Oxidation of 1,2-dihydroquinolines under mild and heterogeneous conditions

Article abstract of DOI:10.1081/SCC-100105895

A combination of NaHSO₄.H₂O and Na₂Cr₂O₇.2H₂O in the presence of wet SiO₂ were used as an effective oxidizing agent for the oxidation of 1,2-dihydroquinolines to their corresponding quinoline derivatives in dichloromethane at room temperature with excellent yields.

Full text of DOI:10.1081/SCC-100105895

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OXIDATION OF 1,2-DIHYDROQUINOLINES UNDER MILD
AND HETEROGENEOUS CONDITIONS
Jafar Asgarian Damavandi a , Mohammad Ali Zolfigol b & Bahador Karami a
a Faculty of Chemistry, University of Teacher Training, Tehran, I.R., Iran
b Chemistry Department, College of Science, Bu-Ali Sina University, Post Box No. 4135,
Hamadan, I.R., 65174, Iran
Version of record first published: 16 Aug 2006.
To cite this article: Jafar Asgarian Damavandi , Mohammad Ali Zolfigol & Bahador Karami (2001): OXIDATION OF 1,2-
DIHYDROQUINOLINES UNDER MILD AND HETEROGENEOUS CONDITIONS, Synthetic Communications: An International Journal
for Rapid Communication of Synthetic Organic Chemistry, 31:20, 3183-3187
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SYNTHETIC COMMUNICATIONS, 31(20), 3183–3187 (2001)
OXIDATION OF
1,2-DIHYDROQUINOLINES
UNDER MILD AND
HETEROGENEOUS CONDITIONS
Jafar Asgarian Damavandi,1 Mohammad Ali Zolfigol,2,
*
and Bahador Karami1
1Faculty of Chemistry, University of Teacher Training,
Tehran, I.R. Iran
2Chemistry Department, College of Science,
Bu-Ali Sina University, Hamadan, 65174,
Post Box No. 4135, I.R. Iran
ABSTRACT
A combination of NaHSO4.H2O and Na2Cr2O7.2H2O in the
presence of wet SiO2 were used as an effective oxidizing agent
for the oxidation of 1,2-dihydroquinolines to their corre-
sponding quinoline derivatives in dichloromethane at room
temperature with excellent yields.
The quinoline skeleton has been used as the basis for the design of
many synthetic antimalarial compounds, of which chloroquine, is one
example. The cyanine dyes are also important commercial quinolines
*Corresponding author. E-mail: zolfi@basu.ac.ir
3183
Copyright & 2001 by Marcel Dekker, Inc.
www.dekker.com

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DAMAVANDI, ZOLFIGOL, AND KARAMI
which are used as sensitizers in photographic emulsions salt with N-ethyl-
quinolinium iodide.1 There are several methods that are reported for the
oxidation of 1,2-dihydroquinolines, but most of them required vigorous
conditions or produce by-products which are difficult to remove from
quinolines.2–4 For oxidation of organic functionalities, one turns often to
high-valent metal oxides or their mineral salts. Classic reagents of this
type are manganese dioxide (MnO2), potassium permangenate (KMnO4),
chromium trioxide (CrO3), potassium chromate (K2CrO4), and potassium
dichromate (K2Cr2O7).5 These are all frequently-used reagents, whether
in the laboratory or in industry, and yet they are be set with multiple
liability. For satisfactory and reproducible results, these oxidants demand
rigorous control of the experimental conditions. The other drawbacks
against such oxidants and their use in multistage organic synthesis, in
spite of their power, are also their lack of selectivity, strong protic and
aqueous conditions, low yields of the products, and tedious work-up.6
Furthermore, the elevated reflux temperatures required by some oxidation
procedures will favor inopportune secondary reactions. Likewise, the pres-
ence of strong acids or bases, which are required adjuncts as catalysts
for some reactions, often leads to detrimental side reactions.7 Therefore,
we were interested to find a new system for the application of chromium
salts to overcome the above limitations. In addition, for our purpose both
clean and easy work-up were also important. Very recently, we among many
others have demonstrated that heterogeneous reagent systems have many
advantages such as simple experimental procedures, mild reaction con-
ditions and minimization of chemical wastes as compared to their liquid
phase counterparts.8–9 Unexpectedly, our reported procedures for the
oxidation of 1,4-dihydropyridines (via in situ generation of NOþ and
NOþ2 under mild and heterogeneous conditions),9 are not suitable for the
oxidation of 1,2-dihydroquinolines. The above facts, encouraged us to seek
a completely heterogeneous system for the oxidation of 1,2-dihydroquino-
lines, and we have investigated a number of different reaction conditions
based upon the in situ generation of H2CrO4 at the surface of SiO2 in
low concentration by very mild inorganic acidic salt [e.g. NaHSO4.H2O,
Pkaꢀ2] and sodium dichromate. This article outlines a convenient system
for the effective oxidation of 1,2-dihydroquinolines (1) using chromium
salts, which appears to circumvent the drawbacks typically associated
with these reagents.
Different types of 1,2-dihydroquinolines (1) were subjected to oxida-
tion reaction in the presence of Na2Cr2O4.2H2O (I), NaHSO4.H2O (II) and
wet SiO2 in dichloromethane (Scheme). The oxidation reactions were per-
formed under mild and heterogeneous conditions at room temperature with
excellent yields (Table).

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Scheme.
Table. Oxidation of 1,2-Dihydroquinolines (1) to Their Corresponding Quinoline
Derivatives (2) with a Combination Na2Cr2O4.2H2O (I), NaHSO4.H2O (II) and Wet
SiO2 in CH2Cl2 at Room Temperature
(Reagent/Substrate)a
Time
Yieldb
(%)
Entry
Substrate
Product
I
II
(Min)
1
3
4
5
6
1a
1c
1d
1e
1f
2a
2c
2d
2e
2f
0.33
0.33
0.33
0.33
0.33
2.66
2.66
2.66
2.66
2.66
10
8
8
10
10
97
99
95
95
97
b
aWet SiO2 : substrate (0.3 g : 1 mmol). Isolated yields.
This oxidation reaction can be readily carried out only by placing
Na2Cr2O4.2H2O (I), NaHSO4.H2O (II), 1, wet SiO2 (50% w/w) and CH2Cl2
as the solvent in a reaction vessel and efficiently stirring the resulting hetero-
geneous mixture at room temperature for 5–10 minutes and the quinoline
derivatives (2) can be obtained by simple filtration and evaporation of the
solvent. The results and reaction conditions are given in the Table.
In conclusion, practical and efficient oxidations of 1,2-dihydroquino-
lines have been achieved by the methodology described. The heterogeneous
nature, low cost and the availability of the reagents, easy procedure and
work-up make this method attractive for the large-scale operations. We

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DAMAVANDI, ZOLFIGOL, AND KARAMI
believe that the present methodology would be an important addition to
existing methodologies.
EXPERIMENTAL SECTION
General: Chemicals were purchased from Fluka, Merck, Riedel-
dehaen AG and Aldrich chemical companies. The oxidation products
were characterized by comparison of their spectral (IR, H-NMR,
13C-NMR), TLC and physical data with the authentic samples. All 1,2-
dihydroquinolines were synthesized by the reported procedures.4,10
Oxidation of 1,2-Dihydroquinoline (1a) to the Corresponding
Substituted Quinoline (2a). A Typical Procedure
A suspension of compound 1a (0.393 g, 3 mmol), Na2Cr2O4.2H2O
[(I) 0.297 g, 1 mmol], wet SiO2 (50% w/w) (0.6 g) and NaHSO4.H2O [(II)
1.104 g, 8 mmol], in dichloromethane (6 ml) was stirred at room temperature
for 10 minutes. The progress of reaction was monitored by TLC and then
filtered. Anhydrous Na2SO4 (5 g) was added to the filtrate. After 15 min
dichloromethane (20 ml) was added to the resulting mixture and filtered.
Dichloromethane was removed by a water bath (40–50ꢁC) and simple dis-
tillation. The yield was 0.379 g, (98%) of quinoline (2a).
ACKNOWLEDGMENTS
Financial support for this work by the research affairs of University of
Teaching Training, Tehran, I. R. Iran, and also Bu-Ali Sina University,
Hamadan, I. R. Iran, are gratefully acknowledged.
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Received in the UK August 23, 2000

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