Silica sulfuric acid as an inexpensive and recyclable solid acid catalyzed efficient synthesis of quinolines

Article abstract of DOI:10.1007/s00706-005-0413-9

Silica sulfuric acid as an inexpensive and recyclable solid acid efficiently catalyzes the Fridedlaender synthesis of quinolines through a condensation reaction of a 2-aminoaryl ketone with an activated α-CH acid compound under solvent-free conditions in

Full text of DOI:10.1007/s00706-005-0413-9

Monatshefte f€ur Chemie 137, 181–184 (2006)
DOI 10.1007/s00706-005-0413-9
Silica Sulfuric Acid as an Inexpensive
and Recyclable Solid Acid Catalyzed
Efficient Synthesis of Quinolines
ꢀ
Ahmad Shaabani , Ebrahim Soleimani, and Zahra Badri
Department of Chemistry, Shahid Beheshti University, P.O. Box 19396-4716, Tehran, Iran
Received August 16, 2005; accepted (revised) September 22, 2005
Published online December 29, 2005 # Springer-Verlag 2005
Summary. Silica sulfuric acid as an inexpensive and recyclable solid acid efficiently catalyzes the
Fridedl€ander synthesis of quinolines through a condensation reaction of a 2-aminoaryl ketone with an
activated ꢀ-CH acid compound under solvent-free conditions in high yields at 100ꢁC.
Keywords. Silica sulfuric acid; Quinolines; Solvent-free.
Introduction
It is well known that quinolines exhibit a wide range of biological activities [1–3]
and are valuable reagents for the synthesis of nano- and mesostructures with
enhanced electronic and photonic properties [4].
The classic version of the Fridedl€ander quinoline synthesis, which combines a
2-aminoaryl ketone and a carbonyl compound containing an activated ꢀ-CH acid
under acidic or basic conditions by refluxing in an aqueous or alcoholic solution to
give quinoline, has been extensively explored [5]. In a typical procedure, the cat-
alyst is varied from strong protic inorganic liquid acids [6], such as HCl, H2SO4,
and polyphosphoric acid, to Lewis acids [7], such as ZnCl2, NaAuCl6, and
AuCl3 ꢂ 3H2O, and transition-metals [8], such as ruthenium and palladium.
However, in spite of their potential utility, these homogeneous catalysts pres-
ent limitations due to the use of toxic and corrosive reagents, the tedious work-up
procedure, the necessity of neutralization of the strong acid media producing
undesired washes, long reactions times, and high temperature. Moreover, the
synthesis of these heterocycles has been usually carried out in a polar solvent
such as THF, DMF, or DMSO leading to complex isolation and recovery proce-
dures. Therefore, the discovery of a novel and inexpensive catalyst, which can be
ꢀ
Corresponding author. E-mail: a-shaabani@cc.sbu.ac.ir

182
A. Shaabani et al.
Scheme 1
easily separated, reused, and does not become contaminated by the products, is of
prime importance.
In connection with our previous work using silica sulfuric acid and other solid
acid catalysts in organic transformations [9], we wish to report the results of
a study of the preparation of quinolines with silica sulfuric acid as an inexpen-
sive and recyclable catalyst, which is easily separated from reaction mixture
(Scheme 1).
Results and Discussion
The procedure gives the product in high yields and avoids problems associated
with solvent and liquid acids use (cost, handling, safety, pollution, corrosiveness,
separation, and recovery). Decreased reaction times are also realized because of the
increased reactivity of the reactant in the solid state and the fact that the other
reaction product, water, evaporates at the reaction temperature of 100ꢁC. The
catalyst is simply prepared by the reaction of chlorosulfonic acid with commer-
cially available silica and the by-product, HCl gas, is easily removed from the
reaction vessel [10].
In a blank experiment no desired product was observed under similar reaction
conditions in the absence of catalyst within 24 h. Catalyst reuse studies were car-
ried out by recycling the catalyst for the reaction of 2-amino-5-chlorobenzophe-
none with ethyl acetoacetate on a 3 mmol scale. This was done by washing the
reactants and product with EtOH and charging the reaction vessel with fresh sub-
strate and then repeating the experiment. Interestingly, the recycled catalyst could
be used for at least four reaction cycles, corresponding to an overall turnover
number of 345 (Table 1, Entry 5). These results clearly show the practical reus-
ability of the silica sulfuric acid catalyst under the described reaction conditions. To
explore the scope and limitations of this catalyst further, we have extended it to
various ꢁ-dicarbonyl compounds and cyclohexanone. As indicated in Table 1, the
reaction proceeds efficiently with various kinds of ꢁ-dicarbonyl compounds, but
the yield of the reaction with cyclohexanone was lower.
In conclusion, we have shown that the silica based solid acid catalyst, which
can be easily prepared from commercially available starting materials efficiently
catalyzes the Fridedl€ander synthesis of quinolines through the condensation of
2-aminoaryl ketone with activated ꢀ-CH acid compounds. Because solid acids
are environmentally friendly with respect to corrosiveness, safety, waste, and ease
of separation and recovery, replacement of liquid acids with solid acids is desirable
in the chemical industry.

Silica Sulfuric Acid
183
Table 1. Silica sulfuric acid catalyzed Fridedl€ander synthesis of quinolines under solvent-free
conditions at 100ꢁC
Entry
1
2-Aminoaryl
ketone
CH-acid
Product
Yield=%
(Time=min)
MP=ꢁC Found
(Reported)
99 (45)
99 (45)
85 (30)
87 (55)
111–112 (115)a
2
3
4
5
154 (151)b
133–135
99–100 (98)a
102–105 (108)b
83 (60)
84 (120)
87 (360)
79 (360)c
86 (120)
6
7
191 (195)a
99 (120)
65 (60)
209–211
8
162–164 (163)b
a
b
Ref. [11]; Ref. [12]; the same catalyst was used for each of the four runs
c
Experimental
Melting points were measured on an Electrothermal 9100 apparatus. Mass spectra were recorded
1
on a FINNIGAN-MAT 8430 mass spectrometer operating at an ionization potential of 70eV. H and
13C NMR spectra were recorded on a BRUKER DRX-300 AVANCE spectrometer at 300.13 and
1
75.47 MHz. H and 13C NMR spectra were obtained on solutions in CDCl3. 2-Aminoaryl ketones
and ketones were obtained from Fluka and Merck and were used without purification.
General Procedure
A mixture of 1.0 mmol 2-aminoaryl ketone, 1.2 mmol ꢀ-CH acid, and 0.16g silica sulfuric acid [10]
(0.4mmol) was heated under solvent-free conditions with stirring at 100ꢁC. After completion of the
reaction as indicated by TLC, the reaction mixture was washed with hot ethanol (2ꢃ10cm3), the
filtrate was concentrated, and the solid product was recrystallised from ethanol.
All products (except entries 3 and 7) are known compounds, which were characterized by IR and
1H NMR spectral data and their mp values were compared with literature reports.

184
A. Shaabani et al.: Silica Sulfuric Acid
6-Chloro-2-methyl-4-phenyl-quinoline-3-carboxylic acid methylester (3, C18H14ClNO2)
White solid, mp 133–135ꢁC; IR (KBr): ꢂꢀ¼ 3070, 2925, 1715 cmꢄ1; MS: m=z (%) ¼ 311 (Mþ, 85),
1
279 (87), 252 (45), 217 (100), 176 (60), 150 (20), 108 (25), 87 (35), 50 (25); H NMR (CDCl3):
ꢃ ¼ 2.69 (s, CH3), 3.51 (s, O–CH3), 7.2–7.96 (m, 8H-arom) ppm; 13C NMR (CDCl3): ꢃ ¼ 23.77 (CH3),
52.31 (O–CH3), 125.26, 125.87, 128.05, 128.52, 128.84, 129.14, 130.54, 131.23, 132.41, 134.93,
145.57, 146.13, 154.93 (arom), 168.66 (C¼O) ppm.
7-Chloro-3,3-dimethyl-9-phenyl-3,4-dihydro-2H-acridin-1-one (7, C21H18ClNO)
Yellow solid, mp 209–211ꢁC; IR (KBr): ꢂꢀ¼ 3065, 2945, 1692 cmꢄ1; MS: m=z (%) ¼ 335 (Mþ, 55),
1
334 (100), 306 (40), 279 (35), 251 (20), 216 (75), 189 (30), 151 (15), 121 (25), 39 (50); H NMR
(CDCl3): ꢃ ¼ 1.09 (s, 2CH3), 2.5, 3.19 (2s, 2CH2), 7.08–7.95 (m, 8H-arom) ppm; 13C NMR (CDCl3):
ꢃ ¼ 28.34 (CH3), 32.27 (C), 48.29, 54.18 (2CH2), 123.30, 126.79, 127.91, 127.99, 128.22, 128.37,
130.18, 132.45, 132.54, 136.80, 147.36, 150.14, 161.45, 154.93 (arom), 197.73 (C¼O) ppm.
Acknowledgements
We gratefully acknowledge the financial support from the Research Council of Shahid Beheshti
University.
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