Phosphonium ionic liquid: A novel catalyst for benzyl halide oxidation

Article abstract of DOI:10.1080/00397910902840835

An environmentally friendly ionic liquid is used for the first time in mild and selective oxidation of aryl halides to corresponding aldehydes using iodoxybenzoic acid as oxidizing agent. Copyright Taylor & Francis Group, LLC.

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Phosphonium Ionic Liquid:
A Novel Catalyst for Benzyl
Halide Oxidation
Satish A. Dake a , Ravibhushan S. Kulkarni a , Vijay
N. Kadam a , Sandesh S. Modani a , Jayant J. Bhale a
, Sumangala B. Tathe a & Rajendra P. Pawar a
a Organic Chemistry Synthesis Laboratory,
Dnyanopasak College, Parbhani, India
Version of record first published: 07 Oct 2009.
To cite this article: Satish A. Dake , Ravibhushan S. Kulkarni , Vijay N. Kadam ,
Sandesh S. Modani , Jayant J. Bhale , Sumangala B. Tathe & Rajendra P. Pawar (2009):
Phosphonium Ionic Liquid: A Novel Catalyst for Benzyl Halide Oxidation, Synthetic
Communications: An International Journal for Rapid Communication of Synthetic
Organic Chemistry, 39:21, 3898-3904
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Synthetic Communications1, 39: 3898–3904, 2009
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397910902840835
Phosphonium Ionic Liquid: A Novel Catalyst
for Benzyl Halide Oxidation
Satish A. Dake, Ravibhushan S. Kulkarni, Vijay N. Kadam,
Sandesh S. Modani, Jayant J. Bhale, Sumangala B. Tathe,
and Rajendra P. Pawar
Organic Chemistry Synthesis Laboratory, Dnyanopasak College,
Parbhani, India
Abstract: An environmentally friendly ionic liquid is used for the first time in
mild and selective oxidation of aryl halides to corresponding aldehydes using
iodoxybenzoic acid as oxidizing agent.
Keywords: Aryl halides, carbonyl compounds, hypervalent iodine, ionic liquids,
selective oxidation
INTRODUCTION
Oxidation of aryl halide to aldehyde is an important transformation in
synthetic organic chemistry. Different reagents have been used for this
transformation.[1,2] However, all these procedures are associated with
certain limitations like long reaction times, poor yields, high tempe-
ratures, and so on. Thus the introduction of a new method for the
oxidation of aryl halides is still in demand.
In past two decades, applications of hypervalent iodine have been
proliferating in synthetic organic chemistry.[3] Among various hyper-
valent iodine reagents, o-iodoxy benzoic acid (IBX) is used as a versatile
oxidizing agent[4] because of its mild, selective, efficient, and ecofriendly
Received December 5, 2008.
Address correspondence to Rajendra P. Pawar, Department of Chemistry,
Deogiri College, Aurangabad-431005, MS, India. E-mail: rppawar@yahoo.com
3898

Benzyl Halide Oxidation
3899
Scheme 1.
properties and operational simplicity. Recently, SIBX (stabilized formu-
lation of IBX) has been used for the efficient and selective oxidation of
alcohol in presence of aq. sodium bicarbonate solution.[5] Oxygen-
mediated oxidation of alkenes using nafion-supported platinum(II)
terpyridyl acetylide complex using a photosensitization reaction has also
been reported.[6]
Nonvolatile ionic liquids have been recognized as environmentally
benign alternatives to volatile organic solvents.[7] They possess high ther-
mal stability and good solvating ability. Their immiscibility with organic
solvents permits development of a heterogeneous system to carry out the
reaction. The products formed are recovered by a liquid–liquid extraction
process.[8] Recently, the oxidation of alcohols and diols has been reported
using imidazoliun ionic liquids.[9,10]
In the present work, for first time we introduce a new methodology
for the oxidation of aryl halide to corresponding carbonyl compounds
using IBX oxidant in the presence of phosphonium salt ionic liquid
and water (Scheme 1). The reaction completes in a short time, affording
the product in excellent yield.
RESULTS AND DISCUSSION
In a typical condensation reaction, a mixture of aryl halide 1 (1 equiv.),
IBX 2 (1.5 equiv.), phophonium ionic liquid (1 ml), and water (2 ml) was
heated at 50ꢀC for the appropriate time (Table 1). After completion of
the reaction as indicated by thin-layer chromatography (TLC), by usual
workup the corresponding pure carbonyl compound 3 was obtained in
excellent yield. The aqueous layer was dried at 120ꢀC, and the residual
ionic liquid was recycled and reused several times. When carried out in
water without phosphonium ionic liquid, the same reactions do not
proceed even after 5 h of heating at 50ꢀC (Table 2).
The oxidation of aryl halides has been done using IBX as oxidizing
agent. The reaction, carried out in water medium, avoids the hazardous
organic solvents, which not only reduces the cost of the product but is

3900
S. A. Dake et al.
Table 1. Conversion of aryl halides to carbonyl compounds in ionic liquid
IBX
(eq.)
Temp
(ꢀC)
Yield
(%)
Time
(Min)
Substrate
Product
1.5
1.5
50
50
92
88
20
20
1.5
1.5
50
50
89
92
30
55
1.5
50
91
40
1.5
1.5
50
50
92
91
40
45
1.5
50
89
35
also helpful for environmental safety. In this transformation, we used
trihexyl (tetradecyl) phosphonium tetrafluroborate as a phosphonium
ionic liquid. The advantage of phosphonium ionic liquid is its easy
availability and relatively low cost. This ionic liquid is thermally
and chemically stable. It is recycled and reused about 10 to 20 times
with slight change in its efficiency (0.5–1%). Trihexyl (tetradecyl)

Benzyl Halide Oxidation
3901
Table 2. Conversion of aryl halides to carbonyl compounds without ionic liquid
in water
IBX
(eq.)
Temp
(ꢀC)
Time
(min.)
Substrate
Yield (%)
—
Product
1.5
1.5
50
50
300
300
Trace
amount
1.5
1.5
1.5
50
50
50
—
—
—
300
300
300
1.5
1.5
50
50
—
—
300
300
1.5
50
—
300
phosphonium tetrafluroborate ionic liquid is less dense than water. Thus,
just decanting aqueous streams can easily separate the product. In addi-
tion, the imidazolium ionic liquids are slightly acidic, which may result in

3902
S. A. Dake et al.
carbene formation. However, the phosphonium ionic liquid does not
possess any acidic protons, so no carbene formation takes place. Hence,
100% utility of this ionic liquid can be achieved by using it in organic
transformations.
The oxidation of nonsubstituted aryl halide to the carbonyl compound
completes in a short time. However, the conversion of substituted aryl
halide into the carbonyl compound requires somewhat longer time. The
product formation in this transformation is excellent (85–92%, Table 1).
The reaction of electron-rich aryl halide proceeds via unimolecular
nucleophilic substitution (SN1) mechanism and leads to the addition
product by salt formation. However, the electron-deficient aryl halide
reaction proceeds via a bimolecular nucleophilic substitution reaction
(SN2) mechanism. Further, the addition product decomposes to carbonyl
compound, forming iodoso benzoic acid (IBA) as a by-product.
In accordance with the mechanism, only 1 equivalent of IBX is
sufficient for the oxidation of benzyl bromide. Excess IBX addition does
not afford great yield of the product. Benzyl chloride reacted only slug-
gishly, leading to poor isolated yield. Observation of the reaction shows
initially generated HX is quite detrimental to IBX. The IBX in reaction
reduces to o-iodoso benzoic acid in saturated hydrogen halide (HX). A
further complication is that the o-iodoso benzoic acid, the product of
partial IBX reduction, may compete with benzyl halide to afford unde-
sirable ethers. The conversion is extremely good when the halide iodine
is taken. It increases the reaction rate, which obviates the reduction of
IBX in situ–generated HX.
EXPERIMENTAL PROCEDURE OF BENZALDEHYDE
SYNTHESIS
A mixture of benzyl bromide (0.171 g; 1 mmol), iodoxybenzoic acid
(0.420 g; 1.5 mmol), trihexyl (tetradecyl) phosphonium tetraflurobo-
rate (1 ml) and water (2 ml) were stirred at 50ꢀC under a nitrogen
atmosphere for the appropriate time. After completion of reaction
as determined by TLC, the reaction mixture was diluted with water
and then extracted with ethyl acetate solvent (3 Â 10 ml). The solvent
ethyl acetate was evaporated, and the crude was purified by silica-gel
chromatography using petroleum ether–ethyl acetate (3:2) mixture as
an eluent. Similarly, the other aldehydes were also prepared using the
same method (Table 1).
The aqueous layer was further heated at 120ꢀC to remove the
water, and the residual ionic liquid was recycled and reused several times
without any change in their efficiency (Table 3).

Benzyl Halide Oxidation
3903
Table 3. Recovery of ionic liquid in carbonyl compound synthesis
Yield (%) of recovered ionic liquid
Aryl Time
Entry halide Product (min)
Cycle 1
Recycle 1
Recycle 2
1
2
1a
1b
3a
3b
20
20
93
90
93
89
92
89
SPECTRAL DATA
1H NMR and 13C NMR spectra were recorded at room temperature on a
Varian Inova spectrometer in CDCl3 using tetramethylsilane (TMS) as
internal standard.
2-Nitro Benzaldehyde
1H NMR (CDCl3): d 7.7 (dd, 1 H, Ar-H), 7.8 (dd, 1 H, Ar-H), 7.9 (dd,
1 H, Ar-H), 8.1 (dd, 1 H, Ar-H), 10.4 (s, 1 H, CHO). 13CNMR (CDCl3):
124.2 (2C, Ar), 130.4 (2C, Ar), 139.9 (1C, Ar), 150.9 (1C, Ar), 190.3 (1C,
CHO).
3-Hydroxy-4-methoxy Benzaldehyde
1H NMR (CDCl3): d 3.9 (s, 3 H, OCH3), 6.0 (s, 1 H, OH), 6.9 (dd, 1 H,
Ar-H), 7.2 (s, 1 H, Ar-H), 7.4 (dd, 1 H, Ar-H), 9.85 (s, 1 H, CHO).
13CNMR (CDCl3): 56.1 (1C, OCH3), 110.1 (1C, Ar), 113.9 (1C, Ar),
124.6 (1C, Ar), 130.4 (1C, Ar), 146.1 (1C, Ar), 151.8 (1C, Ar), 191.1
(1C, CHO).
CONCLUSIONS
In summary, we have demonstrated an efficient and mild protocol for the
selective oxidation of aryl halides to corresponding aldehyde using IBX
as oxidizing agent in the presence of phosphonium ionic liquid. The
method offers several advantages such as good yields, shorter reaction
time, cleaner reaction profiles, and simple experimental and workup
procedures.

3904
S. A. Dake et al.
ACKNOWLEDGMENT
The authors are thankful to the principal, Dr. P. L. More, Dnyanopasak
College, Parbhani, India, for encouragement during the process of this
work.
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