NOTE
Carbon-Based Solid Acid as An Efficient and Reusable Catalyst
for One-Pot Synthesis of Tetrasubstituted Imidazoles under
Solvent-Free Conditions
Tavakoli-Hoseini, Niloofar*
Davoodnia, Abolghasem
Department of Chemistry, Faculty of Sciences, Islamic Azad University, Mashhad Branch, Mashhad, Iran
Carbon-based solid acid catalyst was found to be highly efficient, eco-friendly and recyclable heterogeneous
catalyst for the multicomponent reaction of benzil, aromatic aldehyde, primary amine and ammonium acetate, giv-
ing rise to 1,2,4,5-tetrasubstituted imidazoles in good to excellent yields. The present methodology offers several
advantages, such as high yields, short reaction time, mild reaction condition and a recyclable catalyst with a very
easy work up.
Keywords carbon-based solid acid, heterogeneous catalysis, multicomponent reaction, tetrasubstituted imidazole,
solvent-free condition
Introduction
has some advantages such as ease of products separation,
recycling of the catalyst and environmental acceptability
as compared to liquid acid catalyst.10 Carbon-based solid
acid catalyst has many advantages. It is insoluble in
common organic solvents, causes low corrosion, and
shows environmental acceptability. Also the products
could be easily separated from the reaction mixture and
the catalyst is recoverable without decreasing its activity.
Therefore, it can be successfully used instead of sulfuric
acid as catalyst.11-15 To the best of our knowledge there
are no examples on the use of carbon-based solid acids
as catalyst for the synthesis of tetrasubstituted imida-
zoles.
As an important member of the five-membered ring
heterocycles, imidazole moiety is present in a wide
range of naturally occurring molecule.1 Compounds
with imidazole moiety have many pharmaceutical ac-
tivities.2 The biological importance of the imidazole
ring system has made it a common structure in numer-
ous synthetic compounds, such as fungicides,3 herbi-
cides,3 plant growth regulators4 and therapeutic agents.5
There are several methods reported in literature for the
synthesis of tetrasubstituted imidazoles, such as hetero-
Cope rearrangement,6 four component condensation of
1,2-diketones, aromatic aldehydes, primary amines and
ammonium acetate in the presence of several catalysts
such as K5CoW12O40•3H2O,7a BF3-SiO2,7b HY Zeolite,7c
silica gel/NaHSO4,7d molecular iodine,7e HClO4-SiO2,7f
and InCl3•3H2O,7g reaction of N-(2-oxo)amides with
ammonium trifluroracetate8 and condensation of a
1,2-diketone with an aryl nitrile and primary amine un-
der microwave irradiation.9 However, many of the syn-
thetic protocols for imidazoles reported above, suffer
from one or more disadvantages, such as harsh reaction
conditions, poor yields, prolonged reaction time period,
use of hazardous and often expensive acid catalysts.
Therefore, the development of simple, efficient,
high-yielding, and environmentally friendly methods
using new catalysts for the synthesis of these com-
pounds is still necessary.
In continuation of our previous works on the appli-
cations of reusable acid catalysts in the synthesis of or-
ganic compounds,16-18 herein we wish to report the new
efficient synthesis of tetrasubstituted imidazoles cata-
lyzed by carbon-based solid acid (Scheme 1).
Experimental
All chemicals were commercially available and used
without further purification. The catalyst was synthe-
sized according to the literature. Melting points were
recorded on an electrothermal type 9100 melting point
apparatus. The IR spectra were obtained on a 4300
1
Shimadzu spectrophotometer as KBr disks. The H
NMR (500 MHz) spectra were recorded on Bruker
DRX500 spectrometer.
Replacement of conventional toxic and pollutant
Bronsted and Lewis acid catalysts with environmentally
benign and reusable solid heterogeneous catalysts is an
active area of current research. Using solid acid catalysts
Preparation of carbon-based solid acid
The carbon-based solid acid was prepared according
to the reported procedure by Hara and co-workers.11
*
E-mail: niloofartavakoli@mshdiau.ac.ir; Tel.: +98-511-8416015; Fax: +98-511-8424020
Received April 24, 2010; revised July 16, 2010; accepted September 6, 2010.
Chin. J. Chem. 2011, 29, 203— 206
© 2011 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
203