Silica gel catalyzed synthesis of quinophthalone pigments under solvent-free conditions using microwave irradiation

Article abstract of DOI:10.3390/70200135

Condensations of anhydrides and quinaldine derivatives are accelerated by microwave irradiation under solvent free conditions in the presence of silica gel as catalyst.

Full text of DOI:10.3390/70200135

Molecules, 2002, 7, 135-139
molecules
ISSN 1420-3049
http://www.mdpi.org
Silica gel Catalyzed Synthesis of Quinophthalone Pigments Under
Solvent-Free Conditions Using Microwave Irradiation
Hossein Loghmani-Khouzani*, Majid M. Sadeghi and J. Safari
Department of Chemistry, Faculty of Sciences, University of Isfahan, Isfahan 81744, I. R. Iran.
Fax: (+98) 311 6689732.
* Author to whom correspondence should be addressed; e-mail: h.log119@sci.ui.ac.ir
Received: 28 October 2001; in revised form: 28 January 2002 / Accepted: 1 February 2002 / Published:
28 February 2002
Abstract: Condensations of anhydrides and quinaldine derivatives are accelerated by
microwave irradiation under solvent free conditions in the presence of silica gel as catalyst.
Keywords: Quinophthalone; Pigments; Silica gel; Solvent-Free Conditions; Microwaves.
Introduction
Quinophthalones have achieved great significance in organic synthesis [1-4]. Many studies have shown
these compounds to be rich sources of innovative chemistry as a number of pharmaceutical products and
dyestuffs contain this ring system [5-11]. Their ability to act as ligands that form stable complexes with
different cations is also well known [12]. Several methods for the preparation of quinophthalones have
been reported such as treatment of phthalic anhydrides and a mixture of quinaldine with nitrobenzene
[13], ZnCl2 [14-17] or sodium hydride [18] and reactions of quinoline N-oxides/β-diketones [19]. Some of
the methods mentioned above suffer from various drawbacks including long reaction times, low yield of
products, difficult operating conditions and tedious work-up. Recently the application of microware
irradiation has gained popularity in organic synthesis [20-25], particularly in chemical reactions with
reagents impregnated on solid mineral supports, especially those that are efficient in dry media, due to
their selectivity, high reaction rates, cleaner formation of products and operational simplicity. In this
connection, we now report a simple synthetic procedure for the preparation of quinophthalone pigments
under microwave irradiation catalyzed by silica gel (Scheme 1).

Molecules, 2002, 7
136
Scheme 1
O
O
R2
R1
R1
M.W .
Silicagel
O
+
O
CH3
N
H
N
R2
O
2
3
1
80-97%
Results and Discussion
To choose the best among the various common solid supports such as silica gel, alumina and
montmorillonite K-10 clay, etc., a comparative study was done using phthalic anhydride and 2-methyl-
quinoline (1) as a representative starting material (Table 1). Samples were irradiated with microwave
radiation (700 W) in the presence of a variety of solid supports. The results indicated that silica gel was
clearly the best catalyst for our purposes and consequently this substance was selected as the support for
subsequent experiments.
Table 1. Quinophthalone preparation in the presence of different catalysts.
Quinophthalone
Time
Run
Catalyst
yield (%)
(min.)
15
35
30
97
8
5
5
2
None
1
2
3
4
Alumina
K-10 Clay
Silica gel
Next a variety of quinophthalones were prepared from a variety of phthalic anhydrides and 2-methyl-
quinolines under microwave irradiation (700 W) in the presence of silica gel. All reactions were easily
performed in a beaker and product formation was noted observed after 2 min. The results are summarized
in Table 2.
In all the 1H-NMR spectra of the quinophthalone products the NH group appeared in the δ 14-16 region
as a broad singlet. The integral of this signal was proportional to one proton and no signal was observed in
the aliphatic region corresponding to a C-H. From this data we conclude that the compounds are
13
exclusively in the enaminone form. The C-NMR data was also consistent with the proposed structures
in all cases. In the IR spectra the C=O/C=C groups were observed around 1635-1670 cm-1 [26-32].

Molecules, 2002, 7
137
Table 2. Preparation of quinophthalones from phthalic anhydrides and 2-methyl-
quinolines under microwave irradiation.
Yield b
(%)
97
m. p.
(found) (oC)
240-241
>345
m. p. (lit.)
(oC)
Entry
R1
R2
Producta
H
H
H
Cl4
240 (13)
>310 (13)
1
2
3a
3b
95
O
O
H
94
>370
>396 (33)
3
3c
5,6
O
H
5, 6-(COOMe)2
91
85
87
86
85
93
82
362-363
241-242
270-271
266-267
>350
202-203
360
363 (33)
241 (13)
270 (31)
267 (13)
>310 (13)
203 (16)
361 (32)
4
5
3d
3e
3f
4-Me
4-Ph
3-OH
3-OH
6-Me
H
H
H
H
6
7
3g
3h
3i
Cl4
H
1 or 4-NO2
8
9
c
10
3j
a All products exhibited spectral data (IR, 1H-and 13C-NMR) consistent with their proposed structures.
b Yields refer to isolated pure products.
c The exact location of the NO2 group is not clear.
Conclusions
We believe the reported method offers a mild, simple and efficient route for the preparation of
quinophthalone pigments. Its ease of work up, high yields and short reaction times make it a useful
addition to modern synthetic methodologies.
Acknowledgments
The authors are thankful to Isfahan University Research Council for financial support of this work.
Experimental
General
All products are known compounds and their physical data, IR and NMR spectra were essentially
identical with those of authentic samples. IR spectra were recorded as KBr pellets on a Shimadzu IR-470
spectrometer. 1H-NMR spectra were determined on a Bruker AC80 or JEOL EX-90 instruments.

Molecules, 2002, 7
138
A typical experimental procedure
The preparation of quinoline yellow (3a), is illustrative of the general procedure employed. To a
mixture of 2-methylquinoline (1 mmol, 143 mg) and phthalic anhydride (1 mmol, 148 mg) placed in an
open glass container, silica gel (silica gel 60, 230-240 mesh Merck, 300mg) was added and the reaction
mixture was irradiated in a microwave oven at 700 W power for 2 minutes. Upon completion of the
reaction, as followed by TLC, the product was extracted into ethanol (3 x 10 mL). The solvent was
evaporated and the resulting crude material was purified on a silica gel plate to afford 2-(2-quinolinyl)-
1H-indene-1,3-(2H)-dione (3a) in 97% yield (cf. Table 2). 1H-NMR (CDCl3) δ 7.41-8.61 (m, 11H,
aromatic H), 14.18 (s, 1H, NH); 13C-NMR (CDCl3) δ 99.45-151.21 (aromatic C), 190.80, 194.87
(carbonyl C); IR (KBr) 1710, 1650, 1610, 1565 cm-1.
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Samples Availability: Compounds 1-10 are available from MDPI.
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