Clean chemical synthesis of 2-amino-chromenes in water catalyzed by nanostructured diphosphate Na2CaP2O7

Article abstract of DOI:10.1039/c0gc00387e

A versatile, alternative and environmentally benign three-component strategy to the synthesis of a series of 2-amino-chromenes has been successfully performed in water, using nanostructured diphosphate Na₂CaP ₂O₇ (DIPH) as

Full text of DOI:10.1039/c0gc00387e

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PAPER
www.rsc.org/greenchem | Green Chemistry
Clean chemical synthesis of 2-amino-chromenes in water catalyzed by
nanostructured diphosphate Na2CaP2O7†
Abderrahim Solhy,*a Abdelhakim Elmakssoudi,b Rachid Tahir,b Mohammed Karkouri,b Mohamed Larzek,b
Mosto Bousminaa,c and Mohamed Zahouily*a,b
Received 2nd August 2010, Accepted 27th August 2010
DOI: 10.1039/c0gc00387e
A versatile, alternative and environmentally benign three-component strategy to the synthesis of a
series of 2-amino-chromenes has been successfully performed in water, using nanostructured
diphosphate Na2CaP2O7 (DIPH) as a basic catalyst. The products are obtained in high recoveries
from the one-pot multi-component reaction procedure involving carbonyl compounds,
malononitrile and 1-naphtol, resulting in a green synthetic protocol with high atom economy. The
catalyst is waste-free, easily prepared, and efficiently re-used. The prepared 2-amino-chromenes, in
presence of acetic anhydride, have been easily converted to benzo(a)anthracene derivatives.
advantages of MCRs.9 Thus, they are perfectly amenable to
Introduction
automation for combinatorial synthesis.10
Green or sustainable chemistry has now attained the status
The third issue to be addressed in the context of durable
of a major scientific discipline1 and the studies in this area
development and Green Chemistry is heterogeneous catalysis.
have led to the development of cleaner and relatively benign
These concepts are at the centre of the chemical activity,
chemical processes with many new technologies being developed
and the research on high selectivity is the driving force for
each year. Among them, much effort has been devoted to the use
the conception of all new catalytic processes. At present, it
of non-traditional solvents for chemical synthesis. In addition
is well known that a heterogeneous catalyst must have three
to solvent-free medium,2 these unconventional media include
characteristics: high activity, selectivity and stability (separation,
water,3 supercritical CO2,4 perfluorinated solvents5 and ionic
recovery, recycling). The idea of the new generation of catalysts
liquid.6 The use of water as a medium for organic reactions is
should include these three aspects. In this context, and in order
one of the latest challenges for modern organic chemists. Conse-
to achieve a high control in the preparation of catalytic systems,
quently, it is highly desirable to develop environmentally benign
nanotechnologies constitute an invaluable tool in the hands
processes that can be conducted in aqueous media. During
of chemists. The scientific interests as well as economic, are
the past two decades, many reactions that were conventionally
great, and there is no doubt, that the community of catalysis
believed to occur only in organic solvents have been developed
to run in water.7
Another window of green chemistry to take into consideration
follows closely the development of nanotechnology. Attempts
to exploit the breakthroughs already exist, especially in order
to achieve optimum activity, selectivity and stability in catalytic
is the development of the one-pot multi-component reactions
reactions. Some progress has been accomplished, in particular in
(MCRs) which are one of the best tools in the synthesis of
the conception of active sites and the control of the surrounding
organic compounds.8 Recently, MCRs have emerged as a highly
of these sites, but many challenges that deal with the control
valuable synthetic tool in the context of modern drug discovery.
of the localization of active site and environment in the atomic
The atom economy and convergent character, the simplicity
scale still exist.11
of a one-pot procedure, the possible structural variations, the
2-Amino-chromenes represent an important class of com-
accessible complexity of the molecules, and the very large
pounds being the main components of many naturally occurring
number of accessible compounds are among the described
products, and have been of interest in recent years due to
their useful biological and pharmacological aspects.12 The most
straightforward synthesis of this heterocyclic system involves a
three-component coupling of aromatic aldehyde, malononitrile
and activated phenol. Traditionally, this reaction was catalyzed
by a basic catalyst such as piperidine.13 Many homogeneous
catalysts have been employed for this condensation including
ammonium salts,14 NaOH,15 K2CO3,16 I2/K2CO3,17 TiCl4,18
InCl3,19 heteropolyacid20 and Et3N.21 However, some of the
reported methods require prolonged reaction time, reagents in
stoichiometric amounts, toxic solvents, and generate moderate
recoveries in the final product. On the other hand, a few hetero-
geneous catalysts have been used for this transformation. Very
aINANOTECH (Institute of Nanomaterials & Nanotechnology),
MAScIR Foundation (Moroccan Advanced Science, Innovation and
Research), ENSET, Av. De l¢Arme´e Royale, Madinat El Irfane, 10100,
Rabat, Morocco. E-mail: a.solhy@inanotech.mascir.com; Fax: +212
537570880; Tel: +212 537576193
bLaboratoire de Catalyse, Chimiome´trie et Environnement, URAC 24,
Faculte´ des Sciences et Techniques, Universite´ Hassan II, Mohammedia
B. P. 146, 20650, Morocco. E-mail: m.zahouily@inanotech.mascir.com;
Fax: +212 537570880; Tel: +212 537576188
cHassan II academy of Science and Technology, Rabat, Morocco
† Electronic supplementary information (ESI) available: Full experi-
mental procedures and characterization data for catalyst and organic
products. See DOI: 10.1039/c0gc00387e
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recently, nano-sized magnesium oxide and Mg/Al hydrotalcite
have also been shown as effective catalysts for this reaction.22
In continuation of our progressive program to develop
efficient and environmentally benign protocols for the synthesis
of various products, we report herein the synthesis of a series of
2-amino-chromenes in water using nano-structured diphosphate
(Na2CaP2O7) (Scheme 1) as well as the results of the preparation
of the corresponding benzo[a]anthracenes.
˚
Fig. 1 Projection view of Na2CaP2O7 on the (100) plan (d1 = 4.16 A,
˚
˚
˚
d2 = 6.90 A) and (d3 = 4.04 A, d4 = 5.93 A).
Scheme 1
Results and discussion
The synthesis of the nano-structured diphosphate Na2CaP2O7
(DIPH) (Scheme 2) in powder state has been carried out
from Na2CO3, CaCO3 and NH4H2PO4 in 1 : 1 : 2 proportion,
respectively (purity of starting materials greater than 99%).
These materials were mixed together in an agate mortar and
heated in porcelain crucible progressively from 383 to 873 K. The
X-ray diffraction (XRD) pattern of the final product was
obtained on a Siemens D-500 diffractometer using Cu-Ka
radiation in the Bragg-Brentano geometry (q–2q) (See ESI†).
DIPH crystallizes in the triclinic system with the space group
¯
P1. The lattice parameters of the prepared Na2CaP2O7 are in
˚
˚
excellent agreement with standard data: a = 5.361 A, b = 7.029 A
˚
and c = 8.743 A, with all the registered characteristic peaks
of DIPH being present. In Fig. 1, the projection view of the
structure along the crystallographic plane (100) was shown.
Hence, the presence of tunnels of similar dimension along the
three directions [100], [010] and [001] was noted. Two kinds of
tunnels are present and both running along the [100] direction.
They are built up from P2O7 groups and CaO6 octahedral
(Fig. 2). Their dimension could be estimated by the respective
Fig. 2 Structure of pyrophosphate.
a field of symmetrical vibrations: (996 cm-1, 1031 cm-1) and a
field going from 1130 cm-1 to 1278 cm-1. Scanning Electronic
Microscopy (SEM) was used to study the morphology of the
surface of the Na2CaP2O7 (Fig. 3). The analysis of the obtained
picture shows clearly the porosity of the surface of Na2CaP2O7.
A homogeneous microstructure made up of layers of various
sizes and forms was observed. Specific surface area of the DIPH
by BET method from the adsorption-desorption isotherm of
nitrogen at its liquid temperature at 77 K was found to be
S = 2.4 m2g-1. This is confirmed by the TEM micrograph
(Fig. 4), which also shows that nanoparticles of a parallelepiped
shape can form relatively large agglomerates. The powder forms
irregular grains with lateral size of 90–150 nm.
˚
˚
˚
˚
distances (d1 = 4.16 A, d2 = 6.90 A) and (d3 = 4.04 A, d4 = 5.93 A).
This space is not totally free because it is reduced by the presence
of sodium cations which exhibit two kinds of crystallographic
sites. The FTIR of Na2CaP2O7 is represented on the figure S2
(ESI†) and the attributions suggested are regrouped in the table
S1 (ESI†). The existence of P2O7 groups is confirmed by the band
of the symmetrical vibration of P–O–P at 720 cm-1. The band
of the anti-symmetric vibration of this group is at 893 cm-1. The
relating vibrations of PO4 groups are shared between two fields:
Our initial study focused on the development of the optimal
reaction conditions for this transformation. Therefore, 4d was
randomly chosen to carry out the study which includes catalyst
screening, the influence of the catalyst weight, and the optimum
volume of water. First, p-chlorobenzaldehyde 1d, malononitrile
2 and 1-naphthol 3 were mixed and left a period of time
without catalyst, and then tested in presence of three different
Scheme 2
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Table 1 Catalysts screening for the synthesis of product 4d
Entry
Catalyst
Yield of 4d (%)a
1
2
3
4
—
24
45
58
90
Na4P2O7
K2CaP2O7
Na2CaP2O7
a The time of all reactions is 4 h.
Fig. 3 SEM of the Na2CaP2O7.
Fig. 4 TEM images of Na2CaP2O7 nanopowder.
Fig. 6 Weight effect of the catalyst Na2CaP2O7 in the three-component
coupling of carbonyl compound, malononitrile and 1-naphthol (2-
amino-chromene 4d).
phosphate catalysts including K2CaP2O7, Na4P2O7, and
Na2CaP2O7 (Table 1). Initially, the reaction can start without
any catalyst leading to poor yield in the final product (24%).
The comparison of the reactivity of the three heterogeneous
catalysts used for the synthesis of 2-amino-chromene 4d, showed
clearly that Na2CaP2O7 (DIPH) is the best of the three phosphate
catalysts with a 90% recovery in product 4d. On the other hand,
the use of the DIPH catalyst showed a significant improvement
of yield as well as reaction time when compared to the non-
catalyzed reaction(Fig. 5).
to the increase in the quantity of catalyst. In order to use the
minimum mass of the catalyst, we have been limited to 0.3 g for
the continuation of this study.
To understand the effect of water on the three-component
reaction catalyzed by DIPH, different volumes of water were
added to the reaction mixture (Fig. 7). In the presence of 1 mL
of water, only 47% of the desired product was obtained in 4
hrs reaction. The yield was increased significantly by adding
various volumes of water. Thus, in presence of 10 mL water, the
yield reached 90% in 4d. As clearly shown in Fig. 7, the yield
Fig. 5 Kinetic curves of the synthesis of 2-amino-chromene 4d
catalyzed by Na2CaP2O7.
To determine the best weight of the catalyst, the synthesis of
4d was carried out in presence of various amounts of catalyst in
water. Fig. 6 shows a remarkable increase in outputs according
Fig. 7 Effect of the quantity of water in the synthesis of 2-amino-
chromene 4d (yields after 4 h).
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increases at around 10 mL of added water. Beyond this quantity,
a notable decrease in yield was observed. This phenomenon can
be attributed to the dispersion of the reagents when large volume
of water is used.
A heterogeneous catalyst is more interesting when it can be
easily recovered and re-used. For this purpose, the synthesis of
4d was carried out using fresh and recovered DIPH catalyst for
a 4-cycle run. After 4 h reaction, the product 4d was isolated
and identified. The recovered catalyst was washed with acetone
and calcined at 600 ◦C. The obtained yield after the 4-cycle run
is almost stable and unchangeable (90, 87, 85 and 84%) which
demonstrates that DIPH can be easily recovered and re-used
without any loss of its activity.
A large number of 2-amino-chromene derivatives have been
found to have very interesting biological activities and of
which some are already synthesized on an industrial scale. In
the objective to validate the developed method, we applied
the optimum conditions as previously determined to a large
variety of substrates. The three-component reactions of carbonyl
compounds 1, malononitrile 2 and 1-naphthol 3 were carried
out in presence of DIPH (0.3 g), and refluxed in water to
prepare multiple 2-amino-chromenes (Scheme 1). The products
were isolated, purified by re-crystallization and analyzed by
Scheme 3
1
melting points, H and 13C NMR, IR spectroscopy and mass
Scheme 4
spectrometry (MS). The results obtained with various substrates
are summarized in Table 2. Thus, seven products were easily
prepared in quantitative yields.
sex pheromones,30 and display central nervous system (CNS)
activity.32
The a-cyanocinnamonitrile 3¢a possessing a phenyl group
reacted efficiently with 1-naphthol 3 to afford the 2-amino-2-
chromene 4a, in high yield (entry 1). In addition, aromatic-
cyanocinnamonitriles 3¢d–e containing electron-withdrawing
groups afforded 4d–e, respectively, in essentially quantitative
yields (entries 4–5). Even, the a-cyanocinnamonitriles 3¢b–c
containing electron-donating groups at the para position were
converted smoothly to the 2-amino-2-chromenes 4b–c in 72 and
76% yields, respectively (entries 2–3). In the case of 1g bearing
dimethylamino group at the para position, the reaction stopped
at the alkene 3¢g demonstrating that the nature of the substituent
has a great influence on the orientation of the reaction.
Experimental
Catalyst preparation
The diphosphate Na2CaP2O7 was prepared as a powder by solid-
solid method starting from Na2CO3, CaCO3 and NH4H2PO4
in high purity grade and in 1 : 1 : 2 proportion, respectively
(Scheme 1 and Figure S1†). These materials were mixed together
in an agate mortar and heated in porcelain crucible progressively
to 873 K.
The mechanism of this reaction can be seen as sequential
reactions involving Knoevenagel reaction, Michael addition
and an intra-molecular cyclization that may take place in the
formation of the final product. The probable mechanism is
given in Scheme 3. The first step which is not limiting, it is
the formation of the alkene. Indeed, it is well known that the
first step, if carried out in protic solvents like water, does not
require any catalyst.23 The second phase is the ortho C-alkylation
of 1-naphthol. It is to be noted that this phase requires the
intervention of a catalyst.24 The last step is completely regio-
selective, the addition of 1-naphthol 3 to a-cyanocinnamonitrile
derivative 3¢ in presence of DIPH catalyst gives the regio-isomer
4 (Scheme 3).
General procedure for the synthesis of 2-amino-chromenes
A mixture of the aldehyde 1 (1 mmol), malononitrile 2 (1 mmol,
66 mg), 1-naphthol 3 (1 mmol, 144 mg) and Na2CaP2O7 catalyst
(0.3 g) was refluxed in 10 mL water. After an appropriate
time of reaction, the catalyst was removed by filtration and
washed with acetone or ethyl acetate. After concentration of
the filtrate, the residue was purified by re-crystallization using
an eco-compatible solvents (EtOH or EtOH/water) leading to
2-amino-chromene 4. The structures of the prepared products
were assigned on the basis of their spectral data in comparison
with those reported in the literature.
The importance of the 2-amino-chromenes prepared can be
seen through their reactions with acetic anhydride to produce the
corresponding benzo(a)anthracenes (5a–d) (Scheme 4). These
products are of biological interest as they show antimicro-
bial activity,25 inhibit influenza virus sialidases,26 and show
DNA stand-breaking activity and mutagenicity.27 Also, they
are antiviral,28 antiproliferative29 and antitumor compounds,31
General procedure for the synthesis of benzo(a)anthracenes 5a–d
A solution of 4a–d (10 mmol) in acetic anhydride (20 mL) was
refluxed for 6 h. The solid formed was filtered, washed with
cooled ethanol, dried and re-crystallized from suitable solvent
to give 5a–d (Table 3).
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Table 2 A one-pot multi-component reaction of carbonyl compounds, malononitrile and 1-naphtol for synthesis of 2-amino-chromenes in water
Entry
1
Aldehyde
Product
Yield% (Time h)a
81 (5)
2
3
4
72 (5)
76 (5)
90 (4)
5
94 (4)
6
84 (5)
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Table 2 (Contd.)
Entry
7
Aldehyde
Product
Yield% (Time h)a
91 (5)
a All compounds were identified by melting points,1H NMR, 13C NMR, IR and MS.
Table 3 Synthesis of benzo(a)anthracenes 5a–d
Entry
1
2-Amino-chromenes
Benzo(a)anthracenes
Yield of 4d (%)a
84
2
78
3
80
4
74
a The time of all reactions is 6 h.
trile 2 and 1-naphthol 3 using a catalytic amount of an efficiently
re-usable synthetic DIPH catalyst in water as green solvent.
These 2-amino-chromenes, in presence of acetic anhydride, have
been easily converted to benzo(a)anthracene derivatives. The
developed methodology is highly efficient and environmentally
Conclusion
In conclusion, we described a practical and efficient procedure
for the preparation of 2-amino-chromenes through the one-pot
three-component reactions of aromatic aldehydes 1, malononi-
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benign in terms of product recoveries, atom economy and solvent
usage. The success of this methodology would push us to use
it for the synthesis of other biologically active molecules that
would be published soon and thereafter the contribution to the
development of new and green organic syntheses.
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