Chemoselective oxidation of benzylic alcohols and hydroquinones with bis-(2,4,6-trimethylpyridinium) dichromate (BTMPDC) as an efficient and new oxidizing agent

Article abstract of DOI:10.1080/10426500802191183

A simple, efficient, and chemoselective oxidation of benzylic alcohols to the corresponding aldehydes and ketones and also hydroquinones to the benzoquinones using bis-(2,4,6-trimethylpyridinium) dichromate (BTMPDC) in acetonitrile is described. A good range of primary and secondary benzylic alcohols and a limited range of hydroquinones were selectively oxidized under reflux conditions in reasonable yields.

Full text of DOI:10.1080/10426500802191183

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Chemoselective Oxidation
of Benzylic Alcohols and
Hydroquinones with bis-
(2,4,6-Trimethylpyridinium)
Dichromate (BTMPDC) as an
Efficient and New Oxidizing
Agent
Lotfi Shiri a & Arash Ghorbani-Choghamarani a
a Department of Chemistry, Faculty of Science , Ilam
University , Ilam, Iran
Published online: 04 Feb 2009.
To cite this article: Lotfi Shiri & Arash Ghorbani-Choghamarani (2009)
Chemoselective Oxidation of Benzylic Alcohols and Hydroquinones with bis-(2,4,6-
Trimethylpyridinium) Dichromate (BTMPDC) as an Efficient and New Oxidizing Agent,
Phosphorus, Sulfur, and Silicon and the Related Elements, 184:2, 492-498, DOI:
10.1080/10426500802191183
To link to this article: http://dx.doi.org/10.1080/10426500802191183
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Phosphorus, Sulfur, and Silicon, 184:492–498, 2009
Copyright © Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426500802191183
Chemoselective Oxidation of Benzylic Alcohols and
Hydroquinones with bis-(2,4,6-Trimethylpyridinium)
Dichromate (BTMPDC) as an Efficient and New
Oxidizing Agent
Lotfi Shiri and Arash Ghorbani-Choghamarani
Department of Chemistry, Faculty of Science, Ilam University,
Ilam, Iran
A simple, efficient, and chemoselective oxidation of benzylic alcohols to the cor-
responding aldehydes and ketones and also hydroquinones to the benzoquinones
using bis-(2,4,6-trimethylpyridinium) dichromate (BTMPDC) in acetonitrile is de-
scribed. A good range of primary and secondary benzylic alcohols and a limited
range of hydroquinones were selectively oxidized under reflux conditions in reason-
able yields.
Keywords Alcohols; bis-(2,4,6-trimethylpyridinium) dichromate; hydroquinone;
oxidation
INTRODUCTION
Oxidation of alcohols is the foundation of synthetic chemistry and a fun-
damental transformation in organic synthesis. The conversion of pri-
mary and secondary alcohols to the corresponding carbonyl compounds
is essential for the preparation of fragrances and food additives as well
as of many synthetic intermediates in organic chemistry.1 It is of inter-
est to use heterogeneous systems for performing oxidation in industry
and technologies.2,3 A number of methods for the oxidation of alcohols
have emerged in the recent years,4–24 but low selectivity, long reaction
times, low yields of products, expensive reagents, and overoxidation to
carboxylic acid are still serious obstacles for this transformation.
Received 3 March 2008; accepted 9 May 2008.
Financial support for this work by the research affair, Ilam University, Ilam, Iran, is
gratefully acknowledged.
Address correspondence to L. Shiri and A. Ghorbani-Choghamarani, Department of
Chemistry, Faculty of Science, Ilam University, P.O. Box 69315516, Ilam, Iran. E-mail:
lshiri47@yahoo.com (L. Shiri) or arashghch58@yahoo.com (A. Ghorbani-Choghamarani).
492

Chemoselective Oxidation of Benzylic Alcohols and Hydroquinones
493
RESULTS AND DISCUSSIONS
Consequently, we decided to introduce a new reagent to overcome
the above limitations. In continuation of our ongoing program on
the application of new oxidizing reagents in organic functional group
transformations,25–32 we were interested to synthesize bis-(2,4,6-
trimethylpyridinum) dichromate (BTMPDC) I as a new and efficient
oxidizing reagent. It is interesting to note that the production of BTM-
PDC is easy and clean without difficult workup procedure, and pure
reagent can be obtained only with crystallization in water (Scheme 1).
Me
Me
NH
Me
Me
O Cr O Cr O HN
O
O
H2O
+ CrO3
Me
Me
Me
Me
N
O
O
Me
TMPDC
SCHEME 1
In addition, there is current research of general interest in hetero-
geneous systems33–39 because of the importance such systems have in
industry and in developing technologies. Thereforeherein we disclose
an effective and mild procedure for the oxidation of a variety of pri-
mary and secondary benzylic alcohols to the corresponding carbonyl
compounds using BTMPDC under reflux conditions in acetonitrile.
Initially, we screened different solvents for the oxidation of 2-
chlorobenzyl alcohol to 2-chlorobenzaldehyde, as a typical example,
using BTMPDC and found that acetonitrile is the best solvent in term
of activity and selectivity (Table I).
Eventually, different types of primary and secondary benzylic al-
cohols 1 were subjected to the oxidation reaction in the presence of
bis-(2,4,6-trimethylpyridinium) dichromate (BTMPDC) in acetonitrile
under reflux conditions (Scheme 2); the results for this transformation
are summarized in Table II.
O
OH
R2
BTMPDC
R1
R2
R1
MeCN
reflux
R1, R2: Aryl, Vinyl or H
SCHEME 2

494
L. Shiri and A. Ghorbani-Choghamarani
TABLE I Oxidation of 2-Chlorobenzyl
Alcohol Using BTMPDC in Different
Solvents Under Reflux Conditionsa
Entry
Solvent
Time (Min) Yield (%)b
1
2
3
4
5
6
Acetonitrile
Acetone
Chloroform
Dichloromethane
Tetrahydrofurane
Diethyl ether
10
100
50
65
26
30
12
120
120
120
120
120
a2-chlorobenzyl alcohol : BTMPDC = 1 mmol : 1
mmol. b Yield was determined by 2,4-dinitrophenyl
hydrazone derivatives.
To show the chemoselectivity of the described system, cinnamyl alco-
hol was converted to the cinnamaldehyde in good yields and a carbon–
carbon double bond was intact in the course of the reaction (Table II,
entry 13).
It is of interest that this oxidizing system chemoselectively oxi-
dized all primary benzylic alcohols to the corresponding aldehydes,
and overoxidation did not occur (Table II, entries 1–7, Scheme 3).
COOH
CH2OH
CHO
+
BTMPDC
MeCN
reflux
X
X
X
100 %
0%
SCHEME 3
The oxidation of phenols constitutes an interesting issue from the
biological and chemical point of view.40 Various phenol derivatives ex-
hibit biological properties and are important intermediates in indus-
trial chemical and pharmaceutical synthesis.41–43
Therefore, to show the capability of BTMPDC as an effective oxidiz-
ing agent, three hydroquinones were subjected to the oxidation reaction
to produce the corresponding benzoquinones under same conditions
(Scheme 4).
We hoped that BTMPDC would be an excellent oxidant reagent for all
of the oxidation reactions. In summary, the cheapness and availability
of the reagents, easy and clean workup, good conversion of alcohols, and
acceptable yields make this an attractive methodology. We believe that

Chemoselective Oxidation of Benzylic Alcohols and Hydroquinones
495
TABLE II Oxidation of Primary and Secondary
Benzylic Alcohols 1 to Their Corresponding Carbonyl
Compounds 2 with BTMPDC Under Reflux Conditions
in Acetonitrile
Entry
Substratea
Product
Time (Min) Yield (%)b
1
2
10
10
78
80
3
4
5
5
75
82
70
20
60
6
7
8
10
10
60
85
74
82
9
40
80
10
11
12
45
35
30
82
70
75
13
30
96
asubstrate : BTMPDC (1 : 1 mmol), bIsolated yields.
the present methodology could be an important addition to existing
methodologies.

496
L. Shiri and A. Ghorbani-Choghamarani
OH
OH
O
R1
R2
R1
R2
R1= R2 = H 70%
BTMPDC
R1= R2 = Me 83%
R1= H, R2 = t-Bu 90%
MeCN
reflux
5 Min
O
SCHEME 4
EXPERIMENTAL
Chemicals were purchased from Fluka, Merck, and Aldrich chemical
companies. The oxidation products were characterized by comparison
1
of their spectral (IR, H NMR, and 13C NMR) and physical data with
authentic samples.
General Procedure
Preparation of bis-(2,4,6-trimethylpyridinium) dichromate
A 100 mL round-bottomed flask was equipped with a dropping fun-
nel containing 2,4,6-trimethylpyridine (26.8 mL, 0.2 mol). The flask
was charged with CrO3(20 g, 0.2 mol) and H2O (20 mL). Then 2,4,6-
trimethylpyridine was added dropwise for 10 min at room tempera-
ture. After the addition, the mixture was shaken for 15 min then kept
in the freezer for 24 h. Orange jelly sediment was obtained. The jelly
reagent was discolored with active charcoal and crystallized in water
then dried. Bis-(2,4,6-trimethylpyridinum) dichromate as orange solid
was obtained quantitatively.
Oxidation of Benzyl Alcohol to the Benzaldehyde as a Typical
Procedure
A suspension of benzyl alcohol (0.108 g, 1 mmol) and bis-(2,4,6-
trimethylpyridinium) dichromate (1 mmol, 0.460 g) in MeCN (15 mL)
was stirred under reflux conditions for 10 min (the progress of the re-
action was monitored by TLC). After reaction completion, the reaction
mixture was cooled down to room temperature then filtered off. The
residue was washed with diethyl ether (3 × 10 mL), then solvents were
removed using rotary evaporation. The product was purified by short
column chromatography using diethyl ether as elution. Benzaldehyde
was obtained in 78% yield (0.083 g).

Chemoselective Oxidation of Benzylic Alcohols and Hydroquinones
497
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