Supported ionic-liquid films (SILF) as two-dimensional nanoreactors for enantioselective reactions: Surface-mediated selectivity modulation (SMSM)

Article abstract of DOI:10.1002/chem.200600557

Supported ionic-liquid films (SILF) of nanometric thickness containing bis(oxazoline)-copper complexes can be used as recoverable catalysts for enantioselective cyclopropanation reactions. When a thin film of ionic liquid is supported on a clay, the system behaves as a nearly two-dimensional nanoreactor in which the restrictions in rotational mobility and the close proximity to the surface support produce variations in the stereo- and enantioselectivities, leading to a complete reversal of the overall selectivity of the reaction. As an example, the cyclopropanation of styrene changes from a preference for the (1S,2S)-trans isomer in bulk solution to the (1R,2S)-cis isomer in the SILF. This variation is strongly dependent on both the thickness of the film and the nature of the support. Only layered solids with negative charges in the layers (clays) give rise to this type of behaviour, showing that the formation of ion pairs may be a decisive factor. This unexpected effect also shows the existence of differences between bulk and supported ionic liquids.

Full text of DOI:10.1002/chem.200600557

FULL PAPER
DOI: 10.1002/chem.200600557
Supported Ionic-Liquid Films (SILF) as Two-Dimensional Nanoreactors for
Enantioselective Reactions: Surface-Mediated Selectivity Modulation
(SMSM)
María R. Castillo, Leslie Fousse, JosØ M. Fraile, JosØ I. García, and JosØ A. Mayoral*[a]
Abstract: Supported ionic-liquid films
(SILF) of nanometric thickness con-
taining bis(oxazoline)–copper com-
plexes can be used as recoverable cata-
lysts for enantioselective cyclopropana-
tion reactions. When a thin film of
ionic liquid is supported on a clay, the
system behaves as a nearly two-dimen-
sional nanoreactor in which the restric-
tions in rotational mobility and the
close proximity to the surface support
produce variations in the stereo- and
enantioselectivities, leading to a com-
plete reversal of the overall selectivity
of the reaction. As an example, the cy-
clopropanation of styrene changes
from a preference for the (1S,2S)-trans
isomer in bulk solution to the (1R,2S)-
cis isomer in the SILF. This variation is
strongly dependent on both the thick-
ness of the film and the nature of the
support. Only layered solids with nega-
tive charges in the layers (clays) give
rise to this type of behaviour, showing
that the formation of ion pairs may be
a
decisive factor. This unexpected
effect also shows the existence of dif-
ferences between bulk and supported
ionic liquids.
Keywords: asymmetric catalysis
·
enantioselectivity · ionic liquids ·
surface effects
Introduction
many possibilities for immobilization through the formation
of covalent bonds with the support,[3,4] although in many
cases the required synthetic steps can prove to be difficult.
Similar difficulties are encountered when modifications of
the ligand are required in order to increase the solubility of
the complex in the immiscible solvent of the biphasic
system. This problem is overcome by the use of ionic liquids
as the solvent phase for the catalyst,[5] for which even com-
mercially available ligands and catalysts can be used, as is
the case for bis(oxazoline) ligands.[6]
However, the use of ionic liquids has two major draw-
backs, namely price and viscosity. The first issue makes it
important to reduce the amount of solvent required, where-
as the second makes it of interest to seek new methods that
enable the easy handling of the ionic liquids. One way to
solve these drawbacks would be the immobilization of an
ionic-liquid phase on a solid support, a strategy that has al-
ready been reported in the literature. In some cases imida-
zolium salts have been anchored onto solids and the ionic-
liquid phase added to these modified supports.[7] However, it
has also been shown that it is possible to form films of ionic-
liquid phases directly on nonmodified supports, such as
silica[8] or diatomaceous earth,[9] and more recently ionic-
liquid phases have been occluded during the synthesis of
porous solids to give nanoliquids.[10] However, reports con-
cerning applications of supported ionic liquids for asymmet-
Asymmetric heterogeneous catalysis[1] is conceptually very
interesting due to the combination of the inherent practical
advantages of heterogeneous over homogeneous catalysis
and the enantioselectivity required for many products in the
fine chemicals or specialities industries, such as pharmaceut-
icals or agrochemicals. Although the concept of heterogene-
ous catalysts is usually associated with solid catalysts, bipha-
sic catalysis[2] in two immiscible liquid phases should be also
included in the concept, because the catalyst is located in a
phase different from that containing the reagents and prod-
uct, thus making the catalyst easier to recover and reuse.
The most studied method to prepare asymmetric heteroge-
neous catalysts is probably the immobilization of their ho-
mogeneous counterparts on solid supports.[3] In this regard,
the chiral organic ligand of the catalytic complex offers
[a] M. R. Castillo, L. Fousse, Dr. J. M. Fraile, Dr. J. I. García,
Dr. J. A. Mayoral
Departamento de Química Orgµnica
Instituto de Ciencia de Materiales de Aragón
Instituto Universitario de Catµlisis HomogØnea
Universidad de Zaragoza—C.S.I.C., 50009 Zaragoza (Spain)
Fax : (+34)976-762-272
E-mail: mayoral@unizar.es
Chem. Eur. J. 2007, 13, 287 – 291
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
287

ric catalysis are scarce[11,12] and precedents on the use of la-
mellar solids as supports do not exist.
In this paper we present our results on the application of
supported ionic-liquid films (SILF) for enantioselective cy-
clopropanation reactions and describe for the first time how
this approach can be used to obtain a surface-mediated se-
lectivity modulation (SMSM).
Several factors were studied: ionic-liquid/support ratio,
catalyst concentration (copper/ionic-liquid ratio), amount of
catalyst (diazoacetate/copper ratio), nature of the support
and extraction solvent.
Effect of the amount of ionic liquid: First of all it was cru-
cial to find the limit to which the SILF can be reduced with-
out significant changes to the results. Different ionic-liquid/
support ratios were tested, while the other parameters were
kept constant. The results of these experiments are gathered
in Table 1. Entries 2–4 also show the effect of catalyst recy-
cling.
Results and Discussion
Preparation and use of SILF: Laponite (a generous gift
from Rockwood Additives Ltd.) was chosen as the support.
5,5’-Isopropylidenebis[(4R)-4-phenyl-4,5-dihydro-1,3-oxazole]
When the result in the homogeneous ionic-liquid phase
(entry 1) is compared with those obtained with SILF it ap-
Table 1. Effect of film thickness on cyclopropanation reactions between
styrene (1a) and ethyl diazoacetate (2) catalyzed by SILF.[a]
Entry
A
IL/support
Yield[b] trans/cis[b] ee trans[c] ee cis[c]
[%]
[%]
[%]
1
2
0.076
0.025
homogeneous 42
67/33
60/40
60/40
60/40
60/40
58/42
57/43
57/43
58/42
58/42
58/42
49/51
50/50
50/50
55/45
41/59
37/63
33/67
54
43
47
49
53
43
46
49
51
52
54
32
40
50
53
27
28
25
45
0
2
5
10
À10
À7
À2
3
7
13
À36
À28
À17
7
À53
À50
À56
0.402
0.268
18
29
38
32
18
28
28
26
28
31
12
19
33
32
3
Figure 1. Structure of the catalytic supported ionic-liquid film (SILF).
3
4
0.050
0.076
(Figure 1) was used as the chiral ligand to compare the re-
sults obtained with the SILF and those obtained in bulk
ionic liquids[6] as well as with the heterogeneous catalysts
prepared by cationic exchange on the same support.[13]
The complex was prepared by mixing CuCl, chiral ligand
and the ionic liquid—in most cases the commercially availa-
0.134
ble
1-butyl-3-methylimidazolium
hexafluorophosphate
5
0.076
0.076
0.076
0.067
0.067
0.067
[bmim]
ACHTREUNG
6[d]
7[e]
16
37
ion in cyclopropanation reactions carried out in molecular
solvents leads to much worse results due to the influence of
the counterion on the geometry of the transition state.[14]
However, we have shown that, in ionic liquids, the anion of
the solvent plays the role of counterion.[6] Dichloromethane
was used to facilitate complex formation and the dispersion
of the SILF on the laponite surface. After the addition of
the support, the suspension was stirred for 2 h and the
chlorinated solvent was removed under vacuum. The result-
ing solids were free-flowing powders and were used as heter-
ogeneous catalysts in solvent-free cyclopropanation reac-
tions between alkenes and ethyl diazoacetate. The reaction
between styrene (1a) and ethyl diazoacetate (2) was used as
a benchmark test (Scheme 1).
[a] [Catalyst]=mmol CumLÀ1 [bmim]
[PF6]. IL/support=mLgÀ1
.
Reac-
tion conditions: 1.5 equiv styrene, 1% catalyst, ethyl diazoacetate slowly
added (2 h), room temperature, 24 h. Extraction: hexane (5”3”3”
3 mL). [b] Determined by GC. [c] Determined by GC (cyclodex-B
column). 3Sa and 4Sa are the major enantiomers. Results with a minus
sign indicates that 4Ra is obtained as the major cis enantiomer. [d] Ex-
traction with diethyl ether. [e] 3% catalyst.
pears that the presence of the support has an influence on
the selectivity of the process. A decrease in the amount of
ionic-liquid solvent on the surface leads to a decrease in
yield and in trans/cis selectivity. Furthermore, both enantio-
selectivities, that is, for trans and cis isomers, are modified
with thinner films. The preference for the enantiomers 3Ra
and 4Ra is reduced and the enantioselectivity for cis iso-
mers is even reversed, from 45% ee for 4Ra to 53% ee for
4Sa. This result shows the existence of a surface-mediated
selectivity modulation (SMSM) when the mobility of the
catalytic complex in the SILF is inhibited by reducing the
thickness of the film.
Taking into account the lamellar nature of the laponite
support and the measured surface area (370 m2 gÀ1), the
thickness of a uniform film can be estimated from the
Scheme 1. Cyclopropanation reactions of styrene and related substrates.
288
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ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2007, 13, 287 – 291

Supported Ionic-Liquid Films
FULL PAPER
volume of ionic liquid. This thickness varies from 10.8 Š
(entry 2) to 1.8 Š (entry 5), dimensions that seem to be too
small to accommodate the complex. This may indicate that
only the external surface of the clay tectoids is covered by
the ionic-liquid phase, making it difficult to calculate the
film thickness. X-ray diffraction patterns of oriented samples
were taken from the starting laponite and the support with
ionic liquid at different ratios (Figure 2). As can be seen, a
of the SILF by accumulation of byproducts from the reac-
tion or the presence of residual hexane in the ionic-liquid
phase. In fact, the increase in yield observed could also be
due to this accumulation, which would allow the extraction
of larger amounts of products.
In an attempt to increase the extraction of products, di-
ethyl ether was used as an extraction solvent instead of
hexane (entry 6). As expected, the yield obtained under the
conditions shown for entry 4 in Table 1 was significantly
higher (16% vs. 3% with hexane) and the same selectivities
were observed. This extraction method was used in the sub-
sequent catalytic tests.
In spite of these interesting effects on the stereoselectivi-
ty, it should be noted that cyclopropanation yields are very
low with these thin films, although 37% yield can be ob-
tained when the amount of copper catalyst is increased to a
3% molar ratio without modification of the stereochemical
results.
Effect of the nature of the support and the liquid phase: We
have so far demonstrated the existence a support effect on
the selectivities of cyclopropanation reactions when laponite
is used as the support for a SILF. In another set of experi-
ments we aimed to study the requirements in terms of the
nature of the support to obtain this type of effect. Laponite,
as a clay, is a solid with a lamellar structure and with nega-
tive charges in the layers. We tried to ascertain whether
both properties were necessary for a support effect. Two
more clays were tested, bentonite and K10 montmorillonite,
together with a tridimensional microporous aluminosilicate
(Y zeolite, a nonlamellar solid with negative charges), silica
gel (an amorphous uncharged support) and two more lamel-
lar solids—graphite (an uncharged support) and a hydrotal-
cite (a positively charged solid). The results of these experi-
ments are gathered in Table 2.
Figure 2. XRD patterns: a) laponite, b) laponite with ionic liquid
0.134 mLgÀ1, c) laponite with ionic liquid 0.402 mLgÀ1
.
larger amount of ionic liquid does not produce any increase
in the basal spacing of the support but does lead to a de-
crease in the intensity of the (001) refraction line. From this
result we can conclude that a greater level of disorder in the
stacking of the clay layers is induced by the ionic liquid and,
as a consequence, part of the inaccessible interlamellar sur-
face becomes accessible.
Another interesting aspect of heterogeneous catalysts is
the possibility of recovery and reuse. The solids, after ex-
traction with hexane, were reused under the same conditions
and the results are shown in Table 1 and Figure 3. A sus-
tained increase in enantioselectivity is observed for trans
isomers, whereas for the cis isomers the enantioselectivity
tends to zero or is even reversed, albeit with low values.
This behaviour may be due to an increase in the thickness
Entries 1–5 show the effect of the support with the same
IL/support ratio, even below the lowest level used with la-
Table 2. Effect of the nature of the support on cyclopropanation reac-
tions between styrene (1a) and ethyl diazoacetate (2) catalyzed by
SILF.[a]
Entry Support
IL/sup- Yield[b]
trans/
ee trans[c] ee cis[c]
port
[%]
cis[b]
[%]
[%]
1
2
3
4
5
6
7
8
9
bentonite
graphite
hydrotalcite 0.05
zeolite Y
silica
bentonite
K10
0.05
0.05
76
26
48
7
26
36
24
26
19
28
57/43
68/32
66/34
48/52
64/36
22/78
36/64
23/77
67/33
36/64
47
54
51
38
54
6
27
5
56
11
4
45
43
20
38
À62
À42
À61
48
0.05
0.05
0.03
0.03
0.01
0.01
0.01
K10
graphite
10
K10[d]
À51
[a] [Catalyst]=0.076 mmol CumLÀ1 [bmim][PF6]. IL/support=mLgÀ1
.
A
Reaction conditions: 1.5 equiv styrene, 1% catalyst, ethyl diazoacetate
slowly added (2 h), room temperature, 24 h. Extraction: diethyl ether (5”
3”3”3 mL). [b] Determined by GC. [c] Determined by GC (cyclodex-B
column). 3Sa and 4Sa are the major enantiomers. Results with a minus
sign indicates that 4Ra is obtained as the major cis enantiomer.
Figure 3. Evolution of enantioselectivity with recovery of SILF with dif-
ferent ionic-liquid/support ratios. Open symbols represent % ee for trans
À1
^
cyclopropanes and solid symbols % ee for cis isomers. ( ) 0.402 mLg
,
À1
À1
~
&
(
) 0.268 mLg , ( ) 0.134 mLg
.
[d] [Oct3NMe][OTf] as ionic liquid.
A
Chem. Eur. J. 2007, 13, 287 – 291
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289

J. A. Mayoral et al.
ponite, just to ensure that the thickness is sufficiently low to
observe any effect. It can be seen that graphite, hydrotalcite
and silica give rise to the same selectivity results as the ho-
mogeneous catalyst, both in trans/cis ratio and enantioselec-
tivity. In contrast, zeolite Y and bentonite show a slight
effect, with reduced trans/cis ratio and enantioselectivity, al-
though the yield is greatly reduced with the zeolitic support.
This seems to indicate that the anionic nature of the support
is important to obtain any effect, whereas the lamellar struc-
ture is only important in combination with this anionic
nature. The microporous nature of the zeolite may introduce
an additional complication to this interpretation, as the abil-
ity of the catalytic complex to enter the micropores is doubt-
ful but the ionic liquid probably fills the void space of the
support. As the yield is also very low, the subsequent tests
were carried out with bentonite, but K10 montmorillonite
was also studied as an example of a delaminated clay.
The support effect can be noticeably increased by reduc-
ing the amount of ionic-liquid phase (entry 6), as occurred
in the case of laponite. However, with bentonite the selectiv-
ity towards the cis-cyclopropane 4Ra is even higher, a con-
sequence of a higher cis preference and a higher enantiose-
lectivity (62% ee). In the case of K10 the same effect was
observed, but a smaller amount of SILF was necessary to
obtain the same selectivity levels (entry 8). As further con-
firmation of the inability of the uncharged lamellar graphite
to produce this effect on selectivity, the same low IL/support
was used and identical results in selectivity were obtained.
The nature of the ionic liquid only has a slight influence
on the catalytic results (entry 9). This influence is also ob-
served in the bulk ionic-liquid phase[6] and is not due to dif-
ferent surface effects.
which is rather surprising for an ionic liquid, in which strong
ion pairs are not expected. This phenomenon has been ex-
perimentally demonstrated in this reaction, as complexes
prepared with copper triflate and copper chloride display
the same results when used in ionic liquids,[6] but not in di-
chloromethane.[14,16]
Surface effect in other cyclopropanation reactions: In an at-
tempt to show the generality of this effect on selectivities,
four different alkenes were tested. The results of these ex-
periments are gathered in Table 3. The two cycloalkenes
Table 3. Effect of the surface on cyclopropanation reactions of different
alkenes catalyzed by SILF.[a]
Entry Alkene Support
Yield[b]
[%]
trans/
ee trans[c] ee cis[c]
cis[b]
[%]
[%]
1
2
3
4
5
6
7
8
5a
5a
5b
5b
1b
1b
1c
1c
homogeneous 43
laponite
homogeneous 21
laponite 13
homogeneous 56
laponite 16
homogeneous 64
laponite 20
96/4
–
–
–
–
–
–
–
45
9
58/42
80/20
35/65
68/32
40/60
53/47
35/65
–
66
28
52
15
À33
47
À37
IL/support=
[a] [Catalyst]=0.076 mmol CumLÀ1
[bmim]
[PF6].
0.01 mLgÀ1. Reaction conditions: 1.5 eq alkene, 1% catalyst, ethyl diazo-
acetate slowly added (2 h), room temperature, 24 h. Extraction: diethyl
ether (5”3”3”3 mL). [b] Determined by GC. [c] Determined by GC
(cyclodex-B column). 3S and 4S are the major enantiomers. Results with
a minus sign indicates that 4R is obtained as the major cis enantiomer.
were symmetrical and therefore did not give rise to any
enantioselectivity (Scheme 2). These compounds were tested
to assess the effect on trans/cis selectivity of an aliphatic
The results show how the close proximity of the support
surface, controlled by the thickness of the SILF, can be used
to modify the stereochemical results of the reaction due to
changes in the energies of the different transition states in-
duced by the steric influence of the support. Under these
conditions, the minor product in solution becomes the major
product in the supported phase. If changes in enantioselec-
tivity for the cis products are considered, the influence of
the surface modifies the relative energy of the transition
states by about 1.4 kcalmolÀ1, a significant effect when com-
pared with the subtle energy differences involved in selectiv-
ity.
Scheme 2. Cyclopropanation reactions with cycloalkenes.
cyclic chain. As can be seen, the preference is reversed in
Although the steric influence of the support is the ulti-
mate reason for this behaviour, the support should fulfil two
requirements, namely a negative charge located on a planar
surface. The clay-SILF seems to act as an almost two-dimen-
sional nanoreactor in which rotational mobility is strongly
hindered so that the reactor wall, that is, the clay surface, is
an active component in the process. The necessity for a
charged surface may be related to the cationic nature of the
catalyst. In fact we had previously observed this influence in
reactions carried out in solvents with very low dielectric
constants, a situation that induces the formation of strong
catalyst–surface ion pairs.[15] It seems that the interaction be-
tween catalyst and surface is due to the presence of charges,
the SILF with cyclooctene (5b) (entry 4), from 80:20 in so-
A
cyclohexene (5a) (entry 2). This result is probably due to
the really high trans preference in solution, showing that the
interaction between the incoming cyclohexene ring and the
ester of the diazoacetate moiety in the carbene intermediate
is much stronger than in the case of cyclooctene. As the
final selectivity is a balance between this interaction and
that between the incoming alkene and the catalyst surface,
the very high trans selectivity in solution (96:4) demon-
strates that the steric repulsion is strong in that case and the
alkene–surface interaction can only reduce the preference
to 58:42.
290
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Supported Ionic-Liquid Films
FULL PAPER
erated at 40 kV and 80 mA, and the CuKa radiation was selected using a
graphite monochromator.
In the case of an elongated styrene with a methoxy group
in the para position (1b, Scheme 1), a higher interaction
with the surface was expected and, consequently, a higher
cis selectivity. However, this was not the case (entry 6) and
this fact may be due to the approach of the alkene not being
completely parallel to the surface, a situation that would
reduce the influence of the para substituent. The enantiose-
lectivities follow the same trend as observed with styrene,
with reversal of the induction for the cis cyclopropanes.
However, in these cases the enantiomeric excesses are
lower.
Finally, an additional methyl group was introduced in the
alpha position of the styrene (1c, Scheme 1) in order to
assess the level of the steric effects of both substituents
(phenyl and methyl). As expected (entry 8) the trans/cis se-
lectivity in solution is lower than in the case of styrene and
is reversed with the SILF. The enantioselectivities follow the
same trend observed with the other styrene derivatives in
that it is reduced for the trans isomers and reversed for the
cis cyclopropanes, although the level is only moderate
(37% ee, with 4Rc as the major enantiomer).
Cyclopropanation reactions: Styrene was added to the stirred solid cata-
lyst and ethyl diazoacetate was slowly added (2 h) with a syringe pump
(see Tables 1–3 for amounts). The solid was stirred for 24 h and the prod-
ucts were extracted with hexane or diethyl ether (5”3”3”3 mL). n-
Decane (100 mg) was added as an internal standard and the results were
determined by GC.[18]
Acknowledgements
This work was made possible by the generous financial support of the
CICYT(project CTQ2005-08016 and CSD2006-0003). M.R.C. is indebted
to DGA for a grant.
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Conclusion
We have shown that clay–SILFs of suitable thickness can es-
sentially act as nearly bidimensional nanoreactors. The re-
sults of the reactions taking place in these new reactors are
greatly influenced by the confinement effects, which in turn
depend on the ionic nature of the support surface and the
thickness of the ionic-liquid film. The modifications, and
even reversals, in both trans/cis and enantioselectivity ob-
served with different alkenes show the generality of these
surface effects, which are dependent on the substituents on
the alkene. An understanding of the surface–complex–
alkene interactions should allow the design of new, more ef-
ficient systems that are able to give products that are stereo-
chemically different to those obtained in solution.
Experimental Section
[14] J. M. Fraile, J. I. García, M. J. Gil, V. Martínez-Merino, J. A. Mayor-
al, L. Salvatella, Chem. Eur. J. 2004, 10, 758.
[15] A. I. Fernµndez, J. M. Fraile, J. I. García, C. I. Herrerías, J. A. May-
oral, L. Salvatella, Catal. Commun. 2001, 2, 165.
[16] J. M. Fraile, J. I. García, J. A. Mayoral, T. Tarnai, J. Mol. Catal. A
1999, 144, 85.
[17] A. Cornejo, J. M. Fraile, J. I. García, M. J. Gil, V. Martínez-Merino,
J. A. Mayoral, E. Pires, I. Villalba, Synlett 2005, 2321.
Supported ionic-liquid films: The support was dried under vacuum for
24 h prior to use. The required amount of dried support was mixed with
anhydrous dichloromethane (5 mL) and this suspension was added to a
mixture of 5,5’-isopropylidenebis[(4R)-4-phenyl-4,5-dihydro-1,3-oxa-
zole],[17] copper chloride and anhydrous ionic liquid (see Tables 1–3 for
amounts). After stirring for 2 h, the solvent was evaporated under re-
duced pressure to give a free flowing powder, which was dried under
vacuum overnight.
[18] A. Cornejo, J. M. Fraile, J. I. García, M. J. Gil, S. V. Luis, V. Martí-
nez-Merino, J. A. Mayoral, J. Org. Chem. 2005, 70, 5536.
Two samples without copper and chiral ligand were prepared in the same
way. Step-scanned X-ray diffraction patterns of these two oriented sam-
ples were collected at room temperature from 38 in 2q up to 608, using a
D-max Rigaku system with a rotating anode. The diffractometer was op-
Received: April 20, 2006
Published online: September 27, 2006
Chem. Eur. J. 2007, 13, 287 – 291
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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