Microwave-assisted oxidation of alcohols using zinc polyoxometalate

Article abstract of DOI:10.1055/s-0034-1379211

In the presence of 30% aqueous hydrogen peroxide and [WZn₃(H₂O)₂][ZnW₉O₃₄)₂]^12- polyoxometalate as a catalyst, primary and secondary alcohols were oxidized to carboxylic acids and ketones, respectively, in short reaction times (ca. 15 min) under microwave-pressurized conditions.

Full text of DOI:10.1055/s-0034-1379211

LETTER
▌2757
letter
Microwave-Assisted Oxidation of Alcohols Using Zinc Polyoxometalate
Microwave-Assisted Oxidation of Alcohols
Wiktor Kasprzyk,* Mateusz Galica, Szczepan Bednarz, Dariusz Bogdał
Department of Biotechnology and Physical Chemistry, Faculty of Chemical Engineering and Technology, Cracow University of Technology,
Warszawska 24 St., 31-155 Krakow, Poland
Fax +48(12)6342425; E-mail: jumper.wk@gmail.com
Received: 16.07.2014; Accepted after revision: 01.09.2014
amines, aniline derivatives, as well as epoxidation of
Abstract: In the presence of 30% aqueous hydrogen peroxide and
12–
alkenes and allyl alcohols conducted in the presence of
[
WZn (H O) ][ZnW O ) ] polyoxometalate as a catalyst, prima-
3 2 2 9 34 2
1
4
POMZn revealed impressive selectivities and yields.
ry and secondary alcohols were oxidized to carboxylic acids and ke-
tones, respectively, in short reaction times (ca. 15 min) under
microwave-pressurized conditions.
Moreover, the chemical structure and high activity of this
catalyst are retained during many cycles of reaction. An-
other advantage of POMZn as a catalyst is related to its
equal activity when prepared in situ or separately since ni-
tric acid used during POMZn synthesis can then be fully
Key words: oxidation, hydrogen peroxide, catalysis, alcohols, mi-
crowave irradiation
1
5
neutralized. Finally, the catalyst is soluble in water
which is crucial for its separation from the reaction mix-
ture having great impact on the economics of the whole
Despite various environmental restrictions, many pro-
cesses of organic synthesis are still conducted using clas-
sical stoichiometric and often waste-producing methods.
1
6
process.
Therefore, in recent few decades the fine-chemicals in- Since reactions under microwave activation are the main
1
7
dustry has been searching for alternative, ‘green’ path- interest of our laboratory, we decided to employ the
1
ways. Thus, oxidation processes with H O have gained POMZn catalyst for the oxidation of alcohols under pres-
2
2
much attention since it is one of the most sustainable oxi- surized microwave conditions. The tremendous influence
dants and water is the side product,1 In order to make of microwave irradiation on the acceleration of many or-
c,2
1
8
such oxidation processes even more sustainable, new re- ganic reactions has been extensively described. In this
action media have been proposed including fluorinated manner, microwave-assisted oxidation reactions have
3
4
5
19
solvents, supercritical CO , and ionic liquids. Many also been exemplified. Our group has also been studying
2
catalytic systems that are able to activate oxidation trans- oxidation and epoxidation reactions and microwave acti-
5
d,20
formations with H O have been described, such as vation of these systems.
In this communication, we
2
2
6
Na WO –H PO –quaternary ammonium salts. Another wish to report the fast and facile microwave-assisted and
2
4
3
4
group of promising catalysts for H O -mediated oxida- accelerated oxidation of both primary alcohols to carbox-
2
2
tions are the so-called polyoxometalates (POM). These ylic acids and/or aldehydes as well as secondary alcohols
are anionic oxygen clusters consisting of tungsten, vana- to ketones.
dium, niobium, tantalum, or molybdenum atoms and tran-
The catalyst for these reactions was prepared according to
the well-known procedure. The chemical formula and
7
sition-metal derivatives thereof. Ishii et al. described the
13
3
–
first catalytic features of [PW O ] in oxidation reac-
1
2
40
structure of the POMZn so prepared was confirmed by
means of FT-IR and UV spectroscopic analysis and by
tions of alkenes, alcohols, diols, alkynes, amines, and sul-
8
fides. Further studies proved that catalytic activity of
21
comparison with data from the literature.
these compounds is related to their peroxide form
3
–9
The oxidation reactions were carried out under monopha-
{
PO [WO(O ) ] } which had been synthesized earlier
4 2 2 4
1
0
sic conditions using 30% aqueous H O in the presence of
by Venturello et al. Therefore, these catalysts are often
called Venturello–Ishii systems. Many other POM sys-
tems have also been characterized as catalyst for oxidation
2 2
POMZn catalyst (Scheme 1). The experimental procedure
involved simply mixing the alcohol, POMZn, 30% aque-
2– 11
ous H O , and acetonitrile in 1:0.004:3:11 molar ratio in a
reactions, such as the Prandis systems ({[WO(O ) ] O} )
2 2
2
2 2
sealed Teflon vessel (total volume of the reactants was ca.
15 mL). Then the reaction mixture was placed in a pres-
surized microwave reactor (MAGNUM II, ERTEC, Po-
or polyfluorooxometalates exhibiting the Wells–Dawson
9
– 12
structure ([Ni(H O)NaH W O F ] ). However, the
2
2
17 55 6
most efficient catalysts derived from polyoxometalates
1
2–
are [WZn (H O) ][ZnW O ) ] exhibiting a sandwich-
like structure (POMZn).
3
2
2
9
34 2
1
3
O
O
MW, 135 °C, 3 bar, 15 min
and/or
R
OH
H O -mediated oxidation reactions of primary and sec-
2
2
H2O2, [WZn3(H2O)2][ZnW9O34)2]12– R1
OH
R
ondary, cyclic and aliphatic alcohols, diols, cyclic tertiary
OH
R1
O
MW, 135 °C, 3 bar, 15 min
SYNLETT 2014, 25, 2757–2760
Advanced online publication: 07.10.2014
12–
R1
R2
R1
H O , [WZn (H O) ][ZnW O ) ]
2
2
3
2
2
9
34 2
0
9
3
6
-
5
2
1
4
1
4
3
7
-
2
0
9
6
DOI: 10.1055/s-0034-1379211; Art ID: st-2014-d0600-l
Georg Thieme Verlag Stuttgart · New York
Scheme 1
©

2758
W. Kasprzyk et al.
LETTER
land) and heated for 15 minutes at 135 °C and pressure (3 tones. Both cyclic and aliphatic secondary alcohols were
bar). Upon completion of the reaction, the organic materi- oxidized to ketones in almost 100% yield with equally
al was extracted with CH Cl . The progress of the reac- high selectivity. Furthermore, diols with two secondary
2
2
tions was monitored by GC-FID (Table 1) and LC–ESI- hydroxyl groups were converted into their corresponding
MS (see Table S1 in the Supporting Information, where diketones in very high yield. In the case of oxidation of a
1
the isolated yields and H NMR spectra are also given).
diol with one primary and one secondary hydroxyl group
the main product was 2-ethyl-3-oxo-1-hexanol. This re-
sult implies chemoselectivity of this protocol to oxidize
secondary alcohols. For the oxidation of 1,2-cyclohexane-
diol cyclic mono- and diketones as well as adipic acid
were observed.
Under these conditions, primary alcohols were oxidized
mainly to their corresponding acids, but minor quantities
of aldehydes were also determined. However, in case of
benzyl alcohol, the ratio of aldehyde to acid was 60:40. In
turn, this method proved highly effective for the conver-
sion of secondary alcohols into their corresponding ke-
Table 1 Results of the Oxidation of Alcohols with 30% Aqueous Hydrogen Peroxide in the Presence of POMZn
Alcohol
Conversion (mol%)
Product
Selectivity (mol%)
60
OH
O
1
00
OH
O
40
OH
O
1
00
100
100
OH
OH
O
O
1
1
00
00
100
9
OH
O
O
8
7
9
1
OH
OH
O
O
8
5
12
8
8
OH
OH
O
1
1
00
00
100
100
OH
O
OH
O
1
00
98
OH
O
OH
O
1
1
00
00
80
20
OH
OH
OH
OH
OH
O
Synlett 2014, 25, 2757–2760
© Georg Thieme Verlag Stuttgart · New York

LETTER
Microwave-Assisted Oxidation of Alcohols
2759
Table 1 Results of the Oxidation of Alcohols with 30% Aqueous Hydrogen Peroxide in the Presence of POMZn (continued)
Alcohol
Conversion (mol%)
Product
Selectivity (mol%)
O
O
3
5
O
O
OH
45
OH
OH
O
1
00
3070
HO
HO
O
O
7
0
We have also conducted the oxidation reactions under aqueous hydrogen peroxide, low costs and ease of the syn-
conventional heating conditions, and comparable conver- thesis of the catalyst, easy separation and possible reuse of
sions were achieved after 1–5 hours, depending on the the catalyst. These all have a positive impact on the safety
substrate. It should be noted that the conversions of reac- and economics of the protocol at the laboratory scale.
tions conducted without POMZn under similar conditions
were less than 5% in the most cases.
Supporting Information for this article is available online
The possibility of recycling the catalyst was also studied. at
The catalyst was separated from the reaction mixture by 10.1055/s-00000083.SunogIpimrfiantoSuIpg
http://www.thieme-connect.com/products/ejournals/journal/
n
ori
m
evaporating the water–acetonitrile mixture after ex-
traction with CH Cl . Then, new portions of the substrate,
2
2
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