Journal of Molecular Catalysis A: Chemical 246 (2006) 276–281
Highly efficient, mild and chemo- and stereoselective synthesis of
enaminones and enamino esters using silica supported
perchloric acid under solvent-free conditions
Biswanath Das
∗
, Katta Venkateswarlu, Anjoy Majhi, Majjigapu Ravinder Reddy,
Kuravallapalli Nagabhushana Reddy, Yerra Koteswara Rao,
Krishnan Ravikumar, Balasubramanian Sridhar
Indian Institute of Chemical Technology, Hyderabad 500007, India
Received 27 October 2005; received in revised form 22 November 2005; accepted 23 November 2005
Available online 28 December 2005
Abstract
Silica supported perchloric acid (HClO
4
·SiO
2
) has been utilized as a heterogeneous recyclable catalyst for a highly efficient and chemo- and
stereoselective conversion of -dicarbonyl compounds by treatment with amines at room temperature into -enaminones and -enamino esters
under solvent-free conditions.
© 2005 Elsevier B.V. All rights reserved.
Keywords: -Dicarbonyl compound; -Enaminone; -Enamino ester; HClO
4
·SiO
2
; Recyclable heterogeneous catalyst; Solvent-free conditions
1. Introduction
-Enaminones and -enamino esters are useful synthones
for the synthesis of various pharmaceuticals [1] and bioactive
heterocycles [2]. They have been utilized for the preparation
of different important antibacterial [3a,b], anticonvulsant [3b],
anti-inflamatory [3b] and antitumour agents [3b,c]. They are the
intermediates for the synthesis of several aminoacids [4a–c],
aminols [4b], peptides [4d] and alkaloids [4e,f]. Classically, -
enaminones are prepared by direct condensation of -dicarbonyl
compounds with amines under reflux in an aromatic solvent with
azeotropic removal of water [5]. Several other improved meth-
ods for the preparation of -enaminones and -enamino esters
have been reported to utilize Al
2
O
3
[6a], SiO
2
/microwaves [6b],
montmorillonite K-10 [6c], NaAuClO
4
[6d], Bi(TFA)
3
[6e],
Zn(ClO
4
)
2
·6H
2
O [6f], CeCl
3
·7H
2
O [6g], etc. However, most
of the methods suffer from certain drawbacks including long
reaction times, unsatisfactory yields, low selectivity, lack of
Part 64 in the series “Studies on novel synthetic methodologies.” IICT Com-
munication No. 051114.
∗
Corresponding author. Tel.: +91 40 27160512; fax: +91 40 27160512.
E-mail address: biswanathdas@yahoo.com (B. Das).
general applicability, higher temperatures, applications of non-
available and costly reagents, uses of hazardous solvents and
tedious experimental procedures. Thus, there is still a need to
develop a suitable method for the synthesis of enaminones and
enamino esters conveniently.
In continuation of our work [7] on the synthetic applications
of heterogeneous catalysts we have recently observed that silica-
supported perchloric acid (HClO
4
·SiO
2
) is a highly efficient
catalyst for the preparation of -enaminones and -enamino
esters from -dicarbonyl compounds by treatment with amines
(Scheme 1).
The generality of the method has been shown by the prepara-
tion of a series of -enaminones and -enamino esters using
various -dicarbonyls and amines (Table 1). The conversion
proceeded at room temperature and under solvent-free con-
ditions. The reaction took place within only a few minutes
to afford the product in excellent yields. As for an exam-
ple, acetylacetone reacted with aniline in the presence of
HClO
4
·SiO
2
to form the corresponding -enaminone (entry
3a) in 14 min (yield 98%) while in the presence of Bi(TFA)
3
[6e], Zn(ClO
4
)
2
·6H
2
O/MgSO
4
[6f] and CeCl
3
·7H
2
O [6g] the
reaction was reported to undergo in 1 h (yield 64%), 4 h (yield
95%) and 35min (yield 76%), respectively. The -dicarbonyl
1381-1169/$ – see front matter © 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2005.11.045