Selective deoxygenation of styrene oxides under a CO atmosphere using silver nanoparticle catalyst

Article abstract of DOI:10.1016/j.tetlet.2010.08.031

Deoxygenation of styrene oxide derivatives into the corresponding alkenes was efficiently catalyzed by inorganic materials of hydrotalcite-supported silver nanoparticles (Ag/HT) using CO/H₂O as a reductant. The Ag/HT catalyst was reusable without loss of activity or selectivity.

Full text of DOI:10.1016/j.tetlet.2010.08.031

Tetrahedron Letters 51 (2010) 5466–5468
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Selective deoxygenation of styrene oxides under a CO atmosphere using silver
nanoparticle catalyst
a
a
a
a
a
Yusuke Mikami , Akifumi Noujima , Takato Mitsudome , Tomoo Mizugaki , Koichiro Jitsukawa ,
a,b,
⇑
Kiyotomi Kaneda
a
Department of Materials Engineering Science, Graduate School of Engineering Science, Osaka University, 1-3 Machikaneyama, Toyonaka, Osaka 560-8531, Japan
Research Center for Solar Energy Chemistry, Osaka University, 1-3 Machikaneyama, Toyonaka, Osaka 560-8531, Japan
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Deoxygenation of styrene oxide derivatives into the corresponding alkenes was efficiently catalyzed by
Received 2 July 2010
Revised 31 July 2010
Accepted 10 August 2010
Available online 14 August 2010
inorganic materials of hydrotalcite-supported silver nanoparticles (Ag/HT) using CO/H
The Ag/HT catalyst was reusable without loss of activity or selectivity.
2
O as a reductant.
Ó 2010 Elsevier Ltd. All rights reserved.
Keywords:
Deoxygenation
Epoxide
Carbon monoxide
Silver nanoparticles
Metal nanoparticles (NPs) often show unique catalytic proper-
ties that can dramatically differ from those of metal complex cata-
lysts. Among the various metal NPs, silver nanoparticles (Ag NPs)
have a distinguished catalytic ability for the industrial production
production of a large amount of waste and/or the need for inert
reaction conditions. Therefore, effective catalytic methodology for
the deoxygenation of epoxides into the corresponding alkenes
has been desired from an environmental standpoint, but these re-
1
12,13
of ethylene oxide from ethylene in the gas phase . However, Ag
ports have been few.
NPs are less active catalysts for other organic reactions under
liquid phase conditions. Recently, we re-evaluated the catalytic
potential of Ag NPs and discovered that inorganic materials-sup-
ported Ag NPs exhibited significantly unique and higher catalytic
activities than other NPs for several functional transformations,
We attempted to extend our deoxygenation methodology using
Ag NPs catalyst with CO/H
ygenation of epoxides. We herein report that inorganic materials of
hydrotalcite [HT; Mg Al (OH)16CO ]-supported Ag NPs exhibited
2
O as a reductant to the selective deox-
6
2
3
catalytic ability for the selective deoxygenation of styrene oxide
derivatives into the corresponding alkenes (Scheme 1). This cata-
lyst system has the following advantages: (1) excellent selectivities
2
such as dehydrogenation of alcohols, aqueous oxidation of silanes
3
to silanols, and hydration of nitriles to amides under liquid phase
4
conditions. Notably, we found the outstanding reduction ability of
2
for alkenes; (2) CO as the sole byproduct, which can be easily re-
Ag NPs for the deoxygenation of nitroaromatic compounds to ani-
moved from the reaction system; (3) no need for additives; and (4)
reusability of the Ag/HT catalyst.
Ag/HT was prepared according to our previous Letter. Initially
Ag/HT was examined for deoxygenation of trans-stilbene oxide (1)
5
lines using CO/H
2
O as a reducing reagent. For example, 3-nitrosty-
5
rene was chemoselectively reduced to 3-vinylaniline in over 99%
selectivity without reduction of the C@C double bond. In the deox-
ygenation, a Ag-hydride species was generated in situ from the
at 110 °C in the presence of H
was converted into trans-stilbene (2) in 99% yield for 3 h accompa-
nied by the generation of CO without formations of 1,2-diphenyl-
2
O under 9 atm of CO. Compound 1
2
reaction of H O with CO, which led to the highly chemoselective
reduction.
2
Epoxides are regarded as efficient protecting groups for alkene
ethane, 1,2-diphenylethanol, or benzyl phenyl ketone through the
hydrogenation and/or isomerization of 1 (Table 1, entry 1).
6,7
functionalities in organic synthesis. The deprotection of epoxides
has been achieved by stoichiometric reactions using an excess of
8
9
10
various reducing reagents, such as phosphines, silanes, iodides,
1
1
and heavy metals. These reaction systems have suffered from the
⇑
Corresponding author.
E-mail address: kaneda@cheng.es.osaka-u.ac.jp (K. Kaneda).
2
Scheme 1. Catalytic deoxygenation of epoxide using Ag/HT with CO/H O.
0
040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.08.031

Y. Mikami et al. / Tetrahedron Letters 51 (2010) 5466–5468
5467
Table 1
Table 2
a
a
Deoxygenation of trans-stilbene oxide
Deoxygenation of various epoxides using Ag/HT
O
R2
R1
R1
R2
Time Conv.b,c Sel.b (%)
Conv.b (%)
Sel.b (%)
Entry Substrate
Product
Entry
1
Catalyst
Solvent
(
h)
(%)
Ag/HT
Ag/HT
Ag/HT
Ag/HT
Ag/HT
Ag/HT
Ag/HT
THF
THF
THF
Toluene
Water
1,4-Dioxane
DMA
THF
THF
THF
THF
THF
THF
THF
THF
THF
99
99
99
95
92
10
0
62
33
25
39
4
>99
>99
>99
>99
>99
>99
—
>99
>99
>99
>99
>99
—
c
2
3
d
O
1
2
3
99 (95) >99
4
5
e
O
6
7
8
9
>
99
3
99 (94)
(E/Z = 4/1)
Ag/Al
2
O
3
Ag/MgO
O
O
10
11
12
13
14
15
16
Ag/TiO
Ag/TiO
2
2
f
3
4
5
3
2
2
99 (94) >99
99 (92) >99
95 (90) >99
Ag/SiO
HT
2
0
0
0
0
AgNO
Ag
Ag powder
3
—
—
—
2
O
O
a
Reaction conditions: trans-stilbene oxide (0.5 mmol), catalyst (metal:
.037 mmol), solvent (5 mL), H O (0.1 mL), 110 °C CO (9 atm), 3 h.
0
2
O
b
Determined by LC using internal standard technique.
Reuse 1.
Reuse 2.
Water (4 mL), 150 °C, 3 h.
6
7
8
9
1
1
2
2
92 (87) >99
90 (85) >99
92 (88) >99
91 (86) >99
99 (85) >99
85 (80) >99
c
d
e
f
F
F
O
2 3
Na CO (1.5 mmol) was added.
Cl
Cl
O
O
O
O
O
O
O
OEt
OEt 12
Notably, 2 was not hydrogenated even when the reaction time was
extended beyond the complete conversion of 1.14 Among the vari-
ous solvents tested, THF and toluene afforded the excellent yields
of 2 while 1,4-dioxane and DMA inhibited the reaction (entries 1
and 4 vs 6 and 7). Interestingly, Ag/HT could be used under aque-
ous conditions without organic solvent. Thus, the deoxygenation of
O
O
12
0
1
NH2
NH2 12
1
using Ag/HT in water gave 2 in 92% isolated yield (entry 5). The
O
effect of supports on the catalytic activity of Ag NPs was investi-
gated in the deoxygenation of 1 using a series of inorganic sup-
12
NC
NC
ports, such as Al
best support, followed by other base supports of Al
entries 8 and 9), whereas Ag/TiO and Ag/SiO
2 2
2
O
3
, MgO, SiO
2
, and TiO
2
. HT was found to be the
and MgO
had low activities
entries 10 and 12). Interestingly, the addition of Na CO as a base
reaction system improved the yield of 2 (en-
try 10 vs 11). The parent HT or the silver compounds of AgNO
Ag O, or Ag powder did not promote the deoxygenation at all (en-
O
O
2 3
O
1
1
2
3
24
24
Trace
Trace
—
—
(
(
2
3
to the above Ag/TiO
2
1
5
a
3
,
Reaction conditions: Ag/HT (Ag: 0.037 mmol), substrate (0.5 mmol), THF (5 mL),
O (0.1 mL), 110 °C, CO (9 atm).
Determined by GC and LC using internal standard technique.
H
2
b
2
tries 13–16). These results indicate that a combination of Ag NPs
and base support of HT is essential to obtain high catalytic activity
for the deoxygenation of 1.
c
Values in parentheses are the yields of the isolated products.
The catalyst was filtered off from the reaction mixture after 50%
conversion of 1. Further stirring of the filtrate under identical reac-
tion conditions did not afford any products. ICP analysis of the fil-
trate confirmed that the leaching of Ag species into the solution
was not detected (detection limit: 0.007 ppm). The above phenom-
enon confirms that the present reaction proceeds on the solid Ag/
HT catalyst. The Ag/HT catalyst was separated from the reaction
mixture by simple filtration after the deoxygenation of 1, and then,
product 2 could be easily isolated by evaporation of the solvent.
The recovered Ag/HT was reusable without any loss of its high cat-
alytic activity or selectivity: 99% yields were obtained in the two
recycling reactions of 1 (entries 2 and 3). EXAFS and TEM analysis
of Ag/HT revealed that the size and oxidation state of the Ag NPs on
HT did not change significantly; the particle sizes of fresh and re-
used Ag/HTs were 9.5 nm and 9.8 nm, respectively,17 which is con-
sistent with the retention of the catalytic activity of Ag/HT during
the reuse experiments.
The scope and limitations of the Ag/HT-catalyzed deoxygen-
ation of epoxides were explored next (Table 2). Styrene oxides
substituted with electron-donating or electron-withdrawing
groups were completely converted into the desired alkenes (en-
tries 5–7). Notably, epoxides bearing other reducible functional-
ities were chemoselectively deoxygenated. The ester, ketone,
amide, and cyano moieties remained intact during the deoxygen-
ation of epoxides (entries 8–11). Benzylic and aliphatic epoxides
did not show any reactivity (entries 12 and 13). This high activity
of Ag/HT toward the aromatic oxirane ring was in contrast to that
1
2
of previous catalyst systems. In addition, Ag/HT was also applica-
ble for a preparative scale deoxygenation reaction, for example,
40 mmol of 1 was smoothly converted into 2 in 95% isolated yield
(Scheme 2).
In conclusion, we have developed a highly efficient catalytic
deoxygenation using HT-supported silver nanoparticles with CO/
H O as a reducing reagent. Various styrene oxide derivatives were
2

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Y. Mikami et al. / Tetrahedron Letters 51 (2010) 5466–5468
3
4
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Scheme 2. A 40 mmol-scale deoxygenation of trans-stilbene oxide using Ag/HT.
smoothly converted into the corresponding alkenes in high yields
without reduction of other reducible functionalities. Ag/HT was
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4
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4. The time profile for the reduction of 1 is shown in Fig. 1S in Supplementary
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1
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support was attributed to promote the formation of a metal-hydride species;
see: Refs. 13,16.
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17. See Supplementary data for details.