Application of natural phosphate modified with sodium nitrate in the synthesis of chalcones: A soft and clean method

Article abstract of DOI:10.1016/S0021-9517(02)00017-9

The solid obtained by impregnation of natural phosphate (NP) with a solution of sodium nitrate, followed by calcination at 900°C, is a strongly basic catalyst that is easily prepared from cheap precursors. The catalytic activity of this solid in the Claisen-Schmidt condensation was studied and high yields were obtained with small amounts of catalyst. The reaction rate depends on the substitution in both benzaldehyde and acetophenone derivatives. The effect of the solvent, as well as the addition of water and ammonium salt, was investigated as well. The catalyst can be easily recovered and efficiently reused.

Full text of DOI:10.1016/S0021-9517(02)00017-9

Journal of Catalysis 213 (2003) 1–6
www.elsevier.com/locate/jcat
Application of natural phosphate modified with sodium nitrate
in the synthesis of chalcones: a soft and clean method
Saïd Sebti,a,∗ Abderrahim Solhy,a Rachid Tahir,a Smahi Abdelatif,a Saïd Boulaajaj,a
José A. Mayoral,b,∗ José I. García,b José M. Fraile,b Abdelali Kossir,c and Hammou Oumimoun c
a
Laboratoire de Chimie Organique Appliquée et Catalyse, Université Hassan II, Faculté des Sciences Ben M’Sik, BP 7955, Casablanca, Morocco
Departamento de Química Orgánica, Instituto de Ciencia de Materiales de Aragón, Facultad de Ciencias, Universidad de Zaragoza–C.S.I.C.,
b
E-50009 Zaragoza, Spain
c
Centre d’Etudes et de Recherches sur les Phosphates Minéraux (CERPHOS), Groupe Office Chérifien des Phosphates (OCP), 37 Bd My Ismail,
Casablanca, Morocco
Received 18 February 2002; revised 5 July 2002; accepted 19 September 2002
Abstract
◦
The solid obtained by impregnation of natural phosphate (NP) with a solution of sodium nitrate, followed by calcination at 900 C, is
a strongly basic catalyst that is easily prepared from cheap precursors. The catalytic activity of this solid in the Claisen–Schmidt condensation
was studied and high yields were obtained with small amounts of catalyst. The reaction rate depends on the substitution in both benzaldehyde
and acetophenone derivatives. The effect of the solvent, as well as the addition of water and ammonium salt, was investigated as well. The
catalyst can be easily recovered and efficiently reused.
2003 Elsevier Science (USA). All rights reserved.
Keywords: Natural phosphate; Sodium nitrate; Heterogeneous catalysis; Chalcones; Claisen–Schmidt condensation; Ammonium salt; Solvent effect;
Water effect
1. Introduction
activation [6], antimalarial [7], antimicrobial [8], antimuta-
genic [9], radioprotective [10], and anti-inflammatory [11]
effects. Furthermore, chalcones have attracted much atten-
tion as synthetic intermediates in the preparation of other
compounds [12].
Heterogeneous catalysts are clearly advantageous over
homogeneous ones in large-scale preparations. In this re-
gard, catalysis of organic reactions by inorganic solids is
an important new dimension in preparative organic chem-
istry. Among the different inorganic solids, natural phos-
phate (NP) has advantages in that it is cheap, readily avail-
able, nontoxic, and not a pollutant. During the past seven
years we have studied the use of natural phosphate to pro-
mote organic transformations [1] and have shown that its
mild basic and acidic properties can be exploited in many
synthetic applications [2].
The Claisen–Schmidt condensation is the most useful
reaction in the synthesis of chalcones and, for this reason,
several excellent methods have been developed to carry
out the reaction in the homogeneous phase. The use of
heterogeneous basic catalysts, such as alumina [13], barium
hydroxide [14], hydrotalcite and zeolite [15], facilitates
the separation of the catalyst from the reaction medium.
Furthermore, in many cases the catalyst may be reused.
We previously reported that natural phosphate is capable
of catalyzing the Claisen–Schmidt condensation, but that
a large amount of catalyst is required [16]. We have also
shown that doping with sodium nitrate increases the activity
of natural phosphate [17].
Chalcones represent one of the most abundant and ubiq-
uitous groups of natural products [3]. In the last few years
they have been shown to possess interesting biological prop-
erties, including anti-invasive [4], anticancer [5], inhibitory
In this paper we report a mild and convenient method
for the heterogeneous catalysis of the Claisen–Schmidt
condensation using NP modified by sodium nitrate.
*
Corresponding authors.
E-mail address: saidsebti@yahoo.fr (S. Sebti).
0021-9517/03/$ – see front matter
PII: S0021-9517(02)00017-9
2003 Elsevier Science (USA). All rights reserved.

2
S. Sebti et al. / Journal of Catalysis 213 (2003) 1–6
2. Methods
state). In sedimentary rocks, phosphates are formed from
compounds derived from apatite by partial isomorphic sub-
stitution: Ca2+ ions by Na+, Mg2+, Co2+, Fe3+, or Al3+
All commercial reagents and solvents were used with-
out further purification. NP was purchased from CERPHOS
(OCP). X-ray diffraction (XRD) patterns of the catalysts
were obtained on a Philips 1710 diffractometer using Cu-
Kα radiation. Surface areas were determined at 77 K using
a Coulter SA 31000 instrument with an automated gas vol-
umetric method employing nitrogen as the adsorbate. NMR
spectra were recorded on a Bruker ARX 300 spectrometer.
Mass spectra were recorded on a VG Autospec spectrome-
ter. FTIR spectra were recorded on an ATI Mattson–Genesis
Series spectrophotometer using the KBr disc method.
,
PO4 ions by VO43−, SO42−, CO3 or MnO4−, and F−
by −OH or Cl−. These different substitutions cause distor-
tions of the structure which depend on the nature and the
radii of the ions involved. Natural phosphate used in this
work was obtained from the Khouribga region (Morocco).
Prior to use this material requires initial treatments such
as crushing and washing. For use in organic synthesis, the
natural phosphate is treated by techniques involving attri-
tion, sifting, calcinations (900 ◦C), washing, and recalcina-
tions [19]. These treatments lead to a fraction between 100
and 400 µm that is rich in phosphate and has the follow-
ing chemical composition: P2O5 (34.24%), CaO (54.12%),
CO2 (1.13%), MgO (0.68%), SiO2 (2.24%), Al2O3 (0.46%),
Fe2O3 (0.36%), F− (3.37%), SO3 (2.21%), Na2O (0.92%),
K2O (0.04%) and several metals (Cd, Zn, Cu, U, Cr, V) in
the ppm range. The surface area of NP is 1.4 m2/g and its
total pore volume is VT = 0.0055 cm3/g.
3−
2−
2.1. General procedure for the Claisen–Schmidt
condensation with NaNO3/NP catalyst
NaNO3/NP was prepared by addition of natural phos-
phate (10 g) to an aqueous sodium nitrate solution (50 ml,
1.17 M). The mixture was stirred at room temperature for
30 min and the water was evaporated under reduced pres-
sure. The resulting solid was calcined under air at differ-
ent temperatures for 2 h. NaNO3/NP (0.1 g) was added to
a solution of aldehyde 1 (2.5 mmol) and acetophenone 2
(2.5 mmol) in methanol (1 to 3 ml) and the mixture was
stirred at room temperature for the appropriate time. CH2Cl2
(20 ml) was added, the catalyst was filtered off, and the so-
lution concentrated to give a residue, which was purified by
distillation under vacuum and recrystallisation. The products
The modified natural phosphate (NaNO3/NP) was pre-
pared by adding NP (m1) to an aqueous solution of sodium
nitrate (m0), followed by low-pressure water evaporation
and calcination. The catalysts were tested in the Claisen–
Schmidt condensation of arylaldehydes 1 with acetophe-
nones 2, a reaction that gives chalcones 3 (Scheme 1).
In order to determine the best ratio of r = NaNO3/NP,
we carried out the synthesis of chalcone 3a (R1 = R2 = H)
by condensation between benzaldehyde and acetophenone
at room temperature in methanol (1 ml) using NaNO3/NP
(0.1 g) as a catalyst. The yields obtained after reaction times
of 24 h were 98, 78, 67, 8, and 2% on using NaNO3/NP with
weight ratio r = 1/2, 1/3, 1/5, 1/8, and 1/15, respectively
(Fig. 1). The best result was obtained when r = 1/2. We
also investigated the use of other calcination temperatures.
Treatment at 150, 300, and 500 ◦C led to catalysts with poor
activities (Table 1). This is in agreement with the results of
thermal analysis, with a weight loss between 600 and 800 ◦C
indicating that the solid phase reaction takes only place at
high temperature. With regard to the sodium salt, solids
3 (Scheme 1) were identified by H NMR, 13C NMR, and
1
IR spectroscopy and by mass spectrometry. The catalyst was
reactivated by drying at 150 ◦C or, alternatively, by washing
with acetone, drying at 100 ◦C and calcination at 900 ◦C for
1 h.
The same procedure was used for the reactions carried
out in presence of BTEAC (0.03 g). The products were
washed with water to eliminate the ammonium salt before
purification.
3. Results and discussion
3.1. Effect of catalyst preparation and reaction conditions
Natural phosphate exists under several mineralogical
classes [18] which generally belong to the family of phos-
phocalcic apatites (Ca10(PO4)6F2 for fluoroapatite in its pure
Fig. 1. Effect of m /m ratio on the yield of 3a.
Scheme 1.
0
1

S. Sebti et al. / Journal of Catalysis 213 (2003) 1–6
3
Table 1
Table 2
◦
Condensation of benzaldehyde and acetophenone catalyzed by 0.1 g of solid
(various catalysts)
Most intense diffraction lines for NaNO /NP calcined at 900 C in com-
parison with reference CaO
3
◦
Entry
Catalyst
Calcination
temp. ( C)
Reaction
time (h)
Yield (%)
NaNO /NP (900 C)
Calcium oxide
3
◦
◦
◦
2θ ( )
Intensity
2θ ( )
Intensity
1
2
3
4
5
6
7
8
9
10
11
12
13
Without
–
–
48
48
24
24
24
24
18
24
24
24
24
24
24
0
0
2
4
3
2
98
0
0
0
a
32.21
37.37
53.87
64.16
67.37
78
32.23
37.38
53.90
64.21
67.44
36
100
54
16
16
NaNO
NP
3
100
60
21
900
150
300
500
900
150
150
150
900
900
900
NaNO /NP
3
NaNO /NP
3
14
NaNO /NP
3
a
◦
NaNO /NP
Line overlapped with that of NP at 32.27 .
3
NaCl/NP
Na SO /NP
2
4
In the X-ray diffraction pattern of NaNO3/NP calcined
at 500 ◦C, lines corresponding to NP and pure NaNO3
(2θ = 29.36, 38.96 and 47.90◦) are present. These lines
disappear after calcination at 900 ◦C (Fig. 2), showing
again the decomposition of nitrate, whereas a large number
of new lines appears with modification in the intensity
of the NP lines. The most intense new lines correspond
to CaO, as shown in Table 2, demonstrating that a solid
phase reaction takes place between the supported sodium
nitrate and the calcium phosphate of the support during the
nitrate decomposition process. It is assumed that sodium
phosphate or any type of calcium sodium mixed phosphate
is the resulting species when part of the calcium is released
from the crystalline framework to form CaO. However, the
remaining lines do not fit with any reference composed by
Na, Ca, P, N, H, and O. The only remarkable fact is that
Na CO /NP
2
3
NaCl/NP
Na SO /NP
0
0
0
2
4
Na CO /NP
2
3
prepared by doping NP with NaCl, Na2SO4, and Na2CO3
did not promote the reaction (Table 1).
◦
The best catalyst (NaNO3/NP calcined at 900 C) was
studied further. The FT-IR spectrum of NaNO3 shows a very
strong and broad band with a maximum around 1375 cm−1
.
The same intense band is present at 1365 cm−1 in the
spectrum of NaNO3/NP calcined at 500 ◦C, demonstrating
the stability of the nitrate species under those conditions.
However, this band almost disappears after calcination at
900 ◦C, because of the decomposition of nitrate.
◦
Fig. 2. X-ray diffraction patterns of (a) NP and (b) NaNO /NP calcined at 900 C.
3

4
S. Sebti et al. / Journal of Catalysis 213 (2003) 1–6
the main modifications in the pattern are produced in the
range of 2θ = 32–35◦, with new lines at 33.68◦ and 34.34◦
together with an increase in the intensity of other lines in
that zone. These lines may correspond to sodium phosphate
species, given that the most intense lines in the references
are 34.1◦ for γ -Na3PO4, 34.2◦ for β-Na3PO4, and 31.9◦ for
Na2HPO4. The formation of very small crystals, partially
amorphous materials or mixed phosphates with different
stoichiometry may account for the lack of intense bands
of sodium phosphate species. It has been shown that in
other base-catalyzed reactions [17b] this insoluble sodium
phosphate may be the active species.
The natural phosphate shows a very low surface area
(1.4 m2/g) together with a low total pore volume (0.0055
cm3/g). In the case of NaNO3/NP calcined at 900 ◦C
the surface area is even lower (0.66 m2/g) with similar
total pore volume. It is then rather surprising that this
solid has a very high catalytic activity and basicity was
determined by phenol adsorption. Solutions of phenol were
prepared in cyclohexane with concentrations in the range
5–200 mM and n-octane was added as internal standard.
Variable amounts of the solids (15–45 mg) were stirred at
room temperature with different volumes of those solutions
(3–9 ml) in order to study the adsorption equilibrium. The
phenol remaining in solution was analyzed by GC. Values
of adsorbed phenol were in the range of 0.5–1.8 mmol/g.
These values are surprisingly high for a solid with such a low
surface area; in fact they represent a surface concentration of
4.5–16.4 molecules/Å2. This concentration cannot be due
to chemisorption and it is difficult to find an explanation.
However, the high polarity of the surface might be the origin
of this behavior. In any case, it is clear that the solid is
basic, as indicated by its catalytic activity, observed in the
epoxidation of electron-deficient alkenes with H2O2 [17b],
a reaction that needs the presence of basic centers.
the catalytic activity was observed in subsequent reactions.
The yields obtained are 98, 75, 60, and 35% using the fresh
catalyst, cycles 1, 2, and 3, respectively. However, the activ-
ity was almost completely recovered when the catalyst was
washed with acetone and calcined at 900 ◦C. Thus, the yields
obtained are 98, 94, and 92% with fresh catalyst, cycles 1
and 2, respectively. The strong adsorption of some byprod-
ucts may be responsible for this behavior.
3.2. Scope of NaNO3/NP
In order to determine the scope and limitations associated
with this new catalyst, the optimum conditions for the
Claisen–Schmidt condensation between benzaldehyde and
acetophenone were applied to other substrates (Table 3). All
of the reactions proceeded efficiently at room temperature.
Although all products are obtained in high isolated yields,
longer reaction times are required in the synthesis of some
chalcones. The presence of an electron-donor group in the
aldehyde (entry 4) or the ketone (entry 6) made the reaction
less favorable. The worst result was obtained when both
reagents contained donor methoxy groups (entry 8).
3.3. Effect of addition of water
Having shown that NaNO3/NP is an active catalyst for a
wide variety of Claisen–Schmidt condensations, we tried to
further improve the catalytic activity of this solid. We have
reported previously that addition of small amounts of water
to reaction mixtures can increase the catalytic activity of
natural and synthetic phosphates [16,20–22]. We tested this
approach in the synthesis of chalcone 3a using NaNO3/NP
in the presence of different amounts of water in conjunction
with methanol or ethanol as solvents. When the reaction was
carried out with 1 ml of methanol, the addition of water led
to a decrease in the yield. Thus, the yields obtained after 6 h
are 65, 57, 47, 35, and 28% in the presence of 0.05, 0.1, 0.3,
0.5, and 1 ml of water, respectively, whereas the yield is 57%
with anhydrous solvent. In contrast, the behavior in ethanol
is completely different in that the addition of small amounts
of water has a positive effect. The yields obtained are 13,
21, 55, 42, and 23% using 0.05, 0.1, 0.3, 0.5, and 1 ml of
water respectively. Only 4% of product was isolated with the
anhydrous solvent. The best result was then obtained with
0.3 ml of water in 1 ml of ethanol.
The condensation of the benzaldehyde and the acetophe-
none was used as a benchmark reaction to study the influ-
ence of the volume of the methanol. The best yield (98%) is
obtained after 24 h with a volume of 1–3 ml. An increase in
the volume up to 5 ml slightly decreases the reaction yield
(88%), and this drops further to 40% when a volume of 10 ml
is used. In absence of the solvent, only a 10% yield is ob-
tained. This behavior indicates that some solvent is needed
to facilitate the contact between the reagents and the active
sites, but a large volume reduces the concentration and, as a
consequence, the reaction rate.
We proceeded to study the solvent effect in the syn-
thesis of chalcone 3a using the NaNO3/NP catalyst. In
dichloromethane, THF, hexane, and butanol the reaction did
not take place. The use of isopropanol gave only a 12% yield
of 3a after 24 h, but 63% of 3a was isolated when the re-
action was carried out in ethanol. It can be concluded that
methanol is the best solvent for this reaction.
3.4. Effect of addition of an ammonium salt
In previous work it was shown that the addition of an am-
monium salt to natural phosphate [16], fluorapatite [20], or
hydroxyapatite [21] increases the activity of these catalysts.
Therefore, we carried out the synthesis of chalcone 3a with
NaNO3/NP in methanol using different amounts of benzyl-
triethylammonium chloride (BTEAC). The results obtained
after 6 h show that the best yields are obtained with 0.03–
0.06 g of BTEAC. Moreover, the kinetic curves for the syn-
To complete this preliminary study, the issue of catalyst
recovery was also considered. When the catalyst was sepa-
◦
rated by filtration and dried at 150 C, a rapid decrease of

S. Sebti et al. / Journal of Catalysis 213 (2003) 1–6
5
Table 3
Synthesis of chalcones by Claisen–Schmidt condensation using NaNO /NP in methanol
3
a
Entry
Substituents in reagents
Product
Yield (%) [time (h)]
1
2
R
R
Without BTEAC
73(10)
With BTEAC
1
H
H
3a
95(10)
98(18)
30(6)
2
3
4
p-Cl
H
H
H
3b
3c
3d
90(6)
95(6)
94(16)
38(6)
m-NO
2
94(16)
40(12)
84(24)
91(36)
84(12)
94(24)
p-MeO
H
37(12)
90(24)
82(12)
93(24)
5
p-MeO
3e
74(24)
93(48)
6
7
8
p-Cl
p-MeO
p-MeO
p-MeO
3f
3g
3h
97(24)
92(24)
93(24)
55(24)
81(48)
m-NO
2
40(24)
70(48)
p-MeO
H
b
72(6)
b
9
10
11
p-NO
3i
3j
98(6)
2
b
92(16)
b
80(6)
b
p-Cl
p-NO
94(6)
2
b
94(16)
b
58(6)
b
84(6)
b
m-NO
p-NO
3k
3l
70(12)
2
2
b
98(12)
b
95(24)
b
74(6)
b
12
a
p-MeO
p-NO
89(6)
2
b
93(16)
Isolated yields.
Reaction carried out in the presence of 3 ml of methanol.
b
thesis of chalcone 3a with or without BTEAC clearly show
the enhancement of the catalytic activity of NaNO3/NP by
the addition of the ammonium salt. The yields obtained after
1, 6, 12, and 16 h are 10, 58, 81, and 92% without BTEAC
and 27, 85, 98, and 98% in the presence of BTEAC, respec-
tively.
the product 3 in high yields. The addition of BTEAC caused
a significant increase in the reaction rate. For example, after
6 h the yield in 3b (entry 2) ranged from 30% without
BTEAC to 90% with the ammonium salt. Similar effects
were observed for the other chalcones, even in the synthesis
of 3h, the worst case without BTEAC. It is important to note
that BTEAC itself has not catalytic activity, so that it seems
to act as a phase transfer catalyst.
Finally, this method was extended to the synthesis of
several chalcones (Table 3). In all cases the reaction afforded

6
S. Sebti et al. / Journal of Catalysis 213 (2003) 1–6
4. Conclusions
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Natural phosphate modified by impregnation with sodium
nitrate and calcination at 900 C is an efficient basic cat-
◦
alyst for the Claisen–Schmidt condensation. Several chal-
cones can be synthesized with high yields using catalytic
amounts of NaNO3/NP. The results depend on several fac-
tors, with the nature and amount of solvent playing a deci-
sive role. The advantages of this solid over other basic het-
erogeneous catalysts are the high activity, the simplicity of
the preparation method, the ready availability, the low price
of the precursors, and the ease of reuse. The presence of an
ammonium salt increased the reaction rate for all chalcones
synthesized and the products were obtained in high yields.
These results open the way to the use of this inexpensive
solid as a basic catalyst for other applications.
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Acknowledgments
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Financial assistance from the Ministry of Education,
Government of Morocco (PROTAS, P2T3/59), Office Chéri-
fien des Phosphates (OCP), CICYT, Spain (Project MAT99-
1176), and AECI (Project 2001 MA0005) is gratefully ac-
knowledged.
[17] (a) S. Sebti, A. Solhy, R. Tahir, S. Boulaajaj, J.A. Mayoral, J.M. Fraile,
A. Kossir, H. Oumimoun, Tetrahedron Lett. 42 (2001) 7953;
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[18] A.G. Fischer, D. Jérôme, in: Environmental Phosphorus Handbook,
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