Microwave-assisted oxidation of alcohols using Magtrieve

Article abstract of DOI:10.1016/S0040-4020(02)01533-8

Primary and secondary alcohols can be selectively oxidized under microwave irradiation into the corresponding aldehydes and ketones within 5-30min using commercially available and magnetically retrievable Magtrieve.

Full text of DOI:10.1016/S0040-4020(02)01533-8

Tetrahedron 59 (2003) 649–653
Microwave-assisted oxidation of alcohols using Magtrievee
*
D. Bogdal, M. Lukasiewicz, J. Pielichowski, A. Miciak and Sz. Bednarz
Department of Chemistry, Politechnika Krakowska, ul. Warszawska 24, 31-155 Krakow, Poland
Received 19 September 2002; revised 4 November 2002; accepted 28 November 2002
Abstract—Primary and secondary alcohols can be selectively oxidized under microwave irradiation into the corresponding aldehydes and
ketones within 5–30 min using commercially available and magnetically retrievable Magtrievee. q 2003 Elsevier Science Ltd. All rights
reserved.
1. Introduction
electromagnetic energy into heat according to the dielectric
heating mechanism. For example, the irradiation of
Magtrievee (2.5 g) with a continuous power of microwave
reactor (30%) in an open vessel (5 cm diameter) led to quick
heating of the material up to 3608C within 2 min. As it was
expected, the temperature recorded by means of a thermo-
vision camera had the highest value in the center of the
reaction vessel (Fig. 1). All the attempts to measure the
temperature of Magtrievee with a thermocouple after
switching off the microwave power showed much lower
temperature of ca. 80–1008C which leads to an important
conclusion that the only valid temperature measurement
must be done during microwave experiments while the
material is irradiated with a continuous power.
The oxidation of alcohols to carbonyl compounds is a
fundamental transformation in organic chemistry since
carbonyl compounds are widely used as intermediates
both in manufacturing and research.1 Although the oxi-
dation of organic compounds under non-aqueous conditions
has become an effective technique for modern organic
synthesis,2 the methods still suffer some disadvantages
including the cost of preparation, long reaction time, and
tedious work-up procedures. In recent years, oxidation
processes have received much attention, especially in the
search for selective and environmentally friendly oxidants.3,4
Microwave synthesis is a new technique for conducting
chemical reactions. Acceleration of organic reactions by
microwaves has been largely proven elsewhere, and in
many cases, microwave techniques have become more
effective than conventionally conducted reactions.5 More-
over, in a number of applications, reactions under
microwave conditions can provide pure products in high
yield.6
When toluene (15 mL), which is a weak microwave
absorber, was introduced into the reaction vessel, the
temperature of Magtrivee reached ca. 1408C within 2 min
and was more uniformly distributed (Fig. 2).
2. Results and discussion
Magtrievee is DuPont’s trademark for the oxidant based on
tetravalent chromium dioxide (CrO2).7 In our research on
oxidation processes, we chose Magtrievee as an oxidant,
because it has been proven to be a useful oxidant in some
reactions8 including the oxidation of alcohols.9 Magtrievee
as an oxidant is a very well suited reagent for microwave
synthesis, because as an ionic and magnetically retrievable
material, it carries a benefit of efficient conversion of
Keywords: Magtrievee; alcohols; oxidation; microwave irradiation.
*
Figure 1. The temperature profile for Magtrievee irradiated by
microwaves (2 min of irradiation).
Corresponding author. Tel.: þ(48)(12)628-25-72; fax: þ(48)(12)628-20-
38; e-mail: pcbogdal@cyf-kr.edu.pl
0040–4020/03/$ - see front matter q 2003 Elsevier Science Ltd. All rights reserved.
PII: S0040-4020(02)01533-8

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D. Bogdal et al. / Tetrahedron 59 (2003) 649–653
can be oxidized to ketones under the same conditions
(Scheme 1).
The experimental procedure involves a simple mixing of
an alcohol with toluene in a round-bottomed flask followed
by the addition of Magtrievee in 5:1 weight ratio for both
primary and secondary alcohols.
The excess of the oxidant is required since only its surface is
reduced. Then the reaction mixtures were placed inside a
multimode microwave reactor (Plazmatronika, Poland) and
irradiated under a reflux condenser, which allowed carrying
out the process in a refluxing solvent. After the reaction,
which was monitored by GC/MS analysis, the mixtures
were cooled to room temperature. Then Magtrievee was
retrieved with a magnet, and the solution was concentrated
to give a crude product. The results are summarized in
Table 1.
Figure 2. The temperature profile for Magtrievee in a toluene solution
irradiated by microwaves (2 min of irradiation).
It is worth stressing that even though the temperature of the
solid material was higher than the boiling point of toluene,
we did not observe boiling in the reaction vessel. It leads to
another important conclusion that during reactions in
heterogeneous systems (solid support/organic solvent) in
which microwaves have been proved to be the most
effective10 the temperature of solid material can be higher
than the bulk temperature of solvent and measurements of
temperature with a pyrometer, thermocouple, or fiber optics
thermometer does not give correct values. In such a case, the
higher temperature of the solid support leads to higher
conversion of reactants or/and higher reaction rates, which
in turn might be a reasonable explanation for the so-called
non-thermal microwave effect, i.e. an increase in reaction
rates that cannot be explained by an increase in the
temperature of reaction medium.11 On the other hand, the
application of microwave irradiation is the only simple way
to maintain the temperature of the solid support higher than
the bulk temperature of the reaction mixture, which implies
that such a process might be more energy efficient than other
conventional processes.
The carbonyl compounds were obtained with satisfactory
yield and in a short time (Table 1). The primary alcohols
were turned into the desired aldehydes without over
oxidizing them to carboxylic acids. The reactivity of the
secondary alcohols seems to be slightly lower, but the
addition of higher amount of the oxidant did not increase
the yield of desired ketones. In the case of 1-octen-3-ol, we
have shown that the microwave protocol, like the conven-
tional one, retained the alcohol/olefin chemoselectivity. For
example, the oxidation of 1-octen-3-ol resulted in 1-octen-
3-one; even small amounts of epoxy compounds were not
detected.
Some attempts have been made to carry out the oxidation of
alcohols without a solvent or in the presence of a small
amount of a solvent (toluene) under microwave irradiation.
As was stated above, the reactions resulted in non-uniform
heating of the reaction mixture and overheating of
Magtrievee (up 3608C). An open flame within a reaction
flask was noticed in some cases! Thus, the application of a
solvent was necessary to maintain the reaction temperature
and carry out safely the oxidation of alcohols in the presence
of Magtrievee during the microwave experiment.
All the reactions were carried out under heterogeneous
conditions using Magtrievee as an oxidant and toluene as
an organic phase.
3. Conclusion
Our earlier study on microwave modification12 of Noyori’s
procedure of alcohol oxidation3 showed that in the case of
primary alcohols the oxidation by hydrogen peroxide is so
effective that it cannot be stopped at the aldehyde stage; the
equivalent carboxylic acids are the only product.
In conclusion, we have developed a mild, fast, and efficient
method for the selective oxidation of aliphatic and benzylic
alcohols to their corresponding carbonyl compounds by
employing the solid and magnetically retrievable oxidant,
Magtrievee, under microwave conditions. Moreover, in
comparison to the original work by Lee and Donald,9 the
amount (excess) of the oxidant was substantially reduced
under microwave conditions from 10 to 5-fold excess while
the reaction yield remained at the same level.
In order to solve this drawback, we attempted to apply
Magtrievee to the oxidation of primary alcohols to
aldehydes. Furthermore, we show that secondary alcohols
The advantages of this protocol include a simple reaction
set-up, application of commercially available reagents as
well as oxidant. As was shown in the experiments with the
thermovision camera, high product yields and short reaction
times are the results of higher temperature of Magtrivee
during microwave experiments in comparison with conven-
tional processes.
Scheme 1. Oxidation of alcohols using Magtrievee.

D. Bogdal et al. / Tetrahedron 59 (2003) 649–653
651
Table 1. Microwave-assisted oxidation of primary and secondary alcohols using Magtrievee under microwave irradiation (Plazmatronika reactor)
Alcohol
Carbonyl compound
Microwave
Conventional
Time (min)
Yielda (%)
Time (min)
Yielda (%)
Primary alcohols
5
67
77
85
90
5
10
15
20
26
41
58
67
81
10
15
20
150
5
45
60
73
84
99
5
10
15
20
25
120
18
34
40
51
–
10
15
20
25
75
5
10
15
41
52
60
5
10
15
120
27
36
45
63
5
67
70
73
75b
5
10
15
20
23
29
37
45
57
10
15
20
150
5
96
120
78
Secondary alcohols
10
20
30
49
57
73
10
20
30
180
33
53
69
91
30
20
85
65
120
150
70
69
20
10
98
95
110
75
80
90
The power of the microwave reactor was set to 90% during first 2 min of irradiation and 70% to the end of experiment.
a
Isolated yield.
The amount of Magtrievee was doubled.
b
4. Experimental
4.1. General method
introducing a fibre-optical sensor (ReFlex, Nortech) which
was used to control temperature during microwave
experiments.
The reactions were carried out in a multimode microwave
reactor with a continuous power regulation (PLAZMA-
TRONIKA, Poland), which is equipped with magnetic
stirrer and two inlets on the top and one side of the reactor.
The inlets allowed applying an upright condenser and
The temperatures of Magtrivee during microwave experi-
ments were recorded by means of the thermovision camera
VIGO SYSTEM (V-20).
IR spectra were recorded on FT-IR BIORAD FTS-165

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D. Bogdal et al. / Tetrahedron 59 (2003) 649–653
spectrophotometer as liquids on NaCl disks. 1H NMR
spectra were collected on Tesla 487C (80 MHz) spectro-
meter using TMS as an internal standard. GC/MS spectra
were determined on GC/MS 5890 SERIES II HEWLETT–
PACKARD gas chromatograph equipped with Ultra 2
(25 m£0.25 mm£0.25 mm) column with HEWLETT–
PACKARD 5971 Series Mass Selective Detector.
4.2.8. 2-Octanone. Yield¼0.72 g (73%) liquid; bp 175–
1778C (lit.13 170–1728C). Spectroscopic data consistent
with that found in the literature.
4.2.9. Cyclohexanone. Yield¼0.83 g (85%) liquid; bp
152–1548C (lit.13 154–1568C). Spectroscopic data consist-
ent with that found in the literature.
4.2. Starting materials
4.2.10. Acetophenone. Yield¼0.64 g (65%) solid; mp
18–208C (lit.13 19–208C). Spectroscopic data consistent
with that found in the literature.
All the chemicals were purchased from Aldrich and used as
received.
4.2.11. 1-Octene-3-one. Yield¼0.96 g (98%) liquid; bp
45–488C/10 Torr (lit.13 56–608C/16 Torr). Spectroscopic
data consistent with that found in the literature.
All the reactions were carried out according to the oxidation
procedure given for 1-octanol, which was representative of
the general procedure employed for primary alcohols, and
the oxidation procedure given for 2-octanol, which was
representative of the general procedure employed for
secondary alcohols.
Acknowledgements
This work was undertaken as part of the EU sponsored D10
COST Program (Innovative Methods and Techniques for
Chemical Transformations).
4.2.1. 1-Octanal. A solution of 1-octanol (1.0 g, 7.7 mmol)
in 15 mL of toluene was prepared in a 100 mL round-
bottom reaction flask. Then the 5.0 g of Magtrievee was
added, and the mixture was stirred and irradiated to reflux
under an upright condenser in the microwave reactor
(PLAZMATRONIKA) for the time indicated in Table 1.
At the end of the exposure to microwaves, the mixture was
cooled to room temperature and Magtrievee was com-
pletely retrieved with a magnet. Then toluene was evapo-
rated to afford crude octanal, which was purified by
Kugelrohr distillation. Yield 0.97 g (99%).
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4.2.2. 2-Octanal. A solution of 2-octanol (1.0 g, 7.7 mmol)
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