Oxidation of benzylic alcohols to their corresponding carbonyl compounds using ceric ammonium nitrate (CAN)/broensted acidic ionic liquid

Article abstract of DOI:10.1007/BF03249071

A chemoselective and efficient procedure for the oxidation of alcohols to the corresponding carbonyl compounds is reported using ceric ammonium nitrate in the presence of 3-methylimidazolium hydrogensulfate as Broensted acidic ionic liquid ([Hmim]HSO₄) as the solvent under mild conditions. The use of non-toxic and inexpensive materials, straightforward procedure, short reaction times and good yields of the products are the major advantages of this method.

Full text of DOI:10.1007/BF03249071

J. Iran. Chem. Soc., Vol. 8, No. 2, June 2011, pp. 382-387.
JOURNAL OF THE
Iranian
Chemical Society
Oxidation of Benzylic Alcohols to Their Corresponding Carbonyl Compounds Using
Ceric Ammonium Nitrate (CAN)/Brönsted Acidic Ionic Liquid
A.R. Hajipoura,b,*, L. Khazdoozb and A.E. Ruohoa
aDepartment of Pharmacology, University of Wisconsin, Medical School, 1300 University Avenue, Madison, 53706-
1532, WI, USA
bPharmaceutical Research Laboratory, College of Chemistry, Isfahan University of Technology, Isfahan 84156, IR
Iran
(Received 14 February 2009, Accepted 13 April 2009)
A chemoselective and efficient procedure for the oxidation of alcohols to the corresponding carbonyl compounds is reported
using ceric ammonium nitrate in the presence of 3-methylimidazolium hydrogensulfate as Brönsted acidic ionic liquid
([Hmim]HSO4) as the solvent under mild conditions. The use of non-toxic and inexpensive materials, straightforward procedure,
short reaction times and good yields of the products are the major advantages of this method.
Keywords: Oxidation, Alcohols, Ceric ammonium nitrate, Brönsted acidic ionic liquid
INTRODUCTION
([bmim][ClAlCl3]) [6a], and 1-butyl-3-methylimidazolium
bisulfate ([bmim][HSO4]) [6b], 3-methylimidazolium
hydrogensulfate ([Hmim]HSO4) [7] have been used as acid
catalysts and provide a useful medium under solvent-free
conditions because of their polar nature.
Ionic liquids (IL) have frequently been used as a green
solvent in place of classical organic solvents in modern
synthetic chemistry [1-4]. Ionic liquids are superior to
conventional organic solvents due to their extremely low
vapor pressure, excellent thermal stability, reusability and
ability to dissolves many organic and inorganic substrates [4].
The application of ionic liquids as solvent and catalyst has
been reported for a variety of functional group transformations
but their use as acid catalysts under solvent-free conditions
requires more attention [1-4]. Ionic liquids with Brönsted
acidic counter ions such as 1-hexyl-3-methylimidazolium
bisulfate ([hmim][HSO4]) [5], 1-butyl-3-methylimidazolium
dihyrogen phosphate ([bmim][H2PO4]) [5], 1-[2-(2-hydroxy-
The oxidation of alcohols to the corresponding aldehyde or
ketone is one of the most important functional group
transformations in organic synthesis. Despite the using many
reagents for this transformation [8] still there are many
demands for mild and efficient methods. Tetravalent cerium
salts are strong one-electron oxidizing agents that can be used
as reagents for a wide variety of organic transformations [9].
The most extensively used cerium(IV) reagent in organic
chemistry is cerium(IV) ammonium nitrate (CAN). The
reasons for its general acceptance as a one-electron oxidant
may be attributed to its large reduction potential value of
+1.61 V vs. NHE (normal hydrogen electrode), also low
toxicity, ease of handling, straightforward experiment, and
solubility in a number of organic solvents. CAN is proved to
ethoxy)ethyl]-3-methylmidazolium
[HSO4]) [5], 1-butyl-3-methylimidazolium chloroaluminate
bisulfate
([heemim]
*Corresponding author. E-mail: haji@cc.iut.ac.ir

Hajipour et al.
Table 1. Conversion of 4-Nitrobenzylcohol to 4-Nitrobenz-
[Hmim]HSO4
neat, 80 ºC
CHO
OH
CAN
+
aldehyde in Different Organic Solvents and in the
Presence of Different Ionic Liquids
X
X
Scheme 1
Entry
Solvent
Yield (%) Time (h)
1a
2a
3a
4a
5a
6b
7b
Acetonitrile
Ethylacetate
Cyclohexane
n-Hexane
1,2-Dichloroethane
[bmim]Br-
30
45
5
Trace
Trace
50
4
4
4
4
4
4
4
be very useful reagent for synthetic organic chemists over four
decades.
The enormous growth for application of this reagent is
evidenced by the publication of a large number of research
papers and several reviews concerning CAN-mediated
reactions [10]. The ability of cerium to display stable adjacent
oxidation states +3 and +4 makes it so unique among the
lanthanide elements. Unfortunately oxidation with this reagent
suffers from various disadvantages such as using high amount
of reagent (CAN) and long reaction times [11,12]. This
reagent is not effective for oxidation of benzyl alcohol in
refluxing acetonitrile and after 2.5 h, benzyl alcohol isolated
without oxidation [13]. In continuation of our ongoing
program to develop efficient reagents for oxidation of organic
compounds [14-20], in this communication we report a
synergy between ceric ammonium nitrate (CAN) and 3-
methylimidazolium hydrogensulfate ([Hmim]HSO4) as
effective Brönsted acidic ionic liquid as solvent media which
able to oxidize various benzylic alcohols (Scheme 1).
[Hmim]HSO4
85
aThe reaction was carried out in 5 ml of solvents under
reflux conditions. bThe reaction was carried out in 2 mmol
of ionic liquid at 80 °C.
of 4-nitrobenzylcohol (1 mmol) to 4-nitrobenzaldehyde in the
presence of ceric ammonium nitrate (1 mmol) in various
solvents and also in ionic liquids as a solvent. As shown in
(Table 1), in comparison to conventional methods the yields of
the reaction in the presence of Brönsted acidic ionic liquid
([Hmim]HSO4) as the solvent are higher and no side products
were obtained (Table 1).
EXPERIMENTAL
We also studied the oxidation of 4-nitrobenzylcohol to 4-
nitrobenzaldehyde with other metal nitrates and various non-
nitrates oxidants using Brönsted acidic ionic liquid
([Hmim]HSO4) at 80 °C (Table 2). The results show that most
of these metal nitrates are not effective oxidants for this
transformation under mentioned conditions, and ceric
ammonium nitrate is the best oxidant under these conditions.
Our results for the oxidation of a variety of benzylic
alcohols are summarized in (Table 3). The oxidation of
various benzylic alcohols gave the carbonyl compounds in
high yields and short to moderate reaction times. The
competing reaction such as over-oxidation of aldehydes to the
corresponding carboxylic acids, nitration of benzene ring or
benzyl nitrite was not observed in any of the cases under
above conditions. We observed that aliphatic alcohols did not
oxidize with this system, even in long reaction time. Hydroxyl
benzyl alcohol allylic and propargylic alcohols were converted
to unidentified polymeric materials with explosion, therefore
General Procedure for the Oxidation of Benzylic
Alcohol
Alcohol (1 mmol) and ceric ammonium nitrate (1 mmol)
and 3-methylammonium disulfate [Hmim]HSO4(2 mmol) was
added to a round-bottomed flask. The reaction mixture was
placed in an oil bath at 80 ºC and stirred for the specified time.
The reaction was followed by TLC (EtOAc-cyclohexane,
20:80). After completion of the reaction, the product was
extracted with ethylacetate (2 × 5 ml) and washed with water.
The solvent was evaporated under reduced pressure to give the
corresponding pure products. Purification of the residue using
flash column chromatography (silica gel, EtOAc-cyclohexane,
20:80) provided the pure carbonyl compounds.
RESULTS AND DISSCUTION
For optimizing the reaction conditions, we tried conversion
383

Oxidation of Benzylic Alcohols to Their Corresponding Carbonyl Compounds
Table 2. Oxidation of 4-Nitrobenzylcohol Using Various Metal Nitrates and Oxidants in the
Presence of Brönsted Acidic Ionic Liquid ([Hmim]HSO4) at 80 °Ca
Entry
Metal nitrate
4-Nitrobenzaldehyde
4-Nitrobenzoic acid
Time
(h)
(%)
45
25
55
50
61
51
85
Trace
0
(%)
10
20
10
16
20
20
-
Trace
0
20
10
-
1
2
3
4
5
6
7
8
NaNO3
NH4NO3
3.0
2.5
2.0
1.0
1.0
1.0
4.0
2.0
3.0
2.0
2.0
3.0
Zn(NO3)2.6H2O
Ni(NO3)2.6H2O
Cu(NO3)2.3H2O
Cd(NO3)3.9H2O
Ce(NH4)2(NO3)6
Ca(OCl)2
KIO4
KBrO3
K2S2O8
CrO3
9
10
11
12
20
10
5
aReaction conditions: metal nitrates or oxidants (1 mmol), [Hmim]HSO4 (2 mmol) and 4-
nitrobenzylcohol (1mmol) were mixed and stirred at 80 °C.
Table 3. Oxidation of Various Alcohols Using Ceric Ammonium Nitrate in the Presence of Brönsted
Acidic Ionic Liquid ([Hmim]HSO4) at 80 °Ca,b
Entry
1
Substrate
Product
Time (min)
25
Yield (%)
90
CHO
OH
2c
7
91
OCH3
OCH3
CHO
OH
3c
4c
7
94
90
H3CO
CHO
H3CO
H3CO
OH
OH
CHO
CHO
45
H3CO
Me
5c
20
20
91
90
OH
OH
Me
6c
H3CO
H3CO
CHO
H3CO
H3CO
OCH3
OCH3
OCH3
OCH3
7c
20
92
CHO
OH
384

Hajipour et al.
Table 3. Continued
Cl
Cl
8
55
88
CHO
OH
Cl
CHO
Cl
9
50
90
OH
OH
CHO
10
45
90
Cl
Cl
Br
Br
CHO
CHO
11
45
45
88
92
OH
OH
12
Br
Br
O2N
O2N
CHO
CHO
13
4h
86
OH
OH
14
4h
30
85
90
O2N
O2N
OH
O
15
O
OH
16
30
91
MeO
Cl
MeO
O
OH
17
30
35
89
88
Cl
OH
O
18
19
4h
90
OH
O
O
O
20d
21d
22d
3h
3h
3h
-
-
-
OH
CHO
O
OH
CHO
OH
aThe yields refer to the isolated pure products. bThe products were characterized from their spectral data
(IR, 1H NMR) and compared with authentic samples. The reaction was carried with catalytic amount
c
of CAN (30 mol%). dThe yields refer to GC analysis.
385

Oxidation of Benzylic Alcohols to Their Corresponding Carbonyl Compounds
a
Table 4. Competitive Oxidation of Benzylic Alcohols by Using CAN/[Hmim]HSO4
1
CH2OH
CHO
CAN (1 mmol), [Hmim]HSO4- (2 mmol)
neat, 80 ºC, 15 min
90%
0 %
1 mmol
1 mmol
CH2CH2OH
CH2CHO
2
CHO
CH2OH
CAN (.3 mmol), [Hmim]HSO4- (2 mmol)
neat, 80 ºC, 5 min
80 %
1 mmol
OH
O
O
O
5 %
1 mmol
3
4
O2N
CH2OH
O2N
CHO
CAN (1 mmol), [Hmim]HSO4- (2 mmol)
neat, 80 ºC, 15 min
1 mmol
trace
CH2OH
CHO
1 mmol
OH
90 %
O
CAN (1 mmol), [Hmim]HSO4- (2 mmol)
neat, 80 ºC, 30 min
1 mmol
OH
90 %
O
O
O
1 mmol
5 %
aBased on GC and TLC analysis.
we do not recommend using this system for oxidation of
aliphatic, allylic, propargylic and hydroxyl benzyl alcohols
derivatives.
To show the chemoselectivity of this method, the
competitive reactions have been carried out as shown in Table
4. In comparison to benzylic-alcohols, oxidation of aliphatic
ones with this method did occur with lower yields and longer
reaction time. The donor substituted group in the benzene ring
benzylic alcohols accelerates the reaction rate and the
withdrawing groups reduced the reaction rate dramatically.
CONCLUSIONS
In conclusion, we have introduced a straightforward and
efficient method for chemoselective oxidation of alcohols to
their corresponding carbonyl compounds using cerric
ammonium nitrate in the presence of Brönsted acidic ionic
-
liquid ([Hmim]HSO4 ) at 80 °C. The use of non-toxic and
inexpensive materials, stability of the oxidation system, simple
method, short reaction times, good yields of the products and
mild reaction conditions are the advantages of this method.
386

Hajipour et al.
ACKNOWLEDGEMENTS
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We gratefully acknowledge the funding support received
for this project from the Isfahan University of Technology
(IUT), IR Iran (A.R.H.), and Grants GM 033138, MH 065503,
NS 033650 (A.E.R.) from the National Institutes of Health.
Further financial support from Center of Excellency in Sensor
Research (IUT) is gratefully acknowledged.
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