Use of electrochemistry to provide efficient SmI2 catalytic system for coupling reactions.

Article abstract of DOI:10.1021/cs400587s

Samarium metal has been used for the first time as electrode material to perform as an efficient and versatile SmI₂ catalytic system assisted by electrochemistry. The established electrocatalytic procedure that excludes any metal additives was successfully applied in various transformations mediated by this useful reagent.

Full text of DOI:10.1021/cs400587s

Letter
pubs.acs.org/acscatalysis
Use of Electrochemistry to Provide Efficient SmI2 Catalytic System for
Coupling Reactions.
Linhao Sun, Kamar Sahloul, and Mohamed Mellah*
́ ́
Laboratoire de Catalyse Moleculaire, UMR 8182− ICMMO, Universite Paris Sud, Orsay F-91405, France
S
* Supporting Information
ABSTRACT: Samarium metal has been used for the first time
as electrode material to perform as an efficient and versatile
SmI2 catalytic system assisted by electrochemistry. The
established electrocatalytic procedure that excludes any metal
additives was successfully applied in various transformations
mediated by this useful reagent.
KEYWORDS: samarium diiodide, catalysis, electrochemistry, samarium cathode, coupling reactions
amarium diiodide (SmI2), first described by Kagan and
favor ligand redistribution, and the Sm(II) species regenerated
S
Namy in 1977 is recognized to be one of the most useful
during the catalytic process could be different from SmI2. This
is supported by the fact that lower chemoselectivities and
stereoselectivities were generally observed in reactions
involving catalytic SmI2 by comparison with the stoichiometric
system.1d We recently reported a new method to generate SmI2
by direct electrochemical dissolution of a samarium metal
anode.10 Its efficient application in organic electrosynthetic
procedures encouraged us to pursue our investigations toward
the development of catalytic reactions with the aid of
electrochemistry. The objective herein is to exclude the use
of the metal additives required for the regeneration of the
samarium divalent active reagent by replacing them with an
inert electrode to achieve a more efficient and versatile SmI2
catalytic system (Scheme 1).
and versatile reducing agent in organic synthesis.1a Its
importance and application has been highlighted by several
important reviews, which demonstrate the prominent role of
SmI2 in synthesis.1 However, the majority of organic reactions
mediated by samarium diiodide (SmI2) as reductive reagent
require a stoichiometric amount or even a large excess of SmI2.
This drawback tends to decrease the synthetic potential of the
reactions promoted by SmI2 in large scale. Several research
groups have investigated potential solutions that would allow
the use of substoichiometric amounts of SmI2 by employing
magnesium metal2−4 or zinc amalgam (Zn(Hg))5 as
coreductants associated with oxophilic additives TMSCl or
TMSOTf, respectively.6,7 Namy and co-workers developed an
alternative catalytic system using mischmetal as an original
coreductant without the need for oxophilic additives because
the lanthanides included in mischmetal (Ce, La, Nd, Pr, ...) are
themselves oxophiles, and the resulting catalytic system was
successfully applied in various reactions mediated by SmI2.8
Alternatively, an electrochemical procedure based on the use of
SmCl3 as precatalyst associated with a Mg or Al sacrificial anode
Scheme 1. The Use of Coreductant To Achieve the Catalytic
Cycle of SmI2
was proposed by Dunach and co-workers.9 Although this
̃
approach has been applied in several coupling reactions, it
appears that the formation of samarium divalent species during
the electrolysis is difficult to highlight.2 In addition to the fact
that the proposed metals coreductant for catalytic applications
represent significant advances, they have not been extensively
used, probably because of the incompatibility between the
coreductants used in large excess and the substrates commonly
encountered in SmI2-induced reactions. In addition, the
presence of additional metals in the reaction medium may
Received: July 22, 2013
Revised: September 12, 2013
Published: October 4, 2013
© 2013 American Chemical Society
2568
dx.doi.org/10.1021/cs400587s | ACS Catal. 2013, 3, 2568−2573

ACS Catalysis
Letter
For this purpose, we have investigated the properties of
several electrodes to reduce the trivalent into divalent samarium
species.11 All attempts to reduce a suspension of SmI3 using
cathodes based on platinum, carbon, nickel, lead, or stainless
steel were not conclusive. Indeed, the initial SmI3 suspension
remained unchanged after the electrolysis (Table 1, entries 1−
the electrode, which is made from the same metal, that ensures
a perfect electron transfer to reduce the trivalent complexes.
Moreover, it was found that with an amount of current over
that required to reduce totally the Sm(III) suspension into
Sm(II), the solution color begins to fade. This indicates, on one
hand, that the SmI2 formed could be totally reduced to
samarium metal (Sm (0)) onto the cathode, on the other hand,
prove that the electrode does not produce SmI2 under these
electrochemical conditions.13 We have also verified that no
reaction occurred for 48 h in the absence of imposed current
intensity.
Table 1. Screening of Cathode Materials for Reduction of
a
SmI3 Salts in THF/nBu4NPF6
M cathode
SmI3 ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→⎯ SmI2 + I−
THF,nBu4NPF
6
After demonstrating that Sm(II) can be efficiently generated
by electrochemical reduction of Sm(III) at a samarium cathode,
we turned our attention to checking the validity of our method
in a benchwork reaction, such as the pinacol coupling of
carbonyl derivatives, which has been widely studied with this
reagent.15 For the implementation, benzaldehyde and cyclo-
hexanone were chosen as substrates, and electrolysis experi-
ments were successively performed with 10 mol % of SmI2 in
the presence of TMSCl (1.5 equiv). To our delight, both
carbonyl substrates led to the expected pinacol products that
were obtained in 83% and 59% yields, respectively (Table 2,
entries 3 and 7).
entry
cathode
current density (mA/cm2)
UV−vis (λmax, nm)
1
2
3
4
5
6
Pt
C
1.52
2.10
2.87
1.80
8.06
1.25
nd
nd
Pb
Ni
nd
nd
b
SS
nd
Sm
612−557
a
Conditions: electrolyte 0.04 M NBu4PF6 in THF, 0.1 mmol of SmI3, i
b
= 5·10−2 A, standard vitreous carbon anode. SS: stainless steel
contains 72% Fe, 18% Cr, and 10% Ni.
5). Interestingly, we discovered that the use of a samarium
metal as cathode effectively reduces SmI3 to provide divalent
samarium species (Table 1, entry 6). We found that when
applying a current density of 1.25 mA/cm2, the yellow
suspension of SmI3 in THF changes immediately from yellow
to homogeneous dark blue, the characteristic color of low-
valent samarium species.1a Moreover the UV−vis analysis of the
resulting solution showed absorptions bands at 612 and 557 nm
(Figure 1) attributed to the formation of the SmI2 complex in
THF (Table 1, entry 6).12
Table 2. Additional Experiments To Identify the Mechanism
of Pinacolization
a
b
b
conv
pinacol
(%)
alcohol
(%)
entry substrate
additives
(%)
1
2
3
4
1
1
1
1
63
73
93
5
36
22
83
5
27
51
10
0
c
c
SmI2
SmI2 , TMSCl
c
SmI2 , TMSCl,
HMPA
5
6
7
8
2
2
2
2
34
33
99
23
4
22
59
5
30
11
40
18
c
SmI2
c
SmI2 , TMSCl
c
SmI2 , TMSCl,
HMPA
a
Conversion determined by gas chromatography using undecane as
b
c
internal standard relative to the substrate engaged. Isolated yield. 10
mol % of SmI2.
Figure 1. UV−vis analysis of samarium divalent species solutions.
Spectra of the SmI2 solution prepared (a) by electroreduction of SmI3,
(b) by electroreduction of SmCI2, and (c) chemical SmI2 solution.
SmI2 concentration ≈ 10 mM.
Additional experiments were carried out to identify the
mechanism of the pinacolization reaction (see Table 2 and
Table S1 in the Supporting Information). Thus, it was found
that the pinacolization takes place without SmI2 initially
introduced in the case of benzaldehyde. Indeed, the pinacol
adduct was isolated in only 36% yield and probably results from
the direct reduction of benzaldehyde at the samarium cathode
surface (Table 2, entry 1). This effect was amplified by the
presence of TMSCl, since the pinacol was recovered in 83%
yield. These results indicate that the samarium electrode surface
is probably activated by the presence of TMSCl, as described by
Honda and Katoh.16 In contrast, when the electrolysis is
performed in the presence of cyclohexanone, an aliphatic
ketone more difficult to reduce, pinacolization does not occur
in the absence of SmI2. Cyclohexanone was converted to
The complex formed was also well characterized by
electrochemical analysis. The cyclic voltammogram of the
resulting solution is in total accordance with those previously
described10 (see Figure S4 in the Supporting Information).
Samarium metal turned out to be the best cathode material
with respect to the desired reduction. The screening of
electrode material shows clearly that the nature of the cathode
is crucial and suggests that there is likely some affinity between
the Sm(III) complex free in solution and the samarium cathode
itself. This efficiency is probably due to a better interaction
between the complexes based on samarium and the surface of
2569
dx.doi.org/10.1021/cs400587s | ACS Catal. 2013, 3, 2568−2573

ACS Catalysis
Letter
pinacol with 59% yield only in the presence of 10 mol % of
SmI2 and TMSCl in the electrochemical cell (entry 7, Table 2).
Moreover, as observed in a previously reported procedure that
uses magnesium as a coreductant,2−4 addition of TMSCl is
required to cleave the SmIII−O bonds. These results are in
good agreement with the proposed catalytic cycle. Moreover,
addition of 4 equiv of hexamethylphosphoramide (HMPA)
with respect to the catalytic amount of SmI2 to reduce the
formation of the alcohol was not effective. In the presence of
HMPA, the alcohol resulting from the direct reduction of
cyclohexanone is always observed as a byproduct, and the
electrolysis efficiency is greatly diminished (only 23% of
conversion instead of 99% (entry 7 and 8, Table 2). This effect
was also observed when michmetal was used as coreductant,8a
and as suggested by Namy and co-workers, the addition of
HMPA probably impedes the reduction of Sm(III) species into
Sm(II). This result is also in agreement with the work of Hoz
and Farran, who have also observed that HMPA dramatically
reduces the reactivity of ketyl radical intermediates in the
reduction of carbonyl compounds.17
The procedure was then completed by in situ electro-
generating the catalyst directly from the samarium electrode
used as anode in a first step (pre-electrolysis).10 This
modification avoids any manipulation of the sensitive catalyst
out of the electrochemical cell. Reactions were carried out in an
undivided cell simply flushed by argon, fitted with a Sm anode,
carbon cathode, and charged with degassed anhydrous THF
containing nBu4NPF6 and nBu4NI as the iodide source.
The pre-electrolysis was achieved at a constant current
intensity of 5·0 × 10−2 A during the designated time to prepare
10 mol % SmI2 with respect to the substrate, which is
introduced in a second step. The characteristic deep blue
solution of SmI2 in THF is obtained after the first step. To
perform the second electrolysis step, the carbonyl substrate and
1.5 equiv of TMSCl are added, and the polarity of the
samarium electrode is inverted to act as the cathode during the
whole course of the electrolysis. The catalysis was achieved at a
constant current intensity of −5·10−2 A and followed by GC
analysis. The results of the reductive coupling of various
ketones and aldehydes are listed in Table 3.
For other aromatic aldehydes, the reaction is effective only if
the catalyst is generated. Without SmI2, their electroreduction
led to the corresponding pinacols in low isolated yields (see
Table S1 in the Supporting Information). In the presence of
catalytic amounts of SmI2, all pinacol derivatives were obtained
in moderate to good isolated yields (Table 3); however,
compared with the stoichiometric version,10 yields for reactions
of aldehydes are generally affected by the formation of the
alcohols resulting from direct reduction of the aldehydes at the
samarium cathode surface. Diastereoselectivities are generally
the same as those measured for reactions performed under
stoichiometric conditions (meso/dl ratio close to 1:1); these
results are also in good agreement with a pinacolization
mediated by SmI2.17
Compared with the cyclic voltammograms obtained for the
SmI2 solution, those of Sm(II) species prepared by electro-
chemical reduction of SmI3 and SmI2Cl are different (see the
Supporting Information). This reflects the possibility that the
electroregenerated Sm(II) reductant could be a mixture of
species with probably an equilibrium between these various
samarium divalent species during the electrocatalytic process.
This hypothesis is highly probable because it is well-established
and supported by the UV−vis spectra and electrochemical
analysis (Figure 1 and Figure 2)18 that chloride has a higher
affinity for Sm than iodide.
Table 3. Catalytic Homocoupling of Carbonyl Compounds
a
Using SmI2/Sm Cathode Catalytic System
Figure 2. Cyclic voltammetry performed at a vitreous carbon disk
electrode (d = 2 mm) at 25 °C in THF containing nBu4NPF6 (0.04
M) as the supporting electrolyte. Reduction of SmI2 (0.1 mM, dashed
line) at a scan rate of 50 mVs−1. Reduction in the presence of
increasing amounts of 1 (1, 2, 2.5, 3, and 3.5 equiv).
b
c
c
entry
substrate
benzaldehyde
conv (%)
yield (%)
meso/dl
We have investigated this catalytic process by electrochemical
analyses. The cyclic voltammogramm of SmI2 in THF (0.1
mM) at a scan rate of 0.05 V s−1 exhibited a quasireversible
redox system (Figure 2, dashed line). Benzaldehyde and
TMSCl analyzed as a solution in THF were not reduced until
−2.0 V versus the standard calomel electrode (SCE). The SmI2
redox signal disappeared upon addition of 1 equiv of
benzaldehyde as carbonyl substrate in the presence of TMSCl
(benzaldehyde/TMSCl: 1/1.5) (blue line). This is consistent
with the complete reaction of SmI2 with the carbonyl substrate.
Two new reduction peaks were observed at Ep = −0.95 V
and Ep = −1.35, and their current increased with increasing
1
2
3
4
5
6
93
90
63
83
95
99
83
51
46
42
62
59
45:55
55:45
30:70
50:50
30:70
50:50
4-methylbenzaldehyde
4-methoxybenzaldehyde
3-methoxybenzaldehyde
acetophenone
cyclohexanone
a
Electrolytic conditions: THF, nBu4NPF6 (0.04 M), 4 mmol of
b
carbonyl substrate, charge of 2F/mol. Conversion determined by gas
chromatography using dodecane as internal standard relative to the
substrate engaged. Isolated yields, meso/dl ratio determined by H
c
1
NMR.
2570
dx.doi.org/10.1021/cs400587s | ACS Catal. 2013, 3, 2568−2573

ACS Catalysis
Letter
a
amounts of the benzaldehyde/TMSCl mixture without
reoxidation (from 1 to 4 equiv; Figure 2). From these
experiments, one deduces that the one-electron reduction of
SmX3 species, generated during the chemical transformation, in
the diffusion layer at the electrode, thus gives rise to a catalytic-
reduction current, which is observed (Figure 2). In addition,
one deduced therefrom by the observation of two reduction
peaks that various Sm(III) species are generated during the
catalytic process. These electrochemical experiments provide
direct evidence for support of the proposed mechanism.
Moreover, under these electrolytic conditions, we assume that
the most likely half reaction that can occur at the anode is the
oxidation of halide released during the catalytic cycle to
produce Cl2 or I2.13
Table 4. Catalytic Allylation of Carbonyl Compounds
The possible reaction pathway of the SmI2-catalytic
pinacolization involves the reduction of carbonyl compounds
by electrogenerated SmI2 to give Sm(III) pinacolate, followed
by metal exchange with TMSCl to provide the corresponding
silyl ether and SmI2Cl. Subsequent electron transfer from the
Sm cathode regenerates Sm(II)X2 from Sm(III)ClI2 (Scheme
2). This last step was evidence by the direct electroreduction of
Scheme 2. Proposed Catalytic Cycle for Coupling Reactions
Mediated by Electrogenerated SmI2 and Using
Electrochemistry As Co-reductant
a
Electrolytic conditions: THF, nBu4NPF6 (0.1 M), 2 mmol of
carbonyl substrate, and 4 mmol of allyl iodide; charge of 2F/mol and
b
TMSCl introduced by Syringe pump. Conversion determined by gas
chromatography using dodecane as internal standard relative to the
c
d
substrate engaged. Isolated yields. Alcohol detected as major
e
byproduct around 20%. Corresponding pinacol detected as major
byproduct (40% yield).
Carbonyl substrates and allyl halide are introduced after the
pre-electrolysis step, which delivers the catalytic samarium
divalent species. A short survey of allyl halides highlights allyl
iodide as the most effective, so it was subsequently used for all
reactions. Under optimized conditions, the trimethylsilyl
chloride and carbonyl compound diluted in dry THF were
introduced dropwise by a syringe pump during the electrolysis.
The corresponding homoallylic alcohols were obtained in good
yields similar to those obtained under stoichiometric conditions
(Table 4).10 It is to be noted that the dienes arising from a
Wurtz coupling were generally observed as byproducts. We also
verified that no reaction was observed in the absence of
samarium diiodide, which strongly suggests the coupling
reaction proceeds through the formation of an organometallic
species in a catalytic manner. A possible reaction pathway
involves a coupling reaction mediated by electrogenerated SmI2
through the formation of an allylsamarium derivative that reacts
with the carbonyl compound to afford Sm(III) alcoholate,
followed by metal exchange with TMSCl to provide the
corresponding silyl ether and SmI2Cl. Samarium diiodide could
be regenerated by direct electron transfer from the samarium
cathode, as described earlier for the pinacolization reaction
(Scheme 2).
a previously generated SmClI2. Indeed, when a THF
suspension of SmClI2 is reduced onto a samarium cathode,
the color changes from orange-yellow to dark blue. UV−vis
analysis of the resulting solution shows the characteristic
absorptions bands at 603 and 562 nm attributed to the
formation of the SmI2 complex in THF (Figure 1, curve b).1a,12
A similar experiment could also be conducted from SmI2Br to
generate a THF solution of divalent samarium halides. These
experiments indicate that SmI2 could be regenerated from the
trivalent species SmI2X (X = Cl or Br) intermediate under the
electrochemical conditions described in Table 1, and the
resulting colored solution may suggest that SmI2 is the
prevailing reducing species during the catalytic cycle (Scheme
2).
This catalytic approach was also applied in the allylation of
carbonyl compounds. Reactions were performed according to a
Barbier-type procedure in which the formation of an organo-
samarium compound is generally admitted and confirmed by
several mechanistic studies (Table 4).19 Pre-electrolysis and
electrolysis were performed under the conditions established
for the pinacolization reaction.
To extend the scope of application of this new SmI2/Sm
cathode catalytic system, other reactions mediated by samarium
diiodide were performed (Scheme 3). This electrocatalytic
procedure could also be applied to the reductive coupling of
2571
dx.doi.org/10.1021/cs400587s | ACS Catal. 2013, 3, 2568−2573

ACS Catalysis
Letter
Scheme 3. Other Reactions Mediated by Catalytic SmI2 in Electrochemistry-Assisted Procedure
Notes
imines, as exemplified by the coupling of N-benzylideneaniline,
which leads to the isolations of the corresponding diamine
(meso/dl = 52/48) in 48% yield (Scheme 3). The
diastereoselectivity was similar to the one observed under
stoichiometric conditions.10,20 Reformatsky type reactions
between tert-butylbromoacetate and 4-phenyl-2-butanone gave
the desired cross-coupling product in 60% yield (Scheme 3).
Finally, as a last illustration of the synthetic potential of this
electrochemically assisted catalytic procedure, the cross-
coupling between acetophenone and methyl acrylate was
tried. Under the reaction conditions described in Scheme 3,
the reaction between acetophenone and methyl acrylate leads
to the corresponding lactone in 30% isolated yield. It should be
noted that the efficiency in this case is strongly reduced because
the pinacol product derived from acetophenone is also formed
(up to 48% of isolated product). In all cases, we also verified
that yields are close to those obtained under stoichiometric
conditions, and no reaction occurred in the absence of
samarium diiodide.
In summary, we have discovered a novel and reliable SmI2
catalytic system based on the use of a samarium electrode as
coreductant. SmI2 can act as a catalyst through the use of a
samarium cathode, which effectively reduces the trivalent
samarium, formed during the chemical reaction, to regenerate
the divalent samarium catalytic species. Various transformations
mediated by this useful reagent were conducted under the
established electrocatalytic conditions, and this method
potentially provides an approach to catalytic Sm(II)-based
reductants and reductive coupling reactions. We are currently
exploring the potential of its application with a special focus on
enantioselective transformations.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work is dedicated to the memory of Prof. Jean-Louis
Namy. We thank the University of Paris-Sud, the CNRS and
Labex CHARM3AT for their financial support.
REFERENCES
■
(1) (a) Namy, J. L.; Girard, P.; Kagan, H. B. New J. Chem. 1977, 1,
5−7. (b) Nicolaou, K. C.; Ellery, S. P.; Chen, J. S. Angew. Chem., Int.
Ed. 2009, 48, 7140−7165. (c) Molander, G. A. Organic Reactions;
Paquette, L. A., Ed.; John Wiley & Sons, Inc.: New York, 1994; Vol.
46, pp 211−367. (d) Procter, D. J.; Flowers, R. A., II; Skrydstrup, T.
Organic Synthesis using Samarium Diiodide; Royal Society of Chemistry
Publishing: Cambridge, UK, 2010; ISBN: 978-1-84755-110-8.
(e) Kagan, H. B. Tetrahedron 2003, 59, 10351−10372. (f) Procter,
D. J.; Szostak, M. Angew. Chem., Int. Ed. 2012, 51, 9238−9256.
(g) Molander, G. A. Chem. Rev. 1992, 92, 29−68. (h) Molander, G. A.
Chem. Rev. 1996, 96, 307−338. (i) Edmonds, D. J.; Johnston, D.;
Procter, D. J. Chem. Rev. 2004, 104, 3371−3403. (j) Molander, G. A.;
Harris, C. R. Tetrahedron 1998, 54, 3321−3354.
(2) Nomura, R.; Matsuno, T.; Endo, T. J. Am. Chem. Soc. 1996, 118,
11666−11667.
(3) Aspinall, H. C.; Greeves, N.; Vallas, C. Org. Lett. 2005, 7, 1919−
1922.
(4) Orsini, F.; Lucci, E. M. Tetrahedron. Lett. 2005, 46, 1909−1911.
(5) Corey, E. J.; Zang, G. Z. Tetrahedron. Lett. 1997, 38, 2045−2048.
(6) Oxophilic additives are needed to transform the Sm(III)
pinacolate, resulting from the first reduction step, into its silyl ether,
in addition to the formation of SmI2X, which could be reduced during
the catalytic transformation.
(7) (a) Furstner, A.; Hupperts, A. J. Am. Chem. Soc. 1995, 117,
̈
4468−4475. (b) Furstner, A.; Shi, N. J. Am. Chem. Soc. 1996, 118,
2533−2534. (c) Hirao, T.; Hasegawa, T.; Muguruma, Y.; Ikeda, T. J.
Org. Chem. 1996, 61, 366−367.
(8) (a) Hel
(b) Lannou, M.-I.; Hel
10551−10565.
(9) (a) Hebri, H.; Dunach, E.; Per
Commun. 1993, 499−500. (b) Espanet, B.; Dunach, E.; Per
̈
ASSOCIATED CONTENT
■
S
* Supporting Information
́
ion, F.; Namy, J. L. J. Org. Chem. 1999, 64, 2944−2946.
Detailed experimental procedures, UV−vis analysis, electro-
chemical analysis, and additional experiment data. This material
is available free of charge via the Internet at http://pubs.acs.org.
́
ion, F.; Namy, J. L. Tetrahedron 2003, 59,
́
́
ichon, J. J. Chem. Soc., Chem.
ichon, J.
ri, H.; Dunach, E.;
̃
́
̃
Tetrahedron Lett. 1992, 33, 2485−2488. (c) Heb
Perichon, J. Synlett 1992, 293−294. (d) Hebri, H.; Dunach, E.;
Perichon, J. Syn. Commun. 1991, 21, 2377−2382. (e) Heb
Dunach, E.; Heintz, M.; Troupel, M.; Perichon, J. Synlett 1991, 901−
́
̃
AUTHOR INFORMATION
■
́
́
̃
Corresponding Author
*E-mail: mohamed.mellah@u-psud.fr.
́
́
ri, H.;
́
̃
2572
dx.doi.org/10.1021/cs400587s | ACS Catal. 2013, 3, 2568−2573

ACS Catalysis
Letter
902. (f) Leo
Commun. 1989, 276−277.
́ ́
nard, E.; Dunach, E.; Perichon, J. J. Chem. Soc., Chem.
̃
(10) Sahloul, K.; Sun, L.; Requet, A.; Chahine, Y.; Mellah, M.
Chem.Eur. J. 2012, 18, 11205−11209.
(11) A typical procedure was as follows: In an undivided three
electrode cell equipped with vitreous carbon anode (20 cm2 area), a
saturated calomel electrode (SCE) as reference and containing a
magnetic stirring bar, was introduced 0.04 M of nBu4NPF6 in
anhydrous THF and the trivalent samarium salts as a suspension in 4
mL of electrolyte solution. The solution was flushed with argon before
and during the electrolysis performed at 25 mA. After the electrolysis,
the resulting solution was directly analyzed by UV−vis spectropho-
tometry.
(12) (a) Girard, P.; Namy, J.-L.; Kagan, H. B. J. Am. Chem. Soc. 1980,
102, 2693−2698. (b) Miller, R. S.; Sealy, J. M.; Shabangi, M.;
Kuhlman, M. L.; Fuchs, J. R.; Flowers, R. A., II. J. Am. Chem. Soc. 2000,
122, 7718−7722.
(13) An additional control experiment was performed by the
reduction of 0.5 mmol SmI3 using the samarium cathode. After
complete color disappearance, if the electrolysis is prolonged, the
solution becomes brown, indicating the probable formation of I2.
Upon quenching with a solution of thiosulfate, the resulting brown
solution experiences an instantaneous bleaching, confirming the
presence of I2 at the end of this electrolysis experiment. Moreover,
the weight of the electrode is increased to about 75 mg, which suggests
that samarium metal was deposited onto the electrode.
(14) Electrolyses were carried out in an undivided electrochemical
cell equipped with a vitreous carbon anode and samarium cathode and
charged with 10 mol % of SmI2 chemically prepared in presence of
chlorotrimethylsilane (TMSCl). Electrolyses were performed at
constant current intensities of 5·10−2 A.
(15) (a) Lebrun, A.; Namy, J. L.; Kagan, H. B. Tetrahedron Lett. 1993,
34, 2311−2314. (b) Gartner, P.; Knollmuller, M.; Broker, J. Monatsh.
̈
̈
̈
Chem. 2003, 134, 1607−1615.
(16) Honda, T.; Katoh, M. Chem. Commun. 1997, 369−370.
(17) Farran, H.; Hoz, S. Org. Lett. 2008, 10, 865−867.
(18) Shabangi, M.; Flowers, R. A., II. Tetrahedron Lett. 1997, 38,
1137−1140.
(19) (a) Curran, D. P.; Fevig, T. L.; Jasperse, C. P.; Totleben, M. J.
Synlett 1992, 943−961. (b) Curran, D. P.; Totleben, M. J. J. Am. Chem.
Soc. 1992, 114, 6050−6058. (c) Wipf, P.; Venkatraman, S. J. Org.
Chem. 1993, 58, 3455−3459. Flowers, R. A., II. Synlett 2008, 10,
1427−1439. Prasad, E.; Flowers, R. A., II. J. Am. Chem. Soc. 2002, 124,
6895−6899. Prasad, E.; Flowers, R. A., II. J. Am. Chem. Soc. 2002, 124,
6357−6361. Choquette, K. A.; Sadasivam, D. V.; Flowers, R. A., II. J.
Am. Chem. Soc. 2010, 132, 17396−17398.
(20) (a) Lebrun, A.; Namy, J. L.; Kagan, H. B. Tetrahedron Lett. 1993,
́
34, 2311−2314. (b) Kim, M.; Knettle, B. W.; Dahlen, A.; Hilmerssonb,
G.; Flowers, R. A., II. Tetrahedron 2003, 59, 10397−10402.
(c) Machrouhi, F.; Namy, J. L. Tetrahedron Lett. 1999, 40, 1315−1318.
2573
dx.doi.org/10.1021/cs400587s | ACS Catal. 2013, 3, 2568−2573