Electrochemical generation of low-valent lanthanides

Article abstract of DOI:10.1016/S0040-4039(01)01692-6

A new method is presented to produce divalent lanthanides (including the popular reductant samarium(II) iodide) from the corresponding trivalent triflate salts. This novel route to low-valent lanthanides suggests the development of a catalytic system empl

Full text of DOI:10.1016/S0040-4039(01)01692-6

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 7767–7770
Electrochemical generation of low-valent lanthanides†
J. D. Parrish and R. Daniel Little*
Department of Chemistry and Biochemistry, University of California, Santa Barbara, CA 93106, USA
Received 20 August 2001; accepted 10 September 2001
Abstract—A new method is presented to produce divalent lanthanides (including the popular reductant samarium(II) iodide) from
the corresponding trivalent triflate salts. This novel route to low-valent lanthanides suggests the development of a catalytic system
employing the cathode as a co-reductant. © 2001 Published by Elsevier Science Ltd.
Samarium(II) iodide has become one of the most popu-
lar single-electron reductants in organic chemistry.1
Recently, much work has been published concerning its
electrochemical properties.2 We reasoned that prepara-
tive-scale reductive electrolysis of samarium(III) salts
would afford access to active samarium(II) species.3
This approach contrasts with the popular method of
oxidizing samarium metal to the divalent oxidation
state. We decided to initiate our investigations employ-
ing the inexpensive and readily available Sm(OTf)3
salt.4 We report (a) a new means of electrochemically
generating SmI2 and YbBr2 in situ, (b) a dramatic effect
of the triflate counter-ion upon the redox potential of
the metal, and (c) that using in situ electrochemically
generated SmI2 or YbBr2 as a mediator, reductive
coupling reactions can be conducted at significantly
lower (less negative) potentials than is possible when
the same substrates are reduced directly.
The effects of Lewis basic co-solvents and ligand varia-
tion on the reduction potential of samarium have been
studied,5 but, to our knowledge, the redox properties of
the readily available triflate salts of samarium and
ytterbium are unknown. Therefore, we performed cyclic
voltammetry studies on Sm(OTf)3 in acetonitrile. The
salt showed a quasi-reversible reduction for the 3+/2+
couple in the presence of a triflate-containing elec-
trolyte. Under these conditions, the reduction potential
of the samarium was −2.1 V.6 When utilizing an iodide-
containing electrolyte, the reduction potential shifted to
−1.7 V (Fig. 1). The voltammetric data are similar to
those already reported for SmI2 in acetonitrile.2c
Attempts to generate data with bromide and chloride
electrolytes were unsuccessful.7
This change in reduction potential suggests that the
triflate salt undergoes a ligand-exchange reaction with
the supporting electrolyte to produce the corresponding
samarium(III) halide (Scheme 1).
The chemistry described provides a general method for
the development of electron-transfer reagents. Our
approach uses the acquisition of electroanalytical data
in the form of a voltammogram to provide a simple and
systematic means of screening the properties of redox-
active species. The data provide mechanistic insight and
assist in determining which reagents are best suited for
further investigation. Observations described in this
paper suggest that one may be able to tune the proper-
ties of the metal to fit the redox needs of the problem at
hand.2a
* Corresponding author. Tel.: +1-805-893-3693; fax: 1-805-893-4120;
e-mail: little@chem.ucsb.edu
A preliminary account of this work was presented at the 220th
†
Figure 1. Cyclic voltammetry of samarium(III) triflate with
varying electrolytes.
National Meeting of the American Chemical Society, Washington,
DC, August, 2000.
0040-4039/01/$ - see front matter © 2001 Published by Elsevier Science Ltd.
PII: S0040-4039(01)01692-6

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J. D. Parrish, R. D. Little / Tetrahedron Letters 42 (2001) 7767–7770
controlled potential of −2.7 V.6,10 Using samarium trifl-
ate as a mediator in an iodide-containing electrolyte, we
were able to perform the same reduction at −1.8 V,
nearly one volt less negative than the conditions
required for the direct electrolysis. The controlled-
potential conditions ensure that the low-valent lan-
thanide, not the cathode, is reducing the substrate.
These milder reaction conditions could potentially
allow for the appendage of electrochemically active
groups (such as benzyl ethers) to similar substrates.
n-Bu4NX
SmX3
+ n-Bu4NOTf
Sm(OTf)3
CH3CN
Scheme 1. Ligand metathesis to produce samarium halides.
The voltammetric data support the observation that
SmI3 can be reduced with rather mild reductants, such
as zinc, aluminum, and magnesium,8 whereas Sm(OTf)3
requires more powerful reductants, such as Grignard
reagents or organolithium species, to generate
Sm(OTf)2.9
OH
O
CO2CH3
CO2CH3
In accord with our voltammetry data, we were unable
to produce samarium(II) on a preparative scale in the
presence of bromide- or chloride-containing elec-
trolytes. However, reduction in the presence of iodide-
containing electrolytes allowed reduction of the
samarium salt to produce a deep-blue colored solution
characteristic of SmI2 (Scheme 2).
-1.8 V
2
1
E
pc - 2.7 V6,11
(54 %, 1.4:1 trans/cis)
Scheme 3. Reductive cyclization mediated by samarium(II)
iodide. Conditions: Sm(OTf)3 (2.2 equiv.), t-BuOH (1.2
equiv.) in 0.05 M n-Bu4NI/CH3CN, 2 e−. The cis-hydroxy
ester was isolated as the corresponding lactone.
e-, -1.8 V
SmI2
n-Bu4NX
(X = I)
Sm(OTf)3
SmX3
CH3CN
In addition, we investigated the use of low-valent ytter-
bium-based reagents. While Yb(OTf)3 has received
much attention for its use as a Lewis acid, there has
been surprisingly little investigation into the use of
ytterbium(II) reagents as mild reductants.12 We found
that, in analogy to our results with samarium, one can
generate solutions of ytterbium(II) electrochemically,
and that the reducing ability of the metal is strongly
dependent on the counterion (Fig. 3).
e-
SmX2
(X = Cl, Br)
Scheme 2. Electrochemical generation of samarium(II)
halides.
The identity of the samarium(II) species was confirmed
by UV–vis spectroscopy. An aliquot of the electro-
chemically produced samarium(II) species had a spec-
trum almost identical to that of SmI2 and markedly
different than Sm(OTf)2 (Fig. 2), both formed by
known methods.1,9
Figure 3. Cyclic voltammetry of ytterbium(III) triflate with
varying electrolytes.
Figure 2. UV–vis spectrum of electrochemically produced
SmI2 in comparison to SmI2 and Sm(OTf)2 both prepared by
known methods. The spectra are stacked for clarity.
For ytterbium, the reduction potential of the triflate
and iodide salts are similar (ca. −1.2 V). However, in
contrast to samarium, it is possible to successfully
reduce ytterbium using bromide-containing electrolytes.
This occurs at a more negative potential of −1.7 V
(Scheme 4).
To further verify the identity of the electrogenerated
species, we explored the use of this species in a prepar-
ative transformation known to occur with SmI2. Our
model reaction was the intramolecular cyclization of
aldehyde/unsaturated ester 1 (Scheme 3). This com-
pound can be directly reduced to afford alcohol 2 at a
As a test reaction designed to verify the reducing
properties of the electrogenerated ytterbium(II) reagent,

J. D. Parrish, R. D. Little / Tetrahedron Letters 42 (2001) 7767–7770
7769
e-, -1.2 V
organics were washed with brine (1×100 mL), dried
(MgSO4), filtered and concentrated. Products were iso-
lated using column chromatography and analyzed by
1H and 13C NMR to determine product ratios. All
products gave satisfactory spectral data.
YbI2
(X = I)
n-Bu4NX
Yb(OTf)3
YbX3
e-, -1.7 V
CH3CN
YbX2
(X = Br)
Scheme 4. Electrochemical generation of ytterbium(II)
halides.
Acknowledgements
we investigated the pinacol cyclization of dione 3
(Table 1). This method cleanly affords the cyclic diol
(4) with complete stereoselectivity. The stereoselectivity
can be rationalized by coordination of the metal
between the two carbonyl units of the dione starting
material.13 It should be noted that direct electrochemi-
cal reduction of dione 3 in acetonitrile affords a
diastereomeric mixture of diols.14
J.D.P. is grateful to the John H. Tokuyama fellowship
for support during the completion of this work.
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O
O
see Table
HO
Ph Ph
OH
Ph
Ph
3
4
Conditions
Potential (V)6
Yield (%)
de (%)
Direct electrolysis14
SmI2 reduction8b
Yb(OTf)3-mediated
electrolysis
−2.1
–
−1.8
67
82
83
68
\95
\95
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can effectively induce preparative-scale cyclizations
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of a catalytic system using the cathode as a co-reduc-
tant.15 Work on developing such a system is underway.
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mmol) were added to the catholyte. The cell was
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ytterbium), was applied until 1.1 F of charge was
consumed for the one-electron reduction of the lan-
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ytterbium, respectively. The catholyte was concentrated
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