Substituent effects on the catalytic activity of a series of manganese meso-tetra(aryl)porphyrins: (2-, 3-, 4)-Pyridyl, 4-sulfonatophenyl and 3-sulfonato-4-methoxyphenyl groups compared to phenyl and 4-methoxyphenyl ones

Article abstract of DOI:10.1016/j.molcata.2012.06.003

Catalytic activity and oxidative stability of a series of Mn-porphyrins (Mn-pors) with pyridyl (2-, 3- or 4-), 4-sulfonatophenyl, 3-sulfonato-4- methoxyphenyl, phenyl and 4-methoxyphenyl in oxidation of olefins with tetra-n-butylammonium periodate has been investigated in a comparative study. While the complexes with pyridyl substituents, MnT(py)P(OAc) are generally more stable than those with the other aryl groups toward oxidative degradation in reaction conditions, their catalytic activity is usually lower than these Mn-pors. MnT(4-py)P(OAc) was found to be unusually more stable than MnT(2- or 3-py)P(OAc). The order of catalytic performance of the used Mn-pors and their oxidative stability are less correlated with the electronic properties of the meso substituents. On the basis of indirect evidence obtained in competition oxidation of cis- and trans-stilbene, the involvement of a six coordinate (IO₄)Mn(III)(Porphyrin)(ImH) and a high valent Mn-oxo species has been proposed. However, in the case of MnT(py)P(OAc), the latter seems to be more dominant than the former.

Full text of DOI:10.1016/j.molcata.2012.06.003

Journal of Molecular Catalysis A: Chemical 363–364 (2012) 153–158
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journal homepage: www.elsevier.com/locate/molcata
Substituent effects on the catalytic activity of a series of manganese
meso-tetra(aryl)porphyrins: (2-, 3-, 4)-Pyridyl, 4-sulfonatophenyl and
3
4
-sulfonato-4-methoxyphenyl groups compared to phenyl and
-methoxyphenyl ones
Saeed Zakavi , Aida Ghanbelanie Mojarrad , Saeed Rayatib
a,∗
a
a
Department of Chemistry, Institute for Advanced Studies in Basic Sciences (IASBS), Zanjan 45137-66731, Iran
Department of Chemistry, K.N. Toosi University of Technology, P.O. Box 16315-1618, Tehran 15418, Iran
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Catalytic activity and oxidative stability of a series of Mn-porphyrins (Mn-pors) with pyridyl (2-, 3- or 4-),
Received 11 February 2012
Received in revised form 2 June 2012
Accepted 4 June 2012
4
-sulfonatophenyl, 3-sulfonato-4-methoxyphenyl, phenyl and 4-methoxyphenyl in oxidation of olefins
with tetra-n-butylammonium periodate has been investigated in a comparative study. While the com-
plexes with pyridyl substituents, MnT(py)P(OAc) are generally more stable than those with the other aryl
groups toward oxidative degradation in reaction conditions, their catalytic activity is usually lower than
these Mn-pors. MnT(4-py)P(OAc) was found to be unusually more stable than MnT(2- or 3-py)P(OAc). The
order of catalytic performance of the used Mn-pors and their oxidative stability are less correlated with
the electronic properties of the meso substituents. On the basis of indirect evidence obtained in competi-
tion oxidation of cis- and trans-stilbene, the involvement of a six coordinate (IO4)Mn(III)(Porphyrin)(ImH)
and a high valent Mn-oxo species has been proposed. However, in the case of MnT(py)P(OAc), the latter
seems to be more dominant than the former.
Available online 13 June 2012
Keywords:
meso-Tetra(pyridyl)porphyrins
Sulfonated-Meso-tetra(aryl)porphyrins
Oxidation of olefins
Manganese porphyrins
Catalysis
©
2012 Elsevier B.V. All rights reserved.
1
. Introduction
and co-catalyst are possible reactions occur in addition to the
oxidation of organic substrates in reaction catalyzed by metal-
loporphyrins [11,14]. Accordingly, the catalytic performance of
Mn-porphyrins (Mn-pors) may be greatly influenced by the groups
attached to the meso or ␤ positions of the ligand. It is generally
accepted [2,15] that electron-deficient metalloporphyrins are more
efficient than the electron-rich ones in the catalysis of the oxida-
tion of organic compounds with different terminal oxidants. On
the other hand, there are many reports which show comparable or
higher catalytic activities for the electron-rich metalloporrphyrins
in comparison with the electron-deficient ones [5,7–9,16]. We
have very recently reported the oxidation of olefins with tetra-
n-butylammonium periodate (TBAP) and iodosylbenzene (PhIO)
catalyzed by a series electron-rich and electron-deficient Mn-
pors, indicating the comparable or higher catalytic performance
of the electron-rich Mn-pors. Also, the relative activity of dif-
ferent metalloporphyrins has been shown to change from one
olefin to another one [7,8]. In the present work, catalytic activ-
ity of the Mn(III) complexes of meso-tetra(pyridyl)porphyrins,
meso-tetr(4-sufonatophenyl)porphyrin, meso-tetr(4-methoxyl-3-
sulfonatophenyl)porphyrin (Fig. 1) in oxidation of olefins with
TBAP (Scheme 1) has been studied and compared with that of
meso-tetraphenylporphyrinatomanganese(III) acetate and meso-
tetra(4-methoxyphenyl)porphyrinatomanganese(III) acetate.
Metalloporphyrins catalyzed oxidation of organic compounds
with different terminal oxidants has been extensively studied from
different points of view since 1979 [1]. The stereoelectronic effects
of the substituents on the porphyrin periphery are among the main
factors influencing the catalytic activity of metalloporphyrins [2–9].
The redox properties of porphyrins and metalloporphyrins may be
tuned by the introduction of different peripheral substituents at the
meso and ␤ positions of the aromatic macrocycle [4–10]. Also, the
stability of metalloporphyrins toward oxidative degradation by dif-
ferent terminal oxidants and/or the active oxidizing species formed
in metal-catalyzed oxidation reactions is influenced by the steric
and electronic effects of the substituents attached to the meso and
␤
positions [2,6–9,11]. High valent (porphyrin)Metal-oxo and six
coordinate (porphyrin)Metal(III)(oxidant)(co-catalyst) species are
usually considered as the active intermediates in oxidation reac-
tions catalyzed by manganese and iron porphyrins [1,3,5,12,13].
Oxidation of the metal center or the porphyrin ligand of the cat-
alyst precursor by the active oxidants and oxidation of solvent
∗ Corresponding author. Tel.: +98 241 4153202; fax: +98 241 4153232.
E-mail address: zakavi@iasbs.ac.ir (S. Zakavi).
1
381-1169/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molcata.2012.06.003

1
54
S. Zakavi et al. / Journal of Molecular Catalysis A: Chemical 363–364 (2012) 153–158
Ar
2. Experimental
2.1. Instrumental
1H NMR spectra were obtained on a Bruker Avance DPX-
00MHz spectrometer. The absorption spectra were recorded on
a Pharmacia Biotech Ultrospec 4000 UV–Vis spectrophotometer.
NH
N
N
4
2
3
Ar
Ar
1
4
HN
2
.2. Preparation of porphyrins
meso-Tetra(pyridyl)- [19,20], meso-tetraphenyl- [21], meso-
tetra(4-OCH phenyl)-[21],
meso-tetra(4-sulfonatophenyl)-
meso-tetra(3-sulfonato-4-methoxyphenyl)porphyrin
22,23], were prepared and purified according to the literature
3
Ar
[
[
22,23],
Ar =
Porphyrins
2
Mn-porphyrins
MnTPP(OAc)
methods. The sulfonated meso-tetra(aryl)porphyrins were isolated
as the tetra-n-butylammonium salt by extraction of the sulfonated
Phenyl,
H TPP
4
4
3
2
3
4
-sulfonatophenyl
-methoxyphenyl,
-sulfonato-4-OCH , H T(4-OCH )PPS
H TPPS
2
MnTPPS (OAc)
2
4
4
H T(4-OCH )PP
MnT(4-OCH )PP(OAc)
porphyrin into dichloromethane using tetra-n-butylammonium
3
3
MnT(4-OCH )PPS (OAc)
bromide as the phase transfer catalyst. The UV–Vis and 1H NMR
3
2
3
4
3
4
-pyridyl
-pyridyl
-pyridyl
H T(2-py)P
MnT(2-py)P(OAc)
2
spectral data of the used porphyrins are as follows:
H T(3-py)P
MnT(3-py)P(OAc)
MnT(4-py)P(OAc)
2
H T(4-py)P
2
1
H TPP: H NMR (CDCl , 400 MHz): −2.77 (2H, br, s, NH), 7.75–7.77
2
3
Fig. 1. Porphyrins and Mn-pors used in this study.
m p o
␤
(8H and 4H , m), 8.21–8.24 (8H , d), 8.84 (8H , s); UV–Vis (ꢀ/nm)
in CH Cl : 417 (Soret), 512, 545, 586, 647.
2
2
Using TBAP as a moderately weak oxidant, compared to the
1
H T(4-OCH )PP: H NMR (CDCl , 400 MHz): −2.75 (2H, br, s, NH),
2
3
3
terminal oxidants such as hydrogen peroxide, Oxone and PhIO facil-
itates the comparison of the catalytic performance of Mn-pors with
different electron-withdrawing and electron-donating groups at
the meso-positions. Competitive oxidation of cis- and trans-stilbene
in the presence of the Mn-pors has been used to provide indi-
rect evidence on the nature of active intermediates of the catalytic
cycles [5,17,18].
8
.11–8.15 (8Ho, d), 7.28–7.31 (8Hm, d), 8.93 (8H , s), 4.10 (4H,
␤
OCH , s); UV–Vis (ꢀ/nm) in CH Cl : 421 (Soret), 518, 5565, 594,
3
2
2
6
46.
(tetra-n-butylammonium salt), 1H NMR (CDCl3,
00 MHz): −2.85 (2H, br, s, NH), 8.15–8.17 (8Hm, d), 8.32–8.33
8Ho, d), 8.82 (8H , s), the protons of tetra-n-butylammonium
H TPPS
4
(
2
4
␤
appear at ı 1.01, 1.42, 1.60 and 3.38 ppm; UV–Vis (ꢀ/nm) in
O
Mn(III)porphyrins/imidazole
+
(n-butyl) NIO4
4
CH Cl , Room temp., 4 h
2
2
O
(Major)
Ph
Ph
Ph
Ph
Ph
O
(
Minor)
Ph
O
OH
O
Major
Minor
OH
O
Scheme 1. Product distribution in catalytic oxidation of different olefins with TBAP.

S. Zakavi et al. / Journal of Molecular Catalysis A: Chemical 363–364 (2012) 153–158
155
CH Cl : 418 (Soret), 512, 543, 582, 645. The signal due to the
2
2
1
para-protons of H TPP (ı 7.75–7.77) disappeared in the H NMR
2
spectrum of H TPPS . The ratio of integration values for the ␤,
ortho and meta proton chemical shifts is as ca. 1:1:1 which shows
2
4
NH
N
NH
N
N
the exclusive formation of para-tetrasulfonated H TPP. It should
2
be noted that chlorosulfonic acid as the sulfonating agent has
been used in excess.
N
H T(4-OCH )PS (tetra-n-butylammonium salt): 1H NMR (CDCl3,
2
3
4
4
8
00 MHz): −(2.77–2.80) (2H, br, d, NH), 7.29–7.34 (4Hm, d),
.63–8.64 (4Ho, d), 8.19–8.23 (4Ho, dd), 8.75–8.89 (8H , s), the
␤
protons of tetra-n-butylammonium appear at ı 1.01, 1.42, 1.60
and 3.38 ppm; UV–Vis (ꢀ/nm) in CH Cl : 417 (Soret), 522, 559,
2
2
5
98, 653.
1
H t(2-py)P. H NMR (CDCl , 400 MHz): −2.83 (2H, br, s, NH),
2
3
8
7
.20–8.22 (4Hm, dd), 8.08–8.12 (4Hm, dt), 9.13–9.15 (4Ho, dd),
.70–7.74 (4Hp, qn), 8.87 (8H , s); UV–Vis (ꢀ/nm) in CH Cl : 417
NH
N
N
␤
2
2
(
Soret), 512, 545, 586, 642.
1
H t(3-py)P. H NMR (CDCl , 400 MHz): −2.83 (2H, br, s, NH),
2
3
7
9
.77–7.80 (4Hm,dd), 9.46 (4Ho, br, s), 8.52–8.54 (4Ho, br, d),
.07–9.08 (4Hp, dd), 8.87 (8H , s); UV–Vis (ꢀ/nm) in CH Cl : 418
␤
2
2
(
Soret), 512, 547, 588, 646.
1
H t(4-py)P. H NMR (CDCl , 400 MHz): ı −2.93 (2H, br, s, NH),
2
3
8
.15–8.17 (8Hm, dd), 9.06–9.07 (8Ho, dd), 7.90 (4H, m), 8.87 (8H ,
␤
s); UV–Vis (ꢀ/nm) in CH Cl : 418 (Soret), 512, 547, 588, 646.
Fig. 2. ␲-Resonance between the pyridyl substituents and porphyrin core.
2
2
2
.3. Preparation of metalloporphyrins
absorbance at the ꢀmax of the metalloporphyrin has been compared
with that of the initial one to measure the extent of degradation of
the metalloporphyrin in a 4 h reaction time.
Metallation of the porphyrins with Mn(OAc) ·4H O, following
2
2
the literature method, gave the corresponding Mn(III) complexes
[
22–24]. The Soret (and visible bands, the data in the paren-
theses) of MnT(2-py)P(OAc), MnT(3-py)P(OAc), MnT(4-py)P(OAc),
MnTPP(OAc), MnT(4-OCH )PP(OAc), MnTPPS (OAc) and MnT(4-
3. Results and discussion
3
4
OCH )PS (OAc), appears at 468 (ꢀmax) (570, 598), 469 (ꢀmax) (572,
3.1. Electronic effects of the meso substituents
3
4
6
6
06), 467 (ꢀmax) (572, 604), 468 (ꢀmax) (574, 608), 468 (ꢀmax) (574,
08), 470 (ꢀmax) (576, 610), 480 (ꢀmax) (583, 624) nm, respectively.
The electronic effects of the groups introduced at the meso posi-
tions of meso-tetraarylporphyrins are attributed to the resonance
interactions between the ␲ system of these groups and the a2u
orbital of the porphyrin core [25]; electron-donating substituents
are expected to raise the energy of this orbital by increasing its
electron density. On the other hand, formal replacement of a
CH in benzene by N causes far-reaching changes in the chem-
ical properties of the aromatic compound; pyridines are much
less reactive toward the electrophilic substitution reactions than
benzene and much more susceptible to nucleophilic attack [26].
The relative success of the electrophilic substitution correlates
with the electron density distribution at the various positions
of the pyridine ring; electrophilic attack at pyridine occurs at
the meta positions relative the heteroatom. The directive effect
is suggested to be due to the less deactivation of this posi-
tion relative to the other ones [26–28]. It may be demonstrated
that in any of the three pyridyl groups, electron donation to the
a2u orbital [20] is accompanied with the significant destabiliza-
tion of the pyridyl groups due to the appearance of a positive
charge either on the nitrogen atom or even on the carbon adja-
cent to this atom (Fig. 2). Accordingly, the pyridyl groups are
reasonable to be considered as weaker electron-donating groups
to the porphyrin core compared to the phenyl ones. Also, the
manganese complexes of meso-tetra(pyridyl)porphyrins should be
more electron-deficient than meso-tetra(phenyl)porphyrin. On the
basis of the relative ␲-donation of the pyridyl groups to the meso
position, meso-tetra(3-pyridyl)porphyrin is predicted to be less
electron-deficient with respect to meso-tetra(2-pyridyl)- and meso-
tetra(4-pyridyl)porphyrin; in meso-tetra(3-pyridyl)porphyrin, the
pyridyl group is attached to the meso carbon of porphyrin from the
meta position with respect to the heteroatom i.e. the carbon atom
with the highest electron density in pyridyl substituent.
2
.4. General oxidation procedure
Stock solutions of the Mn-porphyrins (0.003 M) and ImH
(
0.5 M) were prepared in dichloromethane. In a typical reaction,
alkene (0.25 mmol), Mn-porphyrin (0.003 mmol, 1 ml) and ImH
0.03 mmol, 60 ␮l) were added into a 10 ml round bottom flask con-
(
taining 1 ml of dichloromethane. Then, 0.5 mmol (0.217 g) TBAP has
been added. The mixture was stirred thoroughly for 4 h at ambi-
ent temperature. The molar ratios of catalyst:ImH:alkene:TBAP are
1
:10:85:170 which were previously optimized by Mohajer et al. [5].
With the exception of cis- and trans-stilbene, the progress of reac-
tion was followed using TLC and GC analysis. After the required
time, 5 ml diethyl ether was added to the flask and the reaction
mixture passed through a short silicagel column to remove the
unreacted tetra-n-butylammonium periodate and any remaining
Mn-pors [7]. Low solubility of TBAP in ether leads to the precipi-
tation of unreacted oxidant following the addition of ether to the
reaction mixture. Also, the used Mn-porphyrins are slightly solu-
ble in ether and are separated by chromatography through a short
silica gel column. The resulting solution was analyzed by GLC. 1
NMR has been used to analyze the products in the case of cis- and
trans-stilbene. All reactions were repeated at least three times.
H
2
.5. Degradation of metalloporphyrins
Degradation of metalloporphyrins was followed by UV–Vis
spectroscopy at their ꢀmax. General procedure and the molar ratios
are as mentioned above. At different time intervals a small aliquot
was removed with a micro syringe, diluted with dichloromethane
in a UV cell and was used to record the UV–Vis spectrum. Finally, the

1
56
S. Zakavi et al. / Journal of Molecular Catalysis A: Chemical 363–364 (2012) 153–158
Table 1
Table 2
Degradation of the Mn-pors in the presence of styrene, ImH and TBAP in
dichloromethane at room temperature after a reaction time of 4 h.
Competitive oxidation of cis- and trans-stilbene with TBAP catalyzed by various
Mn-pors.
a,b
a,b
Mn-pors
Degradation (%)
Mn-pors
cis–trans isomerizationc
cis/trans epoxided
MnT(2-py)P(OAc)
MnT(3-py)P(OAc)
MnT(4-py)P(OAc)
MnTPP(OAc)
MnT(4-OCH3)P(OAc)
MnTPPS4(OAc)
54.0
55.0
5.0
71.0
70.0
43.0
60.0
MnT(2-py)P(OAc)
MnT(3-py)P(OAc)
MnT(4-py)P(OAc)
MnTPP(OAc)
MnT(4-OCH3)P(OAc)
MnTPPS4(OAc)
10.0
5.0
12.0
10.7
10.0
6.5
6.2 (7.0)
3.3 (3.5)
3.4 (4.0)
8.3 (9.4)
6.2 (7.0)
5.3 (5.7)
11.1 (11.7)
MnT(4-OCH3)PS4(OAc)
MnT(4-OCH3)PS4(OAc)
5.0
a
a
The molar ratios of catalyst:ImH:styrene:oxidant are 1:10:85:170.
On the basis of the absorbance changes (ꢁA) at the ꢀmax of the Mn-pors (ꢁA/A).
The molar ratios for (cis-stilbene, trans-stilbene):oxidant:ImH:catalyst are: (500,
b
500):170:10:1.
b
Analyzed by 1H NMR.
Obtained by control reaction using cis-stilbene:TBAO:ImH:catalyst in
c
Sulfonation of meso-tetra(phenyl)porphyrin and meso-tetra(4-
500:170:10:1.
d
The data in parentheses show the corrected cis/trans stilbene oxide considering
the cis to trans isomerization upon the oxidation of cis-stilbene.
methoxyphenyl)porphyrin leads to the replacement of a hydrogen
−
atom of each aryl group by a SO3 one which is an electron-
withdrawing substituent; it is well known that in the electrophilic
substitution reactions of benzene, the SO H group withdraw the
provides evidence in favor of the involvement of a high valent Mn-
oxo species as the main active oxidant, running the reaction in the
presence of MnT(2-py)P(OAc) and the other Mn-pors gave values
of 5.7–11.7 for the relative reactivity of cis-stilbene compared to
the trans isomer. These ratios seem to be more compatible with the
3
electrons from benzene ring through resonance effect, reduces
the electron density at the benzene ring and directs the incom-
ing group to meta positions [29]. Accordingly, the sulfonation of
meso-tetra(phenyl)porphyrins should lead to an increase in the
electron-deficient character of the porphyrins and the correspond-
ing metalloporphyrins.
dominant contribution of a (IO )Mn(III)(Porphyrin)(ImH) species
4
over a high valent Mn-oxo one in the catalytic cycle.
3.2. Stability of metalloporphyrins toward oxidative degradation
3.4. Catalytic activity of the Mn(III) complexes with pyridyl
substituents
The catalytic activity of metalloporphyrins with different
oxidants in the presence or the absence of imidazole (ImH) is
influenced by their stability toward oxidative degradation in reac-
tion conditions. The stability of different Mn-pors decreases
Oxidation of olefins with TBAP usually gives the epoxide as the
sole product [5–7]. Table 3 shows the results for oxidation of several
cyclic and acyclic alkenes with TBAP in the presence of different
electron-rich and electro-deficient Mn(III) porphyrins.
The order of catalytic activity of manganese porphyrins with
pyridyl substituents, MnT(2 or 3 or 4-py)P(OAc), in oxidation of
different olefins is as follows:
in
the
order
MnT(4-py)P(OAc) ꢀ MnTPPS (OAc) > MnT(2-
4
py)P(OAc) ≈ MnT(3-py)P(OAc) > MnT(4-OCH )PS (OAc) > MnT(4-
3
4
OCH )PP(OAc) ≈ MnTPP(OAc). Interestingly, MnT(4-py)P(OAc)
3
is unusually more stable than MnT(2-py)P(OAc) and MnT(3-
py)P(OAc). According to Table 1, stability of MnTPP(OAc) and
MnT(4-OCH )PP(OAc) increased by sulfonation of the aryl
Cyclohexene:
py)P(OAc).
Indene:
py)P(OAc).
Styrene:
MnT(4-py)P(OAc) ∼ MnT(3-py)P(OAc) > MnT(2-
MnT(4-py)P(OAc) ∼ MnT(2-py)P(OAc) > MnT(3-
MnT(4-py)P(OAc) ꢀ MnT(3-py)P(OAc) > MnT(2-
3
substituents which correlates with the electron-withdrawing
−
properties of the SO3 groups. Also, the Mn-pors with pyridyl
groups at the meso positions are more stable than those with
phenyl or 4-methoxypheny.
The observed order of stability of the used Mn-pors seems to
be in accord with the relative electron with-drawing ability of the
meso substituents.
py)P(OAc).
˛-Methylstyrene: MnT(4-py)P(OAc) ꢀ MnT(3-py)P(OAc) ∼ MnT(2-
py)P(OAc).
4
-Methoxystyrene:
MnT(4-py)P(OAc) ∼ MnT(2-
3.3. The nature of active oxidant
py)P(OAc) ꢀ MnT(3-py)P(OAc).
4
-Methylstyrene: MnT(4-py)P(OAc) ∼ MnT(3-py)P(OAc) ꢀ MnT(2-
In reactions catalyzed by metalloporphyrins, the competitive
py)P(OAc).
1-Hexene:
py)P(OAc).
oxidation of cis- and trans-stilbene may be used to provide indi-
rect evidence about the nature of active oxidants in the absence
of steric hindrance at the meso positions caused by mono- or di-
ortho substitution of aryl groups [5,18]. The observed ratios of cis- to
trans-stilbene oxide in the presence of the used metalloporphyrins
MnT(3-py)P(OAc) ∼ MnT(4-py)P(OAc) ≥ MnT(2-
The results indicate lower catalytic activity of MnT(2-py)P(OAc)
with respect to MnT(4-py)P(OAc) and MnT(3-py)P(OAc) in oxi-
dation of the olefins with the exception of 4-methoxystyrene
and indene. MnT(4-py)P(OAc) shows much higher activity than
MnT(3-py)P(OAc) in oxidation of styrene, ␣-methylstyrene and 4-
methoxystyrene which is in accord with the observed order of
stability of the used MnT(py)P(OAc) in reaction conditions. In the
case of other olefins the two catalysts show comparable activities.
However, the results clearly demonstrate that the order of catalytic
activity and the extent of difference between the performance of
MnT(2 or 3 or 4-py)P(OAc) in oxidation of a given olefin cannot
be extended to the other ones. Also, the comparison of MnT(2
(
Table 2) have been in the range of 3.3–11.1. However, the cis- to
trans isomerization associated with the oxidation of cis-stilbene
18] should be considered in the evaluation of this ratio. Running
[
the reaction of cis-stilbene with TBAP in the presence of different
Mn-pors (Table 2) demonstrated partial cis to trans isomerization
of the product (5–12%). Accordingly, the corrected values of cis- to
trans epoxide (Table 2, the data in parentheses) are 3.5–11.7 which
suggests the involvement of a high valent metal oxo species in com-
bination with a (IO )Mn(III)(Porphyrin)(ImH) one as the reactive
4
intermediates responsible for oxygen atom transfer.
While the result of competitive oxidation in the presence
of MnT(3-py)P(OAc) and MnT(4-py)P(OAc) was the same and
or 3 or 4-py)P(OAc) with MnTPP(OAc) and MnT(4-OCH )PP(OAc)
demonstrates the higher performance of the metalloporphyrins
3

S. Zakavi et al. / Journal of Molecular Catalysis A: Chemical 363–364 (2012) 153–158
157
Table 3
Oxidation of different olefins with TBAP in the presence of meso-tetra(aryl)porphyrinatomanganese(III) acetate in dichloromethane at room temperature.a,b
Olefins
meso-Tetra(aryl)porphyrinatomanganese(III) acetate aryl=
2-Pyridyl
3-Pyridyl
4-Pyridyl
4-OCH3-3-SO3phenyl
4-SO3phenyl
Phenyl
4-OCH3phenyl
Styrene
11.0c (100)d
28.0 (100)
50.0 (100)
6.0 (100)
42.0 (100)
6.0 (100)
–
18.5 (100)
31.0 (100)
32.0 (100)
17.6 (100)
25.0 (100)
14.0 (96)
0.6 (4)
42.0 (100)
52.0 (100)
52.0 (100)
20.6 (100)
43.0 (100)
15.0 (95.5)
0.7 (4.5)
8.0 (100)
9.0 (100)
16.5 (100)
6.4 (100)
32.0 (100)
2.0 (100)
–
53.0 (100)
60.0 (100)
69.0 (100)
52.5 (100)
73.0 (100)
41.0 (98)
1.0 (2)
54 (100)
58 (100)
64 (100)
39(100)
64 (100)
45 (96)
2.0 (4)
51.0 (100)
63.0 (100)
69.0 (100)
27.0 (100)
74.0 (100)
31.0 (97)
0.90 (3)
␣
-Methylstyrene
4
4
-Methoxylstyrene
-Methylstyrene
Indene
Cyclohexene
1
-Hexene
2.7 (66)
1
3.5 (39)
5.4 (61)
3.5 (52)
3.2 (48)
Trace
Trace
14.0 (95)
7.0 (5)
4.6 (62)
2.8 (48)
1.8 (53)
1.6 (47)
.4 (34)f
e
a
b
c
d
e
f
The molar ratios of Mn-porphyrin:ImH:Olefin:TBAP are 1:10:83:170.
All reaction were repeated at least for three times, analyzed by GC and averaged.
Epoxide yield (%).
Epoxide selectivity (%, the data in parentheses).
Yield of alcohol and ketone (%), only in the case of cyclohexene and 1-hexene.
Alcohol and/or ketone selectivity.
with phenyl and 4-methoxylphenyl substituents relative to those
with pyridyl ones for all the alkenes studied.
3.6. The product distribution
According to Table 3, with the exception of cyclohexene and 1-
hexene, oxidation of the other olefins proceeded with the formation
of the corresponding epoxide as the only reaction product. Forma-
tion of cyclohex-2-en-1-ol and cyclohex-2-en-1-one in oxidation of
cyclohexene has been attributed to the allylic oxidation in competi-
tion with the epoxidation of cyclohexene by high valent manganese
or iron porphyrin species as the reactive intermediate responsible
for oxygen atom transfer [32] which may be used to explain the
formation of 1-hexene-3-ol in oxidation of 1-hexene. Interestingly,
no product due to the allylic oxidation of indene has been detected.
While oxidation of indene and cyclohexene with TBAP resulted in
almost exclusive formation of the corresponding epoxides, in the
case of 1-hexene, significant shift of the selectivity toward the prod-
ucts of allylic oxidation was observed. We propose that the higher
selectivity toward allylic oxidation can be explained considering
the low reactivity of the terminal double bonds in manganese por-
phyrin catalyzed oxidation of olefins with different oxygen donors
[5,7,8,33,34] which allows the competition between the C H allylic
oxidation and the epoxidation reaction. It should be noted that 1-
hexene is much less reactive toward oxidation compared to the
other olefins used in this study.
3
.5. Catalytic activity of the sulfonated Mn-pors
In spite of the comparable catalytic activity and stability
of MnT(4-OCH )PP(OAc) and MnTPP(OAc) (Table 3) in oxida-
tion of different olefins, large differences have been observed
between those of the corresponding sulfonated counterparts. Also,
MnTPPS (OAc) has been significantly more efficient than MnT(4-
OCH )PPS (OAc) in oxidation of different olefins. Interestingly,
while the introduction of OCH3 groups at the para positions has
little effect on the catalytic activity of MnTPP(OAc), the presence
of these groups leads to considerable decrease in the activity and
stability of MnTPPS (OAc). The electron-withdrawing inductive
effects of the SO3 groups are suggested to cause an increase in
3
4
3
4
4
−
the electron-donation from the methoxy groups to the aryl sub-
stituents and the porphyrin core.
It seems that the well-known electronic effects of these sub-
stituents, have been used in organic chemistry to predict the
product distribution and the kinetics of reactions as well as the
linear free energy relationship, cannot be simply generalized to
the groups attached to the meso- or ␤-positions of the manganese
porphyrins; in a recent study, using high level ab initio and DFT cal-
culations, we could demonstrate that in spite of the better electron
donating ability of OCH3 to benzene ring compared to that of -
4. Conclusions
SCH , meso-(p-SCH )phenyl group is a clearly better electron donor
3
3
In summary, while the catalytic activity of MnT(4-py)P(OAc) is
to the porphyrin core than meso-(p-OCH )phenyl one [30]. While in
3
usually larger than that of MnT(2 or 3-py)P(OAc) in oxidation of
olefins with TBAP, the relative preference of the Mn(III) complexes
of different meso-tetra(pyridyl)porphyrins has been shown to be
dependent on the type of olefin. However, the order of catalytic
activity and the extent of difference between the performance of
these Mn-porphyrins cannot be simply explained by the difference
between the electronic effects of 2-, 3- and 4-pyridyl substituents.
Also, MnTPP(OAc), MnTPPS (OAc) and MnT(4-OCH )PP(OAc) have
porphyrin free bases, the plane of meso aryl substituents is nearly
perpendicular to the porphyrin mean plane [31], in porphyrin
diprotonated species the dihedral angle between the meso-aryl
groups and the porphyrin mean plane significantly decreases rel-
ative to the free base porphyrins, and consequently the relative
␲
-donor ability of the meso groups may be properly compared.
In that study, we have calculated the distribution of electron den-
sity on different positions of the porphyrin ring of the dication of
4
3
been found to be more efficient than MnT(4-OCH )PPS (OAc) and
3
4
porphyrins with CF COOH. Indeed, we suggest that the order of
3
the Mn-porphyrins with pyridyl substituents at the meso-
electron donor ability of substituents at the periphery of porphyrin
ligand of the metalloporphyrins can be completely different from
that found in many organic reactions and transformations espe-
cially in the electrophilic substitution of aromatic compounds. At
present, computational studies on a wide range of ␤ and/or meso
substituted manganese porphyrins are in progress to calculate the
distribution of electron density on different positions of the por-
phyrin ring and metal center; this enable us to predict the relative
stability of the porphyrin ligand toward oxidative degradation in
addition to the Lewis acid strength of the metal centers.
positions. Stability of the used Mn-porphyrins decreases
in the order of MnT(4-py)P(OAc) ꢀ MnTPPS (OAc) > MnT(2-
4
py)P(OAc) ≈ MnT(3-py)P(OAc) > MnT(4-OCH )PS (OAc) > MnT(4-
3
4
OCH )PP(OAc) ≈ MnTPP(OAc).
MnT(4-py)P(OAc)
shows
an
3
unusually high stability toward oxidative degradation relative to
the other Mn-porphyrins. The results of this work clearly show that
the relative stability and catalytic activity of Mn-porphyrins with
different substituents at meso-positions are less correlated with
the electronic properties of these substituents. The competitive
oxidation of cis- and trans-stilbene gave indirect evidence for the

1
58
S. Zakavi et al. / Journal of Molecular Catalysis A: Chemical 363–364 (2012) 153–158
involvement of a high valent Mn-oxo species and a six coordinate
IO )Mn(III)(Porphyrin)(ImH) one as the active oxidant, though in
the case of MnT(py)P(OAc) the former seems to be more dominant
than the latter.
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