A new MCM-41 supported HPF6 catalyst for the library synthesis of highly substituted 1,4-dihydropyridines and oxidation to pyridines: Report of one-dimensional packing towards LMSOMs and studies on their photophysical properties

Article abstract of DOI:10.1039/c3gc40441b

A new heterogeneous MCM-41 silica supported HPF₆ catalyst has been synthesized and characterized using an array of sophisticated analytical techniques like BET, XRD, HRTEM, EDX, ²⁹Si MAS NMR, TGA, FTIR, and pH measurement. The broad

Full text of DOI:10.1039/c3gc40441b

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A new MCM-41 supported HPF6 catalyst for the library synthesis of
highly substituted 1,4-dihydropyridines and oxidation to pyridines:
report of one-dimensional packing towards LMSOMs and studies on
their photophysical properties
Suman Raya, Mike Brownb, Asim Bhaumikc, Arghya Duttac and Chhanda Mukhopadhyaya*
a: Department of Chemistry, University of Calcutta, 92 APC Road, Kolkata-700009, India
b: Bruker Bio Spin, 2700 Technology Forest Drive, Woodlands, Texas 77381, USA
c: Department of Materials Science, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700
032, India.
*cmukhop@yahoo.co.in
New MCM-41/HPF6 catalyst for library synthesis of 1,4-dihydropyridines and oxidation to pyridines
O
O
R1
Ar1
(NH4)2CO3
Ar2
O
O
O
H
O
(4)
1 mmol
(1)
(3)
1 mmol
(1)
1 mmol
(2)
1 mmol
mmol
MCM-41/HPF6
H2O2 (aqueous)
MCM-41/HPF6
Stirring at r.t. in water
Ar1
O
Ar1
O
R1
R1
Ar2
N
Ar2
N
H
(5)
(6)
19 New compounds
23 New compounds

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ARTICLE TYPE
A new MCM-41 supported HPF6 catalyst for the library synthesis of
highly substituted 1,4-dihydropyridines and oxidation to pyridines:
report of one-dimensional packing towards LMSOMs and studies on
their photophysical properties
5
Suman Raya, Mike Brownb, Asim Bhaumikc, Arghya Duttac and Chhanda Mukhopadhyaya*
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
A new heterogeneous MCMꢀ41 silica supported HPF6 catalyst has been synthesized and characterized
using an array of sophisticated analytical techniques like BET, XRD, HRTEM, EDX, 29Si MAS NMR,
10 TGA, FTIR, and pH measurement. The broad applicability of silicaꢀHPF6 was probed through a library
synthesis of highly substituted 1,4ꢀdihydropyridines (1,4ꢀDHPs). The 1,4ꢀDHPs are further oxidized to
pyridines with aqueous H2O2 as stoichiometric oxidant which is considered as an abundant and green
oxidant. The pyridines spontaneously generate low molecular mass selfꢀaggregated organic materials
(LMSOMs). Their oneꢀdimensional packing and interesting photophysical properties are reported.
15 Synthesis using water as solvent is of paramount importance for its sustainable green impact. Further,
carbon dioxide and water were the only byꢀproducts, which added to its attractiveness. This silicaꢀHPF6
catalyst retained its activity even after having it exposed to ambient atmosphere for 10 days.
substantial decrease in the activity and selectivity of the
immobilized catalyst is frequently observed due to the
heterogeneous nature of support materials in reaction media.10a,b
A great proportion of active species are deep inside the
50 supporting matrix and thus reactants have limited access to
catalytic sites which is another serious drawback of supported
heterogeneous catalysts.10 Therefore, there is still scope to
develop an efficient heterogeneous catalyst to fulfill the ideology
of green chemistry. Open framework MCMꢀ41 mesoporous
55 molecular sieves with high surface area and large pore size and
pore volume favours an easy accessibility of the organic
functions within the insoluble solid. Therefore they appear
attractive for their development and uses as insoluble solid
matrix, particularly for liquid–solid phase catalytic reactions.
60 Moreover, silica has some additional advantages over the other
heterogeneous solid supports, the most important of which is the
relatively easy covalent modification of the surface of silica with
organic, inorganic or organometallic moieties due to presence of
surface silanol groups.9 Their high surface area provides a large
65 concentration of active sites.10c They have tunable pore structures,
excellent physicochemical properties, nonꢀtoxic nature and are
inert under reaction and processing.
Introduction
Multicomponent reactions1 are important for generating high
20 levels of diversity because they allow more than two building
blocks to be combined in a practical, timesaving, oneꢀpot
operation to give rise to complex structures by simultaneous
formation of two or more bonds.1h One crucial aspect of
multicomponent reactions is the choice of solvent. The solvent
25 not only allows a better contact between reactants, stabilizes or
destabilizes intermediates and/or transition states but also
determines the choice of work up procedures and recycling or
disposal strategies.2 In the last decade, many promising media
have appeared as innocuous solvents,2 such as ionic liquids,3
30 supercritical fluids,4 polyethylene glycol,5 glycerol,6 and
perfluorinated solvents.7 Although fascinating results have been
reported, however the use of these solvents is still limited by
many problems: high prices and lack of data about the toxicity
and bioꢀcompatibility for ionic liquids; sophisticated equipment is
35 usually required when perfluorinated solvents are employed.2 For
these reasons, the development of synthetically useful
multicomponent reactions using water as the green reaction
medium has gained considerable interest.
The exploration of privileged structures in drug discovery is a
rapidly emerging theme in medicinal chemistry.1,4ꢀ
70 Dihydropyridines, as the important “privileged scaffold” of low
molecular mass, are very attractive targets for combinatorial
library synthesis due to their wide range of pharmaceutical
activities. These molecules have their therapeutically success due
to the effective binding to the active site of the receptors of
The development of green processes and pollution abatement
40 catalysts has acquired great importance.8 In this regard; the
advancement of heterogenized alternatives such as nanoparticleꢀ
supported, mesoporous materialꢀsupported and various
organic/inorganic polymer supported catalysts has engrossed
immense attention because they combine the advantages of both
45 homogeneous and heterogeneous catalytic systems.9 However, a
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calcium channels, regulating the transport of ions calcium
a very large specific surface area of approximately 1000 m2gꢀ1.
This property makes MCMꢀ41 very interesting to be used as a
through the biological membranes, providing a relaxation effect
on the contractions of the cardiac muscle.11 Their 60 support material for heterogeneous catalysts.
pharmacological properties include neuroꢀ and radio protective
Characterization of the catalyst
5
effects,12,13 noncompetitive inhibition of topoisomerases,14
anticancer activity,15 and HIV protease inhibition,16 among
Nitrogen adsorption analysis
N2 adsorption isotherms of different samples are recorded. The
irreversible type IV adsorption isotherms of MCMꢀ41 with H1
others. Due to the particular molecular structure, certain Hantzsch
dihydropyridines are considered as mimics of the biological
redox system NAD/NADH+ and have emerged as hydrogen
10 transfer agents in biomimetic reductions17 and also as photoactive
compounds.18 Consequently, it has attracted considerable interest
for synthetic chemists and stimulated the development of
methods for their preparation.
Benzylic oxidation of 1,4ꢀDHP to pyridine has attracted
15 considerable attention in recent years as Bocker has demonstrated
that metabolism of those drugs involves a cytochrome Pꢀ450
catalyzed oxidation in the liver.19 Also the pyridines are
prominent synthons in supramolecular chemistry, with their Πꢀ
stacking ability along with directional Hꢀbonding capacity.20 In
20 addition, the excellent thermal stabilities of these pyridines have
instigated a growing interest for their use as monomeric building
blocks in thin films and organometallic polymers.21 These
molecules have been found to be useful for the synthesis of DNA
binding ligands, in particular targeting Gquadruplex DNA which
25 has recently received much attention as a possible target in cancer
therapy.22 Benzylic oxidations are generally carried out with a
large excess of metal oxidants such as chromium and manganese
reagents.23 There are reports on benzylic oxidation including,
65
Scheme 1. Preparation of MCMꢀ41 supported HPF6 catalyst.
hysteresis loop defined by IUPAC are observed, that is a typical
feature of mesoporous materials (Figure 1, panel a).31,32 The
Brunnauer–Emmett–Teller surface area of the MCMꢀ41 material
heterogeneous
chromium
catalysts/tꢀBuOOH,24
30 heteropolyoxometalate/O2,25
Nꢀhydroxyphthalimide/O2,26
70 obtained by using N2 adsorption–desorption isotherm was found
out to be quite high 1057 m2gꢀ1. Pore volume is also very high
(1.79 ccgꢀ1). Pore size distribution (PSD) plot employing non
local density functional theory (NLDFT) suggested narrow
distribution with peak pore dimension of ca. 3.56 nm (Panel b).
75 Sharp increase in N2 uptake at high P/P0 suggested the presence
of interparticle mesopores. We can find that the surface area of
the MSPA-10 decreases when compared to pure MCMꢀ41 (Panel
c), pore volume decreases and pore diameter increases (Panel d).
Therefore, HPF6 is grafted compactly and securely inside the
80 hexagonal pores while not fully occupying the total available
space, therefore still leaving room for N2 adsorption and
CoCl2/O2,27 Mn(salen)/NaOCl,28 RuCl2(PPh3)3/tꢀBuOOH,29 and
CeO2/NaBrO3.30 However owing to the toxic effects and
deactivation associated with metalꢀleaching during the reaction,
increasing interests are being paid for the development of
35 oxidizing methods devoid of metal ions.
Results and Discussion
With the aim of finding efficient and recyclable silicaꢀsupported
catalysts with enhanced selectivity for 1,4ꢀdihydropyridines, we
undertook the preparation of several mesoporous silica MCMꢀ41
40 supported hexafluorophosphoric acid catalysts MSPA with
different catalyst loadings by adding x mmol HPF6 (x = 5, 10,
20, 30) in 2g of MCMꢀ41 according to scheme 1 and denoted as
MSPA-x. Among them MSPA-10 performs most efficiently as
an acid catalyst for the product selective synthesis of efficient
45 assembly of 1,4ꢀdihydropyridines (5) and pyridines (6) in water at
room temperature. To gather insight about the surface area, pore
volume, pore diameter and morphology of MSPA-10 catalyst, it
was characterized by N2 adsorption analysis, powder Xꢀray
diffraction, high resolution transmission electron microscopy
50 (HRTEM), EDX with elemental analysis, solid state 29Si MAS
NMR spectra, Thermo gravimetric analysis, FT IR analysis and
pH measurement. The most striking fact about the material
MCMꢀ41 is that, although composed of amorphous silica, it
displays an ordered structure with uniform mesopores arranged
55 into a hexagonal, honeycombꢀlike lattice. MCMꢀ41 has a very
large void fraction, due to the presence of the mesopores, and
concomitantly a rather low density. As a result MCMꢀ41 displays
Figure 1. (a) N2 adsorption isotherm of MCMꢀ41, (b) PSD of MCMꢀ41,
(c) N2 adsorption isotherm of MSPA-10, (d) PSD of MSPA-10.
2
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molecular transport. The total pore volume of the catalyst can be
calculated approximately by the plateau displayed by the
Isotherm.32 MSPA-10 was found to be most effective catalyst in
terms of selectivity towards the desired products. For MSPA-x,
with x =5 and 20 the selectivity towards 5 was comparatively
lower than MSPA-10. The textural parameters of catalysts with
different HPF6 loadings were listed in Table 1. We have also used
commercially available macroporous 60ꢀ120 mesh silica. For x =
10 specific surface area was found out to be 199 m2/g and the
5
10 pore diameter was 7.4 nm. Unfortunately any of the catalysts
derived from commercial silica did not exhibit any selectivity for
the upcoming reaction protocol.
40
Figure 3. EDS analysis of MSPA-10.
Table1. Textural parameters of different silica supported HPF6 catalysts.
limited number of reflections, all situated at low angles. The
MCMꢀ41 material is highly ordered (Figure 4, panel a); showing
three strong diffractions for the 100, 110 and 200 planes at 2θ
equal to 2.3°, 3.9°, 4.5° corresponding to 3Dꢀhexagonal
1
1
Entry
Sample
MCMꢀ41
MSPA-5
MSPA-10
MSPA-20
MSPA-30
Commercial
Silicaꢀ10
SBET (m2g
1057
900
)
Vp (cm3g
1.7880
1.200
0.6504
0.3421
0.1133
0.4021
)
Dp (nm)
3.56
3.80
4.00
4.50
5.90
7.4
1
2
2
3
4
5
739
292
25
199
45 mesophase.33 These sharp signals indicate the longꢀrange orders
of the uniform hexagonal mesoporous structure, which is the
extraordinarily characteristic of MCMꢀ41 material. On the other
hand in MSPA-10 material the 100 peak is rather wide and 110
and 200 peaks come together as a broad peak (panel b). These
50 facts clearly indicate that the mesoporous structure is affected by
loading of HPF6. This can be attributed to the fact that because of
the very large mesopore surface area the pore walls of MCMꢀ41
are thin, resulting in the collapse of the some portion of the thin
walls upon exposure to mineralizing agents, i.e. fluoride and
55 hydroxide ions etc.
HRTEM
15 TEM image analysis of the catalyst revealed that the MSPA-10
material is composed of uniform ca. 25ꢀ30 nm size mostly
spherical particles (Figure 2). HRTEM image shows arrangement
of mesopores. Dimension of the pores of ca. 4.1 nm.
Figure 4. (a) XRD pattern of MCMꢀ41, (b) XRD pattern of MSPA-10.
29Si MAS NMR spectra
20
Figure 2. HRTEM image of MSPA-10.
EDX and elemental analysis
Due to the low concentration of the modifier part on the silica
surface, the intensities of the new peaks attesting the presence of
modifier elements are weak. Therefore, the EDX spectra are
The 29Si MAS NMR spectral pattern for MSPA-10 catalyst is
60 shown in figure 5. This pattern is quite broad like amorphous
mesoporous silica. Three major peaks in this sample at −96.0,
−101.0 and −111.0 ppm were observed. These peaks have been
assigned to tetrahedral Q2, Q3 and Q4 silica species, respectively
where Qn = Si(OSi)n(OH)4ꢀn, n = 2–4. High Q4 percentage
25 useful only to confirm the existence of the bonded species. In the
EDX spectra (Figure 3) of MSPA-10 the small peaks for
phosphorous and fluorine clearly indicate that HPF6 is definitely
incorporated within the channels of silica. Elemental analysis
showed that the phosphorous and fluorine content to be 2.74%
30 and 8.41% from which a loading of catalyst of 0.89 mmol/g is
obtained.
XRD analysis
Because of the geometry of its lattice MCMꢀ41 can be indexed
with a hexagonal unit cell, with a = b and c = ∞. Because the
35 parameters a and b are in the order of nanometers instead of
tenths of nanometers, as usually encountered in crystals, Xꢀrays
are diffracted over small angles only. Therefore, characterisation
of MCMꢀ41 with Xꢀray diffraction yields a diffractogram with a
δ (ppm)
65
Figure 5. (a) 29 Si NMR spectra of MSPA-10.
(b)
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indicated highly condensed network.33 Such
a
high Q4
reacted with dimedone affording intermediate bishydroxy
concentration is of paramount importance for the catalytic 60 compound (8b) (Figure 6) which subsequently reacted with
activity of silica based catalyst, since reduction of surface silanols
introduces high hydrophobicity (and thus more affinity towards
organic substrates).33
ammonia to produce the acridine derivatives (7).39 During the
four component reaction involving dimedone, ketone, aldehyde,
and ammonia to form 1,4ꢀDHPs competitive formation of the
acridine was found to be a potential problem and an issue not
65 addressed/attended so far and the present study highlights the
influence of the MSPA-10 catalyst in controlling the selective
formation of 1,4ꢀdihydropyridine.
5
TGA analysis
TG curve showed a threeꢀstage weight loss (Not shown here).
First 0.7024% of which considered to be the removal of
o
physically absorbed water (up to nearly 100 C).34,35 The second
10 6.1284% weight loss may be explained by decomposition of
surface modifier material and also removal of some water by
condensation of adjacent silanol groups (170ꢀ427 oC)
respectively. The third 10.7093% weight loss is mainly due to
O
O
R1
Ar1
(NH4)2CO3
Ar2
O
O
O
H
o
O
condensation silanol groups (427ꢀ598 C). DTA analysis further
(4)
1 mmol
(1)
(3)
1 mmol
(1)
1 mmol
(2)
1 mmol
15 demonstrates the presence of the various species, the endothermic
peak at lower temperature represents the removal of physically
absorbed water from the material, while the endothermic peak at
higher temperature represents solely the decomposition of the
silica HPF6 and endothermic peak at much higher temperature is
20 for condensation of silanol groups. These results also suggested
the presence of target group in the catalyst.36a
mmol
MCM-41/HPF6
H2O2 (aqueous)
MCM-41/HPF6
IR analysis
Stirring at r.t. in water
In the IR spectra of the unmodified supports a strong and broad
band in the range 3500ꢀ3400 cmꢀ1 corresponds to the hydrogen
25 bonded SiꢀOH groups and adsorbed water, another broad band at
1637 cm―1 is also due to OꢀH vibration of adsorbed water.34 The
sharp features around 1092 cm−1 and the absorption peak at 468
cm−1 are assigned to asymmetric stretching and bending vibration
of Si–O–Si.34 In the IR spectra (Not shown here) of silicaꢀHPF6
30 apart from those peaks the additional peaks at 1040 cm−1 might
be attributed to the PꢀO stretching modes37 and the peak lies in
901 cm−1 may correspond to the PꢀF stretch.38 It implied that
phosphate moiety was tightly anchored with the silica surface,
probably through a covalent linkage with the oxygen atoms of
35 silicon oxide.
O
Ar1
O
Ar1
R1
R1
Ar2
N
Ar2
N
H
(5)
(6)
19 New compounds
23 New compounds
Scheme 2. Synthesis of 1,4ꢀdihydropyridines and pyridines.
Ar1
H
Ar1
O
(b)
(a)
O
(c)
O
O
H
O
Ar1
O
N
H
OH HO
pH analysis
(7)
(8a)
(8b)
70
The amounts of acid groups after synthesis of silicaꢀ
phosphoric acid catalyst were measured by means of pH
measurement. Ionꢀexchange capacities of the phosphoric acid
40 group were determined using aqueous solution of sodium
chloride as exchange agents. In a typical experiment 1.0 g of the
Figure 6. (a) Acridine as one of the possible side products, (b) and (c) 2ꢀ
arylidenedimedone (8a) and bisꢀhydroxy compound (8b) as the possible
intermediates leading to acridine.
With the optimized conditions in hand, to delineate this approach,
catalyst was added to 100 ml saturated solution of NaCl. The 75 the scope and generality of this protocol was next examined by
resulting suspension was allowed to equilibrate for 48 h and then
pH of the dropped to 3.08 since ion exchange occurred between
45 sodium ions and protons. The acidic site loading was found to be
0.83 mmol/g which was very close to that obtained from
elemental analysis.
employing various aromatic aldehydes and ketones. For
precursors 2 and 3 bearing either electronꢀdonating or electronꢀ
withdrawing substituents on the aroyl group (for 3) or on the
aromatic ring (for 2), the reactions all proceeded very smoothly to
80 provide the corresponding 1,4ꢀdihydropyridine derivatives (5)
(scheme 3). Acidꢀsensitive heteroaryl ketone, and methoxy
substituted aryl aldehyde also reacted very efficiently with no
side reactions. Therefore, the present silicaꢀhexafluorophosphoric
acid catalyzed protocol has a general applicability accomodating
85 a variety of substitution pattern. Sterically bulky 2ꢀacetylfluorene
reacted quantitatively under mild condition (5t). The products
with 2ꢀphenylacetophenone and propiophenone contained five
sterically large substituents (5a-5k). To our surprise despite
distinct steric hindrance and ring tension cyclic ketones reacted
90 smoothly under this present condition (5u). Therefore, this very
strong solid acid catalyst showed an excellent distribution of acid
sites, suggesting that the catalyst possesses a higher number of
Synthetic application of MSPA-10
We have already mentioned that MSPA-10 performs efficiently
50 as an acid catalyst for the synthesis of 1,4ꢀdihydropyridines (5)
and pyridines (6) in water at room temperature (Scheme 2).
Synthesis of 1,4ꢀdihydropyridine compounds from dimedone (1),
aldehyde (2), ketone (3) and ammonia (4) is not straightforward.
The earlier quest39 to construct compound 5 was disappointing
55 producing only acridine derivative (7) (Figure 6). The reason may
be attributed to the stability of intermediate 2ꢀarylidenedimedone
(8a) (Figure 6). The intermediate, 2ꢀ arylidenedimedone (8a),
displayed a higher reactivity and instability, which preferably
4
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NO2
CH3
NO2
CH3
surface active sites compared to its homogeneous analogues. The
synthetic route is facile, convergent, and allows easy placement
of a variety of substituents around the periphery of the
heterocyclic ring system. The wide substrate scope for this
present transformation is displayed in figure 7 and 8.
O
O
H
N
H
N
H
5
H
H
5v (89%)
5w (94%)
O
Ar1
O
10
Scheme 3. Library of 1,4ꢀDHPs synthesized.
R1
R1
40 mg MSPA-10
Ar1
r.t.
(NH4)2CO3
O
O
O
O
O
Ar2
O
H Ar2
O
N
H
Stirring in H2O
O
(5)
Ph
(2)
(3)
(1)
(4)
1 mmol
O
O
1 mmol 1 mmol
O
1 mmol
H
Br
Cl
NO2
Figure 7. Substrate scope for 1,3ꢀdiketones.
O
O
O
O
O
O
O
O
CH3
Cl
CH3
CH3
N
N
H
N
H
NO2
MeO
H
5c (99%)
5a (98%)
5b (94%)
CN
O
O
O
O
NO2
CH3
CH3
O2N
CH3
CH3
CH3
O
O
O
S
N
H
N
N
Figure 8. Substrate scope for ketones.
H
H
5e (95%)
5f (98%)
5d (99%)
Br
CN
NO2
15 Worth mentioning the structure of the 1,4ꢀDHP was confirmed
from Xꢀray single crystal analysis of a compound (5d) (Figure 9)
(CCDC 853740).
O
O
O
CH3
CH3
CH3
N
H
N
H
N
H
5h (98%)
Cl
5g (92%)
5i (93%)
OMe
NO2
H
O
O
O
CH3
CH3
N
N
H
N
H
H
5l (99%)
5j (89%)
5k (94%)
Cl
OMe
NO2
H
O
O
O
H
H
S
S
N
H
N
H
N
H
NO2
5o (86%)
5m (95%)
5n (84%)
Figure 9. Ortep diagram of 5d showing the crystallographic numbering
NO2
OMe
20
(CCDC 853740).
OMe
NO2
H
Plausible mechanism for 1,4-dihydropyridine formation
O
O
O
H
S
H
Literature survey40 reveals that dimedone (1) is not only a
Knoevenagel reagent but it also adds easily to electronꢀpoor
alkenes in the Michael addition fashion leading to the bisꢀ
N
H
N
H
N
H
OMe
OMe
5p (83%)
5q (90%)
5r (99%)
25 hydroxy compound 8b. Thus, cascade reactions of addition,
elimination and addition could be achieved, but the intermediate
alkene i.e., 2ꢀarylidenedimedone (8a) was not available even in
1:1 experiments according to the present reaction conditions.
Therefore if the reaction would commence through the initial
30 formation of Knoevenagel intermediate (8a) then it would
certainly yield some amount of bisꢀhydroxy compound (8b) and
also the acridine (7) in further step. However neither the
compound 8b nor 7 were isolated in any single occasion rather
1,4ꢀdihydropyridine (5) was solely obtained in all occasions.
35 Again when we heated the reaction mixture at 90ꢀ100°C no
Br
NO2
NO2
O
O
O
H
H
N
H
N
H
N
H
5u (74%)
5t (88%)
5s (89%)
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xanthene was obtained by the elimination of water from 8b which
Therefore, besides the Bronsted acidity of the catalyst (MSPA-
also discarded the possibility of formation of 2ꢀ 45 10) the Lewis acidity of PF5 must be considered for the catalytic
arylidenedimedone (8a) or the bisꢀhydroxy compound (8b). The
foregoing discussions clearly establish that the reaction is not
going through the Knoevenagel condensation of aldehyde with
dimedone instead any other suitable pathway. To investigate the
actual mechanistic pathway followed we become interested to
take the 1H NMR spectra of crude reaction mixture. The
performance. Probably the carbonyl oxygen may coordinate with
the Lewis acid PF5 increasing the electrophilicity of the carbonyl
carbon and thereby enabling the reaction to occur at room
temperature.
5
O
O
Ar1
40 mg MSPA-10
R1
R1
H2O
Ar1
(NH4)2CO3
enaminoketone (9) and chalcone (10) were the only isolable
r.t., stirring
1
Ar2
O
H
Ar2
(3)
O
10 intermediate as confirmed from the H NMR spectrum (Figure
10) of the crude reaction mixture obtained by quenching the
reaction forming 5q after 1 hour. There were three detectable
O
N
(2)
(1)
(4)
H
(5)
1 mmol
1 mmol 1 mmol
1 mmol
1
compounds in the crude H NMR spectra: the enaminoketone 9
(indicated with blue colour), chalcone 10 (indicated in pink) and
15 the product itself 5 (green). Also the compound (9) could be
obtained through the reaction between (1) and (4) in the presence
of MSPA-10 at room temperature. This compound, on stirring at
room temperature in water in the presence of MSPA-10 with
intermediate (10) (obtained by reaction of aromatic ketone with
20 aldehyde) afforded the products (5). Thus the intermediacy of 9
and 10 in this transformation is clearly established. On the basis
of the results obtained above, a plausible reaction scenario for
this oneꢀpot fourꢀcomponent reaction is outlined in Scheme 4. It
Ar1
Ar1
O
O
Ar1
O
H
R1
R1
R1
H
Ar2
N
Ar2
O
NH2
O
Ar2
NH2
OH
H
(12)
(9)
(10)
(11)
50
Scheme 4. A plausible mechanistic pathway to explain the formation of
1,4ꢀdihydropyridines.
Being inspired by the above results, it was thought worthwhile to
replace the dimedone with 1,3ꢀindandione in order to get some
55 newly substituted 1,4ꢀDHPs and to show versatility of this
protocol. For this aim, we have examined the reaction of 1,3ꢀ
indandione (13), ketone, aldehyde, and ammonium carbonate
under the same previous reaction conditions. Quite surprisingly,
instead of the anticipated 1,4ꢀDHPs derivative, we obtained an
Ar1
O
O
R1
40 mg MSPA-10
R1
Ar1
r.t.
(NH4)2CO3
Stirring in H2O
Ar2
Ar2
O
H
O
N
O
(3)
1 mmol
(4)
(13)
(14)
(2)
1 mmol
1 mmol
1 mmol
R1 = H, CH3, Ph
60
Me
NO2
NO2
H
25
Figure 10. Crude 1H NMR spectra for the reaction forming 5q.
O
O
O
H
H
was also proved that the catalyst played its crucial role in the acid
promoted dehydrative cyclization of (11) to produce 5. The
cyclization of the intermediate (11) was initiated through
protonation by silicaꢀhexafluorophosphoric acid on its carbonyl
N
N
14b (95%)
N
OMe
Cl
Cl
14c (98%)
14a (91%)
OMe
NO2
NO2
30 oxygen. The electrophilicity of the corresponding carbonyl
carbon was thereby increased. Concomitant intramolecular
nucleophilic attack by the nearby NH2 group led to cyclization to
give 12 which underwent dehydration to give 5. Subsequent
dehydration was assisted by the conjugate base of silicaꢀ
35 hexafluorophosphoric acid through proton abstraction from 12
O
O
O
H
H
N
N
N
N
Cl
OMe
14d (89%)
14e (97%)
14f (91%)
Cl
NO2
CH3
when the product
5 was formed. As a result, silicaꢀ
O
O
O
H
hexafluorophosphoric acid was regenerated and the catalytic
cycle reꢀoperated. Therefore, the attenuated acidity of the MSPA-
10 was found to be crucial for cyclization but insufficient to
40 affect the acidꢀsensitive moieties.
H
N
N
N
Cl
Cl
14i (82%)
14h (82%)
14g (82%)
It is also worth mentioning that the HPF6 is a binary conjugate
BronstedꢀLewis super acidic system in which the Lewis acid PF5
is only weakly coordinated with the Bronsted acid HF.41
Scheme 5. Synthesis of indenopyridine.
6
| Journal Name, [year], [vol], 00–00
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Green Chemistry
Page 8 of 15
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NO2
unexpected pyridine (14) in excellent yields (scheme 5). The fully
NO2
H
NO2
CH3
oxidized pyridine (14) that is generated in case of 1,3ꢀindandione
is most probably due to the very high conjugation in the product
(14) which is not possible for the product (5) obtained from
dimedone.
O
O
O
H
5
N
N
N
In case of reaction of 1,3ꢀindandione, ketone, aldehyde and
ammonium carbonate the oxidation of 1,4ꢀdihydropyridine is
highly beneficial since the resulted fully oxidized pyridine is an
extended conjugated system. Therefore the intermediate 1,4ꢀ
10 dihydropyridine is oxidized by the molecular oxygen present in
the air to get benefit of the extended conjugation so fast that it
becomes impossible to isolate the 1,4ꢀdihydropyridine
intermediate.
6l (91%)
6j (94%)
6k (88%)
Cl
Cl
Cl
NO2
H
NO2
H
O
O
O
H
N
N
N
6n (98%)
6o (86%)
6m (94%)
S
OMe
NO2
Cl
Br
NO2
Till date, only a very few fully oxidized pyridines (6) from
15 dimedone, ketone, aldehyde and ammonia are reported. Kumar et
al. reported the synthesis of only three fully oxidized pyridine by
oxidative aromatization of 1,4ꢀdihydropyridines using very
expensive Human hemoglobin (HbA) as a catalyst with hydrogen
peroxide in phosphate buffer.42a Therefore, exploration of more
20 general, efficient, rapid, and viable catalytic process for the
oxidative aromatization of 1,4ꢀDHPs is another challenge in
current organic trend which was not addressed before. As an
extension of the chemistry of H2O2 as a stoichiometric green
oxidant,42bꢀj we envisioned that it might also be utilized to
25 promote the oneꢀpot oxidative synthesis of pyridine from same
dimedone, ketone, aldehyde and ammonia according to scheme 6.
The excellent conversions show that the H2O2 in combination
with MSPA-10 catalyst has very good activity for oxidative
synthesis of pyridine.
O
O
O
H
H
H
N
N
N
6r (86%)
6q (88%)
S
S
6p (78%)
S
O2N
N
6s (86%)
Scheme 6. Oxidative synthesis of pyridine.
conversion occurred for R1=H. For R1=CH3, despite difficulties to
accommodate two substituents in adjacent carbon in a near planar
40 arrangement (6g-j) moderately high yield was obtained in all
cases. Both the structure was confirmed from Xꢀray analysis
(Figure 11 and 12). When R1=Ph, pyridine was not obtained in
any single cases.
30 To discuss the scope and limitation of this reported protocol we
envisioned that a wide range of Ar1 and Ar2 groups (aromatic and
heteroꢀaromatic) were well tolerated providing a functional
handle for further manipulation. In scheme 6 quantitative
Ar1
O
O
40 mg MSPA-10
R1
R1
Ar1
Aq. H2O2
(NH4)2CO3
H Ar2
O
N
Ar2
O
r.t.
O
(4)
(2)
(3)
(1)
Stirring in H2O
(6)
1 mmol 1 mmol 1 mmol 1 mmol
OMe
NO2
NO2
O
O
O
H
H
H
N
N
N
6c (87%)
6a (90%)
6b (88%)
OMe
OMe
Br
Cl
45
Figure 11. Ortep diagram of of (6i) showing the crystallographic
numbering (CCDC 884043).
NO2
H
O
O
O
H
H
N
N
N
6e (95%)
6f (96%)
6d (94%)
Br
CN
NO2
O
O
O
CH3
CH3
CH3
N
N
N
6h (93%)
6g (92%)
6i (95%)
Figure 12. Ortep diagram of of (6r) showing the crystallographic
numbering (CCDC 853739).
35
This journal is © The Royal Society of Chemistry [year]
Journal Name, [year], [vol], 00–00 | 7

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It was necessary to know whether the formation of pyridine (6) is
mediated via oxidation of 1,4ꢀDHP (5) with H2O2 or it follows
different pathway with MSPA-10 and H2O2 forming fully
crude reaction mixture isolated after 30 min showed the
proportions of different species: bisꢀhydroxy compound (8b)
(58.0%), the acridine (7) (14.2%) and small amount of the
oxidized pyridine. For this purpose when compound 5g was 30 unreacted dimedone (14.2%) and aldehyde (13.6%). Again the
5
stirred at room temperature with H2O2 in presence of MSPA-10
the product isolated was mainly the fully oxidized pyridine (6g)
(Scheme 7). Therefore, the formation of pyridine (6) goes via 1,4ꢀ
DHP (5) intermediate is firmly established.
crude 1H NMR spectra (Not shown here) taken after 45 min
showed mostly the acridine (62.6%) with small amount of bisꢀ
hydroxy compound (8b) (37.4%). The product isolated after 1h
was the only acridine (7) in excellent yield. No enaminoketone
35 (9) was detected from the crude reaction mixture isolated at
different times. Furthermore in a control experiment when the
bisꢀhydroxy compound (8b) was stirred at room temperature with
equivalent amount of ammonium carbonate it immediately
yielded the acridine (7). Therefore, though both the pathways are
40 probable, in this particular instance the reaction advances through
the bisꢀhydroxy intermediate (8b) as evidenced from the
foregoing discussion.43
Br
Br
40 mg MSPA-10
O
O
H2O2
CH3
CH3
r. t.
N
N
Stirring in H2O
H
6g
5g
10
Scheme 7. Oxidation of 1,4ꢀDHP by H2O2.
Synthesis of acridine
Again the MSPA-10 catalyst offers a new protocol to construct
acridine derivatives when it is the desired compound with
advantages like convergence, operational simplicity, and facile
15 automation, rendering the transformations green (scheme 8).
Acridines are a kind of enaminoketones extensively used in
biocatalysis and organic chemistry.
Plausible mechanism for acridine formation
The reaction cascades may proceed following any of the two
20 possible pathways as represented in scheme 9. Therefore, to
investigate the actual mechanistic pathway for the formation of
acridine (7) the reaction forming 7g was chosen as
a
representative example. The 1H NMR spectra (Figure 13) of the
Figure 13. Crude 1H NMR spectra for the reaction forming 7g.
O
O
Ar1
O
O
40 mg MSPA-10
45 Probable reason behind different mechanism of 1,4-DHP (5)
and acridine (7)
(NH4)2CO3
Ar1
H
r.t.
O
N
H
Stirring in H20
1h
We have already discussed that dimedone (1) is not only a
Knoevenagel reagent but is also a Michael donor and thus leads
to the formation of bisꢀhydroxy compound (8b). This
50 intermediate 8b reacts subsequently with ammonia forming
acridine (7). However, in case of 1,4ꢀdihydropyridine synthesis if
Knoevenagel intermediate would form in the reaction then certain
amount of bisꢀhydroxy compound (8b) and also the acridine (7)
would be detected in the crude reaction mixture. Since none of
55 them was isolated in any single occasion therefore the reaction
was not going through the Knoevenagel intermediate 2ꢀ
(1)
2 mmol
(2)
1 mmol
(4)
1 mmol
(7)
OMe
NO2
OMe
O
NO2
O
O
O
O
O
H
H
H
N
H
N
H
N
H
H
7c (93%)
7b (91%)
7a (98%)
CN
NO2
O
1
arylidenedimedone (8a). With the help of crude H NMR spectra
N
O
O
O
O
O
we have instituted that formation of 1,4ꢀdihydropyridine initiated
with the initial formation of enaminoketone (9). This compound 9
60 then reacted with chalcone (10) (formed by the reaction of
aldehyde with ketone) in further steps affording 1,4ꢀ
dihydropyridines (5). The aforementioned discussions clearly
establishe that the presence of ketone is the key parameter to
decide the mechanistic pathway for 1,4ꢀdihydropyridine
65 formation. In case of 1,4ꢀdihydropyridine formation (Scheme 4)
the aldehyde reacts with ketone to produce chalcone (10) instead
of reacting with dimedone to produce 2ꢀarylidenedimedone (8) in
view of the fact that chalcone (10) is much more stable since it is
a greater conjugated system than 2ꢀarylidenedimedone (8).
N
H
N
H
N
H
7f(98%)
7d (85%)
7e (95%)
Cl
O
O
N
H
7g(97%)
25
Scheme 8. MSPA-10 catalyzed efficient protocol for acridine synthesis.
8
| Journal Name, [year], [vol], 00–00
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Another point is that after enaminoketone (9) formation there is
present in substantial amount which drives the formation of
no excess of dimedone in the reaction mixture instead ketone is
chalcone (10).
O
O
O
Ar1
O
H
Kn
Enaminone
H
oeve
nage
l
Ar1
H
O
H
(NH4)2CO3
H
formation
NH2
co
nden
sati
on
O
1 mmol
2 mmol
1 mmol
(9)
(8a)
(1)
(2)
(4)
+
+
O
Ar1
O
H
H
H
O
O
(8a)
(1)
Ar1
H
O
O
Ar1
H
H
O
O
O
Ar1
N
O
(NH4)2CO3
-H2O
-H2O
O
NH2
OH HO
H
(8b)
(15)
(7)
5
Scheme 9. An anticipated mechanistic pathway to explain the formation of acridine derivative.
Therefore when ketone is present 1,4ꢀdihydropyridine formation
is the only observable fact instead of acridine formation is also
40 Table 2. Performance of other catalysts on the said reaction.a
CN
CN
10 evident from table 2. In table 2 when we revisited the reported
methods it was revealed that though all of these reported
procedures led to the mixture of 1,4ꢀDHP (5) and acridine (7)
however in most of the cases 1,4ꢀDHP (5) was obtained in more
than 80% selectivity. Compared to these reported catalysts the
15 hexafluorophosphoric acid derived catalyst system MSPA-10
afforded better results with 97:3 selectivities of 5 and 7. Other
solid acids used were NaHSO4ꢀSiO2, 3ꢀmercaptopropylsilica
(MPS), celluloseꢀsulfuric acid, PPAꢀSiO2, and HClO4ꢀSiO2 for
the reaction between dimedone, 2ꢀphenylacetophenone, 4ꢀ
20 cyanobenzaldehyde and ammonium carbonate. Starting materials
were mostly unaffected with NaHSO4ꢀSiO2. In other cases,
compound (5) was the major product but this was formed less
compared to MSPA-10. It is important to note that with NaPF6ꢀ
silica or NH4PF6ꢀsilica compound 5 was practically not formed.
25 Therefore, PF6‾ had no effect towards the cyclization reaction.
This comparison was vital to establish the efficacy, selectivity,
environment compatibility and broaden the applicability of the
proposed process. Therefore, the efficacy and superiority of
silicaꢀhexafluorophosphoric acid with respect to other solid acid
30 catalysts have thus been firmly established.This observation is
rare and very interesting in synthetic organic chemistry.
CHO
O
O
O
O
Catalysts
(NH4)2CO3
O
N
H
O
N
H
CN
(1)
1 mmol
(2)
1 mmol
(3)
1 mmol
(4)
1 mmol
(5e)
(7e)
Catalysts
Time Temp % of 5eb % of 7eb
Ref
(h)
4
2
1
2.5
5
1ꢀ2
10
5
10
5
5
(°C)
30
30
80
80
80
80
80
50
50
50
50
80
MSPA-10
MCMꢀ41
HPF6
PTSA
None
97
20
10
83
81
80
―
81
70
80
82
―
3
80
90
17
19
20
<10
19
30
20
18
<10
Our work
ꢀ
ꢀ
42a
44
45
ꢀ
None
NaHSO4ꢀSiO2
MPS
celluloseꢀsulfuric
PPAꢀSiO2
HClO4ꢀSiO2
NaPF6ꢀSiO2
ꢀ
ꢀ
ꢀ
ꢀ
24
ꢀ
aReaction conditions: Dimedone (1 mmol), 2ꢀphenylacetophenone (1
mmol), 4ꢀcyanobenzaldehyde (1 mmol), ammonium carbonate (1 mmol);
45 different catalysts (40 mg having 10 mmol of catalytic reagent per 2 gm
of support in case of supported catalyst), different temperature, stirring in
water.
The percentage of the two competitive products as presented in
1
table 2 was evaluated from the crude H NMR spectra. One such
b Percentage was calculated by 1H NMR (300MHz).
crude NMR spectra showing the proportions of 5e (30%) and 7e
35 (70%) is shown here for convenience (Figure 14). In the crude
NMR spectra we have successfully identified the NMR peaks of
both 1,4ꢀDHP (5e) (indicated with pink colour) and acridine (7e)
(indicated with green colour). The equation used to evaluate the
selectivity of the catalysts for 1,4ꢀDHP (5) is represented as
Idhp
x
% Selectivity =
100
Idhp + Iac
50 Where, Idhp and Iac are the peak intensities of the –NH protons of
5e and 7e as determined from crude NMR mixture.
This journal is © The Royal Society of Chemistry [year]
Journal Name, [year], [vol], 00–00 | 9

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(b)
(c)
(a)
(f)
(d)
(e)
Figure 15. SEM image of the nanostructured materials of compound 6d.
Figure 14. Crude 1H NMR spectra showing the proportion of 5e and 7e.
We realized that this is the first report on the unaddressed issue of
competitive formation of acridine (7) during the four component
reaction involving dimedone, ketone, aldehyde, and ammonia and
highlights that MSPA-10 is very effective catalyst for selective
formation of 1,4ꢀDHP (5) at room temperature. The new catalytic
procedure for the synthesis of 1,4ꢀDHP fulfil the triple bottomꢀ
line philosophy of green chemistry and are important addition to
5
10 the toolbox of medicinal chemists.
Application of the compounds
Carbonꢀrich, highly conjugated organicꢀbased molecules have
been the subject of considerable study in recent years,46,47
resulting in advanced materials that hold great promise for
15 successful applications in many optoelectronic devices, such as
organic lightꢀemitting diodes (OLEDs),48 organic fieldꢀeffect
transistors (OFETs),49 and photovoltaic cells.50 The organization
of objects at the mesoscopic scale is a current paradigm of
materials science because it allows the synthesis of structures
20 with novel properties derived from the cooperative interactions of
the individual components.51 Optical and optoelectronic
properties of the low molecular mass organic nanostructured
materials LMSOM are fundamentally different from those of
inorganic and macrocyclic compounds due to the absence of the
25 unfavorable intermolecular forces like magnetic and other
interactions. The main operating forces in the LMSOM are the
noncovalent types of interactions, e.g., hydrogen bonding, πꢀ
stacking, van der Waals forces, and charge transfer.52
50
Figure 16. Comparison between solid state and solution phase UV
spectra of 5d and 6d.
3000
Emission spectra of 4d
2500
CHCl
3
THF
2000
1500
1000
500
400
450
500
550
600
Wavelength (nm)
Figure 17. Emission spectra of 5d.
However, researchers continue to face great challenges in the
30 construction of LMSOM53 wherein macrocyclic compounds or
optically insensitive alkyl chains/steroidal groups substituted
compounds have usually been utilized for the fabrication of 1D
organic nanomaterials.54,55 We disclose herein the preliminary
observations that we have found in our experiments for
35 construction of the novel selfꢀaggregated architectures of wellꢀ
defined shape and size from compound 6d and also their
interesting photophysical properties. Scanning electron
microscope (SEM) imaging of the thin film of 6d from CHCl3
reveals fabrication of ultralong nano tube (panels a, Figure 15),
40 nano rod like arrangement (panel b, c), self aggregated in ethyl
acetate to ultralong fibrous bundles (panel d), in THF to petal like
nano structure. In an attempt to make nanostructured materials of
6d in cyclohexane, surprisingly we have found self aggregated
solid material of several micrometer widths. Probably the
45 crystallization process of the nanobuilding blocks proceeds very
fast in the apolar solvent like cyclohexane and thus unidirectional
molecular packing is disturbed and as a result the broken
55 macrostructured material is observed in the SEM image. The
large red shift in UV of 5d measured in the selfꢀaggregated solid
state (panel a, Figure 16) compared to their dilute solutions (panel
b) is probably due to their unidirectional πꢀπ stacking.56 Similar
observations were found in UV spectra of compound 6d (panel c
60 and d). The spectral properties of compound 5d are further
emphasized in the fluorescence study. Maximum wavelengths
(λem = 422–427 nm) were observed for compound 5d in CHCl3
and EtOAc (Figure 17). Photoluminescent organic materials have
unique potential for highꢀtech applications as light emitting
65 diodes,57 flat panel devices,58 biological probes,59 and
chemosensors.60
Other parameters effecting the yields of 1,4-dihydropyridines
The effect of HPF6 loading on the selectivity of catalyst was also
examined and the results are presented in table 3 taking the
10 | Journal Name, [year], [vol], 00–00
This journal is © The Royal Society of Chemistry [year]

Green Chemistry
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reaction forming 5e as an example. It was observed that MSPA-
10 performed most efficiently as an acid catalyst for the synthesis
of 1,4ꢀdihydropyridines (5). For MSPA-x, with x =5 and 20 the
yield of 5e was comparatively lower than MSPA-10.
The reactions were carried out using different amounts of the
catalyst and the optimum amount has been determined, as
presented in Table 5. Determination of this optimum amount to
achieve maximum yield was very essential to establish the
30 efficacy and broaden the applicability of the proposed process.
For this purpose the reaction forming 5e was chosen as a test
reaction.
5
Table 3. Effect of HPF6 loading on the yield of 1,4ꢀDHPs.a
CN
40 mg
MCM-41/HPF6
CHO
O
Table 5. Optimization of catalyst loading.a
r.t.
O
(NH4)2CO3
H2O
Stirring
CN
O
O
CN
N
CHO
O
MSPA-10
(1)
(2)
H
(5e)
(3)
(4)
r.t.
O
(NH4)2CO3
1 mmol
1 mmol
1 mmol 1 mmol
H2O
Stirring
O
Entry
Acid
Vp (cm3gꢀ
Time
% Conversion of
O
CN
1
N
Catalysts
)
(h)
1.0
2.0
3.0
4.0
3.2
2.5
3.0
5eb
10
20
52
97
57
13
10
(1)
(2)
H
(5e)
(3)
(4)
1
2
3
4
5
6
7
HPF6
1 mmol
1 mmol
1 mmol 1 mmol
Yieldsb (%)
MCMꢀ41
MSPA-5
1.79
1.200
Entry
Amount of
TON
TOF/h
MSPA-10
MSPA-20
MSPA-30
Commercial
Silicaꢀ10
0.6504
0.3421
0.1133
0.4021
catalyst (mg)
1
2
3
4
5
6
7
0
5
0
9
―
―
21.69
24.10
24.70
24.90
28.92
25.70
10.84
12.05
12.34
12.45
14.46
12.85
10
20
30
40
45
20
41
62
95
96
aReaction conditions: Dimedone (1 mmol), 2ꢀphenylacetophenone (1
mmol), 4ꢀcyanobenzaldehyde (1 mmol), ammonium carbonate (1 mmol),
40 mg MSPA-x, room temperature, stirring in water.
35 aReaction conditions: Dimedone (1 mmol), 2ꢀphenylacetophenone (1
mmol), 4ꢀcyanobenzaldehyde (1 mmol), ammonium carbonate (1 mmol),
different amount of MSPA-10, room temperature, stirring in water for 4h.
10 b Percentage was calculated by 1H NMR (300MHz).
The effect of the nitrogen source was evaluated in carrying out
the reaction forming 5e by replacing (NH4)2CO3 with other
ammonium salts (Table 4). Most excitingly, NH4OAc and
(NH4)2CO3 are the most effective nitrogen sources for the
15 conversion (Table 4). However, for the development of green and
pollution abatement process we choose (NH4)2CO3 for carbon
dioxide and water were the only byꢀproducts instead of acetic
acid in case of NH4OAc.
bIsolated yield.
Choice of solvent
40
The present silicaꢀhexafluorophosphoric acidꢀcatalyzed
condensation reaction was also studied in different solvents.
Detailed results were presented in Table 6. It is interesting to note
that the aldehyde was consumed in most of the organic solvents
but yield of 1,4ꢀdihydropyridine (5) increased with increasing
Table 4. The effect of various ammonium salts as the nitrogen source on
20 the four component reaction to form 5e.
45 polarity of the solvent. Therefore, water came out as the best
choice of solvent.53,61
CN
Table 6. Effect of solvent polarity on the yield of 1,4ꢀdihydropyridines
(5).a
CHO
O
40 mg MSPA-10
r.t.
O
Nitrogen
source
CN
H2O
O
O
40 mg
Stirring
CHO
CN
O
MSPA-10
N
r.t.
O
(1)
(2)
H
(5e)
(NH4)2CO3
(3)
1 mmol
(4)
1 mmol
Solvents
Stirring
1 mmol
1 mmol
O
O
Entry
Nitrogen
source
(NH4)2CO3
NH4OAc
Conversion (%) of 5e Yield of 5e (%)
CN
N
(1)
(2)
H
(5e)
(3)
(4)
1
2
3
4
5
6
7
8
9
97
98
92
85
86
80
78
45
41
38
95
96
87
80
62
60
59
43
40
37
1 mmol
1 mmol
1 mmol 1 mmol
Entry Solvents (4 ml) Time (h) % Conversion of 5eb
HCO2NH4
NH4HCO3
(NH4)4[Ce(SO4)4]
NH4PF6
(NH4)2HPO4
NH4Cl
1
2
3
4
5
6
7
8
9
Hexane
Et2O
THF
Acetone
EtOAc
DCM
MeOH
EtOH
H2O
3
3
3
3
3
3
1
1
4
15
20
26
40
44
20
80
80
95
Urea
10
Thiourea
aReaction conditions: Dimedone (1 mmol), 2ꢀphenylacetophenone (1
mmol), 4ꢀcyanobenzaldehyde (1 mmol), different nitrogen source (1
mmol), 40 mg MSPA-10, room temperature, stirring in water for 4h.
50 aReaction conditions: 2ꢀphenylacetophenone (1 mmol), dimedone (1
mmol), 4ꢀcyanobenzaldehyde (1 mmol) and ammonium carbonate (1
mmol), 40 mg MSPA-10, different solvents, room temperature stirring.
25 bPercentage was calculated by 1H NMR (300MHz).
This journal is © The Royal Society of Chemistry [year]
Journal Name, [year], [vol], 00–00 | 11

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Green Chemistry
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b Percentage was calculated by 1H NMR (300MHz).
renewable resources. In the light of this, the present highly
efficient catalytic process can also be considered as a green
45 technology. Using this robust green protocol we have synthesized
UV and fluorescent active LMSOM forming tunable 1D stacking
suitable for designing biosensors and optoelectronic nanodevices.
The unprecedented catalytic performance demonstrated by
MSPA-10 holds a significant promise for the achievement of
50 novel catalyst systems.
Catalyst stability and recycling
The pH of the reaction medium after isolation of the product
was found to be 5.75. Therefore HF can be leached in aqueous
solution since HPF6 is a binary conjugate BronstedꢀLewis super
acidic system in which the Lewis acid PF5 is only weakly
coordinated with the Bronsted acid HF. 40 The concentrations of
the residual H+ on the recovered catalyst after successive
experiments were measured and furnished in Table 7. However
5
Notes and references
10 marginal loss of H+ was observed. It implied that HF was only
slightly leached in aqueous solution. Therefore the dissolved CO2
may contribute to the lowering of pH of the aqueous medium
since we have used (NH4)2CO3 and only ammonia is consumed in
the reaction. Therefore, only a slight decrease in yield was
15 observed and the reaction time was also slightly extended with
recovered catalyst. In the presence of silicaꢀhexafluorophosphoric
acid (preheated at 100°C for 4 hours) the reaction between 2ꢀ
phenylacetophenone, dimedone, 4ꢀcyanobenzaldehyde and
ammonium carbonate (Table 7) occurred with 95% yield of 5e.
20 The same reaction in the presence of silicaꢀhexafluorophosphoric
acid after having it exposed to ambient atmosphere for 10 days
produced similar observation. Obviously, there was no
deteriorating effect of aerial oxygen or moisture towards activity
of the catalyst. Therefore, the catalyst had considerable stability
25 towards heat and moisture which also provided evidence for
efficient recycling.
aDepartment of Chemistry, University of Calcutta, 92 APC Road, Kolkata
ꢀ700009, India. Eꢀmail: cmukhop@yahoo.co.in; Tel: +91ꢀ9433019610
bBruker Bio Spin, 2700 Technology Forest Drive, Woodlands, Texas
55 77381, USA
cDepartment of Materials Science, Indian Association for the Cultivation
of Science, Jadavpur, Kolkata 700 032, India. Eꢀmail: msab@iacs.res.in
† Electronic Supplementary Information (ESI) available: For
experimental procedure, spectroscopic data and crystallographic data in
60 CIF or other electronic format see DOI: 10.1039/b000000x/
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CHO
O
40 mg MSPA-10
r.t.
O
(NH4)2CO3
H2O
O
O
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CN
N
80
(e) N. S. Kus, Tetrahedron, 2012, 68, 949.
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F content P content
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of residual solid
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(%)
95
93
93
92
90
(%)
8.41
8.41
8.38
8.36
8.35
(%)
2.74
2.73
2.70
2.68
2.65
85
1
2
3
4
5
4.0
4.0
4.2
4.2
4.5
0.83
0.82
0.80
0.80
0.78
7 W. Zhang, Green Chem., 2009, 11, 911.
8 A. Dandia, V. Parewa, A. K. Jaina and K. S. Rathore, Green Chem.,
2011, 13, 2135.
90
aReaction conditions: 2ꢀphenylacetophenone (1 mmol), dimedone (1
30 mmol), 4ꢀcyanobenzaldehyde (1 mmol) and ammonium carbonate (1
mmol), 40 mg MSPA-10 catalyst, room temperature stirring in water.
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,
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cIsolated yield.
95
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,
The XRD pattern of the recovered the catalyst showed a wide
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structure of the silica host material still existed after recycling.
Elemental analysis reproduced almost similar results as that of
original catalyst as furnished in table 7. Therefore, no significant
leaching of the HPF6 occurred under the experimental condition.
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40 Conclusions
This present protocol minimizes the dispersal of the harmful
chemicals in the environment and maximizes the use of
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This journal is © The Royal Society of Chemistry [year]

Green Chemistry
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