recent advances in ionic liquids
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Recent advances in ionic liquidsTRANSCRIPT
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Recent advances in ionic liquids: green unconventionalsolvents of this century: part ISuresh a & Jagir S. Sandhu aa Department of Chemistry , Punjabi University , Patiala, 147002, Punjab, IndiaPublished online: 19 May 2011.
To cite this article: Suresh & Jagir S. Sandhu (2011) Recent advances in ionic liquids: green unconventional solvents of thiscentury: part I, Green Chemistry Letters and Reviews, 4:4, 289-310, DOI: 10.1080/17518253.2011.572294
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RESEARCH REVIEW
Recent advances in ionic liquids: green unconventional solvents of this century: part I
Suresh and Jagir S. Sandhu*
Department of Chemistry, Punjabi University, Patiala 147002, Punjab, India
(Received 23 October 2009; final version received 11 March 2011)
The present overview describes ionic liquids as alternate, attractive solvents of today and tomorrow in organic
synthesis. Since this subject is too wide and developments have been too fast, only a recent account is presentedon indispensable carbon-carbon bond forming named reactions such as Knoevenagel, Michael Aldol, BiginelliReaction, and so on, which has never been done before exclusively.
Keywords: ionic liquids; Aldol reaction; Knoevenagel reaction; Michael reaction; Biginelli reaction; green
chemistry
Introduction
There have been fast developments in all spheres of
our society and these are interwoven with problems
related to environmental pollution. Certainly, access
to advanced materials and their preparation processes
were great contributions to these developments (1).
A good amount of contributions to these problems
come from chemical/material developments. Thus,
overall alarming environmental pollution prompted
the government of the United States (2, 3) to issue an
act (Pollution Prevention Act, 1990). This provided
an impetus to other countries and arose worldwide
concern and in the scientists playing a significant role
in developing pollution-free methodologies/processes
(4, 5). Significant approaches are listed below:
. Supercritical liquids
. Biodegradable materials
. Catalysts selection/developments
. Biological process
. Ionic liquids
. Last but not least avoiding organic solvents
All the above areas contributed significantly to the
development (6�33) of green chemistry. Twelveprinciples of Green Chemistry were laid down by
Paul Anastas and John Warner (34) as follows:
(1) Prevention: It is better to prevent waste than to treat
or clean up waste after it has been created.
(2) Atom economy: Synthetic methods should be de-
signed to maximize the incorporation of all materials
used in the process into the final product.
(3) Less hazardous chemical syntheses: Wherever prac-
ticable, synthetic methods should be designed to use
and generate substances that possess little or no
toxicity to human health and the environment.
(4) Designing safer chemicals: Chemical products should
be designed to effect their desired function while
minimizing their toxicity.
(5) Safer solvents and auxiliaries: The use of auxiliary
substances (e.g. solvents, separation agents, etc.)
should be made unnecessary wherever possible and
innocuous when used.
(6) Design for energy efficiency: Energy requirements of
chemical processes should be recognized for their
environmental and economic impacts and should be
minimized. If possible, synthetic methods should be
conducted at ambient temperature and pressure.
(7) Use of renewable feedstocks: A raw material or
feedstock should be renewable rather than depleting
whenever technically and economically practicable.
(8) Reduce derivatives: Unnecessary derivatization (use
of blocking groups, protection/deprotection, tem-
porary modification of physical/chemical processes)
should be minimized or avoided if possible, because
such steps require additional reagents and can
generate waste.
(9) Catalysis: Catalytic reagents (as selective as possible)
are superior to stoichiometric reagents.
(10) Design for degradation: Chemical products should be
designed so that at the end of their function they
break down into innocuous degradation products
and do not persist in the environment.
(11) Real-time analysis for pollution prevention: Analyti-
cal methodologies need to be further developed to
allow for real-time, in-process monitoring and con-
trol prior to the formation of hazardous substances.
*Corresponding author. Email: [email protected]
Green Chemistry Letters and ReviewsVol. 4, No. 4, December 2011, 289�310
ISSN 1751-8253 print/ISSN 1751-7192 online
# 2011 Taylor & Francis
http://dx.doi.org/10.1080/17518253.2011.572294
http://www.tandfonline.com
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(12) Inherently safer chemistry for accident prevention:
Substances and the form of a substance used in a
chemical process should be chosen to minimize the
potential for chemical accidents including releases,
explosions, and fires.
Ionic liquids fulfill all conditions of green chemistry
required of green solvents; prominently, they are not
volatile and several of them are recyclable and do not
leave any hazardous wastes on work-up.
Ionic liquids (ILs) background
Keeping in mind the above principles of green
chemistry, ILs have attracted much attention in the
scientific community (chemists, biologists, and other
related workers) during the past two decades or so.
Here, in this mini review we are presenting recent
developments (e.g. 1999 to 2009 also if scanty work is
there deviations are made) in selected carbon-carbon
bond forming reactions such as Knoevenagel,
Michael Aldol, Biginelli Reaction, and so on. Before
we proceed further, we provide some background
information about these new solvents promoted as
future green solvents of the present century. Histori-
cally, ILs are mentioned as molten salts and this dates
back to 1914 or even before (35), when the first ionic
liquid was reported. However, its earliest use was as a
propellant in warfare specifically � ethylammoniumnitrate. Though there are no hard and fast rules layed
down, there are considered to be ionic salts sub-
stances having a melting point up to 1008C. They arecertainly advocated to have the following properties,
which have generated a voluminous body of research
(36�45):
(1) Unlike conventional solvents, they are not volatile
and do not have any vapor pressure;
(2) They are stable over a long temperature range;
(3) They can be called universal solvents, as they can
dissolve a range of organic compounds;
(4) They can dissolve even gases like H2, CO, O2, and
CO2. They can be used even under supercritical CO2;
(5) In ILs, the solubility determining factors are cations
and anions of which these are composed;
(6) They do not participate in co-ordination with metal
complexes, macrocycles like enzymes, etc;
(7) Mainly, the ionic character of ILs accelerates the
rate of reaction even under MW irradiations;
(8) They are stable and can be stored without decom-
position for a long time;
(9) ILs have found extensive use in the control of
stereoselectivity.
(10) The viscosity of ILs derived form imidazoles can be
manipulated by variations in branching.
Because of these attractive properties, ILs are
employed in a broad area of applications listed below
(46�71):
(1) Solvent extraction (46);
(2) Physico-chemical processes (47);
(3) ILs as media for nucleophilic substitution reactions(47);
(4) As mobile phase modifier in HPLC (48);
(5) Electrodeposition of metals and semiconductors inILs (49);
(6) Chemical analysis (50);
(7) Dye-sensitized solar cells (51, 52);
(8) ILs for the nuclear fuel cycle: electrodeposition andextraction (53);
(9) Nuclear-based separations (54);
(10) Oil shale processing (55);
(11) Separation of petrochemical relevance (56);
(12) Synthesis of functional nanoparticles and other
inorganic nanostructures (57);
(13) ILs as solvents for electrochemistry (58);
(14) ILs as solvents for polymerization processes (59);
(15) Chemical and biochemical transformations (60);
(16) materials chemistry (61);
(17) Biocatalysts in ILs (62�71).
There are many types of ILs available commercially
and some of these that are conveniently available
and used in organic synthesis. The following selected
classes are given below as a reference for the readers
and some of these are often used in the reactions
presented in this paper.
Ionic liquids classification
Ionic Liquids-AnionsA) Borate:I. Tetracyanoborate TCB: [B(CN)4]�
II. Tetrafluoroborate TFB: [BF4]�
III. Oxalatoborate BOB: [B(C2O4)2]�
B) Dicyanamide DCN: [N(CN)2]�
C) Halide: Br�, Cl�, F�, I�
I. Bromide: BrII. Chloride: Cl�
III. Fluoride: F�
IV. Iodide: I�
D) Bis(trifluoromethylsulfonyl)imide NTF:
[N(SO2CF3)2]�
E) Nonaflate NON: [C4H9SO3]�
F) PhosphateI. AlkylphosphateII. Fluoroalkylphosphate FAP: [(C2F5)3PF3]
�
III. Hexafluorophosphate HFP: [PF6]�
G) Sulfate HSO4: [HSO4]�, and Alkylsulfate:
[RSO4]�
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H) SulfonateI. Methanesulfonate MSO: [CH3SO3]
�
II. Tosylate TOS: [CH3C6H4SO3]�
III. Trifluoromethanesulfonate OTF:[CF3SO3]
�
I) Thiocyanate SCN: [SCN]�
J) Tricyanomethide TCC: [C(CN)3]�
http://www.merck-chemicals.com/1-butyl-3-methylimidazolium-tricyanomethane/MDA_CHEM-490330/p_Klmb.s1OrfMAAAEj0FgLFwSw
Ionic Liquids-CationsA) Ammoniums N(R,R1,R2,R3):
N+
R1
R
R3 R2B) Guanidiniums GUA:
C NH2
NH2
NH2
+
C) Imidazoles MIM:I. Disubstituted Imidazoles
N+
NRCH3
II. Trisubstituted Imidazoles
N+
NRCH3
CH3
D) Morpholines MOEMMO:
N+
O
CH3O
CH3
E) Phosphoniums PH3T:
P+
H3C
H3C
CH3
CH3
[ ]4
[ ]4
[ ]4
[ ]12
F) Piperidines MOEMPIP:
N+
CH3O
CH3
G) Pyridiniums PYR:
N+
R R
H) Pyrrolidines MPL:
N+H3CR
I) Sulfones
S+
CH3
CH3CH3
Apart from the areas mentioned in the forgoing
pages there are special reviews and even journals
devoted to developments in organic synthesis (72�85). The authors are presenting the latest develop-
ments using ILs in the following named reactions of
organic chemistry, which has never been done by
previous reviewers exclusively. This paper also in-
cludes the authors’ own work in the area of green
chemistry
. Aldol reaction
. Knoevenagel reaction
. Michael reaction
. Biginellli and Hanstzch reaction (combination ofKnoevenagel and Michael)
. Other related ones like Doebnor modification
Aldol reaction
Among carbon-carbon bond forming reactions in
organic chemistry, the Aldol reaction (Scheme 1) is
the most popular for this process; it was discovered
by Charles-Adolphe Wurtz and A. P. Borodin
R
H
O
+MeMe
O
IL
R Me
OH O
R = Ph, 4-O2NC6H4, 2-O2NC6H4, 4-BrC6H4, 4-MeC6H4, i-Pr, 3-O2NC6H4.
Scheme 1. Simple Aldol reaction using Ils.
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independently in 1872 and thus have a long history(86�88). Aldol, when dehydrates, yields ab-unsatu-rated ketones called chalcones and is known as theClaisen�Schmidt reaction.In both these reactions the present authors, while
looking for greener procedures/processes, have devel-oped the use of Al(III) and Bi(III) (89). However, theuse of ILs in this reaction is of latest interest and isdiscussed below.
Knoevenagel reaction
This reaction represents an efficient way of producingcarbon-carbon double bonds and was discoveredback in 1894 by Knoevenagel (101�103). The con-ventional Knoevenagel reaction (Scheme 2) has beenreported in several reviews and monographs; how-ever, the use of ILs in this reaction is recent. In thepursuit of developing green chemistry we did makeconsiderable efforts for solvent free and also milderLewis acids catalysts developments such as LiBr,CdI2, BiCl3, KI, Alum (104�108), and so on.Evidently, these were milder processes than the earlierreported ones employing strong Lewis acids or strongorganic bases. No doubt that ILs are of currentinterest for use in this reaction and developments aregiven below.
Doebner condensation/modification
Doebner condensation is a modified version ofKnoevenagel reaction and pyridine or piperidinetype catalysts have always been used to produceunsaturated acid from aldehydes and malonic acids(Scheme 3 and 4).Evidently, these catalysts are harmful to humans.
In pursuit of green processes, the present authorsused BiCl3 as a mild catalyst (141). In IL this reaction
was used, though less studied but there are a few cases
such as this reaction, being carried (142) in ionic
liquids [Bmim]BF4 and [Bpyr]BF4 (I & II) to synthe-
size ab-unsaturated carboxylic acid.
Michael reaction
The discovery of this reaction dates back to 1883; it
was generalized by and named after Arthur Michael(143�145). The reaction is very similar to the Knoe-venagel reaction and in conventional chemistry, in
both the reactions, similar solvents and catalysts have
been employed and volumous research work reported
and both seem complementary to each other. Pre-cisely, Knoevenagel is 1-2 and Michael is 1-4 con-
jugate addition (Scheme 5) on to carbonyl and
electron deficient alkenes, respectively. In pursuit of
green chemistry, here also the authors made con-
siderable contributions in developing some environ-mentally benign protocols using non-polluting
catalysts and replacing volatile organic solvents
(VOCs) under solvent-free conditions. Already re-
ported by the authors are catalysts like BiCl3, Cu(II),
alumina, and so on (146�151), efforts to obtainsolvent-free mild catalyst are much reported by theauthors. ILs being solvents of the present, their recent
progress is given here.
Biginelli and Hanztsch reactions
Development of environmental benign protocols can
be briefly characterized as time saving, pollution
free, and reducing reaction steps (novel nomencla-
ture for these is multi-component reactions MCRs);clearly these are prominent steps toward green
chemistry. These two reactions are selected since
they are now three well-developed component reac-
tions and also they are a combination of previously
discussed two reactions, viz., Knoevenagel followedby Michael so both are tandem/cascade reactions
(Scheme 6). The following scheme presenting a
plausible reaction scheme for the Biginelli reaction
is accepted today.Here, urea, acetoacetic ester, and aldehydes are
employed (see Scheme 7) as a three component one
RCHO +EWG1
EWG2RHC
R2
R1
Cat.
-H2O
Scheme 2. Acid catalyzed Knoevenagel reaction.
OH
O
OH
O
H
O
R1
R2
R3R1
R2
R3
+H
OH
O
Cat.
Scheme 3. Base or acid catalyzed Doebner modification.
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pot synthesis. For detailed accounts of both thesereactions see the most recent accounts (161, 162).Though conventional Biginelli involves aldehydes,urea, and active methylene compounds, authorsdemonstrated that even ketones can participate inthis reaction affording unconventional Biginelli com-pounds in excellent yields (163).
As usual, organic reactions are named after their
discoverers. In conventional preparative methods of
Biginelli compounds and Hanstzch pyridines, hun-
dreds of papers have been published employing green,
mild Lewis acids. The authors made considerable
efforts in developing green process/procedures for
these reactions as well. As such, the authors made
significant contribution for finding green reactions/
protocols and demonstrated the efficacy of LiBr, Cu,
Ni, Ga(III), lactic acid, and their salts in solvent-free
conditions too (164�168). Certainly, these weremilder Lewis acids than the ones used conventionally
earlier, as there were no corrosive side products or
waste formed during aqueous work-up so these were
Scheme 4. IL catalyzed Doebner condensation.
X Y
O O
R
+ H2C EWGIL/Catalyst
O
YOC
R
EWGX
Scheme 5. Michael reaction employing a variety ofcatalysts.
I & II
NH
NH
R
O
NH
NH
O
R
H3C
H3C
H3C
H3C
OH
NH
OO
H2N
R
R'O2C
R'O2C
R CH
N+
NH2
O
H
+OR'
O O
-H2O
89
10 2
Scheme 6. A plausible reaction pathway of Biginelli reaction.
CHO
+ NH2H2N
O
CH3O
+ NH
NH
OR
R
Catalysts
Solvents
Scheme 7. Ketone instead of 1,3-dicarbonyl employed Biginelli reaction.
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greener protocols. The use of ILs in combination withmild Lewis acids is a fertile area of research andsystematic developments are given below.
Hantzsch reactions
Historically 1, 4-dihydropyridine 1 preparation isdescribed by Arthur Hantzsch more than a centuryago (190, 191). Exploration of these pyridines initiallywere quite slow, later it picked up very fast because oftheir structural resemblance to reduced nicotinamideadenine dinucleotide (NADH) 2, which is an estab-lished hydrogen transferring agent in biologicalprocesses (192). Clearly, the Hantzsch pyridines area subset of the co-enzyme 2.
NH
R
OR1
OO
R1O
Me Me
NN
N
N
N
NH2
OCH2O
OHOH
P
O
OH
O
OH
O
O
PH2C
O
H
OH
H
OH
CONH2
H H
HH H
H
1 2
These pyridines are commonly called Hantzschpyridines and the reaction is called Hantzsch reac-tion. The original synthesis reported by Hantzsch isthree components coupling reaction in refluxingethanol, which involved acetoacetic ester, benzalde-hyde, and ammonia (Scheme 8).
Evidently, the first step isKnoevenagel followed byMichael, and later on cyclization incorporating in thering system (Scheme 9). For a detailed account andmechanism see (193). Since these molecules had greatsignificance to biologists, they are taken to be privi-leged molecule of chemistry (possessing more than oneactivity) and their use in clinical practice attractedmuch attention of chemists to develop greener produc-tion process. Present authors also made effectivecontribution in this direction (194, 195). Some selectedexamples of these molecules in clinical use are pre-sented below. As such there are more than two dozendrug molecules in clinical use from this family. Need-less to say, a similar number is under developmentstage as anti-hypertensive agents (196, 197). For aselection see below:Lercanidipine(Lerdip, Recordati, Italy, 1997)Aranidipine
(Bec/Sapresta, Maruko Seiyaku, Japan, 1996)
Cilnidipine
(Cinalong or Siscard, Fujirebio, Japan, 1995)
Efonidipine Hydrochloride Ethanol
(Landel, Nissan chemical, Japan, 1994)
NH
CH3 CH3
H3CO2C
NO2
O
OCH3CH3
N
CH3
. ClH
NH
CO2CH3
CH3CH3
O
OCH3
O
NO2
NH
CH3CH3
O
OO
OMeO
NO2
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Nilvadipine
(Nivadil, Fujisawa, Japan, 1989)
Isradipine
(Prescal, Sandoz, Switzerland, 1989)
Felodipine
(Plendil, Astra, Sweden, 1988)
Nifedipine
(1977)
Naturally, because of commercial significance ofHantzsch 1,4-dihydropyridine (DHP), ILs are used
NH
Ar
OC2H5
OO
CH3H3C H3C
H5C2O H5C2O
O
O
OC2H5
CH3
O
O
+
Ar
CHO
NH4OH
1
Scheme 8. Synthesis of Hantzsch pyridine.
ArCH
O
OOO
4
Ar OO
OO
3
NH3
H2N
OO Ar
OH O
NH2
Ar OO
ONH
OC2H5
OC2H5OC2H5
OC2H5
OO
H5C2O
H5C2O
H3C
H3C
H3C
CH3
CH3
CH3
OC2H5
OC2H5
H5C2O
H5C2O
H3C
H3C
CH3
CH3
Ar
- H2O
- H2O
7
81
Scheme 9. A plausible reaction pathway of Hantzsch 1,4-dihydropyridine.
NH
O
O
CH3
CH3CH3
O
OCH3
CH3
N
O
N
NH
O
O
CH3
CH3CH3
O
OCH3
Cl
Cl
NH
NO2
CH3 CN
O
O
CH3
O
OCH3
CH3
NH
P
CH3 CH3
O
O
N
O
O
CH3
CH3
O
NO2
HCl EtOH..
NH
NO2
COOCH3H3COOC
CH3 CH3
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in this important reaction and these developments are given in Table 1:
Table 1. ILs used in DHP as catalysts or/and solvents.
Catalyst used Reaction conditions References
Without catalyst 1,1,3,3-N,N,N?,N?-tetramethylguanidiniumtrifluoroacetate, ultrasound irradiation, r.t., 1.45�2.30 h
N H2
N
NCH3
CH3
CH3
CH3+
CF3COO-
(198)
3,4,5-Trifluorobenzeneboronicacid/ InCl3
1-butyl-3-methylimidazolium chloride, r.t.
N+
NCH3 C4H10
Cl-
(199)
Without catalyst 1-n-butyl-3-methylimidazolium hexafluorophosphate or1-n-butyl-3-methylimidazolium tetrafluoroborate, r.t.,4.5�12 h
N+
NCH3 C4H10
PF6-
or
N+
NCH3 C4H10
BF4-
(200)
Without catalyst 1-methylimidazolium trifluoroacetate, r.t.
N+
NHCH3
CF3COO-
(201)
1-Butyl-3-methylimidazoliumhydroxide
N+
NCH3 C4H10
OH-
solvent-free, r.t., 0.5�1.5 h (202)
1-Butyl-3-methylimidazoliumhydroxide
N+
NCH3 C4H10
OH-
solvent-free, r.t., 0.5�4.0 h (203)
Without catalyst 1-n-butyl-3-methylimidazolium tetrafluoroborate
N+
NCH3 C4H10
BF4-
(204)
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Catalyst used
Without catalyst
Reaction conditions
1-n-butyl-3-methylimidazoliumsaccharinate
N N+
CH3 C4H9
N -
S
OO
O
References
(205)
Table 2. ILs used in Aldol reaction act as catalyst or solvent or both catalyst and solvent.
Catalyst used Reaction conditions References
1-methyl-3-{2-[(2S)-pyrrolidine-2-carbonyloxy]-ethyl}-1H-imidazol-3-ium trifluoroacetate
N+
N
CH3
O
Pro-HCF3COO-
Solvent-free, r.t. 25 h (90)
1(R),2(R)-bis((S)-prolinamido)cyclohexane or(Rax)-2,2?-bis((S)-prolinamido)-1,1?-binaphtyl
NHNHOO
NHNH
or
1
NH
NH
O
O
NH
NH2
(1-butyl-3-methylimidazolium) tetrafluoroborate-water(1:1 v/v), 08C or 268C; 4�10 h or 20�25 h
N+
NCH3 C4H10
BF4-
. H2O
(91)
Imidazolium bis(trifluoromethylsulfonyl)imide-substituted proline and butyldimethylammoniumbis(trifluoromethylsulfonyl) imide-substituted
praline
1-butyl-3-methylimidazoliumBis(trifluoromethylsulfonyl) imide, r.t. 24 h
N+
NCH3 C4H10
N(SO2CF3)2-
(92)
Piperidine
NH
1-butyl-2,3-dimethylimidazolium tetrafluroborate, r.t.
N+
NC4H10
CH3
H3C
BF4-
(93)
Choline hydroxide Solvent-free, r.t.
(94)
Table 1. (Continued)
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Table 2 (Continued)
Catalyst used Reaction conditions References
N-methyl-3-aminopropylated silica (NHMe�SiO2) 1-butyl-3-methylimidazolium hexafluorophosphate, r.t.
N+
NCH3 C4H10
PF6-
(95)
1-butyl-3-methylimidazolium tetrafluoroborate
N+
NCH3 C4H10
BF4-
Solvent-free, r.t. (96)
L-proline
NH
O
OHH
1-butyl-3-methylimidazolium tetrafluoroborate
N+
NCH3 C4H10
BF4-
(97)
Morpholine
O
NH
1-butyl-3-methylimidazolium tetrafluoroborate,808C, 8�20 h
N+
NCH3 C4H10
BF4-
(98)
NaOH1-butyl-3-methylimidazolium hexafluorophosphate,
408C
N+
NCH3 C4H10
PF6-
(99)
Hydrotalcites 1-butyl-3-methylimidazolium hexafluorophosphate
and 1-butyl-3-methylimidazolium tetrafluoroborate,608C
N+
NCH3 C4H10
PF6-
orN
+N
CH3 C4H10
BF4-
(100)
Table 2 (Continued)
Table 3. ILs used in Knoevenagel reaction act as catalyst or solvent or both catalyst and solvent.
Catalyst used Reaction conditions References
Hydroxyapatite-encapsulated g-Fe2O3 nanoparticlesN-(3-propyltrimethoxysilane)imidazole
Water, 308C, 1h (109)
glycine
NH2
OHO
1,3-dimethylimidazolium methyl sulfate, r.t., 100 min
N+
N
[CH3SO4]-
(110)
[H3N��CH2�CH2�OH][CH3COO_] Solvent-free, r.t. 1 h (111)
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Table 3 (Continued)
Catalyst used Reaction conditions References
1-butyl-3-methylimidazolium imidazolide
N N+
NN
_
Water, r.t. 20 min (112)
Tri-(2-hydroxyethyl)ammonium acetate
N+
O-
O
OH
OH
OH
Tri-(2-hydroxyethyl) ammonium acetate, 808C, ½ h (113)
1-butyl-3-methylimidazolium hexafluorophosphatewith alkali metal hydroxide
N+
NCH3 C4H10
PF6-
. KOH or NaOH
Ethanol, r.t. 6 h (114)
Methoxyl propylamine acetate Solvent-free, 508C, 3�8 h (115)1-butyl-3-methyl imidazolium hydroxide
N+
NCH3 C4H10
HO-
Water, r.t. (116)
Metal carbonate 1-butyl-3-methylimidazolium hexafluorophosphate or1-butyl-3-methylimidazolium bromide- benzene
N+
NCH3 C4H10
PF6-
or
N+
NCH3 C4H10
Br-
.C6H6
(117)
Hexadecyltrimethyl ammonium chloride andethylmethyl imidazolium chloride
N+
C10H25
CH3
CH3
CH3
Cl- N+
NCH3
CH3
Cl-
Tetrahydrofuran, 808C, 12 h (118)
Hydrotalcites or without this catalyst
1-butyl-3-methylimidazolium hexafluorophosphate and1-butyl-3-methylimidazolium tetrafluoroborate, r.t.
N+
NCH3 C4H10
PF6-
or N+
NCH3 C4H10
BF4-
(119)
Glycine
NH2
OHO
see IL, 45�558C, 22 h (120)
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Table 3 (Continued)
Catalyst used Reaction conditions References
1-butyl-3-methyl imidazolium hydroxide
N+
NCH3 C4H10
HO-
CH3CN, r.t. 10�15 h (121)
1-butyl-3-methyl imidazolium hydroxide
N+
NCH3 C4H10
HO-
Solvent-free r.t. (122)
Hunig’s base tethered ammonium ionic liquids
N+
CH3
CH3
CH3
O
N
CH3
CH3
CH3
CH3
n
Solvent-free, 30 min. r.t. (123)
Potassium carbonate KCO3 1-n-butyl-3-methylimidazolium bromide, MWI
N+
NCH3 C4H10
Br-
(124)
1-methyl-3-(3-triethoxysilylpropyl imidazoliumchloride
Solvent-free, r.t. h; 1208C (125)
1-butyl-3-methylimidazolium hydroxide
N+
NCH3 C4H10
HO-
Solvent-free, r.t. 10�30 min (126)
L-proline
NH
O
OHH
1-n-butyl-3-methylimidazolium tetrafluoroborate,
12�48 h, 508C(127)
1-n-butyl-3-methylimidazolium tetrafluoroborate
N+
NCH3 C4H10
BF4-
Solvent-free, MWI/grinding, r.t. (128)
1-aminoethyl-3-methylimidazolium hexafluoropho-sphate
N+
NCH3 NH2
PF6-
Water, r.t. (129)
Table 3 (Continued)
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Table 3 (Continued)
Catalyst used Reaction conditions References
Piperidine
NH
1-ethyl-3-methylimidazolium tetrafluoroborate,CH3COOH, r.t.
N+
NCH3
BF4-
(130)
Without catalyst 1-methylimidazolium trifluoroacetate, r.t. 4�30 min
N+
NHCH3
CF3COO-
(131)
Without catalyst Ethylammonium nitrate, r.t. (132)
Piperidine
NH
1-butyl-3-methylimidazolium tetrafluoroborate, r.t.
N+
NCH3 C4H10
BF4-
(133)
1-hexyl-3-methylimidazolium tetrafluoroacetate
N+
NCH3 C6H15
CF3COO-
Solvent-free (134)
2-hydroxyethylammonium acetate
N+
H
H
H
OH
CH3COO-
Solvent-free, r.t. (135)
Without catalyst Ethylammonium nitrate, r.t. (136)
Without catalyst EAN, 1-butyl-3-methylimidazolium hexafluoropho-sphate and 1-butyl-3-methylimidazolium tetrafluorobo-
rate, r.t.
N+
NCH3 C4H10
PF6-
& N+
NCH3 C4H10
BF4-
(137)
1,1,3,3-tetramethylguanidium lactate
N H2
N
NCH3
CH3
CH3
CH3+
Lac-
Solvent-free (138)
Table 3 (Continued)
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Table 3 (Continued)
Catalyst used Reaction conditions References
NaOMe 1-butyl-3-methylimidazolium hexafluorophosphate,3�4.5 h 958C
N+
NCH3 C4H10
PF6-
(139)
GaCl3 Triethylammonium acetate (TEAA), 658C, 30�45 min (140)
Table 4. ILs used in Michael reaction act as catalyst, solvent, or both.
Catalyst used Reaction conditions References
(S)-pyrrolidine sulfonamide
NH
NHS
N+
N
O
O
CH3
CH3
BF4-
Isopropyl alcohol, r.t. (152)
Copper nanoparticles 1-H-tetrazole-5-acetic acid, r.t., 5�15 min
N
N
N+
N
NC
CH3
OH
O
Br -
(153)
Triethylammonium acetate (TEAA) Solvent-free, 258C, 1�20 min (154)
Piperidine
NH
Ionic liquid, r.t. 2 h
N+
NCH3 C2H5
ETSO4-
(155)
N-methylimidazole
N
N
CH3
Imidazolium-based ILs, r.t. (156)
Table 3 (Continued)
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Table 4 (Continued)
Catalyst used Reaction conditions References
Pyrrolidine-based functionalized chiral ionic liquids
N N+
SNH
CH3
O
O
NH
NTf2 -
Methanol, r.t. 3 days (157)
Pyrrolidine-based chiral pyridinium ionic liquids
NH
N+
X-
X = BF4, PF6, NTf2
Solvent-free, 16�48 h, 48C (158)
1-methyl-3-butylimidazolium hydroxide
N+
NCH3 C4H10
HO-
Solvent-free (159)
Metal carbonate 1-butyl-3-methylimidazolium hexafluorophosphate and1-butyl-3-methylimidazolium bromide � benzene
N+
NCH3 C4H10
PF6-
&
N+
NCH3 C4H10
Br-
.C6H6
(160)
Table 4 (Continued)
Table 5. ILs used in Biginelli reaction act as catalyst, solvent, or both.
Catalyst used Reaction conditions References
1-butyl-3-methylimidazolium FeCl4
N+
NCH3 C4H10
FeCl4-
Solvent-free, 908C,2�3 h
(169)
L-prolinium sulfate
N+ O
OHH
HHSO3-
Tetrahydrofuran, r.t.21�30 h
(170)
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Table 5 (Continued)
Catalyst used Reaction conditions References
1-hexyl-3-methylimidazolium HSO4
N
N
CH3
Solvent-free, r.t.15�55 min
(171)
Task-specific ionic liquids
N+
R
R
R
SO3H
A-
Solvent-free, 908C,10�15 min
(172)
1-butyl-3-methyl-imidazolium hydrogen sulfate
N+
NCH3 C4H10
HSO4-
Solvent-free, MWI,1408C, 4.4�8 min
(173)
1-butyl-3-methylimidazolium tetrafluoroborate immobilized Cu(acac)2
N+
NCH3 C4H10
BF4-
. Cu(acac)2
Solvent-free, 508C (174)
1-n-butyl-3-methylimidazolium saccharinate
N N+
CH3 C4H9
N -
S
OO
O
Solvent-free, r.t. (175)
Alkylammonium and alkylimidazolium perhaloborates, phosphates,and aluminates
N+
NCH3 Bu
A-
I
N+
N
CH3
CH3 Bu
II
NET3
+A-
A-
III
Et3NH+ A-
Bu4N+ PF6 -
IV
V
A = BF4 -, PF6 -, AlCl4, Al2Cl7
Solvent-free, 1208C,1 h
(176)
1-butyl-3-methylimidazolium tetrafluoroborate
N+
NCH3 C4H10
BF4-
Solvent-free (177)
1-methylimidazolium hydrogen sulfate, 1-methylimidazolium trifluoroacetate
N+
NH CH3
HSO4-
, N+
NH CH3
CF3COO-
Solvent-free, 40�90 min,508C
(178)
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Table 5 (Continued)
Catalyst used Reaction conditions References
1-butyl-3-methyl-imidazolium hydrogen sulfate
N+
NCH3 C4H10
HSO4-
Solvent-free, r.t. (179)
Polymer-supported IL
N+
NCH3
.
.n
X-
X = BF4 -, PF6 -
Acid Acetic, 2�4 h 1008C (180)
1-butyl-3-methylimidazolium hexafluorophosphate and
1-butyl-3-methylimidazolium tetrafluoroborate
N+
NCH3 C4H10
PF6-
,N
+N
CH3 C4H10
BF4-
Solvent-free, r.t. (181)
1-butyl-3-methylimidazolium chloride 2AlCl3
N+
NCH3 C4H10
Cl-
.2AlCl3
Solvent-free, r.t. (182)
1-n-butylimidazolium tetrafluoroborate
N+
NH C4H10
BF4-
Ultrasound, 40�90 min,308C
(183)
1-butyl-3-methylimidazolium hexafluorophosphate and
1-butyl-3-methylimidazolium tetrafluoroborate
N+
NCH3 C4H10
PF6-
,N
+N
CH3 C4H10
BF4-
Solvent-free (184)
1-n-butyl-3-methylimidazolium saccharinate
N N+
CH3 C4H9
N -
S
OO
O
Solvent-free, r.t. (185)
1-n-butyl-3-methylimidazolium saccharinate
N N+
CH3 C4H9
N -
S
OO
O
Solvent-free (186)
Table 5 (Continued)
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Conclusions
Ionic liquids are well-known alternative solvents
replacing VOCs, but still some questions remain to
be satisfactorily answered by readers and actual
practioners. Are these really non-toxic recyclable
and economically viable alternatives to VOCs used
specifically in industrial processes? The future of
these solvents seems to be very bright in view of
human health and climate change. Certainly, much
more is needed to be done to see them as really viable
alternatives, in spite of a large number of research
studies being published. ILs hold a very bright future
in various fields of sciences. In this presentation, the
authors have made every effort to be precise and if in
this attempt, unknowingly, any author’s fruitful work
is left out, this is sincerely regretted.
Acknowledgements
Authors wish to thank the Council of Scientific and
Industrial Research (CSIR), New Delhi, India and Indian
National Science Academy (INSA), New Delhi, India for
financial support for this research project.
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