Oxidation of benzylic alcohols to carbonyl compounds with potassium permanganate in ionic liquids

Article abstract of DOI:10.1081/SCC-200026242

The selective oxidation of primary and secondary benzylic alcohols with potassium permanganate in ionic liquids to give corresponding aldehydes and ketones in 83-97% yields has been described.

Full text of DOI:10.1081/SCC-200026242

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Oxidation of Benzylic Alcohols
to Carbonyl Compounds with
Potassium Permanganate in
Ionic Liquids
Anil Kumar a , Nidhi Jain a & S. M. S. Chauhan a
a Department of Chemistry , University of Delhi ,
Delhi, 110 007, India
Published online: 10 Jan 2011.
To cite this article: Anil Kumar , Nidhi Jain & S. M. S. Chauhan (2004) Oxidation
of Benzylic Alcohols to Carbonyl Compounds with Potassium Permanganate in
Ionic Liquids, Synthetic Communications: An International Journal for Rapid
Communication of Synthetic Organic Chemistry, 34:15, 2835-2842, DOI: 10.1081/
SCC-200026242
To link to this article: http://dx.doi.org/10.1081/SCC-200026242
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SYNTHETIC COMMUNICATIONSw
Vol. 34, No. 15, pp. 2835–2842, 2004
Oxidation of Benzylic Alcohols to Carbonyl
Compounds with Potassium Permanganate
in Ionic Liquids
*
Anil Kumar, Nidhi Jain, and S. M. S. Chauhan
Department of Chemistry, University of Delhi, Delhi, India
ABSTRACT
The selective oxidation of primary and secondary benzylic alcohols with
potassium permanganate in ionic liquids to give corresponding aldehydes
and ketones in 83–97% yields has been described.
Key Words: Ionic liquid; Potassium permanganate; Benzyl alcohol;
Carbonyl compound.
INTRODUCTION
The selective oxidation of alcohols to carbonyl compounds without over
oxidation to corresponding carboxylic compounds is of importance in
*
Correspondence: S. M. S. Chauhan, Department of Chemistry, University of Delhi,
Delhi 110 007, India; E-mail: smschauhan@chemistry.du.ac.in.
2835
DOI: 10.1081/SCC-200026242
0039-7911 (Print); 1532-2432 (Online)
www.dekker.com
Copyright # 2004 by Marcel Dekker, Inc.

2836
Kumar, Jain, and Chauhan
synthesis of natural products.[1] Different reagents, such as DMSO/DCC or
oxalyl chloride,[2,3] clayfen,[4] 1,1,1-triacetoxy-1,1-dihydro-1,2-benziodoxol-
3H-one (DMP),[5] o-iodoxybenzoic acid (IBX),[6] tetravalent chromium
[7]
oxideTM
,
and hypochlorite/tetrabutylammonium salts,[8] have been used
for oxidative transformation of alkyl and benzyl alcohols to corresponding
aldehydes and ketones. The activity of KMnO4 has been modified in phase
transfer conditions as compared to acidic and basic aqueous conditions
where it is non-selective oxidant.[9] Further, the reactivity of KMnO4 has
also been modified with the use of solid supports[10] such as KMnO4 supported
.
on CuSO4 5H2O. The use of quaternary ammonium permanganates, like
benzyltriethylammonium permanganate and tetra-n-butylammonium perman-
ganate in above transformations, is restricted because of violent explosion
encountered in their use at high temperature.[11]
1,3-Dialkylimidazolium salts are important class of ionic liquids and have
been of considerable interest as environmentally benign, high-tech reaction
media due to their unique chemical and physical properties such as excellent
chemical and thermal stability with ease of reuse, miniscule vapour pressure,
unique miscibilities, and non-flammability.[12–15] They have been employed
as reaction media for various organic and enzymatic transformations,[16–18]
for liquid–liquid separation[19] and catalyst recovery in organometallic cata-
lysis.[20–23] Recently, the oxidation of alcohols to corresponding aldehydes
using IBX in ionic liquid has been reported.[24] In a program towards develop-
ment of chemical models for enzymes and materials from metallo 5,10,15,20-
tetraaryl porphyrins, various types of aromatic aldehydes are needed.[25]
Hence, oxidations of different benzylic alcohols to corresponding carbonyl
compounds have been studied with potassium permanganate in ionic liquids
(Sch. 1).
RESULT AND DISCUSSION
The oxidation of benzyl alcohol with potassium permanganate in 1-butyl-
3-methylimidazolium tetrafluoroborate [bmim][BF4] ionic liquid at room
temperature gave benzaldehyde in 90% yield in 1 hr. Similarly, other benzylic
Scheme 1.

Oxidation of Benzylic Alcohols to Carbonyl Compounds
2837
alcohols were oxidized to the corresponding aldehydes and ketones in excel-
lent yields (Table 1). Primary and secondary aliphatic alcohols, however,
required longer time and higher temperature. Further, their conversion to
corresponding carbonyl compounds was very low and thus benzylic alcohols
could be selectively oxidized to carbonyl compounds in the presence of
primary and secondary aliphatic alcohols with potassium permanganate in
ionic liquids. In a mixture of benzyl alcohol and 2-phenylethanol, selective
oxidation of benzyl alcohol was observed to give benzaldehyde in 88%
yield and 97% of 2-phenylethanol was recovered. Further, the oxidation of
40-(100-hydroxyethyl)-2-phenylethanol gave 4-(20-hydroxyethyl)acetophenone
in 85% yield (Table 1, entry 12), where only benzylic alcohol is oxidized in
presence of both benzylic and aliphatic hydroxyl groups in the same molecule.
The oxidation of benzyl alcohol with potassium permanganate in dichloro-
methane in presence of cetyltrimethylammonium bromide (CTAB) as phase
transfer catalysts gave 58% of benzaldehyde and 23% benzoic acid in 5 hr.
The yield of the product was low and reaction time longer as compared
with those in ionic liquids. Further, over oxidation of benzaldehyde to car-
boxylic acid was also observed in phase transfer conditions. The presence
of electron withdrawing substituents, such as chloro and nitro, on the
benzene ring decreased the reaction rates but not the yields of the products.
Recycling experiments were examined for the oxidation of benzyl alcohol
with potassium permanganate in [bmim][BF4]. Thus, after first run, which
gave benzaldehyde in 90% yields, the ionic liquid was filtered, washed with
dichloromethane, and concentrated. The recovered ionic liquid was subjected
to a second cycle with addition of 1 equiv. of alcohol and 1 equiv. of potassium
permanganate, which gave benzaldehyde in 91% yield. Similarly, in third,
fourth, and fifth cycles, yield of benzaldehyde was 88%, 85%, and 84%,
respectively (Table 2).
In conclusion, a mild, fast, and efficient method for the chemo-selective
oxidation of benzylic alcohols to their corresponding carbonyl compounds
in high yields by an environmentally friendly oxidant potassium permanga-
nate in imidazolium ionic liquids has been developed.
EXPERIMENTAL
The 1H and 13C NMR spectra were recorded on Brucker Heaven
(300 MHz) spectrophotometer using TMS as internal standard and CDCl3 as
solvent and chemical shift were expresssed in ppm. The IR spectra were
recorded using KBr pellets on Perkin Elmer Spectrum 2000 infrared and
nmax were expressed in cm21. GC analysis was performed on Shimazdu
GC-14B using packed column SE-30. The melting points were taken on

2838
Kumar, Jain, and Chauhan
Table 1. Oxidation of benzyl alcohols with potassium permanganate in [bmim][BF4]
at room temperature.a
Time Yieldb
(hr) (%)
Mp/bp
(lit.)[26]
Run
1
Alcohol
Product
1
90c 177/750 mmHgd
(179/750 mmHg)
2
1
97 246–249d (248)
3
4
1.5
88 49 (47)
2
86 107 (106)
5
6
1.5
96 202–203d
(204–205)
1
97 46 (44)
7
8
2
2
87 63–64/13 mmHgd
(62–63/13 mmHg)
85 95–96/26 mmHgd
(97–99/26 mmHg)
(continued)

Oxidation of Benzylic Alcohols to Carbonyl Compounds
2839
Table 1. Continued.
Time Yieldb
(hr) (%)
Mp/bp
Run
9
Alcohol
Product
(lit.)[26]
2
83 80–85/16 mmHgd
(82–83/16 mmHg)
10
1
87 200/760 mmHgd
(202/760 mmHg)
11
12
1.5
1
89 94–96 (95)
85 128/0.4 mmHgd
(128–130/
0.3 mmHg)[27]
aReaction conditions: alcohol (1.0 mmol), KMnO4 (1.0 mmol), and [bmim][BF4]
(2.5 mL).
bYields refer to isolated pure products and the products were characterized by IR and
1H NMR spectral data.
cYield of benzaldehyde is 58% after 5 hr under phase transfer condition and 23%
benzoic acid is also formed.
dBoiling point.
Thomas HOOVER capillary melting point apparatus and all the melting points
and boiling points are uncorrected. The ionic liquid 1-n-butyl-3-methylimida-
zolium tetrafluoroborate f[bmim][BF4]g was prepared by slight modification
1
of literature procedure[28] and characterized by H and 13C NMR data.
Representative Procedure for the Oxidation of Benzyl Alcohol
with KMnO4 in Ionic Liquid
Powdered potassium permanganate (1.0 mmol) was added to benzyl
alcohol (1.0 mmol) dissolved in [bmim][BF4] (2.5 mL) and the reaction
mixture was stirred at room temperature for 1 hr. The product was extracted
with ethyl acetate (3 ꢀ 5 mL) and concentrated under reduced pressure to

2840
Kumar, Jain, and Chauhan
Table 2. Oxidation of benzyl alcohol with KMnO4 in
recycled ionic liquid [bmim][BF4].
Run
Yield (%)a
1
90
2
91
3
88
4
85
5
84
aYields based on GC-analysis.
give benzaldehyde, which was purified by column chromatography over silica
gel and characterized by IR, NMR spectroscopic data and comparison of Bp
with the authentic sample. Similarly, other benzylic alcohols were oxidized
to give corresponding carbonyl compounds and the yields and mp or bp are
given in Table 1.
ACKNOWLEDGMENTS
The financial assistance from Department of Biotechnology (DBT) is
gratefully acknowledged. The authors (AK) and (NJ) are thankful to Univer-
sity Grant Commission (UGC) and Council of Scientific and Industrial
Research (CSIR) respectively for their senior research fellowships (SRF).
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Received in India April 7, 2004