Facile, mild and selective silica sulfuric acid catalyzed oxidation of benzylalcohols to benzaldehyde derivatives by potassium peroxodisulfate

Article abstract of DOI:10.4314/bcse.v27i1.14

An efficient, facile, and mild oxidation of a variety of primary benzylic alcohols to the corresponding aldehydes with potassium peroxodisulfate in the presence of a catalytic amount of sodium chloride and silica sulfuric acid (SSA) in acetonitrile as solvent is reported. It is a renowned fact that potassium peroxodisulfate acts as a powerful oxidizing agent and the control of conditions is difficult. For this purpose, SSA as a mild, efficient and reusable solid acid catalyst was used to afford the carbonyl compounds in excellent yields and short time. The structure of all of the resulting products was confirmed by FT-IR spectroscopy.

Full text of DOI:10.4314/bcse.v27i1.14

Bull. Chem. Soc. Ethiop. 2013, 27(1), 131-136.
Printed in Ethiopia
DOI: http://dx.doi.org/10.4314/bcse.v27i1.14
ISSN 1011-3924
 2013 Chemical Society of Ethiopia
SHORT COMMUNICATION
FACILE, MILD AND SELECTIVE SILICA SULFURIC ACID CATALYZED
OXIDATION OF BENZYLALCOHOLS TO BENZALDEHYDE DERIVATIVES BY
POTASSIUM PEROXODISULFATE
1
1
1
Rashid Badri , Alireza Alizadeh-haddad , Maryam Adlu and Mohammad Kazem Mohammadi
2*
1
Islamic Azad University, Khuzestan Science and Research Branch, Ahvaz, Iran
Departman of Chemistry, Ahvaz Branch, Islamic Azad University, Ahvaz, Iran
2
(
Received November 8, 2011; revised December 4, 2012)
ABSTRACT. An efficient, facile, and mild oxidation of a variety of primary benzylic alcohols to the
corresponding aldehydes with potassium peroxodisulfate in the presence of a catalytic amount of sodium chloride
and silica sulfuric acid (SSA) in acetonitrile as solvent is reported. It is a renowned fact that potassium
peroxodisulfate acts as a powerful oxidizing agent and the control of conditions is difficult. For this purpose, SSA
as a mild, efficient and reusable solid acid catalyst was used to afford the carbonyl compounds in excellent yields
and short time. The structure of all of the resulting products was confirmed by FT-IR spectroscopy.
KEY WORDS: Potassium peroxodisulfate, Silica sulfuric acid, Benzylic alcohol, Oxidizing agent
INTRODUCTION
The oxidation of benzylic alcohols to carbonyl compounds is a fundamental transformation in
synthetic organic chemistry. Many methods using a variety of reagents have been reported over
the years, most notably the Oppenhauer and Swern oxidations and reactions utilizing high valent
metal compounds such as manganese dioxide, ammonium perruthenates and pyridinium
chlorochromate. However, the majority of known methods generally suffer from one or more
disadvantages such as difficulty in manipulation, long reaction times, and utilization of toxic
reagents [1-3]. Therefore, it is still attractive to develop a new efficient oxidant with properties
of high stability, low toxicity, ready availability and high selectivity for the benzylic oxidations
and overcome typical problems that occur during oxidation and accept wide rang of alcohols
with improved selectivity. In addition, the oxidation of alkylarene compounds and benzylic
alcohols to their corresponding carbonyl compounds is of significant importance in synthetic
organic chemistry for basic research and industrial manufacturing, because of the complexities
in natural product structures. The international annual production of carbonyl compounds is
7
over 1×10 tons and the largest part of these compounds are produced from the oxidation of
alkylarenes and alcohols [4-8].
Peroxodisulfate ion is an excellent and versatile oxidant; used mostly for the oxidation of
synthetic and natural organic and inorganic compounds in aqueous solution and it has also been
employed as an initiator for the polymerization of vinyl monomers [9-17]. In spite of the great
2
-
2 8
convenience of using S O , many oxidations by peroxodisulfate do not proceed at a convenient
2-
-
rate. This can be largely attributed to the rate-limiting homolysis (S
activation energy of approximately 30 kcal/mol.
2
O
8
4
→2 SO ), which has the
Thus certain limitations may be observed with this reagent. The decomposition of the
peroxodisulfate ion requires strong mineral acids and heavy metal ions as catalysts [18]. For this
purpose, silica sulfuric acid (SSA), as a mild, inexpensive, efficient, reusable, environmentally
_
*
_________
Corresponding author. E-mail: mkmohamadi@yahoo.com

132
Mohammad Kazem Mohammadi et al.
safe and solid heterogeneous acid catalyst or promoter, could be used. SSA have frequently
been used for many organic transformations because of its low cost, ease of preparation, catalyst
recycling, ease of handling and easy separation from reactants and products [19-24]. SSA can
enhance the reactivity and selectivity of many types of reaction, such as oxidation [25], carbon-
carbon bond formation for example aldol and Knovenagel condensation [26, 27], Friedel-Crafts
alkylation [28], Beckmann rearrangement [29], cycloaddition [30], aromatization [31]
protection [32] and deprotection [33]. It is easily prepared by treatment of silica gel with
chlorosulfonic acid at room temperature [34]. It must be noted that the preparation method is
simple, clean, and without work-up procedure, because hydrogen chloride gas is evolved from
the reaction vessel immediately. In this paper we wish to report a simple, mild, convenient and
selective procedure for the oxidation of benzylic alcohols to their corresponding aldehyde
compounds with peroxodisulfate in the presence of sodium chloride (NaCl) and SSA. This
method is shown to be of considerable utility for the oxidation of benzylic alcohols at benzylic
position.
EXPERIMENTAL
General. Chemicals were purchased from Merck Co. (Darmstadt, Germany) and were used
without further purification. Solvents were dried over BaO and then were distilled under
reduced pressure. FT-IR spectra were recorded on Perkin Elmer Spectrum Rx
spectrophotometer. The spectra of solids were obtained using KBr pellets. The vibrational
1
-
1
transition frequencies are reported in wavenumbers (cm ). Band intensities are assigned as
weak (w), medium (m), strong (s) and broad (br). Purity determination of the products and
reaction monitoring were accomplished by TLC on silica gel polygram SILG/UV254 plates. All
yields refer to isolated products.
Preparation of SSA. SSA was prepared by the reaction of chlorosulfonic acid with silica gel
35].
[
General procedure for peroxodisulfate oxidation of benzylic alcohols. To a solution of alcohol
1 mmol) in CH CN (8 mL) in a 50 mL round-bottomed flask-equipped with a condenser and a
(
3
magnetic stirrer was added potassium peroxodisulfate (1 mmol) (0.27 g), NaCl (1 mmol = 0.06
g NaCl) and SSA (0.07 g) in three portions. The reaction mixture was stirred magnetically under
reflux condition for the appropriate time indicated in Table 1. The progress of the reaction was
monitored by TLC. The reaction mixture was filtered and the solid was washed with ether (5-15
mL). The filtrates were evaporated and the residue was purified on a silica-gel plate with
appropriate eluent. Pure carbonyl compounds were obtained in 72-96% yields (Table 1).
Characterization of products. The structure of all products was confirmed by FT-IR
-
spectroscopy. All of these carbonyl compounds exhibiting absorption at 1660-1750 cm range
1
are commonly attributed to the stretching vibration of carbonyl groups. They also exhibiting
-
1
absorption at 2850-2750 cm range are commonly attributed to the aldehyde C-H. The
absorption band with near medium intensity with absorption band of carbonyl group is a good
testimony for existence of aldehyde. FT-IR data for all of the resulting aldehydes are listed in
Table 1.
Bull. Chem. Soc. Ethiop. 2013, 27(1)

Short Communication
133
Table 1. FT-IR spectral data for some of synthesized compounds.
Product
PhCHO
Spectral data
FT-IR (KBr, cm ): 3080 (m, br), 2850 (m), 2735 (m), 1702 (s), 1580 (m),
455 (s), 1288 (s), 1200 (s), 1017 (w), 826 (s), 743 (s), 685 (s), 614 (m).
-
1
1
-
1
FT-IR (KBr, cm ): 2935 (m, br), 2865 (m), 2724 (m), 1705 (s), 1604 (s),
506 (s), 1455 (w), 1268 (s), 1204 (m), 1155 (s), 1124 (m), 846 (s).
p-CH
3
OC
6
H
4
CHO
CHO
1
-1
FT-IR (KBr, cm ): 3084 (m, br), 2912 (m, br), 2825 (m), 2716 (w), 1736 (s),
603 (s), 1487 (m, br), 1276 (s), 1171 (m), 1037 (m), 780 (m), 727 (m).
m-CH
3
OC
6 4
H
1
-1
FT-IR (KBr, cm ): 2965 (m, br), 2865 (m), 2732 (w) , 2672 (m), 2552 (m),
2285 (w), 1660 (s), 1610 (s), 1570 (m), 1423 (s), 1289 (s), 1188 (s), 1015
3 6 4
p-Me CC H CHO
(
m), 942 (m, br), 856 (m), 779 (m), 708 (m).
-1
FT-IR (KBr, cm ): 3106 (m, br), 2872 (m, br), 2755 (m), 1701 (s), 1604 (m),
564 (w), 1527 (s), 1455 (w),1392 (w), 1343 (s), 1196 (s), 864 (m), 783 (m).
o-NO
2
C
6
H
4
CHO
1
-1
FT-IR (KBr, cm ): 3068 (m, br), 2878 (m), 2720 (m), 1706 (s), 1616 (s),
535 (w), 1352 (w), 1203 (s), 1077 (m), 933 (m), 811 (s), 729 (s).
m-NO
2
C
6
H
4
CHO
1
-1
FT-IR (KBr, cm ): 3107 (m, br), 2849 (m), 2778 (m) ,1710 (s), 1606 (s),
540 (s), 1325 (m), 1198 (s), 1104 (m), 1008 (m), 850 (s), 816 (s), 739 (s).
p-NO
o-BrC
p-BrC
2 6
C H
4
CHO
1
-1
FT-IR (KBr, cm ): 3063 (m, br), 2859 (m, br), 2752 (m), 1702 (s), 1591 (s),
449 (m), 1396 (m), 1265 (s), 1168 (w), 1033 (m), 963 (w), 822 (s), 766 (m).
6
H
4
4
CHO
CHO
1
-1
FT-IR (KBr, cm ): 3015 (m, br, 2859 (m, br), 2763 (w), 1690 (s), 1588 (s),
479 (m), 1384 (m), 1205 (s), 1065 (s), 1248 (m), 835 (m), 810 (m).
6
H
1
-
1
FT-IR (KBr, cm ): 3071 (m, br), 2862 (s),2744 (m), 1754 (m), 1675 (s), 1592
(s), 1460 (m), 1268 (w), 1200 (s), 1052 (s), 956 (w), 1077 (w), 824 (s), 765
o-ClC
6
H
4
CHO
(
s), 715 (s).
FT-IR (KBr, cm ): 2922 (m, br), 2857 (m) ,2715 (w) ,1699 (s), 1567 (m),
467 (w), 1432 (w), 1378 (s), 1295 (w),1058 (w), 997 (m), 945 (m), 781 (s).
-
1
m-IC
6
H
4
CHO
CHO
1
-1
FT-IR (KBr, cm ): 2972 (m, br), 2871 (m, br), 2748 (w), 1700 (s), 1604 (s),
p-C
6
H
5
C
6
H
4
1
566 (w), 1459 (w), 1214 (m), 1176 (w), 828 (w), 767 (s), 709 (m).
RESULTS AND DISCUSSION
As development of our studies on the oxidation reaction with peroxodisulfate, we wish to report
a simple and efficient method for the mild and selective oxidation of a series of benzylic
alcohols to their corresponding aldehydes as shown in Scheme 1. In spite of the great
convenience of using
2 2 8
K S O and relatively high oxidation potential, oxidation by
peroxodisulfate generally does not proceed at a convenient rate. Thus certain limitations may be
observed with this reagent. Hence, SSA was used as catalyst which could carry out the oxidation
reaction. Thermal conditions should be adjusted for using of inorganic salts, because these
compounds decompose at high temperature. Consequently, the temperature was controlled
during the oxidation reaction. The solubility of the oxidant was investigated in several solvents
such as dimethyl sulphoxide (DMSO), dimethylformamide (DMF), acetonitrile (CH
carbontetrachloride (CCl ), dichloromethane (CH Cl ), chloroform (CHCl ), diethyl ether and
acetone. The experimental results showed that CH CN was the best choice for this purpose. At
3
CN),
4
2
2
3
3
first, p-methoxybenzyl alcohol was selected as a typical compound and reacted in the presence
of oxidant, NaCl and SSA. In order to find the optimum time of reaction and the best solvent,
the effect of solvent and reaction time on the yields of the products was studied (Scheme 1).
Bull. Chem. Soc. Ethiop. 2013, 27(1)

134
Mohammad Kazem Mohammadi et al.
K S O / NaCl / SSA
2 2 8
CH
2
OH
CHO
CH CN / Reflux
3
R
R
4-90 min
R= H, Alkyl, Aryl, Halide, NO
2
Scheme 1
Table 2. The effect of solvent on the typical oxidation time and product yield of p-methoxybenzyl in the
presence of K /NaCl/SSA.
2 2 8
S O
Solvent
DMSO
DMF
Time (min)
Yield (%)
Solvent
CH Cl
CHCl
Et
Aceton
Time (min)
Yield (%)
20
38
15
80
57
40
95
30
2
2
60
80
60
35
30
50
38
45
3
CH
3
CN
2
O
CCl
4
Table 2 shows that the optimum result was obtained after 15 min in CH CN. In other
solvents, reaction gave low yields. The carbonyl compounds prepared in this solvent at a special
time and their yields are listed in Table 3.
3
2 2 8 3
Table 3. SSA/NaCl-catalyzed oxidation of benzylic alcohols with K S O in refluxing CH CN.
Entry
Substrate
PhCH OH
p-CH OC CH
m-CH OC CH
p-Me CC CH
o-NO CH
m-NO CH
p-NO CH
Time (min)
Yield (%)a
Product b
PhCHO
p-CH OC
m-CH OC
CC
1
2
3
4
5
6
7
8
9
2
5
4
85
96
90
83
74
86
72
85
86
82
95
93
3
6
H
4
2
OH
3
6 4
H CHO
3
6
H
4
2
OH
OH
7
3
6
4
H CHO
3
6
H
4
2
7
p-Me
3
H CHO
6 4
2
C
6
H
4
2
OH
90
45
60
7
o-NO C H CHO
2 6 4
2
C
6
H
4
2
OH
2
m-NO C
H
6 4
CHO
2
C
6
H
4
2
OH
p-NO
2
C
6
4
H CHO
o-BrC
6
6
H
4
CH
2
OH
OH
OH
o-BrC
6
H
4
4
CHO
CHO
p-BrC
H
4
CH
2
9
p-BrC
6
H
1
1
1
0
1
2
o-ClC
m-IC
p-C
6
H
4
CH
CH
CH
2
6
o-ClC
m-IC
p-C
6
H
4
CHO
CHO
CHO
6
H
4
2
OH
16
50
6 4
H
H
6 5
C
6
H
4
2
OH
6
H
5
C
6
H
4
a
b
Yields refer to isolated yields. Products were characterized by comparison of their physical and spectra
data with authentic samples.
As shown in Table 3, various types of benzylic alcohols were reacted efficiently with
2 2 8
K S O to afford corresponding aldehydes in good to excellent yields (entries 1-12). Further
oxidation of aldehydes to their carboxylic acids and also fragmentation products were not
observed in this method. In order to show the better chemo selectivity of the described method,
an equimolar mixture of benzyl, allyl and alkyl alcohols was treated with this reagent in
refluxing CH CN for 3 h. We found that p-methoxybenzyl alcohol was oxidized selectively in
3
the presence of alkyl alcohols. This selectivity is of vital importance in modern synthetic
methodologies. The mechanism of reaction was been seemed to be the hydrogen attraction of
+
–
4
hydroxyl group in the bezylic alcohol by the K SO OH (produced in path 2) (Scheme 2) that
followed with radically attack of hydroxyl radical group to the α-H and then formation of keto
group. The rule of NaCl was seemed to be for the increase of reaction ionic power and then
better developing the oxidation reaction procedure.
Bull. Chem. Soc. Ethiop. 2013, 27(1)

Short Communication
135
Scheme 2
The noteworthy advantages of this reagent lie in its reusability. Thus Table 4 demonstrates
the reusability of the oxidant after 4 replicate oxidations of p-methoxy benzyl alcohol to p-
methoxy benzaldehyde. The oxidant was recovered by stirring of the white solid, which
2 2 8
obtained after oxidation of alcohols, with K S O for 2 h.
a
Table 4. Reusability of oxidant .
Experimental
st
Yield (%)
1
96
93
95
nd
2
rd
3
a
All reactions are carried out under similar conditions.
CONCLUSIONS
In this work, a series of benzyl aldehyde compound were synthesized via mild oxidation of
benzylic alcohols with potassium peroxodisulfate as an oxidant in the presence of NaCl and
SSA as heterogeneous catalyst. It is evident from the results presented in Table 2 that this
procedure for oxidation of benzylic alkohols was a mild, convenient, and versatile method and
is broadly applicable for the conversion of benzyl alcohol compounds into their aldehyde
derivatives. The said conditions have many advantages such as: i) short time of oxidation, ii)
high selectivity of oxidant in the presence of NaCl/SSA, iii) mild condition, iv) stability of
oxidant and v) easy preparation of oxidant.
ACKNOWLEDGMENTS
We gratefully acknowledge the support of this work by Islamic Azad University, Science and
Research Branch, Ahvaz, Iran.
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