The first synthesis of organic-inorganic hybrid materials with chiral bis(oxazoline) ligands

Article abstract of DOI:10.1039/b507739g

Condensation of tetraethoxysilane with silane-functionalized bis(oxazolines) in the presence of dodecylamine leads to hybrid materials whose textural and catalytic properties depend on both the ligand and the spacer structure. The Royal Society of Chemist

Full text of DOI:10.1039/b507739g

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The first synthesis of organic–inorganic hybrid materials with chiral
bis(oxazoline) ligands{
Jose´ M. Fraile, Jose´ I. Garc´ıa, Clara I. Herrer´ıas and Jose´ A. Mayoral*
Received (in Cambridge, UK) 1st June 2005, Accepted 2nd August 2005
First published as an Advance Article on the web 23rd August 2005
DOI: 10.1039/b507739g
on the synthesis of chiral ORMOSILS from chiral bis(oxazoline)
ligands.
Condensation of tetraethoxysilane with silane-functionalized
bis(oxazolines) in the presence of dodecylamine leads to hybrid
materials whose textural and catalytic properties depend on
both the ligand and the spacer structure.
The bis(oxazoline) derived from indane was hydroxymethylated
and reacted with 3-isocyanatopropyltriethoxysilane to obtain the
precursor 1 (Scheme 1).13 However, the hydroxymethylation
reaction was unsuccessful with the bis(oxazoline) bearing phenyl
groups. The silanized precursor of this ligand (2) was prepared
from the diallyl derivative11 by hydrosilylation with HSi(OEt)3 and
Karstedt’s catalyst14 (Scheme 1).
Organically modified silicas (ORMOSILS) are interesting mate-
rials for different fields given that they combine the properties of
the organic polymers with higher thermal and chemical stability. In
the case of catalysts, most such systems are prepared by grafting
the organic moiety onto a preformed support.1 This is particularly
significant for enantioselective catalysts, whose chiral ligand is
normally tethered to the silica surface.2 In the case of purely
organic supports, both the grafting of the chiral ligand onto a pre-
formed polymer and the polymerization of a conveniently
functionalized ligand are routinely applied strategies, with
completely different results obtained with a good number of
asymmetric catalysts.3 In analogy with the purely organic supports,
the enantioselective silica-based catalysts could also be prepared by
‘‘inorganic polymerization’’ of a suitably functionalized chiral
monomer. Although this strategy has been applied in the
preparation of numerous non-chiral catalysts,4 the application of
this approach to chiral catalysts is limited to some Rh–diamine
complexes immobilized on low-surface area materials.5 Only
recently has the preparation of high-surface area hybrid chiral
materials been described, with the chiral groups either ‘‘hanging
from’’6 or ‘‘incorporated into’’7 the silica walls. However, to date
the catalytic results have been much poorer than those obtained
with the grafted analogues, mainly with regard to enantioselec-
tivity. One additional drawback of this approach is the low
stability in aqueous solution of some of the most efficient families
of chiral ligands, which is a clear limitation regarding the synthesis
of siliceous materials. Bis(oxazolines) represent one of those
families of highly efficient but water-sensitive chiral ligands and,
for this reason, the development of strategies allowing their
incorporation onto ORMOSILS is an attractive and interesting
goal. Our group has been working on the immobilization of chiral
bis(oxazoline)–metal complexes by different methods, including
cationic exchange,8 ionic liquid solutions,9 polymerization,10,11 and
grafting,11–13 with the aim of comparing the scope and limitations
of the different methods. In this work we present our first results
Bis(oxazoline) ligands are hydrolysis-sensitive in aqueous
acidic media and Brønsted acids were therefore ruled out as
catalysts for the sol–gel process. The ligands can also form
strong complexes with a wide variety of Lewis acids and these were
also unsuitable. Basic catalysis was preferred and the method using
a long alkyl amine, which acts both as a catalyst and a template,
was considered appropriate for our purpose. This method was
initially developed for fully siliceous materials,15 although it has
since been used for the preparation of organic–inorganic hybrid
materials with simple (non-chiral) organic precursors.16–21 The
chiral precursor was reacted with Si(OEt)4 in a solution of
dodecylamine in ethanol–water{ (Scheme 2) in order to obtain a
good dispersion of the ligand on the surface of the resulting
material and give isolated catalytic sites. A white solid was
immediately formed in all cases after the addition of the two
silanes. After ageing, the solids were filtered off, washed and
dried.
The presence of the ligand was detected by 13C-CP-MAS-NMR.
The spectrum of Si-1 is presented in Fig. 1. All the carbon atoms
can be assigned in the spectrum, as shown in the figure. The
presence of the imine and aromatic groups, together with the
signals of the other carbon atoms in the oxazoline rings are key to
demonstrating the integrity of the ligand.
Departamento de Qu´ımica Orga´nica, Instituto de Ciencia de Materiales
de Arago´n and Instituto Universitario de Cata´lisis Homoge´nea, Facultad
de Ciencias, Universidad de Zaragoza-C.S.I.C., E-50009 Zaragoza,
Spain. E-mail: mayoral@unizar.es; Fax: +34 976 762077;
Tel: +34 976 762077
{ Electronic supplementary information (ESI) available: N2 adsorption
isotherms and pore size distribution of the three materials. See http://
dx.doi.org/10.1039/b507739g
Scheme 1 Synthesis of bis(oxazoline) precursors.
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Scheme 2 Synthesis of the hybrid materials.
Fig. 2 SEM micrographs of the purely siliceous material (top) and Si-1
(bottom).
Table 1 Characterization of the hybrid materialsa
S/
Vp/
Vfp/
Vtp/
Cu content/
mmol g21
m2 g21 cm3 g21 cm3 g21 cm3 g21 Dp/A
˚
Solid
Siliceous 1253
1.90
0.14
0.21
0.70
0.05
0.13
1.20
0.09
0.08
20 + 850 —
Si-1
Si-2
a
40
241
—
22
0.076
0.148
Fig. 1 13C-CP-MAS-NMR spectrum of Si-1.
S = BET surface area. Vp = total pore volume (at P/P0 = 0.98).
Vfp = framework pore volume (at P/P0 , 0.5). Vtp = textural pore
volume (Vtp = Vp 2 Vfp). Dp = mean pore diameter (BJH).
SEM micrographs (Fig. 2) of the resulting materials show that
the purely siliceous solid (Fig. 2A) presents large spherical particles
of about 15 mm together with aggregates of smaller particles (in the
range 0.5–2 mm). These characteristics are consistent with the
reported morphology for materials prepared by the same
method.18 In contrast, the hybrid materials do not lead to the
formation of such large particles, as shown in the micrograph of
Si-1 (Fig. 1B) and Si-2 (not shown), and only aggregates of small
(,0.5 mm) primary particles can be observed. With regard to
textural properties (Table 1), the purely siliceous material shows
the expected high values of surface area and pore volume. The
framework confined porosity (0.70 cm3 g21, 37% of the total
porosity) is in agreement with values reported by other authors,
either in purely siliceous or in simple organic–inorganic hybrid
materials.16,19 The large textural porosity is a consequence of a
significant degree of particle intergrowth, as shown in the SEM
micrographs. The bimodal character of the porosity is apparent
from the pore size distribution (see ESI{). The maximum in the
material prepared from the indane-derived bis(oxazoline), Si-1,
shows a very low surface area and porosity, as if the presence of
this rather large precursor disturbs the formation of the
micelles necessary to obtain framework porosity. The hybrid
material Si-2 presents completely different textural properties and
has a moderate surface area (240 m2 g21) and pore volume
(0.21 cm3 g21). However, the framework porosity contributes 62%
of the total pore volume, a value much higher than in the other
hybrid material, and the low textural volume is confirmed by the
lack of a maximum in the distribution at large pore size. Another
interesting feature is the slight, but significant, increase in the mean
˚
pore diameter to 22 A, in spite of the large organic moiety that
must mainly be located in the framework pores. This result seems
to indicate the good formation of micelles in the case of precursor
2, which are even larger than in the case of the purely siliceous
material. Finally, if we consider the copper loading obtained after
complexation with Cu(OTf)2, it seems that the chiral ligand is
much more accessible in Si-2 than in Si-1, probably as a result of
the larger surface area and pore size.
˚
distribution at 20 A must correspond to the framework porosity,
and this is in good agreement with the results reported by
other authors.16,19,20 The other maximum detected, in the range
˚
800–900 A, may be due to the textural porosity.
These properties are drastically modified by the introduction of
the chiral precursors and the final texture of the hybrid materials
depends largely on the nature of the chiral bis(oxazoline). The
The copper(II) complexes were tested as catalysts in the
benchmark cyclopropanation reaction between styrene and ethyl
diazoacetate (Table 2). Both solids showed the same catalytic
4670 | Chem. Commun., 2005, 4669–4671
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siliceous material was prepared under the same conditions with only
tetraethoxysilane (20 mmol) as the precursor.
Table 2 Catalytic results in the cyclopropanation reactiona
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Solid Run Yield (%) trans : cis ee trans (%)b ee cis (%)b
Si-1
Si-2
a
1
2
1
2
31
3
33
30
58 : 42
59 : 41
64 : 36
62 : 38
43 (88)c
9
51 (83)c
14
53 (60)d
50
45 (50)d
43
Reaction conditions: catalyst (150 mg), styrene (5 mmol),
dichloromethane (5 ml), slow addition of ethyl diazoacetate
(5 mmol), room temperature. Results determined by GC. The
b
result in homogeneous phase with an analogous chiral ligand is
given in parentheses. The major isomers have S configuration in
c
d
C1. The major isomers have R configuration in C1.
6 J. M. Fraile, J. A. Mayoral, J. Serrano, M. A. Perica`s, L. Sola` and
D. Castellnou, Org. Lett., 2003, 5, 4333.
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activity and almost the same enantioselectivity. However, the
enantioselectivity with Si-1–Cu, up to 51% ee, is still far from that
obtained in the homogeneous phase with the dihydroxymethylated
precursor (83–88% ee). This result confirms the general observa-
tion that lower enantioselectivities are obtained when the siliceous
support is prepared with a silanized chiral precursor.5–7 In
contrast, the enantioselectivity obtained with Si-2–Cu is close to
that obtained in solution with the diallylated precursor. The most
important difference between the two solids is probably the
behaviour in terms of recycling. Unfortunately Si-1–Cu is not
recoverable and loses both activity and enantioselectivity upon
reuse. However, this deactivation is not due to copper leaching, a
fact confirmed by the lack of activity for cyclopropanation in the
final solution and also by elemental analysis. As in other
heterogeneous catalysts,8–11 the deactivation may be ascribed to
poisoning of the catalytic sites by irreversible coordination of the
reaction by-products—namely maleate and fumarate derivatives
from diazoacetate dimerization and subsequent reactions. Thus,
Si-2 is the best support with regard to textural properties and
catalytic performance.
8 J. M. Fraile, J. I. Garc´ıa, J. A. Mayoral, T. Tarnai and M. A. Harmer,
J. Catal., 1999, 186, 214; J. M. Fraile, J. I. Garc´ıa, M. A. Harmer,
C. I. Herrer´ıas, J. A. Mayoral, O. Reiser and H. Werner, J. Mater.
Chem., 2002, 12, 3290; J. M. Fraile, J. I. Garc´ıa, C. I. Herrer´ıas,
J. A. Mayoral and M. A. Harmer, J. Catal., 2004, 221, 532; J. M. Fraile,
J. I. Garc´ıa, C. I. Herrer´ıas, J. A. Mayoral, O. Reiser, A. Socue´llamos
and H. Werner, Chem. Eur. J., 2004, 10, 2997.
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M. Vaultier, Tetrahedron: Asymmetry, 2001, 12, 1891; J. M. Fraile,
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Green Chem., 2004, 6, 93; J. M. Fraile, J. I. Garc´ıa, C. I. Herrer´ıas,
J. A. Mayoral, O. Reiser and M. Vaultier, Tetrahedron Lett., 2004, 45,
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and J. A. Mayoral, Org. Lett., 2000, 2, 3905; E. D´ıez-Barra, J. M. Fraile,
J. I. Garc´ıa, E. Garc´ıa-Verdugo, C. I. Herrer´ıas, S. V. Luis,
J. A. Mayoral, P. Sa´nchez-Verdu´ and J. Tolosa, Tetrahedron:
Asymmetry, 2003, 14, 773.
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In conclusion, we have demonstrated that it is possible to
prepare organic–inorganic hybrid materials with silanized chiral
precursors as hydrolysis-sensitive as bis(oxazolines). The textural
properties and the catalytic performances of the resulting solids are
dependent on the nature of both the substituents of the chiral
ligand and the spacer. These results open the way for the synthesis
of higher performance catalysts and further work is being carried
out to obtain better solid supports for enantioselective catalysis.
This work was made possible by the generous financial support
of the C.I.C.Y.T. (Project PPQ2002–04012) and the Diputacio´n
General de Arago´n.
12 Grafting has also been used by other groups to immobilize bis(oxazo-
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Notes and references
{ Tetraethoxysilane (3.76 g, 18 mmol) and the bis(triethoxysilyl)-bis(oxazo-
line) (1 mmol) were added separately, but simultaneously, to a stirred clear
solution of 1-dodecylamine (1.01 g, 2.5 mmol) in a mixture of ethanol
(9.2 ml) and water (11 ml). The mixture was vigorously stirred for 18 h at
room temperature. The solid was filtered off, thoroughly washed with
ethanol, and dried under vacuum at 50 uC for 24 h. An analogous fully
21 L. Mercier and T. J. Pinnavaia, Chem. Mater., 2000, 12, 188.
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