Catalytic epoxidation of cis-stilbene and naphthalene by tetra-n-butylammonium hydrogen monopersulfate in the presence of manganese(III) tetraarylporphyrins and various anionic co-catalysts

Article abstract of DOI:10.1142/S1088424611004415

The epoxidation of cis-stilbene by n-Bu₄NHSO₅ was studied in the presence of two different manganese porphyrin complexes, [MnTPFPP(Cl) and MnTPP(Cl)], and various n-Bu₄NX (X = OAc, F, Cl, Br, OCN, NO₃, BF₄). In general direct correlation was found between stereoselectivity of the epoxidation reaction and the nucleophilic properties of these anionic co-catalysts. Also epoxidation of naphthalene was carried out in high yield and good selectivity by n-Bu₄NHSO ₅ in the presence of MnTPFPP(Cl) in association with n-Bu ₄NOAc or n-Bu₄NF co-catalysts.

Full text of DOI:10.1142/S1088424611004415

Journal of Porphyrins and Phthalocyanines
J. Porphyrins Phthalocyanines 2012; 16: 149–153
DOI: 10.1142/S1088424611004415
Published at http://www.worldscinet.com/jpp/
Catalytic epoxidation of cis-stilbene and naphthalene
by tetra-n-butylammonium hydrogen monopersulfate
in the presence of manganese(III) tetraarylporphyrins
and various anionic co-catalysts
Zahra Solati*a, Majid Hashemia, Ahmad Keshavarzia and Ezzat Rafieeb
aChemistry Department, Faculty of Sciences, Persian Gulf University, Bushehr 75168, Iran
bChemistry Department, Razi University, Kermanshah, Iran
Received 31 August 2011
Accepted 16 October 2011
ABSTRACT: The epoxidation of cis-stilbene by n-Bu4NHSO5 was studied in the presence of two
different manganese porphyrin complexes, [MnTPFPP(Cl) and MnTPP(Cl)], and various n-Bu4NX (X =
OAc, F, Cl, Br, OCN, NO3, BF4). In general direct correlation was found between stereoselectivity of
the epoxidation reaction and the nucleophilic properties of these anionic co-catalysts. Also epoxidation
of naphthalene was carried out in high yield and good selectivity by n-Bu4NHSO5 in the presence of
MnTPFPP(Cl) in association with n-Bu4NOAc or n-Bu4NF co-catalysts.
KEYWORDS: co-catalyst, epoxidation, catalysis, mangenese porphyrin.
by manganese porphyrins, an Mn(V)-oxo species
INTRODUCTION
leading to high stereoselectivity and an Mn(IV)-oxo
Metalloporphyrins as a model for cytochrome P450
species leading to non-streoselectivity in the products.
enzymes are known to mediate the oxygen atom transfer
These complexes were characterized by UV-vis and
to alkenes from various oxidants such as hydrogen
1H NMR spectroscopy [33–35]. It was mentioned
peroxide [1–3], periodates [4–8], iodosylarenes [9,
that the involvement of these high-valent manganese-
10], peracids [11, 12], sodium hypochlorite [13, 14],
oxo species as oxidizing intermediates are strongly
potassium monopersulfate [15, 16], tetrabutylammonium
depended on the nature of axial ligand [7, 36, 37]. In
hydrogen monopersulfate [17, 18] in a catalytic way.
most studies, the axial ligands used are neutral ligands,
However the search for developing and designing stable,
mostly imidazoles and pyridines, and there are little
selectiveandefficientoxidizingcatalyticsystemshasbeen
articles about the co-catalytic effects of anionic axial
continuously increasing. The most important strategy to
ligands on the epoxidation reactions of olefins catalyzed
improve this sense is the application of co-catalysts such
by manganese porphyrins [18, 20].
as imidazole, pyridine, acetate, etc. [19–29].
Cis-stibene is an interesting substrate that can be used
to prove the nature of active intermediates in the metal-
catalyzed oxidation reactions [30–32].
It has been shown that two different Mn-oxo
species could be involved as intermediates in the
In this work, we have studied the effects of some
different anionic co-catalysts on the yield and stereo-
selectivity of the epoxidation reaction of cis-stilbene
by tetra-n-butylammonium hydrogen monoper-sulfate
(n-Bu4NHSO5) in the presence of manganese(III)
meso-tetrakis (pentaflurophenyl)porphyrin chloride
[MnTPFPP(Cl)] or manganese(III) meso-tetraphenyl
epoxidation of olefins by different oxidants catalyzed
porphyrin chloride [MnTPP(Cl)]. Also MnTPFPP(Cl)/
n-Bu4NHSO5 was used for the selective epoxidation
of naphthalene in the presence of OAc- and F- anionic
*Correspondence to: Zahra Solati, email: solati@pgu.ac.ir,
co-catalysts.
fax: +98 771-4541494
Copyright © 2012 World Scientific Publishing Company

150
Z. SOLATI ET AL.
then bonding between this ion and the harder donors
are stronger. Perhaps it causes the oxidation reaction
proceeds through Mn(V)-oxo species that leads to the
greater stereoselectivity. Also, Br- may be oxidized by
HSO5- leading to lower conversion.
RESULTS AND DISCUSSION
Table 1 shows the effects of different anionic
co-catalysts on the epoxidation of cis-stilbene by
n-Bu4NHSO5 catalyzed by MnTPFPP(Cl). The oxidation
reaction does not proceed in the absence of a co-catalyst
(entry 1). Presence of non-coordinating or very weak
Table2showstheresultsofoxidationofcis-stilbenewith
n-Bu4NHSO5 catalyzed by MnTPP(Cl) in the presence of
different anionic co-catalysts. Again, the oxidation
reaction does not proceed in the absence of a co-catalyst.
-
-
coordinating anions (BF4 and NO3 ) has no special
effect on the yield and stereoselectivity of the reaction
(entries 2, 3). Coordinating anions (OCN-, OAc-, F-, Cl-,
Br-) have great effects on the yield and stereoselectivity
of the reaction (entries 4, 5, 6, 7, 8). The efficiency of
the epoxidizing system considering both the yield and
stereoselectivity increases in following order: F- > OAc- >
OCN- > Cl- > Br-. The fluoride anion improved the yield
and the stereoselectivity of the reaction more than others
(the conversion is 93% and the cis:trans ratio is 22:1),
OAc- as an oxygen donor ligand (conversion = 98% and
cis:trans ratio = 16) is better than OCN-(conversion =
30% and cis:trans ratio = 8) and Cl-(conversion = 30%
and cis:trans ratio = 5) is better than Br-(conversion =
25% and cis:trans ratio = 2).
-
In the presence of BF4 and NO3-, the MnTPP(Cl) catalyst
is inactive. Epoxidation of cis-stilbene in the presence of
F- resulted in 10% conversion and cis- to trans-stilbene
oxide ratio was 1.2. Epoxidation of cis-stilbene in the
presence of OAc- resulted in 24% yield and cis- to trans-
stilbene oxide ratio was 1. Epoxidation of cis-stilbene
in the presence of OCN- proceeds with 12% yield and
cis:trans ratio was 0.5. When Cl- was used as co-catalyst,
the yield was 10% and cis:trans ratio was 0.1. The yield
of the reaction in the presence of Br- was 9% and trans-
stilbene oxide is obtained as the major product. Again the
results are in accordance to hardness of these co-catalysts,
but because the MnTPP(Cl) is softer than MnTPFPP(Cl),
it seems that proceeding the reaction through Mn(IV)-
oxo species is most probable and leads to greater amount
of trans product, the conversion data are closer together
and because the stability of the catalyst is lower, the
conversions are generally lower.
The epoxidation of polynuclear aromatic hydrocarbons
was demonstrated to be a particularly difficult process,
and the oxidation to the diepoxides was only obtained
with peroxyacids in very low yields [28, 38, 39].
Cavaleiro and coworkers described the formation of
diepoxides from naphthalene and anthracene in high
yields using hydrogen peroxide as oxidant in the presence
Interpreting the trends in the yield and the
stereoselectivity of the reaction based on the values of
intrinsic basicity of these coordinating anions (pka values)
is unsuccessful. But considering the trends in the hardness
of these anions, there is a great correlation between the
hardness of these coordinating anions and the yield
and the stereoselectivity of the oxidation reaction. The
most electronegative species (the hardest one), F-, gives
the most cis:trans ratio while the least electronegative
species (the softest one), Br-, gives the least cis:trans
ratio. According to symbiosis principle, existence of
fluoride substituents on phenyl rings of MnTPFPP(Cl)
makes Mn(III) in the center of the complex harder,
Table 1. Epoxidation of cis-stilbene with n-Bu4NHSO5 catalyzed by MnTPFPP(Cl) in the presence
of various anionic co-catalysts in CH2Cl2
Entry
Co-catalyst
pka
Conversion, % Cis, % Trans, % Cis/Trans Time, min
1
2
3
4
5
6
7
8
—
—
—
—
30
98
93
30
25
—
—
—
10
10
10
10
5
n-Bu4NBF4
0.5
n-Bu4NNO3 -1.3
n-Bu4NOCN
n-Bu4NOAc
n-Bu4NF
3.7
4.76
3.17
-6.3
-8.7
24
80
89
25
17
3
5
4
5
8
8
16
22
5
5
n-Bu4NCl
n-Bu4NBr
10
10
2
Reactions were run at least in triplicate under air at 25 ± 2 °C, and the reported values are the
average of the measured values. The molar ratio for catalyst:co-catalyst:substrate:n-Bu4NHSO5
is 1:20:100:190. 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. The values
for pka are for the corresponding acids (HA).
Copyright © 2012 World Scientific Publishing Company
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CATALYTIC EPOXIDATION OF CIS-STILBENE AND NAPHTHALENE
151
Table 2. Epoxidation of cis-stilbene with n-Bu4NHSO5 catalyzed by MnTPP(Cl) in the
presence various anionic co-catalysts in CH2Cl2
Entry
Co-catalyst
Conversion, % Cis, % Trans, % Cis/Trans Time, min
1
2
3
4
5
6
7
8
—
—
—
—
—
4
—
—
10
10
10
10
10
10
10
10
n-Bu4NBF4
n-Bu4NNO3
n-Bu4NOCN
n-Bu4NOAc
n-Bu4NF
12
24
10
10
9
8
12
4
0.5
12
6
1
1.2
n-Bu4NCl
n-Bu4NBr
1
9
0.1
trace
9
Only trans
Reactions were run at least in triplicate under air at 25 2 °C, and the reported values
are the average of the measured values. The molar ratio for catalyst:co-catalyst:substrate:
n-Bu4NHSO5 is 1:20:100:190. 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.
of manganese porphyrins and ammoniumacetate. They
suggested the involvement of an oxo species [Mn(V)-
oxo] in the reaction pathway [28, 40].
shift could relate to the displacement of Cl- counter
ion by fluoride and stronger interaction of fluoride
with metal center and perhaps to the formation of a six
coordinate complex [MnTPFPP(F)2]-. Upon addition
of n-Bu4NHSO5 to this solution, this Soret band
spectrum (437 nm) was subsequently transformed
within 100 s into a new band at about 430 nm. The
band is assigned to an Mn-oxo intermediate species
[41]. In the absence of alkene substrate, this species
was very stable. However, upon addition of an excess
of alkene, this Soret band decayed quickly and
concomitantly the Soret band at 454 nm increased
(Fig. 1).
We used MnTPFPP(Cl)/n-Bu4NHSO5 oxidizing
system in the presence of OAc- and F- anions to check
its efficiency and its selectivity on the oxidation of
naphthalene. As the results in Table 3 show it can
epoxidize naphthalene to diepoxy naphthalene with high
efficiency and selectivity in 1 h.
To check the probability of coordination of these
anions to Mn center and involvement of Mn-oxo species
in the reaction pathway, we followed the variations in
the UV-vis spectra of the manganese porphyrins during
the epoxidation reactions. Involvement of an Mn-oxo
species in the epoxidation of alkenes by n-Bu4NHSO5 in
the presence of MnTPFPP(Cl) catalyst and n-Bu4NOAc
has been already reported [18].
Addition of tetrabutylammonium salts of C1-, Br-,
OCN- to a CH2Cl2 solution containing MnTPFPP(Cl)
caused no observable change in the UV-vis spectrum.
Addition of n-Bu4NHSO5 to this solution caused a slowly
decrease in the Soret band at 474 nm with concomitant
increase in the absorbance at 418 nm ascribed to Mn-oxo
species.
By addition of a dichloromethane solution of
n-Bu4NF to the solution of MnTPFPP(Cl) in CH2Cl2,
the Soret band at 474 nm shifted to 437 nm. This blue
Table3.Epoxidationofnaphthalenewithn-Bu4NHSO5 catalyzedbyMnTPFPP(Cl)
in the presence of n-Bu4NOAc and n-Bu4NF in CH2Cl2 at room temperature
Entry
Co-catalyst Conversion, % Diepoxy, % Diepoxy selectivity, %
1
2
n-Bu4NOAc
n-Bu4NF
97
70
89
54
92
77
The molar ratio for catalyst:co-catalyst:substrate:n-Bu4NHSO5 is 1:100:100:200.
The solvent was removed in vacuo, the products and unreacted naphthalene
were extracted with n-hexane. Characterization of the reaction products and
determination of substrate conversion (based on starting naphthalene) and product
selectivity carried out by 1H NMR and GC-MS spectroscopy.
Copyright © 2012 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2012; 16: 151–153

152
Z. SOLATI ET AL.
CONCLUSION
The coordinating strength of an anionic
co-catalyst could have a great influence on the
yields and selectivities of oxidation reactions
of olefins with n-Bu4NHSO5 catalyzed by
manganese porphyrins. The hardest anionic
axial ligand bond the metal center stronger
than others, leading to higher conversion
and greater stereoselectivity in the reaction.
The importance of the effects of anionic
co-catalysts is more pronounced when their
relative activities are examined in the presence
of manganese porphyrins with electron
withdrawing substituents on the porphyrin
Fig. 1. UV-vis absorption spectra recorded at room temperature: (a) for
MnTPFPP(Cl) (6 × 10-6 mmol) in CH2Cl2 (0.5 mL); (b) after the addition of periphery.
n-Bu4NF (6 × 10-4 mmol); (c) after the addition of n-Bu4NHSO5 (1.2 × 10-3
mmol) to this mixture; (d) immediately after the addition of alkene (6 × 10-3
Acknowledgements
mmol) to the last mixture
This work was supported by the Persian
Gulf University Research Council.
EXPERIMENTAL
REFERENCES
The free-base porphyrins TPPH2 [42], TPFPPH2
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MnTPP(Cl) was synthesized using MnCl2·4H2O
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