Efficient and substrate-specific hydration of nitriles to amides in water by using a CeO2 catalyst

Article abstract of DOI:10.1002/chem.201101576

CeO₂ acted as a reusable and effective catalyst for the hydration of various nitriles to amides in water under neutral conditions at low temperature (30-100 °C). CeO₂ showed notable substrate specificity for nitriles that have a heteroatom adjacent to the α-carbon atom of the CN group (see scheme).

Full text of DOI:10.1002/chem.201101576

DOI: 10.1002/chem.201101576
Efficient and Substrate-Specific Hydration of Nitriles to Amides in Water by
Using a CeO2 Catalyst
Masazumi Tamura,[a] Hiroko Wakasugi,[a] Ken-ichi Shimizu,*[b] and Atsushi Satsuma[a]
CeO2 has been reported to catalyze various organic reac-
tions at high temperature (150–4008C).[1–15] In view of organ-
ic synthesis, it is important to study CeO2-catalyzed reac-
tions at lower temperature. Hydration of nitriles to the cor-
responding amides is an important transformation from
both organic chemistry and industrial points of view. Con-
ventional methods of nitrile hydration with strong acid or
base catalysts have drawbacks such as over hydrolysis of
amides into carboxylic acids and the formation of salts after
neutralization of the catalysts.[16] Various catalytic methods
using homogeneous metal catalysts under neutral conditions
have been reported.[17] However, they have disadvantages
including difficulty in catalyst/product separation, use of or-
ganic solvent, high price, and low activity for hydration of
heteroaromatic nitriles because of their strong coordination
to the metal centers. Use of a recyclable heterogeneous cat-
alyst in water is an ideal system from environmental and
practical viewpoints. However, recent successful systems
suffer from high reaction temperature and use of expensive
noble metals.[18]
tionalized mesoporous SiO2,[24] and metal complexes on mo-
lecular-imprinted SiO2.[25] Knowing that these systems are
driven by repulsive or attractive forces in small pores, we
expected that development of a nonporous inorganic solid
for substrate-specific catalysis opens a possibility of design-
ing new catalysts for organic synthesis. Here we report that
CeO2, an inexpensive metal oxide, acts as a highly active
and reusable catalyst for hydration of nitriles in water,
under neutral conditions at low temperature (30–1008C). In-
terestingly, CeO2 as a nonporous inorganic solid catalyzes
the highly substrate-specific hydration of nitriles. Combined
with kinetic results, a reaction mechanism of the substrate-
specific hydration of nitriles by CeO2 is discussed.
First, the catalytic activity for the hydration of 2-cyano-
pyridine to 2-picolinamide in water at 608C was studied
with various metal oxides (Table 1). Among 16 kinds of
metal oxides, CeO2, MnO2, TiO2, CaO, Y2O3, La2O3, and
ZrO2 produced the amide, and CeO2 showed more than two
Table 1. Hydration of 2-cyanopyridine by various metal oxides.[a]
A satisfactory level of substrate specificity is one of the
most important goals in the design of synthetic catalysis, but
it is still a challenging goal. Based on the fact that enzymes
show excellent substrate specificity, an enzyme-mimetic ap-
proach has been adapted by chemists to improve substrate
specificity in artificial catalysis, employing molecular recog-
nition binding sites of the organic components for selective
binding of a substrate molecule.[19] Unlike these organically
modified molecular catalysts, successful examples of un-
modified inorganic catalysts, such as metal oxides, are still
very rare, except for the shape-selective catalysis of zeo-
lites,[20] microporous heteropolyacids,[21] TiO2-incorporated
porous SiO2,[22] microporous titanosilicate,[23] organic-func-
[b]
[c]
Catalyst
CeO2
CeO2
TiO2
t [h]
0.05
0.17
2
2
18
18
18
18
18
18
18
18
18
18
18
18
18
18
SBET [m2 gÀ1
]
Yield [%]
V [mmolgÀ1 hÀ1
]
81
27
47
45
59
69
18
72
300
28
53
124
19
54
9
34
43
23
22
21
19
12
7
0
0
0
0
0
0
0
0
0
0
261.3
81.5
4.9
4.0
0.4
0.4
0.2
0.1
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
[d]
MnO2
CaO
Y2O3
La2O3
ZrO2
SiO2
SnO2
Fe2O3
Al2O3
MgO
Nb2O5
Er2O3
Dy2O3
Pr6O11
[a] M. Tamura, H. Wakasugi, Prof. A. Satsuma
Department of Molecular Design and Engineering
Graduate School of Engineering, Nagoya University
Nagoya 464-8603 (Japan)
51
34
–
[e]
Ce
N
[b] Dr. K.-i. Shimizu
Catalysis Research Center
Hokkaido University, N-21, W-10
Sapporo 001-0021 (Japan)
Fax : (+81)11-706-9163
[a] Reaction conditions: 2-cyanopyridine (1.0 mmol), H2O (3.0 g), metal
oxide (30 mg). Yield of 2-picolinamide was determined by GC. [b] Sur-
face area of the sample estimated by N2 adsorption at À1968C (BET
method). [c] Initial rate of the formation of 2-cyanopyridine measured
under the conditions in which conversions were below 50%. [d] CeO2
(JRC-CEO3) calcinated in air at 10008C for 1 h. [e] Result of a soluble
E-mail: kshimizu@cat.hokkai.ac.jp
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201101576.
Ce catalyst, CeACTHNUTRGNE(UNG NO3)3·6H2O (63.5 mg).
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Chem. Eur. J. 2011, 17, 11428 – 11431

COMMUNICATION
orders of magnitude higher activity than the other metal
oxides. Note that the reaction rate per surface area for stan-
dard CeO2 (3.2 mmolmÀ2 hÀ1) was nearly the same as that
for a low surface area CeO2, prepared by calcination of the
standard CeO2 at 10008C (3.0 mmolmÀ2 hÀ1). CeO2 from a
different source (Rhodia) showed nearly the same activity
(not shown), which indicated that the activity did not
depend on the type of ceria. Other metal oxides were totally
inactive. An aqueous solution of cerium nitrate did not
show any activity, which indicated that soluble Ce species
were inactive.
This is one of the rare examples of heterogeneously cata-
lyzed hydration of nitriles at room temperature.[27] The reac-
tion at 608C finished in 15 min (Table 2, entry 2). 2-Cyano-
pyridine derivatives (Table 2, entries 4, 6, and 8), 2-furancar-
bonitrile, and methoxyacetonitrile (entries 10 and 11) were
effectively hydrated to the corresponding amides in water at
808C. Substituent effects could not be quantitatively esti-
mated in H2O because of the low solubility of 2-cyanopyri-
dine derivatives. To study the substituent effects, hydration
of 2-cyanopyridine derivatives (Table 2, entries 3, 5, 7, and
9) were tested under the water/ethanol 1:1 solution condi-
tion in which all derivatives could be dissolved completely.
Substituents with electron-withdrawing groups were more
reactive than those with electron-donating groups. It is nota-
ble that pyrazinecarbonitrile was hydrated at 308C within
1 h, and pyrazinecarboxamide (a medicine for tuberculosis)
was obtained in 98% yield (Table 2, entry 12). A gram-scale
experiment of this reaction (Scheme 1) was carried out on
pyrazinecarbonitrile (3.15 g, 30 mmol) in water (10 g) with
CeO2 (30 mg, Ce=0.58 mol%) at 1008C for 1 h to give
>99% yield of the amide and high reaction rate
(990 mmolgÀ1 hÀ1). To the best of our knowledge, these
values are highest among the previous systems reported, in-
cluding enzymes (ꢀ464 mmolgÀ1 hÀ1)[26] and a heterogene-
ous catalyst (69 mmolgÀ1 hÀ1).[18e] Based on the number of
Ce cations on the surface of CeO2 (1.067 mmolgÀ1),[28] CeO2
showed a turnover number (TON) of 928 and a turnover
frequency (TOF) of 928 hÀ1. The catalyst was easily re-
trieved from the reaction mixture by centrifugation. After
washing with water, followed by drying in air at 1008C
(10 min), the recovered catalyst was reused at least three
times without marked loss of its catalytic activity (see Fig-
ure S1 in the Supporting Information).
Among various solvents, H2O gave the highest activity
(see Table S1 in the Supporting Information). Next, the
scope of nitriles for CeO2-catalyzed hydration was examined
(Table 2). Hydration of 2-cyanopyridine at 308C for 12 h
gave 2-picolinamide quantitatively (Table 2, entry 1). After
the reaction, the solution was evaporated under vacuum,
and the obtained solid consisted of 2-picolinamide without
1
any byproducts as confirmed by GC and H NMR analyses.
Table 2. Hydration of various nitriles under various conditions by using
CeO2.[a]
Nitrile
Solvent
T [oC]
t [h]
Yield [%]
1
H2O
H2O
H2O/EtOH
1:1
30
60
60
12
0.25
12
>99
93
98
(>99[b]
ACHTUNGTERNNUNG )
2[c]
3
4
5
H2O
H2O/EtOH
1:1
80
60
6
12
>99
90
6
7
H2O
H2O/EtOH
1:1
80
60
6
12
97
>99
8
9
H2O
H2O/EtOH
1:1
80
60
6
12
95
>99
10
H2O
H2O
80
80
24
24
90
77
Scheme 1. Scaleup reaction of the selective hydration of pyrazinecarboni-
trile by CeO2
11[d]
12
H2O
30
1
98
The results in Table 2 clearly show that a slight difference
of a molecular structure dramatically changes the reactivity
of nitriles. Highly reactive nitrile compounds shown in the
above section have a common structural feature; a heteroa-
tom (N or O) is located adjacent to the a carbon of the CN
group (Table 2, entries 1–12). In contrast, heteroaromatic ni-
triles with a heteroatom adjacent to the b or g carbon of the
CN group (3-cyanopyridine and 4-cyanopyridine) showed
low reactivity (Table 2, entries 15 and 16). Nitriles without
heteroatoms (benzonitirle and valeronitirle) were complete-
ly inactive (Table 2, entries 13 and 14). These results suggest
that the presence of heteroatoms adjacent to the a carbon
of the CN group is indispensable for the progress of the re-
action. This hypothesis is verified by comparing the reactivi-
13
14
H2O
H2O
100
100
24
24
0
0
15
H2O
100
24
5
16
17
H2O
H2O
100
100
24
24
4
0
[a] Reaction conditions: nitrile (2.0 mmol), CeO2 (30 mg), solvent (3.0 g).
Yield of amide was determined by GC. [b] Yield of the isolated product.
[c] Reaction conditions: nitrile (1.0 mmol), CeO2 (30 mg), H2O (3.0 g).
[d] CeO2 calcinated at 6008C was used.
Chem. Eur. J. 2011, 17, 11428 – 11431
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K.-i. Shimizu et al.
ty of methoxyacetonitrile (Table 2, entry 11) and methoxy-
propionitrile (Table 2, entry 17); a linear nitrile with a
heteroACHTUNGTRENNUNGatom adjacent to the b carbon of the CN group
hydration involves a negative charge at the a-carbon atom
adjacent to the pyridine ring. Taking the Michaelis–Menten-
type kinetics (see Figure S3 in the Supporting Information)
and the absence of the H2O/D2O kinetic isotopic effects into
account, the positive 1 value indicates that nucleophilic ad-
dition of OHdÀ on CeO2 to a nitrile carbon atom through a
negatively charged transition state is a rate-determining
step. Based on the above-mentioned mechanistic results, a
possible reaction mechanism of nitrile hydration by CeO2 is
shown in Scheme 2. The catalytic cycle starts with dissocia-
tion of H2O on CeO2 to give Hd+ and OHdÀ (Step 1). Nitrile
and CeO2 are in equilibrium with the nitrile–CeO2 adsorp-
tion complex (Step 2). This nitrile–CeO2 complex will under-
go an addition of OHdÀ to a nitrile carbon atom to give the
amide, accompanied by a regeneration of the adsorption site
on CeO2 (Step 3). Step 3 is the rate-determining step.
(Table 2, entry 17) was completely inactive, whereas that
with a heteroatom adjacent to the a carbon of the CN
group (Table 2, entry 11) was hydrated to the corresponding
amides with good yield (77%). Taking all results into ac-
count, we concluded that CeO2 acts as a highly substrate-
specific catalyst for the hydration of nitriles; the catalyst is
effective only for the nitriles with a heteroatom (N or O)
adjacent to the a carbon of the CN group. To the best of
our knowledge, our results provide the first example of the
highly substrate-specific catalysis by an unmodified and non-
porous metal oxide surface. To discuss the origin of this sub-
strate specificity, Arrhenius parameters for CeO2-catalyzed
reactions of 2-cyanopyridine and 4-cyanopyridine were com-
pared (see Figure S2 in the Supporting Information). These
activation energies for 2-cyanopyridine (81.7 kJmolÀ1) and
4-cyanopyridine (80.7 kJmolÀ1) were identical within the ex-
perimental error, which indicated the same reaction mecha-
nism for these nitriles. On the other hand, surprisingly, the
frequency factor A for 2-cyanopyridine (lnA=31.7) was five
orders of magnitude larger than that for 4-cyanopyridine
(lnA=19.3), which indicated that the observed difference in
the reactivity of 2-cyanopyridine and 4-cyanopyridine is ex-
plained by a larger frequency factor of 2-cyanopyridine.
To discuss the mechanism, we carried out kinetic studies.
First, the dependence of the reaction rate on the 2-cyanopyr-
idine concentration was examined (see Figure S3 in the Sup-
porting Information). Initially, the rate of amide formation
linearly increases with an increase in the concentration of
the nitrile (0.1–0.4m) and then levels off at higher concen-
tration. A good linear correlation is observed in a Linewea-
ver–Burk plot, indicating that the reaction follows Michae-
lis–Menten-type kinetics. This suggests that the free nitrile
and an adsorption site on CeO2 are in equilibrium with the
nitrile–CeO2 adsorption complex as a reaction intermediate,
which is then irreversibly converted to give the product ac-
companied by regeneration of the adsorption site on CeO2,
and the latter step is the rate-determining step. The reaction
in H2O or D2O at 308C showed the absence of the kinetic
isotopic effect (kH/kD =0.95), which indicated that H2O dis-
sociation is fast on CeO2 and is not involved in the rate-de-
termining step. Considering the ionic nature of CeO2, one
assumes that H2O is highly polarized on CeO2 to yield Hd+
and OHdÀ adspecies. Note that the reaction rate at 308C did
not depend on the O2 concentration in the reaction atmos-
phere; the rates under O2 (18.7 mmolgÀ1 hÀ1), under air
(20.1 mmolgÀ1 hÀ1), and under N2 (18.8 mmolgÀ1 hÀ1) were
nearly constant. This indicates that O2 is not involved in the
catalytic cycle. Using the results in Table 2 (entries 3, 5, 7,
and 9), we examined the relationship between the relative
rates and the Hammett parameter (s) for the hydration of
2-cyanopyridines (see Figure S4 in the Supporting Informa-
Scheme 2. Proposed mechanism for hydration of nitriles with CeO2.
In Scheme 2, we assume that in the 2-cyanopyridine–
CeO2 adsorption complex the heteroatom (N) adjacent to
the a carbon of the CN group is coordinated to the surface
Ce site. This adsorption model is reasonable from the fol-
lowing discussions. A pyridine adsorption IR experiment[29]
showed bands at 1440, 1485, and 1593 cmÀ1 due to coordina-
tively bound pyridine on the Lewis acid (surface Ce cation).
No bands due to a pyridinium ion, formed by the adsorption
of pyridine on a Brønsted acid, were observed. Although
the N atom of the CN group can be also adsorbed on the Ce
cation, adsorption of the N atom of the pyridine ring should
be more preferable due to the higher basicity of the latter
species. Taking into account the above reaction mechanism,
the origin of substrate-specific catalysis of CeO2 is discussed
as follows. From the results in Table 2, we concluded that
the catalyst shows higher reactivity to the nitriles with a het-
eroatom adjacent to the a carbon of the CN group (such as
2-cyanopyridine) than the other nitriles (such as 4-cyanopyr-
idine: a nitrile with a heteroatom adjacent to the g carbon
of the CN group). The 2-cyanopyridine–CeO2 adsorption
complex, in which the heteroatom (N) adjacent to the a
carbon of the CN group is coordinated to the surface Ce
site (Scheme 2), should make the position of the CN group
of 2-cyanopyridine closer to the surface active sites of CeO2
than that of 4-cyanopyridine. This difference in the adsorp-
tion complex should lead to the larger frequency factor for
tion). There was a fairly good linearity between logACTHNUTRGNE(UNG kX/kH)
and s which gave a positive slope (1=0.58), and indicated
that a transition state in the rate-determining step of nitrile
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Efficient and Substrate-Specific Hydration of Nitriles to Amides
COMMUNICATION
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In conclusion, CeO2 acts as an effective heterogeneous
catalyst for the hydration of nitriles that have a heteroatom
(N or O) adjacent to the a carbon of the CN group. The
substrate specificity is derived from an enormous difference
of frequency factor. For the hydration of pyrazinecarboni-
trile to pyrazinecarboxamide (a medicine for tuberculosis)
in water, CeO2 shows higher activity than the state-of-the-
art catalysts including enzymes. A Michaelis–Menten-type
mechanism is proposed, which involves 1) H2O dissociation
on CeO2, 2) adsorption of the nitrile on CeO2, and 3) nucle-
ophilic attack of a hydroxyl species to the adsorbed nitrile.
Our result may open the possibility of designing all-inorgan-
ic and nonporous enzyme-mimetic catalysts for organic syn-
thesis.
Keywords: amides · cerium · hydration · nitriles · synthetic
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under a flow of He.
Received: May 23, 2011
Published online: August 29, 2011
Chem. Eur. J. 2011, 17, 11428 – 11431
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
11431