P2W18O62·24H2O as an efficient and recyclable catalyst for the ecofriendly preparation of β-aminocrotonates

Article abstract of DOI:10.1139/cjc-2012-0157

H₆P₂W₁₈O₆₂·24H ₂O bulk-and silica-supported catalysts were found to be efficient and recyclable catalysts for the synthesis of a series of β-aminocrotonates using toluene as the solvent or under no solvent reaction conditions. Several substituted β-aminocrotonates can be prepared in very good yields and purity by direct reaction of amines and 1,3-dicarbonyl compounds in the presence of a catalytic amount of bulk-and silica-gel-supported H₆P ₂W₁₈O₆₂·24H₂O. The title compounds were prepared in very good yields using both methodologies, and no secondary products were detected. The procedure using no reaction solvent resulted in a clean and useful alternative, which has the advantages of a greener methodology with operative simplicity, the use of a reusable and noncorrosive solid catalyst, soft reaction conditions, low reaction times, and good yields.

Full text of DOI:10.1139/cjc-2012-0157

137
ARTICLE
P2W18O62·24H2O as an efficient and recyclable catalyst for the
ecofriendly preparation of ␤-aminocrotonates
Laura M. Sanchez, Ángel G. Sathicq, Graciela T. Baronetti, Horacio J. Thomas, and Gustavo P. Romanelli
Abstract: H6P2W18O62·24H2O bulk- and silica-supported catalysts were found to be efficient and recyclable catalysts for the
synthesis of a series of ␤-aminocrotonates using toluene as the solvent or under no solvent reaction conditions. Several
substituted ␤-aminocrotonates can be prepared in very good yields and purity by direct reaction of amines and 1,3-dicarbonyl
compounds in the presence of a catalytic amount of bulk- and silica-gel-supported H6P2W18O62·24H2O. The title compounds were
prepared in very good yields using both methodologies, and no secondary products were detected. The procedure using no
reaction solvent resulted in a clean and useful alternative, which has the advantages of a greener methodology with operative
simplicity, the use of a reusable and noncorrosive solid catalyst, soft reaction conditions, low reaction times, and good yields.
Key words: ␤-aminocrotonates, supported heteropolyacids, Wells–Dawson, solvent-free.
Résumé : On a observé que le H6P2W18O62·24H2O seul ou sur un support de silice est un catalyseur efficace et réutilisable pour
la synthèse d'une série de ␤-crotonates, dans des conditions sans solvant ou en présence de toluène comme solvant. On a ainsi
été en mesure de préparer plusieurs ␤-aminocrotonates, propres et avec de bons rendements en procédant a` une réaction
d'amine avec des composés 1,3-dicarbonylés, en présence d'une quantité catalytique de H6P2W18O62·24H2O seul ou sur un support
de silice. Les produits mentionnés dans le titre ont été préparés avec de très bons rendements en utilisant l'une ou l'autre des
deux technologies et il n'a pas été possible de détecter la présence de produits secondaires. L'utilisation de la variante de la
réaction sans solvant est une alternative utile et propre qui présente l'avantage d'être plus écologique, d'opération simple, faisant
appel a` un catalyseur solide réutilisable et non corrosif, a` des conditions réactionnelles douces, de faibles temps de réaction et
de bons rendements. [Traduit par la Rédaction]
Mots-clés : ␤-aminocrotonates, hétéropolyacides sur un support, Wells–Dawson, sans solvant.
pyridines, pyrimidines, and pyrazoles,9–12 1,4-dihydropyridines and
Introduction
3,4-dihydropyrimidines,13,14 tetrasubstituted thiophenes,15 pyr-
Organic transformations using heterogeneous acid catalysis are
very important for the development of a sustainable chemical pro-
cedure because of cost-effectiveness, better selectivity, and opera-
roles,16 and ␤-amino acid derivatives.17 Also, ␤-aminocrotonates have
been previously evaluated as bioactive compounds with potent oral
anticonvulsant activity similar to phenytoin, carbamazepine, and
tional simplicity.1 Heterogeneous solid acids are advantageous over
lamotrigine.18
conventional homogeneous acid catalysts as they can be easily recov-
ered from the reaction mixture, thereby eliminating the need for
The most general preparation of ␤-aminocrotonates involves
the reaction of a carbonyl compound with an amine in an aro-
separation through distillation or extraction. In many cases, they can
matic hydrocarbon solvent, with azeotropic removal of formed
be reused without appreciable loss of the catalytic activity.2
water.19,20 Other procedures include the use of iodine under
Catalysis by heteropolyacids (HPAs) and related compounds is a
solvent-free conditions,21 and montmorillonite clay K10, silica, or
field of increasing importance worldwide. Numerous develop-
ments are being carried out in basic research as well as in fine
chemistry processes. HPAs possess a very strong acidity, and they
exhibit the characteristic features of bulk-type catalysis (pseu-
p-toluenesulfonic acid (PTSA) under microwave irradiation.22
Also, some HPAs have been used under heterogeneous and
solvent-free conditions.2,23
Continuing with our studies on green methodologies using HPAs
doliquid bulk type), in which the reagent molecules are absorbed
as recyclable catalysts, we now present our findings about two new
between the polyanions (rather than in a polyanion) in the ionic
methodologies for the catalytic preparation of ␤-aminocrotonates
crystal by replacing crystallization water or expanding the lattice,
using a bulk- and silica-supported Wells–Dawson HPA with toluene
and the reaction occurs there.3 We recently reported the use of
as solvent and under no solvent reaction conditions (Scheme 1).
HPA catalyst H6P2W18O62·24H2O with a Wells–Dawson struc-
ture to different organic transformation reactions. Among
Experimental
them, phenol methoxymethyl ether4 and acylal deprotection,5
alcohol, phenol, amine, and thiol acylation,6 and heterocyclic
synthesis such as flavones, chromones, coumarins, dihy-
drocoumarins,7 and pyridines.8
Preparation of catalysts
The employed catalyst, H6P2W18O62·24H2O (WD), was prepared
by the Drechsel method from an ␣/␤ K6P2W18O62·10H2O isomer
mixture.24 This Dawson-type salt was prepared according to the
technique reported by Lyon et al.25 The WD silica-supported cata-
On the other hand, ␤-aminocrotonate derivatives are versatile in-
termediates for the synthesis of many heterocycles, for example,
Received 4 May 2012. Accepted 4 September 2012.
L.M. Sanchez, Á.G. Sathicq, and G.P. Romanelli. Centro de Investigación y Desarrollo en Ciencias Aplicadas “Dr. J.J. Ronco” (CINDECA), Departamento de Química,
Facultad de Ciencias Exactas, UNLP-CCT-CONICET. Calles 47 N° 257, B1900AJK La Plata, Argentina.
G.T. Baronetti. Departamento de Ingeniería Química, Facultad de Ingeniería, Universidad de Buenos Aires, Ciudad Universitaria, C1428BG Buenos Aires, Argentina.
H.J. Thomas. Planta Piloto Multipropósito PlaPiMu, UNLP-CICPBA, Camino Centenario y 508, CP1987, Manuel B. Gonnet, Argentina.
Corresponding author: Gustavo P. Romanelli (e-mail: gpr@quimica.unlp.edu.ar).
Can. J. Chem. 91: 137–142 (2013) dx.doi.org/10.1139/cjc-2012-0157
Published at www.nrcresearchpress.com/cjc on 18 January 2013.

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Can. J. Chem. Vol. 91, 2013
Scheme 1. General preparation of ␤-aminocrotonates.
Results and discussion
Catalyst characterization
In previous papers,26–29 bulk and supported catalysts were char-
acterized by 31P MAS-NMR, FTIR, and X-ray diffraction (XRD) mea-
surements. The obtained results have been summarized in the
following paragraph. It is known that pure WD acid has two equiv-
alent phosphorus atoms and, consequently, it has only one peak
(at –12.8 ppm) in the 31P MAS-NMR spectrum.26 In the case of
supported catalysts, a main peak with a chemical shift of
–12.7 ppm and two new small signals close to –12 and –11 ppm were
detected in the 31P MAS-NMR spectra. Those results indicate that
WD keeps its heteropolyoxoanion structure after the impregna-
tion and drying steps when it is supported on SiO2 using a WD
loading in the range of 0.1–0.6 g WD/g SiO2. The two new signals
could be related to the presence of H6P2W18O62 species strongly
interacting with the Si–OH groups of the support, and to lacunary
species such as P2W21O716–, respectively.27,28
The FTIR characterization also shows that after impregnation,
the acid maintains its Dawson structure. The characteristic bands
of the HPA with a Dawson structure are 1091 (stretching frequency
of the PO4 tetrahedron), 963 (W=O terminal bonds), 911, and
778 cm–1 (“inter” and “intra” W–O–W bridges, respectively). It can
be observed that catalysts supported on SiO2 using a WD loading
in the range of 0.1 0.6 g WD/g SiO2 display the same characteristic
bands. Nevertheless, for these supported samples, a broadening of
the band at 1091 cm–1 (stretching frequency of the PO4 tetrahe-
dron) must be noted. This fact can be due to a loss of tetrahedron
symmetry because of the interaction between WO6 octahedra and
silanol groups of the support. A slight shift of the 778 cm–1 band
(intra W–O–W bridges) could also be observed, which could be
attributed to the same effect.29
lyst was obtained by wet impregnation of Grace Davison silica
(grade 59, specific area 250 m2/g) with an aqueous solution of the
synthesized WD: Catalysts containing 0.1, 0.2, 0.4, or 0.6 g/g of WD
acid were prepared on SiO2 (0.1WDSiO2, 0.2WDSiO2, 0.4WDSiO2,
or 0.6WDSiO2, respectively). Then, samples were dried at room
temperature in a vacuum desiccator for 8 h.3
Catalyst characterization
Bulk- and silica-supported catalysts were characterized by IR
spectroscopy with Fourier transform infrared (FTIR) Bruker IFS
66 equipment. 31P magic-angle spinning nuclear magnetic reso-
nance (MAS-NMR) measurements were recorded with Bruker MSL-
300 equipment operating at frequencies of 121.496 MHz. A sample
holder with a 5 mm diameter and 17 mm height was used. The
spin rate was 2.1 kHz and several hundred pulse responses were
collected. Chemical shifts were expressed in parts per million
(ppm) with respect to 85% H3PO4 as an external standard for 31P
NMR.
Catalytic tests
We used a bulk catalyst in solvent-free conditions and a sup-
ported catalyst using toluene as the solvent for the synthesis of a
representative ␤-aminocrotonate from aniline and methyl ace-
toacetate. For that purpose, four silica-supported catalysts with
different loadings of Wells–Dawson acid were tested. Measure-
ments at different reaction temperatures were also made. The
variation of the product yield with the molar ratio of materials
was analyzed, and additional runs were made to see whether it
was possible to reuse the catalysts.
Catalytic tests
A range of catalytic tests were performed to analyze the effect of
the reaction temperature, charges of WD on SiO2, molar ratio of
reactants, amount of catalyst and its reuse on the reaction yield
and its selectivity. With that purpose, the general reaction pre-
sented in Scheme 1 was taken, for both methods, with aniline and
methyl acetoacetate as starting materials.
General procedures for the preparation of
␤-aminocrotonates
All starting materials are commercial products. The reactions
were monitored by thin-layer chromatrography (TLC) on pre-
coated silica gel plates. All the yields were calculated from pure
products. All products were identified by comparison of physical
Effect of reaction temperature
It is well-known that procedures without reaction solvent usu-
ally show higher yields than reactions with solvent. To determine
the ideal reaction temperature for each method, the effect of
reaction temperature was evaluated. Figure 1 shows that, in both
cases, the conversion is higher as temperature increases. Accord-
ing to the results obtained, the reactions using toluene presented
good product yields at 110 °C, and the reactions carried out with-
out reaction solvent presented excellent product yields at 80 °C.
1
data (TLC and NMR) with those reported. 13C NMR and H NMR
spectra were recorded at room temperature on a Bruker AC 200
using tetramethylsilane (TMS) as an internal standard.
Supported catalyst using toluene as solvent
A mixture of 1 mmol of aniline or substituted anilines, 1 mmol of
ethyl or methyl acetoacetate, with 3 mL toluene and 1 mmol% of
catalyst (0.4WDSiO2) was stirred at reflux temperature for the appro-
priate amount of time. The progress of the reaction was checked by
TLC on silica with a fluorescent indicator, monitoring the fading out
of the aniline when the developed chromatogram was exposed to UV
light (254 nm). The reaction mixture was filtered and dried with
anhydrous Na2SO4 (0.01 g). Evaporation of the solvent followed by
chromatography on silica gave the pure substituted or nonsubsti-
tuted methyl and ethyl ␤-arylaminocrotonates.
Effect of the charges of WD on SiO2
The catalytic activity of catalysts containing 0.1, 0.2, 0.4, or
0.6 g/g of WD acid supported on SiO2 was evaluated at the ideal
reaction temperature. As can be observed in Fig. 2, performing the
reaction with toluene at 110 °C and in a reaction time of 2 h,
an increase in WD loading produces a conversion increment
following the sequence of 0.1WDSiO2 < 0.2WDSiO2 < 0.6WD-
SiO2 < 0.4WDSiO2. These results are similar to those obtained by
using these catalysts in flavones synthesis.28 The best catalytic
conversion by a gram of catalyst was shown by 0.4WDSiO2, and it
was used to study the effect of other parameters on the catalyst
activity.
Bulk catalyst using no reaction solvent
A mixture of 1 mmol of aniline or substituted anilines, 1 mmol
of ␤-ketoester, and 1 mmol % of catalyst (WD) was stirred at 80 °C
for the appropriate time. The progress of the reaction was
checked by TLC. After completion of the reaction, hot toluene was
added (2.5 mL) and the catalyst was filtered. The extracts were
combined; their solvents were evaporated and then concentrated
in vacuum. Chromatography on silica gave the pure substituted or
nonsubstituted methyl and ethyl ␤-aminocrotonates.
Effect of the amount of catalyst
The effect of different amounts of catalyst on the product yield
(%) was studied. The catalytic activity of bulk WD was evaluated
under no solvent reaction conditions at 80 °C. On the other hand,
0.4WDSiO2 was evaluated under solvent conditions of 110 °C in
Published by NRC Research Press

Sanchez et al.
139
Fig. 1. The effect of temperature on methyl ␤-aminocrotonate yield with toluene and under no solvent reaction conditions. Method A
reaction conditions: aniline, 1 mmol; methyl acetoacetate, 1 mmol; bulk WD, 1 mmol %; toluene, 3 mL; reaction time: 120 min. Method B
reaction conditions: aniline, 1 mmol; methyl acetoacetate, 1 mmol; bulk WD, 1 mmol %; without reaction solvent; reaction time: 15 min. WD,
H6P2W18O62·24H2O.
Fig. 2. The effect of silica-supported catalyst on methyl
␤-aminocrotonate yield. Reaction conditions: aniline, 1 mmol;
methyl acetoacetate, 1 mmol; supported WD, 1 mmol % “active
phase”; temperature, 110 °C; toluene, 3 mL; reaction time: 120 min.
WD, H6P2W18O62·24H2O.
Table 1. Effect of the molar ratio of substrates on
methyl ␤-aminocrotonate yield.
Entry
Molar ratioa
Yield (%)b
1
1:3
1:1.5
1:0.1
1.5:1
3:1
84
82
79
79
81
2
3
4
5
Note: Reaction conditions: aniline, 1 mmol; methyl ace-
toacetate, 1 mmol; 0.4WDSiO2 “active phase”, 1 mmol %;
temperature, 110 °C; toluene,
3 mL; reaction time:
120 min. WD, H6P2W18O62·24H2O.
aAniline : methyl acetoacetate.
bIsolated yield.
Fig. 4. Catalyst reuse on methyl ␤-aminocrotonate synthesis.
Method A reaction conditions: aniline, 1 mmol; methyl acetoacetate,
1 mmol; 0.4WDSiO2 “active phase”, 1 mmol %; temperature, 110 °C;
toluene, 3 mL; reaction time: 120 min. Method B reaction
conditions: aniline, 1 mmol; methyl acetoacetate, 1 mmol; bulk WD,
1 mmol %; temperature, 80 °C; without reaction solvent; reaction
time: 15 min. WD, H6P2W18O62·24H2O.
Fig. 3. The effect of the amount of active phase catalyst on methyl
␤-aminocrotonate yield. Method A reaction conditions: aniline,
1 mmol; methyl acetoacetate, 1 mmol; 0.4WDSiO2 “active phase”,
1 mmol %; temperature, 110 °C; toluene, 3 mL; reaction time:
120 min. Method B reaction conditions: aniline, 1 mmol; methyl ace-
toacetate, 1 mmol; bulk WD, 1 mmol %; temperature, 80 °C; without
reaction solvent; reaction time: 15 min. WD, H6P2W18O62·24H2O.
Effect of the molar ratio of substrates
To optimize the molar ratio between starting materials, this
relationship was evaluated with toluene and 0.4WDSiO2 at 110 °C.
In general, the attained yields employing different molar ratios of
substrates were very similar. Therefore, a molar ratio of 1:1 (Table 1,
entry 3) is appropriate for the synthesis of ␤-aminocrotonates.
toluene. The results are shown in Fig. 3. For supported catalysts
the difference between 1 and 3 mmol % of 0.4WDSiO2 is less than
5% and, in the case of using no reaction solvent, there is no differ-
ence between 1 and 3 mmol % of the bulk catalyst.
Recycling of the catalyst
Recycling of bulk WD and 0.4WDSiO2 was studied under two
conditions: using toluene as the reaction solvent and using no
reaction solvent. After the completion of each reaction, the cata-
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Can. J. Chem. Vol. 91, 2013
Table 2. Synthesis of methyl and ethyl ␤-aminocrotonates.
Method A
Time (min)
120
Method B
Entry
1
Aniline
Aniline
Acetoacetate
Methyl
Product
Yield (%)a
79
PMI
19.8
Time (min)
15
Yield (%)a
98
PMI
12.99
2
3
Ethyl
120
120
77
81
19.05
18.13
15
93
60
12.77
19.84
4-Methylaniline
Methyl
210
4
5
6
Ethyl
120
79
—
82
17.46
—
—
70
—
4-Chloroaniline
Methyl
Ethyl
—
180
15.57
180
15.51
—
7
8
9
2,6-Dimethylaniline
Methyl
Ethyl
240
240
120
76
75
84
18.19
17.31
15.05
15
—
15
93
—
95
12.06
2-Naphthylamine
Methyl
10.83
Note: Method A reaction conditions: aniline, 1 mmol; methyl acetoacetate, 1 mmol; 0.4WDSiO2 “active phase”, 1 mmol %; temperature, 110 °C; toluene, 3 mL. Method
B reaction conditions: aniline, 1 mmol; methyl acetoacetate, 1 mmol; bulk WD, 1 mmol %; temperature, 80 °C; without reaction solvent. PMI, process mass intensity;
WD, H6P2W18O62·24H2O.
aIsolated yield.
Published by NRC Research Press

Sanchez et al.
141
Scheme 2. Plausible mechanism for the synthesis of ␤-aminocrotonates using Wells–Dawson heteropolyacid.
lyst was recovered and reused three times. As is shown in Fig. 4,
the catalysts maintain their activity under both methods.
without reaction solvent reduced reaction times and increased
yields. Also, the method without a reaction solvent uses less en-
ergy than the method with toluene as the solvent. It is possible to
say that method B is greener than method A, and it is easily
demonstrated by their PMI values.
In recent experiments, it was found that it is possible to synthe-
size more sophisticated molecules, such as nonsymmetrical 1,4-
dihydropyridines, from ␤-aminocrotonates with good yields and
in short reaction times.
Preparation of ␤-aminocrotonates
Finally, the reaction was extended to various amines and
␤-dicarbonyl compounds as substrates by using the catalytic amount
(1 mmol %) of bulk WD and 0.4WDSiO2 as active catalysts. Bulk WD
was used without reaction solvent at 80 °C, and 0.4WDSiO2 was used
in toluene at 110 °C. The molar ratio between reactants was 1:1. The
results are summarized in Table 2. In both cases, the desired prod-
ucts were obtained with high selectivity, almost free of secondary
products. Although both methods showed rate enhancements, high
yields, and relatively short reaction times, bulk WD under no solvent
reaction conditions appears to be more convenient. To quantify how
much “greener” one method is than the other one, the process mass
intensity (PMI) was calculated for each reaction product, for both
methods. Results presented in Table 2 advertise that method B is
greener than method A.
The probable mechanism for the synthesis of ␤-aminocrotonates
has been described in refs. 30 and 31. A general mechanism has
been proposed for the synthesis of ␤-aminocrotonates using WD
as the catalyst (Scheme 2). The nucleophilic attack of substituted
aniline to the ␤-dicarbonyl compound is catalyzed by acidic sites;
this addition is followed by elimination of water and further re-
generation of protons. In our case, the acidic sites are provided by
the WD HPA.
Supplementary data
Supplementary data are available with the article through the
journal Web site at http://nrcresearchpress.com/doi/suppl/10.1139/
cjc-2012-0157.
Acknowledgments
The authors thank Consejo Nacional de Investigaciones Cienti-
ficas y Técnicas (CONICET), Agencia Nacional de Promoción
Científica y Tecnológica (Argentina), and Universidad Nacional de
La Plata for financial support. AGS, GPR, GTB, and HJT are mem-
bers of CONICET.
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