A highly efficient reduction of group 14 heteroatom-chlorine single bonds by using samarium diiodide-mediated reaction systems

Article abstract of DOI:10.1016/j.jallcom.2004.12.085

The reduction of group 14 heteroatom-chlorine single bonds by using samarium diiodide-mediated reduction systems in tetrahydrofuran has been investigated in detail. When the reduction of chlorostannanes and chlorogermanes are conducted by employing excess amounts of samarium diiodide (4 equiv.) at the THF refluxing temperature, the corresponding distannanes and digermanes are obtained in good yields, respectively. By the combination with magnesium metal or samarium metal, a novel samarium diiodide-catalyzed reduction of chlorostannanes and chlorogermanes takes place successfully. In contrast, the reduction of chlorosilanes with SmI₂ in THF does not occur at all, because the silylative ring-opening of THF with silyl iodode (formed in situ from chlorosilanes and SmI₂) proceeds in preference to the desired reduction of chlorosilanes.

Full text of DOI:10.1016/j.jallcom.2004.12.085

Journal of Alloys and Compounds 408–412 (2006) 437–440
A highly efficient reduction of group 14 heteroatom-chlorine single bonds
by using samarium diiodide-mediated reaction systems
a
b
a
Ikuyo Kamiya , Koichiro Iida , Nami Harato ,
c
d
c,∗
Zhi-fang Li , Yuri Tomisaka , Akiya Ogawa
a
Department of Chemistry, Faculty of Science, Nara Women’s University, Kitauoyanishi-machi, Nara 630-8506, Japan
b
Department of Applied Chemistry, Faculty of Engineering, Osaka University, Suita, Osaka 565-0871, Japan
c
Department of Applied Chemistry, Faculty of Engineering, Osaka Prefecture University, 1-1 Gakuen-cho, Sakai, Osaka 599-8531, Japan
Advanced Materials Research Lab, KRI Inc., Kyoto Research Park, 134, Chudoji Minami-machi, Shimogyo-ku, Kyoto 600-8813, Japan
d
Received 29 July 2004; received in revised form 8 December 2004; accepted 8 December 2004
Available online 27 June 2005
Abstract
The reduction of group 14 heteroatom-chlorine single bonds by using samarium diiodide-mediated reduction systems in tetrahydrofuran
has been investigated in detail. When the reduction of chlorostannanes and chlorogermanes are conducted by employing excess amounts of
samarium diiodide (4 equiv.) at the THF refluxing temperature, the corresponding distannanes and digermanes are obtained in good yields,
respectively. By the combination with magnesium metal or samarium metal, a novel samarium diiodide-catalyzed reduction of chlorostannanes
and chlorogermanes takes place successfully. In contrast, the reduction of chlorosilanes with SmI
2
in THF does not occur at all, because the
silylative ring-opening of THF with silyl iodode (formed in situ from chlorosilanes and SmI
of chlorosilanes.
2
) proceeds in preference to the desired reduction
©
2005 Elsevier B.V. All rights reserved.
Keywords: Samarium diiodide; Reductive dimerization; Chlorostannane; Chlorogermane; Chlorosilane
1
. Introduction
of its adequate reducing ability and suitable solubility in
organic solvent such as THF [2]. Although samarium diio-
dide itself can reduce aldehydes, ketones, alkyl bromides, and
iodides, the reduction often requires long period of heating
and stirring. To employ the potential reducing ability of low-
valent samarium species, therefore, several unique methods
have been developed recently, which include tuning ligands
[3], combination with other metals [4], and photoexcitation
[5]. Recently, we have developed new reduction systems by
the combination of samarium diiodide and samarium metal
or magnesium metal, which are effective for the reduction of
carbon–halogen single bonds of organic halides [6,4(b)–(d)].
Along our continuing interest about the reduction of group
14 element-halogen linkages, we report here a novel samar-
ium diiodide-catalyzed reduction of group 14 heteroatom-
halogen single bonds in the presence of samarium metal or
magnesium metal (Eq. (1)) [4(a),7].
The reductive coupling of chlorosilanes, chlorogermanes,
and chlorostannanes is one of the most important synthetic
methods for the construction of Si–Si, Ge–Ge, and Sn–Sn
linkages, and alkali metals and related low-valent metals are
usually employed as the reducing agents for this purpose
[1]. However, these reactions often suffer from further reduc-
tions of functional groups on side-chains and great difficulty
in controlling the reactions, because these low-valent metals
bear powerful reducing abilities and are generally insoluble
in organic solvents.
Samarium diiodide (SmI2) is widely employed as a useful
single-electron reducing agent in organic synthesis, because
∗
Corresponding author. Tel.: +81 72 254 9290; fax: +81 72 254 9290.
E-mail address: ogawa@chem.osakafu-u.ac.jp (A. Ogawa).
0
925-8388/$ – see front matter © 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2004.12.085

4
38
I. Kamiya et al. / Journal of Alloys and Compounds 408–412 (2006) 437–440
2.2. Samarium diiodide-catalyzed reduction of
chlorogermanes with samarium metal
After drying with a heat gun under the reduced pressure,
samarium powder (1 mmol) and 1,2-diiodoethane (0.3 mmol)
were added under an argon atmosphere in a 20 mL two-
necked flask with a magnetic stirring bar. Then, freshly dis-
tilled (sodium/benzophenone ketyl) THF (3 mL) was added
to the mixture. The mixture was stirred at ambient temper-
ature for 30 min, resulting in the formation of a dark blue
solution of SmI2 in THF (cat. SmI2/Sm-mixed system). After
the addition of chlorotriethylgermane (1 mmol) to the solu-
tion, the reaction mixture was stirred for 24 h. The resulting
mixture was poured into 1.5N hydrochloric acid (30 mL), and
the products were extracted with diethyl ether (30 mL × 3).
The combined extracts were washed each two times with sat.
NaHCO3 aq. (30 mL) and then sat. NaCl aq. (30 mL) and
dried over MgSO4. After filtration and evaporation of the
solvent, the products were purified by recycling preparative
HPLC (Japan Analytical Industry, Co. Ltd., Model LC-908),
equipped with JAIGEL-1H and -2H columns (GPC) with
CHCl3 as eluent.
2
. Experimental
2
.1. Samarium diiodide-catalyzed reduction of
chlorostannanes with magnesium metal
Samarium powder in oil (99.9%) was purchased from
High Purity Chemical Co., Ltd., and was used after wash-
ing with dry n-pentane, followed by drying for 4 h under the
reduced pressure. 1,2-Diiodoethane was purified by wash-
ing its ether solution with sat. sodium thiosulfate, followed
by drying under the reduced pressure. Representative pro-
cedure for the reductive dimerization of chlorostannanes
with cat. SmI2/Mg-mixed system is as follows: In a 20 mL
two-necked flask with a magnetic stirring bar was placed
under an argon atmosphere magnesium metal (commercially
available for Grignard synthesis, 6 mmol) and then the reac-
tion vessel was heated with a heat gun under the reduced
pressure. After cooling to room temperature, the flask was
filled with argon, and samarium powder (0.3 mmol) and 1,2-
diiodoethane (0.3 mmol) were added in this order into the
flask. Freshly distilled (sodium/benzophenone ketyl) tetrahy-
drofuran (THF, 3 mL) was added to the mixture by syringe
technique. The mixture was stirred at ambient temperature for
about 30 min, resulting in the formation of a dark blue solu-
tion of SmI2 in THF (cat. SmI2/Mg-mixed system). After
the addition of chlorostannanes (1 mmol) to the solution, the
reaction mixture was stirred for 24 h. The resulting mixture
was poured into 1.5N hydrochloric acid (30 mL), and the
products were extracted with diethyl ether (30 mL× 3). The
combined extracts were washed each two times with sat.
NaHCO3 aq. (30 mL) and then sat. NaCl aq. (30 mL) and
dried over MgSO4. After filtration and evaporation of the
solvent, the products were purified by recycling preparative
HPLC (Japan Analytical Industry, Co. Ltd., Model LC-908),
equipped with JAIGEL-1H and -2H columns (GPC) with
CHCl3 as an eluent.
3. Results and discussion
3.1. Samarium diiodide-catalyzed reduction of
chlorostannanes
Table 1 shows the results of the reduction of chlorostan-
nanes (1) with samarium diiodide in the presence (or absence)
of magnesium metal (or samarium metal). By using excess
amounts of samarium diiodide (4 equiv.) at the THF reflux-
ing temperature for 8 h, chloro(tri-n-butyl)stannane could be
reduced by SmI2 itself (entry 1). However the same reduction
did not proceed at room temperature for 24 h.
We previously reported that the combination of SmI2 and
Sm metal extensively enhanced the reducing ability of samar-
ium reagents compared with those of their single systems.
n
Thus, the reduction of Bu3SnCl by employing a Sm/SmI2-
Table 1
Reductive dimerization of chlorostannanes
a
Entry
R
Cat. (equiv.)
M (equiv.)
Temperature
Yield (%)
2
1
1b
2
3
4
5
6
a
n
Bu
SmI2 (4.0)
SmI2 (0.3)
SmI2 (0.3)
–
Cp2TiCl2 (0.3)
SmI2 (0.3)
–
Reflux
r.t.
r.t.
r.t.
r.t.
76
85
82
21
3
Trace
n
n
n
n
Bu
Bu
Bu
Bu
Sm (0.7)
Mg (6.0)
Mg (6.0)
Mg (6.0)
Mg (6.0)
No reaction
No reaction
Ph
r.t.
58
Trace
Isolated yield.
b
8
h.

I. Kamiya et al. / Journal of Alloys and Compounds 408–412 (2006) 437–440
439
Table 2
Catalytic reductive dimerization of a chlorotriethylgermane
a
Entry
SmI2 (equiv.)
Metal (equiv.)
Temperature
Yield (%)
4
5
1
2
3
4
5
6
a
1.0
6.0
0.3
–
0.3
–
–
–
r.t
Reflux
r.t
r.t
r.t
No reaction
84
73
Trace
5
Sm (0.7)
Sm (0.7)
Mg (6.0)
Mg (6.0)
No reaction
No reaction
74
6
r.t.
Isolated yield.
mixed system was examined, which successfully provided
the distannane (2) in good yields even at room temperature
metal nor magnesium metal could work at room temperature
for the reductive dimerization of Et3GeCl (3) (entries 4 and
6).
(
entry 2).
More remarkable is the observation that magnesium metal
A possible catalytic cycle for the present SmI2/Sm or Mg-
mixed system is shown in Scheme 1. Chlorotriethylgermane
(3) undergoes single-electron transfer from Sm(II) species to
give the corresponding digermane (4) and thus the formed
Sm(III) species is reduced by samarium metal or magne-
sium metal to regenerate Sm(II) species in situ. However,
the reduction of chlorogermane (3) in the presence of samar-
ium metal or magnesium metal gave the reduced products
in good yields (entries 3 and 5), whereas the same reduction
using only SmI2 did not proceed at all (entry 1). These facts
are in conflict with the possible pathway (Scheme 1), where
samarium metal and magnesium metal are used only for the
reduction of Sm(III) species to Sm(II) species. On the other
hand, we previously suggested that Sm(I) species is formed
as a powerful reducing agent by the proportionation between
Sm(II) species and samarium metal [4(a),(b)]. To elucidate
the presence of Sm(I) species in the this reduction system,
especially, SmI2/Mg-mixed system, further detailed mecha-
nistic experiments are required.
2
+
(
Mg /Mg = −2.37 V), which has a reducing power similar to
3+
that of samarium metal (Sm /Sm = −2.41 V), also effected
n
the reductive coupling of Bu3SnCl in the presence of cat-
alytic amounts of SmI2 (entry 3). In the absence of SmI2
n
or in the presence of Cp2TiCl2, the reduction of Bu3SnCl
with magnesium metal did not proceed at all (entries 4 and 5,
respectively). Thus, SmI2 is essential for this reductive cou-
n
pling of Bu3SnCl. Chlorotriphenylstannane also underwent
reductive coupling by use of a cat. SmI2/Mg-mixed system
(entry 6). The same reduction also proceeded by using a cat.
SmI2/Sm-mixed system (62%, r.t., 24 h).
3
.2. Samarium diiodide-catalyzed reduction of
chlorogermanes
The results of the reduction of chlorotriethylgermane (3)
with samarium diiodide and related reducing agents are
shown in Table 2. As can be seen from entries 1 and 2, an
equimolar amount of samarium diiodide could not reduce
chlorotriethylgermane (3) at room temperature, whereas the
use of excess amounts of SmI2 in refluxing THF led to the
reductive dimerization efficiently, providing the correspond-
ing digermane (4) in good yield. In the presence of samarium
metal or magnesium metal, however, the reductive coupling
using a catalytic amount of SmI2 took place efficiently even at
room temperature (entries 3 and 5). Noteworthy is that small
amounts of the corresponding trigermane (5) was obtained
in both reactions. In the absence of SmI2, neither samarium
3.3. Attempted reduction of chlorosilanes with
samarium diiodide in THF
Similarly as the samarium diiodide-mediated reduction of
chlorostannanes and chlorogermanes, the reductive coupling
of chlorosilanes (6) was examined. However, the desired dis-
ilanes (R3SiSiR3) were not obtained at all, and instead silyla-
tive ring-opening products of THF (7 and 8) were formed as
the main products (Eq. (2)).
Scheme 1.

4
40
I. Kamiya et al. / Journal of Alloys and Compounds 408–412 (2006) 437–440
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1
(
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895–6899.
(
6
[4] For SmI2–Sm system:
A possible reaction pathway for the formation of 7 and
is shown in Eq. (2), which may include the following: (i)
(a) A. Ogawa, N. Takami, M. Sekiguchi, I. Ryu, N. Kambe, N.
Sonoda, J. Am. Chem. Soc. 114 (1992) 8729–8730;
8
(
b) A. Ogawa, T. Nanke, N. Takami, Y. Sumino, I. Ryu, N. Sonoda,
Chem. Lett. 23 (1994) 379–380;
c) A. Ogawa, T. Nanke, N. Takami, M. Sekiguchi, N. Kambe, N.
the reaction of chlorosilane (6) with SmI2 generates in situ
silyl iodide, which is well-known as a powerful silylating
reagent for cyclic ethers [8]; (ii) silylative ring-opening of
THF provides 7; (iii) further reduction of 7 with SmI2 leads
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(
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(
(
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For SmI2–Fe system: 2(a):
4
. Conclusions
(
(
a) G.A. Molander, J.A. McKie, J. Org. Chem. 58 (1993) 7216–7227;
b) G.A. Molander, S.R. Shakya, J. Org. Chem. 59 (1994) 3445–3452;
We have developed a samarium diiodide-catalyzed reduc-
For SmI2–NiI2 system:
a) F. Machrouhi, B. Hamann, J.-L. Namy, H.B. Kagan, Synlett (1996)
33–634;
b) F. Machrouhi, J.-L. Namy, Tetrahedron Lett. 40 (1999) 1315–1318;
(c) G.A. Molander, C.R. Harris, J. Org. Chem. 62 (1997) 7418–7429.
5] (a) A. Ogawa, Y. Sumino, T. Nanke, S. Ohya, N. Sonoda, T. Hirao,
J. Am. Chem. Soc. 119 (1997) 2745–2746;
tive coupling of chlorostannanes and chlorogermanes by the
combination of samarium metal or magnesium metal, which
provides a useful tool to distannanes and digermanes. In con-
trast, the samarium diiodide-mediated reduction of chlorosi-
lanes failed, because in situ formed silyl iodide worked as a
silylating reagent for THF. Further detailed investigations of
the reductive coupling of chlorosilanes are now underway.
(
6
(
[
(
b) A. Ogawa, Y. Sumino, T. Nanke, I. Ryu, N. Kambe, N. Sonoda,
Kidorui (1995) 338–339;
c) A. Ogawa, T. Hirao, Y. Sumino, N. Sonoda, Kidorui (1996)
98–299;
d) T. Imamoto, Y. Tawarayama, T. Kusumoto, M. Yokoyama, J.
Synth. Org. Chem. Jpn. 42 (1984) 143–152;
e) W.G. Skene, J.C. Scaiano, F.L. Cozens, J. Org. Chem. 61 (1996)
918–7921;
f) G.A. Molander, C. Alonso-Alija, J. Org. Chem. 63 (1998)
4366–4373;
(
2
(
Acknowledgements
(
7
(
We wish to express our sincerest gratitude to Professor
Toshikazu Hirao for his helpful suggestions. This work was
supported in part by a Grant-in-Aid for Scientific Research
from the Ministry of Education, Culture, Sports, Science and
Technology, Japan.
(
(
g) G.A. Molander, M. Sono, Tetrahedron 54 (1998) 9289–9302;
h) G.A. Molander, F. Machrouhi, J. Org. Chem. 64 (1999)
4119–4123;
(
(
i) G.A. Molander, C. K o¨ llner, J. Org. Chem. 65 (2000) 8333–8339;
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