Oxidation of alcohols to carbonyl compounds, benzylic carbons to their ketones, and arenes to their quinones with molecular oxygen in subcritical water

Article abstract of DOI:10.1007/s00706-006-0555-4

Molecular oxygen is used as an efficient oxidant for the conversion of alcohols into carbonyl compounds, benzylic carbons to their ketones, and arenes to their quinones in subcritical water in the absence of catalysts. The procedure utilizes water and does not require support materials and metal salts. Springer-Verlag 2006.

Full text of DOI:10.1007/s00706-006-0555-4

Monatshefte f €u r Chemie 137, 1597–1600 (2006)
DOI 10.1007/s00706-006-0555-4
Short Communication
Oxidation of Alcohols to Carbonyl
Compounds, Benzylic Carbons
to their Ketones, and Arenes
to their Quinones with Molecular Oxygen
in Subcritical Water
Recep Ozen and Nermin S. KusÃ
Ã
Department of Chemistry, Mersin University, Mersin, Turkey
Received May 25, 2006; accepted June 17, 2006
Published online November 6, 2006 # Springer-Verlag 2006
Summary. Molecular oxygen is used as an efficient oxidant for the conversion of alcohols into car-
bonyl compounds, benzylic carbons to their ketones, and arenes to their quinones in subcritical water
in the absence of catalysts. The procedure utilizes water and does not require support materials and
metal salts.
Keywords. Alcohols; Oxidation; Ketones; Subcritical water; Molecular oxygen.
Introduction
Oxidation of alcohols to carbonyl compounds is one of the most important reac-
tions in organic chemistry [1]. Chromium(VI) reagents are widely used in organic
chemistry for the oxidation of alcohols to carbonyl compounds [2]. The use of
these oxidants as heterogeneous catalyst for various organic reactions is well
known. It has been recently shown that alcohols are oxidized when treated under
heterogeneous conditions, such as they occur with Collins reagent [3], pyridinium
chlorochromate [4], pyridinium bromochromate [5], Cerium(III) bromate [6], or
quinolinium fluorochromate [7]. There are several examples using organic mole-
cules on solid support as oxidizing agents, such as silica gel or montmorillonite
K10 [8]. However, several of the reported reagents suffer from disadvantages,
such as instability, hygroscopicity, low selectivity, long reaction time, difficulty of
à Corresponding authors. E-mail: rozen@mersin.edu.tr, simner@mersin.edu.tr

1
598
R. Ozen and N. S. Kus
preparation, and need for a large excess of the reagent. Thus, a mild, more selec-
tive, and inexpensive reagent is still in demand.
Since oxygen is highly soluble in subcritical water, it is one of the most attrac-
tive oxidants [9]. The increasing environmental concern surrounding the use of
toxic and dangerous oxidants has impelled organic chemists to develop oxidation
methodologies using molecular oxygen. In our previous study, we have reported
the oxidation of toluenes to aromatic aldehydes with molecular oxygen in subcri-
tical water [10]. In continuation of our recent work, oxidation of thiols to disulfides
with molecular oxygen in subcritical water [11], we now wish to report conversion
of alcohols to their corresponding carbonyl compounds without using metal salt
catalysis in subcritical water.
Results and Discussion
In the present work, molecular oxygen is used to oxidize alcohols to their cor-
responding carbonyl compounds in subcritical water. The dramatic decrease in
solvent viscosity and an increase in substrate solubility suggest that subcritical
water may be useful as a replacement for environmentally unacceptable solvents
for a number of organic reactions [12]. The oxidation of alcohols to carbonyl com-
ꢀ
pounds in subcritical water was carried out as isothermal experiment at 120 C
using different amounts of oxygen (Scheme 1). As shown in Table 1, all alcohols
were converted to the corresponding carbonyl compounds, which were isolated
in excellent yield. Because aliphatic and aromatic alcohols, benzylic corbons,
and arenes were oxidized to their corresponding carbonyl drivatives in the ab-
sence of an organic solvent and metal salts=complexes, our method is indeed very
green chemistry.
The amount of dissolved oxygen in water at atmospheric pressure was de-
ꢀ
termined according to Henry’s Law [13] (at 25 C the solubulity of O is of
2
À3
1
.296Â10 mol=kg H O). This value was used initially as shown in Table 1 as
2
entry A. The amount of oxygen was regulated by the oxygen pressure. All oxida-
3
tions were performed by adding 200 cm of water, one equivalent of substrate, and
two equivalents of oxygen. Since longer reaction times did not improve the yields,
the time was different for all oxidations. On the other hand, when oxygen pressure
was increased, the yields increased not beyond 20 bar. On the contrary, an increase
in the amount of oxygen led to decomposition of the starting materials converting
them into tars. All the products were characterized by their spectral data and
comparison with reported data.
In conclusion, we developed a practical procedure for the oxidation of alcohols
to their corresponding carbonyl compounds, in subcritical water with molecular
Scheme 1

Oxidation with Molecular Oxygen in Subcritical Water
1599
Table 1. Oxidation of organic substrates with molecular oxygen in subcritical water (total pressure
ꢀ
5
0 bar, temperature 120 C)
a
Entry
Substrate
Product
Time=h [Ref.]
Yield=%
PO =bar
b
2
c
A
5
10 15 20
1
2
3
4
5
6
7
8
9
n-Butanol
Isoamylalcohol
1-Octanol
Butanal
3-Methylbutanal
Octanal
2 [14]
3 [14]
2 [14]
1 [14]
2 [14]
3 [14]
3 [14]
4 [14]
2 [14]
3 [14]
2 [14]
1 [14]
1 [14]
1 [14]
75 77 80 90 90
73 77 80 85 85
70 80 92 95 95
85 85 93 95 95
90 92 95 95 95
88 90 95 95 95
75 78 85 90 90
80 85 90 95 95
82 85 85 90 90
75 80 80 85 85
72 75 80 85 85
70 73 75 75 80
80 85 88 90 90
72 75 85 85 85
Benzyl Alcohol
2-Pentanol
Benzaldehyde
2-Pentanone
Benzhydrol
Cyclohexanol
Cyclopentanol
Cholesterol
1-Menthol
Benzophenone
Cyclohexanone
Cyclopentanon
3-Oxocholesterol
1-Menthone
1
1
1
1
1
0
1
2
3
4
Indane
1-Indanone
Tetralin
Tetralone
Anthracene
Phenanthrene
9,10-Anthraquinone
Phenanthrene-9,10-quinone
a
Yields refer either to 2,4-DNP derivatives identified by melting points or to isolated products
b
for which melting points were taken directly; double equivalents of molecular oxygen, 5 bar:
À3
À3
À3
À3
À2
À3
À3
4
1
.63Â10 mol dm , 10 bar: 9.26Â10 mol dm , 15 bar: 1.38Â10 mol dm , 20 bar: 1.85Â
À2
À3
c
À3
0
mol dm
;
A: dissolved oxygen in water at atmospheric pressure: 1.26Â10 mol dm
oxygen in the absence of metal salt. This method is useful because it is simple,
economic, and environmental.
Experimental
+
Mps were determined on an Electrothermal 9100 apparatus. IR spectra were recorded on a Win
+
1
First Satellite model spectrophotometer. H NMR spectra were obtained using a 400 MHz Bruker
+
DPX instrument.
General Procedure
ꢀ
3
Oxidations were carried out at 120 C in a 215 cm stainless steel pressure reactor equipped with N
2
and O inlet, pressure gauge, safety valve, digital temperature reader, heater, and magnetic stirrer. The
2
total pressure was kept at 50 bar by N . A glass vessel was inserted into the reactor to avoid corrosion.
2
3
The reactor was charged with substrates and 200 cm H O. All the valves of the reactor were tightly
2
closed during preheating. N was supplied through a tube into the liquid phase directly. Then the
2
desired oxygen pressure was applied to the vessel through a stainless steel tube into the liquid phase
3
directly. After completion of the reaction it was extracted with ether (3Â20cm ). The combined
organic layer was dried over MgSO and evaporated on a rotary evaporator under reduced pressure.
4
Then the product was chromatographed over silica gel using ethyl acetate:n-hexane (1:4) as the eluent
to separate the product. After evaporation of the solvent, the fairly pure solid were crystallized. The
liquid carbonyl compounds were derivatized with 2,4-dinitrophenylhydrazine. The melting points of
solid compounds such as benzil, benzophenone, 3-oxocholestorol, 1-menthone, 1-indanone, tetralone,
1
0-anthraquinone, and phenanthrene-9,10-quinone were checked and all compounds were identified by
1
spectral data like IR and H NMR.

1
600
R. Ozen and N. S. Kus: Oxidation with Molecular Oxygen in Subcritical Water
Acknowledgement
We are greatful to Mersin University Research Council and TUBITAK (The Scientific and Technical
Council of Turkey) for supporting this work (Grant No: TBAG-2235).
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Austria. – Redaktion: Getreidemarkt 9=163-OC, 1060 Wien, Austria. – Satz und Umbruch: Thomson Press Ltd., Chennai, India. –
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