The first manganese N-confused porphyrins catalyzed oxidation of alkene

Article abstract of DOI:10.1016/j.molcata.2014.08.016

A series of N-methyl N-confused porphyrin manganese complexes (3a-3f) were synthesized and characterized by UV-vis, XPS, HR-MS and cyclic voltammetry. These complexes were utilized to catalyze the styrene oxidation. It turned out their catalytic activities were comparable with manganese tetraphenylporphyrin (MnTPP). Among all investigated manganese N-confused porphyrins, the most electron deficient 3f exhibited the best catalytic activity for the alkene oxidation. The proposed mechanism for the catalytic oxidation has also been described.

Full text of DOI:10.1016/j.molcata.2014.08.016

Journal of Molecular Catalysis A: Chemical 395 (2014) 180–185
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Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
The first manganese N-confused porphyrins catalyzed
oxidation of alkene
Su-Hong Peng, Mian HR Mahmood, Huai-Bo Zou, Shu-Bao Yang, Hai-Yang Liu∗
Department of Chemistry, South China University of Technology, Guangzhou 510641, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of N-methyl N-confused porphyrin manganese complexes (3a–3f) were synthesized and char-
acterized by UV–vis, XPS, HR-MS and cyclic voltammetry. These complexes were utilized to catalyze the
styrene oxidation. It turned out their catalytic activities were comparable with manganese tetraphenyl-
porphyrin (MnTPP). Among all investigated manganese N-confused porphyrins, the most electron
deficient 3f exhibited the best catalytic activity for the alkene oxidation. The proposed mechanism for
the catalytic oxidation has also been described.
Received 27 March 2014
Received in revised form 2 August 2014
Accepted 5 August 2014
Available online 15 August 2014
Keywords:
N-confused porphyrin
Manganese
© 2014 Elsevier B.V. All rights reserved.
Catalytic oxidation
1. Introduction
transfer reactions [14]. While numerous manganese complexes of
porphyrins [15–18], corroles [19–23], phthalocyanines [24] and
Transition-metal complexes of porphyrins and related macro-
cycles have attracted increased attention recently due to their
promising applications as biomimetic catalysts for cytochrome
P450 [1–3]. The most widely studied catalysts in this field are
manganese porphyrins, which exhibit very high conversion and
good selectivity for alkene oxidation [4]. N-confused porphyrin or
so-called inverted porphyrin, an isomer of porphyrin having one
of the four pyrrole nitrogen atoms lying outside and serving as
peripheral nitrogen, was firstly isolated as minor product from
the normal porphyrin synthesis by Latos-Graz˙ yn´ ski [5] and Furuta
[6] independently in 1994. An improved synthesis of N-confused
porphyrin was achieved by Lindsey et al. in 1999 [7], which sig-
nificantly accelerated the development of N-confused porphyrin
chemistry [8]. The presence of an external nitrogen may impart
some peculiar characteristics to N-confused porphyrin. For exam-
ple, it may involve in the electrophilic reaction and results in the
formation of the alkylation products [9,10]. Unlike N-confused por-
phyrin, it’s alkylation products do not undergo NH-tautomerism
and have only one stable form, which can form manganese(III)
N-confused porphyrin having a direct carbon–manganese (Mn–C)
bond [11]. The metal complexes of N-confused porphyrins have
been demonstrated to be potent catalysts in cyclopropanation
of styrene with high trans-selectivity [12,13] and oxygen atom
related macrocycles [25] have been extensively studied as catalysts
in the oxidation of organic substrates, to the best of our knowl-
edge, no report was found for manganese N-confused porphyrins
catalyzed oxidation of organic substrates so far. Herein, we wish to
report the synthesis and spectroscopic characterization of various
manganese(III) N-methyl N-confused tetraarylporphyrins MnNC-
TArPs (Scheme 1 3a–3f), and their catalytic activity for styrene
oxidation.
2. Experimental
2.1. Materials and methods
All reagents and solvents were of analytical grade and obtained
commercially. The purity of styrene was checked by GC analysis.
UV–vis spectra were obtained with a Hitachi U-2450 spectropho-
tometer. The 1H NMR spectra were recorded at room temperature
on a Bruker Avance III 400 spectrometer. Mass spectra were
taken on Bruker Esquire HCT plus mass spectrometer (ESI/MS)
and Bruker maxis impact mass spectrometer with an ESI source
(HR-MS). X-ray photoelectron spectroscopy (XPS) was performed
using an Axis Ultra DLD spectrometer. All cyclic voltammograms
(CV) were performed in acetonitrile solutions containing 0.1 M
TBAP (tetrabutylammonium perchlorate) using an Ingens Model
1030 and MnNCTArPs (1 mM) under nitrogen atmosphere at
ambient temperature. A three-electrode system consisting of
a glassy carbon working electrode, a platinum wire counter
∗
Corresponding author. Tel.: +86 020 22236805; fax: +86 020 22236805.
E-mail address: chhyliu@scut.edu.cn (H.-Y. Liu).
http://dx.doi.org/10.1016/j.molcata.2014.08.016
1381-1169/© 2014 Elsevier B.V. All rights reserved.

S.-H. Peng et al. / Journal of Molecular Catalysis A: Chemical 395 (2014) 180–185
181
C49H38MnN4O4 801.2274, found 801.2268, with an isotope distri-
bution pattern the same as the calculated one.
3e. Yield 52%. UV–vis (CH2Cl2, nm, ε × 10−4 M−1 cm−1): 335
(2.359), 392 (2.714), 465 (2.403), 504 (3.191), 577 (0.679), 631
(0.440), 738 (0.398), 802 (0.530). HR-MS (ESI) ([M-Cl]+): calcd for
C49H38MnN4 737.2477, found 737.2471, with an isotope distribu-
tion pattern the same as the calculated one.
3f. Yield 45%. UV–vis (CH2Cl2, nm, ε × 10−4 M−1 cm−1): 338
(3.202), 392 (3.471), 461 (2.560), 505 (4.797), 579 (0.791), 637
(0.508), 743 (0.465), 807 (0.639). HR-MS (ESI) ([M-Cl-H]+): calcd for
C45H26Cl4MnN4 817.0292, found 817.0287, with an isotope distri-
bution pattern the same as the calculated one.
2.3. Catalytic oxidation
A mixture of styrene (1.0 mmol), oxidant (0.1 mmol) and cat-
alyst (3a–3f) (1.0 ␮mol) in 2 mL of solvent was stirred in a 10 mL
glass flask at room temperature. After an appropriate reaction time,
chlorobenzene (1 ␮L) was added to this reaction mixture as internal
standard. The products were analyzed on an Echrom A90 gas chro-
matograph equipped with HP-5 capillary column (30.0 m × 320 ␮m
ID; 0.25 ␮m film thickness) coupled with FID detector. The carrier
gas was nitrogen and the chromatographic conditions were as fol-
lows: the oven temperature was increased at a rate of 10 ◦C/min
(from 60 to 250 ◦C); the injector temperature was set 230 ◦C while
the detector temperature was kept 250 ◦C. The injection volume
of the filtrated reaction mixture was 1.0 ␮L and the products were
confirmed by the retention time using standard samples under the
same GC conditions. The yields of products were reported with
respect to the amount of oxidant used.
Scheme 1. Synthesis of manganese N-confused tetraarylporphyrins (3a–3f).
electrode and SCE (saturated calomel reference electrode) were
employed. The scan rate was 100 mV/s. Half-wave potentials (E1/2
for reversible or quasi-reversible redox processes were calculated
as E1/2 = (Epa + Epc)/2, where Epa and Epc represent the anodic
and cathodic peak potentials, respectively. The E1/2 value for the
ferrocenium/ferrocene couple under these conditions was 0.40 V.
)
2.2. Synthesis
3. Results and discussion
5,10,15,20-Tetraphenylporphyrin and its manganese (III) com-
plex (MnTPP) were prepared by traditional method [26,27]. All
free base N-confused porphyrins were synthesized according to
procedure described in references [7,10] and well characterized
(Supporting information). Manganese complexes of N-methyl N-
confused porphyrin (3a–3f) were prepared as follows:
3.1. Synthesis of manganese N-confused tetraarylporphyrins
Manganese complexes of N-confused porphyrin were previ-
ously prepared by the direct reaction of free base with MnBr2 or
Mn2(CO)10 and found to be less stable [28,29]. However, man-
ganese N-alkyl N-confused porphyrins are relatively more stable
and can be easily prepared [11,30]. This phenomenon triggered our
interest in the synthesis of manganese N-methyl N-confused por-
phyrins (3a–3f). Manganese N-confused porphyrins (3a–3f) were
prepared by refluxing the corresponding free base (2a–2f) and
Mn(OAc)2·4H2O in dichloromethane/methanol (1/50, V/V) [11].
A
mixture
of
N-methyl
N-confused
5,10,15,20-
tetraarylporphyrin (2a–2f) (0.08 mmol) in CH2Cl2 (2 mL) and
Mn(OAc)2·4H2O (0.4 mmol) in methanol (100 mL) was refluxed for
5 h. After evaporation of the solvent, the residue was dissolved in
100 mL of CH2Cl2 and washed with saturated aqueous solution of
NaCl. The organic layer was collected and dried over anhydrous
Na2SO4. The filtrate was concentrated and the crude product was
purified on a silica gel (300–400 mesh) using CH2Cl2/ethyl acetate
(8/2, V/V) as eluent. Green colored product was obtained after
recrystallization from CH2Cl2/ethyl acetate (1/5).
3.2. UV–vis spectroscopy
UV–vis absorption maxima and molar extinction coefficient of
free base N-confused porphyrins (1a–1f and 2a–2f) (Supporting
information, Fig S1 and S2) and their manganese complexes 3a–3f
are summarized in Experimental Section. Free base N-confused
porphyrins 1a–1f are characterized by a Soret-band at 437–441 nm
and four Q-bands at 532–734 nm. N-methylated freebases 2a–2f
exhibit an extra N-band at 355–377 nm, a Soret-band between
437 and 441 nm and only two Q-bands between 647–722 nm.
The maxima of these absorption bands is related to the position
and electronic properties of the substituents. In the presence of
methoxy or methyl groups, the soret-band is apparently red shifted.
For example, the Soret-band of 1a and 2a is located at 437 and
444 nm, respectively, and is red shifted by 4 and 9 nm with respect
to the Soret-band of 1d (441 nm) and 2d (453 nm). This indicates
that substitution by electron-donating groups at the para-position
of the meso-phenyl groups leads to a decrease of the HOMO-LUMO
gap as compared to 1a or 2a [31]. The difference in the Soret-bands
of 1b–1d and 2b–2d might be explained by the steric effect of
ortho-methoxy group, which may result in a red shifted bands [32].
3a. Yield 49%. UV–vis (CH2Cl2, nm, ε × 10−4 M−1 cm−1): 337
(2.734), 393 (3.086), 458 (2.132), 505 (4.831), 580 (0.652), 637
(0.414), 743 (0.424), 809 (0.628). HR-MS (ESI) ([M-Cl]+): calcd for
C45H30MnN4 681.1851, found 681.1845, with an isotope distribu-
tion pattern the same as the calculated one.
3b. Yield 60%. UV–vis (CH2Cl2, nm, ε × 10−4 M−1 cm−1): 338
(3.107), 393 (3.384), 458 (2.785), 505 (4.647), 578 (0.881), 628
(0.627), 743 (0.517), 804 (0.627). HR-MS (ESI) ([M-Cl]+): calcd for
C49H38MnN4O4 801.2274, found 801.2268, with an isotope distri-
bution pattern the same as the calculated one.
3c. Yield 58%. UV–vis (CH2Cl2, nm, ε × 10−4 M−1 cm−1): 338
(2.053), 390 (2.118), 458 (1.721), 505 (2.127), 577 (0.621), 633
(0.451), 733 (0.262), 798 (0.300). HR-MS (ESI) ([M-Cl]+): calcd for
C49H38MnN4O4 801.2274, found 801.2268, with an isotope distri-
bution pattern the same as the calculated one.
3d. Yield 47%. UV–vis (CH2Cl2, nm, ε × 10−4 M−1 cm−1): 334
(2.856), 398 (3.466), 457 (3.472), 506 (3.472), 574 (0.814), 636
(0.592), 727 (0.516), 809 (0.572). HR-MS (ESI) ([M-Cl]+): calcd for

182
S.-H. Peng et al. / Journal of Molecular Catalysis A: Chemical 395 (2014) 180–185
2.5
2.0
1.5
1.0
0.5
0.0
60
3a
MnTPP
MnTPP+Fc
3b
+
3b
3c
3d
3e
3f
E1/2(Fc /Fc)= 0.4V
40
20
0
+
Mn3+/Mn4+
Fc/Fc
Mn2+/Mn3+
Mn4+/Mn3+
Fc+/Fc
-20
-40
Mn3+/Mn2+
400
600
800
-1.5
-1.0
-0.5
0.0
0.5
1.0
Fig. 1. Absorption spectra of manganese N-confused porphyrins 3a–3f in
dichloromethane.
E/V vs. SCE
Fig. 3. Cyclic Voltammograms of MnTPP and MnTPP + ferrocene and 3b in acetoni-
trile containing 0.1 M TBAP, Scan rate = 0.1 V/s.
All manganese N-confused porphyrins 3a–3f exhibit an N-band
(334–338 nm), a split Soret-band (392–398 and 457–465 nm) and
four weaker Q-bands (Fig. 1). The absorption at 504–506 nm can
be attributed to metal to ligand charge transfer transition (MLCT)
band, similar to MnTPP. The relative intensity of this band is usually
related with the electron-withdrawing or -donating ability and the
steric hindrance of the peripheral substituents on the macrocyle
[31].
3.4. Electrochemistry
The electrochemical characteristics of manganese complexes
3a–3f were investigated by cyclic voltammetry (CV) in CH3CN
solutions and the data were summarized in Table S2. Fig. 3 rep-
resents the cyclic voltammograms of MnTPP, MnTPP + ferrocene
and 3b in acetonitrile containing 0.1 M TBAP. All the manganese
N-confused porphyrins show a single one-electron reversible or
quasi-reversible oxidation and a series of irreversible reduction
within the scan window (−1.8 to 1.0 V). Reference the elec-
trochemistry of other manganese macrocycles [36–40], the first
redox process of 3a–3f may be assigned to manganese oxidation
(Mn3+/Mn4+) and the three irreversible reduction process with
the potentials in the range of −0.680 to 1.32 V may be attributed
to the ligand reduction. The first half wave oxidation potential
of 3a–3f revealed E1/2 values in the range of 0.47–1.08 V for
Mn4+/Mn3+ analogous to other manganese macrocyclic complexes
[36–40]. As expected, the E1/2 for the first oxidation of the man-
ganese N-confused porphyrins shifts positively with increase in the
electron-withdrawing ability of the substituent, that is, 3f is 81 mV
more difficult to be oxidized than 3a (Table S2). Compared with the
other manganese N-confused porphyrins, 3f has a greater electron
withdrawing ability and is more difficult to lose electron. As a result,
3f has larger oxidation potential. This trend fits with what would
be predicted on the basis of simple substituent’s effect. This implies
that 3f has greater ability to stabilize the higher valent oxidation
states of metal ions.
3.3. X-ray photoelectron spectroscopy (XPS)
XPS was used to directly probe the oxidation state of manganese
in all manganese complexes 3a–3f. Mn 2p region for MnTPP and
3a–3f are shown in Fig. 2 and the data is given in Table S1 (Sup-
porting information). The peak at 196–203 eV in 3a–3f is attributed
to Cl 2p3/2 which indicates that the axial position is occupied by Cl
atom [33]. The binding energies of Mn in 3a–3f is almost similar
to MnTPP. Both components of Mn 2p (2p3/2, BE = 641.2–642.4 eV;
2p1/2, BE = 652.4–654.2 eV) are in agreement with the +3 oxidant
state of manganese ion [34,35]. As shown in Fig. 2, the binding
energy of Mn 2p3/2 for 3f is little higher than that of MnTPP but is
lower for 3a–3e, indicating that 3f has better stability and is more
difficult to be oxidized.
MnTPP
4000
3a
3c
3e
3b
3d
3f
3.5. Catalytic activity
P3/2
3600
3200
2800
2400
PhIO was often used as terminal oxidant in the manganese
porphyrin catalyzed oxidation of alkenes [41]. It was found the cat-
alytic oxidation of styrene by manganese N-confused porphyrins
using PhIO gave benzaldehyde (BA), phenylacetaldehyde (PA),
styrene epoxide (SO) and 1-phenylethanone (PE) (Scheme 2). Time-
dependent conversion yield for the catalytic oxidation of styrene
P1/2
660
655
650
645
640
Binding Energy (eV)
Fig. 2. X-ray photoelectron spectra of MnTPP and 3a–3f.
Scheme 2. Catalytic oxidation products of styrene by MnNCTArPs.

S.-H. Peng et al. / Journal of Molecular Catalysis A: Chemical 395 (2014) 180–185
183
N-methyl N-confused porphyrin 3a exhibits catalytic activity in
styrene oxidation. It should be noted manganese acetate Mn(OAc)2
has no catalytic oxidation activity under the same conditions
(Table 1, Entry 12). When the amount of 3a was increased from
0.5 ␮mol to 1.0 ␮mol, the conversion of styrene increased remark-
ably and no significant improvement with further increasing the
amount of 3a, so the optimized amount of catalyst was set to
1.0 ␮mol.
Significant effect of solvent on the catalytic activity of man-
ganese complexes of porphyrins and corroles have been reported
recently [18,42]. To determine the effect of solvent on the cat-
alytic oxidation by 3a, the reaction was carried out in different
solvents (Table 1, Entry 5–10). Acetonitrile gave the highest total
conversion yield and styrene epoxide selectivity. Under argon
atmosphere, the yield and selectivity of styrene epoxide increase
slightly (Table 1, Entry 10, 11), while the significant decrease
in the yield of benzaldehyde was observed. This evidenced the
involvement of oxygen in the catalytic oxidation under aero-
bic conditions. Similar results were observed for manganese(III)
5,10,15,20-tetraphenylporphyrin (MnTPP) catalyst (Table 1, Entry
13, 14). Whether under aerobic or anaerobic conditions, styrene
epoxide was found as the major product. The total yield of oxida-
tion products by 3a was almost same as that of MnTPP, suggesting
that manganese N-methyl N-confused porphyrins have comparable
catalytic activity with manganese porphyrins.
The effect of various oxidants on the styrene oxidation by 3a
and MnTPP was investigated under the same reaction conditions.
There was almost no conversion when using air or H2O2 as oxi-
dant (Table 1, Entry 17–20). When using tert-Butyl hydroperoxide
(TBHP) as oxidant, the main product was benzaldehyde (Table 1,
Entry 15–16). This may be attributed to free radical mechanism
[43–46]. Two pathways have been envisaged for the formation
of metallo-oxo species using TBHP, i.e. the heterolytic and the
homolytic cleavage of the O O bond in the MnIII-O-O-tBu interme-
diate. Homolytic cleavage results in the formation of tBuO. radical
along with the formation of poor reactive intermediate MnIV-OH,
responsible for the low yield of styrene epoxide [45]. The tBuO rad-
ical then initiates a free radical reaction that is propagated by the
involvement of oxygen, leading to the formation of benzaldehyde
[46]. From Table 1 (Entry 11 and 14), it is clear that 3a and MnTPP
are efficient for the selective epoxidation of styrene in the presence
BA
PA
SO
PE
BA
PA
SO
PE
40
30
20
10
0
40
30
20
10
0
(b)
Tot.
Tot.
(a)
0
2
4
6
8
10
12
0
2
4
6
8
10
12
Fig. 4. The effect of reaction time on the catalytic oxidation of styrene by 3a (b, inset,
3a/PhIO/styrene = 1/100/1000) and the control experiment without catalyst (a).
by 3a using PhIO in dichloromethane is depicted in Fig. 4. Without
catalyst, only a small amount of benzaldehyde could be detected
(less than 5% in 12 h), while the other products were negligible
(less than 2%, Fig. 4b). In the presence of manganese N-methyl
N-confused porphyrin 3a, the conversion yield increased sharply,
which indicated that 3a can catalyze the oxidation reaction sig-
nificantly. The main oxidation products were styrene oxide and
benzaldehyde, while small amounts of phenylacetaldehyde (PA)
and 1-phenylethanone (PE) were also detected (less than 4% in 12 h,
Fig. 4a). The total conversion yield increased with the increasing of
reaction time in the first 6 h, and after that, the increase in the con-
version yield was much slower. Therefore, the optimized reaction
time was selected to be 6 h.
The effects of catalyst concentration, solvents, oxidants and
atmosphere on the catalytic oxidation are summarized in Table 1.
The conversion efficiency of styrene and the selectivity of styrene
epoxide increase significantly with the increasing concentration of
3a (Table 1, Entry 1–4), which further confirms that manganese
Table 1
The influence of different factors on the oxidation of styrene.a
Entry
Catalyst
Catalyst conc. (␮mol)
Solvent
Oxidant
Atmosphere
Yield (%)b
BA
Tot. Conv. (%)
PA
0.36
SO
1.35
PE
0.26
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
3a
3a
3a
3a
3a
3a
3a
3a
0
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
Toluene
EtOAc
Acetone
Methanol
DMAc
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
PhIO
PhIO
PhIO
PhIO
PhIO
PhIO
PhIO
PhIO
PhIO
PhIO
PhIO
PhIO
PhIO
PhIO
TBHP
TBHP
H2O2
H2O2
Air
Air
Air
Air
Air
Air
Air
Air
Air
Air
Air
Argon
Air
Air
Argon
Argon
Argon
Argon
Argon
–
2.99
5.52
12.02
8.58
5.91
4.82
5.60
9.01
5.64
12.92
7.53
1.08
3.55
2.12
23.69
32.73
0.82
0.84
–
4.96
12.33
30.53
32.42
13.86
18.66
19.77
13.08
16.04
41.95
41.53
1.80
46.60
44.33
29.15
50.46
0.82
0.5
1.0
2.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.03
3.21
4.29
1.16
1.94
2.10
1.08
0.45
5.54
8.965
0.19
12.79
10.69
0.32
0.65
0
5.40
14.31
18.68
6.20
11.02
11.38
2.25
9.33
22.05
24.93
0.53
29.67
31.17
1.83
0.66
0
0.38
0.99
0.86
0.59
0.88
0.69
0.74
0.62
1.44
0.42
0
0.59
0.35
3.31
10.42
0
3a
3a
3a
Mn(OAc)2
MnTPP
MnTPP
3a
MnTPP
3a
MnTPP
3a
MnTPP
0
–
–
0
–
–
0
–
–
0.84
–
–
Air
–
–
a
Reaction condition: 1.0 mmol styrene, 0.10 mmol oxidant, 1.0 ␮mol of catalyst in 2 mL solvent at room temperature. Reaction time: 6 h.
Conversion was determined by GC.
b

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S.-H. Peng et al. / Journal of Molecular Catalysis A: Chemical 395 (2014) 180–185
MnTPP
3f
5 min afer adding PhIO
10min afer adding PhIO
15 min afer adding PhIO
20 min afer adding PhIO
1.5
1.2
0.9
0.6
0.3
0.0
5 min after adding PhIO
10 min after adding PhIO
15 min after adding PhIO
20min after adding PhIO
1.2
0.9
0.6
0.3
0.0
400
500
600
700
400
500
600
700
800
Fig. 5. UV–vis spectra obtained by adding PhIO into the acetonitrile solution of MnTPP (a) and 3f (b). [MnTPP] = 10 ␮mol/L, [MnTPP]/[PhIO] = 1/10, [3f] = 25 ␮mol/L,
[3f]/[PhIO] = 1/5.
of PhIO. This is because PhIO is a single-oxygen donor and favors
the formation of the high-valent (oxo)manganese species MnV(O),
which is responsible for oxygen insertion into the carbon–carbon
double bond of styrene [47]. As manganese porphyrin (MnTPP) can
catalyze the epxoidation of styrene using PhIO oxidant under anaer-
obic conditions via MnV(O) species, the same mechanism might be
expected for manganese N-confused porphyrins.
It is well known that manganese porphyrins and related
macrocycles possess very high catalytic activity in alkene
epoxidation [48–51]. Generally, epoxides are formed via oxy-
gen atom transfer (OAT) from high-valent (oxo)manganese(V)
intermediate to carbon–carbon double bond [52,53]. At least
three types of active (oxo)manganese intermediates have
been postulated for manganese porphyrin catalyzed oxida-
tion reactions: (oxo)manganese(IV)-porphyrin [PorMnIV(O)],
(oxo)manganese(IV)-porphyrin ␲-radical cation [PorMnIV(O+•)]
and the most plausible active intermediate (oxo)manganese(V)-
porphyrin [PorMnV(O)] [54–57]. To explore the active intermediate
in the present catalytic system. The direct oxidation of manganese
N-confused porphyrin by PhIO in the absence of organic substrates
was performed. Fig. 5 shows the UV–vis spectral changes of 3f
and MnTPP upon addition of large excess of PhIO in acetonitrile.
For MnTPP, addition of PhIO resulted in the disappearance of the
absorption band at 475 nm. At the same time, the bands at 419
and 522 nm were observed (Fig. 5a), consistent with the typical
UV–vis spectra of [PorMnV(O)] species [57]. The reaction between
3f and PhIO also exhibited a similar spectral changes (Fig. 5b), i.e.
disappearance of the band at 505 nm and appearance of the new
bands at 471 and 775 nm. Similar spectral changes suggest the
existence of MnV O intermediate in both cases.
Catalytic oxidation activities of all synthesized Mn(III) N-methyl
N-confused porphyrins (3a–3f) under the same conditions were
summarized in Table 2. The conversion efficiency of styrene and
the selectivity of styrene epoxide for 3f is obviously higher than 3a
and other Mn-NCTArPs containing phenyl groups substituted with
electron-donors (o-OMe, m-OMe, p-OMe, p-Me). The total yield of
oxidation products by 3f is also a little higher than that of MnTPP
under the same conditions. The improvement of the conversion
efficiency of styrene and the selectivity of styrene epoxide for 3f
may be due to its higher first oxidation potential E1/2 (Mn4+/Mn3+
)
as compared to other Mn-NCTArPs, which favors the formation of
more reactive Mn(V)-oxo species.
Table 2
The catalytic activities of all synthesized manganese N-confused porphyrins on the
oxidation of styrenea.
Based on the results described during aforementioned styrene
oxidation catalyzed by manganese N-confused porphyrin using
PhIO as oxygen source, the suggested mechanism is depicted in
Scheme 3. The rection between PhIO and manganese N-confused
porphyrin to form MnV O intermediate (2), which subsequently
transfers its oxygen to the carbon–carbon double bond of styrene to
form the products SO, PA and PE. Another pathway is free-radical
involved mechanism, the reaction of oxygen and radical species 3
to form the peroxy radical intermediate 4. Species 3 intermediate
can act as radical propagation initiator to generate more alkyl
peroxy radicals, which in turn oxidize styrene to benzaldehyde
Catalyst
Yield (%)
BA
Tot. Conv. (%)
PA
9.99
8.37
9.97
9.81
11.78
7.08
SO
PE
MnTPP
3a
3b
3c
3d
1.90
8.88
6.00
6.43
7.76
11.69
4.35
32.66
25.50
25.54
23.06
22.10
17.40
28.69
0.20
0.53
0.77
0.74
0.80
1.16
0.52
44.75
43.28
42.28
40.04
42.44
37.33
46.36
3e
3f
12.80
a
Reaction condition: 1.2 mmol styrene, 0.10 mmol PhIO, 1.0 ␮mol of catalyst in
2 mL solvent at room temperature under argon. Reaction time: 6 h.
Scheme 3. Proposed mechanism for the catalytic oxidation of styrene by MnNCTArPs using PhIO oxidant.

S.-H. Peng et al. / Journal of Molecular Catalysis A: Chemical 395 (2014) 180–185
185
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We have presented the synthesis of a series of new manganese
N-methyl N-confused porphyrins and their characterization by
UV–vis, HR-MS and X-ray Photoelectron Spectroscopy as well as by
cyclic voltammetry. The first catalytic oxidation of alkene by man-
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can catalyze the oxidation of styrene in the presence of iodosylben-
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the potential use of manganese N-confused porphyrin complexes
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manganese N-confused porphyrins with the manganese porphyrin
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Appendix A. Supplementary data
Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/j.molcata.
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