Highly efficient and metal-free oxidation of olefins by molecular oxygen under mild conditions

Article abstract of DOI:10.1016/j.tet.2007.05.038

Highly efficient and metal-free aerobic oxidations of cyclohexene and styrene were successfully performed under mild conditions in the presence of 1,4-diamino-2,3-dichloro-anthraquinone and N-hydroxyphthalimide. When cyclohexene was oxidized, an 89% conversion and 71% selectivity for 2-cyclohexen-1-one was obtained under 0.3 MPa at 80 °C for 5 h. In the oxidation of styrene, a 77% conversion and 69% selectivity for benzaldehyde was obtained for 10 h. Furthermore, more olefins were efficiently oxidized to corresponding oxygenated products under mild conditions. All kinds of factors that affected cyclohexene oxidation were well investigated, and the possible reaction mechanism was provided.

Full text of DOI:10.1016/j.tet.2007.05.038

Tetrahedron 63 (2007) 7634–7639
Highly efficient and metal-free oxidation of olefins by molecular
oxygen under mild conditions
*
Xinli Tong, Jie Xu, Hong Miao, Guanyu Yang, Hong Ma and Qiaohong Zhang
State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Graduate School of the Chinese Academy of Sciences,
Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, PR China
Received 25 January 2007; revised 28 April 2007; accepted 10 May 2007
Available online 13 May 2007
Abstract—Highly efficient and metal-free aerobic oxidations of cyclohexene and styrene were successfully performed under mild conditions
in the presence of 1,4-diamino-2,3-dichloro-anthraquinone and N-hydroxyphthalimide. When cyclohexene was oxidized, an 89% conversion
ꢀ
and 71% selectivity for 2-cyclohexen-1-one was obtained under 0.3 MPa at 80 C for 5 h. In the oxidation of styrene, a 77% conversion and
69% selectivity for benzaldehyde was obtained for 10 h. Furthermore, more olefins were efficiently oxidized to corresponding oxygenated
products under mild conditions. All kinds of factors that affected cyclohexene oxidation were well investigated, and the possible reaction
mechanism was provided.
Ó 2007 Elsevier Ltd. All rights reserved.
1
. Introduction
extensively used in homogenous or heterogenous catalytic
systems. With these catalytic systems, cyclohexene was gen-
erally oxidized to 2-cyclohexen-1-one, 2-cyclohexen-1-ol,
and cyclohexene oxide. Besides, in the oxidation of styrene,
benzaldehyde and styrene epoxide are desirable and valuable
products in fine chemicals. Recently, some efficient catalysts
containing transition metal have also been reported for the
Selective oxidation process plays an important role in mod-
ern organic chemistry. Thereinto, the oxidation of olefin is
a crucial transformation, which is utilized in natural product
synthesis and production of fine chemicals. For example,
the oxygenated products of cyclohexene and their derivatives
are very important in organic synthesis owing to the exis-
tence of a highly reactive a,b-unsaturated carbonyl group,
which are extensively used in the preparation of a range of
chemical intermediates and products. Traditionally, some
inorganic oxidants containing chromium were employed
1
–3
2
0–29
oxidation of styrene. Furthermore, the catalytic system
containing N-hydroxyphthalimide (NHPI) and metal com-
pound was especially attractive because of its high efficiency
and mild reaction conditions in the aerobic oxidation,
which was also used in oxidation of some olefins.
3
0,31
3
2
4
,5
for the oxidation of cyclohexene. However, when chro-
mium compounds were used, there existed several obvious
disadvantages, for example, the typical separation problem
of products, disposal of toxic solid and liquid wastes, etc.
In recent years, a variety of studies on the direct oxidation
of cyclohexene have been reported which employed the per-
oxide (such as tert-butylhydroperoxide or hydrogen perox-
ide, etc.) or molecular oxygen as terminal oxidant for both
economical and environmental benefits. These oxidation
processes were mainly catalyzed by metallic compounds.
Although these catalyst systems containing metal element
could improve the oxidation of olefin, these systems suffered
from relatively harsh conditions or poor conversion and
selectivity. On the other hand, the disadvantage of metallic
toxicity and the high expense could not be avoided. Thus,
metal-free and more efficient catalytic systems for oxidation
of olefin with molecular oxygen become particularly
desirable.
6
These metallic catalysts generally contained iron, manga-
chromium, cop-
etc., which were
Catalysis with organic molecules is a significant research
orientation in modern catalytic chemistry. Our research
group has reported several organocatalytic systems for
7
–10
11
12,13
14
33,34
nese, ruthenium, vanadium,
per, nickel and cobalt element,
1
5
16–19
3
5–38
efficient oxidation of aromatic hydrocarbons.
Herein,
extending the former work, we report an efficient and
metal-free catalytic process for oxidations of cyclohexene
and styrene by molecular oxygen in the presence of NHPI
and 1,4-diamino-2,3-dichloro-anthraquinone (DADCAQ).
The promising conversions and selectivities are obtained.
Keywords: Metal-free; Oxidation; Cyclohexene; Styrene; N-Hydroxyphthal-
imide.
*
Corresponding author. Tel./fax: +86 411 84379245; e-mail: xujie@dicp.
ac.cn
0
040–4020/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2007.05.038

X. Tong et al. / Tetrahedron 63 (2007) 7634–7639
7635
a
2
. Results and discussion
Table 1. Oxidation of cyclohexene by different catalyst combinations
b
Entry
Anthraquinone
(derivatives)
Conversion
(%)
Product distribution (%)
2
.1. Oxidation of cyclohexene
2
3
4
5
Others
Cyclohexene (1) was efficiently oxidized by dioxygen under
mild conditions in the presence of NHPI and anthraquinone
O
1
2
3
75
76
50
57
9
18
4
12
13
11
(AQ) or derivates (shown in Scheme 1). Based on GC and
GC–MS analysis, the major product was 2-cyclohexen-1-
one (2). In most cases, the by-products were 2-cyclohexen-
1-ol (3), 2-cyclohexen-1-hydroperoxide (4), and cyclohex-
ene oxide (5).
O
O
Cl
Et
61 10 12
43 18 15
39 11 19
63 11 10
4
1
1
4
5
5
O
O
O
OH
OOH
metal-free
catalytic system
+
+
+
O
0
.3MPa O2, 70 °C, 5 h
1
2
3
4
5
O
O
O
OH
OH
Scheme 1. Oxidation of cyclohexene by molecular oxygen.
4
5
6
7
8
52
71
79
69
17
12
15
12
2
.1.1. The investigation of catalyst combinations. The
oxidation of cyclohexene was investigated with the catalyst
combination of different anthraquinone compounds and
NHPI. The reaction results are summarized in Table 1. It
could be seen that 75% conversion of 1 and 57% selectivity
of 2 were obtained when anthraquinone (AQ) and NHPI
were employed; especially, selectivity of 4 also reached
O
O
NH2
Br
Br
O
OH
1
8%. Then, different substituted AQ compound, which
OH
was substituted by chlorine, bromine, hydroxyl, sulfonic
group, amino or their combination, was investigated. As a
result, it was found that the combination of 1,4-diamino-
61
65
67
8
8
8
11
10
7
O
O
SO3H
NH2
2
,3-dichloro-anthraquinone (DADCAQ) and NHPI was
most efficient and highly selective. Under 0.3 MPa at
5 C, an 83% conversion and 67% selectivity for 2 were
Br
ꢀ
7
obtained for 5 h (entry 8). Here, it was indicated that the
amino and chlorine groups could promote the decomposi-
tion of hydroperoxide through the electron effect and
O
O
OH
NH2
3
9
Cl
Cl
molecular structure functioning. Otherwise, it was found
that 1 was just oxidized with 25% conversion without any
catalyst (entry 9), and only 29% conversion or 47% conver-
sion of 1 was, respectively, obtained in the absence of NHPI
or DADCAQ (entries 10 and 11). It was also seen that com-
pound 4 was the major product instead of compound 2
when oxidation of 1 was carried out without any catalyst
or in the presence of only NHPI (entries 9 and 11). This
phenomenon was probably explained that the existence of
AQ could facilitate the decomposition of cyclohexene
hydroperoxide and have a notable effect upon the product
distribution.
83
25
7
5
11
11
O
NH2
NH2
c
9
No
21 18 45
O
Cl
Cl
c
10
29
47
65 11
7
7
4
10
13
O
NH2
11
No
33 12 38
a
Reaction conditions: cyclohexene was performed on a 2 mL scale, in the
presence of NHPI (5.0 mol %), anthraquinone (or derivatives)
Then the effects of different catalyst contents were investi-
gated, and it was found that the oxidation of cyclohexene
could efficiently and selectively proceed when the content
of NHPI–DADCAQ was 4.0–1.0 mol %.
(
1.25 mol %), in 10 mL CH
ꢀ
3
CN, pressure¼0.3 MPa, time¼5 h, temper-
ature¼75 C.
b
c
The data were obtained by GC and GC–MS analysis using toluene as an
internal standard.
Oxidation was performed in the absence of NHPI.
2
.1.2. Effect of temperature. The effect of temperature on
oxidation of 1 was investigated and summarized in Table
. It was shown that the conversion was increased and the
selectivity for 2 was first increased then decreased along
2.1.3. Effect of reaction time. The effect of reaction time
was investigated and outlined in Figure 1. It was indicated
that the selectivity of 2 almost kept unchanged and the con-
version gradually increased during the oxidation of cyclo-
hexene from 30 min to 300 min. The reaction speed
sharply increased during 150–180 min, which matched
with the chain propagation process in the radical oxidation.
2
ꢀ
ꢀ
with the increase of temperature from 60 C to 90 C.
ꢀ
8% selectivity for 2 were obtained in the presence of
.0 mol % NHPI and 1.0 mol % DADCAQ.
When the temperature was 80 C, an 87% conversion and
6
4

7636
X. Tong et al. / Tetrahedron 63 (2007) 7634–7639
a
a
Table 2. The effect of reaction temperature on oxidation of cyclohexene
Table 4. The effect of solvent in the oxidation of cyclohexene
ꢀ
b
Entry Temperature ( C) Conversion (%)
b
Entry Solvent Conversion (%)
Product distribution (%)
Product distribution (%)
2
3
4
5
Others
2
3
4
5
Others
1
2
3
4
60
70
80
90
44
80
87
91
53 14
65 10
3
5
5
9
19 11
1
2
3
PhCF
CCl
DMAc 84
3
71
81
61
63
69
15
12
14
3
4
5
11
10
4
10
11
8
9
7
7
11
11
12
4
68
64
9
8
a
Reaction conditions: cyclohexene was performed on a 2 mL scale, in the
presence of NHPI (4.0 mol %), DADCAQ (1.0 mol %), in 10 mL solvent,
a
Reaction conditions: cyclohexene was performed on a 2 mL scale, in the
presence of NHPI (4.0 mol %), DADCAQ (1.0 mol %), in 10 mL CH CN,
ꢀ
3
temperature¼80 C, pressure¼0.3 MPa, time¼5 h.
b
pressure¼0.3 MPa, time¼5 h.
The data were obtained by GC and GC–MS analysis using toluene as an
internal standard.
b
The data were obtained by GC and GC–MS analysis using toluene as an
internal standard.
2
.1.5. Effect of solvent. Different solvents, including polar
Conversion
Selectivity for 2
9
8
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
0
0
(CH CN, DMAc) and apolar ones (CCl , PhCF ), were em-
3 4 3
ployed on the oxidation of cyclohexene (entry 3 in Tables 2
and 4). Here, 89% conversion or 84% conversion with
CH CN or DMAc as a solvent was superior to 71% conver-
3
sion or 81% conversion with PhCF or CCl as a solvent. So,
3
4
it was shown that polar solvent was more suitable to this
reaction system, which could be attributed to better solubil-
ity of catalyst in the polar one.
2
.1.6. Mechanism of oxidation of cyclohexene. Here, a pos-
sible reaction mechanism was proposed for cyclohexene
oxidation (Scheme 2). Firstly, NHPI can be converted to
phthalimide N-oxyl radical (PINO) through electron and
proton transfer with the assistance of DADCAQ in the begin-
ning of the reaction (step 1). The following step involves the
hydrogen atom abstraction from cyclohexene by PINO, and
the resulting cyclohexene radical being trapped by molecu-
lar oxygen provides peroxy radical, which is eventually con-
verted into products through cyclohexene hydroperoxide
0
50
100
150
200
250
300
Reaction time / min
Figure 1. The effect of reaction time in oxidation of cyclohexene (Reaction
ꢀ
conditions: 4.0 mol % NHPI, 1.0 mol % DADCAQ, temperature¼80 C,
pressure¼0.3 MPa).
(step 2). Furthermore, the better result was obtained when
acetic acid or acetic anhydride was added as an additive
probably because hydrogen bonding between NHPI and
this additive increased the O–H bond dissociation energy,
Moreover, it should be mentioned that the conversion hardly
increased and the selectivity gradually decreased if the oxi-
dation was continuingly preceded to 330 min or longer reac-
tion time, so 300 min is considered as a preferable reaction
time.
4
0
and the conversion sharply fell after a little pyridine was
added probably because pyridine could accelerate decay of
4
1
PINO in acetonitrile. Further investigation about reaction
mechanism, especially including more direct information
of cyclohexene radical and peroxy radical in the oxidation,
is underway.
2
.1.4. Effect of additive. The reaction exhibited an interest-
ing effect after an additive was added. In Table 3, it could be
clearly seen that acidic additives could improve the selectiv-
ity of 2 and conversion was appreciably decreased (entries 1
and 2); thereinto, 89% conversion and 71% selectivity for 2
were obtained when acetic acid was used as an additive.
However, the additionof pyridine made the conversionlessen
sharply (entry 3) and only 34% conversion of 1 was obtained.
2
.2. Oxidation of styrene and other olefins
It could be seen from Table 5 that styrene was mainly oxi-
dized to benzaldehyde and styrene epoxide with this cata-
lytic system. When NHPI and AQ were employed as
a catalyst combination, a 51% conversion and 71% selectiv-
ity for benzaldehyde were obtained (entry 1). Similarly,
a 55% conversion and 72% selectivity for benzaldehyde
were obtained when AQ was replaced by DADCAQ (entry
a
Table 3. The effect of additive in the oxidation of cyclohexene
b
Conversion (%)
Entry Additive
Product distribution (%)
2
). Further investigations indicated that the conversion of
2
3
4
5
Others
styrene could be increased and the selectivity was decreased
little along with the increase of reaction time. When styrene
was oxidized for 10 h, the conversion was increased to 77%
and the selectivity for benzaldehyde was 69% (entry 3). To
our surprise, when 1 mL acetic acid was added as an addi-
tive, the selectivity for benzaldehyde was improved, but
the conversion was acutely decreased (entry 4). It was also
found that a 19% conversion of styrene and 78% selectivity
for benzaldehyde were obtained in the absence of any
1
2
3
Acetic acid
Acetic anhydride 85
Pyridine 34
89
71 11
71 12
75 10
4
4
2
5
3
5
9
10
8
a
Reaction conditions: cyclohexene was performed on a 2 mL scale, in the
presence of NHPI (4.0 mol %), DADCAQ (1.0 mol %), and the additive
ꢀ
(
1 mL), in 10 mL CH
3
CN, temperature¼80 C, pressure¼0.3 MPa,
time¼5 h.
b
The data were obtained by GC and GC–MS analysis using toluene as an
internal standard.

X. Tong et al. / Tetrahedron 63 (2007) 7634–7639
7637
O
O
NH2
O
OH NH2
O
Cl
Cl
Cl
N
O
+
(1)
(2)
N OH +
O
O
O
NH2
NH2
PINO
HDADCAQ
O
OH
OO
OOH
+
PINO + O2
+ HDADCAQ
+
1
2
3
4
+
1
O
5
Scheme 2. Proposed reaction mechanism for cyclohexene oxidation.
a
Table 5. Oxidation of styrene by different catalyst combination
O
O
catalytic system
.3MPa O2, 80°C
H
+
0
1'
2'
3'
b
Conversion (%)
Entry
Catalyst
Reaction time (h)
Product distribution (%)
0
0
2
3
Others
1
2
3
NHPI+AQ
5
5
10
10
5
51
55
77
25
19
32
21
71
72
69
89
78
74
79
12
13
13
1
5
11
7
17
15
18
10
17
15
14
NHPI+DADCAQ
NHPI+DADCAQ
NHPI+DADCAQ
No
NHPI
DADCAQ
c
4
5
6
7
5
5
a
Reaction conditions: styrene was performed on a 2 mL scale, in the presence of NHPI (4.0 mol %), AQ or DADCAQ (1.0 mol %), in 10 mL CH
ꢀ
3
CN, temper-
ature¼80 C, pressure¼0.3 MPa.
b
c
The data were obtained by GC and GC–MS analysis using toluene as an internal standard.
1
mL acetic acid was added as an additive.
catalyst (entry 5), and the conversion was, respectively, 32%
or 21% in the presence of only NHPI or DADCAQ (entries 6
and 7). These results indicated that oxidation of styrene
could be possibly different from oxidation of cyclohexene,
and oxidation of styrene could be performed through the ad-
dition and decomposition processes of radicals.
conversion and 67% selectivity for 4-methoxybenzadehyde
were obtained when 4-methoxystyrene was oxidized;
4-chlorostyrene was oxidized with 69% conversion and
73% selectivity for 4-chlorobenzadehyde in the presence
of NHPI and DADCAQ. Moreover, when 3-methylstyrene
was oxidized, 51% conversion and 69% selectivity for
3-methylbenzaldehyde were obtained. All these results
In addition, oxidations of other olefins were investigated and
the results are shown in Table 6. It was found that an 89%
showed that the oxidation efficiency was relative with the
structure of olefins. On the other hand, we also investigated
the oxidation of aliphatic chain olefins; however, the prod-
ucts were various and the selectivity was low. For example,
when 1-octene was oxidized, the oxygenated products were
hexanal, 2-hexyloxirane, oct-1-en-3-one, hept-2-enal, etc.
So, it could be concluded that this metal-free catalytic sys-
tem was more favorable to the oxidation of alicyclic and
aromatic olefins.
Table 6. Oxidation of some olefins by the combination of NHPI and DAD-
CAQ
a
b
Entry Substrate
Conversion
%)
Product distribution (%)
(
c
Aldehyde Epoxide Others
d
1
2
3
4-Methoxystyrene 85
67
73
69
11
14
14
22
13
17
4-Chlorostyrene
3-Methylstyrene
69
51
a
Reaction conditions: substrates were performed on a 1 mL scale, in the
presence of NHPI (4.0 mol %), DADCAQ (1.0 mol %), in 10 mL
3. Conclusion
ꢀ
CH
The data were obtained by GC and GC–MS analysis using 1,2-dichloro-
benzene as an internal standard.
3
CN, temperature¼80 C, pressure¼0.3 MPa, time¼10 h.
b
c
d
It was demonstrated that various alicyclic and aromatic
olefins could be efficiently oxidized by molecular oxygen
under mild conditions in the presence of DADCAQ and
NHPI. Thereinto, when cyclohexene was oxidized, an 89%
conversion and 71% selectivity for 2-cyclohexen-1-one
The ordinal aldehydes were 4-methoxybenzaldehyde, 4-cholorobenzalde-
hyde, and 3-methylbenzaldehyde.
The ordinal epoxides were 4-methoxystyrene oxide, 4-chlorostyrene ox-
ide, and 3-methylstyrene oxide.

7638
X. Tong et al. / Tetrahedron 63 (2007) 7634–7639
ꢀ
were obtained under 0.3 MPa at 80 C for 5 h. In oxidation
of styrene, a 77% conversion and 69% selectivity for benz-
Supplementary data
ꢀ
aldehyde were obtained under 0.3 MPa at 80 C for 10 h.
The detailed investigations about reaction mechanisms and
radical intermediates are underway.
It contains general procedure for the detailed GC measure-
ment conditions, explanation to several phenomena, and
GC–MS diagrams for all the products. Supplementary data
associated with this article can be found in the online ver-
sion, at doi:10.1016/j.tet.2007.05.038.
4. Experimental
4
.1. Reagents and equipments
References and notes
NHPI, anthraquinone (AQ), and anthraquinone derivates
were purchased from commercial source and purified before
they were employed.
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2
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0
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3
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