Efficient oxygenation of hydrocarbons with tetrabutylammonium monopersulfate catalyzed by manganese meso-tetraphenylporphyrin in the presence of imidazole

Article abstract of DOI:10.1016/S0040-4039(02)00110-7

Oxidation of hydrocarbons, into their corresponding epoxide, alcohol and/or ketone, was achieved with tetrabutylammonium monopersulfate in the presence of manganese(III) meso-tetraphenylporphyrin catalyst and axial imidazole ligand in CH₂Cl₂ in low to very high yields (13-100%) and good to excellent selectivities (73-100%) in less than 5 min at room temperature (～25°C).

Full text of DOI:10.1016/S0040-4039(02)00110-7

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 43 (2002) 1881–1884
Efficient oxygenation of hydrocarbons with tetrabutylammonium
monopersulfate catalyzed by manganese
meso-tetraphenylporphyrin in the presence of imidazole
Daryoush Mohajer* and Abdolreza Rezaeifard
Department of Chemistry, Shiraz University, Shiraz 71454, Iran
Received 8 October 2001; revised 2 January 2002; accepted 14 January 2002
Abstract—Oxidation of hydrocarbons, into their corresponding epoxide, alcohol and/or ketone, was achieved with tetra-
butylammonium monopersulfate in the presence of manganese(III) meso-tetraphenylporphyrin catalyst and axial imidazole ligand
in CH Cl2 in low to very high yields (13–100%) and good to excellent selectivities (73–100%) in less than 5 min at room
2
temperature (ꢀ25°C). © 2002 Elsevier Science Ltd. All rights reserved.
The challenging selective oxyfunctionalization of
hydrocarbons with synthetic models of cytochrome P-
GLC and the identity of the products were confirmed
1
by IR and H NMR spectral data.
4
50 under mild conditions has been of extreme interest
1–6
to chemists.
To achieve this goal a number of
This catalytic system is very efficient for the epoxida-
tion of alkenes in very short times (Table 1). It leads to
95% conversion of cyclooctene with formation of epox-
ide (92%) in less than 1 min at room temperature.
Styrene is 100% converted within 1 min with 95% yield
of epoxide. Similarly, conversion and yield of cyclohex-
ene are 100 and 86%, respectively. It seems that the
efficiency of oxidation in this catalytic system is very
dependent on the steric requirements of the substrates.
Epoxidation of trans-stilbene proceeds with absolute
stereospecificity and the epoxide yield is 55%. Whereas
epoxidation of cis-stilbene is quantitative (100%) and
leads to a mixture of cis-stilbene oxide (84%) and
trans-stilbene oxide (16%) in the same period. The
lower activity of trans-stilbene relative to the cis-stil-
bene seems to be related to the greater phenyl–phenyl
non-bonded interactions between the catalyst and
biomimetic oxygenation systems composed of a variety
of oxygen donors such as PhIO, NaOCl,
and periodates,
metalloporphyrin catalysts have been employed. How-
7,8
9,10
11–13
H O
2 2,
1
4–18
in combination with different
ever, despite the promising employment of KHSO5
®
(
Oxone , 2KHSO ·KHSO ·K SO ) in the oxygenation
5
4
2
4
of hydrocarbons in the presence of manganese por-
1
9–21
phyrins,
attempts at using Bu NHSO (TBAO) as
4 5
an oxygen source in association with porphyrin cata-
lysts were unsuccessful.
22
In this report, we describe for the first time the effective
activation of TBAO with simple Mn(TPP)OAc (TPP=
meso-tetraphenylporphyrin dianion) as a single oxygen
donor for oxygenation of some hydrocarbons in the
presence of imidazole (Im) in CH Cl . The general
2
2
7
23
trans-stilbene. Formation of trans-stilbene oxide in the
procedure for oxidation consisted of adding TBAO
0.3 mmol) to a solution containing hydrocarbon (0.6
mmol), Mn(TPP)OAc (0.006 mmol) and imidazole
0.12 mmol) in CH Cl (2 ml). The solution was stirred
oxidation of cis-stilbene suggests a competition between
closure of the epoxide ring and rotation around the
CꢀC bond during the oxygen transfer step from the
active oxidizing species to the alkene. Interestingly, in
the oxidation of cis-stilbene with sterically more
demanding Mn(TDCPP)OAc (TDCPP=meso-tetrakis
(
(
2
2
at a constant speed, under air, at room temperature.
The consumption of the starting hydrocarbons and
formation of oxidation products were monitored by
(
2,6-dichlorophenylporphyrin) dianion) catalyst,
a
higher yield of cis-stilbene oxide (97%) is obtained, thus
reflecting the importance of the steric effects of the
catalyst. Furthermore, it would be expected that the
electron rich 1-methylcyclohexene would be more active
towards epoxidation, than cyclohexene. However, a
much longer reaction time for the former (5 min) than
Keywords: hydrocarbons; oxidation; tetrabutylammonium monoper-
sulfate; manganese; catalysis; porphyrin.
24
*
Corresponding author. Tel.: 0711-2284822; fax: 0711-2280926;
e-mail: dmohajer@chem.susc.ac.ir
0
040-4039/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(02)00110-7

1
882
D. Mohajer, A. Rezaeifard / Tetrahedron Letters 43 (2002) 1881–1884
a
Table 1. Epoxidation of alkenes with TBAO catalyzed by Mn(TPP)OAc in the presence of Im
the latter (<1 min) again confirms the influence of steric
effects in this catalytic system. Epoxidation of the ter-
minal alkene (oct-1-ene) proceeded in moderate yield
oxygenation of cyclohexene were explored (Table 3).
Our results demonstrate that imidazole is by far the
best co-catalyst in comparison with pyridines in terms
of conversions and epoxide yields.
(
30%) and good selectivity (73%), compared to the
oxygenation of the other alkenes. The present catalytic
system allows the oxygenation of various saturated
CꢀH bonds to a mixture of their corresponding alco-
hols and ketones (except for ethylbenzene which pro-
duces only acetophenone), with combined yields of
An increase in the Im/Mn-porphyrin molar ratio up to
20 remarkably improved the rate of conversion and
selectivity of cyclohexene epoxidation (Table 4). How-
ever, a further increase in the ratio did not affect the
results.
12–72% and excellent selectivities (100%), in less than 3
min at room temperature (Table 2). In all these oxida-
tions much higher yields of ketones rather than alco-
hols were achieved.
In conclusion, the low to excellent yields and good to
very high selectivities of hydrocarbon oxidations
achieved within a few minutes using the homogeneous
TBAO–Mn(TPP)OAc–Im system in CH Cl at room
temperature, make this simple biomimetic system a very
convenient method for rapid monooxygenation of
hydrocarbons from a synthetic point of view. A notable
In a similar manner to other biomimetic porphyrin
catalytic systems, the presence of an axial ligand dra-
2
2
25
matically improves the efficiency of the oxidations.
The effects of three commonly used axial ligands upon

D. Mohajer, A. Rezaeifard / Tetrahedron Letters 43 (2002) 1881–1884
1883
a
Table 2. Hydroxylation of saturated hydrocarbons with TBAO catalyzed by Mn(TPP)OAc in the presence of Im
a
Table 3. Epoxidation of cyclohexene with TBAO catalyzed by Mn(TPP)OAc in the presence of axial ligands
Axial Ligand
Conversion (%)b
Epoxide yield (%)b
Selectivity (%)
Imidazole
Pyridine
93.5
66.5
12
77
42
8
82
63
67
4
-tert-Butylpyridine
None
13.8
2
14.5
a
All reactions were run at room temperature for 5 min. The molar ratio for alkene:oxidant:axial ligand:Mn(TPP)OAc was 100:100:10:1.
GLC yields are based on the starting alkenes.
b
a
Table 4. Effect of various Im/Mn(TPP)OAc molar ratios on cyclohexene epoxidation rate and selectivity by TBAO
Im/Mn(TPP)OAc Ratio
Conversion (%)b
Epoxide yield (%)b
Selectivity (%)
c
1
22.6
13
77
86
86
57.5
82
86
c
1
2
1
0
93.5
d
0
100
d
00
100
86
a
b
c
All reaction conditions were the same as described in Table 3 except for different Im/Mn(TPP)OAc ratios.
GLC yields are based on the starting alkenes.
Reactions were run for 5 min.
d
Reaction times were <1 min.
feature of this catalytic system is its relative stability
Research Council of Iran (NRCI) under the grant
number 844, and by the Shiraz University Research
Council.
2
6,27
towards oxidative degradation.
While the total
turnover numbers for epoxidation of styrene and
cyclohexene are 420 and 460, respectively, the lower
yields of oxidation products with the less reactive
hydrocarbons were due to competing destruction of
the catalyst. Further work with other Mn-porphyrin
catalysts is under way in this area.
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