Bi-functionalized PEG1000 ionic liquid [Imim-PEG 1000-TEMPO][CuCl2-]: An efficient and reusable catalytic system for solvent-free aerobic oxidation of alcohols

Article abstract of DOI:10.1039/c4nj00778f

An efficient and reusable catalytic system for aerobic oxidation of alcohols with bi-functionalized PEG₁₀₀₀ ionic liquid ([Imim-PEG ₁₀₀₀-TEMPO][CuCl₂^-]) was described. This ionic liquid shows catalytic propertie

Full text of DOI:10.1039/c4nj00778f

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Bi-functionalized PEG1000 ioniÀc liquid
[Imim-PEG1000-TEMPO][CuCl2 ]: an efficient
and reusable catalytic system for solvent-free
aerobic oxidation of alcohols†
Cite this: New J. Chem., 2014,
38, 4149
Zhao-gang Wang, Yong Jin, Xiao-hua Cao and Ming Lu*
An efficient and reusable catalytic system for aerobic oxidation of alcohols with bi-functionalized
PEG1000 ionic liquid ([Imim-PEG1000-TEMPO][CuCl2À]) was described. This ionic liquid shows catalytic
properties similar to those of its non-supported counterpart in terms of activity and selectivity affording
the target products in good yield, moreover, [Imim-PEG1000-TEMPO][CuCl2À] could be recycled and
reused without significant loss of catalytic activity after five runs. Notably, the protocol presented here
provides an efficacious strategy to achieve both catalyst recycling and product isolation from a
viewpoint of green chemistry.
Received (in Porto Alegre, Brazil)
14th May 2014,
Accepted 19th June 2014
DOI: 10.1039/c4nj00778f
www.rsc.org/njc
for oxidation of alcohols employing copper salts has been
reported quantitatively for the oxidation of alcohols under the
Introduction
The selective oxidation of primary and secondary alcohols into the aerobic conditions.14–23 In 2004, Gamez24 et al. reported the
corresponding aldehydes or ketones is one of the most important selective and very mild aerobic oxidation of primary alcohols to
and challenging transformations in organic chemistry.1–3 A large aldehydes using the [copper(II)–(N ligand)n] complex with TEMPO
variety of methods have been reported to accomplish this and a base as a co-catalyst at room temperature. More recently,
transformation efficiently and selectively.4,5 These methods Hoover25 et al. reported a new (bpy)CuI/TEMPO/NMI catalytic
usually require stoichiometric amounts of chemical oxidants, system that enables efficient and selective aerobic oxidation of
however, most of these oxidants are toxic, hazardous, or a large a broad range of primary alcohols, including allylic, benzylic,
amount of waste formed. From an economical and environ- and aliphatic derivatives, to the corresponding aldehydes using
mental viewpoint, catalytic oxidations are particularly promising, readily available reagents, at room temperature with ambient air as
and many efforts have been devoted to the development of the oxidant. However, the deficiencies of solubility of substrates,
efficient procedures employing O2 (or air) and hydrogen peroxide product isolation and catalyst recovery hinder the development
as the ultimate oxidants over the last few years. These attractive of these systems, therefore developing a recyclable catalytic
catalytic systems display high atom economy and release only system would be a promising project.
innocuous by-products such as H2O. Many transition metals such
Room temperature ionic liquids (RTILS) have drawn increasing
as ruthenium, rhodium, palladium, gold have been utilized to studies over the last few decades due to excellent advantages like
catalyze this reaction, either on their nanoparticles6,7 or salts with negligible volatility, thermal stability, remarkable solubility and
the assistance of co-catalysts such as stable free nitroxyl radical a variety of available structures.26–30 Notably, functionalized ILs
2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO), which has proved for specific-task have drawn much interest in the synthesis
to be very effective for the aerobic oxidation of alcohols.8–13 fields. In 2005, Miao31 et al. reported an effective catalytic system
Copper as one of the abundant and cheap transition metals is comprising 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) func-
an essential element in the human body, which is found to be very tionalized imidazolium salt ([Imim-TEMPO]+XÀ), a carboxylic
effective in the binding of molecular oxygen and highly selective acid substituted imidazolium salt ([Imim-COOH]+XÀ), and
aerobic oxidative transformations. Recently, utilization of TEMPO sodium nitrite (NaNO2) which was developed for the aerobic
oxidation of aliphatic, allylic, heterocyclic and benzylic alcohols to
the respective carbonyl compounds with an excellent selectivity
School of Chemical Engineering, Nanjing University of Science and Technology,
of up to 499% under ambient conditions. In 2010, Fall32 et al.
Xiaolingwei 200, Nanjing, Jiangsu Province, China. E-mail: luming302@126.com;
described an efficient synthesis of an ionic liquid-supported
Fax: +86-025-84315030; Tel: +86-025-84315030
TEMPO ([Imim-TEMPO][BF4À]) which was used for the oxidation
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c4nj00778f
of alcohols to aldehydes and ketones. The predictable solubility
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of ionic liquids allows an easy separation of the oxidation products. reused without significant loss of catalytic activity. The protocol
Furthermore, the IL-supported TEMPO can be recycled and used presented here provides an efficacious strategy to achieve both
several times without the loss of efficiency. Wu33 et al. also described catalyst recycling and product isolation from a viewpoint of
a novel 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) radical bearing green chemistry.
an ionic liquid-type appendage and its catalytic activity for the
selective oxidation of alcohols to the corresponding carbonyl
Results and discussion
compounds under ionic liquid–aqueous biphasic conditions.
The ionic liquid-supported TEMPO radical shows catalytic proper-
The synthesis steps of bi-functionalized PEG1000 ionic liquid
ties similar to those of its non-supported counterpart in terms of
[Imim-PEG1000-TEMPO][CuCl2À] are as depicted in Scheme 1. As
activity and selectivity, and can be easily recycled and reused
can be seen in Fig. 1, the O–H stretching vibrations of 4-hydroxy-
without loss of activity and selectivity.
TEMPO at 3390 cmÀ1 have disappeared revealing that PEG1000
Polyethylene glycols, as commercially available, non-toxic,
ionic liquid has been functionalized by TEMPO (Fig. 1b and c).
and recyclable polymers, have been widely used in organic
With benzyl alcohol as the model substrate, the bi-functionalized
transformation as environmentally benign solvents and soluble
PEG1000 ionic liquid ([Imim-PEG1000-TEMPO][CuCl2À]) was designed
supports for liquid phase synthesis, which are hopeful to sub-
for the aerobic oxidation of alcohols. The exploratory experiments
stitute volatile organic solvents and display an excellent effect for
were started by screening the metal salts of the catalyst system and
promoting many transformations,34–41 such as N-arylation of
amines, cross-coupling reaction, asymmetric dihydroxylation,
the results are summarized in Table 1. It can be seen that among the
salts tested, copper salts showed much higher catalytic activity than
aldol reaction, Baylis–Hillman reaction, multi-component con-
densation. In 2005, Pozzi et al.42 have reported poly(ethylene
glycol)-supported TEMPO and its catalytic activity in the chemo-
selective oxidation of alcohols with stoichiometric amounts
of organic or inorganic oxidants. The new metal-free catalyst
exhibits high activity and easy removal from the reaction mixture
by filtration. Recently, Zhi and Hu43,44 have reported a new
polyethylene glycol supported ionic liquid (PEG1000-DIL), which
exhibits good compatibility of the tandem catalyst/substrate, an
excellent catalytic effect in promoting reaction and easy catalyst
recovery from the reaction system. As our interest for environ-
mentally benign protocols for reactions, hererin, a novel catalytic
system with bi-functionalized PEG1000 ionic liquid [Imim-PEG1000
-
TEMPO][CuCl2À] was described in our work (Scheme 1) and has
been proved to be an effective catalyst for selective aerobic
oxidation of primary and secondary alcohols into the corre-
sponding aldehydes or ketones without the additive of base.
Moreover, it was delightful that the catalytic system afforded
excellent durability, which could be successfully recovered and
Fig. 1 IR spectra of (a) PEG-1000 (b) 4-hydroxy-TEMPO (c) [Imim-PEG1000
-
TEMPO][CuCl2À].
Scheme 1 Synthesis of bi-functionalized PEG1000 ionic liquid.
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Table 1 Effects of metal salt on bi-functionalized PEG1000 IL catalyzed
oxidation of benzyl alcohol under solvent-free conditionsa
Table 2 Reaction conditions for bi-functionalized PEG1000 IL catalyzed
oxidation of benzyl alcohol under solvent-free conditionsa
Entry
Metal salt
Conv.b (%)
Yieldc (%)
1
2
3
4
5
6
7
8
CuCl
CuBr
CuI
CuCl2
CoCl3
NiCl2
Mn(OAc)2
Cu(OAc)2
100
69
60
79
—
36
41
80
96
64
55
72
0
34
38
71
T
Conv.b Yieldc
Entry Catalysts (mol%)
Solvent (1C) (%)
(%)
1
—
—
60 —
0
2
3
[Imim-PEG1000-TEMPO][CuCl2À] (5) CH3CN 60 499 95
CuCl (5)/TEMPO (5)
CH3CN 60
92 90
60 53
49 47
31 31
89 84
4
5
CuCl (5)/TEMPO (5)
CuCl (10)/TEMPO (5)
DMF
DMSO
60
60
a
Reaction conditions: benzyl alcohol (10 mmol), [Imim-PEG1000
-
b
TEMPO] (0.5 mmol), metal salt (0.5 mmol), 60 1C. Determined by
HPLC. Isolated yield.
6
7
8
9
10
11
12
13
14
CuCl (5)/TEMPO (5)
Toluene 60
c
[Imim-PEG1000-TEMPO][CuCl2À] (1)
[Imim-PEG1000-TEMPO][CuCl2À] (5)
—
—
60
60 100 96
60 100 95
60 100 95
rt
40
80 100 90
100 100 78
[Imim-PEG1000-TEMPO][CuCl2À] (10) —
[Imim-PEG1000-TEMPO][CuCl2À] (20) —
the other transition metal salts with 60–100% conversion of benzyl
alcohol (Table 1, entries 1–4 and entry 8). CuCl was found to be the
best catalyst with 100% conversion, and 96% isolated yield (Table 1,
entry 1). The Co containing catalyst showed no catalytic activity
(Table 1, entry 5) and the Mn and Ni salts showed moderate catalytic
activity with 36–41% conversion (Table 1, entries 6 and 7).
[Imim-PEG1000-TEMPO][CuCl2À] (5)
[Imim-PEG1000-TEMPO][CuCl2À] (5)
[Imim-PEG1000-TEMPO][CuCl2À] (5)
[Imim-PEG1000-TEMPO][CuCl2À] (5)
—
—
—
—
54 50
90 85
a
Reaction conditions: benzyl alcohol (10 mmol), [Imim-PEG1000
-
TEMPO][CuCl2À] (0.5 mmol), 60 1C. b Determined by HPLC. Isolated yield.
c
This ionic liquid [Imim-PEG1000-TEMPO][CuCl2À] shows catalytic
properties similar to those of its non-supported counterpart in
terms of activity and selectivity affording the target products in carbonyl compounds under ambient conditions. The activities
good yield (Table 2, entries 2 and 3). In this respect, control of primary benzylic alcohols were best, and the activities of those
experiments were carried out in other aprotic solvents like primary benzylic alcohols were also not significantly affected by
DMSO, DMF and toluene (Table 2, entries 4–6). The solubility the electronic properties and steric hindrance of the substituents
of the catalyst would likely be more important for this reaction. on the benzene ring (Table 3, entries 1–8). Interestingly, the
As expected, DMSO, DMF and toluene could not well dissolve the designed oxidation system proved to be chemoselective. In the
catalyst and demonstrate a poor performance under identical case of a double bond containing substrate, the double bond
conditions, whereas [Imim-PEG1000-TEMPO][CuCl2À] is capable of was not influenced by this method (Table 3, entry 9). Further-
forming a homogeneous system to enhance the reaction. One more, secondary benzylic alcohols such as benzhydrol could
explanation was the stabilisation of Cu(I) species during the catalytic give moderate conversion by prolonging the reaction time
cycle caused by the coordination of the N-methylimidazole molecule (Table 3, entries 10–12). Heterocyclic alcohols such as furfuryl
and the formed Cu(I) complex would intend to bind with the alcohol alcohol would be also suitable to the system (Table 3, entries 13
molecule, which is more favorable for the reaction. The other and 14). Unfortunately, aliphatic alcohol such as dodecyl
reason may be that the bi-functionalized PEG1000 ionic liquid alcohol gave lower yields when the alcohol was added to the
exhibits good compatibility of the catalyst and the substrate. It system (Table 3, entry 15).
was reported that organic solvents were usually used to improve
The recyclability of the catalytic system was also investi-
the solubility of substrates,24 however, our catalytic system has gated. After completion of the oxidation reaction, the mixture
a good catalytic performance without adding organic solvents. was allowed to cool to room temperature for extraction with
As we know, PEGs are amphiphatic molecules due to their ether. The upper layer containing product was removed by
special long chain structure. The –CH units of PEG could decantation. The lower layer left was dried to remove a small
increase the solubility of organic compounds which is derived amount of water by vacuum and fresh substrates were then
from the gathering effect of the hydrocarbon chain. Moreover, recharged to react once again. With benzyl alcohol as a model
the effect of the catalyst amount was evaluated. The reaction substrate, IL was proved to be efficient after five times reutiliza-
was operative in the range of the catalyst loading from 1 to tion both in the view of yield and selectivity (Fig. 2). There was
20 mol%. The best transformation was completed within 12 h no great loss of activity or apparent reduction of weight during
in the presence of a 5 mol% catalyst (Table 2, entries 7–10). the recycling process.
Notably, the oxidation gave the best performance at 60 1C, and
also could be carried out under mild conditions even at room proposed catalytic cycle. Firstly, species I are generated by the
temperature (Table 2, entries 11–14). TEMPO-mediated oxidation of Cu(I) to Cu(II) species. Then,
The TEMPO might act as a hydrogen acceptor during the
Encouraged by those results, some other alcohols were further alcohol coordinates to the copper(II) species I to lead to alkoxy
examined to extend the substrate scope of this methodology and species II. The transfer of b-hydrogen to TEMPO and intra-
the results are shown in Table 3. To our delight, the catalytic molecular one-electron transfer leads to the aldehyde, TEMPOH
system was effective and highly selective for the oxidation of and Cu(I) species. The catalytic cycle is completed by the aerobic
benzylic, heterocyclic and allylic alcohols to the respective oxidation of the TEMPOH to TEMPO (Scheme 2).
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Table 3 Bi-functionalized PEG1000 IL catalyzed oxidation of various alcoholsa
Entry
1
Substrate
Product
Time (h)
12
Conv.b (%)
100
Yieldc (%)
96
2
3
10
12
499
499
92
98
4
12
499
96
5
6
7
8
9
10
13
12
12
14
499
99
98
95
93
88
85
98
91
90
10
11
12
16
20
79
81
83
78
75
20
20
80
88
13
14
90
88
16
16
87
72
15
76
a
b
c
Reaction conditions: alcohols (10 mmol), [Imim-PEG1000-TEMPO][CuCl2À] (0.5 mmol), 60 1C. Determined by HPLC. Isolated yield.
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performance to promote reaction effectively. Moreover, it could
be recycled and reused without significant loss of catalytic
activity after seven runs. For this environmentally catalytic
system, it would find a wider application in various reactions,
which is an ongoing project.
Experimental section
All starting materials were purchased from commercial sources and
used without further treatment. Analytical thin layer chromato-
graphy (TLC) was performed on precoated silica plates. Yields of
the products refer to purification by silica-gel column chromato-
graphy. IR spectra were recorded on a Nicolet IS-10 Fourier
transform infrared spectrometer. 1H NMR spectra were recorded
on a Bruker Advance III (500 MHz) spectrometer with tetra-
methylsilane (TMS) as an internal standard. High performance
liquid chromatography experiments were performed on a liquid
chromatograph (Shimadzu LC-20AT, Japan).
Synthesis of bi-functionalized PEG1000 ionic liquid [Imim-
À
PEG1000-TEMPO][CuCl2
]
Fig. 2 Cycling reaction of (a) the reaction mixture (b) adding ether when
cooling to room temperature.
To a solution of PEG-1000 (0.06 mol, 60 g) in toluene (150 mL),
pyridine (0.12 mol, 9.6 mL) was added, followed by the addition
of thionyl chloride (0.12 mol, 8.7 mL) within 30 min under a N2
atmosphere at 0 1C. The resulting slurry was stirred for 24 h at
room temperature and filtered. The toluene was then removed
by rotator evaporation under reduced pressure to give inter-
mediate 1 as a gray viscous liquid. In the next step, 4-hydroxy-
TEMPO (8.6 g, 0.05 mol) was added to a suspension of Na
(1.15 g, 0.05 mol) in benzene (100 mL) and the resulting slurry
was stirred for 8 h at room temperature. Intermediate 1 (0.025 mol)
was then added and stirring resumed for 24 h at room temperature.
The suspension was then filtered and the filtrate concentrated
under reduced pressure. This solution was then added to ether
(100 mL) that was stirred vigorously and the precipitate was
collected by filtration, washed twice with ether (50 mL), and
dried under vacuum. The product 2 was obtained as orange red
viscous solid. Then, intermediate 2 (0.025 mol) was added to
N-methylimidazole (2.05 g, 0.025 mol) solution which was
dissolved in toluene, and the mixture was stirred at 70 1C for
20 h. The reaction was determined by HPLC and evaporated
under reduced pressure to give intermediate 3 as an orange red
oil with a yield of 98.3%. In the last step, the intermediate 3 was
mixed with anhydrous CuCl together under a N2 atmosphere
with a mechanical stirring at room temperature. The obtained
red brown liquid was washed with ether, evaporated by reduced
pressure and dried in a vacuum successively. Then, the ionic
liquid ([Imim-PEG1000-TEMPO][CuCl2À]) was thus obtained.
Scheme 2 The mechanism of aerobic oxidation of alcohols.
Conclusions
General procedure for oxidation of alcohols
In conclusion, we have developed an efficient and reusable
catalytic system for the aerobic oxidation of alcohols catalyzed
To a 10 mL round-bottom flask containing the [Imim-PEG1000
-
TEMPO][CuCl2À] (0.5 mmol), benzyl alcohol (10 mmol) was
successively added under a constant vigorous stirring. The reaction
was proceeded at a temperature of 60 1C and monitored by TLC.
Upon completion, the reaction mixture was cooled to room
by bi-functionalized PEG1000 ionic liquid ([Imim-PEG1000
-
TEMPO][CuCl2À]) with good yields. Owing to the high efficiency
and good compatibility, the catalytic system had a good catalytic
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´
18 I. E. Marko, P. R. Giles, M. Tsukazaki, S. M. Brown and
temperature and extracted three times by adding ether. The
organic phase was dried over anhydrous MgSO4, evaporated
under reduced pressure and to give benzaldehyde (96% yield)
with spectral data consistent with the assigned structures
for the products. The next run was performed under identical
reaction conditions.
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