Oxidation of alkanes catalyzed by manganese(III) porphyrin in anionic liquid at room temprature

Article abstract of DOI:10.1016/j.tetlet.2003.09.225

Efficient oxidation of alkanes is achieved by using an electron-deficient manganese(III) porphyrin catalyst in combination with iodobenzene diacetate in an ionic liquid at room temperature; a high-valent manganese-oxo porphyrin complex (Mn^V=O) was considered as the reactive oxidation intermediate.

Full text of DOI:10.1016/j.tetlet.2003.09.225

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 44 (2003) 9229–9232
Oxidation of alkanes catalyzed by manganese(III) porphyrin in
an ionic liquid at room temperature
Zhen Li, Chun-Gu Xia* and Chuan-Zhi Xu
State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics,
Chinese Academy of Sciences, Lanzhou 730000, China
Received 2 June 2003; revised 12 August 2003; accepted 22 September 2003
Abstract—Efficient oxidation of alkanes is achieved by using an electron-deficient manganese(III) porphyrin catalyst in combina-
tion with iodobenzene diacetate in an ionic liquid at room temperature; a high-valent manganese-oxo porphyrin complex
(MnVꢀO) was considered as the reactive oxidation intermediate.
© 2003 Elsevier Ltd. All rights reserved.
In view of today’s environmental consciousness, the use
of environmentally benign reaction mediums is very
important. In this connection, room temperature ionic
liquids that are air and moisture stable have received in
recent years a good deal of attention as potential
cleaner solvents.1–3 More recently, it has been reported
that classical organic oxidation can be performed in
ionic liquids with great advantages (yield and selectiv-
ity) as compared to those performed in organic sol-
vents.4,5 We have described the epoxidation of several
alkenes catalyzed by a manganese porphyrin 1, carrying
four pentafluorophenyl substituents, in an ionic liquid
[bmim]PF6 at room temperature.6 Epoxidation, using
iodobenzene diacetate [PhI(OAc)2] as the oxygen
source, of various alkenes led to the corresponding
epoxides with good to excellent yields. PhI(OAc)2,
which is soluble in most organic solvents, safe to use,
and commercially available, has been employed as an
efficient oxidant in metal complex-catalyzed oxidation
reactions.7,8 Since the oxidation of saturated hydrocar-
bons under mild reaction conditions is of importance in
both synthetic chemistry and industrial processes,
biomimetic catalysts based on metalloporphyrins asso-
ciated with various oxidants have been used as
cytochrome P-450 models and their potential for satu-
rated hydrocarbons oxidation has received considerable
interest in recent years.9–12 In this paper, we hope to
further report the development of this MnTFPPCl-
[bmim]PF6 catalyst system for the oxidation of the inert
carbonꢁhydrogen bond of alkanes.
a
Table 1. Catalytic oxidation of alkanes with metalloporphyrins and PhI(OAc)2 in [bmim]PF6/CH2Cl2
Entry
Substrate
Axial ligandc
Conv. (%)
Products yield (%)b
Ketone
Alcohol
1
2
3
4
2a
2b
5
–
38 (9)
81
56 (16)
57
96 (90)
91
90 (9)
55
24 (2)
36
49 (3)
51
1 (31)
49
90 (7)
54
13 (7)
45
7 (13)
5
95 (59)
42
<1 (2)
<1
Imidazole
–
Imidazole
–
Imidazole
–
Imidazole
8
a All reactions were performed in capped vials in 2.5 mL of [bmim]PF6/CH2Cl2 (3/2, v/v) using a 225/150/1 oxidant/substrate/catalyst ratio.
b The yields are based on the substrate and are determined by GC. Results obtained in the absence of ionic liquid are given in parentheses.
c Molar ratio of axial ligand to catalyst is 50/1.
Keywords: oxidation; ionic liquid; manganese porphyrin.
* Corresponding author.
0040-4039/$ - see front matter © 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2003.09.225

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Z. Li et al. / Tetrahedron Letters 44 (2003) 9229–9232
of the catalyst is very accessible to organic substrates
because of the non-coordination of [bmim]PF6. Fur-
thermore, ionic liquids are generally chemically inert
towards both catalysts and reactive intermediates,
meaning that catalyst stability is not a problem. Unfor-
tunately, up until recently, the nature of interactions
between ionic liquids and different types of solute is not
clear.
It is well known that the catalytic ability of manganese
porphyrins can be improved by the use of nitrogen
bases as an axial ligand.13,14 To study the effect of an
axial ligand on the oxidation of alkanes catalyzed by 1
in [bmim]PF6/CH2Cl2 with PhI(OAc)2, we chose imida-
zole as the axial ligand. The results of Table 1 showed
that the conversion was greatly dependent on the sub-
strate structure. It is clear that the yield of hydroxyla-
tion products was improved in the presence of
imidazole in the case of cyclohexane and cyclooctane.
The proportions of alcohol and ketone were found to
be 4:5 for cyclohexane and 10:1 for cyclooctane. This is
because the ligand favors the formation of the high-
valent oxo-manganese species, OMnV(P), which is
responsible for oxygen insertion into the substrates.
However, in the case of adamantane and tetra-
hydronaphthalene, the conversion was reduced, espe-
cially for the oxidation of adamantane. This happens
probably due to the fact that the approach of the bulky
adamantane to the active site of the catalyst is difficult
in the presence of imidazole.
The catalytic oxidation of alkanes by
1 using
PhI(OAc)2 as the oxidant was carried out in
[bmim]PF6/CH2Cl2 (3:2, v/v) at room temperature. The
results of the oxidation of alkanes were summarized in
Table 1. It was shown that 1 functioned as a good
catalyst in the reaction of PhI(OAc)2 with various
alkanes such as cyclohexane (2a), cyclooctane (2b),
tetrahydronaphthalene (5) and adamantane (8). Trans-
formation of alkanes to their corresponding alcohols
and ketones was found to proceed with moderate to
very high yields depending on the alkane structures
(Fig. 1).
Cyclohexane was oxidized to cyclohexanol (3a) and
cyclohexanone (4a) in yields of 24% and 13% after 2 h
under identical conditions. In the oxidation of cyclooc-
tane (2b) and adamantane (8), alcohols were yielded as
the major products with the formation of small
amounts of ketone products (Table 1, entries 2 and 4).
In the case of tetrahydronaphthalene (5), the main
product detected was ketone (7) (Table 1, entry 3).
One of the primary aims of our study was to investigate
the recycling of the catalyst 1. Clearly this is important
in the context of economic feasibility and sustainable
development. Thus, at the end of the reactions the
products were removed from the catalytic system by
extraction with n-hexane and the recovery and recy-
clability of the brown–red oily ionic liquid phase con-
taining the catalyst were examined. The catalyst was
tested for its activity in the oxidation of cyclohexane
and cyclooctane using PhI(OAc)2 as oxidant leading to
five recycling steps without decrease of activity. More-
over, no manganeseporphyrin was detected in the
extracted solution when analyzed by UV–vis
spectroscopy.
The catalysis in the absence of the ionic liquid, gave
much lower conversions (between 9 and 16%, see values
in parentheses in Table 1) except for tetrahydro-
naphthalene in which 90% conversion was obtained.
As for the role of ionic liquids played during the
catalytical oxidation, we considered that the active site
The UV–vis analysis revealed that the Soret band of
metalloporphyrin
1
in mixed [bmim]PF6/CH2Cl2
showed a blue shift relative to the Soret band at 474 nm
as obtained in CH2Cl2. This change could be attributed,
in part, to the possibility that [bmim]PF6 is more dipo-
lar than CH2Cl2. The reaction of 1 with PhI(OAc)2 in
[bmim]PF6/CH2Cl2 and CH2Cl2 was also monitored by
UV–vis spectroscopy (Fig. 2). In CH2Cl2, addition of
PhI(OAc)2 to a solution containing 1 caused immediate
generation of a new species with a broad Soret band at
422 nm indicating the formation of a MnIVꢀO deriva-
tive, as shown in Figure 2c.15–17 This derivative was
very stable and unchanged even after one day. When
cyclooctane was added to the solution, after 2 h, GC
analysis of the reaction mixture showed little formation
Figure 1. The oxidation of alkanes by PhI(OAc)2 catalyzed by
1 in [bmim]PF6/CH2Cl2.

Z. Li et al. / Tetrahedron Letters 44 (2003) 9229–9232
9231
According to the above discussion, we assumed that in
mixed [bmim]PF6/CH2Cl2 the oxygen atom transfer
occurs via the in situ formation of the oxo-manganese
complex (MnVꢀO). In order to confirm the proposed
MnVꢀO reactive intermediate in this oxidation reaction,
the reaction of MnTFPPCl with PhI(OAc)2 (molar
ratio=1:225) in mixed [bmim]PF6/CH2Cl2 was ana-
lyzed by stopped-flow spectrophotometry at 20°C (Fig.
3). An intermediate with the characteristic Soret maxi-
mum at ca. 428 nm could be observed 5 s after mixing
by difference spectroscopy with clear isosbestic points
at 447 and 384 nm. Then the Soret band of MnTFPPCl
at 458 nm was fully restored. The band at 428 nm has
been reported in the literature and was assigned to a
MnVꢀO intermediate species.18–20
In conclusion, we have shown for the first time that an
electron-deficient manganese(III) porphyrin complex
was able to catalyze the oxidation of alkanes by
PhI(OAc)2 in an ionic liquid at room temperature. The
use of an ionic liquid should provide a way for a simple
recycling of the catalyst. By the stopped-flow spec-
trophotometry we observed a higher active and shorter
life-time intermediate MnVꢀO during the reaction of 1
with PhI(OAc)2 in mixed [bmim]PF6/CH2Cl2. However,
in CH2Cl2 the same reaction only produced a less active
intermediate MnIVꢀO.
Figure 2. (a) UV–vis spectrum of manganese porphyrin 1 in
[bmim]PF6/CH2Cl2 (3/2, v/v). (b) In CH2Cl2. (c) Spectrum of
1 in the presence of PhI(OAc)2 in CH2Cl2.
of oxidation products. On the other hand, for a solu-
tion of 1 in [bmim]PF6/CH2Cl2, no change in the spec-
trum was observed after the addition of PhI(OAc)2 with
the Soret band at 458 nm being constant and no new
absorption appearing. However, in the presence of
cyclooctane the catalytic system gave excellent yields.
Apparently an active oxidizing species was formed
under the experimental conditions but not detected due
to its higher reactivity and shorter life time.
Acknowledgements
This work was supported by the National Natural
Science Foundation of China (29773056/29933050).
Figure 3. (a) Spectrum of manganese porphyrin 1 (24 mM) in [bmim]PF6/CH2Cl2 (3/2, v/v). (b) Spectrum of manganese porphyrin
1 in 5 s upon addition of PhI(OAc)2 (5400 mM) in [bmim]PF6/CH2Cl2 (3/2, v/v). Inset: Difference spectra define for the first 5 s
of the same reaction.

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Z. Li et al. / Tetrahedron Letters 44 (2003) 9229–9232
11. Nam, M.; Kim, I.; Kim, Y.; Kim, C. Chem. Commun.
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