Efficient, solvent-free oxidation of organic compounds with potassium dichromate in the presence of Lewis acids

Article abstract of DOI:10.3390/61100900

The synthetic utility of potassium dichromate in the presence of Lewis acids under solid phase conditions is described. This reagent efficiently oxidizes alcohols, acyloins, oximes and semicarbazones to their corresponding carbonyl compounds, while trimethylsilyl and tetrahydropyranyl ethers, ethylene acetals and ketals undergo oxidative deprotection to produce carbonyl compounds efficiently.

Full text of DOI:10.3390/61100900

Molecules 2001, 6, 900-908
molecules
ISSN 1420-3049
http://www.mdpi.org
Efficient, Solvent-Free Oxidation of Organic Compounds with
Potassium Dichromate in the Presence of Lewis Acids
Iraj Mohammadpoor-Baltork*, Majid M. Sadeghi and Abol-Hassan Adibi
Department of Chemistry, Isfahan University, Isfahan 81744, Iran, Fax: (+98) 311 6689732.
*
Author to whom correspondence should be addressed; e-mail: imbaltork@sci.ui.ac.ir
Received: 9 August 2001; in revised form 25 September 2001 / Accepted: 22 October 2001 /
Published: 31 October 2001
Abstract: The synthetic utility of potassium dichromate in the presence of Lewis acids
under solid phase conditions is described. This reagent efficiently oxidizes alcohols,
acyloins, oximes and semicarbazones to their corresponding carbonyl compounds, while
trimethylsilyl and tetrahydropyranyl ethers, ethylene acetals and ketals undergo oxidative
deprotection to produce carbonyl compounds efficiently.
Keywords: Oxidation, Organic compounds, Dichromate, Lewis acids, Solvent-free
conditions.
Introduction
The total syntheses of complex molecules demand new methods in different areas of organic
chemistry. Therefore, the development of reagents is always rewarding to synthetic organic chemistry.
Oxidation is one of the most important classes of organic reactions from different points of view and
effecting them under non-aqueous and aprotic conditions has found valuable applications in modern
organic syntheses [1].
Potassium dichromate has been used extensively for the oxidation of different organic compounds
under varied aqueous acidic conditions [1-6], but some of these methods suffer from disadvantages
such as high reaction temperature, overoxidation of primary alcohols to carboxylic acids, long reaction
times, low yields of the products and tedious workup. Its use as an oxidant under neutral conditions is
also limited by its very low solubility in most organic solvents.

Molecules 2001, 6
901
Heterogeneous reactions facilitated by reagents supported on various solid inorganic surfaces have
gained special attention in recent years [7-9]. The advantages of these methods over conventional
homogeneous reactions are typically higher selectivity, enhanced reaction rates, milder reaction
conditions, cleaner products and manipulative simplicity. As a continuation of our ongoing work on
development of environmentally benign methods using solid supports [10], we now report a
convenient method for the oxidation of different classes of organic compounds with potassium
dichromate in the presence of Lewis acids under solid phase conditions.
Results and Discussion
Potassium dichromate is a readily available and inexpensive reagent. The catalytic effects of several
Lewis acids upon the activity of this oxidant were thoroughly studied. For this purpose the oxidation of
benzhydrol to benzophenone was investigated in the presence of AlCl , FeCl , BiCl , NiCl , CeCl ,
3
3
3
2
3
ZrCl , SnCl .2H O, CuCl .2H O, MnCl .4H O and CoCl .6H O. The experimental results show AlCl
4
2
2
2
2
2
2
2
2
3
to be the most effective catalyst for this purpose (Table 1).
Table 1. Percent Conversion of Benzhydrol to Benzophenone with K Cr O in
2
2
7
the Presence of Different Lewis Acids Under Solid Phase Conditions
Time (Min)
5
10
-
15
-
30
-
Lewis acid
AlCl3
100
65
FeCl3
75
77
80
BiCl3
NiCl2
CeCl3
ZrCl4
80
30
50
70
85
50
65
80
90
60
75
85
90
60
80
85
SnCl .2H O
30
60
25
60
60
75
50
65
85
80
75
70
90
90
80
75
2
2
CuCl .2H O
2
2
MnCl .4H O
2
2
CoCl .6H O
2
2
Reagent ratio: Ph CHOH: K Cr O : AlCl = 1:1:1
2
2
2
7
3
Primary and secondary benzylic alcohols are oxidized to their corresponding aldehydes and ketones
with potassium dichromate in the presence of AlCl in high yield (Table 2). No overoxidation of
3
primary alcohols to the corresponding carboxylic acids was observed under these conditions.

Molecules 2001, 6
902
Table 2. Oxidation of Alcohols to Carbonyl Compoundsa
R1
R2
R1
R2
K Cr O /AlCl
2
2
7
OH
O
1
2
b
Entry
R
R
Time (Min)
Yields (%)
1
2
3
4
5
6
7
8
9
C H5
2-MeOC H4
3-MeOC H4
4-MeOC H4
H
H
H
H
H
H
H
H
H
5
3
4
3
2
30
30
25
10
15
4
10
15
15
10
3
95
95
90
98
95
72
74
75
93
87
88
95
93
90
90
98
6
6
6
6
3,4-(MeO) C H
2
6
4
4
4
3
2-NO C H
2
6
3-NO C H
2
6
4-NO C H
2
6
4-ClC H4
6
1
0
1
2
3
4
5
6
7
8
9
0
1
4-BrC H4
H
H
6
1
1
1
1
1
1
1
1
1
2
2
5-MeFuryl
C H5
CH3
CH3
CH3
CH3
CH3
C H
6
4-ClC H4
6
4-BrC H4
6
4-PhC H4
6
3,4-(MeO) C H
2
6
3
c
C H5
5
93
6
6
5
2-Pyridyl
C H
15
30
10
3
80
6
5
d
C H CH=CH
H
65
6
5
C H5
C H CO
95
96
6
6
5
4-CH OC H
4-CH OC H CO
3
6
4
3
6
4
2
2
10
90
2
3
30
30
10
10
24
CH (CH )
H
3
2 6
a
All of the products are known compounds and were identified by comparison
b
c
with authentic samples. Isolated yields. In the absence of AlCl , benzhydrol
3
was converted to benzophenone in only 10% yield after 30 min with K Cr O .
2
2
7
d
1
0% of benzaldehyde was obtained from the reaction mixture.

Molecules 2001, 6
903
α-Hydroxy ketones are converted to α-diketones in excellent yields without any carbon-carbon bond
cleavage (entries 20, 21). Saturated alcohols such as cyclohexanol and 1-heptanol are resistant towards
oxidation with this reagent and the corresponding carbonyl compounds are obtained in poor yields
(
entries 23, 24). The oxidation of larger amounts (5-10 mmol) of some alcohols was also investigated.
The results were comparable to those of the small scale experiments, therefore, it seems that the
methodology is also applicable for medium to large scale operations. In order to stress the selectivity of
this method, an equimolar mixture of 4-methoxybenzyl alcohol and 1-heptanol was treated with an
equimolar of potassium dichromate and aluminium chloride. The experimental results show that only
4
-methoxybenzyl alcohol was oxidized selectively. It is noteworthy that many of the other chromium
(VI) based oxidants either do not display such selectivity [11-19] or they require much longer reaction
times [20, 21]. Therefore, this selectivity represents a useful practical achievement in such oxidation
reactions.
Potassium dichromate in the presence of aluminium chloride is also able to convert benzylic oximes
and semicarbazones to their corresponding aldehydes and ketones in high yields (Table 3).
Table 3. Conversion of Oximes and Semicarbazones to Carbonyl Compoundsa
R1
R2
R1
R2
K Cr O /AlCl
2
2
7
N-G
O
1
2
b
Entry
R
R
G
Time(Min) Yield(%)
1
2
3
4
5
6
7
8
9
C H5
2-MeOC H4
4-MeOC H4
H
H
H
H
H
H
H
CH3
CH3
CH3
OH
OH
OH
OH
OH
OH
OH
OH
OH
OH
OH
OH
OH
10
5
5
92
90
95
95
75
85
85
90
95
91
85
86
85
6
6
6
3,4-(MeO) C H
5
2
6
4
4
3
3-NO C H
30
30
10
15
15
15
5
2
6
4-NO C H
2
6
5-MeFuryl
C H5
6
4-BrC H4
6
10
11
12
13
4-ClC H4
6
4-BrC H4
CH Br
6
2
4-PhC H4
CH3
C H
15
15
6
C H5
6
6
5

Molecules 2001, 6
904
1
3
0
0
77
10
1
1
4
5
OH
OH
16
17
18
19
20
21
22
23
24
25
26
27
C H5
H
H
H
H
H
NHCONH2
NHCONH2
NHCONH2
NHCONH2
NHCONH2
NHCONH2
NHCONH2
NHCONH2
NHCONH2
NHCONH2
NHCONH2
NHCONH2
10
5
30
10
5
90
95
77
87
95
80
88
85
85
92
93
90
6
4-CH OC H
3
6
4
4-NO C H
2
6
4
4-ClC H4
6
3,4-(MeO) C H
2
6
3
5-MeFuryl
H
5
C H5
CH3
CH3
CH3
10
10
15
10
20
10
6
4-BrC H4
6
4-ClC H4
6
4-BrC H4
CH Br
6
2
4-PhC H4
CH3
C H
6
C H5
6
6
5
2
2
8
NHCONH2
NHCONH2
30
30
88
5
9
a
All of the products are known compounds and were identified by comparison
b
with authentic samples. Isolated yields.
Further oxidation of aldehydes to their carboxylic acids was not observed. Under the same reaction
conditions, saturated oximes and semicarbazones are converted to their corresponding carbonyl parent
compounds in poor yields (Table 3, entries 15, 29).
In contrast to the method described in this paper, deoximation using pyridinium chlorochromate
(
PCC) suffers from long reaction times (12-94 h), low yields and low selectivity [22]. PCC-H O
2
2
system is not sutiable for aldoximes and overoxidation products are usually produced and show low
selectivity as well [23]. Therefore, this method is superior to PCC and PCC-H O system in terms of
2
2
reaction times, yields and selectivity.
In order to further assess the capabilities of this reagent, we decided to also perform oxidative
deprotection of trimethylsilyl and tetrahydropyranyl ethers, ethylene acetals and ketals. The treatment
of a variety of TMS and THP ethers, ethylene acetals and ketals with potassium dichromate in the
presence aluminium chloride under solid phase conditions afforded the corresponding carbonyl
compounds in high yields (Tables 4 and 5).

Molecules 2001, 6
Table 4. Conversion of TMS and THP Ethers to Carbonyl Compoundsa
R1
R1
R2
R2
Time(Min) Yield (%)
905
K Cr O /AlCl
2
2
7
OR3
O
1
2
3
b
Entry
R
R
R
1
2
3
4
5
6
7
8
9
C H5
2-MeOC H4
4-MeOC H4
H
H
H
H
H
H
TMS
TMS
TMS
TMS
TMS
TMS
TMS
TMS
TMS
TMS
TMS
TMS
TMS
15
15
10
30
30
30
20
25
25
25
10
25
20
90
95
95
80
81
65
82
85
90
77
96
95
90
6
6
6
2-NO C H
2
6
4
4-NO C H
2
6
4
c
C H CH=CH
6
5
C H5
CH3
CH3
CH3
CH3
CH3
C H
6
4-ClC H4
6
4-BrC H4
6
1
0
1
2
3
4-PhC H4
6
1
1
1
3,4-(MeO) C H
2
6
3
C H5
6
6
5
4-ClC H4
C H
6
6
5
1
4
TMS
15
85
1
5
TMS
30
5
16
17
19
20
21
22
23
24
25
C H5
H
H
H
H
H
H
H
THP
THP
THP
THP
THP
THP
THP
THP
THP
5
5
3
30
30
10
3
10
10
95
95
96
75
80
90
96
87
80
6
2-CH OC H
3
6
4
4-CH OC H
3
6
4
3-NO C H
2
6
4
4-NO C H
2
6
4
4-ClC H4
6
3,4-(MeO) C H
2
6
3
C H5
CH3
CH3
6
4-ClC H4
6
26
27
28
4-PhC H4
CH3
C H
THP
THP
THP
15
5
15
90
87
90
6
C H5
6
6
5
4-ClC H4
C H
6
6
5
2
9
THP
30
5
a
All of the products are known compounds and identified by comparison with authentic
b
c
samples. Isolated yields. 15% of benzaldehyde was obtained from the reaction mixture.

Molecules 2001, 6
Table 5. Conversion of Ethylene Acetals and Ketals to Carbonyl Compoundsa
906
R1
R2
O
O
R1
K Cr O /AlCl
2
2
7
3
O
R2
1
2
b
Entry
R
R
Time(Min)
Yields(%)
1
2
3
4
5
6
7
8
9
C H5
3-MeOC H4
4-MeOC H4
H
H
H
H
H
5
5
3
2
30
10
10
15
15
20
3
92c
90
95
98
75
90
95
90
86
90
96
92
88
6
6
6
3,4-(MeO) C H
2
6
3
3-NO C H
2
6
4
4-ClC H4
H
6
C H5
CH3
CH3
CH3
CH3
CH3
6
4-ClC H4
6
4-BrC H4
6
10
11
12
13
4-PhC H4
6
3,4-(MeO) C H
2
6
3
4-BrC H4
CH Br
10
20
6
2
C H5
6
C H
6
5
1
4
15
85
1
5
30
15
a
All of the products are known compounds and identified by
b
c
comparison with authentic samples. Isolated yields. 2-phenyl-
,3-dioxolane remained intact after 30 min in the
presence of K Cr O without AlCl
1
2
2
7
3
Conclusions
We present new methodology for the oxidation of organic compounds under solid phase conditions.
In addition, the commercial availability and low cost of the reagent, high yields of the products, mild
reaction conditions, easy workup and short reaction times are noteworthy advantages of this method
and make this reagent practical bench-top oxidant.
Acknowledgements
We are thankful to the Isfahan University Research Council for partial support of this work.

Molecules 2001, 6
907
Experimental
General
All of the starting materials used in this work are commercially available or were prepared
according to published procedures [24-27]. All of the products are known compounds and were
identified by comparison of their physical and spectral data with those of authentic samples. Reported
yields refer to isolated pure products.
General Procedure for the Oxidation of Organic Compounds
A mixture of substrate (1 mmol), potassium dichromate (1-2 mmol) and aluminium chloride (1
mmol) in a mortar was ground with a pestle for the time specified in Tables 2-5. The progress of the
reaction was monitored by TLC or GLC. The mixture was extracted with CH Cl . The solvent was
2
2
evaporated and the resulting crude material was purified on a silica-gel plate or silica-gel column with
appropriate eluent to afford the pure product (Tables 2-5). In oxidation of alcohols, 1 mmol K Cr O
2
2
7
and in other cases, 2 mmol K Cr O was used
2
2
7
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