A novel protocol for solvent-free synthesis of 4,6-diaryl-3,4- dihydropyrimidine-2(1H)-ones catalyzed by metal oxide-MWCNTs nanocomposites

Article abstract of DOI:10.1016/j.molstruc.2014.05.012

A Biginelli-like condensation is described using acetophenone as active methylene compound with aldehydes and urea to furnish pyrimidinone analogues under solvent-free conditions. In this paper, besides the preparation of nanocomposites based on MWCNTs, o

Full text of DOI:10.1016/j.molstruc.2014.05.012

Accepted Manuscript
A novel protocol for solvent free synthesis of 4,6-diaryl-3,4-dihydropyrimi-
dine-2(1H)-ones catalyzed by metal oxide-MWCNTs nanocomposites
Javad Safari, Soheila Gandomi-Ravandi
PII:
S0022-2860(14)00484-0
DOI:
http://dx.doi.org/10.1016/j.molstruc.2014.05.012
MOLSTR 20611
Reference:
To appear in:
Journal of Molecular Structure
Received Date:
Revised Date:
Accepted Date:
15 January 2014
18 March 2014
5 May 2014
Please cite this article as: J. Safari, S. Gandomi-Ravandi, A novel protocol for solvent free synthesis of 4,6-diaryl-3,4-
dihydropyrimidine-2(1H)-ones catalyzed by metal oxide-MWCNTs nanocomposites, Journal of Molecular
Structure (2014), doi: http://dx.doi.org/10.1016/j.molstruc.2014.05.012
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A novel protocol for solvent free synthesis of 4,6-diaryl-3,4-dihydropyrimidine-2(1H)-
ones catalyzed by metal oxide-MWCNTs nanocomposites
Javad Safari*, Soheila Gandomi-Ravandi
Laboratory of Organic Compound Research, Department of Organic Chemistry, College of Chemistry,
University of Kashan, P.O. Box: 87317-51167, Kashan, I.R. IRAN
Tel.: +98-(0)361-591-2320; Fax: +98-(0)361-591-2397, e-mail: Safari@kashanu.ac.ir
Abstract
A Biginelli-type condensation is described using acetophenone as active methylene compounds with aldehydes
and urea to furnish pyrimidinone analogues under solvent free conditions. In this paper, besides the preparation
of nanocomposites based on MWCNTs, we also focus on investigating the catalytic efficiency of CNT/metal
oxide composites. The requisites of a good catalyst are high activity, selectivity, reusability, reasonable cost and
long lifetime. The application of solvent free conditions and transition metal oxides decorated-CNTs (MOx-
CNTs) nanocomposites as attractive, effective and reusable catalysts, lead to the efficient synthesis of 4,6-
diaryl-3,4-dihydropyrimidin-2-(1H)-ones. This recyclable heterogeneous catalytic system provides a simple
strategy to generate a variety of pyrimidinones under solvent free conditions. Utilization of easy reaction
condition, recyclable green catalyst, reduced environmental impacts and simple work-up make this methodology
as an interesting option for the eco-friendly synthesis of Biginelli-type compounds.
2
Keywords: Nanocomposites, TiO -CNTs, Heterocycle, Heterogeneous catalyst, Pyrimidinone
1
.
Introduction
Nitrogen heterocyclic compounds have attracted considerable attention to design biologically active molecules,
because their structural subunits exist in many natural products as well as pharmaceuticals [1]. Pyrimidine
moiety is an important class of heterocycles including nitrogen and is responsible for the properties of many
pharmaceutically significant natural products and synthetic compounds [2]. The pyrimidine ring is a key
structural moiety of a series of natural compounds (antibiotics, nucleic acids, hormones, alkaloids and vitamin
1

B
1
) [3,4], life-supporting substances (vitamins, coenzymes, and uric acid) [5], synthetic medicines (Veronal,
Sulfadiazine, Fluorouracil, Glivec, Rosuvastatin, and barbiturates) [3,5], chemotherapeutic drugs (fluorouracil)
[
(
3], herbicides, dyes, and many more compounds [4]. Preparation of these non-planar dihydropyrimidinones
DHPMs) is now recognized as a powerful heterocyclic synthesis with many important applications and has
been the subject of several reviews [3,6-8]. With regard to the pharmacological profile and medicinal
applications of functionalized dihydropyrimidinones and -thiones, Biginelli reaction to synthesize these
compounds has received renewed interest and this cyclocondensation process was enlarged by the variation of
CH-acidic carbonyl compound building blocks [9]. Very recently, simple alkyl acetoacetates are employed as
the active methylene building blocks for novel synthesis of Biginelli-like scaffold [7], but other types of 3-
oxoalkanoic esters, benzoylacetic esters or thioesters can also be used successfully [6]. Also, cycloalkanones
[10], β-keto carboxylic acid [11], cyclic ketones and substituted α-keto acids [12] or acyclic and cyclic ketones
[
13] can be used instead of esters to produce pyrimidinones. Also, primary, secondary and tertiary
acetoacetamides [6] and nitroacetone [14] serve as viable substrates and good building blocks. In addition,
condensations can be achieved by employing β-diketones, cyclic β-diketones such as cyclohexane-1,3-dione
[15], β-ketolactones [16], cyclic β-diesters [17] or β-diamides [17,18], benzocyclic ketones [19], and other
cyclic β-dicarbonyl compounds [20]. In addition, the Biginelli-type reaction was recently investigated using
acetophenones instead of 1,3-dicarbonyl compounds in a few publications [21,22]. This could be a significant
and useful extension of the Biginelli reaction, allowing access to a large number of multifunctionalized
pyrimidinone derivatives [6,7,23].
In recent years, special attention has been paid to decorate carbon nanotubes, i.e. filling of the inside of CNTs
[
24] or supporting of the outside of CNTs [25], owing to further improves of their properties. The unique
properties such as the large surface areas and high electric conductivity make carbon nanotubes an ideal and
valuable material to support metal oxide nanoparticles in many potential applications [25]. CNTs-inorganic
composites, including CNT-supported transition metals and/or metal oxides are an important family of inorganic
heterostructures [26]. Studies have shown that metal NPs/CNTs nanocomposites exhibited good catalytic
behaviour under various chemical reactions and considerably improved activity, stability, and selectivity in
heterogeneous catalysis [27]. The high catalytic efficiency was achieved by the effective charge transferring
from metal to CNTs. Chemical and physical properties of metal NPs/CNTs composites naturally depend on the
type of the metal. Numerous synthetic methodologies have been developed to prepare CNT-based
nanocomposites by linking active inorganic components including noble metals [28], transition metals and their
2

oxides and sulphides [29,30], on the surface of CNTs. These approaches include impregnation method [31],
hydrothermal and solvothermal process [32,33], sol–gel technique [34], electron-beam evaporation [35] and
chemical vapour deposition [36]. In general, surface modification of CNTs with strong acid creates functional
groups on the surfaces. This increase the surface binding sites, which usually leads to anchoring the precursors
of metal nanoparticles, good dispersion of metal NPs and small metal nanoparticles [37].
Developing a simple, efficient and green methodology is needed for the Biginelli-type reactions. Herein, the
present investigation deals a novel solid phase and heterogeneous neat strategy towards DHPMs. Our new
approach reports to use metal oxides based on multi-walled carbon nanotubes as catalysts for the Biginelli-like
condensation reaction under solvent free conditions. For this purpose, the reaction of aromatic aldehydes,
x
acyclic ketones (acetophenone) and urea was investigated over MO -CNTs as an effective acid catalytic system
(
Scheme 1).
O
O
H N
2
HN
O
NH2
O
NH
3
MO
X
-CNTs
R'
R'
Solvent Free
H
4
1
2
R
R
X
Scheme 1.MO -CNTs promoted preparation of Biginelli-like compounds under solvent free conditions
2
.
Results and discussion
We prompted to prepare 3,4-dihydropyrimidin-2(1H)-one scaffolds with the solvent free condensation of
acetophenone, aromatic aldehydes and urea in the Biginelli-type reaction. The employed synthetic is
outlined in Scheme 1 to generate 4,6-diaryl-3,4-dihydropyrimidin-2(1H)-ones. The cyclocondensation of
acetophenone (1), benzaldehyde (2) and urea (3) yielding the dihydropyrimidone (4) was used as benchmarking
reaction. The reactions have developed in the presence of a number of transition metal oxides supported on
2 3 4 2
CNTs (MnO -CNTs, NiO-CNTs, Fe O -CNTs and TiO -CNTs) as green and recyclable catalysts or
promoters. The nanocomposites can be easily prepared according to literature procedures. These supported
catalysts promote the reactions due to its potential Lewis acidity. Initially, the activities of the catalysts in the
solvent free synthesis of various substituted 3,4-dihydropyrimidin-2(1H)-ones were found to decrease in the
2 2 3 4
following order: TiO -CNTs > MnO -CNTs > Fe O -CNTs > NiO-CNTs (Table 1). The results show that all of
2
the above nanocomposites could promote the reaction, but TiO -CNTs catalyst is significantly more effective
3

than other acid catalysts in the Biginelli-type reaction and it provides better results with high yields and short
times. The most important advantage of titanium is the more number of unoccupied d-orbitals of the metal
nanoparticles resulting stability.
a
Table 1. Effect of various promoter/catalysts on the Biginelli-type reaction
Entry
Catalyst
Time (min) Yield (%)
1
2
3
4
5
6
7
8
MWCNTs-COOH 10
20
87
69
80
98
48
40
45
55
MnO
NiO-CNTs
Fe -CNTs
TiO -CNTs
MnO NPs
NiO NPs
Fe NPs
TiO NPs
2
-CNTs
10
10
10
10
10
10
10
10
3 4
O
2
2
3 4
O
9
2
a
1
.5:1:1 mmol urea:acetophenone:benzaldehyde
In order to optimize the reaction conditions, the catalytic efficiency of nanocomposites was studied with various
amounts of TiO -CNTs in the model reaction. The results reveal that 0.03 g of nanocatalyst provided the best
2
effects in terms of reaction time, economy of catalyst charge and purity of products. Higher amount of catalyst
did not improve the rate considerably. This behaviour was explained by this fact that catalyst active sites exist in
a certain concentration more than that is required for the reactant molecules, and hence the additional amount of
nanocomposites does not increase the rate of the reaction. The results are presented in Table 2.
2
Table 2. Optimization of amount of TiO -CNTs catalyst
Catalyst (g)
Time (min)
Yield (%)
0
0.01
10
0.02
10
0.03
10
0.04
10
10
61
76
90
98
98
After the success of the reaction, we carried out the condensation reaction under solvent free conditions at 100
4

ºC and in various solvents with varying polarity and protic nature such as ethanol, dioxane, propanol,
acetonitrile, toluene, THF, cyclohexane, and DCM. As indicated in Table 3, the polar solvents such as ethanol,
methanol and acetonitrile were much better than non-polar solvents. The results could be explained with the
much better solubility of the reagents and the much better dispersion of the catalyst in the polar solvents. We
also carried out the model reaction under solvent free conditions. Thus, in the present system, we selected
solvent free conditions as optimized reaction medium from points of view of the reaction yields and
environmental damage.
Table 3. Optimization of reaction medium for the Biginelli-like reaction
Entry Solvent
Time (min) Yield (%)
1
2
3
4
5
6
7
8
9
EtOH
10
10
10
10
10
90
55
86
35
44
15
88
20
98
Dioxane
MeCN
Toluene
THF
Cyclohexane 10
MeOH
DCM
10
10
Solvent free 10
Encouraged by the remarkable obtained results, then we investigated the scope and versatility of the process.
Aromatic aldehydes substituted with either electron-donating or electron-withdrawing group reacted
successfully with urea and acetophenone and gave the products of 4,6-diaryl-3,4-dihydropyrimidin-2(1H)-ones
in high yields. The results are listed in Table 4, which clearly indicates the generality of the reaction.
Apparently, the nature of the substituents does not affect significantly on the reaction time and yield for the
employed reaction conditions.
5

2
Table 4. Solvent free synthesis of 4,6-diaryl-3,4-dihydropyrimidin-2(1H)-ones using TiO -CNTs
M.p (ºC)
Entry
R
Product Time (min)
Yield (%)
Found
Reported
1
2
3
4
5
6
7
8
9
H
4a
4b
4c
4d
4e
4f
10
25
20
15
30
20
15
20
10
35
15
10
25
98
85
80
88
77
86
89
86
92
74
89
90
83
229–231 228–230 [38]
272–274 271–274 [39]
259–262 259–261 [39]
261–263 264–265 [39]
249–251 248–250 [39]
275–277 274–276 [39]
257–259 257–258 [39]
242–244 243–245 [39]
256–258 258–259 [38]
255–257 257–258 [38]
265–267 266–267 [39]
268–269 267–269 [38]
267–269 267–268 [39]
2,4-Cl
2
4-OMe
2-Cl
4-Me
2,6-Cl
2
3-OMe
3,4-(OMe)
3-Br
4g
4h
4i
2
10
11
12
4-OH
4j
2-OMe
4-Cl
4k
4l
3
-OMe-4-
13
4m
OH
One of the most important features of the present protocol is the recyclability and reusability of the catalyst.
The reusability of the catalyst was also investigated after the completion of the same model reaction under
optimized conditions. For this purpose, hot ethanol was added to the mixture and the solid catalyst was filtered,
then was washed in ethyl acetate for several times and was dried in oven for the next reactions. It is interesting
to note that the supported catalyst could be recyclable for five consecutive times without any significant loss of
activity (Figure 1).
6

Figure 1. Recyclability of catalyst in one-pot synthesis of dihydropyrimidinones
The physical morphologies of the composites were examined using scanning electron microscopy (SEM) and
transmission electron microscopy (TEM). Figure 2 shows SEM results about details on the interaction of CNTs
and the deposited metal oxide nanoparticles. From the TEM micrograph, the TiO nanoparticles were dispersed
2
completely homogeneous in the MWCNTs. Therefore, it is confirmed that TiO -MWCNT composites were
2
successfully prepared.
7

Figure 2. SEM of the a) TiO
2 3 4 2
–CNT, b) NiO-CNT, c) Fe O -CNT, d) MnO -CNT and e and f) TEM images
obtained from TiO –CNT catalysts.
2
The EDX data revealed the presence of the main elements such as C, O and metal (Ti, Ni, Fe and Mn) and other
impure elements (Figure 3). The Au signal originates from the substrate used for coating. The results of EDX
elemental microanalysis of the nanocomposites are listed in Table 5.
8

2 3 4 2
Figure 3. EDX elemental microanalysis of the a) TiO /MWCNTs, b) NiO-CNTs, c) Fe O -CNTs, and d) MnO -
CNTs nanocomposites.
Table 5. EDX elemental microanalysis of metal-CNTs composites
Sample
Element W%
O
Element A%
O C
M
C
M
TiO
NiO-CNTs
Fe -CNTs
2
-CNTs
17.72 17.49 40.86
24.63 13.63 40.24
13.66 17.17 58.78
7.38 21.81 67.84
8.84 17.95 70.61
3
O
4
3.91 17.13
78.12
9

2
MnO -CNTs
5.70 10.02 66.73
1.63 9.82
87.15
The structures of the prepared TiO
patterns of oxidized CNTs and titanium oxide composites are shown in Figure 4. The observed diffractions
confirm the presence of both the MWCNT (25.92º, 43.24º) and TiO in the samples.
2
-CNT composites were investigated using X-ray diffraction (XRD). XRD
2
2
Figure 4. XRD spectra of MWCNT-COOH and TiO /MWCNT nanocomposites
Figure 5 shows the Raman spectra of acid treated MWCNTs and metal oxide nanoparticles supported on acid
−1
treated MWCNTs. In Figure 5a, signals in 1596 and 1307 cm are corresponding to acid treated MWCNTs. In
−1
−1
2
Figure 5b, signals correlated to TiO can be shown at approximately 164-608 cm . The signals at 222−603 cm
are corresponding to Fe
O
3 4
in the Figure 5c. Also, the vibration modes correlated to NiO is presented at
−
1
−1
2
2
20−1139 cm (Figure 5d). As shown in Figure 5e, signals at 311−609 cm are related to MnO .
1
0

2 3 4
Figure 5. Raman spectra of a) MWCNTs-COOH, b) TiO /MWCNTs, c) Fe O -CNTs, d) NiO-CNTs and e)
MnO -CNTs nanocomposites
2
Fig. 6 shows the FT-IR of oxidized MWCNTs-COOH, Fe
3 4 2 2
O -CNTs, TiO -CNTs, MnO -CNTs and NiO-CNTs
−1
−1
composites. The broad absorptions appearing at around 3436 cm and 591 cm are attributed to the O–H
stretching and bending vibrations, respectively. Attachment of the carboxylic groups to the surface of CNTs
−1
−1
resulting peak at 1720 cm assigned to C=O stretching vibrations. The bands at 1158 and 1627 cm originated
from the graphitic component and stretching vibration of C=C of MWCNTs, respectively. Compared to the
−1
oxidized CNTs, absorption bands at 583, 450 cm is owing to the stretching vibrations of Fe–O (Fig 6b). From
−1
Fig 6c the stretching vibrations of Ti–O appeared at 616 and 1428 cm . The bands at 428 and 567 are
corresponding to the stretching vibrations of Mn–O (Fig 6d). Also, the bands at 404, 517, 574 and 720 are
correlated to the stretching vibrations of Ni–O Fig 6e).
1
1

3 4 2 2
Figure 6. FT-IR measurements of a) acid treated MWCNTs, b) Fe O -CNTs, c) TiO -CNTs, d) MnO -CNTs and e) NiO-
CNTs composites
The possible mechanism of the reaction is showed in Scheme 2. In this process, the presence of CNTs leads to
good dispersion and small size of metal oxide NPs. The nanocomposites act as a Lewis acid, catalyzed
condensation between the aldehyde and ketone to generate adduct I. The key step of the present reaction is the
formation of the chalcone II thought cross-aldol condensation of aldehyde and ketone. The Michael addition of
urea to chalcone II produces ureide III, which subsequently eliminates water and cyclizes to form
dihydropyrimidinone [40].
O
X
O -CNT
X
O -CNT
M
H
X
MO -CNT
O
H
M
O
O
O
O
O
H
Ar
Ar'
CH
3
Ar'
C
H
Ar
Ar'
C
H
Ar
- H O
2
Ar'
CH
2
Ar
2
I
II
O
Ar'
Ar'
Ar'
H
2
N
NH
2
NH
NH
NH
O
Ar
O
-
2
H O
Ar
N
H
O
Ar
N
H
O
2
H N
OH
H
X
MO -CNT
III
Scheme 2. Possible mechanism for the Biginelli-type reaction
1
2

Thus the use of TiO
technique for the Biginelli-type condensation. These results not only provide a new aspect for solvent free
organic synthesis catalyzed by TiO -CNTs, but also extend the utility of ketones with single carbonyl group
providing 5-unsubstituted 3,4-dihydropyrimidin-2-(1H)-ones in Biginelli reactions. All the products were
2
-CNTs nanocomposites as a green and heterogeneous catalyst is an environmentally clean
2
1
13
identified by comparing their physical (mps) and spectral data ( H NMR, C NMR and IR) with authentic
samples.
3.
Experimental
All chemicals were purchased with high purity from Merck and Fluka chemical companies. MWCNTs were
purchased from Nanotech Port Co. (Taiwan). These MWCNTs were produced with outer diameter between 10
and 20 nm via the chemical vapor deposition (CVD, or sometimes called catalytic pyrolysis). The progress of
the reactions was monitored by TLC on silica-gel plates of 2 mm thickness, which was visualized with UV light
(
solvent: petroleum ether: ethyl acetate 7:3 v/v). The melting points were determined on an Electrothermal MK3
1
13
apparatus were expressed in ºC and are uncorrected. H and C NMR spectra were recorded using a Bruker
DRX–400 spectrometer. Chemical shifts (δ) are reported in ppm downfield from TMS, coupling constants (J)
values are given in Hz. Spin multiplicities are given as s (singlet), d (doublet), t (triplet) and m (multiplet) as
well as b (broad). FT-IR spectra were determined on a Perkin Elmer FT–IR 550 spectrophotometer using KBr
–1
pellets and absorbencies are reported in cm . The nanocomposites materials were studied by scanning electron
microscopy (SEM, TESCAN model VEGA II operated at a 15 kV accelerating voltage) and Transmission
electron microscopy (TEM, Zeiss EM10C operated at 80 kV accelerating voltage). XRD patterns were obtained
on a Holland Philips Xpert X-ray diffraction (XRD) diffractometer (CuK, radiation, λ= 0.154056 nm and 40 kV
voltage), at a scanning speed of 2º/min from 10º to 100º (2θ). The Raman spectra were measured on a Bruker
–1
SENTERRA spectrometer with spectral range: 200-3500cm , CCD detector and Laser wavenumber 785 nm
-
1
(
High spatial & spectral resolution (Spectral Resolution: < 3cm ).
3
.1. Preparation of composites
The nanocomposites were prepared according to reported procedures in the literature. In brief, CNTs was
functionalized by adding 100 mL acid solution containing 2.5 M HNO and 0.5 M H SO at 100 ºC for 6 h.
Then, the acid-treatment CNTs were obtained by rinsing with deionized water and by drying in the air at 100 ºC.
MnO -CNTs were prepared in a direct redox reaction with mixing aqueous solution of KMnO and modified
3
2
4
2
4
1
3

CNTs under ultrasound irradiation. Then, acetic acid was added until pH 2 and was refluxed at 70 ºC for 3 h.
Then, the mixture was separated by centrifuging and was washed with deionized water and acetone and was
dried at 70 ºC for 12 h in a vacuum oven [41]. NiO/MWNTs composites were prepared by mixing NiCl
aqueous solution with acid treated MWCNTs. Next, NH OH was dropped into the suspension to pH 8 under
sonication for 20 min and the mixture was soaked in the solution for 5 h. Finally, intermediate product
Ni(OH) /MWNTs) was separated by centrifuging and were washed with deionized water and was calcinated at
00 ºC for 2 h to be achieved NiO/MWNTs nanocomposites [42]. Fe O /CNT composites were prepared by
2 2
·6H O
4
(
2
4
3
4
chemical coprecipitation of aqueous solution (NH
4
)
2 4
Fe(SO )
2
.6H
2
O (0.4 mmol) and NH
4
4
Fe(SO )
2
2
.12H O (0.8
mmol) at 50 ºC in purified CNTs under sonication for 10 min. Next, to precipitate the iron oxides, 5 M NH
4
OH
aqueous solution was added (pH 11–12) and the reaction was continued at 50 ºC for 30 min under mechanical
stirring. Then, the samples were separated from the suspension by magnet and were washed with doubly
2
distilled water and were dried in a vacuum oven at 100 ºC for 24 h [43]. TiO -CNTs were prepared by adding
TiCl solution consist of a little HCl to acid treated MWCNTs under sonication for 2 h. Then, the suspension
4
was stirred at room temperature for 22 h and up to 80 ºC for 3 h. At last, the mixture was centrifuged and was
calcinated in furnace at 370 ºC for 3 h [44].
3
.2. General method for the synthesis of Biginelli-like products under solvent free technique
2
TiO -CNTs (0.03 g) was added as a suitable catalyst to a mixture of benzaldehyde (1 mmol), acetophenone (1
mmol), and urea (1.5 mmol). The substrates were mixed under solvent free conditions for the needed time. After
completion (monitored by TCL), the reaction mixture was cooled to room temperature and after adding ethanol,
catalyst was separated by simple filtration due to its heterogeneous nature. Then, the filtrate was poured onto
crushed ice and was stirred for 5-10 min. The precipitate was filtered through a sintered funnel after stirring, and
was dried. For further purification, the product was recrystallized from ethanol.
4
.
Conclusions
We expanded the synthetic scope of Biginelli condensation to ketones other than active methylene compounds
to synthesize interesting Biginelli-type scaffolds. Therefore, a simple and new catalytic method was introduced
in the synthesis of 5-unsubstituted 3,4-dihydropyrimidin-2-(1H)-ones in a single operation by cyclocondensation
reaction with substituted aromatic aldehydes, acetophenone and urea using several metal oxides-CNTs as
catalyst under solvent free conditions. It revealed that application of solvent free conditions in the presence of
1
4

x 2
MO -CNTs nanocomposites would be ideally suited to synthesize this privileged heterocycles. Moreover, TiO -
CNTs as a green heterogeneous catalyst can be used with considerable advantages such as high catalytic
efficiency, mild reaction conditions, operational simplicity, green sustainable conditions, and therefore as an
alternative is more attractive than above mentioned nanocomposites and many Lewis acids. We believe this
methodology is superior to existing methodologies to prepare 3,4-dihydropyrimidin-2(1H)-ones.
Acknowledgment
We gratefully acknowledge the financial support from the University of Kashan.
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Scheme 1.MO
Figure 1. Recyclability of catalyst for one-pot synthesis of dihydropyrimidinones
Figure 2. SEM of the a) TiO –CNT, b) NiO-CNT, c) Fe -CNT, d) MnO -CNT and e and f) TEM images
obtained from TiO –CNT catalysts.
X
-CNTs promoted preparation of Biginelli-like compounds under solvent free conditions
2
3
O
4
2
2
2 3 4 2
Figure 3. EDX elemental microanalysis of the a) TiO /MWCNTs, c) NiO-CNTs, d) Fe O -CNTs, and e) MnO -
CNTs nanocomposites.
Figure 4. XRD spectra of MWCNT-COOH and TiO
Figure 5. Raman spectra for a) MWCNTs-COOH, b) TiO
MnO -CNTs nanocomposites
2
/MWCNT nanocomposites
2
/MWCNTs, c) Fe O -CNTs, d) NiO-CNTs and e)
3 4
2
3 4 2 2
Figure 6. FT-IR measurements of a) acid treated MWCNTs, b) Fe O -CNTs, c) TiO -CNTs, d) MnO -CNTs and e) NiO-
CNTs composites
Scheme 2. Possible mechanism for the Biginelli-type reaction
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8

Highlights
•
•
•
easy reaction condition, recyclable green catalyst, reduced environmental impacts
The catalyst is high activity, selectivity, reusability, reasonable cost, and long lifetime.
2
TiO -CNTs catalyst is significantly more effective than other acid catalysts in the Biginelli-type
reaction.
1
9