CeO2-catalysed one-pot selective synthesis of esters from nitriles and alcohols

Article abstract of DOI:10.1039/c2gc16424h

Thirteen kinds of metal oxides were tested for one-pot selective synthesis of esters from nitriles and alcohols. Ceria (CeO₂) showed more than two orders of magnitude higher activity than the other oxides. CeO₂ acted as a reusable and effective catalyst for the ester synthesis from various nitriles and alcohols under neutral and solvent-free conditions at 160 °C. This method provides a rare example for the synthesis of heteroaromatic esters, which have been difficult to synthesize by conventional catalytic esterification methods. Valuable esters such as picolinic acid alkyl esters and niacin benzyl esters were synthesized, demonstrating a practical aspect of the present method. Kinetic studies suggested the following reaction mechanism: (1) H₂O and ROH dissociate on CeO₂, (2) nucleophilic attack of hydroxyl species (OH^δ-) to the adsorbed nitrile on CeO₂, leading to the formation of the primary amide, (3) nucleophilic attack of alkoxide species (OR^δ-) to the amide as the rate-limiting step.

Full text of DOI:10.1039/c2gc16424h

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PAPER
CeO2-catalysed one-pot selective synthesis of esters from nitriles and
alcohols†
Masazumi Tamura,a Takuya Tonomura,a Ken-ichi Shimizu*b and Atsushi Satsumaa
Received 9th November 2011, Accepted 13th January 2012
DOI: 10.1039/c2gc16424h
Thirteen kinds of metal oxides were tested for one-pot selective synthesis of esters from nitriles and
alcohols. Ceria (CeO2) showed more than two orders of magnitude higher activity than the other oxides.
CeO2 acted as a reusable and effective catalyst for the ester synthesis from various nitriles and alcohols
under neutral and solvent-free conditions at 160 °C. This method provides a rare example for the
synthesis of heteroaromatic esters, which have been difficult to synthesize by conventional catalytic
esterification methods. Valuable esters such as picolinic acid alkyl esters and niacin benzyl esters were
synthesized, demonstrating a practical aspect of the present method. Kinetic studies suggested the
following reaction mechanism: (1) H2O and ROH dissociate on CeO2, (2) nucleophilic attack of hydroxyl
species (OHδ−) to the adsorbed nitrile on CeO2, leading to the formation of the primary amide, (3)
nucleophilic attack of alkoxide species (ORδ−) to the amide as the rate-limiting step.
The use of carboxylic acid as a starting material can cause cor-
Introduction
rosion of the reaction vessel. There is a continuous demand for
the development of alternative ester formation methods. One-pot
esterification of nitriles with alcohols may provide an attractive
alternative method of ester formation. Compared with carboxylic
acids, nitriles are storage-stable and less corrosive reagents and
various nitrile compounds can be easily obtained by the Sand-
meyer reaction, ammoxidation and other methods in laboratory
and industry.7 Moreover, in the view of organic synthesis, the
conversion of nitriles into esters is potentially useful.8 However,
conventionally, two step reaction of Pinner reaction/hydrolysis9
or multistep reaction including enzyme reactions10 have been
applied to ester synthesis from nitriles (Scheme 1), but these
multistep methods have disadvantages such as low yield and
high cost. Direct reaction of nitriles with alcohols will give an
environmentally friendly route to esters. Although there are
some reports on nucleophilic reaction of nitriles with alcohols
Esters are important intermediates in organic chemistry, and
making them efficiently is of paramount importance.1 Esters are
conventionally produced by dehydrative condensations between
carboxylic acids and alcohols using sulfuric acid, p-toluenesulfo-
nic acid or phosphoric acid.2 Recently, Lewis acid catalysts such
as complexes of Ti, Hf, Zn, Zr, Fe and Ir have been reported.3
However, these homogenous catalysts are toxic, corrosive,
environmental hazards and/or hard to be removed from the reac-
tion medium. To overcome these problems, heterogeneous cata-
lysts have been reported, but most of them suffer from limited
scope, use of solvent, low yield or necessity of complicated and
costly catalyst preparation.4 Heteroaromatic esters are important
intermediates for pharmaceuticals.5 However, heteroaromatic
substrates having basic functional groups react with homo-
geneous catalysts to potentially produce salts or coordinated
complexes, which lead to catalyst deactivation. To the best of
our knowledge, there are only two reports on the esterification of
heteroaromatic carboxylic acids by the homogenous catalyst,
TiO(acac)2, but this catalyst has drawbacks such as use of
solvent, low yield and limited scope.3g,k There are few hetero-
geneous catalysts for efficient esterification of heteroaromatic
carboxylic acids.6 These facts imply that the formation of hetero-
aromatic esters from carboxylic acids is difficult.
aDepartment of Molecular Design and Engineering, Graduate School of
Engineering, Nagoya University, Nagoya 464-8603, Japan
bCatalysis Research Center, Hokkaido University, N-21, W-10, Sapporo
001-0021, Japan. E-mail: kshimizu@cat.hokudai.ac.jp; Fax: +81-11-
706-9163; Tel: +81-11-706-9240
†Electronic supplementary information (ESI) available: See DOI:
10.1039/c2gc16424h
Scheme 1 Ester formation from nitrile.
984 | Green Chem., 2012, 14, 984–991
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giving esters,8d,11 most reactions are performed under strong
acidic conditions at high temperature, which bring about the
competitive hydration of nitriles and extensive salt formation. To
overcome these disadvantages, homogenous12 and hetero-
geneous transition metal catalysts13 have been developed. Mura-
hashi et al. first reported an efficient Ru complex catalyst for
direct esterification of nitriles under neutral conditions.12a Het-
erogeneous catalysts have been reported, but they have problems
such as low activity (high reaction temperature) and necessity of
the solvent.13
CeO2 has acid–base and redox properties and has been used
as an effective catalyst for various organic reactions14 such as
the dehydration of alcohols,15 the alkylation of aromatic com-
pounds,16 dimerization of alcohol,17 aldehyde, ester17e and car-
boxylic acid18 to ketones, cyclization of diols,19 reduction of
benzoic acid.20 Most CeO2-catalyzed reactions are carried out at
high temperatures (typically 200–500 °C). From the point of
view of organic synthesis and green chemistry, it is important to
study CeO2-catalyzed organic reactions further at lower tempera-
tures (<200 °C). As for CeO2-catalyzed reactions at low tempera-
tures (<200 °C), Tomishige et al. recently reported the carbonate
synthesis from CO2 and alcohols at 150 °C.21 Our research
group found that CeO2 acted as a highly active catalyst for sub-
strate-specific hydration of nitriles at low temperature
(30–100 °C).22 Kinetic studies showed a mechanism involving
nucleophilic attack of OHδ− species to nitriles as a key step.
Based on these findings, we hypothesized that reaction of nitriles
and alcohols would proceed directly to afford esters. In this
work we wish to report that CeO2 is a highly active catalyst for
one-pot ester formation from various alcohols and nitriles,
including heteroaromatic nitriles.
respect to 2-cyanopyridine) was added to the mixture of 2-cya-
nopyridine (1.0 mmol), n-octanol (10.0 mmol) and H2O
(1.0 mmol) in a reaction vessel equipped with a condenser under
N2 and molecular sieve 4 Å(1.0 g) were placed on the top of the
reaction vessel (Fig. S1†). The resulting mixture was vigorously
stirred at 160 °C. Note that the reaction vessel should be opened
in the draft owing to effusion of NH3 gas. The reaction mixture
was analyzed by GC. Conversion and yield of the products were
determined based on 2-cyanopyridine and picolinic acid octyl
ester using tridecane as an internal standard. The products were
identified by GCMS or by comparison with commercially pure
products.
In-situ FTIR
In situ FTIR spectra were recorded at room temperature on a
JASCO FT/IR-6100 equipped with a quartz IR cell connected to
a conventional flow reaction system. The sample was pressed
into a 25–80 mg of self-supporting wafer, having a surface area
of 3.14 cm2 on each face, and mounted into the quartz IR cell
with CaF2 windows. Spectra were measured accumulating 35
scans at a resolution 4 cm−1 in a flow of He. A reference spec-
trum of the catalyst wafer under He was subtracted from each
spectrum. For the introduction of methanol to the IR disc, the
liquid compound was injected under a He flow preheated at
100 °C which was fed to the in situ IR cell. Then, the IR disk
was purged with He, and IR measurement was carried out.
Results and discussion
Catalytic properties
First, the catalytic activity for ester formation from 2-cyanopyri-
dine and n-octanol at 160 °C was examined with various metal
oxides (Table 1). Ester formation hardly proceeded without a cat-
alyst (entry 14). For various metal oxides (entries 1–13), initial
rate of the formation of picolinic acid octyl ester was measured
under the condition in which conversions were below 40%.
Among these catalysts, CeO2 showed the highest reaction rate.
The rate for CeO2 was two orders of magnitude higher than the
other metal oxides.
Experimental
General
The GC (Shimadzu GC-14B) and GCMS (Shimazu
GCMS-QP5000) analyses were carried out with Rtx-65 capillary
column (Shimazu) or DB-1 capillary column (Agilent J&W)
using nitrogen as the carrier gas. All the chemicals for organic
reactions were analytical reagents from chemical products cor-
poration and were used without further purification.
In order to verify whether the observed catalysis is derived
from solid CeO2 or leached Ce species in the solution, the ester
formation reaction was carried out under the above conditions
for 45 min to achieve 45% yield of the product, and then CeO2
was removed from the reaction mixture by hot filtration (Fig. 1).
Then, the reaction was carried out with the filtrate under the
same conditions. In this case, the reaction was completely
stopped. Furthermore, inductively coupled plasma (ICP) result
confirmed that the Ce content in the solution was below the
detection limit (<0.1 ppm). These results clearly show that cata-
lytically active species are not eluted at all from CeO2 under the
reaction conditions and the observed catalysis is truly hetero-
geneous in nature. A further advantage of this solid catalytic
system is its reusability. We studied the reuse of CeO2 for the
ester formation reaction (Fig. 2). The catalyst can be easily
retrieved from the reaction mixture by centrifugation or filtration.
For each successive use, the catalyst was washed with acetone
three times to remove the products, followed by calcination in air
Catalyst
CeO2 (JRC-CEO3, surface area = 81 m2 g−1) pre-calcined at
600 °C before being supplied from the Catalysis Society of
Japan was used as a standard catalyst. CeO2 (140 m2 g−1) was
pre-calcined at 400 °C before being purchased from Rhodia
Electronics Catalysis. Other metal oxides were commercially
available or supplied from the Catalysis Society of Japan. Acidic
oxides (SiO2, SnO2, MnO2 and TiO2) were used without calcina-
tion before the reaction. The other catalysts (basic oxides) were
calcined at 500 °C for 3 h before the reaction to remove CO2
adsorbed on the surface.
Typical procedures for esterification of nitriles
A typical procedure for esterification of 2-cyanopyridine with
n-octanol is as follows. CeO2 (50 mg, 29.0 mol% Ce with
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Table 1 Esterification of 2-cyanopyridine with n-octanol by various metal oxidesa
Entry
Catalyst
SBET/m2 g−1
t/h
Yield (%)
V/mmol h−1 g−1b
1
2
3
4
5
6
7
8
CeO2
MnO2
TiO2
81
45
47
28
9
69
124
51
54
34
300
19
72
—
0.5
68
68
68
74
74
74
74
74
74
68
74
74
68
30.0
34.7
26.8
16.6
19.2
17.3
15.9
14.6
11.4
9.5
1.6
1.2
0.0
0.1
12.00
0.10
0.08
0.05
0.05
0.05
0.04
0.04
0.03
0.03
0.00
0.00
0.00
—
SnO2
Er2O3
Y2O3
Al2O3
Dy2O3
Nb2O5
Pr6O11
SiO2
9
10
11
12
13
14
MgO
ZrO2
Blank
a Reaction conditions: 2-cyanopyridine (1.0 mmol), n-octanol (10.0 mmol), H2O (1.0 mmol), CeO2 (50 mg), molecular sieve (1 g), T = 160 °C, N2.
Yield of picolinic acid octyl ester was determined by GC. b Initial rate of the formation of picolinic acid octyl ester measured under the condition in
which conversions were below 40%.
Fig. 2 Reusability of CeO2 catalyst. Reaction conditions: 2-cyanopyri-
dine (1.0 mmol), n-octanol (10.0 mmol), H2O (1.0 mmol), CeO2
(50 mg), molecular sieve (1 g), T = 160 °C, t = 24 h, N2.
Fig. 1 Leaching test of CeO2 on ester formation of 2-cyanopyridine
with n-octanol. Without removal of CeO2 (●); an arrow indicates the
removal of CeO2 by filtration (○). Reaction conditions: 2-cyanopyridine
(1.0 mmol), n-octanol (10.0 mmol), H2O (1.0 mmol), CeO2 (50 mg),
afford the corresponding esters in high yields (90 to >99%). To
our knowledge, this is the first example for direct esterification
of various heteroaromatic nitriles under neutral and solvent-free
conditions. Without molecular sieves, the corresponding ester
from 2-cyanopyridine and n-octanol was obtained in 44% yield
(entry 2). The reaction with 1 : 1 mixture of 2-cyanopyridine and
n-octanol resulted in low yield (entry 3). Benzonitrile and benzo-
nitrile derivatives having either an electron-donating or an elec-
tron-withdrawing group in the para-position (entries 8–11) were
also reacted to afford the corresponding esters in high yields. It
should be noted that less reactive unconjugated nitriles, benzyl-
nitrile (entry 12) and octanenitrile (entry 13), reacted to afford
the corresponding esters in high yields.
molecular sieve (1 g), T = 160 °C, N2.
at 300 °C for 1 h. After this treatment, the recovered catalyst was
reused at least three times without marked loss of its high cataly-
tic efficiency.
Next, we investigated the scope of nitriles and alcohols on
CeO2-catalyzed ester formation. Ester formation from nitriles
and n-octanol was examined by CeO2 (Table 2). Heteroaromatic
nitriles such as 2-cyanopyridine, 3-cyanopyridine, 4-cyanopyri-
dine, cyanopyrazine, 2-cyanofurane (entries 1, 4–7) reacted to
986 | Green Chem., 2012, 14, 984–991
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a
Table 2 Esterification of various nitriles with n-octanol by CeO2
Entry
Nitrile
Product
t/h
Yield (%)
1
24
24
24
98
44
29
2b
3c
4
5
6
48
24
24
96
>99
94
7
8
24
48
>99
83
9
48
94
10
11
48
48
96
89
12
13
24
19
83
98
a Reaction conditions: nitrile (1.0 mmol), n-octanol (10.0 mmol), H2O (1.0 mmol), CeO2 (50 mg), molecular sieve (1 g), T = 160 °C, N2. Yield of the
product was determined by GC. b Without molecular sieves. c 2-Cyanopyridine/n-octanol = 1.0 mmol/1.0 mmol.
Reaction mechanism
Ester formation from 2-cyanopyridine or 3-cyanopyridine with
various alcohols was examined (Table 3). Using 2-cyanopyridine
as a nitrile, aliphatic alcohols (entries 1, 2) and benzyl alcohol
(entry 3) reacted to afford the corresponding esters in high
yields. Using 3-cyanopyridine as a nitrile, benzyl alcohol deriva-
tives reacted to afford the corresponding esters in high yields
(entries 4, 5). The present method is applicable in the synthesis
of pharmacologically important products. Picolinic acid alkyl
esters (entries 1, 2) are known to show high apoptosis inducing
effects for human leukemia cells.5a and niacin benzyl esters
(entries 4, 5) are rubefacients and prodrugs for improving skin
oxygenation, inducing dermatitis artefacta, causing hyperemia,
stimulating percutaneous absorption and inducing dermal vaso-
dilation.5b–e
To investigate the reaction mechanism of the CeO2-catalysed
ester synthesis, we studied kinetic experiments. Fig. 3 shows the
time-yield profile for ester formation from 2-cyanopyridine and
n-octanol by CeO2. First, 2-cyanopyridine was immediately con-
sumed to produce the corresponding primary amide, picolina-
mide, and then picolinamide was converted slowly to the final
product, picolinic acid octyl ester. As an example of other
nitriles, time-yield profile for the reaction of benzonitrile and n-
octanol is shown in Fig. S2.† The reaction pattern is basically
the same as that in Fig. 3. First, benzonitrile reacted to produce
benzamide as an intermediate and then benzamide was converted
to benzoic acid octyl ester. These results indicate that ester
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a
Table 3 Esterification of 2-cyanopyridine or 3-cyanopyridine with various alcohols by CeO2
Entry
1
Nitrile
Alcohol
Product
t/h
Yield (%)
88
24
2
3
4
5
24
48
24
24
98
84
>99
67
a Reaction conditions: nitrile (1.0 mmol), alcohol (10.0 mmol), H2O (1.0 mmol), CeO2 (50 mg), molecular sieve (1 g), T = 160 °C, N2. Yield of the
product was determined by GC.
Scheme 2 Reaction path of ester formation reaction.
esterification of picolinamide and esterification of picolinic acid
were 557.2, 6.5 and 1.5 mmol h−1 g−1, respectively. Esterifica-
tion rate from carboxylic acid is much smaller than that from the
amide, indicating that the present reaction proceeds by direct
esterification of the amide. The rate of hydration of 2-cyanopyri-
dine is 86 times larger than that of esterification of picolinamide,
which elucidates that esterification of picolinamide is the rate-
limiting step. The reaction pathway via the primary amide is
different from that proposed for a Ru-based homogeneous cata-
lyst by Murahashi et al.,12a where they proposed that esterifica-
tion proceeds via imidate produced by nucleophilic addition of
Fig. 3 Time–yield diagram for ester formation from 2-cyanopyridine
with n-octanol by CeO2. ●: 2-Cyanopyridine, ○: picolinamide, ■:
alcohol to nitrile, followed by hydrolysis of imidate to the corre-
sponding ester. In our previous report we proposed the following
picolinic acid octyl ester. Reaction conditions: 2-cyanopyridine
mechanism for hydration of nitriles by CeO2.22 First, the cataly-
(1.0 mmol), n-octanol (10.0 mmol), H2O (1.0 mmol), CeO2 (50 mg),
tic cycle starts with dissociation of H2O on CeO2 to give Hδ+
molecular sieve (1 g), T = 160 °C, N2.
and OHδ−. Then, nitrile and CeO2 are in equilibrium with the
nitrile–CeO2 adsorption complex. Finally, this nitrile–CeO2
complex undergoes an addition of OHδ− to the nitrile carbon
formation reaction proceeded by two consecutive steps:
hydration of nitrile to the primary amide and esterification of the
amide (Scheme 2). The carboxylic acid was not detected by GC,
but we cannot eliminate a possibility that the reaction proceeds
via a carboxylic acid produced by over-hydration of the primary
amide. Scheme 3 shows reaction rates of three elementary pro-
cesses. The reaction rates for hydration of 2-cyanopyridine,
atom to give the amide and CeO2. In the present ester formation
reaction, the same mechanism is applicable to the formation of
the primary amide intermediate, which may be produced. Thus,
we studied the mechanism for the reaction of the primary amide
with alcohol as the rate-determining step of the present ester
formation reaction.
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Scheme 3 Rate comparison of elementary processes.
Scheme 4 Image of methanol adsorption on CeO2.
Fig. 5 Hammett plot for esterification of p-substituted benzonitrile
with CeO2. Reaction conditions: nitrile (1.0 mmol), n-octanol
(10.0 mmol), H2O (1.0 mmol), CeO2 (50 mg), molecular sieve (1 g),
T = 160 °C, N2. Slope (ρ) = 1.12 (R2 = 0.98).
methoxy species on Lewis acid site (exposed Ce4+) and H+ on
Lewis base site (exposed O2−) as shown in Scheme 4. To
examine the correlation between the number of the Lewis acid–
base pair site and catalytic activity, we studied catalytic tests and
FTIR experiment of methanol adsorption for the standard CeO2
(JRC-CEO3, calcined at 600 °C) and various CeO2 samples pre-
pared by calcining CeO2 at different temperatures
(500–1100 °C). The FTIR results (Fig. S3†) showed that adsorp-
tion of methanol onto CeO2 resulted in the formation of
methoxy species.23 The number of methoxy species was esti-
mated from the v(OC) bands area (970–1130 cm−1) and the
Fig. 4 The reaction rate of esterification of 2-cyanopyridine as a func-
tion of adsorbed methanol of CeO2 (●: Rhodia, ○: JRC- CEO3). The
values in the graph are calcination temperatures in °C. Reaction con-
ditions: 2-cyanopyridine (1.0 mmol), n-octanol (10.0 mmol), H2O
(1.0 mmol), CeO2 (50 mg), molecular sieve (1 g), T = 160 °C, N2.
Initial rate of the formation of picolinic acid octyl ester measured under
the condition in which conversions were below 50%.
Lavalley et al.23 studied the adsorption state of methanol on
CeO2 and showed that methanol is dissociated by coordinatively
unsaturated Lewis acid–base pair site on CeO2 to produce
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Scheme 5 Reaction mechanism of ester formation reaction.
absorption coefficient of methoxy species (6.9 cm μmol−1
)
reported by Lavalley et al.23a Fig. 4 shows a plot of the reaction
rate versus the number of methoxy species on CeO2 (Rhodia)
samples calcined at different temperatures (500–1100 °C) and
the standard CeO2 (JRC-CEO3, calcined at 600 °C). For a series
of CeO2 catalysts, there is fairly good linearity between reaction
rate and the number of methoxy species (R2 = 0.99). Taking into
account that the number of methoxy species corresponds to that
of coordinatively unsaturated Lewis acid–base pair sites on
CeO2, the result indicates that Lewis acid–base pair sites on
CeO2 are active species in the present esterification reaction and
alkoxide (ORδ−) adspecies on the Lewis acid–base pair site
should participate in the reaction.
Conclusion
We have demonstrated that CeO2 acts as an effective hetero-
geneous catalyst for the direct ester synthesis from various
nitriles and alcohols under neutral conditions. This novel catalyst
provides a clean, convenient and practical route for the direct
ester synthesis in view of the following advantages. (1) The reac-
tion proceeds smoothly and selectively in the presence of moist-
ure under solvent-free conditions, producing various esters in
high yield. (2) The commercial catalyst is stable, reusable and a
non-polluting solid that offers easy handling and work-up. (3)
The present method is applicable in the synthesis of pharmaco-
logically important products (such as picolinic acid alkyl esters
and niacin benzyl esters). (4) Compared with carboxylic acids,
nitriles are storage-stable and less corrosive reagents. From a
mechanistic viewpoint, the catalytic cycle consists of three steps:
(1) H2O and ROH dissociate on CeO2, (2) nucleophilic attack of
hydroxyl species (OHδ−) to the adsorbed nitrile on CeO2,
leading to the formation of the primary amide, (3) nucleophilic
attack of alkoxide species (ORδ−) to the amide as the rate-
limiting step.
Using the results in Table 2 (entries 8–11), Fig. 5 shows a
relationship between log(kX/kH) and the Hammett parameter (σ)
for esterification of benzonitriles with an electron-donating or an
electron-withdrawing substituent. The order of reactivity for
esterification of benzonitriles was p–F > p–H > p–CH3 > p–
OCH3. There was fairly good linearity between log(kX/kH) and σ
giving a positive slope (ρ = 1.12, R2 = 0.98). The positive ρ
value indicates that the reaction is favored by the presence in the
para-position of electron-withdrawing groups and that a tran-
sition state in the rate-determining step of nitrile esterification
has a negative charge at the α-carbon atom adjacent to the
benzene ring. Taking into account the FTIR results, the positive
ρ value suggests that nucleophilic addition of ORδ− adspecies to
the amide carbon atom via a negatively charged transition state
is the rate-limiting step. This proposal is consistent with the
result of time-conversion profile (Fig. 3).
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Based on the above discussions and the proposal in our pre-
vious report,22 a possible reaction mechanism of one-pot ester
synthesis by CeO2 is shown in Scheme 5. The catalytic cycle
starts with dissociation of H2O and R2OH on CeO2 to give Hδ+
and OHδ−, Hδ+ and OR2δ−, respectively (step 1). Nitrile is
adsorbed on CeO2. Nitrile–CeO2 adsorption complex will
undergo an addition of OHδ− to a nitrile carbon atom to give the
primary amide and CeO2 via a negatively charged transition state
(step 2). Amide–CeO2 adsorption complex will undergo an
addition of OR2δ− to the amide carbon atom to give the ester and
CeO2 via a negatively charged transition state (step 3).
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