Triethylammonium acetate (TEAA): A recyclable inexpensive ionic liquid promotes the chemoselective aza- and thia-Michael reactions

Article abstract of DOI:10.1007/s00706-008-0886-4

A new, highly efficient, inexpensive, recyclable, mild, convenient, and green protocol for chemoselective aza/thia-Michael addition reactions of amines/thiols to α,β-unsaturated compounds using triethylammonium acetate (TEAA) ionic liquid was developed. The catalyst can be recycled ten times and obviate the need for toxic and expensive catalysts.

Full text of DOI:10.1007/s00706-008-0886-4

Monatsh Chem 139, 1041–1047 (2008)
DOI 10.1007/s00706-008-0886-4
Printed in The Netherlands
Triethylammonium acetate (TEAA): a recyclable inexpensive ionic liquid
promotes the chemoselective aza- and thia-Michael reactions
Akhilesh K. Verma, Pankaj Attri, Varun Chopra, Rakesh K. Tiwari, Ramesh Chandra
Green Organic Chemistry Research Laboratory, Dr. B.R. Ambedkar Center for Biomedical Research,
University of Delhi, Delhi, India
Received 28 August 2007; Accepted 23 December 2007; Published online 18 February 2008
# Springer-Verlag 2008
Abstract A new, highly efficient, inexpensive, recy-
clable, mild, convenient, and green protocol for che-
moselective aza=thia-Michael addition reactions of
amines=thiols to ꢀ,ꢁ-unsaturated compounds using
triethylammonium acetate (TEAA) ionic liquid was
developed. The catalyst can be recycled ten times
and obviate the need for toxic and expensive catalysts.
ionic liquid [6, 7], water [8], ꢁ-CD in water [9], ce-
rium ammonium nitrate (CAN) [10], HClO4–SiO2
[11], DBU [12], neat [13], and indium(III) chloride
[14]. Unfortunately, many of these processes suffer
from limitations, such as the use of expensive re-
agents, harsh conditions, relatively long reaction
times, high catalyst loading, low selectivity, presence
of side reactions, and tedious work-up procedures for
their separation, recycling, or disposal problems and
effluent pollution. All these limitations forced us to
explore a new catalyst, which has more efficiency
and less limitation. So we tried to explore the new
catalyst that has certain properties, such as good
thermal and mechanical stabilities of supported re-
agents, is easy to handle, of low toxicity, non-corro-
sive, easy to separate from reaction mixture through
filtration, and feasible for reuse.
Ionic liquids as new reaction media and catalysts
have been experimentally and theoretically recog-
nized and accepted [15]. The application of ionic
liquids as novel media may provide convenient solu-
tions to both the solvent emission and catalyst reuse
problem [16]. A great deal of attention has been
given to imidazolium ionic liquids in the past several
years [17]. Recently, Li-Wen Xu and coworkers [18]
have used basic ionic liquid [bmim][OH] to catalyze
the aza-Michael reaction of aromatic amines in long
reaction times of 8–24 h affording the addition prod-
uct in 42–98% yields. However, industrial applica-
tion of these ionic liquids is limiting because of the
Keywords Triethylammonium acetate (TEAA); Green; Ionic
liquid; Aza=thia-Michael reaction.
Introduction
The Michael reaction has been studied for more than
one century. The conjugate addition of amines to
carbon–carbon double bonds is a useful protocol in
synthetic organic chemistry [1]. It is used extensive-
ly in the synthesis of pharmaceutical intermediates,
peptide analogues, antibiotics, and other biologically
active molecules and drugs [2–4]. In the past few
years, a number of alternative procedures have been
developed for the conjugate addition of amines to
ꢀ,ꢁ-unsaturated nitriles and carbonyl compounds.
In particular, various Lewis acid catalyzed reactions
have been reported [5]. Recently, there were also
some reports of this reaction conducted in Cu(acac)2=
Correspondence: Akhilesh K. Verma, Green Organic Chem-
istry Research Laboratory, Dr. B.R. Ambedkar Center for
Biomedical Research, University of Delhi, Delhi 110007,
India. E-mail: akhilesh682001@yahoo.com

1042
A. K. Verma et al.
etc.), which cannot react in water even under forc-
ible condition [11]. We designed a new protocol with
TEAA as the catalyst=reaction medium system for
carrying out the Michael reaction of amines and
thiols with ꢀ,ꢁ-unsaturated compounds to obtain
the desired products (minutes) with excellent yield
and purity. The product with 1 mmol scale of re-
actands needed not any purification with column
chromatography. TEAA when added as a reaction
medium, the reaction rates as well as yields of the
reaction improved dramatically. This is probably due
to the higher solubility of amines=thiols in the ionic
liquid. This protocol was successfully applied to cat-
alyze the Michael addition of various substituted
piperazines, aliphatic, aromatic amines, imidazoles,
and thiols with ꢀ,ꢁ-unsaturated nitriles and carbonyl
compounds. Results of the reactions are summarized
in Tables 1 and 2. The (4-nitrophenyl)piperazine
(1g), (4-amino-phenyl) piperazine (1h), and other
substituted piperazines with various Michael accep-
tors, i.e., acrylonitrile, methyl acrylate, and cyclo-
hexenone at 25ꢀC gave the corresponding adduct
in excellent yields up to 99% after 1 min of addition
of TEAA to the reaction system (Table 1).
Scheme 1
high price of imidazolium ionic liquids and also due
to low recyclability [19]. Low-cost ionic liquids,
such as ammonium ionic liquids have drawn much
attention in the recent time for their use in synthesis
methodology. Recently, Ganeshpure and co-workers
[20] used a simple ammonium ionic liquid for the
Fischer esterification. By using this methodology we
try to explore the aza=thia-Michael reaction using the
triethylammonium acetate ionic liquid.
The product thus formed was separated from
the reaction mixture by extraction using diethyl
ether. The product was formed in pure form with-
out any side product and did not need any further
purification. The reaction of aromatic amines, e.g.,
anilines and various other substituted anilines, pro-
ceeded within 30 min after the addition of cata-
lyst to the reaction system containing the Michael
acceptor.
In order to extend the scope of this methodology,
N-heterocycles 1v–1x, e.g., imidazole and substitut-
ed imidazoles were used for aza-Michael reaction
with ꢀ,ꢁ-unsaturated compounds. It is noticed that
the reaction took more time to complete (analysis by
TLC and GC) as compared with other amines, which
may be due to the steric hindrance in heterocycles.
Similar to the aza-Michael reaction there is also dra-
matical increase in the reactivity and yield by addi-
tion of TEAA (Table 2).
Thus, a probable mechanism for the reaction
using TEAA as a catalyst for both the thia-Michael
and the aza-Michael reaction due to the enhanced
reactivity in the triethylammonium acetate ionic
liquid attributed to the inherent Brønsted and Lewis
acidily of the [Et3NH]þ cation, which makes the
NH and SH bond weaker, enhancing the nucleo-
Results and discussion
As a part of our research on developing on economi-
cal, environmentally friendly route for C–N and C–
S bond forming reaction [21], herein we report the
conjugate addition of amines and thiols to ꢀ,ꢁ-un-
saturated nitriles and carbonyl compounds using the
TEAA ionic liquid (Scheme 1).
The report by the B.C. Ranu group [8] showed the
potential of water as the catalyst=reaction medium
for the aza-Michael reaction with aliphatic amines,
but with aromatic amines it was not successful.
One more report by the Gopal L. Khatik group [8]
showed the potential of water for thia-Michael reac-
tion with simple substrates. However, there has been
no straightforward synthesis of these uncomplicated
enones(e.g., chalcones, trans-4-phenyl-3-buten-2-one,

Trichlorogermyl dioic acids
1043
Table 1 Aza-Michael reaction of amines with ꢀ,ꢁ-unsaturated compounds catalyzed by [Et3NH][CH3COO] at room temperature
Entry
Comp.
Amine
Michael accceptor
Time=min
Yield=%b
Ref.
1
2
3
1a
1b
1c
Piperazines (Pip)
1-MePip
1
1
1
99
99
99
[21]
[21]
[21]
1-PhPip
4
5
1d
1e
1-BhPip
1-PhPip
1
1
98
[21]
98c
6
1f
1-(4-ClPh)Pip
1-(4-NO2Ph)Pip
1-(4-NH2Ph)Pip
1-(BDO CH2)Pipa
1-[2(MeO)Ph]Pip
1-(2-MePh)Pip
(n-C4H9)2NH
1
1
1
1
1
1
5
5
1
95c
96c
94c
93c
98
7
1g
1h
1i
8
9
10
11
12
13
14
1j
[21]
[21]
[26]
[26]
[26]
1k
1l
97
98
1m
1n
(n-C4H9)2NH
98
Morpholine
96
15
16
1o
1p
Morpholine
1
1
97
97
[26]
[26]
PhCH2NH2
17
18
19
1q
1r
1s
PhNH2
20
20
20
96
95
95
[26]
[26]
[26]
PhNH2
4-ClPhNH2
20
21
1t
1u
4-MePhNH2
4-(MeO)PhNH2
20
20
90
98
[26]
[26]
22
23
24
1v
1w
1x
Imidozole (lm)
4-NO2lm
20
30
30
98
96
95
[25]
[25]
[25]
4-NO2lm
a BDO 1,3-Benzodioxol-5-yl
b Isolated compounds
philicity of sulfur for addition to electron-deficient
alkenes (Scheme 2).
An experiment was performed for showing the
chemoselectivity between thiols and aniline using
acrylonitrile in TEAA (Scheme 3).
The ionic liquid was recycled and reused for 10
times without apparent loss of reactivity and yield in
the reaction of N-phenylpiperazine (1c) with acrylo-
nitrile (Fig. 1). TEAA is air and water stable and easy
to synthesize from triethylamine and acetic acid,
which are relatively cheap. It is noteworthy that
the preparation of TEAA ionic liquid is direct, sim-
ple, high yielding, and environment friendly, elimi-
nating the need of volatile organic solvents, such as
dichloromethane and acetonitrile used in some re-
ported ionic liquids [22].
In the first condition, the Michael acceptor was
used in 1.2 equivalents, whereas in second condition
the Michael acceptor was used in 2.4 equivalents.
The progress of reaction under both conditions was
monitored after 1 min by TLC and GC analysis. The
aniline remained unreacted while the thiol reacted
completely with 100% conversion to the addition
product 3. After 20 min, the TLC and GC analysis
found that both thiol and aniline were reacted com-
pletely to 3 and 4. This experiment could be applied

1044
A. K. Verma et al.
Table 2 Conjugate addition of aromatic thiols to conjugated alkenes
Entry
Comp.
Enones
R
Time=min
Yield=%a
Ref.
[11]
1
2a
4-MePhSH
1
96
2
3
4
5
6
7
8
2b
2c
2d
2e
2f
PhSH
1
1
1
1
1
1
1
96
99
98c
96
99c
99
99
[11]
[11]
PhCH2SH
HS–(CH2)7CH3
4-NO2PhSH
4-MePhSH
PhSH
[11]
2g
2h
[11]
[11]
4-MePhSH
9
2i
PhSH
1
1
1
1
95
[7c]
10
11
12
2j
2k
2l
2-MePhSH
4-MePhSH
HS–(CH2)7CH3
97c
96c
99c
a Isolated yield
Scheme 2
This leads to the conclusion that if we take the
Michael acceptor in about equimolar quantity it
gave the mono-addition product (Table 1, 1a), but
if we take it in excess it will give the bis-addition
product.
Experimental
All reagents used were AR grade. Melting points were de-
termined using a Thomas Hoover melting point apparatus
Fig. 1 Catalytic cycles versus yield
1
and are corrected. H (300 MHz) and 13C NMR (75 MHz)
spectra were recorded on a Bruker 300 NMR spectrometer in
CDCl3 (with TMS for 1H and chloroform-d for 13C as inter-
nal references) unless otherwise stated. Mass spectra were
recorded on a Hybrid Quadrupole-TOF LC\MS\MS mass
spectrometer (Q. Star XL). Microanalyses were obtained
in various synthesis applications. Thus, if we treat
1 mmol piperazine with 2.4 mmol acrylonitrile the di-
alkyl addition product with addition of TEAA was
obtained (Scheme 4).

Trichlorogermyl dioic acids
1045
Scheme 3
Scheme 4
Table 3 Comparison of different catalyst
Entry
Catalyst
Time=h
Yield=%
Ref.
1
2
3
4
5
6
H2O
24
84
24
8
24
0.3
0
96
78
88
2
[8]
[18]
[9]
[10]
[13]
[bmim][OH]
CAN
ꢁ-CD
solvent free
TEAA
96
with an Elemental Analysensysteme GmbH VarioEL V3.00
element analyzer. The results were in good agreement with
the calculated values. The reactions were monitored by thin
layer chromatography (TLC) using aluminum sheets with
silica gel 60 F254 (Merck).
mixture was stirred for 2 h at 80ꢀC to ensure that the reaction
had proceeded to completion. The reaction mixture was then
dried at 80ꢀC in high vacuum (5mm Hg) until the weight of
the residue remained constant. The yield of TEAA was 98%.
1H NMR (DMSO-d6): ꢂ ¼ 1.18 (t, 9H), 2.10 (s, 3H), 3.10 (m,
6H), 9.0 (s, 1H) ppm.
Modified procedure of triethylammonium ionic liquid (TEAA)
synthesis [23]
Typical procedure for aza-Michael reaction of N-alkyl-,
N-arylpiperazines, aromatic amines, and heterocycles
with ꢀ,ꢁ-unsaturated nitriles and carbonyl compounds
using TEAA ionic liquid
The synthesis of ionic liquid was carried out in a 250 cm3
round-bottomed flask, which was immersed in a water-bath
and fitted with a reflux condenser. Acetic acid (1.5mol, 90.1g,
and 86.03 cm3) was dropped into 101.2 g triethylamine (1 mol,
139.4cm3) at 70ꢀC within 1 h. After the addition, the reaction
A solution of 1 mmol amine and 1.2 mmol ꢀ,ꢁ-unsaturated
nitriles and carbonyl compounds was added to 2 cm3 TEAA,

1046
A. K. Verma et al.
and the mixture was stirred at 25ꢀC for 1–30 min. The
completion of the reactions was monitored by TLC. The
product formed in the one-phase system, was extracted
with 2ꢁ25 cm3 ether. The combined organic layer was
separated, dried (Na2SO4), and evaporated under reduced
pressure to afford the addition products. The aqueous layer
consisting of the IL was subjected to distillation at 80ꢀC
and 5 mm Hg for 2 h to remove water, leaving behind
the IL, which was further recycled. The conversion and
the yield were not reduced after being reused 10 times. The
products thus isolated, were pure and recrystallized from
EtOAc:petroleum ether (single spot on TLC).
3.05 (t, J ¼ 4.6Hz, 4H), 2.76 (t, J ¼ 6.7 Hz, 2H), 2.65 (t, J ¼
4.5 Hz, 4H), 2.56 (t, J ¼ 6.7 Hz, 2H) ppm; 13C NMR (75 MHz,
CDCl3): ꢂ ¼ 144.1, 140.2, 118.6, 116.0, 53.2, 52.7, 50.7, 30.6,
15.7ppm; LC-MS: m=z (%) ¼ 230.15 (100, M þ 1).
3-[4-[(Benzo[1,3]dioxol-5-yl)methyl]piperazin-1-yl]propio-
nitrile (1i, C15H19N3O2)
Mustard colour oil; 1H NMR (300MHz, CDCl3): ꢂ ¼ 6.83 (s,
1H), 6.73 (s, 2H), 5.93 (s, 2H), 3.40 (s, 2H), 2.68 (t, J ¼
6.98Hz, 2H), 2.51–2.46 (m, 10H) ppm; 13C NMR (75 MHz,
CDCl3): ꢂ ¼ 151.2, 148.5, 138.0, 122.0, 109.3, 107.7, 100.7,
62.4, 53.1, 52.5, 31.7, 15. 6 ppm; LC-MS: m=z (%) ¼ 273.13
(100, M þ 1).
General procedure of the thia-Michael reaction
A solution of 1 mmol thiols and 1.2 mmol ꢀ,ꢁ-unsaturated
nitriles and carbonyl compounds in 2 cm3 TEAA was stirred
at 25ꢀC for 1 min. The completions of the reactions were
monitored by TLC. The product formed in one phase sys-
tem was diluted with 20 cm3 H2O and extracted with
2ꢁ25 cm3 ether. The combined organic layer was separat-
ed, dried (Na2SO4), and evaporated under reduced pressure
to afford the addition products. The aqueous layer consist-
ing of the IL was subjected to distillation at 80ꢀC and
5 mmHg for 2 h to remove water, leaving behind the IL,
which was further recycled. The conversion and the yield
were not reduced after being reused 10 times. The products,
thus isolated, were pure and recrystallized from EtOAc:
petroleum ether (single spot on TLC).
3-(Octylsulfanyl)cyclohexanone (2d, C14H26OS)
1
Colorless oil; H NMR (300 MHz, CDCl3): ꢂ ¼ 0.89 (t, J ¼
6.6 Hz, 3H), 1.26–1.36 (m, 10H), 1.51–1.59 (m, 2H), 1.66–
1.75 (m, 2H), 2.11–2.17 (m, 2H), 2.33–2.35 (m, 4H), 2.52
(t, J ¼ 7.35 Hz, 2H), 3.05–3.57 (m, 1H) ppm; 13C NMR
(75 MHz, CDCl3): ꢂ ¼ 14.06, 22.61, 24.26, 25.65, 28.93,
29.14, 29.69, 30.53, 31.65, 31.76, 40.95, 42.74, 48.25,
209.02 ppm; LC-MS : m=z (%) ¼ 242.17 (100, M þ 1).
3-(p-Tolylsulfanyl)cyclohexanone (2f, C13H16OS)
Colorless oil; 1H NMR (300MHz, CDCl3): ꢂ ¼ 1.61–1.77 (m,
2H), 2.01–2.69 (m, 9H), 2.63–3.36 (m, 1H), 7.09–7.14 (m,
2H), 7.32–7.39 (m, 2H) ppm; 13C NMR (75 MHz, CDCl3):
ꢂ ¼ 21.1, 24.1, 31.3, 40.8, 46.5, 47.8, 128.5, 129.1, 129.8,
133.9, 138.2, 208.9 ppm; LC-MS: m=z(%) ¼ 220.1 (100, M þ 1).
3-(4-Phenylpiperazin-1-yl)cyclohexanone (1e, C16H22N2O)
Colorless oil: 1H NMR (300 MHz, CDCl3): ꢂ ¼ 7.29 (m, 2H),
7.92 (m, 3H), 3.20 (t, J ¼ 4.95Hz, 4H), 2.8–2.6 (m, 6H), 2.5–
2.3 (m, 3H), 2.2–2.0 (m, 2H), 1.76–1.72 (m, 2H) ppm; 13C
NMR (75 MHz, CDCl3): ꢂ ¼ 210.5, 151.2, 129.1, 119.8,
116.1, 63.2, 49.5, 48.9, 44.4, 41.2, 28.1, 22.4ppm; LC-MS:
m=z (%) ¼ 258.17 (100, M þ 1).
3-(2-Methoxyphenylsulfanyl)butyraldehyde (2j, C11H14O2S)
Colorless oil; 1H NMR (300MHz, CDCl3): ꢂ ¼ 2.52–2.72 (m,
3H), 3.89 (s, 3H), 6.82–6.95 (m, 3H), 7.24–7.34 (m, 1H)
ppm; 13C NMR (75 MHz, CDCl3): ꢂ ¼ 21.0, 35.8, 50.2, 55.7,
110.9, 121.0, 121.3, 127.6, 129.4, 159.1, 200.9ppm; LC-MS:
m=z (%) ¼ 210.1 (100, M þ 1).
3-[4-(4-Chlorophenyl)piperazin-1-yl]propionitrile
(1f, C13H16ClN3)
3-p-Tolylsulfanyl-butyraldehyde (2k, C11H14OS)
Colorless oil; 1H NMR (300 MHz, CDCl3): ꢂ ¼ 1.35–1.37 (d,
J ¼ 6.90 Hz, 3H), 2.42 (s, 3H), 2.56–2.75 (m, 2H), 3.64–
3.75 (m, 1H), 7.12–7.41 (m, 4H), 9.75 (s, 1H) ppm; 13C
NMR (75 MHz, CDCl3): ꢂ ¼ 20.8, 21.0, 36.9, 50.2, 126.5,
126.7, 127.5, 130.5, 132.6, 133.3, 140.2, 200.4 ppm; LC-
MS: m=z (%) ¼ 194.0 (100, M þ 1).
Light yellow solid, mp 98–100ꢀC; 1H NMR (300MHz,
CDCl3): ꢂ ¼ 7.20 (d, J ¼ 6.9 Hz, 2H), 6.83 (d, J ¼ 6.9 Hz,
2H), 3.17 (t, J ¼ 4.6 Hz, 4H), 2.74 (t, J ¼ 6.9 Hz, 2H), 2.66
(t, J ¼ 4.6 Hz, 4H) 2.55 (t, J ¼ 6.8 Hz, 2H) ppm; 13C NMR
(75 MHz, CDCl3): ꢂ ¼ 149.6, 128.9, 124.7, 118.5, 117.3, 52.5,
49.0, 15.9ppm.
3-(Octylsulfanyl)propionitrile (2l, C11H21NS)
1
3-[4-(4-Nitrophenyl)piperazin-1-yl]propionitrile
(1g, C13H16N4O2)
Colorless oil; H NMR (300 MHz, CDCl3): ꢂ ¼ 0.88 (t, J ¼
6.0 Hz, 3H), 1.27–1.40 (m, 10H), 1.54–1.69 (m, 2H), 2.56–
2.66 (m, 4H), 2.75–2.80 (m, 2H) ppm; 13C NMR (75 MHz,
CDCl3): ꢂ ¼ 14.09, 18.93, 22.42, 22.64, 27.40, 28.55, 29.24,
29.94, 31.81, 32.04, 118.24 ppm; LC-MS: m=z (%) ¼ 199.14
(100, M þ 1).
Yellow solid, mp 92–94ꢀC; 1H NMR (300MHz, CDCl3): ꢂ ¼
8.10 (d, J ¼ 9.06 Hz, 2H), 6.81 (d, J ¼ 9.12 Hz, 2H), 3.44 (d,
J ¼ 4.5 Hz, 4H), 2.77–2.54 (m, 8H) ppm; 13C NMR (75 MHz,
CDCl3): ꢂ ¼ 154.6, 138.5, 125.8, 118.4, 112.7, 53.0, 52.0,
46.8, 16.0ppm.
3-[4-(4-Aminophenyl)piperazin-1-yl]propionitrile
(1h, C13H18N4)
Acknowledgements
Brown solid, mp 92–94ꢀC; 1H NMR (300 MHz, CDCl3): ꢂ ¼
We gratefully acknowledge financial support from Department
of Science and Technology, New Delhi, for this work.
6.80 (d, J ¼ 8.2 Hz, 2H), 6.64 (s, 8.3, 2H), 3.29 (s, br, 2H),

Trichlorogermyl dioic acids
1047
Jessica JL, Loanna N, Kim L, Trans JK, Kristin WJ,
David FC, Davis JH (2002) J Am Chem Soc 124:5962; d)
Wasserscheid P, Keim W (2000) Angew Chem Int Ed
39:3773; e) Hanke CG, Afamas NA, Lynden-Bell RM
(2002) Green Chem 4:107; f) Wang Y, Li H, Han S (2005)
J Chem Phys 123:174501; g) Wang Y, Li H, Han S (2006)
J Chem Phys 124:044504
References
1. a) Han LB, Tanaka M (1999) Chem Commun:395;
b) Arredondo VM, Tian S, MacDonald FE, Marks TJ
(1999) J Am Chem Soc 121:3633
2. Hayao S, Schut RN (1961) J Med Chem 26:3414
´
3. Orjales A, Alonso-Cires L, Labeaga L, Corcostegui R
(1995) J Med Chem 8:1273
4. Ahmad YE, Laurent E, Maillet P, Talab A, Teste JF,
Dohkan R, Tran G, Ollivier V (1997) J Med Chem 40:952
5. a) Loh TP, Wei LL (1998) Synlett:975; b) Bartoli G,
Bosco M, Marcantoni E, Petrini M, Sambri L, Torregiani
E (2001) J Org Chem 66:9052; c) Matsubara S, Yashioka
M, Utimoto K (1994) Chem Lett:827; d) Wabnitz TC,
Spencer JB (2002) Tetrahedron Lett 43:3891; e) Jenner G
(1995) Tetrahedron Lett 36:233; f) Srivastava N, Banik
BK (2003) J Org Chem 68:2109; g) Varala R, Alam MM,
Adapa SR (2003) Synlett:720; h) Aziz N, Saidi MR
(2004) Tetrahedron 60:383
16. a) Liu FC, Abrams MB, Baker RT, Tumas W (2001)
Chem Comm:433; b) Bates ED, Mayton RD, Ntai I,
Davis JH (2002) J Am Chem Soc 124:926; c) Earle
MJ, McCormac PB, Seddon KR (2000) Green Chem
2:261; d) Olah GA, Mathew T, Goeppert A, Torok B,
Bucsi I, Li XY, Wang Q, Marinez ER, Batamack P,
Aniszfeld R, Prakash GKS (2005) J Am Chem Soc
127:5964
17. Bradaric CJ, Downard A, Kennedy C, Robertson AJ,
Zhou Y (2003) Green Chem 5:143
18. Yang L, Xu LW, Zhou W, Li L, Xia CG (2006) Tetrahe-
dron Lett 47:7723
6. Kantam CM 3, Neeraja V, Kavita B, Neelima B, Chaudhuri
MK, Hussain S (2005) Adv Synth Catal 347:763
7. a) Yadav JS, Reddy BVS, Basak AK, Narsaiah AV (2003)
Chem Lett 32:988; b) Xu LW, Li JW, Zhou SL, Xia CG
(2004) New J Chem 28:183; c) Ranu BC, Dey SS, Hajra
A (2003) Tetrahedron 59:2417
8. a) Khatik GL, Kumar R, Chakraborti AK (2006) Org Lett
8:433; b) Ranu BC, Dey SS, Hajra A (2007) Tetrahedron
Lett 48:141
19. a) Weyershausen B, Lehmann K (2005) Green Chem
7:15; b) Weyershausen B, Hell K, Hesse U (2005) Green
Chem 7:283
20. Ganeshpure PA, George G, Das J (2007) Arkivoc
(viii):273
21. Verma AK, Kumar R, Chaudhary P, Saxena A, Shankar R,
Mozumdar S, Chandra R (2005) Tetrahedron Lett 46:5229
22. a) Wu W, Li W, Han B, Zhang Z, Jiang T, Liu Z (2005)
Green Chem 7:701; b) Hobrey JD, Reichert WM,
Swatloski RP, Broker GA, Pitner WR, Seddon KR,
Rogers RD (2002) Green Chem 4:407
9. Chaudhuri MK, Hussain S, Kantam CM 3, Neelima B
(2005) Tetrahedron Lett 46:8329
10. Surendra K, Krishnaveni NS, Sridhar R, Rao KR (2006)
Tetrahedron Lett 47:2125
11. Khatik GL, Sharma G, Kumar R, Chakraborti AK (2007)
Tetrahedron 63:1200
23. a) Wang C, Guo L, Li H, Wang Y, Weng J, Wu L (2006)
Green Chem 8:603; b) Weng J, Wang C, Li H, Wang Y
(2006) Green Chem 8:96; c) Jiang H, Wang C, Li H,
Wang Y (2006) Green Chem 12:1076
12. Yeom CE, Kim MJ, Kim BM (2007) Tetrahedron 63:904
13. a) Ranu BC, Dey SS, Hajra A (2002) Arkivoc (vii):76;
b) Movassagh B, Shaygan P (2006) Arkivoc (xii):130
14. Ranu BC, Dey SS, Samanta S (2005) Arkivoc (iii):44
15. a) Rogers RD, Seddon KR (2003) Science 302:792; b)
Sheldon R (2005) Green Chem 7:267; c) Amanda CC,
24. Kamal A, Reddy DR, Rajendar (2006) Tetrahedron Lett
47:2261
25. Xu JM, Qian C, Liu BK, Wu Q, Lin XF (2007) Tetrahe-
dron 63:986
26. Zhang H, Zhang Y, Liu L, Xu H, Yang Y (2005) Synthesis
13:2129