Molybdatophosphoric acid/NaNO2/wet SiO2 as an efficient system for the aromatization of 1,2-dihydroquinolines under mild and heterogeneous conditions

Article abstract of DOI:10.1080/00397910701196553

A combination of molybdatophosphoric acid and NaNO2 in the presence of wet SiO2 was used as an effective oxidizing agent for the oxidation of 1,2-dihydroquinolines to their corresponding quinoline derivatives in dichlomethane at room temperature with excellent yields. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/00397910701196553

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Molybdatophosphoric Acid/NaNO2/Wet SiO2
as an Efficient System for the Aromatization
of 1,2‐Dihydroquinolines under Mild and
Heterogeneous Conditions
Khodabakhsh Niknam a , Bahador Karami b , Saghar Molazadeh c & Azar
Pourkhosro c
a Department of Chemistry, Faculty of Sciences , Persian Gulf University ,
Bushehr, Iran
b Department of Chemistry , Yasouj University , Yasouj, Iran
c Islamic Azad University , Gachsaran, Iran
Published online: 13 Oct 2010.
To cite this article: Khodabakhsh Niknam , Bahador Karami , Saghar Molazadeh & Azar Pourkhosro
(2007) Molybdatophosphoric Acid/NaNO2/Wet SiO2 as an Efficient System for the Aromatization of
1,2‐Dihydroquinolines under Mild and Heterogeneous Conditions, Synthetic Communications: An
International Journal for Rapid Communication of Synthetic Organic Chemistry, 37:7, 1091-1096, DOI:
10.1080/00397910701196553
To link to this article: http://dx.doi.org/10.1080/00397910701196553
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Synthetic Communicationsw, 37: 1091–1096, 2007
Copyright # Taylor & Francis Group, LLC
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910701196553
Molybdatophosphoric Acid/NaNO2/Wet
SiO2 as an Efficient System for the
Aromatization of 1,2-Dihydroquinolines
under Mild and Heterogeneous Conditions
Khodabakhsh Niknam
Department of Chemistry, Faculty of Sciences, Persian Gulf University,
Bushehr, Iran
Bahador Karami
Department of Chemistry, Yasouj University, Yasouj, Iran
Saghar Molazadeh and Azar Pourkhosro
Islamic Azad University, Gachsaran, Iran
Abstract: A combination of molybdatophosphoric acid and NaNO2 in the presence of
wet SiO2 was used as an effective oxidizing agent for the oxidation of 1,2-dihydroqui-
nolines to their corresponding quinoline derivatives in dichlomethane at room tempera-
ture with excellent yields.
Keywords: aromatization, 1,2-dihydroquinolines, heterogeneous conditions, molybda-
tophosphoric acid, oxidation, quinoline
One of the most important objectives now is to adapt classical processes so
that pollution effects are kept to a minimum, with both a reduction in
energy and consumption of raw materials. In this respect, heterogeneous
systems are promising, and a new approach has been undertaken using hetero-
poly acid (HPA) chemistry. Catalysis by HPAs and related polyoxometallates
Received in Poland August 22, 2006
Address correspondence to Khodabakhsh Niknam, Department of Chemistry,
Faculty of Sciences, Persian Gulf University, Bushehr 75169, Iran. E-mail:
khniknam@gmail.com or niknam@pgu.ac.ir
1091

1092
K. Niknam et al.
is an expanding field of active research being perused worldwide.[1–4] HPAs
are environmentally benign solid acid catalysts, which offer several advan-
tages in terms of catalytic performance, strong acidic and redox sites, and
selectivity to particular reaction products by selective stabilization of
reaction intermediates.[2,3] HPAs, by virtue of their strong acid sites and
redox charactrizatics, have been used as catalysts under homogeneous as
well as heterogeneous conditions.[2,3] The advantage of using HPAs under
homogeneous condition lies in their high solubility in polar solvents such as
water, methanol, and acetonitrile. After completion of the catalytic cycles,
they can be easily isolated from the organic reaction media. These used
catalysts can be subsequently recrystallized and reused for successive
cycles. As far as catalysis is concerned, Keggin-type HPAs have been
widely investigated because of their high structural and thermal stability
with well-defined acidic and redox properties. The acidic properties of
Keggin-type HPAs have been exploited for several important reactions,
such as hydration of olefins, estrification, and condensation reactions of
chemically diverse organic substrates.[1–5] In recent years, there have been
extensive efforts to utilize the potential of the HPAs in synthetic organic
chemistry, such as deprotection of t-butyldimethylsilane,[6] regioselective
aerobic oxygenation of nitrobenzene to 2-nitrophenol,[7] and oxidation of
aliphatic, benzylic, and allylic alcohols using dimethyl sulfoxides as
oxygyen transfer agents.[8]
The quinoline skeleton has been used as the basis for the design of
many synthetic antimalarial compounds, of which chloroquinoline is one
example. The cynine dyes are also important commercial quinolines that
are used as sensitizers in photographic emulsion salt with N-ethylquinolinium
iodide.[9] Also, quinolines and alkylquinolines are widely used in metallurgy,
polymers, corrosive inhibitors, and toxicology, as analytical reagents, and in
the synthesis of agrochemicals.[10] There are several methods reported for the
oxidation of 1,2-dihydroquinolines, but most of them required vigorous con-
ditions or produce by-products that are difficult to remove from quino-
lines.[10–12] 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 permanganate (KMnO4),
chromium trioxide (CrO3), potassium chromate (K2CrO4), potassium dichro-
mate (K2Cr2O7),[13] and a combination of sodium hydrogen sulfate and
sodium dichromate.[14] These are all frequently used reagents, whether in
the laboratory or in industry, and yet they are be set with multiple liabilities.
For satisfactory and reproducible results, these oxidants demand rigorous
control of the experimental conditions. The other drawbacks of such
oxidants and their use in multistage organic synthesis, in spite of their
power, are their lack of selectivity, strong protic and aqueous conditions,
low yields of the products, and tedious workup. Furthermore, the elevated
reflux temperatures required by some oxidation procedures favor inopportune
secondary reactions. Likewise, the presence of strong acids or bases, which

Molybdatophosphoric Acid/NaNO2/Wet SiO2
1093
are required adjuncts as catalysts for some reactions, often leads to detrimen-
tal side reactions.[15]
Recently, we reported that molybdatophosphoric acid as an environmen-
tally benign catalyst for the aromatization of 1,4-dihydropyridines,[16]
N-nitrozation of secondary amines,[17] methoxymethylation of alcohols,[18]
and synthesis of bis-(indolyl)methanes.[19] In continuation of our studies on
HPAs, we now report a new system for the application of molybdatophospho-
ric acid/NaNO2 to overcome these limitations. In addition, for our purposes,
both clean and easy workup were also important.
This article outlines a convenient system for the effective oxidation of
1,2-dihydroquinolines (1) using H3PMo12O40 . xH2O/NaNO2/wet SiO2,
which appears to circumvent the drawbacks typically associated with these
reagents.
Several oxidation reactions of 1,2-dihydroquinolines were performed
with different amounts of reagents to find the most adequate conditions
to optimize the oxidation reaction under various conditions. In a prelimi-
nary study, the effect of the amount of molybdatophosphoric acid on
the yield of oxidation was investigated with the reaction of 1,2-
dihydroquinoline as a model compound. As shown in Table 1, 0.15 g of
H3PMo12O40 . xH2O is the best amount of acid catalyst for this oxidation
reaction in CH2Cl2 under heterogeneous conditions. The required molar
ratio, 1:2:0.15, of 1,2-dihydroquinoline, NaNO2, and catalyst at room
temperature in CH2Cl2 under heterogeneous conditions was found to be
optimal.
Different types of 1,2-dihydroquinolines (1) were subjected to the
oxidation reaction in the presence of molybdatophosphoric acid (I), NaNO2
(II), and wet SiO2 in dichloromethane (Scheme 1). The oxidation reactions
were performed under mild and heterogeneous conditions at room temperature
with excellent yields (Table 2).
The oxidation reaction did not occur in the absence of wet SiO2. This
observation suggests that the water molecule is essential for such processes.
The presence of wet SiO2 thus provides an effective heterogeneous surface
Table 1. Oxidation reaction of 1,2-dihydroquinoline (1 mmol) with NaNO2/wet SiO2
in the presence of acidic catalysts at room temperature in various conditions
Time
Conditions NaNO2/wet (conversion)
Amounts
of acid
Entry
Acid
SiO2/solvent
min (%)
1
2
3
4
5
H2SO4
H2SO4
1 mmol
1 mmol
0/0.2/CH2Cl2
2 (mmol)/0.2/CH2Cl2
0/0.2/CH2Cl2
1 (mmol)/0.2 g/CH2Cl2
2 (mmol)/0.2 g/CH2Cl2
300 (25)
120 (100)
200 (40)
200 (85)
45 (100)
H3PMo12O40 . xH2O 0.15 g
H3PMo12O40 . xH2O 0.1 g
H3PMo12O40 . xH2O 0.15 g

1094
K. Niknam et al.
Scheme 1.
area for insitu generation of NOX. It also eases the reaction workup. Oxidation
reactions using H3PMo12O40 . xH2O and without applying NaNO2 proceeded
slowly (Table 1, entries 1, 3).
This oxidation reaction can be readily carried out only by placing
compound 1, molybdatophosphoric acid (I), NaNO2 (II), wet SiO2 (50% w/w),
and CH2Cl2 as the solvent in a reaction vessel and efficiently stirring the
resulting heterogeneous mixture at room temperature for 35–45 min
Quinoline derivatives (2) can be obtained by simple filtration and evaporation
of the solvent. The results and oxidation conditions are given in Table 2.
In conclusion, practical and efficient oxidations of 1,2-dihydroquinolines
have been achieved by the methodology described. The notable advantages of
this method are mild reaction conditions, high yield, low cost, safety, and
ecofriendliness of the catalyst.
Table 2. Oxidation of 1,2-dihydroquinolines (1) to their corresponding quinoline
derivatives (2) with a combination of molybdatophosphoric acid I (0.15 g), NaNO2
II (2 mmol), and wet SiO2 (0.2 g) in CH2Cl2 at room temperature
Entry
Substrate
Time (min)
Product
Yield (%)a
1
2
3
4
5
6
7
8
1a
1b
1c
1d
1e
1f
45
35
35
45
45
35
35
45
2a
2b
2c
2d
2e
2f
89
90
90
91
90
87
88
90
1g
1h
2g
2h
aIsolated yield.

Molybdatophosphoric Acid/NaNO2/Wet SiO2
EXPERIMENTAL
1095
General
Chemicals were purchased from Fluka, Merck, and Aldrich chemical
companies. IR spectra were run on a Shimadzu infrared spectroscope IR-
1
435. H NMR was run on JEOL NMR spectrometer JNM-EXP 90A. With
TLC, using silica-gel SILG/UV 254 plates, the progress of reaction was
followed. The quinoline products were characterized by comparison of their
spectral (IR, 1H NMR), TLC, and physical data with authentic samples.[12,20]
General Procedure
A suspension of compound 1 (1 mmol), molybdatophosphoric acid I (0.15 g),
NaNO2 II (2 mmol), and wet SiO2 (0.2 g) in dichloromethane (5 mL) was
stirred at room temperature for the specified time in Table 2. The progress of
reaction was monitored by TLC and then filtered. The filtrate was washed
with CH2Cl2 (2 Â 5 mL), then water (10 mL) was added, and it was neutralized
with NaHCO3 (10%). The organic layer was dried over anhydrous Na2SO4
(3 g), and evaporation of the solvent on a rotary evaporator afforded a
residue, which was passed through a short pad of silica gel using a mixture
of ethyl acetate and n-hexane as eluent to afford the quinoline product.
Typical Procedure for the Preparation of 2-Methylquinoline (2b)
A suspension of compound 1b (1 mmol, 0.145 g), molybdatophosphoric acid I
(0.15 g), NaNO2 II (2 mmol, 0.14 g), and wet SiO2 (0.2 g) in dichloromethane
(5 mL) was stirred at room temperature for 35 min. The progress of reaction
was monitored by TLC and then filtered. The filtrate was washed with
CH2Cl2 (2 Â 5 mL), then water (10 mL) was added, and it was neutralized
with NaHCO3 (10%). The organic layer was dried over anhydrous Na2SO4
(3 g), and evaporation of the solvent on a rotary evaporator afforded a
residue, which was passed through a short pad of silica gel using a mixture
of ethyl acetate and n-hexane as eluent to afford 2-methylquinoline
(0.128 g, 90% yield).
IR (KBr): 3090, 2990, 2852, 1600, 1560, 1490, 1375, 840, 800 cm21. 1H
NMR (CDCl3, 90 MHs): d 2.2 (s, 3H), 6.9–7.3 (m, 3H), 7.7 (m, 2H), 8.6 (d, 1H).
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