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Greener Media in Chemical Synthesis and Processing.

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Highlights
DOI: 10.1002/anie.200504285
Green Solvents
Greener Media in Chemical Synthesis and Processing**
Martn Avalos, Reyes Babiano, Pedro Cintas,* Jos L. Jimnez, and
Juan C. Palacios
Keywords:
eutectic solvents · green chemistry · ionic liquids ·
polarity
It is hoped that a greener approach will
be adopted in both academic and industrial chemistry in the 21st century. A
series of green principles has become
well established[1] and can be summarized conveniently as follows: a) environmentally benign strategies (e.g. catalytic
systems, atom-economy protocols, or
use of renewable raw materials), b) activation techniques (e.g. microwaves or
ultrasound) that accelerate certain
transformations, thereby avoiding the
formation of side products,[2] and c) lesshazardous solvents.[3] The last criterion
would ideally be fulfilled by water or,
better yet, by solvent-free reactions that
do not involve any further solvent-based
workup.[4]
Prominent among polar fluids are
salts of 1,3-dialkylimidazolium cations,[5]
which may generate a wide variety of
room-temperature ionic liquids (RTILs)
with weakly coordinating anions. Their
“greenness” is usually linked to a negligible vapor pressure, thus they have the
potential to replace more-volatile organic solvents in chemical processes.[6]
The non-innocent behavior of ILs under
certain conditions that causes decomposition of both cations and anions has also
been described.[7] Moreover, the toxi[*] Dr. M. Avalos, Dr. R. Babiano, Dr. P. Cintas,
Dr. J. L. Jim1nez, Dr. J. C. Palacios
Departamento de Qu4mica Org5nica
Facultad de Ciencias-UEX
06071 Badajoz (Spain)
Fax: (+ 34) 924-271-149
E-mail: pecintas@unex.es
[**] This work was performed under the auspices of the Spanish Network of Sustainable Chemistry (REQS). We thank the
Ministry of Education and Science (grants
BQU2003-05946 and BQU2005-07676) for
financial assistance.
3904
cology of ILs remains unclear and
further studies are required to assess
their sustainability.[8, 9]
Greener alternatives would be mixtures composed of biodegradable constituents that exist as liquids over a large
range of temperatures yet exhibit high
thermal and chemical stabilities. One
such emerging low-freezing liquid results when choline chloride and urea are
combined in a molar ratio of 1:2.[10] The
resulting liquid freezes at 12 8C, which is
indeed remarkable when it is considered
that the pure precursors melt at 302 8C
and 133 8C, respectively. Formation of
this liquid phase is guided by the same
principle that governs the melting point
of ionic compounds: The larger the ions
and the smaller the charge, the less
energy is needed to break the bond,
which causes a depression of the freezing point. Whilst alkaline and alkalineearth halides (e.g. NaCl, NH4Cl, or
CaCl2) are fluids at only very high
temperatures,[11] large quaternary ammonium ions are more difficult to fit
into a lattice. As a result of the lower
lattice energy, such ionic substances may
exist as liquids at ambient temperatures.
Like conventional ILs, the choline chloride/urea mixture exhibits non-flammability and conductivity, although to distinguish RTILs from such mixtures the
term “deep eutectic solvent” has now
been adopted for the latter.[12] In this
context, the reactivity of two solids, once
thought to be controlled solely by diffusion, often proceeds through the formation of a liquid (melt) phase.[13]
A eutectic mixture of choline chloride and urea dissolves a wide range of
transition-metal oxides,[14] a fact that
could be exploited for industrial use,
such as in the recovery and extraction of
metals from mineral ores. Eutectic mixtures with large depressions of freezing
point can equally be obtained from
metal halides and choline chloride or
other quaternary ammonium halides.[15–17] Ionic liquids composed of
choline chloride and ZnCl2 (or SnCl2)
in a molar ratio of 1:2 show interesting
catalytic properties, and, moreover, such
solvent systems are not moisture-sensitive and can be recycled without appreciable decrease in activity.[18] For example, Diels–Alder cycloadditions occur
smoothly in this mixture at room temperature (Scheme 1), although the accelerating effect results from the Lewis
acid. These reactions catalyzed by zinccontaining choline chloride are twophase processes: The dienophile dissolves in the ionic mixture, whereas the
less-polar diene remains in a separate
phase. At the end, the product also
forms a separate phase on top of the
ionic solvent, thereby allowing easy
separation.[19]
A variety of synthetically useful
transformations can be conducted in a
similar way. The Fischer indole annula-
Scheme 1. Diels–Alder cycloadditions in eutectic mixtures.[19]
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2006, 45, 3904 – 3908
Angewandte
Chemie
tion proceeded with one equivalent of a
1:2 mixture of choline chloride and
ZnCl2, and the products could be easily
recovered by sublimation from the ionic
liquid.[20] Exclusive formation of 2,3disubstituted regioisomers was observed
with substrates derived from alkyl methyl ketones (Scheme 2). A supported
heterogeneous catalyst composed of
choline hydroxide on MgO was shown
to be an excellent catalyst for aldol
reactions between aldehydes and ketones.[21] This system exhibited better
catalytic efficiency than other common
basic catalysts and is more ecologically
compatible.
Scheme 2. Fischer indole annulation.[20]
As choline-based ionic liquids dissolve not only metal salts but also
hydrogen-bonded substrates, transformations of certain organic compounds,
such as unprotected sugars and even
cellulose (Scheme 3), can advantageously be carried out. For example, selective
acylation of primary hydroxy groups in
cellulose was reported.[22]
The fact that eutectic liquids may be
generated from choline chloride and
metal salts provides a promising outlook
for two relevant electrochemical processes: electropolishing and electroplating.[12] In the former a metal is removed,
representing a way of increasing resistance to corrosion in certain materials
such as stainless steel. Abbott et al.
demonstrated that an environmentally
friendly eutectic mixture of choline
chloride and ethylene glycol dissolves
metal cations on the surface of the steel
to form a complex that can further be
precipitated and filtered.[23] The eutectic
fluid and the metal particles can also be
recycled, in contrast to the polluting
acid-based technology presently employed. In electroplating, the opposite
process, a metal is deposited onto a
surface, such as in the massive industry
of chromium electroplating. To this end,
electrolysis of an ionic
liquid containing choline
chloride
and
chromium(III)
chloride hexahydrate in a
1:2 molar ratio that
freezes at 14 8C deposits a crack-free coating
of chromium with high
current efficiency.[15] This fluid is much
less toxic than CrVI salts in acid media.
Moreover, hydrogen production is negligible as no aqueous solutions are
involved.
A more innovative application of
choline-based eutectic mixtures was recently disclosed by Morris and co-workers.[24] They showed that choline chloride/urea as well as imidazolium-containing ionic liquids act as the solvent
and the template for the synthesis of
microporous crystalline zeolites and that
the resulting framework depends on the
type of ionic mixture employed as well
Scheme 3. Selective acylation of primary hydroxy groups in cellulose.[22]
Angew. Chem. Int. Ed. 2006, 45, 3904 – 3908
as the presence of other ions and water
(Scheme 4). Thus, 3-ethyl-1-methyl-imidazolium bromide provided four different aluminophosphate zeotypes. On
heating at 150 8C, a novel structure of
formula (C6H11N2)3[Al8(PO4)10H3] consisting of hexagonal prismatic units was
obtained. The pores contain the imidazolium templates that counterbalance
the negative charge resulting from terminal PO bonds. The eutectic mixture
based on choline chloride and urea gave
rise to another novel framework of
formula (NH4)3[Al2(PO4)3] in which
ammonium ions, released by partial
decomposition of the urea, template
the aluminophosphate zeolite and balance the ionic charge. Further experiments revealed that NHþ4 ions can also
be interchanged with other metal cations such as CuII.
Addition of fluoride ion or excess
water to the ionic mixtures also resulted
in condensed zeotypes with channel
structures.[24] Owing to the lack of volatility of the ionic solvents, some syntheses could be conducted in open
vessels, thus avoiding safety risks of
hydrothermal reactions in sealed vessels. Moreover, the authors noted that
the ionic liquids dissolved the raw materials at the reaction temperature, thus
confirming that formation of zeolites
occurs by crystallization from solution
and not by a solid-to-solid reaction. The
eutectic mixture of choline chloride and
urea was similarly used for the synthesis
and crystallization of coordination polymers such as (NH4)[Zn(O3PCH2CO2)],
in which ammonium ions direct the
structure of the framework.[25]
Besides urea, other hydrogen-bond
donors form eutectic complexes with a
quaternary ammonium cation, which
enables the formation of related green
solvents. For instance, choline chloride
forms room-temperature eutectic solvents with carboxylic acids, although a
significant depression of freezing point
is observed only with low-molecularweight acids.[26] The lowest melting point
of these antifreeze mixtures may occur
at different eutectic compositions. Although the minimum freezing point
occurs when the ratio of salt to hydrogen-bond donor is 1:2 for the choline
chloride/urea mixture, polyfunctional
molecules form a eutectic mixture at
lower ratios. Thus, oxalic acid
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
3905
Highlights
Scheme 4. Syntheses of novel zeolite analogues by using ionic mixtures as both solvents and structure-directing agents. Al blue; O red; P pink-red
(left) or yellow (right). The ammonium ions present in the pores of the first zeotype are represented as black spheres. Adapted from
reference [24].
(H2OCCO2H) forms a eutectic mixture
at 50 mol % acid and, as expected, citric
acid (a tricarboxylic derivative) forms a
eutectic mixture at 30–35 mol % acid. In
contrast, phenylacetic or phenylpropionic acids reveal their eutectics at
67 mol % acid, similar to that of the
choline chloride/urea mixture.
Some properties of these mixtures,
including data for refractive indexes,
which are related to the polarizability
of the medium, are summarized in
Table 1. Notably, these values are similar to those of conventional ionic liquids. The surface tensions are larger
than those of most organic solvents,
although they are comparable to those
of imidazolium-based ionic liquids. The
surface tension of the eutectic formed
from choline chloride and CrIII chloride
has a value similar to that of molten salts
such as KBr. Low-freezing liquids can
also be generated from mixtures of fatty
acids such as lauric acid/myristic acid or
lauric acid/palmitic acid in different
molar ratios. These materials exhibit
good thermal properties and are suitable for energy storage and solar-heating applications.[27]
Low-melting eutectic mixtures from
sugars, so-called “sweet solutions”, can
be generated from commercially available sugars (e.g. glucose, fructose, or
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Table 1: Properties of some eutectics formed with choline chloride.
Molecular donor [mol %]
M.p.
[8C]
Refractive
index
Surface
tension
[103 N m1]
urea (67)
malonic acid (50)
oxalic acid (50)
citric acid (30–35)
phenylacetic acid (67)
phenylpropionic acid (67)
CrCl3·6 H2O (67)
anhydrous ZnCl2 (67)
anhydrous SnCl2 (67)
12
10
34
69
25
20
14
23–25
43–45
1.504
1.478
1.480
1.502
1.526
1.522
1.547
–
–
52.02
65.68
–
–
41.86
–
77.68
50.90
–
sorbitol), urea, and an inorganic salt
(CaCl2 or NH4Cl).[28] The three-component mixture is significantly green, yet
clear and viscous solutions can only be
obtained between 65 8C and 75 8C. Notably, this depression avoids a common
drawback of sugars upon heating at
higher temperatures, namely browning
with concomitant formation of caramel.
“Sweet” eutectics were employed as
media for Diels–Alder reactions with
similar results to those obtained using
ILs and supercritical CO2. Despite their
inherent chirality, no asymmetric induction was observed. This solvent system is
as polar as N,N-dimethylformamide
(DMF) and could potentially replace
toxic, high-boiling solvents.[35]
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Elsewhere, the search for ionic fluids
devoid of toxic anions culminated in the
preparation of a benign system with
reversible polarity control. The liquid is
a
mixture
of
1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) and 1-hexanol.[29] When CO2 is bubbled through
the liquid, the DBU salt of 1-hexylcarbonate is generated. Bubbling of an
inert gas, such as N2 or Ar, removes
the CO2, and the ionic liquid reverts to
the nonpolar state (Scheme 5). Both
processes occur at ambient temperature,
although the reverse step proceeds faster at 50 8C. The nonionic liquid is as
nonpolar as CHCl3, whereas the ionic
phase is as polar as DMF or a carboxylic
acid. Thus, n-decane is miscible with the
Angew. Chem. Int. Ed. 2006, 45, 3904 – 3908
Angewandte
Chemie
Scheme 5. Reversible polarity control in a eutectic mixture of DBU and 1-hexanol.[29]
DBU/alcohol mixture under N2 and
before exposure to CO2. The alkane,
however, separates out after bubbling
CO2.
Nature reminds us that living organisms may offer biologically inspired
alternatives; they have developed molecular tools, through natural selection,
to lower the freezing point of their body
fluids, thus allowing their survival in
extreme environments or simply allowing them to remain active in the cold
winter. Some insects use dimethyl sulfoxide as an antifreeze; other organisms
live in briny pools, such as in Antarctica,
containing about one molecule of calcium chloride for every two water molecules and do not freeze until 45 8C. In
this environment there is likely to be a
microflora that continues the metabolism at least down to 23 8C.[30] More
sophisticated strategies use antifreeze
proteins (AFPs), which were first discovered in fishes and versions of which
are now known in bacteria, plants, and
insects.[31] AFPs work by binding to the
surfaces of ice crystals as they start to
form, inhibiting further growth.[32] A
recent study described a species of snow
flea, a wingless insect, that has developed two glycine-rich proteins, which
are among the most effective AFPs ever
observed, lowering the freezing point of
its body fluids by nearly 6 8C. These
proteins are also quite different from the
threonine-rich AFPs found in two other
insects, the beetle and the moth.[33]
These findings shed light on the evolution of AFPs in organisms and could be
instrumental in developing proteinbased antifreeze agents with applications ranging from cryosurgery to frostresistant crops.[34]
Angew. Chem. Int. Ed. 2006, 45, 3904 – 3908
Chemistry and biology, as well as
other applied sciences, will almost certainly benefit from molecular systems
that serve as greener reaction media and
scaffolds for low-temperature applications.
Published online: May 19, 2006
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Highlights
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Angew. Chem. Int. Ed. 2006, 45, 3904 – 3908
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