Mild and efficient CO-mediated eliminative deoxygenation of epoxides catalyzed by supported gold nanoparticles

Article abstract of DOI:10.1039/c0cc02783a

Supported gold nanoparticles (NPs), which are well-known epoxidation catalysts, were found to be exceptionally active for the selective deoxygenation of epoxides into alkenes using cheap and easily accessible CO and H ₂O as the reductant. The Royal Society of Chemistry 2011.

Full text of DOI:10.1039/c0cc02783a

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Mild and efficient CO-mediated eliminative deoxygenation of epoxides
catalyzed by supported gold nanoparticlesw
Ji Ni, Lin He, Yong-Mei Liu, Yong Cao,* He-Yong He and Kang-Nian Fan
Received 24th July 2010, Accepted 17th September 2010
DOI: 10.1039/c0cc02783a
Supported gold nanoparticles (NPs), which are well-known
epoxidation catalysts, were found to be exceptionally active
for the selective deoxygenation of epoxides into alkenes using
cheap and easily accessible CO and H2O as the reductant.
interesting feature of the Au–CO/H2O-catalyzed reaction is
that it tolerates a diverse range of reducible functional groups
including alkenes, alkynes, halogens, ketones, aldehydes,
esters, and nitriles. Furthermore, this reaction can be carried
out under atmospheric CO at room temperature. In view of
the prominent efficiency of the gold system for deoxygenation
of the nitro functionality, we envisioned that the CO/H2O-
mediated Au-catalyzed reduction strategy could afford a green
and efficient protocol for the removal of oxygen from epoxides
under mild conditions.
Catalytic epoxidation is a well-documented reaction widely
used in organic synthesis and industry.1 Its prominence is due,
in part, to our reliance on petroleum-based feedstocks for the
preparation of a number of versatile intermediates that
contain heteroatom functionality.2 In contrast, the reverse
reaction, deoxygenation of epoxides to make olefins, is much
less developed. The deoxygenation of epoxides to olefins
allows the use of the oxirane ring as a protecting group for
double bonds, which is an important protocol to control olefin
stereochemistry,3 for conversion of biomass-derived substrates
to useful organic compounds4 and also for structural analysis
of natural products.5 Deoxygenation of epoxides is traditionally
carried out with a variety of stoichiometric reducing reagents6
such as phosphines, silanes, iodides and heavy metals, mostly
by multi-step procedures. In spite of their utility, clear
drawbacks of these methods are the concomitant formation
of undesired waste, difficult work up procedures and harsh
reaction conditions, which may affect other sensitive
functional groups in the molecule. Therefore, the development
of simple and efficient catalytic deoxygenation methods is of
high interest. Unfortunately, relevant reports are limited to a
few homogeneous systems7 which are not practically useful
because of their low catalytic efficiency, the problem of
reusability and the indispensable use of vast amounts of
hazardous reductants.
Herein, we report for the first time that very small gold NPs
supported on titania (Au/TiO2-VS) can deoxygenate epoxides
exclusively into olefins using cheap and easily accessible
CO and H2O as the reductant.z Notably, the reaction
can even proceed effectively under a CO atmosphere at
sub-ambient temperatures. Compared with those conventional
catalytic deoxygenation processes,6,7 the present Au-catalyzed
CO-mediated deoxygenation system has the following
advantages: (i) unprecedented high activity under very mild
conditions; (ii) high selectivity and functional group tolerance;
(iii) the use of safe and easy-to-handle catalysts and reducing
reagents; and (iv) a simple workup procedure, namely catalyst/
product separation by filtration. The discovery of this unique
catalysis of gold NPs can open new routes to selective
functional transformations in organic synthesis.
We initially studied the deoxygenation of styrene epoxide
(1) under a mild CO atmosphere (2 atm) at room temperature
over a gold catalyst comprising very small gold NPs (ca. 1.9 nm)
Table 1 Deoxygenation of styrene epoxide to styrene with CO/H2O
using various catalysts at 25 1Ca
In recent years, supported gold nanoparticles (NPs) have been
shown to facilitate catalysis in many organic transformations8
including selective aerobic epoxidation of olefins using molecular
oxygen (O2) as the sole oxidant.9 Following the pioneering
work by Corma et al.10 that supported Au NPs are highly
selective for the reduction of nitroaromatics by molecular
hydrogen (H2) at elevated pressures, we have recently
described a more efficient gold-catalyzed approach that can
allow rapid and chemoselective reduction of polysubstituted
nitro and carbonyl compounds under very mild conditions.11
The reductant used in this case was CO and H2O, where the
transient Au0–H formed by the CO-induced H2O reduction is
believed to be the key active species for nitro reduction. An
Entry Catalyst
CO/atm Time/h Conv. (%) Sel. (%)
1
Au/TiO2-VS
2
2
2
2
2
2
2
2
2
2
2
5
1.2
15
99
96
>99
99
>99
—
—
—
—
—
>99
—
2b
3c
Au/TiO2-VS
Au/TiO2
Pt/TiO2
Pd/C
Ag/TiO2
Ir/TiO2
Ru/Al2O3
Au/Fe2O3
Au/C
1.2
1.2
1.2
1.2
1.2
1.2
1.2
1.2
1.2
1.0
0.7
1.2
53.2
trace
trace
trace
trace
trace
6.5
trace
11.4
96
4c
5c
6c
7c
8c
9c
10c
11c
12
13
14d
Au/CeO2
Au/TiO2-VS
>99
>99
>99
99
Au/TiO2-VS 10
Au/TiO2-VS
97
23.6
—
a
Reaction conditions: 1 mmol styrene epoxide, metal (0.5 mol%),
b
acetone (1 mL), H2O (0.05 mL), 25 1C. At 5 1C, Au (1 mol%).
Au/TiO2, Au/Fe2O3, Au/C provided by the World Gold Council.
Au/CeO2, Pt/TiO2, Ag/TiO2, Ir/TiO2 prepared according to ref. 10c
Shanghai Key Laboratory of Molecular Catalysis and Innovative
Materials, Department of Chemistry, Fudan University,
Shanghai 200433, P. R. China. E-mail: yongcao@fudan.edu.cn;
Fax: (+86-21) 65643774
c
d
and 11, respectively. Pd/C, Ru/Al2O3 provided by Alfa Aesar. H2
w Electronic supplementary information (ESI) available: Experimental
section, TEM and XPS data. See DOI: 10.1039/c0cc02783a
(2 atm).
c
812 Chem. Commun., 2011, 47, 812–814
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supported on TiO2 (Au/TiO2-VS, see preparation details in
ESIw). Selective deoxygenation of 1 occurred to afford styrene
(2) in 99% yield without any by-products such as ethylbenzene
and 1- or 2-phenylethanol (Table 1, entry 1). Au/TiO2-VS
successfully promoted the deoxygenation reaction even at a
sub-ambient temperature of 5 1C (Table 1, entry 2). The
reaction does not occur in the absence of the catalyst, or in
the presence of just TiO2. When using gold supported on TiO2
(Au/TiO2) supplied by the World Gold Council (WGC) as the
catalyst, which has larger average Au particles (ca. 3.5 nm,
Fig. S1 in ESIw) relative to Au/TiO2-VS, the catalytic activity
and product yield were much lower (Table 1, entry 3). The use
of other metals (Pt, Pd, Ag, Ir and Ru) instead of Au did not
yield any product (Table 1, entries 4–8). The benefit of using
TiO2 as a support for gold becomes obvious when comparing
the activity of Au/TiO2-VS and Au/TiO2 with the activity
of Au/Fe2O3, Au/C, and Au/CeO2 (Table 1, entries 9–11).
Subsequent experiments focused on investigating the effect of
pressure revealed that the reaction time was greatly shortened
from 1.2 to 0.7 h as the PCO was raised from 2 to 10 atm
(Table 1, entries 1, 12 and 13). Among the solvents tested,
acetone was the best. Changing the solvent to THF, ethyl
acetate and toluene lowered conversion of 1 to 77%, 31.5%
and 3.2% (Table S1 in ESIw), respectively.
Having the optimized conditions in hand, we examined the
scope of the deoxygenation of epoxides with Au/TiO2-VS. As
depicted in Table 2, the Au/TiO2–CO/H2O protocol was
surprisingly versatile. Various structurally diverse epoxides,
such as aromatic (Table 2, entries 1, 5–7), alicyclic (Table 2,
entries 8–10) and aliphatic (Table 2, entries 11–12) epoxides,
could be transformed into the corresponding alkenes in high
yields. Halogen-substituted styrene epoxide was cleanly
reduced to the corresponding chlorostyrene without any
dehalogenation (Table 2, entry 5). Carbonyl and hydroxy
groups also remained unaffected during deoxygenation of
the corresponding epoxides by this procedure (Table 2, entries
9 and 10). The Au/TiO2-VS catalyst was stable and could be
easily reused in the next reaction without any loss in either
activity or selectivity between the first and fifth runs (Table 2,
entries 1 and 2). The deoxygenation of epoxide using the
Au/TiO2–CO/H2O protocol was also applicable to a preparative
scale reaction (Table 2, entry 3). For example, the deoxygenation
of styrene epoxide (20 mmol) with Au/TiO2-VS (0.01 mol% Au)
gave styrene in 96% yield, wherein the turnover frequency
(TOF) was up to 400 hꢀ1 at 60 1C. This value compares
favorably with 270 hꢀ1 for the deoxygenation of 1 to afford 2
achieved at 150 1C reported recently on Au/HT under
2-propanol-driven transfer-hydrogenation conditions.12
To obtain information concerning the reaction pathways
and mechanism of the CO-mediated epoxide deoxygenation, a
couple of test reactions were undertaken. The reduction did
not occur for other unsaturated substrates, such as styrene,
acetophenone, and benzaldehyde under the conditions
described above (Table S2w), which permits a rationalization
of the unique reaction exclusivity as seen in Table 2. An almost
equimolar amount of CO2 was formed for every mole of
styrene oxide conversion, whereas no styrene was formed
when the reaction was conducted in the presence of anhydrous
acetone under a CO atmosphere. Based on the above
results, a plausible mechanism for the Au-VS/TiO2-catalyzed
Table 2 Deoxygenation of various epoxides with CO/H2Oa
Conv. Sel.
(%)
Entry Substrate
Product
Time/h (%)
1
0.7
0.7
24
99
99
96
>99
>99
>99
2b
3c
4d
1.3
97
>99
5
2
96
>99
6
7
2
2
97
97
>99
>99
8e
5
5
99
97
>99
>99
9e
10e
11e
4
96
>99
10
10
99
98
>99
>99
12e
a
Reaction conditions: substrate (1 mmol), Au/TiO2-VS (0.5 mol%
Au), acetone (1 mL), H2O (0.05 mL, 2.8 mmol), 25 1C, CO
(10 atm). Fifth run. Conditions: substrate (20 mmol), Au
b
c
(0.01 mol%), acetone (20 mL), H2O (1 mL), 60 1C, CO (20 atm).
d
e
H2O (9 mL, 0.5 mmol). Conditions: substrate (0.3 mmol), Au
(3.3 mol%).
Scheme
1 Proposed mechanism for the Au/TiO2-VS-catalyzed
deoxygenation of epoxides to alkenes by CO/H2O.
c
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Chem. Commun., 2011, 47, 812–814 813

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deoxygenation of an epoxide by CO/H2O is given in Scheme 1.
In compliance with our recent proposal for gold-catalyzed
carbonylative reduction of nitro groups, a facile CO-induced
reduction of H2O at the Au-support interface results in the
formation of transient Au0–H species, which is considered as
the key intermediate for the deoxygenation reaction
(Scheme 1). Once formed, the Au0–H species would be rapidly
consumed together with a final formation of the olefin product
without liberation of molecular hydrogen. Taking into
account this model, one may assume that the deoxygenation
of styrene epoxide can proceed with complete conversion even
under water-deficient conditions (Table 2, entry 4), owing
to the fact that water could be re-generated during the
deoxygenation process. Considering the result in Table 1 that
the deoxygenation with molecular hydrogen shows a significantly
decreased oxygen removal rate as compared to that in the CO
atmosphere (Table 1, entry 14), hydrogen gas generation
from the low temperature water gas shift (WGS) reaction
(CO + H2O = CO2 + H2) and subsequent reductive
deoxygenation of the epoxides with H2 is unlikely.
open new routes to selective functional transformations in
organic synthesis and have important implications in the
fundamental understanding of the catalytic origin of Au
nanoparticles for low temperature CO oxidation.
This work is supported by the National Natural Science
Foundation of China (20633030, 20721063, 20873026, 21073
042), National Basic Research Project of China (2009CB623
506), New Century Excellent Talents in the University of
China (NCET-09-0305), and Science & Technology Commission
of Shanghai Municipality (08DZ2270500).
Notes and references
z Note: During the course of our study, catalytic deoxygenation of
epoxides to olefins using alcohol as a reductant with gold NPs
deposited on hydrotalcite (HT) was reported by Kaneda and
co-workers.12 However, the reaction requires elevated temperature
(>100 1C) and environmentally harmful organic solvents to achieve
high yields of the products. The potential of CO/H2O as intermediates
for Au-catalyzed deoxygenation of epoxides is further demonstrated
by the same group’s work that appeared during revision of this
communication.13
It is well known that supported gold NPs have shown their
efficiency for a variety of CO-involving reactions, the most
prominent one being the low-temperature CO oxidation. The
distinguished catalytic ability of gold compared to that of
other metal NPs should be attributed to its unique capability
toward CO activation. To gain an insight into the origin of the
enhanced deoxygenation activity achieved by using titania as
support, the use of O2 as the hydrogen acceptors in place
of epoxides, namely, CO oxidation under similar reaction
conditions, has been investigated for Au NPs deposited on
different supports (Fig. S2 in ESIw). It is revealed that the CO
oxidation over Au/TiO2 occurred with much higher rates than
that over other catalysts, which implies that the CO species
adsorbed on the Au–TiO2 interface could be more active for a
relevant transformation. Moreover, a systematic study of the
influence of the size of the gold particles in Au/TiO2 shows
that gold NPs with a smaller particle size exhibited higher
intrinsic CO oxidation activity (Fig. S3 in ESIw), which
showed an excellent correlation with their catalytic activity
for epoxide deoxygenation. Therefore, in line with the broad
literature documenting the catalytic activity of supported gold
NPs, the fact that small gold NPs in combination with
the TiO2 support can substantially facilitate the crucial CO
activation in association with H2O leading to Au–H formation
appears to be a key factor for achieving high activity in the
CO-mediated epoxide deoxygenation.
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In conclusion, we have presented a novel and practical
application of TiO2 supported gold nanoparticles as excellent
and reusable catalysts for the deoxygenation of a wide range of
epoxides to the corresponding olefins using CO and H2O as
reducing agents under near ambient conditions. A preliminary
study using N-oxides and sulfoxides showed that the present
Au/TiO2–CO/H2O protocol is not limited to the deoxygenation
of epoxides (See Table S3 in ESIw). To the best of our knowledge,
this CO/H2O-mediated gold catalysis represents the most
efficient, simple, and eco-friendly catalytic system for the
selective deoxygenation of oxygenated compounds to date.
We believe that the reaction chemistry described herein can
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c
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This journal is The Royal Society of Chemistry 2011