Electrocatalytic reduction of alkyl iodides in tetrahydrofuran at silver electrodes

Article abstract of DOI:10.1002/poc.1133

Recent interest in the electrocatalytic activity of silver towards the reduction of alkyl iodides has led us to investigate whether the effect is observed in tetrahydrofuran (THF) at room temperature. Using platinum electrodes in THF for the reduction of alkyl halides at 298 K has been hampered by the solvent window, which 'obscures' the voltammetric signals of interest. In order to overcome these problems, voltammetry has been performed at low temperature and was shown to extend the voltammetric window, leading to accurate electrochemical analyses and even novel changes in mechanism(s) of the reactive species following electron-transfer (ET). Herein, it is shown that for a primary and tertiary alkyl iodide in THF, electroreduction at silver leads to a significant shift in the reduction potential to more positive values compared to platinum. In addition, following reduction, a characteristic series of oxidation peaks are observed and are shown to be due to the specific activity of iodide ions towards silver following reductive cleavage of the parent alkyl iodide. This characteristic feature is not observed with other halide ions: bromide and chloride. Preparative electrolyses at controlled-potential have suggested that the reduction of the above alkyl iodides is a one-electron concerted process. The 'free' iodide ions act as a monitor of reaction progression, and the carbon-centred radical either dimerises and/or abstracts a hydrogen atom from the electrolyte/solvent; 1-iodoadamantane giving percentage yields of 58% adamantane and 39% 1,1'-biadamantane, the primary alkyl iodide, prepared in-house, giving 67% R-H and 25% R-R. Copyright

Full text of DOI:10.1002/poc.1133

JOURNAL OF PHYSICAL ORGANIC CHEMISTRY
J. Phys. Org. Chem. 2007; 20: 115–121
Published online 5 February 2007 in Wiley InterScience
(www.interscience.wiley.com) DOI: 10.1002/poc.1133
Electrocatalytic reduction of alkyl iodides
in tetrahydrofuran at silver electrodes
1
2
2
1
*
Christopher A. Paddon, Farrah L. Bhatti, Timothy J. Donohoe and Richard G. Compton
1Physical and Theoretical Chemistry Laboratory, University of Oxford, South Parks Road, Oxford OX1 3QZ, UK
2Chemistry Research Laboratory, University of Oxford, Mansfield Road, Oxford OX1 3TA, UK
Received 18 September 2006; revised 24 October 2006; accepted 25 October 2006
ABSTRACT: Recent interest in the electrocatalytic activity of silver towards the reduction of alkyl iodides has led us
to investigate whether the effect is observed in tetrahydrofuran (THF) at room temperature. Using platinum electrodes
in THF for the reduction of alkyl halides at 298 K has been hampered by the solvent window, which ‘obscures’ the
voltammetric signals of interest. In order to overcome these problems, voltammetry has been performed at low
temperature and was shown to extend the voltammetric window, leading to accurate electrochemical analyses and even
novel changes in mechanism(s) of the reactive species following electron-transfer (ET). Herein, it is shown that for a
primary and tertiary alkyl iodide in THF, electroreduction at silver leads to a significant shift in the reduction potential
to more positive values compared to platinum. In addition, following reduction, a characteristic series of oxidation
peaks are observed and are shown to be due to the specific activity of iodide ions towards silver following reductive
cleavage of the parent alkyl iodide. This characteristic feature is not observed with other halide ions: bromide and
chloride. Preparative electrolyses at controlled-potential have suggested that the reduction of the above alkyl iodides is
a one-electron concerted process. The ‘free’ iodide ions act as a monitor of reaction progression, and the
carbon-centred radical either dimerises and/or abstracts a hydrogen atom from the electrolyte/solvent;
1-iodoadamantane giving percentage yields of 58% adamantane and 39% 1,1’-biadamantane, the primary alkyl
iodide, prepared in-house, giving 67% R-H and 25% R-R. Copyright # 2007 John Wiley & Sons, Ltd.
KEYWORDS: electrocatalysis; alkyl iodides; silver electrode; tetrahydrofuran (THF); adsorption
INTRODUCTION
molecular structure of the organic halide,17,18 the type of
halogen atom,17 the surface morphology of the elec-
trode,19 and adsorption of the reagents and their reduction
products especially halide ions,20–22 all play a role in the
catalytic process. More recently, the effect of the solvent
used in the electrocatalytic reduction of organic bromides
on silver has been investigated.23
Herein, we report on the electrocatalytic reduction of
two organic iodides in tetrahydrofuran (THF); a ‘model’
tertiary alkyl halide: 1-iodoadamantane, I and a primary
alkyl halide, II, prepared in-house, as shown in Fig. 1.
Compound II, being a primary alkyl iodide, provided us
with an inert electrochemical backbone which acted as a
molecular ‘tag’ following ET, thus allowing spectro-
scopic detection and chromatographic separation follow-
ing bulk-electrochemical procedures not seen with
straight chain alkyl halides such as 1-iodopentane.
THF is used as the solvent of choice as it used frequently
in synthetic single electron transfer (SET) reductions as a
‘neutral solvent’ that is, electrophilic attack of inter-
mediates is much lowered in this solvent. The results of
voltammetric analyses using silver electrodes in THF at
298 K were surprising, showing not only a significant shift
The electrocatalytic reduction of organic halides has
received much attention in the last few decades.1–3 Silver
as an electrode material has been found to possess
extraordinary electrocatalytic properties4,5 which have
been utilized in synthetic6–9 and environmental appli-
cations.10–12 The reduction of carbon–halogen bonds has
also attracted much interest as a model process for the
investigation of dissociative electron transfers (DETs)
which occur at ‘inert’ electrodes (for example at platinum
and glassy carbon (GC)).13–15 A catalytic surface such as
silver has been shown to modify the reaction scheme
possibly through the formation of a more favourable
activated complexe(s) and/or interaction of the metal with
the organic halide and its reduction intermediate(s) and
product(s)16 and it has been reported that the electron
transfer and dissociation are concerted yielding directly a
radical and anion. In addition, it has been noted that the
*Correspondence to: R. G. Compton, Physical and Theoretical Chem-
istry Laboratory, University of Oxford, South Parks Road, Oxford OX1
3QZ, UK.
E-mail: Richard.Compton@chem.ox.ac.uk
Copyright # 2007 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2007; 20: 115–121

116
C. A. PADDON ET AL.
(TBABr), and tetra-n-butylammonium chloride (TBACl)
O
O
I
I
(Fluka), were used as received without any further
purification. Anhydrous tetrahydrofuran was purified by
filtration through two columns of activated alumina
(grade DD-2) as supplied by Alcoa, employing the
method of Grubbs et al.34 Trans-1-(3-iodopropoxy)-
4-(methoxymethoxy)cyclohexane, II (Fig. 1) was pre-
pared in-house and a brief description of its synthesis is
given below:
Potassium carbonate (560 mg, 4.10 mmol) and sodium
iodide (2.53 g, 16.9 mmol) were added to a solution of the
mesylate: 3-{[trans-4-(methoxymethoxy)cyclohexyl]oxy}
propyl methanesulfonate, (1.00 g, 3.40 mmol) in acetone
(30 ml) and stirred at 40 8C for 6 h. The reaction was
concentrated in vacuo and extracted from saturated
aqueous sodium thiosulfate (50 ml) with diethyl ether
(2 ꢁ 50 ml). The combined organic extracts were dried
and evaporated under reduced pressure to give II (980 mg,
88 %) as a colourless oil.
O
I
II
Figure 1. Structures of 1-Iodoadamantane, I, and trans-
1-(3-iodopropoxy)-4-(methoxymethoxy)cyclohexane, II
of the reduction potential to more positive values as
described by other workers, but also a unique and
characteristic pattern of oxidative iodine-electrochemistry
following reductive cleavage of the alkyl iodide. In
particular, we observed a specific adsorption of iodide
ions24,25 not seen at ‘inert’ electrodes such as platinum.
These are shown to be due to the activity of iodide (Iꢀ),
tri-iodide (Iꢀ3 ) and iodine (I2) towards the silver surface.
Adsorption has important implications in many fields,
such as corrosion, galvanic deposition, electrosynthesis
and of course, electrocatalysis.25 These features were
observed only when using silver electrodes and appeared
regardless of the type of alkyl iodide used. In addition,
this characteristic feature of iodide ions was not observed
with other halide anions such as bromide and chloride. To
the best of our knowledge, this follow-up electrochem-
istry in THF has not been reported thus far, in contrast to
the extensive investigations of iodide/tri-iodide and
iodine electrochemistry at platinum electrodes in organic
solvents26–30 and in aqueous solutions.31–33 In fact, in
previous reports using aprotic solvents such as aceto-
nitrile, N,N-dimethylformamide, dimethylsulfoxide, ace-
tone, and propylene carbonate, no mention has been given
as to whether or not any follow-up electrochemistry
occurs within the full potential window.23
Trans-1-(3-iodopropoxy)-4-(methoxymethoxy)
1
cyclohexane. H NMR dH(400 MHz, CDCl3) 4.67
(2H, s, OCH2OCH3), 3.54 (3H, m, cyclohexane CH and
OCH2), 3.37 (3H, s, OCH2OCH3), 3.29 (3H, m, cyclohe-
xane CH and CH2I), 2.02 (6H, m, cyclohexane CH2 and
OCH2CH2), 1.36 (4H, m, cyclohexane CH2); 13C NMR
dC(100MHz, CDCl3) 94.7, 76.7, 74.4, 67.4, 55.2, 33.6,
29.7, 29.3, 4.0; MS m/z (CIþ) 346 (100%, [MþNH4]þ);
HRMS C11H22O3I (MH) requires 329.0614, MHþ found
329.0607 (ꢀ2.1 ppm); IR nmax (film)/cmꢀ1 2936, 1454,
1376, 1107, 1043.
Cyclic voltammetry (CV)
Voltammetric measurements were carried out on an
Autolab PGSTAT20 (Eco-Chemie, Utrecht, Netherlands)
potentiostat. A three-electrode arrangement was used in
an air-tight, three-necked electrochemical cell. The cell
with solid electrolyte, was dried in vacuo overnight before
solvent addition and electrochemical experiments. The
working electrodes employed were a 1.71 mm (diameter)
silver electrode (housed in a TeflonTM insulating case),
and a 1 mm (radius) platinum electrode (Cypress
Systems, Inc., KS) with a large area, shiny platinum
wire (Goodfellow Cambridge Ltd, Cambridge) as the
counter electrode. The silver electrode was carefully
polished before use on a clean polishing pad (Kemet, UK)
using 3 mm and 1 mm diamond spray (Kemet, UK). The
platinum electrode was polished using 1.0 mm and 0.3 mm
aqueous-alumina slurries (Beuhler, Lake Buff, IL). All
working electrodes were then rinsed in de-ionised and
doubly filtered water of resistivity no greater than
18 MV cm, taken from an Elgastat filter system (Vivendi,
Bucks, UK) and carefully dried prior to use. A Fc/FcþPF6ꢀ
Finally, controlled-potential, bulk electrolyses are used
to determine the number of electrons, n, involved in the
reduction process at silver and also identify additional
products that are formed and not being electroactive
within the potential window. The ‘free’ iodide ions act as
a monitor of reaction progression, and the carbon-centred
radical either dimerises and/or abstracts a hydrogen atom
from the electrolyte/solvent; I giving percentage yields of
58% adamantane and 39% 1,1’-biadamantane, and II
giving 67% R-H and 25% R-R.
EXPERIMENTAL
Reagents
1-Iodoadamantane, I (Fig. 1) (Aldrich, 98%), tetra-n-
butylammonium perchlorate (TBAP), tetra-n-butylam-
monium iodide (TBAI), tetra-n-butylammonium bromide
Copyright # 2007 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2007; 20: 115–121
DOI: 10.1002/poc

ELECTROCATALYTIC REDUCTION OF ALKYL IODIDES
117
reference electrode was developed for use in THF and at
low temperatures.35 The temperature was monitored and
controlled by an external system (Julabo FT902,
JULABO Labortechnik GmbH, D-77960 Seelbach/
Germany) and in all reactions was kept constant at
295 K ꢂ1 K. Typically, the solutions were degassed
vigorously for 5 min using impurity-free argon (BOC
gases, Guildford, Surrey, UK) to remove any trace oxygen
and an inert atmosphere was maintained throughout all
analyses. All solutions were prepared under an atmos-
phere of argon using oven-dried glassware such as
syringes and needles used for the transfer of moistur-
e-sensitive reagents. All voltammetric measurements
were performed inside a faraday cage in order to
minimise any background noise.
micromass LCT (resolution ¼ 5000 FWHM) using a
lock-spray source.
RESULTS AND DISCUSSION
Cyclic voltammetry of 1-iodoadamantane I
and trans-1-(3-iodopropoxy)-
4-(methoxymethoxy)cyclohexane II
First, a blank voltammogram was obtained to determine
the potential window in THF (0.1 M TBAP) at a silver
electrode. It was observed that the cathodic window
extended from <ꢀ0.1 V to ca. ꢀ3.5 V relative to Fc/
FcþPFꢀ6 . Following this, the voltammetric response of I
and II were recorded in separate experiments and the
voltammograms are shown in Figs 2 and 3 respectively.
Cyclic voltammetry is consistent with an n ¼ 1 electron
process with values of the transfer coefficient, a in the
range: 0.5 to 0.3. This is in good agreement with previous
literature reports of a values where the breaking of a
carbon–halogen bond is concerted with the electron
transfer process’.16,23 As a function of scan rate, I was
additionally analysed and the voltammograms are shown
in Fig. 4. The reduction of both substrates (peak A) was
observed to occur at more positive cathodic potentials in
comparison to platinum being used as the electrode
material. In the case of substrate II, Fig. 3 shows a small
pre-peak which may be due to the adsorption of the parent
iodide to the electrode surface.16 Upon reversing the
potential i.e., anodically scanning, a series of oxidation
peaks was seen and are shown again by Figs 2 and 3.
Indeed, this characteristic series of peaks is seen in both
cases regardless of whether I or II is analysed. It was
hypothesised that these were due to iodide ions formed
Preparative electrolysis
Controlled-potential, bulk electrolyses were carried out in
a two-compartment cell, the catholyte and anolyte being
separated by a sintered glass frit. The cathode was a
rectangular silver plate (area ¼ 6.25 cm2) (Goodfellow,
UK), and the anode, a platinum mesh housed within the
separate compartment with 0.1 M TBAP in THF. A Fc/
FcþPFꢀ6 reference electrode was used and a shiny silver
wire (250 mm, diameter) (Goodfellow) served as an
‘in-situ’ monitor of the reaction progression. All
electrolyses were vigorously stirred using a magnetic
stirrer bar and the temperature maintained at 295 K ꢂ1 K
using an external system (Julabo FT902, JULABO
Labortechnik GmbH, D-77960 Seelbach/Germany).
Saturated aqueous ammonium chloride (2 ml) followed
by water (10 ml) was added to the electrolysed solution
upon reaction completion. Diethyl ether (20 ml) was then
added and the two fractions separated. The organic
fraction was then washed with aqueous hydrochloric acid
(1 M 6 ꢁ 10 ml), saturated aqueous sodium thiosulfate1
(10 ml), saturated sodium bicarbonate (10 ml), and
saturated aqueous sodium chloride (10 ml). The organic
layer was dried over magnesium sulphate and then
carefully evaporated under reduced pressure especially in
the case of 1-iodoadamantane electrolyses where
adamantane as a product, is known to sublimate in the
rotary evaporator.
The products were identified by a combination of
1H NMR, 13C NMR (Bruker AV400 (400 MHz)
spectrometer), gas chromatography (GC), thin-layer
chromatography (TLC), flash chromatography (acetone/
petrol gradient elution) and mass spectrometry in which
mass spectra (m/z) were recorded in electrospray
ionisation (ESI) mode on a Fisons Platform II. Accurate
mass (HRMS) were recorded under conditions of ESI on a
Figure 2. Voltammetric response of 1-iodoadamantane, I,
(6.7 mM) in THF (0.1 M TBAP) using a silver electrode
(1.71 mm (diam.)), recorded at a scan rate of 75 mV sꢀ1 at
295 K. Start scan: ꢀ1.0 V, fiþrst reverse: ꢀ2.2 V and second
reverse: ꢀ0.1 V versus Fc/Fc PF6ꢀ
1Saturated aqueous sodium thiosulfate was added to remove the iodine
which was observed to accumulate significantly in the organic layer
during extractions.
Copyright # 2007 John Wiley & Sons, Ltd.
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DOI: 10.1002/poc

118
C. A. PADDON ET AL.
n-butylammonium iodide (TBAI) was used as a source of
iodide ions. In addition, it was also of interest to
investigate further the report that the addition of iodide
ions negatively ‘shifts’ the reduction peak potential of a
‘probe’ organic halide molecule, induced by progressive
additions of halide ions.20,36 The voltammograms
following sequential additions of iodide ions are shown
in Fig. 5. It can be seen that no ‘shift’ occurs in the
reduction peak potential (peak A) of the alkyl iodide, II.
However, the oxidation peaks observed previously now
increase with successive additions of iodide ions (peaks
B, C, D). In addition, the cathodic scans show not only a
sharp stripping peak (peak E), but the emergence of an
additional peak, F. From these analyses, a complex
pattern of reactivity can be deduced and so in an attempt
to ‘simplify’ the voltammetry, we used an ‘inert’
electrode material, platinum, in order to remove the
specific interactions of silver with halide ions.
Figure 3. Voltammetric response of trans-1-(3-iodopropoxy)-
4-(methoxymethoxy)cyclohexane, II, (3 mM) in THF (0.1 M
TBAP) using a 1.71 mm (diam.) silver electrode, recorded at a
scan rate of 50 mV sꢀ1 at 295 K. Start scan: ꢀ1.0 V, first
reverse: ꢀ3.0 V and second reverse: 0 V versus Fc/FcþPF6ꢀ
Cyclic voltammetry of tetra n-butylammonium
iodide (TBAI) at platinum
following the reductive cleavage of the alkyl iodide. In
order to characterise these, a source of iodide ions was
used.
The voltammetric response of a solution of TBAI in THF
using a platinum electrode is shown in Fig. 6. It can be
seen that only two anodic peaks are seen within the
potential window (peaks W and X). These peaks
correspond well to those seen at silver i.e., peaks B
and C and also to previous literature reports on iodide
Cyclic voltammetry of tetra n-butylammonium
iodide (TBAI) at silver
In order to provide evidence for iodide electroactivity
following the reductive cleavage of the alkyl iodide, tetra
Figure 5. Characterisation of iodide ions: Voltammetric
response of 1-iodoadamantane, I, (6.7 mM) in THF (0.1 M
TBAP) using a silver electrode (1.71 mm (diam.)), recorded at
a scan rate of 75 mV sꢀ1 at 295 K. Start scan: ꢀ1.0 V, first
reverse: ꢀ2.2 V and second reverse: ꢀ0.1 V versus Fc/
FcþPFꢀ6 . Tetra n-butylammonium iodide (TBAI) was added
sequentially and the voltammetric response recorded. TBAI
concentrations: (i) 0.14 mM, (ii) 2.83 mM, (iii) 5.67 mM,
(iv) 1.13 mM, (v) 1.58 mM, (vi) 2.11 mM, (vii) 2.59 mM,
(viii) 3.16 mM, (ix) 3.71 mM and (x) 4.75 mM
Figure 4. Voltammetric response of 1-iodoadamantane, I,
(6.7 mM) in THF (0.1 M TBAP) using a silver electrode
(1.71 mm (diam.)), recorded at various scꢀa1n rates: 30, 40,
50, 75, 100, 150, 200, 250 and 300 mV s at 295 K. Start
scan: ꢀ1.0 V, firstþreverse: ꢀ2.2 V and second reverse:
ꢀ0.1 V versus Fc/Fc PF6ꢀ
Copyright # 2007 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2007; 20: 115–121
DOI: 10.1002/poc

ELECTROCATALYTIC REDUCTION OF ALKYL IODIDES
119
of specifically adsorbed iodide is not observed until much
more positive potentials:
ꢀ
2Iaꢀds ꢃ ꢃ ꢃ Ag ꢀꢀ2!e I2ðadsÞ ꢃ ꢃ ꢃ Ag
The origin of peak E is likely due to adsorption of iodine
by the silver or indeed the substrate iodide being
specifically adsorbed as reported by other workers.20–22
With increasing iodide concentrations, peak F is observed
which is most likely due to the increased concentrations
of I2 being generated which can be reduced to Iꢀ3 and then
reduced to Iꢀ. This latter step now competing with the
former.22
Cyclic voltammetry of tetra
n-butylammonium–bromide
and chloride at silver
Figure 6. Voltammetric response of iodide ions (8.5 mM
TBAI) using a platinum electrode (1 mm (diam.)) in THF
(0.1 M TBAP) at 295 K. Scan rate: 100 mV sꢀ1. Start scan
ꢀ2.0 V, first reverse: 0.2 V, second reverse: 0.5 V versus Fc/
FcþPFꢀ6
To provide further support to the specific affinity of silver
toward iodide ions salts of the bromide and chloride were
analysed voltammetrically at a silver cathode. The cyclic
voltammograms of tetra n-butylammonium bromide and
chloride were recorded and clearly showed only two
anodic peaks contrasting to the case with iodide where
three peaks were observed.
activity at platinum in non-aqueous solvents26–29 and also
bromide activity in room temperature ionic liquids
(RTILs).37,38 However, no peak D was observed, even
at lower temperatures, where theþanodic potential window
was extended relative to Fc/Fc PFꢀ6 . The two voltam-
metric peaks for the oxidation of iodide ions are shown
below:
Preparative electrolyses at silver
As the product(s) other than iodide ions are likely to be
redox inactive within the potential window we used
controlled-potential, bulk electrolyses in order to identify
these. In addition, this provided information as to the
number of electrons involved in the reductive process.
Electrolyses were performed on I and II at the potentials
of ꢀ1.7 V and ꢀ2.2 V respectively. The results are
summarised in Table 2 and experimental details outlined
in section 2.3. Using a silver wire, the voltammetric
response of the electrolysis reaction was monitored
in situ. This showed the decreasing response due to the
alkyl iodide and an increasing and specific response due
to the presence of ‘free’ iodide ions. By using a
chronocoulometric analysis and monitoring, the substrate
peak height as a function of Coulombs passed, the number
of electrons, n, involved in the reduction process was
shown to be 1 in both cases. Electrode filming as reported
by some workers21 was not observed, although extrac-
tions following electrolyses were tailored to incorporate
an aqueous saturated sodium thiosulphate wash to remove
the significant quantities of iodine by-product. The
products were identified by various spectroscopic means;
I gave percentage yields of adamantane (58%) and the
dimer, 1,1’-biadamantane (39%), and II yielded a similar
mixture of RH (67%) and R-R (25%). No starting
material or other significant product(s) were recovered,
each starting material showing a full electrochemical
ꢀ2eꢀ
3Iꢀ ꢀ! I3ꢀ
ꢀ2eꢀ
2I3ꢀ ꢀ! 3I2
Therefore peaks B and C are consistent with those
observed using platinum electrodes. Peak D however,
must be specific to silver. Analysis of the area under peak
D for TBAI at silver using selected scan rates was
performed and the data are summarised in Table 1. The
values for surface coverage and molecules per cm2 are not
inconsistent with monolayer coverage. Therefore, it is
likely that due to silver having a high affinity for iodide
ions, these are stabilized sufficiently so that the reduction
Table 1. Analysis of peak D at various scan rates following
reduction of TBAI only, in THF (0.1 M TBAP) at 298 K
Scan rate
)
A.U.P Da
(mC)
Surface coverage
)
Molecules
(cmꢀ2
(mV sꢀ1
(mol cmꢀ2
)
ꢀ9
ꢀ9
5
9.2
5.6
1.5
1.3
2.1 ꢁ 10
1.3 ꢁ 1015
0.8 ꢁ 1015
0.2 ꢁ 1015
0.1 ꢁ 1015
10
50
75
1.3 ꢁ 10
ꢀ10
ꢀ10
3.3 ꢁ 10
2.9 ꢁ 10
a Area under peak D.
Copyright # 2007 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2007; 20: 115–121
DOI: 10.1002/poc

120
C. A. PADDON ET AL.
Table 2. Results following preparative bulk electrolysis at controlled-potential
Compound
(starting material)
Number of
electron(s) (n)
Percentage
yield (%)
Electrolysis no.
Product(s)
1
2
I
1
1
Adamantane:
1,1’-biadamantane
RH:
58
39
67
25
II
R-R
A silver plate was used (A ¼ 6.25 cm2) as the working electrode in a two compartment cell separated by a sintered glass frit in which was housed the counter
electrode, a platinum mesh. The reference electrode was a Fc/FcþPF6ꢀ in THF. THF was the solvent and TBAP (0.1 M) was used as the electrolyte. All reactions
were performed at 295 K under constant stirring using a magnetic stirrer bar and their progress monitored at various points after the passage of X number of
coulombs of charge.
Scheme 1
conversion. This contrasts well with the previous attempts
at bulk electrolyses using platinum electrodes in THF at
298 K in which a significant fraction of charge passed is
consumed by the solvent electroactivity.
response. Finally, preparative electrolyses at controlled-
potential have shown that a mixture of reduced monomers
and dimers are formed following the passage of charge
required for a one-electron reduction.
CONCLUSIONS
Acknowledgements
A summary of the electrochemical activity of alkyl
iodides is shown in Scheme 1 where SH is either solvent
and/or supporting electrolyte in which a hydroge-
n-abstraction reaction may occur. It has been shown that
reduction potentials of alkyl iodides (I and II) in THF at
silver are significantly shifted to more positive values
compared with platinum, consistent with the current
observation of electrocatalytic activity of silver toward
organo-halides. Following a one-electron reduction
process, a unique and characteristic pattern of iodide
electrochemistry is seen. This corresponds to the
oxidation of Iꢀ and I3ꢀ species in solution, whilst at
more positive potentials, a specific oxidation peak is seen
corresponding to the oxidation of adsorbed iodide ions,
this potential being much more positive than that of iodide
ions in solution due to the high affinity of silver towards
halide ions. A cathodic peak is seen on potential reversal,
which is likely due to the reduction of adsorbed molecular
iodine—again the stabilisation inferred by silver respon-
sible for this. Comparisons with TBAI at platinum have
shown that such specific effects are indeed, only
obseraved at silver. In addition, other halide ions such
as bromide and chloride do not show this specific
C. A. P and F. L. B wish to thank EPSRC for a joint
project studentship (Grant GR/T05011/01).
REFERENCES
¨
1. Peters DG, Bard AJ, Stratmann M, Schafer HJ (eds). Organic
Electrochemistry: Chapter 8 Oxidation and Reduction of Halogen-
Containing Compounds, Vol. 8, 1st ed., Wiley: New York, 2004.
2. Peters DG, Lund H, Hammerich O (eds). Organic Electro-
chemistry (4th edn). Marcel Dekker: New York, 2001; Ch 8. pp.
341–377.
´
3. Saveant J-M. Single Electron Transfer and Nucleophilic Substi-
tutions, Vol. 26, Academic Press: London, 1990.
4. Rondinini S, Mussini PR, Crippa F, Petrone M, Sello G. Collect.
Czechoslovak Chem. Commun. 2000; 65: 881.
5. Rondinini S, Mussini PR, Crippa F. Sello G. Electrochem. Com-
mun. 2000; 2: 491.
6. Isse AA, Ferlin MG, Gennaro A. J. Electroanal. Chem. 2005; 581:
38.
7. Isse AA. Gennaro A. Chem. Commun. 2002; 23: 2798.
8. Rondinini S, Mussini PR, Cantu G, Sello G. Phys. Chem. Chem.
Phy. 1999; 1: 2989.
9. Guerrini M, Mussini PR, Rondinini S, Torri G, Vismara E. Chem.
Commun. 1998; 1575.
10. Rondinini S, Vertova A. Electrochimica. Acta. 2004; 49: 4035.
11. Rondinini S, Mussini PR, Specchia M, Vertova A. J. Electrochem.
Soc. 2001; 148: D102.
Copyright # 2007 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2007; 20: 115–121
DOI: 10.1002/poc

ELECTROCATALYTIC REDUCTION OF ALKYL IODIDES
121
12. Delli E, Kouloumtzoglou S, Kyriacou G, Lambrou C, Chem.
Commun. 1998; 16: 1693.
24. Foresti ML, Innocenti M, Forni F, Guidelli R. Langmuir. 1998; 1:
7008.
´
13. Andrieux CP, Gallardo I, Saveant J-M. J Am. Chem. Soc. 1989;
111: 1620.
25. Magnussen OM. Chem. Rev. 2002; 102: 679.
26. Datta J, Kundu KK. Bullet. Electrochem. 1991; 7: 4.
27. Kamau GN, Rusling JF. J. Electroanalyt. Chem. 1990; 292: 187.
28. Behl W, Chin DT. Proc. Electrochem. Soc. 1987; 441.
29. Macagno VA, Giordano MC. Electrochimica. Acta. 1969; 14: 335.
30. Kolthoff IM, Jordan J. J. Am. Chem. Soc. 1953; 75: 1571.
31. Akkermans RP, Fulian Q, Roberts SL, Suarez MF, Compton RG.
J. Phys. Chem. B. 1999; 103: 8319.
´
14. Saveant J-M. J. Am. Chem. Soc. 1987; 109: 6788.
15. Andrieux CP, Saveant J-M, Su KB. J. Phys. Chem. 1986; 90: 3815.
´
16. Isse AA, Falciola L, Mussini PR, Gennaro A. Chem. Commun.
2006; 3: 344.
17. Rondinini S, Mussini PR, Muttini P, Sello G. Electrochimica. Acta.
2001; 46: 3245.
18. Bellomunno C, Falciola BDL, Longhi M, Mussini PR, Doubova
LM, Di Silvestro G. Electrochimica. Acta. 2005; 50: 2331.
19. Ardizzone S, Cappelletti G, Doubova LM, Mussini PR, Passeri
SM, Rondinini S. Electrochimica. Acta. 2003; 48: 3789.
20. Falciola L, Mussini PR, Trasatti S, Doubova LM. J. Electroanalyt.
Chem. 2006; 593: 185.
21. Mussini PR, Ardizzone S, Cappelletti G, Longhi M, Rondinini S,
Doubova LM. J. Electroanalyt. Chem. 2003; 552: 213.
22. Ardizzone S, Cappelletti G, Mussini PR, Rondinini S, Doubova
LM. J. Electroanalyt. Chem. 2002; 532: 285.
32. Breiter MW. Electrochimica. Acta. 1963; 8: 925.
33. Perichon J, Rene B. Bulletin de la Societe Chimique de France
1967; 10: 3707.
34. Pangborn AB, Giardello MA, Grubbs RK, Rosen RK, Timmers RJ.
Organometallics. 1996; 15: 1518.
35. Paddon CA, Compton RG. Electroanalysis. 2006; 19: 1919.
36. Fedurco M, Sartoeretti CJ, Augustynski J. Langmuir. 2001; 17:
2380.
37. Allen GD, Buzzeo MC, Villagran C, Hardacre C, Compton RG.
J. Electroanalyt. Chem. 2005; 575: 311.
23. Falciola L, Gennaro A, Isse AA, Mussini PR, Rossi M.
J. Electroanalyt. Chem. 2006; 593: 47.
38. Allen GD, Buzzeo MC, Davies IG, Villagran C, Hardacre C,
Compton RG. J. Phys. Chem. B. 2004; 108: 16322.
Copyright # 2007 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2007; 20: 115–121
DOI: 10.1002/poc