Development of hydrogelator-based gel-entrapped base catalysts (GEBCs) as heterogeneous basic catalysts for the synthesis of 3-acetylcoumarins

Article abstract of DOI:10.1039/c7nj02342a

New gel entrapped base catalysts (GEBCs) have been prepared by entrapping some organic and inorganic bases into a solid aqueous gel matrix. Sodium alginate, gelrite and carboxymethyl chitosan were studied for the preparation of GEBCs. The phase behaviour of binary systems (gelator + water) is well studied, but ternary systems (gelator + water + base) are not much studied. Addition of a base to a (gelator + water) system drastically affected the behaviour of the gel. The phase behaviour of ternary systems (gelator + water + base) was studied by developing the corresponding ternary phase diagrams. From this study, the promising homogeneous viscous phases were converted into hard, irreversible gel beads by cross-linking the gelator with divalent cations Ca²⁺, Ba²⁺ and Sr²⁺. These gel beads were studied for base entrapment efficiency and leaching of the base. The GEBCs were used for the condensation of salicylaldehydes and methyl/ethyl acetoacetate to obtain 3-acetylcoumarins.

Full text of DOI:10.1039/c7nj02342a

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Development of hydrogelator-based gel-entrapped
base catalysts (GEBCs) as heterogeneous basic
catalysts for the synthesis of 3-acetylcoumarins†
Cite this:DOI: 10.1039/c7nj02342a
Nilesh N. Korgavkar
and Shriniwas D. Samant
*
New gel entrapped base catalysts (GEBCs) have been prepared by entrapping some organic and inorganic
bases into a solid aqueous gel matrix. Sodium alginate, gelrite and carboxymethyl chitosan were studied for
the preparation of GEBCs. The phase behaviour of binary systems (gelator + water) is well studied, but ternary
systems (gelator + water + base) are not much studied. Addition of a base to a (gelator + water) system
drastically affected the behaviour of the gel. The phase behaviour of ternary systems (gelator + water + base)
was studied by developing the corresponding ternary phase diagrams. From this study, the promising
homogeneous viscous phases were converted into hard, irreversible gel beads by cross-linking the gelator
with divalent cations Ca2+, Ba2+ and Sr2+. These gel beads were studied for base entrapment efficiency and
leaching of the base. The GEBCs were used for the condensation of salicylaldehydes and methyl/ethyl
acetoacetate to obtain 3-acetylcoumarins.
Received 5th July 2017,
Accepted 16th September 2017
DOI: 10.1039/c7nj02342a
rsc.li/njc
reactions in continuous flow in a fixed bed reactor.3 But the
interaction between the undissociated K2CO3 and the substrate
1. Introduction
The emphasis on sustainable and environmentally benign present in the solution is not satisfactory in all cases. For many
processes has led to several changes in the chemical world. reactions we need a base catalyst which can give a combined
The development and use of heterogeneous, reusable catalysts effect of both homogeneous and heterogeneous catalysts.
in various chemical processes is also a part of these changes.1 Entrapment of bases in porous polymeric matrix materials can
Heterogeneous catalysts have specific advantages over their accomplish this requirement.4
homogeneous counterparts, such as easy separation of the
The concept of entrapment of enzymes into polymeric gel
catalyst and products, reusability, etc. Many solid bases have matrices is well documented.5–7 But there are few reports on
been reported in the literature, like metal oxides, supported gel-entrapped acid–base catalysis. In 1975, Hines entrapped
alkalis or alkaline metals, anion exchange resins, hydrotalcites palladium complexes in poly(hydroxyethylmethacrylate) hydrogel
and immobilised organic bases on polymeric backbones.2 and used the catalysts for hydrogenation.8 David reported sol–gel
However, these heterogeneous bases have many limitations. entrapped catalysts and reagents using silica and alumina gels.9–14
The basic strength of these bases is comparatively less, and
We initiated work on gel entrapped base catalysts (GEBCs)
hence the reactions need vigorous conditions, leading to an using a hydrogelator agar agar, and entrapping alkalis and
increase in the energy consumption. Still there are no efficient organic bases in the corresponding hydrogels.15,16 Such GEBCs are
heterogeneous and reusable alternatives for classical bases like easy to prepare and handle, can be prepared from biodegradable
NaOH, KOH, piperidine and pyridine. Deactivation of solid matrix materials and are efficient, economical and completely
bases under atmospheric conditions is also a major problem environmentally benign. Agar agar is a complex polysaccharide
and the catalysts require calcination at high temperature for and forms a gel in water. At certain compositions of (agar agar–
reactivation. The lower basic strength of solid bases also makes water–alkali) the gel formed is a solid which can be cut into
them ineffective for flow processes. In some processes inorganic pieces. We used NaOH- and KOH-entrapped GEBCs for base-
bases like K2CO3 have been used in organic solvents to perform catalyzed organic reactions. There are few other references on
agar agar based GEBCs.17–19 Agar agar based GEBCs have some
Department of Chemistry, Institute of Chemical Technology, N.M. Parekh Road,
limitations like poor gelation and poor stability.
In the present work we used hydrogelators, sodium alginate
(Na Alg), gelrite (gellan gum), chitosan and carboxymethyl chitosan,
Matunga, Mumbai 400 019, India. E-mail: samantsd.ict@gmail.com;
Tel: +91 022-33612606
† Electronic supplementary information (ESI) available: Additional information
to overcome the limitations of agar agar. These gelators have been
used in several other areas as matrix materials.20 Sodium alginate
about gelation and GEBC, and characterisation of 3-acetylcoumarin derivatives.
See DOI: 10.1039/c7nj02342a
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(Na Alg) and chitosan have been used to prepare some grafted by filtration. The filtrate was diluted to 100 mL with DW. The
co-polymers which can be used as base catalysts.21–23 Chitosan amount of base present in the filtrate was determined
is used as a catalyst in some reports.24
conductometrically.
In this work, our aim was to entrap organic and inorganic bases
in aqueous gel matrices of sodium alginate, gelrite and chitosan to
prepare GEBCs, and to investigate the effect of bases on the
gelation of these gelators, their base entrapment efficiency and
the ability of GEBCs prepared from these gelators to catalyze
organic reactions. To evaluate the utility of the GEBCs, a model
base catalysed reaction of salicylaldehydes and methyl acetoacetate
to 3-acetylcoumarins was carried out. 3-Acetylcoumarins are
required in the synthesis of dyes25 and chemosensors.26
2.5 Reaction of salicylaldehyde and methyl acetoacetate in the
presence of GEBC
2.5.1 Batch reaction. Salicylaldehyde (1a, 4.0 mmol),
methyl acetoacetate (2a, 4.0 mmol) and methanol (20 mL) were
taken in a round bottom flask (50 mL). The GEBC beads (1.0 g)
were added and the reaction mixture was stirred at room
temperature for 1 h. The solution was filtered and the filtrate
was concentrated and chilled to obtain the solid product. The
product was recrystallised from ethanol.
2.5.2 Flow reaction. A glass column (i.d. = 8 mm and l = 30 cm)
was packed with GEBC beads (10.0 g). A solution of salicylaldehyde
(0.2 M), methyl acetoacetate (0.2 M) in methanol (100.0 mL) was
passed through the column at a flow rate of 1.0 mL minÀ1 using
a peristaltic pump. Fractions of 20 mL each were collected and
analysed for product formation.
2. Experimental
Sodium alginate (food grade, sodium polymannuronate, minimum
assay 91%) was purchased from Loba Chemie Pvt. Ltd, gelrite
(deacetylated gellan gum) was purchased from Sisco Research
Laboratories Pvt. Ltd, and chitosan (degree of deacetylation
Z75%) was purchased from HiMedia Laboratories Pvt. Ltd. The
other reagents and substrates were purchased from Sigma Aldrich,
Alfa acer and Loba Chemie Pvt. Ltd and were used as such. Melting
points were recorded using an Analab melting point apparatus and
are uncorrected. The IR spectra were recorded using a Perkin Elmer
Spectrum-two spectrometer in ATR mode. Thermogravimetric ana-
lysis (TGA) was performed using a Perkin Elmer STA 6000 with a
heating rate of 20 1C minÀ1. The FESEM analysis was carried out
using a Tescan MIRA 3 model. Beads were coated with Pt using a JFC
1600 Auto fine coater and then images were recorded. The 1H NMR
spectra were recorded using a Bruker 400 MHz NMR spectrometer.
3. Results and discussion
Among Na Alg, gelrite and chitosan, chitosan turned out to be
ineffective as a gelator as it was insoluble in neutral and basic
aqueous solutions. Hence, in order to make it soluble in
aqueous solution, chitosan was converted to carboxymethyl
chitosan by the reported procedure using monochloroacetic
acid.27,28 It formed a colloidal viscous solution in an aqueous
basic medium, but could not form a hard gel. Na Alg and gelrite
formed promising viscous homogeneous colloidal solutions.
Divalent cations Ca2+, Ba2+ and Sr2+ could cross-link the gelators
in the solution to give hard gels. Hence, further study was carried
out with these two gelators.
2.1 Construction of ternary phase diagrams
Different compositions of gelator and base were taken, water
was added and solution was stirred to equilibrate. The change
in the phase was noted.
3.1 Ternary phase diagrams
2.2 Preparation of gel entrapped base catalysts (GEBCs)
The behaviour of a hydrogelator in water is affected signifi-
cantly by the polarity of the medium, pH and the temperature
of the system. The phase behaviour of the hydrogelators in
aqueous systems is well studied. However, GEBC systems
containing these gelators and bases have not been studied.
Addition of a base to a (gelator + water) mixture affects
noticeably the swelling and gelation properties of the resulting
system. Hence, we studied the phase behaviour of (gelator +
water + base) ternary systems by constructing the corresponding
ternary phase diagrams (Fig. 1A–E and 2A–E). The different phases
obtained in the phase diagrams are as follows.
Viscous solutions of appropriate phases of (gelator + water + base)
systems were injected into aqueous solutions of CaCl2, BaCl2 or
SrCl2 (0.5 M). The gel beads were separated by filtration and air
dried in a desiccator over anhydrous CaCl2 for 5 h.
2.3 Determination of the base entrapment efficiency of the
GEBCs
The GEBC beads (3.0 g) were added to distilled water (DW,
30 mL) in a glass beaker. The beads were crushed completely
and the mixture was stirred vigorously for 1 h. The solution was
filtered. The filtrate was diluted to 100 mL in a volumetric flask
and the amount of base present in the solution was determined
conductometrically.
Phase A – Heterogeneous solid mass (heterogeneous solid
rich phase)
Phase B – Biphasic mixture of solid and liquid (hetero-
geneous liquid rich phase)
2.4 Leaching of the entrapped base
Phase C – Viscous gel not free flowing (homogeneous solid
The GEBC beads (3.0 g) were taken in methanol (10 mL) and rich phase)
stirred in a glass tube (dia = 22 mm) using a magnetic bar with a
stirring rate of 200 rpm at 27–30 1C. The beads were separated
Phase D – Viscous sol (homogeneous liquid rich phase)
Phase E – Low viscosity sol
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Fig. 1 Ternary phase diagrams of (Na Alg–water–base) systems.
Fig. 2 Ternary phase diagrams of (gelrite–water–base) systems.
3.1.1 Na Alg–water–base ternary systems. The phase behaviour triethylamine, sodium hydroxide, and potassium hydroxide, was
of Na Alg–water with five bases, morpholine, piperidine, studied. In the phase diagram for the (Na Alg–water–morpholine)
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system (Fig. 1A), all the five phases were observed. A 40 : 60
mixture of Na Alg–morpholine formed heterogeneous solid phase A,
but further addition of morpholine led to a heterogeneous biphasic
mixture of solid and liquid (B). Addition of water to both these
heterogeneous phases, A and B, converted them into homogeneous
phases C–E. Phase C was homogeneous but, more viscous, and not
free flowing. Addition of water to phase C converted it to phase D,
which was a free flowing homogeneous sol. Further addition of
water led to phase E which had very low viscosity. Similar phase
diagrams were obtained for piperidine (Fig. 1B) and triethylamine
(Fig. 1C).
Fig. 3 Monomer units of Na Alg, 1,4-linked mannuronate and guluronate.
solution and when it was slowly injected into CaCl2 solution,
spherical hard gel beads were obtained. Phase E did not give a
hard gel. Various compositions of phase D were used to prepare
hard gels. The (Na Alg–water–piperidine) system showed the same
behaviour and its phase D formed spherical hard gel beads. The
phase D of (Na Alg–TEA–water) system formed soft gels; possibly due
to less solubility of TEA in water. In the (Na Alg–NaOH–water)
system phase D was almost negligible (Fig. 1D) and in the
(Na Alg–KOH–water) system phase D was not observed (Fig. 1E).
The homogeneous phase E was poorly viscous and did not form
gel beads. Drops of phase E dissolved in Ca2+ solution.
3.2.2 Gelrite–water–base ternary systems. In this system,
phases A and B were dominant and, being heterogeneous,
could not be used for making hard gels. The phase C was
highly viscous and could not be converted into solid gels. With
TEA, NaOH and KOH the favourable phase D could not be
formed. NaOH and KOH gave poorly viscous phase E which also
could not be converted into a hard gel. Only morpholine and
piperidine gave the required phase D which could be converted
into hard gels.
The phase behaviour of Na Alg in aqueous solutions of
sodium hydroxide and potassium hydroxide was completely
different to that with organic bases [Fig. 1D and E]. Na Alg did
not swell in NaOH solution and remained insoluble (Fig. 1D).
Thus, most of the area was occupied by heterogeneous phases A
and B, and only a small part showed homogeneous free flowing
phase D. The observation was different for KOH (Fig. 1E); it
showed four phases A, B, C, and E. The phase D was not
observed.
3.1.2 Gelrite–water–base ternary systems. More swelling of
gelrite in an aqueous medium resulted into much less area under
the homogeneous phases compared to Na Alg (Fig. 2A–E). The
maximum amount of gelrite that could be taken to form a
homogeneous phase was 5–6%. Morpholine and piperidine con-
taining systems showed four phases A, B, C and D (Fig. 2A and B).
In the case of triethylamine, phase D was absent. It formed either a
viscous mass or a heterogeneous mixture (Fig. 2C). With NaOH it
formed a homogeneous phase, but it was less viscous (phase E),
and similar behaviour was observed with KOH.
The basicity and solubility of the base had a profound effect
on the phases. Thus there was a marked effect on phases A and B.
There are reports on degradation of sodium alginate in alkaline
solution. This might be the reason for the formation of poorly
viscous homogeneous phase E in the case of Na Alg–KOH–water,
gelrite–NaOH–water and gelrite–KOH–water. In the case of the Na
Alg–NaOH–water system the homogeneous phase was negligible
and it appeared only at NaOH amounts less than 0.5%; Na Alg did
not swell or dissolve in NaOH solution.
Swelling of gelrite in the aqueous solution was higher as
compared to that of Na Alg. Hence the area of the homogeneous
free flowing phase D was lower with morpholine and piperidine
(Fig. 2A and B), and absent in TEA (Fig. 2C). It formed a hetero-
geneous mixture or a homogeneous thick paste in most cases. With
NaOH and KOH, gelrite formed poorly viscous homogeneous phase
E and sticky thick paste phase C (Fig. 2D and E). Degradation of
gelrite might have taken place in alkaline solution.
As the phase D in the case of gelrite was very small the
maximum amount of base which can be entrapped was around
4–6% and the same in the case of Na Alg was around 22–24%.
The capacity of the gelrite system to entrap the base was much
lower. Hence, for further work (Na Alg–water–morpholine) and
(Na Alg–water–piperidine) systems were selected. They were
cross-linked with Ca2+ ions to obtain hard gel beads, which
were named as (Ca Alg–Mor) and (Ca Alg–Pip) gels.
3.2 Gelation study
The best gels obtained in the above study were still soft and
when stirred in a non-aqueous solvent could not maintain their
integrity. It is known that divalent and trivalent cations, parti-
cularly Ca2+ cations, cross-link the gelators through –COOH
groups29 (Fig. 3) and form a strong internal network which
leads to a hard gel. Hence, promising gels from the above
systems were treated with 5% aqueous CaCl2 to make hard gels
for catalysis. The gelation with Ba2+ and Sr2+ was also studied
(see the ESI,† Table S1).
3.3 Characterisation of GEBCs
The gel beads (Fig. S1, ESI†) were white. They had diameters
3.2.1 Na Alg–water–base ternary systems. To begin with the in the range of 2.50–3.00 mm, and the mass per bead was in
(Na Alg–water–morpholine) ternary system was studied. Phases the range of 50.0–60.0 mg. The FT-IR spectra were recorded for
A and B were heterogeneous and did not form a gel. Phase C Ca–Alg gel beads, (Ca Alg–Mor) and (Ca Alg–Pip) in ATR mode.
was very viscous (paste) and did not pass through the syringe The Ca–Alg gel bead showed bands at 3348 (broad, strong),
and when added in small portions to 5% aq. CaCl2, only a soft 1604 (strong), 1418 (medium), 1031 (strong) and 791 (weak) cmÀ1
gel was formed. Phase D was a homogeneous free flowing In the case of (Ca Alg–Mor) and (Ca Alg–Pip) beads one extra
.
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Fig. 4 FT-IR spectra of Ca–Alg and those of (Ca Alg–Mor) and (Ca Alg–Pip)
gels in ATR mode.
Fig. 5 TGA of Ca–Alg and those of (Ca Alg–Mor) and (Ca Alg–Pip) gels
and DTG of Ca Alg.
band appeared at 873 cmÀ1 which was because of out-of-plane
bending vibrations of N–H (Fig. 4). In thermogravimetric
analysis (TGA), a significant weight loss for all gels was
observed in the range of 90–150 1C (Fig. 5). The DTG graph
showed two peaks one at around 100 1C which was for loss of
water and bases, and a second at 200 1C which suggested
decomposition of Ca–alginate (Fig. 5).
The FE-SEM images of Ca–Alg systems are shown in Fig. 6.
The vacuum applied during Pt coating removed all the solvent
from the gel beads, which caused a reduction in the diameter of
the Ca–Alg gel beads of around 1 mm. The surface morphology of
the Ca–Alg gel beads was different from that of the (Ca Alg–Mor)
and (Ca Alg–Pip) beads.
Fig. 6 FESEM images of [A] Ca–Alg gel beads, [B] the surface of the
Ca–Alg gel beads, [C] Ca Alg–Mor gel beads, [D] the surface of the
Ca Alg–Mor gel beads, [E] Ca Alg–Pip gel beads and [F] the surface of
the Ca Alg–Pip gel beads.
titration against HCl solution. The GEBCs were prepared from
(6.0% Na Alg + 81% water + 13.0% morpholine) and using
aqueous solutions of BaCl2, SrCl2 and CaCl2 for cross-linking.
There was no significant change in EE (Table S2, ESI†). Similar
results were obtained for the GEBCs from (6.0% Na Alg + 85%
water + 9.0% piperidine) and using the above three cross-
linkers (Table S2 ESI,† entries 9–11). As there was no significant
effect of cross-linking cations on EE, we selected CaCl2 for
further study. We tested the effect of Ca2+ concentration on the
entrapment efficiency, using 5, 10, 15 and 20% CaCl2 solutions.
3.4 Base entrapment efficiency of the gel beads (EE)
As the viscous solution of (gelator–water–base) was added to 10% CaCl2 showed the highest EE.
aq. CaCl2 one would expect the amount of base present in the
Around 25–28% of the base was retained in the GEBCs in the
hard gel to be nearly same as that in the respective phase D. process of formation of a hard gel. Some combinations of (Na
However, in the preliminary study, we found that the total base Alg–water–morpholine) and (Na Alg–water–piperidine) systems
present in the hard gel was less than the amount of base (Fig. 1A, B and Fig. S2, ESI†) forming phase D were tested for
present in the solution of phase D. To find out the amount of entrapment efficiency and (6.0% Na Alg + 81% water + 13%
base entrapped in the gel beads, the beads were crushed and morpholine) and (6% Na Alg + 85% water + 9.0% piperidine)
stirred in water at room temperature (27–30 1C) to extract the were selected for further study, as they showed maximum
base. The amount of base was determined by conductometric entrapment efficiency (Table S3, ESI†).
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Table 1 Reaction of 1a (R1 = R2 = R3 = H) and 2a/b in the presence of GEBCs
2, Me/Et
Entry aceto-acetate Solvent
Reaction Yielda
time (min) of 3a (%)
Base
1
2
3
4
2a
2b
2a
2b
Methanol Ca10 Alg6–Mor13 50
Ethanol Ca10 Alg6–Mor13 60
Methanol Ca10 Alg6–Pip9 50
Ethanol Ca10 Alg6–Pip9 60
82
85
82
86
Reaction conditions: 1a (4.0 mmol), 2a/b (4.0 mmol), GEBC (1.0 g),
a
methanol/ethanol (20 mL), rt (27–30 1C). Isolated yield.
Table 2 Reaction of 1a–j and 2a (R–CH3) using GEBCs
Fig. 7 Decrease in the amount of base present in the GEBCs w.r.t. time
when stirred in methanol (GEBC beads 3.0 g, MeOH 10 mL, stirring with a
magnetic bar at 200 rpm, temp 27–30 1C).
Yielda of
Product (3) 3a–j (%) m. p. (1C)
Entry R1
R2
R3
H
1
2
3
4
5
6
7
8
9
10
H
H
H
H
H
H
H
H
H
3a
82
92
90
55
74
70
62
50
52
60
117–118
165–166
217–218
195–196
221–222
204–205
215–216
148–149
240–241
181–182
OMe 3b
Br
NO2
Cl
Br
NO2
H
H
H
H
3c
3d
3e
3.5 Leaching of base from GEBCs into methanol
For this study GEBCs (Ca10 Alg6–Mor13) and (Ca10 Alg6–Pip9)
were used. As methanol is a common polar solvent for organic
reactions and as we selected methanol for the reaction of
salicylaldehyde and methyl acetoacetate, the tendency of the
base to leach from the GEBCs into methanol was studied by
stirring the GEBCs in methanol at room temperature.
The morpholine and piperidine slowly leached out in methanol
(Fig. 7). The dark line shows the amount of morpholine and the
faint line shows the amount of piperidine present in the
GEBC beads.
OMe 3f
OMe 3g
H
H
N(CH2CH3)2
OH
3h
3i
3j
H
2-Acetyl-3H-
benzo[ f ]chromen-3-one
Reaction conditions: 1a–j (4.0 mmol), 2a (4.0 mmol), methanol (20 mL),
GEBC (Ca10 Alg6–Mor13) (1.0 g), rt (27–30 1C), time 1 h. a Isolated yield.
Table 3 Reaction of 1a–b and 2a (R–CH3) using (Ca10 Alg6–Mor13) in
continuous mode
Yielda (%)
3.6 Knoevenagel reaction of salicylaldehydes and
methyl/ethyl acetoacetate in the presence of the GEBCs to
obtain 3-acetylcoumarins
Fraction
3a (R1 = R2 = R3 = H)
3b (R1 = R2 = H, R3 = OMe)
1
2
3
4
85
73
55
33
95
81
62
42
The reaction of salicylaldehyde (1a) and methyl acetoacetate (2) in
the presence of a base gives 3-acetylcoumarin. This reaction has
been carried out using different catalysts like piperidine, L-proline,
pyridopyrimidine, L-lysine, and heterogeneous acids.30–34 There is
also a report with biocatalyst alkaline protease from Bacillus
licheniformis.35 Reaction of 1a and 2 (Scheme 1) was carried using
GEBCs (Ca10 Alg6–Mor-13) and (Ca10 Alg6–Pip-9) (Table 1). Both
effectively catalysed the reaction in methanol and ethanol. The
conversion was 100% in methanol and ethanol in 1 h and an
excellent yield of 3a was obtained. The results were equivalent to
those obtained with these amines under homogeneous conditions
(Table S4, ESI†). In other solvents (ethyl acetate, chloroform, THF,
toluene) even after 3 h, the conversion of 1a was not complete.
Under optimum conditions, a series of salicylaldehydes were
reacted with 2a (Table 2). To see the recyclability of the catalyst,
the GEBC was filtered from the reaction mass and used as
such for the next cycle. The very poor yield obtained in the
Reaction conditions: 1a–b (0.2 M) and 2a (0.2 M) in MeOH, reactor
volume = 5.5–6.0 mL, flow rate-1.0 mL minÀ1, rt (27–30 1C), Product
isolated from fractions of 20.0 mL, GEBC beads (10 g), (Ca10 Alg6–Mor13)
cross-linked with 10% aqueous CaCl2. a Isolated yield.
second cycle was possibly due to the leaching of the base from
the GEBC.
The reaction was also attempted in continuous mode by
passing a solution of 1a–b (0.2 M) and 2a (0.2 M) in methanol at
a rate of 1.0 mL minÀ1 through a glass column (inner diameter
8 mm, length 30 cm, Fig. S3, ESI†) filled with GEBC (Ca10
Alg6–Mor13) at rt (27–30 1C) using a peristaltic pump. Under
these conditions the yield of product was noted in each fraction
of 20.0 mL; the results are given in Table 3.
In order to see the differential effect of the reaction catalysed
by GEBC and that by the base leached out from the GEBC,
20 mL methanol was passed through the GEBC column and the
eluent was collected. The reaction was attempted in the eluent
collected. Poor yield of the product (30%) was obtained in
3 h. On the other hand, the continuous mode reaction gave
excellent yield (85%) in 15 min. Therefore, the reaction was
mainly assisted by the base present in the gel beads.
Scheme 1 Reaction of salicylaldehyde and acetoacetate ester.
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9 F. Gelman, J. Blum and D. Avnir, New J. Chem., 2003,
27, 205.
10 F. Gelman, J. Blum, D. Avnir and R. V. August, J. Am. Chem.
Soc., 2000, 122, 11999.
4. Conclusion
(Na Alg–water–base) and (gelrite–water–base) systems were studied
for phase behaviour. Na Alg systems with morpholine and piperidine
bases gave promising viscous homogeneous phase. When these
phases were treated with Ca2+, Ba2+ or Sr2+ ions, cross-linking took
place among the gelator molecules and hard gels were formed in
which the base was trapped. These solid GEBCs were useful hetero-
geneous base catalysts for reaction in a non-aqueous medium. The
carboxymethyl chitosan and gelrite could not be used to prepare
GEBCs. Triethylamine, sodium hydroxide and potassium hydroxide
could not be entrapped in the Ca–Alg gel. From this study,
GEBCs (Ca10 Alg6–Mor13) and (Ca10 Alg6–Pip9) were found
to be very promising. They could be used for catalysing
Knoevenagel reaction of salicylaldehydes and methyl acetoacetate
to form 3-acetylcoumarins. These catalysts did not require any pre-
treatment and were convenient to use for flow reactions. This new
strategy of preparing solid GEBCs can be extended to different
gelators to prepare gels with entrapped organic and inorganic bases
(GEBCs) which can be used for various base-catalysed reactions.
For this type of gel-entrapped catalyst, leaching of the
entrapped base from the gel phase to the solution phase
appears to be a concern. Detailed study w.r.t. partitioning of
the entrapped base in the gel phase and solution phase at
equilibrium needs to be studied. A combination of a suitable
gelator and a suitable solvent for the reaction phase to retain
the catalyst in the gel phase is required for such a reaction.
´
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Conflicts of interest
There are no conflicts to declare.
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Acknowledgements
NNK is thankful to the University Grant Commission (UGC),
New Delhi, India for UGC/SAP/DRS fellowship. Authors are
thankful to GJUST Hissar, India for NMR facility.
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