Nano TiO2-supported Preyssler-type heteropolyacid (nanoTiO 2/H14[NaP5W30O110]): An efficient and re-usable catalyst for the one-pot synthesis of N-acetyl-2-aryl-1,2-dihydro-(4H)-3,1-benzoxa

Article abstract of DOI:10.1007/s13738-013-0242-4

An efficient procedure has been developed for one-pot synthesis of N-acyle-2-aryl-1,2-dihydro-(4H)-3,1-benzoxazin-4-ones in the presence of Preyssler-type heteropolyacid acid modified nano-sized TiO₂ as catalyst. The reactions proceed smoothly

Full text of DOI:10.1007/s13738-013-0242-4

J IRAN CHEM SOC
DOI 10.1007/s13738-013-0242-4
ORIGINAL PAPER
Nano TiO2-supported Preyssler-type heteropolyacid (nanoTiO2/
H14[NaP5W30O110]): an efficient and re-usable catalyst
for the one-pot synthesis of N-acetyl-2-aryl-1,2-dihydro-(4H)-3,
1-benzoxazin-4-ones
•
Davood Azarifar Seyed-Mola Khatami
•
•
Mohammad Ali Zolfigol Davood Sheikh
Received: 25 November 2012 / Accepted: 13 February 2013
Ó Iranian Chemical Society 2013
Abstract An efficient procedure has been developed for
one-pot synthesis of N-acyle-2-aryl-1,2-dihydro-(4H)-3,
1-benzoxazin-4-ones in the presence of Preyssler-type
heteropolyacid acid modified nano-sized TiO2 as catalyst.
The reactions proceed smoothly at 25 °C to afford the
products in high yields. The catalyst is easily separated and
re-used for the next successive reactions without significant
loss of its activity.
have been evolved for the synthesis of N-substituted-1,
2-dihydro-(4H)-3,1-benzoxazin-4-ones [19–21]. Wiklund
and co-workers reported a method for the synthesis of
N-acetyl-1,2-dihydro-4H-3,1-benzoxazin-4-one via the
reaction of N-acylated anthranilic acids with paraformal-
dehyde in refluxing AcOH [22]. However, most of the
reported methods are subject to certain drawbacks, such as
longer reaction times, low yield, and harsh reaction con-
ditions. Therefore, development of new and benign
approaches still appears as important experimental chal-
lenge. Of the most commonly used catalysts, heteropoly-
acids (HPAs), as supported or in bulk form, have exhibited
high activities as solid acid catalysts in various reactions
[23]. Preyssler-type heteropolyacid-mediated-nano TiO2
has been proved as a very efficient and environmentally
benign solid acid catalyst to effect various chemoselective
reactions in the liquid phase [24]. Recently, we have
reported an efficient and facile ultrasound-accelerated one-
pot synthesis of novel derivatives of N-acetyl-2-aryl-1,2-
dihydro-(4H)-3,1-benzoxazin-4-ones [25].
Keywords Preyssler-type heteropolyacid ꢀ Nano-TiO2 ꢀ
N-acetyl-2-aryl-1,2-dihydro-(4H)-3,1-benzoxazin-4-one ꢀ
Heterogeneous catalyst
Introduction
4H-3,1-Benzoxazin-4-ones are an important class of het-
erocyclic compounds [1], which are of significant biolog-
ical activities used as potent inactivators of chymotrypsin
[2–4], inhibitors of human leukocyte elastase [5, 6], and
HSV-1 protease [7]. They also act as key intermediates in
synthetic organic reactions [1]. In addition, these com-
pounds are valuable precursors for the synthesis of various
heterocycles, such as 2,3-disubstituted quinazolin-4(3H)-
ones [8–15]. A wide variety of such heterocyclic com-
pounds occur in nature [16, 17], and are employed as
linking units in polymer chemistry [18]. The chemistry of
benzoxazin-4-one is anticipated to play a vital role in the
coming years in the synthesis of medicinal and natural
products [1]. To the best of our knowledge, only few routes
The aim of this study is to investigate the catalytic
capability and enhancement of the efficiency of the
nano-sized TiO2 for the synthesis of N-acetyl-2-aryl-1,
2-dihydro-(4H)-3,1-benzoxazin-4-ones by supporting the
Preyssler-type heteropolyacid on TiO2 nanoparticles.
Results and discussion
Effect of the catalyst particle size on the reaction
In continuation of our interest in exploring new and con-
venient methods for the synthesis of various heterocyclic
compounds, and also regarding the lack of sufficient
reports on the synthesis of substituted 1,2-dihydro-(4H)-
D. Azarifar (&) ꢀ S.-M. Khatami ꢀ M. A. Zolfigol ꢀ D. Sheikh
Faculty of Chemistry, Bu-Ali Sina University, Zip Code 65178,
Hamedan, Iran
e-mail: azarifar@basu.ac.ir
123

J IRAN CHEM SOC
15 nm
3,1-benzoxazin-4-one [21], herein, we are encouraged to
study the hitherto unreported application of Preyssler-type
heteropolyacid-mediated-nano TiO2 as a convenient and
recyclable heterogeneous catalyst for the synthesis of
N-acetyl-2-aryl-1,2-dihydro-(4H)-3,1-benzoxazin-4-ones at
25 °C. We preliminary examined the condensation reaction
of benzaldehyde with anthranilic acid in the presence of
acetic anhydride as the model reaction (Scheme 1).
To optimize the conditions of the reaction, we prepared
TiO2 of various particle sizes via sol–gel method according
to the reported procedure [26]. The particle size distribu-
tion obtained was estimated to be 15, 30 and 50 nm as
determined by transmission electron microscopy (TEM)
images.
100
75
50
25
0
30 nm
50 nm
Micro & No Catalyst
0
1
2
3
4
Time (h)
First, we examined the effect of minimizing the particle
size of nano-TiO2 on the rate and yield of the model
reaction and the results are schematically shown in Fig. 1.
As shown in this figure, the reaction rate and yield are both
found to be improved with decreasing the particle size of
the nano-TiO2 catalyst. When the reaction was performed
using micro TiO2 or in the absence of this catalyst, no
completion of conversion was observed even after 4 h.
Fig. 1 The effect of TiO2 particle size on the rate and yield of the
model reaction. All the reactions were carried out with anthranilic
acid (1 mmol), benzaldehyde (1 mmol) and TiO2 (20 mg) in Ac2O
(1 mL) at 25 °C
with intermittent shaking. Finally, the content of each flask
was centrifuged and the separated solution was titrated
against 0.01 N aqueous NaOH solution using phenol-
phthalein with taking three readings in each case. It is
previously shown that no significant differences were
observed in the TEM images, and also the IR spectrum of
the Preyssler acid-modified nano-sized TiO2 clearly shows
the characterization bands [24].
Effect of supporting the Preyssler acid
(H14[NaP5W30O110]) on nano-TiO2
As previously shown [24], supporting the homogeneous
Preyssler-type acid may alter the efficiency of the catalyst
as to improve the reaction yield. This could be possibly
attributed to the addition of Brøncted acid character of
Preyssler heteropolyacid to Lewis acid character of nano-
TiO2 and enhancement of the acidic character of the
combined catalyst in the present work and we supported
the Preyssler-type acid on the nano-TiO2 of the favorite
particle size (15 nm). To find the most effective molar ratio
of the Preyssler acid to nano TiO2, we examined the
thermodynamic properties of the Preyssler adsorption on
nano-TiO2 of the favorite particle size.
The amount of the adsorbed Preyssler acid per unit mass
of the adsorbent TiO2 (mg/g) was calculated using the
Eq. (1).
qe ¼ ðC0 ꢁ CeÞV=m
where Co and Ce are initial and final concentrations of the
ð1Þ
Preyssler acid, respectively.
Adsorption isotherms
The extent of adsorption of the Preyssler acid on nano TiO2
is represented by the adsorption isotherm. Among the
several possible isotherm equations available, two have
been commonly applied for this study, i.e. the Freundlich
and Langmuir isotherms. The experimental data were fitted
nonlinearly to Freundlich and Langmuir isotherms. At a
given temperature, the mass of a solute adsorbed on a solid
adsorbent at various concentrations is calculated from the
Freundlich Eq. (2) [27]:
Equilibrium studies
Different volumes of Preyssler acid and water (30 mL)
were placed in seven separate conical flasks (Table 1). To
each flask was added the adsorbent nano-TiO2 (1.0 g) with
strongly shaking the flask. Then, these conical flasks were
sealed with rubber stoppers and let to stand for almost 24 h
Scheme 1 Preyssler acid-
O
O
O
mediated nano-TiO2-catalyzed
formation of N-acetyl-2-phenyl-
1,2-dihydro-(4H)-3,1-
Preyssler acid-mediated nano-TiO2 (20 mg)
Ac2O (1ml, 10 mmol), rt
O
OH
H
+
N
benzoxazin-4-one
NH2
H
O
123

J IRAN CHEM SOC
0.06
0.05
0.04
0.03
0.02
0.01
0
Table 1 The calculated and experimental parameters obtained for
different initial Preyssler acid weights
exp
Langmuir
Freundlich
Entry Initial
weight of
Amount of
the adsorbed
Preyssler acid
(mmol/g of
TiO2)
Weight
of the
adsorbent
(g)
Ce
(g/l)
Q
(mg/g)
Preyssler
acid (g)
1
2
3
4
5
6
7
0.2
0.5
1.0
2.0
4.5
6.5
9.0
0.0012
0.0024
0.0041
0.0050
0.0056
0.0059
0.0061
1.0
1.0
1.0
1.0
1.0
1.0
1.0
6.36
9.0
16.07 17.80
32.31 30.50
65.43 37.00
148.61 41.60
215.20 43.90
298.50 45.00
0
100
200
Ce (g/L)
300
400
Fig. 2 The experimental and theoretical plots of Preyssler-acid
adsorption on nano-TiO2
q ¼ K:Ce1=n
ð2Þ
where K and n are Freundlich constants. The calculated and
experimental values of q and Ce obtained at different initial
Preyssler concentrations are shown in Table 1.
Table 3 Screening the catalyst loading and catalyst activity of the
nano TiO2-supported-Preyssler acid on the model reaction
Entry Catalyst (mmol/g)
Time (h) Yield (%)
The Langmuir adsorption model [28] is based on the
assumption that the maximum adsorption corresponds to a
saturated monolayer of solute molecules on the adsorbent
surfaces. The nonlinear expression of the Langmuir model
is given by the Eq. (3) as follows:
1
2
3
4
5
6
7
8
9
None
3
3
3
3
3
3
3
3
3
20
75
50
75
80
87
92
93
95
Nano-TiO2 (20 mg)
Preyssler acid (20 mg)
Preyssler acid/nano-TiO2 (0.0012)
Preyssler acid/nano-TiO2 (0.0024)
Preyssler acid/nano-TiO2 (0.0041)
Preyssler acid/nano-TiO2 (0.005)
Preyssler acid/nano-TiO2 (0.0056)
Preyssler acid/nano-TiO2 (0.0061)
q ¼ abCe=ð1 þ bCeÞ
ð3Þ
where a and b are the Langmuir constants related to the
capacity and energy of adsorption respectively, and can be
determined from nonlinear fitting. The results summarized
in Table 2 and shown in Fig. 2 indicate that the experi-
mental data reported in Table 1 fit better with the Langmuir
isotherm.
Conditions: benzaldhyde (1 mmol), anthranilic acid (1 mmol), Ac2O
(1 mL, 10 mmol), solid catalyst (20 mg), 25 °C
reaction when supported on nano-TiO2 in comparison with
the yield (75 %) obtained using the neat nano-TiO2 as the
catalyst (entry 2). In addition, the importance of the cata-
lyst in this reaction was approved with conducting the
reaction under the catalyst-free condition or using the neat
Preyssler acid that resulted in very low yields of 20 and
50 %, respectively (entries 1 and 3). The best result in
terms of the reaction yield (95 %) was obtained when the
reaction was conducted with using the Preyssler acid/nano-
TiO2 catalyst in 0.0061 mmol/g ratio (entry 9).
Optimizing the reaction conditions
To establish the reaction conditions, the effect of catalyst
loading on the model reaction was studied under conven-
tional condition in the presence of acetic anhydride (1 ml)
at 25 °C using the neat nano TiO2 as well as the nano-
TiO2-supported Preyssler acid with different molar ratios
(Table 3).
The results summarized in Table 3 clearly indicate the
enforcing effect of the Preyssler acid on the yield of the
To establish the generality of the catalyst and to develop
the scope of the reaction, we conducted the reaction with
a series of aromatic aldehydes 1a–r carrying different
substituents under the determined optimum conditions
(Scheme 2).
Table 2 Values of Langmuir and Freundlich constants and regres-
sion coefficients
Freundlich constants
Langmuir constants
All the reactions proceeded smoothly to afford the
respective products 2a–r in quantitative yields as summa-
rized in Table 4. As seen in Table 4, we have noticed
that the aldehydes bearing electron withdrawing groups
K (mg/g)
1/n
R2
9.22
a (mg/g)
b (l/g)
R2
49.05
0.2925
0.8870
0.0417
0.9892
123

J IRAN CHEM SOC
Scheme 2 Preyssler acid-
mediated nano-TiO2-catalyzed
formation of N-acetyl-2-aryl-
1,2-dihydro-(4H)-3,1-
O
O
Preyssler acid-mediated nano-TiO2 (20 mg)
Ac2O (1 ml), rt
O
OH
+ ArCHO
Ar
benzoxazin-4-ones 2a-r
N
H
NH2
1 a-r
2 a-r
O
attached to the ring at positions para and/or meta to the
aldehyde functional group undergo nucleophilic attack by
amino group of the anthranili acid more rapidly to afford
improved yields. The electron releasing groups, on the
other hand, cause the aldehydes to react more slowly with
reduced yields. The possible explanation for the activating
effect of the electron-withdrawing groups rests on the
dispersal of the negative charge of the carbonyl oxygen
over the intermediate anion produced by nucleophilc
addition to carbonyl carbon atom. In contrast, the electron-
releasing groups tend to intensify the negative charge on
the carbonyl oxygen, destabilize the negative ion adduct,
and thus cause slower reaction. These activating or deac-
tivating effects of the substituent groups occur through
their resonance and/or inductive effects. Moreover, the
substituent groups of all kinds situated at position ortho to
the aldehyde group, generally retard the reaction because
of their sterical hindrance that makes the carbonyl group
less accessible to nucleophilic attack (cf. entries 2g–2i,
2j–2k, and 2m–2o). However, the groups such as –OH
attached to ortho position which can involve in hydrogen-
bonding with carbonyl oxygen exhibit activating effect by
increasing the tendency of the aldehyde towards nucleo-
philic attack (cf. entry 2c with entry 2e).
synthesis of N-acetyl-2-aryl-1,2-dihydro-(4H)-3,1-benzox-
azin-4-ones from the one-pot reaction of aromatic alde-
hydes with anthranilic acid in the presence of acetic
anhydride at 25 °C. The reactions proceed under mild and
solvent-free conditions to furnish the products in short
reaction times and quantitative yields.
Experimental
Material and instruments
Chemicals used in this work were purchased from Fluka
(Buchs, Switzerland) and Merck (Hohenbrunn, Germany)
companies and used without purification. Preyssler-type acid
was prepared as reported in the literature [29]. IR spectra
were recorded on a Perkin Elmer GX FT IR spectrometer
from KBr pellets. 1H and 13C NMR spectra were measured
for samples in CDCl3 with a JEOL FX 90Q instrument at 90
and 22.5 MHz, respectively, using Me4Si as an internal
standard. Mass spectra were recorded with a spectrometer
Finnigan-MAT 8430 operating at an ionization potential of
70 eV. Melting points were measured on a SMPI apparatus.
Elemental analyses for C, H, and N atoms were performed
using a Perkin–Elmer 2400 series analyzer.
With respect to the aforementioned effects on the
reaction parameters, this reaction can possibly proceed
through the intermediacy of a corresponding imine deriv-
ative (A) produced from the reaction of anthranilic acid
with catalyst-activated aromatic aldehyde 1 as depicted in
Scheme 3. The subsequent acetylation of this intermediate
with the n-TiO2-activated Ac2O followed by cyclization to
provide the product 2. To approve this mechanism, the
imine (A) was obtained in a separate experiment as a stable
compound from the reaction of anthranilic acid with
2-pyrrolecarbaldehyde 2r as the test compounds in MeOH
both with using the Preyssler-mediated nano-TiO2 catalyst
and without using the catalyst. It was noticed that, the use
of the catalyst can cause considerable enhancement in the
yield and rate of the reaction. The intermediate A was
subsequently treated with Ac2O in the presence of the acid-
mediated n-TiO2 catalyst at room temperature and the
formation of the expected product 2r in *80 % yield was
resulted after crystallization from EtOH.
Typical procedure for the synthesis of N-acetyl-2-aryl-1,
2-dihydro-(4H)-3,1-benzoxazin-4-ones (2)
A mixture of aldehyde 1 (1 mmol), anthranilic acid
(1 mmol), acetic anhydride (1 mL) and Preyssler acid-
mediated-nano TiO2 (20 mg) was stirred at room temper-
ature for an appropriate time (Table 2), until the reaction
was completed as monitored by TLC (n-hexane/EtOAc;
2:1) analysis. The reaction mixture was then centrifuged to
separate the solid catalyst, the residue was treated with iced
water and filtered to leave a solid product, which was
crystallized from ethanol to yield pure product. All the
products were characterized by their physical and spectral
data (IR, MS, 1H NMR, 13C NMR) and elemental analysis
that were in accord with the reported data [25, 30].
Typical synthesis of imine (A)
In summary, we have explored the application of
Preyssler heteropolyacid-mediated nano-TiO2 as a suitable
heterogeneous catalyst in a reliable protocol for the
A mixture of anthranilic acid (0.137 g, 1 mmol) and
2-pyrrolecarbaldehyde 1r (0.095 g, 1 mmol) in MeOH
123

J IRAN CHEM SOC
Table 4 Preyssler acid-mediated nano-TiO2-catalyzed synthesis of N-acetyl-2-aryl-1,2-dihydro-(4H)-3,1-benzoxazin 4-ones 2a–r
Entry
ArCHO 1
Product 2
Time (h)
4
Yield (%)a
2a
95, 95b, 93c, 92d
O
O
N
H
O
O
H
O
O
2b
2c
2d
2e
2f
4
90
95
95
80
85
83
H
O
Me
Me
N
H
O
3.5
OH
O
O
N
H
O
O
H
AcO
O
5
OH
O
HO
H
O
N
H
OAc
AcO
O
5
OMe O
O
N
H
O
H
MeO
O
4
O
O
OMe
OMe
OMe
MeO
MeO
H
O
N
H
O
OMe
O
2g
3
Br
O
H
O
N
H
Br
O
123

J IRAN CHEM SOC
Table 4 continued
Entry
ArCHO 1
Product 2
Time (h)
2
Yield (%)a
90
2h
O
O
O
N
Br
H
H
O
H
Br
O
2i
2
91
86
97
96
85
94
O
O
Br
Br
N
H
O
2j
3
Cl
O
O
N
H
O
O
H
Cl
O
2k
2
O
H
O
Cl
F
Cl
F
N
O
H
O
O
2l
2
O
H
O
N
H
2m
4
NO2
O
O
N
H
O
H
O2N
O
2n
1.5
O
O
O2N
H
O
N
H
NO2
O
123

J IRAN CHEM SOC
Table 4 continued
Entry
ArCHO 1
Product 2
Time (h)
1
Yield (%)a
96
2o
O
O
N
H
O
NO2
O2N
H
O
2p
6
80
O
H
O
OH
O
N
H
AcO
O
2q
5
75
O
O
S
S
O
H
N
O
H
O
2r
4
83
O
N
H
HN
O
H
N
H
O
Conditions: anthranilic acid (1 mmol), aldehyde (1 mmol), Ac2O (1 ml), catalyst (20 mg), 25 °C
a
Isolated yields
b,c,d
Yields of the first, second and third consecutive runs respectively for the recycled catalyst
Scheme 3 Possible mechanism
of the Preyssler acid-mediated
nano-TiO2- catalyzed formation
of N-acetyl-2-aryl-1,2-dihydro-
(4H)-3,1-benzoxazin-4-ones 2
-OAc
n-TiO2 (cat.)
H
O
O
O
O
Ar
H
N
O
O
O
OH
NH2
O
OH
(A)
Ac2O, rt
H
Ar
Ar
H
N
n-TiO2 (cat.)
Ac
AcOH
O
N
O
Ar
H
2
O
(10 mL) was left to stir at room temperature. Following
completion of the reaction within 90 min as monitored by
TLC, the solvent was removed under reduced pressure to
leave a yellow solid which was crystallized from EtOH to
yield pure corresponding imine (A) in 80 % yield. It is
interesting to note that the use of acid-mediated nano-TiO2
catalyst in this reaction resulted in the highly increased
reaction rate and a moderate improvement in the yield
(90 %) of the intermediate (A). The physical and spectral
data for (A) are given below:
123

J IRAN CHEM SOC
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Acknowledgments The authors wish to thank the Research Council
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