A straightforward one-pot synthesis of bioactive: N -aryl oxazolidin-2-ones via a highly efficient Fe3O4@SiO2-supported acetate-based butylimidazolium ionic liquid nanocatalyst under metal- and solvent-free conditions

Article abstract of DOI:10.1039/c7gc01414g

In the present study, we report the fabrication and characterization of novel acetate-based butylimidazolium ionic liquid immobilized silica-coated magnetic nanoparticles (IL-OAc@FSMNP). The synthesized nanocomposite proves its supremacy as an environmentally benign catalyst in the reaction of aniline and its derivatives with ethylene carbonate to form bioactive N-aryl oxazolidin-2-ones under metal-, ligand-, and solvent-free conditions. The catalyst offers excellent assemblies of hydrogen-bond donors and acceptors, which activate the substrates, thereby delivering good-to-excellent product yields with a conversion and selectivity of more than 99%. Additionally, mild reaction conditions, wide substrate scope, effortless catalytic recovery and recyclability of the catalyst up to eight consecutive cycles offer the potential for scale-up in various pharmaceutical applications.

Full text of DOI:10.1039/c7gc01414g

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ARTICLE
Straightforward one-pot synthesis of bioactive N-aryl oxazolidin-
2-ones via highly efficient Fe3O4@SiO2-supported acetate-based
butylimidazolium ionic liquid nanocatalyst under metal- and
solvent-free conditions
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Radhika Gupta, Manavi Yadav, Rashmi Gaur, Gunjan Arora and Rakesh Kumar Sharma*
www.rsc.org/
In the present study, we report the fabrication and characterization of novel acetate-based butylimidazolium ionic liquid
immobilized silica-coated magnetic nanoparticles (IL-OAc@FSMNP). The synthesized nanocomposite proves its supremacy
as an environmentally benign catalyst in the reaction of aniline and its derivatives with ethylene carbonate to form
bioactive N-aryl oxazolidin-2-ones under metal-, ligand-, and solvent-free conditions. The catalyst offers excellent
assemblies of hydrogen-bond donors and acceptors which activate the substrates, thereby delivering good-to-excellent
product yields with conversion and selectivity of more than 99%. Additionally, mild reaction conditions, wide substrate
scope, effortless catalytic recovery and recyclability of the catalyst upto eight consecutive cycles offer the potential for
scale-up
in
various
pharmaceutical
applications.
Introduction
Antimicrobial resistance (AMR) is
a
relentlessly growing
concern in modern medicine rendering even the newest
generation of antibiotics ineffective towards the treatment of
numerous microbial infections.1 N-substituted oxazolidin-2-
ones have shown considerable promise against Gram-positive
bacterial
pathogens
such
as
methicillin-resistant
Staphylococcus aureas (MRSA) and vancomycin-resistant
Enterococci (VRE) (Fig. 1a).2-4 They are important structural
motifs of many bioactive compounds (neuroleptic agents,5
anti-tubercular agents,6 blood thinners (Fig. 1b),3 antiblastic
agents in chemotherapy (Fig. 1c)3 and inhibitors of mono
amine oxidase A,7, tyrosine kinase,9 HIV 1 protease10) and
agrochemical drugs11.
8
Fig. 1 Chemical structures of bioactive N-aryl oxazolidin-2-ones.
During the past few years, numerous synthetic methods and
catalysts have been developed for the formation of N-aryl
oxazolidin-2-ones. Various palladium-catalyzed cross coupling
reactions of oxazolidones with aryl halides and tosylated
pyridines have been reported.12-14 Nevertheless, none of the
methods were environmentally benign because of the
inevitable use of toxic and unstable phosphine ligands.
Therefore, many CuI/ligand/base catalyzed synthetic methods
and C, N-chelated organotin (IV) compounds have been
established.15-17 Other conventional strategies include reaction
of N-2-hydroxyalkylanilines with phosgene in the presence of
pyridine18 and cycloaddition reaction of isocyanates with
epoxides19-21. Although, significant conversion and selectivity
was observed, yet, employment of non-green carbonyl
precursors raises key environmental concern. To solve these
issues, many alternative synthetic approaches have been well-
documented that involve insertion of carbon dioxide into
aziridine moiety. However, use of such approaches requires
drastic conditions such as high CO2 pressure, long reaction
times, high catalytic loading and are associated with
undesirable side reactions.22 Besides this, all the above
mentioned reactions rely heavily on the use of metal
complexes as catalysts. Consequently, they suffer from
multiple competitive disadvantages such as high cost, reservoir
exhaustion, metal contamination in the product stream and
generation of toxic waste caused by metal catalyst residues. In
current scenario, metal-free organic reactions have become
contemporary challenge for chemists. On these grounds,
recently, Seo and Chung have reported metal-free conversion
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of epoxides, amines and carbon dioxide to 3-aryl-2- fabrication
Journal Name
and
characterization
of
acetate-based
oxazolidinones using K3PO4 as a catalyst under ambient butylimidazolium ionic liquid immobilized silica-coated
conditions, but it incorporates non-green DMF as a solvent.23 magnetic nanoparticles as novel nanocatalyst for the synthesis
Therefore, there is still an increasing need to establish metal-, of N-aryl oxazolidin-2-ones. The newly fabricated nanocatalyst
ligand-, and solvent-free sustainable synthetic protocols.24, 25
exhibits remarkable catalytic activity under solvent-, ligand-,
In this regard, catalysis through explicit hydrogen bonding and metal-free conditions. To the best of our knowledge, this
interactions offers another course of action to metal-catalyzed is the first report wherein a solid supported ionic liquid has
reactions and to accomplish this, ionic liquids (ILs) have been been used as a heterogeneous catalyst for the synthesis of a
explored since past few years due to their designable wide range of N-aryl oxazolidin-2-ones.
structures and tunable properties such as low volatility, high
ionic conductivity, good thermal and chemical stability, simple
functionality and ability to form plentiful hydrogen bonds.26, 27
The cooperative effects of their cation-anion simultaneously
activate the starting materials by hydrogen-bond interaction
between substrates and catalysts which enhance reaction
rates, catalytic activity and selectivity.28-30 Therefore, taking
into account their increased importance, various ILs have been
explored as green alternative catalytic species for the synthesis
of N-aryl oxazolidin-2-ones such as imidazolium ILs,30 aza-
crown ether complex cation ILs,31 binary ILs,32 1,8-
Results and discussion
Preparation of IL-OAc@FSMNP
The catalyst was prepared as per Scheme 1. Magnetite
nanoparticles (MNP) were synthesized using co-precipitation
technique.70 The Fe3O4 nanoparticles were coated with a layer
of silica by sol-gel approach to form silica-coated MNPs
(SMNP).71 Further, the SMNPs were functionalized using 3-
chloropropyltrimethoxysilane (CPTMS) to get functionalized
SMNPs (FSMNP). These were reacted with N-butylimidazole to
quaternize the imidazole nitrogen so as to obtain chloride-
based FSMNPs (IL-Cl@FSMNP). The obtained nanoparticles
were finally treated with excess sodium acetate to achieve
acetate-based butylimidazolium ionic liquid immobilized on
FSMNPs (IL-OAc@FSMNP).
Diazabicyclo[5.4.0]undec-7-ene
(DBU)
-derived
ILs,33
[Bmim]OAc IL,34, 35 amine functionalized ILs,36 and cross-linked
polymerized ILs37. Although, good-to-excellent yields were
achieved, yet the overall processes remain unacceptable from
both environmental and practical viewpoint as they suffer
from some major drawbacks such as generation of large
amount of waste ILs, slow diffusion of substrates due to high
viscosity of ILs, product purification and tedious energy
consuming protocols for recycling pure ILs.38 Also, some ILs
undergo decomposition when desiccated at high temperatures
during recycling processes.32
O
Si
O
O
O
O
Si
Cl
O
(
)
O
O
O
Si
Cl
Ethanol
NH4OH
60 oC, 6 h
Dry toluene
reflux, 24 h
N2 atm
O
To address these issues, immobilization of ILs has been proven
MNP
SMNP
FSMNP
to be an effective approach for improving the catalytic
39
This
efficiency by facile recovery and recycling.38,
N
N
Dry acetonitrile
reflux, 24 h
N2 atm
immobilization procedure transfers the desired catalytic
properties of ILs on solid supports and thus combines the
advantages of ILs with those of heterogeneous solid supports.
Among other preferred carrier materials,39-52 silica-coated
magnetic nanoparticles (SMNPs) have emerged as powerful
catalytic support due to their exceptional properties such as
O
O
NaOAc
O
Si
N
O
Si
N
N
N
Cl
O
OAc
Ethanol
40 oC, 24 h
O
IL-OAc@FSMNP
IL-Cl@FSMNP
(
= acetate)
(
excellent
thermal
stability,
easy
synthesis
and
= chloride)
functionalization, high surface area to volume ratio, low
toxicity and effortless separation from the reaction media
through magnetic forces.53-58 Besides screening the magnetic
interactions, the silica coating on bare magnetic nanoparticles,
offers numerous other advantages such as ease of
functionalization, high binding strength between active species
and magnetic support, good dispersion of catalytic species in
reaction media and provides resistance towards organic
solvents.59, 60
Scheme 1 Schematic illustration for the formation of IL-OAc@FSMNP catalyst.
Characterization of IL-OAc@FSMNP
Fourier Transform-Infrared spectroscopy
Thus, in continuation of our research work on the
development of silica-based organic-inorganic hybrid
nanomaterials and their application as catalysts in various
organic transformations,53,
59, 61-69
we herein describe the
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Fourier transform-infrared spectroscopy was used to
investigate the successful immobilization of various
functionalities at every step of catalyst preparation. In Fig. 2,
all absorption bands at 3450-3200 cm−1 correspond to O-H
stretching mode. The presence of Fe3O4 can be identified by
the appearance of a band near 570 cm-1 which is attributed to
Fe-O vibration of bulk Fe3O4. This band shifts towards higher
wavenumber and splits into two peaks when the size is
reduced to nanoscale dimension. In Fig. 2a, the peaks at 623
and 578 cm-1 justify the formation of Fe3O4 nanoparticles.72, 73
The immobilization of silane polymer on MNP surface was
confirmed by the appearance of peaks at 469, 802, 954 and
1098 cm-1 which are assigned to the Si-O-Si bending, Si-O
bending, Si-OH stretching and Si-O-Si stretching respectively
(Fig. 2b).74 The relatively weak bands at 2925 and 2854 cm-1
are ascribed to the two C-H stretching bands, representing the
asymmetric and symmetric stretching modes of the methylene
group (Fig. 2c).75 The peak at 1648 cm-1 is due to C=C and C=N
vibrations of the imidazolium moiety of the ionic liquid (Fig.
2d).76, 77 Finally, the exchange of chloride with acetate ions is
confirmed by the appearance of two sharp and intense peaks
at 1564 and 1419 cm-1 (Fig. 2e).
Fig. 3 XRD pattern of (a) MNP, (b) SMNP, and (c) IL-OAc@FSMNP.
peaks were detected in SMNPs (Fig. 3b) and in the final
catalyst IL-OAc@FSMNP (Fig. 3c), which indicates retention of
the crystalline magnetite core structure throughout the
grafting procedure. Other than this, a weak broad hump at 2θ
= 20º-25º was observed in SMNP and in the final catalyst which
justifies the formation of amorphous silica coating around
Fe3O4 nanoparticles. Besides, no other peaks were detected in
the rest of the XRD pattern, depicting that no crystallinity
changes occurred during the catalyst formation. In order to
estimate the mean crystallite size of the catalyst from the XRD
pattern, Scherrer equation (Dhkl= kλ/βhkl cosθ) was used where
Dhkl is the crystallite size in the direction perpendicular to the
lattice plane, hkl are the Miller indices of the plane being
analysed, k is a dimensionless crystallite-shape factor with a
typical value of 0.89 for spherical particles, λ is wavelength of
the radiation (0.15418 nm for Cu Κ-alpha), βhkl is the full line
width at half the maximum intensity (FWHM) in radians and θ
is the Bragg diffraction angle in degrees. Here, the diffraction
peaks at various 2θ values were chosen and hence the
corresponding FWHM and crystallite size was recorded (Table
S1, ESI†). As observed, the size of the catalyst is found to be in
the nanometer range of 17.04-22.52 nm.
Fig. 2 FT-IR spectra of (a) MNP, (b) SMNP, (c) FSMNP, (d) IL-Cl@FSMNP, and (e) IL-
OAc@FSMNP.
X-ray diffraction studies
Field emission-scanning electron microscopic analysis
To determine the crystallographic structure and chemical
composition of the so formed nanocomposites, XRD analysis of
MNP, SMNP and IL-OAc@FSMNP was carried out (Fig. 3). The
position and relative intensities of all the peaks in the XRD
pattern of bare MNP was found consistent with the standard
XRD pattern of cubic inverse spinel structure of magnetite
(Joint Committee on Powder Diffraction Standards (JCPDS)
card no. 19-0629). Six characteristic Bragg’s diffraction peaks
To study the changes in morphology of the nanoparticles, FE-
SEM analysis was done at various steps of catalyst preparation.
Fig. 4a shows narrow size distribution and uniform spherical
morphology of the as-synthesized magnetite nanoparticles.
Upon addition of TEOS, surface of the nanoparticles became
slightly rough (Fig. 4b) due to the coating of amorphous silica
on MNPs. The FE-SEM image of the final catalyst (Fig. 4c) also
shows nearly spherical nanoparticles, keeping the
morphological properties of SMNPs intact.
were observed at 2θ = 30.23º, 35.59º, 43.21º, 53.68º, 56.80º
and 62.81 which attribute to their corresponding indices (2 2
º
0), (3 1 1), (4 0 0), (4 2 2), (5 1 1) and (4 4 0) (Fig. 3a). Similar
Transmission electron microscopy analysis
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Journal Name
(b)
(a)
(c)
200 nm
100 nm
100 nm
Fig. 4 FE-SEM images of (a) MNP, (b) SMNP, and (c) IL-OAc@FSMNP.
(a)
(b)
(c)
Silica coating
~6-7 nm
50 nm
Fig. 5 TEM images of (a) MNP, (b) SMNP, and (c) IL-OAc@FSMNP.
To determine the size and morphology of the synthesized The thermostability of the ionic liquid functionalized SMNPs
nanocomposites, MNP, SMNP and IL-OAc@FSMNP were was investigated by using TGA. The TGA curve of the catalyst
subjected to transmission electron microscopy (TEM). As shows noteworthy weight loss in three steps (Fig. 6). The initial
shown in Fig. 5a, Fe3O4 nanoparticles displayed nearly weight loss (of about 7%) from room temperature to 300 ºC is
spherical shape with the average particle size of 8-9 nm (Fig. due to the removal of adsorbed water molecules and organic
S1a, ESI†). Fig. 5b shows the TEM image of SMNP with dark solvents which were used during the preparation of the
MNP core coated by grey silica shell of ~6-7 nm thick. Besides catalyst. An additional weight loss from 300 to 520 ºC is due to
screening the magnetic interactions in bare MNPs, this type of the decomposition of butylimidazolium and other organic
silica coating provides grafting sites for the immobilization of groups. Since these functionalities are present as ionic liquid
IL. As can be seen from Fig. 5c, the size and morphology of coating on SMNPs, the catalyst decomposes after 520 ºC.
SMNPs did not change significantly upon anchoring of CPTMS Further, decomposition of silica modified MNPs occurs very
and acetate-based butylimidazolium IL in the final catalyst. little at 780 ºC due to the ability of silica to withstand high
This suggests the existence of a thin monolayer of IL onto the temperatures.79
silica shell.78 Also, the size distribution diagram of catalyst
shows that the average particle size of IL-OAc@FSMNP is in Elemental analysis
the range of 22-24 nm (Fig. S1c, ESI†) which is similar to that of The SEM-coupled energy dispersive X-ray analysis (EDAX)
SMNP (20-22 nm) (Fig. S1b, ESI†). These results are found to confirmed the presence of the expected elements in the final
be in good accord with XRD results.
catalyst, such as Fe, Si, C, N and O. (Fig. S2, ESI†)
Thermo gravimetric analysis
Vibrating sample magnetometric analysis
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capable of providing solvent-less reaction conditions.80 To
begin the investigation, some control experiments were
performed using aniline and ethylene carbonate as model
substrates (Fig. 8). Results showed 69% of aniline conversion
even in the absence of any catalyst. Although various products
were identified using GC-MS including mono- and bis-N-
(hydroxyl)ethyl
derivatives
of
aniline
(62%),
N-
100
90
80
70
60
50
40
30
20
10
0
Fig. 6 TGA of IL-OAc@FSMNP catalyst.
The magnetic properties of the synthesized nanoparticles were
investigated using vibrating sample magnetometer with a field
from -5000 Oe to 5000 Oe at room temperature (Fig. 7). It can
be seen that the saturation magnetization value (Ms)
decreases from MNP (66.27 emu/g) to SMNP (27.71 emu/g) to
Fig.
8 Control experiments showing the formation of 3-phenyl-2-oxazolidinone.
Reaction conditions: aniline (2 mmol), ethylene carbonate (10 mmol), catalyst (15 mg),
IL-OAc@FSMNP (19.98 emu/g) which can be attributed to the 140 ºC, 14 h.
grafting of diamagnetic silica and other organic matter on
Fe3O4. However, the Ms value of the catalyst (IL-OAc@FSMNP)
phenylmorpholine (5%) and other side-products (2%),
however, no evidence for 3-phenyl-2-oxazolidinone was found
which showed that presence of catalyst is very important for
its formation. When MNP, SMNP and FSMNP were added as
catalysts,
a
mixture of desired product 3-phenyl-2-
oxazolidinone (15-18%) along with above mentioned products
were obtained (67-69%). With IL-Cl@FSMNP, although the
conversion percentage reached to 94%, yet the selectivity for
3-phenyl-2-oxazolidinone was only improved to 31%. When
chloride ions were exchanged with acetate in IL-Cl@FSMNP,
both the conversion and selectivity reached to their maximum
levels which showed that acetate-based catalyst (IL-
OAc@FSMNP) was more efficient than chloride-based for the
selective formation of 3-phenyl-2-oxazolidinones. These
results are supported by previous reports according to which
acetate ions activate the substrates by hydrogen-bond
formation due to its higher hydrogen-bond basicity than
chloride ions and is thus responsible for exhibiting better
catalytic activity.34
Fig. 7 Magnetization curves of (a) MNP, (b) SMNP, and (c) IL-OAc@FSMNP.
is sufficient to ensure its facile recovery from the reaction
media via external magnetic forces. Also, no hysteresis loop,
coercivity and remanence were observed in any of the curves
which confirm the superparamagnetic nature of synthesized
nanoparticles.
To get the maximum yield of the desired product, various
reaction parameters such as catalyst loading, temperature,
time, and substrate molar ratio were optimized.
Catalyst loading
Effect of amount of catalyst on the conversion of aniline and
the yield of 3-phenyl-2-oxazolidinone is shown in Fig. 9. It can
be seen that with respect to the uncatalyzed process, the yield
of 3-phenyl-2-oxazolidinone was significantly larger even with
the addition of small quantities of catalyst. Aniline was
detected in trace amounts (3%) with just 6 mg of the catalyst.
This unusual catalytic enhancement gives strong indication
that the cation and anion present in the IL-OAc@FSMNP
strongly activate the substrates. With increasing amount of the
Catalytic activity test
For the synthesis of N-aryl-2-oxazolidinones, a variety of
reactants have already been explored. Many of the recently
reported synthetic methods have used ethylene oxide as the
starting material. However, it is considered as a highly toxic,
flammable and carcinogenic compound. In the present study,
ethylene carbonate is used as an alternative to its parent
oxide. This offers numerous practical benefits such as non-
toxicity, non- flammability and it is a low-melting solid, thus, is
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Journal Name
100
90
80
70
60
50
40
30
20
10
0
100
80
Aniline
60
3-phenyl-2-
40
oxazolidinone
20
Other products
0
1
2
4
6
9
12
14
0
2
4
6
8
10
12
14
16
Time/h
Amount of catalyst/mg
3-phenyl-2-oxazolidinone
Other products
Fig. 11 Effect of reaction time. Reaction conditions: aniline (2 mmol), ethylene
carbonate (10 mmol), IL-OAc@FSMNP (10 mg), 120 ºC.
Fig. 9 Effect of amount of catalyst. Reaction conditions: aniline (2 mmol), ethylene
carbonate (10 mmol), 140 ºC, 14 h.
catalyst, the yield of the product also increases. Reaction gave carbonate. Herein, we describe direct synthesis for the
best results when 10 mg of the catalyst was used.
formation of 3-aryl-2-oxazolidinones, thus avoiding additional
steps for the synthesis and purification of 2-
(arylamino)ethanol.32
Reaction temperature
Further experiments were continued with 10 mg of the
catalyst and here, the temperature was varied (Fig. 10). When
Substrate ratio
Substrate ratio also plays an important role in determining the
yield of a compound in a given reaction. It was observed that
the higher the amount of ethylene carbonate in the reaction
mixture, the higher the yield of desired product obtained (Fig.
12). The observation holds in accordance with the previous
reports which suggests that stronger H-bond interaction
between cyclic carbonate and 2-H of imidazolium ion occurs
when excess amount of cyclic carbonate is used.35 Therefore,
as per the below graph, 4:1 (ethylene carbonate: aniline) was
found to be the optimum substrate ratio for the reaction.
100
90
80
70
60
50
40
30
20
10
0
25
40
80
100
120
140
Reaction temperature/ºC
100
95
90
85
80
75
70
65
60
Fig. 10 Effect of reaction temperature. Reaction conditions: aniline (2 mmol), ethylene
carbonate (10 mmol), IL-OAc@FSMNP (10 mg), 14 h.
the temperature was lowered to ambient conditions (25 and
40
º
temperature from 80 to 120
C), no conversion was obtained. With increase in
C, the conversion of aniline
º
increases to 100% and the reaction proceeded in favour of the
product. Best results were obtained when the reaction was
1:1
2:1
3:1
4:1
5:1
carried out at 120 ºC and thus was used in further studies.
Ethylene carbonate : aniline mmol ratio
Reaction time
Fig. 12 Effect of substrate ratio. Reaction conditions: IL-OAc@FSMNP (10 mg), 120 ºC,
At 120 ºC, going from 1 to 12 h reaction times, a substantial
12 h.
increase in product yield was observed (15-100%) (Fig. 11). The
yield of other products also decreased smoothly and finally
disappeared from the mixture after 9 h. Within just 6 h, more
than 50% of the desired product was detected. As the reaction
proceeded further, aniline was efficiently transformed to 3-
phenyl-2-oxazolidinone and 100% yield was obtained after 12
h. It is worth mentioning here that Gao et al.34 have performed
the synthesis of 3-aryl-2-oxazolidinones in 1 h, but they utilized
2-(arylamino)ethanol as a starting material which is a possible
intermediate of the reaction of aryl amines and ethylene
Substrate scope
Based on the results of abovementioned optimization studies,
the developed catalyst was utilized to synthesize different 3-
aryl-2-oxazolidinones (Table 1). A mixture of aniline (2 mmol),
ethylene carbonate (8 mmol) and IL-OAc@FSMNP (10 mg) was
set to react at 120 ºC for 12 h and the products were
confirmed by GC-MS (Spectra in ESI†). Aniline and substituted
anilines were easily converted into the corresponding
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oxazolidinones upon treatment with ethylene carbonate in the methoxy also gave excellent yield of >98% of the
presence of catalytic amount of IL-OAc@FSMNP. Aniline gave corresponding N-aryl-2-oxazolidinone as sole product. Even
100% 3-phenyl-2-oxazolidinone. Anilines bearing electron the dichloro- and dimethoxy- substituted anilines gave
donating groups such as chlorine, bromine, methyl and
Table 1 Scope of catalytic activity of IL-OAc@FSMNP in the synthesis of N-aryl-2-oxazolidinones.a
Entry
1.
Substituted aniline
Product
Conversionb (%)
Selectivityb (%)
100
100
2.
3.
4.
5.
6.
7.
100
100
100
100
100
100
>99
>99
>99
98
>99
>99
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Journal Name
8.
100
90
9.
100
95
10.
90
95
11.
12.
13.
14.
80
87
80
85
84
90
86
88
aReaction conditions: substituted aniline (2 mmol), ethylene carbonate (8 mmol), IL-OAc@FSMNP (10 mg), 120 ºC, 12 h. bConversion and selectivity were determined
by GC-MS.
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Scheme 2 Possible reaction mechanism.
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Table 2 Comparison of the catalytic efficiency of IL-OAc@FSMNP nanocatalyst with other previously reported catalysts for the synthesis of N-phenyl-2-oxazolidinones.
Reaction
conditions
Product
Yield (%)
S. No.
1.
Reactants
Catalyst
Ref.
Pd2(dba)3:xantphos: NaOBut =
0.05:0.05:1.4
120 ºC, 16 h,
toluene
13
63
LCNPh2SnCl (10 mol%),
[LCNSn(OC(O)CF3)]2 (µ-O)(µ-
OC(O)CF3)2 (5 mol%)
90 ºC, 17h
100,
91
83
2.
110 ºC, 72h,
K3PO4 in
DMF/dioxane (1:9)
CuI (10 mol%) + 1,1,1-Tris
(hydroxymethyl) ethane (10 mol%)
16
15
84
3.
4.
5.
92
95
99
CuI (0.1 mmol) + 2-
isobutyrylcyclohexanone (0.2
mmol)
80 ºC, 20h
acetonitrile,
Cs2CO3,
82 ºC, 24h, DMF,
Cs2CO3, molecular
sieves
Cu2O (5 mol %) + Chxn-Py-Al (20
mol %)
30
34
6.
7.
[Bmim]BF4 (0.2 mmol)
[Bmim]OAc (0.1 mmol)
130 ºC, 9h
95
130 ºC, 1h, N2
atmosphere
100
DBU (0.4 mmol) + HDBUBr (0.1
mmol)
130 ºC, 1h, (pCO2 =
33
32
85
8.
9.
93
94
99
2.5 MPa)
[Bmim]Br (0.2 mmol) + [Bmim]OAc 140 ºC, 9h, (pCO2 =
(0.2 mmol)
2.5 MPa)
Acetonitrile, 45
min, (pCO2 =
1atm)
DBU/n-Bu3P/DBAD
(0.1/2.1/2.1eq.)
10.
37
11.
12.
P[VBIM] [AA]- 10%
[Bmim]OAc (0.5 mmol)
IL-OAc@FSMNP (10 mg)
140 ºC, 9h
100 ºC, 3h
120 ºC, 12h
99
99
35
Present
work
13*.
100
*Heterogeneous catalyst.
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Materials and reagents
100
80
All reagents were commercially available and used without
further purification. Ferric sulfate hydrate and ferrous sulfate
heptahydrate were purchased from Central Drug House and
Thomas Baker respectively. TEOS (tetraethyl orthosilicate),
60
CPTMS
((3-chloropropyl)trimethoxysilane)
and
1-
40
butylimidazole were procured from Sigma Aldrich. Anhydrous
sodium acetate, anhydrous sodium sulfate, ethyl acetate,
ethanol and liquid ammonia (25%) were obtained from Merck.
All other organic substrates were of analytical grade and
acquired from Alfa Aesar and Merck. Double deionized water
was used throughout the study.
20
0
1
2
3
4
5
6
7
8
Reaction runs
The catalyst was analyzed through various techniques. Fourier
transform infrared spectra were recorded on PerkinElmer
Spectrum 2000 using KBr pallet method with a scanning range
of 4000-400 cm-1. The presence of ionic liquid was further
supported by SEM-coupled Ametek EDAX system. To
Fig. 13 Reusability of IL-OAc@FSMNP. Reaction conditions: aniline (2 mmol), ethylene
carbonate (8 mmol), IL-OAc@FSMNP (10 mg), 120 ºC, 12 h.
reused catalyst undergoes no morphological changes even
aꢀer
8 runs (Fig. S3c and S3d, ESI†). Moreover, the
determine
nanocomopsites, X-ray diffractograms were obtained using
the
crystallinity
of
the
as-prepared
magnetization value of the recovered IL-OAc@FSMNP was
sufficient for its effective separation from the reaction medium
using external magnet (Fig. S3b, ESI†). Altogether, the catalyst
fulfils all desirable features for various large scale industrial
applications.
Rigaku Cu (Kα) diffractometer with a scan rate of 4 degree min-
1
and 2θ range of 15-70 degree. Field emission scanning
electron microscopy (FE-SEM) images were collected on a
Tescan MIRA3 FE-SEM microscope. To prepare the samples,
double sided carbon tape was applied on metal stubs. Further,
the sample was placed onto the tape and coated with gold to
make electrical contact using JEOL JEC-3000 FC auto fine
coater gold sputtering machine. Transmission electron
microscopy (TEM) images were acquired using FEI TECHNAI G2
T20 microscope operated at 200 kV by dispersing the
nanoparticles in ethanol and casting them on a copper grid
coated with an amorphous carbon film. ImageJ software was
used for particle size determination. The catalyst thermal
stability was assessed using PerkinElmer Pyris diamond
TGA/DTA by heating the sample under N2 atmosphere from
Conclusions
In this work, we have developed novel acetate-based
butylimidazolium ionic liquid immobilized SMNPs. The
resulting nanocomposite showed its efficiency as a stable
catalyst for the formation of a diverse range of industrially and
pharmaceutically important
N-aryl-2-oxazolidinones with
excellent conversion and selectivity percentage. Some of the
green features include metal-, ligand- and solvent-free
conditions, use of non-toxic precursors and formation of water
as a sole by-product. The system was also found to be
economical when compared to the other earlier reported
catalysts in terms of superior reaction conditions, lesser
reaction time, low catalytic amount and broad substrate
scope. Besides, the catalyst can be easily recovered from the
reaction media using an external magnet and it possess
excellent recyclability for eight consecutive runs without
appreciable loss in catalytic efficiency. In conclusion, this work
may provide a simple, effective and sustainable catalytic
room temperature to 1000 ºC at a heating rate of 10 °C/min
and gas flow of 200 mL/min. Magnetization measurements
were carried out using vibrating sample magnetometer (VSM,
model 7304, Lakeshore Cryotronics Inc., USA). The synthesized
organic products were characterized by interpretation of
fragmentation pattern of the mass spectra using literature and
library data of Agilent gas chromatograph (6850 GC) having a
HP-5MS capillary column (stationary phase: 5% phenyl methyl
siloxane; column length: 30 m; internal diameter: 0.25 mm;
Film thickness: 0.25 µm) and a quadrupole mass filter
equipped 5975C mass selective detector (MSD) using helium
system to obtain bioactive N-aryl-2-oxazolidinones.
as
characterized by H (400 MHz) and 13C NMR (100 MHz) using
JEOL JNM-EXCP 400. Spectra are reported in units downfield
a carrier gas. All new organic compounds were
1
Experimental section
δ
from tetramethylsilane (internal standard) and data has been
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written as: chemical shift (multiplicity, coupling constant,
integration).
General reaction procedure for the synthesis of N-aryl
oxazolidin-2-ones
Catalyst preparation
For the N-arylation reaction, a 10 mL reaction vessel was
charged with aniline derivative and ethylene carbonate along
with the synthesized catalyst. The vessel was then heated to
the desired temperature and the mixture was kept stirring
using a magnetic stirrer for the required time. The reaction
was continuously monitored using thin layer chromatography.
After completion of reaction, the catalyst was effortlessly
separated from the reaction mixture using an external magnet.
For the analysis of product formed, the mixture was first
extracted with ethyl acetate, dried over anhydrous sodium
sulfate and further characterized using GC-MS.
Synthesis of silica-coated magnetic nanoparticles (SMNPs)
Iron oxide magnetic nanoparticles (MNPs) were prepared
according to the reported co-precipitation procedure. For this,
6.0 g of ferric sulphate hydrate and 4.2 g of ferrous sulfate
heptahydrate was dissolved in 250 mL of double distilled water
and stirred at 60 ºC till the solution became yellowish-orange
in color. Furthermore, 15 mL of ammonia solution (25%) was
added with vigorous stirring to adjust the pH around 10. The
color of the solution changed instantaneously to black and
stirring was continued for another half an hour. The MNPs so
precipitated were separated magnetically, washed several
times with deionized water and ethanol to make the resulting
solution neutral and finally dried in a vacuum oven. The
coating of silica layer on the surface of bare MNPs was
achieved by the sol-gel method. For this, 0.5 g of MNPs were
activated in 2.2 mL of 0.1 M HCl. To this, 200 mL of ethanol
and 50 mL of H2O was added and the mixture was kept under
sonication. Next, 5 mL of ammonia solution (25%) and 1 mL of
TEOS was added drop wise to the well dispersed MNPs
solution at room temperature. Then, the solution was stirred
Acknowledgements
One of the authors, Radhika Gupta, gratefully acknowledges
University Grant Commission, Delhi, India for awarding Junior
Research Fellowship. Also, the authors thank USIC (University
of Delhi), Department of Physics and Astrophysics (University
of Delhi) and TERI (Gurgaon, India) for providing
instrumentation facilities.
at 60 ºC for 6 h. The resulting SMNPs were collected by an
external magnet, washed with water and ethanol until the Notes and references
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Graphical Abstract
Straightforward one-pot synthesis of bioactive N-aryl oxazolidin-2-ones via highly efficient
Fe3O4@SiO2-supported acetate-based butylimidazolium ionic liquid nanocatalyst under metal- and
solvent-free conditions
Radhika Gupta, Manavi Yadav, Rashmi Gaur, Gunjan Arora and Rakesh Kumar Sharma*
Green Chemistry Network Centre, Department of Chemistry, University of Delhi, Delhi- 110007, India
rksharmagreenchem@hotmail.com
A supported ionic liquid-based magnetic nanocatalyst has been fabricated for the formation of
pharmaceutically important N-aryl oxazolidin-2-ones.