Chiral enantiopure organosilane precursors for the synthesis of periodic mesoporous organosilicas

Article abstract of DOI:10.1016/j.tetasy.2017.10.032

The manuscript describes synthesis of new chiral organosilica networks starting from modified readily available enantiopure substances such as sugars and amino acids. We report on the successful preparation of robust all-chiral organosilicas by polymerization of the homochiral monomers. When the homochiral organosilane monomers were polymerized in mixtures of polar organic solvents and water in the presence of hydrochloric acid or tetrabutylammonium fluoride as catalysts, mainly spherical microparticles were obtained due to emulsification of the hydrophobic monomers in these mixtures. Polycondensation of the chiral organosilanes in the presence of Pluronic P123 as a template produced ordered mesoporous networks. The new all-chiral materials were characterized by SEM, STEM, BET, SAXS, IR, NMR and TGA.

Full text of DOI:10.1016/j.tetasy.2017.10.032

Tetrahedron: Asymmetry xxx (2017) xxx–xxx
Contents lists available at ScienceDirect
Tetrahedron: Asymmetry
journal homepage: www.elsevier.com/locate/tetasy
Chiral enantiopure organosilane precursors for the synthesis of
periodic mesoporous organosilicas
⇑
⇑
Orit Cohen, Raed Abu-Reziq , Dmitri Gelman
Institute of Chemistry, The Hebrew University, Edmond Safra Campus, Givat Ram, 91904 Jerusalem, Israel
a r t i c l e i n f o
a b s t r a c t
Article history:
The manuscript describes synthesis of new chiral organosilica networks starting from modified readily
available enantiopure substances such as sugars and amino acids. We report on the successful prepara-
tion of robust all-chiral organosilicas by polymerization of the homochiral monomers. When the
homochiral organosilane monomers were polymerized in mixtures of polar organic solvents and water
in the presence of hydrochloric acid or tetrabutylammonium fluoride as catalysts, mainly spherical
microparticles were obtained due to emulsification of the hydrophobic monomers in these mixtures.
Polycondensation of the chiral organosilanes in the presence of Pluronic P123 as a template produced
ordered mesoporous networks. The new all-chiral materials were characterized by SEM, STEM, BET,
SAXS, IR, NMR and TGA.
Received 25 May 2017
Revised 2 October 2017
Accepted 30 October 2017
Available online xxxx
Ó 2017 Elsevier Ltd. All rights reserved.
1. Introduction
monomers or their co-condensation with Si(OR)4 (Scheme 1).
However, such organic–inorganic hybrids prepared via the sol–
Organic–inorganic hybrids play an important role in the design
of advanced functional materials with the desired mechanical,
electric, optical or catalytic properties.1–6 The research in this field
is mainly fueled by the need to control not only surface, but also
bulk properties of the designed materials. The control is more effi-
ciently achieved in highly ordered systems, where the organic and
inorganic counterparts are not simply blended, but covalently
linked via stronger or weaker chemical bonds.2,5
Functionalized organoalkoxysilane compounds RnSi(OR)4Àn are
versatile bridging units between inorganic and organic compo-
nents as they contain at least one CASi bond (organic component)
and can be easily polymerized via SiOH–HOSi condensation
(sol–gel process) to form porous but still robust frameworks.
Chemical and physical properties of these materials can be
diversified and controlled by modifying organic precursors and
reaction conditions.2 For example, properties such as hydrophobic-
ity,7 acidity/basicity,8 rigidity9 and porosity10 are directly related to
the nature of the organosilanes.
gel process are amorphous, while structural periodicity is highly
desired in order to attain uniformity in the bulk properties.
Periodic mesoporous organosilica (PMO) materials were discov-
ered in 1999 and reported independently by three groups.14–16
PMOs are synthesized under mild conditions by hydrolysis and
condensation of bridged organo-alkoxysilane precursor com-
pounds, (OR)3SiARASi(OR)3, in the presence of surfactants or block
copolymers which assist to create uniform pores in the size of 2–
30 nm (Scheme 2).17–25 The surface area of PMOs can reach up to
1800 m2/g, which makes these materials attractive for the various
applications including drug release,26 catalysis,27,28 optical devices
and adsorbents.17,18 The PMOs contain organic groups distributed
homogeneously within the inorganic walls, which can diminish
the pore blockage and by variation of the organic groups, it is pos-
sible to tune the surface properties such as polarity, hydrophilicity
and hydrophobicity.
Over recent years, diverse chiral organosilicas for different
applications have been described in the literature.29–36 A plethora
of amorphous chiral frameworks made solely of chiral organosilox-
anes or those co-condensed with tetralakoxysilanes have been
reported in the context of catalysis37–45 and chromatographic sep-
aration.46–48 Despite intense research activity in the field, highly
ordered PMO materials composed solely of chiral organosiloxanes
are rare.49–55 Herein we present the preparation and characteriza-
tion of several chiral enantiopure organosilane precursors based on
readily available sugars, amino- or hydroxyl acid derivatives
There are two main approaches to incorporate organic func-
tional groups into organosilicas:11–13,5 (i) post-condensation of a
functional trialkoxyorganosilane with the surface SiAOH groups;
and (ii) self-condensation of the functional organosilane
⇑
Corresponding author.
E-mail addresses: raed.abu-reziq@mail.huji.ac.il (R. Abu-Reziq), dmitri.gelman@
mail.huji.ac.il (D. Gelman).
https://doi.org/10.1016/j.tetasy.2017.10.032
0957-4166/Ó 2017 Elsevier Ltd. All rights reserved.
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O. Cohen et al. / Tetrahedron: Asymmetry xxx (2017) xxx–xxx
Scheme 1. Two approaches to incorporate functionality to organosilica materials: (i) post-synthetic and (ii) by co-condensation.
Scheme 2. Synthesis of periodic mesoporous organosilicas by a surfactant-directed self-assembly approach.
containing multiple polymerizable alkoxysilane sites. We also
report about the preparation and characterization of organic–inor-
ganic hybrid materials based on these new compounds as well as
chiral mesoporous materials including PMO.
ance of the carbamate or carbamide signals, correspondingly (two
typical NAH peaks were observed at 3340 and 1536 cmÀ1 in addi-
tion to the carbonyl signal at 1696 cmÀ1 (Fig. 1).
Since the organosilane silane precursors 7–12 are moisture sen-
sitive and have high molecular weight, their isolation in highly
pure form is problematic. The most likely by-products in this reac-
tion were product-derived species with partially hydrolyzed tri-
ethoxysilyl groups (according to inaccurate peak integration by
1H NMR). Attempted purification by high-vacuum distillation or
filtration through dried silica led to no improvement. Nevertheless,
the crude reaction mixture contained the desired product in ca.
90% purity after evaporation of the solvents; therefore, the materi-
als were used without further purification. Identity of the com-
pounds 7–12 was obvious by the appearance of signals around
155 ppm in 13C NMR which are characteristic of the carbamate
and carbamide carbons.
2. Results and discussion
2.1. Preparation and characterization of chiral organosilane
precursors
All the organosilane monomers (Scheme 3) were prepared by
linking triethoxysilyl groups through a quantitative carbamate or
carbamide click between 3-(triethoxysilyl)propyl isocyanate with
hydroxyl- or amine-possessing chiral organic core molecules. We
have chosen only cheap and commercially available precursors
suitable for the synthesis on large scale:
cosamine 2, -isoleucinol 3, Boc- -tyrosinol 4, Boc-
and dimethyl -tartrate 6.
D
-maltose 1, Boc-
D-glu-
L
L
L-threoninol 5
2.2. Chiral organosilica materials characterization
L
In all these cases, the reaction was quantitative according to 1H
and 13C NMR (Supplementary information) and FT-IR monitoring
(disappearance of the isocyanate signal at 2270 cmÀ1) and appear-
The prepared chiral organosilane precursors were employed for
the construction of new chiral organosilica materials. Initially, we
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O. Cohen et al. / Tetrahedron: Asymmetry xxx (2017) xxx–xxx
3
attempted preparation of the solid chiral sol–gel matrices by
hydrolysis and polycondensation reactions of the chiral monomers
in the presence of acidic (HCl) or nucleophilic catalysts (tetrabuty-
lammonium fluoride, TBAF). The sol–gel matrices prepared by
O
C
O
O
Si
O
pyridine
60°C
XH
C
O
O
Si
O
O
N
X
N
H
polycondensation of silane monomers derived from
(M), dimethyl -tartrate (T) or -tyrosinol (Boc-Tyr) were obtained
as glassy materials. On the other hand, polycondensation of the
chiral silane monomers derived from -isoleucinol (Ile), N-Boc-
glucosamine (Boc-G) or -threoninol (Boc-Thr) produced powders.
D-maltose
X=O,NH
OH
L
L
O
C
D
D-
N
H
O
O
O
Si
O
HO
HO
OH
L
O
O
OH
O
SEM images of the chiral materials obtained in the presence of both
types of catalysts are shown in Figure 2.
The SEM images indicate that mostly spherical solid micropar-
ticles are obtained. It seems that the type of a chiral silane mono-
mer and the catalyst applied in the polycondensation reaction can
affect significantly the morphology of the final materials. For
RO
O
OR
OH
HO
RO
OR
OH
O
O
RO
OR
1
OR
O
C
example, the polycondensation of the monomers derived from
D-
OH
O
N
O
O
O
Si
O
H
maltose, N-Boc- -glucosamine, -isoleucinol and N-Boc- -tyrosinol
D
L
L
using hydrochloric acid as a catalyst, gave mainly bulk materials or
aggregated microspheres. However, well-separated microspheres
formed using TBAF as catalyst. In opposite, polycondensation of
HO
HO
OH
O
O
RO
RO
OR
HN
O
HN Boc
the monomer derived from N-Boc-L-threoninol using TBAF as a cat-
alyst yielded bulk material. But for the same monomer resulted in
the formation of spherical microparticles under the HCl catalysis.
2
For the dimethyl L-tartrate-based monomer, both acid and TBAF
O
catalyst resulted in separated microspheres. We believe that the
spherical microparticles form via emulsification of the hydropho-
bic silane monomer as droplets dispersed in the reaction medium.
Solid NMR and FT-IR analyses were recorded to characterize the
obtained chiral materials (Figs. 3 and 4).The disappearance of the
band assigned to the SiAOCH2CH3 group at 900 or 954 cmÀ1 and
broadening of the peak centered at 1013–1032 cmÀ1 in all the
FT-IR spectra indicated condensation of the organosilanes mono-
mers (highlighted for M and Ile in Fig. 12).56
O
Si
O
N
H
OH
N
H
O
NH2
RO
3
R
O
O
OH
O
O
Si
O
HN
O
NH
HO
N
H
O
Boc
O
In addition, the 29Si CP–MAS NMR spectra confirm formation of
the organosilica matrices: the major peak at À66 ppm belongs to
the T3 (SiO3C) species and evidences complete condensation of
the silyl groups, while the minor peak at À59 ppm refers to the
T2 signal and points out at the presence of partially condensed
4
O
O
HO
OH
O
O
Si
R
HN
O
N
O
O
H
HN
O
O
5
O
O
O
O
Si
O
OH
O
C
N
H
O
O
O
O
O
O
OH
O
O
O
R
6
O
C
R=
N
H
O
Si
O
O
Figure 1. FT-IR spectra for chiral organosilane precursors (A)
N-Boc- -glucosamine-silane, (C) -isoleucinol-silane, (D) N-Boc-
(E) N-Boc- -threoninol-silane, (F) dimethyl -tartrate-silane.
D
-maltose-silane, (B)
Scheme 3. New chiral bridged organosilane precursors based on
-glucosamine 2, -isoleucinol 3, Boc- -tyrosinol 4, Boc- -threoninol 5 and dimethyl
-tartrate 6.
D-maltose 1, Boc-
D
L
L-tyrosinol-silane,
D
L
L
L
L
L
L
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O. Cohen et al. / Tetrahedron: Asymmetry xxx (2017) xxx–xxx
Figure 2. SEM images of organosilicas matrices condensed in presence of hydrochloric acid (1) and TBAF (2) for different precursors: (A)
(C) -isoleucinol, (D) N-Boc- -tyrosinol, (E) N-Boc- -threoninol, (F) dimethyl -tartrate.
D-maltose, (B) N-Boc-D-glucosamine,
L
L
L
L
Figure 4. 29Si CP–MAS NMR spectra of the chiral sol–gel matrices obtained by the
polycondensation of chiral silane monomers derived from: (A) -maltose, (B) N-
Boc- -glucosamine, (C) -isoleucinol, (D) N-Boc- -tyrosinol, (E) N-Boc- -threoninol,
(F) dimethyl -tartrate.
D
D
L
L
L
L
Figure 3. FT-IR spectra of the chiral sol–gel matrices obtained by the polyconden-
sation of chiral silane monomers derived from: (A) -maltose (M), (B) N-Boc-
tyrosinol (Boc-Tyr), (C) N-Boc- -glucosamine (Boc-G), (D) -isoleucinol (Ile), (E) N-
Boc- -threoninol (Boc-Thr), (F) dimethyl -tartrate (T).
D
L-
D
L
L
L
The thermal stability of the chiral matrices was determined by
thermal gravimetric analysis (TGA was performed under the nitro-
gen atmosphere (Fig. 6). In all TGA curves, two weight loss stages
were observed. The first (minor, of 0.7–4% between 60 and 94 °C)
represents desorption of some physically adsorbed volatile com-
pounds such as water, ethanol and acetone. The major weight loss
occurs between 200 and 550 °C, which is assigned to the decompo-
sition of the organic segments of the organosilica materials. The
theoretical percentages of the inorganic part of the matrices
correlate well with the values attained by the TGA analysis. The
inorganic portions of the matrices were 28%, 31%, 33%, 34%, 35%
and 38% for chiral materials prepared by the polymerization of
silanol groups (SiAOH).57 As expected, no Q type signals character-
istic to underivatized silica was detected.
13C CP–MAS spectra also supported identity of the desired
materials (Fig. 5). For example, signals at ca. 157 ppm characteris-
tic of the carbamate carbon appeared clearly for all the matrices,
while the signal centered at ca. 28 ppm that is assigned to the t-
Boc protecting groups (t-Boc-G, t-Boc-Tyr and t-Boc-Thr) appears
only in the corresponding cases. In the spectrum F (Fig. 5), exis-
tence of the ester groups derived from dimethyl L-tartrate appear
as two signals at 169 and 52 ppm. For the organosilicas based on
sugars (M and t-Boc-G), typical signals are found between 60 and
100 ppm.
the chiral silane monomers derived from N-Boc-L-tyrosinol,
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O. Cohen et al. / Tetrahedron: Asymmetry xxx (2017) xxx–xxx
5
was examined by SEM and STEM (Fig. 7) indicating mesoporous
structure. The SEM and STEM analyses showed that the -mal-
D
tose-based PMO contains short channels that cross one another
and form a coral-like shape (Fig. 7). Pore diameter of 5.0 nm was
determined by Barrett-Joyner-Halenda (BJH) pore size and volume
analysis.
In the case of the isoleucinol-based silane precursor, the con-
densation gave spherical nanoparticles with structures that look
like channels on their surface (Fig. 8). To analyze the structure of
the isoleucinol-based nanoparticles, the nanospheres were embed-
ded in Agar 100 resin and sectioned with a diamond knife on an
LKB 3 microtome. The resulting ultrathin sections (80 nm) of the
nanospheres were analyzed by STEM that clearly indicated the
presence of the channels not only on their surface but also in the
bulk (Fig. 9).
Specific surface areas of both materials were determined by
BET. The maltose-based PMO showed a surface area higher than
the surface area of the isoleucinol-based nanospheres by Langmuir
model (419 m2/g and 102 m2/g, respectively). In addition, the pore
size distributions were calculated by the maximum of the Barrett–
Joyner–Halenda (BJH) plots and the results exhibit pore diameter
of 5.0 nm for the maltose-based PMO and 3.4 nm for the isoleuci-
nol-based nanospheres.
Figure 5. 13C CP–MAS NMR spectra of the chiral sol–gel matrices obtained by the
polycondensation of chiral silane monomers derived from: (A) -maltose, (B) N-
Boc- -glucosamine, (C) -isoleucinol, (D) N-Boc- -tyrosinol, (E) N-Boc- -threoninol,
(F) dimethyl -tartrate.
D
D
L
L
L
L
N-Boc-L-threoninol,
L-isoleucinol, dimethyl L-tartrate, N-Boc-D-glu-
cosamine and
D
-maltose, respectively.
To examine periodical structure of the maltose-based PMOs,
small-angle X-ray scattering (SAXS) analysis was employed
(Fig. 10).58 Fit2D program was used for integration, and automatic
background subtraction and fitting to 2D hexagonal phase were
done by X+ program as was reported earlier.59 The results sup-
ported periodic structure of the matrix exhibiting a typical distance
between the centers of the channels of 14.0 nm with short correla-
tion length of 5–8 repetitive units. The brag peaks resulting from
the hexagonal structure are relatively wide but we propose coral-
like shape with short correlation length. We presume that the
order in the material is local with ordered units crossing one
another and forming the shape demonstrated in the Figure 7B
which explains to macroscopic non-crystallinity and broadening
of the peaks.
Furthermore, the materials were characterized by the solid
state NMR and FT-IR. The 13C CP–MAS and 29Si CP–MAS spectra
for both sol–gel matrices and mesoporous structures are shown
in Figure 11. Expectedly, there is no difference between the two
NMR spectra. No difference was also detected between the IR spec-
tra of the mesoporous material and the related amorphous matri-
ces (the Fig. 12 demonstrates the IR spectra of the maltose- or
isoleucinol-based precursors and chiral mesoporous organosilicas
materials—It shows disappearance of the band representing the
SiAOCH2CH3 group at 954 cmÀ1, and broadening of the peak cen-
tered at 1024 cmÀ1 related to the SiAOASi bond).
2.3. Chiral mesoporous materials characterization
Chiral mesoporous organosilica materials were successfully
synthesized by polycondensation of the chiral organosilane precur-
sors derived from D-maltose (M–PMO) or L–isoleucinol (MPP–Ile),
in water in the presence of Pluronic P123 as a templating agent.
It is worth mentioning that the mesoporous chiral materials were
obtained by polycondensation of the chiral silane monomers alone
without addition of an achiral silane component. The template was
further removed by extraction, and the resulting PMOs were char-
acterized by scanning electron microscopy (SEM), scanning trans-
mission electron microscopy (STEM), small angle X-ray scattering
(SAXS), thermogravimetric analysis (TGA), solid-state 29Si MAS
and 13C MAS NMR, infra-red (IR) spectroscopy and surface area
was determined by the Brunauer–Emmett–Teller method (BET).
When
D-maltose-based silane monomer was polymerized
under the templating conditions, a powder-like material was
obtained at the end of the process. After removal the directing
agent by soxhlet extraction with ethanol, the resulting material
Finally, the thermal stability of the mesoporous materials was
evaluated by TGA analysis under nitrogen (Fig. 13). The curve of
the maltose-based PMO (Fig. 13A) exhibit two major weight loss
stages. The first (15%) centered at 68 °C represents desorption of
physically adsorbed volatile molecules such as water and ethanol.
The second (57%) centered at 359 °C is ascribed to the decomposi-
tion of the organic segments in the organosilicas. This analysis
indicates that M-PMO comprises 28% of inorganic material. Here
again, these percentages correlate well with the theoretically esti-
mated percentage of the inorganic part in organosilane monomers.
The TGA curve of isoleucinol-based mesoporous nanospheres
(Fig. 13B) shows a similar pattern at 69 °C (4%) and 246 °C.
3. Conclusion
Figure 6. TGA curves of chiral organo-silicas sol gel matrices based on (A)
maltose, (B) N-Boc- -glucosamine, (C) -isoleucinol, (D) N-Boc- -tyrosinol, (E) N-
Boc- -threoninol, (F) dimethyl -tartrate.
D-
Herein we have described the preparation and characterization
of new chiral organosilane monomers by a simple synthetic proce-
D
L
L
L
D
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O. Cohen et al. / Tetrahedron: Asymmetry xxx (2017) xxx–xxx
Figure 7. (A) and (B) SEM images of maltose PMO; (C) and (D) STEM images of maltose PMO.
Figure 8. SEM images of mesoporous isoleucinol particles (MPP-Ile).
dure. These chiral organosilane precursors undergo polycondensa-
tion using acid or fluoride ions as catalysts to give enantiopure chi-
ral organosilica materials. In addition, new chiral PMO and
mesoporous materials could be obtained by the condensation reac-
tion of the maltose- or isoleucinol-based silane precursors in the
presence of a directing agent, Pluronic P123. The applications of
the new chiral materials as inducers of conformational deracem-
ization of liquid crystal phases will be communicated soon.
4. Experimental
4.1. Instrumentation
The optical rotation angle of the chiral organosilane precursors
were measured by AUTOPOL
Rudolph research analytica. Scanning electron microscopy (SEM)
I automatic polarimeter from
Figure 9. STEM images of sliced Ile PMO. The arrows point out the presence of
channels.
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O. Cohen et al. / Tetrahedron: Asymmetry xxx (2017) xxx–xxx
7
were performed using Shottky-type field emission source with
through-lens SE and BSE detector (TLD) and retractable annular
BF/DF STEM detectors. The images were scanned at voltages of
20 kV and 25 kV. Small angle X-ray scattering (SAXS) were per-
formed in the Raviv in-house setup, the X-ray generator, Micro-
Max-007HF (Rigaku Corporation), is a rotating anode operating at
40 kV and 30 mA and has a copper target producing Κ
a
photons
focused
with an energy of 8 keV (wavelength of 1.54 Å).
A
monochromatic beam is obtained using Confocal Max-Flux optics
consisting of a CMF-12-100Cu8 focusing unit (Osmic Inc., a Rigaku
Company.) A MAR345 image-plate detector (Marresearch GmbH)
is stationed at the end of this flight path on a motorized plate. A
more detailed description of our setup the given elsewhere.59 Ther-
mogravimetric analysis (TGA) was performed on Mettler Toledo TG
50 analyzer. Measurements were carried out at a temperature
range that extended from 25 to 950 °C at a heating rate of 10 °C/
min under N2 atmosphere. The 1H NMR spectra were recorded
with Bruker DRX-400 instrument. Solid-state 29Si MAS and 13C
MAS NMR spectra were recorded with Bruker DRX-500 instrument.
Infra-red (IR) spectra were collected by PerkinElmer Spectrum 65
FT-IR spectrometer. Surface area was determined by the N2 Bru-
nauer–Emmett–Teller method (BET) (NOVA-1200e). The pore vol-
ume and radius were measured using the BJH model.
Figure 10. SAXS pattern of maltose-based PMO material.
images were acquired by high resolution scanning electron micro-
scope (HR SEM) Sirion from the FEI company. The experiments
were performed using high stable Schottky field Emission source
with through-lens SE detector. The images were scanned at volt-
ages of 5 kV. Scanning electron microscopy (SEM) and Scanning
transmission electron microscopy (STEM) images were acquired
by extra high resolution scanning electron microscopy (XHR-
SEM) MagellanTM 400L from the FEI company. The experiments
4.2. Synthesis of chiral organosilane precursors
4.2.1.
-Maltose monohydrate 1 (2 g, 5.6 mmol) was dissolved in dry
pyridine (15 mL), under nitrogen and 8 equiv of 3-triethoxysilyl-
D-Maltose-silane precursor 7
D
Figure 11. Comparison of the maltose (A) and isoleucinol (B) sol gel matrices (black) 13C-CP MAS (1) and 29Si-CP MAS (2) solid state NMR spectra, to the corresponding
mesoporous material (red) spectra.
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O. Cohen et al. / Tetrahedron: Asymmetry xxx (2017) xxx–xxx
Figure 12. Infra-red spectra of for (A) organo-silane precursor and (B) mesoporous material.
Figure 13. TGA curve of (A) maltose PMO (M-PMO) and (B) mesoporous isoleucinol particles (MPP-Ile).
propyl isocyanate (11 mL, 0.04 mol) were added. The reaction mix-
ture was heated to 60 °C for 2 days and the solvent was removed
under reduced pressure. The desired material was produced as yel-
low oil (11 ml). No additional purification steps were performed.
1H NMR (400 MHz, pyridine d5): d 0.65–0.74 (16H, m), 1.20 (72H,
t, J = 6.97 Hz), 1.66–1.76 (16H, m), 3.21 (16H, t, J = 6.67 Hz), 3.83
(48H, q, J = 6.97 Hz), 3.90–4.00 (2H, m), 4.04–4.08 (1H, m), 4.08–
4.20 (3H, m), 4.30–4.34 (2H, m), 4.37–4.63 (5H, m), 5.21(0.5H, d,
J = 7.64 Hz), 5.87 (0.5H, d, J = 3.62 Hz) ppm. 13C NMR (100 MHz,
pyridine d5): d 7.97, 18.63, 25.57, 58.71, 59.46, 62.25, 62.84,
71.96, 74.62, 75.36, 75.62, 76.49, 76.96, 78.20, 81.83, 99.00,
4.70–4.78 (1H, m), 5.88 (1H, d, J = 3.56 Hz) ppm. 13C NMR (100
MHz, pyridine d5): d 9.10, 19.77, 26.70, 29.84, 46.84, 59.46,
59.85, 64.54, 74.43, 74.57, 75.10, 79.39, 94.25, 158.31 ppm. FT-IR:
3342, 2974, 2928, 2891, 1709, 1534, 1249 1070, 950 and 770
cmÀ1. [
a]589 = +13 (c 10, THF).
20
4.2.3.
L-Isoleucinol-silane precursor 9
L
-Isoleucinol 3:
L
-Isoleucinol was prepared following a reported
procedure61 with slight modifications. Under nitrogen, 2.5 equiv of
Lithium aluminium hydride (7.2 g, 0.19 mol) was stirred in dry THF
(120 mL). Then L-isoleucine (10 g, 0.08 mol) was added in portions
and the suspension was refluxed for 20 h. After cooling to room
temperature, ethyl acetate was added and the reaction mixture
was poured carefully to concentrated sodium hydroxide solution.
The organic layer was extracted with water and dried with sodium
sulphate. 4.5 g of yellow liquid isoleucinol was obtained after
removing the solvents. The product was used without further
purification. 1H NMR (400 MHz, CDCl3): d 0.75–0.90 (6H, m),
1.01–1.17 (1H, m), 1.22–1.36 (1H, m), 1.37–1.50 (1H, m), 2.48
(2H, br s), 2.50–2.65 (1H, m), 3.19–3.29 (1H, m), 3.57 (1H, dd, J =
3.27, 10.64 Hz) ppm.
103.24, 157.58, 159.51 ppm. FT-IR: 3337, 2974, 2928, 2886, 1709,
1539, 1244, 1070, 954, 770 and 729 cmÀ1. [
a]589 = +23 (c 10, THF).
20
4.2.2. N-Boc-
The -glucosamine protection was carried out following a
reported procedure.60 1 g of Boc-
-glucosamine 2 (3.6 mmol) was
D-glucosamine silane precursor 8
D
D
dissolved in 8 mL of dry pyridine and 4 equiv of 3-triethoxysilyl-
propyl isocyanate (3.54 mL, 14 mmol) were added. The mixture
was heated under nitrogen to 60 °C for 2 days. After removing
the solvent under reduced pressure, 4.8 g of oily product were
obtained. No additional purification steps were performed. 1H
NMR (400 MHz, pyridine d5): d 0.65–0.73 (8H, m), 1.17–1.22
(36H, m), 1.46 (9H, s), 1.65–1.76 (8H, m), 3.20 (8H, t, J = 6.55 Hz),
3.79–3.87 (24H, m), 3.90–4.05 (2H, m), 4.10–4.26 (2H, m), 4.35
(1H, dd, J = 6.01, 11.19 Hz), 4.41–4.55 (2H, m), 4.55–4.67 (1H, m),
L
-Isoleucinol-silane 9: 1.5 g of L-isoleucinol 3 (13 mmol) was
dissolved in 4 mL of dry pyridine and 2 equiv of 3-triethoxysilyl-
propyl isocyanate (6.3 mL, 26 mmol) were added. The mixture
was heated under nitrogen to 60 °C for one day. After removing
the solvent under reduced pressure, 6.7 g of oily product were
Please cite this article in press as: Cohen, O.; et al. Tetrahedron: Asymmetry (2017), https://doi.org/10.1016/j.tetasy.2017.10.032

O. Cohen et al. / Tetrahedron: Asymmetry xxx (2017) xxx–xxx
9
obtained. No additional purification steps were performed. 1H NMR
(400 MHz, pyridine d5): d 0.66–0.79 (4H, m), 0.83 (3H, t, J = 7.44
Hz), 0.98 (3H, d, J = 6.87 Hz), 1.16 (9H, t, J = 7.00 Hz), 1.19 (9H, t,
J = 7.00 Hz), 1.18–1.25 (2H, m), 1.62–1.75 (2H, m), 1.77–1.86 (2H,
m), 1.86–1.93 (1H, m), 3.20 (2H, d, J = 6.64 Hz), 3.39–3.54 (2H,
m), 3.80 (6H, q, J = 7.00 Hz), 3.83 (6H, q, J = 7.00 Hz), 3.93–3.97
(2H, m), 4.06–4.15 (1H, m), 6.21 (1H, br s), 6.30 (1H, d, J = 8.67
Hz), 6.57 (1H, t, J = 5.57 Hz) ppm. 13C NMR (100 MHz, pyridine
d5): d 9.09, 9.44, 13.05, 17.31, 19.76, 19.79, 25.91, 26.70, 27.07,
37.84, 44.63, 46.83, 57.88, 59.72, 59.84, 64.56, 161.07 ppm. FT-IR:
until all the solid dissolved. After cooling the solution in an ice
bath, 18.5 g (0.08 mol) of di-tert-butyl dicarbonate was added.
The reaction was stirred overnight in room temperature, and then
the solution was concentrated by reducing pressure followed by
addition of 50 mL of ethyl acetate and a concentrated potassium
hydrogen sulphate solution until the pH reached 2. The aqueous
phase was extracted twice with ethyl acetate and the combined
organic phases were dried with sodium sulphate. The solvent
was removed under reduced pressure. 13.5 g (84% yield) of oily
N-Boc-L
-threonine was obtained. 1H NMR (400 MHz, CDCl3): d
3346, 2974, 2928, 2882, 1635, 1562, 1244, 1079, 954, 913, 770
1.22 (3H, d, J = 6.37 Hz), 1.43 (9H, s), 4.22–4.27 (1H, m), 4.31–
4.44 (1H, m), 5.74 (1H, d, J = 8.49 Hz), 6.99 (2H, br s) ppm.
and 729 cmÀ1. [
a]589 = À13 (c 10, THF).
20
N-Boc-L-threoninol 5: N-Boc-L-threonine (3 g, 14 mmol) was
4.2.4. N-Boc-
N-Boc- -tyrosine. The
lowing a reported procedure62 with some modifications. 10 g
(0.06 mol) of -tyrosine were stirred in 250 ml dioxane and water
L
-tyrosinol-silane precursor 10
dissolved under nitrogen in 100 mL of dry diethyl ether. Then
2.5 equiv of Lithium aluminium hydride (1.3 g, 30 mmol) were
added in portions and the suspension was refluxed overnight. After
cooling to room temperature, ethyl acetate was added and the
reaction mixture was poured carefully to concentrated sodium
hydroxide solution while stirring. The organic layer was extracted
with water and dried with sodium sulfate. 1.12 g oily material was
obtained after removing the solvents. 1H NMR (400 MHz, CDCl3): d
1.21 (3H, d, J = 6.47 Hz), 1.45 (9H, s), 2.84 (2H, br s), 3.49 (1H, br s),
3.81 (2H, d, J = 4.39 Hz), 5.36 (1H, d, J = 8.74 Hz) ppm.
L
L-tyrosine protection was performed fol-
L
mixture (1:1) and 75 mL of 1 M sodium hydroxide solution was
added until all the solid dissolved. After cooling the solution in
an ice bath, 12 g (0.06 mol) of di-tert-butyl dicarbonate was added.
After stirring for 7 h in room temperature, the reaction was con-
centrated by reducing pressure followed by addition of 50 mL of
ethyl acetate and a concentrated potassium hydrogen sulphate
solution until the pH reached 2. Then the aqueous phase was
extracted twice with ethyl acetate and the combined organic
phases were dried with sodium sulphate. After removing the sol-
vent under reduced pressure a 13 g (84% yield) of pinkish solid
were obtained. 1H NMR (400 MHz, DMSO-d6): d 1.31 (9H, s), 2.67
(1H, dd, J = 9.90, 13.85 Hz), 2.86 (1H, dd, J = 4.95, 13.85 Hz), 3.92–
4.02 (1H, m), 6.63 (2H, d, J = 7.73 Hz), 7.01 (2H, d, J = 7.73 Hz),
9.18 (1H, s), 12.5 (1H, br s) ppm.
N-Boc-L-threoninol-silane 11: 1.3 g of N-Boc-L-threoninol 5 (6.4
mmol) was dissolved in 5 mL of dry pyridine and 2 equiv of 3-tri-
ethoxysilylpropyl isocyanate (3.1 mL, 13 mmol) were added. The
mixture was heated under nitrogen to 60 °C for 2 days. After
removing the solvent under reduced pressure, the oily product
was obtained in a quantitative yield. No additional purification
steps were performed. 1H NMR (400 MHz, pyridine d5): d 0.66–
0.74 (4H, m), 1.20 (18H, t, J = 7.00 Hz), 1.44 (3H, d, J = 6.35 Hz),
1.49 (9H, s), 1.66–1.77 (4H, m), 3.05–3.15 (1H, m), 3.21 (4H, t, J
= 6.61 Hz), 3.84 (12H, q, J = 7.00 Hz), 4.10–4.24 (3H, m), 4.24–
4.36 (1H, m), 4.54–4.72 (1H, m), 7.04 (1H, d, J = 6.38 Hz) ppm.
13C NMR (100 MHz, pyridine d5): d 9.10, 16.23, 19.77, 22.28,
26.78, 46.84, 58.60, 59.76, 59.85, 79.53, 158.26, 158.36, 158.79
N-Boc-L-tyrosinol 4: 1 g of N-Boc-L-tyrosine (3.6 mmol) was dis-
solved under nitrogen in 80 mL of dry diethyl ether. Then lithium
aluminium hydride (0.4 g, 0.01 mol) was added in portions. The
suspension was heated to reflux overnight. After cooling to room
temperature, ethyl acetate was added and the reaction mixture
was poured carefully to concentrated sodium hydroxide solution
while stirring. The organic layer was extracted with water and
dried with sodium sulphate. 0.4 g oily material was obtained after
removing the solvents. 1H NMR (400 MHz, CDCl3): d 1.37 (9H, s),
2.69 (2H, d, J = 6.51 Hz), 3.47 (1H, dd, J = 5.27, 10.85 Hz), 3.57
(1H, dd, J = 3.75, 10.85 Hz), 3.76 (1H, s), 5.06 (1H, d, J = 7.75 Hz),
6.70 (2H, d, J = 8.37 Hz), 6.97 (2H, d, J = 8.37 Hz), 7.70 (1H, br s)
ppm.
ppm. FT-IR: 3341, 2974, 2928, 2882, 1703, 1520, 1246, 1072,
950, 920, 774 and 728 cmÀ1. [
a]589 = +0.6 (c 10, THF).
20
4.2.6. Dimethyl-
L-tartrate-silane precursor 12
1 g of dimethyl-
L
-tartrate 6 (5.7 mmol) was dissolved in 3 mL of
dry pyridine and 2 equiv of 3-triethoxysilylpropyl isocyanate (2.8
mL, 11 mmol) were added. The mixture was heated under nitrogen
to 60 °C for one day. After removing the solvent under reduced
pressure, the oily product was attained in a quantitative yield.
No additional purification steps were performed. 1H NMR (400
MHz, pyridine d5): d 0.64–0.73 (4H, m), 1.20 (18H, t, J = 6.99 Hz),
1.64–1.76 (4H, m), 3.21 (4H, t, J = 6.76 Hz), 3.68 (6H, s), 3.83
(12H, q, J = 6.99 Hz), 5.10 (2H, s), 7.71 (2H, s) ppm. 13C NMR
(100 MHz, pyridine d5): d 9.08, 19.74, 26.79, 45.34, 53.17, 59.83,
N-Boc-L-tyrosinol-silane 10: 1 g of Boc-L-tyrosinol 4 (3.7 mmol)
was dissolved in 7 mL of dry pyridine and 2 equiv of 3-triethoxysi-
lylpropyl isocyanate (1.9 mL, 7.4 mmol) were added. The mixture
was heated under nitrogen to 60 °C for 2 days. After removing
the solvent under reduced pressure, 2.9 g of oily product were
obtained. No additional purification steps were performed. 1H
NMR (400 MHz, pyridine d5): d 0.66–0.74 (4H, m), 1.20 (18H, t, J
= 7.01 Hz), 1.45 (9H, s), 1.65–1.77 (4H, m), 3.05–3.15 (1H, m),
3.17–3.26 (5H, m), 3.84 (12H, q, J = 7.01 Hz), 3.95–4.01 (2H, m),
4.30–4.42 (1H, m), 7.08 (2H, d, J = 8.30 Hz), 7.35 (2H, d, J = 8.30
Hz), 7.45(1H, d, J = 8.49 Hz) ppm. 13C NMR (100 MHz, pyridine
d5): d 9.11, 19.78, 26.71, 29.83, 38.64, 46.85, 57.88, 56.57, 59.86,
64.75, 79.29, 117.39, 131.33, 132.30, 157.80, 158.59 ppm. FT-IR:
75.33, 157.66, 158.67, 174.27 ppm. FT-IR: 3356, 2974, 2928,
20
2886, 1769, 1727, 1525, 1230, 1074, 954 and 766 cmÀ1. [
a
]
=
589
À10 (c 9.6, THF).
4.2.7. Chiral organosilicas matrices by sol–gel technique
All organosilane precursors were condensed using acid catalyst
(HCl) or fluoride ions (TBAF- tetrabutyl ammonium fluoride) as
nucleophilic catalyst.
3337, 2974, 2928, 2891, 1704, 1690, 1534, 1502, 1249, 1212,
1166, 1074, 950 and 770 cmÀ1. [
a]
589 = À0.9 (c 10, THF).
20
4.2.8. Acid catalysis route
4.2.5. N-Boc-
N-Boc- -threonine: The
same manner as the tyrosine with some modifications. 10 g (0.08
mol) of -threonine were stirred in 240 mL dioxane and water mix-
ture (1:1) and 80 mL of 1 M sodium hydroxide solution was added
L
-threoninol- silane precursor 11
1 ml of chiral organosilane precursor was dissolved in a glass
vial with 1.5 mL ethanol and was stirred to 1200 rpm while adding
2 mL of distilled water (DW) and 1 ml of 1 M HCl. The solution was
stirred for one or two days for the organosilica formation. The solid
was filtered, washed with acetone and dried overnight in 53 °C.
L
L-threonine amine was protected in the
L
Please cite this article in press as: Cohen, O.; et al. Tetrahedron: Asymmetry (2017), https://doi.org/10.1016/j.tetasy.2017.10.032

10
O. Cohen et al. / Tetrahedron: Asymmetry xxx (2017) xxx–xxx
4.2.9. Fluoride ion catalysis route
mL) for 48 h. 1.05 g of L-isoleucinol-based particles was obtained
1 mL of chiral organosilane precursor was dissolved in a glass
vial with 2 mL tetrahydrofuran and was stirred to 1200 rpm while
adding 0.2 mL of 0.1 M TBAF aqua solution and 2 mL of DW. The
solution was stirred for one or two days for the organosilica forma-
tion. The solid was filtered, washed with acetone and dried over-
night in 53 °C.
as a white powder.
FT-IR: 3324, 2962, 2932, 2878, 1698, 1546, 1108, 1026 and 916
cmÀ1 13C CP–MAS (125 MHz): d 10.98, 14.33, 22.76, 38.10, 42.13,
.
42.13, 54.28, 59.56, 157.37 ppm. 29Si CP-MAS (99 MHz):
d
À67.34, À60.66 ppm.
The typical weight of organosilica materials was 0.53–0.8 g.
Acknowledgements
D
-Maltose-based sol–gel (M): 0.80 g in HCl or 0.72 g in TBAF. FT-
IR: 3312, 2927, 2873, 1683, 1528, 1013 cmÀ1 13C CP–MAS (125
.
The studies were supported by ISF (Israel Science Foundation)
Grants N917/15 and 1384/13. The authors declare that they have
no conflict of interest.
MHz): d 9.73, 14.26, 22.67, 42.66, 61.71, 71.01, 91.54, 95.43,
100.31, 157.52 ppm. 29Si CP-MAS (99 MHz): d À66.59, À58.47
ppm.
N-Boc-
D-glucosamine-based sol–gel (Boc-G). 0.53 g in HCl or
A. Supplementary data
0.60 g in TBAF. FT-IR: 3316, 2927, 7686, 1524, 1248, 1032 cmÀ1
.
13C CP–MAS (125 MHz): d 9.49, 14.46, 23.53, 28.08, 43.11, 54.39,
Supplementary data associated with this article can be found, in
the online version, at https://doi.org/10.1016/j.tetasy.2017.10.032.
60.78, 70.79, 89.78, 157.14 ppm. 29Si CP–MAS (99 MHz):
d
À66.04, À58.49 ppm.
L
-Isoleucinol-based sol–gel (Ile): 0.76 g in HCl or 0.70 g in TBAF.
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Please cite this article in press as: Cohen, O.; et al. Tetrahedron: Asymmetry (2017), https://doi.org/10.1016/j.tetasy.2017.10.032