Tungstate ions (WO4 =) supported on imidazolium framework as novel and recyclable catalyst for rapid and selective oxidation of benzyl alcohols in the presence of hydrogen peroxide

Article abstract of DOI:10.1002/aoc.3597

Tungstate salt with imidazolium framework is found to be a recoverable and heterogeneous system favouring the highly selective oxidation of primary benzylic alcohols to corresponding aldehydes with 30% H₂O₂ as a green oxidant under neutral aqueous reaction conditions. Furthermore, in order to demonstrate the recyclability of the catalyst, it was recovered and efficiently reused in seven succeeding reaction cycles without any significant loss. The use of green solvent, very short reaction time with excellent yields and recyclability of the catalyst make this protocol highly advantageous.

Full text of DOI:10.1002/aoc.3597

Received: 2 July 2016
DOI 10.1002/aoc.3597
Revised: 31 July 2016
Accepted: 3 August 2016
F U L L PA P E R
=
Tungstate ions (WO4 ) supported on imidazolium framework as
novel and recyclable catalyst for rapid and selective oxidation of
benzyl alcohols in the presence of hydrogen peroxide
*
*
Fereshteh Hosseini Eshbala | Farajollah Mohanazadeh | Alireza Sedrpoushan
Institute of Industrial Chemistry, Iranian Research
Tungstate salt with imidazolium framework is found to be a recoverable and
heterogeneous system favouring the highly selective oxidation of primary benzylic
alcohols to corresponding aldehydes with 30% H2O2 as a green oxidant under
neutral aqueous reaction conditions. Furthermore, in order to demonstrate the
recyclability of the catalyst, it was recovered and efficiently reused in seven
succeeding reaction cycles without any significant loss. The use of green solvent,
very short reaction time with excellent yields and recyclability of the catalyst make
this protocol highly advantageous.
Organization for Science and Technology, Tehran,
Iran
Correspondence
Farajollah Mohanazadeh or Alireza Sedrpoushan,
Institute of Industrial Chemistry, Iranian Research
Organization for Science and Technology, Tehran,
Iran.
Email: fmohanazadeh@yahoo.com;
alirezasedrpoushan@gmail.com
KEYWORDS
alcohol oxidation, heterogeneous catalyst, hydrogen peroxide, tungstate
1
|
INTRODUCTION
etc., were well explored for activation of H2O2. Also, to
achieve high efficiency and selectivity in the oxidation of
Selective oxidation of alcohols to carbonyl compounds is a
fundamental synthetic transformation from both synthetic
and industrial points of view. This is because the correspond-
ing carbonyl compounds are versatile intermediates of
valuable compounds, such as pharmaceuticals, agricultural
chemicals and fine chemicals.[1,2]
alcohols, catalytic systems based on tungsten have received
a great deal of attention.[17]
Catalyst systems based on tungsten that mediate the
oxidation of alcohols in the presence of H2O2 can be traced
to the pioneering work by Jacobsen et al.[18] Later on, several
research groups reported important progress in this area.[19]
Among the seminal reports, that of Noyori et al. on the first
use of homogeneous Na2WO4 has indeed highlighted inter-
esting priority of the contemporary, practical, efficient, mild
and environmentally acceptable oxidation through using
H2O2.[20] Recently use of tungsten peroxide, which is pre-
pared from H2O2 and catalytic amount of sodium tungstate
dihydrate (Na2WO4⋅2H2O), has been reported as a practical
methodology to catalyse the oxidation.[21] Attention has been
particularly directed to a method of green oxidation using
tungsten peroxide.[22,23] Paradoxically, this system operates
under strong acidic conditions, requires an expensive/com-
mercially unavailable phase transfer catalyst, where the
recycling of homogeneous Na2WO4 is basically hampered.
Heterogeneous catalysis, compared to homogeneous
catalysis, has greater promise for use in large‐scale applica-
tions because of the simple separation and recycling of the
catalyst, as well as the minimization of metal contamination
This transformation has traditionally been carried out
using stoichiometric amounts of high‐valent transition metal
salts, notably chromium(VI)‐ and manganese(VII)‐based
reagents,[3] or organic‐based oxidants, such as Swern[4]
(oxalyl chloride), Oppenauer,[5] Dess–Martin[6] reagents,
etc. In view of stringent environmental concerns, green chem-
istry and specially atom efficiency, a substantial amount of
research has recently been dedicated to developing new and
efficient catalytic protocols based on molecular oxygen[7] or
hydrogen peroxide (H2O2)[8] as terminal oxidant. In this
manner, transition metal‐catalysed oxidation of alcohols
using H2O2 has been extensively investigated owing to the
fact that H2O2 is a cheap and environmentally benign
oxidant, which produces water as the only by‐product.[9] In
those investigations, some homogeneous and heterogeneous
catalytic systems including molybdenum,[10] manganese,[11]
iron,[12] palladium,[13] rhenium,[14] ruthenium,[15] copper,[16]
Appl. Organometal. Chem. 2016; 1–6
wileyonlinelibrary.com/journal/aoc
Copyright © 2016 John Wiley & Sons, Ltd.
1

2
HOSSEINI ESHBALA ET AL.
in the final products. The development of heterogeneous
catalytic systems using H2O2, O2 or air as an oxidant, which
are cheap and safe, producing water as the sole by‐product,
would contribute to the establishment of green and sustain-
able chemical processes.[24–27] Therefore, it is highly desir-
able to develop heterogeneous catalysts for these oxidation
reactions.
Although these approaches provide substantial improve-
ments to some extent, catalyst efficiency and selectivity,
especially in the case of alcohol oxidations, still remain
challenging. Based on our knowledge, there is currently no
report of employing imidazolium–tungstate salt (WO4=) as
catalyst in the oxidation reaction of alcohols with 30% H2O2.
(s, 3H, CH2 octyl), 4.31 (s, 3H, CH2 octyl), 7.13 (s, 6H,
CH imidazolium), 8.08 (d, J = 16.4 Hz, 3H, CH
imidazolium). 13C NMR (500 MHz, DMSO, ppm): 14.1
(CH3 octyl), 22.5 (7‐octylC), 27.2 (6‐octylC), 29.9
(4,5‐octylC), 30.0 (3‐octylC), 31.2 (2‐octylC), 47.6
(1‐octylC), 122.8 and 123.5 (4,5‐imiC), 137.5 (2‐imiC),
167 (N─C═N). Anal. Calcd for C36H60Cl3N9 (%): C,
59.61; H, 8.33; N, 17.38. Found (%): C, 55.76; H, 7.87; N,
18.03.
2.4 | Preparation of (3,3′,3″‐(1,3,5‐Triazine‐2,4,6‐triyl)
=
tris(1‐octyl‐1H‐imidazol‐3‐ium))2(WO4
)
3
((1,3,5‐Triazine‐2,4,6‐triyl)tris(1‐octyl‐1H‐imidazol‐3‐ium))2
(WO4=)3 was synthesized using a simple ion exchange tech-
nique according to a previous procedure.[29] Typically,
3,3′,3″‐(1,3,5‐triazine‐2,4,6‐triyl)tris(1‐octyl‐1H‐imidazol‐3‐
ium)⋅3Cl− (0.04 g) was added to 2 ml of deionized water and
stirred for at least 15 min. Na2WO4⋅2H2O (0.043 g,
0.05 mmol) in 3 ml of deionized water was added to the sus-
pension and stirred at room temperature for 4 h. The tungstate
solution was filtered, washed with deionized water and dried
under vacuum. The total tungstate loading of the materials
was estimated using inductively coupled plasma atomic
emission spectroscopy to be 11%.
2
| EXPERIMENTAL
2.1 | Materials
Imidazole, octyl bromide and Na2WO4⋅2H2O were purchased
from Merck. Imidazole was first recrystallized from distilled
CH2Cl2 and dried in a desiccator under vacuum over dry
P2O5 for three days at room temperature.
2.2 | Preparation of 1‐Octyl‐1H‐imidazole
Octyl‐1H‐imidazole (known compound) was prepared by the
reaction of imidazole with octyl bromide. A mixture of imid-
azole (6.81 g, 0.1 mol) and octyl bromide (10.05 g, 0.05 mol)
in toluene (11 ml) was heated under reflux for 4 h. The white
precipitate was filtered and the crude 1‐octylimidazole liber-
ated by addition of NaOH (3.8 g) and water (5 ml). The oily
product was extracted into CH2Cl2 (3 × 7 ml) and the extract
was dried over anhydrous Na2SO4. CH2Cl2 was removed,
and the resulting oil distilled under vacuum to afford
1‐octylimidazole. Yield 6.42 g (70%); b.p. 130–132°C.[28]
IR (KBr, cm−1): 3106, 2926, 2856, 1563, 1510, 1464,
2.5 | General procedure for oxidation of alcohols to
corresponding aldehydes with 30% H2O2
Alcohol (0.5 mmol) and 30% H2O2 (0.034 g, 1 mmol) were
added to water (2 ml). Then, catalyst (0.4 g, 0.21 mmol)
was added to the solution and the resulting mixture was
stirred at room temperature for a requisite time. The reaction
was monitored by TLC and no over‐oxidation was detected.
On completion of the reaction, the catalyst was successfully
filtered and dried under vacuum. The recovered catalyst was
used in the recycling procedure in the same manner as
reported in the first run. All reaction products were known
and characterized by melting point or boiling point,
comparing with those obtained from authentic samples.
A search of the literature revealed that cyanuric chloride
(1) is a well‐known and very important accessible compound,
which has been extensively used in 1,3,5‐s‐triazine synthesis.
Various kinds of nucleophiles such as imidazole, pyrazole,
piperidine and morpholine can react with cyanuric
chloride.[30] Albright briefly mentioned cyanuric chloride,
p‐toluenesulfonyl chloride, p‐toluenesulfonic anhydride and
methanesulfonic anhydride with dimethylsulfoxide as mild
oxidative reagents for oxidizing alcohols to carbonyl
derivatives in hexamethylphosphoramide at −20°C.[31]
The combination of the triazine‐tris‐octylimidazolium
1
1377, 1079, 743, 665. H NMR (500 MHz, CDCl3, ppm):
0.60 (s, 3H, CH3), 0.99 (s, 10H, 5 × CH2), 1.45 (s, 2H,
CH2), 3.59 (s, 2H, CH2), 6.77 (d, J = 26 Hz, 2H, CH
imidazolium), 7.18 (s, 1H, CH imidazolium).
2.3 | Synthesis of 3,3′,3″‐(1,3,5‐Triazine‐2,4,6‐triyl)tris
(1‐octyl‐1H‐imidazol‐3‐ium) Chloride
1‐Octyl‐1H‐imidazole (0.9 g, 5 mmol) in acetonitrile (9 ml)
was added dropwise to a solution of cyanuric chloride in
acetonitrile (0.184 g, 1 mmol in 5 ml at 0°C), followed by
formation of a precipitate. The suspension was stirred for
30 min at room temperature. Then, the process was continued
for another 30 min at 60°C, and refluxed for 4.5 h at 110°C.
The resulting precipitate was filtered, washed with (hot)
acetonitrile and dried in vacuo. Yield 0.36 g (50%); m.p.
227–230°C. IR (KBr, cm−1): 3121, 2925, 2855, 1587,
=
cation with the WO4 anion should therefore lead to
1
1440, 1329, 1094, 750. H NMR (500 MHz, DMSO, ppm):
0.80 (s, 9H, CH3CH2), 1.20 (s, 3 × 12 H, CH2 octyl), 1.87
(3,3′,3″‐(1,3,5‐triazine‐2,4,6‐triyl)tris(1‐octyl‐1H‐imidazol‐3
‐ium))2(WO4=)3.

HOSSEINI ESHBALA ET AL.
3
We envisaged that the inherent phase transfer feature of
imidazolium within the (3,3′,3″‐(1,3,5‐triazine‐2,4,6‐triyl)
tris(1‐octyl‐1H‐imidazol‐3‐ium))2(WO4=)3 framework might
not only enhance the catalytic performance of WO4=, but also
combine the advantages of using 30% H2O2 as a green oxi-
dant under neutral reaction conditions with the possibility
of recycling of the catalyst.
In this article, we aim to demonstrate the feasibility of
oxidizing benzylic alcohols to the corresponding aldehydes
with a new, mild and practical oxidation method that uses
only commercially available reagents. The tungstate salt with
imidazolium framework, which is built from octylimidazol
and cyanuric chloride (Scheme 1), can effectively act as an
efficient and recoverable catalyst in the oxidation of alcohols
to the corresponding aldehydes without over‐oxidation to
acid with 30% H2O2 as green oxidant in aqueous medium.
TABLE 1 Effect of solvent on oxidation of alcoholsa
Entry
Solvent
Time (min)
Yield (%)b
1
No solvent
Dichloromethane
Ethyl acetate
Acetone
80
40
62
45
73
25
63
67
73
69
60
40
50
60
70
72
99
2
3
90
4
88
5
n‐Hexane
100
25
6
CH3CN
7
C2H5OH
127
113
70
8
CHCl3
9
CH3COOH
Isopropanol
DMF
10
11
12
13
14
15
110
90
CH3OH
80
CH2Cl2:CH3OH (1:1)
Acetonitrile:H2O (1:1)
H2O
60
3
| RESULTS AND DISCUSSION
28
5
In continuation of our ongoing research on the development
of more efficient and environmentally friendly procedures
for some important transformations in organic synthesis,[32]
aReaction conditions: 0.21 mmol catalyst, 0.5 mmol benzyl alcohol, 1 mmol 30%
H2O2, 2 ml solvent at room temperature.
bIsolated yield of aldehydes.
herein
3,3′,3″‐(1,3,5‐triazine‐2,4,6‐triyl)tris(1‐octyl‐1H‐
imidazol‐3‐ium)⋅3Cl− was synthesized by alkylation of
imidazole and condensation of octylimidazol and cyanuric
chloride. Nucleophilic reaction on cyanuric chloride was
carried out to give 1,3,5‐triazine derivative with an excellent
yield. For preparation of the catalyst, the resulting 3,3′,3″‐
(1,3,5‐triazine‐2,4,6‐triyl)tris(1‐octyl‐1H‐imidazol‐3‐ium)⋅
3Cl− was then functionalized, through replacing chloride
ions with tungstate ion, using a simple ion exchange
technique.
0.21 mmol, respectively, in water at room temperature
(Table 1, entry 15).
A low reaction temperature could facilitate access to
higher reaction selectivities. Under this condition, benzylic
alcohols were selectively transformed into corresponding
benzaldehydes without any detectable over‐oxidation to
benzoic acid. Having the optimal reaction conditions in hand,
=
the scope of the WO4 salt, in the oxidation of various pri-
mary alcohols, was investigated (Scheme 2). In this context,
it was found that substituted benzyl alcohols, bearing either
electron‐releasing or electron‐withdrawing groups, were also
oxidized selectively and affordably to the corresponding alde-
hydes in excellent yields, under the optimized reaction condi-
tions (Table 2, entries 2–20). In addition, 4‐nitrobenzyl
alcohol, 3‐nitrobenzyl alcohol and 2‐nitrobenzyl alcohol
were also converted to corresponding aldehydes in excellent
yields without any over‐oxidation to corresponding carbox-
ylic acids, under the same reaction conditions (Table 2,
entries 4, 13 and 15).
The catalytic activity of the catalyst was studied in the
oxidation of various alcohols using 30% H2O2 as a green
oxidant. The oxidation of benzyl alcohol as a model was first
investigated for various solvents, in order to determine the
optimized conditions (Table 1). The oxidation of benzyl
alcohol (0.5 mmol) in the presence of 0.21 mmol of catalyst
and 1 mmol of 30% H2O2 at room temperature under
solvent‐free conditions gave benzaldehyde in 62% yield
(Table 1, entry 1). Moreover, it was found that CH2Cl2,
methanol and their mixture are not suitable media for this
oxidation (Table 1, entries 2, 12 and 13). After screening of
various solvents, the best conditions were found to be those
in which 30% H2O2 and catalyst contents were 1 and
The recyclability of heterogeneous catalysts is of para-
mount importance and a critical advantage over homoge-
neous counterparts. As a consequence, the recyclability of
=
tris‐imidazolium WO4 was also studied for the oxidation
of benzyl alcohol. In this regard, after each run, the catalyst
was isolated and then dried under vacuum. The recycled
SCHEME
1
Synthesis of (3,3′,3″‐(1,3,5‐triazine‐2,4,6‐triyl)tris(1‐octyl‐
1H‐imidazol‐3‐ium) chloride
SCHEME 2 Catalytic oxidation of alcohols

4
HOSSEINI ESHBALA ET AL.
TABLE 2 Oxidation of substituted benzylic alcohols to aldehydes using
imidazolium WO4 salt as catalysta
=
B.p./m.p. (°C)
Yield
(%)
Time
(min)
Entry
R
Found
Lit.
1
H
99
93
95
94
95
93
97
87
86
90
94
96
87
92
92
95
85
96
86
88
5
7
177–178
246–248
47–48
178[33]
248[34]
48[33]
106[33]
205[35]
55–58[36]
56–58[37]
61–62[38]
180[33]
211–213[39]
130[35]
74[40]
57–59[41]
169[33]
43[42]
212–213[43]
197[44]
43[45]
69–70[46]
42–43[47]
2
4‐OCH3
4‐Cl
3
7
4
4‐NO2
4‐CH3
4‐Br
10
11
13
8
104–105
204–206
56–58
5
6
7
4‐Ph
57–59
8
4‐CO2CH3
4‐F
3
59–61
9
13
16
11
5
178–180
213–215
128–130
73–75
10
11
12
13
14
15
16
17
18
19
20
3‐Cl
4‐(tert‐Butyl)
4‐N(CH3)2
3‐NO2
3‐OPh
2‐NO2
2‐Cl
14
14
12
14
15
10
15
10
58–60
168–169
43–45
210–215
196–198
40–42
2‐OH
3,4‐Cl
2,4‐Cl
3,4‐OCH3
71–73
42–44
aReaction conditions: 0.21 mmol catalyst, 0.5 mmol substrate, 1 mmol 30% H2O2,
2 ml solvent at room temperature.
SCHEME 3 The interaction of ((1,3,5‐triazine‐2,4,6‐triyl)tris(1‐octyl‐1H‐
imidazol‐3‐ium))2 (WO4=)3 (4) with H2O2
catalyst was then successfully used in at least seven further
reaction cycles with only slight decreases in the catalytic
performances (Figure 1).
and H2O2 gives (3,3′,3″‐(1,3,5‐triazine‐2,4,6‐triyl)tris(1‐
octyl‐1H‐imidazol‐3‐ium))2[W(O2)4]3 (5). Next, coordina-
tion of alcohol occurs, the alkoxide ligand in 6 is
dehydrogenated by hydroperoxo ligand and finally aldehyde
is produced (Scheme 4). We believe that the coordination
step is a key step for this oxidation process.
Oxidation of alcohol using ((1,3,5‐triazine‐2,4,6‐
=
triyl)tris(1‐octyl‐1H‐imidazol‐3‐ium))2(WO4
) was carried
3
out in water. To date, the oxidation of alcohols using this
catalyst has not been reported. We propose a reaction mech-
A comparison was made of the oxidation of benzyl
alcohol with H2O2 in the presence of various catalysts. The
data are summarized in Table 3.
anism as shown in Scheme 3. Reaction of ((1,3,5‐triazine‐
=
2,4,6‐triyl)tris(1‐octyl‐1H‐imidazol‐3‐ium))2(WO4
)
(4)
3
FIGURE 1 Recyclability of the catalyst in selective oxidation of benzyl
alcohol to benzaldehyde with 30% H2O2
SCHEME 4 Proposed mechanism for oxidation of alcohol to corresponding
aldehyde catalysed by tris‐imidazolium tungstate

HOSSEINI ESHBALA ET AL.
5
TABLE 3 Oxidation of benzyl alcohol with H2O2 in the presence of various catalysts
Entry
Catalyst
H2O2 (eq.)
Solvent
Temp. (°C)
Time (min)
Yield (%)
Ref.
[20]
[17]
[21]
[19]
[17]
[8]
1
2
3
4
5
6
7
8
9
Q+HSO4−/Na2WO4
[WO(O2)2(QO)]−
Na2WO4⋅2H2O
MNP@PILW
1.5
4
No solvent
CH3CN
90
78
90
90
90
r.t.
80
r.t.
r.t.
300
900
60
87
89
90
95
75
98
91
82
99
1.2
4
DMA
No solvent
CH3CN:H2O
No solvent
Toluene
CH3CN
90
WO4=@PMO‐IL
5
720
360
45
[PEO‐didodecylimidazolium]3[PW12O40 2
]
2
[8]
RuCl3⋅3H2O/DDAB
2.7
5
[48]
[FeCl3(P‐DPEphos)(P,P‐DPEphos)]
((triazine‐2,4,6‐triyl)tris(1‐octyl‐1H‐imidazol‐3‐ium))2(WO4
360
5
=
)
2
H2O
This work
3
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4
|
CONCLUSIONS
We have shown that tris‐octylimidazolium tungstate salt
prepared through a simple ion exchange technique is a highly
recoverable and effective catalyst for selective oxidation of
alcohols to their corresponding carbonyl compounds under
base‐ or acid‐free conditions without the need to any external
phase transfer additives. The studies show that primary aro-
matic alcohols are selectively converted to the corresponding
aldehyde without any over‐oxidation to the corresponding
carboxylic acids under the reaction conditions. The catalyst
is successfully recovered from the reaction mixture and can
then be reused for at least seven subsequent reaction cycles
with only slight decreases in its activity and selectivity. It is
believed that the imidazolium moieties not only provide a
means of immobilizing tungstate species through simple ion
exchange techniques and preventing their leaching into the
reaction medium, but also they can operate as a phase transfer
catalyst. The mild reaction conditions, simple experimental
procedure, rapid conversion, good yields and reusability of
the catalyst are notable advantages of the method. Further
research on the practical applications of heterogeneous
catalysts is underway in our laboratory.
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ACKNOWLEDGMENTS
We acknowledge the Iranian Research Organization for
Science and Technology (IROST) for supporting this work.
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SUPPORTING INFORMATION
Additional supporting information can be found in the online
version of this article at the publisher’s website.
How to cite this article: Hosseini Eshbala, F.,
Mohanazadeh, F., and Sedrpoushan, A. (2016), Tung-
state ions (WO4=) supported on imidazolium frame-
work as novel and recyclable catalyst for rapid and
selective oxidation of benzyl alcohols in the presence
of hydrogen peroxide, Appl. Organometal. Chem.,
doi: 10.1002/aoc.3597
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