336
A. AGHABALI AND N. SAFARI
bond in the intermediate and the formation of metal-oxo
species. This phenomenon leads to better single oxygen
donation to substrates and more selective epoxide forma-
tions [35–37].
An extensive range of oxidants have been used as
oxygen atom transfer reagents to metalloporphyrins,
such as PhIO, H2O2, m-CPBA, hydroperoxides and air,
[5, 19]. Furthermore, particular attention has been drawn
to potassium monopersulfate, oxone [38]. HSO-5 has a
non-symmetrical O-O bond that encourages metal-oxo
formation [9].
Other examinations have shown that solvent can have
a predominant effect on the mechanism and yield of the
catalytic oxidation reactions. In this regard, insertion of
protic solvents like alcohol in the epoxidation reaction
media affects the total yields in catalytic oxidations with
metalloporphyrins. Protic solvents act as general acid
catalysts and facilitate the O-O bond cleavage. Hydrogen
bonding between the alcohol and coordinated oxidant
can also facilitate O-O cleavage and eases the formation
of (Por)M=O species [31, 39–41].
cis-stilbene, styrene, 4-chloro styrene, 4-methoxy styrene,
α-methyl styrene, 1-octene and 1-heptene) were all pur-
chased from Fluka or Merck.
Since commercial potassium salt of oxone is not sol-
uble in organic solvents [44], the corresponding soluble
salt, tetrabutylammonium monopersulfate, was prepared
using the following procedure. Tetra-n-butylammonium
hydrogen sulfate (2.0 mg, 5.9 mmol) was dissolved in
water (20 mL). Potassium monopersulfate (2 g, 6.5 mmol)
was added to this solution and stirred until a colorless
solution was obtained. This product was extracted with
CH2Cl2 (40 mL) and the organic phase was dried on dehy-
drated Na2SO4 and filtered. After the evaporation of the
solvent, the residue was washed with n-hexane (10 mL)
and dried in vacuum. Because of the reducing oxidation
ability of this oxidant and in order to obtain reproducible
results, only freshly produced oxidant was used and it
was kept in a refrigerator.
Tetra-n-butylammonium hydrogen periodate (n-
Bu4NIO4) was also prepared following the procedure in
the literature [45] with some modifications.
The present work reports the results of alkenes epoxi-
dation by n-Bu4NHSO5 with different manganese meth-
oxy porphyrins as the catalysts and the nitrogenous base
of imidazole as the co-catalyst. The effects of the position
and plurality of the methoxy groups on the yields of epox-
idation and the stability of catalyst were investigated. In
the end, we tried to suggest the probable activated form
of porphyrin while adding different amounts of alcohol
to the reaction solution.
Oxidation reactions
Stock solutions of manganese porphyrins (3 × 10-3 M),
imidazole (0.1 M) and alkenes (0.05 M) were prepared
in CH2Cl2. In a 10 mL round-bottom flask the follow-
ing were added in order: alkene (0.05 mmol, 1 mL),
porphyrin catalyst (6 × 10-4 mmol, 0.2 mL), imidazole
(from 10 to 100 mmol) and tetra-n-butylammonium
hydrogen monopersulfate (0.104 mmol, 0.043 g) and the
reaction solution was stirred for an appropriate amount
of time and 0.5 µl of it was injected to GC. To iden-
tify the parent and by-products, authentic samples and
standard solutions were used in internal standard
method.
EXPERIMENTAL
Materials
Free-base porphyrins; tetraphenylporphyrin (TPPH2),
(5,10,15,20-tetrakis(2,4,6-trimethoxyphenyl)porphyrin),
(5,10,15,20-tetrakis(2,3-dimethoxyphenyl)porphyrin),
(5,10,15,20-tetrakis(3,4-dimethoxyphenyl)porphyrin),
(5,10,15,20-tetrakis(2,6-dimethoxyphenyl)porphyrin) and
(5,10,15,20-tetrakis(4-methoxyphenyl)porphyrin) were
prepared using the Lindsey method [42]. Using this
method, distilled pyrrol (7.2 mmol, 0.5 mL) and appropri-
ate methoxy benzaldehyde (1.5 mmol) were added to a
round-bottom flask containing one liter dried CH2Cl2 as the
solvent and equipped with condenser and N2 atmosphere.
Boron trifluoride etherate (75 µl, 0.06 mmol) was added
as the catalyst to the mixture. It was stirred for 24 hours at
room temperature. To oxidize the produced porphyrino-
gen, the solution was refluxed with p-chloranil (5.5 mmol,
1.35 g) for two hours. Porphyrin formation was confirmed
with UV-vis. All the synthesized porphyrins were purified
with neutral alumina column chromatography. Porphy-
rins were metalated by Mn(OAc)2·4H2O according to the
Adler procedure [43]. Imidazole was used as the nitrog-
enous base; related alkenes (cyclooctene, cyclohexene,
Instrumentation
Gas chromatography was performed on a Trace GC
ultra from the Thermo Company equipped with FID
detector and Rtx®-1 capillary column. UV-vis spec-
tra were recorded in CH2Cl2 by a Shimadzu 2100
spectrophotometer.
RESULTS AND DISCUSSION
Alkene epoxidation
The metalloporphyrins presented in Fig. 1 were used
for alkene epoxidation by n-Bu4NHSO5. Due to the sig-
nificant role of the axial base in these reactions, the best
molar ratio of imidazole to catalyst was optimized in the
reaction condition (Fig. 2) and the molar ratio of 80:1 for
imidazole:T(2,3-OMeP)PMnOAc was obtained. To be
assured of the discovered optimum ratio, Im:Cat ratios
were also examined with our other catalysts. Blockage of
Copyright © 2010 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2010; 14: 336–342