Porous calcium hydroxyapatite as an efficient catalyst for synthesis of pyrazolines via 1,3-dipolar cycoloaddition under solvent-free microwave irradiation

Article abstract of DOI:10.1080/00397910500330619

Adsorbed on porous calcium hydroxyapatite, 1,3-dipolar cycloaddition of diphenylnitrilimine on olefins is readily catalyzed under solvent-free microwaves irradiation. The pyrazolines are obtained in few minutes with high yields. Specific surface of porous

Full text of DOI:10.1080/00397910500330619

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Porous Calcium Hydroxyapatite as an
Efficient Catalyst for Synthesis of Pyrazolines
via 1,3‐Dipolar Cycoloaddition Under
Solvent‐Free Microwave Irradiation
R. Atir a , S. Mallouk a , K. Bougrin a , M. Soufiaoui a & A. Laghzizil b
a Laboratoire de Chimie des Plantes et de Synthèse Organique et
Bioorganique, Université Mohammed V‐Agdal, Faculté des Sciences, Morocco
b Laboratoire de Chimie Physique Générale, Université Mohammed V‐Agdal,
Faculté des Sciences, Morocco
Published online: 21 Aug 2006.
To cite this article: R. Atir , S. Mallouk , K. Bougrin , M. Soufiaoui & A. Laghzizil (2006): Porous Calcium
Hydroxyapatite as an Efficient Catalyst for Synthesis of Pyrazolines via 1,3‐Dipolar Cycoloaddition Under
Solvent‐Free Microwave Irradiation, Synthetic Communications: An International Journal for Rapid
Communication of Synthetic Organic Chemistry, 36:1, 111-120
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Synthetic Communicationsw, 36: 111–120, 2006
Copyright # Taylor & Francis LLC
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910500330619
Porous Calcium Hydroxyapatite as an
Efficient Catalyst for Synthesis of
Pyrazolines via 1,3-Dipolar Cycoloaddition
Under Solvent-Free Microwave Irradiation
R. Atir, S. Mallouk, K. Bougrin, and M. Soufiaoui
`
Laboratoire de Chimie des Plantes et de Synthese Organique et
´ ´
Bioorganique, Universite Mohammed V-Agdal, Faculte des
Sciences, Morocco
A. Laghzizil
´ ´ ´
Laboratoire de Chimie Physique Generale, Universite Mohammed
´
V-Agdal, Faculte des Sciences, Morocco
Abstract: Adsorbed on porous calcium hydroxyapatite, 1,3-dipolar cycloaddition of
diphenylnitrilimine on olefins is readily catalyzed under solvent-free microwaves
irradiation. The pyrazolines are obtained in few minutes with high yields. Specific
surface of porous calcium hydroxyapatite and microwaves effects are discussed.
Keywords: Microwave irradiation, porous hydroxyapatite, pyrazolines, solvent-free
INTRODUCTION
The 1,3-dipolar cycoladdition reaction is a powerful method for the prep-
aration of five-membered heterocyclic compounds.[1] For example, pyrazo-
lines are obtained by addition of nitrilimines with active alkenes. They are
Received in the UK May 23, 2005
Address correspondence to K. Bougrin, Laboratoire de Chimie des Plantes et de
`
´
´
Synthese Organique et Bioorganique, Universite Mohammed V-Agdal, Faculte
des Sciences, B.P. 1014, Morocco. Fax: (þ212) 37 77 54 40; E-mail: kbougrin@
yahoo.fr or bougrin@fsr.ac.ma
111

112
R. Atir et al.
generally considered useful synthons in organic synthesis, and this heterocycle
has often been used as a key intermediate for bioactive product synthesis.[2]
In recent years, it has been shown that various apatites have a catalytic
application in organic synthesis[3] such as openings of epoxides and
oxetanes,[3a] synthesis of phenol from benzene,[3b] oxidation of alcohols,[3c,3d]
dehydrogenation of indolines,[3e] 2-butanol,[3f] and ethane,[3f] Knoevenagel
condensation,[3g] Claisen–Schmidt condensation,[3h] and Friedel–Crafts alky-
lation.[3i] More recently, the hydroxyapatite has been used as excellent catalyst
of three-component coupling reaction[3j] epoxidation of olefins under ultra-
sound irradiation.[3k] After revision of the literature, the applications on
1,3-dipolar cycloaddition reactions are not cited.
The application of microwave (MW) irradiation as a nonconventional
energy source for activation of reactions has now become a very popular
and useful technology in organic chemistry.[4] The combination of
microwave irradiation and solvent-free reaction conditions leads to
enhanced reaction rates, higher yields of pure products, easier workup, and,
sometimes, to selective conversions with several advantages of the eco-
friendly approach in the framework of green chemistry.[5]
Considering these reports and our continued interest in devising new
solvent-free cyclization methods,[6] we report herein a microwave-enhanced
method to obtain pyrazolines using a new catalyst and solvent-free conditions.
Recently, we have developed an efficient method of 1,3-dipolar cycloaddition
of diphenylnitrilimine (DPNI) with some alkenes using KF/g-alumina[6a] or
alumina,[6d] demonstrating its potential utility as a versatile protocol for the
synthesis of pyrazolines. We were consequently encouraged to apply a
solvent-free microwave-assisted procedure to this reaction for the sake of
enhanced reactivity and environmental benefits. Herein, we report a
microwave-assisted 1,3-dipolar cycloaddition of DPNI with alkenes
catalyzed by porous calcium hydroxyapatite support [Ca10(PO4)6(OH)2]
(p-HAP) (Scheme 1). The effects of microwave activation and surface of
the support on the rate enhancement were also reported.
RESULTS AND DISCUSSION
In our experiment, in the absence of solvent, the reagents 1 and indene 2a are
previously impregnated on different supports including p-HAP100,[7]
p-HAP300,[7] p-HAP800,[7] KF/NP (KF/natural phosphate),[8] KF/g-
alumina,[6a] and a-alumina under the same conditions, with the use of
microwave heating for 3 min to afford 1,3-diphenyl-indano[3,2-d] pyrazoline
3a (Scheme 1 and Table 1). The DPNI is generated in situ by heating hydra-
zonoyl chloride with support as base.[6a,9] The porous calcium hydroxyapatites
used in our work were prepared by the chemical wet method and heated at
different temperatures (100, 300, and 8008C) as reported elsewhere.[7] The

Porous Calcium Hydroxyapatite
113
Scheme 1. 1,3-Dipolar cycloadditions reaction (1).
molar ratio of the reagents, irradiation time, and microwave power levels were
optimized to achieve higher yields.
The best result of pyrazoline 3a is obtained in the case of indene 2a; the
optimum conditions employed were 1 mmol of diphenylnitrilimine and
1 mmol of 2a using 1 g of p-HAP300 or p-HAP100 as catalysts. However,
under the same conditions used for p-HAP300 and p-HAP100, the catalyst
KF/g-Al2O3 is more reactive (Fig. 1). The power level of Pmax (continuous
irradiation) was found to be the most appropriate for cycloaddition reaction.
The activity for reaction (1) was affected by two major factors, the effect of
specific surface area and the chemical nature of surface (basic or acidic
Table 1. Effect of several supports on 1,3-dipolar cycloaddition of hydrazonoyl
chloride 1 with 2a under microwave irradiation (max. power ¼ 1250 W)
Specific surface
21
Tf (8C)b
area (m2
g
)
Basicity
Yield (%)c
.
Support
t (min)
p-HAP100a
p-HAP300a
p-HAP800a
KF/NP
KF/g-Al2O3
a-Al2O3
3
142
181
235
159
89
Basic
Basic
Low
88
3
3 (10)
3 (10)
98
95 (93)
86 (88)
170
20 (25)d
8 (10)d
89
8.9
Basic
High
Acid
3
3
High
Low
84
0
ap-HAP is prepared and heated at different temperatures (1008C, 3008C, and 8008C)
according to the described procedure.[7]
bFinal temperature was measured by immersing the digital thermometric probe in the
reaction mixture at the end of exposure during the microwave experiment.
cIsolated yield in pure product.
dReaction is not complete after 10 min under microwave irradiation.

114
R. Atir et al.
surfaces) (Table 1). The increase of p-HAP300 or p-HAP100 quantity in the
cycloaddition reaction of 1 with 2a decreases the reactivity. This phenomenon
is probably related to the high dilution of reagents 1 and 2a onto porous apatite
surface. The use of p-HAP300 is particularly interesting because it is regener-
ated by heating at 3008C under reduced pressure for 20 min. After ten succes-
sive recoveries, product 3a was obtained with the same yield. The best result
for pyrazoline 3a (89%) is also obtained with KF/g-Al2O3, but this hyperbasic
solid is very expensive and it is not regenerative. We notice that the existence
of fluoride ions in KF/g-Al2O3 caused a linkage of the solid on the recipient
during its preparation or when it is used in the reaction.
Once suitable conditions have been determined, we extended this reaction to
other olefins (2a–h) and the main results are summarized in Table 2. The
reactions were carried out in a multimode domestic microwave oven. Reactions
were monitored by TLC; the end of the reaction is indicated by the appearance
of the pyrazolines 3 identified by its intense fluorescence at l ¼ 365 nm.
The pyrazolines 3 were obtained with a mixture of two regioisomers in
the case of trans-olefins (2e–h), but only one regioisomer with the olefins
(2a–d). In all cases, the important role of p-HAP300 in the reaction of 1
with 2 was confirmed by the result readily obtained with pyrazolines 3 to
afford a good yield (87–95%) during a short reaction time (3 min). To
check the possibility of intervention of specific microwave effects, the
reaction of 1 with 2 using p-HAP300 was also examined using a preheated
oil bath for the same time and final temperature (104–1818C), as measured
at the end of exposure during the MW-assisted synthesis (Table 2).
However, the adducts 3 were obtained only with poor yields (0–5%) after
3 min and with incomplete consumption of the starting materials.
We then turned our attention to more comprehensive MW-specific effect
on the 1,3-dipolar cycloaddition reaction. For the sake of comparison and to
Figure 1. Catalytic activity of p-HAP100, p-HAP300, p-HAP800, KF/g-Al2O3, and
KF/NP in the synthesis of pyrazoline 3a via reaction (1).

Table 2. 1,3-Dipolar cycloaddition of hydrazonoyl chloride 1 with olefin 2 (via Scheme 1) on p-HAP300 during 3 min under microwave irradiation
(power ¼ 1250 W)
Isolated yield (%)
Regioisomers
Olefin (2)
T (8C)a
Adduct (3)
MWb
Dc
ratio
2a
181
3a
94
5
100/0
2b
2c
112
104
3b
3c
92
94
5
4
100/0
100/0
2d
2e
116
132
3d
3e
87
95
0
0
100/0
93/7
(continued)

Table 2. Continued
Olefin (2)
Isolated yield (%)
Regioisomers
ratio
T (8C)a
Adduct (3)
MWb
Dc
2f
115
3f
95
0
95/5
95/5
91/9
2g
2h
145
123
3g
3h
89
90
0
0
aFinal temperature was measured by immersing a digital thermometric probe in the reaction mixture immediately after microwave irradiations.
bYields in pure products obtained under microwave irradiation.
cYields determined by classical heating in the same reaction conditions (3 min, final temperature, vessel).

Porous Calcium Hydroxyapatite
117
check the efficiency of MW activation in 1,3-dipolar cycloaddition reactions
as well as the possible intervention of nonthermal MW-specific effects,
control experiments were conducted using a thermostatted oil bath (D ¼
conventional heating) under identical reaction conditions (time, temperature,
vessel). The main results are given in Table 2. As illustrated in Table 2, non-
thermal microwave specific effects were revealed in all cases; for instance, the
reaction (1) was conducted under the same reaction conditions in the
preheated oil bath, comparing the temperature profiles during the reaction
(Fig. 2 and Table 2) to afford 3a with only 5% yield. Microwave irradiation
can therefore improve this reaction by enhancement of the dipole–dipole
stabilization of the transition state when compared to the ground state.[10] In
addition, it was observed that 120 min were required to achieve the adduct
3 by classical heating with the same yield, compared to 3 min under MW
irradiation. This phenomenon can be explained with the weak thermal conduc-
tivity of p-HAP300 observed under classical heating. The same observation is
described in the literature with other oxydes in dry media[11] such as silica,
alumina, KF-alumina, and clay supports.
Although most experiments were performed on a 1-mmol scale, these
reactions can also be performed with higher amounts of reagents (up to
5 mmol) with comparable yields. Additionally, we found that the MW-
assisted reactions are more efficient, more convenient, and cleaner.
CONCLUSION
We have demonstrated that it is possible to perform the synthesis of pyrazo-
lines via 1,3-dipolar cycloaddition on the surface of porous calcium
Figure 2. Temperature profiles monitored during the reaction of 1 and 2a under
microwave irradiation or classical heating at 1818C.

118
R. Atir et al.
hydroxyapatite under solvent-free conditions with the help of MW irradiation
from a domestic microwave oven. The method is fast, operationally simple,
and allows rapid access to a variety of pyrazoline derivatives. Nonthermal
MW-specific effects were also demonstrated. Efforts to extend this catalyst
to other heterocyclic reactions are in progress.
EXPERIMENTAL
General Methods
A microwave of frequency 2.45 GHz, 1250 W, equipped with thermometer
and timer (Philco) was used in all irradiation experiments. Melting points
were determined on a Kofler melting-point apparatus and are uncorrected.
Reactions were monitored by TLC on 0.2-mm silica-gel plates (Merck)
using hexane/ethylacetate (80/20 v/v) mixtures as eluents, and compounds
were visualized with UV light (254 and 365 nm) and iodine. Elemental
analyses were performed by an LEO CHNS-93S apparatus. NMR spectra
were determined on Bruker WP spectrometer at 250 MHz (1H NMR) and at
62.89 MHz (13C NMR). Chemical shifts are reported as d values relative to
the solvent peak of CDC13 set at d ¼ 7.27 (1H NMR) or d ¼ 77 (13C NMR)
with tetramethylsilane as internal standard (d) and were measured in ppm.
Mass spectra (MS) were obtained with AEI MS-50 spectrometer by
chemical ionization (IC) using isobutane as the reagent gas. All solvents
used in reactions were freshly distilled from appropriate drying agents. All
other reagents were recrystallized or distilled when necessary. Column flash
chromatography was carried out on silica gel 60 (70–230 mesh). Completion
of the reaction was generally revealed by TLC and indicated by the appear-
ance of the pyrazolines 3, identified by its intense fluorescence at
l ¼ 365 nm. The resulting solid is analytically pure pyrazoline derivatives
3a–d and the mixture of the two regioisomeric cycloadducts 3e–h. Most
adducts described herein were previously reported in the literature,[6a,12]
with exception of adducts 3c and 3d.
General Procedure for the Synthesis of Pyrazolines 3
A mixture of hydrazonoyl chloride (0.230 g; 1 mmol) and olefin (1 mmol) was
added to p-HAP300 (1 g) in CH2Cl2 (10 mL). The whole mixture was stirred
for 1 min. The solvent was evaporated under reduced pressure in moderate
condition (408C) and then was irradiated by microwave (Philco, operating
at 2.45 GHz, Pmax ¼ 1250 W) for the period of time and temperature (in
Tables 1 and 2). The reaction progress was followed by TLC using
n-hexane/ethyl acetate (8 : 2) as eluent. After 3.0 min, the reaction vessel
was cooled, and the product was washed with 2 ꢀ 10 mL of CH2Cl2 or

Porous Calcium Hydroxyapatite
119
THF. The solvent was evaporated under reduced pressure and the residue was
purified by recrystallization in EtOH. The product was analysed by 1H NMR,
13C NMR, IR, and mass spectrometry, and compared to samples cited in
literature.[6a,12]
5-(1,3-benzodioxol-5-ylmethyl)-1,3-diphenyl pyrazoline (3c): Mp
1
114–1168C (EtOH). H NMR (CDCl3): d 3.1 (m, 2H, CH2), 3.4 (m, 2H,
CH2-C ¼ N), 4.2 (m, 1H, CH), 5.8 (s, 2H, OCH2O), 6.7–7.4 (m, 13H,
Harm). 13C NMR (CDCl3): d 37.0 (CH2), 44.3 (CH2-C ¼ N), 64.6 (CH),
101.0 (OCH2O), 108.5–147.7 (Carm), 162.0 (C ¼ N). CIMS (isobutane,
reagent gas): m/z 357 [M þ H]. Anal. calcd. for C23H20N2O2: C, 77.51; H,
5.66; N, 7.86. Found C, 77.45; H, 5.60; N, 7.89.
N-[1,3-diphenyl-pyrazolinyl methyl] saccharin (3d): Mp 110–1128C
1
(EtOH). H NMR (CDCl3): d 3.9 (m, 2H, CH2-C ¼ N), 4.2 (m, 1H, CH-N),
4.5 (m, 2H, CH2-N-SO2), 5.8 (s, 2H, OCH2O), 6.9–8.0 (m, 14H, Harm).
13C NMR (CDCl3): d 36.3 (CH2-N-SO2), 42.7 (CH2-C ¼ N), 63.8 (CH-N),
111.9–148.3 (Carm), 164.8 (C ¼ N), 166.0 (C ¼ O). CIMS (isobutane,
reagent gas): m/z 418 [M þ H]þ. Anal. calcd. for C23H20N2O2: C, 66.17;
H, 4.59; N, 10.07. Found C, 66.48; H, 4.55; N, 10.13.
ACKNOWLEDGMENTS
Financial assistance of Ministry of Education, Government of Morocco
(PROTARS III, D13/57) and CNRST-Morocco are gratefully acknowledged.
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