Two catalytic systems of l-proline/Cu(II) catalyzed allylic oxidation of olefins with tert-butyl hydroperoxide

Article abstract of DOI:10.1039/c6ra11784h

The nontoxic and water-soluble l-proline combined with two different forms of copper(ii): recoverable Cu-Al hydrotalcite-like compounds (Cu-Al HTLcs) and water-soluble CuCl₂, as a heterogeneous catalytic system (l-proline/Cu-Al HTLcs) and two-phase catalytic system (l-proline/CuCl₂) to catalyze allylic oxidation with tert-butyl hydroperoxide. The results showed that l-proline/Cu(ii) is highly active for oxidizing isophorone (IP) into ketoisophorone (KIP). Maximum catalytic effects were afforded respectively under the optimal reaction conditions, which obtained 77.9% IP conversion with 74.3% KIP selectivity catalyzed by l-proline/Cu-Al HTLcs and 73.5% conversion with 81.6% selectivity by l-proline/CuCl₂. Recycling experiments of the two catalytic systems of l-proline/Cu(ii) showed they are stable and recyclable for at least six cycles with appreciable catalytic activity. And various hydrocarbons could be smoothly transformed into corresponding ketones with satisfactory conversion and selectivity by the two catalytic systems.

Full text of DOI:10.1039/c6ra11784h

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Two catalytic systems of L-proline/Cu(II) catalyzed allylic
oxidation of olefins with tert-butyl hydroperoxide
Yin Zhou, Yingwei Yang, Ruiren Tang*
School of Chemistry and Chemical Engineering, Central South University, Hunan, 410083,
China
*Corresponding author. Tel.: 86-731-88836961 Fax: 86-731-88879616
*E-mail: trr@mail.csu.edu.cn
Abstract: The nontoxic and water-soluble L-proline combined with two different
forms of copper (II): recoverable Cu-Al Hydrotalcite-like compounds (Cu-Al HTLcs)
and water-soluble CuCl2, respectively as heterogeneous catalytic system
2
(Lꢀproline/Cu-Al HTLcs) and two-phase catalytic system (Lꢀproline/CuCl ) to
catalyze allylic oxidation with tert-butyl hydroperoxide. The results showed that the
L-proline/Cu(II) is highly active for oxidizing isophorone (IP) into ketoisophorone
(
KIP) by the two systems. Their maximum catalytic effects were afforded respectively
under the optimal reaction conditions, which obtained 77.9% IP conversion with 74.3%
KIP selectivity catalyzed by Lꢀproline/Cu-Al HTLcs and 73.5% conversion with 81.6%
selectivity by Lꢀproline/CuCl . Recycling experiments of the two catalytic systems of
2
L-proline/Cu(II) showed they are stable and recyclable for at least six cycles with
appreciable catalytic activity. And various hydrocarbons could be smoothly
transformed into corresponding ketones with satisfactory conversion and selectivity
by the two catalytic systems.
Keyword: Allylic oxidation; Isophorone; L-proline; Hydrotalcite-like compounds;
Aqueous media; Copper catalyst
1
Introduction
The selective allylic oxidation of olefins is a pivotal transformation in synthesis
1
-3
chemistry
. The corresponding α,β-enones can be extensively employed as
4
-6
intermediates for the manufacturing of pharmaceuticals and agrochemicals . Given
1

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its significant value and influence, this functional transformation has been
7, 8
9
continuously researched over the past decades . For instance, Li et al reported
Cu(II) 2-quinoxalinol salen complex as catalyst could efficiently catalyze olefin
substrates to the corresponding enones with tert-butyl hydroperoxide (TBHP). Allylic
carbonylation of olefin substrates could be converted into the corresponding
α,β-enones using phosphomolybdic acid as catalyst in DMSO with appreciable
1
0
11
conversion and selectivity . Catino et al employed dirhodium(II) caprolactamate
as an exceptional catalyst for allylic oxidation of olefins affording acceptable yield.
Although these investigations have reported promising results, most of them turn out
to be impractical in large-scale industrial applications due to sophisticated, noble or
unrecoverable catalysts, poisonous solvents or rigorous reaction condition were
indispensable in the catalytic oxidation reactions. One thing to note is that developing
simple, eco-friendly, recoverable and practical catalytic system remain one of the
challenges to carry out selective allylic oxidation of olefins with satisfactory yield in
12-14
moderate conditions
.
In previously vast reports, organometallic catalyst, such as phthalocyanine metal
1
5
16
17
,
metalloporphyrin
and schiffbase complex , has been received considerable
attention for their excellent activity and selectivity in redox reaction. Particularly, they
18-20
are high-efficiency catalysts for allylic oxidation
. However, the organometallic
catalysts are complicated to synthesize, expensive and low stability. By contrast,
many small-molecule heteroatoms N,N- or N,O-dentate ligands combined with
transition metal catalysts are more economic and applicative as efficient
21-23
23
organometallic catalyst
. Dornan et al
employed small-molecule N,O-ligated
Pd(II) complexes to catalyze alcohol oxidation into corresponding ketone achieving
24
remarkable yield. Liu reported Cu(I)/1,10-phenanthroline as efficient and reusable
catalytic system to catalyzed oxidative decarboxylative homocoupling of arylpropiolic
acids. Apparently, organometallic catalysts containing small-molecule heteroatoms
ligands would have a wide variety of applications in the catalysis field.
In recent years, a miraculous N,O-didentate ligand, L-proline, with simple chiral
structure has been paid more and more attention and widely applied in the catalyst
2

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field on account of its low-cost, nontoxic, unique stereoselectivity and excellent water
2
5-27
solubility
. Intriguingly, Lꢀproline/copper was validated as a highly active catalytic
28-30
system reported in many chemical transformations
. While catalytic reaction
utilizing Lꢀproline/copper for allylic carbonylation is sluggish. From the perspective
of green chemistry, this paper employ low-cost and water-soluble L-proline combined
with two different forms of copper (II) to investigate the catalytic activity of
L-proline/Cu(II) for allylic carbonylation of olefins.
Firstly, heterogeneous catalyst has been increasingly considered to be a better
choice in catalysis field, and is dominating in industrial production of chemical
transformation due to they are easier to prepare and handle, particularly, reactant,
31, 32
produce and catalyst can be easily separated at the end of the reaction
. Here, we
use L-proline combined with simple to prepare, stable, and recoverable
copper-aluminium hydrotalcite-like compounds (Cu-Al HTLcs) as a novel
heterogeneous catalytic system for the allylic oxidation.
Furthermore, water as the solvent for organometallic catalysis has developed into
33-35
an important research field
. Organic reactions in aqueous media have been
received considerable attention since water is cheaper, safer and nontoxic than the
highly flammable and volatile organic solvent, and these procedures could be
36, 37
conveniently separated and simply recovered after the end of reaction
. In this
paper, we also developed water-soluble L-proline/CuCl in aqueous media as an
2
unprecedented two phase catalytic system to catalyze allylic oxidation to
corresponding ketone.
2
2
Experimental
.1 Preparation of Catalysts
All the chemicals were analytical grade and used without further treatment.
Deionised water was used throughout the experiments. The preparation of
metal-aluminum hydrotalcite-like compounds (M-Al HTLcs, M=Co, Ni,Cu) was
based on the synthesis of a typical hydrotalcite material via co-precipitation under
38
low supersaturation reported by Guida et al . Detail synthetic process and structure
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39
characterization were presented in our previous paper .
2
.2 Typical Procedure for allylic oxidation by L-proline/Cu-Al HTLcs (Catalytic
system I)
15 mg (1 mmol) Lꢀproline, 150 mg Cu-Al HTLcs and 10 mL acetonitrile were
1
placed in a 50 mL round bottom flask and the mixture was stirred vigorously for 10
minutes to create a finely dispersed heterogeneous system. Sequentially, 1.38 g
isophorone (10 mmol) and 70% TBHP (50 mmol) were added slowly in flask. The
reaction mixture was heated required time in a thermostatic oil bath. Then mixture
reaction was cooled and filtered. Then the filtrate was neutralized with saturated
NaHSO
3×10 mL). Organic phase was washed successively with saturated EDTA solution
and saturated NaCl solution, and dried over anhydrous Na SO . The solvent was
3
solution to remove any extra TBHP and was extracted with ethyl acetate
(
2
4
removed by vacuum and the resulting residue was analyzed by GC-7890II gas
chromatograph with an OV-1701 column in flame ionization detector and the
conversion and selectivity were calculated by area normalization.
Recycled catalysts could be simply and conveniently recovered by separating
40
from the reaction mixture by filtration . After each catalytic experiment, the residue
was washed in solvent, saturated Na CO solution and warm water in succession, and
2
3
then dried in the oven at 105 °C for 24 h. Recycling experiments were performed in
the same reaction condition and the reaction mixture was processed in the same above
method after the end of reaction.
2
2
.3 Typical Procedure for allylic oxidation by L-proline/CuCl in water (Catalytic
system II)
Catalytic reactions were implemented in a 50 mL round flask equipped with
condenser, magnetic stirrer and thermostatic oil bath. L-proline (115 mg, 1 mmol),
2
CuCl (171 mg, 1 mmol) in water (10 mL) were placed in the reactor, then the
mixture was stirred vigorously for 5 minutes to create a clear blue Lꢀproline/copper
water phase. Sequentially, cetyltrimethylammonium bromide (CTMAB) (18.2 mg,
4

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0
.05 mmol), isophorone (1.38 g, 10 mmol), 70% TBHP (50 mmol) was added and the
reaction mixture was heated required time. Cooling and stewing the reaction mixture
for stratification. Water phase was washed with ethyl acetate (3×10 mL) and recycled.
The organic phase was merged and then processed in same method of Catalytic
system I (2.2). And the product was analyzed by GC.
Recycling experiments were performed in the same reaction condition. Recycled
blue water phase was use directly and placed in the reactor, then CTMAB (18.2 mg,
0
.05 mmol), isophorone (1.38 g, 10 mmol), 70% TBHP (50 mmol) were sequentially
added and the reaction mixture was heated required time. The reaction mixture was
processed in the same above method.
3
3
3
Results and discussion
.1 Allylic oxidation by L-proline/Cu-Al HTLcs (Catalytic system I)
.1.1 Effect of Various Catalysts
The catalysis of L-proline combined with different M-Al HTLcs material for the
3
allylic oxidation were examined in CH CN by performing the IP oxidation, and the
results are shown in Table 1. Isophorone (IP) was mainly converting into
ketoisophorone (KIP), and small amount of epoxy isophorone (OIP), hydroxyl
isophorone (HIP) and formyl isophorone (FIP) was detected as coproduct (Scheme 1).
Scheme 1. The allylic oxidation of IP
Compared to low activity of Lꢀproline or Cu-Al HTLcs as the sole catalyst
(Entries 1-2), Lꢀproline combined with Cu-Al HTLcs could efficiently catalyze
isophorone oxidation with a maximum IP conversion (65.2%) and acceptable KIP
selectivity (76.0%), followed by Lꢀproline/Co-Al HTLcs (33.5%, 74.1%) and
Lꢀproline/Ni-Al HTLcs (17.3%, 71.6%) (Entries 3, 5, 6). However, the catalytic
reaction over Lꢀproline/Cu-Al HTLcs in water was unfavorable (Entry 4).
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Subsequently, various small-molecule N,Oꢀdentate ligands are examined. Replacing
Lꢀproline by Lꢀphenylalanine, Lꢀhistidine, 8-hydroxyquinoline or 2-Picolinic acid, the
IP conversion reduced to 29.7%, 38.5%, 43.2%, 32.5%, respectively (Entries 5-8).
Compared with other heteroatoms ligands, apparently, Lꢀproline might be much more
desirable to combine with Cu-Al HTLcs. Therefore, Lꢀproline/Cu-Al HTLcs in
3
CH CN was confirmed as an ideal catalytic system for allylic oxidation of
isophorone.
Table 1. Effect of various catalysts on IP oxidation by Catalytic system I.
Selectivity%
Entry
Catalyst
Conversion%
KIP
58.0
72.5
76.0
63.7
71.6
74.1
72.6
61.8
58.7
69.4
OIP
7.6
4.4
5.5
3.5
6.3
4.2
5.8
4.6
7.7
5.4
HIP
19.4
11.1
8.4
FIP
9.9
9.6
7.7
6.8
8.1
9.5
7.6
1
2
3
4
5
6
7
8
9
Lꢀproline
Cu-Al HTLcs
4.2
21.6
65.2
24.4
17.3
33.5
29.7
38.5
43.2
32.5
Lꢀproline/Cu-Al HTLcs
a
Lꢀproline/Cu-Al HTLcs
Lꢀproline/Ni-Al HTLcs
21.2
10.4
9.3
Lꢀproline/Co-Al HTLcs
Lꢀphenylalanine/Cu-Al HTLcs
Lꢀhistidine/Cu-Al HTLcs
11.3
16.8 10.8
19.2 11.5
8-hydroxyquinoline/Cu-Al HTLcs
2-Picolinic acid/Cu-Al HTLcs
1
0
12.4
8.3
Reaction conditions: 10 mmol IP, 1 mmol N,Oꢀdentate ligand, 100 mg M-Al HTLcs, 50 mmol
TBHP, 10 mL CH CN, 80 ºC, 24 h. Analyzed by GC.
3
a. Water was used as solvent.
3
.1.2 Effect of reaction time and termperature
Table 2 shows the effect of reaction temperature and time on the isophorone
oxidation. At below 80 ºC, a positive impact of reaction temperature on the
transformation was clearly observed. The IP conversion increased noticeably from
2
5.4 % to 65.2 % with gradual increase of reaction temperature from 60 ºC to 80 ºC
(Entries 1-4), whereas the conversion of isophorone decreased with further increase of
temperature (Entries 5-6). This could be possibly due to TBHP is easier for thermal
decomposition in high temperature. A slightly increase in the selectivity of KIP was
observed during the course of the reaction. In addition, reaction time also has a
significant effect for allylic oxidation of isophorone (Table 2). The IP conversion
increased noticeably with increase of reaction time and superior conversion (65.2%)
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was observed at 24 h (Entry 4), while IP conversion just increased a little with further
extending reaction time (Entries 10-11). It was found that there was no significant
change in KIP selectivity with prolonging reaction time. Hence, based on the IP
conversion and KIP selectivity of different reaction temperature and time, a reaction
temperature of 80 ºC and time of 24 h were selected as optimum.
Table 2. Effect of temperature and time on IP oxidation by Catalytic system I.
Selectivity %
Entry
T/ºC
t/h
Conversion %
KIP
60.5
66.8
72.4
76.0
76.9
78.3
69.7
73.9
75.2
74.1
76.8
OIP
8.4
9.0
6.7
5.5
4.8
3.4
8.7
6.5
6.2
4.5
4.2
HIP
17.3
14.6
9.9
FIP
7.4
8.8
9.1
7.7
8.5
11.4
6.2
7.4
8.4
9.1
10.5
1
2
3
4
5
6
7
8
9
50
60
70
80
90
100
80
80
80
80
80
24
24
24
24
24
24
6
25.4
42.8
54.7
65.2
63.8
56.3
24.9
43.7
57.2
68.5
70.2
8.4
7.9
7.3
13.9
11.6
9.2
12
18
30
36
10
1
9.6
1
7.3
Reaction conditions: 10 mmol IP, 100 mg Cu-Al HTLcs, 1 mmol Lꢀproline, 50 mmol TBHP, 10
mL CH CN, 80 ºC, 24 h. Analyzed by GC.
3
3
.1.3 Effect of amount of TBHP
The effect of amount of TBHP for catalytic performance is summarised in Figure
. It can be observed clearly from the figure that the IP conversion kept going up with
1
concomitant gradual increase of amount of TBHP in its research scope. The KIP
selectivity was no significant variation with increase of amount of TBHP until it
reached 50 mmol, whereas drop sharply when more TBHP was added because of
over-oxidation. Therefore, 50 mmol of TBHP was identified as the optimal oxidant
dosages to get satisfactory IP conversion and KIP selectivity.
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Figure 1. Effect of the amount of TBHP on IP oxidation by Catalytic system I.
Reaction conditions: 10 mmol IP, 1 mmol Lꢀproline, 100 mg Cu-Al HTLcs, 10 mL CH
3
CN, 80 ºC,
2
4 h. Analyzed by GC.
3
.1.4 Effect of Catalyst Amount
Table 3. Effect of catalyst amount on IP oxidation by Catalytic system I.
Cu-Al HTLcs
Lꢀproline
Selectivity %
Entry
Conversion %
(
mg)
10
(mmol)
KIP
58.5
69.4
76.0
74.3
72.4
66.4
69.0
71.6
74.7
72.6
OIP
8.3
6.4
5.5
4.6
7.1
6.5
8.5
7.3
6.6
4.9
HIP
19.2
12.2
8.4
FIP
7.1
6.7
7.7
8.4
10.3
12.0
8.7
6.0
7.4
9.5
1
2
3
4
5
6
7
8
9
1
1
38.6
49.8
65.2
77.9
73.3
67.3
26.2
59.8
80.1
83.9
50
100
150
200
250
150
150
150
150
1
1
6.8
1
8.3
1
8.5
0.1
0.5
2
9.4
8.7
7.6
10
5
10.4
Reaction conditions: 10 mmol IP, 50 mmol TBHP, 10 mL CH CN, 80 ºC, 24 h. Analyzed by GC.
3
The reaction results using different amount of Lꢀproline and Cu-Al HTLcs at the
above optimum condition are presented in Table 3. As indicated in the table, the IP
conversion improved markedly from 38.6 to 77.9% with gradual increasing amount of
Cu-Al HTLcs from 10 to 150 mg (Entries 1-4), but it decreased unexpectedly with
further increase in the amount of Cu-Al HTLcs (Entries 5-6). In addition, the variation
tendency of KIP selectivity was in accord with the IP conversion with gradual
increasing amount of Cu-Al HTLcs. It may be presumably because the coke or carbon
4
1
deposition of excess catalyst hindered its activity . Moreover, with gradual increase
of Lꢀproline, IP conversion increased simultaneously at below 1 mmol and the
8

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optimum yield was achieved when employing 1 mmol Lꢀproline (Conversion 77.9 %,
Selectivity 74.3 %), whereas the IP conversion and KIP selectivity remained almost
unchanged if more of Lꢀproline was added (Entries 9–10). Consequently, 150 mg
Cu-Al HTLcs and 1 mmol Lꢀproline were selected as optimum dosage of catalyst for
further studies.
3
.1.5 Recycling experiments
Experiments of repeatability and restorability were carried out using 150 mg
recycled Cu-Al HTLcs, 1 mmol L-proline and 50 mmol TBHP at 80 ºC for 24 h and
the results were given in Figure 2. More interestingly, only a slight loss of catalytic
activity of recycled catalyst was found after repeated 6 times, which reaction
conversion of isophorone only decreased by 15.6% and KIP selectivity decreased by
9
.9%. After that, catalytic activity of Cu-Al HTLcs significantly decreased. Therefore,
it can be inferred that Cu-Al HTLcs is a relatively stable heterogeneous catalyst and
can combine with L-proline as an efficient catalytic system to catalyze allylic
oxidation at least about six times.
Figure 2. Reused of Cu-Al HTLcs.
Reaction conditions: 10 mmol IP, 1 mmol Lꢀproline, 150 mg recycled Cu-Al HTLcs, 50 mmol
3
TBHP, 10 mL CH CN, 80 ºC, 24 h. Analyzed by GC.
3
.1.6 Proposed Oxidation Mechanism
4
2-45
Considering the above experimental results and related reports
, a possible
oxidation mechanism catalyzing by Lꢀproline/Cu-Al HTLcs for allylic oxidation of
isophorone was put forward and shown in Scheme 2. Small-molecule Lꢀproline could
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coordinate with copper atom of Cu-Al HTLcs to activate catalytic activity of Cu-Al
HTLcs. The lamellar Complex A between the Lꢀproline ligand and the copper atom
reacted with TBHP to afford a Cu-based species B, which having two active sites B
and B . The two active sites can both promote the functional transformation. Active
site B and B could abstract a hydrogen atom from IP forming IP radical and
1
2
1
2
respectively release a t-BuOH molecule and a H
2 1
O moleculethen, while active site B
and B reconverted into A. Synchronously, another B or B combined with TBHP
2
1 2
could capture the IP radical forming tert-butylperoxy ethers, which finally
decomposed into oxygenated products, and B reconverted into A. In addition, the
formations of mainly co-product such as OIP, HIP and FIP were given in Scheme 2.
Scheme 2. A possible catalysis mechanism by Catalytic system I.
3
3
.2 Allylic oxidation by L-proline/CuCl in water (Catalytic system II)
2
.2.1 Effect of Various Catalysts
10

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Lꢀproline combined with the first transition metal chloride in water was
investigated for oxidation of isophorone at above optimum condition of catalytic
system I, and the results are shown in Table 4. Lꢀproline assisting in MnCl
2 2
, FeCl or
CoCl led to moderate conversions of isophorone (Entries 1-3). Amusingly, when
2
switching the metal ion to CuCl , the conversion reached 70.5% with 81.6%
2
selectivity of KIP (Entry 4). However, the low activity was observed in the absence of
CTMAB under same reaction condition (Entry 5). This was implied that CTMAB is
essential as phase transfer catalyst to promote the formation of the third catalyst-rich
phase in the whole system. Lꢀproline/CuCl
CH CN was used as solvent (Entry 6). Subsequently, various soluble Cu-salts
including CuCl, CuSO , Cu(OAc) or Cu(NO are examined (Entries 7-10). All of
2
was presented moderate activity when
3
4
2
3 2
)
Cu(II)-salts under this catalytic system could efficiently catalyze allylic oxidation of
IP, but CuCl combined with Lꢀproline in water only demonstrated plain catalytic
performance. There is no doubt that isophorone afforded low conversion in the
absence of CuCl or Lꢀproline (Entries 11-12). Thus, we achieved another ideal
2
2
catalytic system of Lꢀproline/Cu (II) with Lꢀproline assisting in CuCl , CTMAB as a
phase transfer catalyst and water as solvent for the oxidtation of isophorone.
Table 4. Effect of various catalysts on IP oxidationby Catalytic system II
Selectivity %
Entry
Catalyst
Conversion %
KIP
65.5
70.8
77.4
81.6
62.9
73.7
64.2
78.1
73.6
79.8
58.0
54.1
OIP
4.7
6.9
5.8
3.7
6.0
5.8
8.2
5.7
3.9
1.6
7.6
13.4
HIP
18.6
12.9
6.4
FIP
8.9
7.1
6.9
5.2
8.3
6.2
4.6
4.8
7.2
6.7
9.9
10.8
1
2
3
4
5
6
7
8
9
Lꢀproline/MnCl2
44.6
36.1
51.8
73.5
18.3
35.1
37.4
71.9
70.1
69.5
4.2
Lꢀproline/FeCl
2
Lꢀproline/CoCl
2
2
Lꢀproline/CuCl
7.1
a
Lꢀproline/CuCl
Lꢀproline/CuCl2
Lꢀproline/CuCl
Lꢀproline/CuSO
2
21.5
10.2
20.3
8.2
b
4
Lꢀproline/Cu(OAc)
2
10.4
6.3
10
Lꢀproline/Cu(NO
3 2
)
11
Lꢀproline
19.4
23.8
12
CuCl
2
9.6
Reaction conditions: 10 mmol IP, 1 mmol N,Oꢀdentate ligands, 1 mmol metal chloride, 50 mmol
TBHP, 10 mL water, 80 ºC, 24 h. Analyzed by GC.
a. CTMAB was omitted.
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3
b. CH CN was used as solvent.
3
.2.2 Effect of Catalyst Amount
The variation of catalytic activity for isophorone using different amount of
Lꢀproline and CuCl were examined over optimized reaction time, temperature and
2
oxidant dosage in catalytic system I, and the results are shown in Table 5.
Interestingly, an obvious escalation of IP conversion was observed with increase of
amount of Lꢀproline and superior conversion (70.5%) with 81.6% KIP selectivity was
observed when employing 1 mmol Lꢀproline (Entries 1-4). After that, the IP
conversion and KIP selectivity were almost constant (Entries 4-6). In addition, the
conversion of IP and selectivity of KIP increased noticeably to 73.5% and 81.6%,
respectively, with increase of amount of CuCl
2
from 0.05 mmol to 1 mmol (Entry 4),
but the KIP selectivity decrease slightly if more of Lꢀproline was added due to side
reactions intensify (Entries 10-11). Hence, 1mmol Lꢀproline and 1 mmol CuCl
selected as optimum dosage.
2
were
Table 5. Effect of catalyst amount on IP oxidation by Catalytic system II
Lꢀproline
CuCl2
Selectivity %
Entry
Conversion %
mmol
mmol
KIP
56.5
69.4
76.0
81.6
80.9
82.4
63.5
69.0
74.7
78.6
72.3
OIP
8.3
7.3
5.6
3.7
3.2
4.1
6.3
6.9
5.6
4.8
5.3
HIP
FIP
9.1
6.7
7.2
5.2
7.3
5.8
9.7
8.4
7.4
6.9
7.1
1
2
3
4
5
6
7
8
9
0.05
0.1
0.5
1
1
1
26.1
45.8
59.2
73.5
74.3
76.3
29.2
47.8
64.1
76.9
81.3
19.2
12.2
9.4
1
1
7.1
2
1
6.3
5
1
6.5
1
0.05
0.1
0.5
2
15.4
12.7
9.6
1
1
10
1
8.4
11
1
5
10.9
Reaction conditions: 10 mmol IP, 0.05 mmol CTMAB, 50 mmol TBHP, 10 mL water, 80 ºC, 24 h;
Analyzed by GC.
3
.2.3 Recycling experiments
For evaluating the reusability of catalytic system, blue water phase was simply
and conveniently recovered by stewing the reaction mixture for stratification and
immediately used in the subsequent run. Obviously, isophorone afforded appreciable
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conversion and KIP selectivity remained almost constant for the first eight run. From
ninth run, the conversion of IP started to significantly reduce (Figure 3). It is inferred
that Lꢀproline/CuCl is a feasible water solubility catalyst, which can be recycled
2
conveniently and efficiently catalyze for allylic oxidation at least about eight times.
Figure 3. Recycling experiments of Catalytic system II.
Reaction conditions: 10 mmol IP, Recycled water phase, 0.05 mmol CTMAB, 50 mmol TBHP, 80
ºC, 24 h.
3
.2.4 Reaction mechanism
O
O
O
tBuOOH
O
O
.
IP
KIP
organic phase
water phase
N
Br
Cl
Cl
Cl
(
I)
Cu
Cu
(
II)
tBuOOH
HCl
N
O
O
N
D
A
O
O
tBuOH
tBuOOH
HCl
OtBu
tBuOH
Cl
O
Cl
(II) Cu
O
O
Cu (II)
B
O
O
N
N
O
O
C
O
Scheme 3. A possible catalysis mechanism by Catalytic system II.
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In view of the above favorable experimental results and relevant published
9
, 44, 46, 47
reports
catalyze allylic oxidation was put forward (Scheme 3). The onset of catalytic reaction
was the formation of water-soluble complex A between the Lꢀproline and CuCl
, a possible mechanism using Lꢀproline/CuCl catalytic system to
2
2
,
which reacted with TBHP to afford an active Cu-superoxide species B using CTMAB
as phase transfer catalyst. The active species B abstracted a hydrogen atom from
isophorone to release a tBuOH molecule and form unstable intermediate C, which can
decompose into isophorone oxyl radical and Cu(I) species D by hemolytic
decomposition. The former can be back to organic phase and directly converted to
carbonyl product, and Cu(I) species D can be oxidized by TBHP and regenerate A.
3
.3 Comparison of the catalytic activity of different catalysts
We compared the catalytic activity of the Lꢀproline/Cu(II) with other catalysts
that reported for the allylic oxidation of isophorone. As shown in Table 6, the
Lꢀproline/Cu(II) catalyst performed excellent catalytic performance with the
considerable conversion and selectivity in mild condition.
Table 6. Comparison of the catalytic activity of different catalysts for allylic oxidation of IP
Conversion
%
KIP
Selectivity %
70.3
Entry
Catalytic system
Oxidant T/ºC, t/h
Reference
1
2
3
4
5
6
7
8
PMA/DMSO/t-BuOK
O2
115, 24
60, 48
100, 10
66, 24
21, 18
70, 12
60, 10
130, 5
99.1
70
[10]
[41]
[48]
[49]
[50]
[51]
[52]
[53]
CuMgAl-13/CH CN
TBHP
100
3
CuCl
Fe(III)/SiO
ID/AA/TEA/CH
2
/acetylacetone
/pyridine
CN
O
2
94.7
98.7
94.1
49
67.8
2
Air
Air
O2
86.5
3
83.7
TEMPO/pyridine
NHPI
85
O2
15.4
74.8
77.2
NHPI/g-C
Lꢀproline/Cu-Al
HTLcs/CH CN
Lꢀproline/CuCl /water
N
3 4
/CH
3
CN
O
2
44.4
9
TBHP
TBHP
80, 24
80, 24
77.9
73.5
74.3
81.6
This work
This work
3
10
2
3
.4 Oxidation of various hydrocarbons catalyzed by two catalytic systems of
L-proline/Cu (II)
The generality of the two catalytic systems of Lꢀproline/Cu(II) were researched
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for a series of various hydrocarbon substrates under the optimized (Table 6).
Cyclohexene were both converted completely into corresponding
-cyclohexene-1-one with satisfactory selectivity of 86.1%, 92.0% using respectively
2
catalytic system I and catalytic system II (Entry 1). Lꢀproline/Cu-Al HTLcs and
Lꢀproline/CuCl were found to be ordinary catalytic system for oxidizing tetralin to
2
tetralone only affording the modest selectivity of 67.4%, 72.6% respectively (Entry 2).
In addition, ethylbenzene, indane and fluorene were effectively oxidized into their
corresponding carbonyl compounds with the two catalytic systems, but afford higher
2
selectivity using Lꢀproline/CuCl catalytic system in water (Entries 3-5).
Table 7. Oxidation of different hydrocarbons catalyzed by the two catalytic systems of
Lꢀproline/Cu(II).
a
Entry Substrate
Main product
Catalyst system
I
Conversion%
100
Selectivity%
86.1
1
II
I
100
92.0
67.4
95.4
2
3
4
II
I
96.1
95.5
94.2
72.6
93.6
97.1
II
I
II
I
94.6
93.7
96.4
86.5
86.7
91.4
93.5
99.3
5
II
Reaction conditions of Catalytic system I: 10 mmol substrate, 150 mg Cu-Al HTLcs, 1 mmol
Lꢀproline, 50 mmol TBHP, 10 mL CH CN, 80 ºC, 24 h.
Reaction conditions of Catalytic system II: 10 mmol olefin, 1 mmol Lꢀproline, 1 mmol CuCl
.05 mmol CTMAB, 50 mmol TBHP, 10 mL water, 80 ºC, 24 h.
a. Determined by GC-MS
3
2
,
0
4
Conclusion
In conclusion, two catalytic systems of Lꢀproline/Cu(II) were investigated for
catalyzing allylic oxidation with tertꢀbutyl hydroperoxide (TBHP) as oxidant. The
results showed that the L-proline/Cu(II) is highly active for oxidizing isophorone (IP)
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into ketoisophorone (KIP) in different systems. After screened and optimized,
Lꢀproline/Cu-Al HTLcs could effectively catalyzed carbonylation of isophorone with
acceptable selectivity of ketoisophorone under moderate reaction conditions
(Conversion 77.9%, Selectivity 74.3%). Recycling experiments show that it is a stable
and recoverable heterogeneous catalytic system in CH CN. And various hydrocarbons
3
were smoothly transformed into corresponding ketones with appreciable conversion
2
and selectivity by this catalytic system. It is heartening that Lꢀproline/CuCl in
aqueous media as another catalytic system of Lꢀproline/Cu(II) could also catalyze
various olefins affording appreciable conversion and higher selectivity. And its
repeatability and restorability inspiringly demonstrated Lꢀproline/CuCl
2
is an
economic, efficient and eco-friendly two-phase catalyst for the practical and industrial
application of allylic oxidation. The possible catalysis mechanisms were proposed
respectively over the two catalytic systems of Lꢀproline/Cu(II). The two catalytic
systems precisely employ low-cost and water-soluble L-proline instead of expensive
and water-fast organic ligands combine with different forms of copper (II) to catalyze
hydrocarbons efficiently converting into their corresponding carbonyl compounds and
seem to be intriguing methods conforming to the concept of green chemistry and
realizing the industrialized production.
Acknowledgements: This work was financially supported by the National Natural
Science Foundation of China (No. 21071152).
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Graphical abstract:
Allylic oxidation of olefins by the two catalytic system of L-proline/Cu(II).