A green and convenient approach for the one-pot solvent-free synthesis of coumarins and β-amino carbonyl compounds using Lewis acid grafted sulfonated carbon@titania composite

Article abstract of DOI:10.1007/s00706-016-1752-4

Abstract: This paper reports an efficient protocol for the synthesis of coumarins via Pechmann reaction, and β-amino carbonyl compounds via aza-Michael reaction using catalytic amount of solid Lewis acid catalyst, C@TiO₂–SO₃–SbCl₂. Six different catalysts were prepared by covalent immobilization of homogeneous Lewis acids onto sulfonated carbon@titania composite derived from amorphous carbon and nano-titania. Among various catalysts tested, C@TiO₂–SO₃–SbCl₂ showed superior catalytic activity. The catalyst could be recycled without significant loss of its catalytic activity and demonstrated versatile catalysis for a wide range of substrates. Graphical abstract: [Figure not available: see fulltext.]

Full text of DOI:10.1007/s00706-016-1752-4

Monatsh Chem
DOI 10.1007/s00706-016-1752-4
ORIGINAL PAPER
A green and convenient approach for the one-pot solvent-free
synthesis of coumarins and b-amino carbonyl compounds using
Lewis acid grafted sulfonated carbon@titania composite
Manmeet Kour1 Satya Paul1
•
Received: 21 January 2016 / Accepted: 29 March 2016
Ó Springer-Verlag Wien 2016
Abstract This paper reports an efficient protocol for the
synthesis of coumarins via Pechmann reaction, and b-
amino carbonyl compounds via aza-Michael reaction using
catalytic amount of solid Lewis acid catalyst, C@TiO2–
SO3–SbCl2. Six different catalysts were prepared by
covalent immobilization of homogeneous Lewis acids onto
sulfonated carbon@titania composite derived from amor-
phous carbon and nano-titania. Among various catalysts
tested, C@TiO2–SO3–SbCl2 showed superior catalytic
activity. The catalyst could be recycled without significant
loss of its catalytic activity and demonstrated versatile
catalysis for a wide range of substrates.
Introduction
Nowadays, organic synthesis has been oriented towards the
development of new environmentally benign procedures so
as to achieve the goals of ‘‘green chemistry’’ [1, 2].
Catalysis has been known to be an important contributor to
enhance the synthetic efficiency in addition to the reduction
of chemical waste generation. Lewis acids represent an
important class of catalysts that show remarkable catalytic
activity in several important organic transformations [3, 4].
The use of conventional Lewis acids, however, requires the
presence of strictly anhydrous conditions as the Lewis
acids get decomposed or completely deactivated even in
the presence of trace amounts of water and are also
accompanied with the production of corrosive acid waste
that requires additional neutralization step [5]. In compar-
ison with homogeneous analogues, heterogeneous Lewis
acid catalysts have several advantages for the development
of more sustainable protocols in terms of easy catalyst
recovery from reaction media and recyclability up to sev-
eral catalytic runs.
Graphical abstract
Pechmann
aza-Michael
R1
OH
O
O
NH
X
R2
H3C
O
CH3
X
Solvent-free, 60 ˚C
Solvent-free, 120 ˚C
CH3
R1
X
N
R2
O
O
X
C@TiO2-SO3-SbCl2
Coumarins have been known to occupy an important
place in the sphere of natural products and synthetic
organic chemistry. Natural and synthetic derivatives of
coumarins have been shown to possess significant array of
pharmacological properties [6–10] including antimicrobial,
anticancer, antioxidant, anticoagulant, and anti-inflamma-
tory. Typically, several reactions like Pechmann, Perkin,
Knoevenagel, Reformatsky, Wittig, C–C coupling, and
cycloaddition reactions have been applied for the synthesis
of the coumarin skeleton [7–17]. However, acid catalyzed
Pechmann reaction is the simple and most commonly used
method for the synthesis of coumarins from activated
phenols and b-ketoesters. The aza-Michael addition reac-
tion constitutes an important methodology for the synthesis
Keywords Green chemistry Á Heterogeneous catalysis Á
Lewis acid Á One-pot synthesis Á Recyclability
Electronic supplementary material The online version of this
article (doi:10.1007/s00706-016-1752-4) contains supplementary
material, which is available to authorized users.
& Satya Paul
paul7@rediffmail.com
1
Department of Chemistry, University of Jammu, Jammu
180 006, India
123

M. Kour, S. Paul
of b-amino carbonyl compounds that are biologically
essential products and also used as key intermediates for
the preparation of various b-amino alcohols, chiral auxil-
iaries, antibiotics, b-amino acids and other nitrogen-
containing compounds [18, 19]. Various Bronsted and
Lewis acid catalysts have been reported in literature for
Pechmann and aza-Michael reaction [20–44] however,
such methods suffer from one or more disadvantages like
high cost, long reaction times, the requirement of excess
reagents or catalysts, harsh reaction conditions, and the use
of noxious solvents. Thus, the development of simple,
recyclable, and environmentally friendly approach that can
be performed under mild conditions for Pechmann and aza-
Michael reaction is still desirable.
and high resolution transmission electron micrographs
were used to study the morphological changes and fine
structure of C@TiO2–SO3–SbCl2. The thermal stability of
C@TiO2–SO3–SbCl2 was analysed in the temperature
range of 40–700 °C by thermal gravimetric analysis (see
Supporting information for FTIR, SEM, TEM, HRTEM,
EDX, XRD, and TGA). The TGA of C@TiO2–SO3–SbCl2
shows the initial weight loss of 2.7 % due to the release of
physisorbed water and solvent trapped onto the surface of
C@TiO2–SO3–SbCl2. Further, a major weight loss of 39 %
was observed in the temperature range of 320–600 °C,
which can be attributed to the decomposition of Lewis acid
and organic moieties (Fig. S6). Thus, it is concluded that
C@TiO2–SO3–SbCl2 can be used safely below 300 °C.
As part of our ongoing efforts to explore the synthesis
and applicability of solid Lewis acids for various organic
transformations [45], we have investigated the utility of
C@TiO2–SO3–SbCl2 for the efficient one-pot synthesis of
coumarins via Pechmann reaction and b-amino carbonyl
compounds via aza-Michael addition of amines to a,b-
unsaturated carbonyl compounds and nitriles.
Catalytic testing for the one-pot synthesis
of coumarins via Pechmann reaction
Initially, we examined the catalytic activity of various solid
Lewis acids on the model reaction of resorcinol and ethyl
acetoacetate for the synthesis of corresponding coumarin
derivative. As is clear from the results presented in
Table 1, the catalytic performance of different solid Lewis
acids screened follows the order: C@TiO2–SO3–
SbCl2 [ C@TiO2–SO3–FeCl2 [ C@TiO2–SO3–AlCl2 [
C@TiO2–SO3–BiNO3 [C@TiO2–SO3–SnCl2 [C@TiO2–
SO3–Cu(OAc). The above order can be accounted for con-
sidering the difference in Lewis acid strength of various Lewis
acids grafted on the sulfonated carbon@titania composites.
The stronger Lewis acids lead to undesirable side reactions
whereas the weak ones were less effective, thereby the mild
Lewis acid, SbCl3 was foundto be most effectivein thepresent
case. Thus, C@TiO2–SO3–SbCl2 was selected to carry out the
Pechmann reaction for the synthesis of coumarin derivatives.
Further, to establish the optimized reaction conditions, the
model reaction was carried out in the presence of various
amounts of catalyst and a range of temperature and solvents for
appropriate times to give corresponding coumarin derivative.
To study the effect of catalyst amount, the model reaction was
performed in the presence of variable catalyst amounts. It was
found that with 0.05 g catalyst (0.5 mol% Sb), the reaction
yield was only 60 % which increased considerably to 94 % on
increasing the catalyst amount to 0.1 g (1 mol% Sb). On fur-
therincreasing theamountofcatalyst, only amarginal increase
in the reaction yield was observed (Table 2, entries 1–3).
Therefore, 0.1 g C@TiO2–SO3–SbCl2 (1 mol% Sb) was
identified to be optimum for carrying out the Pechmann con-
densation. Next we investigated the effect of solvents, by
carrying out the reaction using a range of protic/aprotic sol-
vents including ethanol, toluene, acetonitrile, water and also
under solvent-free conditions. As is clear from the results
shown in Table 3,thebestresultswereachievedundersolvent-
Results and discussion
Characterization of Lewis acid grafted sulfonated
carbon@titania composites
TiO2, being one of the potential semiconductor photocat-
alysts, is widely used for the degradation of organic
pollutants due to several advantages, like low cost, non-
toxicity, and stability [46–48]. Besides, it also finds broad
applicability as heterogeneous catalyst support due to its
high chemical and thermal stability and large specific
surface area. It is known to enhance the activity in many
cases due to the strong interaction between the active phase
and the support [49]. Titania has been reported to form
hybrid composite with carbon materials exhibiting
enhanced catalytic activity [50–53]. The carbon material
possesses high surface area due to porous structure and thus
the resulting composite material can combine the proper-
ties of macroporous carbon and titania nanoparticles for
applicability as efficient catalyst support for grafting Lewis
acids. The detailed characterization of various Lewis acid
grafted sulfonated carbon@titania composites has been
reported in our previous publication [45]. The surface
functionalization and elemental composition has been
determined by FT-IR and CHNS analysis. ICP-AES and
EDX analysis were used to determine the amount of metal
loaded on the composite material. The phase and crys-
tallinity of nano-titania was determined by powder X-ray
diffraction (XRD) studies. Scanning electron micrographs
123

A green and convenient approach for the one-pot solvent-free synthesis of coumarins and…
Table 1 Comparison of catalytic activities of different Lewis acid grafted sulfonated carbon@titania composites for the one-pot synthesis of
coumarins and b-amino carbonyl compounds
Entry
Catalyst
Pechmann reactiona
Aza-Michael reactionb
Time/h
Yieldc/%
Time/h
Yieldd/%
1
2
3
4
5
6
a
C@TiO2–SO3–AlCl2
C@TiO2–SO3–FeCl2
C@TiO2–SO3–SbCl2
C@TiO2–SO3–SnCl
C@TiO2–SO3–Cu(OAc)
C@TiO2–SO3–Bi(NO3)2
2
90
92
94
85
80
90
6
5
3
8
8
6
90
92
93
90
88
90
1
0.75
3
3
2
Reaction conditions: resorcinol (1 mmol), ethyl acetoacetate (1 mmol), Lewis acid grafted sulfonated carbon@titania composites (1 mol%
metal) at 120 °C under solvent-free conditions
b
Reaction conditions: aniline (1 mmol), acrylonitrile (1 mmol), Lewis acid grafted sulfonated carbon@titania composites (1 mol% metal) at
60 °C under solvent-free conditions
c
Isolated yields
d
Column chromatography yields
Table 2 Effect of catalyst amount on Pechmann and aza-Michael reaction
Entry
Catalysta/mol%
Pechmann reactionb
Aza-Michael reactionc
Time/h
Time/min
Yieldd/%
Yielde/%
1
2
3
4
a
0.5
1
45
45
45
35
60
94
95
95
10
5
90
92
93
94
2
3
3
3
Refers to antimony chloride grafted sulfonated carbon@titania composite
b
c
d
e
Reaction conditions: resorcinol (1 mmol) and ethyl acetoacetate (1 mmol) at 120 °C under solvent-free conditions
Reaction conditions: aniline (1 mmol) and acrylonitrile (1 mmol) at 60 °C under solvent-free conditions
Isolated yields
Column chromatography yields
free conditions. Further, it was observed that the conversion of
phenol to corresponding coumarin increased considerably on
increasing the reaction temperature from 60 to 120 °C
(Table 3, entries 11–14) under solvent-free conditions and any
further increase in reaction temperature did not affected the
product yield appreciably. Thus, 120 °C was selected as the
optimum reaction temperature. We then investigated substrate
generality of substituted phenols and ethyl acetoacetate under
optimized reaction conditions (Table 4). Pyrogallol,
phloroglucinol, and resorcinol afforded excellent yields in
shorter reaction times. In general, compared to unsubstituted
phenol, substrates with electron-donating groups para to the
site of electrophilic substitution afforded good yields of cor-
responding coumarin derivatives, whereas no product
formationwas observed in case ofphenols possessingelectron-
withdrawing groups. Longer reaction time was needed in case
of 1-naphthol and phenol probably due to the presence of
another phenyl ring in case of 1-naphthol and unactivated ring
in case of phenol.
Catalytic testing for the one-pot synthesis of b-amino
carbonyl compounds via aza-Michael reaction
To explore the catalytic activity of different Lewis acid
catalysts, the model reaction of aniline and acrylonitrile
was investigated for the synthesis of corresponding b-
amino carbonyl compound. As can be seen from the
results summarised in Table 1, C@TiO2–SO3–SbCl2
again displayed higher catalytic activity compared to
other Lewis acid catalysts. Further, to determine the effect
of catalyst loading, the model reaction was carried out in
the presence of different amounts of catalyst and it was
observed that the variation of C@TiO2–SO3–SbCl2
amount had an effective influence on the reaction. We
123

M. Kour, S. Paul
Table 3 Effect of different solvents and reaction temperature on C@TiO2–SO3–SbCl2 catalyzed Pechmann condensation and aza-Michael
reaction
Entry
Solvent
Temp/°C
Pechmann reactiona
Time/h
Aza-Michael reactionb
Time/h
Yieldc/%
Yieldd/%
1
Ethanol
60
–
–
3
–
3
–
–
3
3
–
8
3
3
–
–
–
–
82
–
2
Ethanol
Reflux
60
4
64
–
3
Acetonitrile
Acetonitrile
Toluene
–
86
–
4
Reflux
Reflux
60
4
52
80
–
5
2
–
6
DCM
–
70
79
–
7
Water
60
–
–
8
Water
Reflux
rt
24
10
–
Trace
Trace
–
9
Solvent-free
Solvent-free
Solvent free
Solvent-free
Solvent-free
Solvent-free
Solvent-free
45
88
93
–
10
11
12
13
14
15
a
50
60
3
40
84
90
94
95
90
3
100
120
130
2
–
0.75
0.66
–
–
Reaction conditions: resorcinol (1 mmol), ethyl acetoacetate (1 mmol), and 0.1 g C@TiO2–SO3–SbCl2 (1 mol% Sb)
b
c
d
Reaction conditions: aniline (1 mmol), acrylonitrile (1 mmol), and 0.2 g C@TiO2–SO3–SbCl2 (2 mol% Sb)
Isolated yields
Column chromatography yields
Table 4 C@TiO2–SO3–SbCl2 catalyzed one-pot synthesis of coumarins
OH
CH3
O
O
O
C@TiO2-SO3-SbCl2
(0.1 g, 1 mol% Sb)
120 °C, solvent-free
H
3C
O
CH3
O
X
X
1
2
3
Entry
X (Product 3)
Time/h
Yielda/%
1
2
7-OH
0.75
1
94
88
90
92
93
90
91
92
88
85
8-OH
3
7,8-(OH)2
5,7-(OH)2
6-CH3
7-CH3
7-OCH3
6-OH
0.75
0.75
1.5
1.5
1.5
1
4
5
6
7
8
9
7,8-Benzo
H
2.5
3.5
10
Reaction conditions: amine (1 mmol), alkene (1 mmol), and C@TiO2–SO3–SbCl2 (0.2 g, 2 mol% Sb) at 60 °C under solvent-free conditions
a
Isolated yield
123

A green and convenient approach for the one-pot solvent-free synthesis of coumarins and…
observed that 0.2 g of the catalyst could catalyze the aza-
Michael reaction efficiently affording the desired product
in 93 % yield (Table 2, entry 3); increasing or decreasing
the amount of the catalyst did not lead to any improve-
ment in the yields. The model reaction was then carried
out in several solvents such as EtOH, CH3CN, DCM,
H2O and under solvent-free conditions (Table 3, entries 1,
3, 6, 7 and 9–11) to investigate the efficiency of the
catalyst. In this study, it was found that conventional
heating at 60 °C under solvent-free conditions is more
efficient than using organic solvents, with respect to
reaction time and yield of the desired b-amino carbonyl
compound. Using the optimized reaction conditions, sev-
eral structurally varied amines with electron-donating and
electron-withdrawing groups were coupled with a,b-un-
saturated carbonyl compounds and nitriles, and the results
are summarized in Table 5. Both aromatic and aliphatic
amines gave the corresponding b-amino carbonyl com-
pounds in good yields. Aliphatic amines due to their high
nucleophilicity in comparison to aromatic amines under-
went the aza-Michael addition in shorter reaction times.
In addition, the aza-Michael reaction of various substi-
tuted amines with ethyl acrylate was found to be faster in
comparison to acrylonitrile due to high reactivity of ethyl
acrylate in comparison to acrylonitrile.
In order to compare the catalytic activity of C@TiO2–
SO3–SbCl2, catalytic experiments were conducted using
various homogeneous and heterogeneous acid catalysts
under the same reaction conditions as well as in the
absence of any catalyst. As expected, the condensation did
not proceed without catalyst (Table 6, entry 1). It can be
seen that homogeneous catalysts such as H2SO4 and p-
toluenesulfonic acid (Table 6, entries 2, 3) are less effi-
cient. Further, it was also observed that poor yields of the
corresponding coumarin derivative and aza-Michael pro-
duct were obtained when nano-titania, amorphous carbon,
and carbon@titania were employed as catalysts (Table 6,
entries 4, 5, and 6). The homogeneous SbCl3 showed
marked catalytic activity but suffers from the hazards
associated with the use of conventional homogeneous
Lewis acids, such as requirement of more than stoichio-
metric amount of Lewis acid, decomposition during work-
up, non-recovery, and reusability of the catalyst. Remark-
ably, it can be seen that C@TiO2–SO3–SbCl2 with lower
catalyst loading exhibits high activity in Pechmann reac-
tion and aza-Michael reaction under the same reaction
conditions along with the added advantages of catalyst
recovery and reusability (Table 6, entry 9). Further, to
study the merits of the current protocol for the synthesis of
coumarins and b-amino carbonyl compounds,
a
Table 5 C@TiO2–SO3–SbCl2 catalyzed one-pot synthesis of b-amino carbonyl compounds via aza-Michael reaction
C@TiO2-SO3-SbCl2
(0.2 g, 2 mol% Sb)
60 °C, solvent-free
R1
R2
R1
R2
X
NH
X
N
Entry
Amine
X (Alkene)
Time/h
Yielda/%
1
Aniline
CN
3
93
90
89
95
92
90
88
89
92
90
93
90
85
87
2
4-Methylaniline
4-Methoxyaniline
4-Chloroaniline
4-Fluoroaniline
4-Bromoaniline
Morpholine
CN
3
3
CN
3.5
3.5
3
4
CN
5
CN
6
CN
3.5
1.5
1.5
1
7
CN
8
Piperidine
CN
9
Aniline
COOEt
COOEt
COOEt
COOEt
COOEt
COOEt
10
11
12
13
14
4-Methoxyaniline
4-Chloroaniline
4-Bromoaniline
Morpholine
2
2
2
2
Piperidine
2.5
Reaction conditions: amine (1 mmol), alkene (1 mmol), and C@TiO2–SO3–SbCl2 (0.2 g, 2 mol% Sb) at 60 °C under solvent-free conditions
a
Column chromatography yield
123

M. Kour, S. Paul
Table 6 Comparison of
catalytic activity of C@TiO2–
SO3–SbCl2 with various
homogeneous and
Entry Catalyst
Pechmann reactiona
Aza-Michael reactionb
Time/h
Yieldc/%
Time/h
Yieldd/%
1
2
3
4
5
6
7
8
9
a
None
3
NR
30
40
10
10
20
35
93
94
7
–
–
5
5
5
5
3
3
Trace
–
heterogeneous catalysts for
Pechmann and aza-Michael
reaction
H2SO4
2
p-TSOH
2
–
Nano-titania
2
15
12
20
60
88
93
Amorphous carbon
Non-sulfonated carbon@titania
Sulfonated amorphous carbon
Anhyd. SbCl3
2
2
2
0.5
0.75
C@TiO2–SO3–SbCl2
Reaction conditions: resorcinol (1 mmol), ethyl acetoacetate (1 mmol), and catalyst (1 mol% for entries
2 and 3, 0.1 g for entries 4–7, 9, 1 mol% SbCl3 for entry 8) at 120 °C under solvent-free conditions
b
Reaction conditions: aniline (1 mmol), acrylonitrile (1 mmol), and catalyst (1 mol% for entries 2 and 3,
0.1 g for entries 4–7, 9, 1 mol% SbCl3 for entry 8) at 60 °C under solvent-free conditions
c
Isolated yields
d
Column chromatography yields
Effect of pH on the stability and activity
of C@TiO2–SO3–SbCl2
comparison of the efficacy of C@TiO2–SO3–SbCl2 with
some of the reported catalytic systems in the literature was
done. As is clear from the results presented in Table 7, the
present catalytic system affords high product yield in
shorter reaction times with low loading amounts of the
Lewis acid catalyst in comparison to others reported in
literature (Table 7). In addition, the present protocol
comparatively affords a truly green process using benign
reaction media and heterogeneous Lewis acid catalyst.
The effect of pH on the stability of C@TiO2–SO3–SbCl2 was
examined by dispersing 50 mg of the catalyst in four dif-
ferent solutions having pH 3, 6, 8, and 10. These suspensions
were allowed to remain as such for 24 h and afterwards
magnetically stirred and filtered to separate the catalyst. The
supernatant was examined by ICP-AES analysis to
Table 7 Comparison of the catalytic activity of C@TiO2–SO3–SbCl2 with reported catalytic systems for the Pechmann and aza-Michael
reaction
Reaction
Catalyst
Reaction conditions
Yieldc/%
Time/h
Pechmann reactiona
Zeolite BEA [54]
ZAF-16 [55]
PhNO2, 130 °C, catalyst (0.2 g)
Toluene, 120 °C, catalyst (1.0 g)
85
4
65.1
78.3
95
4
SO42-/SnO2 [56]
PANF-PAMSA [57]
C@TiO2–SO3–SbCl2e
FeCl3/mont. K10 [58]
SbCl3-HAP [59]
Solvent-free, 120 °C, catalyst (1.0 g)
Toluene, 110 °C
Solvent-free, 120 °C, C/TiO2–SO3H (0.1 g, 1 mol% Sb)
Solvent-free, 60 °C, catalyst (10 wt%)
CH3CN, N2, 50 °C, 1.6 mol% catalyst
H2O, rt, catalyst (1 mmol)
2
4
94
0.75
5
Aza-Michael reactionb
85d
NR
90d
80d
94d
10
4
[DPPA] [HSO4] [60]
TMSCl [61]
C@TiO2–SO3–SbCl2e
Solvent-free, 60 °C, TMSCl (10 mol%)
Solvent-free, 60 °C, C/TiO2–SO3H (0.2 g, 2 mol% Sb)
3
0.25
a
Reaction conditions: resorcinol (1 mmol), ethyl acetoacetate (1 mmol), and 0.1 g C@TiO2–SO3–SbCl2 (1 mol% Sb) at 120 °C under solvent-
free conditions
b
Reaction conditions: aniline (1 mmol), acrylonitrile (1 mmol), and 0.2 g C@TiO2–SO3–SbCl2 (2 mol% Sb) at 60 °C under solvent-free
conditions
c
Isolated yields
d
Column chromatography yields
e
Our work
123

A green and convenient approach for the one-pot solvent-free synthesis of coumarins and…
determine any possibility of leaching of the Lewis acid. It
was observed that no significant leaching was detected for
solutions with pH 3, 6, and 8, whereas only a small amount of
leached antimony was detected for solution with pH 10.
These observations indicate that catalyst is stable in acidic
and weakly basic conditions, whereas its stability is affected
in the presence of strongly basic conditions. Further, to
evaluate the effect of pH on catalytic activity of C/TiO2–
SO3–SbCl2, the test reaction in case of Pechmann reaction
and aza-Michael addition (Table 4, entry 1; Table 5, entry 1)
was carried out under the optimized conditions in the pres-
ence of C@TiO2–SO3–SbCl2 separated from the respective
suspensions (Table 8). The results indicated that there is no
significant effect on the catalytic activity of C@TiO2–SO3–
SbCl2 in acidic and weakly basic conditions. Whereas, in
strongly basic condition, the reaction time was enhanced as
well as the yield of the products in both the reactions was also
decreased (Table 8, entry 4). Thus, it can be concluded that
in stronglybasic conditions the catalytic stability and activity
is affected to a small extent, whereas in acidic and weakly
basic conditions, the catalyst maintains its activity and
stability.
Recyclability
The stability and sustained catalytic activity of a catalyst is
an important aspect to be considered from economic point
of view. The catalyst reusability contributes to reduce the
cost of the practical application processes. The reusability
of the catalyst was thus investigated in the Pechmann and
aza-Michael reaction. The catalyst was easily recovered
after the reaction by simple filtration and washed with
distilled water, followed by drying and then reused for
further reactions (Table 4, entry 1; Table 5, entry 1). To
our delight, the catalytic activity of the catalyst had only a
slight change after five catalytic runs (Fig. 1), suggesting
Table 8 Effect of pH on the catalytic activity of C@TiO2–SO3–SbCl2 for the Pechmann and aza-Michael reaction
Entry
pH
Pechmanna
Aza-Michaelb
Time/h
Yieldc/%
Time/h
Yieldd/%
1
2
3
4
a
3
0.75
0.75
0.75
1.25
93
94
92
88
3
3
3
4
94
94
92
85
6
8
10
Reaction conditions: resorcinol (1 mmol), ethyl acetoacetate (1 mmol), and 0.1 g C@TiO2–SO3–SbCl2 (1 mol% Sb) at 120 °C under solvent-
free conditions
b
Reaction conditions: aniline (1 mmol), acrylonitrile (1 mmol), and 0.2 g C@TiO2–SO3–SbCl2 (2 mol% Sb) at 60 °C under solvent-free
conditions
c
Isolated yields
d
Column chromatography yields
Fig. 1 Recyclability of
C@TiO2-SO3-SbCl2. Reaction
conditions: resorcinol (1 mmol),
ethyl acetoacetate (1 mmol),
0.1 g C@TiO2-SO3-SbCl2
(1 mol% Sb) at 120 °C under
solvent-free conditions; aniline
(1 mmol), acrylonitrile
(1 mmol), 0.2 g C@TiO2-SO3-
SbCl2 (2 mol% Sb) at 60 °C
under solvent-free conditions
123

M. Kour, S. Paul
the high chemical stability of the heterogeneous Lewis acid
catalyst.
Conclusion
Further, the thermal stability of the catalyst after five
reaction cycles was determined by carrying out the TG
analysis. It was observed that the reused catalyst exhibited
almost the same TG curve as the fresh one (Fig. 2)
depicting that the present heterogeneous system shows high
thermal stability after successive reaction cycles. Mean-
while, there was no significant decrease in the amount of
metal after five cycles as determined from ICP analysis of
heterogeneous Lewis acid catalyst.
To sum up, we have introduced an efficient and clean
procedure for the excellent one-pot synthesis of coumarins
via Pechmann reaction, and b-amino carbonyl compounds
via aza-Michael reaction using C@TiO2–SO3–SbCl2 as the
heterogeneous Lewis acid catalyst. Low catalyst loading
(1 mol%), quantitative yields of products, short reaction
times, recyclability up to five consecutive runs without
significant loss of activity, and easy separation are salient
features of this catalytic system. The applicability of the
Fig. 2 TGA of a fresh C@TiO2-SO3-SbCl2; b C@TiO2-SO3-SbCl2 after five catalytic runs
123

A green and convenient approach for the one-pot solvent-free synthesis of coumarins and…
present catalytic system is being extended to other acid
catalyzed organic reactions.
in the ratio of 1:1.2 in a round-bottom flask and then heating
the mixture at 353 K for 10 h. Afterwards, the incomplete
carbonization of resulting mixture was done by heating it
strongly at 673 K under N2 atmosphere for 10 h leading to
the formation of amorphous carbon@titania composites.
The sulfonation of the as prepared composite was done by
adding concentrated sulfuric acid ([96 wt%) and further
heating it at 423 K for 10 h under N2 atmosphere. The sul-
fonated composite thus obtained was then washed repeatedly
with hot distilled water ([353 K)until sulfate anionswereno
longer detected in the filtered water. Sulfonated car-
bon@titania composite was dried in an oven at 373 K for 2 h
(7 g) and then converted into Lewis acid grafted sulfonated
carbon@titania composites through anion metathesis. The
solid acid (2 g) was treated with different Lewis acids
(0.5 mmol) viz. AlCl3, FeCl3, SbCl3, SnCl2, Cu(OAc)2, or
Bi(NO3)3 in 10 cm3 acetonitrile at reflux temperature for
24 h. After cooling to room temperature, the mixture was
filtered, washed with acetonitrile and dried under vacuum
and finally kept at 90 °C overnight. The catalysts were
conditioned by refluxing for 12 h each in xylene at 130 °C
(2 9 2 h), ethanol at 78 °C (2 9 2 h), and acetonitrile at
80 °C (2 9 2 h), and finally dried in oven at 90 °C
overnight.
Experimental
The chemicals used were purchased from Aldrich chemical
company and Merck, and used without further purification.
The products were characterized by their spectral data and
comparison of their physical data with those of known
samples. The 1H and 13C NMR data were recorded in
CDCl3 or DMSO-d6 or CDCl3 ? DMSO-d6 or CD3COCD3
on Bruker Avance III (400 MHz) spectrometer. The FT-IR
spectra were recorded on Thermo Nicolet, Avatar 370
spectrophotometer, XRD was recorded in 2h range of 10°–
80° on Bruker AXS D8 Advance and mass spectral data on
Bruker Esquire 3000 (ESI). CHNS analysis was recorded
on ThermoFinnigan FLASH EA 1112 series. SEM images
were recorded using JEOL Model JSM-6390LV Scanning
Electron Microscope, Transmission Electron Micrographs
(TEM) were recorded on Philips CM-200. EDX analysis
was carried out using JEOL Model JED-2300 and TGA
was recorded on Perkin Elmer, Diamond TG/DTA.
General procedure for the synthesis of Lewis acid
grafted sulfonated carbon@titania composites
General procedure for the C@TiO2–SO3–SbCl2
catalyzed synthesis of coumarins via Pechmann
reaction
The general procedure for the synthesis of Lewis acid grafted
sulfonated carbon@titania composites is represented in
Scheme 1. Initially carbon@titania composites were pre-
pared by taking 10 g of a mixture of starch and nano-titania
In a 25 cm3 round bottom flask, 0.1 g C@TiO2–SO3–SbCl2
(1 mol% Sb) was added to the mixture of phenolic
Scheme 1
OH
Nano-
OH
HO S
3
SO
OH
SO
3
H
HO
Titania
OH
OH
OH
Partial
Carbonization
Sulfonation
Conc. H SO
423 K, 10 h, N
HO
HO
HO
HO
2
4
,
673 K, 10h, N
2
Starch
2
3
H
SO H
3
OH
Sulfonated carbon@titania
composite
Carbon@titania
composite
Lewis acid,
Acetonitrile,
reflux, 24h
OH
MO S
3
SO
3M
OH
HO
HO
SO M
3
SO M
3
Lewis acid grafted sulfonated
carbon@titania composite
M = AlCl
2
, FeCl
2
, SbCl ,
2
SnCl, Cu(OAc), Bi(NO
3)2
123

M. Kour, S. Paul
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compound (1 mmol) and ethyl acetoacetate (1 mmol) and
the reaction mixture was stirred for the appropriate time at
120 °C under solvent-free conditions. The progress of the
reaction was monitored by TLC. After completion of the
reaction, the reaction mixture was diluted with 30 cm3 hot
ethanol and the catalyst was separated by simple filtration
to obtain the crude product. Compounds with purity below
95 % were further purified using column chromatography
on silica gel and then recrystallized from hot ethanol to
afford the pure coumarin derivatives.
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