Periodic mesoporous organosilicas with trans-(1R,2R)-diaminocyclohexane in the framework: A potential catalytic material for asymmetric reactions

Article abstract of DOI:10.1016/j.jcat.2006.01.018

With benzyl group as a linker, trans-(1R,2R)-diaminocyclohexane was incorporated into the framework of mesoporous silica through one-step co-condensation of tetramethoxysilane with N,N′-bis[4-(trimethoxysilyl) benzyl]-(-)-(1R,2R)-diaminocyclohexane using cetyltrimethylammonium bromide as a structure-directing agent under basic conditions. All materials were fully characterized by X-ray diffraction, N₂ sorption isotherms, transmission electron microscopy, and ¹³C and ²⁹Si cross-polarization magic-angle spinning nuclear magnetic resonance spectroscopy. Coordinated with [Rh(cod)Cl]₂, the material exhibited a TOF up to ～414 h^-1 with 30% ee for the asymmetric transfer hydrogenation of acetophenone. Various ketones were hydrogenated with different activities and enantioselectivities. An enantioselectivity of about 61% ee was observed in the case of 2-acetylnaphthalene. Moreover, a comparison of the catalytic properties of the materials with benzyl and propyl groups as linkers indicates the importance of the rigidity and electron-withdrawing ability of the linker in the high reaction rate of the catalysts.

Full text of DOI:10.1016/j.jcat.2006.01.018

Journal of Catalysis 239 (2006) 65–73
www.elsevier.com/locate/jcat
Periodic mesoporous organosilicas with trans-(1R,2R)-diaminocyclohexane
in the framework: A potential catalytic material for asymmetric reactions
Dongmei Jiang, Qihua Yang ∗, Hong Wang, Guiru Zhu, Jie Yang, Can Li ∗
State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, China
Received 30 November 2005; revised 17 January 2006; accepted 18 January 2006
Available online 9 February 2006
Abstract
With benzyl group as a linker, trans-(1R,2R)-diaminocyclohexane was incorporated into the framework of mesoporous silica through one-step
ꢀ
co-condensation of tetramethoxysilane with N,N -bis[4-(trimethoxysilyl)benzyl]-(−)-(1R,2R)-diaminocyclohexane using cetyltrimethylammo-
nium bromide as a structure-directing agent under basic conditions. All materials were fully characterized by X-ray diffraction, N sorption
2
13
29
isotherms, transmission electron microscopy, and C and Si cross-polarization magic-angle spinning nuclear magnetic resonance spectroscopy.
−1
Coordinated with [Rh(cod)Cl] , the material exhibited a TOF up to ∼414 h with 30% ee for the asymmetric transfer hydrogenation of acetophe-
2
none. Various ketones were hydrogenated with different activities and enantioselectivities. An enantioselectivity of about 61% ee was observed
in the case of 2-acetylnaphthalene. Moreover, a comparison of the catalytic properties of the materials with benzyl and propyl groups as linkers
indicates the importance of the rigidity and electron-withdrawing ability of the linker in the high reaction rate of the catalysts.
© 2006 Elsevier Inc. All rights reserved.
Keywords: Periodic mesoporous organosilicas; Chirality; Diaminocyclohexane; Asymmetric transfer hydrogenation of ketones
1. Introduction
complex) exhibit low reaction rate (with reactions usually per-
formed for several days) for the asymmetric transfer hydro-
genation of ketones because of their low surface areas and small
pore volumes. The synthesis of mesoporous materials function-
alized with chiral ligands is highly desired. Recently, it was
reported that periodic mesoporous organosilicas (PMOs) can
be synthesized using (RꢀO)3Si–R–Si(ORꢀ)3 as precursors in the
presence of surfactant under either basic or acidic conditions
[13–15]. However, the organic molecules bridged in the frame-
work of most reported PMOs, including methylene, ethylene,
phenylene, biphenylene, thiophene, ferrocene, and cyclic CH2,
are nonchiral [16,17]. Very few examples of PMOs with chiral
ligands in the framework have been reported [18–20]. The in-
corporation of chiral ligands into the framework of mesoporous
materials will make the materials useful for the heterogeneous
asymmetric catalysis.
Asymmetric catalysis is of critical importance in the pro-
duction of modern pharmaceuticals and agrochemicals. Re-
cently, heterogeneous asymmetric catalysis has attracted much
research attention because of the easy recovery and separation
of the catalyst from the reaction system. Most of the heteroge-
neous asymmetric catalysts were synthesized through grafting
or immobilization of homogeneous catalyst onto the solid sup-
ports [1–7]. There is an increasing demand for the design and
synthesis of novel chiral solid materials for the heterogeneous
asymmetric catalysis.
trans-1,2-Diaminocyclohexane is among the most com-
monly used chiral ligands [8]. Its derivatives have been incorpo-
rated into the network of amorphous silica through hydrolysis
and condensation of organo-bistrialkoxysilanes [9–12]. These
silica-based amorphous materials (coordinated with rhodium
Recently, we reported the synthesis and catalytic activ-
ity of mesoporous organosilicas with ethane bridged in the
wall and trans-(1R,2R)-diaminocyclohexane (linked by propyl
group) protruding into the channel [21]. After coordination to
[Rh(cod)Cl]2, the PMOs can effectively catalyze the transfer
hydrogenation of acetophenone; however, they do not exhibit
*
Corresponding author. Fax: +86 411 84694447.
E-mail addresses: yangqh@dicp.ac.cn (Q. Yang), canli@dicp.ac.cn
(C. Li).
0021-9517/$ – see front matter © 2006 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcat.2006.01.018

66
D. Jiang et al. / Journal of Catalysis 239 (2006) 65–73
Scheme 1. Molecular structure of the precursors.
desirable catalytic properties in terms of reaction rate and enan-
tioselectivity. Consequently, a new precursor with benzyl group
as a linker, N,Nꢀ-bis[4-(trimethoxysilyl)benzyl]-(−)-(1R,2R)-
diaminocyclohexane (Bbenzyl), was designed and synthesized
(Scheme 1). For comparison, the precursor (Bpropyl) using
propyl group as a linker was also prepared. The PMOs with
trans-(1R,2R)-diaminocyclohexane in the framework were
synthesized using Bbenzyl or Bpropyl as the precursor. It was
found that the chiral mesoporous material with benzyl group as
a linker (coordinated with [Rh(cod)Cl]2) shows a quite high re-
action rate and moderate enantioselectivity in the asymmetric
transfer hydrogenation of ketones.
pumped off from the filtrate. The residue was extracted with
75 mL of freshly distilled pentane. The solvent was eliminated
in vacuo, leaving a yellowish liquid corresponding to Mbenzyl
1
(Scheme 1). H NMR (CDCl3): δ 0.99–1.11, 1.18–1.29, and
1.86–2.09 (m, 8H, –NH–(CH–(CH2)4–CH)–NH2), 2.09–2.38
(m, 2H, –NH–(CH–(CH2)4–CH)–NH2), 3.61(s, 9H, OCH3),
3.68–3.97 (d, 2H, CH2Ph), 7.32–7.65 (m, 4H, Harom).
2.2.2. Synthesis of N,Nꢀ-bis[4-(trimethoxysilyl)benzyl]-
(−)-(1R,2R)-diaminocyclohexane (Bbenzyl
)
In a Schlenk tube, 6.34 g (19.6 mmol) of Mbenzyl was dis-
solved in THF (25 mL). Then 2.43 g (9.8 mmol) of 4-(chlo-
romethyl)phenyltrimethoxysilane in THF (5 mL) was added
dropwise at room temperature to the above solution. The mix-
ture was stirred at room temperature for 1 h and then heated
under stirring to 60 ◦C for 24 h. After filtration, THF was
pumped off from the filtrate. The residue was extracted with
75 mL of freshly distilled pentane. The solvent was elimi-
nated in vacuo, leaving a yellow viscous liquid correspond-
2. Experimental section
2.1. Chemicals
The solvents were of analytical quality and dried by standard
methods. Other materials were analytical grade and used as pur-
chased without further purification. (3-Chloropropyl)triethoxy-
silane (97%), cetyltrimethylammonium bromide (CTAB, 99%),
and acetophenone (99%) were purchased from Sigma–Aldrich.
4-(Chloromethyl)phenyltrimethoxysilane (97%) was purchased
from Gelest. Tetramethoxysilane (TMOS, >98%), tetraethoxy-
silane (TEOS, 99%), and other reagents were obtained from
Shanghai Chemical Reagent. trans-(1R,2R)-Diaminocyclo-
hexane was obtained by the resolution of a commercially avail-
able mixture of cis- and trans-diaminocyclohexane (30/70) as
described previously [22]. N,Nꢀ-bis[4-(triethoxysilyl)propyl]-
(−)-(1R,2R)-diaminocyclohexane (Bpropyl) was prepared as
described previously [9].
1
ing to Bbenzyl (Scheme 1). H NMR (CDCl3): δ 1.07–1.09,
1.20–1.25, 1.74, and 2.15 (m, 8H, –NH–(CH–(CH2)4–CH)–
NH–), 2.18–2.33 (m, 2H, –NH–(CH–(CH2)–CH)–NH–), 3.61
(s, 18H, OCH3), 3.71–3.95 (d, 4H, CH2Ph), 7.30–7.69 (m, 8H,
H
arom). 13C NMR (CDCl3): δ 25.27 (–NH–(CH–CH2–(CH2)2–
CH2–CH)–NH–), 31.74 (–NH–(CH–CH2–(CH2)2–CH2–CH)–
NH–), 51.09 (OCH3), 51.36 (–NH–(CH–CH2–(CH2)2–CH2–
CH)–NH–), 61.61 (–CH2Ph), 126.09, 127.69, 128.59, 135.75
(Carom).
2.2.3. Synthesis of the mesoporous silicas with
trans-(1R,2R)-diaminocyclohexane in the framework using
the benzyl group as a linker (TBbenzyl-n-N)
2.2. Synthesis procedures
For a typical synthesis, CTAB (0.44 g), NH3 · H2O (25
wt%, 5.45 g), EtOH (3.0 g), and deionized water (20.5 g)
were mixed at 20 ◦C under stirring. A mixture of TMOS and
Bbenzyl (10 mmol of Si) in ethanol (1.6 g) was added to the
above solution. The reaction mixture was stirred at 20 ◦C for
12 h and then heated at 60 ◦C under stirring for 4 days. Af-
ter filtration, the powder product was washed thoroughly with
deionized water and dried under vacuum at 60 ◦C. The surfac-
tant was extracted by stirring 1 g of as-synthesized material
in 200 mL of ethanol containing 12 M HCl (1.5 g) at 70 ◦C
for 7 h. The products containing protonated trans-(1R,2R)-
2.2.1.
Synthesis of N-[4-(trimethoxysilyl)benzyl]-(−)-(1R,2R)-
diaminocyclohexane (Mbenzyl
)
In a Schlenk tube, a solution of 4-(chloromethyl)phenyltri-
methoxysilane (9.5 g, 41.5 mmol) in THF (5 mL) was added
dropwise at room temperature to 25 mL of THF containing
trans-(1R,2R)-diaminocyclohexane (9.5 g, 83 mmol). The re-
action mixture was stirred at room temperature for 1 h and then
heated under stirring to 40 ◦C for 24 h. After filtration, THF was

D. Jiang et al. / Journal of Catalysis 239 (2006) 65–73
67
diaminocyclohexane were stirred in 0.1 M tetramethylammo-
nium hydroxide aqueous solution for 30 min at room tempera-
ture to obtain the free base. After filtration, the powder product
was washed with copious amounts of water and ethanol, then
dried under vacuum at 60 ◦C. The materials are denoted as
TBbenzyl-n-N, whereas n (n = 10, 20, 30, 40) is the mol% of
2Bbenzyl/(2Bbenzyl + TMOS).
2.2.4. Synthesis of the mesoporous silica with
trans-(1R,2R)-diaminocyclohexane in the framework using
the propyl group as a linker (TBpropyl-20-N)
The synthetic method of TBpropyl-20-N was similar to that
of TBbenzyl-20-N, but with TEOS and Bpropyl used instead
of TMOS and Bbenzyl, with 20 mol% of 2Bpropyl/(2Bpropyl
TEOS).
+
2.2.5. Coordination of [Rh(cod)Cl]2 with
trans-(1R,2R)-diaminocyclohexane bridged in the mesoporous
framework (TBbenzyl-n-NRh and TBpropyl-20-NRh)
Fig. 1. Powder XRD patterns of TB
-n-N [n is the mole percent of
benzyl
2B
benzyl
2B
propyl
/(2B
/(2B
+ TMOS)] and TB
-20-N [20 is the mole percent of
benzyl
propyl
propyl
+ TEOS)].
In a Schlenk tube, [Rh(cod)Cl]2 in 15 mL of dry ethanol
was added to TBbenzyl-n-N or TBpropyl-20-N with the mole ratio
of trans-(1R,2R)-diaminocyclohexane:[Rh(cod)Cl]2 = 1:0.75.
After stirring for 48 h under argon atmosphere at room tem-
perature, a yellow powder product was obtained. The powder
was filtered and washed with THF to eliminate the free rhodium
complex, then dried under vacuum. The samples are denoted as
TBbenzyl-n-NRh or TBpropyl-20-NRh.
distilled isopropanol (12 mL) and i-PrOK (0.1 mmol). The sus-
pension was stirred for 1 h at room temperature, and then ketone
(2 mmol) was added with a syringe. The reaction mixture was
stirred at 83 2◦C for a specified time under argon atmosphere.
The catalytic activity and enantiomeric excess (ee) were mea-
sured on an Agilent 6890 gas chromatograph equipped with a
flame ionization detector and an HP-Chiral 19091G-B213 cap-
illary column (30 m × 0.32 mm × 0.25 µm).
2.3. Characterization
3. Results and discussion
Powder X-ray diffraction (XRD) patterns were recorded on a
Rigaku D/Max 3400 powder diffraction system using Cu-Kα ra-
diation (40 kV and 30 mA). Transmission electron microscopy
(TEM) was done using a JEM-2010 at an acceleration volt-
age of 120 kV. Nitrogen sorption isotherms were measured at
77 K on an ASAP 2000 system in a static measurement mode.
The sample was outgassed at 373 K before the measurement.
Pore diameter was calculated from the desorption branch of the
isotherm using the BJH method. Solid-state 13C (100.5 MHz)
and 29Si (79.4 MHz) CP-MAS NMR were obtained on a Bruker
DRX-400 spectrometer with the following experimental para-
meters: for 13C CP-MAS NMR experiments, 8-kHz spin rate,
3-s pulse delay, 4-min contact time, and 1000 scans; for 29Si
CP-MAS NMR experiments, 8-kHz spin rate, 3-s pulse delay,
10-min contact time, and 1000 scans. Tetramethylsilane was
used as a reference. N and Rh elemental analyses were done
on an Elementar Vario EL III and a Plasma-spec-II (Leeman
Laboratories), respectively. Diffuse reflectance UV–vis spectra
were collected on a JASCO V-550 UV–vis spectrophotometer
using BaSO4 as the reference.
3.1. Structural characterization of the mesoporous materials
with trans-(1R,2R)-diaminocyclohexane in the framework
The mesoporous organosilicas with chiral ligands in the
framework were prepared using a direct co-condensation meth-
od through the surfactant-assembling pathway. XRD patterns
of TBbenzyl-n-N are presented in Fig. 1. The XRD patterns of
TBbenzyl-n-N (n = 10, 20) exhibit three well-resolved diffrac-
tion peaks (d100, d110 and d200), which can be assigned to the
well-ordered two-dimensional hexagonal structure. TBbenzyl
-
30-N displays a sharp d100 diffraction peak and a weak d110 dif-
fraction peak, whereas TBbenzyl-40-N exhibits a broad diffrac-
tion peak. The intensity of d100 decreases with the amount of
Bbenzyl in the initial gel mixture, indicating that the mesostruc-
tural order of the materials is decreased. A gradual decrease of
the d100 spacing from TBbenzyl-10-N to TBbenzyl-40-N is also
observed, possibly due to the strong interaction between the or-
ganic groups and the tails of the surfactant molecules [23]. The
interaction between the organic groups and the tails of the sur-
factants becomes stronger with increasing amounts of organic
groups in the initial gel mixture. The organic groups can be
pulled further into the surfactant micelles by the stronger in-
teraction, leading to the decrease in the d100 spacing. These
results imply that larger amounts of organic groups were incor-
porated into the mesoporous framework from TBbenzyl-10-N to
TBbenzyl-40-N.
2.4. Catalytic reaction
Asymmetric transfer hydrogenation of ketones was carried
out under argon atmosphere. The rhodium-containing meso-
porous material (7.36 µmol Rh) in a 50-mL flask was degassed
for 1 h at room temperature, followed by the addition of freshly

68
D. Jiang et al. / Journal of Catalysis 239 (2006) 65–73
Nitrogen adsorption–desorption isotherms of TBbenzyl-n-N
The BET surface area and pore volume decrease gradually
from TBbenzyl-10-N to TBbenzyl-30-N (Table 1). TBbenzyl-40-N
has the lowest BET surface area, pore volume, and pore diame-
ter among the materials studied.
TBpropyl-20-N with propyl group as a linker was synthesized
similarly to TBbenzyl-n-N by co-condensation of TEOS with
Bpropyl under basic conditions. A sharp d100 diffraction peak
is observed in the XRD pattern of TBpropyl-20-N, suggesting
that the material has an ordered mesoporous structure (Fig. 1).
TBpropyl-20-N and TBbenzyl-10-N have comparable surface ar-
eas, pore diameters, pore volumes, and wall thicknesses (Ta-
ble 1 and Fig. 2).
are shown in Fig. 2. TBbenzyl-n-N (n = 10, 20, 30) shows a typ-
ical type IV isotherm, which is characteristic of mesoporous
material with an ordered arrangement of cylindrical pores. The
capillary condensation step is less developed for the materials
with higher amounts of organic groups. TBbenzyl-40-N exhibits
a type I isotherm, indicating that TBbenzyl-40-N has a microp-
orous structure. It is noteworthy that an additional uncommon
type-H4 hysteresis loop at P/P0 above 0.4 is observed for
TBbenzyl-30-N. The existence of this type-H4 hysteresis loop
may indicate that some structural defects are formed in the
hexagonal channel matrix of TBbenzyl-30-N [24,25]. The TEM
image further clarifies that void defects are irregularly dispersed
throughout the ordered pore wall of TBbenzyl-30-N, whereas no
3.2. Compositional analysis of the mesoporous materials with
trans-(1R,2R)-diaminocyclohexane in the framework
void defects are observed in other samples, including TBbenzyl
-
10-N (Fig. 3). The presence of void defects in the mesostruc-
tured framework of TBbenzyl-30-N can explain the uncommon
hysteresis loops seen in the N2 sorption isotherms [24,25]. In
our previous work on thioether-functionalized PMOs, we also
found that void defects can be formed on the materials when
the amount of organic precursor in the initial gel mixture is in
the range of certain values [26]. The existence of void defects
in TBbenzyl-30-N will facilitate diffusion of the guest molecules
through the nanochannels of the mesoporous material [27,28].
The integrity of the organic group in the mesoporous ma-
terial was identified by 13C CP-MAS NMR spectroscopy.
For TBbenzyl-10-N, the 13C CP-MAS NMR spectrum (Fig. 4)
displays signals corresponding to cyclic CH2 at 25.2 and
29.8 ppm [11]. The signals centered at 53.7 and 59.8 ppm can
be assigned to NCH and NCH2, respectively [11]. The exis-
tence of the phenyl group is identified by the signals at 128.3,
134.6, and 141.6 ppm [11]. For TBpropyl-20-N, the 13C CP-
MAS NMR spectrum clearly shows the signals corresponding
to cyclic CH2 and the inner carbon atom of n-propyl groups in
the range of 20.0–29.8 ppm and NCH/NCH2 in the range of
50.0–60.5 ppm. The sharp signal at 9.9 ppm is ascribed to the
carbon atom bonded to silicon [21]. In the 13C CP-MAS NMR
spectrum of TBpropyl-20-N, the absence of a chemical shift at
67–69 ppm confirms that the surfactant is removed almost com-
pletely using the EtOH–HCl extraction method [29].
29Si CP–MAS NMR provides direct evidence for the pres-
ence of organic moieties as a part of the silica wall of the meso-
porous materials (Fig. 5). For TBbenzyl-10-N, and TBpropyl-20-
N, the signals at about −110 and −100 ppm can be assigned to
Q4 [Si(OSi)4] and Q3 [Si(OSi)3(OH)] silicon species, respec-
tively. The signals in the range of −50 to −80 ppm are from
silicon species (T sites) bridged by the organic group. The ex-
istence of T sites confirms that the organic moiety is covalently
bonded to the inorganic silicate framework. The 13C and 29Si
Fig. 2. Nitrogen adsorption–desorption isotherms of TB
-n-N [n is the
benzyl
mole percent of 2B
mole percent of 2B
/(2B
/(2B
+ TMOS)] and TB
-20-N [20 is the
propyl
benzyl
benzyl
propyl
+ TEOS)].
propyl
Fig. 3. TEM images of TB
-10-N and TB
-30-N [n is the mole percent of 2B
/(2B
benzyl
+ TMOS)].
benzyl
benzyl
benzyl

D. Jiang et al. / Journal of Catalysis 239 (2006) 65–73
69
c
Table 1
Physico-chemical data for the mesoporous silicas with trans-(1R,2R)-diaminocyclohexane in the framework
a
b
Sample
d spacing
(nm)
BET surface
area (m /g)
Pore volume
(cm /g)
Pore diameter
(nm)
a
Wall thickness
(nm)
0
2
3
(nm)
TB
TB
TB
TB
TB
-10-N
-20-N
-30-N
-40-N
-20-N
3.8
3.5
3.6
3.2
3.7
1071
908
604
260
910
0.55
0.42
0.40
0.15
0.53
2.3
2.0
2.1
< 2
2.4
4.3
4.0
4.2
3.7
4.3
2.2
2.0
2.1
–
benzyl
benzyl
benzyl
benzyl
propyl
1.9
a
b
c
Total pore volume obtained from the volume of N adsorbed at P/P = 0.99.
2
0
√
/ 3.
100
a
0
is the lattice parameter, a = 2d
0
Wall thickness = a − pore diameter.
0
13
Fig. 4. Solid state C CP-MAS NMR spectra of TB
-10-N [10 is the mole percent of 2B
/(2B
+ TMOS)] and TB
-20-N [20 is the mole
propyl
benzyl
benzyl
benzyl
percent of 2B
/(2B
+ TEOS)].
propyl
propyl
29
Fig. 5. Solid state Si CP-MAS NMR spectra of TB
-10-N [10 is the mole percent of 2B
/(2B
+ TMOS)] and TB
-20-N [20 is the mole
benzyl
benzyl
benzyl
propyl
percent of 2B
/(2B
+ TEOS)].
propyl
propyl
CP-MAS NMR spectra demonstrate that the trans-(1R,2R)-
diaminocyclohexane moiety is successfully incorporated in the
framework of mesoporous silica.
The content of chiral ligand in the materials was calculated
from N elemental analysis (Table 2). With increasing amounts
of Bbenzyl in the initial gel mixture, more Bbenzyl can be incor-
porated into the mesoporous silica. According to the theoretical
estimation, ∼65 to ∼73 mol% of Bbenzyl or Bpropyl in the initial
gel mixture could be incorporated into the mesoporous silicas.
TBbenzyl-20-N shows two sharp peaks in the range of 200–
250 nm originating from the phenyl group in the material. Com-
plexing with [Rh(cod)Cl]2, TBbenzyl-20-NRh exhibits a new
peak at 380 nm. A peak at 230 nm along with a shoulder peak in
the range of 300–400 nm is observed in the UV–vis spectrum of
TBpropyl-20-N. For the material complexed with [Rh(cod)Cl]2,
the shoulder peak at 300–400 nm disappears, and a new peak
appears at 380 nm. These findings confirm that the rhodium
complex coordinates with trans-(1R,2R)-diaminocyclohexane
in the materials [11].
3.3. Catalytic properties of the mesoporous organosilicas
coordinated to [Rh(cod)Cl]2
3.3.1. Comparison of the catalytic performance of different
mesoporous materials coordinated with [Rh(cod)Cl]2
Table 2 lists the Rh content and Rh/chiral ligand ratio for all
of the catalysts studied. The Rh/chiral ligand ratio is a direct in-
dication of how much chiral ligand could be coordinated by the
The catalysts were prepared by complexing TBbenzyl-n-N or
TBpropyl-20-N with [Rh(cod)Cl]2. The coordination of rhodium
complex with trans-(1R,2R)-diaminocyclohexane was charac-
terized by diffuse-reflectance UV–vis spectroscopy (Fig. 6).

70
D. Jiang et al. / Journal of Catalysis 239 (2006) 65–73
Table 2
The catalytic results of asymmetric transfer hydrogenation of acetophenone
Rh-containing catalyst
+
+
ꢀ
i-PrOK
a
b
Catalyst
Chiral ligand
(mmol/g)
Rh
(mmol/g)
Rh/chiral ligand
(%)
Conversion
(%)
ee (%)
(S)
TOF
(h
−1
)
TB
TB
TB
TB
TB
TB
-10-NRh
-20-NRh
-30-NRh
-40-NRh
-20-NRh
0.45 (0.68)
0.75 (1.14)
1.02 (1.48)
1.21 (1.74)
0.93 (1.28)
0.93 (1.28)
0.38 (1.36)
–
0.10
0.29
0.17
0.10
0.13
0.13
0.24
–
22
39
17
8
14
14
63
–
97
93
96
39
16
36
95
45
26
27
30
22
8
8
22
21
414
272
336
106
22
–
benzyl
benzyl
benzyl
benzyl
propyl
propyl
c
-20-NRh
d
EM-4-NRh
–
–
B
benzyl
a
The quantity of chiral ligand in the solids is calculated from elemental analyses. The value in the parenthesis is the theoretical estimation of chiral ligand in the
solids, which is calculated based on the formula (O SiRSiO
)
(SiO )
, while R is the chiral ligand and n = 10, 20, 30, 40.
1.5
1.5 2/n
2 (100−n)
b
Conversion is based on acetophenone. Reaction conditions: catalysts (7.36 µmol Rh), i-PrOH (12 mL), i-PrOK (0.1 mmol), acetophenone (2 mmol), reaction
◦
temperature (83 2 C), reaction time (3 h). Turnover frequencies (TOF) were determined after 0.5 h.
c
Reaction time (22 h).
d
From Ref. [21] and reaction time (22 h).
Fig. 6. UV–vis spectra of [Rh(cod)Cl] (solid powder), TB
-20-N [20 is the mole percent of 2B
/(2B
+ TMOS)] and TB
-20-N [20 is the mole
2
benzyl
benzyl
benzyl
propyl
percent of 2B
/(2B
+ TEOS)] before and after coordinating with [Rh(cod)Cl] .
propyl
propyl
2
[Rh(cod)Cl]2 complex. The Rh content and Rh/chiral ligand
ratio do not increase linearly with incorporation of increasing
amounts of chiral ligand into the materials. TBpropyl-20-NRh
gives a Rh/chiral ligand ratio of 14%, and TBbenzyl-20-NRh
gives the highest Rh/chiral ligand ratio, 39%. TBpropyl-20-
NRh and TBbenzyl-10-NRh have similar mesoporous structures
with comparable BET surface areas, pore diameters, pore vol-
umes, and wall thicknesses (Table 1), but TBpropyl-20-NRh has
a higher ligand content than TBbenzyl-10-NRh. The different
Rh/ligand ratios observed for the two kinds of materials is re-
lated mainly to the different linkers used for binding the chiral
ligand. Because propyl group is softer than benzyl group, trans-
(1R,2R)-diaminocyclohexane with propyl group as a linker has
more freedom to interact with the hydrophobic chain of the
surfactant compared with trans-(1R,2R)-diaminocyclohexane
with benzyl group as a linker. Therefore, more chiral ligands
are buried in the mesoporous wall in TBpropyl-20-N than in
TBbenzyl-10-N. This leads to lower accessibility of chiral lig-
ands in TBpropyl-20-N.
The structural order also has a significant influence on the
accessibility of the organic functionality in the mesoporous
framework. TBbenzyl-10-N and TBbenzyl-20-N, with high BET
surface areas and pore volumes, display a high Rh/chiral ligand
ratio, whereas TBbenzyl-30-N, with moderate surface area and
pore volume, has a moderate Rh/chiral ligand ratio. TBbenzyl
-
40-N has a very low Rh/chiral ligand ratio (8%), because of its
microporous structure. The elemental analysis results indicate
that the amount of [Rh(cod)Cl]2 coordinated with the ligands
in the mesoporous framework depends on the rigidity of the or-
ganic functionality and on the structural order of the materials.
The prepared catalysts were first tested for the asymmetric
transfer hydrogenation of acetophenone (Table 2). 2-Phenethyl
alcohol is the only product detected. The TBpropyl-20-NRh
with propyl group as a linker shows a conversion of 16%
with only 8% ee for 3 h. The conversion is increased to
only 36% with the same ee by further increasing the reaction
time to 22 h. TBbenzyl-n-NRh (n = 10, 20, 30) with benzyl
group as a linker exhibits 93–97% conversion with 26–30%

D. Jiang et al. / Journal of Catalysis 239 (2006) 65–73
71
ee. TBbenzyl-n-NRh (n = 10, 20, 30) exhibits much higher
activity and ee than its homogeneous counterpart (45% con-
version with 21% ee) and TBpropyl-20-NRh. The solid cata-
lyst is superior to the homogeneous catalyst in our case, due
mainly to site isolation in the solid catalyst. The lower ac-
tivity and ee of TBpropyl-20-NRh compared with TBbenzyl-n-
NRh (n = 10, 20, 30) are due in part to the fact that fewer
active sites in TBpropyl-20-NRh can be accessed by the reac-
tant molecules. This is consistent with the results of elemental
analysis. Furthermore, from our previous work [21], we know
that EM-4-NRh (trans-(1R,2R)-diaminocyclohexane function-
alized mesoporous ethanesilica with propyl group as a linker
in the channel) has lower activity and ee than TBbenzyl-n-
NRh (n = 10, 20, 30) (Table 2). Moreover, EM-4-NRh has
a higher Rh/ligand ratio than TBbenzyl-n-NRh. Therefore, in-
troduction of benzyl group into the N atom of trans-(1R,2R)-
diaminocyclohexane contributes mainly to the unique catalytic
property of TBbenzyl-n-NRh.
The foregoing results demonstrate that a linker with spa-
tial rigidity and electron-withdrawing ability is favorable for
high catalytic activity and enantioselectivity. The nitrogen atom
becomes a stereogenic center after binding to rhodium com-
plex. The chiral microenvironment in the catalysts can be sub-
tly modified by substituting one hydrogen atom with a differ-
ent organic group on the nitrogen atom in the trans-(1R,2R)-
diaminocyclohexane. The subtle modification of the chiral lig-
and will change the catalytic activity and enantioselectivity of
the resulting catalyst. Moreau et al. [11] reported that, com-
pared with alkyl substituents on the nitrogen atom, the bulky
benzylic group leads to higher ee values, which is consistent
with our results.
(414 h−1) > TBbenzyl-30-NRh (336 h−1) > TBbenzyl-20-NRh
(272 h−1) ꢁ TBbenzyl-40-NRh (106 h−1) (Table 2). TBbenzyl
-
10-NRh shows the highest initial rate because of its high BET
surface area and large pore volume. Interestingly, TBbenzyl-30-
NRh, with a lower BET surface area, has a faster initial rate
than Rh-TBbenzyl-20-N. This can be explained by the existence
of void defects in TBbenzyl-30-N. The void defects amid the
mesoporous channels make the reactants diffuse quickly within
the porous framework of the catalyst, which compensates for
its lower BET surface area and results in a higher reaction
rate. TBbenzyl-40-NRh shows the lowest initial rate because
of its low BET surface area and microporosity. These results
again demonstrate the important role of the ordered mesoporous
structure in the catalytic performance of the catalyst.
3.3.3. Asymmetric transfer hydrogenation of ketones catalyzed
by TBbenzyl-20-NRh
To investigate the scope of the solid catalysts, asymmet-
ric transfer hydrogenation of a number of different ketones
was carried out under identical conditions. The catalytic re-
sults are summarized in Table 3. All of the ketones can be
converted to their corresponding alcohols on TBbenzyl-20-NRh.
Acetophenones bearing electron-withdrawing substituents give
a faster reaction rate than those with electron-donating sub-
stituent, whereas enantioselectivity varies in the range of 18–
42%. The conversion of the more crowded 1-acetylnaphthalene
and 2-acetylnaphthalene is similar; however, the enantioselec-
tivity is quite different (24% ee for 1-acetylnaphthalene and
61% ee for 2-acetylnaphthalene). The homogeneous catalyst
exhibits similar results, excluding the possibility that pore con-
finement of the mesoporous materials leads to the different ee
values for the two reactions. This result indicates that TBbenzyl
-
3.3.2. Relationship between mesoporous structure and
catalytic performance
20-NRh is sensitive to the different reaction substrates, for
example, the different substitution position of the ketone func-
tional group on an aromatic ring. The slower reaction rates of
1-acetylnaphthalene and 2-acetylnaphthalene may be due to the
difficult diffusion of large molecules in the channel of the meso-
porous material.
The relationship between the structural properties and cat-
alytic activity of the catalysts can be clearly seen from the
time profiles of the catalytic process. Fig. 7 shows the con-
version of acetophenone as a function of reaction time cat-
alyzed by TBbenzyl-n-NRh (n = 10, 20, 30, 40). The initial rate
of TBbenzyl-n-NRh is in the following order: TBbenzyl-10-NRh
3.3.4. Hot filtration reaction
To confirm that the catalytic reaction indeed occurs on the
solid catalyst, the TBbenzyl-30-NRh catalyst was removed from
the reaction mixture when the conversion of acetophenone
reached 46% (Fig. 8), and then the filtrate was stirred continu-
ously at the same reaction temperature to check whether the ho-
mogeneous catalyst (possibly leaching from the solid catalyst)
could contribute to the reaction. An additional 6% conversion of
acetophenone with the same ee value is observed after another
7 h, whereas 96% conversion can be obtained with TBbenzyl-30-
NRh after 3 h. Rh elemental analysis of the filtrate after reaction
showed that ∼1.6% of the Rh in the solid material leached into
the solution after the first reaction cycle. (The Rh in the filtra-
tion may also come from the nanosized catalyst, which cannot
be separated by filtration.) It is obvious that asymmetric trans-
fer hydrogenation of acetophenone is catalyzed mainly by the
solid catalyst.
Fig. 7. Asymmetric transfer hydrogenation of acetophenone. Conversion of ace-
tophenone as a function of reaction time on TB
-n-NRh [n is the mole
benzyl
percent of 2B
/(2B
+ TMOS)] catalysts.
benzyl
benzyl

72
D. Jiang et al. / Journal of Catalysis 239 (2006) 65–73
Table 3
Asymmetric transfer hydrogenation of aromatic ketones on TB
-20-NRh
benzyl
a
Substrate
Conversion (%)
93
ee (%) (S)
27
91
96
42
18
Fig. 8. Asymmetric transfer hydrogenation of acetophenone on the
TB -30-NRh [30 is the mole percent of 2B /(2B
+ TMOS)]
benzyl
benzyl
benzyl
catalyst: (P) conversion of acetophenone as a function of reaction time
(TB -30-NRh); (1) conversion of acetophenone as a function of reaction
benzyl
time (TB
-30-NRh was removed after 0.25 h).
benzyl
mesoporous channel further improve the reaction rate of the
catalyst. The chiral mesoporous material combined with Rh is
also an active catalyst for the reduction of a wide range of ke-
tones, with enatioselectivity as high as 61% for the transfer hy-
drogenation of 2-acetylnaphthalene. These chiral mesoporous
organosilicas are potential catalytic materials for asymmetric
reactions. The enantioselectivity of these novel mesoporous
materials could be further improved through modification of
the chiral ligand bridged in the mesoporous framework.
89
63
23
20
Acknowledgments
62 (94)
61 (28)
24 (26)
61 (55)
Financial support for this work was provided by the Na-
tional Natural Science Foundation of China (grants 20303020,
20321303, and 20503028), the National Basic Research Pro-
gram of China (grant 2003CB615803), and the Talent Science
Program of the Chinese Academy of Sciences.
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