Substitution effects on the catalytic activity of Mn(III)-porphyrins in epoxidation of alkenes with iodosylbenzene: A comparison between the electron-rich and electron-deficient porphyrins

Article abstract of DOI:10.1016/j.poly.2011.04.015

Oxidation of different olefins with iodosylbenzene in the presence of Mn(III) complexes of meso-tetra(para-tolyl)porphyrin, meso-tetra(ortho-tolyl) porphyrin, meso-tetra(thien-2-yl)porphyrin and β-hexaboromo-meso- tetra(thien-2-yl)porphyrin as catalyst has been studied. Oxidation of cis- and trans-stilbene in a competitive reaction strongly suggests the involvement of a high valent (porphyrin)MnO as the active oxidant intermediate, in the case of each catalyst. Clear observation of the band relevant to a (porphyrin)Mn(IV)O species in the presence of excess amounts of styrene shows the stability of this moiety towards reaction with olefins. Although, the stability of metalloporphyrins towards oxidative degradation decreases in the order MnT(o-tolyl)P(OAc) > MnT(thien-2-yl)PBr₆(OAc) > MnT(p-tolyl)P(OAc) ≥ MnT(thien-2-yl)P(OAc), a complex pattern of catalytic activity and product (epoxide) selectivity has been found for the Mn-porphyrins in oxidation of various alkenes.

Full text of DOI:10.1016/j.poly.2011.04.015

Polyhedron 30 (2011) 1732–1738
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Substitution effects on the catalytic activity of Mn(III)-porphyrins in epoxidation
of alkenes with iodosylbenzene: A comparison between the electron-rich
and electron-deficient porphyrins
⇑
Saeed Zakavi , Leila Ebrahimi
Department of Chemistry, Institute for Advanced Studies in Basic Sciences (IASBS), Zanjan 45137-66731, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
Oxidation of different olefins with iodosylbenzene in the presence of Mn(III) complexes of meso-tetra-
(para-tolyl)porphyrin, meso-tetra(ortho-tolyl)porphyrin, meso-tetra(thien-2-yl)porphyrin and b-hexabo-
romo-meso-tetra(thien-2-yl)porphyrin as catalyst has been studied. Oxidation of cis- and trans-stilbene
in a competitive reaction strongly suggests the involvement of a high valent (porphyrin)Mn@O as the
active oxidant intermediate, in the case of each catalyst. Clear observation of the band relevant to a
(porphyrin)Mn(IV)@O species in the presence of excess amounts of styrene shows the stability of this
moiety towards reaction with olefins. Although, the stability of metalloporphyrins towards oxidative
degradation decreases in the order MnT(o-tolyl)P(OAc) > MnT(thien-2-yl)PBr6(OAc) > MnT(p-tolyl)-
P(OAc) P MnT(thien-2-yl)P(OAc), a complex pattern of catalytic activity and product (epoxide) selectiv-
ity has been found for the Mn-porphyrins in oxidation of various alkenes.
Received 28 October 2010
Accepted 6 April 2011
Available online 13 April 2011
Keywords:
Electron-rich and electron-deficient
porphyrins
Iodosylbenzene
Mn-porphyrins
Oxidation
Ó 2011 Elsevier Ltd. All rights reserved.
Alkenes
1. Introduction
as well as the catalytic activity of the corresponding metallopor-
phyrins in oxidative conditions, there are many reports that show
Peripheral substitution of the porphyrin macrocycle with the
aim of preparing more stable ‘‘second- and third-generation’’
porphyrins with halogen, nitro or sulfone substituents has been
used as an effective strategy to increase the stability of metallopor-
phyrins towards oxidative degradation and develop new porphy-
rin-based catalysts with enhanced catalytic activity for oxidation
reactions [1]. The stabilization of HOMO level of the so called
electron-deficient porphyrins, causes the improved activity as well
as the stability of the corresponding metalloporphyrins under
reaction conditions [2–4]. On the other hand, resonance interac-
comparable or higher catalytic activity of electron-rich metallopor-
phyrins relative to the electron-deficient ones in oxidation of
hydrocarbons with periodate, iodosylbenzene or meta-chloroper-
benzoic acid (m-CPBA) as oxidant [8,10–14].
In the present work, catalytic activity of a series of electron-rich
and electron-deficient Mn(III)-porphyrins with five and six mem-
bered rings at meso-positions and H or Br atoms at b-positions in
oxidation of alkenes with iodosylbenzene (PhIO) has been studied
in a comparative manner. Also, attempts have been made to
elucidate the nature of the active intermediates of the oxidation
reaction in the presence of the title Mn-porphyrins. It should be
noted that using excess amounts of organic substrate relative to
oxidant is among the main strategies have been utilized to over-
come the problems associated with the stability of metalloporphy-
rins [15–17]. In the present work, the molar ratio of alkene to PhIO
is ca. 17:1 which enables us to compare the catalytic activity of
different metalloporphyrins in a systematic study.
tions between the meso-aryl substituents and the porphyrin
p
system lead to the destabilization of the a2u orbital [5]. Large dihe-
dral angles between the porphyrin mean plane and the aryl groups
induced by introduction of bulky substituents at ortho positions of
aryl groups hinder the effective resonance interactions [6,7].
[Mntmp]OAc (tmp = meso-tetramesitylporphyrin) has been found
to show higher catalytic activity relative to Mn(tpp)OAc in the
oxidation of sterically unhindered cycloalkanes and aryalkanes
with periodate as oxidant [8]. In a recent work, similar catalytic
activities for Ru(IV)(tmp)Cl2 and Ru(IV)(tdcpp)Cl2 (tdcpp = meso-
tetra(2,6-dichlorophenyl)porphyrin) have been reported [9].
Although it is generally accepted that the presence of electron-
withdrawing substituents on the macrocycle enhances the stability
2. Experimental
2.1. Preparation of porphyrins
Meso-tetra(para-tolyl)porphyrin, H2T(p-tolyl)P [18], meso-tetra-
(ortho-tolyl)porphyrin, H2T(o-tolyl)P [18], meso-tetra(thien-
2-yl)porphyrin, H2T(thien-2-yl)P [19] (Fig. 1) were synthesized,
characterized and purified according to the literature methods.
⇑
Corresponding author. Tel.: +98 241 4153202; fax: +98 241 4153232.
E-mail address: zakavi@iasbs.ac.ir (S. Zakavi).
0277-5387/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2011.04.015

S. Zakavi, L. Ebrahimi / Polyhedron 30 (2011) 1732–1738
1733
Ar =
1
X=
Ar
X
Porphyrin =
X
8H,
S
3
2
X
H2T(thien-2-yl)P
X
CH3
NH
N
N
Ar
H2T(o-tolyl)P
H2T(p-tolyl)P
Ar
HN
CH3
X
X
S
6Br, 2H
H2T(thien-2-yl)PBr6
X
X
Ar
Fig. 1. Porphyrin used in this study.
Bromination of H2T(thien-2-yl)P with excess amounts of
N-bromosuccinimide (freshly recrystallized from hot water and
dried at 80 °C under vacuum) [20] leads to the formation of b-hex-
136/2300 Â 100% = 5.91 mol% is corresponding to a conversion of
100% based on the starting oxidant. Other values are calculated
considering the value of 5.91% for the 100% conversion. In the case
of cis- and trans-stilbene, the products were analyzed by 1H NMR.
All reactions were repeated at least three times.
aboromo-meso-tetra(thien-2-yl)porphyrin,
H2T(thien-2-yl)PBr6;
H2T(thien-2-yl)P (0.5 mmol) was dissolved in CHCl3 (80 ml).
N-bromosuccinimide (918 mg, 5.0 mmol) was added and the solu-
tion was stirred for 24 h at room temperature. Evaporation of the
solvent gave a solid which was washed with methanol to remove
any soluble succinimide impurities. The product, H2T(thien-
2-yl)PBr6 was recrystallized from CHCl3/n-hexane (3:1, v/v) and
dried under vacuum. The degree of bromination of porphyrin has
been determined by 1H NMR spectrometry based on the ratio of
the intensities of the signals corresponding to the protons from
Hb and meso-substituents (Hb/H3 ratio). H2T(thien-2-yl)PBr6: 1H
NMR (d ppm) (CDCl3, 250 MHz): À2.62 (2H, br, s, NH); 7.52–7.53
(4H, m); 7.68–7.70 (4H, m); 7.90 (4H, m); 8.99 (8H, s, b); UV–Vis
(k/nm) in CH2Cl2: 458 (Soret), 553, 612, 711.
2.5. Degradation of metalloporphyrins
General procedure and the molar ratios are as mentioned in
Section 2.4. After the required time, sodium bisulfite has been
added to quench the reaction. The dichloromethane phase was
separated and dried over anhydrous magnesium sulfate. The absor-
bance of the final solution has been compared with that of the ini-
tial one at the kmax of the metalloporphyrin to measure the extent
of degradation of initial metalloporphyrin.
2.6. Instrumentation
2.2. Metallation of porphyrins
1H spectra were obtained in CDCl3 solutions with a Bruker
Avance DPX 250 MHz spectrometer. The absorption spectra were
recorded on a Pharmacia Biotech Ultrospec 4000 UV–Vis spectro-
photometer. The reaction products of oxidation were analyzed by
a Varian-3800 gas chromatograph equipped with a HP-5 capillary
Metallation of the porphyrins with Mn(OAc)2Á4H2O was carried
out following the literature methods [21]. The kmax of MnT(thien-
2-yl)P(OAc), MnT(p-tolyl)P(OAc), MnT(o-tolyl)P(OAc) and MnT(thien-
2-yl)PBr6(OAc) appears at 478, 479, 479 and 503 nm, respectively.
2.3. Preparation of PhIO
1.2
0.8
0.4
0
479 nm
a
PhIO has been prepared and purified according to the method
developed by Sharefkin et al. [22].
430 nm
2.4. General oxidation procedure
Stock solutions of the metalloporphyrins (0.003 M) were
prepared in dichloromethane. In
d
a typical reaction, alkene
(1.15 mmol) and metalloporphyrin (0.0005 mmol) were added into
a 5 ml round bottom flask containing 3 ml of dichloromethane.
Reaction was conducted by adding PhIO (0.068 mmol) to the solu-
tion. The molar ratios for catalyst:alkene:PhIO were 1:2300:136.
The mixture was stirred thoroughly for 3 h at ambient tempera-
ture. The progress of reaction was followed using TLC and GC anal-
ysis. After the required time, the reaction was quenched by adding
sodium bisulfite. The dichloromethane phase was separated and
dried over anhydrous magnesium sulfate. The resulting solution
was analyzed by GLC using n-octane as internal standard. It should
be noted that in a non-stoichiometric reaction, the limiting reagent
(here, PhIO) should be used to calculate the expected yields and
conversions. Accordingly, the molar ratio of PhIO to the olefin i.e.
b
c
350
400
450
500
550
λ (nm)
600
650
700
Fig. 2. Spectral changes upon addition of PhIO to
a solution of (a) MnT
(o-tolyl)P(OAc) in dichloromethane in the absence of alkene (b) after 1 min, (c)
after 3 min and (d) after the addition of sodium bisulfite. The molar ratio for MnT
(o-tolyl)P(OAc):PhIO are 1:136.

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S. Zakavi, L. Ebrahimi / Polyhedron 30 (2011) 1732–1738
3.1. Stability of metalloporphyrins
1.6
1.4
1.2
1
Stability of the metalloporphyrins towards oxidative degrada-
tion in the presence and the absence of alkene (styrene) have been
studied. Addition of PhIO to the solution of MnT(o-tolyl)P(OAc) in
dichloromethane in the absence of styrene led to the appearance
of a band at 430 nm and gradual disappearance of the band at
the kmax of MnT(o-tolyl)P(OAc) (Fig. 2).
0.8
0.6
0.4
0.2
0
A similar spectral pattern was observed in the presence of
styrene, but the rate of disappearance of the band at kmax decreased.
Upon the addition of sodium bisulfite to the reaction mixture, the
band at 430 nm disappeared and the band due to the
metalloporphyrin has been observed (Fig. 2, curve d). Using other
metalloporphyrins led to the appearance of an absorption band at
432 nm, 440 nm and 459 nm for MnT(p-tolyl)P(OAc), MnT(thien-
2-yl)P(OAc) and MnT(thien-2-yl)PBr6(OAc), respectively. Although
the competitive oxidation of cis- and trans-stilbene strongly
suggests the involvement of a high valent manganese oxo interme-
diate in the catalytic cycle (vide infra), the absorption bands at
430–459 nm can not be attributed to the formation of a Mn(V)–
oxo species; The band due to a Mn(V)–oxo species appears at ca.
410 nm in dichloromethane/alcohol mixed solvent and disappears
rapidly in the presence of alkenes [19,23]. However, in the present
work, the band at 430–459 nm (for different Mn-porphyrins) has
been observed as a strong band in the presence as well as the
absence of styrene which is probably due to the formation of stable
Mn(IV)–oxo species [24].
1
2
3
4
Catalyst
Fig. 3. The absorbance change at the kmax of (1) MnT(thien-2-yl)P(OAc) (478 nm),
(2) MnT(p-tolyl)P(OAc) (479 nm), (3) MnT(thien-2-yl)PBr6(OAc) (503 nm) and (4)
MnT(o-tolyl)P(OAc) (479 nm) relative to the initial absorbance (see the text).
Table 1
Competitive oxidation of cis- and trans-stilbene in the presence of different Mn-
porphyrins at room temperature.a
Catalyst
Cis-epoxide
Trans-epoxide
Cis/trans
ratio
(%)b
(%)b
The stability of metalloporphyrins (Fig. 3) decreases in the order
MnT(o-tolyl)P(OAc) > MnT(thien-2-yl)PBr6(OAc) > MnT(p-tolyl)-
P(OAc) P MnT(thien-2-yl)P(OAc).
MnT(thien-2-yl)P(OAc)
MnT(p-tolyl)P(OAc)
MnT(thien-2-yl)PBr6(OAc)
MnT(o-tolyl)P(OAc)
19.6
26.4
5.5
12.8
25.5
5
1.53
1.03
1.1
16.5
16.3
1.01
The observed pattern of stability may be explained by electronic
and steric effects of substituents. The substitution of meso-tetra-
phenylporphyrins with methyl groups in ortho positions gives
porphyrins with increased dihedral angles between the meso-
substituted aryls and the porphyrin mean plane [6]. This in turn,
hinders the effective resonance interactions between the aryl
groups and the porphyrin core and leads to a decrease in the elec-
tron donation from the aryl groups to the porphyrin core [6,7]. On
the other hand, introduction of bulky substituents on the aryl
groups, especially at the site adjacent to the meso positions, has
been used as a strategy to reduce intermolecular catalyst interac-
tions and so prevent oxidation of resting catalyst by the active
intermediate [1,25].
a
The molar ratios for catalyst:alkene:PhIO are 1:(500:500):167.
Based on the starting PhIO for 3 h reaction time.
b
column (phenylmethyl siloxane 30 m  320
lm  0.25
lm) and
flame-ionization detector.
3. Results and discussion
Mn(III) complexes of the porphyrins (Fig. 1) have been used as
catalyst in oxidation of alkenes with PhIO. Cyclic and acyclic olefins
with hindered and unhindered double bonds have been used to
study the comparative catalytic activity of different metallo-
porphyrins.
Higher oxidative stability of MnT(o-tolyl)P(OAc) relative to
MnT(thien-2-yl)PBr6(OAc), seems to be due to the smaller size of
100
80
60
40
20
0
1
2
3
4
Fig. 4. Efficiency of MnT(thien-2-yl)PBr6(OAc) (series 3), MnT(p-tolyl)P(OAc) (series 2), MnT(thien-2-yl)P(OAc) (series 1) and MnT(o-tolyl)P(OAc) (series 4) in oxidation
of cyclohexene. (conversion:dotted column; Yield of epoxide: dashed column; epoxide selectivity:blank column).

S. Zakavi, L. Ebrahimi / Polyhedron 30 (2011) 1732–1738
1735
the five membered thienyl ring with respect to phenyl one which
allows for increased coplanarity between thienyl groups and
porphyrin mean plane leading to better electron donation to the
porphyrin core [26]. Also, the less hindered meso-positions of
MnT(thien-2-yl)PBr6(OAc) relative to those of MnT(o-tolyl)P(OAc)
should be taken into consideration.
Oxidation of styrene with PhIO in the presence of imidazole
(ImH) and the catalyst has been achieved. Interestingly, no absorp-
tion band relevant to a Mn(IV)–oxo species has been observed in
UV–Vis spectra. It should be noted that in the absence of ImH
the band due to the high valent Mn(IV)–oxo was observed clearly
during the course of reaction which disappeared after the addition
100
80
60
40
20
0
1
2
3
4
Fig. 5. Results of oxidation of indene (see the caption of Fig. 4).
100
80
60
40
20
0
1
2
3
4
Fig. 6. Results of oxidation of styrene (see the caption of Fig. 4).
140
120
100
80
60
40
20
0
1
2
3
4
Fig. 7. Results of oxidation of
a-methylstyrene (see the caption of Fig. 4).

1736
S. Zakavi, L. Ebrahimi / Polyhedron 30 (2011) 1732–1738
100
80
60
40
20
0
1
2
3
4
Fig. 8. Results of oxidation of 3-chloro-2-methyl-1-propene (see the caption of Fig. 4).
100
80
60
40
20
0
1
2
3
4
Fig. 9. Results of oxidation of cis-stilbene (see the caption of Fig. 4, cis-epoxide selectivity:blank column; trans-epoxide selectivity:black column).
of sodium bisulfite. Furthermore, addition of ImH to the reaction
mixture (up to 1:20 M ratio of catalyst/ImH) has little effect on
the catalytic activity of the metalloporphyrins.
the first three complexes. However, the best selectivity for
cyclohexene oxide was obtained with MnT(o-tolyl)P(OAc). Inter-
estingly, MnT(thien-2-yl)PBr6(OAc) is the least efficient catalyst
of the four. Also, MnT(o-tolyl)P(OAc) which is more stable than
the other Mn-porphyrins in the order of stability, shows less
activity relative to MnT(p-tolyl)P(OAc) and MnT(thien-2-yl)P(OAc).
The results of oxidation of other olefins have been shown in
Figs. 5–11.
In oxidation of indene (Fig. 5), comparable epoxide selectivity is
observed for the four catalysts but, the best conversion and yield of
epoxide are obtained in the presence of MnT(thien-2-yl)P(OAc)
and MnT(thien-2-yl)PBr6(OAc).
3.2. Oxidation of olefins
3.2.1. Competitive oxidation of cis- and trans-stilbene
Oxidation of cis- and trans-stilbene, in a competitive reaction,
has been shown to be a useful probe to elucidate indirectly the nat-
ure of active oxidant in the metalloporphyrin catalyzed epoxida-
tion of olefins in the absence of nonbulky ortho-substituents on
the phenyl groups of meso-tetraphenylporphyrins [11,27]. The re-
sults of competitive epoxidation of the two compounds with PhIO
in the presence of different Mn-porphyrins have been summarized
in Table 1. The observed ratios strongly suggest the involvement of
a high valent Mn–oxo species as the sole active oxidant in the pres-
ence of the used Mn-porphyrins.
Ph
δ+
Ph
3.2.2. Comparative catalytic activity of the Mn-porphyrins
O
The results of oxidation of different olefins with PhIO catalyzed
by the four Mn-porphyrins were summarized in Tables S1–S8.
Fig. 4 shows the efficiency of different catalysts in oxidation of
cyclohexeneasastericallyunhinderedalkene[28,29]. Conversionde-
creased in the order MnT(p-tolyl)P(OAc) > MnT(thien-2-yl)P(OAc) >
MnT(o-tolyl)P(OAc) >> MnT(thien-2-yl)PBr6(OAc). Although, the
maximum yield of epoxide has been obtained in the presence of
MnT(o-tolyl)P(OAc), but there is no significant difference between
δ−
N
N
Mn
N
N
Fig. 10. Cis to trans isomerization in oxidation of cis-stilbene.

S. Zakavi, L. Ebrahimi / Polyhedron 30 (2011) 1732–1738
1737
120
100
80
60
40
20
0
1
2
3
4
Fig. 11. Results of oxidation of trans-stilbene (trans-epoxide selectivity:blank column, see the caption of Fig. 4).
100
80
60
40
20
0
1
2
3
4
Fig. 12. Results of oxidation of 1-hexene (see the caption of Fig. 4).
Oxidation of styrene (Fig. 6) and sterically more hindered
methylstyrene (Fig. 7) was carried out with similar efficiency and
selectivity in the presence of the four Mn-porphyrins.
Catalytic efficiencies of MnT(p-tolyl)P(OAc), MnT(thien-2-yl)-
P(OAc) and MnT(o-tolyl)P(OAc) in epoxidation of 3-chloro-2-
methyl-1-propene are comparable (Fig. 8). In the case of
MnT(thien-2-yl)PBr6(OAc), the selectivity to epoxide is remarkably
lower than the other catalysts.
Catalytic activity of the Mn-porphyrins in selective oxidation of
cis-stilbene (Fig. 9) to the epoxide product decreases in the order
MnT(thien-2-yl)PBr6(OAc) >> MnT(o-tolyl)P(OAc) > MnT(p-tolyl)-
P(OAc) > MnT(thien-2-yl)P(OAc). Epoxidation of cis-stilbene with
different oxidant is usually accompanied with the cis to trans
isomerization [11,27]. Interestingly, the isomerization significantly
increased in the presence of MnT(o-tolyl)P(OAc) as catalysts. It is
notable that due to the steric hindrance caused by the methyl
groups in the position ortho to the meso-carbons, which prevent
the free rotation about the C–C bond in the common intermediate,
lower extent of isomerization would be expected [11]. However,
this observation shows the significant cleavage of the C@C double
bond in the intermediate. Apparently, the increased electrophilic-
ity of the Mn(V)@O moiety of O@Mn(V)T(o-tolyl)P(OAc) (see
Section 3.2.2) relative to that of the other Mn-porphyrins led to
the enhanced electron donation from the double bond to the
Mn(V)@O species and significant cleavage of the double bond
(Fig. 10).
a
-
Oxidation of sterically demanding trans-stilbene [11] gives
trans-stilbene oxide as the sole product. The order of catalytic
activity of the Mn-porphyrins (Fig. 11) correlates well with the
relative stability of the catalysts towards the active oxidant (see
Section 3.1).
MnT(thien-2-yl)PBr6(OAc) shows poor catalytic activity and
product (epoxide) selectivity compared with the other catalysts
in the oxidation of 1-hexene (Fig. 12). The efficiency of the other
Mn-porphyrins is nearly the same.
4. Conclusions
In summary, the competitive oxidation of cis- and trans-stilbene
in the presence of the Mn-porphyrins strongly suggests that a high
valent manganese oxo complex is involved as the reactive interme-
diate responsible for oxygen atom transfer. A complex order of
catalytic activity for the Mn-porphyrins has been observed in the
oxidation of various alkenes with PhIO, so that the electron rich
Mn-porphyrins showed higher catalytic efficiencies than the elec-
tron-deficient ones in oxidation of most of the used alkenes. Differ-
ent orders of catalytic performance of the used Mn-porphyrins in
the oxidation of alkenes of various types can not be simply ex-
plained by the steric and electronic effects of the substituents at-
tached to the double bond as well as the oxidative stability of
the Mn-porphyrins.

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[11] D. Mohajer, G. Karimipour, M. Bagherzadeh, New J. Chem. 28 (2004)
Acknowledgements
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Financial Support of this work by the Institute for Advanced
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