Selective deoxygenation of epoxides to alkenes with molecular hydrogen using a hydrotalcite-supported gold catalyst: Aconcerted effect between gold nanoparticles and basic sites on a support

Article abstract of DOI:10.1002/anie.201007679

(Chemical Equation Presented) A picky catalyst: Hydrotalcite-supported gold nanoparticles (Au/HT) efficiently catalyze the deoxygenation of epoxides to alkenes using molecular hydrogen as an ideal reductant. Various epoxides have been deoxygenated to the corresponding alkenes (see picture) with over 99% selectivity. The high selectivity is based on the concerted effect between basic sites of HT and the gold nanoparticles.

Full text of DOI:10.1002/anie.201007679

Communications
DOI: 10.1002/anie.201007679
Heterogeneous Catalysis
Selective Deoxygenation of Epoxides to Alkenes with Molecular
Hydrogen Using a Hydrotalcite-Supported Gold Catalyst: AConcerted
Effect between Gold Nanoparticles and Basic Sites on a Support**
Akifumi Noujima, Takato Mitsudome, Tomoo Mizugaki, Koichiro Jitsukawa, and
Kiyotomi Kaneda*
Direct conversion of epoxides into the corresponding alkenes
is an important reaction because it allows the use of oxirane
rings as protecting groups for carbon–carbon double bonds.[1]
This transformation also occurs in the production of vita-
min K in the human body[2] and is useful for quantification of
epoxide moieties in graphite epoxide or oxygenated carbon
nanotubes.[3] Traditionally, the deoxygenation of epoxides to
alkenes has been conducted using stoichiometric amounts of
reagents, which results in the production of large amounts of
undesirable waste. To date, several catalytic deoxygenations
using PPh3, Na/Hg, and NaBH4 as reductants have been
reported. These catalysts, however, suffer from low activity,
low atom efficiency, and tedious work-ups with moisture-
sensitive reaction conditions.[4] An ideal “green” protocol for
the catalytic deoxygenation of epoxides is the use of
molecular hydrogen (H2) as a reducing reagent because,
theoretically, water is the only by-product. However, the use
of H2 often causes nonselective reduction of epoxides to yield
alcohols and alkanes as by-products through hydrogenation
of the epoxides and overhydrogenation of the desired alkenes,
respectively.[5] Although there are a few successful reports on
the selective deoxygenation of epoxides using H2, selectivity
for alkenes is restricted to low conversion levels and a limited
range of substrates.[5,6] Therefore, the development of an
efficient catalytic system for the selective deoxygenation of
epoxides to the corresponding alkenes using H2 is highly
desired.
Recently, we discovered that heterogeneous gold and
silver nanoparticle (NP) catalysts have high activities for the
deoxygenation of various epoxides to alkenes with > 99%
selectivity, using 2-propanol used as an environmentally
friendly reductant.[7] Furthermore, CO/H2O was found to
work as an alternative reductant for the selective deoxyge-
nation of epoxides to alkenes in water under mild reaction
conditions.[8]
Herein, we demonstrate that gold NPs supported on
hydrotalcite [HT: Mg6Al2(OH)16CO3·nH2O] (Au/HT) can act
as a highly efficient heterogeneous catalyst for the deoxyge-
nation of epoxides to alkenes with H2 used as an ideal
reductant. Au/HT is applicable to various epoxides, and
selectivities for alkenes are over 99% at high conversions.
After the reaction, solid Au/HT can be easily recovered from
the reaction mixture and reused with no decrease in its
catalytic efficiency.
The deoxygenation of styrene oxide (1a) using various
inorganic-materials-supported Au NPs was carried out in
toluene at 808C under 1 atm of H2 (Table 1). Among the Au
NP catalysts tested,[9] Au/HT exhibited the highest activity
toward this deoxygenation to afford styrene (2a) in 95% yield
with a small amount of the overhydrogenated product
ethylbenzene (3a; Table 1, entry 1). Au/CeO2 and Au/Al2O3
also converted 1a, but selectivities for 2a were much lower
than that of Au/HT (Table 1, entries 4 and 5). Interestingly,
Au/TiO2 showed the highest selectivity for 2a, although the
conversion of 1a was low (Table 1, entry 6). Au/SiO2 did not
have any catalytic activity for this reaction (Table 1, entry 7).
Notably, when the reaction temperature was lowered to 608C,
Au/HT produced 2a as the sole product in quantitative yield
with > 99% selectivity (Table 1, entry 2). Moreover, the
carbon–carbon double bond of 2a was completely intact when
the reaction time was prolonged (Table 1, entry 3).
Next, various HT-supported metal NPs were examined in
this reaction (Table 1, entries 8–13).[11] Ag/HT, Ru/HT, Rh/
HT and Cu/HT did not function as catalysts (Table 1,
entries 10–13). In the case of Pd/HT and Pt/HT, hydrogena-
tion of 1a occurred to give 2-phenylethanol (4a), but no
deoxygenated product was obtained (Table 1, entries 8 and 9).
These results clearly revealed that the combination of Au NPs
and HT had the best catalytic activity and selectivity toward
the deoxygenation of epoxides to alkenes using H2.
[*] A. Noujima, Dr. T. Mitsudome, Dr. T. Mizugaki, Prof. Dr. K. Jitsukawa,
Prof. Dr. K. Kaneda
Department of Materials Engineering Science
Graduate School of Engineering Science
Osaka University, 1-3, Machikaneyama
Toyonaka, Osaka 560-8531 (Japan)
Fax: (+81)6-6850-6260
E-mail: kaneda@cheng.es.osaka-u.ac.jp
Prof. Dr. K. Kaneda
Research Center for Solar Energy Chemistry
Osaka University, 1-3, Machikaneyama
Toyonaka, Osaka 560-8531 (Japan)
[**] We thank Dr. Uruga, Dr. Tanida, Dr. Nitta, Dr. Taniguchi and Dr.
Hirayama (SPring-8) for X-ray absorption fine structure (XAFS)
measurements. The TEM experiments were carried out at a facility
of the Research Center for Ultrahigh Voltage Electron Microscopy,
Osaka University. A.N. thanks the JSPS Research Fellowships for
Young Scientists. He also expresses his special thanks to The Global
COE (Center of Excellence) Program “Global Education and
Research Center for Bio-Environmental Chemistry” of Osaka
University.
Scheme 1 shows the hydrogenation of 2a in the presence
or absence of p-methylstyrene oxide (1b) using Au/HTor Au/
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201007679.
2986
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Angew. Chem. Int. Ed. 2011, 50, 2986 –2989

Table 1: Deoxygenation of styrene oxide using H2.[a]
results show that immobilizing
small Au NPs (< 3 nm) is the key
to promoting the selective deoxy-
genation of epoxides to alkenes.
The applicability of this Au/HT-
Entry
Catalyst
Conversion [%][b]
Selectivity
Selectivity
Selectivity
H2 system was explored using vari-
ous epoxides (Table 3). In all cases,
the epoxides were deoxygenated to
the corresponding alkenes with
over 99% selectivity, and hydro-
genation of the alkene products did
not occur. For example, aromatic
epoxides were good substrates
(Table 3, entries 1, 4–8). trans-Stil-
bene oxide was transformed to
trans-stilbene with retention of its
stereochemistry (Table 3, entry 7),
while cis-stilbene oxide gave a mix-
ture of Z/E-alkene stereoisomers
(Table 3, entry 8). Aliphatic epox-
ides also converted smoothly into
for 2a [%][b]
for 3a [%][b]
for 4a [%][b]
1
2[c]
3[d]
4
5
6
7
8
9
Au/HT
Au/HT
Au/HT
Au/CeO2
Au/Al2O3
Au/TiO2
Au/SiO2
Pd/HT
97
>99
>99
64
82
26
<1
>99
87
97
>99
>99
81
36
>99
–
3
0
0
19
64
<1
–
0
0
–
–
0
0
0
0
0
0
–
99
99
–
0
0
–
–
–
–
Pt/HT
10
11
12
13
Ag/HT
–
–
–
–
Rh/HT
Ru/HT
Cu/HT
–
–
–
–
–
[a] Reaction conditions: Catalyst (M: 0.9 mol%), toluene (5 mL), 1a (0.5 mmol), 808C, 6 h.
[b] Determined by GC using an internal standard technique. [c] 608C, 8 h. [d] 608C, 24 h.
Table 2: Deoxygenation of styrene oxide using H2.[a]
Particle
diameter [nm]
Conversion [%][b]
Selectivity
Selectivity
for 2a [%][b]
for 3a [%][b]
2.7
5.8
7.9
12
97
72
51
24
10
97
95
80
59
38
3
5
20
41
62
20
Scheme 1. Hydrogenation of styrene in the presence or absence of p-
methylstyrene oxide.
[a] Reaction conditions: Au/HT (Au: 0.9 mol%), toluene (5 mL), 1a
(0.5 mmol), 808C, 6 h. [b] Determined by GC using an internal standard
technique.
TiO2, which could selectively deoxygenate 1a to 2a (Table 1,
entries 1 and 6). Neither Au/HT nor Au/TiO2 hydrogenated
2a in the presence of 1b. Notably, Au/HT did not show any
catalytic activity for the hydrogenation of 2a even in the
absence of 1b, but in the case of Au/TiO2, hydrogenation of
2a to 3a occurred. These phenomena indicate that metal–
hydride species generated on Au/HT are effective for the
deoxygenation of epoxides, but are completely inactive for
the corresponding alkenes in high yields, although an aliphatic
internal epoxide of trans-2-octene oxide was not intact
(Table 3, entries 9 and 11 vs. 10). An epoxides bearing a
hydroxy group was deoxygenated to the corresponding
alkene without affecting the functional group (Table 3,
entry 12). After the deoxygenation of 1a, Au/HT could be
separated from the reaction mixture and reused without any
loss of catalytic efficiency (Table 3, entries 2 and 3).
Bearing in mind that basic ligands of transition-metal
complexes promote heterolytic cleavage of H2 to give metal–
hydride species,[14] we propose a concerted effect between the
basic sites of HT and the Au NPs. Namely, heterolytic
cleavage of H2 occurs at the interface between Au NPs and
HT as a macroligand of Au NPs to give [Au-H]À and [HT-H]+
species (Scheme 2). These hydrogen species would selectively
deoxygenate the epoxides to form the corresponding alkenes
and water,[15] which is supported by the fact that Au/HT is
=
the hydrogenation of C C bonds. On the other hand, the high
selectivity of Au/TiO2 for alkenes in the deoxygenation of
epoxides (Table 1, entry 6) is attributed to the preferential
adsorption of epoxides over alkenes, which is a similar
phenomenon to the previous report that the nitro group in 3-
=
nitrostyrene was chemoselectively reduced while C C bonds
were unaffected by a Au/TiO2 catalyst.[12]
We next carried out the above deoxygenation using Au/
HTs with different sized Au particles (Table 2).[13] Larger Au
NPs (> 3 nm) showed not only lower activities but also lower
selectivities for 2a because of the formation of 3a. Interest-
ingly, it was found that the yield and selectivity of 2a
increased as the average size of Au NPs decreased. These
=
inactive for the hydrogenation of C C bonds of alkenes (see
Scheme 1). Furthermore, bands assigned to the [Au-H]À and
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Communications
Table 3: Deoxygenation of various epoxides using Au/HT.[a]
Entry
Substrate
Product
Time [h]
Yield [%][b]
Selectivity [%][b]
1
8
8
8
>99
97
97
>99
>99
>99
2[c]
3[d]
4
5
6
12
12
24
81
84
85
>99
>99
>99
7[e]
12
12
98
96
>99
>99
E/Z=2/3
8[e ]
9
24
24
84
>99
10
<1
–
11
12
24
24
89
92
>99
>99
[a] Reaction conditions: Au/HT (0.1 g), toluene (5 mL), substrate (0.5 mmol), 608C. [b] Determined by GC using an internal standard technique.
[c] Reuse 1. [d] Reuse 2. [e] At 1008C.
In conclusion, we have found that Au/HT selectively
catalyzed the deoxygenation of epoxides in the presence of H2
as an ideal reducing reagent to give alkenes. With this system,
various epoxides were transformed to alkenes with over 99%
selectivity and high atom efficiency. The selective formation
of [Au-H]À and [HT-H]+ species by using H2 obtained through
heterolytic dissociation will be applied to the reduction of
other compounds. Moreover, Au/HT could be reused without
loss of activity or selectivity.
Received: December 7, 2010
Published online: February 24, 2011
Scheme 2. Heterolytic dissociation of H2 at the interface between basic
sites (BS) of HT and Au NPs in the deoxygenation of epoxides (BS
Keywords: deoxygenation · epoxides · gold ·
heterogeneous catalyst · hydrogen
represents OHÀ or CO32À).
.
[HT-H]+ species appeared at around 1748 and 3200 cmÀ1,
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active [Au-H]À and [HT-H]+ species are formed in the
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2988 www.angewandte.org
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Angew. Chem. Int. Ed. 2011, 50, 2986 –2989

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[15] Deoxygenation of 1a using D2 in place of H2 was carried out
under identical reaction conditions. GC–MS analysis revealed
that all the hydrogen atoms in 2a were retained and D2O was
formed.
[7] T. Mitsudome, A. Noujima, Y. Mikami, T. Mizugaki, K.
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[9] Au/HT and other supported Au NPs were synthesized by the
previously reported method. See Ref. [10]. The mean diameters
of the Au particles for the Au/HT, Au/CeO2, Au/Al2O3, Au/TiO2,
and Au/SiO2 were 2.7, 3.8, 3.6, 3.7, and 14 nm, respectively.
[10] T. Mitsudome, A. Noujima, T. Mizugaki, K. Jitsukawa, K.
Kaneda, Green Chem. 2009, 11, 793.
[17] We previously reported that when Au/HT was treated with CO
and H2O vapor at 298 K, a new band attributed to the formation
of a [Au-H]À species appeared at 1750 cmÀ1. See Ref. [8a].
[18] When the spectra were collected in a D2 atmosphere, bands from
Au-D and HT-D were at detected around 1368 and 2518 cmÀ1
respectively.
,
[11] The preparation of HT-supported metal NPs of Ag/HT, Pd/HT,
Ru/HT, Rh/HT, and Cu/HTwas reported in a previous study. See
Ref. [8a].
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