Generation of SmII reductants using high intensity ultrasound

Article abstract of DOI:10.1002/ejic.200800876

Ultrasound (20 kHz) was used to prepare samarium(II) diiodide (SmI ₂), samarium(II) dibromide (SmBr₂), and samarium(II) triflate (SmOTf)₂. The method described herein provides a straightforward and rapid approach to the synthesis of SmII compounds not accessible by other means. Of particular importance is the fact that this approach can be used to produce Sm^II-based reductants in a wide range of solvents including alcohols. Wiley-VCH Verlag GmbH & Co. KGaA, 2008.

Full text of DOI:10.1002/ejic.200800876

SHORT COMMUNICATION
DOI: 10.1002/ejic.200800876
II
Generation of Sm Reductants Using High Intensity Ultrasound
[
a]
[b]
[b]
Joseph A. Teprovich Jr., P. K. Sudhadevi Antharjanam, Edamana Prasad,
[a]
[a]
Esther N. Pesciotta, and Robert A. Flowers II*
Keywords: Samarium / Ultrasound / Synthesis
Ultrasound (20 kHz) was used to prepare samarium(II) di-
lar importance is the fact that this approach can be used to
produce Sm -based reductants in a wide range of solvents
II
iodide (SmI
2
), samarium(II) dibromide (SmBr
2
), and samari-
um(II) triflate (SmOTf)
2
. The method described herein pro-
including alcohols.
vides a straightforward and rapid approach to the synthesis
of Sm compounds not accessible by other means. Of particu-
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2008)
II
of a range of SmII reductants. As a part of our research
program directed towards mechanistic studies of Sm-based
reductants, we have been examining alternative approaches
Introduction
Although initially considered an esoteric reagent, sama-
rium diiodide (SmI ) is now a standard reductant in most
organic laboratories and its use in synthesis is featured in
nearly 100 publications a year. The unique place held by
2
II
to the synthesis of Sm compounds that are efficient and
can be carried out with routine methods. Herein we report
the synthesis of SmI , SmBr and Sm(OTf) in a range of
2
2
2
SmI in the arsenal of synthetic chemists is a result of its
2
solvents using high intensity ultrasound as shown in
Scheme 1.
versatility in promoting numerous fundamentally important
reactions in organic synthesis including reductions, re-
ductive couplings, and tandem reactions.[1] One of the fea-
tures that makes SmI useful is its ease of synthesis and
2
availability from commercial sources. The preparation of
SmI typically involves treatment of excess Sm metal with
2
iodine, 1,2-diiodoethane, or diiodomethane and reactions
[2]
take up to two hours for completion. While there are nu-
II
merous other Sm -based reductants including samari-
um(II) triflate [Sm(OTf) ] and samarium(II) dibromide
2
(
SmBr ), their preparation involves a number of steps that
2
are in some cases non-routine and as a result, the use of
these reagents is limited in synthesis.
Recently, it has been shown that microwave-assisted heat-
ing[3] and sonication in an ultrasonic cleaning bath[4] can be
Scheme 1. Procedures for the generation of SmI
Sm(OTf) via high-intensity ultrasound.
2 2
, SmBr , and
II
2
used to reduce the preparation time of Sm -based reduc-
tants to under 10 min. Concellon’s work,[ utilizing an
4]
ultrasonic cleaning bath to catalyze the formation of the
II
Sm species, is particularly interesting since it is a mild
Results and Discussion
technique that does not require the use of specially designed
containers. On the basis of previous precedent, it is prob- Generation of SmI
2
able that a high-intensity ultrasonic probe, specifically de-
signed for liquid processing, could catalyze the formation
The first detailed report on the synthesis of SmI using
2
conventional cleaning-bath sonication utilized Sm powder
in combination with a range of iodine sources including
iodoform, diiodomethane, 1,2-diiodoethane, or iodine.[
[
[
a] Department of Chemistry, Lehigh University,
Bethlehem, PA 18015, USA
4]
Fax: +1-610-759-6536
The rate of SmI production ranged from 5 min to an hour
2
E-mail: rof2@Lehigh.edu
b] Department of Chemistry, Indian Institute of Technology
Madras,
depending on the iodine source. Low and co-workers re-
ported that the degree of surface activation of a metal using
a cleaning bath is sometimes ineffective for complete con-
Chennai, India
Supporting information for this article is available on the
WWW under http://www.eurjic.org or from the author.
[
5]
version of Sm and an iodine source to SmI2. To study this
Eur. J. Inorg. Chem. 2008, 5015–5019
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
5015

R. A. Flowers II et al.
SHORT COMMUNICATION
2
supposition in more detail, high-intensity ultrasound was Table 1. Percent yield, concentration, and λmax for SmI in each
solvent.
examined as a means for producing SmI in THF from a
2
slight excess of Sm and either iodine or 1,2-diiodoethane.
A submersible ultrasonic probe, operating at a fixed fre-
quency of 20 kHz, was used as the source of the ultrasound.
Solvent[a]
Yield[a]
Concentration[b]
[]
λ
[nm]
max
THF
91%
42%
75%
36%
26%
53%
0.12
0.05
0.03
0.02
0.02
0.04
618
678
632
536
531
555
When Sm metal was combined with either iodine source in CH
3
CN
DME
THF and sonicated, the deep blue color attributed to SmI2
was observed after only 1 min. The maximum concentra-
tion (0.12  as determined by iodometric titration after fil-
tration through a glass-frit funnel) was obtained after 6 to
2
2
2
-Propanol
-Methyl-2-propanol
-Heptanol
[
[
a] Percent yield based on amount of iodine used in SmI
b] Concentration determined by iodometric titration of the filtered
2
synthesis.
7
0
min of sonication. The reported concentration value of
.12  for SmI in THF corresponds to a supersaturated
2
solution.
solution, because maximum concentrations of 0.1  are ob-
tained from traditional methods.
Encouraged by these results, the same method employing
iodine was successfully used to generate SmI in CH CN, Generation of SmBr
2
3
2
DME, 2-propanol, 2-methyl-2-propanol, and 2-heptanol.
Next the sonochemical generation of SmBr was exam-
ined. A range of bromine sources including molecular bro-
2
The concentration of SmI in CH CN obtained (0.05 ) is
comparable to existing methods and the concentration of
2
3
mine were tested but 1,1,2,2-tetrabromoethane was by far
SmI2 obtained in DME (0.03 ) is approximately 33%
[3,10]
the most effective.
Sonication of 1,1,2,2-tetrabromo-
higher than previous reports.[
6–7]
SmI generated in neat 2-
2
ethane in the presence of a slight excess of Sm for 7–10 min
proved to be an effective means for the generation of the
sparingly soluble SmBr2 reductant. During sonication,
color was observed in the solution consistent with the for-
mation of the SmBr2 species (violet/blue: THF, green:
propanol (0.02 ), 2-methyl-2-propanol (0.05 ), and 2-
heptanol (0.04 ) are new species that have not been re-
ported in the literature to date. Figure 1 shows the UV/Vis
spectrum for SmI generated in each solvent using high-
2
intensity ultrasound. The changes in absorption maxima
and relative intensity of the absorption bands in each sol-
[
10,11]
CH CN, blue/black: DME).
A large amount of pre-
3
cipitate with the same color as the solution was produced
in each case. The UV/Vis spectrum was also recorded for
each solution (see Supporting Information) and was consis-
[
7]
vent clearly show that each SmI solvate is unique. Al-
2
though the synthesis of SmI in alcohols may appear some-
2
what surprising, recent work in our group has shown that
tent with the generation of SmBr in THF as previously
2
SmI is relatively stable even in the presence of high concen-
2
[3,11]
reported.
The UV/Vis spectrum of each species re-
trations of water and glycols.[
8,9]
Table 1 contains the con-
sembles those of SmI in each solvent except that the ab-
2
centrations (as determined by iodometric titration), λmax
,
sorption maximum of the SmBr species is slightly blue-
2
and yields for the synthesis of SmI in each solvent. In every
2
shifted (≈ 20 nm).
case, the solution was filtered through a glass frit before
examining the concentration. Precipitate of the same color
as the solution remained in each case indicating that solu-
tions were saturated.
Due to the poor solubility of SmBr in organic solvents,
2
the determination of the concentration and percent yield of
the process is difficult through standard iodometric ti-
tration. To overcome this problem the method developed
II
by Hilmersson to determine the active amount of Sm re-
[
3]
II
ductant was utilized. This method uses a Sm /H O/amine
2
system to rapidly and quantitatively reduce 2-heptanone to
2
-heptanol. The active concentration of SmBr is related to
2
the amount of 2-heptanol produced, determined by GC
using n-decane as an internal standard. The percent yield,
concentration of SmBr and λ
for each solvent is shown
2
max
in Table 2.
Table 2. Percent yield, concentration, and λmax of SmBr
CH CN, and DME.
2
in THF,
3
Solvent
Yield[a]
Concentration
[
λ
[nm]
max
][
b]
THF
58%
62%
58%
0.06
0.07
0.06
599
659
584
3
CH CN
DME
Figure 1. UV/Vis spectrum of SmI
2
. (a) 2-propanol, (b) 2-methyl-
[a] Percent yield based on amount of 1,1,2,2-tetrabromoethane. [b]
Concentration of soluble and precipitated SmBr
2-propanol, (c) 2-heptanol. Inset (d) THF, (e) DME, (f) CH CN.
3
2
.
5016
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© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2008, 5015–5019

Generation of SmII Reductants Using High Intensity Ultrasound
Generation of Sm(OTf)2
was determined by an iodometric titration as shown in
Table 3. The UV/Vis spectrum of each Sm(OTf) solution
2
Next, attention was focused on the preparation of is shown in Figure 2.
Sm(OTf) . This reagent is typically prepared by the reaction
2
of Sm(OTf) with 1 equiv. of an organolithium or organo-
3
2
Table 3. Percent yield, concentration, and λmax for Sm(OTf) in
each solvent.
magensium reagent in THF.[ Although, the role of the
Li or Mg counterion in followup reactions of Sm(OTf)2
is not known, s-BuLi is generally more effective than s-
12]
Solvent
Yield[a]
Concentration[b]
]
λ
max
[nm]
[
[
13]
BuMgCl.
veloped for the synthesis of Sm(OTf)2,
require the use of reagents that are difficult to prepare or
A number of other approaches have been de-
[
14]
THF
85%
87%
44%
60%
30%
8%
0.09
0.07
0.03
0.04
0.02
0.01
0.02
0.01
563
662
568
513
547
553
517
529
but often times
3
CH CN
DME
require extended periods of time. As a result, the original 2-Propanol
protocol developed by Fukuzawa is the preparative 2-Butanol
[
12,13]
Trifluoroethanol
method of choice.
Inanaga reported that Sm(OTf)2
2
2
-Methyl-2-propanol 30%
-Heptanol 15%
could be prepared from triflic acid (TfOH) and Sm metal
but the reaction in THF was sluggish and provided rela-
[
14c]
[a] Percent yield based on amount of Sm(OTf)
sis of Sm(OTf) . [b] Concentration determined by iodometric ti-
tration of the filtered solution.
3
used in the synthe-
tively low yields.
Although this approach is the least
2
productive method for the preparation of Sm(OTf) under
2
standard reaction conditions, if it could be initiated and
carried out efficiently with high intensity ultrasound, it
would be a viable alternative for the preparation of a salt-
free reductant.
A combination of a slight excess of Sm metal with
TfOH was placed in THF and sonicated using conditions
identical to those employed for the preparation SmI . A
2
dark purple color began to appear immediately upon son-
ication and Sm(OTf)2 was produced. Although this
method did generate Sm(OTf) , the concentration and
2
yields were relatively low. Through trial and error it was
determined that the sonication of Sm(OTf) , Sm metal,
3
and a catalytic amount of iodine (1–5 mol-%) was the best
protocol for the synthesis of the Sm(OTf) in all solvents
2
examined. Sm(OTf) was rapidly generated in CH CN and
2
3
DME in 7 min using this approach. The procedure was
also attempted without iodine crystals but Sm(OTf) gen-
2
eration was sluggish. Although the exact role of the iodine
has not been determined, it is possible that iodine activates Figure 2. UV/Vis spectrum of Sm(OTf)
2
. a) 2-Propanol, (b) 2-buta-
nol, (c) trifluoroethanol, (d) 2-methyl-2-propanol, (e) 2-heptanol.
Inset: (f) THF, (g) CH CN, (h) DME.
the surface of the metal in a manner similar to the ad-
dition of iodine to magnesium in a Grignard reaction. Re-
gardless of the mechanism, this method is useful since it
is efficient and does not have Li or Mg countercations ions
3
While the sonochemical preparation of Sm(OTf) from
2
present necessary for generation of Sm(OTf)2 from the Sm metal and Sm(OTf) worked well in a range of solvents
3
combination of Sm(OTf) and alkyllithium or Grignard this method did not work for the generation of Sm(OTf)2
3
[
12–13]
reagents.
in THF. After numerous failed attempts to develop a high-
The sonochemical synthesis of Sm(OTf)2 from Sm- yielding one-step method for the synthesis of Sm(OTf) in
2
(
OTf) and Sm metal was successful in a broad range of THF, an alternative method was developed. First, SmI was
3
2
solvents, including 2-propanol, 2-butanol, 2-methyl-2-buta- generated sonochemically in THF. Next, 2 equiv. of potas-
nol, 2-heptanol, and trifluoroethanol. Sm(OTf) in CH CN sium triflate was added to the solution and then sonicated
2
3
was a dark green/black solution while all of the other again to generate Sm(OTf) . Presumably, the precipitation
2
Sm(OTf)2 solutions were a dark purple/red color. Sm- of potassium iodide drives the ligand exchange between io-
(
OTf) was also found to be more soluble and stable in dide and triflate. Filtration of potassium iodide provided
2
CH CN than SmI . Solutions of SmI in CH CN usually Sm(OTf) . The UV/Vis spectrum for the Sm(OTf) in THF
3
2
2
3
2
2
degrade over time (a few days) whereas we have found the (contained in Figure 2) was identical to that formed
Sm(OTf) -CH CN system to be stable for well over 6 through a salt-free (no Mg or Li salts) procedure using 1,5-
2
3
months in an inert atmosphere with stirring. In the prepara- dithioniabicyclo[3.3.0]octane bis(trifluoromethanesulfon-
[
14b]
tion of each Sm(OTf) solution, a precipitate with the same ate) as the triflate source.
The concentration of the fil-
2
color as the bulk solution was formed. This precipitate was tered Sm(OTf) was determined by an iodometric titration
2
filtered off with a glass-frit funnel before the concentration and is displayed in Table 3.
Eur. J. Inorg. Chem. 2008, 5015–5019
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
5017

R. A. Flowers II et al.
SHORT COMMUNICATION
Determination of the Total Amount of SmII Produced
Through Sonication
and reactivity of Sm . If this supposition is correct, there
II
II
is likely to be a rich and unexplored chemistry of Sm re-
ductants readily available through selective changes in sol-
vent.
In many of the sonochemical preparations, a precipitate
the same color as the bulk solution was observed, suggest-
ing insoluble Sm reductant was also produced in the pro-
II
cess. To test this hypothesis, the method developed by Hil-
mersson was used to determine the total amount of SmII
produced in the sonochemical process. In an initial test, the
concentration of a freshly sonicated and unfiltered solution
[3]
Experimental Section
Materials and General Procedures: Tetrahydrofuran (THF), aceto-
nitrile (CH
ing a Solvent Purification System by Innovative Technology Inc.
Newburyport, MA). Alcohols were distilled twice and dried with
3
CN), and dimethoxyethane (DME) were purified utiliz-
of SmI in THF was determined. This experiment showed
that the total concentration of SmI was 0.13  making this
a quantitative reaction. Similar results were obtained for
the preparation of SmI in DME. For the preparation of
2
2
(
activated molecular sieves. All the sonochemical experiments were
performed in a VCX-750 W model (Sonics & Materials Inc., New-
town, CT) with a 750-Watt 1/2” full-wave probe, working under a
2
2
Sm(OTf) in DME, the total concentration of Sm(OTf)2
was found to be 0.04 , which is higher than determined fixed frequency of 20 kHz and 40% of maximum amplitude. The
from iodometric titration of the filtered solution. The re- experiments were conducted in an Innovative Technology Inc. dry-
sults shown in Table 4 indicate that the active amount of box containing argon. Experiments were carried out at room tem-
II
perature, with a pulser mode on for 55 s and off for 5 s to limit the
amount of solvent evaporation. UV/Vis experiments were per-
formed with a Shimadzu UV-1601 UV/Vis Spectrophotometer con-
trolled by UV Probe (version 1.11) software. In all experiments,
the ligand source (I-, Br-, and -OTf) was the limiting reagent and
samarium powder was used in excess.
Sm generated is approximately 10% greater than the solu-
tion concentration. While the Hilmersson method worked
well in aprotic solvents, it did not work for the alcoholic
solvents examined in this study. The presence of insoluble
II
Sm in each of the of the alcohols suggests that the total
yield of reductant is likely higher than reported in Table 3.
2
Generation of SmI : Samarium powder (40 mesh size, 0.62 g,
Table 4. Determination of the active amount of SmII reductant.
4 mmol) and iodine (0.64 g, 2.52 mmol) were added to 20 mL of
THF at 25 °C in a flame-dried vial. The mixture was sonicated for
7 min with pulser mode. See Supporting Information for the
II
Solution concentration of SmII[a] Total SmII [][b]
Sm /Solvent
[c]
[]
(theoretical yield)
amount of starting materials used for the other SmI
tems.
2
solvent sys-
SmI
SmI
Sm(OTf)
2
/THF
0.12
0.03
/DME 0.03
0.13 (0.13)
0.04 (0.04)
0.04 (0.07)
2
/DME
2
Generation of SmBr : Samarium powder (40 mesh size, 0.25 g,
2
a] Concentration of a filtered SmII solution as determined by iodo-
1.66 mmol) and 1,1,2,2-tetrabromoethane (0.2 g, 0.58 mmol) were
added to 10 mL of dry solvent (THF, DME, or CH CN) in a
3
flame-dried vial. The solution was then sonicated for 7–10 min in
[
II
metric titration. [b] Total amount of Sm generated (soluble and
II
insoluble) as determined by the Sm /H
2
O/amine mediated re-
duction of 2-heptanone.[ [c] Theoretical yield is based on the
8]
pulsar mode. The SmBr precipitates out of solution in 20 min.
2
3
amount of added iodine or Sm(OTf) .
Generation of Sm(OTf)
mmol), Sm(OTf) (0.60 g, 1 mmol), and iodine (0.03 g, ca. 5 mol-
relative to the total amount of Sm added) were added to 15 mL
2
: Samarium powder (40 mesh size, 0.15 g,
1
3
%
3
of dry CH CN in a flame-dried vial. The solution was then
Conclusions
sonicated for 6–7 min in pulsar mode. For the preparation of
in THF, SmI was first sonochemically generated Ϫ from
in
15 mL of THF Ϫ then potassium triflate (0.6 g, 3.2 mmol) was
added to the solution and sonicated for 4–5 additional minutes to
generate Sm(OTf) in THF. See Supporting Information for the
The use of high-intensity ultrasound provided rapid ac- Sm(OTf)
2
2
II
cess to a number of synthetically useful Sm -based reduc- 0.40 g (2.6 mmol) of Sm powder and 0.40 g (1.57 mmol) of I
2
II
tants. The Sm species were generated in a few minutes in
a wide range of solvents including alcohols. The sonochemi-
cal method is atom-efficient and utilizes commercially avail-
able starting materials. This is advantageous, since some
2
amount of starting materials used for the other Sm(OTf)
systems.
2
solvent
II
preparations of Sm reductants require a number of steps
that are in some cases non-routine. One of the other advan- Iodometric Titration: 300–500 µL of the filtered SmII solution was
diluted with 3 mL of solvent and mixed rapidly with a stir bar.
Next, a solution of iodine of known concentration was added to
the rapidly stirred solution until the endpoint was reached.
tages of this approach is that it can be used for small or
larger scale production of Sm and we have routinely made
solutions ranging in volume from 10 to 150 mL.
II
II
II
The generation of Sm in new solvents is interesting Determination of the Active Amount of Sm Reductant: To a freshly
II
since solvent is known to effect the outcome of certain reac- sonicated Sm solution, 3 equiv. of triethylamine, 4 equiv. of water,
[
7,15]
tions.
More recent work has shown that changes in sol- and 2 equiv. of 2-heptanone were added with stir-bar mixing. Upon
work-up, the amount of 2-heptanol produced was determined by
vent have a large impact on the interaction of additives with
GC or GC-MS using decane as an internal standard.[
3]
II
Sm leading to significant changes in the physical proper-
ties and reactivity of the reductant.[ These findings sug-
16]
Supporting Information (see footnote on the first page of this arti-
II
gest that the propensity for coordination between compet- cle): UV/Vis spectra of Sm /solvent combinations and other exper-
ing additives and solvents significantly alters the stability imental details are also included.
5018
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© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2008, 5015–5019

Generation of SmII Reductants Using High Intensity Ultrasound
[
[
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Received: September 1, 2008
Published Online: October 16, 2008
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