Polymer-anchored [Fe(III)Azo] complex: An efficient reusable catalyst for oxidative bromination and multi-components reaction for the synthesis of spiropiperidine derivatives

Article abstract of DOI:10.1016/j.jorganchem.2018.01.001

A heterogeneous catalyst was prepared by attaching Fe(III) into organically modified chloromethylated polytstyrene and characterized by AAS, IR, PXRD, TG-DTA, UV–Vis, and SEM studies. In presence of H₂O₂ and KBr as bromine source, the catalyst showed remarkably high conversion with para-selectivity towards the oxidative bromination reaction in acetic acid medium. The catalyst was also very active and highly efficient for the production of spiropiperidine derivatives through multi-component reaction in isopropyl alcohol at room temperature. The catalyst was not leached during the catalytic reactions, moreover, after five cycles the catalytic activity and selectivity of the catalyst were not decreased very significantly.

Full text of DOI:10.1016/j.jorganchem.2018.01.001

Accepted Manuscript
Polymer-anchored [Fe(III)Azo] complex: An efficient reusable catalyst for oxidative
bromination and multi-components reaction for the synthesis of spiropiperidine
derivatives
Resmin Khatun, Surajit Biswas, Swarbhanu Ghosh, Sk Manirul Islam
PII:
S0022-328X(18)30001-9
10.1016/j.jorganchem.2018.01.001
JOM 20239
DOI:
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 30 July 2017
Revised Date: 30 December 2017
Accepted Date: 2 January 2018
Please cite this article as: R. Khatun, S. Biswas, S. Ghosh, S.M. Islam, Polymer-anchored [Fe(III)Azo]
complex: An efficient reusable catalyst for oxidative bromination and multi-components reaction for
the synthesis of spiropiperidine derivatives, Journal of Organometallic Chemistry (2018), doi: 10.1016/
j.jorganchem.2018.01.001.
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ACCEPTED MANUSCRIPT
Polymer-anchored [Fe(III)Azo] complex: An efficient reusable catalyst for
oxidative bromination and multi-components reaction for the synthesis of
spiropiperidine derivatives
Resmin Khatun, Surajit Biswas, Swarbhanu Ghosh, Sk. Manirul Islam*
Department of Chemistry, University of Kalyani, Kalyani, Nadia, 741235, W.B., India.
ABSTRACT: A heterogeneous catalyst was prepared by attaching Fe(III) into organically
modified chloromethylated polytstyrene and characterized by AAS, IR, PXRD, TG-DTA,
UV-Vis, and SEM studies. In presence of H2O2 and KBr as bromine source, the catalyst
showed remarkably high conversion with para-selectivity towards the oxidative bromination
reaction in acetic acid medium. The catalyst was also very active and highly efficient for the
production of spiropiperidine derivatives through multi-component reaction in isopropyl
alcohol at room temperature. The catalyst was not leached during the catalytic reactions,
moreover, after five cycles the catalytic activity and selectivity of the catalyst were not
decreased very significantly.
Keywords: Heterogeneous Catalyst, polymer support, oxidative bromination, spiropiperidine
derivatives.
________________________________________________________________
Department of Chemistry, University of Kalyani, Kalyani, Nadia, 741235, W.B., India.
Prof. Sk Manirul Islam, Fax: +91-33-2582-8282, E-mail: manir65@rediffmail.com

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1.
Introduction
During several years, appreciable attention has been given to the improvement of the
green environmental influences of the chemical process in the industry [1]. The
heterogeneous catalysts can execute a pivotal role in the development of greener
technologies. It is well accepted that heterogeneous catalysts can show high catalytic activity,
can be separated very easily from the product mixture, stability and selectivity is higher in
comparison to the unsupported metal catalysts [2] and the industrial needs of recyclability for
steady operation can also be achieved here. A great attention has been given to the
improvement of the heterogeneous catalyst because of these enormous advantages over the
homogeneous catalyst [3]. Different types of supports have been used including organic
resins [4] and polymers [5, 6] as well as inorganic carriers such as clays [7], silica [8],
zeolites [9], and molecular sieves [10] to generate heterogeneous catalysts. In recent years, a
great interest has been paid to the application of polymer-anchored metal catalysts for organic
transformations [11], and the development of functionalized polymers supported catalytically
active metal has produced a magnificent interest [12].
The bromination of organic substances is a significant reaction as the brominated products
have been exclusively used as an important precursor for cross-coupling and substitution
reactions which produced many pharmaceuticals, agricultural, and specialty chemicals [13].
Some brominated products are also very useful as pesticides, herbicides, flame retardants,
and bioactive materials [13]. Numerous protocols for bromination reaction of organic
compounds have been published previously [14]. In the classical method of direct
bromination, elemental bromine is used and half of the bromine is consumed for the
production of hydrogen bromide waste (eq. 1). Moreover, the elemental bromine is a health
hazard material and a pollutant.
Ar-Br + HBr
Ar-H + Br2
(1)

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Therefore, it will produce an economical and various environmental problems for large-scale
synthesis. That’s why development of environmental friendly bromination process has been
focused on to avoid the use of elemental bromine [15]. Some reagents have been developed
as a substitute for Br2, such as N-bromosuccinimide, [K+18-crown-6]Br3, methylimidazolium
tribromide,
1-butyl-3-methylpyridinium
tribromide,
ZrBr4/diazene,
1-butyl-3
methylimidazolium tribromide, ethylene bis(N-methylimidazolium) ditribromide and 3-
pentylpyridinium tribromide [16]. Some bromine containing reagent systems have also been
used like: Br2/Ag2SO4, Br2/SbF3/HF, Br2/SO2Cl2/Zeolite, Br2/ Zeolite, Br2/H2O2,
Br2/H2O2/Layered Double Hydroxide-WO4, Br2/tetrabutylammonium peroxydisulphate, etc
[17]. But those reagents are expensive, they have poor recovery and recycling performance
and release large amounts of HBr waste on disposal. At some stage the synthesis of those
reagents involves liquid bromine, as a result, increasing the cost of the end-product.
Therefore, the catalytic oxidative bromination reaction has been still gaining a considerable
importance to develop a more efficient and suitable method for the synthesis. Catalytic
oxidative bromination is a process of generation of electrophilic bromine using different
oxidants in presence of catalyst. From greener point of view environmentally benign oxidant
H2O2 and O2 are considered as best oxidants for oxidative bromination as only water is the
waste product generated here [18].
Many metal complexes show catalytic activity towards oxidative bromination reaction along
with other oxidation reactions [19]. However; these metal complexes usually used as a
homogeneous catalyst and during the catalytic reaction they decomposes, as a result, it was
very difficult to recover them from the reaction mixture. One of the important ways to solve
this problem and make the catalyst heterogeneous is to immobilize the catalytically active
metals onto organically modified polymer supports. The most widely used macromolecular
support is Chloromethylated polystyrene [20]. The polystyrene supported materials offer

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advantageous characteristics of heterogeneous catalysis such as selectivity, recyclability,
thermal stability, and their separation from the catalytic reaction mixture [21]. Though
diverse materials have been explored as catalysts for oxidative bromination, polystyrene
supported transition metal complexes have rarely been used [22]. Moreover, the application
of polystyrene supported Fe(III) complex catalyst for oxidative bromination has been hardly
studied [23].
Multicomponent reactions forming heterocyclic compounds are powerful tools in the drug
discovery process. In a single method, they can provide the convenient synthesis of libraries
of drug-related compounds [24]. The importance of multi-component reactions is simplicity,
less costly, a high degree of variations, high atom economy, good yields, short reaction
durations, high bond-forming efficiency [25], environmentally friendly methods, and
ignorance of multiple steps with high-value purification processes [26].
Spiropiperidine derivatives conquer an important position in the field of heterocyclic
chemistry as they are commonly found in numerous natural and synthetic products along with
useful biopharmaceutical, physiopharmaceutical, and pharmaceutical activities. These
derivatives are potent σ receptor ligands which can be utilized in the depression treatment,
epileptic disorders, abuse of cocaine, and 5-HT2B receptor antagonists [27].
Owing to the importance of the spiropiperidine compounds in medicinal field, unexpectedly,
only a few procedures for the synthesis of these compounds are available in the literature
[28]. However, the procedures mainly involve the use of homogeneous catalysts, long
reaction time and toxic solvents. Therefore, development of an efficient and eco-friendly
procedure which involves heterogeneous catalyst is highly expectable for the synthesis of
spiropiperidine derivatives. For this purpose, here we developed an eco-friendly and cost-
effective protocol in which spiropiperidine derivatives can be synthesized very easily within
very short reaction time using a polymer supported heterogeneous metal catalyst.

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In this paper, we report the preparation, characterization of a polymer-anchored iron(III) azo
catalyst and the investigation of its catalytic activities for oxidative bromination and one pot
multiple components reaction for the synthesis of spiropiperidine derivatives.
2.
Experimental
2.1.
Materials
Chloromethylated polystyrene was purchased from Sigma-Aldrich Company (USA) and
other chemicals were available from Merck (India). The reagents were used as received
without any purification. Reagent grade solvents were dried and distilled prior to use.
2.2. Characterization techniques
Elemental analysis (C, H and N) was performed on Perkin-Elmer 2400 C elemental
analyzer. FTIR spectra of the samples were carried out on a Perkin-Elmer FTIR 783
spectrophotometer using KBr pellets. Thermo gravimetric (TGA) analysis was done on
Mettler Toledo TGA/SDTA 851 instrument. The morphology of samples was analyzed using
a scanning electron microscope (SEM) (ZEISS EVO40, England) equipped with EDAX
facility. NMR spectra were carried out using tetramethylsilane as internal standard on a
Bruker AMX- 400 NMR spectrophotometer and UV-Vis spectra were done on Thermo
Scientific (Model no. GENESYS 10S UV-VIS) spectrophotometer. AAS analysis for the
loading of iron on polymer support was measured on Spectra- AA 240 (Agilent
Technologies). The powder X-ray diffraction (PXRD) data were taken on a Bruker D8
Advance X-ray diffractometer using Cu-Kα radiation (λ = 1.5418 Å) operating at 40 kV and
40 mA. The XRD patterns were recorded in the 2θ range of 3-50 using a LynxEye detector
(1D mode) with a step size of 0.02 and a dwell time of 1 s per step.

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2.3.
Preparation of the Catalyst
Synthesis of polymer-anchored iron(III)Azo (PS-[Fe(III)Azo]) complex is depicted in
the following Scheme 1.
Scheme 1. Schematic presentation for the preparation of PS-[Fe(III)Azo] catalyst.
2.3.1. Preparation of polymer-anchored azo (PS-Azo) ligand (B)
PS-Azo ligand (B) was synthesized by two steps syntheses procedure. In the first step,
in a 50 mL round-bottomed flask 1 g of chloromethylated polystyrene was dissolved in 10
mL dry THF. Then 0.7 mL ethylenediamine was added dropwise to this mixture and stirred it
for 48 h at room temperature [29]. The obtained white solid bead (A) was filtered and washed
thoroughly with deionized water and followed by methanol. The product was dried under
vacuum.
In the second step, 1 g of (A) was suspended in 50 mL methanol in a 250 mL round bottom
flask. Then 50 mL methanolic solution of 1-nitroso-2-naphthol (1 g) was added dropwise to
the suspension of (A). After that, the reaction mixture was refluxed for 24 h under stirring
condition. The colour of the reaction mixture was changed from white to deep brown. The
mixture was cooled to room temperature. Finally, PS-Azo Ligand (B) was obtained by
filtering and washing thoroughly with methanol and drying it under vacuum.

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2.3.2. Preparation of PS-[Fe(III)Azo] Catalyst (C)
In a 100 mL round bottom flask, 1 g of PS-Azo Ligand (B) was taken in 20 mL dry
ethanol. Under constant stirring, 5 mL 1% (w/v) dry ethanolic solution of iron (III) chloride
was added dropwise to the solution. Then the solution was refluxed for 24 h under stirring
condition. The obtained complex was filtered and washed properly with methanol and then it
was dried under vacuum.
2.4.
General procedure for the catalytic oxidative bromination reaction of organic
compounds by PS-[Fe(III)Azo] catalyst
To a 50 mL two-necked round-bottomed flask 50 mg of catalyst, the substrate (5 mM)
and KBr (2.2 mM) were taken in 5 mL acetic acid. Then 30% H2O2 (5 mM) was added
dropwise to the reaction mixture and stirred at room temperature (Scheme 2). The progress of
the reaction was monitored by thin layer chromatography (TLC). After completion of the
reaction, 10 mL water was added to the reaction mixture. Then the catalyst was filtered and
washed thoroughly with ether. To the filtrate, saturated sodium bicarbonate solution was
added and the organic layer was extracted. Anhydrous sodium sulphate was used to dry the
organic layer and solvent was evaporated under reduced pressure. The obtained products
were analyzed by Varian 3400 gas chromatograph equipped with a 30 m CP-SIL8CB
capillary column and a Flame Ionization Detector.
Scheme 2. Schematic presentation of PS-[Fe(III)Azo] catalysed oxidative bromination reaction.

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2.5.
General process for the preparation of spiropiperidine derivatives
In 50 mL round-bottomed flask, 2 mM of amines (1a-d), 4 mM of active methylene
compounds (2a or 2b), 6 mM of 37% aqueous formaldehyde solution and 50 mg of
PS-[Fe(III)Azo] catalyst were added to 15 mL isopropyl alcohol. The reaction mixture was
stirred at room temperature for the specified period of time (Table 7). The precipitation of
solid compound indicated the formation of products. After the stipulated time of completion
of the reaction (monitored by TLC), 20 mL water was added to reaction mixture and 10 ml
ethyl acetate was then added to extract the products. The solid products were obtained by the
evaporation of organic layer under reduced pressure. Recrystallization of the solid products
from ethanol solution gave pure compounds. The catalyst in the aqueous phase was filtered,
washed thoroughly with the mixture of ether and methanol (1:10), dried and reused.
Scheme 3. General schematic presentation of synthesis of spiropiperidine derivatives.
3
Results and Discussion
3.1
Characterization of PS-[Fe(III)Azo] catalyst
The polymer-supported metal catalyst is generally insoluble in common organic
solvents; so, its structural characterization was bounded to their chemical analysis, SEM,
EDX, FTIR, PXRD, TGA, UV-Vis spectroscopic analysis and physicochemical properties.
The elemental analysis data is given in Table 1. The metal loading on the catalyst was
measured by atomic absorption spectroscopy (AAS).

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Table 1
Chemical composition of Ps-Azo ligand and its iron(III) Catalyst.
Compound
PS-Azo ligand
PS-[Fe(III)Azo]Catalyst dark brown 75.60 7.12
Colour
C (%) H (%) N (%) Cl (%)
Fe (%)
˗
brown
84.13 7.24
4.14
3.98
1.20
2.94
1.25
(1.24)a
(1.17)b
(0.72)c
a Reused catalyst after 1 cycle, bReused catalyst after fourth cycle, cReused catalyst after tenth cycle.
The loading of metal to the organically modified polymer support was confirmed by
comparison of the FT-IR spectra (Fig. 1) of polymer-anchored ethylenediamine (PS-en), PS-
Azo ligand and PS-[Fe(III)Azo] catalyst. Pure chloromethylated polystyrene shows an IR
active absorption band at 1,261 cm-1 due to the C-Cl bond stretching frequency. This band is
absent in the ligand as well as in the catalyst. The FTIR spectra of the PS-en (A) shows one
band near 3410 cm−1 for N–H stretching vibration of attached secondary amine. Due to N–H
bending vibration of primary amine, PS-en (A) also gives a moderate peak at 1600 cm−1.
When (A) reacted with 1-nitroso-2-naphthol to form PS-Azo ligand (B) the peak at 1600
cm−1 was disappeared. The band at 1510 cm−1 of PS-Azo ligand (B) may be for the stretching
vibrations of the azo group (N=N). This band is shifted to 1495 cm-1 in the PS-[Fe(III)Azo]
catalyst (C) indicating coordination of the PS-Azo ligand to Fe through the azo N [30]. The
peak observed at 2921 cm−1 in all spectra may be due to C-H stretching vibration of ethylene
group [31]. In case of PS-Azo ligand, a new band is appeared at 3205 cm−1 due to OH
stretching frequency, which is absent in the spectra of the catalyst that confirmed the
coordination of the phenolic oxygen atom with Fe metal. In addition, a new absorption band
is observed at 560 cm−1 due to the formation of M–N bonds between metal ions and PS-Azo
ligand [23]. The complexation of Fe(III) metal with PS-Azo ligand was further evidenced by

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the absorption band at 455 cm−1 due to the formation of the bond between metal and phenolic
oxygen (M–O) [23].
Fig. 1. FT-IR spectra of PS-en (a), PS-Azo ligand (b) and PS-[Fe(III Azo] Catalyst (c).
The metal loading in the catalyst was measured by AAS which suggested 1.25% (w/w) of Fe
present in the immobilized metal catalyst.
Fig. 2 showing the powder XRD pattern of PS-Azo ligand and PS-[Fe(III)Azo] catalyst. Both
ligand and catalyst exhibit a broad band at the range of 2θ (10-30) which clearly indicates the
amorphous nature of ligand as well as the catalyst [32]. Thus there was no nano-crystalline
material of iron is formed after the introduction of metal onto the ligand.
Fig. 2. PXRD of PS-Azo ligand and PS-[Fe(III)Azo] catalyst.

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The field emission scanning electron micrographs of PS-Azo ligand (a) and its iron complex
(b) are shown in Fig. 3. There is a clear change observed in the morphological pattern of the
PS-Azo ligand after insertion of iron metal by the SEM image which confirms the loading of
metal on the functionalized polymer surface. EDX analysis of PS-[Fe(III)Azo] catalyst
further confirms the loading of iron on organically modified polymer support (Fig. 4).
Fig. 3. FE SEM image of PS-Azo ligand (a) and PS-[Fe(III)Azo] catalyst (b).
Fig. 4. EDX images of PS-Azo ligand (a) and PS- [Fe(III)Azo] Catalyst (b).
UV-Vis spectrum (Fig. S1) of the PS-Azo ligand and PS-[Fe(III)Azo] catalyst were taken
after ultra-sonicating the samples in MeOH medium. The PS-Azo ligand in MeOH shows two
intense band at 225 and 293 nm due to n-π* and π-π* transitions respectively [23]. These
bands shifted at 213 and 281 nm respectively in PS-[Fe(III)Azo] catalyst. A very weak new
band has appeared near at 400-410 nm in the spectra of the catalyst. This band is for the spin
forbidden d-d transition of the Fe complex [33]. The shifting of bands and appearing in new

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band for d-d transition clearly indicates the formation of the iron complex with the PS-Azo
ligand.
Thermo gravimetric analysis of PS-Azo ligand and PS-[Fe(III)Azo] catalyst (Fig. 5) were
performed in open atmosphere in between temperature range from 30 ºC to 600 ºC with 10 ºC
min-1 heating rate. The higher thermal stability of the PS-[Fe(III)Azo] catalyst compares to
PS-Azo ligand clearly evidences the loading of iron metal to the polymer surface. Both the
metal catalyst and ligand were thermally stable up to ca. 400 ºC; beyond the temperature,
they decomposed.
Fig. 5. TGA curves for PS-Azo Ligand and PS-[Fe(III)Azo] catalyst.
3.2
Catalytic Activity
3.2.1 Catalytic oxidative bromination reaction of organic compounds
The catalytic activity of PS-[Fe(III)Azo] catalyst was tested for oxidative bromination
reaction of different organic compounds under mild conditions. The effects of different salts
were investigated for this reaction taking salicylaldehyde as a model substrate. The polymer-
catalyst PS-[Fe(III)Azo] was used with oxidant H2O2 and metal bromide (like NaBr, KBr and
LiBr) as a bromine source for the bromination reaction (Table 2). Potassium bromide was

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seemed to be the most powerful bromine source among the three bromide salts and its
application gave the mono-selective product. The regioselective product was not obtained by
the use of sodium bromide and lithium bromide was less efficient than Potassium bromide.
Scheme 4. PS-[Fe(III)Azo] catalyzed oxidative bromination of salicyaldehyde.
Table 2
Effect of various bromide salts on the bromination reaction of salicylaldehyde using PS-[Fe(III)Azo] catalysta.
Entry
1
Bromide salt
% of conversionb
75
Product selectivityb, c
Sodium bomide
5-Bromo-2-hydroxy benzaldehyde (60) +
3, 5-dibromo-2-hydroxy benzaldehyde (40)
5-Bromo-2-hydroxy benzaldehyde (100)
5-Bromo-2-hydroxy benzaldehyde (100)
2
3
Lithium bromide
92
Potassium bromide
100
aConditions: Salicylaldehyde (5 mM); Metal bromide (2.2 mM); glacial AcOH (5 mL); H2O2 (2.2 mM); 50 mg
catalyst; room temperature. bConversion and selectivity were checked by GC.
The effect of temperature was also monitored for the same bromination reaction. But there is
no influence of temperature on the transformation of salicylaldehyde. The conversion of
salicylaldehyde with raise in temperature remains almost same. Different solvents like
hexane, carbon tetrachloride, ethanol, acetonitrile, acetic acid, chloroform, and methanol
were used to study the solvent effects on the catalytic bromination reaction of
salicylaldehyde. Among the different solvents, acetic acid worked as the best solvent. The
function of hydrogen peroxide (H2O2) as an oxidant was established by conducting a blank
test where no production of bromo derivative was found.
The oxidative bromination of various organic compounds was extensively carried out
maintaining the reaction conditions as optimized for salicylaldehyde (Table 3). Selective

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mono-brominated products were obtained from the bromination reaction of substrates with
the activated aromatic ring. Phenol, resorcinol and anisole gave the para-selective product
with the excellent high conversion. But in case of the deactivated aromatic substrate, like
nitrobenzene, did not show any conversion under the similar reaction conditions. Under same
reaction conditions, benzene showed a low conversion after long reaction times. 2-
Substituted organic compounds were selectively converted to 4-bromo derivatives and vice
versa.
Table 3
PS-[Fe(III)Azo] catalysed oxidative bromination reaction of different organic substratesa.
Entry
1d
Substrate
Salicylaldehyde 100, 99, 98, 96, 93,
89, 80, 71, 62, 54, 40 (3.0)
% of Conversion b (time in h)
Product selectivityb (%)
5-Bromo-2-hydroxy benzaldehyde (100) (447/149)e,
TONc/TOFd
(444/148)f,
(310/103)g
2
3
4
5
6
7
8
Phenol
100 (2)
4-Bromophenol (100)
2-Bromo-4-aminophenol (100)
4-Bromo-1,3-dihydroxybenzene (100)
2-Bromo-4-nitrophenol (100)
4-Bromo-2-nitrophenol (100)
4-Bromoanisole (100)
4-Bromoaniline (97)
448/224
393/131
402/134
348/58
4-Aminophenol 88 (3)
Resorcinol
4-Nitrophenol
2-Nitrophenol
Anisole
90 (3)
78 (6)
65 (6)
288/48
100 (2.5)
98 (3)
445/178
438/146
Aniline
2-Bromoaniline (3)
9
4-Nitroaniline
Benzene
88 (3)
14 (6.0)
-
2-Bromo-4-nitroaniline (100)
Bromobenzene (100)
393/131
60/10
10
11
Nitrobenzene
-
aConditions: substrate (5 mM); KBr (2.2 mM); glacial AcOH (5 mL); 30% aq H2O2 (2.2 mM), catalyst (50 mg), room
b
d
c
temperature. Conversion and selectivity were determined by GC. Catalytic runs to test recyclability. TON (turn over
d
number) = moles of substrate converted/moles of active sites of catalyst, TOF (turn over frequency) = TON/time (in h);
eTON/TOF calculated after first cycle based on AAS data of fresh catalyst; TON/TOF calculated after fifth cycle based on
f
AAS data of reused catalyst after fourth cycle; g TON/TOF calculated after fifth cycle based on AAS data of reused catalyst
after tenth cycle.

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Table 4
Comparison of the catalytic activity of PS-[Fe(III)Azo] catalyst with other reported systems towards oxidative
bromination.
Sl.
No
1
Catalyst
Substrate % conversion Product Selectivity (%)
TOF
Reference
23
PS-[Fe(III)teta]
Aniline
96.0
4-bromoaniline (85.0)
2-bromoaniline (15.0)
4-bromoanisole (100.0)
4-bromoaniline (95.6)
2-bromoaniline (4.4)
4-bromoanisole (100.0)
4-Bromoaniline (97.0)
2-Bromoaniline (3.0)
4-bromoanisole (100.0)
118
2
3
PS-[Fe(III)teta]
Fe(opbmzl)2-Y
Anisole
Aniline
94.0
84.4
96
77
23
34
4
5
Fe(opbmzl)2-Y
PS-[Fe(III)Azo]
Anisole
Aniline
72.0
98.0
-
35
146
This work
6
PS-[Fe(III)Azo]
Anisole
100.0
178
This work
3.2.2 Catalytic synthesis of spiropiperidine derivatives by PS-[Fe(III)Azo] catalyst
At room temperature, different control reactions were carried out in isopropyl alcohol
using dimedone, formaldehyde and aniline as model substrates to ensure that Fe sites bound
to PS-Azo ligand create the catalytic activity. In absence of any catalyst no product was
obtained, whereas using PS-Azo ligand (10 M %) as a catalyst gave disappointing results.
Then the reaction was performed by using FeCl3·6H2O (10 M %) as a catalyst, a good result
with product yield of 64% after 3 h was observed. This result confirmed that the Fe sites are
responsible for the catalytic activity, but FeCl3·6H2O act as a homogeneous catalyst under
this reaction conditions, as a result, the recyclability of the catalyst becomes complicated.
Then the reaction was carried out in presence of PS-[Fe(III)Azo] catalyst (50 mg), an
incredible yield of 95% product in a short range of time period was obtained. Effects of
different solvents and the catalyst loading were monitored to optimize the reaction
conditions. The reaction between dimedone (4 mM), formaldehyde (6 mM) and aniline (2
mM) under different reaction conditions was used as a model reaction for all experiments. At

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room temperature, the effects of different solvents like H2O, MeOH, EtOH, isopropyl
alcohol, PEG-600, PEG-400 and PEG-200 on the model reaction were explored using 50 mg
of the catalyst (Table 5). The best result was obtained when isopropyl alcohol was used as a
solvent compared to others.
Table 5
Solvents effect on the model reaction.
Entry
1.
Solvent
methanol
isopropyl alcohol
PEG-400
PEG-600
PEG-200
ethanol
Timea (min)
% of Yieldb,c
60
30
60
60
60
60
60
90
95
2.
3.
74
4.
72
5.
77
6.
92
7.
Water
trace
aReaction of aniline (2 mM), demidone (4 mM and formaldehyde (6 mM), room temperature. Progress of reaction was
monitored by thin layer chromatography. bIsolated yield. ccatalyst amount 50 mg.
Table 6
Effects of the different amount of catalyst on the model the reaction.
Entry
1.
Amount of catalyst (mg)
Reaction time (min)a
% of yieldb
20
30
40
50
60
90
60
30
30
30
73
85
90
95
95
2.
3.
4.
5.
aReaction of aniline (2 mM), indane-1,3-dione (4 mM) and HCHO (6 mM) in isopropyl alcohol (10 mL) at room
temperature. Progress of the reaction was monitored by TLC. bIsolated yield.

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Under the optimized conditions, various spiropiperidine derivatives were synthesized and the
results are summarized in Table 7. The result of Table 7 clearly indicates that the catalyst is
very efficient for the synthesis of spiropiperidine derivatives. For further confirmation of the
catalytic efficacy of the catalyst a comparison with the reported catalysts is shown in Table 8.
Table 7
Synthesis of different spiropiperidine derivatives using amines, active methylene compounds and formaldehyde
in presence of PS-[Fe(III)Azo] Catalysta.
Entry
1.
Amines
(1a-1d)
Active methylene
Productsb
(3a-3h)
Time (min)c
Yield (%)d
TOFe
compounds (2a-2b)
30
95
340
1a
1b
2a
2a
3a
3b
2.
3.
30
30
85
90
304
322
2a
1c
1d
1a
3c
4.
5.
30
30
88
92
314
328
2a
2b
3d
3e

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6.
7.
30
83
89
296
318
2b
1b
3f
30
30
2b
1c
1d
3g
3h
8.
88
314
2b
aReaction of amine (2 mM), dikitone (4 mM) and HCHO (6 mM), catalyst (50mg) in isopropyl alcohol (10 mL) at room
b
1
c
d
e
temperature. Compounds characterized by H NMR spectra. Reaction progress monitored by TLC. Isolated yield. TOF
(turn over frequency) = no. of moles of the starting materials being converted per mole of active site of the catalyst /time (in
h).
Table 8
Catalytic efficacy of PS-[Fe(III)Azo] catalyst for the synthesis of spiropiperidine derivatives with other
reported protocols.
Entry
1
Catalysts
Conditions
solvents
Products
Yield
(%)
Time
reference
STA
R. T.
DCM
93
4 h
35(a)
2
3
Fe3O4
80 °C
solvent-free
ethanol
73
5 h
35(b)
35(d)
reflux (5 min)/
overnight/R. T.
62
12 h

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4
5
6
Ce/chitosan
PS-[Fe(III)Azo]
PS-[Fe(III)Azo]
R. T.
R. T.
R. T.
PEG-200
94
95
89
38 min
30 min
30 min
36
isopropyl
alcohol
This work
This work
isopropyl
alcohol
7
PS-[Fe(III)Azo]
R. T.
isopropyl
alcohol
85
30 min
This work
3.2.3.
Reaction Mechanism
The probable mechanism of the synthesis of spiropiperidine derivatives is depicted in
Scheme 5. The derivatives are seemed to derive through Knoevenagel condensation, Michael
addition and double Mannich reactions [35]. In the very first step, PS-[Fe(III)Azo] catalyst
activates formaldehyde and dimedone via Knoevenagel condensation to produce intermediate (II).
Then Michael addition occurs to the intermediate (II) by another mole of activated iron-dimedone
intermediate (I) through the double bond to form intermediate (III). In the next step, the Mannich
reaction between intermediate (III), aniline and activated iron-formaldehyde gives intermediate
(IV). Finally, intermediate (IV) and activated iron-formaldehyde take part in another Mannich
reaction to generate final product 1a, releasing PS-[Fe(III)Azo] catalyst for successive cycles.

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Scheme 5. Proposed reaction mechanism for the syntheses of spiropiperidine derivatives
.
4.
Tests for Heterogeneity
After catalytic reaction, the leaching of iron from PS-[Fe(III)Azo] catalyst was assured
through IR, UV-Vis, AAS analysis of the utilized catalyst and the product mixtures. The
analysis did not show any considerable loss of iron loading compares to the unused catalyst.
We got similar results from IR spectrums of the reused and fresh catalyst. AAS of used
catalyst after first cycle (Table 1) showed the metal loading remains almost same in the fresh
and reused catalyst but AAS data of reused catalyst after fourth cycle showed some small
decrease (~ 6.4 %) in metal loading of the catalyst. AAS analysis of product mixtures
confirmed the absence of iron in the product mixture. After catalytic reaction, a UV-Vis
spectrum of product mixture was taken but no characteristics absorption peak of catalyst was
observed. All these analyses clearly confirm that there was very low leaching of iron takes
place during the course of each recycling reaction. All the above mention observations
definitely support the heterogeneous nature of PS-[Fe(III)Azo] catalyst.

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5.
Recycling of Catalyst
The recycling experiment of the catalyst was carried out using the model reaction of
salicyaldehyde (5 mM); KBr (2.2 mM); glacial AcOH (5 mL); 30% aq H2O2 (2.2 mM),
catalyst (50 mg) at room temperature. After completion of the reaction, 20 mL H2O was
added to the final reaction mixture and 10 ml ethyl acetate was then also added to extract the
products. The solid product was obtained by evaporating the organic layer under reduced
pressure. The catalyst was filtered from the remaining aqueous phase, washed thoroughly
with ether and methanol (1:10) mixture, dried and reused. Even after five cycles, the catalyst
shows its good catalytic efficiency (Fig. 6). The catalytic conversion by the catalyst becomes
almost half after ten cycle. After eleven cycle we got only 40% conversion of salicyaldehyde.
AAS data showed that the catalyst contains 0.72% of iron after tenth cycle. So the metal
loading decreases nearly 43% after tenth cycle and the TON decreased from 447 to 310. The
leaching of metal from support and catalysis poison is responsible for this decrease of
catalytic efficiency.
Fig. 6. Chart showing the recyclability of the PS-[Fe(III)Azo] Catalyst.
5.
Conclusion
An organically modified polymer-anchored [Fe(III)Azo] catalyst was synthesized and
characterized properly. The catalyst showed magnificent catalytic activity towards oxidative
bromination reaction of different organic substrates. This bromination protocol offers simple

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reaction conditions of commercially available reagents and product yield is excellent without
generation of hydrogen bromide waste product. It is a green alternative method in compare to
the hazardous method of classical bromination and thus carrying an opportunity of easy
production to a variety of bromo-derivatives.
The catalyst was also very effective in one pot three components synthesis of spiropiperidine
derivatives. An environmentally benign simple and efficient method for the preparation of
spiropiperidine derivatives by the efficient PS-[Fe(III)Azo] heterogeneous catalyst at room
temperature has been reported here. The catalyst was recyclable and stable under the reaction
conditions. The reusability of this catalyst is high (i.e. five times) without the significant
decrease in its activity.
Acknowledgements
SMI gratefully acknowledges the West Bengal State Council of Science and
Technology, WB-DST (Sanc.)/ST/P/S&T/4G-8/2014, dated 04/01/2016; for financial
support. S.B. is thankful to the University Grant Commission (UGC) for funding in the form
of Dr. D.S. Kothari post-doctoral research fellowship (Award letter no. F.4-2/2006
(BSR)/CH/16-17/0026. RK acknowledges the University Grants Commission for the
financial support provided through MANF. We acknowledge Department of Science and
Technology (DST) and University Grant Commission (UGC) New Delhi, India for providing
support to the Department of Chemistry, the University of Kalyani under PURSE, FIST and
SAP program.

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Research Highlights
•
A polymer supported catalyst, PS-[Fe(III)Azo] has been synthesized and
characterized .
•
•
The oxidative bromination of organic substrates was carried out by PS-[Fe(III)Azo].
The catalyst showed excellent activities towards oxidative bromination of organic
substrates under mild reaction conditions.
•
•
The catalyst was also useful for the syntheses of spiropiperidine derivatives.
The catalyst is reusable upto five times without diminishing of its activity.