H2O2/phosphonium ionic liquid: An efficient and simple approach for benzyl halides oxidation

Article abstract of DOI:10.2174/157017810791824838

An efficient oxidation of benzyl halide has been done using aqueous hydrogen peroxide (30%) in trihexyl(tetradecyl)phosphonium-tetrafluroborate ionic liquid the protocol is simple mild offering excellent yield of product.

Full text of DOI:10.2174/157017810791824838

Letters in Organic Chemistry, 2010, 7, 491-494
491
H O /Phosphonium Ionic Liquid: An Efficient and Simple Approach for
2
2
Benzyl Halides Oxidation
1
1
1
1
Satish A. Dake , Ravibhushan S. Kulkarni , Ambadas B. Rode , Pravin S. Shinde , Sushil K.
1
1
,2
Ghumbre , Rupali L. Magar and Rajendra P. Pawar*
1
Organic Chemistry Synthesis Laboratory, Dnyanopasak College, Parbhani-431401, MS, India
2
Department of Chemistry, Deogiri College, Aurangabad-431 005, MS, India
Received February 19, 2010: Revised April 23, 2010: Accepted April 23, 2010
Abstract: An efficient oxidation of benzyl halide has been done using aqueous hydrogen peroxide (30%) in
trihexyl(tetradecyl)phosphonium-tetrafluroborate ionic liquid the protocol is simple mild offering excellent yield of
product.
Keywords: Benzyl halides, carbonyl compounds, aq. hydrogen peroxide (30%), ionic liquids.
INTRODUCTION
halide oxidation to corresponding carbonyl compounds using
aq. hydrogen peroxide (30%) in the presence of
phosphonium ionic liquid trihexyl(tetradecyl)phosphonium-
tetrafluroborate [PhosIL-BF ].
4
Benzyl halides oxidation is an important tool in synthetic
organic chemistry. Different reagents have been used for this
transformation [1, 2]. In last two decades applications of
hypervalent iodine have been proliferating in synthetic
organic chemistry [3], among various hypervalent iodine
reagents o- iodoxy benzoic acid (IBX) is used as versatile
oxidizing agent [4], because of its mild, selective, efficient
and eco-friendly properties and operational simplicity.
Oxygen mediated oxidation of alkenes using Nafion
supported Platinum (II) Terpyridyl Acetylide complex is also
used in Photosensitization reaction [5].
O
Br
PhosIL-BF4
H
+
(30%) aq. H
2
O
2
+ HBr + H O
2
R
50 ˚C
2
R
1
3
Scheme 1.
Non-volatile ionic liquids have been recognized as
environmentally benign alternatives to volatile organic
solvents [6]. They possess high thermal stability and good
solvating ability. These ionic liquids not only work as
solvent, but also enhance the rate of the reactions as like of
catalyst. Products formed in these reactions are recovered by
liquid-liquid extraction process [7]. Recently the oxidation of
alcohols and diols has been reported using imidazoliun ionic
liquids [8,9], peroxovanadate [10] and IBX [11], molecular
POSSIBLE MECHANISM
Step 1
HO-O-H + PhosIL-BF4
+
PhosIL-HBF4
HO-O
Step 2
H
C
H
H
O
OH
Br
+
Iodine/H
Hydrotalcite-supported SnO
2
O
2
[12], H
2
O
2
/nitrile using Mg/Al hydrotalcite [13].
catalyst was found to be
C
H
Br + HO-O
2
R
effective in BV oxidations with hydrogen peroxide [14].
However; all these procedures are associated with certain
limitations like long reaction time, hazardous conditions,
poor yield, high temperature, etc. Thus, the introduction of
new method for the oxidation of benzyl halides is still in
high demand.
R
-
H O
2
H
C
O
Step 3
+
2
H O
In the continuation of our work, we report a new
environment friendly method for the oxidation of benzyl
halide to corresponding carbonyl compounds (Scheme 1).
The reaction completed in short time, affording the product
in excellent yield. The results showed, method is compatible
to the reported methods. Herein, we wish to report benzyl
R
PhosIL-HBF4 + Br
PhosIL-BF4 + HBr
RESULT AND DISCUSSION
*
Address correspondence to this author at the Department of Chemistry,
Phosphonium ionic liquids (ILs) are highly basic in
nature [15]. The general formula of these ionic liquids is
Deogiri College, Aurangabad-431 005, MS, India; Tel: +91-240-2334577;
Fax: 91-240-2359940; E-mail: rppawar@yahoo.com
1
570-1786/10 $55.00+.00
© 2010 Bentham Science Publishers Ltd.

4
92 Letters in Organic Chemistry, 2010, Vol. 7, No. 6
Dake et al.
[
PR
3
R’] X, where both R and R’ are alkyl groups and X is
petroleum ether: ethyl acetate (3:2) mixture as an eluent.
halide [16]. Their large radius and polarizable lone pair
makes them more nucleophilic. The structural formula of
ionic liquid trihexyl(tetradecyl)phosphonium-tetrafluroborate
Similarly, the other aldehydes were also prepared using the
same method (Table 1).
O
The aqueous layer was further heated at 120
C
[
PhosIL-BF
4
] is as shown in Fig. (1).
temperatures to remove the water and the residual ionic
liquid was recycled and reused for several times without any
change in their efficiency (Table 2).
C6H13
BF4
P
C H
C14H29
6
13
SPECTRAL DATA
C6H
13
1
13
H NMR and C NMR spectra were recorded at room
Fig. (1). Structure of trihexyl (tetradecyl) phosphonium-
tetrafluroborate [PhosIL-BF ].
temperature on a varian Inova Spectrometer in CDCl
TMS as internal standard.
3
using
4
Phosphonium salts are thermally stable than ammonium
salts. Phosphonium-based ionic liquids have higher
viscosities than the ammonium counterparts at room
temperature. However, on heating at 70-100 C, their
viscosities generally decrease. Whereas, the addition of
reactants or catalysts further reduces the viscosity [17].
3-Hydroxy-4-methoxy benzaldehyde (3c)
1
3 3
H NMR (CDCl ): ꢀ 3.8 (s, 3H, OCH ), 6.2 (s, 1H, OH),
O
6
9
.8 (dd, 1H, Ar-H), 7.5 (s, 1H, Ar-H), 7.4 (dd, 1H, Ar-H),
.8 (s, 1H, CHO). C (CDCl ): 56.4 (1C, OCH ), 112.1 (1C,
3 3
13
Ar), 113.5 (1C, Ar), 125.1 (1C, Ar), 130.8 (1C, Ar), 146.2
(1C, Ar), 152.3 (1C, Ar), 191.2 (1C, CHO).
Oxidation of benzyl halides has been done using (30%)
2 2
aq. H O as an oxidizing agent. The reaction carried out in
water medium shows avoidance of the hazardous organic
solvents which not only reduces the cost of the product but
also is helpful for environmental safety. In this
transformation we used “trihexyl (tetradecyl) phosphonium-
3
-Nitro benzaldehyde (3f)
1
3
H NMR (CDCl ): ꢀ 7.6 (dd, 1H, Ar-H), 7.8 (dd, 1H, Ar-
H), 7.9 (dd, 1H, Ar-H), 8.2 (dd, 1H, Ar-H), 10.4 (s, 1H,
CHO). C (CDCl
(1C, Ar), 151.2 (1C, Ar), 190.5 (1C, CHO).
1
3
tetrafluroborate [PhosIL-BF
4
]” as a phosphonium ionic
3
): 123.7 (2C, Ar), 131.1 (2C, Ar), 139.9
liquid. Advantages of the use of phosphonium ionic liquid
are 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 a slight change in its efficiency
4
-methoxybenzaldehyde (3i)
1
(
[
0.5-1%). trihexyl(tetradecyl)phosphonium-tetrafluroborate
PhosIL-BF ] ionic liquid is less dense than water. Thus, just
3 3
H NMR (CDCl ): ꢀ 3.7 (s, 3H, OCH ), 6.9 (dd, 2H, Ar-
13
H), 7.7 (dd, 2H, Ar-H), 9.8 (s, 1H, CHO). C (CDCl
(1C, OCH ), 115.2 (2C, Ar), 130.5 (2C, Ar), 127.2 (1C, Ar),
165.5 (1C, Ar), 191.5 (1C, CHO).
3
): 56.0
4
by decanting the aqueous streams the product is separated. In
addition, the imidazolium ionic liquids are slightly acidic,
which may result in carbene formation. However, the
phosphonium ionic liquids do not possess any acidic proton,
so no carbene formation takes place. Hence 100% utility of
this ionic liquid can be achieved in organic transformations.
Oxidation of non-substituted benzyl halide is completed in
short time. However, the oxidation of substituted benzyl
halide requires somewhat longer time. Electron rich
substituted benzyl halide rates of oxidation were found to be
higher as compared to electron poor substituted benzyl
halides.
3
4
-(benzyloxy)-3-methoxybenzaldehyde (3j)
1
H NMR (CDCl
3
): ꢀ 3.7(s, 3H, -OCH
3
), 7.2(s, 1H, Ar-H),
),
): 56.3 (1C,
), 150.5 (1C, Ar), 115.2 (1C, Ar), 130.5 (1C, Ar),
123.1 (1C, Ar), 115.7 (1C, Ar), 155.7 (1C, Ar), 71.3(1C, -
CH -Ar), 141.5(1C, Ar), 127.1(2C, Ar), 129.1(2C, Ar),
7.3(dd, 1H, Ar-H), 6.8(dd, 1H, Ar-H), 5.1(s, 2H, -OCH
2
1
3
7.1(s, 5H, Ar-H), 9.8(s, 1H, CHO). C (CDCl
OCH
3
3
2
127.5(1C, Ar), 191.1 (1C, CHO).
The product formation in this process is excellent 85-92
%
(Table 1).
3,4-dimethoxybenzaldehyde (3b)
1
3 3
H NMR (CDCl ): ꢀ 3.8 (s, 6H, -OCH ), 7.2 (s, 1H, Ar-
H), 6.9 (dd, 1H, Ar-H), 7.1(dd, 1H, Ar-H), 9.9 (s, 1H, -CH0).
GENERAL PROCEDURE OF BENZALDEHYDE
SYNTHESIS
3
,4-bis(benzyloxy)benzaldehyde (3d)
A mixture of benzyl bromide (1 mmol.), (30%) aq.
hydrogen peroxide (30%) (3 mmol.), trihexyl (tetradecyl)
1
H NMR (CDCl
), 7.2 (s, 1H, Ar-H), 6.9 (dd, 1H, Ar-H), 7.1 (dd, 1H,
Ar-H), 9.7 (s, 1H, -CHO).
3
): ꢀ 7.1 (s, 10H, Ar-H), 5.2 (s, 4H, -
4
phosphonium-tetrafluroborate [PhosIL-BF ] (1 ml.) was
OCH
2
O
stirred at 50 C temperature under nitrogen atmosphere for
appropriate time. After completion of reaction as confirmed
on thin layer chromatography (TLC), 25 ml. distilled water
was added. The mixture was extracted with ethyl acetate
solvent (3 x 10ml). Solvent ethyl acetate evaporated and the
crude was purified by silica gel chromatography using
4
-Nitro benzaldehyde (3e)
1
3
H NMR (CDCl ): ꢀ 8.0 (dd, 2H, Ar-H), 8.9 (dd, 2H, Ar-
H), 9.9 (s, 1H, -CHO).

H
2
O
2
/Phosphonium Ionic Liquid
Letters in Organic Chemistry, 2010, Vol. 7, No. 6
493
Table 1. Conversion of Aryl Halides to Carbonyl Compounds in Ionic Liquid
Entries
Substrate (1)
(30%) aq. H
2
O
2
(Eq.) (2)
Yield (%)
Time (Min)
Products (3)
O
Br
a
3
90
17
H
O
O
O
O
Br
Br
Br
Br
H
H
H
H
b
c
3
3
89
88
40
50
MeO
MeO
BnO
MeO
MeO
BnO
OMe
OMe
OH
OH
d
e
3
3
90
92
30
55
OBn
OBn
O N
2
O N
2
O
Br
H
f
3
3
87
86
60
30
NO2
NO2
O
Br
g
H
BnO
BnO
O
Br
Br
H
h
3
3
87
85
120
160
H
O N
O N
2
2
O
O
Br
i
H
MeO
MeO
Br
CHO
j
3
92
230
MeO
MeO
OBn
OBn

4
94 Letters in Organic Chemistry, 2010, Vol. 7, No. 6
Dake et al.
Table 2. Recovery of Ionic Liquid in Carbonyl Compound Synthesis
Yield(%) of Recovered Ionic Liquid
Entry
Aryl Halide
Product
Time(min)
Cycle 1
Recycle 1
Recycle 2
1
2
1a
1b
3a
3b
17
40
90
89
89
87
88
87
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-(benzyloxy)benzaldehyde (3g)
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3
H NMR (CDCl ): ꢀ 7.1 (s, 5H, Ar-H), 6.9 (dd, 2H, Ar-
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CHO).
2
), 9.8 (s, 1H, -
[
5]
6]
4
-Nitrobenzene-1,2-dialdehyde (3h)
[
1
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1
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CONCLUSIONS
In summary we demonstrated an efficient and mild
protocol for the selective oxidation of benzyl halides to
corresponding aldehydes using aq. hydrogen peroxide (30%)
as an agent in the presence of phosphonium ionic liquid. The
method offers several advantages such as high yields, shorter
reaction time, cleaner reaction profiles and simple
experimental and work up procedure.
[
9]
[10]
ACKNOWLEDGEMENT
[
11]
12]
The authors are thankful to the Principal Dr. P. L. More,
Dnyanopasak College, Parbhani, for the encouragement
during the process of carrying out this work.
[
[
13]
Llamas, R. Sanchidrian, C. J.; Ruiz, J. R. Heterogeneous Baeyer-
Villiger oxidation of ketones with H O /nitrile, using Mg/Al
2 2
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