Efficient and convenient oxidation of alcohols to aldehydes and ketones with H2O2/(NH4)6Mo7O 24·4H2O regulated by PEG1000-DIL/ methylcyclohexane temperature-dependent biphasic system

Article abstract of DOI:10.1007/s13738-012-0179-z

A simple and efficient method for the oxidation of primary and secondary alcohols to the corresponding aldehydes and ketones with H₂O ₂/(NH4)₆Mo₇O_24·4H ₂O using the PEG₁₀₀₀-DIL/methylcyclohexane temperature-dependent biphasic system has been developed. The product can be easily isolated by a simple decantation, and the catalytic system can be recycled or reused without loss of catalytic activity.

Full text of DOI:10.1007/s13738-012-0179-z

J IRAN CHEM SOC (2013) 10:453–460
DOI 10.1007/s13738-012-0179-z
ORIGINAL PAPER
Efficient and convenient oxidation of alcohols to aldehydes
and ketones with H2O2/(NH4)6Mo7O24Á4H2O regulated
by PEG1000-DIL/methylcyclohexane temperature-dependent
biphasic system
•
Yu-Lin Hu Ming Lu Xiu-Tao Ge
•
Received: 13 July 2012 / Accepted: 3 October 2012 / Published online: 17 January 2013
Ó Iranian Chemical Society 2013
Abstract A simple and efficient method for the oxidation
of primary and secondary alcohols to the corresponding
aldehydes and ketones with H2O2/(NH4)6Mo7O24Á4H2O
using the PEG1000-DIL/methylcyclohexane temperature-
dependent biphasic system has been developed. The
product can be easily isolated by a simple decantation, and
the catalytic system can be recycled or reused without loss
of catalytic activity.
MnO2, CrO3, SeO2, Br2, RuO4, etc.) [3–5]. However, these
protocols are generally associated with one or more dis-
advantages, such as high cost, low yield, harsh or delicate
reaction conditions, and a large amount of waste byprod-
ucts. Aerobic oxidation of alcohols using transition metal
complexes as catalysts is another important method for this
transformation, and a variety of reagents, such as cobalt
[6, 7], copper [8–10], gold [11], nickel [12], iron [13],
osmium [14, 15], palladium [16–18], ruthenium [19, 20],
molybdenum [21], vanadium [22, 23], and other complexes
[24–26], have beendevelopedwith different degrees ofsuccess.
However, most of the procedures still suffered from the use
of expensive reagents, difficulties in work up, environ-
mental hazards, and difficulties in recycling of the catalyst.
Besides molecular oxygen, hydrogen peroxide (H2O2) is an
another environmentally friendly and atom-efficient oxi-
dant which leads to only water after the reaction [27]. This
reagent has, however, high activation energy, making
catalysis necessary [28], and a number of catalytic oxida-
tion processes based upon the combination of transition
metals or organocatalysts have been developed for such a
conversion [29–32]. However, most of these methods are
invariably suffered from limitations such as lower yields,
poor recovery of expensive metal catalysts and laborious
workup procedures. Consequently, there is a great need to
develop new and environment-benign procedures that
address these drawbacks for such a conversion.
Keywords Alcohols Á Oxidation Á H2O2/
(NH4)6Mo7O24Á4H2O Á PEG1000-DIL Á Thermoregulated
ionic liquid biphasic system
Introduction
Aldehydes and ketones are important classes of chemicals
that have been used extensively as synthetic intermediates
in the preparation of a variety of fine or special chemicals
such as drugs, vitamins, fragrances, etc. [1]. Up to now,
many publications in the open literature have been found in
synthesis of these type of compounds, and a well-known
method for such a synthesis constitutes the oxidation of
alcohols [2]. Traditional methods for performing such a
transformation generally involve the use of stoichiometric
amount of the strongest oxidizing reagents (i.e., KMnO4,
Ionic liquids (ILs) have recently gained recognition as
environmentally benign alternative solvents, because of their
favorable properties such as negligible volatility and non-
flammability under ambient conditions, large liquid range,
high thermal stability, wide electrochemical window, and
strong ability to dissolve many chemicals [33]. Therefore,
ILs have found wide applications in chemical synthesis
[34–36], biocatalytic transformations [37], electrochemistry
Y.-L. Hu (&) Á X.-T. Ge
College of Material Science and Chemical Engineering,
Chuzhou University, Chuzhou 239000,
People’s Republic of China
e-mail: huyulin1982@163.com
M. Lu
College of Chemical Engineering, Nanjing University of Science
and Technology, Nanjing 210094, People’s Republic of China
123

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J IRAN CHEM SOC (2013) 10:453–460
H2O2, (NH4)6Mo7O2. 4H2O
Scheme 1 Oxidation of
alcohols and synthesis of
PEG1000-DIL. Reagents and
conditions: (i) toluene,
triethylamine (2 equiv.),
CH3SO2Cl (2 equiv.), 30 °C,
4 h. (ii) toluene, methyl
imidazole (2 equiv.), 80 °C,
18 h
OH
R2
O
PEG1000-DIL, methylcyclohexane, 50 oC
R1 = aryls, alkyls; R2 = H, aryls, alkyls
R1
R1
R2
i
HO
O
H3CO2SO
O
n
O
OH
O
OSO2CH3
n
2
1
N
O
n
ii
O
N
N
N
2CH3SO3
CH3
H3C
PEG1000-DIL
3
[38], and analytical and separation science [39]. In view of
both the advantages and disadvantages of homogeneous and
heterogeneous catalysts, and to improve catalyst recovery,
multiphase systems, such as phase-transfer catalysis [40, 41],
thermoregulated phase transfer catalysis [42], and liquid–
liquid biphasic catalysis [43], have been studied. Some novel
temperature-dependent ionic liquid biphasic catalytic sys-
tems have been reported recently [44–47], and found that
these temperature-dependent ILs showed some advantages
such as high conversions and selectivity, stability at high
temperatures, easy handling and separation, excellent reus-
ability in the reaction, etc., which provide a novel route for
the homogeneous catalysis and the separation and recycling
of catalysts. In 2009, we developed an efficient synthetic
method for the dichloromethylation of some aromatic
hydrocarbons in [C12minPEG800]Br/methylcyclohexane
temperature-dependent phase-separation system in aqueous
media [48]. Based on the concept of thermoregulated ionic
liquid biphasic system (TRILC) and in continuation of our
interest in exploring green synthetic methods using ionic
liquids, we herein report an efficient and convenient proce-
dure for the oxidation of primary and secondary alcohols to
the aldehydes and ketones using aqueous H2O2 as oxidant
and ammonium heptamolybdate ((NH4)6Mo7O24Á4H2O) as
the catalyst in the PEG1000-based dicationic ionic liquid
(PEG1000-DIL) (Scheme 1).
grade. The ionic liquid was synthesized according to the
literature procedure [47, 48]. High performance liquid
chromatography (HPLC) experiments were performed on a
liquid chromatograph (Dionex Softron GmbH, America),
consisting of a pump (P680) and ultraviolet–visible light
detector (UVD) system (170U). The experiments were
performed on discovery C18 column, ø 4.6 9 250 mm. 1H
NMR spectra were recorded on a Bruker 400-MHz spec-
trometer using CDCl3 as the solvent with tetramethylsilane
(TMS) as an internal standard. The melting points were
determined in open capillaries on a Buchi apparatus and are
uncorrected. Mass spectra were measured on an HP1100LC
(Agilent Technologies, USA). Elemental analysis was per-
formed on a Vario EL III instrument (Elmentar Anlalysensy
Teme GmbH, Germany).
Typical procedure for the preparation of ionic liquid
PEG1000-DIL
(1) Preparation of intermediate 2: a mixture of PEG-1000
(1, 0.1 mol), triethylamine (0.2 mol), and toluene (300 mL)
was stirred in 500 mL round flask at 0–5 °C, then CH3SO2
Cl (0.2 mol) was added dropwise slowly, after that the
mixture was stirred for another 4 h at 30 °C. The reaction
progress was monitored by TLC and HPLC. Upon com-
pletion, the mixture was cooled to 10 °C, the organic phase
(top layer) was appeared and then separated by decantation,
the bottom layer was extracted with toluene (2 9 10 mL).
The combined organic phases were the toluene solvent of
2. (2) Preparation of ionic liquid PEG1000-DIL 3: methyl
imidazole (0.2 mol) was added dropwise into the toluene
solvent of intermediate 2 at room temperature, with vig-
orous stirring, then the mixture was stirred for another 18 h
at 80 °C. The reaction progress was monitored by HPLC
Experimental
Apparatus and reagents
All the used chemicals were from commercial sources
without any pretreatment. All reagents were of analytical
123

J IRAN CHEM SOC (2013) 10:453–460
455
and 1H NMR. After the reaction completed, the mixture was
cooled, the solvent was recovered under reduced pressure
to give PEG1000-DIL as a viscous yellowish-brown liquid,
yield 85 %.
Calcd. for C11H14O: C, 81.38; H, 8.71; O, 9.85. Found: C,
81.44; H, 8.70; O, 9.86.
3,4,5-Trimethoxybenzaldehyde (Table 2, entry 5) White
solid, mp 74–76 °C (lit. 75–78 °C) [49–52]. 1H NMR
(400 MHz, CDCl3) d = 3.87 (s, 3H, CH3O), 7.16 (s, 2H,
Ar–H), 9.89 (s, 1H, CHO) ppm. Anal. Calcd. for C10H12O4:
C, 61.21; H, 6.14; O, 32.61. Found: C, 61.22; H, 6.16; O,
32.62.
Intermediate 2 1H NMR (400 MHz, CDCl3) d = 3.1 (s, 6H,
CH3 9 2), 3.65–3.80 (m, 92H, (OCH2CH2)n), 4.39–4.43
(t, 4H, CH2 9 2) ppm.
PEG1000-DIL 1H NMR (400 MHz, CDCl3) d = 2.76 (s,
6H, CH3 9 2), 3.58–3.73 (m, 92H, (OCH2CH2)n), 4.03 (s,
6H, NCH3 9 2), 4.57 (t, 4H, CH2 9 2), 7.34 (s, 4H,
CH 9 4), 7.69 (s, 2H, CH 9 2) ppm. 13C NMR (400 MHz,
CDCl3) d = 36.3, 39.7, 49.3, 69.1, 70.2, 70.5, 123.4, 137.4,
137.7 ppm. IR (KBr, cm-1): 627, 772, 854, 1108, 1321,
1410, 1457, 1528, 1837, 2876, 3105, 3445. MS (EI, 70 eV)
m/z (%): 448, 470, 492, 514, 558, 580, 602, 624, 646, 668,
690, 712, 153 (100).
3,4-(Methylenedioxy)benzaldehyde (Table 2, entry6) White
solid, mp 36–38 °C (lit. 35–36 °C) [49–52]. 1H NMR
(400 MHz, CDCl3) d = 6.04 (s, CH2, 2H), 6.86–7.53
(m, Ar–H, 3H), 9.86 (s, CHO, 1H) ppm. Anal. Calcd for
C8H6O3: C, 63.99; H, 4.01; O, 31.98. Found: C, 64.00; H,
4.03; O, 31.97.
4-Fluorobenzaldehyde (Table 2, entry 7) Colorless oil, bp
180–182 °C/760 mm (lit. 181–183 °C/760 mm) [49–52].
4-Nitrobenzaldehyde (Table 2, entry 8) Light yellow
General procedure for oxidation reaction
solid, mp 104–106 °C (lit. 106–107 °C) [49–52].
To a stirred solution of alcohol (10 mmol) and (NH4)6Mo7
O24Á4H2O (6 mmol) in ionic liquid PEG1000-DIL (5 mL) and
methylcyclohexane (10 mL) was added aqueous H2O2
(30 %, 10 mL) at room temperature. The reaction mixture
was stirred at 50 °C for the appropriate time (Table 2), the
progress of the reaction was monitored by HPLC. After the
completion of the reaction, the reaction mixture was cooled
to room temperature and the organic phase was separated by
decantation, and then rinsed with water (3 9 10 mL). The
organic phase was dried with sodium sulfate, filtered, and the
solvent was removed, and the residue was passed through a
silica gel pad to afford the pure product. The next run was
performed under identical reaction conditions. All the
products are known compounds and their structures were
characterized by elemental analysis, 1H NMR or compared
with their authentic samples. The physical and spectral data
of the products are given below.
2-Hydroxy-1-phenylethanone (Table 2, entry 9) White
solid, mp 86–87 °C (lit. 87–90 °C) [49–52]. 1H NMR
(400 MHz, CDCl3) d = 3.78 (s, OH, 1H), 4.67 (s, CH2,
2H), 7.52–7.61 (m, Ar–H, 3H), 7.79–7.83 (m, Ar–H, 2H)
ppm. Anal. Calcd for C8H8O2: C, 70.53; H, 5.89; O, 23.47.
Found: C, 70.57; H, 5.92; O, 23.50.
Acetophenone (Table 2, entry 10) White solid, mp
18-19 °C (lit. 19–20 °C) [49–52].
trans-Cinnamaldehyde (Table 2, entry 11) Colorless oil,
bp 247–249 °C/760 mm (lit. 251–252 °C/760 mm) [49–52].
1H NMR (400 MHz, CDCl3) d = 6.65 (d, J = 16.7 Hz,
CH, 1H), 6.69 (dd, J = 6.5, 16.7 Hz, CH, 1H), 7.36–7.51
(m, Ar–H, 5H), 9.65 (d, J = 6.5 Hz, CHO, 1H) ppm. Anal.
Calcd for C9H8O: C, 81.79; H, 6.10; O, 12.11. Found: C,
81.79; H, 6.10; O, 12.11.
3,7-Dimethyl-2,6-octadienal (Table 2, entry 12) Color-
less oil, bp 227–230 °C/760 mm (lit. 229–232 °C/
760 mm) [49–52].
Benzaldehyde (Table 2, entry 1) Colorless oil, bp
178–179 °C/760 mm (lit. 178–180 °C/760 mm) [49–52].
2-Methylbenzaldehyde (Table 2, entry 2) Colorless oil, bp
3-Phenylpropanal (Table 2, entry 13) White solid, mp
200–202 °C/760 mm (lit. 199–200 °C/760 mm) [49–52].
45–47 °C (lit. 47–48 °C) [49–52].
4-Methoxybenzaldehyde (Table 2, entry 3) Colorless oil,
bp 246–247 °C/760 mm (lit. 248–250 °C/760 mm) [49–
52]. 1H NMR (400 MHz, CDCl3) d = 3.89 (s, 3H, CH3O),
7.02 (m, 2H, Ar–H), 7.86 (m, 2H, Ar–H), 9.96 (s, 1H,
CHO) ppm. Anal. Calcd. for C8H8O2: C, 70.53; H, 5.94; O,
23.47. Found: C, 70.57; H, 5.92; O, 23.50.
Hexanal (Table 2, entry 14) Colorless oil, bp 126–129
°C/760 mm (lit. 128–130 °C/760 mm) [49–52].
Cyclohexanone (Table 2, entry 15) Colorless oil, bp
157–160 °C/760 mm (lit. 156–158 °C/760 mm) [49–52].
Results and discussion
4-Tert-butylbenzaldehyde (Table 2, entry 4) Colorless
oil, bp 128–129 °C/25 mm (lit. 130–132 °C/25 mm) [49–
52]. H NMR (400 MHz, CDCl3) d = 1.31 (s, CH3, 9H),
7.53–7.79 (m, Ar–H, 4H), 9.96 (s, CHO, 1H) ppm. Anal.
1
The initial study was carried out using 4-methoxybenzyl
alcohol as the substrate to optimize the reaction conditions,
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J IRAN CHEM SOC (2013) 10:453–460
Table 1 Optimization of the reaction conditions for oxidizing
4-methoxybenzyl alcohol to 4-methoxybenzaldehyde
Entrya
(NH4)6Mo7O24Á4H2O (equiv.)
Time (h)
Yield (%)b
1
2
3
4
5
6
7
a
–
8
4
3
2
2
2
5c
68
77
86
92
95
95
74
0.1
0.3
0.5
0.6
0.7
0.6
Reaction conditions: 4-methoxybenzyl alcohol (10 mmol), H2O2
(30 %, 10 mL), PEG1000-DIL (5 mL), methylcyclohexane (10 mL),
50 °C
Fig. 1 Influences of temperature on the oxidation. Reaction condi-
tions: 4-methoxybenzyl alcohol (10 mmol), H2O2 (30 %, 10 mL),
(NH4)6Mo7O24Á4H2O (6 mmol), PEG1000-DIL (5 mL), methylcyclo-
hexane (10 mL), 2 h
b
Isolated yield
c
Reaction was carried out in the absence of the PEG1000-DIL/
methylcyclohexane biphasic system
layer of methylcyclohexane (oil layer), containing product,
by decantation and concentration of the under layer of
catalytic system (aqueous layer) by removing the water
through a water knockout drum. Fresh substrates and
methylcyclohexane were then recharged to the recovered
catalytic system and the mixture was heated to react once
again.
and the results are summarized in Table 1. The oxidation
with 30 % H2O2 was investigated first in the presence and
absence of the catalyst (NH4)6Mo7O24Á4H2O (Table 1).
4-Methoxybenzyl alcohol was oxidized with H2O2 in the
absence of the catalyst to produce the 4-methoxybenzal-
dehyde; however, the reaction proceeded slowly, the yield
was only 68 % after 8 h (Table 1, entry 1). When the
oxidation was catalyzed by (NH4)6Mo7O24Á4H2O, the yield
increased to 77 % in a shorter time (4 h) when only 0.1
equiv. was used (Table 1, entry 2). The yield increased
with the increase in the amount of (NH4)6Mo7O24Á4H2O
(Table 1, entries 2–5), and it reached maximum when
0.6 equiv. of the catalyst was used (Table 1, entry 5).
However, on further addition of the amount of (NH4)6
Mo7O24Á4H2O, the yield was not enhanced significantly
(Table 1, entry 6). In addition, the PEG1000-DIL/methyl-
cyclohexane temperature-dependent biphasic system is
crucial for this oxidation, and the lack of it leads to a lower
yield and a slower reaction rate (Table 1, entry 7). Fur-
thermore, no over-oxidized product (4-methoxybenzoic
acid) was detected by HPLC analysis of the crude reaction
mixtures in all the cases.
With these results in hand, the catalytic system was then
applied to various alcohols, as summarized in Table 2. It is
clear that various types of benzylic, allylic and aliphatic
alcohols, both primary and secondary, have been success-
fully oxidized to the corresponding aldehydes and ketones in
good to high yields (Table 2). Various functionalities such as
alkyl, alkoxy, alkene double bonds, fluoro, and nitro groups
can tolerate the oxidation. However, the aliphatic alcohols
were less reactive, and longer reaction time and elevated
temperature were needed to reach good yields (Table 2,
entries 13–15). It was also observed that the electron defi-
ciency and nature of the substituents on the aromatic ring
have influence on the oxidation rate, and benzylic alcohols
Figure 1 shows the influences of reaction temperature
on the oxidation. The catalytic activity increased with the
temperature to 50 °C; however, the yield was decreased
with the further increase of the temperature, which is due to
the formation of the over-oxidized product (4-methoxy-
benzoic acid), which was confirmed by HPLC. The yield
reached maximum at 50 °C, these results show that the
moderate temperature, such as 50 °C enhanced the
oxidation.
In addition, the catalytic system could be typically
recovered and reused for subsequent reactions with no
appreciable decrease in yields and reaction rates (Fig. 2).
The recycling process involved the removal of the upper
Fig. 2 Repeating reactions using recovered catalytic system. Reac-
tion conditions: 4-methoxybenzyl alcohol (10 mmol), H2O2 (30 %,
10 mL), methylcyclohexane (10 mL), recovered catalytic system
(PEG1000-DIL and (NH4)6Mo7O24Á4H2O), 50 °C, 2 h
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457
Table 2 Oxidation of alcohols
to aldehydes and ketones
Entrya
1
Substrate
Productb
Time (h)
3
Yield (%)c
94
2
3
2
2
93
95
4
2
94
5
6
7
2
2
4
97
96
94
8
4
92
9
2
2
93
94
10
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J IRAN CHEM SOC (2013) 10:453–460
Table 2 continued
Entrya
11
Substrate
Productb
Time (h)
2
Yield (%)c
96
12
2
97
a
Reaction conditions: alcohol
(10 mmol), H2O2 (30 %,
10 mL), (NH4)6Mo7O24Á4H2O
(6 mmol), PEG1000-DIL
(5 mL), methylcyclohexane
(10 mL), 50 °C
13
14
5d
5d
90
85
b
All products were known and
identified by comparing their
HPLC spectra with those of
commercial materials
15
5d
88
c
Isolated yield
d
The reaction was carried out
at 70 °C
having electron-withdrawing groups on the aromatic
ring (Table 2, entries 7, 8) exerted a little inactivation for
the reaction contrast to those having electron-donating
groups (Table 2, entries 2–6). Obviously, the PEG1000-DIL/
(NH4)6Mo7O24Á4H2O catalytic system was found to be more
effective in oxidation of benzylic and allylic alcohols than
that of aliphatic alcohols, which might be attributed to the
different reaction abilities of a-carbon groups when in the
oxidation of benzylic, allylic and aliphatic alcohols with
H2O2 (oxidant). The reaction rate of oxidation depended on
what was being oxidated, under the same conditions, the
substrate molecule which contained better abilities of
a-carbon groups (e.g., benzylic and allylic alcohols) would
lead to a much easier reaction attack, and a faster reaction
rate and a higher yield were obtained.
The excellent reaction results of the catalytic system
suggest that the oxidation among alcohols, H2O2, (NH4)6
Mo7O24Á4H2O, methylcyclohexane and PEG1000-DIL has a
particular catalytic process, which is schematically depic-
ted in Fig. 3. Before the oxidation, there exists an obvious
oil–water biphasic system, and the under layer (aqueous
phase) consists of PEG1000-DIL, (NH4)6Mo7O24Á4H2O and
H2O2, and PEG1000-DIL is dissolved completely in the
aqueous medium, the upper layer (oil phase) consists of
methylcyclohexane and alcohols (substrates). During
the process of oxidation, the oil–water biphasic system dis-
appears and a homogeneous reaction medium is formed, and
the oxidation proceeds homogeneously. After the comple-
tion of the reaction, a complete phase-separation is formed
again after being cooled to room temperature, the next
Fig. 3 Thermoregulated ionic
liquid biphase catalytic process
for the oxidation
Room temperature
Reaction temperature
Room temperature
Sub.
OH
Pro.
Homogeneous
oxidation
O
H
g
n
e
R1
R2
a
R1
R2
i
l
t
i
n
o
g
o
C
Methylcyclohexane
H2O2
Sub.
Methylcyclohexane
Cat.
PEG1000-DIL
PEG1000-DIL
(NH4)6Mo7O24.4H2O
Cat.
Cat.
(NH4)6Mo7O24.4H2O
30% H2O2
Pro.
Catalyst Recycling
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J IRAN CHEM SOC (2013) 10:453–460
459
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cyclohexane (oil layer), containing product, by decantation
and concentration of the under layer of catalytic system
(aqueous layer) by removing the water through a water
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In conclusion, we have developed an efficient method
for oxidation of primary and secondary alcohols to the
corresponding aldehydes and ketones with H2O2/(NH4)6
Mo7O24Á4H2O using the PEG1000-DIL/methylcyclohexane
temperature-dependent biphasic system. Mild reaction
conditions, easy workup, high yields, stability, easy isola-
tion of the compounds, good thermoregulated biphasic
behavior of the IL, and excellent recyclability of the cata-
lytic system are the attractive features of this methodology.
Further aspects of the catalysis and the application to other
organic synthesis are under investigation.
Acknowledgments We thank the National Basic Research Program
(973) of China and Natural Science Foundation of Jiangsu Province
for support of this research.
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