New application of decaniobate salt as basic solid in the synthesis of 4H-pyrans by microwave assisted multicomponent reactions

Article abstract of DOI:10.1007/s11164-018-3440-y

This study focuses on the search of new applications of polyoxometalates known as decaniobate ions [Nb₁₀O₂₈]^6?. The decaniobates can be used as basic solid catalysts in multicomponent reactions to obtain good yields to 4H-pyrans (> 95%) under microwave radiation and solvent-free conditions, using several aldehydes with different electron-withdrawing or electron donor substituents. The synthesis of (TMA)₆ [Nb₁₀O₂₈]6H₂O was performed following a simple protocol (HPNb), which was modified to decrease the number of hexaniobate species that are formed as impurities (HPNb-HF).

Full text of DOI:10.1007/s11164-018-3440-y

Res Chem Intermed
https://doi.org/10.1007/s11164-018-3440-y
New application of decaniobate salt as basic solid
in the synthesis of 4H-pyrans by microwave assisted
multicomponent reactions
1
2
1
1
•
Luisa F. Gutierrez
2
•
Eliana Nope
•
Hugo A. Rojas
2
•
•
Jairo A. Cubillos
1
´
•
Angel G. Sathicq Gustavo P. Romanelli Jos e´ J. Mart ´ı nez
Received: 14 January 2018 / Accepted: 18 April 2018
Ó Springer Science+Business Media B.V., part of Springer Nature 2018
Abstract This study focuses on the search of new applications of polyoxometalates
6
known as decaniobate ions [Nb O ] . The decaniobates can be used as basic
-
1
0 28
solid catalysts in multicomponent reactions to obtain good yields to 4H-pyrans
([ 95%) under microwave radiation and solvent-free conditions, using several
aldehydes with different electron-withdrawing or electron donor substituents. The
synthesis of (TMA) [Nb O ]6H O was performed following a simple protocol
6
10 28
2
(HPNb), which was modified to decrease the number of hexaniobate species that are
formed as impurities (HPNb-HF).
Keywords Decaniobate ions Á 4H-pyrans Á Microwave Á Multicomponent
reactions
Electronic supplementary material The online version of this article (https://doi.org/10.1007/s11164-
018-3440-y) contains supplementary material, which is available to authorized users.
&
&
1
Gustavo P. Romanelli
gpr@quimica.unlp.edu.ar
Jos e´ J. Mart ´ı nez
jose.martinez@uptc.edu.co
Escuela de Ciencias Qu ´ı micas, Facultad de Ciencias, Universidad Pedag o´ gica y Tecnol o´ gica de
Colombia UPTC, Avenida Central del Norte, Tunja, Boyac a´ , Colombia
2
Departamento de Qu ´ı mica, Centro de Investigaci o´ n y Desarrollo en Ciencias Aplicadas ‘‘Dr. J.J.
Ronco’’ (CINDECA), Facultad de Ciencias Exactas, UNLP-CCT-CONICET, Calles 47 No 257,
B1900 AJK La Plata, Argentina
123

L. F. Gutierrez et al.
Introduction
Polyoxometalates (POMs) are anionic metal-oxide clusters based on octahedral
MO units sharing their edges or corners where M represents the metal atom. The
6
diverse applications of polyoxometalates in chemistry and the pharmaceutical
industry have led to the study of less common polyoxometalates; an example is the
6
decaniobate ion ([Nb O ] ), where the more representative compound is
-
1
0 28
?
TMA [Nb O ] (TMA : tetramethyl ammonium ion) that is stable at pH 5–8
6
10 28
and is insoluble in nonaqueous solvents [1]. The possibilities of the applications of
this class of polyoxometalates have been explored in electrocatalysts [2] and
photocatalysts [3, 4].
The decaniobates possess more negative charge than other POMs of group IV
Mo, W) due to the difference in their maximum oxidation states, thus this property
(
should be more exploited [5]. Recently, Tsukuda and colleagues demonstrated that
6
decaniobate clusters [Nb O ] can act efficiently as a new base catalyst for aldol-
-
1
0 28
type condensation reactions, including Knoevenagel and Claisen–Schmidt reactions
6], and in CO fixation [7]. The basic catalysis was ascribed to the negative charge
[
2
on the surface oxygens, which was demonstrated by density functional theory
calculations. However, other types of basic reactions should be explored for new
applications in the field of catalysis using this type of POMs.
One interesting reaction requiring the use of base catalysts is the synthesis of 4H-
pyrans, where a reaction between an aldehyde, malononitrile and a b-dicarbonyl
compound is involved (Fig. 1). The reaction mechanism initially proceeds by
Knoevenagel condensation where the strong basic sites of the catalyst promote the
reaction by abstracting a proton from the active methylene compound, which reacts
with the aldehyde forming an alkene intermediate followed by a Michael-type
addition affording the 4H-pyran compound [8, 9]. Generally, this multicomponent
reaction (MCR) is carried out with piperidine, dibutylamine, morpholine or metal
alkoxides, using volatile solvents leading to long reaction times and tedious catalyst
recovery [8–12]. Different types of basic solids or hybrids such as hydrotalcites,
mesoporous Ca-MCM or oxides have been explored in this reaction [13, 14].
6
-
Although decaniobates ([Nb O ] ) can also act as basic solids, to date the reports
1
0 28
on this solid are scarce in the organic synthesis field [6].
Although MCRs are a valuable synthetic tool in the synthesis of heterocyclic
compounds and have advantages such a high atom economy, alternative sources of
heating are being considered in order to reduce reaction times [15, 16]. One of the
Fig. 1 Synthesis of 6-amino 4H-pyran derivative via a three-component coupling of aldehyde,
malononitrile and b-dicarbonyl compound
1
23

New application of decaniobate salt as basic solid in the…
most widely used in chemistry is microwave radiation, which has shown that it can
reduce the reaction times by generating a direct energy transfer to the reactants,
provoking instantaneous superheating [17–19]. The aim of this work was to study a
new application of decaniobates in MCR, to establish a clean and efficient method
for the synthesis of 4H-pyran derivatives with excellent yields under microwave
radiation at short reaction times.
Materials and methods
Preparation of decaniobate salts
A decaniobate salt [N(CH ) ] [Nb O ].6H O (HPNb) was synthesized according
3
4 6
10 28
2
to the procedure reported by Casey et al. [1]. In a typical procedure, Nb O
2
5
(
2.1 mmol) with an alcoholic solution of 3.3 mmol of tetramethylammonium
hydroxide pentahydrate [N(CH ) ]OH.5H O was heated to 120 °C in a Teflon-lined
3
4
2
Parr vessel for 18 h, after which it was allowed to cool to room temperature and
subsequently filtered, and the obtained solid was washed with acetone. To avoid the
formation of hexaniobate phases, Nb O was treated with HF [20] and then the
2
5
decaniobate salt was prepared. The final solid was denoted as HPNb-HF.
Characterization
Raman spectroscopy experiments were carried out at room temperature using a
Confocal Raman Microscope (Witec, alpha 300) with a 509 objective lens, a Nd:
YAG green laser with 532 nm wavelength and 800 scans. Spectra for each solid
-
1
-1
were taken over the range of 20 and 3000 cm , scanning at a step size of 1.0 cm
with an integration time constant of 10 s.
Thermogravimetric analysis was performed using a Setaram thermobalance. The
decaniobate powder (40.0 mg) was transferred to an alumina crucible and heated
-
1
using a 5 °C min heating rate up to 800 °C, under an N atmosphere flowing at
2
3
0 cm min
-1
2
.
The X-ray diffraction measurements were carried out with a Rigaku Miniflex II
˚
using Cu Ka radiation (k = 1.54056 A).
The XRD patterns were studied in the 2h range of 10°–60°, using a count time of
1
s and a step size of 0.05°.
IR spectra were obtained with a Nicolet iS50 spectrometer by the ATR method.
-
IR spectra were processed with a resolution of 4 cm and a spectral range of
1
-
1
4
000–400 cm
Temperature-programmed desorption (TPD) of CO was performed to study the
.
2
materials basicity. The equipment used was a Micromeritics AutoChem 2920. In
addition, volumetric titration with benzoic acid was carried out to measure the
basicity of the solid catalysts using the method reported by Tanabe et al. [21]. In this
method, for 0.25 g of catalyst, a benzoic acid solution 0.1 N and 1 mL of
bromothymol blue as indicator were used.
123

L. F. Gutierrez et al.
General procedure for the synthesis of 4H-pyrans
The 1,3-Diketo compound, malononitrile and aromatic aldehydes were used in the
synthesis of 4H-pyrans. All chemicals were purchased from Aldrich and used
without further purification. A mixture of ethyl acetoacetate or methyl acetoacetate
(
(
1 mmol), aldehyde (1 mmol), and malononitrile (1 mmol), and the catalysts
100 mg) were placed in a microwave tube containing a magnetic stirrer under
solvent-free conditions. This was heated under microwave irradiation at 80 °C. The
equipment used was an Anton Paar Monowave 400.
The reaction progress was monitored by TLC (1:2 EtOAc: petroleum ether as
mobile phase). The catalyst was recovered by filtration after precipitation of
decaniobate in acetone (2 9 1 mL). The crude product was recrystallized using hot
ethanol and no further recrystallization was required. The yield was expressed as the
ratio of moles of products/product to moles of initial aldehyde (Table 1).
All the products were identified by comparison of melting point (mp) and nuclear
1
13
magnetic resonance (NMR) data. H and C NMR spectra were performed on a
1
3
Bruker (400 MHz) spectrometer using TMS as internal reference. C NMR spectra
were recorded at 100 MHz.
Reuse and stability tests of the catalyst were carried out by running five
consecutive experiments, under the same reaction conditions. After each test, the
catalyst was separated from the reactions mixture by filtration, washed with acetone
(
2 9 1 mL), dried under vacuum, and thn reused.
Results and discussion
To confirm the presence of decaniobate phase ([Nb O ] ), different techniques
6
-
1
0 28
such as Raman spectra, XRD, TGA and FTIR were used. In the synthesis of (TMA)6
Nb O ]6H O (HPNb) HF was also incorporated with the aim of avoiding the
[
1
0
28
2
presence of hexaniobate phase that is generated as impurity by excess of basic
medium in the synthesis process.
Figure 2 shows the Raman spectra of HPNb and HPNb-HF in the region of
-
1
2
00–1200 cm . For comparison purposes, the Raman spectrum of amorphous
Nb O used for the synthesis of HPNb is shown. Figure 2a displays the band at
2
5
-
1
694 cm related to the Nb O T-phase consisting of 4 9 4 blocks forming the
2 5
corner-shared octahedral NbO ; each block is connected, sharing the edges of the
6
-
1
octahedron. Additionally, the band observed around 170 cm is associated with O–
Nb–O stretch vibrations [22]. The spectra of HPNb (Fig. 2b) evidenced the
-
1
appearance of a new signal around 935 cm
indicating the formation of
-
1
decaniobate species [23]. A small band near 200 cm indicates the presence of
the hexaniobate phase [23]. However, when the solid is synthesized in the presence
-
1
-1
of HF, the band at 200 cm disappears but the band at 935 cm decreases
significantly due to that at lower pH, as can be seen in Fig. 2c. In the presence of
HF, the decaniobate phase dissociates rapidly, leaving niobic acid [23]. Other bands
-
1
?
near 420 and 700 cm are related to TMA ions [24].
1
23

New application of decaniobate salt as basic solid in the…
a
Table 1 Decaniobate catalyzed synthesis of 4H-pyrans derivatives
Entry
1
R
R1
Yield (%)
100 (91)b
Time (h)
1 (2)b
CH
3
3
CH
2
–
2
CH
CH
CH
2
–
–
92
0.5
NO
2
3
4
5
CH
3
2
90
98
92
0.5
2
NO
2
CH
3
3
CH
2
2
–
H
3
C
CH
CH
–
2
H
3
CO
6
7
CH
3
3
CH
2
2
–
–
87
88
2
HO
Cl
CH
CH
0.5
8
9
CH
3
3
–
–
100
99
1
2
CH
H
3
C
1
0
1
CH
3
3
–
100
100
0.5
1
Cl
O
1
CH
CH
2
–
O
S
1
2
3
CH
3
3
–
100
100
1
1
1
CH
CH
2
–
–
14
N
CH
3
CH
2
95
2
a
Reaction conditions aldehyde (1 mmol), malononitrile (1 mmol), b-ketoester (1 mmol), 100 mg of
HPNb, solvent-free, microwave radiation and 80 °C
b
Aldehyde (1 mmol), malononitrile (1 mmol), b-ketoester (1 mmol), 100 mg of HPNb, solvent-free,
conventional heating and 80 °C
In all entries, the selectivity was 100%. No side products were detected
123

L. F. Gutierrez et al.
Fig. 2 Raman spectrum of
2 5
a amorphous Nb O ; b HPNb;
c HPNb-HF
Fig. 3 X-ray diffraction
patterns. a HPNb (Theoretical);
b HPNb-HF, c HPNb
The X-ray diffraction patterns of HPNb and HPNb-HF are shown in Fig. 3. The
powder XRD pattern of HPNb agrees well with that of (TMA) [Nb O ].6H O
6
10 28
2
reported previously [1]. The X-ray diffraction pattern reported by Ohlin et al. [1]
was initially considered (code CCDC 722216). Furthermore, the signals around
2
is composed of six NbO octahedral systems in a 2 9 3 arrangement attached to two
h = 10° and 20° are related to the reflection angles of the decaniobate phase, which
6
NbO octahedra above and two more sharing the edges below [25]. However, the
6
signals at 2h = 10.1° and 10.99° decrease when the decaniobate is prepared in the
presence of HF. This confirms that in an acid medium, the decaniobate structure is
unstable, which both promotes the dissociation towards niobic acid and loss of the
decaniobate crystalline phase.
Figure 4 shows the FTIR spectra of HPNb and HPNb-HF. The FTIR bands at
-
1
1
TMA , while the bands at 1000 and 911 cm correspond to Nb=O vibrations,
000 and 1400 cm are characteristic of CH and CH stretching bending of
2 3
?
-1
-
1
partly overlapped with the band of TMA ion [26]. The bands at 826 and 750 cm
1
23

New application of decaniobate salt as basic solid in the…
Fig. 4 FTIR spectra of
a HPNb; b HPNb-HF
were assigned to Nb–O asymmetric stretching. Two symmetric stretching bands of
-
1
Nb–O bond appear at 587 and 546 cm [26], which are indistinguishable in Fig. 4.
However, a shift to higher frequencies is evidenced, which is related to the large
cluster size, suggesting an increase in the number of Nb–O coordination species
[
27].
In our spectrum a signal appears at 1177 cm , which was not detected by Ohlin
-
1
et al. [26]. In order to clarify this point, various experiments were done. At the
beginning, the catalyst was rigorously washed with water (more than eight times) in
order to remove traces of TMAOH. The absence of TMAOH was evaluated by
FTIR (Fig. S2). Although there is a slight decrease in the intensity of the bands
-
1
- 1
between 1000 and 1400 cm , the band at 1177 cm
the absence of TMAOH in the structure. Moreover, in comparison with the
is retained, which confirms
?
spectrum of TMAOH (source of TMA ), it was found that those bands belong to the
?
TMA ion. However, the intensity of these bands in the spectrum of HPNb is much
?
greater than in TMA-OH (Fig. S3), given that the TMA ions are present as
stabilizing agents of the solid structure, thus discarding the presence of TMA-OH
species, which can have an important catalytic effect.
Fig. 5 Thermogram of
a HPNb, b HPNb-HF
123

L. F. Gutierrez et al.
Figure 5 displays the TGA curves of HPNb and HPNb-HF. In the decaniobate
salt, the first signal in the range 80–150 °C can be attributed to loss of physically
adsorbed water, while the second signal between 210 and 380 °C can be associated
?
with loss of TMA ligands. However, given the low concentration of the
decaniobate phase, this weight loss is less evident for the HPNb-HF solid. Finally,
the third mass loss is associated with decomposition, which occurs as a consequence
of the collapse of decaniobate structure and its transformation to oxide Nb O at a
2
5
temperature above 560 °C [28].
TPD-CO studies were used to compare the basicity of decaniobate solids.
Figure 6 depicts the temperature-programmed desorption spectra of two catalysts
2
(
HPNb and HPNb-HF). In the case of HPNb (Fig. 6a), the peak observed at around
?
3
decaniobate, in agreement with thermogravimetric analysis results. Indeed, the
00 °C could be assigned to TMA cations originating from the structure of
quantification of basicity strength using this technique is hard since at this
?
temperature the loss of TMA also occurs. A new peak in the desorption patterns of
HPNb appears at 380 °C, indicating a distinct behavior in comparison with HPNb
due to the desorbed CO .
2
As a comparative method, the basicity of the solids was determined by the
volumetric titration method with benzoic acid. It was found that the basicity of the
decaniobate HPNb (0.12 mmol of benzoic acid/g) is three times greater than that of
the HPNb-HF (0.04 mmol of benzoic acid/g), which confirms the low concentration
of decaniobate phase.
To optimize the reaction temperature in the microwave equipment, three
temperatures (60, 80, 120 °C) were tested in the reaction model between
malononitrile, ethyl acetoacetate benzaldehyde and HPNb. At lower temperature
(
60 °C), the reaction does not yield 4H-pyran but only the Knoevenagel
intermediate, while at 120 °C the reaction yields other unidentified products. Thus,
the chosen temperature was 80 °C. Table 1 summarizes the catalytic activity results
of the basic solid catalysts to obtain 4H-pyrans using a solvent-free reaction medium
and microwave radiation heating at 80 °C. In all cases, the desired products were
obtained with a selectivity of 100%, without side products and recovering, almost
2
Fig. 6 TPD-CO profiles of
a HPNb; b HPNb-HF
1
23

New application of decaniobate salt as basic solid in the…
Table 2 Catalyst reuse in the synthesis of 4H-pyrans
Catalytic cycle
Yield (%)
1
2
3
4
100
96
96
95
Reaction conditions benzaldehyde (1 mmol), malononitrile (1 mmol) and ethyl acetoacetate (1 mmol),
00 mg of HPNb, solvent-free, microwave radiation and 80 °C
1
quantitatively, the unchanged starting materials. The selectivity was 100%. No side
products were detected (Table 1).
No remarkable differences were attained using HPNb or HPNb-HF in the
synthesis of ethyl-6-amino-5-cyano-2-methyl-4-phenyl-4H-pyran-3-carboxylate
(
yield: 96%, selectivity: 100%), indicating that the reaction occurs regardless of
the amount of decaniobate phase, since a low basicity is enough to promote the
reaction. In the absence of catalyst at 80 °C, only the product of condensation
between benzaldehyde and malononitrile (Knoevenagel intermediate) was obtained
(
Table 2).
Several aldehydes with different substituents were tested with HPNb, and the
results (entries 2–14) show that aldehydes exhibiting electron-withdrawing
substituents favor shorter reaction times (entries 2, 3, 7 and 10) compared to the
aldehydes having electron donor groups [29] (entries 4, 5, 6, and 9). Although the
reaction systems show very similar yields, it is well known that the reduction of the
reaction time is a parameter of great importance in organic synthesis. This protocol
offers several advantages, such as using a reusable and efficient heterogeneous basic
catalyst.
The reuse of the catalyst was evaluated in four cycles of reaction using HPNb.
For this purpose, after completion of the reaction, the resulting mixture was stirred
with acetone and centrifuged for recovering the insoluble catalyst, which was
subsequently dried under vacuum and reused under the same procedure as described
before. The catalyst was easily isolated from the reaction mixture due to the
insolubility of the decaniobate salt in acetone and dichloromethane. This solid was
characterized by FTIR (Fig. S1). It can be seen that after the reuse experiments the
most characteristic signals of the decaniobate are preserved but in minor intensity,
which is possibly due to the adsorption of the substrate on the catalyst surface.
Conclusion
A new application of decaniobates in the synthesis of 4H-pyran derivatives was
studied. Although the synthesis of decaniobate modified with HF decreases the
formation of hexaniobate phases, it is not convenient since the decaniobate phase
dissociates rapidly to niobic acid. However, the reaction to afford 4H-pyrans was
123

L. F. Gutierrez et al.
successfully promoted regardless of the type of solid employed and it was favored
under microwave irradiation with short reaction times (1 h) and under solvent-free
conditions. After completion of the reaction, the recovered catalyst was reused, and
it retained its initial catalytic activity after four cycles.
Acknowledgements We thank COLCIENCIAS for the financial support under Project No.
110965843004. EN, GPR and AGS thank to UNLP, CONICET, MYNCIT and ERANET-1.
References
1
2
3
4
5
6
7
8
9
. C.A. Ohlin, E.M. Villa, W.H. Casey, Inorg. Chim. Acta. 362, 1391 (2009)
. Y. Ye, C. Chen, H. Feng, J. Zhou, J. Ma, J. Chen, Open J. Inorg. Chem. 3, 59 (2013)
. L. Shen, Y.-Q. Xu, Y.-Z. Gao, F.-Y. Cui, C.-W. Hu, J. Mol. Struct. 934, 37 (2009)
. P. Huang, C. Qin, Z.-M. Su, Y. Xing, X.-L. Wang, K.-Z. Shao, J. Am. Chem. Soc. 134, 14004 (2012)
. M. Nyman, Dalton Trans. 40, 8049 (2011)
. S. Hayashi, S. Yamazoe, K. Koyasu, T. Tsukuda, RSC Adv. 6, 16239 (2016)
. S. Hayashi, S. Yamazoe, K. Koyasu, T. Tsukuda, Chem. An Asian J. 12, 1635 (2017)
. R. Pagadala, S. Maddila, S. Jonnalagadda, J. Heterocycl. Chem. 52, 1226 (2015)
. H. Valizadeh, A. Azimi, J. Iran. Chem. Soc. 8, 123 (2011)
1
1
1
1
1
0. R.M.N. Kalla, M.R. Kim, I. Kim, Tetrahedron Lett. 56, 717 (2015)
1. G.P. Lu, C. Cai, J. Heterocycl. Chem. 48, 124 (2011)
2. S. Balalaie, S. Ramezanpour, M. Bararjanian, J.H. Gross, Catal. Commun. 38, 1078 (2008)
3. S. Sadjadi, M.M. Heravi, V. Zadsirjan, V. Farzaneh, Appl. Surf. Sci. 426, 881 (2017)
´
4. E. Nope, J.J. Mart ´ı nez, H.A. Rojas, A.G. Sathicq, G.P. Romanelli, Res. Chem. Intermed. 43, 2103
(2017)
1
1
1
1
1
2
2
2
5. E. Ruijter, R. Scheffelaar, R.V. Orru, Angew Chem. Int. Ed. 50, 6234 (2011)
6. L.M. Sanchez, H.J. Thomas, G.P. Romanelli, Mini Rev. Org. Chem. 12, 115 (2015)
7. N.R. Guha, D. Bhattacherjee, P. Das, Tetrahedron Lett. 55, 2912 (2014)
8. E.D.J.M. Prieto, B. Rivas, J. S a´ nchez, Cienc en Desarro. 4, 219 (2013)
9. K. Badamali, R. Luque, J.H. Clark, S.W. Breeden, Catal. Commun. 10, 1010 (2009)
0. R. Das, J. Ray, P. Pramanik, J. Mater. Res. 15, 2273 (2000)
1. K. Tanabe, T. Yamaguchi, J. Res. Inst. Catal. Hokkaido Univ. 11, 179 (1964)
2. P. Chagas, H.S. Oliveira, R. Mambrini, M. Le Hyaric, M.V. de Almeida, L.C. Oliveira, Appl. Catal.
A Gen. 454, 88 (2013)
23. M. Aureliano, C.A. Ohlin, M.O. Vieira, M.P. Marques, W.H. Casey, L.A. de Carvalho, Dalton Trans.
45, 7391 (2016)
2
2
2
2
2
2
4. A.L. Shiguihara, M.A. Bizeto, V.R. Constantino, J. Braz. Chem Soc. 21, 1366 (2010)
5. E.J. Graeber, B. Morosin, Acta Crystallogr. Sect B. 33, 2137 (1977)
6. J.-H. Son, C.A. Ohlin, W.H. Casey, Dalton Trans. 42, 7529 (2013)
7. A. Fielicke, G. Meijer, G. Helden, J. Am. Chem. Soc. 125, 3659 (2003)
8. J. Niu, X. Fu, J. Zhao, S. Li, P. Ma, J. Wang, Cryst. Growth Des. 10, 3110 (2010)
9. S.W. Kshirsagar, N.R. Patil, S.D. Samant, Synth. Commun. 41, 1320 (2011)
1
23