Metal-Free: An efficient and selective catalytic aerobic oxidation of hydrocarbons with oxime and N-Hydroxyphthalimide

Article abstract of DOI:10.1002/adsc.200900509

A non-metal catalytic system consisting of dimethylglyoxime (DMG) and N-hydroxyphthal-imide (NHPI) for the selective oxidation of hydrocarbons with dioxygen is described. The synergistic effect of DMG and NHPI ensures its efficient catalytic ability: 82.1% conversion of ethylbenzene with 94.9% selectivity for acetophenone could be obtained at 80°C under 0.3 MPa of dioxygen in 10h. Several hydrocarbons were efficiently oxidized to their corresponding oxygenated products under mild conditions. aerobic oxidation; catalyst design; dimethylglyoxime ; hydrocarbons ; N-hydroxyphthalimide

Full text of DOI:10.1002/adsc.200900509

FULL PAPERS
DOI: 10.1002/adsc.200900509
Metal-Free: An Efficient and Selective Catalytic Aerobic Oxida-
tion of Hydrocarbons with Oxime and N-Hydroxyphthalimide
Gengxiu Zheng,a,* Chunhong Liu,a Qiufen Wang,a Mingyu Wang,a
and Guanyu Yangb,*
a
School of Chemistry and Chemical Engineering, University of Jinan, Jinan 250022, Peopleꢀs Republic of China
Department of Chemistry, Zhengzhou University, Zhengzhou 450052, Peopleꢀs Republic of China
b
Fax : (+86)-531-8276-5841; phone: (+86)-531-8276-5841; e-mail: chm_zhenggx@ujn.edu.cn or yangguanyu@zzu.edu.cn
Received: July 22, 2009; Published online: October 28, 2009
Abstract: A non-metal catalytic system consisting of tained at 808C under 0.3 MPa of dioxygen in 10 h.
dimethylglyoxime (DMG) and N-hydroxyphthal- Several hydrocarbons were efficiently oxidized to
ACHTUNGTRENNUNGimide (NHPI) for the selective oxidation of hydro- their corresponding oxygenated products under mild
carbons with dioxygen is described. The synergistic conditions.
effect of DMG and NHPI ensures its efficient cata-
lytic ability: 82.1% conversion of ethylbenzene with Keywords: aerobic oxidation; catalyst design; dime-
94.9% selectivity for acetophenone could be ob- thylglyoxime; hydrocarbons; N-hydroxyphthalimide
Introduction
Results and Discussion
The selective catalytic oxidation of hydrocarbons, Catalytic Activity of Various Oximes in the Presence
such as ethylbenzene, cyclohexane, p-xylene and tolu- of NHPI
ene, with molecular oxygen is a primary and essential
tool for organic synthesis and industrial chemistry.[1] The catalytic properties of various oximes were exam-
For this purpose, many efficient catalysts containing ined in the aerobic oxidation of ethylbenzene at 808C
transition metals have been reported.[2] Although under 0.3 MPa O2 for 6 h. The oxidation results are
these catalyst systems could improve the oxidation of listed in Table 1. It was found that 10 mol% acetal-
hydrocarbons, the disadvantages of metallic toxicity doxime in combination with 10 mol% NHPI realized
and the high expense could not be avoided. Yet im- 43.9% conversion of ethylbenzene with the selectivi-
proving the catalytic performance of such aerobic oxi- ties of 59.4% for acetophenone (AcPO), 16.5% for 1-
dations remains an industrial and scientific challenge phenylethanol (PEA) and 24.1% for 1-phenylethyl
because of increasing energy costs and growing envi-
ronmental concerns. In recent years, N-hydroxy-
ACHTUNGTRENNUNGphthalimide (NHPI) has been used as a valuable cata-
Table 1. Oxidation of ethylbenzene catalyzed by different
oximes and NHPI.[a]
Entry Catalyst
lyst for the efficient aerobic oxidation of various or-
ganic compounds under mild reaction conditions in
the presence of some metallic compounds.[3] Interest-
ingly, some non-metallic compounds, such as a,a-azoi-
sobutyronitrile,[4] aldehydes,[5] anthraquinones,[6] xan-
Conversion
[%]
Selectivity [%]
AcPO PEA PEHP
1
acetaldoxime/
NHPI
43.9
59.4
16.5 24.1
thone/tetramethylammonium
chloride,[7] acridine
yellow/Br2,[8] and quaternary ammonium bromides,[9]
can accelerate the NHPI-based oxidation. Herein we
report that an oxime, especially the inexpensive dime-
thylglyoxime (DMG), in combination with NHPI,
serves as a metal-free catalytic system for the aerobic
oxidation of hydrocarbons which provides an unpre-
cedented and efficient catalytic method for the trans-
formation of hydrocarbons to their oxygen-containing
compounds under moderate conditions.
2
3
acetoxime/NHPI 50.3
56.5
51.7
14.8 28.7
15.6 32.7
cyclohexanone
oxime/NHPI
DMG/NHPI
DMG
45.7
4
5
6
74.5
trace
23.2
74.9
–
54.8
17.4 7.7
–
–
NHPI
12.0 33.2
[a]
Reaction conditions: 16 mmol ethylbenzene, 10 mol%
oxime, 10 mol% NHPI, 10 mL CH3CN, 0.3 MPa O2,
808C, 6 h.
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Metal-Free: An Efficient and Selective Catalytic Aerobic Oxidation
FULL PAPERS
Table 2. The oxidation of ethylbenzene with different
hydroperoxide (PEHP) (Table 1, entry 1). And acet-
ACHTUNGTRENNUNGoxime/NHPI and cyclohexanoneoxime/NHPI gave
amounts of DMG and NHPI.[a]
50.3% and 45.7% conversions, respectively (entries 2
and 3). To our delight, by using 10 mol% DMG and
10 mol% NHPI together, the conversion of ethylben-
zene increased to 74.5%, and the selectivity of AcPO
also increased to 74.9% (entry 4). GC and GC-MS
measurements indicated that the products in all oxi-
dations were AcPO, PEA and PEHP without any ap-
preciable benzoic acid, the overoxidation product. In
sharp contrast, the oxidation could not occur by using
DMG alone (entry 5). And in the absence of DMG,
NHPI realized only 23.2% conversion (entry 6),
which was in accordance with the result in the previ-
ous literature.[11] Therefore, it clearly showed that the
combination of oximes and NHPI, DMG and NHPI
in particular, could significantly promote the aerobic
oxidation of ethylbenzene due to their synergistic
effect.
Entry DMG/NHPI
[mol%/mol%]
Conversion
[%]
Selectivity [%]
AcPO PEA PEHP
1
2
3
4
5
6
7
8
9
10/10
5/5
74.5
51.6
54.3
61.2
56.9
55.3
71.2
75.4
75.5
74.9
71.3
69.5
68.9
70.1
70.4
72.2
67.8
61.5
17.4 7.7
12.5 16.2
15.3 15.2
14.2 16.9
16.2 13.7
17.7 11.9
13.3 14.5
18.2 14.0
19.5 19.0
5/10
10/5
5/15
15/5
10/15
15/10
15/15
[a]
Reaction conditions: 16 mmol ethylbenzene, 10 mL
CH3CN, 0.3MPa O2, 808C, 6 h.
with the oxidation using 10 mol% NHPI and
It had been reported that several organic com- 10 mol% DMG (Table 2, entry 1), the conversion of
pounds could mediate the NHPI-based oxidation. ethylbenzene markedly decreased when the amounts
Among these mediators, a,a-azoisobutyronitrile[4] and of both the catalytic components were contracted to
aldehydes[5] play the sole role of a radical initiator to 5 mol% (entry 2). With the amount of one component
induce the formation of phthalimide N-oxyl radical decreased to 5 mol%, the conversions still remained
(PINO), and they are consumed during the reactions low in spite of increasing the amount of the other
and are not recoverable. Noticeablely, purely organic component to 15 mol% (entries 3–6). On the other
substituted anthraquinones,[6] the couple of xanthone hand, when the amounts of either one component or
and tetramethylammonium chloride,[7] and the couple both the components were increased to 15 mol%, the
[8]
of acridine yellow and Br2 can work as the cocata- conversions of ethylbenzene stayed near to the result
lytic components for the NHPI-based oxidation. By of using 10 mol% (entries 7–9). These means that the
comparison, the simple and inexpensive DMG/NHPI catalytic ability was not improved obviously by in-
exhibits nearly the same catalytic activity as the cata- creasing the amounts of the catalysts further above
lytic systems mentioned above, and avoids the corro- and beyond 10 mol%.
sive component.
The effect of temperature on the oxidation of ethyl-
benzene was investigated and is summarized in
Table 3. As expected, with the reaction temperature
rising, the conversion of ethylbenzene increased with
the concomitant gradual increase of AcPO and the
decrease of PHEP in the selectivities. When the tem-
Influence of Reaction Conditions on Ethylbenzene
Oxidation Catalyzed by DMG/NHPI
Due to the poor solubility of NHPI in organic sol- perature was 808C, 74.5% conversion and 74.9% se-
vents, the NHPI-catalyzed aerobic oxidation was lectivity of AcPO were obtained in the presence of
often conducted in polar solvents such as acetonitrile
or acetic acid. In our tests using 10 mol% NHPI and
10 mol% DMG, the ethylbenzene (16 mmol) oxida-
tion at 808C under 0.3 MPa O2 for 6 h in 10 mL di-
chloromethane and tetrachloromethane solution gave
rather low conversions of 20.6% and 15.7%, respec-
tively. On the other hand, in polar solvents such as
acetonitrile, acetic acid, trifluoroacetic acid and tetra-
hydrofuran, the conversions reached 74.5%, 44.9%,
40.7% and 67.3%, respectively. Therefore, acetonitrile
seems the superior choice over the other solvents
screened.
The catalytic performances of DMG/NHPI in ace-
tonitrile were examined in detail as shown in the fol-
lowing paragraphs. Table 2 displays the oxidation re-
sults using different catalyst amounts. In comparison
Table 3. The effect of reaction temperature on oxidation of
ethylbenzene.[a]
Entry Temperature
Conversion
[%]
Selectivity [%]
[8C]
AcPO PEA PEHP
1
2
3
4
5
6
40
60
70
80
90
100
36.2
56.6
68.3
74.5
74.8
75.7
59.7
68.9
70.5
74.9
76.7
76.8
18.9 21.4
19.1 12.0
18.8 10.7
17.4 7.7
15.8 7.5
16.0 7.2
[a]
Reaction conditions: 16 mmol ethylbenzene, 10 mol%
DMG, 10 mol% NHPI, 10 mL CH3CN, 0.3 MPa O2, 6 h.
Adv. Synth. Catal. 2009, 351, 2638 – 2642
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Gengxiu Zheng et al.
FULL PAPERS
Table 4. The oxidation of ethylbenzene under different di-
oxygen pressures.[a]
Entry Pressure
[MPa]
Conversion
[%]
Selectivity [%]
AcPO PEA PEHP
1
2
3
4
5
0.1
0.2
0.3
0.5
0.7
47.7
68.3
74.5
76.5
77.9
71.2
72.7
74.9
76.1
75.7
19.4 9.4
18.3 9.0
17.4 7.7
15.0 8.9
15.2 9.1
[a]
Reaction conditions: 16 mmol ethylbenzene, 10 mol%
DMG, 10 mol% NHPI, 10 mL CH3CN, 808C, 6 h.
(Table 5). 88.1% of indane was oxidized to afford in-
danone with 75.3% selectivity at 808C under 0.5 MPa
O2 for 8 h (entry 1). Toluene and fluorene (entry 2, 3)
were also converted into the benzoic acid and fluor-
Figure 1. Time course of the oxidation of ethylbenzene by
~
!
&
the DMG/NHPI system: ( ) ethylbenzene; ( ) AcPO; (
)
AHCTUNGTREGUNeNN none with satisfactory conversions (57.8% and
*
PEA; ( ) PEHP. Reaction conditions: 16 mmol ethylben-
zene, 10 mol% DMG, 10 mol% NHPI, 10 mL CH3CN, 808C,
0.3 MPa O2.
67.3%), respectively. Diphenylmethane was converted
in 66.1% conversion with 90.6% selectivity of benzo-
phenone at 808C for 6 h (entry 4). Tetralin could also
10 mol% NHPI and 10 mol% DMG during a 6 h re- be oxidized with the corresponding ketone as the
action period. However, the oxidations changed only main product.
very slightly even at higher temperatures (entries 4–
6).
The influence of reaction time on the catalytic oxi- Suggested Catalytic Mechanism of Oxidation
dation of ethylbenzene by DMG/NHPI is shown in
Figure 1. As illustrated, the conversion of ethylben- As the data above show, for the aerobic oxidation of
zene increased with prolonging the reaction time. Si- ethylbenzene, DMG had no catalytic effect, NHPI
multaneously, the selectivity of AcPO continuously had only a slight effect, and the combination of NHPI
increased, and the selectivities of PEA and PEHP de- and DMG had a high catalytic ability. In order to fur-
creased due to the continuous decomposition of ther understand this finding, several experiments
PEHP to PEA, which was further oxidized to yield were carried out. Firstly, hydroquinone (5 mol%), a
more AcPO. However, after 10 h the conversion was free radical scavenger, was added into the oxidation
changed slowly and became stagnated near 80%. As a of ethylbenzene catalyzed by DMG/NHPI. The result
result, in a 10 h reaction course, 82.1% ethylbenzene that no oxidation occurred indicated that the oxida-
was oxidized with 94.9% selectivity of AcPO.
tion followed a radical pathway. Secondly, dimethyl-
Table 4 shows the oxidation results under the differ- glyoxime ester was used to replace DMG in the
ent pressures of molecular oxygen. Below 0.3 MPa a above catalytic oxidation of ethylbenzene. When the
positive influence of pressure on the reaction rate was oxidation of ethylbenzene was carried out at 808C for
observed clearly, the pressure increase leading to an 6 h under the pressure of 0.3 MPa, it only gave the
increase in conversion (entries 1–3). Above 0.3 MPa conversion of 23.5%, which was consistent with the
the pressure influence is no longer obvious with no result of using NHPI alone under the same conditions.
more than 78% conversion (entries 4 and 5). Howev- This implied that dimethylglyoxime ester had no cata-
er, the oxidation still reached a higher conversion on lytic effect, and that the N-hydroxy groups of DMG
prolonging the reaction time under 0.3 MPa played a key role in the catalytic oxidation.
(Figure 1). So 0.3–0.5 MPa of molecular oxygen seems
to be enough for the oxidation.
Accordingly, the NHPI-catalyzed aerobic oxida-
tions follow a free radical route, and are initiated and
propagated by a nitroxyl radical, phthalimide N-oxyl
radical (PINO), which comes from the hydrogen ab-
straction of N-hydroxy of NHPI by the used medi-
Oxidation of Different Hydrocarbons Catalyzed by
DMG/NHPI
AHCTUNGTRENNUNG
ators or their oxidized species.[3] Ishii reported that
ethylbenzene could be efficiently oxidized by molecu-
In order to further probe the potential, this non-metal lar oxygen in the presence of NHPI and Co-
catalytic system was employed to catalyze the oxygen-
(OAc)2.[3b,12] Generally, the bond dissociation energies
ation of various substrates under mild conditions of the O H unit of oximes are lower than that of the
AHCTUNGTRENNUNG
À
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Metal-Free: An Efficient and Selective Catalytic Aerobic Oxidation
FULL PAPERS
Table 5. Oxidation of various hydrocarbons by the NHPI/DMG system.[a]
Entry
Hydrocarbon
Time [h]
Conversion [%]
Main Products and Selectivity [%]
1
8
88.1
75.3
83.1
10.4
10.7
2
3
8
57.8
67.3
10
88.7
11.3
4
6
6
66.1
84.2
90.6
82.2
9.4
9.7
5
[a]
Reaction conditions: 2 mL liquid substrate or 2 mmol solid substrate, 10 mol% NHPI, 10 mol% DMG, 10 mL CH3CN,
808C, 0.5 MPa O2.Coversion and selectivity were determined by GC.
[13]
À
O H of NHPI, meaning that oximes undergo ab- Conclusions
straction of hydrogen more easily to form the corre-
sponding N-oxyl radical. However, at 808C under Summarising, we have developed a novel catalytic
0.3 MPa O2 for 6 h, the combination of DMG and Co- system consisting of DMG and NHPI for the aerobic
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
facts indicated that the corresponding N-oxyl radical corresponding oxygenated compounds with high con-
of DMG could not work like PINO to promote the versions and high selectivities under mild conditions.
radical aerobic oxidation, even though it was formed This metal-free catalytic oxidation method has an en-
in the reaction.
vironmentally friendly feature and provides an attrac-
So a possible catalytic mechanism of oxime/NHPI tive method which has long been desired in the chem-
in the aerobic oxidation was proposed and is dis- ical industry.
played in Scheme 1. The first step of the reaction was
thought to involve the corresponding N-oxyl radical
of oximes, which was formed by the reaction of oxime
and dioxygen. And then this N-oxyl radical abstracted
Experimental Section
hydrogen from NHPI to recover to oxime. Simultane-
All starting materials and catalysts were purchased from
ously, NHPI was converted to PINO, which abstracted
commercial sources and used without further treatment.
a hydrogen atom from the substrates to form alkyl
All the catalytic reactions were performed in an 80-mL
stainless steel autoclave equipped with a magnetic stirrer.
radicals, thus propagating the autoxidation chain.
Scheme 1. A possible reaction mechanism for the catalytic cycle of the DMG/NHPI system.
Adv. Synth. Catal. 2009, 351, 2638 – 2642
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Gengxiu Zheng et al.
FULL PAPERS
Typically, 2 mL (16 mmol) ethylbenzene, 10 mL acetonitrile,
the desired amount of DMG and NHPI were added into the
autoclave. The reactor was flushed three times with O2 and
pressurized to 0.3 MPa, and then heated to the desired tem-
perature with stirring. During the reaction the O2 pressure
was kept constant by supplying dioxygen.
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The oxidation products were identified by GC-MS and
quantified by GC. The conversion of ethylbenzene, and the
selectivities of acetophenone (AcPO) and 1-phenylethanol
(PEA) were determined by calibration curves. The yield of
1-phenylethyl hydroperoxide (PEHP) could not be directly
measured by GC due to its decomposition to AcPO in GC
analysis.[10] In addition, PEHP can be converted quantita-
tively to PEA by excess Ph3P at room temperature.[6,10] Thus
the amount of PEHP can be accurately quantified by treat-
ment of the mixture with excess Ph3P for 1 h and a second
GC measurement. For all the other substrates, the conver-
sions and selectivities were calculated by GC calibration
curves.
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Acknowledgements
We gratefully acknowledge the financial support from the De-
velopment Programs in Science and Technology of Shan
Dong (2008GG10002021).
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