An active catalyst for Diels-Alder reaction of indol with o-benzoquinone

Article abstract of DOI:10.14233/ajchem.2017.20427

The Diels-Alder reaction of indol with masked o-benzoquinone was studied. The stereo selectivity of this reaction by heterogeneous catalysis is improved. This catalyst was prepared by surface modification of nanoporous silica gel with cerium ion in aqueou

Full text of DOI:10.14233/ajchem.2017.20427

Asian Journal of Chemistry; Vol. 29, No. 4 (2017), 908-910
A
SIAN
J
OURNAL OF HEMISTRY
C
https://doi.org/10.14233/ajchem.2017.20427
An Active Catalyst for Diels-Alder Reaction of Indol with o-Benzoquinone
*
S. SEDAGHAT and A. ZEIN
Department of Chemistry, Islamic Azad University-North Tehran Branch, Tehran 19136-74711, Iran
*Corresponding author: E-mail: s_sedaghat@iau-tnb.ac.ir
Received: 29 November 2016;
Accepted: 4 January 2017;
Published online: 31 January 2017;
AJC-18271
The Diels-Alder reaction of indol with masked o-benzoquinone was studied. The stereo selectivity of this reaction by heterogeneous
catalysis is improved. This catalyst was prepared by surface modification of nanoporous silica gel with cerium ion in aqueous ammonia.
The structures of products were characterized by spectroscopic techniques.
Keywords: Silica support, Cerium, Diels-Alder reactions, o-Benzoquinones, Indol, Catalyst.
Alder diastereoselective reaction of indol and masked o-benzo-
quinone were investigated. For more charge and high cerium
ion concentration on the surface of silica, a high endo/exo
diastereoselectivity could be observed.
INTRODUCTION
The Diels-Alder reaction produced six-membered rings
which are versatile building-blocks for the synthesis of numerous
natural products. A simple access could be provided through
these reactions to a number of poly heterocyclic quinones with
antitumour activity [1,2].
EXPERIMENTAL
Among the most easily accessible cyclohexa-2,4-dienones
types, masked with o-benzoquinones (MOBs) can be mentioned
trapped with dienophiles like indol in the Diels-Alder reaction.
The adducts could be used as intermediates in a large diversity
of organic materials synthesis [3-5].
Silica gel, in comparing to the other inorganic material,
is an appropriate support for the heterogenization of various
catalysts types, as it is affordable and accessible, with conve-
nient adsorption properties because of their high surface area,
nano porosity and providing physico-chemical stabilities
during the reaction process. There are two kinds of functional
groups for the silica surface including siloxane (Si-OSi) and
silanol group (Si-OH). The behaviour of silanol groups on
silica surface would be similar to a weak acid (pKa = 9.4). By
increasing the metal charge and concentration and adding a
metal ion, surface acidity was changed [6,7].
ZnCl2, AlCl3, FeCl3, as Lewis acid catalysts can have a
considerable influence on Diels-Alder reactions rate and endo
selectivity. The interaction between nitrogen group of indol
and the metal ions declines the dienophiles LUMO energy
level and provides a more effective overlapping with diene
HOMO [8,9].
In this work, considering the ion exchange approach of
cerium ion(IV) in alkali conditions, different catalysis concen-
trations on the surface of silica gel were obtained. The Diels-
Ce(SO4)2·4H2O, indol, ammonium solution, silica (particle
size 63-200 µm, kieselgel 60), di acetoxyiodo benzene (DAIB),
Acetone and 2-methoxyphenols were purchased from Merck
company.
Preparation of support: First of all, the ammonium
solution 1 M was prepared by adding silica gel (20 g) for 5 min.
To the obtained NH4+-SiO2, the solution of Ce(SO4)2·4H2O with
various concentration [0.05, 0.06, 0.07, 0.08 M] was added
in room temperature with stirring for 1 h. The ion-exchange
between NH4+ and cerium ion occurs at this time. With 30 mL
acetone, the solids were filtered and washed and then dried in
+
oven for 2 h. For removal of none ion exchanged NH4 and
reducing the silanol surface, all solids were heated at 350 °C,
under vacuum for 2 h.As support, until used, all obtained solids
were stored in polypropylene bottle.
Diels-Alder reaction: Considering vigorous solid stirring
in a stopped flask, indol solution in dry methanol (10 mmol)
was added to cerium supports catalysis with various cerium
concentration. A solution of 2-methoxyphenols (10 mmol)
in the dry methanol was cooled to 0 °C under inert gas and
diacetoxyiodo benzene (DAIB) was added as solid in one
portion. The temperature of flux was controlled. For 0.5 h,
the reaction mixture was stirred and cooled up to -20 °C and
then with vigorous stirring, the newly prepared masked
o-benzoquinone was added to the indol mixture. The reaction

Vol. 29, No. 4 (2017)
An Active Catalyst for Diels-Alder Reaction of Indol with o-Benzoquinone 909
mixture was stirred at room temperature for 2-4 h. The product
was afforded the chromatography (silica gel column, ethyl
acetate cyclohexane [20:80, v: v]). Using GC, the reaction
yield was determined. The products structures were determined
considering the IR, 1H and 13C NMR analysis (Scheme-I).
m.p. 77-79 °C; FTIR (cm–1): 3406, 2965, 1770, 1760,
increased. In contrast, the yield and endo/exo diastereo-
selectivity are higher for Ce(IV), which may be because of its
properties of relative strong acidic.
The FT-IR spectra of indol on the Ce(IV) modified silica
were shown that the NH symmetry stretching frequency for
indol is observed at 3406 cm-1. By the loading of indol on the
modified silica of Ce(IV), the wave number of the nitrogen
frequency changes about 20 cm-1. This NH frequency shift to
low wavenumber is relevant to indol coordination with cerium
ions on the surface of silica. It can be found in the literature,
that this band shifted to the lower wavenumber, when Zn(II)
was the metal ion [10]. Cerium(IV) is a more stronger Lewis
acid in comparison with Zn(II), consequently the frequency
shift of NH for cerium ion will shifts to the lower number of
wave region compared to zinc ion [10]. However, the band
width is relevant to hydrogen bonding with the Si-OH of silica
surface. Vesselovskii and coworker suggested that within
each reactant, by gathering the reacting moieties into closer
proximity, the reactants adsorption on the silica particles
surface may facilitate the pre-reaction complexes formation
and stabilization [10,11]. A similar study has been conducted
by Menger, who proposed that the functional groups rigid
anchoring can stable reactants in adjacent together lower
than the distance of critical bonding, consequently leading to
enhancement significant rate [12]. Another study performed
by Parlar and Baumann [13] proposing that only a small
reactants fraction are really effectively adsorbed on the surface
of silica and that the improved rates and selectivities are as a
result of symmetry controlled interactions of secondary orbital
between effectively adsorbed species and ones that are free to
move on the surface. However, in this study, the mechanism
of catalytic is suggested by the Lewis acid adsorption and
coordination site of cerium ion with the indol nitrogen and
perhaps the dienophiles hydrogen bonding or protonation by
acidic SiOH groups on the surface of silica and then the
reaction of Diels-Alder performs with facility [14-16].
As these results, the support could be recycled and reused
for several times. The blank diastereoselective reaction was
55 % without support and solvent. The reaction in solvent had
56 % diastereoselectivity without support so the solvent have
no influence on the diastereoselectivity improvement. To prove
1
1622, 1456, 1374, 1246, 1050, 743. H NMR, δ ppm (500
MHz, CDCl3): 6.65 (dd, J3,4 = 10 Hz and J3,5 = 3 Hz, 1H, H-3),
6.48 (dd, J5,4 = 10 Hz and J5,6 = 3 Hz, 1H, H-5), 6.4 (dd, J4,5
=
10 Hz and J4,6 = 3 Hz, 1H, H-4), 6.32 (dd, J6,5 = 10 Hz and J6,4
= 3 Hz, 1H, H-6), 6.03 (brt, J11,10 = 12 Hz, 1H, H-11), 5.7 (d, J
= 10 Hz, 1H, H-10), 3.97 (d, J = 13 Hz, 1H, H-2), 3.92 (d, J =
13
13 Hz, 1H, H-8), 2.7 (br s, 6H, CH3O), 1.95 (brs, H-1). C
NMR (500 MHz, CDCl3): 136 (CO), 129-120 (aromatic carbon
signal + impurities signals), 115 (C-10), 111(C-11), 60.8 (C-2)
56.4 (C-8), 32 (C-9), 30.1 (CH3O), 23 (C1), 14.5 (C-12).
RESULTS AND DISCUSSION
The obtained results of the reactions of Diels-Alder between
masked o-benzoquinone and indol reported in the existence
of different prepared supports. The reaction exhibited suitable
yield, but less and poor diastereoselective reaction. As shown
in Table-1, regarding the support with high cerium ion concen-
tration on silica surface, led to the increase of yield and parti-
cularly diastereoselectivity.
TABLE-1
DIASTEREOSELECTIVITY AND YIELD OF DIELS-ALDER
REACTION OF INDOL AND MASKED o-BENZOQUINONE IN
THE PRESENCE OF Ce(IV) LOADED ON SILICA AS SUPPORT
Concentration of
mmol Ce/g
Yield (%)
Endo/exo (%)
Ce(IV)
Without catalyst
0.05
0
0.25
0.3
0.35
0.4
72
96
97
99
99
56/34
93/7
96/4
97/3
98/2
0.06
0.07
0.08
As mentioned above, by the increase of metal ion concen-
tration on surface, the Lewis acid sites on the silica surface
will be increased. The high concentration of cerium ion on
the surface of silica, the diastereoselective reaction will be
H
5
H
6
4
H
H
H
H
H
7
NH
H
H
O
3
H
H
H
Cis
Cis
H
8
Cis
2
H
H
H
9
OCH3
N
H
10
H
12
OCH3
O
H
Dieneophile
11
13
H
1
H
Diene
OCH3
H3CO
Scheme-I

910 Sedaghat et al.
Asian J. Chem.
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completed. The extraction was performed only by product
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The cerium ion on the surface of silica catalysts was
prepared by ion exchange method which has been proved for
high diastereoselective Diels-Alder reaction of indol and
dienophiles found to be efficient heterogeneous catalysts. The
diastereoselective reaction highly depends on the cerium ion
concentration as hard Lewis acid on the surface of silica. There
is a positive correlation between cerium ion concentration and
charge on the surface of silica and Diels-Alder diastereoselec-
tive reaction. The characteristic FT-IR band for NH which
stretches frequency showed that with cerium ions on the silica
surface the indol was coordinated. The catalytic mechanism
was proposed by indol adsorption and nitrogen group coordi-
nation with cerium ion on the surface of silica which would be
crucial in the Diels-Alder diastereoselective reactions.
14. H.M. Najafi, M. Ghandi and F. Farzaneh, Chem. Lett., 358 (2000);
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ACKNOWLEDGEMENTS
15. A. Badiei, G. Mohammadi Ziarani, P. Norouzi and F. Tousi, Iranian
Int. J. Sci., 5, 31 (2004).
16. R.C. Mishra, N. Tewari, S.S. Verma, R.P. Tripathi, M. Kumar and P.K.
Shukla, J. Carbohydr. Chem., 23, 353 (2004);
This work was supported by the Research Department of
Islamic Azad University, Tehran, Iran.
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