Preparation, characterization, and use of poly(vinylpyrrolidonium) hydrogen phosphate ([PVP-H]H2PO4) as a new heterogeneous catalyst for efficient synthesis of 2-amino-tetrahydro-4H-pyrans

Article abstract of DOI:10.1007/s11164-015-2312-y

A novel Br?nsted solid acid named poly(vinylpyrrolidonium) hydrogen phosphate ([PVP-H]H₂PO₄) has been prepared by using H₃PO₄ and poly(vinylpyrrolidone) as green and commercially available reagents. The prepared

Full text of DOI:10.1007/s11164-015-2312-y

Res Chem Intermed
DOI 10.1007/s11164-015-2312-y
Preparation, characterization, and use
of poly(vinylpyrrolidonium) hydrogen phosphate
([PVP-H]H PO ) as a new heterogeneous catalyst
2 4
for efficient synthesis of 2-amino-tetrahydro-4H-pyrans
1
1
•
Farhad Shirini
Maryam Akbari
•
Omid Goli-Jolodar
•
2
1
Mohadeseh Seddighi
Received: 20 June 2015 / Accepted: 3 October 2015
Ó Springer Science+Business Media Dordrecht 2015
Abstract A novel Brønsted solid acid named poly(vinylpyrrolidonium) hydrogen
phosphate ([PVP-H]H PO ) has been prepared by using H PO and poly(vinylpyrroli-
2
4
3
4
done) as green and commercially available reagents. The prepared reagent was char-
acterized using a series of techniques including Fourier-transform infrared (FT-IR)
spectroscopy, thermogravimetric analysis (TGA), scanning electron microscopy (SEM),
X-ray diffraction (XRD) analysis, energy-dispersive X-ray analysis (EDAX), pH anal-
ysis, and Hammett acidity function. The mentioned solid acid with wide acidic func-
tional group sites can be utilized as a highly efficient, heterogeneous, and reusable
catalyst for preparation of 2-amino-3-cyano-5-oxo-5,6,7,8-tetrahydro-4H-benzopyrans
using aldehydes, dimedone, and malononitrile. Also, this catalyst can be reused for
several times without loss of catalytic activity.
Keywords Brønsted solid acid Á Poly(vinylpyrrolidone) Á H PO Á Pyran Á
3
4
2
-Amino-4H-pyrans
Introduction
Mineral liquid acids such as H SO , HCl, H PO , HNO , etc. have many
2
4
3
4
3
applications in chemical industry for synthesis of organic and inorganic chemical
materials. Large quantities of these acids, especially sulfuric acid, nitric acid, and
hydrochloric acid, are manufactured for commercial use in vast plants. Furthermore,
liquid acids have a degree of instability in the presence of air and moisture [1]. Also,
H PO is utilized in chemical production, metal treatment, refractories, ceramics,
3
4
&
Farhad Shirini
shirini@guilan.ac.ir
1
2
Department of Chemistry, College of Science, University of Guilan,
Post Box 1914, Rasht 41335, I. R. Iran
Department of Chemistry, University of Guilan, University Campus 2, Rasht, I. R. Iran
123

F. Shirini et al.
miscellaneous industries, and for food and beverages. In spite of the fact that
phosphoric acid is an efficient green acid, its application suffers from various
problems such as high cost, large consumption, and difficult recovery. Also
separation of products and large-scale application are difficult with this reagent.
To avoid these problems, solid acids could be used as easily recyclable and
ecofriendly catalysts. These materials have many other advantages such as
operational simplicity, ease of recoverability, and noncorrosive nature, making
them more efficient than traditional mineral liquid acids.
One of the most efficient heterogeneous and polymeric supports for numerous
reagents and catalysts is cross-linked poly(vinylpyrrolidone) (PVP). This is because
of its commercial availability, stability, reasonably high loading capacity, good
physicochemical structure, and facile functionalization. PVP is also one of the most
widely used excipients in pharmaceuticals, for example, as a binder in tablets,
granules, and capsules. Other applications of this reagent include as a stabilizer for
suspensions, ophthalmic preparations, sugar coating, film coating, and miscella-
neous other applications [2]. In addition, it is an attractive support to immobilize
mineral acids because of the basic nature of the pyrrolidonyl group. Therefore,
various PVP-supported reagents have been designed to catalyze some organic
reactions [3–7].
The 4H-pyran group and its derivatives belong to a major class of natural
heterocyclic compounds used in cosmetics and pigments, and as potentially
biodegradable agrochemicals [8–10]. Also, they may contribute to potential
biological and pharmacological activities such as spasmolytic, diuretic, anticoag-
ulant, anticancer, antianaphylactic, antioxidant, and anti-human immunodeficiency
virus (HIV) effects [9, 11–16]. 2-Amino-4H-pyrans are a series of 4H-pyran
derivatives with an improved structural scaffold and many biological activities [17,
1
8] as well as photochemical reactivity [19]. These derivatives of 4H-pyran can be
synthesized via condensation of aromatic aldehydes, 1,3-cyclohexanedione deriva-
tives, and malononitrile in a one-pot three-component condensation reaction.
In view of the biological, industrial, and synthetic applications of 4H-pyran and
2
-amino-4H-pyran derivatives, a variety of catalysts such as nano-ZnO [20], NaBr
[
21], I [22], CeCl Á7H O [23], tetrabutylammonium fluoride (TBAF) [24], 1,4-
2
3
2
diazabicyclo[2.2.2]octane (DABCO) [25], hexadecyldimethylbenzyl ammonium
bromide (HDMBAB) [26], PEG-1000-based dicationic acidic ionic liquid
(
PEG1000-DAIL) [27], MeCO H [28], SiO -Pr-SO H [29], polyphosphoric acid
2 2 3
(
formin-modified silica-coated magnetic nanoparticles (MNPs) (Fe O /SiO2-Met)
PPA–SiO ) [30], N(Et) ClO [31], Caro’s acid–silica gel (CA-SiO ) [32], met-
2
4
4
2
3
4
2
-
[
urea:choline chloride (as a deep eutectic solvent) [37], Fe O @SiO /DABCO [38],
33], RuBr (PPh ) [34], 2,2,2-trifluoroethanol (TFE) [35], SO /MCM-41 [36],
2 3 4 4
3
4
2
potassium phthalimide-N-oxyl (POPINO) [39], and silica-bonded N-propylpiperazine
sodium n-propionate (SBPPSP) [40] have been used to facilitate this reaction.
Although these procedures improve synthesis of these compounds, some of them
suffer from disadvantages such as long reaction time, harsh reaction condition, need
for excess amount of reagent, low product yield, and catalyst nonrecoverability.
Therefore, introducing simple, efficient, and mild procedures with easily separable
and reusable solid catalysts to overcome these problems is still in demand.
1
23

Preparation, characterization, and use of…
Experimental
Chemicals were purchased from Fluka, Merck, and Aldrich chemical companies.
All yields refer to isolated products. Products were characterized by comparison of
their physical constants, and infrared (IR) and nuclear magnetic resonance (NMR)
spectra with authentic samples and those reported in literature. Purity determination
of substrate and reaction monitoring were carried out by thin-layer chromatography
(
TLC) on silica gel poly gram SILG/UV 254 plates.
FT-IR spectra were obtained on a VERTEX 70 (Bruker, Germany). Thermo-
gravimetric analysis (TGA) was performed on a Polymer Laboratories PL-TGA
thermal analysis instrument (UK). Samples were heated from 25 to 600 °C at a
ramp rate of 10 °C/min under N atmosphere. Scanning election microscopy (SEM)
2
was carried out using a Philips XL30. X-ray diffraction (XRD) measurements were
performed at room temperature on an XRD 6000 X’Pert Pro diffractometer (Philips)
˚
using Co K radiation (K = 1.7890 A) with beam voltage and beam current of
a
4
0 kV and 30 mA, respectively.
Preparation of [PVP-H]H PO : Phosphoric acid (1.2 mL, 18 mmol) was added to
suspension of powdered poly(vinylpyrrolidone) (2.0 g) [cross-linked
2
4
a
poly(vinylpyrrolidone) with MW [ 1,000,000] in 10 mL dry CH Cl over a period
2
2
of 15 min. The mixture was stirred at room temperature for 5 h and then filtered.
The solid residue was washed with ethyl acetate (20 mL) and dried at 80 °C to
afford [H-PVP]H PO as a pale-yellow powder (Scheme 1).
2
4
General procedure for preparation of 2-amino-3-cyano-5-oxo-5,6,7,8-tetrahydro-
H-benzopyrans: A mixture of aldehyde (1 mmol), dimedone and/or 1,3-cyclo-
4
hexadione (1 mmol), malononitrile (1.1 mmol), and [PVP-H]H PO (10 mg,
2
4
2
.5 mol%) in EtOH:H O (1:1) (4 mL) was heated at 80 °C. Progress of the
2
reaction was monitored by TLC. Upon completion of the reaction, the solvent was
evaporated, EtOH was added, and the catalyst was separated by filtration. The
related product was purified by recrystallization from aqueous ethanol.
1
13
Spectral ( H and C NMR) data of new compounds are presented below:
0
4
,4 -(1,3-Phenylene)bis(2-amino-7,7-dimethyl-5-oxo-5,6,7,8-tetrahydro-4H-chro-
mene-3-carbonitrile) (s): m.p. 243–244 °C; IR (neat) m = 3436, 3330, 3180, 2964,
-1, 1
196, 1653, 1600, 1365, 1213, 1150, 1030 cm
2
H NMR (DMSO-d , 400 MHz):
6
d = 0.955 (s, 6H, CH ), 1.057 (s, 6H, CH ), 2.045 (d, J = 17.4 Hz, 2H), 2.8 (d,
3
3
AB
JAB = 17.4 Hz, 2H), 2.38 (d, JAB = 17.4 Hz, 2H), 2.59 (d, JAB = 17.4 Hz, 2H),
.113 (s, 2H), 6.08 (s, 1H, Ar), 6.921 (s, 4H, NH), 7.01 (2H, dd, J = 7.6 Hz,
4
J2 = 1.6 Hz), 7.192 (t, 1H, J = 7.6, Ar) ppm; C NMR (DMSO-d , 100 MHz):
1
1
3
6
n
n
n
H PO , (85-90%)
3
4
N
N
O
O
N
OH
H PO
2 4
CH Cl , r.t., 5 h
2
2
2 4
Scheme 1 Preparation of catalyst [PVP-H]H PO
123

F. Shirini et al.
d = 19, 26.9, 29.3, 32.1, 35.7, 50.3, 56.5, 58.6, 113.2, 120.1, 125.1, 126.2, 128.3,
1
45.5, 159, 162.9, 195.9.
0
4
,4 -(1,4-Phenylene)bis(2-amino-7,7-dimethyl-5-oxo-5,6,7,8-tetrahydro-4H-chro-
mene-3-carbonitrile) (t): m.p. 266–268 °C; IR (neat) m = 3448, 3327, 3189, 2960,
-1, 1
194, 1655, 1602, 1363, 1210, 1148, 1029 cm
2
H NMR (DMSO-d , 400 MHz):
6
d = 0.985 (s, 3H, CH ), 1.035 (s, 3H, CH ), 2.16 (d, J = 16.4 Hz, 1H), 2.23
3
3
AB
(
1
d, J = 16.4 Hz, 1H), 2.50 (s, 2H), 4.14 (s, 1H), 6.98 (s, 2H), 7.04 (s, 2H) ppm;
AB
3
C NMR (DMSO-d , 100 MHz): d = 27.4, 27.6, 28.6, 32.3, 35.4, 50.4, 58.8, 113.1,
6
1
20.2, 127.4, 143.3, 159.0, 163.1, 196.1 ppm.
Results and discussion
Catalyst characterization
The FT-IR spectra of H PO , PVP, and [PVP-H]H PO are presented in Fig. 1.
2
3
4
4
Successful loading of H PO onto the surface of PVP led to an increased number of
3
4
Fig. 1 FT-IR spectra of H
3
PO
4
2 4
, PVP, and [PVP-H]H PO
1
23

Preparation, characterization, and use of…
vibration modes. The overlap of some peaks results in broadening of the [PVP-
H]H PO spectrum. In the case of [PVP-H]H PO , the broad band between 2700
2
4
2
4
-
and 3750 cm , centered at 3410 cm , can be attributed to OH stretching of
1
-1
-
H PO group. The bands appearing at 1170, 1105, 992, and 822 cm can be
1
2
4
assigned to P=O stretching, P–O bending, and stretching vibrations, respectively
-
1
[
(
41]. Furthermore, in this spectrum, the bands at 1429 (C–N) and 650 cm
-
N–C=O) disappeared and a moderate absorption at 1648 cm corresponding to
1
internal imine groups present in the pendant rings of the polymer appeared [7].
The samples of PVP and [PVP-H]H PO were also analyzed by scanning
2
4
electron microscopy (SEM) at various magnifications to determine the size
distribution, particle shape, surface morphology, and diagram of surface, as
presented in Fig. 2. These images show that, with chemical modification, the
primary morphology of PVP changed completely and particles were localized by the
modification process. Both size reduction and increased aggregation resulted in
swelling of the surface area of the catalyst. This increased the catalytic activity of
the catalyst in the reaction [42]. As seen in the diagram of surface (Fig. 2c, d), PVP
showed an almost smooth surface, but after modification the surface became
uneven.
EDAX spectra were recorded to analyze the elemental constitution qualitatively
for the modified PVP. Figure 3 depicts the EDAX spectra for [PVP-H]H PO . The
2
4
presence of phosphor peak confirms the presence of P in the lattice of [PVP-
H]H PO .
2
4
2 4
Fig. 2 SEM micrographs of PVP (a, c) and [PVP-H]H PO (b, d)
123

F. Shirini et al.
2 4
Fig. 3 EDAX spectrograph of [PVP-H]H PO
To gain insight into the thermal changes in the PVP and [PVP-H]H PO system,
2
4
the thermal stability was probed by thermogravimetric analysis. Figures 4 and 5
display typical TGA and differential TGA (DTGA) spectra for native PVP and
[PVP-H]H PO , using weight percent for the TGA curve and differential weight for
2 4
the DTGA curve.
As can be seen from these figures, both PVP and [PVP-H]H PO showed one-
4
2
step thermal decomposition below 120 °C, corresponding to loss of moisture.
Furthermore, the surfaces of PVP almost loaded with H PO because [PVP-
H]H PO have a completely different curves rather than PVP [43]. For PVP, the
3
4
2
4
maximum weight loss occurred at 350–470 °C (Tmax = 428 °C), corresponding to
degradation of the polymer backbone, but in the case of [PVP-H]H PO the
2
4
maximum weight loss (due to decomposition of the backbone) occurred at lower
temperature of 230–315 °C (Tmax = 282.5 °C).
Figure 6 presents the X-ray diffraction (XRD) patterns of the PVP and [PVP-
H]H PO samples. Incorporation of H PO led to reduced intensity and some
2
4
3
4
changes in the diffractogram of PVP. The XRD pattern of PVP reveals a couple of
broad bands located at 2h = 13.5° and 23.5°, clearly indicating the amorphous
nature of the host PVP [44]. After modification of PVP by H PO , the first peak (2h
3
4
around 13.5°) disappeared whereas the broad peak at 2h around 23.5° increased
slightly. These observations imply that PVP lost its primal structure after reaction
with H PO .
3
4
To 25 mL of an aqueous solution of NaCl (1 M) with a primary pH 5.2, [PVP-
H]H PO (0.5 g) was added and the resulting mixture was stirred for 2 h at room
temperature. The pH of the solution decreased to 1.3. This is equal to a loading of
2
4
?
2
.5 mmol H /g [45].
1
23

Preparation, characterization, and use of…
Fig. 4 TGA curves for PVP and [PVP-H]H
2
PO
4
Fig. 5 DTGA curves for PVP and [PVP-H]H
PO
2 4
The Hammett acidity function can efficiently express the acid strength of an acid
in organic solvents [46, 47]. It can be calculated by the equation below:
À
Á
þ
s
H0 ¼ pKðIÞ þ log ½IŠ =½IH Š ;
aq
s
?
where ‘‘I’’ depicts the indicator base (mainly substituted dinitroanilines), and [IH ]
s
and [I] are the molar concentrations of the protonated and unprotonated forms of
s
the indicator, respectively. The pK(I)aq values are already known [for example, the
pK(I)aq value of 4-nitroaniline is 0.99] and can be obtained from literature.
?
According to the Lambert–Beer law, the value of [I] /[IH ] can be determined and
s
s
calculated from UV–Vis spectra. In our experiment, 4-nitroaniline was chosen as
the basic indicator and CCl4 as the solvent. The maximal absorbance of the
123

F. Shirini et al.
2 4
Fig. 6 XRD patterns of PVP in comparison with [PVP-H]H PO
unprotonated form of 4-nitroaniline was observed at 330 nm in CCl . Figure 7
4
compares the absorbance of [PVP-H]H PO and also recovered catalyst after five
2
4
runs.
As shown in Fig. 7, in comparison with blank solution, the absorbance of the
unprotonated form of the indicator in [H-PVP]H PO was weak compared with the
sample of the indicator in CCl , showing that the indicator was partially in the form
2
4
4
?
of [IH ]. The obtained results are listed in Table 1, which shows the acid strength of
[
PVP-H]H PO .
2 4
Fig. 7 Absorption spectra of 4-nitroaniline (indicator) (a) recovered [PVP-H]H
b) and [PVP-H]H PO (catalyst) (c) in CCl
2 4
PO after five runs
(
2
4
4
1
23

Preparation, characterization, and use of…
Table 1 Calculation of Hammett acidity function (H
0
) for [PVP-H]H
[I]
2 4
PO
?
[IH ]
Catalyst
A
max
s
%
s
%
H
0
–
2.2093
1.4471
1.5541
100
0
–
[PVP-H]H
2
PO
4
65.5
35.5
29.65
Recovered after five runs
70.3435
Condition for UV–Vis spectrum measurement: solvent: CCl
4
, indicator: 4-nitroaniline [pK(I)aq = 0.99],
-4
1
.44 9 10 mol/L (10 mL); catalyst: [PVP-H]H PO (10 mg), 25 °C
2 4
2 4
Table 2 Optimization of reaction conditions catalyzed by [PVP-H]H PO
a
Entry
Catalyst (mg)
Solvent
Temperature (°C)
Time (min)
Yield (%)
1
2
3
4
5
6
7
a
20
20
20
10
30
30
0
EtOH
Reflux
80
60
20
12
8
87
H
2
O
85
EtOH:H
EtOH:H
EtOH:H
EtOH:H
EtOH
2
2
2
2
O
O
O
O
80
93
80
94
80
15
30
120
95
60
94
Reflux
Trace
Isolated yields
O
O
O
Ar
O
CN
CN
R
R
[PVP-H]H PO (10 mg)
CN
2
4
Ar
H
EtOH:H2O (1:1), 80 °C
R
R
O
NH2
R= H, CH3
PO
Scheme 2 Synthesis of 2-amino-4H-pyran derivatives in presence of [PVP-H]H
2
4
Catalytic activity
On the basis of the information obtained from the studies on [PVP-H]H PO , we
4
2
anticipated that this reagent could be used as a solid acid catalyst for promotion of
reactions requiring an acidic catalyst for speed-up. So, we were interested in
investigating the applicability of this reagent for promotion of synthesis of 2-amino-
4
H-pyran derivatives.
First and for optimization of the reaction conditions, the reaction of 4-chloroben-
zaldehyde, dimedone (5,5-dimethyl-1,3-cyclohexanedione), and malononitrile to
obtain corresponding product was selected as a model reaction under various
conditions (Table 2). Different solvents such as EtOH, H O, and EtOH:H O were
2
2
used to choose the reaction medium, the best results being obtained in EtOH:H O
2
(
1:1). Having optimized the reaction medium, further experiments were performed
123

F. Shirini et al.
2 4
Table 3 Preparation of 2-amino-4H-pyran derivatives catalyzed by [PVP-H]H PO
Time
Yield
(%)a
Mp (°C)
Entry
Substrate
Product
(min)
Found
Reported [ref.]
CHO
O
1
10
92
91
94
231-233
231-233 [15]
CN
O
O
O
NH2
CHO
Cl
Cl
O
O
2
3
15
208-210
232-234
213-215 [30]
233-234 [18]
CN
NH2
CHO
NO2
NO2
CN
5
NH2
CH3
CHO
O
O
4
5
20
92
93
204-206
212-214
198-200 [34]
212-214 [20]
CN
CH3
NO2
O
NH2
NO2
CHO
3
CN
O
NH2
Cl
CHO
6
O
8
94
208-210
206-207 [24]
Cl
CN
O
NH2
Br
CHO
7
O
9
95
201-203
200-203 [26]
Br
CN
NH2
O
1
23

Preparation, characterization, and use of…
Table 3 continued
Time
min)
Yield
(%)a
Mp (°C)
Entry
Substrate
Product
(
Found
Reported [ref.]
NO2
CHO
8
O
O
O
3
96
94
93
178-180
196-198
203-205
177-178 [15]
O N
CN
2
O
OMe
NH2
CHO
9
25
25
195-197 [21]
204-205 [21]
MeO
CN
O
OH
NH2
CHO
1
0
1
HO
CN
O
O
NH2
CHO
CHO
1
1
O
10
35
93
91
258-260
189-191
258-260 [42]
CN
NH2
2
O
O
183-185 [33]
CN
O
O
NH2
O
HN
O
1
3
15
92
93
296-298
212-214
295-297 [43]
212-214 [20]
O
CN
N
H
NH2
NO2
CHO
O
1
4
3
CN
NO2
O
NH2
123

F. Shirini et al.
Table 3 continued
Time
Yield
(%)a
Mp (°C)
Entry
Substrate
Product
(min)
Found
Reported [ref.]
224-226 [20]
Cl
CHO
CHO
CHO
1
1
1
5
O
O
O
10
93
91
95
234-236
Cl
CN
O
OMe
NH2
6
7
30
196-198
235-237
192-194 [21]
234-236 [20]
MeO
CN
O
NO2
NH2
3
O N
CN
2
O
O
NH2
CHO
1
1
8
15
94
251-253
254-255 [42]
O
CN
NH2
H N
2
NC
O
CHO
O
9
25
93
266-268
243-244
—
—
O
OHC
O
CN
NH
2
CHO
O
O
O
NH2
2
0
25
92
CN
CN
O
CHO
NH
2
a
Isolated yields
1
23

Preparation, characterization, and use of…
H2N
NC
O
CH3
CH3
O
O
CHO
CN
O
H3C
H3C
H3C
O
NH2
O
H3C
CHO
O
H2N
NC
or
CHO
CH3
CN
CN
CH3
OHC
O
O
O
H3C
H3C
CN
NH2
(
Table 2, entry 19 and 20)
Scheme 3 Condensation of terephthalaldehyde with dimedone and malononitrile catalyzed by [PVP-
H]H PO
2
4
+
H from [PVP-H]H2PO4
O
O
Ar
H
O
CN
R
R
Ar
H
CN
O
OH
NC
CN
R
R
IV
III
I
II
Ar
O
O
O
Ar
H
CN
O
Ar
H
CN
NH
CN
R
R
R
R
N
O
VI
R
O
NH2
R
V
+
Product
H from [PVP-H]H2PO4
R= H, CH3
Scheme 4 Proposed mechanism for reaction of aldehyde, dimedone, and malononitrile using [PVP-
H]H PO
2
4
to delineate the best reaction. Finally, the best result was obtained using 10 mg
PVP-H]H PO in EtOH:H O (1:1) at 80 °C (Scheme 2).
After optimization of the reaction conditions and to establish the effectiveness
[
2
4
2
and suitability of this method, we explored the protocol with a variety of simple,
123

F. Shirini et al.
1
00
8
6
4
2
0
0
0
0
0
1
8
94
2
8
93
3
9
93
4
9
92
5
11
90
Time (min)
Yield (%)
Run
2 4
Fig. 8 Reusability of [PVP-H]H PO
readily available substrates under the optimal conditions (Table 3). Under the
determined conditions, it was observed that a wide range of aromatic aldehydes
containing electron-withdrawing as well as electron-donating groups such as Cl, Br,
CH , OCH , NO , and OH in ortho, meta, and para positions of the benzene ring
3
3
2
easily converted to corresponding 2-amino-4H-pyrans in short reaction time with
good to excellent isolated yield (Table 3, entries 1–10). Cinnamaldehyde, 2-naph-
thaldehyde, and isatin, as a,b-unsaturated, polycyclic aromatic, and heterocyclic
substrates, respectively, also provided the desired products in good yields (Table 3,
entries 11–13). Furthermore, when 1,3-cyclohexadione was employed in place of
dimedone, the corresponding products were also obtained during short reaction
times in high yields (Table 3, entries 14–18).
This method was also found to be useful for dialdehydes (Table 3, entry 19 and
0). In this reaction, 2 equivalents of dimedone and malononitrile successfully
2
condensed with 1 equivalent of isophthalaldehyde and terephthalaldehyde with high
yield at short time, showing the practical synthetic efficiency of this reaction
(
Scheme 3).
The proposed mechanism for synthesis of 2-amino-3-cyano-5-oxo-5,6,7,8-
tetrahydro-4H-benzopyrans in the presence of [PVP-H]H PO as catalyst is shown
2
4
in Scheme 4. At the first step in this mechanism, a-cyanocinnamonitrile (III) forms
initially by Knoevenagel condensation between aldehyde and malononitrile in the
presence of [PVP-H]H PO . Then, Michael addition of enolizable dimedone (IV)
2
4
on intermediate (III), followed by intramolecular cyclization and final tautomer-
ization of intermediates, affords the desired product.
To check the reusability of the catalyst, the reaction of 4-chlorobenzaldehyde,
dimedone, and malononitrile under the optimized reaction conditions was studied
again. When the reaction completed, the solid was filtered off, ethanol was added,
and the catalyst was separated by filtration. The recovered catalyst was washed with
ethyl acetate, dried, and reused for the same reaction. This process was carried out
1
23

Preparation, characterization, and use of…
Fig. 9 FT-IR spectra of 2-amino-3-cyano-4-(4-chlorophenyl)-7,7-dimethyl-5-oxo-5,6,7,8-tetrahydro-
2 4
4H-benzopyran using fresh (a) and recovered (b) (after five runs) [PVP-H]H PO
over five runs, and all reactions led to the desired products without significant
change in reaction time or yield, clearly demonstrating the practical recyclability of
this catalyst (Fig. 8).
Furthermore, the FT-IR spectra of 2-amino-3-cyano-4-(4-chlorophenyl)-7,7-
dimethyl-5-oxo-5,6,7,8-tetrahydro-4H-benzopyran using fresh (a) and recovered
(
b) (after five runs) [PVP-H]H PO are shown in Fig. 9. The presence of typical
2 4
absorption bands in the FT-IR spectra of products demonstrates that, in the course of
recovery, the quality of the products was retained.
To highlight the merits of our newly developed procedure, we compared our
results for synthesis of 2-amino-3-cyano-4-(4-nitrophenyl)-7,7-dimethyl-5-oxo-
5
,6,7,8-tetrahydro-4H-benzopyran using the [PVP-H]H PO catalyst with other
2 4
results reported in literature for the same transformation. As shown in Table 4, the
obtained results show that the catalytic activity of [PVP-H]H PO is convincingly
2
4
123

F. Shirini et al.
Table 4 Comparison of results obtained for synthesis of 2-amino-3-cyano-4-(4-nitrophenyl)-7,7-dime-
thyl-5-oxo-5,6,7,8-tetrahydro-4H-benzopyran in presence of [PVP-H]H PO with those obtained using
2 4
other procedures
Entry
Catalyst
Cat. loading
mol%)
Reaction
conditions
Time
(min)
Yield
a
(%)
Ref.
(
1
2
3
4
5
6
7
8
9
Nano-ZnO
TBAF
10
10
10
12
R.t./EtOH:H
2
O
3 h
30
2 h
7 h
60
15
8
91
75
94
93
93
93
85
93
75
96
[20]
Reflux/H
2
O
O
[24]
DABCO
Reflux/H
2
[25]
HDMBAB
PEG1000-DAIL
90 °C/H
2
O
[26]
160
5
80 °C/toluene
Reflux/EtOH:H
[27]
SiO
PPA-SiO
CA-SiO
RuBr (PPh
[PVP-H]H
2
-Pr-SO
3
H
2
O
O
[29]
2
5
Reflux/H
Reflux/EtOH:H
Reflux/CH OH
80 °C/EtOH:H
2
O
[30]
2
16
5
2
15
12
3
[32]
2
3
)
4
3
[34]
1
0
2
PO
4
2.5
2
O
This work
a
Isolated yields
superior to other reported procedures in terms of catalyst loading, reaction time, and
yield.
Conclusions
We have demonstrated an efficient procedure for synthesis of 2-amino-tetrahydro-
4H-pyrans as biologically and pharmacologically active products using a new
heterogeneous polymeric catalyst [PVP-H]H PO . Moreover, this functionalized
2
4
solid acid catalyst can be readily synthesized and easily used as an efficient and
recyclable catalyst in the desired reactions. In addition, this methodology offers
significant improvements with regard to product yield and reaction time.
Acknowledgments The authors are grateful to the Guilan University Research Council for partial
support of this work.
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