The catalytic performance of Preyssler's anion, [NaP5W 30O110]14-, in the oxidation of benzyl alcohols

Article abstract of DOI:10.1080/10426500600865004

Under mild conditions, monosubstituted benzyl alcohols were oxidized to benzaldehydes and benzoic acids in the presence of sodium 30- tungstopentaphosphate (Preyssler's anion), [NaP₅W₃₀O ₁₂₀]^14-, and hydrogen peroxide as an oxidant. This polyanion with high hydrolytic stability (pH = 0-12), high thermal stability, and high acidic strength shows good activities. The effects of various parameters on the yield of the products, including a catalyst type, a nature of the substitutents, and temperature, were studied. Comparison between Keggin's heteropolyacids, H₃[PW₁₂O₄₀], H ₃[PMo₁₂O₄₀], H₄[SiW ₁₂O₄₀], and H₄[SiMo₁₂O ₄₀], and Preyssler's anion shows that this polyanion reacts similar to Keggin's acids whitout any degradation of the structure. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/10426500600865004

Phosphorus, Sulfur, and Silicon, 181:2833–2841, 2006
Copyright © Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426500600865004
The Catalytic Performance of Preyssler’s Anion,
1
4−
[
NaP W O110] , in the Oxidation of Benzyl Alcohols
5
30
Fatemeh F. Bamoharram
Mina Roshani
Department of Chemistry, Islamic Azad University, Mashhad Branch,
Mashhad, Iran
Majid M. Heravi
Department of Chemistry, Alzahra University, Vanak, Tehran, Iran
Shahla Safaie
Department of Chemistry, Islamic Azad University, Mashhad Branch,
Mashhad, Iran
Under mild conditions, monosubstituted benzyl alcohols were oxidized to ben-
zaldehydes and benzoic acids in the presence of sodium 30-tungstopentaphosphate
(
Preyssler’s anion), [NaP5W30 O120] 1 , and hydrogen peroxide as an oxidant. This
4−
polyanion with high hydrolytic stability (pH = 0–12), high thermal stability, and
high acidic strength shows good activities. The effects of various parameters on
the yield of the products, including a catalyst type, a nature of the substitu-
tents, and temperature, were studied. Comparison between Keggin’s heteropoly-
acids, H [PW
O
], H [PMo
O
], H [SiW
O
], and H [SiMo
O
], and
3
12 40
3
12 40
4
12 40
4
12 40
Preyssler’s anion shows that this polyanion reacts similar to Keggin’s acids whitout
any degradation of the structure.
Keywords Benzaldehyde; benzyl alcohol; catalyst; heteropolyacid; Preyssler’s anion
INTRODUCTION
In light of the importance of oxidation in industry, there is still a need
for new environmentally friendly methods for the oxidation of primary
alcohols to carbonyl compounds, especially in the presence of other func-
tional groups.
Received; accepted.
Address correspondence to Majid M. Heravi, Alzahra University, Department of
Chemistry, School of Sciences, Vanak, Tehran, Iran. E-mail mmh1331@yahoo.com
2833

2
834
F. F. Bamoharram et al.
Carbonyl compounds in the fine-chemicals industry are precursors
with wide applicability from drugs to fragrances. The oxidation of al-
cohols to carbonyl products is an important transformation in organic
1
chemistry, which has received much attention over the years, espe-
2
cially in finding versatile catalysts for catalytic processes. There are
many methods for the oxidation of alcohols to the corresponding aldehy-
des, but there still exists a need for introducing new methods. The main
factors, which should be considered, are the simplicity of the method, ef-
fectiveness, and mildness of the reaction conditions. Most of the current
industrial oxidation processes are using traditional oxidants in solution
like the heavy metals Cr(VI) or Mn(VI), and metal nitrates on various
inorganic supports.3 According to stringent environmental laws, in-
dustries are under pressure to replace them with catalytic processes
using clean, inexpensive, and environmentally friendly oxidants.
Heteropolyacids are currently of considerable interest as catalysts
for a variety of organic oxidations with the evironmentally acceptable
−5
hydrogen peroxide as cooxidant.6 The importance and attractive-
ness of heteropoly catalysts is due to their variety and high catalytic
potential.
−8
It has been known that tungesten and molybdenum compounds are
9
,10
efficient catalysts for oxidations with hydrogen peroxide.
Tungesten
and molybdenum heteropoly acids also catalyze various oxidations of
1
organic substances by hydrogen peroxide. Peroxo polyoxometalates
1
1−13
have been shown to be the active intermediates in these reactions.
Peroxo polyoxometalates are obtained by the degradiation of
polyoxometalates.
Although among the wide variety of heteropolyacids much attention
has been devoted to the catalytic behavior of Keggin’s type polyacids
and their derivatives,1
4−19
the application of Preyssler’s anion has been
overlooked and very limited.20
Preyssler’s heteropolyacid is remarkable due to its exclusive physic-
ochemical properties. They include a strong Bronsted acidity, reversible
transformations, solubility in polar and nonpolar solvents, as well as
high hydrolytic and high thermal stability, which are very important in
catalytic processes.
Nevertheless, for this polyanion no catalytic activity was reported in
2
1
some cases. The structure of Preyssler’s anion is shown in Figure 1.
The anion has an approximate D5h symmetry and consists of a cyclic
assembly of five PW6O22 units, each derived from the Keggin’s anion
3
−
[
PW12O40]
by the removal of two sets of three corner-shared WO6
octahedra. A sodium ion is located within the polyanion on the fivefold
◦
axis and 1.25 A above the pseudomirror plane, which contains the

The Catalytic Performance of Preyssler’s Anion
2835
FIGURE 1 The structure of Preyssler’s anion.
five phosphorus atoms.22 Recently, we have reported on the catalytic
behavior of Preyssler’s anion.2
3,24
Armed with these experiences and in continuation of our investiga-
2
5
tions on the application of heteropolyanions in organic syntheses; on
the development of applications for Preyssler’s anion; and also due to
the importance of aromatic benzaldehydes in the perfumery, pharma-
2
6
ceutical, dyestuff and agrochemical industries; we describe in this
article the catalytic ability of this anion in the oxidation of benzyl
alcohols.
RESULTS AND DISCUSSION
The homogeneous catalytic oxidation of monosubstituted benzyl al-
cohols with electron- donating (CH3, OCH3, OH) and electron-
withdrawing groups (Cl, Br, NO2) by Preyssler’s anion as the catalyst
and hydrogen peroxide as the oxidant are reported. In all cases, the
reactions were carried out in mixed solvents including acetonitrile and

2
836
F. F. Bamoharram et al.
◦
water over 8 h at 70 C. Benzaldehydes are the major and benzoic acids
are the minor reaction products.
All products were isolated, and their purities were checked by their
melting points and spectral data. Experiments in the absence of the cat-
alyst yielded no substantial quantity of benzaldehyde or benzoic acid.
THE EFFECT OF THE SUBSTITUENT
The effects of various substituents on the yield of the products have been
examined in the presence of Preyssler’s catalyst. The results are shown
in Table I. The data in Table I indicates that while substrates with
electron-withdrawing substituents are converted to the corresponding
carboxylic acids and benzaldehydes with good yields, low yields are ob-
served for the oxidation of hydroxy-, methoxy-, and methyl-substituted
benzyl alcohols. This behavior may reflect inductive and resonance ef-
fects. In hydroxy-substituted benzaldehydes, because of the existance
of lone pairs at the oxygen atom, there is a possibility of resonance with
the carbonyl group; the ring is stabilized, and poor yields are observed.
The results show that both the nature of the substituent and its posi-
tion are important. Electron-withdrawing substituents result in higher
yields than electron-donating substituents. A comparision of the influ-
ence of substitutents in the ortho, meta, and para position shows that
the largest effect is found for substitutents in the para position. These
results indicate that not only the nature of the substituent but also
its position has an influence on the yield. The yield of benzaldehydes
TABLE I Yields of the Catalytic Oxidation of
Benzyl Alcohols With H14[NaP W O ]
5 30 110
Aldehyde
Yield (%)
Benzoic acid
Yield (%)
Substrate
4
4
2
4
3
2
2
2
2
-bromo benzyl alcohol
-chloro benzyl alcohol
-chloro benzyl alcohol
-nitro benzyl alcohol
-nitro benzyl alcohol
-nitro benzyl alcohol
-methyl benzyl alcohol
,3,4-methoxy benzyl alcohol
,3,4-hydroxy benzyl alcohol
44.5
37.3
35.3
34.5
33.0
31.7
6.7
10.0
30.0
15.0
12.8
28.1
5.0
7.3
4.9
4.9
5.2
7.3
−
5
Reaction conditions: catalyst 10 mol, substrate
−
3
10
mol, solvent acetonitrile (2 mL), H2O (3 mL),
hydrogen peroxide (0.03 mol) reflux for 8 h.

The Catalytic Performance of Preyssler’s Anion
2837
(
major products) increased in the order of substitutents:
4
-Br > 4-Cl > 2-Cl > 4-NO2 > 3-NO2 > 2-NO2
2, 3, 4-(OH)3, 2, 3, 4-(CH3)3, and 2, 3, 4-(OCH3)3.
>
THE EFFECT OF CATALYST TYPE
In order to compare the Keggin type catalysts with Preyssler’s catalyst
in the oxidation of benzyl alcohols to benzaldehydes, we selected the
V
IV
VI
VI
Keggin series including Hn[XM12O40] (X = P , Si , M = W , Mo ,
n = 3, 4). The results are shown in Figure 2 (data for products with
poor yields are not given). The yield of benzaldehyde decreases in the
following order:
H3[PW12O40] > H3[PMo12O40] > H14[NaP5W30O110]
>
H4[SiW12O40] > H4[SiMo12O40].
In the Keggin series the results show that the silicon heteropolyacids
are far less effective than the phosphorus heteropolyacids. Slightly
lower yields were achieved with Preyssler’s heteropolyacid.
The low of reactivity of Keggin’s acid with X = Si may either re-
flect the known lower rate of degradation of this complex by aqueous
hydrogen peroxide and the lower lability in general relative to
FIGURE 2 A comparison between Preyssler’s catalyst and Keggin’s catalysts
◦
at 70 C.

2
838
F. F. Bamoharram et al.
the phosphorus derivatives or indicate that the silicon analogue of
3
−
{
PO4[WO(O )2]4} does not form. A comparision between H3[PW12O40]
2
and H3[PMo12O40] shows for H3[PW12O40] a higher activity. When tung-
sten is replaced by molybdenum, the negative charge on the oxygen
atoms increases, which leads to a decrease in acidity.
2
7,28
It is also interesting to consider that by replacing one of the tungsten
atoms of Keggin’s acid (X = Si) with molybdenum the yield decreases
again.
It is clear that by replacing tungsten with molybdenum the symmetry
decreases, and this distortion may affect the nature and acidity of the
heteropolyacid along with its catalytic properties.
Although in the Keggin’s catalysts the active intermediate is con-
sidered to be a peroxo complex, a detailed mechanism in the case of
Preyssler’s catalyst has not yet been elucidated. It is notworthy that
under our conditions Preyssler’s catalyst is not degraded during the re-
action, which is proven by spectroscopic evidence. The IR spectrum at
the end of reactions shows that the catalyst is safe and unchanged.
Preyssler’s anion proves to be an efficient catalyst in these reactions
with an activity similar to that of the Keggin’s acids without any degra-
dation of the structure. The interesting feature of this polyanion com-
pared to Keggin’s heteropolyacids is its hydrolytic stability (pH = 0–12),
which is very important in catalytic processes. In addition, this polyan-
ion is more stable than the Keggin’s catalysts under thermal conditions,
which makes high temperature reactions possible.
There is a significant decomposition of H2O2 accompanying the oxi-
dation of the benzyl alcohols under reflux conditions. It is proposed that
Preyssler’s anion catalyzes the oxidation of benzyl alcohols to the corre-
sponding benzaldehydes by H2O2 in acetonitrile and water as a mixed
◦
solvent at 70 C, with the hydrogen peroxide decomposition being the
main reaction. A mechanism for this reaction, including the oxidation
of benzaldehydes by HO and HO2 radicals formed via the hydrogen
peroxide decomposition, is suggested.
◦
Usually at temperatures higher than 60 C, HO and HO2 radicals act
as oxidants.
2
9–31
THE EFFECT OF TEMPERATURE
The effect of temperature was studied by carrying out reactions at dif-
ferent temperatures in the presence of Preyssler’s catalyst. The results
showed that this polyanion catalyzes the reaction at r.t. It was observed
that the yield inereased as the reaction temperature was raised. The re-
◦
sults at 70 C and r.t. (highest and lowest temperature) are compared in
Figure 3. At higher temperatures, decomposition of hydrogen peroxide

The Catalytic Performance of Preyssler’s Anion
2839
FIGURE 3 A comparison between the yields at r.t. and under reflux conditions
◦
(
70 C).
may take place, which is undesirable. As expected, with an increase
of temperature, the reaction time decreases. Thus, the yield of prod-
ucts is also a function of time: The longer the time, the higher the
yield.
EXPERIMENTAL
Chemicals and Apparatus
Keggin’s heteropolyacids and benzyl alcohols were commercially avail-
2
3
able. H14[NaP5W30O110] was prepared according to the literature.
Acetonitrile and sulfuric acid were provided from a commercial source
and were used without further purification. Hydrogen peroxide was ob-
tained from Merck Company and was standardized by methods found
online (Hydrogen Peroxide Product Information Manual, Analytical
32
Procedure, Eka Company) .
1H NMR spectra were recorded on a Brucker Aspect 3000 instru-
ment at 100 MHz. IR spectra were recorded on a Buck 500 scientific
spectrometer (KBr pellets). Mass spectra were obtained with a Massens
POEKTRO METER CH-7A VARIN MAT BREMEN spectrometer.
The Oxidation of Benzyl Alcohols: General Procedure
The oxidation of benzyl alcohols was carried out in a two-necked round-
bottom flask equipped with magnetic stirrer, reflux condenser, and
thermometer.
−
5
In all cases, to a stirred solution of the catalyst (10 mol), acetonitrile
−
3
(
2 mL) and water (3 mL) were added the benzyl alcohol (10 mol). In
regular intervals, 30% hydrogen peroxide (0.03 mol) was added in small

2
840
F. F. Bamoharram et al.
◦
portions. The reaction mixture was stirred and refluxed for 8 h at 70 C.
The amount of benzaldehyde formed was determined by the reported
2,4-dinitrophenyl hydrazine method. The quantitative product analysis
was carried out with the 2,4-Dinitrophenylhydrazone (DNP) formed.
The aldehyde was isolated as the DNP, vaccum dried, weighed, re-
crystallized from ethanol, and weighed again. The DNP obtained was
found to have an identical melting point and mass spectrum as the DNP
of the corresponding benzaldehyde reported in the literature. Benzoic
acids were isolated by adding a 5% aqueous solution of NaHCO3 and 2M
HCl, respectively. The obtained solid was washed with water, vaccum
dried, and weighed.
CONCLUSIONS
For the first time, the oxidation of benzyl alcohols with electron-
withdrawing and electron-donating groups to the corresponding ben-
zaldehydes and benzoic acids was achieved using an inexpensive the
easily prepared sodium 30-tungstopentaphosphate H14[NaP5W30O110]
as a catalyst with good yields. The results show that the nature and
positions of the substituents as well as temperature are important fac-
tors. Compared to catalytic procedures using the Keggin’s catalysts,
H14[NaP5W30O110] offers the advantages of a higher hydrolytic and
thermal stability and of retention of the structure, while its activ-
ity is similar to that of a Keggin’s catalysts. The salient features of
Preyssler’s anion are availability, nontoxicity, and reusability. We be-
lieve this methodology will find usefulness in organic synthesis.
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