A samarium "soluble" anode: A new source of SmI2 reagent for electrosynthetic application

Article abstract of DOI:10.1002/chem.201201390

An electrochemical method to prepare solutions of samarium diiodide in THF is reported. The simple electrolysis of a samarium rod provides a rapid and straightforward in situ synthesis of SmI₂. The electrogenerated complex catalyzes various C-C bond formations. The reagent is produced continuously (see scheme) and leads to efficient organic electrosynthesis with significantly smaller amounts of solvent than usually required. Copyright

Full text of DOI:10.1002/chem.201201390

COMMUNICATION
DOI: 10.1002/chem.201201390
A Samarium “Soluble” Anode: A New Source of SmI2 Reagent for
Electrosynthetic Application
Kamar Sahloul, Linhao Sun, Alexandre Requet, Youhana Chahine, and
Mohamed Mellah*[a]
Since the seminal reports of Kagan, dedicated to the prep-
aration of lanthanide iodides and their usefulness in organic
synthesis,[1,2] SmI2 have became one of the most important
reducing agents available to the synthetic organic chemist,
promoting a multitude of radical and anionic reactions.[3,4]
Generally this reagent is synthesized by the oxidation of
metallic samarium by 1,2-diiodoethane in THF under an
inert atmosphere.[3] Molecular iodine[5] or diiodomethane[6]
have also been used and, more recently, rapid preparations
of SmI2 in THF were described under sonication[7,8] or mi-
crowave irradiation.[9] These recent improvements have
indeed reduced the time of preparation significantly. The
dark blue solution is generally prepared and stored under
inert atmosphere. However, the major limitation remains
the concentration of SmI2 in THF (around 0.1m). Therefore,
for synthetic applications, all these routes require large
amounts of solvents and a strictly inert atmosphere; this pre-
vents industrial developments. In this context, we are inter-
ested in developing a convenient method to continuously
produce SmI2 species for applications in organic synthesis.
Herein we disclose our current studies toward this goal and
report an original electrochemical method for the rapid and
easy in situ preparation of SmI2 in THF by direct electro-
chemical oxidation of a samarium metallic rod for synthetic
purposes. Preliminary results on the synthetic application of
this process in several coupling reactions are also described.
To the best of our knowledge, although the electrochemi-
cal characteristics of SmI2 have been widely studied, among
others by Flowers et al.,[10–13] the electrochemical synthesis
of SmI2 by oxidation has never been described and only
very few electrochemical studies have been dedicated to
electrogenerated samarium derivatives. Indeed, only two ap-
proaches have been described to generate divalent samari-
um by electrochemical reduction for synthetic applications.
E. DuÇach et al. have described an electrochemical proce-
dure based on the use of Mg or Al “sacrificial” anodes by
using SmCl3 as precatalyst, which is reduced onto a nickel
sponge cathode to generate a SmII active species. With a cat-
alytic amount of initial SmCl3 (10–20 mol%), this procedure
was applied to perform the homocoupling of carbonyl com-
pounds and the cross-coupling of carbonyl compounds with
chloroesters.[14,15] More recently, Parrish and Little described
the electrochemical production of SmI2 by reduction starting
from SmACTHUNGRTNEUNG(OTf)3 in the presence of nBu4NI. SmI3 was gener-
ated by ligand exchange and reduced under a constant po-
tential (E=ꢀ1.8 V). This procedure was applied to the re-
ductive SmI2-mediated cyclization.[16]
To probe the feasibility of the generated SmI2 species by
oxidation of a samarium rod, the electrochemical oxidation
of a Sm anode was first investigated in detail. THF is a priv-
ileged solvent for samarium diiodide chemistry and was
chosen as a solvent for this study. First of all, the oxidation
potentials of samarium metal in this solvent were deter-
mined. For implementation, a home-made samarium elec-
trode (S=1.0 cm2) was designed and used.[17] Electrochemi-
cal experiments were performed under argon in an electro-
chemical cell, sealed and fitted with electrodes to prepare in
situ samarium complexes and analyze their redox behavior.
The electrochemical oxidation of the samarium rod was in-
vestigated in anhydrous THF containing nBu4NPF6
(5.0·10ꢀ2 m). Several tetraalkylammonium salts were tested
as soluble electrolyte supports in THF and nBu4NPF6 was
ꢀ
the best. Indeed, unlike other anions, PF6 might not alter
the structure of SmI2, as the dark-blue characteristic color of
samarium diode is maintained in the presence of this ion.[1]
Scanning-potential measurements of the oxidation of the sa-
marium anode between ꢀ2.4 and 0.0 V revealed the exis-
tence of two oxidative waves (Figure 1). The first wave,
which starts at about E=ꢀ1.4 V versus SCE, is attributed to
the formation of SmII [Eq. (1)]. The more intense second
wave, around E=ꢀ0.4 V, was attributed to the direct oxida-
tion of the samarium anode in SmIII [Eq. (2)].
From 1:4 to ꢀ0:9 V Sm0 ¼ SmII þ 2eꢀ
For E > ꢀ0:4 V Sm0 ¼ SmIII þ 3eꢀ
ð1Þ
ð2Þ
[a] K. Sahloul, L. Sun, A. Requet, Y. Chahine, Dr. M. Mellah
Equipe de Catalyse Molꢀculaire
Univ Paris-Sud, ICMMO, UMR 8182
Orsay, 91405 (France)
In presence of nBu4NI as iodide source, the first oxidation
wave increased significantly (see the Supporting Informa-
tion, Section 3.2). The increase in intensity might be due to
the coordination between samarium and iodide, promoting
the formation of SmI2, which is soluble and easily liberated
Fax : (+33)169158046
E-mail: mohamed.mellah@u-psud.fr
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201201390.
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cepts another electron from the
electrode to give an alkyl car-
banion, nBuꢀ, which is proto-
nated by a Hofmann elimina-
tion of the supporting electro-
lyte to form butene and
butane.[18] nBu3N was always
detected
after
electrolyses.
Iodide could not oxidize at the
anode during the electrolysis,
because the value of the oxida-
tion potential (E= +0.1 V vs.
SCE) of nBu4NI in THF is
much higher than the potential
measured at the samarium
anode during all the electrolytic
processes (E=ꢀ0.5 V vs. SCE).
The dark-blue color of the
solution does not disappear
when an anodic current was ap-
plied to a chemically prepared
solution of SmI2 introduced
Figure 1. Electrochemical oxidation of the samarium anode (S=1.0 cm2) in THF solution containing
nBu4NPF6 (5.0·10ꢀ2 m). Potential scan rate v=100 mVsꢀ1. The inset shows the cyclic voltammogram of electro-
generated SmI2 ([SmI2]=0.02m) in THF with 0.1m nBu4NPF6. At glassy carbon disc (F=1 mm) working elec-
trode and at scan rate of 0.1 Vsꢀ1 versus SCE.
in the medium. The variation in concentration of nBu4NI
between 10ꢀ4 and 10ꢀ1 m do not modify the oxidation-poten-
tial values. Hexamethylphosphoramide (HMPA) has an im-
portant and significant effect on the redox properties of
SmI2.[12] However, in this electrochemical oxidative process,
the addition of up to ten equivalents of HMPA do not sig-
nificantly alter the potential values for SmII/Sm0 and SmIII/
Sm0 in THF.
When the electrolysis was performed by applying a con-
stant potential of ꢀ0.5 V, the anodic current reached rapidly
a value of about 1.5 mA and remained stable during the
Scheme 1. Schematic diagram showing the electrolytic preparation of
whole electrolytic process. A deep-blue coloration appears
instantaneously near the electrode, similar to the original
color reported by Kagan et al. for a THF solution of chemi-
cally generated SmI2.[1] This observation might suggest the
formation of samarium diiodide species at the surface of the
electrode, which then accumulate in the electrolytic
medium. These samarium species were next studied by
cyclic voltammetry on a glassy carbon electrode. The elec-
trochemical analyses of these species provided a quasi-rever-
sible voltammogram (inset, Figure 1) in accordance with
those obtained from chemically prepared SmI2 in THF.[8b]
These analyses undeniably confirmed the generation of
SmI2 in the THF/nBu4NPF6/nBu4NI electrolytic medium
from oxidation of the samarium anode according to in
Scheme 1. nBu4NI plays a double role in this process. First it
provides iodide anions essential to the formation of samari-
um diiode from the oxidation of Sm0 into SmII at the anode.
Secondly, it provides the counter cation, which is reduced at
the Pt cathode with liberation of iodides. We hypothesize
that the nBu4N+ ion accepts one electron from the electrode
SmI2 from samarium anode in THF/nBu4NPF6 electrolytic medium.
into the electrochemical cell. This experiment indicates that
under these conditions SmI2 in the solution cannot be over-
oxidized at the samarium anode. Preparative electrolyses of
SmI2 were next conducted under galvanostatic conditions.
Intensities of 0.05 to 0.2 A were applied to continuously pro-
duce the deep-blue complex in the electrolytic medium. The
concentration of electrogenerated SmI2 was followed by io-
dometric titration (see the Supporting Information, Sec-
tion 6).[19] Samples, to be assayed by iodometric titration,
were taken throughout the electrolysis. The concentration
increase was linear and slightly lower than the theoretical
concentration calculated by considering that the electro-
chemical production of SmI2 is a process at two Faraday per
mole. This progress is consistent with an effective electro-
chemical production of SmI2 at two electrons with a faradic
yield of 70%. The faradic yield observed reflects the proba-
ble formation of trivalent samarium. However, this monitor-
ing during electrolysis led us to rule out the hypothesis of a
chemical formation of this reagent.
C
to give a neutral radical, nBu4N , which undergoes homolytic
rupture of a carbon–nitrogen bond to form the butyl radical,
C
nBu , and tributylamine, nBu3N. Then the alkyl radical ac-
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SmI2 Reagents for Electrosynthetic Applications
COMMUNICATION
complete carbonyl conversion was reached. This protocol
was also successfully applied to the homocoupling of various
aldehydes and ketones and results are gathered in Table 1.
Pinacolisation products were isolated in good yields (70–
95%) despite good to moderate diastereoselectivities
(Table 1, entries 1–5). Octan-2-one, as an aliphatic ketone,
Table 1. Homocoupling of aromatic aldehydes, ketones, and imines in-
duced by electrogenerated SmI2.
Figure 2. UV spectra of the SmI2 solution prepared by electrolysis (black
curve) and the SmI2 solution prepared by a chemical procedure in pres-
ence of nBu4NPF6 (dotted curve). SmI2 concentration=20 nm for both
solutions.
Entry
X
R1
R2
Yields [%][a]
dl/
meso[b]
1
2
3
4
5
6
7
8
O
O
O
O
C6H5
H
H
H
CH3
CH3
H
H
H
80
70
79
95
82
96
81
77
42
82
70/30
60/40
70/30
80/20
50/50
52/48
67/33
80/20
80/20
43/57
p-CH3C6H5
o-MeO-C6H5
C6H5
C6H13
C6H5
p-CH3C6H5
p-N(Me)2-C6H5
p-OMe-C6H5
C6H13
O
The UV spectrum of the electrogenerated SmI2 solution
was next recorded and compare with the UV spectrum of a
THF solution of SmI2 chemically prepared by the reaction
of samarium powder and diiodoethane at a defined concen-
tration of 20 nm. The two spectra with characteristic broad
bands at 556 and 617 nm almost match perfectly and are
consistent with the generation of SmI2 in THF as previously
reported (Figure 2).[8] The addition of nBu4NPF6 salt to the
solution of chemically prepared SmI2 do not modify the UV
spectrum and confirms that it does not influence the struc-
ture in THF.
To afford a synthetic application for this electrochemical
process, various sp3–sp3 carbon–carbon bond-forming reac-
tions induced by SmI2 as single-electron reducing agent
were investigated. The electrolysis experiment was conduct-
ed either in a potentiostatic or a galvanostatic mode. To ini-
tially probe the feasibility of such an approach, a solution of
electrogenerated SmI2, at an estimated concentration, was
prepared as described above, then the electrode was re-
moved and the corresponding aldehydes and ketones for the
coupling reaction were introduced into the electrochemical
cell. For both cases, coupling products were obtained in
good yields of up to 95%. To improve the synthetic useful-
ness of this methodology, the carbonyl derivative was initial-
ly added into the reaction cell before the electrolysis process
was started. In this single pot experiment, the in situ gener-
ated SmI2 reacts directly with the substrate present in the
medium until its total consumption. The duration of elec-
trolysis depend only on the initial amount of substrates in-
troduced in the electrochemical cell and on the current den-
sity. For example, when a current density of 2.5 Acmꢀ2 was
applied, 4.5 h of electrolysis were needed for a total conver-
sion of 5 mmol of carbonyl derivative into its corresponding
pinacol. In this experiment, it was possible to visually moni-
tor the completion of the reaction. Indeed, throughout the
electrolysis, the solution remained colorless or slightly
yellow, and then turned blue, indicating that SmI2 begins to
accumulate in the medium and that the current required for
NPh
NPh
NPh
NPh
NPh
9
10
H
CH3
1
[a] Isolated yields after 100% conversion. [b] Determined by H NMR.
was also converted into the corresponding diol with 82%
yield, but with poor selectivity (Table 1, entry 5). This result
showed that in our conditions, the divalent samarium species
synthesized by electrochemistry are particularly reactive, be-
cause the coupling of aliphatic ketones is a difficult process
with a classical SmI2 solution.[20]
Substitution of the samarium anode by a platinum anode
ruled out the possible direct electrochemical reduction of
substrates at the cathode surface. Indeed, less than 10%
yield of the corresponding pinacol was produced for aceto-
phenone under the same electrolysis conditions. This elec-
trosynthetic procedure was also applied to the reductive
coupling of imines. Various imines were used as substrates
with good efficiency (Table 1, entries 6–10). For example,
for the coupling of N-benzylideneaniline (entry 6), the cor-
responding diamine was isolated in 96% yield. The diaster-
1
eoselectivity was determined by H NMR analyses and was
similar to the one observed with classical SmI2 solu-
tions.[21,22]
The same electrosynthetic conditions were also successful-
ly applied to a SmI2-mediated Barbier reaction. The reac-
tions of allylic halides (X=I or Br) were tested with a varie-
ty of aldehydes, ketones, and imines as electrophiles
(Table 2, entries 1–5). Without any additives the correspond-
ing allylic products were obtained in low to moderate yields.
Despites the use of two equivalents of allylic halides, the ho-
mocoupling products were obtained (31–80%).
A major advantage of this one-pot protocol is that SmI2 is
in situ and continuously produced by electrolysis during the
course of the reaction and consumed as it is formed. This
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M. Mellah et al.
Table 2. Barbier-type reactions mediated by electrogenerated SmI2.
General procedure for ketones and aldehydes homocoupling: Under
argon, reactions were carried out in a one-compartment cell containing a
magnetic stirring bar, a samarium anode (1.27 cm diameter x 5 cm), and
a
platinum grid cathode (20 cm2 area). Corresponding substrates
(4 mmol), nBu4NI (10 mmol) and nBu4NPF6 (1.0 mmol) were introduced
in 50 mL THF. The electrolysis was performed at 50 mA and was stopped
when the substrate had been completely consumed (monitored by TLC
or GC, 4.5 h, 2.1 Fmolꢀ1). After the electrolysis, THF was removed on a
rotary evaporator. The mixture was then quenched with HCl (1m) and
extracted with ether (2ꢂ10 mL). The combined extracts were washed
with sodium thiosulfate and brine. The organic layer was dried over
MgSO4, and the solvent removed under reduced pressure. The crude ma-
terial was purified by flash chromatography on silica gel. The compounds
were identified by comparison of their physical and spectral data with
those given in the literature.
Entry
X
R1
R2
Yields [%][a]
1
1
2
3
4
5
O
O
O
O
O
NPh
C6H5
C6H5
G
H
H
CH3
CH3
CH3
H
80
64
45
59
31
56
CHTUNGTRENNUNG
[a] Isolated yields after 100% conversion.
greatly reduces the amount of solvent required for the ex-
periment compared with that used for the same experiment
conducted under classical conditions. Indeed, the concentra-
tion of SmI2 prepared chemically is limited to around to
0.1m in THF. Undeniably, this electrochemical preparation
of divalent samarium is solvent economical, because in
50 mL of THF it was possible to reduce up to 20 mmol of
substrate instead of only 5 mmol with 50 mL of a classical
SmI2 solution. Finally, this way more than 75% of solvent
was saved compared with the classical procedure.
Acknowledgements
Dr Jacqueline Collin is acknowledged for helpful discussions. We thank
the University of Paris-Sud 11 and the Centre National de la Recherche
Scientifique (CNRS) for funding.
Keywords: allylation · electrochemistry · pinacolisation ·
soluble anodes · SmI2
In summary, we have described the first electrochemical
preparation of samarium diiodide by direct oxidation of a
“sacrificial” samarium anode. This alternative route for the
synthesis of SmII species is particularly efficient and can be
carried out with routine methods in a galvanostatic mode.
The electrogenerated SmI2 was well characterized by elec-
trochemistry and UV/Vis analysis. The beneficial effects of
this new methodology in terms of reactivity and solvent
economy have been highlighted in several organic reactions
induced by SmI2 as reducing reagent. Further work is on-
going to demonstrate the potential of this method, in partic-
ularly for selective cross-coupling reactions and catalytic ap-
plications. This original in situ synthesis of SmI2 by electro-
chemistry also offers important perspectives to prepare vari-
ous sensitive lanthanide reagents for organic applications.
[1] J.-L. Namy, P. Girard, H. B. Kagan, New J. Chem. 1977, 1, 5–7.
[2] P. Girard, J.-L. Namy, H. B. Kagan, J. Am. Chem. Soc. 1980, 102,
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[3] a) K. C. Nicolaou, S. P. Ellery, J. S. Chen, Angew. Chem. 2009, 121,
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Molander in Organic Reactions, Vol. 46 (Ed.: L. A. Paquette), Wiley,
New York, 1994, pp. 211–367; c) D. J. Procter, R. A. Flowers, T.
Skrydstrup, Organic Synthesis using Samarium Diiodide, Royal Soci-
ety of Chemistry Publishing, Cambridge, 2010, p. 204; d) H. B.
Kagan, Tetrahedron 2003, 59, 10351–10372; e) G. A. Molander,
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[4] a) A. Krief, A.-M. Laval, Chem. Rev. 1999, 99, 745–778; b) G. A.
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[8] a) J. A. Teprovich, Jr., P. K. S. Antharjanam, E. Prasad, E. N. Pes-
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Enemærke, T. Hertz, T. Skrydstrup, K. Daasbjerg, Chem. Eur. J.
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Experimental Section
[9] A. Dahlꢀn, G. Hilmersson, Eur. J. Inorg. Chem. 2004, 3020–3024.
[10] For the determination of the electrochemical reduction potential of
samarium diiodide in THF (E=ꢀ1.33ꢁ0.01 V vs. Ag/Ag+), see
a) M. Shabangi, R. A. Flowers II, Tetrahedron Lett. 1997, 38, 1137–
1140; b) M. Shabangi, J. M. Shealy, J. R. Fuchs, R. A. Flowers II, Tet-
rahedron Lett. 1998, 39, 4429–4432.
Materials: THF was distilled from sodium metal/benzophenone before
use. All commercially available chemicals were used without further puri-
fication. 1H NMR and 13C NMR spectra were recorded on either
a
Bruker AM 360 (360 MHz), AM 300 (300 MHz), or AM 250 (250 MHz)
instrument with samples dissolved in CDCl3. Mass spectra were recorded
on a micrOTOF-q Bruker Daltonics spectrometer. UV/Vis spectroscopy
experiments were performed on a Shimadzu UV-1601 UV-visible spec-
trophotometer controlled by a UV Probe (version 1.11) software. Cyclic
voltammograms for SmI2 in THF were measured with a EG&G Prince-
ton Applied Research (model 362) scanning potentiostat equipped with
an IFELEC (IF 2502) recorder, in an undivided three-electrodes cell con-
taining a standard glassy carbon electrode working electrode, a Pt coun-
ter electrode and a saturated Ag/AgNO3 electrode as reference. Cyclic
voltammograms for samarium anode analysis, preparation of the SmI2
solution in THF, and organic electrolysis were realized with a EG&G
Princeton Applied Research potentiostat/galvanostat (model 273).
[11] For the determination of the formal potential of SmI2+/SmI2 couple.
(E=ꢀ0.89 V vs. SCE, ꢀ1.41 V vs. Fc+/Fc) in THF by cyclic voltam-
metry see R. J. Enemærke, K. Daasbjerg, T. Skrydstrup, Chem.
Commun. 1999, 343–344.
[12] For studies on the effect of hexamethylphosphoramide on the reduc-
tion potential of SmI2, see a) J. B. Shotwell, J. M. Sealy, R. A. Flow-
ers II, J. Org. Chem. 1999, 64, 5251; b) R. S. Miller, J. M. Sealy, M.
Shabangi, M. L. Kuhlman, J. R. Fuchs, R. A. Flowers II, J. Am.
Chem. Soc. 2000, 122, 7718–7722; c) D. V. Sadasivam, P. K. S. An-
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7228–7229; d) K. A. Choquette, D. V. Sadasivam, R. A. Flowers II,
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SmI2 Reagents for Electrosynthetic Applications
COMMUNICATION
J. Am. Chem. Soc. 2010, 132, 17396–17398; e) A. Dahlꢀn, G. Hil-
mersson, Eur. J. Inorg. Chem. 2004, 3393–3403; f) R. A. Flowers II,
Synlett 2008, 10, 1427–1439.
homemade and are stored under inert atmosphere when they are
not used. All the samarium rods are purchased from Alfa Aesar
(99.9% Sm metals basis excluding Ta).
[13] The oxidation potential of the samarium anode depended strongly
on the nature of the solvent. For example, in acetonitrile a value of
ꢀ1.5 V versus SCE was determined for the first oxidation wave.
M. L. Kuhlman, R. A. Flowers II, Tetrahedron Lett. 2000, 41, 8049–
8052.
[14] E. Lꢀonard, E. DuÇach, J. Pꢀrichon, J. Chem. Soc., Chem. Commun.
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[16] J. D. Parrish, R. D. Little, Tetrahedron Lett. 2001, 42, 7767–7770.
[17] The electrode used for the analytic determination of the oxidation
potentials of samarium metal is not commercially available. It was
manufactured by using a samarium rod of 2 mm in diameter set in
Teflon and connected by copper wire. To prepare a SmI2 solution
for electrosynthesis the electrode is based on a samarium rod of
12.7 mm in diameter, 99.9% without Teflon and directly connected
to a copper wire to ensure current conductivity. Both electrodes are
[18] a) G. R. Davies, J. B. Woodhall, J. Appl. Electrochem. 1971, 1, 137–
139; b) C. E. Dahm, D. G. Peters, J. Electroanal. Chem. 1996, 402,
91–96.
[19] Simple weighing of the samarium rod before and after the electroly-
sis experiment was also conducted to estimate the concentration of
SmI2.
[20] The theoretical concentration was calculated by considering the
electrochemical formation of samarium diiodide at 2 F per mole.
[21] A. Lebrun, J. L. Namy, H. B. Kagan, Tetrahedron Lett. 1993, 34,
2311–2314.
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s II, Tetrahedron 2003, 59, 10397–10402; b) F. Machrouhi, J. L.
Namy, Tetrahedron Lett. 1999, 40, 1315–1318.
Received: April 23, 2012
Published online: && &&, 2012
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M. Mellah et al.
An electrochemical method to prepare
solutions of samarium diiodide in THF
is reported. The simple electrolysis of
a samarium rod provides a rapid and
straightforward in situ synthesis of
SmI2. The electrogenerated complex
Electrochemistry
K. Sahloul, L. Sun, A. Requet,
Y. Chahine, M. Mellah* ..... &&&& — &&&&
A Samarium “Soluble” Anode: A
New Source of SmI2 Reagent for Elec-
trosynthetic Application
ꢀ
catalyzes various C C bond forma-
tions. The reagent is produced continu-
ously (see scheme) and leads to effi-
cient organic electrosynthesis with sig-
nificantly smaller amounts of solvent
than usually required.
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