Pyridiniumporphyrazinato oxo-vanadium tribromomethanide as a new source of Br+ catalyst for the chemo and homoselective oxidation of sulfides and benzylic alcohols

Article abstract of DOI:10.1016/j.poly.2019.05.007

The present study describes the design and synthesis of novel nano N-bromo porphyrazin (N-bromo tetra-2,3-pyridiniumporphyrazinato oxo-vanadium tribromomethanide [VO(TPPABr)] CBr₃) as an efficient, recyclable and thermal stable heterogeneous catalyst for chemo and homoselective oxidation of sulfides to sulfoxides and benzyl alcohols to benzaldehydes. This ecofriendly heterogeneous catalyst was fully characterized by FT-IR spectra, UV–Vis spectra, X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and thermal gravimetric analysis (TGA), energy-dispersive X-ray spectroscopy (EDX) and elemental analysis (CHN). The synthesized catalyst exhibited a high-performance and considerable reusability.

Full text of DOI:10.1016/j.poly.2019.05.007

Accepted Manuscript
Pyridiniumporphyrazinato oxo-vanadium tribromomethanide as a new source
of Br+ catalyst for the chemo and homoselective oxidation of sulfides and ben-
zylic alcohols
Maliheh Safaiee, Mahtab Moeinimehr, Mohammad Ali Zolfigol
PII:
S0277-5387(19)30318-3
https://doi.org/10.1016/j.poly.2019.05.007
POLY 13931
DOI:
Reference:
Polyhedron
To appear in:
Received Date:
Revised Date:
Accepted Date:
1 March 2019
21 April 2019
2 May 2019
Please cite this article as: M. Safaiee, M. Moeinimehr, M.A. Zolfigol, Pyridiniumporphyrazinato oxo-vanadium
tribromomethanide as a new source of Br+ catalyst for the chemo and homoselective oxidation of sulfides and
benzylic alcohols, Polyhedron (2019), doi: https://doi.org/10.1016/j.poly.2019.05.007
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Pyridiniumporphyrazinato oxo-vanadium tribromomethanide as a new
source of Br+ catalyst for the chemo and homoselective oxidation of sulfides
and benzylic alcohols
a
a
b

Maliheh Safaiee , Mahtab Moeinimehr , Mohammad Ali Zolfigol
a Department of Medicinal Plants Production, University of Nahavand, Nahavand, 6593139565, Iran.
b Department of Organic Chemistry, Faculty of Chemistry, Bu-Ali Sina University, Hamedan 6517838683, Iran.
*Corresponding Author: Fax: (+98)-813 349 3009.
E-mail: azalia_s@yahoo.com & msafaiee@nahgu.ac.ir.
Abstract
The present study describes the design and synthesis of novel nano N-bromo porphyrazin
(N-bromo tetra-2,3-pyridiniumporphyrazinato oxo-vanadium tribromomethanide [VO(TPPABr)]
CBr3) as an efficient, recyclable and thermal stable heterogeneous catalyst for chemo and
homoselective oxidation of sulfides to sulfoxides and benzyl alcohols to benzaldehydes. This
ecofriendly heterogeneous catalyst was fully characterized by FT-IR spectra, UV-Vis spectra, X-
ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy
(TEM), and thermal gravimetric analysis (TGA), energy-dispersive X-ray spectroscopy (EDX)
and elemental analysis (CHN). The synthesized catalyst exhibited a high-performance and
considerable reusability.
Keywords: Chemoselective oxidation, Homoselective oxidation, molten salt, Tetra-2,3-
pyridiniumporphyrazinato oxo-vanadium tribromomethanide, Oxidation of sulfides, Oxidation of
benzyl alcohols.
1. Introduction
1

The development of heterogeneous catalysts with the possibility of facile separation and
recycling increase in their catalytic activity, as minimization of undesired toxic wastes have
increasingly gained attention.
Selective oxidation of organic compounds are important from both industrial and green
chemistry and play a crucial role in organic chemistry. In particular, the oxidation of alcohols to
aldehydes or ketones and sulfide to sulfoxide has been widely investigated. Recently, a large
number of oxidants such as copper (II) phthalocyanine [1], zirconium tetrachloride [2], Fe3O4/PEG-SO3H
[3], o-methyl-substituted IBX [4], HNO3/FeBr3 [5], heteropolyacids [6], Triflic Anhydride [7], VO(acac)2
[8], PMI-Ru [9], CoFe2O4@glycine-M [10],
CoFe2O4@SiO2‐CPTES‐Guanidine‐Cu(II) [11]
Fe3O4@MCM-41@VO-SPATB [12], Co(III)@Fe3O4@SiO2 salen complex [13] etc. have been applied
in this area. Due to the several disadvantages, such as high activity, hazardous volatility, the bromine as
an efficient catalyst [14–17] has been reported for a wide range of oxidation reactions but nowadays its
application has been restricted. Therefore, the introduction of the new sources of bromine with the lack of
mentioned problems is necessary [18,19]. Organic N-Br reagents which had been reported for the in
situ generation of Br+ [20–22] are as N-bromosuccinimide (NBS) [23,24], 1,3-dibromo-5,5-
dimethylhydantoin (DBH) [25], 2,4,4,6-tetrabromohexa-2,5-dienone (TBCO) [26–28],
pyridinium tribromide [16] and poly(1-vinyl-3-dodecylimidazolium tribromide)/TBN [29], Br3-
TEDETA@Boehmite [30]. These reagents had been used as efficient oxidizing agents for the
various oxidation reactions. Nevertheless, production of hazardous compounds or stoichiometric
amounts of the corresponding organic wastes is inevitable when the above-mentioned reagents
are used. Moreover, final products must be purified by using column chromatography because of
the solubility of the reagents in the common organic solvents.
In the past decades, metallophthalocyanines (MPcs) as the efficient biomimetic catalysts
with a high potential oxidation have been extensively reviewed due to their rather cheap, facile
2

preparation in a large scale, thermal and chemical stability and easy access [31–34]. In addition,
poor solubility of metallophthalocyanines in common organic solvents can be considered as the
most prominent point in their applications as the heterogeneous catalysts. Regarding to the
outlook of chemical processes is moving towards the goal of green chemistry principles, the
design, synthesis, and applications of safe and recyclable catalysts have been the serious
challenges for chemists and chemical engineers since many years ago. As a result, rational
design, synthesis and applications of the new heterogeneous and reusable
metallophthalocyanine-based molten salt which is susceptible for in situ generation of bromium
(Br+) are our main interest. Thus, pyridiniumporphyrazine as a subclass of phthalocyanine (Pc)
aza analogs in which all four benzene rings are replaced by pyridines was synthesized on the first
step (Scheme 1). Then novel nano molten salt N-bromo tetra-2,3-pyridiniumporphyrazinato oxo-
vanadium tribromomethanide [VO(TPPABr)]CBr3 as an efficient heterogeneous catalyst was
synthesized (Scheme 2) and applied for the selective oxidation of sulfides and benzyl alcohols to
their corresponding sulfoxides and aldehydes via in situ generation of Br+ in the presence of
H2O2 as an oxidant at room temperature.
3

Scheme 1. Synthesis of novel nano N-bromo tetra-2,3-pyridiniumporphyrazinato oxo-vanadium
tribromomethanide [VO(TPPABr)]CBr3
4

Scheme 2. Oxidation of sulfides and benzyl alcohols to the corresponding sulfoxides and
aldehydes using N-Bromo nano vanadium oxo molten salt [VO(TPPABr)]CBr3
2. Experimental Section
2.1. General information
All chemicals were purchased from Merck, Sigma-Aldrich and Across Organic Chemical
Companies. Solvents were dried, distilled and stored over molecular sieves. All known
compounds were identified by comparing their melting points and spectral data with those
reported in the literature. Progress of the reactions was monitored by thin layer chromatography
(TLC) using silica gel F254 plates. Melting points were measured with a Thermo Scientific
apparatus. Centrifugation procedures were carried out in an Anke TDL80-2B. Fourier transform-
infrared spectra (FT-IR) were recorded on a Perkin-Elmer FT-IR spectrometer 17259 using KBr
disks. Ultraviolet-visible spectra were investigated using
a
Perkin-Elmer UV-Vis
Spectrophotometer 35. The samples (10 mg) for UV-Vis study were prepared in aqueous ethanol
solutions (EtOH/H2O: 10/1) by dispersing process. The 1H NMR (400 MHz) and 13C NMR (100
MHz) were obtained using a Bruker DRX-400 using deuterated DMSO as a solvent, unless
5

otherwise stated. Chemical shifts are given in δ (parts per million) and the coupling constants (J)
1
in hertz. Data for HNMR are reported as follows: chemical shift (ppm), multiplicity (s, singlet;
d, doublet; t, triplet; q, quartet; m, multiplet; and br, broad), coupling constant (Hz), and
integration. Thermal gravimetric analyses (TGA) were performed on a Rheometric Scientific
STA 1500 under a nitrogen atmosphere at 25 °C and by heating rate of 20 °C min-1 up to 700 °C.
Transmission electron microscopy (TEM) and Scanning electron microscopy (SEM) images
were performed by a Zeiss-EM10C-100 KV and ZEISS SIGMA VP-500 which was equipped
with an X-ray detector Oxford Instrument for microanalysis (EDX) respectively. X-ray
diffraction (XRD) analyses were carried out by a Panalytical X’ Pert Pro apparatus using a
monochromatized Cu Kα (λ= 0.154nm) X-ray source in the range 5° < 2θ < 80°. Inductive
coupling plasma mass spectrometer (ICP-MS) was performed with an Agilent 7700x apparatus
equipped with HMI (high matrix introduction).
2.2. General procedure for the synthesis of a [VO(TPPABr)]CBr3
[VO(TPPA)] was prepared via the procedure described by Yokote et al. [35]
Urea (13 g, 2/145 mol), ammonium molybdate [(NH4)6Mo7O24.4H2O] (92 mg, 0.074 mmol), 2,3-
pyridine-dicarboxylic acid (2 g, 11.96 mmol), and V2O5 (200 mg, 10.99 mmol) were mixed
under solvent-free condition until a homogeneous mixture was achieved. The mixture was placed
in a heating mantle, sealed and heated at 200-210 oC for 5 h. Then, the mixture was cooled up to
the room temperature. The crude blue solid product (8.93 g) was crushed and washed repeatedly
o
with warm water (80 C), a warm aqueous NaOH solution (5% w/v), warm water, a warm
diluted aqueous hydrochloric acid (2.5% v/v), and warm water respectively. Finally, the obtained
o
crude product was dried at 100 C under vacuum and purified by dissolving in concentrated
sulphuric acid and then pouring into ice-water. The precipitated product was collected via
6

centrifugation (ca. 200 rpm /10 min) and transferred to a fine sinter glass funnel where the mass
was carefully washed with hot water, a warm aqueous sodium carbonate solution and finally
o
with warm water. [VO(TPPA)] was dried at 100 C under vacuum (isolated yield 65%; M.p.:
o
288-290 C) (Scheme 1). The vanadium content of the [VO(TPPA)] was measured using ICP
analysis, which showed value of about 1.251 wt% .
In the next step, to a round-bottomed flask (50 mL) including a solution of
tetrabromomethane (4 mmol; 1.327 g) in CH3CN (20 mL), [VO(TPPA)] (1.251 mg) was added
and stirred over a period of 5 h under reflux conditions[36]. The achieved greenish blue solid
[VO(TPPABr)]CBr3 product was washed three times with diethyl ether (3×5 mL), and then dried
under vacuum (isolated yield 91%) (Scheme 1). [VO(TPPABr)]CBr3 was fully characterized by
UV-Vis, FT-IR, elemental analysis (CHN), ICP, EDX, TGA, DTG, SEM and TEM.
2.3. Oxidation of benzyl phenyl sulfide to benzyl phenyl sulfoxide
A mixture of methyl phenyl sulfide (0.124 g, 1 mmol), [VO(TPPABr)]CBr3 (10 mg), and
H2O2 (30 % aqueous, 6 mmol, 0.204 g, 0.183 mL) in CH3CN (3 mL) was stirred for 120 min at
room temperature. At the end of reaction, the catalyst was filtered and the solvent was
evaporated to afford the crude sulfoxide product. Finally, the product was purified by column
chromatography using ethyl acetate/hexane (1.5:8.5) as eluent to give methyl phenyl sulfoxide as
a white solid in 93% yield.
2.4. Oxidation of benzyl alcohol to benzaldehyde
Benzyl alcohol (0.103 mL, 0.108 g, 1 mmol), H2O2 (30 % aqueous, 6 mmol, 0.204 g,
0.183 mL), and [VO(TPPABr)]CBr3 (5 mg) were added to a flask in CH3CN (3 mL) and then
7

stirred for 120 min at room temperature and the reaction was followed by TLC. After completion
of the reaction, the catalyst was separated from the mixture of reaction and it was reused in the
six same reactions with no dramatic changes in its reactivity.
2.5. Selected representative spectral data
1
2l: IR (KBr) 690, 1048, 1203, 1422, 2920, 3298-3365 cm-1 ; H NMR (400 MHz, CDCl3) δ
(ppm) 1.71 (t, 2H, OH), 2.80 (t, 2H), 2.81 (t, 2H), 2.82 (t, 2H), 3.04 ( t, 2H), 3.82 (q, 2H), 4.2 (q,
2H); 13C NMR (CDCl3) δ (ppm) 30.7, 34.4, 54.3, 54.5, 61.6, 62.
1
2d: IR (KBr) 749, 1471, 1610, 3242, 3315 cm-1 ; H NMR (400 MHz CDCl3) δ (ppm) 6.63 (s,
1H, NH), 6.89 ( d, 2H), 7.08 (t, 2H), 7.18 ( t, 2H), 7.37 (d, 2H); 13C NMR (CDCl3) δ (ppm)
116.4, 116.6, 121.6, 122.3, 132.6, 137.8.
1
4c: IR (KBr) 833, 1286, 1454, 1598, 1667, 3207 cm-1; H NMR (400 MHz, CDCl3) δ (ppm)
6.23(t, 1H, OH), 7.01 (d, 2H), 7.85 (d, 2H), 9.89 (s, 1H); 13C NMR (CDCl3) δ (ppm) 116, 129.9,
132.5, 162.4, 191.2.
3. Results and discussion
Scientific-based development of solid and stable bromine sources are in great demand.
As part of our interest on developing of efficient and convenient methods and exploring
applications of reagents for in situ generation of bromium (Br+) [37–39], metallophthalocyanine-
based catalysts [40-42], and chemo and homo selective oxidation [43–48] herein, we would like
to synthesis, heterogeneous bromo catalyst for in situ generates Br+ to prevent using various
substrate for production of these species, and avoid using bromine as a starting material.
Therefore, phthalocyanine with thermal and chemical stability and low solubility was chosen
8

accordingly. Hydrogen peroxide was used as an eco-friendly oxidant for the selective oxidation
of sulfides and benzylic alcohols to their corresponding sulfoxides and aldehydes respectively in
the presence of [VO(TPPABr)]CBr3 as a reusable catalyst at room temperature. (Scheme 2)
3.1. Characterization of [VO(TPPABr)]CBr3
[VO(TPPA)] was prepared by the process which is described by Yokote et al. [35], using
urea, tetrahydrated ammonium molybdate [(NH4)6Mo7O24.4H2O], 2,3-pyridine-dicarboxylic acid
and vanadium pentaoxide. Then, [VO(TPPABr)]CBr3 was prepared by refluxing [VO(TPPA)]
with tetrabromomethane in acetonitrile as the solvent. Characterization of [VO(TPPABr)]CBr3
was carried out using various techniques such as UV-Vis, FT-IR, ICP, EDX, TGA, DTG, XRD,
SEM, and TEM.
3.1.1. UV-Vis analysis
UV-Vis spectra of [VO(TPPA)] and [VO(TPPABr)]CBr3 in ethanol are shown in Fig. 1.
The Q band of [VO(TPPA)] is observed at 831 nm. A clear absorbance which is located at 381
nm is attributed to the B band. Also, the band at 591 is probably due to charge transfer (Figure
1). We followed the appearance in UV-Vis spectra after conversion of [VO(TPPA)] to
[VO(TPPABr)]CBr3. The blue-shift at ~110 nm of the Q band and its split has been attributed to
[VO(TPPABr)]CBr3. Moreover, the bands at 652 and 572 have been completely disappeared.
According to these observations synthesis of porphyrazine molten salt was confirmed.
9

Fig. 1. UV-Vis spectra of [VO(TPPA)] (a), and [VO(TPPABr)]CBr3 (b).
3.1.2. FT-IR results
FT-IR spectra of [VO(TPPABr)]CBr3 is shown in Fig. 2. the absorption bands at 654 cm-
and 997 cm-1 are related to the stretching vibrational modes of V=O and also the absorption
1
bands at 697 cm-1 and 654 cm-1 are attributed to the stretching vibrational modes of N–Br and C–
Br. The absorption bands at 1690 cm-1 and 1498 cm-1 are considered to C=N, and C=C
stretching, respectively. Also, the absorption bands at 3313 cm-1, 3069 cm-1 and 2920cm-1 are
related to the stretching vibrational modes of N-H in pyridinium ring. The correct synthesis of
[VO(TPPABr)]CBr3 was proven by comparing of FT-IR results of [VO(TPPABr)]CBr3 and
[VO(TPPA)] and starting materials.
10

Fig. 2. FT-IR spectra of [VO(TPPA)] (a), CBr4 (b), and [VO(TPPABr)]CBr3 (c)
3.1.3. CHN and EDX data
Elemental analysis from the synthesized catalyst [VO(TPPABr)]CBr3 and [VO(TPPA)] are
presented in Table 1. The anticipated elements in the structure of these compounds are C, H and
N.
Table 1. Elemental analysis of [VO(TPPABr)]CBr3 and [VO(TPPA)]
Element [VO(TPPABr)]CBr3 [VO(TPPA)]
C
H
N
40.29
2.55
37.92
2.13
26.65
25.80
11

The EDX spectrum of [VO(TPPABr)]CBr3 shows the presence of the expected elements in
the structure of this compound. The results are as follows: C (37%), N (29.18%), V (4.24%), Br
(7.64%) within the [VO(TPPABr)]CBr3 and synthesis of the catalyst is approved as it is
illustrated in Fig. 3
2500
2000
N K
1500
1000
BrL
VL
O K
500
VK
VK
BrK
BrK
keV
15
0
0
5
10
Fig. 3. EDX analysis of [VO(TPPABr)]CBr3
3.1.4. TG study
TGA and DTG analysis of [VO(TPPA)] and [VO(TPPABr)]CBr3 are shown in (Fig. 4 a
and b). Based on the TGA curve of [VO(TPPA)] and [VO(TPPABr)]CBr3 profile the first mass
loss of below the 100 °C is related to the residual physisorbed water and organic solvents which
have been used for the preparation of catalyst (Fig. 4, a and b). The second mass loss around 300
°C is related to [VO(TPPA)], and at much lower temperature and in two steps related to
[VO(TPPABr)]CBr3. The DTG analysis diagrams display a general negative downward slope
between 25 °C to 600 °C in which, decomposition of [VO(TPPABr)]CBr3, [VO(TPPA)] and
their precursor in nitrogen atmosphere were exothermic. The comparison between the TGA and
DTG results of [VO(TPPABr)]CBr3 and [VO(TPPA)] can be the demonstrative evidence for the
correct synthesis of the catalyst.
12

Fig. 4. TGA and DTG analysis of [VO(TPPA)] (a) and [VO(TPPABr)]CBr3 (b)
3.1.5. Morphological features
The particle size, shape and morphology of [VO(TPPABr)]CBr3 and [VO(TPPA)] were
investigated by XRD, SEM and TEM analysis (Figures 5-7). The X-ray diffraction patterns of
[VO(TPPABr)]CBr3 and [VO(TPPA)] are shown in Fig 5. Scherrer equation [D = Kλ/(β cosθ)]
can estimate the average crystalline size of [VO(TPPABr)]CBr3 and [VO(TPPA)]. On the other
hand the averaged inter planar distance of these compounds were calculated according to the
Bragg equation: dhkl = λ/(2 sinθ) and summarized in Table 2. Investigation of patterns of XRD
for [VO(TPPABr)]CBr3 and [VO(TPPA)] which are results of XRD analysis, can be confirmed
the successful synthesis of the catalyst.
Morphological properties and surface texture of the catalyst were studied and presented by
SEM in Fig. 6. According to the SEM analysis, the morphology of [VO(TPPABr)]CBr3 is
fibrillar with appropriate monodispersity. Also, the particle size of this compound has been
calculated between 30-36 nm.
13

Fig. 5. XRD patterns of [VO(TPPABr)]CBr3 (a) and [VO(TPPA)] (b)
Table 2. XRD data collection of [VO(TPPABr)]CBr3 and [VO(TPPA)]
Entry
2θ
Peak width [FWHM]
(degree)
Size
Inter planer distance
[nm]
[nm]
[VO(TPPABr)]CBr3
1
19.073
27.358
31.884
33.905
0.14
0.33
0.18
0.08
57.41
24.64
45.58
102.95
0.46494
0.325728
0.280450
2.64180
2
3
4
[VO(TPPA)]
1
6.47
1.1
14
7.23
13.65388

26.54
31.890
34.010
2.40
0.24
0.21
3.4
3.35598
2.80397
2.63389
34.42
39.56
(b)
(a)
Fig. 6. SEM images of [VO(TPPABr)]CBr3 (a and b)
15

(b)
(a)
Fig. 7. TEM images of [VO(TPPABr)]CBr3 (a and b)
3.2. Application of [VO(TPPABr)]CBr3 in the oxidation reactions
3.2.1. Oxidation of sulfides to sulfoxids
At the beginning, we made a conscious effort to develop a heterogeneous catalytic system
for the oxidation of sulfides. For this purpose, oxidation of methyl phenyl sulfide was chosen as
a model reaction and different reaction parameters such as solvent, amount of catalyst and
hydrogen peroxide were studied. The best results are summarized in (Tables S1).
Scheme 3. Oxidation of sulfides to sulfoxides using [VO(TPPABr)]CBr3
16

As, it can be seen in (Tables S1), 10 mg of catalyst, and 6 mmol of H2O2 were used for
oxidation reaction of methyl phenyl sulfide to its corresponding sulfoxide successfully in
acetonitrile at room temperature.
Table 3. Oxidation of sulfides (1) to the corresponding sulfoxides (2) using H2O2 (30%), and a
catalytic amount of [VO(TPPABr)]CBr3 in acetonitrile at room temperature
Time Yielda
MP (° C) /
Entry
a
Sulfides
Sulfoxide
(h)
(%)
MP (° C) [Reported]
Oil [49]
2
93
b
3
63
145-147 / not found
c
8
93
87
117-119 / 115-117 [50]
200-201 / not found
d
12
e
f
4
48
55
100-102 / not found
68-70 / 68-70 [51]
38
17

134-136 / 133-135 [51]
89-91 / not found
g
h
33
23
91
90
149-151 / 150-152 [51]
Oil [52]
i
j
22
23
22
79
97
77
k
127-129 / not found
l
23
33
61
96
95-97 / not found
Oil [51]
m
n
44
72
158-160 / not found
o
p
47
32
39
29
203-204 / not found
Oil [51]
Reaction conditions: sulfide (1 mmol), H2O2 (0.204 g, 6 mmol, 0.183 mL), CH3CN as a solvent, stirring at room
temperature.
aIsolated yield
18

Therefore, we decided to apply [VO(TPPABr)]CBr3 protocol for the selective oxidation
of a wide range of sulfides (1) to their corresponding sulfoxides (2). According to the obtained
results that are collected in Table 3, the procedure is general and effective. The most important
advantage of this catalyst is selective oxidation of sulfides to sulfoxides and lack of the over-oxidation to
their corresponding sulfones. On the other word, the described system has an appropriate
chemoselectivity. In order to prove this claim, substrates with various functional groups such as
COOCH3, COOH, OH, pyridine and carbon-carbon double bond were applied for the oxidation
reaction. The result of the reaction confirmed this fact that only oxidation of sulfur atom
occurred without any extra structural changes in their other functional groups (Scheme S1).
Scheme 4. Dual selectivity (chemo and homoselective) oxidation of 1,8-dihyroxy-3,6-
ditiaoctan using [VO(TPPABr)]CBr3
19

To investigate the reusability of [VO(TPPABr)]CBr3, the catalyst was filtered from the
reaction mixture and washed with CH3CN. The catalyst was capable of being applied in 6 cycles
for the oxidation of methyl phenyl sulfide to its corresponding sulfoxide without any loss of
catalyst activity (Fig. 8). SEM and EDX of [VO(TPPABr)]CBr3 after the sixth run, confirmed the
stability of its structure in the course of reaction and during the recycling procedure (Fig. 9).
There was also no change in the morphology and size during the reaction.
Fig. 8. Recyclability study of [VO(TPPABr)]CBr3 in the oxidation of methyl phenyl
sulfide
20

Fig. 9. EDX (a) and SEM (b) analysis of [VO(TPPABr)]CBr3 after recycling
21

Scheme 5. A plausible mechanism for the oxidation of sulfides to sulfoxides
A proposed mechanism for the oxidation of sulfide to sulfoxide is shown in Scheme 5. The
catalyst acts as a N-bromo reagent for in-situ generation of Br+. Sulfide attacks to the Br+ and the
S-Br bond is formed. After adding H2O2 and going through the unstable species II, the desired
sulfoxide was formed.
3.2.2. Oxidation of alcohols to aldehydes
22

After the successful oxidation of a series of sulfide with high efficiency and selectivity, the
catalytic activity of [VO(TPPABr)]CBr3 was also investigated for the oxidation of alcohols to
aldehydes. In order to optimize the oxidation conditions, we investigated the oxidation of benzyl
alcohols as a model substrate in various conditions (different temperatures, amounts of catalyst
and solvents). As indicated in (Tables S2) the best results in terms of reaction time and yield are
obtained when 5 mg of the catalyst, 6 mmol of H2O2 and acetonitrile as a solvent are applied at
room temperature.
Scheme 6. Oxidation of benzyl alcohols to benzaldehydes using [VO(TPPABr)]CBr3
As it can be seen in (Tables S2), 5 mg of the catalyst and 6 mmol of H2O2 provided
oxidation reaction of benzyl alcohols to benzaldehyde successfully in acetonitrile at room
temperature. Based on the results which are indicated within the (Tables S1), similar reactions
were used for a series of benzyl alcohol derivatives. The results are presented in Table 4.
23

Table 4. Oxidation of benzyl alcohols 3 to the corresponding benzaldehydes 4 using H2O2 (30%)
and a catalytic amount of [VO(TPPABr)]CBr3 in acetonitrile at room temperature
Benzyl alcohols
Time Yielda
MP (° C) / MP (°
C) [Refrence]
Entry
Benzaldehydes
(mmol)
(h)
2
(%)
90
Oil
Oil
a
5
4
85
89
b
c
113-115 / 117b
6
9
76
65
47-50 / 48 [53]
d
e
146-147 / 147.5b
Reaction conditions: benzyl alcohols (0.103 mL, 0.108 g, 1 mmol), H2O2 (6 mmol, 0.204 g, 0.183 mL), catalyst (5
mg), CH3CN as a solvent, stirring at room temperature.
aIsolated yield.
b PubChem
According to the results which are summarized in Table 4, the oxidation reaction of
benzyl alcohol derivatives to the corresponding aldehydes was done well. Also, in order to
investigate the chemoselectivity in the oxidation of various alcohols, aliphatic primary,
secondary and benzylic alcohol have been examined using the described reaction conditions. The
oxidation reaction appears to be selective for the oxidation of benzyl alcohols, and aliphatic
primary and secondary hydroxyl group remained intact during the reaction under the optimized
conditions.
24

Scheme 7. Chemoselective oxidation of benzyl alcohol in the presence of propanol and
cycloxexanol
Based on our observations, the proposed mechanism for the oxidation of benzyl alcohol
is as following. At the beginning, the electron lone pair of oxygen attack to the catalyst and O-Br
band (I) is formed. After adding H2O2 and eliminating of HOBr and going through the unstable
species (II), the intermediate III undergoes degradation to form the corresponding aldehyde VI .
25

Br
Br
Br
Br
N
N
Br
Br
N
Br
Br
Br
Br
Br
O
V
N
N
N
N
N
Br
Br
N
N
Br
Br
Br
Br
Br
N
Br
N
Br
N
OH
O
N
Br
Br
Br
Br
H
Br
Br
Br
Br
Br
H
O
O
Br
N
Br
O
H
Br
OH
(IV)
O
Br
O
Br
Br
H
(III)
Br
H
Br
Br
HOBr
H2O2
HOBr
+
+
Br
N
(II)
Br
H
H2O
Br
Br
O
O
O
Br
Br
H
H
Br
Br
Br
Br
N
Br
N
Br
Br
(I)
O
H
O
H
N
Scheme 8. A proposed mechanism for the oxidation of benzyl alcohols
Reusability and recyclability of [VO(TPPABr)]CBr3 is one of the most important
advantages which makes catalyst useful for the commercial applications. In this regard, the nano
catalyst was examined in subsequent catalytic cycles using benzyl alcohol as a substrate. After
26

completion of the oxidation, the catalyst was filtered carefully, washed with acetonitrile and
finally dried. This process was repeated six more times, giving the desired products in good
yields without any significant loss of its initial catalytic reactivity (Fig. 10). The simple
experimental and product isolation procedures, ease of recovery and reusability in the reaction
medium is anticipated to contribute to the progress of a green method for this reaction.
Fig. 10. Reusability investigation of [VO(TPPABr)]CBr3 in the oxidation reaction of benzyl
alcohol.
In order to showing, the role of Br+ in the N-Br catalyst, ([VO(TPPABr)]CBr3 ) and [VO(TPPA)]
were used in the oxidation reaction for a model substrate. The results demonstrate that the
oxidation of methyl phenyl sulfide and benzyl alcohols using [VO(TPPABr)]CBr3 as a catalyst
carried out with higher efficiency and shorter reaction time than [VO(TPPA)] under the same
conditions. The results are presented in the Table 5. In another study, to show the efficiancy of
the [VO(TPPABr)]CBr3, methyl phenyl sulfide was subjected to the oxidation reaction in the
27

presence of 1 mol% of CBr4, 6 mmol of H2O2 and CH3CN as a solvent at room temperature but
the reaction did not proceed even after 2 days .
Scheme 9. Oxidation of sulfides using CBr4 as a catalyst
Table 5. Comparision of oxidation of methyl phenyl sulfide and benzyl alcohol using H2O2
(30%) in acetonitrile at room temperature
Time (h)
[VO(TPPA)]/
Yield (%)a,b
[VO(TPPA)]/
Entry
1
Substrate
[VO(TPPABr)]CBr3
[VO(TPPABr)]CBr3
10/2
4/2
57/93
88/90
2
Reaction conditions: substrate (0.124 g, 1 mmol), H2O2 (6 mmol, 0.204 g, 0.183 mL) catalyst (5 mg), CH3CN as a
solvent, stirring at room temperature.
aIsolated yield
bCatalyst (5 mg)
3.3. Comparison with other catalysts for the oxidation of methyl phenyl sulfide
In order to evaluate the catalytic reactivity of [VO(TPPABr)]CBr3, the results were compared
for the oxidation of ethyl phenyl sulfide and benzyl alcohols with other those previously reported
methods in the literature. The results are presented in Tables 6-7.
[VO(TPPABr)]CBr3 has uniquely enhanced the synthesis of desired product in different terms
such as: reaction time, amount of catalyst and isolated yield. As depicted, it can be deduced that
the previously reported methods suffer from elevated temperatures, longer reaction times, or
harsh reaction conditions (Table 6).
28

Table 6. Comparison of [VO(TPPABr)]CBr3 with other those reported catalysts for the oxidation
of methyl phenyl sulfide
Catalyst amount/ Temperature
Yield
(%)
Entry
Catalyst
Time (min)
(oC)
KBr in the presence wet SiO2 (50%
w/w)
0.3 mmol / rt
1
2
3
4
14 h
5
55 [19]
87 [39]
95 [43]
90 [54]
70 mg / rt
DPTBE in CH2Cl2/ H2O
KBr in the presence wet SiO2 (50%
w/w)
0.05 mmol, 200 mg / rt
20
755 mg. 400 mg / 40
850 mg / 40
NaBrO3-NH4Cl
MMPP
180
60
5
6
7
100 [55]
95 [56]
93 [57]
1 mmol / 25
10
2,6- DCPCC
10 mol% / 35-40
NBS under Neutral Baffered
5 h
NBS in the presence of β-
1 mmol / rt
8
9
120
30
95 [58]
70 [4]
cyclodextrin in water
1mmol / reflux
Me-IBX
0.005 (2000) mmol (S/C) / 50
Na2WO4, C6H5PO3H2 and PTC
120
150
97 [59]
97 [29]
10
11
Poly[VDIM]Br3 / TBN
25 mg / 25
7 mg / 40
CoFe2O4@SiO2‐
CPTES‐Guanidine‐Cu(II)
20
20
95 [60]
93 [29]
25 [7]
12
Br3-TEDETA@Boehmite
40 mg / rt
1.7 gr / 20
13
14
Triflic Anhydride Complex
[VO(TPPABr)]CBr3
25
93 our
work
10mg/rt
15
2(h)
29