Remarkable effect of bases on core-shell AgNP@CeO2 nanocomposite-catalyzed highly chemoselective reduction of unsaturated aldehydes

Article abstract of DOI:10.1246/cl.130132

A highly dispersed coreshell silver nanoparticleceria nanocomposite catalyst (AgNP@CeO₂-D) was prepared. The addition of bases was found to enhance the catalytic efficiency of AgNP@CeO₂-D significantly in the chemoselective reduction of diverse unsaturated aldehydes to the corresponding unsaturated alcohols.

Full text of DOI:10.1246/cl.130132

doi:10.1246/cl.130132
660
Published on the web May 24, 2013
Remarkable Effect of Bases on Core-Shell AgNP@CeO2 Nanocomposite-catalyzed
Highly Chemoselective Reduction of Unsaturated Aldehydes
Takato Mitsudome,1 Motoshi Matoba,1 Masaaki Yamamoto,1 Tomoo Mizugaki,1 Koichiro Jitsukawa,1 and Kiyotomi Kaneda*1,2
1Graduate School of Engineering Science, Osaka University, 1-3 Machikaneyama, Toyonaka, Osaka 560-8531
2Research Center for Solar Energy Chemistry, Osaka University, 1-3 Machikaneyama, Toyonaka, Osaka 560-8531
(Received February 18, 2013; CL-130132; E-mail: kaneda@cheng.es.osaka-u.ac.jp)
NO2
NH2
R2
A highly dispersed core-shell silver nanoparticle-ceria
nanocomposite catalyst (AgNP@CeO2-D) was prepared. The
addition of bases was found to enhance the catalytic efficiency
of AgNP@CeO2-D significantly in the chemoselective reduction
of diverse unsaturated aldehydes to the corresponding unsatu-
rated alcohols.
AgNP
CeO2
R
O
R
CeO2
AgNP@CeO2-D
AgNP@CeO2
or
R2
R1
R1
H2
R
O
R
OH
The selective reduction of a targeted functional group in a
molecule having several functional groups is of great utility in
organic synthesis.1 In this context, the chemoselective reduction
of unsaturated carbonyl compounds while maintaining the
reducible C=C bonds is an important and challenging objective,
because the corresponding unsaturated alcohols often serve as
valuable intermediates for fragrances, pharmaceuticals, agri-
chemicals, and resins.2 Chemoselective reduction has been
performed by using stoichiometric reagents such as metal
borohydrides,3 diisobutylaluminum hydride,4 and zinc metal,5
which produce large amounts of waste. For the replacement of
such traditional stoichiometric reactions, much effort has been
devoted to the development of catalytic systems.6 Among these
catalytic methods, heterogeneous catalyst systems using mo-
lecular hydrogen (H2) as a highly atom-efficient reductant
represent ideal green protocols. However, the heterogeneous
catalysts reported previously still suffer from insufficient
selectivity at high conversions, limited substrate scope, and
low reusability.7,8
Recently, we reported the synthesis of a core-shell silver
nanoparticle-ceria nanocomposite (AgNP@CeO2) consisting of
AgNPs with diameters of 10 nm in the core and assembled CeO2
NPs with diameters of 3-5 nm in the shell. AgNP@CeO2 acted
as a highly efficient reusable catalyst for the completely
chemoselective reductions of nitrostyrenes to aminostyrenes
and epoxides to alkenes using H2, where over 99% selectivity
for the targeted products was attained.9 AgNP@CeO2 highly
dispersed on CeO2 (AgNP@CeO2-D) was also developed and
was found to be applicable to the selective reduction of
unsaturated aldehydes to the corresponding unsaturated alcohols
(Scheme 1).10 The obtained selectivity for unsaturated alcohols
was much greater than those of previously reported heteroge-
neous catalyst systems. In these reactions, the combination of
AgNPs and a base of CeO2 is the key. Wrapping AgNPs with
CeO2 can maximize the interfacial interaction between the active
AgNP species and the basic sites of CeO2. The core-shell
interface induces the heterolytic cleavage of H2 to produce an
ionic Ag-hydride species and a proton associated with the basic
site of CeO2, which enables the selective reduction of polar
functional groups. In a continuing attempt to achieve higher
chemoselectivity using AgNP@CeO2-D, we discovered that the
addition of bases such as alkali metal salts and amines to the
Scheme 1. Chemoselective reductions with AgNP@CeO2 or
AgNP@CeO2-D.
AgNP@CeO2-D catalyst system enhanced significantly both the
catalytic activity and selectivity for the reduction of unsaturated
aldehydes to the unsaturated alcohols.
Inspired by our previous finding that bases exhibited
cooperative catalysis with AgNPs in chemoselective reduc-
tions,9-13 we initially focused on various bases as additives for
the AgNP@CeO2-D-catalyzed reduction of cinnamaldehyde (1)
to cinnamyl alcohol (2) under 6 atm of H2 at 110 °C for 6 h.14
The results are summarized in Table 1. Interestingly, among the
bases tested, the addition of Cs2CO3 significantly enhanced the
catalytic efficiency to afford over 99% selectivity for 2 with full
conversion (Entry 1 vs. 11). Rb2CO3 and triethylamine also had
a positive effect on the reduction (Entries 6 and 7), and the
weaker bases of Na2CO3, K2CO3, pyridine, and imidazole
improved the selectivity slightly (Entries 4, 5, 9, and 10).
Notably, when the reaction time of Entry 1 was prolonged,
hydrogenation of the olefin moiety of 2 was not observed at all
(Entry 2). Furthermore, even at 60 °C, AgNP@CeO2-D worked
well in the presence of Cs2CO3, giving 2 in 96% yield with
excellent selectivity, while AgNP@CeO2-D without Cs2CO3
hardly promoted the reduction (Entry 3 vs. 12). Next, non-
encapsulated AgNPs on CeO2 (AgNP/CeO2), which had a
similar AgNP size to that of AgNP@CeO2-D, were synthe-
sized,15 and the effect of Cs2CO3 was investigated. The addition
of Cs2CO3 also improved the AgNP/CeO2 catalysis (Entry 13
vs. 14); however, the catalytic performance of AgNP/CeO2 was
inferior to that of AgNP@CeO2-D (Entry 1 vs. 13). These
results revealed that the combination of AgNP@CeO2-D and
Cs2CO3 provided an extremely high efficiency for the chemo-
selective reduction of 1 to 2 with H2.
The substrate scope of the catalytic system consisting of
AgNP@CeO2-D and Cs2CO3 in the chemoselective reduction
of aldehydes was investigated next (Table 2). For all the
aldehydes tested, Cs2CO3 enhanced the activity and selectivity
of AgNP@CeO2-D significantly, affording excellent selectivity
for unsaturated alcohols with high conversions. Cinnamalde-
hydes, which have electron-withdrawing and -donating groups,
were converted efficiently to cinnamyl alcohols (Entries 3
and 4). p-Chlorocinnamyl alcohol was obtained with over 99%
Chem. Lett. 2013, 42, 660-662
© 2013 The Chemical Society of Japan
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661
Table 1. Chemoselective reduction of 1 using AgNP@CeO2-D
Table 2. Chemoselective hydrogenations of unsaturated alde-
hydes using AgNP@CeO2-D with Cs2CO3
a
with basesa
AgNP@CeO2-D
Entry
Product
t/h
Conv./%b
>99
Sel./%b
OH
O
CeO2
H2
O
OH
CH2OH
+
+
1
6
>99 (92)
1
2
3
4
Sel./%b
CH2OH
Entry Base
t/h Conv./%b
2c
3
6
>99
>99
>99
93
2
3
4
1
2
3c
4
5
6
7
8
9
10
11
12c
13d
14d
Cs2CO3
Cs2CO3
Cs2CO3
Na2CO3
K2CO3
Rb2CO3
triethylamine
6
12
24
6
>99
>99
96
>99 trace trace
>99 trace trace
>99 trace trace
CH2OH
10
MeO
Cl
58
94
95
99
5
2
1
1
3
trace
CH2OH
CH2OH
6
56
4
5
3
6
>99
>99
>99
6
78
6
67
>99 trace trace
>99 trace trace
triethylamine 12
>99
38
25
>99 (96)
pyridine
imidazole
®
6
6
93
95
92
2
2
2
5
3
6
6d
7d
OH
OH
8
8
>99
98
98
6
43
®
Cs2CO3
®
24
6
1
96
trace trace trace
96 (91)
94
73
2
16
4
11
8d
9d
8
8
8
97
98
95
97
97
6
38
OH
aReaction conditions: 1 (0.25 mmol), AgNP@CeO2-D (Ag:
6 mol %), THF (5 mL), base (1 mmol), H2 (6 atm), 110 °C.
OH
OH
c
bDetermined by GC using an internal standard. H2 (30 atm),
10d
>99
60 °C. dAgNP/CeO2 (Ag: 6 mol %) was used in place of
AgNP@CeO2-D.
11d
12
8
8
>99
>99
95
97
OH
CH2OH
selectivity, and the chloro moiety as well as the C=C bond
remained intact during the reduction (Entry 4). Aliphatic
conjugated aldehydes including terpenes could also be used in
this catalyst system (Entries 6-14). For example, citral gave the
corresponding allylic alcohols in quantitative yield (Entry 13).
Moreover, unconjugated aldehydes were also found to be good
substrates (Entries 15-18). Vinylbenzaldehyde was reduced
chemoselectively to vinylbenzyl alcohol with complete retention
of the olefinic bond (Entry 15).16
OH
13e
14
6
>99
>99
>99 (94)
98
CH2OH
18
After the reduction of 1, AgNP@CeO2-D was readily
separated from the reaction mixture by filtration, and then, the
used AgNP@CeO2-D catalyst and Cs2CO3 were applied to the
next run. AgNP@CeO2 could be reused without loss of activity
or chemoselectivity in the reuse experiments (Entry 2).
OH
OH
15
6
>99
>99
>99 (93)
96
16d
12
The dramatic effect of the base additives on both the activity
and selectivity of AgNP@CeO2-D may be due to an increase in
the exclusive formation of the ionic hydrogen species. That is,
base additives promote the heterolytic cleavage of H2 to [Ag-
OH
17e
6
>99
>99
¹
H] and [H-base]+ and also assist in the cooperative action
¹
between AgNP and CeO2 to generate [Ag-H] and [CeO2-H-
OH
18
6
>99
98
base]+. These polar hydrogen species prefer polar aldehyde
groups over C=C bonds, enhancing the activity and chemo-
selectivity in the reduction of unsaturated aldehydes to unsat-
urated alcohols.
aReaction conditions: Substrate (0.25 mmol), AgNP@CeO2-D
(Ag: 6 mol %), THF (5 mL), Cs2CO3 (1 mmol), H2 (6 atm),
110 °C. Determined by GC using an internal standard. Values
in parentheses are isolated yields. For the isolation experiments,
the reactions were carried out under the following conditions.
Substrate (10 mmol), AgNP@CeO2-D (Ag: 6 mol %), THF
(25 mL), Cs2CO3 (5 mmol). c2nd reuse. dH2 (15 atm), 60 °C.
eAgNPs@CeO2-D (Ag: 3 mol %), H2 (15 atm), 150 °C.
b
In conclusion, we discovered that the addition of bases
greatly enhanced the catalytic efficiency in AgNP-catalyzed
chemoselective reduction reactions. The catalyst system con-
sisting of AgNP@CeO2-D and Cs2CO3 provided highly chemo-
selective reduction of diverse unsaturated aldehydes to the
corresponding unsaturated alcohols with excellent selectivity.
Chem. Lett. 2013, 42, 660-662
© 2013 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

662
Furthermore, the AgNP@CeO2-D catalyst was separable and
reusable without loss of efficiency.
Vankelecom, D. E. De Vos, Appl. Catal., A 2009, 355, 176.
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This work was supported by JSPS KAKENHI Grant
Numbers 23686116, 23360357, 24246129, and 22360339.
T. M. thanks the Japan Association for Chemical Innovation
and JGC-S Scholarship Foundation for funding.
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Chem. Lett. 2013, 42, 660-662
© 2013 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/