TEMPO-based ionic liquid with temperature-dependent property and application for aerobic oxidation of alcohols

Article abstract of DOI:10.1080/00397911.2012.677927

A novel temperature-dependent ionic liquid based on 2,2,6,6- tetramethylpiperidine-1-oxyl and polyethylene glycol 600 was developed and used in oxidation of alcohols. Various alcohols were oxidized into their corresponding aldehydes or ketones with good yield and selectivity. Owing to the temperature-dependent property of the catalyst, easy recovery was achieved by simple decantation. Supplemental materials are available for this article. Go to the publisher's online edition of Synthetic Communications to view the free supplemental file.

Full text of DOI:10.1080/00397911.2012.677927

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TEMPO Based Ionic Liquid with Temperature-
Dependent Property and Its Application for Aerobic
Oxidation of Alcohols
Jie Zhu a , Peng-cheng Wang a & Lu Ming b
a School of Chemical Engineering, Nanjing University of Science and Technology, Nanjing,
China
b Chemical Engineering College, Nanjing University of Science and Technology, Nanjing,
China
Accepted author version posted online: 02 Jan 2013.
To cite this article: Jie Zhu , Peng-cheng Wang & Lu Ming (2013): TEMPO Based Ionic Liquid with Temperature-Dependent
Property and Its Application for Aerobic Oxidation of Alcohols, Synthetic Communications: An International Journal for Rapid
Communication of Synthetic Organic Chemistry, DOI:10.1080/00397911.2012.677927
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ACCEPTED MANUSCRIPT
TEMPO Based Ionic Liquid with Temperature-dependent Property and its
Application for Aerobic Oxidation of alcohols
Jie Zhu1, Peng-cheng Wang1, Lu Ming2
1School of Chemical Engineering, Nanjing University of Science and Technology,
Nanjing, China, 2Chemical Engineering College, Nanjing University of Science and
Technology, Nanjing, China
Abstract
A novel temperature-dependent ionic liquid based on TEMPO and PEG-600 was
developed and used in oxidation of alcohols. Various alcohols were oxidized into their
corresponding aldehydes or ketones with good yield and selectivity. Owing to the
temperature-dependent property of the catalyst, easy recovery was achieved by simple
decantation.
Supplemental materials are available for this article. Go to the publisher’s online edition
of Synthetic Communications® to view the free supplemental file.
INTRODUCTION
The selective oxidation of primary and secondary alcohols into the corresponding
aldehydes or ketones is one of the most important and challenging transformations in
modern organic chemistry [1–2]. Recently, utilization of the stable nitroxyl radical
2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) for oxidation of alcohols employing a
co-oxidant including m-CPBA [3–4], hypochlorite [5], or molecular oxygen [6–7], together
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with a catalytic amount of metal-salt [8–9], bromide [10], persulfate [11] and so on attracts
great attention in view of catalytic efficiency and green chemistry. Whichever co-oxidant
is used, however, product isolation and TEMPO recovery remain key issues. Moreover,
since TEMPO is quite expensive, although low catalyst concentrations are required
(typically 1-2.5 mol %), it is thus highly desirable to separate and reuse the catalyst after
the oxidation reaction.
To address these problems, several TEMPO catalyst systems have been developed
including silica-supported TEMPO [12–13], MCM-41 supported TEMPO [14–15], sol-gel
entrapped TEMPO [16–17], and polyamine immobilised piperidinyl oxyl (PIPO) [18]
,
Poly(vinyl alcohol)-graft-poly(ethylene glycol) resins supported TEMPO [19], to afford
heterogeneous catalysts, which are readily separated from the reaction mixtures but are
usually far less versatile than the homogeneous TEMPO. A novel emerged option is the
immobilization of TEMPO onto ionic liquids, which makes a homogeneous process and
the IL-supported TEMPO can be recycled and used several times without great loss of
efficiency [20–21]. Then very recently magnetic Fe was introduced into ionic liquid with
TEMPO to allow an easy separation of the oxidation products from reagents, which
proved to be an effective catalyst for selective aerobic oxidation of aromatic alcohols as
well [22–23]
.
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Recently, a possible strategy to circumvent the problem is the study of
temperature-dependent property [24–25], which can achieve homogeneous system and easy
separation as well. And polyethylene glycol (PEG), with its unique solubility, has been
widely used [26–27]. Herein, we designed a new temperature-dependent biphasic system,
including recoverable novel PEG-600 and TEMPO based ionic liquid 3
([Imim-PEG600-TEMPO][OMS], Scheme 1) and mixed solvent of cyclohexane and
carbon tetrachloride, for the oxidation of a serious of alcohols. Improved reaction
efficiency with facilitated separation of the catalyst and products were achieved without
tedious extraction process.
RESULTS AND DISCUSSIONS
[Imim-PEG600-TEMPO][OMS] IL is a hygroscopic viscous red liquid (ρ = 1.21 g/mL,
25℃), and is stable to air and heat. Treatment of it under vacuum at 100℃ for 72 h
resulted in no obvious loss of weight. The solubility properties of
[Imim-PEG600-TEMPO][OMS] IL in some traditional solvents at different temperatures
were investigated and the results are shown in Table 1. It can be seen from Table 1 that
[Imim-PEG600-TEMPO][OMS] is insoluble in several normal organic solvents even when
the temperature is close to their boiling points (entries 8-11). On the other hand, it
exhibited excellent solubility in water and polar organic solvents (entries 1-5). And the
transition state of solubility between cyclohexane and carbon tetrachloride is worthy of
note (entries 6-7). So we hypothesized that there is a certain ratio of cyclohexane to
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carbon tetrachloride, which enables the IL nearly insoluble in the mixed solvent at low
temperature but miscible at high temperature. Experiments show that the optimal
proportion of cyclohexane to carbon tetrachloride is 4:6 for volume (note as mixed
solvent in the following paragraphs), and the phase transformation temperature is about
60℃.
An initial experiment of application of the task-specific temperature-depedent
imidazolium salts for oxidation was carried out by using benzyl alcohol as substrate and
the reaction procedure is demonstrated in Fig. 1.
The oxidation of benzyl alcohol was further studied in different reaction media and the
compared results are summarized in Table 2. It was found that almost no product was
obtained in the mixed solvent only without any catalyst (entry 1, Table 2). The yield was
only 58% in [Imim-PEG600-TEMPO][OMS]/ cyclohexane media, as it is still a biphasic
catalytic system at 60℃ (entry 2, Table 2). Although similar catalytic activity was
achieved when [Imim-PEG600-TEMPO][OMS]/CCl4 or [Imim-TEMPO][Cl] was used
with NaNO2 as co-catalyst and O2 as oxidant [15], longer time and complicated extraction
process, however, were still needed. In comparison, [Imim-PEG600-TEMPO][OMS]/
mixed solvent gave the best result (entry 4, Table 2).
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To screen the reaction conditions, exploratory experiments were conducted with benzyl
alcohol as the model substrate (Table 3). NaNO2, O2 and
[Imim-PEG600-TEMPO][OMS]/mixed solvent were essential to the oxidation reaction.
Preliminary results indicate that good solubility and catalytic performance of the catalyst
were achieved at 60℃. Furthermore, NaNO2 and IL 3 of catalytic amount was enough to
obtain good yield. And from the view of easy separation, 1mL IL was used in the
following experiments.
To examine the utility and generality of this methodology for the oxidation of alcohols,
we applied the present catalyst system to a variety of alcohols as shown in Table 4.
Obviously, all primary benzylic alcohols were converted into their corresponding
aldehydes in high yields (entries 1-7). It is noteworthy that a type of heterocyclic alcohol
(entries 8-10), being less active in many reported systems, worked well in the
[Imim-PEG600-TEMPO][OMS]/mixed solvent/NaNO2/O2 system. And the reactivity
follows the order 2-furfuryl alcohol > 2-thiophenemethanol > 2-pyridinylmethanol. And
it was found that the oxidative efficiency was not affected with the existence of double
bond (entry 11) or substituent group on benzene ring (entries 2-7). Furthermore,
secondary benzylic alcohols such as benzhydrol can give moderate conversion by
prolonging the reaction time (entries 12-13). Unfortunately, in the case of aliphatic
alcohols such as 1-C8H17OH and isooctyl alcohol, the results were unsatisfactory even
after elongating the reaction time (entries 14-15). Interestingly, the designed oxidation
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system proved to be chemoselective. When an equimolar mixture of primary and
secondary alcohols was tested in the system, primary alcohols 4-methoxybenzyl alcohol
or furfuryl alcohol was converted to their corresponding aldehydes while the secondary
alcohols remained almost completely (entries 16-17).
TEMPO-functionalized PEG-6000 combined with copper(I) chloride
PEG6000-(TEMPO)2/CuCl was reported to catalyze the aerobic oxidation reaction of
alcohols in compressed CO2 [29], which prompted us to investigate the effect of molecular
weight of PEG. PEG-200 and PEG-1000 was tried instead of PEG-600 in our
temperature-control system and it was found that [Imim-PEG200-TEMPO][OMS]
couldn’t dissolve well in the mixed solvent even at 60℃, while
[Imim-PEG1000-TEMPO][OMS] was partly soluble in the mixed solvent at room
temperature. Once homogeneous phase was formed, the reaction efficiency had nothing
to do with the molecular weight of PEG.
The recyclability of the catalytic system was also investigated. After completion of the
oxidation reaction, the mixture was allowed to cool to room temperature. The upper layer
of solvent, containing product, was removed by decantation. After vacuum drying to
remove small amount of water, fresh substrates, solvent, NaNO2 and H2SO4 were then
recharged to the residual [Imim-PEG600-TEMPO][OMS] and the mixture was heated to
react once again. When benzyl alcohol was used as a model substrate, the procedure was
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successfully repeated for at least 5 times without great loss of catalytic activity (Fig. 2).
After several times of recycle, however, the IL 3, with an affinity for water, became less
viscous and the temperature-dependent property varied gradually, which undermined the
system.
Based on previous studies [28,30–31], a possible mechanism for the aerobic oxidation of
alcohols in the system was proposed as shown in Scheme 2. Imim-PEG-TEMPO cation
(nitrosonium) oxidized alcohols into their corresponding aldehydes or ketones and was
turned into hydroxylamine (Imim-PEG-TEMPOH) itself. NO and NO2 were released
-
from NO2 under acidic condition and the oxidation of NO into NO2 could proceed
smoothly with the help of O2. The reaction between NO2 and hydroxylamine leaded to
the completion of the recycle of Imim-PEG-TEMPO cation. In other word, the
oxidation process was completed by the cycle between NO and NO2 and the cycle
between TEMPO cation and hydroxylamine.
EXPERIMENTAL SECTION
Materials Remarks
All the PEG samples employed in the synthesis of 3 were heated to 80℃ under vacuum
for 30 min before use to remove traces of moisture. The solvents, toluene and THF, were
distilled by the known procedure prior to use to be anhydrous. The other organic
compounds were obtained from commercial sources and used without further treatment.
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General Procedures For The Synthesis Of PEG 600 Mesylate 1
To a solution of PEG-600 (0.06 mol) in toluene (350 mL) triethylamine was added (0.12
mol), followed by the addition of mesyl chloride (0.12 mol) within 30min under N2
atmosphere at 0℃. The resulting slurry was stirred for 3h at room temperature and
filtered. The toluene was then removed by rotator evaporation under reduced pressure to
give PEG-600 mesylate 1 as a white viscous liquid.
General Procedures For The Synthesis Of TEMPO Based PEG 600 Mesylate 2
To a suspension of NaH (1 g, 0.025 mol) in THF (150 mL) was added
4-hydroxy-TEMPO (4.21 g, 0.025 mol) and the resulting slurry was stirred for 1 h at
room temperature under N2 atmosphere. PEG-600 mesylate 1 (18.92g, 0.025 mol) was
then added and stirring resumed for 48 h at room temperature. The suspension was then
filtered and the filtrate concentrated under reduced pressure. This solution was then added
to diethyl ether (100 mL) that was stirred vigorously and the precipitate collected by
filtration, washed twice with diethyl ether (2-50 mL), and dried under vacuum. The
product 2 was obtained as orange red liquid.
General Procedure For The Synthesis Of [Imim-PEG600-TEMPO][OMS] 3
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19-methyl-1H-Imidazole (1.64 g, 0.02 mol) was added into the solution of 2 (16.96 g,
0.02mol) in toluene (150 mL) and the mixture was stirred at 80℃ for 24 h. The final
solution was evaporated under reduced pressure to remove toluene. Then the yielding
liquid was extracted with petroleum ether to give ionic liquid
[Imim-PEG600-TEMPO][OMS] 3 as a viscous brown liquid.
General Procedure For The Oxidation Reaction
A 25 mL round-bottom flask containing the IL (1 mL), cyclohexane (4 mL), CCl4 (6 mL),
and the alcohol substrate (1.85 mmol) was kept under a constant temperature of 60℃
with reflux. Then NaNO2 (5% mol of the substrate) were added. And oxygen was
inputted into the bottom of the suspension with the flowing rate 0.004 m3/h. The reaction
was allowed to proceed under vigorous stirring after which the contents of the reactor
were cooled down at atmosphere to give two phases. The organic phase was separated,
washed with water, dried over MgSO4, filtered, and finally analyzed by GC-FID.
General Procedure For The Recovery Of The Catalyst
After completion of the oxidation reaction, the final mixture was then cooled to room
temperature and the organic phase was separated by decantation, and the solution was
allowed to stand for purification followed by GC analysis. The layer of PEG600-TEMPO
IL can be reused without any treatment or vacuum drying is adopted to remove small
amount of water if necessary to achieve better results.
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Complete experimental details are available online in the Supplementary Materials.
CONCLUSION
In conclusion, we developed a novel type of temperature-dependent catalytic system
comprised of a TEMPO functionalized imidazolium salt
[Imim-PEG600-TEMPO][OMS]/cyclohexane/CCl4/NaNO2/O2 for the selective oxidation
of alcohols. The present homogeneous catalyst system was recyclable, highly efficient
and selective for a wide set of aliphatic, allylic, heterocyclic and benzylic alcohols.
[Imim-PEG600-TEMPO][OMS] could be efficiently recovered by simple decantation after
reaction without any apparent loss of catalytic activity and little loss of weight even after
5 times recycling. This present task-specific ionic liquids thus offers improved reaction,
facilitated separation of the catalyst and products, and safe operation for the oxidation of
alcohols into aldehydes without the need of a transition metal.
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Table 1 Solubility of [Imim-PEG200-TEMPO][OMS] IL in some solvents a
entry solvent
T/
25 40
60 80 100
1
H2O
s
s
s
s
s
s
s
s
s
2
EtOH
3
CH2Cl2
PhNO2
PhMe
4
s
s
s
s
s
s
s
s
5
6
CCl4
ps ps
7
cyclohexane
Acetone
Benzene
AcOEt
Petroleum
Ether
i
i
i
i
i
i
i
i
i
i
i
i
i
8
9
10
11
i
a s: soluble; i: insoluble; ps: partly soluble.
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Table 2 Oxidation of benzyl alcohol in different media
Entry
Solvent
T/
t/h Conv Yield
(%) (%)
12 trace Trace
1
2
mixed solvent a
60
60
[Imim-PEG600-TEMPO][OMS]/
cyclohexane a
12 58
56
a
3
4
5
[Imim-PEG600-TEMPO][OMS]/CCl4
60
60
3
8
3
98
97
98
97
97
97
b
[Imim-TEMPO][Cl]/H2O/CO2
[Imim-PEG600-TEMPO][OMS]/mixed 60
solvent a
a Reaction conditions: benzyl alcohol (1.85mmol), [Imim-PEG600-TEMPO][OMS] 1mL,
cyclohexane 4mL, CCl4 6mL, NaNO2 (5% mmol of benzyl acohol), 10% H2SO4 0.1mL,
O2 was imputed at 0.004 m3/h.
b according to ref. [21]
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Table 3 Effects of catalyst components on the oxidation of benzyl alcohol a
Entr IL 3 NaNO2 O2
Tim Conv
Yield
(%)
y
(mL) (mmol) (m3/h e (h) (%)c
)
1
2
3
4
5
6
0
0.1
0.1
0.1
0.1
0.2
0.1
0.004 12
12
0.004 2
0.04
trace
trace
98
trace
trace
97
1
0
1
6
2
98
97
1
0.004 2
0.004 2
98
97
0.1
98
97
a Reaction conditions: benzyl benzyl alcohol (1.85mmol), cyclohexane 4mL, CCl4 6mL,
60 .
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Table 4 Oxidation of various alcohols a
Entry
Alcohol
Products
t/h
Conv Yield
(%)
98
(%)
97
1
2
2
2
95
95
3
4
4
2
79
90
77
89
5
1.5
97
97
6
7
6
6
81
88
80
85
8
5
96
92
83
47
53
64
7
92
90
81
46
52
63
7
9
10
24
12
10
10
10
11
12
13
14
O
Ph CH3
1-C8H17OH 1-C7H13CHO 24
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15
24
6
83
+
6
82
+
16
4
+
+
+
+
2
2
17
7
91
+
90
+
3
3
a Reaction conditions: alcohol (1.85mmol), [Imim-PEG600-TEMPO][OMS] 1mL,
cyclohexane 4mL, CCl4 6mL, NaNO2 (5% mmol of benzyl acohol), 10% H2SO4 0.1mL,
O2 was imputed at 0.004 m3/h, 60 .
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Scheme 1 synthesis of [Imim-PEG600-TEMPO][OMS]
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Scheme 2 A possible mechanism for the aerobic oxidation of alcohols.
recycle
ionic liquid
heating
ionic liquid
cooling
homogeneous
organic solvent
organic solvent
+
+
substrate
product
-
NO2
H+
R1
OH
O
Imim - PEG600
Imim - PEG600
N O
R2
NO
O2
R1
R2
N
ΟΗ
NO2
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Figure. 1 Reaction procedure in [Imim-PEG600-TEMPO][OMS]/mixed solvent: (a)
[Imim-PEG600-TEMPO][OMS]/mixed solvent at room temperature; (b) homogeneous
phase at 60 ; (c) phase-separation along with cooling; (d) complete phase-separation
after being cooled to room temperature.
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Figure. 2 Recycling of [Imim-PEG600-TEMPO][OMS].
100
80
60
40
20
0
1
2
3
4
5
Cycle
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