Oxidation of olefins and sulfides with different oxidants catalyzed by meso-tetra(n-propyl)porphyrinatomanganese(III) acetate: Comparison with meso-tetra(phenyl)porphyrinatomanganese(III) acetate

Article abstract of DOI:10.1007/s13738-014-0439-1

In this study, the catalytic activity of meso-tetra(n-propyl)porphyrinatomanganese(III) acetate, MnT(n-pr)(OAc) in oxidation of olefins and sulfides with tetra-n-butylammonium Oxone (TBAO), tetra-n-butylammonium periodate (TBAP), aqueous hydrogen peroxide, sodium periodate and Oxone in the presence of imidazole (ImH) has been studied. The comparison of catalytic performance of MnT(n-pr)P(OAc) and MnTPP(OAc) in oxidation of olefins with TBAP shows that while the latter is four times more efficient than the former, the extent of oxidative degradation of the former is ca. 3.5 times greater than the latter. The use of excess amount of styrene resulted in only a ca. 10 % increase in the catalyst stability, suggesting a mainly intramolecular mechanism for the catalyst degradation. On the other hand, in the case of TBAO, the oxidative degradation of the former is four times greater than the latter, but the catalytic performance of the latter for the oxidation of cyclohexene was only ca. 2 times larger than the former. This observation shows that the decreased catalytic performance of MnT(n-pr)P(OAc) relative to MnTPP(OAc) is essentially due to the high degree of degradation of the former. Due to the high degree of catalyst degradation, oxidation of olefins with periodate and Oxone in the presence of the two manganese porphyrins in aqueous solution (or with hydrogen peroxide in dichloromethane) gave little or no product. Oxidation of sulfides with TBAO and TBAP in the presence of MnT(n-pr)P(OAc) showed a conversion of ca. 15 % for the catalytic oxidation of sulfides to sulfones.

Full text of DOI:10.1007/s13738-014-0439-1

J IRAN CHEM SOC
DOI 10.1007/s13738-014-0439-1
ORIGINAL PAPER
Oxidation of olefins and sulfides with different oxidants catalyzed
by meso-tetra(n-propyl)porphyrinatomanganese(III) acetate:
comparison with meso-tetra(phenyl)porphyrinatomanganese(III)
acetate
•
Saeed Zakavi Mahdieh Fathi
Received: 28 September 2013 / Accepted: 18 February 2014
Ó Iranian Chemical Society 2014
Abstract In this study, the catalytic activity of meso-
tetra(n-propyl)porphyrinatomanganese(III) acetate, MnT
(n-pr)(OAc) in oxidation of olefins and sulfides with tetra-
n-butylammonium Oxone (TBAO), tetra-n-butylammo-
nium periodate (TBAP), aqueous hydrogen peroxide,
sodium periodate and Oxone in the presence of imidazole
(ImH) has been studied. The comparison of catalytic per-
formance of MnT(n-pr)P(OAc) and MnTPP(OAc) in oxi-
dation of olefins with TBAP shows that while the latter is
four times more efficient than the former, the extent of
oxidative degradation of the former is ca. 3.5 times greater
than the latter. The use of excess amount of styrene
resulted in only a ca. 10 % increase in the catalyst stability,
suggesting a mainly intramolecular mechanism for the
catalyst degradation. On the other hand, in the case of
TBAO, the oxidative degradation of the former is four
times greater than the latter, but the catalytic performance
of the latter for the oxidation of cyclohexene was only ca. 2
times larger than the former. This observation shows that
the decreased catalytic performance of MnT(n-pr)P(OAc)
relative to MnTPP(OAc) is essentially due to the high
degree of degradation of the former. Due to the high degree
of catalyst degradation, oxidation of olefins with periodate
and Oxone in the presence of the two manganese porphy-
rins in aqueous solution (or with hydrogen peroxide in
dichloromethane) gave little or no product. Oxidation of
sulfides with TBAO and TBAP in the presence of MnT(n-
pr)P(OAc) showed a conversion of ca. 15 % for the cata-
lytic oxidation of sulfides to sulfones.
Keywords Meso-tetra(n-propyl)porphyrinatomanganese
(III) acetate Á Oxidation Á Sulfides Á Olefins Á Periodate Á
Oxone Á Hydrogen peroxide
Introduction
In past decades, many iron and manganese porphyrins were
synthesized and used to mimic the reactions of cytochrome
p450 enzymes (CYP) [1–3]. In spite of the absence of any
aryl substituents on the porphyrin ring of the active site of
CYP, the majority of designed model compounds are
metalloporphyrins with aryl substituents on the periphery
of the macrocycle [4]. While the oxidation of hydrocarbons
with different terminal oxidants in the presence of the
Mn(III) and Fe(III) complexes of meso-tetraarylporphyrins
has been extensively studied [1–3, 5–10], the study of
catalytic performance of metalloporphyrins with alkyl
substituents at the meso positions was limited to an iron
porphyrin with –n–C3F7 substituents [11]. In 2010, we have
reported the oxidation of olefins with tetra-n-butylammo-
nium hydrogen monopersulfate catalyzed by the Mn(III)
complex of
a
meso-tetra(alkenyl)porphyrin (alke-
nyl = cinnamyl) as catalyst [12]. Very recently, we have
used the Mn(III) and Fe(III) complexes of meso-tetra(n-
propyl)porphyrin, H2T(n-Pr)P, as catalyst for oxidation of
olefins with TBAP as a moderately weak oxidant [13]. To
our knowledge, it was the first report on using the metal
complexes of a nonhalogenated meso-tetraalkylporphyrin
in metalloporphyrin catalyzed oxidation of olefins. How-
ever, an excess amount of olefin (the 1:6 molar ratio of
olefin to oxidant) has been used to overcome the problems
associated with the low oxidative stability of the catalyst
[13, 14]. In the present work, oxidation of sulfides and
olefins with TBAP, tetra-n-butylammonium Oxone
S. Zakavi (&) Á M. Fathi
Department of Chemistry, Institute for Advanced Studies in
Basic Sciences (IASBS), 45137-66731 Zanjan, Iran
e-mail: zakavi@iasbs.ac.ir
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J IRAN CHEM SOC
Fig. 1 Manganese porphyrins
used in this study
Ar
R
OAc
OAc
N
N
N
N
Mn
Ar
Ar
Mn
R
R
N
N
N
N
Ar
R
R = n-Propyl
MnT(n-Pr)P(OAc)
Ar = Phenyl
MnTPP(OAc)
O
electron density on the meso, b and a positions of the
porphyrin ligands.
Experimental
O
O
Preparation and metallation of H2T(n-Pr)P and H2TPP
H2T(n-Pr)P was prepared and purified according to the
procedure proposed by Neya et al. [15]. Freshly distilled
pyrrole (4.2 ml, 0.0605 mol) and butyraldehyde (3.0 ml,
0.0333 mol) were added to 300 ml of propionic acid con-
taining distilled water (12 ml) and pyridine (1 ml) at ca.
100 °C. After 30 min, an additional 1.5 ml (0.0166 mol) of
butyraldehyde was added, and the solution was stirred for
another 2 h at this temperature. After this time, dichloro-
methane (300 ml) was added to the cooled solution, and the
mixture was washed with water (300 ml 9 2), 50 mM
sodium hydroxide (300 ml 9 2), water (300 ml) to remove
propionic acid. It should be noted that dichloromethane
was used instead of chloroform, utilized by Neya et al. [13]
for the preparation of meso-tetra(alkyl)porphyrins.
O
O
S
S
S
+
H3C
Ph
S
H3C
Ph
H3C
Ph
O
S
S
O
O
O
S
O
O
S
H3C
CH3
H3C
CH3
Scheme 1 Oxidation of olefins, sulfides and dimethyl sulfoxide with
TBAP, TBAO, hydrogen peroxide, sodium periodate and Oxone in
the presence of MnT(n-Pr)P(OAc) and MnTPP(OAc)
After evaporation of dichloromethane, the residue was
repeatedly washed with methanol until colorless on a
centrifuge (5,000 rpm, 3 min). The solid product was
chromatographed once on a silica gel column with
dichloromethane. Dark blue crystals of H2T(n-Pr)P were
obtained by dropwise addition of methanol to the solution
of H2T(n-Pr)P in dichloromethane.
1H NMR (400 MHz, CDCl3, TMS): d 9.48 (s, 8H, b-H),
4.93 (t, 8H, –CH2CH2CH3), 2.54 (m, 8H, –CH2CH2CH3),
1.34 (t, 12H, –CH3) and –2.66 (br-s, 2H, NH).
(TBAO), aqueous hydrogen peroxide, sodium periodate
and Oxone in the presence of MnT(n-Pr)P(OAc) (Fig. 1)
and the manganese complex of meso-tetra(phenyl)porphy-
rin, MnTPP(OAc) was studied using stoichiometric
amounts of the oxidant and the results were compared
(Scheme 1). It is noteworthy that in oxidation of olefins
with manganese meso-tetra(aryl)porphyrins, MnTPP(OAc)
has been found to be a more efficient catalyst than the
electron-deficient counterparts with pentafluorophenyl,
mesityl, 2,6-dichlorophenyl and pyridyl at the meso posi-
tions [7, 9]. Accordingly, MnTPP(OAc) was used in this
study to compare the catalytic efficiency of the title com-
plex with manganese porphyrins with meso-aryl substitu-
ents. Also, the 13C NMR spectra of H2T(n-Pr)P, H2TPP and
the corresponding dications with CF3COOH were used to
investigate the difference between the distribution of
In the case of H2TPP [16], the solution of pyrrole
(5.6 ml, 0.08 mol) and benzaldehyde (8 ml, 0.08 mol) in
300 ml propionic acid was refluxed for 30 min. After
cooling, the solution was filtered and the filter cake was
washed thoroughly with methanol and hot water, respec-
tively. The product was chromatographed once on a neutral
alumina column with dichloromethane to obtain purple
crystals of H2TPP.
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J IRAN CHEM SOC
Results and discussion
1.0
0.5
0.0
1.4
0.7
0.0
Oxidation of olefins
The effect of different parameters such as the molar ratio of
olefin to oxidant, the type of oxidant and solvent and the
reaction temperature on the efficiency of MnT(n-Pr)OAc
has been studied.
350
450
550
650
350
450
λ (nm)
550
650
λ (nm)
Fig. 2 The UV–Vis spectra of MnT(n-Pr)P(OAc) (left) and
MnTPP(OAc) (right) in dichloromethane
Effect of the molar ratio of olefin to oxidant
1H NMR (400 MHz, CDCl3, TMS): d -2.76 (br, 2H,
NH); 8.85 (s-br, 8H, b-H,); 8.21–8.24 (m, 8H, Hortho);
7.75–7.77 (12H, multiplet, Hmeta,para).
Oxidation of styrene with TBAP was performed by use of
the olefin and oxidant in 1:1 to 1:4 molar ratios (Table 1)
and the 1:3 ratio was found to be the best one. According to
the data summarized in Table 1, the extent of degradation
of the catalyst is nearly the same for the 1:1, 1:2 and 1:3
molar ratios. The use of very large excess amounts of
organic substrate is one of the strategies commonly
employed with the aim of reducing catalyst degradation
during metalloporphyrin catalyzed oxidation of organic
compounds [14]. Interestingly, the use of excess amount of
styrene resulted in only a ca. 10 % increase in the catalyst
stability as compared to the 1:1 and even 1:3 ratio. This
observation suggests an intramolecular rather than an
intermolecular mechanism [18] for the oxidative degrada-
tion of MnT(n-Pr)OAc in the reaction conditions. In a
recent work, it has been shown that the rate of epoxidation
of olefins with TBAP remarkably increased in water-satu-
rated dichloromethane as compared to the dry solvent [18].
On the basis of the data of Table 1, the maximum epoxide
yield was obtained in dry dichloromethane which is in
contrast to that observed in the case of MnTPP(OAc) and
Mn(TPFPP)(OAc) [19]. This observation may be caused by
higher degree of degradation of MnT(n-Pr)OAc in water-
saturated dichloromethane (Table 1). In other words,
MnT(n-Pr)OAc is probably more sensitive to the oxidative
degradation than MnTPP(OAc) and Mn(TPFPP)(OAc). It
should be noted that the presence of water or alcohols in
dichloromethane is in favor of the formation of a high
valent manganese oxo species as the active oxidant [7, 20].
The manganese complexes of H2TPP and H2T(n-Pr)P
were prepared by refluxing the free-base porphyrin and
Mn(OAc)2.4H2O (in ca. 1:2 molar ratios) in dimethyl-
formamide [13, 17]. The progress of metallation reaction
was followed by UV–Vis spectroscopy. The addition of
excess amounts of manganese acetate was continued until
the disappearance of the Soret band due to the free-base
porphyrin at ca. 418 nm. Cooling the reaction mixture and
addition of chilled water led to the precipitation of man-
ganese porphyrin. Further purification of the product was
achieved by column chromatography on neutral alumina by
dichloromethane and dichloromethane/ethyl acetate. The
UV–vis spectrum of MnTPP(OAc) (kmax = 478) and
MnT(n-Pr)P(OAc) (kmax = 473) is shown in Fig. 2.
General oxidation procedure
Oxidation of olefins was carried out using the procedures
developed by Mohajer et al. [7, 13]. Stock solutions of
MnT(n-Pr)OAc (0.003 M) and ImH (0.5 M) were prepared
in CH2Cl2. In a typical reaction, alkene (0.06 mmol), cat-
alyst (0.003 mmol, 1.0 ml), and ImH (0.012 mmol, 24 ll)
were added to a 10 ml round-bottom flask containing 1 ml
of CH2Cl2, successively. TBAO (0.15 mmol, 0.060 g) was
then added to the reaction mixture at room temperature.
The solution was stirred thoroughly for 2 min at ambient
temperature. All catalytic reactions performed in air. The
progress of reactions was monitored by a Varian-3,800 gas
chromatograph equipped with a HP-5 capillary column
(phenylmethyl siloxane 30 m 9 320 lm 9 0.25 lm) or a
packed column (Chromosorb WHP 80-100 Mesh,
4 mm 9  9 2 m) and a flame-ionization detector. The
reaction was quenched with an aqueous sodium bisulfite
solution and the dichloromethane phase was dried over
sodium sulfate. In the case of TBAP, addition of 10 ml of
freshly distilled ether to the reaction mixture led to the
precipitation of unreacted TBAP. The reaction mixture was
passed through a short silica gel column to remove the
remaining metalloporphyrin before injection into the GC.
All reactions were repeated three times.
Catalytic activity of MnT(n-Pr)OAc compared
to MnTPP(OAc)
Catalytic activity as well as the oxidative stability of
metalloporphyrins is largely determined by the steroelec-
tronic properties of the meso and b substituents [1–3, 9, 10].
To investigate the influence of substitution of meso-phenyl
groups with n-propyl ones on the catalytic performance of
the catalyst, oxidation of styrene and cyclohexene with
TBAP was performed in the presence of MnTPP(OAc) and
MnT(n-Pr)OAc (Table 2). It is observed that the former is
ca. 4 times more efficient than the latter. However, the
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J IRAN CHEM SOC
Table 1 Oxidation of styrene with TBAP in the presence of MnT(n-
Pr)OAc and ImH in CH2Cl2 at room temperature
Table 3 Effect of solvent and temperature on the catalytic activity of
MnT(n-Pr)OAc in the oxidation of styrene with TBAP
Catalyst:ImH:
Styrene:TBAP
Conversion
(%)a
Catalyst
degradation
(%b
Epoxide
selectivity
(%)
Conversion Epoxide
(%)
TON Solvent
Temperature
(°C)
selectivity (%)
7.5
100
100
375
765
CH3CN
60–65
Room
1:10:1,000:170
1:10:50:50 (100)c
1:10:50:150
2.7
8.5
64
72
74
95
82
100
100
100
100
100
15.3
CHCl3 or
CH2Cl2
15.3
9.0
16.5
10.2
100
100
825
510
CH2Cl2
CHCl3
40a
70a
1:10:50:150d
1:10:50:200
13.8
See the footnotes of Table 1 for details
a
Under reflux condition
All reaction were repeated at least three times, analyzed by GC and
averaged
a
Based on styrene for a reaction time of 4 h
Table 4 Oxidation of olefins with TBAO catalyzed by MnT(n-
Pr)OAc in CH2Cl2 at room temperature
b
On the basis of the absorbance changes (DA) at the kmax of the
manganese porphyrin (DA/A)
c
MnT
(n-Pr)
Olefins
Conversion Epoxide
(%)a,b
TON
Similar results were obtained in the case of the 1:10:50:100 molar
selectivity (%)
ratios
d
OAc:ImH:
Olefin:TBAO
For the reaction performed in CH2Cl2 saturated with water
1:20:50:50
1:20:50:50
1:20:50:150
Styrene
13.3
100
100
100
100
665
930
Table 2 Oxidation of styrene and cyclohexene with TBAP in the
presence of MnT(n-Pr)OAc and MnTPP(OAc) in CH2Cl2 at room
temperature
Cyclohexene 18.6
Cyclohexene 41.0
2,045
3,730
1:20:50:150c Cyclohexene 74.6
Catalyst
Olefin
Conversion Epoxide
(%)
TONa
a
Average of three runs (GC Yields)
Selectivity
(%)
b
For a reaction time of 2 min
c
For the reaction catalyzed by MnTPP(OAc)
MnT(n-
pr)P(OAc)b
MnTPP(OAc)b
Styrene
Styrene
15.3
56.0
100
765
the rate of reaction was observed upon the increase of
reaction temperature from 25 to 40 °C, further increase of
the temperature resulted in a decrease in the catalytic
activity of the catalyst. Apparently, the enhanced degra-
dation of the catalyst at higher temperatures led to the
negative effect of temperature on the rate of the oxidation
reaction. Acetonitrile as a coordinating solvent may com-
pete with imidazole in coordination to the metal center of
the metalloporphyrins. It should be noted that the coordi-
nation of imidazole to the metal center of manganese
porphyrins plays a crucial role in the catalytic activity of
the complexes [5–10, 13].
100
100
100
2,800
825
MnT(n-pr)P(OAc) Cyclohexene 16.5
MnTPP(OAc) Cyclohexene 68.0
3,400
See the footnotes of Table 1 for details. The optimized molar ratio of
catalyst:ImH:Olefin:TBAP, i.e., 1:10:50:150 was used
a
Turnover number (moles of epoxide per mole of the catalyst)
b
The extent of degradation of MnT(n-pr)P(OAc) and MnTPP(OAc)
was 74 and 19 %, respectively
difference between the conversions, as a criterion for the
efficiency of the catalysts, is greater in the case of cyclo-
hexene as an electron-deficient olefin as compared to that
observed in the oxidation of styrene as an electron-rich one.
Also, the extent of oxidative degradation of the latter is ca.
3.5 times greater than the former. In other words, the
observed difference between the catalytic efficiency of
MnT(n-Pr)OAc and MnTPP(OAc) arises mainly from the
higher degree of oxidative degradation of the former.
Oxidation of olefins with tetra-n-butylammonium Oxone
(TBAO)
TBAO has been used as a strong terminal oxidant for
oxidation of olefins and sulfides in the presence of metal-
loporphyrins [21–23]. The results of oxidation of olefins
with TBAO are summarized in Table 4. Oxidation of
cyclohexene and styrene with TBAO in 2 min gave similar
results to those obtained in the case of TBAP (Table 2)
after 4 h. Interestingly, the use of 1:3 molar ratio of olefin
to TBAO resulted in a conversion of 41 % in the oxidation
Effect of temperature and solvent
Oxidation of styrene with TBAP at room and higher tem-
peratures was studied (Table 3). While a small increase in
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J IRAN CHEM SOC
Oxidation of sulfides
Table 5 Oxidative degradation of MnTPP(OAc) and MnT(n-
pr)P(OAc) in oxidation of styrene with TBAO in CH2Cl2 at room
temperature
Oxidation of sulfides with TBAP
Catalyst
After 6 min
After 12 min
Oxidation of sulfides with TBAP in the presence of MnT(n-
pr)P(OAc) led to the formation of sulfone as the major
product (Table 7). Also, in the case of dimethyl sulfoxide,
the corresponding sulfone was formed with a conversion of
60 %. Accordingly, over-oxidation of the sulfoxide to
sulfone occurs in the presence of MnT(n-pr)P(OAc). It is
noteworthy that the reaction does not occur in the absence
of the catalyst.
MnTPP(OAc)
18.0
70.0
19.0
72.0
MnT(n-pr)P(OAc)
The molar ratio of catalyst, ImH, styrene and TBAO is 1:10:50:150,
all were used in concentrations ca. 0.1 times lower than those have
been used for catalytic oxidation of alkenes with TBAO (Table 4) for
controlling the reaction rate; the oxidation reactions were performed
in 15 ml dichloromethane
Oxidation of methyl phenyl sulfide with TBAO
of cyclohexene. Under the same conditions, a conversion
of 75 % was achieved for the reaction performed in the
presence of MnTPP(OAc). It is noteworthy that the maxi-
mum conversion achievable in the oxidation of cyclohex-
ene with TBAP was 16.5 and 68 % for reaction catalyzed
by MnT(n-pr)P(OAc) and MnTPP(OAc), respectively. The
oxidative degradation experiments of the catalysts
(Table 5) demonstrate that the former is about four times
less stable than the latter and, therefore, the observed
conversion of cyclohexene to cyclohexene oxide may be
considered as an acceptable result for reaction performed in
the presence of MnT(n-pr)P(OAc).
MnT(n-pr)P(OAc) catalyzed oxidation of methyl phenyl
sulfide with TBAO gave the corresponding sulfone as the
major product (Table 8). In comparison with the reaction
performed in the absence of the catalyst, the use of MnT(n-
pr)P(OAc) led to a ca. 12 % increase in the total
conversion.
13C NMR spectra of the porphyrins and dications
The oxidative degradation of metalloporphyrins depends
on the steric and electronic effects of the substituents
attached to the meso and b positions which determine the
distribution of electron density on different positions of the
aromatic macrocycle [2, 8–10, 24, 25]. According to the
four orbital model of porphyrins [26], for the a1u orbitals
the electron densities are largest on the pyrrole carbon
atoms (Ca [ Cb) and the methane bridges lie on nodal
planes. On the other hand, the a2u electron densities are
largest on the methene carbons (Cmeso) and the pyrrole
nitrogens. Therefore, the positions with highest electron
densities, i.e., the a and meso ones are expected to be
involved in oxidative degradation of metalloporphyrins
[24, 25]. However, cleavage at the b positions has been
also reported as a degradation pathway [27]. The 13C NMR
Oxidation with hydrogen peroxide, sodium periodate
and Oxone
Oxidation of cyclohexene and styrene with aqueous H2O2
(35 %), sodium periodate and Oxone in the presence of
MnT(n-pr)P(OAc) and MnTPP(OAc) yielded little or no
detectable product (Table 6). In these reactions, sodium
periodate and Oxone were dissolved in minimum water and
added to the solution of catalyst, ImH and olefin in
dichloromethane. However, as was expected (see Sect.
3.1.1; Table 1), significant degradation of MnT(n-
pr)P(OAc) occurred.
Table 6 Oxidation of cyclohexene with aqueous H2O2, Oxone and sodium periodate in dichloromethane at room temperature
Catalyst:ImH:Olefin:Oxidant
1:20:50:50b
Catalyst
Conversiona
Degradation
Epoxide selectivity
MnT(n-pr)P(OAc)
MnTPP(OAc)
0.3
1.9
0.6
1.6
1.7
3.3
90
65
90
60
70
42
100
100
100
100
100
100
1:10:83:83c
1:10:83:83d
MnT(n-pr)P(OAc)
MnTPP(OAc)
MnT(n-pr)P(OAc)
MnTPP(OAc)
In the case of Oxone and sodium periodate, the oxidant was dissolved in minimum water and added to the reaction mixture in CH2Cl2
a
Analyzed by GC
b, c, d
For the oxidation performed with aqueous H2O2 (in 20 min), Oxone (in 2 min) and sodium periodate (in 4 h), respectively
123

J IRAN CHEM SOC
respect to the corresponding signals of H2T(n-Pr)P(CF3-
COOH)2 seems to provide evidence for higher electron
density on the a, b and meso positions of the former. In
other words, H2TPP may be considered as a more electron-
rich ligand as compared to H2T(n-Pr)P. It should be noted
that while the phenyl groups of H2TPP donate electron
density into the p system of porphyrin core through p-
resonance interactions, weak hyperconjugative interactions
are responsible for the shift of electron densities from the
meso alkyl residues to the porphyrin core [30]. The reso-
nance interactions have been shown to be strongly signif-
icant even for those instances where other physical data
suggest that the phenyl and porphyrin p systems are very
noncoplanar [30]. According to the above discussion, very
low stability of MnT(n-pr)(OAc) toward oxidative degra-
dation is suggested to be due to lack of steric hindrance at
the periphery of the porphyrin ring. Also, in the case of the
formation of a high valent manganese(IV)-oxo porphyrin p
cation radical as the active oxidant which requires more
electron donation from the meso substituents to the por-
phyrin core, the weak electron donor ability of the n-propyl
substituents would lead to low stability of the species and
the involvement of an intramolecular mechanism of the
catalyst degradation.
Table 7 Oxidation of sulfides and dimethylsulfoxide with TBAP
catalyzed by MnT(n-pr)(OAc) in CH2Cl2 at room temperature
Sulfides
Conversiona Sulfoxide
Sulfone
(%)
Catalyst
degradation
(%)
(%)
(%)
Methyl phenyl 15.9 (25)
sulfide
1.5 (4.0)
14.4 (21.0) 14 (22)
Diallyl sulfide 14.3 (22)
0.3 (Trace) 14.0 (22.0) 18 (33)
60 22 (28)
Dimethyl
sulfoxide
60.0 (40)
–
No reaction was observed in the absence of the catalyst
a
The catalyst, ImH, sulfide and TBAP were used in 1:10:85:85 molar
ratios or in 1:10:85:170 ones (the data in parentheses)
Table 8 Oxidation of methyl phenyl sulfide with TBAO catalyzed by
MnT(n-pr)(OAc) in CH2Cl2 at room temperature
Catalyst:ImH:
Sulfide:TBAO (%)
Conversiona Sulfoxide Sulfone Catalyst
(%)
(%)
degradation
(%)
83:83b
44.9
58.3
57.0
57.4
55.9
2.8
42.1
55.0
57.0
55.1
53.1
–
1:10:83:83
1:15:83:83
1:20:83:83
1:25:83:83
a
3.3
40
35
28
27
Trace
2.3
2.8
Analyzed by GC (Average of three runs)
b
In the absence of MnT(n-pr)(OAc) and ImH
Conclusions
spectra of H2T(n-Pr)P and H2TPP may be used to compare
the distribution of electron densities on different positions
of the porphyrins. On the other hand, the spectra of por-
phyrin dications seem to be more convenient for this
comparison, due to enhanced coplanarity between the
meso-phenyl groups and the porphyrin mean plane upon
the formation of porphyrin dications [28]. The decreased
meso-aryl group-porphyrin dihedral angles would lead to
more effective resonance interactions between the por-
phyrin and phenyl p systems. Also, the diprotonation of
H2T(n-Pr)P seems to be associated with increased contri-
bution of hyperconjugation between the porphyrin p sys-
tem and the C–H bonds of the n-propyl substituents [29] as
compared to that in the free-base porphyrin.The 13C NMR
spectral data of H2T(n-Pr)P, H2TPP and the dications with
CF3COOH (Fig. 3) are summarized in Table 9. The 13C
NMR spectrum of porphyrin core consists of a very broad
resonance, a sharp signal and a broad one due to the a,
meso and b carbon atoms, respectively [30]. Accordingly,
the very broad Ca resonance is not observable in the
spectrum of the free-base porphyrins, but diprotonation of
porphyrins sharpens both the Ca and Cb signals [30]. The
upfield shift of the Cb and Cmeso of H2TPP relative to those
of H2T(n-Pr)P (Table 9) as well as the upfield shift of the
Ca, Cb and Cmeso resonances of H2TPP(CF3COOH)2 with
In summary, the catalytic activity of meso-tetra(n-pro-
pyl)porphyrinmanganese(III) acetate, MnT(n-pr)(OAc) in
oxidation of olefins and sulfides with different terminal
oxidants in the presence of ImH was studied and found
that: (i) while in the oxidation of olefins with TBAP
MnTPP(OAc) is four times more efficient than MnT(n-
pr)P(OAc), the extent of oxidative degradation of the latter
is ca. 3.5 times greater than the former; (ii) the small
decrease of the catalyst degradation caused by the use of
large excess of styrene compared to the 1:1 and even 1:3
molar ratios of olefin to TBAP suggests a mainly
intramolecular mechanism for the catalyst degradation; (iii)
while the increase of reaction temperature from 25 to 40 °C
enhanced the rate of oxidation reaction of olefins, further
increase in the temperature resulted in a decrease in the
catalytic activity of the catalyst; (iv) in oxidation reaction
performed with TBAO, the oxidative degradation of the
former is four times greater than the latter, but the catalytic
performance of the latter for the oxidation of cyclohexene
was only ca. 2 times larger than the former, suggesting that
the decreased catalytic performance of MnT(n-pr)P(OAc)
relative to MnTPP(OAc) is essentially due to the high
degree of degradation of the former; (v) although, the
oxidation of dimethyl sulfoxide with TBAP led to the
formation of dimethyl sulfone with a conversion of ca.
123

J IRAN CHEM SOC
Fig. 3 13C NMR spectra of
a H2TPP,
b H2TPP(CF3COOH)2,
c H2T(n-Pr)P and d H2T(n-
Pr)P(CF3COOH)2 in CDCl3
123

J IRAN CHEM SOC
Table 9 13C NMR spectra of H2T(n-pr)P, H2TPP and the corre-
sponding dications with CF3COOH in CDCl3
4. S.D. Black, M.J. Coon, in Cytochrome P-450, structure, mecha-
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1010 (2011)
9. S. Zakavi, A.G. Mojarrad, S. Rayati, J. Mol. Catal. A: Chem.
363–364, 153 (2012)
Compounds
Caa
Cba
Cmeso
H2T(n-pr)P
_
129.70
126.14
-3.55
131.50
127.70
-3.80
118.52
121.79
3.67
H2T(n-pr)P(CF3COOH)2
144.74
Dd
_
H2TPP
_
120.55
122.10
1.55
H2TPP(CF3COOH)2
145.20
_
Dd
Chemical shifts (ppm) are downfield from CDCl3 (76.90–77.92 ppm)
°
10. S. Zakavi, T. Mokary Yazdeli, J. Mol. Catal. A Chem. 367, 108
(2013)
11. T. Wijesekera, J.E. Lyons, P.E. Ellis, M.V. Bhinde, US Patent
5,770,728 (1998)
at 25 C
a
For the free-base porphyrins Ca is indistinguishable from the
baseline noise and Cb gives a broad signal
12. S. Zakavi, A.S. Ashtiani, S. Rayati, Polyhedron 29, 1492 (2010)
13. S. Zakavi, S. Talebzadeh, S. Rayati, Polyhedron 31, 368 (2012)
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60 %, oxidation of sulfides with TBAO and TBAP in the
presence of MnT(n-pr)P(OAc) demonstrated a conversion
of ca. 15 % for the catalytic oxidation of sulfides to sulf-
ones; (vi) the comparison of 13C NMR spectra of H2TPP,
H2T(n-pr)P and the corresponding dications with CF3-
COOH shows little difference between the distribution of
electron density on the a, b and meso positions of the two
porphyrins. Accordingly, the low stability of MnT(n-
pr)P(OAc) has been attributed to the absence of bulky
substituents at the meso positions and the low stability of
the active oxidants of MnT(n-pr)P(OAc) towards the
intramolecular decomposition.
23. S. Rayati, S. Zakavi, P. Jafarzadeh, O. Sadeghi, M.M. Amini, J.
Porphyrin Phthalocyanines 16, 260 (2012)
24. S. Zakavi, R. Omidyan, L. Ebrahimi, F. Heidarizadi, Inorg.
Chem. Commun. 14, 1827 (2011)
Acknowledgments Financial Support of this work by the Institute
for Advanced Studies in Basic Sciences (IASBS) is acknowledged.
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Van Calcar, K. Winkler, A.L. Balch, J. Am. Chem. Soc. 120,
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