. Diethyl ether (20 mL)
was then added and the two fractions separated. The
organic fraction was then washed with aqueous hydro-
chloric acid (1 M 6 T10 mL), saturated sodium bicarbonate
(10 mL), and saturated aqueous sodium chloride (10 mL).
The organic layer was dried over magnesium sulfate and
then evaporated under reduced pressure. Products were
initially identified by 1H NMR (Bruker AV400 (400 MHz)
spectrometer) and then separated using TLC/flash chroma-
tography (petrol:diethyl ether elutions). Yields for isolated
RH and PhSSPh were 7.1 mg (19%) and 3.5 mg (4%),
respectively. Analytical data for these compounds were
identical to literature data and are given below.
Voltammetric studies of starting compounds:
naphthalene and RSPh
A 6.5 mM solution of naphthalene in THF was prepared
and cooled to 201 ꢁ 1 K. A platinum microelectrode was
used to record a voltammetric response, and a reduction
wave with a well-defineþd plateau was observed at
E ¼ ꢀ3.11 V versus Fc/Fc PFꢀ6 reference electrode, as
shown in Fig. 2. The number of electrons transferred
in the reduction, n and the diffusion coefficient, D of
naphthalene were obtained using microdisk potential
step chronoamperometry.7,24 Figure 3 depicts a typical
chronoamperometric curve with the corresponding fitting
applied to deduce the required parameters. Experiments
were repeated a number of times and the average D and n
trans-1-(Methoxymethoxy)-4-propoxycyclohexane
(RH)7. 1H NMR (400 MHz, CDCl3): dH ¼ 4.66 (2H,
s, OCH2OCH3), 3.43 (1H, m, cyclohexane CH), 3.38
(2H, t, J 6.8, OCH2), 3.36 (3H, s, OCH2OCH3), 3.25
(1H, m, cyclohexane CH), 1.99 (4H, m, cyclohexane CH2),
1.57 (2H, m, OCH2CH2), 1.33 (4H, m, cyclohexane CH2),
0.90 (3H, t, J 7.4, CH3); 13C NMR (100 MHz, CDCl3):
dC ¼ 94.7, 76.5, 74.6, 70.1, 55.2, 30.0, 29.5, 23.3, 10.6; MS
(CIþ): m/z 220 (40%, [M þ NH4]þ), 203 (15%, [M þ H]þ);
HRMS C11H23O3 (MH) requires 203.1647, MHþ found
203.1656 (þ4.5 ppm); IR (film) per cmꢀ1: nmax ¼ 2937,
1455, 1377, 1106, 1045.
1
Diphenyl disulfide (PhSSPh)22. H NMR (200 MHz,
CDCl3): dH ¼ 7.19–7.36 (m, 6H), 7.48–7.53 (m, 4H);
13C NMR (100 MHz, CDCl3): dC ¼ 127.1, 127.5, 129.0,
.
137.0, MS (%B): m/z 218 (M þ, 100), 185 (M-SH, 13),
.
154 (C12H1þ0 , 32þ), 109 (C6H5Sþ, 64); IR (KBr):
n ¼ 1573, 1473, 1436, 736, 686 cmꢀ1
.
Figure 2. Voltammetric response of naphthalene (6.5 mM)
in THF (0.1 M TBAP), using a 5 mm (radius) plꢀa1tinum micro-
electrode recorded at a scan rate of 10 mV s
1
Analytical techniques. H NMR spectra were
recorded using a Bruker AV400 (400 MHz) spectrometer
Copyright # 2007 John Wiley & Sons, Ltd.
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CRYOELECTROCHEMICAL MEDIATED REDUCTION OF A PHENYL SULFIDE
147
corresponding value for naphthalene (E ¼ ꢀ3.11 V vs. Fc/
FcþPFꢀ6 ) by some 500 mV. We were interested in whether
we could use naphthalene as a catalytic mediator at low
temperature to, not only allow reduction at a lower
potential, but also to determine whether or not the
product(s) following reduction might be altered.
Voltammetric study of catalytic process
involving reduction of RSPh via mediation
using naphthalene
Figure 5(a) shows the wave for a 1 mM solution of
naphthalene in THF at low temperature. Next, three
sequential additions of RSPh were made and are shown in
Fig. 5. In the Fig. 5, similar increases caused by additions
are clearly visible. All the curves are observed at the same
potential which is lower than the reduction potential for
RSPh itself and almost the same as for naphthalene. This
effect may be explained by a mediation-type process
involving naphthalene – a type of EC’ mechanism as
shown by Scheme 1.
Figure 3. Experimental ((black) circles) and fitted theoretical
(solid (red) line) chronoamperometric curve for the one-electron
reduction of 6.5 mM naphthalene in THF (0.1 M TBAP), at a
5mm platinum microelectrode and temperature of 196 ꢁ 1K.
Potential was stepped to ꢀ3.50 V versus Fc/FcþPF6ꢀ in the
plateau region of the reductive wave (Figure 2. Fitting by
OriginTM sꢀo1ftware. Diffusion coefficient, D¼ 2.4 (ꢁ0.1) ꢂ
10ꢀ6 cm2 s
The first step of the process involves the one-electron
reduction of naphthalene forming the anion radical. The
mediator then transfers the electron to a molecule of
RSPh, under slow scan rates with the formation of
corresponding anion radical and regeneration of naphtha-
lene (hence catalysis). Under such slow timescales, any
naphthalene regenerated is immediately re-reduced and,
thus, the wave height for naphthalene is increased. The
resulting catalytic waves were then simulated and in
addition, with no clear way of determining n transferred
to RSPh (e.g., RSPh may react with two radical anions),
bulk electrolysis was performed.
used. The diffusion coefficient was found to be 2.5
(ꢁ0.1) ꢂ 10ꢀ6 cm2 sꢀ1 and the number of electrons
transferred was found to correspond to 1.0 (ꢁ0.1).
Previously7 under the conditions described above, the
steady-state voltammetric wave for a 3 mM solution of
RSPh was recorded and is shown again in Fig. 4.
Comparing both curves for naphthalene and RSPh, it is
easily noticed that the reduction potential of RSPh
(E ¼ ꢀ3.58 V vs. Fc/FcþPFꢀ6 ) is more negative than the
Figure 4. Voltammetric response of [(3-{ [trans-4- (methox-
ymethoxy)cyclohexyl]oxy}propyl)thio]benzene, (RSPh) (3mM)
in THF (0.1M TBAP), using a 5 mm (radius) platinum
microelectrode recorded at a scan rate of 10 mV sꢀ1 at
196 ꢁ 1 K
Figure 5. Voltammetric response of (a) naphthalene
(1.0 mM) in THF (0.1 M TBAP), using a 5 mm (radius) platinum
microelectrode recorded at a scan rate of 10 mV sꢀ1 and with
RSPh (2) (b) 1.8 mM, (c) 3.5 mM, and (d) 5.1 mM
Copyright # 2007 John Wiley & Sons, Ltd.
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148
A. V. BURASOV ET AL.
Figure 6. Overlay of (a) the theoretical simulation result
(dotted line) with (b) the RSPh response (1.77 mM, scan
rate 10 mV sꢀ1, 5.0 mm (radius)). Parameters used for
the simulations were as follows: T ¼ 201 K, a ¼ 0ꢀ.51,
D ¼ 2.5 ꢂ 10ꢀ6 cm2 sꢀ1, ks ¼ 1 cm sꢀ1, kf ¼ 5 ꢂ 105 L mol
sꢀ1
Scheme 1. Mechanism of the process includes initial
one-electron reduction of naphthalene, followed by catalytic
cycle with formation of RSPh anion radical and regeneration
of naphthalene
additional electrode sizes were simulated. Keeping all the
parameters unchanged except electrode radius, good
fittings for 1.2 and 12.5 mm electrodes were achieved as
shown in Figs. 7 and 8, respectively. Table 1 contains the
observed limiting currents for sets of various electrode
sizes and concentrations for both experimental curves and
theoretical results. It is obvious from the table that values
are consistent.
Simulations of voltammograms for RSPh
reduction mediated by naphthalene
If the proposed mechanism follows a well-known
electrochemical EC’ mechanism, it may be successfully
simulated using DIGISIM software.25 In subsequent
simulations, the standard EC’ mechanism and hemi-
spherical diffusion were assumed as conditions. Diffusion
of all species was assumed to be the same as for
naphthalene. Therefore, the diffusion coefficient deter-
mined from chronoamperometric data for naphthalene
(Section ’Voltammetric studies of starting compounds:
naphthalene and RSPh’) were used. Having constant
values such as diffusion coefficient (2.5 ꢂ 10ꢀ6 cm2 sꢀ1),
radius of electrode (5 mm), initial concentrations of
species (1 mM for naphthalene, 1.77 mM for RSPh, and
zero for others), temperature (201 K), and scan rate
(10 mV sꢀ1), other parameters of simulation E, k0, and kf
(half wave potential, heterogeneous, and homogeneous
rate constants, respectively) were systematically varied,
until the best fit of the experimental curve was obtained
as shown by Fig. 6. It is clearly visible from the
figure that assigning k0 and kf values of ca. 1 cm sꢀ1 and
5 ꢂ 105 L molꢀ1 sꢀ1, respectively, and using the measured
E of naphthalene, gives satisfactory fitting with deviation
of the theoretical limiting current from experimental
result, not exceeding 0.1 nA. In order to ensure that the
parameters obtained from simulations and the proposed
mechanism are correct, the voltammetric curves for two
Figure 7. Overlay of (a) the theoretical simulation result
(dotted line) with (b) the RSPh response (5.0 mM, scan
rate 10 mV sꢀ1, 12.5 mm (radius)). Parameters used for
the simulations were as follows: T ¼ 201 K, a ¼ 0.5,
D ¼ 2.5 ꢂ 10ꢀ6 cm2 sꢀ1, ks ¼ 1 cm sꢀ1, kf ¼ 5 ꢂ 105 L molꢀ
1 sꢀ1
Copyright # 2007 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2007; 20: 144–150
DOI: 10.1002/poc

CRYOELECTROCHEMICAL MEDIATED REDUCTION OF A PHENYL SULFIDE
149
required to remove TBAP. Specifically, we isolated the
dimer, diphenyl disulfide (PhSSPh) which suggests that
RSPh is accepting a single electron homogeneously via a
mediated electron transfer and the resulting radical anion
cleaves to give carbon anion and the aromatic sulfur
radical. In this case, no addition of the produced radical
on the aromatic catalyst was observed. In addition, these
results show that reduction of RSPh may be conducted
indirectly at a much lower potentials than that under
direct methods.7 The following mechanism of decompo-
sition of RSPh anion radical formed in EC’-process
(Scheme 1) may be proposed (Scheme 2):
In contrast to direct two-electron reduction, where
anion radical of RSPh or its decomposition products
underwent further reduction at the same potential with
formation of dianion, the first step of this mechanism is
decomposition of RSPh, with formation of corresponding
anion and radical. The anion may remove a proton from
any acidic species, within the electrolytic medium or from
the aqueous acid work up, following bulk electrolysis to
yield RH. The phenyl sulfide radical thus formed, can
dimerise forming diphenyl disulfide. In general, the
overall process consumes only one electron per molecule
of RSPh and this is in accordance with mechanism
proposed from voltammetric data.
Figure 8. Overlay of (a) the theoretical simulation result
(dotted line) with (b) the RSPh response (1.77 mM, scan
rate 10 mV sꢀ1, 1.2 mm (radius)). Parameters used for
the simulations were as follows: T ¼ 201 K, a ¼ 0ꢀ.51,
D ¼ 2.5 ꢂ 10ꢀ6 cm2 sꢀ1, ks ¼ 1 cm sꢀ1, kf ¼ 5 ꢂ 105 L mol
sꢀ1
Preparative reduction of RSPh in presence of
naphthalene as mediator
The electrolysis was carried out at the potential of
naphthalene reduction (ꢀ3.1 V vs. Fc/FcþPF6ꢀ) in THF at
low temperature (196 ꢁ 1 K), until a total of 10 8C was
passed. Addition of naphthalene was performed at low
temperature, due to the slow decomposition of naphtha-
lene at room temperature in solution. As a result, two
products were isolated although in low yield (Table 2).
The low yield was a result of both the long timescales
required for charge passage through solution at such low
temperatures and also lengthy work-up procedures
CONCLUSIONS
It has been shown that the mediated reduction of the
phenyl sulfide at low temperature proceeds via a single
electron mechanism (Scheme 1). The mediator used is
naphthalene whose potential is ca. 500 mþV less negative
than that of phenyl sulfide versus Fc/Fc PFꢀ6 . Previous
studies into the reduction of thioether have shown that
the direct reduction involves two electrons and occurs
at much more negative potentials. Mediated electron
transfer, therefore, provides a useful electrosynthetic
method of reducing functional groups, without the need
for extreme potentials and/or electrolyzing at the solvent
breakdown limit. Electrocatalytic experiments have been
performed in which phenyl sulfide, in the presence of
small concentrations of naphthalene, leads to an increase
in the wave height, with subsequent additions of the
sulfide. Various electrode sizes have been used to gain
kinetic information by simulations using DIGISIM. In
addition, preparative electrolysis at low temperature has
allowed the isolation of the dimer, diphenyl disulfide,
although only in low yield. This suggests the transfer of a
single electron from the mediator to the phenyl sulfide
(Scheme 1). Finally, mediated experiments were only
possible at low temperature, due to the decomposition of
the mediator at room temperature and the solvent
breakdown window versus Fc/FcþPF6ꢀ. Overall, at low
temperature, not only does voltammetric resolution of
waves become possible, but a ‘clean’ mediated electron
Table 1. Comparison of limiting currents, observed at var-
ious electrodes and concentrations for RSPh with simulated
currents
Radius
per mm
RSPh
per mM
I
lim(experimental)
per nA
Ilim(simulated)
per nA
5.0
1.8
3.5
5.1
6.6
0.9
1.3
1.8
4.2
5.8
3.4
5.0
6.6
8.0
9.4
1.3
1.8
2.5
2.6
0.266
0.280
0.296
0.375
0.485
6.1
1.3
2.0
2.6
3.1
0.196
0.221
0.242
0.335
0.382
5.4
1.2
12.5
7.1
9.7
10.7
12.4
7.3
9.0
10.7
12.2
Copyright # 2007 John Wiley & Sons, Ltd.
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DOI: 10.1002/poc

150
A. V. BURASOV ET AL.
Table 2. Summary of preparative electrolysis experiment in which the potential was held constant at 3.1 V versus Fc/FcþPF6ꢀ,
until a total of 10 8C was passed
Starting material
I
Mediator
Number of electron(s), n
Product(s) isolated
Yield of product(s) isolated/recovered
Naphthalene
1
RH
7.1 mg (19%)
3.5 mg (4%)
Diphenyl disulfidea
a Thiophenol was not observed in the products and to ensure that this was not aerial oxidized to diphenyl disulfide, a sample of thiophenol was left overnight in
solution, under normal air conditions. 1H NMR revealed the thiol to be stable under these conditions.
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DOI: 10.1002/poc