Oxidation of olefins using molecular oxygen catalyzed by a part per million level of recyclable copper catalyst under mild conditions

Article abstract of DOI:10.1039/c6gc02186g

Copper catalysts with an imidazole salt tag ([Cu-Imace-R-H][X], X^- = F^-, Cl^-, Br^-, I^-, CF₃CO₂^-, HSO₄^-, NO₃^-, PF₆^- or BF₄^-; R = H or CH₃) show quite high reactivity for the oxidation of non-aromatic olefins with good selectivity for epoxides. The reactions perform well with a part per million (ppm) catalyst loading at mild temperature and ambient pressure. The highest turnover frequency (TOF) reaches up to 900:000 h^-1. The catalytic activity is easy to control by changing the anion of [Cu-Imace-R-H][X]. This catalyst is effective for a series of substrates, including internal and terminal olefins, tri- and tetra-substituted olefins and aromatic olefins. In addition, the copper catalyst can be conveniently separated from the reaction system and reused for at least six cycles without any obvious loss of catalytic activity.

Full text of DOI:10.1039/c6gc02186g

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Oxidation of olefins using molecular oxygen catalyzed by part-per-
million level of recyclable copper catalyst under mild conditions
Received 00th August 2016,
Accepted 00th August 2016
Guoqiang Yang, Huiyuan Du, Jia Liu, Zheng Zhou, Xingbang Hu*, Zhibing Zhang*
-
-
-
-
-
-
-
-
-
-
DOI: 10.1039/x0xx00000x
Copper catalyst with an imidazole salt tag ([Cu-Imace-R-H][X], X =F , Cl , Br , I , CF
) show quite high reactivity for the oxidation of nonaromatic olefins with good selectivity to the epoxides.
The reactions perform well at part-per-million (ppm) catalyst loading under mild temperature and ambient pressure. The
3 2
4 3 6 4
CO , HSO , NO , PF , or BF ;
R=H or CH
3
www.rsc.org/
-1
highest turn over frequency (TOF) could reach up to 900000 h . The catalytic activity are easy to be controlled by changing
the anion of [Cu-Imace-R-H][X]. This catalyst is effective for a series of substrates, include internal and terminal olefins, tri-
and tetra-substituted olefins, aromatic olefins. In addition, the copper catalyst can be conveniently separated from the
reaction system and reused for at least six cycles without obvious loss of catalytic activity.
2
2-31
P450 active site represent the most effective one
reactivity of metalloporphyrins can greatly reduce the loading
amount of catalyst, and the concentration of
. The high
Introduction
Oxidation of olefins is a dominant reaction both in industrial
and academia, because the epoxides have been widely used in
the production of epoxy resins, paints, surfactants, and so on.
Besides, they are highly valued as synthetic intermediates in
metalloporphyrins used in the oxidations can be as low as part-
per-million (ppm) level (for examples, ratio of catalyst to
46
47
25
26
substrate: 0.1 ppm ,100 ppm , 500 ppm , 1000 ppm ).
Although metalloporphyrins show high catalytic activity, their
preparation processes are complicated and need to consume
1
-6
the industrial processes . Thus, the development of effective
methods for the selective oxidation of olefins has drawn much
1
22-31
-8
large amounts of organic solvents
.
attention over the past several years
.
7
Various oxygen donors such as oxone , sodium
We have developed a series of effective catalysts modified
8-51
4
8
9
10, 11
by ionic tags for the oxidation reaction
. Among these
hypochlorite , sodium periodate , iodosylbenzene
, tert-
and urea
have been used in the catalytic
oxidation of olefins. As an oxidant, molecular oxygen is cheap,
1
2-14
14-18
compounds, the synthesis of the copper catalyst with an
butyl hydroperoxide
, hydrogen peroxide
1
9, 20
imidazole salt tag are very simple and environmentally
hydrogen peroxide
4
9
friendly . The whole synthesis process of this catalyst
performs quite well in water (Scheme 1). Herein, we found
that these [Cu-Imace-R-H][X] catalysts are super reactive for
the oxidation of olefins. 1ppm of catalyst can well promote the
epoxidation under mild condition. Besides, this catalyst is easy
2
1
clean, readily available, and environmentally friendly .
However, it is relatively unreactive toward the strong C–H and
2
2, 23
C=C bond unless it is activated by highly efficient catalysts
.
Otherwise, high reaction temperature or pressure are required,
which is against to the general requirement of green chemistry.
Many catalysts have been developed for the epoxidation of
olefins with molecular oxygen as oxidant, such as
to be separated and reused without obvious loss of efficiency.
H
R
X
N
R
N
2
2-33
34
HX
metalloporphyrins
, metal phthalocyanines , transition
41,
, heteropoly acid
N
COOH
N
COOH
R
3
5
metals oxides , salen-metal complexes
36-40
H
2
O
Imace-R
[
Imace-R-H][X]
4
2
43-45
and so on
.
Among these catalysts reported,
R
X
H
X
H
metalloporphyrins which are the mimics of the Cytochrome
Cu(OAc)
2
2
O
O
2
O
O
1N
N1
2
H O
N3
Cu
N
3
[Cu-Imace-R-H][X]
Scheme 1 Generation method for preparing [Cu-Imace-R-H][X]
-
-
-
-
-
-
-
-
-
-
(
3 2 4 3 6 4 3
X =F , Cl , Br , I , CF CO , HSO , NO , PF , or BF ; R=H or CH )
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ARTICLE
a
Table 1 The performance of different catalysts.
b
Selectivity (%)
Entry
Catalyst
Conv. (%)
CHO
Cyol
Cyone
1
2
3
4
5
6
7
8
9
[Cu-Imace-H-H][F]
[Cu-Imace-H-H][Cl]
41
43
(96)
50
(98)
56
36
(97)
23
(92)
18
53
60
83
82
(76)
82
(75)
81
81
(80)
84
(82)
81
87
84
4
4
(4)
4
(4)
5
4
(3)
2
(3)
6
2
3
10
11
(14)
10
(17)
9
13
(11)
12
(11)
8
10
9
(9)
13
11
10
[Cu-Imace-H-H][Br]
[Cu-Imace-H-H][I]
[
Cu-Imace-H-H][NO
3
]
[Cu-Imace-H-H][HSO
4
]
10
11
12
13
14
15
16
17
3 2
[Cu-Imace-H-H][CF CO ]
[Cu-Imace-H-H][PF
[Cu-Imace-H-H][BF
6
]
]
4
(96)
45
17
(82)
81
80
(2)
5
6
[Cu-Imace-CH
[Cu-Imace-H-CH
[Imace-H-H][BF
Cu-Imace-H-H][BF
Imace-H-H][BF
Cu(AcO)
Cu(CF COO)
Cu(acac)
3
-H][Cl]
3
][I]
4
]
2
88
1
[
4
]+
18
63
81
1
10
[
4
]
19
20
21
2
37
28
17
72
82
80
4
4
3
13
10
11
3
2
2
a
Reaction condition: cyclohexene (0.2mol), CH CN (50mL), isobutyraldehyde (0.2mol), and
3
6
2
catalyst-to-substrate molar ratios is 1:10 (1ppm), temperature: 60 ℃, O bubbling (1atm),
reaction time: 3 hours. Values in parentheses were obtained with 800 ppm catalyst in 6 hours.
b
The products were analysed by GC and GC-MS. The selectivity were determined by GC using o-
dichlorobenzene as internal standard. Selectivity = (the yield of target product)/(the conversion
of reactant)×100%.
mainly produced epoxycyclohexane (CHO) with a little of 2-
cyclohexen-1-ol (Cyol) and 2-cyclohexen-1-one (Cyone)
(Scheme 2). The byproducts include 1,2-cyclohexanediol, 2,3-
epoxycyclohexanol, 2,3-epoxycyclohexanone based on GC-MS
analysis. Besides, there has less than 1% nonvolatile
byproducts which may be oligomers and polymers. Using 1
ppm catalysts, the reaction can almost complete in 10 hours
under mild condition (Fig. 1).
OH
O
[
Cu-Imace-R-H][X]
O
+
+
O2
CHO
Cyol
Cyone
Scheme 2 The oxidative transformations of cyclohexene
Results and discussion
The conversions of cyclohexene range from 17 to 60% by
using [Cu-Imace-R-H][X] with different anions after 3h (Table
Cyclohexene epoxidation catalyzed by catalysts with different
anions
1
). It suggests that the reactivity of these catalysts can be easy
controlled by introducing different anion. Changing the anion
Eleven simple copper catalysts with an imidazole salt tag
can induce the electron rearrangement on the active metal
48, 50
-
-
-
-
-
-
-
-
-
[Cu-Imace-R-H][X], X =F , Cl , Br , I , CF CO , HSO , NO , PF , or
(
3
2
4
3
6
and results in catalysts with diverse reactivity
. It is
-
4 3
BF ; R=H or CH ) were synthesized according to the our
interesting to find that, for the catalysts modified by halogen
-
anion, their catalytic reactivity increase in the following order I
4
9
previous report (See ESI for details) . The oxidation of
cyclohexene catalysed by [Cu-Imace-R-H][X] was selected as a
probe to investigate the activity of these catalysts. Under the
-
Br >Cl >F . Among these four anions, F has strongest electron
-
-
-
>
-
withdrawing ability, whereas I is the weakest one. This
reaction conditions investigated, the oxidation of cyclohexene observation further proves our previous theoretic results
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ARTICLE
about the relation between the electron affinity and catalytic Table 1). It suggests that the active center is the copper chelated by
5
0
activity of these catalysts modified by ionic tags . Inspiring by carboxylic acid.
this idea, the catalysts modified by weakly coordinating anions
The trace [Imace-R-H][X] residual in the [Cu-Imace-R-H][X]
compound cannot assist the reaction because the catalytic
-
-
BF
4 6
and PF were synthesized. It is exciting to find that these
two catalysts show quite higher catalytic reactivity (Entries 12
reactivity of [Cu-Imace-H-H][BF
4
4
]+[Imace-H-H][BF ] is almost the
and 13, Table 1). At the same time, [Cu-Imace-H-H][PF
the highest CHO selectivity.
6
] give
same to that of [Cu-Imace-H-H][BF
4
] (Entries 13 and 18, Table 1).
Further more, almost no reaction happens when using [Imace-H-
H][BF ] alone (Entry 17).
4
To explore the role of the active N-H part in [Cu-Imace-R-H][X]
,
a
N-methylation catalyst [Cu-Imace-H-CH ][I] was synthesized
3
according the procedure reported in the ESI. It is interesting to find
that the catalytic ability of [Cu-Imace-H-CH ][I] is obviously lower
3
than these of [Cu-Imace-H-H][I] and [Cu-Imace-H-H][BF4] (Entries 6,
1
3 and 16, Table 1). However, almost no reaction happens when
[
Imace-H-H][BF ] was used without the present of cooper (Entry
4
17). It suggests that the active N-H can assist the active center
copper promote the reaction.
Among the catalysts investigated here, [Cu-Imace-H-H][BF4]
is the best choice from the viewpoint of reactivity and CHO
selectivity. This compound was used for further investigation.
Cyclohexene oxidation catalyzed by trace amount of catalyst (ppm
level)
Fig. 1. Profile of the conversion of cyclohexene oxidation using
different amounts of [Cu-Imace-H-H][BF (catalyst-to-
substrate molar ratios, ppm): substrate (0.2mol),
4
]
It is exciting to find that [Cu-Imace-H-H][BF ] is super active
4
isobutyraldehyde (0.2mol), CH
60 ℃.
3
CN (50mL), O
2
bubbling(1 atm), so that the cyclohexene epoxidation can perform quite well at
ppm catalyst loading (Fig. 1). The conversion of cyclohexene
can reach 90% in 10 hours promoted by 0.1ppm [Cu-Imace-H-
H][BF ] under mild condition, and the corresponding turn over
4
-
1
frequency (TOF) value is 900000
h . It is worth noticing that
almost no reaction happens (conversion < 2%) without catalyst
at the same temperature and O2 pressure. When the
concentration of catalyst was increased to 1ppm, the
conversion of cyclohexene can reach almost 100% in 10 hours
-
1
(TOF: 100000
h ).
No matter how much catalyst was used, the total selectivity
of CHO, Cyol and Cyone is larger than 94% with a preference to
the epoxide CHO (Fig. 2). Low concentration of catalyst is
benefit for the produce of the epoxide. A slight decrease of the
CHO selectivity was observed as the increasing of catalyst
loading from 0.1 to 10 ppm.
Indeed, both the concentration and anions of the catalyst
have influence on the reaction selectivity. Many investigators
reported the epoxidation involves a free-radical process
Fig. 2. The product selectivity of cyclohexene epoxidation
using different amounts of [Cu-Imace-H-H][BF ] (Reaction
4
21, 41, 52,
time: 3h).
53
-
.
Previous researches have shown that halogen anions (Cl
-
and Br ) can react with the radical to produce a halogen-
intermediate which can be further oxidized to produce deep
To further understand the catalytic ability of these copper
catalyst with an imidazole salt tag. Cu(OAc) , Cu(O CCF ) and
54
oxidation products . High catalyst concentration is favourable
for the collision between radical and anion. When the
concentration of catalyst was increased to 800 ppm, catalysts
2
2
3 2
Cu(acac) were used as bench mark under the optimum conditions
2
(
Entries 19-21, Table 1)
compounds is lower than that of [Cu-Imace-H-H][BF4]. Besides,
the selectivity of Cu(OAc) is not good. To eliminate the possibility
.
The catalytic reactivity of these three
-
-
with Cl and Br as anion show lower CHO selectivity and higher
Cyone selectivity comparing with other catalysts (Entries 3 and
5, Table 1). Cyone is an over-oxidation product of Cyol. Though
2
-H the radical was also produced in the reaction using catalysts
2
of a Cu-carbene system serving as catalyst, the hydrogen of C
was replaced by methyl and a methylated catalyst [Cu-Imace-CH
H][Cl] was synthesized. The catalytic ability of [Cu-Imace-CH
4 3 4
-H][Cl] intermediate modified by HSO , NO , or BF group (As far as
-
-
-
3
-
with HSO , NO , or BF as anion, it is quite difficult to form an
4 3 4
-
-
-
3
we known, there is little compound forming covalent bond
is quite similar to that of [Cu-Imace-H-H][Cl] (Entries 2 and 15,
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Journal Name
-
-
-
with HSO
4
, NO
3
, or BF
4
group). Hence, the Cyone selectivity
The direct reaction of dioxygen with singlet organic
molecules is a spin-forbidden process, since the ground state
of molecular oxygen is a triplet with two unpaired electrons
-
-
of these catalysts are higher than these using Cl and Br as
anion.
6
1
Table 2 Epoxidation of cyclohexene under different
conditions
having parallel spins . Several aerobic reactions involving the
a
combined use of dioxygen with reducing agents such as Zn
4, 40, 65-68
6
2
63
64
power , NaBH , 2-propanol and aldehyde
conv.
Selectivity (%)
have
4
Entry
Solvent
T(℃)
(
%)
CHO Cyol Cyone
been reported. Co-oxidation of cyclohexene and aldehyde
which is essential for the activation of molecular oxygen, leads
to the formation of CHO (main product) and appropriate acid
under the investigated conditions. Five kinds of aldehyde have
been investigated in the epoxidation of cyclohexene (Entry 5
and Entries 12-15). 2-ethylbutyraldehyde and 1-
pentanecarbaldehyde as co-reductant provide better
selectivity of CHO than isobutyraldehyde. However, better
conversion of cyclohexene could be obtained when
isobutyraldehyde as co-reductant.
1
2
3
4
5
6
7
8
9
CH
3
CN
20
30
40
50
60
70
60
60
50
60
60
60
60
60
60
60
60
60
60
6
90
88
85
83
82
71
40
76
44
<1
74
88
88
80
42
5
2
2
2
2
2
2
8
7
5
9
1
2
7
4
5
18
5
3
2
5
CH CN
34
55
72
99
>99
33
3
5
3
CH
CH
CH
CH
3
3
3
3
CN
CN
CN
CN
6
6
11
18
15
16
19
15
3
THF
CH
3
OH
CH
3
COCH
3
6
The molar ratio of isobutyraldehyde-to- cyclohexene plays a
key role in the epoxidation of cyclohexene using molecular
oxygen. In the absence of isobutyraldehyde, allylic oxidation is
more favorable (Entry 16). Thus the epoxidation with various
molar ratios of isobutyraldehyde-to-cyclohexene were
examined. When the substrate-to-isobutyraldehyde molar
ratio was increased from 0 to 1.0 (Entry 5 and Entries 16-18),
the conversion of cyclohexene and the selectivity of CHO could
be significantly increased, and the rate of epoxidation could be
accelerated simultaneously. However, no significant difference
was observed when the isobutyraldehyde/substrate molar
ratio is increased to 1.2 (Entry 19).
1
1
0
1
CCl
4
74
2
DMSO
b
1
2
CH CN
77
11
18
5
8
3
c
1
1
1
3
CH
CH
CH
CH
CH
3
3
3
3
3
CN
CN
CN
CN
CN
5
d
e
f
4
5
11
54
42
17
15
12
1
1
1
1
6
41
85
98
>99
f
f
f
7
8
9
63
73
82
CH CN
3
3
CH CN
a
Reaction condition: cyclohexene (0.2mol), isobutyraldehyde
0.2mol), solvent (50mL), [Cu-Imace-H-H] [BF ] as catalyst and
catalyst-to-substrate molar ratios is 1:1250, O2 bubbling
1atm), reaction time: 10 hours; 2-Ethylbutyraldehyde as co-
reducant; 1-pentanecarbaldehyde as co-reducant; 1-
Butalyde as co-reducant; 2-Furanaldehyde as co-reducant;
the molar ratio of isobutyraldehyde to cyclohexene of Entries
6-19 are 0, 0.6, 0.8, 1.2. Products formed in the epoxidation
(
4
Catalyst recycling experiment
b
(
c
d
The stability and recyclability of a catalyst is an important
5
2
e
issue for its practical application . The catalyst used here is
ionic compound and its solubility in the reaction solvent is very
poor. After the oxidation reaction catalyzed by [Cu-Imace-H-
f
1
reactions with olefins were identified by GC and GC-MS.
H][BF ] finished, the catalyst can be simply filtrated and reused
4
without any regeneration process. After six runs, there is no
The effects of reaction temperature, Solvent and co-reductant
As shown in Table 2, the conversion of cyclohexene
increased with the increasing of reaction temperature (Entries
significant loss of the reactivity and selectivity (Fig. 3).
1-6). Low temperature is favorable for the producing of CHO.
As the temperature increases, the selectivity of over-oxidation
product Cyone also increases.
The oxidation of cyclohexene is sensitive to solvent (Entry 5
and Entries 7-11). Being similar to most of the cyclohexene
1
4, 21, 55-58
oxidation catalyzed by other catalysts
acetonitrile as solvent give the best conversion. Because no
MeCONH was detected in the catalytic epoxidation, the
, reaction using
2
possibility of involvement acetonitrile as a co-reactant that
through peroxyimidic as an oxygen donor was not
2
1, 59
recognized
. Possible that low coordinating ability and
aprotic nature of acetonitrile play the key roles in improving
4, 21, 60
1
the activity of the catalyst in acetonitrile
. The catalyst
Fig. 3. Reuse of the catalyst in the oxidation of cyclohexene.
almost does not work in methanol, acetone, and DMSO.
Radical inhibition reaction
4
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It is widely accepted that the epoxidation of olefins in the (see Fig. S1 in the ESI). The result indicates that radicals are
2
1, 69
liquid phase is a radical reaction
. In order to verify whether involved in the catalytic cycle. The epoxidation of cyclohexene
the epoxidation reaction catalysed by [Cu-Imace-R-H] [X] was with dioxygen does not proceed without the presence of
via radical intermediate or not, a radical inhibitor 3, 5-di-tert- copper catalyst or without isobutyraldehyde (Fig. 1), which
butyl-p-hydroxytoluene (BHT) was added into the catalytic indicates that the chain reaction is initiated by the interaction
system. It was found that the epoxidation of cyclohexene between isobutyraldehyde and copper catalyst.
reaction was quenched immediately after the adding of BHT
a
4
Table 3 Epoxidation of different olefins catalyzed by [Cu-Imace-H-H][BF ]
-
1
L
oading Time Conv. TOF (h )
c
Entry substrates
b
Main products and selectivity (%)
(
ppm)
(h)
(%)
1
2
1
1
8
99
123750
120000
2
2
a
c
97(92)
8
96
91
98(87)
2b
3
1
8
113750
98
d
4
5
10
10
6
8
99
93
16500
11625
2
d
98
99
96
99
2
e
6
7
10
1
8
98
93
12250
93000
2
f
10
2
g
8
10
12
12
89
63
7416
525
2
h
9
6
9
1
100
1
2i
97
0
6
89
42
148333
350
2
j
9
8
1
1
1
2
100
100
12
2
2
2
k
l
9
5
12
10
53
76
442
95
55
54
45
46
8
00
100
00
100
00
1
1
3
4
12
66
550
67
32
m
8
10
12
10
92
41
70
115
342
88
69
54
55
31
24
22
2
n
8
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a
Reaction condition: substrate (0.2mol), isobutyraldehyde (0.2mol), CH
3
CN (50mL), temperature: 60 ℃, O
2
b
bubbling (1atm). TOF=(the initial moles of substrate × conversion)/(the moles of loading catalyst × reaction time).
c
The products were analyzed by GC and GC-MS. The yields were determined by GC using o-dichlorobenzene as
d
internal standard. Values in parentheses are isolated yield. Reaction temperature: 40℃.
should be recycled. There have many papers concerning the
reduction of acid to aldehyde, which makes the reusing of
7
1, 72
Epoxidation of various substrates catalyzed by [Cu-Imace-H- isobutyraldehyde feasible
.
H][BF4]
Under the optimized conditions, oxidation of various olefins
Acknowledgements
were investigated (Table 3). The substrates include internal
and terminal olefins, tri- and tetra-substituted olefins, and We gratefully acknowledge the grants from the National
aromatic olefins. The catalyst is reactive for all the substrates Natural Science Foundation of China (No.21476105, 21676134
investigated here. But the conversions and epoxides yields and 21176110).
depend strongly on the olefin structures. The conversion of 2a-
2
g are larger than 90% using 1 or 10 ppm catalysts within 10
hours. For the oxidations of ketene (2i and 2k), only 63% and
2% conversions were achieved in 12 hours even when 100
Notes and references
School of Chemistry and Chemical Engineering, Nanjing
University, Nanjing 210093, PR China. Fax: +86 2583 336599; Tel:
+86 2583 596665;
E-mail: huxb@nju.edu.cn; zbzhang@nju.edu.cn.
4
ppm catalyst was used. Aromatic olefins seem to be relatively
low activity, they need higher catalyst loading and longer
reaction time for satisfied conversion. Comparing with styrene,
1
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-methylstyrene has a better conversion due to the electron-
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The substrate investigated here can be divide into two
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7
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21
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9
1
.
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We demonstrated that the copper catalyst with an 11.
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-
-
-
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imidazole salt tag ([Cu-Imace-R-H] [X], X =F , Cl , Br , I , CF
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CO
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,
-
-
-
-
1
1
2.
3.
4 3 6 4 3
HSO , NO , PF , or BF ; R=H or CH ) were efficient catalysts
1
973, 95, 6136-6137.
for the oxidation of olefins by dioxygen. Under mild
temperature and atmospheric pressure, the catalytic system
displays high reactivity by using ppm level of catalyst. The TOF
of cyclohexene epoxidation reaction could reach up to 900000
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222-1225.
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stochiometrically transformed to isobutyric acid. To make the
process more environmentally benign, isobutyraldehyde
6
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Green Chemistry
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DOI: 10.1039/C6GC02186G
Table of contents entry:
O
2
as oxidant, ppm catalyst loading, mild reaction condition, good selectivity, and
wonderful recyclability of the catalyst for the oxidation of olefins.