Synthesis and application of o-xylylenebis(triphenylphosphonium peroxymonosulfate) for selective oxidation of benzylic alcohols and hydroquinones

Article abstract of DOI:10.1080/104265090921155

This article describes the oxidation of benzylic, allylic alcohols and hydroquinones under solvent-free conditions using o- xylylenebis(triphenylphosphonium peroxymonosulfate), which has been prepared by mixing an aqueous solution of o-xylylenebis(triphenylphosphonium bromide) with oxone at room temperature. Copyright Taylor & Francis Inc.

Full text of DOI:10.1080/104265090921155

Phosphorus, Sulfur, and Silicon, 180:2431–2437, 2005
Copyright © Taylor & Francis Inc.
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/104265090921155
Synthesis and Application of
O-Xylylenebis(triphenylphosphonium
Peroxymonosulfate) for Selective Oxidation of Benzylic
Alcohols and Hydroquinones
Mahmoud Tajbakhsh
Moslem-Mansor Lakouraj
Khadijeh Yadoolahzadeh
University of Mazandaran, Babolsar, Iran
This article describes the oxidation of benzylic, allylic alcohols and hydroquinones
under solvent-free conditions using o-xylylenebis(triphenylphosphonium peroxy-
monosulfate), which has been prepared by mixing an aqueous solution of o-
xylylenebis(triphenylphosphonium bromide) with oxone at room temperature.
Keywords benzylic alcohols; hydroquinones; oxidation; O-Xylylenebis(triphenylphos-
phonium peroxymonosulfate); solvent-free
INTRODUCTION
Organic reactions were found to occur efficiently in the solid state. Or-
ganic solid state reactions are usually carried out by keeping a mix-
ture of finely powdered reactant and reagent at room temperature. In
some cases, these reactions are accelerated by heating, shaking, irra-
diation with ultrasound, or grinding of the reaction mixture using a
mortar and pestle. Since the chemistry of solid state organic reactions
has only a very short history, it is not easy to uniformly interpret such
reactions. Theoretical treatments of solid state reactions have many
problems because theory, which has been developed to describe the so-
lution reaction, is not simply applicable to solid state reactions. On the
other hand, heterogeneous reactions that are facilitated by supported
reagents on various solid inorganic surfaces have received attention in
Received September 30, 2004; accepted October 26, 2004.
We are thankful to the Mazandaran University Research Council for the partial sup-
port of this work.
Address correspondence to Mahmoud Tajbakhsh, Department of Chemistry, School
of Sciences, University of Mazandaran, PO Box 453, Babolsar, Iran. E-mail: tajbaksh@
umz.ac.ir
2431

2432
M. Tajbakhsh et al.
recent years.1−4 The advantage of these methods over conventional ho-
mogeneous reactions is that they provide greater selectivity, enhanced
reaction rates, cleaner products, and manipulative simplicity.5−9
The oxidation of alcohols to the corresponding carbonyl compounds
is a transformation that is fundamental to synthetic organic chemistry,
and numerous reagents and methods have been developed to carry out
this important reaction.10−11
O-Xylylenebis(triphenylphosphonium peroxymonosulfate) 1 was
prepared readily from an aqueous solution of oxone and o-
xylylenebis(triphenylphosphonium bromide) in a quantitative yield at
room temperature. This compound is a white and stable solid mate-
rial, which could be stored for months without losing its activity. The
structure of this reagent was confirmed by spectral data accompanied
by elemental analysis and titrimetric methods (Scheme 1).
SCHEME 1
In connection with our recent work on the solvent free oxidations,12
herein we report the oxidation of benzylic alcohols, allylic alcohols, and
hydroquinones with 1 in the presence of Lewis acids under solvent free
conditions as a new system which offers a simple, efficient, and selective
oxidative method, for the preparation of carbonyl compounds. In this
method, the reagent was carefully added to the substrate, and the mix-
ture in a mortar was ground for the time specified in Table I until TLC
analysis indicated a complete reaction. Most reactions are carried out
at room temperature. In general, the oxidations were completed within
5–15 min. The effectiveness of various Lewis acids such as AlCl3, ZnCl2,
FeCl3, and BiCl3 showed that AlCl3 was the most effective catalyst for
this purpose. The reaction in the presence of other Lewis acids proceeds
with lower efficiency even with a higher (1–2) molar ratio of the oxidant.
The oxidation of alcohols with 1 in the presence of AlCl3 proceeds
well under solid phase conditions (Scheme 2).
SCHEME 2

Oxidation of Benzylic, Allylic Alcohols
2433
TABLE I Oxidation of Benzylic Alcohols and Hydroquinones with 1
at Room Temperature
Substrate/
Oxidant
Entry
R1
R2
Time (min)
Yield (%)ab
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
C6H5
H
H
H
H
H
H
H
H
H
H
H
1/1
”
”
”
”
”
”
”
”
1/2
”
2/1
”
”
1/2
”
”
”
3
6
6
98
96
96
95
80
90
90
96
98
85
70
98
95
92
80
90a
N.R
”
2-ClC6H4
4-ClC6H4
4-MeOC6H4
3-MeOC6H4
2-MeC6H4
2-NH2C6H4
2-BrC6H4
4-BrC6H4
3-NO2C6H4
2-NO2C6H4
C6H5
C6H5
C6H5
C6H5CH CH
C6H5C
C6H5CH2
C7H15
Phenol
α-Naphthol
Hydroquinone
tert-Butyl
hydroquinone
6
15
10
5
5
5
15
15
5
CH2CH3
Me
C6H5
H
C6H5
H
H
—
—
—
5
10
15
10
30
”
”
”
3
5
O
”
”
1/1
”
”
”
98
95
—
aProducts were characterized by their physical constants, spectral characteristics
(IR, 1H NMR), and comparison with authentic samples.
bYields are of pure isolated products.
RESULTS AND DISCUSSION
The results were shown in Table I. Primary and secondary benzylic
alcohols are oxidized quantitatively in a short time (entries 1–9, and
12–14). However, aliphatic alcohols are unaffected even when using a
higher molar ratio of an oxidant and a higher temperature (entries 17,
18).
Benzylic alcohols bearing strong electron-withdrawing groups on the
aromatic ring need higher molar ratios of an oxidant and give lower
yields (entries 10, 11). A noteworthy advantage of this reaction lies in
its ability to the selective oxidation of benzylic and allylic alcohols in the
presence of the other functional groups, such as a double bond. When
we treated α, β-unsaturated alcohols with 1, only the OH group was

2434
M. Tajbakhsh et al.
selectively oxidized to the corresponding carbonyl compounds, and the
double bond remained intact (entry 15). α-Hydroxy ketone was oxidized
to the corresponding diketone using 2 equivalents of the reagent 1 at
45◦C (entry 16). Phenols and α-naphthol were unreactive towards this
reagent even at a higher molar ratio of oxidant and higher temperature
(entries 19, 20).
Hydroquinones were oxidized with one equivalent of this agent and
gave excellent yields of the corresponding benzoquinones (entries 21,
22).
In order to evaluate the selectivity of this reagent, the competitive
reactions shown in Eqs. (1)–(3) were carried out. When an equimo-
lar amount of 2-phenylethyl alcohol and benzyl alcohol was treated
with the reagent 1 in the presence of AlCl3, only benzyl alcohol
was selectively oxidized (Eq. (1)). Treatment of 4-methoxybenzyl al-
cohol with one mole ratio of reagent 1 in the presence of equimolar
amounts of 2-nitrobenzyl alcohol led to the exclusive oxidation of 4-
methoxybenzyl alcohol (Eq. (2)). Similarly, when we treated one equimo-
lar amount of benzyl alcohol with reagent 1 in the presence of an
equimolar amount of benzaldehydsemicarbazone, only the benzyl al-
cohol was selectively oxidized and semicarbazone remained unchanged
(Eq. (3)). Interestingly, in the oxidation of alcohols, the overoxidation
of products to the corresponding carboxylic acids was not observed
at all.
1
PhCH2CH2OH + PhCH2OH −−−−−−−−−→ PhCH2CH2OH + PhCHO (1)
AlCl3,5 min
(unchanged)
95%
2-NO2C6H4CH2OH + 4-MeOC6H4CH2OH
1
−−−−−−−−→ 2-NO2C6H4CH2OH + 4-MeOC6H4CHO
(2)
(3)
AlCl3,5 min
(unchanged)
90%
PhCH NNHCONH2 + PhCH2OH
1
−−−−−→ PhCH NNHCONH2 + PhCHO
AlCl3,5 min
(unchanged)
90%
It has also been found that this reagent has certain advantages
over oxidizing agents in terms of amounts of oxidant, short reaction
times required, higher product yields, and especially milder conditions
(Table II).
In summary, we report here the preparation of o-xylylenebis-
(triphenylphosphonium peroxymonosulfate) 1 as a mild, inexpensive,

Oxidation of Benzylic, Allylic Alcohols
2435
TABLE II Oxidation of Alcohols by 1 in Comparison With Other
Oxidants
Ratio of Oxidant:
Substrate
Reagent
Substrate
Time
Conditions
Yield (%) Ref.
PhCH2OH (1)a
p-MeOC6H5
1:1
1:1
1.5:1
1.5:1
12.2:1
4 min Solvent-free, r.t.
5 min
98
98
97
100
77
83
90
90
100
95
—
15
PhCH2OH 4-APCCb
p-MeOC6H5
2 h
Support on silica-gel
1.5 h
48 h
48 h
PhCH2OH MnO2
p-MeOC6H5
Solvent-free, r.t.
16
PhCH2OH PFCc
p-MeOC6H5
1:1
1:1
1:1
1:1
45 min CH2Cl2, r.t.
50 min
45 min CHCl3, reflux
45 min
17a
17b
n-BTPPDC
PhCH2OH
p-MeOC6H5
ao-Xylylenebis(triphenylphosphonium peroxymonosulfate).
b4-Aminopyridinium chlorochromate.
c
Pyridinium fluorochromate.
and stable oxidizing agent. The reagent is soluble in acetonitrile and
DMSO and is slightly soluble in carbon tetrachloride, ether, and hex-
ane. This is an efficient, rapid, mild, and inexpensive oxidant for the
oxidation of benzylic, allylic alcohols, and hydroquinones to the corre-
sponding carbonyl compound under solvent-free conditions in the pres-
ence of a catalytic amount of AlCl3. This method is superior to previ-
ously reported methods in terms of high yields, purity of products, facile
workup, cheaper reagent, and shorter reaction times.
EXPERIMENTAL
All yields refer to isolated products after purification. Products were
characterized by comparison with authentic samples (IR and 1H-NMR
spectrum, thin layer chromatography, m.p. and b.p.). All 1HNMR were
recorded at 90 MHz in CCl4 relative to TMS, and IR spectra were
recorded on a Shimadzu 435 IR spectrophotometer. Spectra of solids
were carried out using KBr pellets. All reactions were carried out un-
der solvent-free conditions.
Preparation of O-Xylylenebis(triphenylphosphonium
Peroxymonosulfate)1
A solution of o-xylylenebis(triphenylphosphonium bromide)13 (24.2
mmol, 20.7 g) in 100 mL of water was prepared, then oxone (48.4 mmol,
29.7 g) (2 KHSO5. KHSO4. K2SO4) in water (100 mL) was added drop-
wise to the above solution and stirred for 48 h at room temperature.

2436
M. Tajbakhsh et al.
The resulting precipitate was filtered and washed with cooled distilled
water (50 mL) and dried in a desiccator under vacuum over calcium
chloride to afford a white powder (95%), which was decomposed at 203–
205◦C to a dark brown material. 1HMR: δ 7.90–6.85 (m. 40 H), δ 4.7 (d.
4 H, CH2P). The content of active oxidizing agent (HSO−) was deter-
5
mined by the titrimetric method.14 Anal. calcd. for C44H40O10P2S2: C,
61.73; H, 4.65. Found: C, 61.8; H, 4.7.
General Procedure for Oxidation of Alcohols (2)
and Hydroquinones with Reagent 1
A mixture of alcohols (2) (1 mmol) or hydroquinones (1 mmol), oxidizing
agent 1 (1 mmol), and 0.1 molar ratio of AlCl3 in a mortar was ground for
the time specified in Table I. When TLC showed complete disappearance
of alcohol, the mixture was extracted with CH2Cl2. Evaporation of the
solvent gave the corresponding carbonyl compound (3). The product
was purified by column chromatography on silica gel using a mixture
of ethyl acetate and hexane as an eluent (10:90).
Competitive Oxidation: Typical Procedure
A mixture of benzyl alcohol (1 mmol), 2-phenylethyl alcohol (1 mmol),
oxidizing agent 1 (1 mmol, 0.855 g), and 0.1 molar ratio of AlCl3 was
ground in a mortar. The mixture was ground until TLC showed complete
disappearance of benzyl alcohol (5 min). The other competitive reactions
see (Eqs. (1)–(3) were carried out in the same manner as above.
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