Vanadyl β-tetrabromoporphyrin: Synthesis, crystal structure and its use as an efficient and selective catalyst for olefin epoxidation in aqueous medium

Article abstract of DOI:10.1039/c8ra09825e

We hereby report the synthesis, characterization and catalytic applications in the epoxidation of alkenes by a vanadyl porphyrin having bulky bromo substituents at the β-positions viz. vanandyltetrabromotetraphenylporphyrin (1). The synthesized porphyrin was characterized by various spectroscopic techniques like UV-visible, FT-IR, EPR, MALDI-TOF mass spectrometry and single crystal X-ray analysis. Porphyrin 1 has a nonplanar structure as indicated by its X-ray structure, DFT and electrochemical studies. 1 was analyzed for its catalytic application in the epoxidation of various alkenes. The catalytic reactions were carried out in CH₃CN/H₂O mixture in 3:1 (v/v) ratio. 1 displayed good efficiency in terms of mild reaction conditions, lower reaction temperature and minimal catalyst amount consumption. 1 exhibited excellent selectivity, high conversion efficiency and huge TOF (7600-9800 h^-1) in a significantly low reaction time of 0.5 h. Catalyst 1 was regenerated at the end of various catalytic cycles making it reusable and industrially important.

Full text of DOI:10.1039/c8ra09825e

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Vanadyl b-tetrabromoporphyrin: synthesis, crystal
structure and its use as an efficient and selective
catalyst for olefin epoxidation in aqueous medium†
Cite this: RSC Adv., 2019, 9, 10405
Tawseef Ahmad Dar,a Reshu Tomar,a Rasel Mohammad Mian,b
a
a
*
*
Muniappan Sankar
and Mannar Ram Maurya
We hereby report the synthesis, characterization and catalytic applications in the epoxidation of alkenes by
vanadyl porphyrin having bulky bromo substituents at the b-positions viz.
a
vanandyltetrabromotetraphenylporphyrin (1). The synthesized porphyrin was characterized by various
spectroscopic techniques like UV-visible, FT-IR, EPR, MALDI-TOF mass spectrometry and single crystal X-ray
analysis. Porphyrin 1 has a nonplanar structure as indicated by its X-ray structure, DFT and electrochemical
studies. 1 was analyzed for its catalytic application in the epoxidation of various alkenes. The catalytic reactions
were carried out in CH3CN/H2O mixture in 3 : 1 (v/v) ratio. 1 displayed good efficiency in terms of mild reaction
conditions, lower reaction temperature and minimal catalyst amount consumption. 1 exhibited excellent
selectivity, high conversion efficiency and huge TOF (7600–9800 hꢀ1) in a significantly low reaction time of
0.5 h. Catalyst 1 was regenerated at the end of various catalytic cycles making it reusable and industrially important.
Received 29th November 2018
Accepted 7th March 2019
DOI: 10.1039/c8ra09825e
rsc.li/rsc-advances
this functionalization, porphyrins can be actually tuned to attain
very good thermal and chemical stabilities.32,33
Introduction
For epoxidation reaction of alkenes, several catalysts of
different structures and natures are already known. The rst
satisfactory method for asymmetric alkene epoxidation was
developed by Sharpless et al. and involved titanium tetraiso-
propoxide as one of the components.34 Since then a large
number of catalysts for epoxidation of alkenes based on chro-
mium and manganese have been developed.35 Other transition
metals that have been exploited as good candidates for epoxi-
dation reactions include rhenium,36 iron,37 tungsten,38 tita-
nium39 and molybdenum.40 In earlier times the ‘oxygen
provider’ species were mainly molecular oxygen,41 organic
hydroperoxides,39 peracids and oxiranes.42 However, in recent
times, H2O2, because of its environment friendly nature has
emerged as one of the best oxidizing species for epoxidation of
alkenes.43–45 Most recently it has been found that presence of
bicarbonate ions along with H2O2 speeds up the epoxidation
reactions by many folds.46–48 Other than the general coordina-
tion complexes,49–51 several metalloporphyrins have also been
used as epoxidation catalysts in the presence of H2O2.52–54 We
have earlier reported the catalytic efficacy of an b-octa-
chlorovanadylporphyrin complex towards alkene epoxidation.55
In continuity of our efforts to develop versatile catalysts for
alkene epoxidation, we hereby report the facile synthesis,
characterization, spectral properties, X-ray structure and cata-
lytic applications of vanadyl tetrabromoporphyrin VOTPPBr4
(1). We have also observed that steric (bulky) and electron
withdrawing factors play a major role in determining the effi-
ciency of the catalysts for epoxidizing alkenes.
Despite rapid progress in synthetic organic chemistry over the
last several decades, epoxidation of alkenes remains an area of
focus owing to the use of its products in further reactions in
industry and commercial materials.1,2 Different types of catalysts
ranging from pure metals/elements to very complex molecules
speed up several useful and important reactions.3,4 Transition
metals are more specic catalysts and catalyze many organic
reactions like hydrogenation,5 epoxidation,6 polymerization,7
hydrogen transfer,8 hydroxylation,9 halogenation10,11 and some
other reactions.12,13 Porphyrins act as multifaceted ligands coor-
dinating with various metals to form metalloporphyrins that have
many applications.14–16 Porphyrins have additionally been found
to possess remarkable properties in the elds of solar energy,17
medicine,18 sensing,19 catalysis,20 non-linear optics and the like.21
In living systems also, porphyrins perform many vital functions
like oxygen transport and storage, photosynthesis, electron
transfer and as cofactors to many enzymes.22–24 Unsubstituted
porphyrins generally tend to undergo degradation25 in oxidation
type reactions and thus b- and meso-functionalization of
porphyrins serves well in tunability as well as stability.26–31 Due to
aDepartment of Chemistry, Indian Institute of Technology Roorkee, Roorkee, 247667,
India
bDepartment of Chemistry, Graduate School of Science, Tohoku University, 6-3 Aza-
Aoba, Aramaki, Sendai 980-8578, Japan
† Electronic supplementary information (ESI) available. CCDC 1880991. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/c8ra09825e
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The UV-visible spectrum of 1 exhibits one Soret band at
435 nm (3, 1.94 ꢁ 105) which is almost similar to H2TPPBr4 (436
nm) whereas it shows only two Q-bands: 560 nm (3, 1.94 ꢁ 104)
and 604 nm (3, 6.32 ꢁ 103) as compared to three in case of
H2TPPBr4 (533, 612 and 685 nm). Thus, the rst Q-band in case
of 1 is 27 nm red shied and the second Q-band is 8 nm blue
shied relative to H2TPPBr4 due to insertion of vanadium metal
as VO2+. The FTIR spectrum of 1 was recorded using KBr pellets
(Fig. S1, ESI†). In the FTIR spectrum of 1, we found character-
istic peaks corresponding to V]O (stretching) at 1010 cmꢀ1 and
C–Br (stretching) at 619 cmꢀ1. Notably, the N–H stretching
frequency at around 3300 cmꢀ1 was not observed which is
generally found in non-metallated (free base) porphyrins. The
calculated MALDI-TOF mass value for 1 is in good agreement
with the observed value (Fig. S2, ESI†). Single crystals for XRD
studies were obtained by diffusing methanol slowly into the
saturated toluene solution of 1 (vide infra).
Results and discussion
Synthesis and characterization
Vanadyl tetrabromoporphyrin VOTPPBr4 (1) was synthesized by
making certain modications to existing literature methods.
H2TPPBr4 was prepared from H2TPP using NBS.56 Target
porphyrin VOTPPBr4 (1) was then prepared from H2TPPBr4 using
VOSO4 and DMF by reuxing under argon atmosphere (Fig. 1).55,57
Different spectrometric techniques like UV-visible, FTIR,
MALDI-TOF-MS, EPR and single crystal XRD were used to
characterize the target compound 1. The UV-visible spectrum of
1 in CH2Cl2 is shown in Fig. 2 and the corresponding data is
shown in Table S1 in the ESI.†
Electrochemical studies
Cyclic voltammogram of 1 in distilled CH2Cl2 was recorded using
Pt working electrode, Ag/AgCl reference electrode and Pt wire as
counter electrode at 298 K. TBAPF6 recrystallized twice from
ethanol and vacuum dried for 6 hours was used as the supporting
electrolyte. 1 exhibited two reversible oxidation peaks at 1.28 V
and 1.46 V as shown in Fig. 3 and Table S2 in the ESI.† The rst
oxidation peak corresponds to the generation of porphyrin cation
radical due to loss of a single electron while the second oxidation
peak corresponds to the generation of porphyrin dication due to
loss of second electron. Similarly the rst and second reversible
reduction peaks were obtained at ꢀ0.84 V and ꢀ1.06 V, which
correspond to the generation of porphyrin anion radical and
porphyrin dianion respectively. Vanadium metal does not
undergo any redox changes during cyclic voltammetry. The
HOMO–LUMO energy gap (DE1/2) on the basis of electrochemical
Fig.
1 Molecular structure of synthesized vanadyl b-tetra- studies was found to be 2.12 V. Cyclic voltammetry supports the
bromoporphyrin 1.
non-planar nature of 1 and the electron decient nature of
porphyrin core as compared to VOTPP.58,59
Thermogravimetric analysis
Thermogravimetric analysis of 1 was carried out using alumina
as the reference powder with a heating rate of 10 ꢂC minꢀ1
Fig. 2 UV-visible spectrum of 1 in CH2Cl2 at 298 K at a conc. of 5.68 ꢁ Fig. 3 Cyclic voltammogram of 1 in distilled CH2Cl2 at 298 K using
10ꢀ6 M.
TBAPF6 as the supporting electrolyte.
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under normal aerial atmosphere. The thermogravimetric prole planar, distorted saddle conformation due to bulkier bromo
of 1 is given in Fig. S3 in the ESI.† 1 displayed excellent thermal groups. The optimized geometry of 1 is shown in Fig. 5.
˚
stability till 637 K despite the labile nature of C–Br bonds having
D24 in case of 1 was found to be ꢃ0.383 A. Likewise, DCb
bond energy of 285 kJ molꢀ1 only. The encouraging thermal (carbon atoms having –Br substituents were taken as b) value
˚
0
stability of 1 can be very useful in terms of industrial was calculated to be ꢃ0.855 A for 1. Similarly DCb (carbon
applications.
atoms attached to –H atoms were taken as b0) value for 1 was
˚
found to be as ꢃ0.725 A. Likewise, DM value was found to be as
˚
0.538 A. Various bond distance values in 1 were calculated as
˚
˚
˚
EPR spectral studies
0
0
1.593 A (V]O), 1.379 A (Cb–Cb) and 1.372 A (Cb –Cb ) respec-
tively. The full list of various bond lengths and bond angles for 1
is given in Table S3 in the ESI.† It has to be noted that bond
X-band EPR spectrum of 1 was recorded in toluene. 1 exhibited
a typical EPR spectrum corresponding to the paramagnetic
species of vanadium(IV) at 120 K as shown in Fig. 4. The number
of lines in the EPR spectrum of 1 were found to be equal to eight
corresponding to the I value of vanadium metal (I ¼ 7/2).
length between b-carbon atoms (Cb–Cb) having bromo substit-
uents is longer than bond length between b -carbon atoms (Cb –
Cb ) devoid of any substituents, which is a direct consequence of
bulkier bromo substituents on these carbon atoms. Similarly,
the deviation of carbon atoms having bromine substituents
0
0
0
The g value for 1 was found to be 1.96 while the corre-
k
sponding gt value was observed to be 1.99. Similarly, the A and
k
˚
from the mean plane (DCb ¼ ꢃ0.855 A) is slightly higher than
the At values obtained for 1 were found to be 158.43 ꢁ
10ꢀ4 cmꢀ1 and 56.00 ꢁ 10ꢀ4 cmꢀ1, respectively. The above
calculated g and A values fall well within the permissible range
for paramagnetic vanadium complexes and highlight that
vanadium exists in +4 oxidation state in VOTPPBr4 (1).
0
carbon atoms devoid of bromine substituents (DCb ¼ ꢃ0.725
˚
A), which highlights the distortion caused due to bromo
substituents.
The frontier molecular orbitals obtained from the optimized
geometry of 1 are displayed in Fig. 6. Both the HOMO and the
LUMO orbitals are mainly concentrated on the porphyrin core.
The energy gap (DE) between the HOMO and the LUMO orbitals
was calculated to be 2.51 eV on the basis of DFT optimized studies.
DFT optimization studies
DFT optimization studies using B3LYP functional and LANL2DZ
basis set was carried out for 1 in gas phase. 1 exhibited non-
Single crystal XRD studies
X-ray quality single crystals for 1 were obtained by slow diffu-
sion of methanol into a saturated solution of 1 in toluene
having a few drops of added chloroform (to aid solubility). 1
crystallizes into a m-oxo dimeric structure where the two
porphyrin units are linked together by a bridging oxygen atom
(m-oxo dimer) resulting into a monoclinic crystal system.
Repeated attempts to obtain the monomeric structure of 1
failed. It is possible that the m-oxo dimer is formed in saturated
solutions at low temperatures in presence of methanol which is
converted into the monomeric form at room temperature or
above. The crystal structure information of the m-oxo dimer of 1
is given in Table S4 in the ESI† and the corresponding bond
lengths and bond angles are listed in Table S5 in the ESI.† The
ORTEP images with top and side views for m-oxo dimer of 1 are
shown in Fig. 7.
Fig. 4 X-band EPR spectrum of 1 in toluene at 120 K. EPR parameters:
microwave frequency, 9.10 GHz; incident microwave power, 0.99
mW.
Fig. 5 B3LYP/LANL2DZ set optimized geometry of 1 in gaseous phase
showing (a) top view and (b) the side view, where b-hydrogens and Fig. 6 Frontier molecular orbitals (a) HOMO and (b) LUMO as obtained
meso-phenyl substituents have been removed for clarity.
from DFT optimized geometry of 1 using B3LYP/LANL2DZ basis set.
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analyzed for its catalytic efficiency for the epoxidation of alkenes
in presence of H2O2 (oxidant) and NaHCO3 (promoter) to
generate corresponding epoxides. For performing the catalytic
epoxidation of different alkenes, various reaction parameters
like catalyst amount, solvent, temperature etc. were rst opti-
mized by performing many control experiments using styrene
as the model substrate. The details of epoxidation reactions
under different conditions are listed in Table 1 and are
described in detail in the Experimental section (vide infra).
The optimized conditions on the basis of Table 1 were
utilized during selective epoxidation of different alkenes with
VOTPPBr4 (1) catalyst. The best suited conditions for selective
epoxidation of alkenes for efficient conversion rates are:
VOTPPBr4 (1) catalyst 1 mmol (1.00 mg), substrate 5 mmol, 30%
H2O2 10 mmol (1.13 g), NaHCO3 1.5 mmol (0.126 g), tempera-
ture 60 ꢂC, reaction time 0.5 h and solvent mixture CH3-
CN : H2O in 3 : 2 (v/v) ratio (entry 8, Table 1). The observed
conversion rates and turnover frequency TOF (hꢀ1) for catalyst 1
under the optimized conditions are shown in Table 2. The
vanadium porphyrin catalyst, VOTPPBr4 (1) exhibited very good
conversion rates, 100% selectivity and high turnover frequency
as compared to VOTPP which is devoid of bromo substituents.
Under the above optimized conditions VOTPP exhibited
a conversion rate of only 54% and a TOF of 3668 hꢀ1 using
styrene as the substrate.
Fig. 7 ORTEPs showing (a) top view and (b) side view of m-oxo dimer
of 1. Hydrogen atoms have been removed for clarity.
XRD studies clearly revealed the non-planar, distorted nature
˚
of the porphyrin units with D24 ¼ ꢃ0.324 A. The distortion from
the mean plane for b-carbon atoms bearing bromo substituents
˚
was calculated as 0.663 A which is much larger than the
distortion found in b0-carbon atoms devoid of bromo substitu-
˚
ents which was found to be 0.535 A. Thus bulkier bromo
substituents are mainly responsible for the observed distortion
in the porphyrin rings. The distance between the mean plane
and the metal center in the porphyrin rings was found to be
˚
0.538 A. Different bond distances in m-oxo dimer of 1 were
˚
˚
˚
0
0
calculated as 1.769 A (V–O), 1.356 A (Cb–Cb) and 1.342 A (Cb –Cb )
respectively. Finally, the distance between the two metal centers
Mechanism of epoxidation
˚
of the porphyrin m-oxo dimer was calculated as 3.534 A. It has to
The reaction between H2O2 and NaHCO3 generates perox-
ymonocarbonate ions by the following reaction;
be noted that different values of bond lengths and bond angles
calculated from DFT studies and XRD structures for monomeric
and m-oxo dimeric form of 1 are in good agreement with each
other.
ꢀ
ꢀ
HCO3 + H2O2 / HCO4 + H2O
Catalytic epoxidation of olens
Peroxymonocarbonate ions are very strong oxidizing species
The tendency of vanadium metal to undergo easy changes in in aqueous solutions.60 Peroxymonocarbonate ions transfer
oxidation sate from (IV) to (V) during the course of a reaction oxygen to the vanadium center (V]O) in the porphyrins to
has made it a rst choice catalyst for many reactions. VOTPPBr4 generate oxidoperoxido species. The formation of such
(1) having electron withdrawing and bulky bromo substituents oxidoperoxido/hydroperoxido (oxidoperoxiodo species are more
in addition to the vanadium metal (VO2+) center was therefore stable) species with different metal centers is well documented
Table 1 Epoxidation of styrene (0.52 g, 5 mmol) using VOTPPBr4 (1) as the catalyst in presence of 30% H2O2 under different conditions with
a reaction time of 0.5 h
Entry no.
Catalyst [mg (mmol)]
30% H2O2 [g (mmol)]
NaHCO3 [g (mmol)]
Temp. [ꢂC]
MeCN/H2O [ml]
Conv. [%]
Selectivity
1
2
3
4
5
6
7
8
1.5 (1.50)
1.5 (1.50)
1.5 (1.50)
1 (1.00)
2 (2.00)
1 (1.00)
1 (1.00)
1 (1.00)
1 (1.00)
1 (1.00)
1 (1.00)
—
1.69 (15)
1.69 (15)
1.69 (15)
1.69 (15)
1.69 (15)
0.565 (5)
1.13 (10)
1.13 (10)
1.13 (10)
1.13 (10)
1.13 (10)
1.13 (10)
1.13 (10)
1.13 (10)
0.168 (2)
0.168 (2)
0.168 (2)
0.168 (2)
0.168 (2)
0.168 (2)
0.168 (2)
0.126 (1.5)
0.084 (1)
0.126 (1.5)
0.126 (1.5)
0.126 (1.5)
—
60
60
60
60
60
60
60
60
60
50
40
60
60
60
2 : 1
3 : 2
4 : 3
3 : 2
3 : 2
3 : 2
3 : 2
3 : 2
3 : 2
3 : 2
3 : 2
3 : 2
3 : 2
3 : 2
93
99
95
98
98
78
97
96
93
82
36
29
9
100
100
100
100
100
100
100
100
100
100
100
100
100
100
9
10
11
12
13
14
—
1 (1.00)
—
18
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Table 2 Selective epoxidation of different alkenes using VOTPPBr4 (1) as the catalyst, at 60 ꢂC in 0.5 h of reaction time in CH3CN : H2O (3 : 2, v/v)
solvent
S. no.
1
Substrate
Product
% Conv.
96
TOF (hꢀ1
9600
)
2
98
9800
3
4
96
97
9600
9700
5
6
94
95
9400
9500
7
8
96
93
9600
9300
9
76
82
7600
8200
10
in literature.50,51,61–63 The oxidoperoxido species are more effi- hence it was expected to be an efficient catalyst which was
cient and more selective in transferring oxygen to the alkenes conrmed by its excellent selectivity, good conversion and high
which selectively converts the alkenes into epoxides and also TOF (hꢀ1) as shown in Table 2. Another important factor that
regenerates the original catalysts in the process. The generation determines the efficiency is the size of the substituents which
of oxidoperoxido vanadium(V) species [VO(O2)TPPBr4]ꢀ in case causes distortion or non-planarity in the porphyrin (macrocy-
of 1 was conrmed by 51V NMR studies in DMSO-d6 showing clic) core. This can be clearly seen in the efficiency of 1 because
a clear peak at ꢀ1298 ppm as shown in Fig. S5, ESI.† The the bulkier bromo groups cause more distortion in the
presence of oxidoperoxido species was alternatively conrmed porphyrin core making the reactive site easily accessible. The
by adding a small amount of cyclohexene to the above solution catalyst was recovered at the end of respective catalytic cycles as
which led to decrease in peak intensity. On adding excess indicated by its absorption spectrum (Fig. S4, ESI†). Thus the
amount of cyclohexene the peak from 51V NMR spectrum of the catalyst can be reused again and again for further catalysis of
compound disappeared completely because the vanadium(V) alkenes.
center is reduced back to vanadium(IV) which is 51V NMR
inactive.
Experimental section
Chemicals and materials
Since the vanadium(V) porphyrin intermediate involved in
the epoxidation reaction is an oxidoperoxido negatively charged
species, hence electron withdrawing substituents are expected
to stabilize it. 1 contains electron withdrawing –Br substituents
Propionic acid, P2O5, and sodium sulfate were purchased from
Thomas Baker, India and used as received. Benzaldehyde,
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VOSO4, HCl and NaHCO3 were purchased from Himedia, India 995.24. Anal. calcd for C44H24Br4N4OV: C, 53.10; H, 2.43; N,
and used without any further processing. Pyrrole was purchased 5.63. Found: C, 52.99; H, 2.46; N, 5.22.
from SRL Chemicals, India and used without further purica-
tion. DMF and CH3CN were purchased from Merck, India and
used as such. Various alkenes used in this study were purchased
from Alfa Aesar, UK and used as received. H2O2 and silica gel
(100–200 mesh) were purchased from Rankem, India and used
as such. N-Bromosuccinimide was also purchased from Hime-
dia, India but used aer recrystallizing it from hot water and
vacuum drying. Various solvents like chloroform, hexane and
methanol were purchased from Molychem, India and used aer
drying over P2O5. Electrochemical studies were carried out in
distilled CH2Cl2 using TBAPF6, recrystallized two times from
ethanol and vacuum dried for 6 hours, as the electrolyte.
Details of the catalytic studies
The catalytic role of the synthesized vanadyl porphyrin 1 was
investigated for the epoxidation of various alkenes by opti-
mizing the reaction conditions initially by taking 1 as the
representative catalyst. Styrene was chosen as the representative
alkene to optimize the conditions for the best conversion rates
and selectivity of the products formed. In a model reaction,
styrene (5 mmol, 0.52 g), 30% H2O2 (10 mmol, 1.14 g), NaHCO3
(1.5 mmol, 0.126 g) and catalyst (1) (1.5 mmol, 0.0015 mg) were
mixed in aqueous MeCN 5 ml (3 : 2: v/v) in 50 ml two neck round
bottomed ask and reuxed with vigorous stirring at 60 ꢂC for 1
hour. The reaction progress was examined by taking out a drop
of sample from the reaction mixture at intervals of 15 minutes.
The products formed were extracted with hexane and injecting
0.25 ml of the same into GC/GCMS. The products were identied
by using GC-MS (Clarus 500). The reaction was completed in
0.5 h only. The GC traces corresponding to some of the alkene
substrates and the reaction mixtures at different stages of the
epoxidation reaction are given in Fig. S6–S15 in the ESI.†
A series of reactions were performed to optimize the condi-
tions best for the reactant conversion and product selectivity.
Irrespective of the reaction conditions 1 gave only styrene
epoxide as the exclusive product as shown in Scheme 1.
Instrumentation
UV-visible spectra were recorded using an Agilent Cary 100
spectrophotometer along with a pair of quartz cells with 3.5 mL
volume and 1 cm path length. IR spectra were recorded using
a mid IR range 4000–400 cmꢀ1 Perkin-Elmer spectrophotometer
by making KBr pellets. For MALDI-TOF spectra we have used
Bruker UltraeXtreme-TN MALDI-TOF-MS spectrometer and
HABA matrix. Cyclic voltammetric measurements were per-
formed on a CHI 620E instrument, using a three electrode
assembly which consisted of Ag/AgCl as reference electrode, Pt
disk working electrode and Pt-wire as counter electrode along
with TBAPF6 as the supporting electrolyte. TGA was carried out
using a SII EXSTAR 6300 instrument. Single crystal X-ray
diffraction data was collected at 100 K on a Bruker APEX-II
diffractometer with APEX-II CCD detector and JAPAN thermal
Engineering Co. Ltd. Cryo system, DX-CS190LD. The crystal
structure was solved by using direct methods (SHELXS-97). The
products obtained from the epoxidation reactions were identi-
ed using a Shimadzu 2010 plus gas-chromatograph endowed
with an Rtx-1 capillary column (30 m ꢁ 0.25 mm ꢁ 0.25 mm)
and an FID detector. The product identities were conrmed
using a Perkin-Elmer GC-MS (Clarus 500).
Keeping in view our earlier reported results55 and the general
solubility of porphyrins, the best solvent for this study was taken
to be a mixture of CH3CN and H2O. The solvent (5 ml) ratio
(CH3CN : H2O) was xed by performing three simultaneous
ꢂ
reactions at 60 C for 0.5 h involving 0.52 g (5 mmol) styrene,
1.5 mg (1.50 mmol) catalyst 1, 1.69 g (15 mmol) 30% H2O2 and
0.168 g NaHCO3 (2 mmol) in 2 : 1, 3 : 2 and 4 : 3 solvent mixtures
as shown in entries 1, 2 and 3 respectively in Table 1. Substrate
conversions corresponding to 93%, 99% and 95% were obtained
in case of 2 : 1, 3 : 2 and 4 : 3 (CH3CN : H2O; v/v) respectively.
Thus 3 : 2 (CH3CN : H2O; v/v) was chosen as the ideal solvent for
the epoxidation of alkenes in the current study.
The amount of catalyst was optimized by performing three
parallel reactions with 1.5 mg (1.50 mmol), 1 mg (1 mmol) and
2 mg (2 mmol) of catalyst 1 in 5 ml CH3CN : H2O (3 : 2; v/v)
Synthetic procedure
Synthesis
of
vanadyl-2,3,12,13-tetrabromo-5,10,15,20-
ꢂ
solvent at 60 C keeping all other parameters constant; 0.52 g
tetraphenylporphyrin (1). 2,3,12,13-Tetrabromo-5,10,15,20-
tetraphenylporphyrin (H2TPPBr4), 300 mg, 0.322 mmol was
dissolved in 75 ml DMF. To this solution, 15 eq. VOSO4 (788 mg,
4.83 mmol) was added. The mixture was then stirred for 10
minutes and reuxed for 24 h under argon atmosphere. Aer
completion of the reaction, the mixture was cooled to room
temperature. 150 ml distilled water was added to it and the
product mixture was allowed to settle as precipitate. The
product was then ltered using a G-4 crucible and dried under
vacuum for 8 h. The crude product was redissolved in chloro-
form and puried over silica (100–200) mesh using chloroform
as the eluent. The target compound 1 was obtained in pure form
by rota-evaporating the solvent in 79% yield (252 mg, 0.254
mmol). UV/vis (CH2Cl2): lmax (nm) (log 3): 435 (5.28), 560 (4.00),
(5 mmol) styrene, 1.69 g (15 mmol) 30% H2O2 and 0.168 g
NaHCO3 (2 mmol): 0.5 h reaction time as shown in entries 2, 4
and 5 in Table 1. Substrate conversions obtained were as 99%,
98% and 98% for catalyst amounts 1.5 mg (1.50 mmol), 1 mg (1
mmol) and 2 mg (2 mmol) respectively. Thus, the ideal amount of
catalyst 1 was subsequently taken as 1 mg for the current study.
The amount of oxidant (30% H2O2) was optimized by per-
forming a parallel set of three reactions in 5 ml CH3CN : H2O
(3 : 2; v/v) solvent taking 15 mmol (1.69 g), 5 mmol (0.565 g) and
604 (3.80). MALDI-TOF-MS (m/z): found 995.99 [M]+, calcd Scheme 1 Selective epoxidation of styrene.
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10 mmol (1.13 g) of 30% H2O2 at 60 ꢂC maintaining other reac-
To follow the reaction kinetics better, the epoxidation of one
tion conditions constant; 0.52 g (5 mmol) styrene, catalyst 1 mmol of the alkenes (e.g. cyclohexene) was monitored more closely by
(1 mg) and 0.168 g NaHCO3 (2 mmol): 0.5 h reaction time as taking sample aliquots at different intervals ranging from 0 to
depicted in entries 4, 6 and 7 in Table 1. 10 mmol (1.13 g) of 30% 40 minutes. The reaction prole for epoxidation of cyclohexene
H2O2 gave the most feasible conversion (entry 7, Table 1) and was with 1 as the catalyst is given in Fig. 8.
chosen as the ideal amount of oxidant for the current study.
The reaction was initiated by taking cyclohexene (0.410 g; 5
The amount of NaHCO3 (working as promoter by gener- mmol) in 5 ml of the ideal solvent (CH3CN : H2O; 3 : 2 v/v) in
ating peroxymonocarbonate ions)46,47 was also optimized by a two neck 50 ml round bottom ask along with the catalyst,
setting up three parallel reactions taking 2 mmol (0.168 g), H2O2 and promoter under the optimized reaction conditions as
1.5 mmol (0.126 g) and 1 mmol (0.084 g) of NaHCO3 in 5 ml shown in entry 8, Table 1. The progress of the reaction was
CH3CN : H2O (3 : 2; v/v) solvent at 60 ꢂC keeping other reaction monitored by withdrawing small aliquots from the reaction
parameters unchanged: 0.52 g (5 mmol) styrene, catalyst 1 mixture aer 0 min, 10 min, 20 min, 30 min and 40 min. The
mmol (1 mg) and 1.13 g H2O2 (10 mmol): 0.5 h reaction time as aliquots from the reaction mixture were injected into and
shown in entries 7–9 in Table 1. 1.5 mmol (0.126 g) of NaHCO3 analyzed via gas-chromatography for percentage conversions.
was therefore chosen as the ideal amount of promoter (entry 8, Immediately aer mixing all the reactants and just shaking,
Table 1).
15% reactant conversion was obtained. As the reaction time
Finally, the effect of temperature on the conversion rate was increases the amount of the product (cyclohexene epoxide)
studied by setting up three parallel reactions at 60 ꢂC, 50 ꢂC and progressively increases with a simultaneous decrease in the
40 ꢂC in 5 ml CH3CN : H2O (3 : 2; v/v) solvent maintaining other amount of the reactant (cyclohexene) as shown in Fig. 8. The
conditions constant: 0.52 g (5 mmol) styrene, catalyst 1 mmol (1 reaction progress was monitored till 40 min (99% reactant
mg), 1.13 g (10 mmol) H2O2 and 0.126 g (1.5 mmol) NaHCO3: conversion), however, no signicant changes were observed
0.5 h reaction time as shown in entries 8, 10 and 11 respectively aer 30 min of reaction time (96% reactant conversion). From
in Table 1. Henceforth, the idꢂeal temperature selected for the this we can conclude that the reaction proceeds to completion
current study was taken as 60 C (entry 8, Table 1).
in only 0.5 h of reaction time.
A control reaction in 5 ml CH3CN : H2O (3 : 2; v/v) solvent at
The conversion rates of the reactant alkenes into epoxides
60 ꢂC without the catalyst was performed to underline the was additionally quantied by using dodecane as the internal
importance of the catalyst keeping other reaction conditions standard in one of the key reactions i.e. conversion of cyclo-
unchanged: 0.52 g (5 mmol) styrene, 1.13 g (10 mmol) H2O2 and hexene into cyclohexene epoxide. Same amount of dodecane (an
0.126 g (1.5 mmol) NaHCO3: 0.5 h reaction time. The absence of internal standard) as that of reactant substrate was added to the
catalyst markedly affected the conversion rate lowering it to just reaction mixture to conrm the conversion rates, for example,
29% (entry 12, Table 1). A similar reaction without catalyst and 5 mmol of dodecane was added to the reaction mixture which
without NaHCO3 was performed under identical conditions. A was started with 5 mmol of cyclohexene. The conversion rates
very poor conversion rate amounting to 9% was achieved (entry 13, obtained from the two reaction mixtures were very close to each
Table 1). Finally, the reaction in presence of catalyst but without other viz. 93% (with internal standard) and 96% (without
the presence of NaHCO3 gave a conversion rate of only 18% internal standard) respectively, signifying the higher selectivity
highlighting the role the promoter (entry 14, Table 1). The full of the porphyrin catalyst 1 (Fig. S16, ESI†).
details regarding the experimental conditions utilized during the
optimization of 1 towards alkene epoxidation are listed in Table 1.
Conclusions
Vanadyl b-tetrabromoporphyrin VOTPPBr4 (1) having electron
withdrawing –Br substituents was synthesized through a facile
procedure in quantitative yields and characterized by various
spectrometric methods. 1 displayed high thermal stability and
encouraging selectivity with high TOF (hꢀ1) during epoxida-
tion of alkenes with H2O2 and NaHCO3. 1 is highly non-planar
due to bulkier –Br substituents which makes it a more efficient
catalyst as the reactive site is easier to access. The catalyst 1
exhibited excellent selectivity, high conversion efficiency and
high TOF (7600–9800 hꢀ1) in signicantly less reaction time of
0.5 h in aqueous medium (CH3CN : H2O; 3 : 2 v/v). The
porphyrin catalyst was regenerated at the end of respective
catalytic cycles making it reusable and thus industrially
important.
Conflicts of interest
Fig. 8 Reaction profile showing progress of epoxidation of cyclo-
hexene with catalyst 1 at different time intervals.
There are no conicts to declare.
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Acknowledgements
TAD is thankful to MHRD, India for senior research fellowship.
We sincerely thank Science and Engineering Research Board
(EMR/2016/004016) for nancial support.
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