1,3-Dipolar cycloaddition reaction of aryl nitrile oxides with alkenes using imidazole and pyridine containing reusable polymeric base catalysts

Article abstract of DOI:10.1080/00397911.2017.1399207

Cross-linked poly-1-(4-vinylbenzyl)imidazole and cross-linked poly-4-vinylpyridine have been prepared from respective vinyl monomers using different quantities of divinylbenzene as a cross-linker by radical polymerization. The polymeric bases were charact

Full text of DOI:10.1080/00397911.2017.1399207

Synthetic Communications
An International Journal for Rapid Communication of Synthetic Organic
Chemistry
ISSN: 0039-7911 (Print) 1532-2432 (Online) Journal homepage: http://www.tandfonline.com/loi/lsyc20
1,3-Dipolar cycloaddition reaction of aryl nitrile
oxides with alkenes using imidazole and pyridine
containing reusable polymeric base catalysts
Nilesh N. Korgavkar & Shriniwas D. Samant
To cite this article: Nilesh N. Korgavkar & Shriniwas D. Samant (2018) 1,3-Dipolar
cycloaddition reaction of aryl nitrile oxides with alkenes using imidazole and pyridine
containing reusable polymeric base catalysts, Synthetic Communications, 48:4, 387-394, DOI:
10.1080/00397911.2017.1399207
To link to this article: https://doi.org/10.1080/00397911.2017.1399207
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SYNTHETIC COMMUNICATIONS®
2018, VOL. 48, NO. 4, 387–394
https://doi.org/10.1080/00397911.2017.1399207
1,3-Dipolar cycloaddition reaction of aryl nitrile oxides with
alkenes using imidazole and pyridine containing reusable
polymeric base catalysts
Nilesh N. Korgavkar and Shriniwas D. Samant
Department of Chemistry, Institute of Chemical Technology, Matunga, Mumbai, India
ABSTRACT
ARTICLE HISTORY
Received 10 August 2017
Cross-linked poly-1-(4-vinylbenzyl)imidazole and cross-linked poly-4-
vinylpyridine have been prepared from respective vinyl monomers
using different quantities of divinylbenzene as a cross-linker by radical
polymerization. The polymeric bases were characterized by FT-IR, TGA,
field emission-scanning electron microscopy (FE-SEM), and Brunauer-
Emmett-Teller (BET) surface area analysis. Their swelling behavior was
also studied. They were used as base catalysts for 1,3-dipolar
cycloaddition reaction of aryl nitrile oxides, generated in situ from
N-hydroxyimidoyl chloride, with N-phenylmaleimide and ethyl
acrylate, to obtain 3-arylisoxazolines. Both the bases gave excellent
results. These polymers were reusable, safe to use, and provided
good alternative for organic nonreusable bases like pyridine and
triethylamine which are hazardous.
KEYWORDS
1,3-Dipolar cycloaddition;
nitrile oxide; poly-4-
vinylpyridine; polymeric
base catalyst
GRAPHICAL ABSTRACT
Introduction
Isoxazoline unit is a part of many natural products. Many of its derivatives show broad
spectrum of biological activity.[1,2] This unit is also important in synthetic chemistry
as a precursor for many useful compounds.[3–5] One of the routes for the synthesis of
this heterocycle is the 1,3-dipolar cycloaddition reaction of nitrile oxides (1,3-dipole)
with alkenes (dipolarophiles).[6–8] Alkenes like 3-methylene oxindoles,[9] chalcones,[10]
α,β-unsaturated carbonyl compounds,[11,12] acrylates,[13] N-Boc-Δ3-pyrroline,[14]
N-phenylmaleimide,[15] allyl amines,[16,17] allyl alcohols,[18] styrenes,[19] and fullerenes[20,21]
CONTACT Shriniwas D. Samant
samantsd.ict@gmail.com
Department of Chemistry, Institute of Chemical
Technology, N.M. Parekh Road, Matunga, Mumbai 400 019, India.
Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/lsyc.
Supplemental data for this article can be accessed on the publisher’s website.
© 2018 Taylor & Francis

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N. N. KORGAVKAR AND S. D. SAMANT
have been used as dipolarophiles in this reaction. Unsaturated compounds like alkynes,
imines,[22] in situ generated benzyne,[23] and 1,3-dicarbonyl compounds in enol form[24]
also react with nitrile oxides.
Nitrile oxides are unstable and dimerize to form furoxans (1,2,5-oxadiazole-2-oxide)
and hence are usually prepared in situ. Dehydrohalogenation of N-hydroxyimidoyl chlor-
ide (chloroxime), in the presence of a base, is one of the commonly used methods for the in
situ generation of nitrile oxides. Nitrile oxides of aromatic nitriles are comparatively more
stable than those of aliphatic nitriles.[25] The steric shielding of the dipole and the electron
delocalization provide the stability. To avoid the dimerization, the concentration of in situ-
generated nitrile oxide is maintained minimum. In one such effort, a syringe pump has
been used for slow addition of N-hydroxyimidoyl chloride.[23] Heterogeneous mixtures
of inorganic bases with organic solvents have also been used.[14]
The reaction usually requires stoichiometric amount of a base. The bases used in the
reaction are not reusable. Further, the reaction requires 2–4 equiv. of hydroxyimidoyl
chloride, and there is a possibility of dimerization of the nitrile oxide. Taking these points
into consideration, there is a need to develop a protocol which would require lesser amount
of hydroxyimidoyl chloride and a recyclable weak base.
Organic polymeric amines are heterogeneous recyclable weak bases for organic reac-
tions. We have been working on poly-4-vinylpyridine (PVP) and cross-linked PVP and
demonstrated successful use of these bases in some reactions.[26–28] This prompted us to
prepare polymeric bases containing pyridine and imidazole and use them for in situ
generation of nitrile oxides from N-hydroxybenzimidoyl chlorides and subsequent reaction
of these nitrile oxides with N-phenylmaleimide and ethyl acrylate. Cross-linking is required
to make the polymer insoluble in organic solvents and thus makes the base reusable.
Hence, divinylbenzene (DVB) was used as a cross-linker. DVB cross-linked poly-1-
(4-vinylbenzyl)imidazole (CPVBIm) polymers are not reported. There are few reports on
imidazole-based polymers used as base catalysts[29,30] and in electrochemical studies.[31,32]
Cross-linked poly-4-vinylpyridine (CPVP) has been used as base catalyst in some
reactions.[33–36] These cross-linked polymers swell rapidly on interaction with certain
solvents, which increase the accessibility of the basic sites. Polymers swell differently in
different solvents. Hence we studied the gelling behavior of the polymers.
Results and discussion
Polymeric catalysts, CPVP-n and CPVBIm-n (n ¼ mol% of DVB) were prepared from
4-vinylpyridine and 1-(4-vinylbenzyl)imidazole, respectively, by radical polymerization
using AIBN as a radical initiator and DVB as a cross-linker. Both CPVP-n and
CPVBIm-n were amorphous white solids. The amount of cross-linker affected the bulk
density and swelling property of the polymers.
Characterization of polymers
The polymers were characterized as follows.
FT-IR
FT-IR spectra were recorded in ATR mode. FT-IR of CPVP showed characteristic bands at
1,598, 1,416, and 821 cm−1, while CPVBIm showed characteristic bands at 1,511, 1,100,

SYNTHETIC COMMUNICATIONS®
389
900, and 817 cm−1. The bands at 1,230, 1,076, and 1,030 cm−1 were common to both
(Fig. S1).
Thermal analysis
Thermogravimetric analysis (TGA) showed that CPVP-3, 5, and 7 and CPVBIm-3, 5, and 7 were
stable up to 350 °C and at higher temperatures the polymers decomposed. The amount of cross-
linker had no effect on the thermal stability. But, about 2–3% more mass loss was observed
around 100 °C with an increase in cross-linker amount. This increase in mass loss suggested
the presence of some water and solvent in the polymers having more amount of DVB. This
observation was in consonance with the increase in porosity of the material with an increase
in the amount of DVB. TGA graphs are given in the supporting information (Figs. S2, S3).
FE-SEM images
FE-SEM images of CPVP-5 (Fig. S4) showed clusters of spherical particles with average
diameter 2.5 µm. For CPVBIm-5, irregular shaped particles were observed (Fig. S5). The
images showed typical porous polymer morphology for CPVBIm-5.
Swelling behavior of the basic polymers
The materials were highly porous and showed good swelling property. The swelling helps
helps to increase the accessibility of the active sites to the substrate molecules. However, a
swollen polymer occupies more volume of the reaction vessel and thickens the reaction
mass. Thus, it requires more amount of solvent for efficient mixing. The swelling of
polymers was tested in various solvents which are commonly used in the synthesis and
polymerization. CPVP and CPVBIm polymers showed high swelling in methanol, ethanol,
chloroform and DMF. In DMSO, CPVP dispersed throughout the volume of the solvent
and it was difficult to separate it from the solvent. Thus, an increase in mass and volume
of CPVP-5 in DMSO could not be measured. Swelling of CPVBIm polymers was compara-
tively higher than that of CPVP (Fig. 1). Both the polymers floated on chloroform,
nitrobenzene, 1,2-dichloroethane, and chlorobenzene.
The two polymers were studied with respect to BET surface area analysis. Both the
polymers showed type-IV isotherm (Fig. S6). The hysteresis loop at relative pressure
0.5 <P/Po < 0.9 indicated the mesoporous nature of CPVBIm.[37] The BET surface areas
of CPVBIm-5 and CPVP-5 were 274.97 and 36.48 m2 g−1, respectively, and the pore
volumes of both the polymers were 0.323 and 0.046 cm3 g−1, respectively (Table S3). It is
reported that the surface area increases with an increase in the amount of DVB.[36] Thus,
the surface area for CPVP-5 was also in agreement with the previous reports.
1,3-Dipolar cycloaddition reaction of N-hydroxybenzimidoyl chloride and
N-phenyl-maleimide in the presence of CPVP and CPVBIm
=
We selected the reaction between N-phenylmaleimide chloride (R H, 1a) and
N-phenylmaleimide (2) as a model reaction (Scheme 1) and optimized the reaction
conditions. In this reaction, 1 undergoes in situ base-catalyzed elimination of HCl to form
aryl nitrile oxide.
Effect of base
The reaction was performed in ethanol, DMF, and acetonitrile using TEA as a base. Good
yield of 3a was obtained in all solvents. The results in ethanol were comparatively better.

390
N. N. KORGAVKAR AND S. D. SAMANT
Figure 1. Swelling study: (a) swelling of CPVP-5 (b) swelling of CPVBIm-5.
But, the poor solubility of 3a restricted the use of ethanol under heterogeneous condition.
Considering the ease in workup and recovery of solvent, we performed the reaction using
polymeric bases in acetonitrile (Table 1). The reaction took place smoothly at room
temperature and gave good yield of 3a within 1 h. For comparison, the reaction was also
performed using classical organic bases, TEA, pyridine, imidazole; when comparable
results were obtained.
There was no difference in the yield of 3a using CPVBIm-3, 5, and 7 (Table 1,
entries 1–3) and similar observation was made using CPVP-3, 5, and 7 (Table 1, entries
4–7). The elemental analysis showed the nitrogen content in the range of 11–12% for
Scheme 1. 1,3-Dipolar cycloaddition reaction of 1a–h with 2 in the presence of polymeric bases.

SYNTHETIC COMMUNICATIONS®
391
Table 1. Effect of catalyst on the reaction of 1a and 2.
Sr. No.
Catalyst (wt%)
Yield of 3aa (%)
1
2
CPVBIm-3b
CPVBIm-5b
CPVBIm-7b
CPVP-3b
80
80
78
74
75
75
85
78
85
80
3
4
5
CPVP-5b
6
CPVP-7b
7
TEAc
8
Pyridinec
Imidazolec
N–Me imidazolec
9
10
CPVBIm, cross-linked poly-1-(4-vinylbenzyl)imidazole; CPVP, cross-linked poly-4-vinylpyridine.
=
Reaction conditions: 1a (R H, 3.4 mmol), 2 (1.7 mmol), solvent—acetonitrile (10 mL), rt (27–30 °C),
reaction time—1 h.
aIsolated yield calculated w.r.t. 2.
bPolymeric base 100 wt% w. r. t. 2.
c1.7 mmol of homogeneous base.
CPVBIm-3, 5, and 7 and in the range of 10–11% for CPVP-3, 5, and 7. As there was no
difference, the 5% cross-linked polymers were selected for further studies.
Effect of solvent and molar ratio of 1a and 2
The effect of solvent was studied using CPVBIm-5 as a base catalyst in ethanol, methanol,
acetonitrile, ethyl acetate, THF, DMF, and chloroform. Acetonitrile (80%) and DMF (78%)
were the best solvents. The 2:1 molar ratio of 1a to 2 (yield ¼ 80%) was selected; increasing
the amount of 1a did not increase the yield (Table S4).
Effect of catalyst loading
The catalyst was increased from 50 to 150 wt% w.r.t N-phenylmaleimide (2). 120 wt% of
catalyst was found to be satisfactory (Table S5). The results are also comparable to
homogeneous bases under identical conditions (Table 1, entries 7–10). However, polymeric
bases are safe to handle, free of odor, and recyclable.
Reaction of 1a–h with N-phenylmaleimide (2)
To study the scope of the new protocol, the reaction of substituted N-hydroxyimidoyl
chlorides (1a–h) with N-phenylmaleimide was performed (Table 2).
Table 2. The reaction of 1a–h with 2.
Yield of 3a–ha (%)
Sr. No
Reactant 1 ‘R’
With CPVBIm-5
With CPVP-5
1
2
3
4
5
6
7
8
H
85
80
82
78
74
85
68
68
72
72
77
70
65
78
58
60
2-Cl
4-Me
4-Cl
4-OMe
4-NO2
3-Br
3-NO2
Reaction conditions: 1a–h (3.4 mmol), 2 (1.7 mmol), solvent—acetonitrile (10 mL), CPVBIm-5 and
CPVP-5 (120 wt% w.r.t 2), rt (27–30 °C), reaction time 1 h.
aIsolated yield, calculated w.r.t. 2.

392
N. N. KORGAVKAR AND S. D. SAMANT
Scheme 2. Reaction of 1a–h with ethyl acrylate (4).
Table 3. Reaction of 1a–h with 4.
Sr. No
Reactant 1 “R”
Yield of 5a–ha (%)
1
2
3
4
5
6
7
8
H
74
80
82
78
74
85
68
68
2-Cl
4-Me
4-Cl
4-OMe
4-NO2
3-Br
3-NO2
Reaction conditions: 1a–h (3.4 mmol), 4 (1.7 mmol), solvent—acetonitrile (10 mL), CPVBIm-5
(120 wt% w.r.t. 4), rt (27–30 °C), reaction time 6 h.
aIsolated yield, calculated w.r.t. 4.
1
The structures of 3a–h were confirmed by FT-IR and H NMR spectroscopy, and the
data were compared with the reported characterization data of same compounds for
verification.[38–42] The 3a–h were obtained as the racemic mixtures.
Reaction of 1a–h with ethyl acrylate (4)
The reaction of nitrile oxides with ethyl acrylate (Scheme 2) is known to give 5-substituted
isoxazolines (5).[13] We performed the reactions of 1a–h with ethyl acrylate (4) using
CPVBIm-5 in acetonitrile (Table 3). This reaction was slower as compared to the reaction
of 1a with N-phenylmaleimide.
1
The structures for 5a–h were confirmed by FT-IR and H NMR spectroscopy, and data
were also compared with the previous literature.[40,42–45] The 5a–h were obtained as the
racemic mixtures. Spectral data for all the compounds (3a–h and 5a–h) have been given
in the Supporting information.
Recyclability study
The recyclability of CPVBIm-5 was studied for both the reactions. The spent catalyst was
washed with aqueous saturated sodium bicarbonate solution and then with acetonitrile
before being used in the next cycle. The recyclability was studied up to five cycles and there
was no significant decrease in the yield of the product (Figs. S7, S8). Only decrease of about
4–5% in the isolated yield after the fifth recycle was observed. Some basic sites on surface
may be blocked with CO2 or water, or some basic sites reacted with HCl in reaction
medium (acetonitrile) and are difficult to regenerate.
Experimental
Starting materials were purchased from Sigma-Aldrich, Loba Chemie, and Alfa Acer
and used as such. TGA was performed on PerkinElmer STA 6000 with heating rate

SYNTHETIC COMMUNICATIONS®
393
20 °C min−1. FE-SEM analysis was performed using Tescan MIRA 3 model. Nitrogen
adsorption–desorption isotherms were recorded on BET-PMI sorptometer (porous
material integrated Pvt. Ltd., USA). Samples were degassed at 150 °C for 5 h before
analysis. FT-IR spectra were recorded on a PerkinElmer Spectrum Two spectrophotometer.
1H NMR spectra were recorded on Agilent 400 MHz NMR spectrometer. Elemental
analysis (C, H, N) was performed on vario MICRO select elemental analyzer. Experimental
procedures for the preparation of CPVP-n and CPVBIm-n have given the Supporting
information.
1,3-Dipolar cycloaddition reaction of N-hydroxybenzimidoyl chloride (1a) and
N-phenylmaleimide (2)
N-hydroxybenzimidoyl chloride (3.4 mmol) and N-phenylmaleimide (1.7 mmol) were
added to acetonitrile (10.0 mL) in a round-bottom flask. The polymeric base (0.3 g) was
added to it. The mixture was stirred at 27–30 °C for 1 h. The base was separated from
the reaction mixture by filtration. Acetonitrile was removed under reduced pressure at
50 °C. Ethanol (3.0 mL) was added to the reddish brown oily residue and was allowed to
stand at ambient temperature (27–30 °C) for 20–30 min. A white solid product was
precipitated, which was isolated by filtration, and dried at 80 °C.
The same procedure was followed with ethyl acrylate. The reaction time in this case was
6 h. The products were purified by column chromatography with silica as a stationary
phase and ethyl acetate (0–5% v/v): PET ether as a mobile phase.
Conclusion
Cross-linked basic polymers were prepared by the polymerization of 4-vinylpyridine
(4-VP) and 1-(4-vinylbenzyl)imidazole using 3, 5, and 7 mol% of DVB as a cross-linker.
The resulting polymers CPVP-n and CPVBIm-n were mesoporous and excellent
heterogeneous catalysts for in situ generation of aryl nitrile oxides from N-hydroxyimidoyl
chlorides and their 1,3-dipolar cycloaddition reaction with N-phenylmaleimide and with
ethyl acrylate. CPVBIm-5 showed excellent results compared to CPVP-5. The much high
activity of CPVBIm-5 can be explained on the basis of high basicity of imidazole than
pyridine, and much high surface area and porosity of CPVBIm-5. These parameters should
be kept in mind for further developments of related catalysts. The main advantages of this
protocol were replacement of homogeneous, pungent smelling, hazardous organic bases
with recyclable polymeric bases.
Acknowledgment
Author Nilesh N. Korgavkar is thankful to the University Grant Commission (UGC), New Delhi,
India for UGC/SAP/DRS fellowship.
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