Rapid and highly selective epoxidation of styrene by meta-chloroperbenzoic acid catalyzed by manganese meso-tetrakis(pentafluorophenyl)porphyrin in the presence of tetrabutylammonium acetate and bromide

Article abstract of DOI:10.1007/s10562-010-0460-7

Epoxidation of styrene with m-CPBA in the presence of Mn(TPFPP)Cl was improved to high yield (98-100%) and excellent selectivity (100%) by using n-Bu₄NOAc or n-Bu₄NBr as co-catalysts in less than 5 min. Also comparison of co-catalytic activity of n-Bu₄NOAc, n-Bu ₄NBr and imidazole in epoxidation of cis-stilbene and styrene was carried out at similar conditions.

Full text of DOI:10.1007/s10562-010-0460-7

Catal Lett (2011) 141:163–167
DOI 10.1007/s10562-010-0460-7
Rapid and Highly Selective Epoxidation of Styrene
by meta-Chloroperbenzoic Acid Catalyzed by Manganese
Meso-tetrakis(Pentafluorophenyl)Porphyrin in the Presence
of Tetrabutylammonium Acetate and Bromide
•
Zahra Solati Majid Hashemi Leila Ebrahimi
•
Received: 11 August 2010 / Accepted: 28 September 2010 / Published online: 16 October 2010
Ó Springer Science+Business Media, LLC 2010
Abstract Epoxidation of styrene with m-CPBA in the
presence of Mn(TPFPP)Cl was improved to high yield
(98–100%) and excellent selectivity (100%) by using
n-Bu4NOAc or n-Bu4NBr as co-catalysts in less than 5 min.
Also comparison of co-catalytic activity of n-Bu4NOAc,
n-Bu4NBr and imidazole in epoxidation of cis-stilbene and
styrene was carried out at similar conditions.
co-catalysts such as pyridines, imidazoles and quaternary
ammonium salts results in a significant change of catalytic
activity of metalloporphyrins, in particular Manganese(III)
porphyrins catalysts [19–30].
The epoxidation of olefins with Manganese(III)(TPFPP)
(TPFPP = meso-tetrakis(pentafluorophenyl)porphyrin)
and n-Bu4NHSO5 were carried out in the presence of tet-
rabutylammonium salts. The best yields and selectivities
have been achieved by the addition of n-Bu4NOAc as
co-catalyst. Yields and epoxide selectivities were low when
n-Bu4NBr was used as co-catalyst [31].
Keywords Epoxidation Á Styrene Á Catalysis Á
Metalloporphyrin Á Co-catalytic effect
Here we described the complete conversion and highly
selective epoxidation of styrene with meta-chloroperben-
zoic (m-CPBA) catalyzed by Mn(TPFPP)Cl (TPFPP =
meso-tetrakis(pentafluorophenyl)porphyrin) in the presence
of n-Bu4NOAc or n-Bu4NBr as co-catalyst in CH2Cl2 in
less than 5 min at room temperature. Also co-catalytic
activities of n-Bu4NOAc and n-Bu4NBr were compared to
imidazole (ImH) (as a nitrogen donor ligand) in the
epoxidation of cis-stilbene and styrene using m-CPBA and
Mn(TPFPP)Cl.
1 Introduction
Metalloporphyrin catalysts have been shown to be useful in
olefin epoxidation by various oxidants such as PhIO [1, 2],
NaOCl [3, 4], periodates [5–7], H2O2 [8–10], hydroper-
oxides [11], percarboxylic acids [12, 13], and n-Bu4NHSO5
(TBAO) [14]. meta-Chloroperbenzoic acid is used widely
as an oxidant in organic synthesis and is often preferred to
other proxy acids because of its relative ease of handling.
Among olefins epoxidation of styrene to styrene oxide is of
particular interest from both scientific and commercial
point of view [15, 16]. However the formation of desired
product (styrene oxide) is always accompanied by the
oxidative cleavage of double bond to benzaldehyde and
isomerisation of styrene oxide to phenyl acetaldehyde [17,
18]. Achieving high selectivities, excellent yields and
mechanism elucidations are of interest for these catalytic
epoxidations. Also it was found that the employment of
2 Experimental
2.1 Material
The free base porphyrins TPPH2 [32], TPFPPH2 [33], were
prepared and purified as reported previously. Mn(TPP)Cl
was synthesized using MnCl2Á4H2O according to the pro-
cedure of Adler et al. [34]. The synthesis of Mn(TPFPP)Cl
was based on the procedure given by Kadish et al. [35].
Imidazole was purchased from Merck. n-Bu4NOAc was
prepared by adding tetrabutylammonium hydrogen sulfate
(6.5 mmol) to a solution of sodium acetate (32.5 mmol) in
Z. Solati (&) Á M. Hashemi Á L. Ebrahimi
Chemistry Department, Faculty of Sciences,
Persian Gulf University, 75168 Bushehr, Iran
e-mail: solati@pgu.ac.ir
123

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Z. Solati et al.
water (40 mL), the mixture was stirred for 30 min and then
extracted with CH2Cl2 (80 mL) and the extract was dried
over magnesium sulfate. After filtration and evaporation of
the solvent, the remaining paste was washed with hexane
(10 mL) and dried under vacuum. Styrene was purified by
passing through a column of silica gel. m-CPBA (as 70%
solution in water) and other substrates were purchased from
Aldrich or Fluka and were used as received.
styrene oxide (conversion = 51%; epoxide selectiv-
ity = 35%). Addition of Mn(TPFPP)Cl alone had no
noticeable effect on the conversion and selectivity of sty-
rene epoxidation reaction (conversion = 55%; epoxide
selectivity = 34%). Using m-CPBA/Mn(TPFPP)Cl in asso-
ciation of a tetrabutylammonium salt with a noncoordi-
nating anion (n-Bu4NBF4) as co-catalyst had a little effect
on the conversion and selectivity of this reaction (conver-
sion = 59%; epoxide selectivity = 43%). But using tetra-
butylammonium salts with the coordinating anions
(n-Bu4NBr and n-Bu4NOAc) as co-catalyst remarkably
improved the conversion and selectivity of this reaction
(for n-Bu4NOAc conversion = 100% in 1 min and for
n-Bu4NBr conversion is 98% in 5 min; epoxide selectivity
for both of them is 100%). The relative co-catalytic
activities of OAc- and Br- could be rationalized based on
their intrinsic basicities and steric properties. pKa for
HOAc and HBr are 4.76 and -9.00 respectively. Steric
bulk for Br is more than O regarding their atomic radii. So
OAc- should be a better co-catalyst compared to Br-. The
total turnover numbers for epoxidation of styrene with
m-CPBA/Mn(TPFPP)Cl in association of n-Bu4NOAc and
n-Bu4NBr were 907 and 903, respectively.
2.2 General Oxidation Procedure
Stock solutions of MnPor catalysts (0.003 M) were pre-
pared in CH2Cl2. In a typical reaction 0.001 mmol
(0.3 mL) of catalyst, 0.1 mmol of alkene and x mmol (as
required) of co-catalyst were dissolved in 0.2 mL of
CH2Cl2. m-CPBA (0.2 mmol) was then added to the
reaction solutions at 25 °C. The reaction solutions were
stirred for the required times. In the case of styrene, the
reaction solutions were analyzed immediately by GLC. For
cis-stilbene, the solvent of the reaction solution was
removed under reduced pressure, n-hexane (2 mL) was
added, the resulting mixture was filtered and n-hexane was
removed under vacuum. The identification and the quan-
1
tification of the products were done by H NMR spec-
When the same reaction was performed in the presence
of ImH as co-catalyst under the co-catalyst/Mn(TPFPP)Cl
molar ratio mentioned in Table 1, the selectivity was 100%
but the conversion was to some extent lower relative to
non-catalyzed reaction (conversion reaches 42% in 1 min
and was nearly unchanged after 2 h).
troscopy (in CDCl3).
3 Results and Discussion
3.1 Comparison of Co-catalytic Effects of ImH
and Tetrabutylammonium Salts on the Styrene
Epoxidation Using Mn(TPFPP)Cl and Mn(TPP)Cl
Table 2 shows the effect of different co-catalyst/
Mn(TPFPP)Cl molar ratios on the conversion and selec-
tivity of styrene epoxidation reaction. When n-Bu4NOAc
was used as co-catalyst, the conversion was excellent
(100%) at all co-catalyst/Mn(TPFPP)Cl molar ratios
(10–150), but the epoxide selectivity was improved with
increasing the co-catalyst/catalyst ratio (100% at the molar
In Table 1 we compiled the data for the epoxidation of
styrene with m-CPBA. m-CPBA alone yields a mixture of
9% benzaldehyde, 24% phenylacetaldehyde and 18%
Table 1 Epoxidation of styrene with m-CPBA catalyzed by Mn(TPFPP)Cl in the presence of different co-catalyst
Entry
Co-catalyst
Conversion (%)
Styrene
oxide (%)
Benzaldehyde (%)
Phenylacetaldehyde (%)
Epoxide
selectivity (%)
1a
2
–
51
18
9
24
23
30
–
35
–
55
19
13
4
34
3
4b
n-Bu4BF4
ImH
59
25
43
(42) 44
(85) 98
(100)
(42) 44
(85) 98
(100)
–
(100) 100
(100) 100
(100)
5
n-Bu4Br
n-Bu4OAc
–
–
6
–
–
Reactions were run at least in triplicate under air at 25 2 °C, and the data represent an average of these reaction with 10%. The molar ratio
for catalyst–co-catalyst–styrene–oxidant were 1:100:100:200 and [Mn(TPFPP)Cl] = 9 9 10-4. Conversions and yields are determined after a
reaction time of 5 min and results corresponding to 1 min are put in parentheses. The GC conversions (%) to the products and the epoxide yields
(%) were measured relative to the starting alkene
a
The data are obtained by m-CPBA alone
b
Styrene conversion ends after 5 min
123

Rapid and Highly Selective Epoxidation of Styrene
165
to Mn(TPFPP)Cl under these experimental conditions.
Furthermore, in the case of Mn(TPP)Cl the degradation of
the catalyst occurred before the completion of olefin con-
version and the existence of these co-catalysts has no
noticeable effect on the stability of this catalyst [41–43].
Table 2 Styrene epoxidation
co-catalyst in association with Mn(TPFPP)Cl catalyst under different
co-catalyst/catalyst ratios
% for ImH and n-Bu4NOAc as
Ratios Conversion Epoxide Phenylacetaldehyde Epoxide
(%)
(%)
(%)
selectivity (%)
10
100 (93)
100 (86)
100 (76)
100 (42)
100 (27)
44 (32) 56 (61)
57 (33) 43 (53)
98 (37) 2 (39)
44 (34)
57 (37)
3.2 Comparison of Co-catalytic Effects of ImH,
n-Bu4NOAc and n-Bu4NBr on the cis-stilbene
Epoxidation in the Presence of Mn(TPFPP)Cl
20
70
98 (49)
100
150
100 (42)
100 (27)
–
–
100 (100)
100 (100)
Epoxidation of cis-stilbene with m-CPBA catalyzed by
Mn(TPFPP)Cl was performed as a probe [44–46] for better
comparison of co-catalytic activities of imidazole, n-
Bu4NBr and n-Bu4NOAc (Table 4). In the presence of
imidazole, cis-stilbene oxide (conversion = 41%) was
obtained predominantly with only a trace amount of trans-
stilbene oxide. The epoxidation was preceded with lower
stereoselectivity in the presence of both n-Bu4NOAc and
n-Bu4NBr. Cis-stilbene was disappeared completely with
m-CPBA under the reaction conditions and led to similar
cis/trans-stilbene oxide in the presence of n-Bu4NOAc
(92/8) and n-Bu4NBr (96/4). It seems that the active oxi-
dizing species are the same for n-Bu4NOAc and n-Bu4NBr
and are different for imidazole.
All the reaction conditions were the same as those described in
Table 1 in 1 min. The GC conversions (%) or epoxide yields (%)
were measured relative to the starting alkene. The data outside of the
parentheses refer to n-Bu4NOAc co-catalyst and those inside the
parentheses relate to ImH
ratio 100). When ImH was used as co-catalyst, although
increasing the co-catalyst/catalyst ratio improved the
selectivity (34% at co-catalyst/catalyst ratio 10; 100% at
co-catalyst/catalyst ratio 100), but it had a deleterious
effect on styrene conversion (93% at co-catalyst/catalyst
ratio 10; 27% at co-catalyst/catalyst ratio 150).
Therefore we concluded that ImH should have been
oxidized in a competitive reaction at higher concentra-
tions, although, the possibility of the formation of cata-
lytically inactive six-coordinate manganese complex,
[Mn(TPFPP)(ImH)2]?, cannot be ruled out [36–40].
n-Bu4NOAc clearly showed better co-catalytic activity
than ImH at all co-catalyst/Mn(TPFPP)Cl molar ratios
considering both conversion % and selectivity %.
3.3 Active Oxidant
In the preliminary studies on the oxidation of manganese
porphyrins by acylhydroperoxide, Groves described the
observation of a (acylperoxo)manganese(III) complex and
the corresponding several different high valent oxoman-
ganese species at a very low temperature [12]. In our study
for the oxidation of Mn(TPFPP)Cl by m-CPBA, UV–Vis
spectroscopy revealed a different spectrum evolution in the
presence of ImH, n-Bu4NOAc or n-Bu4NBr at room tem-
perature. Addition of ImH (6 9 10-4 mmol) and then m-
CPBA (1.2 9 10-3 mmol) to a solution of Mn(TPFPP)Cl
(6 9 10-6 mmol in 0.5 mL CH2Cl2) showed no change in
Epoxidation of styrene by m-CPBA in association of
Mn(TPP)Cl (TPP = meso-tetraphenylporphyrin) with a
non electron deficient porphyrin ligand was carried out in
the presence of n-Bu4NOAc, n-Bu4NBr and ImH (Table 3).
Although the existence of these co-catalysts improves the
selectivity of epoxidation reaction, especially in the case of
n-Bu4NOAc, they have no considerable effects on the con-
version. Mn(TPP)Cl was less effective catalyst compared
Table 3 Epoxidation of styrene with m-CPBA catalyzed by
Mn(TPP)Cl in the presence of n-Bu4NOAc, n-Bu4NBr and ImH in
CH2Cl2
Table 4 Epoxidation of cis-stilbene with m-CPBA catalyzed by
Mn(TPFPP)Cl in the presence of n-Bu4NOAc, n-Bu4NBr and ImH in
CH2Cl2
Co-catalyst
Conversion
(%)
Styrene
oxide (%)
Epoxide
selectivity (%)
Co-catalyst
Conversion
(%)
Cis-stilbene
oxide (%)
Trans-stilbene
oxide (%)
ImH
50
54
49
53
26
49
26
17
51
90
53
31
ImH
41
100
100
41
92
96
Trace
n-Bu4NOAc
n-Bu4NBr
–
n-Bu4NOAc
n-Bu4NBr
8
4
All the reaction conditions were the same as those described in
Table 1 in 1 min for n-BuN4OAc and 5 min for ImH and n-BuN4Br.
The solvent was removed in vacuo, the products and unreacted alkene
were extracted with n-hexane. The isomer ratios were determined by
1H NMR spectroscopy
All the reaction conditions were the same as those described in
Table 1 in 5 min. The GC conversions (%) or epoxide yields (%)
were measured relative to the starting alkene. Conversions and yields
are determined after a reaction time of 5 min
123

166
Z. Solati et al.
Also ImH could help improving the selectivity of alkene
epoxidation by this catalytic system, but has a negative
effect on the conversion. This catalytic system shows
excellent stability toward its oxidation.
Acknowledgments This work was supported by the Persian Gulf
University Research Council.
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Fig. 1 UV–Vis absorption spectra recorded at room tempreature:
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