Oxidation of benzylic alcohols to their corresponding carbonyl compounds using KIO4 in ionic liquid by microwave irradiation

Article abstract of DOI:10.1080/00397910600781539

A chemoselective, straightforward, and rapid method for oxidation of alcohols to the corresponding carbonyl compounds using microwave irradiation and a catalytic amount of ionic liquid is reported. The reaction has been carried out with excellent yields and short reaction times. Copyright Taylor & Francis Group, LLC.

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Oxidation of Benzylic Alcohols to Their
Corresponding Carbonyl Compounds using
KIO in Ionic Liquid by Microwave Irradiation
4
a
a
b
Abdol Reza Hajipour , Fatemeh Rafiee & Arnold E. Ruoho
a
Pharmaceutical Research Laboratory, Department of Chemistry , Isfahan
University of Technology , Isfahan, Iran
b
Department of Pharmacology , University of Wisconsin Medical School ,
Madison, Wisconsin, USA
Published online: 27 Oct 2006.
To cite this article: Abdol Reza Hajipour , Fatemeh Rafiee & Arnold E. Ruoho (2006) Oxidation of Benzylic
Alcohols to Their Corresponding Carbonyl Compounds using KIO in Ionic Liquid by Microwave Irradiation,
4
Synthetic Communications: An International Journal for Rapid Communication of Synthetic Organic Chemistry,
36:17, 2563-2568, DOI: 10.1080/00397910600781539
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Synthetic Communications , 36: 2563–2568, 2006
Copyright # Taylor & Francis Group, LLC
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910600781539
Oxidation of Benzylic Alcohols to Their
Corresponding Carbonyl Compounds using
KIO in Ionic Liquid by Microwave
4
Irradiation
Abdol Reza Hajipour and Fatemeh Rafiee
Pharmaceutical Research Laboratory, Department of Chemistry,
Isfahan University of Technology, Isfahan, Iran
Arnold E. Ruoho
Department of Pharmacology, University of Wisconsin Medical School,
Madison, Wisconsin, USA
Abstract: A chemoselective, straightforward, and rapid method for oxidation of
alcohols to the corresponding carbonyl compounds using microwave irradiation and
a catalytic amount of ionic liquid is reported. The reaction has been carried out with
excellent yields and short reaction times.
Keywords: Oxidation, aldehydes, ketones, ionic liquid
INTRODUCTION
The use of ionic liquids (ILs) as solvent for chemical reactions offers several
advantages from an environment perspective. These solvents are nonflam-
mable and thermally stable, exhibit negligible vapor presser (nonvolatile),
[1–4]
and also offer the potential for reclability.
Microwave-assisted organic transformation coupled with ILs have
attracted attention because of enhanced reaction rates, easier workup, and
Received in Poland February 2, 2006
Address correspondence to Abdol Reza Hajipour, Pharmaceutical Research Labora-
tory, Department of Chemistry, Isfahan University of Technology, Isfahan 84156,
IR Iran. E-mail: haji@cc.iut.ac.ir
2
563

2
564
A. R. Hajipour, F. Rafiee, and A. E. Ruoho
facilitated purification. The dipole characteristics of IL translate into rapid
[5]
excitation by microwaves and consequently faster reactions.
Oxidation of alcohols to carbonyl compounds is a fundamental type of
[6]
reaction encountered at all levels and aspects of organic synthesis. Most
oxidants suffer at least from one of the following disadvantages: long reaction
time, hygroscopicity, high acidity, highly toxicity, instability, dangerous
procedure, low yields of products, photosensitivity, high cost of preparations,
[7–9]
overoxidation to carboxylic acids, environmental harm, and mess.
RESULTS AND DISCUSSION
In continuation of our ongoing program to develop environmentally benign
[
6–9]
methods,
microwave-accelerated oxidation of alcohols to their corresponding
we report an extremely clean, fast, green, and facial
carbonyl compounds using potassium periodate (KIO ) in the presence of
4
catalytic amount of tetraethylammonium bromide as ionic liquid. Potassium
periodate (KIO ) is a mild and inexpensive oxidizing agent, which cannot
4
easily be used in ordinary organic solvent. However, using this reagent in
the presence of tetraethylammonium bromide (Et NBr) as ionic liquid and
4
with microwave irradiation accelerates the reaction rate of oxidation of
alcohols (Scheme 1).
To evaluate the synergy of KIO /tetraetylammonium bromide (Et NBr)
4
4
and microwave irradiation for oxidation of various alcohols, several exper-
iment have been done. Oxidation of alcohols with KIO /Et NBr without
4
4
microwave irradiation was unsuccessful. Also, we performed the oxidation
of benzyl alcohol, 2-chlorobenzyl alcohol, and 4-methoxybenzylalcohol by
using KIO without Et NBr with microwave irradiation and observed that in
4
4
all cases the yield was les than 15%. When we treated the benzyl alcohol,
-chlorobenzyl alcohol, and 4-methoxybenzylalcohol with KIO4 in the
2
presence of Et NBr without microwave irradiation at 808C for 3 h, the
4
yields were not more than 25%, even after a long reaction time. The results
for the oxidation of a variety of benzylic alcohols are summarized in
Scheme 1.

Oxidation of Benzylic Alcohols
2565
a
4 4
Table 1. Oxidation of alcohols by KIO /Et NBr by microwave irradiation
Time
(min)
Yield
(%)
Mp or bp
(8C)/(torr)
Entry
Substrate
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
a
b
c
d
e
f
PhCH
2-MeOC
3-MeOC H CH OH
2
OH
2
3
2
3
4
4
4
3
1
3
4
5
3
6
6
5
5
8
8
98
90
90
85
70
75
97
85
98
90
90
80
97
57
52
40
65
20
15
177–179
34–35
6 4 2
H CH OH
100–102/10
247–249
42–43
6
4
2
4-MeOC H CH OH
6
4
2
3,4-(MeO)
2,5-(MeO)
2
C
C
6
H
3
CH
CH
2
2
OH
OH
2
6
H
3
48–50
g
h
i
2-ClC H CH OH
210–212
212–214
45–47
6
6
6
4
4
4
2
2
2
3-ClC
4-ClC
H
H
CH
CH
OH
OH
j
4-MeC H CH OH
2
106–108/15
47–49
6
4
k
l
PhCHOHPh
4-ClC CHOHPh
4-ClC H CHOHCH
6
H
4
75–77
m
n
o
p
q
230–232
105–107
56–59
6
4
3
4-NO
3-NO
2
C
C
6
H
H
4
CH
CH
2
2
OH
OH
2
6
4
PhCHOHCOPh
PhCH CH OH
93–95
47–50
2
2
b
r
Cyclohexanol
CH (CH OH
197
159–162
b
s
3
2
)
3
a
Yields based on the isolated pure products after chromatography and confirmed by
comparison with authentic samples (TLC, IR, and NMR).
b
4 4 3
Oxidation by using KIO /Et NBr (0.25 mmol)/AlCl (5 mol%).
Table 1. The aliphatic alcohols did not oxidize with this system at all;
however, these alcohols only oxidized with KIO /Et NBr /AlCl (5 mol%,
4
4
3
0
.006 g) with long reaction times and moderate yields.
To show the chemoselectivity of this method, the competitive reaction
have been carried out as shown in Scheme 2. In comparison to benzylic
alcohols, oxidation of aliphatic ones with this method occurs with lower
yields and longer reaction time. The donor substituted group in the benzene
ring of benzylic alcohols accelerates the reaction rate and the withdrawing
groups reduce the reaction rate dramatically.
In this method, IL is recyclable. After 4 recycling the IL four times, we did
not observe any significant loss of this IL capability, and benzyl alcohols were
oxidized with the same yields as first run. We also did not observe any over-
oxidation of producing aldehydes to the corresponding carboxylic acids.
In conclusion, we have developed a simple and efficient method for
oxidation of benzylic alcohols to their corresponding carbonyl compounds
using potassium periodate (KIO ) in the presence of a catalytic amount of
4
tetraethylammonium bromide as ionic liquid by microwave irradiation with
high yields and short reaction times.

2
566
A. R. Hajipour, F. Rafiee, and A. E. Ruoho
Scheme 2.
EXPERIMENTAL
General
Yields refer to isolated pure products. The products were characterized by
1
comparison their spectral data ( H NMR, IR) and melting and boiling
points with authentic samples (authentic samples prepared by reported
1
methods). All H NMR spectra were recorded at 250 MHz in CDCl3
relative to TMS (0.00 ppm). All of the reactions were carried out in a hood
with strong ventilation. IR spectra were recorded on Shimadzu 435 IR
spectrophotometer. Spectra of solids were carried out using KBr pellets.
Oxidation of 2-Methoxybenzylalcohol 1b: A Typical Procedure
A mixture of 2-methoxybenzyl alcohol (1 mmol, 0.138 g), KIO (1 mmol,
4
.0230 g), and Et NBr (0.25 mmol, 0.05 g) was ground thoroughly in
4
a mortar. The mortar was covered with a watch glass and put inside a
Samsung microwave (2450 MHz, 900 W). The completion of the reaction
was monitored by TLC (EtOAc–cyclohexane 25 : 75). After completion of
the reaction, the mortar was removed from the oven, and the mixture was
cooled to room temperature. The resulting aldehydes (with no traces of
carboxylic acids) and ketenes were isolated by straightforward extraction
with diethyl ether and filtered. The filtrate was evaporated under reduced
pressure, and the resulting residue was purified by flash chromatography on
SiO (eluent EtOAc–cyclohexane, 20 : 80). The yield was 0.124 g (90%) of
2

Oxidation of Benzylic Alcohols
2567
1b, mp 34–358C. The ionic liquid was further washed with ether and reused
several times.
ACKNOWLEDGMENTS
We gratefully acknowledge the funding support received for this project from
the Isfahan University of Technology (IUT), I.R. Iran (A. R. H.), and Grants
GM 033138, MH 065503, and NS 033650 (A. E. R.) from the National Insti-
tutes of Health. USA. Further financial support from Center of Excellency in
Chemistry Research (IUT) is gratefully acknowledged.
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