Flower-like mesoporous silica: A bifunctionalized catalyst for rhodium-catalyzed asymmetric transfer hydrogenation of aromatic ketones in aqueous medium

Article abstract of DOI:10.1039/c3gc40547h

Functionalized flower-like mesoporous silica with a chiral organorhodium functionality incorporated within its silica framework is prepared through an assembly of chiral 4-((trimethoxysilyl)ethyl)phenylsulfonyl-1,2- diphenylethylenediamine and tetraethoxysilane under a cooperative dual-template approach followed by complexation with organorhodium complexes. Structural characterization discloses its mesostructure and well-defined single-site chiral organorhodium functionality, while electron microscopy analyses reveal the uniformly distributed three-dimensional spherical flowers constructed by the stacking of leaf-shaped nanoflakes. In particular, as a bifunctionalized heterogeneous catalyst, it shows excellent catalytic activity and high enantioselectivity in the asymmetric transfer hydrogenation of aromatic ketones in aqueous medium (more than 99% conversion and up to 97% ee). The superior catalytic performance is attributed to the synergistic effect of the salient cetyltrimethylammonium bromide phase-transfer function and confined chiral organorhodium catalytic nature. Furthermore, this heterogeneous catalyst could be recovered easily and reused repeatedly (ten times) without affecting its ee value, showing a practical application in asymmetric synthesis.

Full text of DOI:10.1039/c3gc40547h

Green Chemistry
View Article Online
View Journal
PAPER
Flower-like mesoporous silica: a bifunctionalized
catalyst for rhodium-catalyzed asymmetric transfer
hydrogenation of aromatic ketones in aqueous
medium†
Cite this: DOI: 10.1039/c3gc40547h
Fei Gao, Ronghua Jin, Dacheng Zhang, Quanxi Liang, Qunqun Ye and Guohua Liu*
Functionalized flower-like mesoporous silica with a chiral organorhodium functionality incorporated
within its silica framework is prepared through an assembly of chiral 4-((trimethoxysilyl)ethyl)phenylsulfo-
nyl-1,2-diphenylethylenediamine and tetraethoxysilane under a cooperative dual-template approach
followed by complexation with organorhodium complexes. Structural characterization discloses its
mesostructure and well-defined single-site chiral organorhodium functionality, while electron microscopy
analyses reveal the uniformly distributed three-dimensional spherical flowers constructed by the stacking
of leaf-shaped nanoflakes. In particular, as a bifunctionalized heterogeneous catalyst, it shows excellent
catalytic activity and high enantioselectivity in the asymmetric transfer hydrogenation of aromatic
ketones in aqueous medium (more than 99% conversion and up to 97% ee). The superior catalytic per-
formance is attributed to the synergistic effect of the salient cetyltrimethylammonium bromide phase-
transfer function and confined chiral organorhodium catalytic nature. Furthermore, this heterogeneous
catalyst could be recovered easily and reused repeatedly (ten times) without affecting its ee value,
showing a practical application in asymmetric synthesis.
Received 23rd March 2013,
Accepted 13th May 2013
DOI: 10.1039/c3gc40547h
www.rsc.org/greenchem
MCM-41,2 SBA-15,3 SBA-164 and PMO5 have been developed
and some catalysts have exhibited high reactivity and enantio-
Introduction
Immobilization of homogeneous chiral organometallic cata- selectivity,2,4a,6 most of them still suffer from a low catalytic
lysts onto mesoporous materials is a well-known method of efficiency relative to their homogeneous counterparts. Accord-
solving the problem of expensive transition-metal recycling, ingly, exploration of new silica-based supports and appli-
while the green catalytic process is environmentally friendly. cations in environmentally friendly catalytic systems is still a
Recently, the exploration of organometallic-functionalized challenge in asymmetric catalysis.
mesoporous silica materials for green asymmetric catalysis has
Three dimensional mesoporous silica with unique mor-
attracted a great deal of interest.1 The general features of easy phologies have a wide range of applications in drug delivery
separation, large specific surface area and pore volume are and bimolecular separations.7 In particular, functionalized
beneficial to enhance loadings and to improve the dispersibil- flower-like mesoporous silica (FFMS), as a new type of silica-
ity of active species, resulting in a high catalytic performance. based support,8 shows significant advantages in asymmetric
Furthermore, tunable pore dimensions and additional con- catalysis that have not yet been explored. Due to the special
finement effects can adjust the chiral microenvironment of preparation strategy, the self-assembly of chiral functionalized
the active center, presenting superior stereocontrol perform- materials often requires cetyltrimethylammonium bromide
ance. In addition, the high thermal and mechanical stability (CTAB) as a structure-directing template reagent, which in-
in catalytic processes often leads to a high recycling efficiency, evitably leaves parts within the FFMS materials. This behavior
providing the opportunity for use in industrial applications. can provide additional phase transfer function and may facili-
Although various silica-based mesoporous materials including tate an asymmetric reaction in a two-phase catalysis system.
More importantly, a unique mesoporous morphology with
ultrathin aligned mesoporous nanoflakes has a relatively short
Key Laboratory of Resource Chemistry of Ministry of Education, Shanghai Normal
nanochannel and special flower-like cavum that is beneficial
University, Shanghai, 200234, China. E-mail: ghliu@shnu.edu.cn;
for the accessibility/diffusion of substrates and improves
Fax: +86-21-64321819; Tel: +86-21-64321819
the catalytic efficiency. In particular, the asymmetric transfer
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
hydrogenation of ketones catalyzed by
a
N-sulfonylated
c3gc40547h
This journal is © The Royal Society of Chemistry 2013
Green Chem.

View Article Online
Paper
Green Chemistry
diamine-based organorhodium complex9,10 is a typical type of 0.50 mmol) was added and underwent hydrolytic decompo-
aqueous catalytic system. Taken together, it is reasonable to sition for 1 min, then tetraethoxysilane (TOES) (1.08 g,
expect that the self-assembly of the chiral functionality within 5.2 mmol) was rapidly added via injection (pH = 11.8). After
the FFMS and utilization of the additional phase transfer stirring for an additional 2 h, the reaction mixture was trans-
function of cetyltrimethylammonium bromide will contribute ferred into a Teflon-lined autoclave and aged at 383 K under
to high catalytic efficiency in an aqueous reaction system.
static conditions for 72 h. The surfactant was extracted by
We are interested in mesoporous silica-supported chiral refluxing with a HCl–ethanol solution for 8 h. The resulting
heterogeneous catalysts,11 especially, in an effort to develop products were collected by filtration and dried at 333 K
highly recyclable heterogeneous catalysts for cleaner and more in vacuum overnight to afford 2 (0.65 g) as a white powder. IR
benign chemical processes.12 In this contribution, we develop (KBr) cm−1 (s: strong peak, m: medium peak and w: weak
a flower-like chiral TsDPEN-functionalized mesoporous silica peak): 3427.2 (s), 2978.4 (w), 2926.6 (w), 1625.6 (m), 1495.9 (w),
via a cooperative dual-template approach and prepare a bifunc- 1459.5 (w), 1402.3 (w), 1384.1 (w), 1078.8 (s), 949.6 (s), 800.6
tionalized heterogeneous catalyst, in which the residual CTAB (s), 700.8 (s), 668.6 (s) 575.4 (w), 462.5 (s). Elemental analysis
functionality, as a phase transfer catalyst, and the chiral (%): C 12.01, H 1.73, N 1.01, S 0.71. SBET: 486.67 m2 g−1, Vpore
Cp*RhTsDPEN (Cp*
pentamethyl cyclopentadiene and 0.34 cm3 g−1, dpore: 5.34 nm. 29Si MAS NMR (300 MHz): T2 (δ =
TsDPEN
:
=
=
4-methylphenylsulfonyl-1,2-diphenylethylenedi- −59.1 ppm), T3 (δ = −69.2 ppm), Q2 (δ = −92.9 ppm), Q3 (δ =
amine) functionality, as a chiral promoter, are combined −102.0 ppm), Q4 (δ = −110.0 ppm). 13C CP MAS NMR
together within this material. As expected, this heterogeneous (161.9 MHz): 136.2 (Ar-C), 128.0 (Ar-C), 76.3 (NH–CH–Ph), 60.2
catalyst presents excellent catalytic efficiency in the asym- (OCH3 and OCH2CH3), 36.9 (CH2-Ar), 29.2 (OCH2CH3), 17.9
metric transfer hydrogenation of aromatic ketones in aqueous (CH2–Si) ppm.
medium due to its bifunctional features, in which the CTAB
Preparation of the heterogeneous catalyst 3. In a typical syn-
functionality can greatly enhance the catalytic performance thesis, to a stirred suspension of 2 (1.0 g) and NEt3 (1.00 mL,
while the enantioselectivity can be maintained. Furthermore, 16.5 mmol) in 20 mL dry CH2Cl2 was added [Cp*RhCl2]2
the heterogeneous catalyst can be readily recycled and reused (82.0 mg, 0.15 mmol) at room temperature. The resulting
at least 10 times without reducing the catalytic efficiency. In mixture was stirred at room temperature for 12 h. The mixture
particular, as a novel immobilized strategy, this research does was then filtered through filter paper and rinsed with excess
not only open up a new route to immobilize homogeneous CH2Cl2. After Soxhlet extraction in CH2Cl2 solvent to remove
chiral catalysts, but also promotes the catalytic efficiency as a homogeneous and unreacted start materials for 24 h, the solid
result of the synergistic effect of the salient phase-transfer was dried at 333 K in vacuum for 12 h to afford the catalyst 3
function and the confined chiral organorhodium nature.
(1.06 g, 73.2% relative to 2) as an orange powder. ICP analysis
shows that the Rh loading-amount was 27.18 mg (0.26 mmol)
per gram catalyst. IR (KBr) cm−1: 3426.9 (s), 2978.4 (w), 2926.6
(w), 1625.1 (m), 1495.9 (w), 1459.5 (w), 1402.3 (w), 1384.1 (w),
1078.8 (s), 948.6 (s), 769.6 (s), 700.8 (s), 569.4 (w), 459.5 (s).
Experimental
General
Elemental analysis (%): C 13.99, H 2.06, N 0.95, S 0.66. SBET
:
232.45 m2 −1, Vpore: 0.19 cm3 −1, dpore: 4.25 nm. 29Si MAS
g
g
All experiments, which were sensitive to moisture or air, were
carried out under an Ar atmosphere using standard Schlenk
techniques. 2-(4-Chlorosulfonylphenyl)ethyltrimethoxysilane,
4-(methylphenylsulfonyl)-1,2-diphenylethylenediamine [(S,S)-
TsDPEN], the surfactant cetyltrimethylammonium bromide
(CTAB), tetraethoxysilane (TEOS) and [Cp*RhCl2]2 were
purchased from Sigma-Aldrich Company Ltd. and used as
received. Compound [(S,S)-DPEN-SO2Ph(CH2)2Si(OMe)3] and
the pure material was synthesized according to the reported
literature.12b
NMR (300 MHz): T3 (δ = −68.7 ppm), Q2 (δ = −93.1 ppm), Q3
(δ = −102.0 ppm), Q4 (δ = −110.3 ppm). 13C CP MAS NMR
(161.9 MHz): 136.2 (Ar-C), 127.9 (Ar-C), 94.4 (Cp*-C), 76.3
(NH–CH–Ph), 61.1 (OCH3), 36.9 (CH2-Ar), 29.2 (OCH2CH3),
17.5 (CH2–Si), 9.3 (CpCH3) ppm.
Characterization
The amount of Rh loaded in the catalyst was analyzed using
an inductively coupled plasma optical emission spectrometer
(ICP, Varian VISTA-MPX). Fourier transform infrared (FT-IR)
spectra were collected on a Nicolet Magna 550 spectrometer
Catalyst preparation
Preparation of TsDPEN-functionalized-FFMS (2). A typical using KBr method. X-Ray powder diffraction (XRD) was carried
procedure was as follows: 0.20 g of CTAB was dissolved in out on Rigaku D/Max-RB diffractometer with Cu-Kα
a
96.0 g of deionized water under stirring at room temperature. radiation. Scanning electron microscopy (SEM) images were
Then 0.10 g of CaO (mol ratio of Ca/Si = 0.3) was added into obtained using a JEOL JSM-6380LV microscope operating at
the solution, the resulting mixture was ultrasonically irradiated 20 kV. Transmission electron microscopy (TEM) images were
for 1 h under ambient conditions with a ultrasonic cleaning performed on a JEOL JEM2010 electron microscope at an
instrument (KUDOS, SK3300H, 59 kHz) immersed directly in acceleration voltage of 220 kV. X-Ray photoelectron spectro-
the solution (the power used was 160 W). The solution was scopy (XPS) measurements were performed on a Perkin-Elmer
raised to 353 K. Then, TsDPEN-derived silica (1) (0.25 g, PHI 5000C ESCA system. All the binding energies were
Green Chem.
This journal is © The Royal Society of Chemistry 2013

View Article Online
Green Chemistry
Paper
calibrated using the contaminant carbon (C1s = 284.6 eV) as a silane (1) using the cooperative dual-template reagents of an
reference. Thermal gravimetric analysis (TGA) was performed inorganic layered calcium silicate hydrates template and a
with a Perkin-Elmer Pyris Diamond TG analyzer under an liquid crystal template8 gave TsDPEN-functionalized-FFMS (2)
air atmosphere with a heating ramp of 5 K min−1. Solid-state in the form of a white powder. Finally, the direct complexation
13C (100.5 MHz) and 29Si (79.4 MHz) CP MAS NMR were of 2 with (Cp*RhCl2)2 followed by trimming of the nanopore
obtained on a Bruker DRX-400 spectrometer.
via thorough Soxhlet extraction afforded the catalyst 3 in the
form of an orange powder (see ESI in Fig. S1†). The inductively
coupled plasma (ICP) optical emission spectrometer analysis
showed that the Rh loading was 21.18 mg (0.21 mmol) per
gram catalyst while the elemental analysis disclosed that the
amount of S was 0.21 mmol per gram of catalyst, calculated
from the mass % of S atoms (S 0.66%), in which the 1 : 1 mole
ratio of Rh to S atoms suggested the generation of a single-site
Cp*RhTsDPEN functionality within the FFMS silicate network.
The incorporation of the Cp*RhTsDPEN functionality in
the silicate network could be further confirmed by the solid-
state 13C cross polarization (CP) MAS NMR spectra. As shown
in Fig. 1, besides the general peaks of the chiral TsDPEN
ligand, it was easily found that the catalyst 3 displayed
clear characteristic peaks around 94 ppm corresponding to the
C atoms of the Cp rings, and the peaks around 9 ppm corres-
ponded to the C atoms of the methyl groups connected to the
Cp rings, in which all these peaks were strongly similar to
those of its homogeneous counterpart Cp*RhTsDPEN,13 con-
firming that the catalyst 3 possessed the same well-defined
single-site active center as its homogeneous counterpart. It
is worth mentioning that the peaks around 14 and 30 ppm
correspond to the C of the methyl (or methylene) groups
without connection to a nitrogen atom in the CTAB molecule,
and the peaks around 60 ppm correspond to the C atoms of
the methyl (or methylene) groups connected to nitrogen atoms
in the CTAB molecule; in which these peaks, the parts of the
peaks in the TsDPEN moiety and the peak of the nonhydro-
lyzed methoxide groups (CH3O–)14 were overlapped together as
marked in Fig. 1. In addition, other peaks denoted by asterisks
were the rotational sidebands that often appear in the CP MAS
high speed rotation process.15
General procedure for the asymmetric transfer hydrogenation
of aromatic ketones
A typical procedure was as follows: the heterogeneous catalyst
3 (7.58 mg, 2.00 μmol of Rh based on ICP analysis), HCO2Na
(0.68 g, 10.0 mmol), ketone (0.20 mmol) and 2.0 mL water
were added to a 10 mL round bottom flask in turn. The
mixture was allowed to react at 313 K for 0.5–5 h. During that
time, the reaction was monitored constantly by TLC. After
completion of the reaction, the heterogeneous catalyst was
separated via centrifuge (10 000 r min−1) for the recycling
experiment. The aqueous solution was extracted by Et2O
(3 × 3.0 mL). The combined Et2O was washed with brine twice
and dehydrated with Na2SO4. After the evaporation of Et2O,
the residue was purified by silica gel flash column chromato-
graphy to afford the desired product. The conversion and the
ee value could be determined by chiral GC using a Supelco
β-Dex 120 chiral column (30 m × 0.25 mm (i.d.), 0.25 μm film)
or a HPLC analysis with a UV-Vis detector using a Daicel OD-H
chiralcel column (Φ 0.46 × 25 cm).
Results and discussion
Synthesis and structural characterization of the heterogeneous
catalyst
The flower-like organorhodium-functionalized mesoporous
silica, abbreviated as Cp*RhTsDPEN-FFMS (3), was prepared
via an assembly strategy followed by an in situ complexation
approach as outlined in Scheme 1. Firstly, the TsDPEN-derived
silica (1) was synthesized according to our previous report.11b
The assembly of tetraethoxysilane (TOES) and TsDPEN-derived
In addition, the 29Si magic angle spinning (MAS) NMR
spectra of the catalyst 3 revealed two groups of typical signals
Scheme 1 Synthesis of the heterogeneous catalyst 3.
Fig. 1 The 13C CP MAS NMR spectra of 2–3.
This journal is © The Royal Society of Chemistry 2013
Green Chem.

View Article Online
Paper
Green Chemistry
Fig. S2†), it was found that 16.3% of the Cp*RhTsDPEN func-
tionality was loaded within its FFMS silicate network, which
was nearly consistent with 27.18 mg (0.26 mmol) of the Rh
loading per gram catalyst detected by ICP analysis.
In order to investigate the electronic state of the rhodium
center, the X-ray photoelectron spectroscopy (XPS) of the cata-
lyst 3 and its homogeneous counterpart were investigated.
As shown in Fig. 4, the XPS spectra of the catalyst 3 gave a
Rh 3d5/2 binding energy of around 309.43 eV, indicating that
the rhodium center was in the trivalent state.19 More impor-
tantly, it was found that catalyst 3 had almost the same
Rh 3d5/2 electron binding energy as the parent Cp*RhTsDPEN
(309.43 eV versus 309.38 eV), suggesting that the rhodium
center within the FFMS silicate network retained its original
chiral coordination microenvironment.
The mesostructure and morphology of catalyst 3 were
further characterized using a nitrogen adsorption–desorption
technique, along with scanning electron microscopy (SEM)
and transmission electron microscopy (TEM). As shown in
Fig. 5, the nitrogen adsorption–desorption isotherm revealed
that catalyst 3 was mesoporous due to the typical type IV
Fig. 2 The 29Si CP MAS NMR spectrum of 2–3.
(Q and T signals) distributed broadly from −50 to −150 ppm,
as marked in Fig. 2, in which the up-field Q signals originate
from the inorganic molecules of the TEOS functionality while
the downfield T signals are derived from the organic molecules
of the silylether groups. Typical isomer shift values were
−91.5/−101.5/−110 ppm for the Q2/Q3/Q4 signals {[(HO)2Si-
(OSi)2]/[(HO)Si(OSi)3]/[Si(OSi)4]}
and
are
−48.5/−58.5/
−67.5 ppm for the T1/T2/T3 signals {[R(HO)2SiOSi]/[R(HO)Si-
(OSi)2]/[RSi(OSi)3]}.16 As compared with these typical signals,
the strongest Q3 peak in the Q series suggested that the cata-
lyst 3 possessed a mainly inorganosilicate framework of (HO)-
Si(OSi)3, while the strongest T3 peak in the T series indicated
that the organosilicate [RSi(OSi)3] (R = Cp*RhTsDPEN func-
tionality) was the main part of the silica wall in its FFMS
silicate network.17
Fig. 3 shows the TG/DTA curves of the catalyst 3 treated in
air. It was found that an endothermic peak around 355 K, with
weight loss of 10.2%, could be attributed to the release of
physically adsorbed water. Furthermore, two exothermic peaks
around 636 K and 701 K with a total weight loss of 23.3%
could be assigned to the oxidation of the organic moieties
(including Cp*RhTsDPEN functionalities and the residual
CTAB surfactants18). When eliminating the contribution of
water, the total weight loss of organic moieties was 25.9%. In
sharp contrast to the TG/DTA curve of the pure FMS material,
with a 9.6% weight loss for the residual CTAB18 (see ESI in
Fig. 4 The XPS spectra of Cp*RhTsDPEN and 3.
Fig. 3 The TG/DTA curves of catalyst 3.
Fig. 5 The nitrogen adsorption–desorption isotherm of the catalyst 3.
Green Chem.
This journal is © The Royal Society of Chemistry 2013

View Article Online
Green Chemistry
Paper
isotherms with a H1 hysteresis loop and a visible step at P/P0 = salient phase transfer feature of the residual CTAB functional-
0.40–1.0, which could be further confirmed by the powder ity, this asymmetric transfer hydrogenation catalyzed by cata-
X-ray diffraction (see ESI in Fig. S3†). Furthermore, the SEM lyst 3 was carried out in the absence of a Bu4NBr system, in
image (Fig. 6a) clearly shows that catalyst 3 was composed of which HCO2Na worked as a hydrogen source and 1.0% mol
uniformly distributed spherical flowers with a particle size of of FFMS silica acted as a catalyst, according to the reported
about 750 nm, in which a spherical flower was constructed as methods.10a Taking acetophenone as an example, catalyst 3
a result of the stacking of leaf-shaped nanoflakes as building gave (S)-1-phenyl-1-ethanol with more than 99% conversion
blocks. The TEM images (Fig. 6b and 6c) further demonstrated and 97% ee, in which the conversion was obviously higher
that catalyst 3 had worm-like mesostructural nanoflakes with than that of its homogeneous counterpart (Table 1, entry 1
a thickness of about 25 nm on the mesoporous ridge. It versus entry 1 in bracket) while the ee value was comparable to
was worth mentioning that the TEM image with a chemical that of its homogeneous counterpart and even to that of its
mapping technique (Fig. 6d) displayed the uniform distri- homogeneous counterpart with Bu4NBr as a phase transfer cata-
bution of the Rh centers within the silicate network, which lyst.10a In addition, this asymmetric reaction of acetophenone
would play an important role in dominating its catalytic could also be completed with the same result on a large scale
performance, as discussed below. Based on the above studies, using 10.0 mmol of acetophenone as the substrate. Of particu-
a safe conclusion could be drawn, in which Cp*RhTsDPEN- lar note, was that the reaction could be run at a high ratio
functionalized mesoporous silica with a uniform distribution of substrate to catalyst without affecting its ee value, as
of Rh centers could be readily obtained in this case.
exemplified by the asymmetric transfer hydrogenation of
acetophenone with a substrate/catalyst ratio = 500 (Table 1,
entry 2).
Catalytic properties of the heterogeneous catalyst
Based on the above excellent catalytic efficiency, catalyst 3
was investigated systematically in the asymmetric transfer
hydrogenation of various substrates in the absence of Bu4NBr.
As shown in Table 1, excellent conversions, no side products,
and high enantioselectivities were obtained under similar con-
ditions. It was worth mentioning that the asymmetric reac-
tions for all tested substrates could be completed within a
short reaction time relative to those of its counterpart in the
As
a
type of highly active catalyst, homogeneous
Cp*RhTsDPEN catalysts are used extensively in the asymmetric
transfer hydrogenation of ketones in aqueous media.9,10 Due
to its typical two-phase catalysis system in the asymmetric reac-
tion, it often needs Bu4NBr to act as a phase transfer catalyst
to enhance its catalytic efficiency. In this case, because of the
Table 1 Asymmetric transfer hydrogenation of aromatic ketonesa
Entry
Ar
Time [h]
Conv.b [%]
eeb [%]
1
2
3
4
5
6
7
8
Ph
Ph
Ph
Ph
0.5
4.0
0.5
0.5
0.5
0.5
0.5
0.5
2.0
4.0
0.5
0.5
0.5
0.5
24.0
>99 (88)
86
97
97c (96)
97d
95e
96f
92
94
94
97
94
94
95
89
94
>99
>99
>99
>99
>99
>99
97
>99
>99
>99
>99
46 (79)
4-FPh
4-ClPh
4-BrPh
3-BrPh
4-MePh
4-OMePh
3-OMePh
4-CNPh
4-CF3Ph
4-NO2Ph
1-Pyrenyl
9
10
11
12
13
14
15
86
55c (81)
a Reaction conditions: catalyst (7.58 mg, 2.00 μmol of Rh based on ICP
analysis), HCO2Na (0.68 g, 10.0 mmol), ketone (0.20 mmol) and
2.0 mL water, reaction temperature (313 K), reaction time (0.5–5.0 h).
b Determined by chiral GC or HPLC analysis (see ESI in Fig. S4†). c Data
were obtained using the homogeneous Cp*RhTsDPEN catalyst without
the addition of Bu4NBr in aqueous medium. d Data was obtained
with 0.2% mol of catalyst. e Data were obtained using TsDPEN-
functionalized-FFMS (2) plus (Cp*RhCl2)2 as a catalyst. f Data were
obtained using the pure FMS material plus Cp*RhTsDPEN as a
catalyst.
Fig. 6 (a) The SEM image of 3, (b) the TEM image of 3, (c) the TEM image of
the mesoporous ridges and (d) a chemical mapping of 3 showing the distri-
bution of Si (white) and Rh (red).
This journal is © The Royal Society of Chemistry 2013
Green Chem.

View Article Online
Paper
Green Chemistry
Table 2 Reusability of catalyst 3 for the asymmetric transfer hydrogenation of acetophenonea,b
Run time
1
2
3
4
5
6
7
8
9
10
Conv.[%]
ee [%]
99.9
97.5
99.9
97.5
99.9
97.5
99.9
97.4
99.9
97.4
99.9
97.0
99.7
97.4
99.6
97.2
99.1
97.0
98.3
96.0
a Reaction conditions: catalyst (7.58 mg, 2.00 μmol of Rh based on ICP analysis), HCO2Na (0.68 g, 10.0 mmol), ketone (0.20 mmol) and 2.0 mL
water, reaction temperature (313 K), reaction time (0.5–4.0 h). b Determined by chiral GC analysis (see ESI in Fig. S5†).
absence of Bu4NBr, demonstrating its bifunctionalized feature.
Important features to include in the design of any hetero-
Notably, the superior catalytic activity could be attributed to geneous catalyst is easy separation via simple filtration, and
the salient phase-transfer functions of the residual CTAB func- the retention of catalytic activity and enantioselectivity in the
tionality in a two-phase catalysis system. The comparable recovered catalyst after multiple cycles. As shown in Table 2,
enantioselectivity should be due to the fact that the active the heterogeneous catalyst 3 was recovered easily and reused
rhodium center within its FFMS silicate network could main- repeatedly when acetophenone was chosen as the substrate.
tain its original chiral microenvironment, as verified by XPS Remarkably, in ten consecutive reactions, the recycled catalyst
spectra in Fig. 4.
3 still afforded (S)-1-phenyl-1-ethanol with 98% conversion
To gain better insight into the nature of the heterogeneous and 96% ee. The high recyclability could be due to the fact
catalysis and to eliminate non-covalent adsorption during the that the highly dispersed Cp*RhTsDPEN active centers via
catalytic process, two parallel experiments were carried out strong covalent-bonding immobilization within the FFMS
using TsDPEN-functionalized-FFMS (2) plus (Cp*RhCl2)2 and network, as verified by the TEM chemical mapping, can
the pure FFMS material plus homogeneous Cp*RhTsDPEN as efficiently decrease the leaching of Rh. Evidence to support
the heterogeneous catalysts. It was found that the asymmetric this reasoning came from the ICP analysis, in which the
reaction in the former afforded the corresponding alcohol with amount of Rh after the tenth recycle was 26.16 mg per gram
97% conversion and 95% (Table 1, entry 3), while that in the catalyst and only 4.8% of Rh was lost, demonstrating that the
latter gave the alcohol with more than 99% conversion and high recyclability could be due to the low leaching of Rh.
96% ee (Table 1, entry 4). The former suggested that the cata-
lyst synthesized by the in situ complexation method could
result in a catalytic performance. The slightly lower catalytic
Conclusions
activity and enantioselectivity than those observed for catalyst
3 could be ascribed to the fact that the small parts of
(Cp*RhCl2)2 were not coordinated during the catalytic process.
The loss of Rh could be detected by ICP analysis in solution.
The comparable conversion and enantioselectivity in the latter
case indicated that the homogeneous catalyst, via the non-
covalent physical adsorption, retained its original catalytic
efficiency. However, when this catalyst in the latter case was
underwent a process of Soxhlet extraction, the reused catalyst
gave only a small products. This fact ruled out the role of the
non-covalent physical adsorption, demonstrating that the
nature of the heterogeneous catalysis was indeed derived from
the heterogeneous catalyst itself.
Also, it was noted that catalyst 3 exhibited size selectivity
toward some substrates. Due to the size constraint and
reduced accessibility of the activation sites, a large size 1-acetyl-
pyrene (Table 1, entry 15), was difficult to be transformed
completely even after a prolonged reaction time, in which only
46% conversion and 55% ee was observed within 24 hours. To
further confirm the size selectivity, a comparable experiment
was investigated, in which the reaction of a mixture of catalyst
3 and two equivalent mole reactants (small acetophenone and
large 1-acetylpyrene) was carried out under similar conditions.
It was found that acetophenone could be transformed quanti-
tatively, while only 8% conversion of 1-acetylpyrene was
obtained within 0.5 hour, confirming the presence of the size
selectivity.
In conclusion, we have developed organorhodium-functiona-
lized flower-like mesoporous silica, which was constructed by
ultrathin perpendicularly aligned mesoporous nanoflakes via a
cooperative dual-template approach. The developed bifunctio-
nalized heterogeneous catalyst, exhibits excellent catalytic
activity and high enantioselectivity in the asymmetric transfer
hydrogenation of aromatic ketones in aqueous medium. More
importantly, the residual surfactant within its mesoporous sili-
cate network acted as a phase transfer catalyst that could
enhance greatly the catalytic activity, while the confined chiral
organorhodium complex bonded within its mesoporous sili-
cate network could maintain a similar enantioselectivity to its
homogeneous counterpart. Furthermore, this heterogeneous
catalyst could be recovered easily and reused repeatedly ten
times without any obvious affects on its catalytic performance,
showing a good potential for use in industrial applications. In
particular, this immobilized strategy does not only highlight a
new way to immobilize various chiral homogeneous catalysts
but also promotes the catalytic efficiency via a synergistic
effect of the salient phase-transfer function and the catalytic
nature of the confined chiral organorhodium.
Acknowledgements
We are grateful to the China National Natural Science
Foundation (20673072), the Shanghai Sciences and Techno-
logies Development Fund (10DJ1400103, 10JC1412300 and
Green Chem.
This journal is © The Royal Society of Chemistry 2013

View Article Online
Green Chemistry
Paper
12nm500500), the CSIRT (IRT1269) and the Shanghai Munici-
pal Education Commission (12ZZ135) for financial support.
Lett., 2009, 11, 847; (c) R. Malacea, R. Poli and E. Manoury,
Coord. Chem. Rev., 2010, 254, 729.
10 (a) X. F. Wu, X. H. Li, A. Zanotti-Gerosa, A. Pettman, J. Liu,
A. J. Mills and J. L. Xiao, Chem.–Eur. J., 2008, 14, 2209;
(b) X. F. Wu, J. Liu, D. D. Tommaso, J. A. Iggo, C. R. Catlow,
J. Bacsa and J. L. Xiao, Chem.–Eur. J., 2008, 14, 7699;
(c) P. N. Liu, J. G. Deng, Y. Q. Tu and S. H. Wang,
Chem. Commun., 2004, 2070; (d) N. A. Cortez, G. Aguirre,
M. Parra-Hake and R. Somanathan, Tetrahedron Lett., 2009,
50, 2228.
11 (a) Y. Q. Sun, G. H. Liu, H. Y. Gu, T. Z. Huang, Y. L. Zhang
and H. X. Li, Chem. Commun., 2011, 47, 2583; (b) G. H. Liu,
J. Y. Wang, T. Z. Huang, X. H. Liang, Y. L. Zhang and
H. X. Li, J. Mater. Chem., 2010, 20, 1970; (c) G. H. Liu,
M. Yao, F. Zhang, Y. Gao and H. X. Li, Chem. Commun.,
2008, 347; (d) W. Xiao, R. H. Jin, T. Y. Cheng, D. Q. Xia,
H. Yao, F. Gao, B. X. Deng and G. H. Liu, Chem. Commun.,
2012, 48, 11898; (e) J. Long, G. H. Liu, T. Y. Cheng, H. Yao,
Q. Q. Qian, J. L. Zhuang, F. Gao and H. X. Li, J. Catal.,
2013, 298, 41; (f) H. S. Zhang, R. H. Jin, H. Yao, S. Tang,
J. L. Zhuang, G. H. Liu and H. X. Li, Chem. Commun., 2012,
48, 7874; (g) Y. L. Xu, J. Long, K. T. Liu, Q. Q. Qian,
F. Gao, G. H. Liu and H. X. Li, Adv. Synth. Catal., 2012,
354, 3250.
12 (a) G. H. Liu, Y. Q. Sun, J. Y. Wang, C. S. Sun, F. Zhang and
H. X. Li, Green Chem., 2009, 11, 1477; (b) G. H. Liu, Y. Gao,
X. Q. Lu, F. Zhang, M. M. Liu and H. X. Li, Chem. Commun.,
2008, 3184; (c) J. L. Huang, F. X. Zhu, W. H. He, F. Zhang,
W. Wang and H. X. Li, J. Am. Chem. Soc., 2010, 132, 1492;
(d) X. S. Yang, F. X. Zhu, J. L. Huang, F. Zhang and H. X. Li,
Chem. Mater., 2009, 21, 4925; (e) K. T. Liu, R. H. Jin,
T. Y. Cheng, X. M. Xu, F. Gao, G. H. Liu and H. X. Li,
Chem.–Eur. J., 2012, 18, 15546.
References
1 (a) P. J. Walsh, H. M. Li and C. A. de Parrodi, Chem. Rev.,
2007, 107, 2503; (b) C. E. Song, in Handbook of Asymmetric
Heterogeneous Catalysis, ed. K. L. Ding and Y. Uozumi,
Wiley-VCH, Weinheim, 2009, p. 25; (c) J. M. Thomas and
R. Raja, Acc. Chem. Res., 2008, 41, 708; (d) M. Heitbaum,
F. Glorius and I. Escher, Angew. Chem., Int. Ed., 2006, 45,
4732; (e) C. Li, H. D. Zhang, D. M. Jiang and Q. H. Yang,
Chem. Commun., 2007, 547; (f) M. Bartók, Chem. Rev., 2010,
110, 1663.
2 (a) J. M. Thomas and R. Raja, Acc. Chem. Res., 2008, 41,
708; (b) R. Raja, J. M. Thomas, M. D. Jones,
B. F. G. Johnson and D. E. W. Vaughan, J. Am. Chem. Soc.,
2003, 125, 14982.
3 (a) D. M. Jiang, J. S. Gao, Q. H. Yang, J. Yang and C. Li,
Chem. Mater., 2006, 18, 6012; (b) P. N. Liu, P. M. Gu,
F. Wang and Y. Q. Tu, Org. Lett., 2004, 6, 169;
(c) D. M. Jiang, Q. H. Yang, J. Yang, L. Zhang, G. R. Zhu,
W. G. Su and C. Li, Chem. Mater., 2005, 17, 6154.
4 (a) H. Q. Yang, L. Zhang, L. Zhong, Q. H. Yang and C. Li,
Angew. Chem., Int. Ed., 2007, 46, 6861; (b) H. Q. Yang, J. Li,
J. Yang, Z. M. Liu, Q. H. Yang and C. Li, Chem. Commun.,
2007, 1086.
5 (a) A. Thomas, Angew. Chem., Int. Ed., 2010, 49, 8328;
(b) N. Mizoshita, T. Taniab and S. Inagaki, Chem. Soc. Rev.,
2011, 40, 789; (c) F. Hoffmann and M. Fröba, Chem. Soc.
Rev., 2011, 40, 608.
6 (a) S. Y. Bai, H. Q. Yang, P. Wang, J. S. Gao, B. Li,
Q. H. Yang and C. Li, Chem. Commun., 2010, 46, 8145;
(b) H. Q. Yang, L. Zhang, P. Wang, Q. H. Yang and C. Li,
Green Chem., 2009, 11, 257; (c) P. Y. Wang, X. Liu, J. Yang,
Y. Yang, L. Zhang, Q. H. Yang and C. Li, J. Mater. Chem.,
2009, 19, 8009.
13 J. Mao and D. C. Baker, Org. Lett., 1999, 1, 841.
14 P. F. W. Simon, R. Ulrich, H. W. Spiess and U. Wiesner,
Chem. Mater., 2001, 13, 3464.
15 T. Posset and J. Blumel, J. Am. Chem. Soc., 2006, 128,
8394.
16 O. Krőcher, O. A. Kőppel, M. Frőba and A. Baiker, J. Catal.,
1998, 178, 284.
17 (a) H. X. Li, F. Zhang, Y. Wan and Y. F. Lu, J. Phys. Chem. B,
2006, 110, 22942; (b) H. X. Li, Q. F. Zhao, Y. Wan, W. L. Dai
and M. H. Qiao, J. Catal., 2006, 244, 251.
7 (a) M. E. Davis, Nature, 2002, 417, 813; (b) L. Zhang,
S. Z. Qiao, Y. G. Jin, H. G. Yang, S. Budihartono, F. Stahr,
Z. F. Yan, X. L. Wang, Z. P. Hao and G. Q. Lu, Adv. Funct.
Mater., 2008, 18, 3203; (c) H. Zhang, Z. Li, P. Xu, R. Wu and
Z. Jiao, Chem. Commun., 2010, 46, 6783.
18 V. Cauda, A. Schlossbauer, J. Kecht, A. Zürner and T. Bein,
J. Am. Chem. Soc., 2009, 131, 11361.
8 L. Zhang, Y. N. Guo, J. Peng, X. Liu, P. Yuan, Q. H. Yang
and C. Li, Chem. Commun., 2011, 47, 4087.
19 L. Wang, J. R. Sowa, C. Wang, R. S. Lu, P. G. Gassman and
T. C. Flood, Organometallics, 1996, 15, 4240.
9 (a) T. Ikariya and A. J. Blacker, Acc. Chem. Res., 2007, 40,
1300; (b) J. E. D. Martins, G. J. Clarkson and M. Wills, Org.
This journal is © The Royal Society of Chemistry 2013
Green Chem.