Manganese dioxide and N-hydroxyphthalimide. An effective catalytic system for oxidation of nitrotoluenes with molecular oxygen

Article abstract of DOI:10.1002/adsc.200700183

The inexpensive manganese dioxide has been proven to be an efficient auxiliary for oxidizing N-hydroxyphthalimide (NHPI) to form the phthalimide N-oxyl radical via reduction and reoxidation. The combination of manganese dioxide and NHPI could catalyze effectively the oxidation of nitrotulenes by molecular oxygen. Thus, the oxidation of p-nitrotoluene with molecular oxygen (0.4 MPa) in the presence of manganese dioxide (10 mol%) and NHPI (10 mol%) in acetic acid at 110° for 4 h proceeded with 97 % conversion, and gave p-ni-trotoluene in 89 % isolated yield.

Full text of DOI:10.1002/adsc.200700183

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DOI: 10.1002/adsc.200700183
Manganese Dioxide and N-Hydroxyphthalimide. An Effective
Catalytic Systemfor Oxidation of Nitrotoluenes with Molecular
Oxygen
a,
a
a
a
Guanyu Yang, * Liwen Zheng, Guanghui Wu, Xuesong Lin,
a,
and Maoping Song *
Department of Chemistry, Zhengzhou University, Zhengzhou, 450052, Peopleꢀs Republic of China
a
Fax : (+86)-371-6776-3927; e-mail: yangguanyu@zzu.edu.cn
Received: April 14, 2007
Abstract: The inexpensive manganese dioxide has
been proven to be an efficient auxiliary for oxidiz-
ing N-hydroxyphthalimide (NHPI) to form the
phthalimide N-oxyl radical via reduction and reoxi-
dation. The combination of manganese dioxide and
NHPI could catalyze effectively the oxidation of ni-
trotulenes by molecular oxygen. Thus, the oxidation
of p-nitrotoluene with molecular oxygen (0.4MPa)
in the presence of manganese dioxide (10 mol%)
and NHPI (10 mol%) in acetic acid at 1108C for
sequently, there are many methods reported of
NHPI-catalyzed oxygenations, however, PINO gener-
ation always relies on the assistance of various auxil-
iaries (Scheme 1). The reported auxiliaries could in-
clude generally two sorts: the cocatalyst and the radi-
[
5]
cal initiator. The transition metal ions, including
[
2a]
[6]
[7]
[8]
Co, Mn, V, Ce, etc., are well-documented co-
catalysts. Notably, the combination of NHPI with Co
or with Co and Mn together affords an effective cata-
lytic system, known as the Ishii system, for the au-
tooxidation of a broad range of organic substrates.
For example, the oxidation of toluene in the presence
4
h proceeded with 97% conversion, and gave p-ni-
trotoluene in 89% isolated yield.
of NHPI (10 mol%) and Co(OAc) (0.5 mol%) in
A
H
R
U
G
2
Keywords: aerobic oxidation; catalysis; N-hydroxy-
phthalimide; manganese dioxide; nitrotoluene
acetic acid afforded benzoic acid with 96% selectivity
in 84% conversion even at ambient temperature and
[
2a,b]
pressure.
Based on the assumption that redox-
active organic compounds could oxidize NHPI to
PINO, we found that anthraquinones can serve as
purely organic cocatalysts for NHPI-catalyzed oxy-
[
9a,b]
The oxyfunctionalization of alkylarenes with molecu- genations.
On the other hand, some radical initia-
[
10]
[11]
lar oxygen as an ultimate oxidant is a key technology tors, such as alkyl hydroperoxide, AIBN, and al-
[
12]
for converting petroleum-based feedstocks to useful dehyde, can also mediate the formation of PINO in
primary and special chemicals. The success of these the NHPI-based oxidation, although they are con-
oxygenations always depends largely on the develop- sumed during the reactions and are not recoverable.
ment of catalysts to promote productivity, the rate of
The investigations revealed that the methyl groups
[
1]
reaction, as well as the selectivity of products. Due on nitroarenes, due to the strong electron-withdraw-
to its very highly catalytic efficiencies even under ing effect of the nitro substituent, are very difficult to
mild conditions in the aerobic oxidation of a wide va-
riety of organic substrates, N-hydroxyphthalimide
[
2]
(
NHPI) is attracting continuous attention. The gen-
eral consensus for the NHPI-catalysis mechanism is
that the one-electron oxidation of NHPI to the phtha-
[
3]
limide N-oxyl radical (PINO) must take place first.
The formed PINO abstracts the hydrogen of a CÀH
bond in organic substrates to produce carbon-cen-
tered radicals, and then propagates the autoxidation.
[
4]
In 1983, Masui et al. had documented that the oxi-
dation of NHPI to PINO occurs on the anode when
they employed NHPI as mediator in the electrochem-
ical oxidation of secondary alcohols to ketones. Sub- Scheme 1. NHPI-catalyzed aerobic oxidation.
Adv. Synth. Catal. 2007, 349, 2445 – 2448
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2445

Guanyu Yang et al.
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be oxidized with O . For example, using the wide- NHPI to PINO, by acting as a radical initiator, and
2
used Amoco MC method (cobalt salt, manganese salt also serve as a cocatalyst for NHPI-catalyzed aerobic
and bromide ion as the catalysts and acetic acid as a oxidation. Hence, it is of interest to suppose that the
solvent), the aerobic oxidation of nitrotoluenes to ni- double function of MnO would enhance the efficien-
2
trobenzoic acids was carried out only under high pres- cy of NHPI catalysis. Herein, selecting the more inert
sure of air and higher reaction temperature (around nitrotoluenes as substrates, we investigate the perfor-
[
13]
1
758C). Despite the fact that the Ishii system dis- mance of the inexpensive MnO in combination with
2
[
14]
played more efficiency in this oxidation, affording NHPI in the aerobic oxidation.
p-nitrobenzoic acid in 78% yield when 10 mol% The initial experiment was carried out using p-ni-
NHPI, 0.5 mol% Co(OAc) and 0.05 mol% Mn- trotoluene (8 mmol) as a test substrate with 10 mol%
(OAc) were employed as catalytic system in acetic NHPI and 20 mol% MnO in acetic acid (20 mL)
AHCTREUNG
2
ACHTREUNG
2
2
acid solution at 1308C, the reaction time still needed under 0.3MPa of dioxygen at 1008C for 1 h, affording
as long as 14 h and a pressure as high as 1.0MPa of 58% conversion and 46% isolated yield of p-nitrobe-
air. Under the same conditions, the oxidation of o-ni- zoic acid (Table 1, entry 1). In sharp contrast, the re-
trotoluene gave as little as 3% conversion. An inter- action could not occur by using MnO alone. And
2
esting find by Ishii is that the addition of NO (20 NHPI alone, realizing 37% conversion, was of low ef-
2
mol%) into the oxidation obviously promoted the ficiency. Prolonging the reaction time to 6 h increased
[
14]
catalytic efficiency of the Ishii system. Using NO2, the conversion to 85% and the isolated yield to 77%
with Co(OAc) and Mn(OAc) together, the oxidation with MnO /NHPI (entry 4). Furthermore, optimiza-
A
C
H
T
R
E
U
N
G
ACHTREUNG
2
2
2
of o-nitrotoluene gave 52% conversion and 46% tion experiments showed that the catalytic system
yield of o-nitrobenzoic acid at 1508C; without NO2 could exhibit much better performance. Using 10
the reaction gave only 20% conversion and 11% mol% NHPI and 10 mol% MnO at 1108C under
2
yield. The author suggested that NO would smoothly 0.4MPa for 4 h, the conversion reached 97% and the
2
abstract the hydrogen atom from NHPI to form isolated yield reached 89% (entry 7). Comparably,
PINO in the first step, playing a similar role to a radi- Co/Mn/NHPI gave 76% conversion and 65% isolated
cal initiator, and then was reoxidized with dioxygen yield at 1108C under 1.0MPa of air for 14 h with a
[
14]
to accomplish the redox cycle, acting as a cocatalyst. same loaded amount of NHPI. To probe further its
This double function means that when cocatalyst and potential, the new catalyst system was employed to
radical initiator are used together, the NHPI catalysis catalyze aerobic oxidations of o- and m-nitrotoluenes.
should be more powerful.
The oxidation of m-nitrotoluene also gave an excel-
It is known that manganese oxides could be em- lent result, affording 96% conversion and 88% the
ployed as oxidation catalysts based on two transfor- isolated yield (entry 9). Unfortunately, the conversion
mations: the reduction of Mn species of higher va- of o-nitrotoluene reached only 19% (entry 10). o-Ni-
lence to lower ones and the reoxidation of the lower trotoluene is known to be reluctant to oxidation due
[
15,16]
ones to the higher ones by dioxygen.
Further- to the steric hindrance and the strong deactivation of
more, a recent study revealed that MnO could stoi- the adjacent bulky NO . At 1308C and 1508C under
2
2
chiometrically oxidize hydroxylamines to the corre- 1.0MPa for 14 h the conversion with Co/Mn/NHPI
sponding nitrones with good to excellent yields even was as low as 3% and 20%, respectively; An interest-
[
17]
in an ice-cooled solution.
These observations in- ing fact was that the conversions could be promoted
spired us to suggest that MnO could in situ oxidize to 19% at 1308C and 25% at 1508C, respectively, by
2
Table 1. Aerobic oxidation of nitrotoluenes with MnO and NHPI in acetic acid.
2
Entry
Nitrotoluene
NHPI
mol%]
MnO2
[mol%]
T
[ C]
Time
[h]
P
Conversion
[%]
Isolated Yield
[%]
o
[
[MPa]
1
2
3
4
5
6
7
8
9
p-
p-
p-
p-
p-
p-
p-
p-
m-
o-
o-
10
0
10
10
0
20
10
0
20
20
10
10
7
100
100
100
100
100
110
110
110
110
110
150
1
1
1
6
6
4
4
4
4
0.3
0.3
0.3
0.3
0.3
0.4
0.4
0.4
0.4
0.4
1.0
58
0
37
85
0
94
97
87
96
19
37
46
–
–
77
–
87
89
77
88
–
7
10
10
10
10
10
10
10
10
10
1
4
12
1
33
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Adv. Synth. Catal. 2007, 349, 2445 – 2448

Manganese Dioxide and N-Hydroxyphthalimide
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the addition of the radical initiator AIBN (1 mol%)
into the Co/Mn/NHPI-catalyzed oxidation of o-nitro-
[14]
toluene. Similarly, by promoting the reaction tem-
perature to 1508C, MnO /NHPI realized obvious in-
2
creases of the conversion to 37% and the isolated
yield to 33% in a period of 12 h (entry 11). This con-
Scheme 2. The proposed route of MnO -mediated formation
of PINO in aerobic oxidation.
2
trast showed that the performance of MnO looks like
2
a combined use of cocatalyst and radical initiator in
the NHPI-catalyzed oxidation. Hence, it could be
concluded that MnO /NHPI is an effective system to mechanism of this new catalytic oxidation can be de-
2
catalyze the aerobic oxidation of methyl groups on ni- scribed possibly as in Scheme 2. Both reduction and
troarenes.
reoxidation of Mn species take place together to con-
Recognizing the successful performance of the first struct the redox cycle in the oxidation. The reduction
combination of MnO and NHPI, we sought to find is less fast than the reoxidation, therefore, MnO2
2
the role which MnO played in the oxidation. We no- could be consumed completely in the case of low
2
ticed that the residual amount of MnO after reaction load.
2
is obviously less than the loading in the case of using
In conclusion, we have successfully developed a
over 10 mol% of MnO . On decreasing the loading highly efficient catalytic process for the aerobic oxida-
2
amount of MnO to 7 mol%, the conversion still was tion of nitrotoluenes. The oxidation catalyzed by a
2
good (entry 8), but black MnO disappeared after re- mixture of MnO /NHPI is carried out under compara-
2
2
action, giving a brown solution. Adding a small tively mild conditions in acetic acid. Mechanistically,
amount of H SO into the solution, the brown color the concurrence of reduction and reoxidation con-
2
4
immediately became light yellow. The reduction of structs the redox cycle of Mn species. For inducing
MnO2 by organic compounds, such as ascorbic PINO formation, MnO plays the double role of the
2
[18a]
[18b]
[18c]
acid,
fructose,
oxalic acid
etc., has been well cocatalyst and the radical initiator in the NHPI-cata-
studied, revealing that, in these reactions, Mn con- lyzed oxidation. Moreover, the ready availability and
verts finally into Mn via an intermediate Mn fol- low price of MnO makes it feasible for industrial ap-
4+
2+
3+
2
lowing a one-step, one-electron transfer mechanism. plications. Further studies will focus on the use of this
It is especially notable that the one-electron oxidation novel catalytic system in the aerobic oxidation of
property of ascorbic acid is similar to that of NHPI. other substrates.
The color change and disappearance of MnO in our
2
reaction are quite similar to the above reduction of
MnO . Therefore, the assumption could be reasonably Experimental Section
2
made that the reaction between MnO and NHPI fol-
2
lows the same route as the reaction between MnO2 General Remarks
and ascorbic acid, i.e., PINO is formed in the every
Gas chromatography measurements were conducted using
step of Mn reduction. In order to confirm the reactivi-
an Agilent Technologies 6890N Network GC system with a
ty of MnO2 and NHPI, the reaction of 0.8 mmol
flame ionization detector and a DB-1 capillary column
MnO2 (equal to 10 mol% load in reaction) and
(30 m”0.535 mm”3.0 mm). Agilent Chemstation software
.6 mmol NHPI (equal to 20 mol% load) in 20 mL was used for spectra acquisition and processing. The conver-
1
acetic acid was run at 1108C. The disappearance of sions of substrates were estimated with calibration curves
MnO required only 2 h. Under the same conditions using chlorobenzene as an internal standard. The products
2
were characterized by IR and HPLC in comparison with the
authentic sample, respectively. All starting materials and
catalysts were purchased from commercial sources and used
without further treatment.
no change was observed in the mixture of MnO and
acetic acid even after 5 h. These results clearly sup-
2
port our assumption. The role of MnO is similar to
2
that of a radical initiator for NHPI-based catalysis.
However, if MnO takes the sole role of radical initia-
2
tor, in consideration of stoichiometric relation, the
load amount of 10 mol% cannot result in as high as
General Procedure for the Oxidation
The reaction was carried out in a 150-mL autoclave. Typical-
ly, a mixture of p-nitrotoluene (8 mmol), NHPI (0.8 mmol),
97% conversion. In association with the above reac-
tion phenomena, it could be concluded that Mn spe-
cies with low valences were reoxidized to ones with
higher valences during the course of the oxidation.
The oxidants for low-valence Mn species were sug-
MnO (0.8 mmol), and acetic acid (20 mL) was stirred and
2
heated to 1108C after the atmosphere over the mixture was
replaced with O2 for three times. The pressure of O was
kept under 0.4MPa for the desired reaction time. After
cooling to room temperature, the mixture was diluted with
2
gested to be O , the intermediate alkyl hydroperox-
2
[
18d]
ide, as well as MnO2 . As a consequence, although ethanol to a volume of 100 mL, and samples were taken for
all Mn species are not known in detail, the general GC measurement. Then, the ethanol solution was distilled
Adv. Synth. Catal. 2007, 349, 2445 – 2448
ꢁ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
2447

Guanyu Yang et al.
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under reduced pressure to dryness. A dilute solution of
NaOH was added, the mixture stirred for 30 min, and fil-
tered to remove the insoluble solid. The filtrate was acidi-
fied to a pH above 1, and then was filtered to afford p-nitro-
benzoic acid in 89% yield.
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Adv. Synth. Catal. 2007, 349, 2445 – 2448