Oxidation of alcohols with hydrogen peroxide catalyzed by a new imidazolium ion based phosphotungstate complex in ionic liquid

Article abstract of DOI:10.1016/j.jcat.2004.12.022

A new catalyst based on imidazolium and tungstate ion (tris(imidazolium)- tetrakis(diperoxotungsto)phosphate (3-)) has been synthesized and characterized by FT-IR spectroscopy. An efficient and environmentally friendly procedure is described for the catalyst recycling and easy product isolation for the oxidation of alcohols with hydrogen peroxide catalyzed by imidazolium ion-based phosphotungstate complex in ambient-temperature ionic liquid.

Full text of DOI:10.1016/j.jcat.2004.12.022

Journal of Catalysis 230 (2005) 436–439
www.elsevier.com/locate/jcat
Oxidation of alcohols with hydrogen peroxide catalyzed by a new
imidazolium ion based phosphotungstate complex in ionic liquid
∗
Bhupender S. Chhikara, Ramesh Chandra, Vibha Tandon
Medicinal Chemistry Research Laboratory, Dr. B.R. Ambedkar Center for Biomedical Research, University of Delhi, Delhi, India
Received 15 October 2004; revised 13 December 2004; accepted 16 December 2004
Abstract
A new catalyst based on imidazolium and tungstate ion (tris(imidazolium)-tetrakis(diperoxotungsto)phosphate (3-)) has been synthesized
and characterized by FT-IR spectroscopy. An efficient and environmentally friendly procedure is described for the catalyst recycling and
easy product isolation for the oxidation of alcohols with hydrogen peroxide catalyzed by imidazolium ion-based phosphotungstate complex
in ambient-temperature ionic liquid.

2004 Elsevier Inc. All rights reserved.
Keywords: Oxidation; H O ; Ionic liquid; Alcohol; Green chemistry
2
2
1
. Introduction
Carbonyl compounds constitute an important group of
research on alternative reaction media. More attention is be-
ing given to the reusability of solvents and catalysts of the
reaction systems for the development of cost-effective pro-
tocols.
molecules in organic chemistry, as carbonyl groups are
present as an essential constituent of pharmaceuticals, dyes,
fragrances, industrially important chemicals, and natural
products [1,2]. The oxidation of alcohols to the corre-
sponding carbonyl groups is a fundamental transformation
in organic chemistry and industrially important [3]. Be-
cause of the advantageous properties of hydrogen peroxide,
a number of useful procedures have been developed for
H2O2-mediated oxidation of alcohols catalyzed by tungsten
systems such as tungstic acid [4], quaternary ammonium
tetrakis(diperoxotungsto)phosphates [5], sodium tungstate
There is considerable interest in the use of room temper-
ature ionic liquids as promising substitutes for volatile or-
ganic solvents [8]. These ambient-temperature ionic liquids,
especially those based on 1,3-dialkylimidazolium cations,
have been emerging as promising green solvents in recent
decades [9–11]. Their nonvolatile nature, without any de-
tectable vapor pressure, gives them a significant advantage in
minimizing solvent consumption. They have been employed
as reaction media for several organic reactions, namely alky-
lation [12], hydrogenation [13], oxidation [14–16], and het-
erocyclization [17]. Because of their tunable polarity and
hydrophobicity, they can solvate various organic, inorganic,
and polymeric compounds, and have been used as green
solvents for liquid–liquid separations, extractions, and recy-
cling in homogeneous catalysis [18]. Here we report the syn-
thesis of imidazolium-based phosphotungstate catalyst (1)
and its catalytic function in the oxidation of alcohols with
H2O2 in ionic liquid [bmim][BF4] [19].
+
[(n-C4H9)4N]Cl [6], and sodium tungstate + quaterna-
ryammoniumhydrogen sulfate [7]. These reported proce-
dures have used chlorohydrocarbon as a solvent, and in most
of the cases the catalyst was not recovered. The recent in-
creased awareness of the detrimental effects of these organic
solvents in the environment has led to rapid growth in the
*
Corresponding author.
E-mail address: vibhadelhi@hotmail.com (V. Tandon).
0021-9517/$ – see front matter  2004 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcat.2004.12.022

B.S. Chhikara et al. / Journal of Catalysis 230 (2005) 436–439
437
Table 1
Oxidation of secondary alcohols with H O catalysed by 1 in ionic liquid
2
2
a
Entry
Substrate
Time
h)
Yield
(%)
(
Fig. 1. Phosphotungstate catalyst 1.
1
2
3
98
b
(96)
2
. Experimental
All chemicals and reagents used in the present study
2
3
4
97
93
96
were of analytical grade and procured from Sigma Aldrich
and Spectrochem Pvt Ltd, India. Organic solvents were
freshly distilled and purified before use. IR spectra were
recorded on a FT-IR Perkin–Elmer Spectrum BX spec-
trophotometer with either NaCl plates or KBr pallets. NMR
spectral characterization was carried out with a Bruker 300
MHz instrument operating near 300 (1H) and 100 (13C)
MHz, with tetramethylsilane as an internal standard for
the chemical shifts measurement. The FAB mass spec-
tra were recorded from Central Drug Research Institute
3
3
(
Lucknow, India) on a Jeol SX 102/DA-6000 mass spec-
trometer with an m-nitrobenzyl alcohol matrix. Gas chro-
matographs were recorded with a Shimadzu GC-14B in-
strument, a flame ionization detector, and a Varian CP-Sil
5
3
78
5
CB capillary column and packed column, with nitrogen
as the carrier gas. The required ionic liquid [bmim][BF4]
was prepared according to the reported procedure by the
alkylation of 1-methylimidazole with 1-bromobutane fol-
lowed by substitution of bromide anion with tetrafluorob-
6
7
8
9
4
2
3
4
84
86
92
90
1
orate in acetone [20]. [ H NMR (CDCl3, 300 MHz, δ ppm):
8
.98 (s, 1H, C–2H), 7.49 (s, 2H, C–4H and C–5H), 4.21
t, J = 7.11 Hz, 2H, NCH2CH2CH2CH3), 4.01 (s, 3H,
NCH3), 1.85 (p, J = 7.20 Hz, 2H, –NCH2CH2CH2CH3),
(
1
.35 (sexet, J = 7.20 Hz, 2H, NCH2CH2CH2CH3), 0.91
13
(t, J = 7.12 Hz, 3H, NCH2CH2CH2CH3). C NMR
(CDCl3, 75 MHz, δ ppm): 13.27, 19.23, 31.87, 36.13, 49.57,
1
22.49, 123.79 and 136.12.]
2
.1. Synthesis of phosphotungstate complex 1
Tungstic acid (5 g, 20 mmol) was added portion wise to
a 30% solution of H2O2 (14 mL) with stirring and heated to
◦
6
5 C, and mixture was stirred for 2 h. The reaction mixture
was filtered through a sintered funnel (G3). H3PO4 (0.6 mL
of 80% sol., calc. 2.5 mmol) was added to the filtrate at
room temperature. [bmim]Br (2.2 g, 2 mmol) in CH2Cl2
1
0
1
3
4
96
98
(
20 mL) was added dropwise to the above solution. A brown-
ish yellow solid separated out with shaking. The supernatant
solvent was decanted away, and solid settled at the bottom
was washed with water to obtain the light brownish yellow
1
−1
viscous solid product (4 g). [IR (KBr, cm ): 3435, 3152,
a
3
1
096, 2921, 2858, 1626, 1560, 1458, 1348, 1167, 1094,
082, 1023, 961, 872, 816, 737, 618, 523, 471.]
Yield from GC.
Yield in recovered ionic liquid and catalyst 1.
b

438
B.S. Chhikara et al. / Journal of Catalysis 230 (2005) 436–439
Fig. 2. FT-IR spectrum of catalyst 1.
H2O
4
H2WO4 + H3PO4 + 8H2O2 + 3[bmim]Br −−−→
CH2Cl2
ꢀ
ꢁ
PO4(W(O)(O2)2)4 3 + 3HBr + 12H2O
−
Scheme 1. Synthesis of catalyst 1.
Scheme 2. Oxidation of alcohols with catalyst 1.
2
.2. General procedure for the oxidation of alcohols
1
were assigned to P–O stretching vibrations [24]. A strong
peak at 961 indicated the presence of W=O, and peaks at
872, 816 (O–O) and 618 (W–O–O asym), 523 (W–O–O
sym) confirmed the tungstate structure [22]. Absorption at
Presaturated phosphotungstate catalyst 1 (0.05 mmol)
was dissolved in the ionic liquid [bmim][BF4] (3 mL) at
◦
9
0 C with stirring. Alcohol (1 mmol) was added, followed
by the dropwise addition of H2O2 (1.5 mmol) with contin-
uous stirring. After completion of the reaction, the reaction
mixture was extracted with diethyl ether (4 × 1 mL), dried
over Na2SO4, and concentrated to obtain the respective ke-
tones in 82–98% yield (Table 1).
2
921, 2858, 1560, and 1458 is due to an organic imida-
zolium moiety. Interestingly this phosphotungstate catalyst
was found to be soluble in dimethylsulfoxide and ionic liq-
uid [bmim][BF4] and remained insoluble in most of the other
laboratory solvents.
Catalyst 1 was dissolved in [bmim][BF4] by simply mix-
ing at ambient temperature, and oxidation of different alco-
hols was carried out in homogeneous medium. A mixture
of diphenyl carbinol and 1 in [bmim][BF4] was stirred for
3
. Results and discussion
The catalyst tris(imidazolium)-tetrakis(diperoxotungsto)-
1
20 min after the addition of hydrogen peroxide to give ben-
phosphate (3-) (1) was synthesized from tungstic acid,
hydrogenperoxide, phosphoric acid, and 1-butyl-3-methyl-
imidazolium bromide by slight modification of the reported
procedure [21–23] (Scheme 1).
zophenone in 98% yield. Similarly, oxidation of other sec-
ondary alcohols with H2O2 catalyzed by 1 in [bmim][BF4]
was carried out; the results are shown in Table 1. The ke-
The catalyst was purified, and its structure was confirmed
by FT-IR spectroscopic data (Fig. 2) (see Section 2). The ab-
sorption in the region 1050 (peaks at 1094, 1082, and 1023)
1
−1
Peaks in the region of 1050 cm has been observed with tungstophos-
phates.

B.S. Chhikara et al. / Journal of Catalysis 230 (2005) 436–439
439
Scheme 3. Oxidation of benzyl alcohol.
tones were extracted with diethyl ether from the ionic liquid
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2
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7
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