A facile approach of Diels-Alder reaction in imidazolium-based ionic liquids at room temperature

Article abstract of DOI:10.13005/ojc/300333

A Diels-Alder reaction between anthracene and 1- p-tolyl-2,5-dione was conducted in imidazolium-based ionic liquids. Imidazolium cation was utilised with counter anions, [BF 4] and [PF6], as the solvents to carry out the desired Diels-Alder reaction. In this work, the diene and dienophile were introduced into the ionic liquids for 72 hours at room temperature. The desired cycloadduct was prepared by a green chemistry procedure in high yield. The expected cycloadduct was characterized on the MS as well as FTIR and NMR spectroscopy. Herein we report, only the [Bmim][BF4] ionic liquid gave the desired cycloadduct in 86% yield compared to [Bmim][PF6]. Diels-Alder reaction, anthracene, imidazolium-based ionic liquids. We would like to thank the School of Chemical Sciences and Food Technology, Faculty of Science and Technology and Centre for Research and Instrument (CRIM), University Kebangsaan Malaysia for instrumental facilities as well as grants GUP- 2012-073 and GGPM-2013-038 for funding the project and the Ministry of Higher Education.

Full text of DOI:10.13005/ojc/300333

ISSN: 0970-020 X
CODEN: OJCHEG
2014, Vol. 30, No. (3):
Pg. 1191-1196
ORIENTAL JOURNAL OF CHEMISTRY
An International Open Free Access, Peer Reviewed Research Journal
www.orientjchem.org
A Facile Approach of Diels-Alder Reaction in
Imidazolium-based Ionic Liquids at Room Temperature
NUR LIYANA SAKINAH JOHARI, NUR HASYAREEDA HASSAN*
and NURUL IZZATY HASSAN
School of Chemical Sciences & Food Technology, Universiti
Kebangsaan Malaysia, 43600 UKM Bangi, Selangor Darul Ehsan, Malaysia.
*Corresponding author E-mail: syareeda@ukm.edu.my
http://dx.doi.org/10.13005/ojc/300333
(Received: July 30, 2014; Accepted: September 05, 2014)
ABSTRACT
A Diels-Alder reaction between anthracene and 1-p-tolyl-2,5-dione was conducted in
imidazolium-based ionic liquids.Imidazolium cation was utilised with counter anions, [BF4] and [PF6],
as the solvents to carry out the desired Diels-Alder reaction. In this work, the diene and dienophile
were introduced into the ionic liquids for 72 hours at room temperature.The desired cycloadduct was
prepared by a green chemistry procedure in high yield.The expected cycloadduct was characterized
on the MS as well as FTIR and NMR spectroscopy. Herein we report, only the [Bmim][BF4] ionic
liquid gave the desired cycloadduct in 86 % yield compared to [Bmim][PF6].
Key words Diels-Alder reaction, anthracene, imidazolium-based ionic liquids.
INTRODUCTION
mixture of isomers, a process which is not always
selective 15. It is well known that the selectivity and
reactivity of the Diels-Alder reaction are strongly
influenced by solvent effects. Consequently, the
Diels-Alder reaction has been investigated in various
mediums such as water, surfactants, lithium amides,
borane-THF complexes and others.Due to that, rate
enhancements also have been explored in these
Recently, there has been increased
attention on the use of ionic liquids as an alternative
medium in organic reactions such as the Friedel-
Crafts reaction, isomerization and hydrogenation
1-3. In addition, the Diels-Alder reaction performed in
ionic liquids has attracted growing interest to meet
the awareness of green and environmentally friendly
synthesis 4-7.More recently works on the Diels-Alder
reaction in imidazolium-based ionic liquids has been
well documented 8-14. The Diels-Alder reaction is a
pericyclic reaction, and provides cycloadducts as a
solvents 16-19
.
As mentioned from previously reported
work, ionic liquids have great potential to influence
the cycloadducts of the Diels-Alder reaction. In

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JOHARI et al., Orient. J. Chem., Vol. 30(3), 1191-1196 (2014)
addition, ionic liquids also promote the efficiency of were purchased from Sigma-Aldrich, Acros Organic,
the catalyst in terms of recyclability, as well as being and Friendemann Schmidt.
a catalyst itself in the Diels-Alder reaction 20, 21. The
use of ionic liquids in the Diels-Alder reaction was Starting materials
first reported by Jaeger and Tucker in 1989. They
1-p-tolyl-2,5-dione, [Bmim][BF4], and
used ethylammonium nitrate as the solvent in the [Bmim][PF6] were synthesized using previously
Diels-Alder reaction between cyclopentadiene and reported synthetic procedures 27,28. Anthracene
methyl acrylate. Their work successfully yielded a is commercially available and was recrystallized
6.7:1 mixture of endo and exo isomers. The same prior to use. 2-methylfuran, N-methylmaleimide and
reaction was conducted in various ionic liquids, [Hmim][BF4] are commercially available from the
including [emim][BF4], [emim][PF6], [emim][NO3], supplier.All other chemicals such as tetrahydrofuran,
[emim][ClO4] and [emim][CF3SO3] 22-24
.
dichloromethane, diethyl ether, zinc bromide, HMDS,
MgSO4, HCl, activated charcoal, basic alumina and
We herein report the synthesis of acidic alumina used in this work were commercially
the imidazolium-based ionic liquids, 1-butyl-3- available and used with no further purification.
methylimidazolium tetrafluoroborate [Bmim][BF4] and
1-butyl-3-methylimidazolium hexafluor-ophosphate 1-p-Tolyl-2, 5-dione
[Bmim][PF6], and their application as solvents in
In a two-neck round bottom flask, p-toluidine
the Diels-Alder reaction between anthracene and was dissolved in tetrahydrofuran and mixed with
1-p-tolyl-2,5-dione. These imidazolium-based ionic maleic anhydride. Mixture was stirred at room
liquids are purposely replacing molecular solvents temperature under an inert atmosphere overnight.
in Diels-Alder reaction. There are some previous The resulting precipitate was filtered and used in
reports that work on the Diels-Alder reaction by the next step without further purification. The crude
applying other imidazolium-based ionic liquids as p-maleimide acid was suspended in dry acetonitrile.
solvent and using maleimide as the dienophile Zinc bromide and HMDS were added to the solution
25, 26. However, we believe that this cycloadduct and the resulting reaction mixture was heated at
that formed by cycloaddition of anthracene and reflux (90°C) for 2 h.The reaction mixture was cooled
1-p-tolyl-2,5-dione was the first time developed in down at room temperature and filtered. Water was
this imidazolium-based ionic liquids, [Bmim][BF4] added and the pH of the solution was adjusted
and [Bmim][PF6], a novel synthetic method for this to pH1 using 1M HCl. The solution was washed
cycloadduct.
with ethyl acetate, the layers separated and the
organic phase dried (MgSO4) 27. The solution was
filtered and concentrated to give a yellow powder.
Recrystallization from ethyl acetate gave 1-p-tolyl-
MATERIALS AND METHOD
All melting points were obtained using 2,5-dione 84 % as yellow needles. IR (KBr) 3450.67
a Barnstead Electrothermal 9100 melting point (OH), 2967.11 (sp3 CH),1702.41 (C=O) 1041.54 (C-
apparatus. IR spectra were recorded as KBr disks N) cm–1;1H NMR (400 MHz, acetone-d6) ´7.27 (2H, d,
on a Perkin Elmer Spectrum GX spectrometer. J= 8.08 Hz, ArCH), 7.22 (2H, d, J= 8.44 Hz, ArCH),
1
The H NMR spectra were obtained using a Jeol 7.00 (2H, s, HC=CH), 2.35 (3H, s, CH3); 13C NMR
spectrometer operating at 400MHz with TMS (100 MHz, acetone-d6) ´169.8 (C=O), 137.4 (C=C),
as an internal standard. 13C NMR spectra were 134.4 (quaternary C), 129.5 (aromatic C=C), 129.3
determined using TMS as an internal standard with (aromatic C=C), 126.5 (quaternary C), 20.0 (CH3);
a Jeol spectrometer operating at 100 MHz. High mp:150.0–150.5 °C;MS (EI+) m/z 187.1 (M+, 100 %).
resolution MS spectra were measured using a Bruker
spectrometer in ESI mode. GCMS spectra were 1-Butyl-3-methylimidazolium tetrafluoroborate
obtained using an Agilent 7890A GC coupled with [Bmim][BF4]
Agilent 5975C MSD.TLC was carried out using Merck
Tetrafluoroborate acid was added
Kieselgel 60 PF254 plates. Column chromatography dropwise to a cold aqueous solution of 1-butyl-3-
was performed using Merck Kieselgel 60 (0.063– methylimidazolium chloride, [Bmim][Cl] in a two-
0.200 mm). All organic solvents used in this work neck round bottom flask. The reaction mixture was

JOHARI et al., Orient. J. Chem., Vol. 30(3), 1191-1196 (2014)
1193
stirred at room temperature for 72 h under an inert
atmosphere.The resulting ionic liquid was extracted
from the aqueous phase using dichloromethane and
the layers separated. The dichloromethane layer
was washed using distilled water to the excess
unreacted acid and chloride salt. The aqueous
layer was tested with litmus paper and silver nitrate
whilst the dichloromethane layer was concentrated
to give a yellow liquid.The yellow liquid was directly
treated with activated charcoal and filtered through
basic alumina to give 1-butyl-3-methylimidazolium
tetrafluoroborate (28 %) as a colorless liquid 28. IR
(KBr) 3565.66 (OH), 1635.22 (C=C), 1064.73 (CN)
DMSO-d6) ´ 136.9 (NCN), 124.0 (C-4), 122.6 (C-
5), 49.1 (NCH2CH2CH2CH3), 36.1 (NCH3), 31.8
(NCH2CH2CH2CH3), 19.2 (NCH2CH2CH2CH3),
13.5 (NCH2CH2CH2CH3); MS (ESI+) m/z 423.21
[2(C4C1im) + PF6] and 139.12, [C4C1im], MS (ESI-)
m/z 429.05 [2(PF6) + (C4C1im)] and 144.96 [PF6].
Procedure for the preparation of cycloadducts
1 and 2
Reaction between 1-p-tolyl-2,5-dione and
anthracene in [Bmim][BF4] at rt
In a two-neck round bottom flask,
anthracene and 1-p-tolyl-2,5-dione were added
directly into [Bmim][BF4] and the resulting reaction
mixture was stirred at room temperature for 72 h
under an inert atmosphere. The desired product
was removed from the ionic liquid by extracting the
solution with diethyl ether. The diethyl ether was
removed under vacuum and the residual powder
obtained was purified by column chromatography
using a mixture of hexane: ethyl acetate (9:1) as the
eluent to give the cycloadduct 1 (86%) as a colorless
powder. IR (KBr) 3422.16 (OH), 2928.43 (sp3
1
cm–1; H NMR (400 MHz, DMSO-d6) δ 9.01 (1H,
s, CH-2), 7.70 (1H, s, CH-4), 7.63 (1H, s, CH-5),
4.15 (2H, t, J= 7.32 Hz, NCH2CH2CH2CH3), 3.88
(3H, s, NCH3), 1.76 (2H, m, NCH2CH2CH2CH3),
1.25 (2H, m, NCH2CH2CH2CH3), 0.88 (3H, t, J=
7.32 Hz, NCH2CH2CH2CH3); 13C NMR (100 MHz,
DMSO-d6) ´ 136.9 (NCN), 124.0 (C-4), 122.7 (C-
5), 49.0 (NCH2CH2CH2CH3), 36.1 (NCH3), 31.8
(NCH2CH2CH2CH3), 19.1 (NCH2CH2CH2CH3),
13.6 (NCH2CH2CH2CH3); MS (ESI+) m/z 365.25
[2(C4C1im) + BF4] and 139.12 [C4C1im]; MS (ESI-)
m/z 313.12 [2(BF4) + C4C1im].
1
CH),1650.51 (C=O), 1045.41 (C-N)cm–1; H NMR
(400 MHz, CDCl3) δ 7.41 (2H, d, J= 8.40 Hz, ArCH),
7.33 (2H, d, J= 8.80 Hz, ArCH), 7.19 (4H, m, ArCH),
7.08 (2H, d, J= 8.04 Hz, ArCH), 6.36 (2H, d, J= 8.08
Hz, ArCH), 4.87 (2H, s, sp CH), 3.36 (2H, s, sp CH),
2.27 (3H, s, CH3); 13C NMR (100 MHz, CDCl3) ´
176.3 (C=O), 141.3 (aromatic C=C), 138.8 (aromatic
C=C), 136.9 (aromatic C=C), 129.8 (aromatic
C=C), 128.8 (aromatic C=C), 127.2 (aromatic
C=C), 126.9 (aromatic C=C), 126.2 (aromatic
C=C), 125.2 (aromatic C=C), 124.4 (aromatic
C=C), 123.4 (aromatic C=C), 121.8 (aromatic
C=C), 50.0 (quaternary C), 47.1 (quaternary C),
45.9 (quaternary C), 36.5 (quaternary C), 31.9
(quaternary C), 29.7 (quaternary C), 28.9 (HC=C),
19.4 (HC=C), 13.4 (CH3); mp 270.0- 270.2°C; MS
(EI+) m/z 365.4 (M+, 100%).
1-Butyl-3-methylimidazolium hexafluoro
phosphate [Bmim][PF6]
Lithium hexafluorophosphate was mixed
with 1-butyl-3-methylimidazolium chloride, [Bmim]
[Cl] in a two-neck round bottom flask. The reaction
mixture was stirred in dry dichloromethane at room
temperature for 72 h under an inert atmosphere.
The reaction mixture was filtered and the excess
lithium chloride was washed with dichloromethane.
The combined organic extraction was washed with
distilled water until the aqueous layer was free
from halide, determined using the silver nitrate test.
The dichloromethane layer was concentrated to
give a yellow liquid. The yellow liquid was treated
with activated charcoal and filtered through acidic
alumina to give 1-butyl-3-methylimidazolium
hexafluorophosphate (27%) as a colorless liquid
28. IR (KBr) 3392.27 (OH), 1649.54 (C=C), 1008.56
Reaction between 1-p-tolyl-2,5-dione and
anthracene in [Bmim][BF4] at reflux (90 °C)
Inatwo-neckroundbottomflask,anthracene
and 1-p-tolyl-2,5-dione were added directly into
[Bmim][BF4] and the resulting reaction mixture was
heated at refluxed (90 °C) for 2 h under an inert
atmosphere.The desired product was removed from
the ionic liquid by extracting the solution with diethyl
ether.The diethyl ether was removed under vacuum
1
(CN) cm–1; H NMR (400 MHz, DMSO-d6) δ 9.00
(1H, s, CH-2), 7.67 (1H, s, CH-4), 7.61 (1H, s, CH-
5), 4.14 (2H, t, J= 7.32 Hz, NCH2CH2CH2CH3), 3.83
(3H, s, NCH3), 1.76 (2H, m, NCH2CH2CH2CH3),
1.26 (2H, m, NCH2CH2CH2CH3), 0.89 (3H, t, J=
7.32 Hz, NCH2CH2CH2CH3); 13C NMR (100 MHz,

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JOHARI et al., Orient. J. Chem., Vol. 30(3), 1191-1196 (2014)
and residual powder obtained was purified by column
chromatography with mixture of hexane:ethyl acetate
(9:1) as the eluent to give the cycloadduct 2 (51%) as
RESULTS AND DISCUSSION
Two ionic liquids were synthesized using
a colorless powder;IR (KBr) 3421.90 (OH), 2928.43 two synthetic approaches involving acid-based and
1
(sp3 CH), 1685.02 (C=O), 1010.62 (C-N)cm–1; H metathesis methods. [Bmim][BF4] was prepared
NMR (400 MHz, CDCl3) δ 7.41 (2H, d, J= 8.80 Hz, via an acid-based method whilst a metathesis
ArCH), 7.33 (2H, d, J= 8.80 Hz, ArCH), 7.19 (4H, m, method was used to synthesize [Bmim][PF6]. In
ArCH), 7.08 (2H, d, J= 8.40 Hz, ArCH), 6.36 (2H, d, order to get pure ionic liquids, all of the ionic liquids
J= 8.44 Hz, ArCH), 4.87 (2H, s, sp CH), 3.36 (2H, s, were treated using activated charcoal to remove
sp CH), 2.27 (3H, s, CH3);13C NMR (100 MHz;CDCl3) residual organic materials from the earlier steps of
´176.3 (C=O), 141.3 (aromatic C=C), 138.8 (aromatic their synthesis. For ionic liquids prepared via the
C=C), 137.2 (aromatic C=C), 129.8 (aromatic C=C), acid-based method, they must be filtered through
128.7 (aromatic C=C), 127.2 (aromatic C=C), basic alumina in order to remove the residual acidic
126.9 (aromatic C=C), 126.2 (aromatic C=C), materials from the tetrafluoroboric acid reagent.
125.2 (aromatic C=C), 124.4 (aromatic C=C), Whilst for the metathesis method, the ionic liquids
123.3 (aromatic C=C), 121.7 (aromatic C=C), 50.0 must be filtered through acidic alumina to remove
(quaternary C), 47.1 (quaternary C), 45.9 (quaternary the residual imidazole starting material.
C), 36.5 (quaternary C), 31.9 (quaternary C), 29.7
(quaternary C), 21.2 (HC=C), 19.4 (HC=C), 13.4
These two ionic liquids were applied in
(CH3); mp 270.0- 270.2 °C; MS (EI+) m/z 365.4 (M+, the Diels-Alder reaction because of the ability of
100%).
the imidazolium cation to act as a hydrogen bond
donor, whilst the BF4 and PF6 counter anions can
O
N
CH3
O
[Bmim][BF4],
rt, 72 h, N2
[Bmim][BF4],
reflux (90oC), 2 h, N2
CH3
CH3
O
O
N
N
O
O
1
2
86 %
51 %
Scheme 1: Schematic diagram for the reaction of anthracene and 1-p-tolyl-2,5-dione in ionic liquids

JOHARI et al., Orient. J. Chem., Vol. 30(3), 1191-1196 (2014)
1195
act as hydrogen bond acceptors. It is well known
that Lewis acid catalysts significantly influence the
selectivity and reaction rates of the Diels-Alder
reaction; the formation of a hydrogen bond between
the imidazolium ionic liquid and the 1-p-tolyl-2,5-
dione substrate is known as the Lewis acid-base
interaction.
proved and better yields were obtained at room
temperature.We believe that this Diels-Alder reaction
with the use of catalyst will provide better yields and
we are looking forward in developing this reaction.
CONCLUSION
We have shown that Diels-Alder reaction
between anthracene and 1-p-tolyl-2,5-dione in
[Bmim][BF4] was best in room temperature condition
with 86% yield. The ionic liquids that used in this
reaction can be potentially recycled after purification
process up to five times.
The Diels-Alder reaction between
anthracene and 1-p-tolyl-2,5-dione was carried
out in the ionic liquids, [Bmim][BF4] and [Bmim]
[PF6]. Unfortunately, when using [Bmim][PF6] as the
reaction solvent, no cycloadducts were observed.
Theoretically, the ionic liquid [Bmim][PF6] gave higher
yields compared to [Bmim][BF4] due to the lower
basicity of PF6. Therefore, further investigation into
why no cycloadducts were formed in [Bmim][PF6]
is warranted. In contrast, the Diels-Alder reaction
carried out at room temperature using [Bmim][BF4]
as the reaction solvent gives the cycloadducts in 86%
yield, whilst conducting the reaction at reflux gives
the cycloadducts in a lower 51% yield.The ability of
[Bmim][BF4] to act as a solvent in this reaction was
ACKNOWLEDGEMENTS
We would like to thank the School of Chemical
Sciences and Food Technology, Faculty of Science
and Technology and Centre for Research and
Instrument (CRIM), University Kebangsaan Malaysia
for instrumental facilities as well as grants GUP-
2012-073 and GGPM-2013-038 for funding the
project and the Ministry of Higher Education.
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