Wacker oxidation of terminal olefins in a mixture of [bmim][BF4] and water

Article abstract of DOI:10.1016/j.tetlet.2005.09.013

A simple and efficient PdCl₂/CuCl catalyzed oxidation of alkenes has been successfully developed using a mixture of water and the ionic liquid [bmim][BF₄] as solvent. Starting from various types of terminal olefins, the corresponding ketones have been prepared under mild reaction conditions and obtained in good to excellent yields after a simple extraction with diethyl ether. Furthermore, it was possible to recycle and reuse the ionic liquid and the catalytic system.

Full text of DOI:10.1016/j.tetlet.2005.09.013

Tetrahedron
Letters
Tetrahedron Letters 46 (2005) 7507–7510
Wacker oxidation of terminal olefins in a mixture of
[bmim][BF4] and waterI
I. A. Ansari,a,* Sipak Joyasawal,a Manoj K. Gupta,a J. S. Yadava and R. Greeb,*
aDivision of Organic Chemistry-I, Indian Institute of Chemical Technology, Hyderabad 500 007, India
bUniversite de Rennes 1, Synthese et Electrosynthese Organiques, CNRS UMR 6510 Avenue du General Leclerc,
35042 Rennes Cedex, France
Received 29 June 2005; revised 22 August 2005; accepted 2 September 2005
Available online 19 September 2005
Abstract—A simple and efficient PdCl2/CuCl catalyzed oxidation of alkenes has been successfully developed using a mixture of
water and the ionic liquid [bmim][BF4] as solvent. Starting from various types of terminal olefins, the corresponding ketones have
been prepared under mild reaction conditions and obtained in good to excellent yields after a simple extraction with diethyl ether.
Furthermore, it was possible to recycle and reuse the ionic liquid and the catalytic system.
Ó 2005 Elsevier Ltd. All rights reserved.
The Pd(II) chloride and copper(I) chloride oxidation of
alkenes to ketones and aldehydes is an extension of the
well-known Wacker process.1,2 The solvent of the origi-
nal Wacker reaction is water containing HCl. It affords
excellent results in the production of acetaldehyde from
ethylene and it is one of the most important industrial
process employing transition metal catalysis. However,
higher alkenes have very poor solubility in water
and therefore exhibit lower rates for the oxidation and
furthermore they afford different types of by-products
due to the presence of the acid. Therefore, mixtures of
water and organic solvents (such as DMF, NMP or sul-
folane) have been studied in order to solve this prob-
lem.1,2 In the recent literature, various alternative
solutions have been described. They involve the use of
surfactants,3 cyclodextrins4 or calixarenes,5 supported
catalysts,6 nonclassical solvents such as fluorous phases7
or polyethylene glycols (PEGs).8 During the last decade,
room temperature ionic liquids (RTILs) have attracted a
lot of interest as a novel type of reaction medium9 and
they have proved to be especially useful in the case of
catalytic reactions.10 Therefore, ionic liquids have also
been considered for Wacker oxidations. A Pd-catalyzed
selective oxidation of styrene to acetophenone, using
H2O2 in imidazolium type ionic liquids, has been re-
ported.11 This has been further extended to super critical
(sc) CO2 and mixtures of ionic liquids and sc CO2.12
During the last few years, we have been interested in
the use of ionic liquids in synthesis, including oxidation
reactions. For instance, we have demonstrated recently
that ionic liquids are excellent solvents for the aerobic
oxidation of alcohols to carbonyl compounds.13 As an
extension of this study, we demonstrate in this letter that
a combination of the ionic liquid [bmim][BF4] and water
is an excellent solvent system for the Wacker oxidation
of different types of olefins using the classical Pd/Cu cat-
alysts under an oxygen atmosphere. The corresponding
ketones, bearing various substituents as well as different
chain lengths, were obtained in good to excellent yields
and it was possible to recycle and reuse the ionic liquid
and the catalyst (Scheme 1).
A key issue for the success of such a reaction was to
obtain the best possible miscibility of all components.
The imidazolium ionic liquids are reported to be good
solvents for metal salts and therefore they appeared as
O
PdCl2 -CuCl / O2 / H2O
R
CH3
R
[bmim][BF4]
Keywords: Wacker oxidation; Ionic liquid; PdCl2; CuCl; Alkenes.
q IICT Communication No. 050807.
R = aryl, phenoxyalkyl, alkyl
*
Corresponding authors. Fax: +91 40 27160512 (I.A.A.); e-mail
addresses: ansari78655@yahoo.com; Rene.Gree@univ-rennes1.fr
Scheme 1.
0040-4039/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2005.09.013

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I. A. Ansari et al. / Tetrahedron Letters 46 (2005) 7507–7510
obvious candidates. However, it has also been well estab-
lished that their miscibility with water was strongly
dependent upon the nature of the counter anion.14 The
[bmim][BF4] ionic liquid was selected for this study since
it has an excellent miscibility with H2O. Styrene was se-
lected as a convenient model alkene to optimize the reac-
tion conditions using such an ionic liquid–water mixture.
quantities of water gave more degradation products
while lower quantities gave less conversion to the desired
product. Under these optimized reaction conditions, we
found that 95% of the styrene was consumed after 25 h
and the isolated yield of carbonyl compound was 90%,
just 5% of the starting material being left unreacted.
It is interesting to note that under these conditions it was
not necessary to add any acid, contrary to the classical
Wacker process. This may be due to the hydrolysis of
In pure [bmim][BF4], the reaction was very slow, afford-
ing only traces of acetophenone. Then, water was added
and after a careful optimization of the quantity, it was
found that the best ratio was 2/1 by volume. Higher
À
BF4 liberating HF during the reaction, which may
act as the catalyst.15
Table 1. Ionic liquid mediated Wacker oxidation of terminal olefins
Entry Alkenes
Productsa
Conversionb (%) Reaction time/h GC ratio ketone/aldehyde Yieldc (%)
O
a
95
90
88
92
90
95
90
90
88
25
24
24
24
28
30
33
25
25
92:6.5
90
88
85
80
85
82
70
78
80
Me
O
Me
b
97.5:2.4
96.2:3.1
80:18
Cl
Cl
O
Me
c
Br
Br
O
Me
d
H3C
H3C
MeO
Cl
O
e
95.1:3.5
94.1:4.2
83.4:8.8
90.2:6.9
91:7.5
Me
MeO
O
f
Me
Cl
O
O
O
g
Me
O
O
O
h
Me
Me
Me
O
O
i
O
Me
Me
j
85
84
86
84
45
50
48
58
81.1:13.3
72:13.6
70
60
62
60
O
Me
O
k
l
Me
72.1:11.5
73.1:10.3
O
O
m
TBDMSO
TBDMSO
Me
Me
THPO
n
85
64
74:12.6
62
THPO
O
a ,b All products were characterized by GC analysis and 1H NMR.
c Yield refers to the pure ketones after column chromatography.

I. A. Ansari et al. / Tetrahedron Letters 46 (2005) 7507–7510
7509
Table 2. Reuse of the solvent system for the Wacker oxidation of methyl styrene to ketone
Entry
Olefin
Run
Conversion (%)
Ketone
(%)a
Aldehyde
(%)a
a
b
c
d
e
f
g
h
1
2
3
4
5
6
7
8
92
96
95
94
94
90
89
85
80
76
78
82
85
90
92
96
18
15
14
12
11
7
O
O
H
Me
Me
Me
Me
—
—
a GC ratio of the ketone taken from 100% product ratio.
These reaction conditions could be extended to various
types of alkenes as indicated in Table 1. These included
various styrenes (entries a–f), as well as aliphatic
derivatives (entries j–l). Several examples (entries g–i
and m–n) demonstrated that functional groups could
be tolerated in this process. In each case, the conversions
were excellent (84–95%). Furthermore, these reactions
afforded excellent (in the case of aromatic) to good
yields (in the case of aliphatic derivatives) as shown in
Table 1.
Wacker oxidation of higher and functionalized alk-
enes.16 Under these reaction conditions, the classical cat-
alytic system of PdCl2/CuCl and O2 can be used. The
catalyst/solvent system can be recycled several times
with only a slight decrease in the yield of the expected
ketones. Extension of these reactions and their applica-
tions in total synthesis are under active study in our
laboratories.
Acknowledgements
The possibility of recycling the solvent and the catalysts
is one of the key advantages in the use of ionic liquids as
solvents. This was demonstrated using 4-methylstyrene
as a model (Table 1, entry d). It was possible to recycle
the system up to eight times with only a slight decrease
in the conversion (96–85%). However, at the same time,
an increase in the selectivity was observed since the
excess of the ketone increased from 70% to 96% (Table
2) with less tolylaldehyde being obtained. At this stage,
there is no clear explanation for this intriguing result.
S.J. and M.K.G. thank CSIR, New Delhi, for financial
assistance. We thank CNRS, France, for the award of
a research associate position to I.A.A. We also thank
IFCPAR (New Delhi) and CEFISO/IFCOS, for
support.
References and notes
1. Tsuji, J. In Comprehensive Organic Synthesis; Trost, B. M.,
Fleming, I., Eds.; Pergamon: Oxford, 1991; Vol. 3, p 449
and references cited therein.
2. Tsuji, J. Synthesis 1984, 369–383 and references cited
therein.
In addition to recycling of the solvent and catalyst for
the same substrate, it was found suitable for being re-
used for different types of substrates. This was demon-
strated in different reactions using the same solvent–
catalyst system and taking either aromatic alkenes (such
as entries a, d and e) or aliphatic derivatives (such as en-
tries j and k, Table 1), as shown in Scheme 2. In each
case, the yields were equal to previous ones and NMR
and GC analysis of the products indicated no contami-
nation due to the earlier product. In all cases, the alde-
hyde yield decreased.
3. Alandis, N.; Rico-Lattes, I.; Lattes, A. New J. Chem. 1994,
18, 1147.
4. Monflier, E.; Tilloy, S.; Blouet, E.; Barbaux, Y.; Mor-
treux, A. J. Mol. Catal. A: Chem. 1996, 109, 27–35.
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J. Mol. Catal. A: Chem. 2004, 217, 59–67.
6. Arhancet, J. P.; Davies, M. E.; Hanson, B. E. Catal. Lett.
1991, 11, 129; Tang, H. G.; Sherrington, D. C. J. Mol.
Catal. 1994, 94, 7–17; Ahn, J.-Y.; Sherrington, D. C.
Macromolecules 1996, 29, 4164.
In conclusion, a mixture of [bmim][BF4] and water ap-
pears to be an efficient solvent system for the aerobic
7. Betzemeier, B.; Lhermitte, F.; Knochel, P. Tetrahedron
Lett. 1998, 39, 6667–6670.
8. Haimov, A.; Neumann, R. Chem. Commun. 2002, 876–
877.
9. Wasserscheid, T.; Welton, T. Ionic Liquids in Organic
Synthesis; Wiley-VCH: Weinheim, 2003.
O
PdCl2 -CuCl / O2 / H2O
Me
[bmim][BF4]
10. Welton, T. Chem. Rev. 1999, 99, 2071–2084; Wasserscheid,
P.; Keim, W. Angew. Chem., Int. Ed. 2000, 39, 3772–3789;
Sheldon, R. Chem. Commun. 2001, 2399–2407 and refer-
ences cited therein.
11. Namboodiri, V. V.; Varma, R. S.; Sahle-Demessie, E.;
Pillai, U. Green Chem. 2002, 4, 170–173.
12. Jiang, H.; Jia, L.; Li, J. Green Chem. 2000, 2, 161–
163; Hou, Z.; Han, B.; Gao, L.; Jiang, T.; Liu, Z.; Chang,
Y.; Zhang, X.; He, J. New J. Chem. 2002, 26, 1246–
1248.
then
recycled solvent
O
and catalyst
R
Me
R
O2 / H2O
R = aryl, phenoxyalkyl, alkyl
Scheme 2.
13. Ansari, I. A.; Gree, R. Org. Lett. 2002, 4, 1507–1509.
´

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I. A. Ansari et al. / Tetrahedron Letters 46 (2005) 7507–7510
14. Seddon, K.; Stark, A.; Torres, M. Pure Appl. Chem. 2000,
12, 2275.
(MgSO4) and concentrated under vacuum. Acetophenone
was isolated by flash chromatography on SiO2 and fully
characterized by NMR and by comparison with an
authentic sample. Similar reactions were performed with
other alkenes. The corresponding ketones had spectro-
scopic data (IR, 1H NMR) in agreement with the
literature. A further comparison by GC analysis was also
performed in the case of commercially available ketones.
Recycling and reuse of the ionic liquid and catalyst: After
extraction of the product, the ionic liquid containing
PdCl2 and CuCl was dried at 60 °C under vacuum for 4 h
before the next run. Then, 1 mL of H2O was added and
the mixture was stirred at 60 °C for 1 h under an oxygen
atmosphere before the slow addition of the olefin as
described previously.
15. Anbar, M.; Guttmann, S. J. Phys. Chem. 1960, 64, 1896.
16. Typical procedure: A mixture of PdCl2 (36 mg, 0.2 mmol,
10 mol %), CuCl (200 mg, 2 mmol) in ionic liquid [bmim]-
[BF4] (2 mL, 8.4 mmol) and H2O (1 mL, 55.5 mmol) was
stirred at 60 °C for 1 h under an oxygen atmosphere (the
colour of the reaction mixture changed from greenish to
dark and then again to greenish). Then styrene (208 mg,
2 mmol) was added dropwise and slowly over 30–40 min.
After complete addition, the reaction mixture was further
stirred at the same temperature under an oxygen atmos-
phere and the progress of the reaction was monitored by
GC. After completion of the reaction, the product was
extracted with ether. The organic phase was dried