An ionic manganeseporphyrin catalyst for alkene epoxidations: The significant roles of the cationic trimethylbenzylammonium functionalities and the anionic mercaptopropionate counterions

Article abstract of DOI:10.1055/s-2008-1032062

An ionic manganeseporphyrin, bearing cationic trimethylbenzylammonium functionalities and mercaptopropionate [HS(CH₂)₂CO ₂^-] counteranions, was synthesized and applied as a catalyst for alkene epoxidations using iodosylbenzene (PhIO) as an oxidant. The reactions proceeded with high activity and oxide selectivity, and the catalyst exhibited relatively good recyclability. It was found that the presence of thiol (SH) group and quaternary ammonium substituents contributed greatly to the improved activity and stability of this ionic manganeseporphyrin. This was due to the synergistic effects of HS(CH₂)₂CO₂ ^- as an axial ligand, and the suppression of oxidation degradation by the strongly electron-withdrawing nature of the quaternary ammonium cations. Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-2008-1032062

LETTER
453
An Ionic Manganeseporphyrin Catalyst for Alkene Epoxidations: The
Significant Roles of the Cationic Trimethylbenzylammonium Functionalities
and the Anionic Mercaptopropionate Counterions
I
Y
onic Manganese
p
u
orphyrin
C
at
e
alyst for Al
q
kene
E
poxid
i
a
tions n Cai, Yong Lu, Ye Liu,* Ming-Yuan He, Qingxia Wan
Shanghai Key Laboratory of Green Chemistry and Chemical Processes, East China Normal University, Shanghai 200062, P. R. of China
Fax +86(21)62233424; E-mail: yliu@chem.ecnu.edu.cn
Received 28 August 2007
R
Abstract: An ionic manganeseporphyrin, bearing cationic trimeth-
ylbenzylammonium functionalities and mercaptopropionate
–
[
HS(CH ) CO ] counteranions, was synthesized and applied as a
2
2
2
catalyst for alkene epoxidations using iodosylbenzene (PhIO) as an
oxidant. The reactions proceeded with high activity and oxide selec-
tivity, and the catalyst exhibited relatively good recyclability. It was
found that the presence of thiol (SH) group and quaternary ammo-
nium substituents contributed greatly to the improved activity and
stability of this ionic manganeseporphyrin. This was due to the syn-
N
N
N
R
Mn (III)
R
X
N
–
ergistic effects of HS(CH ) CO as an axial ligand, and the sup-
2
2
2
pression of oxidation degradation by the strongly electron-
withdrawing nature of the quaternary ammonium cations.
Key words: manganese porphyrins, catalysis, epoxidations, sulfur,
alkenes
R
–]
+
2 2 2
) CO
–]
1
2
[MnIIITTMAPP]5+[HS(CH
2
)
2
CO
2
5
R = N Me
3
, X = 5 [HS(CH
, X = [I–] [OAc–]5–n n = 0–5mm
n
Epoxidations of alkenes catalyzed by metalloporphyrins
have been studied in the past decades as biomimetic reac-
tions of cytochrome P-450 due to their high reactivities
[MnIIITTMAPP]5+[I–]
[OAc–]5–n
+
n
R = N Me
R = NMe
3
3 [MnIIITDMAPP]5+[OAc–]
, X = OAc–
2
Figure 1
1
–3
and selectivities.
Simple metalloporphyrins such as
MnTPP are easily destroyed mainly due to self-aggrega-
ligand like pyridine and imidazole or their derivatives.2,27
However, this strategy suffered from the drawback of pro-
pensity to form inactive six-coordinate bisligated species
IV
tion into an uncreative m-oxo Mn -porphyrin dimer and
4
,5
oxidation degradation. In this aspect, it has been recog-
nized that bulky and/or electronegative substituents intro-
duced in the porphyrin ring can improve the stability of
2
8
in the presence of excess axial ligands. A feasible meth-
od to circumvent such drawback could be the introduction
of a covalently attached axial ligand into metalloporphy-
rins.2
6
–14
these catalysts.
For example, polyhalogenated metal-
loporphyrins were robust catalysts for alkene epoxida-
tions with high turnover number in homogeneous
9–32
1
3,14
Herein we described the synthesis of a new ionic manga-
neseporphyrin, manganese tetrakis(4-N,N,N-trimethyl-
processes.
In addition, the attachment of metallopor-
phyrins onto solid supports enhanced the catalyst stability
towards oxidative degradation through efficient site isola-
tion, and made the catalyst readily separable from the re-
ammoniumbenzyl)porphyrin
mercaptopropionate
III
5+
–
[
Mn TTMAPP] [HS(CH ) CO ] (1, Figure 1), and its
2 2
2
5
1
5
application in the epoxidation of alkenes. In such ionic
action mixture with high reusablility. Therefore, some
solid-supported metalloporphyrins, such as mesoporous
–
manganeseporphyrin, the HS(CH ) CO2 anion func-
2 2
1
6,17
tioned as an axial ligand as well as the counterion. The key
idea here was to modify the neutral metalloporphyrins
into an ionic one, which could offer more available sites
to incorporate the required functional groups in the form
of cations and/or anions. In comparison with the neutral
metalloporphyrin, the ionic 1 has the following special
features. Firstly, the strong electron-withdrawing effect of
quaternary ammonium in metalloporphyrin could de-
crease the electron density of the porphyrino ring. Sec-
ondly, the repulsive force between the porphyrin cations
could avoid the self-aggregation of metalloporphyrins.
molecular sieve (MCM-41),
Merrifield’s peptide res-
1
8
19
20
in, highly crossing-linked polymer, PEG, and
2
1–23
others
have been developed to address this issue. Nev-
ertheless, the heterogeneous catalysis was generally slug-
gish, probably owing to the biphasic nature of the reaction
systems. Another important influence on the nature of
metalloporphyrins was the axial ligand bound to metal
2
4–26
IV
center.
The formation of uncreative m-oxo Mn -por-
phyrin dimer can be prevented by adding an exogenous
–
2
SYNLETT 2008, No. 3, pp 0453–0457
x
x.
x
x.
2
0
0
8
Thirdly, the axial ligand [HS(CH ) CO ] incorporated
2
2
Advanced online publication: 16.01.2008
DOI: 10.1055/s-2008-1032062; Art ID: W15307ST
into 1 stoichiometrically could obviate the propensity to
form six-coordinate bisligated species.28 The consider-
©
Georg Thieme Verlag Stuttgart · New York

4
54
Y. Cai et al.
LETTER
Table 1 Styrene Epoxidation under Different Conditionsa
no product could be detected. On the basis of these results,
the reaction conditions were optimized in terms of molar
ratio of PhIO to styrene, reaction time, and temperature.
Under the optimal reaction conditions (1.5 equiv of PhIO,
O
1
1
h, 30 °C), the conversion of styrene reached a maximum
of >96% with excellent epoxidation selectivity (>98%) in
MeCN (Table 1, entries 7 and 10). The higher catalytic ac-
tivity was partially attributed to good solubility of ionic 1
and PhIO in the polar solvent MeCN. Further increase in
the amount of PhIO or in the reaction temperature led to
decreased conversions due to oxidative destruction of the
porphyrino ring.
Entry Solvent
Oxidant
NaIO4
Temp Conv Selectivity
b
b
(
°C) (%) (%)
1
2
3
4
5
6
7
8
9
1
1
1
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
30
30
30
–
–
e
H O
–
–
2
2
NaClOe
–
–
PhI(OAc)2 30
9
97
93
97
98
95
96
99
96
95
97
It was found that the presence of water in the reaction sys-
tem led to a notable decreased activity of 1, although 1 it-
self was very stable in water. Meanwhile, the reaction
solution was bleached quite rapidly with increasing
amount of water because of the oxidative degradation of
MCPBA
PhIO
30
20
30
40
30
30
30
30
30
76
83
96
76
86
99
59
40
32
PhIO
1
, which was supported by the UV-Vis analysis. These re-
PhIO
sults indicated that the oxidative degradation of 1 was par-
V
tially caused by the interaction of the reactive Mn =O
CH Cl2
PhIO
2
intermediate with water to form the species with hydroxy,
0
1
2
3
anhyd MeCNc
PhIO
which was apt to be oxidatively broken down at meso-car-
35
bons of the porphyrino ring. As a result, the conversion
d
MeCN–H O (20:1) PhIO
2
of styrene was substantially lower in the mixture of
d
MeCN–H O than that in anhydrous MeCN (Table 1, en-
MeCN–H O (10:1) PhIO
2
2
tries 10–13). Evidently, without additional involvement
of axial ligands, the ionic 1 was a highly efficient catalyst
for epoxidation of styrene in MeCN under mild conditions
d
1
MeCN–H O (5:1)
PhIO
2
a
Styrene (0.5 mmol), catalyst 1 (0.5 mol%), oxidant (0.65 mmol), sol-
–
vent (2 mL), 1 h.
(Table 1, entries 7, 9, 10), implying that HS(CH ) CO
2
2
2
b
Determined by GC.
MeCN was distilled over CaO before use.
Volume ratio.
Oxidant: 5 mmol.
could definitely play the role of axial ligand to activate
c
V
and stabilize the active Mn =O porphyrin intermediate.
d
V
e
As we know, the reactive Mn =O porphyrin intermediate
was an unstable (short-lived) species. It could only be de-
tected by the rapid-mixing, stopped-flow (time-resolved)
3
5
38
UV-visible spectrophotometer or isolated at –70 °C. In
ation for selecting the appendage SH group in carboxyl
anion as an axial ligand was based on the idea to mimic P-
–
our work, when HS(CH ) CO2 was applied as the axial
2 2
ligand and the counterion dully, the reactive intermediate
4
50 enzymes in which the Fe ion was bound to a peptide
V
Mn =O could be identified by the UV-visible spectropho-
backbone via a thiolate bridge of a cysteine residue. The
sulfur atom was found to be a favorable ligand to metal
owing to its electron-donating nature and the better mix-
ing of its orbital with that of the heme.
Two other structurally similar manganeseporphyrins,
manganese tetrakis(4-N,N,N-trimethylammoniumben-
zyl)porphyrin iodide [Mn TTMAPP] [I ] [OAc ]
tometer at room temperature. It was found that the maxi-
mum absorbance of 1 in MeCN was observed at 472 nm
III
2
4
as a typical Soret band of Mn -porphyrin. Whenever
PhIO was added into the solution of 1, a new absorbance
peak at 440 nm appeared immediately, which was identi-
V
fied as the reactive Mn -oxo porphyrin intermediate ac-
III
5+
–
–
n
5–n
35
cording to the reported work. Subsequently the formed
(
n = 0–5) (2), and manganese tetrakis(4-N,N-dimethyl-
V
Mn =O porphyrin intermediate decayed gradually with a
III
+
–
aminobenzyl)porphyrin [Mn TDMAPP] [OAc ] (3)
were also synthesized
clear isosbestic point at 440 nm when the reaction time
was extended. These results definitely proved that the for-
3
3,34
and used as comparisons in the
epoxidation study. The ionic catalyst 1 was prepared from
V
mation and stabilization of the reactive Mn =O porphyrin
III
5+
–
–
[
Mn TTMAPP] [I ] [OAc ]
and HS(CH ) CO K.
2 2 2
n
5–n
intermediate was improved in 1–MeCN system with
1
The structure was confirmed by H NMR, FT-IR, and
UV-Vis spectroscopy.
–
2
HS(CH ) CO anion as the axial ligand.
2
2
On the other hand, the oxidation of axial ligands like pyr-
idines and imidazoles (derivatives) often happened in
To demonstrate the utility of 1 as a catalyst for alkene ep-
oxidations, styrene was chosen as a model substrate for
epoxidation with various oxidants including PhIO,
PhI(OAc) , NaIO , NaClO, H O , and MCPBA. Iodosyl-
36
metalloporphyrin-catalyzed oxidation processes. In con-
–
sideration of the side reaction of HS(CH ) CO ion with
2
2
2
2
4
2
2
–
+
PhIO, HS(CH ) CO K was treated with excess PhIO at
2
2
2
benzene was found to be the most suitable one (Table 1,
entries 1–7). When the blank reaction was carried out only
with PhIO as the oxidizer without the participation of 1,
3
0 °C for one hour. The FT-IR spectra before and after re-
–1
action showed a weak absorption band at 582 cm (S–S
Synlett 2008, No. 3, 453–457 © Thieme Stuttgart · New York

LETTER
Ionic Manganeseporphyrin Catalyst for Alkene Epoxidations
455
Table 2 Effect of Concentration of Catalyst 1 on Styrene Epoxida-
tiona
Entry
Cat. 1
mol%)
Conv. (%)b Selectivity TON (h–1)
b
(
(%)
1
2
3
4
5
0.005
0.01
0.05
0.1
36
48
73
75
92
99
98
98
97
96
97
100
7200
4800
1260
750
0.25
0.5
368
6
198
a
Styrene (0.5 mmol), PhIO (0.65 mmol), MeCN (2 mL), 30 °C, 1 h.
Determined by GC.
b
Figure 2 Recycling uses of manganeseporphyrins 1–3 for styrene
epoxidation
stretching vibration). The FT-IR analysis indicated that
the thiol group (–SH) might be oxidized to a disulfide,
which was quite stable against further oxidation, and then teracted the p–p stacking of the metalloporphyrin. These
could act as the axial ligand without degraded impact on facts suggested that the ionic catalyst 1 having quaternary
3
7
–
the activity of 1.
ammonium substituents and HS(CH ) CO anion into
2 2 2
one metalloporhyrin unit indeed possessed additional ad-
vantages over the neutral metalloporphyrin in terms of ac-
tivity, stability, and recyclability.
The high efficiency of 1 also could be seen in Table 2.
Even the concentration of 1 was decreased to 0.005 mol%,
a conversion of 36% was obtained with turnover rate
–
1
(
turnover number per hour, TON) of 7200 h . As expect- To examine the scope of this method, the oxidation of dif-
ed, increasing the catalyst concentration improved the ferent substrates was investigated (Table 3), in which a
conversion of styrene until reaching 99% at 0.5 mol% in wide array of styrene derivatives with different electronic
anhydrous MeCN.
and steric effects was examined. Obviously, the electronic
character of substituents of styrene showed a significant
impact on conversion and selectivity. With the electron-
withdrawing group at the para position, the conversions
of the substrate and the selectivities for epoxides were re-
markable (Table 3, entries 1–4). If the substituting group
was of electron-donating nature (Table 3, entry 5), the in-
creased electron density at the a-C of the C=C bond re-
sulted in the formation of a rearranged product (phenyl
Many studies have implicated the importance of the elec-
tronic effect as well as axial ligands on activity and stabil-
6
–15
ity of metalloporphyrins.
In order to evaluate the
activity and stability of 1, the recycling uses of the differ-
ent manganeseporphyrin catalysts 1–3 were compared
(Figure 2). It was shown that the stability and activity of
the catalysts were in the ranking of 1 > 2 > 3. As we ex-
pected, the best results were obtained by using 1 as the
catalyst, which could be recycled four times with excel-
lent epoxide selectivity and without much loss of activity.
As to the catalysts not bearing SH, such as ionic 2 or neu-
tral 3, even the fresh catalyst gave relatively lower activi-
ty, further implying the efficient function of
1
3
acetaldehyde) through a radical mechanism. For the sub-
strates with an internal C=C bond (Table 3, entries 7–11),
the epoxide was the only product, implicating the facial
V
13
access of the substrate to Mn =O porphyrin domain.
The formation of benzaldehyde through the bond cleav-
age at a-C and b-C of styrene could not be observed in any
case. It was also noted that the trans-substituted substrates
gave the trans-epoxides as the only products (Table 3, en-
tries 7–10). However, with cis-substituted substrates like
cis-stilbene, a mixture of cis-epoxide (62%) and trans-ep-
oxide (38%) was obtained with high conversion, indicat-
ing a large loss of stereochemistry (Table 3, entry 11). In
the case of p-vinylphenol, no products corresponding to
the C=C bond oxidation were found though 100% conver-
sion of p-vinylphenol was observed. The unidentified
product compositions resulted from the more facile oxida-
tion propensity of phenol to quinines, nitro compounds,
and others, by PhIO under the reaction conditions. Hence,
chemoselectivity cannot be achieved for catalyst 1 when-
ever OH and C=C groups are present together in the sub-
strate.
–
HS(CH ) CO as an axial ligand. In consideration of sta-
2
2
2
bility, the ionic catalysts 1 and 2 were much superior to
that of the neutral one, due to the introduction of strong
electron-withdrawing groups of quaternary ammonium
cations.
As for the ionic manganeseporphyrin of 1, the presence of
the four strong electron-withdrawing groups of quaterna-
ry ammonium cations in the porphyrino ring decreased the
electron density at the meso-carbons, and thus improved
1
3–15
the oxidation tolerance of metalloporphyrin.
Mean-
while the SH groups present in the counteranions behav-
ing as the coordinating axial ligands also contributed to
the activity and stability of metalloporphyrin synergisti-
cally. On the other hand, the repulsive force between the
positive charges in the porphyrino ring presumably coun-
Synlett 2008, No. 3, 453–457 © Thieme Stuttgart · New York

4
56
Y. Cai et al.
LETTER
Table 3 Epoxidation of Styrene and Derivatives Catalyzed by 1a
quaternary ammonium cations, the prevention of p–p
stacking of porphyrino ring by the repulsive force be-
tween the positive charges. In addition, another special
feature of using 1 was the easy isolation of products. The
catalyst 1, endowed with high polarity due to its ionic na-
ture, was readily separable from the reaction mixture by
precipitation with cyclohexane and could be used in the
next run without further treatment.
Entry Substrate
1
Time Conv. Selectivity
b
b
(
h)
(%)
(%)
1
1
96
98
2
99
99
98
97
91
99
98
99
99
Cl
3
1
1
1
1
Br
Acknowledgment
The research was financially supported by the National Natural Sci-
ence Foundation of China (No. 20533010, 20590366, 20673039),
the Science & Technology Commission of Shanghai Municipality
4
NC
(
No. 06JC14023, 06SR07101), Shanghai Leading Academic Disci-
pline Project (B409), and China Postdoctoral Science Foundation
No. 44021220).
5
HOOC
(
6
74 (26)c
100
Me
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Synlett 2008, No. 3, 453–457 © Thieme Stuttgart · New York

LETTER
Ionic Manganeseporphyrin Catalyst for Alkene Epoxidations
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