Sm(II)-mediated reduction in water: Importance of the additive in the proportionation of SmCl3 and Sm

Article abstract of DOI:10.1080/00397910903079615

Mechanistic studies of the SmCl₃-Sm system in water were carried out. The addition of N,N-dimethylacetoamide (DMA) enhanced the reactivity of the one-electron reduction. According to the ultraviolet-visible (UV-vis) analysis, DMA was found to accelerate proportionation of SmCl₃ and Sm.

Full text of DOI:10.1080/00397910903079615

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Sm(II)-Mediated Reduction in Water:
Importance of the Additive in the
Proportionation of SmCl and Sm
3
a
a
a
S. Matsukawa , K. Ichikawa & Y. Ogura
a
Department of Science Education, Faculty of Education, Ibaraki
University, Mito, Ibaraki, Japan
Published online: 07 Apr 2010.
To cite this article: S. Matsukawa , K. Ichikawa & Y. Ogura (2010) Sm(II)-Mediated Reduction in Water:
Importance of the Additive in the Proportionation of SmCl and Sm, Synthetic Communications: An
3
International Journal for Rapid Communication of Synthetic Organic Chemistry, 40:9, 1345-1349, DOI:
10.1080/00397910903079615
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Synthetic Communications , 40: 1345–1349, 2010
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397910903079615
Sm(II)-MEDIATED REDUCTION IN WATER:
IMPORTANCE OF THE ADDITIVE IN THE
PROPORTIONATION OF SmCl AND Sm
3
S. Matsukawa, K. Ichikawa, and Y. Ogura
Department of Science Education, Faculty of Education, Ibaraki University,
Mito, Ibaraki, Japan
3
Mechanistic studies of the SmCl -Sm system in water were carried out. The addition of
N,N-dimethylacetoamide (DMA) enhanced the reactivity of the one-electron reduction.
According to the ultraviolet–visible (UV-vis) analysis, DMA was found to accelerate
3
proportionation of SmCl and Sm.
Keywords: Additive; aqueous media; reduction; samarium
INTRODUCTION
The use of low-valent samarium (Sm) as a single-electron-transfer reagent in
organic synthesis has been developed, and numerous examples, such as coupling,
reduction, cyclization, and the Barbier reaction, have been widely studied and
[
1]
utilized. It is well known that additives, such as hexamethylphosphoric triamide
0
(HMPA), N,N -dimethylpropyleneurea (DMPU), tetramethylguanidine (TMG),
N,N-dimethylacetoamide (DMA), amine, inorganic bases, and water (H O), acceler-
2
[
ate the reduction using SmI in tetrahydrofuran (THF). Mechanistic studies on the
2]
2
[3]
roles of additives have also been reported. However, the use of water as a solvent
for organic reactions has been attracting attention because of its advantages with
regard to environmental concerns, and numerous successful reactions have been
[
4]
reported.
developed. Recently, we have reported Sm(II)-mediated one-electron reduction
Samarium-mediated reactions in aqueous media have also been
[
5]
[
of a carbonyl compound in water using SmCl and Sm. In this reaction, proportio-
6]
3
nation of SmCl and Sm occurred. To apply this method to other reactions, we
3
examined the effect of the additive in this water-mediated reaction.
RESULTS AND DISCUSSION
First, the effect of the additive in the proportionation of SmCl and Sm was
3
[
analyzed using ultraviolet–visible (UV-vis) spectroscopy. Figure 1 shows the
7]
Received March 19, 2009.
Address correspondence to S. Matsukawa, Department of Science Education, Faculty of
Education, Ibaraki University, Mito, Ibaraki 310-8512, Japan. E-mail: smatsuka@mx.ibaraki.ac.jp
1
345

1346
S. MATSUKAWA, K. ICHIKAWA, AND Y. OGURA
Figure 1. UV-vis spectra of SmCl
H
3 2 2 3
and samarium powder in H O, H O-DMA (8 eq. to SmCl ) and
2
O-DMPU (8 eq. to SmCl ).
3
UV-vis spectra in water, water–DMPU (8 eq. to SmCl ), and water–DMA (8 eq. to
3
SmCl ). Interestingly, stronger absorption of Sm(II) (about 560 nm) was observed in
3
the addition of DMA (Fig. 1a). Slightly weaker absorbance was observed when a
reduced amount of DMA (4 eq. to SmCl ) was added (Fig. 1b). No further changes
3
occur in the absorption spectrum at excess of DMA (12 and 16 eq.). This strong
absorption remained after 30 min and continued up to 60 min (Fig. 1c). This charac-
teristic peak of Sm(II) disappeared when PhCHO was added to this mixture. How-
ever, the spectra in water–DMPU showed almost the same spectra as that without
additive after 5 min, and intensity of the peak decreased after 30 min. These results
indicate that DMA accelerates the proportionation of Sm and SmCl and stabilizes
3
the formed Sm(II) species.
To confirm the effect of the additive, several one-electron reductions were
examined upon addition to 8 eq. of DMA to Sm and SmCl in water. As expected,
3
the products were obtained in 70–90% yield upon addition to 8 eq. of DMA in the

Sm(II)-MEDIATED REDUCTION IN WATER
1347
reduction of imines, ketone, and acid chloride. Lesser yields of the products were
obtained when the reaction was carried out without DMA. Modest anti-selectivity
was observed in the reaction with b-hydroxyketone, although a high level of
[
8]
diastereoselectivity was observed using SmI2. Unfortunately, a moderate yield of
the product was obtained in the reduction of nitrobenzene, even with the reaction
longer time.
In conclusion, we show that the addition of DMA enhanced the reactivity of
the one-electron reduction in the SmCl -Sm system in water (Table 1).
3
Representative Experimental Procedure
Sm powder (450 mg, 3 mmol) was added to a solution of SmCl (364 mg,
3
1
without degassing the dissolved oxygen. After 5 min, N-benzylidenebenzylamine
mmol) in H O (4 mL) and dimethylacetoamide (743 ml, 8 mmol). Water was used
2
(97 mg, 0.5 mmol) was added to the resultant solution. The reaction was monitored
by thin-layer chromatography (TLC). After 8 h, the resultant yellow-green suspen-
sion was treated with 2 M HCl (10 ml) and extracted with ether. The organic layer
a,b
2
Table 1. SmCl -Sm–mediated reduction of various substrates with additives
Entry
Substrate
Additive
Condition
Product
Yield (%)
1
2
None
rt, 8h
rt, 8h
15
55
c
DMPU
c
3
4
5
6
7
DMA
rt, 8h
rt, 8h
rt, 8h
rt, 4h
rt, 4h
91
18
90
25
84
None
DMA
None
DMA
d
8
9
None
DMA
rt, 1h
rt, 1h
54
e
98
1
0
1
None
DMA
rt, 4h
rt, 4h
34
73
1
f
1
1
2
3
None
DMA
rt, 34h
31
55
f
rt, 34h
a
b
c
The reaction was carried out in H
Sm:SmCl
:substrate ¼ 3:1:0.5.
eq. to SmCl
2
O unless otherwise noted.
3
8
3
.
d
Syn:anti ¼ 44:56.
e
Syn:anti ¼ 41:59.
f
2
The reaction was carried out in the mixture of H O and MeOH (4:1).

1348
S. MATSUKAWA, K. ICHIKAWA, AND Y. OGURA
was washed with NaHCO (twice) and brine, dried, and concentrated in vacuo. The
3
crude product was passed through flash-column chromatography, and the coupling
product was obtained in 91% yield.
Selected Data
0
N,N -dibenzylamine. IR (neat) 3340, 3050, 2820, 1500, 1470, 1110, 740,
ꢀ
1
1
13
6
90 cm . H NMR (400 MHz, CDCl ) d 3.82 (s, 4H), 7.26–7.37 (m, 10H).
C
3
NMR (100 MHz, CDCl ) d 53.5, 126.5, 128.2, 128.6, 140.6.
3
[
7b]
ꢀ1
IR (neat) 3410, 2960, 1480, 1120, 750, 690 cm .
1-Phenyl-butan-13-diol.
1
H NMR (400 MHz, CDCl ) syn-isomer: d 1.24 (d, J ¼ 6.2 Hz, 3H), 1.73–1.94 (m,
3
2
7
H), 3.15 (br, 1H), 3.32 (br, 1H), 4.13 (m, 1H), 4.93 (dd, J ¼ 2.8, 9.8 Hz, 1H),
.28–7.45 (m, 5H). Anti-isomer: d 1.22 (d, J ¼ 6.4 Hz, 3H), 1.73–1.94 (m, 2H), 3.15
(br, 1H), 3.32 (br, 1H), 4.05 (m, 1H), 5.04 (dd, J ¼ 3.8, 7.8 Hz, 1H), 7.28–7.45 (m, 5H).
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