Direct synthesis of porous organosilicas containing chiral organic groups within their framework and a new analytical method for enantiomeric purity of organosilicas

Article abstract of DOI:10.1039/b714163g

Organosilica porous solids containing chiral organic moieties in the framework with an enantiomeric purity of 95% ee, estimated by eluting organic constituent units from chiral organosilicas, were synthesized from a newly designed chiral (R)-(+)-1,2-bis(t

Full text of DOI:10.1039/b714163g

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COMMUNICATION
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Direct synthesis of porous organosilicas containing chiral organic
groups within their framework and a new analytical method for
enantiomeric purity of organosilicas{
Shinji Inagaki,*ab Shiyou Guan,{a Qihua Yang,§a Mahendra P. Kapoor"a and Toyoshi Shimada*bc
Received (in Cambridge, UK) 13th September 2007, Accepted 11th October 2007
First published as an Advance Article on the web 17th October 2007
DOI: 10.1039/b714163g
mesoporous ethylene-silicas from 100% chiral borated ethylene-
bridged disilane precursors prepared by enantioselective hydro-
boration. However, borated ethylene-bridged precursors are
limited in that they only yield mesoporous organosilicas functio-
nalized with amino and hydroxy groups. A variety of sol–gel
precursors having highly controlled asymmetric centers would
open a wide range of opportunities for the development of chiral
organosilicas. The development of a simple analytical method for
determining the enantiomeric purity of chiral organosilicas is of
utmost importance as well.
Organosilica porous solids containing chiral organic moieties in
the framework with an enantiomeric purity of 95% ee,
estimated by eluting organic constituent units from chiral
organosilicas, were synthesized from a newly designed chiral
(R)-(+)-1,2-bis(trimethoxysilyl)phenylethane precursor via
a
surfactant-mediated self-assembly approach.
The concept of periodic mesoporous organosilicas (PMOs)
possessing a homogeneous distribution of organic and silica
moieties in its framework emerged in the late 1990’s with the
expansion of conventional mesoporous materials.1 PMOs, synthe-
sized from 100% or less organic-bridged organosilane precursors
[(R9O)3Si–R–Si(OR9)3], are widely studied because they offer
multiple advantages in designing materials for diverse applica-
tions.2 The introduction of chirality into PMOs is one of the most
exciting topics. Several groups reported the synthesis of PMOs
from bridged organosilanes containing various chiral organic
groups such as bulky vanadyl Schiff base complexes,3 a binaphthyl
group,4 and diaminocyclohexane groups.5,6 The PMOs showed
optical activity and enhanced activity for asymmetric reactions.
They showed the higher potential of the chiral mesoporous
organosilicas for applications such as chromatography of optical
isomers or enantioselective catalysis. Previous studies have
typically employed a large amount of pure silica precursor such
as tetraethoxysilane (60–95 mol%) for co-condensation to stabilize
the porous framework because the bridging organic groups are too
bulky to accommodate in the framework. However, the introduc-
tion of a higher amount of chiral organic groups in the framework
of mesoporous materials is highly promising from the standpoint
of improving the performance of chiral organosilicas. Recently,
Polarz et al.7 and Ide et al.8 reported the synthesis of functionalized
In this communication, we present a novel synthesis of chiral
porous organosilicas via surfactant-mediated self-assembly from a
newly designed chiral (R)-(+)-1,2-bis(trimethoxysilyl)phenylethane,
1, of 95% ee (Scheme 1), and describe a simple, widely applicable
method for determining the enantiomeric purity of organosilica
solids.
The hydrosilylation of (E)-1-phenyl-2-trichlorosilylethene
(14.3 g) with trichlorosilane in the presence of 0.3 mol%
palladium–(R)-3 as catalyst, followed by the addition of methanol
and triethylamine, gave 18.9 g of 1 in 91% yield and 95% ee
(Scheme 2, Supporting Information{).9
Further, in a typical synthesis of organosilica, 0.63 g of
octadecyltrimethylammonium chloride (C18TMACl) was dissolved
in 9 g of deionized water and 4.5 g of HCl (y36%) was added to
obtain a homogeneous aqueous acid solution. Then, 0.87 g of 1
was slowly introduced, and the mixture was stirred vigorously
under ambient conditions for 42 h. After heating at 80 uC for 8 h
under static conditions, the gel was cooled and kept under ambient
conditions for around 16 h, followed by further heating at 80 uC
for another 7 h under static conditions. The resultant precipitate
was recovered by filtration and suspended in a C18TMACl
solution of NH4F at 70 uC for 6 h to stabilize the framework
structure. The surfactant was removed by extraction using an
ethanol–HCl solution as described elsewhere.1
aToyota Central R&D Labs., Inc., Nagakute, Aichi 480-1192, Japan.
E-mail: inagaki@mosk.tytlabs.co.jp; Fax: +81-561-63-6507;
Tel: +81-561-71-7393
bCore Research and Evolution Science and Technology (CREST),
Japan Science and Technology (JST), Japan
In the basic synthesis procedure, 0.91 g of precursor 1 was
added to a homogeneous solution of C18TMACl surfactant
cDepartment of Chemical Engineering, Nara National College of
Technology, 22 Yata-cho, Yamatokoriyama, Nara 639-1080, Japan.
E-mail: shimada@chem.nara-k.ac.jp; Fax: +81-743-55-6154;
Tel: + 81-743-55-6154
{ Electronic supplementary information (ESI) available: Experimental
details. See DOI: 10.1039/b714163g
{ Current address: Sanyo Chemical Industry, Ltd., Higashiyama, Kyoto,
605-0995, Japan. E-mail: s.guan@sanyo-chemical.com
§ Current address: Dalian Institute of Chemical Physics, Chinese Academy
of Sciences, Zhongshan Road, Dalian 116023, China.
E-mail: yangqh@dicp.ac.cn
" Current address: Taiyo Kagaku Co. Ltd., 1-3 Takaramachi, Yokkaichi,
Mie 510-0844, Japan. E-mail: mkapoor@taiyokagaku.co.jp
Scheme 1
202 | Chem. Commun., 2008, 202–204
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conditions showed a broad peak centered with a d-spacing of
˚
31.4 A in the lower angle region. Meanwhile, only the lamellar
phase was observed when the synthesis was performed under basic
conditions (Table 1 entry 2, XRD not given). The organosilicas
derived from the mixture of 1 and 2 (entry 4) showed sharp peaks
˚
at a lower angle (d = 45.0 A) and also several peaks in the medium
˚
angle region at d-spacings of 7.6, 3.8 and 2.5 A (Fig. 1A inset)
confirming the molecular scale periodicity in the pore walls, in
addition to the ordered mesostructure.10 Transmission electron
microscopy confirmed the 2D-hexagonal mesoporous structure in
the organosilicas (Fig. 1B). Although the mesostructure ordering
was not so significant for both organosilicas, as evidenced by the
single peak in the low angle realm of the XRD patterns, the TEM
result evidently confirmed the structural ordering, however, its
proportion was modest in the materials.
Scheme 2
(0.83 g), 6 N NaOH (1.2 g) and H2O (15 g) and stirred vigorously
for 9 h under ambient conditions. The gel was kept under ambient
temperature for 18 h statically and heated at 45 uC for 33 h in a
closed vessel. Finally, the solvent was evaporated from the open
vessel over a period of 63 h. The material was filtered, recovered
and dried prior to characterization.
Nitrogen adsorption–desorption isotherms confirmed the pre-
sence of uniform micropores (entry 1) or mesopores (entries 3 and
4) in the organosilicas (Fig. 2). The BJH (Barrett–Joyner–Halenda)
Further, the synthesis of mesoporous hybrid solids with crystal-
like pore walls10 was attempted using a combination of chiral
precursor 1 and 1,4-bis(triethoxysilyl)benzene, 2, in basic medium.
A mixture of precursors 1 and 2 was added into a homogeneous
solution of the surfactant C18TMACl and sodium hydroxide (3 N
NaOH). The molar ratio of the initial gel was (1 + 2) : C18TMACl
: NaOH : H2O = 1 : 0.48 : 1.2–1.4 : 280–400. The suspension was
stirred vigorously at ambient temperature for 18 h followed by
heating at 95 uC for 20 h. The white precipitate obtained was
filtered, washed and dried before the surfactant was removed by
extraction using an ethanol–HCl solution. Materials were
synthesized with 20 and 40 mol% 1. Details of the organosilica
solids studied are listed in Table 1.
˚
pore diameters were 9.1 and 26 A for the organosilicas derived
from 100% 1 (entry 1) and a mixture of 1 and 2 (entry 4),
respectively. One should notice that the pore size estimated by the
BJH method is well under-estimated.11
Fig. 3 shows 29Si and 13C MAS NMR spectra. A broad
resonance centered at 264.1 ppm was observed for the
X-Ray diffraction patterns revealed the formation of ordered
mesostructures in the organosilicas produced (Fig. 1). The
organosilica derived from 100% precursor
1 under acidic
Table 1 Microstructural properties of organosilicas prepared from 1
& 2
Entry
no.a
BJH pore
Surface
ee
Product
diameter/A area/m2 g21 (%) structure
˚
Precursor
1
2
3
4
a
1
1
9.1
—
1180
—
780
770
95
—
—
0
microporous
lamellar
mesoporous
mesoporous
Fig. 2 N2 adsorption–desorption isotherms and BJH pore size distribu-
tion curves (inset) of organosilicas from 1 (entry 1) and 1 and 2 (entry 4).
1 + 2 (2 : 8) 28
1 + 2 (4 : 6) 26
Entry 1 in acidic medium, entries 2, 3 and 4 in basic media.
Fig. 1 (A) X-Ray diffraction patterns of organosilicas from (a) 1 (Table 1
entry 1) and (b) 1 and 2 (entry 4). (B) Transmission electron micrograph of
mesoporous organosilica (entry 4).
Fig. 3 (A) 29Si MAS NMR and (B) 13C CP-MAS NMR spectra of
organosilicas from (a) 1 (entry 1) and (b) 1 and 2 (entry 4). [Spinning side
bands are indicated with an asterisk (*).]
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organosilica solid derived from 100% precursor 1. This can be
assigned to the combined resonance due to the T2 and T3 silicons
bridged by a phenylethane moiety. Three resonances were
observed at a chemical shift of 258.9, 271.8 and 281.3 ppm
for the material derived from the mixture of 1 and 2. The
resonance at 258.9 ppm is attributable to the T2 silicon bridged by
the phenylethane moiety, while the resonance at 271.8 ppm is due
to the combination of T3 and T29 silicon bridged by phenylethane
and benzene moieties. The strong resonance at 281.3 ppm is due
to the T39 silicon bridged by the benzene moiety. Due to the
crystal-like pore walls in the latter materials, the silicon moieties
are well arranged in the system, resulting in the sharp resonances in
the 29Si MAS NMR spectrum.
present in the resultant material. These results clearly indicate that
acidic conditions are more suitable for the synthesis of chiral
porous hybrid solids.
In summary, chiral porous organosilica was synthesized from
100% organosilane containing bridging organic chiral groups with
high enantiomeric purity (95% ee). For the first time, the
enatiomeric purity of organic groups in chiral organosilica solids
was determined by eluting the organic groups from the solids.
These chiral porous organosilicas can be used in chromatographic
separation and enantioselective catalysis by attaching active
functional species to the framework of the phenyl groups.
We thank Professor Tamio Hayashi for fruitful discussions.
All the resonances observed in the 13C CP-MAS NMR
spectrum of the organosilica material derived from 100% 1 are
assignable and shown in Fig. 3. Meanwhile, the 13C NMR
spectrum of the material derived from the mixture of precursors 1
and 2 displayed signals for both phenylethane and benzene
moieties. The signal at 133.4 ppm is due to the aromatic carbons of
the bridged benzene, while the nearby accompanying resonances
at 127.6, 28.0 and 15.9 ppm can be attributed to the phenyl group
in phenylethane. Apart from these, the signals at 15.9–16.0 and
28.0–28.1 ppm are due to the carbons of the CH2– and CH(Ph)–
groups, respectively, and confirm the formation of the (O1.5Si–
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Decomposition of the silica network due to cleavage of siloxane
bonds (Si–O–Si) in chiral organosilanes by HF treatment, followed
by Tamao oxidation,12 was successfully applied to elute organic
constituent units of 1-phenyl-1,2-ethanediol for the determination
of enantiomeric purity. The analysis resulted in 95% ee for the
organosilica prepared from 100% 1 and showed no racemization
during self-assembly. However, the enantiomeric excess of the
mesoporous materials derived from the mixture of 1 and 2 was
totally lost, indicating that racemization of the chiral center
occurred during hydrolysis and condensation processes under
basic conditions. Comparison of the 13C NMR spectra of the two
materials clearly showed that chemical moieties from 1 were also
11 M. Kruk and M. Jaroniec, Chem. Mater., 2001, 13, 3169.
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204 | Chem. Commun., 2008, 202–204
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