SCHEME 1
Rea ction s on a Solid Su r fa ce. A Sim p le,
Econ om ica l a n d Efficien t F r ied el-Cr a fts
Acyla tion Rea ction over Zin c Oxid e (Zn O)
a s a New Ca ta lyst
Mona Hosseini Sarvari and Hashem Sharghi*
a typical Friedel-Crafts acylation reaction, an aromatic
compound undergoes electrophilic substitution with an
acylating agent in the presence of more than 1 equiv of
acid catalyst (e.g., anhydrous AlCl3) as a result of its
complexation by the ketone product. The catalyst used
cannot be easily recovered and recycled. Therefore, a
large amount of toxic waste is generated. To solve this
problem, some catalytic Freidel-Crafts acylations have
recently been developed. Ln(OTf)3-LiCLO4,3 TiCl(OTf)3-
TfOH,4 Re-Br(CO)5,5 LiClO4-acylhydride complex,6 FeCl3
over K10,7 and clay catalysts8 have been reported as
catalysts for Friedel-Crafts acylation. In addition, the
use of inorganic solid supported reagents9 or solid acids10
as catalysts, resulting in higher selectivity, easier work-
up, and environmentally safe reactions, have been re-
ported. For instance, HZSM-5 zeoilte has been reported
to promote the liquid-phase acylation of anisole with
carboxylic acids.11 However, it presents limitations with
regard to generality and efficiency. Recently, it was also
shown that zinc powder promotes the acylation of acti-
vated and unactivated aromatics under microwave ir-
radiation.12
Although numerous methods to achieve Friedel-Crafts
acylation are known, newer methods continue to attract
attention for their experimental simplicity and effective-
ness. We have been interested in the development of
methods for Friedel-Crafts acylation that (a) would avoid
the use of added acids and bases, (b) would avoid aqueous
workup and chromatographic purification, (c) are easy
to perform, and (d) are economical for application to
large-scale preparations. In this pursuit, we have recently
reported on the use of ZnO for Beckmann rearrange-
ment.13 During the course of our studies aimed at
developing solvent-free procedures,13,14 we have now
discovered that ZnO alone promotes a very efficient
Friedel-Crafts acylation of activated and unactivated
Department of Chemistry, Faculty of Science,
Shiraz University, Shiraz 71454, I.R.Iran
shashem@chem.susc.ac.ir
Received April 3, 2004
Abstr a ct: Zinc oxide (ZnO) brings about a rapid Friedel-
Crafts acylation of a range of activated and unactivated
aromatic compounds such as anisole and chlorobenzene with
acid chlorides in solvent-free conditions at room tempera-
ture. The ZnO powder can be reused up to three times after
simple washing with dichloromethane.
Synthetic chemists continue to explore new methods
to carry out chemical transformations. One of these new
methods is to run reactions on the surface of solids. As
the surfaces have properties that are not duplicated in
the solution or gas phase, entirely new chemistry may
occur. Even in the absence of new chemistry, a surface
reaction may be more desirable than a solution counter-
part, because the reaction is more convenient to run or
a high yield of product is attained. For these reasons,
surface synthetic organic chemistry is a rapidly growing
field of study.
Experiments using these solid-phase catalysts gener-
ally have the following features: (i) it is often easy to
isolate the products and to separate the catalyst; (ii)
comparing the reaction conditions with those of related
homogeneous reactions, they are so mild that a high yield
of specific products and suppression of byproduct forma-
tion are expected; and (iii) selectivity and activity of the
catalysts are often comparable to those of enzymes.1
Several classes of solids have commonly been used for
surface organic chemistry, including aluminas, silica gels,
clays, etc.2 Zinc oxide (ZnO), an inexpensive and com-
mercially available inorganic solid, is certainly one of the
most interesting of these solids. In this paper we de-
scribed our work to reach a successful ZnO catalyst for
Friedel-Crafts acylation reactions.
(3) Kawada, A.; Mitamura, S.; Kobayashi, S. J . Chem. Soc., Chem.
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The Friedel-Crafts acylation of aromatic compounds
is an important transformation in organic synthesis. In
* To whom correspondence should be addressed. Tel: +98-711-
2284822. Fax: +98-711-2280926.
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10.1021/jo0494477 CCC: $27.50 © 2004 American Chemical Society
Published on Web 08/28/2004
J . Org. Chem. 2004, 69, 6953-6956
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