Spontaneous product segregation from reactions in ionic liquids: Application in Pd-catalyzed aliphatic alcohol oxidation

Article abstract of DOI:10.1039/b920813p

A methodology is introduced to separate polar reaction products from ionic liquids without the need for organic solvent extraction or distillation. We investigated product isolation after an alcohol oxidation performed in ionic liquids. Suitable ionic liquids were selected based on their mixing or demixing with a range of alcohols and the derived ketones. The aim was to obtain complete miscibility with the alcohol substrate at reaction temperature and a clear phase separation of the derived ketone product at room temperature. Six imidazolium based ionic liquids displayed this desired behaviour and were sufficiently stable to oxidation. These ionic liquids were then employed in the oxidation of non-activated aliphatic alcohols with molecular oxygen in the presence of palladium(ii) acetate. In 1-butyl-3-methylimidazolium tetrafluoroborate, 2-ketone yields of 79 and 86% were obtained for, respectively, 2-octanol and 2-decanol. After cooling to room temperature the ionic liquid expels the immiscible ketone and the product phase can be isolated by decantation. In addition, the ionic liquid acts as an immobilization medium for the palladium catalyst, allowing efficient catalyst recycling.

Full text of DOI:10.1039/b920813p

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PAPER
www.rsc.org/pccp | Physical Chemistry Chemical Physics
Spontaneous product segregation from reactions in ionic liquids:
application in Pd-catalyzed aliphatic alcohol oxidationw
a
a
a
b
Charlie Van Doorslaer, Yves Schellekens, Pascal Mertens, Koen Binnemans
a
and Dirk De Vos*
Received 5th October 2009, Accepted 19th November 2009
First published as an Advance Article on the web 21st December 2009
DOI: 10.1039/b920813p
A methodology is introduced to separate polar reaction products from ionic liquids without the
need for organic solvent extraction or distillation. We investigated product isolation after an
alcohol oxidation performed in ionic liquids. Suitable ionic liquids were selected based on their
mixing or demixing with a range of alcohols and the derived ketones. The aim was to obtain
complete miscibility with the alcohol substrate at reaction temperature and a clear phase
separation of the derived ketone product at room temperature. Six imidazolium based ionic
liquids displayed this desired behaviour and were sufficiently stable to oxidation. These ionic
liquids were then employed in the oxidation of non-activated aliphatic alcohols with molecular
oxygen in the presence of palladium(II) acetate. In 1-butyl-3-methylimidazolium tetrafluoroborate,
2
-ketone yields of 79 and 86% were obtained for, respectively, 2-octanol and 2-decanol. After
cooling to room temperature the ionic liquid expels the immiscible ketone and the product phase
can be isolated by decantation. In addition, the ionic liquid acts as an immobilization medium for
the palladium catalyst, allowing efficient catalyst recycling.
Introduction
important drawbacks, related to the specific nature of DMSO,
remain. DMSO is toxic and difficult to fully separate from the
product, leading to a problematic recycling of the catalyst and
solvent. These problems could be overcome by applying ILs,
of which the polarity is comparable with DMSO.
The selective oxidation of alcohols is an important transformation
in chemical industries owing to the abundant use of the derived
carbonyl compounds as intermediates in the production of fine
chemicals and pharmaceuticals. Numerous oxidizing agents are
Ionic liquids (ILs) receive increasing interest as green alter-
1–3
available for this key reaction. Unfortunately, these reagents
are generally consumed stoichiometrically and they often have a
toxic nature, which demands costly purification processes to
natives for the conventional volatile organic media owing to
10–14
their unique solvent properties.
Recently, numerous transition
metal catalyzed systems in ILs for alcohol oxidations were
15–21
4–7
obtain non-noxious end products. In view of the ever-growing
environmental concern, more sustainable processes, e.g. pro-
ductive catalytic transformations with safe reagents, became a
principal R&D focus in all chemical industries. Therefore, the
development of selective oxidation processes that use environ-
mentally benign oxidants represents a significant challenge. The
palladium catalyzed oxidation with molecular oxygen is truly a
worthy alternative for traditional processes with metal oxides and
introduced.
In some cases the IL even served as solvent
and catalyst, e.g. with a catalytic metal species part of the IL’s
22–24
anion.
While such catalytic systems performed well for
allylic or benzylic alcohols, they were not very active for non-
activated aliphatic alcohols. Moreover, the inevitable need for
an extraction step to separate the reagent/product from the IL
phase makes this approach less attractive.
We previously reported on a recyclable IL, based on the
spontaneous expulsion of the product from the IL, for the
25
5,7
metal salts. Moreover, molecular oxygen is a relatively cheap
8
oxidant and only water is formed as a by-product (Scheme 1).
catalytic hydrogenolysis of aromatic ketones. In line with
In 1998, Peterson and Larock reported the palladium
9
catalyzed oxidation of alcohols in DMSO with O2. Given the
our previous work, we now present a catalytic IL system for
the alcohol oxidation which allows ketone product isolation,
without an extracting solvent. Applying the concept of
temperature dependent miscibility of ILs and organic com-
ponents, a selection of ILs was made which formed one
homogeneous phase with the substrate at reaction tempera-
ture, while after cooling to room temperature, phase separation
occurs. This enables product isolation by simple decantation.
favourable oxidation potential in DMSO for the reoxidation
2
of palladium, an extra co-oxidant such as CuCl proved
unnecessary. Although this system shows several advantages,
a
Centre for Surface Chemistry and Catalysis,
Department of Microbial and Molecular Systems, K.U. Leuven,
Kasteelpark Arenberg 23, box 2461, 3001 Leuven, Belgium.
E-mail: dirk.devos@biw.kuleuven.be
b
Laboratory of Coordination Chemistry, Department of Chemistry,
K.U. Leuven, Celestijnenlaan 200F, box 2404, 3001 Leuven, Belgium
w Electronic supplementary information (ESI) available: NMR
spectra of ILs before and after oxygen treatment are provided. See
DOI: 10.1039/b920813p
Scheme 1 The palladium catalyzed oxidation of secondary alcohols
using molecular oxygen.
This journal is ꢀc the Owner Societies 2010
Phys. Chem. Chem. Phys., 2010, 12, 1741–1749 | 1741

In the class of alcohols, aliphatic alkanols were selected as
substrates, as they are non-activated and not easily oxidized to
ketones in good yields. Furthermore, the IL acts as an
immobilization medium for the catalyst, which leads to an
efficient catalyst recycling.
compositions representative of two specific stages (0 and
70% substrate conversion) in the course of the oxidation
reaction. In order to mimic the initial reaction conditions, a
100 mol% alcohol/0 mol% ketone mixture (‘100/0’) was
prepared consisting of alcohol (5 mmol) and IL. The mass
ratio IL/alcohol was 1/1. The conditions at 70% conversion
were simulated with a 30 mol% alcohol/70 mol% ketone
mixture (‘30/70’) containing alcohol (1.5 mmol), ketone
Experimental
(
3.5 mmol), water (3.5 mmol) and the same amount of IL as
Materials
used in the ‘100/0’ mixture. The mixtures were introduced
in a viewing cell and were heated at the reaction temperature
Abbreviaton list of ILs: [MOct N][Tf N] (methyltrioctyl-
2
3
ammonium bistriflimide with bistriflimide = bis(trifluoromethyl-
sulfonyl)imide); [MOct N][CF CO ] (methyltrioctylammo-
(
90 1C) for 4 h while stirring (1000 rpm). The mixing behaviour
3
3
2
of the different alcohol/(ketone, water)/IL combinations at the
reaction temperature was evaluated visually. Afterwards, the
mixtures were cooled to room temperature, allowed to stand
overnight, and the mixing behaviour was re-evaluated.
nium trifluoroacetate); [H
3
TP][N(CN)
2
] (trihexyltetradecyl-
] (1-methylimidazo-
N] (1-methylnitrile-
-methyl-imidazolium bistriflimide); [EMIM][BF ] (1-ethyl-
-methylimidazolium tetrafluoroborate); [EMIM][Tf N] (1-ethyl-
-methylimidazolium bistriflimide); [EMIM][CH
phosphonium dicyanamide); [MIM][HSO
lium hydrogensulfate); [CNMMIM][Tf
4
2
3
3
3
4
Oxidation experiments
2
3
CO
CO
2
]
]
Oxidation reactions were performed in stainless steel auto-
(
(
1-ethyl-3-methylimidazolium acetate); [EMIM][CF
3
2
claves (10 mL) under O atmosphere (10–30 bar) at 90–140 1C
2
1-ethyl-3-methylimidazolium trifluoroacetate); [EMIM]-
for 2–24 h. The reaction mixtures were stirred magnetically
[
EtSO ] (1-ethyl-3-methylimidazolium ethylsulfate); [EMIM]-
4
(
1000 rpm) and the reaction temperature was controlled with a
[
MeSO3] (1-ethyl-3-methylimidazolium methanesulfonate);
thermocouple. A manometer was used to monitor the pressure
changes during the reaction. A typical starting mixture con-
sisted of the alcohol (10 mmol), the same mass of ionic liquid
[
EMIM][TsO] (1-ethyl-3-methylimidazolium tosylate); [BMIM][BF4]
1-butyl-3-methylimidazolium tetrafluoroborate); [BMIM][Tf N]
1-butyl-3-methylimidazolium bistriflimide); [BMIM][TfO]
1-butyl-3-methylimidazolium triflate); [BMIM][Cl] (1-butyl-
(
2
(
(
2
and 1 mol% Pd(OAc) with respect to the alcohol. After
reaction, the mixtures were allowed to cool down to room
temperature. The upper layer was analyzed with a Shimadzu
3
-methylimidazolium chloride); [BMMIM][BF
4
]
(1-butyl-
N]
2
,3-dimethylimidazolium tetrafluoroborate); [BMMIM][Tf
2
2
014 GC equipped with an apolar CP-Sil 5 CB column and a
(
1-butyl-2,3-dimethyl-imidazolium bistriflimide); [BMMIM]-
CH CO ] (1-butyl-2,3-dimethylimidazolium acetate); [DMIm]-
TfO] (1-decyl-3-methylimidazolium triflate); [BMPyr][CF CO ]
FID detector. The identity of the reaction products was
verified by reference compounds and GC-MS. An Agilent
[
[
3
2
3
2
6
890 gas chromatograph, equipped with a HP-5MS column,
(
(
(
1-butyl-1-methylpyrrolidinium trifluoroacetate); [BMPyr][N(CN)2]
coupled to a 5973 MSD mass spectrometer was used.
1-butyl-1-methylpyrrolidinium dicyanamide); [BMPyr][Tf N]
2
1-butyl-1-methylpyrrolidinium bistriflimide); [CNBMPyr]-
NMR spectroscopic measurements
[
Tf
2
N] (1-butylnitrile-1-methylpyrrolidinium bistriflimide);
N] (1-ethylnitrile-1-methylpyrrolidinium bis-
N] (choline bistriflimide); [Chol][CH CO
choline acetate); [Chol][Ace] (choline acesulfamate); [Hbet]-
NMR Measurements were carried out with a Bruker AMX-300
1
at 300 MHz ( H) in deuterium oxide (D O).
[
CNEMPyr] [Tf
2
2
triflimide); [Chol][Tf
2
3
2
]
(
[
Tf
2
N] (betainium bistriflimide).
Results and discussion
2
The Pd(OAc) catalyst and all alcohol reagents and ketone
Miscibility of ionic liquids with alcohols and ketones
reference compounds were obtained from commercial sources.
All imidazolium-based ILs except for [EMIM][Tf N],
2
When performing oxidation reactions in ILs, it is required
that: (1) the IL has a melting point lower than the reaction
temperature; (2) the IL is chemically stable towards oxygen;
(3) in order to simplify the product separation after the
oxidation reaction, the IL should be miscible with the alcoholic
substrates at a typical reaction temperature (e.g. 90–120 1C)
and immiscible with the ketone products at room temperature.
The strong resemblance between the polarity of the reagent
and the product makes the selection of an IL with the desired
phase behaviour difficult. In a previously published paper it
was already found that the simplest reaction type to obtain the
desired phase behaviour is one in which a more polar reagent
[
[
[
[
BMIM][Tf N], [BMMIM][CH CO ], [BMMIM][Tf N] and
2 3 2 2
CNMMIM][Tf N] were obtained from IoLiTec or Merck.
2
MOct N][Tf N],
3
[H TP][N(CN) ],
3
[BMPyr][CF CO ],
3 2
2
2
BMPyr][N(CN)
2 2
] and [BMPyr][Tf N] were purchased from
IoLiTec. The other ILs were synthesized in our laboratories
2
6–28
based on synthesis procedures reported in literature.
The
water content of the ionic liquid batches used in the oxidation
reactions was less than 100 ppm for the hydrophobic ILs and
less than 1000 ppm for the halide containing ionic liquids. In
non-halide ILs, such as bistriflimides or acetates, the halide
content was typically less than 100 ppm.
2
5
is transformed into a more apolar product. Indeed, ILs will
more readily dissolve more polar compounds. The ideal ILs
thus lead to one homogeneous reaction phase at the reaction
temperature and phase separation at room temperature
enabling simple product isolation by decantation (Scheme 2).
Miscibility tests
The miscibility of reagents and reaction products with ILs was
evaluated by a simple visual test under optical magnification.
Mixtures were prepared to simulate two reaction mixture
1
742 | Phys. Chem. Chem. Phys., 2010, 12, 1741–1749
This journal is ꢀc the Owner Societies 2010

2
cation, as in [CNMMIM][Tf N], did not result in sufficient
miscibility with the model substrate either (entry 6). This IL
would have been highly valuable in view of the stabilizing
effect of the coordinating nitrile group on metal ions or
3
0
colloids. On the other hand, several 1-ethyl-3-methylimidazo-
lium ([EMIM]) ILs led to the appropriate mixing behaviour in
the case of 2-octanol/2-octanone (entries 7–13), except for
cases like 1-ethyl-3-methyl imidazolium tetrafluoroborate
(
[EMIM][BF
bis(trifluoromethylsulfonyl)imide ([EMIM][Tf
Most probably, the combination of a rather short alkyl chain
on the cation and a more hydrophobic anion like [BF ] or
Tf N] in the IL enhances its miscibility with both 2-octanol
4
]) (entry 7) and 1-ethyl-3-methylimidazolium
Scheme 2 Recyclable catalytic ionic liquid system using the concept
of temperature dependent miscibility of ionic liquids and organic
components.
2
N]; entry 8).
4
[
2
To mimic the initial conditions, 100 mol% alcohol/0 mol%
ketone mixtures (‘100/0’) were prepared in various ILs. The
reaction mixture composition at 70% conversion was
simulated with a 30 mol% alcohol/70 mol% ketone mixture
and 2-octanone and hence no phase separation was observed
upon cooling of the 30/70 mixture to room temperature.
However, the combination of a short alkyl chain cation with
a more polar anion substantially decreased the IL’s miscibility
with 2-octanone at room temperature without affecting the
solubility of the alcohol at reaction temperature. This is
evidenced in Table 1 where EMIM combined with the more
polar anions acetate ([MeCO ]), ethyl sulfate ([EtSO ]), tosylate
(
‘30/70’), adding extra water in an amount equimolar to that
of the ketone used. Water was added to closely resemble the
actual reaction conditions, since water is generated stoichio-
metrically (Scheme 1). For this screening phase, 2-octanol and
2
4
1
-phenylethanol were selected as model substrates for aliphatic
([TsO] or methane sulfonate [MeSO ] (entries 9, 11-13)
3
and aromatic alcohols respectively. Mixtures that appeared as
a single, clear phase without any visible interface or cloudiness
were considered as miscible (‘1’ in Tables 1 and 2). Other ILs
proved immiscible with the alcohol or ketone compounds,
resulting in a distinct interface between the two phases (‘2’ in
Tables 1 and 2). The results of these miscibility tests are
summarized in Table 1 (abbreviations of the ILs are given in
the Experimental section). Suitable ILs form a homogeneous
phase with the alcohol at reaction temperature (‘1’ in
exhibited the desired phase behaviour: mixing with the
2-octanol on reaction temperature but phase separating with
the 2-ketone product at room temperature. Unexpectedly,
after replacing the acetate anion with trifluoroacetate (entry 10),
the IL remained miscible with the ketone, pointing to the
complexity of the IL selection.
The mixing behaviour of the 1-butyl-3-methylimidazolium
([BMIM]) solvent series was slightly different as compared to
that of the EMIM based ILs (entries 14–17), and comparing
both series also allows to draw some conclusions on the role of
1
00/0 column) but exhibit phase separation with the ketone
‘2’ in 30/70 column) upon cooling to room temperature. It
should be noted that ILs containing the [PF ] anion were not
(
the anion. While the behaviour of [Tf N] based ILs seems
2
6
independent of the cation (entries 8, 15, 19 and 23), this was
assessed because of their pronounced sensitivity to hydrolysis.
Firstly, the results for 2-octanol will be discussed. As shown
not the case for ILs containing the tetrafluoroborate anion.
Both [EMIM][Tf N] and [BMIM][Tf N] proved miscible with
2
2
in Table 1, the ammonium-based ILs of the [MOct N][X] type
3
2-octanone (entries 8, 15). 1-Butyl-3-methylimidazolium tetra-
fluoroborate [BMIM][BF ], in contrast with [BMIM][Tf N]
did not display the desired mixing behaviour, since no phase
separation was observed for the 30/70 mixtures after cooling
to room temperature (Table 1, entries 1 and 2). Presumably,
van der Waals interactions between the ILs’ octyl chains and
4
2
and [BMIM][TfO], exhibited phase separation with the ketone
upon cooling to room temperature (entries 14–16). The same
phase behaviour was observed for 1-butyl-3-methylimidazo-
lium chloride ([BMIM][Cl]) (entry 17). Note that at 90 1C, the
2
9
2
-octanone enhance the miscibility of the IL with the ketone.
Similar behaviour was observed for other classes of ILs bearing
longer alkyl chains: both trihexyltetradecyl-phosphonium
mixture with [BMIM][BF ] did not form one homogenous
4
phase. Nevertheless, the volume of the IL phase strongly
dicyanamide ([H
3
TP][(CN) N]) (entry 3) and 1-decyl-3-methyl-
2
increased due to dissolution of the alcohol substrate. Note
that upon further increasing to 120 1C the miscibility enhanced
and nearly one phase was formed. It is assumed that sufficient
contact between the substrate and the catalyst in the IL will be
established to perform the catalytic oxidation. Interestingly,
introducing a methyl group on the C-2 position of the
imidazolium ring of the IL, e.g. in 1-butyl-2,3-dimethylimidazo-
imidazolium triflate ([DMIM][TfO]) (entry 4) remained
miscible with the ketone at room temperature which can also
be ascribed to van der Waals interactions. In the case of
[
H TP][(CN) N], where the 100/0 mixture formed one clear
3 2
phase, the 30/70 mixture did not, which can be explained with
the presence of water in the 30/70 mixture. The fact that a
mixture 100/0 turned from clear to cloudy upon water addi-
tion corroborates this hypothesis. This could be expected as
the long alkyl chains make this IL behave like an aliphatic
hydrocarbon, with poor affinity for water.
lium tetrafluoroborate [BMMIM][BF ], decreased the miscibility
4
of 2-octanol and 2-octanone with the IL dramatically (entry 18).
However in combination with other anions, like acetate and
[Tf N], the [BMMIM] based ILs were completely miscible with
2
Next, imidazolium-based ILs functionalized with short alkyl
chains were assessed (entries 4–19). The Brønsted acid
both alcohol and ketone (entries 19–20).
Next, 1-butyl-1-methylpyrrolidinium ([BMPyr]) based ILs
were tested (entries 21–23). Irrespective of the anion, the
pyrrolidinium ILs proved not suitable. Although they formed
[
MIM][HSO ] IL proved immiscible with 2-octanol at reaction
4
temperature (entry 5). Introducing a nitrile group into the
This journal is ꢀc the Owner Societies 2010
Phys. Chem. Chem. Phys., 2010, 12, 1741–1749 | 1743

Table 1 Miscibility of ammonium, phosphonium, imidazolium, pyrrolidinium, and choline- and betainium-based ionic liquids with 2-octanol/
2-octanone and 1-phenylethanol/acetophenone mixtures
a,b
2
-Octanol 2-octanone
1-Phenyl ethanol acetophenone
Entry
Ionic liquid
100/0
30/70
100/0
30/70
1
2
3
4
5
6
7
8
9
[MOct
[MOct
3
N][Tf
2
N]
N][CF CO
1
1
1
1
2
2
2
1
1
1
1
1
1
1
1
1
1
—
—
1
—
—
1
2
—
1
1
1
1
1
1
1
2
2
1
—
1
1
—
1
1
—
—
—
1
1
—
—
e
e
3
3
2
]
c
e
e
[H
3
TP][(CN)
2
N]
[DMIM][TfO]
[MIM][HSO
[CNMMIM][Tf
[EMIM][BF
[EMIM][Tf
[EMIM][CH
[EMIM][CF
[EMIM][EtSO
[EMIM][TsO]
[EMIM][MeSO
[BMIM][BF
[BMIM][Tf
[BMIM][TfO]
[BMIM][Cl]
[BMMIM][BF
[BMMIM][Tf
[BMMIM][CH
[BMPyr][CF CO
[BMPyr][(CN) N]
[BMPyr][Tf N]
[CNBMPyr][Tf
[CNEMPyr][Tf
[Hbet][Tf
[Chol][Tf
1
2
1
2
1
2
1
2
2
2
2
1
1
2
2
1
1
1
1
1
1
2
—
4
]
e
e
2
N]
4
]
2
1
1
1
1
1
1
1
1
2
N]
CO
CO
3
2
]
2
]
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
3
4
]
3
]
d
4
]
1
2
N]
1
1
1
2
1
1
1
1
1
2
2
1
1
1
2
f
2
—
e
e
4
]
1
1
—
2
N]
e
e
3
CO
2
]
3
2
]
1
1
—
—
2
e
e
2
e
e
e
2
N]
N]
—
—
1
1
2
2
e
e
e
2
—
f
2
2
N]
N]
2
1
—
—
e
e
[Chol][CH CO
3
[Chol][Ace]
2
]
e
e
a
Conditions: 100/0 mixture: alcohol (5 mmol), ionic liquid, mass ratio IL/ketone = 1/1, 120 1C; 30/70 mixture: 1.5 mmol alcohol (1.5 mmol),
b
ketone (3.5 mmol), water (3.5 mmol), ionic liquid, same amount of IL as for 100/0 mixture, 20 1C. ‘1’: Mixtures that appeared as a single, clear
phase without any visible interface or cloudiness; ‘2’: Mixtures that exhibited a distinct interface between the two separated phases. Upon cooling
c
d
to room temperature, the mixture appeared as one but cloudy phase, probably due to the water present. At reaction temperature the volume of
e
the ionic liquid phase is increased by >50% due to dissolution of the alcohol in the ionic liquid. ‘—’: solubility experiment not performed.
f
Mixture consists of 2 phases, but the volume of the ionic liquid phase is slightly (B10%) increased due to dissolution of alcohol.
a,b
Table 2 Miscibility of selected imidazolium IL with a series of different alcohol/ketone mixtures
2
-Butanol 2-butanone 2-Hexanol 2-hexanone 2-Decanol 2-decanone 2-Dodecanol 2-dodecanone 1-Octanol 1-octanal
Entry
Ionic liquid
[EMIM][CH CO
100/0
30/70
100/0
30/70
100/0
30/70
100/0
30/70
100/0
30/70
1
2
3
4
5
6
3
2
] 1
1
1
1
1
2
1
1
1
2
1
1
1
1
1
1
1
2
2
2
2
2
2
1
1
1
1
1
1
2
2
2
2
2
2
1
1
1
2
2
1
2
2
2
2
2
2
1
1
1
1
1
1
1
2
2
2
2
1
[EMIM][EtSO
4
]
[EMIM][TsO]
[EMIM][MeSO
3
]
c
c
d
[BMIM][BF
[BMIM][Cl]
4
]
1
a
b
c
d
Conditions: Table 1. ‘1’ and ‘2’ as in Table 1. Phase separation occurred, but only when the mixture contained less than 50 mol% ketone. At
reaction temperature the volume of the ionic liquid phase is increased by >50% due to dissolution of the alcohol in the ionic liquid.
one phase with 2-octanol at reaction temperature, no phase
separation with the 2-ketone occurred upon cooling to room
temperature. Introduction of a nitrile function in the IL’s
cation led to the same effect as observed for the imidazolium
based ILs: the IL is not able to dissolve the model substrate at
reaction temperature anymore. Indeed, as reported in
Table 1, neither 1-butylnitrile-1-methylpyrrolidinium bistriflimide
Next, other classes of ILs were evaluated, more specifically
the betaine and choline based ILs (entries 26–29). The betaine
based ILs are of interest because of their carboxylic acid
group. In various Pd-catalyzed oxidation reactions, it has been
found that a carboxylic acid function can facilitate the
3
1
palladium reoxidation. As evidenced in Table 1, the betaine
and choline based ILs containing the hydrophobic [Tf N]
2
(
[CNBMPyr][Tf
bistriflimide ([CNEMPyr][Tf
with 2-octanol (entries 24–25).
2
N]) nor 1-ethylnitrile-1-methylpyrrololidinium
anion proved miscible with 2-octanone (entries 26, 28). How-
ever, when choline is combined with a more polar anion, such
as acetate or acesulfamate, phase separation with the ketone at
2
N]) formed a homogeneous phase
1
744 | Phys. Chem. Chem. Phys., 2010, 12, 1741–1749
This journal is ꢀc the Owner Societies 2010

room temperature was observed (entries 28–29). In the case of
choline acetate, a homogeneous phase with 2-octanol was
formed. The acesulfamate anion decreased however the
miscibility with both the alcohol and ketone making this IL
less favourable. In general, it seems that irrespective of the
2
exposing them to 30 bar O for 24 h at 120 1C in the presence
of palladium(II) acetate. Oxygen resistance was checked at the
1
molecular level by H NMR as well as macroscopically by
investigating colour and viscosity changes. For the six selected
ILs no colour or viscosity changes could be observed. Further-
1
more, the H NMR spectra of the ILs before and after
cation, the ILs containing the [Tf N] anion did not exhibit the
2
right phase behaviour.
oxidation were identical, proving their oxidative stability
For the aromatic alcohol substrates, with 1-phenylethanol
(NMR spectra are provided in the ESI).w
as the model, the [Tf
effect on the phase behaviour of the IL. Both [BMIM][BF
and [BMIM][Tf N] were miscible with 1-phenylethanol at
2
N] anion did not have the same negative
Catalytic oxidation of aliphatic alcohols in selected ionic liquid
solvents
4
]
2
reaction temperature but did not mix with acetophenone at
room temperature (Table 1, entries 14–15). When triflate as
anion was assessed, the miscibility with the aromatic ketone
increased dramatically (entry 16). A large number of ILs
mixed perfectly not only with 1-phenylethanol at reaction
temperature but also with acetophenone at room temperature,
including the ammonium and imidazolium based ILs with
long alkyl chains (entries 1-4), and most EMIM, BMMIM and
BMPyr based ionic liquids (entries 8–13 and 18–22).
Influence of the nature of the ionic liquid. The alcohol
oxidation was studied applying the IL media which displayed
the appropriate mixing behaviour. Firstly, the influence of the
type of IL on the catalytic oxidation of 2-octanol, as model
aliphatic alcohol substrate, was explored using Pd(OAc)
as catalyst. Fig. 1 demonstrates that [BMIM][BF ] and
2
4
[EMIM][TsO] are the better reaction media, with 2-ketone
yields of, respectively, 79 and 61%, compared to [EMIM][EtSO4]
and [EMIM][MeSO ]. Using these ILs, phase separation was
3
Summarizing, the IL screening revealed that seven ILs
exhibit the ideal mixing behaviour with the model substrate
quick and spontaneous, allowing straightforward product
analysis. Note that all oxidations performed in the various
IL media were 100% selective towards the 2-ketone. No side-
products such as ethers were observed. Remarkably, when the
alcohol oxidation was performed in [BMIM][Cl] and
[EMIM][CH CO ], no phase separation occurred after cooling
2
-octanol and its 2-ketone oxidation product: [EMIM][CH
EMIM][EtSO ], [EMIM][TsO], [EMIM][MeSO ], [BMIM][Cl],
BMIM][BF ] and [Chol][CH CO ]. In the case of aceto-
3 2
CO ],
[
[
4
3
4
3
2
phenone, only two ILs proved suitable: [BMIM][BF ] and
4
3
2
[
BMIM][Tf N].
2
the reaction mixture to room temperature. Even upon extra
ketone addition to the reaction mixture, the product was not
expelled from the IL.
The 7 ILs suitable for 2-octanol oxidation were then
subjected to a more in-depth miscibility test with a series of
alcohols with varying chain length between C4 and C10.
Mixing with the primary alcohol 1-octanol and the corres-
ponding aldehyde was studied as well (Table 2). Due to the
Optimization of reaction conditions. After identification of
4
[BMIM][BF ] as the optimal IL, the other reaction conditions
were optimized for the 2-octanol oxidation. Fig. 2 shows
that for a molar alcohol to catalyst ratio of 40 a reaction
temperature of at least 120 1C is required to obtain acceptable
presence of an alcohol function, [Chol][CH
not stable towards oxidation and hence this IL will not be
2
addressed further. Table 2 reports that [EMIM][CH CO ]
3 2
CO ] is probably
3
2
-octanol conversion rates. Recall that octanol dissolution in
exhibits the desired phase behaviour with the whole range of
secondary aliphatic alcohols (entry 1). The other [EMIM]
based ILs form 2 phases with the ketone upon cooling to
room temperature in the series between 2-hexanol to 2-dodecanol
[BMIM][BF ] is not complete; rather the alcohol swells the IL
4
phase. At 120 1C the alcohol was selectively converted for 79%
towards 2-octanone. Presumably, the too low solubility of
2-octanol in the IL at temperatures below 120 1C, leading to
slow kinetics, accounts for the low yields of 10 and 17%
(
entries 2–4). However, they are not suitable for the catalytic
oxidation of 2-butanol, due to the observed mixing of all
ILs with butanone at room temperature. The same phase
behaviour was observed for [BMIM][Cl] (entry 6). On the
4
other hand, [BMIM][BF ] demixes with all the investigated
ketones at room temperature. Nevertheless, it did not form a
homogeneous phase with 2-dodecanol at reaction temperature
(
entry 5). It should also be noted that in the case of 2-butanol
and 2-hexanol, if the alcohol is converted for more than 50%,
the ketone is not expelled spontaneously from the IL anymore,
whereas at lower conversions phase separation occurred. For
the primary 1-octanol, four ILs exhibited the desired
phase behaviour with both alcohol and the corresponding
ketone: [EMIM][EtSO
and [BMIM][BF (entries 2–5). [EMIM][CH
BMIM][Cl] formed one phase with octanal at room tempera-
ture (entries 1, 6).
4
], [EMIM][TsO], [EMIM][MeSO
3
]
4
]
3
CO and
2
]
[
Fig. 1 Influence of the nature of the ionic liquid on the alcohol
oxidation: Y, ketone yield; TON = total turnover number (reaction
conditions: 2-octanol (10 mmol), ionic liquid, mass ratio IL/alcohol = 1,
The six ILs meeting the solubility criteria for 2-octanol were
then evaluated for their chemical stability towards oxygen. A
standard oxidation-stability test was designed for the ILs, by
Pd(OAc)
2
(molar ratio substrate/catalyst = 40, 120 1C, 10 bar O
2
, 24 h).
This journal is ꢀc the Owner Societies 2010
Phys. Chem. Chem. Phys., 2010, 12, 1741–1749 | 1745

4
[BMIM][BF ] at 120 1C compared to 90 1C, leading to a better
substrate-catalyst contact. Indeed, whereas at 90 1C the ionic
liquid phase is swollen but no complete mixing with the
substrate takes place, nearly one homogeneous IL-substrate
phase is formed at 120 1C. Thus with the substrate-catalyst
contact at 120 1C not being dependent on the IL/A ratio, the
positive effect of the ionic liquid on the reaction kinetics
(
e.g. catalyst stability) becomes more important as more ionic
liquid is introduced into the reaction system. Interestingly, by
increasing the amount of catalyst the effect of the IL/A ratio
was reduced. Nevertheless the highest alcohol conversion was
still observed for an IL/A ratio of 1, viz. 79% combined with a
full 2-ketone product selectivity (entries 9–10). For 2-decanol
Fig. 2 Influence of the reaction temperature on the Pd-catalyzed
oxidation of 2-octanol (reaction conditions: 2-octanol (10 mmol),
[
BMIM][BF ], mass ratio IL/alcohol = 1, Pd(OAc)
4
2
(molar ratio
oxidation in [BMIM][BF ], the effect was not as straight-
4
substrate/catalyst = 40), 10 bar O , 24 h; Y = yield; TON = turnover).
2
forward as for the 2-octanol, although the conversion of
2
-decanol improved when the IL/A ratio was increased from
2
-ketone at 90 and 110 1C, respectively. Similar observations
were made for a substrate to catalyst ratio of 100. Interestingly,
0.5 to 1 or 2 (entries 11–15). The optimal IL/A ratio identified
was 0.82 in this case, yielding 87% of 2-decanone. By lowering
the IL/A ratio to 0.25 the reaction mixture remained one
homogeneous phase. It appears that the amount of IL is too
small to spontaneously expel the ketone. Taking into account
this result, the affinity of the IL for the ketone was checked, to
investigate the leaching of IL into the product phase. The
when further increasing the temperature from 120 1C to
140 1C, reaction rates decreased. This is probably due to the
aggregation of the Pd clusters, which becomes more prominent
at higher temperatures.
In Fig. 3 the evolution of the overall average turnover
number (TON) with time is shown for a reaction at 120 1C.
After high activity in the first few hours, the TON keeps
steadily increasing, showing that stable activity can be
maintained by the Pd catalyst dispersed in the IL phase, even
at this high temperature.
4
oxidation of 2-decanol (10 mmol) in [BMIM][BF ] (IL/A = 1)
at 120 1C was chosen as model reaction. The product phase,
which was spontaneously expelled upon cooling of the
reaction mixture to room temperature, was analyzed by
NMR. Spiking tests indicated that the amount of IL leached
in the product phase was less than 0.01%. Next, the effect of
the catalyst loading was investigated. Table 3 (entries 3, 16–17)
shows that at 90 1C and with an IL/A mass ratio of
As reported in Table 3, at 90 1C the 2-octanone yield
increased from 4 to 22% and 36% when decreasing the molar
IL to alcohol ratio (IL/A) from 1 to 0.5 and 0.1, respectively
À1
(
entries 1-5). Apparently the poor substrate-catalyst contact at
0 1C due to too low substrate solubility in the IL was
mitigated when lowering the IL/A ratio. This finding contrasts
0.5, a substrate to catalyst molar ratio (S C ) of 40 resulted
9
in the highest 2-octanone yield (57%). A higher catalyst
À1
loading (S C = 20) probably provokes the aggregation of
with the results obtained at 120 1C, at which temperature
Pd particles, explaining the lower yield. As can be derived
from Table 3 the oxygen pressure is also a significant reaction
2
-octanol dissolves much more readily in [BMIM][BF ]. In
4
that case, an increase of the IL/A ratio had a positive influence
on the conversion of the alcohol: employing an IL/A ratio of
parameter (entries 18–28). For [BMIM][BF ] at 120 1C, using
4
À1
a S C
molar ratio of 100, the conversion of 2-octanol
0
.25 gave a 2-octanone yield of 37%, where for a molar IL/A
was close to zero when no external O2 was introduced
(entry 18). This could be expected because nearly no oxygen
is available for the Pd re-oxidation. The yield of 2-octanone
ratio of 1 the ketone product yield was 76% (entries 6–8). The
opposite trend in function of the IL/A ratio observed at 120 1C
could be explained by a better solubility of 2-octanol in
increases significantly when the O
1 and 76% yield for 0, 5 and 10 bar of O
entries 8, 18–19). A further increase of the oxygen pressure to
0 bar and in particular 30 bar remarkably inhibited the
reaction, with a ketone yield limited to 41% at 30 bar (entries
0–21). This is most likely due to palladium oxide formation,
2
pressure was increased: 2,
5
2
, respectively
(
2
2
becoming more prominent at higher oxygen concentrations.
À1
The same observations were made when a S C molar ratio
of 40 was applied (entries 10, 22–23). For reactions in
4
[EMIM][EtSO ], a similar oxygen pressure-catalytic activity
relation was found (entries 24–28). However, in this IL the
optimum oxygen pressure for 2-octanol oxidation was 30 bar.
By further increasing the oxygen pressure, the ketone yield
dropped to just 5%. In the case of [EMIM][MeSO
the ketone was obtained in only 9% yield, whereas at
0 bar, a ketone yield of 23% was obtained. Increasing the
3
], at 5 bar
O
2
Fig.
reaction conditions: 2-octanol (10 mmol), [BMIM][BF
IL/alcohol = 1, Pd(OAc) (molar ratio substrate/catalyst = 40),
3
Evolution of the average turnover number in time
1
(
4
], mass ratio
oxygen pressure to 20 and 30 bar did not further improve the
yields (entries 29–32).
2
2
120 1C, 30 bar O ).
1
746 | Phys. Chem. Chem. Phys., 2010, 12, 1741–1749
This journal is ꢀc the Owner Societies 2010

Expanding the substrate scope. After optimizing the reaction
conditions for 2-octanol and 2-decanol, the oxidation of other
alcohols was investigated. First, secondary aliphatic alcohols
Table 4 Influence of the alkyl chain length on the catalytic alcohol
oxidation
a
b
c
Entry
Alcohol
P(O
2
)/bar
Y(%)
TON
were used. At 120 1C, 10 bar of O and after 24 h of reaction,
2
1
2
3
4
5
6
7
2-Butanol
2-Hexanol
2-Octanol
2-Decanol
2-Dodecanol
2-Octanol
2-Decanol
10
10
10
10
10
30
30
11
16
76
83
21
66
64
11
16
76
83
21
66
64
the 2-ketone yields were determined for substrates with
varying alkyl chain length. As reported in Table 4, 2-octanol
and 2-decanol were the most reactive substrates, with ketone
yields of 76 and 83%, respectively. Isolated yields of 2-butanol
and 2-hexanol were substantially lower. One should however
note that the difference in reactivity between the substrates
may be overestimated because the oxidation experiments
were performed under optimal conditions for 2-octanol and
d
a
Reaction conditions: alcohol (10mmol), [BMIM][BF
4
], mass ratio
IL/alcohol = 1, Pd(OAc)
b
2
(molar ratio substrate/catalyst = 100),
d
c
1
performed under constant pressure (in TOP multireactor).
20 1C, 24 h. Yield of ketone. Turnover number. Reaction
2
-decanol. In some reactions of 2-butanol and 2-hexanol, the
overall conversion was higher than for the reactions shown in
Table 4; but as no phase separation occurred (cf. Table 2),
these results are not included. In the case of 2-dodecanol, the
low yields can be explained in terms of low miscibility of
performing the 1-phenylethanol oxidation in the above
mentioned ILs, no phase separation was observed upon cooling
to room temperature or even to 0 1C. This can be ascribed to
one of the following reasons: (1) the conversion may have been
lower than 70%; note that 70% conversion corresponds with
the standard mixture tested in the miscibility experiments; (2)
side-products were formed which inhibit phase separation.
Upon adding extra ketone to the mixture, no phase separation
occurred, indicating that the too low conversion was not the
cause for the monophasic nature of the reaction mixture
containing the IL.
2
-dodecanol with [BMIM][BF
4
]. As shown in Table 2, even at
1
20 1C the 2-dodecanol and the IL form 2 phases without a
significant swelling of the IL phase. With the catalyst in the IL
phase, there is a pronounced substrate shortage in the
catalyst phase.
Secondly, aromatic and primary aliphatic alcohol substrates
were tested. The miscibility experiments indicated that two
ILs, viz. [BMIM][BF ] and [BMIM][Tf N] exhibited the right
4
2
phase behaviour with 1-phenylethanol. Remarkably, when
a
Table 3 Optimization of the reaction conditions for the catalytic oxidation of aliphatic alcohols
b
À1c
d
e
Entry
Ionic liquid
IL/A
P(O
2
)/bar
T/1C
S/C
Y(%)
TON
1
2
3
4
5
6
7
8
9
[BMIM][BF
[BMIM][BF
[BMIM][BF
[BMIM][BF
[BMIM][BF
[BMIM][BF
[BMIM][BF
[BMIM][BF
[BMIM][BF
[BMIM][BF
[BMIM][BF
[BMIM][BF
[BMIM][BF
[BMIM][BF
[BMIM][BF
[BMIM][BF
[BMIM][BF
[BMIM][BF
[BMIM][BF
[BMIM][BF
[BMIM][BF
[BMIM][BF
[BMIM][BF
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
]
]
]
]
]
]
]
]
]
]
]
]
]
]
]
]
]
]
]
]
]
]
]
0.1
0.2
0.5
0.75
1
0.25
0.5
1
0.5
1
0.25
0.5
0.82
1
10
10
10
10
10
10
10
10
10
10
30
30
30
30
30
10
10
0
90
90
90
90
100
100
100
100
100
100
100
100
40
40
40
40
40
40
40
40
20
100
100
100
100
40
36
23
22
19
4
37
47
76
72
79
36
23
22
19
4
37
47
76
29
32
90
120
120
120
120
120
120
120
120
120
120
90
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
f
f
f
f
f
g
g
Np
Np
55
87
64
78
57
21
2
51
71
41
71
66
11
13
12
21
5
22
35
26
31
23
4
2
0.5
0.5
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
90
120
120
120
120
120
120
120
120
120
120
120
120
120
120
120
2
5
51
71
41
71
26
4
5
5
9
2
20
30
20
30
5
10
20
30
40
5
40
40
40
40
40
40
40
40
[EMIM][EtSO
[EMIM][EtSO
[EMIM][EtSO
[EMIM][EtSO
[EMIM][EtSO
[EMIM][MeSO
[EMIM][MeSO
[EMIM][MeSO
[EMIM][MeSO
4
4
4
4
4
]
]
]
]
]
3
3
3
3
]
]
]
]
9
4
9
10
9
10
20
30
23
25
23
40
40
a
b
c
2
Reaction conditions: 2-octanol (10mmol), ionic liquid, Pd(OAc) , 24h. Ionic liquid to alcohol mass ratio. Substrate to catalyst molar
d
e
f
g
ratio. Yield of ketone. Turnover number. 2-decanol used as substrate. No phase separation occurred upon cooling the reaction mixture to
room temperature.
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Phys. Chem. Chem. Phys., 2010, 12, 1741–1749 | 1747

In the case of 1-octanol, a product phase was spontaneously
expelled from [BMIM][BF ] after cooling to room temperature
Table 5, entry 1). It should be noted that the oxidation did not
Table 6 Catalytic alcohol oxidation under optimized reaction
a
conditions
4
(
b
c
Entry
Alcohol
IL/A
P(O
2
)/bar
Y(%)
stop at the aldehyde stage but further oxidation towards the
carboxylic acid and ester (1-octyl octanoate) took place. The
acid and ester were formed with a yield of 11 and 45%,
respectively. This acid and ester formation from 1-octanol
contrasts with results obtained for benzyl alcohol by Seddon
et al., who found that even in hydrophilic ionic liquid
d
1
2
3
4
5
2-Hexanol
2-Octanol
2-Octanol
2-Decanol
2-Decanol
1
1
0
1
0
10
10
10
30
30
48
79
63
87
39
e
e
a
Reaction conditions: alcohol (10mmol), [BMIM][BF
4 2
], Pd(OAc)
b
(molar ratio substrate/catalyst=40), 120 1C, 10 bar O , 24 h. Ionic
liquid to alcohol mass ratio. Yield of ketone. 5 mmol 2-hexanol.
2
[
BMIM][BF
4
] no over-oxidation of benzyl alcohol due to the
c
d
presence of water takes place, even upon extra introduction of
e
1
5
Reaction performed in absence of ionic liquid.
water. [EMIm][TsO] remained miscible with the product
when the reactor was only loaded with 5 and 10 bar of oxygen;
however further increasing the pressure to 20 and 30 bar
caused phase separation, leading to acid and ester yields of
liquid phase was reused in the oxidation reaction. First,
2-octanol (10 mmol) was oxidized for 24 h at 120 1C in
[BMIM][BF ] under oxygen in the presence of Pd(OAc) .
1
2 and 83% at 30 bar. Performing oxidation in [EMIM][MeSO3]
using 10 bar of oxygen, led to ester yields of 32%. Note that
EMIM][EtSO ] did not phase separate from the products,
4
2
After allowing the reaction mixture to cool down to room
temperature, phase separation occurred, and the product layer
was isolated by decantation (see Fig. 4).
[
4
irrespective of the oxygen pressure.
In Table 6, the results obtained under optimized reaction
After removing the upper layer, the bottom IL layer was
recycled. In the recycling experiment 2-decanol was oxidized.
Different alcohols were used to exclude cross-contamination
errors between the different recycles. As can be seen in Table 7,
the selectivity remained 100% after recycling, while the con-
version decreased from 79 to 70%. The decrease in conversion
is not necessarily due to a decrease in catalytic activity since,
unlike 2-octanol, 2-decanol is less reactive at 10 bar of oxygen
compared to 30 bar O2 (see Table 3). Additionally, the
leaching problem in the recycled catalytic system was
addressed by investigating the exact amount of palladium
leaching by ICP (inductively coupled plasma) atomic emission
analysis. After the recycling, leaching of palladium into the
product phase amounted to less than 0.1% of the total amount
of Pd supplied to the reaction.
4 2
conditions are shown. In [BMIM][BF ] at 120 1C, 10 bar of O
À1
and a S C molar ratio of 40, 2-hexanol could be converted
towards 2-hexanone with a yield of 48%. In the case of 2-octanol,
2
4
-octanone was obtained in 79% yield in [BMIM][BF ], com-
pared to 63% in the absence of IL. Also for 2-decanol, the IL
catalytic system was more active than for the neat alcohol solvent:
at 120 1C and 30 bar of oxygen, 87% of ketone yield in
[
BMIM][BF ] compared to only 39% in pure 2-decanol. These
4
product yields are to the best of our knowledge the highest
reported in literature for the oxidation of aliphatic alcohols in
ILs. Our group previously published on the catalytic oxidation of
alcohols in ILs, reporting a 2-octanol conversion of 43% in
tetramethyl ammonium hydroxide and in the presence of
19
3
.
RuCl
As mentioned in the Introduction, most of the IL
catalytic systems in the literature only dealt with the more reactive
aromatic and allylic alcohols. Nevertheless two research groups
investigated the activity towards non-activated secondary
alcohols. Wang et al., applying NaClO as the oxygen source,
Conclusions
In conclusion, we developed a unique recyclable system for the
oxidation of non-activated alcohols towards ketones. The
choice of the ionic liquid was primarily governed by its ability
to spontaneously expel the product, rather than by its effect on
activity. From miscibility tests it emerged that only 6 out of 29
ionic liquids displayed the appropriate mixing behaviour for
the oxidation reaction. Two ionic liquids resulted in high
activity and selectivity values in the aliphatic alcohol oxidation
17
reported a conversion of 2-octanol of only 19%. De Souza et al.
converted 2-butanol with a yield of 61% in the presence of RuCl
however a reaction time of 70 h was required. In the case of
3
;
16
1-octanol, no conversion was observed at all.
Recycling and product separation. In order to demonstrate
the catalyst recycling, the catalyst immobilized in the ionic
in the presence of Pd(OAc)
2
: 2-octanone yields of 79 and 61%
] and [EMIM][TsO] are to our
a
Table 5 Catalytic oxidation of the primary alcohol 1-octanol
in, respectively, [BMIM][BF
4
b
c
Entry Ionic liquid
2
P(O )/bar Y(COOH) (%) Y(ester) (%)
1
2
3
4
5
6
[BMIM][BF
4
]
10
5
10
20
30
10
11
Np
Np
1
12
0
45
d
d
d
d
[EMIm][TsO]
[EMIm][TsO]
[EMIm][TsO]
[EMIm][TsO]
[EMIM][MeSO
Np
Np
14
83
32
3
]
a
Reaction conditions: 1-octanol (10 mmol), ionic liquid, mass ratio
(molar ratio substrate/catalyst = 40),
IL/alcohol = 1, Pd(OAc)
2
b
c
1
20 1C, 24 h. Yield of carboxylic acid. Yield of 1-octyl octanoate.
d
No phase separation occurred upon cooling the reaction mixture to
room temperature.
Fig. 4 Phase separation after catalytic oxidation of 2-octanol.
This journal is ꢀc the Owner Societies 2010
1
748 | Phys. Chem. Chem. Phys., 2010, 12, 1741–1749

Table 7 Recycling of catalyst and ionic liquid in catalytic oxidation
of alipathic alcohols
5 J. Muzart, Tetrahedron, 2003, 59, 5789.
6 S. S. Stahl, Angew. Chem., Int. Ed., 2004, 43, 3400.
a
7
M. J. Schultz, S. S. Hamilton, D. R. Jensen and M. S. Sigman,
J. Org. Chem., 2005, 70, 3343.
B. M. Trost, Science, 1991, 254, 1471.
K. P. Peterson and R. C. Larock, J. Org. Chem., 1998, 63,
3
b
c
Run
Alcohol
C(%)
S(%)
8
9
1
2
2-Octanol
2-Decanol
79
70
100
100
185.
0 N. V. Plechkova and K. R. Seddon, Chem. Soc. Rev., 2008, 37,
23.
11 K. R. Seddon, Kinet. Catal., 1996, 37, 693.
a
1
Reaction conditions: alcohol (10 mmol), [BMIM][BF
4
], mass ratio
1
IL/alcohol = 1, Pd(OAc)
20 1C, 120 1C, 10 bar O , 24 h. Conversion of alcohol. Selectivity
to ketone.
2
(molar ratio substrate/catalyst = 40),
b
c
1
2
12 V. I. Parvulescu and C. Hardacre, Chem. Rev., 2007, 107, 2615.
13 T. Welton, Coord. Chem. Rev., 2004, 248, 2459.
14 V. Neff, T. E. Muller and J. A. Lercher, Chem. Commun., 2002,
9
5 K. R. Seddon and A. Stark, Green Chem., 2002, 4, 119.
16 R. F. de Souza, J. Dupont and J. E. L. Dullius, J. Braz. Chem.
Soc., 2006, 17, 48.
06.
knowledge the highest yields for aliphatic alcohols reported in
ionic liquids. In the case of 2-decanol, even a yield of 86%
could be achieved. The ionic liquids were miscible with the
alcoholic substrates at reaction temperature, while a distinct
phase separation of the corresponding ketone products was
observed at room temperature. This mixing behaviour enabled
the isolation of the reaction products by decantation without
the need for an extracting solvent. Moreover, the ionic liquid
mixture functioned as an immobilization medium for the
catalyst which allowed efficient catalyst recycling.
1
1
7 M. Rong, C. Liu, J. Han and H. Wang, Catal. Commun., 2009, 10,
62.
18 N. Jiang and A. J. Ragauskas, Tetrahedron Lett., 2007, 48,
3
273.
9 A. Wolfson, S. Wuyts, D. E. De Vos, I. F. J. Vankelecom and
P. A. Jacobs, Tetrahedron Lett., 2002, 43, 8107.
0 A. Shaabani, E. Farhangi and A. Rahmati, Appl. Catal., A, 2008,
338, 14.
1 C. Comminges, R. Barhdadi, A. P. Doherty, S. O’Toole and
M. Troupel, J. Phys. Chem. A, 2008, 112, 7848.
2 B. S. Chhikara, R. Chandra and V. Tandon, J. Catal., 2005, 230,
1
2
2
2
2
2
4
36.
Acknowledgements
3 W. X. Qian, E. L. Jin, W. L. Bao and Y. M. Zhang, Angew. Chem.,
Int. Ed., 2005, 44, 952.
4 H. B. Xie, S. B. Zhang and H. F. Duan, Tetrahedron Lett., 2004,
Financial support by K. U. Leuven (project IDO/05/005), by
IWT (Institute for the Promotion of Innovation by Science and
Technology in Flanders) and by K. U. Leuven (CECAT grant)
is gratefully acknowledged. The contacts with IoLiTec
4
25 C. Van Doorslaer, I. Wahlen, P. G. N. Mertens, B. Thijs,
5, 2013.
P. Nockemann, K. Binnemans and D. E. De Vos, ChemSusChem,
008, 1, 997.
2
(
Denzlingen, Germany) are greatly appreciated. This research
has been performed within the Scientific Research Network
‘Ionic Liquids’’ of the Research Foundation - Flanders (FWO).
2
6 B. Thijs, Task-Specific Ionic liquids for Solubilizing Metal
Compounds, Katholieke Universiteit Leuven, 2007.
‘
27 P. Nockemann, B. Thijs, S. Pittois, J. Thoen, C. Glorieux, K. Van
Hecke, L. Van Meervelt, B. Kirchner and K. Binnemans, J. Phys.
Chem. B, 2006, 110, 20978.
2
8 P. Nockemann, B. Thijs, K. Driesen, C. R. Janssen, K. Van Hecke,
L. Van Meervelt, S. Kossmann, B. Kirchner and K. Binnemans,
J. Phys. Chem. B, 2007, 111, 5254.
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This journal is ꢀc the Owner Societies 2010
Phys. Chem. Chem. Phys., 2010, 12, 1741–1749 | 1749