Tetrahedron Letters
Michael addition of indoles to b-nitrostyrenes catalyzed by HY zeolite
under solvent-free conditions
Mariappan Jeganathan a, Kuppusamy Kanagaraj a, Amarajothi Dhakshinamoorthy a,b, Kasi Pitchumani a,b,
⇑
a School of Chemistry, Madurai Kamaraj University, Madurai 625 021, India
b Centre for Green Chemistry Processes, School of Chemistry, Madurai Kamaraj University, Madurai 625 021, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Michael addition of indoles to b-nitrostyrenes is reported using HY zeolite as catalyst under mild reaction
conditions. This methodology allows the synthesis of various indole derivatives in good to high yields at
50 °C under solvent-free conditions. The short reaction time and achieving high yield of the desired
products are the main advantages of the present work. The catalyst can be easily recovered and reused
for six successive runs without considerable changes in yields. This Michael addition catalyzed by HY
zeolite is operationally simple and can be considered as a greener protocol as it avoids the use of
corrosive acids and toxic reagents.
Received 4 January 2014
Revised 22 January 2014
Accepted 24 January 2014
Available online 1 February 2014
Keywords:
Michael addition
HY zeolite
Ó 2014 Elsevier Ltd. All rights reserved.
Indole
b-Nitrostyrenes
C–C bond formation
Michael addition is one of the most important reactions for the
construction of carbon–carbon bonds in organic reactions.1 Among
the Michael acceptors, nitroolefins are very attractive, since the
nitro moiety is a strong electron-withdrawing group that can be
readily transformed into a range of different functionalities.2,3
Indole and many of its derivatives are relevant units in many
naturally occurring compounds, because of their pharmacological
and biological properties.4 Due to the increased nucleophilic
reactivity of C3 position of indole ring, it is often used for the
subsequent transformations leading to different indole alkaloids.
In general, indole 3-derivatives were prepared by the Michael
addition of indoles in the presence of samarium(III) iodide,5
and efficient method is always desirable for the conjugate addition
of indoles to electron-deficient olefins.
Zeolites are microporous, crystalline aluminosilicates built from
infinitely extending three-dimensional network of SiO4 and AlO4
tetrahedra that are linked together through oxygen bridges. By vir-
tue of their structure, crystallinity, and variable stoichiometry, zeo-
lite catalysts have well defined pore size distributions, adjustable
acidity, very high surface area and good thermal stability.22 Zeo-
lites are also efficient inorganic solid acids, which are cheap and
show high activities in organic reactions under heterogeneous
reaction conditions.23 They have been extensively used as solid
heterogeneous catalysts in organic synthesis24 due to their
non-corrosive and non-toxic nature. In particular, HY zeolite has
been reported as catalyst for the synthesis of heterocyclic
compounds like imidazoles,25a tetrahydrocarbazoles,25b bis
(indolyl)methanes,25c benzimidazoles,25d polyhydroquinolines,25e
1,4-dihydropyridines,25f and quinzolin-4(3H)ones.25g In continua-
tion of our interest on the development of heterogeneous catalysts
for organic transformations,26 herein we report a new and efficient
method for the Michael addition of indoles to b-nitrostyrenes in
the presence of HY zeolite as a catalyst under mild and solvent-free
conditions. Although zeolites have been used as catalysts for the
synthesis of many heterocycles as mentioned above, this is the first
report where the synthesis of indole derivatives being carried out
in the presence of HY zeolite.
CeCl3Á7H2O–NaI/SiO2,6 Yb(OTf)3,7 InCl3,8 InBr3,9 iodine,10 sulfamic
14
acid,11 boric acid,12 Bi(OTf)3,13 H3PW12O40
,
basic alumina,15
carbohydrate-based tolylsulfonyl hydrazines,16 K10 montmorillon-
ite,17 b-cyclodextrin,18 silica sulfuric acid,19 and Sc(OTf)3 as
20
catalysts. Recently catalyst-free conjugate addition of indole to
b-nitrostyrene was reported in aqueous medium21 at 100 °C with
5–15 h of extended reaction time. Many of these reported proce-
dures involve strongly acidic conditions, expensive reagents, and
longer reaction times, and involve tedious procedures for the
isolation of product. Also in some cases lower yield of products
were observed due to dimerization of indole or polymerization of
the nitroalkene. To overcome these problems, developing a simple
The reaction conditions were optimized for the Michael addi-
tion between indole and b-nitrostyrene under various parameters
⇑
Corresponding author. Tel.: +91 452 2456614; fax: +91 452 2459181.
0040-4039/Ó 2014 Elsevier Ltd. All rights reserved.