Supported silver nanoparticle catalyst for selective hydration of nitriles to amides in water

Article abstract of DOI:10.1039/b902469g

Hydroxyapatite-supported silver nanoparticles (AgHAP) acted as a highly efficient reusable heterogeneous catalyst for hydration of diverse nitriles, including heteroaromatic ones, into amides in water.

Full text of DOI:10.1039/b902469g

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COMMUNICATION
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Supported silver nanoparticle catalyst for selective hydration of nitriles
to amides in waterw
Takato Mitsudome,a Yusuke Mikami,a Haruhiko Mori,a Shusuke Arita,a Tomoo Mizugaki,a
Koichiro Jitsukawaa and Kiyotomi Kaneda*ab
Received (in Cambridge, UK) 5th February 2009, Accepted 9th April 2009
First published as an Advance Article on the web 5th May 2009
DOI: 10.1039/b902469g
Hydroxyapatite-supported silver nanoparticles (AgHAP) acted
as a highly efficient reusable heterogeneous catalyst for hydration
of diverse nitriles, including heteroaromatic ones, into amides
in water.
appeared to date, and these hydrations often suffer from low
activities.10a,c
In the present report, it was found that hydroxyapatite
(HAP)-supported silver NPs (AgHAP)11 could catalyze
hydration of nitriles to amides in water with high efficiency.
Particularly, AgHAP showed distinguished activities for the
hydration of a wide range of heteroaromatic nitriles containing
nitrogen, oxygen and sulfur atoms. This hydration method,
using a reusable silver catalyst with water as the solvent, can
make a significant contribution not only to disclose the
catalytic potential of silver NPs but also to establish a more
environmentally-benign and industrially-acceptable process.10
AgHAP was synthesized as follows: 2.0 g of Ca5(PO4)3(OH)
(HAP) was soaked in a 150 mL aqueous solution of AgNO3
(6.7 Â 10À3 M) and stirred at room temperature for 6 h. The
obtained slurry was filtered, washed, and dried at room
temperature in vacuo. Reduction with an aqueous solution of
KBH4 yielded HAP-supported AgHAP (Ag 3.3 wt%). The
X-ray diffraction (XRD) peak positions of AgHAP were
similar to those of the parent HAP, and transmission electron
microscopy (TEM) showed that silver NPs with a mean
diameter of 7.6 nm and a narrow size distribution (standard
deviation of 1.8 nm) were formed on the surface of the HAP
support.11
Metal nanoparticles (NPs), which reside in the size range
between bulk and monomeric metal species, are applied in a
wide range of technologies from electronic, optic and magnetic
devices, to advanced catalytic materials.1 Currently, metal NP
catalysts are receiving much attention for use in organic
synthesis under liquid-phase conditions. For example, gold
NPs have been shown to facilitate catalysis in many
organic reactions.2 On the other hand, there have been
few studies on the prominent catalytic activity of silver
NPs for other organic reactions,3 except for the gas-phase
epoxidation of ethylene.4 Our group has focused on the
catalytic potential of silver NPs, and found that supported
silver NPs show high catalytic activity for the dehydro-
genation of alcohols5 and the selective oxidation of silanes
to silanols using water6 under liquid-phase conditions. We
think that the further exploration of new and intrinsic catalytic
ability of silver NPs combined with inorganic materials is a
very important research target.
Hydration of nitriles into the corresponding amides is of
great importance in organic synthesis, because amides are
versatile synthetic intermediates used in the production of
pharmacological products, polymers, detergents, lubricants
and drug stabilizers.7 However, traditional catalyst systems
have required organic solvents in the presence of homogeneous
strong acid and base catalysts, which causes over-hydrolysis of
amides into undesirable carboxylic acids,8 and the formation
of a large amount of salts after neutralization of the catalysts.
Therefore, much effort has been expended on the development
of effective homogeneous metal-catalysts for the hydration
of nitriles.9 Due to practical and environmental concerns,
current research has focused on catalysts in water under
neutral conditions.10 However, few promising methodologies
using reusable examples of excellent hydration systems have
The catalytic activity of Ag0 NPs formed on different
supports was tested for the hydration of benzonitrile (1) under
aqueous conditions without organic solvents.11,12 AgHAP was
an effective catalyst, affording benzamide as a sole product in
99% yield (Table 1, entry 1). The use of Ag/TiO2 in place of
AgHAP showed a relatively high conversion of 1; however,
benzoic acid was formed as a side product via over-hydrolysis
of benzamide. Ag/MgO, Ag/SiO2 and Ag/C were significantly
less active. The hydration reaction did not proceed using Ag0
powder (average particle size = 106 mm), HAP and Ag+HAP
without a reduction treatment (Table 1Sw). After filtration of
the reaction mixture containing AgHAP at a 40% conversion
of 1, further stirring of the filtrate at 140 1C for 3 h did not
yield any additional products, and no silver species was
detected in the filtrate by inductively coupled plasma spectro-
scopy (ICP) analysis. These results show that the combination
of Ag0 NPs with HAP is essential for efficient hydration, and
the hydration proceeds at the silver NPs on the surface
of HAP.
a Department of Materials Engineering Science, Graduate School of
Engineering Science, Osaka University, 1-3, Machikaneyama,
Toyonaka, Osaka 560-8531, Japan.
E-mail: kaneda@cheng.es.osaka-u.ac.jp; Fax: +81-6-6850-6260;
Tel: +81-6-6-6850-6260
b Research Center for Solar Energy Chemistry Osaka University 1-3,
Machikaneyama, Toyonaka, Osaka 560-8531, Japan
w Electronic supplementary information (ESI) available: NMR data,
spectroscopic characterization, IR spectra, TEM image, kinetic data.
See DOI: 10.1039/b902469g
The scope of nitrile reactants for AgHAP-catalyzed
hydration was surveyed. As exemplified in Table 1, AgHAP
was efficient for the hydration of nitriles, except for aliphatic
ones (entries 14 and 15). Various benzonitrile derivatives
ꢀc
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Table 1 Scope of nitriles for AgHAP-catalyzed hydrationa
and no accompanying carboxylic acids were detected. For
example, hydration of 2-cyanopyridine, 2-furancarbonitrile,
and 2-thiophenecarbonitrile afforded the corresponding
amides in quantitative yields (entries 1, 5, and 7). Even a very
water-insoluble nitrile, such as 3-quinolinecarbonitrile, was
also hydrated to 3-quinolinecarboxamide in a 95% yield
(entry 4). It is notable that pyrazinecarbonitrile (2) was
hydrated within only 10 min, and the corresponding pyrazine-
carboxamide, which is used as a medicine for tuberculosis, was
obtained in 99% yield (entry 8), moreover, 2 was converted
quantitatively even at 40 1C (entry 9). The AgHAP catalyst
system was also applicable for scaled-up conditions; 2
(100 mmol; 10.5 g) was successfully converted to the amide
(97% isolated yield; 12.0 g) and the turnover number (TON)
reached over 10 000 (entry 10), and this value is at least two
orders of magnitude higher than those previously reported
heterogeneously catalyzed hydrations.10a–c To the best of our
knowledge, such specifically enhanced reactivity of hetero-
aromatic nitriles compared with other nitriles has not been
reported.13,14 Furthermore, AgHAP was easily separated by
centrifugation after hydration of 2, and could be reused four
times for the hydration of 2 without loss of its catalytic activity
and selectivity (entries 11–14).
Entry Nitrile
1
Amide
T/1C t/h Yieldb (%)
140
3
99 (94)
2
3
4
180
160
140
6
2
2
99 (94) (o)
99 (94) (m)
99 (96) (p)
5
6
7
8
140
140
140
160
2
1
1
2
99 (95)
99 (95)
99 (97)
97 (92)
Interactions between the AgHAP surface and nitriles were
examined using Fourier transform infrared (FTIR) spectro-
scopy. 1, 2-cyanopyridine (3) and hexanenitrile (4) were
treated with AgHAP, respectively, and each absorption
band assigned to a CRN stretching vibration of the adsorbed
nitriles was shifted to higher frequencies with respect to
their liquid forms,11 which indicates side-on coordination of
the nitrile groups on silver NPs.15 Furthermore, the nitriles
adsorbed onto AgHAP were also exposed to water vapor
at 298 K. Time-resolved IR spectra showed that the nitrile
band of 3 gradually decreased in intensity with the increase of
a new band indicating CQO stretching vibration (Fig. 5Sw).11
The production of amide was also confirmed by mass
spectrometry, while the intensity of the nitrile IR band of 1
slightly decreased and that of 4 was hardly changed.11
The order of reactivity of the adsorbed nitriles with water
vapor is 3 4 1 4 4, which is consistent with the results
of catalytic hydration of the nitriles using AgHAP, as shown
in Tables 1 and 2.
9
10
11
180
160
160
6
2
2
47 (o)
99 (96) (m)
96 (92) (p)
12
140
180
6
6
98 (93)
84 (79)
13
14
180
180
6
6
2
2
15
a
Reaction conditions: nitrile (1 mmol), AgHAP (0.1 g, Ag:
b
0.03 mmol), water (3 mL). The values in parenthesis are isolated yields.
were hydrated in high yields with over 99% selectivity for
the corresponding amides (entries 1–12). Interestingly, the
substituents bearing electron-withdrawing groups were more
reactive than that bearing electron donating groups, which is
in sharp contrast with the previously reported reactivities
using metal complex catalysts.10b The steric effect of
ortho-substituted nitriles on the reaction rates was observed
(entries 2 and 9). The hydration of cinnamonitrile proceeded
to afford cinnamamide with an intact CQC double bond
(entry 13).
A possible mechanism involving the coordination of water
and an aromatic nitrile on the silver NP surface is proposed.
Aromatic nitriles are strongly activated on silver NPs of
AgHAP through the dual activation of cyano and aromatic
groups. Subsequently a nucleophilic OHÀ from H2O, which is
generated on the silver surface,16 easily attacks the proximal
nitrile carbon atom to form the corresponding amide through
an iminol transition state.17
In conclusion, novel catalytic properties of silver NPs for
selective hydration of nitriles into amides were discovered.
HAP-supported silver NPs act as a highly active and reusable
solid catalyst for the hydration of aromatic nitriles especially
heteroaromatic nitriles in water.
Usually, the hydration of heteroaromatic nitriles is more
difficult and the reaction rates are much lower than those of
common nitriles because of their strong coordination to the
metal centers. The hydration of various heteroaromatic nitriles
was next tried using the AgHAP catalyst, as summarized in
Table 2. Remarkably, many of the heteroaromatic nitriles
containing nitrogen, oxygen, and sulfur atoms were effectively
converted into the corresponding amides within only 1 h,
This work was supported by Grant-in-Aid for Scientific
Research on Priority Areas (No.18065016, ‘‘Chemistry of
Concerto Catalysis’’) from Ministry of Education, Culture,
Sports, Science and Technology, Japan. A part of the present
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Table 2 Hydration of various heteroaromatic nitriles using AgHAPa
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New York, 1991; Ullmann’s Encyclopedia of Industrial Chemistry,
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5 T. Mitsudome, Y. Mikami, H. Funai, T. Mizugaki, K. Jitsukawa
and K. Kaneda, Angew. Chem., Int. Ed., 2008, 47, 138.
Entry
1
Nitrile
Amide
t/min
Yieldb (%)
99 (94)
15
2
3
4
5
6
30
20
60
10
30
95 (93)
98 (94)
95 (91)
99 (95)
99 (95)
6 T. Mitsudome, S. Arita, H. Mori, T. Mizugaki, K. Jitsukawa and
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8 J. C. Westfahl and T. L. Gresham, J. Am. Chem. Soc., 1955, 77,
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D. Barton and W. D. Ollis, Pergamon, Oxford, 1979, vol. 2,
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Interscience, New York, 1992, p. 383.
9 Selected examples for hydration of nitriles using homogeneous
metal catalysts: (a) S.-I. Murahashi, S. Sasao, E. Saito and
T. Naota, Tetrahedron, 1993, 49, 8805; (b) T. Ghaffar and
A. W. Parkins, J. Mol. Catal. A: Chem., 2000, 160, 249;
(c) W. K. Fung, X. Huang, M. L. Man, S. M. Ng, M. Y. Hung,
Z. Lin and C. P. Lau, J. Am. Chem. Soc., 2003, 125, 11539;
(d) T. Oshiki, H. Yamashita, K. Sawada, M. Utsunomiya,
K. Takahashi and K. Takai, Organometallics, 2005, 24, 6287;
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7
8
20
10
98 (96)
99 (96)
9c
10d
11e
12f
13g
14h
2880
2880
10
10
10
94
99 (97)
99
99
99
99
10 Solid catalysts using water as
a solvent; (a) K. Mori,
K. Yamaguchi, T. Mizugaki, K. Ebitani and K. Kaneda, Chem.
Commun., 2001, 461; (b) K. Yamaguchi, M. Matsushita and
N. Mizuno, Angew. Chem., Int. Ed., 2004, 43, 1576;
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S. M. Pillai, V. K. Kaushik and M. Ravindranathan, Appl. Catal.,
A, 2005, 290, 175; (d) F. Bazi, H. EI Badaoui, S. Tamani, S. Sokori,
A. Solhy, D. J. Macquarrie and S. Sebti, Appl. Catal., A, 2006, 301,
211.
10
a
Reaction conditions: nitrile (1 mmol), AgHAP (0.1 g, Ag:
b
0.03 mmol), water (3 mL), 140 1C. The values in parenthesis are
c
d
isolated yields. Nitrile (0.5 mmol), 40 1C. Nitrile (100 mmol),
AgHAP (0.03 g, Ag: 0.009 mmol), water (35 mL). 1st reuse. 2nd
h
reuse. 3rd reuse. 4th reuse.
e
f
g
11 See the ESIw.
12 The use of organic solvents in the presence of AgHAP gave 2 in low
yield. The yields of 2 were as followed: THF, 2%; 1,4-dioxane, 4%;
DME, 11%; EtOH, 5%.
13 Same or slightly lower reactivities for heteroamoratic nitriles
than benzonitrile were reported in other papers. See: ref. 9b,d,f,
10a, and b.
experiments were carried out by using a facility in the
Research Center for Ultrahigh Voltage Electron Microscopy,
Osaka University. We thank Professor Tsunehiro Tanaka and
Professor Tetsuya Shishido for support of IR study.
14 This is the first report of the hydration of pyrazinecarbonitrile to
pyrazinecarboxamide.
15 B. N. Storhoff and H. C. Lewis, Jr, Coord. Chem. Rev., 1977,
23, 1.
Notes and references
16 Microwave dielectric studies on the dynamics of water provided
information for the strong interaction of H2O with the surface of
the silver NPs to generate a nucleophilic OHÀ species. See the ESI
(Fig. 4S)w.
17 The effect of the substituent on reactivity; a r value of 1.69 in
a Hammett plot with s+ value supports the formation of a
negatively charged transition state. See the ESI (Fig. 9S)w.
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ꢀc
This journal is The Royal Society of Chemistry 2009
3260 | Chem. Commun., 2009, 3258–3260