Nanostructured N doped TiO2efficient stable catalyst for Kabachnik-Fields reaction under microwave irradiation

Article abstract of DOI:10.1039/d0ra04533k

Herein, we report nitrogen-doped TiO2 (N-TiO2) solid-acid nanocatalysts with heterogeneous structure employed for the solvent-free synthesis of α-aminophosphonates through Kabachnik-Fields reaction. N-TiO2 were synthesized by direct amination using trieth

Full text of DOI:10.1039/d0ra04533k

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Nanostructured N doped TiO efficient stable
2
catalyst for Kabachnik–Fields reaction under
Cite this: RSC Adv., 2020, 10, 26997
microwave irradiation†
Sachin P. Kunde, Kaluram G. Kanade,d *ac Bhausaheb K. Karale,
ab
a
a
a
a
Hemant N. Akolkar, Sudhir S. Arbuj,
Mubarak H. Shaikh and Aniruddha K. Kulkarni
Pratibha V. Randhavane, Santosh T. Shinde,
a
e
Herein, we report nitrogen-doped TiO
employed for the solvent-free synthesis of a-aminophosphonates through Kabachnik–Fields reaction.
N-TiO were synthesized by direct amination using triethylamine as a source of nitrogen at low
temperature and optimized by varying the volume ratios of TiCl , methanol, water, and triethylamine,
under identical conditions. An X-ray diffraction (XRD) study showed the formation of a rutile phase and
the crystalline size is 10 nm. The nanostructural features of N-TiO were examined by HR-TEM analysis,
2 2
(N-TiO ) solid-acid nanocatalysts with heterogeneous structure
2
4
2
which showed they had rod-like morphology with a diameter of ꢀ7 to 10 nm. Diffuse reflectance
spectra show the extended absorbance in the visible region with a narrowing in the band gap of 2.85 eV,
and the high resolution XPS spectrum of the N 1s region confirmed successful doping of N in the TiO
lattice. More significantly, we found that as-synthesized N-TiO showed significantly higher catalytic
activity than commercially available TiO for the synthesis of a novel series of a-amino phosphonates via
2
2
Received 21st May 2020
Accepted 7th July 2020
2
Kabachnik–Fields reaction under microwave irradiation conditions. The improved catalytic activity is due
to the presence of strong and Bronsted acid sites on a porous nanorod surface. This work signifies N-
DOI: 10.1039/d0ra04533k
rsc.li/rsc-advances
2
TiO is an efficient stable catalyst for the synthesis of a-aminophosphonate derivatives.
immense interest in synthetic organic chemistry. It has been
demonstrated that on incorporation of heterocycles such as
In recent years, organophosphorus compounds have received thiophene, benzothiazoles, thiadiazoles, and pyrazole into
1
Introduction
9
10
11
12
much attention due to their widespread applications in the a-aminophosponates scaffold, the resulting compounds
1,2
medicinal and agriculture industries. a-Aminophosphonates exhibited interesting biological activities. Pyrazole derivatives of
are one such biological important framework that are structural a-aminophosponates have been rarely reported in the litera-
13,14
mimics of amino acids. For example, glyphosate (N-(phospho- ture,
thus synthesis of novel pyrazole derivatives of a-ami-
nomethyl)glycine) is extensively utilized in agriculture as nophosponates is important to research.
a systemic herbicide and Alafosfalin is used as an antibacterial
Although several protocols for the synthesis of a-amino-
3
agent (Fig. 1). The bioactivity of these molecules such as anti- phosponates are reported, one of the most important is the
4
5
6
15,16
microbial, antioxidant, anti-inammatory, enzyme inhibi- Kabachnik–Fields reaction.
This involves a one-pot three-
7
8
tors and antibacterial is one of the reasons for them to be of component coupling of a carbonyl compound, an amine and
alkylphosphite. These protocols has been accomplished in
1
7
18
presence of a variety of catalyst such as TiCl , CuI, hex-
4
a
PG and Research Centre, Radhabai Kale Mahila Mahavidyalaya, Ahmednagar, 414
19
20
anesulphonic sodium salt,
triuoroacetic acid (TFA),
In(OTf) , BiCl , Cu(OTf)2, SbCl /Al O , InCl3, LiClO4,
0
01 India. E-mail: kgkanade@yahoo.co.in
21
22
23
24
25
26
3
3
3
2 3
b
PG and Research Centre, Mahatma Phule Arts, Science and Commerce College,
Panvel, 410 206, India
27
28
29
30
ZrOCl
2
,
TsCl, Mg(ClO
4
)
2
,
and Na
2
CaP
2
O
in presence or
c
PG and Research Centre, Yashavantrao Chavan Institute of Science, Satara, 415 001
India
d
Centre for Materials for Electronics Technology (C-MET), Department of Electronics
and Information Technology (DeitY), Government of India, Panchavati, Off Pashan
Road, Pune-411 008, India
e
Dr. John Barnabas School for Biological Studies, Department of Chemistry,
Ahmednagar College, Ahmednagar-414 001, India
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/d0ra04533k
Fig. 1 Some biological active a-aminophosphonate.
1
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the ability of oxidizing and reducing ability under suitable
irradiation makes promising greener alternative approach
towards important organic transformations compared to other
expensive, toxic, transition metal oxides. Moreover, the phase
composition and the degree of crystallinity of the titania sample
Scheme
carbaldehyde.
1
Synthesis of 1-phenyl-5-(thiophen-2-yl)-1H-pyrrole-3-
8
plays an important role in catalytic activity. In the past several
organic transformations such as oxidation of primary alco-
3
7
38
39
hols, synthesis of xanthenes, Friedel–Cras alkylation,
40
Beckmann rearrangement efficiently utilizes TiO
2
as a hetero-
geneous reusable catalyst. In the literature several reports have
been debated to inuence nitrogen doping on photocatalytic
activity of nanocrystalline TiO2. However, the effect is unre-
vealed for catalytic applications in organic synthesis. Recently,
Hosseini-Sarvari explored the use of commercial TiO
2
in the
synthesis of a-aminophosponates via Kabachnik–Fields
41
reactions.
In present investigation, we have prepared nanostructured N
doped TiO and also investigation emphasis was given on the
2
synthesis of a series of a novel diethyl(1-phenyl-3-(thiophen-2-
yl)-1H-pyrazol-4-yl)(phenylamin) methylphosphonates under
microwave irradiation.
2
Fig. 2 X-ray diffraction patterns of (a) TN0 (TiO ), (b) TN1, (c) TN2 (d)
TN3 (e) TN4.
2
Experimental sections
2.1 Synthesis of N doped TiO
2
nanorods
The nanostructured N-TiO were synthesized by previously re-
2
42,43
even in the absence of a solvent. However, most of these existing ported method with some modication.
In a typical proce-
procedures are sluggish, require long reaction times, use of dure, 0.5 mL of titanium tetrachloride (TiCl ) was added in
4
strong acidic conditions, give unsatisfactory yields and also absolute methanol (25 mL) with constant stirring at room
suffer from the formation of many side products. Moreover, in temperature. To this solution requisite quantity a 0.1–2 M
all alternatives microwave reaction proved to be a kind of aqueous triethylamine solution is injected rapidly. The result-
31
promising medium for such reaction.
ing solution was reuxed for 24 h with constant stirring. The
In the last few years, the application of transition metal white precipitate formed was collected and washed with ethanol
oxides gained particular interest as a heterogeneous catalyst for several times followed by centrifugation (10 000 rpm for 20
32
various organic synthesis. Among all transition metal oxides min). The precipitate was dried at 473 K for 24 h. To control the
the use of nanocrystalline titania (TiO ) has been grown exten- nal morphologies of samples, the sample were synthesized as
2
sively owing to their outstanding physiochemical properties, function of volume ratio of TiCl , methanol, water, and trie-
4
3
3
which furnished their wide applications in sensors,
thylamine. The sample prepared in volume ratio 1 : 10 : 50 : 0,
34
35
36
pigments, photovoltaic cells, and catalysis. Also, the use of 1 : 10 : 50 : 1, 1 : 10 : 50 : 2, 2 : 10 : 50 : 2, and 2 : 10 : 50 : 4
potential titania catalyst attracted in organic synthesis due to its were denoted as TN0 (pure TiO ), TN1, TN2, TN3 and TN4
2
environmental compatibility, inexpensive, safe, stable, reusable respectively.
and earth-abundant. It has been proven the desired property of
2
TiO was attained by fullling requirements in terms of unique
2.2 Synthesis of 1-phenyl-5-(thiophen-2-yl)-1H-pyrrole-3-
morphology, high crystallinity and mixed-phase composition,
carbaldehyde
1-Phenyl-5-(thiophene-2-yl)-1H-pyrrole-3-carbaldehyde were ob-
tained via the Vilsmeier–Haack reaction of the appropriate
phenylhydrazones, derived from the reaction of 2-acetyl thio-
phene with phenylhydrazine (Scheme 1).
Table
1 Phase composition and crystallite size of as-prepared
44
samples from analysis of XRD
Crystallite size
(nm)
2
.3 Synthesis of diethyl(1-phenyl-3-(thiophene-2-yl)-1H-
Sample
Rutile
Anatase
pyrazole-4-yl)(phenylamino)methylphosphonates
TN0
TN1
TN2
TN3
TN4
100
98
94
95
91
0
2
6
5
9
25
19
16
12
9
In a typical procedure, the pyrazolealdehyde 1 (1 mmol), aniline
2
(1 mmol), triethyl phosphite 3 (1.1 mmol) and N-TiO2
(12 mol%) were taken in a round bottom ask equipped with
a condenser and subjected to microwave irradiation for (10–15
min) using 420 W (RAGA's Microwave system) (Scheme 3). The
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Fig. 5 Nitrogen (N
TiO ), (b) TN2 (N-TiO
sponding pore size distributions.
2
) adsorption–desorption isotherms of (a) TN0
(
2
2 2
), (c) TN4 (N-TiO ). Insets shows their corre-
accelerating voltage of 300 kV. The Brunauer–Emmett–Teller
BET) surface area of nanocatalysts was examined using the
Fig. 3 HR-TEM images of (a–c) TN0, (d–f) TN1, and (d–f) TN2; inset c,
f and h SAED pattern of TN0, TN2 and TN3 respectively.
(
Quantachrome v 11.02 nitrogen instrument. The optical prop-
erties of the powder samples were studied using UV-vis diffuse
reectance absorption spectra (UV-DRS) were recorded on the
Perkin-Elmer Lambada-950 spectrophotometer in the wave-
length range of 200–800 nm. Powder samples were used for XPS
measurements. The XPS measurements of powdered samples
were carried out on a VG Microtech ESCA3000 instrument.
Fourier transform infrared (FTIR) spectra of prepared samples
were recorded on a Shimadzu Affinity 1-S spectrophotometer in
progress of the reaction was monitored by TLC. Aer the reac-
tion was completed, the reaction mixture extracted using ethyl
acetate and insoluble catalyst separated by ltration. The crude
product was puried by silica gel column chromatography
using n-hexane/ethyl acetate as eluent. The product structure
1
was determined by FTIR, H NMR, and LS-MS.
ꢂ1
1
over a range of 400–4000 cm . H NMR was recorded in DMSO-
d solvent on a Bruker Advance-400 spectrometer with tetra-
6
2.4 Samples characterization
methylsilane (TMS) as an internal reference.
The phase purity and crystallinity were examined by X-ray
diffraction (XRD) technique (Advance, Bruker AXS D8) using
˚
Cu Ka1 (1.5406 A) radiation with scanning 2q range from 20 to
ꢁ
3 Results and discussions
80 . For FETEM analysis samples were prepared by evaporating
dilute solution on carbon-coated grids. FE-TEM measurements 3.1 Structural study
were carried using the JEOL SS2200 instrument operated at an
Nanostructured TiO2 and N doped TiO were synthesized by
2
a simple reuxing method. The phase purity and phase
formation of as-synthesized material were analysed by powder
X-ray diffraction pattern. Fig. 2 compares powder XRD patterns
of TiO
2
and N doped TiO
2
samples. The peak position and peak
powder can be indexed into rutile
intensity of the pure TiO
2
phases (Fig. 2). Further, it is observed that an increase in the
amount N-dopant (triethylamine) the intensity of the diffraction
Table 2 BET specific surface area and pore size distribution of TiO
2
and N-TiO
2
Surface area
Pore volume
Pore radius
(A)
2
ꢂ1
3
ꢂ1
˚
Sample
(m g
)
(cm g
)
TN0
TN2
21.956
40.359
53.589
0.051
0.215
0.101
18.108
30.811
18.041
Fig. 4 HR-TEM images of (a–c) TN3 and (d–f) TN4; inset c, and f SAED TN4
pattern of TN3, and TN4 respectively.
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Fig. 6 UV-DRS spectra of (a) TN0 (TiO
2
), (b) TN1 (c) TN2 (d) TN3 (N-
and N-TiO samples.
2
TiO ), (e) TN4. Insets shows Tauc plot of TiO
2
2
Fig. 8 (a and b) High resolution spectrum of N 1s region (c) high
peaks of the rutile phase decreases, while that of anatase phase resolution spectrum of Ti 2p region (d) high resolution spectrum of O
1
s region.
increases, indicating that the fraction of the anatase phase
gradually increases at the expense of the rutile phase during this
condition (sample TN2–TN4). The phase composition of rutile
the incorporation of nitrogen in the TiO crystal structure. The
and anatase phase of TiO evaluated from the peak intensity
2
2
crystallite size is calculated from each (1 1 0) peak in the XRD
using the following equation,
39
pattern using the Sherrer formula. The average crystalline size
are 25, 19, 16, 12 and 9 nm for TN0, TN1, TN2, TN3 and TN4
respectively (Table 1). From, XRD analysis it is clear that with an
increase in the concentration of nitrogen in TiO , fraction of
where f is the fraction of the anatase phase, and I and I are anatase increases phase and crystalline size decreases.
1
f
A
¼
K ¼ 0 : 79; f
A
. 0:2; K1=40 : 68; f
A
# 0:2
,
1
I
R
A
1
þ
K I
2
A
A
R
the intensities of the anatase (1 0 1) and rutile (1 1 0) diffraction
peaks, respectively. The higher molar concentration of trie-
thylamine is favourable for the transformation from rutile to
3
.2 Surface and morphological study
45,46
Transmission electron microscopy (TEM) and high-resolution
transmission electron microscopy (HRTEM) analysis were per-
formed to study morphology and crystallinity of as-synthesized
anatase.
2
Therefore, the phase composition of TiO samples,
i.e. the fraction of anatase and rutile, can be facilely controlled
through adjusting the concentration of triethylamine. The
slight shi of rutile (1 1 0) diffraction peaks towards a higher
angle with an increase in the amount of N dopant suggesting
pure and N doped TiO materials (Fig. 3). The pure TiO (TN0)
2
2
sample seems owerlike nanostructures (Fig. 3a). At high-
resolution it reveals that each ower microstructure consist-
ing several nanorods. The length of nanorods are in the range of
50–70 nm and diameter is about 10–15 nm (Fig. 3b). Fig. 3c
shows the lattice fringes of the material with interplanar
spacing d spacing 0.33 nm matches well (1 0 0) plane of rutile
TiO . Fig. 3c inset shows a selected area diffraction pattern in
2
which bright spots observed that conrm the TiO nanorods are
2
in nanocrystalline nature. It was observed that addition of N
dopant, resulting sample TN1 and TN2 grows into new super-
structure consisting nanorods of length 30–50 nm and spheres
2 2
Fig. 7 FTIR spectra of (a) TN0 pure (TiO ), (b) TN1 (N-TiO ), (c) TN2, (d)
TN3 and (e) TN4.
Scheme 2 Standard model reaction.
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a
Table 3 Comparative study of catalysts used for the synthesis of a- Table 4 Optimization of the concentration of catalyst
aminophosphonate
a
Concentration of catalyst
(mol%)
b
b
Entry
Catalyst
Time (minutes)
Yield (%) Sr. no.
Yield (%)
1
2
3
4
5
6
7
8
9
—
20
20
15
15
10
10
10
10
10
Trace
30
20
30
72
73
76
85
95
1
2
3
4
5
3
6
9
12
15
69
76
86
95
95
Acetic acid
Commercial ZnO
Commercial TiO
TN0
TN1
TN2
TN3
TN4
2
a
Reaction condition: aldehyde (1a) (1 mmol), aniline (1 mmol),
triethylphosphite (1.1 mmol), N-TiO
2
catalyst, MW power 420 watt.
b
Isolated yield.
a
Reaction condition: aldehyde(1a) (1 mmol), aniline (1 mmol),
b
triethylphosphite (1.1 mmol), catalyst, MW power 420 watt. Isolated
yield.
completion of the rst monolayer. The BET surface area of pure
2
ꢂ1
TiO is found to be 21.956 m g . The pore size distribution of
2
prepared samples was investigated by Barrett–Joyner–Halenda
(
BJH) method Fig. 5(a)–(c) insets. The average pore diameter of
pure TiO nanoparticles is 18 nm which demonstrates the
material is mesoporous nature. Further, it is observed that the
incorporation of nitrogen in TiO nanoparticles the surface area
of diameter 20–30 nm, particles size is obviously smaller than
2
TN0 (Fig. 3d and h). HRTEM results are consistent with XRD
˚
results. The d-spacing is about 0.325 A between adjacent lattice
2
planes of the N doped TiO
2
.
shis towards higher values. The adsorption–desorption
It was revealed that with doubling concentration of TiCl
4
,
isotherms of nitrogen-doped TiO samples display the type II
2
sample TN3 and TN4 were grown into very ne agglomerated
nanorods (Fig. 4). Further, it is observed that these nanorods
having size in length 30–40 nm and diameter is around 7–10 nm
46
isotherm according to IUPAC classication. The specic BET
2
ꢂ1
surface area of samples TN
5
2
and TN
3.589 m g respectively (Fig. 5b and 4c). This observation
species a decrease in the particle size of TiO nanoparticles
4
are 40.359 m g and
2
ꢂ1
which is lower than pure TiO . Fig. 4f inset shows selected area
2
2
diffraction pattern shows, surprisingly, ring-like pattern unlike
specic surface area increases which are in consisting of XRD
and TEM results. The Brunauer–Emmett–Teller (BET) specic
surface areas, pore volumes and mean pore and mean pore
diameters of samples TN0, TN2, and TN4 are summarized in
Table 2.
TiO
2 2
, indicates N-TiO nanorods are in polycrystalline nature.
From HR-TEM results it is concluded that increase in concen-
tration of TiCl
nanorods.
4
and triethylamine reduces the size of the
The specic surface area of as-prepared samples was studied
by (N ) nitrogen gas adsorption–desorption measurement at 77
2
K using the Brunauer–Emmett–Teller (BET) method. The N2
3
.3 Optical and electronic property studies
adsorption–desorption isotherm of N-TiO
2
nanoparticles is
The optical property of the as-synthesized material was analyzed
by UV-Vis diffuse absorbance spectra as shown in Fig. 6. Fig. 6
shown in Fig. 5. The pure TiO shows type IV isotherm
2
47
according to IUPAC classication, which are typical charac-
teristics of a material with pore size in the range of 1.5–100 nm
displays the comparative UV-DRS spectra of pristine TiO
a series of N doped TiO samples. The absorption edge for the
pure TiO (TN0) is observed at around 410 nm (Fig. 6a), which is
consistent with the band gap of the rutile phase. The N doped
TiO nanostructures show strong absorption in the visible
2
and
2
3
Fig. 5a. The shape of the hysteresis loop is H type may asso-
ciates due to the agglomeration of nanoparticles forming slit-
2
45
like pores, reected in TEM images. At higher relative pres-
0
2
sure (p/p ) the slope shows increased uptake of adsorbate as
region (410–600 nm). The redshi clearly indicates the
pores become lled; inection point typically occurs near
a
Table 5 Screening of solvents
b
Entry
Solvent
Yield (%)
1
2
3
4
6
7
Ethanol
85
87
55
58
60
95
Methanol
Dichloromethane
THF
Toluene
Neat
Fig. 9 (A) Progress of reaction (a) TN0 (b) TN1 (c) TN2 (d) TN3 and (e)
TN4. (B) Reusability of catalyst TN4; reaction condition: aldehyde (1a)
a
Reaction condition: aldehyde (1a) (1 mmol), aniline (1 mmol),
triethylphosphite (1.1 mmol), N-TiO
watt. Isolated yield.
2
catalyst, solvent, MW power 420
b
(
1 mmol), aniline (2a) (1 mmol), triethylphosphite 3 (1.1 mmol), N-TiO
2
(
12 mol%), MW power 420 watt.
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2
successful doping of N in the lattice of TiO . Moreover, as the
concentration of triethylamine increases redshi of N-TiO also
2
increases which conrms higher nitrogen doping and a higher
fraction of absorption of photons from the visible region. The
band gap of as-synthesized material calculated by using the
Tauc plot shown in Fig. 6 (insets). The band gap (E
sample TN0, TN1, TN2, TN3, and TN4, were observed to 3.15,
.09, 3.07, 3.03 and 2.85 eV respectively. The decrease in the
g
) for the
3
Fig. 10 XRD of sample TN4 (a) before reaction (b) after reaction.
band gap is attributed to higher mixing of the (O/N) 2p level is
developed in the Ti-3d level falls at the top of the VB, therefore,
band gap reduced compared to the pristine TiO nanostructure.
2
gave better results than acetic acid, commercial ZnO and
commercial TiO
Inspiring these results, we further studied the progress of
2
.
3.4 FT-IR spectroscopy
Fig. 7 shows comparative FTIR spectra for pure and N doped reaction at different time intervals, we observed the sample N-
ꢂ1
TiO . The absorption peak signal in the range of 400–1100 cm
2
doped TiO catalyzes efficiently than undoped TiO2, and this
2
is characteristic of the formation of O–Ti–O lattice. The may be attributed to the higher surface area (Fig. 9A).
ꢂ1
ꢂ1
ꢂ1
ꢂ1
absorption at 668 cm , 601 cm , 546 cm and 419 cm
The optimum concentration of the catalyst was investigated
48,49
corresponds to Ti–O vibrations.
TN1–TN3 the IR bands centred at 1400–1435 cm indicates such as 3, 6, 9, 12 and 15 mol%. The reaction yielded in 69, 76,
Further, for the sample by performing the model reaction at different concentrations
ꢂ1
nitrogen doping in the TiO
2
sample. The band located at 86, 95 and 95% yields respectively (Table 4). This shows that
is adequate for the reaction by considering the
ꢂ1
1
070 cm is attributed to Ti–N bond vibrations. Also, it is 12 mol% of TN
4
ꢂ1
observed that the band at 1335 cm for pure TiO is shied yield of the product.
2
ꢂ1
towards longer wavenumber 1430 cm supports for the claim
To evaluate the effect of solvents, different solvents such as
of N doping in TiO
of the minor the peaks of pure TiO
2
lattice. Further it is also observed that some ethanol, methanol, dichloromethane, THF, 1,4-dioxane and
are rather different than the toluene were used for the model reaction in presence of N-TiO
2
2
N-doped TiO
TiO
2
, this indicates the incorporation of nitrogen in catalyst. The reaction proceed with better yield in polar protic
lattices. The peak centered at 1600–2180 cm is ascribed solvent (Table 5, entries 1, 2). However it was observed that the
ꢂ1
2
due to –OH stretching frequency. From, IR spectra it is clear usage of solvents slows down the rate of reaction and gives the
that N is successfully incorporated in the lattice of TiO2.
desired product in lower yields than that for neat condition
(Table 5, entries 1–6).
3.5 X-ray photoelectron spectroscopy
The recyclability of the catalyst was then examined and the
outcomes are shown in Fig. 9B. Aer the completion of reaction,
the reaction mixture was extracted with ethyl acetate. The
residual catalyst was washed with acetone, dried under vacuum
at 100 C and reused for consequent reactions. The recovered
catalyst could be used for 5 times without obvious loss of
catalytic activity.
The difference between the XRD of fresh catalyst and reused
catalyst shown in Fig. 10.
The usefulness of optimized reaction condition for model
reaction (12 mmol % of catalyst, solvent-free, MWI) was
extended for the synthesis of a series of novel a-amino-
phosphonates (4a–l) by reacting pyrazoldhyde (1a–c), anilines
The XPS were used for chemical identication and electronic
state of dopant nitrogen in sample TN2 and TN4. The high
resolution XPS spectra of N 1s on deconvolution shows two
different peaks at 399.6 and 401.5 eV indicates nitrogen present
in two different electronic state (Fig. 8a and b). The peak at
ꢁ
3
99.6 is attributed to presence of interstitial N or N–Ti–O
43
linkage. The result is consistent with previous reports. The
peak at 401.5 is attributed to presence of N in oxidized state as
NO or NO
and TN4 are 2.8% and 3.4% respectively. Fig. 8c shows the peak
at 458.8 and 458.3 is attributed to Ti 2p3/2 and Ti 2p2, in good
2
. The concentration of nitrogen on surface of TN2
1
agreement the presence of Ti(IV) in TiO . The peak at binding
2
(2a–d) and triethylphosphite (3) in excellent yields (Scheme 3).
energy 530.1 and 530.2 eV of sample are attributed to O 1s
(Fig. 8d).
3.6 Catalytic study in synthesis of a-aminophosponates
In order to nd out the best experimental condition, the reac-
tion of pyrazolaldhyde 1a, aniline 2a and triethylphospite 3
under microwave irradiation is considered as standard model
reaction (Scheme 2).
In the absence of a catalyst, the standard model reaction
gave a small amount of product (Table 3 entry 1). These results
specify catalyst is required to occur reaction. In order to check
the catalytic utility, the model reaction carried out in the pres-
Scheme 3 Optimized reaction condition for synthesis of diethyl(1-
phenyl-3-(thiophen-2-yl)-1H-pyrazol-4-yl)(phenylamino)
ence of a variety of catalysts (Table 3 entry 2–9). The N-TiO
2
NRs methylphosphonates
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Table 6 Microwave assisted synthesis of novel diethyl(1-phenyl-3- Table 6 (Contd.)
a
(
thiophen-2-yl)-1H-pyrazol-4-yl)(phenylamino)methylphosphonates
ꢁ
b
Entry
Product
M.P. ( C)
Yield (%)
ꢁ
b
Entry
Product
M.P. ( C)
Yield (%)
4
a
218
220
95
79
4i
195
81
4
b
4
4
j
120
210
190
76
89
75
4
4
4
c
208
216
180
92
79
86
k
d
e
4
l
a
Reaction condition: aldehyde (1 mmol), aniline (1 mmol),
triethylphosphite (1.1 mmol), N-TiO
Isolated yield.
2
(12 mol%), MW power 420 watt.
b
4
f
200
82
The obtained product 4a–l was characterized by spectroscopic
techniques (Table 6).
The spectroscopic data of synthesized compounds are given
in ESI (S-2 to S-26).†
4
4
g
162
190
85
71
4
Conclusions
In summary, we have prepared N doped TiO2 nanorods by
thermal hydrolysis method using triethylamine as the source of
nitrogen at relatively low temperatures. The XRD analysis
showed that with varying composition molar ratios of TiCl4,
h
3 2 2 5 3
CH OH, H O, and (C H ) N, phase composition of rutile to
anatase also tunes. FTIR spectra show the chemical environ-
ment of doping by the formation of the N–Ti–O and Ti–O–Ti
bond. The morphological study performed by the FE-TEM
technique shows the formation of well-developed nanorods of
size in length 30–40 nm and diameter is around 7–10 nm, which
2 2
is lower than pure TiO . Further, BET analysis N-TiO shows the
2
ꢂ1
maximum specic surface area 53.4 m g which is 2.5 times
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higher than pure TiO
2
. The as-synthesized materials were 19 K. S. Niralwad, B. B. Shingate and M. S. Shingare, Ultrason.
employed for the synthesis of a-aminophosphonates via
Kabachnik–Fields reaction under microwave irradiation. The N- 20 T. Akiyama, M. Sanada and K. Fuchibe, Synlett, 2003, 10,
TiO shows remarkable catalytic activity for aminophosphonate 1463–1464.
Sonochem., 2010, 17(5), 760–763.
2
derivatives compared with TiO
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Conflicts of interest
2
There are no conicts to declare.
24 K. S. Ambica, S. C. Taneja, M. S. Hundal and K. K. Kapoor,
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5 B. C. Ranu, A. Hajraa and U. Jana, Org. Lett., 1999, 1, 1141–
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SPK is gratefully acknowledged UGC New Delhi for the award of 26 N. Azizi, F. Rajabi and M. R. Saidi, Tetrahedron Lett., 2004,
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