Electron deficient nonplanar β-octachlorovanadylporphyrin as a highly efficient and selective epoxidation catalyst for olefins

Article abstract of DOI:10.1039/c5dt02349a

We have synthesized 2,3,7,8,12,13,17,18-octachloro-meso-tetraphenylporphyrinatooxidovanadium(iv) (VOTPPCl₈) and characterized by various spectroscopic (UV-Vis, IR and EPR) techniques, MALDI-TOF mass spectrometry and elemental analysis. The DFT optimized structure of VOTPPCl₈ in CH₃CN exhibited a highly nonplanar saddle shape conformation of the porphyrin macrocycle. The cyclic voltammogram of VOTPPCl₈ showed a 500 mV anodic shift in the first ring reduction potential and 220 mV in the first ring oxidation potential compared to VOTPP indicating the electron deficient nature of the porphyrin π-system and further proving the existence of a nonplanar conformation of the macrocycle in solution. Further, VOTPPCl₈ exhibited very high thermal stability till 390 °C as indicated in its thermogram. The oxidation state of the metal ion (V^IV) was confirmed by EPR spectroscopy and VOTPPCl₈ exhibited an axial spectrum which corresponds to the axially compressed d_xy¹ configuration. VOTPPCl₈ was utilised for the selective epoxidation of various olefins in good yields with very high TOF numbers (6566-9650 h^-1) in the presence of H₂O₂ as an oxidant and NaHCO₃ as a promoter in a CH₃CN/H₂O mixture. The oxidoperoxidovanadium(v) species is expected to be the intermediate during the catalytic reaction which is probed by ⁵¹V NMR spectroscopy and MALDI-TOF mass analysis. Notably, VOTPPCl₈ is stable after the catalytic reaction and doesn't form a μ-oxo dimer due to the highly electron deficient nonplanar porphyrin core and can be reused for several cycles.

Full text of DOI:10.1039/c5dt02349a

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Electron deficient nonplanar β-octachlorovanadyl-
porphyrin as a highly efficient and selective
epoxidation catalyst for olefins†
Cite this: DOI: 10.1039/c5dt02349a
Ravi Kumar, Nikita Chaudhary, Muniappan Sankar* and Mannar R. Maurya*
We have synthesized 2,3,7,8,12,13,17,18-octachloro-meso-tetraphenylporphyrinatooxidovanadium(IV)
(
VOTPPCl
mass spectrometry and elemental analysis. The DFT optimized structure of VOTPPCl
a highly nonplanar saddle shape conformation of the porphyrin macrocycle. The cyclic voltammogram of
VOTPPCl showed a 500 mV anodic shift in the first ring reduction potential and 220 mV in the first ring
8
) and characterized by various spectroscopic (UV-Vis, IR and EPR) techniques, MALDI-TOF
8
in CH CN exhibited
3
8
oxidation potential compared to VOTPP indicating the electron deficient nature of the porphyrin
π-system and further proving the existence of a nonplanar conformation of the macrocycle in solution.
Further, VOTPPCl
8
exhibited very high thermal stability till 390 °C as indicated in its thermogram. The oxi-
IV
dation state of the metal ion (V ) was confirmed by EPR spectroscopy and VOTPPCl
8
exhibited an axial
1
spectrum which corresponds to the axially compressed dxy configuration. VOTPPCl
8
was utilised for the
selective epoxidation of various olefins in good yields with very high TOF numbers (6566–9650 h− ) in
1
2 2 3 3 2
the presence of H O as an oxidant and NaHCO as a promoter in a CH CN/H O mixture. The oxidoper-
Received 21st June 2015,
Accepted 7th September 2015
oxidovanadium(V) species is expected to be the intermediate during the catalytic reaction which is probed
51
8
by V NMR spectroscopy and MALDI-TOF mass analysis. Notably, VOTPPCl is stable after the catalytic
DOI: 10.1039/c5dt02349a
reaction and doesn’t form a µ-oxo dimer due to the highly electron deficient nonplanar porphyrin core
www.rsc.org/dalton
and can be reused for several cycles.
and flexible architectural modification to tailor redox pro-
Introduction
perties and conformational features. Apart from catalysis, they
6
Selective oxidation of olefins to epoxy compounds has always are widely utilized in dye-sensitised solar cells (DSSCs), photo-
7
8
been an interesting area of research for chemists owing to dynamic therapy (PDT), anion sensing and nonlinear optical
9
their great importance in the production of highly valued com- studies. The β-functionalization of meso-tetraphenylporphyrin
modity chemicals such as polyurethanes, unsaturated resins, (TPP) is of great interest since the electronic properties of the
1
glycols, surfactants, and other products. Transition metal porphyrin π-system can be altered by tuning the size, shape
1
0
complexes play a vital catalytic role in various organic trans- and electronic nature of the β-substituents compared to the
formations e.g. epoxidation, hydroxylation, C–H activation, substituents at the meso-aryl positions.
hydrogenation, polymerisation, carbon–carbon coupling reac-
Iron and manganese porphyrin complexes have been widely
tions, halogenation, dehalogenation and so on in homo- utilized as effective homogeneous catalysts for such oxygen-
2
,3
2,3,11
geneous and heterogeneous media.
Among them, ation reactions in the last few decades.
The major draw-
metalloporphyrins have been found to be efficient catalysts for backs of homogeneous MTPP-based catalysts are (i) the
4
,5
alkene epoxidation using various oxygen atom donors due to macrocyclic ring which is liable to oxidative self-destruction;
their high chemical and thermal stability, interesting physico- (ii) aggregation of metalloporphyrins through π–π interactions;
chemical properties, strong absorption in the visible region (iii) lower turnover frequency (TOF) and (iv) poor product
III
selectivity. Recently, a highly efficient and stable Mn por-
phyrinic framework (MOF) was demonstrated for selective
1
2
Department of Chemistry, Indian Institute of Technology Roorkee, Roorkee-247667,
India. E-mail: sankafcy@iitr.ac.in, rkmanfcy@iitr.ac.in; Fax: +91-1332-273560;
Tel: +91-1332-28-4753/5327
epoxidation of olefins in heterogeneous media.
Among
IV
various metalloporphyrins, vanadyl (V O) porphyrins merit
special attention owing to their use as anti-HIV agents, as
compared to other vanadium complexes, as potential thera-
peutics, 3D supramolecular assemblies and catalysts for
13
†
Electronic supplementary information (ESI) available: IR and mass spectra of
VOTPPCl and a table containing bond lengths and bond angles of the DFT opti-
8
1
4
15
mized structure of VOTPPCl
c5dt02349a
8
and CV and DPV traces. See DOI: 10.1039/
1
5,16
oxidation reactions.
Notably, VOTPP was utilised for oxi-
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Fig. 1 Molecular
structure
of
β-octachlorovanadylporphyrin
(
8
VOTPPCl ) employed in this study.
1
6
dation of cyclohexene under homogeneous
and hetero-
1
7
Fig. 2 Electronic absorption spectrum of VOTPPCl8 in CH Cl at 298 K.
2
2
geneous conditions, leading to mixture of products (viz.
epoxide, alcohol and ketone) with very low TOF. So, there is a
quest for highly efficient vanadylporphyrin catalysts for selec-
tive oxidation of olefins. Besides dioxygen, aqueous H
2 2
O is
the most widely used oxidant since it is inexpensive, environ-
1
8–20
mentally benign and biologically important.
NaHCO as a promoter and H as a terminal oxidant has
been evolved as a highly efficient method for epoxidation of
The use of
3
2 2
O
1
8,19,21,22
olefins catalyzed by W, Mo and Mn complexes.
H O
2
2
2
−
and HCO
3
2
are used together to increase the activity of H O
by in situ generation of peroxymonocarbonate ions which are
more nucleophilic than H O and thus contributes to the
2
2
1
8–22
enhanced oxidizing activity of the metal complexes.
To the
best of our knowledge, there is no report on selective epoxi-
dation of olefins in almost quantitative yields catalyzed by oxi-
dovanadium porphyrin complexes in homogeneous media. side views of VOTPPCl
Fig. 3 B3LYP/LANLD2Z optimised geometry showing (a) top and (b)
in CH CN. In the side view, β-chloro and meso-
phenyl substituents are omitted for clarity.
8
3
Herein, we present the selective epoxidation of olefins using a
biologically important oxidant (i.e., H O ) catalyzed by robust
2
2
8 3
VOTPPCl (Fig. 1) using NaHCO as a promoter with very high
turnover frequency (TOF).
disappeared upon metallation and the appearance of a new
peak at ∼1003 cm− for VvO stretching confirmed the inser-
1
1
5
tion of VO into the porphyrin core (Fig. S1 in the ESI†).
Further, MALDI-TOF mass analysis showed a molecular ion
peak at 954.95 (m/z) which is in agreement with the corres-
ponding calculated mass number (m/z = 954.96) as shown in
Results and discussion
Synthesis and characterisation
We have synthesised free base β-octachloro-meso-tetraphenyl- Fig. S2 in the ESI.† Our attempts to obtain X-ray quality single
2
3
porphyrin (H TPPCl ) using the reported procedure.
crystals failed. So, we have carried out DFT studies using the
with 10 equiva- B3LYP functional with the LANLD2Z basis set. Fig. 3 rep-
in refluxing DMF for 16 hours under argon resents the top and side views of optimised geometries of
2
8
VOTPPCl
lents of VOSO
atmosphere. Then the porphyrin was precipitated by adding VOTPPCl
excess water, filtered and air dried. The crude porphyrin was dal geometry with the axial oxido ligand. From the side view
purified on a silica column using CHCl
as the eluent. The (Fig. 3b), it is clear that VOTPPCl adopts a nonplanar saddle
yield was found to be 79%. In general, our vanadium metalla- shape conformation due to steric hindrance between peri-
tion procedure is much simpler as compared to literature pheral β-chloro and meso-phenyl groups.
was characterised by average bond angles and bond lengths of VOTPPCl are listed
optical absorption, IR and EPR spectroscopy techniques, mass in Table S1 in the ESI.† Herein, the V ion resides 0.515 Å
8 2 8
was prepared by reacting H TPPCl
4
IV
8
3
in CH CN. The V ion is present in square pyrami-
3
8
2
3
The selected
1
5,24
methods reported so far.
VOTPPCl
8
8
IV
spectrometry and elemental analysis. Fig. 2 represents the above the porphyrin mean plane formed by the 24 atom core.
UV-Vis spectrum of VOTPPCl
a characteristic UV-Vis spectrum for a metalloporphyrin
which is blue shifted as compared to that of H TPPCl
. The large deviation of 24 atom core (Δ24 = 0.535 Å) and β-pyrrole
IR spectrum of H TPPCl
exhibited a characteristic peak at carbons, (ΔC = ±1.128 Å) from the porphyrin mean plane
ν ∼3328 cm− for the NH stretching frequency which (Table S1, ESI†). This is further supported by the longer C
–C
8 2 2 8
in CH Cl . VOTPPCl
exhibited The VvO distance is slightly shorter (1.60 Å) than β-octaethyl-
2
5
26
8
vanadylporphyrin (1.62 Å). Further, VOTPPCl exhibited a
2
8
2
8
β
1
β
β
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8
Fig. 4 CV of VOTPPCl (1 mmol) in dichloromethane containing 0.1 M
6
TBAPF at 298 K. Potentials are measured with respect to the Ag/AgCl
electrode.
β α m
bond length (1.375 Å) and the increment in the C –C –C
angle (∼128°) with the concomitant decrement in the N–Cα–
Cm angle (∼123.8°) as compared to the reported quasi planar
2
6
vanadyl porphyrin. Furthermore, DFT analysis of free base Fig. 5 X-band EPR spectrum of VOTPPCl8 was recorded in toluene at
H TPPCl in CH CN slightly increases nonplanarity (Fig. S3, 120 K (bottom). EPR parameters: microwave frequency, 9.453 GHz; inci-
2
8
3
dent microwave power, 0.189 mW; modulation frequency, 100.0 kHz;
modulation amplitude, 5.0 G; receiver gain, 1 × 10 . Simulated EPR spec-
ESI†) as reflected by the increment in ΔC (±1.185 Å) and Δ24
β
4
(
±0.674 Å) as compared to VOTPPCl
8
(Table S2, ESI†). The
trum of VOTPPCl
8
is shown at the top.
2
+
insertion of vanadyl (VO ) into the H TPPCl core slightly
2
8
alters the nonplanarity of the macrocycle.
Electrochemical studies of VOTPP have been examined in
non-aqueous media. Fig. 4 represents the cyclic voltammo- 172 × 10 cm and 61 × 10 cm−1, respectively. The g|| < g
2
7
4
−1
4
⊥
gram (CV) of VOTPPCl which is measured in dichloromethane and A|| ≫ A
relationships are normal for the axially com-
8
⊥
1
24c,28
containing TBAPF
6
as the supporting electrolyte at 298 K. It pressed d xy configuration.
These results clearly indicate
exhibited ring-centred two one electron oxidations and that vanadium is in the IV oxidation state. Further, we couldn’t
reductions which are further supported by the differential observe well resolved superhyperfine splitting from porphyrin
pulse voltammogram (DPV) of VOTPPCl
peaks with almost the same amplitude (Fig. S4, ESI†). Notably, bonding orbital pointing away from these N atoms which are
8
showing four redox N atoms since the unpaired electron resides on a σ-non-
2
8
VOTPPCl8 exhibited a remarkable anodic shift (ΔEred
=
present in the equatorial (xy) plane.
5
00 mV) in the first ring reduction as compared to VOTPP
whereas in oxidation only a 220 mV anodic shift was observed
due to a nonplanar conformation of the macrocycle which
destabilizes the HOMOs and makes oxidation easier relative to
reduction (Fig. S5, ESI†). The anodic shift in redox potentials
is due to the strong electron withdrawing effect of eight
Catalytic studies
To explore the catalytic potential of the synthesized vanadium
porphyrin complex (VOTPPCl ), we conducted a series of
experiments. Catalytic oxidation of cyclohexene by VOTPPCl
using H O as an oxidant and NaHCO as the promoter gave
9
8
8
2
2
3
β-chloro groups. The thermogram of VOTPPCl
8
exhibited
three products: cyclohexene epoxide (1), 2-cyclohexen-1-ol (2)
and 2-cyclohexen-1-one (3) (Scheme 1).
The catalytic activity of VOTPPCl was initially investigated
for cyclohexene as a representative oxidation substrate and
almost a flat line till 390 °C indicating the high thermal stabi-
lity of this porphyrin (Fig. S6, ESI†). The high stability of
VOTPPCl is possibly due to sterically hindered porphyrin
8
periphery (nonplanar saddle shape conformation) and electron
8
different reaction parameters were optimized for 5 mmol
withdrawing β-chloro groups which prevent the oxidative
(
0.410 g) of cyclohexene in 6 h of reaction time. All the
4
a
decomposition. To the best of our knowledge, this is the first
example of metalloporphyrin showing very high thermal stabi-
lity (till 390 °C) which prompted us to perform catalytic oxi-
dation studies using this vanadylporphyrin complex.
catalytic experiments were carried out in a 50 mL round
bottom flask containing cyclohexene (0.410 g, 5 mmol), an
We have recorded the X-band EPR spectrum of VOTPPCl in
8
toluene (Fig. 5). VOTPPCl has exhibited an axial EPR spec-
8
5
1
trum with well resolved V hyperfine lines at 120 K. The spin
Hamiltonian parameters were obtained from the simulated
EPR spectrum which is in close agreement with the experi-
mentally observed one. The obtained g|| and g
⊥
values
are 1.965 and 1.985, respectively and A|| and A⊥ values are Scheme 1 Oxidation of cyclohexene catalyzed by VOTPPCl8.
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appropriate amount of aqueous 30% H
2
O
2
, and NaHCO
3
in
The amount of promoter NaHCO
3
also affects the oxidation
5
mL of desired solvent and fitted with a condenser. At first, of cyclohexene. This effect has been examined by conducting
the effect of different types of solvents viz. acetonitrile, metha- experiments with three different NaHCO amounts (1, 2 and
nol, dichloromethane and toluene (5 mL each), on the oxi- mmol) while maintaining other reaction parameters
dation of 5 mmol (0.410 g) of cyclohexene was studied using (1.04 µmol of catalyst, 10 mmol of 30% H O , 5 mL of CH CN,
3
3
2
2
3
1
0 mmol (1.13 g) of 30% H
2
O
2
and 1 mmol (0.084 g) of temperature 60 °C) constant. An increase in the conversion
CN was found to be the from 73% to 82% was achieved by increasing the amount of
NaHCO as a promoter at 60 °C. CH
3
3
most suitable solvent with the maximum of 61% conversion sodium bicarbonate from 1 mmol to 2 mmol. Using 3 mmol of
followed by 57% conversion of cyclohexene using methanol sodium bicarbonate did not increase the conversion signifi-
as a solvent (data not shown). The possible reason for cantly (85% conversion, as presented in Table 1) and therefore,
the increased conversion is due to the enhanced formation of 2 mmol NaHCO amount was optimized for further optimiz-
3
oxidohydroperoxidovanadium- or oxidoperoxidovanadium(V) ation (entry no. 6, Table 1).
species. Therefore, CH
3
CN was used as a solvent for the optim-
The solvent (CH
3
CN) amount is also an important factor in
ization of remaining reaction conditions.
terms of percentage conversion of cyclohexene. The effect of
The catalyst amount was optimized by taking three different amounts of solvent is shown in Table 1 under the
different amounts of catalyst (0.0005 g, 0.0010 g and 0.0015 g) above-mentioned optimized conditions i.e. catalyst (0.0010 g,
while keeping other reaction parameters constant such as 30% 1.04 µmol), 30% H O (1.31 g, 10 mmol), and NaHCO3
2
2
H
2
O
2
(1.13 g, 10 mmol), NaHCO
3
(0.084 g, 1 mmol), aceto- (0.168 g, 2 mmol) at 60 °C reaction temperature. While using
mL of acetonitrile 82% of cyclohexene conversion
nitrile (5 mL) and reaction temperature (60 °C). The conversion
5
increased from 61% to 73% by increasing the catalyst amount was obtained which was reduced to 68% for 7 mL of CH CN
3
from 0.0005 g to 0.0010 g as presented in Table 1 (entry no. 2). possibly due to the dilution effect. The conversion was
Thereafter, no considerable improvement in conversion was slightly decreased (81%) when 3 mL of solvent amount was
obtained (77%) upon increasing the catalyst amount up to employed. Considering all these facts, 5 mL of CH CN
3
0
.0015 g and hence the catalyst amount 0.0010 g (1.04 µmol) was chosen as the optimal solvent amount for cyclohexene
was considered as the optimized catalyst amount.
oxidation.
To understand the dependence of cyclohexene oxidation on
The effect of oxidant amount i.e. aq. 30% H O was
2
2
measured by varying its amount. For this purpose, three reaction temperature, reactions have been carried out at
different oxidant amounts viz. 5, 10 and 15 mmol were chosen different temperatures i.e. room temperature (30 °C), and 50,
and reactions were carried out by taking 1.04 µmol of catalyst, 60 and 70 °C temperature under the optimized conditions
1
6
5
3 3
mmol of promoter (i.e. NaHCO ) and 5 mL of CH CN at for 6 h. The oxidation of cyclohexene was found to be the
0 °C. The cyclohexene conversion was lowest (59%) for slowest at room temperature (30 °C) with only 35% conversion.
mmol of 30% H O and comparable conversions were The catalytic activity of the catalyst VOTPPCl increased signifi-
2
2
8
obtained while increasing H
2
O
2
amounts up to 10 mmol (77%) cantly on increasing temperature from room temperature to
and 15 mmol (78%). Considering these observations, 10 mmol 70 °C. A conversion of 51% was obtained for 50 °C which was
of 30% H O was concluded the best for the maximum oxi- also further increased up to 82% and 86% for 60 °C and 70 °C,
2
2
dation of cyclohexene (entry no. 2, Table 1).
respectively (Table 1).
Table 1 Oxidation of cyclohexene (0.41 g, 5 mmol) using VOTPPCl
8
as a catalyst on a 6 hour time scale under different reaction conditions
Product selectivity
3
(g, mmol) CH CN (mL) Temp. (°C) Conv. (%) -oxide -ol -one
Entry no. Catalyst (mg, µmol) 30% H
2
O
2
(g, mmol) NaHCO
3
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
0.5, 0.52
1.0, 1.04
1.5, 1.57
1.0, 1.04
1.0, 1.04
1.0, 1.04
1.0, 1.04
1.0, 1.04
1.0, 1.04
1.0, 1.04
1.0, 1.04
1.0, 1.04
1.0, 1.04
—
1.13, 10
0.084, 1
0.084, 1
0.084, 1
0.084, 1
0.084, 1
0.168, 2
0.252, 3
0.168, 2
0.168, 2
0.168, 2
0.168, 2
0.168, 2
—
5
5
5
5
5
5
5
7
3
5
5
5
5
5
5
60
60
60
60
60
60
60
60
60
70
50
30
60
60
60
61
73
77
78
59
82
85
68
81
86
51
35
58
16
5
25
28
29
21
23
22
19
28
15
20
26
16
19
11
9
55
47
40
34
58
42
46
33
60
49
38
64
42
58
56
20
25
31
45
19
36
35
39
25
31
36
20
39
31
35
1.13, 10
1.13, 10
1.69, 15
0.57, 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
1.13, 10
1.13, 10
0
1
2
3
4
5
a
0.168, 2
—
—
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Details of all experimental conditions are presented in ing equilibria and is a more active (more nucleophilicity)
Table 1. Thus, the optimized reaction conditions for the oxi- oxidant than H O .
2
2
dation of 5 mmol of cyclohexene are (entry no. 6, Table 1):
catalyst [VOTPPCl ] (0.0010 g, 1.04 µmol), 30% aqueous H
1.13 g, 10 mmol), CH CN (5 mL) and temperature 60 °C. The
H2O2 þ HCO3ꢀ Ð HCO4ꢀ þ H2O
8
2 2
O
(
3
selectivity of different oxidation products under the optimized
Later, this bicarbonate-activated peroxide (BAP) system, was
reaction conditions is: 22% cyclohexene epoxide (1), 42% adopted for olefin epoxidation using H O as the terminal
2
2
1
8,19
2
-cyclohexen-1-ol (2) and 36% 2-cyclohexen-1-one (3). To oxidant in CH CN solvent
and in the water/co-solvent
3
2
2
investigate the effect of both catalyst (VOTPPCl
8
) and promoter mixture. All these facts and our previous results showing a
3
(NaHCO ) on the oxidation of cyclohexene, three types of con- respectable amount of selectivity towards epoxide (i.e. 22%,
trolled experiments were performed under the optimized con- Table 2, entry no. 1) under the optimized conditions led us to
ditions (Table 1): (i) the control experiment in the absence investigate how the solvent mixture of water with CH CN or
3
of catalyst (VOTPPCl
optimized conditions gave 16% of conversion (entry no. 14), were obtained with methanol) affects the cyclohexene con-
ii) the control experiment in the absence of the promoter version and epoxide selectivity.
NaHCO but with the catalyst (VOTPPCl ) under optimized
To understand the effect of these solvent mixtures, the
8 3
3
) but with the promoter NaHCO under CH OH (as we have stated previously that comparable results
(
3
8
conditions gave 58% of conversion (entry no. 13) and, (iii) experiments were conducted under the above stated optimized
the control experiment in the absence of both promoter conditions. The use of a CH CN/H O mixture (3 mL/2 mL, v/v)
3
2
3 8
NaHCO and catalyst (VOTPPCl ) under optimized condi- at 60 °C dramatically improved the catalytic efficiency of the
tions gave only 5% of conversion after 6 h of reaction time catalyst (VOTPPCl ) and almost all cyclohexene was converted
8
(entry no. 15).
into epoxide (99% selectivity) within half an hour of reaction
These control experiments demonstrate that the catalyst time (Table 2, entry no. 3). Using the CH CN/H O (3 mL/2 mL,
3
2
itself is moderately efficient in catalytic oxidation of cyclo- v/v) mixture at room temperature yielded 51% of conversion in
hexene using 30% H as an oxidant with adequate con- 2 h of reaction time with reduced 30% selectivity for cyclo-
2 2
O
version of 58% even in the absence of any promoter. But the hexene epoxide (1) and increased 44% and 26% selectivity for
product selectivity for cyclohexene epoxide decreased from the allylic oxidation products 2-cyclohexen-1-ol (2) and 2-cyclo-
2
2% to 13% (Table 2, entry no. 2), whereas, only 5% of cyclo- hexen-1-one (3), respectively. Similarly, in a methanol/water
hexene was converted in the absence of both catalyst and pro- (3 mL/2 mL, v/v) mixture 97% conversion was obtained on a
moter showing their catalytic roles in the oxidation (entry no. 0.5 hour time scale at 60 °C but with poor cyclohexane epoxide
1
5, Table 1). However, 16% of cyclohexene conversion was (1) selectivity i.e. 7% (Table 2, entry no. 7) and a maximum of
observed using NaHCO alone in the reaction. Surely the use 45% conversion was obtained at room temperature with 18%
of NaHCO3 as the promoter activated the oxidant H O and selectivity towards epoxide. A similar trend of epoxide selecti-
3
2
2
resulted in the increased catalytic efficiency of the catalyst vity for acetonitrile and methanol was observed throughout
VOTPPCl ) towards the oxidation of cyclohexene in respect of the series of experiments (i.e. the epoxide selectivity is always
both the conversion and selectivity of epoxide.
Such type of activation of H with the bicarbonate
(
8
1
8,19,21,22
higher if CH CN is used as the solvent alone or with water).
3
2
O
2
Finally, optimized conditions both in respect of percentage
2
9
ion has been demonstrated by Drago and co-workers and conversion and epoxide selectivity were achieved for the cyclo-
Richardson et al. in sulfide oxidations using a solvent hexene oxidation as follows: catalyst (VOTPPCl ) (0.0010 g,
mixture of alcohol/water. Richardson et al. have reported 1.04 µmol), 30% aqueous H O (1.13 g, 10 mmol), CH CN/
, which is an essential component in such systems, H O (3 mL/2 mL, v/v) and temperature 60 °C. Under these
forms a peroxymonocarbonate ion, HCO4 and exists as follow- optimised conditions, we have carried out the epoxidation
3
0
8
2
2
2
2
3
−
that HCO
3
2
−
Table 2 Product distribution for the oxidation of cyclohexene (5 mmol) using VOTPPCl
oxidant under different reaction conditions
8
2 2
as the catalyst (1 µmol) and 10 mmol of 30% H O as an
Product selectivity
TOF (h−
1
)
-oxide (1)
-ol (2)
-one (3)
Entry no.
Solvent (mL, v/v)
NaHCO
3
(g, mmol)
Temp. (°C)
Time (h)
Conv. (%)
1
2
3
4
5
6
7
8
CH
CH
CH
CH
CH
CH
CH
CH
3
3
3
3
3
3
3
3
CN (5)
CN (5)
0.168, 2
—
0.168, 2
0.168, 2
0.168, 2
—
60
60
60
6
6
0.5
2
6
6
0.5
2
82
58
100
51
80
56
653
462
9560
1219
637
446
9273
1076
22
13
99
30
3
1
7
42
58
—
44
11
43
52
42
36
29
—
26
86
57
41
40
CN/H
2
O (3/2)
O(3/2)
a
CN/H
2
30
OH (5)
OH (5)
60
60
60
OH/H
OH/H
2
O(3/2)
O(3/2)
0.168, 2
0.168, 2
97
45
a
2
30
18
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Table 3 Oxidation of different olefins using 1.04 µmol of VOTPPCl
8
as the catalyst, 30% H
2
O
2
as terminal oxidant and sodium bicarbonate as pro-
3 2
moter using the CH CN/H O solvent mixture under the optimized reaction conditions
0
:002 mol% VOTPPCl8; 2 equiv: 30% H2O2
Olefins 0 ꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀ!
Epoxides
:4 equiv: NaHCO3; CH3CN=H2O; 60 °C
TOF (h−
9560
1
)
Entry no.
1
Substrate (5 mmol)
Product
Time (h)
0.5
Conv. %
% Selectivity
99
100
2
3
0.5
0.5
92
98
8884
9463
98
100
4
5
0.5
0.5
85
100
99
8208
9560
100
6
7
0.5
0.5
91
73
100
98
8787
7049
8
0.5
68
99
6566
reactions of various olefins and the results are summarized in
Table 3.
Catalytic mechanism
We tried UV-Visible spectral titration of VOTPPCl
ence of H /NaHCO in the CH CN/H O mixture and found
that no spectral changes are observed. So, we carried out
NMR experiments in order to ascertain the intermediate
8
in the pres-
2
O
2
3
3
2
5
1
V
−
formed during the catalytic reaction. H
2
O
2
and HCO
3
are
used together to increase the activity of H O by the in situ
2
2
−
generation of HCO
4
2 2
which is more nucleophilic than H O
and thus contributes to the enhanced oxidizing activity of the
1
8–22
metal complexes and to speed up the reaction.
the presence of H and NaHCO , the solution of VOTPPCl
in DMSO-d is converted into oxidoperoxidovanadium(V), [VO-
Hence, in
2
O
2
3
8
5
1
Fig. 6
V NMR spectra of VOTPPCl
8
(5.2 µmol) in DMSO-d
and 0.3 mmol of NaHCO
0 min after addition, a peak appeared at −618.4 ppm; (b) after
8 hours; (c) 1.2 mmol of cyclohexene was added and heated for 5 min
hexene decreases the peak intensity at −618.4 ppm by heating at 50 °C; (d) large excess of cyclohexene was added and heated for
0 minutes at 50 °C.
6
in the
6
−
presence of: (a) 0.36 mmol of 30% aq. H
2
O
2
3
,
(
O )TPPCl ] with a resonance at −618.4 ppm and it remains
2
8
1
4
stable for two days as shown in Fig. 6. The addition of cyclo-
1
to 50 °C for 5 minutes (Fig. 6c). Further, the peak at
618.4 ppm is completely diminished by adding excess of
cyclohexene and heating at 50 °C for 10 minutes (Fig. 6d).
−
5
1
32,33
The peak appearing at −618.4 ppm in V NMR indicates peroxidovanadium(V), [VO(O–OH)TPPCl
the formation of oxidoperoxidovanadium(V) species.
In general, oxidoperoxidovanadium(V) species,
VO(O )TPPCl ] is more stable as compared to oxidohydro- (70–100%) from olefin to epoxide by the VOTPPCl
8
] species
in which
Hence,
1
8,19,31–35
18,19,31–35
the hydroperoxide ion binds in side-on mode.
the 100% selectivity for epoxide formation in good yields
catalyst is
1
8,19,31,33–35
−
[
8
2
8
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π-system by showing a remarkable anodic shift in redox poten-
tials compared to VOTPP. VOTPPCl showed very high thermal
8
stability till 390 °C due to the electron deficient nonplanar por-
phyrin core. Further, VOTPPCl
epoxidation of various olefins in good yields in the presence of
/NaHCO in a CH CN/H O mixture. The formation of the
oxidoperoxidovanadium(V) intermediate during the catalytic
8
was utilized for the selective
H
2
O
2
3
3
2
5
1
cycle was probed by V NMR studies and MALDI-TOF mass
analysis. This porphyrin catalyst (VOTPPCl ) has higher ther-
8
mochemical stability and recyclability. However, anchoring of
these catalysts in polymers or mesoporous materials (hetero-
Scheme 2 Plausible catalytic mechanism for the conversion of olefins genization) in high concentrations would be important for
to epoxides using the VOTPPCl
8
catalyst.
commercial applications. Currently, we are working on the
said topic and the results will be reported in the near future.
possibly due to the formation of the oxidoperoxidovanadium(V)
−
Experimental section
intermediate, [VO(O
2
)TPPCl
8
]
which converts olefin to
1
8,19
epoxide selectively.
TOF mass analysis to identify the catalytic intermediate.
VOTPPCl (0.02 mol%) was heated in the presence of H
2 equiv.) and NaHCO3 (0.4 equiv.) in CH CN at 60 °C for
Further, we have carried out MALDI- Chemicals and materials
CH CN and DMF employed in the present work were of
analytical grade and distilled before use. VOSO and NaHCO
3
8
2 2
O
4
3
(
3
were obtained from HiMedia, India and used as received.
Various alkenes used in this study were purchased from Alfa
Aesar and used as received. Silica gel (100–200 mesh) used for
column chromatography was purchased from Rankem, India
and used as received. TBAPF was obtained from Alfa Aesar,
6
India and recrystallized twice from ethanol followed by drying
under vacuum at 60 °C for 10 h.
3
0 minutes. Then the reaction mixture was cooled to RT and
subjected to MALDI-TOF mass analysis using the HABA
matrix. The peak at m/z 989.3 clearly shows the formation of
oxidoperoxidovanadium(V) species (Fig. S7 in the ESI†). Cyclo-
hexene (0.410 g, 5 mmol) was added to the reaction mixture
containing oxidoperoxidovanadium(V) species and heated to
6
0 °C for 30 minutes. In MALDI-TOF mass analysis, the dis-
appearance of the peak at m/z 989.3 shows the consumption of
oxidoperoxidovanadium(V) species (Fig. S8 in the ESI†). This
experiment further proves the existence of oxidoperoxidovana-
Instrumentation and methods
2 2
UV-Vis absorption spectra were recorded in distilled CH Cl
using an Agilent Cary 100 spectrophotometer. IR spectra were
recorded in the mid IR range of 4000–400 cm− on a Perkin-
Elmer spectrophotometer by making KBr pellets. Elemental
analysis was carried out using an Elementar Vario EL II instru-
5
1
dium(V) species as the reaction intermediate in addition to
NMR studies (Scheme 2).
V
1
The reason for very high TOF numbers (6566–9650 h− ) is
1
8
due to high thermal and chemical stability of VOTPPCl which
5
1
ment. V NMR spectra were recorded on a JEOL ECX 400 MHz
spectrometer using DMSO-d as a solvent containing 0.03%
resists thermal as well as chemical oxidative degradation and
further bulkiness prevents the bimolecular attack. At the end,
the obtained epoxide product was distilled off and the catalyst
was recovered. The isolated yields are close to GC yields. For
example, the isolated yields of 1-octene epoxide, trans-stilbene
epoxide and cis-cyclooctene epoxide were found to be 94%,
6
TMS (v/v). MALDI-TOF-MS spectra were recorded using a
Bruker UltrafleXtreme-TN MALDI-TOF/TOF spectrometer using
HABA as a matrix. Cyclic voltammetric measurements were
carried out using a CHI 620E instrument in triple distilled
2 2 6
CH Cl containing 0.1 M TBAPF as the supporting electrolyte
8
4% and 93%, respectively. The UV-Vis absorption spectral
under an argon atmosphere. A three electrode assembly was
used consisting of a Pt disk working electrode, Ag/AgCl as a
reference electrode and a Pt-wire as a counter electrode. Elec-
tron paramagnetic resonance spectra were recorded on a
Bruker EMX EPR spectrometer in toluene solvent at 120 K. The
spin Hamiltonian parameters were obtained from the simu-
lated EPR spectrum. Thermal analyses were performed with a
SII EXSTAR 6300 instrument. DFT studies were carried out
features of VOTPPCl didn’t change before and after the cata-
lytic reaction indicating its high thermal and chemical stability
as shown in Fig. S9 in the ESI.†
8
Conclusions
8
We have synthesized VOTPPCl in good yield and characterized using the B3LYP functional with the LANLD2Z basis set. The
by various spectroscopic techniques. The DFT optimized oxidation products were quantified using a Shimadzu 2010
structure of VOTPPCl in CH CN exhibited a highly nonplanar plus gas-chromatograph equipped with an Rtx-1 capillary
8
3
saddle shape conformation and the oxidation state of the column (30 m × 0.25 mm × 0.25 μm) and an FID detector.
IV
metal ion (V ) was confirmed by EPR spectroscopy. CV studies GC-MS analyses were carried out using a Perkin-Elmer GC-MS
revealed the electron deficient nature of the porphyrin (Clarus 500).
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Synthesis of 2,3,7,8,12,13,17,18-octachloro-meso-
tetraphenylporphyrinato vanadium(IV) (VOTPPCl8)
B. M. Trost, I. Fleming and S. V. Ley, Pergamon, Oxford,
1991, vol. 7, pp. 357–436.
2
(a) I. Bauer and H. J. Knölker, Chem. Rev., 2015, 115, 3170–
2 8
H TPPCl (0.18 g, 0.202 mmol) was dissolved in 40 mL of
3
1
387; (b) W. Liu and J. T. Grooves, Acc. Chem. Res., 2015, 48,
727–1735; (c) J. Y. Lee, O. K. Farha, J. Roberts,
DMF. To this, 10 equiv. of VOSO4 (0.518 g, 2.02 mmol)
was added and refluxed for 16 h under argon atmosphere.
At the end of this period the reaction mixture was cooled to
room temperature and then 120 mL of distilled water was
added. Porphyrin was precipitated out and filtered through a
G-4 crucible. The crude product was purified on a silica
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009, 38, 1450–1459; (d) A. H. Chughtai, N. Ahmad,
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2
2
T. C. Brunold, M. I. Davis, J. N. Kemsley, S. K. Lee,
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3
0
.152 g (0.16 mmol, 79%).
UV-Vis (CH Cl ): λ (nm) (log ε) 354 (4.01), 452 (5.15), 578
max
2
2
−
1
(
(
4.00), 618 (sh); IR (KBr, cm ) 1003 (νVvO); MALDI-TOF-MS
m/z): found 954.95 [M] , calcd 954.96; elemental analysis
+
calcd for C H N Cl VO: C, 55.32%; H, 2.11%; N, 5.87% and
4
4
20
4
8
2315–2344.
found: C, 55.23%; H, 2.28%; N, 5.98%.
3
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Catalytic activity studies
The catalytic efficiency of VOTPPCl
8
was tested for the oxi-
dation of various alkenes. In a typical catalytic experiment,
5
1
mmol (0.410 g) cyclohexene, 10 mmol 30% H
2
O
2
(1.13 g),
−
4
mmol NaHCO
3
(0.084 g) and 5.23 × 10 M (0.0005 g) of
VOTPPCl were added into 5 mL of CH CN in a round bottom
8
3
flask fitted with a condenser. The flask was placed in an oil
bath and the temperature was maintained at 60 °C throughout
the reaction time with 500 RPM mechanical stirring. After the
reaction was completed, the reaction mixture was filtered and
its 1 mL portion was subjected to multiple heptane extraction.
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injected into the GC/GC-MS. The oxidation products were
quantified using a Shimadzu 2010 plus gas-chromatograph
equipped with an Rtx-1 capillary column (30 m × 0.25 mm ×
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9
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