CoFe2O4–SiO2–SO3H nanocomposite as a magnetically recoverable catalyst for oxidative bromination of alkynes

Article abstract of DOI:10.1080/00397911.2017.1376333

A hybrid catalyst has been prepared by incorporating sulfonic acid onto cobalt ferrite magnetic nanoparticles. The catalyst was successfully applied for rapid (20 min) synthesis of α,α-dibromoketones directly from alkynes and NBS. The reaction works well in the presence of 10 wt% of the catalyst at room temperature to produce the desired products in high yield. The catalyst could be recovered using an external magnet and reused without appreciable change in activity.

Full text of DOI:10.1080/00397911.2017.1376333

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
CoFe O -SiO -SO H nano composite as a
2
4
2
3
magnetically recoverable catalyst for oxidative
bromination of alkynes
Mintu Maan Dutta, Kamal Krishna Rajbongshi & Prodeep Phukan
To cite this article: Mintu Maan Dutta, Kamal Krishna Rajbongshi & Prodeep Phukan (2017):
CoFe O -SiO -SO H nano composite as a magnetically recoverable catalyst for oxidative
2
4
2
3
bromination of alkynes, Synthetic Communications, DOI: 10.1080/00397911.2017.1376333
To link to this article: http://dx.doi.org/10.1080/00397911.2017.1376333
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Date: 17 September 2017, At: 22:50

CoFe O -SiO -SO H nano composite as a magnetically
2
4
2
3
recoverable catalyst for oxidative bromination of alkynes
Mintu Maan Dutta
Department of Chemistry, Gauhati University, Guwahati, Assam, India
Kamal Krishna Rajbongshi
Department of Chemistry, Gauhati University, Guwahati, Assam, India
Prodeep Phukan
Department of Chemistry, Gauhati University, Guwahati, Assam, India
Address correspondence to Prodeep Phukan. E-mail: pphukan@yahoo.com
ABSTRACT
A hybrid catalyst has been prepared by incorporating sulfonic acid onto cobalt ferrite
magnetic nanoparticles. The catalyst was successfully applied for rapid (20 min) synthesis of
α,α-dibromoketones directly from alkynes and NBS. The reaction works well in the presence of
1
0 wt % of the catalyst at room temperature to produce the desired products in high yield. The
catalyst could be recovered using an external magnet and reused without appreciable change in
activity.
Graphical abstract
1

KEYWORDS: cobalt ferrite MNPs, catalyst, α,α-dibromoketones, NBS, sulfonic acid
Introduction
In recent years, functionalized magnetic nanoparticles (MNPs) have emerged as an
efficient catalyst for a variety of organic transformations.[1–4] Spinel ferrites, with the general
formula of MFe
2
4
O , where M is a divalent cation, offer more interesting catalytic activities
compared to the corresponding single component metal oxides.[5] CoFe
O showed remarkable
2
4
O [
6,7]
Easy recovery of the catalyst from
chemical stability as compared to both Fe
3
O
4
and γ -Fe
2
3
reaction medium using an external magnet and high surface area to volume ratio make the
magnetic nanocatalysts superior over other catalysts like MCM, zeolite and silica.[8] They can be
functionalized to accommodate a wide variety of organic and organo-metallic moiety. Other
advantages such as high catalyst loading capacity, high dispersion, and outstanding stability have
widened the application potential of this class of materials. This is evident from a number of
publications appeared in the literature on the use of this type of catalyst for various transition
metal-catalyzed reactions in the recent past.[1–4] After initial report on sulfonic acid
functionalized magnetic nanoparticle by Jones,[9] Naeimi and his co-workers reported the
application of Fe
3
O
@silica sulfonic acid[10,11] catalyst for the synthesis of 1,8-dioxo-
4
octahydroxanthene and 1-substituted 1H-tetrazoles. There are also few reports of MNPS
supported catalysts for the photo-degradation of organic pollutants under UV and visible
irradiation.[12,13]
2

α,α-Dibromoketones are important intermediates in organic synthesis.[14] They are
valuable synthons in the field of pharmaceutical chemistry.[15–23] A number of methods have
been reported for the synthesis of α,α-dibromoketones using various brominating reagents
including molecular bromine, NBS, benzyltrimethylammoniumtribromide, dioxane dibromide
and hexabromocyclopentadiene have been successfully utilized for their synthesis.[24] However,
the use of silica as catalyst failed to give α,α-dibromoketones as a major product.[25]
Oxyhalogenation of alkynes has emerged as an important alternative process for the
preparation of α,α-dihaloketones. A common strategy for the catalytic synthesis of α,α–
dibromoketones from alkyne is the use of KBr in presence of an oxidant.[26–28] However, this
reaction suffers from formation of undesired by-products.[27] Other reagents like aqueous HBr
–hν[29] and NaBrO
-NaHSO
[30]
have been developed to afford α,α-dibromoketones
(
48%)–O
2
3
3
from alkyne. Unfortunately such protocols require use of strong acid or alkali mediators as well
as longer reaction time. Comparatively milder protocols has been achieved by utilizing CTAB-
NaBr, K
2
S
2
O
-NaBr[31] NBS[32,33] and NN,-dibromo-p-toluene sulphonamide[34] as the
8
brominating reagent. Nevertheless there is a great need for well developed catalytic protocols for
the synthesis of α,α-dibromoketones. While heterogeneous catalysis is very important in organic
transformations, there is no such approach for this particular transformation till date.
In continuation of our work on the applications of CoFe
2
4
O -based magnetic nano-
catalyst,[35] we report herein the preparation and utilization of sulfonic acid functionalized
magnetic nano-composite as a reusable catalyst for the synthesis of α,α-dibromoketones from
alkynes using NBS as the brominating reagent (Scheme 1). To the best of our knowledge, this is
the first heterogeneous catalytic approach for the conversion of alkynes to corresponding α,α-
dibromoketones.
3

Results and Discussion
Synthesis of CoFe
2
O
4
@SiO
2
–SO H catalyst
3
CoFe
2
O
4
@SiO
2
–SO H was prepared by a simple, convenient method using cobalt ferrite
3
as a support that has been prepared by a co-precipitation method that has been reported earlier
Scheme 2).[13,35] Initially, CoFe
O was prepared using the co-precipitation method under
(
2
4
ultrasonic irradiation. Then, the magnetic nanoparticles were coated with a thin layer of silica by
hydrolysing tetraethylorthosilicate in presence of aqueous ammonia.[35] Further surface
functionalization of CoFe
2
O
4
@SiO
2
–nanocomposite was carried out by treating the material with
@SiO –SO H catalyst. In
chlorosulfonic acid in dimethoxyethane to produce the final CoFe
2
O
4
2
3
this step, the amount of chlorosulfonic acid was found to be crucial for the formation of stable
nanoparticles. It was found that the addition of chlorosulfonic acid at a slightly higher
concentration than 0.1 mL (per 125 mg in 20 mL of solvent) causes agglomeration of the particle
and the material turns into a sticky mass. The nanocomposite was then characterized using FT-
IR, XRD, SEM, EDX, TEM, TGA and VSM (in Supporting Information).
-
1
In the IR spectrum (Figure 1), the absorption peak at ̴ 1100 cm belongs to the S-
-
1
O stretching vibrations, also a broad peak at around 3300–3500 cm is related to the
stretching of OH group of sulfonic acid.[10,11]
XRD patterns of pure CoFe
2
O
4
, CoFe
2
O
4
-SiO
2
and CoFe
2
O
4
-SiO
2
-SO H are
3
shown in (Figure 2). Both XRD patterns (b) and (c) have a broad peak between 15° and
30°, centered at 23.7°, typical for the silica. Thus the appearance of broad XRD peaks at
2
θ = 23.7° confirms the silica coating and sulfonic acid functionalization over the
[
9]
CoFe
2
O
4
From the XRD pattern of the CoFe
2
O
4
@SiO
2
–SO H, average crystalline size
3
4

of the nanoparticles was calculated using Debye-Scherrer’s equations. Thus the crystallanite size
of the CoFe -SiO -SO H was found in the range of 7–13 nm.
2
O
4
2
3
Thermogravimetric analysis (Figure S1 in Supporting Information) showed that the
sulfonic acid modified MNPs displayed several mass loss regions. First region centered at 100
°
C attributed to the desorption of moisture and volatile components from the sample. Above this
temperature, slightly different thermal behaviours depending on the surfactant used were
observed. A weight loss occurred at 400 °C which may be due to be thermal decomposition of
the sulfonic acid.[10] Thus the sample synthesized was thermally stable up to 250 °C. The
morphology of CoFe
2
O
4
@SiO
2
-SO H was analyzed by using SEM (Figure 3) and it is obvious
3
from the image that the major portion of the material is an aggregation of small spherical
particles. This may be due to the magnetic interaction of the cobalt ferrite particles with the silica
layers of Tetraethyl orthosilicate (TEOS).[12] The distribution of elements in the product was
determined by EDX analysis and it was found to be Co = 0.55%, Fe = 0.95%, O = 51.62%, S =
0
.12%, C = 35.48% and Si = 11.52% which confirms the successful synthesis of composite.
Thus the iron/cobalt ratio in the nanocrystals by EDX was found to be 1.73 which is very much
close to the atomic ratio in the formula CoFe . The higher percentage of carbon and oxygen in
the EDX analysis is primarily due to the coating over CoFe using TEOS. The EDX analysis
for CoFe @SiO –SO H is shown in (Figure S2 in SI).
2
O
4
2
O
4
2
O
4
2
3
The magnetic properties of the synthesized CoFe
2
O
4
@SiO
2
-SO H MNPs were
3
investigated by VSM measurement. Incorporation of SiO and –SO
2
3
H group greatly affects the
magnetic property of the core material. (Figure S3 in SI) shows the M–H loop taken at room
temperature with a maximum applied field of ±2 T. From the hysteresis loop both saturation
magnetization (Ms) coercivity values (Hc), and retentivity (Mr) were determined. Ms values
5

were found to be 35.304, 6.192, and 3.6116 emug-1 for CoFe
CoFe @SiO -SO H respectively. Similarly, coercivity, Hc values of 514.41, 237.94 and
40.61 G and the Mr values of 7.4479, 1.1114, 0.8869 emug were obtained for CoFe
CoFe @SiO , and CoFe @SiO -SO H respectively. The incorporation of silica and sulfonic
O
4
, CoFe
O
4
@SiO , and
2
2
2
2
O
4
2
3
-1
4
2
4
O ,
2
O
4
2
2
O
4
2
3
acid over cobalt ferrite had very little effect on coercitivity, whereas magnetization and
retentivity decreased rapidly. Thus the synthesized nanoparticle showed coercitivity value of
4
40.61 G with the corresponding saturation magnetization value of 3.6116 emug-1 which
indicates the ferromagnetic property of the nanoparticles. The size of the CoFe @SiO
SO H was examined by a high resolution transmission electron microscopy, (Figure S4a
2
O
4
2
-
3
in SI) shows that the nanoparticles formed (10 nm) are well dispersed and evenly
distributed in all directions. It is also very clear that most of the particles are spherical in
shape. The related selected area electron diffraction pattern (SAED) in (Figure S4b in SI)
reveals the crystalline nature of CoFe
2
O
4
@SiO
2
-SO H. The multiple lattice fringes in the
3
high resolution TEM image suggest the high crystallinity of the particles.
Catalytic activity of CoFe
dibromoketones
2
O
4
@SiO
2
-SO H catalyst for the synthesis of α,α
3
The magnetic nanoparticles exhibits potential use as a heterogeneous catalysts for
various organic transformations and addresses solutions to various issue associated with
the homogeneous catalysis.[35] After successful synthesis of thermally stable, Cobalt
ferrite silica-sulfonic acid functionalized MNP, we intended to examine its catalytic
efficiency for the synthesis of α,α-dibromoketones from alkynes using NBS. To start
with, a study was carried out using phenylacetylene as the model substrate to find out the
optimum reaction condition (Table 1). In a typical reaction, 10 wt% of the catalyst was
6

added to a mixture of phenylacetylene (entry 1) and 3 equivalent of NBS in acetonitrile
and water (3 mL, 30:1 ratio). Delightedly we have observed a very fast reaction and
isolated 93% of the desired product in the first attempt. Inferior results were observed
when the ratio of solvents was varied (entry 2). Thereafter, some other binary solvent
systems such as THF:H
provided lower yields. The reaction did not proceed at all, when the reaction was
examined in DCM:H O (entry 5). Further experiments using varying amount of catalyst
2
O and CH
3
COCH
3
:H
2
O (entry 3–4), were screened which
2
could not improve the reaction efficiency (entry 6–7). Lowering the concentration of NBS
to 2 equivalents resulted in a rapid decrease in the yield to 63% (entry 8). When the
reaction was carried out in absence of the catalyst, only 12% of the desired α,α-
dibromoketones was isolated. The efficiency of the catalyst was compared with that of
cobalt ferrite and cobalt ferrite-silica under the standardised condition. The catalytic
efficiency for both the compounds is found to be significantly low. While the use of
cobalt ferrite could produce only 26% yield of the desired product, a marginal
improvement of yield (33%) was observed in case of cobalt ferrite-silica catalyst even
after continuing the reaction for 24h (Table 1, entry 10, 11).
With the optimum reaction conditions in hand, the scope of the reaction was extended to
various alkynes (Table 2). Various alkynes underwent the transformation efficiently under the
standardised protocol to produce corresponding α,α-dibromoketones. Aromatic alkynes (Table 2,
entry 1b–8b) could be transformed in high yields regardless of the presence of electron activating
and deactivating group on the benzene ring. However, the reaction yield was found to be
relatively low in case of aliphatic alkynes (Table 2, entry 9b–11b).
Proposed reaction mechanism
7

A plausible mechanism for the formation of α,α-dibromoketones is proposed in
Scheme 3. The magnetic acid catalyst (CF@SiO -SO H) helps in generating Br + ion
2
3
from NBS. Treatment of alkyne with electrophilic brominating reagent (NBS) generates
the three membered cyclic bromonium intermediate I. The intermediate undergoes ring
opening by the attack of water molecule to generate intermediate vicinal bromo alkeneol
II. This intermediate further reacts with NBS to form corresponding α,α-dibromoketone
(
III) under the catalytic influence of CoFe
2
O
4
@SiO
2
-SO H.
3
Catalyst reusability
The synthesized nanoparticles are magnetic in nature and can be separated from
the reaction mixture by using an external magnet. We have examined the reusability of
the catalyst for the synthesis of α,α-dibromoketones from phenyl acetylene. To reuse, the
catalyst was separated using an external magnet after completion of the reaction, washed
with ethyl acetate and methanol, dried under vacuum, and then kept in a desiccator
overnight before reuse. It was observed that the catalyst could be reused at least six times
without significant loss in its activity (Figure 4).
Conclusion
In conclusion, we have prepared a cobalt ferrite silica sulfonic acid functionalized
magnetic nanoparticles using an ultrasound assisted co-precipitation method. The catalyst
was characterised using IR, XRD, TEM, SEM, VSM and TGA analysis. It was further
demonstrated that the nanocatalyst serves as a reusable catalyst for the synthesis of α,α-
dibromoketones from alkynes with NBS as bromine source. The reaction proceeds with
appreciable efficiency and the catalyst could be recovered with the help of an external
8

magnet and reused for successive reactions without significant loss in the catalytic
activity.
Experimental
The chemicals used are of analytical grade and purchased from Himedia
Chemicals (India). Compounds were purified using flash column chromatography over
silica gel (230–400 mesh, SRL chemicals). Wide angle X-ray powder diffraction (XRD)
patterns were recorded to characterize surface fictionalization of CoFe
2
O
4
in CoFe
2
O
4
-SiO and
2
CoFe -SiO -SO H using a Bruker D-8 Advance X-ray diffractometer with Cu-Kα radiation at
2
O
4
2
3
wavelength 1.5418 Å operated at voltage 40 kV and current 30 mA. The XRD data were
collected in the 2Ө range of 5–80°. Affinity-1 SHIMADZU, FT-IR spectrophotometer was used
for recording the FT-IR spectra of CoFe
pallets. Scanning electron micrographs (SEM) and energy dispersive X-ray spectrometer (EDX)
of synthesized CoFe -SiO -SO H sample were taken for investigation of particle morphology
and quantitative elemental analysis by capping the sample with gold, using a JEOL JSM-6360
scanning electron microscope. The particle size of the CoFe -SiO -SO H composite was
2
O
4
-SiO
2
and CoFe
2
O
4
-SiO
2
3
-SO H in the form of KBr
2
O
4
2
3
2
O
4
2
3
investigated by transmission electron microscopy (200 KV TEM, JEOLJEM 2100). Thermo
gravimetric analysis (TGA) of the parent and sulfonic acid functionalized samples were recorded
on Mettler Toledo TGA/DSC 1, STAR system analyzer in the temperature range 300K–1023K
with heating rate of 7 K min-1 under nitrogen atmosphere. The magnetic properties of the
synthesized CoFe
2
O
4
, CoFe
2
O
4
@SiO
2
, and CoFe
2
O
4
@SiO
2
-SO
3
H nanoparticles were
1
13
determined by vibrating sample magnetometer (Lakeshore 7410). H and C-NMR Spectra were
obtained in CDCl using Bruker 300 MHz instrument.
3
9

Synthesis of CoFe
2
4
O nanoparticles
CoFe
2
O
4
and silica-coated MNP (CoFe
2
O
4
@ SiO ) were synthesized by following
2
previously reported method.[13]
Synthesis of the CoFe
2
O
4
@ SiO nanocomposite
2
Approximately 200 mg of as prepared hydrophilic CoFe
2
4
O nanoparticles were dispersed
in 40 mL of deionised water under continuous ultrasonic irradiation for 1h and then mixed with 8
mL of aqueous ammonia solution (25 wt %) and 100 mL of ethanol. Next, 3.2 mL of TEOS was
added drop wise into the above solution under ultrasonic irradiation and kept for another 5h
under ultrasonication. The mixture was then placed under vigorous stirring at room temperature
for 24h. The products were separated by centrifugation and washed several times with water. The
final product (CoFe
2
O
4
@SiO ) was collected, dried at 50 °C for 30 min and placed in desiccator
2
overnight.
Synthesis of CoFe
2
O
4
@SiO
2
-SO H
3
CoFe
2
O
4
@SiO nanoparticles (0.125 g) were dispersed in dimethoxyethane (20 mL)
2
under ultrasonication for 60 min. The dispersion was cooled to 0 ºC (ice bath) and chlorosulfonic
acid (0.1 mL, density 1.753) was added drop-wise over a period of 30 min.[10] After completion
of the addition, the mixture was stirred for 5h at room temperature to allow for the complete
dissipation of HCl from the reaction vessel. The resulted MNPs were separated via centrifugation
and washed with ethanol and water before being dried in an oven at 70 °C to get
CoFe
2
O
4
@SiO
2
-SO H as a dark brown powder.
3
General Procedure for α,α-dibromoketones using CoFe
2
O
4
@SiO
2
-SO H catalyst
3
1
0

In a typical reaction, alkyne (1 mmol), NBS (3 mmol), and CoFe
2
O
4
@SiO
2
-SO H
3
catalyst (10 wt %) were taken in a round bottom flask in acetonitrile-water mixture (30:1). The
reaction mixture was placed under continuous stirring for 20 min at room temperature. The
reaction was monitored using TLC from time to time to ascertain the completion of the reaction.
After completion of the reaction, the catalyst was separated via magnetic decantation. The crude
product was purified by flash column chromatography using 5% petroleum ether-ethyl acetate as
eluent. The catalyst was magnetically separated, washed with excess of ethyl acetate, methanol
and dried in oven and kept in desiccator for further use. Spectroscopic data for the some of the
compounds are given below.
[
28,36]
2
,2-dibromo-1-phenylethanone (1b)
-1
Yield: 92%; Colourless oil; IR (KBr, cm ): υmax 3460, 3055, 3026, 2928, 1692, 1450,
1
1
257, 1188, 981, 804; H NMR (CDCl
3
, 300 MHz) δ (ppm): 8.08 (d, 2H, J = 7.8 Hz), 7.48-7.64
, 75 MHz) δ (ppm): 185.8, 134.4, 130.6, 129.5, 128.8,
1
3
(
m, 3H), 6.76 (s, 1H); C NMR (CDCl
3
3
9.8.
2
,2-dibromo-1-p-tolylethanone (3b)[28,36]
-1
Yield: 86%; Pale yellow solid, m.p. 96–98°C; IR (KBr, cm ): υmax. 3452, 3026, 3014,
1
2
7
928, 2850, 1708, 1684, 1426, 1271, 993, 966, 758, 689; H NMR (CDCl
3
, 300 MHz) δ (ppm):
1
3
.98 (d, 2H, J = 9 Hz), 7.31 (d, 2H, J = 6 Hz) 6.71 (s, 1H), 2.45 (s, 3H); C NMR (CDCl , 75
3
MHz) δ (ppm): 185.6, 145.7, 129.8, 129.6, 128.1, 39.9, 21.8.
Acknowledgment
1
1

Financial support from DST (Grant No. SR/NM/NS-18/2011(G)) is gratefully
acknowledged. MD thanks UGC for a research fellowship. We would like to
acknowledge the support from Department of Chemistry, Gauhati University, SAIF-GU,
SAIF-NEHU, CIF-IITG for all the analytical facilities during the course of investigations.
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1
4

Table 1. Optimization of the model reaction of phenylacetylene with NBS catalyzed by
CoFe @SiO -SO H catalyst by varying the reaction parameters.
2
O
4
2
3
Entry
2a Equiv.
Time(min)
Catalyst (Wt Solvent (mL)
Yield (%)a
%)
1
2
3
4
5
6
7
8
3
3
3
3
3
3
3
2
20
20
20
20
20
20
20
20
10
10
10
10
10
20
5
CH
3
CN-H
0:1
CN-H
0:1
2
O
O
93
77
60
65
nr
3
CH
3
2
1
THF-H
2
O
3
0:1
CH
3
COCH
3
-
H
2
O 30:1
DCM: H
2
O
3
0:1
CH
CH
CH
3
3
3
CN-H
0:1
CN-H
0:1
CN-H
2
2
2
O
90
46
63
3
O
O
3
10
1
5

3
0:1
CN-H
0:1
CN-H
0:1
CN-H
0:1
9
3
3
3
24 h
24 h
24 h
–
CH
CH
CH
3
3
3
2
2
2
O
O
O
12
26
33
3
1
0
1
10b
10c
3
1
3
a
Reaction condition: Phenylacetylene (1mmol), NBS (3 mmol), catalyst (10 wt%), CH
3
CN:H
2
O (3 mL), rt, 20 min. Isolated
b
c
yield after column chromatography, Cobalt ferrite catalyst, Cobalt ferrite-silica catalyst.
1
6

Table 2. Synthesis of α,α-dibromoketones from alkynes using sulphonic acid functionalized
cobalt ferrite
Reaction condition: Alkynes (1mmol), NBS (3 mmol), catalyst (10 wt%), CH
Yields correspond to isolated products after column chromatography,
3 2
CN:H O (3 mL, 30:1), rt, 20 min.,
1
7

Figure 1. FTIR Spectra of CoFe
2
O
4
MNPs and CoFe
2
O
4
@SiO
2
–SO H MNPs
3
1
8

Figure 2. XRD pattern of (a) CoFe
CoFe @SiO –SO H MNPs.
2
O
4
MNPs (b) CoFe
2
O
4
@SiO MNPs and (c)
2
2
O
4
2
3
1
9

Figure 3. SEM image of CoFe
2
O
4
@SiO
2
–SO H MNPs
3
2
0

Figure 4. Reusability of the catalyst
2
1

Scheme 1. Synthesis of α,α-dibromoketones.
2
2

Scheme 2. Synthesis of sulfonic acid functionalized Cobalt ferrite MNPs.
2
3

Scheme 3. Plausible reaction mechanism.
2
4