A novel and selective oxidation of benzylic alcohols with polymer-supported periodic acid under mild aprotic conditions

Article abstract of DOI:10.1016/j.cclet.2011.07.022

A new polymeric oxidizing reagent was prepared by supporting periodic acid on poly(1,4-phenylene-2,5-pyridine dicarboxyamide). This polymeric reagent was used for the selective oxidation of primary benzylic alcohols to the corresponding benzaldehydes in CH₃CN at reflux conditions. Excellent selectivity was observed between primary benzyl alcohols and secondary ones as well as non-benzylic alcohols in the oxidation reactions. Allylic alcohols were also converted to the corresponding aldehydes with good yields.

Full text of DOI:10.1016/j.cclet.2011.07.022

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Chinese Chemical Letters 23 (2012) 21–24
www.elsevier.com/locate/cclet
A novel and selective oxidation of benzylic alcohols with
polymer-supported periodic acid under mild aprotic conditions
*
Ali Reza Pourali , Mehrosadat Tabaean, S. Mohamad Reza Nazifi
School of Chemistry, Damghan University, Damghan 36715/364, Iran
Received 26 May 2011
Available online 8 November 2011
Abstract
A new polymeric oxidizing reagent was prepared by supporting periodic acid on poly(1,4-phenylene-2,5-pyridine dicarbox-
yamide). This polymeric reagent was used for the selective oxidation of primary benzylic alcohols to the corresponding
benzaldehydes in CH3CN at reflux conditions. Excellent selectivity was observed between primary benzyl alcohols and secondary
ones as well as non-benzylic alcohols in the oxidation reactions. Allylic alcohols were also converted to the corresponding
aldehydes with good yields.
# 2011 Ali Reza Pourali. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
Keywords: Oxidation; Benzylic alcohol; Polymer-supported reagent; Periodate
The selective oxidation of benzylic alcohols in the presence of saturated alcohols is very important in organic
chemistry and can be achieved by many reagents such as MnO2, DDQ, bis(benzyltriethylammonium)dichromate and
K2FeO4 under phase-transfer conditions [1,2]. Also, many supported reagents [3] or catalysts [4] have been used for
selective oxidation of benzylic alcohols. Recently palladium-loaded oxidized diamond catalysis for the selective
oxidation of alcohols has been reported by Suzouki et al. [5]. Periodic acid has been used as oxidant in several selective
oxidations of alcohols [6]. Pyridinium chlorochromate [6a], pyridinium fluorochromate [6b], chromium trioxide [6c],
chromium (III) acetylacetonate [6d], copper on boehmite [6e], Fe(III)/2-picolinic acid [6f], [Ru(acetylacetona-
te)2(CH3CN)2]PF6 [6g] have been used as catalysts for the oxidation of alcohols with periodic acid. All of these reactions
utilized transition metal contained catalysts that have some disadvantages such as toxicity, difficulty in the preparation of
catalyst or using expensive catalysts. The immobilization of oxidizing reagents onto polymer supports provides potential
for extending the benefits of heterogeneous catalysts to homogeneous systems in organic syntheses [7].
To continue our previous works on the preparation and application of polymer-supported reagents in organic
synthesis [8], we now report a novel method for the selective oxidation of alcohols using polymer-supported periodic
acid as a new polymeric oxidizing agent under aprotic conditions. We first supported periodic acid on the poly(1,4-
phenylene-2,5-pyridinedicarboxyamide) by reacting them in water. It was stirred at room temperature for 48 h. Then it
was filtered and washed with H2O to remove unsupported periodates. The obtained polymeric reagent was dried to
afford polymer-supported periodic acid (Scheme 1). By having weight of the polymeric reagent, we found that each
* Corresponding author.
E-mail address: pourali@du.ac.ir (A.R. Pourali).
1001-8417/$ – see front matter # 2011 Ali Reza Pourali. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
doi:10.1016/j.cclet.2011.07.022

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A.R. Pourali et al. / Chinese Chemical Letters 23 (2012) 21–24
Scheme 1. Structure of polymer-supported periodic acid.
Scheme 2. Oxidation of benzylic alcohols.
gram of the reagent contains 1.75 mmol HIO4. Thus, we used 0.60 g (1.05 mmol) of the polymeric reagent for the
oxidation of 1 mmol alcohol.
We have studied the selective conversion of benzylic alcohols to the corresponding carbonyl compound using this
polymeric reagent under mild conditions in CH3CN as an aprotic solvent (Scheme 2).
We selected p-methoxybenzyl alcohol as a model and the oxidation was carried out in different solvents and
conditions (Table 1). Reaction progress was monitored by thin layer chromatography (TLC). We used 0.3 g of
polymer-supported periodic acid for the conversion of 0.5 mmol of p-methoxybenzyl alcohol. When the reaction
mixture was stirred in acetonitrile at reflux conditions for 3 h, p-methoxy benzaldehyde was obtained in excellent
(98%) isolated yield (Table 1, entry 5).
We used the same conditions for the conversion of other alcohols to the corresponding carbonyl compounds in good
to excellent yields (Table 2). Primary benzyl alcohols without strong electron withdrawing groups such as NO2 had
excellent isolated yields (Table 2, entries 1–8), while non-benzylic alcohols (Table 2, entries 10–12) and secondary
benzyl alcohols (entries 13–15) had poor yields. Allylic alcohols were also converted to the corresponding aldehydes
with good yields (Table 2, entries 16–17).
For investigating the selectivity of polymeric agent between primary and secondary benzyl alcohols, a competition
reaction was performed between benzyl alcohol (1 mmol) and benzhydrol (1 mmol) with 0.6 g polymer-supported
periodic acid in acetonitrile at reflux conditions (Scheme 3). After 2.25 h, benzyl alcohol was converted to
benzaldehyde with 90% isolated yield while benzylhydrol was recovered completely.
Also this polymeric reagent was showed another excellent selectivity between primary benzyl alcohols and non-
benzylic alcohols in the reaction of benzyl alcohol (1 mmol) and 2-phenyl ethanol (1 mmol) with 0.6 g polymer-
supported periodic acid in the same conditions (Scheme 4). After 2 h, benzyl alcohol was converted to benzaldehyde
with 95% isolated yield while 2-phenyl ethanol was recovered completely.
Table 1
Oxidation of 4-methoxybenzyl alcohol to 4-methoxybezaldehyde by polymer-supported periodic acid in different solvents.
Entry
Solvent
Condition
Time (h)
Conversion (%)
1
2
3
4
5
Diethyl ether
Dichloromethane
n-Hexane
Reflux
Reflux
Reflux
r.t.
3
3
20
40
30
20
98
20
6
Acetonitrile
Acetonitrile
Reflux
3

A.R. Pourali et al. / Chinese Chemical Letters 23 (2012) 21–24
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Table 2
Selective oxidation of alcohols to carbonyl compounds withpolymer-supported periodic acid in acetonitrile at reflux conditions.
Entry
Substrate
Product
Time (h)
Isolated yield (%)
1
4-MeOC6H4CH2OH
C6H5CH2OH
4-MeOC6H4CHO
C6H5CHO
3
98
95
96
95
85
90
95
90
40
20
10
5
2
1.6
1.25
1.25
6
3
2-MeC6H4CH2OH
2-t-BuC6H4CH2OH
3-MeOC6H4CH2OH
2-ClC6H4CH2OH
4-ClC6H4CH2OH
2,4-ClC6H3CH2OH
4-NO2C6H4CH2OH
C6H5CH(CH3)CH2OH
C6H5CH2CH2OH
2-MeC6H4CHO
2-t-BuC6H4CHO
3-MeOC6H4CHO
2-ClC6H4CHO
4-ClC6H4CHO
2,4- ClC6H3CHO
4-NO2C6H4CHO
C6H5CH(CH3)CHO
C6H5CH2CHO
C6H5CH2CH2CHO
C6H5COCH3
4
5
6
1.15
1
7
8
1.25
1
9
10
11
12
13
14
15
16
17
1
13
C6H5CH2CH2CH2OH
C6H5CH(CH3)OH
C6H5CH(OH)C6H5
C6H5CH(OH)CH2CH3
CH2 CHCH2OH
C6H5CH CHCH2OH
0.75
2
40
30
50
85
70
C6H5COC6H5
1.75
1.5
1.5
2
C6H5COCH2CH3
CH2 CHCHO
C6H5CH CHCHO
In conclusion, various primary benzylic alcohols were efficiently converted to the corresponding aldehydes using
polymer-supported periodic acid under mild conditions in CH3CN as an aprotic solvent. This method is a novel,
selective and efficient route for preparation of carbonyl compounds. Furthermore, it has other advantages such as re-
generability of the reagent, mildness, simple work up and simple experimental procedure.
1. Experimental
Chemicals were obtained from Merck and Fluka chemical companies. All compounds were known and are
identified by comparison of their physical and spectroscopic data with those of authentic samples. Melting points were
determined in open capillary tubes with a Buchi 510 apparatus. FT-IR spectra were recorded on a PerkinElmer RXI
spectrometer. NMR spectra were recorded on a Brucker Advance DPX 250 MHz instrument.
Preparation of poly(1,4-phenylene-2,5-pyridinedicarboxyamide) was accomplished using 2,5-pyridine dicar-
boxylic acid, thionyl chloride and p-phenylenediamine according to literature [9].
To a solution of p-methoxy benzyl alcohol (0.5 mmol, 0.069 g) in acetonitrile (5 mL), was added polymer-
supported periodic acid (0.3 g). The mixture was refluxed and reaction progress was monitored by thin layer
Scheme 3. Selective oxidation of benzyl alcohol in the presence of benzhydrol.
Scheme 4. Selective oxidation of benzyl alcohol in the presence of 2-phenyl ethanol.

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A.R. Pourali et al. / Chinese Chemical Letters 23 (2012) 21–24
chromatography (TLC) using mixture of ethyl acetate and n-hexane (1:4, v/v) as solvent. After 3 h, the mixture was
filtered and polymeric reagent was washed with acetone (10 mL). The organic phases was mixed and evaporated by
rotary evaporator at reduced pressure. The obtained crude product was purified by short column chromatography using
silica-gel with n-hexane and ethyl acetate (4:1, v/v) as eluent, then obtained p-methoxy benzaldehyde (0.066 g, 98%);
b.p. 247-249 8C (Lit [3a] b.p. 247–250 8C). The used reagent can be re-generated by reacting with a solution of HIO4
as preliminary preparation of the reagent.
Acknowledgment
We are grateful to The Damghan University Research Council for the partial support of this work.
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