Halogenated tetraphenyl porphyrin; Iodosylbenzene an efficient catalytic system for olefin epoxidation

Article abstract of DOI:10.1142/S1088424615500935

Olefin epoxidation using o-phenyl and b-pyrrole substituted tetraphenyl porphyrin complexes of Fe(III) and Mn(III) as catalysts and yIodosylbenzene as oxidant was studied. Excellent yields of epoxide and secondary oxidation products were observed. Effect of pyridine, imadizole, sodium lauryl sulphate, and solvent variation was also studied. Effect of substituents on catalytic activity of the porphyrin complex is also explained. o-phenyl substituted complexes gave better yield of epoxide as compared to b-pyrrole substituted complexes.

Full text of DOI:10.1142/S1088424615500935

Journal of Porphyrins and Phthalocyanines
J. Porphyrins Phthalocyanines 2015; 19: 1–7
DOI: 10.1142/S1088424615500935
Published at http://www.worldscinet.com/jpp/
Halogenated tetraphenyl porphyrin; iodosylbenzene
an efficient catalytic system for olefin epoxidation
a
b◊
Dau D. Aggrawal and Daisy Bhat*
a
SOS Chemistry Jiwaji University, Gwalior (M.P) 474011, India
R.D Foundation Group of Institutions NH-58, Ghaziabad (U.P) 201204, India
b
Received 3 August 2015
Accepted 4 September 2015
ABSTRACT: Olefin epoxidation using o-phenyl and b-pyrrole substituted tetraphenyl porphyrin
y
complexes of Fe(III) and Mn(III) as catalysts and Iodosylbenzene as oxidant was studied. Excellent
yields of epoxide and secondary oxidation products were observed. Effect of pyridine, imadizole, sodium
lauryl sulphate, and solvent variation was also studied. Effect of substituents on catalytic activity of the
porphyrin complex is also explained. o-phenyl substituted complexes gave better yield of epoxide as
compared to b-pyrrole substituted complexes.
KEYWORDS: epoxidation, porphyrin catalysis, olefins.
be very effective catalysts for hydrocarbon epoxidation,
INTRODUCTION
hydroxylation and other controlled oxidation reactions
16, 17] besides being excellent biomimetic models,
Epoxidation of olefins continues to be an important
reaction in organic synthesis, due to the feasibility of
conversion of the epoxide into number of useful products
[
Iron, mangenes, ruthenium metalloporphyrins have
been used efficiently for synthesis of a range of organic
molecules and continue to amuse researchers all over
the world [18–20]. Keeping in mind, the importance
of metalloporphyrins, commercial applications of
epoxidation and our continued interest in metal complex
catalysts [21], we studied epoxidation of cycloolefins
using metalloporphyrins as catalysts. This research
paper presents the investigation of catalytic behavior
of some sterically hindered metallo porphyrins having
substituents at b-pyrrole and o-phenyl positions. The
possible effect on calatytic activity viz-a-viz substituents
is also explained.
[
1–3]. The controlled oxygenation of alkanes alkenes,
and aromatic hydrocarbons is one of the most important
technologies to convert petroleum products into useful
commodities [4–8]. The heme protein, cytochrome P-450
can very efficiently catalyze these complex reactions
using aerial oxygen [7–8], but isolation and synthesis
of cyto-chrome P-450 is very difficult and expensive.
Hence development of efficient catalytic systems that
mimic action of cytochrome P-450 and other such natural
oxidation catalysts has always been and continues to
attract much attention [9–12]. Transition metal complexes
like Schiff bases, metalloporphyrins show remarkable
catalytic activity towards epoxidation, due to their ability
to bring the substrate and oxidant within the coordination
sphere. The oxygen transfer rate is enhanced many fold
as a result of face-to face proximity of the substrate and
oxidant in presence of these catalysts [13–15]. Synthetic
metalloporphyrin/oxidant systems have been found to
RESULTS AND DISCUSSION
Epoxidation of cyclohexene gave cyclohexene oxide,
cyclohexenol and cyclohexenone as the products (Table 1,
Scheme 1) In the case of Mn(OBP)Cl acetonitrile was
found to give better yield while in the case of Mn(2,6-Cl
PP)Cl and Mn(OCOBP)Cl dichloromethane solvent gave
better results. The yield of cyclohexene epoxide followed
the order M(OCOBP)Cl > M(2,6-Cl TPP)Cl > M(OBP)Cl
(where M = Mn and Fe). This shows that substitution at
◊
SPP full member in good standing
*
Correspondence to: Daisy Bhat, email: daisybhat@yahoo.
co.in
Copyright © 2015 World Scientific Publishing Company

2
D.D. AGGRAWAL AND D. BHAT
Table 2. Effect of catalyst concentration on epoxidation of
cyclohexene using Mn(III) porphyrin complxes as catalyst
ITI
+
Mn
X
+
PhIO
OH
O
S. No
Catalyst
Yield of epoxide, mmol
O
+
+
Mn OCOBP
Mn(OBP)Cl
1
2
3
4
5
0.00025 g
0.0005 g
0.00075 g
0.001 g
0.646
0.676
0.731
0.521
0.571
0.272
0.317
0.348
0.266
0.255
epoxide
cyclohexenol
cyclohexenone
Scheme 1. Epoxidation of cyclohexene
orthoposition in mesophenyl ring makes the porphyrin
catalyst more stable compared to that obtained after
brominating the b-pyrrole positions.
0.002 gm
*
Yields are based on amount of PhI generated.
The effect of change in catalyst concentration using
Mn(III) derivatives was studied. It was observed that
Mn(2,6-Cl TPP)Cl and Mn(OCOBP)Cl exhibited no
significant effect on the yield of epoxide when quantity
of catalyst was increased from 0.00025 to 0.00075 g. The
yield of epoxide increased in case of Mn(OBP)Cl with
increase in quantity of catalyst from 0.00025 to 0.00075 g.
but further increase in concentration, decreased the yield
of epoxide (Table 2).
o-phenyl position can reduce the m-oxo-dimer formation
considerably, so that the catalyst is not wasted in side
reaction even on increasing the concentration. On the
other hand, substitution at b-pyrrole position decreased
the electron density of metal centre and increases the
rate of reaction as compared to Mn(TPP)Cl but it could
not be effective in reducing the formation of m-oxo-
dimer. Similar behavior was observed with Fe(III)
complexes. Here also the decrease in epoxide yield at
higher concentration may be attributed to formation of
catalytically inactive, µ-oxo-dimer species.
Nolte et al. [22] while studying some Mn(III) comple-
xes,havesuggestedtheformationofanoxo-manganese(V)
species as the rate-determining step in epoxidation.
Formation of high valent oxo-iron(IV) porphyrin cation
+
o
radical (Fe IV=O ) as reactive intermediate for enzymatic
oxidation reaction for iron porphyrin models has been
proposed by previous workers [16, 18]. Formation of
similar type of species can explain the results observed
with the system, under investigation. It can be suggested
that the oxo-manganese(V) species responsible for
epoxidation, reacts with the Mn,(III) complex, resulting
in the formation of a m-oxo-dimer, as the concentration of
Mn(OBP)Cl increases. This m-oxo-dimer is catalytically
inactive. Thus at higher concentration of Mn(OBP)Cl, the
oxo-manganese(V) species is wasted in the unproductive
side reaction. Since the increase in concentration of
Mn(OCOBP)Cl and Mn(2,6-Cl TPP)Cl had no significant
effect on epoxidation, it suggests that substitution at
EPOXIDATION VS. HYDROXYLATION
Cyclohexene on epoxidation also gave cyclohexenol
and cyclohexenone as allylic oxidation products (Table 1)
along with cyclohexene oxide. Comparison with Fe(TPP)
X and Mn(TPP)X, studied by T.G. Taylor et al. [23]
shows that substitution at o-phenyl position ofTPP brings
greater selectivity towards epoxidation, suggesting the
involvement of cyclohexenyl redical [a] or direct electron
transfer [b] while substitution of bromine at b-pyrrole
position did not improve selectivity towards epoxidation
suggesting that radical abstraction [c] is responsible
for allylic oxidation products; (Scheme 2). This shows
Table 1. Epoxidation of cyclohexene catalyzed by Fe(III) and Mn(III)
porphyrin complexes
S. No
Catalyst
Product, mmol*
Cyclohexenone
Epoxide
Cyclohexenol
1
2
3
4
5
6
Mn(OBP)Cl
0.23
0.57
0.59
0.18
0.42
0.37
0.29
0.12
0.14
0.22
0.19
0.19
0.13
0.14
0.17
0.07
0.17
0.21
Mn(OCOBP)Cl
Mn(2,6-ClTPP)Cl
Fe(OBP)Cl
Fe(OCOBP)Cl
Fe(2,6-ClTPP)Cl
*
Yields are based on amount of PhI generated.
Copyright © 2015 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2015; 19: 2–7

HALOGENATED TETRAPHENYL PORPHYRIN; IODOSYLBENZENE AN EFFICIENT CATALYTIC SYSTEM
3
Solv.
+
+
HOM+
+
M = O
M = O
(
b)
(a)
(c)
+
OH
H
H
OM
OM
O
+
Scheme 2.
CHO
a
O
Catalyst
PhIO
O
+
+
+
O
b
Scheme 3. Epoxidation of norbornene
Table 3. Epoxidation of norbornene using Fe(III) and Mn(III) porphyrin as
catalyst
S. No
Catalyst
Epoxide yield, mmol*
Relative
Exo/endo ratio
Exo epoxide Endo epoxide
1
2
3
4
5
6
Mn(OBP)Cl
0.11
0.12
0.02
0.14
0.10
0.14
0.70
0.94
0.95
0.32
0.54
0.68
1:6.6
1:7.9
1:50.1
1:2.3
1:5.5
1:4.9
Mn(OCOBP)Cl
Mn(2,6-ClTPP)Cl
Fe(OBP)Cl
Fe(OCOBP)Cl
Fe(2,6-ClTPP)Cl
*
Yields are based on amount of PhI generated.
that location of substituent plays an important role in
product formation.
The epoxidation of norbornene using Mn(III)
and Fe(III) porphyrin complexes gave exo and endo
epoxide along with traces of rearrangement products
cyclohexene-4 carboxaldehyde (a) and norcamphor (b)
exo/endo ratio of epoxide, using the b-pyrrole substituted
porphyrins is lesser than M (TTP)X type of complexes.
The exo/endo ratio is 7.0 for Mn(OBP)Cl) 50.0 for
[Mn[2.6 ClPP]Cl and 8.0. for Mn(OCOBP)Cl, which is
much less than 450 reported for Mn(TPP)X [23]. This
shows that exo/endo epoxide ratio decreases as the
electron density in the porphyrin decreases. The addition
of bromine at b-pyrrole positions has marked effect on
exo/endo ratio compared to the effect by addition of
chlorine at 2,6-positions of phenyl ring. This suggests
that substitution of electronegative atom near the metal
center has pronounced effect compared to that in phenyl
ring. The same behavior was observed using F(III)
catalysts. The exo/endo ratio increased markedly from
Fe(III) complexes to Mn(III) porphyrin complexes.
(
Table 3, Scheme 3). Formation of exo and endo-epoxide
shows that rotation of bond is taking place leading to
endo and exo epoxide. This finding suggest that either
carbocation or carbon radical intermediate is involved
in the reaction, as metalloxetane, ion pair and concerted
insertion of oxygen will lead to exo epoxide formation
only. Rearrangement of exo-epoxide can be suggested for
+
formation of endo expoxide. It is known that M-O-C-C
canexplaintheformationofsmallamountofcyclohexene-
4
-carboxaldehyde and norcamphor [24]. These results
The effect of pyridine on the epoxidation of norbornene
was studied (Table 4). Addition of pyridine increased the
yield of epoxide with Mn(OBP)Cl. It can be suggested
that pyridine helps in rapid transfer of oxygen to the
olefin. pyridine gets co-ordinated to the oxo-manganese
complex and thus (i) makes the oxygen of oxo-manganese
complex less negatively charged, (ii) the oxygen atom
suggest a nucleophilic attack of the olefin on the oxo
metal and development of positive charge on unbound
cabon atom. Thus the most probable intermediate being
involved is the carbocation intermediate.
Comparison of data for exo/endo ratio for norbornene
among the porphyrin catalysts (Table 3) shows that the
Copyright © 2015 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2015; 19: 3–7

4
D.D. AGGRAWAL AND D. BHAT
results in faster rate of epoxidation. Addition of pyridine
also increased the exo/endo ratio in Mn(OBP)Cl and
Mn(OCOBP)Cl catalyzed reactions. This increase can be
explained by the fact that in these complexes the addition
of pyridine pulls the metal atom down towards the plane
of the ring. This increases the steric hindrance for olefin
as a result, the rotation of C–C bond is reduced and
consequently the ratio of exo/endo epoxide is increased.
Addition of bromine atoms at b-pyrrole positions makes
the ring puckered and the metal has to go out of plane to
stabilize itself, this gives freedom to C–C bond rotation
and hence a low value of exo/endo ratio. Mn(2,6-Cl TPP)
Cl did not exhibit significant change in exo/endo ratio on
addition of pyridine.
In case of Fe(III) porphyrin complexes (Table 5) the
pyridine addition was found to increase the over all yield
of epoxide. This behavior is in contrast to that observed
using Fe(TPP)Cl/NaOCl system [26] where pyridine
inhibits catalytic activity as coordination of pyridine at
distal position is faster than that at proximal position. This
shows that addition of the electron withdrawing group
favours the pyridine addition at the axial position thus
promoting the yield of epoxide. The addition of pyridine
decreased exo/endo ratio. This observation is in contrast
to that observed with Mn(III) complexes, indicating that
in oxo iron porphyrin, axial ligation of pyridine leads to
some geometrical changes. The most conspicuous is the
M-O bond length and shifting of metal atom to the other
side of the porphyrin plane. This can explain the high
yields of epoxide. It also suggests that Fe(III) porphyrins
are more sensitive to electronic effects whereas Mn(III)
porphyrins are more sensitive to steric effect.
Fig. 1. Visible spectrum of cyclohexene epoxidation using
Mn(OBP)Cl, PhIO, pyridine
becomes a better acceptor of p electron density and (iii).
The oxo-manganese bond is weakened [25]. The l-max
of Mn(OBP)Cl (Fig. 1) catalysed epoxidation did
not show any remarkable change after completion of
reaction. This shows that pyridine reduces destruction of
catalyst by reducing formation of m-oxo dimer, All these
factors help in promoting the yield of epoxide. In case
of Mn(OCOBP)Cl and Mn(2,6-ClTPP)Cl, addition of
pyridine did not effect the overall yield of epoxidation
showing that substitution by chlorine or bromine makes
the metal centre fairly electron deficient, which inturn
Addition of imidazole, increased the yield of epoxide
in Fe(OBP)Cl (Table 4) manyfold, but decreased exo/
endo ratio. The increase can be attributed to direct
co-ordination of imidazole to active metal centre
(proximal effect) as well as its action as a base (distal
effect). The decrease in exo/endo ratio shows that
coordination of imidazole to metal-oxo species makes
Table 4. Effect of additives on the epoxidation of norbornene using Mn(III) porphyrin complexes as catalysts
S.No
Additive
Product, mmol
Mn(OCOBP)Cl
Mn(OBP)Cl
Mn(2,6-ClTPP)Cl
Endo
Exo
Exo/endo
Endo
Exo
Exo/endo Endo
Exo
Exo/endo
ratio
ratio
ratio
1
2
3
4
5
6
No addition
pyridine
Imidazole
Triton x-100
CTABr
0.106
0.036
Nil
0.694
0.896
0.231
0.364
0.283
0.694
1:6.6
0.120
0.032
0.012
0.063
Nil
0.942
0.946
0.317
0.866
0.707
0.585
1:7.85
1:29.5
1:26.4
1:13.75
1:0.70
1:5.18
0.019 0.952
1:50.11
1:27.35
1:20.55
1:15.20
1:0.89
1:24.9
1:0.23
1:0.36
1:0.28
1:5.83
0.034
0.93
0.033 0.678
0.059 0.897
Nil
Nil
Nil
0.897
SLS
0.119
0.113
0.095 0.580
1:6.11
Copyright © 2015 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2015; 19: 4–7

HALOGENATED TETRAPHENYL PORPHYRIN; IODOSYLBENZENE AN EFFICIENT CATALYTIC SYSTEM
5
Table 5. Effect of additives on epoxidation of norbornene
using Fe(OBP)Cl as catalysts
bromide (CTABr), a cationic surfactant substantially
reduced the endo and exo epoxide yield. This shows that
positively charged micelles of CTABr retard epoxidation
in general but endo epoxide in particular. These results
suggest the presence of a carbocation intermediate in this
system. Fe(III) porphyrin complexes too present a similar
behavior.
S. No
Additive
Endo epoxide, Exo epoxide,
mmol
Mmol
1
2
3
4
5
6
No addition
pyridine
Imidazole
Triton x-100
CTABr
0.140
0.428
0.828
0.147
Nil
0.316
0.480
0.458
0.207
0.039
0.270
The results of individual and competitive epoxidation
(Table 6) of cycloolefins follow the order norbornene
>
cyclo-heptene cyclohexene, suggesting that
>
coordination of olefin to metal center is not the rate
determining step because coordination constant of
norbornene is less than cyclohexene, yield of epoxide
should be less in norbornene. This observation too
discards the oxometallacycle intermediate formation.
This shows that only relief in bond angle strain play an
important role in anticipating the relative rate for cyclic
olefins. All these studies suggest the involvement of
carbocation type intermediate in the reaction.
SLS
0.193
it stable, enough to undergo rotation which results in
formation of endo epoxide. In case of Mn(III) complexes,
a general decrease in yield of epoxide was observed with
all catalysts (Table 4). Although imidazole is known to
increase efficiency of Metal porphyrin catalysts towards
olefin oxidation [27, 28]. The possible rationale for this
decrease can be oxidative destruction of imidazole in
given reaction conditions resulting in unproductive side
engagement of the oxidant.
Addition of methanol increased the yield of
norbornene epoxide when Mn(OBP)Cl and Mn(2,6-
ClTPP)Cl catalyst were used. Decrease in exo/endo
ratio was observed in case of Mn(OCOBP)Cl catalyzed
reaction. This shows that addition of methanol increased
the polarity of the medium leading to charge separation
with the formation of carbocation type intermediate
which rotates to give endo epoxide. The exo/endo ratio
decresed in Fe(III) catalyzed reactions as well, but over
all yield remained unaltered.
Mechanism
On the basis of aforementioned studies a general
mechanism shown in (Scheme 4) can be proposed. It
is known that Mn(TPP) X/PhIO [23] system gives oxo-
manganese [V] species which, in turn aids formation of
epoxide and allylic oxidation products. The epoxidation
of norbornene gave exo-and endo epoxy norbornene
showing that in norbornene the two O–C bonds are
formed simultaneously but at different rates. Under the
experimental conditions. the internediate so formed finds
sufficient time to undergo rotation which explains the
formation of observed products in present system.
In order to distinguish between carbon radical and
carbocation intermediate the effect of other surfactants
EXPERIMENTAL
(Table 4) on the epoxidation of norbornene was studied.
Addition of sodiumlaurylsulphate, an anionic surfactant,
promoted the endo epoxide formation. This suggests that
being an anionic surfactant SLS stabilize the carbocation
intermediate giving it enough time to undergo C–C
bond rotation, resulting in the increase of yield of endo
epoxide. The addition of non-ionic surfactant triton x-100
decreasedtheyieldofepoxideshowingthatmicellesretard
epoxidation. The addition of cetyl trimethyl-ammonium
IodosyIbenzene was synthesized by Iiterature [29]
procedure and stored at -20°C. The olefins, (cyclohexene,
cycloheptene, norbornene) and dichoromethane were
purified before use [30]. Standard samples of the
various epoxides were prepared by literature procedures
[31]. Sodium lauryl sulphate [SLS], cetyl trimethyl
ammonium bromide [CTABr], Triton x-100 imidazole
and pyridine used were of high purity. The epoxidation
Table 6. Competitive epoxidation of cycloolefins
S. No
Catalyst
Cyclohexene
Cycloheptene
Norbornene
1
Mn(OBP)Cl
1.0
1.0
1.0
1.0
1.0
1.0
4.63
2.11
1.45
5.39
1.76
1.28
3.73
2.39
1.96
7.20
1.90
1.96
2
3
4
5
6
Mn(OCOBP)Cl
Mn(2,6-ClTPP)Cl
Fe(OBP)Cl
Fe(OCOBP)Cl
Fe(2,6-ClTPP)Cl
Copyright © 2015 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2015; 19: 5–7

6
D.D. AGGRAWAL AND D. BHAT
O
of electronegative atoms at b-pyrrole and O-phenyl
position has a remarkable effect on their catalytic activity
as well as selectivity in product formation.
III
III
V
Mn
Cl
+ PhIO
Mn
CI
O
+
Mn
CI
OH
III
Mn
Cl
Acknowledgements
e-
I
The authors are thankful to Jiwaji Univ. Gwalior.
CSIR New Delhi & R.D Foundation Group of Institutions
Ghaziabad for financial assistance and providing facility.
+
+
Mn
O
O
IV
Mn
Mn
O
µ-oxo dimer
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