Chiral ruthenium catalyst immobilized within magnetically retrievable mesoporous silica microcapsules for aqueous asymmetric transfer hydrogenations

Article abstract of DOI:10.1002/ejic.201403212

The preparation of magnetically separable silica microcapsules that incorporate in their inner shell a chiral catalyst and their application in asymmetric transfer hydrogenation reactions are described. The preparation method is based on the emulsification of an oil phase containing chloroform, a modified Noyori Ru-TsDPEN catalyst, tetraethoxysilane (TEOS), and hydrophobic magnetic nanoparticles in water in the presence of an appropriate surfactant, followed by an interfacial polycondensation process under basic conditions to generate a silica shell around the oil droplets. The resulting catalytic microreactors can be considered a "quasi-homogeneous" system because the immobilized chiral catalyst reacts in a homogeneous zone, the microcapsule core filled with an organic solvent. The catalytic activity was tested in the asymmetric transfer hydrogenation of ketones in an aqueous medium. The catalytic reactions took place only in the presence of surfactants. In addition, the judicious selection of the surfactant plays a crucial role in enhancing the reaction progress through the emulsion-solid transfer (EST) approach. The catalytic activity of the Ru-TsDPEN catalyst immobilized within the silica microcapsules was superior to the same catalyst supported on silica microspheres or linked to the backbone of a silica sol-gel matrix, which indicates the importance of the homogeneous zones for the reactions. We report the design of a "quasi-homogeneous" catalytic system, based on a modified Noyori Ru-TsDPEN catalyst confined within mesoporous silica-based microreactors, for the efficient asymmetric transfer hydrogenation of ketones in water. The new system demonstrates excellent reactivity and enantioselectivity characteristic of homogeneous catalysts, but can be easily recovered.

Full text of DOI:10.1002/ejic.201403212

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DOI:10.1002/ejic.201403212
Chiral Ruthenium Catalyst Immobilized within
Magnetically Retrievable Mesoporous Silica Microcapsules
for Aqueous Asymmetric Transfer Hydrogenations
[
a]
[a]
[a]
Amani Zoabi, Suheir Omar, and Raed Abu-Reziq*
Keywords: Asymmetric catalysis / Hydrogenation / Microreactors / Sol–gel processes / Nanoparticles
The preparation of magnetically separable silica microcap- in a homogeneous zone, the microcapsule core filled with an
sules that incorporate in their inner shell a chiral catalyst and
their application in asymmetric transfer hydrogenation reac-
tions are described. The preparation method is based on the
emulsification of an oil phase containing chloroform, a modi-
fied Noyori Ru-TsDPEN catalyst, tetraethoxysilane (TEOS),
and hydrophobic magnetic nanoparticles in water in the
presence of an appropriate surfactant, followed by an interfa-
organic solvent. The catalytic activity was tested in the asym-
metric transfer hydrogenation of ketones in an aqueous me-
dium. The catalytic reactions took place only in the presence
of surfactants. In addition, the judicious selection of the sur-
factant plays a crucial role in enhancing the reaction progress
through the emulsion-solid transfer (EST) approach. The
catalytic activity of the Ru-TsDPEN catalyst immobilized
cial polycondensation process under basic conditions to gen- within the silica microcapsules was superior to the same cata-
erate a silica shell around the oil droplets. The resulting cata-
lytic microreactors can be considered a “quasi-homogen-
eous” system because the immobilized chiral catalyst reacts
lyst supported on silica microspheres or linked to the back-
bone of a silica sol–gel matrix, which indicates the impor-
tance of the homogeneous zones for the reactions.
Introduction
by implementing different strategies based on the covalent
or noncovalent anchoring of homogeneous catalysts onto
During the last few decades, the manufacture of enantio-
merically pure fine chemicals such as pharmaceuticals, ag-
rochemicals, or flavors has attracted considerable research
interest in different fields of life sciences.[ The asymmetric
transfer hydrogenation of ketones is among the most popu-
lar facile routes for the production of optically active sec-
ondary alcohols, which are essential intermediates for the
construction of many active compounds.[ Since the discov-
ery of Noyori’s catalyst, based on the ruthenium(II) com-
plex of (1R,2R)- or (1S,2S)-N-(p-tolylsulfonyl)-1,2-diphen-
ylethylenediamine (TsDPEN), significant progress in this
area has been achieved. The application of homogeneous
Noyori-type catalysts in a variety of enantiomeric transfor-
mations has come to be regarded as a highly effective and
valuable advancement in multiple fields.[ However, the dif-
ficulties associated with the separation and recycling of
such homogeneous catalysts have limited their application.
From an industrial point of view, the immobilization of
homogeneous chiral catalysts onto solid supports is of cen-
tral interest in the field of catalysis.[ This can be achieved
[5]
[6]
[7]
[9]
organic polymers,
ionic liquids, and organic/inorganic hybrid composites
inorganic matrices,
dendrimers,
[8]
[10]
among others.
Such heterogenized homogeneous asym-
1]
metric catalysts have been developed as alternative catalytic
[11]
systems for more effective and sustainable processes.
Efforts are continuously being made to promote more
convenient and retrievable systems by utilizing different
tools and materials. Accordingly, the development of mag-
netically separable catalysts has recently received a great
2]
[12]
deal of attention. In addition to their attractive features,
such as low cost, negligible toxicity, and high surface area,
magnetic nanoparticles have greatly enhanced both the
catalytic behavior and recyclability of catalysts in many
[13]
catalytic applications.
Consequently, considerable effort
3]
has recently been devoted to the immobilization of highly
[14]
expensive chiral homogeneous catalysts,
metric N-heterocyclic-carbene-based catalysts
Noyori-type catalysts,
such as asym-
[15]
and
onto magnetic nanoparticles.
[16]
These unique systems could significantly enhance catalyst
behavior as well as their reusability, the main requirement
for both academic and industrial interests.
4]
[
a] Institute of Chemistry, Casali Center for Applied Chemistry,
The Center for Nanoscience and Nanotechnology, The Hebrew
University of Jerusalem,
In recent years the design and fabrication of silica-based
[17]
materials through emulsion/sol–gel methods
have at-
Jerusalem 91904, Israel
tracted considerable attention in various fields, including
E-mail: Raed.Abu-Reziq@mail.huji.ac.il
[18]
[19]
[20]
drug delivery,
bioencapsulations,
and catalysis.
http://nanoscience.huji.ac.il/researchers/AbuReziq.html
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201403212.
Among the many appealing approaches and morphologies,
hollow microcapsules with porous silica-based shells pre-
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pared from oil-in-water emulsions (O/W) are of particular
interest for catalytic applications due to the tailorability and
facile fine-tuning of the chemical and physical properties of
[
21]
both the porous silica framework and the active core.
Encapsulation occurs as the silicon precursors polymerize
through an interfacial process to yield a silica shell, which
confines the oil droplets. Both porosity and morphology are
highly dependent on the polymerization conditions as well
as on the incorporated silane monomer type.[ Recently,
the development of new catalytic silica microcapsules with
palladium nanoparticles and ionic liquids entrapped in
their cores was reported by our group.[ These new materi-
als exhibited good catalytic activity and desirable chemo-
selectivity in the hydrogenation of alkynes. In addition, the
distinctive structure of these silica microcapsules could
render unique properties that are commonly attributed to
22]
23]
ionic liquids, accompanied by easy handling and the pos- Scheme 1. Preparation of the catalytic silica-based magnetic micro-
sible reusability of the catalyst.
reactors.
Herein we describe the preparation of magnetically re-
coverable catalytic silica microcapsules. The shells of the
capsules were synthesized by the co-polymerization of tetra-
ethoxysilane (TEOS) with chiral Noyori’s Ru-TsDPEN cat-
alyst functionalized with a trimethoxysilane group. The
hydrophobic nature of the core ensured the location of the
catalyst as well as the hydrophobic magnetite nanoparticles
within the cores of the microcapsules. The catalytic activity
of these microreactors was tested in the asymmetric transfer
hydrogenation of ketones in an aqueous medium. The
microreactors were easily recovered by simply applying an
external magnetic field.
relates well with the results of dynamic light scattering
(DLS) measurements, which revealed an average size of
295 nm (see Figure S1 in the Supporting Information).
SEM analysis also showed the stable dispersion state of
these microreactors, the protective silica shell limiting the
tendency to self-aggregate (Figure 1, a). The encapsulation
of magnetite nanoparticles within the capsules was con-
firmed by transmission electron microscopy (TEM) analysis
(Figure 1, b). TEM analysis also revealed the location of
the magnetic nanocrystals exclusively in the interior of the
microcapsules (Figure 1, b).
Results and Discussion
Preparation of Magnetically Separable Silica Microcapsules
Containing the Chiral Ruthenium Catalyst Ru-TsDPEN
(Ru-TsDPEN/MNP@SiO2)
The preparation of the microreactors was initiated by the
synthesis of hydrophobic magnetic nanoparticles. Fe O4
3
nanocrystals were synthesized by the co-precipitation of
Figure 1. a) SEM and b) TEM images of the Ru-TsDPEN/
MNP@SiO microreactors.
2
II
III
Fe and Fe salts in basic solution at 85 °C according to
Massart’s method.[
24]
These magnetite nanoparticles
(MNP) were coated with oleate groups and dispersed into
In addition, scanning transmission electron spectroscopy
chloroform. The hydrophobically modified magnetic nano- (STEM) in conjunction with energy-dispersive X-ray spec-
crystals in chloroform were mixed with TEOS and the tri- troscopy (EDS) evidenced the presence of both iron and
methoxysilane-modified Ru-TsDPEN catalyst and then ruthenium species in the Ru-TsDPEN/MNP@SiO2 micro-
transferred into an aqueous solution containing cetyl- capsules (Figure 2). The composition and structure of the
trimethylammonium chloride (CTAC) as surfactant microcapsules were analyzed by the EDS mapping method
to produce an emulsion. Spontaneous polymerization (Figure 3). The analysis confirmed that both the magnetic
generated magnetic Ru-TsDPEN/MNP@SiO2 microcap- nanoparticles and the ruthenium complex exist in the silica
sules (Scheme 1).
microcapsules. Moreover, the analysis indicated that the
Scanning electron microscopy (SEM) images exhibited magnetic nanoparticles and the chiral ruthenium complex
silica microcapsules with significant shrinkage (Figure 1, a). are dispersed almost homogeneously within the microcap-
Because the SEM analysis was performed under vacuum, sules.
evaporation of the volatile organic solvent from the core of
The powder XRD pattern of the microreactors is consis-
the microcapsules could occur resulting in the collapse of tent with the presence of unchanged Fe O and amorphous
3
4
their structures. The SEM analysis indicated that the SiO (Figure 4). It shows the characteristic peaks of magne-
2
average size of the microcapsules is 300–350 nm, which cor- tite nanoparticles at 2θ = 30.1, 35.4, 43.4, 53.4, 57.2, and
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Figure 4. Powder XRD pattern of the Ru-TsDPEN/MNP@SiO
microreactors.
2
Figure 2. STEM/EDS analysis of the Ru-TsDPEN/MNP@SiO
microreactors.
2
teristic aromatic C=C stretching band at 1635 cm–1 can also
be observed in the spectrum, which indicates the presence
of the ruthenium catalyst in the silica microcapsules (see
Figure S3 in the Supporting Information). One of the two
characteristic signals of the SO2 group appears at
–1
1
458 cm , with a second band overlapping with the peak
of the siloxane groups. The N–H stretching vibrations of
the TsDPEN amine groups could not be detected clearly
because their band overlaps with the peak arising from the
silanol groups. These results prove the successful incorpora-
tion of the chiral Ru-TsDPEN catalyst as well as the entrap-
ment of magnetite nanoparticles within the prepared silica
microcapsules.
The composition of the silicate framework was also con-
firmed by solid-state 13C and Si NMR spectroscopy after
treatment with HCl to remove the magnetite nanoparticles
from the core. The 29Si CP-MAS NMR spectrum shown in
Figure 5 presents two major upfield resonance peaks corre-
sponding to native silica absorptions of siloxane groups
29
Figure 3. EDS elemental mapping of the Ru-TsDPEN/MNP@SiO
microreactors.
2
4
3
(Q , δ = –111 ppm) and single silanol groups (Q , δ =
–
–
101 ppm). The chemical shifts in the range δ = –69 to
57 ppm have been assigned to T and T organosilica spe-
6
2.4° in addition to a broad peak at 2θ = 15–28°, which has
3
2
been attributed to amorphous silica.
Furthermore, the total surface area of these amorphous
silica microcapsules was evaluated by means of a Brunauer–
Emmet–Teller (BET) analysis in conjunction with Barrett–
Joyner–Halenda (BJH) calculations; a surface area of
2
2
09.4 m /g with a pore size of 6.89 nm was determined (see
Figure S2 in the Supporting Information).
The structural composition was further determined by
an IR analysis in which the magnetic silica microreactors
were compared with the prepared Si-Ru-TsDPEN catalyst.
The IR spectrum of the Ru-TsDPEN/MNP@SiO micro-
2
capsules shows well-defined absorption bands at 1160 and
–
1
3
069–3620 cm , which have been assigned to the asymmet-
rical stretching of siloxane groups (Si–O–Si) and Si–OH
stretching vibrations, respectively (see Figure S3 in the Sup-
porting Information). In addition, bands arising from the
oleate groups coating the magnetite nanoparticles appear at
–
1
2851–2930 and 1645 cm , which have been attributed to
Figure 5. 29Si CP-MAS NMR spectrum of the Ru-TsDPEN/
C–H and C=C stretching, respectively. Moreover, a charac- MNP@SiO
2
microreactors.
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cies, respectively. These results indicate that the main micro- is of central importance to assist in transporting the organic
4
3
3
2
structure is composed of Q , Q T , and T and clearly ver- substrates to the heart of the reactor.
ify the successful anchoring of the silylated ruthenium com-
plex in the silicate network of the microcapsules.
Upon examination of various surfactants in the asym-
metric transfer hydrogenation of acetophenone, we found
Further evidence for the integration of Ru-TsDPEN that the surfactant plays a crucial role in this reaction
13
within the silica microcapsules was obtained by C CP- (Table 1). Thus, the reaction did not occur in the absence
1
3
MAS NMR analysis. The C CP-MAS NMR spectrum in of surfactant (Table 1, entry 1). Based on previous work re-
[
25]
Figure 6a clearly shows the characteristic signals of carbon ported by Avnir and Blum and co-workers, the transport
species in the catalyst. The peaks at δ = 6.1–32.2 ppm corre- of a hydrophobic substrate from the aqueous medium to
spond to SiCH and SiCH CH Ph, the characteristic peak an entrapped catalyst in sol–gel matrices requires a suitable
2
2
2
at δ = 56–62 ppm has been attributed to the C atoms of the surfactant. This is described as an emulsion-solid transport
NCHPh groups, and the peaks at δ = 127–148 ppm have (EST) process. We believe that the asymmetric reaction per-
been assigned to the phenyl C atoms in the TsDPEN moi- formed in aqueous media using our magnetic catalytic
1
3
ety. Comparison of this C CP-MAS NMR spectrum with microreactors follows the same process. Accordingly, a mi-
the 1 C NMR spectrum of TsDPEN functionalized with celle carrying a hydrophobic substrate can be broken after
trimethoxysilane groups (Figure 6, b) or the Si-Ru- adsorption onto the surface of a microreactor and the ad-
TsDPEN complex (Figure 6, c) reveals that the carbon peak sorbed surfactant can facilitate the transport of the sub-
of the methoxy groups is not present. This demonstrates strate into the core of the reactor because it converts the
that the Si-Ru-TsDPEN complex has been successfully teth- hydrophilic silica surface into a hydrophobic surface. After
ered to the silica shell. On the other hand, the spectrum the reaction, the products can be extracted into the micelles
does not show any peak arising from the p-cymene group, that form upon desorption of surfactant molecules. This
which indicates that this moiety dissociates during the treat- process is illustrated in Scheme 2.
3
ment with hydrochloric acid to remove magnetite nanopar-
ticles. However, the presence of carbon peaks from modi-
Table 1. Asymmetric transfer hydrogenation of acetophenone in
water in the presence of different surfactants.
1
3
fied TsDPEN in the C CP-MAS NMR spectrum indicates
the successful insertion of the modified Ru-TsDPEN cata-
lyst within the silica network.
[a]
Entry
Surfactant
Yield [%][b]
ee [%][c]
1
2
3
4
5
6
7
8
9
None
CTAB
CTAC
SDS
0
Ͼ99
Ͼ99
57
85
95
88
89
84
Ͼ99
95
–
95
95
91
90
94
92
93
97
91
91
SDBS
Tween 80
Bu-PVP
Brij 78
Pluronic P123
Aliquat 336
DDAC
10
1
1
[
a] Dispersion of silica microreactors containing 0.005 mmol of
ruthenium catalyst, 0.529 mmol of acetophenone, 2.6 mmol of so-
dium formate, and 0.125 mmol of surfactant, 24 h, 45 °C. [b] Deter-
mined by H NMR and GC analyses. [c] Determined by HPLC
1
using a chiral OJ-H column.
Figure 6. a) 13C CP-MAS NMR spectrum of the Ru-TsDPEN/
microreactors, b) 13C NMR spectrum of TsDPEN
MNP@SiO
2
The results presented in Table 1 demonstrate the different
reactivity and enantioselectivity of the catalytic microreac-
tors depending on the nature of the surfactant. Although
cationic surfactants gave high yields and enantioselectivities
in the asymmetric transfer hydrogenation of acetophenone
13
functionalized with trimethoxysilane groups, and c) C NMR
spectrum of the complex Si-Ru-TsDPEN.
(
Table 1, entries 2, 3, 10, and 11), anionic surfactants pro-
Catalytic Activity in the Asymmetric Transfer
Hydrogenation of Ketones
vided low yields (Table 1, entries 4 and 5). On the other
hand, the reactivity of the catalyst in the presence of non-
Having the magnetic microreactors in hand, we investi- ionic surfactants was moderate (Table 1, entries 6–9). These
gated their catalytic activity in the asymmetric transfer results further strengthen our assumption that the reactions
hydrogenation of aromatic ketones in water. The choice of follow the EST process. In this process, the employment of
water as a reaction medium was not casual; we hypothe- different cationic, anionic, non-ionic, or polymeric surfac-
sized that a faster and more efficient diffusion of organic tants can affect the reaction progress differently due to their
substrates through the reactor shell into the hydrophobic different abilities to be adsorbed effectively onto the silica
core can be achieved by minimizing substrate–solvent inter- surface of the microreactors and make it hydrophobic,
actions. On the other hand, a catalytic amount of surfactant which can be monitored by ζ-potential measurements. The
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0.529 mmol of the desired ketone, 0.125 mmol of CTAC
(these amounts correspond to a catalyst/surfactant/ketone
ratio of 1:2:100), and 2.6 mmol of sodium formate (5 equiv.)
with stirring for 24 h at 45 °C. For example, under these
optimal conditions, the reduction of acetophenone is driven
Table 2. Representative results of the asymmetric transfer hyd-
rogenation of aromatic ketones within the silica-based microreac-
tors.[
a]
Scheme 2. Illustration of the transport of the hydrophobic reactants
from the water medium to the core of the catalytic microreactors
and extraction of the products from these microreactors.
ζ potential of the microreactors before the addition of sur-
factants was –26.4 mV, which indicates that the silica sur-
face is negatively charged. When cetyltrimethylammonium
bromide (CTAB) was applied as the surfactant, a ζ poten-
tial of +44.4 mV was detected, which demonstrates that the
surface of the silica microreactors is positively charged.
This ζ potential could be obtained only if the CTAB mol-
ecules were adsorbed onto the surface and organized in a
double layer. The double layer of CTAB generates a hydro-
phobic surface and promotes the transport of acetophenone
into the catalytic core, as reflected by the high yield ob-
tained in its asymmetric reduction (Table 1, entry 2). On the
other hand, in the presence of anionic surfactants the abil-
ity to form an organized double layer is impaired because
of repulsive interactions between the anionic surfactant and
the negatively charged silica surface. For example, in the
presence of sodium dodecyl sulfate (SDS) as surfactant, the
ζ potential of the microreactors was –41.2 mV. In this case,
a lower yield was obtained, which demonstrates that the
transport of the substrate to the catalytic sites is less ef-
ficient (Table 1, entry 4). The non-ionic surfactants utilized
in this study can be adsorbed onto the silica surface
through hydrogen bonding with the silanol groups. After
adsorption, these surfactants can mask the deprotonated
silanol groups leading to a decrease in the negative value of
the ζ potential. For example, when butylated poly(vinylpyr-
rolidone) (Bu-PVP) was employed as the surfactant in the
asymmetric reaction, a ζ potential of –3.2 mV was deter-
mined. Although the non-ionic surfactants could be ef-
ficiently adsorbed onto the microreactor surface, lower
yields were obtained in their presence in comparison with
the cationic surfactants.
Consequently we decided to study the scope and possible
limitations of the new catalytic systems. The results of this
study are presented in Table 2. Experiments were typically
carried out by using 2 g of microcapsules (containing
[
a] Dispersion of silica microreactors containing 0.005 mmol of the
ruthenium catalyst, 0.529 mmol of ketone, 2.6 mmol of sodium for-
mate, and 0.125 mmol of CTAC, 24 h, 45 °C. [b] Determined by
1
H NMR and GC analyses. [c] Determined by HPLC using a chiral
0.005 mmol of the Ru catalyst) suspended in water, OJ-H column.
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to full conversion and the desired 1-phenylethanol can be
isolated in excellent yield and enantioselectivity (Ͼ99 and
95%, respectively, Table 2, entry 1). For comparison pur-
poses, the reduction of acetophenone was also performed
under homogeneous conditions by using the Si-Ru-
TsDPEN catalyst in addition to an azeotropic mixture of
formic acid and triethylamine. In this case, full conversion
and an enantioselectivity of 98% was observed. Notably,
these results prove that the Ru-TsDPEN/MNP@SiO cata-
2
lyst has almost the same reactivity and selectivity as the
homogeneous catalyst. When the cyclic ketone α-tetralone
was applied as substrate, complete conversion and a high
enantioselectivity were achieved (Table 2, entry 2). Very ef-
ficient transformations of electron-rich (Table 2, entries 3
and 4), electron-neutral (Table 2, entries 1, 2, and 5), and
electron-poor (Table 2, entries 6–11) acetophenone deriva-
tives were demonstrated. In all these cases the reactions
gave full conversions to the desired 1-arylethanols after 24 h
at 45 °C and the enantiomeric excesses were in the range
2
Figure 7. Recyclability of Ru-TsDPEN/MNP@SiO microreactors
in the asymmetric transfer hydrogenation of acetophenone.
82–99%. Remarkably, our catalyst was relatively insensitive
to the steric hindrance of substrates and very good enantio-
selectivities were demonstrated with ortho-substituted ace-
tophenones (Table 2, entries 3 and 7) and the cyclic ketone
α-tetralone (Table 2, entry 2).
Very facile work-up, isolation of the product, and recov-
ery of the catalyst are highlights of the described method.
Typically, the catalyst was separated from the reaction mix-
ture by applying an external magnetic field and the microre-
actors were washed with a small amount of dichlorometh-
ane to ensure that the entire product was washed out. We
also demonstrated that the catalyst can be successfully re-
used after isolation three times in the asymmetric transfer
hydrogenation of acetophenone without a significant de-
crease in its enantioselectivity (Figure 7). However, a slight
decrease in its reactivity was observed after the third cycle,
which can be attributed to some extent to pore blockage in
the silica shell and inaccessibility of the catalyst. To investi-
gate whether leaching of the catalyst occurs during the reac-
tion, the microcapsules were separated after the second and
third cycles by using an external magnetic field and the fil-
trates were analyzed by inductively coupled plasma (ICP).
There was no detectable ruthenium in either of the filtrates,
which indicates that the decrease in activity is not a result
of catalyst leaching.
Figure 8. a) Reference magnetite-containing capsules with the cata-
lyst anchored to the silica shell and b) reference sol–gel anchored
catalyst.
To highlight the difference between this microreactor ap-
proach and more conventional anchoring methods we per-
formed several blank experiments. First, we prepared cata-
lyst-free emulsion-templated mesoporous silica-based mag- Figure 9. SEM image of the Si-Ru-TsDPEN catalyst covalently en-
netite-containing spheres according to the protocol de- capsulated in the sol–gel matrix.
scribed in this article and anchored the trimethoxysilane-
modified Ru-TsDPEN catalyst to the external part of the
The catalytic activities of these systems were compared
silica shell by using a standard condensation method (Fig- with the activity of the microreactors using 4-bromoaceto-
ure 8, a). Secondly, a sol–gel-anchored catalyst was pre- phenone as a model substrate. The results are summarized
pared by the co-polymerization of tetramethyl orthosilicate in Table 3. As can be clearly seen, the efficiencies and
(TMOS) and the trimethoxysilane-modified Ru-TsDPEN enantioselectivities of the blank systems are incomparable
species (Figure 8, b). The SEM image of this catalyst (Fig- to the catalytic microreactors. This indicates that the cataly-
ure 9) exhibits a smooth surface and the typical structure sis indeed takes place within the hydrophobic zones of the
of a sol–gel matrix.
capsules and is not interfered with by residual species en-
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trapped within the silica shells or by species that have equipped with dispersing aggregates 3030/4EC. Enantiomeric ex-
cesses were determined by HPLC (Thermo Scientific, Chiral OJ-H
column, 5 μm, 4.6ϫ 250 mm). Surface areas were determined by
the N Brunauer–Emmett–Teller method (NOVA-1200e). The pore
2
volume and radius were measured by using the BJH model. Powder
XRD analysis was performed by using a D8 Advance X-ray dif-
fractometer. Inductively coupled plasma (ICP) mass spectra were
recorded with a 7500cx spectrometer (Agilent) using external stan-
dard calibration.
leaked out of the reactors. The sluggish reactivity of the Si-
Ru-TsDPEN catalyst anchored to the outer surface of the
silica microcapsules can be attributed to the inability of 4-
bromoacetophenone to approach the catalytic sites sur-
rounded by surfactant molecules due to steric hindrance. In
the case of the Ru-TsDPEN/MNP@SiO microreactors, the
2
catalyst is located inside the microreactors and can be ac-
cessed by the substrate but not by surfactant molecules. The
low reactivity of the sol–gel-anchored catalyst is ascribed
to the inefficient EST process in hydrophilic silica sol–gel
Materials: Tetraethyl orthosilicate (TEOS), tetramethyl orthosili-
cate (TMOS) cetyltrimethylammonium chloride (CTAC), poly(oxy-
ethylene) (20) stearyl ether (Brij 78), the triblock co-polymer poly-
[
25c]
matrices, as has been observed in previous studies.
(
ethylene glycol)-poly(propylene glycol)-poly(ethylene glycol) (Plu-
ronic P123), sodium dodecyl sulfate (SDS), trioctylmethylammon-
ium chloride (Aliquat 336), butylated poly(vinylpyrrolidone) (Bu-
PVP), and didecyldimethylammonium chloride (DDAC) were con-
tributed by the FMC Corporation. (1R,2R)-1,2-Diphenylethane-
Table 3. Comparison of the catalytic activities of the microreactors
and the sol–gel covalently entrapped and anchored catalysts.[
a]
Entry
Catalyst
Yield [%][b] ee [%][c]
1
,2-diamine (DPEN), acetophenone, 4-methoxyacetophenone, 2,5-
1
2
3
Microreactors
Empty capsule anchored catalyst
Sol–gel-anchored catalyst
90
8
41
87
N/A
76
dimethoxyacetophenone, and 4-chloroacetophenone were pur-
chased from Sigma–Aldrich. Cetyltrimethylammonium bromide
(
3
CTAB), poly(oxyethylene) (20) sorbitan monooleate (Tween 80),
-nitroacetophenone, 4-bromoacetophenone, 2-chloroacetophen-
[
a] Heterogeneous catalyst containing 0.005 mmol of the ruthenium
catalyst, 0.529 mmol of 4-bromoacetophenone, 2.6 mmol of so-
one, 4-chloroacetophenone, 3-iodoacetophenone, and 3-bromo-
acetophenone were obtained from Across Organics. Sodium for-
mate was purchased from BDH Chemicals. Sodium dodecylbenz-
enesulfonate (SDBS) was purchased from TCI chemicals. 2-(4-
Chlorosulfonylphenyl)ethyltrimethoxysilane was purchased from
Acros Organics. Di-μ-chlorobis(p-cymene)chlororuthenium(II) was
obtained from Strem Chemicals.
dium formate, and 0.125 mmol of CTAC, 24 h, 45 °C. [b] Deter-
mined by 1H NMR and GC analyses. [c] Determined by HPLC
using a chiral OJ-H column.
Conclusions
Synthesis of Magnetite Nanoparticles Coated with Oleate: Distilled
water (400 mL) was degassed for 15 min under nitrogen. Then
We have designed and prepared quasi-homogeneous
catalytic microreactors. Catalysis by this system takes place
in a homogeneous zone confined within a protective meso-
porous silica shell permeable to organic substrates and the
catalyst can be easily recovered from the reaction mixture
FeCl
3
·6H
2
O
(11.6 g, 42.9 mmol) and FeCl
2
·4H
2
O
(4.3 g,
21.6 mmol) were added and the mixture heated at 85–90 °C. Am-
monium hydroxide (28%, 15 mL) was then added and the mixture
heated for an additional 30 min. Then oleic acid (18 mL, 57 mmol)
by applying simple physical separation methods. In particu- was added and a black precipitate was obtained. The mixture was
lar, we have developed a highly efficient catalytic system stirred for an additional 10 min and then cooled to room tempera-
for the enantioselective transfer hydrogenation of aromatic ture. The black precipitate was collected by using an external mag-
netic field, washed with water (5ϫ) and acetone (5ϫ), and then
dispersed in chloroform (50 mL).
ketones in water catalyzed by magnetically recoverable
microreactors. The accessibility of the substrates to the cat-
alyst within the microreactors occurs through an emulsion- N-[(1R,2R)-2-Amino-1,2-diphenylethyl]-4-[2-(trimethoxysilyl)-
solid transport approach in which a micelle carrying the ethyl]benzenesulfonamide:[ (1R,2R)-1,2-Diphenylethane-1,2-di-
substrate transfers its cargo to the microreactors after ad- amine (DPEN) (0.48 g, 2.31 mmol) and dry triethylamine (0.8 mL)
6b]
were dissolved in CH
ethyltrimethoxysilane (0.75 g, 2.31 mmol) in CH
then added dropwise at –20 °C. The reaction mixture was then
warmed to room temperature slowly and stirred for 24 h. After
removing the solvent in vacuo, dry diethyl ether was added, and a
white solid precipitated. The filtrate was then evaporated to give a
2
Cl
2
(20 mL) and 2-(4-chlorosulfonylphenyl)-
Cl (20 mL) was
sorption of the surfactant molecules onto the surface of the
silica shell. Importantly, this concept can be generalized and
applied to different catalytic systems, substrates, and reac-
tion media.
2
2
beige solid that was then dried under reduced pressure for 2 h, yield
Experimental Section
1
57.9% (0.67 g). H NMR (500 MHz, CDCl
3
): δ = 0.9–0.94 (m, 2
H), 2.65–2.7 (m, 2 H), 3.57 (s, 9 H), 3.95 (d, J = 7.2 Hz, 1 H), 4.3
General: NMR spectra were recorded with 400 and 500 MHz
Bruker spectrometers. SEM images were acquired by using a Sirion
(
1
d, J = 7.2 Hz, 1 H), 7.02–7.03 (d, J = 8.3 Hz, 2 H), 7.12–7.19 (m,
1
3
0 H), 7.34–7.36 (d, J = 8.3 Hz, 2 H) ppm. C NMR (100 MHz,
): δ = 11.0, 28.6, 90.9, 50.6, 60.5, 63.1, 126.5, 127, 127.3,
27.5, 127.8, 128.2, 128.4, 128.5, 137.4, 139.1, 141.4, 148.8 ppm.
(FEI Company) high-resolution scanning electron microscope, a
CDCl
1
3
Schottky-type field emission source, and a secondary electron (SE)
detector. The images were scanned at an acceleration voltage of
5
kV. Transmission electron microscopy (TEM) and electron dif-
Silyl-Functionalized Ru-TsDPEN:[26] A mixture of N-[(1R,2R)-2-
fraction spectroscopy (EDS) were performed with a (S) Tecnai F20
G instrument (FEI company) operated at 200 kV. Dynamic light sulfonamide (64.8 mg, 0.13 mmol), [RuCl
amino-1,2-diphenylethyl]-4-[2-(trimethoxysilyl)ethyl]benzene-
2
6
2
(η -p-cymene)]
2
(40 mg,
scattering (DLS) measurements were performed by using a
ZEN3600 Malvern Zetasizer Nano instrument. Emulsification was
performed by using a PT-6100 Kinematica Polytron homogenizer
0.065 mmol), and NEt
stirred at room temperature for 30 min under nitrogen. Then the
mixture was washed with water (10 mL), extracted with dichloro-
3
(0.27 mL, 2 mmol) in CH
2
Cl
2
(10 mL) was
Eur. J. Inorg. Chem. 0000, 0–0
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/KAP1
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Pages: 10
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FULL PAPER
methane, and dried with anhydrous MgSO
vent gave a yellow solid catalyst powder, yield 92% (0.09 g).
NMR (500 MHz, CDCl ): δ = 0.87–0.93 (m, 2 H), 1.36–1.38 (m, 6 filtration, the solvent was evaporated. The conversions were deter-
H), 2.34 (s, 3 H), 2.54–2.61 (m, 2 H), 3.09–3.15 (m,1 H), 3.36–3.43
4
. Removal of the sol-
The mixture was stirred for 24 h at 45 °C. The products were ex-
1
H
tracted with diethyl ether (3ϫ5 mL). After drying over MgSO and
4
3
1
mined by H NMR and GC analyses. The enantiomeric excesses
(m, 1 H), 3.60 (s, 9 H), 3.69–3.73 (m, 1 H), 5.73–5.85 (m, 4 H), were determined by HPLC using a chiral OJ-H column.
6
6
1
1
1
.29–6.28 (m, 2 H), 6.54 (t, J = 8.3 Hz, 2 H), 6.57–6.73 (m, 5 H),
Supporting information (see footnote on the first page of this arti-
cle): DLS measurements, nitrogen adsorption/desorption isotherms
of the silica microreactors, IR analysis, HPLC chromatograms, and
NMR spectra of the products
.95–7.04 (m, 5 H) ppm. 13C NMR (125 MHz, CDCl
): δ = 11.2,
3
8.9, 22.2, 22.5, 28.3, 30.5, 50.6, 69.1, 71.8, 79.9, 82, 85.3, 94.5,
04.1, 125.9, 126.3, 126.5, 126.7, 127.2, 127.4, 127.9, 129.06, 138.7,
39.5, 143.6, 145.1 ppm.
Preparation of Magnetically Separable Catalytic Silica-Based
Microreactors: The microcapsules were produced by an interfacial
polymerization technique. Thus, an oil phase containing tetraethyl
Acknowledgments
orthosilicate (TEOS; 9 g, 43.2 mmol), CHCl
3
(21 g) containing
The authors gratefully acknowledge the financial support of this
study by the Deutsche Forschungsgemeinschaft (DFG) through
grant SCHO687/8-2 and the Niedersachsen–Israeli Research Co-
operation Program.
hydrophobic magnetic nanoparticles (1.3 g), and silyl-function-
alized Ru-TsDPEN (0.04 g, 0.055 mmol) was emulsified with dis-
tilled water (63.9 g) containing cetyltrimethylammonium chloride
(
CTAC 25%; 6.1 g) by shearing with a homogenizer at 10000 rpm
for 4 min. The resulting emulsion was then stirred mechanically for
4 h at room temperature. The microcapsules formed were sepa-
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Received: December 19, 2014
Published Online: ᭿
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© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Job/Unit: I43212
/KAP1
Date: 18-03-15 15:17:33
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FULL PAPER
Immobilized Chiral Catalysts
A. Zoabi, S. Omar, R. Abu-
Reziq* ............................................. 1–10
We report the design of a “quasi-homo-
geneous” catalytic system, based on a
modified Noyori Ru-TsDPEN catalyst con-
fined within mesoporous silica-based mic-
roreactors, for the efficient asymmetric
transfer hydrogenation of ketones in water.
The new system demonstrates excellent re-
activity and enantioselectivity character-
istic of homogeneous catalysts, but can be
easily recovered.
Chiral Ruthenium Catalyst Immobilized
within Magnetically Retrievable Meso-
porous Silica Microcapsules for Aqueous
Asymmetric Transfer Hydrogenations
Keywords: Asymmetric catalysis / Hydro-
genation / Microreactors / Sol–gel pro-
cesses / Nanoparticles
Eur. J. Inorg. Chem. 0000, 0–0
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© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim